Medical Biochemistry, Fourth Edition

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MEDICAL BIOCHEMISTRY FOURTH EDITION

This Page Intentionally Left Blank

MEDICAL BI0C HEMISTRY FOURTH EDITION

N. V. BHAGAVAN Department of Biochemistry and Biophysics John A. Burns School of Medicine University of Hawaii

San Diego San Francisco New York Boston London Sydney Tokyo

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Copyright 9 2002 by HARCOURT/ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887-6777 Academic Press A Division of Harcourt, hw. 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.academicpress.com Academic Press Harcourt Place, 32 Jamestown Road, London NWI 7BY, UK http://www.academicpress.com H a r c o u r t / A c a d e m i c Press A Division of Harcourt, hw. 200 Wheeler Road, Burlington, Massachusetts 01803 http://www.harcourt-ap.com Library of Congress Catalog Card Number: 2001090826 International Standard Book Number: 0-12-095440-0 PRINTED IN CANADA 02 03 04 05 06

FR

9

8

7

6

5

4

3

2

CONTENTS

Contributors Preface

1.4

xxv

H+ Concentration and pH

15

xxvii

Acknowledgments

CHAPTER

Supplemental Readings and References 16

xxix

Properties of Water 16 Acid-Base Chemistry and Respiratory Function of Hemoglobin 16 Nuclear Magnetic Resonance and Magnetic Resonance Imaging 16

1

Water, Acids, Bases, and Buffers 1.1

Properties of Water

1

Hydrogen Bonding l Physical Properties 2 Solutes, Micelles, and Hydrophobic Interactions 2 Colligative Properties 3 Dissociation of Water and the pH Scale

1.2

CHAPTER

Amino Acids

3

2.1

L-(x-AminoAcids: Structure

2.2

Classification

4

Measurement of pH

11

Nuclear Magnetic Resonance and Magnetic Resonance Imaging 11 Gibbs-Donnan Equilibrium 13

Glycine

18

Alanine

20

18

Valine, Leucine, and Isoleucine Phenylalanine

20

Tryptophan

20

Methionine

21

Proline

21

Acidic Amino Acids Aspartic Acid Glutamic Acid

17

17

Nonpolar Amino Acids

Buffers

Henderson-Hasselbalch Equation 5 Buffer Systems of Blood and Exchange of O2 and CO2 6 Blood Buffer Calculations 9 Nonbicarbonate Buffers in Blood 10

1.3

2

23 23

23

20

vi

Contents Basic Amino Acids Lysine


23

23

Histidine

24

Arginine

24

Neutral Amino Acids 24 Serine

CHAPTER

Threonine

25

Tyrosine

25

Glutamine

Three-Dimensional

25

Cysteine

Asparagine

4.1

25 26

Electrolyte and Acid-Base Properties

2.4

Chemical Reactions of Amino Acids

27

4.2

Primary Structure Peptide Bond

31 4.3


53

53

Secondary Structure

53

or-Helix 54 /3-Pleated Sheet 55 /3-Turns 56 Random Coil 56 Determination of Secondary Structure by Using Circular Dichroism (CD) Spectroscopy 56 Other Types of Secondary Structure 56

3

Protein Isolation and Determination of Amino Acid Sequence

3.1

Quantitative Determination of Proteins 35

3.2

Determination of Primary Structure

3.3

Separation of Proteins 36

36

Separation by Molecular Size 37 Separation by Chromatography 37 Affinity Tag Chromatography 39 Separation by Electrophoresis 39

4.4

Tertiary Structure

4.5

Quaternary Structure

4.6

Denaturation

4.7

Protein Folding and Associated Diseases 59

57

58

58

Supplemental Readings and References 64 Protein Folding and Its Defects 64 Alzheimer's Disease, p53, and Prions

Capillary Electrophoresis 41 Separation by Solubility 41

3.5

Amino Acid Composition 42

3.6

Amino Acid Sequence Determination 43 Identification of the C-Terminal Residue 44 Selective Hydrolysis Methods 45 Peptide Sequence Confirmation 46

Identification of the N-Terminal Residue

Fmoc Solid-Phase Peptide Synthesis

Attractive and Repulsive Forces in Proteins 52 Attractive Forces 52 Repulsive Forces 52

2.3

3.4

Structure

of Proteins

Unusual Amino Acids 26 Amino Acids Used as Drugs 26

CHAPTER

4

24

48

43

CHAPTER

5

Thermodynamics,

Chemical Kinetics,

and Energy Metabolism

5.1

64

Methods of Altering the Rate of Reactions 67

Contents

vii

5.2

Thermodynamics

5.3

Standard Free Energy of Hydrolysis of ATP 73

5.4

Chemical Kinetics

5.5

EnergyMetabolism

5.6

Obesity

68

6.5

Kinetics of Ligand-Receptor Interaction 104

6.6

Mechanisms of Enzyme Action Coenzymes, Prosthetic Groups, and Cofactors 106

75 77


82

Biochemical Mediators of Obesity

82


CHAPTER

7

Enzymes CHAPTER

6

Enzymes

I" G e n e r a l P r o p e r t i e s ,

Nomenclature

6.2

Catalysis

Types of Regulation 109

7.2

AIIosteric Enzyme Regulation 111 Kinetics of Allosteric Proteins 111 Examples of Allosteric Proteins 112 Theoretical Models for Allosteric Effect

85

117


86

Specificity of Enzyme Catalysis 86 Active Site and Enzyme-Substrate Complex 86 Factors Governing the Rate of Enzyme-Catalyzed Reactions 86

Enzyme Regulation (General) 119 Allosteric Properties of Aspartate Transcarbamoylase and Hemoglobin Aspartate Transcarbamoylase 119 Hemoglobin 119

Effect of Temperature 87 Effect of pH

II" R e g u l a t i o n

7.1

Kinetics, and Inhibition

6.1

105

87

Effect of Concentration of Enzyme and Substrate 88

CHAPTER

Michaelis-Menten Treatment of the Kinetic Properties of an Enzyme 88

Enzymes

Linear Plots for Michaelis-Menten Expression 91

8.1

6.3

Kinetics of Enzymes Catalyzing Two-Substrate Reactions 92

6.4

Inhibition

92

III: Clinical Applications

Diagnosis and Prognosis of Disease Factors Affecting Presence and Removal of Intracellular Enzymes from Plasma 122 Measurement of Enzyme Activity 124

8.2

Reversible Inhibition 92 Competitive Substrates in Treatment of Some Intoxications 96

Irreversible Inhibition 98 Inactivation and Reactivation of Cytochrome Oxidase 99 Proteinase Inhibitors and Their Clinical Significance 102

8

Serum Markers in the Diagnosis of Tissue Damage 126 Myocardium 126 Pancreas 127 Liver 127

8.3

Enzymes as Analytical Reagents

128

8.4

Enzymes as Therapeutic Agents

130

121

viii

Contents


Molecular Mimicry 171 Disorders of Red Blood Cell Membrane Skeleton 171

CHAPTER9

Simple Carbohydrates Heteropolysaccharides II" Proteoglycans and Peptidoglycans

9.1 Classification 133 Monosaccharides 133 Some Physiologically Important Monosaccharide Derivatives 139 Sugar Alcohols Sugar Acids

139

Glycosides

Polysaccharides

174

Turnover of Collagen and Tissue Repair 142

Elastin

142

Turnover of Elastin

144

Proteoglycans

147

178

179

Structure and Function

143

Disaccharides

173


141

Sugar Phosphates Deoxy Sugars

Collagen

173

Collagen Types 173

140

Amino Sugars

11.1 Protein Fibers and Proteoglycans

179

181

182

Types, Structures, and Functions of Glycosaminoglycans 182


Turnover of Proteoglycans and Role of Lysosomes 186 Mucopolysaccharidoses

11.2 Peptidoglycans

187

188

Lysis of Peptidoglycans by Lysozymes

Heteropolysaccharides I- Glycoproteins and Glycolipids

193

11.3 Lectins 194 Supplemental Readings and References 195

10.1 Glycoproteins 153 10.2 Cell Membrane Constituents 156 10.3 Cell-Surface Glycoproteins 161 Red Blood Cell Membrane and Membrane Skeleton Proteins 163 Blood Group Antigens 166

10.4 Serum Glycoproteins

168

10.5 Molecular Mimicry of Oligosaccharides and Host Susceptibility 170 Supplemental Readings and References 170 Extracellular Matrix 170 Blood Group Antigens 170

Gastrointestinal Digestion and Absorption 12.1 Anatomy and Physiology of the GITract 197 Mouth and Esophagus 197 Stomach 198 Small Intestine 199 Formation, Secretion, and Composition of Bile 199 Exocfine Pancreatic Secretion 201

ix

Contents

Composition of Pancreatic Juice Large Intestine 202

202

Source and Entry of Glucose into Cells Reactions of Glycolysis 226

225

Phosphorylation of Glucose 226

12.2 Gastrointestinal Hormones 202

Isomerization of Glucose-6-Phosphate to Fructose-6-Phosphate 229

Gastrin 203 Peptic Ulcer Disease 207 Cholecystokinin 208 Secretin 208 Gastric Inhibitory Peptide 208

Phosphorylation of Fructose-6-Phosphate to Fructose-l,6-Bisphosphate 229 Cleavage of Fructose-l,6-Bisphosphate into Two Triose Phosphates 229 Isomerization of Dihydroxyacetone Phosphate to Glyceraldehyde 3-Phosphate 229

12.3 Digestion and Absorption of Major Food Substances 208 Carbohydrates

Dehydrogenation of Glyceraldehyde 3-Phosphate 230

208

Phosphorylation of ADP from 1,3-Bisphosphoglycerate 231

Digestion of Starch 209 Brush-Border Surface Hydrolysis

211

Isomerization of 3-Phosphoglycerate to 2-Phosphoglycerate 231

Transport of Monosaccharides into the Enterocyte 211 Na+,K+-ATPase 212

Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate 232

Disorders of Carbohydrate Digestion and Absorption 212

Phosphorylation of ADP from Phosphoenolpyruvate 232

Proteins

214

Digestion

Reduction of Pyruvate to Lactate

214

Absorption of Amino Acids and Oligopeptides Disorders of Protein Digestion and Absorption

Lipids

215 216

216

Intraluminal Phase

216

Intracellular (Mucosal) Phase

233

Alternative Substrates of Glycolysis 234 Role of Anaerobic Glycolysis in Various Tissues and Cells 235 Glycolytic Enzyme Deficiencies in Erythrocytes 235

218

13.2 Pyruvate Metabolism

Secretion 218 Disorders of Lipid Digestion and Absorption General Malabsorptive Problems

218

D-Lactic Acidosis

218

235

Lactic Acidemia and Lactic Acidosis 236

Oxidation of Pyruvate to Acetyl-CoA

12.4 Absorption of Water and Electrolytes

222

Disorders of Fluid and Electrolyte Absorption 222

12.5 Thermic Effectof Food 224 Supplemental Readings and References 224

236

Regulation of Pyruvate Dehydrogenase Activity

239

Abnormalities of Pyruvate Dehydrogenase Complex 240

13.3 TricarboxylicAcid (TCA)Cycle Reactions of TCA Cycle

241

241

Condensation of Acetyl-CoA with Oxaloacetate to Form Citrate 241 Isomerization of Citrate to Isocitrate

241

Oxidative Decarboxylation of lsocitrate to ~-Ketoglutarate 243

13 Carbohydrate Metabolism I: Glycolysis and TCA Cycle 13.1 Glycolysis

236

225

Oxidative Decarboxylation of ot-Ketoglutarate to Succinyl-CoA 243 Conversion of Succinyl-CoA to Succinate Coupled to Formation of GTP 243 Dehydrogenation of Succinate to Fumarate

244

X

Contents

Hydration of Fumarate to Malate

244

Dehydrogenation of Malate to Oxaloacetate 244 Stereochemical Aspects of the TCA Cycle 244 Amphibolic Aspects of the TCA Cycle 244

Base Substitution Mutations 268 Transfer RNA (tRNA) Mutations 269 mtDNA Deletions and Duplications 270

14.7 Other Reducing-Equivalent Transport and Oxygen-Consuming Systems 270

Regulation of the TCA Cycle 245 Energetics of the TCA Cycle 245



15 Carbohydrate Metabolism II" Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways

Electron Transport and Oxidative Phosphorylation 14.1 Mitochondrial Structure and Properties 248 Submitochondrial Particles

15.1 Gluconeogenesis 275

251

Components of the Electron Transport Chain 251

Electron Transport Complexes Complex I

251

251

Complex H 253 Complex III

254

Complex IV

255

Carboxylation of Pyruvate to Oxaloacetate 279 Conversion of Oxaloacetate to Phosphoenolpyruvate 280

Organization of the Electron Transport Chain 256

Conversion of Fructose-l,6-Bisphosphate to Fructose-6-Phosphate 280

14.2 Oxidative Phosphorylation 257 Mechanisms of Oxidative Phosphorylation Uncoupling Agents of Oxidative Phosphorylation 261

257

14.3 Mitochondrial Energy States 263 Energy-Linked Functions of Mitochondria Other Than ATP Synthesis 263 Transport of Cytoplasmic NADH to Mitochondria 264

14.4 The Mitochondrial Genome 266 Mitochondrial Biogenesis 267 Expression of mtDNA 267

14.5 Nuclear Control of Respiratory Chain Expression 267

Metabolic Role 275 Gluconeogenic Pathway 276 Gluconeogenic Precursors 278 Regulation of Gluconeogenesis 279

Conversion of Glucose-6-Phosphate to Glucose 281

Abnormalities of Gluconeogenesis

15.2 Glycogen Metabolism

283

Glycogen Synthesis 283 Glycogen Breakdown 285 Regulation of Glycogen Metabolism 286 Muscle 286 Control of Glycogen Synthase

286

Control of Glycogen Phosphorylase

288

Integrated Regulation of Muscle Glycogen Metabolism 289

Liver

290

Control of Glycogen Synthase

14.6 Mitochondrial Diseases 268

282

290

Control of Glycogen Phosphorylase

290

Contents

xi


Integrated Regulation of Liver Glycogen Metabolism 290

Glycogen Storage Diseases

291

15.3 Alternative Pathways of Glucose Metabolism and Hexose Interconversions 291 Glucuronic Acid Pathway 291 Fructose and Sorbitol Metabolism 296 Galactose Metabolism 297 Metabolism of Amino Sugars 298 Pentose Phosphate Pathway 298 Oxidative Phase

General 329 Glycoproteins 329 GPI-Anchored Proteins 329 Glycosphingolipids 329 Glycosaminoglycans and Proteoglycans 329 Peptidoglycans, Antibiotics, and Resistance

330

300

Nonoxidative Phase

300

Protein and Amino Acid Metabolism

Pentose Phosphate Pathway in Red Blood Cells 301

17.1 Essential and Nonessential Amino Acids 331

Glucose-6-Phosphate Dehydrogenase Deficiency 302

Phagocytosis and the Pentose Phosphate Pathway 304


Nitrogen Balance 332 Quality and Quantity of Dietary Protein Requirement 332 Protein Energy Malnutrition 333 Transport of Amino Acids into Cells 333 General Reactions of Amino Acids 335 Deamination

335

Dehydrogenation of L-Glutamate Transamination

Carbohydrate Metabolism III: Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans 16.1 Biosynthesis of Glycoproteins

Penicillins and Cephalosporins

327

337

Role of Specific Tissues in Amino Acid Metabolism 338

17.2 Metabolism of Ammonia Urea Synthesis

311

N-Glycan Asn-Linked Glycoproteins 312 Phosphorylation of Oligosaccharide Chains on Lysosomal Enzymes 315 Inhibitors of Glycoprotein Biosynthesis 315 O-Glycan Ser(Thr)-Linked Oligosaccharides 317 Biosynthesis of GPI-Anchored Proteins 318 Biosynthesis of Glycosphingolipids 320 Biosynthesis of Glycosaminoglycans 322

16.2 Biosynthesis of Peptidoglycans

336

324

340

340

Formation of Carbamoyl Phosphate Formation of Citrulline

341

342

Formation of Argininosuccinate

342

Formation of Arginine and Fumarate Formation of Urea and Ornithine Energetics of Ureagenesis Hyperammonemias

342

343

343

343

17.3 Metabolism of Some Individual Amino Acids 345 Arginine 345 Metabolism and Synthesis of Nitric Oxide

345

lsoforms (Also Known as Isozymes) of Nitric Oxide Synthase 346

xii

Contents Signal Transduction of NO Glycine

346

18.2 Metabolism of Ketone Bodies

347

Disorders of Glycine Catabolism Creatine and Related Compounds

Physiological and Pathological Aspects of Metabolism of Ketone Bodies 376

348 348

Use of Creatine as a Dietary Supplement Serine 349 Proline 349 Histidine

349

351

Branched-Chain Amino Acids 352 Sulfur-Containing Amino Acids 353 Methionine Cysteine

353

354

Abnormalities Involving Sulfur-Containing Amino Acids 354 Homocysteine

354

Phenylalanine and Tyrosine Phenylketonuria (PKU) Melanin

358

Abnormalities of Tyrosine Metabolism

18.3 Metabolism of Ethanol 377 18.4 Synthesis of Long-Chain Saturated Fatty Acids 379 Functional Organization of Fatty Acid Synthase 383 Sources of NADPH for Fatty Acid Synthesis Source and Transport of Acetyl-CoA 384 Regulation of Fatty Acid Synthase 384 Fatty Acid Elongation 385

18.5 Metabolism of Unsaturated Fatty Acids 386

356

360

Tryptophan

374

360

Structure and Nomenclature of Unsaturated Fatty Acids 386 Functions of Unsaturated Fatty Acids 386

361

18.6 Nonessential Fatty Acids Supplemental Readings and References 363

18,7 trans-Fatty Acids

386

388

18.8 Essential Fatty Acids

388

Deficiency of Essential Fatty Acids

18.9 Metabolism of Eicosanoids

Lipids I: Fatty Acids and Eicosanoids 18.1 Oxidation of Fatty Acids

366

Activation of Fatty Acids 366 Transport of Acyl-CoA to Mitochondrial Matrix 367 /3-Oxidation 368 Energetics of/3-Oxidation 371 Regulation of Fatty Acid Oxidation 372 Peroxisomal Fatty Acid Oxidation 372 Other Pathways of Fatty Acid Oxidation 373 Propionyl-CoA Oxidation m-Oxidation

374

Oxidation of Mono- and Polyunsaturated Fatty Acids 374

389

Biological Properties of Prostanoids Leukotrienes 396

395


19 Lipids lI- Phospholipids, Glycosphingolipids, and Cholesterol

373

or-Oxidation 373

389

19.1 Phospholipids

401

Phosphatidylcholines 401 Other Glycerophospholipids Phosphosphingolipids 406

402

384

Contents

xiii

19.2 Phospholipids and Glycosphingolipids in Clinical Medicine 406 Pulmonary Surfactant Metabolism and Respiratory Distress Syndrome 406 Biochemical Determinants of Fetal Lung Maturity 408 Catabolism and Storage Disorders of Sphingolipids 409 Alterations in Cell Surface Glycosphingolipids 414

20.3 Lipoprotein-Associated Disorders 440 Hyperlipidemias 440 Hypertriacylglycerolemias 440 Hypercholesterolemias 441 Hypolipidemias 442 Atherosclerosis and Coronary Heart Disease Lipid-Lowering Methods 448


19.3 Cholesterol 414 Conversion of Acetyl-CoA to HMG-CoA 415 Conversion of HMG-CoA to Mevalonate 416 Conversion of Mevalonate to Isoprenyl Pyrophosphate 419 Condensation of Isoprenyl Pyrophosphate to Form Squalene 420 Conversion of Squalene to Lanosterol 420 Conversion of Lanosterol to Cholesterol 421 Utilization of Cholesterol 421

19.4 Bile Acids

423

Regulation of Bile Acid Synthesis 424 Disposition of Bile Acids in the Intestines and Their Enterohepatic Circulation 425 Bile Acid Metabolism and Clinical Medicine 426


21.1 Muscle Systems 454 Structure and Development of Skeletal Muscle 454 Myofibrils 457 Thin Myofilaments 458 Thick Myofilaments 460 Organization and Properties of Muscle Fibers 462 Contractile Properties 462 pH Dependence of Myosin ATPase Activity 462 Metabolic Profile 463 Multigene Families Encode Muscle Proteins 463

21.2 Mechanism of Muscle Contraction: Overview 464

20 Lipids III: Plasma Lipoproteins 20.1 Structure and Composition 429 20.2 Metabolism

Muscle and Nonmuscle Contractile Systems

Mechanism of Contraction: Excitation/Contraction Coupling 464 Mechanism of Contraction: Activation of Contraction 466 Mechanism of Contraction: Cross-Bridge Cycling 466

433

Chylomicrons 434 Very-Low-Density Lipoproteins 435 Low-Density Lipoproteins 437 High-Density Lipoproteins 438

21.3 EnergySupply in Muscle

468

Phosphocreatine Shuttle 471 Regulation of Smooth and Cardiac Muscle 472

444

xiv

Contents

21.4 Inherited Diseases of Muscle

476

Appetite, Hunger, and Control of Food Intake 497

Degenerative Syndromes 478 Dynamic Syndromes 478

22.5 Carbohydrate Homeostasis 497

21.5 Nonmuscle Systems 478 Actin 478 Cilia 480

21.6 Drugs Affecting Microtubules Immotile Cilia Syndrome Kinesins 484

22.4 Stored Fuels 496

483

483


Carbohydrate as a Food 497 Disposition of High Glucose Intake 498 Glucose Tolerance 499 Glucose Homeostasis during Fasting 499 Utilization of Hepatic Glycogen 500 Utilization of Skeletal Muscle Glycogen 501 Gluconeogenesis 502 Regulation of Gluconeogenesis

502

22.6 Lipid Homeostasis 504

Metabolic Homeostasis 22.1 Metabolic Homeostasis 485 22.2 Metabolic Roles of Organs 487 Liver 487 Adipose Tissue 487 Skeletal Muscle 488 Brain 488 Heart 489 Kidneys 489 Gastrointestinal System 489 Blood and Other Body Fluids 489 Albumin

490

Lipoproteins

Protein Synthesis and Proteins as Energy Source 508 Nitrogen Balance 508 Ammonia Toxicity 509 Nitrogen Transfer between Compounds and Tissues 509

Structure and Synthesis

Reactions in Which Ammonia Is Released 509 Reactions That "Fix" Ammonia

509

Disposition of Dietary Intake of Protein 509 Protein Catabolism during Starvation 510

490

Secretion

22.7 Protein Synthesis and Nitrogen Homeostasis 508

Methods for Directly Transferring Nitrogen 509

490

22.3 Endocrine Pancreas and Pancreatic Hormones 490 Insulin

Lipid Digestion and Absorption 504 Disposition of Absorbed Triacylglycerol 504 Production of Triacylglycerol from Carbohydrate 505 Release of Lipid from Adipose Tissue Stores 505 Tissue Utilization of Fatty Acids 506 Ketone Body Production and Utilization 506

490

492

Biological Actions of lnsulin Insulin Receptor

494

Glucagon 495 Somatostatin 496 Pancreatic Polypeptide

494

22.8 Abnormalities of Metabolic Homeostasis 511 Diabetes Mellitus

511

Etiologic Classification of Diabetes Mellitus

496

Obesity

515

512

Contents

XV

23.8 DNAVaccines

22.9 Metabolic Homeostasis during Exercise 517

Antibodies to DNA

Exercise Generating Maximum Power 517 High-Intensity Endurance Exercise 517 Low-Level Nonfatiguing Exercise 518


522

DNA Replication, Repair, and Mutagenesis

Tautomerization of Bases 523 Methylation of Bases 524

23.2 Physical and Chemical Structure of DNA 524 The Watson-Crick DNA Structure 524 Alternative DNA Structures 526 Plasmid DNA 526 Circular and Supercoiled DNA 526

528

23.5 Repetitive DNASequences 23.6 Degradation of DNA Nucleases 530 Restriction Enzymes

23.10 Genomics and Proteomics 542

521

Base Pairing and Base Composition

23.4 Renaturation of DNA

538

23.9 The Human Genome Project

Structure of DNA 542 Sequencing DNA 542 Properties of DNA 542 Gene Therapy 543 Human Genome Project 543

23.1 Componentsof Nucleic Acids

527

537


Nucleic Acid Structure and Properties of DNA

23.3 Denaturation of DNA

537

24.1 DNA Replication

546

Problems of Replication 546 Semiconservative Replication 546 Origin and Direction of DNA Replication Replicons 546 Discontinuous DNA Replication

547

24.2 Enzymologyof DNA Replication DNA Polymerases

530

Polymerase III

548

548

Exonuclease Activities of Polymerase I

530

546

550

551

Eukaryotic Polymerases

552

24.3 The Replication Fork 552 530

23.7 Diagnostic and Clinical Applications of DNA 532 DNA Probes 532 Southern Blot Analysis 532 Polymorphisms 533 Forensic DNA Analysis 535 Sequencing DNA 535 Gene Therapy 536

Inhibitors of DNA Replication 553 Topoisomerase I Inhibitors 553

24.4 Chromosome Replication

554

Replication of DNA in Chromosomes 555 Replication at the Ends of Chromosomes 555

24.5 DNA Repair

556

Mismatch Repair and Methylation of DNA 556

xvi

Contents

Glycosylases 556 Alterations of DNA Molecules 557 General Mechanisms for Repair of DNA 557 Photoreactivation Excision Repair

557

25.9 Ribosomes 575

558

559

Chemical Composition of Prokaryotic Ribosomes 575 Ribosomes Are Ribozymes 575 Chemical Composition of Eukaryotic Ribosomes 576

Human Diseases and DNA Repair Deficiency 559

24.6 DNA Mutation

559

Types of Mutations 559 Triplet Repeats and Fragile Sites

560

RNA and Protein Synthesis 25.1 Structure of RNA

564

Ribosomal RNA (rRNA) 564 Transfer RNA (tRNA) 564

25.11 Collagen Biosynthesis and Its Disorders 585

564

25.3 Enzymatic Synthesis of RNA 25.4 Prokaryotic Transcription Lifetime of Prokaryotic mRNA

566

566 568

25.5 Transcription in Eukaryotes 568 Eukaryotic RNA Polymerases 568 RNA Polymerase II Promoters 569 RNA Polymerase III Promoters 569 Eukaryotic mRNA Synthesis 569 Capping and Polyadenylylation 569 Splicing of RNA in Eukaryotes Splicing and Ribozymes

25.10 Protein Synthesis 576 Stages of Protein Synthesis 576 Role of GTP 579 Posttranslational Modification of Proteins 579 Coupled Transcription and Translation 580 Endoplasmic Reticulum 580 Compartment Disorders 581 Inhibitors of Protein Synthesis and Related Disorders 584


25.2 Messenger RNA

25.8 Initiator tRNA Molecules and Selection of Initiation Codon 574

557

Recombination Repair SOS Repair

25.7 Attachment of Amino Acid to tRNA Molecule 574

570

571

Transcription and Translation of Collagen Polypeptides 586 Intracellular Posttranslational Modifications 587 Hydroxylations of Selected Prolyl and Lysyl Residues 587 Glycosylation of Hydroxylysyl Residues 588 Formation of Disulfide Linkages and Assembly of Procollagen Polypeptides into a Triple Helix 588 Translocation and Secretion of Procollagen 589 Extracellular Posttranslational Modification 589 Formation of Collagen Fibrils from Ordered Aggregation of Collagen Molecules 589

25.6 Genetic Code 571 "Universal" Genetic Code 571 Genetic Code of Mitochondria 572


xvii

Contents

Formation of One-Carbon Derivatives of Folate 617

Regulation of Gene Expression 26.1 Regulation of mRNA

593

26.2 Gene Regulation in Prokaryotes

594

Housekeeping Genes and RNases Gene Families 600

619

27.3 Biosynthesis of Purine Nucleotides

620

De Novo Synthesis

620 Salvage Pathways 622 Dietary Purines 623

Lactose (lac) Operon 594 Tryptophan (trp) Operon 595 Temporal mRNA Regulation in Phage Systems 598 Regulons 599

27.4 Conversion of Nucleoside Monophosphates to Diphosphates and Triphosphates 624

599

26.3 Gene Regulation in Eukaryotes

27.2 Formation of 5-Ph osp hori bosyl- 1- Pyrophosphate

27.5 Formation of Purine Deoxyribonucleotides

600

624

27.6 Regulation of Purine Biosynthesis

26.4 Mechanisms of Gene Regulation in Eukaryotes 601 Transcription Factors 601 Steroid Receptors 603 G-Protein Diseases 604 Regulation of Transcription by Methylation 604 Genomic Imprinting 606 Regulation of RNA Processing 606 Alternative RNA Splicing and Editing 607 Regulation of Iron Utilization in Cells 607 Polyproteins 608 Translational Regulation 608 Regulation of Protein Activity 608

26.5 Regulation of Cell Death: Apoptosis

609

26.6 Regulation of Cell Proliferation: Oncogenes 609 26.7 Retroviruses and AIDS

27.7 Inhibitors of Purine Biosynthesis

612

Inhibitors of Folate Biosynthesis 626 Inhibitors of Formation of IMP 626 Inhibitors of Formation of AMP and GMP 626 Inhibitors of Multiple StepsmPurine Analogues 627 Inhibition of Conversion of Ribonucleoside Diphosphate to Deoxyribonucleoside Diphosphate 628

27.8 Catabolism of Purine Nucleotides

Overproduction of PRPP

628

Treatment

631

631

Dietary and Lifestyle Factors

Nucleotide Metabolism 615

Inhibitors of Dihydrofolate Reductase

626

Xanthine Oxidase Reaction 629 Disorders of Purine Nucleotide Metabolism Gout 630


27.1 One-Carbon Metabolism

625

PRPP Synthetase Reaction 625 Amidophosphoribosyltransferase Reaction 625 Regulation of Formation of AMP and GMP from IMP 626

617

633

Lesch-Nyhan Syndrome 633 Adenine Phosphoribosyltransferase (APRT) Deficiency 634 Adenosine Deaminase (ADA) Deficiency and Purine Nucleoside Phosphorylase (PNP) Deficiency 634 Myoadenylate Deaminase Deficiency 636 Xanthine Oxidase Deficiency 637

630

XVIII

Contents

27.9 Biosynthesis of Pyrimidine Nucleotides 637 Salvage Pathways 638 De Novo Synthesis 638 Formation of UMP 638 Formation of Other Pyrimidine Nucleotides 639 Synthesis of Cytidine Nucleotides

640

Synthesis of Thymidine Nucleotides

640

Pyrimidine Analogues 641 Regulation of de Novo Pyrimidine Biosynthesis 641

27.10 Coordination of Purine and Pyrimidine Nucleotide Biosynthesis 642

ot-Thalassemias 660 Thalassemias of the/3-Globin Gene Family 662 3-/3 Thalassemias, Lepore Hemoglobins, and Hereditary Persistence of Fetal Hemoglobin (HPFH) 662 Hemoglobinopathies 664 Molecular Pathology 664 Unstable Hemoglobins 667 Secondary Polycythemia Syndromes 667 Congenital Methemoglobinemias and Cyanosis 668 Hemoglobin S and Sickling Disorders 668 y- and 6-Globin Mutants 670 Screening and Prenatal Diagnosis 670

28.4 Derivatives of Hemoglobin 671

27.11 Catabolism of Pyrimidine Nucleotides 643

Carbon Monoxide-Hemoglobin Carbaminohemoglobin 672 Methemoglobin 672 Sulfhemoglobin 673 Cyanmethemoglobin 673 Glycated Hemoglobins 674

27.12 Abnormalities of Pyrimidine Metabolism 644 Supplemental Readings and References 644

67]


Hemoglobin 28.1 Structure of Hemoglobins 645

Metabolism of Iron and Heme

Globin Chains 645 Heme Group 646

29.1 Iron Metabolism

28.2 Functional Aspects of Hemoglobin 646 Oxygen Transport 646 Mechanism of Oxygenation 650 Function, Metabolism, and Regulation of Organic Phosphates in Erythrocytes 652 CO2 Transport 655 Nitric Oxide (NO) Binding to Hemoglobin

Erythropoietin

656

28.3 Inherited Disorders of Hemoglobin Structure and Synthesis 657 Normal Hemoglobins Thalassemias 659

657

656

675

Absorption of Iron from the Diet 675 Plasma Iron Transport 679 Storage of Iron 679 Coordinate Regulation of Iron Uptake and Storage in Non-Erythroid Cells 679 Alterations of Plasma Transferrin Concentration 680 Disorders of Iron Metabolism 681 Iron Deficiency Anemia

681

Iron-Storage Disorders

682

Hereditary Hemochromatosis

683

29.2 Heme Biosynthesis 684 Formation of ~-Aminolevulinic Acid

684

xix

Contents

Formation of Porphobilinogen 684 Formation of Uroporphyrinogen III 685 Formation of Coproporphyrinogen III 685 Formation of Protoporphyrinogen IX

685

Formation of Protoporphyrin IX and Heme Disorders of Heme Biosynthesis

Hepatic Porphyrias 687 Erythropoietic Porphyrias

685

686

688

29.3 Heme Catabolism 689 Formation of Bilirubin 690 Circulatory Transport of Bilirubin 691 Hepatic Uptake, Conjugation, and Secretion of Bilirubin 692 B ilirubin in the Intestinal Tract 694 Disorders of Bilirubin Metabolism 694 Unconjugated Hyperbilirubinemias 694 Conjugated Hyperbilirubinemias 694 Neonatal Hyperbilirubinemia 696


Endocrine Metabolism I: Introduction 30.1 Hormonal Amines

700

30.2 Peptide, Protein, and Glycoprotein Hormones 701

30.4 Mechanism of Hormone Action

30.5 Types of Hormone Receptors 710 Nuclear Receptors 710 Thyroid Hormone Receptors 710 Steroid Hormone Receptors 711 Cell Surface Receptors 713 G-Protein-Coupled Adenylate Cyclase-cAMP System 713 Abnormalities in Initiation of G-Protein Signal 717 G-Protein-Coupled Phosphatidylinositol- Ca 2+ Pathway 718 Mechanism of the Calcium Messenger System 718 Receptors for Insulin and Growth Factors 721 Receptors for Growth Hormone and Prolactin 722

30.6 Organization of the Endocrine System Supplemental Readings and References 726

Endocrine Metabolism II: Hypothalamus and Pituitary 31.1 Hypothalamus 729 Hypophysiotropic Peptides (Table 31-1) Neurohypophyseal Peptides 733 Neuroregulatory Peptides 733 Brain-Gut Peptides

30.3 Steroid Hormones 702 The/X 4 Pathway: 3/3HSD 705 Directional Flow Valve: CYP17 705 The Corticosteroid Pathway Is Initiated by 21-Hydroxylase (CYP21) 705 The Sex Steroid Pathway Is Initiated by 17ot-Hydroxylase/17,20-Lyase (CYP17) 706 Synthesis of Androgens: 17,20-Lyase 706 Synthesis of Estrogens: Aromatase (CYP19) 706 Steroid-Binding Serum Proteins 707 Eicosanoids 709

709

Endogenous Opiates

730

733 735

31.2 Pituitary Gland (Hypophysis) 736 Somatomammotropin Family 737 Growth Hormone (GH Somatotropin) 737 Actions of GH 738 Regulation of GH Release 739 Insulin-like Growth Factors (IGFs) 739 Disturbances in GH and IGF 740 Prolactin 741 Disturbances in Prolactin 742

722

XX

Contents

The Opiomelanocortin Family 742 Glycoprotein Hormones 744 Neural Regulation of Anterior Pituitary Function 745

Mechanism of Action

763

Dopamine Receptors

763

Adrenergic Receptors

763

Biological Effects of Epinephrine and Norepinephrine (Table 32-3) 765 Cardiovascular, Pulmonary, and Renal Effects 765


Pulmonary Respiratory System Renal Urinary System

32


32.1 Adrenal Cortex 749 Synthesis of Corticosteroids (See Chapter 30) 750 Regulation of Corticosteroid Secretion 752

33

Regulation of Aldosterone Secretion 752

Endocrine Metabolism IV: Thyroid Gland

753

Metabolism of Corticosteroids 754 Synthetic Corticosteroids 754 Biological Actions of Aldosterone 754 Mechanism of Action

33.1 Structure-Activity of Thyroid Hormones 769

754

Physiological Effects of Aldosterone

Biological Actions of Cortisol Mechanism of Action

33.2 Thyroid Hormone Synthesis 770

755

Regulation of Thyroid Hormone Synthesis

755

Thyroid-Stimulating Hormone (TSH)

755

Physiological Effects of Cortisol

Iodine

755

756

Pharmacological Effects of Glucocorticoids Adrenal Androgen, Dehydroepiandrosterone (DHEA) 757

Disturbances in Adrenocortical Function Deficiency

757

757

32.2 Adrenal Medulla

Regulation of Release 760 Synthesis of Epinephrine 761 Regulation of Catecholamine Secretion Regulation of Release

33.3 Transport and Metabolism of Thyroid Hormones 774 33.4 Biological Actions of Thyroid Hormones 776

776

Intermediary Metabolism

761

Growth and Maturation Reproductive System

763

Transport and Metabolism of Catecholamines 763 Biological Actions of Catecholamines

773

Cardiovascular System

761

777 777

778

Pharmacological Effects

763

772

772

Mechanism of Action and Latency Period Physiological Effects 776

760

Regulation of Synthesis

765

Metabolic, Endocrine, and Thermogenic Effects 766 Disturbances in Adrenal Medullary Function 767

Endocrine Metabolism IIIAdrenal Glands

Regulation of Cortisol Secretion

765

778


776

xxi

Contents

Endocrine

Metabolism

Reproductive

Molecular

V:

Immunology

System

35.1 Innate Immunity 803 34.1 Testes 782 Regulation of Spermatogenesis: Sertoli-Neuroendocrine Axis 782 Regulation of Testicular Steroidogenesis: Leydig-Neuroendocrine Axis 784 Metabolism of Testosterone 785 Biological Effects of Androgens 785 Reproductive (Androgenic) Effects 787 Nonreproductive Effects 787 Pharmacological Inhibition of Spermatogenesis or Fertility (Male Contraception) 789

34.2 Ovaries 790 Menstrual Cycle 790 Endocrine Control of Folliculogenesis 790 Hormonal Control of Follicle Growth Hormonal Control of Luteal Function 792 Pregnancy 792 Human Chorionic Gonadotropin (hCG) Placental Steroids

Relaxin

35.3 Antibody-Dependent Cell-Mediated Cytotoxicity 811 35.4 Distinguishing Self from Nonself 811 35.5 Molecules and Chemical Processes of the Immune System 811 35.6 Immunoglobulin Structure and Function 812 Antigen-Antibody Reactions 818 Monoclonal Antibodies: Diagnostic Tools and Therapeutic Agents 819 Recognition of Infected Cells by Cell Receptors 820

35.7 B-Cell Clonal Selection and Proliferation 823 792

793

Human Placental Lactogen (hPL) Decidual Prolactin

790

35.2 Acquired or Adaptive Immunity 808

793

794

35.8 Antibody Diversity and Immunoglobulin Genes 824 Class (Isotype) Switching

826

794

Parturition 794 Lactation 794 Biological Effects of Estrogens 795 Metabolism of Estrogen 797 Selective Estrogen Receptor Modulators (SERM) 797 Biological Effects of Progesterone 798 Pharmacological Enhancement of Fertility 799 Pharmacological Prevention of Pregnancy (Female Contraception) 799


35.9 Major Histocompatibility Complex (MHC) Genes 827 35.10 Complement 827 The Alternative Pathway 83 l The Classical Pathway 831 The Lectin Pathway 832

35.11 Cytokines 832 35.12 Vaccines

833


xxii

Contents Anticoagulant SubsystemmProteinase Inhibitors 858 Mechanism of Action of Heparin as a Therapeutic Anticoagulant 859 Inhibitors of the Contact Phase Proteinases 859

Biochemistry of Hemostasis 36.1 Primary Hemostasis 840 36.2 Secondary Hemostasis 841

36.9 Fibrinolytic Subsystem 859

Clot DissolutionmFibrinolysis 841 Thrombosis: A Dark Side of Hemostatic System Function 842

Plasminogen Activation 859 Degradation of Fibrin (Fibrinolysis)

36.3 Functional Properties and Structures of the Hemostatic System Factors(Proteins) 842 Proteinase Precursors 842 Proteinase Domain Structures 848 Amino Terminal Domain Structures and Structural Motifs 848 Cofactor Proteins 848 Proteinase Inhibitors 849 Other Proteins of the Hemostatic System Membrane Phospholipid Surfaces 851

36.11 Coagulation Factor Measurements

850

36.5 The Procoagulant Subsystem of Coagulation 852 Activation Complexes 852 The Prothrombin Activation Complex (Prothrombinase) 853 Activation of Prothrombin 853

854

Initiation of the Procoagulant Subsystem and Activation of Factor VII 854 Activation of Factor X by Factor VII(VIIa) 855

36.7 The Intrinsic Pathway

36.10 Vitamin K, Oral Anticoagulants, and Their Mechanisms of Action 861 Action of Warfarin and Other Vitamin K Antagonists 862

36.4 Clotting 851

36.6 The Extrinsic Pathway

861

856

Activation of Factor X 856 Activation of Factor IX by Factor VIIa and Tissue Factor 856 Activation of Factor IX by Factor XIa 856 The Contact Phase of the in Vitro Intrinsic Pathway of Coagulation 857

36.8 Anticoagulant Subsystem--Activation of Protein C and Inactivation of Factors Va and Villa 857

863

Deficiencies 863 Laboratory Assessment of Coagulation System Functions 864 Prothrombin Time 864 Thrombin Time 864 Activated Partial Thromboplastin Time 864 Specific Factor Assays 870 Assay of Heparin 870

36.12 Case Studies 870 Fibrinogen

870

Factor XIII

870

Activated Protein C Resistance

870

Antibiotic-Induced Vitamin K Deficiency Hemophilia

871

871

Von Willebrand Disease

871

Laboratory-Created Artifacts

871


Mineral Metabolism 37.1 Calcium and Phosphorus 873 Distribution and Function 873 Bone Structure, Formation, and Turnover

875

xxiii

Contents

Calcium and Phosphate Homeostasis Calcium and Phosphate in the Diet Vitamin D Metabolism and Function

876

37.2 Magnesium

Function

876 880

Parathyroid Hormone-Related Protein Disorders of Calcium and Phosphorus Homeostasis 888

887

890

37.3 Nonessential Trace Elements Cadmium 891 Lead 894 Aluminum 894 Other Trace Elements

37.4 Metallothioneins

Absorption, Transport, and Metabolism

891

915

915

Hypo- and Hypervitaminosis

915

Riboflavin (Vitamin B2) 915 Pyridoxine (Vitamin B6) 916 Cobalamin (Vitamin B12) 918 Folic Acid (Pteroylglutamic Acid) 922 Niacin 924 Pantothenic Acid (Pantoyl-fl-Alanine) 924 Biotin 924 Ascorbic Acid (Vitamin C) 925

38.3 Vitamin-Responsive Inherited Metabolic Disorders 926

894

894

38.4 Vitamin-like Substances 927

37.5 Essential Trace Elements

894


Copper 895 Zinc 897


Water, Electrolytes, and CHAPTER

Acid-Base Balance

38

39.1 Water Metabolism

Vitamin Metabolism 38.1 Fat-Soluble Vitamins Vitamin A

39.2 Homeostatic Controls 930

901

General Considerations

39.3 Water and Osmolality Controls 931

904

904

39.4 Electrolyte Balance

Nutrition and Chemistry

904

Absorption, Transport, and Metabolism Function

Vision and Vitamin A

Vitamin E

39.5 Acid-Base Balance

912

913

Hypo- and Hypervitaminosis E

913

38.2 Water-Soluble Vitamins 914 Thiamine (Vitamin B 1) 9t4 Nutrition and Chemistry

914

934

Disorders of Acid-Base Balance 935

912

Nutrition and Chemistry

933

Sodium 933 Potassium 934 Chloride 934

907

908

Absorption, Transport, and Metabolism Function

905

906

Hypo- and Hypervitaminosis A

929

913

Supplemental Readings and References 938 Appendix I. Median Heights and Weights and Recommended Energy Intake 939 Appendix II. Weights for Heights of Adults in the United States 941

xxiv Appendix II1. Weight and Height of Males and Gemales up to 18 Years in the United States 943 Appendix IV. Recommended Daily Dietary Allowances 945 Appendix V. Classification of Selected Enzymes with Clinical Importance According to the Enzyme Commission (EC) and Their Common Names 947 Appendix VI. Serum Protein Electrophoresis and Its Diagnostic Significance 949

Contents

Appendix VII. Laboratory Evaluation of Hemoglobin Disorders 957 Appendix VIII. Acronyms and Abbreviations Found in This Text 963 Appendix IX. Reference Intervals for Clinical Laboratory Parameters 967

Index

979

CONTRIBUTORS

Gordon Edlin, Ph.D. (Chapters 23, 24, 25, 26) Department of Biochemistry and Biophysics, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii

Richard J. Guillory, Ph.D. (Chapter 14) Department of Biochemistry and Biophysics, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii Craig M. Jackson, Ph.D. (Chapters 35, 36) Hemo SAGA Corporation, San Diego, California Conrad A. Hornick, Ph.D. (Chapters 18, 19, 20) Departments of Physiology and Pathology, Louisiana State University, New Orleans, Louisiana

David A. Lally, Ph.D. (Chapter 21) Department of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii Walter K. Morishige, Ph.D. (Chapters 30, 31, 32, 33, 34) Department of Physiology, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii James R. Wilson, Ph.D. (Chapter 16) Department of Biology, La-Sierra University, Riverside, California

XXV


PREFACE

In keeping with the previous editions, the primary purpose of Medical Biochemistry, Fourth Edition, is to present the fundamentals of biochemistry and related materials in a way that is useful to students pursuing medical and other health-related careers. The book was conceived and written with the hope that it would generate interest and enthusiasm among these students, particularly because biochemistry has a crucial role in human health and disease. Since it is assumed that most students in medicine and health-related fields eventually will apply biochemical principles to the art of healing, discussion of the factual information is integrated with frequent use of clinical examples and applications. A vast number of constituents of the human body m cells, enzymes, hormones, sugars, salts, vitamins, and so forthmvary in normal and abnormal states of human health. Understanding the metabolic and regulatory processes that underlie metabolism is essential to any practice of the healing arts and the relief of human suffering. I have tried to keep this idea foremost. Progress in biochemistry, molecular biology, cellular biology, endocrinology, and other disciplines has been so rapid and profound in the past 20 years that biochemistry texts obsolesce rapidly. To maximize the usefulness of this book, authors actively involved in research have written several chapters dealing with the most rapidly changing fields. The overall organization of topics is designed to lead the student logically through the biochemical organization of cells. Emphasis is placed on the structures and functions of the molecular components of cells and on metabolic controls. The text begins with a discussion of water, acids, bases, and buffers, amino acids, proteins, and thermodynamics (Chapters 1-5). This is fol-

lowed by a detailed discussion of important aspects of enzymology (Chapters 6-8). The broad subject of carbohydrate chemistry (Chapters 9-11, 13, 15, 16) is integrated with chapters that discuss gastrointestinal digestion (Chapter 12), oxidative phosphorylation (Chapter 14), and protein metabolism (Chapter 17). Three chapters on lipids (Chapters 18-20) are integrated with chapters coveting muscle systems (Chapter 21) and metabolic homeostasis (Chapter 22). The principles of molecular biology including nucleic acid chemistry and the regulation of gene expression and protein synthesis are presented in Chapters 23-26. These chapters are followed by ones on nucleotide, hemoglobin, and heme metabolism (Chapters 27-29). The endocrine system and its organs are discussed in Chapters 30-34. Molecular immunology is presented in Chapter 35, and the biochemistry of blood coagulation appears in Chapter 36. The last section discusses mineral and vitamin metabolism and electrolyte balance (Chapters 37-39). Nine appendices contain tables of nutritional values and clinical laboratory measurements that are important in diagnosis. The importance of human nutrition is emphasized th~ughout the text and has not been relegated to a single ~apter. Likewise, hereditary disorders are discussed throughout, along with other clinical examples that relate the relevant biochemistry to diagnosis and treatment. This book is not small, although conciseness consistent with clarity was a primary goal. Detailed discussions of experiments have, for the most part, been omitted, and discussion of more subtle points has been minimized. References provided at the end of each chapter will lead interested students deeper into particular topics and case studies. xxvii


ACKNOWLEDGMENTS

I am grateful to the authors for their contributions and for sharing their knowledge and insights. I also appreciate their cooperation during the lengthy writing and production process. I am indebted to Gordon Edlin, who participated in and supported all aspects of the publication of this book, to Alan E Goldstein, who reviewed many chapters and helped integrate basic science and clinical medicine, and Chung-eun Ha for spending tireless hours in the preparation of the manuscript during its many developmental stages. I am especially appreciative to the following individuals, who contributed to this book as reviewers of selected chapters, as writers of subsections, by offering constructive suggestions, or by providing encouragement and support during difficult times: R. A. Dubanoski, J. M. Hardman, K. H. Higa, S. A. A. Honda, K. Harohalli, D. K. Kikuta, W. Lee, A. A. Manoukian, H. E Mower, D. S. Park, C. E. Petersen, C. N. Rios, R. T. Sakaguchi, C. E. Sugiyama, and L. E. Takenaka. My thanks also to the following reviewers for their valuable chapter critiques: Dr. James W. Campbell, Department of Biochemistry and Cell Biology, Rice University; Dr. David Daleke, Department of Biochemistry, Indiana University; Dr. Beverly Delidow, School of Medicine, Marshall University; Dr. JoAnne Flynn, School of Medicine, University of Pittsburgh; Dr. Jennifer A. Pietenpol, School of Medicine, Vanderbilt University; Dr. Connie Prosser, Department of Laboratory Medicine, University of Alberta; Dr. Kevin D. Sarge, Department of Biochemistry, University of Kentucky;

Dr. Peter B. Smith, School of Medicine, Wake Forest University; Dr. Terry Stoming, Department of Biochemistry and Molecular Biology, Medical College of Georgia; Dr. Francis Vella, International Union of Biochemistry and Molecular Biology; and, Dr. D. Eric Walters, Department of Biochemistry and Molecular Biology, The Chicago Medical School. Dr. Craig M. Jackson acknowledges the images shown in Chapters 35 and 36 were created using RasMol 2.6, Molecular Graphics Visualization Tool by Roger Sayle, Bio-Molecular Structures Group, Glaxo Research & Development, Greenford, Middlesex, UK. RasMol can be obtained at http://www.umass.edu/microbio/rasmol/. The coordinates were obtained from the Protein Data Bank at Brookhaven National Laboratory, as described in F. C. Bernstein, T. E Koetzle, G. J. B. Williams, E. F. Meyer, Jr., M. D. Brice, J. R. Rodgers, O. Kennard, T. Shimanouchi, and M. Tasuxni, "The Protein Data Bank: A Computer-Based Archival File for Macromolecular Structures," J. Mol. Biol. 112, 535-542 (1977). Gene information has been obtained from GenBank, as described in C. Burks, M. Cassidy, M. J. Cinkosky, K. E. Cumella, E Gilna, J. E-D. Hayden, G. M. Keen, T. A. Kelley, M. Kelly, D. Kristofferson, and J. Ryals GenBank. Nucl. Acids Res. 19(Suppl), 2221-2225 (1991). For the coordinates of the molecules for which structures are shown, see http://www.rcsb.org/. The author thanks James G. White, M. D., Regents Professor, University of Minnesota, Minneapolis, Minnesota, for the photographs of platelets, and Meir Rigbi,

xxix

XXX

Professor Emeritus, Hebrew University of Jerusalem, for his many helpful comments on the manuscript. I also express my thanks to C. Agbayani, J. Gerber, and N. Mead for unfailing assistance in the preparation of

Acknowledgments

the manuscript. Finally, I greatly appreciate the assistance and advice provided to me by the editorial staff of Harcourt/Academic Press, in particular J. R. Hayhurst, N. M. Donaghy, and R. L. Orbegoso.

CHAPTER

1

Water, Acids, Bases, and Buffers

1.1 Properties of Water Acid and base concentrations in living systems are carefully regulated to maintain conditions compatible with normal life. Biochemical reactions involving acids and bases occur in the body water, whereas buffer systems protect the body from significant variations in the concentrations of acids and bases. This chapter introduces basic concepts of the properties of water, acids, bases, and buffers, and Chapter 39 presents a detailed discussion of both normal and pathological aspects of acid-base metabolism. Life cannot be sustained without water. Water constitutes 45-73% of total human body weight. It is distributed in intracellular (55%) and extracellular (45%)compartments and provides a continuous solvent phase between body compartments. As the biological solvent, water plays a major role in all aspects of metabolism: absorption, transport, digestion, and excretion of inorganic and organic substances as well as maintenance of body temperature. The unique properties of water are due to its structure.

Hydrogen Bonding Water (H20) is a hydride of oxygen in which the highly electronegative oxygen atom attracts the bonding electrons from two hydrogen atoms. This leads to polar H-O bonds in which the hydrogen atoms have a slightly positive

charge (~+) and the oxygen atom has a slightly negative charge (~)-, (Figure 1-1). "Water molecules have a relatively high dipole moment because of the angle (104.5 ~ of the H - O - H bond and the polarity of the bonds. Neighboring liquid water molecules interact with one another to form an extensive lattice-like structure similar to the structure of ice. The intermolecular bonding between water molecules arises from the attraction between the partial negative charge on the oxygen atom and the partial positive charge on the hydrogen atom of adjacent water molecules. This type of attraction involving a hydrogen atom is known as a hydrogen bond (Figure 1-2). Hydrogen bonds contain a hydrogen atom between two electronegative atoms (e.g., O and N). One is the formal hydrogen donor; the other is the hydrogen acceptor. The amount of energy required to break a hydrogen bond (bond energy) is estimated to be 2-5 kcal/mol (8.4-20.9 kJ/mol) in the gas phase. Covalent bonds have bond energies of 50-100 kcal/mol (209-418 kJ/mol). The cumulative effect of many hydrogen bonds is equivalent to the stabilizing effect of covalent bonds. In proteins, nucleic acids, and water, hydrogen bonds are essential to stabilize overall structure. In ice, each water molecule forms a hydrogen bond with four other water molecules, giving rise to a rigid tetrahedral arrangement (Figure 1-2). In the liquid state, water maintains a tetrahedrally coordinated structure over short ranges and for short time periods.

CHAPTER1 Water, Acids, Bases, and Buffers T A B L E 1-1

5 +

Physical Properties of Water*

m

25

m

F I G U R E 1-1 Structure of the water molecule.

Physical Properties Other properties of water uniquely suited to biological systems include melting point, boiling point, heat of vaporization (quantity of heat energy required to transform 1 g of liquid to vapor at the boiling point), heat of fusion (quantity of heat energy required to convert 1 g of solid to liquid at the melting point), specific heat (the amount of heat required to raise the temperature of 1 g of substance by I~ and surface tension (Table 1-1). All these values for water are much higher than those for other lowmolecular-weight substances because of the strong intermolecular hydrogen bonding of water. These properties contribute to maintenance of temperature and to dissipation of heat in living systems. Thus, water plays a major role in thermoregulation in living systems. The optimal body temperature is a balance between heat production and heat dissipation. Impaired thermoregulation causes either hypothermia or hyperthermia and has serious metabolic consequences; if uncorrected, impaired thermoregulation may lead to death (Chapter 39). Water freezes to form ice at 0~ but its maximum density is at 4~ Aquatic

H

J O"

H

= H

I ~O,~

H~

o/

I

H

__E~H

"',,,o,/

~

I

I

0

H ~H

F I G U R E 1-2 Tetrahedral hydrogen-bonded structure of water molecules in ice. The tetrahedral arrangement is due to the fact that each water molecule has four fractional charges: two negative charges due to the presence of a lone pair of electrons on the oxygen atom and two positive charges, one on each of the two hydrogen atoms. In the liquid phase this tetrahedral array occurs transiently.

Density (at 4~ Molecular weight Liquid range Melting point Boiling point Heat of fusion Heat of vaporization Dipole moment Dielectric constant (E) Solid/liquid density ratio

1.0 g/mL 18 0~176 0~ 100~ 80 cal/g 540 cal/g 1.86 Debye unit 78.4 0.92

*Some of these properties are measured at 1 atm pressure.

organisms survive cold winters because ice floats over and insulates liquid water from sub-zero-degree temperature. If ice were denser than liquid water--which is the case for most liquid-to-solid transformations~the solid form would sink and the entire amount of fluid would solidify rapidly in freezing weather. During hot weather, deep lakes and oceans remain cool because heat generated by sunlight can be dissipated by evaporation of surface water. Water is transported across cell membranes in one of two ways: 1. by simple diffusion through the phospholipid bilayer and 2. by the action of membrane-spanning transport proteins known as aquaporins. Thus, the concentration of water is in thermodynamic equilibrium across the cell membrane. In the renal collecting duct, water is reabsorbed through a specific aquaporin channel protein (aquaporin 2). This reabsorption of water is regulated by the antidiuretic hormone (also known as vasopressin). A defect or lack of functional aquaporin 2, vasopressin, or its receptor leads to enormous loss of water in the urine, causing the disease known as diabetes insipidus (Chapter 39). Water plays a significant role in enzyme functions, molecular assembly of macromolecules, and allosteric regulation of proteins. For example, the effect of protein solvation in allosteric regulation is implicated in the transition of deoxyhemoglobin to oxyhemoglobin. During this process about 60 extra water molecules bind to oxyhemoglobin (Chapter 28). Solutes, Micelles, a n d H y d r o p h o b i c Interactions Water is an excellent solvent for both ionic compounds (e.g., NaC1) and low-molecular-weight nonionic polar

SECTION1.1 Properties of Water

compounds (e.g., sugars and alcohols). Ionic compounds are soluble because water can overcome the electrostatic attraction between ions through solvation of the ions. Nonionic polar compounds are soluble because water molecules can form hydrogen bonds to polar groups (e.g., -OH). Amphipathic compounds, which contain both large nonpolar hydrocarbon chains (hydrophobic groups) and polar or ionic groups (hydrophilic groups) may associate with each other in submicroscopic aggregations called micelles. Micelles have hydrophilic (water-liking) groups on their exterior (bonding with solvent water), and hydrophobic (water-disliking) groups clustered in their interior (Figure 1-3). They occur in spherical, cylindrical, or ellipsoidal shapes. Micelle structures are stabilized by hydrogen bonding with water, by van der Waals attractive forces between hydrocarbon groups in the interior, and by energy of hydrophobic reactions. The last is the stabilization energy that would be lost if each hydrocarbon group were transferred from the hydrophobic medium to the polar aqueous solvent. As with hydrogen bonds, each hydrophobic interaction is very weak, but many such interactions result in formation of large, stable structures. A micelle may contain many hundreds of thousands of amphipathic molecules. The interior molecular organization of micelles has been likened to a "liquid hydrocarbon droplet." However, a recent model departs from this conventional concept and suggests that because of severe constraints in the space-filling requirements of hydrocarbon chains in the interior as well as because of micellar

"~~ ~ jHydrophobicchain Z J . ~ ./Hydrophilicgroup

3

geometry, the chain ends are not uniformly distributed throughout the micelle but tend to be clustered between the center of the micelle and the outer surface, implying that many of the hydrocarbon side chains are bent back upon themselves (Figure 1-3). In this model, there appears to be a progression from ordered (as in crystals) to disordered (as in liquids) structures proceeding from the center of the micelle to the periphery. Hydrophobic interaction plays a major role in maintaining the structure and function of cell membranes, the activity of proteins, the anesthetic action of nonpolar compounds such as chloroform and nitrous oxide, the absorption of digested fats, and the circulation of hydrophobic molecules in the interior of micelles in blood plasma.

Colligative Properties The colligative properties of a solvent depend upon the concentration of solute particles. These properties include freezing point depression, vapor pressure depression, osmotic pressure, and boiling point elevation. The freezing point of water is depressed by 1.86~ when 1 mol of nonvolatile solute, which neither dissociates nor associates in solution, is dissolved in 1 kg of water. The same concentration of solute elevates the boiling point by 0.543~ Osmotic pressure is a measure of the tendency of water molecules to migrate from a dilute to a concentrated solution through a semipermeable membrane. This migration of water molecules is termed osmosis. A solution containing 1 mol of solute particles in 1 kg of water is a 1-osmolal solution. When 1 mol of a solute (such as NaC1) that dissociates into two ions (Na + and C1-) is dissolved in 1 kg of water, the solution is 2-osmolal. Measurement of colligative properties is useful in estimating solute concentrations in biological fluids. For example, in blood plasma, the normal total concentration of solutes is remarkably constant (275-295 milliosmolal). Pathological conditions (e.g., dehydration, renal failure) involving abnormal plasma osmolality are discussed in Chapter 39.

Dissociation of Water and the pH Scale Water dissociates to yield a hydrogen ion (H +) and a hydroxyl ion (OH-). F I G U R E 1-3 A geometrical representation of a spherical micelle showing the hydrocarbon chains in the micelle. The hydrophilic groups are attracted to water, and the hydrophobic chains are within the micelle. The ends of the hydrocarbon chains are nonuniformly distributed, and many are located in the middle. The degree of disorder is much higher in the periphery than in the center of the micelle.

H20 ~ H + + OH-

(1.1)

The H + bonds to the oxygen atom of an undissociated H20 molecule to form a hydronium ion (H30+). H 2 0 if- H 2 0

~

H3 O + -+- O H -

CHAPTER1 Water, Acids, Bases, and Buffers

Thus, water functions both as an acid (donor of H + or proton) and as a base (acceptor of H + or proton). This description of an acid and a base follows from the Br6nstedLowry theory. According to the Lewis theory, acids are electron pair acceptors and bases are electron pair donors. The equilibrium constant, K, for the dissociation reaction in Equation (1.1) is K =

[H+][OH -]

(1.2)

[H20]

where the square brackets refer to the molar concentrations of the ions involved. K can be determined by measurement of the electrical conductivity of pure water, which has the value of 1.8 x 10 -16 M at 25~ indicative of a very small ion concentration, where M (molar) is the units of moles per liter. Therefore, the concentration of undissociated water is essentially unchanged by the dissociation reaction. Since 1 L of water weighs 1000 g and 1 mol of water weighs 18 g, the molar concentration of pure water is 55.5 M. Substitution for K and [H20] in Equation (1.2) yields

TABLE 1-2 The pH Scale [H+], M

pH

[OH-l, M

10.0 1.0 0.1 0.01(10 -2 ) 10-3 10-4 10-5 10 -6 10-7 < 10-8 10-9 10-10 10- l l 10-12 10-13 10-14 10-15

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

10-15 10-14 10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 0.01 0.1 1 10

Acidic

Neutral

T 1

Basic

[H+][OH -] = (55.5 M) x (1.8 x 10 -16 M) [H+][OH -] = 1.0 • 10 -14 M 2 = Kw Kw is known as the ion product of water. In pure water, [H +] and [OH-] are equal, so that

discussions of acid-base problems in human biochemistry, it is often preferable to express H + concentration as nanomoles per liter (nmol/L).

[ O H - ] = [H + ] = 1.0• 10 - 7 M . pH is employed to express these ion concentrations in a convenient form, where the "p" of pH symbolizes "negative logarithm (to the base 10)" of the concentration in question. Thus, pH - - loglo[H +] -- log

[H + ]

Similarly, pOH - - loglo[OH-] -- log

[OH-I

Therefore, for water,

1.2 Buffers Buffers resist change in pH in solutions when acids or bases are added. They are either a mixture of a weak acid (HA) and its conjugate base (A-) or a mixture of a weak base (B) and its conjugate acid (HB+). EXAMPLE 1 Acetic acid (CH3COOH) and carbonic acid (H2CO3) are weak acids. Ammonia (NH3) is a weak base. CH3COOH/CH3COO-, H2CO3/HCO 3, and NH3/NH~- constitute buffer systems.

log[H +] -4- log[OH-] -- log 10 -14 or

pH + pOH -- 14. The pH value of 7 for pure water at 25~ is considered to be neutral, and values below 7 are considered acidic and above 7 basic. Table 1-2 illustrates the pH scale extending from - 1 to + 15. It is important to recognize that as the pH decreases, [H +] increases. A decrease in one pH unit reflects a 10-fold increase in H + concentration. In

A buffer solution functions in the following manner to resist changes in acidity or alkalinity. In an acetic acid/sodium acetate buffer system, the species present in solution are CH3COOH, CH3COO-, Na +, and H20. Amounts of H + and O H - are initially assumed to be small. When acid is added to the buffer, almost all of the H + ions react with acetate ions to produce weakly ionized acetic acid ( H + + CH3COO- Z CH3COOH). The H + ions are thereby prevented from appreciably changing the pH.

SECTION1.2 Buffers

5

weak base (B) and water, the ions O H - and HB + (the conjugate acid) are produced. B + H20 Z

HB + + O H -

The concentration of the conjugate base (or acid) generated from a weak acid (or base) is small, since, by definition, weak acids and bases are only slightly dissociated in aqueous solution. Examples of weak acids are organic acids (e.g., acetic) and of strong acids are mineral acids (e.g., hydrochloric and sulfuric).

Henderson-Hasselbalch Equation

F I G U R E 1-4 Titration profile of acetic acid (CH3COOH) with sodium hydroxide (NaOH). Maximum buffering capacity is at pH = pK ', at which point minimal change in pH occurs upon addition of acid or base.

When O H - is added, almost all of the hydroxyl radicals react with acetic acid molecules to produce more acetate ions and w a t e r ( O H - + CH3COOH ~ CH3COO- + H20). The additional O H - is thus consumed with little increase in pH. Adding H + or O H - to a buffer causes only slight pH changes provided there is excess salt (CH3COO-) or acid (CH3COOH). If all of the acid is converted to the salt form by the addition of a large amount of O H - , the solution can no longer behave as a buffer. Adding more O H - will cause the pH to rise rapidly, as if the solution contained no buffer or only salt. The maximum buffering capacity exists when the molarities of the salt and acid are equal, i.e., when pH = pK' (or - log K'). The pK t is at an inflection point on the titration curve and hence is the point of minimum slope or minimum change in pH for a given addition of acid or base (Figure 1-4). In a generalized weak acid buffer reaction, H20 +

HA ~

K =

[H+][A -] [HA]

(1.4)

where K is the equilibrium constant at a given temperature. For a defined set of experimental conditions, this equilibrium constant is designated as K' (K prime) and referred to as an apparent dissociation constant. The higher the value of K t, the greater the number of H + ions liberated per mole of acid in solution and hence the stronger the acid. K t is thus a measure of the strength of an acid. Rearrangement of Equation (1.4) yields [H +] =

K'[HA] [A-]

(1.5)

Taking logarithms of both sides of Equation (1.5) and multiplying throughout by - 1 gives - log[H +] = - log K' - log[HA] + log[A-]

(1.6)

Substituting pH for - log[H + ] and pK' for - log K' yields

H30 + + A-

a hydronium ion, H3 O+, is formed by the association of a hydrogen ion with a water molecule. In dilute solutions, the concentration of water changes very little when HA is added; therefore, by convention, the dissociation reaction equation is usually written as HA ~-- H + + A -

The Henderson-Hasselbalch equation was developed independently by the American biological chemist L. J. Henderson and the Swedish physiologist K. A. Hasselbalch, for relating the pH to the bicarbonate buffer system of the blood (see below). In its general form, the Henderson-Hasselbalch equation is a useful expression for buffer calculations. It can be derived from the equilibrium constant expression for a dissociation reaction of the general weak acid (HA) in Equation (1.3):

(1.3)

A weak acid, HA, does not readily dissociate, owing to the high affinity of the conjugate base, A - , for the hydrogen ion. Similarly in the hydrolysis reaction of a

[A-] pH = pK' + log ~ [HA]

(1.7)

or

pH = pK ~+ log

[conjugate base] [acid]

(1.8)

This relationship is represented by the HendersonHasselbalch equation. Since a buffer is intended to give only a small change in pH with added H + or O H - , the best buffer for a given pH is the one that gives the smallest change. As may be

6


TABLE 1-3

TABLE 1-4

Percent Unprotonated Species and Ratio of Unprotonated Forms Relative to the Difference between thepH andpK'*

pH Values of Human Body Fluids and Secretions

%A-

[A-]/[HA]

Log [A-] / [HA] ( p H - pK')

"100" 99 98 96 94 92 91 90 80 70 60 50 40 30 20 10 8 6 4 2 1 "0"

999/1 99/1 98/2 96/4 94/6 92/8 91/9 90/10 80/20 70/30 60/40 50/50 40/60 30/70 20/80 10/90 8/92 6/94 4/96 2/98 1/99 1/999

3.00 2.00 1.69 1.38 1.20 1.06 1.00 0.95 0.60 0.37 0.18 0.00 - 0.18 -0.37 -0.60 -0.95 - 1.06 -1.20 -1.38 -1.69 -2.00 -3.00

*Reproduced, with permission, from J. N. Aronson: The HendersonHasselbalch equation revisited. Biochemical Education 11(2), 68 (1981).

seen from the Henderson-Hasselbalch equation, when the pH of the solution equals the pK' of the buffer, [conjugate base] = [acid], and the buffer can therefore respond equally to both added acid and added base. It also follows from Equation (1.7) that when the pH of the solution is one pH unit above or below the pK' value, the solution contains approximately 9% unprotonated or protonated species, respectively. Similarly, if the pH of the solution is two units above or below the pK' value, the solution contains almost entirely (99%) unprotonated or protonated species, respectively. Table 1-3 provides percent unprotonated species and the corresponding unprotonated/protonated ratios for selected (pH-pK') values. Buffer Systems of Blood and Exchange of 02 and CO2 If the H + concentration departs significantly from its normal value in blood, the health and survival of the human

Body Fluid or Secretion Blood Milk Hepatic bile Gall bladder bile Urine (normal) Gastric juice (parietal secretion) Pancreatic juice Intestinal juice Cerebrospinal fluid Saliva Aqueous humor of eye Tears Urine (range in various disease states) Feces Muscle cell, resting (at 37~ extracellular pH = 7.4)

pH 7.4 6.6-6.9 7.4-8.5 5.4-6.9 6.0 0.87 8.0 7.7 7.4 7.2 7.2 7.4 4.8-7.5 7.0-7.5 6.94-7.06 (intracellular)

body are in jeopardy. H + is the smallest ion, and it combines with many negatively charged and neutral functional groups. Changes of [H+], therefore, affect the charged regions of many molecular structures, such as enzymes, cell membranes, and nucleic acids, and dramatically alter physiological activity. If the plasma pH reaches either 6.8 or 7.8, death may be unavoidable. Despite the fact that large amounts of acidic and basic metabolites are produced and eliminated from the body, buffer systems maintain a fairly constant pH in body fluids (Table 1-4). The major metabolic product from oxidation of ingested carbon compounds is CO2. Hydration of CO2 dissolved in water yields the weak acid H2CO3 (carbonic acid). Depending on the type of food ingested and oxidized, 0.7-1.0 mol of CO2 is produced per mole of O2 consumed. This results in the metabolic production of about 13 mol of hydrated CO2 each day in a normal person. For efficient transport of relatively insoluble CO2 from the tissues where it is formed to the lungs where it must be exhaled, the buffers of the blood convert CO2 to the very soluble anionic form HCO 3 (bicarbonate ion). The principal buffers in blood are bicarbonate-carbonic acid in plasma, hemoglobin in red blood cells, and protein functional groups in both. The normal balance between rates of elimination and production of CO2 yields a steady-state concentration CO2 in the body fluids and a relatively constant pH.

SECTION1.2 Buffers

7

F I G U R E 1-5 Schematic representation of the transport of CO2 from the tissues to the blood. Note that the majority of CO2 is transported as HCO~- in the plasma and that the principal buffer in the red blood cell is hemoglobin. Solid lines refer to major pathways, and broken lines refer to minor pathways. Hb = hemoglobin.

Other acids that are products of metabolism are lactic acid, acetoacetic acid, fi-hydroxybutyric acid, phosphoric acid, sulfuric acid, and hydrochloric acid. The organic acids (e.g., lactate, acetoacetate, and fi-hydroxybutyrate) are normally oxidized further to CO2 and H20. The hydrogen ions and anions contributed by mineral acids and any unmetabolized organic acids are eliminated via the excretory system of the kidneys. Thus, although body metabolism produces a large amount of acid, a constant pH is maintained by transport of H + ions and other acid anions in buffer systems and by elimination of CO2 through alveolar ventilation in the lungs and through excretion of aqueous acids in the urine. Metabolic activities continuously release CO2 to the blood (Figure 1-5), and the lungs continuously eliminate CO2 (Figure 1-6). As oxygen is consumed in peripheral tissues, CO2 is formed and its pressure (Pco2) builds to about 50 mm Hg, whereas the blood entering the tissue capillaries has a Pco2 of about 40 mm Hg. Because of this difference in Pco2 values, CO2 diffuses through the cell membranes of the capillary endothelium and the blood Pco2 rises to 45-46 mm Hg. Despite this increase in Pco2, the blood pH value drops by only about 0.03 during the flow from the arterial capillary (pH 7.41) to the venous capillary (pH 7.38) as a consequence of buffering. About 95% of the CO2 entering the blood diffuses into the red blood cells. Within the red blood cells, the enzyme carbonic anhydrase catalyzes conversion of most of the

C 0 2 to H 2 C O 3 : C O 2 -Jr- H 2 0 ~

H2CO3

H2CO3 dissociates to H + and HCO 3. Although H2CO3 is a weak acid, its dissociation is essentially 100% because of removal of H + ions by the buffering action of hemoglobin. The presence of CO2 and the production of H + cause a reduction in the affinity of hemoglobin for oxygen. Oxyhemoglobin (HbO2)consequently dissociates into oxygen and deoxyhemoglobin (Hb). This effect of pH on the binding of O2 to hemoglobin is known as the Bohr effect (Chapter 28). Oxygen diffuses into the tissues because the Po2 in blood is greater than the Po2 in tissue cells and because protonated deoxyhemoglobin (HHb) is a weaker acid than HbO2 and thereby binds H + more strongly than HbO2. When purified HbO2 dissociates at pH 7.4 to yield oxygen and Hb, the Hb binds 0.7 mol of H + per mole of oxygen released. However, under physiological conditions in whole blood, the Hb combines 0.31 mol of H + per mole of oxygen released. This process is reversible. The remainder of the H + is buffered by phosphate and proteins other than hemoglobin. The major buffering group involved in the transport of H + is an imidazolium group of a histidine residue in hemoglobin. The imidazolium group has a pK' value of about 6.5 (Figure 1-7 depicts the reactions). The difference in acidbase properties between the two forms of hemoglobin molecules is explained by the conformational change that


F I G U R E 1-6 Schematic representation of the transfer of CO2 from the alveolus (and its loss in the expired air in the lungs) and oxygenation of hemoglobin. Note that the sequence of events occurring in the pulmonary capillaries is the opposite of the process taking place in the tissue capillaries (Figure 1-5). Solid lines indicate major pathways and broken lines indicate minor pathways. Hb - hemoglobin.

accompanies conversion from HbO2 to HHb + (Chapters 7 and 28). As the concentration of HCO 3 (i.e., of metabolic CO2) in red blood cells increases, an imbalance occurs between the bicarbonate ion concentrations in the red blood cell and plasma. This osmotic imbalance causes a marked efflux of HCO 3 to plasma and consequent influx of C1- from plasma in order to maintain the balance of electrostatic charges. The latter osmotic influx, known as the chloride shift, is accompanied by migration of water to red blood cells. Thus, transport of metabolic CO2 in the blood occurs primarily in the form of plasma bicarbonate formed after CO2 diffuses into red blood cells.

F I G U R E 1-7 Buffer function of the imidazole/imidazolium functional groups of histidine residues in protein.

A small percentage of C 0 2 entering the red blood cells combines reversibly with an un-ionized amino group (-NH2) of hemoglobin: hemoglobin-NH2 + CO2 ~ hemoglobin-NH-COO- + H + Hemoglobin-NH-COO-, commonly known as carbaminohemoglobin, is more correctly named hemoglobin carbamate. Formation of this compound causes a lowering of the affinity of hemoglobin for oxygen. Thus, an elevated concentration of CO2 favors dissociation of oxyhemoglobin to oxygen and deoxyhemoglobin. Conversely, CO2 binds more tightly to deoxyhemoglobin than to oxyhemoglobin. All of these processes occurring in the red blood cells of peripheral capillaries are functionally reversed in the lungs (Figure 1-6). Since alveolar P02 is higher than that of the incoming deoxygenated blood, oxygenation of hemoglobin and release of H + occur. The H + release takes place because HbO2 is a stronger acid (i.e., has a lower pK') than deoxyhemoglobin. The released bicarbonate, which is transported to the red blood cells with the corresponding efflux of CI-, combines with the released H + to form H2CO3. Cellular carbonic anhydrase catalyzes dehydration of H2CO3 and release of CO2 from the red blood cells. Thus, red blood cell carbonic anhydrase which catalyzes the reversible hydration of CO2, plays a vital role in carbon dioxide transport and elimination. Carbonic anhydrase is a monomeric (M.W. 29,000) zinc metalloenzyme and is

9

SECTION 1.2 Buffers

present in several different isoenzyme forms. The most prevalent red blood cell isoenzyme of carbonic anhydrase is type I (CAI). Zinc ion, held by coordinate covalent linkage by three imidazole groups of three histidine residues, is involved in the catalytic mechanism of carbonic anhydrase. Water bound to zinc ion reacts with CO2 bound to the nearby catalytic site of carbonic anhydrase to produce H2CO3. The action of carbonic anhydrase is essential for a number of metabolic functions. Some include the formation of H + ion in stomach parietal cells (Chapter 12), in bone resorption by the osteoclasts (Chapter 37) and reclamation of HCO~ in renal tubule cells (Chapter 39). In osteoclasts and in the renal tubule cells, the isoenzyme CAII catalyzes the hydration reaction of CO2. A deficiency of CAII caused by an autosomal recessive disorder consists of osteopetrosis (marble bone disease), renal tubular acidosis, and cerebral calcification. The diffusion of CO2 from venous blood into the alveoli is facilitated by a pressure gradient of CO2 between the venous blood (45 mm Hg) and the alveoli (40 mm Hg) and by the high permeability of the pulmonary membrane to CO2. Blood leaving the lungs has a Pco2 of about 40 mm Hg; thus, essentially complete equilibration occurs between alveolar CO2 and blood CO2.

log

1.3

Taking antilogarithms, [HCOy]

20

proton acceptor

[H2CO3]

1

proton donor

This ratio is large because the pH is greater than the pK' (see Table 1-3). At pH 7.4, the bicarbonate system is a good buffer toward acid (i.e., it can neutralize large amounts of acid) but a poor buffer for alkali. However, blood H2CO3 is in rapid equilibrium with a relatively large (about 1000 times as much) reservoir of cellular CO2 and can function as an effective buffer against increases in alkalinity. The HCO3-/H2CO3 ratio in blood is coupled to the partial pressure of CO2, i.e., to the metabolic production of CO2 and to the loss of CO2 during respiration. In the equilibrium expression for the bicarbonate-carbonic acid buffer system at pH 7.4, the carbonic acid term can be replaced by a pressure term because the carbonic acid concentration is proportional to Pco2 in the blood. pH -- 6.1 + log

[Hco ]

(1.15)

a Pco2

[H2CO3] -- aPco2

Carbonic acid has the following pK' values: H + 4- HCO~-

HCO3- ~ CO 2- + H +

[H2CO3]

where a, a proportionality constant, is defined by the equation

Blood Buffer Calculations

H2CO3 ~

[Hco3 ] =

pK'I -- 3.8

(1.9)

pK~ -- 10.2

(1.10)

It is apparent from the pK' values that neither equilibrium can serve as a buffer system at the physiological pH of 7.4. However, carbonic acid (the proton donor) is in equilibrium with dissolved CO2, which in turn is in equilibrium with gaseous CO2: H20 + CO2(aqueous) +~- H2CO3

(1.11)

The hydration reaction (1.11), coupled with the first dissociation of carbonic acid (1.9), produces an apparent pK' of 6.1 for bicarbonate formation. Thus, the summation of Equations (1.9) and (1.11) yields H20 + CO2 ~ H + + HCO3pK' (apparent) -- [HCO~-] [H +] = 6.1 [H2CO3]

(1.12) (1.13)

The ratio of HCO~- to H2CO3 at a physiological pH of 7.4 can be calculated by using the Henderson-Hasselbalch relationship: 7.4 -- 6.1 4- log

[Hco3 ] [H2CO3]

(1.14)

(1.16)

The numerical value of a depends on the solvent and the temperature. For normal plasma at 37~ a 0.0301 mmol of dissolved CO2 per liter of plasma per mm Hg of CO2 pressure. Assuming that 37~ (310 K) is approximately the same as standard temperature, 25~ (298 K), and that [H2CO3] in Equation (1.16) includes both carbonic acid and dissolved CO2, the value of a can be derived from two facts: 1. At 37~ and 760 mm Hg of CO2 pressure, 521 mL of CO2 will dissolve per liter of normal plasma; 2. At standard temperature and pressure, 1 mol of dry CO2 occupies a volume of 22.26 L (not 22.4 L since CO2 is not an ideal gas). a--

521 mL CO2/L of plasma 760 mm Hg x 22.26 mL/mmol of CO2 mmol CO2/L of plasma = 0.0301 mm Hg of CO2 pressure

The equation form of the Henderson-Hasselbalch expression [Equation (1.15)] can be further modified by substituting another expression for the bicarbonate term. When excess strong acid is added to plasma, CO2 is stoichiometrically released from dissolved CO2, carbonic

10


acid, bicarbonate ions, and carbonate. Carbonate concentration is negligible; thus, Total [CO2] - [HCO3] + dissolved[CO2] + [H2CO3] But Dissolved[CO2] + [H2CO3] = 0.0301Pco2 so that [HCOy ] -- total[CO2] - 0.0301Pco2 Finally, pH -- 6.1 + log

(total [CO2] - 0.0301 Pco2 ) [0.0301Pco2 ] (1.17)

Equation (1.17) is useful for calculating the pH from the total [CO2] and Pco2. The HCO~-/H2CO3 buffer system effectively maintains a constant blood pH of 7.4 if bicarbonate and H2CO3 concentrations are maintained at a ratio of 20:1. The concentration of HCO~- is regulated by its selective excretion and reclamation by the membranes of the renal tubular epithelial cell. Pco2 and [H2CO3] in the blood can be altered by changes in the rate and depth of respiration. For examples, hypoventilation (slow, shallow breathing) leads to increased blood Pco2, whereas hyperventilation (rapid, deep breathing) has the opposite effect. Pco2 changes mediated by the lungs are more rapid than [HCO~-] changes affected through the kidneys (Chapter 39). Nonbicarbonate Buffers in Blood

Other important nonbicarbonate blood buffers are protein and phosphate. The predominant buffer system in the red blood cells is hemoglobin. Protein amino acid side chains (R-groups) that act as buffers are carboxylate groups of glutamate and aspartate and the weakly basic groups of lysine, arginine, and histidine. To be effective, the pK' value of a buffer should be close to the pH of the system to be buffered. Except for the R-group of histidine, which is an imidazolium group (Figure 1-7), the pK' values of the other amino acids mentioned above are not close enough to the physiological pH of blood to be effective buffers. The imidazolium group has a pK' value of 6.5 but it can vary from 5.3 to 8.3 depending on differences in electrostatic environment either within the same protein molecule or in different proteins. Another potential buffering group in protein is the a-amino group of the amino acid residues at the amino terminus of the protein. This group has a pK' value ranging from 7.8 to 10.6, with a typical value of about 8 (acid-base properties of amino acids and proteins are discussed in detail in Chapters 2 and 3). In plasma,

F I G U R E 1-8 Titration profile of phosphoric acid (H3PO4) with sodium hydroxide (NaOH). The three pK' values correspond to three buffer regions. The physiological buffering occurs at the pK' region with H2PO 4 (acid) and HPO 2- (conjugate base) ionic species.

the protein buffer system has a limited role; the principal plasma buffer is the bicarbonate-carbonic acid system. Compared with hemoglobin in the red blood cells and HCO3/HzCO3 in plasma, phosphates (both organic and inorganic) play minor roles in physiological buffering. Phosphoric acid (H3PO4) has three dissociable protons: H3PO4 Z H2PO 2 + H +

p K ' ~ - 1.9

(1.18)

H2PO 2 ~ n P O 2- + n +

pK~ -- 6.8

(1.19)

pK~ = 12.4

(1.20)

HPO42- = PO43- + n +

The titration profile of phosphoric acid with NaOH is shown in Figure 1-8. The principal dissociation expression functioning at a given pH depends on which pK' is closest to the pH. At a plasma pH of 7.4, the important conjugate pair is HPOZ-/HzPO4. The HendersonHasselbalch equation can be used to obtain the value of the ratio HPOZ-/HzPO4 at pH 7.4: [HPO 2- ] 7.4 -- 6.8 + log [H2PO2 ] [HPO2-] = 0.6 log [H2PO 4 ]

and

(1.21)

[HPO2-] =4 [H2PO 4 ]

As was the case for the bicarbonate-carbonic acid system, the conjugate base form (HPO 2-) of the phosphate buffer is present in large (fourfold) excess compared to the acid form (HzPO4) and provides acid buffering capacity. Since the body metabolism produces more acid than

SECTION1.3

Measurementof pH

11

H2N\ NH~

base, this ratio assists in neutralizing acid and in maintaining a constant pH. The HPOZ-/HzP04 buffering system plays a minor role in plasma because of the low concentrations of these ions, but it is important in raising the plasma pH through the excretion of HzPO 4 via the kidney (Chapter 39). In summary, hemoglobin absorbs a major portion of the hydrogen ions produced by the dissociation of H2C03 generated by the hydration of CO2--the most important buffer system in the blood. However, since hemoglobin and carbonic anhydrase are present only in red blood cells, the HCO3/HzCO 3 system in the plasma is an indispensable intermediary in transporting the acid. Thus, the principal method of C02 transport is in the form of HCOf in blood plasma.

_

(CH2)s

y

I

I ?H 2

.,

/

oooGlycine

I

HCNH~ I

COOArginine

J~ Arginine:glycine ~~midinotransferase

/

..;

~ H2N\ //NH~

(OH=)s

C I NH

I

HCNH~

I

I

CH2

COOOrnithine

I

COOGuanidinoacetate

S-adenosylmethionine --~ Guanidinoacetate S-adenosylhomocysteine,~ Methyltransferase(GMT)

1.3 Measurement of pH Blood and urine pH can be measured easily by means of a calibrated glass electrode, whereas pH measurement inside the metabolizing cells is not easily accomplished. Techniques for estimating intracellular pH include glass electrode measurements on homogenates, calorimetric or fluorometric analysis of intracellular distribution of indicator dyes, and microelectrode methods.

NH;

C '1 NH

§

H2 N+\\ / HN-pO2" C

I

Creatine Kinase

HNCH 3 cH2l I COOPhosphocreatine

44 nM), the condition is called acidemia. Conversely, if the pH rises above pH 7.44 ([H +] < 36 nM), the condition is called alkalemia. The suffix -emia refers to blood and usually to an abnormal concentration in blood. Over the pH range of 7.20-7.50, for every change of 0.01 pH unit, there is a change of approximately 1 nM [H +] in the opposite direction. Since the Henderson-Hasselbalch expression uses pH terms, its utility in clinical situations is less than optimal. Kassirer and B leich have derived a modified HendersonHasselbalch expression that relates [H+], instead of pH, to P c o 2 and HCO;, as follows: H2CO3

Clinical applications of Equation (1.23) are discussed in Chapter 39.

Supplemental Readings and References Properties of Water M. E Colombo, D. C. Rau, and V. A. Parsegian: Protein solvation in allosteric regulation: A water effect on hemoglobin. Science 256, 655 (1992). M. A. Knepper: Molecular physiology of urinary concentrating mechanism: Regulation of aquaporin water channels by vasopressin. American Journal of Physiology 272 (Renal Physiology 41), F3 (1997). M. A. Knepper, J. G. Verbalis, and S. Nielsen: Role of aquaporins in water balance disorders. Current Opinion in Nephrology and Hypertension 6, 367 (1997). M. D. Lee, L. S. King, and E Agre: The aquaporin family of water channel proteins in clinical medicine. Medicine 76, 141 (1997). R. E Rand: Raising water to new heights. Science 256, 618 (1992). P. M. Wiggins: Role of water in some biological processes. Microbiological Reviews 54, 432 (1990).

~ H + + HCO 3

Acid-Base Chemistry and Respiratory Function of Hemoglobin

for which

[HC03 ] [H + ] K! =

[H2CO3]

Substituting a x Pco2 for [H2CO3], as in Equation (1.16), yields

H. J. Androgue and N. E. Madias: Management of life-threatening acid-base disorders. New England Journal of Medicine; First of two parts 338, 26 (1998); Second of two parts 338, 107 (1998). C. C. W. Hsia: Respiratory function of hemoglobin. New England Journal of Medicine 338, 239 (1998).

K' = [HCO3] [H+]

Nuclear Magnetic Resonance and Magnetic Resonance Imaging

a x Pc02

and H+ = K'a

x Pco 2

[HCO;]

(1.22)

In Equation (1.22), K' and a are constants, and the numerical value of K'a is 24 when Pco2 is expressed in mm Hg, [HCO3] in mM, and [H+] in nM. Therefore, H+ = 24 • Pco2

[HCO;]

(1.23)

The above formulation expresses the interdependence of three factors; if two of them are known, the third can be readily calculated. For example, at a blood [HCO3] of 24 nM and a Pco2 of 40 mmHg, [H +] is 40 mM.

R. R. Edeman and S. Warach: Medical progress: Magnetic resonance imaging. New England Journal of Medicine; First of two parts 328, 708 (1993); Second of two parts 328, 785 (1993). S. Gilman: Medical progress: Imaging of the brain. New England Journal of Medicine; First of two parts 338, 812 ( 1998); Second of two parts 338, 889 (1998). S. J. Knoury and H. L. Weiner: Multiple sclerosis. Archives of Internal Medicine 158, 565 (1998). L. A. Moulopoulos and M. A. Dimopoulos: Magnetic resonance imaging of the bone marrow in the hematologic malignancies. Blood 90, 2127 (1997). A. Schulze, T. Hess, R. Wevers, et al.: Creatine deficiency syndrome caused by guanidinoacetate methyltransferase deficiency: Diagnostic tools for a new inborn error of metabolism. Journal of Pediatrics 131,626 (1997). S. St6ckler-Ipsiroglu: Creatine deficiency syndromes: A new perspective on metabolic disorders and a diagnostic challenge. Journal of Pediatrics 131, 510 (1997).

CHAPTER

2

Amino Acids

Proteins are the most abundant class of organic compounds in the healthy, lean human body, constituting more than half of its cellular dry weight. Proteins are polymers of amino acids and have molecular weights ranging from approximately 10,000 to more than one million. Biochemical functions of proteins include catalysis, transport, contraction, protection, structure, and metabolic regulation. Amino acids are the monomeric units, or building blocks, of proteins joined by a specific type of covalent linkage. The properties of proteins depend on the characteristic sequence of component amino acids, each of which has distinctive side chains. Amino acid polymerization requires elimination of a water molecule as the carboxyl group of one amino acid reacts with the amino group of another amino acid to form a covalent amide bond. The repetition of this process with many amino acids yields a polymer, known as a polypeptide. The amide bonds linking amino acids to each other are known as peptide bonds. Each amino acid unit within the polypeptide is referred to as a residue. The sequence of amino acids in a protein is dictated by the sequence of nucleotides in a segment of the DNA in the chromosomes and the uniqueness of each living organism is due to its endowment of specific proteins.

proteins have an amino group, a carboxyl group, a hydrogen atom, and an R-group attached to the a-carbon (Figure 2-1). Proline is an exception because it has a cyclic structure and contains a secondary amine group (called an imino group) instead of a primary amine group (called an amino group). Amino acids are classified according to the chemical properties of the R-group. Except for glycine (R = H), the amino acids have at least one asymmetrical carbon atom (the a-carbon). The absolute configuration of the four groups attached to the a-carbon is conventionally compared to the configuration of L-glyceraldehyde (Figure 2-2). The D and L designations specify absolute configuration and not the dextro (right) or levo (left) direction of rotation of planepolarized light by the asymmetrical carbon center. In organic chemistry, the assignment of absolute configurations of an asymmetrical center is made by the R and S classification of isomers. This system prioritizes the substituents linked to the asymmetrical carbon atom (e.g., decreasing atomic number or valence density) and the assignment is based upon the clockwise (R) or the counterclockwise (S) positioning of the three higher priority groups.

2.2 Classification 2.1 L-(x-Amino Acids: Structure Almost all of the naturally occurring amino acids in proteins are L-a-amino acids. The principal 20 amino acids in

A useful classification of the amino acids is based on the solubility (i.e., ionization and polarity) characteristics of the side chains (R-groups). The R-groups fall into four classes:

17

18

CHAPTER2 Amino Acids

COOH

CHO

(~-Carbon

CHO

I

R

OH H

CH2OH

FIGURE 2-1 Basic structureof an or-aminoacid.

H OH

CH2OH

CHO 1. 2. 3. 4.

Nonpolar (hydrophobic), Polar, negatively charged (acidic), Polar, positively charged (basic), and Polar, neutral (un-ionized).

Within each class, R-groups differ in size, shape, and other properties. Figure 2-3 shows the structure of each amino acid according to this classification with the Rgroup outlined. Ionizable structures are drawn as they would exist at pH 7.0. The three-letter and one-letter abbreviations for each amino acid are given in Table 2-1. A -yl ending on amino acid residue indicates that the carboxyl group of an amino acid is linked to another functional group (e.g., in a peptide bond). The eight essential amino acids (Table 2-1) are those which humans cannot synthesize and which must be supplied in the diet. The remaining amino acids are synthesized in the body by various biochemical pathways (Chapter 17).

I

H~C~OHI

CHO

I

OH--?~H

CH2OH D-Glyceraldehyde

CH2OH L-Glyceraldehyde

CHO

CHO

9

9

H " - C--- OH 9

H0 - - C.,--'.H 9

CH,OH

CH,OH

Fischer perspective formulas CHO

CHO OH

H O .......I

CH2OH

H

CH2OH

Fischer projection formulas Nonpolar Amino Acids

COOH

COOH

9

Glycine Glycine is the smallest amino acid and has an H atom as its R-group. It is the only u-amino acid that is not optically active. The small R-group provides a minimum of steric hindrance to rotation about bonds; therefore, glycine fits into crowded regions of many peptide chains. Collagen, a rotationally restricted fibrous protein, has glycyl residues in about every third position in its polypeptide chains. Glycine is used for the biosynthesis of many nonprotein compounds, such as porphyrins and purines. Glycine and taurine are conjugated with bile acids, products derived from cholesterol, before they are excreted into the biliary system. Conjugated bile acids are amphipathic and are important in lipid absorption (Chapter 12). Glycine also is a neurotransmitter; it is inhibitory in the spinal cord and excitatory in the cerebral cortex and other regions of the forebrain. Nonketotic hyperglycinemia (NKH) is an inborn error of glycine degradation in which a large amount of glycine accumulates throughout the body. NKH causes

~

9

H--C-=" NH2

H,N-- r

~

(-,H3 D-Alanine

CH3 L-Alanine

FIGURE 2-2 Different representations of the configurational stereoisomersof glyceraldehydeand of alanine.

severe consequences in the central nervous system (CNS) and leads to death (Chapter 17).

CO0 I +H3N--C--H I H Glycine

SECTION2.2 Classification

FIGURE 2-3 Classification of commonly occurring L-a-amino acids based on the polarity of side chains (R-groups) at pH 7.0.

19

20


TABLE 2-1 Amino Acid Abbreviations and Nutritional Property

Amino acid

Abbreviation

Designation letter

Nutritional property*

Alanine Arginine

Ala Arg

A R

Asparagine Aspartic acid Cysteine Glutamic acid Glutamine

Asn Asp Cys Glu Gln

N D C E Q

non-essential conditionally essential non-essential non-essential non-essential non-essental conditionally essential non-essential conditionally essential essential essential essential essential essential non-essential non-essential essential essential non-essential essential

Glycine Histidine

Gly His

G H

Isoleucine Leucine Lysine Methionine Phenylalanine Proline Serine Threonine Tryptophan Tyrosine Valine

lie Leu Lys Met Phe Pro Ser Thr Trp Tyr Val

I L K M F P S T W Y V

*The eight essential amino acids are not synthesized in the body and have to be supplied in the diet. The conditionally essential amino acids, although synthesized in the body, may require supplementation during certain physiological conditions such as pregnancy. The non-essential amino acids can be synthesized from various metabolites.

Valine, Leucine, and Isoleucine These branched-chain aliphatic amino acids contain bulky nonpolar R-groups and participate in hydrophobic interactions. All three are essential amino acids. A defect in their catabolism leads to maple syrup urine disease (Chapter 17). Isoleucine has asymmetrical centers at both the o~- and/3-carbons and four stereoisomers, only one of which occurs in protein. The bulky side chains tend to associate in the interior of water-soluble globular proteins. Thus, the hydrophobic amino acid residues stabilize the three-dimensional structure of the polymer.

COOCOO-

l

+H3N~C~H

I

I

+H3N~C~H

CH2

I

/ \

I

CH

H3C

OH 3 H3C

Valine

/ \

COO-

I

+H3N~C~H

I

H~C~CH 3

I

CH

CH2 OH3

Leucine

i

OH 3

Isoleucine

Phenylalanine A planar hydrophobic phenyl ring is part of the bulky R-group of phenylalanine. It is an essential amino acid whose metabolic conversion to tyrosine is defective in phenylketonuria (Chapter 17). Phenylalanine, tyrosine, and tryptophan are the only a-amino acids that contain aromatic groups and consequently are the only residues that absorb ultraviolet (UV) light (Figure 2-4). Tryptophan and tyrosine absorb significantly more energy than phenylalanine at 280 nm, the wavelength generally used to measure the concentration of protein in a solution.

CO0 -

Alanine

I

The side chain of alanine is a hydrophobic methyl group,--CH3. Other amino acids may be considered to be chemical derivatives of alanine, with substituents on the fl-carbon. Alanine and glutamate provide links between amino acid and carbohydrate metabolism (Chapter 22).

+H3N~C~H

I

COO -

Phenylalanine

I

+H3N~C~H Tryptophan

,~CarrbOn.~CiH3

Alanine

A bicyclic nitrogenous aromatic ring system (known as an indole ring) is attached to the /~-carbon of alanine to form the R-group of tryptophan. Tryptophan is a precursor of serotonin, melatonin, nicotinamide, and many

21

SECTION 2.2 Classification

FIGURE 2-4 UV absorption spectra of phenylalanine, tyrosine, and tryptophan.

naturally occurring medicinal compounds derived from plants. It is an essential amino acid. The indole group absorbs UV light at 280 n m - - a property that is useful for spectrophotometric measurement of protein concentration (Figure 2-4). Tryptophan and tyrosine both show fluorescence; however, tryptophan absorbs more intensely. When molecules are raised to a higher energy state by absorption of radiant energy, they generally are unstable and return to the ground state. The energy released in this process manifests as heat (radiation energy) or light. The process of light emission is known as fluorescence. The quantum of energy re-emitted as fluorescence is always less than that of absorbed energy. Thus, the fluorescent light always appears at a longer wavelength (lower energy) than the original absorbed light energy (Figure 2-5). Tryptophan fluorescence studies can provide valuable information regarding protein and protein-ligand conformations due to the effects of surrounding amino acid residues. COO -

I + H3N---C--- H

I CH2 I

N/CH Tryptophan

Methionine This essential amino acid contains an R-group with a methyl group attached to sulfur. Methionine serves as donor of a methyl group in many transmethylation reactions, e.g., in the synthesis of epinephrine, creatine, and

F I G U R E 2-5 Tryptophan fluorescence spectrum. The emission spectrumappears at longer wavelengths as compared to the absorption spectrum.

melatonin. Almost all of the sulfur-containing compounds of the body are derived from methionine. COO -

I +H3N~C---H I CH2 I CH2 I S I CH3

Methionine Proline Proline contains a secondary amine group, called an imine, instead of a primary amine group. For this reason, proline is called an imino acid. Since the three-carbon R-group of proline is fused to the ol-nitrogen group, this compound has a rotationally constrained rigid-ring structure. As a result, prolyl residues in a polypeptide introduce restrictions on the folding of chains. In collagen, the principal protein of human connective tissue, certain prolyl residues are hydroxylated (Figure 2-6). The hydroxylation occurs during protein synthesis and requires ascorbic acid (vitamin C) as a cofactor. Deficiency of vitamin C causes formation of defective collagen and scurvy (Chapters 25 and 38). COO -

I C~H I /OH2

+H2N ! H2C ~CH2

Proline

22

CHAPTER2 Amino Acids COOH

1COOH

HN"

COOH

H2N--C--H

2~C--H

H2C~ 4~CH2 ~C~ H OH 4-Hydroxyproline

CH2 HC--OH CH2

Present in collagen, a fibrous protein. COOH

I H2N--C--H I CH2 I CH~_ I CH2 I

O O-Phosphoserine

NH2 5-Hydroxylysine

COOH

I I

H=N--C--H COOH

CH=

I I CH2 I

r

H2N--C--H

C

H3C--N~~N

I

t

-C

I

ECH2

I

CH2 ,

I

(H3C),N--C--H

N

H a-Methylhistidine Present in myosin, a muscle protein.

H ~CH3 ~-N-Methyllysine The N-trimethyl derivative of lysine is involved in the synthesis of carnitine.

COOH

I I CH2 I

COO -

I H=N ~ C ~ o Diphthamide

A novel derivative of histidine present only in the eukaryotic protein elongation factor 2 (EF-2), which participates in the elongation step of protein biosynthesis. Diphtheria toxin inhibits eukaryotic protein synthesis by catalyzing a covalent modification of diphthamide (see Chapter 25).

H2N--C--H

I

HN - - - - - - - C - - H

I

Phosphorylationand dephosphorylation of selectedserine residuesin a variety of proteinsplayan importantrole in the regulationof metabolismand are mediatedby some hormones.

I I CH2 I O I HO--P--OH II

H2N--C--H

CH2

I

O---G~ ~CH2 C H2

CH ,OOC "~ ~COOH 7-Carboxyglutamic acid Present in certain blood-clotting proteins

Pyrrolidone car boxylate (pyroglutamate) Present in some proteins and peptides as N-terminal amino acid residue. H2N~

/COOH

c~ ;H~3 H2N~

fNH2

4z=.~~

HOoc/CH-- (CH,)~I

~3

NH2

(CRy#__ CH ~ C O O H

HOOc./CH -- (CH=)

(OH=)E--CH--COOH (CH=)F--CH--COOH 'I"

,H~,

(?H=),

H2NfC%ooH Desmosine

H~C%ooH Isodesmosine Desmosineand isodesmosineare formedfrom lysylresidues of the polypeptidechainsof elastin,a fibrousprotein.

FIGURE 2-6 Modified derivatives of certain amino acids are found in proteins.

I

NH2

23


Acidic Amino Acids

Aspartic Acid The ]3-carboxylic acid group of aspartic acid has a pK' of 3.86 and is ionized at pH 7.0 (the anionic form is called aspartate). The anionic carboxylate groups tend to occur on the surface of water-soluble proteins, where they interact with water. Such surface interactions stabilize protein folding. COO -

I

residues are present in a number of blood coagulation proteins (factors II, VII, IX, and X) and anticoagulant proteins C and S (Chapter 36). Osteocalcin, a protein present in the bone, also contains v-carboxyglutamate residues (Chapter 37). A cyclic, internal amide derivative of glutamic acid is pyrrolidone carboxylic acid (also known as pyroglutamic acid or 2-oxoproline). Some proteins (e.g., heavy chains of immunoglobulins; Chapter 35) and peptides (e.g., thyrotropin-releasing hormone; Chapter 33) have pyroglutamic acid as their N-terminal amino acid residue.

+ H3N---~C~H COO -

I ~CH2

I

I +H3N--(zC--H

I ~CH2 I

yCH2

Aspartate

I O//'C~o Glutamate

Glutamic Acid The v-carboxylic acid group of glutamic acid has a pK' of 4.25 and is ionized at physiological pH. The anionic groups of glutamate (like those of aspartate) tend to occur on the surfaces of proteins in aqueous environments. Glutamate is the primary excitatory neurotransmitter in the brain. Its levels are regulated by clearance that is mediated by glutamate transfer protein in critical motor control areas in the CNS. In amyotrophic lateral sclerosis (ALS) glutamate levels are elevated in serum, spinal fluid, and brain; glutamate excitotoxicity is implicated in the progression of the disease. ALS is a progressive disorder affecting motor neurons in the spinal cord, brain stem, and cortex. The precise molecular basis of the disease is unknown; however, factors involved are glutamate excitotoxicity, genetics, oxidative stress, and diminished neurotrophic factors. Two drugs that provide neuroprotection against glutamate excitotoxicity are riluzole and gabapentin. Gabapentin is an amino acid structurally related to the neurotransmitter v-aminobutyrate (GABA). GABA, an inhibitory neurotransmitter in the CNS, is produced by the decarboxylation of glutamate by glutamate decarboxylase, a pyridoxal phosphate dependent enzyme. GABA, when bound to its receptors, causes an increase in permeability to chloride ions in neuronal cells. A group of tranquilizing drugs known as benzodiazopines enhance the membrane permeability of chloride ions by GABA. In some proteins, the v-carbon of glutamic acid contains an additional carboxyl group. Residues of v-carboxyglutamic acid (Gla) bear two negative charges and can strongly bind calcium ions. v-Carboxylation of glutamic acid residues is a posttranslational modification and requires vitamin K as a cofactor, v-Carboxyglutamate

COO -

I

+H3NmCmH

COO -

I C--H I OH2

HN I

O---C

~c~

H2 Pyrrolidone carboxylate (pyroglutamate)

NH2 I

I

CH2

CH 2

I CH2 I

I

CH I O--c/C~c=o l I OO-

CH2 I COO-

},-amino butyrate (GABA)

y - C a r b o x y glutamate

Basic Amino Acids

Lysine Lysine is an essential amino acid. The long side chain of lysine has a reactive amino group attached to the s-carbon. The s-NH2 ( p K ' = 10.53) is protonated at physiological pH. The lysyl side chain forms ionic bonds with negatively charged groups of acidic amino acids. The s-NH2 groups of lysyl residues are covalently linked to biotin (a vitamin), lipoic acid, and retinal, a derivative of vitamin A and a constituent of visual pigment. In collagen and in some glycoproteins, g-carbons of some lysyl residues are hydroxylated (Figure 2-6), and sugar moieties are attached at these sites. In elastin and collagen, some s-carbons of lysyl residues are oxidized to reactive aldehyde (-CHO) groups, with elimination of NH3. These aldehyde groups then react with other s NH2 groups to form covalent cross-links between polypeptides, thereby providing tensile strength and insolubility to

24


protein fibers. Examples of cross-linked amino acid structures are desmosine, isodesmosine (Figure 2-6), dehydrolysinonorleucine, lysinonorleucine, merodesmosine, and dehydromerodesmosine (Chapter 10). Lysyl R-groups participate in a different type of cross-linking in the formation of fibrin, a process essential for the clotting of blood. In this reaction, the s-NH2 group of one fibrin polypeptide forms a covalent linkage with the glutamyl residue of another fibrin polypeptide (Chapter 36).

by diphenhydramine and pyrilamine. Hi-receptor antagonists are used in the treatment of allergic disorders. Secretion of HC1 by the stomach (Chapter 12) and an increase in heart rate are mediated by H2 receptors. Examples of Hz-receptor antagonists of histamine action are cimetidine and ranitidine, agents used in the treatment of gastric ulcers.

Arginine COO -

The positively charged guanidinium group attached to the 6-carbon of arginine is stabilized by resonance between the two NH2 groups and has a pK' value of 12.48. Arginine is utilized in the synthesis of creatine and it participates in the urea cycle (Chapter 17). The nitrogen of the guanidino group of arginine is converted to nitric oxide (NO) by nitric oxide synthase. NO is unstable, highly reactive, and has a life span of only a few seconds. However, NO affects many biological activities, including vasodilation, inflammation, and neurotransmission (Chapter 17).

+H3N~=C~H /YCH2 yCH2 6CH2 sCH2 NH3 + Lysine

Histidine

COO-

The imidazole group attached to the/~-carbon of histidine has a pK' value of 6.0. The pK' value of histidyl residues in protein varies depending on the nature of the neighboring residues. The imidazolium-imidazole buffering pair has a major role in acid-base regulation (e.g., hemoglobin). The imidazole group functions as a nucleophile, or a general base, in the active sites of many enzymes and may bind metal ions. Histidine is nonessential in adults but is essential in the diet of infants and individuals with uremia (a kidney disorder). Decarboxylation of histidine to yield histamine occurs in mast cells present in loose connective tissue and around blood vessels, basophils of blood, and enterochromaffin-like (ECL) cells present in the acid-producing glandular portion (oxyntic cells) of the stomach. The many specific reactions of histamine are determined by the type of receptor (HI or H2) present in the target cells. The contraction of smooth muscle (e.g., gut and bronchi) is mediated by Hi receptors and antagonized

l

+H3N--CmH

I

CH2

I CH2

I 6CH2

I

NH

I

C~'NH2 ~ Guanidinium I

,

NH~.. . . . . !

Neutral

Amino

Acids

Serine The primary alcohol group of serine can form esters __.:*.L

wit.

._1_

I . . . .

." _

p.u~p.ull~

~

_

". _1

[11 ~'.

. . . . . .

,~l

,

+H3N--C~H

I CH2

I

I

C

CH

C

I

I

I

N,.

~c / H Histidine

NH

N,.

"~c j H Histamine

_1

_

1_

_

"_ _1 . . . . .

:a_l_

-

I

H3N~C~H

I

._

+H3NmC--H

I

CH2

~

I

H

I

f'~

a~lu t l " l g u l c z.-U) allu gly~u~luc~ wluJ COO

COO -

group

Arginine

Hm C - - O H

I CH

H Serine

I

NH

sugars. The phosphorylation and dephosphorylation processes regulate the biochemical activity of many proteins. Active centers of some enzymes contain seryl hydroxyl


25

groups and can be inactivated by irreversible derivatization of these groups. T h e - O H group of serine has a weakly acidic pK' of 13.6.

Threonine This essential amino acid has a second asymmetrical carbon atom in the side chain and therefore can have four isomers, only one of CO0 -

I

+H3N~C~H

I

H~C~OH

I

CH3 Threonine

which, L-threonine, occurs in proteins. The hydroxyl group, as in the case of serine, participates in reactions with phosphoric acid and with sugar residues.

Cysteine The weakly acidic ( p K ' = 8.33) sulfhydryl group (-SH) of cysteine is essentially undissociated at physiological pH. Free-SH groups are essential for the function of many enzymes and structural proteins. Heavy metal ions, e.g., Pb 2+ and Hg 2+, inactivate these proteins by combining with their-SH groups. Two cysteinyl-SH groups can be oxidized to form cystine. A covalent disulfide bond of cystine can join two parts of a single polypeptide chain or two different polypeptide chains through cross-linking of cysteine residues. These - S - S - bonds are essential both for the folding of polypeptide chains and for the association of polypeptides in proteins that have more than one chain, e.g., insulin and immunoglobulins.

Tyrosine The phenolic hydroxyl group of this aromatic amino acid has a weakly acidic pK' of about 10 and therefore is un-ionized at physiological pH. In some enzymes, the hydrogen of the phenolic hydroxyl group can participate in hydrogen bond formation with oxygen and nitrogen atoms. The phenolic hydroxyl group of tyrosine residues in protein can be sulfated (e.g., in gastrin and cholecystokinin; see Chapter 12) or phosphorylated by a reaction catalyzed by the tyrosine-specific protein kinase that is a product of some oncogenes (Chapter 26). Tyrosine kinase activity also resides in a family of cell surface receptors that includes receptors for such anabolic polypeptides as insulin, epidermal growth factor, plateletderived growth factor, and insulin-like growth factor type 1. All of these receptors have a common motif of an external ligand binding domain, a transmembrane segment, and a cytoplasmic tyrosine kinase domain (Chapter 22). Tyrosine accumulates in tissues and blood in tyrosinosis and tyrosinemia, which are due to inherited defects in catabolism of this amino acid. Tyrosine is the biosynthetic precursor of thyroxine, catecholamines, and melanin. Tyrosine and its biosynthetic precursor, phenylalanine, both absorb UV light (Figure 2-4). COO -

I

+H3N--C--H

I CH2

C) OH Tyrosine

Asparagine The R-group of this amide derivative of aspartic acid has no acidic or basic properties but is polar and participates in hydrogen bond formation. It is hydrolyzed to aspartic acid and ammonia by the enzyme asparaginase. In glycoproteins, the carbohydrate side chain is often linked through the amide group of asparagine.

CO0 -

I

+H3N~C~H

I

CH2

I

SH Cysteine COO

-

I

+H3N~C~H CO0 -

COO-

I

I

+H3N~C~H

+H3N~C~H

I

CH2~S~S Cystine

I

CH2

I

CH2

I O//'C~NH 2 Asparagine

2fi

CHAPTER 2 AminoAcids Glutamine

This amide of glutamic acid has properties similar to those of asparagine. The 7-amido nitrogen, derived from ammonia, can be used in the synthesis ofpurine and pyrimidine nucleotides (Chapter 27), converted to urea in the liver (Chapter 17), or released as NH3 in the kidney tubular epithelial cells. The last reaction, catalyzed by the enzyme glutaminase, functions in acid-base regulation by neutralizing H + ions in the urine (Chapter 39). Glutamine is the most abundant amino acid in the body. It composes more than 60% of the free amino acid pool in skeletal muscle. It is metabolized in both liver and gut tissues. Glutamine, along with alanine, are significant precursors of glucose production during fasting (Chapter 15). It is a nitrogen donor in the synthesis of purines and pyrimidines required for nucleic acid synthesis (Chapter 27). Glutamine is enriched in enteral and parenteral nutrition to promote growth of tissues; it also enhances immune functions in patients recovering from surgical procedures. Thus, glutamine may be classified as a conditionally essential amino acid during severe trauma and illness. CO0 -

I +H3N---C~H

I CH2

I

CH2

I O~/'C~NH 2 Glutamine

Unusual Amino Acids

Several L-amino acids have physiological functions as free amino acids rather than as constituents of proteins. Examples are as follows: 1. ~-Alanine is part of the vitamin pantothenic acid. 2. Homocysteine, homoserine, omithine, and citrulline are intermediates in the biosynthesis of certain other amino acids. 3. Taurine, which has an amino group in the ~-carbon and a sulfonic acid group instead of COOH, is present in the CNS and as a component of certain bile acids participates in digestion and absorption of lipids in the gastrointestinal tract. 4. y-Aminobutyric acid is an inhibitory neurotransmitter. 5. Hypoglycin A is present in unripe akee fruit and produces severe hypoglycemia when ingested.

6. Some D-amino acids are found in polypeptide antibiotics, such as gramicidins and bacitracins, and in bacterial cell wall peptides.

Amino Acids Used as Drugs D-Penicillamine (D-fl,/3-dimethylglycine), a metabolite of penicillin, was first isolated in the urine specimens from patients treated with penicillin with liver disease. It is an effective chelator of metals such as copper, zinc, and lead. It is used in the chelation therapy of Wilson's disease, which is characterized by excess copper accumulation leading to hepatolenticular degeneration (Chapter 37).

CH3

I I

H3C ~ C ~ CH - - CO0"

I

SH NH + D-penicillamine

N-Acetylcysteine is administered in the acetoaminophen toxicity. It replenishes the hepatic stores of glutathione (Chapter 17). N-Acetylcysteine is also used in the treatment of pulmonary diseases including cystic fibrosis (Chapter 12). In patients with chronic renal insufficiency, prophylactic oral administration of N-Acetylcysteine have been used in the prevention of further renal impairment due to administration of radiographic contrast agents. In this setting presumably N-Acetylcysteine functions as an antioxidant and augments the vasodilatory effect of nitric oxide via the formation of S-nitrosothiol (Chapter 17). COOH

O

I

II

HS~CH2---CH--NH ~C~CH

3

N-acetylcysteine

Gabapentin is y-aminobutyrate covalently linked to cyclohexane to make it lipophilic and to facilitate its transport across the blood-brain barrier. It is used as an anticonvulsant and in amyotrophic lateral sclerosis (ALS).

H3N'~COO Gabapentin

SECTION2.3 Electrolyte and Acid-Base Properties

27

H /o

2.3 Electrolyte and Acid-Base Properties

I

Nonpolar neutral

Amino acids are ampholytes, i.e., they contain both acidic and basic groups. Free amino acids can n e v e r occur as neutral nonionic molecules:

H

0

I

II

R--C--C--OH

I

= R--C--CI

R-group

I NH3

~"0

Zwitterion

Zwitterions are electrically neutral and so do not migrate in an electric field. In acidic solution (below pH 2.0), the predominant species of an amino acid is positively charged and migrates toward the cathode: H

O

I

II

R--C--C--OH

I NH3 +

FIGURE 2-7 Titration profile of glycine, a monoaminomonocarboxylicacid.

Resonance,-- stabilized carboxylate anion

Ammonium cation

NH2

Instead, they exist as neutral z w i t t e r i o n s that contain both positively and negatively charged groups:

e

28


In basic solution (above pH 9.7), the predominant species is negatively charged and migrates toward the anode:

H I

O H

RmC_C_O I NH2

FIGURE 2-8 Titration profile of histidine.

-

The isoelectric point (pI) of an amino acid is the pH at which the molecule has an average net charge of zero and therefore does not migrate in an electric field. The pI is calculated by averaging the pK' values for the two functional groups that react as the zwitterion becomes alternately a monovalent cation or a monovalent anion. At physiological pH, monoaminomonocarboxylic amino acids, e.g., glycine and alanine, exist as zwitterions.

SECTION 2.3

Electrolyte and Acid-Base Properties

That is, at a pH of 6.9-7.4, the oe-carboxyl group (pK' 2.4) is dissociated to yield a negatively charged carboxylate ion (-COO-), while the ol-amino group (pK' 9.7) is protonated to yield an ammonium group (-NH~-). The pK' value of the o~-carboxyl group is considerably lower than that of a comparable aliphatic acid, e.g., acetic acid (pK' 4.6). This stronger acidity is due to electron withdrawal by the positively charged ammonium ion and the consequent increased tendency of a carboxyl hydrogen to dissociate as an H +. The or-ammonium group is corre-

FIGURE 2-9 Titrationprofileof lysine.

29 spondingly a weaker acid than an aliphatic ammonium ion, e. g., ethylamine (pK' 9.0) because the inductive effect of the negatively charged carboxylate anion tends to prevent dissociation of H +. The titration profile of glycine hydrochloride (Figure 2-7) is nearly identical to the profiles of all other monoaminomonocarboxylic amino acids with nonionizable R-groups (Ala, Val, Leu, Ile, Phe, Ser, Thr, Gln, Asn, Met, and Pro). The titration of glycine has the following major features. The titration is initiated with glycine hydrochloride,

30


F I G U R E 2-11} Titration profile of aspartic acid.

CI-(H~-NCH2COOH), which is the fully protonated form of the amino acid. In this form, the molecule contains two acidic functional groups; therefore, two equivalents of base are required to completely titrate 1 mol of glycine hydrochloride. There are two pK' values: pK'1 due to reaction of the carboxyl group and pK~ due to reaction of the ammonium group. Addition of 0.5 eq of base to 1 mol of glycine hydrochloride raises the pH 2.34 (PKtl), whereas addition of 1.5 eq further increases the pH to 9.66 (pK~). At low pH values (e.g., 0.4), the molecules are

predominantly cations with one positive charge; at pH values of 5-7, most molecules have a net charge of zero; at high pH values (e.g., 11.7), all of the molecules are essentially anions with one negative charge. The midpoint between the two pK' values [i.e., at pH = (2.34 + 9.66)/2 = 6.0] is the pI. Thus, pI is the arithmetic mean of pK'1 and pK~ values and the inflection point between the two segments of the titration profile. The buffering capacities of weak acids and weak bases are maximal at their pK values. Thus, monoaminomono-

SECTION 2.4

Chemical Reactions of Amino Acids

31

TABLE 2-2 pK'and p l Values of Selected Free Amino Acids at 25~ * Amino Acid

pI

pK' 1 (a-COOH)

pK' 2

pK' 3

Alanine

2.34

9.69 (a-NH3+)

Aspartic acid

2.09

3.86 (/3-COOH)

9.82 (a-NH 3+)

298(pKI+pK21) 2

Glutamic acid

2.19

4.25 (7-COOH)

9.67 (tx-NH3+)

3.22 ( pK'I +2pK'2)

Arginine

2.17

9.04 (a-NH3+)

Histidine

1.82

6.00 (Imidazolium)

Lysine

2.18

Cysteine

6.00

10.76

pK' + PK'3) 2

9.17 (NH3+)

7.59

(eKe+eKe) 2

8.95 (a-NH3+)

10.53 (e-NH3 +)

9.74

(PK'2 + PK'3) 2

1.71

8.33 (-SH)

10.78 (~-NH3+)

5.02 ( pK'I +2pK'2 )

Tyrosine

2.20

9.11 (a-NH3 +)

10.07 (Phenol OH)

5.66 ( pK'I +2pK'2 )

Serine

2.21

9.15 (a-NH3 +)

13.6 (Alcohol OH)

5.68 ( pK'I +2pK'2)

12.48 (Guanidinium)

*ThepK' valuesfor functionalgroupsin proteinsmay varysignificantlyfromthe valuesfor free aminoacids.

carboxylic acids exhibit their greatest buffering capacities in the two pH ranges near their two pK' values, namely, pH 2.3 and pH 9.7 (Figure 2-7). Neither these amino acids nor the a-amino or ot-carboxyl groups of other amino acids (which have similar pK' values) have significant buffering capacity in the neutral (physiological) pH range. The only amino acids with R-groups that have buffering capacity in the physiological pH range are histidine (imidazole; pK' 6.0) and cysteine (sulfhydryl; pK' 8.3). The pK' values for R-groups vary with the ionic environment. The titration profile of histidine is shown in Figure 2-8. The pI is the mean of pK~ and pKg. Titration profiles of the basic and acidic amino acids lysine and aspartic acid are shown in Figures 2-9 and 2-10. The R-groups are ionized at physiological pH and have anionic and cationic groups, respectively. The pI value for aspartic acid is the arithmetic mean of pK'1 and pK~, whereas for lysine and histidine the pI values

are given by the arithmetic mean of pK~ and pKg. The pK' and pI values of selected amino acids are listed in Table 2-2.

2.4 Chemical Reactions of Amino Acids The reactions of amino acids with ninhydrin, carbon dioxide, metal ions, and glucose are described below. The last three are of physiological importance. Ninhydrin (triketohydrindene hydrate) reacts with o~amino acids to produce CO2, NH3, and an aldehyde with one less carbon than the parent amino acid. In most cases, a blue or violet compound (proline and hydroxyproline give a yellow color) is formed owing to reaction of the liberated NH3 with ninhydrin, as shown in Figure 2-11. Color and CO2 production provide a basis for the quantitative determination of amino acids.

32

CHAPTER2 AminoAcids O

II

H

C\ /OH

I

RmCRCOOH

7~OH

I

NH2

II

o Ninhydrin

Amino acid H +

CO2

+

H20

O

II

7 ~NH 2 I! o

o II Ninhydrin

II

o o

o-

QL I!

I

cT=N-\

§ 2.20

II

II

o o Purple-coloredion FIGURE 2-11 Reaction of an o~-aminoacid with ninhydrin.Two moleculesof ninhydrin and the nitrogenatomof the aminoacid are involvedin the productionof the purpleproduct.

Ammonia, some amines, and some proteins and peptides will also yield a colored product but will not generate CO2. Thus, the colorimetric analysis is not specific for amino acids unless CO2 release is measured or the amino acids are purified and freed from interfering materials (the usual procedure). The color reaction with ninhydrin is used extensively in manual and automated procedures. CO2 adds reversibly to the un-ionized amino group of an amino acid. The product is a carbamate (or carboxyamino) derivative.

0

0

C/LO ~C

H2NaC--H

I j

R

H =

Cl[110N--C--H

I O / / , C . o _R

+

H+

This type of reaction accompanies transport of C02 in the blood (Chapter 1). In tissue capillaries, CO2 combines with free o~-amino groups of hemoglobin to form carbaminohemoglobin; in pulmonary capillaries, this reaction is reversed to release CO2 into the alveoli. This mode of transport is limited to only about 10% of the carbon dioxide transported in the blood. Metal ions can form complexes with amino acids. Metal ions that function in enzymatic or structural biochemical systems include those of iron, calcium, copper, zinc, magnesium, cobalt, manganese, molybdenum, nickel, and chromium. The functional group that binds a metal ion is called a ligand. Ligands are electron donors that form noncovalent bonds with the metal ions, usually two, four, or six ligand groups per ion. When four ligand groups bind a metal ion, the complex is either a plane or a tetrahedron; when six ligand groups participate, octahedral geometry results. The term chelation is applied to a metal-ligand interaction when a single molecule provides two or more ligands (e.g., chelation of iron with four nitrogens in one porphyrin molecule; see Chapter 14). Metal ions can also react with amino acid functional groups to abolish the biological activity of proteins. Heavy metal ions that form highly insoluble sulfides (e.g., HgS, PbS, CuS, AgzS) characteristically react with sulfhydryl groups of cysteinyl residues. If the reactive-SH group is involved in biological activity of the protein, the displacement of the hydrogen and the addition of a large metal atom to the S atom usually cause a major change in protein structure and loss of function. Hence, heavy metals are often poisons. In contrast, amino acid residues in proteins may undergo nonenzymatic chemical reactions that may or may not alter biological activity. An example of this type of reaction is the formation of glycated proteins. The amino groups of proteins combine with carbonyl groups of sugars (glucose) to form labile aldimines (Schiff bases), which are isomerized (Amadori rearrangement) to yield stable ketoamine (fructosamine) products (Figure 2-12). The degree of glycation achieved in a protein is determined by the concentration of sugar in the environment of the protein. In glycated hemoglobin, a Schiff-base adduct forms between the sugar and the N-terminal group of the /~-chains of hemoglobin. The Amadori sugar-amino acid residue adducts in proteins are produced with prolonged hyperglycemia and undergo progressive nonenzymatic reactions involving dehydration, condensation, and cyclization. These compounds are collectively known as advanced glycosylation end products and are involved in the chronic complications of diabetes mellitus (cataracts and nephropathy) (Chapter 22).

SupplementalReadingsand References

33

~2COH O

.of~ Glucose -H20

RNH2

Aminogroup of a protein

H OH

1[

H2COH

H2COH OH HR

H pNR

-_ ~ H(

H

H

H

/

1

, ,

OH Glucosylamine H+[

OH Schiff base [ Amadorirearrangement

H2COH H2R H(

H

H2c~O"~ ' ~

OH

OH

[02]

H~//(~H2NHR

OH 1-Amino-1-deoxy2-keto compound

HN

I (?H2)3 CH

I

((~H2)4 CH I~

I~ H2N COON H2N COOH Pentosidine

F I G U R E 2-12 Nonenzymatic reaction between the glucose and a free amino group of a protein.

Supplemental Readings and References D. G. Dyer, J. K. Blackledge, S. R. Thorpe, and J. W. Baynes: Formation of pentosidine during nonenzymatic browning of proteins by glucose. Journal of Biological Chemistry 266, 11654 (1991). M. E. Gurney, E B. Curring, E Zhai, and others: Benefit of vitamin E, riluzole, and gabapentin in a transgenic model of familial amyotrophic lateral sclerosis. Annals of Neurology 39, 147 (1996). R. G. Hankard, M. W. Haymond, and D. Darmaun: Role of glutamine as a precursor in fasting humans. Diabetes 46, 1535 (1997). S. J. Kuhl and H. Rosen: Nitric oxide and septic shock. West Journal of Medicine 168, 176 (1998). L. Lacomblez, G. Bensimon, E N. Leigh, et al.: Dose-ranging study of riluzole in amyotrophic lateral sclerosis. Lancet 347, 1425 (1996). J. Loscalzo and G. Welch: Nitric oxide and its role in the cardiovascular system. Progress in Cardiovascular Diseases 38, 87 (1995).

R. G. Miller, D. Moore, L. A. Young, et al.: Placebo-controlled trial of gabapentin in patients with amyotrophic lateral sclerosis. Neurology 47, 1383 (1996). R. Safirstein, L. Andrade, and J. M. Vieira: Acetylcysteine and nephrotoxic effects of radiographic contrast agents--a new use for an old drug. The New England Journal of Medicine 343, 210 (2000). M. Tepel, M. van der Giet, C. Schwarzeld, and others: Prevention of radiographic-contrast-agent-induced reductions in renal function by acetylcysteine. The New England Journal of Medicine 343, 180 (2OOO). H. Vlassara: Recent progress on the biological and clinical significance of advanced glycosylation end products. J. of Laboratory and Clinical Medicine, 124, 19 (1994). T. R. Ziegler, K. Benfele, R. J. Smith, et al.: Safety and metabolic effects of 1-glutamine administration in humans. Journal of Parenteral and Enteral Nutrition 14, 1375 (1990).


CHAPTER

3


To understand the structure and function of a protein, one must know the number and kinds of amino acids present in the protein and their order (called sequence or primary structure). This information is necessary to understand the effects of mutations, mechanisms of enzyme-catalyzed reactions, and chemical synthesis of species-specific peptides that may eliminate undesirable hypersensitivity reactions. Studies of amino acid sequences in proteins have aided in the understanding of the evolutionary development of living systems. Another application of amino acid sequence determination is in recombinant DNA technology. A desired DNA coding for a given polypeptide can be constructed from knowledge of the precise sequence of that polypeptide (Chapter 23). Through the use of available technologies of sequence determination and peptide synthesis, peptide fragments have been produced that are identical to part of large proteins present on the surface of a virus or other pathogen. In the appropriate host, such peptides elicit antibody production that is effective against the active pathogen and are potentially useful in the development of synthetic vaccines (Chapter 35). Viruses for which immunogenic peptides are being synthesized and clinically applied are hepatitis B, influenza, rabies, mouse leukemia, and hoof-and-mouth disease. Bovine insulin (M.W. 5,700) was the first protein to be completely sequenced (Sanger, 1955). Sanger was also the first to deduce the base sequence of a DNA molecule obtained

from phage 4~X 174 (a bacterial virus; see Chapter 23). Bovine insulin comprises two peptide chains of 21 and 30 amino acids each, linked by two interchain disulfide bonds. In 1960, Hirs, Moore, Stein, and Anfinsen described the first primary structure of the enzyme ribonuclease (M.W. 13,700), which has a single peptide chain of 124 amino acid residues and four intrachain disulfide bonds. These investigators established many of the techniques still used in sequence analysis, such as the use of ion exchange resins for separation of peptides and amino acids.

3.1 Quantitative Determination of Proteins UV absorption at 280 nm is an inaccurate method of protein determination because proteins have different amounts of tyrosine and tryptophan residues, and nucleic acids also absorb at 280 nm. In the biuret test, the sample is treated with an alkaline copper sulfate reagent that produces a violet color and requires a peptide with at least two peptide bonds. The violet color is produced through formation of a coordination complex (between peptide nitrogen atoms and cupric ion) that is analogous to the structure of the complex of biuret with cupric ion, as shown below:

35

36

CHAPTER3 Protein Isolation and Determination of Amino Acid Sequence NH2

I

O H

C=O

2

I

NH

HH

, , . . ./ %/ ~\N

+

I

C--O

I NH2 Biuret

Cu 2+

,NH:

C--N"

J/J Un

H,O

\WL-d\ /

_..'% m

+.

l H31~I -

A

il

1 B

I

1 C

COO -

I

;NH

"N--C MI-I m O

% .I \,,\\

Violet complex

In the Folin-Ciocalteu reaction, the protein reacts with the phosphomolybdotungstic acid reagent to give a blue color through reaction with tyrosyl residues. The sensitivity of this method depends on the amino acid composition of proteins in each sample. Protein content, particularly in urine or cerebrospinal fluid, may also be estimated by methods based on precipitation using sulfosalicylic acid (an anionic protein precipitant) or heat. The turbidity, which is a measure of protein concentration, can be quantitated by spectrophotometric absorbance methods or light scattering analysis. Absorbance of a hydrophobic indicator dye that binds to protein and changes color is also used. Specific proteins in biological fluids can be estimated by more discriminating methods, such as electrophoresis, specific binding techniques, or immunochemical methods.

Assume that cleavage of the above polypeptide with a specific cleaving agent yields four peptides, A, B, C, and D. These peptides are separated and sequenced but it is yet not known whether the correct order is ABCD or ACBD. A second hydrolytic procedure gives rise to three peptides: E, F, and G.

----E +

+---F .... i

A-B break

i B-C break

-+ I

Gl C-D break

--

_ COO

-

By comparison of the amino acid sequence of fragments E and G, the terminal fragments A and D can be matched with their respective adjacent peptides, B and C. Alternatively, the order of the peptides B and C can be deduced from the overlapping sequence in E Purification of a protein is indispensable for establishing its amino acid sequence. Mixtures of proteins will yield mixtures of peptides and ambiguous amino acid positions; a unique amino acid sequence can be obtained only from a pure protein. Protein purification techniques exploit differences in size, shape, charge, solubility, and specific binding affinity of the proteins. The optimal combination of techniques is usually reached by trial and error.

3.2 Determination of Primary Structure The determination of the primary structure of a protein consists of the following steps: 1. Obtain a pure protein. 2. Determine the amino acid composition and molecular weight of the pure protein. From amino acid composition and molecular weight data, calculate the number of residues of each amino acid present per protein molecule to the nearest whole number. 3. Reduce disulfide bonds to sulfhydryl groups. 4. Determine amino terminal (N-terminal) and carboxy terminal (C-terminal) amino acids. (A unique residue for each terminus suggests that the native protein contains only one peptide chain.) 5. Fragment aliquots of the polypeptide by enzymatic or chemical hydrolysis and separate the peptide mixtures into individual fragments. (This process will yield overlapping sets of smaller peptides.) 6. Sequence each fragment and, by analyzing the overlapping parts, assemble the sequence of the original protein. The logic for arranging overlapping peptides is as follows:

3.3 Separation of Proteins Proteins are separated on the basis of differences in their size, shape, charge, solubility, and binding affinity for other molecules. Most separations begin with proteins in solution. However, when proteins are an integral part of an organelle's structure, it is first necessary to extract them from membranous elements. Chemicals used to extract proteins from membranous particles include dissociating agents (e.g., urea and mercaptoethanol), chelating agents (e.g., ethylenediaminetetraacetic acid [EDTA]), and organic detergents (e.g., sodium deoxycholate, sodium lauryl sulfate, and Triton X-100). Since the total quantity (or activity) of a protein in a tissue sample is difficult to determine, the original (100%) quantity of protein at the start of a purification is usually based on measurements made on an aliquot of the initial homogenate. Each step in a purification process should remove extraneous protein and retain most of the protein of interest. A pure protein preparation is operationally defined as one that maintains a high activity per gram of protein following several purification steps, i.e., the optimum

SECTION3.3 Separation of Proteins

specific activity of the protein. Alternatively, a pure protein cannot be further subdivided by the methods described below, e.g., chromatography or electrophoresis. The initial purification steps usually separate proteins according to general classification, e.g., fibrous (insoluble) or globular (soluble). The fibrous and globular designations are related to shape and solubility. Globular proteins are spherical or ellipsoidal and make up the majority of known proteins. Fibrous proteins contain one or more polypeptide chains. Their molecules are elongated and asymmetrical with lengths that may be many times their diameters. Lateral cross-linking between adjacent polypeptides, by a variety of types of chemical bonding, confers mechanical strength and water insolubility to fibrous proteins; consequently, they are found in connective, elastic, and contractile tissues as well as in hair and skin. An initial aqueous extraction procedure tends to partition globular proteins into the soluble fraction and fibrous proteins into the "insoluble pellet" remaining after centrifugation.

Separation by Molecular Size Protein can be separated on the basis of their size by g e l f i l t r a t i o n , and m e m b r a n e f i l t r a t i o n . Small molecules (e.g., NaC1, amino acids, and sucrose) originally present or added during the separation of organelles can be removed by dialysis through a semipermeable membrane. Dialysis membranes are prepared from cellophane or collodion and contain pores that permit passage of solute molecules whose molecular weight is less than ~5000. Thus, proteins of high molecular weight are retained within a dialysis bag, whereas low-molecularweight solutes diffuse through the pores into the fluid (dialysate) outside of the bag. Complete removal of lowmolecular-weight solutes requires repeated changes of dialysate. Rapid removal of low-molecular-weight solutes and simultaneous concentration of the protein solution can be accomplished by applying pressure to the dialysis solution (or vacuum to the dialysate). Such a process is known as u l t r a f i l t r a t i o n (Figure 3-1). The principles of membrane filtration are the same as those of dialysis except that synthetic membranes with specified pore sizes are used. In gel filtration (or molecular sieving), a column is packed with hydrated, insoluble gel particles with known pore sizes; the protein solution is passed through the column and the effluent solution is collected in fractions that will contain solutes of different sizes (Figure 3-2). The volume of the column is essentially divided into two phases: the gel phase (within the pores) and the solvent phase (outside the gel particles). As a solution migrates in the column, the solute molecules that can penetrate the pores of the gel are distributed both within the gel and outside it. The particles that are larger than the pore size are excluded

37 Hydrodynamic force [

Solvent Large,

--impermeable molecules :::!~ ::!:!::ii:::::~i:::ii~.:iii'~.:i

Small,

. . . . . . . . ............... 9 ~i:!~!.::~!;::~:@: Semi. ::i~.?~-:..~)# :.~:L~"~ - - permeable :i::::i:: i iI :i:i:i:iil ::i!::i:::iiiiiii: membrane

6

i .9.....

6

..:. ~....."

!:i#iiii!iiiii!iiiii',iiiii:i :;::!::;::i~i!!Oi::~iii!!O::iiiiiii::~:~:#l

J

.....::::i:!.........!~!~~::~:::~:i i~:~::~i........ :~

F I G U R E 3-1 Separation and concentration of high-molecular-weight solutes from low-molecular-weight solutes by application of hydrodynamic force over the solution above a semipermeable membrane (ultrafiltration).

dialysis,

from the gel phase flow through the column more rapidly and appear first in the effluent. Commonly used gel particles are inert cross-linked dextrans or agarose, which are commercially available with a wide range of exclusion limits. Molecular sieving is effective in the purification of macromolecules and if the column has been calibrated by elution of solutes of known molecular weight, it can also be used in the estimation of molecular weights of proteins or other solutes.

Separation by Chromatography Chromatographic separations of proteins are based upon the differential partitioning of solute molecules due to their differences in affinity between a moving solvent phase and a fixed or supportive phase. In gas chromatography, the mobile phase is a gas, whereas in liquid chromatography it is a liquid. Gas chromatography is not useful in protein purification because proteins cannot be converted to gases without decomposition. Liquid chromatography of proteins is performed on a variety of mechanically different stationary phases, e.g., paper, finely divided particles coated onto a glass or a plastic surface (thin-layer chromatography), or beads packed in a column. Many chemically different stationary phases are used in liquid chromatography of proteins. Ion exchange c h r o matography uses an ion exchange resin, and the proteins are eluted with buffer solutions differing in ionic strength

38

CHAPTER 3

OOOo 9_oJ Sam~,eI O gO0 O O'WOq~' 9 9O 9

zone

DOg O e,_,_[ :ii !i !i i !~;ii!Qii !i i i !I!i~i~:


,..,0 0

ilJi!iiiiiiiiiiiiiiilQiiiiii!iiiii~il il

ii!i!ii!iiii!! !ii!i] i

.ii :.:,:.,~::?.!',!',!i....... i i i ,.,,i, i !i',i':~!~:..[

i~~~i 9!!ii i!~i~lid ', G:~!i ::.i ",i!i i::~i%!i'.':....... ~:::.'.::

Ge,

,~i~.:!::,::Gi;~.!,,ii!!ii:,..:,:.,::.~ pa,~de~ ,

:~i!i;i:,ii~:~:~:!:iiiiiil;i!iiii',i;iiiil :~i!ii~:::Qi:iUIiI',Q .

.

.

.

.

The pores of the gel particle exclude the larger solutes but admit smaller ones. Individual solute molecules are represented by the open circles.

......................................

9iii!i;~;i::::;~iiii:ii!i!~i~!i',~i:!iii'i i ,!

iii!iiiiiiiii;iiii!iiiiiiii !;i !iii!ill ii!!iiiii;i!ii!ii iii!iiiiiiiii!i i

!ii!i!!i!ii!iiii!i!i! F I G U R E 3-2 Gel filtration for separation of solutes by size. Solute molecules smaller than the diameter of the pores of the gel particles enter them and are retained for a longer time. Larger solute molecules cannot penetrate the pores of the gel particles and are eluted off the column first.

and pH. The resins are inert polymers to which ionizable groups have been attached; resins with negative charges are cation exchangers, and those with positive charges are anion exchangers. Two ion exchange resins frequently used in protein purification are diethylaminoethylcellulose (DEAE-cellulose, an anion exchanger) and carboxymethylcellulose (CM-cellulose, a cation exchanger). A protein with a net positive charge at a given pH will combine with the negative groups of a cation exchange resin, and its flow will be retarded; a protein with a net negative charge will migrate through the cation exchange resin unimpeded. Cationic proteins in a mixture going through the column will compete with one another for binding to the negatively charged groups of the resin. The relative migration rates of different molecules depend on three factors: their individual affinities for the charged sites on

the resin, the degree of ionization of the functional groups attached to the resin, and the chemical properties and concentrations of competing low-molecular-weight ions, e.g., potassium and sodium. For example, NaC1 at the appropriate concentration can displace a cationic protein through competition between sodium ions (Na+) and the positively charged groups of the protein for the negatively charged sites on the resin (Figure 3-3). Changing the pH of the eluting buffer can also bring about desorption of proteins from the resin through neutralization of the ionizable R-groups of amino acid residues. Decreasing pH will elute proteins in the order of their decreasing is 9 point values during anion exchange chromatography. Affinity chromatography takes advantage of specific affinities between protein molecules and analogues of biological molecules that are covalently bound to the column

SECTION 3.3

39

S e p a r a t i o n of Proteins Adsorption

(':h

Affinity Tag Chromatography ~.i -. '~. .~J~: ~ . ~

F I G U R E 4-2 Geometry of a peptide (amide) linkage. For the peptide bond, bond angles and bond lengths indicate that carbon-nitrogen bonds have a significant amount of double-bond character and that the C, O, N, and H atoms all lie in the same plane. The 05 and ~ refer to rotations about the single bonds connecting the e-carbon with the o~-nitrogen and the o~-carbonyl carbon, respectively.

The folding of polypeptide chains into ordered structures maintained by repetitive hydrogen bonding is called secondary structure. The chemical nature and structures of proteins were first described by Linus Pauling and Robert Corey who used both fundamental chemical principles and experimental observations to elucidate the secondary structures. The most common types of secondary structure are the right-handed c~-helix, parallel and antiparallel ~-pleated sheets, and ~-turns. The absence of repetitive hydrogen-bonded regions (sometimes erroneously called "random coil") may also be part of secondary structure. A protein may possess predominantly one kind of secondary structure (a-keratin of hair and fibroin of silk contain

~

CHAPTER4 Three-Dimensional Structure of Proteins

mostly u-helix and/3-pleated sheet, respectively), or a protein may have more than one kind (hemoglobin has both or-helical and non-hydrogen-bonded regions). Globularproteins usually have mixed and fibrous proteins have predominantly one kind of secondary structure. c~-Helix The rod-shaped right-handed or-helix, one of the most common secondary structures found in naturally occurring proteins, consists of L-or-amino acids (Figure 4-4). In the right-handed c~-helix the helix turns counterclockwise (C-terminal to N-terminal) and in the left-handed it turns clockwise. The left-handed c~-helix is less stable than a right-handed or-helix because its carbonyl groups and the R-groups are sterically hindered. The helical structure is stabilized by intrachain hydrogen bonds involving each - N H a n d - C O group of every peptide bond. These hydrogen bonds are parallel to the axis of the helix and form between the amido proton of the first residue and the carbonyl oxygen of the fourth residue, and so on, producing 3.6 amino acid residues per turn of the helix. The rise per residue is 0.15 nm and the length of one turn is 0.54 nm (Figure 4-5). In some proteins, or-helices contribute significantly to the secondary structure (e.g., ct-keratin, myoglobin, and hemoglobin), whereas in others, their contribution may

F I G U R E 4-4 Hydrogen bonds in the a-helix. (a) Each peptide group forms a hydrogen bond with the fourth peptide group in each direction along the amino acid chain. (b) Coiling of an amino acid chain brings peptide groups into juxtaposition so that the hydrogen bonds shown in (a) can form. The multiple hydrogen bonds (indicated by the three dots) stabilize the helical configuration.

0.51 nm

26 ~

m1 . . . . 0.54-nm pitch (3.6 residues)

...... l 0.15 nm

Rise per amino acidresidue

f F I G U R E 4-5 Average dimensions of an a-helix. Only the atoms of the a-carbon, the carbonyl carbon and the nitrogen of the peptide bonds are shown. The rise per residue and the length of one turn are 0.15 and 0.54 nm corresponding to minor and major periodicity, respectively.

be small (e.g., chymotrypsin and cytochrome c) or absent (e.g., collagen and elastin). Whether a polypeptide segment forms an c~-helix depends on the particular R-groups of the amino acid residues. Destabilization of an or-helix may occur for a variety of reasons: electrostatic repulsion between similarly charged R-groups (Asp, Glu, His, Lys, Arg); steric interactions due to bulky substitutions on the/3-carbons of neighboring residues (Ile, Thr); and formation of side-chain hydrogen or ionic bonds. Glycine residues can be arranged in an a-helix; however, the preferred and more stable conformation for a glycine-rich polypeptide is the/3-pleated sheet because the R-group of glycine (-H) is small and gives rise to a large degree of rotational freedom around the a-carbon of this amino acid. Prolyl and hydroxyprolyl residues usually create a bend in an a-helix because their a-nitrogen atoms are located in rigid ring structures that cannot accommodate the helical bonding angles. Moreover, they do not have an amido hydrogen and therefore can form neither the necessary hydrogen bond nor the usual planar peptide bond. However, some proteins such as rhodopsin do contain proline residues embedded in a-helical segments. In some proteins, the a-helices twist around each other to form rope-like structures (coiled coils) to give rise to a supersecondary structure. Examples of such proteins are the ot-keratins, which are major protein components of

SECTION4.3 Secondary Structure

/c-.

SS

/c-.

/c-.

/

99 9 ..

9

. 9 9

R--C/k 9

. H ~.

9

7%0 9 .H./%a \ .

/C%o H~N\

.

CmR

C~R

/

N/'H"

"

R--C\/

R--C\/

"" . H . / ' O .

N / H "

CmR

" "

R--C(

/~O.

.
0, the reaction will not occur by itself. It is A G, however, that determines whether or not a reaction will occur under conditions different from the standard state, such as those existing within a cell, for two reasons. 1. Most solutes are present at fairly low concentrations (< 0.1 M) and water is present at high concentration (55.6 M for pure water); the water concentration is thus assumed to be constant. If water enters into a reaction, it is incorporated into Keq; if Keq is determined at pH 7.0, it is indicated by K'eq" 2. If H + or O H - participates in a reaction, its concentration influences AG ~. Since most biological reactions occur in systems buffered to pH "- 7, it is useful to define AG ~ as the value of AG ~ measured at pH -- 7.0, i.e., where [H +] -- 10 -7 M. AG ~ values can be compared to each other but not to values of AG ~ Therefore, for biochemical systems, Equation (5.2) can be written as !

AG ~ -- - 2 . 3 0 3 R T log Keq

(5.3)

Substituting for R -- 1.987 x 10 -3 kcal. mol - l . deg-l (8.314 x 10 .3 kJ. mol-1 d e g - l ) and T -- 298 K (i.e., 273 + 25~ .

!

~

'

.

[in kcal/mol]AG ~ -- - 1.36 log Keq, t __ 0-AG~ o r Keq 1

[in kcal/mol]AG ~ -- - 5 . 6 9 log Keq, o r g e 'q

m

10 -AG~

(5.4)

From Equation (5.4), at 25~ each - 1.36 kcal/mol value of A G ~ corresponds to a factor of 10 in the equilibrium constant in favor of product formation (Table 5-1). As Keq increases, the value for A G ~ decreases and the tendency for reactions to occur increases spontaneously. Thus, we I can calculate A G ~ by knowing Keq and the temperature at

T A B L E 5-1

Relationship between Standard Free Energy Change (AG ~ and Equilibrium Constant (Keq ) at p H 7.0 and 25~ * AG O"

K' eq

kcal/mol

kJ/mol

10 -6 10 -5 10 -4 10 -3 10 -2 10 - i

8.18 6.82 5.46 4.09 2.73 1.36

1

0

0

-1.36 -2.73 -4.09 -5.46 -6.82 - 8.18

-5.69 -11.42 -17.11 -22.84 -28.53 -34.22

10 102 103 104 105 106 ,

34.22 28.53 22.84 17.11 11.42 5.69

1.36 K , eq = 1 0 - A G 0 ' when AG O' is expressed in kcal/mol at 25~ K,eq = 10_AGO"5.69 when AG o" is expressed in kJ/mol at 25~

I which Keq is determined. In living systems, chemical reactions rarely reach complete equilibrium states but do attain near-equilibrium and nonequilibrium conditions. These conditions are maintained by utilizing energy from the external source. In a metabolic pathway, one of the nonequilibrium reactions usually becomes the rate-determining step of that pathway and its regulation provides directionality to the pathway. In a reaction sequence

A ~-~-B ~- C ~- . . . the reaction A --+ B can be prevented from reaching near-equilibrium by removing B (converting it to C) as fast as it can be made from A. However, the conversion of B to C may attain near-equilibrium. The nearequilibrium reactions are reversible and allow for reversal of the biochemical pathway. All reactions in the body are interrelated and the system as a whole is in a steady-state condition, with individual reactions operating in either near- or nonequilibrium conditions. A change in concentration of any component (product of one reaction used as reactant in another reaction) shifts the concentration of all other components linked to it by means of a sequence of chemical reactions, resulting in the attainment of a new steady state. The value of A G for a nonequilibrium system can be determined from A G ~ , and the actual concentrations of

SECTION 5.2

Thermodynamics

71

products and reactants in the cell can be determined by use of the common logarithm analogue of Equation (5.1)" AG -- AG ~ + 2.303R T log [products]. [reactants] If A G is negative, then u n d e r c e l l u l a r c o n d i t i o n s the reaction proceeds to form products. The AG ~ value for a given chemical reaction may be positive suggesting immediately that the reaction cannot occur (without external assistance), yet the AG value may be negative indicating that the reaction can occur under the prevailing concentrations of reactant and products. The reaction of glycolysis, catalyzed by the enzyme aldolase (Chapter 13), furnishes a good illustration. Fructose 1,6-bisphosphate ~- glyceraldehyde 3-phosphate + dihydroxyacetone phosphate The Ke~qfor this reaction at 25~ is 6.7 x 10 -5 M. We can calculate AG ~ from Equation (5.3)"

!

AG ~ -- - 2 . 3 0 3 R T log Keq = - 2 . 3 0 3 x 1.98 x 10 -3 x 298 x log (6.7 x 10 -5) -- +5.67 kcal/mol (or + 23.72 kJ/mol). The positive value for AG ~ indicates that the reaction is not favored in the forward direction but can proceed in the reverse direction. Now we calculate the A G value using the concentration of 50/zmol/L (or 50 x 10 -6 M) for both reactant and product, which is close to their physiological concentrations: AG-

AG ~ + 2.303RT

glyceraldehyde] [ dihydroxyacetone] 3-phosphate I I phosphate j • log i

~

of reactants and products (formation of fructose 1,6bisphosphate from glyceraldehyde 3-phosphate and dihydroxyacetone phosphate during gluconeogenesis). The same enzyme catalyzes both the forward and the reverse reactions. The concept of A G is further clarified by an example. Consider a self-operating heat engine that functions by taking in an agent (e.g., steam) at temperature T1 and releasing it at T2 (T1 > T2). The heat extracted (Q) is converted as efficiently as possible into useful work. If W represents the maximum useful work available, W-

rl

= - 0 . 1 8 kcal/mol(or - 0.75 kJ/mol). The negative value of A G suggests that under physiological conditions the forward reaction is favored, despite the fact that AG ~ is positive. Thus, under physiological conditions A G values predict better than AG ~ values whether a reaction can occur spontaneously. Similarly, whereas under certain conditions of reactants and products the conversion of reactant to product is favored (e.g., the conversion of fructose 1,6-bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate during glycolysis), the direction of this reaction can be reversed when appropriate changes occur in the concentrations

T2

-e-eg

where 7'i and Te are absolute (Kelvin) temperatures. Unless T2 = 0 or T1 = oo, the useful work is always less than the total energy supplied by a factor of Q ( T 2 / T 1 ) = (Q / T1) T2. The ratio Q / T1 is the entropy (S) of the system, and the amount of energy unavailable for useful work (T2 S) is that which is lost in the process of energy transfer; it can be thought of as the amount of randomness or disorder introduced into the system during the transfer. For chemical systems, G can be defined by the equation G = H - TS. Here, G (free energy) is analogous to W, the maximum useful work available; H (enthalpy) is analogous to Q, the heat content of the system at constant pressure; S (entropy) is analogous to ( Q / T 1 ) , the wasted heat energy; and T is again the absolute temperature. This relationship G = H - T S is more commonly used to describe c h a n g e s in these quantities. If a system goes from state I (with G = GI, H = HI, S = SI) to state II (G = GII, H -- HII, S -- Sn) at constant temperature,

i]~55b~i

= +5.67 kcal/mol + 2.303 • 1.98 • 10 -3 • 298 [(50 • 10 -6) X (50 X 10-6)] • log (50 • 10 -6) -- 5.67 kcal/mol - 5.85 kcal/mol

Q (T1 - T2)

GII - GI = (HII - HI) - T (Sn - SI) or, simply, A G-

AH-

T AS.

Since entropy (S) is a measure of disorder (randomness), it is the energy that is unavailable for useful work. Entropy values of denatured molecules are high relative to those of native structures (as in protein denaturation). In a living system, entropy is kept at a minimum by utilization of free energy (G) from outside and by increase in entropy of the surroundings. Enthalpy (H) is related to the internal energy of a system by the following equation, where E is the internal energy, P the external pressure on the system, and V the volume of the system. In terms of changes between states, the equation becomes AH

-- A E + A ( P V ) -

AE + PAV

+ VAP.

~

CHAPTER5 Thermodynamics, Chemical Kinetics, and Energy Metabolism

At constant pressure, A P - - 0 and A H -- A E 4- P A V. If, in addition, the volume is constant, then A V -- 0 and A H - A E . In many biological reactions, A V - A P = 0. Consequently, for a change of state, the maximum amount of energy that can be released as heat ( A H ) is equal to A E , the change in the internal energy of the system. A H can be measured by burning the particular compound in a bomb calorimeter at constant pressure. The heat released is measured as the rise in temperature of a large water bath surrounding the combustion chamber. The values so obtained indicate the total energy available in a compound when it is completely oxidized. Factors that usually contribute to A H under these conditions are the heat of fusion (melting), the heat of vaporization (boiling), and the heat of combustion (bond making and breaking). The first two factors are important, for example, if at the standard temperature (25~ a solid is combusted to a combination of liquids and gases, as in the examples below. Just as with A G, A H for a process is equal to - A H for the reverse process: A H (liquid--+ g a s ) - - A H

(gas--~ liquid)

A H (liquid --+ solid) -- - A H (solid --+ liquid)

Nemst equation:

Eh = E ~ 4-

2.303 R T nF

log

(electron acceptor) (electron donor)

where Eh = observed potential, E ~ = standard redox potential, R = gas constant (8.31J deg -1. mol), T = temperature in Kelvin, n = number of electrons being transferred, and F = the Faraday constant (96,487 J/V). It is evident from the Nernst equation that Eh = E ~ when [electron acceptor] -- [electron donor]. Thus, the E ~ value represents the midpoint in a redox reaction, which is analogous to the measurement of the pK' value of a weak acid, HA, when [A-] = [HA] (see discussion of the HendersonHasselbalch equation in Chapter 1). In this manner, E ~ values for many redox reactions of biochemical importance have been determined (Chapter 14). A half-reaction is a reaction in which electrons or hydride ions are written explicitly. Values of E ~ and E ~ are tabulated with the half-reactions for which they are measured. For example, NADH --+ NAD + 4- H -

E ~ = -0.32 V

and 1 H20---+ ~ 0 2 4- 2H + + 2e-

E ~ -- - 0 . 8 1 6 V

A H (making a bond) - - A H (breaking the same bond). A H values for various food substances and their application to human energy metabolism are discussed later in this chapter. A number of processes in living systems result in the transfer of electrons (oxidation and reduction) and can be understood in thermodynamic terms. Oxidation is the loss of electrons or hydride ( H - ) ions (but not hydrogen [H +] ions) by a molecule, atom, or ion. Reduction is the gain of electrons or hydride ( H - ) ions by a molecule, atom, or ion. Transfer of one hydride ion results in the transfer of two electrons. The amount of work required to add or remove electrons is called the electromotive potential or force (ernf ) and is designated E. It is measured in volts (joules per coulomb, where a coulomb is a unit of electric charge or a quantity of electrons). The standard emf, E ~ (or, at pH 7, E~ is the emf measured when the temperature is 25~ and the materials being oxidized or reduced are present at concentrations of 1.0 M. In biological systems, E ~ is most commonly used. 1 The emf measured for an oxidation-reduction (redox) reaction under nonstandard conditions, Eh, is mathematically related to the emf measured for the same reaction under standard conditions, E ~ through the !

l Some texts use the notation E0 for the standard oxidation-reduction potential.

are half-reactions because they show electrons (e-) or hydride ions (H-). NADH and NAD + are abbreviations for reduced and oxidized forms of nicotinamide adenine dinucleotide (whose structure is given in Chapter 6 and whose metabolic functions are discussed elsewhere). Alternatively, these half-reactions can be written as NAD + 4 - H - ~ N A D H E~ 1 2e- + 2H + + ~ 0 2 ~ H20 E ~ -- +0.816 V. Reversing the direction of a reaction changes the sign of the potential charge (just as with AG and AH). The oxidation potential is a measure of the ease of removal of electrons from a material compared to the ease of removal of electrons from hydrogen in the half-reaction: H2 ~ 2H + 4- 2e-

E ~ -- 0.00 V.

E ~ in this reaction is defined as zero when H2 gas is at 1.0 atm, [H +] is 1.0 M (i.e., pH 0), and the temperature is 25~ thus fixing the scale of E ~ value for other reactions. If it is easier to remove electrons from a material, E ~ will be negative; if it is harder, E ~ will be positive. E ~ for the hydrogen half-reaction is used as the zero point even when reference is made to E ~ values. Thus, H2 ~ 2H + 4- 2e-

E ~ ---=- 0 . 4 2 V.

It is easier to remove electrons from hydrogen (and produce H +) when [H + ] -- 10 -7 M than when [H +] - 1.0 M.

SECTION 5.3

Standard Free Energy of Hydrolysis of ATP

The choice of a negative sign for the standard emf to signify "easier to remove electrons" is arbitrary. 2 Since free electrons combine rapidly with whatever is at hand, half-reactions never occur by themselves. Something must accept electrons as fast as they are released. The substance releasing electrons (or H - ) is the reductant, or reducing agent (because it is oxidized), and the substance accepting electrons is the oxidant, or oxidizing agent (because it is reduced). Two half-reactions when combined give a redox reaction. When balanced, such reactions never show free (uncombined) electrons. For example, 1 H + + NADH + ~02 ~ NAD + + H20

A E ~ -- - 0 . 3 2 - (+0.816)-- - 1 . 1 3 6 V. A E ~ for this reaction is calculated accordlng to AE ~

A E ~ ( r e d u c t a n t ) - A E ~ (oxidant).

Under standard conditions (and pH -- 7.0), the reaction will occur as written if A E ~ < 0. Otherwise the reaction will proceed from right to left (the reverse of the way it is written). When A E ~ in volts is converted to calories per mole, the result is A G ~ for the redox reaction being considered. This conversion is accomplished by means of the equation AG~

n F 9A E ~ 4.184

= (23.061)n. A E ~

where, as before, F is Faraday's constant -- 96,487 J/V; n is the number of electrons transferred per mole of material oxidized or reduced; A E ~ is E ~ (reductant) - E ~ (oxidant) in volts; and AG ~ is the standard free energy change in calories per mole of material oxidized or reduced (since there are 4.184 J/cal). In the case of the reaction just considered 1 H + + NADH + ~02 ---> NAD + + H20 AE ~

73

As in all reactions, the actual value of AG ~ depends on the reactant or product for which it is calculated. The significance of free-energy changes and redox reactions lies in the fact that life on earth depends on the redox reaction in which CO2 is reduced by H20 to yield (CH20), using sunlight as the energy source: Reduction (photosynthesis)

CO2 + H20 + energy
02 + (CH20)

Oxidation (respiration)

where (CH20) represents carbohydrates. The A E ~ for the forward reaction is about + 1.24 V, or AG ~ is about 114.3 kcal/(CH20). Thus, for glucose formation, AG ~ is +685.8 kcal. In plants, energy from the sun is absorbed by the chloroplasts of green plants, causing water to be oxidized and carbon dioxide reduced, producing oxygen and carbohydrate [represented by ( C H 2 0 ) m ] . In oxidation reactions, carbohydrate (or lipids) and oxygen are consumed, and energy, water, and carbon dioxide are released. The energy is captured primarily as adenosine triphosphate (ATP), which is used as an immediate source of energy in most cellular endergonic processes.

5.3 Standard Free Energy of Hydrolysis of ATP In the living organism, ATP functions as the most important energy intermediate, linking exergonic with endergonic processes. Exergonic processes (e.g., oxidation of glucose, glycogen, and lipids) are coupled to the formation of ATR and endergonic processes are coupled to the expenditure of ATP (e.g., biosynthesis, muscle contraction, active transport across membranes). ATP is called a "high-energy compound" because of its large negative free energy of hydrolysis

- 1 . 1 3 6 V, ATP 4- + H20 --+ ADP 3- + HPO 2- (or Pi) + H +

n equals 4 gram-equivalents of electrons per mole of O2 or 2 gram-equivalents of electrons per mole of H20, NAD +, or NADH. Then, AG ~

23.061n x ( - 1 . 1 3 6 ) = -26.197 x 2-

-26.197n

-52.394

calories/mol of H20 or NAD + formed or NADH converted -- - 2 6 . 1 9 7 x 4 -- - 1 0 4 . 7 8 8 calories/mol of 02 transformed 2Although the International Union of Pure and Applied Chemistry recommends the opposite convention, the older convention is retained here because it is commonly encountered in medically and biologically related publications.

AG~

kcal/mol ( - 3 0 . 5 kJ/mol).

The reason for the large release of free energy associated with hydrolysis of ATP is that the products of the reaction, ADP and Pi, are much more stable than ATR Several factors contribute to their increased stability: relief of electrostatic repulsion, resonance stabilization, and ionization. At pH 7.0, ATP has four closely spaced negative charges with a strong repulsion between these charges (Figure 5-4). Upon hydrolysis, the strain in the molecule due to electrostatic repulsion is relieved by formation of less negatively charged products (ADP s- and Pi2 - ). Furthermore, since these two products are also negatively charged, their

74

CHAPTER 5

Thermodynamics, Chemical Kinetics, and Energy Metabolism

NH,

Adenine I

II

II

O-

-

N

O-

O---P-tO--P-r-O--P-

oIII /

II

o

II

o

D-Ribose

I

OH

I

OH

Adenosine Adenosine monophosphate(AMP 2-) I

Adenosine diphosphate (ADP ~) FIGURE 5-4 Structural formula of adenosine triphosphate (ATP) at pH 7.0. The three phosphate groups are identified by Greek letters o~,/5, and y. The y- and E-phosphate groups are linked through phosphoanhydride bonds and their hydrolysis yields a large negative AG ~ whereas the u-phosphate linked by a phosphate ester bond has a much lower negative AG ~ In vivo most ATP is chelated to magnesium ions via two of the anionic oxygens (Mg 9ATP 2-).

tendency to approach each other to re-form ATP is minimized. The number of resonance forms of ADP and Pi exceeds those present in ATP so that the products are stabilized by a larger resonance energy. Ionization of HzPO 4 ~- HPO2-+ H + has a large negative AG ~ contributing the AG ~ for ATP hydrolysis. The free energy of hydrolysis of ATP is also affected by Mg 2+ and the physiological substrate is Mg 2+ 9ATP4(or Mg. ATP 2-). The presence of magnesium ions favors ATP hydrolysis because both ADP and Pi have higher affinity (about six times) for Mg 2+ than ATE In vivo, the A G value for ATP hydrolysis is probably much higher than AG ~ owing to the prevailing intracellular concentration of the reactants and products (in erythrocytes, - 12.4 kcal/mol). In some energy-consuming reactions, ATP is hydrolyzed to AMP and pyrophosphate, with a value of free energy of hydrolysis comparable to that of ATP to ADP and Pi ATP4- + H20--+ AMP 2- + HP20~- + H + AG ~ = - 7 . 7 kcal/mol ( - 3 2 . 2 kJ/mol) In vivo, pyrophosphate is usually hydrolyzed to inorganic phosphate, which also has a large negative A G ~ HP2073- + H20--+ 2 H P O 2- + H § AG ~ = - 7 . 1 7 kcal/mol ( - 3 0 kJ/mol)

The pyrophosphate hydrolysis, although not coupled to any particular endergonic reaction, still ensures completion of the forward reaction or process (e.g., activation of fatty acids and amino acids, synthesis of nucleotides and polynucleotides). The hydrolysis of AMP to adenosine and Pi does not yield a high negative AG ~ because the phosphate is linked in a normal ester bond, as opposed to the other two linkages (fl and y), which are phosphoanhydride bonds (Figure 5-4). The AG ~ values of hydrolysis of other nucleoside triphosphates (e.g., UTP, CTP, GTP) are similar to that of ATP and they are utilized in the biosynthesis of carbohydrates, lipids, and proteins (discussed elsewhere). In addition to undergoing hydrolysis, ATP may act as a donor of phosphoryl (e.g., in the formation of glucose 6-phosphate; see Chapter 13), pyrophosphoryl (e.g., in the formation of phosphoribosylpyrophosphate; see Chapter 27), adenylyl (e.g., in the adenylylation of glutamine synthetase; see Chapter 17), or adenosyl groups (in the formation of S-adenosylmethionine; see Chapter 17). Since ATP synthesis from ADP and Pi is energydependent, it requires compounds or processes that yield larger negative AG ~ values than ATP hydrolysis does. Some of these compounds are organic phosphates (Table 5-2). Other high-energy compounds are thioesters,

75

SECTION5.4 Chemical Kinetics TABLE

t2 is the slope,

5-2

Standard Free Energy o f Hydrolysis (AG O") o f Some OrganophosphaCes A G O'

Compound / Hydrolyzed

kcal/mol (kJ/mol)

Product

Phosphoenolpyruvate --->Pyruvate 1,3-Bisphosphoglycerate ---> 3-Phosphoglycerate Phosphocreatine --->Creatine ATP --->ADP ATP --->AMP ADP ~ AMP Glucose 1-Phosphate --->Glucose Fructose 6-Phosphate --->Fructose AMP --->Adenosine Glucose 6-Phosphate --->Glucose Glycerol 3-Phosphate ~ Glycerol

v (-49.3) (-43.1) (-30.5) (-32.2) (-27.6) (-20.9) ( - 15.9) ( - 14.2) (-13.8) ( - 9.2)

[B]2 - [B]l

At

t2 - - t l

The instantaneous velocity, V, at some time t' (7~ to) is expressed as follows"

-14.8 (-61.9) -11.8 -10.3 -7.3 -7.7 -6.6 -5.0 -3.8 -3.4 -3.3 -2.2

A[B]

d[B (t)]

dt

evaluated at t'.

The equation d[B(t)]/dt is the first derivative of [B(t)] with respect to t. It is the limit (lim) of the slope A[B]/At as At --+ 0, or v--

lim ~ - -

A[B]

lim

At~O At

At--,0

derivatives,

and

sugar

5.4 Chemical Kinetics The study of chemical reaction rates is called chemical kinetics. Whereas thermodynamics deals with the relative energy states of reactants and products, kinetics deals with how fast a reaction occurs and with the chemical pathway (mechanism) it follows. The conversion A ~ B (where k is the rate constant) can be represented diagrammatically by the increase in concentration of product, [B], with respect to time (Figure 5-5). The average velocity, ~, for the period tl to

A[B (t)]

dt

.

Alternatively, one can consider the change in concentration of the reactants. Thus,

d[A(t)] dt

d[B (t)]

dt

aminoacyl esters, sulfonium nucleoside diphosphates.

--

since the reactant concentration, [A], decreases at the same rate at which the product concentration, [B], increases. The expressions [B(t)] and [A(t)] imply a detailed knowledge of the way in which the concentrations of A and B vary with time and the way in which the reaction progresses. Since the preceding is generally not the case, [B] and [A] will be used instead of [B(t)] and [A(t)]. The rate of formation of [B] should increase if [A] increases. For the reaction n A ~ B, it is appropriate to write A[B] =

k[A]"

At

where k is again the rate constant, and n -- 0, 1, 2, . . . is the order of the reaction with respect to A. (The order is said to be zero, first, second, etc., with respect to A.) The rate constant expresses the proportionality between the rate of formation of B and the molar concentration of A and is characteristic of a particular reaction. The units of k depend on the order of the reaction. For zero order, they are moles liter -1 time -1 (time is frequently given in seconds). For first order, they are time -1 , and for second order, liters moles -1 time -1, etc. The units of k are whatever is needed for d[B]/dt to have the unit of moles liters -1 time -1. If n - 0, v - k; this means that in a zero order reaction, [B] changes at a constant rate independent of the concentration of reactants, which is especially important in enzyme kinetics. A plot of [B] versus t for such a reaction is a straight line. A somewhat more complicated example is A+B--~ C+D

F I G U R E 5-5 Time course of a chemical reaction.

d[C] v

--

dt

diD] =

dt

d[products] =

dt

=

k[A][B]

~6 This reaction is first order with respect to A, first order with respect to B, and second order overall. Most reactions are zero, first, or second order. The concept of reaction order can be related to the number of molecules that must collide simultaneously for the reaction to occur. In a first-order reaction, no collisions are required (since only one reactant molecule is involved). Every molecule with sufficient free energy to surmount the activation barrier may spontaneously convert to products. In a second-order reaction, not only must two molecules have enough free energy but they also must collide with each other for the products to form. A third-order reaction requires the simultaneous meeting of three molecules, a less likely event. Reactions of orders higher than second are seldom encountered in simple chemical conversions. Higher apparent orders are encountered, however, in some cases in which an overall rate equation is written for a process that proceeds in several steps via one or more intermediates. In such situations, the individual steps seldom involve a third- or higher order process. Therefore, for a chemical reaction consisting of multiple steps, the order of the reaction provides information about the overall rate equation but does not provide information regarding the number of atoms or molecules that must actually collide to form products during each step. This information is obtained by knowing the molecularity of the reaction, which is defined as the number of molecules that come together in a reaction. In a single-step reaction, the molecularity (which is the same as the order of reaction) determines the exponents in the rate equation. In a multistep reaction sequence, each step has unique molecularity; knowing this provides information about the reaction mechanism. Zero-order reactions can be explained in two ways. In a process that is truly zero order with respect to all reactants (and catalysts, in the case of enzymes), either the activation energy is zero or every molecule has sufficient energy to overcome the activation barrier. This kind of reaction is rare in homogeneous reactions in gases or solutions. Alternatively, a reaction can be zero order with respect to one or more (but not all) of the reactants. This kind of reaction is important in enzyme kinetics and assays of enzymes of clinical importance (Chapter 8). If one of the reactants (or a catalyst, such as an enzyme) is limited, then increasing the availability (concentrations) of the other reactants can result in no increase in the velocity of the reaction beyond that dictated by the limiting reagent. Hence, the rate is independent of the concentrations of the nonlimiting materials and zero-order with respect to those materials.


Theoretically all reactions are reversible. The correct way to write A --+ B, then, is ki

AZB k-1

where kl is the rate constant for the conversion of A to B, and k_l is the rate constant for the reverse reaction, i.e., conversion of B to A. The reaction is kinetically reversible if kl ~ k_l; it is irreversible for all practical purposes (i.e., the reverse reaction is so slow that it can be ignored) if k-1 6CO2(g) nt- 6HzO(e)

At 20~

A H ~ = - 6 7 3 kcal/mol or - 2 8 1 6 kJ/mol.

Oxidation of 1 mol of palmitic acid (a fatty acid) to carbon dioxide and water: C16H3202(s) + 2302(8)----> 16CO2(g) q-- 16H20(1)

At 20~ A H ~ 2380 kcal/mol or - 9 . 9 6 MJ/mol (1 MJ -- 1 megajoule -- 1000 kJ). The average A H values in kilocalories per gram are as follows: carbohydrate, 4.1; lipid, 9.4; and protein, 5.6. These values vary within each class of food depending on chemical structure. For example, the value for starch is 4.1 kcal/g, whereas for glucose it is 3.8. Furthermore, in humans the end product of protein metabolism is urea instead of CO2 and nitrogen, which are the end products obtained after complete oxidation in the bomb calorimeter. Therefore, a realistic estimate of energy derived from protein in the body, taking into account incomplete oxidation and specific dynamic action, is 4.1 kcal/g. In human nutrition, the commonly used values for energy yield in kilocalories (or kilojoules) per gram are as follows: carbohydrate, 4 (16.7); lipid, 9 (37.7); and protein, 4 (16.7). During the metabolism of food substances in the body, oxygen is consumed and CO2 is produced. The molar ratio of CO2 produced to oxygen consumed is known as the respiratory quotient (RQ) and is characteristic of a given substrate. For example, in the complete oxidation of glucose presented above, RQ-

volume of CO2 produced volume of 0 2 consumed

=

6CO2 602

= 1.0.

For the corresponding oxidation of palmitic acid, RQ -

16CO2 2302

= 0.7.

The RQ for protein is difficult to measure because it is not oxidized completely in the body and part of its carbon, hydrogen, and oxygen is lost in the urine and feces. However, the RQ is usually taken to be 0.8. The measurement of RQ thus provides a means for assessing the type of food that is being metabolized. An overall RQ of 0.85 is obtained in a normal adult who consumes a mixed diet. This value is enhanced by carbohydrate feeding and decreased by lipid feeding. Heat production can be calculated from oxygen consumption and RQ as follows. For glucose oxidation: C 6 H 1 2 0 6 -Jr- 6 0 2 -- 6 C O 2 -Jr- 6 H 2 0 180 g 192 g (134L)

Since the energy yield of 1 g of glucose is 3.8 kcal, 180 g of glucose yields 180 x 3.8 or 684 kcal. The energy yield per liter of oxygen consumed equals (180 x 3.8)/134 5.1 kcal (21.3 kJ). Again, for palmitate oxidation, C16H3202 + 2302 --+ 16CO2 + 16H20. 256 g

736 g

(513.7 L)

~8 Since the energy yield of 1 g of fat is 9 kcal, 256 g of fat yields 256 x 9, or 2304 kcal. The energy yield per liter of oxygen consumed equals (256 x 9)/513.7 = 4.5 kcal (18.8 kJ). In the body, the energy derived from food is released as body heat and also used in the synthesis of ATP. The energy captured in ATP is then transformed into other forms, i.e., chemical (synthesis of new compounds), mechanical (muscle contraction), electrical (nerve activity), electrochemical (various ion pumps), thermal (maintenance of body temperature), and informational (base sequences in nucleic acids, amino acids in proteins). In general, the energy of food provides for the specific dynamic action of food, the maintenance of the body's basal metabolism, and the energy expenditure associated with various types of activity. The term specific dynamic action (SDA) describes the thermogenic effect of food, which consists of the rise in the metabolic rate during the assimilation of food above the metabolic rate during fasting. The mechanism of the thermogenic effect is not understood, but it may be due to the work performed in the assimilation of food and in its preparation for energy storage or energy yield. These processes require energy, which is derived from the body's energy stores. The values for SDA vary with the type of food substance ingested. The SDA value for protein is about 30% of its energy value because protein must undergo many changes before the carbon skeletons of its constituent amino acids can be used appropriately. SDA values for carbohydrates and lipids are, respectively, 5% and 4% of their energy value. The energy utilized as a result of SDA is wasted and appears as heat. It is maximal with protein intake. Thermodynamically, all energy derived from the metabolism of substrates to CO2 and H20 must of course eventually appear as heat. Basal metabolism (determined by the basal metabolic rate [BMR]) is an expression of the body's vital energy needs during physical emotional, and digestive rests. It represents the energy required for the maintenance of body temperature, muscle tone, the circulation of blood, the movement of respiration, and the glandular and cellular activities of a person who is awake and not involved in physical, digestive, or emotional activities. The BMR is measured in an individual at rest after a 12-hour fast and in comfortably warm, pleasant, quiet surroundings. Although the BMR could be obtained directly by measurement of the amount of heat produced, an indirect method that measures the volume of oxygen consumed and carbon dioxide evolved per unit time is less cumbersome and provides acceptable values. Oxygen consumption is measured in a respirometer. For every liter of oxygen consumed, the energy equivalent is 4.83 kcal


(20.2 kJ). This value is based on the average value for the oxidation of carbohydrates and lipids (see above). BMR values are standardized and usually expressed as kilocalories per square meter of body surface per hour. The metabolic rate that occurs during sleep is approximately equal to the BMR. Less stringent conditions often used for hospitalized patients provide the resting metabolic rate, which is about 3% less than the BMR. The normal values for the resting rate of energy expenditure are shown in Table 5-3. It is clear that the resting metabolic energy requirements are closely related to lean body mass (as the body fat increases, the energy requirement decreases). Muscle, endocrine glands, and organs such as the liver are metabolically more active (i.e., consume a larger amount of oxygen per unit weight) than adipose tissue and bone. The B MR for women (who have a greater proportion of fat per total body weight than men) is lower than that for men. In both sexes, the development of muscle tissue as a result of increased exercise increases the metabolic rate and the basal metabolic needs. Body fat is rapidly determined with calipers by skinfold measurement at several different sites. In normal young men and women, the average body fat constitutes about 12% and 26% of total body weight, respectively; if these values are exceeded by more than 20% and 30% of the average values respectively, they indicate the presence of obesity. The BMR changes with age. It is high at birth, increases up to 2 years of age, then gradually decreases except for a rise at puberty. This general decline in the BMR (approximately 2% per decade after age 21 years) is proportional to the reduction in muscle tissue (or lean body mass) and the accompanying increase in body fat. B MR is also affected by menstruation, pregnancy, and lactation. The B MR is altered in a number of pathological states. The BMR is increased in hyperthyroidism, fever (approximately 12% elevation for each degree Celsius above normal body temperature), Cushing's syndrome, tumors of the adrenal gland, anemia, leukemia, polycythemia, cardiac insufficiency, and injury. B MR is decreased in hypothyroidism, starvation, malnutrition, hypopituitarism, hypoadrenalism (e.g.,Addison's disease), and anorexia nervosa. To calculate energy requirements in an individual, it is necessary to take into account BMR, physical activity (muscular work), age, sex, height, and weight (Appendix I). The energy requirements of muscular work can be measured. Table 5-4 lists the energy expenditure for various physical activities. Maintenance of desirable body weight at any age depends on the balance between energy intake and energy output or requirement. During growth, pregnancy, or recovery from illness, energy demands are greater; hence,

SECTION5.5 Energy Metabolism

79

TABLE 5-3 Normal Values for the Resting Rate of Metabolic Energy Expenditure of Adults Expressed in kcal/min (kJ/min)*

Men

Percent Body Women Fat

Thin Average

Weight in kg (lbs) 45(99)

50(110)

55(121)

60(132)

65(143)

70(154)

75(165)

80(176)

5

0.98 (4.10)

1.05 (4.39)

1.12 (4.69)

1.20 (5.02)

1.27 (5.31)

1.32 (5.52)

1.40 (5.86)

10

0.94 (3.93)

1.00 (4.18)

1.08 (4.52)

1.15 (4.81)

1.22 (5.10)

1.27 (5.31)

1.34 (5.61)

Plump

Thin

15

0.82 (3.42)

0.89 (3.72)

0.96 (4.02)

1.04 (4.35)

1.11 (4.64)

1.18 (4.94)

1.22 (5.10)

1.30 (5.44)

Fat

Average

20

0.79 (3.30)

0.84 (3.51)

0.90 (3.77)

0.98 (4.10)

1.05 (4.39)

1.12 (4.69)

1.18 (4.94)

1.24 (5.19)

Plump

25

0.79 (3.31)

0.86 (3.60)

0.94 (3.93)

1.00 (4.18)

1.08 (4.52)

1.12 (4.69)

1.20 (5.02)

Fat

30

0.82 (3.43)

0.89 (3.72)

0.96 (4.02)

1.04 (4.35)

1.08 (4.52)

1.15 (4.81)

*Data from S. Davidson, R. Passmore,J. F. Brock, and A. S. Truswell:HumanNutrition and Dietetics, 7th ed. (ChurchillLivingstone, 1979).

intake should be higher than output. Obesity and cachexia (a disorder characterized by general physical wasting and malnutrition) are extreme examples of problems affecting the energy stores of the body. The appetite and satiety centers in the central nervous system normally are

sensitive regulators that adjust energy consumption to energy requirements. Obesity is rarely caused by damage to those centers. Obesity results from an excess of energy intake over physiological needs. Factors predisposing or contributing to the development of obesity are heredity,

TABLE 5-4

Examples of Rate of Energy Expenditure during Various Types of Physical Activities of Adults* Rate of Energy Expenditure, kcal/min (kJ/min) Type of Activity

Man, 70 kg

Woman, 58 kg

Very light: Sitting, standing, painting, driving, working in a laboratory, typing, playing musical instrument, sewing, ironing.

up to 2.5 (up to 10.5)

up to 2.0 (up to 8.3)

Light: Walking on level (2.5-3 mph), tailoring, pressing, working on automobiles, washing, shopping, golfing, sailing, playing table tennis or volley ball.

2.5-4.9 (10.5-20.5)

2.0-3.9 (8.37-16.3)

Moderate: Walking (3.5-4 mph), plastering, weeding and hoeing, loading and stacking bales, scrubbing floors, shopping with heavy load, cycling, skiing, playing tennis, dancing.

5.0-7.4 (20.9-31.0)

4.0-5.9 (16.7-24.7)

Heavy: Walking uphill with load, tree felling, working with pick and shovel, basketball, swimming, climbing, playing football.

7.5-12.0 (31.4-50.2)

6.0-10.0 (25.1-41.8)

*Adapted from RecommendedDietaryAllowances, 9th ed. (Food and Nutrition Board, National Research CouncilINational Academy of Sciences, Washington, D.C., 1980).

~0


endocrine disorders, behavior, lifestyle and physical activity, eating habits, and culture. Obesity is the most common nutritional disorder in developed countries. A sedentary lifestyle is a significant contributing factor in insidious weight gain. Sedentary lifestyles may not only predispose individuals to obesity but also lead to degenerative arterial diseases. A sound exercise program coupled with good nutrition can help maintain health and quality of life. Obese individuals who wish to lose weight by dietary restriction and increased physical activity should be under medical supervision. The human body needs a wide variety of nutrients to function optimally. Recommended Dietary Allowance (RDA), published by the Food and Nutrition Board of the National Academy of Sciences National Research Council (USA), provides a broad guideline of the amounts of some of the well-established nutrients (protein, vitamins, and minerals) that should be consumed daily to maintain normal health (RDA values are given in Appendix IV). To use the RDA properly, keep in mind the following: 1. RDA values are overestimates of nutrient requirements; minimum quantities of nutrients required to maintain normal function and health in humans are either unknown or incompletely known. 2. RDA values relate primarily to populations of developed countries and are intended to meet the needs of virtually all healthy individuals. Thus, the allowances are far higher than demanded by physiological needs, and for many populations of the world, the RDA estimates may be inappropriately high. If the intake of a given nutrient is below the RDA value, nutritional inadequacy does not necessarily result. However, if a particular deficiency is accompanied by biochemical and clinical abnormalities, then corrective action is needed.

Clinical conditions include trauma and stress, prematurity, inherited metabolic disorders, disease states and rehabilitation, and use of certain drugs. 3. Since the RDA applies to healthy people, it does not provide requirements in disease states or in drug-nutrient interactions. 4. RDA does not address the fact that some nutrients possess pharmacological activities unrelated to nutrient function. Excessive intake of such nutrients can have toxic effects. 5. RDA does not provide values for all essential nutrients, and therefore the daily requirement should be met by selection from a wide variety of foods. No single food source is nutritionally complete. Foods are categorized into four groups: 1) milk and milk products, 2) meat and meat alternatives (e.g., legumes, nuts, and milk products), 3) fruits and vegetables, and 4) breads and cereals. Table 5-5 lists the key nutrients in some representative foods in each group. Not only is consumption of adequate amounts of nutrients essential but so is consumption of appropriate quantities of foods that provide amounts of energy commensurate with physical activity and the physiological state. Knowledge of a given nutrient concentration (also known as nutrient density) per unit of energy (e.g., 1000 kcal) in a specific food is useful in adjusting the diet to individual need. Dietary fiber, although not nutritionally essential, is important in maintaining good health. Dietary fiber consists of polymers of sugars and other indigestible substances. Fiber affects nutrient absorption and makes the contents of the gut bulky and soft. In terms of the four food groups (Table 5-4), fiber can be obtained from the second group by selection of dry beans, peas, and nuts in place of (or in addition to) meat, poultry, and fish

TABLE 5-5

Food Groups*

Group 1. Milk and milk products (e.g., milk, yogurt, cheese, ice cream) 2. Meat (e.g., beef, pork, poultry, fish) and meat alternatives (e.g., legumes, nuts, milk products) 3. Fruits and vegetables (e.g., green leafy, deep yellow, or orange vegetables, citrus fruits) 4. Breads, cereals, and grains

Major Nutrients Provided Calcium, riboflavin, protein Protein, niacin, iron, thiamine (vitamin B l), Vitamins A and C Carbohydrate, thiamine, iron, niacin

*A well-balanced diet includes a variety of foods on a daily basis, selected from each of the four food groups. With proper planning a vegetarian diet can be nutritionally adequate. Special foods consumed by ethnic groups should be considered in diet planning. Basic principles of a good diet include consumption of a variety of foods; consumption to maintain ideal body weight; moderation in intake of foods or beverages rich in saturated fats, cholesterol, sugar, and sodium salts; and consumption of adequate quantities of fruits, vegetables, and whole grains.

SECTION5.5 Energy Metabolism

81

TABLE 5-6

BMI Values Corresponding to Different Height-Weight Combinations* BMI -->

19

20

21

22

23

24

25

Weight**

Height* ft.in,

m

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

4'10" 4'11" 5'0" 5'1" 5'2" 5'3" 5'4" 5'5" 5'6" 5'7" 5'8" 5'9" 5'10" 5'11" 6'0" 6'1" 6'2" 6'3" 6'4"

1.473 1.499 1.524 1.549 1.575 1.600 1.626 1.651 1.676 1.702 1.727 1.753 1.778 1.803 1.829 1.854 1.880 1.905 1.930

91 94 97 100 104 107 110 114 118 121 125 128 132 136 140 144 148 152 156

41.28 42.64 44.00 45.36 47.17 48.54 49.90 51.71 53.52 54.89 56.70 58.06 59.88 61.69 63.50 65.32 67.13 68.95 70.76

96 99 102 106 109 113 116 120 124 127 131 135 139 143 147 151 155 160 164

43.55 44.91 46.27 48.08 49.44 51.26 52.62 54.43 56.25 57.61 59.42 61.24 63.05 64.86 66.68 68.49 70.31 72.58 74.39

100 104 107 111 115 118 122 126 130 134 138 142 146 150 154 159 163 168 172

45.36 47.17 48.54 50.35 52.16 53.52 55.34 57.15 58.97 60.78 62.60 64.41 66.23 68.04 69.85 72.12 73.94 76.20 78.02

105 109 112 116 120 124 128 132 136 140 144 149 153 157 162 166 171 176 180

47.63 49.44 50.80 52.62 54.43 56.25 58.06 59.88 61.69 63.50 65.32 67.59 69.40 71.22 73.48 75.30 77.57 79.83 81.65

110 114 118 122 126 130 134 138 142 146 151 160 162 165 169 174 179 184 189

49.90 51.71 53.52 55.34 57.15 58.97 60.78 62.60 64.41 66.23 68.49 72.58 73.48 74.84 76.66 78.93 81.19 83.46 85.73

115 119 123 127 131 135 140 144 148 153 158 162 167 172 177 182 186 192 197

52.16 53.98 55.79 57.61 59.42 61.24 63.50 65.32 67.13 69.40 71.67 73.48 75.75 78.02 80.29 82.56 84.37 87.09 89.36

119 124 128 132 136 141 145 150 155 159 164 169 174 179 184 189 194 200 205

53.98 56.25 58.06 59.88 61.69 63.96 65.77 68.04 70.31 72.12 74.39 76.66 78.93 81.19 83.46 85.73 88.00 90.72 92.99

BMI --~

Weight**

Height* ft.in,

m

40

35

30

29

28

27

26

lb

kg

4'10" 1.473 4'11" 1.499 5'0" 1.524 5'1" 1.549 5'2" 1.575 5'3" 1.600 5'4" 1.626 5'5" 1.651 5'6" 1.676 5'7" 1.702 5'8" 1.727 5'9" 1.753 5'10" 1.778 5'11" 1.803 6'0" 1.829 6'1" 1.854 6'2" 1.880 6'3" 1.905 6'4" 1.930

124 128 133 137 142 146 151 156 161 166 171 176 181 186 191 197 202 208 213

56.25 58.06 60.33 62.14 64.41 66.23 68.49 70.76 73.03 75.30 77.57 79.83 82.10 84.37 86.64 89.36 91.63 94.35 96.62

*Without shoes

**Withoutclothes

lb

kg

129 58.51 133 60.33 138 62.60 143 64.86 147 66.68 152 68.95 157 71.22 162 73.48 167 75.75 172 78.02 177 80.29 182 82.56 188 85.28 193 87.54 199 90.27 204 92.53 210 95.26 216 97.98 221 100.25

lb

kg

134 60.78 138 62.60 143 64.86 148 67.13 153 69.40 158 71.67 163 73.94 168 76.20 173 78.47 178 80.74 184 83.46 189 85.73 195 88.45 200 90.72 206 93.44 212 96.16 218 98.88 224 101.61 230 104.33

lb

kg

138 62.60 143 64.86 148 67.13 153 69.40 158 71.67 163 73.94 169 76.66 174 78.93 180 81.65 185 83.92 190 86.18 196 88.91 202 91.63 208 94.35 213 96.62 219 99.34 225 102.06 232 105.24 238 107.96

lb

kg

lb

kg

lb

kg

143 148 153 158 163 169 174 180 186 191 197 203 209 215 221 227 233 240 246

64.86 67.13 69.40 71.67 73.94 76.66 78.93 81.65 84.37 86.64 89.36 92.08 94.80 97.52 100.25 102.97 105.69 108.86 111.59

167 173 179 185 191 197 204 210 216 223 230 236 243 250 258 265 272 279 287

75.75 78.47 81.19 83.92 86.64 89.36 92.53 95.26 97.98 101.15 104.33 107.05 110.22 113.40 117.03 120.20 123.38 126.55 130.18

191 198 204 211 218 225 232 240 247 255 262 270 278 286 294 302 311 319 328

86.64 89.81 92.53 95.71 98.88 102.06 105.24 108.86 112.04 115.67 118.84 122.47 126.10 129.73 133.36 136.99 141.07 144.70 148.78

82

CHAPTER 5

from the third group; and from the fourth group by selection of whole-grain products (See also Chapter 9).

5.6 Obesity The first law of thermodynamics states that the amount of stored energy equals the difference between energy intake and energy expenditure. The principal storage of energy is that of triglycerides in adipose tissue. Energy stores are essential for survival during times of energy deprivation (Chapters 18 and 22). Energy stores in an adult are maintained at a relatively constant level throughout life. However, even a small imbalance in energy intake over long periods of time will have a significant effect on energy storage. For example, suppose a non obese adult's energy intake exceeds expenditure by about 1% daily for one year. This amounts to an excess of 9,000 kcal and corresponds to a weight gain of about 2.5 lb (1.15 kg) per year. 3,500 kcal of chemical energy is equivalent to 1 lb (0.45 kg) of adipose tissue. Weight gain in most people is attributable to overconsumption of palatable, energy-dense foods (e.g., lipids) and a sedentary lifestyle. Childhood obesity is a risk factor for obesity in adulthood. Obesity is a consequence of a positive energy balance, i.e., input is greater than output. Body mass index (BMI) is the most useful parameter in assessing the magnitude of obesity (Table 5-6). A BMI of 19-25 is considered healthy, 26-29.9 is moderately overweight, and 30 or greater is obese. Obesity is the most common health problem in the developed world, and it is estimated that obesity affects about one third of the U.S. adult population. In developed nations, the prevalence of obesity is higher among economically deprived people, whereas in developing nations, the relatively affluent have a higher risk. A BMI greater than 28 is an independent risk factor (3-4 times higher than the general population) for cardiovascular diseases (Chapter 20), diabetes mellitus type 2 (Chapter 22), and stroke. The prevalence of obesityassociated morbidity depends on the location of fat distribution in the body. Intra-abdominal or visceral fat deposits are associated with higher health risks than gluteofemoral adipose tissue fat accumulation. In experimental animals and possibly in humans, energy intake influences the aging process as well as the onset of aging-associated diseases. Energy restriction in animals, without altering their optimal nutritional status, increases the average life span but not the maximal life

Thermodynamics,Chemical Kinetics, and Energy Metabolism

span. This beneficial biological response to energy restriction has been attributed to the attenuation of oxidative damage to proteins, lipids, and DNA (Chapter 14). During aging, despite increased body fat, there is a linear decrease in food intake and metabolism over the life span.

Biochemical Mediators of Obesity The regulation of energy intake and expenditure is achieved by coordinating the effects of endocrine mediators and neural signals that arise from adipose tissue, endocrine glands, neurological and gastrointestinal systems. All of the information finally is integrated by the central nervous system (Figure 5-6). One of the most significant mediators of the energy store in the adipose tissue is leptin (from the Greek leptos, meaning "thin"). Leptin is a protein of 167 amino acid residues that is synthesized in adipocytes. Its synthesis is increased by insulin, glucocorticoids, and estrogens and is decreased by/3-adrenergic agonists. The role of leptin in obesity comes from studies on rodents. In genetically obese mice (Ob/Ob), the observed gross obesity is due to absence of leptin production in the adipocytes. Leptin's action on energy metabolism is mediated by receptors in many cells and it binds specifically to a receptor in the hypothalamus. The action of leptin involves at least two pathways. During starvation and weight loss, adipose tissue is decreased with consequent low levels of leptin. The low level of leptin leads to production of neuropeptide Y, which is synthesized in the arcuate nucleus of the hypothalamus and is transported axonally to the paraventricular nucleus. Neuropeptide Y binds to its receptor and functions as a potent appetite stimulant. The overall effect is increased appetite, decreased energy expenditure and temperature, decreased reproductive function (infertility), and increased parasympathetic activity. An opposite set of events occurs when the leptin levels rise, except that the effects are mediated by melanocyte-stimulating hormone (MSH) that binds to the melanocortin 4 receptor (MC4-R). The MSH binding to MC4-R initiates several biological responses, including decreased appetite, increased energy expenditure, and increased sympathetic activity (Figure 5-7). In humans, obesity is a complex disease because of the redundancy of systems that regulate energy storage. There are inherited disorders of hyperphagia leading to obesity with associated clinical features such as hypogonadism and mental retardation. One hereditary disorder is PraderWilli syndrome (Chapter 26), which is the most prevalent form of dysmorphic genetic obesity (1 in 10,000-20,000

SECTION 5.6

Obesity

83 Afferent signals increasing appetite or decreasing energy expenditure

Afferent signals decreasing appetite or increasing energy expenditure

Gastrointestinal tract Opioids,neurotensin, growthhormone-releasing hormone,somatostatin

Gastrointestinal tract Glucagon,cholecystokinin, glucagon-likepeptides, bombesmpeptides,glucose

Endocrine system l" Epinephrine(o~-adrenergiceffect), androgens,glucocorticoids, insulin,peptideYY,progesterone

Endocrine system Epinephrine([3-adrenergiceffect), estrogens

~

t ' ~ ~ ] ~ : . 4 ' Adipose . ~ . . = ~ . ;tissue .

~

l

Per,0hera, nervous system Norepinephrine (cz-adrenergiceffect))

Peripheral nervous system l Norepinephrine (13-adrenergiceffect)

} - - ~ -~

Centra, oe~oos s~stem Dopamine,y-aminobutyricacid, serotonin,cholecystokinin

Centra' ne ous system Oa,anin,opioi~s,

~ " ' ~ ' ~ #

Hypothalamic tracts Norepinephrine, serotonin, neuropeptide Y, melanocyte-concentratinghormone, glucagon-like peptide I, corticotropin-releasing hormone

.

growthhormone-releasing hormone,somatostatin . . . .

@

Efferents

Sympathetic nervous system parasympathetic nervous system, thyroid hormones

@

Energy intake and expenditure

FIGURE 5-6 Afferent and efferent pathways of regulation of energy intake and expenditure. [Reproduced with permission from M. Rosenbaum, R. L. Leibel, and J. Hirsch. Obesity. N. Engl. J. of Med. 337:396 (1997).]

Starvation

Overeating (hyperphagia) obesity

Weight loss and decreased adipose tissue

Weight gain and increased adipose tissue

keptin

keptin

I Transport across blood-brain barrier I

I Transport across blood-brain barrier I

Leptin receptor-hypothalamus

Leptin receptor-hypothalamus

Neuropeptide Y and Y5 receptor

Melanocyte-stimulating hormone-

Biological responses: Food intake Energy expenditure Fertility TemperatureS, Parasympathetic activity

Melanocortin receptor

Biological responses: Food intake Energy expenditure Sympathetic activity

FIGURE 5-7 An overview of the action of leptin on food intake, energy expenditure, and biological responses (1" increased and ,l, decreased).

.

84

births). Deletions in chromosome 15 account for a majority of cases of Prader-Willi syndrome. In other cases, a novel genetic mechanism of uniparental disomy is the cause of this syndrome. Congenital human leptin deficiency is associated with early-onset obesity. Homozygous mutations in the human leptin receptor gene result in a truncated leptin receptor lacking both transmembrane and intracellular domains; such mutations are associated with early-onset obesity, absence of pubertal development, and pituitary dysfunction. In most obese humans, the plasma levels of leptin are high due to excess adipose tissue. In spite of abundant leptin, there is continued overeating. This puzzling observation may ultimately be explained by as yet-to-be identified defects in leptin metabolism.

Supplemental Readings and References P. Bj6rntorp: Obesity. Lancet 350, 423 (1997). J. E Caro, J. W. Kolaczynski, M. R. Nyce, et al.: Decreased cerebrospinalfluid/serum leptin ratio in obesity: A possible mechanism for leptin resistance. Lancet 348, 159 (1996).

CHAPTER5 Thermodynamics, Chemical Kinetics, and Energy Metabolism K. Clement, C. Vaisse, N. Lahlou, et al.: A mutation in the human leptin receptor gene causes obesity and pituitary dysfunction. Nature 392, 398 (1998). J. M. Friedman and J. L. Halaas: Leptin and the regulation of body weight in mammals. Nature 395, 763 (1998). C. T. Montague, I. S. Farooqi, J. E Whitehead, et al.: Congenital leptin deficiency is associated with severe early-onset obesity in humans. Nature 387, 903 (1997). J. E. Morley: Anorexia of aging: Physiologic and pathologic. American Journal of Clinical Nutrition 66, 760 (1997). Recommended Dietary Allowances, 10th ed. Food and Nutrition Board, National Research Council-National Academy of Sciences, Washington, DC, (1989). M. Rosenbaum, R. L. Liebel, and J. Hirsch: Obesity. New England Journal of Medicine 337, 396 (1997). M. W. Schwartz, E. Peskind, M. Raskind, et al.: Cerebrospinal fluid leptin levels: Relationship to plasma levels and to adiposity in humans. Nature Medicine 2, 589 (1996). J. Stevens, J. Cai, E. R. Parnuk, et al.: The effect of age on the association between body-mass index and mortality. New England Journal of Medicine 338, 1 (1998). L. A. Tartaglia, M. Dembroski, X. Weng, et al.: Identification and expression of cloning of a leptin receptor, OB-R. Cell 83, 1263 (1995). R. Weindruch and R. S. Sohal: Caloric intake and aging. New Journal of Medicine 337, 986 (1997).

CHAPTER

6

Enzymes I: General Properties, Kinetics, and Inhibition

This chapter deals with enzyme catalysis, kinetics, inhibition, and mechanisms. Enzymes are catalysts and are functional units of cellular metabolism. Enzymes are usually proteins but may include RNA molecules as well. This chapter and the following two chapters discuss protein enzymes. RNA molecules are ribonucleases (ribozymes) that recognize specific nucleotide sequences in the target RNA and hydrolyze phosphodiester bonds. Ribosomal RNA functions as a peptidyl transferase in protein biosynthesis (Chapter 25). Some RNA molecules also undergo a self-splicing process. One example is the transformation of pre-mRNA to mature, functional mRNA by the splicing out of intervening sequences (introns) and ligation of the coding sequences (exons). The splicing-ligation process requires small nuclear ribonucleoprotein particles (snRNPs) and other proteins. Enzymatic RNA also can act on other RNA molecules. An example is ribonuclease P (RNase P), which is involved in the conversion of precursor tRNA to functional tRNA by generating 5'-phosphate and 3'hydroxyl termini. RNase P contains both a catalytic RNA moiety and an associated protein (Chapter 25). Many enzymes are bound either covalently or noncovalently to nonprotein components essential for enzymatic activity. The term prosthetic group is generally reserved for those nonprotein components that are bound tightly, whereas the term coenzyme applies to less tightly bound nonprotein components. The coenzymes

are complex organic compounds, frequently derived from vitamins. The term eofaetor is used for metal ions and simple organic compounds that partipate in enzyme catalysis. The protein portion of an enzyme is the apoenzyme, and the fully functional enzyme with its attached nonprotein component is the holoenzyme (i.e., apoenzyme + coenzyme = holoenzyme).

6.1 Nomenclature Enzymes are generally named for the substrate or chemical group on which they act, and the name takes the suffix -ase. Thus, the enzyme that hydrolyzes urea is named urease. Examples of exceptions to this terminology are trypsin, pepsin, and papain, which are trivial names. Systematic nomenclature for the enzymes has been developed by the Enzyme Commission of the International Union of Biochemistry. This system provides a rational and practical basis for identification of all enzymes currently known as well as for new enzymes. The systematic name describes the substrate, the nature of the reaction catalyzed, and other characteristics. A unique numerical code consisting of four numbers separated by periods (e.g., EC. 4.2.1.1) is designated. The prefix "EC" denotes "Enzyme Commission." The first number in this designation specifies the class to which the enzyme belongs. All enzymes are assigned to one

85

86

CHAPTER6 Enzymes I: General Properties, Kinetics, and Inhibition

of six classes on this basis of the type of reaction they catalyze: 1. 2. 3. 4.

oxidoreductases (oxidation-reduction), transferases (transfer of groups), hydrolases (hydrolysis), lyases (nonhydrolytic and nonoxidative cleavage of groups), 5. isomerases (isomerization), and 6. ligases or synthetases (joining of two molecules with the breaking of a pyrophosphate bond).

The next two numbers in the code indicate the subgroup and the sub-subgroup; the last number is the special serial number given to each enzyme in its sub-subgroup. Consider the systematic nomenclature for the enzyme with the trivial name carbonic anhydrase, which facilitates the transport of CO2 by catalyzing the following reaction (Chapter 1):

(D-glucose, 2-deoxy-D-glucose, D-fructose, and D-mannose) at almost equivalent rates. 9 Phosphatase, which hydrolyzes phosphate groups from a large variety of organic phosphate esters. 9 Esterase, which hydrolyzes esters to alcohols and carboxylic acids, with considerable variation in chain length in both the alkyl and acyl portions of the ester. 9 Proteinase, which hydrolyzes peptide bonds irrespective of the chemical nature of the substrate. Many enzymes show stereoisomeric specificities. For example, human or-amylase catalyzes the hydrolysis of glucose from the linear portion of starch but not from cellulose. Starch and cellulose are both polymers of glucose, but in the former the sugar residues are connected by or(1 --+ 4) linkages, whereas in the latter they are connected by/3(1 --+ 4) linkages (Chapter 9).

H2CO3 (or H + + HCO3) ~-- H 2 0 Jr- CO2

Active Site and Enzyme-Substrate Complex

or H2CO3(or H § -t- HCO3- ) ~ H20 + CO2

or

O II

The systematic name for carbonic anhydrase is carbonate hydro-lyase, and its numerical code is EC. 4.2.1.1. The first number identifies it as a lyase; the second as an enzyme that catalyzes the breakage of a carbon-oxygen bond, leading to unsaturated products; and the third as a hydro-lyase, participating in a reaction involving the elimination of water. The last number is the specific serial number assigned to this enzyme. In this text, the trivial names are used. Names of a selected list of clinically useful enzymes with their EC codes, systematic names, other common names, and abbreviations are given in Appendix V.

6.2 Catalysis Specificity of Enzyme Catalysis Enzymes are highly specific and usually catalyze only one type of reaction. Some enzymes show absolute specificity. For example, pyruvate kinase catalyzes the transfer of a phosphate group only from phosphoenolpyruvate to adenosine diphosphate during glycolysis (Chapter 13). Examples of enzymes that show less specificity are: 9 Hexokinase, which transfers the phosphate group from adenosine triphosphate to several hexoses

An enzyme-catalyzed reaction is initiated when the enzyme binds to its substrate to form an enzyme-substrate complex. In general, enzyme molecules are considerably larger than the substrate molecules. Exceptions are proteinases, nucleases, and amylases that act on macromolecular substrates. Irrespective of the size of the substrate, binding to the enzyme occurs at a specific and specialized region known as the active site, a cleft or pocket in the surface of the enzyme that constitutes only a small portion of the enzyme molecule. Catalytic function is accomplished at this site because various chemical groups important in substrate binding are brought together in a spatial arrangement that confers specificity on the enzyme. Thus, the unique catalytic property of an enzyme is based on its three-dimensional structure and on an active site whose chemical groups may be brought into close proximity from different regions of the polypeptide chain. The stereospecificity of an enzyme for a substrate has been compared to a lock-and-key relationship. This analogy implies that the enzyme has an active site that fits the exact dimensions of the substrate (Figure 6-1). However, after attachment of substrate, the enzyme may undergo conformational changes that provide a more perfect fit between it and the substrate. This process has been described as inducedfit (Figure 6-2).

Factors Governing the Rate of Enzyme-Catalyzed Reactions The overall reaction involving conversion of substrate (S) to product (P) with formation of the enzyme-substrate

SECTION 6.2

Catalysis

8? to the same extent. Thus, enzymes affect only the rate at which equilibrium is established between reactants and products. However, under steady-state conditions, which is the normal state of affairs in the human body, the net effect of the enzyme is to convert substrates to products as rapidly as the products are removed (Chapter 5).

Effect of Temperature

FIGURE 6-1

Schematic diagramof a lock and key relationshipbetween an enzymeand its substrate. The shape of the active site is complementaryto that of the substrate molecule.

complex, ES can be written: kl

E+S

k2

~-- ES ~ E + R k-1

k-2

where kl, k_l, k2, and k_2 are rate constants for the designated steps; the positive subscripts indicate forward reactions, and the negative subscripts indicate backward reactions. Because an enzyme increases the rate of a particular reaction by decreasing the free energy of activation without itself being consumed or permanently altered, a small number of enzyme molecules can convert an extremely large number of substrate molecules to products very rapidly. Enzymes do not alter the equilibrium constant of a chemical reaction because they catalyze the forward and backward reactions

FIGURE 6-2

Schematic diagramof the induced-fitmodel for the relationshipbetween an enzymeand its substrate. The shape of the active site of the enzyme conforms to that of the substrate onlyafter substrate binding induces appropriate conformationalchangesin the enzyme.

The rates of almost all chemical reactions increase with a rise in temperature, which causes both the average kinetic energy and the average velocity of the molecules to increase, resulting in a higher probability of effective reaction collisions. Enzymes, however, undergo denaturation and are inactivated at high temperatures. Below the denaturation temperature, the reaction rate will approximately double for every rise of 10~ The ratio by which the rate changes for a 10~ increase in temperature is known as Q 10; this ratio varies from 1.7 to 2.5. The optimal temperature for most enzymes is close to the normal temperature of the organism at which catalysis occurs at the maximum rate. In humans, most enzymes have an optimal temperature of 37~

Effect of pH The activity of most enzymes depends on pH. The p H enzyme activity profile of most enzymes delineates a bellshaped curve (Figure 6-3), exhibiting an optimal pH at which activity is maximal. This pH is usually the same as the pH of the fluid in which the enzyme functions. Thus, most enzymes have their highest activity between pH 6 and pH 8 (the pH of human blood is about 7.4). However, pepsin, which must function at the low pH of gastric juice, has maximal activity at about pH 2. The pH dependence of enzyme activity is the result of several effects. Ionizable groups in the active site of the enzyme (or elsewhere), in the substrate, or in the enzyme-substrate complex can affect catalysis depending on whether the protons on the reactive groups are dissociated or undissociated. Ionization of these groups depends on their pK values, the chemical properties of surrounding groups, and the pH of the reaction medium. Changes in pH affect the binding of the substrate at the active site of the enzyme and also the rate of breakdown of the enzyme-substrate complex. Thus, it may be possible to infer the identity of an ionizable group that participates at the active site from the pH-activity profile for a given enzyme. The enzymes in living systems function at nearly constant pH because they are in an environment that contains buffers (Chapter 1).

CHAPTER6 Enzymes I: General Properties, Kinetics, and Inhibition

~8

F I G U R E 6-4 Plot of substrate concentration versus initial velocity of an enzymecatalyzed reaction. Segment A: At low substrate concentration, the reaction follows first-order kinetics with respect to substrate concentration; i.e., v = k[S], where k is a reaction rate constant. Segment B: At high substrate concentration, maximum velocity (Vmax) is attained (saturation kinetics), and any further increase in substrate concentration does not affect the reaction rate; the reaction is then zero-order with respect to substrate but first-order with respect to enzyme. Km is the value of [S] corresponding to a velocity of 1 Vmax.

F I G U R E 6-3 pH dependence of the rates of enzyme-catalyzed reactions. (a) Optimal activity at alkaline pH. (b) Optimal activity of acidic pH.

derived for a single substrate-enzyme-catalyzed reaction. In the reaction k~

k2

E + S +--'--ES--+E + P k-i

Effect of Concentration of Enzyme and Substrate The reaction rate is directly proportional to the concentration of the enzyme if an excess of free substrate molecules is present. Thus, enzyme-substrate interactions obey the mass-action law. For a given enzyme concentration, the reaction velocity increases initially with increasing substrate concentration. Eventually, a maximum is reached, and further addition of substrate has no effect on reaction velocity (v) (Figure 6-4). The shape of a plot of v versus [S] is a rectangular hyperbola and is characteristic of all nonallosteric enzymes (Chapter 7). At low substrate concentrations, the reaction rate is proportional to substrate concentration, with the reaction following firstorder kinetics in terms of substrate concentration.

Michaelis-Menten Treatment of the Kinetic Properties of an Enzyme A model for enzyme kinetics that has found wide applicability was proposed by Michaelis and Menten in 1913 and later modified by Briggs and Haldane. The MichaelisMenten equation relates the initial rate of an enzymecatalyzcd reaction to the substrate concentration and to a ratio of rate constants. This equation is a rate equation,

an enzyme E combines with substrate S to form an ES complex with a rate constant kl. The ES complex formed can dissociate back to E and S with a rate constant k-l, or it can give rise to a product P and regenerated enzyme with a rate constant k2. In the latter process, the formation of product is essentially irreversible; the reverse reaction k_'~ (E + P --4 ES) does not occur to any appreciable extent. Thus, k-2 is much less than k2, which corresponds to a large, negative AG in going from ES to E + E In the Michaelis-Menten expression, [ET] represents the total concentration (in moles per liter) of enzyme, equal to [E] + [ES]. Therefore, the concentration of free enzyme [E] is equal to [ET] -- [ES]. The following assumptions are made: 1. In the reaction mixture, [S] is greater than [E]. However, [S] is not so large that all enzyme molecules are in the ES form, but [S] must be sufficiently large that [S] does not rapidly become so small that [S] < [E]. Excess concentration of substrate is the most common condition in enzymatic studies in which the catalyst is present at very low concentration. Under these conditions, [ES] rapidly becomes constant, so that the rate of formation of ES

SECTION 6.2

Catalysis

89

equals its rate of breakdown, and steady-state kinetics then applies. 2. Only initial velocities are measured. Hence, during the period of measurement, [S] is not significantly depleted as [P] increases. 3. [ES] is constant during the period of measurement, which means that (a) enough time has elapsed since mixing E and S for [ES] to build up; (b) not enough time has passed for the rate of formation of ES to decrease owing to substrate depletion; and (C) k2 < k - l , so that [ES] can build up. If k2 > k - l , ES breaks down as rapidly as it is formed, and a steady-state can never be established. Rate of formation of ES --

d[ES]

dt

These expressions can both be solved for k2" k2 --

d[ES]

= k-1 [ES] 4- k2 [ES]

kl ([ET] -- [ES])[S] -- (k_l 4- k2)[ES].

(6.1)

Rearranging the above Equation yields

[ES]

k-1 4- k2 kl

= Km.

(6.2)

Note that Km (the Michaelis constant) is not an equilibrium constant but a ratio of rate constants. Equation (6.2) can be rearranged in the following ways" [S][E7] - [S][ES] -- Km[ES] or

[S][ET] - [ES] (Km + [S]) or

[ S ] - [ES] [ET] (Km 4- [S]).

[ET]

Vmax

V --

v or

[ES]

[ET]

[ES] --

.

Wmax

[E7]

Substituting the latter equation for [ES]/[ET] in Equation (6.3), we obtain V

[S] --

(Km 4- [S]). Vmax

Rearranging and solving for v,

Under steady-state conditions, the rate of formation of ES equals its rate of breakdown, so that

=

[gs]

- kl [E] [S]

dt -- (k-1 4- k2)[ES].

([ET] - [ES])[S]

Vmax

and k2 -

Therefore,

v --

-- kl ([ET] -- [ES])[S].

Rate of breakdown of ES --

V

(6.3)

It is usually difficult to measure [ES] in a reaction mixture. Consequently, Equation (6.3) is not useful experimentally. On the other hand, the velocity (v) and the maximum velocity (Vmax) are readily determined by a variety of methods. As indicated in Figure 6-4, Vmaxis the limiting value that v approaches as [S] --+ oc. In that situation, all enzyme molecules have substrate bound to them, and so [E ] = 0 and [ET] = [ES]. Thus, v = k2 [ES], and Vmax = k2 [ET].

[ S ] Vmax

Km 4- [S]

.

(6.4)

Equation (6.4) is known as the Michaelis-Menten equation, and the following points should be noted. 1. Km is a constant for a particular enzyme and substrate and is independent of enzyme and substrate concentrations. 2. Vmaxdepends on enzyme concentration, and at saturating substrate concentration, it is independent of substrate concentration. 3. Km and Vmax may be influenced by pH, temperature, and other factors. 4. A plot of v versus [S] fits a rectangular hyperbolic function (Figure 6-4). 5. If an enzyme binds more than one substrate, the Km values for the various substrates can be used as a relative measure of the affinity of the enzyme for each substrate (the smaller the value of Km, the higher the affinity of the enzyme for that substrate). 6. In a metabolic pathway, Km values for enzymes that catalyze the sequential reactions may indicate the rate-limiting step for the pathway (the highest Km corresponds roughly to the slowest step). 7. When k_l >> k2, [ES] is assumed to be at equilibrium with [E] and [S], ES is dissociating more often to yield E and S than to yield product. Under this condition, the dissociation constant of the enzyme-substrate complex Ks for this equilibrium (ES ~ E + S) is Ks--

[E] [S]

k-1

[ES]

kl

But K m

k-1 + k e kl

90

CHAPTER 6

so that

Enzymes I General Properties, Kinetics, and Inhibition

T A B L E 6-1

Turnover Numbers of Some Enzymes

k-1 Km

=

kl

=

Ks.

Turnover Number (per second)

Enzyme 8. When k2 ) ) k_l, the rate of dissociation of ES to E and S is low, so that products are usually formed. For practical purposes, the reaction sequence can be thought of as consisting of two irreversible reactions: E + S-~ES-~ E+ P The overall reaction rate is always determined by [ES]. At low values of [S], ES is formed by a second-order reaction whose rate is proportional to [E][S]. At high values of [S], [ES] is constant and the rate is determined by the first-order breakdown of ES to product. 9. When [S] >> Km, the characteristic property of the turnover number for an enzyme can be invoked. This number provides information on how many times the enzyme performs its catalytic function per unit time, or how many times it forms the ES complex and is regenerated (turned over) by yielding product. The turnover number can be determined from Vmax and [ET]. We know that d (P)

dt

- - Vmax - - k 2 [ E S ] -

k2[ET]

under saturation conditions. Therefore, if [ET] is increased n-fold while [S] > [ET] is maintained, then Vmax will also increase n-fold. The rate is proportional to (or first order with respect to) [ET]. The kinetic constant ke, denoted as kcat under saturation conditions, provides the value for the turnover number. For example, if a reaction mixture contains an enzyme at a concentration of x molar, and the product is formed at a rate such that its concentration increases by y molar per second when the enzyme is saturated, Y per second. x

kcat - - -

This means that y/x substrate molecules are converted to product molecules every second. Correspondingly, the time required for a single conversion is x / y seconds. Turnover numbers for most enzymes usually range from 1 to 104 per second. A few enzymes have turnover numbers above 10 5 (Table 6-1). The ability of a cell to produce a given amount of product by an enzymatic reaction during its life span is proportional to the turnover number and the number of molecules of that enzyme in the cell. Because turnover numbers

Catalase Carbonic anhydrase Acetylcholinesterase a-Amylase Lactate dehydrogenase /3-Galactosidase Chymotrypsin Phosphoglucomutase Tryptophan synthase Lysozyme

5 x 10 6 6 x 10 5 2.5 x 10 5 1.9 x 104 1 x 10 3 2 x 10 2 10 2 21 2 0.5

can be measured only for pure enzymes, the activity of an enzyme is expressed as specific activity, defined as micromoles (#mol) of substrate converted to product per minute per milligram (mg) of enzyme protein. The International Union of Biochemistry recommends use of the unit known as katal (kat); one katal is the amount of enzyme that converts one mole of substrate to product per second. In clinical disorders, the activity of a variety of enzymes is measured in biological fluids. For this purpose, the activity is usually defined as that quantity of enzyme which catalyzes the conversion of 1 #mol of substrate to product per minute under a defined set of optimal conditions. This unit, referred to as the International Unit (U), is expressed in terms of U/mL of biological specimen (e.g., serum), or U/L. 10. When [S] 4) glycosidic linkages (Figure 9-21). The conformation of amylose has been elucidated by the use of stable amylose complexes prepared by reacting amylose with iodine. X-ray diffraction studies of such complexes have revealed a helical conformation with six glucose residues per turn of the helix. The amylose-iodine complex has an intense blue color, which provides the basis for the iodine test for starch.

qD

6 o

o

0 o.(1 ---->4)

0

,

o

0

Amylopectin

F I G U R E 9-21 Structures of the starch polysaccharides amylose and amylopectin.

Amylopectin contains glucosyl units joined together in both o~(1 --+ 4) and o~(1 --> 6) linkages, the latter linkages being responsible for branch points (Figure 9-21). Unlike amylose, amylopectin is unable to assume a stable helical conformation because of the branching. Amylopectin complexes with iodine to a much lesser extent than amylose; therefore, the amylopectin-iodine complex has a redviolet color that is much less intense than the blue of the amylose-iodine complex. Starch from different sources contains different amounts of amylose and amylopectin. In most plants, amylopectin is the more abundant form (about 75-80%). Virtually no amylose is found in starch obtained from some waxy varieties of maize (corn) and rice. Starch is easily digested by humans (Chapter 12). Glycogen is the animal equivalent of starch in plants and functions as the main storage polysaccharide in humans. It is a branched polysaccharide of D-glucose and, like amylopectin, contains both or(1 --+ 4) and c~(1 --> 6) linkages, the latter forming branch points (Figures 9-22 and 9-23). Each molecule of glycogen contains one reducing glucose

1~8

CHAPTER9 Simple Carbohydrates

,,

_./0%

Zo..

\

%

b

%

~(1~4) linkages

a(1--'6) linkage Main chain

C~OH

~

CH,OH

O\CH=

06"

CH=OH O

H

o

OH

OH

H

H

OH

OH

F I G U R E 9-22 Linkages of glucose residues in glycogen. Glycogen is structurally similar to amylopectin but more highly branched.

residue (i.e., unsubstituted C 1 hydroxyl group), which is the terminal unit on one of the chains. At each of the other termini, the glucose residue has a free hydroxyl group at C4, while the C~ hydroxyl group participates in the glycosidic linkage. Synthesis and breakdown take place at these termini. A glycogen molecule contains about 105 glucose units. It has no discrete molecular weight, since its size varies NE \

4

NE

/

NE

/

NE NE /

(

/

4

NE 4/

I

F I G U R E 9-23 A diagrammatic representation of a portion of a glycogen molecule. The glucose residues are shown by circles; Q)-Q) indicates oe(l ~ 4) linkages, and Q) --+ Q) indicates c~(1 ~ 6) linkages that are branch points. The ratio of 1,4 to 1,6 linkages varies between 12:1 and 18:1. Note that each molecule of glycogen has many nonreducing (NE) termini; further addition or cleavage of glucose units occurs at the nonreducing ends. Numbers 1 and 4 refer to C1 (reducing end) and C4 (nonreducing end), respectively.

considerably depending on the tissue of origin and its physiological state. In the human body, most glycogen is found in liver and muscle. The functional roles of glycogen in these two tissues are quite different: in muscle, glycogen serves as an energy reserve mostly for contraction, whereas liver glycogen supplies glucose to other tissues via the blood circulatory system. Cellulose, the most abundant carbohydrate on earth, is an unbranched polymer with glucosyl residues joined together in #(1 --+ 4) linkages (Figure 9-24). Cellulose from different sources varies in molecular weight, and the number of glucose units lies in the range of 2,500-14,000. Cellulose is the structural polysaccharide in plants. Cellulose microfibrils are tightly packed aggregates of cellulose molecules, which are chemically inert and insoluble and possess significant mechanical strength. Because of its # linkages, the preferred conformation of cellulose is one in which the ring oxygen of one residue forms a hydrogen bond with the C3 hydroxyl group of the next residue (Figure 9-24). The 40 or more individual cellulose molecules that aggregate to form microfibrils are held together by intermolecular hydrogen bonds. In many fungal cell walls and invertebrates (shells of crustaceans and exoskeletons of insects), the main structural polysaccharide is chitin, which is a polymer of N-acetylglucosamine linked in #(1 --+ 4) glycosidic linkage. In humans, cellulose is not digested in the small intestine but is digested in the large intestine to varying degrees by the microflora to yield short-chain fatty acids,

SECTION 9.1

149

Classification

p0---4) linkage [ CH2OH

CH2OH

).

CH, -O

-o

OH

-O

OH

OH

(a) CH2OH

--

.

~u .....

H~O

-

3

OH

- H ~ O ~ OH

I

4

/-~"O

~

CH2OH

\

~

O ....

--

CH20H

(b) F I G U R E 9-24 Structure of cellulose. (a)/3(1 --~ 4) linkage of glucose residues in cellulose. (b) Conformational formula showing hydrogen bonds ( . . . . . ) between the ring oxygens and the C3 hydroxyl groups.

hydrogen, carbon dioxide, and methane. Undigested cellulose forms a part of the indigestible component of the diet, known as dietary fiber. Ruminants and termites are able to digest cellulose, which is a primary energy source for them, because they harbor microorganisms in their intestinal tract that elaborate cellulase, which catalyzes hydrolysis of the/3(1 --+ 4) glucosidic linkages. Dietary fiber consists of cellulose and other polysaccharides (hemicelluloses, pectins, gums, and alginates) and a nonpolysaccharide component, lignin. These different types of dietary fiber are derived from the cell wall and the sap components of plant cells, and their properties are listed in Table 9-2. The exact role of dietary fiber in human nutrition is not clearly understood. Each component has a different chemical composition and exhibits different actions in the gastrointestinal tract. Dietary fiber has effects on the rate of absorption of nutrients and on the fecal mass. Depending on the amount and type of the indigestible component, gastric emptying is decreased, thus delaying of entry of digestible components into the small intestine, where the nutrients are absorbed. Thus, the absorption of nutrients can be modulated by the presence of fiber, particularly pectins or guar gums. Pectins, guar gums, and lignins may reduce plasma cholesterol levels. Thus, fiber may ameliorate diabetes mellitus (Chapter 22) and atherosclerosis (Chapter 20). Since the degree of satiety is related to the bulk of food that is consumed, the amount of fiber in the diet regulates the total energy consumed. Fiber also affects nutrient density, so that diets high in fiber are normally low in energy content. These aspects are important in the development and management of obesity. The relationship of dietary fiber to colonic cancer has

been the subject of epidemiological studies showing an inverse relationship between fiber consumption and risk of colonic cancer. The postulated mechanism is that the highfiber diet results in rapid intestinal transit, so that potential carcinogens or cocarcinogens have less time to interact with the mucosa. Various components of fiber may also bind carcinogens or dilute their concentrations by increasing the intraluminal bulk. Although dietary fiber is resistant to human digestive enzymes, some fiber is digested by microorganisms that reside in the colon, yielding organic anions such as acetate, propionate, and butyrate. Fiber and the osmotically active anions increase the size and wetness of the stools because they attract water and form gels. The overall effect of a high-fiber diet is a larger fecal mass, decrease in intraluminal pressure, reduction in transit time through the colon, and greater ease and frequency of defecation. These effects are useful in the treatment of Some gastrointestinal disorders. For example, a high-fiber diet has been recommended in the treatment of diverticulosis, in which outpouchings of colonic mucosa occur through the muscularis layer at points of weakness in the muscle wall. The diverticuli can become infected (diverticulitis) and produce bleeding, altered bowel function, and pain. Cross-cultural epidemiological studies link low-fiber diets with diverticulosis, cancer of the colon and rectum, coronary heart disease, diabetes mellitus, appendicitis, varicose veins (dilated tortuous veins), and hemorrhoids (enlarged veins of the lower rectum and anus). Despite gaps in our knowledge about the role of dietary fiber in human nutrition, adequate amounts probably are beneficial in the prevention of some chronic diseases. Consumption of carbohydrates in their

TABLE 9-2

Composition and Properties of Dietary Fiber with Respect to Human Nutrition

Type of Dietary Fiber and Food Sources

Chemical Composition

Properties

Cellulose Whole-wheat flour, Bran, Cabbage family, Dried peaslbeans, Apples, Root vegetables

Polymer of glucose with /3(1-+4) linkages.

Not digested in the small intestine; a significant amount is digested by the bacterial flora of the large intestine.

Hemicellulose Bran, Cereals, Whole grains

Heterogeneous group of polysaccharides; the major group consists of pentose polymers (pentosans), xylans, and arabinosylans; the second group consists of hexose polymers, galactans, and mannans; and the third group consists of uronic acid polymers containing either galacturonic or glucuronic acid.

Associated with cellulose in plant tissues not digested in the small intestine but digested by the bacterial flora of the large intestine. Can be extracted from cell walls by alkaline solutions.

Pectins Apples, Citrus fruits, Strawbemes

Mixture of galactouronan [a polymer of galacturonic acid linked in /3(1-+4) linkages] and galactan and arabinan in varying proportions depending on the source. Carboxylate groups of the uronic acids are either free or esterified with methyl groups.

Function as intercellular cementing material in plant tissues. When solubilized, the product has a characteristic gel texture (viscous and sticky properties). Not digested in the small intestine but digested to a small extent by the bacterial flora of the large intestine. Pectins are extracted by acidic solutions or solutions containing chelating agents.

Gums and Alginates Oatmeal, Dried beans, Other legumes

Heterogeneous group of polysaccharides. Some gums are galactomannans (e.g., guar gum). Alginates are polymannuronic acids.

Gums form viscous solutions; alginates are used as thickening agents and stabilizers in food. Not digested in the small intestine but digested to varying extents by the bacterial flora of the large intestine.

Lignins Mature vegetables, Wheat

Nonpolysaccharide polymer found in woody plant tissues. The nature of the monomers is not completely known, but appears to involve coniferyl and sinapyl or related alcohols.

Totally indigestible even by ruminants.

Supplemental Readings and References native form rather than in a processed state may provide an adequate amount of fiber. Fiber should be derived from a variety of plant foods such as high- and low-fiber breads and cereals, fruits, vegetables, legumes, nuts, and seeds, and eaten as part of normal meals. The recommended dietary fiber intake for adults is 20-25 g/day or 10-13 g per 1000 kcal (4184 kJ) of energy intake. A suggested intake of dietary fiber for children older than 2 years is the age of the child plus 5-10 g/day. The consumption of dietary fiber in childhood is also associated with health benefits, such as normal laxation, and may reduce the future risk of cardiovascular disease, some cancers, and diabetes mellitus type 2.

151

Supplemental Readings and References L. B. Ferzoco, V. Raptopouoos, and W. S. Len: Acute diverticulitis. New England Journal of Medicine 338, 1521 (1998). J. S. Hampl, N. M. Betts, and B. A. Benes: The "age +5" rule: Comparison of dietary fiber intake among 4- to 10-years old children. Journal of the American Dietetic Association 98, 1418 (1998). J. L. Listinsky, G. P. Siegal, and C. L. Listinsky: ot-L-Fucose: A potentially critical molecule in pathologic processes including neoplasia. American Journal of Clinical Pathology 110, 425 (1998). Position of the American Dietetic Association: Health implications of dietary fiber. Journal of the American Dietetic Association 97, 1157 (1997). S. M. Riordan and R. Williams: Treatment of hepatic encephalopathy. New England Journal of Medicine 337, 473 (1997). C. L. Williams (Ed.): The role of dietary fiber in childhood. Pediatrics 96, 985 (1995).


CHAPTER

10

Heteropolysaccharides 1: Glycoproteins and Glycolipids

10.1 Glycoproteins Glycoproteins, a wide range of compounds of diverse structure and function, are components of cell membranes, intercellular matrices, and extracellular fluids, such as plasma. They occur in soluble and membrane-bound forms (Table 10-1). The proportion of carbohydrate varies considerably in glycoproteins derived from different tissues or from various sources. Collagen, for example, contains an amount of carbohydrate that varies with the source: skin tissue collagen, about 0.5%; cartilage collagen, about 4%; and basement membrane collagen, more than 10%. Glycoproteins containing high amounts of carbohydrate include glycophorin, a membrane constituent of human erythrocytes, about 60%, and soluble blood group substances, as much as 85%. In glycoproteins, the protein and the carbohydrate residues are bound in covalent linkage. There are five common types of carbohydrate, four of which are common to human glycoproteins (Figure 10-1). All the linkages are either N- or O-glycosidic bonds. The amino acid residue that participates in the N-glycosidic linkage is asparagine, and the amino acid residues that participate in O-glycosidic linkage are serine, threonine, hydroxylysine, and hydroxyproline. The glycoproteins exhibit microheterogeneity, i.e., glycoproteins with an identical polypeptide sequence

may vary in the structure of theire oligosaccharide chains. Microheterogeneity arises from incomplete synthesis or partial degradation and poses problems in the purification and characterization of glycoproteins. For example, human serum o~1-acid glycoprotein has five linkage sites for carbohydrates and occurs in at least 19 different forms as a result of differences in oligosaccharide structures. The oligosaccharide side chains of glycoproteins consist of only a limited number (about 11) of different monosaccharides. These monosaccharides are hexoses and their derivatives (N-acetylhexosamine, uronic acid, and deoxyhexose), pentoses, and sialic acids derived from neuraminic acid, a nine-carbon sugar. The most common of the many different types of sialic acids is N-acetylneuraminic acid. (All of these sugars are in the D-configuration, unless indicated otherwise.) The sugar residues of the oligosaccharide chains are not present in serial repeat units (unlike the proteoglycans, Chapter 11), but they do exhibit common structural features. For example, the oligosaccharide chains bound in N-glycosidic linkages may be envisaged as consisting of two domains. The inner domain, common to all glycoproteins, is attached to the protein via an N-fl-glycosidic linkage between an asparaginyl residue and N-acetylglucosamine. The inner domain consists of a branched polysaccharide, which is trimannosyl-di-N-acetylglucosamine (Figure 10-2). The peripheral mannose residue of the inner domain is linked

153

154

CHAPTER10 Heteropolysaccharides I: Glycoproteins and Glycolipids

A 13-N-glycosidiclinkage between N-acetylglucosamine and the amide nitrogen of an asparagine residue of the protein (GIcNAc-Asn). Linkage of wide occurrence. ?H20_~HO,,

HOC HO H~I~

O

~

NH2

NH--ICI--CHF --IC-cOOH HI

An o~-O-glycosidic linkage between N-acetylglucosamine and the hydroxyl group of serine (R=H) or threonine (R=CH3) residue of the protein (GalNAc-Ser/Thr). Linkage found in glycoproteins of mucus secretions and blood group substances. HO /

CH2OH

_

H

Some Functions of Glycoproteins in the Body

Function

Examples

Structure Lubrication and protection Transport Endocrine regulation Catalysis Defense against infection Membrane receptors

I I HOOO--?--?--. H,N

T A B L E 10-1

H

A 13-O-glycosidiclinkage between galactose and a hydroxylysine residue of the protein (GaI-Hyl). Linkage found in collagen.

Antigens Cell-cell recognition and adhesion Miscellaneous

COOH

I

HO CH2OH

NH~--Cf--H /

H

CH2

I

NHa

A 13-O-glycosidic linkage between xylopyranose and the hydroxyl group of a serine residue of the protein (XyI-Ser). Linkage found in thyroglobulin and proteoglycans.

-

CH--COOH I NH2 F I G U R E 10-1 Carbohydrate-peptide linkages in glycoproteins.

to an oligosaccharide chain, known as the outer domain. The outer domain is made up of either oligosaccharides consisting of mannose residues (oligomannosidic types)or N-acetyllactosamine units (complex types, Figure 10-3). The latter are disaccharides consisting of galactosyl/3-(1 --+ 4)-N-acetylglucosamine units. Glycoproteins with O-glycosidic linkages (Figure 10-4) do not show the common features of glycoproteins with N-glycosidic linkages. The number of sugar residues may vary from one (e.g., collagen) to many (e.g., blood group substances). Many glycoproteins, however, do show the presence of a common disaccharide constituent, namely, galactosyl-fl-(1 -+ 3)-N-acetylgalactosamine, which is

Collagen Epithelial mucins, synovial fluid glycoproteins Ceruloplasmin (copper carrier), transferrin (iron carrier) Thyrotropin, chorionic gonadotropin, erythropoietin Proteases, nucleases, glycosidases, hydrolases Immunoglobulins, complement proteins, interferon Receptors for hormones (e.g., insulin), acetylcholine, cholera toxin, electromagnetic radiation (e.g., rhodopsin) Blood group substances Fibronectin, laminin, chondronectin Glycophorin (an intrinsic red blood cell membrane constituent), intrinsic factor (essential for absorption of dietary vitamin B I 2 ) , clotting factors (e.g., fibrinogen)

linked to either serine or threonine. In collagens, the O-glycosidic linkages occur via hydroxyproline or hydroxylysine residues, which are unique to collagens. A given glycoprotein may contain oligosaccharide chains of both N- and O-glycosidic types. If more than one N-glycosidic carbohydrate linkage site are present in a glycoprotein, they are usually separated by several amino acid residues. In contrast, O-glycosidic carbohydrate linkages

Man or Man o~(1--,,3)/

Man 13(1"4) GIcNAc l](1--,,4)GIcNAc 13*Asn

or schematically,

F I G U R E 10-2 Structure of the common branched inner domain of oligosaccharides linked in N-glycosidic linkages with asparagine. Man = Mannose T, GlcNAc = N-acetylglucosamine O, A s n = Asparagine. [Adapted with permission from N. Sharon and H. Lis, Chem. Eng. News, p. 28 (March 30, 1981) (~)1981 by the American Chemical Society.]

SECTION10.1 Glycoproteins

155

Oligomannosidic V. . V ~ O m l t ~ /

Humanimmunoglobulin M (IgM), bovine rhodopsin

i V--V~

hickenovalbumin, Sindbis virus

~~V~O~O~ I.

,

Complex

I

~

Inner domain

r'

i

m--A--o--v~ m_A_o_v/v-o-oHumanand rabbit transferrin, rat liver plasma membrane

m-A-o\ m--A--o--v~

I

m~/k~O~V~V--OmO

m--~--o-!\

m_~__o_~

~

V-O-ore

I

Vesicularstomatitus virus

(IgG)Humanimmunoglobulin G

A

m--A--o--v\

,~V~O~O~

I

Bovineimmunoglobulin G (IgG)

F I G U R E 10-3 N-Glycosidically linked oligosaccharides. The oligomannosidic type is rich in mannose residues, whereas the complex type contains carbohydrate residues in the outer domain. Both types have a common core.

f'-

]

,j 9 A = galactose, 9 = mannose, ~ = L-fucose, 9 = N-acetylglucosamine, 9 = N-acetylneuramine acid. [Adapted, with permission, from N. Sharon and H. Lis, Special Report, Chem. Eng. News, p. 28 (March 30, 1981) @ 1981 by the American Chemical Society.]

may be found in adjacent hydroxyamino acid residues, or they may occur in close proximity, e.g., in glycophorin, human chorionic gonadotropin, and antifreeze glycoprotein. This last is found in the blood of Arctic and Antarctic fish species and other species on the eastern coast of North

ZX--

Collagen Ovine submaxillary mucin

A--F-l-m-A-~-

I m

Antifreeze glycoprotein, human immunoglobulin A, (IgA,), rat and rabbit brain glycoproteins Calf fetuin, glycophorin, human chorionic gonadotropin

dmh

I

F-1--AmFlm

A+ porcine submaxillary mucin

F I G U R E 10-4 O-Glycosidically linked oligosaccharides. The sugar residues vary from one to many and are not arranged in any particular pattern. A = Galactose; 8 = L-Fucose; [] = GalNAc; 9 -- N-acetylneuraminic acid. [Adapted with permission from N. Sharon and H. Lis, Special report, Chem. Eng. News, p. 28 (March 30, 1981). 9 1981 by the American Chemical Society.]

America. It contains a very high amount of carbohydrate, since every threonine residue of the glycoprotein is linked with a galactosyl-/3-(1 --+ 3)-N-acetylgalactosamine unit. The protein consists of the repeating tripeptide sequence of alanyl-alanyl-threonine. The presence of antifreeze glycoprotein, along with high concentrations of NaC1 in the blood, prevents water from freezing in blood vessels and permits survival at the low temperatures of polar seawater. This freezing-point depression by antifreeze glycoproteins has been attributed to their highly hydrated and expanded structure, which interferes with the formation of ice crystals. In all glycoproteins, the polypeptide component is synthesized first on the membrane-bound ribosomes of the rough endoplasmic reticulum; carbohydrate side chains are added during passage through the endoplasmic reticulum and Golgi apparatus. The carbohydrate additions involve specific glycosyltransferases and their substrates (uridine diphosphate sugars) and, in some glycoproteins, an oligosaccharide carrier known as dolichol (a lipid). (See also Chapters 16 and 25.) Glycoproteins can also be formed by addition of carbohydrate residues without any of the complex enzymatic pathways of carbohydrate addition. This process, known as nonenzymatic glycation, proceeds by the condensation of a monosaccharide, usually glucose, with certain reactive amino groups on the protein. The initial, labile Schiff base adduct slowly rearranges to the stable ketoamine or fructosamine form (Chapter 2). For example, a small fraction of hemoglobin A, the major hemoglobin of adult humans, is present in the red blood cells as glycated hemoglobin (HbAlc). The glycation of hemoglobin is a continuous process occurring throughout the 120-day life span of the red cell. In HbAac, glucose is incorporated via an N-glycosidic linkage into the N-terminal amino group of valine of each/3-chain. Enhanced levels of HbAlc occur in individuals with diabetes mellitus (Chapter 22), and measurement of glycated hemoglobin has been useful in monitoring the effects of therapy. Human serum albumin, which has a half-life of 19 days, is also subjected to nonenzymatic glycation, producing a stable condensation product known as fructosamine. Fructosamine is a generic term applied to the stable condensation product of glucose with serum proteins, of which albumin is quantitatively the largest fraction. Measurement of fructosamine concentration provides a means by which short-term (1-3 weeks) plasma glucose levels can be estimated, whereas measurement of HbAlc concentration reflects integrated plasma glucose levels over a longer period (2-3 months). Human lens proteins, or-,/3-, and ?'-crystallins, which have much longer life spans than other proteins in the body, also undergo agedependent, nonenzymatic glycation at the s-amino groups

156

CHAPTER 10

of their lysine residues. In diabetics, this process occurs twice as often as in normal individuals of comparable age. Lens cells, like red blood cells, do not require insulin for the inward transport of glucose. However, the extent of nonenzymatic glycation of crystallins is much lower than that of hemoglobin (about 2.4% versus 7.5% at age 50) because

tissues, cells, and intracellular domains. They function as protective barriers and as transducers of extracellular messages carried by the chemical agents because they have recognition sites that interact with metabolites, ions, hormones, antibodies, or other cells in a specific manner. This characteristic selectivity of membranes to interact with specific molecules confers unique properties on a given cell type. Within the cell, the membranes of organelles are highly differentiated and have properties consistent with metabolic function. Examples include electron transport and energy conservation systems in the mitochondrial membrane, protein biosynthesis in the rough endoplasmic reticulum, modification and packaging of proteins for export in the membranes of the Golgi complex, drug detoxification in the smooth endoplasmic reticulum, and light reception and transduction in the disk membranes or retinal cells. The membrane constituents are lipids (phospholipids, glycosphingolipids, and cholesterol; Figure 10-5), carbohydrates, and proteins. The ratio of protein : lipid : carbohydrate on a weight basis varies considerably from membrane to membrane. For example, the human erythrocyte membrane has a ratio of about 49:43:8, whereas myelin has a ratio of 18:79:3. The composition of the normal human erythrocyte membrane is shown in Table 10-2. All membrane lipids are amphipathic (i.e., polar lipids). The polar heads of the phospholipids may be neutral, anionic, or dipolar. The surface of the membrane bears a net negative charge. The distribution of lipid constituents in the bilayer is asymmetrical. For example, in the erythrocyte membrane, phosphatidylethanolamine and phosphatidylserine are located primarily in the internal monolayer, whereas phosphatidylcholine and sphingomyelin are located in the external monolayer. Lipids are organized in bilayers that account for most of the membrane barrier properties. Membrane proteins may be peripheral (extrinsic) or integral (intrinsic). Peripheral proteins are located on either side of the bilayer and are easily removed by ionic solutions, whereas integral proteins are embedded in the bilayer to varying degrees (Figure 10-6). Some integral proteins penetrate the bilayer and are exposed to both external and internal environments. By spanning both external and internal environments of the cell, these proteins may provide a means of communication across the bilayer that may be useful in the transport of metabolites, ions, and water, or in the transmission of signals in response to external stimuli provided by hormones, antibodies, or other cells. Because of hydrophobic interactions, isolation of integral proteins requires harsh methods, such as physical disruption of the bilayer and chemical extraction procedures using

1. The lens cells contain about one sixth the glucose of red cells. 2. The content of lysine is lower in crystallin compared to hemoglobin 3. The lysine residues are inaccessible in crystallins owing to the high content of/3-pleated sheet structures packed in a structured array oriented orthogonally to the lens optic axis, restricting rotational and translational movement. Crystallins contain almost no a-helix, whereas hemoglobin, a globular protein, possesses a high content of a-helical structure and can freely rotate in the fluid media, exposing the lysine residues. Crystallins constitute 90% of the soluble proteins of the lens cells (also called fiber cells). The human lens, a transparent, biconvex, elliptical, semisolid, avascular structure, is responsible for focusing the visual image onto the retina. The lens grows throughout life at a slowly decreasing rate, building layer upon layer of fiber cells around a central core and never shedding the cells. Crystallin turnover is very slow or nonexistent. In addition to nonenzymatic glycation, crystallins undergo other age-dependent, posttranslational modifications in vivo: formation of disulfide bonds and other covalent cross-links, accumulation of high-molecular-weight aggregates, deamidation of asparagine and glutamine residues, partial proteolysis at characteristic sites, racemization of aspartic acid residues, and photo-oxidation of tryptophan. Some of these processes contribute to the increasing amount of insoluble crystallins during aging. Nonenzymatic incorporation of carbohydrates into proteins in vivo can be extensive and may contribute to the pathophysiology of diabetes mellitus and galactosemia.

10.2 Cell Membrane Constituents Various aspects of the cell membrane are discussed throughout this text, and a brief introduction is presented here. The living system's ability to segregate from and protect itself against--and interact with and against---changes in the external environment is accomplished by membranes. In the body, membranes function at the level of

Heteropolysaccharides I: Glycoproteins and Glycolipids

TABLE 10-2

Composition of Normal Human Red Blood Cell Membranes (Ghosts)" Component Proteins and glycoproteins Lipids Phospholipids Sphingomyelin Phosphatidylcholine Phosphatidylethanolamine Phosphatidylserine Phosphatidylinositols Phosphatidic acid Other Cholesterol Glycolipids Free fatty acids

GramsIGhost ( x l0I3) 5.7

Approximate Number of MoleculesIGhost ( x lo6)

% in Outer Half

% in Inner Half

of Bilayer

of Bilayer

80 75 20 0 20 Unknown Unknown -50 100 Unknown

20 25 80 100 80 Unknown Unknown -50 0 Unknown

3.7

*Reproduced with permission from J. B. Stanbury, J. W. Wyngaarden, D. S. Fredrickson, et al., Eds.: The Metabolic Basis of Inherited Disease, 5th ed. (McGraw-Hill, 1983).

158

CHAPTER10 Heteropolysaccharides I" 61ycoproteins and 61ycolipids

(a) A (D

Phosphoglycerides _

CH=----O~C

r Q.

=8 Glycerol moiety

CH - - O - - C . I

.

.

.

.

,,Fatty add ,/residues CH3

I

CH= CHs o~11 I+ -O ~ P'~, O ~ CH~-- CH=----N ~CH=

.m

o/.

' I,

=1 . . . . . .

Phosphatidylcholine (lecithin) Choline moiety

O

li

oH, CH - - O - - C CH= CH=

o%_,io --

I

.../ P.,,< u , O--CH=----CH=-----NH= ~ Ethanolaminem o i e t y r "

"l"

C H - - NH+ ""•O--CHi--J

Phosphatidylethanolamine

Serine moiety

, !

Phosphatidylserine

: . . . . . . . . . .c. .o.o. .-. 8

II i~C~

CH3

CH ~ C

I

OH3

CH=

I

O~..,~O

', ,

~ OH

~

O

H

Phosphatidylinositol Inositol moiety

i

F I G U R E 11)-5 Structures of some membrane lipids.

synthetic detergents or bile salts to disrupt the lipid-protein interactions. Carbohydrate residues are covalently linked (exclusively on the external side of the bilayer) to proteins or lipids to form glycoproteins or glycolipids, respectively, both of which are asymmetrically distributed in the lipid bilayer (Figure 10-7). Fluidity of the membrane structure is determined by the degree of unsaturation of the hydrocarbon chains of the phospholipids and by the amount of cholesterol in the membrane. Hydrocarbon chains

with cis-double bonds produce kinks and allow a greater degree of freedom of movement for the neighboring alkyl side chains; hence, these unsaturated chains give rise to more fluidity than do saturated alkyl chains, which associate in ordered arrays. Cholesterol, an inflexible polycyclic molecule, is packed between fatty alkyl chains, the ring bearing the polar hydroxyl group interacting with the polar groups of phospho- and glycolipids. The presence of cholesterol disrupts the orderly stacking of alkyl side chains, restricts their mobility, and causes increased

159

SECTION 10.2 Cell Membrane Constituents

(b)

Phosphosphingolipids These lipids contain a long unsaturated hydrocarbon chain amino alchohol known as sphingosine: oH

C

I

CH3

HOH,C/I ~C " I

H

C,,I-~NO=

A derivative of sphingosine, in which a fatty acid is linked by an amide linkage, is ceramide: ~

Fatty acid residue

Amide,,,.--------~/C linkage H.N H

I

o

H~./?~cJ Primar~ I alchohol group~'''~" HO

H

~.

CH,

I

H

The product obtained when the alchohol hydroxyl group of sphingosine is esterifled with phosphorylcholine is sphingomyelln .The conformationsof phosphatidylcholine and sphingomyelin are similiar. O IJcI CH3 HN"

I OH ,C.... I ~,

OH,

%_//0 .~CH= H3C/!H"~3CH3

(c)

Glycosphingolipids (cerebrosides)

A derivative of ceramide that contains a monosaccharide unit (either glucose or galactose) linked in a 13-glycosidiclinkage is a cerebroside. These neutral lipids occur most abundantly in the brain and myelin sheath of nerves. The specific galactocerebroside, phrenosine, contains a 2-hydroxy-24-carbon fatty acid residue. Following is a structure of a glucocerebroside. O HN m

CH3

I

"o2"2

o"jCH2

C

c.,

3-Glycosidic linkage

FIGURE 10-5 (Continued) membrane viscosity. Thus, the lipid composition of the membrane at physiological temperatures can have significant effects on fluidity and permeability. Some correlation appears to exist between the concentrations of sphingomyelin and cholesterol in different membranes. Plasma membranes (e.g., red blood cells and myelin sheaths) are rich in both; the inner membrane of mitochondria contains neither.

Membrane proteins show considerable mobility in the plane of the bilayer (lateral motion). There is no evidence that proteins migrate from one side of the bilayer to the other. The frequency of reorientation of lipid components (flip-flop migration) is extremely slow or nonexistent, for thermodynamic reasons. Artificial membrane systems (liposomes) have increased our understanding of the dynamic nature of lipid

160


(d) Cholesterol Cholesterol (3-hydroxy-5,6-cholestene) belongs to a family of compounds derived from a fused, reduced, nonlinear four-ring system of cycIopenta[ (x]-phenanthrene. Bile acids, steroid hormones, and vitamin D metabolites are derived from cholesterol.

H~ ~

Angular methyl

~

orou,:,s~--_____.~''-r ~ \ 1

'F ~ 1T D "]" lg

I

,..-x,, il-/{'-, 13-Hydroxylgroup

~H

21"" . ~ I ~

"

I

|Structural /

l,.,

/4.1

]

~'~ 3GlcNAc/31---> 3Gall31--> 4Glc---> GalNAc~ 1"/" (In type A 2, the linkage between Gal and GlcNAc is/31---> 4.)

B 1 antigen

F u c a l ~ 2Gal/31----> 3GlcNAc/3---> 3Gal/31 ~ 4Glc---> Galo~1,2,

Fucc~l---> 2Gal/31---> 3GlcNAc]31---> 3Gal/31---> 4Glc--+ H or O antigen The difference between the above antigens resides in the absence or presence of either N-acetylgalactosamine or galactose linked to the penultimate galactose residue in o~(1--->3) linkage. The oligosaccharide determinants occur in glycoproteins and in glycosphingolipids.

Lewis (L) Locus Le a antigen

GalI31,N 3GalNAc/31__> 3Gal/31---> 4Glc-Cer Fucoz 1,/"

Le b antigen

Fucc~l---> 2Gall31 "N 3GalNAc]31__> 3Gal/31----> 4Glc-Cer Fucc~ 1,7

The Lewis antigens are not components of developng red blood cells. They are acquired as glycosphingolipidsduring circulation by adsorption from plasma low- and high-density lipoproteins (Chapter 20).

P Locus P antigen pK antigen

GalNAc/31--4 3Galc~l---> 4Gal/31 ~ 4Glci31-Cer Galo~l---> 4Gal/31----> 4Glc]31-Cer

I Locus I antigen

Gall31 ___>4GlcNAc/31"N 6Gal/31---> 4-X* Gall31---> 4GlcNAc/31//"

i antigen

Gal/31 ~ 4GlcNAc]31---> 3Gall31---> 4-X* X* = GlcNAc]31----> 3Gal/31--> 4Glc/31-Cer

Gal=galactose, GalNAc=N-acetylgalactosamine, Glu=glucose, GluNAc=N-acetylglucosamine, Fuc=fucose (6-deoxy-L-galactose), Cer=ceramide (a sphingolipid).

precursor oligosaccharide. The classification of individuals into ABO types is based upon whether A or B antigens are present or not; type A individuals have the A antigen, type B, the B antigen, and type O, the H antigen, respectively. Normally, an antigen and its specific antibody are not present simultaneously. For example, individuals having type A red blood cells possess anti-B antibodies; those of type B possess anti-A antibodies; those of type AB have neither of these antibodies; and those of type O have both anti-A and anti-B antibodies (Table 10-5).

The antigenic variation of blood group substances is due to specific glycosyltransferases (the primary gene products) responsible for synthesis of the oligosaccharide determinants (the secondary gene products). Under appropriate experimental conditions, the blood type of red blood cells can be changed by addition or removal of specific carbohydrate residues. For example, when type 0 red blood cells are incubated with the galactose donor substrate (uridine diphosphate galactose) and the specific ot-galactosyltransferase enzyme, they are

168


TABLE 10-5

ABO Blood Group System and the Agglutination Reaction* Blood Type

Type of Oligosaccharide Antigens Present on Red Blood Cells

Antibody in Serum

Types of Serum That Cause Agglutination When Mixed with Red Blood Cells

O A B AB

H A B A and B

Anti-A, anti-B Anti-B Anti-A None

None O, B O, A O, A, B

*Red blood cells from type O individuals can be donated to individuals with any other cells from type within the ABO system without causing agglutination. Individuals with type AB can accept red blood cells from all other types but cannot donate to individuals with other types except AB. Thus, type O individuals are known as universal donors and AB as universal acceptors.

converted to type B red blood cells. The reverse conversion can be accomplished by incubation of cells in the presence of ot-galactosidase, which removes the terminal galactose residue. Glycophorins A and B possess antigenic determinants for the M and N blood group antigens, respectively. Rh blood group antigens are clinically important due to their involvement in hemolytic disease of the newborn, transfusion medicine, and autoimmune hemolytic anemia. The designation of Rh stands for Rhesus because the antibody specificity was identical to that of antibodies generated in rabbits injected with red blood cells from Rhesus monkeys. The Rh blood group system consists of more than 50 nonglycosylated protein antigens; however, only five are commonly identified; these are encoded by two genes termed RHD and RHCED. Marked racial differences are observed for Rh alleles in different populations. Typically, 85% of all Caucasians are Rho(D)-positive. Anti-Rho(D) immunoglobulin G (IgG) prepared from human plasma is administered intramuscularly as means of passive immunization to Rho(D)-negative individuals exposed to Rho(D)-positive cells. Rh proteins have a molecular weight of about 32,000 and span the red blood cell membrane. Rh proteins are incorporated into the erythrocyte membrane after palmitolyation via thioester linkages involving free sulfhydryl groups of cysteine residues. The exact function of Rh proteins is not known, however, individuals lacking all Rh protein expression exhibit multiple red blood cell abnormalities including abnormal morphology and survival. This condition is known as Rhnull syndrome. A number of protein blood group antigens possessing a variety of functions are shown in Table 10-6. Blood group antigens including both oligosaccharides and proteins are expressed on other cell surface struc-

tures as well as on erythrocytes. These antigens function as receptors for a variety of infectious agents. Examples of oligosaccharide antigens include P antigen, which is the receptor for B 19 parvovirus; individuals who do not express this antigen show no serological evidence of past infections with this common childhood pathogen. Le b antigen is the epithelial receptor for Helicobacter pylori, which is the causative agent of peptic ulcer disease (Chapter 12). Lewis antigens function as receptors for a wide variety of pathogens including S. aureus, N. meningitidis, H. influenzae, N. gonorrhoeae, and

C. albicans. Blood group antigen-bearing proteins also serve as receptors for infection agents (Table 10-7). A notable example is the presence of receptors in the Duffy blood group antigens to Plasmodium vivax and Plasmodium knowlesi. Individuals who do not express Duffy antigens are resistant to infection by these malarial parasites.

10.4 Serum Glycoproteins Almost all serum proteins, with the notable exception of albumin, are glycoproteins. The sugar residues found most commonly in the outer domain of the oligosaccharides of these glycoproteins are galactose, Nacetylhexosamine, and sialic acid. L-Fucose is a minor constituent. The structure of a typical oligosaccharide chain of a serum glycoprotein is shown in Figure 10-14. The liver plays a major role in the synthesis and catabolism of these proteins. Glycoproteins lose their terminal sialic acid residues through the action of neuraminidase (sialidase) during circulation in the blood, which exposes the galactose residues (Figure 10-14). The resulting galactose-terminated glycoproteins, known as

SECTION 10.4

Serum61ycoproteins

169

TABLE 10-6 Functions of Blood Group Protein Antigens of Erythrocytes

Blood Group (abbreviation)

Locus Name

Protein

a. Proteins that contribute to the structural integrity Rhesus (Rh) Gerbich (Ge) Diego (Di), Wright (Wr) XK

RH GYPC AE1 Kx

Rh acylproteins Glycophorins C and D Anion channel protein Kx protein

DAF

Decay accelerating factor (CD55)

Fy

Chemokine receptor

AE1 JK AQP1

Anion channel protein Urea transporter Aquaporin 1 water channel protein

KEL YT

93kD protein, metalloproteinase (?) Acetylcholinesterase

b. Proteins with complement-related functions Cromer (Cr) c. Proteins with receptor functions Duffy (Fy) d. Proteins with transport functions Diego (Di), Wright (Wr) Kidd (Jk) Colton (Co) e. Proteins with enzymatic activity Kell (K,k,Kp,Js) Cartwright (Yt)

asialoglycoproteins, are taken up after binding to receptors on hepatocytes. The bound asialoglycoprotein is internalized by a process known as receptor-mediated endocytosis (Chapter 11) and subjected to lysosomal degradation. In TABLE 10-7 Blood Group Protein Antigens as Microbial Receptors*

Oligosaccharides

P Lewis (Leb) H, A

Escherichia coli, Parvovirus B 19 Helicobacter pylori Candida albicans

Proteins Glycophorin A (MN) Decay accelerating factor (Cromer) CD44 (Indian) Duffy AnWj

Plasmodium falciparum Echovirus, E. coli Poliovirus Plasmodium vivax, Plasmodium knewlesi H. influenzae

*Modified and reproduced with permission from M. J. Telen. Erythrocyte Blood Group Antigens: Polymorphisms of functionally Important Molecules. Seminars in Hematology33, 302 (1996).

liver disease, the plasma levels of asialoglycoproteins are elevated. The internalized glycoproteins are catabolized to their monomeric units by the lysosomal enzymes. The oligosaccharides are degraded sequentially by specified hydrolases, starting from the nonreducing termini. Hereditary deficiency of some of these enzymes has been reported: ot-o-mannosidase in mannosidosis, o~-L-fucosidase in fucosidosis, glycoprotein-specific ot-neuraminidase in sialidosis, and aspartylglycosaminidase in aspartylglycosaminuria. In these disorders, undigested or partially digested oligosaccharides derived from glycoproteins accumulate within the lysosomes. Keratin sulfate (a glycosaminoglycan, Chapter 11) also accumulates, presumably because the above enzymes are required for its degradation. These disorders are inherited as autosomal recessive traits and can be diagnosed prenatally. Clinically, they resemble mild forms of mucopolysaccharidosis (Chapter 11). There is no definitive treatment. Removal of senescent red blood cells from the circulation has been attributed to desialylation of the membrane glycoproteins. In vitro removal of sialic acid from human red blood cells and introduction of the modified cells into the circulatory system result in drastic shortening of their life span. However, aging and removal of red blood

170


Outer domain ,

Inner domain

i

II--~O-~-y~

Protein backbone

,,

I__A__I+I/I--I--I~I

Asparagine--

i

Typical serum glycoprotein 1

+ l~Neuraminidase

/ X - - O - - Y ~ V - - O m O m Asparagine-AmO myJ Asialog lyco protei n F I G U R E 10-14 Oligosaccharide side chain of a serum glycoprotein. The removal of the terminal sialic acid residue by neuraminidase exposes the penultimate galactose residue. The resulting asialoglycoprotein is cleared from the circulation by a galactose receptor-mediated process in the hepatocytes. I = Sialic acid, 9 = N-acetylglucosamine, A = galactose, IIt = mannose.

cells depend on many other factors (e.g., inactivation of enzymes and oxidation of sulfhydryl proteins). If desialylation occurs normally as part of the physiological mechanism in clearing glycoproteins and cells of the circulatory system, the anatomical location of this process is not known. Other clearance systems for glycoproteins have been identified, e.g., one in the reticuloendothelial system that terminates with mannose and N-acetylglucosamine. Information of this type has potential application in targeting biologically active molecules to a specific tissue site. For example, mannose-terminated catalytically active lysosomal enzymes can be targeted to reticuloendothelial cells. Such targeting of lysosomal enzymes could be useful in the treatment of lysosomal enzymedeficiency diseases. The same principle can be applied to targeting drugs to specific tissue sites by coupling the drugs to glycoproteins with the appropriate terminal sugar residues.

and legs leading to paralysis. It is a self-limited autoimmune disease. Most GBS cases have resulted from an antecedent, acute infection of bacterial or viral origin; in children GBS has also been identified following vaccination. One major cause of bacterial gastroenteritis is Campylobacter jejuni and this infection also may be followed by GBS. Some serotypes of C. jejuni possess liposaccharides that contain terminal tetrasaccharides identical to ganglioside GM 1. Antibodies made in response to the bacterial infection also attack GM1 which is widely present in the nervous system. The antibodies directed against the GM1 epitope cause an immune-mediated destruction of nerve fibers. Thus, the sharing of homologous epitopes between bacterial liposaccharides and gangliosides is an example of molecular mimicry, which causes disease. Host factors may also influence a person's susceptibility to GBS. Plasma exchange and intravenous immunoglobulin administration are used for immunomodulation in the therapy of GBS.

Supplemental Readings and References Extracellular Matrix M. H. Ascarelli and J. C. Morrison: Use of fetal fibronectin in clinical practice. Obstetrical and Gynecological Survey 52(Suppl.), S I (1997). R. O. Hynes: Integrins: Versatility, Modulation and Signalling in Cell Adhesion. Cell 69, I1 (1992). K. A. Piez: History ofextracellular matrix: A personal review. Matrix Biology 16, 85 (1997). J. Labat-Robert, M. Bihari-Varga, and L. Robert: Extracellular Matrix. FEBS Letters 268, 386 (1990). W. K. Stadelmann, A. G. Digens, and G. R. Tobin: Physiology and healing dynamics of chronic cutaneous wounds. American Journal of Surgery 176(Suppl. 2A), 265 (1998). W. K. Stadelmann, A. G. Digens, and G. R. Tobin: Impediments to wound healing. American Journal of Surgery 176(Suppl. 2A), 395 (1998).

Blood G r o u p Antigens

10.5 Molecular Mimicry of Oligosaccharides and Host Susceptibility In the previous section we discussed the presence of oligosaccharide and protein blood group substances that cause susceptibility to certain pathogens. In this section we discuss an infectious agent that contains a common antigenic epitope with the host and that elicits antibodies that cause a disease. Guillain-Barr~ syndrome (GBS) is one such disease. GBS is an acute inflammatory neuropathy with progressive weakness in both arms

E Agre and J-E Cartron: Molecular biology of Rh antigens. Blood 78, 551 (1991). C-H. Huang, E Z. Liu, and J. G. Cheng: Molecular biology and genetics of the Rh blood group system. Seminars in Hematology 37, 150 (2000). A. O. Pogo and A. Chaudhuri: The Duffy protein: a malarial and chemokine receptor. Seminars in Hematology 37, 122 (2000). M. Rios and C. Bianco: The role of blood group antigens in infectious disease. Seminars in Hematology 37, 177 (2000). M. J. Telen: Erythrocyte blood group antigens: Polymorphisms of functionally important molecules. Seminars in Hematology 33, 302 (1996). M. J. Telen: Red blood cell surface adhesion molecules: Their possible role in normal human physiology and disease. Seminars in Hematology 37, 130 (2000).

Supplemental Readings and References

Molecular Mimicry T. E. Feasby and R. A. C. Hughes: Campylobacter jejuni, antiganglioside antibodies, and Guillain-Barr6 syndrome. Neurology 51, 340 (1998). A. E Hahn: Guillain-Barr6 syndrome. The Lancet 352, 635 (1998). T. Lasky, G. J. Terraccians, L. Magder, et al.: The Guillain-Barr6 syndromes and the 1992-1993 and 1993-1994 influenza vaccines. New England Journal of Medicine 339, 1797 (1998). J. J. Ma, M. Nishimura, H. Mine, et al.: HLA and T-cell receptor gene polymorphisms in Guillain-Barr6 syndrome. Neurology 51, 379 (1998). A. H. Ropper and M. Victer: Influenza vaccination and Guillain-Barr6 syndrome. New England Journal of Medicine 339, 1845 (1998).

171 K. A. Sheikn, I. Nachamkin, T. W. Ho, et al.: Campylobacter jejuni lipopolysaccharides in Guillain-Barr6 syndrome. Neurology 51, 371 (1998).

Disorders of Red Blood Cell Membrane Skeleton E S. Becker and S. E. Lux: Hereditary Spherocytosis and Hereditary Elliptocytosis. In the Metabolic Bases of Inherited Disease, 7th ed., C. R. Scriver, A. L. Beaudet, W. S. Sly, D. Valle., (Eds.) McGraw-Hill, New York, 1995, p. 3513. S. C. Liu and L. H. Derick: Molecular anatomy of the red cell membrane skeleton: Structure-function relationships. Seminars in Hematology 29, 231 (1992).


CHAPTER

11

Heteropolysaccharides II: Proteoglycans and Peptidoglycans

11.1 Protein Fibers and Proteoglycans Connective tissues are composed of insoluble protein fibers (the glycoprotein collagen and the nonglycoprotein elastin) embedded in a matrix of proteoglycans (ground substance). The connective tissues bind tissues together and provide support for the organs and other structures of the body. Their properties depend on the proportion of different components present. A tissue of very high tensile strength, the Achilles tendon, is composed of about 32% collagen and 2.6% elastin, whereas an elastic tissue, the ligamentum nuchae, is composed of about 32% elastin and 7% collagen. The proteins and proteoglycans are synthesized by connective tissue cells: fibroblasts (generalized connective tissue), chondroblasts (cartilage), and osteoblasts (bone). Connective tissue also contains blood and lymphs vessels and various transient cells including macrophages and mast cells. Adipose tissue is a specialized form of connective tissue consisting of a collection of adipocytes (stores of triacylglycerol) that cluster between the protein fibers.

Collagen Collagens are extracellular proteins of connective tissue and they make up about one third of all body protein. They are a family of related glycoproteins in which hydroxylysyl residues provide the sites for attachment

of glucose, galactose, or an or(1 --+ 2) glucosylgalactose residue via a /3-O-glycosidic linkage (Figure 11-1). In brief, the synthesis of collagen can be considered to occur in two stages: intracellular and extracellular. The intracellular stage consists of the production of procollagen from precursor polypeptide chains that undergo, in sequence, hydroxylation, glycosylation, formation of a triple helix, and secretion. The extracellular stage consists of the conversion of procollagen to tropocollagen by limited proteolysis from the amino and carboxyl termini, self-assembly of tropocollagen molecules into fibrils, and finally cross-linking of the fibrils to form collagen fibers (Chapter 25). Collagen exists predominantly as fibrous protein; however, in the basement membrane of many tissues, including kidney glomeruli and the lens capsule, it is present in a nonfibrous form. The unique property of each connective tissue (e.g., the flexibility of skin, rigidity of bone, elasticity of large arteries, and strength of tendons) depends on the composition and organization of collagen and other matrix components.

Collagen Types More than 16 different types of collagen have been reported. They constitute the most abundant family of proteins in the human body. The collagens are encoded by 28 genes dispersed in at least 12 different chromosomes.

173

174

CHAPTER11 Heteropolysaccharides I1: Proteoglycans and Peptidoglycans HO

"\

H

NH3

OJ

example, human fibroblasts and smooth muscle cells synthesize both types I and III collagens; epithelial and endothelial cells synthesize type IV collagen; and chondroblasts synthesize type II collagen. Structure and Function

II

0 [

I

Polypeptide chain 13-o-Glucosylhydroxylysylresidue

HO

HO

0 HO 0

o~

HO~~

~H'~C/

H

OHo~C I

II o I

Polypeptide chain OH D-Glucosyl ( o~1-4"2)-O-13- D-galactosylhydroxylysylresidue F I G U R E 11-1 Glycosides of the collagen polypeptide chain.

The individual genes are identified by both collagen type and the polypeptide chain. For example, COL1A2 gene codes for type I collagen (COL l) and or2 (A2) polypeptide chain. The tissue distribution of collagens also varies (Table 11-1 ). Type I collagen consists of two identical chains of ot 1(I) and one chain of or2; it is the major connective tissue protein of skin, bone, tendon, dentin, and some other tissues. Type II collagen consists of three identical chains ofc~ 1(II); it is found in cartilage, cornea, vitreous humor, and neural retinal tissue. Type III collagen consists of three identical chains of ot 1(III); it is present, along with type I collagen, in skin, arteries, and uterine tissue. Type IV collagens are composed of ot 1(IV) to ol5(IV) and are found in the basement membranes of various tissues. Unlike collagen types I, II, III, V, and XI which are fibrillar, type IV collagen is not fibrillar in structure. The synthesis of collagens in cultured cells has aided the understanding of collagen biochemistry. The tissue specificity of various types of collagen is also reflected in cultured cells obtained from appropriate tissues. For

Each collagen molecule contains three polypeptide chains coiled around each other in a triple helix. These chains are called or-chains and are designated by Roman numerals according to the chronological order of their discovery. The three polypeptide chains may be identical or may consist of two identical chains and one dissimilar chain. Each ot-polypeptide chain consists of about 1000 amino acid residues, of which every third following a glycine is also a glycine. Thus, the molecular formula of an u-chain may be written as (Gly-X-Y)333, where X and Y represent amino acid residues other than glycine. In mammalian collagens, about 100 of the X-positions are occupied by proline residues, and 100 of the Y-positions are occupied by 4-hydroxyproline residues. At a few X-positions, 3-hydroxyproline residues are present; however, they only occur next to 4-hydroxyproline. Most hydroxyproline residues are present as the trans isomers. Although all collagen polypeptides have the general structure (Gly-X-Y),, differences between the various collagen types are associated with the particular sequences of amino acid residues in the X and Y positions. Hydroxyproline residues are not common in proteins; other than the collagens, hydroxyproline residues are found in elastin, acetylcholinesterase, and the C lq subcomponent of the complement system (Chapter 35). Another unique amino acid residue found in collagen is hydroxylysine, which occurs at the Y-position. The number of hydroxylysine residues per polypeptide chain lies in the range of 5 to 50. The hydroxylysine residues provide the sites for/~-O-glycosidic linkage with galactose or glucose or ct(1 --+ 2) glucosylgalactose. The collagens differ in their ratio of monosaccharide to disaccharide residues, as well as in their total carbohydrate content. For example, the carbohydrate contents of collagen from skin, cartilage, and basement membrane are about 0.5%, 4%, and more than 10%, respectively. Because collagen biosynthesis involves intracellular and extracellular posttranslational modifications (e.g., hydroxylation, glycosylation, fibril formation, and formation of cross-links), a given genetically determined collagen may show a great deal of heterogeneity. This is particularly true for type I collagen. Collagen also contains alanine residues in relatively high quantities. The only amino acid not found in collagen

TABLE 11-1

Types of Collagen, Their Distributions in Tissues and Properties --

Type*

Composition

Distribution in Tissues

Examples of Known Disorders

Bone, Tendon, Skin, Dentin, Fascia, Arteries

Osteogensis Imperfacta (01) and Ehlers-Danlos Syndrome (EDS). Both syndromes are clinically heterogeneous due to genetic defects that affect the biosynthesis, assembly, postranslational modification, secretion, fibrillogenesis, or other extracellular matrix components.

Cartilage, Vitreous humor

Chondrodysplasia: Spondyloepiphysial dysplasia, a chondrogenesis and Stickler Syndrome

I11

[ a 1(111)13

Skin, Blood vessels, Uterus

EDS Type IV: Mutation affects synthesis, structure or secretion

IV

[al(IV)],a,(IV) other forms are also known

Basement membrane

Alport Syndrome: characterized by nephritis and sensorineural deafness

V

[ a 1(V)l3 [(a 1(v)2)12a2(v) a 1(V)a2(V)a3(V)

Skin, Placenta, Blood vessels, Chorion uterus

VII

[a(vII)13

Anchoring fibrils

VIII

Not yet known

Cornea, Blood vessels, Network-forming collagen

IX

[ a 1(IX)a2(IX>a2(IX)

Cartilage, Fibril associated collagen

X

[ a1

Cartilage

XI

a 1(XI)a2(XI)a l(I1)

Cartilage

XI1

[ a 1(XII)13

Soft tissues

*Collagen Types I, 11, 111, V, and XI are fibrillar.

Epidermolysis bullosa

1~6

is tryptophan (an essential amino acid). Collagen consists essentially of four amino acids in abundant quantities and negligible amounts of almost all other amino acids. For this reason, collagen is an inferior protein nutritionally. It is very insoluble in water and, for the most part, indigestible. However, collagen can be converted to soluble and digestible products by hydrolysis of some covalent bonds (by the use of heat or by certain plant proteinases) to yield gelatin. In the human body, collagen is resistant to most proteinases. Polymerized fibrillar collagen is a stable component of the extracellular matrix and its turnover rate is insignificant except in areas where tissue remodeling and repair occur. However, nonfibrillar collagens (e.g., type IV) are susceptible to proteolytic attack. Specific proteases (collagenases) cleave collagen at specific sites. The amino acid sequence of collagen produces its unique secondary and tertiary structures. A tropocollagen molecule contains three polypeptide chains, each coiled into a left-handed helix having about three amino acid residues per turn (Figure 11-2). This type of helix is unique to collagen and differs significantly from the c~-helix in its periodicity and dimensions (Chapter 4). The three helical polypeptides twist tightly around each other, except for the two short nonhelical regions at the C and N termini to form a right handed triple-stranded superhelix. Tropocollagen has a molecular weight of about 300,000, a length of 300 nm, and a diameter of 1.5 nm. The glycine residue, which occurs at every third position, has the smallest R-group and is thus able to fit into the restricted space where the three polypeptide chains are closest to each other. Since the proline and hydroxyproline residues hinder free rotation around the N-C bond (Chapter 4), the polypeptide chain has a rigid and kinked conformation. These stereochemical properties are responsible for the superhelix. Hydrogen bond formation between the NH group of a glycyl residue in one chain and the CO group of a prolyl or other amino acid residue in the X-position of an adjacent chain stabilizes the triple helix. The hydrogen bond between the glycyl residue of one chain and the prolyl residue of another chain is direct. If the prolyl residue is replaced by any other amino acid, the interchain hydrogen bond occurs through a water-bridged structure. These bonds are further stabilized by hydrogen bonding with the hydroxyl group of the trans-4-hydroxyprolyl residue, which occurs at the Y-position. Additionally, the chains are held together by covalent linkages involving lysine residues (Figure 11-3). Polypeptide chains consisting only of glycine, proline, and hydroxyproline residues (in that order) form an extremely stable triple helix. Furthermore, the stability (thermal) of the triple helix decreases in the following order for repeating sequences of the chain: Gly-Pro-Hyp >

CHAPTER11 Heteropolysaccharides I1: Proteoglycans and Peptidoglycans

Gly

~---Onl y Gly

fo) Only Gly ~ --

0.5 nm

(a) F I G U R E 11-2 Structure of tropocollagen. (a) The coiling of three left-handed helices of collagen polypeptides. The dotted lines indicate hydrogen bonds. (b) The right-handed triple-stranded superhelix. The curved arrow indicates the covalent linkage between two chains. [Reproduced with permission from I. Geis and R. E. Dickerson.]

Gly-Pro-Y > Gly-X-Pro > Gly-X-Y. In a given polypeptide chain of native tropocollagen, about one third of the molecule contains the Gly-Pro-Hyp sequence and two thirds involve Gly-X-Y, which decreases the stability of the triple helix. Amino acid residues other than proline and hydroxyproline that occupy the X- and Y-positions decrease helix stability but are essential for the next level of organization of collagen--the formation of microfibrils.

SECTION11.1 Protein Fibers and Proteoglycans Polypeptide chain

f

O

H

II

I

Polypeptidechain (helical region) 0

H

0

II I ~C~N~

H

II I ~C...c~N~

~Al~"~y~ine ~ residue

Lysyl ~C~H oxidase t" ~, ; H2 2H20 2NH4 H.,,,. //.O

II

HO

C--H

H~ ~C /

c./C~N /

II

O

I

O

I

II

~N /

II

H

I

H

Dehydrohydroxylysinonorleucine

N

i

H

N II

-H~O

Allysine residue

O

Polypeptide (a) chain Formation of aldol condensation product--linkage involving nonhelical regions of the collagen molecule.

i

Polypeptide chain (nonhelical region) (b)

Formation ofdehydrohydroxylysinonorleucine linkage.

Polypeptidechains(nonhelicalregions)

Polypeptidechain (helicalregion) O H ,

HO

H

I

i

~H

~H

I~#snd~e

N.../

I

O

H

I

H--N

~N /

II

I

O

H

II

H

II I /%/N~

Hydroxy-

/

II

i

O

c~ H

O H Aldol condensation product

~C

I

~H C / ~C=O ~kk J H

~c/C~

~N ~

H

II

HO-~/

Aldol condensation (-H20)

H

O

/C,,,._/N~

"H

H~

~G /

O

,

,

Lysine residue

H2N

177

,

II

I

/C...~.,..,./N ~ /~H

\

O

H

II

/C~_/N~ /C~H

I

O

H

II

I

/C~c/N ~ ~H

O

\

/

Hydroxy-

'r S n,eue

H2N

~o.

-H20 "

~1 ii

H~c~.O

H-- C

Amadori rearrangement

//o__~

H--N I H--C--H

OH

II

H/C~cH2 ~ L HQ ~

o~O /,/

O

NH2

0

I

Condensation.~

(severalsteps)" L.~..~/

OH Hydroxyallysine H //residue ~c'/C'H'" N/

II

I

O

H

, , Polypeptide chain (nonhelicalregion)

I

OH H~

CH2 OH

H~

~C"/II ~ NI/

o

H

I

(an aldimine)

H~ / : N - - H CH2~C C__ O

~c/C~ II

N/ I

H~

O

H

~N/C~c

II

(a ketamine)

(c)

Formation of dehydrohydroxylysinohydroxynorleucine(I), a n aldimine, and hydroxylysino-5-keto-norleucine(II), a ketamine. The linkage in II is more stable.

I

~

H~

~c/C~ N/

II

H O Polypeptidechain (helicalregion)

J H

II O

(d) Formation of pyridinoline cross-linkage, a polyfunctional cross-linkage.

F I G U R E 11-3 Formation of cross-links in collagen. All cross-links are derived from lysyl and hydroxylysyl amino acid residues. The initial step is the oxidative deamination of the e-NH2 group of two amino acid residues located at certain strategic positions (e.g., short nonhelical segments at both ends of the collagen molecule), with the formation of corresponding aldehyde-containing residues, allysine and hydroxyallysine. This reaction occurs extracellularly and is catalyzed by lysyl oxidase, a copper-dependent enzyme. The aldehyde groups react spontaneously with other aldehyde groups located in the adjacent chain of the same molecule or adjacent molecule. The structures do not show carbohydrate moieties for the sake of clarity.

1~8

Hydroxylysine glycosides occur at the Y-position and may play a role in determining fibril diameter. The side chains of these amino acids project outward from the center of the triple helix, permitting hydrophobic and ionic interactions between tropocollagen molecules. These interactions determine the manner in which individual tropocollagen molecules aggregate to form microfibrils initially, then larger fibrils, and eventually fibers. The microfibril, about 4 nm wide, consists of four to eight tropocollagen molecules that aggregate in a highly ordered and specific manner owing to interactions of amino acid residues at the X- and Y-positions. In this ordered arrangement, each molecule is displaced longitudinally by about one-fourth of its length from its nearest neighbors. The longitudinally displaced tropocollagen molecules are not linked, and there is a gap of about 40 nm between the end of one triple helix and the beginning of the next (Figure 11-4). These holes may provide sites for deposition of hydroxyapatite [Cal0(PO4)6(OH)2] crystals in the formation of bone (Chapter 37). Electron microscopic studies of negatively stained collagen fibrils reveal alternating light and dark regions. The light region, where the stain does not deposit, represents the overlapping of

F I G U R E 11-4 Packing of collagen polypeptides into a fibril. The individual tropocollagen molecules are quarter-staggered with respect to their nearest neighbors by a distance of 68 nm (D). A gap of about 40 nm (0.6 D) known as the hole region comes between the end of one tropocollagen molecule and the beginning of the next. The overlap zones are about 27 nm (0.4 D) in length and do not contain any hole zones. [Modified and reproduced, with permission, from A. Cohen, Ed.: Rheumatology and Immunology (W.B. Saunders, 1979).]


tropocollagen molecules; the dark region corresponds to a hole where the stain is deposited. The tensile strength of collagen fibrils is determined by covalent cross-links involving lysyl and hydroxylysyl side chains. The packing arrangement of tropocollagen molecules provides tensile strength and also prevents sliding of molecules over one another. This organization does not allow for stretching, which does occur in elastin. The nature and extent of crosslinking depend on the physiological function and age of the tissue. With age, the density of cross-linkages increases, rendering connective tissue rigid and brittle. The arrangement of fiber bundles varies with the tissue; it is random in bone and skin, sheet-like in blood vessels, crossed in the cornea, and parallel in tendons.

Turnover of Collagen and Tissue Repair The catabolism of collagen in the connective tissue matrix is carried out by enzymes known as collagenases. Part of the degradation may also involve neutral proteinases. A metalloenzyme specific for collagen catalyzes the cleavage of the triple helix at a single peptide bond, located at a distance from its N terminus corresponding to about three fourths the length of the tropocollagen molecule. The cleavage sites in type I collagen are the peptide bonds of Gly-Ile of the ot 1-chain and of Gly-Leu of the c~2-chain. The role and regulation of collagenase activity in vivo are unclear. The activity of the enzyme may be regulated via the formation of an enzyme-inhibitor complex or by the activation of a proenzyme, or both. A substantial amount (20-40%) of newly synthesized polypeptide chains of collagen undergo intracellular degradation. This degradation, which appears to occur in lysosomes, may be important in regulating the amount of collagen synthesized and in removing any defective or abnormal polypeptide chains that may be synthesized. Turnover of collagen in humans has been estimated by measurement of urinary hydroxyproline, which occurs mostly in the form of a peptide. Hydroxyproline makes up about 9-13 % of collagen residues and is not reutilized. Two aspects of hydroxyproline metabolism affect its use in the assessment of the true rate of collagen turnover: 1. Hydroxyproline is rapidly metabolized in a pathway initiated by the enzyme hydroxyproline oxidase (Chapter 17). 2. Hydroxyproline is present in C lq, which is synthesized at a rate estimated to be about 4.5 mg/kg per day. The first aspect has been overcome by measurement of urinary hydroxyproline in individuals with an inherited deficiency of hydroxyproline oxidase. Such persons excrete

SECTION 11.1

Protein Fibers and Proteoglycans

0.3 g of hydroxyproline per day in urine, which corresponds to about 2.25 g of hydroxyproline-containing proteins, most of which is collagen. Total protein catabolism in a normal well-fed adult is about 300 g; thus, collagen catabolism constitutes only about 0.8% of total protein catabolism. Collagen turnover has important clinical implications. The location, amount, type, and form of collagen depend on the coordinated control of its synthesis and degradation. In tissue injury (physical, chemical, infectious, or radiation), repair process comprises regeneration and fibrous connective tissue formation. Regeneration is the most desirable form of repair: a cut surface of an epidermis is replaced with new epidermis; scattered dead liver cells are replaced with new liver cells. Degradation products of type I collagen, namely N-telopeptides, C-telopeptides, hydroxyproline, and the collagen cross-links pyridinolone and deoxypyridinolone, have been used as markers of osteoclast activity in bone resorption. Collagen metabolite markers used for bone formation reflecting osteoblast activity are procollagen type I carboxyterminal propeptide and procollagen type I N-terminal propeptide. Both of these propeptides are cleaved byproducts generated from the conversion of procollagen to tropocollagen. Osteocalcin and bone-specific alkaline phosphatase are also used as bone formation markers. Biochemical markers of bone turnover are used for assessing and monitoring therapy in a number of diseases of bone including osteoporosis (Chapter 37). The cells of the body fall into three groups according to regenerative capacity: labile, stable, or permanent. The labile cells multiply throughout life and are maintained at an optimal level by continual proliferation of reserve cells. Labile cells are found in all epithelial surfaces, the spleen, and lymphoid and hematopoietic tissues. Stable cells have the potential to regenerate but do not normally undergo replication. However, under appropriate stimuli, they can proliferate rapidly. Stable cells include parenchymal cells of all glandular organs of the body and mesenchymal cells (e.g., fibroblasts, smooth muscle cells, osteoblasts, chondroblasts, and vascular endothelial cells). Permanent cells do not undergo significant replication postnatally. Nerve cells, skeletal muscle, and cardiac muscle cells are permanent cells. In fibrous tissue scar formation (a tough mass of collagen), normal cells are permanently lost. Many tissue injuries are repaired partly by regeneration and partly by scar formation (e.g., healing of dermal cut, improperly united bone fracture, and damaged liver). In tissues with permanent cells, injury and repair result in the formation of scar tissue only (e.g., myocardium). Although the liver normally undergoes repair by regeneration, chronic liver disease

119

resulting from ethanol abuse, some forms of malnutrition, viral infections, or exposure to chemicals can lead to hepatic fibrosis (cirrhosis) with distortion of the liver architecture by newly synthesized collagen fibers. In hepatic fibrosis, decreased collagenolytic activity and increased collagen synthesis may determine the net collagen deposition. A defect in the turnover of collagen may be responsible for the pathogenesis of idiopathic pulmonary fibrosis. This usually fatal disorder of the lungs is characterized by chronic inflammation of alveolar structures (alveolitis) and progressive interstitial fibrosis. An active collagenase in the lower respiratory tract may be responsible for sustained collagen lysis followed by disordered resynthesis. In rheumatoid arthritis, collagenous tissues of the joint (including articular cartilage) are eroded by collagenases and neutral proteinases derived from proliferating cells of the synovial membrane and leukocytes. Therapeutic approaches may depend on regulation of collagen synthesis, breakdown, or both. In a large family with osteoarthritis, which is a disorder of progressive degeneration of joint cartilage, in all affected members the ot(II) collagen contained a single-base mutation converting the codon for arginine at position 519 to a codon of cysteine, an amino acid not found in normal ot(II). In unaffected members of the family, this mutation does not occur. Other disorders of collagen metabolism, heritable or acquired, are discussed in Chapter 25. Elastin


Elastin is a fibrous, insoluble protein that is not a glycoprotein but is present with collagen in the connective tissues. Connective tissues rich in elastic fibers exhibit a characteristic yellow color. Elastic fibers are highly branched structures responsible for physiological elasticity. They are capable of stretching in two dimensions and are found most notably in tissues subjected to continual high-pressure differentials, tension, or physical deformation. Elastin imparts to these tissues the properties of stretchability and subsequent recoil that depend only on the application of some physical force. Tissues rich in elastic fibers include the aorta and other vascular connective tissues, various ligaments (e.g., ligamentum nuchae), and the lungs. Microscopically, elastic fibers are thinner than collagen fibers and lack longitudinal striations. Elastin fibers can be separated into amorphous and fibrillar components. The amorphous component consists of elastin, which is characterized by having 95% nonpolar amino acid residues and two unique lysine-derived amino acid residues, desmosine and isodesmosine.

181]

CHAPTER11 HeteropolysaccharidesI1"Proteoglycansand Peptidoglycans

A major fibrillar component is a glycoprotein called The structure of fibrillin consists of several cysteine-rich motifs and exhibits a multidomain organization similar to epidermal growth factor. The structure is stabilized by disulfide linkages. Fibrillin monomers undergo aggregation extracellularly into supramolecular structures. The fibrillar structures have a diameter of 10-12 nm and surround the amorphous component elastin. During fetal development, the fibrillar component appears first in the extracellular matrix. With increasing fetal age and with maturation of the fibers the amorphous component is deposited within the framework of the microfibrillar bundles producing mature, elastic microfibrils. Fibrillin is encoded by a gene located in the long arm of chromosome 15. Mutations in the fibrillin gene lead to an autosomal dominant trait known as Marfan's syndrome. The incidence of this disorder is 1:10,000, and 15-30% of cases are caused by new mutations in the fibrillin gene. Consistent with the function of fibrillin in the elastic connective tissues, the clinical manifestations present as disorders of cardiovascular, musculoskeletal, and opthalmic systems. For example, dissecting aneurysm of

fibrillin.

the aorta, preceded by a dilatation, is a potentially fatal cardiac manifestation. Mature elastin is a linear polypeptide, tropoelastin, which has a molecular weight of about 72,000 and contains about 850 amino acid residues. Although glycine accounts for one third of the residues, the repeat sequence GlyX - Y characteristic of collagen is not present in elastin. Instead, glycine residues are present in the repeat units Gly-Gly-Val-Pro, Pro-Gly-Val-Gly-Val, and Pro-GlyVal-Gly-Val-Ala. Elastin is relatively rich in the nonpolar amino acids; alanine, valine, and proline. In contrast to collagen, only a few hydroxyproline residues are present in elastin. Elastin contains no hydroxylysine or sugar residues. A feature of mature elastin is the presence of covalent cross-links that connect elastin polypeptide chains into a fiber network. The major cross-linkages involve desmosine and isodesmosine, both of which are derived from lysine residues. Several regions rich in lysine residues can provide cross-links. Two such regions that contain peptide sequences that are repeated several times in tropoelastin have the primary structure-Lys-Ala-Ala-Ala-Lys- and

R3

H I

O II

--N,,~.C--

I 1=14

I O---C~ H--N H ' ~ C ~ 8 I

-

I

H/C'~'-O

/N--H 8r " ~ ' ' ~

1 c e

CH

Xc=o

I

e c :

o

NH, ye 8

XkX~ndensation ?

__cZCH'LN,

II O

I

H

R,

,

Desmosine cross-link

F I G U R E 11-5 Formation of desmosine and isodesmosine covalent cross-links in elastin. Three allysine residues (R2, R3, and R4) and one lysyl residue (R1) condense to give a desmosine cross-link. The allysine residues (e-aldehydes) are derived from the oxidative deamination of lysyl residues. The isodesmosine cross-link is formed similarly, except that it contains a substitution at position 2 rather than at position 4, along with substitutions at 1, 3, and 5 on the pyridinium ring.

SECTION 11.1

Protein Fibers and Proteoglycans

-Lys-Ala-Ala-Lys-. The clustering of lysine residues with alanine residues provides the appropriate geometry for the formation of cross-links. Cross-linking elastin occurs extracellularly. The polypeptides are synthesized intracellularly by connective tissue cells (such as smooth muscle cells), according to the same principles for the formation of other export proteins (e.g., collagen; see Chapter 25). After transport into the extracellular space, the polypeptides undergo crosslinking. A key step is the oxidative deamination of certain lysyl residues, catalyzed by lysyl oxidase (a Cu 2+ protein). These lysyl residues are converted to very reactive aldehydes (ot-aminoadipic acid 6-semialdehyde) known as allysine residues. Through an unknown mechanism, three allysine residues and one unmodified lysine residue react to form a pyridinium ring, alkylated in four positions, which is the basis for the cross-links (Figure 11-5). The cross-links may occur within the same polypeptide or involve two to four different polypeptide chains. Current models of elastin structure suggest that only two polypeptide chains are required to form the desmosine cross-link, one chain donating a lysine and an allysine residue, the other contributing two allysine residues. Elastin polypeptides are also cross-linked by condensation of a lysine residue with an allysine residue, followed by reduction of the aldimine to yield a lysinonorleucine residue (Figure 11-6). These linkages are present to a much lesser extent than in collagen. Formation of cross-linkages in elastin can be prevented by inhibition of lysyl oxidase. Animals maintained on a copper-deficient diet and those administered lathyrogens such as #-aminopropionitrile or o~-aminoacetonitrile develop connective tissue abnormalities. Lathyrogens are so-called because of their presence in certain peas of the genus Lathyrus. They are noncompetitive inhibitors of lysyl oxidase, both in vitro and in vivo. Consumption of certain types of sweet pea (e.g., Lathyrus odoratus) can lead to lathyrism. In osteolathyrism, the abnormalities involve bone and other connective tissues, the responsible compounds being the above-mentioned nitrites. The toxic agent responsible for neurolathyrism, #-N-oxalyl-Loe,#-diaminopropionic acid, has been isolated from Lathyrus sativus but its mode of action is not known. Thus, copper deficiency or administration of lathyrogens in animals prevents the formation of the highly insoluble cross-linked elastin but results in the accumulation of soluble elastin. Soluble elastin obtained in this manner is particularly useful in structural studies. Purification of the insoluble elastin without disruption of the integrity of the polypeptide chains is a formidable task. Several models of the macromolecular structure of elastin have been suggested to account for its elasticity: cross-linked globular elastin subunits, cross-linked

181

I

o=cx

I

c."~

HC ~ , . / n--,~ I

Y 13~

~.C He I I C/ H/' t ~ - - N - - H

e

N 6

..u n,

Lysine residue

~

~

fellYi~ue

1

I O--C_

/c=o

L. I N

I

-

I

C

I"-) H

I

I

,,-,,OH

._N ~ ~

c=o

~

~

__

I]1 H

(Schiff base) -H20

I

O--C~ HC~ ~ H--N/ ~

H ~

I

~

,-,/ ~ N ~ t ~ ~ Oehydrolysinonorleucine cross-link

I

sC---O CH ~N--H

I

/

[ Reduction

I O--C~

H

HC~ H__N /

V

A

A V

H

~,',/ ~N/~'~""L'LN__

HI

I

I .C=O

C~ H

I

Lysino no rleucine cross-link

FIGURE 11-6 Formationof the lysinonorleucinecross-linkbetweenan allysineand a lysine residuein elastin. random elastin chains, the oiled-coil model, and the fibrillar model. The fibrillar model incorporates a unique conformation due to the presence o f a glycine residue followed by an amino acid residue with a bulky R-group (e.g., Pro-Gly-Gly-Val). This type of sequence gives rise to near right angle turns (#-turns; Chapter 4) in the polypeptide backbone. The oiled-coil model assumes that each polypeptide chain is fibrillar and consists of alternating segments of cross-linked regions and "oiled coils," each chain being linked to many others to form a threedimensional network similar to a mattress spring. It is postulated that these coils can stretch up to 2-2.5 times their original length. The driving force for recoil of elastic fibers is due to a change in entropy.

Turnover of Elastin The turnover rate of mature elastin in healthy persons is relatively low. Insoluble elastin in healthy elastic tissue is usually stable and subjected to minimal proteolytic degradation. In several clinical conditions (e.g., emphysema, advanced atherosclerosis, pancreatitis), increased degradation of fragmentation of elastic fibers may play a significant role. The interaction between insoluble elastin and soluble elastolytic enzymes, and the regulation of these enzymes, may shed light on certain cardiovascular diseases, in view of the role of elastin in arterial dynamics.

1~


Elastolytic proteinases (elastases) are found in pancreatic tissue, polymorphonuclear leukocytes, macrophages, and platelets. These enzymes exhibit broad specificity. They catalyze preferential cleavage of peptide bonds adjacent to aliphatic amino acids, namely, glycine, alanine, and valine, which are present in high amounts in elastin. Elastases degrade elastin at neutral or slightly alkaline pH. The pancreas secretes elastase as an inactive precursor (zymogen) known as proelastase, which is converted to its active form by trypsin in the duodenum; elastase takes part in the digestion of dietary proteins (Chapter 12). This enzyme has a structure homologous to that of other pancreatic serine proteinases, including the amino acid sequence at the active site (Chapter 6). Polymorphonuclear leukocyte elastase is also a serine proteinase. Elastases are inactivated by serum otl-proteinase inhibitor and otz-macroglobulin; thus, their proteolytic action may be checked to prevent indiscrimination digestion of elastin-containing tissues (Chapter 6). An enzyme that degrades only soluble forms of elastin and exhibits trypsin-link activity has been described. Its biological significance is unknown. Several

heritable and acquired disorders of elastin are known (see Chapter 25).

Proteoglycans Proteoglycans are high-molecular-weight, complex molecules with diverse structures and functions. They are polyanionic substances containing a core protein to which at least one glycosaminoglycan (also known as mucopolysaccharide) chain is covalently attached. Proteoglycans are major components of connective tissue and participate with other structural protein constituents, namely, collagen and elastin, in the organization of the extracellular matrix.

Types, Structures, and Functions of Glycosaminoglycans Six classes of glycosaminoglycans have been described. All are heteropolysaccharides and contain repeating disaccharide units. The compositions and structures of the repeating disaccharide units are shown in Table 11-2 and Figure 11-7. In five glycosaminoglycans, the

TABI~E 1I-2

Compos#ion, Properties, and Distribution of Glycosaminoglycans Glycosaminoglycan (and range of molecular weight)

Amino Sugar

Uronic Acid

Type of Sulfate Linkage (extent of sulfation is variable)

Hyaluronate (4 to 80 x 106)

D-Glucosamine

D-Glucuronate

None

Chondroitin sulfate (5,000 to 50,000)

D-Galactosamine

D-Glucuronate

4-0- and/or 6-0-sulfate on galactosamine

Dermatan sulfate (15,000 to 40,000)

D-Galactosamine

L-Iduronate, D-Glucuronate

Keratan sulfate (4,000 to 19,000)

D-Glucosamine

None (but contains D-Galactose)

Heparan sulfate (104 to 105 )

D-Glucosamine

Heparin (103 to 106)

D-Glucosamine

D-Glucuronate (major), L-Iduronate (minor) L-Iduronate (major) D-Glucuronate (minor)

4-0-sulfate on galactosamine; 2-0-sulfate on iduronate 6-0-sulfate on both carbohydrate residues 6-0-sulfate and N-sulfate (or N-acetyl) on glucosamine; 2-0-sulfate on iduronate 2-0-sulfate on iduronate; 6-0-sulfate and N-sulfate (or N-acetyl) on glucosamine

Tissue Distribution Connective tissues, cartilage, synovial uid, vitreous humor, umbilical cord Cartilage, bone, skin, cornea, blood vessel walls Skin, heart valve, tendon, blood vessel walls Cartilage, cornea, intervertebral disks Lung, blood vessel walls, many cell surfaces Lung, liver, skin, intestinal mucosa (mast cells)

SECTION11.1 Protein Fibers and Proteoglycans

183

Hyaluronic Acid

~~

co~ \ "xZ-----~O\

HO ----~ O

CH20 H O I Ac

[glucuronate O 1, 3-NacetylglucosamineO 1, 41-

Chondronitin Sulfate / Dermatan Sulfate ~o~ --0

0

no i-

,~,o

o

~u2 \ ~ 0

OR_

oH

~ . SO~

L

"'J'x) / CH

~fi-.x~o~

I Ac [glucuronate-13 1, 3-Nacetylglucosamine-13 1, 4][ 4 or 6 sulfate

Dermatan Sulfate [iduronate-o~ 1, 3 ...]

Keratan Sulfate

OH(S030) dH

/0...~

\'~.s \

-

OH

\

HO

OSOp

/

I

~

I Ac

CH 2

~

\ XO'~'-'O~

[Nacetylglucosamine-[31, 3-galactose-[~ 1, 4]-

I

I

6-sulfate

6-sulfate

Heparan Sulfate/Heparin

IO~

CO ~

b../LLo

OR /

/

~ .o

HO

OH

. . O ~

"o~

CH 2

~2----..L~o

(o, .) . # - Rn /

so~ (orAc) -[glucuronate-~ 1, 3-...]-[iduronate-c~ 1, 4-Nsulfate glucosamine-~ 1, 4]I (acetate) I 2-sulfate 6-sulfate

F I G U R E 11-7 Basic repeating disaccharide units of six classes of glycosaminoglycans. Chondroitin sulfate and dermatan sulfate differ in type of principal uronic acid residue; the former contains glucuronate and the latter iduronate. Heparin and heparan sulfate differ in that heparin contains more iduronate 2-sulfate and N-sulfated glucosamine residues than heparan sulfate. In the sulfate-containing glycosaminoglycans, the extent of sulfation is variable. All of these molecules have a high negative charge density and are extremely hydrophilic. [Reproduced with permission from V. C. Hascall and J. H. Kimura; Proteoglycans: Isolation and characterization. In Meth. Enzymol. 82(A), L. W. Cunningham and D. W. Frederiksen. Eds. Academic Press, San Diego (1982).1

disaccharide units consist of amino sugars (usually D-galactosamine) alternating with uronic acids. In one type, keratan sulfate, the uronic acid is replaced by galactose. The amino sugars are usually present as N-acetyl derivatives. In heparin and heparan sulfate, however, most amino sugars are found as N-sulfates in sulfamide link- 9 age, with a small number of glucosamine residues as N-acetyl derivatives. With the exception of hyaluronate, all glycosaminoglycans contain sulfate groups in ester linkages with the hydroxyl groups of the amino sugar residues.

The presence of carboxylate and sulfate groups provides the glycosaminoglycans with a high negative charge density and makes them extremely hydrophilic. Almost all water in the intercellular matrix is bound to glycosaminoglycans, which assume a random conformation in solution, occupying as much solvent space as is available by entrapping the surrounding solvent molecules. In solution, glycosaminoglycans also undergo aggregation and impart a high viscosity. In order to maintain electrical neutrality, negatively charged (anionic) groups of glycosaminoglycans, fixed to the matrix of the connective tissue, are neutralized by positively charged (cationic) groups (e.g., Na+). Osmotic pressure within the matrix is thereby elevated, contributing turgor to the tissue. Thus, proteoglycans, because they contain glycosaminoglycans, are polyanionic and usually occupy large hydrodynamic volumes relative to glycoproteins or globular proteins of equivalent molecular mass. Usually, they have high buoyant densities, and this property has been utilized in their isolation and characterization. The physical properties of connective tissue also depend on collagen or elastin fibers. Collagen fibers resist stretching and provide shape and tensile strength; the hydrated network of proteoglycans resists compression and provides both swelling and pressure to maintain volume (Figure 11-8). Thus, the properties of these different macromolecules are complementary, and their relative compositions vary according to tissue function. For example, sulfated glycosaminoglycans tend to provide a solid consistency, whereas hyaluronate provides a softer, more fluid consistency. The vitreous space behind the lens of the eye is filled with a viscous, gelatinous solution of hyaluronate free of fibrous proteins and transparent to light. Hyaluronate is not sulfated, and there is no evidence that it is linked to a protein molecule, as are the other glycosaminoglycans. Hyaluronate is also present in synovial fluid in joint cavities that unite long bones, bursae, and tendon sheaths. The viscous and elastic properties of hyaluronate contribute to the functioning of synovial fluid as a lubricant and shock absorber. Hyaluronate is depolymerized by hyaluronidase, which hydrolyzes the fl(1--+ 4) glycosidic linkages between N-acetylglucosamine and glucuronate. Some pathogenic bacteria secrete hyaluronidase, which breaks down the protective barrier of hyaluronate and renders the tissue more susceptible to infection. Hyaluronidase, found in spermatozoa, may facilitate fertilization by hydrolyzing the outer mucopolysaccharide layer of the ovum, thereby enabling the sperm to penetrate it. Hyaluronidase (prepared from mammalian testes) is used therapeutically to enhance dispersion of drugs administrated in various parts of the body.

18~

CHAPTER11 Heteropolysaccharides I1 Proteoglycans and Peptidoglycans

HA

Collagen (type II)

Proteoglycan Collagen (type II)

F I G U R E 11-8 Schematic representation of the molecular organization of structural elements in cartilage matrix. Collagen fibrils provide tensile forces, and proteoglycans, because of their large solvent domains, accommodate reversible compressible forces. LP -- link protein; HA = hyaluronate; KS -- keratan sulfate; CS -- chondroitin sulfate; PC = core protein. [Reproduced with permission from L. C. Junqueira, J. Carneiro, and J. A. Long, Basic Histology, 5th ed. Appleton & Lange, Norwalk, CT, 1986. @ 1986 Appleton-Century-Crofts.]

Heparin is a heterogeneous glycosaminoglycan found in tissues that contain mast cells (e.g., lungs and perivascular connective tissue). Its primary physiological function is unknown, but it is a powerful inhibitor of blood clotting and is used therapeutically for that purpose. The therapeutic anticoagulant action of heparin is due to its ability to produce conformational changes in the proteinase inhibitor antithrombin. Both activated factor X (factor Xa) and thrombin are inactivated by a heparinantithrombin complex (Chapter 36). The antithrombin is homologous in structure with the ot~-antitrypsin family of proteinase inhibitors and is a suicide substrate for factor Xa and thrombin. The proteinase binds to a specific Arg-Ser peptide bond present at the reactive site of antithrombin. This 1:1 antithrombin-thrombin is a stable inactive complex. The function of heparin is catalytic. After the formation of the heparin-mediated antithrombinthrombin complex, heparin is released to initiate another cycle. Factor Xa is inhibited by a specific heparin pentasaccharide bound to antithrombin. However, the inhibition of thrombin requires an antithrombin bound to the heparin consisting of the specific pentasaccharide that consists of chains of at least 18 monosaccharide units.

Heparin therapy has a serious risk. In about 3% of patients on heparin therapy, thrombocytopenia (low platelet levels) develops. This can be life threatening if it leads to thromboembolic complications such as disseminated intravascular coagulation. Thrombocytopenia is caused by a heparin-induced immune-mediated process. The antibody is directed against a cryptic epitope present on a platelet factor (PF4) that appears only after the protein binds to heparin. The multimolecular complex consisting of antibody-heparin-PF4 causes platelet activation by binding to specific receptors (FcTRIIA) on the platelet membrane. A genetic predisposition to this disorder is due to a specific mutation in the FcTRIIA gene changes arginine to histidine at position 131. This amino acid change causes a strong affinity for the Fc region of immunoglobulin IgGl and IgG2. Thus, identification of patients with homozygosity for the Arg 131 --+ His 131 mutation in FcTRIIA and who are on heparin therapy has important therapeutic implications. Heparin-induced thrombocytopenia occurs less frequently with the use of low-molecular-weight heparin (M.W. 5,000) compared to unfractionated heparin (M.W. 3,000-30,000). The treatment of life threatening complications of heparin-induced thrombocytopenia requires cessation of heparin therapy. Heparin's anticoagulant action can be reversed rapidly by intravenous administration of prolamine sulfate. Protamines are basic proteins, isolated from fish sperm and bind tightly to heparin. One of the laboratory tests used in the identification of patients with heparin-induced thrombocytopenia, employs a micro-ELISA plate coated with purified PF4-heparin complex. Upon incubation of patient's plasma, if antibody is present, it binds to the PF4-heparin complex. The bound antibody is detected with goat antihuman antibody coupled to peroxidase, followed by incubation with substrates o-phenylenediamine/H202 (Chapter 8). Tendons have a high content of collagen and sulfated glycosaminoglycans (chondroitin and dermatan sulfates). Tendons are fibrous cords that fuse with skeletal muscle at each end and penetrate bones at the two sides of a joint. Thus, tendons are aligned along their long axis, providing flexible strength in the direction of the muscle pull. Remarkable progress has been achieved in understanding the organization of the macromolecular components of cartilage. Cartilage is one type of dense connective tissue; bone is the other (Chapter 37). Cartilage is less resistant to pressure than is bone, and its weightbearing capacity is exceeded by that of bone. It has a smooth, resilient surface. During growth and development of an embryo, cartilage provides the temporary framework for the formation and development of bone. In postnatal life, it permits the long bones of the extremities to grow until

SECTION11.1 ProteinFibers and Proteoglycans

~

........

~e

4-----------'--- Hyaluronicacid

k '~-~~-~ ~ & ! ' ---

Link protein Keratansulfate Chondroitinsulfate

~ "lw'Ll'~V'~l'~t'~'J;l"l~ Ig ,,l,l,l,lA,~,t,hl

Core protein

,11,11,1i, LI, 11,11,,a_.~ , " - ' - " ' I' I' I ' 1' I ' I ' I ' t ....... e tjl,l,l,l.l,li 1 t.I,I,I,L I I l,l,~-.~el -'-'''~'I'l']'1'1'1

~,t',l"t"t't" ~

|

t

,,ljI,I,I,I,I,L,I,IA

7~T'T'l'l'l'r';'l'[ '' -|~ ~,lJl,ll~,l,l,L,J,l J,l I I I I I 1 1 ol .... ltl'l't'l'l'l'l'l

Subunits

~'-T*T' l ' T ' l ' l ' r ' l ' 1 " - P L , , I , I , J , I , I , I , I t l l,l,l,l,l, l,l,l,, L_el-~"l'l'l' t,[,[qq , r['~'tl'/'/',r',r'r" _IL],I,I,I,I,I,I,I,~,j ~ 1 - , , , 7 , i,l,I, r,i,l,i,FrT ' o -~'~'-~" ' , ,l,i,l,j,j,]

F I G U R E 11-9 Schematic representation of cartilage proteoglycan. The monomers, consisting of glycosaminoglycan chains linked to a core protein by covalent linkage, extend laterally at intervals from opposite sides of a very long filament of hyaluronate. The interaction between the core protein and hyaluronate is noncovalent and is aided by the link protein. The entire structure is highly hydrated and occupies a large volume. [Reproduced with permission from W. J. Lennarz, The Biochemistry of Glycoproteins and Proteoglycans, Plenum Press, New York, 1980].

their longitudinal growth ceases. Its smooth surface provides for freely movable joints (e.g., knee and elbow). The properties of cartilage depend on its content of collagen (or collagen and elastin) and proteoglycans. Cells responsible for the synthesis and maintenance of cartilage are known as chondrocytes. The proteoglycan monomer of cartilage consists of glycosaminoglycan chains of chondroitin sulfate and keratan sulfate covalently linked to a core protein (Figure 11-9) about 300 nm long with molecular weight of 250,000. Each molecule of core protein contains about 80 chondroitin sulfate and 100 keratan sulfate chains. The linkage between the oligosaccharides and the protein can be O-glycosidic between xylose and serine residues, O-glycosidic between N-acetylgalactosamine and serine or threonine residues, or N-glycosidic between N-acetylglucosamine and the amide nitrogen of asparagine. The glycosaminoglycan chains with their negatively charged groups extend out of the core protein to occupy a large volume and to entrap surrounding solvent. One end of the core protein has relatively few or no attached glycosaminoglycan chains and resembles a

185 glycoprotein; at this end, the core protein possesses an active site that binds to a very long filament of hyaluronate by a noncovalent interaction at intervals of 30 nm. The interaction between the core protein and the hyaluronate is aided by a protein (M.W. 50,000) known as link protein. The macromolecular complex is negatively charged and highly hydrated; it interacts electrostatically with basic charges on collagen fibrils, which define tissue space and provide tensile strength. The interspersed proteoglycan aggregates provide a hydrated, viscous gel for resistance to compressive loads. Proteoglycans originating in different tissues and exhibiting differences in molecular structure are presumed to be products of different genes. Synthesis and secretion of proteoglycans consist of several different phases of the internal membrane systems of the cell. Synthesis of the protein component occurs on ribosomes attached to the rough endoplasmic reticulum, with translocation of the newly synthesized polypeptide chains to the cisternal side of the membrane. The polypeptide chains travel through the cisternae of the endoplasmic reticulum to the Golgi apparatus, where the oligosaccharide side chains are synthesized by the addition of one sugar residue at a time. Further modification of the sugar residues (the conversion of selected residues of D-glucuronate to Liduronate, and sulfation) takes place after completion of the oligosaccharide (Chapter 16). The fully synthesized proteoglycans are then transferred from the Golgi apparatus, in the form of vesicles, to the cell membrane for the purpose of secretion into the extracellular spaces. The proteoglycan content in animal tissues appears to change with aging. Tissues rich in keratan sulfate show a continual increase in amount of this proteoglycan throughout life. The amounts of chondroitin sulfate in cartilage and in nucleus pulposus (of intervertebral disk) and of hyaluronic acid in skin decrease with age. Administration of growth hormone to an older animal appears to reverse the pattern of proteoglycan synthesis, making it similar to that of a younger animal. Some of the effects of growth hormone are mediated indirectly by a group of peptides known as somatomedins, which are secreted by the liver (and possibly by the kidney) under the influence of growth hormone (Chapter 31). Somatomedin A (formerly known as sulfation factor) exhibits cartilagestimulating activity, in part by affecting the incorporation of sulfate into proteoglycans. Testosterone, a steroid hormone (Chapter 34), increases the rate of hyaluronate synthesis. Insulin deficiency causes diminished synthesis of proteoglycans in rats that are rendered diabetic experimentally; synthesis is restored to normal following insulin administration. Some chronic complications of diabetes mellitus, namely, accelerated vascular degeneration, poor wound healing, and increased susceptibility to

186 infection, may be partly attributable to altered proteoglycan metabolism.

Turnover of Proteoglycans and Role of Lysosomes Proteoglycans undergo continuous turnover at rates dependent upon the nature of the proteoglycan and tissue location. Their half-life is between one and several days. Degradation of proteoglycans is initiated by proteolytic enzymes that release glycosaminoglycans; the latter are subsequently degraded by lysosomal enzymes. Some of the products of hydrolysis (e.g., dermatan sulfate and heparan sulfate) are excreted in the urine. Lysosomes are subcellular organelles in which a wide range of catabolic enzymes are stored in a closed, protective membrane system. They are major sites of intracellular digestion of complex macromolecules derived from both intracellular (autophagic) and extracellular (heterophagic) sources. Lysosomal enzymes show optimal activity at acidic pH. The pattern of enzymes in lysosomes may depend upon the tissue of origin, as well as upon the physiological or developmental state of the cells. Lysosomal enzymes are synthesized on the ribosomes of the rough endoplasmic reticulum, passed through the Golgi apparatus, and packaged into vesicles. The hydrolyases are glycoproteins, some of which contain mannose 6-phosphate markers necessary for the normal uptake of glycoproteins into lysosomes. Thus, carbohydrates may also serve as determinants of recognition in the intracellular localization of the glycoproteins following their synthesis. In two biochemically related disorders of lysosomal function, I-cell disease (mucolipidosis II) and pseudoHurler's polydystrophy (mucolipidosis III), the lesion is in the posttranslational modification step for acid hydrolases destined to be packaged into lysosomes. In normal cells, these acid hydrolyases are glycoproteins carrying mannose 6-phosphate markers that direct them to lysosomes through a receptor-mediated process. In other words, the presence of phosphomannose residues on the newly synthesized acid hydrolases and of phosphomannose receptors on selected membranes leads to the segregation of these enzymes in the Golgi apparatus, with subsequent translocation into lysosomes. In mucolipidosis II, the acid hydrolases are not phosphorylated; in mucolipidosis III, the enzymes either are not phosphorylated or have significantly diminished phosphate content. The lack of mannose 6-phosphate leads to defective localization of acid hydrolases that, instead of being packaged in lysosomes, are exported outside the cell; thus, the enzyme activity in plasma reaches high levels. These enzymes could cause indiscriminate damage. At least eight acid hydrolases (glycosidases, sulfatases, and cathepsins) appear to be affected


in this manner. However, not all lysosomal enzymes and tissue cells are affected. For example, lysosomal acid phosphatase and/~-glucosidase, and hepatocytes and neurons, appear to be spared from this defect. In mucolipidosis II and III, large inclusions of undigested glycosaminoglycans and glycolipids occupy almost all of the cytoplasmic space in cultured skin fibroblasts. In addition to the phosphorylation defect, the acid hydrolases are much larger than their normal counterparts, presumably owing to lack of the limited proteolysis of the hydrolases that occurs in normal lysosomes. Both disorders are inherited as autosomal recessive traits, affect primarily connective tissue, and are characterized by psychomotor retardation, skeletal deformities, and early death. The segregation of lysosomal enzymes into lysosomes requires carbohydrate recognition markers (phosphomannose in some) and also the formation of coated vesicles into which the enzymes are sequestered. Coated vesicles shuttle macromolecules between organelles and may be responsible for selectivity in intercompartmental transport (e.g., from endoplasmic reticulum to Golgi, from Golgi to lysosomes). The major component of coated vesicles is clathrin, a nonglycosylated protein (M.W. 180,000). It is located on the outer surface (cytoplasmic side) of the coated vesicles. Clathrin and its tightly bound light chains (M.W. 33,000 and 36,000) form flexible lattices that function as structural scaffolds surrounding the vesicles. In addition to their role in intracellular transport, coated vesicles are involved in receptor-mediated endocytosis. This process accomplishes internalization of macromolecules (ligands) by binding them to receptors on the cell membranes located in specialized regions of clathrincontaining coated pits, which invaginate into the cell to form coated vesicles. Inside the cell, the vesicles lose their clathrin coat (which may be reutilized) and fuse with one another to form endosomes, whose contents are acidified by proton pumps driven by free energy of hydrolysis derived from ATE In the endosome, the ligand and receptor undergo dissociation. The endosome fuses with the primary lysosome to form a secondary lysosome. Receptors may also be recycled or degraded. Thus, there are several pathways of receptor-mediated endocytosis. In some cells, the receptors migrate continuously to coated pits and undergo internalization whether or not ligands are bound to them (e.g., receptors for low-density lipoproteins, transferrin, and asialoglycoproteins). In other cells, the receptors are diffusely distributed and do not migrate to coated pits unless they are bound with ligands (e.g., epidermal growth factor). Following internalization, the receptor-ligand complex may be disposed of by one of four routes:

SECTION11.1 Protein Fibers and Proteoglycans

1. Ligand degradation and receptor reutilization (e.g., low-density lipoproteins, Chapter 20; asialoglycoproteins, Chapter 10; transcobalamin II, Chapter 38; some peptide hormones; 2. Both ligand and receptor reutilization (e.g., transferrin, Chapter 29; class I major histocompatibility complex molecules on T cells and class II molecules on macrophages, Chapter 35); 3. Both receptor and ligand degradation (e.g., epidermal growth factor, immune complexes); 4. Both ligand and receptor transportation out of the cell (e.g., maternal IgG, secretory IgA Chapter 35). Receptors for different ligands have been characterized and show multiple functional domains (see low-density lipoproteins, Chapter 20; insulin, Chapter 22). Phagocytosis is a transport system that does not involve receptor-mediated endocytosis coupled to the formation of coated vesicles. This type of internalization consists of forming phagosomes, which are membrane-bound vesicles containing ingested substances (e.g., bacteria or their products, extracellular material). Secondary lysosomes are formed when primary lysosomes fuse with phagosomes; they can also be formed when primary lysosomes fuse with autophagosomes, which contain internally derived defective organelles (e.g., mitochondria or rough endoplasmic reticulum). Within the secondary lysosomes, the ingested macromolecules are broken down by a wide variety of hydrolases into their respective monomeric units (e.g., sugars, amino acids, purines, pyrimidines, fatty acids, and cholesterol). Eventually these molecules appear in the cytoplasm and are usable for metabolic purposes. Undigested material is retained within the lysosomes and forms a residual body. A schematic representation of the formation and some functions of lysosomes is shown in Figure 11-10. In some circumstances, lysosomal enzymes are extruded into the extracellular milieu and cause destruction of the surrounding matrix. This extracellular catabolic process releases materials that are taken up in phagosomes for further breakdown by the cells. This process occurs in the remodeling of bone and cartilage. In the thyroid gland, lysosomes participate in the regulation of hormone release from the cells to the blood circulation. The thyroid hormones, thyroxine and triiodothyronine, are synthesized on a polypeptide, thyroglobulin, and stored within the thyroid follicles. Under appropriate stimuli, the thyroglobulin is internalized (endocytosed) and hydrolyzed by lysosomal hydrolases to release the free hormones, which are then transferred to the blood (Chapter 33). In vitro, estradiol, testosterone, and vitamins A and D have disruptive

187

effects on the lysosomal membrane, causing the release of lysosomal enzymes. The anti-inflammatory steroid hormone cortisol has an opposite effect. The destructive potential of leukocytic lysosomal proteinases is checked by O~l-proteinase inhibitor and otz-macroglobulin in plasma and synovial fluids. Enhanced catabolism of proteoglycans of articular cartilage occurs in rheumatoid arthritis as a result of the action of lysosomal enzymes on articular cartilage. Many pathological conditions occur as a result of a deficiency of lysosomal enzymes (e.g., storage disorders of glycoproteins and glycosphingolipids). At least 41 distinct lysosomal storage diseases are known. Most of these diseases are due to deficiency of a particular enzyme and a few are due to defects in the nonlysosomal proteins that are necessary for lysosomal biogenesis. Lysosomal storage disorders are predominantly autosomal recessive, with the exception of Fabry's disease and mucopolysaccharidosis type II, which are Xlinked recessive disorders. Overall, lysosomal storage disorders are relatively common with a prevalence of 1 per 7700 live births, although some disorders are very rare.

Mucopolysaccharidoses Mucopolysaccharidosis (MPS) encompasses disorders in which undegraded or partly degraded glycosaminoglycans accumulate in the lysosomes of many tissues owing to a deficiency of specific lysosomal enzymes. Table 11-3 lists the missing enzymes and gives relevant clinical and laboratory findings. These disorders, with the exception of Hunter's syndrome, which is an X-linked trait, are inherited in the autosomal recessive manner. All of the deficient enzymes are acid hydrolases except for acetyltransferase in Sanfilippo's syndrome type C. In mucopolysaccharidoses the catabolism of heparan sulfate, dermatan sulfate, and keratan sulfate is affected. Their degradation proceeds from the nonreducing end of the carbohydrate chain by the sequential actions of lysosomal exoglycosidases, exosulfatases, and an acetyltransferase (Figures 11-11 through 11-13). These disorders are rare; collectively, they may occur in 1 in 20,000 live births. Since proteoglycans are widely distributed in human tissues, the syndromes can affect a wide variety of tissues; thus, the clinical features vary considerably. All types are characterized by reduced life expectancy, with the exception of Scheie's syndrome. All types are characterized by skeletal abnormalities, which are particularly severe in types IV and VI. Types IH, IS, IV, VI, and VII usually exhibit clouding of the

188


I ! Heterophagosome

~

Secondary lysosome

lysosome

....'i:~. , . . ' . : .. Iv----:"~ ... ""~ ~' ~;-:--:--3"." :: ~ t ~.~.=-~;--:.... ~1 5 ..ii,t;D,::

1

,

i

.

~

~

Autophagosome

Nucleus'""i:"~"":; " :."..

9

, ,, ~< :".:":. " ":::' " " . ;.::~')~:;,bi ~,'i~:~ ..,............~~~!,~:~!~ 9 .. '

i

~,:.,_.

:

1[: : N U itq.l~';.:~,;.~;~-a~ imr~l,.:~...: .~..:.,,c., ~

~-.:~t~:-;f.'~ :". 4) linkage, is hydrolyzed not in the small intestine but in the colon and is converted to products similar to those derived from lactose fermentation. It has been used in the treatment of patients with liver disease. Normally, ammonia (NH3) produced in the GI tract is converted in the liver to urea (Chapter 17), hence, in patients with severe liver disease, blood ammonia levels increase. Absorption of ammonia can be decreased by administration of lactulose, which acidifies the colonic constituents so that NH3 is trapped as NH4+ ions (Chapter 9).

~1~

CHAPTER12 Gastrointestinal Digestion and Absorption

Proteins Protein is an essential nutrient for human growth, development, and homeostasis. The nutritive value of dietary proteins depends on its amino acid composition and digestibility. Dietary proteins supply essential amino acids, which are not synthesized in the body. Nonessential amino acids can be synthesized from appropriate precursor substances (Chapter 17). In human adults, essential amino acids are valine, leucine, isoleucine, lysine, methionine, phenylalanine, tryptophan, and threonine. Histidine (and possibly arginine) appears to also be required for support of normal growth in children. In the absence from the diet of an essential amino acid, cellular protein synthesis does not occur. The diet must contain these amino acids in the proper proportions. Thus, quality and quantity of dietary protein consumption and adequate intake of energy (carbohydrates and lipids) are essential. Protein constitutes about 10-15% of the average total energy intake. Animal proteins, with the exception of collagen (which lacks tryptophan), provide all of the essential amino acids. Vegetable proteins differ in their content of essential amino acids (Table 12-4), but a mixture of these proteins will satisfy the essential amino acid requirement. For example, the lysine lacking in grains can be provided by legumes. This combination also corrects for the methionine (which is supplied in corn) deficiency of legumes. Such combinations (e.g., lentils and rice, chick-peas and sesame seeds, spaghetti and beans, corn and beans) are widely used in different cultures to provide for an optimal protein requirement. The recommended allowance for mixed proteins in an adult in the U.S.A. is 0.85 g per kilogram of body weight per day (Chapter 5). The allowances

TABLE 12-4 Limiting Essential Amino Acids in Plant Proteins Source of Plant Proteins Grain Maize or corn Millet, oats, wheat, or rice Legumes Beans, immature Beans, mature Peas Peanuts Nuts and oil seeds Coconut Vegetables

Limiting Amino Acids Lysine, tryptophan Lysine, threonine Methionine, isoleucine Methionine, valine Methionine, tryptophan Lysine, threonine Lysine, threonine Lysine, threonine Methionine, isoleucine

are increased during childhood, pregnancy, and lactation (Appendix IV). Besides dietary protein, a large amount of endogenous protein undergoes digestion and absorption. Endogenous protein comes from three sources: 1. Enzymes, glycoproteins, and mucins secreted from the salivary glands, stomach, intestine, biliary tract, and pancreas, which together constitute about 20-30 g/day; 2. Rapid turnover of the gastrointestinal epithelium, which contributes about 30 g/day; and 3. Plasma proteins that normally diffuse into the intestinal tract at a rate of 1-2 g/day. In several disorders of the GI tract (e.g., protein-losing gastroenteropathy), loss of plasma proteins is considerable and leads to hypoproteinemia.

Digestion Protein digestion begins in the stomach, where protein is denatured by the low pH and is exposed to the action of pepsin. The low pH also provides the optimal H + concentration for pepsin activity. The zymogen precursor pepsinogen (M.W. 40,000) is secreted by the chief cells and is converted to pepsin (M.W. 32,700) in the acid medium by removal of a peptide consisting of 44 amino acid residues. This endopeptidase hydrolyzes peptide bonds that involve the carboxyl group of aromatic amino acid residues, leucine, methionine, and acidic residues (Table 12-5). The products consist of a mixture of oligopeptides. Chyme contains potent secretagogues for various endocrine cells in the intestinal mucosa. CCK and secretin cause release of an alkaline pancreatic juice containing trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidases A and B. Activation begins with that of trypsinogen to trypsin by enteropeptidase (previously called enterokinase) present in the brush-border membranes of the duodenum. Enteropeptidase cleaves between Lys-6 and Ile-7 to release a hexapeptide from the N-terminus. Trypsin autocatalytically activates trypsinogen and activates the other zymogens. The importance of the initial activation of trypsinogen to trypsin by enteropeptidase is manifested by children with congenital enteropeptidase deficiency who exhibit hypoproteinemia, anemia, failure to thrive, vomiting, and diarrhea. Trypsin, elastase, and chymotrypsin are endopeptidases. Carboxypeptidases are exopeptidases (Table 12-5). The combined action of these enzymes produces oligopeptides having two to six amino acid residues and free

SECTION 12.3

Digestion and Absorption of Major Food Substances

215

TABLE 12-5

Specificities of the Proteolytic Enzymes Involved in Gastrointestinal Protein Digestion Peptide bond cleaved o

H2N. . . . . N H - C H -

//o

oiI

N H - C H - C . . . . . C\

I R1

I

R2

OH

Enzyme*

Preferentially cleaves peptide bonds in which

Trypsin Chymotrypsin Carboxypeptidase A Carboxypeptidase B Elastase Aminopeptidase Pepsin Dipeptidases

R1R1R1R1R1R1R] =

Arg or Lys; R 2 = any amino acid residue aromatic amino acid (Phe, Tyr, Trp); R 2 - any amino acid residue any amino acid residue; R2 - any C-Terminal residue except Arg, Lys, or Pro any amino acid residue; R2 - Arg or Lys at the C-terminus of a polypeptide Neutral (uncharged) residues; R 2 - a n y amino acid residue Most N-terminal residues of a polypeptide chain; R 2 - any residue except Pro Trp, Phe, Tyr, Met, Leu; R 2 = any amino acid residue H2N-CH - O

NH-CH-C,~ O

I

I

R2

R1

OH

Peptide bond cleaved Tripeptidases

O II

O

H2N-CI H-

N H - C HI- C

R1 R 1, R2, R 3 - any amino acid residue

R2

,O

NH-CH-C// I "

R3

OH

Choice of bond cleaved depends on specific enzyme. In a given molecule, only one bond is cleaved by a tripeptidase.

*Trypsin, chymotrypsin, elastase, and pepsin are endopeptidases (trypsin is the most specific and pepsin is the least specific)" carboxypeptidaseA and B and aminopeptidase are exopeptidases.

amino acids. Hydrolysis of oligopeptides by the brushborder aminopeptidases releases amino acids. Leucine aminopeptidase, a ZnZ+-containing enzyme, is an integral transmembrane glycoprotein with a carbohydrate-rich hydrophilic portion and active site protruding into the luminal side. It cannot degrade proline-containing dipeptides, which are largely hydrolyzed intracellularly. Dipeptidases and tripeptidases are associated with the brush-border membranes, but their functions are not clearly understood. The major products of intraluminal digestion of protein are mixtures of amino acids (30-40%) and small peptides (60-70%). Chymotryptic activity can be measured by oral administration of N-benzoyl-L-tyrosyl-p-amino acid (Figure 12-12). On hydrolysis, p-aminobenzoic acid (PABA) and N-benzoyl-L-tyrosine are released; PABA is absorbed, conjugated to glycine by the liver, and excreted

in the urine to give an index of exocrine pancreatic function.

Absorption of Amino Acids and Oligopeptides Dipeptides and tripeptides that escape brush border membrane peptidases are actively transported against a concentration gradient by Na+-dependent mechanisms. Inside the cell they are hydrolyzed. Free amino acids are transported into enterocytes by four active, carrier-mediated, Na+-dependent transport systems remarkably similar to the system for glucose. These systems transport, respectively, neutral amino acids; basic amino acids (Lys, Arg, His) and cystine; aspartic and glutamic acids; and glycine and imino acids. Some amino acids (e.g., glycine) have affinities for more

216

CHAPTER12 Gastrointestinal Digestion and Absorption OH

1

S/,teo, c,eavaoe by chymotrypsin

mNHmCH uC--NH

COOH

N-Benzoyl-L-tyrosyl-p-aminobenzoicacid H20 OH

I

I--I=N~ C O O H

~

O

II

CH= O

I

II

C--NH --CH --C--OH N-BenzoyI-L-tyrosine

p-Aminobenzoic acid (PABA) Absorbed and excretedin urine as p-aminobenzoylglycine

F I G U R E 12-12 Hydrolysis of a synthetic tripeptide by chymotrypsin in the small intestine. The hydrolysis yields PABA, which is absorbed and eventually excreted in the urine. The concentration of PABA is a measure of exocrine pancreatic function.

than one transport system; within each group, amino acids compete with each other for transport. These systems are specific for L-Amino acids. D-Amino acids are transported by a passive diffusion process. Amino acid transport in the renal cells may use similar systems (Chapter 39). Entry of amino acids into cell compartments elsewhere in the body may require different transport systems (e.g., the y-glutamyl cycle, Chapter 17). In the enterocyte, amino acids may be metabolized or transported to the liver. Glutamate, glutamine, aspartate, and asparagine are metabolized in the enterocyte (Chapter 17). Small amounts of protein (e.g., dietary, bacterial, and viral) may be absorbed intact by nonselective pinocytosis. This absorption may be more common in neonatal life. Absorption of food proteins (or antigenic peptides from them) can cause allergic manifestations, whereas bacterial and viral antigens stimulate immunity by production and secretion of secretory IgA (Chapter 35). Since thyrotropin-releasing hormone (pGlu-His-Pro-NH2) is resistant to hydrolysis, it is effective if taken orally (Chapter 33).

Disorders of Protein Digestion and Absorption The principal causes of protein maldigestion and malabsorption are diseases of the exocrine pancreas and

small intestine. Primary isolated deficiency of pepsinogen or pepsin, affecting protein assimilation, has not been described. Deficiencies of trypsinogen and enteropeptidase are rare. Defects in neutral amino acid transport (Hartnup disease), in basic amino acids and cystine (cystinuria), dicarboxylic aminoaciduria, and aminoglycinuria have been reported. The clinical severity of these disorders is usually minimal and relates to the loss of amino acids or relative insolubility of certain amino acids in the urine. In cystinuria, for example, cystine can precipitate in acidic urine to form stones. In Hartnup disease, severe nutritional deficiencies are uncommon, since the essential amino acids are absorbed as dipeptides or oligopeptides. Tryptophan, a precursor of nicotinamide (a vitamin; see Chapter 38), and NAD + and NADP + (Chapter 17) are lost in this disease; skin and neuropsychiatric manifestations characteristic of nicotinamide deficiency respond to oral nicotinamide supplementation.

Lipids Dietary fat provides energy in a highly concentrated form and accounts for 40--45% of the total daily energy intake (100 g/day in the average Western diet). Lipids contain more than twice the energy per unit mass than carbohydrates and proteins (Chapter 5). The efficiency of fat absorption is very high; under normal conditions, almost all ingested fat is absorbed, with less than 5% appearing in the feces. The predominant dietary lipid is triacylglycerol (previously called triglyceride), which contains three long-chain (16-carbon or longer) fatty acids (Chapter 18). The dietary lipids include essential fatty acids (Chapter 18) and the lipid-soluble vitamins A, D, E, and K (Chapters 36-38). Digestion and absorption of lipids involves the coordinated function of several organs but can be divided into three phases: luminal, intracellular, and secretory. Intraluminal Phase

Digestion of lipid in the mouth and stomach is minimal. However, lipases secreted by lingual glands at the base of the tongue are active at acid pH and initiate the hydrolysis of triacylglycerol without a requirement for bile acids. The fatty acids released'stimulate the release of CCK and a flow of bile and pancreatic juice. The free fatty acids also stabilize the surface of triacylglycerol particles and promote binding of pancreatic colipase. This phase aids in the optimal action of pancreatic lipase and is particularly important in disorders of pancreatic function or secretion (e.g., in prematurity, cystic fibrosis, congenital deficiency of pancreatic lipase).

SECTION12.3 Digestion and Absorption of Major Food Substances

217

O

1

II

H=?2--O--C--R II R--C--O---CH O O

I~

II

0

Pancreatic

H2?--OH

II R--C--O--CH

,lipase

+ 21"120

l

+ 2RCOO- + 2H+

Fatty acid

H2C--OH 2-Monoacylglycerol

H2C--O--C--R

Triacylglycerol

O

1 ~)

II

O

H~?2--O---C-- R,

R~---C--O~H

3

O

II

II

CH3

I+

H --O--P--O-I_ CH2---CH2--'?-CH3 O

H2?--O--C--R, phospholipase A2 + H20

CH 3

Phosphatidycholine

HC--OH O

CH~

II I§ H~C--O--l~yO-- CHF--CHF-- ? --CH3 I

O Lysophosphatidylcholine

+ ~COO-+ H+ Fatty acid

CH 3

F I G U R E 12-13 Hydrolysis of triacylglycerol and phosphatidylcholine by pancreatic lipase and phopholipase A2, respectively.

The major functions of the stomach in fat digestion are to store a fatty meal, to contribute to emulsification by the sheafing actions of the pylorus, and to gradually transfer the partially digested emulsified fat to the duodenum by controlling the rate of delivery. The hydrolysis of triacylglycerol in the duodenum and jejunum requires bile and pancreatic juice. Bile acids are powerful detergents that, together with monoacylglycerol and phosphatidylcholine, promote the emulsification of lipids. Emulsification is also aided by the churning action of the GI tract which greatly increases the area of the lipid-water interface that promotes the action of pancreatic lipase. This triacylglycerol lipase hydrolyzes ester linkages at the 1- and 3-positions to yield 2-monoacylglycerol and fatty acids (Figure 12-13). The products of digestion are relatively insoluble in water but are solubilized in micelles. Micelles also contain lipidsoluble vitamins, cholesterol, and phosphatidylcholine. Pancreatic lipase functions at the lipid-water interface, its activity being facilitated by colipase (M.W. ~ 10,000), also secreted by the pancreas as procolipase activated by tryptic hydrolysis of an Arg-Gly bond in the N-terminal region. Colipase anchors the lipase to the triacylglycerol emulsion in the presence of bile salts by forming a 1:1 complex with lipase and protects lipase against denaturation. Colipase deficiency (with normal lipase) is accompanied by significant lipid malabsorption, as is pancreatic lipase deficiency. Pancreatic juice contains esterases that act on short-chain triacylglycerols and do not require bile salts, as well as cholesteryl esterase. Phosphatidylcholine in the diet (4-8 g/day) and in bile secretions (17-22 g/day) is hydrolyzed to lysophosphatidylcholine and fatty acid by

phospholipase A2, a pancreatic enzyme with an absolute requirement for Ca 2+ ions and for bile acids. The secreted form, prophospholipase A2, is activated by tryptic hydrolysis of an Arg-Ala bond in the N-terminal region. Phospholipase A2 also hydrolyzes fatty acids esterified at the 2-position of phosphatidylethanolamine, phosphatidylglycerol, phosphatidylserine, phosphatidylinositol, and cardiolipin but has no effect on sphingolipids. Lipid absorption in the duodenum and jejunum appears to be a passive diffusion process. Lipid-laden micelles migrate to the microvilli, and the fatty acids, monoacylglycerols, and lysophosphoglycerols are transferred across the membrane according to their solubility within the lipid bilayer of the cell surfaces. Bile acids are not absorbed into the enterocyte but migrate to the ileum, where they are actively absorbed and transferred to the liver via the portal venous system. The bile acid pool is recycled several times daily (enterohepatic circulation) to meet the demands of lipid digestion, and disorders that interfere with this process lead to malabsorption of lipids. A cytoplasmic fatty acid-binding protein with high affinity for long-chain fatty acids transports fatty acids to the smooth endoplasmic reticulum for resynthesis of triacylglycerol. Digestion and absorption of triacylglycerols with medium-chain fatty acids (~ LDH-2

Oxidized NBT ~

Reduced NBT

Elevation of LDH-5

(colored and insolublel

As long as the total activity of LDH is not very high so that the substrates are not limiting, the intensity of the color is approximately proportional to the various isoenzyme activities. Thus, densitometric scanning of an electropherogram provides an estimate of the activities of the individual enzymes as well as quantitation of their relative intensities. LDH-1 is the most negatively charged and fastest moving isoenzyme (mobility comparable to that of the ot 1-globulin region in serum protein electrophoresis). LDH-5 is the least negatively charged and slowest migrating isoenzyme (mobility comparable to that of ),-globulin); the other isoenzymes possess intermediate mobilities. The normal relative intensity of LDH isoenzyme fractions in serum is LDH-1 < LDH-2 > LDH-3 > LDH-4 < = > LDH-5 Since myocardium is rich in LDH-1, injury to that tissue results in the elevation of LDH- 1 and the ratio of LDH- 1 to LDH-2; similarly, since liver is rich in LDH-4 and LDH-5, elevations of these isoenzymes suggests liver disease. Because a particular isoenzyme pattern may result from injury to more than one tissue (Table 13-3), further diagnostic tests are required. Forexampte, measurement of creatine kinase and its isoenzyme determination and troponin I in the serum is helpful in the evaluation of acute injury to the myocardium. Because of differences in halflives in serum, serial determination of total enzyme activity and relative isoenzyme composition aids in the diagnosis and assessment of the magnitude of the injury of the tissue in question (Chapter 8). In patients with acute myocardial infarction, serial analysis every 8-12 hours during the first 48 hours after the onset of symptoms is useful. In vitro, LDH-I(H4) has a low Km for pyruvate and is strongly inhibited by high concentration of pyruvate. This form may favor the oxidation of lactate to pyruvate in aerobic tissues such as the heart. LDH-5(M4) exhibits a higher Km value for pyruvate and is less susceptible to inhibition by high concentrations of pyruvate than is LDH-1.

Elevation of LDH-3, frequently LDH-3 > LDH-2 Elevation of LDH-2 and LDH-3 All isoenzymes elevated

Disorder Myocardial infarction Renal cortical infarction Pernicious anemia Hemolysis Muscular dystrophy (later stages) Liver disease Skeletal muscle damage Some cancers Some neoplastic diseases Lymphoproliferative disorders Platelet-related disorders Pulmonary infarction Widespread tissue injury

*Normal distribution: LDH-I < LDH-2 > LDH-3 > LDH-4 < = > LDH-5

This form may occur in anaerobic tissues, in which lactate is the end product of glycolysis. However, in vitro differences in kinetic properties of the isoenzyme may be inappropriate to explain actual physiological actions for several reasons: differences in kinetic properties between LDH-1 and LDH-5 are less marked at 37~ (body temperature) than at 25~ high intracellular concentrations of the enzyme are present, and differences in the actual ratio of ketopyruvate to enolpyruvate exist (the enol form may be the more potent inhibitor). Furthermore, the occurrence of similar isoenzyme patterns in widely different tissues with divergent metabolic goals (e.g., LDH-5 in liver and muscle; LDH-1 in heart and erythrocytes) points out our tack of understanding of their precise role.

Alternative Substrates of Glycolysis The glycolytic pathway is also utilized by fructose, galactose, mannose, glycogen, and glycerol. The metabolism of the monosaccharides and glycogen is discussed in Chapter 15. Glycogenolysis catalyzed by phosphorylase in a nonequilibrium reaction yields glucose-1-phosphate, which is then converted to glucose-6-phosphate (Chapter 15), bypassing the initial phosphorylation reaction of glycolysis. Therefore, the conversion of a glucosyl unit of glycogen to two lactate molecules yields three ATP, which is 50% more than the yield from a glucose molecule. Glycerol released by hydrolysis of

SECTION 13.2

235

Pyruvate Metabolism

triacylglycerol enters the glycolytic pathway by way of dihydroxyacetone phosphate, as follows: Glycerol + ATP4-

glycerol kinase, Mg2+

> glycerol 3-phosphate2+ ADP3- + H+ glycerol-3-phosphate dehydrogenase

Glycerol 3-phosphate2- + NAD+

dihydroxyacetone phosphate2- + NADH + H+

Role of Anaerobic Glycolysis in Various Tissues and Cells In tissues that lack mitochondria or function under limiting conditions of oxygen (e.g., lens and erythrocytes), glycolysis is the predominant pathway providing ATE On the basis of speed of contraction and metabolic properties, skeletal muscle fibers may be classified into three types: type I (slow-twitch, oxidative), type IIA (fast-twitch, oxidativeglycolytic), and type IIB (fast-twitch, glycolytic). Shortterm, sudden energy output is derived from glycolytic fibers. The glycolytic fibers (particularly type IIB) have few mitochondria, with relative enrichment of myofibrils and poor capillary blood supply. Thus, these fibers can perform a large amount of work in a short period of time, in contrast to slow-twitch fibers, which are rich in mitochondria, have good capillary blood supply, and power sustained efforts (Chapter 21). A high rate of glycolysis also occurs in lymphocytes, kidney medulla, skin, and fetal and neonatal tissues. The anaerobic capacity of lymphocytes is increased for growth and cell division. Kidney medulla (Chapter 39), which contains the loops of Henle and collecting tubules, receives much less blood than the cortex and is rich in glycogen. In contrast, kidney cortex, which contains the glomeruli, proximal tubules, and parts of the distal tubules, receives a large amount of blood through the renal arterioles. The renal cortex derives its energy from oxidation of glucose, fatty acids, ketone bodies, or glutamine to CO2 and H20. Some rapidly growing tumor cells exhibit a high rate of glycolysis. Inadequate oxygen supply (hypoxia) causes tumor cells to grow more rapidly than the formation of blood vessels that supply oxygen. Thus, in hypoxic tumor cells, glycolysis is promoted by increased expression of glucose transporters and most glycolytic enzymes by hypoxia-inducible transcription factor (HIF- 1).

reduce their normal 120-day life span. For example, deficiency of hexokinase, glucose-phosphate isomerase, 6-phosphofructokinase, aldolase, triose-phosphate isomerase, or phosphoglycerate kinase is associated with hemolytic anemia, whereas lactate dehydrogenase deficiency is not. The most common deficiency is that of pyruvate kinase, (PK), which is inherited as an autosomal recessive trait. The PK deficiency occurs worldwide, however, it is most commonly found in kindreds of Northern European ancestry. Erythrocyts of PK individuals have elevated levels of 2.3-DPG and decreased ATP level. Several mutations of the PK gene have been identified. Individuals with PK deficiency suffer from lifelong chronic hemolysis to a varying degree. Splenectomy ameliorates hemolytic process in severe cases. Some enzymopathies of erythrocytes may be associated with multisystem disease (e.g., aldolase deficiency with mental and growth retardation). Individuals with 6-phosphofructokinase deficiency exhibit hemolysis and myopathy and have increased deposition of muscle glycogen (a glycogen storage disease; see Chapter 15). The myopathy is usually characterized by muscle weakness and exercise intolerance. (See also Chapters 10, 15, and 28.)

13.2 Pyruvate Metabolism Pyruvate has several metabolic fates. It can be reduced to lactate, converted to oxaloacetate in a reaction important in gluconeogenesis (Chapter 15) and in an anaplerotic reaction of the TCA cycle (see below), transminated to alanine (Chapter 17), or converted to acetyl-CoA and CO2. Acetyl-CoA is utilized in fatty acid synthesis, cholesterol (and steroid) synthesis, acetylcholine synthesis, and the TCA cycle (Figure 13-6).

ALANINE

GLUCOSE

y/Glycolysis PYRUVATE

OXALOACETATE

-.,_

ACETYL CoA - ~

Gluconeogenesis

Glycolytic Enzyme Deficiencies in Erythrocytes The only pathway that provides ATP in mature erythrocytes is glycolysis. Because these cells lack mitochondria, a nucleus, and other organelles required for protein synthesis, deficiency of glycolytic enzymes may

GLUCOSE

~

~ LACTATE

Fatty acids

~ TCA cycle CO 2 + H20

FIGURE 13-6 Major pathwaysof pyruvatemetabolism. Pyruvateis metabolizedthrough four majorenzymepathways:Lactatedehydrogenase(LDH),pyruvate dehydrogenasecomplex(PDH),pyruvatecarboxylase(PC), and alanine aminotransferase (ALT). Arrowsindicate multiple steps.

~36

CHAPTER13

Conversion of pyruvate to ethanol by certain yeast strains occurs in two steps. It is first decarboxylated to acetaldehyde by pyruvate decarboxylase, which utilizes thiamine pyrophosphate (TPP) as coenzyme. m g 2+

CH3COCOOH - - + CH3CHO + C02 TPP

In the second step, acetaldehyde is reduced to ethanol by alcohol dehydrogenase, an NAD-dependent enzyme. CH3CHO + NADH + H + ~ CH3CH2OH + NAD + The NADH is derived from glyceraldehyde 3-phosphate dehydrogenation. Thus, yeast fermentation yields ethanol rather than lactate as an end product of glycolysis. Small amounts of ethanol are produced by the microbial flora of the gastrointestinal tract. Other types of fermentation using similar reactions occur in microorganisms and yield a variety of products (e.g., acetate, acetone, butanol, butyrate, isopropanol, hydrogen gas). Lactic Acidemia and Lactic Acidosis

As discussed earlier, some tissues produce lactate as an end product of metabolism. The lactate produced is L-lactate and is commonly referred to simply as lactate. As we will see in the following discussion, D-lactate is also produced under certain pathological conditions, which presents a unique clinical problem. Under normal conditions, lactate is metabolized in the liver and the blood lactate level is between 1 and 2 mM. Lactate accumulation in body fluids can be due to increased production and/or decreased utilization. Blood lactate-to-pyruvate ratio below 25 suggests defects in a gluconeogenic enzyme (Chapter 15) or pyruvate dehydrogenase (discussed later). A common cause of lactic acidosis is tissue hypoxia caused by shock, cardiopulmonary arrest, and hypoperfusion. Inadequate blood flow leads to deprivation of oxygen and other nutrients to the tissue cells as well as to the removal of waste products. Oxygen deprivation leads to decreased ATP production and accumulation of NADH, which promotes conversion of pyruvate to lactate. A major cause of acidosis that occurs during inadequate cellular oxygen delivery is continual hydrolysis of the available supply of ATP that releases protons:

Carbohydrate Metabolism I: Glycolysis and TCA Cycle

D-Lactic Acidosis

This unusual form of lactic acidosis is due to increased production and accumulation of D-lactate in circulation. The normal isomer synthesized in the human body is L-lactate but the D-lactate isomer can occur in patients with jejunoileal bypass, small bowel resection, or other types of short bowel syndrome. In these patients, ingested starch and glucose bypass the normal metabolism in the small intestine and lead to increased delivery of nutrients to the colon where gram-positive, anaerobic bacteria (e.g., Lactobacilli) ferment glucose to D-lactate. The D-lactate is absorbed via the portal circulation. A limited quantity of D-lactate is converted to pyruvate by a mitochondrial flavoprotein enzyme D-2-hydroxy acid dehydrogenase. Thus, the development of D-lactate acidosis requires excessive production of D-lactate and an impairment in its metabolism. The clinical manifestations of D-lactic acidosis are characterized by episodes of encephalopathy after ingestion of foods containing carbohydrates. The diagnosis of D-lactic acidosis is suspected in patients with disorders of the small intestine causing malabsorption and when the serum anion gap (Chapter 39) is elevated in the presence of normal serum levels of L-lactate and other organic acids. Measurement of serum D-lactate requires special enzymatic procedures utilizing D-lactate dehydrogenase and NADH. As D-lactate is converted to pyruvate, NADH is oxidized to NAD + which is detected spectrophotometrically (Chapter 8). The treatment of D-lactic acidosis consists of oral administration of antibiotics, limitations of oral carbohydrate intake, and recolonization of the colon by bacterial flora which do not produce D-lactate.

Oxidation of Pyruvate to Acetyl-CoA Pyruvate must first be transported into mitochondria by a specific carrier that cotransports a proton to maintain electrical neutrality. Inside mitochondria pyruvate undergoes oxidative decarboxylation by three enzymes that function sequentially and are present as a complex known as the pyruvate dehydrogenase complex. The overall reaction is physiologically irreversible, has a high negative A G O' ( - 8 . 0 kcal/mol, or - 3 3 . 5 kJ/mol), and commits pyruvate to the formation of acetyl-CoA:

ATP 4- + H20 --+ ADP 3- + HPO 2- + H + Pyruvate + NAD + + CoASH --> Laboratory assessment includes measurements of blood lactate, pyruvate, /~-hydroxybutyrate, and acetoacetate (Chapter 39). The primary treatment of lactic acidosis involves correcting the underlying cause such as reversal of circulatory failure.

acetyl-CoA + NADH + H + + CO2 In the above reaction CoASH stands for coenzyme A (Figure 13-7), and it contains the vitamin pantothenic acid (Chapter 38). Coenzyme A functions as a carrier of

SECTION13.2 Pyruvate Metabolism

237

NH, 4"-Phosphopantetheine

HI

'

I 3 CH

I O-

HS--OH i--'CH [-- N H - - I - - CH T'- CH2"--"N H - - OII - - CI- - C -I - C H ' - - O - ,

I,

O OH CH3 Pantothenic acid

O

I~-Mercaptoethylamine

I ' O-

H

P II - - O - - I - - O - - C H ' , O

N ~NtC'N~CH

O

O

[ 7

Adenine

]

-

H H

Ribose 3"-phosphate

OH

I

O--P--O-

I_

0 F I G U R E 13-7 Coenzyme A. The terminal sulfhydryl group is the reactive group of the molecule.

acetyl (and acyl groups) by formation of thioesters, and the acetyl-sulfur bond has a AG o' comparable to that of the phosphoanhydride bond of ATE Dehydrogenation and decarboxylation of pyruvate occur in five reactions requiring three enzymes and six coenzymes, prosthetic groups, and cofactors (Mg 2+, TPP, lipoic acid, CoASH, flavin adeninedinucleotide [FAD], and NAD+). The mammalian complex (M.W. 7-8.5 x 106) consists of 20 or 30 units of pyruvate dehydrogenase (depending on the source), 60 units of dihydrolipoyl transacetylase (also called dihydrolipoamide acetyltransferase), and 5 or 6 units of dihydrolipoyl dehydrogenase (also called dihydrolipoamide reductase). A kinase and a phosphatase are tightly bound to the pyruvate dehydrogenase subunit and participate in the regulation of the activity. In the first reaction, pyruvate is decarboxylated by reaction with TPP bound to pyruvate dehydrogenase (El), with formation of a hydroxyethyl group attached to the thiazole ring of TPP:

H CH3--C--TPP--E1 +

I

OH E2

H:S~ O---CI ICH

E2

H

I

CH3--C--TPP--E1 + CO2

I

OH

In the second step, also catalyzed by pyruvate dehydrogenase, the hydroxyethyl group is transferred to the oxidized form of the lipoyl-lysyl prosthetic group of dihydrolipoyl transacetylase (E2):

E2

A detailed mechanism of the first two reactions is shown in Figure 13-8. In the third step, catalyzed by dihydrolipoyl transacetylase, the acetyl group attached to E2 is transferred to CoASH, with formation of the dithiol form of the lipoyl group and acetyl-CoA.

S-- --OH 3

O II CH3--C--COO- + TPP--E1

+ E1--TPP

+co,sH . SH

O

II

SH

O

E2

+ CHaC--S--CoA AcetyI-CoA

In the fourth step, catalyzed by the same enzyme, the hydrogen atoms of the dithiol group of E2 are transferred to the FAD prosthetic group of dihydrolipoyl dehydrogenase (E3).

SH + E3__FAD SH E2

;

+ E3__FADH2 E2

CHAPTER13 Carbohydrate Metabolism I: Glycolysis and TCA Cycle

238

NH`, i 9

H3C~, .

(i)~1 TPP , .....CH~_.._CHi___.O~PmO~PmO-

N~-~'" ,"h----- CH2--"--N,

I ""-...

O

O

s

H3C-~~N J

~IT H

(removal of this hydrogen forms an ylide that acts as a nucleophilicreagent)

~

R~N.+

"~Thiazole ring R`, ~

1

\\ ~ .~., S ~ 8 +7 ~ H3C--C--COO ~ Pyruvate

+H+

\ /~ -R~----N~ +/ \~ yS

/ ,o+"~'Y

- R`,

H3C~ C- - C~ O -

OH I

CO,, R._._N~) s

R

~

Ri--

H3CJ'C~OH

o~-HydroxyethyI-TPP (active acetaldehyde)

H3C~ C m OH

/s _/~,s\

~.......~~o

/~

S

*

oII

..CH-- (CH2);--C--N--Enzyme ~C ~ H H= Oxidized lipoic acid

'a`,

a`,

9.

I

s

"s

I

/ CIH ~

H~CH ~

o

(CH`,)----~--N--Enzyme H

S

I

H

S

I

S

I o H`, .CH II CH"~2~(CH`,),---C--N -- Enzyme

I

H S-Acetylhydrolipoyl-enzyme F I G U R E 13-8 Steps involved in the decarboxylation and formation of the S-acetylhydrolipoyl enzyme of the dehydrogenase complex. TPP = Thiamine pyrophosphate.

The structure of FAD that contains the vitamin riboflavin is given in Figure 13-9. In the last step, catalyzed by dihydrolipoyl dehydrogenase, the hydrogens (or reducing equivalents) are transferred to NAD +, with the formation of NADH and the oxidized flavoprotein. E3-FADH2 + NAD + ~ E3-FAD + NADH + H +

Thus, the flow of reducing equivalents in the pyruvate dehydrogenase complex is from pyruvate to lipoyl to FAD to NAD +. Conversion of pyruvate to acetyl-CoA requires four vitamins: thiamine, pantothenic acid, riboflavin, and niacin. In contrast, in glycolysis, niacin is the only vitamin used. NADH generated in this reaction is oxidized to NAD + in the electron transport

SECTION13.2 Pyruvate Metabolism

239 NH=

_

o II

O--P-0I i ~

_

o

I

o--P-=-~

1

I

OH

?

OH

H~OH HCOH I

Riboflavin

H~OH

~

H2

9

10

I

~

~.NH 0

F I G U R E 13-9 Flavin adenine dinucleotide (FAD).

chain, with the production of ATP (Chapter 14). Dihydrolipoyl transacetylase plays a central role in transferring hydrogen atoms and acetyl groups from one enzyme to the next in the pyruvate dehydrogenase complex. This is made possible by the lipoyl-lysyl swinging arm of the transacetylase, which is about 1.4 nm long (Figure 13-10). In a similar way, other o~-keto acids, e.g., ot-ketoglutarate (in the TCA cycle; see below) and branched-chain o~-keto acids derived by transamination from the branched-chain amino acids valine, leucine, and isoleucine (Chapter 17), undergo decarboxylation and dehydrogenation catalyzed by enzyme complexes. These enzyme complexes differ in specificity of E1 and E2, but all contain the same E3 (the dihydrolipoyl dehydrogenase).

Regulation of Pyruvate DehydrogenaseActivity The pyruvate dehydrogenase complex catalyzes an irreversible reaction that is the entry point of pyruvate into the TCA cycle (see below) and is under complex regulation by allosteric and covalent modification of the pyruvate dehydrogenase component of the complex. The end products of the overall reaction (NADH and acetyl-CoA) are potent allosteric inhibitors of the pyruvate dehydrogenase

F I G U R E 13-10 Schematic representation of the relationship between the three enzymes of the pyruvate dehydrogenase complex. The lipoyl-lysyl moiety of the transacetylase delivers the acetyl group to CoA and the reducing equivalents to FAD. TPP = Thiamine pyrophosphate.

component of the complex. They also function as effectors in a non-cAMP-dependent reversible phosphorylation and dephosphorylation cycle of the dehydrogenase. Phosphorylation occurs by an ATP-specific pyruvate dehydrogenase kinase at three serine residues of the o~-subunit of the enzyme and leads to inactivation. The kinase is activated by elevated [acetyl-CoA]/[CoA], [NADH]/[NAD +], and [ATP]/[ADP] and inhibited by increases in [pyruvate], [Ca2+], and [K+]. The phospho enzyme is converted to the active dephospho enzyme by a pyruvate dehydrogenase phosphatase, an MgZ+/CaZ+-stimulated enzyme that is also stimulated by insulin in adipocytes. The kinase and phosphatase are associated with pyruvate dehydrogenase. The regulation of pyruvate dehydrogenase is shown in Figure 13-11. In general, when the levels of ATE NADH, or acetyl-CoA are high, the oxidation of pyruvate to acetyl-CoA is markedly decreased. For example, fatty acid oxidation (Chapter 18) provides all three metabolites and thus decreases the need for pyruvate oxidation. An analogue of pyruvate, dichloroacetate (CHClzCOO-), inhibits pyruvate dehydrogenase kinase and maintains the pyruvate dehydrogenase complex

CHAPTER 13 CarbohydrateMetabolismI: GlycolysisandTCACycle

240

[ATP] [NADH][AcetylCoA] [A~P] [NAD"].~A-]~ ,.m../Pyruvate ~P Oki~naD/ i c h l ~ 1 7 6 a2+ DephosphoPhosphopyruvate pyruvate dehydrogenase dehydrogenase (acti~~ ( i n a c t i v e ) Phospho-PDphosphatase Ca2+,~(~2+ i~nsulin , NADHj+ H+ F I G U R E 13-11 Regulation of pyruvate dehydrogenase (PD) by inactivation and reactivation by a non-cAMP-dependent phosphorylation-dephosphorylation cycle. Although PD kinase phosphorylates three specific seryl residues in the c~-subunit of PD, phosphorylation at any of these sites inactivates PD. The kinase and the phosphatase are under the influence of several regulators, and the dephospho-active PD is also regulated by end products. (~ = Activation; Q = inhibition; E2 -- dihydrolipoyl transacetylase; E3 -- dihydrolipoyl dehydrogenase.

in the active state. In experimental animals and in humans with lactic acidemia due to a variety of causes, dichloroacetate administration lowers the concentration of lactate through promotion of pyruvate oxidation. Compounds like dichloroacetate have potential applications in disorders associated with lactic acidemia (e.g., diabetes mellitus).

Abnormalities of Pyruvate Dehydrogenase Complex Thiamine deficiency causes decreased pyruvate oxidation, leading to accumulation of pyruvate and lactate, particularly in the blood and brain, and is accompanied by impairment of the cardiovascular, nervous, and gastrointestinal systems (Chapter 38). Inherited deficiency of pyruvate dehydrogenase complex is accompanied by lactic acidemia and abnormalities of the nervous system (e.g., ataxia and psychomotor retardation). Pyruvate carboxylase deficiency causes similar abnormalities (Chapter 15). Both inherited disorders of pyruvate utilization are autosomal recessive. Nervous system abnormalities may be attributed in part to diminished synthesis of neurotransmitters rather than to inadequate synthesis of ATE In pyruvate dehydrogenase complex deficiency, diminished levels of acetylCoA cause decreased production of acetylcholine; in pyruvate carboxylase deficiency, decreased production of

oxaloacetate may lead to deficiency of amino acid neurotransmitters (e.g., F-aminobutyrate, glutamate, aspartate). The interrelationships of these amino acids are discussed in Chapter 17. Although no particular therapeutic method is well established in treatment of the inherited disorders of pyruvate metabolism, ketogenic diets have been beneficial in pyruvate dehydrogenase complex deficiency, since they provide the product of the deficient reaction (acetyl-CoA). Administration of large doses of thiamine may be of benefit because mutations of pyruvate dehydrogenase complex may give rise to decreased affinity for thiamine pyrophosphate. In one patient, administration of dichloroacetate was beneficial even in the absence of a ketogenic diet. In pyruvate carboxylase deficiency, administration of diets supplemented with aspartate and glutamate demonstrated sustained improvement in neurological symptoms. These two amino acids cross the blood-brain barrier after amidation to asparagine and glutamine in nonneural tissues. The toxicity of organic arsenicals and arsenite (AsO~-)is due to their abiltiy to bind functional sulfhydryl groups of enzymes. One important target is the dithiol form of the lipoyl group of pyruvate dehydrogenase and ot-ketoglutarate dehydrogenase complexes. In the earlier times arsenicals were used in the treatment of syphilis, however due to their toxicity it has been replaced with better drugs such as penicillin.

SECTION 13.3 Tricarboxylic Acid (TCA) Cycle

13.3 Tricarboxylic Acid (TCA) Cycle

241 SCoA

I

*C=O

I

The tricarboxylic acid (TCA) cycle (also called the citric acid cycle or Krebs cycle) consists of eight enzymes that oxidize acetyl-CoA with the formation of carbon dioxide, reducing equivalents in the form of NADH and FADH2, and guanosine triphiosphate (GTP): CH3COSCoA + 3NAD + + FAD + GDP 3- + HPO 2+ 2H20 --+ CoASH + 2CO2 + 3NADH + FADH2 + GTP 4- + 2H + The reducing equivalents are transferred to the electron transport chain, ultimately to reduce oxygen and generate ATP through oxidative phosphorylation (Chapter 14). Thus, oxidation of acetyl-CoA yields a large supply of ATE Since the intermediates in the cycle are not formed or destroyed in its net operation, they may be considered to play catalytic roles. However, several intermediates are biosynthetic precursors of other metabolites (amphibolic role) and hence may become depleted. They are replenished (anaplerotic process) by other reactions to optimal concentrations. The TCA cycle is the major final common pathway of oxidation of carbohydrates, lipids, and proteins, since their oxidation yields acetyl-CoA. AcetylCoA also serves as precursor in the synthesis of fatty acids, cholesterol, and ketone bodies. All enzymes of the cycle are located in the mitochondrial matrix except for succinate dehydrogenase, which is embedded in the inner mitochondrial membrane. Thus, the reducing equivalents generated in the cycle have easy access to the electron transport chain. TCA cycle enzymes, with the exception of ot-ketoglutarate dehydrogenase complex and succinate dehydrogenase, are also present outside the mitochondria. The overall TCA cycle is shown in Figure 13-12.

~CH 3

AcetyI-CoA

tl

o

II

H2o

+ C--COO-

L

I

+

, HO--C--COO-

I CH 2--0OO

-

OH 2__000

Oxaloacetate

-

Citrate

The condensation reaction is thought to yield a transient enzyme-bound intermediate, citroyl-CoA, which undergoes hydrolysis to citrate and CoASH with loss of free energy. This reaction is practically irreversible and has a AG o' of - 7 . 7 kcal/mol ( - 3 2 . 2 kJ/mol). Formation of citrate is the committed step of the cycle and is regulated by allosteric effectors. Depending on the cell type, succinylCoA (a later intermediate of the cycle), NADH, ATE or long-chain fatty acyl-CoA functions as the negative allosteric modulator of citrate synthase. Citrate formation is also regulated by availability of substrates, and citrate is an allosteric inhibitor. Citrate provides the precursors (acetyl-CoA, NADPH) for fatty acid synthesis and is a positive allosteric modulator of acetyl-CoA carboxylase, which is involved in the initiation of long-chain fatty acid synthesis (Chapter 18). It regulates glycolysis by negative modulation of 6-phosphofructokinase activity (see above). All of the above reactions occur in the cytoplasm, and citrate exits from mitochondria via the tricarboxylate carrier. Isomerization of Citrate to Isocitrate

In this reaction, the tertiary alcoholic group of citrate is converted to a secondary alcoholic group that is more readily oxidized. The isomerization catalyzed by aconitate dehydratase (or aconitase) occurs by removal and addition of water with the formation of an intermediate, cis-aconitate: CH2_CO O-

I HO--C--COOCH2__CO O-

Citrate

"CH2--COO-" H20

)

. CI- - C O 0 II

H20 k._

.

CH--COOcis-Aconitate

Condensation of Acetyl-CoA with Oxaloacetate to

CH2--COO-

Form Citrate

CH--COO-

The first reaction of the TCA cycle is catalyzed by citrate synthase and involves a carbanion formed at the methyl group of acetyl-CoA that undergoes aldol condensation with the carbonyl carbon atom of the oxaloacetate:

CoASH+ H +

I

I

Reactions of T C A Cycle

CH2--~OO-

I I HO--CH--CO0Isocitrate

The cis-aconitate may not be an obligatory intermediate, since the enzyme presumably can isomerize

~2

CHAPTER13 Carbohydrate Metabolism I: Glycolysis and TCA Cycle

F I G U R E 13-12 Reaction of the tricarboxylic acid (TCA) cycle.

citrate to isocitrate directly. Aconitate dehydratase has an iron-sulfur center, the exact function of which is not known. The reaction is considered to be near-equilibrium, even though under physiological conditions the ratio of citrate to isocitrate is about 9:1 owing to rapid conversion of isocitrate to subsequent products of the cycle. Although citrate is a symmetrical molecule, its two -CHzCOOH groups are not identical with regard to the orientation of t h e - O H a n d - C O O H groups. Thus, citrate reacts in an asymmetrical manner with the enzyme, so that only t h e - C H z C O O H group derived from oxaloacetate is modified. A three-point attachment of the enzyme to the substrate accounts for the asymmetrical nature of the reaction.

A powerful competitive inhibitor (highly toxic) of aconitate dehydratase is fluorocitrate, an analogue of citrate:

CHF--COO-

I

HO--CH--COO-

I CH2--CO0-

In vivo, fluorocitrate can be formed by condensation of fluoroacetyl-CoA with oxaloacetate by action of citrate synthase. Fluoroacetyl-CoA itself is synthesized from fluoroacetate by acetyl-CoA synthase.

SECTION13.3

Tricarboxylic Acid (TCA) Cycle

243

Oxidative Decarboxylation of lsocitrate

CH2_CO O-

I

to ~-Ketoglutarate Isocitrate dehydrogenase catalyzes the first of two decarboxylations and dehydrogenations in the cycle. Three different isocitrate dehydrogenases are present: one specific for NAD + and found only in mitochondria, the other two specific for NADP + and found in mitochondria and cytoplasm. The NAD+-specific enzyme is the primary enzyme with regard to TCA cycle operation. All three require Mg 2+ or Mn 2+. The reaction yields o~-ketoglutarate (2-0xoglutarate), NAD(P)H, and CO2 and involves enzyme-bound oxalosuccinate as an intermediate. CH2--COONAD +

I CH--COO

+ CoASH + NAD + - ~

CH2

NADH + H +

-

I HO--CH--COO-

Isocitrate

C H 2__CO O -

I CH--COO-

I O--C--COO -

Oxalosuccinate (enzyme-bound)

H 2C__CO O -

~

I ;

CH2

I O - - C-- C O O -

0r (2-oxoglutarate)

The reaction is nonequilibrium in type and has a AG O' o f - 5 . 0 kcal/mol ( - 2 0 . 9 kJ/mol). Although in mitochondria both NAD +- and NADP+-linked enzymes are involved in the cycle, the NAD+-linked enzyme, which is also under allosteric regulation, is the more predominant. Positive effectors are ADP and Ca 2+, and negative effectors are ATP and NADH. Thus, under conditions of abundance of energy the enzyme is inhibited, and under conditions of low energy the enzyme is stimulated.

Oxidative Decarboxylation of ~-Ketoglutarate to Succinyl-CoA The o~-ketoglutarate dehydrogenase complex is analogous to the pyruvate dehydrogenase complex and consists of three enzymes: o~-ketoglutarate dehydrogenase, dihydrolipoyl transsuccinylase, and dihydrolipoyl dehydrogenase (the latter dehydrogenase is identical in both complexes). The cofactor, prosthetic groups, and coenzyme requirements are identical to those of pyruvate oxidation.The overall reaction is

I O - - - C - - C O O

-

=-Ketoglutarate CH2--COO+ NADH + H +

I

CH2CO--SCoA

The reaction is of the nonequilibrium type, with a AG O' o f - 8 kcal/mol (-33.5 kJ/mol). Unlike the pyruvate dehydrogenase complex, the ot-ketoglutarate dehydrogenase complex does not possess a complex regulatory mechanism involving a kinase and a phosphatase. However, activity is inhibited by high ratios of [ATP]/[ADP], [succinyl-CoA]/[CoASH], and [NADH]/[NAD +] and stimulated by Ca 2+. ot-Ketoglutarate is reversibly transaminated to glutamate and is utilized in the hydroxylation of prolyl or lysyl residues of collagen (Chapter 25).

Conversion of Succinyl-CoA to Succinate Coupled to Formation of GTP In this complex reaction catalyzed by succinylCoA synthase (succinate thiokinase), the energy-rich thioester linkage of succinyl-CoA is hydrolyzed with release of free energy that is conserved in the substrate phosphorylation of GDP with phosphate ~to form GTP: CH2COO-

I

+ G DP3- + Pi2-

CH2CO-SCoA SuccinyI-CoA CH2COOI + GTP 4- + CoASH CH2COO-

Succinate

This reaction is readily reversible (a near-equilibrium reaction) and has a AG o' o f - 0 . 7 kcal/mol ( - 2 . 9 kJ/mol). It proceeds with the formation of intermediates of the enzyme with succinyl phosphate and with phosphate (Pi), the latter being linked to histidyl residue of the enzyme (E): E + succinyl-CoA + p2+ 9

-N-----

E-succinyl phosphate + CoASH E-succinyl phosphate ~-- E-phosphate + succinate E-phosphate + GDP ~- E + GTP

244

CHAPTER 13

GTP is converted to ATP by nucleoside-diphosphate kinase: GTP + ADP ~- GDP + ATP Succinyl-CoA can also be synthesized from propionylCoA by way of methylmalonyl-CoA, which is formed in the oxidation of branched-chain amino acids (e.g., valine, isoleucine) and in the terminal stage of oxidation of oddchain-length fatty acids (Chapter 18). Succinyl-CoA is utilized in the activation of acetoacetate (Chapter 18) and the formation of 6-aminolevulinate, a precursor of prophyrin (Chapter 29).

Dehydrogenation of Succinate to Fumarate Succinate is dehydrogenated to the trans-unsaturated compound fumarate by succinate dehydrogenase, an FAD enzyme: C H 2COO CH2COO-

HCCOO + E--FADH 2 , II -OOCCH

Succinate

Fumarate

I

+ E--FAD

This reaction is the only dehydrogenation reaction of the TCA cycle in which NAD + does not mediate the transport of reducing equivalents. The FAD is covalently linked to a histidyl residue of the enzyme, which is an integral protein of the inner mitochondrial membrane and contains iron-sulfur centers that undergo redox changes (Fe3++ e- ~ FEZ+). Reducing equivalents from FADH2 enter the electron transport chain at the coenzyme Q level, bypassing one of the sites of oxidative phosphorylation (Chapter 14). The enzyme is stereospecific for the trans hydrogen atoms of the methylene carbons of the substrate, so that only the trans isomer is produced (the cis isomer is maleate). Succinate dehydrogenase is competitively inhibited by malonate, the next lower homologue of succinate (Chapter 6).

Hydration of Fumarate to Malate In this reaction, water is added across the double bond of fumarate by fumarate hydratase (fumarase) to yield the hydroxy compound L-malate: HC--COO-

II -OOC--CH Fumarate

HO--CH--COO+

H20 ~

I CH2--COOH L-Malate

The enzyme has absolute specificity for the double bond of the trans-unsaturated acid and for the formation of

Carbohydrate Metabolism I: Glycolysis and TCA Cycle

L-hydroxy acid. The reaction is freely reversible (nearequilibrium).

Dehydrogenation of Malate to Oxaloacetate In the last reaction of the cycle, L-malate is oxidized to oxaloacetate by malate dehydrogenase, an NAD+-linked enzyme: HO--CHCOO-

I

+ NAD +

CH2--COOL-Malate

o II

C--COO-

+ NADH + H +

l

CH2--COO-

Oxaloacetate

Although the equilibrium of this reaction favors malate formation, in vivo the reaction proceeds toward the formation of oxaloacetate, since the latter is rapidly removed by the citrate synthase reaction to initiate the next round of the cycle.

Stereochemical Aspects of the TCA Cycle Aconitate dehydratase, succinate dehydrogenase, and fumarase yield stereospecific products. Labeling experiments with 14C in methyl and carboxyl carbons of acetylCoA or in all of the carbons of oxaloacetate yield the following results in terms of the intermediates or product formed. When only labeled acetyl-CoA is used, the label does not appear in CO2 during the first revolution of the cycle, and upon completion of the first revolution, two of the four carbon atoms of oxaloacetate are labeled. Thus, during the first revolution, neither carbon atom of acetylCoA is oxidized to CO2 and the carbon atoms of the CO2 produced are derived from oxaloacetate. This finding is due to discrimination of the two -CHzCOO- groups of citrate by aconitate dehydratase, so that citrate is treated as a chiral molecule at the asymmetrical active center of the enzyme. However, during the succeeding revolutions of the cycle, the label is randomized because succinate, a symmetrical compound, is treated as such by the enzyme without discriminating between its two carboxyl groups. If only labeled oxaloacetate is used, half of the label is retained in the oxaloacetate at the end of the first revolution of the cycle.

Amphibolic Aspects of the TCA Cycle At each turn of the TCA cycle, oxaloacetate is regenerated and can combine with another acetyl-CoA

245


molecule. The TCA cycle is amphibolic; i.e., it serves as a catabolic and an anabolic pathway. Reactions that utilize intermediates of the cycle as precursors for the biosynthesis of other molecules are listed below. Some of these reactions occur outside the mitochondria. 1. Citrate + ATP + CoA -+ acetyl-CoA + oxaloacetate + ADP + Pi. This reaction takes place in the cytoplasm and is a source of acetyl-CoA for fatty acid biosynthesis. 2. ot-Ketoglutarate + alanine ~- glutamate + pyruvate. 3. ot-Ketoglutarate --+ succinate + CO2. This reaction is involved in the hydroxylation of prolyl and lysyl residues of protocollagen, a step in the synthesis of collagen. 4. Succinyl-CoA + glycine --+ 6-aminolevulnic acid (ALA). ALA is then utilized for the synthesis of heme. 5. Succinyl-CoA + acetoacetate --+ acetoacetyl-CoA + succinate. This reaction is important in the activation of acetoacetate (a ketone body) and hence for its utilization in extrahepatic tissues. 6. Oxaloacetate + alanine ~ aspartate + pyruvate. Mg 2+

7. Oxaloacetate + GTP GDP § CO2.

> phosphoenolpyruvate +

Utilization of intermediates in these reactions leads to their depletion and hence to a slowdown of the cycle unless they are replenished by the replacement or anaplerotic reactions listed below: biotin, Mg 2+

1. Pyruvate § C O 2 § ATP > oxaloacetate § ADP + Pi. This reaction, catalyzed by pyruvate carboxylase, is the most important anaplerotic reaction in animal tissues and occurs in mitochondria. Pyruvate carboxylase is an allosteric enzyme that requires acetyl-CoA for activity (see gluconeogenesis; Chapter 15). 2. Pyruvate § C O 2 § NADPH § H + ~- L-malate § NADP + . 3. Oxaloacetate and o~-ketoglutarate may also be obtained from aspartate and glutamate, respectively, by transaminase reactions. 4. Oxaloacetate obtained in the reverse of reaction (7), above.

Regulation of the TCA Cycle TCA cycle substrates oxaloacetate and acetyl-CoA and the product NADH are the critical regulators. The availability of acetyl-CoA is regulated by pyruvate dehydrogenase complex. The TCA cycle enzymes citrate synthase,

isocitrate dehydrogenase and the ot-ketoglutarate dehydrogenase complex are under regulation by many metabolites (discussed above) to maintain optimal cellular energy needs. Overall fuel homeostasis is discussed in Chapter 22.

Energetics of the TCA Cycle Oxidation of one molecule of acetyl-CoA in the cycle yields three NADH, one FADH2, and one GTP. Transport of reducing equivalents in the electron transport chain from one NADH molecule yields three ATP and from FADH2, two ATP (see Chapter 14). Thus, oxidation of one molecule of acetyl-CoA yields 12 ATE Since one glucose molecule yields two acetyl-CoA, the yield of ATP is 24. In addition, in the oxidation of glucose to acetyl-CoA, two other steps yield four NADH (i.e., glyceraldehyde3-phosphate dehydrogenase and pyruvate dehydrogenase reactions), since one glucose molecule yields two triose phosphate molecules, which accounts for 12 more ATE Oxidation of one molecule of glucose to two of pyruvate yields two ATP (see under glycolysis) by substratelevel phosphorylation. Thus, complete oxidation of one molecule of glucose can yield 38 ATE In cells dependent only on anaerobic glycolysis, glucoseconsumption has to be considerably greater to derive an amount of energy equal to that obtainable from aerobic oxidation (two ATP versus 38 ATP per glucose). However, in cells capable of both aerobic and anaerobic metabolism, glycolysis, the TCA cycle, and oxidation in the electron transport chain are integrated and regulated so that only just enough substrate is oxidized to satisfy the energy needs of a particular cell. In the presence of oxygen, a reduction in glucose utilization and lactate production takes place, a phenomenon known as the Pasteur effect. The depression of rate of flux through glycolysis can be explained in part by the accumulation of allosteric inhibitors (e.g., ATP) of 6-phosphofructokinase and pyruvate kinase, together with the supply of cofactors and coenzymes of certain key enzymes (e.g., glyceraldehyde-3-phosphate dehydrogenase).

Supplemental Readings and References D. C. De Vivo, R. R. Trifiletti, R. I. Jacobson, et al.: Defective glucose transport across the blood-brain barrier as a cause of persistent hypoglycorrhachia, seizures and developmental delay. New England Journal of Medicine 325, 703 (1991). O. N. Elpeleg, W. Ruitenbeek, C. Jakobs, et al.: Congenital lacticacidemia caused by lipoamide dehydrogenase defciency with favorable outcome. Journal of Pediatrics 126, 72 (1995). R. A. Fishman: The glucose-transporter protein and glucopenic brain injury. New England Journal of Medicine 325, 731 (1991).

246 R. S. Hotchkiss and I. E. Karl: Reevaluation of the role of cellular hypoxia and bioenergetic failure in sepsis. Journal of the American Medical Association 267, 1503 (1992). S. J. Hunter and T. Garvey: Insulin action and insulin resistance: Diseases involving defects in insulin receptors, signal transduction, glucose transport effector system. American Journal of Medicine 105, 331 (1998). M. A. Magnuson: Glucokinase gene structure. Diabetes 39, 523 (1990). A. Mattevi, M. Bolognesi, and G. Valentini: The allosteric regulation of pyruvate kinase. FEBS Letters 389, 15 (1996). M. A. Permutt, K. C. Chiu, and Y. Tanizawa: Glucokinase and NIDDM. Diabetes 41, 1367 (1992).

CHAPTER13 Carbohydrate Metabolism I' Glycolysis and TCA Cycle S. J. Pilkis, I. T. Weber, R. W. Harrison, and G. I. Bell: Glucokinase: Structural analysis of a protein involved in susceptibility to diabetes. Journal of Biological Chemistry 269, 2192 (1994). B. H. Robinson: Lactic acidemia. In: The Metabolic and Molecular Bases of Inherited Disease, 7th ed. C. R. Scriver, A. L. Bendet, and S. Sly (Eds). McGraw-Hill, New York, 1995, p. 1479. K. R. Tanaka and D. E. Paglia: Pyruvate kinase and other enzymopathies of the erythrocyte. In: The Metabolic and Molecular Bases of Inherited Disease, 7th ed. C. R. Scriver, A. L. Bendet, and S. Sly (Eds). McGraw-Hill, New York, 1995, p. 3485. J. Uribarri, M. S. Oh, and H. J. Carroll: D-Lactic acidosis. Medicine 77, 73 (1998).

CHAPTER

14

Electron Transport and Oxidative Phosphorylation

The energy requirements of aerobic cells are met by the energy released in the oxidation of carbohydrates, fatty acids, and amino acids by molecular oxygen. In these oxidation processes, the reducing equivalents from substrates are transferred to NAD +, flavin mononucleotide (FMN), or flavin adenine dinucleotide (FAD). The hydrogens, electrons, or hydride (H-) ions are removed catalytically from NADH and reduced flavin nucleotides and transferred through a series of coupled reductionoxidation (redox) reactions to oxygen, which serves as the ultimate electron acceptor. The reduced oxygen is then converted to water. The entire process is known as cell respiration. The numerous coupled redox reactions are tightly linked and take place in the inner mitochondrial membrane. This reaction sequence is called the respiratory chain electron transport. Electrons in substrate or cofactor begin with a high potential energy and end at oxygen with a lower potential energy. During this electron flow, a portion of the free energy liberated is conserved by an energy-transducing system (by which electrical energy is changed to chemical energy). Since the energy is conserved in the terminal phosphoanhydride bond of ATP through phosphorylation of ADP to ATE the overall coupled process is known as oxidative

phosphorylation. The following reactions yield NADH or FADH2 during the breakdown of glucose by glycolysis and in the

TCA cycle (Chapter 13). The NADH-producing reactions are Glyceraldehyde 3-phosphate --+ 1,3-bisphosphoglycerate Pyruvate --+ acetyl-CoA Isocitrate --+ ot-ketoglutarate ot-Ketoglutarate --+ succinyl-CoA Malate --+ oxaloacetate The FADHz-producing reaction is Succinate --+ fumarate Except for the conversion of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate occurring in the cytoplasm, all of the above reactions take place in the mitochondria. Since mitochondria are not permeable to NADH, two shuttle pathways transport the reducing equivalents of cytoplasmic NADH into the mitochondria. The TCA cycle functions for both the oxidation and the production of reducing equivalents from a variety of metabolites such as carbohydrates, amino acids, and fatty acids. In the mitochondria fatty acids undergo successive removal of two carbon units in the form of acetylCoA, which is converted in the TCA cycle to CO2 and reducing equivalents found in NADH and FADH2. The latter two substances are then fed into the electron transport chain. The formation of each acetyl-CoA molecule

247

248

CHAPTER14 Electron Transport and Oxidative Phosphorylation

requires the removal of four H atoms and is catalyzed by two dehydrogenases (Chapter 18). One dehydrogenase yields FADH2; the other yields NADH. The term "one reducing equivalent" means that 1 mol of electrons is present in the form or one gram-equivalent of a reduced electron carrier. One mole of NAD +, when reduced to 1 mol of NADH, utilizes 2 mol of electrons from 1 mol of substrate (Chapter 13):

Inner membrane Intermembrane

~

space

~

Matrix

Cristae

~f

Outer membrane

NAD + + SH2 (reduced substrate) ~NADH + H + + S (oxidized substrate) In this reaction, the 2 mol of electrons is transferred in the form of 1 mol of hydride ion (H:)-.

14.1 Mitochondrial Structure and Properties Mitochondria are present in the cytoplasm of aerobic eukaryotic cells. They are frequently found in close proximity to the fuel sources and to the structures that require ATP for maintenance and functional activity (e.g., the contractile mechanisms, energy-dependent transport systems, and secretory processes). The number of mitochondria in a single cell varies from one type of cell to another; a rat liver cell contains about 1000, while one giant amoeba has about 10,000. In a given cell, the number of mitochondria may also depend on the cell's stage of development or functional activity. The size and shape of mitochondria vary considerably from one cell type to another. Even within the same cell, mitochondria can undergo changes in volume and shape depending on the metabolic state of the cell. In general, they are 0.5-1.0 # m wide and 2-3 # m long and are known to aggregate end to end, forming long filamentous structures. Mitochondria consist of two membranes, one encircling the other, creating two spatial regions: the intermembrane space and the central space, called the matrix. The outer membrane is 6-7 nm thick, smooth, unfolded (Figure 14-1), and freely permeable to molecules with molecular weights below 10,000. It contains a heterogeneous group of enzymes that catalyze certain reactions of lipid metabolism as well as hydroxylation reactions (Table 14-1). The intermembrane space (5-10 nm) contains the enzymes that catalyze interconversion of adenine nucleotides. The inner membrane (6-8 nm thick) has many folds directed toward the matrix. These invaginations, known as cristae, increase the surface area of the inner membrane. The lipid component, almost all of which is phospholipid, constitutes 30-35% by weight of the inner membrane.

Inner

~~ - ~ 1 I ~

II~-.~ ~ ~ n~embra~e sphere I ~-------~-In;emrbran e

Intermembrane

membrane F I ( ; U R E 14-1 Morphology of a mitochondrion (transverse section).

Phospholipids are asymmetrically distributed in the lipid bilayer, with phosphatidylethanolamine predominating on the matrix side and phosphatidylcholine on the cytoplasmic side. Seventy-five percent of the cardiolipin is present on the matrix side of the membrane. The fatty acid composition of the phospholipids depends on the species, tissue, and diet. In all cases, sufficient unsaturated fatty acids are contained in the phospholipids to provide a highly fluid membrane at physiological temperatures. The inner membrane is studded with spheres, each 8-10 nm in diameter, that are attached via stalks 4-5 nm in length. These inner membrane spheres are present on the matrix side (M-side) but absent from the cytoplasmic side (C-side). The components of the inner membrane include respiratory chain proteins, a variety of transport molecules, and a part of the ATP-synthesizing apparatus (the base piece of ATP synthase). The ATP-ADP translocase and

SECTION 14.1

Mitochondrial Structure and Properties

249

TABLE 14-1

Distribution of Enzymes in Mitochondrial Compartments Outer Membrane

Intermembrane Space

NADH cytochrome b 5 reductase Cytochrome b 5 Monoamine oxidase Kynurenine hydroxylase Glycerolphosphate acyltransferase Lysophosphatidyl transferase Phosphatidate phosphatase Phospholipase A Nucleoside diphosphokinase Fatty acid elongation enzyme system

Adenylate kinase (myokinase) Nucleoside diphosphokinase Nucleoside monophosphokinase Sulfite oxidase

Inner Membrane

Matrix

NADH-coenzyme Q reductase Succinate-coenzyme Q reductase Coenzyme QH2_ cytochrome c reductase Cytochrome oxidase Oligomycin-sensitive ATPase /3-Hydroxybutyrate dehydrogenase Pyridine nucleotide transhydrogenase Carnitine palmitoyltransferase Ferrochelatase (heine synthase) Adenine nucleotide carrier and other carrier proteins Carbamoyl phosphate synthetase I Dihydroorotate dehydrogenase (C-side) Glycine cleavage enzyme complex

Pyruvate dehydrogenase complex c~-Ketoglutarate dehydrogenase complex Citrate synthase Aconitase Malate dehydrogenase Isocitrate dehydrogenase [NAD +] Isocitrate dehydrogenase [NADP +] Fumarase Glutamate dehydrogenase Pyruvate carboxylase Aspartate aminotransferase Ornithine carbamoyltransferase Fatty acyl-CoA synthetase Fatty acyl-CoQ dehydrogenase Enoyl hydrase /3-Hydroxyacyl-CoA dehydrogenase /3-Ketoacyl-CoA thiolase Amino acid activating enzymes RNA polymerase DNA polymerase 6-Aminolevulinic acid synthase HMG-CoA synthase HMG-CoA lyase Acetyl-CoA acetyltransferase /3-Ketoacid CoA-transferase Rhodanese (detected in both inner membrane and matrix) *Coproporphyrinogen oxidase *Protoporphyrinogen oxidase

*The exactlocation is not yet known.

the ATP synthase together make up at least two thirds of all protein within the inner mitochondrial membrane. Small uncharged molecules (e.g., water, oxygen, carbon dioxide, ammonia, and ethanol) can diffuse through the inner membrane, but all other molecules that pass through require specific transport systems. The mitochondrial outer and inner membranes are vastly different in their constituents and function:

1. The outer membrane contains two to three times more phospholipids per unit of protein; 2. Cardiolipin is localized in the inner membrane; and 3. Cholesterol is found predominantly in the outer membrane. The permeability of the outer membrane to charged or uncharged substances up to a molecular weight of 10,000

250


F I G U R E 14-2 Formation of submitochondrial particles. These resealed inside-out inner membrane vesicles are formed when mitochondria are subjected to a variety of disruptive forces; they can support both reducing-equivalent transport and phosphorylation of ADP. Removal of the inner membrane spheres from the vesicles leads to the loss of ability to phosphorylate ADP.

is due to pore-like structures that consist of a protein (M.W. 30,000) embedded in the phospholipid bilayer. In vitro, addition of the purified pore protein renders phospholipid bilayers permeable to a variety of substances. An analogous pore protein, known as porin, has been identified in the outer membrane of gram-negative bacteria (Chapter 11). The matrix is viscous and contains all TCA cycle enzymes except succinate dehydrogenase, which is a component of electron transport complex II and is located within

the inner membrane. Enzymes of the matrix or the inner mitochondrial membrane mediate reactions of fatty acid oxidation, ketone body formation and oxidation, and biosynthesis of urea, heme, pyrimidines, DNA, RNA, and protein. Mitochondria are involved in programmed cell death apoptosis, (see Chapter 26). The structure, mechanism of replication of mitochondrial DNA, and processes of transcription and translation are unique in several respects (discussed later).

SECTION 14.1 M i t o c h o n d r i a l

251

S t r u c t u r e and P r o p e r t i e s

Submitochondrial Particles

Complex l

Submitochondrial particles (SMPs) are produced by disruption of the mitochondria by mechanical, osmotic, or sonic shock treatment. The fragmentation results in the release of water-soluble components and inner membrane fragments that re-form into vesicles (Figure 14-2). The components are then separated by differential centrifugation. The membranes of SMPs have the characteristic inner membrane spheres on their outside. SMPs are capable of electron transport and oxidative phosphorylation (i.e., the synthesis of ATP from ADP and phosphate). Removal of the inner membrane spheres by further mechanical treatment, urea, or trypsin results in the dissociation of the electron transport assembly from ATP synthesis. The ability to synthesize ATP resides in the overall structure, which includes the spheres (called F1), the stalks, and a membrane protein subunit (called Fo). The Fo-subunit spans the inner membrane and thus is retained in the vesicles. The spheres removed from SMPs do not support ATP synthesis but do hydrolyze ATP to ADP and phosphate. Thus, ATP synthesis is carried out by Fo/F1-ATPase (ATP synthase). The subscript "o" in Fo indicates that it contains the site at which a potent antibiotic inhibitor, oligomycin, binds and inhibits oxidative phosphorylation. Oligomycin does not bind F1-ATPase and does not inhibit ATP hydrolysis to ADP and phosphate. FoF1-ATPase (ATP synthase; inhibited by oligomycin)

ADP + Pi,

, ATP + H20 F1-ATPase

(Not inhibited by oligomycin)

The fully active SMPs can be reconstituted by adding F~ATPase to depleted vesicles under appropriate conditions.

The electron transport system can be reconstituted into discrete enzyme complexes that catalyze the following four reactions: NADH-coQ reductase

1. NADH + H + + coenzyme Q (Q) NAD + + QH2

>

Succinate-CoQ reductase

> fumarate + QH2 coQ cytochrome c reductase

3. QH2 + 2ferricytochrome c 2 ferrocytochrome c + 2H +

- -

I

Complex II

--" Complex III

/

- - " Cyt a,a3-Cu

I_i O,

Complex IV

F I G U R E 14-3 Diagram of the functional complexes of the electron transport system within the respiratory chain. FNAD ----NADH dehydrogenase flavoprotein; Fs = succinate dehydrogenase flavoprotein; Fe(n.h.) = nonheme iron.

ation procedures, whereas the bonds holding unlike complexes are relatively weak and can be dissociated. The functional organization of the four complexes in the inner mitochondrial membrane is shown in Figure 14-3. In addition to these complexes, the F0/F1-ATPase, which is required for ATP synthesis, may be considered as complex V. The relative ratios of complexes I, II, III, IV, and V have been estimated to be 1:2:3:6:6. Complexes I, II, III, and IV can be combined in the presence of cytochrome c (which separates during fractionation) to form a single unit with all of the enzymatic properties of the intact electron transport system except coupled phosphorylation. The individual electron carriers of the four complexes of the respiratory chain, shown in Figure 14-4, are arranged in accordance with their redox potentials, with the transfer of electrons from NADH to oxygen associated with a potential drop of 1.12 V, and that of succinate to oxygen of 0.8 V. In the electron transport system, the electrons can be transferred as hydride ions (H:)- or as electrons (e.g., in the cytochromes).

Electron Transport Complexes Complex I catalyzes an NADH-CoQ reductase activity, and it contains the NADH dehydrogenase flavoprotein. It has two types of electron-carrying structures: FMN and several iron-sulfur centers. FMN is a tightly bound prosthetic group of the dehydrogenase enzyme, and it is reduced to FMNH2 by the two reducing equivalents derived from NADH: NADH + H + + E-FMN ~-- NAD + + E-FMNH2

>Q + cytochrome c oxidase

4. 2 Ferrocytochrome c + 2H + + 1/202 2 ferricytochrome c + H20

S u c c , r, at,

"'Q//--"

Complex I

Components of the Electron Transport Chain

2. Succinate + Q

NDH - I

,,

>

Each complex can be considered as a functional unit composed of a fixed number of electron carriers. The individual components of the four complexes are firmly bonded together and are not dissociated by mild fraction-

The electrons from FMNH2 are transferred to the next electron carrier, coenzyme Q, via the iron-sulfur centers of the NADH-CoQ reductase. The iron-sulfur centers consist of iron atoms paired with an equal number of acid-labile sulfur atoms. The respiratory chain iron-sulfur clusters are of the FezS2 or Fe4S4 type. The iron atom, present as nonheme iron, undergoes oxidation-reduction cycles (Fe z+ ~ Fe 3+ + e -). In the Fe4S4 complexes, the centers

Complex Ill

Complex I

H

FAD

FIGURE 14-4 Mitochondria1 electron transport system

yoH,

S

u m reductase

SECTION14.1 Mitochondrial Structure and Properties O

NADH+ H +

253

i

H NAD +

\ J

H O N N/=O

.,C N NS=O

J

I

I

R

I

/Protein

O

/cy~

II I_

R= --OH E--~-- C - - ( ~ - - CHIO--O--- p - - OH

/

H

H

FMNH 2

OH OH OH

I

I

R

FMN

I

O

H

/

O

S

Fe

,._.% Proteinm CysmS - --Fe

Iron-sulfur protein complex(Fe4S4) (~)= acid-labile sulfur

/Fe

/

| ~ F e

ys

S = cysteine sulfur

\

Protein

F I G U R E 14-5 Transport of reducing equivalents from NADH to FMN and structure of the iron-sulfur protein complex that mediates electron transport from FMNH2 to CoQ. Both FMN and the iron-sulfur centers are components of NADH-CoQ reductase.

are organized such that iron and sulfur atoms occupy alternate corners of a cube. The four iron atoms are covalently linked via the cysteinyl sulfhydryl groups of the protein (Figure 14-5). Complex I is inhibited by rotenone (a natural toxic plant product), amobarbital (a barbiturate), and piericidin A (an antibiotic) (Figure 14-6). all of which act at specific points and are useful in the study of electron transport.

OCH3

H,CO..~ H ,O

.,...

Rotenone

Complex H Complex II contains succinate dehydrogenase and its iron-sulfur centers. The complex has also been reported to contain a specific cytochrome, cytochrome b558. Cytochromes are heme proteins that undergo oxidationreduction reactions and are differentiated on the basis of their apoprotein structure, heme structure, and optical absorption in the visible spectrum. The mitochondrial electron transport chain contains at least six different cytochromes classified into three groups (a, b, and c). It is usual to indicate the absorption maximum of the cz-band of a particular cytochrome (e.g., cytochrome b558). Succinate dehydrogenase, an FAD-containing enzyme, is part of the TCA cycle and catalyzes the trans elimination of two hydrogens from succinate to form fumarate (Chapter 13).

--

CH2

CH3

OH

H

I

~

C

O

~ OH

H3CO

"N"

-

"i

"-

CH3

~'~ CH3

CH3

CH3

Piericidin A 0 /'~

,,,CH~H3

H

CH3 Amytal

F I G U R E 14-6 Inhibitors of NADH-CoQ reductase: rotenone (a toxic plant product), piericidin A (an antibiotic), and amytal (a barbiturate).

CHAPTER 14

254

cool

coo-

~--o

I ~H~_

(~H 2 _

COO

COO


o

(a)

(c)

(b)

(d)

FIGURE 14-7 Inhibitors of complexII. (a) Oxaloacetate,(b) malonate,(c) thenoyltrifluoroacetone,and (d) carboxin.

The hydrogens are accepted by FAD, which is covalently bound to the apoprotein via a histidine residue. In many flavoproteins, the flavin nucleotide is bound to the apoprotein not covalently but rather via ionic linkages with the phosphate group. The reducing equivalents of FADH2 are passed on to coenzyme Q (CoQ or Q) via the iron-sulfur centers. Thus, the overall reaction catalyzed by complex II is Succinate + FAD--(Fe--S)n--E

Fumarate + FADH2R(Fe--S)n__E

t

-

_Q

FAD--(FeRS)n--E + QH 2

During the terminal stages of electron transfer in complex II, cytochrome b558 is involved; however, its specific function is not understood. Oxaloacetate and malonate are competitive inhibitors of succinate dehydrogenase and compete with the substrate for binding at the active site (Chapters 6 and 13). Carboxin and thenoyltrifluoroacetone (Figure 14-7) inhibit electron transfer from FADH2 to CoQ.

Complex III Complex III contains cytochromes b562 and b566, (collectively called cytochrome b), cytochrome cl, and an o

H3C--O~

H3C-- O

Coenzyme Q, also called ubiquinone because of its ubiquitous occurrence in microorganisms, plants, and animals, is lipid-soluble and not tightly or covalently linked to a protein, although it carries out its electron transport function together with specific CoQ-binding peptides. It plays a central role in the electron transport chain because it collects reducing equivalents from NADH- and FADHz-linked dehydrogenases and passes them on to the terminal cytochrome system. CoQ is a substituted 1,4-benzoquinone containing a polyisoprenoid side chain at C6 (Figure 14-8). In bacteria, CoQ usually contains six isoprenoid units (Q6), whereas in most mammalian mitochondria it has ten (Q]0). The reduction of Q to QH2 (a hydroquinone) requires two electrons and two protons and probably occurs via a one-electron intermediate: le-,1H +

Q~

_ --'~ f+ le

l e - , 1H +

: QH" "

,1H

H3C --O~c/C%(;;--CH3

CH3

.c.L,;g_ic._c.= [

"J,o

-2"rH"-

H3C

CH3

I

OH Coenzyme Q (ubiquinone) (CoQ or Q)

~ _,,r..+ le

~ QH 2

,1H

where the dot associated with QH represents an unpaired electron (a free radical). Antimycin A (a Streptomyces antibiotic) inhibits the transfer of electrons from QH2 to cytochrome c (Figure 14-9). Cytochrome c transfers electrons from complex III to complex IV, and cytochromes a and a3 transfer electrons

I

C3 4 51CI--CH3

II o

QH2 + 2Cyt c (Fe 3+) --+ Q + 2Cyt c (Fe z+) + 2H +

OH

II jC~ C

iron-sulfur protein. Complex III catalyzes the transport of reducing equivalents from coQ to cytochrome c:

Hydroquinone (ubiquinol) (CoQH2or QH2)

FIGURE 14-8 Structure and redox reaction of coenzymeQ. In mostmammaliantissues, CoQ has 10 isoprenoidunits. CoQ collects reducing equivalentsfromNADHdehydrogenaseand fromother flavin-linkeddehydrogenases.

255

SECTION14.1 Mitochondrial Structure and Properties H

H~

I ~C~ C I

O

O

I

CH3

II

O

I

II

--CH3

.C.--C--N,~,,C-O---~C_ _O--C--CHT--CH /

y I(I H/

II

H

~C/---H

OH

H/

. , ~ " H II O~C-

~r ,/

~r,u

I

O H3C

C--(CH2)s--CH3

~C~H

HI

F I G U R E 14-9 Structure of antimycin A, an antibiotic that inhibits electron transport from CoQ to cytochrome c.

to oxygen in complex IV. The structure of the heme prosthetic group (iron-protoporphyrin IX) in cytochromes b, c, and Cl is the same as that present in hemoglobin and myoglobin but differs from the heme group (heme A) of cytochromes a and a3 (Figure 14-10). The heme groups in cytochromes c and c] are covalently linked to the apoprotein by thioether bonds between sulfhydryl groups of two cysteine residues and the vinyl groups of the berne. The heme of cytochromes b and a3 is bound by strong hydrophobic interactions between the heme and the apoprotein. Cytochromes b, Cl, a, and a3 are integral membrane proteins, whereas cytochrome c is a peripheral protein located on the C side of the membrane and is easily isolated from mitochondria. The amino acid sequence of cytochrome c has been established for a wide variety of species and

consists of 100-113 amino acid residues (M.W. 13,000). Cytochrome c has played an important role in our understanding the evolutionary relationships among species. Twenty-eight residues are invariant among 67 species sequenced, presumably because a hydrophobic environment around the heme appears to be essential. Amino acids in other positions vary from one species to another and reflect the times of divergence of the different species. For example, only two amino acid residues differ in duck and chicken, but 48 differences are found between horse and yeast cytochrome c, consistent with their long, divergent evolutionary history. Cytochrome c's are identical in chicken and turkey, and in pig, cow, and sheep.

Complex IV Complex IV, also called cytochrome c oxidase, is the terminal component of the respiratory chain. It consists of four redox centers: cytochrome a, cytochrome a3, and two Cu ions. Like the iron atoms, the copper ions function as one-electron carriers: Jr- e- ~ Cu +

C u 2+

Cytochrome c transfers electrons from cytochrome c l, the terminal component of complex III, to the four redox centers of the cytochrome oxidase complex. The transfer of four electrons from each of the four redox centers of the cytochrome oxidase complex to an oxygen molecule CH3

I OHm-OH--C-CH]:-H

~H2 H3

H~ C ~ O H CH3

CH O N

_

0II

O~C~CH~--CH

U-

"

H~Jc/N~~N ~ ~-

~

H2

H--C

CH3

II CH--CH2 H

~CH

II

oII

- O ~ C ~ CHE---CH2

N/

il

.N- -Fe - N ~

I

oHeme (iron-protoporphyrin IX) F I G U R E 14-10 Structure of heme (present in cytochromes b, c, and c 1) and of heine A (present in cytochromes a and a3).

,,,

II CH3 II

,, H~

--:CH H2

CH3

C--O

/~

C---O

I

o_

Heme A

CH3

OH-OH.

256

CHAPTER

occurs in a concerted manner to yield two molecules of water: 4e- + 02 -t- 4H + --+ 2H20 More than 90% of metabolic oxygen is consumed in the cytochrome oxidase reaction. Oxygen contains an unconventional distribution of its two valence electrons. These two electrons occupy different orbitals and are not spin paired; thus oxygen is a diradical. The reduction of an oxygen molecule with less than four electrons results in the formation of an active oxygen species. One electron transfer yields Superoxide radical (O 2-) and the two electron transfer yields hydrogen peroxide (H202). Hydroxyl free radical (HO ~ formation can take place from hydrogen peroxide in the presence of ferrous iron or cuprous chelates. Both O 2 and HO ~ free radicals are cytotoxic oxidants. In the mitochondrial electron transport system, leakage of electrons at any one of the redox-centers due to aging or pathological conditions results in the formation of superoxide. Antioxidant enzymes, namely superoxide dismutases (SOD), catalase, and glutathione peroxidase participate in the elimination of toxic oxygen metabolites. Three different SODs are present in human cells; they are located in mitochondria, cytosol and extracellular fluid. The importance of mitochondrial SOD (labeled as SOD2), which is a manganese containing enzyme, is exemplified in the homozygous SOD2 knockout mice. These SOD2 knockout mice have low birth weights and they die shortly after their birth from dilated cardiomyopathy. Redox reactions are a required part of normal metabolism. In neutrophils, for example, the killing of the invading microorganisms requires reactive oxygen metabolites (discussed later and also see Chapter 16). Oxidants also are involved in gene expression (e.g., the variety of protein kinases) and in the regulation of redox homeostasis. Perturbation of redox homeostasis causes oxidative stress and may contribute to chronic inflammatory diseases and malignancy. Cytochrome oxidase in inhibited by cyanide (CN-; see Chapter 6), carbon monoxide (CO), and azide (N~).

14 ElectronTransportand OxidativePhosphorylation

TABLE 14-2

Standard Oxidation-Reduction Potential (E ~) of Components oft he Electron-Transport Chain

Redox Component

E~

NADH/NAD + F M N H 2 / F M N (of NADH dehydrogenase) CoQHz/CoQ Cyt b (red)/Cyt b (ox) Cyt c 1 (red)/Cyt c 1 (ox) Cyt c (red)/Cyt c (ox) Cyt aa 3 (red)/Cyt aa 3 (ox) 102-/102~ z (or H 20/1702)

-0.32 -0.11 +0.10 +0.12 +0.23 +0.25 +0.29 +0.82

Complexes I-IV of the respiratory chain are organized asymmetrically in the inner membrane (Figure 14-11) as follows: 1. The flavin prosthetic groups of NADH dehydrogenase and succinate dehydrogenase face the M side of the membrane; 2. CoQ and cytochrome b of complex III are probably inaccessible from either side of the membrane; 3. Cytochrome c interacts with cytochrome cl and cytochrome a, all located on the C side;

Intermembrane space (C-side) ( ~ ~

Matrix (M-side)

Inner membrane

T~~l~hydrog~m'~--.

NADPH

Complexl FM~---- NADH

b

Fe-S ,,,)

~

Organization of the Electron Transport Chain The arrangement of components of the electron transport chain was deduced experimentally. Since electrons pass only from electronegative systems to electropositive systems, the carriers react according to their standard redox potential (Table 14-2). Specific inhibitors and spectroscopic analysis of respiratory chain components are used to identify the reduced and oxidized forms and also aid in the determination of the sequence of carriers.

volts)

O==:Z FIGURE 14-11 Orientation of the components of the electron transport complexeswithin the inner mitochondrial membrane. Fe-S -- Iron-sulfurcenter; b, c, Cl, a, and a3 = cytochromes;Cu : copper ion.

257

SECTION14.2 Oxidative Phosphorylation

4. Complex IV spans the membrane, with cytochrome a oriented toward the C side; copper ions and cytochrome a3 are oriented toward the M side. 5. Nicotinamide nucleotide transhydrogenase, which catalyzes the reaction NADPH + NAD + ~ NADH + NADP +, spans the membrane, but its catalytic site faces the M side. The anisotropic organization of electron carriers across the membrane accounts for the vectorial transport of protons from the inside to the outside of the membrane, which occurs with the passage of electrons. The coupling of this proton gradient to a proton-translocating ATP synthase (also known as ATP synthetase) accounts for the chemiosmotic coupling in oxidative phosphorylation.

14.2 Oxidative Phosphorylation ATP is synthesized from ADP and phosphate during electron transport in the respiratory chain. This type of phosphorylation is distinguished from substrate-level phosphorylation, which occurs as an integral part of specific reactions in glycolysis and the TCA cycle. The free energy available for the synthesis of ATP during electron transfer from NADH to oxygen can be calculated from the difference in the value of the standard potential of the electron donor system and that of the electron acceptor system. The standard potential of the NADH/NAD + redox component is - 0 . 3 2 V and that of H20/102 is +0.82 V; therefore, the standard potential difference between them is A E ~ --

(electron acceptor - electron donor)

= 0.82 - (-0.32) - + 1.14 V The standard free energy is calculated from the expression AG o' -- - n F AE ~ where n is the number of electrons transferred, and F is the which is equal to 23,062 cal/eV. Thus, the standard free energy for a two-electron transfer from NADH to molecular oxygen is Faraday constant,

A G ~ -- - n F

AE ~

-- - 2 x 23,062 x 1.14

= - 5 2 . 6 kcal/mol ( - 2 2 0 kJ/mol) Although 52.6 kcal of free energy is available from the reaction, only 21.9 kcal is conserved in the formation of phosphoanhydride bonds at ATR Formation of each phosphoanhydride bond requires 7.3 kcal (30.5 kJ), and 21.9 kcal accounts for three ATPs synthesized. The remainder of the energy is assumed to be dissipated as heat.

F I G U R E 14-12 Flow of reducing equivalents in the respiratory chain and their relationship to energy availability to drive ATP synthesis. The largest free energy changes occur between NADH and FMN, between cytochromes b and Cl, and between cytochromes (a + a3) and molecular oxygen; ATP formation is coupled to these three sites.

In the mitochondria of brown adipose tissue, very little ATP is synthesized, and most of the energy liberated in the electron transport system is converted to heat. The conservation of energy in the electron transport chain occurs at three sites where there is a large decrease in free energy (Figure 14-12): between NADH and FMN, between cytochromes b and c l, and between cytochromes (a + a3) and molecular oxygen. Because electrons from succinate bypass complex I and enter at the CoQ level, only two moles of ATP are synthesized per mole of succinate. The number of ATP molecules synthesized depends on where the reducing equivalents enter the respiratory chain and is indicated by the ratio (or quotient) of phosphate esterified (or ATP produced) to oxygen consumed (P/O or ATP/O) per two-electron transport. The NAD +linked substrates (e.g., malate, pyruvate, oe-ketoglutarate, isocitrate, fi-hydroxybutyrate, and fi-hydroxyacyl-CoA) have P/O values of 3, and some flavin-linked substrates (e.g., succinate, glycerol phosphate, and fatty acyl-CoA) have P/O ratios of 2. The complete oxidation of 1 mol of glucose yields either 36 or 38 mol of ATE depending on which shuttle pathway is used in the transport of cytoplasmic NADH to mitochondria. A list of the energy-yielding reactions in glucose oxidation is shown in Table 14-3.

Mechanisms of Oxidative Phosphorylation Three hypotheses have been proposed to explain how the mechanism of energy conservation is coupled to electron transport (the energy transduction system): the chemical, conformational, and chemiosmotic hypotheses. The c h e m i c a l h y p o t h e s i s proposes that the energy is conserved by the formation of high-energy intermediates as reducing equivalents pass from one carrier to the

~8


TABLE 14-3

Energy-Yielding Reactions in the Complete Oxidation of Glucose N e t M o l e s of A T P Generated per M o l e of G l u c o s e

Reaction

Glycolysis (phosphoglycerate kinase, pyruvate kinase; two ATPs are expended) NADH shuttle glycerol-phosphate shuttle (or malateaspartate shuttle) Pyruvate dehydrogenase (NADH) Succinyl CoA synthetase (GTP is equivalent to ATP) Succinate dehydrogenase (succinate --~fumarate + FADH 2) Other TCA cycle reactions (isocitrate ~ a-ketoglutarate, a-ketoglutarate ~ succinyl CoA, malate---~ oxaloacetate; total of 3 NADH generated) Total

2

4(6) 6 2 4

18 36(38)

next. The energy is ultimately stored as a phosphoric acid anhydride bond in ATE The formulation, in general terms, is A

~

AH2 + B -4- C ~- A --~ C C -4- ADP + Pi ~- A + C

Sum: AH2 -4- B

+ BH2 + ATP

+ ADP + Pi ~- A + B H2 -4- ATP

where A and B represent the known redox pair, C is a hypothetical ligand, and A ~ C is a hypothetical high-energy intermediate. The above mechanism can be modified to include other phosphorylated intermediates. A model reaction that supports the above mechanism is the glycolytic substrate-linked phosphorylation, which proceeds via a thiol ester prior to the formation of the phosphorylated intermediate (Chapter 13). Although the chemical hypothesis is consistent with the substrate-linked phosphorylation mechanism, it is deficient in explaining the oxidative phosphorylation in mitochondria for two reasons: 1. The postulated high-energy chemical intermediates, either phosphorylated or nonphosphorylated, have never been identified despite many attempts to find them, and 2. The chemical-coupling mechanism does not explain why the inner mitochondrial membrane must be

present as a completely closed vesicle for oxidative phosphorylation to occur. The conformational hypothesis proposes that the energy-yielding steps generate protein conformational changes that are used in ATP synthesis. The conformational changes that occur in the redox catalysts are transmitted to the energy-transducing units via protein-protein interactions, the formation of covalent intermediates, or the proton-motive force. Current opinion holds that the conformational changes are linked with a proton-motive force (see below). Morphological changes do occur in the inner membranes of the mitochondria when active respiration is stimulated by ADP. Fluorescent probes, such as 1aminonaphthalene-8-sulfonate (ANS) and the antibiotic aurovertin, bind either to the inner membrane (ANS) or directly to ATP synthase (aurovertin). The binding enhances or diminishes fluorescence in response to changes in conformation or hydrophobicity of the inner membrane. Resuits support the hypothesis that ATP synthase undergoes conformational changes during respiration and oxidative phosphorylation (discussed later). According to the chemiosmotic hypothesis, developed by Peter Mitchell, an electrochemical gradient (pH gradient), generated across the inner mitochondrial membrane by the passage of reducing equivalents along the respiratory chain provides the driving force for the synthesis of ATP. There are three prerequisites for achieving oxidative phosphorylation according to this hypothesis: I. An anisotropic (direction-oriented) proton-translocating respiratory chain capable of vectorial transport of protons across the membrane; 2. A coupling membrane impermeable to ions except via specific transport systems; and 3. An anisotropic ATP synthase whose catalytic activity is driven by an electrochemical potential. The transport of reducing equivalents in the respiratory chain generates a proton gradient across the membrane by virtue of the specific vectorial arrangement of the redox components within the inner mitochondrial membrane. The proton gradient is generated by ejection of protons from the matrix into the intermembrane space during proton-absorbing reactions, which occur on the M side of the inner membrane, and the proton-yielding reactions, which occur on the C side, to form redox loops (Figure 14-13). According to the chemiosmotic hypothesis, ejection of two or more protons occurs at each of three sites in complexes I, III, and IV. Thus, in the transfer of two reducing equivalents from NADH to oxygen, at least six protons


259

Intermembrane Inner membrane Matrix space

2. A hydrophobic complex of three or four proteins (also called proteolipids), or F0 complex, located in the inner membrane, which is thought to contain a proton-translocating channel; and 3. A stalk consisting of protein components that connect F1 with F0.

~2H

1

2H+~-.2H +

F I G U R E 14-13 Schematic representation of a transmembrane redox loop, in which 2H + are ejected into the intermembrane space as the substrate is oxidized on the matrix side.

(2 x 3) are ejected. Development of an electrochemical potential for protons is the mechanism by which conservation of free energy occurs during transport of reducing equivalents in the respiratory chain. The electrochemical potential is proportional to the proton-motive force (pmf, designated as Ap or A#H+), given by the following equation: A#H+ = &~p -- Z ApH where & ~ is the membrane potential resulting from charge separation, the matrix compartment being negative; &pH is the pH difference across the membrane, due to proton translocation; and Z is equal to -2.303 RT/F, the factor used to convert pH units into millivolts, the unit of the other two terms of the expression; Z equals - 5 9 at 25~ The anisotropic arrangement of the respiratory chain and the vectorial transport of protons are supported by experimental observations. The distribution of the redox carriers required by the chemiosmotic hypothesis is remarkably similar to that derived from studies on enzyme topology of the inner membrane. In respiring mitochondria, the intramembrane space is more acidic than the matrix space by about 1.4 pH units, and the transmembrane potential is about 0.180-0.220 V. Thus, the basic premise of the chemiosmotic hypothesis is Transport of reducing equivalents --+ A/2H+ --+ ATP synthesis The proton-motive force drives ATP synthesis via the reentry of protons. The ATP synthase (Fo/F1 complex) is driven by this vectorial transfer of protons from the intramembrane space into the matrix. ATP synthesis occurs in the inner membrane spheres (the F1 component of the synthase). ATP synthase consists of three parts (Figure 14-14): 1. The catalytic part, F1, composed of five firmly associated subunits with a subunit composition of ot3fi3?'~s;

When the ATP synthase forms a part of the intact membrane system, it catalyzes proton translocation and ATP synthesis. However, when F1 is dissociated from F0, the FI no longer catalyzes ATP synthesis but rather ATP hydrolysis. Thus, F1 becomes an ATPase rather than an ATP synthase. Submitochondrial vesicles devoid of F1 can transport reducing equivalents, since they contain the redox carriers, but are unable to support ATP synthesis. Careful reconstitution of membrane vesicles by addition of FI allows the complex to regain its capacity for ATP synthesis. The antibiotics oligomycin and aurovertin and the reagents N,N'-dicyclohexylcarbodiimide (DCCD) and 4-chloro-7-nitrobenzofurazan (Nbf-C1) inhibit ATP synthesis by binding to different sites of ATP synthase. Oligomycin and DCCD bind to proteolipid (F0) components and block translocation of protons, aurovertin binds to a specific fi-subunit of F1, and Nbf-C1 binds with a specific tyrosine residue of the fi-subunit of F1. The mechanism of ATP formation by the ATP synthase driven by the passage of protons is attributed to conformational changes that occur in the o~fi dimers of the F1 complex (Figure 14-15). The F1 complex has three interacting nucleotide-binding and conformationally distinct olfl domains: 1. Open conformation (O): has very low affinities for substrates and products and is catalytically inactive. 2. Loose conformation (L): binds loosely to nucleotides and is catalytically inactive. 3. Tight conformation (T): binds nucleotides tightly and is catalytically active. In this model, originally proposed by Paul Boyer, the conformational changes of o~fi assemblies are brought about by the energy dependent rotation of-the ?, subunit. The y subunit rotation occurs in discrete steps of 120 ~ and is fuelled by proton passage through F0 channels. In step 1 rotation (Figure 14-15), the conformation change at the T site causes ATP release at the O site. In step 2 rotation, ADP and Pi bound at the L site, now bind at the T site and undergo conversion to ATE Thus, in the conformational cycle, ATP is synthesized only in the T site and is released only in the O site. The energy dependent process is the ATP release step which is accomplished by energy dependent rotation of the g subunit.

260

CHAPTER14 Electron Transport.and Oxidative Phosphorylation

F I G U R E 14-14 Structure of ATP synthase. The Fi complex situated above the membrane consists of three c~/~ dimers and single y, 5, e subunits. The membrane segment F0, also known as a stalk, consists of H + channels. Protons are conducted via the C-subunit channels. The rotation of C-subunits relative to a subunit of the stalk drives the rotation of the V-subunit. [Reproduced with permission from Y. Zhou, T. M. Duncan and R. L. Crass: Subunit rotation in Escherichia coli FoF1-ATP synthase during oxidative phosphorylation. Proc. Natl. Acad. Sci. 94, 10583 (1997). [(~)1997 by the National Academy of Sciences.]

F I G U R E 14-15 Nucleotide binding conformational changes of FI ATP synthase complex. See text for details. [Reproduced with permission from Y. Zhou, T. M. Duncan and R. L. Crass: Subunit rotation in Escherichia coli FoF1-ATP synthase during oxidative phosphorylation. Proc. Natl. Acad. Sci. 94, 10583 (1997). 9 1997 by the National Academy of Sciences.]


261 Intermembrane Inner membrane Matrix space NO2

The central idea of chemiosmosis, that ATP synthesis is driven by the proton-motive force generated during respiration, is well accepted and is supported by several lines of experimental evidence. However, some aspects of the chemiosmotic theory are still to be resolved. For example, the precise location of proton-generating sites, the pathways of proton transport, the mechanism of proton flow, and how many protons are ejected per site are not well understood. It was originally proposed that two protons were ejected at each site, but the number may be more.

~ I

NO~

NO2

().

NO2

OH

OH

NO2

q -

U n c o u p l i n g A g e n t s of O x i d a t i v e P h o s p h o r y l a t i o n Uncoupling agents dissociate ATP synthes!s and other energy-dependent membrane functions from the transport of reducing equivalents in the respiratory chain. Normally, these processes are tightly coupled. These uncoupling agents cause a several-fold stimulation of respiration with unimpeded utilization of substrate and dissipation of energy as heat. A classic example of an uncoupling agent is 2,4-dinitrophenol (Figure 14-16); A group of antibiotics (e.g., valinomycin, nigericin, and gramicidin A) transport cations across the cell membrane. Such agents, known as i o n o p h o r e s , are widely used to probe membrane structure and function. Ionophores uncouple oxidative phosphorylation. Valinomycin, a cyclic peptide (Figure 14-17), forms a lipid-soluble complex with K + that readily passes through the inner membrane, whereas K + by itself does not. In the valinomycin-K + complex, hydrophobic groups, present on the outside, facilitate transport of the complex in the lipid environment;

0

o

FIGURE 14-16 Uncoupling of oxidative phosphorylation by 2,4-dinitrophenol (2,4-DNP). The anionic form of 2,4-DNP is protonated in the intermembrane space, is lipid soluble, and crosses the inner membranereadily. In the matrix, the protonated form dissociates, abolishing the proton gradient established by substrate oxidation. The ionized form of 2,4-DNP is poorly soluble in the membrane lipids and therefore is not easily transported across the membrane (dashed arrow). It is lipophilic and capable of transporting protons from one side of the membrane to the other (aprotonophore), thus abolishing the proton gradient. K +, located on the inside, interacts with the hydrophilic groups. The matrix side of the inner membrane has a negative potential, so the positively charged valinomycin-K + complex is drawn inward. The complex uncouples oxidative phosphorylation by decreasing the membrane potential. Unlike valinomycin, it exchanges H + for K + across the membrane. Its

i?H3 0

(CH~)~cH ~,.---O-CH~c~

/ /CH.va. O%,.,/NH (CH3)2CH /L, H C O-Hyi

L-Lac 2

(CH3)2CH_C H L-Val 11

NH CH(CH3)2 ~CH L-Val \C O 3 ~_ rO OH(OH3)2 ~"

D-Val 5

_Lac

10

L-Lac

CH...n~C\cH o D-Val

/

~ c,,CH

L-Val g D-Hyi 7 .NH U / "NH 8 C H ./ (CH3)2CH o/C~(~H--O-4~OO 'OH(OH3) 2 CH(CH3)2

(a)

L-Vat

~

t.La c

CH -CH(CH3) 2

/ O C=

NH

NO2

NO2

O

__ O~ -O

~ "U--~ ..... ~ O .....

O

o~ ',

0 " ~ "~"~"/' ~ , ~ ~ ~/ ~ O')~" " - "~ "

(b)

FIGURE 14-17 Structure of valinomycin (a) and its complex with K+ (b). Valinomycin, which consists of three identical fragments of D-valyl-L-lactyl-L-valyl-D-o~-hydroxyisovalericacid (D-Val-L-Lac-L-Val-D-Hyi),is a mobile or channel-forming ionophore and an uncoupler of oxidative phosphorylation. Note the hydrophobic exterior and the hydrophilic interior of the complex. [Structure (b) is reproduced, with permission, from B. C. Pressman: Biological application of ionophores. Annu. Rev. Biochem. 45, 501 (1976). ~ 1976 by Annual Reviews Inc.]

~6~

CHAPTER14 Electron Transport and Oxidative Phosphorylation OCH3

O

H~C.~ 0

o

o

H3C / ~ C H

3-

o

-o.

~.., L;N3 H3

H3

I

I1

O / ~ / . ~ O . ~ c

\r.u ,.,n3

v

c--o-

CH3

(a)

o-. o "x

x,

< o ....

(b) F I G U R E 14-18 Structure of nigericin (a) and its complex with K + (b). Nigericin is a mobile or channel-forming ionophore; at one side of the membrane, the anionic form complexes with K + and then migrates to the other side, where K § is discharged and the anionic form is protonated. The protonated form migrates to the opposite side and discharges H +, and the anionic species formed initiates the next cycle. Thus, nigericin exchanges H + for K + across the membrane and uncouples oxidative phosphorylation. Nigericin, a polycyclic ether carboxylate like valinomycin, forms a complex with K + that diffuses through the membrane.

carboxylate group interacts with the cation to form an electroneutral complex. In the exchange of H + for K +, although the membrane potential is not altered, the pH differential is abolished, thereby uncoupling oxidative phosphorylation.

Gramicidin A (Figure 14-19), a linear peptide, forms a head-to-head dimer that creates an aqueous helical channel which allows transport of a variety of monovalent cations (e.g., Na +, K +, and H+). The channel is a helix formed by the coiling of two/3-pleated sheets and is lined with

0 H H II I L L D L D L D L D L D L D L I H--C--N--Va I--GI y--A| a--Leu--Ala--Va l--Va I --Va I--Trp--Leu--Trp--Leu--Trp--Leu--Trp--N--CH2--CH2--OH

(a)

1

15

( (b)

(

F I G U R E 14-19 Structure of gramicidin A (a) and the formation of a helical pore through a lipid bilayer by assembly of two gramicidin A molecules via their formyl groups at the N termini (b). A variety of monovalent cations are transported through the static pore. [Structure (b) is reproduced with permission from Y.A. Ovchinnikov; Physico-chemical basis of ion transport through biological membranes: Ionophores and ion channels. Eur. J. Biochem. 94, 321 (1979).]

SECTION 14.3

Mitochondrial Energy States

263

TABLE 14-4

Energy States o f Mitochondria* State 1

Characteristics ADP level Substrate level Respiration rate Rate-limiting component

Aerobic Low Low-endogenous Slow Phosphate acceptor (ADP)

State 2

State 3

Aerobic High Approaching 0 Slow Substrate

Aerobic High High Fast Respiratory chain

State 4

Aerobic Low High Slow Phosphate acceptor (ADP)

State 5

Anaerobic High High Oxygen

*Reproduced, with permission, from B. Chance and G. R. Williams: Respiratory enzymes in oxidative phosphorylation: the steady state. J. Biol. Chem. 217, 409 (1955).

carbonyl groups of the peptide bonds. Unlike valinomycin and nigericin, gramicidin A forms a static pore in the membrane.

14.3 Mitochondrial Energy States Normal mitochondria can exist in one of five energy states (Table 14-4). In the presence of nonlimiting amounts of respiratory chain components, the rate of oxygen consumption is affected by changes in the NADH/NAD + ratio, the phosphorylation ratio (or phosphate potential, [ATP]/[ADP][Pi]), and pO2. In a resting (unstimulated) mitochondrion, in the presence of nonlimiting concentrations of substrate (reductant), oxygen, and phosphate, respiratory activity is controlled by the availability of ADP (state 4, Table 14-4). In this state, the rate of endogenous energy dissipation is low and is regulated by ADE On addition of ADP, the respiratory rate increases dramatically (state 3) until the ADP supply is depleted. Repeated ADP-induced respiratory cycles between states 4 and 3 cause depletion of substrate (state 2), phosphate, or oxygen. If oxygen is unavailable, respiration ceases (state 5). Dinitrophenol abolishes respiratory control because it uncouples the transport of reducing equivalents in the respiratory chain from the phosphorylation of ADE In the oxidation of carbohydrates, energy is conserved at all three stages: glycolysis, TCA cycle, and oxidative phosphorylation. Each of these stages is regulated at specific sites, and all three are coordinated in such a manner that ATP is synthesized only to the extent that it is needed in the cell. For example, when the [ATP]/[ADP] ratio is high, glycolysis, TCA cycle, and oxidative phosphorylation are inhibited. The key regulatory enzyme, 6-phosphofructokinase, is activated by ADP and inhibited by ATP (see also Chapters 13, 15, and 22.)

E n e r g y - L i n k e d Functions of M i t o c h o n d r i a Other Than ATP Synthesis

The energy derived from the respiratory chain, although primarily coupled to the formation of ATE is also utilized for purposes such as heat production, ion transport, and the transhydrogenase reaction. Brown adipose tissue is responsible for nonshivering thermogenesis, which is important in the arousal of hibernating animals and for maintaining body temperature in hairless neonates of mammals, including humans. Dietinduced thermogenesis also occurs in brown adipose tissue (Chapter 12). Brown adipose tissue, in contrast to white adipose tissue, consists of small cells that are rich in mitochondria and lipid dispersed in the cytoplasm as distinct droplets. Mitochondria of brown adipose tissue are naturally uncoupled, so that oxidation of substrates generates heat rather than a proton-motive force. The inner mitochondrial membrane contains, instead of the normal proton-translocating channel in F0, an H + uniport that causes the dissipation of energy as heat without the concomitant synthesis of ATE When the requirement for thermogenesis ceases, the H + uniport is blocked by a protein called thermogenin (M.W. 32,000). Thermogenin becomes functional after binding to purine nucleotides, of which GDP is the most effective and ADP and ATP are less effective. This protein recouples phosphorylation with the energy released in the respiratory chain. It is located at the entrance to the H + channel on the C side of the inner membrane. Stimulation of thermogenesis in brown adipose tissue is initiated by stimulation of the sympathetic nervous system, which releases norepinephrine at nerve endings. Norepinephrine combines with the fiadrenergic receptors of the brown adipose tissue cells and initiates cAMP-dependent activation of triacylglycerol lipase (Chapter 22), leading to the elevated intracellular concentrations of fatty acids that are oxidized in the mitochondria.

~64


TABLE 14-5

Mitochondrial Metabolite Translocators* Metabolites Dicarboxylates Tricarboxylates o~-Ketoglutarate Phosphate Pyruvate Glutamate Glutamine Ornithine Neutral amino acids Acyl carnitine, carnitine Glutamate, aspartate ADP, ATP

Species Translocated (In ~-- out) Malate 2- ~,--~--phosphate2Citrate3- + H +,.~__~malate 2a-Ke t o g lutarate 2- ~ m al ate 2Phosphate- ~ O H Pyruvate- ~,-~--OHGlutamate- ~,-~--OHGlutamine ~ glutamate- + HOrnithine+~,-~--H+ Neutral amino acids Acyl carnitine ~,-~--carnitine Glutamate- + H + ~,-~--aspartateADP 3- ~ ATP 4-

*Reproduced, with permission, from L. Ernster and G. Schatz: Mitochondria: A historical review. J. Cell Biol. 91,227s (1981). 9 1981 by the Rockefeller University Press.

chain are present. The translocator consists of two identical hydrophobic peptides. The export of ATP 4- is necessarily linked to the uptake of ADP 3-. This transport system is electrogenic owing to the transport of nucleotides of unequal charge, the driving force being the membrane potential. Although made up of two identical subunits, the translocator is asymmetrical in its orientation; on the C side, it binds with ADP, and on the M side, it binds with ATE This asymmetry of nucleotide transport has been demonstrated by use of the inhibitors atractyloside and bongkrekic acid (Figure 14-20). Atractyloside is a glycoside found in the rhizomes of a Mediterranean thistle; bongkrekic acid is a branched-chain unsaturated fatty acid synthesized by a fungus found in decaying coconut meat. The former inhibitor binds to the C side of the translocator at the ADP site, whereas the latter binds to the M side of the translocator at the ATP site.

Transport of Cytoplasmic NADH to Mitochondria A transhydrogenase catalyzes the transfer of reducing equivalents from NADH to NADP + to form NADPH by utilizing energy captured by the energy conservation mechanisms of the respiratory chain without the participation of the phosphorylation system. Similarly, ion translocation occurs at the expense of energy derived from substrate oxidation in the respiratory chain. For example, Ca 2+ is transported from the cytoplasm to the mitochondrial matrix, through the inner mitochondrial membrane, at the expense of proton-motive force. The Ca 2+ efflux from mitochondria is regulated so that levels of cytoplasmic Ca 2+ that are optimal for function are achieved. Increased cytoplasmic Ca 2+ levels initiate or promote muscle contraction (Chapter 21), glycogen breakdown (Chapter 15), and oxidation of pyruvate (Chapter 13). Decreased levels of Ca 2+ have the opposite effect. Translocation systems of the inner mitochondrial membrane are listed in Table 14-5. Anion translocators are responsible for electroneutral movement of dicarboxylates, tricarboxylates, c~-ketoglutarate, glutamate, pyruvate, and inorganic phosphate. Specific electrogenic translocator systems exchange ATP for ADP, and glutamate for aspartate, across the membrane. The metabolic function of the translocators is to provide appropriate substrates (e.g., pyruvate and fatty acids) for mitochondrial oxidation that is coupled to ATP synthesis from ADP and Pi. The ATP/ADP translocator, also called adenine nucleotide translocase, plays a vital role in the metabolism of aerobic cells because mitochondrial ATP is primarily consumed outside the mitochondria to support biosynthetic reactions. The translocator is the most abundant protein in the mitochondrion; two molecules per unit of respiratory

NADH generated in the cytoplasm during glycolysis must be transported into the mitochondria if it is to be oxidized in the respiratory chain. However, the inner mitochondrial membrane not only is impermeable to NADH and NAD + but contains no transport systems for these substances. Thus, the [NAD+]/[NADH] ratio is many times higher in the cytoplasm (about 1000) than in mitochondria (about 8). The high value for this ratio in the cytoplasm favors the glyceraldehyde-phosphate dehydrogenase reaction (which is essential for glycolysis) and contributes to a negative A G; the low value for the ratio in mitochondria favors the oxidation of NADH in the respiratory chain. Impermeability to NADH is overcome by indirect transfer of the reducing equivalents through shuttling substrates that undergo oxidation-reduction reactions. This process consists of the following steps: a reaction in which NADH reduces a substrate in the cytoplasm; transport of the reduced substrate into mitochondria; and oxidation of the reduced substrate in the respiratory chain. Two pathways that transport reducing equivalents from NADH into mitochondria have been characterized and are known as the glycerol phosphate shuttle and the malate-aspartate shuttle. The glycerol-phosphate shuttle is mainly associated with wing muscle mitochondria in insects and is not quantitatively significant in mitochondria of mammalian muscle. However, its activity is higher in mammalian brain and liver cells than in mammalian muscle. The shuttle (Figure 14-21) involves two glycerol-3-phosphate dehydrogenases, one NAD+-linked and present in the cytoplasm, the other FAD-linked and present on the C side of the inner mitochondrial membrane. The process begins in the cytoplasm with the reduction of dihydroxyacetone

SECTION14.3 Mitochondrial Energy States

265 _OCH3

.

CH3

V

H~C

H~

H__

.CH=__

u r'. ~r,__~ / HOOC " ~ _ .../=,-,~,.,, / v - - v ~ , I ~,(.;--U =. L~rl2 / I PI HC-~-C I H

~r,u/ vn 2

,

/

..jH

---U~

C---C:

~u

/

COOH

~

k~

--CH;

/C---C~CH3

9 "C--C: H/ -- ~u

n

I

H~

Ga s

rl

Bongkrekic acid

~CH,

COOH

~H=OH

CH=

H C|..... _O I _SO3 \ / O~ / CL~= / C . ~H_ / gH/ H C CH H

-O3S--O~ \?

I/ ~H

C~Cl

T

H

?

I

(r

"CH CH2

~ L

C~H2 .~CH~ j C H 2 -"'~CH CH=

I

C--CH-- 2

H / ~" OH

13--O OH

(~H=),

Atractyloside

CH3

F I G U R E 14-20 Inhibitors of adenine nucleotide (ATP/ADP) translocation. Bongkrekic acid binds to the ATP binding site, whereas atractyloside binds to the ADP binding site of the translocator.

phosphate to glycerol 3-phosphate by NAD+-dependent glycerol-3-phosphate dehydrogenase: CH2OH

CH2OH

I

C---'O

I

+ NADH + H +

~ CHOH

I

CH2OPO32-

Dihydroxyacetone phosphate

+

NAD +

I

CH2OPO32Glycerol 3-phosphate

Glycerol 3-phosphate in the intramembrane space is oxidized to dihydroxyacetone phosphate by the FAD-linked dehydrogenase. The FADH2 thus generated is then oxidized by CoQ of the respiratory chain. This shuttle differs from the malate-aspartate shuttle in three important respects:

Cytoplasm of cell NAD,+H, Dihydroxyacetone n " - ~ / f phosphate V Cytoplasmic J glycerol-3-phosphate ,/~ dehydrogenase

NA~

~ Glycerol 3-phosphate,

Outer mitochondrial membrane

1. The substrate glycerol 3-phosphate, which carries the reducing equivalents, does not penetrate the inner mitochondrial membrane, since it is oxidized on the C side of the membrane. 2. The oxidation of one molecule of cytoplasmic NADH yields only two molecules of ATE since the reducing equivalents enter the respiratory chain at the CoQ level. 3. The shuttle is bidirectional. The malate-aspartate shuttle is quantitatively more significant in all vertebrate tissues. It is unidirectional, requiring cytoplasmic and mitochondrial malate dehydrogenases and aspartate aminotransferases as well as two membrane-bound carrier systems (Figure 14-22). In this process, the reducing equivalents of NADH are

Inner mitochondrial membrane

Intermembrane space

Matrix of mitochondrion

Electron trar~sport ..... Dihydroxyacetone phosphate *",, F A D H 2 Mitochondrial glycerol-3-phosphatedehydrogenase = Glycerol 3-phosphate J

~

,

FAD

F I G U R E 14-21 Glycerol 3-phosphate shuttle for the transport of cytoplasmic reducing equivalents to the inner mitochondrial membrane. Glycerol 3-phosphate, which carries the reducing equivalents, is oxidized to dihydroxyacetone phosphate by an FAD-linked dehydrogenase located on the C side of the inner membrane.

266

CHAPTER14 Electron Transport and Oxidative Phosphorylation Cytoplasm

Matrix

~~.~OAA"-...,,.~GIo'

NAI~/~

Ma, 1 and 3. 2. 4 and 6. 5.

lf.[

"GIu.

1~2-~1

~OAA__

.,.NADH+H+

--Ma,- " ' ~ NAD+

Malatedehydrogenase

Malate-(x-ketoglutaratetranslocase Aspartateaminotransferase Glutamate-aspartatetranslocase

F I G U R E 14-22 Malate-aspartate shuttle for the transport of cytoplasmic reducing equivalents across the inner membrane of mitochondria. Malate, which carries the reducing equivalents, is oxidized to oxaloacetate with the generation of NADH in the matrix. To complete the unidirectional cycle, oxaloacetate is transported out of the matrix as aspartate. Mal = malate; OAA = oxaloacetate; c~-KG = c~-ketoglutarate; Glu = glutamate; Asp -- aspartate.

transferred in the cytoplasm to oxaloacetate, which is converted to malate. Malate is transported, via the malate-ot-ketoglutarate translocase, to the mitochondrial matrix, where it is oxidized to oxaloacetate, accompanied by the formation of NADH, which, by respiratory chain oxidation, produces three molecules of ATE To complete the shuttle, oxaloacetate must be transported from the matrix to the cytoplasm but there is no such transport system. Instead, oxaloacetate is first converted to aspartate by aspartate aminotransferase (Chapter 17) and then transported out of the mitochondria via the glutamate-aspartate translocase.

14.4 The Mitochondrial Genome The human mitochondrion contains its own genome; a circular double-stranded DNA molecule of 16,569 base pairs. Mitochondrial DNA (mtDNA) is a highly compact molecule and one of its strands has greater density (heavy-, H-strand) than the other (light-, L-strand). The only noncoding region, which consists of about 1 kb of mtDNA, is known as a displacement loop (D-loop). The D-loop contains the replication origin for H-strand replication and promoters for transcription from both strands. The mtDNA consists of 37 genes that encode 13 polypeptides that are components of the respiratory chain complexes I, III, IV, and V. Complex I (NADH: ubiquinone oxidoreductase) contains seven mtDNA gene products. Complex III (ubiquinol: cytochrome c oxidoreductase) incorporates one (cytochrome b); complex IV (cytochrome c oxidoreductase) incorporates three; and complex V incorporates two (Table 14-6). The remainder of the mtDNA genes encode 12S and 16S rRNAs and 22 tRNAs.

TABLE 14-6 The Mitochondrial Genes Contributing to the Complexes of the Mitochondrial Oxidative Phosphorylation Machinery

Complex

Total Subunits

Complex I, NADH: CoQ reductase Complex II, succinate dehydrogenase Complex III, ubiquinone: cytochrome c reductase Complex IV, cytochrome oxidase Complex V, ATP synthase

Encoded by mtDNA

40

7

4

0

10

1

13

3

13

2

Nuclear DNA (nDNA) provides more than 65 oxidative phosphorylation gene products. Overall, oxidative phosphosphorylation requires at least 100 gene products; proteins encoded in nuclear genes are synthesized on cytoplasmic ribosomes and imported and assembled within the mitochondria. A typical human cell contains many mitochondria with each mitochondrion having more than one mtDNA. Cytoplasmic location and multiple mtDNAs give mitochondria special characteristics: 9 The mtDNA is transmitted through the oocyte cytoplasm and is maternally inherited. 9 The occurrence of a mtDNA mutation creates a mixed intracellular population of mutant and original molecules (heteroplasmy). As the heteroplasmic cells divide during mitosis or meiosis, the mutant and original mtDNAs are randomly distributed to the daughter cells. This results in drifting of the mitochondrial genotype. The replicative segregation ultimately results in cells with either mutant or normal mtDNAs (homoplasmy). 9 Severe mtDNA defects reduce cellular energy outputs until they decline below the minimum energy level (energetic threshold) for normal tissue function. Such energetic thresholds differ among tissues, with the brain, heart, muscle, kidney and endocrine organs being most reliant on mitochondrial energy. 9 The mtDNA has a very high mutation rate, some 10-17 times higher than nDNA, which affects both germ line and somatic tissue mtDNAs. Germ line mutations result in maternally transmitted diseases or predispose individuals to late-onset degenerative

SECTION 14.5

Nuclear Control of Respiratory Chain Expression

diseases. Somatic mutations accumulate in somatic tissues and exacerbate inherited oxidative phosphosphorylation defects. The high mtDNA mutation rate may be due to the high concentration of oxygen radicals at the mitochondrial inner membrane, the lack of efficient mtDNA repair mechanisms and/or the absence of histones. 9 Cellular oxygen utilization decreases as a function of age and the ATP generating capacity of the cell is a function of age. The attenuation of ATP synthesis is associated with an age-related increase in somatic mtDNA damage in postmitotic tissue. The "normal" decline in ATP generating capability may facilitate disease occurrence when it is associated with an inherited oxidative phosphorylation mutation.

Mitochondrial Biogenesis Mitochondrial biogenesis incorporates two distinct processes: 1. The synthesis of mitochondrial membranes in which the synthesis of both outer and inner compartments is linked closely to the cell cycle; and 2. Differentiation of the organelle for respiratory chain phosphorylation, requiring coordinated control of both nuclear and mitochondrial genes. This is independent of cell growth and division. The supply of lipids for mitochondrial membrane biosynthesis depends largely on lipids synthesized elsewhere in the cell, especially the endoplasmic reticulum. However, one major lipid component of the inner mitochondrial membrane, cardiolipin (diphosphatidylglycerol), is synthesized within the mitochondria. The formation of functional enzyme complexes within the inner mitochondrial membrane proceeds through the sequential assembly of subunits. In assembly of complex I, nuclear encoded subunits form the inner core of the complex followed by attachment of mtDNA encoded subunits. Mitochondrial proteins synthesized in the cytosol contain information that guide them to the mitochondria (intracellular targeting) and that direct their incorporation to a specific site. This information resides in a signal sequence since mitochondrial proteins lacking all or a part of their sequences are not transported into mitochondria. The signal sequences share the property of having a high content of basic and hydroxylated amino acids, no acidic amino acids, and stretches of hydrophobic sequences. One disorder of mitochondrial transport in humans is due to a failure to transport the methylmalonyl-CoAmutase precursor protein. The mutation responsible for this disorder affects the signal sequence region. An-

267

other disorder has been ascribed to the transport of alanine glyoxylate aminotransferase, an enzyme normally located in peroxisomes of human liver, to mitochondria. The transport of proteins into mitochondria is aided by an outer membrane protein, the general insertion protein (GIP). Proteins that reside in the matrix space, inner membrane, or intermembrane space are routed from the GIP, across the contact site to the matrix space. This transport requires an electrical potential across the inner membrane. After crossing the mitochondrial membrane, precursor polypeptides are converted to mature proteins by the action of proteolytic enzymes that remove signal sequences.

Expression of mtDNA In addition to limited initiation sites for transcription and the presence of overlapping reading flames, mtDNA has other distinctive features, including a codon assignments that differ from those of nDNA. 1. UGA is used as a tryptophan codon (instead of a stop codon). 2. During translation in mitochondria, unusual codon recognition is a "two out of three" base interaction between codon and anticodon. 3. There are no AGA or AGG (arginine) codons in mtDNA genes. Also, no tRNAs are made in mitochondria for these codons. 4. A single tRNA met specifies both methionine tRNA and N-formylmethionine tRNA by secondary modification of the primary transcript. Thus, mitochondria require only 22 tRNA molecules to read the genetic code as compared to 31 required for the cytosolic system.

14.5 Nuclear Control of Respiratory Chain Expression Nuclear genes contribute the majority of respiratory subunits and all of the proteins required for mtDNA transcription, translation, and replication. It is estimated that more than 80% of the genes encoding the subunits of the respiratory chain are located in the cell nucleus. Despite the predominant role for nuclear gene products in the biogenesis and function of the mitochondrial respiratory chain, few nuclear genes have been implicated in mitochondrial genetic disorders. Certain abnormalities of mtDNA are transmitted as Mendelian traits, and it is thought that such characteristics are caused by mutation in identified nuclear genes.

268

CHAPTER 14


TABLE 14-7 Correlations Found between Mitochondrial DNA Mutations and Human Diseases*

A. Nueleotide Substitutions 1. Mildly deleterious base substitutions 2. Moderately deleterious nucleotide substitutions 3. Severe nucleotide substitutions

Clinical Features Familial deafness, Alzheimer's disease, Parkinson's disease Leber's Hereditary Optic Neuropathy (LHON), Myoclonic Epilepsy and Ragged-Red Fiber disease (MERRF) Leigh's Syndrome dystonia

B. (mt)DNA Rearrangements 1. Milder rearrangements (duplications) 2. Severe rearrangements (deletions)

Maternally inherited adult-onset diabetes and deafness Adult-onset, Chronic Progressive External Ophthalmoplegia (CPEO), Kearns-Sayre Syndrome (KSS), Lethal Childhood Disorders, Pearsons Marrow/Pancreas Syndrome

*Modified from D. C. Wallace J. of Bioenergetics and Biomembranes 26 (1994) 241-250.

A number of nuclear-derived transcription factors has been implicated in respiratory chain expression and may play an important role in control of mitochondrial functions. One nuclear respiratory factor (NRF-1) binds to genes encoding respiratory proteins, to the rate-limiting enzyme in heme biosynthesis, and to components of mtDNA transcription and replication. A second respiratory factor (NRF-2) is required for maximal production of cytochrome c oxidase subunit IV (COXIV) and Vb (COXVb). The two regulatory proteins act on a subset of overlapping nuclear genes required for mitochondrial respiratory activity and integrate the expression of nuclear and mitochondrial genes. Complexes III, IV, V, and cytochrome c have at least one subunit whose expression is under NRF control. Another regulatory protein (mtTFA) stimulates transcription initiated by mitochondrial RNA polymerase from heavy (H) and light (L) strand promoters within the mtDNA D-loop. The essential role of mtTFA in the transcription and maintenance of mtDNA makes it a prime candidate for a regulatory function in nuclearmitochondrial interactions. Another nDNA regulatory protein functions in the termination of mitochondrial transcription. The mitochondrial termination factor TERF promotes termination in vitro. The factor binds at the junction between 16S rRNA and leucyl-tRNA genes, a bidirectional termination site. The binding activity is associated with a 34-kDa protein. A point mutation associated with mitochondrial myopathy, encephalomyopathy, lactic acidosis and stroke-like episodes diminishes the termination of 16S rRNA transcription in vitro and reduces the binding affinity of the 34-kDa protein. These findings suggest a potential link between human disease and a nDNA regulatory factor.

14.6 Mitochondrial Diseases The bioenergetic defects resulting from mtDNA mutations may be a common cause of degenerative diseases (Table 14-7). Defects in nuclear-cytoplasmic interaction are generally the resultant of autosomal dominant mutations; complex disease states result from depletion of mtDNAs from tissues. Mitochondrial DNA mutations are associated with a broad spectrum of chronic degenerative diseases with a variety of clinical presentations. Identical mutations are associated with very different phenotypes and the same phenotype can be associated with different mutations.

Base Substitution Mutations Diseases involving mtDNA nucleotide substitution represent a broad array of clinical phenotypes typically featuring nervous system involvement. LHON (Leber's hereditary optic neuropathy), the best studied mtDNA nucleotide substitution disease, is characterized by a rapid, painless, bilateral loss of central vision due to optic nerve atrophy. The disease is maternally inherited, and its occurrence within families is variable, with young adult males most commonly affected. Modulating factors also influence disease expression. LHON is a genetically heterogeneous disease associated with at least 16 different missense mutations in mtDNA. Three of thesemND1 gene, ND4 gene, and ND6 gene--are the primary causes of LHON. The latter mutation is believed to cause not only LHON but also childhood dystonia associated with bilateral basal ganglia degeneration (LHON + dystonia). Two heteroplasmic missense mutations (ATPase G gene) result in highly variable disease phenotypes within

269

SECTION14.6 Mitochondrial Diseases

F I G U R E 14-23 Human mitochondrial genome showing locations of disease-causing mutations. [Reproduced with permission from M.D. Brown and D.C. Wallace: Molecular basis of mitochondrial DNA disease, J. Bioenerg. Biomembranes 26, 273 (1994).]

families. In one, the mutation leads to neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NAPP) while in most individuals the presentation is described as Leigh's disease. These mutations cause replacement of a highly conserved leucine residue resulting in loss of ATP production. The differences in the clinical phenotypes of the various mtDNA substitution mutations (LHON, LHON and dystonia, and NARP/Leigh's disease) illustrate the clinical variability of mtDNA diseases. The LHON group of diseases demonstrates that mildly deleterious mtDNA point mutations can accumulate on a mitochondrial chromosome during evolution, thus creating a "predisposing" mtDNA genotype. Such mutations are relatively benign and require other factors such as age-related decline in oxidative phosphorylation to reduce energy output sufficiently to cause disease. Consequently, these mtDNA mutations are generally associated with late-onset disease and also may predispose individuals to Alzheimer's and Parkinson's disease. Figure 14-23 presents a map of the human mitochondrial genome showing positions of point mutations and deletions.

Transfer RNA (tRNA) Mutations Fourteen mtDNA tRNA mutations have been associated with maternally inherited disease. Such mutations are typically associated with severe mitochondrial myopathies, characterized by "ragged red" skeletal muscle fibers upon Gomori trichrome staining and the accumulation of structurally abnormal mitochondria in muscle. Mutations in tRNAs exemplify the threshold effect whereby (due to replicative segragation) individuals may not exhibit clinical signs until the proportion of mutant mtDNA exceeds 80-90%. Myoclonic epilepsy and ragged red fiber (MERRF) disease, mitochondrial encephalomyopathy lactic acidosis (MELAS), as well as maternally inherited myopathy and cardiomyopathy (MMC) are wellcharacterized mitochondrial diseases. Because oxidative phosphorylation capacity declines with age, individuals with identical mtDNA genotypes may express different clinical signs if they are of different ages. Individuals under the age of 20 with 80% mutant mtDNAs can be asymptomatic; individuals with the same

270

mtDNA mutation over the age of 60 may suffer severe multisystem neurological disease. In general, older individuals have a lower threshold for oxidative phosphorylation dysfunction without clinical consequences. Two tRNA Leu(uuR) point mutations account for the majority of MELAS patients. An A-to-G transition at 3243 has been found in approximately 80% of MELAS patients. The 3243 mutation alters a highly conserved nucleotide within the dihydrouridine loop of the leucine tRNA. Unlike the MERRF variants, the 3243 mutation can result in a number of different clinical syndromes in addition to MELAS. These include diabetes mellitus and deafness, cardiomyopathy and ocular myopathy. Other mutations to tRNA Leu result in mitochondrial myopathy as well as late-onset hypertrophic cardiomyopathy and myopathy. Overall, the mtDNA tRNA mutations affect both CNS and skeletal/cardiac muscle tissue while missense mutations primarily affect nervous tissue. Only one mitochondrial rRNA point mutation has been associated with disease. The homoplasmic A-to-G transition at position 1555 of the 12S rRNA gene is associated with maternally inherited deafness and aminoglycoside-induced deafness.

CHAPTER 14


exercise intolerance). Despite major advances in our understanding of mitochondrial disease, treatment options are limited. Pharmacological therapies have been reported to be of some benefit in isolated cases. In two sisters with NADH-CoQ reductase deficiency who exhibited exercise intolerance, the 31p_NMR study of forearm muscles showed an abnormally rapid decrease in phosphocreatine levels during light exercise and a very slow recovery to normal phosphocreatine and pH levels in the postexercise period. 31p-NMR spectroscopy has also been used to evaluate the bioenergetic capacity of skeletal muscle in a patient with a history of progressive muscle weakness and lactic acidosis. The defect resides between Q and cytochrome c (i.e., in complex liD; in particular, it is related to reduced levels of cytochrome b and to a deficiency of at least four additional polypeptides of complex III. Administration of appropriate redox mediators, menadione (vitamin K3), and ascorbate (vitamin C) bypasses the deficient complex III and allows for the transport of electrons from Q to cytochrome c: NADH ~

CoQ

= Cyt b ~

Cyt c 1 ~ " Cyt cl ~ Deficient Step

mtDNA Deletions and Duplications Large mtDNA deletions account for most cases of ocular myopathy and Pearson's marrow~pancreas syndrome. Ocular myopathy patients can exhibit a variety of clinical symptoms, from mild chronic progressive external ophthalmoplegia (CPEO) to Kearns-Sayre Syndrome (KSS). These diseases are characterized by an early onset of ophthalmoplegia, atypical retinitis pigmentosa, mitochondrial myopathy, and usually cerebellar syndrome and cardiac conduction abnormalities. More than 120 different mtDNA deletions have been identified from patients' tissues. Partial duplications of mtDNA have been detected in ocular myopathy and Pearson's syndrome; however, duplications are much rarer than spontaneous deletions in patients with these conditions. Exactly how partial mtDNA duplications arise is unknown. Patients with mitochondrial disorders may present at any age and show variation in both the severity and kind of symptoms associated with a single genetic abnormality. For example, the tRNA Leu(UuR) mutation is predominantly associated with the neurological syndrome MELAS but may also be manifested as CPEO, myopathy, diabetes, and deafness. Mitochondriopathies associated with severe limitation of aerobic metabolism in such organs as liver, kidney, heart, and brain are probably incompatible with survival. Milder mitochondrial disorders may become clinically observable when cellular energy demand is not satisfied (e.g.,

Cyt c ~

Cyt a + a3 ~

02

l

Menadione

~- Ascorbate

Although the bypass pathway for electron transport theoretically decreases ATP production by 33% of normal mitochondria, in this individual it improves phosphorylation and functional activity.

14.7 Other Reducing-EquivalentTransport and Oxygen-ConsumingSystems In most cells, more than 90% of the oxygen utilized is consumed in the respiratory chain that is coupled to the production of ATP. However, electron transport and oxygen utilization occur in a variety of other reactions, including those catalyzed by oxidases or oxygenases. Xanthine oxidase, an enzyme involved in purine catabolism (Chapter 27), catalyzes the oxidation of hypoxanthine to xanthine, and of xanthine to uric acid. In these reactions, reducing equivalents are transferred via FAD, and Fe 3+ and Mo 6+, while the oxygen is converted to superoxide anion (O2): H20,O2 Hypoxanthine

'

~ H20,O2

Xanthine

H +, 0 2 ~

l,

H +, 0 2 ;- Uric acid

SECTION14.7 Other Reducing-Equivalent Transport and Oxygen-Consuming Systems

D-Amino acid oxidase is a flavoprotein located in peroxisomes. D-Amino oxidases catalyze the oxidation of a D-amino acid to the corresponding keto acid:

271

are powerful microbicides: 0 2 -}-H202 ~ OH" + OH- + 02

and COO-

02

H202

HmCmNH3 + + H20

.~.

I

R o-Amino acid O

II R9m C m C O O - + NH4 + Keto acid

The O 2 and H202 produced in the above two reactions are potentially toxic. Superoxide anion is also produced by oxygen-reducing enzymes of phagocytes (neutrophils, eosinophils, and mononuclear phagocytes) that defend the host against invading organisms by producing reactive oxidants from oxygen. The reducing equivalents are provided by NADPH derived from the hexose monophosphate shunt (Chapter 15). Neutrophils are the most active participants in phagocytosis. Unstimulated neutrophils circulate in the bloodstream with a life span of a few days. Upon bacterial infection, neutrophils actively migrate to the infected site, where they kill bacteria by the process of phagocytosis. During phagocytosis a large amount of oxygen is consumed by the neutrophils in a reaction termed the respiratory burst. The burst was initially thought to represent an energy requirement for phagocytosis; however, it was subsequently found to be insensitive to inhibitors of the mitochondrial respiratory chain (cyanide, antimycin) and associated with an increased turnover of the hexose monophosphate shunt (Chapter 15). The oxygen consumed during phagocytosis is utilized by a unique enzyme system termed the respiratory burst oxidase or NADPH oxidase. The oxidase generates superoxide anion (O2), a one-electron reduced species, driven by intracellular NADPH, NADPH § 202 --+ NADP + + 20 2 § H + Superoxide anion is also the source of a number of other microbicidal oxidants. It functions as an electron donor and is converted to hydrogen peroxide by superoxide dismutase: 20 2 + 2H +----~H202 + 02 A number of highly reactive oxidizing agents are produced from H202 and 0 2, such as the conjugate acid of superoxide, the hydroperoxy radical (H02), hydroxyl radical (OH'), and hypochlorite (OC1-). These reactive agents

C l - + H202 ~ OC1- + H20

The former reaction is promoted by metal ions (e.g., Fe z+), and the latter is catalyzed by myeloperoxidase present in neutrophil granules. By acquiring a single electron, oxygen can give rise to a variety of toxic products. For example, tissue destruction is enhanced when x-ray treatment is used in conjunction with hyperbaric oxygen. The NADPH oxidase consists of five components. In the unstimulated cell, two of the components are membranebound (p22 ph~ and gp91 ph~ and three components are present in cytosol (p47 ph~ p67 ph~ and p40Ph~ The designations gp, phox, and p represent glycoprotein, phagocytic oxidase, and protein, respectively. The membranebound components occur as a heterodimeric flavohemoprotein known as cytochrome b558. The NADPH oxidase complex is dormant in resting phagocytes and becomes assembled and activated for superoxide formation upon bacterial invasion. The respiratory burst is stimulated in vitro as well as in vivo by a variety of reagents, among which are phorbol esters (PMA, phorbol 12-myristate 13-acetate), heat-aggregated IgG, unsaturated fatty acids and analogues of bacterial peptides (FMLP, formylmethyonyl-leucyl phenylalanine). The importance of NADPH oxidase is highlighted by chronic granulomatous disease (CGD), a rare inherited disease in which affected individuals are incapable of mounting a sustained respiratory burst. As a consequence, persons with CGD are incapable of generating the reactive oxygen compounds necessary for the intracellular killing of phagocytized microorganisms. The CGD patient suffers severe and recurrent bacterial and fungal infections. Studies using CGD patients' neutrophils as well as a cellfree system have helped identify the cellular components responsible for O 2 generation. Four different mutations cause CGD and these cause four defects in different components of the NADPH oxidase. These are the two membrane bound components of cytochrome b558 and two cytosolic factors p47 ph~ and p67 ph~ The loci for the four proteins have been mapped in chromosomes X, 16, 7, and 1, respectively. CGD patients are therefore classified as being either of an X-linked type or autosomal recessive. All reported cases of CGD may be explained by abnormalities in the genes coding for one or more of these four components of the oxidase complex. Recent studies have identified an additional cytosolic factor, p40 ph~ and a small G protein (rac) as being involved in the activation of the O 2 generating system.

272

CHAPTER 14

Unsaturated fatty acid residues of membrane lipids form ot-hydroperoxyalkenes by reacting with oxygen:

agents after a myocardial infarction, during reperfusion after kidney transplant, and in the lungs of premature infants. Allopurinol, a xanthine oxidase inhibitor (Chapter 27), is potentially useful in preventing the tissue injury brought about by ischemia followed by reperfusion. During ischemia, hypoxanthine production increases by enhanced adenine nucleotide catabolism (ATP --+ ADP --+ AMP --+ adenosine --+ inosine --+ hypoxanthine; see Chapter 27), providing a larger supply of substrate for xanthine oxidase and thus increased formation of superoxide anions. Complete reduction of O2 by single-electron transfer reactions (the univalent pathway) yields superoxide anion, hydrogen peroxide, and hydroxyl radical as intermediates:

OOH

These hydroperoxides undergo cleavage to give rise to short-chain aldehydes. Peroxidation can also cause damage to DNA and proteins. In the latter, sulfhydryl groups give rise to disulfide linkages. Vitamin E functions as an antioxidant and can prevent oxygen toxicity (Chapter 38). Glutathione peroxidase, a selenium containing enzyme, inactivates peroxides:

I I I HC--O

I Glutathione

H20

HC

,~ GSSG +

II + H202

In contrast, most 02 reduction in aerobic cells occurs by cytochrome c oxidase, which prevents the release of toxic oxygen intermediates:

HC Oxidized glutathione

OH 9 e-.H§ >H20

02 e- 0 2- e-,2H+* H20 2 ~

I

HC--O

2GSH +


I

O2 + 4H + + 4 e- --+ 2H20 Glutathione, which protects sulfhydryl groups of proteins, is regenerated by glutathione reductase, which uses reducing equivalents from NADPH.

Mitochondrial superoxide dismutase maintains intramitochondrial superoxide anion at very low steady-state concentrations. The overall process of reducing oxygen to water can be depicted as follows:

GSSG + NADPH -t- H + --+ 2GSH + NADP + The hydrogen peroxide formed above is decomposed by catalase:

Cytochrome c oxidase

-~ H20 ."

2H202 ~ 2H20 4- 02

l

Aerobic organisms are protected from oxygen toxicity by three enzymes: glutathione peroxidase, catalase, and superoxide dismutase. Superoxide dismutases are metalloenzymes (the cytoplasmic enzyme contains Cu 2+ and Zn 2+, whereas the mitochondrial enzyme contains Mn 2+) widely distributed in aerobic cells. The role of superoxide dismutases in preventing oxygen toxicity is still controversial. For example, some aerobic cells (e.g., adipocytes and some bacteria) lack superoxide dismutase, whereas some strict anaerobes possess this enzyme. Human superoxide dismutase has been prepared in large quantities by recombinant DNA methods in Escherichia coli. It has potential clinical use in preventing oxygen toxicity, for example, during the reestablishment of blood flow through dissolution of a blood clot by thrombolytic

Catalase,. peroxidase

I

02

\

~02-

~-H202

OH

J

Superoxide dismutase

Paraquat, a herbicide that is highly toxic to humans, causes respiratory distress that can lead to death. Damage to membranes of the epithelial cells lining the bronchioles and alveoli that occurs with paraquat poisoning has been ascribed to excessive production of superoxide anion. Paraquat readily accepts electrons from reduced substrates of high negative potential, while the reduced paraquat reacts with molecular oxygen to form superoxide anion and an oxidized paraquat molecule: Oxidized electron-donor

Reduced electron-donor

+

H3C--N

N--CH3 "

Reduced paraquat

H3C--N or

o2-

Oxidized paraquat

--OH 3

SECTION14.7 Other Reducing-Equivalent Transport and 0xygen-Consuming Systems

Oxygenases catalyze reactions in which an oxygen atom or molecule is incorporated into organic substrates. They may therefore be monooxygenases or dioxygenases. An example of a dioxygenase is tryptophan-2,3-dioxygenase (a heme enzyme), which participates in the catabolism of tryptophan (Chapter 17): + NH3

[•N)

I

CH2wCH__COO- + 02

~

~

O

+ NH3

II

I

v

H

~ NHmC'--O

I

H Tryptophan

Formylkynurenine

Monooxygenases, also called hydroxylases or mixedfunction oxidases, catalyze the incorporation of one atom of an oxygen molecule into the substrate, while the other atom is reduced to water. A second reductant is required (e.g., NADH, NADPH or tetrahydrobiopterin); the overall reaction is RH + AH2 -4- 0 2

~

T A B L E 14-8

Cytochrome P-450-Dependent Oxidations Oxidation of carbon atoms Aliphatic hydroxylation RCH2OH 9

R-OH 3

OH

R-CH2-CH

C--CH2--CHmCOO-

I

273

3

I , R-CH-CH

3

Epoxidation R1

RI\c/H

\CH II /CH R2

9

I/O

R2/C\

H

Aromatic hydroxylation

\

/

oH

ROH + A + H20

where AH2 is the second reductant. There are many classes of hydroxylases, depending upon the nature of the second reductant. One group, which

O-, N-, S-dealkylation R-O-CH

3

" ROH + HCHO

H I

R-N-CH 3

" RNH2 + HCHO

R-S-OH 3

" RSH + HCHO

Oxidation of nitrogen atom N-Hydroxylation H O I

II

R1-N-C-R2

F I G U R E 14-24 Cytochrome P-450 monooxygenase system in the hepatic endoplasmic reticulum. P-450-Fe(III) = ferricytochrome P-450; P-450-Fe(II) = ferrocytochrome P-450; bs = cytochrome bs; F p a = NADPH-cytochrome P-450 reductase; Fpb = NADH-cytochrome b5 reductase; XH = substrate. [Reproduced with permission from G. Mannering, K. W. Renton, R. el Azhary, and L. B. Delor, Effects of interferon-inducing agents on the hepa cytochrome P-450 drug metabolizing systems. Ann. N. Y Acad. Sci. 350, 314 (1980).]

OH O I II R9 1 - N - C - R 2

acts on a variety of substrates including foreign compounds (xenobiotics), contains cytochrome P-450. The latter derives its name from the absorption maximum at 450 nm of its carbon monoxide adduct, the P standing for pigment. The cytochrome P-450 monooxygenase system is widely distributed in nature. In humans, it is present in most organs, but the highest concentrations are found in the liver. It is found in membranes of the endoplasmic reticulum (microsomal fraction of the cell), mitochondria, and nucleus. The hepatic microsomal cytochrome P-450 system has been extensively investigated. It consists of a flavin

274 reductase (NADPH-cytochrome P-450 reductase) and one of at least six molecular species of cytochrome P-450. The overall reaction is initiated by combining the substrate (XH) with the ferric form of P-450 to produce the ferrous form by accepting electrons from NADPH-cytochrome P-450 reductase (steps a and b, Figure 14-24). The reductase contains one molecule each of FMN and FAD. The reduced P-450-substrate complex binds molecular oxygen, which becomes activated upon acceptance of an electron from the heme iron (steps c and d). The P-450substrate-oxygen complex accepts a second electron from either NADPH-cytochrome-P-450 reductase (step e) or cytochrome b5 (step f). In the final steps (g-j), one oxygen atom receives two protons to form a water molecule, and the other oxygen forms the hydroxyl group of the substrate. The regenerated ferric form of P-450 initiatesthe next cycle. Supply of the second electron pair is via the cytochrome b5 electron transport system, which is a minor pathway used principally for the desaturation of fatty acids. Cytochrome bs, a microsomal membrane-heme protein, receives electrons from NADH via the flavoprotein NADH-cytochrome b5 reductase. Superoxide anion can also be formed as a byproduct in the cytochrome P-450 system. The hepatic cytochrome P-450 system exhibits a broad substrate specificity, and many lipophilic compounds including drugs, chemicals, and endogenous metabolites are oxidized by it (Table 14-8). The hydroxylated compounds are converted to more polar metabolites by conjugation with glucuronate, sulfate, amino acids, or acetate that is catalyzed by appropriate transferases. The polar metabolites are excreted by either the biliary-intestinal or the renal system.

CHAPTER 14


Occasionally, the cytochrome P-450 system converts some chemicals to reactive species with carcinogenic potential (e.g., polycyclic hydrocarbons). The hepatic microsomal cytochrome P-450 system is inducible by many of its substrates. The cytochrome P-450 of adrenal cortical mitochondria is involved in steroid hydroxylase reactions, and this system contains iron-sulfur (FezS2) proteins.

Supplemental Readings and References T. E. Andreoli: Free radicals and oxidative stress. American Journal of Medicine 108, 650 (2000). B. M. Babior: NADPH oxidase: An update. Blood 93, 1464 (1999). B. M. Babior: Phagocytes and oxidative stress. American Journal of Medicine 109, 33 (2000). H. Beinert, R. H. Holon, and E. Munck. Iron-sulfur clusters: Nature's modulator, multipurpose structures. Science 277, 653 (1997). E D. Boyer: The ATP synthase--a splendid molecular machine. Annual Review of Biochemistry 66, 717 (1997). S. Iwata, J. W. Lee, K. Okada, et al.: Complete structure of the 11 subunit bovine mitochondrial cytochrome bcl complex. Science 281, 64 (1998). N. G. Larsson and D. A. Clayton: Molecular genetic aspects of human mitochondrial disorders. Annual Review of Genetics 29, 151 (1995). G. S. Shadel and D. A. Clayton: Mitochondrial DNA maintenance in vertebrates. Annual Review of Biochemistry 66, 409 (1997). B. L. Trumpower: The proton motive Q cycle. Journal of Biological Chemistry 265, 1409 (1990). L. H. Underhill: Mitochondrial DNA and Disease. New England Journal of Medicine 333, 638 (1995) D. Xi, C. A. Chang, H. Kim, et al.: Crystal structure of the cytochrome bcl complex from bovine heart mitochondria. Science 277, 60 (1997). M. Zeviani, V. Tiranti, and C. Piantadosi: Mitochondrial disorders. Medicine 77, 59 (1998). Y. Zhou, T. M. Duncan, and R. L. Cross: Subunit rotation in Escherichia Coli FoFI-ATP synthase during oxidative phosphorylaton. Proc. Natl. Acad. Sci. 94, 10583 (1997).

CHAPTER

15

Carbohydrate Metabolism II: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways

In this chapter, carbohydrate metabolism is discussed in terms of nondietary sources of glucose and nonglycolytic pathways. Gluconeogenesis and glycogen synthesis and breakdown make up the first category, while the pentose phosphate pathway (also called hexose monophosphate shunt) and the glucuronic acid pathway make up the second. Metabolic pathways of fructose, galactose, and some other sugars belong in both categories.

15.1 Gluconeogenesis Metabolic Role

Gluconeogenesis (literally, "formation of new sugar") is the metabolic process by which glucose is formed from noncarbohydrate sources, such as lactate, amino acids, and glycerol. Gluconeogenesis provides glucose when dietary intake is insufficient to supply the requirements of the brain and nervous system, erythrocytes, renal medulla, testes, and embryonic tissues, all of which use glucose as a major source of fuel. Gluconeogenesis has three additional functions. 1. Control of acid-base balance. Production of lactate in excess of its clearance causes metabolic acidosis, and resynthesis of glucose from lactate is a major

route of lactate disposal. Since glycolysis is almost totally anaerobic in erythrocytes, renal medulla, and some other tissues, even under normal conditions lactate is continually released. Other tissues, particularly muscle during vigorous exercise, can produce large amounts of lactate, which must be removed or lactic acidosis will result (Chapter 21). The continuous conversion of lactate to glucose in the liver and of glucose to lactate by anaerobic glycolysis, particularly in muscle, forms a cyclical flow of carbon called the Cori cycle (Chapter 22). Deamination of amino acids prior to gluconeogenesis in the kidney also provides a supply of NH3 to neutralize acids excreted in the urine (Chapter 39). 2. Maintenance of amino acid balance. Metabolic pathways for the degradation of most amino acids and for the synthesis of nonessential amino acids involve some steps of the gluconeogenic pathway. Imbalances of most amino acids, whether due to diet or to an altered metabolic state, are usually corrected in the liver by degradation of the excess amino acids or by synthesis of the deficient amino acids through gluconeogenic intermediates. 3. Provision of biosynthetic precursors. In the absence of adequate dietary carbohydrate intake, gluconeogenesis supplies precursors for the synthesis of glycoproteins, glycolipids, and structural carbohydrates.

275

CHAPTER15 Carbohydrate Metabolism I1" Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways

~6

Gluconeogenic Pathway Gluconeogenesis from pyruvate is essentially the reverse of glycolysis, with the exception of three nonequilibrium reactions (Figure 15-1). These reactions are Glucose + ATP 4-

hexokinase or glucokinase>

glucose-6-phosphate 2- + ADP 3- + H +

(15.1)

Fructose_6_phosphate 2_ + ATp4_ 6-phosphofructokinase> fructose- 1,6-bisphosphate 4- + ADP 3- + H +

(15.2)

Conversion of pyruvate to phosphoenolpyruvate involves two enzymes and the transport of substrates and reactants into and out of the mitochondrion. In glycolysis, conversion of phosphoenolpyruvate to pyruvate results in the formation of one high-energy phosphate bond. In gluconeogenesis, two high-energy phosphate bonds are consumed (ATP -+ ADP + Pi; GTP ~ GDP + Pi) in reversing the reaction. Gluconeogenesis begins when pyruvate, generated in the cytosol, is transported into the mitochondrion~through the action of a specific carrier~ and converted to oxaloacetate:

Phosphoenolpyruvate 3_ + ADp3_ + H+ pyruvatekinase> pyruvate- + ATP 4-

( 15.3)

In gluconeogenesis, these reactions are bypassed by alternate steps also involving changes in free energy and also physiologically irreversible.

Pyruvate ATP + CO2 + H20 ADP + P, -- >1Pyruvate carboxylase (in mitochondria) Oxaloacetate GTP GDP + CO2------"] Phosphoenolpyruvate carboxykinase (in mitochondria and cytosol) Phosphoenolpyruvate H20-.-.-~ 2-Phosphiglycerate 3-Phosphoglycerate ATP------. ADP-----' 1,3-Bisphosphoglycerate NADH + H+----~ / NAD~(~___, P, Glyceraldehyde 3-phosphate :

, Dihydroxyacetonephosphate

Fructose 1,6-bisphosphate H20 ~ Fructose-1 ,6-bisphosphatase

e,:

q

Fructose 6-phosphate

Glucose 6-phosphate H 2 0 - - - J Glucose-6-phosphatase (endoplasmic reticulum

P,

Glucose

FIGURE 15-1 Pathway of gluconeogenesis from l:)yruvate to glucose. Only enzymes required for gluconeogenesis are indicated; others are from glycolysis. The overall reaction for the synthesis of one molecule of glucose from two molecules of pyruvate is 2Pyruvate + 4ATP 4- + 2GTP 4- + 2NADH + 2 H + + 6 H 2 0 --+ Glucose + 2NAD + + 4 ADP 3- + 2GDP 3- + 6P 2- + 4H +

acetyl-CoA, Mg 2+

Pyruvate- + HCO 3 + ATP 4-

> pyruvate carboxylase

oxaloacetate 2- + ADp3- + p2- + H + Like many CO2-fixing enzymes, pyruvate carboxylase contains biotin bound through the s-NH2 of a lysyl residue (Chapter 18). The second reaction is the conversion of oxaloacetate to phosphoenolpyruvate: Oxaloacetate 2+ GTP 4- (or ITP 4-) phosphoenolpyruvatecarboxylinase(PEPCK)> phosphoenolpyruvate 3- +

C02

+ GDp3-(or IDP 3-)

In this reaction, inosine triphosphate (ITP) can substitute for guanosine triphosphate (GTP), and the CO2 lost is the one fixed in the carboxylase reaction. The net result of these reactions is Pyruvate + ATP + GTP (or ITP) -+ phosphoenolpyruvate + ADP + Pin t- GDP (or IDP) Pyruvate carboxylase is a mitochondrial enzyme in animal cells, whereas PEPCK is almost exclusively mitochondrial in some species (e.g., pigeons) and cytosolic in others (e.g., rats and mice). In humans (and guinea pigs), PEPCK occurs in both mitochondria and cytosol. An interesting consequence of this species differences is that quinolinate, an inhibitor of cytoplasmic PEPCK, causes hypoglycemia in rats but is less active in humans and guinea pigs. In humans, oxaloacetate must be transported out of the mitochondrion to supply the cytosolic PEPCK. Because there is no mitochondrial carrier for oxaloacetate and its diffusion across the mitochondrial membrane is slow, it is transported as malate or asparate (Figure 15-2). The malate shuttle carries oxaloacetate and reducing equivalents, whereas the aspartate shuttle, which does not require a preliminary reduction step, depends on the availability of glutamate and ot-ketoglutarate in excess of tricarboxylic acid (TCA) cycle requirements.

277

SECTION15.1 Gluconeogenesis INNERMITOCHONDRIAL MEMBRANE MATRIX CYTOSOL [ Oxaloacetate Oxaloacetate --NADH + H+ NADH+ H+~,,{3

Cytoplasm Glucose 6-phosphat

~

\

I ~",,...~NAD+ Malate

NAD+---JI

Malate,

H20~

Malate shuttle

Oxaloacetate Glutamate~ [ 7 ot-KetoglutarateJ[ Aspartate--

Glucose6-phosphatase Glucose6-phosphatase Regulatoryprotein(SP)

/61

Glutamate ~ NAD+ ate (z-Keto- - ~ N A D H + H+ glutarate Aspartate ~ NH+

Oxal~

P~ ucose/X~-.J 4 ~7 GI

'

~~ ~

~

Endoplasmic reticulum membrane

Aspartate shuttle

F I G U R E 15-2 Shuttle pathways for transporting oxaloacetate from mitochondria into the cytosol. The shuttles are named for the molecule that actually moves across the mitochondrial membrane. 1 and 3 = malate dehydrogenase; 2 -- malate translocase; 4 and 7 = aspartate aminotransferase; 5 = glutamate dehydrogenase; 6 = aspartate translocase.

The proportion of oxaloacetate carried by each shuttle probably depends on the redox state of the cytosol. If most of the pyruvate is derived from lactate, the NADH / NAD + ratio in the cytosol is elevated. In this situation, there is no need to transport reducing equivalents out of the mitochondria, and the asparate shuttle predominates. However, if alanine is the principal source of pyruvate, no cytosolic reduction occurs, and the glyceraldehyde-phosphate dehydrogenase reaction (which requires NADH) requires transport of reducing equivalents via the malate shuttle. In species in which oxaloacetate is converted in mitochondria to phosphoenolpyruvate (which is readily transported to the cytosol, perhaps via its own carrier system), no transport of oxaloacetate or reducing equivalents is required. In pigeons, which have virtually no cytosolic PEPCK, the rate of gluconeogenesis from pyruvate is much slower than it is from lactate, since there is no source of cytosolic reducing equivalents. Phosphoenolpyruvate is converted to fructose-l,6bisphosphate by reversal of glycolysis in the cytosol via reactions that are at near-equilibrium and whose direction is dictated by substrate concentration. Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate is a nonequilibrium step, catalyzed by fructose-l,6bisphosphatase: Mg +

Fructose- 1,6-bisphosphate 4- + H20

>

fructose_6_phosphate 2- + pZFructose-6-phosphate is then converted to glucose-6phosphate by reversal of another near-equilibrium reaction of glycolysis. In the last reaction in gluconeoge-

F I G U R E 15-3 Schematic representation of the hepatic microsomal glucose-6-phosphatase system. The hydrolysis of glucose phosphate to glucose and inorganic phosphate (Pi) consists of (1) glucose-6-phosphate transport protein (T1), (2) glucose-6-phosphatase with its catalytic site located on the luminal side of the membrane, (3) glucose-6-phosphatase CaZ+-binding stabilizing protein (SP) also known as regulatory protein, (4) phosphate transport protein (T2fl), (5) phosphate, pyrophosphate, and carbamoyl phosphate transport protein (T2fl), and (6) glucose transport protein (GLUT7).

nesis, glucose-6-phosphate is converted to glucose by glucose-6-phosphatase: Glucose-6-phosphate 2- + H20--+ glucose + p2Glucose-6-phosphatase is part of a multi-component integral membrane protein system (Figure 15-3) located in the endoplasmic reticulum of liver, kidney, and intestine but not of muscle or adipose tissue. This system consists of six different proteins. These are 1. Glucose-6-phosphate transport protein (T1), which transports glucose-6-phosphate into the lumen of the endoplasmic reticulum; 2. Catalytic subunit of glucose-6-phosphatase (M.W. 36,500), which hydrolyzes glucose-6-phosphate into glucose and phosphate at the luminal surface; 3. Glucose-6-phosphatase regulating protein (M.W. 21,000), which stabilizes the activity of glucose-6-phosphatase; 4. Microsomal phosphate transport protein (Tzot) which mediates the efflux of Pi, an inhibitor of the glucose-6-phosphatase, from the lumen of the endoplasmic reticulum to the cytosol; 5. Microsomal phosphate/pyrophosphate transport protein (Tzfl, M.W. 37,000), which transports phosphate, pyrophosphate, and carbamoyl phosphate, which are substrates for glucose-6-phosphatase; and 6. Microsomal glucose transport protein (GLUT7), which is a member of the family of facilitative glucose transport proteins (Chapter 13) and which transports glucose into the cytosol.

~

CHAPTER15

CarbohydrateMetabolism I1: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways

These actions of all four transport proteins are reversible. Phosphate transport protein Tzfl can transport pyrophosphate and carbamoyl phosphate as well as Pi, providing substrates and removing products for several other reactions (of unknown metabolic significance), which are also catalyzed by glucose-6-phosphatase. The importance of each member of the glucose-6-phosphate hydrolysis system is reflected in the occurrence of glycogen storage diseases. Deficiency of any of these five proteins leads to glycogen storage diseases (discussed later). Thus, gluconeogenesis requires the participation of enzymes of the cytosol, mitochondrion, and smooth endoplasmic reticulum, as well as of several transport systems, and it may involve more than one tissue. The complete gluconeogenic pathway, culminating in the release of glucose into the circulation, is present only in liver and kidney. Most tissues contain only some of the necessary enzymes. These "partial pathways" are probably used in glycerogenesis and in replenishing tricarboxylic acid (TCA) intermediates. Muscle can also convert lactate to glycogen, but this probably takes place only in one type of muscle fiber and only when glycogen stores are severely depleted and lactate concentrations are high, such as after heavy exercise. Under normal conditions, the liver provides 80% or more of the glucose produced in the body. During prolonged starvation, however, this proportion decreases, while that synthesized in the kidney increases to nearly half of the total, possibly in response to a need for NH3 to neutralize the metabolic acids eliminated in the urine in increased amounts (Chapter 22). Gluconeogenesis is a costly metabolic process. Conversion of two molecules of pyruvate to one of glucose consumes six high-energy phosphate bonds (4ATP + 2GTP --+ 4ADP + 2GDP + 6Pi) and results in the oxidation of two NADH molecules (Figure 15-1). In contrast, glycolytic metabolism of one molecule of glucose to two of pyruvate produces two high-energy phosphate bonds (2ADP + 2Pi ~ 2ATP) and reduces two molecules of NAD +. For gluconeogenesis to operate, the precursor supply and the energy state of the tissue must be greatly increased. Using some gluconeogenic precursors to provide energy (via glycolysis and the TCA cycle) to convert the remainder of the precursors to glucose would be inefficient, even under aerobic conditions. Usually, the catabolic signals (catecholamines, cortisol, and increase in glucagon/insulin ratio) that increase the supply of gluconeogenic precursors also favor lipolysis, which provides fatty acids to supply the necessary ATE When amino acid carbons are the principal gluconeogenic precursors, the metabolic and physiological debts are particularly large compared to those incurred

when lactate or glycerol is used. Amino acids are derived by breakdown of muscle protein, which is accompanied by a loss of electrolytes and tissue water. One kilogram of muscle contains about 150 g of protein, which can be used to form about 75 g of glucose. However, 1 kg of muscle also contains 1,200 mM of K +, 27 mM of phosphate, and 8 mM of Mg 2+, all of which must be excreted by the kidney. The ammonia released from catabolism of the amino acids is converted to urea in an energy-requiring process, and the urea further increases the osmotic load on the kidney. Renal excretion of these solutes necessitates mobilization of at least 2 L of water from other tissues in addition to the 750 mL released from the muscle, and osmotic diuresis may result from catabolism of even a relatively small amount of muscle. Finally, if fat metabolism is stimulated, as in starvation and diabetes mellitus, increased plasma levels of fatty acids and ketone bodies causes acidemia. To prevent severe acidosis, renal proton excretion is increased by increasing secretion of ammonia into the renal tubules. Excretion of nitrogen as ammonia avoids energy consumption due to urea synthesis, but it doubles the osmotic load per nitrogen excreted, causing even greater diuresis. Without a compensatory increase in water intake, profound depletion of blood volume can occur (Chapter 22). Gluconeogenic Precursors Gluconeogenic precursors include lactate, alanine, and several other amino acids, glycerol, and propionate (Chapter 22). Lactate, the end product of anaerobic glucose metabolism, is produced by most tissues of the body, particularly skin, muscle, erythrocytes, brain, and intestinal mucosa. In a normal adult, under basal conditions, these tissues produce 1,300 mM of lactate per day, and the normal serum lactate concentration is less than 1.2 mM/L. During vigorous exercise, the production of lactate can be increased several fold. Lactate is normally removed from the circulation by liver and kidney. Because of its great capacity to use lactate, liver plays an important role in the pathogenesis of lactic acidosis, which may be thought of as an imbalance between the relative rates of production and utilization of lactate (Chapter 39). Alanine, derived from muscle protein and also synthesized in the small intestine, is quantitatively the most important amino acid substrate for gluconeogenesis. It is converted to pyruvate by alanine aminotransferase (Chapter 17): B6-PO4

Alanine + ot-ketoglutarate 2- ~

> pyruvate- + glutamate-

279

SECTION15.1 Gluconeogenesis

Glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate by glycerol-3-phosphate dehydrogenase:

I Cysteine Alanine Glycine Hydroxyproline

Serine

Tryptophan Glucose

I t

Pyruvate

Oxaloacetate

j / Aspartate

,,,,,...Oxaloacetate~~~__._ AcetyI-CoA

CH2OH I CHOH I

+ NAD+ -

CH2OPO3 z-

Glycerol 3-phosphate M te

Citrate

Tyrosine] PhenylalanineJ--* Fumarate

or

1

CH2OH I + NADH+ H+ C=O I CH2OPO32Dihydroxyacetone phosphate

,~

Succiny L;

/ i.,.oA~,,"

~ [ Glutamate Arginine Glutamine

r

Histidine Proline

Isoleucine Methionine Valine Threonine

FIGURE 15-4 Points of entryof aminoacidsintothe pathwayof gluconeogenesis.

where B6-PO4--pyridoxal phosphate. An amino acid is classified as ketogenic, glucogenic, or glucogenic/ ketogenic depending on whether feeding it to a starved animal increases plasma concentrations of ketone bodies (Chapter 18) or of glucose. Leucine and lysine are purely ketogenic because they are catabolized to acetyl-CoA, which cannot be used to synthesize glucose (Chapter 17). Isoleucine, phenylalanine, tyrosine, and tryptophan are both glucogenic and ketogenic, and the remaining amino acids (including alanine) are glucogenic. Entry points of the amino acids into the gluconeogenic pathway are shown in Figure 15-4. Glycerol is more reduced than the other gluconeogenic precursors, and it results primarily from triacylglycerol hydrolysis in adipose tissue. In liver and kidney, glycerol is converted to glycerol 3-phosphate by glycerol kinase: CH2OH I CHOH + ATP 4I CH2OH

Dihydroxyacetone phosphate is the entry point of glycerol into gluconeogenesis. Glycerol cannot be metabolized in adipose tissue, which lacks glycerol kinase, and the glycerol 3-phosphate required for triacylglycerol synthesis in this tissue is derived from glucose (Chapter 22). During fasting in a resting adult, about 210 mM of glycerol per day is released, most of which is converted to glucose in the liver. During stress or exercise, glycerol release is markedly increased (Chapter 22). Propionate is not a quantitatively significant gluconeogenic precursor in humans, but it is a major source of glucose in ruminants. It is derived from the catabolism of isolecucine, valine, methionine, and threonine; from ~-oxidation of odd-chain fatty acids; and from the degradation of the side chain of cholesterol. Propionate enters gluconeogenesis via the TCA cycle after conversion to succinyl-CoA (Chapter 18).

Regulation of Gluconeogenesis Gluconeogenesis is regulated by the production and release of precursors and by the activation and inactivation of key enzymes. Glucagon and glucocorticoids stimulate gluconeogenesis, whereas insulin suppresses it. See Chapter 22 for a discussion of the overall metabolic control and physiological implications of the several seemingly competing pathways.

Carboxylation of Pyruvate to Oxaloacetate

Glycerol CH2OH I CHOH + ADP3- + H+ I CH2OPO32Glycerol 3-phosphate

The pyruvate carboxylase reaction is activated by Mg 2+ and, through mass action, by an increase in either the [ATP]/[ADP] or the [pyruvate]/[oxaloacetate] ratio. It is virtually inactive in the absence of acetyl-CoA, an allosteric activator. The enzyme is allosterically inhibited by glutamate, since oxaloacetate formed in excess would flood the TCA cycle and result in a buildup of ot-ketoglutarate and glutamate. Pyruvate carboxylase

280

CHAPTER 15

Carbohydrate Metabolism I1: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways

activity is indirectly stimulated by the catabolic hormones glucagon (via cAMP) and epinephrine (independent of cAMP), in four ways: 1. By increasing substrate availability through stimulation of mitochondrial respiration. This decreases the intramitochondrial concentration of H + and increases the rate of pyruvate transport. The [ATP]/[ADP] ratio also rises. 2. By increasing the rate of fatty acid oxidation, which results in an increase in the mitochondrial concentration of acetyl-CoA, an allosteric activator of pyruvate carboxylase. 3. By decreasing the mitochondrial concentration of glutamate, an inhibitor of pyruvate carboxylase, through stimulation of the TCA cycle (secondary to the increase in mitochondrial acetyl-CoA) and the aspartate shuttle (secondary to the increase in cytosolic PEPCK induced by glucagon). 4. By decreasing the activity of glycolytic enzymes competing for the same substrate; in this case, by inactivating pyruvate kinase in the cytosol and pyruvate dehydrogenase in the mitochondria (via cAMP stimulation of protein kinase).

Conversion of Oxaloacetate to Phosphoenolpyruvate Short-term regulation of this reaction is accomplished by changes in the relative proportions of substrates and products. Increased concentrations of oxaloacetate and GTP (or ITP) increase the rate, and accumulation of phosphoenolpyruvate and GDP (or IDP) decreases it. The cytosolic PEPCK is also under long-term regulation by hormones. Its synthesis is increased by corticosteroids. Starvation and diabetes mellitus increase the synthesis of cytosolic PEPCK, whereas refeeding and insulin have the opposite effect. The net effect of increasing the ratio ofcytosolic PEPCK to mitochondrial PEPCK is to maintain gluconeogenesis even during periods of increased fatty acid metabolism. When gluconeogenesis is strongly stimulated, lipolysis and intramitochondrial fatty acid oxidation increase with the activity of cytosolic PEPCK. The increase in fatty acid oxidation elevates the [NADH]/[NAD +] ratio within the mitochondria, inhibiting phosphoenolpyruvate formation by channeling oxaloacetate into formation of malate or aspartate. At higher [NADH]/[NAD +] ratios, aspartate synthesis is favored because of increased formation of glutamate by mitochondrial glutamate hydrogenase: c~-Ketoglutarate + NH~- + NADH ~ glutamate + NAD+ + H20

The increase in glutamate favors transamination of oxaloacetate and limits oxaloacetate availability for phosphoenolpyruvate synthesis. When the [NADH]/[NAD +] ratio is low, malate formation occurs more readily. The cytosolic PEPCK is relatively unaffected by the mitochondrial [NADH]/[NAD +] ratio. Once malate and aspartate are transported to the cytosol and they are reconverted to oxaloacetate, cytosolic PEPCK can convert it to phosphoenolpyruvate.

Conversion of Fructose-l,6-Bisphosphate to Fructose-6-Phosphate This reaction is catalyzed by fructose-l,6-bisphosphatase (FBPase-1). This enzyme is a tetramer (M.W. 164,000) of identical subunits. It is inhibited by AMP, inorganic phosphate, and fructose-2,6-bisphosphate, all of which are allosteric activators of 6-phosphofructo1-kinase (PFK-1), the competing glycolytic enzyme (Figure 15-5). This inhibition prevents the simultaneous activation of the two enzymes and helps prevent a futile ATP cycle. However, this futile cycle apparently does operate at a low rate in mammalian muscle, and in the bumblebee it is essential for maintaining the flight muscle at 30~ Activation of the futile cycle occurs after exposure to certain anesthetic agents such as suxamethonium and/or volatile halogenated compounds. In susceptible persons, malignant hyperthermia is characterized by a hypermetabolic state and is accompanied by a catastrophic rise in body temperature (to 42~ or higher), massive increase in oxygen consumption, rhabdomyolysis, (disintegration of muscle, associated with excretion of myoglobin in the urine) and metabolic acidosis. Rhabdomyolysis is assessed by measurement of serum creatine kinase levels (Chapter 8). In susceptible persons, the hypermetabolic events can also be triggered by excessive exercise under hot conditions, infections, and neuroleptic drugs.


(~ AMP ],_ . (~ F 2,6-BPJ I-rucl~

P,

~ PFK-~[QATP IQAuP . ,6.bisphosphat ~,~ase)l /,--ATP H2OJI , " ' A D P (-_)Citrate I(.t)F 2,6-BP Fructose 1,6-bisphosphate

FIGURE 15-5 Regulation of liver6-phosphofructokinaseand fructose-1,6bisphosphatase. Thesemultimodulatedenzymescatalyzenonequilibrium reactions, the formerin glycolysisand the latterin gluconeogenesis.Note the dual action of fructose-2,6-bisphosphate (F-2,6-BP),whichactivates phosphofructokinase (PFK-1)and inactivatesfructose-l,6-bisphosphatase. The activityof F-2,6-BPis underhormonaland substrateregulation (Figure 15-6). Q = positiveeffectors; G = negativeeffectors.

281

SECTION15.1 Gluconeogenesis

The primary biochemical abnormality of malignant hyperthermia is a membrane defect that leads to a rapid and sustained rise in Ca 2+ levels in the cytosol of skeletal muscle cells. Sarcoplasmic reticulum is the main storage site of intracellular Ca 2+ and it is released into the myoplasm by excitation-contraction coupling (Chapter 21). Dantrolene, a drug useful in the treatment of malignant hyperthermia, inhibits excitation-contraction coupling. However, dantrolene does not affect neuromuscular transmission or the electrical properties of skeletal muscle. In the animal model, pigs susceptible for malignant hyperthermia exhibit a genetic defect in a Ca 2+ release channel receptor located in the sarcoplasmic reticulum. This receptor protein is named ryanodine receptor (RYR) because it binds to a plant alkaloid ryanodine and upon binding causes Ca 2+ release. The mutation that alters the receptor protein causes a substitution of cysteine for arginine at position 615. The concentration of fructose-2,6-bisphosphate is controlled by two competing enzyme activities in a single

Insulin

Glucagon

t

A ....~

+ PP~

cAMP

o,

L..

R AMP F-6-P

..,

ATP

Proteinphosphatase

/ f

_ //~,~

Cell membrane

.................p.

' cAMP-dependent protein kinase

|

ou,s,o,

Acti vatiohof adenylata cvclasa

2ATP 2ADP ~. ) ~J

|

1 /

I~

~r~-l.,.

ot-GP Citrate PEP

~

ADP

F-2,6-BP f J l 1 H=O I..

..................................

:_/~_

/o '.,,.e

PEP ot-Gp

o-.or

?o,

F-6-P P~

F I G U R E 15-6 Regulation of fructose-2,6-bisphosphate (F-2,6-BP) concentration in liver. F-2,6-BP is a major factor controlling the relative activities of fructose- 1,6-bisphosphatase (FBPase- 1) and 6-phosphofructo-l-kinase (PFK-1), key enzymes in gluconeogenesis and glycolysis, respectively. 6-Phosphofructo-2-kinase (PFK-2) and fructose-2,6-bisphosphatase (FBPase-2) are separate enzymes expressed by a single multifunctional protein. Insulin can antagonize the effect of glucagon on cAMR but the mechanism is not known. Dotted lines = regulatory factors; @ = inhibitory; @ = stimulatory. F6P = fructose-6-phosphate; PEP = phosphoenolpyruvate; ot-GP = o~-glycerol phosphate; Pi = inorganic phosphate; F-6-P -- fructose-6-phosphate.

multifunctional protein (M.W. 110,000) with two identical subunits (Figure 15-6). Phosphorylation by cAMPdependent protein kinase produces one phosphoserine per subunit, inhibiting 6-phosphofructo-2-kinase (PFK-2) activity and stimulating fructose-2,6-bisphosphatase (FBPase-2) activity. The phosphotransferase inactivation results from an increased Km for fructose-6-phosphate and a small decrease in Vmax. An increase in Vmax and a decrease in Km for fructose-2,6-bisphosphate increases the phosphohydrolase activity. The protein phosphatase that reverses the cAMP-dependent phosphorylation has not yet been characterized. The nomenclature of these enzyme activities (PFK-1, PFK-2, FBPase-1, and FBPase-2) reflects both the order of their discovery and the carbon atom of fructose that is phosphorylated or dephosphorylated. Phosphorylation is regulated by insulin and glucagon. Diabetes mellitus (in which the ratio of glucagon to insulin is increased) and glucagon therapy reduce the hepatic activity of PFK-2 and increase that of FBPase-2. The concentration of fructose-2,6-bisphosphate is thus reduced, thereby stimulating gluconeogenesis. The concentration of fructose-2,6-bisphosphate is also influenced by nonhormonal factors. PFK-2 is allosterically stimulated by inorganic phosphate, AMP, and fructose-6-phosphate and inhibited by citrate, fructose2,6-bisphosphate, phosphoenolpyruvate, and u-glycerol phosphate. FBPase-2, on the other hand, is stimulated by phosphoenolpyruvate and u-glycerol phosphate and inhibited by phosphate and fructose-6-phosphate. Glucose increases the concentration of fructose-2,6-bisphosphate in vivo, probably by increasing the availability of fructose6-phosphate, thereby stimulating PFK-2, the kinase for which this is a substrate and inhibiting the phosphatase, FBPase-2. The effect is to increase glycolysis and inhibit gluconeogenesis. The concentration of u-glycerol phosphate in vivo can be altered by ischemia consumption of ethanol, and feeding of various sugars; it may be an important regulatory molecule in determining the levels of fructose-2,6-bisphosphate. A rise in orglycerol phosphate decreases fructose-2,6-bisphosphate concentration, decreasing glycolysis and increasing gluconeogenesis.

Conversion of Glucose-6-Phosphate to Glucose Glucose-6-phosphatase and the corresponding glycolytic enzyme, glucokinase, are not controlled by the metabolites that affect PFK-1 and FBPase-1. Glucose-6-phosphatase seems to be regulated only by the concentration of glucose-6-phosphate, which is

282

CHAPTER 15


increased by glucocorticoids, thyroxine, and glucagon. The concentration of glucose-6-phosphate, which is also elevated in diabetes mellitus and following ethanol administration, is increased 300-fold after a 48-hour fast. Consumption of a low-protein diet reduces the concentration of glucose-6-phosphate. A futile cycle involving glucokinase and glucose-6-phosphatase does not occur because glucose-6-phosphatase is active only when the concentration of glucose-6-phosphate is high, and glucokinase is active only when glucose-6-phosphate concentration is low. ATP

Glucose ~ ~

yP ~ Glucose 6-phosphate,

Gluconeogenesis

P,

For extrahepatic tissue utilization

Glycogen

Glycolysis

synthesis

Pentose phosphate pathway

Abnormalities of Gluconeogenesis Glucose is the predominant fuel for cells that depend largely on anaerobic metabolism, cells that lack mitochondria, and tissues such as brain that normally cannot use other metabolic fuels. An adult brain represents only 2% of total body weight, but it oxidizes about 100 g of glucose per day, accounting for 25% of the basal metabolism. Brain cannot utilize fatty acids because they are bound to serum albumin and so do not cross the blood-brain barrier. Ketone bodies are alternative fuels, but their concentration in blood is negligible, except in prolonged fasting or diabetic ketoacidosis (Chapters 18 and 22). Brain and liver glycogen stores are small relative to the needs of the brain, and gluconeogenesis is essential for survival. Decreased gluconeogenesis leads to hypoglycemia and may cause irreversible damage to the brain. Blood glucose levels below 40 mg/dL (2.2 raM/L) in adults or below 30 mg/dL (1.7 mM/L) in neonates represent severe hypoglycemia. The low levels may result from increased glucose utilization, decreased glucose production, or both. Increased glucose utilization can occur in hyperinsulinemia secondary to an insulinoma. In a diabetic pregnancy, the mother's hyperglycemia leads to fetal hyperglycemia and causes fetal and neonatal hyperinsulinemia. Insufficiency of glucocorticoids, glucagon, or growth hormone and severe liver disease can produce hypoglycemia by depressing gluconeogenesis (Chapters 22 and 32). Ethanol consumption can cause hypoglycemia, since a major fraction of ethanol is oxidized in the liver by cytosolic alcohol dehydrogenase, and the NADH and acetaldehyde generated inhibit

gluconeogenesis: CH3CH2OH + NAD + -+ CH3CHO + NADH + H + Excessive NADH decreases the cytosolic [NAD+]/ [NADH] ratio, increasing lactate dehydrogenase activity and thereby increasing conversion of pyruvate to lactate. The decrease in pyruvate concentration inhibits pyruvate carboxylase. Acetaldehyde inhibits oxidative phosphorylation, increasing the [ADP]/[ATP] ratio, promoting glycolysis, and inhibiting gluconeogenesis. Hypoglycemia and lactic acidosis are common findings in chronic alcoholics. Pyruvate carboxylase deficiency can cause intermittent hypoglycemia, ketosis, severe psychomotor retardation, and lactic acidosis. The deficiency of either cytosolic or mitochondrial isoenzyme form of phosphoenolpyruvatecarboxykinase (PEPCK), is characterized by a failure of gluconeogenesis. Both these disorders are rare and the mitochondrial PEPCK deficiency is inherited as an autosomal recessive trait. The major clinical manifestations include hypoglycemia, lactic acidosis, hypotonia, hepatomegaly and failure to thrive. The treatment is supportive and is based upon symptoms. Fructose1,6-bisphosphatase deficiency, inherited as an autosomal recessive trait, severely impairs gluconeogenesis, causing hypoglycemia, ketosis, and lactic acidosis. Glucose6-phosphatase deficiency, also an autosomal recessive trait, causes a similar condition but with excessive deposition of glycogen in liver and kidney (discussed later). Hypoglycin A (2-methylenecyclopropylalanine), the principal toxin of the unripe akee fruit, produces severe hypoglycemia when ingested. A less toxic compound, hypoglycin B, is a y-glutamyl conjugate of hypoglycin A. The akee tree is indigenous to western Africa and grows in Central America and the Caribbean. Hypoglycin causes vomiting ("vomiting sickness") and central nervous system depression. In Jamaica, symptoms apparently occur in epidemic proportions during colder months when the akee fruit is unripe and other food sources are limited. The edible portion of the ripe akee fruit, which contains only small amounts of hypoglycin, is a main dietary staple in Jamaica. Hypoglycin A causes hypoglycemia by inhibiting gluconeogenesis. In the liver, hypoglycin A forms nonmetabolizable esters with CoA (shown below) and carnitine, depleting the CoA and carnitine pools, thereby inhibiting fatty acid oxidation (Chapter 18). Since the principal source of ATP for gluconeogenesis is mitochondrial oxidation of long-chain fatty acids, gluconeogenesis is stopped. Intravenous administration of glucose relieves the hypoglycemia.

SECTION15.2 Glycogen Metabolism OH2

NH2

/\ H2C--C

283

,

CHuCH2mCH--COOH

Hypoglycin A

=-Ketoglutarate smination Glutamate

o

II H2C~CH~CH~CH2~C~COOH Methylenecyclopropyl ~-ketopropionic acid

If'-"'~

CoASH -~l~V' Oxidative

k decarboxylation I~._~ NAOH + H + ~ CO2

o

II

H2C'--C

CHCH2C~S~CoA

Methylenecyclopropyl 0c-ketoproprionyI-CoA

Overactivity of gluconeogenesis due to increased secretion of catecholamines, cortisol, or growth hormone or an increase in the glucagon/insulin ratio (Chapter 22) leads to hyperglycemia and causes many metabolic problems.

15.2 Glycogen Metabolism Animals have developed a method of storing glucose that reduces the need to catabolize protein as a source of gluconeogenic precursors between meals. The principal storage form of glucose in mammals is glycogen. Storage of glucose as a polymer reduces the intracellular osmotic load, thereby decreasing the amount of water of hydration and increasing the energy density of the stored glucose. In muscle, glycogen is stored in the cytosol and endoplasmic reticulum as granules, called fl-particles, each of which is an individual glycogen molecule. In the liver, the fi-particles aggregate, forming larger, rosette-shaped a-particles that can be seen with the electron microscope. The average molecular weight of glycogen is several million (10,000-50,000 glucose residues per molecule). The storage granules also contain the enzymes needed for glycogen synthesis and degradation and for regulation of these two pathways. Glycogen is present in virtually every cell in the body, but it is especially abundant in liver and skeletal muscle. The amount stored in tissues varies greatly in response to metabolic and physiological demands, but in a resting individual after a meal, liver usually contains roughly 47% of its wet weight as glycogen and muscle about 1%.

Since the body contains 10 times more muscle than hepatic tissue, the total amount of glycogen stored in muscle is greater than that in liver. Muscle needs a rapidly available supply of glucose to provide fuel for anaerobic glycolysis when, as during bursts of muscle contraction, blood may provide inadequate supplies of oxygen and fuel. The liver, under most conditions, oxidizes fatty acids for this purpose; however, liver is responsible for maintaining blood glucose levels during short fasts, and it integrates the supply of available fuels with the metabolic requirements of other tissues in different physiological states. Liver glycogen content changes primarily in response to the availability of glucose and gluconeogenic precursors. Muscle glycogen stores vary less in response to dietary signals, but they depend on the rate of muscular contraction and of oxidative metabolism (tissue respiration). Glycogenesis (glycogen synthesis and storage) and glycogenolysis (glycogen breakdown) are separate metabolic pathways having only one enzyme in common, namely, phosphoglucomutase. Glycogen synthesis and breakdown are often reciprocally regulated, so that stimulation of one inhibits the other. The control mechanisms are more closely interrelated than are those for glycolysis and gluconeogenesis, perhaps because the glycogen pathways ensure the availability of only one substrate, whereas intermediates of several other metabolic pathways are produced and metabolized in glycolysis and gluconeogenesis (Chapter 13). Glycogen Synthesis Glycogenesis begins with the phosphorylation of glucose by glucokinase in liver and by hexokinase in muscle and other tissues (Chapter 13): Mg 2+

G l u c o s e + ATP 4-

> g l u c o s e - 6 - p h o s p h a t e 2- + A D P 3- + H +

The second step in glycogenesis is conversion of glucose6-phosphate to glucose-1-phosphate by phosphoglucomutase in a reaction similar to that catalyzed by phosphoglyceromutase. The phosphoryl group of the enzyme participates in this reversible reaction in which glucose-1,6bisphosphate serves as an intermediate: E-P at- g l u c o s e - 6 - p h o s p h a t e E + g l u c o s e - l , 6 - b i s p h o s p h a t e ~ E-P + g l u c o s e - l - p h o s p h a t e

In the third step, glucose-1-phosphate is converted to uridine diphosphate (UDP) glucose, the immediate precursor of glycogen synthesis, by reaction with uridine triphosphate (UTP). This reaction is catalyzed by glucose-lphosphate uridylyltransferase (or UDP-glucose pyrophosphorylase):

~4

CHAPTER15 Carbohydrate Metabolism I1: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways

O CH2OH . ,F---o. UTP4-+ Glucose 1-phosphate 2- ,------

H~)~

uo

HN~I

y

Jf L--O--P--O--P~O--CH2

l

+ PPi 4-

-0,

ON ON

The reaction is freely reversible, but it is rendered practically irreversible by hydrolysis of pyrophosphate (PPi) through the action of pyrophosphatase. PPi + H20--+ 2Pi Thus, two high-energy phosphate bonds are hydrolyzed in the formation of UDP-glucose. Nucleoside diphosphate sugars are commonly used as intermediates in carbohydrate condensation reactions. ADP-glucose is the precursor for the synthesis of starch in plants and of other storage polysaccharides in bacteria. In each case, the high-energy bond between the sugar and the nucleoside diphosphate provides the energy needed to link the new sugar to the nonreducing component of another sugar or polysaccharide. The next step in the biogenesis of glycogen is addition of a glucosyl residue at the C-1 position of

UDP-glucose to a tyrosyl group located at position 194 of the enzyme glycogenin, which is a MgZ+-dependent autocatalytic reaction. Glycogenin (M.W. 37,000) extends the glucan chain, again by autocatalysis, by six to seven glucosyl units in or(1 --+ 4) glycosidic linkages using UDPglucose as substrate (Figure 15-7). The glucan primer of glycogenin is elongated by glycogen synthase using UDP-glucose. Initially, the primer glycogenin and glycogen synthase are firmly bound in a 1:1 complex. As the glucan chain grows, glycogen synthase dissociates from glycogenin. The branching of the glycogen is accomplished by transferring a minimum of six or(1 ~ 4) glucan units from the elongated external chain into the same or a neighboring chain by or(1 --+ 6) linkage. This reaction of c~(1 --+ 4) to or(1 --+ 6) transglucosylation creates new non-reducing ends and is catalyzed by a branching enzyme. A mature glycogen particle is spherical, containing one molecule of

lycogenin~-- Tyrosyl Group

Mg 2+ Autocatalysis

(.--UDP-glucose "----~ UDP ~~)f-Glucosyl Unit

Autocatalysis

~ ~UDP-glucose

Glycogen Synthase

I (--UDP-glucose ""---~UDP

Branching Enzyme i

Glycogen Synthase and Branching Enzyme

~' ~ cUDP-glucose "----~ UDP Mature Glycogen (60,000 Glucosyl Units) (I] particle)

FIGURE 15-7 Schematic representation of glycogen biosynthesis. ~, Glucosyl units, (1 ~ 6) linkage. A

285

SECTION15.2 Glycogen Metabolism

C

~~~)/~

a(1 ~ 6) bonds;all other linka~s are a(1 ~ 4)

Glycogen

C~% ~

Inaccessible to

Debranching enzyme

( ~

(al,4 -, al,4 glu.can

.204

16 G-1-P

~i

(~x_

/]

Debranching enzyme

(amy.lo-l,6-glucosidase)

Glycogen

phosphorylase 16P. I

Glucose

Glycogen phosphorylase

Debranching

enzyme H20

10 G-1-P Debranching enzyme

F I G U R E 15-8 Catabolism of a small glycogen molecule by glycogen phosphorylase and debranching enzyme. Pi = inorganic phosphate; G-1-P -- glucose-l-phosphate.

glycogenin and up to 60,000 glucosyl units (/3-particles). In the liver, 20-40/3-particles are aggregated into rosettes, known as (or-particles). Glycogen Breakdown Glycogenolysis is catalyzed by two enzymes unique to the pathway: glycogen phosphorylase and debranching enzyme. The former normally regulates the rate of glucose release from glycogen. The progressive degradation of glycogen is illustrated in Figure 15-8. Glycogen phosphorylase catalyzes the release of glucose-l-phosphate from the terminal residue of a nonreducing end of a glyco-

gen branch, by means of phosphorolysis. A molecule of inorganic phosphate attacks the C1 side of an or(1 --+ 4) glycosidic bond, leaving a hydroxyl group on C4 that remains in the glycogen polymer. The reaction is analogous to hydrolysis, in which water attacks and cleaves bonds. The phosphorolysis reaction is summarized below. (Glucose)n + Pi --+ (glucose)n_l 4-glucose-l-Phosphate The energy stored in the or(1 --+ 4) glycosidic bond during the condensation reaction in glycogen synthesis is sufficient to permit the formation of a glucose-phosphate bond without using ATE

~6

CHAPTER15


Glucose-1-phosphate is next converted by phosphoglucomutase to glucose-6-phosphate. The latter may then enter the glycolytic pathway, but if glucose-6-phosphatase is present, free glucose can be formed. Glycogen phosphorylase sequentially removes the glucosyl residues from a glycogen branch until further action is sterically hindered by a branch point. This occurs when the branch is four residues long from the branch point. Debranching enzyme, a multifunctional protein, first removes the trisaccharide "stump" on the branch and then removes the branch point itself. The or(1 --+ 4) glycosidic bond linking the trisaccharide to the branch point residue is first cleaved, and the trisaccharide is transferred to the nonreducing end of an adjacent branch. This elongated branch can now be cleaved, one residue at a time, by glycogen phosphorylase. The glucose residue that remains, linked by an or(1 --+ 6) glycosidic bond, is then cleaved by hydrolysis to yield free glucose. Thus, one molecule of glucose is released for each branch point removed, even in muscle, which lacks glucose-6-phosphatase. Roughly 7% of the glycosidic bonds in glycogen are or(1 --+ 6) linkages. Under normal conditions, glycogen phosphorylase and debranching enzyme act simultaneously at different regions of the glycogen molecule. Deficiency of either enzyme prevents complete glycogen degradation. Glycogen phosphorylase deficiency leaves the original glycogen molecule untouched. Deficiency of debranching enzyme results in a glycogen molecule smaller than the original, with very short chains on the outer branches but with the inner core unchanged (a limit dextrin).

Regulation of Glycogen Metabolism Metabolism of glycogen in muscle and liver is regulated primarily through control of glycogen synthase and glycogen phosphorylase. The activities of these enzymes vary according to the metabolic needs of the tissue (as in muscle) or of other tissues that use glucose as a fuel (as in liver). Proximal control is exerted on synthase and phosphorylase by phosphorylation/dephosphorylation and by allosteric effectors such as glucose, glucose-6-phosphate, and several nucleotides (ATE ADP, AMP, and UDP). The concept that allosteric effectors are of primary importance in regulating synthase and phosphorylase activities was based mostly on in vitro experiments. For example, inactive (phosphorylated) glycogen synthase can be activated, without dephosphorylation, by glucose-6phosphate. For this reason, the active and inactive forms of this enzyme were formerly called glycogen synthase I (glucose-6-phosphate independent) and glycogen synthase D (glucose-6-phosphate dependent). It now appears that the concentration of glucose-6-phosphate may not

vary widely enough, particularly in muscle, to change synthase activity significantly, although glucose-6-phosphate binding may help determine the basal activity of the enzyme. The two forms are now known as glycogen synthase a (dephosphorylated) and b (phosphorylated). The same type of nomenclature is used for glycogen phosphorylase, except that phosphorylase a, the active form, is phosphorylated, while phosphorylase b is dephosphorylated. The regulation of glycogen metabolism in liver and in muscle differs in several ways. Although the control mechanisms are not completely understood, particularly in liver, the differences probably are due to the receptors in each tissue and to the presence of glucose-6-phosphatase in liver rather than to differences in the intrinsic regulatory properties of the enzymes involved. This aspect of tissue differences in glycogen metabolism between muscle and liver probably also applies to that in brain, myocardium, and other tissues. In all tissues, the rate of glycogen synthesis must be inversely proportional to the rate of glycogenolysis to avoid futile cycling.

Muscle In muscle, glycogen is used as a fuel for anaerobic metabolism during brief periods of high-energy output (e.g., sprinting). Glycogenolysis is initiated and glycogenesis inhibited by the onset of muscle contraction and by factors such as epinephrine that signal a need for muscular activity. Since muscle glycogen is not a source of glucose for other tissues, it is not sensitive to blood glucose levels.

Control of Glycogen Synthase Muscle glycogen synthase (M.W. ~340,000) is a tetramer of identical subunits that exists in several forms, which differ in catalytic activity and degree of covalent modification. Glycogen synthase a, an active dephosphorylated form, can interconvert with several less active, phosphorylated forms, collectively called glycogen synthase b. The enzyme contains at least nine serine residues located near the extremities of the molecule, which can be phosphorylated by protein kinases (Figure 15-9). For the most part, the sites can be phosphorylated in any order. An exception is site C42, which undergoes phosphorylation by glycogen synthase kinase-3 only after phosphorylation at site C46 by casein kinase-2. Once C46 and C42 are phosphorylated in that order, glycogen synthase kinase-3 phosphorylates at sites C38, C34, and C30. In general, phosphorylation reduces synthase activity, and an increase in the number of phosphorylated sites additively decreases the activity. Reduced synthase activity may be manifested as increased Km for UDP-glucose, increased

SECTION15.2 Glycogen Metabolism

287

5. Glycogen synthase kinase-4 (GSK-4; site N7; relatively specific for glycogen synthase). 6. Casein kinase-2 (site C46; broadly specific; physiologically important substrates not yet known). 7. A novel protein kinase that may be activated by cAMP-dependent protein kinase (site N 10).

F I G U R E 15-9 Regulation of glycogen synthase by multisite phosphorylation. The location of phosphorylation sites (*) and the protein kinases that phosphorylate at these sites (boxes) are shown. Phosphorylations occur only at N- and C-terminal regions of the enzymes, as indicated by CB-N and CB-C, respectively. The single-letter abbreviations for amino acids are used (see Chapter 2). cAMP-PK -- cyclic AMP-dependent protein kinase; CAM-MPK = Ca2+/calmodulin-dependent multiprotein kinase; PhK -phosphorylase kinase; GSK -- glycogen synthase kinase; CK -- casein kinase; N10-PK -- A novel protein kinase. [Reproduced with permission from E Cohen, Protein phosphorylation and hormone action. Proc. R. Soc. Lond. (Biol.)234, 115 (1988).]

Ka for glucose-6-phosphate, or decreased Ki for ADR inorganic phosphate, or other inhibitors, or a change in Vmax. At least seven different protein kinases can phosphorylate muscle glycogen synthase in vitro, but whether all are involved in vivo is not established. Some are moderately specific for glycogen synthase, while others phosphorylate a variety of proteins. They are listed below, together with the sites that they phosphorylate (Figure 15-9). 1. cAMP-dependent protein kinase (sites N7, C87, and C100, broad specificity; mediates effects of many hormones). 2. Phosphorylase kinase (site N7; main function in vivo probably is activation of glycogen phosphorylase). 3. CaZ+/calmodulin-dependent multiprotein kinase (sites N7 and C100; relatively specific for glycogen synthase). 4. Glycogen synthase kinase-3 (GSK-3; sites C30, C34, C38 and C42; also phosphorylates inhibitor-2 (see below), thereby activating protein phosphatase-1).

In muscle, cAMP-dependent protein kinase (Chapter 30) mediates the inhibitory effect of epinephrine on glycogenesis by phosphorylating protein phosphatase inhibitor-1 and phosphorylating sites N7, C87, and C100. The inhibitory effect of cytosolic Ca 2+ on glycogen synthesis may be mediated by calmodulin-dependent multiprotein kinase. GSK-3 may be a key control enzyme; it phosphorylates sites on glycogen synthase that have the greatest effect on synthase activity. GSK-3 also activates protein phosphatase-1 by phosphorylating inhibitor-2, however, and these seemingly opposing effects have not been fully explained. Stimulation by insulin of glycogen synthesis in muscle may be mediated by GSK-3 or phosphatases that remove the phosphate group at the GSK-3 sites. Reactivation of glycogen synthase requires removal of the phosphate groups by a phosphatase. Four enzymes that dephosphorylate glycogen synthase or one or more kinases in vitro have been isolated from skeletal muscle. 1. Protein phosphatase-1 (Mg2+/ATP-dependent phosphatase; multisubstrate protein phosphatase; M.W. of catalytic subunit is 35,000; major enzyme in regulation of glycogen metabolism in skeletal muscle; dephosphorylates glycogen phosphorylase,/~-subunit of phosphorylase kinase, and at least three sites of glycogen synthase; regulated by inhibitor-1, inhibitor-2, and GSK-3 + Mg 2+. ATP). 2. Protein phosphatase-2A (polycation-dependent phosphatase, M.W. ~200,000; M.W. of catalytic subunit ~38,000; function unclear). 3. Protein phosphatase-2B (calcineurin; dimer; catalytic subunit of M.W. 60,000 has Ca2+ binding site; CaZ+-binding subunit of M.W. ~ 18,000, partially homologous with calmodulin and troponin C; N-terminal glycine blocked by amide linkage with myristic acid; absolute requirement for Ca2+; regulated by CaZ+/calmodulin; dephosphorylates ot-subunit of phosphorylase kinase and inhibitory but not glycogen phosphorylase or synthase or/~-subunit of phosphorylase kinase). 4. Protein phosphatase-2C (monomer; M.W. 43,000; represents a small fraction of glycogen synthase phosphatase in skeletal muscle; may primarily regulate other metabolic pathway in vivo).

288


Other proteins involved in glycogen synthase control are calmodulin, inhibitor-1, and inhibitor-2. Troponin C, when complexed with Ca 2+, can activate phosphorylase kinase, but phosphorylase kinase may have little or no role in the inactivation of glycogen synthase in vivo. Inhibitor-1 (M.W. 18,600), when phosphorylated on threonine 35 by cAMP-dependent protein kinase, binds to and inhibits protein phosphatase-1. The concentration of protein phosphatase-1 is lower than that of inhibitors, and its activity is sensitive to the latter, suggesting that inhibitor- 1 phosphorylation may be an important regulatory mechanism for this phosphatase. Inhibitor-2 (M.W. 30,500) appears to be an integral part of protein phosphatase1, forming an inactive 1:1 complex, called Mg2+/ATP dependent phosphatase, with the catalytic subunit of this enzyme. Phosphorylation of inhibitor-2 (also at a threonine residue) activates protein phosphatase-1. This phosphorylation, which requires Mg2+-ATR is catalyzed by an "activating factor" (FA), GSK-3.

Control of Glycogen Phosphorylase Regulation of glycogen phosphorylase in muscle is accomplished by many of the same enzymes that control glycogen synthesis. Phosphorylase kinase converts the dimeric phosphorylase from the inactive to the active form by Mg 2+ and ATP-dependent phosphorylation of two identical serine residues. The principal enzyme that removes this phosphate may be protein phosphatase-1 (phosphorylase phosphatase). Phosphorylase kinase is a hexadecamer having four different subunits: c~ (M.W. 145,000) or or' (M.W. 133,000), fl (M.W. 128,000), ?, (M.W. 44,700), and 6 (M.W. 16,700), with the stoichiometry ota(or Of~)j~aY4~ 4 (Figure 15-10). Whether the oe- or oe'-subunit is present depends on the tissue. The subunits apparently differ in Ca 2+ sensitivity, since the 6'-subunit, discussed later, will not bind to the o~'-isozyme. A catalytic site on the ?'-subunit has considerable homology with the catalytic subunit of cAMP-dependent protein kinase (Chapter 30). Evidence for active sites on the or- and fl-subunits is weak. Phosphorylase kinase activity has an absolute requirement for Ca 2+, which binds to the 6-subunit. The amino acid sequence of this subunit is nearly identical to that of calmodulin, with four calcium binding sites, but unlike calmodulin, the 3-subunit is an integral part of the enzyme and does not dissociate from it in the absence of Ca 2+. In the presence of Ca 2+, kinase activity is further increased by phosphorylation of the or- and fi-subunits, catalyzed by cAMP-dependent protein kinase and several other kinases. Phosphorylation may activate the enzyme by disinhibiting

i

G

G

Tmponin C, calmodulin, ) olher related calcium-

)

1

binding proteins

F I G U R E 15-10 Subunit structure of muscle phosphorylase kinase. The oe- and fl-subunits are regulatory proteins containing the sites at which phosphorylation can activate the kinase. The V-subunit contains the catalytic site. The ~-subunit is a Ca2+-binding protein, essentially identical with calmodulin, Ca e+. Calcium binding to the &subunit causes a change in the quaternary structure of the enzyme, allowing binding of the 6'-subunit, thereby increasing kinase activity even in the absence of phosphorylation of the fl-subunit. [Modified and reproduced with permission from A. K. Campbell: Intercellular Calcium: Its Universal Role as Regulator, Wiley, New York, 1983. @ 1983 by John Wiley & Sons, Inc.]

it, since limited proteolysis (which degrades the or- and fl-subunits) and dissociation of the holoenzyme into partial complexes (/3V6 and g6) also cause activation. Activation by covalent modification increases the affinity of the kinase for phosphorylase b. The kinase also is activated by the binding of a 6'-subunit (Ca2+/calmodulin or Ca2+/troponin C) to a site formed by the or- and fl-subunits. There are sites for four 6'-subunits on the holoenzyme. Activation by Ca2+/troponin C is 20-30 times greater than by Ca2+/calmodulin. This route may provide a means of coordinating muscle contraction, initiated by binding of Ca 2+ to troponin C (Chapter 21), with glycogenolysis. Finally, phosphorylase kinase is allosterically activated by the binding of up to 8 mol of ADP. Only 20-30% of isolated phosphorylase kinase is associated with glycogen particles, whereas all glycogen phosphorylase activity is associated with glycogen particles. This fact, together with the subcellular localization of phosphorylase kinase in intact cells and its biochemical properties, suggests that phosphorylase kinase may control other metabolic pathways as well. Suggested phosphorylase kinase targets include the Na +, K +and Ca2+-ATPases in the sacrolemma and sarcoplasmic reticulum, respectively.

SECTION 15.2 Glycogen Metabolism

289 Epinephrine

Nervous stimulus

Outside Plasmamembrane Inside

CaZ+/troponinC (thin filaments)~

Outside Plasma membrane Inside

I/creased

[Ca=+] in cytosol

cAMP + PP~

t

usc,

coo, raO,oo

f

li

r

j

Activationof phosphorylase kinase

ATP

Activationof cAMPdependent proteinkinase

-

Activationof

Activation of calmodulin-

inhibitor-1

dependentmultiprotein kinase

......

Activationof ~ k kate'~cla~e

-.~

t

--

9..

t

. ~

Activation of

glycogenphosphorylase

t Increased glycogen

breakdown

~176 ~176

.....

Inhibitionof

Activation of Inhibition of phosphorylase kinase ........ protein phosphatase-1 .........

%

"'glycogen synthase

l

Reduced

glycogen synthesis

F I G U R E 15-11 Possible mechanism for regulation of glycogen metabolism in skeletal muscle by changes in cytosolic calcium. Increased glycogen breakdown may be coordinated with muscle contractions, as indicated here. The actual control scheme is probably more complicated, since phosphoprotein phosphatases are also involved. Interactions with cAMP-activated reactions, which also may complicate regulation, are not included. Whether glycogen synthase is a substrate for phosphorylase kinase in vivo is unclear. [Modified and reproduced with permission from E Cohen, Protein phosphorylation and the control of glycogen metabolism in skeletal muscle. Philos. Trans. R. Soc. Lond. (Biol.) 302, 13 (1983).]

Integrated Regulation of Muscle Glycogen Metabolism Figures 15-11 and 15-12 provide a model for the regulation of glycogen metabolism in muscle. This model is consistent with many of the known facts, but alternative interpretations are equally plausible. Only two of the external stimuli that affect muscle glycogen metabolism are considered in these figures. 1. Initiation of muscle contraction by action potential from an o~ motor neuron depolarizes the muscle-cell membrane and causes an increase in the myoplasmic [Ca2+] (Chapter 21). The increase in calcium activates phosphorylase kinase and CaZ+/calmodulin-dependent kinase, which inactivate glycogen synthase and active glycogen phosphorylase (Figure 15-11). This step coordinates muscle contraction with glycogenolysis. These steps are reversed by one or more phosphatases at the end of contraction. 2. Binding of epinephrine to an adrenergic receptor away from a motor end plate on the muscle membrane causes glycogenolysis to increase and glycogenesis to decrease. This activates adenylate cyclase, increasing the cytosolic concentration of cAMP and activating

Activation of

o~176176

,,X

glycogenphosphorylase

L

Increased glycogen breakdown

Inhibition of

glycogensynthase

,*****9

? ....... ................................................................................ Reduced

glycogen synthesis

F I G U R E 15-12 Possible mechanism for regulation of glycogen metabolism in skeletal muscle by changes in cyclic AMP. The actual control scheme is probably more complicated, since changes in cytoplasmic calcium concentration are likely also to be important. Whether glycogen synthase is a substrate for phosphorylase ldnase in vivo is unclear (indicated by ?). Dashed arrows from protein phosphatase-1 to glycogen synthase and phosphorylase and phosphorylase kinase indicate that inhibition of dephosphorylation may be less important than activation of phosphorylation in changing the activities of these enzymes. [Modified and reproduced with permission from P. Cohen, Protein phosphorylation and the control of glycogen metabolism in skeletal muscle. Philos. Trans. R. Soc. Lond. (Biol.) 302 (1983).]

the cAMP-dependent protein kinase (Figure 15-12). The net effect in vivo is an increase in phosphorylation of glycogen synthase and phosphorylase. This may occur in several ways, and the relative importance of increased phosphorylation compared to decreased dephosphorylation remains unclear. Insulin also regulates skeletal muscle glycogen metabolism. Skeletal muscle is the major site of insulinstimulated glucose uptake from the bloodstream, most of which is converted to glycogen. The insulin-induced glucose uptake is mediated by the glucose transporter GLUT4 (Chapter 13). Binding of insulin to its cell surface receptor initiates a cascade of phosphorylation reactions followed by dephosphorylation reactions (Chapter 22). Activation of glycogen synthase occurs by dephosphorylation at sites phosphorylated by cAMP-dependent kinase and GSK-3. Protein phosphatase-1 is a key component of the insulin signaling pathway and it activates glycogen synthase; it simultaneously inactivates phosphorylase a and phosphorylase kinase, promoting glycogen synthesis. Inactivation of GSK-3 via the protein kinase B pathway has also

~0


been implicated in insulin-mediated activation of glycogen synthase. Liver In liver, glycogen serves as the immediate glucose reserve for other tissues, and regulation of its metabolism is closely linked to maintenance of blood glucose concentration. An important feature of glucose transport in hepatocytes is its bidirectional flux across the plasma membrane that is mediated by glucose transporter GLUT2. Insulin does not regulate GLUT2 and its function is not rate limiting. Therefore, the concentrations of glucose in the blood and hepatocytes are equal. During energy deprivation for short periods of time such as overnight fasting, moderate exercise, and stress, which cause hypoglycemia (e.g., hypovolemic shock), glycogenolysis is stimulated. During sustained periods of starvation, gluconeogenesis maintains blood glucose homeostasis. During refeeding, hepatocytes replenish their stores of glycogen by using glucose-6-phosphate obtained from either plasma glucose or from the gluconeogenic pathway using amino acids, glycerol, and propionate as precursors. Regulatory factors include glucagon, vasopressin, angiotensin II, and ot-adrenergic agonists (including epinephrine). Glycogen synthesis is stimulated by a rise in insulin or blood glucose concentration.

Control of Glycogen Synthase Hepatic glycogen synthase is similar to the muscle enzyme, although it is encoded by different genes. It is inactivated by phosphorylation and activated by dephosphorylation and may contain 12 mol of alkali-labile phosphate per mole of subunit. The phosphorylation sites have not been mapped, and the specificities of hepatic glycogen synthase kinases are not known. Several enzymes that phosphorylate glycogen synthase (muscle or liver) in vitro have been identified in liver. cAMP-dependent protein kinase, thought to mediate the inhibition of glycogen synthesis by glucagon, is similar to the muscle enzyme. A Ca2+/calmodulin-dependent protein kinase may mediate Ca2+-dependent inactivation of the synthase by vasopressin, angiotensin II, and o~-adrenergic agonists. Casein kinase I and GSK-3 have also been found in liver, and there are undoubtedly other hepatic glycogen synthase kinases. As in muscle, hepatic phosphorylase kinase is probably not important as a glycogen synthase kinase in vivo, despite its ability to readily phosphorylate the synthase in vitro. Muscle and liver thus have major regulatory pathways in common but differ in more subtle aspects of control.

Protein phosphatases-1, 2A, 2B, and 2C occur in mammalian liver and, as in skeletal muscle, possess essentially all of the phosphatase activity toward enzymes and regulatory proteins of glycogen metabolism. In liver, however, the ratios of the activities of phosphatase-2A and 2C to that of phosphatase-1 are seven-fold higher than in muscle. Although protein phosphatase-I sediments with glycogen particles in both tissues, a much smaller fraction is glycogen-associated in liver than in muscle. The specific activity of phosphatase-2B is lower in liver than in muscle. Protein phosphatase inhibitors-1 and 2 have been identified in liver, where they appear to function as they do in muscle. A disinhibitor protein (M.W. ~9,000) of liver can block the effects of inhibitors-1 and 2 on phosphatase-1.

Control of Glycogen Phosphorylase Liver glycogen phosphorylase exists in an inactive, dephosphorylated form and in at least one active, phosphorylated form. Conversion of phosphorylase b to phosphorylase a is catalyzed by phosphorylase kinase, which is activated by vasopressin, angiotensin II, and c~-adrenergic agonists (mediated by Ca 2+) and by glucagon (which elevates cAMP). Glucagon activation of phosphorylase is in some way antagonized by insulin. Dephosphorylation of glycogen phosphorylase a is probably catalyzed by either protein phosphatase-1 or 2A. Binding of free glucose to phosphorylase a makes it a better substrate for the phosphatase, increasing the rate of inactivation of glycogenolysis. Phosphorylase a is a competitive inhibitor of the reaction between glycogen synthase b and phosphatase, suggesting that the same enzyme dephosphorylates the synthase and kinase, activating one and inhibiting the other.

Integrated Regulation of Liver Glycogen Metabolism Glycogen metabolism in liver is regulated by phosphorylation and dephosphorylation of regulatory and metabolic enzymes. Control of the phosphorylation state is mediated by Ca 2+, cAME cytosolic glucose concentration, and perhaps, in the case of insulin, by another mechanism. When the glucose concentration rises in the hepatocyte, the rate of conversion of glycogen phosphorylase a to phosphorylase b increases. This decreases the rate of glycogenolysis and, initially, causes no change in glycogen synthesis. As the concentration of phosphorylase a falls, however, its ability to competitively inhibit glycogen synthase b dephosphorylation decreases. The rate of activation of glycogen synthase then increases, as does the rate of synthesis of glycogen. Glucose can be an effective regulator because its concentration in the hepatocyte varies

SECTION 15.3

Alternative Pathways of 61ucose Metabolism and Hexose Interconversions

with its concentration in the blood. The postprandial rise in blood glucose contributes to the inhibition of hepatic glycogenolysis and to the activation of glycogen synthesis. Following a meal, the blood level of insulin also rises, while that of glucagon falls. As glucagon concentration decreases, activity of the hepatocyte adenylate cyclase decreases and cytosolic cAMP concentration falls owing to degradation by phosphodiesterase. The result is a decreased phosphorylation of glycogen synthase and phosphorylase, activation of the synthase, and inhibition of the phosphorylase. Insulin, which may accelerate this shift, can also directly antagonize the cAMP-mediated effects of glucagon. The mechanisms of insulin action may include activation of a low-Km cAMP-phosphodiesterase, reduction in basal activity of cAMP-dependent protein kinase, or increase in basal activity of glycogen synthase phosphatase. Since the effects on the kinase and phosphatase are on basal activities, insulin may affect the metabolism of glycogen that has not been previously stimulated by glucagon. The net effect of blood glucose, insulin, and glucagon, then, is to coordinate the relative rates of hepatic glycogen synthesis and breakdown (see Chapter 22). In shock, inadequate perfusion of the heart, brain, and other organs is due to a variety of causes. The body's responses include release of epinephrine from the adrenal medulla, release of vasopressin from the neurohypophysis, and activation of the renin-angiotensin system with elevation of angiotensin II. In the liver, epinephrine (acting as an ot-adrenergic agonist), vasopressin, and angiotensin II increase the intrahepatocytic [Ca 2+ ], stimulating glycogenolysis. The consequent rise in blood glucose provides a rapid energy source in times of stress. Since shock is often caused by an absolute (hemorrhage) or relative (vasodilation) decrease in intravascular volume, release of the large amount of water of hydration stored with glycogen may contribute to restoration of blood volume.

Glycogen Storage Diseases This group of diseases is characterized by accumulation of normal or abnormal glycogen due to deficiency of one of the enzymes of glycogen metabolism. Although all are rare (overall incidence of ~1:25,000 births), they have contributed greatly to the understanding of glycogen metabolism. They are summarized in Table 15-1 and Figure 15-13. Infants with type I disease (subtypes Ia, Ib, Ic, and Id) develop hypoglycemia even after feeding because of inability to convert glucose-6-phosphate to glucose. Lactate is produced at a high rate in extrahepatic tissues and is transported to the liver for gluconeogenesis. The low insulin and high glucagon levels caused by hypoglycemia

291

promote glycogenolysis, leading to an unusual metabolic substrate cycle in these infants (Figure 15-14). The small amount of glucose released by the action of debranching enzyme is the only endogenous source of glucose available to these patients. Many characteristics of type I disease are attributed to the attendant hypoglycemia, and patients have been treated with frequent daytime feedings and continuous nocturnal intragastric feeding with a high-glucose formula. This regimen produces substantial improvement in growth, reduction in hepatomegaly, and normalization of other biochemical parameters. Feeding uncooked cornstarch every 6 hours resulted in normoglycemia, resumption of normal growth, and reduction in substrate cycling and liver size. The success of this simple nutritional therapy is thought to depend on slow hydrolysis of uncooked starch in the small intestine by pancreatic amylase, with continuous release and absorption of glucose. (Cornstarch that is cooked or altered in other ways is ineffective, presumably because of too rapid hydrolysis.) The therapy is ineffective in patients with low pancreatic amylase activity due, for example, to prematurity.

15.3 Alternative Pathways of Glucose Metabolism and Hexose Interconversions Glucuronic Acid Pathway The glucuronic acidpathway (Figure 15-15) is a quantitatively minor route of glucose metabolism. Like the pentose phosphate pathway, it provides biosynthetic precursors and interconverts some less common sugars to ones that can be metabolized. The first steps are identical to those of glycogen synthesis, i.e., formation of glucose-6-phosphate, its isomerization to glucose-l-phosphate, and activation of glucose-1-phosphate to form UDP-glucose. UDP-glucose is then oxidized to UDP-glucuronic acid by NAD + and UDP-glucose dehydrogenase. An aldehyde intermediate is formed that remains bound to an amino group on the enzyme as a Schiff base. Since this is a four-electron oxidation, two molecules ofNAD + are used for each molecule of UDP-glucuronic acid formed. UDP-glucuronic acid is bound to the enzyme as a thioester and is released by hydrolysis. UDP-glucuronic acid is utilized in biosynthetic reactions that involve condensation of glucuronic acid with a variety of molecules to form an ether (glycoside), an ester, or an amide, depending on the nature of the acceptor molecule. As in condensation reactions that use nucleotide sugars as substrates, the high-energy bond between UDP and glucuronic acid provides the energy to form the new

TABLE 15-1

Glycogen Storage Diseases Type Ia

Comments

Deficiency

von Gierke's disease (hepatorenal glycogenosis)

Glucose-6-phosphatase in liver, intestine, and kidney

Normal

Glucose-6-phosphate transporter in hepatocyte microsomal membrane Phosphate transporter in hepatocyte microsomal membrane Glucose transporter GLUT-7 Lysosomal a-1,4glucosidase (acid maltase)

Normal

Hypoglycemia; lack of glycogenolysis by epinephrine or glucagon; ketosis, hyperlipidemia, hyperuricemia; hepatomegaly; autosomal recessive. Galactose and fructose not converted to glucose. Clinically identical to type Ia.

Normal

Clinically identical to type Ia.

Normal Normal

Clinically identical to type Ia. How this deficiency leads to glycogen storage is not well understood. In some cases, the heart is the main organ involved; in others, the nervous system is severely affected; autosomal recessive. Hypoglycemia; diminished hyperglycemic response to epinephrine or glucagon; normal hyperglycemic response to fructose or galactose; autosomal recessive. Six subtypes have been defined based on relative effects on liver and muscle and on properties of the enzyme. Rare, or difficult to recognize; cirrhosis and storage of abnormal glycogen; diminished hyperglycemic response to epinephrine; abnormal liver function; autosomal recessive.

Ib

...

Ic

...

Id I1

Glycogen Structure

Eponym

... Pompe's disease (generalized glycogenesis)

I11

Forbes' disease, Cori's disease (limit dextrinosis)

Amylo- l,6-glucosidase (debranching enzyme)

Abnormal; outer chains missing or very short; increased number of branched points

IV

Andersen's disease (branching deficiency; amylopectinosis)

(1,4-+ 1,6)-transglucosylase (branching enzyme)

Abnormal; very long inner and outer unbranched chain

V

McArdle's disease

Muscle glycogen phosphorylase (myophosphorylase)

Normal

VI

Hers' disease

Liver glycogen phosphorylase (hepatophosphorylase)

Normal

VII

Tarui's disease

Muscle phosphofructokinase

Normal

Reduced activation of phosphorylase in hepatocytes and leukocytes

Normal

VIII

...

High muscle glycogen content (2.5-4.1 % versus 0.2-0.9% normal); fall in blood lactate and pyruvate after exercise (normal is sharp rise) with no postexercise drop in pH; normal hyperglycemic response to epinephrine (thus normal hepatic enzyme); myoglobinuria after strenuous exercise; autosomal recessive. Not as serious as glucose-6-phosphatase deficiency; liver cannot make glucose from glycogen but can make it from pyruvate; mild hypoglycemia and ketosis; hepatomegaly due to glycogen accumulation; probably more than one disease; must be distinguished from defects in the glycogen phosphorylaseactivating system. Shows properties similar to type V; autosomal recessive; it is not completely clear why this defect results in increased glycogen storage. Hepatomegaly; increased hepatic glycogen stores; probably X-linked, but there may be more than one type, with some autosomally inherited; must be distinguished from glycogen phosphorylase deficiency.

~9~


F I G U R E 15-14 Lactic acidosis and substrate cycling in glucose-6-phosphatase deficiency. *Urinary lactate output is increased 10- to 300-fold in patients with type I glycogen storage disease.

SECTION 15.3

Alternative Pathways of Glucose Metabolism and Hexose Interconversions

Glucose 6-P .----* ~H2OH

O

/,

CH2OH

O

/ " 2NAD+

10 UDP H 10

o~OH

pyrophosphorylase f'~ ~ PP~" H - - P UTP

H

,.

HO

/ dehydrogenase 1

UDP-glucose COOH

COOH O

O

lc

H

2NADH + 2H+

- - P - - P--U

HO

Glucose-1-P COOH O

H

P,

H

H20

295

tO

lo

H

--P

HO

UM~P

H20

H

Substrate /'~ Glucuronyltransferase

H

HO - - P - - P - - U

"~UDP

HO

D-Glucuronic

D-Glucuronicacid (hemiacetal form)

UDP-glucuronic acid

Glucuronides

acid 1-phosphate

Spontaneous) N~ " d O

I

CH2OH

COOH

I

I

H--C--OH Glucuronate H--C--OH I reductase I HO--C--H /" ~'~," + HO--C--H I NADPH NADP I H--C--OH +H+ H--C--OH

I H--C--OH I

HO--C--H I HO--C--H I ~ '/ H--C--OH

(~

I H--C--OH I

COOH

COOH

["-~NADH + H+ COOH

co,

I HO--C--H O---C

I

~

CH2OH

L-Xylulose

HO--C--H

I

NADPH~1 L-Xylulose + H+ ] reductase (block in NADP§ [ essential pentosuria)

CH2OH 3-Oxo- L-gulonate (3-keto- L-gulonate)

CH2OH

CH2OH

I

CH2OH

I

HO--C--H

L-XylitoI

1

L-Ascorbic acid

I I H--C--OH I

HO--C--H

CH2OH

L-Gulonolactone t (This conversion is absent in humans, other primates, and guinea pigs) 2-Keto- L-gulonolactone

L~Gulonicacid rNAD+

CH2OH

I I

CH2OH

CH2OH

O---C

HO--C--H

I I

HO--C--H i

D-Glucuronicacid (open-chain form)

I I H--C--OH I ~

,,

H--C

I HO--C--H I

i

I H--C--OH

,

I HO--C--H I HO--C--H Spontaneous I

NADPH NAD +

L.._ J

+H + -

HO--C--H I H--C--OH

I

#-m

C--'O I

Lor) H O - - C - - H

I I

I I

O--C

Kinase f ~ ,; ATP ADP

Xylulose 5-P

H--C--OH

CH2OH

CH2OH

D-Xylulose

I I I

Pentose 3hosphate pathway

Oxalate

FIGURE 15-15 Glucuronic acid pathway. The first part consists of synthesis of UDP-glucuronic acid and release of free D-glucuronic acid. The second part is the metabolism of D-glucuronic acid. D-Glucuronic acid is written as both the cyclic hemiacetal and the open-chain aldohexose and two orientations of L-gulonic acid and D-xylulose are shown. P -- phosphate.

bond in the product. UDP is hydrolyzed to UMP (uridine monophosphate) and inorganic phosphate, further ensuring the irreversibility of the coupling reaction. Because glucuronic acid is highly polar, its conjugation with less polar compounds, such as steroids,

bilirubin, and some drugs, can reduce their activity and make them more water-soluble, thus facilitating renal excretion. Glucuronic acid is a component of the structural polysaccharides called glycosaminoglycans (hyaluronic acid and other connective tissue polysaccharides; see

CHAPTER 15


Chapters 11 and 16). Glucuronic acid is usually not a component of glycoproteins or glycolipids. Other metabolic routes available to UDP-glucuronic acid are shown in Figure 15-15. Hydrolysis to free glucuronic acid and NADPH-dependent reduction together lead to L-gulonic acid, which spontaneously cyclizes to L-gulonolactone. In many animals and higher plants, this can be converted to 2-ketogulonolactone, a precursor of ascorbic acid (vitamin C), by gulonolactone oxidase. In humans and other primates and in guinea pigs, absence of this enzyme makes ascorbic acid an essential dietary ingredient (Chapter 38). Gulonic acid is oxidized and decarboxylated to Lxylulose. Entry of this sugar into the pentose phosphate pathway requires isomerization to D-xylulose. This is accomplished by reduction (NADPH) to xylitol via Lxylulose reductase and by oxidation to D-xylulose with reduction of NAD +. D-Xylulose, after conversion of xylulose 5-phosphate, is metabolized in the pentose phosphate pathway or converted to oxalate via formation of xylulose-l-phosphate, glycoaldehyde, and glycolate. In essential pentosuria, a clinically benign inborn error of metabolism, L-xylulose reductase (also known as NADPlinked xylitol dehydrogenase) is abnormal or absent, and large amounts (1-4 g/day) of L-xylulose appear in the urine. Because L-xylulose is a reducing sugar, tests for urinary reducing substances that are intended to detect glucose may result in an erroneous diagnosis of diabetes mellitus. A positive test for urinary reducing substances, especially in the absence of clinical symptoms of diabetes mellitus, should be verified by a method that is specific for glucose. Alimentary pentosuria may follow ingestion of large quantities of fruit, with L-arabinose and L-xylose occurring in high concentrations in urine. Ribosuria may occur in some muscular dystrophy patients. In both conditions, urine is positive for reducing substances. Fructose and Sorbitol Metabolism Fructose is a ketohexose found in honey and a wide variety of fruits and vegetables. Combined with glucose in an or(1 --+ 2)fl linkage, it forms sucrose (Chapter 9). It makes up one sixth to one third of the total carbohydrate intake of most individuals in industrialized nations. Inulin, found in some plants, is a polymer containing about 40 fructose residues connected in/3(2 ~ 1) linkages with some fl(2 --+ 6) branch points. It cannot be hydrolyzed by any human enzyme and has been used for measurement of renal clearance. Sorbitol, a sugar alcohol, is a minor dietary constituent. It can be synthesized in the body from glucose by NADPHdependent aldose reductase (Figure 15-16). It is clinically

Glucose

NADP+--~Aldose .

11

reductase

NADH + H+~I[ Sorbitol I

NAD+-~SorbitoI NADH + H + ~ ehydr~ Fructose

~Fructokinase ADP ATP


L

AldolaseB

1+

i

Dihydroxyacetone 3-phosphate

ATP--~I

Glyceraldehyde

ADP qTriokinase

Glyceraldehyde3-phosphate Fructose-1,6-

bisphosphatealdolase

T Fructose 1,6-bisphosphate

~ Glucone~jenesis

Glycolysis

1

Pyruvate

t

Glucose

FIGURE 15-16 Metabolic pathwayfor sorbitoland fructose. Sorbitoldehydrogenaseis sometimesknownas iditol dehydrogenase.AldolaseB is also called fructose-1-phosphatealdolase, in contrastto fructose-1,6-bisphosphate aldolase.

important because of its relationship to cataract formation in diabetic patients. Fructose and sorbitol are catabolized by a common pathway (Figure 15-16). Fructose transport and metabolism are insulin-independent; only a few tissues (e.g., liver, kidney, intestinal mucosa, and adipose tissue, but not brain) can metabolize it. Fructose metabolism, which is much less tightly regulated, is more rapid than glucose metabolism. The renal threshold for fructose is very low, and fructose is more readily excreted in urine than is glucose. Despite these differences, the metabolic fates of glucose and fructose are closely related because most fructose is ultimately converted to glucose. Fructose metabolism (Figure 15-16) starts with phosphorylation and formation of fructose- 1-phosphate, and this reaction is the rate-limiting step. The Km of hexokinase for fructose is several orders of magnitude higher than that for glucose, and at the concentrations of glucose found in most tissues, fructose phosphorylation by hexokinase is competitively inhibited. In tissues that contain fructokinase, such as liver, the rate of fructose phosphorylation depends primarily on fructose concentration, and an increase in fructose concentration depletes intracellular ATE Essential fructosuria is caused by absence of hepatic fructokinase activity. The condition is asymptomatic but, as with pentosuria, may be misdiagnosed as diabetes mellitus. The next step in fructose metabolism is catalyzed by aldolase B (fructose-l-phosphate aldolase), which cleaves fructose-l-phosphate to the trioses dihydroxyacetone

SECTION 15.3


3-phosphate and glyceraldehyde. Glyceraldehyde is phosphorylated by triokinase, and glyceraldehyde 3phosphate and dihydroxyacetone 3-phosphate can either enter the glycolytic pathway or be combined to form fructose-l,6-bisphosphate by the action of fructose-l,6bisphosphate aldolase. Thus, fructose metabolism bypasses phosphofructokinase, the major regulatory site of glycolysis. Most dietary fructose is converted to glucose by way of gluconeogenesis, through condensation of the triose phosphates to fructose-l,6-bisphosphate. However, administration of large doses of fructose (i.e., by intravenous feeding) may lead to hypoglycemia and lactic acidosis because of saturation of aldolase B, causing accumulation of fructose-l-phosphate, and depletion of intracellular ATP and inorganic phosphate. This situation may be likened to hereditary fructose intolerance, which is caused by inadequate amounts of aldolase B activity. Although normally asymptomatic, individuals with this condition exhibit hypoglycemia, metabolic acidosis, vomiting, convulsions, coma, and signs of liver failure following ingestion of fructose or sucrose. The fructose-induced hypoglycemia arises from accumulation of fructose-l-phosphate, reduction in the [ATP]/[ADP] ratio, and depletion of inorganic phosphate. Fructose-lphosphate depresses gluconeogenesis and promotes glycolysis, while inorganic phosphate depletion inhibits ATP synthesis (Chapter 14). Glycogenolysis is inhibited by fructose-1-phosphate at the level of phosphorylase. If sucrose, fructose, and sorbitol are eliminated from the diet, complete recovery occurs. High levels of fructose also increase purine turnover owing to enhanced ATP utilization, which leads to increased production of purine degradation products--inosine, hypoxanthine, xanthine, and uric acid (Chapter 27). Fructose and sorbitol have been recommended as substitutes for sucrose in diabetic diets because they are much sweeter than sucrose. Foods can be made more palatable even when the total carbohydrate content is reduced by replacing sucrose with fructose or sorbitol. Their insulinindependent metabolism has also been cited as a reason for substituting them for glucose or sucrose in diabetic diets. In severe physical injury, hyperglycemia and insulin resistance are common, and inclusion of fructose and sorbitol in the parenteral nutrition formulations for these patients has been suggested. In small amounts, fructose and sorbitol could benefit both groups of patients. In large amounts, however, they can severely damage the liver by depleting ATP stores, and they can cause hypoglycemia and lactic acidosis. When sorbitol and fructose are taken by mouth, the increase in blood fructose is modulated by their rates of absorption from the intestine, preventing the serious

297

metabolic problems caused by high concentrations of these sugars. Most normal diets do not result in adverse effects, except in patients with hereditary fructose intolerance. However, when sorbitol and fructose are given parenterally, this important modulating action is lost, and serious side effects can occur. Sorbitol and fructose as intravenous nutrients are not widely employed in the United States. Sorbitol has been implicated in cataract formation in diabetics. In general, cataracts are formed when the lens of the eye becomes cloudy, probably because of a change in solubility of lens proteins. Because entry of glucose into the lens does not require insulin, intravenous and intralenticular glucose concentrations increase in parallel. In diabetics, quite high concentrations can be achieved. In the lens, glucose is converted to sorbitol by aldose reductase, an NADPH-dependent enzyme present in many tissues. Unlike glucose, sorbitol does not diffuse readily across the lenticular membrane but accumulates within the lens, increasing osmolarity and causing water retention. A reduction in lens glutathione concentration parallels the increase in water content and may reflect the depletion of NADPH by the aldose reductase reaction and lead to oxidation and denaturation of lens proteins. The lens is particularly susceptible to oxidative damage because it is exposed to a high oxygen concentration. Galactose Metabolism Most galactose ingested by humans is in the form of lactose (Chapter 9), the principal sugar in human and bovine milk. Milk sugar other than lactose is found in the sea lion and marsupials, whose first pouch milk contains a trisaccharide of galactose. Lactose is hydrolyzed to galactose and glucose by lactase, located on the microvillar membrane of the small intestine (Chapter 12). Following absorption, galactose is transported to the liver, where it is converted to glucose (Figure 15-17). The enzymes required are found in many tissues, but the liver is the quantitatively most important site for this epimerization. Galactose is a poor substrate for hexokinase; it is phosphorylated by galactokinase. Galactose-l-phosphate is converted to UDP-galactose by galactose-l-phosphate uridylyltransferase. This enzyme may be regulated by substrate availability, since the normal hepatic concentration of galactose-l-phosphate is close to the Km for this enzyme. The transferase is inhibited by UDP, UTR glucose1-phosphate, and high concentrations of UDP-glucose. Deficiency of galactokinase or transferase can cause galactosemia. Galactose is isomerized to glucose by UDP-galactose4-epimerase in what may be the rate-limiting step in galactose metabolism. The reaction, which is freely

298

CHAPTER15 Carbohydrate Metabolism I1: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways Galactose ATP --'~_!

ADPM~ Galactokinase* Galactose 1-phosphate

~ -UDP-glucose [Galactose- 1-phosphate

luddylyltransferase* Jk.

l Glucose 1-phosphate Phosphoglucomutase Glucose 6-phosphate

I

H20~ Glucose6-phosR'~"[ phatase Glucose

Glucose UDP UDP-ga~actose Lactose synthase -- Lactose iUridine {diphosphogalactose/4-epimerase (NAD') UDP-glucose

~Glycogen synthesis Glycogen Glycogen breakdown Glucose

FIGURE 15-17 Metabolic pathway of galactose. *Inherited defects that lead to galactosemia.

reversible, converts UDP-glucose to UDP-galactose for the synthesis of lactose, glycoproteins, and glycolipids. The epimerase requires NAD + and is inhibited by NADH. It may also be regulated by the concentrations of UDPglucose and other uridine nucleotides. Because of this epimerase, preformed galactose is not normally required in the diet. However, infants with deficiency of epimerase in hepatic and extrahepatic tissues require small quantities of dietary galactose for normal development and growth. An isolated deficiency of epimerase in erythrocytes, which is clinically benign, has been described. Genetically determined deficiencies of galactokinase and galactose 1-phosphate uridylyltransferase cause clinically significant galactosemia. Galactokinase deficiency is a rare, autosomal recessive trait in which high concentrations of galactose are found in the blood, particularly after a meal that includes lactose-rich foods, such as milk and nonfermented milk products. Patients frequently develop cataracts before 1 year of age because of accumulation of galactitol in the lens. Galactose diffuses freely into the lens, where it is reduced by aldose reductase, the enzyme that converts glucose to sorbitol. Galactitol may cause lens opacity by the same mechanisms as does sorbitol. Galactose in the urine is detected by nonspecific tests for reducing substances, as are fructose and glucose. Deficiency of the transferase causes a more severe form of galactosemia. Symptoms include cataracts, vomiting,

diarrhea, jaundice, hepatosplenomegaly, failure to thrive, and mental retardation. If galactosuria is severe, nephrotoxicity with albuminuria and aminoaciduria may occur. The more severe clinical course may be due to accumulation of galactose-l-phosphate in cells. Measurement of transferase activity in red blood cells is the definitive test for this disorder. In about 70% of transferase alleles in the white population, the DNA in cells of transferase-deficient patients possesses an A-to-G transition that leads to the Q186R mutation. Both forms of galactosemia are treated by rigorous exclusion of galactose from the diet. In pregnancies for which the family history suggests that the infant may be affected, the best outcomes have been reported when the mother was maintained on a galactose-free diet during gestation. Condensation of glucose with UDP-galactose, catalyzed by lactose synthase (Figure 15-17), forms lactose, the only disaccharide made in large quantities by mammals. This enzyme is a complex of galactosyltransferase, a membrane-bound enzyme that participates in glycoprotein synthesis (Chapter 16), and t~-lactalbumin, a soluble protein secreted by the lactating mammary gland. Binding of ct-lactalbumin to galactosyltransferase changes the Km of the transferase for glucose from 1-2 mol/L to 10 -3 mol/L. Synthesis of ct-lactalbumin by the mammary gland is initiated late in pregnancy or at parturition by the sudden decrease in progesterone level that occurs at that time. Prolactin promotes the rate of synthesis of galactosyltransferase and ot-lactalbumin. Metabolism of Amino Sugars In amino sugars, one hydroxyl group, usually at C2, is replaced by an amino group. Amino sugars are important constituents of many complex polysaccharides, including glycoproteins and glycolipids (Chapters 10 and 16), and of glycosaminoglycans (Chapters 11 and 16). D e n o v o synthesis of amino sugars starts from glucose6-phosphate, but salvage pathways can also operate. Some reactions involved are shown in Figure 15-18. Incorporation of amino sugars into biological macromolecules is described in Chapter 16.

Pentose Phosphate Pathway The series of cytoplasmic reactions known as the pentose phosphate pathway is also called the hexose monophosphate (HMP) shunt (or cycle) or the phosphogluconate pathway. The qualitative interconversions that take place

299

SECTION15.3 Alternative Pathways of 61ucose Metabolism and Hexose Interconversions -

Glycogen

'"

~ Glucose1-phosphate--~ Glucose~

Glucosamine

I t Fructose 6-phosphate ~,--Glutamine Glucose 6-phosphate

-Glycolysis

. . . . . Aminotransferase[ ATP ADP ["----Glutamate ~ J~ Glucosamine6-phosphate Glucosamine~"-- AceIyl C~ acetyltransferase[,,...,.CoASH

i i

.......................

N-Acetylglucosamine6-phosphate J ~.

N-Acetylglucosaminer phosphomutase[ N-Acetylglucosamine1-phosphate

Acetylgluc~ 2 -epimerase N-Acetylmannosamine6-phosphate

UDP.glucosamineI(-'-- UTP pyrophosphorylase[~ PP, UDP-glu[osamine

Phosphoenoylq Gtycosaminoglycans pyruvate ] N-Acetylneuraminate-9-phosphatesynthase

UDP-acetylglucosamine~- UTP

pyrophosphorylase[,,~_,pp~

P,.--q

N-Acetylneuraminicacid 9-phosphate

UDP-N-acetylglucosamine "I/ Glycoproteins, glycosaminoglycans

Glucosamine1-phosphate

~UDP-N-acetyl\ glucosamine~-epimerase

t

Sialie

acids,

glycoproteins

UDP-N-acetyIgalactosamine

l

Glycosaminoglycans F I G U R E 15-18 Summary of some interconversions and synthetic reactions in which amino sugars participate. Substrates for the pathway can be derived from glucose, glycogen, and gluconeogenesis.

are summarized in Figure 15-19, in which stoichiometry is ignored.

The pentose phosphate pathway can be thought of as two separate pathways: 1. The oxidative conversion of glucose-6-phosphate to ribulose 5-phosphate and CO2 and 2. Resynthesis of glucose-6-phosphate from ribulose 5-phosphate. Because the pathway begins and ends with glucose-6-phosphate, it is a cycle, or shunt. For every six molecules of glucose-6-phosphate that enter the pathway, five molecules of glucose-6-phosphate + 6CO2 are produced. Thus, there is a net loss of one carbon from each glucose-6-phosphate that enters the cycle.

F I G U R E 15-19 Summary of the pentose phosphate pathway. This diagram is intended to show the two major parts of the pathway: oxidation and decarboxylation of glucose-6-phosphate to ribulose 5-phosphate, and resynthesis of the former from the latter. The stoichiometry of the pathway is ignored.

Interpreting the pathway as a means of oxidizing glucose to ribulose 5-phosphate and recycling the ribulose 5-phosphate back to glucose-6-phosphate is too narrow. The pathway performs a variety of functions, the least important of which seems to be that of an alternative pathway for glucose metabolism. Production of ribose 5-phosphate for nucleotide synthesis by d e n o v o and

300

CHAPTER 15


salvage pathways (Chapter 27), generation of NADPH for biosynthetic reactions in the cytosol, and interconversion of pentoses and hexoses are more valuable processes. Thus, the pathway provides a means for generating glucose from ribose and other pentoses that can be converted to ribose 5-phosphate. The pentose phosphate pathway is active in a wide variety of cell types, particularly those which have a high rate of nucleotide synthesis or which utilize NADPH in large amounts. Nucleotide synthesis is greatest in rapidly dividing tissues, such as bone marrow, skin, and gastric mucosa. NADPH is needed in the biosynthesis of fatty acids (liver, adipose tissue, lactating mammary gland), cholesterol (liver, adrenal cortex, skin, intestine, gonads), steroid hormones (adrenal cortex, gonads), and catecholamines (nervous system, adrenal medulla) and in other reactions that involve tetrahydrobiopterin. NADPH is also needed for maintenance of a reducing atmosphere in cells exposed to high concentrations of oxygen radicals including erythrocytes, lens and cornea of the eye, and phagocytic cells, which generate peroxide and superoxide anions during the process of killing bacteria.

H

0

I

H

]

~ilj~H) ~ H

~ O

HO ~

i H

Glucose

OH

I OH

6-phosphate

(?H!OPO32NADP'

~--

NADPH+ H*

~

H~) :

HCOH

6-Phosphogluconp-&-Iactone

H2COPO326-Phosphogluconate

The equilibrium of this reaction lies far to the right. In the third step, 6-phosphogluconate is oxidatively decarboxylated to ribulose 5-phosphate in the presence of NADP +, catalyzed by 6-phosphogluconate dehydrogenase: NADP" -ICO0 -

NADPH + H + .

I

+ ~OO -

H~OH

HCOH

I

Mn2+

HOCH

I

C=O

I

I

HCOH

HCOH

I

6-Phosphogluconate

CO2 + CH2OH

I HCOH

I

H2COPO32-

I

HCOH

I

H2COPO32-

3-Keto-6-phosphogluconate D-Ribulose-5-phosphate

3-Keto-6-phosphogluconate is a probable intermediate. The reaction is similar to those catalyzed by malic enzyme (in gluconeogenesis) and by isocitrate dehydrogenase (in the TCA cycle).

H~ H

HO

I HCOH

I

I

CH2OPO32-

HOOH' I

OH

H2COPO32-

The first reaction in this phase is the oxidation of glucose-6-phosphate to 6-phosphoglucono-6-1actone and reduction of NADP + to NADPH, catalyzed by glucose-6phosphate dehydrogenase (G6PD):

-~

H

HCOH

Oxidative Phase

CO0 I HCOH

CH2OPO32-

O

Nonoxidative Phase I

H

I

OH

6-Phosphoglucono(~-Iactone

The equilibrium of the reaction lies far to the right. Under physiological conditions, G6PD is a dimer of identical subunits of M.W. 55,000. The active enzyme contains a molecule of NADP +, removal of which causes dissociation into inactive monomers. Enzymes from rat, cow, and human are very similar, and active interspecies hybrid enzymes can be formed in vitro. The enzyme is inhibited by NADPH at the concentration normally found in hepatocytes and erythrocytes, by ATP competing with glucose-6-phosphate for binding, and by cyanate. A large reserve capacity of G6PD activity exists in the red blood cell and, presumably, in other tissues. In the second step, 6-phosphoglucono-6-1actone is hydrolyzed to 6-phosphogluconate by 6-phosphogluconolactonase:

In this phase, ribulose 3-phosphate is converted to glucose-6-phosphate. Stoichiometrically, this requires the rearrangement of six molecules of ketopentose phosphate to five molecules of aldohexose phosphate (Figure 15-20). It has been claimed that the pathway shown in Figure 15-20 occurs primarily in fat tissue and that a modified pathway involving arabinose 5-phosphate and octulose 8-phosphate occurs in liver cells. The overall scheme of the nonoxidative phase should be considered tentative. There are only four types of reactions in this part of the pathway: 1. Interconversion of a keto sugar (ribulose 5-phosphate or fructose-6-phosphate) and an aldo sugar (ribose 5-phosphate or glucose-6-phosphate) by an isomerase. 2. Inversion of the optical configuration of an optically active carbon atom, as in a conversion of ribulose 5-phosphate to xylulose 5-phosphate by an epimerase.

SECTION 15.3

301


Epime~

~merase

6 Ribulose5-P (30)

4 Xylulose 5-P (20)

~ T K- 2 Xylulose5-P (10) ' ~

2 Xylulose5-P (10)

Glutamate + Cysteine

'- y-Glutamylcysteine

2 Ribose 5-P (10) 29Sedoheptulose7~14)

ADP + P,~.,,~ v NADP+

?-Glutamylcysteinylglycine (GSH) ROOH or H202-~(~)

2 Glyceraldehyde3-P (6)

ROH or H20*-'" ~ Oxidized glutathione (GSSG)

NADPH + H+

[

2 Erythrose4-P (8) r

'/

TK

2 Fructose 6-P (12)

From pentose phosphate pathway

FIGURE 15-21

.,

GAP (3)

ATP (~)K+~~ K Glycine

Mg2+

2 GAP (6) + 2 Fructose 6-P (12) ~--GAP k. --~ DHAP~(3)

merase

Metabolism of glutathione. (1) y-Glutamylcysteine synthase; (2) GSH synthase; (3) glutathione peroxidase (Se-containing enzyme); 4. glutathione reductase (an FAD enzyme).

AIdolase Fructose 1,6-bisP Phosphatase Fructose 6-P (6)

~

5 Glucose 6-P (30)

FIGURE 15-20 Nonoxidative phase of the pentose phosphate pathway. Numbers in parentheses show the distribution of carbon atoms between the various branches of each reaction. TK = transketolase; GAP -- glyceraldehyde 3-phosphate; DHAP = dihydroxyacetone phosphate; P -- phosphate.

3. Transfer of a two-carbon unit from a 2-keto sugar to the carbonyl carbon (C1) of an aldose by a transketolase, which requires thiamine pyrophosphate and magnesium as cofactors. A covalent enzymesubstrate intermediate is formed similar to the one that occurs during the pyruvate dehydrogenase reaction (Chapter 13). 4. Transfer of carbons 1-3 from a ketose phosphate to the carbonyl carbon (C1) of an aldose phosphate by a transaldolase. The reaction mechanism is similar to that of aldolase, with formation of a Schiff base intermediate involving the e-amino group of a lysyl side chain on the enzyme. The flexibility of this phase of the pathway allows for interconversion of a number of sugars and glycolytic intermediates, in part because of the ready reversibility of the reactions and regulation of the enzymes by substrate availability. Thus, the pathway adjusts the concentrations of a number of sugars rather than act as a unidirectional anabolic or catabolic route for carbohydrates.

Pentose Phosphate Pathway in Red Blood Cells In the erythrocyte, glycolysis, the pentose phosphate pathway, and the metabolism of 2,3-bisphosphoglycerate (Chapter 28) are the predominant pathways of carbohydrate metabolism. Glycolysis supplies ATP for membrane ion pumps and NADH for reoxidation of methemoglobin. The pentose phosphate pathway supplies NADPH to

reduce glutathione for protection against oxidant injury. Glutathione (y-glutamylcysteinylglycine; GSH) is synthesized by y-glutamylcysteine synthase and GSH synthase (Figure 15-21). In the steady state, 99.8% of the glutathione is in the reduced form (GSH), and only 0.2% is in the oxidized form (GSSG), because of theNADPHdependent reduction of oxidized glutathione, catalyzed by glutathione reductase: T-Glutamylcysteinylglycine I

s

NADPH + H + NADP+

I

s

r

I

T-Glutamylcysteinylglycine (GSSG) SH

I 2 T-Glutamylcysteinylglycine (GSH)

Glutathione reductase also catalyzes reduction of mixed disulfides of glutathione and proteins (Pr)" Pr-S-SG + NADPH + H + --+ Pr-SH + GSH + NADP + Thus, the active sulfhydryl groups of hexokinase, glyceraldehyde-phosphate dehydrogenase, glutathione reductase, and hemoglobin (-SH group at the ~-93 position) are maintained in the reduced form. Glutathione reductase is an FAD enzyme composed of two identical subunits encoded by a single locus on the short arm of human chromosome 8. GSH is used in the inactivation of potentially damaging organic peroxides (e.g., a peroxidized unsaturated fatty acid) and of hydrogen peroxide. Hydrogen peroxide is formed through the action of superoxide dismutase (Chapter 14): 20 2 + 2H + --+ H202 -Jr- 02

30~

CHAPTER15


Peroxide inactivation is catalyzed by glutathione peroxidase, a selenium-containing enzyme, in the following reactions: 2GSH -Jr-ROOH --+ GSSG + H20 -k- ROH and 2GSH -Jr-H202 ~ GSSG + 2H20 The concentrations of substrates for glutathione reductase and peroxidase determine the rate at which the pentose phosphate pathway operates in erythrocytes. Genetically determined deficiencies of y-glutamylcysteine synthase and glutathione synthase can cause hemolytic anemia (Chapter 17). Abnormalities of glutathione metabolism can also result from nutritional deficiencies of riboflavin or selenium. Glutathione reductase is an FAD enzyme that requires riboflavin for activity, and riboflavin deficiency can cause hemolytic anemia. An inherited lack of glutathione reductase apoenzyme has been described. Selenium deficiency diminishes activity of glutathione peroxidase and can lead to peroxidative damage. This can be partially ameliorated by vitamin E, an antioxidant (Chapters 37 and 38).

Glucose.6.Phosphate Dehydrogenase Deficiency G6PD deficiency is the most common inherited enzyme deficiency known to cause human disease, occurring in about 100 million people. Most clinical manifestations are related to hemolysis, which results from impaired ability to produce cytosolic NADPH. Over 150 variants of the G6PD structural gene are known, many of which show either abnormal kinetics or instability of the enzyme. The erythrocytes are most severely affected because of their long half-lives and inability to carry out protein synthesis. Since persons with G6PD deficiency can usually make an adequate supply of NADPH under normal conditions, the defect may not become apparent until the patient takes a drug, such as primaquine, that greatly increases the demand for NADPH. The severity of the reaction depends, in part, on the particular inherited mutation. In most persons with G6PD deficiency, hemolysis is observed as an acute phenomenon only after severe oxidative stress, leading to loss of perhaps 30-50% of the circulating red cells. The urine may turn dark, even black, from the high concentration of hemoglobin, and a high urine flow must be maintained to prevent damage to the renal tubules by the high-protein load. Because G6PD activity is highest in reticulocytes and decreases as the cell ages, the older erythrocytes (more than 70 days old) are destroyed. For this reason, measurement of red cell G6PD activity following a hemolytic crisis can lead to a spuriously high value,

TABLE 15-2

Partial List of Drugs and Chemicals That Cause Acute Hemolytic Anemia in Persons with G6PD Deficiency 8-Aminoquinoline derivatives (pamaquine, pentaquine, primaquine)

Phenylhydrazine

Neoarsphenamine

Sulfonamides and sulfones (dapsone, sulfacetamide, sulfamethoxazole, sulfanilamide, sulfapyridine, salicylazosulfapyridine, thiazolesulfone)

Niridazole

Toluidine blue

Nitrofurantoin

Trinitrotoluene

Methylene blue Nalidixic acid Naphthalene

even within the normal range. If the patient survives the initial crisis, with or without transfusions, recovery usually occurs as the reticulocyte count increases. The characteristics of the disorder were elucidated during investigations into the hemolytic crises observed in some patients following administration of 8-aminoquinoline derivatives, such as primaquine and pamaquine, used for prophylaxis and treatment of malaria. The relatively high frequency of such crises in some geographic areas is due to the extensive overlap in the distributions of endemic malaria and G6PD deficiency. The geographic distribution of G6PD variants, like that of sickle cell trait (Chapter 28), suggests that heterozygosity for G6PD deficiency may confer some protection against faleiparum malaria. A number of other drugs (Table 15-2) also cause hemolytic crises. Oxidation of glutathione by these drugs beyond the capacity of the cell to generate NADPH for GSSG reduction causes the acute crisis. Since these drugs do not cause hemolysis in vitro, additional steps must take place in vivo. Following administration of a drug known to promote hemolysis, Heinz bodies are seen in erythrocytes in the peripheral blood. These also occur in some thalassemias and consist of oxidized and denatured forms of hemoglobin known as hemiehromes (Chapter 28). Heinz bodies impair movement of the cells through the splenic pulp and probably are excised there, presumably together with the adjacent piece of plasma membrane, leaving a red cell that is more susceptible to destruction by the reticuloendothelial system. Infection and diabetic ketoacidosis can also cause hemolysis in persons

SECTION 15.3

Alternative Pathways of 131ucose Metabolism and Hexose Interconversions

with G6PD deficiency, possibly through depletion of NADPH. Favism is characterized by acute hemolysis following ingestion of fava beans (Viciafava) by persons with G6PD deficiency. The fava bean is a vegetable staple of the Mediterranean region, an area in which G6PD deficiency is endemic. Infants are especially susceptible to favism. The disorder is frequently fatal unless a large amount of blood is transfused rapidly. Not everyone with G6PD deficiency reacts to fava beans, and ingestion of fava beans does not invariably destroy 51Cr-labeled G6PD-deficient erythrocytes in vivo. Thus, there may be a fundamental difference between favism and the hemolytic response to other agents. It has been suggested that another mutation must also be present for the expression of favism, but the nature of this hypothetical mutation is unknown. Studies of the genetics of human G6PD variants have contributed to the understanding of G6PD deficiency and of more general aspects of human genetics. G6PD deficiency is inherited as an X-linked trait, as are hemophilia (Chapter 36) and color blindness (Chapter 38). If the X chromosome carrying an abnormal G6PD allele is designated X*, then the three possible genotypes containing X are 1. X*Y--hemizygous male, with full phenotypic expression of the abnormal allele; 2. XX*--heterozygous female, with a clinically normal phenotype in spite of the abnormal allele expressed in about half her cells; and 3. X*X*--homozygous female, with full phenotypic expression of the abnormal allele. Sons of affected males are usually normal (because they receive their X chromosome from their mothers), and daughters of affected males are usually heterozygotes (because they receive one X chromosome from their fathers). The rarest genotype is that of the homozygous female, since it requires that both parents have at least one abnormal X chromosome. Females heterozygous for a G6PD variant are phenotypic mosaics. They have two erythrocyte populations, one containing normal G6PD, the other the variant. In fact, in heterozygotes, every tissue has some cells expressing the normal, and some the abnormal, G6PD gene. Random X-chromosome inactivation early in embryonic development causes only one of the two X chromosomes to be active (Lyon's hypothesis). Severity of the disorder in homozygotes and hemizygotes depends on a number of factors. G6PD variants are classified into five groups (class I being the most severe) depending on the presence or absence of chronic anemia and on the amount of enzyme activity

303

present in the erythrocytes. The most common normal activity (class IV) allele is G6PD B. Another common electrophoretic variant with normal activity is G6PD A. Among North American blacks, the gene for the absence of G6PD A (G6PD A - ; class III) has an incidence of about 11%. Hemizygous males have only 515% of normal erythrocyte G6PD activity and exhibit a mild hemolytic anemia following an oxidative insult, such as primaquine administration. Hemolysis may cease even with continued administration of the drug because the reticulocytes, which increase in proportion following hemolysis, have adequate G6PD activity. In persons of Mediterranean and Middle Eastern ancestry, the most common abnormal allele is G6PD M (G6PD Mediterranean; class II) associated with severe hemolysis following administration of an appropriate drug (Table 15-2). The average incidence of this allele is approximately 510%, but a subpopulation of Kurdish Jews is reported to have an incidence of 50%. Enzyme activity in erythrocytes from these patients is often less than 1% of normal, and transfusion is usually required following a hemolytic crisis. The difference in the clinical severity of the diseases associated with G6PD A - and G6PD M can be explained by the characteristics of the two variants. G6PD A - has a normal Km for glucose-6-phosphate (50-70 #mol/L) and for NADP + (2.9-4 #mol/L), but it exhibits an abnormal pH activity curve. The deficiency is due to an accelerated rate of inactivation of G6PD A - protein. Bone marrow cells and reticulocytes have normal amounts of G6PD activity, while activity in older red cells is very low. However, the residual enzyme seems to be more resistant to inhibition by NADPH. In contrast, G6PD M molecules have an intrinsically lower catalytic activity, and both reticulocytes and older red cells have decreased amounts of G6PD activity. Consequently, when one of the drugs in Table 15-2 is given, more cells are susceptible, and hemolysis is greater than with G6PDA-. The low Km of G6PD M for G6P and NADP + may account for the near-normal survival of erythrocytes in the absence of oxidative stress. A number of other enzymopathic substances (e.g., pyruvate kinase, Chapter 13; and pyrimidine-5'-nucleotidase, Chapter 27), abnormal hemoglobins (Chapter 28), and abnormalities of the erythrocyte cytoskeleton (Chapter 10) may cause hemolytic anemia. Because many enzymes in the red cell are identical to those in other tissues, defects in these enzymes may have pleiotropic effects. Thus, in addition to hemolytic anemia, triose phosphate isomerase deficiency causes severe neuromuscular disease, and phosphofructokinase deficiency causes a muscle glycogen storage disease (Chapter 13). Mutations that result in decreased enzyme stability are usually most strongly expressed in erythrocytes because of their inability to synthesize proteins.

~04

CHAPTER15


Phagocytosis and the Pentose Phosphate Pathway The pentose phosphate pathway is crucial to the survival of erythrocytes because of its ability to provide NADPH for reduction of toxic, spontaneously produced oxidants. In phagocytic cells, the pentose phosphate pathway generates oxidizing agents that participate in the killing of bacteria and abnormal cells engulfed by the phagocytes. In humans, the two principal types of phagocytic cells are polymorphonuclear leukocytes (PMN leukocytes, neutrophilic granulocytes, or neutrophils) and those of the mononuclear phagocyte system (MPS). The MPS, also called the reticuloendothelial system (RES), includes peripheral blood monocytes, tissue macrophages (inflammatory macrophages, Kupffer cells, histiocytes, alveolar macrophages, and others), and their precursor cells in bone marrow. These cells are normally active against bacteria, foreign cells, and particulate matter such as asbestos fibers, silicon dioxide particles, and coal dust. Neutrophils are particularly important in protecting against acute bacterial infections. Phagocytosis is the process whereby phagocytes move toward, engulf, and digest foreign material. Attraction of phagocytes is mediated by chemotactic factors such as C5a (derived from complement; Chapter 35), bacterial endotoxins (Chapter 16), kallikrein, fibrinopeptide B (Chapter 36), and leukotrienes (Chapter 18). The contractile systems, which enable the phagocytes to extend pseudopods for movement and engulfment, are discussed in Chapter 21. Following invagination of the phagocyte plasma membrane to surround a particle or microorganism, the edges of the vesicle fuse to form a cytoplasmic vacuole or phagosome. Lysosomes or, in the case of neutrophils, primary (azurophilic) and secondary (specific) cytoplasmic granules then fuse with the phagosome to form a phagolysosome. The contents of these organelles are emptied into the phagolysosome and contribute to the killing and digestion of the phagocytized material. Most of the cytocidal activity of phagocytes is due to the generation of highly reactive forms of oxygen from NADPH and molecular oxygen. In the absence of stimulation, the activity of the pentose phosphate pathway in phagocytes is low. Within 15-60 seconds after a phagocyte is alerted to the presence of foreign material, there is a 20- to 200-fold increase in oxygen consumption, the respiratory burst, and a 10- to 20-fold increase in pentose phosphate pathway activity. These changes are initiated by interactions between receptors on the plasma membranes of the phagocytes and a wide variety of stimuli, including immune complexes, chemotactic peptides, and opsonized bacteria. Phosphatidyl inositol, prostaglandins, cAME and changes in cytosolic [Ca 2+]

appear to be essential for this process. The magnitude of the response is related to the type of inducing signal and to the cell's ability to respond to that signal. Most of the oxygen consumed in the respiratory burst is converted to superoxide anion (O2). This highly reactive oxygen radical anion is one of the substances responsible for oxidative damage to red cells. In erythrocytes, superoxide anion occurs spontaneously, as a byproduct of hemeoxygen interactions. In the phagocyte, it is formed by the NADPH oxidase on the outer surface of the plasma membrane. The reaction involves transfer of a single electron from NADPH to oxygen: NADPH oxidase

NADPH + 202

> 20 2 + NADP + + H +

NADPH oxidase is thought to be an electron transport chain that includes a flavoprotein, and cytochrome b558 (Chapter 14). The NADPH is provided by the pentose phosphate pathway. Glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase are inhibited by NADPH at the concentrations found in the unstimulated phagocyte. As NADPH is consumed by NADPH oxidase, inhibition of the pentose phosphate pathway is reduced, increasing the rate of formation of NADPH. Phagosomes contain some of the activated NADPH oxidase from the cell surface, and superoxide is produced within the phagolysosome. Although superoxide is only bactericidal against organisms that lack superoxide dismutase, it is the substrate for formation of a number of cytotoxic compounds. Formation of phagolysosomes is accompanied by a decrease in the pH of their interior, which causes spontaneous conversion of superoxide anion to oxygen and hydrogen peroxide: 20 2 + 2H + --+ H202 -k-O2 Hydrogen peroxide is a substrate for myeloperoxidase, a multisubunit heme protein of M.W. --~150,000, present in primary neutrophilic granules. The active prosthetic groups are two hemes covalently attached to the apoenzyme. This enzyme catalyzes many kinds of oxidation reactions, but oxidation of halide ions to hypohalite ions appears to be the most important. Hypochlorite ion is the principal compound formed, although Br-, I-, and SCN(a pseudohalide) can also serve as substrates. The reaction catalyzed is C 1 - nu H 2 0 2 ~

C10- +

H20

The hypochlorite ion has an antimicrobial potency 50-fold greater than that of hydrogen peroxide. It oxidizes a variety

305

Supplemental Readingsand References

of biological molecules and inactivates Otl-antitrypsin and other protease inhibitors. In the presence of metals, such as iron, hydrogen peroxide reacts with superoxide anion to produce hydroxyl radicals (OH') and singlet oxygen (102) by the reaction shown below: 0 2 + H202 ~

102-Jr-OH-

-4-OH"

Hydroxyl radicals are highly reactive, oxidizing a variety of biological molecules. Singlet oxygen, which has an excited (high-energy) electronic configuration, can decay spontaneously to O2 or interact with and oxidize some other molecule. In either case, part of the energy of the singlet state is emitted as light. This chemiluminescence, like the respiratory burst, is characteristic of actively phagocytic cells capable of killing microorganisms; it has been used for evaluating the functional capacity of these cells. Persons with chronic granulomatous disease (CGD) suffer from recurrent bacterial and fungal infections. About 300 cases have been reported, of which about 80% were inherited as an X-linked, and 20% as an autosomal, recessive trait. CGD is actually a group of diseases caused by lack of activation of NADPH oxidase due to an abnormality in the enzyme itself or in the activating system. Phagocytes from CGD patients fail to show either a respiratory burst or chemiluminescence. Chemotaxis, recognition, engulfment, degranulation, and the presence of enzymes of the pentose phosphate pathway are normal in the phagocytes from CGD patients. Many infections are caused by bacteria that make catalase. The small amounts of hydrogen peroxide made in these organisms are destroyed by this endogenous catalase. Bacteria that lack catalase release small amounts of H 2 0 2 into the phagolysosome, where myeloperoxidase, the presence of which is normal, can use it as a substrate to generate bactericidal products. Therapy of CGD involves prevention of infections (e.g., by antibiotic prophylaxis) and treatment of infections and their complications. A promising new prophylactic agent is v-interferon, which is synthesized by recombinant DNA technology. This agent, a cytokine, alters the development of very early neutrophil precursors and eventually leads to improvement in neutrophil function. Severe G6PD deficiency ( Orotate )

D~H.~ NADH + H§ Orotate PRPP OPRT PPi

~

AIIopurinol

XO

PRT Oxipurinol ~ ~ OxipurinOlpRppf ~p~ Ribonucleotide 9

Orotidine 5"-Monophosphate (OMP) . . . . -~ Orotidine OMP Decarboxylase

Uridine 5"-Monophosphate

F I G U R E 17-8 The metabolic interrelationship between mitochondrial carbamoyl phosphate synthesis to urea formation and to cytosolic carbamoyl phosphate channeled into pyrimidine biosynthesis. In ornithine transcarbamoylase (OTC) deficiency, mitochondrial carbamoyl phosphate diffuses into the cytosol and stimulates pyrimidine biosynthesis, leading to orotidinuria. Administration of ailopurinol augments orotidinuria by increasing the flux in the pyrimidine biosynthetic pathway. CPS = Carbamoyl phosphate synthase, AT = aspartate transcarbamoylase, D = dihydroorotase, DH -- dihydroorotate dehydrogenase, OPRT = orotate phosphoribosyltransferase, XO = xanthine oxidase, PRT = phosphoribosyltransferase, PRPP = 5-phosphoribosyl-l-pyrophosphate.

of nitrogen to products other than urea is achieved by administration of sodium benzoate or sodium (or calcium) phenylacetate. Administration of benzoate leads to elimination of hippurate (benzoylglycine):

phosphate, catalyzed by mitochondrial glycine synthase (glycine cleavage enzyme). Phenylacetate or phenylbutyrate administration increases excretion of phenylacetylglutamine: O

O ~~'-~nO-

)--CH3--COO- + ATP4- + CoASHActivatingenzyme

CH3MC~SCoA + AMPz- + PPi3-

+ ATP 4- + CoASH Acfivalingenzyme

PhenylacetyI-CoA

Benzoate

CONH2 CONH2 I

O

~~--~mSmCoA

+ AMp2- + PPi 3-

BenzoyI-CoA

Hippurate is rapidly secreted since its clearance is five times greater than its glomerular filtration rate. The glycine nitrogen is derived from ammonia in a complex reaction that uses CO2, NADH, NS,Nl~ (a source of a single carbon unit; Chapter 27) and pyridoxal

O (0H2)2 II I CH3BC~SCoA + H3+N- CH I COOGlutamine

l

0 (CH~)~ II I Conjugatmgenzyme,/g~=.~CH3MC__NBCH

'

H I COO-

+ CoASH+ H +

Phenylacetylglutamine

The excretion of phenylacetylglutamine produces loss of two nitrogen atoms. NAG synthase deficiency cannot be treated by administration of NAG, since NAG undergoes cytosolic inactivation by deacylation and is not readily permeable across the inner mitochondrial membrane. An analogue of NAG,

SECTION 17.3

345

Metabolism of Some Individual Amino Acids

N-carbamoylglutamate, activates CPSI, does not share the undesirable properties of NAG, and has been effective in management of this deficiency. The most common cause of hyperammonemia in adults is disease of the liver (e.g., due to ethanol abuse, infection, or cancer). The ability to detoxify ammonia is decreased in proportion to the severity of the damage. In advanced disease (e.g., cirrhosis), hyperammonemia is augmented by shunting of portal blood that carries ammonia from the intestinal tract and other splanchnic organs to the systemic blood circulation (bypassing the liver) and leads to portalsystemic encephalopathy. In addition to dietary protein restriction, colonic growth of bacteria must be suppressed by antibiotics (e.g., neomycin) and administration of lactulose (Chapter 9), a nonassimilable disaccharide. Enteric bacteria catabolize lactulose to organic acids that convert NH3 to NH4+, thereby decreasing absorption of NH3 into the portal circulation. Catabolism of lactulose also leads to formation of osmotically active particles that draw water into the colon, produce loose, acid stools, and permit loss of ammonia as ammonium ions.

17.3 Metabolism of Some Individual Amino Acids Mammalian tissues synthesize the nonessential amino acids from carbon skeletons derived from lipid and carbohydrate sources or from transformations that involve essential amino acids. The nitrogen is obtained from NH4+ or from that of other amino acids. Nonessential amino acids (and their precursors) are glutamic acid (ot-ketoglutaric acid), aspartic acid (oxaloacetic acid), serine (3-phosphoglyceric acid), glycine (serine), tyrosine (phenylalanine), proline (glutamic acid), alanine (pyruvic acid), cysteine (methionine and serine), arginine (glutamate-y-semialdehyde), glutamine (glutamic acid), and asparagine (aspartic acid). Amino acids may be classified as ketogenic, glucogenic, or glucogenic and ketogenic, depending on whether feeding of a single amino acid to starved animals or animals with experimentally induced diabetes increases plasma or urine levels of glucose or ketone bodies (Chapter 18). Leucine and lysine are ketogenic; isoleucine, phenylalanine, tyrosine, and tryptophan are glucogenic and ketogenic; and the remaining amino acids are glucogenic. Points of entry of amino acids into the gluconeogenic pathway are discussed in Chapter 15.

Arginine

Arginine participates in a number of metabolic pathways depending on the cell type. It is synthesized as an inter-

mediate in the urea cycle pathway and is also obtained from dietary proteins. A number of key metabolites such as nitric oxide, phosphocreatine, spermine and ornithine are derived from arginine. During normal growth and development, under certain pathological conditions (e.g., endothelial dysfunction) and if the endogenous production of arginine is insufficient, a dietary supplement of arginine may be required. Thus, arginine is considered a semiessential amino acid.

Metabolism and Synthesis of Nitric Oxide

Nitric oxide (NO) is a reactive diatomic gaseous molecule with an unpaired electron (a free radical). It is lipophilic and can diffuse rapidly across biological membranes. NO mediates a variety of physiological functions such as endothelial derived relaxation of vascular smooth muscle, inhibition of platelet aggregation, neurotransmission, and cytotoxicity. The pathophysiology of NO is a doubleedged sword. Insufficient production of NO has been implicated in the development of hypertension, impotence, susceptibility to infection, and atherogenesis. Excessive NO production is linked to septic shock, inflammatory diseases, transplant rejection, stroke, and carcinogenesis. NO is synthesized from one of the terminal nitrogen atoms or the guanidino group of arginine with the concomitant production of citrulline. Molecular oxygen and NADPH are cosubstrates and the reaction is catalyzed by nitric oxide synthase (NOS). NOS consists of several isoforms and is a complex enzyme containing bound FMN, FAD, tetrahydrobiopterin, heme complex, and nonheme iron. A calmodulin binding site is also present. NO formation from arginine is a two-step process requiring five-electron oxidations. The first step is the formation of NC-hydroxylarginine (N G denotes guanidinium nitrogen atom): Arginine + O2 + NADPH + H + -+ HO-NC-Arg + NADP + + H20 This step is a mixed-function oxidation reaction similar to the one catalyzed by cytochrome P-450 reductase and there is considerable homology between NOS and cytochrome P-450 reductase. In the second step, further oxidation of NG-hydroxyl arginine yields NO and citrulline:

1 HO-NG-Arg + 02 -~- ~(NADPH + H +) --+ 1 citrulline + NO + H20 -k- - N A D P + 2

346

CHAPTER 17 Protein and Amino Acid Metabolism

The overall reaction is" O

H

II

I

?H 2

"Om C n OH m (0H2)3~ N - - C - - N H

+ 0 2 + NADPH + H §

I NH3+ Arginine Nitric Oxide Synthase

/ 1

FMN, FAD, Tetrahydrobiopterin Fe 2+, Heme complex

o H O II I II -O-- C - - OH - - (OH2) 3 - N -- C - - NH 2 + NO

I

interferon-v), or bacterial lipopolysaccharides can induce and cause expression of NOS in many cell types. Glucocorticoids inhibit the induction of iNOS. In stimulated macrophages and neurophils, NO and superoxide radical (O2) react to generate peroxynitrite, a powerful oxidant, and hydroxyl radicals. These reactive intermediates are involved in the killing of phagocytized bacteria (Chapter 14). Excessive production of NO due to endotoxinemia produces hypotension and vascular hyporeactivity to vasoconstrictor agents, and leads to septic shock. NOS inhibitors have potential therapeutic application in the treatment of hypotensive crisis.

NH3+ Citrulline

The NOS activity is inhibited by NO-substituted analogues of arginine, such as NC-nitroarginine and N cmonomethyl-L-arginine.

Isoforms (Also Known as Isozymes) of Nitric Oxide Synthase There are three major isoforms of Nitric Oxide Synthase (NOS) ranging in molecular size from 130 to 160 kDa. Amino acid similarity between any two isoforms is about 50-60%. Isoforms of NOS exhibit differences in tissue distribution, transcriptional regulation, and activation by intracellular Ca 2+. Two of the three isoforms of NOS are constitutive enzymes (cNOS) and the third isoforin is an inducible enzyme (iNOS). The cNOS isoforms are found in the vascular endothelium (eNOS), neuronal cells (nNOS), and many other cells, and are regulated by Ca 2+ and calmodulin. In the vascular endothelium, agonists such as acetylcholine and bradykinin activate eNOS by enhancing intracellular Ca 2+ concentrations via the production of inositol 1,4,5-trisphosphate, which activates the phosphoinositide second-messenger system (Chapter 30 ). The NO produced in the vascular endothelium maintains basal vascular tone by vasodilation which is mediated by vascular smooth muscle cells. Organic nitrates used in the management of ischemic heart disease act by denitration with the subsequent formation of NO. Sodium nitroprusside, an antihypertensive drug, is an NO donor. Thus, organic nitrates and sodium nitroprusside are prodrugs, and the exact mechanism by which these prodrugs yield NO is not yet understood. Inhaled NO can produce pulmonary vasodilation. This property of NO has been used in the management of hypoxic respiratory failure associated with primary pulmonary hypertension in neonates. NO produced by cNOS in neuronal tissue functions as a neurotransmitter. The inducible class of NOS (iNOS) is found in macrophages and neutrophils and is Ca2+-independent. Bacterial endotoxins, cytokines (e.g., interleukin-1,

Signal Transduction of NO NO is lipophilic and diffuses readily across cell membranes. It interacts with molecules in the target cells producing various biological effects. One mechanism of action of NO is stimulation of guanylate cyclase, which catalyzes the formation of cyclic guanosine monophosphate (cGMP) from GTP, resulting in increased intracellular cGMP levels (Figure 17-9). NO activates guanylate cyclase by binding to heme iron. The elevation of cGMP levels may activate cGMP-dependent protein kinases. O

II ~ "~0\5,2

HN

II

II

O-- I~--0-- P--O--I.P - - O -

H"~H H//~,H F--V-' OH OH

O-

O-

O-

Guanosine Triphosphate (GTP)

J PPi j

GuanylateCyclase ~

~

Derived from Nerve Impulse 7/

NO

Derived from a Ligand-cell interaction

O

HN

H2N : o .,> Smoothmuscle .. relaxation O , Cyclic GMP

~

CH3CH20

Cyclic GMP phosphodiesterase O

/CH 3

HN~N\

....

I /;

N~CH2CH2CH3

O2S\ N" ~

HN

N%,

H2N - ~'N /

-Nk/o

O 5" C -- O-- P-- O-

H, H ,/ ~ H

~.

CH3

Sildenafil

I O-

OH OH 5"-GMP

F I G U R E 17-9 NO-mediated synthesis of cGMP from GTP in the corpus cavernosum that leads to smooth muscle relaxation. Sildenafil potentiates the effects of NO by inhibiting cGMP phosphodiesterase.

SECTION17.3 Metabolism of Some Individual Amino Acids --Glucose

Purines Proteins\\

3-PhosphoglyceratJ/ f ~ Pyruvate ~TCA ,_ | ~ Serin e ~ Ethan~ a m i n e . , ~ cycle-----*CO2 oI line

Urea_ Glutathione~\ \ " ~ ~ " + NH~ '+ " ~ ~GlyciEe \~ "One-carbon" J / ~ " / ~ ~ Bileacidconjugates Hippurate and // ~ ~ otheracyl conjugates Porphyrins

347

Glyoxalate l . Glycolate"

" Phospholipids

Creatine Oxalate

1 1 Creatinine

Phosphocreatine

FIGURE 17-11) Overviewof glycineand serinemetabolism.

These kinases phosphorylate specific proteins that may be involved in removal or sequestration of Ca 2+ or other ions, resulting in physiological stimuli. The physiological actions of cGMP are terminated by its conversion to 5'-GMP by cGMP-phosphodiesterase. Inhibitors of cGMP phosphodiesterase promote the actions of NO. Sildenafil is a selective inhibitor of a specific cGMP phosphodiesterase (type 5) present in the corpus cavernosum. This compound (structure shown in Figure 17-9) is used orally in the therapy of some types of erectile dysfunction. NO is the principal transmitter involved in the relaxation of penile smooth muscle. During central or reflex sexual arousal, NO production is enhanced leading to increased production of cGME Smooth muscle relaxation permits the corpus cavernosum to fill with blood. Since the therapeutic effect of sildenafil potentiates the action of cGMP, the drug is ineffective in the absence of sexual arousal. The relaxation of cavernosal smooth muscle caused by cGMP involves inhibition of Ca 2+ uptake. Prostaglandin E1 (alprostadil) inhibits the uptake of Ca 2+ smooth muscle by a separate mechanism and causes erections in the absence of sexual arousal. Blood flow through corpus cavernosum may also be increased by o~-adrenergic blocking agents (e.g., phentolamine mesylate). Coadministration of NO donor drugs with the NO potentiation drug sildenafil may have severe consequences on the cardiovascular system. Signal transduction of NO by cGMP-independent mechanisms include ADP-ribosylation of glyceraldehyde3-phosphate dehydrogenase (GADPH), an enzyme of the glycolytic pathway (Chapter 13), and interactions with many heme-containing and nonheme iron-sulfur containing proteins. NO activates ADP-ribosyltransferase which catalyzes the transfer of ADP-ribose from NAD + to GADPH. This results in the inactivation of GADPH causing inhibition of glycolysis and decreased ATP production.

The antiaggregability of platelets and the neurotoxicity of NO have been attributed to inhibition of glycolysis by NO.

Glycine Glycine participates in a number of synthetic pathways and is oxidized to provide energy (Figure 17-10). The interconversion of glycine and serine by serine hydroxymethyltransferase is shown below: pyridoxalphosphate NH~--CH2-COO- + N 5,N 10-methylene-FH4 + H20 < > glycine H O H z C - C H N H + - C O O - + FH4 serine

The one-carbon carrier NS,Nl~ is derived from reactions of the one-carbon pool (Chapter 27). [The term one-carbon pool refers to all single-carbon-containing metabolites (e.g.,-CH3,-CHO, N H = C - , etc.) that can be utilized in biosynthetic reactions such as formation of purine and pyrimidine.] These reactions include oxidation of glycine by glycine cleavage enzyme complex (glycine synthase): pyridoxal phosphate

NH+-CH2-COO - nt- FH4 + NAD +

glycine

NH + +

C02

+ NADH + NS,Nl~

This reaction favors glycine degradation, but the formation of glycine may also occur. The enzyme complex is mitochondrial and contains a pyridoxal phosphate-dependent glycine decarboxylase, a lipoic acid-containing protein that is a carrier of an aminomethyl moiety, a tetrahydrofolate-requiring enzyme, and lipoamide dehydrogenase. The reactions of glycine cleavage resemble those of oxidative decarboxylation of pyruvate (Chapter 13).

348

CHAPTER 17 Protein and Amino Acid Metabolism

Glycine is also oxidized by D-amino acid oxidase, an FAD protein:

NH~--CH2-COO- -Jr-02 -~- H20 glycine C H O - C O O - + NH + + H202 glyoxalate Glyoxalate can be transaminated to glycine, reduced to glycolate, converted to ot-hydroxy-/~-ketoadipate by reaction with o~-ketoglutarate, or oxidized to oxalate and excreted in urine. The first three reactions require pyridoxal phosphate, NADH, and thiamine pyrophosphate, respectively. In humans, ascorbic acid (vitamin C) is a precursor of urinary oxalate (Chapter 38). Since calcium oxalate is poorly soluble in water, it can cause nephrolithiasis and nephrocalcinosis due to hyperoxaluria.

Creatine and Related Compounds Phosphocreatine serves as a high-energy phosphate donor for ATP formation (e.g., in muscle contraction; see Chapter 21). Synthesis of creatine (methyl guanidinoacetate) requires transamidination, i.e., transfer of a guanidine group from arginine to glycine, to form guanidinoacetate (glycocyamine) by mitochondrial arginine-glycine amidinotransferase NH2 I ~NH2 I

NH3 +

NH3 +

NH2 I ~NH2+

I

I

(CH2)3

+

NH

I

NH

I

(CH2)3

I

I

CHNH3

I CHNH3 +

COO-

CH2 I

CO0 -

I

+

CH 2

~

'~

I

I

coo-

COO -

Arginine

Disorders of Glycine Catabolism Nonketotic hyperglycinemia is an inborn error due to a defect in the glycine cleavage enzyme complex in which glycine accumulates in body fluids and especially in cerebrospinal fluid. It is characterized by mental retardation and seizures. Glycine is an inhibitory neurotransmitter in the central nervous system, including the spinal cord. Strychnine, which produces convulsions by competitive inhibition of glycine binding to its receptors, gives modest results in treatment but not very effective. Sodium benzoate administration reduces plasma glycine levels but does not appreciably alter the course of the disease. Exchange transfusion may be useful. Ketotic hyperglycinemia also occurs in propionic acidemia but the mechanism has not been established. Primary hyperoxaluria type I is due to a deficiency of cytosolic oe-ketoglutarate-glyoxylate carboligase, which catalyzes the following reaction:

Glycine

Guanidinoacetate

Ornithine

(glycocyamine)

In the next step guanidinoacetate is methylated by Sadenosylmethionine by cytosolic S-adenosylmethionine guanidinoacetate-N-methyltransferase to form creatine. OH 3 I S+.Adenosyl I (0H2)2

I

+

NH2 I C--NH2 + [ NH

I

NH2 I C--NH2 + I N--OH3

~

CH2.

CH2

COO-

COO-

COO-

S-Adenosylmethionine

Guanidino-

Creatine

I

+

I

CHNH3 +

I

S-Adenosyl I (CH2)2 I CHNH3 § I

I

+

H+

coo -

S-Adenosylhomocysteine

acetate

These reactions occur in liver, kidney, and pancreas, from which creatine is transported to organs such as muscle and brain. Creatine synthesis is subject to negative modulation of amidinotransferase by creatine. Phosphocreatine production is catalyzed by creatine kinase:

O

II

NH2

C H O m C O O - + - OOC__CH2~CH2wC..._COO Glyoxalate

0c-Ketoglutarate

Carboligase

Thiaminepyrophosphate

O

OH

II

I

CO2 + - O O C ~ C H 2 m C H 2 - - C ~ C H ~ C O O 0e-Hydroxy-/Y-ketoadipate

The glyoxylate oxalate.

that

accumulates

is

converted

to

NH--PO3 2-

I

I

C--NH2 +

C=NH2 +

I

I

N - - C H 3 + ATP 4- ~

N--CH 3

I

I

CH2

CH 2

+

ADP 3- + H +

I

I

COO-

COO-

Creatine

Phosphocreatine

This kinase is a dimer of M and B (M = muscle, B = brain) subunits produced by different structural genes. Three isozymes are possible: BB (CK-1), MB (CK-2), and MM (CK-3). Another isozyme differs immunologically and electrophoretically and is located in the intermembrane space of mitochondria. Tissues rich in CK-1

349

SECTION17.3 Metabolism of Some Individual Amino Acids

are brain, prostate, gut, lung, bladder, uterus, placenta, and thyroid; those rich in CK-3 are skeletal and cardiac muscle. Cardiac muscle contains significant amounts of CK-2 (25-46% of total CK activity, as opposed to less than 5% in skeletal muscle), so that in myocardial infarction the rise in serum total CK activity is accompanied by a parallel rise in that of CK-2 (Chapter 8). Phosphocreatine undergoes a slow and nonenzymatic cyclization to creatinine.

NH~PO3 2I ~NH2, I N--CH3 +H* I CH2

NH H2N---C , \

j

COOPhosphocreatine

/

~~O /

+ Pj~- + H20

N~CH2

HaC Creatinine

Creatinine has no useful function and is eliminated by renal glomerular filtration and to a small extent by renal tubular secretion. Creatinine clearance approximately parallels the glomerularfiltration rate (GFR) and is used as a kidney function test. It is calculated as follows:

regenerates ATP from ADP, thereby maintaining a high level of ATP required during intense exercise. A large pool of phosphocreatine resides in the skeletal muscle. It has been theorized that in order to maximize phosphocreatine stores in the skeletal muscle to replenish ATP during rapid muscle contractions, an exogenous source of creatine may be beneficial. Double-blind placebo-controlled studies of oral supplementation of creatine in human subjects have shown increased performance during short duration, strenuous, high-intensity exercise. Such activities require that ATP be replenished rapidly from phosphocreatine stores during anaerobic metabolism. These studies usually consisted of ingestion of 20 g of creatine per day for 5 days followed by a maintenance dose of 5-10 g/day. Studies on creatine as an ergogenic aid have not been uniformly positive; some have shown no beneficial effect and still others have been equivocal and indicated that creatine supplementation did not enhance athletic activities. The safety issues of long-term creatine supplementation on kidney, liver, nerve, muscle, and other tissues are not known.

Serine

Creatinine concentrations are measured from a precisely timed urine specimen (e.g., 4-hour, 24-hour) and a plasma specimen drawn during the urine collection period. Excretion of creatinine depends on skeletal muscle mass and varies with age and sex. However, day-today variation in a healthy individual is not significant. Creatinuria, the excessive excretion of creatine in urine, may occur during growth, fever, starvation, diabetes mellitus, extensive tissue destruction, muscular dystrophy, and hyperthyroidism.

Synthesis of serine from 3-phosphoglycerate, an intermediate of glycolysis (Chapter 13), requires oxidation of 3-phosphoglycerate to 3-phosphohydroxypyruvate, transamination of 3-phosphohydroxypyruvate by glutamate, and hydrolysis of 3-phosphoserine to serine (Figure 17-11). This cytosolic pathway is regulated by inhibition of phosphoserine phosphatase by serine. Serine is converted to pyruvate by cytosolic serine dehydratase. More importantly, it is converted in mitochondria to 2-phosphoglycerate by way of hydroxypyruvate and D-glycerate; and the enzymes involved are a transaminase, a dehydrogenase, and a kinase. Serine is interconvertible with glycine (Figure 17-10) and is involved in phospholipid (Chapter 19) and in cysteine synthesis.

Use of Creatine as a Dietary Supplement

Proline

The creatine pool in the human body comes from both endogenous synthesis and the diet which provides 1-2 g. Red meat provides large amounts of dietary creatine and vegetables a limited amount. Using glycine, arginine, and methionine, creatine is synthesized in the liver, pancreas, and kidney. Creatine transported in blood crosses muscle and nerve cell membranes by means of a specific creatine transporter system against a concentration gradient of 200:1. Intracellularly, creatine is converted to phosphocreatine by ATE a reaction catalyzed by creatine kinase. Phosphocreatine, with its high phosphoryl transfer potential,

Proline arises from and gives rise to glutamate. Synthesis is by reduction of glutamate to glutamate-v-semialdehyde by way of an enzyme-bound v-glutamyl phosphate. The v-semialdehyde spontaneously cyclizes to A'-pyrroline5-carboxylate, which is then reduced by NAD(P)H to proline (Figure 17-12). Proline is converted to A'-pyrroline-5carboxylate by proline oxidase, which is tightly bound to the inner mitochondrial membrane in liver, kidney, heart, and brain. A'-Pyrroline-5-carboxylate is in equilibrium with glutamate-v-semialdehyde, which can be transaminated to ornithine or reduced to glutamate (Figure 17-12).

Creatinine clearance =

urine creatinine (mg/L) plasma creatinine (mg/L) • urine volume per unit time

350

CHAPTER17 Protein and Amino Acid Metabolism cool

Phosphoglycerate dehydrogenase ,

Glycolysis-"-"-"-~ HC-- OH

coo-

I

H --O--(~ NAD+ NADH+ H2C--O--(~ 3-Phosphoglycerate 3-Phosphohydroxypyruvate t~nGlutamate

osphoserine saminase ec-Ketoglutarate

COO-

COOPhosphoserine -_ phosphatase(,,.~ -'~ H2N~H

, H3N?H I

CH~----OH L-Serine

P~

HO

F I G U R E 17-11 Synthesis of serine from 3-phosphoglycerate. P = PO 2-"

Pi

--

~C--O--(~ 3-Phosphoserine HPO 2-.

Decarboxylation of ornithine to putrescine by ornithine decarboxylase serves as a source of the polyamines spermidine and spermine. Ornithinemia results from deficiency of ornithine aminotransferase or ornithine

decarboxylase. Ornithine aminotransferase deficiency is associated with gyrate atrophy of the choroid and retina (Chapter 38). Proline and hydroxyproline (produced by posttranslational modification) are major constituents

CO01

ATP (?H2)= ADP,~......-------~ ? HNH+ COONADPH+ H+.~/(~" L-Glutamate

NAOP

)

CHO I (C,H,)~ ~ ICHNH+ ~

._. /

(~/---NADH+ H§ NAD+

ICOO- ~ ~ H Glutamate-7-semialdehyde _ l,,-----Glutamate ( ' ~ F er -Ketoglutarate ?(rH2NH+ (H~ CHNH+ I COOOrnithine Via

reactionsof ureacycle Arginine

|

C02 3 ,

2

0

?HF---NH+ (?Hz), CH=NH+ Putrescine

I-I=? H20 H+~

?Hz

HC~N/CHCOOH A -Pyrroline5"carbOxylate

NAD(P)H+

Flav%iotein

NAD(P)*-~ CH2

~C

I H2C~N/CHCOOH H

Proline

FIGURE 17-12 Metabolism of proline (1) A1-pyrroline-5-carboxylate (P5C) synthase; (2) ornithine aminotransferase; (3) P5C reductase; (4) proline oxidase; (5) P5C dehydrogenase; (6) ornithine decarboxylase.


SECTION 17.3

of collagen (Chapter 25). Hydroxyproline released by collagen turnover undergoes degradation similar to that of proline. Hydroxyproline cleavage initiated by hydroxyproline oxidase eventually yields glyoxylate and pyruvate. In hyperprolinemia type I, proline oxidase is deficient, and in type II, A'-pyrroline-5-carboxylate dehydrogenase is deficient. Hydroxyprolinemia results from hydroxyproline oxidase deficiency. All are clinically harmless autosomal recessive traits.

Histidine Histidine is not essential for adults except in persons with uremia. It is essential for growth in children. Histidine is synthesized from 5-phosphoribosyl1-pyrophosphate and ATE forming N'-l'-phosphoribosylATE catalyzed by the allosteric enzyme ATP phosphoribosyltransferase. This reaction is analogous to the initial reaction of purine nucleotide biosynthesis (Chapter 27). Histamine breakdown produces a one-carbon unit (N 5formiminotetrahydrofolate) and glutamate (Figure 17-13) by a nonoxidative deamination to urocanate, cleavage of the imidazole ring to N-formiminoglutamate (Figlu), and transfer of the formimino group (-CH=NH) to tetrahydrofolate (Chapter 27). Folate deficiency leads to accumulation of Figlu, which is excreted in urine. The excretion is very pronounced after a loading dose of histidine, a test used to detect folate deficiency. More sensitive radioisotopic assays use folate binders to the vitamins. High urinary levels of Figlu may coexist with elevated levels of serum folate. Thus, in vitamin B]2 (cobalamin) deficiency, since cobalamin participates in the following reaction, FH4 is trapped as

NH +

I ---------~CHF-CH--COO-

N~.~/NH

Histidineammonia-lyase ,. (histidase)

NH3

L-Histidine

O

Urocanate

H20-~ Urocanate hydratase

H

II I -O--C--C--CH~--C~COOI HN~c./NH

I

H N-Formiminoglutamate(Figlu) Glutamate~ F H , formiminotransferase NLformino.FH ' O

II

-O-- C-- CH--CH.~H2COOL-Glutamate

FIGURE 17-13 Catabolismof histidine.

I]--C"=CHCOO N~,,.,/.NH

Imidazolone=~oropionase ~0

0~]~ 1

CHF"CH2COO-

N~NH 4-1midazolone-5-propionate

351

NS-methyltetrahydrofolate and is unavailable as a carrier for the formimino group of Figlu. Homocysteine methyltransferase

NS-Methy I-FH 4

(methylcobalamin) /~ Homocysteine

~FH4

Methionine

Similarly, a deficiency of glutamate formiminotransferase leads to accumulation of Figlu and high levels of serum folate. Histidinemia results from deficiency of histidine ammonia-lyase. With a normal diet, histidine (and the products imidazole-pyruvate, imidazole-lactate, imidazoleacetate) accumulates in plasma, cerebrospinal fluid, and urine. This rare autosomal recessive disease may be benign or may manifest with mental retardation and speech defects. Histidine and #-alanine yield the dipeptide carnosine (present in muscle), and histidine and 7-aminobutyrate yield homocarnosine (found in brain). Methylhistidyl residues are found in some proteins (e.g., actin; Chapter 21) as a result of posttranslational modification. Histamine is decarboxylated histidine. H I N~.~,,,,, NH Histidine

COO-

Histidine decarboxylase

(pyridoxal phosphate)

CO2

)[

] CH2--CH2--NH3+

N~,,~,,,/N H Histamine

Histamine occurs in blood basophils, tissue mast cells, and certain cells of the gastric mucosa and other parts of the body (e.g., anterior and posterior lobes of the pituitary, some areas of the brain). Histamine is a neurotransmitter in certain nerves ("histaminergic") in the brain. In mast cells found in loose connective tissue and capsules, especially around blood vessels, and in basophils, histamine is stored in granules bound by ionic interactions to a heparin-protein complex and is released (by degranulation, vacuolization, and depletion) in immediate hypersensitivity reactions, trauma, and nonspecific injuries (infection, burns). Degranulation is affected by oxygen, temperature, and metabolic inhibitors. Release of histamine from gastric mucosal cells is mediated by acetylcholine (released by parasympathetic nerve stimulation) and gastrin and stimulates secretion of hydrochloric acid (Chapter 12). Histamine causes contraction of smooth muscle in various organs (gut, bronchi) by binding to H1 receptors. The conventional antihistaminic drugs (e.g., diphenhydramine and pyrilamine) are Hi-receptor antagonists and are useful in the management of various allergic manifestations. However, in acute anaphylaxis, bronchiolar constriction is rapidly relieved by epinephrine (a physiological antagonist

352

CHAPTER 17

Protein and Amino Acid Metabolism

of histamine). Its effect on secretion of hydrochloric acid is mediated by H2 receptors. Hz-receptor antagonists are cimetidine and ranitidine (Chapter 12), which are useful in treatment of gastric ulcers. Histamine is rapidly inactivated by methylation from S-adenosylmethionine of one of the nitrogen atoms of the imidazole ring (catalyzed by N-methyltransferase) or of the terminal amine group (catalyzed by methyltransferase). Ring-methylated histamine is deaminated by monoamine oxidase to methyl imidazole acetic acid, which is readily excreted. Inactivation also results from deamination of histamine by diamine oxidase. The imidazole acetic acid formed is then excreted as 1ribosylimidazole-4-acetic acid. This reaction is the only known reaction in which ribose is used for conjugation. Branched-Chain Amino Acids

Leucine, isoleucine, and valine are essential amino acids but can be derived from their respective ot-keto acids. A single enzyme may catalyze transamination of all three. The ot-keto acids, by oxidative decarboxylation, yield the acyl-CoA thioesters, which, by ot,/3-dehydrogenation, yield the corresponding ot,fl-unsaturated acyl-CoA thioesters. The catabolism of these thioesters then diverges. Catabolism of leucine yields acetoacetate and acetyl-CoA via fl-hydroxy-fl-methylglutaryl-coenzyme A (HMG-CoA)malso an intermediate in the biosynthesis of cholesterol and other isoprenoids (Chapter 19). Catabolism of isoleucine yields propionyl-CoA (a glucogenic precursor) and acetyl-CoA. Catabolism of valine yields succinyl-CoA (Figure 17-14). Thus, leucine is ketogenic and isoleucine and valine are ketogenic and glucogenic. Oxidative decarboxylation of the c~-keto acids is catalyzed by a branched-chain keto acid dehydrogenase (BCKADH) complex analogous to that of the pyruvate dehydrogenase and c~-ketoglutarate dehydrogenases complexes. BCKADH is widely distributed in mammalian tissue mitochondria (especially in liver and kidney). It requires Mg 2+, thiamine pyrophosphate, CoA-SH, lipoamide, FAD, and NAD + and contains activities of otketo acid decarboxylase, dihydrolipoyl transacylase, and dihydrolipoyl dehydrogenase. Like the pyruvate dehydrogenase complex, BCKADH is regulated by product inhibition and by phosphorylation (which inactivates) and dephosphorylation (which activates). The o~,fl-dehydrogenation is catalyzed by an FAD protein and is analogous to the dehydrogenation of straightchain acyl-CoA thioesters in fl-oxidation of fatty acids (Chapter 18). Methylenecyclopropylacetyl-CoA derived from the plant toxin hypoglycin (Chapters 15 and 18), which inhibits this step in fl-oxidation, also inhibits it in the catabolism of branched-chain amino acids.

FIGURE 17-14 Overviewof the catabolismof branched-chainaminoacids. TPP = thiamin pyrophosphate.

Hypoglycin produces hypoglycemia and metabolic acidosis, which frequently are fatal. Branched-chain ketoaciduria (maple syrup urine disease), an autosomal recessive disorder characterized by ketoacidosis starts early in infancy and is due to a defect in the oxidative decarboxylation step of branchedchain amino acid metabolism. The name derives from the characteristic odor (reminiscent of maple syrup) of the urine of these patients. Five different variants (classic, intermittent, intermediate, thiamine-responsive, and dihydrolipoyl dehydrogenase deficiency) are known, of which the first, which is due to deficiency of branchedchain ot-keto acid decarboxylase, is the most severe. The incidence of maple syrup urine disease in the U.S. population is 1 in 250,000-400,000 live births. In Mennonite populations the incidence is extremely high (1 in 760). Neonatal screening programs consist of measuring leucine levels in dried blood spots using a bacterial inhibition assay. Neonatal screening programs usually include testing for a number of other treatable metabolic disorders such as hypothyroidism, phenylketonuria, galactosemia, and others. If the screening test is positive for a given metabolic disease, a confirmatory test is performed. For maple syrup urine disease, the confirmation requires quantitation of the serum levels of branched-chain amino acids and urine levels of both the branched-chain amino acids and their ketoacids. Long-term management includes dietary restriction of the branched-chain amino acids. Frequent measurement of plasma concentrations of these amino

SECTION 17.3


acids is necessary to monitor the degree of dietary restriction and patient compliance. Many aminoacidurias and their metabolites give rise to abnormal odors, maple syrup urine disease is one example. Some of the others are phenylketonuria (musty odor), tyrosinemia type I (boiled cabbage), glutaric aciduria (sweaty feet), 3-methylcrotonylglycinuria (cat's urine), and trimethylaminuria (fish). In patients with trimethylaminuria the compound responsible for the fish odor is trimethylamine which is a byproduct of protein catabolism by the large intestinal bacterial flora. Normally, trimethylamine is inactivated by hepatic flavin monooxygenases. Several different mutations in the gene for flavin monooxygenases have been identified in trimethylaminuric patients. An inhibitor of flavin monooxygenases is indole-3-carbinol found in dark green vegetables (e.g., broccoli). The amelioration of symptoms of bad odor in trimethylaminuria may be achieved by limiting intake of dark green vegetables and protein, and by administering low doses of antibiotics to reduce intestinal bacterial flora.

353 NH2

NH2

I

I

+

Adenosine ~ S ~ (CH2);--CHmCOOH

Adenosine ~ S ~ (CH=)z~CH~ COOH /

CH3 S-Adenosylhomocysteine S-Adenosylmethionine . Specificmethyltransferase Acceptor (examples)

Acceptor ~ CH, (examples)

1. Guanidinoacetic acid 2. Nicotinamide 3. Norepinephrine 4. Phosphatidylethanolamine 5. N-Acetyl-serotonin

1. Creatine 2. N-Methylnicotinamide 3. Epinephrine 4. Phosphatidylcholine (three cycles of methylation) 5. Melatonin

FIGURE 17-15 Selected methyl transfer reactions involving S-adenosylmethionine.

The methyl group is transferred to appropriate acceptors by specific methyltransferases with production of S-adenosylhomocysteine (Figure 17-15), which is hydrolyzed to homocysteine and adenosine by adenosylhomocysteinase: NH3 +

NH3 +

I AdenosyI--S--(CH2)2--CHmCOO-

H20 Adenosine I ~ J : HS~(CH2)2~ CHmCOO-

S-Adenosylhomocysteine

Homocysteine

Sulfur-Containing Amino Acids

Methionine and cysteine are the principal sources of organic sulfur in humans. Methionine is essential (unless adequate homocysteine and a source of methyl groups are available), but cysteine is not, since it can be synthesized from methionine.

Homocysteine can be recycled back to methionine either by transfer of a methyl group from betaine catalyzed by betaine-homocysteine methyltransferase, or from N 5methyltetrahydrofolate (NS-methyl-FH4) catalyzed by N 5methyl-FH4-methyltransferase, which requires methyl cobalamin: (:)

NH3 +

!

Methionine

HS~(CH2)2~H~COO-

Methionine is utilized primarily in protein synthesis, providing sulfur for cysteine synthesis, and is the body's principal methyl donor. In methylation reactions, S-adenosylmethionine (SAM) is the methyl group donor. SAM is a sulfonium compound whose adenosyl moiety is derived from ATP as follows:

+ (CH3)3+N- CH2~COO-

Homocysteine

Betaine NH3 +

I

H3C__S__(Ch2)2mCH~COO- + (CH3)2N--CH2--COOMethionine

(2)

Dimethylglycine

NH 3+ I H S ~ ( C H 2 ) 2 ~ C H ~ C O O - + NS~MethyI~FH4

NH3 + I

ATP.H~

CH3~S~(CH2)2--CH~COO

Betaine-homocysteine methyltransferase ,

NS_Methyt_FH4.homocysteine methyltransferase (methylcobalamin)

~.spp, + p.

NH3 +

Methionine adenosyltransferase

I

Methionine

H3CmSm(CH2)2--CHmCOO - + FH4

NH2

Betaine (an acid) is obtained from oxidation of choline (an alcohol) in two steps"

NH3 +

I

FAD

- O O C - - C H - - ( C H 2 ) 2 - - ~;

CH2

I

I

(CH3)3+N__CH2--CH2OH Choline

CH3

~

FADH2

J

Cholineoxidase

,(CH3)3+N__CH2mCHO Betaine aldehyde NAD*~I '~

HO OH S-Adenosylmethionine (active methionine)

NADH +

Belaine aldehyde

.,/1 9 dehydr~ H+Wl

(CH3)3N__CH2mCOO Betaine

CHAPTER17 Protein and Amino Acid Metabolism

354

Cysteine In the biosynthesis of cysteine, the sulfur comes from methionine by transsulfuration, and the carbon skeleton and the amino group are provided by serine (Figure 17-16). Cysteine regulates its own formation by functioning as an allosteric inhibitor of cystathionine v-lyase, otKetobutyrate is metabolized to succinyl-CoA by way of propionyl-CoA and methylmalonyl-CoA. Cysteine is required for the biosynthesis of glutathione and of CoA-SH. A synthetic derivative, Nacetylcysteine, is used to replenish hepatic levels of glutathione and prevent hepatotoxicity due to overdosage with acetaminophen. When high concentrations of acetoaminophen are present in the liver, the drug undergoes N-hydroxylation to form N-acetyl-benzoquinoneimine, which is highly reactive with sulfhydryl groups of proteins and glutathione and causes hepatic necrosis. NAcetylcysteine is used as a mucolytic agent (e.g., in cystic fibrosis) because it cleaves disulfide linkages of mucoproteins. Cysteine and cystine are interconverted by NADdependent cystine reductase and nonenzymatically by an appropriate redox agent (e.g., GSH). The major end products of cysteine catabolism in humans are inorganic sulfate, taurine, and pyruvate. Taurine is a/3-amino acid that has a sulfonic acid instead of a L-Methionine M~tATP hionine adenosyltransferase PPI + P0 S-Adenosylmethionine [~,~.( n CH3)in methyltransferase reaction

S-Adenosylhomocysteine nosylhomocysteinase denosine HS~CH~-'CHi--~H-- COONH~

.

L -Hornocysteine

NH+

I/,-.- HOH,C----*CH--COO- L-Serine ~Cystathionine [~-synthase (pyridoxal phosphate)

L"--H,O

~H* *

[.

.

OOC--CHmCl-l=w S--CHi---Cl-.l=m C H ~ C OO-

I

NH+ L-Cystathionine H,O ~Cystathionine ~'-Iyase (pyridoxal phosphate) ["---. NH+ (cystathionase) NH+ OOCmCH--CH/-'-SH L-Cysteine

0

II

+ CH3----CH~---~_,mCOO---.,-~--~ Succinyl-OoA or

F I G U R E 17-16 Biosynthesis of cysteine. The sulfur is derived from methionine, and the carbon skeleton and amino group are derived from serine.

carboxylic acid group. Taurine is conjugated with bile acids in the liver (Chapter 19) and is readily excreted by the kidney. It is a major free amino acid of the central nervous system (where it may be an excitatory neurotransmitter) and the most abundant in the retina; it also occurs in other tissues (e.g., muscle, lung). Sulfate can be converted to the sulfate donor compound 3'-phosphoadenosine-5'-phosphosulfate (PAPS) in a twostep reaction (Figure 17-17). PAPS participates in the sulfate esterification of alcoholic and phenolic functional groups (e.g., in synthesis of sulfolipids and glycosaminoglycans).

Abnormalities Involving Sulfur-Containing Amino Acids Deficiencies of methionine adenosyltransferase, cystathionine fl-synthase, and cystathionine y-lyase have been described. The first leads to hypermethioninemia but no other clinical abnormality. The second leads to hypermethioninemia, hyperhomocysteinemia, and homocystinuria. The disorder is transmitted as an autosomal recessive trait. Its clinical manifestations may include skeletal abnormalities, mental retardation, ectopia lentis (lens dislocation), malar flush, and susceptibility to arterial and venous thromboembolism. Some patients show reduction in plasma methionine and homocysteine concentrations and in urinary homocysteine excretion after large doses of pyridoxine. Homocystinuria can also result from a deficiency of cobalamin (vitamin BI2) or folate metabolism. The third, an autosomal recessive trait, leads to cystathioninuria and no other characteristic clinical abnormality. Hereditary sulfite oxidase deficiency can occur alone or with xanthine oxidase deficiency. Both enzymes contain molybdenum (Chapter 27). Patients with sulfite oxidase deficiency exhibit mental retardation, major motor seizures, cerebral atrophy, and lens dislocation. Dietary deficiency of molybdenum (Chapter 37) can cause deficient activity of sulfite and xanthine oxidases. Cystinuria is a disorder of renal and gastrointestinal tract amino acid transport that also affects lysine, ornithine, and arginine. The four amino acids share a common transport mechanism (discussed above). Clinically, it presents as urinary stone disease because of the insolubility of cystine. In cystinosis, cystine crystals are deposited in tissues because of a transport defect in ATP-dependent cystine efflux from lysosomes (discussed above).

Homocysteine Homocysteine is an amino acid not found in proteins. Its metabolism involves two pathways; one is

355


SECTION 17.3

NH2

~ -

O

Sulfate

O---S--O

adenyl~transferase.-

-

''"~';'~"g~---'"

oli

ATP

-

N

O

II

II

O-- IIS'--O-- P - - O - - C H , / O

PP,

O

O]-

I~ I HO

I OH

Adenosine5"-phosphosulfate

Adenylylsulfate~. ATP

kinase~Mg''~ z/ "----*ADP NH=

O

II

\N~N

O

J

II

-O--S--O--P--O--CH2~O II I_ P~ I o

OH

I O--P--OI O-

PAPS

FIGURE 17-17 Formationof 3'-phosphoadenosine-5'-phosphosulfate(PAPS).

the methylation of homocysteine to methionine using NS-methyltetrahydrofolate (NS-methyl-FH4) catalyzed by a vitamin B12-dependent enzyme. The second is the transsulfuration pathway where homocysteine condenses with serine to form cystathionine; this is catalyzed by cystathionine #-synthase (CBS) which is a pyridoxal5'-phosphate enzyme. End products of the transsulfuration pathway are cysteine, taurine, and sulfate (Figure 17-18). The methyl donor NS-methyl-FH4 is synthesized from NS,Nl~ and the reaction is catalyzed by NS,Nl~ reductase (MTHFR). MTHFR is a FAD-dependent enzyme. Thus, the metabolism of homocysteine involves four water soluble vitamins, folate, vitamin B12, pyridoxine, and riboflavin. Any deficiencies or impairment in the conversion of the four vitamins to their active coenzyme forms will affect homocysteine levels. Severe cases of hyperhomocysteinemia occur due to deficiencies of enzymes in the homocysteine remethylation or transsulfuration pathways. Individuals with a homozygous defect in cystathionine

#-synthase have severe hyperhomocysteinemia (plasma concentrations >50 #M/L) and their clinical manifestations are premature atherosclerosis, thromboembolic complications, skeletal abnormalities, ectopia lentis and mental retardation. In plasma, homocysteine is present as both free (< 1%) and oxidized forms (>99%). The oxidized forms include protein (primarily albumin)-bound homocysteine mixed disulfide (80-90%), homocysteine-cysteine mixed disulfide (5-10%), and homocystine (5-10%). Several studies have shown the relationship between homocysteine and altered endothelial cell function leading to thrombosis. Thus, hyperhomocysteinemia appears to be an independent risk factor for occlusive vascular disease. Five to ten percent of the general population have mild hyperhomocysteinemia. It has been shown that a thermolabile form of MTHFR is a major cause of mildly elevated plasma homocysteine levels, which have been associated with coronary heart disease. The thermolabile MTHFR gene has a

3,~


Folate (F) /i--2 NADPH+2H+ ihydrofolate reductase (DR)

~

~ "~ 2 NADP+ Tetrahydrofolate(FH4) ~ n f e Serine e hydroxymethyl rase oxal phosphate)

~

Glycine

Methylene FH4

/~

PP/+ Pi

//( Methionine / \Adenosyltransferase ( \ATP

I

I\ Methylated . i / \ accept~ 4~'- 1

Methylation

Methionine

MTHFR (FAD)

Methyl (acceptors

S-adenosylmethionine ~

~(~t

S-adenosylhomocysteine

Cycle

ethyl transferase

Methyl FH4 Adenosine ~

y l c o ~ a l a m i Hh i m o c y s t e i n e

enosylhomocysteinase

r gerine

fy

sti~tOhi xO~ipnheSspy~tathea)se

Cystathionine

Transulfuration Pathway

~ c~-Ketobutyrate

Cystathionine3'-lyase (pyridoxal phosphate)

Cysteine

/ / Glutathione

\ \ Taurine

,v_

-~ Sulfate

FIGURE 17-18 Homocysteine metabolism.

mutation of C to T at nucleotide position 677 which causes an alanine-to-valine amino acid substitution in the protein. The mechanism by which homocysteine mediates vascular pathology remains to be understood. The targets for homocysteine damage are connective tissue, endothelial cells, smooth muscle cells, coagulation factors, nitric oxide metabolism, plasma lipids and their oxidized forms (Chapter 20). Vitamin supplementation with B12, folate, and B6 has reduced total plasma homocysteine levels. Vitamin supplementation may decrease the morbidity and mortality from atherosclerotic vascular disease due to hyperhomocysteinemia. However,

further studies are required to assess the utility of vitamin supplementation.

Phenylalanine and Tyrosine Phenylalanine is an essential amino acid. Tyrosine is synthesized by hydroxylation of phenylalanine and therefore is not essential. However, if the hydroxylase system is deficient or absent, the tyrosine requirement must be met from the diet. These amino acids are involved in synthesis of a variety of important compounds, including thyroxine, melanin, norepinephrine, and epinephrine

357

SECTION17.3 Metabolism of Some Individual Amino Acids DIET

I

Dihydrofolate reductase

Dihydr0pteridine

Jl

Tyrosine

[f~(z -Ketoglutarate

I

[Tyrosinetransaminase

i I L_..7,8.Dihydrobiopterin~__~

pNADP* ~"NADPH+ H+

CH2-'---CH-- COO-

HO~

o

II

CH2--.-C--COO-

t

* p-Hydroxyphenylpyruvate

Hydroxylation,shiftp~p-HyC~roxyphenylpyruvic of the side chain I acid oxidase;ascorbic

decarboxylation~.~d~CoU'+

H O ~ O H COO

CH=

m

I CH II HC I

I I CH= I

CH2CO0

C--O

CO0

Homogentisicacid fO,

Homogentisicacid oxidase; ascorbic acid; GSH

CO0

Fumarate Acetoacetate H20[ Fumarylacetoacetase m

OOC

\c /

H

COO-

i

C ?H2 C H . . H/ \C \C / \CO011 II

0

MaleylacetoacetateHC CHz .CH, isomerase \C / \C / \CO0-

0

Fumarylacetoacetate

II

0

It

0

Maleylacetoacetate

F I G U R E 17-20 Conversion of phenylalanine to tyrosine and the oxidative pathway of tyrosine.

6-pyruvoyl tetrahydrobiopterin synthase leads to hyper-

phenylalaninemia. Phenylketonuria (PKU) Deficiency of phenylalanine hydroxylase, tetrahydrobiopterin, or dihydropteridine reductase results in phenylketonuria (PKU), an autosomal recessive trait. Because phenylalanine accumulates in tissues and plasma (hyperphenylalaninemia), it is metabolized by alternative pathways and abnormal amounts of phenylpyruvate appear in urine (Figure 17-22). Phenylalanine hydroxylase deficiency may be complete (classic PKU, type I) or partial

(types II and III). Many mutations of the phenylalanine hydroxylase gene have been identified (missense, nonsense, insertions, deletions, and duplications) leading to PKU or non-PKU hyperphenylalaninemia. The incidence of classic PKU is about 1 in 10,00020,000 live births and exhibits considerable geographic variation (the incidence in Ireland is 1 in 4000, whereas the condition is rare among blacks and Asians). About 2% of hyperphenylalaninemic infants have a deficiency of biopterin or biopterin reductase. The most important clinical presentation is mental impairment. Diagnosis can be made early in the neonatal period by measurement of phenylalanine concentration in blood collected from

SECTION17.3 Metabolism of Some Individual Amino Acids H

I

H~ j N ~ jN~,../NH2 H/~]'~ 8I T "T H .~ II H3C~ ? ~ C /

"N"

"hl"

/

OH OH

H

!

R-group

5,6,7,8-Tetrahydrobiopterin H

I H~,,-N".-,,~ N"-,~NH

H~N~..-N~I,./NH2

O

O

Ortho-quinonoid dihydropteridine

Para-quinonoid dihydropteridine

"Quinonoid"dihydrobiopterins H

I

!~i I~'~NHN~NH2 O 7.8-Dihydrobiopterin F I G U R E 17-21 Structures of biopterin derivatives.

a heel prick onto filter paper. Treatment of phenylalanine hydroxylase deficiency consists of a diet low in phenylalanine but which maintains normal nutrition. This diet is effective in preventing mental retardation, and its con~H+ Transaminase @ CHu--CH-- COO~ (pyridoxal phosphate) y,.-~ " -Ketoglutarate Glutamate Phenylalanine ~ NAD+ ~

~'~

CH2COO-

0 II COOCH;---C--

~

Phenylpyruvate NADH+ H+

NAD+

C02

Phenylacetate

OH Phenyllactate

/

Inthei"'-- Glutamine liver~...--H20 /!

\\

0 H COOII , I

~k__/~---OH2---C--N--(~H

CONH2 Phenylaeelyk:jlutamine F I G U R E 17-22 Formation of metabolites of phenylalanine that accumulate in abnormal amounts and are excreted in phenylketonuria.

359

tinuation throughout the first decade, or for life, may be necessary. Treatment of biopterin and biopterin reductase deficiency consists not only of regulating the blood levels of phenylalanine but of supplying the missing form of coenzyme and the precursors of neurotransmitters, namely, dihydroxyphenylalanine and 5-hydroxytryptophan, along with a compound that inhibits peripheral aromatic decarboxylation. This compound is necessary because the amine products do not cross the blood-brain barrier. Successfully treated females who have reached reproductive age may expose their offspring (who are obligate heterozygotes) to abnormal embryonic and fetal development. These effects include spontaneous abortion, microcephaly, congenital heart disease, and intrauterine growth retardation, and they correlate with the plasma level of phenylalanine of the pregnant mother. Thus, reinstitution of a low-phenylalanine diet during preand postconception periods may be necessary. The diet should also restrict intake of phenylalanine-containing substances, such as the synthetic sweetener aspartame (Laspartyl-L-phenylalanyl methyl ester). Because defective myelination occurs in the brain in PKU, there is an increased incidence of epileptic seizures and abnormal electroencephalograms are common. The biochemical basis for the severe mental impairment is not understood. One factor may be inhibition of glutamate decarboxylase by phenylpyruvate and phenylacetate: NH3 +

I

- OOC--(CH2)2--CH--COO-

Glutamate decarboxylase

CO2

L-Glutamate -

+

OOC--(CH2)3--NH 3 7-Aminobutyrate

The substrate is an excitatory neurotransmitter and the product an inhibitory one in the central nervous system." Abnormal indole derivatives in the urine and low levels of serotonin (a product of tryptophan metabolism) in blood and brain point to a defect in tryptophan metabolism in PKU. 5-Hydroxytryptophan decarboxylase, which catalyzes the conversion of 5-hydroxytryptophan to serotonin, is inhibited in vitro by some of the metabolites of phenylalanine. Phenylalanine hydroxylase is similar to the enzyme that catalyzes the hydroxylation of tryptophan to 5-hydroxytryptophan, a precursor of serotonin. In vitro, phenylalanine is also found to inhibit the hydroxylation of tryptophan. The mental defects associated with PKU may be caused by decreased production of serotonin. High phenylalanine levels may disturb the transport of amino

360 acids into cells. Variations in the clinical manifestations of PKU may reflect differences in this disturbance. Defects in pigmentation of skin and hair (light skin and hair) may be caused by interference of melanin formation by phenylalanine and its metabolites and also by lack of tyrosine.

CHAPTER17 Proteinand AminoAcid Metabolism ~'~--C--COO-

Tyrosinase

--CO0-

[O1 H,O H Tyrosine

N/

3,4-Dihydroxyphenylalanine (dopa) [O]-'~ Tyrosinase

H.O~ 1 Melanin Melanin is an insoluble, high-molecular-weight polymer of 5,6-dihydroxyindole, which is synthesized from tyrosine (Figure 17-23). It is produced by pigment cells (melanocytes) in cytoplasmic organelles (melanosomes). In the epidermis, melanocytes are associated with keratinocytes, which contain melanosomes supplied by melanocytes via dendritic processes. Color variation in human skin reflects the amount of melanin synthesized in melanosomes. Melanin synthesis is apparently under hormonal and neural regulation. The first two steps in the synthesis of melanin are catalyzed by tyrosinase, a copper-containing oxidase, which converts tyrosine to dopaquinone. All subsequent reactions presumably occur through nonenzymatic auto-oxidation, in the presence of zinc, with formation of the black to brown pigment eumelanin. The yellow to reddish brown, high-molecular-weight polymer known as pheomelanin and the low-molecular-weight trichromes result from addition of cysteine to dopaquinone and further modification of the products. Pheomelanins and trichromes are primarily present in hair and feathers.

CO0_---.,---,,--O.,j,~~.,j+I~N/I IH--C--CO0H Dopachrome

co2J zn'+

H Indole-5,6-quinone Cysteine,,......,,I.[

H

5,6-Dihydroxyindole Trichromes

/

/

Pheomelanins

Abnormalities of Tyrosine Metabolism Hepatic cytosolic tyrosine aminotransferase (tyrosine transaminase) deficiency produces tyrosinemia type II, an autosomal recessive trait marked by hypertyrosinemia and tyrosinuria. Clinical manifestations may include corneal erosions and plaques, inflammation (from intracellular crystallization of tyrosine), and mental retardation. Low-tyrosine and low-phenylalanine diets are beneficial. Tyrosinosis is presumably due to fumarylacetoacetate hydrolase deficiency and has a high prevalence in the French-Canadian population of Qu6bec. It is associated with abnormal liver function, renal tubular dysfunction, anemia, and vitamin D-resistant rickets. Transient tyrosinemia of the newborn, particularly in premature infants, is the most common form of tyrosinemia in infancy. Alcaptonuria is a rare metabolic hereditary disease in which homogentisic acid is eliminated in urine, which darkens upon exposure to air owing to oxidation of

Dopaquinone

1

Polymerization [

H N

O

N~c

0

Eumelanins F I ( ; U R E 17-23 Biosynthesis of melanins.

homogentisic acid. The biochemical lesion is homogentisic acid oxidase deficiency. Clinical features include pigmentation of cartilage and other connective tissues (ochronosis) later in life from deposition of oxidized homogentisic acid. Patients nearly always develop arthritis in later years, but the relationship between pigment deposition and arthritis is not understood. Lack of melanin production (hypomelanosis) gives rise to several hereditary disorders collectively known as albinism. Some forms result from deficiency of tyrosinase. The inheritance pattern of albinism varies with type. Affected individuals have increased susceptibility to various

SECTION17.3 Metabolismof Some Individual Amino Acids

361 O

pyrrolase, CH;---CHmCOO- Tryptophan f

E ~

NIH:

II

E~IH~I~--i7

COO

02

I

N-Formylkynurenine

H

f-H,O

Formamidase Formate~ One-carbonpool

_CmCH2--'-CH--COO"~ I'Y ] ~. NH.~ ' 'I , k,.~ ~NH= i;

Z"

3-Hydroxykynurenine

o2~ N A D P H + H +

""~eeb~, ~

Kynureninase H20 (pyridoxal~f'ph~ ~'~Alanine

~

Kynurenine hydroxylase

'

,~"~C--

I-

II

~_,

I~

~

CHs ICHmCOO

".§

....3

"NH2 Kynurenine

o/,. ~J"Ob'o ~e/~ ~O~,o/, -" ~

OH

CO0 ....

NAD+

~NH+ OH

3-Hydroxyanthranilate

I~/,~CO0OH

Xanthurenate (excretedin urine)

FIGURE 17-24 Pathway for the synthesis of NAD from tryptophan.

types of carcinoma (from the effect of solar radiation on DNA). When the eyes are involved, photophobia, subnormal visual acuity, strabismus, and nystagmus may be present.

Tryptophan

Tryptophan is an essential amino acid involved in synthesis of several important compounds. Nicotinic acid (amide), a vitamin required in the synthesis of NAD + and NADP +, can be synthesized from tryptophan (Figure 17-24). About 60 mg of tryptophan can give rise to 1 mg of nicotinamide. The synthesis begins with conversion of tryptophan to N-formylkynurenine by tryptophan pyrrolase, an inducible iron-porphyrin enzyme of liver. N-Formylkynurenine is converted to kynurenine by removal of formate, which enters the one-carbon pool. Kynurenine is hydroxylated to 3-hydroxykynurenine, which is converted to 3-hydroxyanthranilate, catalyzed by kynureninase, a pyridoxal phosphate-dependent enzyme. 3-Hydroxyanthranilate is then converted by a series of reactions to nicotinamide ribotide, the immedi-

ate precursor of NAD. In deficiency of pyridoxal phosphate, 3-hydroxykynurenine accumulates and is converted to xanthurenate, which is excreted in urine. Thus, vitamin B6 deficiency can be diagnosed by measurement of urinary xanthurenate after administration of a standard dose of tryptophan (tryptophan load test). 5-Hydroxytryptamine (serotonin) is found in enterochromaffin cells, brain, and platelets. In the former two, it is produced from tryptophan, whereas in platelets, serotonin is taken up from plasma. Synthesis involves hydroxylation of tryptophan by tryptophan 5-hydroxylase and decarboxylation by aromatic L-amino acid decarboxylase (Figure 17-25). Hydroxylation is the rate-limiting reaction, is analogous to that of phenylalanine, and requires molecular oxygen and tetrahydrobiopterin. Serotonin is a powerful vasoconstrictor and stimulator of smooth muscle contraction. In the brain it is a neurotransmitter, and in the pineal gland it serves as a precursor of melatonin. Synthesis of melatonin requires N-acetylation of serotonin, followed by methylation (Figure 17-26). The role of melatonin in humans is not understood; in frogs, it lightens the color of skin melanocytes and blocks the action of melanocyte-stimulating hormone (MSH) and

362


(.CHCH--COO~H;~

I

H

Tryptophan 02, tetrahydrobiopterin~1 H20, dihydrobiopterin..~Tryptophan-5-hydroxylase

HO~,~CHr--CH--COO

-

I

N

5-Hydrox~ryptophan Aromatic L-aminoacid r,~ .,.,~decarboxylase "'=t(pydd~ phosphate)

HO.

I I

N

5-Hydroxytryptamine (serotonin) FIGURE 17-25 Biosynthesis of serotonin from tryptophan.

H O . ~

II--CHE--CHs NH+ jlJ Serotonin

? H AcelyI'C~ 4 5-Hydrox~ryptamine-

HO~

CoASH~'~N-acetyltransferase

O

II

CHT'-CHu--NH~C~CH 3 N-Acetyl-5-hydroxytryptamine

9 H S-Adenosylmethionine-~

S.Adenosylhomocysteine.~,.[Hydroxyindolemethyltransferase O

j

CHE---CHs

N

I

H

Melatonin (N-acetyl-5-methoxytryptamine) FIGURE 17-26 Biosynthesis of melatonin from serotonin.

"

adrenocorticotropic hormone (ACTH). In rats, melat0nin regulates the breeding cycle, and its secretion is increased by exposure to light via adrenergic stimulation of the pineal gland. N-Acetyltransferase is activated by increased concentrations of cytosolic cyclic AMP and Ca 2+ that is mediated by the activation of adrenergic receptors of the pineal gland. In humans, melatonin synthesis and its release follows a circadian rhythm which is stimulated by darkness and inhibited by light. The blood levels of melatonin increase by passive diffusion from the central nervous system after the onset of darkness, reaching a peak value during the middle of the night and declining during the second half of the night. Melatonin secretion is also regulated endogenously by signals from the suprachiasmatic nucleus. Since melatonin's peak concentration in plasma coincides with sleep, exogenous administration of the hormone can affect the circadian rhythm. Melatonin supplementation has been used to ameliorate subjective and objective symptoms of jet lag caused by travel across time zones. At high levels, melatonin promotes sleep. Short-term and longterm biological effects of melatonin supplementation have yet to be determined. Serotonin is degraded to 5-hydroxyindoleacetic acid (5-HIAA) by monoamine oxidase and aldehyde dehydrogenase acting in sequence. 5-HIAA is excreted in the urine. Its excretion is markedly increased in subjects with carcinoid tumor (found most frequently in the gastrointestinal tract and lungs). Carcinoid tumors are frequently indolent and asymptomatic; however, a significant number of these tumors can manifest as metabolic problems. Although increased production of serotonin is a characteristic feature of the carcinoid tumor, cells also synthesize other substances. These include kinins, prostaglandins, substance P, gastrin, somatostatin, corticotropin, and neuron-specific enolase. Carcinoid tumor cells are known as enterochromaffin cells because they stain with potassium dichromate and also are known as argentaffin cells because they take up and reduce silver salts. The symptoms of the carcinoid tumor are due to synergistic biochemical interactions between serotonin and the above-mentioned active metabolites. Characteristics of the carcinoid syndrome are flushing, diarrhea, wheezing, heart valve dysfunction, and pellagra. Lifestyle conditions that precipitate symptoms include intake of alcohol or spicy foods, and strenuous exercise. The treatment options for the carcinoid syndrome are multidisciplinary, including lifestyle changes, inhibitors of serotonin release and serotonin receptor antagonists, somatostatin analogues, hepatic artery embolization, chemotherapy, and surgery.


Hartnup disease is a disorder of renal tubular and intestinal absorption of tryptophan and other neutral amino acids.

Supplemental Readings and References J. Abrains: The role of nitrates in coronary heart disease. Archives oflnternal Medicine 155, 357 (1995). M. L. Batshaw: Inborn errors of urea synthesis. Annals of Neurology 35, 133 (1994). D. Bohn: Nitric oxide in acute hypoxic respiratory failure: From the bench to the bedside and back again. Journal of Pediatrics 134, 387 (1999). A. Brezezinski: Melatonin in Humans. The New England Journal of Medicine 331, 186 (1997). M. E. Caplin, J. R. Buscombe, A. J. Hilson, et al.: Carcinoid tumour. Lancet 352, 799 (1998). K. J. Carpenter: Protein requirements of adults from an evolutionary perspective. American.Journal of Clinical Nutrition 55, 913 (1992). D. Christensen: What's that smell? Science News 155, 316 (1999). D. T. Chuang: Maple syrup urine disease: It has come a long way. Journal of Pediatrics 132, S 17 (1998). J. E Cooke and E S. Tasao: Arginine: A new therapy for artherosclerosis? Circulation 97, 311 (1997). B. R. Crane, A. S. Arvai, R. Gachhui, et al.: The structure of nitric oxide synthase oxygenase domain and inhibitor complexes. Science 278, 425 (1997). E. L. Dobyns, D. N. Cornfield, N. G. Anas, et al.: Multicenter randomized control trial of the effects of inhaled nitric oxide therapy on gas exchange in children with acute hypoxic respiratory failure. Journal of Pediatrics 134, 406 (1999). E. B. Feldman: Creatine: A Dietary supplement and ergogenic aid. Nutrition Reviews 57, 45 (1999). I. Goldstein, T. E Lue, H. Padma-Nathan, et al.: Oral sildenafil in the treatment of erectile dysfunction. The New England Journal of Medicine 338, 1397 (1998).

363 R. Jalan and E C. Hayes: Hepatic encephalopathy and ascites. Lancet 350, 1309 (1997). S. G. Korenman: New insights into erectile dysfunction. American Journal of Medicine 105, 135 (1998). J. Loscalzo and G. Welch: Nitric oxide and its role in the cardiovascular system. Progress in cardiovascular diseases 38, 87 (1995). N. E. Maestri, D. Clisold, and S. W. Brusilow: Neonatal onset ornithine transcarbamylase deficiency: A retrospective analysis. Journal of Pediatrics 134, 268 (1999). M. M. Malinow, E B. Duell, D. L. Hess, et al: Reduction of plasma homocysteine levels by breakfast cereal fortified with folic acid in patients with coronary heart disease. The New England Journal of Medicine 338, 1009 (1998). A. J. Maxwell and J. E Cooke: Cardiovascular effects of L-arginine. Current Opinion in Nephrology and Hypertension 7, 63 (1998). D. J. Millward: Aging, protein requirements, and protein turnover. American Journal of Clinical Nutrition 66, 774 (1997). J. D. Parker and J. O. Parker: Nitrate therapy for stable angina pectoris. The New England Journal of Medicine 338, 520 (1998). S. M. Riordan and R. Williams: Treatment of hepatic encephalopathy. The New England Journal of Medicine 337, 473 (1997). A. M. Spiekerman: Proteins used in nutritional assessment. Clinical Laboratory Medicine 13, 353 (1993). J. H. Stein and E E. McBride: Hyperhomocysteinemia and atherosclerotic vascular disease. Archives of Internal Medicine 158, 1301 (1998). J. G. Theone: Treatment of urea cycle disorders. Journal of Pediatrics 134, 255 (1999). R. D. Utiger: A pill for impotence. The New England Journal of Medicine 338, 1458 (1998). G. Wagner and I. S. de Tejada: Update on male erectile dysfunction. British Medical Journal 376, 678 (1998). N. J. Wald, H. C. Watt, M. R. Law, et al.: Homocysteine and ischemic heart disease. Archives of Internal Medicine 158, 862 (1998). G. N. Welch and J. Loscalzo: Homocysteine and atherothrombosis. The New England Journal of Medicine 338, 1042 (1998). M. Wyss, R. Kaddurah-Daouk: Creatine and creatine metabolism. Physiological Reviews 80, 1107 (2000).


CHAPTER

18

Lipids I: Fatty Acids and Eicosanoids

Lipids (or fats) are a heterogeneous group of organic compounds defined by their solubility in nonpolar solvents such as chloroform, ether, and benzene and by their poor solubility in water. Lipids may be polar or nonpolar (amphipathic). Polar lipids have limited solubility in water because they are amphipathic, i.e., they possess hydrophilic and hydrophobic regions in the same molecule. Major polar lipids include fatty acids, cholesterol, glycerophosphatides, and glycosphingolipids. Very short chain fatty acids and ketone bodies are readily soluble in water. Nonpolar lipids serve principally as storage and transport forms of lipid and include triacylglycerols (also called triglycerides) and cholesteryl esters. Lipids have numerous functions including the following: thermal insulation, energy storage (as triacylglycerol), metabolic fuels, membrane components (phospholipids and cholesterol; Chapter 10), hormones (steroids and vitamin D metabolites; Chapters 32 and 37, respectively), precursors of prostanoids (discussed on p. 391) and leukotrienes, (vitamins A, C, D, E, and K; Chapters 36-38), emulsifying agents in the digestion and absorption of lipids (bile acids; Chapters 12 and 19), and surfactants in the alveolar membrane (phosphatidylcholine; Chapter 19). The metabolism of fatty acids (saturated and unsaturated) is discussed in this chapter. The metabolism of phospholipids, glycosphingolipids, and cholesterol is considered in Chapter 19.

Fatty acids that contain no carbon-carbon double bonds are known as saturated and those with carbon-carbon double bonds as unsaturated. Fatty acids that contain an even number of carbon atoms and are acyclic, unbranched, nonhydroxylated, and monocarboxylic make up the largest group. The most abundant saturated fatty acids in,animals are palmitic and stearic acids (Table 18-1). The melting point of fatty acids rises with increase in chain length, the even-numbered saturated fatty acids having higher melting points than the odd-numbered. Among the evennumbered, the presence of cis double bonds lowers the melting point significantly. Free fatty acids at physiological pH are ionized (pK ~4.85) and exist only in small quantities; in plasma, they typically are bound to albumin. They are usually present as esters or amides. Digestion and absorption of lipids are discussed in Chapter 12. The Western diet contains about 40% fat, mostly as triacylglycerol (100-150 g/day). Triacylglycerols packaged as chylomicrons in the intestinal epithelial cell are delivered to the blood circulation via the lymphatic system and are hydrolyzed to glycerol and fatty acids by endothelial lipoprotein lipase. Fatty acids are taken up by the cells of the tissue where the hydrolysis occurs, whereas glycerol is metabolized in the liver and kidney (Chapter 15). Another means of triacylglycerol transport is very-low-density lipoprotein (VLDL), which is synthesized in the liver. Its triacylglycerol is also hydrolyzed by endothelial lipoprotein lipase. The metabolism of plasma

365

366

CHAPTER 18


TABLE 18-1

Naturally Occurring Saturated Fatty Acids Common Name Capric Lauric Myristic Palmitic t Stearic t Arachidic Behenic Lignoceric C erotic Montanic

Systematic Name*

Molecular Formula

n-Decanoic n-Dodecanoic n-Tetradecanoic n-Hexadecanoic n-Octadecanoic n-Eicosanoic n-Docosanoic n-Tetracosanoic n- Hexaco s anoic n-Octacosanoic

C 10H2002 C 12H2402 C 14H2802 C 16H3202 C 18H3602 C20H400 2 C22H440 2 C24H4802 C26H5202 C28H560 2

Structural Formula CH3[CH2]8COOH CH3[CH2] loCOOH CH3[CH2] 12COOH CH3[CH2] 14COOH CH3[CH2] 16COOH CH3[CH2] 18COOH CH3[CH2]20COOH CH3[CH2122COOH CH3[CHz]z4COOH CH3[CH2126COOH

Melting Point (~ 31 44 58 63 70 76 80 84 88 92

*Systematic name is based on replacingthe final letter "e" of the parenthydrocarbonwith"oic." tMost abundantfattyacidspresentin animallipids. lipoproteins is discussed in Chapter 20. Fatty acids are released by hydrolysis of triacylglycerol in adipocytes by hormone-sensitive lipase, particularly during starvation, stress, and prolonged exercise (Chapter 22). Interrelationships of tissues in lipid metabolism are discussed in Chapter 22.

fully assembled, mature enzymes. During/~-oxidation of acyl-CoA, the chain length of the substrate is shortened by two carbon atoms (acetyl-CoA) each cycle. Thus, ,6oxidation requires a group of enzymes with chain length specificity.

Activation of Fatty Acids

18.1 Oxidation of Fatty Acids The overall fatty acid oxidation process in mitochondria consists of uptake of fatty acids, their activation to acylCoA, then to thioesters, and finally translocation into mitochondria which involves a carnitine transesterification shuttle and/3-oxidation. Fatty acids released from chylomicrons and VLDLs are transferred across cell membranes by passive diffusion, which depends on the concentration gradient. Fatty acids are also obtained from the hydrolysis of triacylglycerol stored in adipose tissue which are bound to albumin and transported in blood. Fatty acids serve as substrates for energy production in liver, skeletal and cardiac muscle during periods of fasting. Although the brain does not utilize fatty acids for generating energy directly, brain cells can utilize ketone bodies synthesized from acetyl-CoA and acetoacetyl-CoA. The latter two are obtained from/3-oxidation of fatty acids in the liver. All of the enzymes involved in mitochondrial fatty acid/3oxidation are encoded by nuclear genes. After their synthesis in the cytosolic endoplasmic reticulum, the enzymes are transported to mitochondria. The transport of the enzymes into mitochondria, in many instances, requires the presence of N-terminal extensions to guide the protein across the mitochondrial membrane, receptor-mediated ATP-dependent uptake, and proteolytic processing to form

At least three acyl-CoA synthases, each specific for a particular size of fatty acid, exist: acetyl-CoA synthase acts on acetate and other low-molecular-weight carboxylic acids, medium-chain acyl-CoA synthase on fatty acids with 4-11 carbon atoms, and acyl-CoA synthase on fatty acids with 6-20 carbon atoms. The activity of acetyl-CoA synthase in muscle is restricted to the mitochondrial matrix. Medium-chain acyl-CoA synthase occurs only in liver mitochondria, where medium-chain fatty acids obtained from digestion of dietary triacylglycerols and transported by the portal blood are metabolized. Acyl-CoA synthase, the major activating enzyme, occurs on the outer mitochondrial membrane surface and in endoplasmic reticulum. The overall reaction of activation is as follows: RCOO- + A TP 4- + CoASH ~

O II

R C - - S C o A + AMP 2- + PPi 3AcyI-CoA (a thioester)

The reaction favors the formation of fatty acyl-CoA, since the pyrophosphate formed is hydrolyzed by pyrophosphatase: PPi 4-H20--+ 2Pi. Thus, activation of a fatty

367

SECTION18.1 Oxidation of Fatty Acids

acid molecule requires expenditure of two high-energy phosphate bonds. The reaction occurs in two steps (E -enzyme)"

The overall translocation reaction is as follows: O

OH3

II

I

RBCmSCoA + H3C u N +~CH2~CHmCH2--COOH

I

RCOO- + ATP + E AcyI-CoA

Step 1

OH Carnitine

[Carnitpal inemitoyltransferase

O

II

E~AMPmCmR

CoASH~ ~

I

CH3

+

PPi OH3

I

H

H3C--N +--CH2--CmCH2mCOOH + CoASH

Step 2

I

OH3

O

I

O

I

C--O }

II

E + RmC~SCoA + AMP

I R

A mitochondrial acyl-CoA synthase, which utilizes GTP, has also been identified:

Acyl group

The standard free-energy change of this reaction is about zero, and therefore the O-ester bond of acylcarnitine may be considered as a high-energy linkage. Malonyl-CoA, a precursor in the synthesis of fatty acids, is an allosteric inhibitor of CPTI in liver and thus prevents a futile cycle of simultaneous fatty acid oxidation and synthesis. Carnitine is synthesized from two essential amino acids, lysine and methionine. S-Adenosylmethionine donates three methyl groups to a lysyl residue of a protein with the formation of a protein-bound trimethyllysyl. Proteolysis yields trimethyllysine, which is converted to carnitine (Figure 18-2). In humans, liver and kidney are major sites of carnitine production; from there it is transported to skeletal and cardiac muscle, where it cannot be synthesized. Four inherited defects of carnitine metabolism lead to impaired utilization of long-chain fatty acids for energy production. These include defects of plasma

R. COO- + CoASH + GTP 4- --+ RCO. SCoA + GDP 3- + p2Its role is not known.

Transport of AcyI-CoA to Mitochondrial Matrix This transport is accomplished by carnitine (L-/3-hydroxyg-trimethylammonium butyrate), which is required in catalytic amounts for the oxidation of fatty acids (Figure 18-1). Carnitine also participates in the transport of acetylCoA for cytosolic fatty acid synthesis. Two carnitine acyltransferases are involved in acyl-CoA transport: carnitine palmitoyltransferase I (CPTI), located on the outer surface of the inner mitochondrial membrane, and carnitine palmitoyltransferase II (CPTII), located on the inner surface.

Mitochondria

,,k.

f

Cytosol

Outer membrane

O

II

C~

RBC--OH

+ATP

Intermembranal space / /

AcyI-CoA

synthetase

H

AMP + PP~~J~

d

~)

9

I I

Outer surface

1"-

Carnitine palmitoyl

acyltransferaseI

R--C--SCoA 9 --~ (acyI-CoA)

Inner membrane Acyl-carnitine 9 [acylcarniti-

~

Inner surface

Matrix

CoASH Carnitine palmitoyl transferase II

,,---..AcyliCoA 13-Oxidation

F I G U R E 18-1 Role of carnitine in transport of fatty acids into the mitochondrial matrix. Entry of acyl-carnitine is linked to the exit of carnitine, both mediated by a translocase.

368

CHAPTER 18

ec-Ketoglutarate

NH2

I (C~)3~N--[CHJ;-"CH--COOH

Succinate

+ O2~/+

CO2

NH2

I

"~-"Ascorbate, " (CH3~N--[CH2];--~H--CH--COOH Fe2+

Succinate

OH

13-Hydroxy-r

-N-Trimethyllysine

H

ot-Ketoglutarate

+ COz

+ 02

glycine

NADH + H+

NAD+

(C..).---~--CH~~--CHr-COO.-k,,~~ k,,j/ Ascorbate, (CH.).~tC.,],---COOH 3 OH Carnitine


Fe=+

, Pyridoxal

~phosphate %

1C~:~--~--[CHz]~ - CHO

" 7-Butyrobetaine

Y-Butyrobetainealdehyde

FIGURE 18-2 Carnitine biosynthesisin humans.A lysyl residueis trimethylatedby S-adenosylmethionine,with subsequent proteolytic releaseof trimethyllysine,the starting material.The reactionsare catalyzedby (1) trimethyllysine /~-hydroxylase,(2)/3-hydroxy-trimethyllysinealdolase(pyridoxalphosphate),(3) y-trimethylaminobutyraldehyde dehydrogenase,and (4) y-butyrobetainehydroxylase.

membrane carnitine transport, carnitine palmitoyltransferase I (CPTI), carnitine palmitoyltransferase II (CPTII), and carnitine-acylcarnitine translocase. Clinical manifestations of disorders of carnitine metabolism and fatty acid oxidation disorders (discussed later) span a wide spectrum and can be affected by the severity and site of the defect (e.g., muscle, liver, and kidney). The disorders may be characterized by hypoketotic hypoglycemia, hyperammonemia, liver disease, skeletal muscle weakness, and cardiomyopathy. In some instances, dietary intervention brings about marked improvement in the clinical manifestations; for example, patients with carnitine transport defect respond well to carnitine therapy.

/3-Oxidation The major pathway for fatty acid oxidation,/3-oxidation (Figure 18-3), involves oxidation of acyl-CoA at the/3carbon and removal of two carbon fragments as acetylCoA and takes place entirely in the mitochondrial matrix. Oxidation of a saturated acyl-CoA with an even number of carbon atoms to acetyl-CoA requires repeated sequential action of four enzymes. 1. Acyl-CoA dehydrogenase dehydrogenates acyl-CoA at the oe- and/3-carbon atoms to yield the ot,/3-unsaturated acyl-CoA (or AZ-unsaturated acyl-CoA). Each one of four distinct dehydrogenases is specific for a given range of fatty acid chain length. All four are flavoproteins and contain a tightly bound molecule of flavin adenine dinucleotide (FAD). The electrons from the acyl-CoA dehydrogenase are

transferred to the main respiratory chain (Chapter 14) through mitochondrial electron transfer flavoprotein (ETF) and ETF-ubiquinone oxidoreductase (ETF-QO) (Figure 18-4). Both ETF and ETF-ubiquinone oxidoreductase are nuclear encoded proteins. They also mediate transfer of electrons from dimethylglycine dehydrogenase and sarcosine dehydrogenase. Inherited defects in ETF and ETF-QO cause accumulation of organic acids (acidemia) and their excretion in the urine (acidurias) Examples of these disorders are glutaric acidemia type I and type II which are inherited as autosomal recessive traits. Glutaric acid is an intermediate in the metabolism of lysine, hydroxy lysine, and tryptophan. Glutaric acidemia type I is caused by deficiency of glutaryl-CoA dehydrogenase which catalyzes the conversion of glutaryl-CoA to crotonyl-CoA. Glutaric acidemia type H is caused by defects in the ETF/ETF-QO proteins. The clinical manifestations of these disorders are similar to medium-chain acyl-CoA dehydrogenase deficiency (discussed later). The A 2 double bond formed by the acyl-CoA dehydrogenase has a trans configuration. The double bonds in naturally occurring fatty acids are generally in the cis configuration. The oxidation of unsaturated cis-fatty acids requires two auxiliary enzymes, enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. Acyl-CoA dehydrogenase (especially butyryl-CoA dehydrogenase) is irreversibly inactivated by methylene cyclopropylacetyl-CoA through the formation of covalent adduct with the FAD of the enzyme. The inhibitor is derived by transamination


369

O

, II R~CHi---CH2--C--OH

(A fatty acid)

CoASH-----~ / ATP ~ A c y I - C o A ..... M~ ~synthetase ~ M ~ + r'~ ~---[

t~'tE t - t . O~ . O u)

>,__

Many thiokinases are known. They differ in their substrate specificity and occur in the outer membrane of the mitochondria and microsomes.

O R~CH2---CHs

II

ITranslocation to mit(x~hondrial [matri x involving carnitine Mitochondrial matrix [

O

II (AcyI-CoA) _- R R CH2---- CH~----C ~ S ~ C o A A I "" A t ' ~ - - F P ~ ..---* Red. carriers ~

( /

(~

~ - 1 / 2 02, 2ADP, 2Pa

d,~h,,droCYen~a~ ~ (respiratory chain) ~ ~ Y ~ ~"--* FPH2"~" "~ Ox. carriers = 1 .,..._.31-1=O,2ATP trans

R~ CH--CH~C~S~CoA

(0r ~-Unsaturated acyI-CoA or z~?.-unsaturated acyI-CoA)

EnoyI-CoA~- H20 @

hydratase[

O

I

II

R--CHmCH~---C~SmCoA (L (+)-13-hydroxy acyI-CoA)

=

O "-,~

-o ~ 0

OH

,-I-

9

" I " A II (r NAD = --Y t - -.- Red L-13-Hyaroxyacy-L;o . " .carriers . . . ~ . . . .v- - - - 1 / 2 02 ' 3ADP, 3P~ dehydrogenase~ . . . . . . . . . + . j , ~ i,resplralory, c n a l n ) ~ ~ . ~ ~ ----N,,~url + N ux. carriers 4H20, 3ATP

(~

cCL

R--C--

@

CH~--- C ~ S m C o A

i

3-Ketoacyl-CoA

TCA cycle;

L ""*CH~---C~SmCoA

thi~

~

respiratory chai n.... 2CO2 + 12ATP

(acetyI-CoA) ~

)

'-

R-- C~S~CoA Fatty acyi-CoA, two carbons shorter

Ketone bodies (in liver)

"

To cytosoi, for cholesterol synthesis

FIGURE 18-3 Fatty acid activation, transport, and/~-oxidation. The shortened fatty acyl-CoA from one cycle is further oxidized in successive passes until it is entirely converted to acetyl-CoA. Odd-chain fatty acids produce one molecule of propionyl-CoA. Ox. = Oxidized; Red. = reduced; respiratory chain = oxidative phosphorylation and electron transport; = high-energy bond; FP = flavoprotein.

and oxidative decarboxylation of the amino acid hypoglycin (Chapter 15). Ingestion of hypoglycin causes severe hypoglycemia due to the inhibition of /~-oxidation and corresponding decrease in ATP synthesis. Gluconeogenesis, which is important in maintaining fasting glucose levels, is dependent on adequate supplies of ATE The action of hypoglycin thus serves to emphasize the importance of /~-oxidation in gluconeogenesis under normal circumstances.

Among the fatty acid oxidation disorders,

medium-chain acyl-CoA dehydrogenase deficiency (MCAD) is the most common and its frequency is similar to that of phenylketonuria. The disorder can be identified by mutant alleles and some key abnormal metabolites. An A --+ G transition mutation occurs at position 985 of MCAD-cDNA in about 90% of cases. This mutation leads to replacement of lysine with glutamate at position 329 (K329E) of the polypeptide.

370

CHAPTER 18


Fatty acids I Activati~ t~ C~ derivatives } ~ ~ . . Carnitine mediated translocation into mitochondria

ISites of Defects]

Fatty acids CoA Mitochondrial electron transfer flavoprotein (ETF) and ETF-Ubiquinone oxidoreductase ~ - - NADH

Turns in } [3-oxidation Spiral

I /

]Main mitochondrial respiratory chain I ATP

Acetyl CoA

[ In Liver

/

Acetoacetate, ~ ~- ~-hydroxybutyrate ; ~ (Ketone bodies)

~ADH2,NADd

Oxidationin extrahepatic tissues (including brain)

|

f

Energy utilized in: \\ Citrate 1. liver for conversion \ \ / of ammonia into \\ / urea and gluconeogenesis N,x" 8CH3COSCoA + AMP + PPi + 7FADH2 + 7NADH + 7H + Each molecule of acetyl-CoA yields 12 ATP (12 x 8 = 96); FADH2 yields 2 ATP (7 x 2 = 14); NADH yields 3ATP (7 x 3 = 21); and two high-energy bonds are consumed ( - 2 ; ATP --+ AMP + PPi). Thus, net ATP production is 129. The energy yield from total combustion of palmitic acid in a bomb calorimeter (Chapter 5) is

372

CHAPTER18 Lipids I: Fatty Acids and Eicosanoids

TABLE 18-2 Reaction

Direct Consequences of the Reaction

Activation reaction First dehydrogenation Hydration Second dehydrogenation Oxidation of acetyl-CoA

Hexanoic acid --->Hexanoyl-CoA Dehydrogenation of acyl-CoA; 2 (FAD ---->FAD H 2) Hydration of a,/]-unsaturated fatty acyl-CoA Dehydrogenation of/3-hydroxy-acyl-CoA; 2 (NAD +--->NAD H + H +) Formation of 3 mol of acetyl-CoA, and their oxidation in the TCA cycle and electron transport system

Moles of ATP Gained or Lost per Mole of Hexanoic Acid

-2 +4 0 +6 +(3 x 12) = +36

Total

- 2 3 8 0 kcal/mol:

C16H3202 Jr- 2302

--+ 16CO2 + 16H20 at 20~

AH ~ = - 2 3 8 0 kcal/mol In biological oxidation, the energy conserved as ATP is about 942 kcal/mol (129 x 7.3). Thus, the percentage of standard free energy of oxidation of palmitic acid conserved as high-energy phosphate is about 40% (942/2380 x 100). Regulation of Fatty Acid Oxidation Regulation of fatty acid oxidation involves diet, cofactors, competing substrates and hormones of fatty acid mobilization. Adipose tissue triacylglycerol lipolysis is one of the major sites of regulation. The other site is CPTI. The latter is inhibited by malonyl-CoA which is involved in fatty acid biosynthesis (discussed later). Thus, fatty acid oxidation and synthesis do not occur simultaneously. Insulin inhibits fatty acid oxidation by blocking lipolysis in adipose tissue, and it stimulates lipogenesis and synthesis of malonyl-CoA. Glucagon stimulates fatty acid oxidation by inhibiting synthesis of acetyl-CoA carboxylase which leads to decreased synthesis of malonyl-CoA. This causes enhanced activity of CPTI, and promotion of fatty acid oxidation. In the fed state, the glucagon/insulin ratio is low, and fatty acid synthesis is promoted in the liver. In the fasting state, the glucagon/insulin ratio is high and mobilization of free fatty acids from adipose tissue and mitochondrial fatty acid oxidation are augmented. Peroxisomal Fatty Acid Oxidation

Peroxisomes have a single membrane and contain a fairly homogeneous, moderately electron-dense matrix. They

ATP = +44

are present in many mammalian cells and are particularly abundant in liver and kidney. A normal hepatocyte contains about a thousand peroxisomes, whose proliferation is inducible by hypolipidemic drugs such as clofibrate (Chapter 20). Peroxisomes contain HzOz-producing oxidases and also HzOz-inactivating catalase. Oxidation of very long-chain, saturated, unbranched fatty acids (C24-C26) appears to take place mainly, if not exclusively, in peroxisomes after the acyl-CoA derivatives are transported across the membrane without involvement of carnitine. Oxidation is mediated by flavoprotein dehydrogenases that yield H202 and acetyl-CoA and terminates with octanoyl-CoA. Octanoyl- and acetyl-CoA are transferred to mitochondria for further oxidation. Phytinic acid monooxygenase, which initiates the catabolism of phytinic acid (a 20-carbon branched chain fatty acid of dietary origin), is probably a peroxisomal enzyme. Peroxisomal oxidation does not yield ATE All the energy produced appears as heat. Three genetic disorders (Zellweger's syndrome, neonatal adrenoleukodystrophy, and childhood adrenoleukodystrophy) exhibit defective formation of peroxisomes (in Zellweger's syndrome no morphologically detectable peroxisomes are present) or deficiency of one or more constituent enzymes. All three disorders are characterized by a marked accumulation of very long chain, saturated, unbranched fatty acids (tetracosanoic and hexacosanoic acids) in liver and central nervous system tissues, severe neurological symptoms, and early death. Peroxisomes contain dihydroxyacetone phosphate acyltransferase and alkyldihydroxyacetone phosphate synthase, which are involved in synthesis of the plasmalogens (Chapter 19). Peroxisomes may also participate in the biosynthesis of bile acids. The conversion of trihydroxycholestanoic acid to cholic acid (Chapter 19) has been localized to peroxisomes.

373


Other Pathways of Fatty Acid Oxidation

Propionyl-CoA Oxidation /~-Oxidation of fatty acids with an odd number of carbon atoms yields propionyl-CoA. Since the concentration of such fatty acids in the diet is small, little propionyl-CoA is produced. Important sources of propionyl-CoA are the catabolism of isoleucine, valine, methionine, and threonine (Chapter 17). Cholesterol side chain oxidation also yields propionyl-CoA. Thus, propionyl-CoA is derived from the catabolism of lipids and proteins. In ruminants, propionate is largely derived from bacterial fermentation in the rumen. Propionyl-CoA is converted to succinyl-CoA, which is oxidized or converted to glucose by way of oxaloacetate and pyruvate (gluconeogenesis; Chapter 15). SuccinylCoA may also form 3-aminolevulinate, a precursor of porphyrin biosynthesis (Chapter 29). Formation of succinylCoA from propionyl-CoA requires three mitochondrial enzymes and two vitamins (Figure 18-5). 1. Propionyl-CoA carboxylase is a tetramer of nonidentical subunits, ot and/~. The native enzyme (M.W. ~540,000) appears to have the structure (ol/~)4. Biotin is bound through an amide linkage to an e-amino group of a lysyl residue in the o~-subunit. Carboxylation is a two-step reaction similar to that of acetyl-CoA carboxylase (see below). The first step requires ATP and Mg 2+ and fixes CO2 with the

Sources

H2~CH3

Valine, isoleucine ] methionine, threonine, cholesterol side chain, odd-chain fatty acids

O---C~SCoA PropionyI-CoA Cq----ATP___~ PropionyI-CoA ADP + P~~---" carboxylase (biotin) COOH

COOH

I HC3--~H

*MethylmalonyI-COAracemase

I I

HC--CH3

O---C--SCoA

O~---C-- SCoA L-MethymalonyI-CoA

formation of an apoenzyme-biotin-CO2 complex. In the second step, the carboxyl group from the biotinyl complex is transferred to propionyl-CoA to form D-methylmalonyl-CoA. 2. Methylmalonyl-CoA racemase converts o-methylmalonyl-CoA to the L-isomer by labilization of an or-hydrogen atom, followed by uptake of a proton from the medium. 3. Methylmalonyl-CoA mutase utilizes 5'-deoxyadenosylcobalamin (Chapter 38) to catalyze intramolecular isomerization by the migration of the -COSCoA group. The only other cobalamindependent reaction in the mammalian system is methylation of homocysteine to methionine (Chapters 17, 27, and 38). Inborn errors of metabolism may be due to propionylCoA carboxylase deficiency, defects in biotin transport or metabolism, methylmalonyl-CoA mutase deficiency, or defects in adenosylcobalamin synthesis. The former two defects result in propionic acidemia, the latter two in methylmalonic acidemia. All cause metabolic acidosis and developmental retardation. Organic acidemias often exhibit hyperammonemia, mimicking ureagenesis disorders, because they inhibit the formation of N-acetylglutamate, an obligatory cofactor for carbamoyl phosphate synthase (Chapter 17). Some of these disorders can be partly corrected by administration of pharmacological doses of the vitamin involved (Chapter 38). Dietary protein restriction is therapeutically useful (since propionate is primarily derived from amino acids). Propionic and methylmalonyl acidemia (and aciduria) results from vitamin B12 deficiency (e.g., pernicious anemia; Chapter 38). c~-Oxidation

u-Oxidation is important in the catabolism of branchedchain fatty acids. The general reaction, catalyzed by a monooxygenase, requires O2, Fe z+, and either ascorbate or tetrahydropteridine. It has been demonstrated in plants and in microsomes from brain and other tissues.

D-Methylmalonyl-CoA

Methylmalonyi-CoA mutase (5"-deoxyadenosyl-cobalamin)

R--CH2--CH2--COOH + Reduced cofactor + 02 Monooxygenase

COOH

I I CH2 I

CH2

O~--~-C - - SCoA

Metabolized in TCA cycle or used in other metabolic pathways (porphyrin synthesis, ketone body utilization, etc.)

OH

I

R--CH2mCH--COOH

+ Oxidized cofactor + H20

~-Hydroxy fatty acid

SuccinyI-CoA

FIGURE 18-5 Metabolism of propionyl-CoA.

This reaction is also a route for the synthesis of hydroxy fatty acids. The o~-hydroxy fatty acid can be further

374

CHAPTER 18

oxidized and decarboxylated to a fatty acid one carbon shorter than the original. Thus, if an odd-chain-length compound is used initially, an even-chain-length acid is produced that can be further oxidized by fl-oxidation. OH I

R--CH2--CHmCOOH

LipidsI: FattyAcidsand Eicosanoids

w-Oxidation co-Oxidation is oxidation of the carbon atom most remote from the carboxyl group in a fatty acid. The basic reaction, catalyzed by a monooxygenase that requires NADPH, O2, and cytochrome P-450, is shown below. It has been observed in liver microsomes and some bacteria. w-oxidation

H3C-(CH2)n-COOH + 02 + NADPH + H + -

HO-CH2-(CH2)n-COOH -t- NADP + -t- H20

DehydrogenaseL ~ ""-,----~ NADH+ H '

Further oxidation of the co-hydroxy acids produces dicarboxylic acids, which can be fl-oxidized from either end.

o II R--CH2mC~COOH

OxidativLe

decarboxylation~~

Oxidation of Mono- and Polyunsaturated Fatty Acids

C02

Oxidation of unsaturated fatty acids requires A3-cis-,A 2trans-enoyl-CoA isomerase and NADPH-dependent 2,4-

o II R~CH2~C--OH In Refsum's disease, an autosomal recessive disorder, the defect is probably in the ot-hydroxylation of phytanic acid. Phytanic acid is a 20-carbon, branched-chain fatty acid derived from the plant alcohol phytol, which is present as an ester in chlorophyll. Thus, its origin in the body is from dietary sources. The oxidation of phytanic acid is shown in Figure 18-6. The clinical characteristics of Refsum's disease include peripheral neuropathy and ataxia, retinitis pigmentosa, and abnormalities of skin and bones. Significant improvement has been observed when patients are kept on low-phytanic acid diets for prolonged periods (e.g., diets that exclude dairy and ruminant fat).

Phytol CH3

, / ~

CH~

/

CH~

~ " { CH,)

dienoyl-CoA reductase, in addition to the enzymes of fioxidation. The enoyl-CoA isomerase produces the substrate for the hydration step. The reductase catalyzes the reduction of a2-trans-,a4-cis-decadienoyl-CoA to A 3trans-decenoyl-CoA. The latter is isomerized to AZ-trans decenoyl isomerase, which is a normal/3-oxidation intermediate. These reactions are illustrated for oxidation of oleic and linoleic acids in Figures 18-7 and 18-8.

18.2 M e t a b o l i s m

~-Oxidation blocked by the methyl group OOH

Phytanicacid(3,7,11,15-tetramethylhexadecanoic acid) CO2-,~ a-Oxidation (block in Refsum's disease)

CH3 ~

~ C O O H

Pristanic acid Degradation by 13-oxidation as shown byarrowmarks

H=~CHmCOO H _ ( 3 H,O/

+ 3CH3CH=COOH+3CI-~COOH

FIGURE 18-6 Oxidation of phytol and phytanicacid.

of K e t o n e B o d i e s

Ketone bodies consist of acetoacetate, D-fl-hydroxybutyrate (D-3-hydroxybutyrate), and acetone. They are

1

H3C

>

NAD '

SECTION18.2 Metabolism of Ketone Bodies

375

9 10 OleyI-CoA

mSC~

~-Oxidations

3 AcetyI-CoA4

o

4

II

3

C--SCoA

As-cis-EnoyI-CoA

(inactive substrate in ~-oxidation)

[ l~.cis-~.trans-

EnoyI-CoAisomerase o

3

II

C~SCoA

A=.trans-EnoyI-CoA (normal substrate in 13-oxidation)

[

~-Oxidation

6 AcetyI-CoA F I G U R E 18-7 Oxidation of oleic acid.

synthesized in liver mitochondria. The overall steps involved in the formation of ketone bodies include the mobilization of fatty acids by lipolysis from adipose tissue triacylglycerol by hormone-sensitive triacylglycerol lipase, plasma fatty acid transport, fatty acid activation, fatty acid transport into mitochondria (with acylcarnitine as an intermediate), and /3-oxidation. The regulatory reactions are those of lipolysis and of acyl-CoA transport across the mitochondrial membrane (CPTI). Synthesis of ketone bodies from acetyl-CoA consists of three steps: formation of acetoacetyl-CoA, formation of acetoacetate, and reduction of acetoacetate to/3hydroxybutyrate. Nonenzymatic decarboxylation of acetoacetate yields acetone, which is eliminated via the lungs. The pathways of formation of ketone bodies are shown in Figure 18-9. The major pathway of production of acetoacetate is from /3-hydroxy-/3-methylglutaryl-CoA (HMG-CoA). Hydrolysis of acetoacetyl-CoA to acetoacetate by acetoacetyl-CoA hydrolase is of minor importance because the enzyme has a high Km for acetoacetylCoA. HMG-CoA is also produced in the cytosol, where it is essential for the synthesis of several isoprenoid compounds and cholesterol (Chapter 19). The reduction of acetoacetyl-CoA to /3-hydroxybutyrate depends on the mitochondrial [NAD +]/[NADH] ratio. Ketone bodies are oxidized primarily in extrahepatic tissues (e.g., skeletal muscle, heart, kidney, intestines, brain) within mitochondria. /3-Hydroxybutyrate is oxidized

to acetoacetate by NAD+-dependent/3-hydroxybutyrate dehydrogenase by reversal of the reaction that occurred during ketogenesis: CH3CH(OH)CH2CO- + NAD + --+ D-/3-Hydroxybutyrate

CH3COCHzCOO- + NADH + H + acetoacetate

Activation of acetoacetate requires transfer of coenzyme A from succinyl-CoA, derived from the TCA cycle, by succinyl-CoA-acetoacetate-CoA transferase (thiophorase): CH2COSCoA

I

CH2COO- CH2COO- CH2COSCoA

+1

----' I

+1

CH2COO-

COCH3

CH2COO- COCH3

SuccinyiCoA

Acetoacetate

Succinate AcetoacetylCoA

The activation occurs at the expense of conversion of succinyl-CoA to succinate in the TCA cycle and formation of GTP (Chapter 13). Acetoacetyl-CoA is cleaved to two molecules of acetyl-CoA by acetyl-CoA acetyltransferase, the same enzyme involved in the synthesis of acetoacetylCoA (Figure 18-9). Acetyl-CoA is oxidized in the TCA cycle. Thus, formation of ketone bodies in the liver and their oxidation in extrahepatic tissues are dictated by the ratio [substrates]/[products].

376

CHAPTER18 Lipids I" Fatty Acids and Eicosanoids O

13

12

10

9

~SCoA

i

3 Acetyl CoAql3-Oxidations

o

7 . . . .6 . . . 4. .

C---SCoA

3

Z~-cis-Ae-cis-DienoyI-CoA (inactive substrate in 13-0xidation) [ A3-cis-A 2-trans-

EnoyI-CoA isomerase O

II

3 7

C~

SCoA

6

~2. trans.A6.cis.DienoyI.CoA (normal substrate in 13-oxidation)

Acetyl CoAq 13-Oxidation

~

,

cis ~ C - - S C o A 5

4

-cis-enoyI-CoA AcyI-CoA dehydrogenase O

~C~SCoA

cis 5

II

4 ...... 2

z~ - trans-A" -cis DienoyI-CoA 2,4-Dienoyl-CoA redu~ase 4

~

C

~

S

C

o

A

A3.trans.EnoyI.CoA

II 0

EnoyI-CoA isomerase

~

C

~

S

C

o

AL trans'En~176

A

0II

13-Oxidations

5 AcetyI-CoA F I G U R E 18-8 Oxidation of linoleic acid.

Physiological and Pathological Aspects of Metabolism of Ketone Bodies Acetoacetate and fl-hydroxybutyrate are products of normal metabolism of fatty acid oxidation and serve as metabolic fuels in extrahepatic tissues. Their level in blood depends on the rates of production and utilization. Oxidation increases as their plasma level increases. Some extrahepatic tissues (e.g., muscle) oxidize them in preference to glucose and fatty acid. Normally, the serum concentration of ketone bodies is less than 0.3 mM/L. The rate of formation of ketone bodies depends on the concentration of fatty acids derived from hydrolysis of

adipose tissue triacylglycerol by hormone-sensitive lipase. Insulin depresses lipolysis and promotes triacylglycerol synthesis and storage, while glucagon has the opposite effects. Thus, insulin is antiketogenic and glucagon is ketogenic (Chapter 22). Uncontrolled insulin-dependent diabetes may result in fatal ketoacidosis (Chapter 39). Although ketonemia and ketonuria are generally assumed to be due to increased production of ketone bodies in the liver, studies with depancreatized rats have shown that ketosis may also arise from their diminished oxidation. Ketosis can occur in starvation, in ethanol abuse, and following exercise, the last because of a switch in blood

377

SECTION18.3 Metabolism of Ethanol AcetyI-CoA

2CH3COSCoA AcetyI-CoA

acetyltransferase

-

(~,, .................CoASH

9CH~COCI-IzCOSCoA

................

9

..(Acet~ %,

.... "(~ HMG-CoA synthase* -- C~COSCoA AcetyI-CoA ~---*CoASH CH3

I

-OOCCH2~CH2COSCoA OH

J3-Hydroxy- 13-methylglutaryI-CoA (H MG-CoA) HMG-CoA

lyase NADH + H+

CH3COSCoA + CHsCOCH=COO Acetoacetate

CO2~ymati

c

NAD+

~-Hydroxy-

, C~CH(OH)CH=COO-

butyrate dehydrogenase

O-.6-Hydroxybutyrate

CH3COCHs

Acetone F I G U R E 18-9 Ketogenesis in the liver. All reactions occur in mitochondria; the rate-controlling reactions (not shown) are release of fatty acids from adipose tissue and uptake of acyl-CoA into mitochondria, in particular, the CPTI reaction (see Figure 18-1). Acetoacetyl-CoA may regulate ketogenesis by inhibiting the transferase and the synthase. *This enzyme is similar to citrate synthase (Chapter 13) which catalyzes an analogous reaction.

flow. During sustained exercise, the blood flow to the liver, intestines, and kidneys is substantially decreased, with a corresponding increase in blood flow to working muscles, so that more fatty acids mobilized from adipose tissue are delivered to the muscle. Thus, the formation of ketone bodies is severely curtailed. But during the postexercise period, with the resumption of normal blood flow to liver, ketone bodies are generated as a result of increased mobilization of fatty acids. Reduced ketone body utilization in the extrahepatic tissues can occur due to deficiency of either succinyl-CoAacetoacetate-CoA transferase or acetyl-CoA acetyltransferase. These patients are susceptible to attacks of ketoacidosis and the presence of persistent ketone bodies in the urine.

18.3 Metabolism of Ethanol Ethanol is consumed widely. Microbial fermentation in the large intestine of humans can produce about 3 g of ethanol per day. Ethanol is rapidly absorbed throughout the gastrointestinal tract or, when inhaled, through the lungs. It is metabolized in the liver by a process having zeroorder kinetics; i.e., the rate of oxidation is constant with time. The amount metabolized per unit time depends on

liver size (or body weight); the average rate in an adult is about 30 mL in 3 hours. The energy content of alcohol is about 7 kcal/g. Ethanol oxidation begins with conversion to acetaldehyde by alcohol dehydrogenase (M.W. ~85,000), a zinccontaining, NAD+-dependent enzyme that is a relatively nonspecific cytoplasmic enzyme with a Km of about 1 mM/L: CH3CH2OH + NAD + --+ CH3CHO 4- NADH + H + Acetaldehyde is rapidly converted to acetate by NAD +dependent aldehyde dehydrogenase: CH3CHO + NAD + + H 2 0 - + CH3COOH + NADH + H + Ethanol is also oxidized by the mixed-function oxidase of smooth endoplasmic reticulum, which requires NADPH, oxygen, and a cytochrome P-450 electron transport system (Chapter 14): CH3CHzOH + NADPH + H + + 202 --+ CH3CHO + 2H202 -Jr-NADP + Many drugs are metabolized by this enzyme, hence the competition between ethanol and other drugs (e.g.,

378

CHAPTER18

barbiturates). Peroxisomal catalase catalyzes the reaction: CH3CH2OH -+- H202 --+ CH3CHO --F 2H20 The Km for this catalase and for the mixed-function oxidase is about 10 mM/L. The extent to which these two enzymes metabolize ethanol is not known. Acetaldehyde is converted to acetate in the liver by NAD+-linked aldehyde dehydrogenases, one in the cytosol (Km = 1 mM/L) and another in mitochondria ( K m - - 0 . 0 1 mM/L):

CH3COO- + NADH + 2H + Disulfiram (tetraethylthiuram disulfide)

H5C~

/ 02H5

N--CwS--S--CmN H5C2

H

S

for acetaldehyde. Thus, the cause of ethanol sensitivity may be impaired rate of removal of acetaldehyde rather than its excessive formation. Individuals who are predisposed to ethanol sensitivity should avoid ethanol intake in any form. Acetate produced from ethanol is converted to acetylCoA by acetyl-CoA synthase in hepatic and extrahepatic tissues. CH3COO- + ATP 4- + CoASH ---+ CH3COSCoA + AMP 2- + PP~-

CH3CHO + NAD + + H20--+

/


ii

S

\

C2H5

causes irreversible inactivation of these aldehyde dehydrogenases by reacting with sulfhydryl groups, with a buildup of acetaldehyde that produces the acetaldehyde syndrome (vasodilation, intense throbbing, pulsating headache, respiratory difficulties, copious vomiting, sweating, thirst, hypotension, vertigo, blurred vision, and confusion). Disulfiram by itself is relatively nontoxic. It is used in the treatment of chronic alcoholism but does not cure it. Disulfiram provides a willing patient with a deterrent to consumption of alcohol. A shorter acting reversible inhibitor of aldehyde dehydrogenase is calcium carbimide, which causes accumulation of acetaldehyde and unpleasant symptoms. Thus, calcium carbimide can also be used as a deterrent to alcohol consumption. Symptoms similar to the disulfiram-ethanol reaction occur in high proportion in certain ethnic groups (e.g., Asians and Native Americans) who are extremely sensitive to ethanol consumption. The ethanol sensitivity in these populations is accompanied by a higher acetaldehyde steady-state concentration in the blood, which may be due to a rapid rate of formation of acetaldehyde by alcohol dehydrogenase or to a decreased rate of its removal by aldehyde dehydrogenase. Both of these dehydrogenases are present in several isozyme forms and exhibit extensive polymorphism among racial groups. An alcohol dehydrogenase variant found in the ethanol-sensitive populations has a relatively higher rate of activity at physiological pH and may account for more rapid oxidation of ethanol to acetaldehyde. However, a more important cause of acetaldehyde accumulation appears to be deficiency of an isozyme of aldehyde dehydrogenase, which has a low Km

Acetyl-CoA is oxidized in the TCA cycle and is used in liver and adipose tissue for biosynthesis of fatty acids and triacylglycerol. Alcoholism affects about 10% of the drinking population and alcohol (ethanol) abuse has been implicated in at least 20% of admissions to general hospitals. This chronic disease exhibits high mortality due to a wide variety of factors. Ethanol produces effects in virtually every organ system. The biochemical effects of ethanol are due to increased production of NADH that decreases the [NAD+]/[NADH] ratio in the cytoplasm of liver cells at least tenfold from the normal value of about 1000. Increased production of lactate and inhibition of gluconeogenesis (Chapter 15) result. The hyperuricemia associated with ethanol consumption has been attributed to accelerated turnover of adenine nucleotides and their catabolism to uric acid (Chapter 27). Alcohol increases hepatic fatty acid and triacylglycerol synthesis and mobilization of fat from adipose tissue, which can lead to fatty liver, hepatitis, and cirrhosis. These effects are complicated by a deficiency of B vitamins and protein. Alcohol increases the plasma level of VLDL and of HDL cholesterol (Chapter 20). Many actions of ethanol may be attributed to a membrane-disordering effect. Changes in membrane fluidity can affect membrane-bound enzymes (e.g., Na +, K +ATPase, adenylate cyclase) and phospholipid architecture. Alcohol also affects several neurotransmitter systems in the brain. These include dopamine (mediates pleasurable effects), v-aminobutyric acid (GABA), glutamate, serotonin, adenosine, norepinephrine, and opioid peptides. Potential drug therapy for alcohol dependence consists of the use of antagonists and agonists of alcohol affected neurotransmitter systems. For example, naltrexone, a/z-opioid antagonist, inhibits alcohol-induced dopamine release, thus minimizing the pleasurable effect of alcohol and reducing the desire to consume alcohol. Another drug, acamprosate, reduces the craving for alcohol presumably by an agonist activity at GABA receptors

SECTION 18.4

Synthesis of Long-Chain Saturated Fatty Acids

and an inhibitory activity at N-methyl-D-aspartate receptors. A selective antagonist of serotonin receptor 5-HT3, ondansetron, reduces alcohol consumption in patients with early onset alcoholism. The 5-HT3 receptors are densely distributed in mesocorticolimbic neuronal terminals and regulate dopamine release. Attenuation of dopamine release reduces alcohol consumption. In chronic alcoholics, heavy drinking and decreased food intake lead to ketoacidosis. Accelerated lipolysis arising from reduced insulin and increased glucagon secretion caused by hypoglycemia leads to ketosis with a high [flhydroxybutyrate]/[acetoacetate] ratio. Treatment requires normalization of fluid and electrolyte balance (Chapter 39) and of glucose level. Administration of glucose provokes insulin release and depresses glucagon release, thus suppressing the stimuli for ketogenesis. The distinction between diabetic ketoacidosis and alcoholic ketoacidosis may be difficult to determine, and in some patients plasma glucose levels may not discriminate between the two entities (although in diabetic ketoacidosis plasma glucose levels are usually high, whereas in alcoholic ketoacidosis these levels may be low, normal, or marginally elevated). Fluid and electrolyte replacement and glucose administration in ketoacidosis are essential regardless of etiology. Ethanol is a teratogen partly because it inhibits embryonic cellular proliferation. Maternal alcoholism causes fetal alcohol syndrome, which is characterized by abnormal function of the central nervous system, microcephaly, cleft palate, and micrognathia.

18.4 Synthesis of Long-Chain Saturated Fatty Acids The reactions of d e n o v o fatty acid biosynthesis are shown in Figure 18-10. They are carried out by two multienzyme systems functioning in sequence. The first is acetyl-CoA carboxylase, which converts acetyl-CoA to malonyl-CoA. The second is fatty acid synthase, which sequentially joins two-carbon units of malonyl-CoA, eventually producing palmitic acid. Both complexes consist of multifunctional subunits. The various catalytic functions can be readily separated in plant cells and prokaryotes, but in yeasts, birds, and mammals, attempts to subdivide catalytic functions lead to loss of activity. Important features of this system are as follows: synthesis takes place in the cytosol (whereas fatty acid oxidation occurs in mitochondria). 2. All carbon atoms are derived from acetyl-CoA (obtained from carbohydrates or amino acids), and 1. D e n o v o

:379

palmitate (C16) is the predominant fatty acid

3.

4.

5.

6.

produced. Fatty acids longer than 16 carbons, those that are unsaturated, and hydroxy fatty acids are obtained by separate processes of chain elongation, desaturation, or ot-hydroxylation, respectively. The committed (rate-controlling) step is the biotin-dependent carboxylation of acetyl-CoA by acetyl-CoA carboxylase. Important allosteric effectors are citrate (positive) and long-chain acyl-CoA derivatives (negative). Although the initial step requires CO2 fixation, CO2 is not incorporated into fatty acids. The labeled carbon in 14C02 (as H14CO3) is not incorporated into the carbons of fatty acids synthesized. Synthesis is initiated by a molecule of acetyl-CoA that functions as a primer. Its two carbons become C15 and C16 of palmitate. The acetyl group is extended by successive addition of the two carbons of malonate originally derived from acetyl-CoA, the unesterified carboxylic acid group being removed as CO2. In mammalian liver and mammary gland, butyryl-CoA is a more active primer than acetyl-CoA. Odd-chain-length fatty acids found in some organisms are synthesized by priming the reaction with propionyl-CoA instead of acetyl-CoA. Release of the finished fatty acid occurs when the chain length reaches C16 by action of thioester hydrolase, which is specific for long-chain acyl-CoA derivatives. A thioester hydrolase of mammary gland is specific for acyl residues of C8, C10, or C12.

The overall reaction for palmitate synthesis from acetylCoA is 8Acetyl-CoA + 14NADPH 4- 14H + 4- 7ATP 4- H20 -+ palmitate 4- 8CoASH 4- 14NADP + 4- 7ADP 4- 7Pi The reducing equivalents of NADPH are derived largely from the pentose phosphate pathway. Acetyl-CoA carboxylase is a biotin-dependent enzyme. It has been purified from microorganisms, yeast, plants, and animals. In animal cells, it exists as an inactive protomer (M.W. ~400,000) and as an active polymer (M.W. 4-8 million). The protomer contains the activity of biotin carboxylase, biotin carboxyl carrier protein (BCCP), transcarboxylase, and a regulatory allosteric site. Each protomer contains a biotinyl group bound in amide linkage to the s-amino group of a lysyl residue. Citrate shifts the equilibrium from inactive protomer to active polymer. The polymeric form appears as long filaments in electron micrographs.

380

CHAPTER18 Lipids I" Fatty Acids and Eicosanoids O

AcetyI-CoA O carboxylase system * Ii ..e"-" ~ - -1:'-- " HOOC-- CH2--C--SCoA ~ S H - ~ ~,~ / Mg" "~ '-'2 ATP ADP MalonylCoA CoASH~ Acetyl + H20 +P~ L , - S H ~ ,transacylase O transacylasel . . . . . reactions S--C--CH3 CysteinyI-SH group [ O "O

II

4"-ehosphopantetheine ~"'.~

CH3Cm SCoA

NO_ Icl.__C

H ;II - - - C ~ S ~ MalonyI-ACP l

II O

13-Ketoacyl synthase

~

~-Ketoacylsynthase~_..C02 0 0 II II ..... H3C--C~CH2---'C~S" v v v v AcetoacyI-ACP (in successive cycles 13-ketoacyI-ACP)

(~ Condensing reaction (~) First reduction

13.KetoacylF NADPH + H+ reductase~,-.NADP+ I-~C~ CHm C H 2 C - - S ~ D(-) 13-HydroxybutryI-ACP (in successive cycles 13-hydroxyacyI-ACP)

(~) Dehydration

13-Hydroxyacyl dehydratase "-H20 H3C--C-- C - - C ~ S ~

I

H

trans-2-ButenyI-ACP (in successivecycles ct.13-unsaturatedACP) o~.13-UnsaturatedJ~ NADPH + H+ acyI-ACP reductase (enoyl reductase) NADP+ O

(~) Second reduction

II

I--~Cm CH;--- CHz----C--S~ ButyryI-ACP (in successive cycles acyI-ACP) Is tMe acyl group16carbons long yet?

If ,es

If no

~

SH

~-~O

13-Ketoacyl synthase Acyl-group transfer

|

O

Thioester hydrolase

II

(~SmC--CH2C~CH3 Substrate for reaction 2 Palmitic acid (Accepts another malonylgroup, reaction 1, and the malonylACP initiatesthe next cycle of two-carbon extension.) F I G U R E 18-10 Synthesis of fatty acid. ACP = Functional unit of acyl-carrier-protein segment of fatty acid synthase. The cysteinyl-SH group of/3-ketoacyl synthase accepts the acetyl group or the acyl group, and its catalytic site, which is adjacent to the -SH group, catalyzes the condensation reaction.

SECTION 18.4

Synthesis of Long-Chain Saturated Fatty Acids

381 O

a(-)

/L.

~--(i~)wi~)--Ou(~)--O--~+OH

:NH 9

~".....

HO

ATP

\

................ ""O- ..""

/

NH

O

II

(CH2),--C-- NH

I

HO~C---OaBicarbonate

BCCP

Mg carboxylase a(-)

R

0

-OuCmN

'~')~

I

2'~Biotin

ADP + H2PO,-+H+

NH

Carboxylated-BCCP

..................................................................9 '" ~.CmCwSCoA

*>...

Lysyl

5+

R

H"lH OIJ 6-

Transcarboxylase

AcetyI-CoA

O

[ I

o H O

_o lel [

//~

II

C--C--SCoA

I

H

NH

NH

+

R

MalonyI-CoA

Regenerated BCCP

FIGURE 18-11 Mechanismof carboxylationof acetyl-CoA.BCCP= Biotincarboxylcarrierprotein; Ad = adenosine.

The mechanism of the carboxylation reaction consists of two half-reactions: biotincarboxylase ATP + HCO~- + BCCP < > Mg2+ ADP + Pi -Jr-B C C P - C O O and BCCP-COO- + acetyl-CoA Z BCCP + malonyl-CoA The overall reaction is ATP 4- HCO~- 4- acetyl-CoA ~ malonyl-CoA 4- ADP 4- Pi The presumed reaction mechanisms are shown in Figure 18-11. Other biotin-dependent enzymes include propionylCoA carboxylase and pyruvate carboxylase (Chapter 15). The latter, like acetyl-CoA carboxylase, is subject to allosteric regulation. Pyruvate carboxylase, a mitochondrial enzyme, is activated by acetyl-CoA and converts pyruvate to oxaloacetate which, in turn, is converted to glucose via

the gluconeogenic pathway or combines with acetyl-CoA to form citrate. Some of the citrate is transported to the cytosol, where it activates the first step of fatty acid synthesis and provides acetyl-CoA as substrate (see below). Other carboxylation reactions use bicarbonate but are dependent on vitamin K, the acceptor being glutamyl residues of glycoprotein clotting factors II, VI, IX, and X and anticlotting factors Protein C and Protein S (Chapter 36). Acetyl-CoA carboxylase is under short- and long-term control. Allosteric modulation functions as a short-term regulator. Positive modulators are citrate and isocitrate; negative modulators are long-chain acyl-CoA derivatives. The binding of citrate increases the activity by polymerization of the protomers, whereas negative modulators favor dissociation of active polymers to inactive monomers. Acetyl-CoA carboxylase is also regulated by covalent modification by phosphorylation, which inhibits activity, and by dephosphorylation, which restores activity. Phosphorylation can occur by action of cAMP-dependent protein kinase through/~-adrenergic agonists and glucagon or by action of calcium-dependent protein kinase through c~-adrenergic agonists. It is not known whether the

382

CHAPTER18 Lipids I" Fatty Acids and Eicosanoids

kinases act at different sites. Insulin suppresses cAMP levels and promotes activity of acetyl-CoA carboxylase. Insulin may also increase the activity of acetyl-CoA carboxylase phosphatase, which is complexed with the carboxylase. This phosphatase also dephosphorylates glycogen synthase, phosphorylase a, and HMG-CoA reductase. Thus, common mediators (e.g., insulin, glucagon, and catecholamines) regulate fatty acid synthesis and carbohydrate metabolism. Long-term regulation of acetyl-CoA carboxylase involves nutritional, hormonal (e.g., insulin, thyroxine), and other factors. In animals on high-carbohydrate diets, fatfree diets, choline deprivation, or vitamin B 12 deprivation, the activity is enhanced. However, fasting, high intake of fat or of polyunsaturated fatty acids, and prolonged biotin deficiency leads to decreased activity. In diabetes, the enzyme activity is low, but insulin administration raises it to normal levels. Fatty acid synthesis is also carried out by a multienzyme complex and leads from acetyl-CoA, malonyl-CoA, and NADPH to palmitic acid. The overall process is as follows: o

o

II

II

/3-Ketoacyl-ACP synthase (condensing enzyme) has an active cysteine-SH group that forms an acetyl-S (or acylS) intermediate (E -- enzyme). O

II CH3C~S~ACP + E~SH O

II CH3C~S~E + ACP~SH

The condensation occurs with release of CO2" O

O

o

II

II

II

CH3C~S~E + HO~C---CH2~C~S~ACP

O

O

II

II

.~

C H 3 C C H 2 C ~ S ~ A C P + CO2 + E ~ S H

/3-Ketoacyl-ACP reductase catalyzes the first reduction reaction: O

O

II

II

C H 3 C ~ C H 2 C ~ S ~ A C P + NADPH * + H +

.GHa.G--SCoA + 7HOOC--CH2G--SGoA + 14NADPH + 14H +

Acetyl-CoA

OH

MalonyI-CoA

I

C;H3(~H2(CH2CH2)6CH2COOH+ 7CO2 + 14NADP+ + 8CoASH + 6H20

0 II

C H 3 ~ C H C H 2 C ~ S - - - A C P + NADP +

Palmitic acid

The fatty acid synthesis complex contains seven catalytic sites. Acetyl transacylase catalyzes the reaction O

II CH3C~SCoA + A C P ~ S H

;

It is stereospecific, and the product formed is D(--)-flhydroxybutyryl-ACP (or D(-)-fl-hydroxyacyl-ACP, in subsequent reactions). fl-Hydroxyacyl-ACP dehydratase catalyzes the reaction: OH

O

I

II

CH3CHCH2C~S~ACP : O

II

O

C H 3 C ~ S ~ A C P + CoASH

II

trans-CH3CH'--CHC~S~ACP + H20

and malonyl transacylase catalyzes the reaction O

O

II

II

HO~C~CH2~C~SCoA

+ ACP - SH --,

O

O

II

II

HO~C--CH2--C~S--ACP

In this stereospecific reaction, the D(--) isomer is converted to a trans-oe,fl-unsaturated acyl-ACP derivative. Enoyl-ACP reductase catalyzes the second reduction with NADPH. O

Ii CH3CH=CH C ~ S ~ A C P

(ACP-SH is the 4'-phosphopantetheine-SH of the acyl carrier domain.) These two priming reactions load a cysteinyl-SH of one subunit with acetyl-CoA, and t h e - S H of the 4'phosphopantetheine arm of the acyl carrier site on the other subunit with malonyl-CoA.

+ NADPH + H + .-

O

II C H 3 C H 2 C H 2 C ~ S ~ A C P + NADP +

The product formed is a saturated acyl-thioester of ACE

SECTION18.4 Synthesis of Long-Chain Saturated Fatty Acids

383

F I G U R E 18-12 Diagram of a fatty acid synthase dimer with its head-to-tail association of the two multifunctional polypeptides. KS =/5-Ketoacyl synthase; MT = malonyl transacylase; AT = acetyl transacylase; DH = dehydratase; ER = enoyl reductase; KR =/3-ketoacyl reductase; ACP = acyl carrier site; TE -- thioesterase. [Reproduced with permission from S. J. Wakil, J. K. Stoops, and V. C. Joshi, Fatty acid synthesis and its regulation. Annu. Rev. Biochem. 52, 537 (1983). 9 1983 by Annual Reviews Inc.]

Thioester hydrolase catalyzes the removal of palmitate from the 4'-phosphopantetheine arm of the acyl carrier site. O

II CH3(CH2)14C--S--ACP + H20

CH3(CH2)14COOH + ACP--SH

The enzyme has an active serine residue and is specific for long-chain acyl derivatives.

Functional Organization of Fatty Acid Synthase In animal cells, fatty acid synthase (FAS) consists of two subunits, each having a molecular weight of 250,000. Evidence from negative-stain electron microscopy indicates that FAS is a linear polypeptide with a series of globular domains representing areas of catalytic activity. Fatty acid synthase mRNA is large enough to code for the ~2300 amino acids required. Animal FAS is the largest

known multifunctional protein. Although each subunit contains all of the catalytic activities required to synthesize palmitate, the monomer lacks fl-ketoacyl synthase activity. The two subunits must be juxtaposed head to tail to bring the cysteine-SH of the/3-ketoacyl synthase of one close to the 4'-phosphopantetheine-SH of the acyl carrier site of the other to obtain a fully functional dimer. Figure 18-12 presents schematically the active FAS homodimer complex. Synthesis may proceed from either end of the active complex. The assembly of rat liver FAS involves three stages: synthesis of the multifunctional polypeptide chains, formation of the dimer, and attachment of a 4'-phosphopantetheine group by an enzyme-catalyzed reaction. This assembly process is influenced by changes in developmental, hormonal, and nutritional states. The FAS complex provides considerable catalytic efficiency, since free intermediates do not accumulate and the individual activities are present in equal amounts. The central role of the acyl carrier domain is to carry acyl groups from one catalytic site to the next. The

384

CHAPTER 18

4'-phosphopantetheine (20 nm long) derived from coenzyme A is bound as a phosphodiester through the hydroxyl group of a specific seryl residue. The acyl intermediates are in thioester linkage with t h e - S H of the prosthetic group, which serves as a swinging arm to carry acyl groups from one catalytic site to the next. The structure of 4'-phosphopantetheine attached to the serine residue of ACP is 0

Seryl ) residue

OH 3

0

O

II ] II II --O--P--OnCH2mCmCH--C--NmCH2mCHzmC--N--CH2--CH2uSH I I I H H OH I OH CH3

Lipids I Fatty Acids and Eicosanoids

mitochondrial membrane to the cytosol, its carbon atoms are transferred by two transport mechanisms. 1. Transport dependent upon carnitine: Carnitine participates in the transport of long-chain acyl-CoA into the mitochondria and plays a similar role in the transport of acetyl-CoA out of mitochondria. However, carnitine acetyl transferases have a minor role in acetyl-CoA transport. 2. Cytosolic generation of acetyl-CoA ("citrate shuttle"): This pathway is shown in Figure 18-13. Citrate synthesized from oxaloacetate and acetyl-CoA is transported to the cytosol via the tricarboxylate anion carrier system and cleaved to yield acetyl-CoA and oxaloacetate.

Sources of N A D P H for Fatty Acid Synthesis

ATP citrate-lyase

Citrate 3- + ATP 4- + CoA The reducing agent for fatty acid synthesis is NADPH. It is mostly supplied by the pentose phosphate pathway (Chapter 15) in the reactions Glucose-6-phosphate

dehydrogenase

Glucose 6-phosphate

f--,~ NADP §

6-Phosphate-gluconolactone

NADPH + H *

and 6-Phosphogluconate

dehydrogenase

6-Phosphogluconate

/%--.,, NADP *

CO2

Ribulose 5-phosphate

NADPH+ H

> (or citrate cleavage enzyme)

acetyl-CoA + oxaloacetate 2- -I- ADP 3- + p2Thus, citrate not only modulates the rate of fatty acid synthesis but also provides carbon atoms for the synthesis. The oxaloacetate formed from pyruvate may eventually be converted (via malate) to glucose by the gluconeogenic pathway. The glucose oxidized via the pentose phosphate pathway augments fatty acid synthesis by providing NADPH. Pyruvate generated from oxaloacetate can enter mitochondria and be converted to oxaloacetate, which is required for the formation of citrate.

Oxidation of malate also provides NADPH: M a late

Malic enzyme

, NADP *

CO2

, Pyruvate

NADPH + H *

These three enzymes, like the fatty acid synthase complex, are located in the cytosol. Active lipogenesis occurs in liver, adipose tissue, and lactating mammary glands, which contain a correspondingly high activity of the pentose phosphate pathway. Thus, lipogenesis is closely linked to carbohydrate oxidation. The rate of lipogenesis is high in humans on carbohydrate-rich diets. Restricted energy intake, a high-fat diet, and insulin deficiency decrease fatty acid synthesis. Source and Transport of Acetyl-CoA Acetyl-CoA is synthesized in mitochondria by a number of reactions: oxidative decarboxylation of pyruvate, catabolism of some amino acids (e.g., phenylalanine, tyrosine, leucine, lysine, and tryptophan; see Chapter 17), and fl-oxidation of fatty acids (see above). Since acetylCoA cannot be transported directly across the inner

Regulation of Fatty Acid Synthase Like acetyl-CoA carboxylase, FAS is under short- and long-term control. The former is due to negative or positive allosteric modulation or to changes in the concentrations of substrate, cofactor, and product. The latter usually consists of changes in enzyme content as a result of protein synthesis or decreased protein degradation. Variation in levels of hormones (e.g., insulin, glucagon, epinephrine, thyroid hormone, and prolactin) and in the nutritional state affect fatty acid synthesis through short- and long-term mechanisms. In the diabetic state, hepatic fatty acid synthesis is severely impaired but is corrected by administration of insulin. The impairment may be due to defects in glucose metabolism that lead to a reduced level of an inducer or increased level of a repressor of transcription of the FAS gene, or both. Glucagon and epinephrine raise intracellular levels of cAME and their inhibitory effect on fatty acid synthesis may be due to phosphorylation or dephosphorylation of acetyl-CoA carboxylase. They also stimulate the action of hormone-sensitive triacylglycerol lipase and raise intracellular levels of long-chain acyl-CoA.

SECTION18.4 Synthesis of Long-Chain Saturated Fatty Acids

385

FIGURE 18-13 Cytoplasmic generationof acetyl-CoAvia citrate transport and related reactions. PPP -- Pentose phosphate pathway; FAS = fatty acid synthase. ~ Negativeallosteric modifier.~ Positiveallosteric modifier.

As a result, acetyl-CoA carboxylase and citrate synthase are inhibited. The stimulatory effect of prolactin is confined to the mammary gland and may involve synthesis of the enzyme.

Mitochondrial fatty acid elongation occurs primarily when the [NADH]/[NAD +] ratio is high (e.g., anaerobiosis, excessive ethanol oxidation).

Linoleic acid, C18:2(9,12) (from the diet)

Fatty Acid Elongation Cytoplasmic fatty acid synthase yields palmitate. Human triacylglycerol contains fatty acids with 18, 20, 22, and 24 carbon atoms, which are synthesized by elongation of palmitate in endoplasmic reticulum or mitochondria. Elongation in the endoplasmic reticulum occurs mainly in liver and involves C]0_16-saturated and Cl8-unsaturated fatty acids by successive addition of two-carbon groups derived from malonyl-CoA (Figures 18-14 and 18-15). The reductant is NADPH. The intermediates, however, are CoA thioesters. The mitochondrial system uses acetyl-CoA, not malonyl-CoA, by a slightly modified reversal of /3oxidation. The substrates are saturated and unsaturated C12, C14, and C16 fatty acids, and the products are C18, C20, C22, and C24 fatty acids. The first reduction step utilizes NADH, and the enzyme is/3-hydroxyacyl dehydrogenase (a/3-oxidation enzyme); the second reduction step utilizes NADPH, and the enzyme is enoyl reductase.

(octadecadienoic acid) Activation

LinoleoyI-CoA (octadecadienoyI-CoA) H++ NAD(P)H + O2--..I ,+ ] C s desaturase NAD(P) + 2H20.~"~ ~'-LinolenoyI-CoA, C18:3(6,9,12) (octadecatrienoyI-CoA) q~ (MalonyI-CoA, Chain-elongationsystem 2NADPH) Dihomo- ~,-linolenoyI-CoA, C20:3(8,11,14) (eicosatrienoyI-CoA) H++ NAD(P)H + 02--.~ NAD(P~'+ 2H20~ !Cs desaturase ArachidonoyI-CoA, C20:4(5,8,11,14) (eicosatetraenoyI-CoA) FIGURE 18-14 Synthesis of arachidonic acid from linoleic acid. The desaturation and chain elongation occur in microsomes.

386

CHAPTER18 Lipids I: FattyAcids and Eicosanoids

(z-Linolenic acid, C18:3(9,12,15)(from the diet) (octadecatrienoic acid) Activation

o~-LinolenoyI-CoA,C18:3(9,12,15)

I

Desaturation

OctadecatetraenoyI-CoA, C18:4(6,9,12,15)

C2qchain-elongation

system

EicosatetraenoyI-CoA, C20:4(8,11,14,17) Desaturation EicosapentaenoyI-CoA, C20:5(5,8,11,14,17)

c,~

Chain-elongation system

DocosapentaenoyI-CoA, C22:5(7,10,13,16,19)

carbon chain length, number of double bonds, and doublebond positions (in parentheses). Thus, palmitoleic acid is designated C16:1(9) and linoleic acid is C18:2(9,12). The location of the double bond is sometimes indicated by A; for example, a 9 signifies that the double bond is between carbon 9 and carbon 10. In both methods, the carboxyl carbon is carbon 1. The double-bond position can also be related to the co-end of the fatty acid molecule (i.e., the methyl carbon farthest from the carboxyl end); oleic acid is an co-9 acid; linoleic acid has double bonds at o)-6 and co-9 carbons. The structures and names of some naturally occurring unsaturated fatty acids are given in Table 18-3. The presence of a double bond in the hydrocarbon chain gives rise to geometrical isomerism, which is due to restricted rotation around carbon-carbon double bonds and is exemplified by fumaric and maleic acids.

HOOC

Desaturation

DocosahexaenoyI-CoA, C22:6(4,7,10,13,16,19)

H

\/

/\

Synthesis of docosahexaenoic acid from c~-linolenic acid. Desaturation and chain elongation are similar to the use described in Figure 18-13.

18.5 Metabolism of Unsaturated Fatty Acids S t r u c t u r e and N o m e n c l a t u r e of U n s a t u r a t e d Fatty Acids Unsaturated fatty acids contain one or more double bonds. A common method for designating fatty acids gives the

Saturated chain

H

I

C~C ~ I H

trans-Monounsaturatedchain (uncommonin naturally occurringfattyacids)

H H

I I

C-~-C~

cis-Monounsaturated chain

(commonin naturally

occurring fatty acids)

/\ H

COOH

\/

c II c

HOOC

FIGURE 18-15

H

c II c

HOOC

H

Maleic acid

Fumaric acid

( cis-form )

( trans-form )

Almost all naturally occurring, unsaturated, long-chain fatty acids exist as the cis isomers, which are less stable than the trans isomers. The cis configuration introduces a bend (of about 30 ~ in the molecule, whereas the trans isomer resembles the extended form of the saturated chain (Figure 18-16). Arachidonic acid with four cis double bonds is a U-shaped molecule. Some cis isomers are biologically active as essential fatty acids. The trans isomers cannot substitute for them but are metabolized like the saturated fatty acids. F u n c t i o n s of U n s a t u r a t e d Fatty Acids

The cis unsaturated fatty acids provide fluidity of triacylglycerol reserves and phospholipid membranes and many serve as precursors of eicosanoids (prostaglandins, prostacyclins, thromboxanes, and leukotrienes). The importance of membrane fluidity and its relationship to the membrane constituent phospholipids are discussed in Chapter 10. Eicosanoids have numerous functions (see below).

18.6 Nonessential Fatty Acids F I G U R E 18-16 Geometry of saturated, trans monounsaturated, and cis monounsaturated chains.

Palmitoleic and oleic acids, the two most abundant monounsaturated fatty acids of animal lipids, can be

TABLE 18-3

Naturally Occurring Unsaturated Fatty Acids

Common Name ~almitoleict Oleicg Vaccenic ~inoleic* a-Linolenic y-Linolenic ~rachidonic* Nervonic

Systematic Name* 9-Hexadecenoic 9-Octadecenoic trans- 11-0ctadecenoic 9,12-Octadecadienoic 9,12,15-Octadecatrienoic 6,9,12-Octadecatrienoic

5,8,11,14-Eicosatetraenoic 15-Tetracosenoic

Molecular Formula Cl6H3,O, ClsH3,02 C, ,H,,O, C,,H,,O, C1,H,,O, C, ,H3,0, C2,H3,02 C,,H,,O,

Structural Formula CH,(CH,),CH =CH(CH,),COOH CH,(CH,),CH =CH(CH,),COOH CH,(CH,),CH =CH(CH,),COOH CH,(CH,),CH =CHCH,CH =CH(CH,),COOH CH3CH2CH=CHCH,CH =CHCH,CH =CH(CH,),COOH CH,(CH,),CH =CHCH,CH =CHCH,CH =CH(CH,),COOH CH3(CH2),--(CH =CH-CH,),-(CH,),-COOH CH3(CH2),CH =CH(CH,),,COOH

*All double bonds are in the cis geometric configuration, except where indicated. h his series is based on the number of carbon atoms present between the terminal methyl group and the nearest double bond; 0 - 3 and w-6 are essential fatty acids. $Most abundant unsaturated fatty acids in animal lipids.

@-series+

Melting Point ("C)

388

CHAPTER18 Lipids I" Fatty Acids and Eicosanoids

synthesized from the respective saturated fatty acid coenzyme-A esters. Desaturase is a monooxygenase system present in endoplasmic reticulum of liver and adipose tissue. The overall reaction for palmitoleic acid synthesis is

a saturated-fat diet. A diet rich in oleic acid has a lower ratio of LDL cholesterol to HDL cholesterol compared to either of the other diets. The metabolism of lipoproteins and their role in atherosclerosis are discussed in Chapter 20.

Palmitoyl-CoA + NAD(P)H + H + + O2 --+ palmitoleyl-CoA + NAD(P) + + 2H20 One molecule of oxygen accepts two pairs of electrons, one from palmitoyl-CoA and the other from NADPH or NADH. The electrons NAD(P)H are transported via cytochrome-b5 reductase to cytochrome b5 (microsomal electron transport; Chapter 14). An enzyme-bound superoxide radical is responsible for the oxidation of acylCoA. Four desaturases specific for introducing cis double bonds at C9, C6, C5, and C4, respectively, are known. If the substrate is saturated, the first double bond introduced is C9. With an unsaturated substrate, other double bonds are introduced between the carboxyl group and the double bond nearest the carboxyl group. Desaturation yields a divinylmethane arrangement of double bonds (--CH=CH--CHz--CH=CH--). Usually desaturation alternates with chain elongation. Desaturation is inhibited by fasting and diabetes. The oxidation of unsaturated fatty acids occurs in mitochondria.

18.7

trans-Fatty Acids

trans-Fatty acid metabolism is similar to that of saturated fatty acids. During the partial dehydrogenation of vegetable oils (e.g., in the manufacture of margarine), the cis fatty acids are isomerized to trans-fatty acid forms. The "hydrogenated" margarines contain 15-40% of trans-fatty acids. The hypercholesterolemic effect of trans-fatty acids may be due to impairment of the first step in the formation of bile acids from cholesterol. Since the steadystate level of cholesterol depends on its conversion to bile acid (Chapter 19), any perturbation in this process affects cholesterol levels. Both metabolic and epidemiological studies have shown that the consumption of transfatty acids increases the risk of coronary heart disease. This risk appears to be even higher when compared on a per-gram basis with saturated fatty acids. The adverse effects of trans-fatty acids are attributed to the elevation of atherogenic low-density lipoprotein (LDL) cholesterol and a decrease in the antiatherogenic (or cardioprotective) high-density lipoprotein (HDL) cholesterol level. Thus, the ratio of LDL cholesterol to HDL cholesterol is significantly higher with the trans-fatty acid diet compared to

18.8 Essential Fatty Acids Polyunsaturated fatty acids not synthesized in the body but required for normal metabolism are essential fatty acids (EFAs). EFAs are linoleic acid, linolenic acids (or and X), and arachidonic acid. All contain at least one double bond located beyond C-9 or within the terminal seven carbon atoms (Table 18-3). ~-Carbon '~1 2 3 4 5 6 7 8 H3C--CH2--CH2--CH2--CH2mCH2--CH2--CH2m(CH2)n--COOH Introduction of double bonds does not occur in humans

Double bonds can be introduced in this region

A double bond within the terminal seven carbon atoms can be present at ~o-3 or ~-6. y-Linolenic acid is an oJ-6 EFA and ot-linolenic acid an o>-3 EFA. Other co-3 EFA are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DCHA), both abundant in edible fish tissues. Vegetable oils are rich in oJ-6 EFA (Table 18-4). Plants contain otlinolenic acid, which can be converted in the body to EPA and DCHA, but it is found within chloroplast membranes and not in seed oils; hence, it may not be available in significant quantities in the diet. The o~-3 and o>-6 EFA have different metabolic effects (see below). Particularly rich sources of EPA are fishes (e.g., salmon, mackerel, blue fish, herring, menhaden) that live in deep, cold waters. These fishes have fat in their muscles and their skin. In contrast, codfish, which have a similar habitat, store fat in liver rather than muscle. Thus, cod liver oil is a good source of EPA, but it also contains high amounts of vitamins A and D, which can be toxic in large quantities (Chapters 38 and 37, respectively). Shellfish also contain EPA. Plankton are the ultimate source of EPA. Linoleic acid can be converted in mammalian liver to y-linolenic acid and arachidonic acid by the microsomal desaturation and chain elongation process (Figure 18-14). Thus, the requirement for arachidonic acid may be dispensed with when the diet contains adequate amounts of linoleic acid. Similarly, ot-linolenic acid is converted by desaturation and chain elongation to EPA and DCHA (Figure 18-15).

SECTION 18.9

Metabolism of Eicosanoids

Deficiency of Essential Fatty Acids The clinical manifestations of EFA deficiency in humans closely resemble those seen in animals. They include dry, scaly skin, usually erythematous eruptions (generalized or localized and affecting the trunk, legs, and intertriginous areas), diffuse hair loss (seen frequently in infants), poor wound healing, failure of growth, and increased metabolic rate. Abnormalities in ECG patterns may be due to membrane alterations, which may also account for structural and functional abnormalities observed in mitochondria. Surgical patients maintained on glucose-amino acid solutions for prolonged periods develop EFA deficiency, manifested as anemia, thrombocytopenia, hair loss and sparse hair growth, increased capillary permeability, dry scaly skin, desquamating dermatitis, and a shift in the oxygendissociation curve of hemoglobin to the left. Oral or intravenous administration of linoleic acid is necessary to correct these problems. Fat emulsions containing linoleic acid are commercially available for intravenous use. An adult requires 10 g of linoleic acid per day. The recommended dietary allowance for EFA is 1-2% of the total energy intake. EFA deficiency can also occur in infants with highly restricted diets (e.g., primarily skim milk intake), in patients receiving total parenteral hyperalimentation without supplements of unsaturated lipids, and in those with severe malabsorptive defects. In EFA deficiency, oleic acid can be dehydrogenated to yield polyunsaturated fatty acids (PUFAs) that are nonessential and do not substitute for the essential fatty acids. One such PUFA is 5,8,11-eicosatrienoic acid, which occurs in significant amounts in heart, liver, adipose tissue, and erythrocytes of animals fed diets deficient in EFAs but decreases after supplementation with linoleic or linolenic acids. Its appearance in tissues and plasma has been used in the assessment of EFA deficiency. Most vegetable oils are relatively rich in EFAs (coconut oil is an exception), low in saturated fatty acids, and lack cholesterol. Animal fats (except those in fish), on the other hand, are generally low in EFAs, high in saturated fats, and contain cholesterol. The EFA content of body and milk fat of ruminants can be increased by the feeding of EFA encased in formalin-treated casein. The EFA is released at the site of absorption by dissolution of the capsules. These dietary manipulations in ruminants are accompanied by an increase in carcass EFA, a decrease in saturated fatty acids, and no change or an increase in cholesterol content. Table 18-4 summarizes the fatty acid composition of some fats of animal and plant origin. The recommended daily diet does not exceed 30-35% of the total energy intake as fat (current average consumption in North America is

389

40-45%), with equal amounts of saturated, monounsaturated, and polyunsaturated fats, and a cholesterol intake of no more than 300 mg/day (current average consumption in North America is about 600 mg/day). Substitution of co-6 polyunsaturated for saturated fats in the diet lowers plasma cholesterol levels through reduction in levels of VLDL and LDL. Diets rich in polyunsaturated fats lead to higher biliary excretion of sterols, although this effect may not be directly related to reduced levels of plasma lipoproteins. Diets low in EFA (linoleic acid) have been associated with high rates of coronary heart disease. A significantly lower proportion of EFA in the adipose tissue of people dying from coronary heart disease has been reported, and an inverse relationship has been found between the percentage composition of EFA in serum cholesteryl esters and mortality rates from coronary heat disease. Consumption of o9-3 polyunsaturated fatty acids markedly decreases plasma triacylglycerol and, to a lesser extent, cholesterol levels in some hyperlipoproteinemic patients (Chapter 20). Consumption of fish-oil fatty acids decreases the biosynthesis of fatty acids and of VLDL by the liver and also decrease the platelet and monocyte function. These effects of o9-3 fatty acids appear to prevent or delay atherogenesis. Low death rates from coronary heart disease are found among populations with high intake of fish (e.g., Greenland Eskimos, people of fishing villages of Japan, people of Okinawa). Metabolic and functional differences exist between o9-3 and o9-6 fatty acids. They have opposing physiological effects and their balance in the diet is important for homeostasis and normal development.

18.9 Metabolism of Eicosanoids The eicosanoids--prostaglandins (PGs), thromboxanes (TXs), prostacyclins (PGIs), and leukotrienes (LTs)Iare derived from essential fatty acids and act similarly to hormones (Chapter 30). However, they are synthesized in almost all tissues (unlike hormones, which are synthesized in selected tissues) and are not stored to any significant extent; their physiological effects on tissues occur near sites of synthesis rather than at a distance. They function as paracrine messengers and are sometimes referred to as autacoids. The four groups of eicosanoids are derived, respectively, from a 20-carbon fatty acid with three, four, or five double bonds: 8,11,14-eicosatrienoic acid (dihomov-linolenic acid), 5,8,11,14-eicosatetraenoic acid (arachidonic acid), and 5,8,11,14,17-eicosapentaenoic acid (Figure 18-17). In humans, the most abundant precursor is arachidonic acid. Secretion of eicosanoids in response

390

CHAPTER18 Lipids I: Fatty Acids and Eicosanoids

T A B L E 18-4

Fatty Acid Composition of Some Fats of Animal and Plant Origin

Saturated*

C14, C16, C18 (predominantly C16 and C18)

Polyunsaturated*, mostly as linoleic acid (C18:2)

Monounsaturated* C16:1; C18:1 (predominantly C18:1)

Animal Fats Butter Beef Chicken Pork Fish (salmon and tuna)

59 54 40 40 28

37 44 38 46 29

4 2 22 14 23 + 20 t

11 14 14 15 20 29 92 53 16 12

11 26 43 27 45 19 6 37 69 18

78 60 43 58 35 52 2 10 15 70

Vegetable Oils Safflower Corn Sesame Soybean Peanut Cottonseed Coconut Palm Olive Sunflower

*All values expressed as weight percentages of total fatty acids. tOther polyenoic acids, e.g., eicosapentaenoic acid, docosahexaenoic acid.

~

COOH

< . . . t PGE,, PGF,, TXA, LTA~, LTC3, LTD3

8,11,14-Eicosatrienoic acid (dihomo- ~,-linolenic acid)

COOH

PDG2, PGE~, PGF2, PGI2, TXA2 90%) of bile acids in the intestines is reabsorbed by an active transport system into the portal circulation at the distal ileum and transported bound to albumin. They are taken up by the liver, promptly reconjugated with taurine and glycine, and resecreted into bile. Both ileal absorption and hepatic uptake of bile acids may be mediated by Na+-dependent (carrier) transport mechanisms. This cyclic transport of bile acids from intestine to liver and back to the intestine is known as the enterohepatic circulation (Figure 19-20). During a single passage of portal blood through the liver, about 90% of the bile acids are extracted. The bile acid pool size in the enterohepatic circulation is 2-4 g and circulates about twice during digestion

4~6

CHAPTER19 Lipids I1" Phospholipids, Glycosphingolipids, and Cholesterol LIVER

Cholesterol

Rate-limiting step I 7or-Hydroxycholesterol CholyI-CoA + ChenodeoxycholyI-CoA (Synthesis: 0.5 g/d)

t-

k,

O

g

o

Taur,ne-

-O,yc, ne

CoASH,~'[

['--* CoASH

Taurocholic acid

t'-

+ Glyco-CDCA

",,,, ...

? o 0

O

orB-

Giycocholic acid

+ Tauro-CDCA

m

.2

,/

t,D

~? t'I)

Temporary storage of conjugated bile acids

GALLBLADDER

/

\,,..

Tauro- and glyco-

c 0

"1o . .

Tauro- and glyco-

cholic acid CDCA ~GDeconjugation by ~.q acterial enzymes]]

eo m

t,D

8 o~

o_ "10 O

O

lycine, taurine*----"~

Cholic acid

CDCA H7-Dehydr0xylati0n ~ by action of bacterial enzymes

Deoxycholic Lithocholic acid ,,(Secondary bile acids)*, acid

k,

INTESTINE

)

t Fecal excretion (~5-10%, or "-0.5 9 g/d, mostly secondary bile acids)

FIGURE 19-20 Formation, enterohepaticcirculation, and disposition of the bile acids. CDCA-- Chenodeoxycholic acid. of each meal. The amount of bile acids lost in feces is about 0.8-1 g/day and consists mostly of secondary bile acids (particularly lithocholic acid, the least soluble of the bile acids). The loss is made up by synthesis of an equal amount in the liver. Bile Acid Metabolism and Clinical Medicine In liver disorders, serum levels of bile acids are elevated, and their measurement is a sensitive indicator of liver disease. Bile acids are not normally found in urine owing to efficient uptake by the liver and excretion into the intestines. In hepatocellular disease and obstructive jaundice, however, their urinary excretion increases. Lithocholic acid is toxic and can cause hemolysis and fever. Effects associated with hyperbile acidemia include pruritus, steatorrhea, hemolytic anemia, and further liver injury. The main cause of cholelithiasis (presence or formation of gallstones) is precipitation of cholesterol in bile. Elevated biliary concentrations of bile pigments (bilirubin

glucuronides) can also lead to formation of concretions known as pigment stones (Chapter 29). Since biliary cholesterol is solubilized by bile acids and lecithin, an excess of cholesterol along with decreased amounts of bile acids and lecithin, cause bile to become supersaturated with cholesterol with the risk of forming cholesterol stones. The limits of solubility of cholesterol in the presence of bile salts and lecithin has been established by using a ternary phase diagram. If the mole ratio of bile salts and phospholipids to cholesterol is less than 10:1, the bile is considered to be lithogenic (stone forming), but this ratio is not absolute. The pattern of food intake in Western nations, which consists of excess fat, cholesterol, and an interval of 12-14 hours between the evening meal and breakfast, leads to a fasting gallbladder bile that is saturated or supersaturated with cholesterol. In individuals who have gallstones, the problems associated with production of lithogenic bile appear to reside in the liver and not in the gallbladder. When the activities of HMG-CoA reductase and 7o~-hydroxylase were measured in the liver of patients who had gallstones and compared with those

427


of controls, cholesterogenesis was found to be increased in the patients with gallstones, and bile acid synthesis was reduced. Under these conditions, the ratio of cholesterol to bile acids secreted by the liver is increased. The gallbladder provides an environment where concentration of the lithogenic bile triggers formation of gallstones. A genetic predisposition to formation of cholesterol gallstones is common in certain groups (e.g., 70% of Native American women older than 30 years have cholesterol gallstones). In general, the incidence of gallstones in women is three times higher than in men, and this proportion increases with age. Obesity and possibly multiparity are also associated with formation of gallstones. Any disorder of the ileum (e.g., Crohn's disease) or its resection can result in gallstones owing to impaired absorption of bile acids and depletion of the bile acid pool. Agents that increase biliary secretion of cholesterol (e.g., estrogen, oral contraceptives, and clofibrate) and those which prevent bile acid reabsorption in the intestines (e.g., cholestyramine) are also predisposing factors. Cholelithiasis is frequently treated by surgical removal of the gallbladder (cholecystectomy). Oral treatment with ursodeoxycholic (ursodiol) and chenodeoxycholic (chenodiol) acids has effectively solubilized gallstones in a number of patients. These bile acids apparently reduce HMG-CoA reductase activity, thereby lowering cholesterol levels while enriching the bile acid pool. The increased bile acid to cholesterol ratio in the bile apparently aids in solubilizing the stones already present. However, the effect of chenodiol is transient, and therapy may have to be long-term. In addition, the treatment appears to be promising only in patients who have radiolucent gallstones and functioning gallbladders; and because it raises serum transaminase levels, the significance of which is not clear, the measurement of other liver function tests may be necessary. However, ursodiol has few side effects and is effective at a lower dosage. Ursodiol is not a human metabolite and differs from chenodiol only in the orientation of a hydroxyl group at C7 (Figure 19-17). Chenodiol is in the a-orientation, whereas ursodiol is in the /3-orientation. Two other nonsurgical treatments undergoing clinical evaluation are extracorporeal shock-wave lithotripsy to fragment gallstones and direct infusion into the gallbladder with the solvent methyl tert-butyl ether to dissolve cholesterol stones. Oral

treatment with ursodiol in conjunction with extracorporeal shock-wave lithotripsy is more effective than lithotripsy alone.

Supplemental Readings and References S-H. Bae and Y-K. Paik: Cholesterol biosynthesis from lanosterol: Development of a novel assay method and characterization of rat liver microsomal lanosterol A24-reductase. Biochemical Journal 326, 609 (1997). C. S. Baker, T. W. Evans, B. J. Randle, and P. L. Haslam: Damage to surfactant-specific protein in acute respiratory distress syndrome. Lancet 353, 1232 (1999). N. Braverman, P. Lin, E E Moebius, et al.: Mutations in the gene encoding 3fl-hydroxysteroid-AS,A7-isomerase cause X-linked dominant ConradiHtinermann syndrome. Nature Genetics 22, 291 (1999). J. Charrow, J. A. Esplin, T. J. Gribble, et al.: Gaucher disease: Recommendations on diagnosis, evaluation, and monitoring. Archives of Internal Medicine 158, 1754 (1998). S. Y. Cho, J-H. Kim, and Y-K. Paik: Cholesterol biosynthesis from lanosterol: Differential inhibition of sterol AS-isomerase and other lanosterolconverting enzymes by tamoxifen. Molecules and Cells 8, 233 (1998). C. Cunniff, L. E. Kratz, A. Mosher, et al.: Clinical and biochemical spectrum of patients with RSH/Smith-Lemli-Opitz syndrome and abnormal cholesterol metabolism. American Journal of Medical Genetics 68, 263 (1997). J. M. J. Derry, E. Gormally, G. D. Means, et al.: Mutations in a AS-A 7 sterol isomerase in the tattered mouse and X-linked dominant chondrodysplasia punctata. Nature Genetics 22, 286 (1999). S. B. Dubin: The laboratory assessment of fetal lung maturity. American Journal of Clinical Pathology 97, 836 (1992). R. V. Farese, Jr. and J. Herz: Cholesterol metabolism and embryogenesis. Trends in Genetics 14, 115 (1998). J. L. Goldstein and M. S. Brown: Regulation of the mevalonate pathway. Nature 343, 425 (1990). H. P. Haagsman and L. M. G. van Golde: Synthesis and assembly of lung surfactant. Annual Review of Physiology 53, 441 (1991). C. E. M. Hollack, E. P. M. Corssmit, J. M. E G. Aerts, et al.: Differential effects of enzyme supplementation therapy on manifestations of type I Gaucher disease. American Journal of Medicine 103, 185 (1997). R. I. Kelley: RSH/Smith-Lemli-Opitz syndrome: Mutations and metabolic morphogenesis. American Journal of Human Genetics 63, 322 (1998). J. M. Klein, M. W. Thompson, J. M. Snyder, et al.: Transient surfactant protein B deficiency in a term infant with severe respiratory failure. Journal of Pediatrics 132, 244 (1998). M. C. Maberry: Methods to diagnose fetal lung maturity. Seminars in Perinatology 17, 241 (1993). G. Paumgartner and T. Sauerbruch: Gallstones: Pathogenesis. Lancet 338, 1117 (1991). J. A. Porter, K. E. Young, and P. A. Beachy: Cholesterol modification of hedgehog signaling proteins in animal development. Science 274, 255 (1996). M. Trauner, P. J. Meier, and J. L. Boyer: Molecular pathogenesis of cholestasis. Mechanisms of Disease 339, 1217 (1998). K. W. A. Wirtz: Phospholipid transfer proteins revisited. Biochemical Journal 324, 353 (1997).


CHAPTER

20

Lipids 111: Plasma Lipoproteins

Lipids, by virtue of their immiscibility with aqueous solutions, depend on protein carriers for transport in the bloodstream and extracellular fluids. Fat-soluble vitamins and free fatty acids are transported as noncovalent complexes. Vitamin A is carried by retinol-binding protein and free fatty acids on plasma albumin. However, the bulk of the body's lipid transport occurs in elaborate molecular complexes called lipoproteins.

20.1 Structure and Composition Lipoproteins are often called pseudomicellar because their outer shell is in part composed of amphipathic phospholipid molecules. Unlike simple micelles, lipoproteins contain apolipoproteins, or apoproteins, in their outer shell and a hydrophobic core of triacylglycerol and cholesteryl esters. Unesterified, or free, cholesterol, which contains a polar group, can be found as a surface component and in the region between the core and surface (Figure 20-1). Most lipoproteins are spherical. However, newly secreted high-density lipoproteins (HDLs) from the liver or intestine are discoidal and require the action of lecithin-cholesterol acyltransferase (LCAT) in plasma to expand their core of neutral lipid and become spherical. The hydrophobic core of the low-density lipoprotein (LDL) molecule may contain two concentric layers: one of triacylglycerol and another of cholesteryl ester.

The apoproteins are distinct physically, chemically, and immunochemically and have important roles in lipid transport and metabolism (Table 20-1). In keeping with their individual metabolic functions, they have specific structural domains. Amino acid substitutions or deletions in critical domains result in functional abnormalities. The apoproteins share a common structure in the form of an amphipathic helix, in which the amino acid residues have hydrophobic side chains on one face of the helix and hydrophilic polar residues on the other. The hydrophilic face is believed to interact with the polar head groups of the phospholipids, while the hydrophobic residues interact with their fatty acid portions. The laws of mass action govern the interactions of lipids and most apoproteins in lipoproteins, so that as the affinities between surface components change during lipoprotein metabolism, apoproteins may dissociate from one particle and bind to another. In fact, all of the apoproteins, with the possible exception of apoprotein B (apo B), can change their lipoprotein associations. The reason for the unique behavior of apo B remains a mystery. On the basis of their principal transport function, lipoproteins may be divided into two classes according to the composition of their major core lipids. The principal triacylglycerol carriers are chylomicrons and very-low-density lipoproteins (VLDLs), whereas most cholesterol transport occurs via LDLs and HDLs.

429

430

CHAPTER 20

Lipids II1: Plasma Lipoproteins

F I G U R E 20-1 Generalized structure of a lipoprotein molecule showing the distribution of polar components in an outer shell composed of free cholesterol, phospholipids, and amphipathic proteins and in an inner core composed of neutral triacylglycerols and cholesteryl esters. Phospholipids are oriented with polar head groups toward the aqueous environment and hydrophobic tails toward the neutral core, analogous to their positioning in the outer leaflet of the typical cell membrane.

These four major groups of plasma lipoproteins can be separated and characterized by electrophoresis and ultracentrifugation (Table 20-2). Each group is heterogeneous and can be subdivided on the basis of variation in apoprotein and lipid compositions. The groups share several apoprotein components, e.g., apo B occurs in chylomicrons, LDLs, and VLDLs. Some apoproteins belong to families of polypeptides, one or another of which may predominate in a particular lipoprotein group. For example, apo B, when formed in chylomicrons synthesized in intestinal mucosa, is apo B-48, while that in VLDL from the liver is B-100. The designations B-48 and B-100 reflect the relative molecular masses of these proteins, B-48 being 48% of the mass of B-100. Intestinal apo B contains 2152 of the 4563 amino acids that make up the hepatic form. Oligonucleotide hybridization studies have shown the intestinal and hepatic genes to be identical. The shortened intestinal form of apo B is produced by a single nucleotide substitution of uracil for cytosine at position 6457 in the nucleotide sequence of the apo B mRNA. This changes the codon CAA (glutamine) to the termination

codon UAA. The tissue-specific apo B mRNA editing, which consists of site-specific deamination of cytosine to uracil, is mediated by a multicomponent cytidine deaminase. This unique form of mRNA editing eliminates the carboxy terminal portion of apo B to form the B-48 of chylomicrons. Since the carboxy terminal portion of the apo B sequence contains the apo B-binding domain, this deletion ensures a distinct metabolic routing of the chylomicron particle. In a tube of plasma or serum collected from a nonfasting individual and allowed to stand overnight, chylomicrons collect at the top surface in a milky layer because of their low density (d < 0.95). If a pure sample of chylomicron is required, it is better to obtain it from lymph or to separate VLDLs from chylomicrons in plasma by chromatography. Sequential ultracentrifugation is used to obtain various lipoprotein fractions from a single sample. Ultracentrifugation of fasting serum or plasma at its own density (1.006) for 18 hours at 100,000 x g will float the VLDL fraction (d < 1.006). After removal of the VLDL, the density of the remaining serum is raised to 1.063 and

SECTION 20.1

Structure and Composition

431

TABLE 20-1

Properties of Human Apolipoproteins* Plasma Concentration (mg/dL) (approximate)

Molecular Weight

Major Density Class

A-I

100-150

29,016

HDL

A-II

30-50

17,414

HDL

A-IV B-100

15 80-100

44,465 512,723

Chylomicron VLDL, LDL

B-48 C-I C-II C-Ill

95%) of this acid will be dissociated in the physiological range of pH, imposing an almost equimolar H + load on the cell's buffer systems. Intracellular pH in muscle fibers is about 7.0, and decreases 0.4-0.8 pH unit during intense exercise. Lactate efflux from muscle occurs by simple diffusion of undissociated lactic acid and by carrier-mediated lactateproton cotransport. The latter probably accounts for 50-90% of the lactate efflux, depending on fiber type and pH. It was thought that fast glycolytic fibers had an abundant lactate transporter with a low Km, providing rapid efflux to support continued glycolysis, while slow oxidative fibers had a less abundant transporter or one with higher Km, or both, so that lactate would be retained as a redox buffer. Retained lactate maintains an NAD/NADH ratio that stimulates oxidation, and ensures subsequent oxidation or gluconeogenesis in the muscle. There are three or more muscle lactate transporters, one of which has been sequenced in rat muscle and found to have 494 amino acids (M.W. 53,000) and 86% sequence identity with the corresponding protein from human erythrocytes. However, Km for lactate has not been found to differ significantly between fiber types, being around 30 mM. Moreover, the transport capacity is almost twice as great in SO fibers as in FG. The high Km implies that most of the lactate produced will be retained during exercise regardless of fiber type, and the relative amounts of carrier imply that lactate efflux from FG fibers is greater than from SO fibers only because the rate of production is usually much higher, creating greater gradient for efflux. It follows that lactate efflux from FG fibers is not adequate to prevent eventual inhibition at the G3PD step and pronounced acidification of the fibers. The greater lactate transport capacity of oxidative fibers also enhances lactate entry into these fibers when blood lactate rises during moderate-intensity work in which the SO fibers produce little lactate. The fate of the lactate produced is primarily oxidative (55-70%). Lactate released from muscle enters exclusively oxidative tissues in which lactate concentration is low (like heart, diaphragm and brain), is converted back to pyruvate, and is oxidized. Retained lactate is oxidized to generate ATP needed to replenish phosphocreatine stores following exercise and to restore normal distribution of Ca 2+, Na +, and K +. About 15% is used in gluconeogenesis and subsequent glycogenesis. The balance is converted to alanine, glutamate, or other substances. Utilizing amino acids as fuel requires eventual elimination of an equimolar amount of ammonia, which may also require eliminating more water. It also requires a longer recovery time, since replacement of the hydrolyzed proteins may be slow, and requires higher dietary protein intake. In most circumstances, protein is the macronutrient in least

SECTION21.3 Energy Supply in Muscle

supply. For all of these reasons, protein metabolism is not a preferred pathway for energy supply. Nevertheless, protein metabolism contributes 2-3% of the energy requirement in exercise of a few minutes duration and rises to as much as 12% after several hours of physical work. Replenishment of TCA cycle intermediates such as oe-ketoglutarate (derived from glutamate) or oxaloacetate (from aspartate or asparagine) is probably important to offset the loss of TCA cycle intermediates from mitochondria over time. This role of protein catabolism in supporting glucose and lipid oxidation, called anaplerosis, may be more important than its direct contribution to energy supply. Nitrogen transport from muscle to liver seems to be accounted for mainly by release from muscle of alanine synthesized de novo by amination of pyruvate. Liver production of urea, however, is thought to be driven mainly by plasma glutamine and ammonia concentrations. Muscle release of glutamine does not increase during exercise (although ammonia efflux increases), nor does the plasma glutamine concentration, and there is no significant increase in urea production during exercise of moderate intensity and duration. Most alanine reaching the liver in this circumstance is deaminated and used in gluconeogenesis, but the immediate disposition of the amino nitrogen is still unclear. Special significance is often attributed to branchedchain amino acids (BCAAs: leucine, isoleucine, and valine) in the context of muscular performance. Increased BCAA uptake and oxidation by muscle does occur during exercise, and it has been claimed that intramuscular BCAA concentration exerts a regulatory influence on rate of protein synthesis. It is also claimed that, since BCAA and tryptophan (TRP) compete for transport across the blood-brain barrier, decreasing plasma BCAA concentration leads to increased brain TRP uptake and serotonin synthesis and a host of putative effects thereof. Although these hypotheses are attractive and a segment of the supplement industry relies on them, available studies show that BCAA supplementation in humans neither offers any performance benefit over other energy supplements nor alters mood or perception during or after exercise. The purine nucleotide cycle also is involved in muscle energy production. During intense stimulation, or when O2 supply is limited, the high-energy bond of ADP is used to synthesize ATP via the myokinase reaction (Figure 21-12). The resulting AMP can dephosphorylate to adenosine, which diffuses out of the cell. Conversion of AMP to IMP via adenylate deaminase and then to adenylosuccinate helps sustain the myokinase reaction, especially in FG fibers, by reducing accumulation of AMR It may also reduce the loss of adenosine from the cell, since nucleosides permeate cell membranes while nucleotides do not.

471

2ADP

Adenylate kinase (myokinase)

.

ATP

AI~IP Fumarate.,, , / ~ ~-~O Adenylate + Adenylosuccinase/Aeamin~tee~ NH' /Adenylosuccinatek, _/ _ s9nthetase ", Adenylosuccinate ,. ~ ~ ~ . IMP

Adenosine (crosses the cell membrane)

GDP + P~ GTP Aspartate F I G U R E 21-12 Role of purine nucleotides in muscle energy metabolism. The conversion of AMP to IMP prevents loss of adenosine from the cell.

Lack of muscle adenylate deaminase has been found in some muscle disorders in which other abnormalities could not be identified. Failure to produce ammonia during intense effort may be diagnostic. In the heart, AMP accumulation is typically due to ischemia, and the adenosine released as a result is a potent coronary vasodilator. Accordingly, myocardium has less adenylate deaminase than skeletal muscle. However, large losses of adenosine from myocardium are dangerous because they lead to a decrease in ATP concentration that does not respond to dilation, thrombolytic, or oxygen therapy. Lack of adenosine deaminase in lymphoid tissue causes a severe immunodeficiency (Chapter 27).

Phosphocreatine Shuttle Energy transfer between mitochondria and the myofibrillar ATPases is mediated by phosphocreatine (Chapter 17). The phosphocreatine shuttle is illustrated schematically in Figure 21-13. Synthesis of ATP in mitochondria is closely coupled to that of phosphocreatine. Since the reaction ATP + C --> ADP + P ~ C is energetically favored when the ATP/ADP ratio is high, phosphocreatine is rapidly formed from ATP releasing ADP, which stimulates mitochondrial respiration. Phosphocreatine diffuses from the mitochondria to the various sites of energy utilization, where CK reverses this reaction to form ATP and creatine. Creatine diffuses back to the mitochondria for rephosphorylation. Since creatine elicits formation of ADP in mitochondria, the increase in creatine helps stimulate respiration when muscle activity increases. Cellular adenine nucleotides are compartmentalized by their very low diffusibility (due to their size and charge) with pools in the mitochondria, at the myofibrils, SR, and other sites of energy utilization. CK is located at those sites. Phosphocreatine is much smaller and less charged, and therefore much more mobile in cells than ATP. ATP produced by substrate-level phosphorylation in glycolysis may be used to rephosphorylate creatine in the sarcoplasm;

CHAPTER21 Muscle and Nonmuscle Contractile Systems

~ Oxidative P~,~phosphorylationH20 ADP

ATP

ADP

AMP

ADP

ATP

Phosphocreatine

Mitochondria

Creatine

the cytoplasm or bound to SR, sarcolemma, or other sites of ATP utilization. Measurement of serum concentration of total CK and CK-2 (i.e., MB) have long been used to assess the extent of myocardial damage in suspected MI (Chapter 8). Measurement of LDH isozymes has been used similarly. In recent years, however, reliance on cardiac troponin assays has increased and use of CK and LDH assays will probably decrease.

Cytosol

Regulation of Smooth and Cardiac Muscle Phosphocreatine A[~P

R

Creatine ATP

Myofibrils, sarcoplasmic reticulum, etc.

H20

F I G U R E 21-13 Phosphocreatine shuttle. A myokinase (adenylate kinase) cascade between oxidative phosphorylation and creatine kinase (CK) has been postulated. A similar myofibrillary cascade may exist at the myofibrillar ATPase site.

however, CK and glycolytic enzymes are not colocalized. Phosphocreatine can be replenished rapidly only by oxidative metabolism. In addition to its energy transport function, phosphocreatine functions as an intracellular energy store. ATP inhibits mitochondrial respiration, limiting the maximum achievable concentration of ATE but phosphocreatine can accumulate without inhibiting respiration and ATP synthesis. Phosphocreatine provides a reserve of immediately available energy that can be used for brief bursts of activity as in throwing or jumping, and which is able to cover the energy needs for a few seconds at the beginning of sprint-type activity while glycolysis is accelerating. Supplementation with creatine has not been found to enhance aerobic work performance despite measurable increases in muscle creatine and phosphocreatine, suggesting that energy transfer in muscle cells is not normally limited by the total creatine concentration. However, ability to perform intermittent high-intensity work is enhanced, indicating that the energy storage function of PC is increased by creatine supplementation. Creatine kinase (also called creatine phosphokinase, or CPK) is a dimer of subunit molecular weight 40,000. The brain isozyme is a dimer of B subunits. In skeletal muscle, the principal form is a homodimer of M subunits. In cardiac muscle, 80-85% of the CK is MM, the balance is MB. These isozymes are electrophoretically distinct, as is mitochondrial CK. Depending on fiber type, 10-30% of CK activity is on the outer side of the inner mitochondrial membrane, 3-4% is at the M-lines, and the remainder is in

In all the actin-based motility systems, and especially skeletal, cardiac and smooth muscle, contraction is initiated primarily by an increase in cytoplasmic [Ca 2+ ]. However, the major differences in histology and function of these muscle types are associated with great variety in how contraction is controlled. Smooth muscle especially differs from the model presented for skeletal muscle. Skeletal muscle is activated by Ca 2+ released from SR in response to sarcolemmal action potentials. Ca 2+ exerts its effect by binding to troponin on the thin filaments, which reverses the tropomyosin inhibition of cross-bridge formation. Smooth muscle is much more dependent on entry of extracellular calcium. Smooth muscle cells are much smaller, having lengths about equal to the diameter of small skeletal muscle fibers, and so have much bigger surface-to-volume ratios than skeletal muscle cells. Therefore, Ca 2+ entry from the extracellular space can increase cytoplasmic [Ca 2+] much more readily in smooth muscle than in skeletal muscle. Most smooth muscle accordingly need not depend on Ca 2+ release from SR, may not have much SR, and does not need action potentials to trigger SR Ca 2+ release. Smooth muscle exhibits very diverse behaviors depending on which control mechanisms are present. Vascular smooth muscle, for example, lacks fast voltage-dependent Na + or Ca 2+ channels and so does not have action potentials or Ca 2+ spikes. It has slow voltage-dependent Ca 2+ channels that admit calcium in a graded fashion in response to fluctuations in membrane potential induced by humoral or transmitter effects on membrane ion conductances, and it has several membrane receptor-initiated second-messenger cascades that control Ca 2+ entry and Ca 2+ release from its limited SR, and which moderate the effectiveness of Ca 2+. Vascular smooth muscle contraction is thus tonic rather than phasic, and is very dependent on extracellular Ca2+; therefore Ca 2+ channel blockers effectively inhibit contraction. In contrast, gut smooth muscle does have fast voltage-dependent channels sufficient to produce action potentials and more SR than vascular smooth muscle, and also has gap junctions through which ion fluxes can occur. It also has receptor-mediated Ca 2+

SECTION 21.3

EnergySupply in Muscle

control similar to vascular smooth muscle. Therefore, gut smooth muscle can contract tonically or phasically, and depolarization can be directly transmitted from one cell to the next. As the name implies, smooth muscle lacks the highly ordered sarcomere structure of striated muscle, having thick and thin filaments in less orderly arrays with relatively less myosin (one fifth as much) than in striated muscle. Smooth muscle thin filaments have tropomyosin but generally lack troponin. Myosin in smooth muscle is found in monomeric form as well as small thick filaments, and phosphorylation is almost essential for condensation of monomeric myosin into filaments. Thus, the amount of myosin available to cross-bridge with actin may be physiologically adjustable. Like other myosin II types, smooth muscle myosin is a hexamer, and several isoforms of the heavy chains and both light chains are known. The SM-1 isoform (M.W. 204,000) has an unusually long C O O H -

/173 terminal tail not found in SM-2 (M.W. 200,000). Some smooth muscle myosins have an extra 7 amino-acid segment in the head near the aminoterminal which is associated with higher ATPase activity and velocity of shortening. The light chains are called LC17 and LC20, based on their molecular weight, for each of which two forms have been described. The LC17 isoform particularly seems to influence speed of shortening and tension development, the LC17aform being associated with higher speed. In skeletal muscle, disinhibition of actin is necessary for contraction to occur, and control of contraction is said to be actin-linked. In smooth muscle, phosphorylation of myosin light chains (MLCs) is required for contraction. Several mechanisms alter MLC phosphorylation, and so in smooth muscle, control of contraction is primarily myosin-linked. Three control proteins have been identified in smooth muscle: myosin light chain kinase (MLCK); caldesmon (CAD); and calponin (CAP). Figure 21-14

FIGURE 21-14 Pathways activating contraction in vascular smooth muscle. As in other muscle types, increased cytosolic Ca2+ is the most important signal, but control of cytosolic [Ca2+] and the mode of Ca2+ action is more complicated in smooth muscle, and other mechanismsmay contribute. Key: (3, (3TP-binding protein; PLC, phospholipase C; PLD, phospholipase D; PIP2, phosphatidylcholine 4,5-bisphosphate; PC, phosphatidylcholine; IP3, inositol 1,4,5-triphosphate; PA, phosphatidic acid; DA(3,diacylglycerol; SR, sarcoplasmicreticulum; CaK calmodulin; PKC, protein kinase C; MLCK, myosin light chain kinase; LC20, 20-kDa myosin light chain; CaD, caldesmon; CaP, calponin; CaMKII, Ca/calmodulinprotein kinase II; MAPK, mitogen-activatedprotein kinase; MEK, MAP/ERKkinase; (3x, small (3TP-binding protein; CaBP, calcium-binding protein; RyR, ryanodine receptor. Dashed lines indicate pathways not fully defined or that are speculative. The role of membrane potential is not indicated in this figure. [Reproduced with permission from A. Horowitz, et al., Mechanisms of smooth muscle contraction. Physiol. Rev. 76(4), 967-1003 (1996).]

~

summarizes the control mechanisms in smooth muscle that are discussed below. MLCK is a very specific kinase whose only known physiological substrate is MLC in myosin II. The only phosphate donor is Mg-ATR Smooth muscle MLCK (actually found in almost all cells) phosphorylates LC20 at Ser 19. There is also a skeletal muscle-specific MLCK and an embryonic MLCK. The various MLCKs are about 950-1000 amino acids long. MLCK has functional domains for: calmodulin binding, actin binding, substrate (ATP and myosin) binding, a catalytic domain, several target sites for various protein kinases, and a putative autoinhibitory site. The sequence and structure of skeletal and smooth muscle MLCKs are generally similar at these functional sites, but are quite divergent otherwise. MLCK is inactive in the absence of Ca-CaM, and its activity increases with increasing [Ca 2+ ]i such that the phosphorylation of LC20 becomes half-maximal at about 0.3 #M Ca 2+. Phosphorylation of a serine in the CaM binding domain decreases the affinity of MLCK for CaM and causes a decrease in the sensitivity of MLCK activity to increases in [Ca 2+]i. This site appears to be phosphorylated in vivo by Ca-CaM-dependent protein kinase II (CaMKII), but not by protein kinase C (PKC) or cAMP-dependent kinases. However, some studies indicate that this site is phosphorylated by protein kinase A (PKA), which might play a role in relaxation induced by dilators that increase cAME but the evidence is conflicting. Also, it appears that the [Ca2+]i required to activate CaMKII is about three times higher than that required to activate MLCK. This implies that Ca-CaM would bind to MLCK and shield the target serine before CaMKII could act on it, raising doubts about the significance of CaMKII in control of smooth muscle. It is well established only that Ca-CaM activates MLCK. LC20 phosphorylation produces two pronounced effects: it promotes the ability of myosin monomers to assemble into filaments, and it markedly increases (100-fold) the ATPase activity of myosin compared to unphosphorylated filamentous myosin. How unphosphorylated myosin filaments can remain stable is not certain, but may be related to binding to myosin of independently expressed MLCK fragments containing the myosin binding domain of MLCK. How LC20 phosphorylation increases the ATPase activity so dramatically is still unknown. LC20 phosphatase dephosphorylates LC20. This is a trimeric type I protein phosphatase (PP-I) comprising a catalytic subunit (M.W. 37,000), a myosin-binding subunit (M.W. 130,000), and a 20,000-M.W. subunit that also binds myosin. This PP-I may be inhibited by high arachidonic acid, and inhibited or stimulated by phosphorylation


by PKC or cAMP-dependent kinases, respectively. However, since the phosphatase is bound to myosin, and the ratio of phosphatase to myosin is only about 1:50, there is some doubt that LC20 phosphorylation is regulated via the phosphatase. MLCs may spontaneously dephosphorylate at a significant rate. In some smooth muscle (especially vascular smooth muscle), tension elicited by agonists or K+-induced depolarization is often tonically maintained after LC20 phosphorylation and sometimes [Ca 2+ ]i has fallen to near basal levels. These tonic contractions are characterized by very low cross-bridge cycling rates and ATP turnover, and are referred to as the latch phenomenon. It is currently believed that dephosphorylation of attached cross-bridges converts them to a form in which ADP is stably bound, called latch bridges, thereby dramatically slowing the events of the cross-bridge cycle subsequent to the force production step. It also appears that in the complete absence of CaZ+-induced LC20 phosphorylation, the latch state cannot be sustained. Protein kinase C (PKC) may play a role in tonic tension. PKC refers to a family of related serine/threonine kinases, five of which are found in smooth muscle. PKC is activated by diacylglycerol, or DAG. DAG (and also IP3) is liberated from membranes by the action of phospholipase C (PLC) on phosphatidylinositol 4,5-bisphosphate, or by the action of phospholipase D (PLD) on phosphatidylcholine. A number of receptor-mediated events are transduced by activation of these lipases. Some agents that elicit tonic contraction (e.g., angiotensin II) activate PLD, thus producing DAG (but not IP3), which activates PKC with no effect on [Ca2+]i. There are at least three sites on LC20 that can be phosphorylated by PKC, but it is not known which one, if any, of these is involved in the induction of contraction and latch. Caldesmon, or CaD, is a thin filament-linked regulator in smooth muscle. CaD is a single long peptide (M.W. ~90,000). It binds to actin, tropomyosin, myosin, and Ca-CaM. The affinity of CaD binding to thin filaments is much higher in the presence of tropomyosin than in its absence, and in vivo, CaD is probably bound only to tropomyosin-containing filaments. CaD inhibits actinactivated myosin ATPase by competing with myosin for a binding site on actin, and substantially reduces ATPase activity at a CaD/actin ratio of 1:10. CaD displaced by myosin remains attached to actin, however, via a second binding site for which myosin does not compete. It has been found that CaD can also bind to the $2 region of the myosin head, which might play a role in stabilizing thick filaments that have dephosphorylated. Reversal of CaD-induced inhibition can occur by several means. High concentrations of Ca-CaM inhibit CaD

SECTION21.3 Energy Supply in Muscle

binding to actin/tropomyosin. In some species, another calcium-binding protein called caltropin (M.W. ~ 11,000) has also been shown to reverse the CaD inhibition of actin-myosin interaction in a CaZ+-dependent manner. Phosphorylation is another such mechanism. CaD can be phosphorylated by CaMKII, PKC, PKA, casein kinase II, cdc2 kinase, and mitogen-activated protein kinase (MAP kinase). Mammalian CaD is phosphorylated at two sites by MAP kinase, which is probably the physiologically relevant kinase in mammals. The extent to which actin-myosin interaction is normally inhibited by CaD is not clear, and so the importance of these mechanisms regulating CaD in the control of smooth muscle contraction is debated. It has been argued that CaD phosphorylation is required to permit formation of latch bridges. Calponin is another polypeptide monomer (M.W. ~32,000) that can inhibit actin-activated myosin ATPase activity. In contrast to CaD, CaP exerts its effect in the absence of tropomyosin and completely inhibits motility in a 2/3 ratio with actin. CaP inhibits myosin binding to actin, but does so by reducing the affinity of actin for myosin rather than competing for the same site. CaP can be phosphorylated by PKC and CaMKII, both of which reverse CaP's inhibitory activity. As with caldesmon, many questions remain. The ratio of CaP to actin actually observed in smooth muscle is in the range 1:10 to 1:16, far from the 2/3 ratio found to produce near-complete inhibition of motility. Therefore, the importance of CaP and its regulation by phosphorylation is still debatable. Relaxation of smooth muscle (especially airway smooth muscle) by f12 agonists remains a puzzle. It had been thought that G-protein-mediated cAMP production stimulated cAMP-dependent kinases such as PKA, eliciting phosphorylation of MLCK, which prevented Ca-CaM binding and subsequently inhibited MLCK. This would result in decreasing LC20 phosphorylation and loss of tension. It is clear that /~2 stimulation does indeed increase [cAMP] and activity of cAMP-dependent kinases in smooth muscle. However,/3-agonists that relax tracheal smooth muscle can reduce the sensitivity ofLC20 phosphorylation to [Ca2+]i without reducing the Ca 2+ sensitivity of MLCK activity, which is not consistent with a mechanism based on inhibition of Ca-CaM binding to MLCK. /32-agonists generally elicit hyperpolarization by increasing K + conductance, particularly in a K + channel population called large-conductance calcium- and voltage-activated K + (KCa) channels, also called maxi-K channels. The probability of these channels being open is increased by phosphorylation by cAMP-dependent kinases, and a subpopulation may also be directly opened by/3z-receptor activation via a G-protein interaction. Hyperpolarization reduces the open probability of

475

voltage-dependent Ca 2+ channels and reduces [ca2+]i, which reduces MLCK activity. There are several other mechanisms by which ,6 stimulation can alter [Ca 2+ ]i, and there are circumstances in which/3-adrenergic relaxation occurs without hyperpolarization. It has also been claimed that the light chain phosphatase, PP-I, is stimulated by /~-adrenergic pathways. Given the diversity of smooth muscle, it is quite possible that the relative importance of various activating and relaxing mechanisms varies with the specific smooth muscle type and, perhaps, with the recent electrical and chemical history of the cell. Control of cardiac muscle is somewhat similar to gut smooth muscle while its mechanical properties are similar to skeletal muscle. Myocardial cells are stimulated to contract by action potentials generated by a combination of fast Na + and Ca 2+ voltage-dependent channels, and which propagate from cell to cell via gap junctions, as in gut smooth muscle. As in smooth muscle, various transmitters and hormones can alter extracellular Ca 2+ entry, but in myocardium these effects are largely confined to the plateau phase of the action potential. Myocardial cells have large T-tubules and moderately extensive SR, with DHPR and RyR directly apposed to one another as in skeletal muscle. Myocardial DHPR is a better Ca 2+ channel than skeletal DHPR, and calcium-induced calcium release triggered by Ca 2+ entry during the plateau phase plays a significant role in E-C coupling in myocardium, but a fully activating [Ca 2+]i is not normally attained in the absence of sympathetic stimulation. In the absence of action potentials, no normally occurring chemical stimulus induces sufficient /Ca 2+ entry to initiate contraction, and so myocardium is entirely phasic. Phosphorylation of myosin light chains is not required for contraction in myocardium but significantly increases cross-bridge cycling rate. Sympathetic (norepinephrine) stimulation of the heart increases both Ca 2+ entry and light chain phosphorylation, thus increasing both force and speed of contraction. Ca 2+ channel blockers reduce force of contraction and oxygen demand in myocardium, although the reflex increase in sympathetic drive to the heart (in response to the decrease in blood pressure induced by these agents) may largely offset these effects. It has recently become possible to alter the relation between force and [Ca 2+]i in myocardium, a useful trick in congestive heart failure. Pimobendan modestly increases the affinity of cardiac Tn C (cTn) for Ca 2+, thereby increasing the activation of contraction at any given [Ca 2+ ]i. The more potent levosimendan binds to the N-terminal region of cTn C in a CaZ+-dependent manner, and amplifies the effect of Ca 2+, perhaps by increasing the stability of the CaZ+-induced conformational change in cTn C or by enhancing cooperativity in the thin filament.

476

21.4 Inherited Diseases of Muscle Many disorders of skeletal and cardiac muscle due to genetic defects have been described. Taken as a group, the hypertrophic cardiomyopathies are the most common, with a combined prevalence of about 1 in 500. By comparison, the most common muscular dystrophy, Duchenne's muscular dystrophy (DMD), has a prevalence of roughly 1 in 3500 male births. It is not known whether genetic defects specific to myocardium are actually more common than those affecting skeletal muscle, or whether cardiac defects are simply more serious due to the incessant and vital nature of cardiac work. Most of these genetic defects do not affect both skeletal and cardiac muscle because there are cardiac-specific forms of almost all sarcomeric proteins and some enzymes. These disorders fall into the following broad categories: cardiomyopathies, muscular dystrophies, channelopathies (including the myotonias), metabolic diseases, and mitochondrial gene defects (see Chapter 14). The most prevalent examples of each are summarized in Table 21-5. Hypertrophic cardiomyopathy (HCM) is a syndrome characterized by dyspnea and chest pain associated with decreased diastolic compliance and outflow obstruction and left ventricular hypertrophy, usually without dilation. More than 100 different mutations have been found in HCM patients in genes coding for/~-MHC, essential and regulatory MLC, Tn T, Tn I, c~-tropomyosin, and myosinbinding protein C. Most of these are transmitted as autosomal dominant traits with variable penetrance. Those cases due to defects in /3-MHC and Tn T are the most severe and have the worst prognosis, and heavy chain mutations are the most common type. About 15% of HCM is due to mutations in chromosome 11 p I 1.2, which codes for myosin-binding protein C, a structural protein that binds myosin and titin, and may also play a role in mediating the contractility response to adrenergic stimulation. Protein C is 1274 amino acids long, and is coded for by a complex gene comprising 24,000 base pairs in at least 37 exons. Duchenne's muscular dystrophy (DMD), is a muscle degenerative disease due to a recessive mutation in the X chromosome. The mutation involves large defects in or complete deletion of the gene coding for dystrophin. The involved gene was identified by producing probes to the DNA surrounding the site of a large deletion in the X chromosome of a DMD patient. Applying these probes to the DNA of normal individuals, it was possible to isolate DND cDNA. Subsequent determination of the DNA sequence made it possible to identify dystrophin (M.W. 426,000); the gene coding dystrophin has over 2 million bases. Dystrophin has some similarities to spectrin and laminin, which led to the realization that dystrophin is

CHAPTER 21


involved in attaching the cytoskeleton to the extracellular matrix (ECM). That is, dystrophin binds to webs of actin filaments and to transmembrane glycoproteins which, in turn, bind to ECM proteins such as laminin and agrin. Dystrophin is required to transmit force from the myofibrils through the costamere structure of the fiber to the ECM, and ultimately, to the tendons of the muscle (see subsection on Myofibrils, p. 457). Similarly, dystrophin allows forces applied to the sarcolemma via the ECM to be transmitted to the myofibrils and be borne by them rather than the sarcolemma, which cannot support tension by itself. Lack of dystrophin results in mechanical stresses in muscle tearing holes in the sarcolemma, which causes sustained high [Ca2+]i and activation of Ca-dependent proteases such as calpains. This leads to focal destruction in the fiber, migration of polymorphonuclear leukocytes (PMNs) into the site, and either remodeling or total destruction of the fiber. Becker's muscular dystrophy (BMD) is allelic to DMD. Both DMD and BMD patients often have defects or deletions of genes flanking the dystrophin gene, and so present a varied clinical picture in which the muscular dystrophy is usually the most prominent feature. Channelopathies are a varied group of rare hereditary disorders due to defects, usually point mutations, in genes for ion channel proteins. Most of these affect voltage-gated channels. Two that do not are Thomsen's disease (autosomal dominant myotonia congenita) and Becket's disease (autosomal recessive generalized myotonia), which are both due to mutations in a gene coding for a skeletal muscle C1- channel called C1C-1. Since C1- conductance stabilizes the membrane potential by allowing movement of C1- in response to depolarization, loss of 75% or more of the C1- conductance makes membranes unusually susceptible to the generation of action potentials by random depolarizing stimuli and delays repolarization, resulting in the unwanted twitches and contractions and difficulty in releasing voluntary contractions seen in these disorders. The channel is an oligomer, and some mutations in one CIC-1 gene result in a protein that interacts negatively with the normal (wild-type) peptide from the nonmutated gene, suppressing its ion conductance. In such mutations, association of a mutant channel with its normal allele produces a dysfunctional channel, so that only one mutated gene is required to produce symptoms (Thomsen's disease). Mutations that do not affect the properties of the normal protein produce symptoms only in homozygous individuals (Becker's disease). Most channelopathies have some features in common. There are paroxysmal attacks of myotonia or paralysis, migraine, or ataxia precipitated by physiological stressors. They are often suppressed by membrane-stabilizing agents

TABLE 21-5 Inherited Diseases of Muscle

Description

Inheritance*

Hypertrophic cardiomyopathy Dyspnea, chest pain or syncope, usually features ventricular hypertrophy with impingement on LV volume and often LV outflow obstruction. Most cases hereditary, but some are new mutations. Defective genes for/3-MHC, either MLC, TnT, TnI, a-tropomyosin, myosin binding protein C.

Muscular dystrophy Duchene (DMD) and Becker's (BMD) muscular dystrophy. Progressive number and severity of lesions in muscle fibers leading to inflammation and rapid (DMD) or slow (BMD) loss of fibers. DMD due to deletion of most or all of the gene for dystrophin, BMD due to defective dystrophin. See text. Limb girdle (Erb's disease) Either pelvic or shoulder girdle initially, with spread to the other. Variable progression. Facioscapulohumeral (Lou Gehrig's disease) Face and shoulder girdle initially, then pelvic girdle and legs. Progression usually slow. Distal Onset in extremities, progresses proximally. Usually slow progression. Ocular External ocular muscles, some involvement of face, neck, arms. Oculopharyngeal As in ocular, but with dysphagia. Myotonic dystrophy (Steinert's disease) Muscle stiffness and impaired relaxation after contraction, wasting of muscles of face, neck, distal limbs, many other serious symptoms. Defect in gene coding for a serine-threonine kinase, resulting in impaired Na-K ATPase activity and perhaps other problems. Metabolic diseases: Glycogen storage diseases (GSD) GSD type II, Pompe's disease Deficiency of acid maltase causes excess glycogen accumulation in lysosomes. See text. GSD type III, Cori's disease Deficiency of debranching enzyme activity causes accumulation of limit dextrins. See text. GSD type V, McArdle's disease Myophosphorylase deficiency, inability to utilize glycogen. See text. GSD type VII, Tarui 'sdisease Muscle phosphofructokinase deficiency causes pronounced decrease in exercise tolerance.

Other metabolic diseases Carnitine deficiency Impaired/3-oxidation with lipid accumulation, see text, Adenylate deaminase deficiency Impaired anaerobic tolerance. Channelopathies Myotonias (muscle stiffness, impaired relaxation, fasciculation) due to various defects in skeletal muscle chloride channel 1 (CLCN1 or C1C-1): Becker's generalized myotonia (with moderate weakness and atrophy) Myotonia levior Thomsen's myotonia congenita (without weakness, may hypertrophy) Central core storage disease Abnormal SR with myofibrillar degeneration near fiber axis. Due to RyR1 gene defect. Hypokalemic periodic paralysis Due to defective gene for one type of DHPR. Hyperkalemic periodic paralysis Due to defects in a skeletal muscle sodium channel, SCN4A. Paramyotonia congenita and "pure" myotonias Are due to varied (and different) mutations in the gene for SCN4A. Mitochondrial myopathy Due to defects in mitochondrial genes. Mitochondrial myopathy Fiber degeneration, weakness, "ragged red fibers" histological appearance. CPEO (chronic progressive external opthalmoplegia) Paralysis of eye muscles and mitochondrial myopathy. KSS (Kearns-Sayre syndrome) CPEO plus retinal degeneration, cardiomyopathy, hearing loss, diabetes, renal failure. MELAS (mitochondrial encephalopathy, lactic acidosis, and strokelike episodes) Cognitive symptoms, seizures, transient paralysis, and mitochondrial myopathy. *AD= Autosomal dominant, AR= autosomal recessive

Most AD with variable penetrance

X-linked recessive, or new mutations Usually AR, maybe dominant AD AD Most cases AD AD AD

AR AR AR AR

AR AR

AR AD AD 9 AD AD AD See text most cases tRNA gene defects

478

such as mexilitine, and by acetazolamide. Several channelopathies are listed in Table 21-5. Metabolic disorders of muscle include those of glycogen storage, substrate transport and utilization, and electron transport chain and ATP metabolism. Some produce dynamic syndromes with symptoms occurring primarily during exertion, some cause degenerative syndromes, and some produce both. A few are discussed below. Degenerative Syndromes

Acid maltase is a lysosomal enzyme that is not in the energy pathway of the cell, so that its deficiency does not produce dynamic symptoms in muscle. Also called o~-1,4glucosidase, acid maltase hydrolyzes the c~-1,4 bonds in glycogen. Since lysosomes degrade glycogen along with other macromolecules in the normal process of cellular turnover, this deficiency causes marked accumulation of glycogen in lysosomes. This leads to a vacuolar degeneration of muscle fibers. Heart and other tissues are also affected. Deficiency of debranching enzyme (amylo-1,6glycosidase) is another disorder of glycogen metabolism. Since phosphorylase cannot act at or near branch points (Chapter 15), lack of debranching enzyme results in great accumulation of limit dextrins in muscle, liver, heart, and leukocytes, with swelling and functional impairment. Since this enzyme is in the energy pathway, its absence causes dynamic symptoms and, more importantly, vacuolar degeneration. Carnitine deficiency causes a disorder of lipid metabolism. Carnitine is derived both from the diet and from e-N-trimethyllysine produced by catabolism of methylated proteins including myosin, and is required for the transport of fatty acids across the mitochondrial membranes (Chapter 18). If any of the enzymes or cofactors required for carnitine synthesis are deficient or defective, carnitine deficiency may develop. This limits the energy supply available from/3-oxidation, and causes a lipid storage myopathy.

Dynamic Syndromes Myophosphorylase deficiency is the classic example of a carbohydrate-related dynamic syndrome. Affected persons are unable to mobilize glycogen; therefore they cannot perform high-intensity work and must rely extensively on lipid metabolism. Several other defects of glycolysis produce similar symptoms. All are characterized by inability to do anaerobic work and to produce lactate during ischemic exercise, which is the basis for the customary screening test for these disorders. Patients are asked to perform maximal hand grip contractions at the rate of one per second for 60 seconds with the forearm circulation

CHAPTER 21


occluded by inflation of a cuff on the upper arm. Following release of the cuff pressure, venous effluent from the exercised arm is sampled and analyzed for lactate. In addition to reduced anaerobic work capacity, the low flux through glycolysis reduces maximal muscle power output and maximal aerobic power as well. In phosphorylase deficiency, muscular performance can often be improved by glucose infusion, while patients with other defects of glycolysis are dependent on lipid metabolism and show little or no improvement with glucose infusion.

21.5 Nonmuscle Systems Actin Actin is present in all eukaryotic cells where it has structural and mobility functions. Most movement associated with microfilaments requires myosin. The myosin-to-actin ratio is much lower in nonmuscle cells, and myosin bundles are much smaller (10-20 molecules rather than about 500), but the interaction between myosin and actin in nonmuscle cells is generally similar to that in muscle. As in smooth muscle, myosin aggregation and activation of the actin-myosin interaction are regulated primarily by light chain phosphorylation. Myosins involved in transporting organelles along actin filaments are often activated by Ca-CaM. Actin filaments are relatively permanent structures in muscle, whereas in nonmuscle cells microfilaments may be transitory, forming and dissociating in response to changing requirements. The contractile ring that forms during cell division to separate the daughter cells and the pseudopodia formed by migrating phagocytes comprise transient actin filaments. Belt desmosomes in epithelial cells and microvilli on intestinal epithelial cells comprise relatively permanent filaments. The rate-limiting step in actin polymerization appears to be nucleation, the formation of an actin cluster large enough (typically three or four G-actins) for the rate of monomer association to exceed the rate of dissociation. Once filaments of this size form, they continue to grow, and the concentration of G-actin monomers decreases until it is in equilibrium with F-actin. The concentration of monomeric actin at equilibrium is called the critical concentration, Cc. In vitro, Cc is 0.1 raM. The value in vivo is variable, depending in part on the concentration of ATE In the presence of ADP, instead of ATE both ends of the filament grow at the characteristic slow rate. ATP speeds up the rate of polymerization and lowers the effective Cc. If nascent actin filaments anchored to the cytoskeleton (by binding proteins such as those listed in Table 21-6)

Actin-Cross-Linking Proteins**

MW

Rotein*

-

-

Domain Organizationt

-

Lacation

-

Filopodia, lamellipodia, stress fibers

Pseudopodia Filopodia, lamellipodia, s m fibers, microvilli, m s o m d p e s s Acrosomal p c e s s

GROUPI1 Villin

Intestinal and kidney brush border microvilli Erythrocyte corticaI network MicrwilIi, stemcilia, adhesion plaques, yeast actin cables Filopodia, famellipodia, stress fibers, adhesion plaques

Cortical networks

Muscle cortical networks ABP 120

Filamin

280,000

-

Filopodii pseudopodia, stress fibers

*cross-linkingproteinsare placed into Ovee groups- Group I proteins have unique &-binding domain: aad Group IH h e p a h ofa 26,000-MWrctin-binding domain. tCalmadulin-like catcium-binding domains (light blue), actin-binding domains (dark blue).

Actin-Capping and Actin-Severing Pd&s**

Protein gCAP39

MW

-

-

-

Domain Organization*

wm

Gelsolin Villin

-

domains; Group Ilhave a 7.000-MWactin-bindng

Activity - -

CaPPb Capping, severing Capping, severing

9

~

~

Capping, severing, cross-linking

0 -

-

4~0

grow toward the cell membrane and continue to grow at the membrane, they will create a projection of membrane and cytoplasm in the direction of growth, especially if the filaments are connected by short linking proteins into rigid bundles. In this way, microvilli extend from the surfaces of many cell types, including highly specialized structures such as stereocilia in sensory cells of the ear. Projection of the acrosomal process through the zona pellucida is driven in this way, as are the extension of filopodia and lamellipodia at the leading edges of migrating cells. The structure and properties of actin filaments can be regulated by controlling the transformation of G-actin to F-actin or the length of the F-actin filaments, and by modulating the aggregation of actin filaments into bundles or three-dimensional arrays. Proteins that bind actin monomers reduce polymerization. Gelsoin, villin, and other proteins affect actin polymerization and actin filament elongation by capping the growing filament and blocking elongation. Some accelerate nucleation, perhaps by binding to and stabilizing dimers and trimers. Many capping proteins can also sever actin filaments without depolymerizing them, markedly reducing the viscosity of F-actin gels. The severing and capping activities of gelsolin require Ca 2+. Although nucleation and severing increase the number of free ends available for growth, the net effect of these proteins is a greater number of short actin filaments and an increased concentration of monomeric actin. Cytochalasins, drugs that inhibit cellular processes that require actin polymerization and depolymerization (e.g., phagocytosis, cytokinesis, clot retraction, etc.), also act by severing and capping actin filaments. Actin filaments can be stabilized by phalloidin, derived from the poisonous mushroom Amanita phalloides. Assembly of actin filaments into bundles (as in microvilli) and threedimensional networks is accomplished by two groups of cross-linking proteins (Table 21-6). Cilia Tubulin and microtubules occur in all plant, animal, and prokaryotic cells and participate in a number of essential processes. As is the case for actin filaments, microtubules occur in highly organized, relatively permanent forms such as cilia and flagella, and as transient cytoplasmic structures. Other similarities between actin filaments and microtubules are listed in Table 21-1. Cilia are hair-like cell surface projections, typically 0.2-0.3 # m in diameter and 10 #m long, found densely packed on many types of cells. In eukaryotes, they move fluid past cells by a characteristic whip-like motion (Figure 21-15). Eukaryoticflagella are basically elongated


Power stroke 2

~

,

Recovery

5

F I G U R E 21-15 Cycle of ciliary stroke shown for three cilia beating in synchrony. (1) Beginning of the power stroke. The cilia are straight and stiff. (2) Half completion of the power stroke. (3) Start of recovery stroke. The cilia are flexible, and by bending reduce frictional drag. (4) Near completion of the recovery stroke. (5) Start of the next power stroke.

cilia, one or two per cell, that propel the cell through a fluid medium. Flagellar movement is wavelike and distinct from ciliary beating, although the microtubular arrangement and mechanism of movement are quite similar in the two structures. Bacterial flagella are structurally and functionally different from other flagella. Figure 21-16 shows a cross-section of a cilium, showing the "9 + 2" arrangement of microtubules found in the axoneme (core) of cilia and flagella. Nine asymmetrical doublet microtubules are arranged in the periphery, and a symmetrical pair of singlet tubules is in the center. Attached to the A microtubules of each doublet are dynein arms extending toward the B microtubule of the adjacent doublet. The protein tektin, a highly helical protein about 48 nm long, runs along the outside of each doublet where the A and B microtubules join. It is regarded as a structural rather than a regulatory protein despite the similarity to tropomyosin. Three types of links preserve the axoneme structure. The central singlets are linked by structures, called inner bridges, like rungs on a ladder, and are wrapped in a fibrous structure called the inner sheath. Adjacent outer doublets are joined by links of nexin (M.W. 150,000-165,000) spaced every 86 nm. Outer doublets are

SECTION 21.5

Nonmuscle Systems

481

F I G U R E 21-17 Drawing of a microtubule. (a) Cross-sectional view of the 13 protofilaments. (b) Longitudinal view.

F I G U R E 21-16 (a) A microtubule doublet consisting of an A subfiber with 13 protofilaments of tubulin and a B subfiber with 11 protofilaments. (b) A cilium contains nine microtubule doublets and a central pair of microtubules. The dynein arms provide for sliding of one doublet along another during the beating of the cilium. The functions of radial spokes and the projections from the central pair of microtubules have not been as well defned.

joined to central singlets by radial spokes, which have a relatively globular end, or head, that interacts with the central singlet. The radial spokes are complex structures, which seem to have 17 or more constituent proteins, at least 6 of which are in the head. They are arranged in pairs every 96 nm along the microtubules. The inner sheath comprises primarily thin protein arms extending from the central microtubules at roughly 14-nm intervals, and the spoke heads may interact with these. Cilia grow from basal bodies, one of several types of microtubule organizing centers in cells. Each basal body contains nine fused triplets of microtubules that act as nucleation centers for the growth of microtubules down the axoneme. Each triplet contains one complete A microtubule, that is continuous with the A microtubule of the axonemal doublet, and an incomplete B microtubule that is continuous with the B microtubule of the doublet. A second incomplete microtubule, the C microtubule, is fused to the B but does not extend beyond the basal body. The basal body does not have central singlets. There are numerous proteins beside the or- and/3-tubulin of the microtubules in the basal bodies, which are thought to be involved in controlling polymerization, stabilizing the axoneme structure,

and securing the basal body to the cytoskeleton. These include a third type of tubulin, F-tubulin, which is believed to form a ring that serves as the nucleus from which the tubules grow. Microtubules are constructed of protofilaments (Figure 21-17), the axis of each being parallel to the axis of the microtubule. In cilia, the A microtubule and the central singlets have 13 protofilaments, while the B microtubule has 10 protofilaments of its own and shares 4 with the A microtubule (Figure 21-16a). The protofilaments are chains of heterodimers of or- and/~-tubulin arranged end-to-end. There are two models of protofilament arrangement. In one, the heterodimers in adjacent protofilaments are only slightly staggered, forming spiraling rows of orand/3-tubulin in the microtubule wall as in Figure 21-17. In the other, the o~- and /3-tubulin units are staggered a half-unit apart, yielding a checkerboard pattern. The strongest bonds in the structure are clearly along the axis of the filament. In depolymerizing conditions, the dissociating ends of the microtubules have a frayed appearance due to the separation of protomers from one another. The tubulins are globular proteins with a mean diameter of 4 nm. They are about 450 amino acids long, (M.W. 50,000). The human genome contains 15-20 genes for both the or- and/3-forms. Of these, probably five or six of each are expressed in a tissue, developmental, or cell cycle-dependent manner, and the rest are pseudogenes. Tubulin synthesis is regulated at the level of transcription and translation. Monomeric tubulin binds to ribosomes during tubulin synthesis and causes degradation of tubulin mRNA, thus providing a negative feedback on tubulin

48~ synthesis. There is typically about 90% homology between the various tubulins for both the o~- and/~-forms, and the tubulins are functionally quite alike, but some additional diversity can be created by posttranslational modification. Removal of the C-terminal tyrosine from o~-tubulin by a specific carboxypeptidase (detyrosinase) affects primarily ot-tubulin that is incorporated into a microtubule. Tyrosine can be reattached by a second enzyme, tubulintyrosine ligase, which acts preferentially on monomeric o~-tubulin in a reaction that requires hydrolysis of ATE Tyrosinated ~-tubulin is found in all microtubules, while detyrosinated ot-tubulin is found preferentially in the more stable tubular structures such as cilia. In some organisms, acetylation of a lysine in ot-tubulin occurs during or just after incorporation into the microtubule. Acetylation has no known effect on the behavior of microtubules in which it occurs. Some serine residues in fi-tubulin can be phosphorylated, but again the significance is unknown. Oxidation and blocking of sulfhydryl groups can prevent tubule assembly and may promote tubule disassembly. Tubulin is similar to actin and myosin in its ability to spontaneously polymerize in vitro. Like actin, there is a critical concentration, Cc, of ot/~ dimers at which equilibrium between association and dissociation occurs, and the microtubule, like actin, has a polarity and undergoes polymerization and depolymerization more rapidly at one end [(+) end] than at the other [ ( - ) end]. The heterodimers polymerize ct to/~ all along the protomer, so that the protomer has a polarity, ct at one end,/~ at the other, and all of the protomers associate with the same polarity. So one end of a microtubule is ringed with ct-tubulin and the other with fi-tubulin. The or/3 heterodimers each bind two molecules of GTE The ct-tubulin site binds GTP irreversibly and does not hydrolyze it because fi-tubulin shields the site from water, but the/3-tubulin site binds GTP reversibly and hydrolyzes it to GDP after incorporation into the protomer, and especially after additional dimers have been added. This site is called the exchangeable site because GTP can displace GDP from it. Microtubules, then, tend to be capped with GTP dimers, but may be capped with GDP dimers if the microtubule has shortened, exposing GDP-tubulin, or when the microtubule has not grown for such a long time that most GTP has hydrolyzed. When the terminal dimers at the (+) end of the microtubule have GTP at the exchangeable site, the structure grows at dimer concentrations above Cc and slowly depolymerizes below Cc. When the (+) end dimers have GDP, however, depolymerization is rapid below Cc. This raises the surrounding dimer concentration and accelerates the growth of those microtubules that are elongating. Thus, microtubules are inherently unstable unless restrained in a structure like cilia by numerous accessory proteins.


Formation and stabilization of microtubules other than in cilia and flagella usually involves a group of proteins called microtubule-associated proteins (MAPs). One class, called assembly MAPs, cross-link microtubules in the cytosol. These have two large domains, a basic microtubule binding domain and an acidic projection domain, so named because it is a filamentous structure projecting from the microtubule. The projection domain binds to membrane proteins, intermediate filaments, or other microtubules. In the latter case its length determines the spacing between microtubules. MAPs are divided into two groups. Type I MAPsmMAP1A and MAP1Bmare large proteins found mainly in neurons but also in other cell types. Each is derived from a larger precursor which is proteolytically processed to yield one light chain and one heavy chain. Type II MAPs include MAP2, MAP4, and tau protein. These are smaller than type 1, and are characterized by three or four repeats of an 18-aminoacid sequence in the microtubule binding region. MAP2 proteins are found only in dendrites where they connect microtubules to each other and to cytoskeletal IFs. The tau proteins are a group of four or more proteins derived from the same gene by alternative mRNA splicing. They accelerate polymerization and cross-link microtubules. They may play a role in stabilizing the long and quite permanent microtubules found in axons. MAPs generally inhibit depolymerization, and some enhance polymerization. Several MAPs can be phosphorylated on their projection domains by mitogen-activated protein kinase, or MAP kinase, which prevents them from binding microtubules. There is some evidence that MAPs can also be phosphorylated by Ca-CaMKII and that they may be directly influenced by Ca-CaM. Thus, MAPs may be modulated by signal pathways using Ca 2+ or MAP kinase, although specific roles for such control have not been clearly demonstrated. Dynein is the motor protein in cilia and flagella. Dynein arms are arranged in two groups called inner and outer dynein arms, spaced at 24-nm intervals along the A microtubule. Dynein is a large family of microtubule-based motor proteins. Axonemal dyneins are multimers of (very) heavy chains, light chains, and intermediate chains. Each heavy chain has a large globular region with two stalks extending from it. One stalk connects to a cluster of proteins forming a "base" that attaches to the A microtubule. The other stalk projects 10 nm toward the B microtubule and forms a small head that binds to it. The heavy chain is quite large, about 4500 amino acids long with a molecular weight of more than 540,000. It has ATPase activity, especially when bound to the B microtubule, which is believed to be localized to the head, near the tubulin binding site. Most of the smaller chains are clustered at the base

SECTION 21.6

Drugs Affecting Microtubules

483

Dynein arms

L

Base of cilil fm

(a)

(b)

FIGURE 21-18 Sliding of one microtubule along another in an intact cilium causes the cilium to bend, creating a power stroke.

and are believed to form a fixed attachment of the dynein complex to the A microtubule, and they probably play a role in regulating the activity of the dynein, although not much is known about the regulatory mechanisms. As with myosin, there are multiple dynein genes, and the axoneme contains eight or more different heavy chains. The outer dynein arms have two or three heavy chains; the inner arms have two. The scheme of chemomechanical transduction by dynein-tubule systems is similar in a general way to that in myosin-actin systems, but the structure of dynein is very different from that of myosin. Force generation begins by formation of a dynein cross-bridge to the adjacent B microtubule. This is followed by a conformational change in the dynein that pulls the two microtubules past each other. Dynein is a (-)-directed motor, i.e., it tries to pull its base toward the end of the adjacent microtubule, which is toward the basal body in cilia and flagella. Free energy of hydrolysis of ATP is required to release the dynein head and allow the dynein bridge to recycle. As in the case of actomyosin ATPase, it is thought that the coupling of ATP hydrolysis to mechanical movement is effected at the product release step and that this step is rate limiting. Since the microtubules are all fixed at one end, the only way they can slide past one another is to bend (Figure 21-18). Activation of the dynein seems normally to occur from the base to the outer tip.

21.6 Drugs Affecting Microtubules Microtubule systems are used in the cell for many other functions, such as transport of organelles and vesicles, and separating genetic material on the mitotic spindle and other motile events of the cell cycle. Substances that interfere with microtubule growth or turnover, or with microtubule interaction with motor proteins, will disrupt these

functions. The classic example of such a drug is colchicine, an alkaloid derived from the autumn crocus (Colchicum autumnale). Colchicine in high concentration causes cytosolic microtubules to depolymerize. In low concentrations, it does not produce this effect, but binds to tubulin dimers. The tubulin-colchicine complex, even at quite low concentration, can add on to the end of a growing (or at least stable) microtubule and block further reactions at that end. Only one or two colchicine-tubulin units at the end of a microtubule prevents any further addition or removal of dimers at that end. In cells that are replicating, this freezes the cell at metaphase. Drugs that produce such an effect are useful as antineoplastic agents. The well-known effect of colchicine in gouty arthritis is probably due to its inhibiting the migration of granulocytes into the inflammatory area by interfering with a microtubule-based component of their motility. Vinca alkaloids (vincristine, vinblastine, vinorelbine) are derived from the periwinkle plant (Vinca rosea). These agents work by binding to tubulin at a site different than colchicine or paclitaxel. They block polymerization, which prevents the formation of the mitotic spindle, and are used as antineoplastic agents. Taxanes produce a stabilization of microtubules similar to colchicine, but by a different mechanism, and also halt cells in metaphase. Paclitaxel (taxol) is the taxane used clinically. It is derived from the bark of the pacific yew. Taxol disrupts several microtubule-based functions as completely as inhibitors of polymerization, emphasizing the importance of assembly/disassembly balance in microtubule function. Recently, it has been found that paclitaxel also binds to and inhibits the function of a protein called bcl-2, an inhibitor of one or more pathways involved in mediating apoptosis. Paclitaxel's interference with this function promotes apoptosis in addition to its microtubule-related inhibition of cell division.

lmmotile Cilia Syndrome Defects in proteins needed for normal assembly and functioning of microtubules can cause cellular dysfunction. Several inherited disorders of this type have been identified that cause dyskinetic or completely immotile cilia and flagella. Kindreds have been identified in whom dynein arms, radial spokes, central sheath, or one or both central singlets are missing or defective. These disorders may result from a mutation in a gene needed for one of the axonemic structures, or in a regulatory gene controlling assembly of the microtubule system in cilia and flagella. Affected individuals manifest bronchiectasis and chronic sinusitis. Because the cilia of the respiratory epithelium are defective, mucociliary clearance is reduced

484 or absent, leading to frequent pneumonia, colds, and ear infections. These defects were originally described in conjunction with situs inversus (lateral transposition of the thoracic and abdominal viscera). Some embryonic cells have a single flagellum that is presumed to be important for movement to their proper location. The flagellar defect may make the cells unable to migrate properly, giving rise to situs inversus. Only about half of persons with immotile cilia have situs inversus, suggesting that chirality (handedness) of embryonic organization is random in the absence of flagellar function. Association of the three abnormalities (bronchiectasis, sinusitis, situs inversus) is known as Kartagener's syndrome. These defects also cause infertility in males because the sperm are immotile. Affected females have nearly normal fertility despite the probable lack of ciliary activity in the oviducts. The function of microtubules other than in cilia and flagella is presumed to be normal; otherwise cell division could not occur.

Kinesins Kinesins are another large family of motor proteins. Thirteen have been described altogether, 11 in mouse brain. How many occur in humans is uncertain. Cytosolic kinesin is a tetramer of two heavy chains of unit M.W. 124,000 and two light chains of unit M.W. 64,000. The heavy chains have a head region and a tail, similar to myosin except that the tail is largely globular rather than a rod. There are structural similarities between myosin and kinesin heads, and in key functional groups, such as the helices flanking the ATPase site, the sequence homology is high. The light chains are associated with the tail. Typically, kinesin is a (+)-directed motor, i.e., it tries to pull whatever it is attached to toward the (+) end of the microtubule. In an axon, that would be away from the cell body. Cytosolic dynein, which is smaller and simpler than axonemal dynein, is a (-)-directed motor. In most kinesins, the motor region is at the N-terminal half of the molecule, but in some it is in the C-terminal region. Most of these latter kinesins are, like dynein, (-)-directed. Kinesin has in common with cytosolic dynein and transport myosins (e.g., myosin I) a property called proeessivity. This is the term used to refer to the fact that a single motor molecule, when dragging a vesicle or other cargo along a filament or a microtubule, cannot let go of that filament or microtubule lest the motor and its cargo drift away or continually reattach to the same site. Instead the motor must remain bound to one G-actin or tubulin while "reaching for" the next one. A similar argument applies to other motor proteins, such as DNA helicase which crawls along


DNA unzipping the strands, and the ribosomal motors which pull the RNA and the nascent polypeptide through the ribosome. In contrast, myosins whose functional form is filaments, such as myosin II, need not exhibit this property: any given myosin molecule need not be attached to the actin filament because attachment and orientation of the myosin filament to the actin filament will be maintained so long as one or two myosins anywhere in the filament remain attached. Which features of these motor molecules determine the presence or absence of processivity remains a mystery.

Supplemental Readings and References M. J. Ackerman and D. E Clapham: Ion channels--basic science and clinical disease. New England Journal of Medicine 336, 1575 (1997). L. A. Amos and R. A. Cross: Structure and dynamics of molecular motors. Current Opinion in Structural Biology 7, 239 (1997). G. Bonne, L. Carrier, E Richard, B. Hainque, and K. Schwartz: Familial hypertrophic cardiomyopathy: from mutations to functional defects. Circulation Research 83, 580 (1998). R. Cooke: Actomyosin interaction in striated muscle. Physiological Reviews 77, 671 (1997). M. A. Geeves and K. C. Holmes: Structural mechanism of muscle contraction. Annual Review of Biochemistry 68, 687 (1999). A. M. Gordon, E. Honsher, and M. Regnier: Regulation of contraction in striated muscle. Physiological Reviews 80, 853 (2000). N. Hirokawa: Kinesin and dynein superfamily proteins and the mechanism of organelle transport. Science 279, 519 (1998). A. Horowitz, C. B. Menice, R. Laporte, and K. G. Morgan: Mechanisms of smooth muscle contraction. Physiological Reviews 76, 967 (1996). H. Lodish, D. Baltimore, A. Berk, et al.: Microfilaments: Cell motility and control of cell shape. In: Molecular Cell Biology. H. Lodish et al. (Eds)., W. H. Freeman, New York, 1995, p. 991. G. J. Lutz and R. L. Lieber: Skeletal muscle myosin II structure and function. Exercise Sport Science Review 27, 63 (1999). L. A. Sabourin and M. A. Rudnicki: The molecular regulation of myogenesis. Clinical Genetics 57, 16 (2000). S. Sach, E J. Kull, and E. Mandelkow: Motor proteins of the kinesin family. Structures, variations, and nucleotide binding sites. European Journal of Biochemistry 262, 1 (1999). S. Schiaffino and C. Reggiani: Molecular diversity of myofibrillar proteins: Gene regulation and functional significance. Physiology Reviews 76, 371 (1996). C. A. Sewry: Immunocytochemical analysis of human muscular dystrophy. Microsc. Res. Tech. 48, 142 (2000). J. M. Squire and E. P. Morris: A new look at thin filament regulation in vertebrate skeletal muscle. FASEB Journal 12, 761 (1998). M. J. Tanasijevic, C. P. Canon, and E. M. Antman: The role of cardiac troponin-I (CTnI) in risk stratification of patients with unstable coronary artery disease. Clinical Cardiology 22, 11 (1999). R. H. Wade and A. A. Hyman: Microtubule structure and dynamics. Current Opinion in Cell Biology 9, 12 (1997). D. C. Wallace: Mitochondrial DNA in aging and disease. Scientific American 227, 40 (1997). A. Weiss and L. A. Leinwand: The mammalian myosin heavy chain gene family. Annual Review of Cell Devopmental Biology 12, 417 (1996).

CHAPTER

22

Metabolic Homeostasis

22.1 Metabolic Homeostasis The functions of cells and tissues in all organisms require energy that normally is obtained from ingested food containing proteins, lipids, and carbohydrates. Human beings follow an intermittent food/fast schedule; however, energy must be supplied to cells and tissues continuously. Thus, excess chemical substances capable of supplying energy are stored and released as needed to maintain homeostasis, the tendency for biological systems to maintain relatively constant chemical conditions in their internal environments. Some organs, tissues, and cells are metabolic providers; others are users. Each cell membrane acts as a barrier to hold its cellular components and, by expending energy, is able to regulate the gradient of ions and metabolites moving into and out of the cell. This chapter discusses how energy is stored and released by tissues and how homeostasis is maintained. Organs serve as reservoirs or sites for synthesis of the metabolites required for homeostasis; at all levels homeostasis is regulated by the endocrine system. Fluxes of metabolites through biochemical pathways are regulated by stoichiometric need, allosteric control of enzyme activity, modification of regulatory enzymes, and changes in their rate of enzyme synthesis or degradation. Hormones often influence these processes and, in turn, metabolites modulate the secretion of hormones. Thus, many

interconnected feedback loops provide the positive and negative regulation that maintains homeostasis. Human beings possess considerable metabolic versatility for utilization of major fuel classes such as carbohydrate, lipid, and protein to maintain energy requirements. Metabolism of these substances is organized to: 1. Maintain the blood glucose level within narrow limits; 2. Maintain an optimal supply of glycogen; 3. Maintain an optimal supply of protein. If carbohydrates or proteins are ingested in excess of the amounts necessary to maintain optimal supplies of glycogen in tissues or protein, the excess is converted to fat. Conversion of excess carbohydrate and/or protein into fat is biochemically an irreversible process. As a result, the body conserves the compounds that it can interconvert, uses them to supply energy when needed, or converts them to fatty acids when it is more efficient to store the carbon in the form of fat. The body tends to conserve its protein reserves and to draw on fat reserves preferentially in time of energy demand. The tricarboxylic acid (TCA) cycle and /3-oxidation (Chapter 14) are tightly coupled to electron transport via the nicotinamide and flavin nucleotides and by ADP and ATE The TCA cycle in cells functions only when ADP is present (i.e., when ATP is being utilized). Thus,

485

~86

oxidation of primary foodstuffs is determined by energy need and cannot occur simply to convert excess foodstuffs to products that can be readily eliminated. As a consequence, regulation of body weight is determined by three factors: 1. Food intake, 2. Heat loss, and 3. Energy expenditure (i.e., exercise). The metabolic activity of each organ in the body depends on its specific functions and the energy required to perform those functions. The amount of energy needed can be determined from the amount of ATP generated since most of the oxygen is used by mitochondria to produce ATP. At rest, the adult brain (~ 2% of body weight) uses 20% of the oxygen consumed; in contrast, skeletal muscle (~ 40% of body weight) consumes only 30% of the oxygen. During heavy muscular work, oxygen consumption by the body may increase 5-10 fold with a corresponding 10- to 25-fold increase in oxygen consumption by the

CHAPTER22 Metabolic Homeostasis

skeletal muscle. During heavy muscular work, however, oxygen consumption by the brain does not change. These differences between tissues illustrate the metabolic flexibility that can be achieved. The large change in oxygen consumption by muscle tissue is accompanied by large changes in fuel utilization and in production of metabolites while overall homeostasis is maintained. Skeletal muscle has the highest glycolytic capacity reflecting the need to generate ATP rapidly for muscle contraction. Skeletal muscle relies on glycolysis for short bursts of high-intensity work under conditions where oxygen is in short supply (Chapter 21). Other tissues that also possess a high glycolytic capacity are the brain and the heart. However, the heart, which has a much greater capacity than skeletal muscle to derive energy oxidatively from sources other than glucose, rarely relies on glycolysis for energy. The brain, on the other hand, preferentially uses glucose for energy, first converting it to pyruvate and then to CO2. The liver and kidney, which do not metabolize glucose to a significant degree, rely primarily on fatty acid

F I G U R E 22-1 Interrelationships of organs to maintain homeostasis. [Reproduced with permission from R. H. Garrett and C. M. Grisham, Biochemistry, 2nd ed. Harcourt College Publishers, 1995, p. 940.]

SECTION 22.2

Metabolic Roles of Organs

487

oxidation for energy. The rate-limiting enzymes of gluconeogenesis (pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and fructose bisphosphatase) show high activities in the liver and kidney, but very low glycolytic activities reflect the fact that the liver and kidney are glucose producers whereas the other tissues are glucose users. The overall metabolic patterns in the various tissues and organs of the body are illustrated in Figure 22-1.

22.2 Metabolic Roles of Organs Liver The liver (see also Chapter 12) in a normal, 70-kg human adult weighs approximately 1.5 kg, or about 2% of body weight. (Metabolic fluxes quoted throughout this chapter refer to the typical 70-kg human.) Its major cell type, the hepatocyte, is of epithelial origin. Interposed between the intestinal tract and the general circulation, the liver is uniquely related to the endocrine pancreas; insulin and glucagon, released from the pancreas, play a prominent role in metabolic homeostasis. The liver receives a large volume of blood from the intestinal tract via the portal vein and a small volume of blood via the hepatic artery, and it drains via the hepatic veins into the inferior vena cava. It is the first organ to meet nutrients delivered from the intestine, with the exception of lipid, and to meet the secreted insulin and glucagon. By delivering bile (which contains bile acids and cholesterol) into the intestine, the liver predominates in cholesterol homeostasis. It has the greatest metabolic flexibility of any organ and shows tremendous adaptability to fluxes of metabolites and nutrients. It rapidly undergoes changes in size and glycogen and protein content In the fed state, 5-10% of its wet weight consists of glycogen but after a 24-hour fast the glycogen disappears almost entirely. After a day or two on a high-protein diet, the liver shows a large increase in activity of enzymes involved in amino acid metabolism and in gluconeogenesis. A high-carbohydrate diet produces the opposite effect. The liver is the primary site of glycogen deposition and blood glucose maintenance. It also plays a central role in lipid, protein, and nitrogen homeostasis. In the typical adult, the liver exports daily 180 g of glucose, 100 g of triacylglycerol, and 14 g of albumin. Its metabolic energy is derived primarily from fatty acid oxidation.

F I G U R E 22-2 Schematic representation of a white adipocyte.

to that of the liver. The typical adult has 13 kg of adipose tissue located extensively under the skin, in the abdominal cavity, around internal blood vessels, in skeletal muscle, and in mammary glands. Obesity results when this amount increases. White and brown adipocytes, the two types of fat cell, are schematically depicted in Figures 22-2 and 22-3. White adipocytes have a characteristic spherical shape with a large central droplet of triacylglycerol surrounded by a thin rim of cytoplasm, and nucleus, mitochondria, and endoplasmic reticulum situated peripherally. Brown adipocytes have multiple lipid droplets and numerous mitochondria, which impart a

Adipose Tissue Cosmetically, adipose tissue is viewed as an enemy; however its importance in energy homeostasis is second only

FIGURE 22-3 Schematic representationof a brownadipocyte.

488

red-brown color. White adipocytes store triacylglycerol for export as fatty acid to serve as fuel for other tissues. In brown adipocytes, the lipid is oxidized within the tissue to CO2 and leads to the production of heat. Brown adipocytes play a principal role in thermogenesis (Chapter 14). Human adipose tissue is mostly of the white type. It responds rapidly to the supply of and demand for fuel in the organism. Storage of fuel as triacylglycerol is advantageous over storage as glycogen, because triacylglycerols are more highly reduced and release more energy per gram when oxidized; in addition, lipids are stored anhydrously and therefore very compactly, whereas glycogen must have an aqueous environment. As stored, triacylglycerols yield four to five times more energy than do equivalent amounts of glycogen. If the adipose tissue in the typical adult were replaced by an energy-equivalent amount of glycogen, body weight would increase by 60% (from 70 to 115 kg). Adipocytes are highly active metabolically. They synthesize triacylglycerols from glucose or from fatty acids delivered by chylomicrons or very-low-density lipoproteins (VLDLs). Glucose supplies acetyl-CoA for the biosynthesis of fatty acids, the reducing power in the form of NADPH produced by the pentose phosphate shunt, and glycerol phosphate for esterification of fatty acids. Glycerol phosphate is derived from intermediates of glycolysis because adipocytes lack glycerol kinase. Adipocytes hydrolyze the triacylglycerol and release fatty acids. Energy for adipocyte function is derived primarily from fatty acid oxidation and TCA cycle activity. Skeletal Muscle The typical adult has about 35 kg of skeletal muscle (see also Chapter 21), which contains 5-6 kg of contractile protein. Thus, skeletal muscle contains the major portion of the body's nonlipid fuels. In severe wasting diseases or starvation, it can be depleted by about 40%. In the well-fed state, it contains about 1% of its wet weight as glycogen. Because of its mass, muscle contains almost four times as much glycogen as does the liver. Muscle glycogen is not directly available as a source of blood glucose because muscle lacks glucose-6-phosphatase, but when mobilized to support muscular activity, glycogen becomes available, via lactate for conversion into blood glucose in the liver (discussed later). During starvation, muscle protein provides amino acids, which become the primary carbon source for maintenance of glucose homeostasis. Since there is no "storage" form of protein, the protein that is degraded in muscle is the contractile protein. Resting muscle maintains itself mainly on energy


/i•/t•

,,,.....

Gluc~

,..,,,,&

"A=,,,oo,,,r,,

12ili;i) F I G U R E 22-4 Cori cycle. Skeletal muscle anaerobic glycolysis yields lactate, which in the liver is converted to glucose by gluconeogenesis, which provides more glucose for support of muscle energy needs. The 2 tool of ATP generated in muscle occurs at the expense of 6 tool of ATP in liver. Hepatic metabolism is maintained by fatty acid oxidation. Muscle glycogen cannot serve directly as a source of blood glucose because of the absence of glucose-6-phosphatase.

provided by fatty acid oxidation. Muscle contraction can be supported by anaerobic or aerobic glucose utilization and by oxidation of fatty acids and amino acids, depending on the intensity of the exercise and the availability of glucose and glycogen and of oxygen. The density of mitochondria is lower in skeletal muscle than in heart or liver, and severe exercise can result in the oxidative capacity's becoming insufficient. During anaerobic glycolysis, 2 mol of ATP is formed per mole of glucose whereas aerobic oxidation produces 34-36 mol. However, anaerobic glycolysis permits muscle to function quickly at high intensity in the absence of oxygen. The lactate produced can be removed by the circulation and oxidized by the heart or used by the liver for glucose homeostasis. Lactate can be oxidized in muscle during rest following intense exercise. The cyclic process of muscular anaerobic glycolysis maintained by hepatic gluconeogenesis is termed the Cori cycle (Figure 22-4). This cycle is energetically expensive, since gluconeogenesis from lactate requires the expenditure of 6 mol of ATP per mole of glucose formed (Chapter 15). At rest, muscle can be a major contributor of amino acid carbon for gluconeogenesis. During exercise it becomes the predominant user of metabolic fuels.

Brain The brain is a constant user of metabolic fuel, provided by other tissues. It uses ~ 20% of the total oxygen consumed by an adult in the resting state. Perhaps two-thirds of this

SECTION 22.2

Metabolic Roles of Organs

demand is used to maintain the transmembrane potential by action of membrane enzymes such as Na+,K +ATPase. In the well-fed human, the brain uses glucose exclusively for energy (with little or no formation of lactate) and depends on its continual supply, since it stores little glycogen. A decrease in the blood glucose level below ~ 50 mg/L can cause symptoms of dizziness and lightheadedness. More marked drops lead to coma and death if uncorrected.

Heart Although the molecular design of the contractile apparatus of heart muscle is very similar to that of skeletal muscle, the two tissues differ metabolically. Under essentially all circumstances, metabolism in heart muscle is aerobic and uses fatty acids or ketone bodies, lactate, and pyruvate. The heart stores some glycogen but appears to utilize it only when occlusion of a coronary artery prohibits aerobic metabolism. In starvation, cardiac muscle increases its endogenous glycogen stores approximately twofold. The heart can function well on glucose, oxidizing it anaerobically or aerobically, but if presented with glucose and fatty acids, it preferentially uses the fatty acids for its energy needs. Cardiac muscle does not contribute significantly to fuel homeostasis, but its ability to utilize any metabolic fuel makes it a

"scavenger." Kidneys The kidneys (see also Chapter 39) eliminate noxious material while preserving important metabolites; normally, very little glucose, ketone bodies, or amino acids is wasted. Renal tissue is important in amino acid homeostasis (see below) and has the same gluconeogenic capacity per gram of tissue as does liver, although the liver provides a larger amount because of its larger size. About 80% of the total energy produced by the kidneys is utilized in the active transport processes involved in urine formation. It readily oxidizes fatty acids, ketone bodies, glucose, and amino acids.

Gastrointestinal System The principal nutrients of the body are of the same type as those that it uses as stores, namely, complex carbohydrates, triacylglycerol, and protein. These are hydrolyzed within the intestinal tract to produce primarily mono- and disaccharides, amino acids and small peptides, glycerol, and free fatty acids (Chapter 12).

489

Blood and Other Body Fluids Blood distributes metabolic fuels among tissues. Approximately 40-50% of the body fluid of an adult is intracellular and separated from other fluid compartments by the cell membranes whose properties and transport systems determine which metabolites pass across them. Figure 22-5 shows the relationships and volumes of the fluid compartments of the body. If homeostasis is well maintained, the plasma concentration of a metabolite does not change because its utilization by one organ is matched by its release from another; thus, a low plasma concentration does not indicate a low flux. Simple monosaccharides (principally glucose) are the means for transporting carbohydrates. Glycogen and phosphorylated sugars do not occur to any significant extent in plasma. All 20 amino acids used in protein synthesis are present in plasma but at ratios that do not reflect those found in proteins. Particularly high are the levels of alanine and glutamine, which are important carriers of carbon and nitrogen between muscle, liver, and other tissues (Chapter 17). Organic acids present as anions include lactate and pyruvate (from anaerobic glycolysis in muscle and red blood cells), acetoacetate, /~-hydroxybutyrate (produced by oxidation of fatty acids), and several intermediates of the TCA cycle, although little importance has been attached to their interorgan transport. Other constituents of plasma reflect the metabolism, function, and metabolic processes of particular organs. Thus, urea, synthesized in liver, is the major vehicle for eliminating excess nitrogen; bilirubin is a reticuloendothelial cell product of porphyrin degradation; creatinine and its metabolite creatine, which is the primary elimination product, are derived almost entirely from muscle, where phosphocreatine serves as an immediate source of energy for contraction (Chapter 21). These specific plasma constituents can be assayed to evaluate disease processes. Macromolecules found in plasma include simple proteins, metalloproteins, glycoproteins, and lipoproteins. Albumin and the lipoproteins play a special role in metabolic

Intestinal absorption

4:. s1 5%

IINTRACELLULAR

12%

43%

Urinary excretion

FIGURE 22-5 Blood-fluidrelationships. Volumesare expressedas a percentageof body weight.

490


TABLE 22-1

Constituents of Endocrine Pancreas

Cell Type

Hormone

Structure

a

Glucagon

29 amino acids

13

Insulin

Two chains: A = 21 amino acids B = 30 amino acids

6

Somatostatin

Two forms: S- 14 = 14 amino acids S-28=2 8 amino acids 36 amino acids

Pancreatic polypeptide

homeostasis. The protein constituents of plasma do not serve as significant sources of carbon and nitrogen for cells.

Albumin One of the smallest and the most abundant plasma proteins, albumin plays a significant role in osmotic regulation and transport of free fatty acids. Albumin is synthesized in the liver at a rate of approximately 14 g/d, or 10% of the total protein synthesis of the body. Deviations from the normal concentration of albumin in plasma can indicate the state of hepatic function. Albumin is also present in interstitial fluid.

Lipoproteins The plasma lipoproteins play a central role in metabolic homeostasis. As discussed in Chapter 20, a lipoprotein is a complex composed of triacylglycerol, protein, cholesterol (free or esterified), and phospholipids. The phospholipid and protein form an external coat to render the particles soluble in plasma for transport around the body. The roles of lipoproteins in metabolic homeostasis are presented in Chapter 20.

22.3 Endocrine Pancreas and Pancreatic Hormones The endocrine pancreas is pivotal in metabolic homeostasis and an integral component of metabolic regulation (see also Chapter 30). It is composed of 1-2 million islets of Langerhans scattered throughout the organ and forming

Structurally Analogous Hormones Secretin Vasoactive intestinal peptide Gastric inhibitory polypeptide Glicentin IGF-I (somatomedin C) IGF-II Relaxin Nerve growth factor None known

None known

2-3% of its total weight. The islets contain at least four cell types---a, /3, 6, and F--that secrete glucagon, insulin, somatostatin, and pancreatic polypeptide, respectively (Table 22-1). The ot and/3 cells make up 20% and 75% of the total, respectively. Most islets have a characteristic distribution of cells; ot and 3 cells are located at the periphery and fl cells the center, but close intercellular communication exists between different cell types. The islet appears to function as an integrated unit rather than as four independent types of cell. Islets in the posterior portion of the head of the pancreas may contain up to 70% of F cells; those in the other regions of the pancreas may have fewer or none.

Insulin Structure and

Synthesis

The structures of insulin and proinsulin are given in Figure 22-6. Proinsulin is a single polypeptide chain of 86 amino acids that permits correct alignment of three pairs of disulfide bonds. Insulin is composed of an A chain of 21 amino acids and a B chain of 30 amino acids, the chains being held together by two disulfide bonds. A third disulfide bond is present within the A chain. Human insulin differs from porcine insulin by a single amino acid and from beef insulin by three amino acids. These substitutions do not significantly affect activity; hence the widespread use of bovine and porcine insulin in clinical therapy. Human insulin and its analogues have been synthesized by recombinant technology for clinical use. In one analogue, two amino acid residues, namely prolyl and lysyl residues at positions 28 and 29 on the B chain, respectively, are switched. This analogue is called lispro

SECTION22.3 Endocrine Pancreas and Pancreatic Hormones

491

F I G U R E 22-6 Structures of human proinsulin and insulin. Insulin is derived from proinsulin by cleavage at the dipeptides Arg-Arg and Lys-Arg to give A and B chains held together by disulfide bonds. In the pig, B30 is Ala. In the cow, A8 is Ala, A10 is Val, and B30 is Ala. Bovine and porcine insulins are used extensively in clinical practice.

insulin and has the same biological activity as normal human insulin. However, unlike normal human insulin, lispro insulin has a reduced capacity for self-association and thus begins its biological activity within 15 minutes of subcutaneous administration; it has become the insulin of choice in many patients with diabetes (discussed later). Insulin biosynthesis resembles that of other export proteins: gene transcription, processing and maturation of precursor mRNA, translation of mature RNA, translocation of preproinsulin to cisternae of the endoplasmic reticulum with removal of the N-terminal leader or signal sequence of 23 amino acids, folding of proinsulin and formation of the proper disulfide bridges, packaging of proinsulin into secretory granules, and conversion of proinsulin to equimolar amounts of insulin and connecting peptide (C-peptide) by site-specific enzymatic cleavages. The human insulin gene is located on the short arm of chromosome 11 and consists of a 5' untranslated region split by an intron; an exon that codes for the leader peptide; a region coding for the B chain; a region coding for C-peptide that is split by an intron; a region coding for the A chain; and a 3' untranslated region. Conversion of proinsulin to insulin and C-peptide in secretory granules involves site-specific cleavages at the Arg-Arg and Lys-Arg sequences (Figure 22-6); these serve as signals for proteolytic processing of many other proteins. Cleavage occurs at the C-terminal end of each pair by trypsin-like enzymes and is followed by

removal of the basic residues by a carboxypeptidase B-like enzyme. Insulin monomers undergo noncovalent dimerization by formation of antiparallel/3-pleated sheet associations between monomers involving the C-terminal portion of the B chain. As discussed earlier, lispro insulin, in which the B28 and B29 is reversed from the normal prolyl and lysyl sequence, does not dimerize. Three insulin dimers subsequently self-associate to form hexamers in the presence of Zn 2+. The Zn 2+ hexameric array of insulin probably gives the/3-cell granule its unusual morphologic characteristics. Zn 2+ is released when insulin is secreted. Conversion of proinsulin to insulin in the secretory granule is not complete and some proinsulin is also released upon secretion of insulin. Proinsulin has less than 5% of the biological activity of insulin. The C-peptide has no physiological function but assay of C-peptide helps distinguish between endogenous and exogenous sources of insulin. Like genes for other proteins, the insulin gene may undergo mutation and produce an abnormal product. This process may be suspected in individuals who exhibit hyperinsulinemia without hypoglycemia or evidence of insulin resistance. Three abnormal insulins have been documented: one in which Ser replaces Phe at B24, another in which Leu replaces Phe at B25, and a third in which the Lys-Arg basic amino acid pair is replaced by Lys-X (X = nonbasic amino acid). The former two mutations occur in the nucleotide sequence that codes for an invariant

492


Sulfonylurea-., K+ ] _ _ t ' 2 " ~ , ~ ~ Diazoxide -\~'~17 ~

~~l~ Kir6.2

Glucose (G

I v

[

I

I

I ]

I _ ~ I ] [ PLC]

Acetylcholine-----~

}+G

~

~, G-6-P Glucokinase

G~/

P

~

DAG ~ PKC ~

Voltage-sensitive channel

Depolarization of membrane potential

ATP

~--'---~

Ca 2+ ' S

,

/

Ca

Ca2+ ~ / ~~(~ ,(-~'

GLP insnllns g

/'~~PKA

cA ~

/ G~ ~'

t. Insulin secretion

Norepinephrine

FIGURE 22-7 Diagrammatic representation of insulin secretion from pancreatic/3 cells. The sequence of events of insulin secretion coupled to glucose entry into/3 cells consists of glucokinase action, ATP production, inhibition of the ATP-sensitive K+ channel, membrane depolarization, Ca2+ influx, and insulin release. Neurotransmittersacetylcholine and norepinephrine stimulate and inhibit insulin secretion via trimeric G-proteins GQ and Gi, respectively. Giucagon-like peptide (GLP) promotes insulin release via the G-protein G~. Sulfonamides and diazoxide have direct effects on sulfonylurea receptors (SURs); the former promotes insulin release and the latter inhibits insulin release. +, Stimulation; -, inhibition. Other abbreviations are given in the text.

tetrapeptide sequence (B23-B26), and hence the abnormal insulins have greatly diminished biological activity. The third mutation yields an abnormal form of proinsulin in which the C-peptide remains attached to the A chain after processing. It is expressed as an autosomal dominant trait (familial hyperproinsulinemia). Affected individuals show no signs of insulin resistance and may or may not exhibit mild hyperglycemia.

Secretion Multiple factors regulate insulin secretion from the pancreatic fl cells. Glucose, amino acids, glucagon, acetylcholine and fl-adrenergic agents stimulate insulin secretion, whereas somatostatin and ot-adrenergic agents exert inhibitory influences. Most notable of the insulin secretagogue is glucose. The sequence of events that leads to

insulin secretion by the/3 cells as the threshold of blood glucose increases is shown in Figure 22-7. The normal fasting plasma glucose is maintained between 70 and 105 mg/dL (3.89-5.83 mmol/L). Glucose enters the/3 cells by means of glucose transporters GLUT1 and GLUT2 (Chapter 13). GLUT1 is a constitutive glucose transporter and GLUT2 is a low-affinity glucose transporter capable of sensing the glucose concentration in fl cells. Glucose transport is not a rate-limiting step in the fl-cell glucose metabolism because the transporters are present in greater abundance relative to physiological rates of glucose entry. After glucose enters the/3 cells, it is converted to glucose-6-phosphate by glucokinase, which is an isoenzyme of hexokinase. Glucokinase has a low affinity for glucose, is the rate-limiting step for glucose metabolism and is not affected by feedback inhibition. The glucose-sensing device that allows rapid and precise

SECTION 22.3

Endocrine Pancreas and Pancreatic Hormones

quantitation of the ambient glucose level appears to be glucokinase, which is ultimately responsible for glucoseregulated insulin secretion. The oxidation of glucose-6-phosphate via glycolysis, the TCA cycle, and the electron transport system coupled to oxidative phosphorylation leads to increased ATP levels (or ATP/ADP ratio) in the/3 cell. The elevated ATP levels cause a closure of the ATP-sensitive K + channel, leading to inhibition of K + efflux and depolarization of the/3-cell membrane potential. The ATP-sensitive K + channel consists of two different types of protein subunits, namely, the sulfonylurea receptor (SUR) and a potassium channel (Kir6.2). SUR is the regulatory subunit and belongs to the family of ATP-binding cassette proteins. Kir6.2 subunits participate in the actual conduction of K +. It has been suggested that the overall functional K + channel is an octameric complex composed of equal numbers of SUR and Kir6.2 subunits. The depolarization of/3-cell membrane that occurs with the closure of the ATP-sensitive K + channel activates the voltage-sensitive Ca 2+ channel causing Ca 2+ influx, which ultimately leads to release of insulin from stored insulin granules. The exocytosis of insulin involves both phosphorylation and exocytosisrelated ATPases. Along with insulin and C-peptide, a 37-amino-acid peptide known as amylin is also secreted during exocytosis. Amylin is obtained by proteolytic processing of an 89-amino-acid precursor molecule. The precise physiological role of amylin remains to be understood. Mechanisms that alter the level of cytosolic Ca 2+ in /3 cells, other than glucose metabolism, also affect insulin release. For example, a regulatory protein associated with the ATP-sensitive K+-channel, when occupied with sulfonylurea, inhibits K + efflux causing insulin secretion. Sulfonylureas are drugs used in the management of type 2 diabetes mellitus (discussed later). Diazoxide has an opposite effect to that of sulfonylureas. It either prevents the closing or prolongs the open time of the ATP-sensitive K + channel resulting in the inhibition of insulin secretion and thus hyperglycemia. Somatostatin inhibits Ca 2+ influx and causes diminished insulin secretion. Acetylcholine causes elevation of cytosolic Ca 2+ followed by insulin secretion caused by activation of GQ-protein, phospholipase C-inositol trisphosphate-Ca 2+, and protein kinase C (Chapter 30). Norepinephrine and epinephrine depress insulin secretion by binding at the o~-adrenergic receptor sites and by inhibiting adenylate cyclase mediated by the activation of the inhibitory G-protein (Gi). This leads to the inhibition of cAMP production and results in decreased activity of protein kinase A. Decreased protein kinase A levels determine exocytosis-related phosphorylation required for insulin secretion. Release of epinephrine during stress signals the need for catabolic rather than anabolic activity.

493

Depression of insulin secretion during exercise or trauma also is associated with epinephrine (catecholamine) secretion. A gastrointestinal hormone known as glucagon-like peptide (GLP-1) promotes insulin secretion via G-protein activation of the adenylate cyclase-cAMP-protein kinase A system. Gastrointestinal hormones that regulate insulin secretion may act in a feed-forward manner to signal gastrointestinal activity and metabolic fuel intake. Pancreatic glucagon, simply known as glucagon, stimulates secretion of insulin while somatostatin depresses it. Thus, various regulatory inputs to the/3 cells are integrated to maintain secretion of optimal quantities of insulin and to maintain glucose homeostasis. The coordinated activity of three pancreatic hormones (insulin, glucagon, and somatostatin) is essential for fuel homeostasis. Furthermore, the action of insulin is opposed by glucagon and by other counterregulatory hormones, namely, epinephrine, cortisol, and growth hormone. All of these hormones correct hypoglycemia by maintaining adequate levels of glucose in tissues such as brain, which is primarily dependent on glucose as a fuel source. Some amino acids also function as secretagogues of insulin secretion. An example is leucine, which stimulates the release of insulin by allosteric activation of glutamate dehydrogenase. Glutamate dehydrogenase, a mitochondrial enzyme, converts glutamate to ot-ketoglutarate by oxidative deamination (Chapter 17). Glutamate dehydrogenase is positively modulated by ADP and negatively modulated by GTE The ot-ketoglutarate is subsequently oxidized to provide ATP which blocks the ATP-sensitive K + channel, eventually causing insulin release. The overall glucokinase-glucose sensor mechanism as the primary regulator of glucose-controlled insulin secretion of/3 cells has been substantiated by identifying mutations that affect human glucokinase. Both gain-infunction and loss-of-function mutants of glucokinase are known. The activating glucokinase mutation (Va1455Met) with increased affinity for glucose results in hyperinsulinism with fasting hypoglycemia. Other mutations have been identified that impair glucokinase activity. These defects result in hyperglycemia and diabetes mellitus, known as maturity-onset diabetes of the young (MODY), which also occurs as a result of mutations in genes that encode hepatocyte nuclear factors l o~, 4o~, 1/3, and insulin promoter factor 1. Hyperinsulinemic hypoglycemia can be caused by mutations in the SUR/Kir6.2 components of the K + channel. Abnormalities can also result from defects in glutamate dehydrogenase function. Conversion of glutamate to ot-ketoglutarate by glutamate dehydrogenase provides substrates for ATP production and the enzyme is inhibited by GTP at an allosteric site. The importance of the sensitivity of glutamate dehydrogenase to inhibition by

494 GTP is illustrated in patients with hyperinsulinism with episodes of hypoglycemia. All of these patients carry mutations affecting the allosteric domain of glutamate dehydrogenase and result in insensitivity to GTP inhibition. This caused increased c~-ketoglutarate production followed by ATP formation and consequent insulin secretion. In addition to hyperinsulinemia, these patients also have high levels of plasma ammonia (hyperammonemia) that result from hepatic glutamate dehydrogenase activity. In liver mitochondria glutamate is converted to N-acetylglutamate, which is the required positive allosteric effector of carbamoyl-phosphate synthetase. Carbamoyl-phosphate synthetase catalyzes the first step in the conversion of ammonia to urea (see Chapter 17 for ammonia metabolism). Increased activity of glutamate dehydrogenase causes depletion of glutamate required for the synthesis of N-acetylglutamate. The decreased level of N-acetylglutamate impairs ureagenesis in the liver and is accompanied by hyperammonemia.

Biological Actions of Insulin Insulin affects virtually every tissue. Insulin is an anabolic signal and promotes fuel storage in the form of glycogen and triacylglycerols while inhibiting the breakdown of these two fuel stores. Insulin also promotes protein synthesis while inhibiting its breakdown. Regulation of expression of several genes, either positive or negative, is mediated by insulin. The genes involved in the expression of enzymes that participate in fuel storage (e.g., hepatic glucokinase) are induced; those that encode catabolic enzymes (e.g., hepatic phosphoenolpyruvate carboxykinase) are inhibited. The principal effect of insulin on blood glucose is its uptake into muscle and adipose tissue via the recruitment and translocation of glucose transporter 4 (GLUT4). The mechanism of action of insulin is complex and can be divided into three parts. The first part is the binding of insulin to its receptor on the cell membrane; the second consists of postreceptor events; and the third consists of biological responses.


incorporated into the cell membrane with the ot-subunits projecting into the extracellular space and the/3-subunit with its transmembrane subunit projecting into cytosolic space (Figure 22-8). The ot-subunits contain the insulin binding domain, but it is not clear whether insulin binds to both oe-subunits. Insulin binding to one site induces negative cooperativity for the second insulin binding site. The intracellular portion of the/3-subunit has a tyrosine-specific protein kinase (tyrosine kinase) activity. Tyrosine kinase activity initiated by insulin binding to ot-subunits results in autophosphorylation on at least six tyrosine residues in r Other protein substrates also are targets of a series of phosphorylations catalyzed by the activated/3-subunit tyrosine kinase. One of the key proteins that is phosphorylated is insulin receptor substrate- 1 (IRS- 1), which has many potential tyrosine phosphorylation sites as well as serine and threonine residues. Phosphorylated IRS-1 interacts and initiates a cascade of reactions involving other proteins. Phosphotyrosine motifs of IRS-1 have binding sites for SH2 domains (src homology domain 2) contained in signal transducing proteins such as phosphatidylinositol-3-kinase

Insulin binding domain

§

/

\

H3N ~

§ ~

NH3

S-S

Cysteine-rich domain

-OOC + H3N

ot - Subunit

COO S S

S S

NH 3

Transmembrane } domain lntracellular

13 -

Insulin Receptor The insulin receptor is derived from a single polypeptide chain that is the product of a gene located on the short arm of chromosome 19. The proreceptor undergoes extensive posttranslational processing consisting of glycosylation and proteolysis. The proteolytic cleavage yields ot (M.W. 135,000) and/3 (M.W. 95,000) subunits that are assembled into a heterotetrameric ( l Y Z f 1 2 ) complex. The subunits are held together by both disulfide linkages and noncovalent interactions. The heterotetrameric receptor is

+

Subunit

Tyrosine kinase domain

COO-

CO0-

22-8 Model of the insulinreceptor.The receptorcontainstwo a- and two fl-subunits, held togetherby disulfidelinkages.The o~-subunitsare entirely extracellular and containthe insulinbindingsite. The ~-subunitshave transmembraneand intracellulardomains.Bothautophosphorylationand tyrosine kinase activityreside in the/~-subunitand are markedlyenhanced upon insulinbinding. FIGURE

495

SECTION22.3 Endocrine Pancreas and Pancreatic Hormones

(PI-3-kinase). SH2 domains are conserved sequences of about 100 amino acids but the role of PI-3-kinase is not understood. Insulin-mediated IRS-1 phosphorylation also leads to activation of ras proteins. The ras proteins are protooncogene products that mediate signaling pathways from cell membrane receptors, the regulation of cellular proliferation, differentiation, or apoptosis. They are posttranslationally modified by farnesylation at a conserved cysteine residue in the C-terminal CAAX motif; the tripeptide AAX is removed by proteolysis and the newly exposed C terminus is methylated. The ras protein is functionally active when GTP is bound and inactive when GDP is bound. The ras-GTP/GDP cycle is regulated by GTPase-activating proteins and guanine nucleotide exchange factor proteins. The pathway ofIRS-1 to activation ofras protein involves a number of proteins such as growth factor receptor binding protein-2 (GRB-2). Activated ras protein stimulates a cascade of serine/threonine kinases affecting transcriptional activation of specific genes. These pathways also involve proteins with SH2 domains. In summary, responses due to insulin binding to its receptor involve a cascade of phosphorylation and dephosphorylation steps that lead to gene regulation. One of the major effects of insulin is to promote glucose transport from the blood into muscle and adipose tissue cells. This is accomplished by recruiting and localizing GLUT4 receptor molecules in the plasma membrane. In the absence of insulin GLUT4 remains in the intracellular vesicles. Insulin-mediated GLUT4 trafficking to the cell membrane is constitutive, multicompartmental, and involves the participation of several proteins. Impairment in any of the steps of GLUT4 transport can cause insulin resistance and diabetes mellitus. The importance of the biological actions of insulin on target cells is underscored by defects in any of the five steps involved in receptor function. These steps are analogous to the scheme described for LDL receptor gene defects (Chapter 20). Mutations can lead to: 1. 2. 3. 4. 5.

resistance, acanthosis nigricans, and hyperandrogenism. The latter two have been ascribed to toxic effects of insulin on the skin and ovaries. Insulin resistance is also associated with hyperandrogenism and polycystic ovary disease syndrome (diabetes mellitus is discussed later in this chapter). Insulin is catabolized (inactivated) primarily in the liver and kidney (and placenta in pregnancy). Liver degrades about 50% of insulin during its first passage through this organ. An insulin-specific protease and glutathioneinsulin transdehydrogenase are involved. The latter reduces the disulfide bonds with separation of A and B chains, which are subjected to rapid proteolysis.

Glucagon Glucagon is a single-chain polypeptide of 29 amino acids that has a structural homology with secretin, vasoactive intestinal polypeptide (VIP), and gastric inhibitory polypeptide (GIP). The glucagon structure is also contained within the sequence of glicentin ("gut glucagon"). The synthesis of glucagon in the pancreatic ot cells probably involves a higher molecular weight precursor. Glucagon secretion is inhibited by glucose and stimulated by arginine and alanine. Depressed plasma glucagon levels result in depressed hepatic glucose output at times when glucose is available by intestinal absorption. Amino acid-stimulated glucagon secretion counteracts the effects of the coincidental secretion of insulin, which otherwise would provoke hypoglycemia. Secretion of glucagon from the o~ cells and somatostatin from the 3 cells is regulated by the other pancreatic hormones. Figure 22-9 illustrates the coordination of islet hormone secretion. This coordination may occur by the product of one cell type regulating secretion by the others and by direct intercommunication between cells

Impaired receptor biosynthesis, Impaired transport of receptors to the cell surface, Decreased affinity of insulin binding, Impaired tyrosine kinase activity, and Accelerated receptor degradation.

Examples of disorders caused by mutations in the insulin receptor gene are leprechaunism and type A insulin resistance. A severe form of leprechaunism is due to mutations in both alleles of the insulin receptor gene. These patients exhibit insulin resistance, intrauterine growth retardation, and many other metabolic abnormalities. Patients with type A insulin resistance exhibit insulin

FIGURE 22-9 Paracrine regulation of islet cell hormonal secretion. Q, Stimulation; Q, inhibition. [DrawnafterR. H. Unger,R. E. Dobbs, and L. Orci,Insulin, glucagon, and somatostatin secretionin the regulation of metabolism. Annu. Rev.Physiol. 40, 307 (1978). @ 1978by AnnualReviewsInc.]

496

(of the same or different types). Glucagon secretion is also stimulated by gastrin and cholecystokinin, presumably as an anticipatory response to intestinal fuel absorption. Stress and catecholamines provoke glucagon secretion, which increases output of hepatic glucose and adipocyte fatty acids to provide the extra energy requirements. The primary targets for glucagon are the liver and adipose tissue. It stimulates hepatic glycogenolysis, gluconeogenesis, and ketogenesis and inhibits glycogen synthesis. Glucagon stimulates adipocyte lipolysis to provide fatty acids to tissues for which glucose is not the obligatory fuel. In both liver and adipose tissues, glucagon activates adenylate cyclase, increases cAMP level, activates cAMPdependent protein kinase, and increases phosphorylation of the controlling enzymes in the various metabolic pathways (Chapter 15).The initial step in the glucagon signaling pathway involves its binding to the receptor on the cell membrane and activation of G-protein complex (Chapter 30). The major function of insulin and glucagon is to maintain fuel homeostasis. Since glucagon opposes the actions of insulin, the [insulin]/[glucagon] ratio determines the metabolic fate of fuels due to the induction or repression of appropriate enzymes. Enzymes induced by a high [insulin]/[glucagon] ratio and repressed by a low ratio are glucokinase, citrate cleavage enzyme, acetyl-CoA carboxylase, #-hydroxy#-methylglutaryl-CoA (HMG-CoA) reductase, pyruvate kinase, 6-phosphofructo- 1-kinase, 6-phosphofructo-2kinase, and fructose-2,6-bisphosphatase. Enzymes induced by a low [insulin]/[glucagon] ratio and repressed by a high ratio are glucose-6-phosphatase, phosphoenolpyruvate carboxykinase, and fructose-l,6-bisphosphatase. Metabolic implications of these actions are discussed later.


Somatostatin Two forms of somatostatin exist: S 14 and $28, which are single-chain polypeptides of 14 and 28 amino acids, respectively. Somatostatin is synthesized in the 6 cells of the islets, the gut, the hypothalamus, and several other areas of the brain. Its actions appear to be primarily paracrine in nature (i.e., on other cell types in the immediate vicinity of its secretion). In the islets it blocks insulin and glucagon secretion; in the pituitary gland it inhibits growth hormone and thyroid-stimulating hormone (TSH) release; in the gut it blocks secretion of gastrin and motilin, inhibits gastric acid and pepsin secretion, and suppresses gallbladder contraction. The action of somatostatin on the gastrointestinal tract leads to decreased delivery of nutrients to the circulation.

Pancreatic Polypeptide Pancreatic polypeptide, a peptide of 36 amino acids, is secreted in response to fuel ingestion and potentially affects pancreatic exocrine secretion of bicarbonate and protein.

22.4 Stored Fuels Table 22-2 lists the amounts of fuels stored in the human body. The amount of glycogen present would not suffice for even 1 day of normal energy expenditure. Triacylglycerol and protein are the principal energy stores, with lipid being in highest abundance. Although glucose, amino acids, fatty acids, and triacylglycerols are present in body fluids, their amounts would not support life for 1 hour. Glycogen is rapidly mobilized in the first 24-36 hours of fasting; little is mobilized after the fourth day of fast. Protein degradation starts off at a

TABLE 22-2

Metabolic Fuels Stored in a Normal 70-kg Human*

Tissues Triacylglycerol (adipose tissue) Glycogen (liver and muscle) Protein (primarily muscle)

Wt(kg) 15 0.4 6

Available Energy kcal(kJ) 141,000 (589,944) 1,500 (6,276) 24,000 (100,416)

Extraeellular Fluids Glucose Lipid (fatty acid and triacylglycerol) t Amino acids

0.02 0.05 0.03

*Data from G. F. Cahill: Starvation in man. N. Engl. J. Med. 282, 668 (1970). tTotal lipid varies from this level down to 0.005 kg between periods of eating and fasting.

80 (335) 400 (1,674) 120 (502)

SECTION 22.5

CarbohydrateHomeostasis

low level, increases to a maximum by about the fourth, and then decreases to a point below the initial level. Protein is not mobilized rapidly and even at maximum utilization accounts for only ~ 25 % of fuel expenditure. Protein degradation is limited mainly to the maintenance of blood glucose. Starting postprandially, oxidation of lipid accounts for the bulk of fuel utilization for metabolic homeostasis. As fasting progresses, the total energy utilized slowly decreases, reflecting the decreasing energy expenditure, but lipid continues to be the major energy source. Few people in developed countries undergo direct prolonged starvation, but disorders such as alcoholism and anorexia nervosa reflect a similar pattern of fuel utilization. The starvation pattern is very helpful for understanding metabolic control and will be used throughout this chapter. The metabolic controls that function in a long-term fast are similar to those that occur in the normal day-to-day pattern of fasting and feeding.

Appetite, Hunger, and Control of Food Intake The perception that the extent of food intake by humans is poorly controlled is false because most people, once they have matured physiologically, maintain a stable body weight. There appears to be a set point for weight (as there is for body temperature) in each individual. During fasting, body weight decreases. Upon refeeding, an initially greater intake of food returns the body to the original weight. Conversely, if a mammal is force-fed body weight increases, but when force-feeding is stopped, the animal voluntarily decreases food intake until the body has returned to the original weight. Even in most cases of obesity, a set body weight is manifest, albeit at a higher than normal level. A 70-kg adult typically consumes 5-10 tons of food between the ages of 20 and 40. Since body weight will most frequently be maintained at its set point or close to it, food intake is finely controlled to within 4-1% over time. A 1% increase in food intake over this 20-year period would result in a 100-pound gain in weight. Both short-term regulation of starting and stopping of eating and long-term regulation of total food intake occur. Both composition and content are regulated. Different foods have different energy contents. Thus, the amount of food eaten decreases if it has a higher energy content per unit of weight; the result is a constant energy intake. After ingestion, food may either be stored or utilized, with release of energy. Metabolic homeostasis requires a balance between energy input and energy output. Energy output occurs as heat produced, work produced, or maintenance of cellular integrity. These factors are balanced by the signals for hunger, appetite, and satiety. Eating starts and stops voluntarily in a manner that is not related

497 simply to body weight (short-term regulation) but also to the need for energy and specific constituents (long-term regulation). Regulators of food intake, fuel stores, and energy balance are many and involve a complex set of physiological signals. Some of the regulators are blood glucose level, which is maintained by insulin and other counterregulatory hormones, the mass of adipose tissue and the adipocytederived hormone leptin, cerebral cortex and hypothalamic neurons, hypothalamic-pituitary-adrenal axis, distention of the gastrointestinal tract, and release of gastrointestinal and gonadal hormones. The prevalence of overweight and obesity are global health problems. The risk of death from all causes is increased in both conditions and is discussed later.

22.5 Carbohydrate Homeostasis Most tissues can use fatty acids as their primary, if not sole, source of metabolic energy. Brain and other nervous tissues, except in long-term fasting use glucose as the sole energy source; even in long-term fasting they require significant amounts of glucose. Red blood cells can obtain energy only by anaerobic glycolysis. Skeletal muscle at rest uses predominantly fatty acids, but in heavy exercise it also draws on muscle glycogen and blood glucose. Because brain and red blood cells depend on glucose for energy, glucose must always be available. Glucose occurs in plasma and interstitial fluid at a concentration of approximately 80 mg/dL; this value corresponds to about 20 g in the extracellular compartment of a typical person. Approximately 180 g of glucose is oxidized per day. The body must therefore replenish the total blood glucose concentration about nine times a day; nevertheless, the concentration in blood remains remarkably constant. The glucose level following an overnight fast is approximately 80 mg/dL. Following a meal, such as breakfast, the level rapidly rises by 30-50 mg/dL, but within 2 hours it returns to the previous level where it remains until the next meal when the pattern is repeated. The remarkable stability of the blood glucose level is an indication of the balance between supply and utilization. Maintenance of this balance is discussed below.

Carbohydrate as a Food Carbohydrate is essential for the survival of some tissues and as a structural constituent of nucleic acids, glycoproteins, proteoglycans, and glycolipids. The normal adult can synthesize all the needed carbohydrate from noncarbohydrate sources, namely, amino acids and glycerol. Thus,

~98

humans can exist with little or no dietary carbohydrate intake. In Western cultures, the diet generally consists of approximately 45% carbohydrate, 43% lipid, and 10% protein. Of the carbohydrate, about 60% is starch, 30% sucrose, and most of the remainder lactose. In less affluent cultures, the typical diet contains more grains and vegetables, with correspondingly higher levels of carbohydrate. Dietary carbohydrate is digested to glucose (80%), fructose (15%), and galactose (5%). Glucose and galactose are actively transported directly into the blood by the intestinal cells. Fructose is absorbed by a specific system that is distinct from glucose and galactose absorption (Chapter 12). Part of the fructose is metabolized by the intestinal cells, and the rest enters the portal blood. Liver and kidney are the other sites of fructose utilization (Chapter 13).

Disposition of High Glucose Intake A temporary rise in plasma glucose concentration immediately follows a meal, but within 2-3 hours the level returns to the preprandial level. Several factors influence the blood glucose profile. 1. Digestion and absorption of carbohydrate are very rapid. The rate of appearance of glucose in plasma is determined by gastric motility and emptying and by intestinal absorption. The glucose peak is characteristically only 30-50% above the fasting level. If the total amount of glucose in a typical meal (50-100 g) were to enter the blood and not be used immediately, total blood glucose would go up almost fourfold. This rise does not occur because the glucose entering the plasma is rapidly taken up by the tissues. The influx of glucose following a meal is accompanied by a rapid decrease in hepatic secretion of glucose, while utilization by tissues dependent on glucose remains unchanged. Thus the rise in blood glucose level after a meal reflects only a portion of the glucose entering the system from digestion and absorption. During fasting blood glucose remains at a constant level because utilization is matched by formation. 2. The rise in blood glucose level is quickly followed by a rise in the blood insulin level which increases threeto tenfold. An elevated blood glucose level directly stimulates pancreatic insulin release, as do leucine and arginine derived from digestion of protein. Release of gastrin, cholecystokinin, gastric inhibitory peptide, and glicentin appears to stimulate insulin release in an anticipatory manner in addition to regulating other digestive responses. 3. Concomitant with insulin release are changes in glucagon release, whose magnitude and direction


depend on dietary composition. If the diet contains only carbohydrate, glucagon secretion falls precipitously because of direct inhibition of the ot cells by glucose and of the insulin released from /3 cells. On a diet high in protein, glucagon secretion is stimulated as a consequence of amino acid influx. After a meal that contains both carbohydrate and protein, plasma glucagon levels may not change perceptibly. 4. The liver is freely permeable to glucose and typically extracts about half of the carbohydrate load. This glucose is phosphorylated to glucose-6-phosphate by glucokinase and hexokinase. After a glucose load, glucokinase is more important because it has a high Km, is inducible, and is sensitive in a wide range of glucose concentrations. After overnight fasting, most carbohydrate taken up by the liver is converted to glycogen. This regulation is illustrated in Figure 22-10. Insulin promotes dephosphorylation of the inactive glycogen synthase D to the active glycogen synthase I form. Depressed levels of glucagon and elevated levels of insulin decrease hepatic cAMP levels and the cAMP-dependent protein kinase inactivation of glycogen synthase. High levels of glucose promote high levels of glucose-6-phosphate, a feed-forward, allosteric, positive modifier of glycogen synthase, and result in hepatic glucose storage. Glycogen synthesis is affected by the level of glycogen itself, which inhibits glycogen synthase phosphatase. All of these events occur rapidly. Insulin also has a long-term effect on the liver by inducing the synthesis of glucokinase (see also Chapter 15).

Glucose Glucose6-phosphate..,, Glu!ose~,Lphosphate (~ / UDP-glucose 'i', ~Gn-~-~

Glucagon

Phosphorylase(

~ Glycoge,nsynthaseD cA[VIP ~'----Gn+lJ ---(i~""~ - := Protem " kinase. . . . 0-" I. --Q-" Phospatase",$,.--."/ GlycogensynthaseI (active)

FIGURE 22-10 Hepatic glycogensynthesisafter a meal. The permeabilityof the liverto glucose providesthe substrate for hepatic glycogensynthesis.This synthesis is controlled by (1) activationof glycogensynthaseby insulin, (2) allosteric activationof glycogensynthaseby glucose-6-phosphate,and (3) absenceof cAMEblockingthe cAMP-dependentprotein kinase inactivation of glycogensynthase.Synthesisendswhenglycogenstores are high as a result of inhibition of glycogensynthasephosphatase by glycogen.

SECTION22.5 Carbohydrate Homeostasis

5. The liver plays a major role in converting excess glucose into triacylglycerols, packaging them into VLDLs, and exporting them to nonhepatic tissues. Thus, glucose in excess of that needed to restore glycogen levels ends up stored as triacylglycerol in adipocytes. 6. Glucose that is not sequestered in the liver is eventually distributed within the extracellular space and accounts for the increased plasma glucose levels. This glucose is rapidly metabolized by the other tissues. 7. Insulin stimulates uptake of glucose into muscle by recruiting more glucose transporters to the cell membrane. The glucose that is taken up will be used to replenish muscle stores of glycogen through reaction mechanisms similar to those depicted in Figure 22-10 (except that glucagon does not regulate muscle metabolism). Muscle glycogen levels will be restored, while any protein degraded for gluconeogenesis during fasting will be replenished. The signal for increased protein synthesis is insulin. 8. Much of the glucose not taken up by liver and muscle is taken up by adipocytes under the influence of insulin-stimulated transport of glucose into the cell. Esterification of fatty acids for storage as triacylglycerols is dependent on the availability of o~-glycerophosphate derived in situ from glucose. The glucose can also be converted into fatty acids from acetyl-CoA generated via glycolysis and the pyruvate dehydrogenase reaction and from NADPH obtained via the pentose phosphate pathway. 9. With the exception of brain, liver, and blood cells, insulin directly stimulates the entrance of glucose that is used for anabolic processes in most cells of the body. The disposition of a high glucose intake is presented in Figure 22-11. As much glucose as possible is stored in liver and muscle, and the remainder is used for biosynthetic purposes or converted to fatty acids and stored as triacylglycerol. Fatty acid synthesis in liver leads to secretion of triacylglycerols as VLDLs and transport to adipose tissue. Plasma glucose levels do not exceed the renal threshold of 200 mg/dL for glucose. Insulin stimulates a variety of anabolic processes in addition to those of glycogen, protein, and triacylglycerol synthesis by mechanisms that may involve rapid, covalent modification of key regulatory enzymes or regulation of their synthesis. Insulin stimulates glycolysis for provision of anabolic metabolites. Low levels of glucagon, glucocorticoids, and catecholamines, which oppose the action of insulin, also contribute to rapid removal of glucose.

1199

Glucose Tolerance Intake of carbohydrate leads to a characteristic change in blood glucose level that may be affected by many factors, such as abnormalities in insulin secretion or effectiveness. Normal and abnormal glucose tolerance responses are shown in Figure 22-12. The test is performed on a patient who has fasted overnight and is at rest. The patient ingests 75 g of glucose, and the blood glucose level is measured over a 2- to 4-hour period. In a normal individual, the fasting level is constant before and shortly after glucose ingestion. Only values outside the range of normal fasting values (denoted by the shaded bar) are helpful diagnostically. Glucose intolerance is manifested by a slower return to the fasting level. Many factors affect the shape of the curve. After an overnight fast, hepatic glycogen stores will be low, but if the fast is shorter they may be much higher and the plasma glucose levels would remain elevated longer. Tissue sensitivity to insulin is modulated by past dietary composition, obesity, stress, age, and exercise. After a diet rich in carbohydrate, for example, insulin levels would be increased by down-regulation of insulin receptors, tissue insulin insensitivity, and glucose intolerance. After fasting or on a low-carbohydrate diet, insulin responsiveness and glucose tolerance would be greater. Exercise enhances insulin sensitivity. The pattern shown in Figure 22-12 is typical of glucose intolerance observed at the onset of insulin-independent diabetes. The fasting level of glucose is frequently in the normal range in mild diabetes, but after glucose ingestion the level stays higher for a longer period of time. As the plasma glucose level decreases, it falls slightly below the fasting level before returning to normal, since restoration of insulin to the normal fasting level occurs slightly after that of the blood glucose level. This depression can be heightened in "reactive hypoglycemia." If, after a fast of 18-24 hours a small amount of glucose in a highly digestible form is consumed (such as the sucrose in a candy bar), the blood glucose level will be temporarily elevated followed by rapid elevation of insulin level, rapid entry of glucose into tissues, and depression of plasma glucose level below normal.

Glucose Homeostasis during Fasting During most of the day the plasma glucose level remains constant, even after a 24-hour fast. In very prolonged fasts, the plasma glucose level decreases very slightly. During these periods, glucose is being actively metabolized by tissues such as brain and red blood cells, and is replenished. During a 24-hour fast, the body uses approximately 180 g


~00

FIGURE 22-11 Disposition of high glucose intake. FA, Fatty acid; TG, triacylglycerol;VLDL, very-low-densitylipoprotein; LDL, low-density lipoprotein.

of glucose, initially derived from hepatic glycogenolysis but with an ever-increasing contribution from gluconeogenesis.

150 / ~ " -.., , ,,,..,./ G lucose intolerant A

v

o 100

Utilization of Hepatic Glycogen The signal for glycogen breakdown is glucagon secreted in response to hypoglycemia. Glucagon stimulates hepatic glycogenolysis by the adenylate cyclase cascade (Figure 22-13). The hepatic plasma m e m b r a n e is markedly sensitive to glucagon, which stimulates adenylate cyclase with formation of c A M E activation of phosphorylase kinase and phosphorylase, and degradation of glycogen to glucose-1-phosphate, then to glucose-6-phosphate, and finally to blood glucose. In hypoglycemia, insulin levels are low, and no hepatic glycogen synthesis occurs. Thus,

//a-Ul~

" 125

O O

~

"'~

,'ff/ / / / / / ~ . / ~ / .

Norrnal

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75 I

I

I

I

I

0

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2

3

4

Hours after glucose ingestion FIGURE 22-12 Typical glucose tolerance response for a normal individual and one with mild glucose intolerance. The shaded area shows the range for plasma glucose levels in normal fasting individuals. At the peak value, the level for a glucose-intolerantindividual often is not significantlydifferent from that of a normal individual; thus, intolerance is best detected at the later time points when glucose values remain higher.

SECTION22.5 CarbohydrateHomeostasis

501 glucose-producing mechanism. Hepatic glycogenolysis is also regulated by catecholamines. Catecholamine release is less sensitive to hypoglycemia than glucagon release but plays a significant role in stress and high-intensity exercise. Catecholamines act via stimulation of calcium release from endoplasmic reticulum along with allosteric activation ofphosphorylase kinase that results in activation of phosphorylase, degradation of glycogen, and hepatic glucose output.

............... Inositoltriphosphate Adenylatecyclase ATP cAMP~

Cytosolic Ca2% )

cAMP-dependent Phosphorylase / protein kin a s ~ . ~ , / ~ i n a s ~ J Activated by phosph~rylation

Activated allosterically by Ca2+ ......

Phosphorylaseb

U t i l i z a t i o n of Skeletal M u s c l e G l y c o g e n

j

Phosphorylasea

Glycogen

Glucose 1-phosphate

[[

Glucose6-phosphate

t

Glucose FIGURE 22-13

Regulation of hepatic glycogenolysis.Glucagon and epinephrine can activate hepatic glycogenolysisand lead to glucose release. Glucagon acts primarilyby increasing cytosoliclevels of cAMP,whereas epinephrine acts predominantly by increasing the release of calciumfrom endoplasmic reticulum into cytosol. In either circumstance,phosphorylasekinase is activated, cAMP mediatesthe phosphorylation of phosphorylase kinase, while calciumactivatesphosphorylasekinase allosterically.The result is the phosphorylationand activationof phosphorylase b. Someoverlap between the two mechanismsoccurs. glucagon and insulin function to regulate glycogen synthesis and degradation that directly reflects the blood glucose level. In the initial phases of starvation, this is the major

FIGURE 22-14

Coordinated regulation of skeletalmuscle metabolismby nucleotides.

The muscle of a 70-kg human contains about 300 g of glycogen, but this glycogen is not readily available to maintain blood glucose levels because muscle lacks glucose-6-phosphatase. However, the Cori cycle provides a means for muscle to function anaerobically during intense exercise. Thus, muscle glycogen contributes to plasma glucose homeostasis, although its conversion to lactate is regulated by the metabolic demands of muscle contraction. The ATP/ADP cycle links muscle contraction and conversion of muscle glycogen to lactate. Since there are only limited amounts of adenine nucleotides, these processes are tightly coupled. The coordination of muscle glycogen utilization and contraction is presented in more detail in Figure 22-14. Regulation occurs because 1. ADP is a required substrate for glycolysis (stoichiometric regulation), 2. Phosphofructokinase catalyzes an irreversible step in glycolysis and is subject to allosteric inhibition by

502

ATP and activation by inorganic phosphate, 5'-AME and ADP (Chapter 13), and 3. Skeletal muscle phosphorylase b is allosterically activated by 5'-AME Conversion of phosphorylase b to phosphorylase a is affected by epinephrine, through increased levels of cAME in a sequence of reactions similar to that indicated in Figure 22-13. Following depolarization of the muscle cell membrane, calcium is released into the sarcoplasm from the sarcoplasmic reticulum, resulting in the calcium-dependent activation of phosphorylase kinase and conversion of phosphorylase to the "a" form.

Gluconeogenesis As hepatic glycogen stores become depleted, gluconeogenesis becomes increasingly important. Although the kidney is a source of glucose during protracted starvation, during brief fasting at least 90% of total gluconeogenesis occurs in the liver. The three sources for this glucose synthesis are the following. 1. Glycerol: The hydrolysis of triacylglycerol to fatty acids by adipocytes releases glycerol, which is released into the plasma and converted to glucose in the liver. It makes only a minor contribution to the total glucose formed, but it helps to conserve protein. 2. Lactate: In sedentary humans 10-30% of plasma glucose is recycled from lactate that originates from glycolysis in blood cells. During exercise, anaerobic glycolysis in muscle increases leading to enhanced gluconeogenesis. 3. A m i n o acids: Amino acids are derived from proteolysis in muscle. All except leucine and lysine are potentially gluconeogenic in mammals; however, alanine is the primary amino acid released from muscle. This alanine is derived from muscle protein and from the metabolism of many other amino acids.

CHAPTER 22 Metabolic Homeostasis

Lactate Alanine ] Serine

, cine S

Aspartate=

J

> Pyruvate

Glycerol

'

I

Glycerol 3-phosphate

>..Oxaloacetate x.

~

Valine / . ~ 9 . . ] / / S u c c , nate iso~euclne ~=::=~/--1-1-,^ \ Th,~u.,.~ ~ - . . . . Methionine J ~ CYCLE ] _ \ / Glutamate ~ k , ~ - K e t o g l u t a r a t e / / Proline I " ~ J Histidine J

~

~ . Glucose /

Rate-limitingsteps Oxaloacetate -" Phosphoenolpyruvate FDP--* Fructose 6-phosphate Glucose 6-phosphate --*Glucose

FIGURE 22-15 Entry of precursors into the pathway of gluconeogenesis.

to the status of gluconeogenesis include glucagon (an acute modulator), glucocorticoid (a chronic modulator), and absence of insulin. The general effects of these hormones are shown in Figure 22-16. Glucagon is directed toward glycogen degradation, gluconeogenesis, triacylglycerol hydrolysis, and fatty acid oxidation. In the liver, it stimulates glycogen degradation (Figure 22-13) and gluconeogenesis, possibly by inactivation of pyruvate kinase. Glucagon also modulates the levels of fructose-2,6-bisphosphate (Chapter 15). Insulin opposes these effects by stimulating glycogen synthesis, glycolysis, and fatty acid synthesis. The stimulation of glycolysis by insulin is not meant to increase energy production, since glycolysis is amphibolic and provides metabolites for biosynthesis, e.g., acetyl-CoA for fatty

Figure 22-15 shows the points of entry of precursors into hepatic gluconeogenesis. Lactate and alanine enter the gluconeogenic pathway at the pyruvate carboxylase reaction. Glycerol is phosphorylated and then oxidized to dihydroxyacetone phosphate. A number of amino acids are converted to intermediates of the TCA cycle. Conversion of gluconeogenic substrates to glucose is not a direct reversal of glycolysis, since the pathway differs at three rate-limiting steps (Chapter 15).

Regulation of Gluconeogenesis Tissues involved in glucose conservation are liver, skeletal muscle, and adipose. Signals that attune the body

FIGURE 22-16 Effects of glucagon,glucocorticoids,and insulin on the pathwaysof carbohydrateand lipid metabolism.G-6-P,Glucose-6-phosphate; PEP, phosphoenolpyruvate;OA, oxaloacetate;FA, fatty acid; TG, triacylglycerol.

SECTION22.5 CarbohydrateHomeostasis

503 muscle, amino acids from muscle protein, and glycerol from adipocytes. The kidney reabsorbs the glucose in the glomerular filtrate and is a site of gluconeogenesis. In mammals, there is no net conversion of fatty acids to glucose because during oxidation of acetyl-CoA to CO2 via the TCA cycle there is no net production of oxaloacetate. However, fatty acids, play an important role in glucose homeostasis (Figure 22-17). Since brain and red blood cells use glucose, the heart functions well on fatty acids without depleting glucose levels. When glucose levels are low, only tissues that have a requirement for glucose and are insulin-independent use glucose because, with low insulin levels, there is essentially no glucose uptake by insulin-dependent tissues. After fasting for a 24-hour period, 180 g of glucose is produced from glycogen or gluconeogenesis, of which 75% is used by the brain and the remainder by red and white blood cells, etc. From the latter, about 36 g of lactate return to the liver for gluconeogenesis,

acid synthesis. Glucocorticoids stimulate fatty acid breakdown and gluconeogenesis and increase the rate of glycogen synthesis. This action appears counterproductive, but glucocorticoids are a major signal for the degradation of muscle proteins and thus for gluconeogenesis. Glucocorticoid effects are long term (whereas those of glucagon are short term), but they are synergistic and act through induction of enzymes of gluconeogenesis. Regulation by glucocorticoids may involve increases in the amounts of regulatory proteins that are involved in the actions of glucagon. Many allosteric regulators in addition to hormones fine-tune the gluconeogenic pathway at the steps catalyzed by pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-bisphosphatase, and glucose-6phosphatase. A summary of glucose homeostasis during fasting is shown in Figure 22-17. The sources of carbon for hepatic glucose formation are hepatic glycogen, lactate from red blood cells or skeletal

~ R B C

BRAIN

Glucose~

ATP ~'"J" Lactate

KIDNEY LIVER

Glycogen

Glutarni

(systemic blood)

Glucose ~

Lactate.

AA Glycerol

TG

Glycerol FA AA

ADIPOSE TISSUE

~

Protein

Lactate Glycogen1~

MUSCLE

A

co 2

ATP

HEART

FIGURE 22-17 Glucosehomeostasis in the fasting state. TG, Triacylglycerol;RBC,red blood cells; FA, fatty acid; AA, aminoacid.

504

CHAPTER22 MetabolicHomeostasis High-fat diet

High-carbohydratediet Glucose

TG

Glucose ~

/

INTESTINE

e

FA q

( Z O,',o,es,or,.,,,s,ers, . / Chylomicrons (lymph) "~

INTESTINE , ~ Phospholipid J VLDL d/ ~ "Apoprotein/

Chylomicrons ~

(blood)

~

~

J

LIVER

(systemic blood)

TG

Jr Lipoprotein lipase

FA

Glucose

,. Glucose--~ Glycerol~ 3-phosphate

(stor

ADIPOSE TISSUE

FIGURE 22-18 Triacylglycerol storage in high-fat and high-carbohydratediets. FA, Fatty acid; TG, triacylglycerol.

supported by amino acids from 75 g of protein and 16 g of glycerol.

The two principal types of lipid in fuel homeostasis are triacylglycerols and fatty acids. Other lipids serve roles in cellular membrane structure and other functions. Sufficient triacylglycerol normally is stored in humans to support many weeks of fasting (Table 22-2). Triacylglycerols constitute a very compact form of energy and their complete oxidation yields a high amount of energy. They are transported between organs as components of lipoproteins. Free fatty acids are amphiphilic and act as a detergent. For interorgan transport, fatty acids are bound to albumin.

small proportion of the free fatty acids, notably the shortchain acids in milk and milk products, is absorbed directly into the portal blood and transported to the liver. The bulk of the fatty acids are esterified in intestinal cells into triacylglycerol and packaged within the Golgi apparatus into chylomicrons that contain triacylglycerol, phospholipids, cholesterol and cholesterol esters, and apoproteins. Chylomicrons are secreted into lymphatic capillaries that surround the intestinal cells and fuse into larger vessels to form the thoracic duct, which opens into the systemic circulation by a one-way valve at the junction of the left jugular and subclavian veins. Chylomicrons appear in the plasma shortly after a fatty meal and reach a maximum concentration within 4-5 hours. Their large size results in light scattering and gives plasma a turbid appearance termed postalimentary lipemia.

Lipid Digestion and Absorption

Disposition of Absorbed Triacylglycerol

Dietary lipid consists mostly of triacylglycerol from plant and animal sources. It supplies about 45% of the energy in a typical Western diet. Except for the essential polyene fatty acids (Chapter 18), the dietary requirement for lipid can be met by carbohydrate or protein. However, the Eskimo has a satisfactory diet consisting of 80-90% lipid. Digestion of triacylglycerol is discussed in Chapter 12. A

In various tissues of the body, chylomicrons are acted upon by lipoprotein lipase that hydrolyzes the associated triacylglycerol. This lipase is secreted by adipocytes, hepatocytes, and cardiac and mammary tissue, and it becomes associated with the external surface of endothelial cells of the vasculature in response to the need of these tissues for fatty acids. Its half-life is a matter of hours. Its synthesis

22.6 Lipid Homeostasis

505

SECTION22.6 Lipid Homeostasis

and export are stimulated by insulin and glucose and inhibited by catecholamines. Mammary gland lipoprotein lipase activity is elevated maximally at parturition when lipid is needed for milk formation. Cardiac lipoprotein lipase is elevated during starvation. The lipoproteins transport triacylglycerol and cholesterol (Chapter 20).

Production of Triacylglycerol from Carbohydrate Processes involved in triacylglycerol storage are presented in Figure 22-18. When dietary carbohydrate exceeds requirements, it is converted into triacylglycerol within adipocytes or hepatocytes. The liver daily synthesizes 40-100 g of triacylglycerols, partly from glucose and partly from free fatty acids present in excess of its energy needs. Their transport to adipocytes occurs via VLDLs, whose synthesis, secretion, and fate are analogous to those of chylomicrons.

Release of Lipid from Adipose Tissue Stores During fasting or exercise and at times of stress, lipid is used as a source of energy. Hormone-sensitive lipase catalyzes the sequential hydrolysis of triacylglycerol to

,,,. Chylomicron-

Insulin / \ triacylglycerol ~ ~ [ Lipoprotein ~ k,~ / li a / ~

Epinephrine Glucagon (~ .~ ~

TABLE 22-3

Hormonal Regulation of Adipocyte Lipolysis Rapid Stimulators Epinephrine Norepinephrine Glucagon ACTH Secretin Vasopressin

Slow Stimulators Glucocorticoids Growth hormone

lnhibitors Insulin Prostaglandin E 1

yield ultimately 3 mol of fatty acids and 1 mol of glycerol. Hormone-sensitive lipase is regulated by phosphorylation by cAMP-dependent protein kinase. Listed in Table 22-3 are hormones that affect lipase. Epinephrine, glucagon, and the other rapid stimulators release free fatty acids within minutes. The actions of growth hormone and of glucocorticoids require hours, but their sites of action are unknown. Insulin and prostaglandin E1 suppress lipolysis by depressing cAMP levels. The major activities of adipocytes are summarized in Figure 22-19. Glucagon, an indicator of the need for

Glucocorticoid urowth hormone

Glucose

FA-all)umin corn )lex

FIGURE 22-19 Regulationof adipocytetriacylglycerolsynthesisand degradation.FA, Fattyacid; PDE,phosphodiesterase.

506

carbohydrate, increases hepatic glycogenolysis, gluconeogenesis, and ketogenesis at the expense of fatty acid oxidation and enhances adipocyte lipolysis. Epinephrine and glucocorticoids are, respectively, acute and chronic indicators of stress. They increase hepatic glucose output and peripheral tissue fatty acid utilization. Growth hormone regulates many processes associated with growth of the organism. The energy for much of this growth derives from fatty acid oxidation. As fatty acids leave the adipocyte, they form complexes with albumin. The availability of separate pathways for fat deposition and release in these cells permits appropriate regulation under anabolic or catabolic circumstances. The absence of glycerol kinase in the adipocyte decreases the potential for resynthesis of the triacylglycerol and promotes hepatic gluconeogenesis. Triacylglycerols are deposited in the adipocyte when glucose and insulin are readily available.

Tissue Utilization of Fatty Acids The level of plasma albumin-bound fatty acids increases when lipolysis rates are high. The tissue uptake of fatty acids is proportional to their concentration in plasma and is therefore largely dependent on blood flow. During intense exercise, the flow of blood through the splanchnic bed is reduced and more fatty acids are available to skeletal muscle. With the exception of nerve tissue and blood cells, tissues can use fatty acids by/3-oxidation and by the TCA cycle. Fatty acid uptake is not regulated by hormones or intracellular effectors. Free fatty acids readily diffuse across the plasma membrane of cells where they are used strictly in response to supply and demand. This is illustrated for cardiac muscle in Figure 22-20. Fatty acid oxidation requires adequate amounts of NAD and FAD. Oxidation of NADH and FADH2 occurs in mitochondria via electron transport and the use of molecular oxygen. However, the electron transport chain can only function if ADP is present, so that fatty acid breakdown occurs at the rate necessary to maintain cellular levels of ATP which decrease if metabolic or other work is performed. Tight coupling of electron transport to oxidative phosphorylation (Chapter 14) is of considerable importance in fuel economy. Without it, fatty acids would be utilized in an unproductive manner. In the liver, fatty acids in excess of requirements are converted to triacylglycerols and secreted as VLDLs. Many tissues preferentially utilize fatty acids as an energy source. The sparing effect on glucose is illustrated in Figure 22-21 and reflects the tight coupling of oxidation of fatty acids to ATP production. When sufficient fatty acids are available ATP concentrations will be high and ADP concentrations low. ADP


F I G U R E 22-20 Regulation of fatty acid oxidation in cardiac muscle.

is required in two reactions in glycolysis; ATP is an allosteric inhibitor, and AMP is an allosteric activator of phosphofructokinase.

Ketone Body Production and Utilization Fatty acids released from adipose tissue are the source of ketone bodies (Figure 22-22). Fasting, high levels of glucagon and catecholamine, and a low level of insulin result in rapid lipolysis and ready availability of fatty acids. Fatty acids, after being converted to CoA thioesters, are oxidized in the mitochondria. The rate-limiting step in the oxidation process is the transport of fatty acyl-CoA

FIGURE 22-21 Glucose-sparingeffect of fatty acid oxidation. Note the inhibitionof glucose utilizationby high levels of ATP and citrate. OA, Oxaloacetate; F-1, 6-bisP,fructose-1,6-bisphosphate; 1,3-bisPG, 1,3-bisphosphoglycerate; 3-PG, 3-phosphoglycerate;PEP,phosphoenolpyruvate;Pyr, pyruvate.

SECTION22.6 LipidHomeostasis

507

Epinephrine

Glucagon ~ Glucocorticoids -~ acylglycerol ~ ac,os )Fatty~ FA-albumin

Insulin,~

ADIPOSETISSUE

//

/

/

/

MUSCLE

\

Oxidation

/ /

NADH FA

oso ~

t Oxa,oaco, ,

AcetoacetyI-CoA .."

,J \ Oxida;on .........

/ /

~"

D

.

HMG-CoA J

NADH,~,-y, J NAD~ " /

~ 13-Hydroxybutyrate~

I~

A~toacetyI-CoA

-F--''c"

TCA cycle

1"_- ~ Succinate~ ~ GTP\ c --

~

1~ X~

~ HB . _ . . I , ~

,/-'/

,&c

~ NADH+H" ot-Ketoglutarate/ r " NAD" /

HB

/

F I G U R E 22-22 Ketone body production and utilization. Ketone bodies are produced in the liver from fatty acids derived from adipocyte lipolysis. They are released and used as fuel in peripheral tissues. The initial step in acetoacetate metabolism is activation to acetoacetyl-CoA by succinyl-CoA. HMG-CoA,/3-hydroxy-/3-methylglutaryl-CoA; HB,/3-hydroxybutyrate.

across the inner mitochondrial membrane. The transport is a transesterification process and involves carnitine and two enzymes, carnitine palmitoyltransferase (CPT) types I and II. CPTI is inhibited by malonyl-CoA, primary substrate for fatty acid synthesis that occurs in the cytosol. In the presence of a low insulin:glucagon ratio, malonyl-CoA formation is inhibited, leading to active CPTI and promoting fatty acid oxidation and consequent ketone body production. The opposite situation occurs at a high insulin: glucagon ratio. Hepatic fl-oxidation, without oxidation of acetyl-CoA through the TCA cycle, produces a substantial amount of energy. At such a time, liver is actively engaged in gluconeogenesis so that mitochondrial oxaloacetate is depleted, TCA cycle activity is depressed, and acetyl-CoA levels rise. The last reaction in/3-oxidation is conversion of acetoacetyl-CoA to acetyl-CoA, with an equilibrium in favor of high levels of acetoacetyl-CoA. Thus, acetyl-CoA and acetoacetyl-CoA accumulate and form HMG-CoA; cleavage of this last compound yields acetoacetate, which is reduced to /3-hydroxybutyrate. Acetone results from nonenzymatic decarboxylation of acetoacetate. Ketone body formation occurs exclusively in liver (see Chapter 18) and is prominent in starvation and diabetes owing to the

low insulin:glucagon ratio in these conditions. The ketone bodies readily leave the liver and enter the bloodstream. Acetone is eliminated in the urine and exhaled by the lungs. Because of its odor, it has long been diagnostic in urine and in breath in clinical situations. Because production of ketone bodies was associated with starvation and disease, these compounds were not considered to be metabolically useful. During periods of starvation, acetoacetate and /3-hydroxybutyrate make a considerable contribution to fuel homeostasis. The ratio of their plasma concentrations in starvation and diabetes is about 1:5, reflecting high hepatic levels of NADH, which favors the reduction of acetoacetate to/3-hydroxybutyrate. In utilization of ketone bodies in skeletal muscle, /3-hydroxybutyrate is converted to acetoacetate and then to acetoacetyl-CoA, which is cleaved, and the acetyl-CoA formed is oxidized through the TCA cycle. Activation of acetoacetate to acetoacetyl-CoA requires conversion of succinyl-CoA to succinate. Under normal conditions, the brain cannot use ketone bodies because it lacks the enzyme needed to activate acetoacetate. However, this enzyme is induced in brain after about 4 days of starvation, permitting the brain to obtain 40-70% of its energy from ketone body oxidation while

508

needing less glucose from the liver. Two ketone bodies are acids and give rise to acidosis which is decreased by their elimination in urine. In a 24-hour fast, a 70-kg person hydrolyzes approximately 160 g of triacylglycerol, with release of 160 g of fatty acid and 16 g of glycerol. The glycerol is used for gluconeogenesis. Of the fatty acids, approximately 40 g is metabolized to ketone bodies in liver; the remainder and these ketone bodies provide for the energy needs of all other tissues.

22.7 Protein Synthesis and Nitrogen Homeostasis Protein Synthesis and Proteins as Energy Source While carbohydrates and lipids are essential for structural and functional needs of cells and organs, the principal changes in their fluxes reflect fuel homeostasis and energy requirements. Protein homeostasis is different because no protein serves purely as an energy storage form. All proteins--in addition to being potential sources of carbon for energy--serve some structural, catalytic, transport, or regulatory function. Many fluctuations observed in protein metabolism reflect changes in the need for the function of that protein. Thus, in starvation the total rate of skeletal muscle protein synthesis decreases but that of protein degradation increases because the amounts of proteases and enzymes involved in the catabolic routes for amino acids increase. Thus, starvation stimulates the specific synthesis of some proteins. Protein anabolism and catabolism are governed by many forces. Some rules apply: I. Which specific proteins are synthesized in a cell is governed entirely by the needs or functions of the cell at that time. 2. Synthesis of specific proteins is triggered by a specific signal. 3. Synthesis of a specific protein may not occur if the general "anabolic state" of the cell is low. 4. The anabolic state is reflected by metabolite availability and the presence of anabolic or catabolic hormones. 5. Synthesis occurs if the strength of the specific trigger overrides that of catabolic messages or other hormones or metabolites. 6. Degradation of a protein may occur if a) the protein has no function in the cell, b) a specific trigger to initiate degradation is present, or c) the state of the cell is sufficiently catabolic as a result of the presence or absence of specific metabolites or hormones.


Many hormones modulate the amounts and types of proteins synthesized and their rates of degradation. One group functions on very few target cells, e.g., TSH (thyroidstimulating hormone) on the thyroid gland, FSH (folliclestimulating hormone) and LH (luteinizing hormone) on the gonads, or ACTH (adrenocorticotropic hormone) on the adrenals. Another group has widespread action on protein metabolism, e.g., insulin, glucocorticoids, growth hormone, and thyroid hormone. Many sites of action of insulin and glucocorticoids remain to be elucidated. Glucocorticoids act at the gene level and promote peripheral tissue protein degradation, delivery of amino acids for gluconeogenesis, and adipocyte lipolysis. Insulin stimulates peripheral tissue protein synthesis by stimulating amino acid uptake and protein synthesis at the level of translation and by inhibiting protein degradation. At low insulin levels, muscle proteolysis occurs. As the levels increase, proteolysis decreases and protein synthesis is favored. Exercise decreases proteolysis and increases protein synthesis, whereas disuse results in muscle wasting and depressed protein synthesis. Exercise increases sensitivity to insulin, whereas disuse makes the tissue insulinresistant. Obesity, pregnancy, and glucocorticoids also increase insulin resistance.

Nitrogen Balance Unlike most carbohydrate and lipid, protein contains nitrogen required for incorporation into a wide range of compounds (Table 22-4). Although the synthesis of proteins fixes nitrogen into those proteins, they are synthesized in direct response to specific needs. Dietary protein nitrogen is used for the synthesis of nitrogen-containing compounds in response to specific hormones suchas insulin.

TABI,E 22-4

Compounds for Which Amino Acids Serve as Precursors

Amino Acid Precursor Glycine, aspartate, glutamine Aspartate, glutamine Glycine Arginine, methionine Arginine, glycine Tryptophan Tyrosine Histidine Arginine

Product Purines Pyrimidines Porphyrins Polyamines Creatine Serotonin, melatonin Catecholamines, dopamine, melanin Histamine Nitric oxide

SECTION 22.7

509

Protein Synthesis and Nitrogen Homeostasis

Excess nitrogen is excreted primarily as urea (Chapter 17). Since nitrogen is essential but cannot be stored, the body attempts to maintain "nitrogen balance." In positive nitrogen balance, more nitrogen is ingested than is eliminated. This occurs during growth, pregnancy, lactation, and recovery from trauma, such as major surgery. If less nitrogen is ingested than is excreted, the body is in negative nitrogen balance. In times when dietary nitrogen is not available, negative nitrogen balance occurs because some nitrogen is always excreted as urea and ammonia. Of the 20 amino acids in proteins, the body can readily synthesize eight if an appropriate nitrogen source is available. Two others can be synthesized from other amino acids of the diet: tyrosine from phenylalanine and cysteine from methionine. The rest must be provided in the diet (Chapter 17), since the body can synthesize none or an insufficient amount. The dietary requirement depends on several factors. Beside essential amino acids, the diet should provide the nitrogen required for synthesis of the nonessential amino acids.

Ammonia Toxicity Ammonia, a metabolite formed from many nitrogencontaining compounds, is used for biosynthesis. High ammonia levels are toxic. The energy requirements of the brain, which is sensitive to high levels of ammonia, are met almost exclusively by glucose oxidation, for which a source of oxaloacetate is essential. This condition is satisfied by the carboxylation of pyruvate, even though the pyruvate carboxylase is somewhat limiting. For efficient TCA cycle activity, considerable recycling of oxaloacetate is necessary, since de novo synthesis is limited. With high levels of ammonia, the equilibrium for glutamate dehydrogenase favors formation of glutamate and glutamine, which diminishes the ammonia level but decreases TCA cycle activity. The brain is thus deprived of its source of ATP generation. In addition, glutamate and aspartate have neurotransmitter functions. Within the liver, elimination of ammonia occurs via urea synthesis (Chapter 17). Since urea is uncharged, it does not disturb the acid-base balance. Many interorgan relationships in protein and nitrogen homeostasis arose because of the role that the liver plays in excess nitrogen excretion.

Nitrogen Transfer between Compounds and Tissues Nitrogen is eliminated from the body as urea and ammonia. Urea synthesized in the liver is excreted by the kidneys. Urinary ammonia is produced in the kidney. The nitrogen in other tissues is transported to the liver and kidney in

the form of amino acids. The general reactions involved in these processes are described below. Methods for Directly Transferring Nitrogen

Nitrogen for biosynthetic processes is derived from the o~-amino group of amino acids and from the amido group of glutamine and asparagine by transamination and transamidation, respectively (Chapter 17). Because of the wide variety of transaminases, transamination can provide the right balance of all nonessential amino acids and nitrogencontaining compounds derived from them. Transamidation is less ubiquitous. Other reactions utilizing the nitrogen of amino acids include the incorporation of glycine into purines and its partial incorporation into porphyrins. Reactions in Which Ammonia Is Released

Ammonia is produced by oxidative and nonoxidative deaminations catalyzed by glutaminase and glutamate dehydrogenase (Chapter 17). Ammonia is also released in the purine nucleotide cycle. This cycle is prominent in skeletal muscle and kidney. Aspartate formed via transamination donates its or-amino group in the formation of AMP; the amino group is released as ammonia by the formation of IMR Reactions That "'Fix" Ammonia

In muscle, ammonia is used to synthesize other nitrogen-containing compounds or is eliminated. In either case, glutamine and glutamate are formed by glutamine synthetase and glutamate dehydrogenase, respectively. The glutamate dehydrogenase reaction is readily reversible, but that of glutamine synthetase is not because it is driven by the hydrolysis of ATE Glutamine synthesis is the major mechanism for nonhepatic tissues to eliminate ammonia. In liver, ammonia is fixed by the formation of urea (Chapter 17).

Disposition of Dietary Intake of Protein Ingested protein is digested in a stepwise fashion in the stomach, small intestinal lumen, and small intestinal mucosal cells (Chapter 12). Peptides formed in the intestinal lumen are absorbed into the mucosal cells and degraded to free amino acids. The outflow of amino acids to the portal vein does not reflect the amino acid composition of the ingested protein. Thus, alanine levels increase twoto fourfold, and glutamine, glutamate, and aspartate are absent. These changes arise from amino acid interconversions within the intestinal cell. With the exception of the branched-chain amino acids (valine, leucine, and isoleucine), most amino acids

510


released from the intestine are metabolized by the liver. The liver carries out 1. Formation of other molecules from amino acid precursors, e.g., nonessential amino acids, purines, pyrimidines, porphyrins, and plasma proteins (such as albumin, VLDL, and transferrin), for its needs and those of other tissues; 2. Utilization of the amino acid carbon skeletons in gluconeogenesis or glycogen or lipid synthesis; and 3. Formation of urea. After a protein meal, the plasma amino acids are those delivered from the intestine and those synthesized by the liver. The levels of branched-chain amino acids are greatly increased because they bypass the liver, reaching a peak at about 4 hours and returning to normal by 8-10 hours. They account for up to 60% of the total amino acids entering the systemic circulation. After uptake by skeletal muscle and transamination, followed by oxidation of the oe-keto acid derivatives by the pathways described in Chapter 17, they can meet most energy requirements of muscle. The branched-chain amino acids have been called amino fats. Their or-amino nitrogen is converted to alanine, which is transported to liver for elimination as urea. Figure 22-23 summarizes the various pathways for amino acids following protein ingestion. Alanine is formed in intestine and muscle from many of the amino acids. Following a protein meal, insulin secretion increases, which stimulates peripheral tissue uptake of amino acids and net protein synthesis. On a diet high in carbohydrate and low in protein, insulin levels are elevated and glucagon levels are depressed. On a diet high in protein and low in carbohydrate, insulin and glucagon levels are elevated. While amino acids and glucose stimulate insulin release, amino acids stimulate and glucose inhibits glucagon release. Thus, on a high-carbohydrate diet, insulin promotes hepatic and peripheral tissue utilization of glucose without the need for hepatic gluconeogenesis. Because of the low level of glucagon, hepatic glucose formation is depressed. On a high-protein, low-carbohydrate diet, insulin secretion is stimulated by amino acids and results in their uptake by peripheral tissue and utilization for glucose uptake. However, glucose homeostasis is maintained because glucagon stimulates hepatic glucose release.

Protein Catabolism during Starvation During starvation, following depletion of hepatic glycogen, amino acids become the major source for glucose homeostasis because of the decrease in plasma insulin

Alanine + non-BCAA

BCAA N ot-Ketoacids N

Mus

I

~176

X

\

F I ( ; U R E 22-23 Amino acid metabolism following dietary protein intake. Digestion of protein produces amino acids. Within the intestinal cell, amino acid interconversions form alanine, which is delivered by the portal blood to liver, where it serves as the source of c~-amino nitrogen and pyruvate, which is converted to lipid and glucose. Excess nitrogen is converted to urea. Branched-chain amino acids (BCAAs) are not taken up by the liver but enter peripheral tissues such as muscle where they serve as an important fuel source. Their c~-amino nitrogen is transported to liver in the form of alanine.

level and the rise in glucocorticoid level. The pattern of amino acids released by skeletal muscle during starvation does not reflect the composition of muscle protein. Alanine and glutamine account for over half of the amino acids released. Alanine is taken up by liver, its carbon chain converted to glucose, and the nitrogen to urea. In early starvation, the principal site of glutamine metabolism is the gut. One product is alanine. The special role of glutamine in gut may be due to the high demand for glutamine in purine synthesis because of the active shedding of intestinal cells. In long-term starvation, a major site of glutamine metabolism is the kidney, since the excretion of ketone bodies requires NH + as a counterion formed from ammonia produced by glutaminase. The resulting glutamate is

511

SECTION22.8 Abnormalities of Metabolic Homeostasis Glucose production

200-

175-

150-

Glucose 125-

~L~nie

I00-

LIVER

Glutamine

Ala

~

75-

Urea Liver

_

-

~

-

..

-

..

-

-

-

.. . . . . . . . . .

_

_

..

_

..

..

. . . . . . . . . ..

..

~ . _ ~ : . , x . . . . _ . . . . . ~

5o-

(g/24 h)

~

.. . . . . . . . . . . .

25

~~,~ac;,200 mg/dl (11.1 mmol/1). Casual is defined as any time of day without regard to time since last meal. The classic symptoms of diabetes include polyuria, polydipsia, and unexplained weight loss. or 2. FPG >126 mg/dl (7.0 mmol/1). Fasting is defined as no caloric intake for at least 8 h. or 3. 2hPG >200 mg/dl during an OGTT. The test should be performed as described by WHO using a glucose load containing the equivalent of 75-g anhydrous glucose dissolved in water. In the absence of unequivocal hyperglycemiawith acute metabolic decompensation, these criteria should be confirmed by repeat testing on a different day. The third measure (OGTT) is not recommended for routine clinical use. FPG = Fasting plasma glucose; OGTT = Oral glucose tolerance test; 2hPG = 2 hour post load glucose. The diagnostic criteria are based upon the report by the Expert committee on the diagnosis and classification of Diabetes mellitus. Diabetes Care24(suppl. 1), 55 (2001).

in Table 22-5. Subjects who have fasting plasma glucose levels between _>110 mg/dL and 140 mg/dL (7.8 mmol/L), the test is considered positive and requires a diagnostic test consisting of oral ingestion of 100 g of glucose after a fasting for 8 hours, followed by 3-hour oral glucose tolerance test (Table 22-6). In the 50-g challenge test, if the threshold for the abnormality is decreased to _>130 mg/dL (7.2 mmol/L), the sensitivity for identifying patients with gestational diabetes mellitus increases from 80% to 90%. Since diabetes mellitus is an insidious disorder, testing of asymptomatic patients may be desirable under certain conditions, including age 45 years or older; obesity; firstdegree relatives of diabetics; members of high-risk ethnic population (e.g., Native American, Hispanic, AfricanAmerican); women who have delivered an infant weighing more than 9 lb (4.08 kg) or have had gestational diabetes mellitus; hypertension; abnormal lipid studies; recurring

Plasma Glucose

50-g Screening Test

100-g Diagnostic Test

140 mg/dl

180 mg/dl

Fasting

1-h 2-h 3-h

95 mg/dl 155 mg/dl 140 mg/dl

Screening for GDM may not be necessary in pregnant women who meet all of the following criteria: < 25 years of age, normal body weight, no first degree relative with diabetes, and not Hispanic, Native-American, Asian-, or African-American. The 100-g diagnostic test is performed on patients who have a positive screening test. The diagnosis of GDM requires any two of the fourplasma glucose values obtained duringthe test to meet or exceed the values shown above. To convert values for glucose to mmol/L, multiply by 0.05551. The diagnostic criteriais based upon the report by the American Diabetes Association on Gestational Diabetes Mellitus. Diabetes Care 23(suppl. 1), $77 (2000). skin, genital, or urinary tract infections; and previous impaired glucose intolerance. After the diagnosis of diabetes mellitus has been made and with the initiation of appropriate therapy (discussed later), assessment of other biochemical parameters is necessary in the management phase of the disorder to maintain the fasting blood glucose level as close to normal as possible and to prevent long-term complications. These biochemical tests include measurement of a stable form of glycosylated hemoglobin (hemoglobin Alc), determination of urine albumin excretion rate, measurement of serum fructosamine levels, and self-monitoring of blood glucose levels. Hemoglobin Aac levels are used to assess average glucose control over a 2 to 3-month period, since the, re,d hlc~c~d ce,ll'.~ life_span is about 120 days, EntLy~f glucose into red blood cells depends only on the prevailing plasma glucose concentration. (Formation of hemoglobin Alc, which is nonenzymatic, is discussed in Chapters 2 and 10). The normal hemoglobin Ale concentration is about 4-6%; spurious values for hemoglobin Alc levels can occur in uremic states, hemoglobinopathies (Chapter 28), hemolytic anemia and blood transfusion. Fructosamine is a generic term applied to the stable condensation product of glucose with serum proteins (albumin, with a circulating half-life of about 20 days, is the major contributor). Thus, serum fructosamine levels reflect glucose control over a period of 2-3 weeks. One of the chronic complications of diabetes mellitus is diabetic nephropathy, which leads to end-stage renal disease. An initial biochemical parameter of diabetic nephropathy in the asymptomatic state is a persistent

514

increase in urine albumin excretion rate between 20 and 200 #g/min (or 30-300 mg/d). This degree of albumin loss in the urine is called microalbuminuria and is a harbinger of renal failure and other complications of diabetes. It is important to identify individuals with microalbuminuria because with appropriate therapeutic intervention attenuation of loss of renal function can be accomplished. Therapeutic interventions include blood glucose control, treatment for high blood pressure if present, and the inhibition of formation of angiotensin II by use of angiotensinconverting enzyme (ACE) inhibitors. The formation of angiotensin II involves the following steps: release of the protease renin by justaglomerular cells in response to decreased renal perfusion pressure, release of angiotensin I from angiotensinogen by the action of renin, and conversion of angiotensin I to angiotensin II by ACE. Physiologic functions of angiotensin II in the kidney include restoration of normal renal blood flow and glomerular filtration and stimulation of aldosterone secretion, which affects Na + reabsorption (Chapter 32). In diabetes mellitus, renal hemodynamics are altered due to glomerular hyperfiltration. Angiotensin II has been postulated to contribute to the progression of renal disease. Some of the deleterious effects of angiotensin II may involve its action on vasomotor functions and induction of cytokines (e.g., TGF-/3, see Chapter 35) that lead to hypertrophy of mesangial cells. Thus, ACE inhibitors as well as angiotensin II receptor antagonists can prevent or slow the rate of progression of renal disease in diabetes mellitus. Clinical studies have shown that even in normotensive diabetic patients, ACE inhibitors are beneficial in the management of renal disease.

Chronic Complications of Diabetes Mellitus These stem from elevated plasma glucose levels per se and involve tissues that do not require insulin (e.g., lens, retina, peripheral nerve) for the uptake and metabolism of glucose. In these tissues, the intracellular level of glucose parallels that in plasma. The chronic complications, which cause considerable morbidity and mortality, are atherosclerosis, microangiopathy, retinopathy, nephropathy, neuropathy, and cataract. The biochemical basis of these abnormalities may be attributed to increased tissue ambient glucose concentration and may involve the following mechanisms: nonenzymatic protein glycation (Chapter 2), increased production of sorbitol, and decreased levels of myo-inositol. The ramifications of glycation of proteins are not clear. Sorbitol is produced from glucose by NADPH-dependent reduction catalyzed by aldose reductase. Sorbitol may also be converted to fructose by NAD+-dependent oxidation. Sorbitol (and fructose) diffuses poorly across cell membranes and accumulates


inside the cell causing osmosis-induced disturbances (e.g., cataract formation). The myo-inositol depletion may be due in part to the competition of glucose with myo-inositol for its intracellular transport; glucose and myo-inositol are strikingly similar in structure (Chapter 9). Myo-inositol can also be synthesized from glucose-6-phosphate. Decreased myo-inositol may lead to decreased phosphoinositide turnover. The latter yields at least two active catabolites (inositol polyphosphates and diacylglycerol) that function as second messengers (Chapter 30). The diminished activity of Na+,K+-ATPase found in nerve fibers is thought to be related to the altered phosphoinositide turnover. Potential therapeutic use of myo-inositol supplementation and aldose reductase inhibitor administration is being explored.

Management of Diabetes Mellitus The primary goal in the management of all types of diabetes mellitus (type 1, type 2, and GDM) is to maintain nearnormal plasma glucose levels in order to relieve symptoms (polydipsia, polyuria, polyphagia) and to prevent both acute and chronic complications. The glycemic control is assessed by monitoring of glucose (by self and in clinical settings), hemoglobin Alc, fructosamine, and microalbuminuria. In type 1 diabetes mellitus due to /~-cell destruction, administration of insulin is required throughout the person's lifetime. There are many insulin preparations that differ in duration of action (ultrashort, short, intermediate, and long acting) and in their origin (human, porcine, and bovine). Human insulin analogues lispro and aspart (B28) do not undergo polymerization and are rapid acting insulins. The side effects of insulin therapy include hypoglycemia and weight gain. The management of type 2 diabetes mellitus is based on both behavioral changes with lifestyle modifications and pharmacological measures. Diet, exercise, and weight loss (in the obese) are the cornerstone of treatment in maintaining euglycemia. Physical activity stimulates insulin sensitivity, whereas physical inactivity leads to insulin resistance. In the absence or failure of lifestyle and behavioral measures in the management of type 2 diabetes mellitus, pharmacological therapy is employed. Pharmacological agents used in the treatment of type 2 diabetes mellitus include insulins, sulfonylureas, ot-glucosidase inhibitors, biguanidines, and thiazolidinediones (Figure 22-26). Sulfonylureas bind to specific fl-cell plasma membrane receptors and cause closure of the ATP-dependent K + channel with accompanying depolarization. The depolarization leads to influx of Ca 2+ followed by insulin secretion. A benzoic acid derivative (repaglinide) is structurally similar to sulfonylurea without the sulfur

SECTION22.8 Abnormalities of Metabolic Homeostasis

515

Acarbose (an tx-Glucosidaseinhibitor) C,i_i2OH ,,,5.,,~

CH3 0

CH2OH ~" O

CH2OH ~ / O ~ , d "-tOH

HO HO

Glipizide (a sulfonylurea)

H3c-(-

0 [CI--NH--CH2--CH 2 ~

0 802 --NHJCI --NH--~

Metformin (a biguanidine)

Troglitazone (a thiazolidinedione) CH3

NH NH H3C II II H3C> N - - C - - N H - - C - - NH2

)

CH.

/7"'7N

.o-

o

N.

CH3 FIGURE 22-26

Structures of four classes of hypoglycemicagents. group and belongs to the class meglitinides. This drug inhibits a different K + channel than SUR in promoting secretion and has rapid-onset and short-acting actions. An example of an ot-glucosidase inhibitor is acarbose, a nitrogen-containing pseudotetrasaccharide. Acarbose inhibits the breakdown of carbohydrates into glucose by inhibiting intestinal brush-border ot-glucosidase and pancreatic amylase (Chapter 12) and brings about a reduction in postprandial hyperglycemia. Metformin belongs to the class of biguanidines and has many effects on carbohydrates and lipid metabolism. Its mechanism of action may include inhibition of hepatic gluconeogenesis and glycogenolysis and, in muscle, increased insulin receptor tyrosine kinase activity and enhanced GLUT4 transporter system. An adverse effect of metformin is lactic acidosis. Troglitazone is an example of the thiozolidinedione class of antidiabetic agents. It enhances insulin sensitivity in liver, muscle, and adipose tissue. Troglitazone promotes the conversion of non-lipid-storing preadipocytes to mature adipocytes (discussed later) with increased insulin sensitivity. The cellular target for the action of troglitazone, which functions as a transcription factor, is the nuclear receptor known as peroxisome proliferationactivated receptor-F (PPAR-F). PPAR-y upon binding

with troglitazone is activated and forms a heterodimer with retinoid X receptor (RXR). The PPAR-y/RXR heterodimer regulates the transcription of genes encoding proteins that control metabolic pathways (e.g., acetyl CoA-synthetase, lipoprotein lipase, GLUT4, mitochondrial uncoupling protein). An adverse effect of troglitazone is hepatic toxicity; periodic monitoring of liver function by measurement of serum alanine aminotransferase is recommended. When a diabetic patient becomes refractory to monotherapy, combination therapy (e.g., sulfonylurea and insulin) is used to achieve glycemic control.

Obesity Overweight and obesity are defined based on the body mass index (BMI), which is calculated as weight in kilograms divided by the square of height in meters (BMI -- kg/m2). A BMI between 25 and 29.9 is considered as overweight and >_30 is considered obese (Chapter 6). Body weight is determined by a balance between energy intake and energy expenditure. The energy expenditure is required to maintain basal metabolic functions, absorption and digestion of foods (thermic effect of food), physical activity and, in children, linear growth

~16


and development. If there is a net excess energy intake, BMI increases, eventually leading to overweight and ultimately to obesity and morbid obesity. In a population with stable genetic factors, increase in obesity is primarily attributable to consumption of excess foods with high fat content and decreased physical activity. Obesity is a worldwide health problem. It is a risk factor for development of diabetes mellitus, hypertension, and heart disease, all of which cause decreased quality of life and life expectancy. Feeding behavior and energy balance are regulated by a complex set of short-term and long-term physiological signals. Mechanisms that lead to obesity involve interactions between genetic, environmental, and neuroendocrine factors. The short-term regulators of hunger and satiety may include plasma levels of glucose and amino acids, cholecystokinin and other hormones, and body temperature. One of the long-term regulators of food intake and energy expenditure is an adipocyte-derived hormone leptin (Chapter 5). Leptin is a polypeptide of 167-amino acid

PPARy2\ (Active)~

Phosphorylation*

Preadipocytes

residues that functions in the afferent signal pathway of a negative feedback loop in regulating the size of adipose tissue and energy balance. The physiological role of leptin was established by using ob/ob and db/db mice that have identical phenotypes of obesity and diabetes mellitus. In the ob/ob mice, leptin levels are low due to mutations in the leptin gene, and in the db/db mice the leptin receptors are defective due to inactivating mutations. The role of leptin and its receptor was investigated by joining the circulatory systems (parabiosis) of ob/ob and db/db mice. In ob/ob mice, obesity, diabetes, and sterility are corrected by leptin administration. These animal studies have led to understanding of several aspects of obesity in humans. In humans, leptin is secreted in a pulsatile manner, in a nyctohemeral rhythm, and in proportion to size of the adipose tissue. The pathway of leptin's action involves the neuroendocrine system of the hypothalamus (Figure 22-27). When leptin levels are low, appetite increases via the activation of the neuropeptide Y

~ PhosphorylatedPPAR72 (Inactive)

J, Adipocytes

1 l

Leptin*

~

Hypothalamic LeptinReceptor* ~

~

Low Leptin Levels

~

Increased FoodConsumption

.

High Leptin Levels

NeuropeptideY/ AGRP ~ Neuropeptide Y Receptors*

~

~

Inhibition ~

POMC ]

Proteolysis*

MSH + other peptides Melanocortin-4 Receptors* Decreased Food Consumption

FIGURE 22-27 A modelfor the regulationof humanfood consumption.Asterisks(*) indicateobesity-causingmutations.Abbreviations are givenin the text.

SECTION 22.9

Metabolic Homeostasis during Exercise

pathway, eventually leading to stimulation of food intake. Neuropeptide Y contains 36 amino acid residues and has tyrosine at both the N and C termini (hence the designation Y). The actions of neuropeptide Y result in the stimulation of food intake followed by the restoration of depleted adipose stores and inhibition of sympathetic nervous system outflow with a consequent decrease in energy expenditure. When leptin levels are high, food intake decreases; this is mediated by the pro-opiomelanocortin (POMC)-ot-melanocyte stimulating hormone and melanocortin-4 receptor pathway. Inhibition of the melanocortin-4 receptor signal transduction system by the agouti protein leads to obesity in mice. Agouti protein (AGRP) is normally synthesized in hair follicles and antagonizes the action of ot-melanocytestimulating hormone in eumelanin synthesis (Chapter 17) by binding to melanocortin-4 receptor. In autosomaldominant mice that overexpress agouti protein in skin and hypothalamus, the phenotype is obese, yellow mice. In humans, the pathways of regulation of appetite and food consumption have been verified by identifying mutations in genes that participate in regulation of body weight (Figure 22-27). The regulators of conversion of preadipocytes to adipocytes also determine the adipose tissue mass. One of the regulators is a transcription factor known as peroxisome proliferator-activated receptor-F2 (PPAR-F2). Troglitazone, a hypoglycemic agent (previously discussed) that increases insulin sensitivity, is a synthetic ligand for PPAR-F2. However, contrary to studies in rodents, administration of troglitazone in humans does not cause significant weight gain as compared to other oral antidiabetic drugs. Phosphorylation of PPAR-F2 at ser114 makes the receptor less active in converting preadipocytes to adipocytes. A mutation converting proline to glutamine at position 115 blocks phosphorylation at the serine site, maintains the protein in the active state, and leads to obesity. Leptin therapy has corrected obesity in a child with congenital leptin deficiency. In obese individuals, the presence of circulating high levels of leptin has been attributed to resistance or some other defect in the leptin receptors. This apparent paradox of high leptin levels associated with obesity is analogous to insulin resistance seen in type 2 diabetes mellitus. In general, in the vast majority of obese patients, the molecular defects remain unknown. Diet and exercise are the mainstays in the management of obesity.

22.9 Metabolic Homeostasis during Exercise At rest, skeletal muscle utilizes the catabolism of fatty acids and branched-chain amino acids to maintain cellular

517

integrity. In exercise, oxygen utilization by muscle may increase 30-fold, and additional energy can arise from anaerobic glycolysis. The substrates used depend on the intensity of exercise and on training. High-intensity exercise can start almost instantaneously (e.g., the 100-m dash or the clean-and-jerk in weight lifting), but increase of oxygen utilization is limited by the rates of blood flow, respiration, and oxygen delivery to the tissue. After the onset of high-intensity exercise, the body and muscle are initially in oxygen deficit until a new steady-state, increased rate of oxygen uptake can be achieved. When the exercise ends, this oxygen debt is repaid during the recovery phase. The availability of oxygen determines the metabolic fuels utilized. Skeletal muscle has fewer mitochondria per unit weight than heart or liver. A major adaptation induced by endurance exercise training is an increase in the potential for delivery and utilization of oxygen: the heart hypertrophies; total blood volume increases; muscle capillarity increases; and skeletal muscle mitochondrial size, number, and capacity increase (see Chapter 21).

Exercise Generating Maximum Power The substrates for muscular activity are intracellular ATE phosphocreatine, glycogen, plasma glucose, fatty acids, ketone bodies, triacylglycerols, and dietary branchedchain amino acids. In exercise that generates maximum power, the primary sources are ATP and phosphocreatine. The total amount of ATE and of ATP generated by the adenylate kinase reaction (2ADP --+ ATP + AMP), is sufficient for only a fraction of a second. The immediate reserve is ATP produced by the creatine kinase reaction (phosphocreatine + ADP --+ ATP § creatine). Maximumpower-output exercise rapidly depletes phosphocreatine, can be sustained for short periods (15-20 seconds) and ends as phosphocreatine is depleted. Some anaerobic glycolysis from glycogen may occur during such a period but would reach a maximum rate only after the work had stopped. Maximum-power-output work exceeds the capacity of muscle to generate energy oxidatively even if this could be initiated rapidly.

High-Intensity Endurance Exercise This exercise is typified by an individual working in the range of 60-80% of maximum capacity of oxygen uptake (e.g., running distances of 5 km up to a marathon). The exercise is terminated by exhaustion. Phosphocreatine is the initial energy source; glycogenolysis ensues, anaerobically at first, with lactate production, but with increasing aerobic oxidation as oxygen availability increases. Glycogen utilization is the major fuel for the first 15 minutes, but

518 it is superseded by fatty acid and plasma glucose utilization. Most of the plasma glucose used is oxidized entirely but 10-20% is converted to lactate. During the exercise and depending on the past dietary history, branched-chain amino acid oxidation contributes some energy, and alanine is formed to eliminate the or-amino nitrogen. Lactate and alanine are used in the liver for gluconeogenesis and can provide 5-15% of the glucose required by skeletal muscle. The rest of the glucose is derived from glycogenolysis. Thus, in endurance exercise, hepatic glycogen serves as a major fuel for muscle, and muscle glycogen is the initial fuel. As plasma glucose and fatty acids are used, glycogenolysis in muscle diminishes; despite its smaller contribution, muscle glycogen plays a critical role since exhaustion occurs with its total depletion in spite of the availability of fatty acids and plasma glucose. Thus, "glycogen loading" is used to extend endurance. Exercisedepleted glycogen stores can be increased to above-normal levels for about 24 hours by high-carbohydrate refeeding. With elevated levels of tissue glycogen, an individual's endurance ability is increased. Several other phenomena contribute to the pattern of metabolite utilization in highintensity endurance exercise. 1. Although fatty acids are avidly utilized by skeletal muscle during exercise, their plasma concentrations do not differ significantly from those at rest. Their utilization is being balanced by production from adipocyte lipolysis, probably as a result of an elevated catecholamine level. 2. Glucose uptake by skeletal muscle increases with intensity of exercise even though levels of circulating insulin decrease. Either this glucose transport is insulin-independent or exercise enhances insulin sensitivity. 3. The increase in glucose uptake with increased intensity of exercise is matched by hepatic glucose production to the extent that arterial glucose concentrations are elevated by very intense exercise. Hepatic glycogenolysis probably is enhanced by a decrease in the level of insulin and by an increase in levels of glucagon and epinephrine. 4. Exercise may lower insulin levels even though plasma glucose concentrations are increased. This effect may result from ot-adrenergic inhibition of/3-cell secretion.

Low-Level Nonfatiguing Exercise This exercise is characteristic of an individual in normal occupational tasks that could reasonably be continued for 8-12 hours. The principal substrates used are similar to those in long endurance exercise but without depletion


of phosphocreatine and with minimal muscle glycogen utilization. The main source of energy is the aerobic oxidation of fatty acids, glucose, and branched-chain amino acids.

Supplemental Readings and References E H. Bennett, S. Haffner, B. Kasiske, et al.: Diabetic renal disease recommendations. American Journal of Kidney Disease 25, 107 (1995). E. E. Calle, M. J. Thun, J. M. Petrelli, et al.: Body-mass index and mortality in a prospective cohort of U.S. adults. New England Journal of Medicine 341, 1097 (1999). G. W. Cline, K. E Petersen, M. Krssak, et al.: Impaired glucose transport as a cause of decreased insulin-stimulated muscle glycogen synthesis in type 2 diabetes. New England Journal of Medicine 341, 240 (1999). S. Dagogo-Jack and J. V. Santiago: Pathophysiology of type 2 diabetes and modes of action of therapeutic interventions. Archives of Internal Medicine 157, 1802 (1997). R. DeFronzo: Pharmacologic therapy for type 2 diabetes mellitus. Annals of Internal Medicine 131,281 (1999). Expert Committee on the Diagnosis and Classification of Diabetes Mellitus: Report of the expert committee on the diagnosis and classification of diabetes mellitus. Diabetes Care 20, 1183 (1997). I. S. Farooqi, S. A. Jebb, G. Langmack, et al.: Effects of recombinant leptin therapy in a child with congenital leptin deficiency. New England Journal of Medicine 341,879 (1999). E L. Ferris III, M. D. Davis, and L. W. Aiello: Treatment of diabetic retinopathy. New England Journal of Medicine 341,667 (1999). J. M. Friedman and J. L. Halaas: Leptin and the regulation of body weight in mammals. Nature 395, 763 (1998). B. Glaser, P. Kesavan, M. Heyman, et al.: Familial hyperinsulinism caused by an activating glucokinase mutation. New England Journal of Medicine 338, 226 (1998). S. B. Heymsfield, A. S. Greenberg, K. Fujioka, et al.: Recombinant leptin for weight loss in obese and lean adults. Journal of American Medical Association 282, 1568 (1997). E Holleman and J. B. L. Hoekstra: Insulin lispro. New England Journal of Medicine 337, 176 (1997). S. J. Hunter and W. T. Garvey: Insulin action and insulin resistance: diseases involving defects in insulin receptors signal transduction, and the glucose transport effector system. American Journal of Medicine 105, 331 (1998). S. L. Kjos and T. A. Buchanan: Gestational diabetes mellitus. New England Journal of Medicine 341, 1749 (1999). A. Must, J. Sadano, E. H. Coakley, et al.: The disease burden associated with overweight and obesity. Journal of American Medical Association 282, 1523 (1999). O. E. Owen, K. J. Smalley, D. A. D'Alessio, et al.: Protein, fat, and carbohydrate requirements during starvation: anaplerosis and cataplerosis American Journal of Clinical Nutrition 68, 12 (1998). A. Rebollo and C. Martinez-A: Ras protein: recent advances and new functions. Blood 94, 2971 (1999). M. Ristow, D. Miller-Wieland, A. Pfeiffer, et al.: Obesity associated with a mutation in a genetic regulator of adipocyte differentiation. New England Journal of Medicine 339, 953 (1998). E. Ritz and S. R. Orth: Nephropathy in patients with type 2 diabetes mellitus. New England Journal of Medicine 341, 1127 (1999). J. N. Roemmich and A. D. Rogol: Role of leptin during childhood growth and development. Endocrinology and Metabolism Clinics of North America 28, 749 (1999).

Supplemental Readings and References M. W. Schwartz and R. J. Seeley: Neuroendocrine responses to starvation and weight loss. New England Journal of Medicine 336, 1802 (1997). P. R. Shepherd and B. B. Kahn: Glucose transporters and insulin action. New England Journal of Medicine 341,248 (1999). C. A. Stanley, Y. K. Lieu, B. Y. L. Hsu, et al.: Hyperinsulinism and hyperammonemia in infants with regulatory mutations of the glutamate dehydrogenase gene. New England Journal of Medicine 338, 1352 (1998).

519 J. A. Tamada, S. Garg, I. Jovanich, et al.: Noninvasive glucose monitoring. Journal of the American Medical Association 282, 1839 (1999). A. H. Xiang, R. K. Peters, E. Trgo, et al.: Multiple metabolic defects during late pregnancy in women at high risk for type 2 diabetes. Diabetes 48, 848 (1999). J. A. Yanovski and S. Z. Yanovski: Recent advances in basic obesity research. Journal of American Medical Association 282, 1504 (1999).


CHAPTER

23

Nucleic Acid Structure and Properties of DNA

The genetic information in all living cells is carried in molecules of deoxyribonucleic acid (DNA), which is primarily found in chromosomes. However, DNA is also present in cellular organelles such as mitochondria and chloroplasts. Viruses carry genetic information in either DNA or RNA (ribonucleic acid) molecules. When RNA viruses infect cells, the genetic information in RNA is converted to DNA prior to the replication and synthesis of new viral particles. In the case of certain RNA viruses (retroviruses) such as human immunodeficiency virus (HIV), the DNA that is copied from the infecting RNA is permanently integrated into the host chromosomes and the viral genome becomes an integral part of the cells' genetic information. The structure of DNA was elucidated by James Watson and Francis Crick in 1953. The structure they proposed made it apparent for the first time how genetic information in chemically stored in cells and how it is replicated and transmitted from one generation to the next. The proposed DNA structure also provided insight into the chemical nature of mutations and how they might occur during replication. Scarcely fifty years has passed since that monumental discovery and now we are deciphering the complete sequence of a human genome. This will lead to a chemical understanding of thousands of genetic disorders and the ability to diagnose, prevent, and treat many inherited diseases and cancers. In this chapter we discuss the structure and properties of DNA. Subsequent chapters examine the functions of DNA

such as replication, mutation, recombination, repair, and the expression of genes.

23.1 Components of Nucleic Acids A nucleic acid is a polynucleotide~a linear polymer of nucleotides that is defined by its three components (Figure 23-1):

1. A nitrogenous heterocyclic base (either a purine or a pyrimidine) attached to the l'-carbon atom of the sugar by an N-glycosidic bond. In DNA the purine bases are adenine (A) and guanine (G) and the pyrimidine bases are cytosine (C) and thymine (T). The bases in RNA are the same except that uracil (U), a pyrimidine, replaces thymine. 2. A cyclic five-carbon sugar which is ribose for RNA and deoxyribose for DNA. The difference in structure between ribose and 2'-deoxyribose is shown in Figure 23-1. The carbon atoms of the sugar are numbered with a prime to distinguish them from the carbon atoms in the base in the same nucleotide structure. 3. A phosphate group attached to the 5'-carbon atom of the sugar by a phosphodiester linkage. This phosphate group is responsible for the strong negative charge of both nucleotides and nucleic acids. 521

CHAPTER23 Structureand Propertiesof DNA

522

Amononuc~[

Adeninej 1 The nucleoside

Ribose

unit

deoxyribose Adenine A

NH~ I

N'~C%.~N:\\e I

I

HC~N~.C-~N/

CH

I OI

OI

OI

o

o

o

o--P-O-P,,

,,

5'

,

"C,xH, H

[ I

O

H/'C,

!

1

I

"1 X

OH

Ribose

Adenosine or deoxyadenosine

(X

or Deoxyribose

H

--

OH)

(X - H)

1

Adenosine monophosphate or deoxyadenosine monophosphate (AMP or dAMP) Adenosine diphosphate or deoxyadenosine diphosphate (ADP or dADP) Adenosine triphosphate or deoxyadenosine triphosphate (ATP or dATP) F I G U R E 23-1 Structure of the nucleotide in DNA and RNA. The base adenine is used as an example. See Table 27-1 for a complete nomenclature of nucleosides and nucleotides.

A base linked to a sugar is called a nucleoside; a base linked to a sugar linked to a phosphate is called a nucleotide or a nucleoside phosphate. The nucleotides in nucleic acids are joined to one another by a second phosphodiester bond that joins the 5'-phosphate of one nucleotide to the 3'-OH group of the adjacent nucleotide. Such a doubly esterified phosphate is called aphosphodiester group (Figure 23-2).

Base Pairing and Base Composition The molar content of the four bases in DNA (called the base composition) always satisfies the equalities [A] = [T] and [G] = [C] where [ ] denotes molar concentration. These equalities arise because of the base pairing rules in

F I G U R E 23-2 Structure of a phosphodiester group. The vertical arrows show the bonds in the phosphodiester group that are free to rotate. The horizontal arrows show the N-glycosidic bond about which the base can rotate freely. A polynucleotide consists of many nucleotides linked together by phosphodiester groups.

SECTION23.1 Componentsof Nucleic Acids H H~c~N\

\ / N--H---O\ \

523

The overall base composition of DNA varies considerably among organisms. Base composition is expressed as the fraction of all bases in DNA that are GC pairs divided by the total number of base pairs, such as ([G] + [C])/[all bases]. This fraction is termed the GC content or percent GC. For human beings and other primates, the value of the GC content is approximately 0.5. For lower organisms the value can vary widely; the most extreme variation is found in bacteria where the GC content varies from 0.27 to 0.76 from one genus to another; E. coli DNA has a GC value of 0.5, which may reflect its close association and evolutionary history with human beings. The higher the GC content of DNA, the more stable is the doublestranded helical molecule. This is because GC base pairs share three hydrogen bonds whereas AT base pairs share only two (Figure 23-3).

/CH 3

I c--c c--c N~C// \\ / \ / N---H--N C--H Deoxyribose \N-- C / \C--N / \H

O/ /

Adenine

\ Deoxyribose

Thymine

H / H~c~N\ /O---H-- N\ /H I c--c c--c /

~C

Deoxyribose

N--H---N C--H \N-- C / \C--N / k

Guanine

// N-- H- --O

H/

k

Deoxyribose

Cytosine

F I G U R E 23-3 The normal base pairs in DNA. Adenine in one polynucleotide chain pairs with thymine in the complementary chain; guanine pairs with cytosine. A-T base pairs are joined by two hydrogen bonds; G-C base pairs are joined by three hydrogen bonds.

DNA which require that A pair with T and G pair with C in the double-stranded DNA molecule (Figure 23-3). Because of the base pairing rules it follows that [purines] = [pyrimidines] in all DNA molecules.

Adenine H

\

/

T a u t o m e r i z a t i o n of Bases

Although the bases are chemically quite stable, certain hydrogen atoms bound to the bases are able to undergo tautomerization in which they change their locations on the bases (Figure 23-4). The preferred tautomeric forms of adenine and cytosine are the amino configurations, however, with low probability each can assume the imino configuration. The preferred tautomeric forms of guanine and cytosine are the keto configuration. However, again with low probability, each can assume the enol configuration.

Guanine

H

H

\

/

N

N

O

H

O

/H I

I

I

N\

Cytosine H \

/ N

H

\H

Thymine H ~

N

O

Imino

Keto

I

Amino

H

N"

I

/H O'

I

Enol

F I G U R E 23-4 Tautomerization of bases in DNA. The most stable forms of adenine and cytosine are the amino conformations. With low probability these bases can tautomerize into the imino form; if this occurs during replication, an incorrect base pair (a point mutation) may result. The stable forms of guanine and thymine are the keto conformations; the enol conformations also can result in mistakes in base pairing during replication.

524

CHAPTER23 Structure and Properties of DNA

Tautomerization of the bases can occur as the free base and as polynucleotides. If tautomerization of a base should occur at the moment of replication of that region of DNA, an incorrect base may be inserted. For example, the tautomeric form of adenine can pair with cytosine or the tautomeric form of thymine can pair with guanine. During subsequent rounds of DNA replication these mismatched base pairs can result in point mutations in half of the DNA molecules in subsequent cycles of DNA replication and cell divisions. During replication, tautomerization of bases in DNA and ensuing mistakes in base pairing are extremely rare, but experiments show that point mutations do arise in this manner and contribute to genetic variation. However, most mistakes in base pairing that occur during DNA replication are corrected by enzymatic editing or repair functions.

Methylation of Bases After the four bases have been incorporated into DNA, they can be modified by methylation, the addition of methyl groups at various positions (Figure 23-5). The bases that are most frequently methylated are guanine and cytosine. Methylation of cytosine residues influences gene regulation in higher organisms, and about 70% of GC base pairs in mammalian cells are methylated. The pattern of methylation of cytosine residues is inherited and is specific

H

\ / N - - H ""O \\ / CH3 I cc--c N--C//' C~ / \ / N'"H--N \ Deoxyribose \ / /C - - H N--C \ //C N\ H~c~N\

H

O

Deoxyribose

Adenine

H\c~N

I /N~

Deoxyribose

Thymine

\

/ O...H--N // \

C\

\ / N - H... N\

/~-c

//c - c

N ---C

\

/ H Guanine

H H

~C /

C--N

//

N -- H... O

/

-- H

for each species. However, methylation of DNA is not universal; for example, the DNA in the fruitfly Drosophilia is completely unmethylated. Methylation of DNA is primarily the result of two enzymes, Dam methylase and Dem methylase. Dam methylase transfers a methyl group from S-adenosylmethionine to an adenine contained in any GATC sequence in DNA. Dem methylase acts in a similar fashion on cytosine residues in the sequence CCAGG. One or both cytosines in opposite strands of DNA are methylated. Methylated segments of DNA are recognized by proteins that interact with DNA in such processes as replication, recombination, and gene expression. Methylation of DNA also serves an important function in bacteria. Methylation of specific sequences in bacterial DNA protects the bacterial DNA from cleavage by endogenously synthesized restriction endonucleases. Many human genes are methylated differently in maternal and paternal chromosomes at CpG nucleotides, a mechanism that is referred to as imprinting. As a result of genomic imprinting, the expression of genes on maternal and paternal chromosomes differs; the loss of imprinting through mutation or some other mechanism can result in overexpression of critical genes and severe disease. Two inherited diseases that are the result of faulty imprinting are Prader-Willi/Angelmann's syndrome (PWS) and Beckwith-Wiedemann syndrome (BWS). The symptoms of PWS include neonatal hypotonia, hypogonadism, obesity, and short stature; the symptoms of BWS are omphalocele (abdominal wall defect), thickening of long bones, and renal abnormalities. The altered gene in BWS codes for insulin-like growth factor-2 (IGF2) and is located on the short arm of chromosome 15. Usually IGF2 is silent in the maternal chromosome and active in the paternal chromosome. In some cases of BWS, the child receives two copies of chromosome 15 from the father, a condition knows as disomy. These individuals have twice the normal level of IGF2 and suffer from "overgrowth." Low levels of IGF2 may also result from disomy and lead to abnormal "undergrowth." Other inherited diseases and cancers may result from mutations that affect imprinting and normal transmission of chromosomes (Chapter 26).

\

Deoxyribose

Cytosine

F I G U R E 23-5 Methyl groups can be added to bases in DNA. Except for some yeasts and insects, all DNA contains methylated bases. The attachment of methyl groups to bases at specific sequences in DNA acts to regulate the functions of DNA. One function of site-specific methylation in prokaryotes is to protect endogenous bacterial DNA from digestion by restriction endonucleases produced by the bacteria.

23.2 Physical and Chemical Structure of DNA The W a t s o n - C r i c k DNA Structure In DNA, two polydeoxynucleotide strands are coiled about one another in a double-helical structure as originally proposed by the Watson-Crick (W-C) model (Figure 23-6). The important features of the W-C model are as follows:

SECTION 23.2

Physical and Chemical Structure of DNA

525

F I G U R E 23-6 (a) Schematic diagram indicating the dimensions and helical structure of double-stranded DNA. (b) The structure of B-DNA showing the orientation of the atoms and the major and minor grooves in DNA. [From R. E. Dickerson, Sci. Am. December 1983, pp. 100-104; Copyright @1983 by Scientific American, Inc. All rights reserved. Illustration C. Irving Geis.]

1. The sugar-phosphate backbones of the double helix follow helical paths at the outer edge of the molecule. The sugar-phosphate strands are oriented in antiparallel directions such that the 5'-phosphate end of one strand is opposite the 3'-OH end of its partner. As a result of the antiparallel orientation, if one reads a sequence of bases in a 5' to 3' direction on one strand, one is reading the complementary bases on the other strand in a 3' to 5' direction (Figure 23-6a). The overall width of the double helix is 2.0 nm. 2. The two strands in the double helix are complementary because of the base pairing rules that dictate A = T and G = C. Because of the base

pairing rules, the information in DNA is redundant; knowing the sequence of bases in one strand dictates the sequence of bases in the opposite strand. 3. Each base pair in the double helix lies in a plane that is perpendicular to the axis of the helix (Figure 23-6b). Adjacent pairs of bases in DNA are separated by 0.34 nm and rotated with respect to one another so that 10 base pairs occupy each turn of the helix, which repeats every 3.4 nm. 4. Space filling models of DNA reveal two grooves that run the length of the molecule (Figure 23-6b). The major groove is wide and deep and the minor groove is narrow and shallow. These grooves in DNA provide


5~fi

space for other strands of nucleic acids and also for binding of regulatory proteins.

Alternative DNA Structures The W-C DNA structure is a right-handed helix as one looks down either axis of the molecule and the helix rotates in a clockwise direction. This conformation of DNA is called B-DNA and is the form found in solution and inside cells. The double helix is stabilized by a number of forces, including hydrophobic interactions and van der Waals forces, which also help stabilize single polynucleotide chains. Collectively, these two forces are known as stacking interactions because of their contribution to the stacked arrangement of the bases in DNA. Other stabilizing forces in DNA are the hydrogen bonds between the AT and GC base pairs. AT base pairs share two hydrogen bonds and GC base pairs share three. These hydrogen bonds are of sufficient strength to discriminate between insertion of a correct and an incorrect base during DNA replication. Although B-DNA is the physiologically significant form of DNA, two other conformations, A-DNA and Z-DNA, have been observed. When water is removed from solutions of DNA (such as in DNA prepared for x-ray analysis), a structure referred to as A-DNA is observed. While A-DNA is still a right-handed helix, the helix is wider and more condensed than in B-DNA. Also, the bases are tilted with respect to the helix axis and the minor groove in the molecule almost disappears. A-DNA is not believed to have any biological significance. A polynucleotide that consists of repeating CGCGCGCGCG base pairs forms a structure in solution that is a left-handed double helix with only one major groove. Also, because the repeating units are dinucleotide pairs, the phosphate groups in the backbone are rearranged to form a zig-zag configuration; this feature accounts for the term Z-DNA. While it is not known what biological role, if any, Z-DNA plays in cells, the fact that long CG tracts exist in DNA in vivo suggests that segments of DNA may assume different structural configurations and that DNA exists in a dynamic rather than a static state in chromosomes. P l a s m i d DNA Circular DNA molecules that are of great significance in nature as well as in numerous biotechnology applications areplasmids, which are widespread among bacteria. Plasmids are small circular DNA molecules consisting of just a few genes to more than a hundred. Among the important genes carried in plasmids are those coding for resistance to

a wide range of antibiotics and genes that allow plasmids to be transferred to other bacteria, even to other species of bacteria. The process of transfer of plasmid and chromosomal DNA from one bacterium to another is called

conjugation. Transfer of antibiotic resistance genes among bacteria in nature has created serious problems in the treatment of many infectious diseases such as tuberculosis, gonorrhea, pneumonia, staphylococcus, and others. The widespread use of antibiotics in agriculture and in hospitals has resulted in the selection and evolution of pathogenic microorganisms that are resistant to many, sometimes all, of the antibiotics normally used to treat infections by these microorganisms. Plasmids also have been genetically engineered in a multitude of ways so that they can carry and express foreign genes in bacteria. For example, the genes coding for human insulin and human growth hormone have been inserted into plasmids which are then reintroduced into bacteria such as E. coli. The genetically engineered bacteria are then used as biological factories to produce the desired drugs. C i r c u l a r a n d Supercoiled DNA Most DNA molecules isolated from prokaryotes and from some virus particles are circular. A circular molecule may be covalently closed, consisting of two unbroken complementary single strands, or nicked, i.e., having one or more interruptions (nicks) in one or both strands. With few exceptions, covalently closed circles assume the form of supercoils (Figure 23-7). The two ends of a linear DNA helix can be brought together and joined in such a way that each strand is continuous. Consider a molecule in which one of the ends is rotated 360 degrees with respect to the other in the unwinding direction, and then the ends are joined. If the hydrogen bonds re-form, the resulting covalent circle will twist in the opposite sense to form a twisted circle, in order to relieve the strain of underwinding. Such a molecule will resemble a figure 8 and will have one crossover point. If it is underwound by 720 degrees before the ends are joined, the resulting superhelical molecule will have two crossover points. In the case of a 720-degree unwinding of the helix, 20 base pairs must be broken (because the linear molecule has 10 base pairs per turn of the helix). However, such a DNA molecule tries to maintain a right-handed (positive) helical structure with 10 base pairs per turn; it will deform itself in such a way that the underwinding is compensated for by negative (left-handed) twisting of the circle. All naturally occurring, superhelical DNA molecules are initially underwound and, hence, form negative

SECTION 23.3

Denaturation of DNA

527 !

1.40

o

T

1.30

q:

Strand separation

,,i.0 :3 m

m 1.20 > I I I I I I I

> .o m (1.)

rr

1.10

-

/

, I t |

iLJ 1.00

...........

1

30

F I G U R E 23-7 Different states of a covalent DNA circle. (a) A nonsupercoiled covalent DNA having 36 turns of the helix. (b) An underwound covalent circle having only 32 turns of the helix. (c) The molecule in (b), but with four superhelical turns to eliminate the underwinding. In solution, (b) and (c) would be in equilibrium; the equilibrium would shift toward (b) with increasing temperature.

superhelices. In bacteria, the underwinding of superhelical DNA is not a result of unwinding before end joining but is introduced into preexisting circles by an enzyme called DNA gyrase, which is one of a class of enzymes called topoisomerases (Chapter 24). In eukaryotes, the underwinding is a result of the structure of chromatin, a DNA-protein complex of which chromosomes are composed. In chromatin, DNA is wound about specific protein molecules, in a direction that introduces underwinding.

23.3 Denaturation of DNA The three-dimensional structures of DNA, RNA, and proteins are determined by weak noncovalent interactions, principally hydrogen bonds and hydrophobic interactions. The free energies of these interactions are not much greater than the energy of thermal motion at room temperature, so that at elevated temperatures the structures of these molecules are disrupted. A macromolecule in a disrupted state is said to be denatured; the ordered state, which is presumably that originally present in nature, is called the native state. A transition from the native to the denatured state is called denaturation. When double-stranded (native) DNA is heated, the bonding forces between the strands are disrupted and the two DNA strands separate; thus, completely denatured DNA is single stranded. Much information about the structure and stabilizing interactions has been obtained by studying denaturation. Some property of DNA that changes as denaturation

1

:

l,

50

!

l

70

11

1

Tm 90

1

l

100

110

Temperature, ~ F I G U R E 23-8 Melting curve of DNA showing the melting temperature (Tm) and possible molecular conformations for various degrees of melting.

proceeds is measured, e.g., the absorption of ultraviolet light. A change in the ultraviolet absorbance (or some other property) as a function of temperature is called a melting curve (Figure 23-8). Many reagents either break hydrogen bonds or weaken hydrophobic interactions, and are powerful denaturants. Thus, denaturation is also studied by varying the concentration of a denaturant at a constant temperature. For DNA, the simplest way to detect denaturation is to monitor the ability of DNA in a solution to absorb ultraviolet light at a wavelength of 260 nm. The absorbance of DNA at 260 nm, A260, is not only proportional to its concentration (as is the case for most light-absorbing molecules) but also depends on the structure of the molecule; the more ordered the structure, the less light is absorbed. Therefore, free nucleotides absorb more light than a single-stranded polymer of DNA (or RNA), which in turn absorbs more light than a double-stranded DNA molecule. For example, solutions of double-stranded DNA, single-stranded DNA, or free bases, each at 50 #g/mL, have the following A260 values: Double-stranded DNA: Single-stranded DNA: Freebases:

1.00 1.37 A260 -- 1.60

A260 - -

A260 - -

If a DNA solution is heated slowly and the A260 is measured at various temperatures, a melting curve such as that shown in Figure 23-8 is obtained. The following features of this curve should be noted: 1. The A260 remains constant up to temperatures well above those encountered by living cells in nature.

528 2. The rise in A260 occurs over a range of 6-8~ 3. The maximum A260 is about 37% higher than the starting value. The hypothetical state of a DNA molecule in different regions of the melting curve is also shown in Figure 23-8. Before the rise begins, the molecule is fully double stranded. In the region of rapid denaturation, base pairs in various segments of the molecule are broken; the number of broken base pairs increases with temperature. A convenient parameter to characterize a melting transition is the temperature at which the rise in A260 is half complete. This temperature is called the melting temperature, Tin. The value of Tm varies both with base composition and experimental conditions. In particular. Tm increases with increasing percent G + C, which is a result of the hydrogen bonds in a GC pair (three) versus an AT pair (two). A higher temperature is required to disrupt a GC pair than an AT pair. Reagents such as urea and formamide, which can hydrogen-bond with the DNA bases, reduce Tm. These denaturing agents maintain the unpaired state at a temperature at which a broken base pair would normally pair again, so that permanent melting of a section of paired bases requires less thermal energy. Other reagents either enhance the interaction of weakly soluble substances (such as the nucleic acid bases) with water or disrupt the water shell; such substances should weaken hydrophobic interactions. An example of the former type of substance is methanol, which increases the solubility of the bases. Sodium trifluoracetate is an example of the second type. The addition of both these reagents greatly reduces Tm because hydrophobic interactions are also important in stabilizing the DNA structure. In fact, the three-dimensional structure of DNA is one that minimizes contact between bases and water and maximizes the contact of the highly soluble phosphate group with water. Minimization of base-water contact is accomplished by stacking of the bases, which occurs even in singlestranded DNA. The bases of double-stranded DNA are more stacked than those in single-stranded DNA because of the hydrogen bonds between the two strands. Both hydrogen bonds and hydrophobic interactions are weak and easily disrupted by thermal motion. Maximum hydrogen bonding is achieved when all bases are oriented in the right direction. Similarly, stacking is enhanced if the bases are unable to tilt or swing out from a stacked array. Clearly, stacked bases are more easily hydrogen-bonded, and correspondingly, hydrogen-bonded bases, which are oriented by the bonding, stack more easily. Thus, the two interactions act cooperatively to form a very stable structure. If one interaction is eliminated, the other is weakened, which explains why Tm drops so markedly following


addition of a reagent that destroys either type of interaction. When hydrogen bonds and hydrophobic interactions are eliminated, the helical structure of DNA is disrupted and the molecule loses its rigidity. This collapse of the ordered structure is accompanied by complete disentanglement of the two strands. At high pH, the charge of several groups engaged in hydrogen bonding is changed and base pairing is reduced. At a pH greater than 11.3, all hydrogen bonds are eliminated and DNA is completely denatured. When a DNA solution is heated above 90~ the value of A260 increases by 37% and the solution consists entirely of single strands whose bases are unstacked. If the solution is then rapidly cooled to room temperature and the salt concentration is greater than 0.05 mol/L, the value of A260 drops significantly because random intrastrand hydrogen bonds re-form between distant short tracts of bases whose sequences are complementary (or nearly so). After cooling, about two-thirds of the bases are either hydrogenbonded or in such close proximity that stacking is restored and the molecule is very compact. In contrast, if the salt concentration is 0.01 mol/L or less, the electrostatic repulsion due to unneutralized phosphate groups keeps the single strands sufficiently extended that the bases cannot approach one another. Thus, after cooling, no hydrogen bonds are re-formed. At a sufficiently high DNA concentration and in a high salt solution, interstrand hydrogen bonding competes with the intrastrand bonding just mentioned. This effect can be used to re-form native DNA from denatured DNA.

23.4 Renaturation of DNA If a DNA solution is heated to a temperature at which most (but not all) hydrogen bonds are broken and then cooled slowly to room temperature, A260 drops immediately to the initial, undenatured value and the native structure is restored. Thus, if strand separation is not complete and denaturing conditions are eliminated, the helix rewinds. A related observation is that if two separated strands come in contact and form even a single base pair at the correct position in the molecule, the native DNA molecule will re-form. This phenomenon is called renaturation, or

reannealing. Two requirements are necessary for renaturation to occur. 1. The salt concentration must be high enough that electrostatic repulsion between the phosphates in the two strands is eliminated--usually 0.15-0.50 M NaC1 is used.

SECTION23.4 Renaturation of DNA

529

2. The temperature must be high enough to disrupt the random, intrastrand hydrogen bonds described above. However, if the temperature is too high, stable interstrand base pairing will not occur or be maintained. The optimal temperature for renaturation is 20-25~ below the value of Tm. Renaturation is slow compared with denaturation. The rate-limiting step is not the rewinding of the helix but the precise collision between complementary strands such that base pairs are formed at the correct positions. Since two molecules participate in the rate-limiting step, renaturation is a concentration-dependent process requiring several hours under typical laboratory conditions. In particular, the kinetics of association follows a simple second-order rate law with the association rate increasing with DNA concentration. The kinetics are conveniently described by an equation that relates the fraction, f , of single-stranded (dissociated) DNA and the time elapsed after exposing a DNA sample to renaturing conditions (high salt concentration, elevated temperature): f = 1/(1 + kCot) in which Co is the initial DNA concentration, t is the elapsed time in seconds, and k is a rate constant. A plot of f versus the logarithm of the product Cot yields a sigmoid curve commonly called a Cot ("cot") curve. A set of such curves for several DNA samples is shown in Figure 23-9. A notable feature of these curves is that the renaturation rate is related to the molecular weight of the DNA. A useful index for characterizing these curves is Co/tl/2, with the value of Cot corresponding to renaturation of half of the DNA ( f = 1/2). Comparison of the Co/tl/2 values for E. coli DNA (M.W. = 2.7 • 109) and T4 DNA (M.W. = 1.1 • 108) shows that T4 DNA renatures roughly 50 times faster than E. coli DNA. The reason

FIGURE 23-9 Reassociation "Cot" curves for bacteriophage T4 DNA, E. coli DNA, and mouse DNA. [Reproduced with permission from D. M. Prescott: Cells: Principles of Molecular Structure and Function. Jones and Bartlett, London, 1988.]

F I G U R E 23-10 (a) Hypothetical DNA molecule having a base sequence that is 3% of the total length of the DNA and is repeated five times. The dashed lines represent the nonrepetitive sequences: they account for 85% of the total length. (b) Renaturation curve for the DNA in (a). Time is logarithmic to keep the curve on the page.

is that if the two DNA samples have equal molar concentrations of nucleotides, the T4 sample will contain many more DNA molecules than the E. coli sample. Cot analysis is not-usually done with intact DNA molecules but rather with fragments having lower molecular weights. This breakage does not affect the relative values of Co/tl/2 since the number of different kinds of fragments of T4 DNA is smaller and hence their concentration larger than the corresponding number of fragments in the E. coli sample. Studies of a variety of prokaryotic DNAs show that the value of Co/tl/2 is directly related to the total size of the DNA of the organism (the genome size). However, this relation does not apply to the DNA of eukaryotes because of the presence of highly repetitive sequences. The fragmentation of the DNA molecules allows a new feature of base sequences to be seen. Figure 23-10(a) shows a long hypothetical DNA molecule having a repeating base sequence. If the unbroken denatured DNA were allowed to renature, the Cot curve would be a smooth curve in which Co/tl/2 would be proportional to the size of the DNA. However, if the DNA were broken into small pieces, several fragments from each molecule would contain the repeated sequence, and these fragments would renature more rapidly than the bulk of the sequences. For example, if the repeated sequence contains 3% of the total number of bases of the DNA and there are five copies of the repeated sequence in the DNA, the Cot curve would be that shown in Figure 23-10b, in which the more rapidly renaturing component accounts for 15% of the transition; hence, the conclusion that 15% of the DNA (on a weight basis) contains a repeating sequence. Approximately 30% of human DNA contains sequences of bases that are repeated 20 times or more.

530

23.5 Repetitive DNA Sequences Cot curves from various eukaryotic organisms show the presence of repetitive sequences to varying degrees. Unique sequences represent genes, approximately 100,000 in the human genome, but constitute only 1-2% of the total genetic information in the human genome. Some genes may be slightly repetitive if they are related to pseudogenes, sequences of DNA that resemble genes but that are no longer expressed. About one-third of the human genome consists of sequences of bases that are repeated at least 20 times or more. Moderately repetitive refers to sequences that usually contain between 10 and several hundred copies; highly repetitive refers to sequences that are repeated thousands or millions of times throughout the genome. The distinction among various classes of repetitive sequences is frequently blurred and additional categories are sometimes defined. Repetitive sequences are also defined by the number of base pairs in each repeated segment. Sequences that have from 100 to 500 bp are referred to as SINES (short interspersed repeated sequences) and sequences that have several thousand base pairs are referred to as LINES (long interspersed repeated sequences.) Thus, repetitive DNA sequences can be described both by the length of the segment and the degree to which it is repeated. Some examples of moderately repetitive sequences are transposable elements including retroviruses, histone genes that are repeated 30-40 times in the human genome, and genes coding for ribosomal RNAs and transfer RNAs. Most of the moderately repetitive genes do not code for proteins but serve other functions in the cell. Histones are involved in condensation of DNA in chromosomes and both ribosomal RNAs and transfer RNA are involved in protein synthesis. One special class of a highly repetitive sequence is satellite DNA which consists of 6-8 bp (such as ATAAACT) and may account for 10-20% of the total DNA. The term satellite DNA derives from the observation that if fragmented DNA is centrifuged in a density gradient solution that separates DNA by weight, a small band of DNA is observed that is separate from the bulk of the DNA. The base composition of satellite DNA is AT-rich, which explains why it separates from the rest of the DNA. Satellite DNA is thought to play a structural role in chromosomes. Certain satellite DNA sequences are concentrated near the centromeres of chromosomes, the site where spindle fibers attach when sister chromatids are separated. Other satellite DNA sequences are located in

CHAPTER 23

Structure and Properties of DNA

telomeres, structures at the ends of chromosomes that play a critical role in DNA replication (Chapter 24). Another class of highly repetitive sequences is the Alu family, which consist of about 300 bp that are repeated millions of times throughout the genome. Any Alu sequence is at least 85% homologous in base sequence to any other Alu sequence; hence, the family of genes has been highly conserved. Each Alu sequence contains a restriction site that is recognized by the AluI restriction enzyme, from which the name of the family of sequences is derived.

23.6 Degradation of DNA Nucleases A variety of enzymes break phosphodiester bonds in nucleic acids; deoxyribonucleases (DNases) cleave DNA and ribonucleases (RNases) cleave RNA. DNases usually are specific for single- or double-stranded DNA although some DNases can cleave both. DNases can act as exonucleases in which they remove one nucleotide at a time from either the 3' or 5' end of the strand. Other DNases function as endonucleases and are specific for cleaving between particular pairs of bases.

Restriction E n z y m e s An important class of DNA endonucleases are restriction enzymes (restriction endonucleases) that recognize specific sequences of bases in DNA (a restriction site) and make two cuts, one in each strand that generates fragments of double-stranded DNA. Microorganisms use their restriction enzymes to degrade any foreign DNA that may enter the cell in the form of viruses, plasmids, or naked DNA. Hundreds of different restriction enzymes have been isolated from microorganisms; these enzymes generally recognize sequences of four, six, eight, or rarely more bases that have an axis of symmetry (Table 23-1). Restriction sites that have an axis of symmetry are called palindromes, which means that the restriction site can be rotated 180 degrees and the sequence of bases will remain the same. Restriction enzymes can cut DNA in either of two ways. Each strand can be cleaved along the axis of symmetry generating fragments of double-stranded DNA that have blunt (flush) ends. Symmetrical cuts that are staggered around the axis of symmetry generate fragments of doublestranded DNA that have single-stranded cohesive ends (Figure 23-11). Fragments of DNA with cohesive ends

SECTION23.6 Degradation of DNA

531

TABLE 23-1 Some Restriction Endonucleases and Their Cleavage Sites* Name of Enzyme

Microorganism

Target Sequence and Cleavage Sites Axis of symmetry

Generates cohesive ends

Escherichia coli RY 13

EcoRI

Bacillus amyloliquefaciens H

BamHI

Bacillus globigii

BgIII

Haemophilus aegyptius

HaeII

Haemophilus influenzae R d

HindIII

Providencia stuartii

PstI

Streptomyces albus G

SaII

Xanthomonas badrii

XbaI

Thermus aquaticus

TaqI

Nocardia otitdis

Not I

Streptomyces sp.

Sgf I

G C G C

GSA A C T T GZG A C C T AZG A T C T Pu G C PyTC G AsA G T T C C T G GTA C GZT C CzA G T C T A GzA T C A G CSG G G C C C G A G CTT

T C ATG C C GTG C T G?A C $ Py C G Pu C T T G ATA C A$G G T C G A C C TTG A G A T CTT G A CTT C C G G GTC T$C G A G C

T

G

G

C

C

A

A

C

C ~ G

G

T

C

C ~ G

G

C

C

C ~, G

G

G

G

G

G ~ C

C

C

T A T A T A G

C G C G

Generates flush ends

Brevibacterium albidum

Haemophilus aegyptius Serratia marcescens

BalI

HaeI

SmaI

| | | |

*The vertical dashed line indicates the axis of dyad symmetryin each sequence. Arrows indicate the sites of cutting. The enzyme TaqI yields cohesive ends consisting of two nucleotides, whereas the cohesive ends produced by the other enzymes contain four nucleotides. The enzyme HaeI recognizes the sequence GGCC whether the adjacent pair is A .T or T .A, as long as dyad symmetryis retained. Pu and Py refer to any purine and nvrimidin~ rP.~n~tivolv x.,

l

.,

are useful for constructing novel DNA molecules, which is the goal of recombinant DNA (rDNA) technology. Since a restriction enzyme recognizes a unique sequence, the number of cuts and the number of DNA fragments depends on the size of the molecule. In general, restriction sites consisting of four bases will occur more

frequently by chance than sites consisting of six or eight bases. Thus, four-cutters will generate more fragments than six-cutters which, in turn, will generate more fragments than eight-cutters. A particular restriction enzyme generates a unique family of fragments for a particular DNA molecule.

532

CHAPTER 23

(a) Cuts on axis of symmetry

........ ........

i

TCIGA AGjCT

........

I S e p a r a t i o n of fragments

........ ........

TC AG

3 I i i 5

5" GA ........ CT ........ 3"

Flush-end molecules

(b) Cuts symmetrically placed around axis of symmetry

........ ........

I

GAAITTC CTTIAAG

t I Separation of fragments

..... ........

G

3"

........

CTTAA

5" AATTC

........


has a strong interaction with (ideally) a specific DNA target and can be detected after the interaction. DNA probes consisting of 20 bases or fewer usually will have a unique target even in a large set of DNA molecules. The probability that any base will follow any other base in DNA is one in four or 0.25. Therefore, the probability of a specific sequence of 20 bases occurring in a DNA molecule by chance is 0.252~ a vanishingly small number. The use of DNA probes in various aspects of medical diagnostics is increasing rapidly. DNA probes can be used to identify infectious agents if sequences specific to different pathogens are known. Identification of a pathogen by DNA probes can be done in hours as compared to days or weeks by conventional culturing of microorganisms. DNA probes are now used routinely to detect the presence of mutant alleles in fetal cells obtained by amniocentesis, as well as in cells removed from affected adults or carriers. Many inherited disorders, such as sickle cell disease, cystic fibrosis, Huntington's disease, Duchenne's muscular dystrophy, and dozens of other Mendelian (single-gene) disorders, can now be diagnosed in fetuses and adults. In addition to inherited disorders, DNA probes are used to detect the presence of active oncogenes or inactive tumor suppressor genes in cancerous tissues removed from patients (Chapter 26).

G ........ 5"

Cohesive-end

3 molecules

F I G U R E 23-11 Two types of cuts made by restriction enzymes. The arrows indicate the cleavage sites. The dashed line is the axis of symmetry of the sequence.

Figure 23-12a shows the positions of restriction sites in bacteriophage lambda (k) DNA for the restriction enzymes EcoRI and BamHI. This arrangement of restriction sites is known as a restriction map. The family of fragments generated by one or more restriction enzymes is detected by agarose gel electrophoresis that separates DNA molecules according to their molecular weights (Figure 23-12b).

23.7 Diagnostic and Clinical Applications of DNA DNA Probes The ability to isolate specific fragments of DNA containing known sequences of genes gives rise to DNA probes that can be used for a variety of diagnostic, forensic, and therapeutic purposes. DNA probes can be labeled with either radioactive or nonradioactive markers. A DNA probe

Southern Blot Analysis The Southern blot (named for its inventor, E. M. Southern) is a method for hybridizing one or more labeled DNA probes to a large number of DNA fragments and discriminating among them. The procedure depends on the ability of denatured DNA single strands to bind tightly to nitrocellulose under certain conditions. The Southern blot procedure is outlined schematically in Figure 23-13. The DNA to be investigated is digested with several restriction enzymes to generate DNA fragments of varying sizes which are then separated by agarose gel electrophoresis. After the fragments are separated, the gel is immersed in a denaturing solution that converts all DNA to single strands. Then the gel, which is in the form of a broad, flat slab, is placed on top of filter paper supported by a glass plate. A nitrocellulose filter is placed over the gel and covered with another sheet of filter paper. Finally, paper towels and weights are placed on top of the gel. The glass plate supporting the filter papers and gel is adjusted so that the ends of the filter paper are suspended in buffer and act as wicks. The buffer moves upward through the gel and into the wad of paper towels that act to absorb the buffer. The DNA migrates from the gel to the nitrocellulose filter that binds the single-stranded DNA fragments.

Diagnostic and Clinical Applications of DNA

SECTION 23.7

533 EcoRI

EcoRI 0

43.9

53.5

64.9

79.9

91.8

t

t

I

t

I

t

43.9

9.6

15.0

11.4

11.9

BamHI

100 8.2

o

BamHI 0

10.6

45.3

56.5

69.3

83.8

F

i

t

I

I

I

10.6

34.7

11.2

12.8

14.5

100

4 16.2

(a)

5/6

(b)

F I G U R E 23-12 (a) Restriction maps of ~. DNA for EcoRI and BamHI nucleases. The vertical bars indicate the sites of cutting. Thc top numbers indicate the percentage of the total length of DNA measured from the gene A end of the molecule. The bottom numbers are the lengths of each fragment, again expressed as percentage of total length. (b) Gel electrophoretogram of EcoRI and BamHI restriction enzyme digests of ~. DNA. The bands labeled "cohered ends" contain molecules consisting of two terminal fragments, joined by the normal cohesive ends of ~. DNA. Numbers indicate fragments in order from largest 1 to smallest 6. Bands 5 and 6 of the BamHI digest are not resolved.

~ ~_1,.,. ~ - _ ~ . ~

,,.~_~#~-:

DNA restriction fragments

Agarose gel electrophoresis

,ii H I tt/ t . I /III

[/////I

/I I I/ I #I I II m~ I I I I I/1I ~ - -

[

Ill/ I l l

"

~

..........

Film // I

Fragments separated by size

I

.Weight Transfer of ~ towels DNA fragments litrocellulose filter from gels to nitrocellulose ~el filter

1

# , . , .,,... ] /////# ///// / # t /i/~/t / / t t t .

////,1''1/'1/

[

Nitrocellulose filter with DNA fragments

Radioactive D N A probe hybridized to fragments on filter

. Autoradiograph showing -positions of hybrid DNA

F I G U R E 23-13 Procedure for a Southern blot. This analysis allows detection of a specific fragment of DNA by hybridization of a radioactive probe with a complementary sequence.

The nitrocellulose filter is dried and exposed to high temperature to keep the DNA in a denatured state. A labeled DNA probe containing a known sequence or gene is then incubated with the nitrocellulose filter under a variety of hybridization conditions to identify a particular fragment of DNA from the sample. Strands of the DNA probe bind to fragments of the separated DNA that contain similar or identical sequences and hybridize together to form double-stranded DNA. After the hybridization reaction is complete, the solution is cooled and the itrocellulose filter is washed to remove unbound probe DNA and dried. The labeled DNA fragments can be visualized by autoradiography (if the probe was radioactive) or by fluorescence. The DNA fragments of interest also can be eluted from the nitrocellulose filter for further analysis.

Polymorphisms Approximately 25% of all human genes occur among individuals in a population in multiple allelic forms referred to as polymorphisms. For example, the ABO locus, an important factor in blood transfusions, consists of three different alleles (alternative states of a gene) called A, B, and O. Individuals can be homozygous or heterozygous for any pair of A, B, and O alleles. The HLA family

534 of genes that play a vital role in the rejection of tissue transplants consists of hundreds of different alleles (Chapter 35). No two individuals, except for identical twins, share exactly the same set of HLA alleles. For a gene to be polymorphic, alternative alleles must be present in at least 1% of individuals in a population. The relative frequencies of the different alleles at a polymorphic locus usually vary from one human population to another. The remaining 75% of human genes consist of a single allelic form and are said to be monomorphic. For example, all human beings have the same ot and/~ genes for hemoglobin, presumably because the protein structure has been optimized by millions of years of human evolution. All persons, irrespective of race or ethnicity, have hemoglobin genes that are identical in sequence. The only exceptions to the monomorphic state of the hemoglobin genes are the rare individuals who inherit or acquire a mutation in the ot or/3 genes that gives rise to the several hundred characterized human hemoglobinopathies (Chapter 28). Restriction enzyme sites in DNA also are highly polymorphic in human populations. The pattern of DNA fragments produced by digesting DNA from different individuals will usually show a difference if a sufficient number of fragment sizes are examined; again, only identical twins have identical restriction enzyme sites. The different fragments produced by digesting DNA with one or more restriction enzymes are called restrictionfragment length

polymorphisms (RFLPs). RFLPs have been extremely useful in constructing a restriction map of the human genome (which can be correlated with the genetic and physical maps), in screening human populations for the presence of mutant alleles, and for diagnosing hereditary disorders in fetuses and prospective parents. An early success in mapping the human genome was determining the location of more than 10,000 RFLPs on the 23 human chromosomes. Most mutant genes that cause hereditary disorders are linked to specific RFLP differences among affected and unaffected persons that can be detected by using DNA probes carrying the gene in question. For example, the single-base change found in all cases of sickle cell anemia also changes a restriction site located within the/3-globin gene that is recognized by the restriction enzyme MstII (Figure 23-14). When DNA from unaffected and affected individuals is digested with MstII and a Southern blot analysis is performed with the appropriate probe, affected individuals show a different pattern of DNA fragments as compared with unaffected individuals. Huntington's disease (HD) is an autosomal dominant disorder characterized by progressive chorea, dementia,

CHAPTER 23


FIGURE 23-14 Detection of sickle cell anemiaby RFLPanalysisand Southern blot.

and ultimately death. It is a late-onset hereditary disease that usually manifests after age 40. Any son or daughter who had a parent with Huntington's disease has a 50% probability of inheriting the mutant allele from the affected parent. A HindIII restriction site is closely linked to the HD gene in asymptomatic family members who carry the mutant allele but is absent in family members who have not inherited the mutant allele. Southern blot analysis can determine which family members are destined to die of the disease later in life. (Not all families that carry a HD gene mutation have this particular mutation that was characteristic of the original large family investigated.) Genetic testing for susceptibility to Huntington's disease (and many others) raises profound ethical questions for society, for families, and for individuals who may or may not have inherited disease-causing mutant alleles, and who may or may not want to know their disease status. Persons diagnosed with a gene for an inherited disorder may face discrimination in obtaining health insurance, life insurance, job opportunities, and so path. Hundreds of Mendelian (single-gene) disorders can now be detected with appropriate DNA probes (Table 23-2). However, genetic screening generally is not advised, particularly if no medical treatment is available for the particular disease.

SECTION 23.7

Diagnostic and Clinical Applications of DNA

535

TABLE 23-2

A Partial List of Diseases for Which Single Genes Have Been Isolated and Characterized* Familial amyloid polyneuropathies Phenylketonuria Familial hypercholesterolemia Christmas factor deficiency Retinoblastoma, Wilson's disease Type IV collagen deficiencies Hydatidiform moles and choriocarcinomas Tay-Sachs disease Alzheimer' s disease Gaucher' s disease Charcot-Marie-Tooth 1A disease Williams syndrome Von Hippel-Lindau disease Breast cancer (BRCA1) Prostate cancer

Niemann-Pick disease X-linked Charcot-Marie-Tooth disease Familial colon cancer Hereditary retinal degenerative diseases Ototoxic deafness XSCID Compulsive disorders Myotonic muscular dystrophy Familial dysalbuminemic hyperthuroxinemia Long QT syndrome Cerebral autosomal dominant arteriopathy Cancer-associated mar binding protein Breast or ovarian cancer Idiopathic dilated cardiomyopathy Hereditary neuropathy with liability

*Each of these mutant alleles contributes to a greater or lesser degree to the pathology of the disease or inherited disorder. As of December 1997, patents had been issued to universities or companies for the commercial use of these genes.

Forensic DNA Analysis RFLPs are also widely used in forensic pathology, criminology, and cases of contested paternity. A particular set of DNA probes is specific for hypervariable sequences in the human genome that are inherited in a Mendelian pattern identical to the inheritance of genes. Hypervariable sequences are highly polymorphic minisatellite loci that are unique to each individual just as each individual has a unique set of genes. These hypervariable sequences can be detected by a special set of DNA probes called Jeffreys probe after their discoverer. A Southern blot analysis of fragments of DNA from an individual using the Jeffreys probe constitutes a unique "genetic fingerprint." Only monozygous twins have the same pattern of hypervariable sequences and identical genetic fingerprints (Figure 23-15). DNA obtained from a blood stain, a human hair cell, or even a few sperm provides enough material to match with the DNA obtained from a suspect. DNA testing of suspects in rape, murder, and other crimes in which a sample of DNA was obtained at the scene of the crime is now a routine procedure. DNA sequences can be amplified by the polymerase chain reaction (PCR) technique. Only an infinitesimal amount of DNA is needed, e.g., a sample of DNA from a single hair follicle or sperm. The DNA to be amplified is mixed with three other components in an automated PCR procedure:

1. A heat-stable DNA polymerase that is isolated from a thermophilic bacterium, 2. An excess of two short primer DNAs that are complementary to opposite strands of the DNA fragment that is to be amplified, and 3. An excess of deoxyribonucleotide triphosphates. PCR consists of repeated cycling of three reactions: denaturation of the DNA by heating, reannealing of the primers with the target DNA by cooling, and synthesis of new DNA strands. The sequence of reactions is automatically repeated at defined intervals to yield an exponential increase in the amount of DNA. Twenty cycles of PCR amplify DNA by about a factor of 106 and 30 cycles by about 109. PCR amplification of DNA is one of the most widely used techniques in medical research and diagnostics. PCR is used in forensic pathology (to identify human remains), in rapid identification of infectious microorganisms, in diagnosis of inherited diseases, and in archaeology and anthropology where small DNA samples can be recovered.

Sequencing DNA Until the development of automated DNA sequencing machines in the 1990s, two techniques were used to sequence the bases in a segment of DNA. Each of the techniques involves the isolation of a restriction fragment containing a few hundred or a few thousand base pairs. The DNA is denatured and each strand is sequenced separately so

536

CHAPTER 23


radioactivity and used in four separate reactions in vitro in which a single-stranded DNA template is copied. A series of radioactive DNA fragments are separated according to length by agarose gel electrophoresis (fragments differing by only one nucleotide are separated), and the DNA sequence can be read directly from the pattern of radioactive bands in the gel. Another sequencing technique is the Maxam-Gilbert method (developed by Alan Maxam and Walter Gilbert). In this method, a single DNA strand is labeled at the 5' end with radioactive phosphorus (p32). The radioactive DNA is divided into four portions and each one is exposed to different chemical reactions. Each reaction causes a 5' cleavage adjacent to either 1. 2. 3. 4.

AorT, G alone, C or T, or C alone.

The reactions are carried out for a short time so that, on average, only one cleavage occurs in each DNA molecule. This produces a set of DNA fragments (one set for each reaction), whose length identifies that position of a particular base. For example, a fragment containing 19 nucleotides in the G-only reaction mixture identifies G at position 20 from the 5' end. Similarly, a fragment containing 27 nucleotides present in the C or T reaction but not in the C-only reaction indicates that T is at position 28. The lengths of DNA fragments are determined by polyacrylamide gel electrophoresis, a technique that can separate DNA fragments differing in length by only one nucleotide.

Gene Therapy

F I ( ; U R E 23-15 Genetic fingerprint of monozygotic twins (columns 2 and 3) and their parents (columns 1 and 4). The arrows indicate hypervariable sequences inherited from the father. Similar hypervariable sequences can be identified from the mother. [Reproduced with permission from A. J. Jeffreys, V. Wilson, and S. L. Thein: Individual-specific "fingerprints" of human DNA. Nature 316, 76 (1985). @ 1985 by Macmillan Magazines Ltd.]

that the sequences can be compared in order to eliminate errors. One sequencing technique is the Sanger method (developed by Fred Sanger) which uses dideoxynucleotides that stop chain elongation at the site of their incorporation. The four dideoxynucleotide substrates are labeled with

One expected advance in medical treatment is the use of gene therapy to ameliorate or cure a variety of inherited disorders as well as diseases caused by somatic mutations such as cancer. The goal of gene therapy is the replacement of a defective, disease-causing gene in an individual by normal, functioning copies. In essence, gene therapy is a novel form of drug therapy; it uses the biochemical capacity of a patient's cells to synthesize the therapeutic agent from the introduced gene. The first attempts at gene therapy were directed at trying to correct severe combined immunodeficiency (SCID) which is caused, in some cases, by a deficiency of adenosine deaminase (ADA), that is expressed in all tissues. ADA deaminates both adenosine and deoxyadenosine and, in the absence of ADA, deoxyadenosine accumulates in cells. Deoxyadenosine can be phosphorylated by the enzyme deoxcytidine kinase to produce deoxyadenosine

531

SECTION23.8 DNA Vaccines

triphosphate, a toxic substance that kills dividing cells. As a result, individuals with an ADA deficiency are defective in both humoral and cell-mediated immunity. In the mid-1980s, the first clinical gene therapy trials were attempted to correct ADA deficiency in two children with SCID. The ADA gene was cloned into a viral vector and inserted into peripheral T cells removed from the affected children. After a period of growth of the genetically modified cells in vitro, they were reintroduced into the patients. Synthesis of ADA could be detected for a time but the activity disappeared as the introduced cells died. Several other trials of gene therapy for ADA deficiency showed promise, but it was still necessary to maintain the children with SCID on enzyme replacement therapy. Finally, in 2000, successful treatment of an X-linked form of SCID (SCID-X1) by gene therapy was reported by a French medical team. Almost a year after a normal gene was introduced into their cells, two children were still synthesizing the enzyme that they lacked. This was the first success for gene therapy in curing a disease after years of effort. Since the 1980s, hundreds of clinical trials of gene therapy for cystic fibrosis, osteogenesis imperfecta, Gaucher's disease, Fanconi's anemia, and several forms of cancer have been attempted. The problems that need to be overcome in developing a successful strategy for gene therapy are: 1. Development of safe and effective vectors for cloning the gene and inserting it into palients cells, 2. Regulation of expression of the desired gene product so that it is produced at the right time and in the correct amounts in the appropriate tissues, and 3. Stability of the inserted genetic construct in cells so that the product will continue to be produced. Many different viral vectors have been developed to deliver genes to various organs in the body; these include retroviruses that can insert themselves and the genes they carry into the chromosomes of cells; adenovirus, a DNA virus that causes respiratory infections, which has been inactivated by removal of many viral genes; and lentiviruses, slow-growing retroviruses. All of the viral vectors carry with them some risks. In 1999, a gene therapy patient died of complications from the use of adenovirus that was being tested as a therapy for an inherited liver disease caused by a deficiency in the enzyme ornithine transcarbamylase (OTC). Despite the problems that have beset gene therapy trials, it is expected that development of safer vectors and new techniques for delivering genes to cells will eventually make gene therapy a vital part of medical treatment.

23.8 DNA Vaccines An extension of gene therapy, and one that may turn out to be of worldwide importance, is the use of naked DNA as a vaccine to prevent viral diseases. For example, plasmid DNA can be injected into tissues; upon entry, the DNA expresses any cloned gene, such as a viral antigen, that is carried by the plasmid. DNA vaccines have the advantage that the viral protein that is expressed in the cells stimulates both humoral and cell-mediated immunity. Fragments of the synthesized viral protein are carried to the cells' surface where they stimulate CD8 + cytotoxic T cells and, thereby, cell mediated-immunity (Figure 23-16). In experiments with mice, DNA vaccines have proven to be very effective. The gene coding for the core protein of the influenza virus was cloned into a plasmid vector and injected into mice. The mice developed immunity not only to the strain of influenza from which the gene was derived but from other strains of influenza virus as well. Inducing an immune response with a viral core antigen is thought to be more effective than capsid antigens because viral core proteins from related viral strains do not differ much in structure or antigenicity. Viral capsid proteins, on the other hand, evolve rapidly; such changes, for example, account for the different strains of influenza virus that arise each year. Before DNA vaccines can replace conventional vaccines that use inactivated or attenuated viruses, the safety of the plasmid vectors must be rigorously proved. The plasmids might occasionally integrate into the host genome or they might stimulate an immune response to tissues containing the plasmid DNA. Either event might dictate against the widespread use of DNA vaccines.

Antibodies to DNA Both single- and double-stranded DNA are antigenic; antibodies to DNA are normally found in the circulation, but in some individuals overproduction of DNA antibodies causes disease. In particular, patients with systemic lupus erythematosus (SLE) show abnormal levels of antibodies to double-stranded DNA. Antibodies to singlestranded DNA also bind to bases, nucleosides, nucleotides, oligonucleotides, and the ribose-phosphate backbone of RNA. Antibodies to double-stranded DNA also bind to base pairs, chromatin, nucleosomes, type IV collagen, and the deoxyribose-phosphate backbone of DNA. Antibodies to DNA consist of both IgM and IgG classes. Healthy individuals usually have low-affinity IgM antibodies to DNA; however, if these undergo an isotype switch to IgG, they may become pathogenic. Tests for DNA antibodies help establish a diagnosis of SLE,

538

CHAPTER 23


!"1(;1 Ri'~ 2 3 - 1 6 How a DNA vaccine might work. A viral gene coding for a core or capsid protein is inserted into a plasmid. The plasmids are injected into cells where the plasmid migrates to the nucleus. Viral proteins are expressed in the cells producing both a cell-mediated and humoral immune response. [Reproduced with permission from W. M. McDonnel and E K. Ashari, DNA vaccines, N. Engl. J. Med. 314, 44 ( 1996)].

although the level of such antibodies is not always predictive of the severity of the disease. Genetic susceptibility also seems to play a role in the pathogenicity of DNA antibodies in some individuals. Antibodies to double-stranded DNA may cause glomerulonephritis by forming antibody-DNA complexes that become trapped in the glomeruli. In some cases, DNA antibodies can be recovered from damaged tissues in patients with SLE or glomerulonephritis, indicating that the antibodies are involved in tissue damage. The production of DNA antibodies is reduced by immunosuppressive drugs, and immunosuppressive therapy is the accepted treatment for SLE and related diseases caused by antibodies directed against DNA.

23.9 The Human Genome Project A complete human genome, the total genetic information in a haploid set of chromosomes, contains approximately 3.1 billion bases and 50,000-1000,000 genes that encode proteins. The H u m a n Genorne Project was established

with federal funds to obtain the complete sequence of a human genome or, more accurately, the sequence of pooled DNA extracted from cells donated by several anonymous individuals. Initially a public research effort, the sequencing task later was undertaken by a private company, Celera Genomics (Rockville, MD). The ensuing competition led to a joint announcement by the U.S. Government and Celera Genomics on June 26, 2000 that the human genome had essentially been sequenced although there were still gaps and errors in the sequence. The two groups agreed to continue cooperating to complete the sequence of a generic human genome as soon as possible. Any two human genomes are approximately 99.9% identical in sequence. Of course, the 0.1% represents a difference of 3 million bases between any two individuals, and these base differences may have profound effects on disease susceptibility, behavior, intelligence, personality, and other traits. Approximately 75% of all human genes have the same DNA sequence in all individuals except for those with rare mutations. The type of genetic markers employed in the dissection of the human genome include

SECTION 23.9

The Human 6enome Project

1. Restriction sites: sequences of DNA digested by restriction enzymes;

2. Variable-number tandem repeats (VNTRs): sequences of bases that are repeated in tandem almost without change up to 40 copies; 3. Simple sequence repeats (SSRs): sequences of bases, most commonly CA, that are repeated dozens of times and occur at thousands of sites in the genome; and 4. Single-nucleotide polymorphisins (SNPs): sites in human DNA that differ by a single base pair. Because SSRs and SNPs occur so frequently in the genome, they can be used to distinguish one chromosome from another in an individual. An SSR may occur 19 times at one location in a particular chromosome and 21 times in a homologous chromosome. Since an SSR is inherited like a gene, the variability can be followed from person to person in a pedigree. The length of a particular SSR can be determined by gel electrophoresis without any knowledge of the exact sequence. SNP differences occur about once in every thousand bases in DNA. That is, at a position on a chromosome where an A - T base pair occurs, a G-C pair might occur on the homologous chromosome. Because only a small percentage of the total number of nucleotides in the human genome is actually used to encode information for the synthesis of different proteins, techniques have been developed to search for and map genes that are expressed. Knowing the location of these protein-encoding genes is extremely useful as mutated forms of these expressed genes are likely implicated in many diseases. Two techniques have contributed to the identification of many active genes in the human genome. Expressed sequence tags (ESTs) for as many as 50,000 human genes have been generated by isolating messenger RNAs and copying them into DNA. These DNA probes can then be used to identify the location of the expressed genes on the various chromosomes by hybridization. Specific chromosomal sites also can be identified by using PCR primers to amplify a small segment of a chromosomal locus. The sites identified by using PCR amplification are called sequence

tagged sites (STSs). Completing the sequencing of the human genome is only the first step in the revolution in medicine that many are predicting will result in the 21st century. However, knowing the base sequence, chromosomal position, and even the function of the protein that is encoded in the gene is only the first step in using the information to benefit patients with inherited defects (Figure 23-17). The sequence information does not specify how and when a gene is turned or or off, in what cells the gene is active or inactive,

539 or in what stage of development expression of the gene is needed. The expression of most genes is modulated in complex ways and coordinated by numerous intracellular and extracellular factors. How a particular protein affects the biochemistry, physiology, and phenotype of an individual must be understood before therapeutic messures can be developed. Often inherited defects are manifested during fetal development and the damage is irreversible by the time the child is born. Some of the anticipated medical, social, and ethical consequences deriving from the human genome project are described below. Genetic testing: Hundreds of genetic tests are available that can be used to determine if a person carries a defective gene and to estimate the risks of developing a disease as well as the risk of passing the gene on to progeny. In many instances, determining the risk of passing on genes for Hungtington's disease, cystic fibrosis, hemophilia, Duchenne's muscular dystrophy, or sickle cell anemia provides prospective parents with information useful in making reproductive choices. However, an asymptomatic person with a family history of Huntington's disease faces a difficult choice. Finding out while young that you carry a gene that eventually will produce a lethal, neurological disease creates enormous emotional and psychological stress since there is no treatment or cure that can be offered. Faced with the choice of genetic testing for Huntington's disease, many possible carries of the gene opt not to be tested. Genetic tests also can determine if a person carries genes that increase the chances of developing breast cancer, colon cancer, and other polygenic-multifactorial diseases. Again, in most cases, only marginally helpful medical help can be offered to individuals who test positive for cancer-causing susceptibility genes. Some young women who are homozygous for the BRCA1 gene have elected to undergo prophylactic bilateral mastectomy to reduce the risk of breast cancer later in life. If one tests positive for colon cancer susceptibility genes, the only recourse is frequent examination of the colon and immediate removal of any polps that appear. Clearly, the consequences of many of the genetic tests currently available create serious problems for those who discover that they carry disease-causing genes of one kind or another. As hundreds of additional genetic tests become available, patients and physicians will have to make difficult choices regarding the risks and benefits of genetic testing. Genetic discrimination: Profound social, ethical, and financial questions are raised by wide spread use of genetic testing and identification of persons at risk for diseases. Employers and insurance companies would like to have access to such information since it would be in their financial interest to know who is likely to become sick or disabled.

540


F I G U R E 23-17 A sampling of single-gene defects (Mendelian disorders) that have been mapped to specific chromosomal loci. More than 8000 inherited disorders and defects have been identified as stemming from mutations in single genes, and many of the altered base changes have been mapped to specific chromosomal loci. As the analysis of the DNA sequence of the human genome proceeds, it is expected that eventually thousands more mutated alleles that cause inherited disorders and defects will be identified and mapped.

SECTION 23.9

The Human 6enome Project

FIGURE 23-17

(Continued)

5/11

542 On the other hand, such use of personal medical information would generate a society that accepts the concept of genetic discrimination. Since our society has made great efforts to eliminate sex, racial, and age-related discrimination, the overwhelming sentiment is to not allow genetic discrimination. The question for society is how to guarantee that genetic discrimination will not be practiced either legally or illegally as more and more DNA information becomes part of peoples' medical records. Laws against genetic discrimination have been introduced in many states but there is no federal law (as of 2000) that guarantees a person's right to genetic privacy. Drug design: One of the anticipated benefits of the identification of human genes involved in diseases is the development of highly specific drugs that can be targeted to correct the biochemical and physiological consequences of a defective gene and its abnormal protein. It is expected that drugs eventually can be developed that will be targeted to correct the effects of specific mutations that individuals carry. Drug design and drug therapy may evolve to the point where each person's genotype determines the drug that will be used. Pharmacogenomics is the study of the relation between human genetic variability in the activity, toxicity, and metabolism of drugs. Examples include deficiencies of catabolic enzymes dihydropyrimidine dehydrogenase and thiopurine S-methyl transferase which can cause profound toxicity to fluoropyrimidine and mercaptopurine, respectively. These are used in cancer chemotherapy (Chapter 27).

23.10 Genomics and Proteomics Enormous progress has been made in recent years in deciphering the genetic basis of different human phenotypes and clinical diseases that result from inherited mutations. Technological breakthrough have made the identification of genes and their sequencing a relatively straightforward procedure. Genomics is the study of DNA and the genes that it contains. While genomics has resulted in the characterization of many human genes that are responsible for abnormal phenotypes and genetic disorders, identification of the protein corresponding to the gene is often difficult and, in many cases, is still unknown long after the gene has been identified and sequenced. Often the activity of the gene does not correlate with the amount of the protein or its activity in the cell. Posttranslational modifications cannot be determined by genomics, which only allows determination of the primary amino acid sequence as deduced from the nucleotide sequence of the gene. Proteomics is the direct study of proteins produced by both healthy and diseased cells. For example, twodimensional gel electrophoresis can be used to display

CHAPTER 23


and quantify thousands of proteins within a cell. A change in the level of a particular protein, either higher or lower, can be detected by differential protein abundance analysis. When a cell receives a chemical signal, such as the binding of a growth factor or cytokine, the immediate response is at the level of proteins that are unrelated to the target gene. Cell surface receptor proteins are modified in response to the external signal. Also, transmission of information from the cell surface to the nucleus is mediated by the physical movement of proteins. Proteomic technologies have been developed that can detect and analyze such protein changes. In addition, the level of a particular protein in the cell may change dramatically in response to a change in transcriptional regulation of one or more genes, and this can be monitored (Chapter 26). Proteomics is a valuable new technology used in the development of novel and more effective drugs following the identification and characterization of disease-causing proteins. Proteins also provide useful molecular markers for the diagnosis of specific diseases. In essence, proteomics provides the link between genes, proteins, and diseases and complements the information obtained by genomics.

Supplemental Readings and References Structure of DNA B. Alberts, A. B. Bray, J. Lewis, et al.: Molecular Biology of the Cell. New York: Garland (1994). S. Henikoff, E. A. Greene, S. Pietrokovski, et al.: Gene families: The taxonomy of protein paralogs and chimeras, Science 278, 609 ( ! 997). D. W. Ross: The human genome: information content and structure. Hospital Practice, June 15, 49 (1999).

Sequencing DNA L. Alphey: DNA Sequencing. New York: Springer-Verlag (1997). F. R. Blattner, G. Plunkott, C. A. Bloah, et al.: The complete genome sequence of Escherichia coli K-12. Science 277, 1453 (1997). F. S. Collins: Sequencing the human genome. Hospital Practice, January 15, 35 (1997). E S. Collins: Medical and societal consequences of the human genome project, New England Journal of Medicine 341, 28 (1999). N. Rosenthal: Fine structure of a gene--DNA sequencing. New England Journal of Medicine 332, 589 (1995). L. Rowen, G. Mahairas, and L. Hood: Sequencing the human genome. Science 278, 669 (1997).

Properties of DNA B. H. Hahn: Antibodies to DNA. New England Journal of Medicine 338, 1359(1998). D. E. Housman: DNA on trial--The molecular basis of DNA fingerprinting. New England Journal of Medicine 332, 534 (1995).

543

Supplemental Readings and References S. E Naber: Molecular pathology--diagnosis of infectious DNA. New England Journal of Medicine 331, 1212 (1994). W. Reik and M. A. Surami, eds.: Frontiers in Molecular Biology. New York: Oxford University Press (1997). C. W. Schmid, N. Deka, and G. Matera: Repetitive human DNA: The shape of things to come. In Chromosomes: Eukaryotic, Prokaryotic, and Viral. Volume 1. K. W. Adolph, ed., Boca Raton, FL: CRC Press (1990). R. A. Seder and S. Gurunathan: DNA vaccines--Designer vaccines for the 21st century. New England Journal of Medicine 341, 277 (1999).

Gene Therapy H. M. Blau and M. I. Springer: Muscle-mediated gene therapy. New England Journal of Medicine 333, 1554 (1995). M. Cavazzana-Calvo, S. Hacein-Ray, G. de Saint Basile, et al.: Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease. Science 288, 669 (2000). S. J. Eck, ed.: Gene therapy. Hematology/Oncology Clinics of North America 12 (1998).

W. M. McDonnell and E K. Askari: DNA vaccines. New England Journal of Medicine 334, 42 (1996). L-C. Tsui and E Durie: Genotype and phenotype in cystic fibrosis. Hospital Practice, June 15, 15 (1997).

Human Genome Project G. J. Annas: Rules for research on human genetic variation--Lessons from Iceland. New England Journal of Medicine 342, 1830 (2000). E. Green: The Human Genome Project and its impact on the study of human disease. In Metabolic and Molecular Bases of Inherited Disease, C. R. Scriver, ed., 8th edition. New York, McGraw-Hill (2000). V. Hatzimanikatis, L. H. Choe, and K. H. Lee: Proteomics: Theoretical and experimental considerations. Biotech. Prod. 15, 312 (1999). N. A. Holtzman and T. M. Marteau: Will genetics revolutionize medicine? New England Journal of Medicine 343, 141 (2000). G. J. B. van Ommen, E. Bakker, and J. T. den Dunnen: The human genome project and the future of diagnostics, treatment, and prevention. The Lancet 354, si8, (Molecular Medicine) (1999). D. B. Searls: Using bioinformatics in gene and drug discovery. Drug Discovery Today 5, 135 (2000).


CHAPTER

24


DNA is the permanent repository for all genetic information in every cell and, as such, must maintain the fidelity of that information from one cell generation to the next as well as from one human generation to the next. This means, in the case of human beings, accurately copying the approximately 6 x 10 9 base pairs in each diploid cell prior to its undergoing mitosis (somatic cells) or meiosis (germ cells). Over billions of years of biological history, cells have developed intricate mechanisms to ensure faithful replication of DNA molecules and for the detection and repair of errors in the sequence of bases when they do occur. However, errors m u s t occur; the evolution of new organisms and new species depends on a supply of new genomes, the source of genetic diversity on which natural selection acts and which fuels biological evolution. This is the paradox of DNA replication. On the one hand, DNA must replicate itself with the utmost accuracy; on the other hand, it must allow a low frequency of genetic change to occur during each round of replication, particularly during meiosis, which produces the cells (sperm and ova) that pass DNA to succeeding generations. This apparent paradox is resolved by the mechanisms of mutation and recombination that provide for a small but essential amount of genetic change to occur in each

generation. Mutations occur spontaneously during DNA replication because not all of the replication errors or damage to templates are detected and repaired by the enzymes responsible for proofreading and repairing DNA. The rate of mutations can be markedly increased by ionizing radiation (x-rays), mutagenic chemicals (cigarette smoke), or retroviruses that integrate in DNA (HIV). Thus, mutations are essential to the species because they allow for adaptive changes but often are harmful to the individual by giving rise to cells that cause cancer, congenital defects, and other diseases. Recombination occurs primarily in germ cells of eukaryotes and during cell division in partially diploid prokaryotes. In general, recombination does not occur in mitosis of eukaryotic cells. However, recombination is an integral part of meiosis in which recombinant sperm and ova provide new genotypes and phenotypes in each generation which may be favored by natural selection. In this chapter, the basic mechanisms of DNA replication, recombination, repair, and mutation are discussed. Much of our understanding of these mechanisms derived from the study of bacteria. However, as further evidence for the unity of biology, the processes that replicate and modify DNA in bacteria also apply to human DNA with some modifications.

545

546

CHAPTER 24


24.1 DNA Replication

TABLE 24-1

P r o b l e m s of Replication

Comparison o f Parameters o f D N A Replication in E. coli and Human Cells

The replication process requires that each double-helical molecule of DNA produce two identical molecules of DNA. This means that wherever a G-C or A-T base pair occurs in the parental molecule, the identical base pair must occur in the progeny molecules. However, many factors interfere with accurate replication of DNA. If an A should pair with C or G with T as a result of tautomerization (Chapter 23), a point mutation (a change in one base pair) will result. Occasionally, a segment of DNA will be replicated more than once (duplication) or a segment may fail to be replicated (deletion). These and other aberrations in DNA replication do occur, but the mechanism of replication has evolved to minimize such mistakes.

DNA

E. coli

Amount of DNA -~3.9 x 106 bp Rate of replication at -850 bases/sec each replication fork Number of origins of one replication Time for cell division -30 min

Human -3 • 109 bp -6-90bases/sec 103 to 104 --24 hours

The information in each strand of the double helix serves as the template for the construction of a new double-helical DNA molecule; this is called semiconservative replication since one old strand of DNA is paired with one new strand to produce the daughter DNA molecule. All DNA molecules in all organisms replicate semiconservatively. This basic fact was first proved by Matthew Meselson and Frank Stahl who labeled DNA in E. coli by growing cells for several generations in medium containing a heavy isotope of nitrogen (15N). Cells were then transferred to medium containing 14N and continued to grow exponentially. After one generation of growth in 14N medium, the DNA was extracted from a sample of cells and analyzed by cesium chloride density centrifugation. All DNA was of intermediate (hybrid) density; one strand contained JSN and the newly synthesized strand contained 14N. After two generations of growth in 14N medium, 50% of the DNA contained only 14N (light DNA) and 50% is hybrid. This important experiment confirmed a prediction of the Watson-Crick model for DNA. Subsequent experiments also showed that human DNA undergoes semiconservative replication.

from one DNA molecule to another just as genes can be moved by genetic engineering techniques. In both bacterial and plasmid DNA, replication is initiated at a unique origin and proceeds in both directions along the DNA molecule to a termination site that is located approximately 180 degrees from the origin. Thus, replication of DNA is, in most cases, bidirectional replication. The ability of DNA to replicate from many origins and in both directions means that cells can replicate all of their DNA in a fairly short period of time. The shortest generation time for the bacterium E. coli is approximately 30 minutes in rich medium in the laboratory whereas the shortest division time for a human cell is approximately 24 hours (Table 24-1 ). Unwinding a double helix during replication presents a serious mechanical problem. Either the two daughter DNA molecules at the replication fork (the Y-shaped fork) must rotate around one another, or the unreplicated segment of DNA must rotate (Figure 24-1). This necessity for DNA rotation during replication creates topological problems for covalently closed circular DNA in bacteria and for the enormously long condensed and folded DNA in human chromosomes. The unwinding of strands of the DNA double helix is accomplished by a special group of enzymes called helicases; positive and negative coiling of DNA to maintain the topology necessary for replication is the function of another group of enzymes called topoisomerases (discussed in a later section).

Origin and Direction of DNA Replication

Replicons

Another unifying principle of DNA replication is that each molecule of DNA has one or more specific origins ofreplication where DNA synthesis begins. In bacteria that carry their genetic information in a circular DNA molecule, only one origin of replication exists. Plasmid DNA also contains a unique origin of replication. Origins of replication can be moved from one location in DNA to another, or

Much of the basic understanding of DNA replication was gleaned from studying the replication ofplasmids (Chapter 23): small, circular, extrachromosomal DNA elements in bacteria. Some plasmids, such as the F (fertility) factor in E. coli, have an origin of replication but replicate only unidirectionally from that single origin by a rolling-circle mechanism of replication (Figure 24-2). If an F factor

Semiconservative Replication

SECTION24.1 DNA Replication

FIGURE 24-1 Semiconservative replication of DNA. The two replicas consist of one parental strand plus one daughter strand. Each base in a daughter strand is selected by the requirement that it form a base pair with the parental base.

integrates into the bacterial chromosome, it can replicate its own D N A and bacterial D N A as well during conjugation, a process in which bacteria physically join and transfer D N A from the donor bacterium to the recipient bacterium. In this situation, bacteria replicate D N A simultaneously in a bidirectional manner from their own origin of replication (ori c) and unidirectionally from the origin of the integrated F factor (oriF). Such replication clearly raises topological problems during D N A replication (Figure 24-3). Studies of bacterial and plasmid D N A replication led to the concept of a replicon, which is: 1. A site on D N A consisting of a sequence of nucleotides that defines an origin of replication, and 2. Structural genes coding for proteins that recognize and bind to the origin to initiate D N A replication.

FIGURE 24-2 Rolling-circle or cr replication. Newly synthesized DNA is shown in color.

5/17

FIGURE 24-3 Replication of bacterial DNA with an integrated F factor. There are two origins of DNA replication, orir and oriF. Replication is bidirectional from orir and unidirectional from oriF. Somehow the cell solves this topological dilemma when cell division occurs.

These genes and the segment of D N A that is replicated from the origin are collectively defined as a replicon. Thus, a bacterial c h r o m o s o m e or a plasmid is a single replicon, whereas a single h u m a n c h r o m o s o m e may contain thousands of replicons.

Discontinuous D N A Replication In the diagram of replication shown in Figure 24-1, both daughter molecules are shown as having continuous strands. In truth, D N A cannot replicate by copying both strands continuously because D N A polymerases, the enzymes that synthesize new DNA, can only add nucleotides to a T - O H group. Because of the antiparallel nature of the

548

CHAPTER24 DNA Replication, Repair, and Mutagenesis 3'-, ~/5'

l llllllllllll

#

.

I'! l i, l llillll

Ligase seals nick

I i I I ! !11 l l I !I

Site of missing nucleotide

Ligase cannot seal a gap

NOactivity

(a)

(b)

F I G U R E 24-4 Action of DNA ligase. (a) A nick having a 3'-OH and a 5'-P terminus sealed (left panel). (b) If one or more nucleotides is absent, the gap cannot be sealed.

DNA strands, synthesis of one new strand of DNA can be continuous but synthesis along the other strand must occur by discontinuous replication. At each replication fork one DNA strand has a free 3'-OH group; the other has a free 5'-PO 2- group. Since DNA can only elongate in a 5' to 3' direction, synthesis of one strand (the leading strand) is continuous; synthesis of the opposite strand (the lagging strand) is discontinuous. Short fragments of DNA are synthesized in the replication fork on the lagging strand in a 5'to 3' direction; these are called Okazaki fragments after the scientist (Reiji Okazaki) who first demonstrated their existence during replication. These Okazaki fragments are joined in the replication fork by the enzyme DNA ligase that can form a phosphodiester bond at a single-strand break in DNA. DNA ligase joins a 3'-OH at the end of one DNA fragment to the 5'-monophosphate of the adjacent fragment (Figure 24-4). However, if even one nucleotide is missing in the DNA strand, ligase cannot seal the sugar-phosphate backbone.

24.2 Enzymology of DNA Replication The fundamental enzymology of DNA replication derives from both in vivo and in vitro studies with cells and extracts derived from E. coli. Many of the enzymes involved in DNA replication were identified by isolation of conditional lethal mutants of the bacterium, e.g., mutants that are unable to replicate DNA (and unable to grow) at high temperatures (42~ but that replicate and grow normally at low temperatures (30~ The synthesis of DNA is a complex process because of the need for faithful replication, enzyme specificity, and topological constraints. Approximately 20 different enzymes are utilized in bacteria to replicate DNA. In addition to polymerization reactions, DNA replication requires accurate initiation, termination, and proofreading to eliminate errors. DNA Polymerases Three DNA polymerases have been characterized in E. coli and are designated polymerase I, polymerase II, and polymerase III (Table 24-2). Although present in very low concentration in the cell, polymerase III, also called replicase, is the polymerase that elongates both strands of the bacterial DNA in the replication fork. Polymerase I is primarily a DNA repair enzyme and is responsible for excision of the short RNA primer that is required to initiate DNA synthesis on both the leading and lagging strands of DNA during replication. It also can remove mismatched base pairs during replication and fill in gaps in singlestranded DNA that is joined in a double helix. The function of polymerase II is not clear but it probably also has repair functions. All DNA polymerases select the nucleotide that is to be added to the 3'-OH end of the growing chain

T A B L E 24-2

Properties orE. coli DNA polymerases Property

Molecular weight Molecules/cell Nucleotides/sec 3' exonuclease activity 5' exonuclease activity Biological activity

Polymerase I

II

III

105,000

90,000

130,000

-400 -20 yes yes RNA primer excision, DNA repair

-100 --5 yes no SOS DNA repair?

-10 -1,000 no no Replicase

SECTION 24.2

Enzymologyof DNA Replication

549

accomplished by a potent pyrophosphatase, a widely distributed enzyme that breaks down pyrophosphate to inorganic phosphate. Hydrolysis of pyrophosphate has a large negative free energy, so essentially all of the pyrophosphate is rapidly removed. No DNA polymerase can catalyze the reaction between two free nucleotides, even if one has a 3'-OH group and the other a 5'-triphosphate. Polymerization can occur only if the nucleotide with the 3'-OH group is hydrogen-bonded to the template strand. Such a nucleotide is called aprimer (Figure 24-5). The primer can either be a single nucleotide or the terminus of a hydrogen-bonded oligonucleotide. When an incoming nucleotide is joined to a primer it supplies another free 3'-OH group, so that the growing strand itself is a primer. Since polymerization occurs only at the 3'-OH end, strand growth is said to proceed in the 5' --+ 3' direction. All known polymerases (both DNA and RNA) are capable of chain growth only in the 5' --+ 3' direction. This unidirectional feature ofpolymerases complicates the simultaneous replication of both strands of DNA. Polymerization is not confined to addition of a nucleotide to a growing strand in a replication fork. For example, pol I can also add nucleotides to the 3'-OH group at a single-strand (a nick) in a double helix. This activity results from the ability of pol I both to recognize a T-OH group anywhere in the helix and to displace the base-paired strand ahead of the available 3'-OH group.

and catalyze formation of the phosphodiester bond. The substrates for DNA polymerases are the four deoxynucleoside-5'-triphosphates (dATE dCTP, dGTP, and dTTP) and a single-stranded template DNA. The overall chemical reaction catalyzed by all DNA polymerases is Poly(nucleotiden)-3'-OH + dNTP Z poly(nucleotiden+ 1)-T-OH + PPi in which PPi represents pyrophosphate cleaved from the dNTE That is, a reaction occurs between a 3'-OH group at a terminus of a DNA strand and the phosphoryl group (the one linked to the sugar) of an incoming nucleoside triphosphate. Even though the hydrolysis of the nucleoside triphosphates has a large negative A G, the polymerization reaction as written still has a positive A G at concentrations present in a cell and in laboratory reactions (+0.5 kcal/mol = 2.1 kJ/mol). Thus, in the absence of any other reaction DNA polymerases would catalyze depolymerization rather than polymerization. Indeed, if excess pyrophosphate and a polymerase are added to a solution containing a partially replicating DNA molecule, the polymerase acts like a nuclease. In order to drive the reaction to the right, pyrophosphate must be removed, and this is

Product

Template

(a)

P-5'

3'-OH

HO-3'

(b)

[c)

,. . . . . . .

HO-3'

P-5'

3'-OH ,

(e)

NO-3'

3'-OH ...

.

.

P-5' .

.

P-5' HO-3', .

.

5'-P

HO-3'

3'-OH 5'-9

P-5'

(f)

P-5'-5'-P

P-5' HO-3',,L

No synthesis

5'-P

HO-3'w.

(d)

No synthesis

5'-P

.

3'-OH

NO s y n t h e s i s

Nick~

P-5'--

5'-9

NO-3'

5'-P

HO-3'

-

.

3'-OH 5'-P

3'-OH .

~

P-5' -

-

= 5'-P

FIGURE 24-5

Effect of various templates used in DNA polymerizationreactions. A free 3f-OHon a hydrogen-bondednucleotide at the strandterminus and a non-hydrogen-bondednucleotide at the adjacentposition on the template strand are needed for strand growth. NewlysynthesizedDNA is shownin color.

550

CHAPTER24 DNA Replication, Repair, and Mutagenesis

F I G U R E 24-6 Strand displacement and nick translation on linear and circular molecules. In nick translation, a nucleotide is removed by an exonuclease activity for each nucleotide added. The growing strand is shown in color.

This reaction is called strand displacement (Figure 24-6). Not all DNA polymerases can carry out strand displacement. Pol I has several enzymatic activities other than its ability to polymerize; one of these is important in maintaining continuity of the daughter strand, and the other improves the fidelity of replication. These two functions are a 5' --+ 3' exonuclease activity and a 3' --+ 5' exonuclease activity.

Exonuclease Activities of Polymerase I Occasionally DNA polymerase, in error, adds a nucleotide to the 3'-OH terminus that cannot hydrogen-bond to the corresponding base in the template strand. Such a nucleotide would clearly alter the information content of the daughter DNA molecule, and mechanisms exist for correcting such incorporation errors. Once having added an incorrect nucleotide and moved on to the next position, pol I cannot add another nucleotide because the enzyme requires a primer that is correctly hydrogen-bonded. Where such an impasse is encountered, a 3' --+ 5' exonuclease activity, which may be thought of simply as pol I running backward or in the 3' --+ 5' direction, is stimulated and the unpaired base is removed. After removal of this base, the exonuclease activity stops, polymerizing activity is restored, and chain growth resumes. This exonuclease activity is called the proofreading or editing function of polymerase I. Pol I also has a potent 5' ~ 3' exonucleolytic activity (Figure 24-7). This activity is directed against a basepaired strand and consists of stepwise removal of nucleotides one by one from the 5'-P terminus. Furthermore, the nucleotide removed can be either of the deoxyribo or the ribo type. The 5' ~ 3' exonucleolytic activity also can be coupled to the polymerizing activity. Recall that pol I can add nucleotides to a 3'-OH group at a nick and displace

the downstream strand. Under certain conditions, the displacement reaction does not occur and instead the 5' ---> 3' exonuclease acts on the strand that would otherwise be displaced, removing one downstream nucleotide for each nucleotide added to the 3' side of the nick. Thus, the position of the nick moves along the strand; this reaction is called nick translation. It is an important laboratory procedure for producing labeled DNA that can be used as probes; simply by carrying out the reaction in the presence of radioactive or chemically labeled nucleotides, an unlabeled DNA molecule with nicks in both strands can be converted to a labeled molecule. DNA probes are used for many diagnostic and forensic purposes (Chapter 23). The main function of the 5' --+ 3' exonuclease activity is to remove ribonucleotide primers that are used in DNA replication. Pol I also has a 5' --+ 3' endonuclease activity. An endonucleolytic cut is made between two base pairs that follow a 5'-P-terminated segment of unpaired bases, as shown below.

G

J Af l

G

o

"/

,5,~

G

lil

!

AI oI A!

T

T

I

C

I

T

i

~"~

Hydrolysis site

This type of endonucleolytic activity is unimportant in normal replication but is important in excision repair.

551

SECTION 24.2 Enzymology of DNA Replication

Product

Substrate

(a)

5'

3'

3'

5'

3'

3'

5', .

3'

5'

(b)

5' ""

3'

5'

5'

=

3' 5'

.

3' .

5'

.

3'

3'

5'

Nick

(c)

3 ' ~ 5'

5'

'

3'

3' 5'

~-

3'

5'

5'

'

3'

3' 5'

RNA 3'

5'

(d)

~

3'

5. (e)

5'

.

N i c k 3 " ~ , 5, # R N A ,-. .

3'

,....

3'

3'

5'

5'

3 ............

3'

5'

5'

3'

5'

3'

5' 3'

5'

'

3' 5'

3'il~5' 5'

(f)

3'

~

3'

5'

N o exonucleolytic activity

FIGURE 24-7

Several substrates and products alter activity of the polymerase 15' --~ 3' exonuclease. The 5' terminus attackedby the enzyme is shown by the small straight arrow. (a) 5'-Terminalnucleotides are removed from each end of the molecule. (b) A gap is enlarged by cleavage at a 5' terminus. (c) A gap is formed by removal of a 5'-terminal nucleotide at a nick. The gap is then enlarged as in (b). (d) RNA (colored line) at the boundary of a gap is removed by cleavage at the 5' terminus of the RNA. (e) A gap is formed from a nick at a DNA-RNAboundary by removal of 5'-P-terminal ribonucleotides. (f) A non-hydrogen-bonded 5' terminus is resistant to the exonuclease; there is an endonucleolytic activity.

Polymerase 111 Polymerase I plays an essential role in the replication process in E. coli, but it is not responsible for the overall polymerization of the replicating strands. The enzyme that accomplishes this is a less abundant enzyme, polymerase III (pol III). (A D N A polymerase II has also been isolated from E. coli, but it probably plays no role in DNA synthesis.) Pol III catalyzes the same polymerization reaction as pot I but has certain distinguishing features. It is a very complex enzyme and is associated with eight other proteins to form the pol III holoenzyme. (The term holoenzyme refers to an enzyme that contains several different subunits and retains some activity even when one or more subunits is missing.) Pol III is similar to pol I in that it has a requirement for a template and a primer but its substrate specificity is much more limited. For a template pol III cannot act at a nick nor can it unwind a helix and carry out strand displacement. The latter deficiency means that an auxiliary system is needed to unwind the helix ahead of a replication fork. Pol III, like pol I, possesses a 3' --+ 5' exonuclease activity, which performs the major editing function in DNA replication. Polymerase III also has a 3' exonuclease activity, but this activity does not seem to play a role in replication.

Pol I and pol III holoenzyme are both essential for

E. coli replication. The need for two polymerases seems to be characteristic of all cellular organisms but not all viruses, e.g., E. coli. Phage T4 synthesizes its own D N A polymerase, which is capable of carrying out all functions necessary for synthesizing phage DNA. In the usual polymerization reaction, the activation energy for phosphodiester bond formation comes from cleaving of the triphosphate. Since DNA ligase can use a monophosphate, another source of energy is needed. This energy is obtained by hydrolyzing either ATP or NAD; the energy source depends on the organism from which the DNA ligase is obtained. Ligases have two major functions: the sealing of singlestrand breaks produced randomly in DNA molecules by nucleases and the joining of fragments during a particular stage of replication. DNA ligases are enzymes that can form a phosphodiester bond at a single-strand break in DNA, a reaction between a 3'-OH group and a 5'-monophosphate. These groups must be termini of adjacent base-paired deoxynucleotides (Figure 24-4). Bacteria usually contain a single species of ligase. Mammalian cells possess two DNA ligases (I and II) present in very small amounts compared with bacteria. Both

552

CHAPTER 24


TABLE 24-3

Properties of Eukaryotic DNA Polymerases Property

Polymerase a

Location in cell

Associated primase 3' exonuclease Sensitivity to aphidicolin Biological activity

Nucleus

Nucleus

Mitochondria

Nucleus

Nucleus

Yes No High Replication (lagging strand)

No No Low DNA repair

No Yes Low Replication

No Yes High Replication (leading strand)

No Yes Low Replication

eukaryotic ligases are located in the nucleus. Ligase I is predominant in proliferating cells and presumably plays a role in DNA replication; ligase II predominates in resting cells.

Eukaryotic Polymerases Five polymerizing enzymes have been isolated from many mammalian cells (Table 24-3). Threempol or, pol/3, and pol ymfunction in replication. Pol ot is the major polymerase of mammalian cells; it is found in the nuclei and is analogous to E. coli pol III. It is a multisubunit enzyme with a core (4-5 subunits) responsible for polymerization and a holoenzyme form possessing additional subunits and activities. It lacks the 3' --+ 5' exonuclease editing function. An intriguing protein subunit in the holoenzyme enables it to bind AppppA (diadenosine tetraphosphate), a small molecule that stimulates replication of resting mammalian cells and is hypothesized to be a growth signal. The pol ot holoenzyme of rat liver also possesses a DNA primase activity (see below), a feature not found in prokaryotic enzymes. Pol/3 is a nuclear polymerase, probably analogous in function to E. coli pol I. Pol y is found in mitochondria and is responsible for replication of mitochondrial DNA. It functions in the same way as pol III but is a single polypeptide.

24.3 The Replication Fork DNA replication requires not only an enzymatic mechanism for adding nucleotides to the growing chains but also a means of unwinding the parental double helix. These are distinct processes, and the unwinding of the helix is closely related to the initiation of synthesis of precursor fragments. The pol III holoenzyme cannot unwind the helix. In order for unwinding to occur, hydrogen bonds and

hydrophobic interactions must be eliminated, which requires energy. Pol I utilizes the free energy of hydrolysis of the triphosphate for unwinding as it synthesizes a DNA strand in a way that other polymerases cannot; instead, helix unwinding is accomplished by enzymes called helieases. These enzymes hydrolyze ATP and utilize the free energy of hydrolysis for unwinding. Unwinding of the helix by a helicase is not sufficient in itself for advance of the replication fork. Accessory proteins called single-stranded DNA-binding proteins (SSB proteins) are usually needed. As a helicase advances, it leaves in its wake two single-stranded regions: a longer one that is copied discontinuously and a shorter one just ahead of the leading strand. In order to prevent the single-stranded regions from reannealing or from forming intrastrand hydrogen bonds, the single-stranded DNA is protected with SSB proteins (Figure 24-8). SSB proteins bind tightly to both single-stranded DNA and to one another and hence are able to cover extended regions. As the polymerase advances, it must displace the SSB proteins so that base pairing of the added nucleotide can occur. Some phage replication systems utilize a single protein that functions as both a helicase and an SSB protein; the gene-32 protein of phage T4 is the prototype. It binds very tightly to single-stranded DNA and exceedingly tightly to itself, and its binding energy is great enough to unwind the helix. As a replication fork moves along a circular helix, rotation of the daughter molecules around one another causes the individual polynucleotide strands of the unreplicated portion of the molecule to become wound more tightly, i.e., overwound. (This may be difficult to visualize but can be seen by taking two interwound circular strings and pulling them apart at any point.) Thus, advance of the replication forks causes positive supercoiling (Chapter 23) of the unreplicated portion. This supercoiling obviously cannot increase indefinitely because soon the unreplicated portion becomes coiled so tightly that further advance of the

SECTION24.3 The Replication Fork

553

F I G U R E 24-8 Simplified overview of a bacterial DNA replication fork. The leading strand is initiated with an RNA primer and then elongated by a DNA polymerase, pol III (replicase). The lagging strand is synthesized in a series of fragments, each initiated by primase which inserts short sequences of RNA primers that will be elongated by a replicase. The RNA fragments are removed by another DNA polymerase, pol I, and the breaks in the sugar-phosphate backbone are sealed by DNA ligase.

replication fork is impossible. This additional coiling can be removed by atopoisomerase that can introduce negative superhelicity. In E. coli, the enzyme DNA gyrase produces negative superhelicity in nonsupercoiled covalent circles and is also responsible for removing the positive superhelicity generated during replication. The evidence for this comes from in vivo experiments using drugs (discussed later) that inhibit DNA gyrase; addition of any of these drugs to a growing bacterial culture or raising the temperature of cells with a temperature-sensitive gyrase protein inhibits DNA synthesis. Furthermore, in vitro replication of circular DNA can proceed only if DNA gyrase or a similar topoisomerase is present in the reaction mixture.

Inhibitors of DNA Replication Inhibitors of DNA synthesis are used in the laboratory and in treatment of bacterial, viral, and neoplastic diseases. Successful treatment of these conditions requires careful attention to dosage and the fine difference between drug effectiveness and toxicity. Inhibitors of DNA synthesis can be divided into three main classes: 1. Those that prevent or reduce the synthesis of precursors (bases, nucleotides), 2. Those that affect either the template or the priming ability of the growing strand, and 3. Those that act directly on polymerases or other enzymes needed for replication. A variety of inhibitors bind to DNA and thereby eliminate its template activity. Notable among these are the intercalating agents, which slip in between base pairs (e.g., the acridines, phenanthridium derivatives), and the anthracyclines (daunorubicin, doxorubicin, and plicamycin).

Other agents bind to DNA covalently and cause chain breakage (bleomycin, zinostatin) and interstrand crosslinks (alkyl sulfonates, anthramycin, mitomycin, nitrogen mustards). Several of these compounds are also useful antitumor agents. A variety of platinum coordination compounds bind to DNA and inhibit its template activity, apparently by binding to guanine. Substances that prevent extension of the growing chain (2',3'-dideoxyribonucleosides, cordycepin) are incorporated into the growing chain, but since they lack a 3'-OH group, further extension is not possible. Only a few substances act directly on DNA polymerases; they often are effective only on one or a small number of polymerases. For example, acyclovir inhibits the DNA polymerase of herpes simplex. Aphidicolin inhibits pol o~, pol 3 (but not pol/~ or pol V), many viral polymerases, and pol I and pol II of yeast; this compound is extremely valuable in laboratory research on DNA replication. Other components of the replication complex can also be inhibited; for example, 2'-dideoxyazidocytidine is an inhibitor of bacterial primase, and cournermycin, novobiocin, oxolinic acid, and nalidixic acid are effective inhibitors of DNA gyrase in bacteria.

Topoisomerase I lnhibitors A variety of antibiotics and antineoplastic drugs exert their therapeutic effects by interaction with topoisomerase I and disruption of DNA synthesis during phase of dividing cells. Topoisomerase I is essential for DNA replication and cell growth. The enzyme relieves torsional stress in DNA by inducing reversible single-strand breaks. The interaction of topoisomerase I and certain drugs produces double-strand breaks in DNA that are irreversible and can lead to cell death.

55~


F I G U R E 24-9 The basic structure of camptothecin. Analogues of camptothecin that are used to treat various neoplastic diseases involve substitutions at positions C-7, C-9, and C-11 of the basic molecule. All analogues as well as the parent molecule bind to topoisomerase I and interfere with its functions during DNA replication.

A variety of antibiotics and antineoplastic drugs function as topoisomerase I inhibitors; these include the quinolone antibiotics, anthracyclines (doxorubicin), epipodophylotoxins (etoposide), and the camptothecins, which are active in treating lung, ovarian, and colorectal cancers. The camptothecins also are used in the treatment of myelomonocytic syndromes, chronic myelomonocytic leukemia (CMML), acute leukemia, and multiple myeloma. A variety of synthetic analogues of natural camptothecins are being tested clinically for efficacy and safety in the treatment of these aggressive cancers (Figure 24-9). The camptothecins were discovered in extracts from the Chinese tree Camptotheca acuminata. Initial studies showed that camptothecins had antitumor activity, but clinical trials demonstrated severe side effects and toxicity. Numerous camptothecin analogues have been synthesized; several have received FDA approval (irinotecan and topotecan), whereas others are still in clinical trials.

24.4 Chromosome Replication DNA in mammalian cells is organized in complex structures called chromosomes (prokaryotes do not have a nucleus, do not divide by mitosis, and do not, strictly speaking, have chromosomes). The DNA in the chromosomes of human and other eukaryotic cells is intimately associated with two classes of proteins called histones and nonhistones. Collectively, DNA, histones, and nonhistones constitute chromatin, from which the name chromosome is derived. The DNA in a chromosome is an extremely long, linear molecule that must be condensed and organized to fit into the chromosomes in the nucleus. (The DNA in the 46 human chromosomes would be about 1 m long if fully extended.) Histones are responsible for the structural organization of DNA in chromosomes; the nonhistone proteins

F I G U R E 24-10 Structure of a nucleosome. DNA is looped around a core of eight histone proteins (pairs of four different histone proteins) and connected to adjacent nucleosomes by linker DNA and another histone (H 1).

regulate the functions of DNA including replication and gene expression. The positive charge of histones, due to the presence of numerous lysine and arginine residues, is a major feature of the molecules, enabling them to bind to the negatively charged phosphate groups in DNA. The electrostatic attraction is an important stabilizing force in chromatin. If chromosomes are placed in solutions of high salt that break down electrostatic interactions, chromatin dissociates into free histones and DNA. Chromatin also can be reconstituted by mixing purified histones and DNA in concentrated salt solutions and gradually removing the salt by dialysis. Histones share a similar primary structure among eukaryotic species. However, they undergo various posttranslational modifications such as phosphorylation, acetylation, methylation, and ADP ribosylation.The chemical modifications of histones can alter their net charge, shape, and other properties affecting DNA binding. Pairs of four different histones (H2A, H2B, H3, and H4) combine to form an eight-protein bead around which DNA is wound; this bead-like structure is called a nucleosome (Figure 24-10). A nucleosome has a diameter of 10 nm and contains approximately 200 base pairs. Each nucleosome is linked to an adjacent one by a short segment of DNA (linker) and another histone (H1). The DNA in nucleosomes is further condensed by the formation of thicker structures called chromatin fibers, and ultimately DNA must be condensed to fit into the metaphase chromosome that is observed at mitosis (Figure 24-11).

SECTION24.4 Chromosome Replication

555

Replication of DNA in Chromosomes The rate of movement of a replication fork in E. coli is about 105 nucleotides per minute; in eukaryotes the DNA polymerases move only about one-tenth as fast. To replicate all of the DNA in a human cell in a few hours means that replication must be initiated at thousands of origins of replication in each chromosome and move bidirectionally. To accomplish this, mammalian cells have thousands of times more DNA polymerase available than is found in bacteria. As replication proceeds through each nucleosome, the histones must dissociate to allow the replication fork to proceed; after replication, the nucleosome re-forms. The separation of parental and daughter DNA also requires the synthesis of new nucleosomes. Recent studies suggest that DNA is replicated in "replication factories," i.e., fixed sites within the nucleus consisting of the numerous proteins needed for replication. The DNA is replicated by being drawn through the replication factories rather than having the replication proteins move along the DNA.

Replication at the Ends of Chromosomes

F I G U R E 24-11 Hypothetical stages in the condensation of DNA with chromatin to form a chromosome. (a) Double-stranded DNA. (b)-(e) Formation of nucleosome beads and fibers consisting of histones and condensed DNA to form (f) a metaphase chromosome. These proposed intermediates are derived from dissociation reactions with intact chromosomes.

Despite the dense packing of DNA in chromosomes, it must be accessible to regulatory proteins during replication and gene expression. At a higher level of organization, chromosomes are divided into regions called euchromatin and heterochromatin. Transcription of genes seems to be confined mainly to euchromatic regions while DNA in heterochromatic regions is genetically inactive. When DNA is replicated during the S phase of the cell cycle, the histone and nonhistone proteins also are duplicated and combine with the daughter DNA molecules.

Since DNA in chromosomes is a linear molecule, problems arise when replication comes to the ends of the DNA. Synthesis of the lagging strand at each end of the DNA requires a primer so that replication can proceed in a 5' to 3' direction. This becomes impossible at the ends of the DNA and 50-100 bp is lost each time a chromosome replicates. Thus, at each mitosis of a somatic cell, the DNA in chromosomes becomes shorter and shorter. Ultimately, after a limited number of divisions, a cell enters a nondividing state, called replicative senescence, which may play an important role in biological aging. To prevent the loss of essential genetic information during replication, the ends of DNA in chromosomes contain special structures called telomeres that are synthesized by a specific enzyme called telomerase. Intact telomerase consists of an RNA primer and associated proteins so telomerase is actually a reverse transcriptase. The activity of telomerase in replenishing telomeres is regulated by a number of telomere-specific DNA-binding proteins, TRF1 and TRF2. TRF1 regulates the length of telomeres and TRF2 protects the ends. Overexpression of TRFI results in progressive shortening of telomeres; underexpression results in lengthening. Another telomere regulatory protein is tankrase (TRFl-interacting ankyrin-related ADPribose polymerase), which alters the activity of TRF1. Telomeres in human chromosomes consist of tandem repeats of the sequence TTAGGG. In most adult somatic cells, telomerase activity is very low or absent. Even in


556

hematopoietic stem cells that do have residual telomerase activity, telomere shortening is observed at the level of granulocyte and mononuclear cell fractions. Accelerated telomere shortening has been observed in cells from patients with aplastic anemia, suggesting that abnormal telomere shortening is associated with disease and aging. A characteristic of malignant tumor cells is that they can replicate indefinitely. The immortality of tumor cells appears to result, at least in part, from enhanced levels of telomerase that allow them to repair and elongate telomeres at the ends of DNA. This hypothesis is supported by the observation that ectopic expression of the catalytic subunit of telomerase (a product of the hTERT gene) enabled human retinal pigmented epithelial cells and fibroblasts to avoid senescence and to maintain their differentiated state when grown in vitro. Current research is focused on drugs that can promote or inhibit the action of telomerase or telomere-associated regulatory proteins. It is hoped that increasing telomerase activity in cells approaching senescence will retard aging or that decreasing telomerase activity in tumor cells will result in arrested tumor growth.

24.5 DNA Repair DNA can be damaged by external agents and by replication errors. Since maintenance of the correct base sequence of DNA and of daughter DNA molecules is essential for hereditary fidelity, repair systems have evolved that restore the correct base sequence.

5'

3' Template strand

3'4

GAGTCGAATC CTCAGTTTAG

--

5' Newly synthesized strand

Mismatch

excision

GAGTCGAATC CTC AG

Replacement correct base

9

ofI ~ ~

GAGTCGAATC CTCAG CTTAG

(a)

(b)

F I G U R E 24-12 Mismatch repair. (a) Excision of a short segment of a newly synthesized strand and repair synthesis. (b) Methylated bases in the template strand direct the excision mechanism to the newly synthesized strand containing the incorrect nucleotide. The regions in which methylation is complete are black lines; the regions in which methylation may not be complete are shown in color.

an enzyme, DNA methylase, methylates adenine in the sequence GATC. Methylation occurs soon (but not immediately) after the replication fork has synthesized such a sequence in the daughter strand. Thus, the nucleotides in the daughter strand are usually not methylated near the fork, whereas those in the parental strand are always completely methylated (Figure 24-12b). The mismatch repair system recognizes the degree of methylation of a strand and preferentially excises nucleotides from the undermethylated strand. The daughter strand is always the undermethylated strand, so that parental information is retained.

Mismatch Repair and Methylation of DNA DNA polymerases occasionally catalyze incorporation of an "incorrect" base that cannot form a hydrogen bond with the template base in the parental strand; such errors usually are corrected by the editing function of these enzymes. The editing process occasionally fails, so a second system, called mismatch repair, exists for correcting the errors that are not edited out. In mismatch repair, a pair of non-hydrogen-bonded bases (e.g., G . . . T; Figure 24-12a) within a helix are recognized as aberrant and a polynucleotide segment of the daughter strand is excised, thereby removing one member of the unmatched pair. The resulting gap is filled in by pol I, which presumably uses this "second chance" to form correct base pairs; then the final seal is made by DNA ligase. If it is only to correct and not create errors, the mismatch repair system must be capable of distinguishing the correct base in the parental strand from the incorrect base in the daughter strand. Rare methylated bases (methyl-A and methyl-C) provide the basis for this distinction. In E. coli,

Glycosylases Occasionally, uracil or other incorrect bases may become incorporated into new DNA strands. These bases are usually removed by a pathway that starts with the cleavage of the N-glycosidic bond by an enzyme called a glycosylase (Figure 24-13). Many glycosylases are known, and each is base-specific (e.g., uracil N-glycosylase). This enzyme cleaves the N-glycosidic bond and leaves the deoxyribose in the backbone. A second enzyme (AP endonuclease) makes a single cut, freeing one end of the deoxyribose. (AP stands for apurinic acid, a polynucleotide from which purines have been removed by hydrolysis of the N-glycosidic bonds.) This step is followed by removal of the deoxyribose and several adjacent nucleotides (probably by a second enzyme that acts at the other side of the apurinic site), after which pol I fills the gap with correct nucleotides. This sequence, endonuclease-enlargementpolymerase, is an example of a general repair mechanism called excision repair (see below).

557

SECTION24.5 DNA Repair

AGTGACTTAG TCACTGAATC

Ligase

weakened, causing inhibition of advance of the replication fork.

AGTGACTTAG TCACTGAATC I

I Deamination

AGTGACTTAG TCAUTGAATC i

Uracil

j l n

I~,-~

Polymerase

TGA

General Mechanisms for Repair of DNA

AGTGACTTAG TCA TGAATC

tl

N-glycosylase~ AGTGACTTAG TCA TGAATC

_~AP

endonuclease

i

U

FIGURE 24-13 Scheme for repair of cytosine deamination. The same mechanism could remove a uracil that is accidentally incorporated.

Alterations of DNA Molecules Several agents can break phosphodiester bonds. Among the more common are peroxides and various metal ions (e.g., Fe z+, Cu2+). Ionizing radiation also efficiently produces strand breaks. DNases present in cells probably also occasionally break phosphodiester bonds. Double-strand breakage, i.e., two single-strand breaks opposite one another, results from exposure to all forms of ionizing radiation. Single-strand breaks can be repaired by DNA ligases, although sometimes additional enzymes are required. Double-strand breaks rarely are repaired. Bases can be changed into different compounds by a variety of chemical and physical agents. For instance, ionizing radiation can break purine and pyrimidine rings and can cause several types of chemical substitutions, the most common being in guanine and thymine. The best studied altered base is the dimer formed by covalent linkage of two adjacent pyrimidine rings; these are produced by ultraviolet (UV) irradiation. The most prevalent of these dimers is the thymine dimer. The significant effects of the presence of thymine dimers are the following: 1. The DNA helix becomes distorted as the thymines, which are in the same strand, are pulled toward one another (Figure 24-14). 2. As a result of the distortion, hydrogen bonding to adenines in the opposite strand is significantly

CGAT

I I I

GC

I

AAC

I

I

T AG

I I

I

I

T A T T G A T C

UV

'C G A

~

G C T

/ rm \

I

I

I

T

A G

I I

I

A T C

FIGURE 24-14

Ultraviolet radiation damage of two adjacent thymines of DNA. Distortion of the DNA helix caused by two thymines moving closer together when joined in a dimer. The dimer is shown as two joined lines.

Repair of damaged bases was first observed and is best understood in bacteria. It is a widespread and probably universal phenomenon in both prokaryotes and eukaryotes. Some systems that repair dimers repair other types of DNA damage also. Four major pathways for DNA repair exist that can be subdivided into two classes: light-induced repair (photoreactivation) and light-independent repair (dark repair). The latter can be accomplished by three distinct mechanisms: 1. Excision of the damaged nucleotides (excision repair), 2. Reconstruction of a functional DNA molecule from undamaged fragments (recombinational repair), and 3. Disregard of the damage (SOS repair). Pho to reac tivatio n

Photoreactivation is a light-induced (300-600 nm) enzymatic cleavage of a thymine dimer to yield two thymine monomers. It is accomplished by photolyase, an enzyme that acts on dimers contained in single- and doublestranded DNA. The enzyme-DNA complex absorbs light and uses the photon energy to cleave specific C-C bonds of the cyclobutylthymidine dimer. Photolyase is also active against cytosine dimers and cytosinethymine dimers, which are also formed by UV irradiation but much less frequently. Excision Repair

Excision repair is a multistep enzymatic process. Several mechanisms are known, but only two will be described (Figure 24-15). All require an early incision step, in which a nuclease recognizes the distortion produced by a thymine dimer and makes a cut in the sugar-phosphate backbone. Following this, a DNA polymerase mediates a strand displacement step. In E. coli, two cleavages occur; the first is 12 nucleotides from the 5' side of the dimer and the second is 4-5 nucleotides from the 3' side. Each cut produces a 3'-OH and a 5'-P group. The T-OH group of the first cut is recognized by pol I, which then synthesizes a new strand, displacing the dimer-containing DNA strand. When the second cut is reached, the displaced fragment falls away and DNA ligasejoins the 3'-OH and 5'-P groups. In Micrococcus luteus, the first step is breakage of the N-glycosidic bond of the thymine at the 5' end of the dimer (by a dimer-specific glycosylase), leaving a free deoxyribose which is removed, leaving a free 3'-OH group. As

5~8


F I G U R E 24-15 Two modes of excision repair. (a) Escherichia coli mechanism. Two incision steps are followed by gap-filling and displacement by polymerase I. (b) Micrococcus luteus mechanism. A pyrimidine dimer glycosylase breaks an N-glycosidic bond and makes a single incision. Poi I displaces the strand, which is removed by an exonucleolytic event. In both mechanisms, the final step is ligation.

in E. coli, pol I polymerizes from this end and displaces the dimer-containing strand. After the dimer has been displaced, a second cut is made, the displaced strand falls away, and ligase forms the final phosphodiester bond. Repair of dimers in mammalian cells is more complicated and poorly understood, and requires a larger collection of enzymes.

Recombination Repair Recombination repair is a mechanism for generating a functional DNA molecule from two damaged molecules. It is an essential repair process for dividing cells because a replication fork may arrive at a damaged site, such as a thymine dimer, before the excision repair system has eliminated damage. When pol III reaches a thymine dimer, an adenine is added to the growing strand. However, the distortion of the helix caused by the dimer weakens the hydrogen bond and activates the polymerase editing function, and the adenine

is removed. The cycle begins againman adenine is added and then removed--the net result of which is that the replication fork fails to advance. A cell in which DNA synthesis is permanently stalled cannot complete a round of replication and does not divide. However, in a way that is not understood, after a pause of --~5 seconds per dimer, chain growth begins again beyond the thymine dimer block. The result of this process is that the daughter strands have large gaps, one for each unexcised thymine dimer. Viable daughter cells cannot be produced by continued replication alone because the strands having the thymine dimer will continue to turn out defective daughter strands and the first set of daughter strands would be fragmented when the growing fork enters a gap. However, by a recombination mechanism called sister-strand exchange proper doublestranded molecules can be made. The essence of sister-strand exchange is that a singlestranded segment free of any defects is excised from an undamaged strand on the homologous DNA segment at the replication fork and somehow inserted into the gap

559

SECTION24.6 DNA Mutation

created by the thymine dimer. The combined action of polymerase I and DNA ligase joins this inserted piece to adjacent regions, thus filling in the gap. The gap formed in the donor molecule is also repaired. If this exchange and gap filling are done for each thymine dimer, two complete single daughter strands can be formed, and each can serve in the next round of replication as a template for synthesis of normal DNA molecules. The system fails if two dimers in opposite strands are very near one another because no undamaged segments are available. Since recombinational repair occurs after DNA replication, in contrast with excision repair, it is often called postreplicational repair.

SOS Repair SOS repair includes a bypass system that allows DNA chain growth across damaged segments at the cost of fidelity of replication. It is an error-prone process; even though intact DNA strands are formed, the strands are often altered. As described above, activation of the editing system stalls replication at a thymine dimer. In SOS repair, the editing system is relaxed to allow polymerization to proceed across a dimer. Relaxation of the editing system means a loss of the ability to remove "incorrect" bases added to the growing strand. Most of the time, pol III inserts two adenines at a dimer site. However, the distortion increases the error frequency, allowing other nucleotides to be added to the chain. This error-prone repair is the major cause of UV-induced mutagenesis. An important nuclear protein, conserved from yeast to mammals, is the Ku heterodimer. This protein binds to DNA and repairs double strand breaks caused by x-rays and other agents. The Ku heterodimer is essential in maintaining chromosome integrity.

As predicted, nontumor cells cultured from these patients are hypersensitive to x-rays. Fanconi's syndrome, a lethal aplastic anemia, is also due to defective DNA repair. Cells from affected persons cannot repair interstrand cross-links or damage induced by x-rays. Two premature aging disorders (HutchinsonGilford syndrome and Bloom's syndrome) and several other disorders (Cockayne's syndrome and retinoblastoma) are also associated with defects in DNA repair. Cells from patients with some chromosome abnormalities (e.g., Down syndrome) may also show aberrant DNA repair. Several human DNA mismatch repair genes are associated with hereditary nonpolyposis colon cancer (HNPCC). One of these mismatch repair genes (hMSH2) is located on the short arm of chromosome 2; others are located on chromosome 3. Defects in any of these DNA repair genes predispose individuals to colon cancer as well as to other cancers.

24.6 DNA Mutation Mutation refers to any change in the base sequence of DNA. The most common change is a substitution, addition, rearrangement, or deletion of one or more bases. A mutation need not give rise to a mutant phenotype. A mutagen is a physical agent or chemical reagent that causes mutations. For example, nitrous acid reacts with some DNA bases, changing their chemistry and hydrogen bonding properties, and is a mutagen. Mutagenesis is the process of producing a mutation. If it occurs in nature without the action of a known mutagen, it is called spontaneous mutagenesis and the resulting mutations are spontaneous mutations. If a mutagen is used, the process is called induced mutagenesis.

Human Diseases and DNA Repair Deficiency Human disease may result from inability to carry out certain stages of DNA repair. The best studied disease, xeroderma pigmentosum, is a result of mutations in genes that encode the UV excision system. Cells cultured from tissue obtained from affected individuals are killed by much smaller doses of UV light than are normal cells. Furthermore, the removal of thymine dimers in DNA from these cells is very inefficient. People with this disease develop skin lesions when exposed to sunlight and commonly develop one of several kinds of skin cancer. Ataxia telangiectasia is characterized by severe abnormalities in various organ systems and a high incidence of lymphoreticular cancer. Defective DNA repair was suspected when patients developed an unexpected severe or fatal reaction while undergoing radiotherapy for cancer.

Types of Mutations Mutations can be categorized in several ways. One system is based on the nature of the change, specifically on the number of bases changed. Thus, we distinguish a point mutation, in which a single base pair is changed from a multiple mutation, in which two or more base pairs differ from the wild-type sequence. A point mutation may be a base substitution, a base insertion, or a base deletion, but the term most frequently refers to a base substitution. A second system is based on the consequence of the change in terms of the amino acid sequence that is affected. For example, if there is an amino acid substitution, the mutation is a missense mutation. If the substitution produces a protein that is active at one temperature (typically 30~ and inactive at a higher temperature (usually

560


~~:~.~i . !:.-~i~ !i Acridine

i

....y...~ ...

(a)

Tyr

i~;~i~i~=:~:,~~,:~ '1=":"~" C.G

Glu

Thr

Gly

lie

TACGAATCGGGTATT ATGCTTAGCCCATAA

rq+ /

~

c. G

/

\

r Replication in the

\

presence of an acridine iii

TACGAGATCGGGTATT ATGCT CTAGCCCATAA (a)

E r r o r in r e p l i c a t i o n : A . T --~ G

9 C

(b)

Error in incorporation:

G

9C --, A

9T

F I G U R E 24-16 Two mechanisms of 5-bromouracil (BU)-induced mutagenesis. (a) During replication, BU in its usual keto form substitutes for T, and the replica of an initial A-T pair becomes an A-BU pair. In the first mutagenic round of replication, BU in its rare enol form pairs with G. In the next round of replication, the G pairs with a C, completing the transition from an A-T pair to a G-C pair. (b) During replication of a G-C pair, a BU in its rare enol form pairs with a G. In the next round of replication, the BU is again in the common keto form and it pairs with A, so the initial G-C pair becomes all A-T pair. The replica of the A-BU pair produced in the next round of replication is another A-T pair.

the mutation is called a temperature-sensitive (Ts) mutation. Sometimes no amino acid corresponds to a new base sequence (Chapter 25); in that case, termination of synthesis of the protein occurs at that point, and the mutation is called a chain termination mutation or nonsense mutation. These mutations generate one of the three nonsense codons: UAA, UAG, or UGA (Chapter 25). Replication errors can be introduced by base analogues that can be added to a replicating DNA molecule. Such a base analogue must be able to pair with the base on the complementary strand being copied, or the 3' --+ 5' editing function would remove it. However, if it can tautomerize (Chapter 23), or if it has two modes of hydrogen bonding, it will be mutagenic. The substituted base 5-bromouracil (BU) is an analogue of thymine inasmuch as the bromine atom has about the same van der Waals radius as the methyl group of thymine. BU forms a nucleoside triphosphate, and DNA polymerases will add BU to a DNA strand opposite an adenine. However, BU tautomerizes at high frequency to a form that can pair with guanine. Thus, if B U replaces a thymine, in subsequent rounds of replication it occasionally generates a guanine in the complementary strand, which in turn specifies cytosine, resulting in formation of a GC pair (Figure 24-16). A chemical mutagen is a substance that can alter a base that has already been incorporated into DNA and thereby change its hydrogen bonding specificity. Nitrous acid, a 40-42~

(b)

Tyr

Glu

lie

Gly

Tyr

FIGURE 24-17 Mutagenesis by intercalating substances (e.g., acridine). (a) Separation of two base pairs (shown as a box) by an intercalating agent. (b) A base addition resulting from replication in the presence of an acridine. The change in amino acid sequence read from the upper strand in groups of three bases is also shown.

powerful mutagen, converts amino groups to keto groups by oxidative deamination. Thus, cytosine, adenine, and guanine are converted to uracil (U), hypoxanthine (H), and xanthine (X), respectively, which can form the base pairs UA, HC, and XC, respectively. As a consequence, in a later round of replication, GC-to-AT and AT-to-GC mutations can occur. The chemical mutagen hydroxylamine reacts specifically with cytosine and converts it to a modified base that pairs only with adenine; thus, a GC pair ultimately becomes an AT pair. An interesting class of mutagen are intercalating agents, e.g., the acridines. These planar molecules insert between base pairs. When replication occurs in the region of an intercalated molecule, one or both daughter strands are synthesized that either lack one or more nucleotides or have additional ones. These changes alter the reading frame of the base sequence of a gone and hence are called frameshift mutations (Figure 24-17).

Triplet Repeats and Fragile Sites Inherited and acquired (sporadic) mutations account for an enormous diversity of human diseases and disorders. At least 300 genetic disorders include mental retardation as one of the symptoms, implying that many genes can affect brain development and mental functions. In the 1970s, a defect at the tip of the long arm of X chromosomes, called a fragile site, was discovered in a large number of patients with mental retardation. It is now known that a specific mutation is responsible for the fragile site on

SECTION24.6 DNA Mutation

561

TABLE 24-4

Inherited Diseases Involving Repeated Trinucleotides That Are Present in an Excessive Number of Copies* Disease Fragile X syndrome

Huntington's disease

Myotonic dystrophy Spinocerebellar ataxia Type 1

Fredreich's ataxia

Machado-Joseph disease

Trinucleotide repeat CGG 5-54 unaffected 60-230 carriers 230- 4000 affected CAG 11-30 unaffected 36-121 affected CTG CAG 19-36 unaffected 40-81 affected GAA 7-22 unaffected 200-900 affected CAG 12-37 unaffected 61-84 affected

Symptoms Mental retardation

Chorea, progressive dementia, death

Muscle weakness, myotonia Ataxia, dysarthria, rigidity, abnormal eye movements

Ataxia, dysarthria, hypertropic cardiomyopathy, diabetes

Spasticity, dystonia

*The extent of the repeated triplet nucleotide usually determines the severity of the disease and the age of onset. These diseases may be caused by slippage of the DNA polymerase during replication that increases the length of the repeated triplet from generation to generation. The patterns of inheritance are: Fragile X syndrome (X-linked); Huntington's disease, myotonic dystrophy, Spinocerebellar ataxia Type I, MachadoJoseph disease (autosomal dominant); Fredreich's ataxia (autosomal recessive).

the X chromosome and the associated inherited disorder

fragile X syndrome. This particular mutation accounts for about 20% of all mental retardation; about 1 in 1500 boys and 1 in 2500 girls are born with fragile X syndrome. Although fragile X syndrome is classified a Mendelian dominant sex-linked disorder, it does not behave as a classically dominant gene inherited disease. Only about 80% of males and 35% of females who carry the mutation and a fragile site suffer from mental retardation. For many years it remained a mystery how both men and women could carry a fragile X chromosome and be unaffected. In the 1990s, the molecular explanation for fragile X syndrome was revealed. The gene responsible for fragile X syndrome (FMR1) was found to contain a repeat of the trinucleotide CGG. In unaffected individuals, the number of CGG repeats ranges from about 5 to 54 copies. In mentally retarded individuals, the number of CGG repeats is dramatically increased to hundreds or thousands of copies. When the CGG repeat exceeds 230, the DNA becomes abnormally methylated in that region and the gene becomes nonfunctional. Presumably the amplification of the CGG repeat occurs during DNA replication in germ cells prior to meiosis perhaps due to "stuttering" of the

DNA polymerase as it reads the repeating trinucleotides. Why amplification of the CGG repeats occurs in some X chromosomes but not others and in some individuals and not others is unresolved. Triplet repeats account for a growing list of muscularneurologic diseases including Huntington's disease, myotonic dystrophy, Friedreich's ataxia, and others (Table 24-4). Triplet repeats represent an entirely new class of mutations. They are not caused by any external mutagenic agent but are generated during normal DNA replication. Fragile sites are chromosomal regions that are prone to breakage during chromosome movement or recombination; fragile sites can be visualized by a variety of cytological techniques and dyes that bind to metaphase chromosomes. Fragile sites are relatively common in chromosomes and there is a growing suspicion that they may play a greater role in disease, especially cancer, than previously thought. The chromosomes in many tumor cells exhibit fragile sites that are not present in normal cells. What is not yet clear is whether a fragile site initiates the conversion of a normal cell to a cancer cell or is merely the consequence of the altered growth properties of the

562 tumor cell. One possibility is that fragile sites are more susceptible to carcinogenic chemicals and that mutations in fragile sites contribute to the transformation of a normal cell to a tumor cell.

Supplemental Readings and References S. G. Arbuck and C. H. Takimoto: An overview of topoisomerase I-targeting agents. Seminar in Hematogy 3g(Suppl. 4), 3 (1998). S. E. Ball, E M. Gibson, S. Rizzo, et al.: Progressive telomere shortening in aplastic anemia. Blood 91, 3582 (1998). V. A. Bohr: Gene specific DNA repair. Carcinogenesis 12, 1983 (1991). D. Bootsma: The genetic defect in DNA-repair deficiency syndromes. European Journal of Cancer 29, 1482 (1993). C. E. Bronner, S. M. Baker, P. T. Morrison, et al.: Mutation in the DNA mismatch repair gene homologue hMLH1 is associated with hereditary non-polyposis colon cancer. Nature 368, 258 (1994). C. H. C. M. Buys: Telomeres, telomerase, and cancer. New England Journal of Medicine 342 (2000). J. E. Cleaver: Stopping DNA replication in its tracks. Science 285, 212 (1999). A. Durr, M. Cossee, Y. Agid, et al.: Clinical and genetic abnormalities in patients with Friedreich's ataxia. New England Journal of Medicine 335, 1169 (1996).

CHAPTER24 DNA Replication, Repair, and Mutagenesis M. Engelhardt, R. Kumar, J. Albanell, et al.: Telomerase regulation, cell cycle, and telomere stability in primitive hematopoietic cells. Blood 90, 183 (1997). M. Fessel: Telomerase and the aging cell. Journal of the American Medical Association 279, 1732 (1998). E. C. Friedberg, G. C. Walker, and W. Siede: DNA Repair and Mutagenesis. Washington, DC: ASM Press (1995). M. Grunstein: Histone acetylation in chromatin structure and transcription. Nature 389, 349 (1007). J. H. J. Hoeijmakers: Nucleotide excision repair II: from yeast to mammals. Trends in Genetics 9, 211 (1993). B. Kremer, E Goldberg, S. E. Andrew, et al.: A worldwide study of the Huntington disease mutation. New England Journal of Medicine 330, 1401 (1994). C. Lengauer, K. W. Kinzler, and B. Vogelstein: Genetic instabilities in human cancers. Nature 396, 543 (1998). T. Lindahl and R. D. Wood: Quuality conntrol by DNA rpair. Science 286, 1897 (1999). J.-L. Mandel: Breaking the rule of three. Nature 386, 767 (1997). D. W. Ross: Cancer: The emerging molecular biology. Hospital Practice 35, 63 (2000). S. Smith, I. Giriat, A. Schmitt, et al.: Tankyrase, a poly (ADPribose) polymerase at human telomeres. Science 282, 1484 (1998). T. A. Steitz: DNA polymerises: Structural diversity and common mechanisms. Journal of Biological Chemistry 274, 17395 (1999). S. T. Warren: Trinucleotide repetition and fragile X syndrome. Hospital Practices 32, 73 (1997).

CHAPTER

25

RNA and Protein Synthesis

Most proteins are enzymes that catalyze the myriad of chemical reactions in cells that are necessary for life; other proteins form structural functions as in bone and muscle. The information for making proteins resides in the sequence of bases in DNA in chromosomes and in organelles such as mitochondria. Converting the information contained in genes into proteins involves two complex processes. Transcription is the first step in which the sequence of bases in a gene is converted into a complementary sequence of bases in a molecule of RNA. Three chemically identical but functionally quite different molecules of RNA are transcribed from DNA: messenger RNA (mRNA) carries the genetic information contained in a gene; transfer RNA (tRNA) and ribosomal RNA (rRNA) are also transcribed from genes but are used to convert the information in the sequence of bases in mRNA into the corresponding sequence of amino acids in a protein. Translation is the process by which an mRNA is "read" by tRNAs, ribosomes (complex structures consisting of rRNAs and ribosomal proteins), and numerous other enzymes. Each type of cell is programmed to synthesize only those proteins necessary for its particular cellular functions (Chapter 26). The difference between a neuron and a liver cell is the kind of proteins that are synthesized even though both cells contain exactly the same genetic information. Cellular differentiation is due to differential gene expression; a tumor cell invariably is a cell that has lost the ability to regulate and express its genetic information

correctly and generally grows in an unregulated manner, as opposed to normal cells whose growth is regulated. The flow of information in all cells is from DNA to RNA to protein, which is known as the central dogma of molecular biology; it was formulated by Francis Crick shortly after the discovery of the structure of DNA. Information also can flow from DNA to DNA in both cells and among viruses that infect cells. Information also flows from RNA to RNA during the replication of RNA viruses such as the polio virus. The final permitted information transfer is from RNA to DNA, which only occurs in the case of retroviruses such as human immunodeficiency virus (HIV). The only information transfer that is prohibited by the central dogma is from protein to RNA or to DNA. The permitted information transfers in cells (infected or uninfected) is summarized below.

@

A~

>. @ A

~, Protein

The synthesis of a protein in a human cell can be broadly outlined as follows. A mRNA molecule is transcribed from a single strand of DNA (the "sense" strand) in the nucleus. The mRNA is processed by splicing out nontranslatable segments of nucleotides (introns) and rejoining the translatable segments (exons). Additional chemical modifications are made to both ends of the mRNA molecule and it is transported into the cytoplasm. The mRNA is translated

563

564 in the cytoplasm where groups of three bases in the mRNA (codons) are recognized by anticodons in specific tRNAs that carry a particular amino acid. The genetic code consists of 64 different codons that specify all 20 amino acids as well as codons that function to initiate and terminate translation. More than one codon may specify the same amino acid, which is called degeneracy of the genetic code. Finally, every organism from bacteria to human uses the same codons to specify the same amino acids; this is why the genetic code is said to be universal. Amino acids are joined together in a specific order determined by tRNAs, ribosomes, and associated enzymes that translate the mRNA. Each amino acid is joined to its neighbor by a peptide bond. The specific amino acid sequence of a protein specifies its three-dimensional structure. Some proteins require the help of chaperonins to fold into a functional configuration. When synthesis of a polypeptide chain is completed on a ribosome, it is released from the ribosome and may join with one or more similar or different polypeptides to constitute a functional protein.

25.1 Structure of RNA RNA is a single-stranded polynucleotide containing the nucleosides adenosine, guanosine, cytosine, and uridine. Roughly one-third to one-half of the nucleotides are engaged in intrastrand hydrogen bonds, with single-stranded segments interspersed between double-stranded regions that may contain up to about 30 base pairs. The base pairing produces conformations that are important to the function of the particular RNA molecules.

Ribosomal RNA (rRNA) Ribosomes contain several different RNA molecules, three in prokaryotic ribosomes and four in eukaryotic ribosomes. For historical reasons, each class is characterized by its sedimentation coefficient, which represents a typical size. For prokaryotes, the three Escherichia coli rRNA molecules are used as size standards; they have sedimentation coefficients of 5S, 16S, and 23S. The E. coli rRNA molecules have been sequenced and contain 120, 1541, and 2904 nucleotides, respectively. The sizes of the prokaryotic rRNA molecules vary very little from one species of bacterium to another. Eukaryotic rRNA molecules are generally larger and there are four eukaryotic rRNA molecules. Rat liver rRNA molecules are used as standards; the S values and the average number of nucleotides are 5S (120), 5.8S (150), 18S (2100), and 28S (5050), respectively. The eukaryotic 5.8S species corresponds functionally to the prokaryotic

CHAPTER 25


5S species; no prokaryotic rRNA molecule corresponds to the eukaryotic 5S rRNA.

Transfer RNA (tRNA) Transfer RNA molecules range in size from 73 to 93 nucleotides. Since they function as amino acid carriers, they are named by adding a superscript that designates the amino acid carried, e.g., tRNA Ala for alanine tRNA. All tRNA molecules studied contain extensive doublestranded regions and form a cloverleaf structure in which open loops are connected by double-stranded stems. By careful comparison of the sequences of more than 200 different tRNA molecules, common features have been found and a "consensus" tRNA molecule consisting of 76 nucleotides arranged in a cloverleaf form has been defined (Figure 25-1). By convention, the nucleotides are numbered 1 through 76 starting from the 5'-P terminus. The standard tRNA molecule has the following features: 1. The 5'-P terminus always is base-paired, which probably contributes to the stability of tRNA. 2. The 3'-OH terminus always is a four-base single-stranded region having the base sequence XCCA-3'-OH, in which X can be any base. This is called the CCA or acceptor stem. The adenine in the CCA sequence is the site of attachment of the amino acid by the cognate synthetase. 3. tRNA has many "modified" bases. A few of these, dihydrouridine (DHU), ribosylthymine (rT), pseudouridine (~p), and inosine (I), occur in particular regions. 4. tRNA has three large single-stranded loops. The anticodon loop contains seven bases. The loop containing bases 14-21 is called the DHU loop; it is not constant in size in different tRNA molecules. The loop containing bases 54-60 almost always contains the sequence T ~ C and is called the T~pC loop. 5. Four double-stranded regions called stems (or arms) often contain GU base pairs. The names of the stems match the corresponding loop. 6. Another loop, containing bases 44-48, is also present. In the smallest tRNAs it contains four bases, whereas in the largest tRNA molecule it contains 21 bases. This highly variable loop is called the extra arm.

25.2 Messenger RNA Messenger RNA molecules in prokaryotic and eukaryotic cells are similar in some structural aspects but also differ significantly. All messenger RNAs contain the same four nucleotides, A, C, G, and U, and utilize the codon AUG to

565

SECTION25.2 Messenger RNA

F I G U R E 25-1 (a) The currently accepted "standard" tRNA cloverleaf with its bases numbered. A few bases present in almost all tRNA molecules are indicated. (b) Schematic diagram of the three-dimensional structure of yeast tRNA Phe. (Courtesy of Dr. Sung-Hou Kim.)

initiate translation of a polypeptide and the codons UAG, UGG, and UAA to terminate translation. Prokaryotic mRNAs are polycistronic (polygenic) and usually carry information for the synthesis of several polypeptides from a single mRNA. The triplet codons in prokaryotic mRNA are transcribed from the sense strand of DNA and subsequently are translated continuously from the 5 t - P O 4 end of the mRNA to the 3'-OH end. Since prokaryotic DNA is not separated from the cytoplasm by a nuclear membrane, translation begins on mRNA molecules before transcription is completed. Thus, transcription and translation are coupled in prokaryotes. Synthesis of each polypeptide chain in a polycistronic mRNA is determined by an AUG initiation codon and one or more nonsense codons that release the finished polypeptide from the ribosome. Eukaryotic mRNAs differ from prokaryotic mRNAs in several respects (Figure 25-2). Eukaryotic genes invariably contain information for only a single polypeptide but each gene may consist of millions of nucleotides because eukaryotic genes contain introns and exons. The mRNA that is transcribed (primary transcript) is processed in several ways: 1. The introns (intervening sequences) are spliced out of the primary transcript and the exons (expressed

sequences) are joined together. The splicing reactions and removal of introns from the primary transcript are carried out by small nuclear ribonucleoproteins (snRNPs). 2. While transcription is in process, the 5' end of the mRNA is capped with a methyl guanine nucleotide (m y Gppp).

3. After the primary transcript is complete, a polyA tail (-AAAn AoH) is added to the 3' terminus.

Prokaryotic mRNA 5' @

A Gene 1

~

XS~O ~/&G~ Gene 2

~ G : 3'

Gene 3

Eukaryotic mRNA (Primary transcript)

Cap I = intron

AAAn A-3' poly A tail One G e n e

E = oxon

F I G U R E 25-2 Structures of prokaryotic and eukaryotic primary transcripts (mRNAs). Prokaryotic mRNAs are polygenic, do not contain introns or exons, and are short lived in the cell. Eukaryotic mRNAs are monogenic, contain introns and exons, and usually are long lived in the cell.

566

4. Other modifications of the primary transcript are possible such as alternative splicing which produces mRNAs with different sets of exons and RNA editing in which bases are modified or changed in the original transcript. 5. The functional mRNA is transported to the cytoplasm where translation occurs on ribosomes bound to the endoplasmic reticulum of the cell.

25.3 Enzymatic Synthesis of RNA The basic chemical features of the synthesis of RNA are the following (Figure 25-3): 1. The precursors of RNA synthesis are the four ribonucleoside 5'-triphosphates (rNTPs): ATE GTP, CTP, and UTE The ribose portion of each NTP has an -OH group on both the 2' and the 3' carbon atoms. 2. In the polymerization reaction, a 3'-OH group of one nucleotide reacts with the 5'-triphosphate of a second nucleotide; a pyrophosphate is removed and a phosphodiester bond is formed. This same reaction occurs in the polymerization of DNA. 3. The sequence of bases in an RNA molecule is determined by the base sequence of the DNA template strand. Each base added to the growing end of an RNA chain is chosen by base pairing with the appropriate base in the template strand; thus, the bases C, T, G, and A in a DNA strand cause incorporation of G, A, C, and U, respectively, in the newly synthesized RNA molecule. The RNA is complementary to the template DNA strand, which is called the coding (sense) strand or template strand. 4. The RNA chain grows in the 5' ~ 3' direction, which is the same as the direction of chain growth in DNA

CHAPTER25 RNA and Protein Synthesis

synthesis. The RNA strand and the DNA template strand are also antiparallel. 5. RNA polymerases, in contrast with DNA polymerases, can initiate RNA synthesis, i.e., no primer is needed. 6. Only ribonucleoside 5'-triphosphates participate in RNA synthesis, and the first base to be laid down in the initiation event is a triphosphate. Its 3'-OH group is the point of attachment of the subsequent nucleotide. Thus, the 5' end of a growing RNA molecule terminates with a triphosphate. In tRNAs and rRNAs, and in eukaryotic mRNAs, the triphosphate group is removed.

E. coli RNA polymerase consists of five subunits--two identical ot subunits and one each of/3, ~', and o - - h a v i n g a total molecular weight of 465,000; it is one the largest enzymes known. The ot subunit is easily dissociated from the enzyme and, in fact, does so shortly after polymerization is initiated. The term core enzyme describes the o-free unit (Otzflfl'); the complete enzyme is called the holoenzyme (ot2/3/~'~). In this chapter, the name RNA polymerase is used when the holoenzyme is meant. Several different RNA polymerases exist in eukaryotes and are described below. An E. coli cell contains 3000-6000 RNA polymerase molecules; the number is greater when cells are growing rapidly. In eukaryotes, the number of RNA polymerase molecules varies significantly with cell type and is greatest in cells that actively make large quantities of protein, e.g., secretory cells.

25.4 Prokaryotic Transcription The first step in prokaryotic transcription is the binding of RNA polymerase to DNA at a particular region called

FIGURE 25-3 Model of RNA synthesisby RNA polymerasefrom the sense strand of DNA. See text for details.

SECTION25.4 Prokaryotic Transcription

567

F I G U R E 25-4 Segments of the coding strand of conserved regions from various genes showing the common sequence of six bases. The start point for mRNA synthesis is indicated by the heavy letters. The "conserved T" is underlined.

a promoterma specific nucleotide sequence, which for different RNA classes range in length from 20 to 200 nucleotides. In bacteria, a promoter is divided into subregions called t h e - 3 5 sequence (8-10 bases long) and t h e - 1 0 or Pribnow box (6 bases). The bases in t h e - 3 5 sequence are quite variable but the Pribnow boxes of all promoters are similar (Figure 25-4). Both the-35 sequence and the Pribnow box interact with RNA polymerase to initiate RNA synthesis. Following RNA polymerase binding to a promoter, a conformational change occurs such that a segment of the DNA is unwound and RNA polymerase is positioned at the polymerization start site. Transcription begins as soon as the RNA polymerase-promoter complex forms and an appropriate nucleotide binds to the enzyme. RNA polymerase contains two nucleotide binding sites called the initiation site and the elongation site. The initiation site binds only purine triphosphates (ATP and GTP), and one of these (usually ATP) becomes the first nucleotide in the RNA chain. Thus, the first DNA base that is copied from the DNA is usually thymine. The initiating nucleoside triphosphate binds to the enzyme and forms a hydrogen bond with the complementary DNA base. The elongation site is then filled with a nucleoside triphosphate that is selected strictly by its ability to form a hydrogen bond with the next base in the DNA strand. The two nucleotides are then joined together, the first base is released from the initiation site, and initiation is complete. The dinucleotide remains hydrogen-bonded to the DNA. The elongation phase begins when the polymerase releases the base and then moves along the DNA chain. The drug rifampin binds to bacterial RNA polymerases and is a useful experimental inhibitor of initiation of transcription. It binds to the fl subunit of RNA polymerase, blocking the transition from the chain initiation phase to the elongation phase; it is an inhibitor of chain initiation but not of elongation. Actinomycin D also inhibits initiation but does so by binding to DNA. These drugs have limited clinical use because of their toxicity.

F I G U R E 25-5 Base sequence of (a) the DNA of the E. coli trp operon at which transcription termination occurs and of (b) the 3' terminus of the mRNA molecule. The inverted-repeat sequence is indicated by reversed arrows. The mRNA molecule is folded to form a stem-and-loop structure.

After several nucleotides have been added to the growing chain, RNA polymerase holoenzyme changes its structure and loses the cr subunit. Thus, most elongation is carried out by the core enzyme, which moves along the DNA, binding a nucleoside triphosphate that can pair with the next DNA base and opening the DNA helix as it moves. The open region extends over about 30 base pairs. Chain elongation does not occur at a constant rate but slows down or stops at various points along the DNA molecule and, in some cases, may have a regulatory function. Termination of RNA synthesis occurs at specific base sequences within the DNA molecule. Many prokaryotic termination sequences have been determined and most have the following three characteristics (Figure 25-5): 1. An inverted-repeat base sequence containing a central nonrepeating segment; the sequence in one DNA strand would read ABCDEF-XYZ-F'E'D'C'B'A', in which A and A', B and B', and so on, are complementary bases. The RNA transcribed from this segment is capable of intrastrand base pairing, forming a stem and loop. 2. A sequence having a high G + C content. 3. A sequence of AT pairs in DNA (which may begin in the stem) that results in a sequence of 6-8 U's in the RNA. Termination of transcription includes the following steps:

568

CHAPTER 25


F I G U R E 25-6 Transcription cycle of E. coli RNA polymerase showing dissociation of the c~ subunit shortly after chain elongation begins, dissociation of the core enzyme during termination, and re-formation of the holoenzyme from the core enzyme and the o- subunit. A previously joined core enzyme and a subunit will rarely become rejoined; instead, reassociation occurs at random.

1. Cessation of RNA elongation, 2. Release of newly formed RNA, and 3. Release of the RNA polymerase from the DNA. There are two kinds of termination events in prokaryotes: those that are self-terminating (dependent on the DNA base sequence only) and those that require the presence of a termination protein called Rho. Both types of events occur at specific but distinct base sequences. Rho, an oligomeric protein, does not bind to the core polymerase or to the holoenzyme and binds only very weakly to DNA. The action of Rho is poorly understood. Some microorganisms regulate transcription of certain genes by inhibiting Rho thereby allowing transcription to continue into adjacent genes, a process called antitermination. The transcription cycle of E. coli RNA polymerase is shown in Figure 25-6. Various drugs inhibit chain elongation. Cordycepin is converted to a 5'-triphosphate form and then acts as a substrate analogue, blocking chain elongation.

Lifetime of Prokaryotic mRNA All mRNA molecules are subject to attack by RNases, and this degradation is an essential aspect of the regulation of gene expression. Proteins are not made when they are not needed, and the rate of protein synthesis is determined by

a balance between the rates of RNA synthesis and RNA degradation. The half-life of a typical prokaryotic mRNA molecule is only a few minutes, so constant production of a bacterial protein requires continued transcription. In contrast, eukaryotic mRNA molecules have a lifetime of hours to days. Presumably, the reason for the difference is that bacteria must adapt to rapidly changing environments, whereas eukaryotic cells receive a constant supply of nutrients that maintain a uniform environment.

25.5 Transcription in Eukaryotes The chemistry of transcription in eukaryotes is the same as in prokaryotes. However, the promoter structure and the mechanism for initiation are strikingly different.

Eukaryotic RNA Polymerases Eukaryotic cells contain three classes of RNA polymerases, denoted I, II, and III, which are distinguished by their requirements for particular ions and by their sensitivity to various toxins. All are found in the nucleus. Minor RNA polymerases are found in mitochondria and chloroplasts. Polymerase I molecules are located in the nucleolus and are responsible for synthesis of 5.8S, 18S, and 28S rRNA molecules. Polymerase II synthesizes all

SECTION 25.5

Transcription in Eukaryotes

569

F I G U R E 25-7 Sequences found in and near some RNA polymerase II promoters. Only the TATA box is represented in many promoters (some promotors lack this sequence). The CAAT box occurs much less frequently, and the GC box has only occasionally been observed. Upstream sites are very common but are not considered to be part of the promoter.

RNA molecules destined to become mRNA. Polymerase III synthesizes 5S rRNA and the tRNAs. Polymerases II and III are inhibited by ot-amanitin, the toxic product of Amanita phalloides mushrooms, and are identified by their sensitivity to this substance. Rifampin, a powerful inhibitor of bacterial RNA polymerase, is inactive against the eukaryotic nuclear polymerases but inhibits mitochondrial RNA polymerases, although at higher concentrations than needed to inhibit bacterial polymerases. Thus, rifampin is used with other drugs (e.g., isoniazid) in treating tuberculosis. Actinomycin D is a general inhibitor of eukaryotic transcription by virtue of its binding to DNA. It has been useful in the treatment of childhood neoplasms (e.g., Wilms' tumor) and choriocarcinoma. However, it inhibits rapidly proliferating cells of both normal and neoplastic origin and hence produces toxic side effects.

RNA Polymerase II Promoters The structure of eukaryotic promoters is more complex than that of prokaryotic promoters. DNA sequences, hundreds of base pairs (bp) upstream from the transcription start site, control the rate of initiation. Furthermore, initiation requires numerous specific proteins (transcription factors) that bind to particular DNA sequences. Without the transcription factors, RNA polymerase II cannot bind to a promoter. However, KNH polymerase II itself is not a transcription factor. The complexity of initiation may derive, in part, from the fact that eukaryotic DNA is in the form of chromatin, which is inaccessible to RNA polymerases. Many RNA polymerase II promoters have the following features: 1. A sequence, TATAAAT, about 25 bp upstream from the transcription start site, known as the TATA or Hogness box. (Note its similarity to the Pribnow box.) The TATA box probably determines the base that is first transcribed. 2. A common sequence in the-75 region, GG(T/C)CAATCT, in which T and C appear with equal frequency at the third position. This sequence is

called the CAAT box. A third element, recognized ina few promoters, is the GC box, GGGCGG (Figure 25-7). Note: The term "upstream" refers to regions in the DNA that are to the left (or 5') of the start of transcription of a gene; the term "downstream" refers to regions to the right (or 3') of the gene. Regulatory regions affecting gene expression may be located close to the transcriptional start site, thousands of bases or more upstream or downstream from the gene, or in introns.

RNA Polymerase III Promoters RNA polymerase III promoters differ significantly from RNA polymerase II promoters in that they are located downstream from the transcription start site and within the transcribed segment of the DNA. For example, in the 5S RNA gene of the South African toad (Xenopus laevis) the promoter is between 45 and 95 nucleotides downstream from the start point. Thus, the binding sites on RNA polymerase III are reversed with respect to the transcription direction, as compared with RNA polymerase II. That is, RNA polymerase II reaches forward to find the start point, and RNA polymerase III reaches backward. In fact, RNA polymerases can slide in either direction along a DNA template; however, they can only synthesize RNA molecules in a 5' --+ 3' direction.

Eukaryotic mRNA Synthesis Eukaryotic mRNA used in protein synthesis is usually about one-tenth the size of the primary transcript. This size reduction results from excision of noncoding sequences called intervening sequences or introns. After excision, the coding fragments are rejoined by RNA splicing enzymes (Figure 25-8).

Capping and Polyadenylylation Capping occurs shortly after initiation of synthesis of the mRNA and precedes other modifications that protect the mRNA from degradation by nucleases. The poly(A)

5~0

CHAPTER25


FIGURE 25-8

Schematic drawing showing production of eukaryotic mRNA.The primarytranscript is capped before it is released. Then its T-OH end is modified, and finally the interveningregions are excised. MeG, 7-methylguanosine. **Two nucleotides whose riboses may be methylated.

terminus is synthesized by a nuclear enzyme, poly(A) polymerase. However, adenylate residues are not added directly to the 3' terminus of the primary transcript. Instead, RNA polymerase transcribes past the recognition site for addition of poly(A). After synthesis of the complete RNA, endonucleolytic cleavage occurs at the poly(A) recognition site in the RNA, and poly(A) is added. A sequence, AAUAAA, located 10-25 bases upstream from the poly(A) addition site, is also required for enzyme recognition of the site of polyadenylylation.

Splicing of RNA in Eukaryotes A distinguishing feature of most primary transcripts of higher eukaryotes is the presence of untranslated intervening sequences (introns) that interrupt the coding sequence and are excised from the primary RNA transcript. In the processing of RNA in higher eukaryotes, the amount of discarded RNA ranges from 30% to nearly 90% of the primary transcript. The remaining coding segments (exons) are joined together by splicing enzymes to form translatable mRNA molecules. The excision of the introns and the formation of the final mRNA molecule by joining of the exons is called RNA splicing. The 5' segment (the cap) of the primary transcript is never discarded and hence is always present in the completely processed mRNA molecule; the 3' segment is also usually retained. Thus, the number of exons is usually one more than the number of introns. The number of introns per gene varies considerably (Table 25-1). Furthermore, within different genes the introns are distributed differently and have many sizes (Figure 25-9), and introns are usually longer than exons. The splicing reaction is remarkably precise: cuts are made at unique positions in transcripts that contain

thousands of bases. The fidelity of the excision and splicing reaction is extraordinary, for if an error of even one base were made, the correct reading frame would be destroyed. Such fidelity is achieved by recognition of particular base sequences by splicing enzymes. Base sequence studies of the regions adjacent to several hundred different introns indicate that common sequences can be found at each end of an intron. The sites at which cutting occurs are always 5' to GU and 3' to AG. The rule is that the base sequence of an intron begins with GU and ends with AG. Introns are excised one by one, and ligation occurs before the next intron is excised; thus, the number of different nuclear RNA molecules present at any instant is huge. Translation does not occur until processing is complete.

TABI,E 25-1

Translated Eukaryotic Genes in Which Introns Have Been Demonstrated Gene

a-Globulin Immunoglobulin L chain Immunoglobulin H chain Yeast mitochondrial cytochrome b Ovomucoid Ovalbumin Ovotransferrin Conalbumin a-Collagen

N u m b e r of lntrons

2 2 4 6 6 7 16 17 52

*At present the histone and interferon genes are the only known translated genes in the higher organisms that do not contain introns.

571

SECTION25.6 6enetic Code

F I G U R E 25-9 Diagram of the conalbumin primary transcript and the processed mRNA. The 16 introns, which are excised from the primary transcript, are shown in color.

Before RNA splicing was discovered, the nucleus was observed to contain a significant amount of seemingly untranslated RNA. The collection of RNA molecules of widely varied sizes was given the name heterogeneous nuclear RNA (hnRNAL a term that is still sometimes used for nuclear RNA.

Splicing and Ribozymes Different snRNPs are found in eukaryotic cells which function in removing introns from primary RNA transcripts. The association of small RNAs, nuclear proteins, and the introns that they attach to is referred to as a spliceosome. Small nuclear RNAs (designated U1 through U6) provide specificity to different spliceosomes so that they recognize different classes of introns. Introns are distinguished according to their three-dimensional structures and each class of introns is spliced out by a different mechanism. Class I introns were originally discovered in ciliated protozoa and subsequently were found in fungi, bacteriophages, and other organisms. The RNA itself in a class I intron has catalytic activity and class I introns remove themselves from primary RNA transcripts by a self-splicing reaction. Class I introns are not true enzymes in that they function only once. The nucleotides in the intron that is spliced out are recycled in the cell. Class H introns are removed from RNA by a selfsplicing reaction that proceeds through an intermediate structure called a lariat. The removal of class II introns from RNAs also results in the splicing together of exons on either side of the intron. The ability of class II introns to specifically bind to a 5' exon has led to their use as reagents to construct novel RNA molecules. Chemical derivatives of class II introns have been constructed that can carry out the reverse of the splicing reaction. When these introns insert themselves into RNA, they can be used to shuffle sequences or to link one RNA molecule to another. Class II introns, which are found in bacteria, plant organelles, yeast, and fungi, also are capable of reintegrating into DNA after being excised from an RNA molecule. Class II introns encode a multifunctional intron-encoded

protein (IEP) that has reverse transcriptase activity, RNA splicing activity, and DNA endonuclease activity. These three enzymatic activities allow the intron to be excised from RNA, copied into an RNA-DNA heteroduplex, and inserted into DNA at specific target sites recognized by the IEP-intron complex. Recently, class II introns have been modified so that they can be targeted to specific genes in any DNA molecule. This novel molecular mechanism raises the possibility that suitably engineered class II introns can be used therapeutically in gene therapy and possibly to inactivate viruses such as HIV by integration and disruption of essential genes. Ribonuclease P (RNase P) consists of both protein and an RNA component that has catalytic activity. RNase P functions in eukaryotic cells to process the 5' end of precursor tRNA molecules. RNase P also can be directed to cleave any RNA molecule when the target is complexed with a short complementary oligonucleotide called an external guide. Ribozymes refer to catalytic RNA molecules that recognize specific target sequences in other RNA molecules. This activity and specificity makes ribozymes potentially useful therapeutic agents. For example, a ribozyme could be directed to silence the expression of a deleterious gene by destroying the mRNA transcript before it can be expressed. Or ribozymes might be constructed that would inactivate the mRNA or oncogenes or the expression of genes in RNA viruses such as HIV. Laboratory studies have supported the feasibility of ribozyme therapy but its application in clinical practice is still far in the future.

25.6 Genetic Code "Universal" Genetic Code Production of proteins from mRNA requires translation of the base sequence into an amino acid sequence. The collection of base sequences (codons) that correspond to each amino acid and to signals for termination of translation is the genetic code. The code consists of 64 triplets of bases (Table 25-2). The codons are written with the 5' terminus


57~

TABLE 25-2 "Universal" Genetic Code*

First Position (5' end)

Third Position (3' end)

Second Position U

C

A

G

the exception of serine, leucine, and arginine, all synonyms (codons corresponding to the same amino acid) are in the same box and differ by only the third base. For example, GGU, GGC, GGA, and GGG all code for glycine. The fidelity of translation is determined by two features of the system:

U

Phe Phe Leu Leu

Ser Ser Ser Ser

Tyr Tyr Stop Stop

Cys Cys Stop Trp

U C A G

C

Leu Leu Leu Leu

Pro Pro Pro Pro

His His Gln Gln

Arg Arg Arg Arg

U C A G

A

Ile Ile Ile IMetl

Thr Thr Thr Thr

Asn Asn Lys Lys

Ser Ser Arg Arg

U C A G

G

Val Val Val [Val[

Ala Ala Ala Ala

Asp Asp Glu Glu

Gly Gly Gly Gly

U C A G

*The boxed codons are used for initiation. GUG is very rare.

of the codon at the left. The following features of the code should be noted: 1. Sixty-one codons correspond to 20 different amino acids. 2. The codon AUG has two functions. It corresponds to the amino acid methionine when AUG occurs within a coding sequence in the mRNA, i.e., within a polypeptide chain. It also serves as a signal to initiate polypeptide synthesismwith methionine for eukaryotic cells but with N-formylmethionine for prokaryotic cells. How the protein-synthesizing system distinguishes an initiating AUG from an internal AUG is discussed below. The codon GUG also has both functions, but it is only rarely used in initiation. Once initiation has occurred at an AUG codon, the reading frame is established and the subsequent codons are translated in order. 3. Three codonsmUAA, UAG, and U G A m d o not represent any amino acid but serve as signals to terminate the growing polypeptide chain. 4. Except for methionine (AUG) and tryptophan (UGG), most amino acids are represented by more than one codon. The assignment of codons is not random; with

1. Attachment of the correct amino acid to a particular tRNA molecule by the corresponding aminoacyl synthetase, and 2. Correct codon-anticodon pairing. The former results from the specificity of interaction of the enzyme, the amino acid, and the tRNA molecule. The latter is assured by base pairing. A striking aspect of the code is that, with few exceptions, the third base in a codon appears to be unimportant, i.e., XYA, XYB, XYC, and XYD are usually synonymous. This finding has been explained by the base pairing properties of the anticodon-codon interaction. In DNA, no base pairs other than GC and AT are possible because the regular helical structure of double-stranded DNA imposes steric constraints. However, since the anticodon is located within a single-stranded RNA loop, the codon-anticodon interaction does not require formation of a structure with the usual dimensions of a double helix. Model building indicates that the steric requirements are less stringent at the third position of the codon, a feature called wobble. The wobble hypothesis allows for inosine (I), a nucleoside in which the base is hypoxanthine and which is often found in anticodons, to base-pair with A, U, or C. In addition, the wobble hypothesis allows for U to base-pair with G. This explains how a tRNA molecule carrying a particular amino acid can respond to several different codons. For example, two major species of yeast tRNA Ala are known; one responds to the codons GCU, GCC, and GCA and has the anticodon IGC. Recall that the convention for naming the codon and the anticodon always has the 5' end at the left. Thus, the codon 5'-GCU-3' is matched by the anticodon 5'-IGC-3'. The other tRNA Ala has the anticodon CGC and responds only to GCG. The function of three stop codons derives from the fact that no tRNA exists that has an anticodon that can pair with the stop codons. Genetic Code of Mitochondria The genetic code of mitochondria is not the same as the "universal" code, which has implications for the evolution of mitochondria. Human mitochondria contain a set of tRNA molecules that are not found elsewhere in the

SECTION25.6 Genetic Code

573

TABLE 25-3

Genetic Code of Human Mitochondria

First Position (5' end)

Third Position (3' end)

Second Position U

C

A

G

U

Phe Phe Leu Leu

Ser Ser Ser Ser

Tyr Tyr Stop Stop

Cys Cys Trp Trp*

U C A G

C

Leu Leu Leu Leu

Pro Pro Pro Pro

His His Gln Gln

Arg Arg Arg Arg

U C A G

A

IIle I Ile IMet~ IMetl

Thr Thr Thr Thr

Asn Asn Lys Lys

Ser Ser Stop* Stop*

U C A G

G

Val Val Val Val

Ala Ala Ala Ala

Asp Asp Glu Glu

Gly Gly Gly Gly

U C A G

*These entries are found in mitochondriabut not in the univeral code. Boxed codons are used as start codons. The mitochondrial codes of other organisms exhibit further differences.

cell and a circular DNA molecule containing 16,569 base pairs. This DNA molecule encodes some mitochondrial enzymes (Chapter 14) and is the template for synthesis of all mitochondrial tRNA and rRNA molecules. Human mitochondrial DNA sequences contain the genes for 12S and 16S ribosomal RNA, 22 different tRNA molecules, three subunits of the enzyme cytochrome oxidase (whose amino acid sequence is known), cytochrome b, and several other enzymes. The human mitochondrial code is shown in Table 25-3; entries shown with an asterisk differ from the universal code (cf. Table 25-2). The differences are striking in that most are in the initiation and termination codons. That is, in mammalian mitochondria, 1. UGA codes for tryptophan and not for termination. 2. AGA and AGG are termination codons rather than codons for arginine. 3. AUA and AUU are initiation codons, as is AUG. Both AUA and AUG also code for methionine. AUU also codes for isoleucine, as in the universal code. 4. AUA codes for methionine (and initiation, as shown in item 3) instead of isoleucine.

Maize mitochondria use CGG for tryptophan rather than for arginine, and CGU, CGC, and CGA for arginine. Yeast mitochondria use CUX, where X is any base, for threonine rather than for leucine. Both maize and yeast use AGA and AGG for arginine. Evidently, various mitochondrial codes can differ from each other as well as from the universal code. The number of mammalian mitochondrial tRNA molecules is 22, which is less than the minimum number (32) needed to translate the universal code. This is possible because in each of the fourfold redundant sets--e.g., the four alanine codons GCU, GCC, GCA, and GCGNonly one tRNA molecule (rather than two, as explained above) is used. In each set of four tRNA molecules, the base in the wobble position of the anticodon is U or a modified U (not I). It is not yet known whether this U is base-paired in the codon-anticodon interaction or manages to pair weakly with each of the four possible bases. For those codon sets that are doubly redundant--e.g., the two histidine codons CAU and CAC--the wobble base always forms, a G-U pair, as in the universal code. The structure of the human mitrochondrial tRNA molecule is also different from that of the standard tRNA molecule (except for mitochondrial tRNALeuuux). (X = any nucleotide.) The most notable differences are the following: 1. The universal sequence CTgzCXA is lacking in mitochondrial tRNA. 2. The "constant" 7-bp sequence of the TgzC loop varies from three to nine bases. 3. The invariant bases U8, A14, G15, G18, G19, and U48 of the standard tRNA molecule are not invariant in mitochondrial tRNA. In standard tRNA molecules, each of these bases participates in bonds that produce the folded L-shaped molecule. Thus, the mitochondrial tRNA molecule seems to be stabilized by fewer interactions. The threedimensional configurations of these molecules are not known with certainty; possibly they differ from the standard L-shape, and mitochondrial tRNA engages in a different type of interaction with the ribosome than standard tRNA molecules do. Most DNA molecules contain long noncoding segments (spacers) between genes. In mitochondrial DNA, there are either a few bases or none. In each case, there is a start codon (AUG, AUA, or AUU) at the 5' end of the mRNA molecule or within a few bases of it. This arrangement differs from the usual arrangement for eukaryotes in that ordinarily eukaryotic mRNA molecules commence with a short leader sequence thought to be responsible for binding to the ribosome. Such a leader is not present in the mitochondrial mRNA molecules, which suggests that

574


the 5' terminus itself of a mitochondrial mRNA molecule binds to the ribosome. The plethora of diseases and clinical abnormalities caused by mitochondrial mutations discussed in Chapter 14.

H tRNA fMet + Met

O

II

I (CH2)2 O

\\

p

25.7 A t t a c h m e n t of Amino Acid to t R N A M o l e c u l e

I

Methionyl-tRNAsynthetase~ H 3 N ~ C ~ C ~ O ~ t R N A f M e t

H

H

I

~N~

H

I

O

i,

C~C~O~tRNA

S tMet

Transformylase

I (CH2)2

THF

J

fTHF

I

S

I

CH3

When an amino acid is covalently linked to a tRNA molecule, the tRNA is said to be aminoacylated or charged. The notation for a tRNA charged with serine is seryl tRNA. The term uncharged tRNA refers to a tRNA molecule lacking an amino acid. Acylation is accomplished in two steps, both of which are catalyzed by an aminoaeyl tRNA synthetase. In the first, or activation, step an aminoacyl AMP is generated in a reaction between an amino acid and ATP: H

O

H

O

O

I

II

I

II

II

H3N + ~ C ~ C ~ O -

I

R

+ ATP ~

H3N + ~ C ~ C ~ O ~ P ~ R i b o s e ~ A d e n i n e

I

R

+ PP

I

O-

In the second, or transfer, step, the aminoacyl AMP reacts with tRNA to form an aminoacylated tRNA and AMP: Aminoacyl AMP + tRNA ~ aminoacyl tRNA + AMP As in DNA synthesis, the reaction is driven to the right by hydrolysis of pyrophosphate, so the overall reaction is Amino acid + ATP § tRNA + H20 -+ aminoacyl tRNA § AMP § 2Pi Usually only one aminoacyl synthetase exists for each amino acid. However, for a few amino acids specified by more than one codon, more than one synthetase does exist.

25.8 Initiator t R N A M o l e c u l e s Initiation Codon

and Selection of

The initiator tRNA molecule in prokaryotes--tRNA tMetm has several properties that distinguish it from all other tRNA molecules. One feature is that the tRNA is first acylated with methionine, and then the methionine is modified. Acylation is by methionyl tRNA synthetase, which also charges tRNA Met. However, the methionine of charged tRNA fMet is immediately recognized by another enzyme, tRNA methionyl transformylase, which transfers a formyl group from Nl~ (fTHF) to the amino group of the methionine to form

N-formylmethionine (fMet): Transformylase does not formylate methionyl tRNA Met because tRNA Met and tRNA fMet are structurally different. A second feature is its ability to initiate polypeptide synthesis. Eukaryotic initiator tRNA molecules differ from the prokaryotic initiator molecule in several ways. The most striking difference is that whereas eukaryotic organisms produce both a normal tRNA Met and an initiator tRNA, which is also charged with methionine, the methionine does not undergo formylation. In eukaryotes, the first amino acid in a growing polypeptide chain is Met and not fMet. The codon for both kinds of tRNA molecules in eukaryotes is AUG, just as for prokaryotes. The sequence AUG serves as both an initiation codon and the codon for methionine. Since methionine occurs within protein chains, some signal in the base sequence of the mRNA must identify particular AUG triplets as start codons. In eukaryotes, initiation usually begins at the AUG triplet nearest to the 5' terminus of the mRNA molecule, i.e., no particular signal is used (although sequences do play a role in the efficiency of initiation). Presumably, the ribosome and the mRNA interact at the 5' terminus and slide with respect to one another until an AUG is encountered. This AUG determines the reading frame. When a stop codon is encountered, the ribosome and the mRNA dissociate. Only a unique polypeptide is translated from a particular eukaryotic mRNA molecule. In prokaryotes, the situation is quite different. At a fixed distance upstream from each AUG codon used for initiation is the sequence AGGAGGU (or a single-base variant). This sequence, known as the Shine-Dalgarno sequence, is complementary to a portion of the 3'-terminal sequence of one of the RNA molecules in the ribosome (in particular, the 16S rRNA in the 30S particle; see below). Base pairing between these complementary sequences orients the initiating AUG codon on the ribosome. Thus, initiation in prokaryotes is restricted to an AUG on the 3' side of the Shine-Dalgarno sequence. Many prokaryotic mRNA molecules are polycistronic and contain coding sequences for several polypeptides. Thus, a polycistronic mRNA molecule must possess a

575

SECTION25.9 Ribosomes

series of start and stop codons for each polypeptide. The mechanism for initiating synthesis of the first protein molecule in a polycistronic mRNA is the same as that in a monocistronic mRNA. However, if a second protein in a polycistronic mRNA is to be made, protein synthesis must reinitiate after termination of the first protein. This is usually accomplished by the start codon of the second protein being so near the preceding stop codon that reinitiation occurs before the ribosome and the mRNA dissociate. Otherwise, a second initiation signal is needed.

25.9 Ribosomes A ribosome is a multicomponent structure that serves to bring together a single mRNA molecule and charged tRNA molecules so that the base sequence of the mRNA molecule is translated into an amino acid sequence. The ribosome also contains several enzymatic activities needed for protein synthesis. The protein composition of the prokaryotic ribosome is well known, but less is known about the composition of eukaryotic ribosomes. However, both share the same general structure and organization. The proper ties of the bacterial ribosomes are constant over a wide range of species, and the E. coli ribosome has been analyzed in great detail and serves as a model for all ribosomes.

Chemical Composition of Prokaryotic Ribosomes A prokaryotic ribosome consists of two subunits. The intact particle is called a 70S ribosome because of its S (sedimentation) value of 70 Svedberg units. The subunits, which are unequal in size and composition, are termed 30S

and 50S subunits. The 70S ribosome is the form active in protein synthesis. At low concentrations of Mg 2+, ribosomes dissociate into ribosomal subunits. At even lower concentrations, the subunits dissociate, releasing indi vidual RNA and protein molecules. The composition of each subunit is as follows: 30S subunit : one 16S r R N A molecule + 21 different proteins 50S subunit : one 5S r R N A molecule + one 23S r R N A molecule + 34 different proteins

The proteins of the 30S subunit are termed S 1, $2 . . . . . $21 (in this context, S is for "small" subunit); one copy of each is contained in the 30S subunit. The proteins of the 50S subunit are denoted by L (for "large" subunit). Each 50S subunit contains one copy of each protein molecule, except for proteins L7 and L12, of which there are four copies, and L26, which is not considered to be a true component of the 50S subunit. Most ribosomal proteins are very basic proteins, containing up to 34% basic amino acids. This basicity probably accounts in part for their strong association with the RNA, which is acidic. The amino acid sequences of most of the E. coli ribosomal proteins are known. A summary of the structure of the E. coli 70S ribosome is shown in Figure 25-10.

Ribosomes Are Ribozymes Ever since the discovery of the role of ribosomes in protein synthesis there has been uncertainty as to the functional roles of the rRNAs and the many ribosomal proteins. Conventional wisdom held that the rRNAs provided the scafolding (structural role) that oriented the ribosomal proteins so that one or more could carry out the enzymatic

FIGURE 25-10 Dissociation of a prokaryoticribosome.The configurationof two overlappingcircles is used throughoutthis chapter, for the sake of simplicity.

576

CHAPTER25 RNA and Protein

reactions necessary for protein synthesis. It was not until crystallization of the 50S subunit provided an atomic-level view of its structure that it was finally proved that the opposite is actually correct. The ribosomal proteins provide the structural framework for the 23S which actually carries out the peptidyltransferase reaction. Thus, the 23S rRNA is an enzyme and the ribosome is a ribozyme. Although the activity of the 23S rRNA is dependent on the ribosomal proteins, the 50S subunit that is substantially deproteinized still can perform the peptidyltransferase reaction. A crucial feature of the peptidyltransferase reaction is a particular adenine that is conserved in rRNAs extracted from the large ribosomal subunits of thousands of different organisms from all three kingdoms. Sequence analysis shows that this adenine base (2451A) always is present in the active site of the ribosome and acts as a general acidbase catalyst by deprotonating the nucleophilic amine as shown below. ...

2451A

\

25.10 Protein Synthesis Protein synthesis has three stages" 1. Polypeptide chain initiation, 2. Chain elongation, and 3. Chain termination. The main features of the initiation step are binding of mRNA to the ribosome, selection of the initiation codon, and binding of the charged tRNA bearing the first amino acid. In the elongation stage, there are two steps: joining of adjacent amino acids by peptide bond formation, and moving the mRNA and the ribosome with respect to one another so that codons are translated successively (translocation). In the termination stage, the completed polypeptide dissociate from the ribosomes, which are released to begin another cycle of synthesis.

l

.,.

i

,H aa-tRNA--=N:/""~==O

Synthesis

aa-tRNA----N HO 2451A ~tRNA

tRNA m

..,

tetrahedml

intermediate

The positive charge on 2451A in the tetrahedral intermediate is probably transferred to adjacent tRNA nucleotides that undergo tautomeric shifts. The evidence that ribosomal RNAs perform enzymatic functions critical to protein synthesis lends weight to the evolutionary significance of RNA in primordial chemical reactions that eventually led to the development of cells.

Every polypeptide has an amino terminus and a carboxyl terminus. In both prokaryotes and eukaryotes, synthesis begins at the amino terminus. For a protein having the sequence H 2 N - M e t - T r p - A s p . . . Pro-Val-COOH, the fMet (or Met) is the initiating amino acid and Val is the last amino acid added to the chain. Translation of mRNA molecules occurs in the 5' --+ 3' direction.

Chemical Composition of Eukaryotic Ribosomes

Stages of Protein Synthesis

The basic features of eukaryotic ribosomes are similar to those of bacterial ribosomes, except for their larger size. They contain a greater number of proteins (about 80) and an additional RNA molecule. A typical eukaryotic ribosome has a sedimentation coefficient of about 80 and consists of two subunits: 40S and 60S. These sizes may vary by as much as 10% from one organism to another in contrast with the homogeneity of bacteria ribosomes. The components of the subunits of eukaryotic ribosomes are as follows:

The mechanismsof protein synthesis in prokaryotes and eukaryotes differ slightly in detail, but the prokaryotic mechanism is used as a general model"

40S subunit 9 one 18S rRNA molecule + 30-35 proteins 60S subunit 9 one 5S, one 5.8S, and one 28S rRNA molecule + 45-50 proteins

1. Initiation. Protein synthesis in bacteria begins by the association of one 30S subunit (not the 70S ribosome), an mRNA, a charged tRNA fMet, three protein initiation factors, and guanosine 5'-triphosphate (GTP). These molecules make up the 30S preinitiation complex. Association occurs at an initiator AUG codon, whose selection was described above. A 50S subunit joins to the 30S subunit to form a 70S initiation complex (Figure 25-11). This joining process requires hydrolysis of the GTP contained in the 30S preinitiation complex. There are two tRNA

577

SECTION25.10 Protein Synthesis mRNA + 30S subunit + fMet-tRNA fMet + GTP + initiation factors

1 mRNA

I

fMet

50S subunit

L

P site

A site

GDP + P, + initiation factors 70S initiation complex

F I G U R E 25-11 Early steps in protein synthesis: in prokaryotes: formation of the 30S preinitiation complex and 70S initiation complex.

binding sites on the 50S subunit: the aminoacyl(A) site and the peptidyl(P) site; each site consists of several L proteins and 23S rRNA. The 50S subunit is positioned in the 70S initiation complex such that the tRNA fMet, which was previously bound to the 30S preinitiation complex, occupies the P site of the 50S subunit. Positioning tRNA fMet in the P site fixes the position of the anticodon of tRNA fMet so that it can pair with the initiator codon in the mRNA. in this way, the reading frame is unambiguously defined. 2. Binding of a second tRNA to the A site. The A site of the 70S initiation complex is available to any tRNA molecule whose anticodon can pair with the codon adjacent to the AUG complex initiation codon. However, entry to the A site by the tRNA requires the activity of a protein called elongation factor EF-Tu (Figure 25-12). EF-Tu binds GTP and an aminoacyl tRAA, forming a ternary complex, aminoacyl tRNA [EF-Tu-GTP]. This ternary complex enters the A site for the codon-dependent placement of the aminoacyl tRNA at the A site. During this process of placement, GTP is hydrolyzed to GDP and Pi, and EF-Tu is released. EF-Tu-GTP is regenerated from EF-Tu-GDP through the inter action of another protein, EF-Ts, and GTP. 3. Chain elongation (formation of the first peptide bond). After a charged tRNA is positioned in the A site, a peptide bond between fMet and the adjacent amino acid forms. The peptide bond is formed by an enzyme complex called peptidyltransferase, whose active site resides in 23SrRNA of 50S subunit as previously discussed (Figure 25-13). As the peptide bond is formed, the Met is cleaved from the tRNA fMet

in the P site by another ribosomal protein, tRNA deacylase. 4. Translocation of the ribosome. After the peptide bond forms, an uncharged tRNA occupies the P site and a dipeptidyl tRNA is in the A site. At this point three events, which together make up the translocation step, occur: the deacylated tRNA fMet leaves the P site, the peptidyl tRNA moves from the A site to the P site, and the ribosome moves a distance of three bases on the mRNA to position the next codon at the A site. The translocation step requires the activity of another elongation protein, EF-G, and hydrolysis of GTP to provide the energy to move the ribosome. Thus, the total amount of energy expended in the synthesis of one peptide bond comes from the hydrolysis of four high-energy phosphate bonds (equal to about 30 kcal = 7.5 • 4). Recall that the synthesis of each molecule of ammoacyl tRNA consumes two high-energy phosphate bonds (ATP --+ AMP + PPi; PPi + 2Pi). (In these calculations, the one GTP molecule expended in the formation of the initiation complex is not included because of its small contribution to the overall synthesis of a polypeptide.) 5. Refilling of the A site. After translocation has occurred, the A site is again available for a charged tRNA molecule with a correct anticodon. When this occurs, the series of elongation reactions is repeated. 6. Termination. When a chain termination codon is reached, no aminoacyl tRNA is available that can fill the A site, so chain elongation stops. Since the polypeptide chain is still attached to the tRNA occupying the P site, release of the protein is

5~8


FIGURE 25-12 Elongation phase of protein synthesis: binding of charged tRNA, peptide bond formation, and translocation. Note that this phase consumes 2 GTP during each cycle of chain elongation.

accomplished by release factors (RFs), which are proteins that, in part, respond to chain termination codons. In the presence of release factors, peptidyltransferase catalyzes the reaction of the bound peptidyl moiety with water rather than with the free aminoacyl tRNA (Figure 25-14). Thus, the

polypeptide chain, which has been held in the ribosome solely by the interaction with the tRNA in the P site, is released from the ribosome. 7. Dissociation of the 70S ribosome. The 70S ribosome dissociates into 30S and 50S subunits, which may start synthesis of another polypeptide chain.

FIGURE 25-13 Peptide bond formation in prokaryotes. In eukaryotes, the chemistryis the same, although the initiating tRNA is not tRNAfMetbut tRNAinit Met 9

SECTION25.10 Protein Synthesis

579

Peptidyltransferase

OH +

O II H2N~Ala~Phe~His~Thr~C~OH

+ Free 70S ribosome

I Rapid dissociation

30S subunit + 50S subunit

FIGURE 25-14

The steps of termination of protein synthesis. Role of GTP GTR like ATE is an energy-rich molecule. Generally, when such molecules are hydrolyzed, the free energy of hydrolysis is used to drive reactions that otherwise are energetically unfavorable. This does not seem to be the case in protein synthesis. The reaction sequence indicates that GTP facilitates binding of protein factors either to tRNA or to the ribosome. Furthermore, hydrolysis of GTP to GDP always precedes dissociation of the bound factor. Comparison of the structure of the free factor and the factor-GTP complex indicates that the factor undergoes a slight change in conformation when GTP is bound. The function of GTP is to induce a conformational change in a macromolecule by binding to it. Since it is easily hydrolyzed by various GTPases, the use of GTP as a controlling element allows cyclic variation in macromolecular shape. When GTP is bound, the macromolecule has an active conformation, and when the GTP is hydrolyzed or removed, the molecule resumes its inactive form. GTP plays a similar role in hormone activation systems (Chapter 30).

Posttranslational Modification of Proteins The protein molecule ultimately needed by a cell often differs from the polypeptide chain synthesized. Modification of the synthesized chain occurs in several ways: 1. In prokaryotes, fMet is never retained as the NH2, terminal amino acid. In roughly half of all proteins, the formyl group is removed by the enzyme deformylase, leaving methionine as the NH2 terminal amino acid. In both prokaryotes and eukaryotes, the fMet, methionine, and possibly a few more amino

acids, are often removed; their removal is catalyzed by a hydrolytic enzyme called aminopeptidase. This hydrolysis may occur as the chain is being synthesized or after the chain is released from the ribosome. The choice of deformylation versus removal of fMet usually depends on the identity of the adjacent amino acids. Deformylation predominates if the second amino acid is arginine, asparagine, aspartic acid, glutamic acid, isoleucine, or lysine, whereas f Met is usually removed if the adjacent amino acid is alanine, glycine, proline, threonine, or valine. 2. Newly created NH2 terminal amino acids are sometimes acetylated, and amino acid side chains may also be modified. For example, in collagen a large fraction of the prolines and lysines are hydroxylated (see below). Phosphorylation of serine, tyrosine, and threonine occurs in many organisms. Various sugars may be attached to the free hydroxyl group of serine or threonine to form glycoproteins. Finally, a variety of prosthetic groups such as heme and biotin are covalently attached to some proteins. 3. Two distant sulfhydryl groups in two cysteines may be oxidized to form a disulfide bond. 4. Polypeptide chains may be cleaved at specific sites. For instance, chymotrypsinogen is converted to the digestive enzyme chymotrypsin by removal of four amino acids from two different sites. In some cases, the uncleaved chain represents a storage form of the protein that can be cleaved to generate the active protein when needed. This is true of many mammalian digestive enzymes, e.g., pepsin is formed by cleavage of pepsinogen. An interesting precursor is a huge protein synthesized in animal cells infected


580

F I G U R E 25-15 Polysomes. (a) Electron micrograph of an E. coli polysome. (Courtesy of Barbara Hamkalo.) (b) Diagram showing relative movement of the 70S ribosome and the mRNA and growth of the protein chain.

with some viruses; this molecule is cleaved at several sites to yield different active proteins and hence is called a polyprotein. Some cellular enzymatic systems are also formed by cleavage of polyproteins. C o u p l e d Transcription and Translation After about 25 amino acids have been joined together in a polypeptide chain in prokaryotes, the AUG initiation site of the mRNA molecule becomes exposed and a second initiation complex then forms. The overall configuration is of two 70S ribosomes moving along the mRNA at the same rate. When the second ribosome has moved a distance similar to that traversed by the first, a third ribosome is able to attach. This processmmovement and reinitiationmcontinues until the mRNA is covered with ribosomes separated by about 80 nucleotides. This large translation unit is called a polyribosome or polysome (Figure 25-15). Polysomes greatly increase the overall rate of protein synthesis, since 10 ribosomes traversing a single segment of mRNA clearly can make 10 times as many polypeptides per unit of time as can a single ribosome. In prokaryotes, transcription and translation are coupled; this can not occur in eukaryotes since transcription occurs in the nucleus and translation in the cytoplasm.

Endoplasmic Reticulum In most eukaryotic cells two major classes of ribosomes exist: attached ribosomes and free ribosomes. The attached ribosomes are bound to an extensive cytoplasmic network of lipoprotein membranes called the endoplasmic reticulum. The rough endoplasmic reticulum consists of bound ribosomes; the smooth endoplasmic reticulum is devoid of ribosomes. There is no structural difference between a

free and an attached ribosome, and attachment to the membrane occurs after synthesis of particular proteins begins. Most endoplasmic reticulum membranes enclose large, irregularly shaped, discrete regions of the cell called cisternae. In this sense, the membrane system has an inside and an outside, and ribosomes are bound only to the outside. Cells responsible for secreting large amounts of a particular protein (e.g., hormone-secreting cells) have an extensive endoplasmic reticulum. Most proteins destined to be secreted by the cell or to be stored in intracellular vesicles such as lysosomes (which contain degradative enzymes) and peroxisomes (which contain enzymes for eliminating hydrogen peroxide) are synthesized by attached ribosomes. These proteins are primarily found in the cisternae of the endoplasmic reticulum. In contrast, most proteins destined to be free in the cytoplasm are made on free ribosomes. The formation of the rough endoplasmic reticulum and the secretion of newly synthesized proteins through membrane is explained by the signal hypothesis (Figure 25-16). The basic idea is that the signal for attachment of the ribosome to the membrane is a sequence of very hydrophobic amino acids near the amino terminus of the growing polypeptide chain. When protein synthesis begins, the ribosome is free. Then the amino terminal hydrophobic amino acids interact with lipophilic membrane components and somehow direct the association of the large ribosomal subunit to a ribosomal receptor protein on the membrane surface. As protein synthesis continues, the protein moves through the membrane to the cisternal side of the endoplasmic reticulum. A specific protease termed the signalpeptidase cleaves the amino terminal signal sequence. Intracellular compartmentation of the newly synthesized proteins is a complex process, and disorders in this process lead to severe abnormalities (see below).

SECTION25.10 Protein Synthesis

581

F I G U R E 25-16 Signal hypothesis for the synthesis of secretory and membrane proteins. Shortly after initiation of protein synthesis, the amino-terminal sequence of the polypeptide chain binds a signal recognition protein, which then binds to a docking protein. The signal peptide is released from the signal recognition protein as the ribosome binds to a ribosome receptor, which is adjacent to a pore. Translation continues with the signal peptide passing through the pore. Once through the endoplasmic reticulum, the signal sequence is excised by the signal peptidase within the vesicle. When protein synthesis is completed, the protein remains within the vesicle and the ribosome is released.

In summary, the following events take place. When the predominant portion of the protein is synthesized and migrates in the endoplasmic reticulum, the signal sequence is removed by proteolytic cleavage. The finished protein, which is sequestered on the cisternal side of the endoplasmic reticulum, undergoes many posttranslational modifications, one of which is the addition of a series of carbohydrate residues to form glycoproteins (Chapter 16). Glycoproteins are transported to the Golgi complex, where further modification of the carbohydrate residues occurs. The finished glycoproteins are then packaged into lysosomes, peroxisomes, or secretory vesicles. The last fuses with the plasma membrane, discharging its contents into the extracellular fluid. Defects in compartmentation processes may lead to mislocation of the proteins and cause deleterious effects. Many proteins that are destined for various organelles (e.g., mitochondria) are not routed through endoplasmic reticulum and the Golgi complex. These proteins are made on cytosolic polysomes. They possess presequences at amino terminal ends that target them to receptors on appropriate organelles. Import of proteins into cell organelles is aided by chaperone proteins (Chapter 4).

Compartment Disorders Two defects of compartmentation are known. One, mucolipidosis or I-cell disease (Chapter 16), is characterized by mislocalization of lysosomal enzymes (acid hydrolases).

These glycoproteins lack mannose-6-phosphate residues, and consequently they are secreted into extracellular fluids instead of being sequestered into lysosomal vesicles. The mannose-6-phosphate serves as a marker that allows the enzymes to bind with mannose-6-phosphate receptors that direct the enzymes into the vesicles to be packaged into lysosomes. The lysosomal enzymes in the extracellular fluid bring about indiscriminate destruction of tissues. The second example of the compartmentation defect is ~ 1-antitrypsin deficiency (also known as o~1-proteinase inhibitor deficiency). The defect in this disorder is almost exactly the opposite of that in I-cell disease, i.e., I-cell disease is a disorder of a defect in (intracellular) retention of proteins and o~1-antitrypsin deficiency is a defect in the secretion of a protein, ot1-Antitrypsin (ol 1-AT) consists of a single polypeptide chain of 394 amino acid residues with three oligosaccharide side chains (Figure 25-17), all of which are attached to asparagine residues. It contains three/~ sheets and eight o~ helices. It has an overall molecular weight of 51,000. It is a polar molecule and readily migrates into tissue fluids. It is synthesized in hepatocytes and secreted into the plasma, where it has a half-life of 6 days. Its normal serum concentration is 150-200 mg/dL. Although the presence of O~l-AT has been noted in other cell types, the primary source in plasma is the hepatocyte. O~l-AT does not possess a propeptide but does contain a 24-residue signal peptide, which is eliminated during its passage through the endoplasmic reticulum, ot 1-AT is a defense protein, and its synthesis and release into circulation

582


F I G U R E 25-17 Schematic representation of the amino acid sequence of c~l-antitrypsin (PiMM) and some of its variants. The amino acid residues shown are not drawn to scale; CHO represents the oligosaccharide units.

increase in response to trauma and inflammatory stimulus. Thus, it is known as an acute-phase reactant, ot I-AT functions by forming a tight complex with the active site of the target enzyme that inhibits its proteolytic activity. The enzyme-inhibitor complexes are rapidly cleared by the reticuloendothelial cells. Thus, or l-AT is a suicide protein. Many genetic variants of ot ~-AT have been identified by isoelectric focusing. The ot I-AT gene is on chromosome 14, is 10.2 kb long, and contains four introns. The phenotype is based on a codominant pattern with both alleles expressed equally. Variants are classified by a system known as Pi (protease inhibitor). The most common form is PiMM (carried by 95% of the U.S. population). Most polymorphism observed in ot i -AT are due to single-aminoacid substitutions. o~m-AT is a broad-spectrum serine proteinase inhibitor (Chapter 6). Its principal action is to inhibit leukocyte (neutrophil) elastase. This function appears to be most important in maintaining the integrity of the elastic fibers for the elastic recoil of normal lung tissue. Recall that elastic fibers contain an amorphous core of elastin surrounded by an envelope of microfibrils (Chapter 16). The target substrate for neutrophil elastases is elastin. The turnover rate of mature elastin is extremely low, and if it is destroyed, replacement is severely limited. Thus, the risk of development of a lung disease (emphysema) characterized by dilatation of air spaces distal to the terminal bronchiole and destruction of bronchiole walls is high in c~l-AT deficiency. A human phenotype in which a severe deficiency of ot 1-AT (10-15% of normal serum concentration) has been observed is designated PiZ. The risk of development of emphysema in PiZZ (homozygotic ZZ individuals) is 20 times that in PiMM (a normal genotype). In addition, cigarette smoking by PiZ individuals accelerates development of destructive lung disease for several reasons.

1. The levels of c~l-AT are very low, and smoking causes an increase in elastolytic load owing to change in the phagocytic population. 2. A reduction of the proteinase inhibitory activity is due to oxidation of the reactive methionine residue of the active center by the cigarette smoke as well as by oxygen radicals produced by leukocytes and macrophages. Emphysema is a common disease, and its most common cause is cigarette smoking. Only 1-2% of cases are due to genetic deficiency of ot1-AT. The substitution of a lysine for a glutamic acid at position 342 leads to the deficiency of c~I-AT in PiZ individuals (Figure 25-17). This substitution of a basic for an acidic amino acid is consistent with a base change of cytosine to thymine in DNA. The amino acid change prevents normal processing of oligosaccharide side chains of the protein and therefore its secretion. As a result, Z-or ~-AT accumulates in hepatocytes. An association of hepatic disease with severe c~l-AT deficiency has been observed; however, the relationship is not understood. The sequestered a ~-AT may cause damage to hepatocytes or render them more susceptible to injury, toxins, or viruses. Liver transplantation has been attempted with limited success in patients with hepatocellular failure. The therapy for destructive lung disease is administration of an anabolic steroid (danazol), which increases the serum levels of c~l-AT by about 40%, or direct replacement of c~I-AT by transfusion. Another structural variant of ot ~-AT that has been studied in detail is S-or I-AT (PISS). This protein differs from M-or i-AT at position 264, where a glutamic acid residue is substituted by valine (Figure 25-17). The S-or t-AT undergoes normal addition of oligosaccharides, and the protein is secreted into the blood. Affected individuals have sufficiently high levels of S-o~1-AT to protect against neutrophil elastase and so are not at higher risk for developing

583

SECTION25.10 Protein Synthesis C

NH

NH

II

.o

I/c",

II

H2NCNH

-o

NHCNH2 I

CH3

HO

|

O

H3C " ~ H3C\ /OH3

HO

N

I

CH ' 30

0

I

HO ~ 1 / 0 \ 1 CONH2 OH

O

OH

R ~--- C H 3 " " N H --"

O OH Binds to a specific site on the 23S RNA and blocks elongation by interfering with the translocation step.

OH Interferes with pairing between aminoacyl tRNAs and RNA codons, causing misreading.

Erythromycin-

Streptomycin:

Inhibits the binding of aminoacyl tRNAs to the ribosome in bacteria.

Tetracycline:

H3C\ /OH3

e

N

HN

OH

k

H \ /C-----O /C\. H3C--'O~CH

2

NH 3

Puromycin" Causes premature chain termination. The molecule resembles the 3' end of the aminoacylated tRNA and will enter the A site. It transfers to the growing chain causing premature termination. FIGURE 25-18 Antibiotic inhibitors of protein synthesis.

emphysema. Prenatal diagnosis of o~1-AT can be done by the use of either synthetic oligonucleotide probes or restriction fragment length polymorphism analysis. The reactive site of o~1-AT is at the methionine residue, position 358. The sequence around the reactive center of several proteinase inhibitors, including o~~-AT, antithrombin III, and several unrelated plant proteinase inhibitors, is strikingly similar. In fact, ot 1-AT, antithrombin III, and ovalbumin (a protein without known inhibitory function)

share extensive homologies and may all have evolved from a common ancestral protease inhibitor more than 500 million years ago. A fatal bleeding disorder due to substitution of a methionine for an arginine residue at position 358 of o~1-AT has been reported. This particular alteration brought a change in the specificity of the inhibitor. This altered protein (known as O~l-antitrypsin Pittsburgh) exhibits antithrombin activity while losing antielastase activity. Antithrombin III, which has an important regulatory

58~

role in hemostasis (Chapter 36), has a reactive arginine at its active center. Since the o~I-AT and antithrombin III are structurally similar, the substitution of methionine of ot 1-AT for arginine at the active center alters its specificity. This bleeding disorder is further complicated by the fact that the concentration of normal antithrombin III does not increase in response to inflammatory stress as does the level of ot 1-AT. Therefore, with each hemolytic episode, the abnormal o~I-AT increases, creating a cycle resulting in uncontrolled bleeding.

Inhibitors of Protein Synthesis and Related Disorders Many antibacterial agents (antibiotics) have been isolated from fungi. Antibiotics are used both clinically and as reagents for unraveling the details of protein, RNA, and DNA synthesis. Antibiotics that have no effect on eukaryotic translation either fail to penetrate the cell membrane (which is quite common) or do not bind to eukaryotic ribosomes. The differences in effectiveness of antibiotics on the two classes of cells in vivo is the basis of their usefulness as therapeutic agents for bacterial infections (Figure 25-18). Some antibiotics are active against both bacterial and mammalian cells. One example is chloramphenicol, which inhibits peptidyltransferase in both bacterial and mitochondrial ribosomes, although eukaryotic cytoplasmic ribosomes are unaffected. Such a drug may be clinically useful if a concentration range can be maintained in the patient in which the antibacterial action is substantial but toxic effects on host cells are minimal. However, because of the potential for toxicity, such antibiotics are used only in serious infections when other drugs fail. Three antibiotic inhibitors that act only on prokaryotes are streptomycin, tetracycline, and erythromycin. Other antibiotics act primarily on eukaryotic cells (Table 25-4). Eukaryotic protein synthesis can also be inhibited by toxins of bacterial origin. An example is the toxin produced by Corynebacterium diphtheriae bacteria carrying a lysogenic bacteriophage. Uninfected bacteria do not elaborate the toxin, and therefore acquisition of the phage is essential for the toxic effect. Diphtheria is an acute infectious disease usually localized in the pharynx, larynx, and nostrils and occasionally in the skin. Initially, the cytotoxic effects are restricted to those tissues immediately adjacent to the bacterial growth. With increased bacterial growth, production of toxin increases and is disseminated to the blood, lymphatics, and other organ systems (e.g., cardiovascular and nervous systems), where it brings about destructive changes.


The pathogenesis of diphtheria is due entirely to the toxin's inhibition of protein synthesis in the host cells. Diphtheria toxin consists of a single polypeptide chain (M.W. 63,000) with two intramolecular disulfide linkages. Toxin binds to the outer surface of susceptible cells at specific sites and enters by receptor-mediated endocytosis. The toxin is cleaved proteolytically into a small fragment, A (M.W. 21,000), and a larger fragment, B (M.W. 42,000), during its internalization. Fragment A is the biologically active part of the molecule and presumably penetrates the cell membrane with the aid of fragment B. Upon entry into the cytoplasm, fragment A catalyzes the ADP ribosylation of the transfer factor, EF-2, leading to its inactivation and the interruption of protein synthesis. The ADP-ribose group is donated by NAD +. The ADP ribosylation reaction, catalyzed by the toxin, is specific for EF-2 of eukaryotic cells; other proteins of eukaryotic and bacterial cells are not substrates. This specificity is due to an unusual amino acid residue in EF-2, diphthamide, which is the acceptor of the ADP ribosyl group. Diphthamide derives from the posttranslational modification of histidine. The acute symptoms are treated with antitoxin. The bacteria, which are gram-positive, succumb to a variety of antibiotics, including penicillin. Diphtheria is effectively prevented by immunization with toxoid (inactivated toxin) preparations. ADP ribosylation is also involved in the action of cholera toxin and in certain pathogenic strains of E. coli (Chapter 12). Cholera toxin catalyzes the ADP ribosylation of the guanidinium, group of a specific arginine residue in the guanine nucleotide-binding protein of the adenylate cyclase. This activates adenylate cyclase, which catalyzes the formation of cAMP from ATE The cAMP formed stimulates secretion of water and electrolytes from intestinal epithelial cells. Thus, patients infected with Vibrio cholerae secrete enormous quantities (up to 20 L/d) of water and electrolytes. Without adequate and prompt replacement, death can ensue owing to dehydration and electrolyte imbalance. Interestingly, in the case of diphtheria toxin, ADPribosylation leads to inactivation of a protein, whereas with cholera toxin, ADP ribosylation causes activation of the protein. A group of plant lectins, such as abrin, ricin, and modeccin, are highly toxic to eukaryotic cells. Their mode of action consists of inhibition of protein synthesis by enzymatically inactivating the EF-2 binding region of the 60S ribosomal subunit, whereas the diphtheria toxin inactivates the EF-2 protein itself. Ricin is isolated from castor beans and has a molecular weight of 66,000. Like most plant and bacterial toxic proteins, ricin contains two

SECTION 25.11

Collagen Biosynthesis and Its Disorders

585

25.11 Collagen Biosynthesis and Its Disorders

TABLE 25-4

Inhibitors of Protein Synthesis in Eukaryotes Inhibitor

Abrin, ricin Diphtheria toxin

*Chloramphenicol

*Puromycin

*Fusidic acid Cycloheximide Pactamycin Showdomycin Sparsomycin

Action

Inhibits binding of aminoacyl tRNA Catalyzes a reaction between NAD* and EF2 to yield an inactive factor. Inhibits translocation. Inhibits peptydyltransferase of mitochondrial ribosomes. Is inactive against cytoplasmic ribosomes. Causes premature chain termination by acting as an analogue of charged tRNA. Inhibits translocation by altering an elongation factor. Inhibits peptidyltransferase. Inhibits positioning of tRNA Met on the 40S ribosome. Inhibits formation of the elF2-tRNAMet-GTP complex. Inhibits translocation.

*Also active on prokaryotic ribosomes.

polypeptide chains with two different but complementary functions. The A chain possesses enzymatic activity and is responsible for toxicity, and the B chain, which is a lectin, binds to galactose-containing glycoproteins or glycolipids on the cell surface. The A and B chains are linked by a labile disulfide linkage. Upon binding of the ricin molecule to the carbohydrate receptors of the cell surface via the B chain, the A chain enters the cytoplasm, presumably by receptor-mediated endocytosis, where it inhibits protein synthesis by irreversible inactivation of the 60S ribosomal subunit. Toxins have been used to develop highly selective cytotoxic agents targeted against specific cells. For example, ricin A has been coupled to agents that selectively bind to cell surface membrane components. The selective agents may be monoclonal antibodies (Chapter 35), hormones, or other cell surface ligands. These conjugates act as selective cytotoxic agents and may have potential therapeutic applications (e.g., in the treatment of cancer). Potential clinical application for diphtheria toxin may be impossible because of the prevalence of the diphtheria antitoxin in human populations.

Collagen occurs in several genetically distinct forms and is the most abundant body protein; most of the body scaffolding is composed of collagen (Chapter 11). Its structure is uniquely suited for this structural role. It is a fibrillar protein but also exists in a nonfibrillar form in the basement membrane. The basic structural unit of collagen, tropocollagen, consists of three polypeptide chains. Each polypeptide chain has the general formula (-Gly-X-Y-)333. Some of the amino acid residues at the X and Y positions are proline, hydroxyproline, lysine, hydroxylysine, and alanine. Collagen is a glycoprotein and contains only two types of carbohydrate residues, namely, glucose and galactose linked in O-glycosidic bonds to hydroxylysyl residues. In collagen, each polypeptide chain is coiled into a special type of a rigid, kinked, left-handed helix, with about three amino acid residues per turn. The three helical polypeptides in turn are wrapped around each other to form a right-handed triple-stranded superhelix that is stabilized by hydrogen bonding. The collagen molecules are aggregated in an ordered quarter-staggered array to give rise to microfibrils, which in turn combine to give fibrils. Covalent cross-linkages occur at various levels of collagen fiber organization and provide great mechanical strength. Collagen biosynthesis is unusual in that it consists of many posttranslational modifications. The unique posttranslational reactions of collagen biosynthesis are 1. Hydroxylation of selected prolyl and lysyl residues. 2. Glycosylation of certain hydroxylysyl residues. 3. Folding of procollagen polypeptides into a triple helix. 4. Conversion of procollagen to collagen. 5. Self-assembly into fibrils. 6. Oxidative deamination of s-amino groups of strategically located lysyl and hydroxylysyl residues to provide reactive aldehydes. These form cross-linkages between polypeptide chains of the same molecule as well as between the adjacent molecules that give strength and stability to the fibrils. The first three processes take place inside the cell, whereas the last three are extracellular modifications. Collagen disorders can result from primary defects in the structure of procollagen or collagen or from secondary changes that affect collagen metabolism. Collagen disorders are both acquired and inherited. Ehlers-Danlos syndrome and osteogenesis imperfecta are examples of inherited primary collagen diseases; scurvy and various fibrotic processes (e.g., pulmonary fibrosis and cirrhosis)


586

are examples of secondary collagen diseases. The mechanisms that lead to collagen diseases are: 1. Aberration in the control mechanisms that alter the balance between synthesis and degradation. This imbalance can lead either to excessive deposition or depletion of collagen. 2. Synthesis of structurally altered collagen due to defects at the level of DNA transcription, RNA processing, translation, or posttranslational modifications. 3. Imbalance in the relative rates of synthesis of genetically distinct collagens. 4. Abnormalities in the packing of collagen molecules into a fiber or in the interaction of collagen fibers with other extracellular components of the connective tissue.

Transcription and Translation of Collagen Polypeptides In general, the transcription and translation of collagen polypeptides resemble other proteins. The primary RNA transcript is capped and polyadenylated, introns are removed, and the functional mRNA is transported to the cytoplasm. The procollagen mRNA, in addition to being one of the largest mRNAs in eukaryotic cells, contains unusually high amounts of G and C, because these bases are overrepresented in the codons for glycine and proline. As is the case for other secretory proteins, the procollagen mRNA encodes a signal sequence coding for hydrophobic

amino acids at the amino terminal ends. The signal sequences of proot 1 chain are different from the proot2 chain and are unusually long (100 amino acid residues) in comparison with other secreted proteins (15-30 residues). Because of their hydrophobicity, these signal sequences are thought to facilitate binding and transport of the nascent polypeptide into cisternae of the endoplasmic reticulum. The signal sequences are removed by proteolysis within the membrane during the translation. The synthetic rate of proot chain is rather slow and is probably related to the unfolding of the secondary structure of procollagen mRNA, which is rich in GC content. Molecular defects at the levels of transcription and translation of collagen polypeptides have not yet been clearly established but may cause one type of EhlersDanlos syndrome (type IV). This disorder is characterized by decreased synthesis of type III collagen in the aorta, intestinal tract, skin, and probably other tissues. Among the Ehlers-Danlos disorders, type IV is the most severe because of the threat of arterial rupture or gastrointestinal perforation. Ehlers-Danlos syndrome is a group of inherited disorders that share similar clinical symptoms; many have been attributed to specific errors in the structure or metabolism of collagen (Table 25-5). The major phenotypic features of Ehlers-Danlos syndrome include hyperextensible skin, hypermobile joints, easy bruisability, and friability of tissues. The clinical severity of these disorders is highly variable. Defects in splicing, transcription, or translation may also cause osteogenesis imperfecta. In this disease, fibroblasts or tissue samples have a markedly diminished capacity for type I collagen

TABLE 25-5

Ehlers-Danlos Syndrome Type I

Inheritance

VI VII

AD AD AD ADorAR XL XL AR AD

VII IX X

AD SL AR

II III IV V

Biochemical Defect

Collagen Fibril Diameter

Unknown Unknown Unknown Decreased type III collagen synthesis and its intracellular accumulation Unknown Unknown Lysyl hydroxylase deficiency Deletion of exons from the genes of proal(I) and pro a 2 (I) that encode the amino-terminal cleavage site; required for the action of procollagen aminoprotease Unknown Lysyl oxidase deficiency Fibronectin deficiency

AD= Autosomal dominant;AR= autosomal recessive; XL= X-linked.

Increased Increased Increased Heterogeneous Heterogeneous Increased Decreased Heterogeneous

Not determined Increased Not determined

SECTION 25.11

Collagen Biosynthesis and Its Disorders

TABLE 25-6

Osteogenesis Imperfecta Type

Inheritance

I

AD

II

AR

III

AR

IV

AD

Biochemical Defect

Decreased synthesis of pro a 1(I), leading to less production of type I collagen because of several defects in the gene. Markedly diminished synthesis of type I collagen; presence of hydroxylysine-enriched collagens associated with delayed helix formation. Delayed secretion of type I procollagen associated with excess mannose residues in the carboxyterminal propeptide region; absence of a 2 (I) synthesis. Point mutations in or ) gene, resulting in substitution for glycine residues.

AD= Autosomal dominant; AR= autosomal recessive.

synthesis. Osteogenesis imperfecta is a genetically and clinically heterogeneous disorder (Table 25-6). It is one of the most common inherited connective tissue disorders and has an incidence of 1" 15,000. It is characterized clinically by defects in bone, tendon, dentin, ligament, and skin. Intracellular Posttranslational Modifications

Hydroxylations of Selected Prolyl and Lysyl Residues The hydroxylation reactions are catalyzed by three enzymes: prolyl 4-hydroxylase (usually known as prolyl hydroxylase), prolyl 3-hydroxylase, and lysyl hydroxylase. These enzymes are located within the cisternae or rough endoplasmic reticulum; as the procollagen chains enter this compartment, the hydroxylations begin. All three enzymes have the same cofactor requirements: ferrous ion, oe-ketoglutarate, molecular oxygen, and ascorbate (vitamin C). The reducing equivalents required for the hydroxylation reaction are provided by the decarboxylation of equimolar amounts of o~-ketoglutarate to succinate and carbon dioxide. One atom of the 02 molecule is incorporated into succinate while the other is incorporated into the hydroxyl group. The data on enzyme kinetics and mechanism of reaction are consistent with the ordered binding of Fe 2+, o~-ketoglutarate, 02, and the peptide

587

substrate and an ordered release of hydroxylated peptide, CO2, succinate, and Fe z+. The Fe z+ does not necessarily dissociate from the enzyme during each catalytic cycle. The activation of oxygen is required and may involve the formation of superoxide (O 2 ). The requirement for ascorbate is specific with purified enzyme preparations. It is not consumed stoichiometrically during the hydroxylation reaction. The exact role of ascorbate is not known, but presumably it reduces either the enzyme iron complex or the free enzyme. Humans, other primates, and guinea pigs lack the enzyme required for the conversion of gluconate to ascorbate (Chapter 15). However, most mammals do possess this enzyme and are able to synthesize ascorbate from glucose by way of glucuronic acid. Other reducing agents, such as dithiothreitol, L-cysteine, and some reduced pteridines, in high concentrations, can partially replace ascorbate in in vitro assays. The generalized hydroxylation reaction is shown below. COOl COOCH2 I I CH2 Enzyme ~ Substrate-OH + I + CO2 CH2 Substrate-H + 02 + I Fe2* CH2 C ~ - O Ascorbate I I COOCOO 0r Succinate

The substrates for all three hydroxylases are highly specific. Free proline and lysine are not substrates. The residue to be hydroxylated must be in a peptide linkage (the minimum is a tripeptide) and in the correct X or Y position in the chain. The proline hydroxylase and the lysyl hydroxylase catalyze the hydroxylation of only prolyl or lysyl residues in the Y positions of peptides with the s e q u e n c e - X - Y - G l y - , whereas the prolyl 3-hydroxylase catalyzes the hydroxylation of prolyl residues at the X position only if Y is 4-hydroxyproline. The conformation of the substrate is also important for catalytic activity. The substrates have to be nonhelical; triple-helical polypeptides do not function as substrates. Thus, the hydroxylation reactions must occur before triplehelix formation. Many important functions are associated with the hydroxylation of prolyl and lysyl residues. The 4-hydroxyprolyl residues participate in interchain hydrogen bonding that aids in the maintenance of triplehelical structures. For example, nonhydroxylated polypeptide chains cannot form stable triple-helical structures at body temperature. The biological role of 3-hydroxyprolyl residues is not known. The hydroxyl groups of hydroxylysyl residues are sites of attachment of carbohydrate

588

and also participate in the intermolecular collagen crosslinks. The importance of lysyl hydroxylation is seen in patients with the type VI variant of Ehlers-Danlos syndrome (Table 25-5). The collagen in these individuals has a decreased fibril diameter and profound changes in mechanical properties. Skin fibroblasts show virtually no lysyl hydroxylase activity. Furthermore, hydroxylysine formation can be severely affected in some tissues, mildly affected in others, and unaffected in still others (e.g., cartilage). These observations suggest the presence of tissue-specific lysyl hydroxylases.

Glycosylation of Hydroxylysyl Residues The glycosylation occurs as the N-terminal ends of the polypeptide chains move into cisternae at specific hydroxylysyl residues. The carbohydrates found are galactose and the disaccharide, glucosylgalactose. These reactions are catalyzed by two specific enzymes: hydroxylysyl galactosyltransferase and galactosylhydroxylysyl glucosyltransferase. The first enzyme catalyzes the transfer of galactose from UDP-galactose to hydroxylylsyl residues, and the second enzyme transfers a glucose from UDP-glucose to galactosylhydroxylysyl residues. Both enzymes require a bivalent cation, preferably manganese. The substrate has to be in the nonhelical conformation, and the glycosylation ceases when the collagen propeptides fold into a triple helix. Thus, both hydroxylations and glycosylation must occur before triple-helix formation, which is an intracellular process. The exact function of the carbohydrate unit in the collagen molecule is not clear, but it may have some role in the organization of fibrils. For example, an inverse relationship between carbohydrate content and fibril diameter has been observed. Deficiency of galactosylhydroxylysyl glucosyltransferase has been seen in the members of kindred with a dominantly inherited disease known as epidermolysis buflosa simplex. This disease belongs to a group of inherited disorders characterized by blister formation in response to minor skin trauma. Further investigation of this disorder may provide information about the role of glycosylation of collagen. Collagen propeptides contain oligosaccharide units typical of glycoproteins in the proregions of the polypeptide chain (Figure 25-19). These oligosaccharide units may be present in either the N- or C-terminal proregions of the peptide or both. In type I procollagen, the oligosaccharide units are at the C-terminal, whereas in type II procollagen, the units are found at both ends, These oligosaccharicles do not appear in the finished collagen molecule because the proregions are excised. The oligosaccharide units probably are synthesized through

CHAPTER25 RNA and Protein Synthesis _ (Man)n ~' ( G I c N * c ) 2

N-Te rmi nal propeptides Occur in part in globular conformation and in short triplehelical structure

Collagen

C-Terminal propeptides Occur in globular conformation

F I G U R E 25-19 Structure of type I procollagen molecule, which consists of two proot l (I) and one proot2(I) chains. Note the presence of oligosaccharide units at the C-terminal propeptide region. In other collagen types, the oligosaccharide units may be present in either one of the regions or both. The disulfide linkages are also present in these regions. These linkages may be either intra- or interchain linkages and may be involved in the triple-helical formation. In the finished collagen molecule, the N- and C-propeptide regions are absent because they are excised (shown by vertical dotted lines). Glc, Glucose; Gal, galactose; Man, mannose; GlcNAc, N-acetylglucosamine. [Reproduced with permission from D. J. Prockop and K. I. Kivirikko, Heritable diseases of collagen. N Engl J Med 311:376 (1984).1

dolichol intermediates (Chapter 16) and linked to the nascent peptide chains within the rough endoplasmic reticulum. The function of these asparaginyl-linked oligosaccharide units is not known. Since ascorbate is required for hydroxylation reactions, its deficiency causes accumulation of the collagen polypeptides in the endoplasmic reticulum and their eventual secretion. However, these proteins lack the proper modifications and therefore cannot be used for assembly of collagen fibrils. Thus, the role of ascorbate in the hydroxylation of collagen is one of its major physiological roles (see also Chapter 38). Scurvy, a connective tissue disorder, is due to vitamin C deficiency in the diet. Most pathological changes of scurvy can be attributed to failure to synthesize collagen. Scurvy patients have defective blood vessels and poor intravascular support, leading to frequent hemorrhages, defective formation of bone and teeth, and poor wound healing. All of these manifestations can be corrected by administration of vitamin C.

Formation of Disulfide Linkages and Assembly of Procollagen Polypeptides into a Triple Helix The propeptide regions of procollagen polypeptides contain cysteine residues that can form both intra- and interchain disulfide linkages (Figure 25-19). As is the case for other proteins that contain disulfide linkages, it is not known whether synthesis of these linkages requires an enzyme or whether it occurs spontaneously. The assembly of procollagen polypeptides into a triple helix appears to have two requirements:

SECTION25.11 Collagen Biosynthesis and Its Disorders

1. The 4-hydroxylation of at least 100 prolyl residues per prod chain; 2. The formation of interchain disulfide linkages in the C-terminal propeptide. The latter cannot be formed until translation is nearly finished. The rate of disulfide bond and triple-helix formation varies greatly from one cell type to anothermonly minutes in tendon cells that synthesize type I collagen but an hour in cells that synthesize basement membrane collagen. These differences in synthesis time may account for the variations in hydroxylation and glycosylation. Thus, the extent of posttranslational modifications depends not only on levels of enzyme and cofactors but also on the time available.

Translocation and Secretion of Procollagen After the procollagen polypeptides are assembled into a triple helix, they are secreted by the classical route. They pass through the smooth endoplasmic reticulurn and the Golgi complex, where they are packaged into membranous vesicles and secreted into the extracellular space by exocytosis. This process requires ATP and may involve microtubules and microfilaments. The conformation of procollagen markedly affects the rate of secretion. Prevention of the formation of a triple helix (e.g., lack of 4-hydroxylation due to vitamin C deficiency) leads to the accumulation of nonhelical propolypeptides within the cisternae of the rough endoplasmic reticulum and a delayed rate in its secretion.

Extracellular Posttranslational Modification Conversion of procollagen to collagen requires at least two proteases: a procollagen aminoprotease and a procollagen carboxyprotease. The former catalyzes removal of the N-terminal propeptide and the latter removal of the C-terminal propeptide. The two enzymes are endopeptidases, function at neutral pH, require a bivalent cation such as Ca 2+, and show a preference for the helical conformation. There appears to be no preferential order for the cleavage of the propeptides. The conversion of procollagen to collagen by removal of propeptides seems to be essential for the formation of collagen fibrils. This supposition is supported by studies of two heritable diseases, one found in humans and the other in cattle, sheep, and cats. In both, the defect lies in the removal of N-terminal propeptides and results in impaired fibril formation. The human disorder is the type VII variant of Ehlers-Danlos syndrome (Table 25-5). Affected individuals exhibit marked joint hypermobility, dislocation of joints, short stature, and minor changes in skin elasticity. Their skin fibroblasts show normal

589

N-terminal procollagen protease activity; however, the defect resides in the deletions of the exon that encodes the protease cleavage sites, thus preventing normal cleavage of the N-terminal propeptide. In animals, however, the defect is the N-terminal procollagen protease deficiency, which produces skin that is easily torn; thus, the disease is known as dermatesparaxis.

Formation of Collagen Fibrils from Ordered Aggregation of Collagen Molecules The collagen molecules formed by removal of the propeptides spontaneously assemble into fibrils. At this stage, the fibrils are still immature and lack tensile strength, which is acquired by cross-linking. The initial step in cross-link formation is the oxidative deamination of or-amino groups in certain lysyl and hydroxylysyl residues catalyzed by lysyl oxidase. The enzyme is a copper-dependent (probably cupric) protein, and the reaction requires molecular oxygen and pyridoxal phosphate for full activity. Only native collagen fibrils function as substrates.

/ HN \ HC--(CH2)4--NH3 + + 02 NH 3

c/ /%

Lysyl residue

/ HN

\

H

HC--(CH2)3--C-'-O + H20

o/ /% Allysine residue

In a similar reaction, the hydroxyallysine residue is formed from hydroxylysine residues. The aldehyde groups react spontaneously with each other or with amino groups, with formation of intra- and intermolecular linkages. Lysyl oxidase is also involved in the formation of elastin crosslinkages. Several collagen disorders result from defects in the formation of cross-links (Chapter 11). The cross-linking disorders can be due to a hereditary cleficiency of lysyl oxidase, inhibition of lysyl oxidase, deficiency of copper, defects in the formation of cross-links, or defects in their stabilization. The genetic deficiency of lysyl oxidase is characterized by Ehlers-Danlos syndrome type IX (Table 25-5) and some forms ofcutis laxa. Type IX EhlersDanlos syndrome patients exhibit extreme extensibility of the skin, "cigarette-paper" scarring, and easy bruisability. In cutis laxa, the skin is loose and inelastic and appears to be too large for the surface it covers. Some affected individuals exhibit deficiency of lysyl oxidase, presumably

590


limited to skin. The animal counterpart of lysyl oxidase deficiency is seen in several variants of aneurysm-prone mice. All of these disorders are X-linked. Lysyl oxidase undergoes irreversible inactivation when exposed to certain nitriles, fl-Aminopropionitrile (HzN-CHz-CHz-CN), which is found in certain peas (e.g., Lathyrus odoratus), inhibits the lysyl oxidase presumably by binding covalently to the enzyme. Ingestion of these peas results in lathyrism, which is characterized by multiple defects in collagen- and elastin-containing tissues. In experimental animals, lathyrism is produced by administering/3-aminopropionitrile during their active growth period. Lysyl oxidase is also irreversibly inhibited by carbonyl reagents (e.g., hydroxylamine) and copperchelating agents. Impaired cross-linking can also occur as a result of copper deficiency, since lysyl oxidase is a copper-dependent enzyme. In experimental animals, copper deprivation causes skeletal and cardiovascular abnormalities. These abnormalities are similar to a human disorder known as Marfan's syndrome, an autosomal dominant trait whose molecular defect is unknown. A genetic defect affecting the gastrointestinal absorption of copper, known as Menkes' (kinky-hair) syndrome, exhibits neurological, connective tissue, pigmentary, and hair abnormalities. These defects can be explained by deficient activities of various copper-requiring enzymes (Chapter 37). The connective tissue changes are attributed to the deficiency of lysyl oxidase. Cross-linking can be inhibited by agents that can react with the aldehyde groups of allysyl and hydroxyllysine residues. Penicillamine reacts with the aldehyde groups, forming a thiazolidine complex and rendering the aldehyde groups unavailable for cross-link formation.

Penicillamine is an effective chelator of copper, thereby inhibiting lysyl oxidase. Penicillamine is used therapeutically in the treatment of disorders involving copper accumulation (Wilson's disease) or in lead and mercury poisoning. Several inherited disorders of methionine metabolism (Chapter 17) give rise to excessive production ofhomocysteine, HS-CHz-CHzCH(NH~-)COO-, and its excretion in urine. The most common form of homoeystinuria is due to a deficiency of cystathionine synthase (Chapter 17). A major clinical manifestation of homocystinuria is connective tissue abnormalities that are probably due to the accumulation of homocysteine, which either inactivates the reactive aldehyde groups or impedes the formation of polyfunctional cross-links. The structure or the metabolism of collagen is abnormal in a variety of other disorders. For example, in diabetes mellitus (Chapter 22), poor wound healing, atrophy of the skin, and thickening of the basement membranes have been attributed to changes in collagen metabolism. Fibrotic tissue and scars consist primarily of collagen fibrils. The fibrosis is a normal process of wound healing in response to trauma and injury. However, excessive fibrosis in parenchymal tissues (e.g., liver and lungs) can severely limit their function. Thus, attempts are being made to develop specific agents that would inhibit collagen synthesis or increase its catabolism. Both growth and aging involve changes in collagen metabolism. These changes occur in the type, quantity, and quality of collagen. Growth of connective tissues requires the appropriate type and amounts of collagen. It is not known to what extent changes in collagen metabolism are responsible for aging.


H,\ /OH,

N. Ban, P. Nissen, J. Hansen, et al.: The complete atomic structure of the large ribosomal subunit at 2.4 /~ Resolution. Science 289, 905

(2OOO). +

\H

I

H2NmCH

I

COOH

Penicillamine

H,\/OH, S

~

C

CH H

I

COOH Thiazolidine complex

P. Cramer, D. A. Bushnell, J. Fu, et al.: Architecture of RNA polymerase II and implications for the transcription mechanism. Science 288, 640 (2000). D. E. Draper: Protein-RNA recognition. Annual Reviews in Biochemistry 64, 593 (1995). L. Ellgaard, M. Molinari, and A. Helenius: Setting the standards: quality control in the secretory pathway. Science 286, 1882 (1999). R. Green and H. Noller: Ribosomes and translation. Annual Reviews in Biochemistry 66, 40 (1997). H. Guo, M. Karberg, M. Long, et al.: Group II introns designed to insert into therapeutically relevant DNA target sites in human cells. Science 289, 452 (2000). V. Hatzimanikatis, L. H. Choe, and K. H. Lee: Proteomics: theoretical and experimental considerations. Biotechnology Progress 15, 312 (1999). J. W. B. Hershey, M. B. Mathews, and N. Sonnenberg, eds.: Translational Control, Cold Spring Harbor Laboratory Press, 1996.

Supplemental Readings and References E H. von Hippel: An integrated model of the transcription complex in elongation, termination, and editing. Science 281, 660 (1998). M. Ibba and D. $611: Quality control mechanisms during translation. Science 286, 1893 (1999). H. A. James and I. Gibson: The therapeutic potential of ribozymes. Blood 91, 371 1998. G. Kuznetsov and S. K. Nigam: Folding of secretory and membrane proteins. New England Journal of Medicine 339, 1688 (1998). R. Lund and J. E. Dahlberg: Proofreading and aminoacylation of tRNAs before export from the nucleus. Science 282, 2082 (1998). G. W. Muth, L. O-Donnelly, and S. A. Strobel: A single adenosine with a neutral pKa in the ribosomal peptidyl transferase center. Science 289, 947 (2OOO). W. Neuport: Protein transport into mitochondria. Annual Reviews in Biochemistry 66, 863 (1997). V. M. Pain: Initiation of protein synthesis in eukaryotic cells. European Journal of Biochemistry 236, 747 (1996). T. V. Pestova, S. I. Borukhov, and C. U. T. Hellen: Eukaryotic ribosomes require initiation factors I and IA to locate initiation codons. Nature 394, 854 (1998).

591 A. M. Pyle: Ribozymes: a distinct class of metalloenzymes. Science 261, 709 (1993). H. Riezman: The ins and outs of protein translocation. Science 278, 1728 (1997). C. H. Schilling, J. S. Edwards, and B. O. Palsson: Toward metabolic phenomics: analysis of genomic data using flux balances. Biotechnology Progress 15, 288 (1999). E Schimmel and E. Schmidt: Making connections: RNA-dependent amino acid recognition. Trends in Biomedical Science 20, 1 (1995). H. Stark, E. V. Orlova, J. Rinke-Appel, et al.: Arrangement of tRNAs in preand posttranslational ribosomes revealed by electron cryomicroscopy. Cell 88, 19 (1997). M. D. Wang, M. J Schnitzer, H. Yin, et al.: Force and velocity measured for single molecules of RNA polymerase, Science 282, 902 (1998). S. Wikner, M. Maurizi, and S. Gottesman: Posttranslational quality control: folding, refolding, and degrading proteins. Science 286, 1888 (1999). C. L. Will, C. Schnaider, R. Read, et al.: Identification of both shared and distinct proteins in the major and minor spliceosomes. Science 284, 2003 (1999).


CHAPTER

26

Regulation of Gene Expression

The number of molecules of a protein produced per unit time from a particular gene differs from gene to gene, sometimes greatly. This variation can occur in several ways. For some genes, the strength of a promoter or a ribosome binding site is sufficient to regulate the required concentration of a particular protein. However, other gene products are needed only occasionally, e.g., when a particular nutrient is present in the surrounding medium; still other gene products are required only during particular stages of differentiation or in specific cell types. For such genes, the products are usually not present in significant amounts except when gene expression is activated, and the level of gene expression is determined by an on-off switch of some sort. In this chapter, numerous mechanisms are described by which differences in gene expression are achieved.

26.1 Regulation of mRNA Regulation of gene expression is achieved by 1. Regulation of the number of mRNA molecules produced per unit time, 2. Regulation of the translation of mRNA, 3. Regulation of the number of copies of a gene, and 4. Posttranslational modification.

In prokaryotes, mRNA synthesis can be controlled simply by regulating initiation of transcription. In eukaryotes, mRNA is formed from a primary transcript followed by a series of processing events (e.g., intron excision, polyadenylylation). Eukaryotes regulate not only transcription initiation but the various stages of processing as well. An important aspect ofmRNA regulation is determined by the turnover of mRNA molecules, i.e., translation can occur only as long as the mRNA remains intact. In bacteria, mRNA molecules have a lifetime of only a few minutes, and continued synthesis of mRNA molecules is needed to maintain synthesis of the proteins encoded in the mRNAs. In eukaryotes, the lifetime of mRNA is generally quite long (hours or days), thereby enabling a small number of transcription initiation events to produce proteins over a long period of time. Metabolic pathways normally consist of a large number of enzymes; in some cases, the individual enzymes are used in a particular pathway and nowhere else. In these cases, it is efficient to regulate expression of all or none of the enzymes. In bacterial systems, all enzymes of the pathway are encoded in a single polycistronic mRNA molecule, and synthesis of the mRNA produces all the enzymes. In eukaryotes, common signals for transcription of different genes may be used or the primary transcript can be differentially processed to yield a set of mRNA

593

594

CHAPTER26


molecules each of which encodes one protein. Regulation of synthesis of the primary transcripts simultaneously regulates synthesis of all the gene products. In prokaryotes, gene expression fluctuates because bacteria must be able to respond rapidly to a changing environment. However, in the differentiation of cells of the higher eukaryotes, changes in gene expression are usually irreversible, as in the differentiation of a muscle cell from a precursor cell. By changing the number of copies of a gene during differentiation, eukaryotic cells can regulate levels of gene expression.

26.2 Gene Regulation in Prokaryotes Several common patterns of transcription regulation have been observed in bacteria. The particular pattern depends on the type of metabolic activity of the system being regulated. For example, in a catabolic (degradative) pathway, the concentration of the substrate for the first enzyme in the pathway often determines whether all the enzymes in the pathway are synthesized. In contrast, the final product is often the regulatory substance in a biosynthetic pathway. In the simplest mode, absence of an end product stimulates transcription and presence of an end product inhibits it. Even for a gene in which a single type of protein is synthesized from monocistronic mRNAs, the protein may "autoregulate" itself in the sense that the transcriptional activity of the promoter is determined directly by the concentration of the protein. The molecular mechanisms for each regulatory system vary considerably but usually fall in one of two major categories--negative regulation and positive regulation. In negative regulation, an inhibitor, which keeps transcription turned off, is present in the cell and an antiinhibitor--an inducer--is needed to turn the system on. In positive regulation, an effector molecule (which may be a protein, small molecule, or molecular complex) activates a promoter. Negative and positive regulation are not mutually exclusive, and some degradative pathways, such as the utilization of lactose in E. coli, are both positively and negatively regulated.

Lactose (lac) Operon The prototype for negative regulation is the system in

E. coli for metabolizing lactose (lac) (Figure 26-1). The key chemical reaction carried out by the lac system is a cleavage of lactose to galactose and glucose. This reaction enables bacteria to utilize lactose as a carbon source when glucose is not available. The regulatory mechanism of the lac system, known as the operon model, was the

F I ( ; U R E 26-1 (a) Genetic map of the lac operon, not drawn to scale; the p and o sites are actually much smaller than the genes. (b) Diagram of the lac operon in (I) repressed and (II) induced states. The inducer alters the shape of the repressor, so the repressor can no longer bind to the operator.

first system described in detail, and it defines most of the terminology and concepts in current use. The principal regulatory features of the lac operon are as follows: 1. The products of two genes are required for lactose utilization. The lacz gene encodes an enzyme, /3-galactosidase, that degrades lactose; the lacy gene encodes a protein, lac permease, needed to transport lactose and concentrate it within the cell. A third gene, laca, encodes an enzyme that participates in lactose metabolism only in certain circumstances; some lactose-fermenting bacteria do not have the laca gene, and it is not essential for lactose utilization in bacteria. 2. Lac enzymes are encoded in a single polycistronic mRNA molecule (lac mRNA), and enzyme levels are regulated by controlling synthesis of that mRNA. 3. Synthesis of lac mRNA is determined by the activity of a specific regulatory proteinmthe lac repressor. This protein is encoded in the laci gene, which is transcribed in a monocistronic mRNA molecule distinct from lac mRNA. When the repressor is active,

SECTION 26.2

6ene Regulation in Prokaryotes

lac mRNA is not made. This is the negative regulation of the lac operon. Mutants have been isolated that produce inactive repressor proteins; these cells make lac mRNA continuously (constitutive synthesis), even in the absence of lactose. Actually, some lac mRNA is always made at a level of about one or two transcription events per generation because repression is never complete. This basal level synthesis is responsible for a very small amount of the proteins. 4. A sequence of bases (in the DNA)--the operator lac o--is adjacent to the lac mRNA promoter and binds repressor. When the repressor protein is bound to the operator, attachment of RNA polymerase to the promoter is prevented by steric interference, and initiation of transcription of lac mRNA does not occur. A mutation in the operator, which eliminates repressor binding, also leads to constitutive synthesis. 5. An inducer (a small effector molecule) is needed to initiate transcription of lac mRNA. The inducer of the lac operon, which is allolactose, [/~-D-galactopyranosyl-(1 ~ 6)-/3-D-glucopyranose] a structural isomer of lactose formed by basal synthesis of/~-galactosidase, binds to the repressor and alters its three-dimensional structure such that it is unable to bind to the operator. Thus, in the presence of lactose or other inducers the operator is unoccupied and the promoter is available for initiation of mRNA synthesis. It is common to refer to inactivation of the repressor by an inducer as derepression. Thus, the overall regulatory pattern is as follows: A bacterium growing in a medium without lactose does not make either the lac z or lacy product because the repressor that is made is bound to the operator and prevents synthesis of lac mRNA. The cell grows by utilizing whatever other carbon source is available. If lactose is added (and if glucose is absent; see below), basal level lac y and lac z proteins bring the lactose into the cell and convert it to allolactose. As a result, the repressor is inactivated, RNA polymerase binds to the promoter, and synthesis of lac mRNA begins. Lac mRNA synthesis continues until the lactose is exhausted, in which case the inactive repressor would be reactivated and repression reestablished. The lac operon is also positively regulated, presumably because of the role of glucose in general metabolism. The function of/~-galactosidase is to generate glucose by cleaving lactose, so if both glucose and lactose are available, the cell can use glucose and there is no reason for the lac operon to be induced. (The other cleavage product, galactose, is also converted to glucose by enzymes of the galactose operon.) Indeed, when glucose is present in the medium, little or no lac mRNA is made because glucose

595

TABLE 26-1

Concentration of Cyclic AMP in Cells Growing in Media Having the Indicated Carbon Sources Carbon Source Glucose Glycerol Lactose Lactose + glucose Lactose + glycerol

cAMP Concentration Low High High Low High

indirectly prevents RNA polymerase from binding to the lac promoter. This positive regulation is accomplished by cyclic AMP, a regulatory molecule in both prokaryotes and eukaryotes. Cyclic AMP (cAMP) is synthesized from ATP by the enzyme adenylate cyclase, and its concentration is regulated by glucose metabolism. In a bacterial culture that is starved of glucose, the intracellular concentration of cAMP is very high. If a culture is growing in a medium containing glucose, the cAMP concentration is very low. The observation that in a medium containing a carbon source that cannot enter the glycolytic pathway the cAMP concentration is high (Table 26-1) suggests that some glucose metabolite or derivative is an inhibitor of adenylate cyclase. Cyclic AMP does not act directly as a regulator but is bound to a protein called the cAMP receptor protein (CRP), which forms a complex with cAMP (cAMP-CRP). This complex is active in the lac system and in many other operons involved in catabolic pathways. The cAMPCRP complex binds to a base sequence in the DNA in the lac promoter region and stimulates transcription of lac mRNA; thus, cAMP-CRP functions as a positive regulator. The cAMP-CRP complex is not needed for binding of RNA polymerase to the lac promoter, for a free promoter binds the enzyme. However, an open-promoter complex does not form unless cAMP-CRP is also bound to the appropriate DNA sequence. Thus, when glucose is absent, the cAMP concentration is high and there is enough cAMP-CRP to allow an active transcription complex to form; if glucose is present, cAMP levels are low, no cAMPCRP is available, and transcription cannot begin. Thus, the lac operon is independently regulated both positively and negatively, a feature common to many carbohydrate utilization operons (Figure 26-2).

Tryptophan (trp) O p e r o n The tryptophan operon is responsible for the production of the amino acid tryptophan, whose synthesis occurs in

596


CHAPTER 26

that binds to the operator. When the external supply oftryptophan is depleted (or reduced substantially), the operator becomes exposed, and transcription begins. This type of on-off mechanism--activation of an aporepressor by the product of the biosynthetic pathwaymhas been observed in other biosynthetic pathways. When the trp operon is derepressed, which is usually the case unless the concentration of tryptophan in the medium is very high, the optimal concentration of tryptophan is maintained by a modulating system in which the enzyme concentration varies with the concentration of tryptophan. This modulation is effected by F I G U R E 26-2 Three states of the lac operon showing that lac mRNA is made only if cAMP-CRP is present and repressor is absent.

1. Premature termination of transcription before the first structural gene is reached, and 2. Regulation of the frequency of premature termination by the concentration of tryptophan.

five steps, each requiring a particular enzyme. In E. coli, these enzymes are translated from a single polycistronic mRNA. Adjacent to the enzyme coding sequences in the DNA are a promoter, an operator, and two regions called the leader and the attenuator (Figure 26-3). The leader and attenuator sequences are transcribed. Another gene (trpR) encoding a repressor is located some distance from this gene cluster. Regulation of the trp operon is determined by the concentration of tryptophan: when adequate tryptophan is present in the growth medium, there is no need for tryptophan biosynthesis. Transcription is turned off when a high concentration of tryptophan is present and is turned on when tryptophan is absent. The regulatory signal is the concentration of tryptophan itself and, in contrast with lactose, tryptophan is active in repression rather than induction. The trp operon has two levels of regulation~an on-off mechanism and a modulation system. The protein product of the trpR gene~the trp aporepressor--cannot bind to the operator in contrast with the lac repressor. However, if tryptophan is present, the aporepressor and the tryptophan molecule join together to form an active repressor complex

Located between the 5' end of the trp mRNA molecule and the start codon of the trpE gene is a 162-base segment called the leader (a general term for such regions). Within the leader is a sequence of bases (bases 123 through 150) having regulatory activity. After initiation of mRNA synthesis, most mRNA molecules are terminated in this region (except in the complete absence of tryptophan), yielding a short RNA molecule consisting of only 40 nucleotides and terminating before the structural genes of the operon. This region in which termination occurs is a regulatory region called the attenuator. The base sequence around which termination occurs (Figure 26-4) has the usual features of a transcription termination site~namely, a possible stem-and-loop configuration in the mRNA followed by a sequence of eight AU pairs. The leader sequence has an AUG codon that is in-phase with a UGA stop codon; together these start-stop signals encode a polypeptide of 14 amino acids. The leader sequence has an interesting featuremat positions 10 and 11 are two adjacent tryptophan codons. Premature termination of mRNA synthesis is mediated through translation of the leader peptide. The two tryptophan codons make translation of the leader polypeptide

60 Numberof b a s e pairs

,-

"~

i

162 i

~,-

,~,..-.-..-.~ L ~ . . ~

I

po

~r

~~','-

I

t L

I

trpE

1350

1593

1560

-,--'----"-'~-

-,~

I

I

troD

1196 ~,, ~.___...~

trpC

804

300

_,, , ~ - ~ - . - ~

,-~.~

I

trpB

I

J I

trpA !

Spacer

Spacer

Attenuator sequence

(trpa) i~

Regulation

, I~

Enzyme

production

-

F I G U R E 26-3 Escherichia coli trp operon. For clarity, the regulatory region is enlarged with respect to the coding region. The proper size of each region is indicated by the number of base pairs. L is the leader.

SECTION26.2 Gene Regulation in Prokaryotes

597

F I G U R E 26-4 Terminal region of the trp leader mRNA (right end of L in Figure 26-3). The base sequence given is extended past the termination site at position 140 to show the long stretch of U's. The bases (colored lines) form an inverted repeat sequence that could lead to the stem-and-loop configuration shown (segment 3-4 in Figure 26-5).

sequence quite sensitive to the concentration of charged tRNA Trp. If tryptophan is limiting, there will be insufficient charged tRNA Trp and translation will pause at the tryptophan codons. Thus, regulation depends on two characteristics of gene regulation in bacteria. 1. Transcription and translation are coupled. 2. Base-pair formation in mRNA is eliminated in any segment of the mRNA that is in contact with the ribosome. Figure 26-5 shows that the end of the trp leader peptide is in segment 1 and a ribosom is in contact with about 10 bases

in the mRNA past the codons being translated. When the final codons are being translated, segments 1 and 2 are not paired. In a coupled transcription-translation system, the leading ribosome is not far behind the RNA polymerase molecule. Thus, if the ribosome is in contact with segment 2, when synthesis of segment 4 is being completed, then segments 3 and 4 are free to form the duplex region 3 - 4 without segment 2 competing for segment 3. The presence of the 3 - 4 stem-and-loop configuration allows termination to occur when the terminating sequence of seven U's is reached. If exogenous tryptophan is not present or is present in very small amounts, the concentration of charged tRNATrp will be limiting, and occasionally a translating ribosome will be stalled for an instant at the tryptophan codons. These codons are located 16 bases before the beginning of segment 2. Thus, segment 2 will be free before segment 4 has been translated and the 2-3 duplex will form. In the absence of the 3 - 4 stem and loop, termination will not occur and the complete mRNA molecule will be made, including the coding sequences for the trp genes. Thus, if tryptophan is present in excess, termination occurs and little enzyme is synthesized; if tryptophan is absent, there is no termination and the enzymes are made. At intermediate concentrations, the frequency of ribosome pausing will be such as to maintain the optimal concentrations of enzymes. This tryptophan regulatory mechanism is called attenuation and has been observed for several amino acid biosynthetic operons, e.g., histidine and phenylalanine.

Ribosome ~ ,

"l'rp codons

1 ~: ~ 1~2

i ~

~

~

4

(a) Free mRNA. Base pairs between 1 and 2 and between 3 and 4.

End of transc ription

ontinuationof E ' transcripti~

(b) Low concentration of tryptophan. Ribosome .stalled in region 1 permits formation of 2-3 before transcription of region 4 is completed.

F I G U R E 26-5 Model for the mechanism of attenuation in the E. coli trp operon.

3~

(c) Highconcentration of tryptophan. Ribosome reaches region 2 before region 4 is completed, and permits formation of 3-4.

590

CHAPTER26


Roughly halfway through the life cycle of the phage this promoter enters the cell, transcription occurs by T7 RNA polymerase, and the structural proteins and lysis enzymes are synthesized. Lysis does not occur until phage particles have been assembled. Phage T4 has numerous promoters only a few of which can be recognized by E. coli RNA polymerase. However, unlike T7, the late promoters are made available by successive modification of the E. coli enzyme. These modifications are of two types: addition of phage-encoded protein subunits and chemical modification of preexisting subunits. Temporal regulation occurs because the gene responsible for the first modification is encoded in the first set of mRNAs, that for the second modification in the second set, and so on. To ensure that the late mRNA, which encodes the structural proteins and the lysis enzyme, is not synthesized until adequate DNA has been made by the replication system, the template for this late mRNA cannot be parental T4 DNA but must be a replica. E. coli phage A also uses E. coli RNA polymerase throughout its life cycle. With this phage, regulation of transcription is accomplished in two ways: modification of the positive regulatory elements, such as the cAMPCRP complex, needed for recognition of certain promoters; specific repressors are also used to turn off expression of certain genes. Figure 26-6 shows a portion of the genetic map of ~., which includes four regulatory genes (cro, N, Q, and cll), three promoters (pL, pR, and pR2), and five termination sites (tL1, tRl, tR2, tR3, and tR4). Seven mRNA molecules are also shown; the L and R series are transcribed leftward and rightward respectively from complementary DNA strands. The genes O and P encode proteins required for k DNA replication, and the late genes encode the structural proteins of the phage and the lysis enzyme. The basic transcription sequence is: Two early mRNAs, LI and RI, are made that encode the regulatory proteins N and Cro, respectively. N is an antitermination

Some bacterial operons are regulated solely by attenuation without repressor-operator interactions.

Temporal mRNA Regulation in Phage Systems The lac and trp operons are regulated by molecules that turn gene expression on and off in response to the concentration of nutrients. However, some organisms require regulation of genes in time. For example, in the production of phage (bacteriophage or bacterial viruses) in an infected cell, various biosynthetic activities must occur on schedule. When a bacterium is infected by a phage, the enzymes responsible for breaking open the cell and releasing progeny phage must act late in the life cycle after phage DNA replication and packaging has occurred; otherwise, the cell could burst before any phage formed. Almost all phage systems accomplish this temporal regulation by selective and sequential transcription of particular sets of genes. This transcription gives rise to various classes of mRNA which are usually termed early and late mRNA. Three E. coli phage systems, which accomplish temporal regulation in different ways, are described briefly below. Phage T7 contains several promoters, but only one is recognized by E. coli RNA polymerase. Transcription of T7 DNA begins at this promoter. Two important proteins are encoded there by transcript (early mRNA). One phosphorylates and inactivates the E. coli RNA polymerase, thus preventing E. coli from making any bacterial mRNA. The second protein is a new RNA polymerase that does not recognize any E. coli promoters but is active at the remaining phage promoters. In the absence of E. coli RNA polymerase, the early mRNA is no longer made, but the new T7 RNA polymerase makes the second transcript, which encodes the DNA replication enzymes and other proteins needed early in the life cycle. Transcription at the third promoter is delayed because injection of the phage DNA into the bacterium occurs very slowly.

DNA

att

int

xis

red

N tL1

~ ~ nutL pL oL L1

cl ~ oFt

~ cro pR

c,

nutR tR1

Late genes

o

tR2

tR3

pR2

R1 hr

tR4 (qut)

J

R2 J

R4 v

F I G U R E 26-6 Genetic map of the regulatory genes of phage ~. Genes are listed above the line; sites are below the line. The mRNA molecules are heavy lines. The dashed black arrows with thin lines indicate the sites of action of the N, Cro, and Q proteins.

j

SECTION 26.3

6ene Regulation in Eukaryotes

protein and enables E. coli RNA polymerase to ignore certain transcription termination sites and thereby extend synthesis of these mRNAs. It acts together with a bacterial protein, NusA, by binding to a site called nutR in the DNA. When RNA polymerase, which has initiated transcription at pR, reaches this site, it picks up the N-NusA complex and is thereby modified such that it is able to ignore the tR1 and tR2 terminators. A similar site, nutL, is present downstream from the pL promoter. Because of this antitermination effect, in time RI is extended until the DNA replication proteins and another regulatory protein, Q, are made. Q is also an antitermination protein. A small constitutively synthesized RNA, R4, is made from the outset of the infection. The Q protein binds to a site (qut) downstream from the promoter for R4, causing RNA polymerase to antiterminate, and R4 is extended to form an mRNA that encodes the head, tail, and lysis proteins. From the extended R1 mRNA, the gene-Cro protein is made. The concentration of Cro ultimately reaches a value at which the protein dimerizes, producing the active form. The Cro dimer acts as a repressor at both pL and pR; this activity turns off synthesis of early proteins that are no longer needed and prevents excess synthesis of DNA replication proteins. Lambda, like many phages (but not T4 or T7), can engage in two alternative life cycles. In the lytic cycle, progeny phage particles are produced and the cell ultimately lyses, releasing the phage to the surrounding medium. In the lysogenic cycle, injected phage DNA is repressed and becomes inserted into the bacterial chromosome. At a later time, if the bacterial DNA is damaged sufficiently, the phage DNA is excised and a lytic cycle ensues. DNA damage, in a complicated way, leads indirectly to inactivation of the cell repressor and subsequent excision of the phage DNA and production of progeny phage. The int gene (Figure 26-6) encodes the enzyme that causes insertion of the phage DNA into the bacterial chromosome. Neither the cI repressor nor the Int protein is needed in the lytic cycle; however, and both are needed in the lysogenic cycle and are coordinately regulated. The two products, CI and Int, are encoded in different mRNA molecules but synthesis of the mRNA is initiated by a common signal. The product of the gene clI is a positive regulatory element. Like the cAMP-CRP complex in the lac operon, the cH product must be bound to the DNA adjacent to the promoters for the mRNAs encoding the cI and Int proteins. In this way, the choice of the lytic versus the lysogenic cycle depends on the concentration of the cII protein. If the concentration is low, neither cI repressor nor Int is made and the lytic cycle is followed; if the concentration is high, both cI repressor and Int are made and the lysogenic cycle occurs. The concentration

599

of clI protein is regulated in response to environmental influences.

Regulons One fairly well-studied regulon is the heat-shock or hightemperature protection regulon (htp) which synthesizes a variety of proteins when bacteria are exposed to temperatures above 40~ In E. coli, a single protein, the product of the htpR gene, is a positive effector of all mRNAs of the regulon. The C-terminal end of the HtpR protein is homologous to the RNA polymerase cr subunit, which suggests that the heat-shock response involves a reprogramming of RNA polymerase by this cr-like protein, enabling RNA polymerase to initiate transcription at a class of promoters that is not otherwise recognized. The inducible SOS repair system was described in Chapter 24. This system allows the frequency of replication errors to increase when repair is necessary and is regulated in order to keep the normal error frequency low. Since the repair system is needed only following certain types of DNA damage, some feature of the damage may act as the inducer. Several genes representing functions not directly related to repair are also components of this system, which is called the SOS regulon. Common to all mRNAs of the SOS regulon is an operator region to which a repressor, LexA, binds. When DNA is damaged, a protein called RecA binds to the damaged segment. Binding causes a conformational change in the protein and converts it to a specific protease that is active against only a small number of repressor proteins, LexA among them. Cleavage of LexA prevents it from binding to the SOS operator, so transcription of all mRNAs of the SOS regulon occurs, yielding Uvr enzymes, RecA, and SOS repair proteins. LexA normally represses its own synthesis (it is autoregulated). However, the large amount of RecA protein made after the LexA protein has been cleaved continues to be activated for proteolysis and cleaves LexA; thus, all proteins of the SOS regulon continue to be made. Once DNA repair is completed, RecA loses its proteolysis activity, and LexA is no longer cleaved. Without RecA protease activity, newly made LexA rapidly accumulates, binds to the SOS operators, and turns off the SOS regulon; thus, the state of the cell existing before DNA damage occurred is reestablished.

26.3 Gene Regulation in Eukaryotes Regulationof gene expression in eukaryotes proceeds primarily by control of transcription as in prokaryotes. Some systems are also regulated at the translational level.

600 However, synthesis of mRNA in eukaryotes is not a simple matter of initiation at a promoter, as it is in prokaryotes, but includes several steps in which the primary transcript is converted to mRNA (Chapter 25). Control of these processing steps is also used to regulate gene expression in eukaryotes. A variety of differences between the regulatory mechanisms of prokaryotes and eukaryotes are evident. First, transcription of related genes is initiated in response to a single signal. Second, a single polygenic primary transcript may be differentially processed to yield a set of distinct mRNA molecules each encoding one protein. Third, large proteins can be processed into small, active polypeptides. The second and third mechanisms are unique to eukaryotes. Housekeeping Genes a n d RNases Many proteins in eukaryotic cells such as those involved in glycolysis (Chapter 13) are needed continuously; the genes encoding such proteins are called housekeeping genes. A variety of regulatory mechanisms have evolved to ensure a constant supply of these gene products. In some instances the amount of each protein may be regulated by the strength of the promoter and ribosomal binding site. However, in housekeeping genes with strong promoters, the gene product functions as a repressor and binds to a site adjacent to the gene, thus regulating the level of transcription. This mechanism is called autoregulation. If the gene product is in short supply, transcription is activated; as the concentration of the product increases, the level of transcription is reduced. In bacteria, mRNAs are rapidly degraded, which is essential for an organism that must adapt to rapidly changing environments. The RNase E protein (product of the rne gene) is essential in controlling the stability of mRNAs in E. coli. The RNase E mRNA, in turn, is autoregulated; an amino terminal fragment of RNase E acts as a repressor of the rne gene. In human cells an analogous RNase activity has been purified using antibodies against RNase E and both RNases recognize the same sequence (AUUUA). At least four RNase families can be identified in eukaryotes that have significant homology to pancreatic RNases. Overall, it is estimated that there should be as least 100 different eukaryotic RNases. Because of the crucial roles RNases and their structural homologues play in regulation of gene expression, splicing of primary transcripts, organogenesis, and other cellular activities, RNases are often referred to as housekeeping enzymes. RNase activity in serum and cell extracts is elevated in a variety of cancers and infectious diseases. The level of RNases is regulated by both activators and inhibitors. The

CHAPTER 26


TABLE 26-2 Diseases Associated with Elevated Levels o f RNase Activity in Body Fluids or Cell Extracts Disease

Colorectal adenocaricnoma CML monocytes Chronic fatigue syndrome Liver diseases Chronic myeloid leukemia (CML) Pancreatic necrosis Neurological infectious diseases

RNase

RNase L RNase L RNase L poly-C degrading RNase >200 folds increase in endoribonuclease activity pancreatic RNase poly-C degrading RNase

cellular content of RNase depends on both regulation of endogenous synthesis as well as the uptake of enzymes synthesized in the pancreas. Because of the multiple regulatory roles of RNases in cells, elevated levels are associated with a wide spectrum of diseases (Table 26-2) and serve as useful markers in diagnosis. RNases also are being investigated for antifungal, antiviral, and antitumor activity, and drugs are being developed that are based on specific RNase activities. Gene Families

Eukaryotic genes are not arranged in operons, since each mRNA contains only one gene. However, many systems are grouped into gene families either because of their location or, more commonly, because of their related function. For example, the set of tRNA genes, whose transcription is correlated by some unknown mechanism, is an example of a gene family. Another example, whose regulation is particularly simple, is the family of rRNA genes. Eukaryotic ribosomes contain one copy each of 5S, 5.8S, 18S, and 28S rRNA, and these are synthesized in equal numbers. The 5.8S, 18S, and 28S rRNA molecules are components of a single transcript that is cleaved to yield one copy of each molecule. The 5S rRNA is, however, part of another transcript. How the ratio of the 5S rRNA to the others is maintained is not known; possibly, the promoter strengths of the two transcripts are the same. However, this kind of gene organization is not particularly common, and related genes often reside on different chromosomes. Of particular interest are the developmentally regulated gene families. Similar, but not identical, proteins having the same properties are made at different stages of development. A well-studied example is hemoglobin, a tetrameric protein containing two o~ subunits and two /~ subunits.

SECTION 26.4

Mechanisms of Gene Regulation in Eukaryotes

There are several different forms of both o~ and/3 subunits differing by only one or a few amino acids, and the forms that are present depend on the stage of development of an organism (Chapter 28). For example, the following temporal sequence shows the subunit types present in humans at various times after conception. Subunit Embryonic type (< 8 wk)

Fetal Adult (8-41 wk) (birth--+ henceforth)

or-like /3-like

o/ y G and yA

~2 --+ ~1 ~

o/ /~ and

Both the or- and fl-like genes form distinct gene clusters. A property of each cluster is that the order of the genes is the order in which they are expressed in development.

601

TABLE 26-3

The Role of General Transcription Factors in Initiation of Gene Expression in Human Cells Factor

Subunits*

TFIID

13

TFIIA TFIIB TFIIE TFIIF

3 1 2 2

TFIIH

9

Function

Recognizes TATA box; recruits TFIIB (composed of factors TBP and TAFs) Stabilizes TFIID binding Orients RNAPII to start site Recruits TFIIH (helicase) Destabilizes nonspecific RNAPII-DNA interactions Promotes promoter melting by helicase activity

*RNAPII contains nine subunits.

26.4 Mechanisms of Gene Regulation in Eukaryotes Gene expression in human cells is regulated primarily at the level of transcription as it is in prokaryotic cells. However, because transcription is more complex in eukaryotic cells, gene expression can be regulated at many different stages:

1. Initiation of transcription, 2. Processing the primary transcript (this includes capping, splicing out introns, addition of the polyA tail, and RNA editing in which specific nucleotides are changed), 3. Transport of the processed mRNA from the nucleus to the cytoplasm, 4. Translation of the mRNA (synthesis of protein), and 5. Posttranslational modification of the protein(s). Each of these regulatory steps is crucial to the proper functioning of the cell and the organism. Transcription Factors The synthesis of mRNA molecules by RNA polymerase II (RNAPII) is a multistep process requiring the interaction of numerous proteins with DNA in the region of the promoter (Chapter 25). However, before the expression of a gene can be initiated, the transcription factor TFIID must attach to the promoter. This is followed by the attachment of other proteins, called general transcription factors (GTFs), to DNA in the region of the promoter. A description of the functions of some human GTFs is given in Table 26-3. The various GTFs facilitate attachment of RNAPII to the promoter at the correct nucleotide for initiation, destabilize the DNA at the promoter, and initiate transcription; together the GTFs and RNAPII are called

the preinitiation complex. Once the first phosphodiester bond has formed, transcription has been initiated. The large number of GTFs required to initiate transcription does not completely solve the transcription problem. DNA is organized in chromatin and is wrapped around histone proteins and tightly packaged into nucleosomes (Chapter 24). These structures inhibit transcription as well as DNA replication. A distinct class of transcription factors has been identified that modifies chromatin structure so that transcription can occur. A complex of proteins termed RSF (remodeling and spacing factor) facilitates transcription initiation on chromatin templates in vitro. Another protein complex termed FACT (facilitates chromatin transcription) promotes elongation of RNA chains through nucleosomes. Together, RSF and FACT disassemble nucleosomes and permit transcription initiation to occur. In addition to the GTEs that control the initiation of transcription at the TATA box and the protein factors that disassemble nucleosomes, other transcription factors are required to regulate the expression of particular genes or families of genes. Each transcription factor binds to DNA at a specific sequence, which ensures specificity (Table 26-4). The large number of transcription factors have certain structural motifs that facilitate their binding to DNA and, based on structural similarities, fall into four distinct groups (Figure 26-7).

1. Zinc finger: The structure of the zinc finger motif (discovered in TFIIA) consists of two cysteine residues and two histidine residues separated by 12 amino acids. The two cysteine residues are separated by two amino acids and the two histidine residues are separated by three amino acids. The cysteine and histidine residues are linked by a zinc ion and this

60~

CHAPTER 26

Regulationof Gene Expression

TABLE 26-4 Promoter and Enhancer Controlling Elements and Transcription Factors That Bind to Them Response Element

Consensus Sequence*

Factor

TATAAAA

TATA Box

TFIID

GGCCAATCT

CAAT Box

CTG/NF1

GGGCGG

GC Box

SP1

ATTTGCAT

Octamer

Octl, Oct2 H e a t s h o c k factor

CNNGAANNTCCNNG

HSE

TGGTACAAATGTTCT

GRE

Glucocortoid receptor

CAGGGACGTGACCGCA

TRE

Thyroid receptor

CCATATTAGG

SRE

Steroids

*Consensus sequence is for either a promoter or enhancer element. The letter N stands for any nucleotide (A,G, C,T).

NH 2

NH 2

0-10--0---0

9

0

x 0

."

-.

I

I I

0

0

; o

H2N ~ "

/

0

o Zinc finger

~

X

COOH

COOH

Leucine zipper

9"O~cOOH

NH 2

COO

a- Ihelix

c~- helix ~

Helix - loop - helix COOH

Helix - turn - helix F I G U R E 26-7 Structures of transcription factors that facilitate binding to DNA at specific sequences to regulate gene expression.

SECTION26.4 Mechanisms of Gene Regulation in Eukaryotes

motif is repeated nine times in TFIIA protein. The 12 amino acids between the cysteine and histidine residues form a loop that can interact with the major groove in DNA; this loop is called the "zinc finger." 2. Leucine zipper: The structure of the leucine zipper (first discovered in C/EBP, CCAAT, and enhancer binding protein) consists of four leucine residues in an or-helical segment of the protein. Two polypeptides join to form a Y-shaped dimer whose arms can interact with the major groove of DNA. The stem of the Y-shaped structure is known as the "leucine zipper." 3. Helix-turn-helix (HTH): This structure (first discovered in the )~ bacteriophage repressors, cI and cro) consists of two o~-helical regions of a protein joined by an amino acid sequence that allows the o~ helices to turn. The two o~ helices form a dyad axis of symmetry, a structure that binds tightly to the major groove of DNA. 4. Helix-loop-helix (HLH): This structure is similar to the helix-turn-helix in that it consists of two oe-helical segments joined by a long sequence of amino acids that can form a loop. This gives the two o~-helical segments more flexibility so that the proteins can fit into the large groove of DNA at some distance from one another. This may facilitate looping of the DNA to make enhancer or promoter sites more accessible.

603

The serum albumin gene is one example of a gene that is regulated by several transcription factors. Although this gene is present in all tissues, it is only expressed in liver and spleen. The gene is activated by five different transcription factors that bind DNA in a region located between the CCAAT and TATA boxes (Figure 26-8). The transcription factor NF- 1 actually binds to the right of the CCAAT box. Binding sites for transcription factors are determined by changing bases in the DNA one at a time and observing changes in the binding of transcription factors.

Steroid Receptors Steroid hormones perform many functions in cells, one of which is to activate gene expression by binding to steroid receptors, proteins in the cytoplasm that, when activated, act as factors that initiate transcription. All steroid hormones are derived from cholesterol and, as a result, have similar chemical structures. Steroid hormones differ one from another primarily in hydroxylation of particular carbon atoms and by aromatization of the steroid A ring of the molecule. Once a steroid hormone binds to a steroid receptor protein, the complex undergoes a series of structural changes that result in the complex binding to DNA at a particular sequence called a steroid response element (SRE)

FIGURE 26-8 Expression of the human serum albumingene is regulated by five transcription factors, four of which bind to the promoter region.

60~

CHAPTER 26

located at some distance upstream or downstream from the promoter. Steroid receptor proteins are synthesized from a gene family that shows a high degree of homology in the DNA binding region. All steroid receptors belong to one of five classes: androgen receptor (AR), estrogen receptor (ER), glucocorticoid receptor (GR), mineralocorticoid receptor (MR), and progesterone receptor (PR). Also, all receptors contain a zinc finger motif which, if altered by mutation, destroys the steroid receptor's function.

G-Protein Diseases Mutations that alter the functions of trimeric guanine nucleotide-binding proteins (G-proteins) are responsible for wide range of diseases (Table 26-5). While most of these diseases are quite rare, the identification of an altered G-protein in hypertension is estimated to affect as many as 15% of patients with hypertension. G-proteins relay signals from hormones, neurotransmitters, and other extracellular agents to different intracellular effectors that regulate enzymes and ion channels. G-proteins are responsible for signal transfer from more than 1000 cell receptors. Changes in G-proteins cause excessive or insufficient transmission of signals that are essential for proper cellular functioning. G-proteins consist of three subunits: an ot subunit that is loosely bound to a dimer consisting of a 13 subunit that is tightly bound to a V subunit. Human chromosomes contain genes for 16ot, 6fl, and 12y subunits. Thus, a large

TABLE 26-5 Diseases Caused by Mutations That Alter the Signal Activities of G Proteins

Diseases Excess Signal Pituitary and thyroid adenomas Adrenal and ovarian adenomas McCune-Albright syndrome Cholera

G Proteins

G3ot Gi2o~ Gsot G2o~

Deficient Signal Pseudohypoparathyroidism type Ia Nightblindness

G5oL

Deficient or Excessive Signal Pertussis Essential hypertension

GioL

GtoL

133


variety of Got proteins define different G-protein trimers that regulate signaling pathways such as cAMP synthesis, cGMP breakdown, closing of Ca 2+ channels, and opening of K + channels. The activity of G-proteins is regulated by the binding and hydrolysis of guanosine triphosphate (GTP) by the Got subunit. If guanosine diphosphate (GDP) is bound to the ot subunit, it will associate with the fly subunits, but the trimer is inactive. When a cell receptor is activated, the trimer releases GDP and the ot subunit is able to bind GTE After binding GTP the ot subunit dissociates from the/3 V subunit and from the receptor. Either the ot-GTP complex or the/3 9/subunits can then activate downstream effectors. Thus, defects in any of the numerous G-proteins may alter cellular biochemistry and cause disease.

Regulation of Transcription by Methylation Most cells contain several enzymes that transfer a methyl group from S-adenosylmethionine (Chapter 17) to cytosine or adenine in DNA. These DNA methylases are both base-specific and sequence-specific. Another enzyme that methylates cytosine only in particular CpG sequences also is important for transcription. In a few cases, particularly in vertebrates, methylation of CpG sequences prevents transcription of some genes. Evidence for a regulatory role of methylation is: 1. Certain genes are heavily methylated in cells in which the gene is not expressed and unmethylated in cells in which the gene is expressed. 2. In vitro methylation of the upstream site of a cloned v-globulin gene prevents transcription. Methylation outside the upstream sequence does not inhibit transcription. 3. If the base analogue 5-azacytidine is added to cultures of growing cells, newly made DNA contains the analogue, and methylation of CpG sites containing the analogue fails to take place (5-azacytidine is also a general inhibitor of many methylases). Cells in such cultures gain the ability to make proteins whose synthesis is normally turned off (i.e., in cells growing in medium lacking the base analogue). 4. The housekeeping genes, which provide for general cell function and which are continuously transcribed, are rarely methylated in or near their initiation regions. Undifferentiated and precursor cells often replicate. If methylation actually prevents gene expression in some types of cell, an inhibitory methylated site must be inherited as a methylated site in a daughter strand during DNA


605

FIGURE 26-9 Mechanism by which the pattern of methylation in parental DNA is inherited in daughter molecules. The rule is that a C in a CpG sequence can be methylated only if the C in the complementary CpG sequence is already methylated. For clarity only, methyl groups have been drawn outside the sugar-phosphate strands.

replication. This requires methylation of a sequence complementary (rather than identical) to a methylated site. A property of certain DNA methylases shows how a methylated parental strand can direct methylation of the appropriate daughter sequence, i.e., these enzymes only methylate CpG (embedded in certain surrounding sequences) and only when the CpG in the opposite strand is already methylated (Figure 26-9). In this way, the methylation pattern of parental DNA strands is inherited by the daughter strands. The sex chromosome composition of human males and females is XY and XX, respectively. If cells contain more than one X chromosome, all except one are inactivated. The cells of XX females contain a structure called a Barr body, which is a condensed, heterochromatic, transcriptionally inactive X chromosome. The cells of XXY males also contain a Barr body. XXY males suffer from Klinefelter's syndrome; they are usually mentally retarded, sterile, and suffer from physiological and developmental abnormalities. It is thought that extra X chromosomes must be prevented from gene expression to preserve the correct gene dosage in both males and females. Evidence for inactivation of one X chromosome in XX females comes from the observation that females are mosaic for X-linked alleles that are heterozygous. For example, a woman who is heterozygous for a gene that controls production of sweat glands has patches of skin that perspire and patches that do not. Cells in the patches of skin

that do not perspire express the mutant allele, and the wild type is silent in the inactive X chromosome. Cells in the patches of skin that do perspire express the wild type allele, and the mutant allele is silent. X chromosome inactivation occurs early in embryonic female development and the X chromosome that is inactivated is selected at random. Once selected, the same X chromosome is inactivated in subsequent cell divisions, which explains why patches of skin and other tissues differ in the expression of X-linked heterozygous genetic loci. Inactivation of the X chromosome is thought to occur in three phases: initiation, spreading, and maintenance. Initiation involves the choice of chromosome to be inactivated and depends on a unique genetic locus on the X chromosome called the X-inactivation center (X/c). Once one X-inactivation center has been triggered in a cell, the other X chromosome is protected from inactivation by a "blocking signal." The inactivation signal spreads from the Xic locus, eventually to inactivating almost all of the genes on that X chromosome. However, at least 19 genes on inactive X chromosomes have been shown to have some transcriptional activity, so inactivation is not complete. Specific maintenance mechanisms ensure that the inactive X chromosome is clonally transmitted and that it remains inactive in subsequent cell divisions. Methylation of CpG islands at the beginning of genes in the inactive X chromosome appears to play an important role in suppressing gene expression; the corresponding CpG islands in the active X chromosome are usually unmethylated.

606

Genomic Imprinting Approximately two dozen autosomal genes in humans are inherited in a silent state from one parent and in a fully active state from the other parent. Such genes are said to be "imprinted" by the parents during gamete formation and the phenomenon is referred to as genomir imprinting, which does not change the nucleotide sequence of DNA. Rather, it is an epigenetic phenomenon in which the DNA at a particular locus is marked according to gender, and this determines whether or not the locus is expressed during embryonic development. Although regarded as a normal epigenetic phenomenon, two clinically different disorders are characterized by genomic imprinting. Prader- Willi and Angelman syndromes are distinct disorders associated with multiple abnormalities and mental retardation. Both disorders are caused by mutations at the proximal end of chromosome 15 that silence one or more genes. In Prader-Willi syndrome, the maternally inherited chromosome is silent; in Angelman syndrome, the paternally inherited chromosome is silent. For both syndromes, a small deletion is usually responsible for the genomic imprinting that silences the relevant gene(s). Both syndromes can also result from having both copies of chromosome 15 derive from only one parent, a condition called uniparental disomy. Angelman syndrome is caused by mutation in a single gene whereas Prader-Willi syndrome is caused by mutations in more than one gene. Thus, Angelman syndrome can also be caused by mutations in the responsible gene itself, which is not the case for PraderWilli syndrome. Diagnosis of both can be confirmed by analysis of DNA methylation in the respective genes since methylation is the mechanism used for imprinting.

Regulation of RNA Processing Initiation of transcription ultimately leads to production of a primary transcript, which in higher eukaryotes is processed to form an mRNA. Alternative processing patterns can yield different mRNAs. One example comes from chicken skeletal muscle in which two forms of the muscle protein myosin, LC1 and LC3, are produced. The myosin gene has two different TATA sequences that yield two different primary transcripts. These two transcripts are processed differently to form mRNA molecules encoding distinct forms of the protein (Figure 26-10). Another example of regulation of RNA processing is found in adenovirus, which has only a few promoters but which makes numerous primary transcripts and a large number of mRNAs. A fairly small amount of DNA is used efficiently because proteins are translated in all reading

CHAPTER26 Regulation of Gene Expression

frames and different regions of the DNA are, by virtue of distinct RNA splicing patterns, used to form different proteins. The amount of each mRNA is regulated with respect to the time after infection. RNAs 1 and 2 are both formed shortly after the primary transcript is made, although more of mRNA-2 is made. Later in the viral life cycle, mRNA- 1 is not made and mRNA-2 is abundant. If cycloheximide, an inhibitor of protein synthesis, is added to the infected cells before the shift in splicing pattern takes place, the shift does not occur. This inhibition implies that a newly synthesized protein is a positive effector of the shift. Cycloheximide has a similar effect on other mRNA species derived from other primary transcripts at late times. Adenovirus has a single promoter for all RNA made late in the cycle of infection. The primary transcripts terminate at five major polyadenylation sites. Each termination site influences the splicing pattern by allowing particular introns or intron termini to be present or not. The five sets are not used with equal frequency, with the result that most mRNAs encoding various genes are not the same. This is the primary mechanism for determining the relative amounts of the different structural proteins synthesized late in the adenovirus life cycle. Another type of regulation of processing involves choice of different sites of polyadenylation. One example is the differential synthesis of the hormone calcitonin in different tissues; another is the synthesis of two forms of the heavy chain of immunoglobulins (Chapter 35). In both cases, the differential processing includes distinct patterns of intron excision (i.e., splicing), but they are necessitated by an earlier event in which differential poly(A) sites are selected from the primary transcript. That is, when the poly(A) site nearer the promoter is selected, a splice site used in the larger primary transcript is not present, so a different splice pattern results. Thus, slightly different proteins are synthesized. The calcitonin gene consists of five exons and uses two alternative polyadenylation sites that respond to different signals in different tissues. In the thyroid calcitonin is produced by a signal that produces a pre-mRNA consisting of exons 1-4. The introns are then spliced out to give the mature mRNA. However, in neural tissue a different polyadenylation site is activated that produces a pre-mRNA consisting of exons 1-5 and the intervening introns. When this larger pre-mRNA is processed, the introns are spliced out but so is exon 4, producing a mature mRNA consisting of exons 1-3 and 5. This is translated into a growth factor called calcitoninrelated gene peptide (CRGP). Mutations in the calcitonin gene can result in both adrenal and thyroid tumors,


607

F I G U R E 26-10 Chicken LC1/LC3 gene. Two distinct TATA sequences lead to production of different primary transcripts, which contain the same coding sequences. Two modes of intron excision lead to the formation of distinct mRNA molecules that encode proteins having different amino terminal regions and the same carboxy terminal regions.

conditions that are classified as multiple endocrine neo-

plasias. Alternative RNA Splicing and Editing Most primary transcripts in eukaryotic cells derive from complete removal of all introns and complete joining of all exons. This results in only one species of mature mRNA being synthesized from each primary transcript. However, many eukaryotic genes can give rise to many different forms of mature mRNAs using different promoters and different polyadenylation sites (discussed above), as well as by alternative splicing. This form of regulation of RNA processing is especially useful because a single gene can be expressed differently at various developmental stages or in different tissues of the same organism. One example is the troponin T gene that synthesizes the fast skeletal muscle protein. This gene consists of 18 exons; however, only 11 are found in all mature mRNAs. Five of the exons can be included or excluded and two are mutually exclusive; if one is included, the other is excluded. Altogether, 64 different mature mRNAs can be produced by alternative splicing of the primary transcript of the troponin T gene. RNA editing involves processing of RNA in the nucleus by enzymes that change a single nucleotide (insertion, deletion, or substitution). One example is apolipoprotein B (apo B) gene. In the liver this gene produces a 4536-amino-acid protein (apo B100), whereas in the small intestine the same gene produces a 2152-amino-acid protein (apo B48). The truncated protein is identical in

amino acid sequence to the first 2152 amino acids in apo B 100. This occurs because in cells of the small intestine one nucleotide (at position 6666) is edited by deamination of a cytosine residue. The conversion of a cytosine to uracil at this position produces a stop codon that terminates translation and produces the truncated protein. Thus, selective editing of mRNAs prior to translation in specific tissues is used to produce different proteins.

Regulation of Iron Utilization in Cells Iron is an important constituent of human cells and regulates many biochemical functions (Chapter 29). Iron acts as an effector molecule in the translation of several mRNAs by binding to either 3' or 5' stem-loop structures, called iron-response the elements (IREs), that flank the coding sequences for several genes whose expression is regulated by iron. In particular, the inRNAs for the transferrin receptor, light and heavy chains of ferritin, an crythrolytic form of aminolevulinic acid synthetase, and a form of mitochondrial aconitase are regulated by IREs and an IRE-binding protein (IRE-BP). The IRE in the ferritin in RNA is in the 5' flanking sequence. Translation of the mRNA is regulated by binding of IRE-BP whose activity, in turn, is regulated by the concentration of iron in the cell. In contrast, the IRE of transferrin mRNA is in the 3' flanking sequence; binding of IRE-BP to this IRE regulates turnover of the mRNA. This example illustrates the variety of ways that gene expression can be regulated under varying physiological conditions in human cells.

60~

Polyproteins In prokaryotes, coordinated regulation of the synthesis of several gene products is accomplished by regulation of the synthesis of a single polycistronic mRNA molecule encoding all of the products. The analogue to this arrangement in eukaryotes is the synthesis of a polyprotein, a large polypeptide that is cleaved after translation to yield individual proteins. Each protein can be thought of as the product of a single gene. In such a system, the coding sequences of each gene in the polyprotein unit are not separated by stop and start codons but instead by specific amino acid sequences that are recognized as cleavage sites by particular protein-cutting enzymes. Polyproteins have been observed with up to eight cleavage sites; the cleavage sites are cut not simultaneously but rather in a specific order. Use of a polyprotein serves to maintain an equal molar ratio of the constituent proteins; moreover, delay in cutting at certain sites introduces a temporal sequence of production of individual proteins, a mechanism frequently used by animal viruses. Some polyproteins are differentially cleaved in different tissues. An example is proopiomelanocortin, a polyprotein that is the source of several hormones synthesized in the pituitary gland. In the anterior lobe of the pituitary, the polyprotein is cleaved to release/3-1ipotropin and adrenocorticotropic hormone (ACTH). In the intermediate lobe, a different pattern of cleavage forms/~-endorphin and ot-melanotropin (Chapter 31 ).

Translational Regulation Translational regulation refers to the number of times a finished mRNA molecule is translated. The three ways in which translation of a particular mRNA may be regulated are 1. By the lifetime of the mRNA, 2. By the probability of initiation of translation, and 3. By regulation of the rate of overall protein synthesis. The silk gland'of the silkworm Bombyx mori predominantly synthesizes a single type of protein, silkfibroin. Since the worm takes several days to construct its cocoon, it is the amount and not the rate of fibroin synthesis that must be great; hence, the silkworm can manage with a fibroin mRNA molecule that is very long lived. Silk production begins with chromatid amplification in which, over a period of several days, the single cell of which the gland is composed has a ploidy of 106. Each fibroin gene is transcribed from a strong promoter to yield about 104 fibroin mRNA molecules. An "average" eukaryotic mRNA molecule has a lifetime of 3 hours before it is degraded.

CHAPTER26


However, fibroin mRNA survives for several days, during which each mRNA molecule is translated repeatedly to yield 105 fibroin molecules. Thus, the whole unicellular silk gland makes 1015 molecules or 300 mg of fibroin. Production of a large amount of a single type of protein by means of a prolonged mRNA lifetime is common in highly differentiated cells. For example, cells of the chicken oviduct, which makes ovalbumin (for egg white), contain only a single copy of the ovalbumin gene per haploid set of chromosomes, but the cellular mRNA is long lived. Translational and transcriptional control are sometimes combined. For example, insulin (which regulates the synthesis of a large number of substances) andprolactin (another hormone) are required together for production of casein (milk protein) in mammary tissue. Both hormones are needed to initiate transcription but prolactin in addition, increases the lifetime of casein mRNA. The synthesis of some proteins is regulated by direct action of the protein on the mRNA. For instance, the concentration of one type of immunoglobulin is kept constant by self-inhibition of translation. This protein, like all immunoglobulins, consists of two H chains and two L chains. The tetramer binds specifically to H-chain mRNA and thereby inhibits initiation of translation.

Regulation of Protein Activity Many enzymes contain several subunits and are regulated by a process known as allostery. A common arrangement in enzymes is that the binding sites for the molecule that is acted on (the substrate) and the inhibitor (which may be the product) are located on different subunits. If binding of the inhibitor prevents binding of the substrate, the information from a site on one subunit must somehow be transmitted to the other subunit. This can be accomplished by the following subunit interactions. Binding of the inhibitor molecule alters the shape of the subunit to which it is bound, resulting in changes in the reactive sites on other subunits. If the subunits remain in contact, all subunits adjoining the first will undergo a conformational change at their respective subunit interaction sites, altering, in turn, the substrate binding site of other subunits. Proteins capable of undergoing such conformational interactions are called allosteric proteins (Chapter 7). In mammalian cells, cAMP is called a second messenger, because it regulates the activities of many proteins. Furthermore, certain hormones and cAMP work in concert to regulate enzymatic activities. Many hormones regulate metabolic processes, such as glucose metabolism and calcium utilization, through binding to specific receptors in the cell membranes of target cells. However, many

SECTION 26.6

Regulation of Cell Proliferation: Oncogenes

hormones are not capable of penetrating the target cells, and instead the binding of a hormone to a membrane receptor induces intracellular synthesis of cAMP and other second messengers; these second messengers then cause the desired metabolic effects (Chapter 30).

26.5 Regulation of Cell Death" Apoptosis Apoptosis (programmed cell death) is characterized by a complex series of biochemical changes that culminate in cell death without inflammation or swelling, which are signs of necrosis. Embryonic, fetal, and postnatal development involve cell death by apoptosis, which serves to eliminate excessive cell proliferation and migration. Apoptosis is initiated by a variety of external stimuli and molecular events such as oxidative stress, mitochondrial permeability transition, mitochondrial cytochrome c release, activation of caspase proteases, activation of endonucleases, transglutaminase activation, and poly(ADP-ribose) polymerase cleavage. Necrosis is a form of passive cell death that is very distinct from apoptosis and usually occurs in a regional group of cells after a particular event, such as stroke, infarction, hemorrhage, or infection. In contrast to necrosis, apoptosis is an active process requiring RNA synthesis, protein synthesis, and new enzyme activities. Apoptosis usually involves isolated single cells that may undergo programmed cell death in a particular organ at different times. Certain features of a cell undergoing death by apoptosis distinguish it from one dying by necrosis. First, the cell shrinks, the plasma membrane and vesiculates change shape, and phosphatidylserine is redistributed on the cell surface. Specific nuclear changes also are diagnostic of a cell undergoing death by apoptosis. Nuclear fragmentation occurs from chromatin condensation and internucleosomal DNA breakdown. Analysis of the DNA shows a laddering fragment pattern that is the result of abnormally activated endonucleases. Although apoptosis is essential to normal development, it also appears to be the cause of pathogenic changes in several chronic neurological diseases including amyotrophic lateral sclerosis (ALS), Parkinson 's disease, Huntington's disease, and spinocerebellar ataxias. Apoptosis also may play a key role in the development of Alzheimer's disease. Regardless of the activating signal for apoptosis, dying cells follow the same series of events. Caspase proteinases play a central role in apoptosis; they are cysteine proteinases that are activated by apoptotic signals and degrade cellular proteins by cleavage after aspartate residues. The first caspase discovered (caspase-1) in mammalian cells was recognized as a "cell death" enzyme by its similarity

609 to the cell death gene, ced-3, in the nematode Caenorhabditis elegans. Subsequent studies have identified almost a dozen mammalian caspases that function as apoptotic initiators (caspases 2, 8, 9, 10), apoptotic executioners (caspases 3, 6, 7), and cytokine processors (caspases 1,4, 5). Activation of caspases 1 and 3 appears to play a key role in the pathogenesis of chronic neurological diseases resulting from progressive cell death. On the other hand, caspase inactivation appears to promote oncogenesis by allowing cell accumulation. Thus, caspases are prime targets for intervention in a variety of diseases that progress to death. ALS was first described by the French neurologist Chariot in 1869. The disease involves rapid loss of motor neurons in the cortex, brain stem, and spinal cord; death usually follows 3-5 years after diagnosis. ALS is more commonly referred to as Lou Gehrig's disease because it struck down the New York Yankees immortal at the peak of his baseball career. ALS usually occurs in a sporadic form but in rare cases there is a familial factor; this provided the first clues to the molecular basis for ALS. One form of ALS results from mutations in the SOM gene that codes for the cystolic enzyme copper/zinc superoxide dismutase (Cu,Zn-SOD). Mice lacking this enzyme do not develop ALS but mice that overexpress the mutant gene do develop symptoms characteristic of ALS and motor neuron death. It is now known that a mutant SOD1 gene causes apoptotic motor neuron death by activating initiator caspases. Using drugs that inhibit caspase activity in ALS transgenic mice, the life span of the mice has been increased by about 70%. As more molecular details of caspase functions and apoptosis in cancer and neurological diseases are revealed, it eventually may be possible to develop drugs that ameliorate or suppress symptoms of these deadly diseases.

26.6 Regulat!on of Cell Proliferation" Oncogenes In the course of differentiation, most adult cell types of higher eukaryotes lose the ability to divide. Only a small number of cell types, (e.g., cells of the intestinal mucosa, erythropoietic cells, and male germline cells) continue to do so. Many genes and regulatory elements determine whether a cell can or cannot divide. However, a class of genes has been discovered that, in certain circumstances, restores the ability of the cells to divide and, more significantly, to do so with little or no control. These oncogenes have been detected in many tumors and in RNA and DNA oncogenic viruses that cause cancers in infected animals. The first oncogene discovered was src, a gene carried by Rous sarcoma virus that causes sarcomas in infected fowl.

610 LTR

CHAPTER26 Regulation of Gene Expression .

gag

F I G U R E 26-11 Genome of the Rous sarcoma virus (RSV). The single-stranded RNA is about 10 kb. Four proteins are encoded in this retrovirus: gag, a core protein; pol, reverse transcriptase; env, glycoprotein of the envelope; and src, a nonessential viral gene whose product is responsible for cellular transformation. LTR, Long terminal repeat sequences present at the ends of each viral RNA molecule.

This single-stranded oncogenic RNA virus infects chicken cells in which the RNA is copied by reverse transcriptase and converted to DNA (Figure 26-11). The DNA copy integrates into a chromosome, where it becomes a provirus. Integration of viral DNA is an essential step in the production of more virus particles. Expression of the src gene, which is nonessential to multiplication of the virus, causes tumor formation in infected fowl. Infected chicken fibroblast cells cultured in vitro become transformed and grow in an unregulated fashion that is visibly distinct from the growth of uninfected cells. Study of DNA from avian tissue has indicated that the src gene is present in cells from normal tissue. However, the cellular src gene, denoted c-src, contains introns, whereas the viral gene, v-src, does not. This has led to the conclusion that at some time in the distant past, processed v-src mRNA was converted to DNA by reverse transcriptase, a virus-encoded enzyme that makes DNA from a single-stranded RNA template, and the virus picked up this DNA copy by genetic recombination. Similar oncogenes have been found in other viruses, and in each case a related cellular copy of the gene is present. The cellular counterpart is called a p r o t o - o n c o g e n e . The oncogenes and proto-oncogenes are almost never identical because they have evolved separately. The product of the src gene is a tyrosine-specific protein kinase, an unusual enzyme because most protein kinases are active only against serine. This enzyme is capable of phosphorylating a large number of cellular proteins, causing either activation or inactivation. The first human oncogene was discovered by testing DNA fragments isolated from human cancer cells and testing for their ability to transform a culture of mouse cells. This experiment utilized DNA from a line of human bladder cancer cells that was applied to a culture of National Institutes of Health 3T3 cells, an established mouse cell line. Some of the mouse cells became transformed, which suggested that an oncogene was being expressed in the bladder cancer cells. Only a single DNA fragment could induce the transformation; therefore, this fragment contained the human bladder cancer oncogene, which was later named ras. A search for a matching sequence in non-

cancerous cells (in analogy to the src study) uncovered a proto-oncogene that differed from ras by a single base pair in the fragment that was sequenced. That is, ras is a mutant form of a normal gene. ras has also been found in many other viruses and eukaryotes, including yeast. A list of most known oncogenes is given in Table 26-6.Their names are abbreviations or acronyms for the virus or tissue of origin. Among the numerous oncogenes that have been identified in cancer cells, the ras genes are the most common. Expression of the oncogenic or mutant ras gene is found in 50% of human cancers. The protein produced by the ras genes is called p21 (M.W. 21,000) and contains 189 amino acid residues. The oncogenic ras gene proteins differ from the normal protein by a single amino acid substitution in most instances, and these changes may be responsible in some cases for converting a normal cellular gene product to a cancer-promoting one. The normal ras p21 binds GTP and GDP; it also hydrolyzes GTP to GDE The mutant ras p21 proteins show reduced or complete absence of GTPase activity. In this regard, normal p21 appears to be similar to the cellular G (GTP-binding) protein that activates adenylate cyclase and thereby mediates cellular activities. Chromosome changes are the hallmark of many cancer cells. The most common type of change is transloeation, a chromosome alteration in which two chromosomes have exchanged segments. For example, in Burkitt's lymp h o m a , fragments in chromosomes 8 and 14 are exchanged in 90% of affected individuals, between chromosomes 8 and 2 in 5%; and between chromosomes 8 and 22 in another 5%. In each case, a segment of chromosome 8 has been moved. The p r o t o - o n e o g e n e e - m y e is located on chromosome 8, and in each of the translocations, cmyc is relocated adjacent to a gene encoding an antibody. This relocation apparently places the proto-oncogene under control of the genes that regulate antibody synthesis, so that c-myc is made in large quantities. Involvement of a different oncogenic protein has been demonstrated in c h r o n i c m y e l o g e n o u s l e u k e m i a (CML). This form of leukemia is associated with the so-called Philadelphia (Ph) chromosome and is characterized by a translocation from chromosome 9 to a new location on the short arm of chromosome 22. The translocation results in the production of a chimeric abl oncogenic protein. The chimeric abl oncogene has enhanced tyrosine protein kinase activity both in vivo and in vitro that is correlated with the development of leukemia. The translocated segment of DNA is inserted into chromosome 22 in a region consisting of 200 bp called the b r e a k p o i n t cluster region (bcr). Consequently, in different CML patients the bcr/abl p21 proteins differ slightly in amino acid sequence, but all

SECTION 26.6

Regulation of Cell Proliferation: Oncogenes

611

TABLE 26-6 Some Oncogenes Involved in Human Cancers*

Oncogene K-ras N-ras H-ras erb-B erb-B2 C-myc N-myc L-myc bcl-2 cycd- 1 bcr-abl cdk-4 /3-cat hst mdm-2 gll ttg

Protein**

Cancer**

p21 GTPase p21 GTPase p21 GTPase EGF receptor growth factor receptor transcription factor transcription factor transcription factor antiapoptosis protein cyclin-D tryosine kinase cyclin-dependent kinase transcription factor growth factor p53 binding protein transcription factor transcription factor

pancreas, lung, colorectal endometrial, other carcinomas bladder gliomas, carcinomas breast, ovarian, gastric Burkitt's lymphoma, SCLC neuroblastoma, SCLC SCLC B-cell lymphoma B-cell lymphoma, carcinomas ALL (T cell), CML sarcoma melanoma, colorectal gastric sarcoma glioma, sarcoma ALL (T cell)

*Each oncogene must be activated by a mutation. Most are activated by one or more somatic mutations. Germ line mutations that activate oncogenes lead to familial cancers. **Abbreviations: EGF, epidermal growth factor; SCLC, small cell lung carcinoma; ALL, acute lymphocytic leukemia; CML, chronic myelogenous leukemia.

chimeric proteins are characterized by elevated protein kinase activity. If oncogenes are altered forms (or aberrantly expressed forms) of normal genes that usually participate in growth regulation, the number of different oncogenes should be fairly small. Thus, we should expect that if a large number of tumor viruses and tumors are screened for oncogenes, the same ones should appear repeatedly. This is indeed the case. For example, the oncogene in human bladder carcinoma (ras) has been found in human lung and colon carcinomas and in rat mammary carcinoma. A similar sequence but with a different base change was found in a human neuroblastoma and a fibrosarcoma. This variant was called N-ras. How oncogenes cause cellular transformation is unknown. However, when a tumor virus brings into a cell an oncogene whose sequence differs from the cellular protooncogene, the mutant gene product probably causes transformation because it is able to carry out some process that the proto-oncogene fails to do. That is, the protooncogene itself carries out a normal function or is silent; only a mutation or chromosomal rearrangement produces the cancerous cell. Certain biological functions are not determined simply by the presence of a particular gene product but by its

concentration. Thus, the conversion of a proto-oncogene to an active oncogene may occur simply by changes in concentration. The activity of a gene can be changed by altering the adjacent promoter or regulatory sequences (or both). Some oncogenes differ from the normal counterparts in that base changes exist only in the promoter and these changes presumably alter the rate of RNA synthesis. Another important mechanism by which the expression of a proto-oncogene can be altered is by moving the gene to a new location, e.g., by relocating it next to a different promoter, separating it from an adjacent regulatory element, or placing it adjacent to an enhancer. Thus, when the viral DNA is inserted into the chromosome of an infected cell, the viral oncogene may be expressed at a much greater rate than the proto-oncogene, which remains at its normal location. Whereas oncogenes are characterized by gain of function, another class of genes are characterized by loss of function; these are the tumor suppressor genes, w h i c h are often involved in familial (inherited) cancers (Table 26-7). The rare human cancer retinoblastoma provided the first significant insight into tumor suppressor genes. Retinoblastoma affects about 1 in 20,000 children who usually develop tumors in both eyes. Affected individuals have a small deletion in chromosome 13 that

612

CHAPTER26 Regulation of Gene Expression

TABLE 26-7

Some Tumor Suppressor Genes Involved in Human Cancers* Tumor Suppressor Gene Rb- 1 p53 p 16 APC MSH1, MLH2, PMS 1, PMS2 WT- 1 NF-1 VHL BRCA- 1 BRCA-2 PTEN DPC4 E-CAD DCC MEN- 1

Protein E2F1 binding transcription factor cyclin-dependent, kinase inhibitor regulates/3-catenin DNA mismatch repair transcription factor p2 lras-GTPase protein stability regulator ?DNA repair DNA repair tyrosine phosphastase TGF-/3 pathway transmembrane cell-cell adhesion molecule transmembrane receptor undetermined

Cancer retinoblastoma, osteosarcoma Li-Fraumeni syndrome, --50% of all cancers breast, lung, bladder, pancreas, --30% of all cancers familial adenomatous polyposis hereditary non-polyposis colorectal cancer Wilms' tumor melanoma, Neurofibromatosis type 1, neuroblastoma renal (clear cell), von Hippel Lindau syndrome ovarian, familial breast cancer familial breast cancer, pancreas breast, prostate, thyroid, glioma pancreas gastric, breast colorectal, other carcinomas parathyroid, adrenal, pituitary

*Mutations in tumor suppressor genes (many of which code for transcription factors) alter an essential cellular activity that regulates growth. Mutations occur both in somatic cells and in germ line cell and are often the cause of hereditary cancers.

predisposes mutations to occur in the homologous chromosome by mitotic recombination. Thus, in the majority of cases of retinoblastoma, one mutation is transmitted in the germline and the other arises as the result of a somatic mutation. It is estimated that 5-10% of breast cancers are due to germline mutations in the cancer susceptibility genes BRCA-1 and BRCA-2, which are also tumor suppressor genes. Carriers of the BRCA-I mutation have an 80-90% lifetime risk of developing breast cancer; carriers of the BRCA-2 mutation have a lesser risk. Ashkenazi Jews and the population of Iceland carry founder mutations since only one mutation, or a very few mutations, occurs repeatedly in the BRCA-1 and BRCA-2 genes among breast cancer patients in these populations. As a result, it is possible to genetically screen many individuals at high risk for harboring particular alleles of the B R C A - I and BRCA-2 genes. However, even with extensive counseling, learning that one is at high risk for developing breast cancer carries a heavy emotional burden. The only medical intervention offered at present for young, high-risk carriers of BRCA-1 or BRCA-2 genes is prophylactic mastectomy to avoid future development of breast cancer. Tumorigenesis is a complex, multistep process involving acquisition of a number of genetic lesions. Whereas

the mutations in oncogenes can cause unregulated growth, a deletion or a mutation in tumor suppressor genes (also called antioncogenes) can also predispose normal cells to become cancer cells. Thus, induction of a malignant phenotype involves a mutation in oncogenes, tumor suppressor genes, and perhaps other genes. In human colon cancer, defects in both oncogene and tumor suppressor genes are known. The latter have been assigned to chromosomes 5, 17, and 18.

26.7 Retroviruses and AIDS Since 1980 three types of infectious human T-cell lymphotrophic viruses (HTLVs) have been identified. HTLV-I is frequently associated with adult forms of T-cell leukemia-lymphoma, HTLV-II with hairy T-cell leukemia, and HTLV-III (now called human immunodeficiency virus; HIV) with acquired immunodeficiency syndrome (AIDS). Other related retroviruses have been isolated from human and primate populations. Molecular biologists have developed a wealth of information about the biology and genetics of retroviruses, including the complete sequencing of their genomes. Despite this enormous research effort it still

SupplementalReadingsandReferences

613

l

T Nucleocapsid-coreproteins l 5"-LTR

gag

Strong transcriptional activator

Promotes infectivity of cell-free virus

Binding sites for host transcription factors; tat response element

Regulator ofstructural . ~ gene expression

I

Reverse transcriptase, protease, integrase, and ribonuclease

LTR-3"

f env

Weak transcriptional activator

nef

[

repl'Fcati'oof n

Inhibits virus (negative factor) Viral coat proteins mediating

CD4binding and membrane fusion

FIGURE26-12 Genome of the human immunodeficiency virus (HIV, or the AIDS virus).

is not possible to prevent or cure infections by retroviruses. The well-characterized retroviruses, HTLV-I, HTLV-II, and HIV, all exhibit a strong affinity for T cells; HIV specifically recognizes a surface antigen (CD4) on T cells of the immune system. The organization of the HIV genome resembles that of the oncogenic RNA viruses described above but it is considerably more complex. In addition to the gag, pol, and env genes and LTR sequences characteristic of all retroviruses, the HIV genome encodes some additional functions (Figure 26-12). At least four regulatory proteins have been provisionally identified encoded by the vpr, tat, rev, and n e f genes. The envelope protein is synthesized as a single large polypeptide (gp 160), which is subsequently cleaved into two structural components. Most attempts at constructing an effective vaccine utilize part or all of the envelope proteins as antigens. Since these proteins do not elicit a strong antibody response by themselves, efforts are directed at coupling envelope antigens with more potent antigens to stimulate antibody response. The human retroviruses represent a unique class of infectious agents, and the mechanisms whereby they ultimately result in a variety of diseases are unknown. In comparison to infections caused by most other viruses, retroviruses are weak antigens. This failure to evoke a strong immune system response presumably enables them to become established and remain active in cells that they infect. They rarely produce a strong viremia and consequently may remain quiescent in infected individuals for years before clinical symptoms develop. Other yet-to-be-identified extrinsic or intrinsic factors also may be necessary to activate the expression and proliferation of human retroviruses. At present, the only protection against retroviral

infections is avoidance of exposure to these agents that are transmitted primarily through intercourse and exposure to contaminated blood.

Supplemental Readings and References A. A. Antson, E. J. Dodson, G. Dodson, et al.: Structure of the trp RNAbinding attenuation protein, TRAP, bound to RNA. Nature 401, 235 (1999). C. Brenner and G. Kroemer: Mitochondria--the death signal integrators. Science 289, 1150 (2000). L. C. Brody and B. B. Biesecker: Breast cancer susceptibility genes: BRCA1 and BRCA2. Medicine 77, 208 (1998). S. B. Cassidy and S. Schwartz: Prader-Willi and Angelman syndromes: Disorders of genomic imprinting. Medicine 77, 140 (1998). G. M. T. Cheetham, D. Jeruzalini, and T. A. Steitz: Structural basis for initiation of transcription from an RNA polymerase-promoter complex. Nature 399, 80 (1999). D. Chen, H. Ma, N. Hong, et al.: Regulation of transcription by a protein methyltransferase. Science 284, 2174 (1999). P. R. Cook: The organization of replication and transcription. Science 284, 1790 (1999). Z. Farfel, H. R. Bourne, and T. Iiri: The expanding spectrum of G protein diseases. New England Journal of Medicine 340, 1012 (1999). X. Y. Li, A. Virbasivs, and M. R. Green: Enhancement of TBP binding by activators and general transcription factors. Nature 399, 605 (1999). D. A. Haber and E. R. Fearon: The promise of cancer genetics. The Lancet 351, SII 1 (1998). E. Heard, P. Clerc, and P. Avner: X-chromosome inactivation in mammals. Annual Review of Genetics 31, 571 (1997). S. John and J. L. Workman: The facts of chromatin transcription. Science 282, 1836 (1998). J. O. Kahn and B. D. Walker: Acute human immunodeficiency virus type I infection. New England Journal of Medicine 339, 33 (1998). L. S. Honig and R. N. Rosenberg: Apoptosis and neurologic disease. American Journal of Medicine 108, 317 (2000). P. W. Laird and R. Jaenisch: The role of DNA methylation in cancer genetics and epigenetics. Annual Review of Genetics 30, 441 (1996).

61~ G. Larola, R. Cuesta, G. Brewer, et al.: Control of mRNA decay by heat shock-ubiquitin-proteasome pathway. Science 284, 499 (1999). D. S. Latchman: Transcription-factor mutations and disease. New England Journal of Medicine 334, 28 (1996). M. Li, V. D. Ona, C. Guegan, et al.: Functional role of caspase- 1 and caspase3 in an ALS transgenic mouse model. Science 288, 335 (2000). A. J. Lopez: Alternative splicing of pre-mRNA: Developmental consequences and mechanisms. Annual Review of Genetics 32, 279 (1998). R. G. Roeder: The role of general initiation factors in transcription by RNA polymerase II. Trends in Biochemical Sciences 21, 327 (1996). C. H. Schein: From housekeeper to microsurgeon: The diagnostic and therapeutic potential of ribonucleases. Nature Biotechnology 15, 529 (1997). R. Singal and G. D. Ginder: DNA methylation. Blood 93, 4059 (1999). V. Skulachev: Mitochondria in the programmed death phenomena;

CHAPTER26 Regulation of 6ene Expression a principle of biology: "It is better to die than to be wrong." IUBMB LIFE 49, 365 (2000). D. G. Tenen, R. Hromas, J. D. Licht, et al.: Transcription factors, normal myeloid development, and leukemia. Blood 90, 489 (1997). M-J. Tsai and B. W. O'Malley: Molecular mechanisms and action of steroid/thyroid receptor superfamily members. Annual Review of Biochemistry 63, 451 (1994). B. B. Wolf and D. R. Green: Suicidal tendencies: apoptotic cell death by caspase family proteinases. Journal of Biological Chemistry 274, 20049 (1999). J. L. Workman and R. E. Kingston: Alternation of nucleosome structure as a mechanism of transcriptional regulation. Annual Review of Biochemistry 67, 275 (1998). W. S. Yarnell and J. W. Roberts: Mechanism of intrinsic transcription termination and antitermination. Science 284, 611 (1999).

CHAPTER

27

Nucleotide Metabolism

A nucleotide consists of a purine or pyrimidine base, a pentose (or deoxypentose), and a phosphate. The synthesis and degradation of nucleotides are discussed in this chapter. Structural features of nucleotides were discussed in Chapter 23 and how nucleotides are used in the synthesis of DNA and RNA in Chapters 24 and 25, respectively. Table 27-1 gives the nomenclature of purine and pyrimidine nucleosides and nucleotides. Names of purine nucleosides end in -osine, whereas those of pyrimidine nucleosides end in -idine; guanine nucleoside is guanosine and should not be confused with guanidine, which is not a nucleic acid base; thymidine is a deoxyriboside. Nucleotides are synthesized by two types of metabolic pathways: de novo synthesis and salvage pathways. The former refers to synthesis of purines and pyrimidines from precursor molecules; the latter refers to the conversion of preformed purines and pyrimidines~derived from dietary sources, the surrounding medium, or nucleotide catabolism~to nucleotides, usually by addition of ribose5-phosphate to the base. De novo synthesis of purines is based on the metabolism of one-carbon compounds.

27.1 One-Carbon Metabolism One-carbon moieties of different redox states are utilized in the biosynthesis of the purine nucleotides and thymidine monophosphate (also known as thymidylate), the

metabolism of several amino acids (particularly serine and homocysteine), the initiation of protein biosynthesis in bacteria and mitochondria by formylation of methionine, and methylation of a variety of metabolites. These onecarbon reactions utilize coenzymes derived from folic acid, or folate. Folate is a vitamin for humans (and other animals because of their inability to synthesize it) (Chapter 38). Folic acid is the common name for pteroylglutamic acid, a compound consisting of a heterobicylic pteridine, p-aminobenzoic acid (PABA), and glutamic acid (Figure 27-1). The combination of the first two produces pteroic acid. Humans lack the enzymes capable of synthesizing PABA or of linking pteroic acid to glutamate. The antimicrobial activity of sulfonamides is due to their competitive inhibition of the bacterial enzyme that incorporates PABA into dihydropteroic acid (Chapter 6). Folates have a wide biological distribution; a rich dietary source is green leaves. They occur in nature largely as oligoglutamyl conjugates (e.g., in plants, predominantly pteroylheptaglutamate) in which the peptide linkages occur between the v-carboxyl group of one glutamate and the oe-amino group of the next (Figure 27-1). The mechanism of intestinal absorption of folate is not completely understood. The ingested folylpolyglutamates must be converted to folylmonoglutamate prior to absorption. The folylpolyglutamates are rapidly hydrolyzed in the intestines at neutral pH by the brush-border enzyme

615

616

CHAPTER 27

Nucleotide

Metabolism

TABLE 27-1

Nomenclature of Nucleosides and Nucleotides Base

Nucleoside (Base-Sugar)*

Nucleotide* (Base-Sugar Phosphate)

Purines Adenine (6-aminopurine) Guanine (2-amino-6-oxypurine) Hypoxanthine (6-oxypurine) Xanthine (2,6-dioxypurine)

Adenosine Deoxyadenosine Guanosine Deoxyguanosine Inosine Deoxyinosine Xanthosine

Adenosine monophosphate (AMP) or adenylic acid Deoxyadenosine monophosphate (dAMP) Guanosine monophosphate (GMP) Deoxyguanosine monophosphate (dGMP) Inosine monophosphate (IMP) Deoxyinosine monophosphate (dIMP) Xanthosine monophosphate (XMP)

Cytidine Deoxycytidine Thymidine (thymine deoxyriboside) Uridine

Cytidine monophosphate (CMP) Deoxycytidine monophosphate (dCMP) Thymidine monophosphate (TMP) (thymine deoxyribotide) Uridine monophosphate (UMP)

Pyrimidines Cytosine (2-oxy-4-aminopyrimidine) Thymine (2,4-dioxy-5-methylpyrimidine) Uracil (2,4-dioxypyrimidine)

*The sugar residue can be ribose or deoxyribose. If it is deoxyribose, it is identified as such; otherwise it is assumed to be ribose, with the exception of thymidine, which is deoxyriboside. tA nucleotide is a nucleoside monophosphate; the monophosphates are sometimes named as acids.

pteroylpolyglutamate hydrolase (also called conjugase) to pteroylmonoglutamate (folic acid). If the folylpolyglutamates enter the intestinal epithelial cells intact, they may be converted to folylmonoglutamate within lysosomes by lysosomal hydrolase; however, the significance of this pathway appears to be minor. Folate transport in the intestine and the choroid plexus is mediated by a specific carrier, and the disorder hereditary folate malabsorption is associated with a defective folate carrier.

Individuals affected with hereditary folate malabsorption exhibit early onset of failure to thrive, megaloblastic anemia, and severe mental retardation. Therapy requires the administration of large doses of oral and systemic folates. Folate is reduced and converted to NS-methyltetrahydrofolate in intestines and secreted into the circulation. In the plasma, about two-thirds of the folate is proteinbound. Tissue needs for folate are met by uptake from

o N

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.

.

.

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I

Pteroylmonoglutamate(folate) FIGURE 27-1 Structure of folic acid showingits components.The numberedpart participates in one-carbontransferreactions. In nature, folate occurs largelyas polyglutamylderivativesin which the glutamateresidues are attached by isopeptide linkages via the 7-carboxylgroup. The pteridine ring structureis also present in tetrahydrobiopterin,a coenzymein the hydroxylationof phenylalanine, tyrosine, and tryptophan(Chapter 17).

SECTION27.1 One-Carbon Metabolism

H N. N.. NyH

617 N H2N"',,,~ ~

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II

NH2 OH

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reductaseh

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= I HN.

Tetrahydrofolate

FIGURE 27-2 Reduction of folate to tetrahydrofolate.

plasma. Within tissue cells, NS-methyltetrahydrofolate transfers its methyl group to homocysteine with the formation of methionine (Chapter 17). This reaction is catalyzed by homocysteine methyltransferase, a vitamin B 12 coenzyme-dependent enzyme, and appears to be the major site of interdependence of these two vitamins. In the tissues, tetrahydrofolate is converted to polyglutamyl forms by an ATP-dependent synthetase. In the liver, the major form is pteroyl pentaglutamate. Reduced polyglutamyl forms, each substituted with one of several one-carbon moieties, are the preferred coenzymes of folate-dependent enzymes. Reduction of folate (F) to tetrahydrofolate (FH4) occurs in two steps: F is reduced to 7,8-dihydrofolate (FH2), and FH2 is reduced to 5,6,7,8tetrahydrofolate (FH4). Both of these reactions are catalyzed by a single NADPH-linked enzyme, dihydrofolate reductase (Figure 27-2).

Inhibitors of Dihydrofolate Reductase Methotrexate (Figure 27-3), a structural analogue of FH2, is a potent inhibitor of dihydrofolate reductase and is used

Trimethoprim

F I G U R E 27-3 Inhibitors of dihydrofolate reductase. Methotrexate, a structural analogue of dihydrofolate, is effective against intact mammalian cells but ineffective against protozoa and some bacteria owing to permeability barriers. Trimethoprim and pyrimethamine (2,4-diaminopyrimidines) are effective against microorganisms. The former is antibacterial and antimalarial; the latter is primarily antimalarial.

in chemotherapy of neoplastic disease. Methotrexate is not effective against infections from bacteria and protozoa, since these organisms are impermeable to folate and its analogues. However, methotrexate inhibits dihydrofolate reductase of both bacterial and protozoal origin in cell-free preparations. Pyrimethamine is extremely effective against protozoan (e.g., malarial parasite) infections, is ineffective against bacterial infections., and is a mild inhibitor of the mammalian enzyme. Trimethoprim is an effective inhibitor of both bacterial and protozoal enzymes but has minimal inhibitory action against the mammalian enzyme. These selective enzyme inhibitors have been used in the treatment of bacterial and malarial infections.

Formation of One-Carbon Derivatives of Folate There are several one-carbon derivatives of folate (of different redox states) that function as one-carbon carriers in different metabolic processes. In all of these reactions, the one-carbon moiety is carried in a covalent linkage to one or both of the nitrogen atoms at the 5- and 10positions of the pteroic acid portion of tetrahydrofolate. Six known forms of carrier are shown in Figure 27-4. Folinic acid (NS-formyl FH4), also called leucovorin or citrovorum factor, is chemically stable and is used clinically to prevent or reverse the toxic effect of folate antimetabolites, such as methotrexate and pyrimethamine. The formation and interconversion of some metabolites of

618

CHAPTER27 Nucleotide Metabolism H

H2N

N

H

I

N

3. Hydrolysis of NS,Nl~ FH4 by a cyclohydrolase yields NS-formyl FH4 (folinic acid) and Nl~ FH4. The oxidation state of the one-carbon moiety is the same in all three species.

I

H2N~N~

N~

I

I I

1,0

Nl~

N~

H2C

FH4

"R

s lo

NS-MethylFH,

N ,N -MethyleneFH,

H20

H

H

I

H ~ ~,N ~

H2N~ ~ N ~ N ~

I

--H~o

/ /

N

N 5, N l~

FH4

" ~ ~ OH

I

jN~

O-"C

I

ff~

OH

I,o

H

R

H

I

I10

C--O/N~ [ H R H

H20"

~X N5-Formyl FH4

NS-FormylFH,

FH,

H20

(citrovorumfactor)

4. Nl~ FH4 can be formed directly from formate in an ATP-requiring reaction.

?

H

I

H ~ ~"N ~ .,.,.,~N",~

H2N~N ~

N~ , Nl~

FH4 -+- HCOO- -+- ATP OH

II

HC~ N , ,

I,o

o.

~R

NS,N'~

H

FH,

synthetase

>

NS-formyl FH4 + ADP + Pi

c=..

|

FH4

Hj

~R

NS-FormiminoFH,

F I G U R E 27-4 The six one-carbon substituents of tetrahydrofolate. The oxidation state is the same in the one-carbon moiety of NS-formyl, Nl~ and NS,Nl~ FH4. Nl~ FH4 is required for de n o v o synthesis of purine nucleotides, whereas NS,Nl~ FH4 is needed for formation of thymidilic acid.

5. The formimino group (-CH=NH) of formiminoglutamate (Figlu), a catabolite of histidine (Chapter 17), forms formimino FH4. O O II II - O--C--CH2--CH2--CH--C--O-

+ FH4

)

Transferase

I NH

I HC--NH

FH4 are shown below.

Formiminoglutamate (Figlu)

1. Serine is the principal source of one-carbon units. FH4 + serine

+

serine hydroxymethyltransferase

o II

pyridoxal phosphate

NS,Nl~

FH4 -F glycine + H20

2. NS,Nl~ FH4 plays a central role, since it can be reduced to NS-methyl FH4 or oxidized to NS,Nl~ FH4. Red.

NS,Nl~

Ox

NS-Formimino FH4

FH4 -k- NADH + H + reductase, NS-methyl FH4 + NAD + dehydrogenase

NS,Nl~ FH4 -Jr-NADP+ 5 10 N ,N -methenyl FH4 + NADP'H + H +

where "Ox." is oxidation and "Red." is reduction.

H

O II

- O--C--CH2--CH2--C--C--O-

I NH2 Glutamate

NS-Formiminoglutamate undergoes deamination to N 5, Nl~ FH4. If a loading dose of histidine is given to a patient who is deficient in folic acid, urinary excretion of Figlu is increased. This Figlu excretion test is useful in the diagnosis of folate deficiency, which clinically is manifested as a megaloblastic anemia (Chapter 38). The various catalytic roles of folate-mediated onecarbon transfer reactions in anabolic and catabolic

SECTION 27.2

Formation of 5-Phosphoribosyl-l-Pyrophosphate

619

Folic acid (F) NADPH + NADP +

FH, NADPH + NADP + FH,

ATP--~.--'~ HCOOH )

rine biosynthesis

.....-- . . . .

glutamate Glutamate

FH,

Form used in .

l

-~N5 -Formyl FH, ~ ATP ~

FH, + HCOO-

.

qO ~

N'~ ADP t +P, J H,O'~-'l

~ . , - , , - , - - . ~ . ~ + ' i'" r, NADPH T ' ~N"N' +' H+ "~

FH,-""

/

/-

Formiminoglycine / "--,.Glycine

NH3 FH' " ~

FH,

= N"F~176

FH~(~.Glutamat e

NADP+~*I dUMP TMP ~ Formiminoglutamate FH PyridO~xal p~sphate~-- N',N'~ FH, _t,._ 3= F.H, ~'FH, (Figlu),, { "x 7 t,,--'*NADH + H+ I,~--~ NADPH + H+ ' Serine Glycine/ [ + [ + Histidine CO,, N H 3 " ~ NADH + H+ [~"--*NAD ["---~NADP 9 ~,~,^n+

clne" / \ /

FH,

,~r,,.,

NS-MethylFH. I

FH. '

.. Lf~-Homocy steine I Vitamin u , , [ I ["---Methionine [ FH,

.]ReutilizedJ

FIGURE 27-5 One-carbontransferreactionsinvolvingfolate-derivedcarriers.Dashedarrowsindicatemultiple-stepreactionpathways; solid arrowsrepresentdirect, single-stepreactions.

reactions are shown in Figure 27-5. Serine, a nonessential amino acid, is the major source of one-carbon fragments. Some key processes that utilize folate-mediated one-carbon transfer reactions are as follows: 1. Synthesis of purine nucleotides (Nl~ FH4); 2. Methylation of deoxyuridylate to thymidylate (NS,Nl~ FH4) (an essential step in the biosynthesis of DNA; impairment of this reaction is responsible for the main clinical manifestations of folate deficiency); 3. Synthesis of formylmethionyl-tRNA (Nl~ FH4) required for initiation of protein synthesis in prokaryotes (Chapter 25) and in mitochondria; and 4. Conversion of homocysteine to methionine (NS-methyl FH4) (Chapter 17). Inherited disorders of folate transport and metabolism include defects in folate carrier (hereditary folate malabsorption, previously discussed), deficiency of NS,N l~ methylene FH4 reductase (Chapter 17), or functional deficiency of NS-methyl FH4 methyltransferase due to defects

of vitamin B12 metabolism (Chapter 38) or formiminotransferase (previously discussed). Several observational studies have shown that dietary supplementation of folate in women of childbearing age who might become pregnant can reduce the risk of fetal neural tube defects (e.g., spina bifida).

27.2 Formation of 5- Ph os p h o ri b osyl- 1 - Pyro p h osp h ate 5-Phosphoribosyl-l-pyrophosphate (PRPP) is a key intermediate in nucleotide biosynthesis. It is required for de n o v o synthesis of purine and pyrimidine nucleotides and the salvage pathways, in which purines are converted to their respective nucleotides via transfer of ribose 1-phosphate group from PRPP to the base; that is, Purine 4- P-ribose-P-P --+ purine-ribose-P 4- PPi (PRPP) PRPP is synthesized from ribose 5-phosphate by the following reaction:

620

CHAPTER27 Nucleotide Metabolism

_

2-O3POCH2

H

~

H

ATP

H

5-phosphate produced in this pathway is converted to ribose 5-phosphate as follows"

AMP

epimerase

PRPP synthetase

D-Xylulose 5-phosphate

H HO

OH

D-Ribulose 5-phosphate

2-O3POCH2

. .

H o

o

II II " ~~~J' O--P--O--P---OOH OH I I O-

O-

PRPP

In this reaction, the pyrophosphate group of ATP is transferred to ribose 5-phosphate; the product PRPP is a high-energy compound. PRPP synthetase has an absolute requirement for inorganic phosphate (Pi), which functions as an allosteric activator. The enzyme is inhibited by many nucleotides, the end products of the pathway for which PRPP is an essential substrate. The gene for PRPP synthetase is located on the X chromosome. Mutations in this gene have given rise to PRPP synthetase variants with increased catalytic activity, which leads to overproduction of uric acid (discussed below, under "Gout"). The ribose 5-phosphate needed comes from three sources:

1. The pentose phosphate pathway. This pathway is probably the major source of ribose 5-phosphate in liver and in bone marrow. 2. The uronic acid pathway (Chapter 15). Xylulose

>

The contribution of this pathway to the supply of ribose 5-phosphate may be a minor one. The epimerase and isomerase are the same enzymes that participate in the pentose phosphate pathway. 3. Glycolysis. Ribose 5-phosphate can also be produced from intermediates of glycolysis (Figure 27-6). The enzymes involved are those of the nonoxidative phase of the pentose phosphate pathway, that occur in many tissues.

27.3 Biosynthesis of Purine Nucleotides Purine nucleotides can be produced by two different pathways. The salvage pathway utilizes free purine bases and converts them to their respective ribonucleotides by appropriate phosphoribosyltransferases. The de novo pathway utilizes glutamine, glycine, aspartate, Nl~ FH4, bicarbonate, and PRPP in the synthesis of inosinic acid (IMP), which is then converted to AMP and GME

De Novo Synthesis The atoms of the purine ring originate from a wide variety of sources (Figure 27-7). C2 and C8 are derived from

Glucose

(Glycolysis)

/

P

,,

Fructose 6-phosphate + Glyceraldehyde3-phosphate J Transketolase 4-phosphate + Xylulose5-phosphate -~ ransaldolase

Sedoheptulose 7-phosphate + Glyceraldehyde3-phosphate I Transketolase Ribose 5-phosphate + Xylulose 5-phosphate Isomerase 2(Ribose 5-phosphate) -Sum:

isomerase

D-ribose 5-phosphate

Ribose 5-phosphate

~~[;se

>

Epimerase

2(Ribulose 9 5-phosphate)

2(Fructose6-phosphate) + Glyceraldehyde3-phosphate

= 3(Ribose5-phosphate)

FIGURE 27-6 Synthesis of ribose 5-phosphatefromintermediatesof glycolysisby enzymesof the nonoxidativephase of the pentose phosphate pathway.(See Chapter 15.)

SECTION27.3 Biosynthesis of Purine Nucleotides

621

co~

stepwise synthesis of IMP is depicted in Figure 27-8. The following features of purine biosynthesis should be noted.

r'~ ......... is ~-~-~Glycine "~'W--.N/g"~:, 'C ~ ~.~ .=. 11 is I i'"' 8c ~ N~~

Aspartate ~

1. The glycosidic bond is formed when the first atom of the purine ring is incorporated (reaction 1,

FH,

Figure 27-7). Free purines or purine nucleosides are not intermediates of this pathway. In pyrimidine biosynthesis, the pyrimidine ring is formed completely before addition of ribose 5-phosphate. 2. The biosynthesis is accomplished by several ATP-driven reactions. Ring formation (cyclization) is achieved by incorporating nucleophilic amino groups and electrophilic carbonyl groups at the appropriate positions. 3. The first purine nucleotide synthesized is inosinic acid (IMP), which is the precursor for the synthesis of

l, 1|

Amidonitrogenof91utamino F I G U R E 27-7 Sources of the atoms in d e n o v o synthesis of the purine ring. The numbered reactions correspond to the reaction steps in Figure 27-8.

Nl~ FH4. Glycine is incorporated entirely and provides C4, C5, and N7. The or-amino group of aspartic acid provides N1. The amide group of glutamine contributes N3 and N9. Carbon dioxide (or HCO~-) provides C6. The

/NH2

uc~. Glutamine

H20_.P

O

H2

No~Poc~O

Glycine ATP

|

O~P~OH

I

OH

OH

OH

OH

ec-5-Phospho-D-ribosylI-pyrophosphate

OH

ON

13-5-PhosphoribosylI-amine

0

II

|176

H

II

~c..

c~

II

.c.....

OH

5"-Phosphoribosylglycinamide

~

.o.~'=a+~'oo C~NH

I P~R

PmR 5"-Phosphoribosyl5-amino-4-imidazole-

i P~R

5"-Phosphoribosyl5-aminoimidazole

r

5"-Phosphate-ribose

5"-Phosphoribosyl-o~-Nformylglycinamide OH

5"-Phosphodbosyl-a-Nformylglycinamidine

I

coo. ~[o ,;~ I II /C..,N II

O

II

II

/ (+~O) xCH

I

.

9

.~C-.c~ \

O

H2N/C~c/N~

.c-..coo. c~\ I

O

I

... I Aspartic add

H,

FH,

H

H ~ / C ", ~ N /

N'~

~c. ~

P--R

H=N/C~c/% L 2 H

.~...

C

.

(-H,O)

@

=

P--R

pmR

5"-Phosphoribosyl-5amino-4-imidazolesuccinocarboxamide

5"-Phosphoribosyl-5amino-4-imidazolecarboxamide

5"-Phosphoribosyl-5formamido-4-imidazolecarboxamide

F I G U R E 27-8 Biosynthesis of inosinic acid. The reactions are numbered. Enzymes: (1) amidophosphoribosyltransferase; (2) phosphoribosylglycinamide synthetase; (3) phosphoribosylglycinamide formyltransferase; (4) phosphoribosylformylglycinamidine synthetase; (5) phosphoribosylaminoimidazole synthetase; (6) phosphoribosylaminoimidazole carboxylase; (7) phosphoribosylaminoimidazolesuccinocarboxamide synthetase; (8) adenylosuccinate lyase; (9)phosphoribosylaminoimidazolecarboxamide formyltransferase; (10) IMP cyclohydrolase.

OH

OH

Inosine 5"-phosphate (inosinic acid)

622

CHAPTER27 Nucleotide Metabolism L -Aspartate

H O O C - - C m C - - COO H2 i--I......,:p

O

H

N %

NH3! "'"~~

/,

H -ooc--c-cmcoo H2 ...[........ NH ............

0

m

Fumarate

j

N

N/

H

H

i NH2i

-OOC~C--C~CO0-

"............

N

)

N

Adenylosuccinase

GTP, Mg2+ Adenylosuccinate [ synthetase Ribose 5-phosphate

N

I

Ribose 5-phosphate Adenylsuccinate (AMPS)

Inosine monophosphate

(IMP)

I

Ribose 5-phosphate Adenosine monophosphate (AMP)

NAD*K+H20 NADH~*"] IMP dehydr~

O " N

Glutamine

HN

O ~

Glutamate

,

::r.......J

iii~:~;

H

.N

y "~N"

I

"N--

I

Ribose 5-phosphate

Xanthosine monophosphate (XMP)

\

Ribose 5-phosphate Guanosine monophosphate (GMP)

FIGURE 27-9 Biosynthesis of AMP and GMP from IMP.

adenylic acid (AMP) and guanylic acid (GMP) in two different pathways (Figure 27-9). 4. All the required enzymes occur in the cytosol. This is also true for enzymes of salvage pathways, nucleotide interconversion, and degradation. 5. D e n o v o synthesis is particularly active in the liver and placenta. Nonhepatic tissues (e.g., bone marrow) depend on preformed purines that are synthesized in the liver and transported by red blood cells. They are very effective in salvaging the purines and exhibit little or no activity of xanthine oxidase, which oxidizes free purines. Salvage P a t h w a y s Reutilization of purine bases after conversion to their respective nucleotides constitutes salvage pathways. These pathways are particularly important in extrahepatic tissues. Purines arise from several sources: intermediary metabolism of nucleotides, degradation of polynucleotides, and dietary intake. Quantitatively, the first two sources are the more important. Salvage occurs mainly by the phosphoribosyltransferase reaction: Base + PRPP ~-- base-ribose-phosphate + PPi

Human tissue contains two phosphoribosyltransferases (Figure 27-10). Adenine phosphoribosyltransferase (APRT) catalyzes the formation of AMP from adenine. Hypoxanthine guanine phosphoribosyltransferase (HPRT) catalyzes the formation of IMP and GMP from hypoxanthine and guanine, respectively. HPRT also catalyzes the conversion of other purines (6-thiopurine, 8-azaguanine, allopurinol) to their respective ribonucleotides. The Keq of both phosphotransferases favors formation of ribonucleotides. During salvage, only two high-energy bonds are used, whereas in de n o v o synthesis of AMP or GMP, at least six high-energy bonds are expended. Deficiencies of these enzymes are discussed later. A quantitatively less significant salvage pathway uses purine nucleoside phosphorylase and nucleoside kinase: nucleoside phosphoryiase

Base + ribose l-phosphate < Base-ribose + ATP

> base-ribose 4- Pi

nucleoside kinase

> base-ribose-phosphate + ADP

The phosphorylase can catalyze the formation of inosine or deoxyinosine, and of guanosine or deoxyguanosine, but not adenosine or deoxyadenosine. However, the last twoi nucleosides can be converted to inosine and deoxyinosine by adenosine deaminase. The normal function of the phosphorylase appears to be the formation

SECTION27.3 Biosynthesisof PurineNucleotides

623 O

O

HN

PRPP

HN,~ I

PR

N

I H Hypoxanthine

0

_/I .~

"CH

~

I

cI

-O

cI

OH

HO IMP

Hypoxanthine-guanine phosphoribosyltransferase

O

O H N ~ % H~N

H2N~ N

I~N/~ H Guanine

PRPP

.,CH2 / 0

PR

HO__lpl~U

HC\H

i

H)CH

c

-O

cI

OH

HO

GMP NH2 NH2 PRPP

PP~ .

Adenine phosphoribosyltransferase H Adenine

0

I

-O

0

C

" C H

c

cI

OH

HO AMP

F I G U R E 27-10 Salvage pathways of purine nucleotide synthesis. The preformed purines can be converted to mononucleotides in a single step, using PRPP.

of free hypoxanthine and guanine for conversion to uric acid. Deficiency of adenosine deaminase or purine nucleoside phosphorylase results in immunodeficiency diseas (Chapter 35). In muscle, a unique nucleotide reutilization pathway, known as the purine nucleotide cycle, uses three enzymes: myoadenylate deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase. In this cycle, AMP is converted to IMP with formation of NH3, and IMP is then reconverted to AME Myoadenylate deaminase deficiency produces a relatively benign disorder of muscle

(Chapter 21), which is characterized by muscle fatigue following exercise (see Myoadenylate Deaminase Deficiency). Nucleoside kinases specific for inosine or guanosine have been described. Adenosine kinase is widely distributed in mammalian tissues. Dietary Purines

Purines derived from food do not participate significantly in the salvage pathways described above; they are mostly

624

CHAPTER 27 Nucleotide Metabolism

NuCleO:)rotelns Proteolytic enzymes "'--* Amino acids Nucleic acids Nucleases and phosphodiesterases

polymerases, respectively. They are formed from the monophosphates in two stages. Conversion to diphosphates is catalyzed by kinases. These enzymes are basespecific but not sugar-specific. ATP is the usual source of phosphate; in some cases, other triphosphates or dATP may be used. Typical reactions are as follows" guanylate kinase

Nucleotides

GMP -t- ATP

> GDP -t- ADP

(dGMP)

Nucleotidases, phosphatases

(dGDP) adenylate kinase

~'--~Phosphate

AMP + A T P ,

, ADP + ADP

Nucleosides

~UFt Ph~ cleoside phosphorylase ibose 1-phosphate Free bases

Xanthine oxidase Uric acid (Excreted in urine, or catabolized by bacteria in the large intestine)

FIGURE 27-11 Fate of dietary nucleoproteins. Only the predominantreactions are shown. Dietary purines are mostlyconvertedto uric acid. converted to uric acid (Figure 27-11). Dietary nucleic acids, present predominantly as nucleoproteins, are converted in the small intestine to nucleic acids and protein digestive products by the action of proteolytic enzymes (Chapter 12). The liberated nucleic acids are depolymerized to nucleotides by pancreatic nucleases and phosphodiesterases. The nucleotides are hydrolyzed to nucleosides by nucleotidases and phosphatases. The nucleosides are either absorbed or further cleaved to free bases by nucleoside phosphorylase. The free bases may be oxidized to uric acid by xanthine oxidase. Nucleoside phosphorylase and xanthine oxidase are very active in human small intestinal mucosa. Uric acid produced in the small intestine may be absorbed and excreted in urine, or it may be further catabolized by bacteria in the large intestine. In experiments in which normal or gouty subjects ingested 15N-labeled nucleic acids, the labeled purines were converted to uric acid mainly by direct oxidative pathways without prior incorporation in tissue nucleic acids. Dietary purines (and pyrimidines; see below) do not serve significantly as precursors of cell nucleic acids in the body.

The diphosphates are converted to the triphosphates by the ubiquitous enzyme nucleoside diphosphate kinase. Remarkably, the lack of base or sugar specificity applies to the phosphate acceptor and the phosphate donor. Hence, the general reaction is nucleoside diphosphate kinase

dXDP 4- dYTP

(acceptor)

> 4- dXTP 4- dYDP

(donor)

Recall that conversion of ADP to ATP occurs mostly by mitochondrial oxidative phosphorylation coupled to electron transport (Chapter 14).

27.5 Formation of Purine Deoxyribonucleotides Conversion of ribonucleotides to the deoxy forms occurs exclusively at the diphosphate level. Ribonucleoside diphosphate reductase (ribonucleotide reductase) catalyzes the reaction. This enzyme is found in all species and tissues. The immediate source of reducing equivalents is the enzyme (E) itself in which two sulfhydryl groups are oxidized to a disulfide. The general reaction is ribonucleoside diphosphate

Ribonucleoside diphosphate + E(SH)2 reductase > deoxyribonucleoside diphosphate + E(S-S)+ H20 In this reaction, the T-hydroxyl group of ribose is replaced by a hydrogen, with retention of configuration at the 3'-carbon atom: 3-O6P2OCH2 .-.

Base

3-O6P2OCH2 _

2, OH

HI

OH

OH

~'~--~

Base

W1 H

Ribonucleotide~ ' ' j / reductase

27.4 Conversion of Nucleoside Monophosphates to Diphosphates and Triphosphates

f

,,,,

Thioredoxin-(SH)2

Thioredoxin-(S-S)

(red uced)

(oxidized) Thioredoxin reductase

The triphosphates of nucleosides and deoxynucleosides are substrates for RNA polymerases and DNA

\ NADP+

NADPH + H+

+ H20

SECTION 27.6 Regulation of Purine Biosynthesis

Regeneration of the ribonucleotide reductase is accomplished in Escherichia coli and in mammals by thioredoxin, a dithiol polypeptide (M.W. 12,000) coenzyme, which also plays a role in other protein disulfide reductase reactions. In thioredoxin, two cysteine residues in the s e q u e n c e - C y s - G l y - P r o - C y s are converted to cystine. Reduced thioredoxin is regenerated by thioredoxin reductase, a flavoprotein enzyme that uses NADPH + H +. E. coli mutants unable to synthesize thioredoxin are still able to form deoxyribonucleotides. In these bacteria, a related substance, glutaredoxin, and two molecules of glutathione carry out the reduction. In Lactobacillus, the triphosphate is reduced and vitamin B12 is an essential coenzyme. Another example of this use of vitamin B12 is in Euglena, where the diphosphates are reduced. The mammalian system is nearly identical to that of E. coli.

Ribonucleotide reductase from E. coli consists of two subunits, B1 and B2, neither of which possesses catalytic function. B] is a dimer in which each monomer contains a substrate binding site and two types of allosteric effector binding sites. One type of effector site confers substrate specificity and the other is regulatory. B ] contains a pair of sulfhydryl groups that are required for catalytic activity. B2 also is a dimer, contains one nonheme Fe(III), and has an organic free- radical delocalized over the aromatic ring of a tyrosine residue in each of its polypeptide chains. The catalytic site is formed from the interaction of B1 and B2; a free-radical mechanism involves the tyrosyl residues, iron atom of B2, and sulfhydryl groups of B]. An antineoplastic agent, hydroxyurea, inhibits ribonucleotide reductase by inactivating the free radical. Ribonucleotide reductase is regulated so as to ensure a balanced supply of deoxynucleotides for DNA synthesis. For example, if excess dATP is present, decreased synthesis of all the deoxyribonucleotides ensues, whereas ATP stimulates formation of dCDP and dUDE Binding of TTP (also designated as dTTP) stimulates formation of dGDP and hence of dGTP; binding of dGTP stimulates the formation of dADP and hence of dATE In this way, these nucleotide effectors, by binding to various regulatory sites, tend to equalize the concentrations of the four deoxyribonucleotides required for DNA synthesis.

625

dophosphoribosyltransferase reaction, and the steps involved in the formation of AMP and GMP from IME

PRPP Synthetase Reaction PRPP synthetase requires inorganic phosphate as an allosteric activator. Its activity depends on intracellular concentrations of several end products of pathways in which PRPP is substrate. These end products are purine and pyrimidine nucleotides (Figure 27-12). Increased levels of intracellular PRPP enhance de novo purine biosynthesis. For example, in patients with HPRT deficiency, the fibroblasts show accelerated rates of purine formation. Several mutations of PRPP synthetase, which exhibit increased catalytic activity with increased production of PRPP, have been described in gouty subjects.

Amidophosphoribosyltransferase Reaction This reaction is the first and uniquely committed reaction of the de novo pathway and the rate-determining step. Amidophosphoribosyltransferase is an allosteric enzyme and has an absolute requirement for a divalent cation. The enzyme is inhibited by AMP and GMP, which bind at different sites. The enzyme also is inhibited by pyrimidine nucleotides at relatively high concentrations.

aibose 5-phosphate + ATP

e_A_.l-- e ............. ...... P R P P + Glutamine

"| @

5-Phosphodbosylamine

I

.....

,'. . . . . . . . I ! |

' I

of P u r i n e B i o s y n t h e s i s

Regulation of de novo purine biosynthesis is essential because it consumes a large amount of energy as well as of glycine, glutamine, N]~ FH4, and aspartate. Regulation occurs at the PRPP synthetase reaction, the ami-

l

S

......

AMP-S |

i

t

:::

~

--#,

, I

ADP

[

"

ATP

XMP

",.....t '" -'GMP [

AMP

!

Regulation

(Multiple steps)

IMP

-O

,- . . . . . .

27.6

|

,,"

9

."

9

9

'

GDP ........

I

9

I ! .

[

,

9

9 9

GTP

FIGURE 27-12 Feedback regulation of the d e n o v o pathway of purine biosynthesis. Solid lines represent metabolic pathways, and broken lines represent sites of feedback regulation. @, Stimulatory effect; Q, inhibitory effect. Regulatory enzymes: A, PRPP synthetase; B, amidophosphoribosyltransferase; C, adenylosuccinate synthetase; D, IMP dehydrogenase.

626

Inhibition by AMP and GMR is competitive with respect to PRPR The human placental enzyme exists in a small form (M.W. 133,000) and a large form (M.W. 270,000). The small form is catalytically active. Ribonucleotides convert the active form to the large form, whereas PRPP does the opposite. The regulatory actions of PRPP synthetase and amidophosphoribosyltransferase are coordinated. When there is a decrease in the intracellular concentration of adenine ribonucleotides, PRPP synthetase is activated; this results in increased synthesis of PRPP, which in turn converts the inactive form of amidophosphoribosyltransferase to the active form and increases production of purine nucleotides.

Regulation of Formation of AMP and G M P from IMP In the formation of AMP and GMP from IMR ATP is required for GTP synthesis, and GTP is needed to form ATP (Figure 27-9). In addition, adenylosuccinate synthetase is inhibited by AMP, and IMP dehydrogenase is inhibited by GMP (Figure 27-12).

27.7 Inhibitors of Purine Biosynthesis A variety of inhibitors of purine biosynthesis function at different stages and are used as antimicrobial, anticancer, and immunosuppressive agents.


acid (NS-formyl FH4), which decreases damage to bone marrow and prevents the development of leukopenia and thrombocytopenia. Methotrexate also produces mucositis, gastrointestinal symptoms, and liver damage by a direct toxic effect on hepatic cells. Chronic oral administration of methotrexate in psoriasis has been associated with an increased incidence of postnecrotic cirrhosis. The "rescue" of normal, but not tumor, cells from methotrexate toxicity by folinic acid is partly explained by differences in membrane transport. For example, osteogenic sarcoma cells (which do not respond to conventional doses of methotrexate treatment) are not rescued by folinic acid administered after methotrexate, presumably owing to the absence of transport sites for folinic acid in the neoplastic cells. The therapeutic effects of administration of methotrexate and "rescue" with folinic acid are superior to those of methotrexate alone. Resistance to methotrexate can develop from increased activity of dihydrofolate reductase, synthesis of an enzyme having a lower affinity for the inhibitor, decreased transport of the drug into tumor cells, decreased degradation of the reductase, and genetic amplification of the gene for dihydrofolate reductase. Azaserine (L-Serine diazoacetate), isolated from a species of Streptomyces, is a structural analogue of glutamine O § II - N= N~---CH~C~O~CH2~CH~COOH f I Azaserine NH2 O

lnhibitors of Folate Biosynthesis The sulfonamide drugs were the first effective antibacterial agents to be employed systemically in humans. These drugs resemble p-aminobenzoic acid in structure and inhibit utilization of that compound for the synthesis of folate in bacteria. Sulfonamides do not interfere with human metabolism.

Inhibitors of Formation of IMP

Folate analogues, such as methotrexate (Figure 27-3), are folate antagonists. They block production of FH2 and FH4 by dihydrofolate reductase and lead to diminished purine biosynthesis (inhibition of reactions 3 and 9 in Figure 27-8). Methotrexate also affects metabolism of amino acids and pyrimidine (inhibition of thymidylate synthesis) and inhibits DNA, RNA, and protein synthesis. It is effective in the treatment of breast cancer, cancer of the head and neck, choriocarcinoma, osteogenic sarcoma, and acute forms of leukemia. High doses of methotrexate can be tolerated provided that the patient also receives folinic

II

H2N-- C mCH 2mOll 2--CH ~COOH Glutamine

I

NH 2

that inhibits steps 1 and 4 of inosinic acid synthesis (Figure 27-8) and the conversion of XMP to GMP (Figure 27-9). A related substance, 6-diazo-5-oxo-Lnorleucine (DON), is also a glutamine analogue. Both compounds function as alkylating agents and become attached to an essential sulfhydryl group (a nucleophilic group) of the enzyme to form an inactive thioether derivative. The linkage occurs via the electrophilic carbon atom (designated by the arrow) of azaserine and release of N2.

Inhibitors of Formation of AMP and G M P

Hadacidin (N-formyl-N-hydroxyglycine) is an H

OH

I

.~--N--CH2uCOOH

SECTION27.7 Inhibitors of Purine Biosynthesis

627

analogue of aspartic acid isolated from fungi. By competing with aspartate, it inhibits adenylosuccinate synthetase and hence the synthesis of AMP, it is an experimental antineoplastic agent. Mycophenolic acid and ribavarin monophosphate inhibit IMP dehydrogenase and hence GMP synthesis.

I n h i b i t o r s of M u l t i p l e

S t e p s ~ P u r i n e Analogues

6-Mercaptopurine is a structural analogue of hypoxanthine and is converted to thio-IMP in the salvage pathway.

SH SH

6-Mercaptopurine is partially metabolized to 6-thiouric acid (which lacks antitumor activity) by xanthine oxidase. Allopurinol, a xanthine oxidase inhibitor used in the treatment of gout, potentiates the action of 6-mercaptopurine by preventing its conversion to 6-thiouric acid. This effect is taken into consideration in treatment. Mercaptopurines are also inactivated by S-methylation carried out by thiopurine S-methyltransferase (TPMT), particularly in hematopoietic tissues which lack xanthine oxidase. Deficiency of TPMT due to polymorphisms causes profound toxicity with the regular therapeutic regime. This is another example of the use of pharmacogenomics. Azathiopurine, a derivative of 6-mercaptopurine, functions as an antiproliferative agent. Its principal use is as an immunosuppressive agent. It presumably releases 6-mercaptopurine in the body by reacting with sulfhydryl compounds such as GSH. S

i'l

"N~CH3

N

~~

HPRT PRPP

pp,

i

2-O3POCH%o~

Thio-IMP N~~

I

H i:-I~ ! OH

6-Mercaptopurine

O2N

I

H I OH

H Azathiopurine SH

N

Thio-IMP prevents production of AMP and GMP by inhibiting the following reactions" H2N

~~ N t

Glutamine + PRPP

H 6-Thioguanine

| I I I

5-Phosphodbosylamine

,I

...|

,.

I Th,o-,MP I

/ IMP

XMP I

Adenylosuccinate

i Ii

i I i

- GMP

{

-'- AMP

6-Mercaptopurine is used in the treatment of acute leukemias. However, resistant tumor cells develop rapidly, probably because of altered specificity or lack of phosphoribosyltransferases, so that thio-IMP (the active inhibitor) is not formed. In support of this mechanism, resistant cells respond to 6-methylmercaptopurine ribonucleoside, which is phosphorylated to the corresponding nucleotide. Other mechanisms may include altered cell permeability and increased rate of destruction of 6-mercaptopurine.

6-Thioguanine is similar to 6-mercaptopurine in its action. The most active form is 6-thio-GMP, which inhibits guanylate kinase and, at higher concentrations, IMP dehydrogenase. Thio-IMP and thio-GMP also inhibit PRPP amidotransferase. Vidarabine (adenine arabinoside), an analogue of adenosine, does not interfere with purine biosynthesis but affects DNA synthesis. NH2

HOCH2 O/

ON N It is phosphorylated to adenine arabinoside triphosphate, which inhibits viral DNA polymerases, but not the

CHAPTER 27 NucleotideMetabolism

628

Acyclovir O HN~N~,

Valacyclovir O HN%N~

Ganciclovir

O H N ' ~I I IN

NZ

NH2 H2N"~N,"'~ N// H3C~ O _ _ _ . _ ] ~ O ~

HO~...A,..

H3C

HO- ~ 0 ~

0

OH

Penciclovir

Famciclovir

O

HN~N~, O H2N/'~N,,," ~ N HaN'~O

CHa

OH O F I G U R E 27-13 Structure of some of the purine nucleoside analogues used as antiviral agents.

mammalian enzymes by competition with dATE Originally developed as an antileukemia drug, it has proved useful in the treatment of herpes virus infections. Several other purine nucleoside analogues are used as antiviral agents (Figure 27-13); these include acyclovir, valacyclovir, ganciclovir, penciclovir, and famiciclovir. After conversion to their respective triphosphate derivatives, these drugs inhibit viral DNA polymerase. Viruses inhibited by these drugs are herpes simplex, varicella zoster, cytamegalovirus, and hepatitis B. Antiviral agents that are not purine nucleoside analogues are amantadine analogues and or-interferon.

Inhibition of Conversion of Ribonucleoside Diphosphate to Deoxyribonucleoside Diphosphate Hydroxyurea, an antineoplastic agent, acts by destroying an essential free radical in the active center of ribonucleotide reductase. 0 II

H2N~C~NHOH An unexpected finding of hydroxyurea treatment is induction of hemoglobin F (HbF) production in red blood cells. Hydroxyurea therapy results in an increase in both content and the number of red blood cells that contain HbF (known as F cells). This property of hydroxyurea has been

used in the treatment of sickle cell disease because of the antisickling effect of HbF (Chapter 28).

27.8 Catabolism of Purine Nucleotides Degradation of purines and their nucleotides occurs during turnover of endogenous nucleic acids and degradation of ingested nucleic acids (Figure 27-11), during which most of the purines are converted to uric acid. Degradation of purine nucleoside phosphates (AMP, IMP, GMP, and XMP) begins by hydrolysis by 5'-nucleotidase.(Figure 27-14) to produce adenosine, inosine, guanosine, and xanthosine, respectively, and phosphate. Adenosine is converted to inosine by adenosine deaminase (adenosine aminohydrolase). For inosine, guanosine, and xanthosine, the next step is catalyzed by purine nucleoside phosphorylase and involves a phosphorylation and a cleavage to produce ribose 1-phosphate and hypoxanthine, guanine, and xanthine, respectively. (The enzyme also acts on deoxyribonucleosides to release deoxyribose 1-phosphate.) The pentose sugars are metabolized further or excreted. Purine nucleoside phosphorylase functions in purine salvage, and deficiency of this enzyme results in decreased cell-mediated immunity. Hypoxanthine and guanine are converted to xanthine by xanthine oxidase and guanine aminohydrolase, respectively. Thus, all purine nucleosides produce xanthine. Xanthine oxidase is very active in the intestinal mucosa and the

629

SECTION27.8 Catabolismof Purine Nucleotides H20 NH3 AMP j

~/

)iMP

Adenylate aminohydrolase

H20 " ~

H20

~'--

I

Aden!.ln.

H20 ~/

NH3

XMP

H20

5'-Nucleotidase P,

GMP

5'-Nucleotidase

H20 5'-Nucleottdase

Pi

Pi , Ino.in.

Guanoslne

Adenosine aminohydrolase p ~ J Ribose 1-phosphate , ~ t

I

A U .,IL

/ p ,

/

/

/

,,,~ Rlbose1-phosphate "XX~/ ,~, ~ / Purine -~ N -'-7ooc'eos~e HN/ " ~ ~ /phosphorylase

~

Ha" "~"" ~\

Hypoxanthlne

/

Guanine

Xanthine

o, +.,o

Xanthosine

p.~J ' "~Purine nucleoside . . . . . . ~'1 phosphorylase Hioose 1-pnospnate,P--"- I

i "~JPurine nucleoside JJl phosphorylase

5'-Nucleotidase

Pi

/

/

~

~

H20

~

I

NH3 < " I

~

.,o,

J

/

i H

H

lanthine /

02 + H20 " ~ H202 ~

Xanthine oxidase

0

H

H

H

Uric acid

FIGURE 2"7-]4 Pathways for the formation of uric acid from purine nucleotides. [Reproduced with permission from G. Zubay, Biochemistry, 2nd ed. New York: (Macmillan 1988). @ 1988 by Macmillan Publishing Company.]

liver and is present in most tissues. The products of the xanthine oxidase reaction are uric acid and hydrogen peroxide. In primates (humans included) and birds, uric acid is excreted in the urine. In mammals other than primates, a liver enzyme, urate oxidase, converts uric acid to allantoin (Figure 27-15). In lower animals, allantoin is metabolized to allantoic acid (some fish), urea (most fish and amphibia), and ammonia (some marine invertebrates and crustaceans). In humans, uric acid, urea, and creatinine are the end products of nitrogen metabolism. All three are excreted in urine. The primary end product is urea (Chapter 17). Animals in which uric acid is the major end product excrete the rather insoluble uric acid as a semisolid mass, which allows for conservation of water. In humans, uric acid production and excretion is a balanced process. On a daily basis, about two-thirds of the uric acid produced is excreted by the kidneys and the remainder via intestinal

bacterial degradation. In chronic renal failure and hyperuricemia, extrarenal uric acid disposal is enhanced. Renal excretion of uric acid is a multicomponent bidirectional process involving glomerular filtration (100%) and proximal tubular reabsorption (98-100%), followed by proximal tubular secretion (50%) and proximal tubular reabsorption (40-44%). Ultimately, the renal clearance of uric acid is only about 7-10% of creatinine clearance (Chapter 17).

Xanthine Oxidase Reaction Xanthine oxidase is a flavoprotein that contains molybdenum, nonheme iron, and labile sulfur. The enzyme is present in two forms, one with dehydrogenase activity (xanthine dehydrogenase) and the other with oxidase activity. The former is converted to the latter by oxidation of

630

CHAPTER27 Nucleotide Metabolism O

Uric acid (keto form)

&N

t H

Primates, birds uricotelic reptiles, insects (other than diptera)

"N _ _ O

H

Orate oxidasel~H20' 02 O

H

H2N ~--~..~- N \

Mammals (otherthan primates), insects (diptera only)

Allantoin H

H

Disorders of Purine Nucleotide Metabolism

Allantoinase ~ H20

COOH Allantoic acid

Some teleost fish

H

Gout

Allantoicase

~'~Glyoxylic acid O

II

Most fishes, amphibia, some mollusks

2H2N--C-- NH2 U r e a s e ~ H20

Ammonia

Several disorders affect purine metabolism. They are gout and the syndromes associated with deficiency of HPRT, APRT, adenosine deaminase, nucleoside phosphorylase, myoadenylate deaminase, and xanthine oxidase.

H

fH,O

Urea

which can cause tissue destruction. Thus, the xanthine oxidase reaction can cause tissue injury during reperfusion of organs that have been deprived of oxygen (e.g., ischemic myocardium, transplanted organs). The xanthine oxidase reaction is further promoted in hypoxic organs by providing increased substrates owing to the enhanced adenine nucleotide breakdown that occurs during oxygen deprivation. Hydrogen peroxide is inactivated by catalase or by peroxidase. Xanthine oxidase is inhibited by allopurinol (a purine analogue), which is used in the treatment of gout.

Some marine invertebrates, crustaceans

2NH3

F I G U R E 27-15 Metabolic degradation of uric acid in some animals.

thiol groups of the enzyme owing to the presence of high concentrations of oxygen. It is active on purines, aldehydes, and pteridines. The reactions catalyzed on purines are

Hypoxanthine + H20 +

0 2 --+

xanthine + H202

Gout is a heterogeneous group of genetic and acquired diseases characterized by elevated levels of urates in blood (hyperuricemia) and of uric acid in urine (urieuria). Hyperuricemia in men is defined as serum urate concentration greater than 7 mg/dL (420 #mol/L) and in women 6 mg/dL (357 #mol/L,). In gout, hyperuricemia is a common biochemical occurrence; however, many hyperuricemic subjects may not develop clinical gout (asymptomatic hyperuricemia). All clinical symptoms of gout arise from the low solubility of urate in biological fluids. The maximum solubility of urate in plasma at 37~ is about 7 mg/dL. However, in peripheral structures and in the extremities, where the temperature is well below 37~ the solubility of urate is decreased. When urate is present in supersaturated solutions, crystals of monosodium urate monohydrate form easily (Figure 27-16). Deposits of aggregated crystals, known as tophi, in and around joints of

and Xanthine + H20 +

0 2 ---> uric

OH

acid + H202

The reaction mechanism is incompletely understood. Molybdenum, an essential cofactor, is the initial acceptor of electrons during purine oxidation and undergoes reduction from Mo 6+ to Mo 4+. Deficiency of molybdenum can result in xanthinuria. The electrons from molybdenum are passed successively to the iron-sulfur center, to FAD, and finally to oxygen. The oxygen incorporated into xanthine and uric acid originates in water. Xanthine oxidase also yields the superoxide radical, O 2, which is then converted to hydrogen peroxide by superoxide dismutase (Chapter 14). This may yield free radicals,

H202-+-Fe z+ ---> Fe 3+ + OH- + OH"

O

'J...7 -o H Lactim form

H pK" = 5.75

H

Lactam form

pK'= 10.3 OH N

N

H Monosodium urate

F I G U R E 27-16 Structures of tautomeric forms of uric acid. In the lactim form, uric acid has two acidic hydrogens, with pK' values of 5.75 and 10.3. At physiological pH (7.4), the predominant species of the lactim form is urate monoanion.

SECTION 27.8

631

Catabolism of Purine Nucleotides

the extremities initiate an inflammatory foreign body reaction (acute arthritis) that involves leukocytes, complement, and other mediators (Chapter 35). This reaction causes severe pain, swelling, redness, and heat in the affected areas. Initial attacks are usually acute and frequently affect the metatarsophalangeal joint of one big toe. Tophi may also be present in subcutaneous tissues, cartilage, bone, and kidneys. Formation of urinary calculi (urolithiasis) of urate is common. Gout is potentially chronic and disfiguring. Primary gout is a disorder of purine metabolism seen predominantly in men. The condition is multifactorial and involves genetic and nongenetic factors. Occurrence in women is uncommon; when it does occur, it is usually found in postmenopausal women. The blood urate concentration of normal men is ~1 mg/dL higher than that in women, but this difference disappears after the menopause. Thus, in women, the postmenopausal rise in serum urate levels may increase the risk of developing gout. Gout is very rare in children and adolescents. Primary gout may be due to overproduction or underexcretion of uric acid or to a combination of both. Frequently, siblings and other close relatives of afflicted individuals have high levels of uric acid in blood but do not develop gout, indicating that hyperuricemia is not the only factor involved. Primary renal gout is due to underexcretion caused by a renal tubular defect in uric acid transport. Primary metabolic gout is due to overproduction of purines and uric acid. The prevalence of gout is high in some populations (e.g., 10% of adult male Maori of New Zealand). In Europe and the United States, the prevalence rate is 0.13-0.37%. Secondary gout develops as a complication of hyperuricemia caused by another disorder (e.g., leukemia, chronic nephritis, polycythemia). This type of hyperuricemia usually is associated with abnormally rapid turnover of nucleic acids. The rare cases of gout in adolescents and children are usually of this type.

Overproduction of PRPP The mechanism of the hyperuricemia in most individuals who have gout is unknown. Following is a discussion of biochemical lesions that lead to hyperuricemia and may eventually lead to gout. Enhanced PRPP synthesis results from X-chromosome-linked mutants of PRPP synthetase. Several variants show increased Vmax, resistance to feedback inhibition, or a low Km for ribose 5-phosphate. PRPP levels can also be increased as a result of underutilization in purine salvage pathways. Thus, HPRT deficiency (partial or complete) causes hyperuricemia as an X-linked recessive trait. In situations in which ATP is consumed more rapidly than it is synthesized or in which

ATP synthesis is impaired, ADP and AMP accumulate and eventually are converted to uric acid. Hyperuricemia can occur in hypoxic conditions (e.g., adult respiratory distress syndrome), glucose-6-phosphatase deficiency, and acute illness (e.g., severe hemorrhagic shock). Furthermore, hyperuricemia may serve as a marker for cellular energy crises. Ethanol ingestion causes hyperuricemia owing to increased degradation of ATP to AME The latter occurs in ethanol metabolism during the conversion of acetate to acetyl-CoA (Chapter 18). In all these instances of production of uric acid by xanthine oxidase, the formation of cytotoxic byproducts of the reaction, namely, hydrogen peroxide and superoxide radical, also is increased. Patients with glucose-6-phosphatase deficiency (glycogen storage disease type 1; see Chapter 15) exhibit hyperuricemia from infancy and some develop gout later in life. The hyperuricemia is due to decreased excretion and increased production of uric acid. Because of their hypoglycemia, these patients develop marked hyperlactic acidemia, and lactate reduces the renal clearance of uric acid by suppressing its tubular secretion. Renal excretion of urate is complex, comprising glomerular filtration, tubular reabsorption, and tubular secretion. The urate in the urine is thought to arise almost entirely by tubular secretion. The increased production of uric acid in glucose-6-phosphatase deficiency has been attributed to enhanced ATP turnover and consequent adenine nucleotide depletion, with release of feedback inhibition of amidophosphoribosyltransferase, and to acceleration of de novo purine nucleotide synthesis. Diminished levels of ATP and Pi result from the presence of increased levels of phosphorylated glycolytic intermediates (Chapter 15). A similar mechanism has been proposed for the hyperuricemia that results when fructose is administered intravenously to humans. Excessive production of organic acids leads to elevated levels of uric acid. Lactate, acetoacetate, and /3-hydroxybutyrate (the latter two are known as ketone bodies) compete with uric acid for secretion by the kidney tubules. Lactic acidemia can occur in glucose6-phosphatase deficiency and in alcohol ingestion. Ketonemia and ketonuria occur in untreated diabetes mellitus, starvation, glucose-6-phosphatase deficiency, etc. (Figure 27-17). Treatment

A variety of drugs are used in the management of gout in three clinical situations: 1. To treat acute gout arthritis, 2. To prevent acute attacks, and 3. To lower serum urate concentrations.

632

CHAPTER27 Nucleotide Metabolism Glycogen

From-] [ gluconeo- t--- Glucose 6-phosphate Pentose phosphate" Ribose 5-phosphate genesisj _[_(~ pathway [fATP Glucose

(~) - lkPRPP sythetase [ ' ' AMP PRPP + Gutamine Hypoxanthine-. Guanine

..'" . IMP

(~),-':"~"G

/ Hypoxanthine

L Urinary uric -acid

(~ ',

'[ 1AIIop urin~ Xanthine !-" inhibits [ ] these steps Uric acid

Enzyme (~ Glucose-6-phosphatase deficiency

Comments Leads to excess PRPP production enhanced A'FP turnover

(~) PRPP synthetase overactivity

Overproduction of PRPP

(~) Hypoxanthine-guanine phosphoribosyltransferase decrease or absence

Decreased purine salvage; conversion of bases to uric acid; enzyme is completely absent in Lesch-Nyhan syndrome

(~ Overproduction of organic acids

Decreased secretion of uric acid owing to competition from other organic acids in the kidneys

FIGURE 27-17 Compositediagram of the biochemical lesionsthat may lead to hyperuricemia.

Acute gouty attacks commonly affect the first metatarsal joint of the foot. In aspirated joint fluids, birefringent urate crystals can be seen in the polarized light microscope, which is used in the definitive diagnosis. The treatment of acute gout includes the administration of colchicine, nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, adrenocorticotropic hormone (ACTH), and analgesics. Colchicine and NSAIDs also can be used prophylactically to prevent acute attacks in patients with gout. Drugs used to lower serum urate concentrations include probenecid, sulfinpyrazone, and allopurinol. Colchicine depolymerizes microtubules and structures (such as the mitotic spindle) consisting of microtubules. It is effective in decreasing pain and the frequency of attacks but its mechanism of action is obscure. Allopurinol, an analogue of hypoxanthine, inhibits xanthine oxidase and reduces formation of xanthine and uric acid. It is converted by xanthine oxidase to alloxanthine, which binds tightly to the active site by chelation with

Mo 4+. This greatly reduces reoxidation of Mo 4+ to Mo 6+ and affects catalytic activity (Figure 27-18). This type of inhibitor, in which a substrate analogue is converted to an inhibitor and not released from the active site, is known as a suicide enzyme-inactivator or a mechanismbased inhibitor (Chapter 6). Allopurinol also is converted to allopurinol ribonucleotide by HPRT. Uric acid production is decreased through depletion of PRPP. Allopurinol ribonucleotide also inhibits PRPP-amidotransferase allosterically. These effects are illustrated in Figure 27-19. Since xanthine is both a product and a substrate for xanthine oxidase, allopurinol therapy could be expected to cause accumulation of xanthine in the body. Since xanthine is only sparingly soluble in urine (but more soluble than uric acid), this could lead to urinary xanthine crystalluria, or stone formation. This complication has not been observed in patients who receive the drug for treatment of gout or uric acid stones, but it has occurred in a few patients with Lesch-Nyhan syndromeand lymphosarcoma.

SECTION27.8 Catabolism of Purine Nucleotides

633

OH

OH 8

N

~N ~N/

N

J N'~8

H

H

AIIopu ri nol

Hypo xant hi ne

Lesch-Nyhan Syndrome

Xanthine oxidase .-OH

oooO~176

Mo",+'''" N "'H'

H AIIoxanthin~ (chelating Mo at the active site of the enzyme)

FIGURE 27-18 Inhibition of xanthine oxidaseby allopurinol, an analogue of hypoxanthine. N7 and C8 of hypoxanthine are reversedin allopurinol. Allopurinol, a suicide enzymeinactivator,is convertedto alloxanthine, which binds tightly to the active site of the enzymevia M o 4+ and interrupts the reoxidation of M o 4+ to M o 6+ needed to initiate the next catalytic cycle.

Drugs that increase uric acid excretion in humans include probener which is effective in the regulation of hyperuricemia and the resolution and prevention of tophi, and sulfinpyrazone, which has similar effects. Both agents are weak organic acids and probably act as competitive inhibitors of tubular reabsorption of uric acid.

. r AIIoxanthine (suicide enzymeinactivator of xanthine oxidase)

AIIopurinol

PRPP Depletes the supply of PRPP needed for the first step of de novo purine nucleotide biosynthesis

Serum urate levels can be lowered by dietary and lifestyle changes. These include correction of obesity, avoidance of ethanol consumption, and avoidance of highpurine foods (e.g., organ meats). Psuedogout is a disorder caused by deposition of calcium pyrophosphate dihydrate usually in larger joints such as knee, wrist, and ankle.

AIIopurinol ribonucleotide Inhibits the first step of de novo purine nucleotide biosynthesis

FIGURE 27-19 Multiple actions of allopurinol inhibit formation of uric acid.

Lesch-Nyhan syndrome is characterized by virtual absence of HPRT, excessive production of uric acid, and abnormalities of the central nervous system. These abnormalities include mental retardation, spasticity (increased muscle tension resulting in continuous increase of resistance to stretching), choreoathetosis (characterized by irregular, jerky, or explosive involuntary movements, and writhing or squirming, which may involve any extremity or the trunk), and a compulsive form of self-mutilation. The disorder associated with partial deficiency of HPRT also leads to hyperuricemia but lacks the devastating neurological and behavioral features characteristic of the LeschNyhan syndrome. Both disorders are X-linked. The hyperuricemia in Lesch-Nyhan patients is explained, at least in part, on the basis of intracellular accumulation of PRPP leading to increased purine nucleotide biosynthesis de novo and increased production of uric acid. Such patients do not usually develop gouty arthritis early in life but do exhibit uric acid crystalluria and stone formation. In Lesch-Nyhan patients, all tissues are devoid of HPRT. The disorder thus can be detected by an assay for HPRT in erythrocytes and by cultured fibroblasts. The former test has been used in detection of the heterozygous state. HPRT is a 217-amino-acid cytosolic enzyme coded for by a single gene on the X chromosome. Several mutations of the HPRT gene are known (Table 27-2). Mutations include major gene alterations and missense mutations causing either gouty arthritis or LeschNyhan syndrome (Table 27-2). An example of a missense mutation causing Lesch-Nyhan syndrome is HPRTKinston, which has a substitution of asparagine for aspartic acid at position 194 (D194N). The mutant enzyme, although present in a concentration similar to that of the normal enzyme, is catalytically incompetent because of its very high Km values for hypoxanthine and PRPE The mechanism by which deficiency of HPRT causes central nervous system disorders remains unknown. Lesch-Nyhan patients do not show anatomical abnormalities in the brain. In normal subjects, HPRT activity is high in the brain and, in particular, in the basal ganglia where de novo purine biosynthesis is low. This suggests the importance of the purine salvage pathway in this tissue. However, the relationship between HPRT deficiency and

634


TABLE 27-2

Examples of Missense Mutations That Alter Human Hypoxanthine-Guanine Phosphoribsoyl-Transferase (HPRT) and Cause Lesch-Nyhan Syndrome (LN) or Gout

Mutant HPRT Kinston Urangan Mashad Montreal Toowong London Ann Arbor Tokyo Brisbane

Nucleotide change G--->A G--->A A--->T T--->C G--->A C--->T T--->G G--->A C--->T

neurological manifestations is not understood. The most significant abnormality identified in neurotransmitter systems is in the dopaminergic pathway (Chapter 32). There is no known treatment for the central nervous system dysfunction. Allopurinol has been used in the management of the hyperuricemia.

D194N G16S D20V M57T G58R S 110L I 132M G140D T168I

LN Gout Gout LN Gout Gout Gout LN Gout

also involved in conversion of guanosine and deoxyguanosine to guanine. In ADA or PNP deficiency, the appropriate substrates accumulate along with other alternative

NH2

~N N

Adenine P h o s p h o r i b o s y l t r a n s f e r a s e

APRT

(APRT) Deficiency APRT isolated from erythrocytes is a dimer with each subunit having a molecular weight of 19,481: the gene is located on chromosome 16. This autosomal recessive trait results in inability to salvage adenine, which accumulates and is oxidized to 2,8-dihydroxyadenine by xanthine oxidase (Figure 27-20). The main clinical abnormality is the excretion of 2,8-dihydroxyadenine as insoluble material (gravel) in the urine. These stones can be confused with those of urate on routine analysis. Thus, appropriate chemical analyses are required, particularly in the pediatric age group, to identify APRT-deficient individuals. The neurological disorders characteristic of HPRT deficiency are not found in APRT deficiency, indicating that APRT may not play a significant role in the overall regulation of purine metabolism in humans. APRT deficiency is treated with a low-purine diet and allopurinol.

ADA deficiency and PNP deficiency are two autosomal recessive traits that cause immune system dysfunction. The reactions catalyzed by ADA and PNP are shown in Figure 27-21. Both enzymes function in conversion of adenosine and deoxyadenosine to hypoxanthine. PNP is

PRPP

N

N~

N

PP~

N

H

Adenine In the absence Xanthine of APRT oxidase

OH

OH

~N Adenylic acid (AMP)

N

OH

N H 8-Hydroxyadenine Xanthine oxidase

~N N

N

[ Adenosine Deaminase (ADA) Deficiency and Purine Nucleoside Phosphorylase (PNP) Deficiency

Clinical presentation

Amino acid change

HO

~

OH

N> H 2,8-Dihydroxyadenine

FIGURE 27-20 Metabolic pathwaysfor the conversionof adenine. Adenineis normally salvaged by conversionto AMP by adeninephosphoribosyltransferase (APRT). In the absenceof APRT,adenineis oxidizedto the highly insoluble product 2,8-dihydroxyadenineby xanthineoxidase.

SECTIO27.8 N Catabolism of Purine Nucleotides

635

OH

H2o

NH3

,nosino

Adenosine

t/

,g~%

deaminase

Hyp

~N ~ N

.

OH H

2'-Deo~aOenosine

.

thine

,,,-

OH

H

Deo~inosine

Punne o. .//'t'~/N. H2N" ~ N~

?

nucleoside

phosphorylase

P,

o.o.

H NO~ N>~

Ribose-1-P04

N1% H2N"~

N ~

Guanine

M2N I~N~N

HOC~ ~V- ~

Deoxyribose-l-PO4

OH H

Deoxyguanosine

FIGURE 27-21 Reactions catalyzedby adenosine deaminase (ADA) and purine nucleoside phosphorylase(PNP). ADA and PNP participate in the purine catabolic pathway,and deficiency of either leads to immunodeficiencydisease. [Slightly modified and reproduced, with permission, from N. M. Kredich and M. S. Hershfield, Immunodeficiencydiseases caused by adenosine deaminase and purine nucleoside phosphorylasedeficiency.In: The Metabolic Basis of Inherited Disease, 6th ed., C. S. Scriver,A. L. Beaudet, W. S. Sly, and D. Valle, Eds. New York: McGraw-Hill (1989).]

products to cause toxic effects on the cells of the immune system. Patients with ADA deficiency lack both T- and B-lymphocyte-mediated functions, namely, cellular and humoral immunity, respectively, and exhibit a severe combined immunodeficiency (SCID) disorder. Other genetic defects can cause SCID (Chapter 35), but ADA deficiency is responsible for about one-third of patients who have SCID. PNP deficiency is associated only with T-lymphocyte dysfunction. The exact mechanisms responsible for immune system dysfunction are not known. In ADA-deficient T lymphocytes, there is a 50- to 100-fold increase in the

concentration of dATP; similarly, PNP-deficient T lymphocytes exhibit elevation in dGTP concentration. Both dATP and dGTP inhibit ribonucleotide reductase, cause a reduction of other deoxyribonucleotides (e.g., dCTP) required for DNA synthesis, and inhibit cell division. S-Adenosylhomocysteine (SAH) also accumulates and inhibits many methylases required for normal functioning of the cell (Chapter 17). Accumulation of SAH results from the suicide inactivation of SAH hydrolase by deoxyadenosine. Formation and degradation of SAH are shown in Figure 27-22. In the hydrolysis of SAH, the substrate undergoes a temporary oxidation by NAD +, which is tightly coupled to the enzyme. This reaction eventually

636

CHAPTER27 Nucleotide Metabolism ATP~ E~P, +P, Methionine FH4~'',

NS-Methyl

FHj4.1

B12

~ S-Adenosylmethionine Methyl acceptorq Methyltransferas~ Transmethylation Methylated product ,,F I reaction I, S-Adenosylhomocysteine (SAH) SAH hydrolase

I[

(deoxyadenosine inhibits this enzyme)

HomocIsteine + Adenosine

Cysteine F I G U R E 27-22 Formation and degradation of S-adenosylhomocysteine (SAH). In ADA deficiency, accumulation of deoxyadenosine, a suicide substrate, inhibits SAH hydrolase. This inhibition is accompanied by a higher steady-state level of SAH, which in turn inhibits several methyltransferase reactions. Normally, homocysteine is reconverted to methionine or cysteine.

leads to the hydrolysis of the substrate and to the reoxidation of NADH, (Figure 27-23). Deoxyadenosine binds to the enzyme and undergoes initial oxidation to a ketosugar intermediate, which is unstable and decomposes with elimination of adenine, leaving the enzyme in the reduced (NADH) state (Figure 27-23). The catalysis is stopped, and exogenous NAD+/NADH + H + cannot affect the reaction. Thus, inactivation of SAH hydrolase by conversion of enzyme-bound NAD + to NADH is a suicide inactivation, in which no covalent modification of the enzyme is involved. Immune system dysfunction in ADA deficiency has also been ascribed to the inhibition of pyrimidine nucleotide synthesis by adenosine, known as pyrimidine starvation. This may arise from inhibition of conversion of orotic acid to orotidine 5'-monophosphate or from inhibition of PRPP synthesis by excessive synthesis of adenine nucleotides. ADA deficiency causes death from massive infection before the patient reaches the age of 2 years. Some children with ADA or PNP deficiency have benefited from periodic infusions of irradiated erythrocytes (which contain ADA and PNP). Irradiation of erythrocytes is necessary to inactivate any white blood cells that may be present and thereby to reduce the risk of graft-versus-host disease (Chapter 35). PNP deficiency usually causes hypouricemia and hypouricosuria and excretion of inosine, guanosine, deoxyinosine, and deoxyguanosine. Another mode of enzyme replacement therapy is periodic administration of a polyethylene glycol-modified form of bovine intestinal ADA by intramuscular injection. The modified enzyme, which is prepared by conjugating polyethylene glycol with purified ADA (PEG-ADA), possesses a longer half-life as well as reduced immunogenicity. As a consequence of this treatment, the correction

of two biochemical abnormalities, namely, accumulation of toxic phosphorylated metabolites and inhibition of SAH hydrolase, preceded clinical improvement (i.e., the absence of infection and resumption of weight gain). Gene replacement therapy should be possible in both ADA and PNP deficiency, and such a trial was attempted with two patients having ADA deficiency. The patient's T cells were removed and a normal ADA gene was inserted into the T cells by means of a retroviral vector. The modified T cells were reintroduced into the patient's bloodstream. Patients were followed for clinical improvement while they continued to receive PEG-ADA treatment. No permanent cure was achieved.

Myoadenylate Deaminase Deficiency Myoadenylate deaminase (or AMP deaminase) deficiency is a relatively benign muscle disorder characterized by fatigue and exercise-induced muscle aches. This disorder is presumably inherited as an autosomal recessive trait. The relationship between the exercise-induced skeletal muscle dysfunction and AMP deaminase deficiency is explained by an interruption of the purine nucleotide cycle. The purine nucleotide cycle of muscle consists of the conversion of AMP --+ IMP --~ AMP and requires AMP deaminase, adenylosuccinate synthetase, and adenylosuccinate lyase (Figure 27-24). Flux through this cycle increases during exercise. Several mechanisms have been proposed to explain how the increase in flux is responsible for the maintenance of appropriate energy levels during exercise (Chapter 21 ). 1. During muscle contraction AMP deaminase activity increases. Nucleoside triphosphates are negative modulators, whereas nucleoside di- and monophosphates are positive modulators of the enzyme. The increased AMP deaminase activity prevents accumulation of AMP so that the adenylate kinase reaction favors the formation of ATP: ADP + ADP ~- ATP + AME 2. The NH3 produced in the AMP deaminase reaction and the decreased levels of ATP as a result of exercise stimulate phosphofructokinase to enhance the rate of glycolysis (Chapter 13). 3. The increased concentration of IMP may activate glycogen phosphorylase and further enhance glycolysis. 4. The production of fumarate (Figure 27-24; see also Chapter 13) may enhance the TCA cycle when demand for ATP production increases. 5. The formation of IMP may provide a means by which the intracellular purine nucleotide pool is maintained. AMP deaminase deficiency disrupts the purine

SECTION 27.9

Biosynthesisof Pyrimidine Nucleotides

FB I H2G. y / / O ' ~ Ad H"~H

OH

~/'H

Enzyme

C~H

~

S-AdenosyI-L-homocysteine

FB I H2G o ~ O ' ~ Ad RSH2C .y/'/"O" ~ Ad H O~/OH~Ad (H Enzyme - ~ ~ ~ / / "H Enzyme H-\ C H H OH ~ OH ~ C~H -- RSH

(SAH)

o.CO>,

HOH2C~

H20

637

H

OH

Enzyme ~

H

H /~H OH

Enzyme ~

H~~ OH

Enzyme ~-"

HOH2C ~O.~ Ad /~H H O OH

Deoxyadenosine

Ad = Adenine;

OH

Adenosine

(a)

HOH~I3THI~O H H~Ad H H I OH H

H

HOH2C O H~I~ ~ H ~

H

-t- Ad

07

Ketosugar (Unstable intermediate)

R=-O2C-CH- CHE- CH2"-" I NH3+

(b) F I G U R E 27-23 Mechanism of action of S-adenosylhomocysteine hydrolase (enzyme) and its inhibition by deoxyadenosine. (a) The enzyme uses enzyme-bound NAD + to temporarily oxidize substrate and eventually hydrolyze it to adenosine and homocysteine. [Reproduced with permission from R. H. Abeles, Suicide enzyme inactivators. Chem. Eng. News 61(38), 55 (September 19, 1983). @1983 by the American Chemical Society.] (b) Deoxyadenosine, a suicide substrate, is also oxidized by the enzyme with the formation of a ketosugar, which undergoes decomposition, with the product dissociating from the enzyme and leaving the enzyme in the reduced state (NADH).

nucleotide cycle and leads to muscle dysfunction during exercise; however, the cycle operates minimally at rest so that AMP deaminase deficiency should not cause muscle dysfunction during rest. Type

IIa (fast-twitch oxidative) and type I (slow-twitch oxidative) muscle fibers have greater oxidative capacity and are less dependent on the cycle than type IIb (fast-twitch glycolytic) fibers. Thus, gradual exercise programs that lead to production of a greater proportion of type IIa and type I fibers might improve exercise tolerance in AMP deaminase deficiency.

Xanthine Oxidase Deficiency This autosomal recessive trait results in hypouricemia and in increased urinary excretion of hypoxanthine and xanthine. Patients frequently have xanthine stones.

27.9 Biosynthesis of Pyrimidine Nucleotides

F I G U R E 27-24 Purine nucleotide. The cycle plays an important role in energy production in skeletal muscle during exercise.

Pyrimidine nucleotides, in common with purine nucleotides, are required for the synthesis of DNA and RNA. They also participate in intermediary metabolism. For example, pyrimidine nucleotides are involved in the biosynthesis of glycogen (Chapter 15) and of phospholipids

CHAPTER 27 NucleotideMetabolism

638 (Chapter 19). Biosynthesis of pyrimidine nucleotides can occur by a d e n o v o pathway or by the reutilization of preformed pyrimidine bases or ribonucleosides (salvage pathway).

Salvage Pathways Pyrimidines derived from dietary or endogenous sources are salvaged efficiently in mammalian systems. They are converted to nucleosides by nucleoside phosphorylases and then to nucleotides by appropriate kinases. Pyrimidine

/ Ribose5-phosphate4

A~ Phosphatem"

Phosphorylase

Pyrimidine-ribose (nucleoside)

ATP~ Kinase ADPr" ~ Pyrimidine-ribose-phosphate(nucleotide) UMP also can by synthesized from uracil and PRPP by uracil phosphoribosyltransferase. De Novo

Synthesis

The biosynthesis of pyrimidine nucleotides may be conveniently considered in two stages: the formation of uridine monophosphate (UMP) and the conversion of UMP to other pyrimidine nucleotides.

Formation of UMP The synthesis of UMP starts from glutamine, bicarbonate, and ATE and requires six enzyme activities. The sources of the atoms of the pyrimidine ring are shown in Figure 27-25, and the pathway is shown in Figure 27-26. 1. The pyrimidine base is formed first and then the nucleotide by the addition of ribose 5-phosphate from

From amide nitrogen of

glutamine

!

9

J i~--Fr~

[ i From C02

" .......

FIGURE 27-25 Sources of the pyrimidinering atomsin de novo biosynthesis.

PRPE In contrast, in de n o v o purine nucleotide biosynthesis, ribose 5-phosphate is an integral part of the earliest precursor molecule. 2. In the biosynthesis of both pyrimidine and urea (or arginine) (Chapter 17), carbamoyl phosphate is the source of carbon and nitrogen atoms. In pyrimidine biosynthesis, carbamoyl phosphate serves as donor of the carbamoyl group to aspartate with the formation of carbamoyl aspartate. In urea synthesis, the carbamoyl moiety of carbamoyl phosphate is transferred to ornithine, giving rise to citrulline. In eukaryotic cells, two separate pools of carbamoyl phosphate are synthesized by different enzymes located at different sites. Carbamoyl phosphate synthetase I (CPS I) is located in the inner membrane of mitochondria in the liver and, to lesser extent, in the kidneys and small intestine. It supplies carbamoyl phosphate for the urea cycle. CPS I is specific for ammonia as nitrogen donor and requires N-acetylglutamate as activator. Carbamoyl phosphate synthetase II (CPS II) is present in the cytosol. It supplies carbamoyl phosphate for pyrimidine nucleotide biosynthesis and uses the amido group of glutamine as nitrogen donor. The presence of physically separated CPSs in eukaryotes probably reflects the need for independent regulation of pyrimidine biosynthesis and urea formation, despite the fact that both pathways require carbamoyl phosphate. In prokaryotes, one CPS serves both pathways. 3. In mammalian tissue, the six enzymes are encoded by three genes. One gene codes for a multifunctional polypeptide (Pyr 1-3) that is located in the cytosol and has carbamoyl phosphate synthetase II (Figure 27-27), aspartate transcarbamoylase, and dihydroorotase activity. Each subunit of Pyr 1-3 has a molecular weight of 200,000-220,000, and the native enzyme exists as multiples of three subunits. The second gene codes for dihydroorotate dehydrogenase which is located on the outer side of the inner mitochondrial membrane. Dihydroorotate, the product of Pyr 1-3, passes freely through the outer mitochondrial membrane and converted to orotate. Orotate readily diffuses to the cytosol for conversion to UME The third gene codes for another multifunctional polypeptide known as UMP synthase (Pyr 5,6). Pyr 5,6 (M.W. 55,000) contains orotate phosphoribosyltransferase and orotidylate (orotidine-5'-monophosphate) decarboxylase activity. Use of multifunctional polypeptides is very efficient, since the intermediates neither accumulate nor become consumed in side reactions. They are

SECTION27.9 Biosynthesisof Pyrimidine Nucleotides

639

-OOC

O

O

II ~_ = NHE--C--OPO3 +

ATP

(Z)

CO2 Glutamine

+2

-ooc.

(~ #

H2N

HC--NH3

:

~COOCarbamoyl

phosphate

I

CH2

(~

HN -- ~.....~

I/H

O~.-~'C~N-/C'~co OH Carbarnoyl aspartate

Aspartate

I

tO

"N / COOH Dihydroorotate (~ AD+

O

O

HN

~" NADH + H+

HN

O

CO.~

'-O3PO

O

(~

COO- ~ P P

2-O3PO

O

HN

(~

ooH

H~I OH UMP

F H HO

H~ I

OH

I/H

Orotate

I

OH

Orotictine 5"-monophosphate

FIGURE 27-26 pathway of uridine-5'-monophosphate (UMP) synthesis. Enzymes: (1) carbamoyl phosphate synthetase II; (2) aspartate transcarbamoylase; (3)dihydroorotase; (4)dihydroorotate dehydrogenase; (5)orotate phosphoribosyltransferase; (6) orotidine-5'-monophosphate decarboxylase (orotidylate decarboxylase).

De novo

rapidly channeled without dissociation from the polypeptide. Other pathways in eukaryotic cells, such as fatty acid synthesis, occur on multifunctional polypeptides.

4. Conversion of dihydroorotate to orotate is catalyzed by dihydroorotate dehydrogenase, a metalloflavoprotein that contains nonheme-iron atoms and flavin adenine nucleotides (FMN and FAD). In this reaction, the electrons are probably transported via iron atoms and flavin nucleotides that are reoxidized by NAD +. 5. Biosynthesis of purine and pyrimidine nucleotides requires carbon dioxide and the amide nitrogen of glutamine. Both use an amino acid "nucleus"--glycine in purine biosynthesis and aspartate in pyrimidine biosynthesis. Both use PRPP as the source of ribose 1-phosphate. 6. UMP is converted to UTP by uridylate kinase and nucleoside diphosphate kinase. uridylate kinase

UMP + ATP .

, UDP § ADP

nucleoside diphosphate kinase

UDP + ATP. F I G U R E 27-27 Schematic representation of the intracellular location of the six enzymes of UMP biosynthesis in animals. Pyr 1-3 = 1, Carbamoyl phosphate synthetase II; 2, aspartate transcarbamoylase; 3, dihydroorotase 4, dihydroorotate dehydrogenase; Pyr 5,6 = 5, orotate phosphoribosyltransferase; 6, orotidine-5t-monophosphate decarboxylase.

, UDP + ADP

Formation of Other Pyrimidine Nucleotides UMP is the parent compound in the synthesis of cytidine and deoxycytidine phosphates and thymidine nucleotides (which are deoxyribonucleotides).

640

CHAPTER 27

Nucleotide Metabolism

FIGURE 27-28 Synthesisof thymidylicacid (TMP). Fluorodeoxyuridylateinhibitsconversionof dUMPto TMP,and methotrexate inhibits regenerationof the tetrahydrofolatecoenzyme.

Synthesis of Cytidine Nucleotides CTP is synthesized from UTP by transfer of the amide nitrogen of glutamine to C-4 of the pyrimidine ring of UTP. This reaction requires ATP as an energy source.

O

NH2

O

~

4- Ogp3OCH2 H

I

CTP synlhetase

O 4- Ogp3OCH2

curs exclusively by methylation of the C-5 of dUMP (Figure 27-28) by thymidylate synthase. The methylene group of NS,Nl~ FH4 is the source of the methyl group, and FH4 is oxidized to FH2. For sustained synthesis, FH4 must be regenerated by dihydrofolate reductase. Recall that deoxynucleotides are formed at the diphosphate level by ribonucleotide reductase; thus, UDP is converted to dUDP, then to dUTP, dUMP is then generated mainly by dUTPase.

H

H I HO

i

Synthesis of Thymidine Nucleotides De novo synthesis of thymidilic acid (TMP) oc-

OH

OH

i H OH

The deoxycytidine phosphates result from reduction of CDP to dCDP by a mechanism analogous to that described for the purine nucleotides. Then dCDP is converted to dCTP by nucleoside diphosphate kinase.

dUTPase

dUTP

> dUMP + PPi

The dUTPase reaction is very important because the DNA polymerases cannot distinguish dUTP from TTP and catalyze significant incorporation of dUMP into DNA when dUTP is present. Incorporation of the base U into DNA is not deleterious because all cells contain a uracil

SECTION 27.9

Biosynthesisof Pyrimidine Nucleotides

CDP

=dCDP .

-dCTP.

DNA 9 o

CTP

...'

NH~)]dld!Teap ~in.a'se

641 pyrimidines uracil and thymine (see page 643). The deficiency of the initial rate-limiting enzyme dihydropyrimidine dehydrogenase can cause adverse drug reactions in patients receiving standard chemotherapy. This is another use of pharmacogenomics.

[ TTPI

UDP '

"dUDP/-

[

dUTPase

TDP

--dCMP

-dUMP '

: TMP [

Thymidylate synthase

Thymidine

Thymine FIGURE 27-29 Pathwayfor TTP synthesis.The key intermediateis dUTP,whichis convertedto dUMPby dUTPase.

N-glycosylase that removes U from DNA. Figure 27-29 summarizes the pathway for TTP synthesis. Because there is only one pathway for synthesis of TMP, it can be used to synthesize radioactively labeled DNA or to inhibit DNA synthesis selectively. In bacterial thymidylate synthase mutants (thy-), DNA synthesis is not possible without added thymine or thymidine, both of which can be utilized by salvage pathways. Neither thymine nor thymidine engages in synthetic reactions other than production of TMP, so radioactive thymine or thymidine can serve as unique precursors for synthesis of radioactive DNA. The radioactivity usually is in the methyl group; since this group is not metabolized, the appearance of radioactivity in any other compound is avoided. Thymidylate synthase is competitively inhibited by fluorodeoxyuridylate (FUDRP), with formation of a stable ternary complex with methylene FH4. FUDR:P is generated by salvage from exogenous 5-fluorouracil (FU) or fluorodeoxyuridine (FUDR). FUDR is a useful drug in cancer chemotherapy because it inhibits TMP formation in proliferating cells. Thymidine nucleotide deficiency can also be induced by competitive inhibitors of dihydrofolate reductase, e.g., aminopterin (4-aminofolate) and methotrexate (4-amino-10-methylfolate; see Figure 27-3). Fluoropyrimidines are catabolized by enzymes that normally participate in the breakdown of endogeneous

Pyrimidine Analogues This group of compounds (Figure 27-30) includes several drugs that are clinically useful in the treatment of neoplastic disease, psoriasis, and infections caused by fungi and DNA-containing viruses. The mechanism of action of 5-fluorouracil, a halogenated pyrimidine, was mentioned above. An antifungal agent, flucytosine (5-fluorocytosine), acts through conversion to 5-fluorouracil by cytosine deaminase in the fungal cells. Idoxuridine (iododeoxyuridine), another halogenated pyrimidine derivative, is used in viral infections. It is a competitive inhibitor (via phosphorylated derivatives) of the incorporation of thymidylic acid into DNA. Cytarabine (cytosine arabinoside) is an analogue of 2'-deoxycytidine. The T-hydroxyl group of the arabinose moiety is in a trans position with respect to the 3'-hydroxyl group (instead of in a cis position, as in the ribose) and causes steric hindrance to rotation of the base around the nucleoside bond (Chapter 23). Phosphorylated derivatives of cytarabine inhibit nucleic acid synthesis as well as being incorporated into nucleic acids. 6-Azauridine and its triacetyl derivative, azaribine, inhibit pyrimidine biosynthesis. Azaribine is better absorbed than 6-azauridine, but it is converted in the blood to 6-azauridine, which undergoes intracellular transformation to 6-azauridylic acid (6-AzUMP). 6-AzUMP, a competitive inhibitor of orotidylate decarboxylase, blocks formation of UMP (Figure 27-26), resulting in high rates of excretion of orotic acid and orotidine in the urine. 5-Azacytidine, after conversion to a phosphorylated derivative, arrests the DNA synthesis phase of the cell cycle. It also affects methylation of certain bases, which leads to hypomethylation (Chapter 26). Azathymidine, 3'-azido-3'-deoxythymidine (AZT), after conversion to the corresponding 5'-triphosphate, inhibits viral reverse transcriptase. Hence, AZT is used in the treatment of AIDS (Chapter 25).

Regulation of de Novo Pyrimidine Biosynthesis Regulation of the de novo pathway is complex. In prokaryotic cells, aspartate transcarbamoylase, an allosteric protein, is inhibited by the end products of pyrimidine

642


(a) Inhibitors of de novo pyrimidine biosynthesis (by inhibiting orotidylate decarboxylase) O

O

N 6

Azauridine R = m O H O

II

Azaribine R= m O - - C - - C H

I

3

I

R

R

(b) Inhibitors of nucleic acid synthesis; also incorporated into nucleic acids NH2

O HN

N

jCII3

Cytarabine (cytosine arabinoside; 1-13-D-arabi nofurano sylcytosine)

HOG~jO

HOCUO

H ~ ....

H\I + ]

13"

12'

_

OH

H

N=N~N

(c) Inhibitors of thymidylate synthase O

O..,,../~ N j N H

H

3'-Azido3'-deoxythymidine (AZT)

, H

NH2 Flucytosine (5-fluorocytosine; converts to 5-FU)

5-Fluorouracil (5-FU) H

FIGURE 27-30 Pyrimidineanalogues.All of these compoundsrequire conversionto appropriatenucleotidesbefore they become active.

biosynthesis, in particular by CTP (Chapter 7). However, mammalian aspartate transcarbamoylase is not regulated in this manner, and carbamoyl phosphate synthetase II (CPS II) appears to be a primary regulatory site. CPS II is inhibited by UTP and is activated by PRPP and ATE Activation by ATP may be important in achieving a balanced synthesis of purine and pyrimidine nucleotides. PRPP is also an essential (and probably a rate-limiting) substrate for the orotate phosphoribosyltransferase reaction (reaction 5 in Figure 2726) promoting d e n o v o pyrimidine nucleotide synthesis by induction of Pyr 1-3. Another potential site of regulation is orotidine-5-phosphate decarboxylase, which is inhibited by UMP, CMP, allopurinol nucleotide, and oxypurinol.

27.10 Coordination of Purine and Pyrimidine Nucleotide Biosynthesis A balanced synthesis of pyrimidine and purine nucleotides is essential for the biosynthesis of nucleic acids in growing cells. Several mechanisms exist for coordinating nucleotide synthesis. 1. PRPP affects purine and pyrimidine nucleotide biosynthesis. PRPP formation is activated by inorganic phosphate and inhibited by several end products of pathways that use PRPE In the purine d e n o v o pathway, PRPP activates amidophosphoribosyltransferase and is the rate-limiting substrate for the enzyme. In purine

SECTION27.11 Catabolismof PyrimidineNucleotides Cytosine NH2

Uracil O

H20 3.3= Cytosine deaminase

HN O

Thymine O HN

..

H

643

"[l"

L.)

H

H

H+'NADPHfi Dihydropyrimidinep NADPH'H+ NADP~.~ dehydr~ ~NADP + O O HN

HN~"~

Dihydrouracil

CH3

O' ' ~ N)

Dihydrothymine

H H H20"~ Dihydropyrimidinase~/~H20 OOC -OOC [3-Ureidopropionate NH2 ~(~H2 NH2 ~CH --CH3 O,,,,~ N'/CIH2 [3-Ureidoisobutyrate (N-carbamoyl-13-alanine) O"" ~ " N/CH2 H H H'O~ [3-Ureidopropionase1/'-H20 NH,+ + CO, +

CO0

CO0

I

I + CO= + NH,+ CHCH3

~H2

13-AlanineCH2 +NIH3

! CH2 +N]H3 13-Aminoisobutyrate

FIGURE 27-31 Pathways for pyrimidine catabolism. The major end product from cytosine and uracil is/%alanine, from thymine it is /3-aminoisobutyrate.

salvage pathways, PRPP is the substrate for HPRT and APRT. In the pyrimidine pathway, PRPP activates CPS II, may induce Pyr 1-3, and is a rate-limiting substrate for orotate phosphoribosyltransferase. Thus, changes in the levels of PRPP bring about concordant changes, while enhanced utilization by one pathway might result in reciprocal changes in the other. This interrelationship between the synthesis of purine and pyrimidine nucleotides was seen in a patient who had a deficiency of PRPP synthetase and exhibited orotic aciduria and hypouricemia, consistent with decreased synthesis of purine and pyrimidine nucleotides. 2. Coordination of synthesis of purine and pyrimidine nucleotides is affected by activation of CPS II by ATR 3. Cultured mammalian cells (e.g., human lymphocytes) fail to grow when exposed to adenosine and have increased pools of ADR ATR and GTR decreased pools of UDR UTR and CTR and decreased

incorporation of [14C] orotic acid. These effects are reversed by exogenous uridine. These results suggest that adenosine inhibits the conversion of orotic acid to orotidine-5'-monophosphate. Adenosine deaminase reduces the toxic effect of adenosine by converting it to inosine. Increased levels of purine nucleotides may also inhibit PRPP synthesis. This inhibition of formation of pyrimidine nucleotides in the presence of excess purine nucleosides and nucleotides has been termed "pyrimidine starvation."

27.11

C a t a b o l i s m of P y r i m i d i n e N u c l e o t i d e s

Pyrimidine catabolism occurs mainly in the liver. In contrast to purine catabolism, pyrimidine catabolism yields highly soluble end products. Pyrimidine nucleotides are converted to nucleosides by 5'-nucleotidase.

644 Cytidine so formed is converted to uridine by cytidine aminohydrolase, while uridine and thymidine are converted to free bases by pyrimidine nucleoside phosphorylase. In mammalian systems, catabolism of uracil and thymine proceeds in parallel steps, catalyzed by the same enzymes (Figure 27-31). The rate-determining step is reduction to a 5,6-dihydroderivative by dihydropyrimidine dehydrogenase. In the second step, dihydropyrimidinase hydrolyzes cleavage of the dihydropyrimidine rings to/3ureido compounds. In the third step,/3-ureidopropionase hydrolyzes the /3-ureido compounds to/3-alanine or/3aminoisobutyrate (BAIB), with release of ammonia and carbon dioxide. Thus, the major end product of the catabolism of cytosine and uracil is/3-alanine, whereas that of thymine is BAIB. High concentrations of BAIB in urine follow excessive cell turnover or destruction (e.g., owing to leukemias or radiation therapy). High levels of BAIB excretion has been observed in some Asian families. Its significance is not known and the high excretors are otherwise normal. Degradation of/3-alanine and BAIB or their reutilization in various biosynthetic pathways is possible.

27.12 Abnormalities of Pyrimidine Metabolism Hereditary orotic aciduria is a rare autosomal recessive trait. In this disorder, both orotate phosphoribosyltransferase and orotidine-5'-phosphate decarboxylase activities (reactions 5 and 6 in Figure 27-26) are markedly deficient. Recall that these activities occur on the polypeptide Pyr 5,6. Orotic aciduria is characterized by failure of normal growth and by the presence of hypochromic erythrocytes and megaloblastic bone marrow, none of which are improved by the usual hematinic agents (e.g., iron, pyridoxine, vitamin B~2, and folate). Leukopenia is also present. Treatment with uridine (2-4 g/d) results in marked improvement in the hematological abnormalities, in growth and development, and in decreased excretion of orotic acid. These patients are pyrimidine auxotrophs and require an exogenous source of


pyrimidine just as all humans need vitamins, essential amino acids, and essential fatty acids. Deficiency of folate or vitamin B12 can cause hematological changes similar to hereditary orotic aciduria. Folate is directly involved in thymidylic acid synthesis and indirectly involved in vitamin B12 synthesis. Orotic aciduria without the characteristic hematological abnormalities occurs in disorders of the urea cycle that lead to accumulation of carbamoyl phosphate in mitochondria (e.g., ornithine transcarbamoylase deficiency; see Chapter 17). The carbamoyl phosphate exits from the mitochondria and augments cytosolic pyrimidine biosynthesis. Treatment with allopurinol or 6-azauridine also produces orotic aciduria as a result of inhibition of orotidine-5'phosphate decarboxylase by their metabolic products. Dihydropyrimidine dehydrogenase deficiency can cause unexpected toxic effects to 5-fluorouracil administration (see page 641).

Supplemental Readings and References H. H. Balfour, Jr.: Antiviral drugs. New England Journal of Medicine 340, 1255 (1999). R. Curto, E. O. Voit, and M. Cascante: Analysis of abnormalities in purine metabolism leading to gout and to neurological dysfunctions in man. Biochemical Journal 329, 477 (1998). B. T. Emmerson: The management of gout. New England Journal of Medicine 334, 445 (1996). M. D. Harris, L. B. Siegel, and J. A. Alloway: Gout and hyperuricemia. American Family Physician 59, 925 (1999). S. Haste, E. De Clercq, and J. Balzarini: Role of antimetabolites of purine and pyrimidine nucleotide metabolism in tumor cell differentiation. Biochemical Pharmacology 58, 539 (1999). W. L. Nyhan and D. F. Wong: New approaches to understanding LeschNyhan disease. New England Journal of Medicine 334, 1602 (1996). J. R. Pittman and M. H. Bross: Diagnosis and management of gout. American Family Physician 59, 1799 (1999). J. G. Puig, A. D. Michan, M. L. Jimenez, et al.: Clinical spectrum and uric acid metabolism. Archives oflnternal Medicine 151,726 (1991). D. Rush: Periconceptional folate and neural tube defect. American Journal of Clinical Nutrition 59(Suppl), 511 S (1994). A. Saven and L. Piro: Newer purine analogues for the treatment of hairy-cell leukemia. New England Journal of Medicine 330, 691 (1994). V. Serre, G. Hedeel, and X. Liu: Allosteric regulation and substrate channeling in multifunctional pyrimidine biosynthetic complexes: analysis of isolated domains and yeast-mammalian chemeric proteins. Journal of Molecular Biology 281, 363 (1998).

CHAPTER

28

Hemoglobin

The easy availability of blood has resulted in many studies of its constituents. Probably the most extensively studied component is hemoglobin, the predominant protein in the red blood cell and the molecule responsible for transporting oxygen, carbon dioxide, and protons between the lungs and tissues. The study of hemoglobin has led to a detailed knowledge of how oxygen and carbon dioxide transport is accomplished and regulated and has provided insight into the functioning of other allosteric proteins (Chapter 7). Hemoglobin is the first allosteric protein for which molecular details of allosteric effector binding and the mechanism of allosteric action are known. Studies of the genes coding for the globin polypeptide chains have provided a better understanding of many anemias and of the regulation of expression of other eukaryotic genes. Correction of the genetic defects in sickle cell anemia and thalassemia by the introduction of new genetic information into bone marrow cells (gene therapy) is being actively explored.

However, the differences in tertiary structure among them are critical to the functional properties of each subunit. Each globin subunit has associated with it, by noncovalent interaction, an Fe2+-porphyrin complex known as a heme group. Oxygen binding occurs at the heme iron. The predominant hemoglobin in adult erythrocytes is oe2f12, known as hemoglobin A1 (HbA). The structural and functional characteristics of hemoglobin have been worked out almost entirely through studies of HbA and its naturally occurring variants. Each tetramer has a molecular weight of about 64,500, and each oe-like and fl-like chain has a molecular weight of about 15,750 and 16,500, respectively. The subunits are situated at the corners of a tetrahedron (Figure 28-1). The structure changes slightly during the binding and release of oxygen. In a tetramer, dissimilar chains are more strongly joined than similar chains. In dilute solution, oxyhemoglobin dissociates as follows:

28.1 Structure of Hemoglobins Globin Chains Mammalian hemoglobins are tetramers made up of two oe-like subunits (usually or) and two non-oe subunits (usually 13, V, or 8). These subunits differ in primary structure but have similar secondary and tertiary structures.

There is no evidence for the formation of |174 or |174 although in the absence of ol-chains ~4 (HbH) forms. Association involves salt bridges, hydrogen bonds, van

645

646

CHAPTER28 Hemoglobin

Heme Group

(") F I G U R E 28-1 Arrangement of hemoglobin subunits. The four polypeptide subunits are located at the corners of a tetrahedron to give a roughly spherical structure.

der Waals bonds, and hydrophobic forces. About 60% of the contact between subunits does not change during reversible oxygen binding (packing contacts), while about 35% does (sliding contacts), otot and tiff contacts form about 5 % of the total intersubunit contacts, the remainder being olfl contacts. In deoxyhemoglobin, there is no tiff contact. This information was derived principally from x-ray crystallographic studies of Perutz, Kendrew, and coworkers and clarifies how 2,3-bisphosphoglyeerate (BPG) or (2,3-diphosphoglyeerate, 2,3-DPG) regulates oxygen transport (Note: 2,3-bisphosphoglycerate and 2,3-diphosphoglycerate are identical.) If each subunit is labeled, the dissociation in dilute solution becomes clearer:

@ Q(

QQ

It also shows the asymmetry in the tetramer, since the oe~/3 ~ contact differs from the Otlfl 2 contact, and O/2fl I differs from OtZfl 2. However, c~l/31 and Of2fl 2 contacts are similar, as are Otlfl2 and lYZfl I contacts. During oxygenation and deoxygenation, the c~1/31 and ffZf12 contacts do not change, but the O/lf12 and lYZfll c o n t a c t s do. Thus, the dimers or 1/3 l and lYZfl 2 function as a unit and can slide relative to each other. Stable o~/3 dimers can be present as a result of a mutation that prevents the formation of the tetramer. An example in which hemoglobin dissociates into stable dimers in the liganded state is the variant hemoglobin Rothschild, which is due to a mutation affecting the/3 chain (/337 Trp --~ Arg). Figures 28-2 and 28-3 are schematic diagrams of the secondary structures of o~ and/3 chains, respectively. Each chain contains helical and nonhelical segments and surrounds a heme group. Almost 80% of the amino acids exist in an o~-helical conformation.

Heine consists of a porphyrin ring system with an Fe 2+ fixed in the center through complexation to the nitrogens of four pyrrole rings. The porphyrin system is a nearly planar aromatic ring formed from four pyrrole rings linked by = C H - (methene) bridges. The pyrrole rings are substituted so that different porphyrins are distinguished by variations in their side chains (see also Chapters 14 and 29). The heine porphyrin is protoporphyrin IX. Iron has a coordination number of six, i.e., each atom of iron can bond with six electron pairs. In heme, two coordination positions of iron are not occupied by porphyrin nitrogens. When heine is associated with a globin chain, a histidyl nitrogen (from histidine FS, important in the allosteric mechanism) bonds with the fifth coordination position of iron, while the sixth position is open for combination with oxygen, water, carbon monoxide, or other ligands. Portions of the seven or eight helices of a globin chain form a hydrophobic crevice near the surface of the subunit. Heine lies in this crevice, between helices E and F. The low dielectric constant of the crevice prevents permanent oxidation of iron (from Fe 2+ to Fe 3+) by oxygen and is responsible for the reversible binding of oxygen. When Fe 2+ in solution combines with oxygen, it is oxidized to Fe 3+, which does not bind reversibly with oxygen. Evidence from electron paramagnetic resonance studies shows that, in oxyhemoglobin, the oxygen can be considered to oxidize the iron as long as the oxygen is bound. When the oxygen is released, Fe 3+ is again reduced to Fe 2+. Reversible 02 binding is a unique property of hemoglobin. If the Fe 2+ in hemoglobin is permanently oxidized to the ferric state (as in methemoglobin), the Fe 3+ binds tightly to a hydroxyl group, and oxygen will not bind. Heme groups can be removed from hemoglobin by dialysis, indicating that they are not covalently attached. Not counting the proximal histidine [His/392 (F8) or ot87 (F8)], about 60 amino acid residues come within 0.4 nm of one or more atoms of the heme group. This is approximately the maximal length of an effective hydrogen bond or hydrophobic interaction. Of these contacts, only one in the ot subunit and two in the/3 subunit are polar, which emphasizes the highly hydrophobic environment of the hemes. The three polar interactions involve the carboxyl groups of the propionic acid side chains on heme.

28.2 Functional Aspects of Hemoglobin Oxygen Transport The primary function of hemoglobin is to transport oxygen from the lungs to the tissues. Hemoglobin forms a

SECTION 28.2

FunctionalAspects of Hemoglobin

647

FIGURE 28-2 Secondary structure of the c~ chain of human hemoglobin. The helical regions (labeled A-H, after Kendrew), N and C termini, and the histidines located near the heine group are indicated. The axes of the B and C helices are indicated by dashed lines. Note that the o~ chain lacks helix D present in the fl chain. The amino acid residues are numbered by two different methods: from the N terminus of the polypeptide chain and from the N-terminal amino acid residue of each helix. Nonhelical regions are designated by the letters of helices at each end of a region.

dissociable complex with oxygen. Deoxyhemoglobin + 402 ~ oxyhemoglobin This reaction goes to the fight with an increase in oxygen pressure (as in the lungs) and to the left with a decrease in oxygen pressure (as in the tissues), in accordance with the law of mass action. Table 28-1 shows typical partial pressures of oxygen between the atmosphere and tissue mitochondria. Hemoglobin increases the solubility of oxygen in the blood 70-fold (from 0.3 to 20.3 mL of oxygen per milliliter of blood). In Figure 28-4, the oxygen saturation percent for hemoglobin is plotted against oxygen pressure in the gas above the surface of the hemoglobin solution. The curves, known as binding or dissociation curves, are often characterized by their Ps0 value, the oxygen pressure at which the hemoglobin is 50% saturated with oxygen. For

comparison, the dissociation curve for myoglobin is also shown. The hemoglobin curves are sigmoid (S-shaped), whereas the myoglobin curve is a rectangular hyperbola. This difference arises because hemoglobin is allosteric and shows cooperative oxygen binding kinetics (Chapter 7), whereas myoglobin is not allosteric. The binding of each molecule of oxygen to hemoglobin causes binding of additional oxygen molecules. This cooperativity occurs because hemoglobin is a tetramer. The cooperative binding of oxygen by hemoglobin is the basis for the regulation of oxygen and, indirectly, carbon dioxide levels in the body. The molecular understanding of this phenomenon is a major accomplishment of x-ray crystallographic studies of protein structure. Myoglobin is a heme protein found in high concentrations in red muscle, particularly cardiac muscle, where it functions as a storage site for oxygen. At the oxygen pressures found in

648


F I G U R E 28-3 Secondary structure of the/3 chain of human hemoglobin. The helical regions (labeled A-H, after Kendrew), N and C termini, and the histidines located near the heme group are indicated. The axes of the B, C, and D helices are indicated by dashed lines.

tissues, the dissociation curve for myoglobin lies to the left of the hemoglobin curves; consequently, hemoglobin (Hb) can readily oxygenate myoglobin (Mb). In the equation Hb(O2)4 -+- 4Mb ~- Hb + 4MbO2 the equilibrium lies far to the right. In the hemoglobin curves, the change from A to B to C is termed a rightward shift. The farther to the right a curve is shifted, the larger is the Ps0 value and the lower is the oxygen affinity. In this example, the shift is due to an increase in Pco2, since pH, temperature, and 2,3-DPG concentration were held constant. However, the shift could also have been caused by an increase in temperature (as in strenuous exercise or fever), a decrease in pH (as in acidosis or exercise), an increase in 2,3-DPG concentration (see below), or a combination of these variables. For

example, an increase in Pco2 is usually accompanied by a decrease in pH. Substances that cause a rightward shift are called negative allosteric effectorsor allosteric inhibitors

T A B I , E 28-1

Partial Pressures of Oxygen

POz(mm Hg) 158 100 95 40 c. When 02 binds to the iron (b), the iron atom is closer to the heine plane (distance a decreases), the N-Fe bond becomes more nearly perpendicular (o~= 1 degree and b = c), and the heine acquires a less domed conformation. These events pull helix F across the heine group, setting off a series of reactions leading to the T-to-R transition. (Modified and redrawn with permission from I. Geis and R. E. Dickerson.)

FIGURE 28-6

Relative motion of the/3 subunits of hemoglobin upon oxygenation and deoxygenation.

652

His ol122, His/3143, and Lys/3144. The amount each of these groups contributes depends on pH and other conditions prevailing at the time of measurement. A primary allosteric effector of hemoglobin in human erythrocytes, in addition to H + and CO2, is the organic phosphate 2,3-DPG. Deoxyhemoglobin binds 1 mol of 2,3-DPG per mole of hemoglobin, whereas oxyhemoglobin does not. The reason for this differential binding is the location of the binding site (Figure 28-6). The negatively charged 2,3-DPG molecule fits between the/3 subunits, where it binds to positively charged residues (see below). In oxyhemoglobin, the/~ subunits are close together, the cleft between them being too small for entry of the 2,3-DPG molecule. Binding of 2,3-DPG inhibits this movement of the/3 chains and helps to stabilize the deoxy form. This binding is an equilibrium phenomenon, and the amount of 2,3-DPG bound depends on the concentrations of 2,3-DPG, O2, CO2, H +, and hemoglobin. Chloride promotes H + binding to deoxyhemoglobin and acts as an allosteric effector, so its concentration must also be considered. All of these factors interact to determine the amount of oxygen bound to hemoglobin at any particular Po2.

Function, Metabolism, and Regulation of Organic Phosphates in Erythrocytes 2,3-DPG, ATE inositol hexaphosphate (IHP), and other organic phosphates bind to hemoglobin and decrease its affinity for oxygen. IHP, the principal organic phosphate in avian erythrocytes, is the most negatively charged of these compounds and binds the most tightly; however, it is not found in human red cells. 2,3-DPG levels in the erythrocytes help regulate hemoglobin oxygenation (Figure 28-7). Unloading of oxygen at the Po2 in tissue capillaries is increased by 2,3-DPG, and small changes in its concentration can have significant effects on oxygen release. At the pH prevailing in the erythrocyte, the net charge on the 2,3-DPG molecule is - 5 . The binding site between the two/~ chains of hemoglobin contains eight positively charged amino acid residue side chains contributed by the Vall, His2, Lys82, and His143 of each chain (Figure 28-8). These residues are highly conserved in mammalian hemoglobins, indicating their importance for normal hemoglobin function. In placental mammals, a fetus receives its oxygen by diffusion from the maternal circulation, across the placenta, into the fetal circulation. To ensure that the oxygen flow is adequate, the pressure gradient from mother to fetus is increased by increasing the affinity of fetal hemoglobin for oxygen. This decreases the partial pressure of oxygen in the fetal circulation, thereby increasing the rate of transplacental diffusion.


FIGURE 28-7 Effect of 2,3-bisphosphoglycerate (2,3-DPG) on the oxygen saturation curve of hemoglobin. Note that 2,3-DPG decreases the affinity of hemoglobin for oxygen.

Different species increase transplacental diffusion in different ways. In humans and other primates, adult hemoglobin contains two o~ and two/~ chains, while fetal hemoglobin has two c~ and two y chains. Although the amino acid sequences of/~ and y chains are similar, they differ in position 143, which is part of the 2,3-DPG binding site. In the Ychain, His 143 is replaced by Ser, thus reducing the charge in the 2,3-DPG binding site from -+-8to +6. Thus, 2,3-DPG binds less tightly to fetal hemoglobin, the oxygen affinity of which is thereby increased relative to that of HbA at the same concentration of 2,3-DPG. Thus, erythrocytes in fetal primates, despite having 2,3-DPG concentrations equal to those in adult erythrocytes, have higher oxygen affinity than maternal red blood cells, allowing for maternal to fetal oxygen transport. In other mammals, including horse, dog, pig, and guinea pig, fetal hemoglobin is not structurally different from adult hemoglobin, and transplacental diffusion is facilitated by a reduced concentration of 2,3-DPG in fetal erythrocytes. In ruminants, hemoglobin does not bind 2,3-DPG because the /3 chains are too far apart. However, fetal hemoglobin in ruminants has a higher affinity for oxygen than does adult hemoglobin because of other structural differences. These three different solutions to the problem of the need for transfer of oxygen to the fetus are an example of convergent evolution. In humans, 2,3-DPG is the most abundant phosphate compound in the red cell. Its concentration is 5 mmol/L, approximately the same as that of hemoglobin tetramer. The concentration of ATP is also high, 1.3 mmol/L. Although ATP has roughly the same affinity for hemoglobin as does 2,3-DPG, it has little effect on oxygen affinity because it is mostly present as ATP-Mg 2+, which binds weakly to hemoglobin. 2,3-DPG is formed by rearrangement of 1,3-bisphosphoglycerate,

SECTION

28.2 FunctionalAspectsof Hemoglobin

653

13143His [582Lys

>

/--T\ HN,,~NH

-~

132His

~ N H+ 3

131Val

~NH3 +

HN~~NH

-0~ ~Oo~P~o \ ~H 0 2,3-DPG ~C~c :'~'" _ CH2 I -O~ / O

+/ H3N

"

GyAy HPFH

Hb KENYA GyAy(B/~)~ thai

---e J

INV~.RTED

Gy (8~)~ GY ( 8 ~ ) o thai

,(.--

l

(yS~) ~ thai (y8,8)~ thal X~Othal

F I G U R E 28-14 Chromosomal organization of the human/~-globin gene family and deletions that cause thalassemia (thal). In the/~-gene family, the exons are shown as dark boxes and the introns as open boxes. The various deletions are shown by horizontal bars under the/4-gene family. Cross-hatched regions indicate uncertainty in the termination points of the deletion. Arrows indicate unmapped end points of the deletion. [Reproduced with permission from R. A. Spritz and B. G. Forget, The thalassemias: molecular mechanisms of human genetic disease. Am. J. Hum. Genet. 35, 336 (1983). @ 1983 by the University of Chicago Press.]

Presumably, sequences somewhere in that region repress expression of the ygenes; removal of these sequences allows unrestrained expression of the y loci. If the deletion extends too close to the G• locus, however, expression of the G• gene is reduced, producing the more severe phenotype seen in G• Finally, in y-/3-thalassemia, the 6 locus and both g loci are deleted, while the/3 gene is normal; yet there is no synthesis of any of the/f-like globins. The reason for the nonexpression of this normal/3 locus is unknown. Thus, these disorders hint at the presence of regulatory information within the/3gene complex. It may eventually be possible to activate the ygenes to produce HPFH-like states and reduce the morbidity associated with/~-thalassemias and with mutations affecting the function or stability of the/3-globin chain. In normal adults, the small amount of HbF present is distributed nonuniformly among the erythrocytes. The distribution pattern was established by selective extraction of HbA from erythrocytes in a peripheral blood smear. The more poorly soluble HbF left behind stains pink with eosin. In most persons who have HPFH, all erythrocytes contain some HbE

Hemoglobinopathies The term hemoglobinopathies refers to hemoglobin disorders caused by normal synthesis of qualitatively abnormal globin chains. Transcription and translation of mutant genes usually proceed at a normal rate, but the products denature rapidly or function abnormally.

A selected list of mutant hemoglobins is given in Table 28-5. The most common mutations are the singleamino-acid substitutions caused by one nucleotide change in a globin gene. The table also lists several deletion mutations and one nonsense mutation. Globin chains that contain two mutations are extremely rare and appear to have resulted from a second mutation in a common mutant (usually HbS or HbC).

Molecular Pathology The effect of a mutation on the structure and function of hemoglobin is determined by the effects of the amino acid change. Many residues in hemoglobin have been conserved during evolution, probably because they are essential for normal hemoglobin function. The most critical regions for normal structure and function are the heme contacts and oelfll and Ofl~ 2 contacts. Substitutions near the heme group can cause weak or absent heme binding, resulting in loss of the heme group from the affected subunits (Hb Sydney, Hb Hammersmith). They can also stabilize the iron as Fe 3+ and prevent reversible oxygen binding (as in hemoglobin M). The heme group is also less tightly bound in methemoglobin than normal hemoglobin. Substitutions in amino acids involved in subunit contacts can have several effects. In Hb Philly, a Tyr ~ Phe change in the at ot 1/61 interface eliminates a hydrogen bond needed to stabilize deoxyhemoglobin (T-state) and leads to dissociation into monomers, which precipitate. This dissociation decreases cooperativity and increases oxygen

TABLE 28-5 Molecular and Clinical Characteristics of Some Abnormal Hemoglobins Position*

Amino Acid Change

t~-Globin Mutations Memphis 23(B4) Torino 43(CEI)

Glu---~Gln Phe---~Val

Hemoglobin

DNA Codon t Alterations

GAG---~CAG TTC---~GTC

Ann Arbor Chesapeake

80(F1) 92(FG4)

Leu---~Arg CTG---~CGG Arg---~Leu CGG-+CTG

G-Georgia

95(G2)

Pro-+Leu

CCG---~CTG

Leu---~Pro

CTG-+CCG

Glu-+Val Glu-+Ala Glu-+Lys

GAG---~GTG GAG-+GCG GAG-+AAG

Bibba 136(H19) /3-Globin Mutations S 6(A3) G-Makassar 6(A3) C 6(A3) Leiden

Glu---~---

GAG---~---

G-San Jose Saki Savannah E

6 or 7 (A3 or 4) 7(A4) 14(All) 24(B6) 26(B8)

Glu-+Gly Leu---~Pro Gly-+Val Glu-+Lys

GAG---~GCG CTG-+CCG GGT-+GTT GAG---~AAG

Genova S t. Louis

28(B 10) 28(B 10)

Leu-+Pro Leu---~Gln

CTG---~CCG CTG-+CAG

Tacoma Abraham Lincoln Philly

30(B 12) 32(B 14) 35(CI)

Arg-+Ser Leu---~Pro Tyr---~Phe

AGG-+AG(T,C) CTG-+CCG TAC---~TTC

Hammersmith

42(CD1)

Phe-+Ser

TTT-+TCT

Louisville

42(CD1)

Phe--~Leu

TTT-+TT(A,G) or TTC

Tochigi Bristol

56-59 deletion (D7-E3) 67(Ell) Val---~Asp

GTG-+GA(T,G)

Sydney

67(Ell)

Val-+Ala

GTG-+GGG

Seattle Shepherd's Bush

70(E 14) 74(E18)

Ala-+Asp Gly---~Asp

GCC-+GAC GGC-+GAC

Tours

87(F3)

Thr---~---

ACA-+---

Gun Hill

91-95

deletion

KOln Nottingham Kempsey

98(FG5) 98(FG5) 99(G 1)

Val---~Met Val---~Gly A sp---~Asn

GTG-+ATG GTG-+GGG G AT---~AAT

Remarks ~

Reduces sickling when present in HbS homozygotes. Decreases 0 2 affinity; unstable; mild anemia; decreases heme binding. Unstable. Increases 0 2 affinity; R state stabilized; a 1~ z -c~ mutant. Oxy form dissociates to dimers; decreases O 2 affinity; a 1/32-contact mutant. Dissociates. See discussion of sickle cell anemia in this chapter Benign; does not interact with HbS. Crystallizes in red blood cells; enhances sickling when heterozygous with HbS. Deletion decreases O 2 affinity; mild hemolysis; unstable. Compare to HBS; benign. Unstable; Pro disrupts a-helix. Unstable. Mild hemolytic anemia; increases subunit dissociation; normal O z affinity; a 1/31-contact mutant Increases O 2 affinity; unstable. Iron readily oxidized; polar group in heme pocket; increases 0 2 affinity; unstable. Normal O z affinity; decreases Bohr effect; unstable. Also called Perth; unstable. Loss of H bond needed to stabilize T state; high 0 2 affinity; unstable. Decreases O 2 affinity; unstable; poor heme binding severe anemia; inclusion bodies. Decreases 0 2 affinity; unstable; mild anemia; heme misoriented but not lost. Shortening of E-helix; unstable. Unstable; polar group in heme pocket; unusual codon needing 2 mutations. Unstable; mild hemolysis; inclusion bodies; poor heme binding. Unstable; mild hemolysis; decreases O 2 affinity. Unstable; increases 0 2 affinity; negative charge in DPG site; decreases DPG binding. Deletion shortens F-helix; unstable; increases 0 2 affinity. Deletion shortens F-helix; unstable; increases 0 2 affinity. Unstable; increases O z affinity; al/32-contact mutant. Unstable; increases 0 2 affinity; T-state destabilized. Increases 0 2 affinity; R-state stabilized.

(continued)

fi66 T A B L E 28-5


(Continued)

Hemoglobin

Position*

Amino Acid Change

Kansas Beth Israel Yoshizuka Presbyterian San Diego Peterborough Indianapolis D-Los Angeles

102(G4) 102(G4) 108(G10) 108(G10) 109(G11) 1 ll(G13) 112(G14) 121 (GH4)

O-Arab North Shore Syracuse

121 (GH4) 134(H12) 143(H21)

McKees Rocks

145(HC2)

Bethesda Cowtown

1145(HC2) 146(HC3)

DNA Codon r Alterations

Remarks ~

AAC-->ACC "[ Verylow 0 2 affinity; cyanosis. AAC---~AGC J AAC-->GAC '~ AAC--->AA(A,G) ~.Unstable; increases O 2 affinity; a 1/31-contact GTG---~ATG [ mutant. GTC---~TTC J TGT---~CGT Unstable. GAA---~CAA Also called D-Punjab; benign; enhances sickling in heterozygotes with HbS. As for HbD-LosAngeles. Glu---~Lys GAA---~AAA GTG---~GAG Unstable; mild hemolysis; normal O 2 affinity. Val---~Glu CAC---~CCC Increases O 2 affinity, decreases Bohr effect; loss of His--->Pro positive group at DPG binding site. Decreases DPG binding and Bohr effect; increases Tyr-+term. TAT---~TAA O2 affinity; deletes C-terminal residues; eliminates C-terminal bonding. TAT---~CAT "[ Increases O 2 affinity; decreases Bohr effect and Tyr--~His CAC---~CTC J DPG binding; disrupts C-terminal H bond, His---~Leu destabilizes T-state. Methemoglobinemia Asn---~Thr Asn-->Ser Asn---~Asp Asn-->Lys Val-->Met Val-->Phe Cys-->Arg Glu---~Gln

Mutations Causing Congenital o~-Chain: M-Boston 58(E7) Hi s---~Tyr CAC---~TAC M-Iwate 87(F8) Hi s----~Tyr CAC---~TAC /3-Chain: M-Freiburg 23(B5) Val---)--GTT---) .... M-Saskatoon 63 (E7) His---~Tyr C AT---~TAT M-Milwaukee 67(E11) Val---~Glu GTG---~GAG M-Hyde Park 92(F8) His---~Tyr CAC---~TAC

Only known in heterozygotes; all cause a benign cyanosis.

*The first number is the residue position, with 1 being the N-terminal amino acid; Kendrew's helical notation is given in parentheses. *Codon assignments are based on published sequences for human or- and [3-globin genes. Sequences of the alpha-1 and alpha-2 globin genes do not differ from each other at any of the codons used above. In a few instances, the normal codon could have mutated to either of two or three codons; these ambiguities are indicated by: term = termination codon; --- means codon (and amino acid) deleted. The codons shown here would occur on the nontranscribed strand in DNA. SBenign means no observable symptoms.

affinity. In hemoglobins Yoshizuka, Presbyterian, and Peterborough, the c~fl~ interface also is affected, but they exhibit decreased oxygen affinity because the R-state (oxyhemoglobin) is more destabilized than the T-state. Contacts at the O(lfl 2 interface stabilize the interaction of the two oefl dimers through the packing contacts and allow subunit motion during the T Z R transformation at the sliding contacts. Mutations in the /3 chains of Hb Kansas and Hb Beth Israel at this interface eliminate a hydrogen bond found only in the R-state, destabilizing the R-state and decreasing oxygen affinity and dissociation of the tetramer. The oxygen affinities of these variants are so low that they cause cyanosis, usually seen only in methemoglobinemia. Hb Chesapeake has an or-chain mutation

that affects the ot l f12 interface, stabilizing the R-state and increasing oxygen affinity. Other otlfl2 interface mutants, such as HbG Georgia, dissociate into dimers upon oxygenation and reassociate to tetramers when oxygen is removed. The deleterious effect of an amino acid substitution is due to replacement of a useful side chain by one that cannot perform the necessary function. The importance of a side chain may reside in its size, shape, or charge. A side chain that is too small (Hbs Torino, Hammersmith, and Sydney) or too large (Hbs Savannah and Peterborough) can be disruptive. Because 80% of the globin chain has a helical conformation, mutations that introduce proline residues are likely to disrupt this structure because proline

SECTION 28.3

Inherited Disorders of Hemoglobin Structure and Synthesis

cannot fit into an c~ helix (Chapter 4). The disruption can cause severe disease (Hbs B ibba, Abraham Lincoln, and Genova) or be benign (Hb Saki). If proline is introduced at one end of a helix, it does not disrupt it. However, in Hb Syracuse, new proline at the end of the H helix eliminates positive charge needed for 2,3-DPG binding. Loss of a charged or polar group (Hb Syracuse) or introduction of a charged or polar group into the hydrophobic interior of the molecule (Hbs Ann Arbor, St. Louis, North Shore, and Indianapolis) usually distorts the structure and causes altered function. If the mutation occurs at a site where the side chain is not very important or where only the type of side chain is important, the mutation may well be benign. A number of mutations of this type have been detected during routine screening in many populations.

Unstable Hemoglobins Many of the substitutions mentioned above adversely affect the stability of the tetrameric molecule and cause easy denaturation and precipitation within erythrocytes to form Heinz bodies. Deletion of one or more amino acids is also likely to cause instability, precipitation (e.g., Hbs Leiden, Tochigi, Tours, Gun Hill), and membrane damage. Consequently, these variants are associated with intravascular hemolysis, anemia, reticulocytosis, splenomegaly, and, in some patients, intermittent urobilinuria. If the hemoglobin precipitates soon after synthesis, before the cells leave the bone marrow, a thalassemialike syndrome may occur (Hbs Nottingham and Indianapolis). These mutations as well as Hbs Hammersmith, Abraham Lincoln, B ibba, and others produce a severe clinical disorder with massive hemolysis that is not improved by splenectomy. Other mutations (e.g., Hbs Torino, Ann Arbor, St. Louis, Koln, and Shepherd's Bush) in which precipitation occurs in the circulation cause severe hemolysis but are improved by splenectomy, which increases the erythrocyte lifetime. Although sometimes difficult, it is important to distinguish these two groups clinically when deciding whether splenectomy is indicated. Most unstable hemoglobins are associated with only mild hemolysis and occasional hemolytic crises, usually brought on by some stress such as a mild infection or treatment with sulfonamide or other oxidizing drugs (e.g., Hbs Philly, Sydney, North Shore, Leiden, Gun Hill, Seattle, and Louisville). Another group (e.g., Hbs Saki and Tacoma) is clinically benign, and the unstable hemoglobin is discovered incidentally by routine electrophoretic screening. The difference in disease severity among the unstable hemoglobins may reside in the number and time of

667

formation of Heinz bodies. Heinz body formation is believed to start when hemoglobin is oxidized to methemoglobin. The unstable methemoglobins are degraded to hemichromes, a process promoted by their tendency to dissociate (separate subunits form hemichromes more readily than tetramers). The tendency of the chains to form hemichromes decreases in the order c~ >/3 > y. Hemichromes precipitate and form Heinz bodies, which attach hydrophobically to the erythrocyte membrane, increasing membrane permeability and the rate of lysis. Loss of heme and oxidation of sulfhydryl groups do not appear to be necessary for precipitation. The degree of morbidity associated with an unstable hemoglobin is also determined by the effect of the mutation on oxygen affinity. If the oxygen affinity is decreased, delivery of oxygen to the tissues will be higher than normal and compensate, in part, for the lower hemoglobin concentration. This is the case with Hbs Leiden, Seattle, Louisville, and Peterborough. In contrast, Hbs K61n, St. Louis, and Shepherd's Bush, which cause severe hemolytic anemia and Heinz body formation, have increased oxygen affinity. The effect of the anemia is exacerbated by the inability of erythrocytes to unload the oxygen they carry. Both groups have similarly low hemoglobin levels and rapid hemolytic rates. Thus, the phenotype caused by a mutation is the net result of all its effects on hemoglobin structure and function.

Secondary Polycythemia Syndromes Mutations thatincrease oxygen affinity (reduce Ps0) can decrease tissue oxygenation more than expected on the basis of hemoglobin concentration. At low oxygen tension, production of erythropoietin is stimulated, causing a secondary polycythemia that increases oxygen delivery to the tissues and partially compensates for the hemoglobin abnormality. Any mutation that stabilizes the R (oxy) state (Hb Chesapeake) or destabilizes the T (deoxy) state (Hb Kempsey) can cause polycythemia. More than 28 such variants are known. Fourteen of these high-affinity hemoglobins have mutations at the 0/1,8 2 interface. Because the C terminus of the /3 chain supplies a hydrogen bond that is important for stabilizing the T-state, its replacement (Hb Bethesda), deletion (Hb McKees Rocks), or alteration in its environment (Hbs Syracuse and Cowtown) markedly decreases Ps0. Hb San Diego is unique in having high oxygen affinity due to a substitution in the 0/1/81 interface. Mutations that decrease the stability of the hemoglobin and cause precipitation or dissociation can also increase oxygen affinity.

668

Congenital Methemoglobinemias and Cyanosis Cyanosis is a bluishcoloration of the skin caused by the presence of more than 5 g of deoxyhemoglobin or methemoglobin per deciliter of blood in the subcutaneous capillary beds. It occurs in severe cardiopulmonary disease from poor oxygenation of normal hemoglobin or methemoglobin production. An extremely rare cause of cyanosis is the presence of a hemoglobin variant with very low oxygen affinity (e.g., Hb Kansas, Hb Beth Israel). Two groups of genetic defects can cause methemoglobinemia. 1. Defects in oxidoreductase in erythrocytes (methemoglobin reductase, glutathione reductase, glucose-6-phosphate dehydrogenase; Chapter 15). Patients who have these defects usually are not cyanotic unless subjected to oxidative stress (e.g., treatment with primaquine or sulfonamide). 2. Defects in which substitution of an amino acid in the region of the heme pocket increases the stability of Fe 3+ in the heme group and produces methemoglobin. Although heme oxidation occurs in several unstable hemoglobins, cyanosis is principally a manifestation of the Hbs M (Table 28-5). Since a single mutation can affect only one type of subunit (a or fi), half of each tetramer is normal. Most of these hemoglobins undergo reduced T-to-R transitions but with more difficulty. In addition, since only heterozygotes, for the Hbs M have been identified, half the tetramers are normal, and the carriers have no functional impairment. Cyanosis is visible because only 1.5-2 g of methemoglobin are needed to produce the same degree of discoloration as 5 g of deoxyhemoglobin. Diagnosis of patients with Hbs M is important because of the seriousness of the alternative causes of cyanosis.

Hemoglobin S and Sickling Disorders The most common, severe, and best studied hemoglobin mutation is HbS, which causes sickle cell anemia (or disease) in homozygous individuals and sickle cell trait in heterozygous individuals. The characteristic change in shape of erythrocytes from biconcave disk to curved and sicklelike occurs at low oxygen pressures. Carriers of sickle cell trait are asymptomatic, since their red cells do not sickle unless the Po2 drops below about 25 mm Hg; this happens only under unusual circumstances, such as unassisted breathing at high altitudes, some severe forms of pneumonia, and occasionally during anesthesia. The symptoms of sickle cell anemia are due to sequestration and destruction of the abnormal, sickled erythrocytes in the spleen and to the inability of many sickled


cells to pass through capillaries. The normal life span of 120 days for red blood cells is decreased to 10-12 days. Painful vaso-occlusive crises can occur anywhere in the body and are the major complication in sickle cell anemia. These crises are caused by blockage of capillaries in the affected tissue by the deformed red cells, which causes hemostasis, anoxia, further sickling, and eventually infarction. Increased adhesion of sickled erythrocytes to capillary endothelial cells may also contribute to capillary obstruction. Crises occur only when the circulation slows or hypoxia is present because the normal circulatory rate does not keep cells deoxygenated for the approximately 15 seconds required for sickling to begin. In contrast to the splenomegaly usually seen in chronic hemolytic anemias, a small, fibrous spleen is usually seen in adults with sickle cell anemia. This autosplenectomy is due to repeated infarction of the spleen. The clinical complications of sickle cell disease are highly variable. The severe forms of the disease occur in homozygous SS disease and S/fl~ and a milder disease occurs in double heterozygous SC disease and S/fl+-thalassemia. Patients who have a concurrent ot-thalassemia trait or high HbF levels have a mild course of the SS disease. The latter observation has been utilized in the pharmacological approaches that raise HbF levels (discussed later). Some of the clinical consequences in SS disease include megaloblastic erythropoiesis, aplastic crisis, stroke, bone pain crisis, proneness to infection particularly by Pneumococcus, Salmonella, and Haemophilus due to hyposplenism and acute chest syndrome. Prophylactic use of penicillin and antipneumococcal and Haemophilus vaccines has aided in the management of life-threatening infectious complications of SS disease. Neonatal screening has been used in the identification of infants with sickle cell disease so that risk of infection can be modulated by appropriate immunizations and penicillin prophylaxis. The acute chest syndrome characterized by chest pain is due to clogged pulmonary capillaries; in a small number of studies, patients have been treated with inhaled nitric oxide, which dilates blood vessels with clinical improvement. Sickle cell trait is present in about 8% of black Americans and to a much greater extent (as high as 45%) in some black African populations. The homozygous condition causes considerable morbidity and about 60,00080,000 deaths per year among African children. HbS also occurs in some parts of India, the Arabian region, and occasionally in the Mediterranean area. The HbS mutation that occurs in eastern Saudi Arabia and India, known as the Asian haplotype, has different flanking DNA sequences surrounding the fi-globin locus. Thus, the Asian haplotype may represent an independent occurrence of the HbS mutation that is distinct from its African counterpart. The deleterious gene likely has persisted in these

SECTION 28.3

669

Inherited Disorders of Hemoglobin Structure and Synthesis

populations because the HbS that increases resistance to malaria caused by Plasmodium falciparum, which was, until recently, endemic in those areas. A similar explanation has been advanced for the high frequencies of /~-thalassemia and single or-locus genotypes in the same regions. The biological basis of resistance to malaria has been established in laboratory experiments. As schizonts of P. falciparum grow in erythrocytes, they lower the intracellular pH and generate hydrogen peroxide. The lower pH promotes sickling and the hydrogen peroxide damages cell membranes of thalassemic erythrocytes. In both cases, the erythrocyte membranes become more permeable to potassium ions; the resulting decrease in intracellular potassium kills the parasites. The mutation in HbS replaces glutamic acid (a polar amino acid) by valine (a nonpolar residue) at position 6 of the/~ chains. The solubilities of oxy- and deoxy-HbA and oxy-HbS are similar, being about 50 times that of deoxy-HbS. Upon deoxygenation, HbS precipitates, forming long, rigid, polymeric strands that distort and stiffen the red cells. The very high concentration of hemoglobin in the erythrocyte (340 mg/mL), giving an average intermolecular distance of about 1 nm (10 A), minimizes the time necessary for precipitations to occur. Dilution of ribS, as in sickle cell trait (HbS/HbA heterozygotes), reduces the concentration below the point at which sickling readily occurs. Similarly, the mildness of homozygous HbS disease in populations such as those of eastern Saudi Arabia and Orissa, India is attributed to an accompanying elevation of HbF, which reduces the effective concentration of HbS tetramers. Other abnormal hemoglobins can interact with HbS and alter the course of the disease. The most common is HbC, which has a lysine in place of glutamic acid, also at/~6. The gene has a frequency second only to that of HbS in black Americans and in some black African populations. Persons heterozygotic for HbC are asymptomatic, but homozygotic individuals have a mild hemolytic anemia with splenomegaly. Because of its insolubility, crystals of HbC can sometimes be seen in peripheral blood smears from homozygous individuals. As a result of the coincidental distribution of the genes for HbS and HbC, heterozygotes for both hemoglobins are not uncommon. HbSC disease has a severity intermediate between that seen in persons homozygotic for HbS and those homozygotic for HbC. Unlike HbA, HbC copolymerizes with HbS. In contrast, replacement of the/~6 glutamic acid by alanine (HbG-Makassar) or deletion of/36 (Hb Leiden) results in hemoglobin that neither precipitates nor interacts with HbS. Hb San Jose (Glu --+ Gly at residue/37) is a harmless variant. Individuals heterozygous for HbS and HbO Arab or HbD Los Angeles have a hemolytic anemia of a severity

intermediate between that of HbSC disease and sickle cell anemia. Certain or-chain variants (e.g., Hb Memphis), when present in HbS homozygotes, can ameliorate the clinical course of the disease. The severity of HbS-/~thalassemia depends on whether the thalassemia is/3 ~ or /~+ and, if it is/3 +, on how much normal/~ chain is synthesized. Severity also depends on the HbF concentration. A patient with heterozygosity for both HbS and Hb Quebec-chori exhibited clinical symptoms suggestive of sickle cell disease. Hb Quebec-chori, an electrophoretically silent variant at acid and alkaline pH (see Appendix VII) (/~87 Thr --+ Ile), polymerizes with HbS with the stabilization of the polymer under hypoxic conditions leading to sickling of red blood cells. Thus, Hb Quebec-chori provides an example of a hemoglobin that has the potential to polymerize with HbS and causing sickle cell disease in a sickle cell trait condition which is otherwise benign by itself. Determination of the structure of crystalline HbS has shown that in the/3 subunits of oxy-HbA and oxy-HbS, a "hydrophobic pocket" between helices E and F is closed; this opens in the deoxy form. In HbA, the residues at the surfaces of the globin subunits are hydrophilic (polar) and do not interact with this pocket. In HbS, however, the/36 valine is hydrophobic and fits into the hydrophobic pocket (formed by leucine and phenylalanine at/~85 and/388) of an adjacent/3 chain to form a stable structure. Since each /~ subunit in deoxy-HbS has an "acceptor" hydrophobic pocket and a "donor" valine, linear aggregates form (Figure 28-15). Understanding of the sickling process and of the structure of the HbS polymer provides a rational basis for ways of correcting the molecular defect. Thus, dilution of the HbS in the red cells, blockage of the interaction of the /36 valine with the hydrophobic pocket, and decrease of

F I G U R E 28-15

Structure of hemoglobin S (HbS)polymer.The valine at the/~6 position of the deoxy-HbSfits into the hydrophobicpocket formedby leucine and phenylalanine at/~85 and/~88 of an adjacent/~ chain. Sinceeach/~ chain has an "acceptor"pocket and a "donor" valine, the HbS polymerhas a double-stranded, half-staggeredstructure. [Reproducedwith permission from S. Charache, Advancesin the understanding of sickle cell anemia. Hosp. Pract. 21(2), 173 (1986). J. E. Zupko, illustrator.]

670 the deoxy-HbS oxy-HbS ratio should reduce the likelihood of sickling and the severity of the disease. These approaches have been tried but so far have failed to ameliorate the disease. Among Bedouin Arabs and some populations of central and southern India, high HbF levels reduce the severity of sickle cell disease by inhibiting the formation HbS polymers. This observation has led to therapeutic approaches to induce higher levels of HbF in patients with sickle cell disease. The therapeutic agents are hydroxyurea and short-chain fatty acid derivatives. Hydroxyurea is an antineoplastic agent that inhibits ribonucleotide reductase (Chapter 27). It is thought that in bone marrow, hydroxyurea selectively kills many precursor cells but spares erythroblasts that produce HbE Hydroxyurea therapy also results in decreased circulating granulocytes, monocytes, and platelets. These changes, along with increased HbF, reduce vaso-occlusion due to a decreased propensity for sickling and adherence of red blood cells to endothelium. The long-term toxicity of hydroxyurea due to its myelosuppressive and teratogenic effects is not known. Induction of HbF by short-chain fatty acids such as butyrate was discovered in infants of diabetic mothers who had a delay in switching from HbF to HbA in association with elevated serum levels of amino-n-butyrate. This led to studies of HbF induction with several short-chain acids, including butyrate, all of which induce production of HbE The mechanism by which butyrate and other short-chain fatty acids affect gene expression involves transcriptional activation of a v-globin gene at the promoter site. One of the mechanisms of regulation of transcription, which occurs at the promoter site, is due to changes in histone acetylation and deacetylation (Chapter 26). Acetylation of specific histones by acetylases allows increased binding of transcription factors to target DNA that stimulates transcription. Deacetylation by histone deacetylases of histones prevents transcription. Butyrate is an inhibitor of deacetylase and it is thought that this action leads to induction of v-globin gene expression. Since hydroxyurea acts by a different mechanism in the elevation of HbF, butyrate's action can be additive or synergistic. Sustained induction of HbF by pulse butyrate therapy in patients with sickle cell anemia has resulted in up to a 20% increase in HbF along with marked clinical improvement. Hemoglobin E results from the substitution of glutamic acid with lysine at the 26th residue of the/~ chain; it has a high prevalence in Southeast Asia. Neither heterozygous nor homozygous states are associated with significant clinical abnormalities. The homozygous state exhibits a mild anemia, and both states show red blood cell indices resembling those of heterozygous thalassemic states, namely, hypochromic microcytosis. The latter has been attributed

CHAPTER 28

Hemoglobin

to activation of a cryptic splice site in exon 1 of/3-globin mRNA caused by/~E mutation. This leads to abnormal splicing of/~-globin mRNA, producing a less stable and ineffective mRNA. Coinheritance of the HbE gene with different forms of thalassemia genes is also known. 9'- and 6-Globin M u t a n t s The V- and 3-chain mutants are difficult to study because of the small fraction of HbF and HbA2 present in adult erythrocytes. Overt clinical symptoms associated with ?,- and g-chain variants are rare. By routine screening, 14 6-globin variants have been discovered but are of no clinical consequence. Thirty-five mutant V sequences (involving the G• or the A• chains) have been identified. They are all benign except for HbF Poole [G• Tyr ~ Gly], an unstable hemoglobin that causes hemolytic disease in the newborn.

Screening and Prenatal Diagnosis Screening programs are aimed at detection of the most common and most harmful hemoglobin genes and the identification of heterozygotes. Screening can provide a basis for genetic counseling and decrease the number of homozygous individuals conceived. Electrophoresis is used to differentiate among sickle cell trait, sickle cell anemia, and HbSC disease and to detect thalassemia, hereditary persistence of HbF, and HbC trait and HbC disease (see Appendix VII). Prenatal diagnosis of genetic disease often complements programs for detection of heterozygotes. If both parents are known to be carriers, a homozygous fetus can be detected early in pregnancy. The first prenatal tests measured the ability of fetal reticulocytes to direct the synthesis in vitro of normal or abnormal /3 chains. Fetal blood was obtained either by percutaneous aspiration from the placenta or transvaginally by direct aspiration from one of the fetal vessels on the surface of the placenta. The cells were incubated with radioactive amino acids, and the labeled globin chains were separated by chromatography and quantitated. This method detects normal and sickle cell anemia genotypes, absence of or- or /~-globin synthesis (homozygous c~~ or/~~ and some types of/~+-thalassemia. DNA has been isolated directly from cultured fetal cells (obtained by amniocentesis and from trophoblast biopsy) and subjected to restriction endonuclease digestion. (Culturing of fetal cells is necessary, since their number in the amniotic fluid sample is small.) The restriction fragments are separated according to size by electrophoresis and reacted with a labeled DNA probe that is complementary to the gene being sought. The hybridization pattern is compared with

671

SECTION28.4 Derivatives of Hemoglobin

F I G U R E 28-16 Diagram of the 5' region of the human fl-globin gene and its flanking region, showing the cleavage sites of the restriction endonuclease Mst II (vertical arrows). The fragments obtained by digestion of normal and sickle fl-globin genes are shown in the lower part of the diagram. Shaded regions of the gene are the exons, and IVS indicates the introns (intervening sequences). [Reproduced with permission from J. C. Chang and Y. W. Kan, A sensitive prenatal test for sickle cell anemia. N. Engl. J. Med. 307, 30 (1982).]

one from a normal individual (Chapter 23). Differences between the normal and some mutant globin genes at recognition sites for restriction endonucleases will cause differences in the cuts made and, hence, in the sizes of the fragments detected. In the ideal situation, a restriction endonuclease recognizes a site in the DNA that includes the mutated base. Restriction enzymes Mnl I, Dde I, and Mst II recognize part of the coding sequence for amino acids 5, 6, and 7 of the normal fi-globin gene. The mutation in the fi-globin gene in HbS abolishes these recognition sites, converting two small pieces of DNA found in the digest of normal /~-globin DNA into one larger fragment (Figure 28-16). Mst II is the most useful enzyme because it generates larger, more easily detected fragments. Loss of recognition sites due to deletions is seen in most o~-thalassemias and occasionally in fi-thalassemias, since most fi-thalassemias are not caused by deletion. In gene deletion, multiple sites are lost. However, point mutations are more common and therefore of greater importance than deletions. The endonuclease that is most useful in the detection of each disorder must be carefully selected. Because fi-thalassemias are caused by many different mutations with similar phenotypic expressions, it is not possible to find one or even a few restriction endonucleases that can detect all fi-thalassemia genes. What has been accomplished so far is the detection of affected individuals among members of the same kindred or racial group in which a single type of fi-thalassemia occurs. Alternatively, sites adjacent to an abnormal gene may differ from those on normal chromosomes. Such linked polymorphisms have been found for the sickle cell gene and for several fi-thalassemias. Because different individuals having a particular hemoglobin abnormality will not

always have the same linked polymorphism, this method suffers from lack of sensitivity. Under proper conditions, short, synthetic oligonucleotide probes, complementary to the normal DNA sequence in the region that contains the mutation, hybridize with completely homologous sequences but not with sequences that differ by as little as one nucleotide. This method is potentially more powerful than restriction endonuclease mapping because an oligonucleotide probe can be synthesized for any mutation once its sequence is known. Furthermore, multiple mutations can be detected simultaneously by hybridizing the DNA with a mixture of different probes, making this technique useful for screening. This technique has been used successfully to detect the sickle cell gene and several fl-thalassemias that could not be detected by restriction endonuclease mapping. All these methods depend on a satisfactory sample being collected from the fetus. Studies of globin chain synthesis require an adequate volume of fetal blood. Collection methods result in 4-6% fetal death under optimal conditions. Furthermore, fi-globin synthesis cannot be detected until about the middle of the second trimester. Use of amniotic fluid cells obtained by amniocentesis is less dangerous. However, an adequate volume of fluid cannot be obtained until the middle of the second trimester of pregnancy (14-20 weeks); if culturing of the cells is necessary, there is an additional delay of 1-2 weeks. Chorionic villus biopsy can be obtained at about 8-12 weeks of gestation. This allows more time for deliberation regarding termination or continuation of the pregnancy, thereby increasing the margin of safety for the mother, should termination be desired. This technique is no more dangerous than either of the other methods. Some other techniques for laboratory evaluation of hemoglobin disorders are presented in Appendix VII.

28.4 Derivatives of Hemoglobin Carbon Monoxide-Hemoglobin Carbon monoxide, an odorless gas, has an affinity for hemoglobin that is 210 times that of oxygen. Thus, in the equilibrium reaction HbO2 + CO ~ HbCO-4-02

Keq -- 2.1 • 102

the equilibrium lies far to the right. Like oxygen, CO binds to the sixth position of the heme iron. When CO and 02 are present together in appreciable quantities, CO is bound preferentially and oxygen is excluded, effectively causing an anemic hypoxia. Even if some O2 is bound to the hemoglobin, it cannot be released owing to the tight


672 binding of CO and to its action as an allosteric activator, with the result that hemoglobin is trapped in the R-state. Carbon monoxide poisoning may result from breathing automobile exhaust, poorly oxygenated coal fires in stoves and furnaces, or incomplete combustion of a carbon-containing compound. Breathing air containing 1% CO for 7 minutes can be fatal; automobile exhaust contains about 7% CO. Unconsciousness, a cherry-red discoloration of the nail beds and mucous membranes (due to large amounts of R-state carbon monoxide-hemoglobin), and spectrophotometric analysis of the blood are useful for clinical diagnosis. Treatment of carbon monoxide poisoning consists of breathing a mixture of 95% O2 and 5% CO2, which will usually eliminate carbon monoxide from the body in 30-90 minutes, or of breathing hyperbaric oxygen. The enzymatic oxidation of heme produces CO and biliverdin in equimolar amounts (Chapter 29). The CO is transported via the blood to the lungs, where it is released. Although no cases are known in which endogenous CO proved toxic, this carbon monoxide may contribute significantly to air pollution, particularly in crowded and enclosed areas.

Carbaminohemoglobin Some of the C02 in the bloodstream is carried as carbamino compounds (Chapter 1). These compounds form spontaneously in a readily reversible reaction between CO2 and the free c~-amino groups in the N-terminal residues of the Hb chains.

H2N~

NH2 4-4002 ~

H2N

~-

NH2 Hemoglobin

0%

H

I

H

I //o

Ho/C--N~N--C~oH O~c__N//xL'A-'~ N C~ 0 HO/ I H HI- ~OH Carbaminohemoglobin Although Hb can carry as many moles of C02 as of 02, the HCO~- system is a more important way of transporting CO2. When the N-terminal amino groups are blocked by carbamylation with CNO-, no marked degree of acidosis develops. The carbamino groups decrease the affinity of Hb for 02. The Pr thus can affect oxygen affinity independently of any effect it may have on pH and may

influence the position of the oxygen dissociation curve under various conditions.

Methemoglobin In the presence of oxygen, dissolved hemoglobin is slowly oxidized to methemoglobin, a derivative of hemoglobin in which the iron is present in the ferric (Fe 3+) state. In metHb, the ferric ion is bound tightly to a hydroxyl group or to some other anion. The heme porphyrin (protoporphyrin IX) that contains an Fe 3+ ion is known as hemin. A small but constant amount of methemoglobin is produced in the body. It is reduced by specific enzymes (methemoglobin reductases) and NADH, which is generated in glycolysis. Reductases isolated from human red cells also use NADPH but to a lesser extent. Inability to reduce metHb produces methemoglobinemiaandtissue anoxia. Hereditary methemoglobinemia may arise from the following: 1. Deficiency of one or more of the metHb reducing enzymes (usually a recessive trait). MetHb values may range from 10% to 40% of the total Hb (normal = 0.5%). Treatment involves administration of an agent that will reduce the metHb (e.g., ascorbic acid, methylene blue). 2. Defects in the hemoglobin molecule that make it resistant to reduction by metHb reductases and ascorbate or methylene blue. These hemoglobins are collectively called the M hemoglobins (Table 28-5). HbM is inherited as a dominant trait, since homozygosity for an M fl-chain hemoglobin would be lethal. In four types that involve an His ---> Tyr substitution, the phenolic hydroxyl group forms a stable complex with Fe 3+, making it resistant to reduction to Fe z+. In HbM Milwaukee, a glutamic acid residue substitutes for a valine at position 67 near the distal histidine and forms a stable complex with Fe 3+. Although a brownish cyanosis is characteristic of blood containing HbM, in contrast to the purplish cyanosis caused by excess deoxyhemoglobin, diagnosis should be confirmed by the absorption spectrum of either the Hb or its CN- derivative. The spectrum of the cyanide derivative is preferred, since it differs more from that of HbA. Some of the M hemoglobins cannot form CN- derivatives, presumably because a Tyr or Glu blocks the sixth iron position. In these cases, the absorption spectrum of the methemoglobin itself must be examined. Because of the change in the environment of the heme due to the change in the globin structure, the spectra of M

SECTION 28.4

Derivatives of Hemoglobin

673

hemoglobins differ among themselves and from that of normal metHb. Theoretically, hereditary methemoglobinemia could occur if the hemoglobin was altered in that region where methemoglobin reductase binds in performing its function. In such a mutant Hb, the reductase would not bind. Acquired acute methemoglobinemia is a relatively common condition caused by a variety of drugs such as phenacetin, aniline, nitrophenol, aminophenol, sulfanilamide, and inorganic and organic nitrites and nitrates. The condition is less commonly produced by chlorates, ferricyanide, pyrogallol, sulfonal, and hydrogen peroxide. These compounds appear to catalyze the oxidation of Hb by oxygen. Symptoms include brownish cyanosis, headache, vertigo, and somnolence. Diagnosis is based on occurrence of brownish cyanosis and presence of excessive amounts of metHb (measured spectrophotometrically). Treatment usually consists of removal of the offending substance and administration of ascorbic acid (in mild cases) or methylene blue (in severe cases). These reducing agents function according to the reactions below (MB -- methylene blue, oxidized; MBH2 -- methylene blue, reduced). MBH2 ~ MB + Hz(gas) 2Hz(gas) 4- Hb(Fe 3+)4 --+ Hb(Fe 2+) + 4H + (The release of H2 by MBH2 is spontaneous because methylene blue is autoxidizable.) " O---C

I

HO--C

II

HO--C

I

H--C

I

I

T

HO--CH

I CH2OH Ascorbic acid (vitamin C)

O--C

I

j/

4H+

O--C

I

O--C

I

Hb(Fe+3)4

Hb(Fe+2)4

H--C

i

O

I

I

HO--CH

I

CH2OH Dehydroascorbic acid

Sulfhemoglobin Many drugs that cause methemoglobinemia also stimulate production of sulfhemoglobin, a greenish derivative. Methemoglobin and sulfhemoglobin may appear together in poisoning by phenacetin, acetanilid, or sulfanilamide. Dapsone (used to treat leprosy) and exposure to sulfurcontaining compounds either occupationally or from air pollution can also produce sulfhemoglobinemia. The sulfur appears to attach covalently to the porphyrin ring rather than to the iron atom. It is unclear whether the sulfur is derived from hydrogen sulfide generated by anaerobic

metabolism of intestinal bacteria, from glutathione, or from some other source. Because of its color, patients with circulating sulfhemoglobin appear cyanotic like those with methemoglobinemia. However, unlike methemoglobinemia, sulfhemoglobinemia cyanosis becomes clinically apparent at extremely low concentrations and is usually not accompanied by cardiopulmonary pathology. This fact, together with the overlap in the list of causative agents, may lead to underdiagnosis of sulfhemoglobinemia. Both sulfHb and metHb show an absorption peak at about 620 nm that is not present in deoxyHbA (the most common cause of cyanosis). They can be discriminated by changes in this peak on addition of cyanide, carbon monoxide, or dithionite (a reducing agent). Isoelectric focusing of samples treated with the same chemicals is also useful for differential diagnosis. Oxygen does not bind to a subunit having a sulfurated porphyrin ring. The presence of one or two such subunits, together with normal subunits in a hemoglobin tetramer, alters the cooperativity, favors the deoxy structure, and thereby decreases the oxygen affinity of the normal subunits. Thus, at low levels of sulfuration, the percentage of affected hemoglobin tetramers is greater than the percentage of sulfurated monomers. (This is also true of the methemoglobins). In contrast, binding of carbon monoxide to one or two subunits increases the oxygen affinity of the tetramer, since CO stabilizes the oxyhemoglobin conformation. In a person who has HbA as the predominant hemoglobin, the presence of sulfhemoglobin is probably innocuous. Tissue oxygenation is relatively normal because the decrease in the number of binding sites for oxygen is offset by the lower binding affinity. However, since sulfuration stabilizes the deoxy form, sulfhemoglobin, in individuals who have sickle cell anemia, should increase the likelihood of sickling and thereby exacerbate the disease. No way is known to reverse formation of sulfhemoglobin. If the causative agent is removed, sulfhemoglobin will disappear from the circulation at the same rate as that of the erythrocytes that contain it.

Cyanmethemoglobin Cyanide poisoning does not cause production of cyanohemoglobinemia or cyanosis. It does produce cytotoxic anoxia by poisoning cytochrome oxidase and other respiratory enzymes, thereby preventing utilization of O2 by tissues. Cyanide poisoning is detected by the characteristic odor of HCN gas (odor of bitter almonds) on the breath and by laboratory tests (absorption spectra, tests for CN-).

674 Treatment of cyanide poisoning consists of diverting the cyanide into the production of cyanmetHb. First, some of the normal hemoglobin is converted to methemoglobin by intravenous infusion of a solution of NaNO2 or inhalation of amyl nitrite. Once metHb is formed, CN- can replace OH- at position 6 of the iron, since it has a higher affinity of Fe 3+ than does OH-. CyanmetHb is no more toxic than metHb and cells containing it can be eliminated by normal body processes. The cyanide bound to metHb is always in equilibrium with free CN-, and this uncomplexed cyanide is converted to thiocyanate (SCN-; nontoxic) by administration of thiosulfate (Chapter 6).

Glycated Hemoglobins Several minor varieties of hemoglobin are produced by nonenzymatic posttranslational modification (e.g., glycation). Because these minor hemoglobins are present in such small amounts, none is pathological. Both or- and s-amino groups of hemoglobin form amino-l-deoxyfructose adducts upon reaction with glucose (Chapter 2). Other hexoses can give rise to similar adducts (e.g., galactose in galactosemia; Chapter 15). The major sites of in vivo glycation in order of prevalence are fl-Vall, fl-Lys66, c~-Lys61, fl-Lysl 7, and ot-Vall. The adduct formed with the amino terminus of the fl chains is known as HbAic, which makes up about 4-6% of the total hemoglobin in normal red blood cells. Its concentration is increased in uncontrolled diabetics who have hyperglycemia. Glycated hemoglobins are detected as a fast-moving hemoglobin in electrophoresis at alkaline pH. Because HbAlc accumulates within the erythrocyte throughout the cell's normal life span, it is used as an indicator of the success of long-term blood glucose control in diabetics (Chapter 22). Also known are adducts of hemoglobin with glucose-6-phosphate and fructose1,6-diphosphate, which probably compete with 2,3-DPG for binding to the fl chains of deoxyhemoglobin, and a complex between hemoglobin and glutathione, particularly in older erythrocytes.

Supplemental Readings and References M. O. Arcasoy and E G. Gallagher: Molecular diagnosis of hemoglobinopathies and other red blood cell disorders. Seminars in Hematology 36, 328 (1999).

CHAPTER28 Hemoglobin G. F. Atweh, M. Sutton, L. Nassif, et al.: Sustained induction of fetal hemoglobin by pulse butyrate therapy in sickle cell disease. Blood 93, 1790 (1999). H. E Bunn: Pathogenesis and treatment of sickle cell disease. New England Journal of Medicine 337, 762 (1997). H. E Bunn: Induction of fetal hemoglobin in sickle cell disease. Blood 93, 1787 (1999). A. Cao, R. Galanello, and M. C. Rosatelli: Genotype-phenotype correlations in fl-thalassemias. Blood Reviews 8, 1 (1994). M. Cazzola, E Mercuriali, and C. Brugnara: Use of recombinant human erythropoietin outside the setting of uremia. Blood 89, 4248 (1997). D. H. K. Chui, R. Hardison, and C. Riemer: An electronic database of human hemoglobin variants on the World Wide Web. Blood 91, 2643 (1998). S. C. Davies and L. Oni: Management of patients with sickle cell disease. British Medical Journal 315, 656 (1997). B. L. Ebert and H. E Bunn: Regulation of the erythropoietin gene. Blood 94, 1864 (1999). A. Ferster, C. Vermylen, G. Cornu, et al.: Hydroxyurea for treatment of severe sickle cell anemia: a pediatric clinical trial. Blood 88, 1960 (1996). J. R. Girman, Y-L. Chang, S. B. Hayward, et al.: Causes of unintentional deaths from carbon monoxide poisonings in California. Western Journal of Medicine 168, 158 (1998). E Grosveld, E. De Boer, N. Dillon, et al.: Dynamics ofglobin gene expression and gene therapy vectors. Annals of the New York Academy of Sciences 550, 18 (1998). C. C. W. Hsia: Respiratory function of hemoglobin. New England Journal of Medicine 338, 239 (1998). E. Liakopoulou, C. A. Blau, L. Qiliang, et al.: Stimulation of fetal hemoglobin production by short chain fatty acids. Blood 86, 3227 (1995). J. A. Little, N. J. Dempsey, M. Tuchman, et al.: Metabolic persistence of fetal hemoglobin. Blood 85, 1712 (1995). J. M. Manning, A. Dumoulin, X. Lee, et al.: Normal and abnormal protein subunit interactions in hemoglobins. Journal of Biological Chemistry 273, 19359 (1998). P. G. McCaffrey, D. A. Newsome, E. Fibach, et al.: Induction of v-globin by histone deacetylase inhibitors. Blood 90, 2075 (1997). R. T. Means Jr. and S. B. Krantz: Progress in understanding the pathogenesis of the anemia of chronic disease. Blood 80, 1639 (1992). M. Noor and E. Beutler: Acquired sulfhemoglobinemia. Western Journal of Medicine 169, 386 (1998). N. E Olivieri: The fl-Thalassemias. New England Journal of Medicine 341, 99 (1999). J. P. Scott, C. A. Hillery, E. R. Brown, et al.: Hydroxyurea therapy in children severely affected with sickle cell disease. Journal of Pediatrics 128, 820 (1996). G. Serjeant: Sickle-cell disease. Lancet 350, 725 (1997). M. H. Steinberg: Management of sickle cell disease. New England Journal of Medicine 340, 1021 (1999). I. A. Tabbara: Erythr~176 oflnternal Medicine 153, 298 (1993). P. M. Tibbles and J. S. Edelsberg: Hyperbaric-oxygen therapy. New England Journal of Medicine 334, 1642 (1996). D. J. Weatherall and J. B. Clegg: Genetic disorders of hemoglobin. Seminars in Hematology 36, 24 (1999). H. E. Witkowska, B. H. Lubin, Y. Beuzard, et al.: Sickle cell disease in a patient with sickle cell trait and compound heterozygosity for hemoglobin S and hemoglobin Quebec-Chori. New England Journal of Medicine 325, 1150(1991).

CHAPTER

29


Heme, an iron-porphyrin complex, is the prosthetic group of many important proteins. The central role of hemoglobin and myoglobin in oxygen transport and storage was discussed in Chapter 28. Heme proteins or enzymes are involved in redox reactions (e.g., cytochromes) and participate in many oxidation reactions needed for synthesis of metabolically important compounds as well as for degradation and detoxification of waste products and environmental toxins. Ionic forms of iron (referred to hereafter as iron) also participate in a variety of enzymatic reactions as nonheme irons, which are present as iron-sulfur clusters (e.g., mitochondrial electron transport). There are also both storage and transportable forms of iron that are bound to proteins. Under normal physiological conditions only trace amounts of free iron exist. In the body, if iron exceeds the sequestration capacity of the iron-binding proteins present in different physiological compartments, the free iron can cause tissue damage. Cellular injury is caused by reactive oxygen species that are produced from H202 in a reaction catalyzed by iron. Thus, iron homeostasis in the body is in a delicate balance. Either the deficiency or the excess results in abnormalities and presents as a common cause of human diseases.

29.1 Iron Metabolism Total-body iron of a 70-kg adult is about 4.2-4.4 g. The distribution of iron in various body compartments is given in Table 29-1. The key players of iron metabolism include iron-responsive elements of appropriate mRNAs, iron regulatory proteins divalent metal transporter 1, major histocompatibility complex (MHC) class I-like protein designated as HFE protein,/32-microglobulin, transferrin, transferrin receptor, and ferritin.

Absorption of Iron from the Diet The dietary requirement for iron depends on the amount and composition of the food, the amount of iron lost from the body, and variations in physiological state such as growth, onset of menses, and pregnancy. The average North American diet contains about 6 mg of iron per 1000 calories and supplies about 10-15 mg/d. Of that ingested, 8-10% (1-1.5 mg/d) is absorbed. Thus, dietary factors that affect absorption are more important than the iron content of the diet and may be more important for correction of iron deficiency than addition of iron to the diet.

675

CHAPTER29 Metabolism of Iron and Heme

676

TABLE 29-1 Distribution of Iron in a 70-kg Adult I Circulating erythrocytes Bone marrow (erythroid) Muscle myoglobin Heme and non-heme enzymes Liver parenchyma 3 Reticuloendothelial macrophages 4 Plasma transferrin 5

1800 300 300 180 1000 600 3

mg 2 mg mg mg mg mg mg

1 These are approximate values. Premenopausal women have lower iron stores due to periodic blood loss through menstruation. Iron balance in the body is maintained by intestinal absorption of 1-2 mg/day and by loss of 1-2 mg/day. 21 mg = 17.9/xmol 3 Primarily storage forms of iron. 4 Senescent red blood cells are catabolized by the macrophages, the salvaged iron is temporarily stored, and made available via transferrin for erythron and for hemoglobin synthesis. 5 Transportable form of iron.

Iron in food exists mainly in the ferric (Fe 3+) state, complexed to proteins, amino acids, organic acids, or heme. It is absorbed in the ferrous state, reduction being accomplished in the gastrointestinal tract by ascorbate, succinate, and amino acids. Gastric acid potentiates iron absorption by aiding in formation of soluble and absorbable ferrous chelates. In achlorhydria or after partial gastrectomy, absorption is subnormal but is increased by administration of hydrochloric acid. Carbonates, tannates, phosphates, phytates, and oxalates may decrease iron absorption by formation of insoluble complexes, but their effect can be prevented by adequate dietary calcium, which complexes with them and makes them unavailable for reaction with iron. Absorption of heme iron is not affected by these agents. Heme is absorbed intact from food and more effectively than inorganic iron. Antacids, such as aluminum hydroxide and magnesium hydroxide, also decrease iron absorption. In general, foods of animal origin provide more assimilable iron than foods of vegetable origin, since on a weight basis, vegetables contain less iron and more substances (e.g., phytates) that inhibit iron absorption. Foods that contain more than 5 mg of iron per 100 g include organ meats (liver, heart), wheat germ, brewer's yeast, oysters, and certain dried beans. Foods that contain 1-5 mg of iron per 100 g include muscle meats, fish, fowl, some fruits (prunes, raisins), most green vegetables, and most cereals. Foods that contain less than 1 mg of iron per 100 g include milk and milk products and most nongreen vegetables. Ferrous iron is absorbed principally from the mature enterocytes lining the absorptive villi of the duodenum. The amount of iron absorption by these enterocytes is

determined by the prior programming of the duodenal crypt cells based on iron requirements of the body as they undergo maturation. The regulation of intestinal iron absorption is critical because iron excretion from the body is a limiting physiological process (discussed later). The small intestine is also an excretory organ for iron, since that stored as ferritin in the epithelial cells is lost when they are shed and replaced every 3-5 days. Heme iron is transported intact into the mucosal cells and the iron removed for further processing. The mechanism of entry of ferrous iron from the intestinal lumen into the enterocytes and its eventual transport into the portal blood is beginning to be understood. The first step is the programming of the duodenal undifferentiated deep-crypt cells with regard to sensing the iron requirements of the body. The programming of the crypt cells for capacity to absorb iron is thought to occur as follows. A protein (HFE) that spans the cell membrane like an HLA molecule associates with flz-microglobulin like an HLA protein. The HFE protein is coded for by a gene (HFE) located on the short arm of chromosome 6 near the MHC gene loci. Mutations in the HFE gene are associated with an inherited disorder of excessive dietary iron absorption that is known as hereditary hemoehromatosis (discussed later). HFE protein spans the cell membrane of the crypt enterocytes with its N-terminal domain projecting outside. Near the cell membrane a segment of HFE protein, like an HLA protein, associates with /32-microglobulin (Figure 29-1) and stabilizes the HFE protein. Plasma transferrin transports iron in the ferric state and is an indicator of iron stores in the body. Each molecule of transferrin binds with two ferric ions (diferric transferrin) and undergoes receptor-mediated endocytosis when bound to transferrin receptors (discussed later) with the aid of HFE protein/32-microglobulin complex. In the cytosol, iron is released from the endosomes. The level of cytoplasmic iron regulates the translation of mRNA of a protein known as divalent metal transporter-I (DMT1), which participates in iron entry into the enterocytes located on the villus tip (Figure 29-2). Regulation of DMT1 synthesis is coupled to cytoplasmic iron levels and involves the presence of a stem-loop hairpin structure in the 3'-untranslated region that resembles an iron regulatory element (IRE) and its interaction with an iron regulatory protein (IRP1). In the iron-deficient state, IRP binds to IRE and stabilizes the mRNA of DMT1. This stabilization of mRNA leads to increased production of DMT1 and its eventual localization on the cell surface. The transport of ferrous iron across the apical membrane of the villus enterocyte that is mediated by DMT1 occurs through a proton-coupled process. DMT1 also transports a number

SECTION29.1 Iron Metabolism

677

F I G U R E 29-1 A diagrammatic representation of the transmembrane HFE protein. The extracellular portion of the HFE protein consists of three o~ domains. The/32-microglobulin is noncovalently associated with the oe3 domain of HFE protein, stabilizing its structure. The extracellular missense mutations H63D and C282Y are identified in patients with hereditary hemochromatosis. HFE protein is involved in sensing circulating iron concentration and participates in the regulation of gene expression of products involved in iron absorption, transport, or storage. [Reproduced with permission from: A novel MHC class l-like is mutated in patients with hereditary hemochromatosis. J. N.Feder, A. Gnirke, W. Thomas et al. Nature Genetics 13, 399 (1996).]

of divalent metal ions, including Mn 2+, Cu 2+, Zn 2+, Cd 2+, and Pb 2+. At normal levels of iron intake, absorption requires uptake from the intestinal lumen by the mucosa and transfer from the mucosa to the portal blood. Both events are inversely affected by the state of body iron stores. In iron deficiency states, nonferrous metals such as cobalt and manganese, which have an ionic radius similar to that of iron and form octahedral complexes with six-coordinate covalent bonds, also are absorbed at an increased rate. Oral administration of a large dose of iron reduces (or temporarily inhibits) the absorption of a second dose of iron by the absorptive enterocytes even in the presence of systemic iron deficiency. The mechanism of mucosal block, which resists acquiring additional iron by the enterocytes with high amounts of intracellular iron, is not yet understood. It probably involves set points established in the enterocytes for iron recently consumed in the diet (dietary regulator), o Iron absorption also is affected by erythropoiesis. When erythropoiesis is accelerated by bleeding, hemolysis, or hypoxia, iron absorption is increased. Conversely,

diminished erythropoiesis due to starvation, blood transfusion, or return to sea level from a high altitude decreases iron absorption. How the size of body iron stores and the rate of erythropoiesis transmit information to the duodenum is not known. Feedback control seems to be weak or absent, since in iron-deficient subjects enhanced iron absorption continues long after hemoglobin is restored to normal levels. Furthermore, in chronic hemolytic anemia, iron absorption is increased, persists for prolonged periods, and leads to iron overload. The need for dietary iron is ultimately determined by the rate of iron loss from the body and the amount required for maintenance and growth. Iron is tightly conserved once it is absorbed. Its excretion is minimal and unregulated, and facilitated by normal exfoliation from the surfaces of the body (dermal, intestinal, pulmonary, urinary), loss of blood by gastrointestinal bleeding, and loss in bile and sweat. Insignificant amounts are lost in urine, since iron in plasma is complexed with proteins that are too large to pass through the kidney glomerular membrane. Iron in feces is primarily unabsorbed dietary iron. Obligatory iron

6~


F I G U R E 29-2 Intestinal absorption of dietary iron. Ferrous iron is absorbed by the duodenal villus tip enterocytes mediated by divalent metal transporter-1 (DMTI). Iron transport mediated by DMTI of the apical surface and the basolateral transporter at the basolateral surface are coupled to ferric reductase and ferroxidase that change the iron oxidation state, respectively. The degree of iron entry is determined by the level of DMT 1 and its level of expression is programmed in the crypt cells. The programming of the crypt cells is coupled to the body iron stores via transferrin-mediated and HFE protein-modulated iron transport. [Modified and reprinted with permission from B. R. Bacon, L. W. Powell, R C. Adams, et al. Molecular medicine and hemochromatosis: at the cross roads. Gastroenterology 116, 193 (1999).]

loss for a 70-kg man is 0.5-1 mg/d, an amount equal to that normally absorbed from the diet. During growth, menstruation, and pregnancy, the requirement reaches about 2-2.5 mg/d. Recommended daily iron intake for various groups is shown in Appendix IV.

The principal loss of iron in nonpregnant women during the reproducing years is through menstruation. In one study, the mean menstrual blood loss was 43.4 4- 2.3 mL. Since each milliliter of blood from a normal woman contains about 0.5 mg of iron, the amount lost every 27 days is


about 20-23 mg. Increased menstrual flow (menorrhagia) significantly augments iron loss and leads to iron deficiency anemia (see below). In pregnancy use of supplemental iron is recommended. A newborn has about a 3- to 6-month supply of iron in its liver and may require ironrich foods from the sixth month onward, since milk is poor in iron.

Plasma Iron Transport Over 95% of plasma iron is in the Fe 3+ state bound to the glycoprotein transferrin, a monomeric/~ 1-globulin (M.W. 80,000). Electrophoretic studies have revealed the existence of 21 genetic variants. In some, single-aminoacid substitutions account for variation in electrophoretic mobility. Transferrin is synthesized primarily in the liver and appears at the end of the first month of fetal development. Its half-life in humans is about 8 days. Desialylation may be a requirement for its removal from plasma by the liver, as it is for other plasma proteins (Chapter 10). In fact, asialotransferrin is more rapidly cleared from plasma than transferrin. It is not required for intestinal absorption of iron. Each molecule of transferrin can bind two Fe 3+ ions. The binding is extremely strong under physiological conditions, and the binding constants of the two sites are not significantly different. For each Fe 3+ bound, one HCOy ion is also bound and three H + ions are released from the protein. Thus, diferric transferrin gains two net negative charges. 2Fe 3+ + apotransferrin + 2HCO~-

679

and HFE protein. This process of iron release from the complex is inhibited by HFE protein. Thus, dysfunctional HFE protein can cause excessive release of iron from the transferrin-transferrin receptor-HFE protein complex. In the acidified endosomes, DMT1 facilitates iron transport into the cytosol. Both apotransferrin (and a fraction of ironbound transferrin) and transferrin receptor are returned to cell surfaces for reuse. In this type of receptor-mediated endocytosis of transferrin-transferrin receptor complex, the endosomes do not come into contact with lysosomes. The process is therefore unlike that of low-density lipoprotein receptor-mediated internalization (Chapter 20). In the erythroid cells, most of the iron released from the endosomes is transported into mitochondria for heme synthesis (discussed later); in nonerythroid cells, the iron is stored predominantly as ferritin and to some extent as hemosiderin.

Storage of Iron There are two storage forms of iron, ferritin and hemosiderin. Ferritin is the predominant storage form and contains diffusable, soluble, and mobile fractions of iron. Hemosiderin is aggregated deposits resulting from the breakdown of ferritin in secondary lysosomes and its level increases progressively with increasing levels of iron overload. Apoferritin is a protein shell consisting of 24 subunits of two types; a light (L) subunit (M.W. 19,000) and a heavy (H) subunit (M.W. 21,000). The H subunit has ferroxidase activity and the L subunit facilitates nucleation and mineralization of the core made up of hydrated ferric oxide phosphate complex.

[Fe2-transferrin-(HCO3)2] 2- + 6H + The metal binding sites are located in N- and C-terminal domains. The protons released upon binding of each Fe 3+ ion are probably derived from ionization of two tyrosyl residues and of a water molecule bound to Fe 3+ ion. The bulk of transferrin iron is delivered to immature erythroid cells for utilization in heme synthesis. Iron in excess of this requirement is stored as ferritin and hemosiderin. Unloading of iron to immature erythroid cells is by receptor-mediated endocytosis. The process begins in the clathrin-coated pits with the binding of diferric transferrin to specific plasma membrane transferrin receptors that are associated with the HFE protein complex. The next step is the internalization of the transferrintransferrin receptor-HFE protein complex with formation of endosomes. The iron transporter DMT1 present in the cell membrane is also internalized into the endosomes. In the endosomes, a proton pump acidifies the complex to pH 5.4, and by altering conformation of proteins, iron is released from transferrin bound to transferrin receptor

Coordinate Regulation of Iron Uptake and Storage in Non-Erythroid Cells Iron uptake is regulated by transferrin receptor and storage of iron as ferritin which occurs post-transcriptionally for these two proteins. The regulation maintains an optimal intracellular-transit-chelatable iron pool for normal functioning in the body. The regulatory process consists of an interaction between IREs and IRPs 1 and 2. One copy of each IRE has been identified in the 5'-untranslated region of H and L ferritin mRNAs and five copies in the 3'-untranslated region (UTR) of transferrin receptor mRNA. IRE sequences are highly conserved and have a stem-loop structure with a CAGUGN sequence at the tip of the loop. IRPs are RNA-binding proteins that bind to IREs and regulate the translation of the respective mRNAs. During low levels of intracellular chelatable iron, iron storage declines due to inhibition of ferritin synthesis; cellular entry of iron increases due to enhanced transferrin receptor synthesis. An opposing set of events occurs

680


At low cytosolic mobile iron pool 9 IRP levels increase and bind to mRNA-IREs of ferritin and transferrin receptors.

5,~c~f~f~fXj ~ RE Coding Region UTR > Inactive ferritin mRNA

~

An 3'

IRP IRE

5

'

~ UTR

I

Coding Region

~

"',~""'--.~

An 3'

~- '

Active Transferrin receptor mRNA O ~-'" IRE At high cytosolic mobile iron pool : IRP levels decrease causing opposite II effects on the mRNAs. , ~ ~ ~ . J ~ 5 UTR > i Coding Region g ~ F ~ F x ~ ' ~ A n 3' P~ UTR Active ferritin mRNA IRE

5

'

~

Coding Region

An 3' UTR

Inactive transferrin receptor mRNA F I G U R E 29-3 Coordinate translational regulation of ferritin mRNA and transferrin receptor mRNA in nonerythroid cells. Iron regulatory proteins (IRP) are RNA-binding proteins that bind to iron regulatory elements (IREs). IREs are hairpin structures with loops consisting of CAGUGN sequences and are located at the 5'-untranslated region (UTR) and 3'-UTR for ferritin mRNA and transferrin mRNA, respectively.

during intracellular chelatable iron excess or iron-replete states. A coordinated control occurs when IRP binds to IRE at the 5'-UTR of ferritin mRNAs inhibiting ferritin synthesis; simultaneously, the binding of IRP to IRE at the 3'-UTR of transferrin receptor mRNA stimulates transferrin receptor synthesis (Figure 29-3). Intracellular iron regulates the level of IRPs. During the expansion of the iron pool, IRPs are inactivated, leading to efficient translation of ferritin mRNA and rapid degradation of transferrin receptor mRNA. In iron-replete cells, IRP1 acquires iron by the formation of iron-sulfur clusters (4Fe-4S) that bind to IREs with low affinity. During iron deficiency states, IRP 1 lacks a 4Fe-4S cluster and binds to IREs with high affinity. IRP1, when it possesses an iron-sulfur cluster, has aconitase activity, normally TCA cycle enzyme (Chapter 14). Mutations that change IREs can lead to constitutive ferritin biosynthesis. An autosomal dominant disorder of IRE leads to hyperferritinemia without any iron overload. Patients with this inherited disorder also exhibit early onset of cataract that may also be cotransmitted as an autosomal

dominant trait. Other factors also regulate ferritin synthesis. For example, nitric oxide enhances binding of IRP to IRE and inhibits ferritin synthesis. One of the causes of anemia of chronic inflammatory diseases may be due to increased ferritin synthesis in the reticuloendothelial macrophages by inflammatory cytokines interleukins 1 and 6 (IL-I and IL-6), which act by preventing efficient release of iron. Measurement of serum ferritin levels has diagnostic utility. In iron deficiency anemia (discussed later), serum ferritin levels are low; in iron storage disease, the levels are high. However, serum ferritin levels can also be elevated under many other circumstances, including liver diseases and chronic inflammatory diseases. Alterations of Plasma Transferrin Concentration Plasma transferrin levels are commonly measured in the evaluation of disorders of iron metabolism (see below). It is customary to measure transferrin concentration


indirectly from the maximum (or total) iron binding capacity (TIBC) of plasma (reference range for adults, 250-400 #g/dL). It can also be measured directly by immunological methods (reference range for adults, 220-400 mg/dL). Hypertransferrinemia (or increased TIBC) can occur with diminished body iron stores as in iron deficiency anemia or during pregnancy (because of enhanced mobilization of storage iron to supply maternal and fetal demands). Hypertransferrinemia of iron deficiency is corrected by oral iron supplementation, whereas that due to pregnancy is not. Exogenous administration of estrogens (e.g., oral contraceptives) also causes hypertransferrinemia. Hypotransferrinemiacan result from protein malnutrition and accompanies hypoalbuminemia. Since transferrin has a much shorter half-life (8 days) than albumin (19 days), measurement of the transferrin level may be a more sensitive indicator of protein malnutrition than albumin measurement (see also chapter 17). Hypotransferrinemia also results from excessive renal loss of plasma proteins (e.g., in nephrotic syndrome).

Disorders of Iron Metabolism

Iron Deficiency Anemia Iron deficiency anemia is the most prevalent nutritional disorder. Its cause may comprise many overlapping factors: dietary iron deficiency; absence of substances that favor iron absorption (ascorbate, amino acids, succinate); presence of compounds that limit iron absorption (phytates, oxalates, excess phosphates, tannates); lack of iron absorption due to gastrointestinal disorders (malabsorption syndrome, gastrectomy); loss of iron due to menstruation, pregnancy, parturition, lactation, chronic bleeding from the gastrointestinal tract peptic ulceration, hemorrhoids, cancer, colonic ulceration, or hookworm infestation or the genitourinary tract (uterine fibroids); enhanced demand for growth or new blood formation; deficiency of iron transport from mother to fetus; abnormalities in iron storage; deficiencies in release of iron from the reticuloendothelial system (infection, cancer); inhibition of incorporation of iron into hemoglobin (lead toxicity); and rare genetic conditions (transferrin deficiency, impaired cellular uptake of iron by erythroid precursors). In the initial phase of depletion of the iron content of the body, the iron stores maintain normal levels of hemoglobin and of other iron proteins. With exhaustion of storage iron, hypochromic and microcytic anemia becomes manifest. The clinical characteristics of iron deficiency anemia are nonspecific and include pallor, rapid exhaustion, muscular weakness, anorexia, lassitude, difficulty in concentrating,

681 headache, palpitations, dyspnea on exertion, angina on effort, peculiar craving for unnatural foods (pica), ankle edema, and abnormalities involving all proliferating tissues, especially mucous membranes and the nails. The onset is insidious and may progress slowly over many months or years. Physiological adjustments take place during the gradual progression of the disorder, so that even a severe hemoglobin deficiency may produce few symptoms. Iron deficiency may affect the proper development of the central nervous system. Early childhood iron deficiency anemia may lead to cognitive abnormalities. Individuals who have congenital atransferrinemia lack apotransferrin and suffer from severe hypochromic anemia in the presence of excess iron stores in many body sites, susceptibility to infection (transferrin inhibits bacterial, viral, and fungal growth, probably by binding the iron required for growth of these organisms), and retardation of growth. This condition does not respond to administration of iron. Intravenous administration of transferrin normalizes the iron kinetics. A rare congenital defect in uptake of iron by red cell precursors has been reported that leads to severe hypochromic anemia with normal plasma iron and transferrin levels. Microcytic anemia occurs frequently in thalassemia syndromes (Chapter 28), but these patients do not require iron supplementation unless they have concurrent iron deficiency as assessed by measurement of serum iron levels and TIBC. Serum iron concentration exhibits a morning peak and an evening nadir; this pattern is reversed in nightshift workers. The circadian variation is primarily due to differences in rate of release of iron by the reticuloendothelial system. Transferrin levels do not show circadian fluctuation. Iron deficiency anemia can also be assessed from the plasma ferritin concentration (which when decreased reflects depleted iron stores), red cell protoporphyrin concentration (increased because of lack of conversion to heme), and the number of sideroblasts in the bone marrow (which parallels iron stores). Sideroblasts are erythrocyte precursors (normoblasts) containing free ferritin-iron granules in the cytoplasm that stain blue with the Prussian blue reagent. There is a close correlation between plasma iron levels, TIBC, and the proportion of sideroblasts in bone marrow. In hemolytic anemias, pernicious anemia, and hemochromatosis, the serum iron level increases and sideroblast number reaches 70% (normal range, 30-50% of total cells). In iron deficiency, the sideroblasts are decreased in number or absent. Before treatment is initiated, the cause of the negative iron balance must be established. Treatment should correct the underlying cause of anemia and improve the iron balance. In general, oral therapy with ferrous salts is

682

CHAPTER 29

satisfactory; however, sometimes parenteral therapy is preferred, e.g., in proven malabsorption problems, gastrointestinal disease and excessive blood loss, and for patients who cannot be relied on to take oral medication.

Iron-Storage Disorders A type of iron storage disorder characterized by general increase in tissue iron levels without damage to parenchymal cells is known as hemosiderosis. Hemosiderin is a storage form of iron in which ferric hydroxide is present as micelles. It appears as insoluble granules that contain denatured aggregated ferritin, nonferritin proteins, lipids, heme, and other pigments. Hemosiderosis results when iron is present in excessive quantities in a diet that permits maximum iron absorption. For example, the African Bantu eat a diet high in corn (low in phosphate) cooked in iron pots, drink an indigenous beer brewed in iron pots, and suffer from Bantu siderosis. Hemosiderosis can progress to hemochromatosis with hepatic cirrhosis and diabetes mellitus.

Metabolism of Iron and Heine

progressive deterioration in pancreatic, hepatic, gonadal, and cardiac function. Clinical manifestations include cirrhosis, diabetes mellitus, life-threatening arrhythmias, and intractable heart failure. Removal of excess iron produces clinical improvement, particularly of diabetes and congestive heart failure. In iron storage diseases accompanied by normal erythropoiesis (e.g., hereditary hemochromatosis), removal of excessive iron is accomplished by repeated bloodletting (phlebotomy). Therapeutic phlebotomy of a unit of blood (which contains about 250 mg of iron) may be performed up to three times per week. When the iron stores become depleted, reaccumulation of iron is prevented by four to six phlebotomies per year. In asymptomatic patients, periodic determination of serum ferritin provides a measure of storage of iron. In hemochromatosis secondary to refractory anemias (e.g., Cooley's anemia, sickle cell anemia), patients require repeated blood transfusions to survive childhood and adulthood. Therapy consists of administration of ironchelating agents. Deferoxamine, an iron chelator isolated from Streptomyces pilosis, has the structure:

H2N--(CH2)5--N--C--(CH2)2--C--N--(CH2)5--N--C--(CH2)2--C--N--(CH2)s--N--C--CH3 I II II I I II II I I il HO O O H HO O O H HO O

Excessive accumulation of iron (chronic iron overload) can result from the following. I. Defective erythropoiesis (dyserythropoiesis); impaired hemoglobin synthesis leading to lack of utilization and consequent accumulation of iron in mitochondria, e.g., from inhibition of ALA synthase activity by dietary vitamin B6 deficiency; inhibition of heme synthesis by lead; impairment of pyridoxine metabolism in alcoholic patients; familial sideroblastic anemias; and Cooley's anemia. 2. Repeated blood transfusions, e.g., in Cooley's anemia or sickle cell disease. 3. Hereditary hemochromatosis, an autosomal recessive defect in which there is increased rate of absorption of iron in the presence of normal or enlarged iron stores and normal hematopoiesis (discussed later). 4. High dietary iron and substances that enhance its absorption (e.g., Bantu siderosis). 5. Hereditary atransferrinemia. In all of these disorders, the gastrointestinal tract cannot limit absorption of iron to significant extent. Thus, the "mucosal block" responsible for keeping out unneeded iron on a daily basis is susceptible to disruption, perhaps at more than one point. Iron overload leads to

It contains six nitrogen atoms separated by fairly long, flexible stretches of methylene groups. Since each iron atom can bind six ligands, one molecule of deferoxamine is probably capable of occupying all six coordination sites and producing a 1:1 iron-deferoxamine complex. For ferric iron, the Kas~ocof deferoxamine is about 103~ while the Ka~oc for Ca 2+ is about 102. Iron in hemoproteins is not affected by this agent, while the ferric iron of ferritin and hemosiderin is chelated in preference to that found in transferrin. Such selectivity makes the compound useful in treatment of iron storage problems and acute iron poisoning. The deferoxamine-iron complex is excreted in urine. (Iron is not normally excreted by this route.) Deferoxamine given orally complexes with dietary iron, making the drug and the iron unavailable for absorption. The preferred route of administration is by intramuscular injection. Irritation and pain at the site of administration and the need for daily injections make the treatment unpopular. In addition, even with coadministration of large amounts of ascorbic acid, the iron loss produced is far below that necessary to remove all of the iron accumulated during chronic transfusion therapy. Slow, continuous intravenous infusion or continuous subcutaneous administration may be more effective in


establishing negative iron balance and eliminating stored iron. A small, labile (chelatable) iron pool may be in slow equilibrium with a much larger (storage) pool. When deferoxamine is administered, the labile pool is rapidly emptied. Any deferoxamine that remains in the body or is administered thereafter finds no iron to bind. Thus, most of a single intramuscular dose is simply excreted unchanged. If the chelator is given as a continuous infusion, the labile pool is initially depleted and kept empty. As iron is released from storage sites, it is immediately chelated and removed. Removal of up to 180 mg of iron per day has been accomplished by this method, making it as effective as phlebotomy. Massive intravenous injections of deferoxamine have also been reported to produce excretion of large amounts of iron in iron-overloaded patients.

Hereditary Hemochromatosis Hereditary hemochromatosis is a common inherited autosomal recessive disorder of excessive iron accumulation in parenchymal cells of liver, heart, pancreas, endocrine organs, skin, and joints. The term hemochromatosis is used when organ damage has occurred in the presence of impaired function. It occurs predominately in Caucasian populations; about 1 in 200-400 caucasians are at risk for developing clinical symptoms. Individuals with hereditary hemochromatosis absorb about 3-4 mg/d of iron as compared with a normal rate of about 1-2 mg/d. This excess iron, absorbed over several years, causes accumulation of as much as 20-40 g as compared to normal amounts of about 4 g. In untreated patients, progressive iron accumulation can cause organ damage resulting in hepatic dysfunction, diabetes, cardiomyopathy, hypogonadism with infertility, arthritis, and skin hyperpigmentation. Death can occur due to cirrhosis, diabetes, cardiomyopathy or hepatocellular carcinoma. Hereditary hemochromatosis is associated with a gene on the short arm of chromosome 6 near the MHC gene complex. This gene is known as HFE and codes for HFE protein. The roles of HFE protein along with /~2-microglobulin in the regulation of intestinal iron absorption and iron sequestration in the form of ferritin and hemosiderin have been discussed previously. Gene knockout mice for either HFE protein or/~2-microglobulin develop an iron overload disorder similar to human hemochromatosis, thus substantiating the roles of HFE protein and/~2-microglobulin in iron homeostasis. Two missense mutations (C282Y and H63D) in the HFE gene have been identified in hereditary hemochromatosis. The substitution of a tyrosyl residue for a cysteinyl residue at position 282 results in the loss of formation of a

683

disulfide linkage necessary for the proper association with /~2-microglobulin (Figure 29-1). In the absence of this disulfide linkage, HFE protein fails to reach the normal membrane location and is rapidly degraded. Thus, C282Y is a loss of function (knockout) mutation. The H63D mutation has no effect on the HFE protein's association with flz-microglobulin. However, this mutation may compromise the protein regulation of the interaction between transferrin and its receptor. Population studies among Caucasians have shown that about 1 in 10 are heterozygous for the C282Y mutation. Homozygosity of C282Y has been observed in 85-90% of hereditary hemochromatosis patients, In about 4% of the hereditary hemochromatosis patients, heterozygosity of C282Y and H63D has been observed. There are still unanswered questions concerning hereditary hemochromatosis. For example, some C282Y homozygotes exhibit neither biochemical nor clinical evidence of iron accumulation. On the contrary, some hemochromatosis patients do not possess a C282Y mutation. Thus, other yet-to-beidentified genetic and environmental factors must play a role in the development of hemochromatosis. Other forms of hemochromatosis include neonatal and juvenile types in which biochemical defects have not yet been identified. Patients with aceruloplasminemia resulting from mutations in the ceruloplasmin gene exhibit accumulation of iron in neural and glial cells in the brain, in hepatocytes, and in pancreatic islet cells. Ceruloplasmin, a copper-containing protein, has ferroxidase activity and participates in the release of iron from cells. Aceruloplasminemia associated with iron overload is a different disorder from that of Wilson's disease (hepatolenticular degeneration) in which biliary excretion of copper and incorporation of copper into ceruloplasmin are defective (Chapter 37). Porphyria cutanea tarda, a disorder of porphyrin biosynthesis (discussed later), usually is accompanied by iron accumulation. Thirty percent of patients with porphyria cutanea tarda are either homozygous or heterozygous for the C282Y mutation affecting the HFE protein. Treatment of hereditary hemochromatosis is therapeutic phlebotomy (discussed earlier). This method is safe, effective, and life saving, and ideally should begin before symptoms develop. Serum ferritin levels are used as a surrogate marker for estimating total-body iron stores. Morphologic studies and quantitative determination of iron in liver tissue obtained by biopsy have been used in the assessment of early hereditary hemochromatosis and the degree of liver injury. Finally, hereditary hemochromatosis is a treatable disease. Biochemical screening for the identification of the


684

disease in the general population has been suggested. The biochemical tests include the measurement of serum levels of iron, transferrin saturation, and ferritin.

29.2 Heme Biosynthesis The principal tissues involved in heme biosynthesis are the hematopoietic tissues and the liver. Biosynthesis requires participation of eight enzymes, of which four (the first and the last three) are mitochondrial and the rest are cytosolic (Figure 29-4). The reactions are irreversible. Glycine and succinate are the precursors of porphyrins. Formation of 6-Aminolevulinic Acid 6-Aminolevulinic acid (ALA) formation is catalyzed by mitochondrial ALA synthase, which condenses glycine and succinyl-CoA to ALA. The enzyme is located on the matrix side of the inner mitochondrial membrane. It is encoded by a nuclear gene and is synthesized in the cytosol on the free polyribosomes as a

COOH I OH2 (~OOH

I

COOH I OH2 Pyridoxal

I

isonicotinic acid hydrazide; Chapter 17) can prevent heme synthesis and cause anemia. Heme synthesis also requires a functional tricarboxylic acid cycle and an oxygen supply. The primary regulatory step of heme synthesis in the liver is apparently that catalyzed by ALA synthase. The regulatory effects are multiple. The normal end product, heme, when in excess of need for production of heme proteins, is oxidized to hematin, which contains a hydroxyl group attached to the Fe 3+ atom. Replacement of the hydroxyl group by a chloride ion produces heroin. Hemin and heme inhibit ALA synthase allosterically. Hemin also inhibits the transport of cytosolic ALA synthase precursor protein into mitochondria. ALA synthase has a turnover rate of 70 minutes in adult rat liver and is inducible. Its induction is suppressed by hemin and increased by a variety of xenobiotics (e.g., environmental pollutants) and natural steroids. In erythropoietic tissues, where the largest amount of heme is synthesized, regulation of heme biosynthesis may also involve the process of cell differentiation and proliferation of the erythron, which occurs to meet change in requirements for the synthesis of heme. The differentiation and proliferation are initiated by erythropoietin.

Formation of Porphobilinogen ,

I + CH2 phosphate=--CH2 + CO2+ COASH CH2~NH2 I I CmSCoA C~O II I O CH2 I NH2 Glycine Succinyl-CoA 5-Aminolevulinic acid

Two molecules of ALA are condensed by cytosolic zinc containing ALA dehydratase to

COOH I

CH2

I

2

precursor. The precursor protein is processed to active form during its translocation into mitochondria (Chapter 25). Pyridoxal phosphate is the required coenzyme. The reaction mechanism consists of formation of a Schiff base by pyridoxal phosphate with a reactive amino group of the enzyme; entry of glycine and formation of an enzyme-pyridoxal phosphate-glycine-Schiff base complex; loss of a proton from the r carbon of glycine with the generation of a carbanion; condensation of the carbanion with succinyl-CoA to yield an enzyme-bound intermediate (ol-amino-fl-ketoadipic acid); decarboxylation of this intermediate to ALA; and liberation of the bound ALA by hydrolysis. ALA synthesis does not occur in mature erythrocytes. In experimental animals, deficiency of pantothenic acid (needed for CoASH and, hence, succinyl-CoA synthesis), lack of vitamin B6, or the presence of compounds that block the functioning of pyridoxal phosphate (e.g.,

CH2 I C~O I

-2.~o ~,~ ._ -

OH2 I NH2 ALA

COOH/ ICOOH a c i ~ H2 d /

Acetic

acid

Propionic substituent

substituent (A)

L(~H2 I C II

CH2

(P)

I C II

/C H H2C ~ N / C I H NH2 Porphobilinogen yield porphobilinogen (PBG). There are four zinc ions per

SECTION 29.2

Heine Biosynthesis

685

F I G U R E 29-4 Biosynthetic pathway of heme. The pathway consists of eight irreversible reactions, four each in the mitochondrion and the cytosol. The primary site of regulation is the ALA synthase step.

octamer of the enzyme, and they are bound via the reduced thiol groups. Zinc is required for enzyme activity. The reaction mechanism consists of Schiff base formation by the keto group of one molecule of ALA with the s-amino group of a lysyl residue of the enzyme, followed by nucleophilic attack by the enzyme-ALA anion on the carbonyl group of a second ALA molecule with elimination of water. Then, a proton is transferred from the amino group of the second ALA molecule to the s-amino group of the lysyl residue with formation of PBG. Lead is a potent inhibitor of ALA dehydratase, presumably by displacement of zinc by lead because the leadinhibited enzyme can be reactivated by the addition of zinc. ALA dehydratase is inhibited competitively by succinyl acetone (HOOC-CHz-CHz-CO-CHz-CO-CH3), which occurs in urine and blood in hereditary tyrosinemia (Chapter 17). Genetic deficiency of ALA dehydratase is known to occur.

tetrapyrrole, followed by its cyclization to form uroporphyrinogen III. In the absence of uroporphyrinogen III synthase (e.g., in congenital erythropoietic porphyria), the hydroxymethylbilane cyclizes spontaneously to the uroporphyrinogen I isomer, which is not a precursor of heme (Figure 29-5).

Formation of Coproporphyrinogen III Cytosolic uroporphyrinogen decarboxylase catalyzes successive decarboxylation of the four acetic groups to yield four methyl groups (Figure 29-6).

Formation of Protoporphyrinogen IX Mitochondrial coproporphyrinogen oxidase is localized in the intermembrane space and is probably loosely bound to the outer surface of the inner membrane. It catalyzes the successive conversion of propionic acid groups of ring A and ring B to vinyl groups (Figure 29-7).

Formation of Uroporphyrinogen III Uroporphyrinogen III formation occurs in the cytosol and requires the successive action of porphobilinogen deaminase (or methylbilane synthase) and uroporphyrinogen III synthase. Porphobilinogen deaminase catalyzes condensation of four porphobilinogen molecules in a symmetrical head-to-tail arrangement to form a straight-chain tetrapyrrole, hydroxymethylbilane. Uroporphyrinogen III synthase catalyzes the rearrangement of one of the pyrrole rings (ring D in Figure 29-5) to form an asymmetrical

Formation of Protoporphyrin IX and Heine Both of these steps occur in mitochondria (Figure 29-8). Porphyrinogen oxidase removes six hydrogen atoms (four from methane bridge carbons and two from pyrrole nitrogens) from protoporphyrinogen to yield protoporphyrin. The oxidase has an absolute requirement for oxygen. Protoporphyrinogen can also be oxidized nonenzymatically to protoporphyrin at physiological pH, temperature, and aerobic conditions. Protoporphyrin oxidase is bound to the

686

CHAPTER29 Metabolismof Iron and Heme Ac

P

Porphobilinogen NH2

H J

4NH3~-"I

P

PGBdeaminase Ac

[

Ac

P

Hydroxymethylbilane HO-"-z/T"- NH p

~

A

HN~ c

Ac Uroporphyrinogen-III / synthase / A c

P ~pontaneous p "~

P Ac

P

Ac/ ~ ~Ac ,,/ \ r p UroporphyrinogenIII

(biologicallyusefulisomer)

Ac

Ac

P

P

~

A

c

UroporphyrinogenI (biologicallynot usefulisomer)

FIGURE29-5 Synthesis of uroporphyrinogen I and III. The latteris the biologicallyuseful isomer,and its formationrequiresthe action of uroporphyrinogen-IIIsynthase.Ac,-CH2COOH;P,-CH2CH2COOH. inner mitochondrial membrane, and its active site faces the cytosolic side of the membrane. Formation of heme is accomplished by ferrochelatase (or heme synthase), which incorporates Fe 2+ into protoporphyrin and is inhibited by lead. Zinc can function as a substrate in the absence of iron.

Disorders of Heme Biosynthesis The porphyrias are a group of disorders caused by abnormalities in heme biosynthesis. They are inherited and acquired disorders characterized by excessive accumulation and excretion of porphyrins or their precursors. Defects in any one of the eight enzymes involved in heme biosynthesis may cause inherited porphyrin-related disorders (Figure 29-9). Porphyrins have a deep red or purple color (Greek porphyra - purple). Porphyrins are

excreted by different routes, depending on their water solubility. For example, uroporphyrin with its eight carboxylic group substituents is more water-soluble than the porphyrins derived from it and is eliminated in the urine, whereas protoporphyrin (which contains two carboxylic groups) is excreted exclusively in bile. Coproporphyrin has four carboxylic groups and is found in bile and urine. These disorders are associated with acute or cutaneous manifestations (or both). In the acute state, the presentation may include abdominal pain, constipation, hypertension, tachycardia, and neuropsychiatric manifestations. Cutaneous problems consist of photosensitivity (itching, burning, redness, swelling, and scarring), hyperpigmentation, and sometimes hypertrichosis

SECTION 29.2

HeineBiosynthesis

687

Ac

Porphyria maybe classified as hepatic or erythropoietic. However, enzyme defects are sometimes common to both tissues. Porphyrias can be induced by alcohol, stress, infection, starvation, hormonal changes (e.g., menstruation), and certain drugs. These drugs presumably precipitate acute manifestations in susceptible subjects since they are inducers of cytochrome P-450 and increase the need for synthesis of heme as they deplete the mitochondrial pool of free heme. Major hepatic porphyrias include acute intermittent porphyria, variegate porphyria, hereditary coproporphyria, andporphyria cutanea tarda. The principal erythropoietic porphyrias are hereditary erythropoietic porphyria and erythropoietic

P

Ac

P

~ ' ~ n I~H

H~ 1 ~

UroporphyrinogenIII

AC~Ac P

p

Uroporphyrinogen-III decarboxylase "-'4CO2 M

protoporphyria.

P

M B

~ N H

P

Hepatic Porphyrias

P

HN~

CoproporphydnogenIII

P

FIGURE 29-6 Formationof coproporphyrinogenIII from uroporphyrinogenIII. Acetic acid side chains (Ac) are decarboxylatedto methylgroups (M), sequentially, startingclockwisefromring D. P, -CH2CH2COOH. (an abnormally excessive growth of hair). Four porphyrias can manifest as acute disorders: 6-ALA dehydratase deficiency porphyria, acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria.

p.

M

M

V. P

Acute intermittent porphyria is associated with excessive urinary excretion of ALA and porphobilinogen. The lack of polymerization of porphobilinogen is due to deficiency of porphobilinogen deaminase in several cell types (e.g., hepatocytes, erythrocytes, fibroblasts, lymphocytes). Acute clinical manifestations include neuropsychiatric disorders and abdominal pain. The cause of these manifestations is not clear, but accumulation of porphyrin precursors (ALA and porphobilinogen) in pharmacological amounts has been implicated. Since afflicted subjects cannot make porphyrins to any great extent, they are not photosensitive. This disorder is inherited as an autosomal dominant trait. Porphyria cutanea tarda is the most common form. It is inherited as an autosomal dominant trait and is due to deficiency of uroporphyrinogen III decarboxylase. Clinical

M

M

V. P

M

M

V

P

ff-~-'- NH M

M

P

P

CoproporphyrinogenIII

M

HN"----~

M

P

P

Harderoporphyrinogen

p

p

ProtoporphyrinogenIX

FIGURE 29-7 Formation of protoporphyrinogenIX from coproporphyrinogenIII by coproporphyrinogenoxidase. Sequential oxidative decarboxylationof the propionic acid (P) sidechains of rings A and B produces vinyl (V) groups (V = -CH=CH2). The reactionproceeds via the stereospecificloss of one hydrogenatom and decarboxylationof the propionic acid group. Molecularoxygenis the oxidant, and/~-hydroxypropionateis a probable intermediate. M, CH3.

688


V,

M

M

rVrotopor.

/

.......

~NH

M

V, M

V

ox a e. ~~...~.NH -~) phydnogen HN'~

V,

"N_.~

M

M

V

ForrFe~=h: ,a,aso ~ ? e ( N .

~

OH

P

P ProtoporphyrinogenIX

P P ProtoporphyrinIX

P

P Heme

F I G U R E 29-8 Formation of heme. In the reaction catalyzed by protoporphyrinogen oxidase, six hydrogens are removed and the primary electron acceptor is not known, but oxygen is required for enzyme activity. In the terminal step of heme synthesis, only Fee+ is incorporated into protoporphyrin.

manifestations are mild to severe photosensitivity and liver disease. Most affected individuals have increased hepatic iron stores, which can be successfully decreased by phlebotomy. Acute episodes can be precipitated by overindulgence of alcohol or, less frequently, by therapy with estrogen. Hereditary coproporphyria and variegate porphyria are inherited as autosomal dominant traits. They are caused by deficiency of coproporphyrinogen oxidase and protoporphyrinogen oxidase, respectively. In most of these disorders, increased hepatic ALA synthase activity is due to decreased heme synthesis. There are also increased amounts of ALA and porphobilinogen in liver, plasma, and urine and specific

metabolites produced before the metabolic block. ALA synthase is regulated by the heme by a feedback process and by gene repression. Hematin has been used to treat acute attacks with marked success. Although often inadequate, carbohydrate feeding has been associated with improvement in acute intermittent porphyria. This "glucose effect" may depend on repression of the gene for ALA synthase, but the mechanism is not known.

Erythropoietic Porphyrias A defect in synthesis of type III isomers from hydroxymethylbilane, due to deficiency of uroporphyrinogen III

Inheritance & Clinical Manifestations

Glycine + Succinyl-CoA ALAsynthase ALAdehydratase Porphobilinogen deaminase Uroporphyrinogen III synthase Uroporphyrinogen decarboxylase Coproporphyrinogen oxidase ~ Protoporphyrinogen oxidase Ferrochelatase

5-aminolevulinic acid (ALA)

I ALA"yporphyria rataee ciency II acute A" Neur~ episodes

Porphobilinogen

I cute intermittentP~

[Hydroxymethylbilane] ~, UroporphyrinogenIII

I C~

Coproporphyrinogen III

[

II acutel eurovi Y' cera,.ep,soae [

a nerythr~176 d p o r pI [ hsAR' kni yPh~176 ,esions r i a

Porphyria cutanea tarda

[ [AD, Photosensitivityand skin lesions

[ Hereditary c~176176

[ [AD' l eosm se.iNeur~ ~ acuteepisodes skni

ProtoporphyrinogenIX

[

] [AD, l eosm se.iNeurovisceral. o nacute S l i nepisodes skni

Protoporphyrin IX Fe2+ Heme

[Erythropoietic protoporphyria]]AD, Photosensitivity

Variegateporphyria

I

F I G U R E 29-9 Heme biosynthetic pathway and the enzyme defects in various porphyrias. AD, autosomal dominant; AR, autosomal recessive.

I [

[

SECTION29.3 Heme Catabolism

synthase, produces congenital erythropoietie porphyria. Type I porphyrins (principally uroporphyrin I) are formed, accumulate in the tissues, and are excreted in the urine. The deficiency in production of the type III isomer further increases levels of the type I isomers by reducing the regulatory effect on ALA synthase. Excessive amounts of porphyrins in erythrocytes may produce hemolysis. A compensatory increase in hemoglobin formation can then exaggerate the already increased production of type I porphyrins. Their accumulation produces a pink to dark red color in teeth, bones, and urine. Red-brown teeth and urine are pathognomonic. Patients are sensitive to long-wave ultraviolet light and sunlight. The abnormality is transmitted as an autosomal recessive trait. Erythropoietic protoporphyria results from deficiency of ferrochelatase in reticulocytes in bone marrow. It is transmitted as an autosomal dominant trait with variable penetrance and expressivity. In general, it is a benign disorder whose most prominent symptom is photosensitivity. Occasionally, it leads to liver disease. Reduced ferrochelatase activity results in accumulation of protoporphyrin in maturing reticulocytes and young erythrocytes. When a smear of these cells is exposed to fluorescent light, they exhibit red fluorescence. The protoporphyrin appears in the plasma, is picked up by the liver, and is excreted into the bile. Protoporphyrin accumulation in the liver can lead to severe liver disease. In contrast to individuals afflicted by other porphyrias, these patients have normal urinary porphyrin levels. High levels of protoporphyrin are found in erythrocytes, plasma, and feces. The photosensitivity may be caused by stimulation of protoporphyrin in dermal capillaries to an excited (triplet) state by visible light. This in turn converts molecular oxygen to singlet oxygen, which produces cell damage. Oral administration of/3-carotene decreases the photosensitivity, possibly owing to a quenching effect on singlet oxygen and free-radical intermediates.

29.3 Heme Catabolism When heme proteins are degraded in mammals, the polypeptides are hydrolyzed to amino acids while the heme groups are freed of their iron, which is salvaged, and are converted to bilirubin. After transport to the liver, bilirubin is coupled to glucuronic acid and the conjugated bilirubin is excreted into bile as the principal bile pigment. When increased production or decreased excretion of bilirubin causes its plasma concentration to exceed 0.11.0 mg/dL (2-17 #mol/L), it diffuses into tissues and

689 produces jaundice. Although jaundice is relatively harmless unless due to extremely high concentrations of unconjugated bilirubin, it indicates the presence of a disease process that requires medical attention. The yellow coloration of jaundiced skin and sclerae has aroused much interest and has made bilirubin the subject of extensive research. Fractionation and quantitation of serum bilirubin are now widely used for diagnosis and prognosis of hepatobiliary disease. B ilirubin is a waste product and has no known beneficial physiological function. However, both the conjugated and the unconjugated forms of bilirubin show antioxidative properties (e.g., inhibition of lipid peroxidation). The physiological role of the antioxidative property of bilirubin is not known. Bilirubin is a yellow-orange pigment that in its unconjugated form is strongly lipophilic and cytotoxic. It is virtually insoluble in aqueous solutions below pH 8 but readily dissolves in lipids and organic solvents and diffuses freely across cell membranes. Bilirubin toxicity is normally prevented by tight binding to serum albumin. Only when the binding capacity of albumin is exceeded can a significant amount of unconjugated bilirubin enter cells and cause damage. Conjugated bilirubin is hydrophilic and does not readily cross cell membranes, even at high concentrations. Of the 250-300 mg (4,2755,130 #mob of bilirubin normally produced in 24 hours, about 70-80% is derived from hemoglobin. The remainder comes from several sources, including other heme proteins (e.g., cytochromes P-450 and bs, catalase), ineffective hemopoiesis (erythrocytes that never leave the marrow), and "free" heme (heme never incorporated into protein) in the liver. Hemoglobin heme has a life span equal to that of the red cell (about 125 days), whereas heme from other sources (with the exception of myoglobin, which is also quite stable) turns over much more rapidly. Hepatic P-450 enzymes have half-lives of 1-2 days. When radioactively labeled glycine or ALA is injected and radioactivity in fecal bile pigments is monitored, two peaks are seen. The rapidly labeled bilirubin (early bilirubin) peak appears 3-5 days after injection and contains about 15-20% of the injected label. It is increased by drugs that induce hepatic P-450 oxygenases and in erythropoietic porphyria and anemias associated with ineffective erythropoiesis (lead poisoning, thalassemias, and some hemoglobinopathies). Thus, the bilirubin in the early peak is partly derived from these sources. The slowly labeled bilirubin (late bilirubin) peak appears at approximately 120 days, contains about 80-85% of the label, and is due to heine released from senescent erythrocytes.

690

CHAPTER 29

Heme )roteins Proteins-."

Amino[adds (reutilized)

Heme Heine oxygenase system

Mononuclear phagocytic cells of spleen, bone marrow, and liver

Fe=+,~--J" - - C O (reutilized) Biliverdin

NAD(P)H + H+ Urinary urobilinogen (up to ~ rag/d) / Kidney

T

Systemic circulation

Biliverdin reductase "----..BAD(P)+ Bilirqbin (250-300 rag/d) ITransported to the liver complexed to albumin in the plasma

Bilirubinin the hepatocytes (bound predominantly to ligandin)

~

UBG in liver

UDP-glucumnicacid

bin UDPronyltransferase UDP Bilirubin monoglucuronide

UDP-glucuronic acid Bilirubin UDPronyltransferase UDP Secretion into bile

~,~ Enterohepatic circulation

Bilrubin diglucuronide in bile and gall bladder

l

UBG in small bowel

l

Bilirubin diglucuronide in small bowel

l

Bilirubin large bowel ]Metabolism by 1fecal flora ~Ur0bilino~len (UBG) and other compounds Excretion in the feces (50-250 mg of urobilinogen per day)

F I G U R E 29-1(t Catabolic pathway for the heme group from hemoproteins (predominantly hemoglobin).

Formation of Bilirubin A summary of the pathway for bilirubin metabolism and excretion is shown in Figure 29-10. Release of heme from heme proteins and its conversion to bilirubin occur predominantly in the mononuclear phagocytes of liver, spleen, and bone marrow (previously known as the reticuloendothelial system), sites where sequestration of aging red cells occurs. Renal tubular epithelial cells, hepatocytes, and macrophages may also contribute to bilirubin formation under some conditions. Structures of the intermediates in the conversion of heme to bilirubin are shown in Figure 29-11. The initial step after the release of heme is its binding to heme oxygenase, a microsomal enzyme distinct from the microsomal P-450 oxygenases. Heme oxygenase catalyzes what appears to be the rate-limiting step in catabolism of heme. It is induced by


heme and requires 02 and NADPH for activity. The activity of the inducible isoenzyme form of heme oxygenase is highest in the spleen, which is involved in the sequestration of senescent erythrocytes. The constitutive form of heme oxygenase is mainly localized in the liver and brain. After binding, the oe-methene carbon of heme is oxidized (hydroxylated) to ol-hydroxyhemin, which undergoes autoxidation to biliverdin (a blue-green pigment) with consumption of O2 and release of iron and carbon monoxide (derived from oxidation of the ot-methene bridge). Since CO production in mammals occurs primarily by this pathway, measurement of expired CO has been used to estimate heme turnover. Values obtained exceed those derived from plasma bilirubin measurements by about 15%, probably because of bilirubin produced in the liver and excreted into the bile without entering the circulation. A potent competitive synthetic inhibitor of heme oxygenase is tin (Sn) protoporphyrin, which has a potential therapeutic use in treatment of neonatal jaundice (see below). In nonmammalian vertebrates, biliverdin is the final metabolite in heme catabolism. Transport of biliverdin is much easier than that of bilirubin because biliverdin is water-soluble. Conversion of biliverdin to bilirubin may have evolved in mammals because, unlike biliverdin, bilirubin readily crosses the placenta. In this way, the fetus can eliminate heme catabolites via the mother's circulation. However, this explanation may not be complete, since the rabbit (a placental mammal) excretes biliverdin as the major bile pigment. B iliverdin is reduced to bilirubin by NAD(P)Hdependent biliverdin reductase, a cytosolic enzyme that acts at the central methene bridge. Although both molecules have two propionic acid groups, the polarity of biliverdin is greater than that of bilirubin. B ilirubin can form six internal hydrogen bonds between the carboxylic groups, the two lactam carbonyl oxygens, and four pyrrolenone ring nitrogens, and thus prevents these groups from hydrogen-bonding with water (Figure 29-12). Biliverdin cannot form these hydrogen bonds because of the lack of free rotation imposed by the double bond at the central methene bridge. Esterification of the propionyl side chains of bilirubin with glucuronic acid disrupts the hydrogen bonds and increases its solubility and lability. "Activators," such as ethanol and methanol, used in the van den Bergh test to measure "indirect bilirubin," and phototherapy for neonatal jaundice also act by disrupting the hydrogen-bonded structure of unconjugated bilirubin. Hemoglobin and heme released from intravascular hemolysis or blood extravasations (e.g., subcutaneous hematomas) are bound, respectively, by haptoglobin and

SECTION 29.3 HemeCatabolism M

691

M

M

P

V

M

p

V N

' HeNADPY/_lgenase '~ ,O2

.' 9 ..... ~.

~N"'" P~

C

~N-~: H

~

,

M

'

N

I-

M~ Of,-Methene unuge

~N'"'" P~

V

~N~ /

h e m ~ . ~i ~Dn c M M

Heme

OH o~-Hydroxy-

V

Probablyk 02 CO(from~-methene carbon),Fe'+ M M H

nonenzymatic M

M

C P

Central methene bridge, P

V

H2C

NH

V

O Biliverdinreductase ~H~,.,.~ -~~O NAD(P) + r- - - NAD(P)H~ +H+ -~ HN~ N M

M

O Biliverdin _.~O HN

p

V

M

M

V

Bilirubin

FIGURE 29-11 Conversion of heme to bilirubin in the monocytic phagocytic cells. Carbon monoxide and bilirubin are generated. Fe3+ released is conserved and reutilized. Biliverdin and bilirubin are lactams. P, Propionic acid; M, methyl; V, vinyl.

hemopexin V

g H"

M

C--O

"..

~)15

M M

-. M

~0

ii:::o=

................ H / 0

FIGURE 29-12 Conformation of bilirubin showing involutedhydrogenbonded-structure between NH/O and OH/O groups. Despite the presence of polar carboxyl groups, bilirubin is nonpolar and lipophilic. Disruption of hydrogen bonds by glucuronidation or by conversion of bilirubin to configurational or structural isomers yields water-solublepigments.

to form complexes that cannot be filtered by the kidney. This action prevents renal loss of the heme iron and protects the renal tubules from possible damage by precipitated hemoglobin. Haptoglobinhemoglobin and hemopexin-heme complexes are processed in mononuclear phagocytic cells in a way similar to that for hemoglobin. Haptoglobin and hemopexin are glycoproteins synthesized in the liver. The former is an o~2-globulin and an acute-phase reactant (i.e., its synthesis and release into the circulation are augmented during an acute insult to the body); the latter is a ill-globulin but not an acute-phase protein (see also Appendix VI).

C i r c u l a t o r y T r a n s p o r t of Bilirubin B ilirubin formed in extrahepatic tissues is transported to the liver for excretion in bile. Since bilirubin is virtually insoluble in aqueous media, it is transported to the liver bound noncovalently to serum albumin. The bilirubinalbumin complex increases the amount of bilirubin carried

692

per volume of plasma and minimizes diffusion of bilirubin into extrahepatic tissues, thereby preventing bilirubin toxicity. Because of formation of this complex, bilirubin does not normally appear in urine. Urinary bilirubin is almost invariably conjugated bilirubin (see below) and signifies the presence of a pathological process. An albumin molecule binds two molecules of bilirubin at one high-affinity site and at one to three secondary sites. Bilirubin conjugated with glucuronic acid also binds to albumin but with much lower affinity. Another form of bilirubin (probably conjugated), very tightly (probably covalently) bound to albumin, has been described. The mechanism of its formation is not known, although blockage of biliary flow associated with an intact hepatic conjugating system releases a chemically reactive form of bilirubin into the circulation. If the capacity of albumin to bind bilirubin is exceeded because of increased amounts of unconjugated bilirubin or decreased concentration of albumin, bilirubin readily enters extrahepatic tissues. In neonates, this can cause kernicterus, a serious condition associated with permanent neurological damage (see below). Bilirubin can be displaced from binding to albumin by sulfonamides, salicylates (notably aspirin), and cholangiographic contrast media. Use of these substances in jaundiced newborn infants increases the risk of occurrence of kernicterus. Medium-chain fatty acids increase and short-chain fatty acids decrease bilirubin binding to albumin. Binding, at least to the primary site, is independent of pH. Estimation of reserve bilirubin binding capacity has been used to evaluate the risk of bilirubin toxicity in icteric patients.

Hepatic Uptake, Conjugation, and Secretion of Bilirubin Hepatocytes take up bilirubin from the sinusoidal plasma and excrete it after conjugation with glucuronic acid across the canalicular membrane into the bile. The entry and exit steps and the transport of bilirubin within the cell are not completely understood. The following is a plausible interpretation of the available data. Since binding of bilirubin to albumin is usually reversible, a small amount of free bilirubin is present in plasma in equilibrium with albumin-bound bilirubin. It is probably this free bilirubin that is taken up at a rate determined by its plasma concentration. As this free bilirubin concentration decreases, more bilirubin is released from albumin and becomes available for uptake. Alternatively, the albumin-bilirubin complex may bind to specific hepatocyte plasma membrane receptors, and thereby bilirubin is released to enter the cell. Both models are consistent

CHAPTER 29


with the finding that albumin does not accompany bilirubin into the hepatocyte. The entry step seems to be carrier-mediated, is saturable, is reversible, and is competitively inhibited by sulfobromophthalein, indocyanine green, cholecystographic agents, and several drugs. Bile salts do not compete with bilirubin for hepatic uptake. After it enters hepatocytes, bilirubin is transported to the smooth endoplasmic reticulum for glucuronidation bound to a protein. Two cytosolic proteins, Z protein (fatty acid-binding protein) and ligandin (Y protein), bind bilirubin and other organic anions. Ligandin, which constitutes about 2-5% of the total soluble protein in rat and human liver, has lower capacity but higher affinity for bilirubin than Z protein. Ligandin (M.W. 47,000) has two subunits, A and B, which appear to be identical except for a 30-amino-acid extension at the carboxyl terminus of the B subunit. Bilirubin is bound entirely to the A subunit (two molecules per A subunit). Ligandin also has glutathione S-transferase, glutathione peroxidase, and ketosteroid isomerase activities, which depend on both subunits. Glutathione S-transferases catalyze detoxification reactions for a number of substances. Binding of bilirubin and other organic anions to ligandin occurs at sites unrelated to its enzyme activities. Under normal conditions, ligandin is probably the principal hepatic bilirubin-binding protein and may serve intracellularly the same protective and transport functions as albumin in plasma. It may also help limit reflux of bilirubin into plasma, since its affinity for bilirubin is at least five times greater than that of albumin. Z protein (M.W. 11,000) becomes important at high plasma bilirubin concentrations. The concentration of ligandin in the liver does not reach adult levels until several weeks after birth, whereas neonatal and adult levels of Z protein are the same. This lack of ligandin, together with low hepatic glucuronyltransferase activity, is the probable cause of transient, "physiological," nonhemolytic, neonatal

jaundice. Glucuronidation of bilirubin in the endoplasmic reticulum by UDP-glucuronyltransferase produces an ester between the 1-hydroxyl group of glucuronic acid and the carboxyl group of a propionic acid side chain of bilirubin (Figure 29-13). In bile, about 85% of bilirubin is in the diglucuronide form and the remainder is in the monoglucuronide form. Glucuronidation increases the water solubility of several lipophilic substances. There appear to be many UDP-glucuronyltransferases in the endoplasmic reticulum, which differ in substrate specificity. (Biosynthesis of UDP-glucuronic acid was described in Chapter 15.)

SECTIO29.3 N HemeCatabolism

693

COOH

HOc'~~'O

O%C O"~OH "~,COOH OH HO

O%cO /H O / H O~Oc'~O

#

~ H HN~ :u~AUr~

ranSu::"

v@o M

~ H HN-~/~

V%ooX

V

M

Bilirubin

V

Bilirubin diglucuronide

FIGURE 29-13 Formation of bilirubin diglucuronide. Glucuronidation occurs in two steps via formation of monoglucuronide. Monoand diglucuronides are more water-soluble and less lipophilic than bilirubin. Conversion of bilirubin to water-soluble products is obligatory for excretion of bilirubin from hepatocytes. M, Methyl; V, vinyl; UPD-GA, UDP-glucuronic acid.

natural isomer of bilirubin (Figure 29-12). Thus, they are more polar and readily excreted in the bile without the requirement for glucuronidation. Lumirubin, a structural isomer of bilirubin, is formed by light-induced intramolecular cyclization of the vinyl side group of C-3. It contains a seven-membered ring, is stable, is polar, and is excreted without conjugation. These observations explain the mechanism of phototherapy commonly used for treatment of neonatal hyperbilirubinemia.

Secretion across the canalicular membrane into bile appears to be the rate-limiting step in hepatic bilirubin metabolism. It is probably carrier-mediated, requires energy, is saturable, and is unaffected by bile salts. Bilirubin can be made water-soluble by conversion to its configurational isomers. These photobilirubins are formed when bilirubin is exposed to blue light of the 400- to 500-nm wavelength (Figure 29-14). Photobilirubins cannot form the intramolecular hydrogen bonds characteristic of the

HOOC

COOH

H

.o/o \OOH

H

H

H 4E, 15Z

4Z, 15Z (natural

bilirubin bilirubin)

H HOOC

bilirubin

H HOOC

4Z, 15E

COOH

bilirubin

FIGURE 29-14 Photoisomers of bilirubin. The presence of two methene bridges containing double bonds (colored areas) gives rise to configurational (geometrical) isomers of bilirubin. Each double bond can exist in the Z or E configuration. The naturally occurring, most stable, water-insoluble form is the Z, Z isomer. It undergoes photoisomerization to configurational isomers (Z, E; E, Z; and E, E), which are more polar owing to inability to form intramolecular hydrogen bonds and are excretable from the liver without glucuronidation. Some excretion of photoisomers in urine also occurs.

COOH

H

H 4E, 15E bilirubin

H

694

Bilirubin in the Intestinal Tract

CHAPTER 29


The plasma of normal subjects contains 0.1-1 mg of bilirubin per deciliter (2-17 #mol/L), mostly in the unconjugated form. Unconjugated bilirubin is known as indirect-reacting bilirubin and conjugated bilirubin as direct-reacting bilirubin (see Table 29-2). Jaundice occurs when plasma becomes supersaturated with bilirubin (>2-2.5 mg/dL) and the excess diffuses into the skin, sclera, and other tissues. The sclera is particularly affected because it is rich in elastin, which has a high affinity for bilirubin. Reddish yellow pigments, particularly carotene and lycopene, may give a yellowish tinge to the skin but they do not usually produce scleral coloration. Hyperbilirubinemia may result from elevation of unconjugated or conjugated bilirubin levels.

of these cells to store, transport, or conjugate bilirubin. Bilirubinuria does not accompany these disorders. Except in infancy or when pigment gallstones form, unconjugated hyperbilirubinemias are benign. Gilbert's syndrome may be the most common cause of mild, persistent, nonhemolytic, unconjugated hyperbilirubinemia. Serum bilirubin concentration rarely exceeds 5 mg/dL and usually fluctuates between 1.3 and 3 mg/dL. Other liver function tests are normal. The syndrome is usually asymptomatic and is detected during routine laboratory testing or examination for other disease. Family studies suggest that Gilbert's syndrome is an autosomal dominant disorder. The unconjugated hyperbilirubinemia in Gilbert's syndrome is due to decreased UDP-glucuronyltransferase activity resulting from an insertion mutation found in the promoter region of the enzyme. The wild-type promoter [TA]6TAA is mutated to [TA]vTAA. Mutations affecting the coding region of the enzyme, although rare, also occur. In Crigler-Najl'ar syndrome type/activity of hepatic bilirubin UDP-glucuronyltransferase is undetectable and bilirubin conjugates are absent from the serum, bile, and urine, but biliary secretion of sulfobromophthalein and indocyanine green is normal. The disease is apparent shortly after birth, kernicterus develops, and death commonly occurs during the neonatal period. The effectiveness of phototherapy is often transient. The enzyme is not inducible by phenobarbital. This autosomal recessive defect occurs in all races. The Gunn strain of Wister rats has a similar genetic defect and has been used to study the syndrome. Crigler-Najj'ar syndrome type H (Arias syndrome) is milder, usually benign, and caused by partial deficiency of bilirubin UDP-glucuronyltransferase. Jaundice may not appear until the second or third decade of life. The monoglucuronide is the predominant pigment in bile. Phenobarbital induces the enzyme. Dominant and recessive inheritance patterns have been described. An accurate diagnosis of type 1, as opposed to type 2 Crigler-Najjar syndrome, is essential since orthotopic liver transplantation is an important therapy for type 1 patients.

Unconjugated Hyperbilirubinemias

Conjugated Hyperbilirubinemias

Unconjugated hyperbilirubinemias result from imbalance between the rates of production of pigment and of its uptake or conjugation in the liver. Because of the large reserve capacity of the liver for conjugation and excretion of bilirubin, increased production seldom elevates unconjugated serum bilirubin to more than 3-4 mg/dL. If a greater increase occurs, some degree of liver dysfunction probably also occurs. These disorders are usually due to decreased uptake of pigment by hepatocytes or to failure

Conjugated hyperbilirubinemias are due to intra- or extrahepatic reduction to bile flow (cholestasis) with spillage of conjugated bilirubin into the bloodstream, which may occur from injury to the endothelial cells lining bile ductules or from reverse pinocytosis, by the hepatocytes. Since the serum bilirubin is mostly the water-soluble glucuronide, bilirubinuria is usually present. Abdominal tumors, gallstones, strictures, hepatitis, and cirrhosis can mechanically block the biliary canaliculi or

Most bilirubin entering the intestine in bile is in the diglucuronide form, which is very poorly absorbed in the small and large intestines. In the lower small intestine and colon, bacteria remove glucuronic acid residues and reduce bilirubin to the colorless urobilinogen and stercobilinogen. Exposure to air oxidizes these to urobilin and stercobilin, respectively, (i.e., red-orange pigments that contribute to the normal color of stool and urine). Other degradation products of bilirubin are present in minor amounts in feces. Urobilinogen is excreted mostly in the feces, but a small fraction is absorbed from the colon, enters the portal circulation, is removed by the liver, and is secreted into bile. That which is not removed from the portal blood by the liver enters the systemic circulation and is excreted by the kidneys. Urobilinogen excretion in urine normally amounts to 1-4 mg per 24 hours, as opposed to the 40-280 mg (67-470 #mob excreted in feces. Lack of urobilinogen in the urine and feces indicates biliary obstruction; stools are whitish ("clay-colored") owing to the absence of bile pigment. Urinary and fecal urobilinogen excretion increases in hemolytic anemia.

Disorders of Bilirubin Metabolism

695

SECTION29.3 Heme Catabolism

ducts. If obstruction affects only intrahepatic bile flow, hyperbilirubinemia occurs when 50% or more of the liver is involved. Extrahepatic obstruction elevates serum bilirubin only if it increases the pressure in the canaliculi above the maximum secretion pressure of the hepatocytes (about 250 mm Hg). Nonmechanical cholestasis can be caused by bacterial infection, pregnancy, and sex steroids and other drugs, or it may be genetically determined. In cholestasis, bile salts and bile pigments are retained and appear in the circulation, and steatorrhea and deficiencies of fat-soluble vitamins may occur. These deficiencies are often manifested as hypoprothrombinemia (from lack of vitamin K) and osteomalacia (from lack of vitamin D). The magnitude depends on the degree of obstruction. If blockage is complete, urinary urobilinogen will be absent and the stools will have a pale, clay-like color. Familial diseases include Dubin-Johnson syndrome, Rotor's syndrome, and benign familial recurrent cholestasis. Serum bilirubin values in Dubin-Johnson syndrome are the same as those found in Rotor's syndrome (Table 29-2). Very little is known about benign familial recurrent cholestasis. All three disorders are uncommon or rare, and all are benign. The liver in Dubin-Johnson

syndrome appears black on direct examination. The pigment is not identical to melanin but may be a catecholamine (perhaps epinephrine) polymer. The total urinary coproporphyrin excretion is normal but about 80% is the I isomer and the rest is the III isomer, the reverse of the normal ratio. This abnormality seems to be diagnostic for the syndrome, provided the history and physical examination are consistent with the diagnosis. Obligate heterozygotes have urinary coproporphyrin patterns intermediate between those of normal and affected individuals, indicating autosomal recessive inheritance. The hyperbilirubinemia, hepatic pigment, and urinary coproporphyrin abnormality may result from a defect in porphyrin biosynthesis. Studies of sulfobromophthalein clearance show normal storage but greatly decreased secretion. Investigation of a mutation in Corriedale sheep, which causes a similar condition, may help clarify this poorly understood disorder. In Rotor's syndrome, patients lack the hepatic pigment but have urinary coproporphyrin levels 2.5-5 times greater than normal. Coproporphyrin type I is increased relative to the type III isomer but not as much as in Dubin-Johnson syndrome. Intrahepatic storage of sulfobromophthalein is

TABLE 29-2 Serum Bilirubin Levels in Normal and Some Abnormal Conditions

Bilirubin Values in mg/dL of Serum* Condition Newborn 1 day 3 days 7 days Normal (adult) Hemolytic disorders (in adults) Gilbert's disease (probably a heterogeneous group of diseases) Glucuronyltransferase deficiency Type I: Crigler-Najjar syndromem complete deficiency Type II: Crigler-Najjar syndromem partial deficiency Dubin-Johnson syndrome Cholestasis (severe form) Cirrhosis (severe) Hepatitis (acute/severe) *To convert mg/dL to/xmol/L, multiply by 17.1.

Unconjugated (indirect reacting)

Conjugated (direct reacting)

0.1-0.3 0.2-0.4 Normal

(rarely >5)

0.2-0.7 2-3 1.1-2.7 (rarely >5)

15-48

15-48

Negligible

6-22 (up to 40 with physiological stress) 2.5-20 10 11 10

6-22 (up to 40 with physiological stress) < 50% of total 19 56 1.5

Trace

Total 0.2-2.9 0-6 0.3-11 0.1-9.9 0.1-1 2.2-3.4

1.2-3

>50% of total

8.5

696

markedly decreased, but the rate of secretion is only moderately depressed. Inheritance appears to be autosomal recessive.

Neonatal Hyperbilirubinemia Normal neonates are frequently hyperbilirubinemic (Table 29-2). Birth interrupts normal placental elimination of pigment, and the "immature" liver of the neonate must take over. Normally serum bilirubin levels rise on the first day of life, reaching a maximum (rarely greater than 10 mg/dL) by the third or fourth day. This type is mostly unconjugated. If the placenta is functioning normally, jaundice will not be present at birth. If jaundice is present at birth, a cause other than hepatic immaturity must be sought. The primary blocks to bilirubin metabolism are low activity of bilirubin glucuronyltransferase and low concentration of ligandin in the liver at birth. Secretion of conjugated bilirubin into the bile is also reduced. Hepatic immaturity may be partly due to diversion in utero of blood from the liver by the ductus venosus. When this channel closes shortly after birth and normal hepatic blood flow is established, concentrations of a number of substances rise within the hepatocytes and may induce enzymes needed for their metabolism. Accumulation ofbilirubin in plasma may play an important role in hastening the maturation. Although the liver normally matures within 1-2 weeks after birth, hypothyroidism can prolong this process for weeks or months. The neonate is at risk for kernicterus if the serum unconjugated bilirubin level is higher than 17 mg/dL. Kernicterus is characterized by yellow staining of clusters of neuronal cell bodies in the basal ganglia, cerebellum, and brain stem, leading to motor and cognitive deficits or death. Immaturity and perhaps hypoxia make the bloodbrain barrier permeable to bilirubin and contribute to the likelihood of kernicterus. The biochemical basis of bilirubin encephalopathy is due to many causes: inhibition of RNA and protein synthesis, carbohydrate metabolism (both cAMP-mediated and CaZ+-activated), phospholipiddependent protein kinases, enzymes involved in the electron transport system, and impaired nerve conduction. A major complicating factor can be hemolytic anemia such as that of erythroblastosis fetalis caused by Rh incompatibility between mother and child. The hemolysis increases the rate of bilirubin formation, which soon overwhelms the liver and produces severe jaundice and kernicterus. Sickle cell anemia has a similar effect. Congenital absence of bilirubin UDP-glucuronyltransferase (CriglerNajj ar syndrome type 1) usually causes a kernicterus that is fatal shortly after birth. Inhibition of glucuronyltransferase

CHAPTER 29


by various drugs (e.g., novobiocin) or toxins can increase the severity of neonatal jaundice. "Breast milk jaundice" is due to the presence in breast milk of a substance (perhaps pregnane-3o~,20/3-diol) that inhibits bilirubin glucuronyltransferase, although the resulting unconjugated hyperbilirubinemia is seldom serious enough to cause neurotoxicity or to require discontinuation of breast-feeding. Other risk factors for pathologic hyperbilirubinemia include Gilbert's syndrome (discussed earlier) and glucose6-phosphate dehydrogenase deficiency (Chapter 15). Conjugated hyperbilirubinemia is rare during the neonatal period. It can result from impaired hepatocellular function or extrahepatic obstruction. Hepatocellular defects can be caused by bacterial, viral, or parasitic infections, cystic fibrosis, ot 1-antitrypsin deficiency, DubinJohnson and Rotor's syndromes, and other genetic disease. Extrahepatic obstruction can be congenital (biliary atresia) or acquired. Treatment of neonatal jaundice is usually by phototherapy. A decrease in bilirubin production in the neonatal period can also be achieved by inhibiting the rate-limiting enzyme of bilirubin formation from heme, namely, the heme oxygenase. A potent competitive inhibitor of heme oxygenase is the synthetic heme analogue tin (Sn 4+) protoporphyrin. When administered parenterally, the tin protoporphyrin safely decreases bilirubin formation. Exchange transfusions also rapidly decrease plasma bilirubin levels.

Supplemental Readings and References B. R. Bacon, J. K. Olynyk, E. M. Brunt, et al.: HFE genotype in patients with hemochromatosis and other liver disease. Annals oflnternal Medicine 130, 953 (1999). N. C. Andrews: Disorders of iron metabolism. New England Journal of Medicine 341, 1986 (1999). J. D. Arnold, A. D. Mumford, J. O. Lindsay, et al.: Hyperferritinaemia in the absence of iron overload. Gut 41,408 (1997). M. C. Augustine: Hyperbilirubinemia in the healthy term newborn. Nurse Practitioner 24, 24 (1999). B. R. Bacon, L. W. Powell, P. C. Adams, et al.: Molecular medicine and hemochromatosis: at the crossroads. Gastroenterology 116, 193 (1999) J. D. Bancroft, B. Kreamer, and G. R. Gourlev: Gilbert syndrome accelerates development of neonatal jaundice. Journal of Pediatrics 132, 656 (1998). T. H. Bothwell and A. P. MacPhall: Hereditary hemochromatosis: etiologic. pathologic, and clinical aspects. Seminars in Hematology 35, 55 (1998). S. S. Bottomley: Secondary iron overload disorders. Seminars in Hematology 35, 77 (1998). N. Chalasani, N. R. Chowdhury, J. R. Chowdhury, et al.: Kernicterus in an adult who is heterozygous for Crigler-Najjar syndrome and homozygous for Gilbert-type genetic defect. Gastroenterology 112, 2099 (1997). M. E. Conrad: Introduction: iron overloading disorders and iron regulation. Seminars in Hematology 35, 1 (1998). R. W. I. Cooke: New approach to prevention of kernicterus. Lancet 353, 1814(1999).

Supplemental Readings and References E A. Dennery, D. S. Seidman, and D. K. Stevenson: Neonatal Hyperbilirubinemia. New England Journal of Medicine 344, 581 (2001 ). C. Q. Edwards, L. M. Griffen, R. S. Ajioka, et al.: Screening for hemochromatosis: phenotype versus genotype. Seminars in Hematology 35, 72 (1998). G. H. Elder, R. J. Hift, and E N. Meissner: The acute porphyrias. Lancet 349, 1613 (1997). B. M. Gaston: Emissions testing for children, chapter 2: carbon monoxide. Journal of Pediatrics 135, 537 (1999). Y. Haimi-Cohen, E Merlob, T. Marcus-Eidlits, et al.: Dubin-Johnson syndrome as a cause of neonatal jaundice: the importance of coporphyrins investigation. Clinical Pediatrics 37, 511 (1998). R. Kauppinen and E Mustajoki: Prognosis of acute porphyria: occurrence of acute attacks, precipitating factors, and associated diseases. Medicine 71, 1 (1992). Y. Maruo, H. Sato, T. Yamano, et al.: Gilbert syndrome caused by a homozygous missense mutation (Tyr486Asp) of bilirubin UDPglucuronyltransferase gene. Journal of Pediatrics 132, 1045 (1998). J. M. McCord: Iron: free radicals and oxidative injury. Seminars in Hematology 35, 5 (1998). G. Monaghan, A. McLellan, A. McGeehan, et al.: Gilbert's syndrome is a contributory factor in prolonged unconjugated hyperbilirubinemia of the newborn. Journal of Pediatrics 134, 441 (1999).

697 E. Pollitt: Early iron deficiency anemia and later mental retardation. American Journal of Clinical Nutrition 69, 4 (1999). P. Ponka, C. Beaumont, and D. R. Richardson: Function and regulation of transferrin and ferritin. Seminars in Hematology 35, 35 (1998). R. D. Press: Hemochromatosis: a simple "genetic" trait. Hospital Practice 34, 55 (1999). Report of a Meeting of Physicians and Scientists at the Royal Free Hospital School of Medicine: Genetic hemochromatosis. Lancet 349, 1688 (1997). E E Rubaltelli, A. Novello, L. Zancan, et al.: Serum and bile bilirubin pigments in the differential diagnosis of Crigler-Najjar disease. Pediatrics 553, (1994). A. Tefferl, L. Solberg, and R. Ellefson: Porphyrias: clinical evaluation and interpretation of laboratory tests. Mayo Clinic Proceedings 69, 289 (1994). C. G. Uasuf, A. Jatakanon, A. James, et al.: Exhaled carbon monoxide in childhood asthma. Journal of Pediatrics 135, 569 (1999). J. N. Umbreit, M. E. Conrad, E. G. Moore, et al.: Iron absorption and cellular transport: the mobilferrin/paraferritin paradigm. Seminars in Hematology 35, 13 (1998). C. Utzel and M. E. Conrad: Absorption of heme iron. Seminars in Hematology 35, 27 (1998).


CHAPTER

30

Endocrine Metabolism 1: Introduction

Survival of multicellular organisms depends on integration and coordination of differentiated cell functions and the ability to react appropriately to internal and external influences that threaten to disrupt homeostatic conditions. These requirements are fulfilled by a form of intercellular communication in which chemical signals (messengers) released by one cell evoke a receptor-mediated response in another. There are two types of chemical messenger: neurotransmitters and hormones. Neurotransmitters convey signals from one neuron to another or from a neuron to an effector cell, travel very short distances to reach their target sites, and function within the specialized regions of synapses and junctions. Hormones are usually defined as messengers that are transported by the blood to distal target cells. Because they are released into the interstitial space and thence into blood, they are called "endocrine" (ductless; "secreted within") secretions to distinguish them from those which are released into the external environment ("exocrine" or ductal secretions). This terminology and these definitions, though not always appropriate and possibly misleading, are used for want of better alternatives. The term "hormone" comes Endocrine topics not discussed in Chapters 30-34 that are covered elsewhere in the text are as follows: gastrointestinal hormones, Chapter 12; eicosanoids, Chapter 18; pancreatic hormones, Chapter 22; parathyroid hormone and vitamin D, Chapter 37; renin-angiotensin system and antidiuretic hormone, Chapters 32 and 39. A list of expanded acronyms appears in Appendix VIII.

from the Greek hormaein, "to excite." This is a misnomer because some hormones have primarily inhibitory effects, and others may stimulate or inhibit, depending on the target cell type. The term endocrine is misleading because some hormones (steroids, prostaglandins, growth factors) are released via ducts to the external environment, where they affect reproductive activities or wound healing. The designation pheromones may apply to some of these hormones (steroids, prostaglandins) because they act on cells in another organism or individual. The usual definition of a hormone excludes those messengers which affect cells in the vicinity of their release and which do not rely on transport in the blood to reach their destination. For these, the term parahormone (paracrine secretion, local hormone, autacoid) is commonly used. However, if produced in sufficiently large amounts, parahormones diffuse into the vascular compartment and behave as authentic hormones (e.g., somatomedins). Conversely, hormones frequently assume paracrine functions. For example, androgens produced by Leydig cells of the testes diffuse and exert their effect on nearby Sertoli cells. Parahormones, therefore, should be regarded as authentic hormones that exert mainly local effects. Finally, neurotransmitters and hormones differ in respect to the systems that use them. Thus, the nervous and endocrine systems share some messengers (norepinephrine, dopamine, enkephalins, endorphins, cholecystokinin, bombesin, etc.) that exert receptor-mediated effects. The different events that are

699

700

CHAPTER 30

triggered by the same substance clearly manifest the different specialized cell functions that are affected. Hormones and neurotransmitters may have evolved from the same, or similar, ancestral prototypes in unicellular organisms. Substances that resemble mammalian hormones in structure and biological activity serve as neurotransmitters in some invertebrates and lower vertebrates. Several peptide and steroid hormones long considered to be evolutionarily advanced mammalian forms have been found in certain prokaryotes and unicellular eukaryotes. Several mammalian brain-gut peptides occur in the skin of amphibians. Specific receptor sites for mammalian hormones are demonstrable at all stages of phyletic evolution. Thus, the structures of hormones and neurotransmitters and their functions as chemical messengers may have been highly conserved during evolution. The fact that many prototypic messengers in unicellular organisms predate the appearance of nerve cells implies that neurotransmitters may be a specialized form of hormone. Chemically, hormones are of four types:

1. 2. 3. 4.

Endocrine Metabolism I: Introduction

Hormonal amine, Peptide, protein, or glycoprotein, Steroid, and Eicosanoid.

Amine hormones and some agents of the second type have counterparts in the nervous system that function as neurotransmitters.

30.1 Hormonal Amines Hormonal amines are derived from amino acids and, in most cases, represent simple modifications of the parent compound. Table 30-1 lists the important hormonal amines, parent amino acids, major sites of synthesis, and principal actions. All of these amines except the thyroid hormones (Chapter 33) are decarboxylated products that are synthesized both in and out of the nervous system. Within the nervous system, they are important neurotransmitters; outside the nervous system, the cells that produce

TABLE 30-1 Hormonal Amines

Hormone

Parent Amino Acid

Dopamine

Tyr

Hypothalamus; adrenal medulla

Yes

Epinephrine

Tyr

Adrenal medulla

Minor

Norepinephrine

Tyr

Peripheral nerve endings; adrenal medulla

Yes

Histamine

His

Mast cells; basophils; regenerating cells

Yes

Serotonin

Trp

Mast cells; liver; sympathetic nerve endings; stored platelets

Yes

3,5,3'-Triiodothyronine (T3)1

Tyr

Peripheral conversion from thyroxine (T4); thyroid

1An amino acid.

Major Site of Synthesis

Neurotransmitter in Nervous System

Systemic Action

Renal vasodilation (D1) inhibition of anterior pituitary hormone release (D 2); inhibition of aldosterone synthesis (D 1)" Cardiovascular and metabolic effects (via ~- and /3-adrenergic receptors). Cardiovascular and metabolic effects (via c~- and /3-adrenergic receptors). Systemic vasodilation (H l , H2); increased capillary permeability (H l); increased gastric secretion (H2); increased heart rate (H 2). Vasodilation in skin and skeletal muscle, vasoconstriction elsewhere; positive inotropic and chronotropic effects. Many effects, e.g., regulation of metabolism.

SECTION30.2 Peptide, Protein, and Glycoprotein Hormones

them are modified postsynaptic neurons (e.g., adrenal medulla), blood-derived cells (e.g., basophils, mast cells), or APUD 1 cells (e.g., enterochromaffin cells). Regardless of where they are released, hormonal amines exert their effects through specific receptor sites located in various parts of the body. These systemic receptors (Table 30-1) and their subtypes probably have identical counterparts in the central and peripheral nervous systems. All of these amines except T3 exert rapid systemic effects that usually involve smooth-muscle activity. Because the amines are hydrophilic, their receptors are located on the outer surface of target cells, and most if not all of their effects are mediated by intracellular mediators.

30.2 Peptide, Protein, and Glycoprotein Hormones Cells that produce peptide, protein, or glycoprotein hormones are derived embryologically from the entoderm or ectoblast (progenitor of ectoderm and neuroectoderm). More than 40 hormones have been identified, containing from three to over 200 amino acid residues. Table 30-2 lists the important hormones and hormonal candidates, number of amino acid residues, and sites of synthesis. All hormonal peptides and many hormonal proteins are synthesized as part of a larger molecule (preprohormone) that contains a leader sequence (signal peptide) at its amino terminal end. The leader sequence is removed as the nascent precursor enters the lumen of the endoplasmic reticulum, and the resultant prohormone undergoes posttranslational processing after being packaged into secretory granules by the Golgi complex. Posttranslational processing involves proteolytic cleavage at specific sites, usually paired basic amino acid residues (Lys-Arg, ArgLys, Arg-Arg, Lys-Lys), by endopeptidases within the secretory granule. In the synthesis of insulin, for example, removal of the 23-amino-acid leader sequence of preproinsulin results in proinsulin, which has two pairs of basic residues (Lys-Arg and Arg-Arg) that are cleaved to yield insulin and the C-peptide (Chapter 22). Likewise, the endogenous opiates arise from site-specific cleavages of their respective prohormones (Chapter 31). When the cell is stimulated to secrete, all major fragments of the prophormone, active and inactive, are released by calciumdependent exocytosis. 1APUD (amine precursor uptake and decarboxylation) cells are derived from the neuroblast (stem cells that give rise to nerve cells and neural crest cells) or the entoderm. They have the ability to synthesize and release peptide hormones and, as their name implies, take up amine precursors (e.g., dopa) and decarboxylate them, producing hormonal amines (e.g., dopamine).

701 Although several peptide hormones have multiple anatomical sites of synthesis, there is usually only one important source of the circulating hormone. The presence of the same hormone in ancillary sites (Table 30-2) may indicate that it functions as a local hormone at those sites. For example, somatostatin produced by the hypothalamus is transported by blood to the anterior pituitary, where it inhibits the release of growth hormone (GH); somatostatin produced by the hypothalamus also functions as an inhibitory neurotransmitter when released into synapses in the central nervous system; somatostatin produced by pancreatic islet cells acts locally to inhibit the release of insulin and glucagon; and somatostatin produced by the gastrointestinal mucosa acts locally to inhibit the secretion of gastrin, secretin, and gastric inhibitory polypeptide (GIP). Other "brain-gut" peptides(Chapter 31) also exemplify this point, although they are not released into the general circulation in significant amounts. Peptide hormones, because of their high information content,may have evolved from a limited number of ancestral molecules that functioned as intercellular messengers or as extracellular enzymes. Thus, some exhibit homology in their amino acid sequences, suggesting the same (or similar) evolutionary roots. Several families of peptides have been described, including the opiomelanocortin family (endorphin, adrenocorticotropic hormone, melanocyte-stimulating hormone), the somatomammotropin family (growth hormone, prolactin, human placental lactogen), the glycoprotein hormones (thyroid-stimulating hormone, luteinizing hormone, follicle-stimulating hormone, human chorionic gonadotropin), the insulin family (insulin, insulin-like growth factors, somatomedins, relaxin), and the secretin family (secretin, glucagon, glicentin, gastric inhibitory peptide, vasoactive intestinal peptide). Members in a family are related in structure and function. Thus, members of the secretin family stimulate secretory activity in target cells, while those of the insulin family promote cell growth. This preservation of structure and function can be traced to more primitive forms of life. For example, "o~-mating factor," a primitive relative of mammalian gonadotropin-releasing hormone (GnRH) with which it shares 80% sequence homology, functions as a reproductive pheromone in yeast. Moreover, mammalian corticotropin-releasing hormone (CRH) is related to urotensin I of teleosts and to sauvagine of amphibians in structure and in the ability to cause hypotension, vasodilatation, and ACTH release when administered to mammals. Certain strains of bacteria synthesize substances indistinguishable from mammalian hCG and TSH and also synthesize serine proteases that are structurally homologous to the subunits of the glycoprotein hormones. Glycoprotein hormones

702

CHAPTER 30


TABLE 30-2

Peptide, Protein, and Glycoprotein Hormones in Humans Hormone

Released into Blood?l

Major Production Site z

Y N N Y Y

Hypoth CNS, Hypoth CNS, Hypoth Blood Circulation Hypoth

9 10 14 21 28 29 31 32 33 39

Y Y Y N Y Y Y Y Y Y

Hypoth Hypoth Hypoth Blood vessels Heart Islets AP, Hypoth Thyroid C-cell Intestinal mucosa AP

41 43 44 51 57 70 84 191 191

Y Y Y Y Y Y Y Y Y

H ypoth Intestinal mucosa H ypoth Islets Ovary, Placenta Cartilage, liver, et al. Parathyroid glands AP Placenta

198 209 215 231 236

Y Y Y Y Y

AP AP AP Placenta AP

AA Residues

Thyrotropin Releasing Hormone (TRH) Methionine Enkephalin (MENK) Leucine Enkephalin (LENK) Angiotensin II (AII) Antidiuretic Hormone (ADH); Arginine Vasopressin (AVP) Oxytocin (OT) Gonadotropin Releasing Hormone (GnRH) Somatostatin (SS)(SRIH) Endothelin- 1 (ET- 1) Atrial Natriuretic Peptide (ANP) Pancreatic Glucagon /3-Endorphin (/3-END) Calcitonin (CT) Cholecystokinin (CCK) Adrenocorticotropic Hormone (ACTH)(Corticotropin) Corticotropin Releasing Hormone (CRH) Gastric Inhibitory Polypeptide (GIP) GH-Releasing Hormone (GRH) Insulin Relaxin Insulin-like Growth Factor I (IGF-I) Parathyroid Hormone (PTH) Growth Hormone (GH)(Somatotropin) Placental Lactogen (hPL) (Chorionic Somatomammotropin) Prolactin (PRL) Thyroid Stimulating Hormone (TSH)(Thyrotropin) Luteinizing Hormone (LH) Chorionic Gonadotropin (hCG) Follicle Stimulating Hormone (FSH)

Other Production Sites 3

Islets (P)

Hypoth (S)

1y = major route of dissemination to target cells is via blood transport; N = functions mainly as paracrine or autocrine hormone, although it may enter the blood stream. 2Hypoth= hypothalamus; CNS = central nervous system; AP = anterior pituitary (Adenohypophysis); Islets = Islets of Langerhans. 3Restricted to those sites that are physiologically important;P = paracrine function; S = released into blood.

therefore may have evolved from ancestral extracellular proteases.

30.3 Steroid Hormones Four kinds of steroid hormones differ in structure and action; they are the androgens (C19), the estrogens (C18), the progestins (C21), and the corticosteroids (C21). All

are synthesized from cholesterol and are produced by mesoderm-derived cells regulated by at least one peptide hormone. Structures of the steroid hormones are shown in Figure 30-1. The hormonal form of vitamin D3 [1,25(OH)2-D3] is a sterol derived from an intermediate of cholesterol biosynthesis and is discussed in Chapter 37. Synthesis begins with cholesterol, which in most instances is acquired from circulating low-density lipoprotein (Chapter 20); however, all steroidogenic cells are

SECTION 30.3

Steroid Hormones

703

$

Cholesterol (exclusive substrate)

B H C--O

Pregnenolone (intermediate)

~H3

?I-I=OH

c=o

c=o HO

Progesterone (major progestin)

(~H2OH

Cortisol (hydrocortisone) (major glucocorticoid)

%0/"~-o

HO

Aldosterone (major mineralocorticoid) OH

Testosterone (major androgen) F I G U R E 30-1 Steroid structure and nomenclature.

Estradiol (major estrogen)

-

-

-

OH

704


CHAPTER 30

CH3 j

/CH3

CHcI~~~

\CH3

HC-- ell2-- C1"12--CI'~ CH

H O " ~ CHOLESTEROL

CHOLESTEROL O E S M O t ~ S E (cvP~A) ....... ] . . 36-HYDROXYSTEROID

[17=-HYOROXYt~SE (CYP~r~ !Z,20-LYA~EI

PREGNEI~IOLONE-~ = =. ~

'17..OHPREGNENoLONE = =. ~ - D E HANDROSTERONE YDROEPI"-

....... ! ............................ 21-HYOROXYLASE :i16-HYDROXYLASE ' 18-HYDROXYLASE (CYP11B2) 18-OXlDASE (CYP11B2)

IL(CYP~9} ''~~

PROGESTERONE

t ............................... 17-OH PROGESTERONE

....... ! ............................

!

.

.

.

.

.

.

! ....

.

.

.............

11-DEOXYCORTISOL

TESTO

CORTICOSTERONE

CORl~SOLI(CYPllB1} ............... 21 ~;H2OH

I "'"

H ~

:

ERONE

:i .........

~

HO~~

~

t

---

i

OH

OH

"OH

21 CH=OH

0

I .....

ESTRADIOL

"

I01HC=O H CH~

ESTARONE/176-HYOROXYSTEROIO1

J - ..... DIHYDROTESTOSTERONE

HC= O

ALDOSTERONE

: 9 -

ANDROSTENEDIONE ~

DEOXYCORTICOSTERONE

....... t'" 18-OH CORTICOSTERONE ....... t'"

Jl

HO

V

0

F I G U R E 30-2 Biosynthesis of steroid hormones from cholesterol. The different pathways occur to varying extent in adrenals, gonads, and placenta. The systematic names for cytochrome P-450 enzymes, namely CYP followed by a number, are given in parentheses. CYPll B2 and CYP17 possess multiple enzyme activities. [Reproduced with permission from R C. White and R W. Speiser, Congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Endocrine Reviews 21,245 (2000).]

capable of d e n o v o cholesterol synthesis from acetate. Cholesterol either is stored as a fatty acid ester or is immediately utilized in steroidogenesis. The esterified cholesterol is tapped when the cell is stimulated by the activation of cholesterolesterase, a cytoplasmic enzyme that catalyzes the hydrolysis of cholesteryl esters to free cholesterol and fatty acid. The steroidogenic pathway begins in the mitochondria. The unesterified cholesterol is transported from the cytosol to the matrix side of the mitochondrial membrane; this process is mediated by a 37-kDa phosphoprotein known as stetoridogenic acute regulatory (STAR) protein. The importation of cholesterol from cytosol to mitochondria is the rate-limiting step in the steroidogenesis. Trophic stimuli such as ACTH in zona fasciculata and zona reticularis and elevated cytosolic Ca 2+ in zona glomerulosa cause increased synthesis of the StAR protein within minutes. Other proteins in addition to StAR are also involved in cholesterol transfer across the mitochondrial membrane. A number of enzymes in the steroidogenic pathway are cytochrome P-450 proteins that require molecular oxygen and NADPH (Chapter 14). These enzymes

catalyze the clevage of carbon-carbon bonds and hydroxylation reactions. The P-450 enzymes are distinguished by specific notations. For example, C Y P l l A (also known as cholesterol desmolase, side chain clevage enzyme, P450scc) is a mitochiondrial enzyme and catalyzes the conversion of cholesterol (a C27 compound) to pregenolone (a C21steroid). CYP21 refers to the hydroxylase that catalyzes the hydroxyation at carbon 21. The pathways of steroidogenesis are shown in Figure 30-2. A true steroidogenic cell is equipped with the enzyme CYP11A, which catalyzes the initial step of steroidogenesis; the conversion of cholesterol to the steroid intermediate, pregnenolone. CYP1 I A catalyzes the hydroxylation of carbons 20 and 22 of cholesterol, followed by the cleavage of the bond between these two hydroxylated carbons. The resultant steroid, pregnenolone, is a 21-carbon derivative of cholesterol with a two-carbon remnant side chain. It retains the A 5 configuration and the 3/~-hydroxyl group of cholesterol, and is lacking in biological activity; however, it is the universal intermediate in all steroid hormone synthesis. Because mitochondria have no enzyme that recognizes pregnenolone as a

SECTION 30.3

Steroid Hormones

substrate, pregnenolone enters the smooth endoplasmic reticulum (SER) and is processed along whatever steroidogenic pathway is active in the given tissue. Because no single steroidogenic tissue normally expresses all of the steroidogenic enzymes, the presence or absence of enzymes in a given tissue will determine what sort of steroid hormone the tissue can produce from pregnenolone. Pregnenolone serves as a substrate for either of two enzymes; the SER:3/~-hydroxysteroid dehydrogenase/ isomerase (3/3HSD) and 17oe-hydroxylase/17,20-1yase (CYP17). In cells that contain both activities, pregnenolone is first processed by 3/3HSD, followed by CYP17.

T h e / k 4 Pathway: 3/3HSD 3r is a single, nonheme protein that catalyzes a two-step conversion of a aS,3/~-hydroxylated steroid to a A4,3-ketosteroid in the SER of the adrenal cortex, ovary, and testes. When pregnenolone is the substrate, the product is progesterone, a biologically active steroid. In the corpus luteum and the placenta, which are lacking in progesterone-metabolizing enzymes, the nascent progesterone diffuses from the cells and enters the plasma. However, in other tissues progesterone serves as a substrate for at least one other enzyme.

Directional Flow Valve" CYP17 The presence or absence of the enzyme, CYP17, is a major determinant of the fate of pregnenolone (or progesterone). The presence of the enzyme allows access to either the cortisol or the androgenic (sex steroid) pathways but blocks off the aldosterone synthetic pathway. The absence of the enzyme allows for the formation of progesterone and aldosterone but precludes the formation of either cortisol or androgens. (See following subsections on corticosteroid pathway and sex steroid pathway.) CYP17 is a single protein that catalyzes the 17oehydroxylation of either pregnenolone or progesterone in the SER. In some tissues, such as the gonads, this enzyme catalyzes an additional reaction, involving lysis of the bond between carbons 17 and 20 of the 17o~-hydroxylated product. This results in the formation of a 19-carbon androgenic steroid: dehydroepiandrosterone (DHEA) is formed from pregnenolone and A4-androstenedione (A4-A) is formed from progesterone. Thus, the lyase reaction initiates the sex steroid pathway. However, in other tissues that express the CYP17 gene, such as the zona fasciculata of the adrenal cortex, the enzyme does not catalyze the 17, 20-1yase reaction to a significant extent, and the product is a 17oe-hydroxylated pregnenolone or progesterone, which

705

enters the cortisol synthetic pathway. In both tissues, the enzyme is identical, with the same 17oe-hydroxylation activity; and it is not clear what confers the 17,20-1yase activity. In tissues that have CYP17 but are lacking in 3/~HSD, such as the fetal zone and the zona reticularis (Chapter 32), the 17o~-hydroxylation of pregnenolone favors the lyase reaction and the formation of dehydroepiandrosterone (DHEA), probably because no other options are available in the A 5 pathway.

The Corticosteroid Pathway Is Initiated by 2 1 - H y d r o x y l a s e (CYP21) The valve that allows entry of 17-OH progesterone into the corticosteroid pathway is CYP21, a SER enzyme that catalyzes the hydroxylation of carbon 21. In the absence of CYP21, neither aldosterone nor cortisol can be produced. In the presence of this enzyme, two corticosteroid pathways become available, each with a different substrate requirement. Normally, the gene encoding CYP21 is expressed only in the adrenal cortex, primarily in the outer two zones (glomerulosa and fasciculata). The kidney and other tissues are also capable of 21-hydroxylation of steroids; however, these tissues contain an enzyme that is not CYP21 and its activity is too low to interfere with the normal endocrine function of the adrenals. The mineralocorticoidpathway is catalyzed by two enzymes and utilizes progesterone as the initial substrate. The first enzyme, CYP21, catalyzes the 21-hydroxylation of progesterone, which converts it to the mineralocorticoid, 11-deoxycorticosterone (DOC). This enzyme is present in the outer two zones of the adrenal cortex. The second enzyme is zone-specific in that each of the two outer zones of the cortex expresses a different gene for the enzyme. In the zona fasciculata, the CYPllB1 gene expresses a mitochondrial enzyme that catalyzes the 11/~-hydroxylation of DOC to form cortisol; in the zona glomerulosa, another gene, CYPllB2, expresses a mitochondrial enzyme (aldosterone synthase) that catalyzes the three-step conversion of DOC to aldosterone with the formation of the intermediates corticosterone and 18-hydroxycorticosterone. No other tissue in the body normally expresses either of these two genes. Therefore, production of DOC and corticosterone occurs only in the two outer zones of the adrenal cortex, and aldosterone is synthesized exclusively by the zona glomerulosa. The glucocorticoid pathway is confined to the zona fasciculata of the adrenal cortex and it is catalyzed by the same CYP21 enzyme that is involved in the mineralocorticoid pathway in this zone along with CYP11B1. The principal difference between the two pathways in the zona fasciculata is the initial substrate, and this explains

~06

why the intermediates such as DOC and the products (corticosterone and cortisol) are different. Thus, whereas in the mineralocorticoid pathway the initial substrate is progesterone, in the glucocorticoid pathway it is 17o~hydroxyprogesterone. The only difference between the two substrates is the presence of a 17ot-hydroxy group in the latter. Note that this same difference is seen between the two pathways at all three stepsmsubstrate, intermediate, and product. The salient features here are: 1. The presence of 17ot-hydroxy group confers maximal glucocorticoid activity to cortisol (Chapter 32); and 2. The zona fasciculata is the exclusive site of cortisol formation.

The Sex Steroid Pathway Is Initiated by 17~-Hydroxylase/17,20-Lyase ( C Y P I 7) The valve that allows entry of 17-OH prognenolone into the "sex steroid" pathway is the 17,20-1yase component of CYP17, the SER enzyme that catalyzes the 17othydroxylation of steroids. As noted above, the lyase component of the enzyme is active in certain tissues for reasons that are not currently clear. Both A 4 and A 5 steroids are known to feed into the sex steroid pathways although the absence of 3/3HSD precludes the former. It should be emphasized that the formation of androgenic steroids is necessary before estrogen formation is possible; therefore, the initial reaction in the sex steroid pathway invariably produces an androgenic steroid.

Synthesis of Androgens: 17,20-1~yase In the adrenal cortex, the sites of sex steroid production (fetal zone and zona reticularis) are lacking in 3/3HSD, and pregnenolone is processed by both components of CYP 17 to become DHEA, as discussed above. Most of the DHEA is sulfated at the 3-hydroxyl group and the product, DHEAS, is secreted into the plasma. This occurs due to the paucity or absence of any other steroid-modifying enzymes in the tissue. In the Leydig cells of the testes and the theca interna cells of the ovaries, both components of CYP17 are active and will accept either pregnenolone and progesterone as a substrate; radioactive tracer studies indicate that the A 4 substrate (progesterone) is preferred. Progesterone processing by the enzyme results in the formation of androstenedione, which is secreted as such (by the theca interna) or converted to the biologically active androgen, testosterone (T) by a 17/3-hydroxysteroid dehydrogenase (17flSDH)-catalyzed reaction (by the Leydig). Androstenedione and testosterone are 19-carbon steroids are quantitatively the major androgens produced by the

CHAPTER 30


gonads; however, the two biologically active androgens are testosterone and dihydrotestosterone (DHT). Androstenedione is inactive because its 17-keto group is not recognized by the androgen receptor. In the peripheral tissues, androstenedione can be converted to T because of the presence of 17/3SDH and many androgen target tissues can convert either androstenedione or T into DHT because of the presence of 5o~-reductase. CYP17 activity (both 17o~hydroxylase and 17,20-1yase activities) in the ovary and testis is inhibited by estrogen.

Synthesis of Estrogens: Aromatase (CYP19) Formation of estrogen is catalyzed by the enzyme aromatase (CYP19) which is present in the SER of a few tissues (gonads, brain, placenta, and adipose). The enzyme is expressed by a single gene ( c y p l 9 ) on chromosome 15; it catalyzes a multistep reaction in which a 19-carbon androgen undergoes hydroxylations at carbon 19 followed by the removal of carbon 19 and the aromatization of ring A. The overall aromatization reactions require an appropriate ring A as substrate (either testosterone or androst-4-ene-3,17dione), 3 NADPH molecules, and 3 oxygen molecules. The regulation of CYP19 activity differs among tissues; FSH induces the enzyme in Sertoli and granulosa cells of the gonads, whereas glucocorticoids induce the enzyme in adipose tissue. The tissues that contain CYP19 are of two varieties; one that produces estrogen de novo from cholesterol (e.g., corpus luteum), and the other that lacks the ability to produce its own 19-carbon substrate and relies on a supply of androgens from other sources (e.g., Sertoli cells of testis, granulosa cells of ovary, placenta, adipose cells, hypothalamus, liver). CYP19 activity is inhibited by the nonaromatizable androgen DHT and is stimulated by the aromatizable androgen testosterone. Steroidogenic enzymes are located mainly, but not exclusively, in the three tissues that are authentically steroidogenic (i.e., capable of converting cholesterol to pregnenolone): the adrenal cortex, the gonads, and, in pregnancy, the placenta. Steroidogenic enzymes are also present in a few tissues that are not capable of de novo steroid synthesis from cholesterol but are able to take up intermediates from plasma and convert them to active hormones; for example, fat cells and the hypothalamus contain CYP19, which enables them to convert circulating androgens into estrogens. The kidney, one of the few extra-adrenal sites with CYP21, converts progesterone to the mineralocorticoid DOC. A "steroid-modifying" enzyme catalyzes a reaction that alters some structural feature of a steroid, resulting in the formation of an active hormone or an inactive metabolite. These enzymes are present in many tissues, although

707

SECTION30.3 Steroid Hormones

in most the result of steroid modification is inactivation; for example, the liver contains a large number of steroidmodifying enzymes, virtually all of which are involved in steroid inactivation. The kidney is another tissue that contains several steroid-modifying enzymes, most of which serve to inactivate steroids. On the other hand, at least three steroid-modifying enzymes are physiologically important, as demonstrated in the disorders resulting from a deficiency in any one of them: 5t~-Reductase, an enzyme that is present in a few tissues, serves to convert testosterone and other androgens to DHT, a reduced form that is most active in those tissues. There are two types of the enzyme (type 1 and type 2), which are products of different genes. The two types of 5ot-reductase do not differ in substrate specificity but differ in enzyme kinetics and tissue distribution. Tissues that express the type 1 enzyme appear to be those contributing to secondary sex characteristics (the hair follicles, sebaceous glands), while tissues that express the type 2 enzyme are structures of the reproductive system (external genital tissues of the fetus, prostate gland, and seminal vesicles). A deficiency of 5o~-reductase type 2 enzyme results in a disorder of sexual differentiation in the male (Chapter 34). The synthesis of 5o~-reductase activity in prostate and skin is regulated by androgens; activity in liver is regulated by thyroid hormone. 17r dehydrogenase (17~HSD) (also called 17-ketosteroid reductase; 17/3-hydroxysteroid ketoreductase) catalyzes the reversible conversion of unconjugated 17-ketosteroids to 17-hydroxysteroids and supplies the physiologic mechanism for generating estradiol from estrone and testosterone from androstenedione, conversions that lead to the formation of biologically active hormones from inactive precursors. This enzyme is present in the testes and ovaries and its activity is critical for the gonads' ability to produce the biologically active form of androgen (testosterone, DHT) and estrogen (estradiol); a deficiency of this enzyme is known to cause disturbances in sexual differentiation. The gene encoding gonadal and placental 17/3HSD is on chromosome 17. The same enzyme is also present in nonreproductive tissues such as liver, lung, erythrocytes, and platelets, but the gene encoding this enzyme is not subject to the same regulation as its gonadal counterpart. Accordingly, in pseudohermaphroditism due to hereditary 17/3HSD deficiency, nongonadal 17/3HSD activity is normal. ll~-Hydroxysteroid dehydrogenase (ll~HSD) catalyzes the conversion of l l-hydroxysteroids into l lketosteroids, a physiologically important reaction that converts the biologically active cortisol to cortisone, an inactive metabolite. The presence of this enzyme in tissues that respond to aldosterone is critical in the hormonal

regulation of blood volume (Chapter 32). It is not clear whether the reverse reaction, one that converts cortisone to cortisol, is catalyzed by a separate enzyme, 11-oxidoreductase. From the clinical standpoint, this is an important reaction because the effectiveness of cortisone treatment relies on it (Chapter 32). Evidence that 11-oxidoreductase and 11/3HSD are different enzymes comes from the observation that patients with a deficiency of 11-oxidoreductase activity may have normal 11/3HSD activity; however, there is evidence that the same enzyme catalyzes both reactions but that different isoforms of the enzyme exist. The activity of 1 l flHSD (cortisone formation) is inhibited by licorice and by carbenoxolone while the activity of 11-oxidoreductase is inhibited by carbenoxolone. A number of drugs have an inhibitory effect on one or more steroidogenic/steroid modifying enzymes, and some of these are used clinically to inhibit the production of certain steroid hormones. Some of the important drugs used for this purpose are broad spectrum while others are hormone-specific; a few are listed below along with their principle therapeutic use: 9 Aminoglutethimide--inhibits CYP11A and CYP19 (breast cancer, Cushing's syndrome) 9 Trilostaneminhibits 3/3HSD (Cushing's syndrome) 9 Ketoconazole--inhibits CYP 17 enzymes (17ol-hydroxylase and Cl7,20-1yase), particularly those involved in testosterone synthesis. After 6 months of treatment in hirsute women, serum testosterone declines without a change in serum cortisol or DHEAS. 9 Mitotane (o,p'-DDD)minhibits adrenal CYP11A and CYP11B 1, but not CYP11B2 (neoplasms of adrenal cortex) 9 Metyrapone--inhibits CYP11B 1 (hypercorticism due to adrenal neoplasms or ectopic ACTH production) 9 Finasteride--inhibits 5o~-reductase type 2 (prostatic hyperplasia) 9 Testolactone~inhibits CYP19 (breast cancer) 9 Fadrozole~inhibits CYP19 (breast cancer) 9 Spironolactone~primarily an aldosterone antagonist but inhibits CYP17 in both ovary and testis. It is also an inhibitor of androgen binding to the AR (K+-sparing diuretic)

Steroid-Binding Serum Proteins Steroid hormones are hydrophobic molecules that convey information to diverse regions of the body by way of the bloodstream. Because of their very limited solubility in water, however, steroid hormones would precipitate

~08

CHAPTER 30


TABLE 30-3

Steroid-Binding Serum Proteins CBG 1

Albumin

Site of production Molecular weight Normal concentration

Liver 66,000 550/~M

Liver 58,000 710 nM

Cortisol-binding capacity Testosterone-binding capacity Estradiol-binding capacity Steroid binding sites/molecule Affinity (Kd)

>200 mg/L >400 mg/L >3,000 mg/L >10 -2 • 10--4 M (cortisol) ---4 x 10-4 M (testosterone) Glucocorticoids Thyroid hormone

250 ~g/L

Factors that increase concentration

Factors that decrease concentration

Hepatic disease Renal disease Protein malnutrition

TeBG 2

Liver 94,000 25 nM in men 40 nM in women 6/.~g/L 2/zg/L

1

1

3 x 10-8 M (cortisol)

-2• M (testosterone)

Increased estrogens Increased thyroid hormone

Increased thyroid hormone Increased estrogen (5-10• Decreased androgens Stress Aging High carbohydrate diet Prolonged stress Cirrhosis of liver Increased androgens (2• Obesity Hyperprolactinemia Increased GH Menopause Progestins Glucocorticoids

Increased glucocorticoids Septic shock Pernicious anemia High carbohydrate diet

lCorticosteroid-bindingglobulin (also called transcortin). 2Testosterone-estradiol-binding globulin.

out of blood if it there were no means of rendering them water-soluble. Two processes normally operate to solubilize steroids in blood conjugation and protein binding. Steroids that are conjugated, either with sulfate or glucuronic acid, are unable to enter target tissues because of their water-soluble (hydrophilic) property; they are usually destined for excretion by the kidney. Although a few tissues (e.g., placenta) have sulfatases that can liberate a steroid from its sulfoconjugated state, most tissues are lacking in this ability and are therefore unresponsive to the high levels of conjugated steroids in the blood. The physiologically important means by which steroid hormones are solubilized for transport in blood is the use of certain serum proteins that solubilize steroids. The liver produces three serum proteins that serve to solubilize steroid hormones (Table 30-3): albumin, a nonspecific adsorber

of hydrophobic substances that binds steroids with low affinity; corticosteroid-binding globulin (CBG, also called transcortin), which binds Czl steroids but has the greatest affinity for cortisol; and testosterone-estradiol-binding globulin (TeBG, also called sex hormone-binding globulin, SHBG), which binds steroids of the androstane (C19) and estrane (CIs) series but has greater specificity for androgens than estrogens. Because of the high-affinity binding, steroids are not accessible for tissue uptake while bound to either CBG or TeBG; therefore, the greater the fraction of the steroid that is bound to these proteins, the longer the half-life of the steroid. Unlike that of albumin, the concentrations of CBG and TeBG in plasma are normally, near the concentration of the hormone they preferentially bind, and each protein has only one steroid binding site per molecule. The CBG concentration in plasma is

SECTION 30.4

Mechanism of Hormone Action

~0.71 #M, with a cortisol binding capacity of 250 #g/L (mean cortisol levels (_~ 100 #g/L); while the plasma TeBG concentration is ~ 25 nM in men and -~ 40 nM in women (mean testosterone levels in adult men is ~ 22 nM). An increase in steroid hormone production will result in an increase in the fraction of the hormone bound by the protein until all of the sites on the protein are saturated (i.e., maximal binding capacity); any increase in hormone production beyond this will result in a pathological situation of hormone excess because the cells will be exposed to supraphysiological levels of albumin-bound and unbound hormone. The concentrations of CBG and TeBG in plasma have important influences on both the production and bioeffectiveness of steroid hormones; thus, factors that affect the plasma levels of these binding proteins will indirectly affect the physiology of the steroid hormones they bind. Serum albumin is a quantitatively important binder of hydrophobic molecules that makes possible the transport of these otherwise insoluble molecules in blood; however, it binds very loosely, and this enables the molecules (ligands) to dissociate very rapidly. This means that the albumin-bound fraction serves primarily to solubilize the steroid in plasma and does not oppose tissue uptake of the steroid. For this reason, the albumin-bound fraction should be regarded as being functionally "free." Thus, plasma steroid hormones can be viewed as existing in only two fractions: those that are bound to specific binding protein and those that are not (albumin-bound + unbound). For the sake of simplicity, when dealing with specific steroid hormones, the fractions will be designated relative to the specific binding protein only; for example, when dealing with testosterone, the two plasma fractions will be designated "TeBG-bound" and "non-TeBG-bound." Likewise, plasma cortisol fractions will be referred to as "CBGbound" and "non-CBG-bound." Although the unbound form of steroid hormones is the "active" form, it is also the form that is more susceptible to rapid metabolism. The major site of steroid metabolism is the liver, which inactivates steroids by conjugation and by structural modification. Hepatic A4-hydrogenase reduces the double bond at positions 4 and 5; 17-HSD oxidizes the hydroxyl group at position 17, thereby forming 17ketosteroids; 3o~-hydroxysteroid dehydrogenase reduces the 3-keto to a 3~-hydroxy group. The efficacy of this liver function explains why steroid hormones are ineffective when taken by mouth: they are transported via the hepatic portal system to the liver, where they are promptly metabolized. Minor modifications in the steroid structure, however, can render the hormone more resistant to liver inactivation and, hence, orally active. For example, the introduction of a double bond at position 1 of cortisol yields prednisolone, of a 17ot-ethinyl group in estradiol gives

709

ethinyl estradiol, and of a 17o~-ethinyl group and removal of the 19-carbon in testosterone yields the orally active progestin, norethindrone.

Eicosanoids The eicosanoids are discussed in Chapter 18.

30.4 Mechanism of Hormone Action A target cell is equipped with specific receptors that enable it to recognize and bind a hormone selectively and preferentially. Usually there is one type of receptor for a hormone, but in some cases more than one type of receptor may exist. For example, ol 1-, orz-,/71-, and/Jz-adrenergic receptors are available for catecholamines. The number of receptors for a given hormone in a given cell varies from about 10,000 to more than 100,000. Hormone recognition (and binding) by a receptor initiates a chain of intracellular events that ultimately lead to the effect of the hormone. Binding can be described as a lock-and-key interaction, with the hormone serving as the key and the receptor as the lock. The structural attributes of a hormone allow it to bind to its receptor and to unlock the expression of receptor function. The receptor, which frequently is a hormone-dependent regulatory protein, is functionally coupled to key enzyme systems in the cell, such that hormone binding initiates a receptor-mediated activation of enzymatic reactions, or it is functionally coupled to a region on chromatin, such that hormone binding initiates expression of one or more structural genes. Stated in another way, a hormone-responsive cell is programmed to carry out certain functions when a sensor (receptor) receives the appropriate signal (hormone). In general, the number of receptors for a hormone determines how well the cell responds to that hormone. Several factors influence the number of receptors in a cell. 1. The genotype of the cell determines whether the cell is capable of receptor synthesis and, if so, how much and of what type. (Some endocrinopathies involve receptor deficiency or a defect in receptor function.) 2. The stage of cellular development. 3. Hormones themselves are important regulators of the number of receptors in a cell; this regulation may be homologous or heterologous. Homologous regulation occurs when a hormone affects the number of its own receptors. "Downregulation" (decrease in receptor number) is seen when a target cell is exposed to chronically elevated levels of a hormone. Downregulation involves a decrease in receptor synthesis and

710

is a means by which a cell protects itself against excessive hormonal stimulation. Insulin, the catecholamines, GnRH, endogenous opiates, and epidermal growth factor downregulate their receptors. "Upregulation" (increase in receptor number) also occurs. Prolactin, for example, upregulates its receptors in the mammary gland. Heterologous regulation is more widespread and is a mechanism by which certain hormones influence the actions of other hormones. Some hormones diminish the production of receptors for another hormone thereby exerting an antagonistic effect. Growth hormone, for example, causes reduction in the number of insulin receptors. More prevalent, however, is augmentation of receptor number by a heterologous hormone. Estrogen, for example, increases the number of receptors for the progesterone, oxytocin, and LH, while thyroid hormone increases the number of /~-adrenergic receptors in some tissues. Hormone receptors can be classified into three types on the basis of their locations in the cell and the types of hormone they bind: 1. Nuclear receptors, which bind triiodothyronine (T3) after it enters the cell; 2. Cytosolic receptors, which bind steroid hormones as they diffuse into the cell; and 3. Cell surface receptors, which detect water-soluble hormones that do not enter the cell (peptides, proteins, glycoproteins, catecholamines). The mechanism of action of each of these receptor types is different because each is associated with different postreceptor events in the cell.

30.5 Types of Hormone Receptors Nuclear Receptors Receptors for thyroid hormone (TR), 1,25-dihydroxyvitamin D (VDR), and retinoic acid (RAR) are called nuclear receptors because they are located in the nucleus already bound to DNA (nuclear chromatin) even in the absence of their respective hormone or ligand. These nuclear receptors closely resemble the steroid hormone receptors and belong to the same "superfamily" of DNAbinding proteins. The similarities among the members of this superfamily are striking, particularly in their DNA recognition domains and in the corresponding receptor recognition segment of DNA. The receptor molecule consists of three domains: a carboxy terminal that binds the ligand (hormone/vitamin), a central DNA binding domain (DBD), and an amino terminal that may function as a gene enhancer. The ligand binding domain has high specificity for the ligand, and is the trigger that initiates

CHAPTER 30


receptor-mediated regulation of gene expression by the ligand. The receptor is bound to DNA by way of two "zinc fingers" (Chapter 26) in the DBD, which associates with a specific nucleotide sequence in DNA called the "response element" for the ligand. The response element is usually located "upstream" (at the 5' end) of a promoter for the gene that is regulated by the ligand, such that binding of the ligand to the receptor activates the response element (via the DBD), which then activates ("transactivates") transcription of the gene. Transactivation of gene transcription involves the binding of RNA polymerase II to the promoter region of the gene and construction of an RNA transcript (hnRNA) from the DNA gene sequence (Figure 30-3). The hnRNA is then spliced to yield mature mRNA (messenger RNA), which translocates to the cytoplasmic compartment and becomes associated with ribosomes. Ribosomal translation of the mRNA results in the synthesis of a nascent polypeptide, the primary sequence of which is encoded in the gene that was activated by the ligand.

Thyroid Hormone Receptors Thyroid hormone receptors (TRs) are nonhistone proteins that function as transducers of the effects of thyroid hormone on gene expression. There are four isoforms of TR (TRot 1, TRot2, TR/31, TR/32) that are products of two different genes, c-erbAot and c-erbAfl. There is some tissue specificity in the distribution of these isoforms. Three of these isoforms (TRot 1, TRot2, TRill) are found in almost all cells (although in different relative amounts); however, the TR/32 isoform is found only in the brain. The TRot2 is unique because it does not bind thyroid hormone but it does bind to DNA. Its function is unclear. The other three TR isoforms are single proteins with three regions (domains): a carboxy terminal region that binds T3, a central region that binds a specific region of DNA, and an amino terminal region that may function as a gene enhancer. The TR is synthesized on ribosomes and is actively transported through the nuclear membrane pores into the nucleus, where the DBD of the TR binds to a specific segment of DNA called the "thyroid hormone response element" (TRE). The TR binds to a TRE half-site as a monomer, a homodimer (ot/ot, Ot/fl,/3//3, etc.) or a heterodimer, in which a TR isoform dimerizes with another protein, e.g., the retinoid X receptor (RXR). In the absence of thyroid hormone, the TR association with the TRE does not result in any changes in gene expression. Thus, TRs exist in the nucleus, are bound to DNA, and await the arrival of thyroid hormone. Thyroid hormones T4 and T3 arrive at their target cells by transport in plasma, for the most part bound to plasma proteins (Chapter 33). The very small fraction of T4 and

SECTION 30.5

Types of Hormone Receptors

711

Ligand

S

RNA Transcription

DNA

\

v Hormone Response Element

/

Promoter Region

F I G U R E 30-3 Thyroid hormone, 1,25-dihydroxyvitamin D, and retinoic acid receptor regulation of transcription. The hormone receptor (HR) is dimerized at site (3) and is bound to DNA at hormone response element site (2). Without the ligand, transcription is inactive due to the interaction of HR with corepressor at site 4. When the ligand (hormone) binds to HR, the bound corepressor dissociates leading to an interaction between the coactivator and HR. These regulatory changes result in increased transcription.

T3 that is unbound at any given instant is accessible for cellular "uptake," i.e., the diffusional passage of the hydrophobic hormone through the lipid matrix of the cell membrane and into the cell. Once in the cell, the hormone associates loosely with cytoplasmic proteins that prevent its escape back into plasma. Microsomal 5'-deiodinase, which is present in thyroid target cells, catalyzes the removal of the 5'-iodine from T4, converting it to T3. This creates a concentration gradient that favors the movement of hormone into the nuclear compartment, in which TR (monomer or dimers) exists in association with the TRE sequence of DNA. Monomeric TR appears to bind to a TRE half-site, while dimeric (homodimer or heterodimer) TR binds to two half-sites. When activated by T3 binding to TR, the TRE promotes transcription of the gene into hnRNA. After removal of the introns from hnRNA, the resultant mRNA is translated into a protein at the ribosomes. The protein, which may be regulatory or structural, is the final molecular expression of thyroid hormone action. The outcome of T3 binding to TR is not a stimulation of transcription in all cases. In the pituitary thyrotrophs, T3 inhibits transcription of both the TSH-ot and TSH-/3 genes by binding to TR-TRE complexes. The TR mediating this inhibitory effect appears to be a monomer that binds to a TRE half-site, in contrast to the usual dimeric TR that binds to two TRE half-sites and mediates the stimulatory effects of T3 on transcription in other cell types. In some instances, the actual cell response to thyroid hormone is a manifestation of the activity of the induced

protein; therefore, the induced protein would first need to be activated (e.g., by phosphorylation) before the thyroid hormone effect on the cell can be seen. Steroid H o r m o n e Receptors

Steroid hormone receptors belong to a large family of DNA-binding proteins that include receptors for thyroid hormone, retinoic acid, and 1,25(OH)2 vitamin D. There are specific receptors for glucocorticoids (GRs), mineralocorticoids (MRs), estrogens (ERs), androgens (ARs), and progestogens (PRs), all of which are coded for by different genes. Unlike the TR, there appears to be only one functional receptor protein for a given steroid that is encoded on a single gene. Like the TR, the steroid receptor is a single protein with three regions (domains): a carboxy terminal region that binds the steroid specifically, a central DBD, and an amino terminal region that may function as a gene enhancer. The DBD of the steroid receptor projects two zinc fingers that allows both recognition of and binding to the hormone response element (HRE) of DNA. A generic depiction of the mechanism for steroid hormone activity at a target cell is shown in Figure 30-4. The first step consists of dissociation of the hormone from the plasma transport protein and entry into the cell by diffusion across the plasma membrane. In the second step the hormone binds with the receptors in the cytoplasm and nucleus. Receptors for glucocorticoid and aldosterone are found in the cytoplasm and receptors for estrogen and progesterone are found in the nucleus. Recall that receptors for

712

CHAPTER30

Plasma transport ~ = protein with bound steroid hormone Heat shock protein

STEP 1 ,~ . ~ =

-.

Free steroid hormone

~ ,, ' ~ "

~ J

'1- ~

J'~

..

7

~STEP3

+ ,....,fSTEP 4

+

Target cell Nucleus

STEP2 ~ " " O

Non-DNA / binding cytoplasmic receptor


.........

+!~-

~'i" 4 ~ ~ ++.+:'"Activa~on ~!'.i \t;~'; / ::..'-:.~

~~~receptor

~

!1

-:7+

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v t '~!I)~

r

~l ~ ' STEP 6

_

++

+++++l] .":i!!

on"++

~,._J . Trenslocetion ~...';i..... ~ .:~'~ "Activated" ?fece~p?gl%Smipiex~':~:.:.+:.i . I ~ ~ + /

/ \ response /

~ STEP 7 i

mRNA

";~"

mRNA

Protein synthesis ,

.

.

~

j

,

FIGURE 30-4 A genericdepictionof the mechanismof steroidhormoneaction(seetextfor details). [Reprintedwithpermissionfrom A. W.Normanand G. Liwack,Hormones, 2nded. San Diego;AcademicPress, 1997,p. 40.] thyroid hormone, retinoic acid, and 1,25(OH)2 vitamin D are all found in the nucleus. Activation of the receptor occurs in steps 3 and 4. The receptors exist in cells complexed with a dimer of heatshock protein (HSP, 90-kDa), which conserves the functional status of the receptor molecule in the absence of the hormone. In addition, HSPs may function to prevent the unoccupied receptor from binding to DNA by masking the DNA binding domain. HSPs bound to the hormone binding domain of the receptor are displaced by the hormone for which the receptor is specific. Once formed, steroid-receptor complexes aggregate into homodimers, and in this form the receptor is said to be "activated." The activated cytoplasmic receptors are translocated via a nucleopore to the nucleus (step 5). In step 6, the activated receptor triggers (or inhibits) transcription of the hormone-regulated gene. In the event of mRNA production, the newly synthesized mRNA is translocated to the cytoplasm where it is translated (step 7). The biological effect of the hormone reflects the function of the protein (e.g., enzyme, transporter, hormone, receptor), and the duration of the effect is a function of the half-life of the mRNA (minutes to hours) and of the protein (hours to days).

The steroid receptor homodimer may also regulate gene expression by interacting with transcription factors that are associated with either the HRE or the promoter. By binding to certain transcription factors, the activated hormone receptor may 1. Stimulate gene expression by removing an inhibitory transcription factor; 2. Stimulate gene expression by facilitating the activation of the promoter by a transciption factor; or 3. Inhibit gene expression by removing a stimulatory transcription factor. One important transcription factor with which steroid receptors interact is AP-1 (accessory/auxiliary/adapter protein-l), which promotes the expression of genes involved in cell proliferation. The growth-suppressing effect of glucocorticoids is probably mediated by the binding of the hormone-GR complex to one component of AP-1 (c-jun) which, in effect, removes the AP-1-mediated promotion of cell proliferation. Another transcriptional factor involved in steroid hormone action is the cAMP response element binding protein (CREB), which mediates the genomic effects of peptide hormones that utilize the cAMP second messenger system (see below). A

SECTION 30.5


number of steroid-regulated genes are subject to modulation by CREB and are thus subject to the regulatory influence of both steroid and peptide hormones.

Cell Surface Receptors Many hormones, such as the hormonal amines and all peptidic hormones, are unable to penetrate the lipid matrix of the cell membrane, and thus depend on the presence of receptor sites at the surface of target cells. As listed in Table 30-4, there are several types of cell membrane receptors for these hormones, each of which is coupled to a distinct set ofintracellular postreceptor pathways. The surface receptors all initiate postreceptor events that involve the phosphorylation of one or more intracellular proteins, some of which are enzymes whose activities depend on the state of phosphorylation. In two of these cases, an intracellular second messenger is utilized to implement the hormonal action and involves G-protein-coupled receptors. One is coupled to the adenylate cyclase-cAMP system and the other is associated with the phosphatidylinositol-Ca 2+ pathway (IP3 pathway).

G-Protein-Coupled Adenylate Cyclase-cAMP System The effect on adenylate cyclase of ligand binding to specific receptors mediated by heterotrimeric G-proteins is shown in Figure 30-5. The G-proteins belong to a family of regulatory proteins each of which regulates a distinct set of signaling pathways. The cell membrane receptors coupled to intracellular G-proteins share a common seven transmembrane-spanning o~-helical serpentine structure; however, they do not show a common overall sequence homology. The oe-helical segments are linked with alternating intracellular and extracellular peptide loops with the N-terminal region located on the extracellular side and C-terminal region located on the intracellular side (Figure 30-6). Receptors coupled to G-proteins have been divided into three subfamilies and significant sequence homologies do exist within the families, the receptors belonging to the three families are described in Figure 30-6. Extracellular signals detected by the membrane receptors are diverse and include hormones, growth factors, neurotransmitters, odorants, and light. Examples of G-proteins are Gs, Gi, products of ras oncogenes (Chapter 26), and transducin. Transducin is a constituent of the light-activated cGMP-phosphodiesterase system in the retina (Chapter 38). This system is similar to the adenylate cyclase system except that light is the ligand, rhodopsin is the membrane receptor, cGMP-phosphodiesterase is the effector, and the G-protein is the transducer. The two types

713

of receptors, Rs and Ri, can be functionally linked to adenylate cyclase (Figure 30-5). Binding of an appropriate ligand to Rs stimulates the adenylate cyclase whereas binding to Ri inhibits the enzyme. The G-proteins are heterotrimeric consisting of an o~ subunit and a tightly coupled fl ~, subunit. The ot subunit is the unique protein in the trimer. It possesses sites for interaction with cell membrane receptors, fl V subunit, intrinsic GTPase activity, and ADP ribosylation. In the unstimulated state, the G-protein is present in the heterotrimeric (otfi V) form with GDP tightly bound to the ot subunit. Upon activation of the receptor by the bound ligand, GDP is exchanged for GTP on the ot subunit followed by dissociation of Gs~Tp from the f i t subunit. The el subunits Gs and Gi are designated G~ and Gia. The G-protein subunits undergo posttranslational covalent additions of lipids such as palmitoyl, farnesyl, and geranyl groups. These hydrophobic groups linked to G-protein subunits are necessary for proper anchoring in the cell membrane. Figure 30-7 shows the cyclic functioning of the G~ protein. Binding of a hormone to R~ permits binding of GTP to Gs, to form a Gsc~.GTPcomplex and release of the G~ and G• subunits. The Gso~.GTPcomplex activates adenylate cyclase which remains active as long as GTP is bound to Gs~. Binding of GTP to Gs~ also activates a GTPase intrinsic to Gs~ that slowly hydrolyzes the bound GTP (GTP turnover is one per minute), eventually allowing reassociation and formation of the trimer and inactivation of adenylate cyclase. A similar series of events is initiated by binding of a ligand to Ri. In this instance, dissociation of the Gi trimer by formation of Gi~.GTP inhibits adenylate cyclase and may allow the/3,g subunit to bind to Gs~, preventing Gs~ from activating adenylate cyclase. GTP hydrolysis by GTP-activated Gi~-GTPase allows re-formation of the Gi trimer. GDP is displaced from Gi~ by binding of another molecule of GTP with reactivation of Gic~. The inhibitory effect occurs because the target cell contains much more Gi than G~. Gic~ may also interact with membrane calcium transport and the phosphatidylinositol pathway. The function of/3 g dimers is not completely understood but they may participate in the regulation of downstream effectors in the signaling pathway. The G-protein-coupled receptor signaling pathway is regulated by several different mechanisms. Negative regulation of the signaling pathway can occur at both the receptor and G-protein levels. Receptor phosphorylation by G-protein-coupled receptor kinase and arrestin binding can lead to inability of the ligand to activate the signaling system (e.g.,/3-adrenergic receptor-G-protein transducing system). Negative regulation also occurs after the activation of G-protein, by promoting the GTPase activity of G~. This is achieved by regulator of G-protein signaling

TABLE 30-4

Membrane-Bound Receptor Hormone Transduction Systems

Hormones acting via utilization of a cAMP system -

cAMP decrease

CAMPincrease

Hormones acting via a cGMP system

Hormones acting via IP, pathway

Hormones acting via ion channels

Atrial natriuretic hormone

ACTH Angiotensin IHII Cholecystokinin EGF Epinephrine GnRH Histamine (HI) NGF Norepinephrine ( a , adrenergic) PGF Thromboxanes TRH TSH Vasopressin VIP

Acetylcholine Nicotinic receptor (Na+ channel) Nitric oxide (NO) (ca2+ channel)

-

Bradykinin A Norepinephrine(a2-adrenergic) Somatostatin

ATH a-MSH Calcitonin FSH Gastrin Glucagon GnRH Histamine (Hz) LH Norepinephrine ( P, adrene~ic) PGE, PTH Secretin TSH Vasopressin VIP

Reprinted with permission from A. W. Norman and G. Litwack, Hormones 2nd ed. Academic Press 1997, page 32.

SECTION 30.5

Typesof Hormone Receptors

715

Stimulatory Receptor (Rs) ~ _ _ . ~ Hormones

Cell Membrane

~~~j'

Cytoplasm

Gs_Protein

Hormone/

~

Inhibitory Receptor (Ri) ~

C"-

Gi_Protein ~

/Aden,lal GTP

ADP-ribosylation by cholera toxin maintains Gso~in active state

cAMP + PPi

ADP-ribosylation by pertussis toxin results in inactivation of Gi~

Protein Kinase A FIGURE 30-5 Dual control of adenylate cyclase activity by guanine nucleotide-binding proteins (G). Subscripts s and i denote stimulatory and inhibitory species, respectively. + and - indicate activation and inactivation, respectively.

(RGS) proteins, which bind to G~ and promote GTPase activity. This process of converting G~ from an active state to an inactive state is rapid. Transcriptional regulation and posttranslational modification of G-proteins provide other means for regulation of the G-protein-coupled signaling pathway. Understanding of G-proteins has been aided by specific agents that modify Gs or Gi and by mutant mouse lymphoma cell lines deficient in Gs activity. Both Gs~ and Gio~ contain sites for NAD+-dependent ADP ribosylation. Cholera toxin ADP-ribosylates a specific arginine side chain of Gs~ and maintains it in a permanently active state, while islet-activating protein, one of the toxins in Bordetella pertussis, ADP-ribosylates a specific cysteine side chain of Giot, permanently blocking its inhibitory action because it maintains Gioe in the GDP form. Thus, both toxins stimulate adenylate cyclase activity and lead to excessive production of cAME Cholera toxin causes a diarrheal illness (Chapter 12), while toxins of B. pertussis cause whooping cough, a respiratory disease affecting ciliated bronchial epithelium. Response of the adenylate cyclase system to a hormone is determined by the types and amounts of various constituent proteins. Cyclic AMP production is limited by the amount of adenylate cyclase present. When all the adenylate cyclase is fully stimulated, further hormone binding to Rs's cannot increase the rate of cAMP synthesis. In cells having many different Rs's (adipocytes have them for epinephrine, ACTH, TSH, glucagon, MSH, and vasopressin), maximal occupancy of the receptors may not

stimulate cAMP production beyond what can be achieved by full occupancy of only a few of the receptor types. Therefore, the greatest stimulation that can be achieved by a combination of several hormones will not be simply the sum of the maximal effects of the same hormones given singly. A hormone's ability to stimulate cAMP production may depend on the cell type. For example, epinephrine causes large increases in cAMP concentration in muscle but has relatively little effect on liver. The opposite is true for glucagon (see Chapter 15). Within a particular cell type, destruction of one type of Rs does not alter the response of the cell to hormones that bind other stimulatory receptors. In prokaryotic cells, cAMP binds to catabolite regulatory protein (CAP), which then binds to DNA and affects gene expression (Chapter 26). In eukaryotic cells, cAMP binds to cAMP-dependent protein kinase, which contains two regulatory (R) and two catalytic (C) subunits. Upon binding of cAME the catalytic subunits separate, become active, R2C2 -4- 4cAMP ~ 2(R-2cAMP) + 2C (inactive) (active)

and catalyze ATP-dependent phosphorylation of serine and threonine residues of various cell proteins, often altering the activities of these proteins. Some, such as phosphorylase kinase, become activated, whereas others, such as glycogen synthase, are inactivated (Chapter 15). Cyclic AMP-dependent protein kinase remains active while intracellular cAMP concentration, controlled by the

Family A. Rhodopsin/132 adrenergic receptor-like Biogenic amine receptors (adrenergic, serotonin, dopamine, muscarinic, histamine) CCK, endothelin, tachykinin, neuropeptide Y, TRH, neurotensin, bombesin, and growth hormone secretagogues receptors plus vertebrate opsins Invertebrate opsins and bradytdninreceptors Adenosine, cannabinoid, melanocortin, and olfactory receptors. Chemokine, fMLP, CSA, GnRH, eicosanoid, leukotriene, FSH, LH, TSH, fMLP, galanin, nucleotide, opioid, oxytocin, vasopressin, somatostatin, and protease-activated receptors plus others.

Melatonin receptors and other non-classified

Family B. Glucagon/VIP/Calcitonin receptor-like Calcitonin, CGRP and CRF receptors PTH and PTHrP receptors Glucagon, glucagon-like peptide, GIP, GHRH, PACAP,VIP, and sscretin receptors

Latrotoxin

Family C. Metabotropic neurotransmitter/ Calcium receptors

Metabotropic glutamate receptors Metabotropic GABA receptors Calcium receptors Vomeronasal pheromone receptors Taste receptors

F I G U R E 30-6 G-protein-coupled seven-transmembrane receptors (GPCR). These can be divided into three major subfamilies (1). A snake diagram for a prototypic member of each subfamily is shown. Family A receptor (upper panel) can be subdivided phylogenetically into six subgroups as indicated. Family A receptors are characterized by a series of highly conserved key residues (black letter in white circles). In most family A receptors a disulfide bridge connects the second (ECL2) and third extracellular loops (ECL3) (white letters in black circles). In addition, a majority of the receptors have a palmitoylated cysteine in the carboxy terminal tail causing formation of a putative fourth intracellular loop. Family B receptors (middle panel) are characterized by a long amino terminus containing several cysteines that form a network of disulfide bridges. The B receptors contain, like the A receptors, a disulfide bridge connecting ECL2 and ECL3. However, the palmitoylation site is missing. Moreover, the conserved prolines are different from the conserved prolines in the A receptors and the DRY motif at the bottom of TM 3 is absent. Family C receptors (lower panel) are characterized by a very long amino terminus (~600 amino acids). The amino terminal domain contains the ligand binding site. Except for two cysteines that form a putative disulfide bridge, the C receptors do not have any of the key features characterizing A and B receptors. Some highly conserved residues are indicated (black letter in white circles). A unique characteristic of the C receptors is a very short and highly conserved third intracellular loop. [Reproduced with permission from U. Gether, Endocrine Reviews 21, 92 (2000).]

SECTION30.5 Types of Hormone Receptors

717

Activation of Adenylate Cyclase GSo~-GTP iActive)

/

G T P ~

Hormone

-'~G~,/ ~

fG-protein [ I GTP-GDP ~/rCOp e x c n a n[.---Jg uexeh~noe Dled

~

/

reac~on \

. .... / . . t.iH-'aseI I /ilRggu]iatgr(~tGt~)PprOtotien n

GSc~.GDP.13~/ (Inactive) FIGURE 30-7 Activation and inactivation (or return to resting state) of signal transducer trimeric stimulatory G-protein (Gs). Hormone binding to membrane G-protein-coupled stimulatory receptor causes a GTP and GDP exchange reaction in Gs,~ and dissociation of G~• Gs~.GTPactivates adenylate cyclase and initiates the signal transduction pathway. The return of Gsot.GTPto the resting state is mediated by the intrinsic GTPase activity of the Gs~ subunit and also through promotion of GTPase activity by the RGS protein. Activation of Gi protein occurs by analogous reactions of Gs when a hormone binds to its corresponding inhibitory receptor. This leads to the formation of Gio~.GYPwhich inhibits adenylate cyclase. Gain in function or loss in function can result from mutations in genes coding for receptor protein, Gs~, Gia, or RGS protein.

relative rates of cAMP synthesis by adenylate cyclase and of degradation by phosphodiesterase, is elevated above basal levels. The reaction catalyzed by the phosphodiesterase is 3',5'-cAMP + H20 ---->5'-AMP As cAMP concentration decreases, reassociation of catalytic and regulatory subunits of the kinase occurs. The cAMP system also derives some of its specificity from the proteins that are substrates for cAMP-dependent protein kinase. It is an indirect regulatory system, since cAMP modulates the activity of cAMP-dependent protein kinase and the kinase, in turn, affects the activities of a variety of metabolic enzymes or other proteins. This indirectness is the basis for amplification of the hormonal signal. Activation of adenylate cyclase by binding of a hormone molecule to a receptor causes formation of many molecules of cAMP and allows activation of many molecules of the protein kinase, each of which in turn can phosphorylate many enzymes or other proteins. This "amplification cascade" accounts in part for the extreme sensitivity of metabolic responses to small changes in hormone concentrations. Finally, the response to increased cAMP concentrations within a cell usually involves several metabolic pathways and a variety of enzymes or other proteins.

Abnormalities in Initiation of G-Protein Signal Two-well studied bacterial toxins, cholera toxin and pertussis toxin, perturb normal functioning of G~ proteins by ADP ribosylation of specific amino acid residues (discussed earlier). Germline and somatic mutations in the genes coding for G-proteins can cause endocrine disorders in several ways. Mutations can be either activating (gain of function) or inactivating (loss of function). Examples of gain-in-function mutations that result in the ligand-independent activator of Gs~ include some adenomas of pituitary and thyroid, some adenomas of adrenals and ovary, and syndromes of hyperfunction of one or more endocrine glands. In most of these disorders, a point mutation alters an amino acid in Gs~ leading to inhibition of GTP hydrolysis required for the termination of the activity of Gs~. An example of autonomous hyperhormone secretion and cellular proliferation of several endocrine glands is McCune-Albright syndrome. In this disorder, two missense mutations result in the replacement of Arg 2~ ----> His or Arg 2~ --+ Cy in G~ of affected endocrine glands. Some of the clinical manifestations of McCune-Albright syndrome include hyperthyroidism, polyostotic fibrous dysplasia, and hyperpigmented irregularly shaped skin lesions (known as cafd-au-lait lesions). The cutaneous hyperpigmentation

~1~

is caused by hyperfunctioning of MSH receptors coupled to Gs~ in the melanocytes. An example of decreased function due to impaired activation or loss of Gs~ and resistance to hormone action is pseudohypoparathyroidism (Albright's hereditary osteodystrophy). Patients with this syndrome exhibit generalized resistance to the action of those hormones dependent on Gs~ for their function despite elevated serum hormone levels. Several dysmorphic features are also characteristic of these patients. A well-understood disorder of hypofunctioning Gs~ is pseudohypoparathyroidism type la. This disorder exhibits a dominant inheritance pattern with one normal and one abnormal Gs~ allele. In different families affected by pseudohypoparathyroidism Ia, two missense mutations have been identified in G~: Arg 385 --+ His and Arg 231 ~ His. The former mutation prevents receptormediated Gs stimulation and the latter mutation prevents receptor-mediated binding of GTP to Gs. A paradoxical finding consisting of both gain and loss of function due to identical mutations in Gs~ (Ala 366 ~ Ser), in two unrelated boys with pseudohypoparathyroidism Ia. Both of these patients, while exhibiting hormone resistance in some tissues, showed autonomous production of testosterone by testicular Leydig cells due to gain of function in the receptors of luteinizing hormone. This results in precocious puberty (testotoxicosis). This particular mutation in the G~ gene involves a gain in function property in G~ enhancing the rate of release of GDE However, the paradoxical effect is due to temperature stability. The mutant protein is unstable at 37~ which causes pseudohypoparathyroidism; however, the mutant protein is stable at lower temperature and thus is active in testes, which are about 3~176 cooler than the rest of the body. G-protein abnormalities also may arise from mutations in genes encoding proteins that regulate G-protein signaling. Several RGSs have been identified. RGS proteins bind to G~ and accelerate the hydrolysis of GTE The physiological responses of G-proteins regulated by RGSs are fast and include vagal slowing of the heart rate (Gi), retinal detection of photones (Gt), and contraction of vascular smooth muscle (GQ).

G-Protein-Coupled PhosphatidylinositolCa z+ Pathway The calcium system is more complex than the cAMP system and contains a variety of mechanisms for transducing the Ca 2+ signal into changes in cellular function. It is also sensitive and responds to relatively small, transient changes in free Ca 2+ concentration. This sensitivity is desirable from a regulatory point of view, but it also

CHAPTER30 Endocrine Metabolism I: Introduction

reflects the toxicity of even low concentrations of free Ca 2+ (Chapter 37). In a resting cell, the pool of cytosolic calcium is very small, and most of the intracellular Ca 2+ is bound in mitochondria or the endoplasmic reticulum or is bound to the plasma membrane. The cytosolic calcium ion concentration is -~0.1 /~mol/L, in contrast to the high concentration (~ 1000 #mol/L) outside cells. This 10,000-fold gradient is maintained by the plasma membrane, which is relatively impermeable to calcium; by an ATP-dependent Ca 2+ pump (which extrudes Ca 2+ in exchange for H +) in the plasma membrane; and by "pump-leak" systems in the endoplasmic reticulum and inner mitochondrial membranes. A change in the permeability of any of these membranes alters the calcium flux across the membrane and causes a change in cytosolic calcium concentration. This change is thought to act as the messenger in the calcium system. Plasma membranes regulate cytosolic calcium concentration by their role as a diffusion barrier and by containing the ATP-dependent Ca 2+ pump and receptors for extracellular messengers. Activation of some of these receptors increases the Ca 2+ permeability of the plasma membrane and thus the level of cytosolic calcium. In other cases, the activated receptor produces one or more second messengers that increase cytosolic Ca 2+ concentration from the calcium pool in the endoplasmic reticulum. Calcium then becomes a "third messenger." The endoplasmic reticulum is the source of Ca 2+ in the initial phase of cell activation by some hormones in many systems. Since the plasma membrane can bind Ca 2+, the rise in cytosolic calcium following binding of some extracellular messengers may be due to release from this intracellular calcium pool. The inner mitochondrial membrane may function primarily as a calcium sink, taking up excess calcium in the cytosol that results from hormonal activation of the cell. At cytosolic Ca 2+ concentrations greater than 0.6 #mol/L, the mitochondrial calcium pump is activated and stores calcium in the mitochondrial matrix as a nonionic, rapidly exchangeable, phosphate salt. At low cytosolic calcium concentrations, the inner mitochondrial membrane allows Ca 2+ to "leak" into the cytosol. The capacity of the active influx pathway (the pump) is much greater than that of the passive efflux route (the leak). The mitochondrial pumpleak system may serve to fine-tune the cytosolic calcium concentration while the plasma membrane is the principal safeguard against entry of toxic amounts of calcium into the cell.

Mechanism of the Calcium Messenger System Most extracellular messengers that cause a rise in cytosolic Ca 2+ concentration also increase the turnover

SECTION 30.5


719

Receptor . Hormones

~ Cell membrane

DAG

PIP2

Go-Protein

DAG

PS

II

.V.oop os,a ~"-GTP

p

p

Protein Kinase C /P IP3

~ p

p ~

Calmodulin (CoM) Protein Kinase ~

:~[/') }

.)

CaM/Ca2+ ~

,./.,//ReceptOrsmoothforIP3

/////

endoplasmic recticulum(SER)

FI G U R E 30-8 " 2+ pathway. The binding of a hormone at a specific receptor site results in GQ protein-coupled phosphatidylinosito 1-tJa the activation of G-protein which, in turn, activates phospholipase C via GQ~.OTp-protein. The action of phospholipase C on phosphatidylinositol 4,5-bisphosphate (PIP2) yields inositol trisphosphate (IP3) and diacylglycerol (DAG) which, along with phosphatidylserine (PS), activates protein kinase C. IP3 binds to receptors on SER, releasing Ca 2+ which, in turn, activates another set of protein kinases. +, Activation.

of phosphoinositides in the plasma membrane. The pathway is initiated by the binding of extracellular messengers to specific receptors located on the plasma membrane and activating G-proteins (e.g., GO). The activated GQ~.CTp-protein, in turn, activates phospholipase C, which catalyzes hydrolysis of phosphatidylinositide 4,5-bisphosphate, initiating the intracellular secondmessenger effects (Figure 30-8). The action of phospholipase C results in the formation of two products, inositol 1,4,5,-trisphosphate (IP3) and diacylglycerol (DAG), which function as intracellular messengers (Figure 30-9). Most of the DAG released in response to stimulation of calcium-mobilizing receptors contains arachidonic acid esterified to C2 of the glycerol. This arachidonate may

O"

I

O ~ p _ O"

oII

I

H2 - O - C - R1

R2-

O II C - O - CH

I CH 2 - O

i

O H

o

P- O - Cl

I O"

I"

H~?

H

HJ ( ~ / -

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C

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I

I

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\

~.?C

/

H

o

II

0"

.-P-

I

0"

O"

F I G U R E 30-9 Structure of phosphatidylinositol 4,5-bisphosphate. Hydrolysis at the dotted line is catalyzed by phospholipase C and releases diacylglycerol and inositol 1,4,5-trisphosphate, which are the intracellular second messengers.

be released from DAG by diacylglycerol lipase or from phosphatidic acid by phospholipase A2, following phosphorylation of DAG. It can then serve as a substrate for the synthesis of eicosanoids (prostaglandins, prostacyclins, thromboxanes, and leukotrienes; Chapter 18), which also modulate a number of physiological functions. These reactions are part of lipid and phosphoinositide cycles (Figure 30-10). Many extracellular messengers in a variety of cells activate this "leak" pathway. They include acetylcholine (many tissues); epinephrine, angiotensin, and vasopressin (liver); angiotensin (adrenal cortex); glucose (pancreatic islets); photons (photoreceptor cells); and several different substances in neutrophils, leukocytes, and platelets. Many of the membrane receptors linked to phosphoinositide metabolism also activate guanylate cyclase, but the mechanism and function of this activation remain unknown. The starting point of these reactions is phosphatidylinositol 4,5-bisphosphate (PIP2; Figure 30-10), which is synthesized from phosphatidylinositol (PI) in two steps catalyzed by specific kinases. The phosphates are removed by specific phosphomonoesterases setting up futile cycles. Stimulation of phospholipase C by binding of an agonist to its receptor hydrolyzes PIP2, releasing IP3 and DAG (usually 1-stearoyl-2-arachidonyl-sn-glycerol). IP3 stimulates the release of Ca 2+, predominantly from the endoplasmic reticulum, and may increase plasma membrane permeability to Ca 2+ in some cells. In most cells studied, the rise in level of cytosolic Ca 2+ is preceded by an increase in that

720


99 L,p,d c y c l e f . . . . ~

CDP-DG ~

P~ CTP ~-(~)~~.

Futile cycles i

(

\

myo-lno

(~ADP

ATP ATP L ADP L 6 DP Agonist

P'

-,o [P~

itol

..-.

I\ATP I.~---'~--~ MG

H,O

,

P~

H=O

PMG t (C~ I~.Arachidonic acid

]X,R

~

P~

H=O

Inositolphosphate cycle

F I G U R E 30-10 Inositol metabolism. An inositol phosphate cycle converts inositol 1,4,5-trisphosphate (IP3) to free inositol, which can then combine with CDP-diacylglycerol (CDP-DAG) to re-form phosphatidylinositol (PI). Inositol 1,4-bisphosphate (IP2) and inositol l-phosphate (IP) are intermediates in this cycle. The CDP-DAG produced in the lipid cycle returns diacylglycerol for resynthesis of PI. It is uncertain whether arachidonic acid is released by hydrolysis of diacylglycerol, with release of a monoglyceride (MG), or of phosphatidic acid (PA), with release of a phosphomonoglyceride (PMG). The futile cycling of PI, phosphatidylinositol 4-phosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2) maintains a supply of PIP2 to generate diacylglycerol and IP3 in response to agonist binding. The enzymes involved are l, phosphatidylinositol kinase; 2, PIP kinase; 3, PIP2 phosphomonoesterase; 4, PIP phosphomonoesterase; 5, PIP2 phosphodiesterase (phospholipase C); 6, IP3 phosphatase; 7, IP2 phosphatase; 8, IP phosphatase; 9, CDP-diacylglycerol inositol phosphatidate transferase (phosphatidylinositol synthase); 10, diacylglycerol kinase; l I, CTP-phosphatidate cytidyl transferase; 12, diacylglycerol lipase; and 13, phospholipase A2. [Modified and redrawn with permission from M. J. Berridge and R. E Irvine, lnositol triphosphate, a novel second messenger in cellular signal transduction. Nature 312, 315 (1984). @ 1984 by Macmillan Magazines Ltd.]

of IP3, making IP3 the second messenger and calcium the third messenger. In the endoplasmic reticulum, IP3 seems to increase efflux of Ca 2+. Amplification occurs since an estimated 10-20 calcium ions are released from liver endoplasmic reticulum by each IP3 molecule. IP3 is rapidly hydrolyzed to free inositol, which is used for resynthesis of PI (Chapter 19). Removal of the 1-phosphate is blocked by lithium, with accumulation of inositol 1-phosphate and depletion of m y o - i n o s i t o l pools. The effect is most marked in the brain, where lithium also inhibits de n o v o synthesisof m y o - i n o s i t o l and plasma inositol is unable to cross the blood-brain barrier. Lithium carbonate, used to treat bipolar affective disorders, may have its therapeutic effect by inhibition of the phosphatidylinositol system. In several types of cells, stimulation of plasma membrane receptors coupled to phosphoinositide turnover not only triggers a rapid mobilization of intracellular Ca 2+ as discussed above, but also a prolonged mobilization.

The latter is dependent extracellular Ca 2+, and another metabolite of inositol, namely, inositol tetrakisphosphate (IP4),or Ins(1,3,4,5)P4, has been implicated in the transport of extracellular Ca 2+. The molecular mechanism of this process is nuclear. IP4, unlike IP3, cannot mobilize Ca 2+ from endoplasmic reticulum. IP4 metabolism consists of its synthesis from IP3 by a novel, specific ATPdependent kinase and its conversion to inactive metabolite Ins(1,3,4)P3 by a phosphatase. The regulation of kinase and phosphatase activity may be of importance in controlling the intracellular events. Diacylglycerol released by phospholipase C may increase protein phosphorylation. Protein kinase C (C-kinase) is a monomeric enzyme (M.W. 82,000) found free in the cytosol in most tissues except brain, kidney, and liver, where it is predominantly membrane-bound. Diacylglycerol increases both the binding of protein kinase C to the inner surface of the plasma membrane and the sensitivity of the kinase to activation by Ca 2+ and phosphatidylserine. The effects of DAG related to protein phosphorylation are thought to be mediated by this kinase. Tumor-promoting phorbol esters such as 12-O-tetradecanoylphorbol- 13-acetate (ingredient in croton oil that promotes skin tumor production by carcinogens) can directly activate protein kinase C, mimicking DAG and bypassing the receptors. When activated by phorbol esters, protein kinase C phosphorylates cytosolic myosin light-chain kinase (Chapter 21) and other proteins, particularly those related to secretion and proliferation. When activated by DAG, the kinase phosphorylates a number of proteins in vivo, but its physiological substrates are unknown. DAG is removed by phosphorylation to phosphatidic acid or by hydrolysis to monoacylglycerol and free fatty acid. In addition to the effects of DAG and phorbol esters on protein kinase C, some evidence links malignant transformation by some oncogene products to phosphoinositide metabolism. Platelet-derived growth factor, the product of the sis oncogene, stimulates inositol lipid metabolism. Products of the src and ras oncogenes may be phosphoinositide kinases, which increase the concentration of PIP3 and provide more substrate for phospholipase C. The product of the ras gene is a GTP-binding protein (G-protein) involved in receptor activation of PIP2 hydrolysis. A point mutation activates the ras oncogene, and the resulting oncogene product binds GTP but lacks GTPase activity. This abnormal protein could activate PIP2 hydrolysis independently of receptor occupancy. Following these early events initiated by binding of a calcium-mediated hormone to its receptor, a transient rise in cytosolic calcium concentration and a more prolonged increase in DAG concentration occur. There is also a prolonged four- to fivefold increase in Ca 2+ flux across the

SECTION 30.5

Typesof Hormone Receptors

plasma membrane, which may maintain an elevated Ca 2+ concentration at the cytoplasmic face of the membrane. This increase, together with the diacylglycerol, may continue to activate the membrane-bound protein kinase C. In this way, myosin light chains may remain phosphorylated and active during smooth muscle contraction (Chapter 21) despite a fall in myoplasmic Ca 2+ to near-basal levels. The rise in intracellular Ca 2+ level also alters the activity of metabolic pathways by calcium-binding proteins. Of these proteins, calmodulin (M.W. 17,000), found in the cytoplasm of every eukaryotic cell, has received the most attention. Parvalbumin in skeletal muscle, troponin C in skeletal and cardiac muscle, and myosin light chain in smooth muscle serve similar functions. The affinity of these proteins for Ca 2+ is high, as reflected by dissociation constants of 10-6-10 -8 M. Occupation of at least three of four equivalent Ca 2+ binding sites on calmodulin induces a conformational change and, in most cases, increases its affinity for proteins having a calmodulin binding site. A major function of CaZ+-calmodulin is regulation of the phosphorylation states of intracellular proteins. Systems affected include those involved in secretion (of insulin, for example), neurotransmitter release and neuronal function, cellular contractile processes (Chapter 21), and several protein kinases involved in regulation of glycogen and glucose metabolism. A phosphatase activated by Ca 2+ and calmodulin, known as calcineurin, functions as a negative modulator of some of the effects of Ca 2+ related to protein phosphorylation. Calcineurin is a CaZ+-calmodulin-dependent serine/threonine protein phosphotase and it participates in many CaZ+-dependent signal transduction pathways. The immunosuppressant drugs cyclosporin A and FK506, after binding to their respective cytoplasmic binding proteins (cyclophilin and FK506-binding protien), inhibit the action of calcineurin. This blocks the translocation of a factor into the nucleus of activated T lymphocytes and prevents allograft rejection by blocking T cell cytokine gene transcription (Chapter 35).

Receptors for Insulin and Growth Factors Insulin exerts its effects by altering the state of phosphorylation of certain intracellular enzymes by a mechanism that does not involve cAMP but that requires specific binding to surface receptors with tyrosine kinase activity. Insulin exerts acute (minutes), delayed-onset (hours), and longterm (days) effects entirely by way of a single receptor. The insulin receptor is a transmembrane heterotetramer consisting of one pair each of two dissimilar subunits linked by disulfide bridges (Chapter 22). Both subunits

721

are expressed entirely by a single gene, which is located on chromosome 19. The two subunits, ot and/~, are formed posttranslationally from a single proreceptor following glycosylation and proteolytic cleavage by an endopeptidase. The o~ subunit, which projects into the extracellular space, is a glycosylated 135-KDa protein that specifies the insulin binding site (one binding site per o~ subunit). The transmembrane/~ subunit is a glycosylated 95-KDa subunit that contains four domains, one of which has tyrosine kinase activity. The tyrosine kinase activity of the subunit is suppressed by the o~ subunit in the absence of insulin; insulin binding to the ot subunit removes the inhibition and allows the/~ subunit tyrosine kinase to phosphorylate itself (autophosphorylation of the fl subunit). Autophosphorylation is followed by phosphorylation of one or more key intracellular proteins, one of which is insulin receptor substrate-1 (IRS-1) that initiates a cascade of intracellular phosphorylations mediated by a number of intracellular enzymes that are incompletely characterized, but which are critical in bringing about all of the known cellular effects of insulin (Chapter 22). Autophosphorylation of the insulin receptor may also be required for inactivation of the insulin signal ("OFF" signal). Within minutes after autophosphorylation, the insulin-activated receptor complex undergoes lateral movement toward clathrin-coated pits, the site at which a number of activated insulin receptors congregate or "cluster." This clustering stimulates endocytosis of the congregation, a process referred to as "internalization" or "endocytosis," which is followed by fusion of the internalized insulin receptor membrane vesicle (endosome) with a lysosome, i.e., an endolysosome formation. This allows breakdown of both insulin and its receptor by lysosomal proteases. It has been suggested that insulin fragments resulting from this digestion process are mediators of some of the actions of insulin within the cell, but further evidence in support of this is needed. In states of chronically elevated blood levels of insulin, there is a substantive decrease in the density of insulin receptors in insulin-dependent cells due to a decrease in the synthesis of insulin receptors. This phenomenon, which is referred to as "downregulation," represents a means by which a cell autoregulates its receptivity to the hormone, and is also detected by other hormones such as the catecholamines, endorphins, and GnRH. The precise mechanisms involved in downregulation are not known. The insulin receptor is expressed in most cells and tissues examined to date, including the non-insulindependent tissues such as brain, kidney, and blood cells (both red and white). Although no specific cellular response to insulin has been described in these tissues, it is conceivable that insulin regulates one or more intracellular

722

CHAPTER 30


F I G U R E 30-11 Mechanism of prolactin receptor activation. Activation of prolactin receptor consists of ligand-induced sequential receptor homodimerization driven by the two binding sites of prolactin. In the intracellular domain of the homodimer of the ligand-receptor complex, a tyrosine kinase [known as Janus kinase 2 (Jak-2)] is activated. Jak-2 kinase causes autophosphorylation and phosphorylation of the receptor. [Reproduced with permission from M. E. Freeman, B. Kanyicska, A. Lerant, and G. Nagy, Physiological Reviews 80, 1530 (2000).]

activities that are unrelated to glucose uptake and utilization. The IGF-I receptor is a n otzfl 2 heterotetramer that is structurally similar to the insulin receptor but is coded for by a different, single-copy gene located within bands q25-26 of chromosome 15. Like the insulin receptor, with which it shares 50-60% sequence homology, the IGF-I receptor has a cysteine-rich domain in the c~ subunit for ligand binding and a tyrosine kinase domain in the /3 subunit that catalyzes transphosphorylation of contralateral fl-subunit sites. The receptor has highest affinity for IGF-I, a lesser affinity for IGF-II (about 2- to 15-fold less), and low affinity for insulin (about ~ 100- to 1000-fold less) due to the absence of insulin-binding determinants in the ot subunit.

Receptors for Growth Hormone and Prolactin Growth hormone (GH) and prolactin (PRL) belong to the "helix bundle peptide" (HBP) hormone family, which includes human placental lactogen (hPL), erythropoietin (EPO), and many interleukins and cytokines. All members of this family have similar mechanisms of action; they utilize membrane receptors that are straight-chain glycoproteins with extracellular, transmembrane and intracellular domains. The membrane receptors for this group of hormones when bound with their respective receptors undergo dimerization. Each dimerized receptor is bound to one molecule of hormone. The intracellular events of the ligand-mediated activation of the receptor consists of activation of a constitutively associated tyrosine kinase known

as Janus kinase 2 (Jak-2). Jak-2 kinase transphosphorylates itself and phosphorylates the intracellular domains of the receptor. Figure 30-11 illustrates the mechanism of prolactin receptor activation. The signal transduction pathways for GH and PRL are discussed in Chapter 31.

30.6 Organization of the Endocrine System The nervous and endocrine systems function in a coordinated manner to promote growth, homeostasis, and reproductive competence. The need to rapidly adjust physiological processes in response to impending disturbances in homeostasis is met by the nervous system, while the endocrine system effects the more prolonged, fine-tuned adjustments. Together, they effect appropriate organismic adaptation. Some interdependence also exists between the two systems. The nervous system, for example, relies heavily on a continuous supply of glucose, the circulating levels of which are under multiple hormonal control, while maturation of the nervous system is regulated by thyroid hormone, and maintenance of mental acuity in adults depends on the availability of both thyroid hormone and glucocorticoids. On the other hand, hormone production is generally dependent on the nervous system, albeit to varying extents. The synthesis of insulin, glucagon, calcitonin, parathyroid hormone, and aldosterone appears to require little or no neural regulation, whereas that of the hypothalamic peptides and adrenal medullary hormones is totally dependent on it. It may be significant that those hormones which are relatively independent of neural control serve

SECTIO30.6 N Organizationof the EndocrineSystem

723

Nervoussyste m_.~ .... Level I (neuroendocrine)

IHypothalamus[ I Adrena' I\~ ' l " ~ " [medlulla ] ~ ~ CRH ADH. TRH Oxytoc,nEpin!phrine ~ SS/GRH Norepinephrine \

)

LevelII

] Antedor~,~pGHLPituitary ] ACTH TSH

LevelIII

ic-c ,,I Gastro-

~/

' int~rStic~al ~ ' '$ ~'X~ alcitOnin Many \

[ ,,,Paintresati ~i cI' l Paratiyr~ i Insulin

LH

Glucagon

Parathyroid

hormone

]Adrenal'cortexj I mhyroidi [Gonads[

L

Cortisol

1

T, ,Ts

t

Progestins Androgens Estrogens

F I G U R E 30-12 Organization of the endocrine system. The endocrine system can be classified into three levels. At level I are those endocrine tissues that are, or were embryologically derived from, nervous tissue; these include the hypothalamus, the adrenal medulla, the thyroid C-cell, and several of the gastrointestinal mucosal cells. Hormones produced by these tissues resemble neurotransmitters to the extent that their effects are rapid in onset and short term. At level II are those endocrine tissues that are directly or indirectly influenced by the nervous system; these include the anterior pituitary, the pancreatic islets, and the parathyroids. Hormones produced by these tissues, which are polypeptides, proteins, or glycoproteins, exert both short- and long-term effects. At level III are the adrenal cortex, the thyroid, and gonads; these endocrine tissues exhibit strong dependence on the anterior pituitary. Hormones produced by these tissues are small, hydrophobic molecules that ultimately affect gene expression in the target cells; their effects are thus slow in onset but long term. Although the level III tissues are not directly regulated by the central nervous system (CNS), they are, by virtue of their control by the anterior pituitary, indirectly regulated by the hypothalamus. Neural regulation by the CNS is critical; - - , neural regulation by the CNS exists but is not critical; . . . . , little or no regulation by CNS; - - - > , hormonal regulation.

to regulate the level of substances that profoundly affect nerve function (glucose, calcium, sodium, potassium). The organization of the endocrine system can best be described in relation to the central nervous system. Three levels of endocrine tissues can be distinguished on the basis of their association with the central nervous system (Figure 30-12). The first level consists of those that are (or were) derived from nerve cells; these include the hypothalamus, adrenal medulla, thyroid C-cell, and gastrointestinal enterochromaffin cells. The hypothalamus and adrenal medulla still retain their neural connections and can therefore be regarded as endocrine extensions of the nervous system. The C-cell and the gut cells, however, are APUD cells and lack neural connections. These four tissues produce hormonal peptides or amines having, like neurotransmitters, rapid-onset, short-term effects.

The second level of endocrine tissues are the anterior pituitary, pancreatic islets, and parathyroids, which show varying degrees of dependence on neural regulation. Because it is highly dependent on the hypothalamus for control of hormone synthesis and release, the anterior pituitary is highly dependent on the nervous system. On the other hand, the pancreatic islets and parathyroids synthesize and release their hormones in the absence of neural signals; however, the rate of hormone release can be influenced by autonomic nerve stimulation. These three tissues produce polypeptide, protein, or glycoprotein hormones having both short- and long-term effects, often involving the regulation of cellular protein synthesis. The third level of endocrine tissues includes the adrenal cortex, thyroid follicles, and gonads, all of which

724

depend heavily on trophic stimulation by anterior pituitary hormones; therefore, although these peripheral endocrine tissues are anatomically and functionally separated from the central nervous system, they are indirectly dependent on the hypothalamus and, thus, on the nervous system. They produce low-molecular-weight hormones (steroids, iodinated amines) having long-term effects that involve regulation of gene expression. The hierarchical organization of the endocrine system emphasizes the substantial influence of the nervous system on endocrine function; however, the influence of the central nervous system is not always obligatory. Thus, although C-cells and gut cells are of neuroblastic origin, they function well in the absence of neural input; similarly, the neural influence on hormone release from pancreatic islets and parathyroid glands is relatively unimportant. Nevertheless, the hypothalamus and adrenal medulla fail when their neural connections are severed, leading to atrophy of the anterior pituitary, adrenal cortex, thyroid, and gonads. However, two hormones are produced by these tissues in normal or elevated amounts following neural disconnection: prolactin from the anterior pituitary, and aldosterone from the adrenal cortex. Release of aldosterone is regulated by the renin-angiotensin system and by extracellular potassium levels, while that of prolactin normally is under tonic inhibition by the hypothalamus (Chapter 31 ). A unidirectional scheme of endocrine control has been alluded to above in the discussion of three levels of endocrine tissues: the nervous system-hypothalamus controls the anterior pituitary, which in turn controls the peripheral endocrine tissues. Closer examination of how hormonal secretions are regulated reveals that, in many instances, a closed-loop feedback (servo-mechanism) circuit operates to ensure that the correct amount of hormone is released at any given time. In its simplest form, a circuit involves an endocrine cell that can monitor changes in the blood level of the substance it regulates. This sensing ability is coupled to a discriminator with an imprinted "set point" such that if the circulating level deviates significantly from the set point, the hormonal output is appropriately modified to counteract that deviation. This type of simple negative feedback regulation of hormone release-usually involving one endocrine tissue, one hormone, and one substance that is monitored--is the primary means by which secretion of insulin, glucagon, parathyroid hormone, and (to some extent) aldosterone is regulated (Figure 30-13). Another way in which hormonal secretion is regulated involves a neuroendocrine reflex, which differs from a neural reflex in that the efferent neuron is replaced by a

CHAPTER30

[.,~L.'.dl TOO

Endocrine Metabolism I: Introduction ,

i

I substance I

j

Too

,

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F I G U R E 30-13 Simple negative feedback control of hormone release. (Top) A discriminator associated with an endocrine cell is capable of monitoring blood levels of a regulated substance. If the level of the substance deviates from the "set point," the endocrine cell adjusts its hormonal output to restore that level. The hormone may act either on the source of the substance or on its removal. (Middle) If the blood level of glucose rises above the set point imprinted in the # cell of the pancreatic islet, insulin secretion is increased. Insulin acts on the liver (source of glucose) to inhibit ( G ) further production of glucose, thereby diminishing the input of glucose into the blood glucose pool (broken line). Insulin also acts on several tissues (muscle, others) to promote ( ~ ) glucose utilization, thereby facilitating the removal of glucose from the blood glucose pool (solid line). (Bottom) Plasma K + concentration is monitored by the zona glomerulosa of the adrenal cortex, and an elevation will stimulate the release of aldosterone. Aldosterone acts only on the disposal of K +, thereby serving to control the level of K + in blood. ECF, Extracellular fluid.

neuroendocrine cell (Table 30-5). As in a neural reflex arc, a sensory (afferent) neuron conveys, via synaptic connections, signals emitted from a sensory receptor to an efferent (neuroendocrine) cell, feedback which then affects the function of an effector organ or tissue. Release of oxytocin, ADH (vasopressin), and adrenal medullary hormones is regulated in this way (Table 30-5). No direct negative feedback comparable to that in the previous model operates; however, the efferent signal (hormone) counteracts, alleviates, or compensates for the stimulus that initiated the reflex. Several examples follow. By increasing water reabsorption, ADH promotes osmodilution and volume expansion, which counteract the dehydration that triggered distress signals from osmoreceptors and baroreceptors; by causing milk ejection, oxytocin helps appease the suckling infant. The adrenal medullary catecholamines produce

SECTION 30.6

Organizationof the Endocrine System

725

TABLE 30-5 Neuroendocrine Reflex

Sensory receptor

Sensoryneuron

Sensory Receptor

Sensory Neuron

Cardiovascular baroreceptors

Cranial nerves IX and X

Osmoreceptors

? Osmoreceptor neuron Ascending touch pathways Unclear

Touch receptors in nipple Uterine stretch receptors Various receptors of noxious stimuli

Many

o~

Neuroendocrinecell HormoneEffectorcell / ~ ~

Neuroendocrine Cell

Hormone

Effector; Effects

Magnocellular neurons, hypothalamic PVN*, SON* (As above)

ADH

Kidney; water reabsorption

(As above)

(As above)

(As above)

Oxytocin

(As above)

(As above)

Adrenal medulla

Epinephrine, norepinephrine

Mammary gland; milk ejection Uterus; smooth muscle contraction Cardiovascular augmentation and liberation of energy substrates

*PVN = paraventricular nucleus. *SON = supraoptic nucleus.

cardiovascular and metabolic changes that enable the individual to cope with the stressful situation that triggered their release, while the adrenal medullary enkephalins probably serve to suppress the sensation of traumatic pain. The neuroendocrine reflex is usually polysynaptic, i.e., one or more interneurons connect the sensory neuron to the endocrine cell. Thus, the system is more amenable to modulation by other neurons or hormones, serving to fine-tune the system. For example, under conditions of normal extracellular fluid volume and osmolality (i.e., when ADH release is suppressed), the sensation of pain stimulates the release of ADH and promotes water retention. Pain-induced release of ADH may be due to neural discharge of endorphins or enkephalins in the central nervous system, which act on the hypothalamus. The peripheral hormones (level III, Figure 30-12) also influence the neuroendocrine reflex. Cortisol, for example, amplifies the adrenal medullary response to stress by increasing the excitability of interneurons in the reticular formation and by stimulating the synthesis of adrenal medullary phenylethanolamine N-methyltransferase (PNMT), which catalyzes the formation of epinephrine (Chapter 32). Estrogen is an important regulator of hypothalamic pro-oxyphysin synthesis

(Chapter 31) and thus promotes the formation of oxytocin. Some hormones are under complex feedback regulation involving both a neuroendocrine reflex and a negative feedback circuit. Invariably, the neuroendocrine reflex component involves the hypothalamus, which in one instance is also the site of positive feedback (Chapter 34). The "substance" that is monitored and exerts negative feedback may be a substrate (e.g., glucose, amino acid) or the hormone that is being regulated. Generally, two or more hormones are involved, which is not surprising, since most hypothalamic and anterior pituitary hormones subserve the function of regulating the release of another hormone (Figure 30-14). As examples, release of growth hormone is regulated by growth hormonereleasing hormone and somatostatin; that of prolactin by prolactin-releasing factor and prolactin-inhibiting factor (dopamine); that of thyroid hormone by thyrotropinreleasing hormone and thyroid-stimulating hormone; that of cortisol by corticotropin-releasing hormone and corticotropin; and that of the sex steroids by gonadotropinreleasing hormone, luteinizing hormone, and folliclestimulating hormone.

726


F I ( ; U R E 30-14 Complex feedback regulation of hormone secretion. Hormones of level III endocrine tissues are regulated by a complex system having both a neuroendocrine reflex component and a simple negative feedback component. Invariably, the former involves the extrahypothalamic central nervous system (sensory neurons), the hypothalamus (endocrine cell 1), and the anterior pituitary (endocrine cell 2), while the latter involves the anterior pituitary (endocrine cell 2) and a level III endocrine tissue (endocrine cell 3). Hormone 3 exerts negative feedback on endocrine cell 2 and hence on its own secretion. Hormone 3 may also feed back to endocrine cell I and to higher centers, although such cases of feedback may be positive ones.

Supplemental Readings and References M. K. Bagchi, M. J. Tsai, B. W. O'Malley, and S. Y. Tsai: Analysis of the mechanism of steroid hormone receptor-dependent gene activation in cell-free systems. Endocrine Reviews 13, 525 (1992). D. M. Berman and A. G. Gilman: Mammalian RGS proteins: Barbarians at the gate. Journal of Biological Chemistry 273, 1269 (1998). D. L. Bodenner and R. W. Lash: Thyroid disease mediated by molecular defects in cell surface and nuclear receptors. American Journal of Medicine 105, 524 (1998). G. A. Brent: The molecular basis of thyroid hormone action. New England Journal of Medicine 331, 847 (1994). D. L. Cadena and G. N. Gill: Receptor Tyrosine kinases. FASEB Journal 6, 2332 (1992). M. A. Carson-Jurica, W. T. Schrader, and B. W. O'Malley: Steroid receptor family: structure and functions. Endocrine Reviews 11,201 (1990) L. De Vries, B. Zheng, T. Fischer, et al.: The regulator of G protein signaling family. Annual Review of pharmacology and Toxicology 40, 235 (2000).

Z. Farfel, H. R. Bourne, and T. Iiri: The expanding spectrum of G protein diseases. New England Journal of Medicine 340, 1012 (1999). M. E. Freeman, B. Kanyicska, A. Lerant, et al.: Prolactin: structure, function and regulation of secretion. Physiological Reviews 80, 1523 (2000). U. Gether: Uncovering molecular mechanisms involved in activation of G protein-coupled receptors. Endocrine Reviews 21, 90 (2000). C. K. Glass: Differential Recognition of target genes by nuclear receptor monomers, dimers and heterodimers. Endocrine Reviews 15, 391 (1994). M. A. Lazar: Thyroid hormone receptors: Multiple forms, multiple possibilities. Endocrine Reviews 14, 184 (1993). W. L. Miller: Molecular biology of steroid hormone synthesis. Endocrine Reviews 9, 295 (1988). M. Pfahl: Nuclear receptors/AP- 1 interactions. Endocrine Reviews 14, 651 (1993). J. W. Putney, Jr. and G. S. Bird: The inositol phosphate-calcium signalling system in nonexcitable cells. Endocrine Reviews 14, 610 (1993). M. D. Ringel, W. F. Schwindinger, and M. A. Levine: Clinical implications of genetic defects in G proteins: The molecular basis of McCune-Albright

Supplemental Readingsand References Syndrome and Albright Hereditary Osteodystrophy. Medicine 75, 171 (1996). W. Rosner: The functions of corticosteroid-binding globulin and sex hormone-binding globulin: Recent advances. Endocrine Reviews 11, 80 (1990). E Rusnak and P. Mertz: Calcineurin: form and function. Physiological Reviews 80, 1483 (2000).

727 E. R. Simpson, M. S. Mahendroo, G. D. Means, and others: Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocrine Reviews 15, 342 (1994). M. Truss and M. Beato: Steroid hormone receptors: Interaction with DNA and transcription factors. Endocrine Reviews 14, 459 (1993). E C. White and E W. Speiser: Congenital adrenal hyperplasia due to 21hydroxylase defeciency. Endocrine Reviews 21, 245 (2000).


CHAPTER

31

Endocrine Metabolism lI: Hypothalamus and Pituitary

31.1 Hypothalamus The hypothalamus is a small region of the brain in the ventral aspect of the diencephalon. In the adult human, it is about 2.5 cm in length and weighs about 4 g. Ventromedially, it surrounds the third ventricle and is continuous with the infundibular stalk of the pituitary (hypophysis). This cone-shaped region of the hypothalamus, the median eminence, consists mainly of axonal fibers from hypothalamic neurons, which either terminate in the median eminence or continue down into the posterior lobe of the pituitary, and it is perfused by a capillary network (primary plexus) derived from the carotid arteries. Blood from the primary plexus is transported by portal vessels (hypophyseal portal vessels) to another capillary network (secondary plexus) in the anterior lobe of the pituitary (adenohypophysis) (Figure 31-1). The hypothalamus contains a high density of nerve cell bodies clustered into nuclei or areas. Neurons in each of these nuclei tend to send their axons to the same regions in the form of tracts. These nuclei innervate the median eminence, other hypothalamic nuclei, the posterior pituitary, and various structures in the extrahypothalamic central nervous system. All of the hypothalamic neurons are presumably monoaminergic (i.e., they A list of expanded acronyms appears in Appendix VIII.

synthesize and release the neurotransmitter amines norepinephrine, serotonin, or dopamine); however, many are also peptidergic (i.e., they synthesize and release neuropeptides). At least 12 hypothalamic neuropeptides have been identified, and it is e:~pected that many more will be discovered. The hypothalamus is an important integrating area in the brain. It receives afferent signals from virtually all parts of the central nervous system (CNS) and sends efferent fibers to the median eminence, the posterior pituitary, and certain areas of the central nervous system. In the median eminence, a number of peptidergic fibers terminate in close proximity to the primary plexus, which transports their neuropeptide secretions via the portal blood flow to the anterior pituitary. Since these neuropeptides affect the function of the anterior pituitary cells, they are called "hypophysiotropic." Two hypothalamic nuclei, the paraventricular nucleus (PVN) and the supraoptic nucleus (SON), consist of two populations of neurons that differ in size: the parvicellular (small-celled) neurons and the magnocellular (large-celled) neurons. The parvicellular neurons of the paraventricular nucleus produce a peptide (CRH; see below) that is transported by axoplasmic flow to the median eminence and then released into the hypophyseal portal blood. The magnocellular neurons of the paraventricular and supraoptic nuclei send their long axons directly into the posterior pituitary (neurohypophysis),

729

730

CHAPTER31 Endocrine Metabolism I1: Hypothalamus and Pituitary

F I G U R E 31-1 Hypothalamic-pituitary regulatory system. Hypothalamic releasing hormones and release-inhibiting hormones are synthesized in various neurons. These hypothalamic hormones are transported to the anterior pituitary via a portal venous system and the anterior pituitary target cells either release or inhibit the release of specific hormones into the general circulation. The posterior pituitary hormones are synthesized and packaged in cell bodies in the hypothalamus and then are transported to nerve endings in the posterior pituitary. Afterward they are released following appropriate stimuli. The explanation for the abbreviations appears in the text. [Slightly modified and reproduced with permission from A. W. Norman and G. Litwack, Hormones, 2nd ed. New York: Academic Press, 1997, p. 89.1

where their dilated nerve endings are closely positioned next to capillaries that drain the tissue. Peptides synthesized in these magnocellular neurons are transported to the neurohypophysis by axoplasmic flow, then released into the general circulation. These neuropeptides are called "neurohypophyseaL" Other hypothalamic neurons make synaptic contact with other neurons, and their neuropeptide products function as neurotransmitters or as neuromodulators. They are called "neuroregulatory."

Hypophysiotropic Peptides (Table 31-1) Thyrotropin-releasing hormone (TRH) is a tripeptide amide with the following structure:

o O H

o

C--N--C--C--N H I CH 2

C--O

I

NH2

Pyroglutamic acid

Histidine

Proline amide

All of its components are essential for activity. TRH is resistant to intestinal peptidase action and is active when taken orally. TRH is produced by the parvicellular neurons of the PVN and the periventricular nucleus, and is

TABLE 31-1

Hypophysiotropic Hormones of the Human Hypothalmus

NO.A A ~ (MW)

Hormone

Gene (human)

GH Releasing Hormone (GHRH) Somatostatin SS (SRIH)

G~~-PLcP-C~~+-PKC

PVNpaw

Stimulates POMC synthesis and processing (fi ACTH secretion)

Gsa-AC activation

20

TIDA (Arcuate, VMN)

Stimulates GH secretion

Gsa-AC activation

3q28

PeriVN, PVNpaw, PON PVNpaw, PeriVN

Inhibits GH secretion, also inhibits TSH secretion Stimulates TSH secretion, also stimulates PRL release

Gia-AC inhibition

TIDA (Arcuate, VMN)

Inhibits PRL release, also indirectly inhibitsLH, FSH release

Thyrotropin Releasing Hormone (TRH) na

Prolactin InhibitingHormone PIH (Dopamine) Prolactin Releasing Factor (PRF) --

unknown

Mechanism of Action

Stimulates LH and FSH secretions

8q13

Corticotropin Releasing Hormone (CRH)

Functions

PON, OVLT

8~21-q11.2

Gonadotropin Releasing Hormone GnRH (LHRH)

Origin1 (Prod. Site)

G ~ ~ - P L-c~~+-PKc @ Gia-AC inhibition (D2)

Stimulates PRL release

-

'PeriVN= periventrlcular nucleus, PVNpan, = parvlcellular paraventricular nucleus; PON = preoptic nucleus; OVLT = organum vasculosum of the lamlna termlnalls, VMN = ventromedial nucleus, TIDA = tuberolnfundlbulardopamlnerglcsystem, 2~~ = amino acid residue, MW = molecular weight, na = not applicable

~32

released at the median eminence. TRH principally stimulates the synthesis and release of thyroid-stimulating hormone (TSH, thyrotropin) in the anterior pituitary, and also stimulates the release of prolactin (PRL). Both effects are mediated by membrane receptors coupled to the GQ~-phospholipase C-/~-calcium-protein kinase C second-messenger system (Chapter 30). Gonadotropin-releasing hormone (GnRH) or luteinizing hormone-releasing hormone (LHRH) is a decapeptide which, like TRH, has a pyroglutamic acid residue in its N terminus. The presence of histidine and tryptophan at positions 2 and 3, respectively, is required for biological activity, while the replacement of glycine at position 6 by D-Trp increases activity up to 100-fold. The potent synthetic GnRH agonist shown below lacks the glycinamide terminal and has instead an ethylamide residue and D-Trp at position 6:

CHAPTER31

EndocrineMetabolism I1: Hypothalamus and Pituitary

anterior pituitary. CRH stimulates the release of ACTH and/~-endorphin by the anterior pituitary corticotrophs by stimulating the synthesis and posttranslational processing of their prohormone, pro-opiomelanocortin (POMC). Growth hormone-releasing hormone (GHRH) is a 44amino-acid polypeptide produced in the tuberoinfundibular dopaminergic system (TIDA) of the hypothalamus, which includes the arcuate and ventromedial nuclei. GHRH stimulates the synthesis and release of GH in anterior pituitary somatotrophs by a cAMP-mediated mechanism. The efficacy of GHRH in promoting GH release is strongly modulated by hypothalamic somatostatin.

Somatostatin(growth hormone release-inhibiting hormone) is a tetradecapeptide with an intrachain disulfide bridge. Somatostatin neurons are located in the parvicellular neurons of the PVN, the periventricular nuclei

O I! pGlu l~His2~Trp3~Ser4~Tyr5~D-Trp6~Leu7~Arg8~ Pr09~C~ NH~CH2~CH3 GnRH-producing neurons are located in the preoptic area and in the organum vasculosum of the lamina terminalis but are known to have migrated to these areas from the olfactory placode during fetal development. Failure of these neurons to migrate into the hypothalamus during embryogenesis results in infertility secondary to GnRH deficiency (Kallmann's syndrome). GnRH stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), the two gonadotropic hormones produced by the pituitary. GnRH also occurs in other regions of the brain and in the placenta. Its pivotal role in the regulation of gonadotropin release, and thus in reproductive competence, has led to the formulation of GnRH antagonists for contraceptive purposes. Paradoxically, however, synthetic GnRH agonists, when administered chronically in humans, cause a downregulation of GnRH receptors that results in suppression of gonadal steroid production. GnRH action on pituitary gonadotropes begins by occupancy of GnRH receptors by GnRH and involves intracellular mobilization of calcium ions through the action of G-protein, phospholipase C, and phosphatidylinositol pathways (Chapter 30). This suppression of gonadal function has been referred to as "medical castration." Corticotropin-releasing hormone (CRH) is a 41amino-acid polypeptide produced in the parvicellular neurons of the paraventricular nucleus and released into the hypophyseal portal system in the median eminence. Antidiuretic hormone (ADH, AVP) is also synthesized and released by the same neurons, but in humans, unlike the situation in rats, the amount of ADH released at the median eminence is too small to influence the function of the

and the preoptic nuclei. Somatostatin inhibits the synthesis and release of GH from the somatotropes by neutralizing the effect of GHRH, an effect mediated by Giot inhibition of adenylyl cyclase. Within the hypothalamus, somatostatin inhibits the release of GHRH by the same mechanism and therefore exerts an inhibitory effect at two levels of GH control. Somatostatin is also produced outside of the CNS, particularly in the D cells of the islets of Langerhans and in the gastrointestinal tract. Somatostatin produced at these extra hypothalamic sites does not normally influence GH secretion by the anterior pituitary because it is greatly diluted by systemic blood. Prolactin-inhibiting hormone (PIH), also known as dopamine, is not a peptide (Chapter 17). It functions as a neurotransmitter in the CNS and as a precursor of norepinephrine and epinephrine in the adrenal medulla. In the hypothalamus, it originates in the TIDA and is released at the median eminence. Dopamine is a potent inhibitor of PRL release by the lactotropes (and mammosomatotropes) of the anterior pituitary, and this effect is mediated by D2 receptors that are coupled to Gia inhibition of adenylate cyclase. The lactotropes are unique in that they do not require stimulation by the hypothalamus to secrete PRL; in fact, blockage of the blood flow from the hypothalamus to the anterior pituitary results in elevated serum levels of PRL, due to withdrawal of dopamine. Thus, unlike somatostatin, the effectiveness of dopamine does not depend on the presence of a stimulating hormone (Chapter 34). The existence prolactin-releasing factor (PRF), a yetto-be-characterized stimulator of PRL release, remains controversial, particularly in view of the fact that TRH subserves this role. What is clear, however, is that the

733

SECTION31.1 Hypothalamus

TABLE 31-2

Neurohypophyseal Hormones Structure

Hormone S-S Antidiuretic hormone (ADH) (arginine vasopressin)

I ]6 -Pro 7-Arg 8-Gly 9- N H 2 Cysl-TyrZ-Phe3-Gln4-AsnS-Cys

Oxytocin

C~sl -TyrZ-Ile3-Gln4-AsnS-Cys S-S ]6 -Pro 7-Leu 8-Gly 9- N H 2

principal effect of the hypothalamus on PRL secretion is inhibitory. A stimulating hormone from the hypothalamus may be important in tempering the inhibition by dopamine.

Neurohypophyseal Peptides The magnocellular neurons of the paraventricular and supraoptic nuclei synthesize antidiuretic hormone (ADH, vasopressin, or arginine vasopressin) and oxytocin, neuropeptides that are released into the general circulation in significant amounts (Table 31-2). These neuropeptides are synthesized in separate cells and transported by axoplasmic flow to nerve endings in the neurohypophysis. Cholinergic signals to the magnocellular neurons stimulate the release of ADH and oxytocin via nerve impulse propagation along the axon and CaZ+-dependent exocytosis of secretory granules. Release mechanisms for ADH and oxytocin are under separate control. ADH is a nonapeptide that contains an intrachain disulfide bridge. ADH is synthesized as part of a precursor glycoprotein (propressophysin), whose gene is located on chromosome 20. Propressophysin is packaged into secretory granules and transported to the neurohypophysis. During transit, propressophysin is split by proteases in the granule wall, yielding ADH and a "carrier" polypeptide called neurophysin II or "nicotine-sensitive neurophysin" (NSN), whose release is stimulated by nicotine. Upon neural stimulation, ADH and NSN are released in equimolar amounts. NSN has no known biological activity. In the collecting ducts of the kidney, ADH acts by a Gs-protein coupled cAMP-mediated mechanism to increase the permeability of ductal cells to water by mobilizing aquaporin proteins to the apical membrane which prevents diuresis (see also Chapter 39). At relatively high concentrations, ADH is a potent vasoconstrictor. Such concentrations are attained after massive hemorrhage, when constriction of blood vessels prevents further blood loss. Major stimuli for ADH release are an increase in plasma osmolality, monitored by hypothalamic osmoreceptors, and a decrease in blood volume, monitored by baroreceptors in the carotid sinus and in the left atrial wall. Afferent

fibers to the vasomotor center in the hindbrain sense a fall in blood volume, and diminished noradrenergic signals from this center to the hypothalamic magnocellular neurons stimulate ADH release. Because norepinephrine inhibits ADH release, decrease of this inhibition constitutes a stimulation. Oxytocin is a nonapeptide that differs from ADH at the residues in positions 3 and 8. It is synthesized in separate magnocellular neurons from a gene that codes for prooxyphysin, the precursor of oxytocin and its "carrier" (neurophysin I or estrogen-sensitive neurophysin). Neurons in the paraventricular and supraoptic nuclei synthesize prooxyphysin, which is packaged and processed during transit to the neurohypophysis. The principal action of oxytocin is ejection of milk from the lactating mammary gland ("milk let-down"), and it also participates in parturition (Chapter 34). Oxytocin is released by a neuroendocrine reflex mechanism and stimulates contraction of estrogenconditioned smooth muscle cells. The mechanism of action of oxytocin does not involve cAMP but may involve regulation of increased intracellular Ca 2+. The oxytocin receptor belongs to a seven-membrane-spanning receptor family.

Neuroregulatory Peptides Neuronal projections from the hypothalamus to other regions of the brain relay important output information that influence blood pressure, appetite, thirst, circadian rhythm, behavior, nociception (pain perception), and others factors. Although many of these neurons release neurotransmitter amines at synapses, some of them are known to release neurotransmitter peptides. These include, among others, peptides that closely resemble hormones formed in the gastrointestinal system as well as the endogenous opiates (Table 31-3).

Brain-Gut Peptides Gut hormone-like neurotransmitters have been detected in the brain and are believed to function in a manner that

TABLE 31-3

Neuroregulatory Peptides of the Hypothalamus Hypothalamic Peptide ~eurotransmitter',~

No. AA's

Functions in CNS Inferred or Postulated

GI Counterpart

Functions in Gut

--

Cholecystokininoctapeptide (CCK-8)" Gastrin*

8 17

SubstanceP*

11

Vasoactive Intestinal Polypeptide (VIP)* Gastrin-Releasing Peptide(GRP) (mammalianBornbesin)* Somatostatin*

CCK-33; "I" cell in duodenum and jejunum Gastrin- 17; "G"cel1s in stomach, duodenum

Suppressionof appetite Opiate antagonist Appetite regulation?

Nociception

28

Neural crest-derived cells, sensory C fibers Enteric nerve endings

27

Stomach. duodenum

14

Stomach, pancreatic islet, myenteric plexus

Thermoregulation Nociception Suppressionof GHRH release (?) Suppressionof TRH release (?)

Neurotensin* p-Endorphin

Met-Enkephalin (MENK)*

"N" cells of Ileum ?

5

Myenteric neurons (AS and C fibers) Myenteric neurons (AS and C fibers)

Dynorphin 1-8

* = A "Brain-Gut"

8

Probably vasomotor

Gallbladder contraction; Pancreatic enzyme secretion Gastric acid secretion via histamine release from APUD cells in gastric mucosa (enterochromaffin-like(ECL) cells) (H2 antagonist-sensitive) Vasoconstriction (local effect) Mediates immune response to inflammation(?) Probably vasomotor Gastrin release from G cells in stomach Suppresses release of glucagon, insulin, gastrin,CCK, secretin, VIP, GIP Suppressionof acid productionby parietal cells, pancreaticenzyme and bicarbonate release Suppressionof proliferativegrowth in most cells Not known ?

Suppresses appetite Suppresses avoidancelalarmbehavior (p ) Suppressesbehavioral reactiveness to pain, trauma, stresses (pl) Stimulatesappetite (p?) Suppressionof GI motility (p2) Suppressesnociception(a)( pl) Stimulationof sphinctertone (p2) Substance Pantagonism(S)(pI) Depresses medulla respiratory center ( p 2 ) Suppressionof GI motility(p2) Suppresses nociception(S)( p I ) Stimulationof sphinctertone b 2 ) Substance P antagonism (a)( p Depresses medulla respiratory center w 2 ) Stimulatesappetite ( K ) ? ? StimulatesADH release (p)(CNS) InhibitsADH release (~)(neurohypophysis)

peptide; has a counterpart in the gastrointestinal tract. Many are also produced in other regionsof the CNS. For example, the enkephalin pentapeptides are produced mainly in the periaqueductal gray (mesencephalon), the raphe nucleus (pons), and in the dorsal horn of the spinal cord, with minor amounts in the hypothalamus. In contrast, fl-endorphin is produced mainly in the adenohypophysis and hypothalamus.

735

SECTION31.1 Hypothalamus

_a

IGF-II00

10 mg/day 200 ng/ml Nonpulsatile Steady, nonvariable 3 (types 1,3,4) 12-15 h Muscles, connective tissue, cartilage, bone Plasma membrane heterotetramer (a 2132) IGF-I > IIl0 >> InSl000

Ins inhib GH inhib

IGF-I inhib Starvation inhib

differences of IGFs with insulin and among themselves are summarized in Table 31-5. Six serum proteins produced mainly in the liver that bind IGFs in the circulation have been identified; these are designated IGF-binding proteins (IGFBPs) 1 through 6. IGFBP- 1 appears to retard target tissue uptake of IGF-I, while IGFBP-3 appears to enhance it. The latter accounts for most of the IGF found in blood. About 80-90% of the IGF-I in circulation is bound to IGFBP-3, along with an 88-kDa acid-labile subunit (ALS); this complex confers protection on the IGF and prolongs its half-life to 12-15 hours. About 5% of the IGF is unbound, and the remaining 5-15% is bound to IGFBP-I, 2, or 4 as smaller complexes. Unlike insulin and other peptide hormones that are released from storage granules, IGFs are released as they are produced. Most tissues produce IGFs in small amounts that are sufficient for local (paracrine/autocrine) effects and do not contribute significantly to the circulating pool of IGFs. The highest concentration of IGF-I and II is found in circulation and is primarily derived from the liver. GH and insulin are positive regulators of IGF-I synthesis in the nonfasting state. During mild starvation when GH levels increase, there is a decline in hepatic production

IGF-I I

+

13 mg/day 700 ng/ml Nonpulsatile Steady, nonvariable 6 (types 1-6) 15h Muscles, connective tissue, cartilage, bone IGF-I receptor; plasma membrane IGF-II/Man-6-P receptor (monomeric) IGF-II >> I500 IGF-I inhib (IGF-IR); Starvation inhib (IGF-IR); Insulin, IGF-I stim (IGF-II R)

of IGF-I due, in part, to the decline in insulin. Refeeding, which causes insulin to rise and GH to fall, promotes IGF-I production only if the diet is adequate in caloric and protein content. Note that circulating IGFI does not mediate the growth-promoting effect of GH but is an important feedback regulator of GH. The indispensability of IGF-I for linear growth and the fact that it mediates this effect of GH are firmly established. IGF-I, locally produced in endochondral bone in response to GH, promotes skeletal growth by stimulating clonal chondrocyte expansion of the distal proliferative zone of the epiphyseal plate. Under physiological conditions, IGF-I is a relatively minor regulator of fuel homeostasis. It does not mediate the effects of GH on intermediary metabolism; in fact, many of the effects of IGF-I resemble those of insulin, not GH. IGF-I promotes production and actions of erythropoietin and is responsible for the increased packed volume of blood that results from elevated GH levels. Disturbances in G H and I G F

Only rarely has a condition of IGF deficiency or excess been described that is not accompanied by a disturbance in

SECTION31.2 Pituitary 61and (Hypophysis)

741

FIGURE 31-6 Structure of human prolactin. [Reproduced with permission from R. K. Murray, D. K. Granner, P. A. Mayes, and V. W. Rodwell, Harper's Biochemistry, 21st ed. Norwalk: Appleton & Lange, 1988. @1988 Appleton & Lange.]

GH production or responsiveness. Thus, a GH deficiency results in IGF-I deficiency, whereas a GH excess results in IGF-I excess. GH deficiency in children results in reduced growth velocity and growth retardation (dwarfism) due to secondary IGF-I deficiency, whereas in adults there are no dramatic signs or symptoms. Adults who are deficient primarily in GH have decreased lean body mass, increased adiposity, and are at increased risk of cardiovascular disease. GH hypersecretion in children causes increased growth rate and can result in gigantism. However, GH excess occurs infrequently in childhood, and most frequently in middle-aged adulthood, which leads to acromegaly (acral -- extremities + megas -- large), a condition in which the cartilaginous tissues proliferate, resulting in distorted overgrowth of the hands, feet, mandibles, nose, brow, and cheek bones. Because epiphyseal cartilage is absent in long bones, there is no gain in height. Acromegaly promotes insulin resistance and cardiovascular complications. Resistance to the effects of GH results in a condition similar to GH deficiency. The major cause of GH resistance is a genetic defect in the growth hormone receptor (GHR) and the resultant condition is known as Laron-type dwarfism. This is an autosomal recessive disorder characterized by normal to high levels of serum growth hormone and low levels of IGF-I. Analysis of the GHR gene

in Laron-type dwarfism has shown point mutations, deletions, and splicing defects. Growth hormone deficiency in children can be treated by administration of recombinant GH or, in some cases, growth hormone-releasing hormone. Potential targets for recombinant GH that are undergoing clinical investigation include children with idiopathic short stature, persons with wasting syndrome associated with human immunodeficiency virus infection, critically ill patients, and elderly individuals. Excess production of GH due to somatotroph adenomas may be treated by surgical resection, irradiation, or in some cases with somatostatin analogues (e.g., octreotide) that suppress GH secretion, or by a combination of these above.

Prolactin Human prolactin (PRL) contains 199 amino acid residues (M.W. 23,500) and three intramolecular disulfide bridges (Figure 31-6). In healthy adults the anterior pituitary releases very little PRL under nonstressed conditions, primarily because PRL release is under hypothalamic inhibition. This inhibition is exerted by dopamine or PIH. The hypothalamus also secretes PRF but the major regulator appears to be PIH.

742

CHAPTER 31

Endocrine Metabolism I1: Hypothalamus and Pituitary

TABLE 31-6

Opiomelanocortin Family Site of Secretion

Peptide Length

Location of Gene

Adrenocorticotropi c Hormone (ACTH) (Corticotropin)

Anterior Pituitary (Corticotroph)

39 aa

2p25

/3-Endorphin

Anterior Pituitary (Corticotroph)

31 aa

2p25

Hormone

The biological actions of prolactin are initiated by its membrane receptor, followed by the associated intracellular signal transduction pathways. These actions are similar to growth hormone (discussed previously; also see Chapter 30). In humans, the function of PRL may be restricted to promotion of lactation, but there is some evidence that PRL suppresses gonadal function in females. There is no consensus regarding a physiological role for PRL in males. During late pregnancy, the maternal pituitary releases increasing amounts of PRL in response to rising levels of estrogen, a stimulator of PRL synthesis. Elevated levels of PRL stimulate milk production in the mammary gland (Chapter 34). After parturition, PRL promotes milk secretion via a neuroendocrine reflex that involves sensory receptors in the nipples (Chapter 34). In mammary tissue, it binds to alveolar cells and stimulates the synthesis of milk-specific proteins (casein, lactalbumin, and lactoglobulin) by increasing production of their respective mRNAs. Accordingly, there is a lag of several hours before this effect of PRL is seen. In the liver, PRL stimulates the synthesis of its own receptors. PRL receptors occur in the mammary gland, liver, gonads, uterus, prostate, adrenals, and kidney.

Disturbances in Prolactin In women, hyperprolactinemia is often associated with amenorrhea, a condition that resembles the physiological situation during lactation (lactational amenorrhea). Excess PRL may inhibit menstrual cyclicity directly by a suppressive effect on the ovary or indirectly by decreasing the release of GnRH (Chapter 34). In men, hyperprolactinemia is not associated with altered testicular function but is often attended by diminished libido and impotence. This finding suggests that PRL may serve an important role in regulating certain behavior patterns. Prolactinomas are the

Regulation of Secretion

Function Stimulates steroidogenesis in all three zones of the adrenal cortex, thereby increasing secretion of cortisol, DHEAS, and aldosterone. Possible role in stress analgesia,

Stimulated by CRH and inhibited by cortisol.

Stimulated by CRH and inhibited by cortisol.

most common hormone-secreting pituitary adenomas and, in addition to the above-mentioned clinical characteristics, the patient may exhibit visual field defects. Prolactinoma may be treated by surgery, radiotherapy, or pharmacotherapy. This last method consists of using a dopamine D2 receptor agonist such as bromocriptine or cabergoline to suppress prolactin secretion.

The Opiomelanocortin Family All members of this family are derived from a single prohormone, pro-opiomelanocortin (POMC). The prohormone molecule contains three special peptide sequences: enkephalin (opioid), melanocyte-stimulating hormone (MSH), and corticotropin (ACTH). These result from posttranslational cleavage of the prohormone but do not share the same biological actions (Table 31-6). In humans, the gene encoding POMC is normally expressed in the adenohypophysis, hypothalamus, and brain, as well as in other sites. It may also be expressed as a result of neoplastic transformation, notably in the lung. POMC is a large glycoprotein that contains three MSH sequences and one MENK sequence. Several paired basic amino acid residues exist along the molecule and are sites of potential cleavage by proteases during posttranslational processing (Figure 31-7). In humans, the important products of POMC that are secreted by the adenohypophysis are ACTH and fl-endorphin, although other products (e.g., v3-MSH, y-LPH) may also be significant (see later). After translation POMC is processed by proteases contained in tissue. In the adenohypophyseal corticotrophs there is no further processing of ACTH and fi-endorphin, whereas in the intermediate lobe of other species, ACTH can be processed to form ot-MSH, and v-LPH can be processed to form fi-MSH. The human hypothalamus

SECTION31.2 Pituitary Gland (Hypophysis)

743

=m

<E I

I

I

I

L,,

i

_J !

I

Pro-opiomelanocortin (PMOC) f f f

230+

if)

3"

>E= NE

D2

DA>>E= NE

Bromocriptine

a l4

E>NE>>I

Methoxamine Phenylephrine

E g NE>>I

I>E= NE

I>E >>NE

L33

Metoclopramide

Clonidine

Dobutamine

Terbutaline Salbutamol Soterenol

I= NE>E, Low Fenoterol affinity receptor

Phentolamine Phenoxybenzamine

Yohimbine

Gs: AC stimulation Increased CAMP Gi: AC inhibition Decreased cAMP Gp:PLC:IP3:DAG Ca+2-~rotein Kinase

Butoxamine

none8

Physiologic Ligand3

Effect

Vascular smooth muscle (kidney, coronary, mesenteric) Lactotrope (anterior pituitary)

DA

Relaxation (low levels)

DA

Inhibition of prolactin release

Vascular smooth muscle (skin)

NE

Constriction

Vascular smooth muscle (arterioles) Vascular smooth muscle (abdominal viscera except liver) Intestinal smooth muscle Intestinal sphincters Urinary smooth muscle (ureter, sphincter of bladder) Iris (radial muscle) Skin (piloerector muscle) Skin (sweat gland) (palm of hands, sole of Hypothalamus fi cell Islet of Langerhans

NE NE

Constriction Constriction

NE NE NE NE NE NE

Relaxation Contraction Increased ureter tone, contraction of sphincter Contraction (mydriasis (pupil dilation)) Contraction (pilorection) Secretion

NE NE, E

Stimulation of ADH release Inhibition of insulin secretion

Vascular smooth muscle (skin) Vascular smooth muscle (veins) Hypothalamus

E NE, E NE

Gs: AC stimulation Increased CAMP

Heart (SA node, AVnode, His-Purkinjie system, ventricles)

NE, E


Juxtaglomemlar appartus of kidney Adipose tissue (white) Vascular smooth muscle (skeletal muscle)

NE E E

Constriction (vasoconstriction) Constriction Decreased SRIH andlor increased GRH release (results in increased GH release from pituitary) Increased firing rate; increased conduction velocity; increased contractility, increased heart rate, increased cardiac output Stimulation of renin release Lipolysis Relaxation (vasodilation)

Vascular smooth muscle (coronary) Bronchiolar smooth muscle Skeletal muscle acells Islets of Langerhans Intestinal smooth muscle Genitourinary smooth muscle Liver White adipose tissue (visceral > subcutaneous) Brown adipose tissue

E E E NE, E

Relaxation (vasodilation) Relaxation (bronchodilation) Glycogenolysis; Increased contractility? Stimulation of glucagon release

E NE, E

Glycogenolysis; Gluconeogenesis Lipolysis

Gi:AC inhibition Decreased cAMP

Metoprolol Atenolol

Tissue


NE (and E) Thennogenesis

' a 1 and a 2 each have 3 subtypes, information about which can be found in: Bylund (1992) Subtypes of a l - and a 2 - a d r e n e ~ i creceptors. FASEB J 6:832-839. 'DA= dopamine; E = epinephrine: NE = norepinephrine; I = isoproterenol (a non-selective P-adrenergic agonist: stimulates all 3 subtypes of 0-adrenoceptors). 3NE = norepinephrine as neurotransmitter released from sympathetic nerve endings at synaptic junctions. acting upon postjunctlonal adrenergic receptors; E = plasma-bome epinephrine that bind to extrajunctional adrenergic receptors that are not asyociated with nerve terminals. 4~ocalizedentirely postsynaptically 5Most of the sweat glands are innervated by cholineq~csympathetic fibers that release AChol at synaptic junctions and respond to sympathetic stimulation with an increase in secretion (increased sweating).The exceptions are the sweat glands in the palm of the hands and sole of the feet. which are innervated by adrenergic fibers that release NE at synaptic junctions. Emotional excitement activates these adrenergic nerves, and causes sweating in the hands and feet C'adrenergic sweating"). Emotional excitement usually has no effect on sweating in other reglans because the cholinergic sympathetic nerves are mainly controlled by the thermoregulatory center of the hypothalamus, and are not affected by behavioral reactiveness. Note, however. that all sweat glands haveal adrenoceotors. and are therefore caoable of resoonding to olasma eoineohrine (and NE?) with an increase in sweat secretion. This would result in a more generalized sweating. , 6Located both pre- and~ostsynaptically. ' 7Primarily involved in neuronal functions. * ~ e t o p r o l oisl a non-selective antagonist.

.

- .

. .

SECTION32.2 Adrenal Medulla

accessible to both circulating (E) and neurotransmitted (NE) catecholamines. In fact, thedistribution of the subtypes appears to be appropriate for the ligand that is prevalent in a given region. For example, the /~2-adrenergic receptors (E > NE) in bronchiolar smooth muscles are located in regions of the respiratory system that are poorly supplied by sympathetic nerve fibers but are accessible to agents that arrive by way of the blood supply. These receptors are most likely to respond to circulating E than to neural NE, and are responsive to inhaled synthetic adrenergic agonists (e.g., albuterol, salmeterol) that diffuse through the mucosal surface. On the other hand, the/~ 1-adrenergic receptors in the juxtaglomerular apparatus (JGA) of the kidney, the site of renin release that is richly supplied by sympathetic nerve fibers but is also well vascularized, tend to respond preferentially to neural NE rather than to plasma E, despite the fact that this subtype is known to have equal affinity for both catecholamines (Table 32-3). This suggests that the/31 receptors of the JGA are localized in the synaptic clefts as postjunctional binding sites and are not accessible to plasma-derived E. To facilitate the discussion on the effects of catecholamines (below), Table 32-3 includes a list of the "physiological ligand" (i.e., neural NE or plasma E) for the adrenergic receptor subtype in some of the tissues, based on their efferent sympathetic nerve supply. The different tissue distribution of adrenergic receptor subtypes indicates the genetic influence on the kinds of receptors; however, the density of the receptor subtypes also exhibits tissue variation. For example, although white adipose tissue contains both/31 and/33 receptors, the latter is in greater quantity in visceral adipose tissue. The density of adrenergic receptors is strongly influenced by hormones. The adrenergic catecholamines exert an important regulation on their own receptors, causing downregulation when the concentrations are raised. All subtypes of adrenergic receptors except the/33 subtype is subject to downregulation. Thyroid hormone increases the density of/3-adrenergic receptors in some tissues (heart, fat, skeletal muscle), but decreases it in the liver. Glucocorticoids (GC) increase the density of/~2-adrenergic receptors in vascular and bronchiolar smooth muscle, and has been shown to be effective in reversing agonist-induced desensitization of/32-adrenergic receptors in asthmatic patients.

Biological Effects of Epinephrine and Norepinephrine (Table 32-3) Although NE is present at higher concentrations in plasma than E (Table 32-1), plasma NE is physiologically ineffective under most conditions. Plasma NE levels above

765

100 ng/dL are required for a systemic response, and such concentrations usually are produced only during very stressful situations (e.g., strenuous exercise or myocardial infarct). In contrast, E is capable of activating adrenergic receptors at threshold concentrations of 10-15 ng/dL, levels that are attained with relatively mild stimuli (e.g., cigarette smoking or hypoglycemia). E exerts important cardiovascular, pulmonary, renal, metabolic, endocrine, and thermogenic effects, most of which are supported or complemented by neural effects of NE that are exerted at the same time.

Cardiovascular, Pulmonary, and Renal Effects E increases cardiac output by increasing both the strength and the rate of ventricular contractions (/~1). This effect is augmented by the constrictive effect of E (ot2) and NE (ot2) on the great veins, which increases venous return to the heart and leads to increased ventricular preload and increased stroke volume. The increased cardiac output is selectively directed to certain organs by the combined effects of circulating E and neurotransmitted NE. Thus, blood flow to the skin (subcutaneous) is reduced by the combined vasoconstrictive effects of E (oe2) and NE (oil), while flow to the gastrointestinal tract, spleen, pancreas and kidney are reduced by sympathetic (NE) stimulated vasoconstriction (oil). At the same time, E redirects the flow of blood through skeletal and cardiac muscles by promoting dilatation of the arterioles of the skeletal muscle (/32) and the coronary arteries of the heart (/32). Because the arteries in the brain, lungs, and liver are not directly affected by the adrenergic catecholamines, these tissues passively receive an increase in blood flow as a result of the augmented cardiac output and increased blood pressure secondary to increased peripheral resistance.

Pulmonary Respiratory System E promotes air flow through the bronchioles by causing relaxation of their smooth muscle (bronchodilatation) (/32), and thus allows for increased alveolar ventilation. The increased blood flow through the lungs (see above) and the increased alveolar ventilation ensure maximal oxygenation of blood.

Renal Urinary System Autonomic discharge results in stimulation of ADH release due to a neurotransmitted NE effect (oe1) on the magnocellular neurons in the hypothalamic paraventricular nuclei. This promotes increased retention of water, which, in turn, promotes the effective blood volume. In addition, autonomic discharge causes the release of neurotransmitter

766

NE at the JGA of the kidney and this stimulates the release of renin (/~), ultimately resulting in aldosteronemediated sodium retention at the nephron. The overall result is preservation and promotion of vascular volume due to isotonic fluid retention, decreased urine flow, and decreased glomerular filtration rate, all of which are attributable to neurotransmitted NE. Paradoxically, elevated levels of E and NE can cause a reduction in plasma volume that can lead to hemoconcentration and poor tissue perfusion; however, the mechanism of this effect is not known. This is said to be the cause of orthostatic hypotension in untreated patients with pheochromocytoma, a tumor of chromaffin tissue that produces excessive amounts of NE; most of these patients have reduced plasma volume but exhibit hypertension while in the reclining position.

Metabolic, Endocrine, and Thermogenic Effects E promotes the production and release of glucose from the liver by two related mechanisms: 1. A direct stimulatory effect on hepatic glycogenolysis (/32) mediated by cAMP-dependent phosphorylation of phosphorylase. This effect is dependent on prior storage of glycogen in the liver; therefore, both insulin and cortisol serve to condition the liver for this effect. 2. A direct activation of gluconeogenesis in the liver (/~2) via cAMP-dependent phosphorylation of the key gluconeogenic enzymes. This effect requires prior induction and maintenance of enzyme concentrations by cortisol. These two glucose-generating actions of E on the liver are enhanced in an additive fashion by glucagon which, like E, depends on cAMP mediation and cortisol conditioning. The fact that neural NE simultaneously causes a fall in the insulin/glucagon ratio indicates that the sympathetic nervous system has a supportive role. Neural NE and plasma E are both stimulators of adipocyte lipolysis, an effect that is mediated by/~ l- and /~3-adrenergic receptors and involves cAMP-mediated phosphorylation of hormone-sensitive lipase (HSL). There are regional differences in /33-adrenergic receptor density (visceral fat has more than subcutaneous fat), which determines that catecholamine is physiologically important. Note that/~3 receptors are low-affinity receptors that require high concentrations of catecholamines. They respond better to neural stimulation because of the higher local concentration of NE at the fat tissue site. Cortisol, thyroid hormone, and GH support this effect of the catecholamines, presumably by promoting the synthesis of one or more components of the lipolytic pathway.

CHAPTER32 Endocrine Metabolism II1: Adrenal Glands

This results in the release of free fatty acids (FFAs) and glycerol. Glycerol is taken up by the liver and is used for the production of glucose via gluconeogenesis (/~2). FFAs are taken up by cardiac muscle, skeletal muscle, kidney, and liver, and are oxidized for energy which obliges these tissues to consume less glucose. In the liver, where FFA uptake is related to circulating FFA levels, the oxidation of acyl-coA also is shunted to ketogenesis; ketone body production increases as a result (Chapter 18). E can promote FFA utilization and ketogenesis in the liver by inhibiting acetyl-CoA carboxylase activity (/32); this results in unrestricted entry of FFA into the mitochondria for oxidation and subsequent ketogenesis. Ketone bodies are consumed by extrahepatic tissues that possess succinyl-CoAacetoacetate-CoA transferase, also known as thiophorase (cardiac muscle, skeletal muscle, intestines, kidney, and the brain during starvation), and this also has a glucosesparing effect. E stimulates skeletal muscle glycogenolysis (/~2) and thus promotes glycolysis. The effects of E may depend on skeletal muscle activity because contraction alone is a stimulus of both glycogenolysis and glycolysis; the effects may also be influenced by the muscle fiber type (types I, IIa, or IIb). In resting muscle and presumably in type I (slow-twitch) fibers, there is preferential use of fatty acids and ketone bodies for energy, which also inhibits glycolysis; thus, the glycogenolytic and glycolytic effects of E may be minimal or overriden. In the contracting muscle and presumably in fast-twitch (types IIa and IIb) fibers, E may potentiate the active glycogenolytic and glycolytic pathways and promote the release of both lactate and alanine. Indirectly, E can inhibit skeletal muscle glycolysis by increasing the supply of FFAs and ketone bodies, which are glucose-sparing energy fuels for this tissue; however, the increased turnover of ATP in the contracting muscle probably allows the tissue to accommodate both glucose and fatty acids as fuels (Chapter 21). A potentially harmful effect is the stimulation by E of potassium ion movement from plasma into skeletal muscle and liver (/~2), which can lead to hypokalemia. This effect is accompanied by a decrease in potassium ion excretion by the kidney. The ability of E to stimulate plasma membrane Na+,K+-ATPase activity in skeletal muscle (/~2) may partially explain this hypokalemic action, as well as the thermogenic effect of the hormone (see below). The direct adrenergic innervation of the islets of Langerhans in the pancreas allows NE to stimulate the release of glucagon from the o~ cells (/~2) and to inhibit the release of insulin from the /~ cells (o~2), causing the insulin:glucagon (I:G) ratio to fall. Circulating E also exerts these effects and is probably important in maintaining the low I:G ratio following neural stimulation. Because the

767


effects of a lowered I:G ratio are similar to those of NE and E on glycogenolysis, gluconeogenesis, lipolysis, and ketogenesis, the outcome is a great enhancement of the metabolic response to adrenergic discharge. In the neonate, neural NE promotes "nonshivering thermogenesis," i.e., heat production via stimulation of lipolysis and fatty acid oxidation in brown adipose tissue (/33) with the support of thyroid hormone. Brown adipose tissue contains a high density of mitochondria with an "uncoupling protein" that allows the cells to oxidize fatty acids and generate heat as a major product; NE released at nerve endings stimulates cAMP-mediated lipolysis in these cells and promotes thermogenesis by this fl3-mechanism (Chapter 14). In the adult, nonshivering thermogenesis is probably a minor source of body heat production because brown adipose tissue is sparse and limited to only a few regions of the body. Adults depend on neurogenically induced shivering thermogenesis, which does not depend on adipose lipolysis. However, even in the resting state, catecholamines stimulate thermogenesis by promoting the thermogenic effect of thyroid hormone. Thus, when administered to adults, E increases resting body temperature and oxygen consumption (fl), in part due to stimulation by E of skeletal muscle Na+,K+-ATPase activity (f12).

Disturbances in Adrenal Medullary Function Because there is considerable overlap in the functions of the sympathetic nervous system and the adrenal medulla, elimination of the adrenal medulla would be tolerated as long as the autonomic nervous system remains functionally intact. However, overproduction of adrenal medullary hormones would be disruptive. Such catecholamine excesses are seen in patients with tumors of the adrenal medullary chrornaffin cells and/or tumors of chromaffin tissue located outside of the adrenal gland. These tumors are called pheochromocytomas, and, although their incidence is rare, the pathophysiology should be understood for full appreciation of adrenergic catecholamine functions under normal conditions. One site at which pheochromocytomas often develop is the organ of Zuckerkandl, a collection of pheochromocytes at the bifurcation of the aorta. Familial predisposition to pheochromocytoma can occur due to activating mutations of the RET

proto-oncogene that cause multiple endocrine neoplasia type II (Chapter 37) and mutations in the von HippelLindau tumor suppressor gene. Pheochromocytoma is usually characterized by intermittent to permanent hypertension with potentially life-threatening consequences. The biochemical diagnosis in patients suspected of pheochromocytoma consists of measuring epinephrine, norepinephrine, and their metabolitesmnamely, metanephrine, normetanephrine, dihydroxyphenylglycol, and vanillylmandelic acidmin a 24-hour urine specimen. Measurement of plasma levels of total free metanephrine has a very high sensitivity for detecting pheochromocytoma. Tumors of the pheochromocytoma can be surgically removed.

Supplemental Readings and References E.-E. Baulieu: Dehydroepiandrosterone (DHEA): a fountain of youth? Journal of Clinical Endocrinology and Metabolism 81, 3147 (1996). S. R. Bornstein, C. A. Stratakis, and G. R Chrousos: Adrenocortical tumors: recent advances in basic concepts and clinical management. Annals of Internal Medicine 130, 759 (1999). R R. Casson, S. A. Carson, and St. J. E. Buster: Replacement dehydroepiandrosterone in the elderly: rationale and prospects for the future. Endocrinologist 8, 187 (1998). K. C16ment, C. Vaisse, B. S. J. Manning, et al.: genetic variations in the fl3adrenergic receptor and an increased capacity to gain weight in patients with morbid obesity. New England Journal of Medicine 333, 352 (1995). A. Ganguly: Primary aldosteronism. New England Journal of Medicine 339, 1828 (1998). T. L. Goodfriend, M. E. Elliott, and K. J. Catt: Drug therapy: angiotensin receptors and their antagonists. New England Journal of Medicine 334, 1649 (1996). R A. Insel: Adrenergic receptors--evolving concepts and clinical implications. New England Journal of Medicine 334, 580 (1996). M. New: The prismatic case of apparent mineralocorticoid excess. Journal of Clinical Endocrinology and Metabolism 79, 1 (1994). W. Oelkers: Current concepts: adrenal insufficiency. New England Journal of Medicine 335, 1206 (1996). D. N. Orth: Cushing's syndrome. New England Journal of Medicine 332, 791 (1995). T. M. Penning: Molecular endocrinology of hydroxysteroid dehydrogenases. Endocrine Reviews 18, 281(1997). K. Pacak, W. M. Linehan, G. Eisenhofer, et al.: Recent Advances in Genetics, Diagnosis, localization, and Treatment of Pheochromocytoma. Annals of Internal Medicine 134, 315 (2000). R M. Stewart: Mineralocorticoid hypertension. Lancet 353, 1341 (1999). R C. White: Disorders of aldosterone biosynthesis and action. New England Journal of Medicine 331, 250 (1994). E C. White and R W. Speiser: Congenital adrenal hyperlpasia due to 21hydroxylase deficiency. Endocrine Reviews 21,245 (2000).


CHAPTER

33


The thyroid gland consists of two lobes connected by an isthmus and is positioned on the ventral surface of the trachea just below the larynx. It receives adrenergic fibers from the cervical ganglion and cholinergic fibers from the vagus and is profusely vascularized by the superior and inferior thyroid arteries. Histologically, thyroid tissue is composed of numerous follicles lined by a single layer of epithelial cells around a lumen filled with proteinaceous material called colloid. The follicle cells, called thyrocytes, produce the thyroid hormones and are derived from the entodermal pharynx. Interspersed between follicles are specialized APUD cells derived from the neural crest, called C-cells or parafollicular cells. These cells produce calcitonin (CT), a polypeptide hormone discussed in Chapter 37.

33.1 Structure-Activity of Thyroid Hormones The generic term "thyroid hormones" refers to the iodinated amino acid derivatives T3 (3,3',5-triiodo-Lthyronine) and T4 (3,3',5,5'-tetraiodo-L-thyronine), the only iodinated hormones produced endogenously. T3 is the biologically active hormone and is, for the most part, produced from T4 in extrathyroidal tissues. T4 lacks significant bioactivity and is a hormone precursor; however, A list of expanded acronyms appears as Appendix VIII.

since thyroidal production of T4 and its handling by the body are the pivotal determinants of thyroid hormone effects, the term "thyroid hormone" includes T4. The basic structure of T3 and T4 is that of thyronine, a substituted tyrosine [0-(4-hydroxyphenyl)tyrosine] (Figure 33-1). Iodine residues at positions 3 and 5 of the inner phenolic ring confer on the outer ring a preferred orientation approximately 120 ~ to the plane of the inner ring (Figure 33-2). Hormonal activity is maximal when the following requirements are met: 1. Iodine is present at positions 3 and 5 of the inner ring (required for receptor binding). A natural analog of T3, called reverse T3 (rT3, 3,3',5'-triiodo-L-thyronine), is inactive. Substitution of iodine by bromine or a methyl group almost completely abolishes the activity. 2. A substituent is present at position 3', but it need not be iodine. One of the most potent synthetic thyroid hormones has an isopropyl group at position 3'. 3. A hydroxyl group is present at position 4'. This group forms a hydrogen bond with the receptor protein. 4. A substituent is absent at position 5'. This permits full expression of the activity of the hormone. A substituent at this position would cause steric hindrance in binding with receptor and interfere with hydrogen bond formation. Thus, T4 lacks significant activity whereas T3 has about six times the activity of T4.

769

770

CHAPTER33 Endocrine Metabolism IV: Thyroid Gland

Diphenylether I

i

NH=

,/ I

\

I

Alanine

I

Diiodotyrosine

F I G U R E 33-1 Structure of 3,3',5,5'-tetraiodo-L-thyronine (T4). T4 is a substituted tyrosine or a diphenylether derivative of alanine. Positions in the outer phenolic ring have prime designations, in contrast to those in the inner ring. T4 is iodinated at positions 3 and 5 in both rings. [Modified and reproduced with permission from V. Cody, Thyroid hormone interactions: molecular conformation, protein binding, and hormone action. Endocr. Rev. 1, 140 (1980). @ 1980 by the Endocrine Society.]

5. The molecule must be the L form. D Isomers have minimal biological activity; for example, D-T3 has about 7% the activity of L-T3.

33.2 Thyroid Hormone Synthesis Two substrates are required in the synthesis of thyroid hormones. The intrinsic substrate is thyroglobulin (Tg),

F I G U R E 33-2 Skewed conformation of T4. The molecule shown at the top is viewed in parallel to the plane of the inner phenolic ring, while that shown at the bottom is viewed perpendicularly to the plane of the inner ring. The ether linkage between the two rings is angled at about 120 ~ [Reproduced, with permission from V. Cody, Thyroid hormone interactions: molecular conformation, protein binding, and hormone action. Endocr. Rev. 1, 140 (1980). @ 1980 by the Endocrine Society.]

a homodimeric glycoprotein (M.W. 669,000) synthesized in the rough endoplasmic reticulum of the thyrocytes and secreted into the follicular lumen by exocytosis. Tg contains 134 tyrosyl residues, only 25-30 of which undergo iodination and only four of which become a hormonogenic segment of the molecule. The extrinsic substrate is elemental iodine, present in food as inorganic iodide. Iodide is readily absorbed via the small intestine; it is almost entirely removed from the general circulation by the thyroid and kidney. Iodide that is taken up by salivary and gastric glands is secreted in salivary and gastric fluids and is returned to the plasma iodide pool by intestinal reabsorption. A small amount of iodide is taken up by the lactating mammary gland and appears in milk. The synthesis and release of thyroid hormone involves a number of steps (Figure 33-3).

1. Uptake oflodide.Iodide (I-) is actively taken up by the follicular cells against electrical and concentration gradients. The active uptake of I- is mediated by the Na+/I - symporter, an intrinsic membrane protein with approximately 13 transmembrane segments. The Na + and I- bound symporter releases both Na + and I- on the cytoplasmic side. The empty symporter then returns to its original conformation exposing binding sites on the external surface of the cell. The internalized Na + is pumped out of the cell by the ATP-dependent Na +, K+-ATPase (oubain sensitive) to maintain an appropriate ion gradient. In the normal gland, the limiting step of thyroid hormone synthesis is uptake of I-. Both I- and thyroid-stimulating hormone (TSH) regulate the Na+/I - symporter function. TSH promotes I- uptake whereas excess Idecreases I- uptake (discussed later). Anions of similar charge and ionic volume (e.g., C10 4, BF-, TcO 4, SCN-) compete with I- for transport in the follicular cell. 2. Activation and Organification of lodide. Iodide that enters thyrocytes is "activated" by oxidation that is catalyzed by thyroperoxidase (TPO). Hydrogen peroxide is required and is supplied by an H202-generating system which may be an NADPH (NADH) oxidase system similar to that of leukocytes (Chapter 15). Thyroperoxidase is a glycosylated heme enzyme that is bound to the apical plasma membrane of the thyrocyte. Thyroperoxidase exists in two molecular forms (M.W. 105,000 and 110,000) and its catalytic domain faces the colloid space. In thyroid autoimmune disorders, one of the major microsomal antigenic components is thyroperoxidase. In vitro, iodide peroxidase catalyzes the reaction 2H+§ 21-+ H202 -+ 12 -4- 2H20. However, in vivo

SECTION 33.2

Thyroid Hormone Synthesis

771

33-3 Synthesis of thyroidhormonein the thyrocyte.Tg, Thyroglobulin.

FIGURE

the enzyme probably forms an enzyme-bound iodinium ion (E-I +) or a free radical of iodine. These activated derivatives iodinate the tyrosyl residues of Tg ("organification of iodide") by the action of thyroperoxidase and H202. Thionamides (e.g., thiourea, thiouracil, propylthiouracil, methimazole) inhibit this organification of iodide without affecting iodide uptake. The first position iodinated is position 3, which forms monoiodotyrosyl-Tg (MIT-Tg). The second is position 5, which forms diiodotyrosyl residues (DIT-Tg). Iodination is accompanied by structural changes (e.g., cystine formation) and includes a conformational change in the Tg molecule (illustrated diagrammatically in Figure 33-3). Activation and organification occur at the cell-colloid interface where thyroperoxidase activity is prevalent. 3. Coupling Reaction and Storage as Colloid. Iodide peroxidase or a "coupling enzyme" catalyzes the "coupling reaction" at the cell-colloid interface by intramolecular coupling of two iodotyrosyl residues with formation of an iodothyronyl residue. Coupling of DIT residues is favored; thus, formation of T4

residues predominates. The coupling reaction requires that both substrates be iodinated and that one of the substrates be DIT-Tg. For this reason, no To, T1, or T2 residues are formed. Under normal conditions less than half of the iodotyrosyl residues in Tg undergo coupling such that of the total iodinated residues in Tg, 49% are MIT, 33% DIT, 16% T4, 1% T3, and a trace amount rT3. The ratio of T4 to T3 is > 10 on a typical American diet, but it rises or falls with dietary iodine content. The coupling reaction appears to be the most sensitive to inhibition by the thionamides (propylthiouracil, methimazole), being inhibited at doses that do not inhibit the organification reactions. As a final note, although proteins other than Tg are iodinated in nonthyroid tissues that take up iodine (e.g., salivary gland, mammary gland, intestinal mucosa), no coupling of the iodinated residues occurs. Thus, the requirements for the coupling reaction may be more stringent than previously believed. 4. Processing of TG and Release of Thyroid Hormone. Thyroglobulin, with its tetraiodothyronyl residues, is a pre-prohormone stored in the follicular lumen.

772

CHAPTER 33

When follicle cells are stimulated by TSH or TSI (see below), their luminal border ingests colloid (and hence Tg) into endocytotic vesicles. These vesicles fuse with lysosomes to form phagolysosomes, which, during transit toward the basolateral surface of the cell, hydrolyze Tg and release T4, DIT, MIT, and a small amount of T3. These substances are released into the cytosol, and T4 and T3 diffuse into the blood. MIT and DIT do not enter the circulation in significant amounts because they are rapidly deiodinated by iodotyrosine deiodinase, an enzyme complex that contains ferredoxin, NADPH: ferredoxin reductase, and a deiodinase containing flavin mononucleotide (FMN). This reaction promotes recycling of iodide within the follicle cell. Some of the iodide, however, diffuses into plasma and constitutes the daily "iodide leak," estimated to be about 16 #g/d. By mechanisms that are not clear, a small amount of intact Tg also leaks out of the thyroid and can be found circulating in plasma in normal individuals. Tg leakage at a rate of 100 #g/d has been reported in euthyroid individuals, and a concentration of 15-25 #g Tg per liter of serum is considered to be normal. The route of Tg leakage is by way of the lympathic drainage and is increased when the gland is excessively stimulated. Increased entry of Tg into the circulation may result in an immune response because of the antigenic nature of this glycoprotein. The release of thyroid hormone following endocytosis of colloid is inhibited by iodide and is known to be reduced with high dietary intake of iodine. This inhibition is due to the inverse relationship between the iodide content of Tg and the digestibility of iodo-Tg by lysosomal peptidase; that is, poorly iodinated Tg is more readily digested than richly iodinated Tg.

Regulation of Thyroid

H o r m o n e Synthesis

Thyroid-Stimulating Hormone (TSH) TSH (also called thyrotropin), which is secreted by the pituitary, plays a central role in the regulation of growth and function of the thyroid gland. TSH receptors are functionally coupled to G-proteins; thus, the extracellular stimulus by TSH is transduced into intracellular signals mediated by a number of G-proteins. Activation of Gs-protein results in the stimulation of the adenylate cyclase-cAMP-protein phosphorylation cascade. Other G-proteins coupled to TSH-receptor activation include GQ-protein, which mediates the phospholipase C-phosphatidylinositol 4,5-bisphosphate-Ca 2+ signaling pathway (see Chapter 30 for a detailed discussion).


The TSH receptor is a glycoprotein that consists of three major domains: a long amino-terminal extracellular segment that confers binding specificity, a transmembrane segment, and a carboxy-terminal intracytoplasmic segment that mediates G-protein interactions (see Figure 30-6). TSH-receptor activation promotes iodide trapping, organification of iodide, and endocytosis and hydrolysis of colloid by a mechanism that involves cAME Thyroid disorders consisting of either hypofunction or hyperfunction can result form constitutive inactivating or activating mutations, respectively, in the TSH receptor. Similarly, Gs~-inactivating (loss of function) or activating (gain of function) mutations can result in hypothyroidism and hyperthyroidism. An example of the former is Albright hereditary osteodystrophy and the latter is McCune-Albright syndrome. Both syndromes are discussed in Chapter 30. All of the effects of TSH on the thyroid gland are exaggerated in hyperthyroidism due to excessive stimulation of the TSH receptor whether or not the ligand is TSH. For example, in Grave's disease hyperactivity of the thyroid gland is due to a thyroidstimulating antibody that binds the TSH receptor and, in so doing, activates it. This thyroid-stimulating immunoglobulin (TSI) or long-acting thyroid stimulator (LATS) is an expression of an autoimmune disease that may be hereditary, although in some cases it may be the result of a normal immune response. For example, certain strains of bacteria contain a membrane protein which is homologous to the TSH receptor and which elicits an immune response in infected individuals. The antibodies generated resemble TSI. Thus, some cases of hyperthyroidism may be due to cross-recognition across phylogenetic lines (molecular mimicry). Human chorionic gonadotropin (hCG), a placental hormone, has some TSH-like activity, hCG synthesis is initiated during the first week after fertilization and reaches its highest level near the end of the first trimester, after which levels decrease. The action ofhCG on thyrocytes may help meet the increased requirement of thyroxine during pregnancy. The total thyroxine pool is increased in pregnancy due to elevated levels of thyroxine-binding globulin. However, the serum free T4 level, which represents the biologically active form, remains the same compared to nonpregnant status. During the first trimester, the fetal requirement for thyroxine is met by maternal circulation until the fetal pituitary-thyroid axis becomes functional late in the first trimester. The maternal-fetal transport of thyroxine which occurs throughout pregnancy minimizes the fetal brain abnormalities in fetal hypothyroidism (discussed later). Maternal hypothyroidism (e.g., due to women consuming iodine-deficient diets) is associated with defects in the neurological functions of offspring.

SECTION

33.2 Thyroid Hormone Synthesis

773

Cold

Stress

Other] P'~--,O/ stimuli/7 Adrenergicneural network J i Norepinephrine i ,,

(9

: I Hypothalamus

TRH

Som~Starvation

an

Estrogens

(chronic)

1

TSI

TSH

[Thy;oidi r4

Peripheral tissues

; l~

J

Starvationand Non-thyroidal illness

"1"3----.-Targetcells

: Activation of metabolism,required for growth differentiation and mental development

FIGURE 33-4 Schematicrepresentationof the major stepsin the regulationof thyroidhormonesecretionsand metabolismat five levels, namely,brain, hypothalamus,pituitarythyrotropes,thyroid, and peripheraltissues.

Release of TSH from the anterior pituitary is stimulated by hypothalamic TRH and inhibited by thyroid hormone (Chapter 31). These opposing signals to the anterior pituitary determine the magnitude of TSH secretion. Many other factors influence secretion of TSH and thyroid hormone (Figure 33-4). For example, starvation reduces conversion of T4 to T3 in peripheral tissues but not in the anterior pituitary; consequently, TSH secretion is not suppressed but circulating T3 levels fall. Stress, somatostatin, estrogens, and cold exposure (in infants) also affect TSH release, but except for cold stress in infants, they are not regarded as important regulators of TSH and thyroid hormone secretion. Iodine

The thyroid gland is capable of adjusting its synthetic activity to the supply of iodine. When the iodine supply

is low, the thyroid makes maximal use of iodide; when the iodide supply is abundant, the thyroid defends itself against hormone overproduction by reducing iodide uptake. The thyroid gland autoregulates its iodine supply by an internal feedback mechanism that controls the intraglandular handling of iodide and the thyroid response to TSH. The exact nature of this feedback is not known, but the amount of the organified iodide pool is believed to have an inhibitory effect on the rate of iodide uptake, intraglandular T4/T3, the completeness of thyroglobulin hydrolysis, and the responsiveness to TSH. This autoregulatory mechanism is not prominent when iodine intake is within the normal range but becomes physiologically significant when there is a moderate deficiency or excess of iodine. On an ordinary diet in which the iodine intake ranges between 300 #g and 1000 #g/d, T4 production increases linearly with increased consumption

774

CHAPTER 33

of iodine. With lower iodine intake (e.g., 50-150 #g/d), T4 production increases due to increased efficiency in iodide processing and to a greater response to TSH; in fact, although plasma TSH levels are within the normal range, a mild enlargement of the gland is not an uncommon consequence (e.g., nontoxic goiter). On the other hand, with a diet enriched in iodine (e.g., 1-2 mg/d), the increase in T4 production as a function of iodine consumption is subnormal, due to negative feedback of the organified iodide pool on T4 production and the thyroid response to TSH; as a result, plasma TSH levels are maintained within the normal range. Abnormal thyroid function is seen at the extremes of iodine intake: at the lower end, less than 50 #g/d; at the upper end, 5 mg/d or more. At the lower end, frank hypothyroidism occurs because there is an insufficient supply of iodine to meet the T4 production demands despite the efficiency with which the limited iodide is used. In this condition, the iodide deficiency creates an organified iodide pool consisting of MITs and a paucity of DITs, and, consequently, little or no coupling occurs because substrate requirements are not met. Products of Tg digestion are mainly MITs, small amounts of DITs, and possibly some T3 and T4. The decrease in thyroid hormone increases output of TSH which, in its futile attempt to increase T4 production, creates glandular overgrowth due to thyrocyte hypertrophy and colloid accumulation ("hypothyroid goiter," "colloid goiter," or "endemic goiter"). At the upper end, further increase in iodine intake causes a shut-down of thyroid gland function and acutely reduces the thyroid response to TSH; consequently, there is a paradoxical decline in T4 production as a function of iodine consumption accompanied by a dramatic reduction in vascular supply to the gland. This phenomenon is called the Wolff-Chaikoff effect and is observed only in


thyroid glands that are stimulated (e.g., with TSH or in hyperthyroidism).

33.3 Transport and Metabolism of Thyroid Hormones Thyroid hormones in blood are mainly bound to serum proteins. The major thyroid hormone-binding protein in blood is thyroxine-binding globulin (TBG), a glycoprorein (M.W. 63,000) of electrophoretic mobility between that of ot 1- and ot2-globulins. TBG exhibits a high affinity for T4 (Kd -- 50 pM) and a moderate affinity for T3 (Ko = 500 pM). Approximately three-fourths of circulating T4 is firmly bound to TBG, while about one-half of circulating T3 is TBG-bound (Table 33-1). Thyroxine-binding prealbumin (TBPA), more properly known as transthyretin, which functions in the transport of retinol, has about 1% of the affinity of TBG for T4, but it accounts for about 20% of circulating T4 and does not bind T3. These differences in binding can be partially explained, since T4 is almost completely ionized at physiological pH, whereas T3 is not. The 4'-hydroxyl group of T4 probably exists as a 4'-phenoxide ion, which optimizes binding to serum proteins. However, the 4'-hydroxyl group is required for receptor binding. Binding to albumin accounts for much of the circulating T4 and T3 that is not bound to TBG and TBPA. About 0.03% of T4 and about 0.3% of T3 is unbound, so that only a small fraction is accessible for target cell uptake. The albumin-bound hormone dissociates rapidly in the vicinity of target cells. The cellular uptake of T4 and T3 is mediated by carrier-mediated processes at stereospecific binding sites. At high-affinity sites the uptake is energy, temperature, and often Na +dependent. T3 is metabolized about six times more

TABLE 33-1

Source, Transport, and Metabolism of the Thyroid Hormones

T4 3, 5, 3, 5'-Tetraiodo-L-thyronine Molecular weight Production rate (pg/day) % Produced by thyroid gland Half-life Total plasma concentration % Free % Bound to plasma proteins % Bound to TBG % Bound to albumin % Bound to transthyretin

777 90 100 7 days 51-142 nmol/L (4-11 gg/dl) 0.02% 99.98% 70-75 5-10 15-20

3, 5,

T3 3'-Triiodo-L-thyronine

651 35 20 0.75 day 1.2-3.4 nmol/L (75-220 ng/dl) 0.3% 99.7% 70-75 20-25 0-5

SECTION 33.3 Transport and Metabolism of Thyroid Hormones

775

rapidly than T4 and has a metabolic clearance rate (MCR) 25 times as high as that of T4 (Table 33-1). Mutations in a human serum albumin gene that substitute either histidine or proline for arginine at position 218 increase binding affinity for T4. These mutations are autosomal dominant and occur with relatively high frequency. Carriers of these mutations have high levels of total serum T4 but their free-T4 and TSH concentrations are within the normal range. Individuals are euthyroid (normal thyroid) and their conditions are known as familial dysalbumine-

mic hyperthyroxinemia (FDH). Two reactions account for the metabolic fate of about 80% of the T4 in plasma: about 40% is converted to T3 via 5'-deiodination (activation), and another 40% of the T4 is converted to rT3 by 5'-deiodination (inactivation). These two reactions are catalyzed by three enzymes designated types I, II, and III iodothyronine deiodinases (Figure 33-5 and Table 33-2). Types I and II both catalyze the 5'-deiodination reaction but differ with respect to substrate specificity, tissue distribution, and regulation. Type III is a 5-deiodinase, which catalyzes the removal of iodine from position 5 of the inner ring. Type I is deiodinase selenocysteine-containing microsomal enzyme present in the liver, kidney, and thyroid, with specificity for

HO-- ii>--O __

thyronines bearing iodine at position 5'. It is responsible for catalyzing the formation of most of the circulating T3 and is a high-capacity (Km ~1 #M) processor ofT4. A unique feature of the type I enzyme is that it is sensitive to inhibition by propylthiouracil (PTU), a known inhibitor of thyroid gland function that has little or no effect on the activity of the other deiodinases. It is also inhibited in conditions such as starvation, caloric restriction, and nonthyroid-related illnesses, which may deplete the cofactor supply and/or depress enzyme synthesis. The type II deiodinase is less abundant and has a higher specificity and a higher affinity for T4 than type I. The type II enzyme is present in the pituitary, brain, brown fat, and skin, and is responsible for intracellular T3 formation that leads to a cellular response; whereas the type I enzyme produces T3 for export to other cells, and the type II enzyme is selfserving, producing its own supply of intracellular T3. A unique feature of the type II enzyme is that it is sensitive to inhibition by T4 by an unknown mechanism. The activity of the type II enzyme is increased in hypothyroidism and is depressed in hyperthyroidism, whereas these conditions have the opposite effect on the activity of the other two deiodinases. The type III deiodinase is mainly involved with the degradation of T3 and the inactivation of

H2 CHE--'CH---COOH

If (thyroxine)

5"-Deiodinase/

(Type I and II)

~5-Deiodinase

/

HO' / _I . ~

.... y-c.

c

(Type III)

cooH

i/

i/

r~ (inactive)

T3 (active hormone) 5-Deiodinase (Type III)

COOH 3,3"-T2 (inactive) FIGURE 33-5 Activating and inactivatingpathwaysof T4.

776

CHAPTER 33


TABLE 33-2

The Iodothyronine Deiodinases Type I Reaction catalyzed Substrate preference Tissue distribution Affinity (Km) for T4 Effect of: Iopanoic acid* Amiodarone** Fetal life Propylthiouracil (PTU) Selenium deficiency Starvation Caloric restriction Glucocorticoids$ Excess glucocorticoids Nonthyroid illnesses High carbohydrate diet Hypothyroidism Hyperthyroidism Propranolol

5'-deiodination (outer ring) rT3 > T4 > T3 Liver, kidney, thyroid, et al. Low (--1 gM) Decrease Decrease Decrease Decrease Decrease Decrease Decrease Increase Decrease Decrease Increase Decrease Increase Decrease

Type II

Type III

5'-deiodination (outer ring) 5-deiodination (inner ring) T4 > rT3 T3 > T4 Brain, pituitary, skin, brown fat Liver, kidney, et al. High (-~1 nM) High (-40 nM) Decrease Decrease Decrease None None None None None None None None Increase Decrease None ?

Decrease Decrease ? None

Decrease Increase ?

*Iopanoic acid (3-amino-a-ethyl-2,4,6-triiodo-hydrocinnamicacid) is a radiopaque iodine compound used as a contrast medium in cholecystography. **Amiodarone (2-butyl-3-benzofuranyl) [4- [2- (diethylamino)ethoxyl]-3,5-diiodo-phenyl] methanone) is a coronary vasodilator used in the control of ventricular arrhythmias, and in the managementof angina pectoris. ;The physiologicrise in cortisol levels at about the time of birth stimulates 5'-deiodinase and causes a rise in T3 and fall in rT3. The effect of physiologic levels of cortisol on 5'-deiodinase in adults is unclear.

T4, both involving the removal of the inner ring 5-iodide. The type III enzyme is widely distributed among tissues and is responsible for the formation of almost all (95%) of the circulating rT3. Its activity is not altered by most of the factors or conditions that inhibit the activity of types I or II deiodinases, and this explains why there is a reciprocal relationship between the plasma levels of T3 and rT3. Not all metabolic processing of iodothyronines involves deiodination, although it is the major metabolic route for T4, T3, and rT3, accounting for about 80% of the fate of each compound. The remaining 20% is conjugated in the liver with sulfate or glucuronide, and/or processed by oxidative deamination and decarboxylation to form thyroacetic acid derivatives with minimal biological activity (Figure 33-6). Inorganic iodide liberated from deiodinations enter the extracellular compartment and is excreted in the urine (~488 #g/d) or stool (~12 #g/d via bile), or is reabsorbed by the thyroid gland (~108 #g/d) for resynthesis of thyroid hormones. At the average daily intake of ~500 #g inorganic iodide, this amounts to no net change in the iodide status, although the small amount of iodide that

is lost through the skin and gastrointestinal tract would place the body in a negative iodide balance unless intake is adjusted accordingly.

33.4 Biological Actions of Thyroid Hormones M e c h a n i s m of Action and Latency Period All but a few of the thyroid effects that have been identified occur at the level of gene transcription, mediated by nuclear thyroid hormone receptors (Chapter 30). These effects have a longer latency period than for most steroids; some of the relatively early responses show a latency period of several hours.

Physiological Effects

Cardiovascular System Thyroid hormone enhances cardiac contractility and exerts a positive chronotropic effect on the heart, increasing heart rate by a mechanism that may involve more than a potentiation of the fi-adrenergic effect. In the

SECTION 33.4

Biological Actions of Thyroid Hormones

Hydrolysis of conjugate Intestine*---- Bile :

777

= (Enterophepatic circulation)

Glucumnide/SO~=- ~ conjugation (liver) ~ "

TRIAC o (+ active)

~20%

5-Deiodination

To ~

Deaminationdecarboxylation

3"-T, ~ (inactive)

Liberated I -

L

3,3"-% (inactive)

- Thyroids or urine

Stool

Hydrolysis ----* (Enterophepatic of conjugate circulation) Intestine ~

Bile ~

Glucuromde/SO4 conjugation

(liver)

/

.,.

~

~20%

IT J

~80%

i

Deamination-

~50Olo

decarboxylation

5"-Deiodination

-30o/o 5"-Deiodination

L

TETRAC : - - - ' TRIAC (_+active) "'-., rTRIAC

(inactive)~ Undergoes rapid /

5"deiodinatio~)

To" Stool

3"-T~ (inactive)

3,3"-% " (inactive)

(active) ,~, [T3 P~176

IT, contributes to the % pool]

FIGURE 33-6 Metabolism of T3 and T4. To, Thyronine; 3,3'-T2, 3,3~-diiodothyronine; 3~-T, 3'-iodothyronine; TETRAC, 3,T, 5,5~-tetraiodothyroacetic acid; TRIAC, 3,3~,5-triiodothyroacetic acid.

ventricular myocardium, thyroid hormone stimulates the synthesis of myosin heavy chain o~(MHCot), while inhibiting the synthesis of MHC/3, by regulating the expression of the respective genes. The result is that myocardium with a higher MHCo~ content has higher calcium and actin-activated ATPase activities and increased velocity of muscle fiber shortening. Thus, the thyroid hormoneconditioned ventricle is capable of increased contractile performance. Intermediary Metabolism

Thyroid hormone increases both lipolysis and lipogenesis, although lipogenesis is stimulated before lipolysis, due to early induction of malic enzyme (malate dehydrogenase), glucose-6-phosphate dehydrogenase, and fatty acid synthase. Thyroid hormone lowers serum cholesterol levels by maintaining low-density lipoprotein (LDL) receptor density, by maintaining hepatic lipase activity which catalyzes conversion of intermediate density lipoproteins

(IDL) to LDL, and probably also by maintaining lipoprotein lipase activity which promotes triglyceride clearance. In liver, kidney, skeletal muscle, cardiac muscle, and adipose tissue, thyroid hormone stimulates Na+,K+-ATPase gene expression and promotes thermogenesis. Growth and Maturation

Thyroid hormone stimulates production of IGF-I directly (liver) and indirectly (via increased growth hormone, GH). In adults with hypothyroidism, basal serum GH levels are normal, but the GH responses to provocative stimuli, nocturnal GH secretion, and serum IGF-I are all subnormal. In cretinism (infantile hypothyroidism), linear growth is severely retarded and the resultant dwarfism is characterized by a retention of the high upper to lower body ratio of infancy. The growth retardation in persons with cretinism is primarily due to delayed appearance of ossification centers in long bone, and secondarily to a deficiency in growth factors.

CHAPTER33 Endocrine Metabolism IV: Thyroid Gland

~8

A deficiency of thyroid hormone during fetal development due to untreated or undertreated maternal hypothyroidism results in a neurological deficit in the offspring. In congenital hypothyroidism, a normal maternal thyroid can meet the fetal requirements for thyroid hormone. However, in the postnatal period these infants require a prompt thyroid hormone replacement therapy throughout their life, beginning in the first few weeks of life. If untreated, they inevitably develop growth and mental retardation. Thyroid hormone is essential for maturational development of the CNS and is required for the development of axonal projections and myelination. One of the important effects of thyroid hormone is to promote the synthesis of myelin basic protein. Congenital hypothyroidism in Western populations occurs in about 1 in 4000 births. Congenital hypothyroidism is caused by embryogenic defects leading to thyroid agenesis (50-55%), dysgenesis (30-35%), errors in the synthesis of thyroid hormone ( 10-15 %), and defects in the pituitary-hypothalamic axis (4%). Neonatal screening for congenital hypothyroidism is performed using blood collected on filter paper from newborns older than 12 hours. The laboratory test consists of measuring T4, TSH, or both. Abnormal values must be reconfirmed and additional tests include other parameters (e.g., TBG).

Reproductive System Thyroid hormone increases total plasma androgen levels by increasing the production of testosterone-binding globulin (TeBG) by the liver. Hyperthyroidism is associated with increased plasma TeBG levels, higher total testosterone levels, but normal free testosterone levels in adult men. There is a high incidence of gynecomastia in hyperthyroid men (40-83%) due to a higher circulating level of free estradiol in plasma. The elevated free-estradiol level may be explained by the lower affinity of estrogen for TeBG and also by increased conversion of androgen to estrogen in hyperthyroidism. In prepubertal boys, but not in older boys or adult men, hypothyroidism is associated with a high incidence of increased testicular growth (macroorchidism) that is not accompanied by any change in testosterone levels. Elevated follicle-stimulating hormone levels are found in most cases, but there is no testicular maturation. The mechanism of this disorder is not known.

Pharmacological Effects The effects of thyroid hormone may persist for up to a week because of its long half-life in the circulation, its long-acting effect on the genome of the target cell, and the slow rate of recovery of some target tissues (e.g., brain).

Thyroid hormone excess depletes body energy stores and imparts hypersensitivity of tissues to catecholamines. Protein synthesis is inhibited, protein catabolism is accelerated, and the antianabolic actions of cortisol are enhanced. During the period of growth, these factors would lead to growth suppression, muscle weakness, weight loss, and depletion of liver and muscle glycogen stores. Gluconeogenesis is fueled by elevated substrate levels (amino acids, glycerol, lactate, and pyruvate), and this contributes to the blood glucose pool; however, since peripheral glucose utilization is also increased, hyperglycemia is not severe. Thyroid excess also leads to depletion of triacylglycerol stores, mainly due to its action on catecholamine-induced lipolysis. Tissue consumption of free fatty acids and ketones is also increased. Thus, serum lipid levels fall, including the level of serum cholesterol. In essence, the storage forms of glucose, amino acids, and fatty acids are depleted, and these substrates are rapidly metabolized, resulting in the need for increased food intake. Because thyroid hormone increases tissue responsiveness to catecholamines, many symptoms of thyroid hormone excess are those that characterize catecholamine excess. Effects of T3 on cardiovascular hemodynamics consist of increased tissue thermogenesis, decreased systemic vascular resistance, decreased effective arterial filling volume, increased renal sodium reabsorption, increased blood volume, increased cardiac inotropy and chronotropy leading to increased cardiac output. Behavioral changes (such as nervousness, restlessness, short attention span, and emotional lability) are common, some of which require a longer period for decay than do the metabolic changes following normalization of thyroid hormone levels. Clearly, the central actions of thyroid hormone involve more than potentiation of adrenergic neurotransmission. Thyroid disorders. Disturbances in thyroid metabolism can occur at any level of the hypothalams-pituitarythyroid-peripheral tissue axis. Several of these disorders have been discussed previously. Hyperthyroidism is more prevalent in women than men. The three most common causes of hyperthyroidism are Graves' hyperthyroidism, toxic multinodular goiter, and toxic adenoma. The clinical features of hyperthyroidism include hyperkinesis, weight loss, cardiac anomalies (e.g., atrial fibrillation), fatigue, weakness, sweating, palpitations, and nervousness. The typical biochemical laboratory parameters are increased serum free T4 and decreased serum TSH. The most common cause of hypothyroidism is failure of the thyroid gland; this is known as primary hypothyroidism. In adults, the cause of primary hypothyroidism is often spontaneous autoimmune disease (e.g., Hashimoto's thyroiditis) or destructive therapy for hyperthyroid states


(e.g., Graves' disease). In adults, hypothyroidism has an insidious onset with a broad range of symptoms. The typical biochemical laboratory parameters include the measurement of serum free T4 and TSH, which are decreased and increased, respectively. In addition, measurement of serum autoantibodies for thyroglobulin and thyroperoxidase is also utilized.

Supplemental Readings and References D. L. Bodenner and R. W. Lash: Thyroid disease mediated by molecular defects in cell surface and nuclear receptors. American Journal of Medicine 105, 524 (1998). S. C. Boyages: Clinical review 49: Iodine deficiency disorders. Journal of Clinical Endocrinology and Metabolism 77, 587 (1993). G. A. Brent: The molecular basis of thyroid hormone action. New England Journal of Medicine 331, 847 (1994). H. B. Burch and L. Wartofsky: Graves' ophthalmopathy: current concepts regarding pathogenesis and management. Endocrine Review 14, 747 (1993). G. N. Burrow: Thyroid function and hyperfunction during gestation. Endocrine Review 14, 194 (1993). G. N. Burrow, D. A. Fisher, and P. R. Larsen: Mechanisms of disease: maternal and fetal thyroid function. New England Journal of Medicine 331, 1072 (1994). I. J. Chopra: Clinical review 86: Euthyroid sick syndrome: is it a misnomer? Journal of Clinical Endocrinology and Metabolism 82, 329 (1997). D. Glinoer: The regulation of thyroid function in pregnancy: pathways of endocrine adaptation from physiology to pathology. Endocrine Review 18, 404 (1997). M. A. Lazar: Thyroid hormone receptors: multiple forms, multiple possibilities. Endocrine Review 14, 184 (1993). J. H. Lazarus: Hyperthyroidism. Lancet 349, 339 (1997). R. S. Lindsay and A. D. Toft: Hypothyroidism. Lancet 349, 413 (1997). S. M. McLachlan and B. Rapoport: The molecular biology of thyroid peroxidase: cloning, expression and role as autoantigen in autoimmune thyroid disease. Endocrine Review 13, 192 (1992).

779 G. Medeiros-Neto, H. M. Targovnik, and G. Vassart: Defective thyroglobulin synthesis and secretion causing goiter and hypothyroidism. Endocrine Review 14, 165 (1993). A. J. Mixson, R. Parrilla, S. C. Ransom, et al.: Correlations of language abnormalities with localization of mutations in the B-thyroid hormone receptor in 13 kindreds with generalized resistance to thyroid hormone: identification of four new mutations. Journal of Clinical Endocrinology and Metabolism 75, 1039 (1992). J. C. Morris: The clinical expression of thyrotropin receptor mutations. Endocrinologist 8, 195 (1998). R. Paschke and M. Ludgate: Mechanism of disease: the thyrotropin receptor in thyroid disease. New England Journal of Medicine 337, 1675 (1997). S. P. Porterfield and C. E. Hendrich: The role of thyroid hormones in prenatal and neonatal neurological development--current perspectives. Endocrine Review 14, 94 (1993). S. Refetoff, R. E. Weiss, and S. J. Usala: The syndromes of resistance to thyroid hormone. Endocrine Review 14, 348 (1993). E. Roti, R. Minelli, E. Gardini, and L. E. Braverman: The use and misuse of thyroid hormones. Endocrine Review 14, 401 (1993). V. Singh and J. P. Catlett: Hematologic manifestations of thyroid disease. Endocrinologist 8, 87 (1998). M. I. Surks and R. Sievert: Drug therapy: drugs and thyroid function. New England Journal of Medicine 333, 1688 (1995). A. D. Toft: Thyroxine therapy. New England Journal of Medicine 331, 174 (1994). Y. Tomer and T. E Davies: Infection, thyroid disease, and autoimmunity. Endocrine Review 14, 107 (1993). J. I. Totrtns and H. B. Burch: Serum thyroglobulin measurement: utility in clinical practice. Endocrinologist 6, 125 (1996). G. Vassart and J. E. Dumont: The thyrotropin receptor and the regulation of thyrocyte function and growth. Endocrine Review 13, 596 (1992). A. De La Vieja, O. Dohan, O. Levy, et al.: Molecular analysis of the sodium/iodide symporter: impact on thyroid and extrathyroid pathophysiology. Physiological Reviews 80, 1083 (2000). A. P. Weetman and A. M. McGregor: Autoimmune thyroid disease: further developments in our understanding. Endocrine Review 15, 788 (1994). I. Klein and K. Ojamaa: Thyroid Hormone and the Cardiovascular System. New England Journal of Medicine 344, 501 (2001).


CHAPTER

34

Endocrine Metabolism V: Reproductive System

The sex of an individual is the outcome of genetic determinants (genotypic sex) and of hormonal effects that confer structural features characteristic of a given sex (phenotypic sex). Both genetic and hormonal determinants normally operate only at two phases of life: during fetal development and at puberty. Genotypic sex is either homogametic (XX) or heterogametic (XY). In humans and other mammals, the homogametic (XX) pairing programs ovarian development and oocyte formation (oogenesis), while the heterogametic (XY) pairing leads to testicular development and spermatogenesis. In both genotypes, the embryonal gonads develop from the epithelium and stroma of the urogenital ridge, a thickening of the coelomic (ventromedial) aspect of the mesonephros that emerges at about the second or third week of pregnancy. Both the epithelium and stroma of the urogenital ridge are derived from the intermediate mesoderm of the embryo; however, invading this structure at about week 4 are primordial germ cells from the yolk sac, which take residence in association with the epithelial cells of the developing gonad and replicate. During the germ cell invasion, the epithelial cells of the gonads undergo proliferation and begin entering the stromal spaces as cord-like projections, called "primary sex cords." Until about week 6, the gonads are undifferentiated and uncommitted; that is, the structure has the potential to develop into either ovaries or testes. A list of expanded acronyms appears as Appendix VIII.

The fetal gonads develop in parallel with the Wolffian (mesonephric) duct and the Mtillerian (paramesonephric) duct, both of which are positioned bilaterally to the gonads within the urogenital ridge. The Wolffian duct begins forming from about week 4, starting at the mesonephros and terminating caudally at the cloaca. The Mtillerian duct begins forming from about week 6 and merges caudally with the urogenital sinus. The Wolffian duct is the primordium of the male reproductive tract, whereas the Mtillerian duct is the primordium of the female reproductive tract. Until about week 8, both ducts are present and undifferentiated. In the absence of a Y chromosome, a gene on the X chromosome directs the development of fetal gonads into ovaries. This critical sex-determining gene located on the X chromosome is known as the DAX-1 gene. Subsequent to the action of this gene, the Mtillerian ducts develop into a female genital tract and the Wolffian duct regresses. In the presence of a Y-chromosome, the SRY gene (sexdetermining region of the Y-chromosome) antagonizes the action of the DAX-1 gene (the SRY gene was previously known as testis-determining factor, TDF). Mutations in the SRY gene can result in an XY female with gonadal dysgenesis. Testis differentiation may also require the participation of a gene located on chromosome 17 (SOX9 gene). The precise interaction between SOX9 and SRY in gonadal development is not yet understood; however, both genes code for DNA-binding proteins. Mutations in the SOX9 gene can result in sex reversal in XY individuals

781

782

with camptomelic dysplasia. The latter is a severe form of skeletal dysplasia associated with dysmorphic features and cardiac defects. After the fetal gonads are converted into testes by SRY, testosterone and Mtillerian-inhibiting substance (MIS) control subsequent sexual differentiation. MIS causes the regression of the Mtillerian duct, which would otherwise develop into female genital tracts. Testosterone promotes the development of the Wolffian duct into male internal genitalia, including epididymis, vas deferens, and seminal vesicles. Testosterone also causes differentiation of the urogenital sinus into the prostate gland and masculinizes the external genitalia; however, these tissues must first convert testosterone into dihydrotestosterone (DHT), a reaction catalyzed by cytoplasmic 5ot-reductase type 2. The effects of testosterone on the Wolffian duct and urogenital sinus are dependent on the presence of androgen receptors and 5ot-reductase, which are expressed by genes on the X chromosome and chromosome 2, respectively, and are present in both genotypic sexes. This explains why exposure of the fetus to androgens during the critical period may lead to masculinization of the internal and external genitalia of genotypic female fetuses. It also explains why lack of expression of either gene can lead to feminization in genotypic male fetuses. The gonad has two interrelated functions: gametogenesis and hormone production. Gametogenesis depends on gonadal hormone production, which is influenced by gametogenesis. Both processes are controlled by luteinizing hormone (LH) and follical-stimulating hormone (FSH), which are released in response to hypothalmic gonadotropin-releasing hormone (GnRH) and to the levels of circulating gonadal hormones. In addition, gametogenesis is regulated by paracrine actions of the gonadal hormones. The obvious difference between the sexes in gametogenesis is formation of ova (ootids) in the female and of sperm (spermatozoa) in the male. Spermatogenesis becomes operational from about the time of puberty and continues throughout life. The process takes about 74 days. Oogenesis occurs in three phases. The initial phase involves proliferation of the stem cells (oogonia) and occurs only in fetal life. The second phase involves the first maturational division (formation of the secondary oocyte) and occurs about the time of ovulation (see below). The third phase involves the second and final maturational division (formation of ova) and occurs at fertilization. Because the number of oogonia is determined before birth, the number of fertilizable ova that can be produced in a woman's lifetime is limited and is greatly reduced by normal degenerative processes (atresia), so that of the estimated seven million oogonia in the fetal ovaries,

CHAPTER 34


only about 300-500 ova ultimately develop. Unlike the continuous generation of sperm in the testes, the ovaries generally produce only one secondary oocyte every 22-28 days. The fertile lifetime in the average woman is about 35-40 years. Ovaries and testes produce identical steroid hormones, but the amounts and their patterns of secretion are different (Table 34-1). Although testosterone is present in both males and females, its level in the male is about 18 times that in the female; conversely, circulating levels of estradiol in the female are about 3-15 times those in the male. In either sex, the major sex steroid originates in the gonads, whereas in the opposite sex, this steroid is generated in sustantial amounts by the adrenal cortex or by peripheral conversion of another steroid. For example, almost all of the circulating estradiol in the female comes from the ovaries, whereas in the male, only about one-third comes from the tests, the rest being generated by peripheral conversion of androgenic precursors. Finally, the pattern of gonadal steroid secretion is different. In the male, the secretion of gonadal steroids is fairly constant, although minor fluctuations occur as a result of circadian rhythms. In the adult female, gonadal steroid secretion undergoes dramatic, cyclic changes at about monthly intervals. These cyclic changes, which are dictated by processes regulating oogenesis, are referred to as the menstrual cycle.

34.1 Testes Regulation of Spermatogenesis: Sertoli-Neuroendocrine Axis Sertoli cells are epithelial cells that line the seminiferous tubules of the testes. At their basal aspects, these cells form the basement membrane and tight junctions that make up the highly selective "blood-testes barrier," which normally prevents entry of immune cells into the lumen. Their function is to provide nutritional and hormonal support for cells undergoing spermatogenic transformation (i.e., spermatocytes and spermatids). For this reason, they are often referred to as "nurse cells." Sertoli cells require FSH stimulation for their maintenance and specifically for production of androgen-binding protein (ABP). ABE which has a high affinity for testosterone, maintains high levels of testosterone in the seminiferous tubules and thereby helps maintain spermatogenesis. FSH also stimulates aromatase activity in the Sertoli cell, thus stimulating conversion of testosterone into estradiol. This FSH-sensitive Sertoli cell activity accounts for most of the testicular output of estradiol. FSH secretion by the anterior pituitary is held in check by negative feedback from the testes. This feedback is

TABLE 34-1

Circulating Levels and Origin of Sex Steroids

Steroid Testosterone (T) Dihydrotestosterone (DHT) Androstenedione (A) Dehydroepiandrosterone (DHEA) DHEA sulfate (DHEAS) Estradiol (E,)

Plasma Level (W/dL)

Adult Men

Adult Women

Approximate Contribution (%) of

Approximate Contribution (%) of Cycle Plasma Level Phase* (ng/dL)

Testes

Adrenal

Periphery

700 40

90 20

5 -

5 (A, DHEA) 80 (T, A)

-

30 (A, DHEA) 100 (A, T)

115 640

60 10

30 85

10 (DHEA, T) 5 (DHEAS)

-

10 (DHEA) 5 (DHEAS)

Progesterone (P) *EF = Early follicular phase; LF = late follicular phase; ML= midluteal phase; F = follicular phase; L = luteal phase

-

Ovary

Adrenal

Periphery

784

CHAPTER34 Endocrine Metabolism V: Reproductive System (a) A4 Pathway

?H:3

(b) As Pathway

?H3 I

I

C~O

C---O

Ho.C

4

Pregnenolone

Progesterone

(~H3

4

17(x-Hydroxypregnenolone

17or 3

O

022; Androstenedione

Dehydroepiandrosterone

41

OH

Testosterone

OH

AS-Androstenediol

FIGURE 34-1 Testosterone biosynthetic pathways. (a) A4 Pathway (preferred pathway in human testes). (b) A5 Pathway(all enzymes located in the endoplasmic reticulum). Enzymes: 1,3/4-hydroxysteroiddehydrogenase and AS,A4-isomerase;2, 17c~-hydroxylase(CYP 17)"3, Ci7_20-1yase;4, 17/4-hydroxysteroiddehydrogenase.

achieved in part by a polypeptide produced by Sertoli cells called inhibin, which specifically inhibits FSH release. Presumably, the output of inhibin is proportional to the intensity of spermatogenesis and maintains spermatogenesis at a relatively constant rate. FSH release also is modulated by estrogens. Because the anterior pituitary lacks aromatase and therefore cannot convert testosterone to estradiol, it is possible that

testicular estrogen exerts a negative feedback on FSH release.

Regulation of T e s t i c u l a r S t e r o i d o g e n e s i s : L e y d i g - N e u r o e n d o c r i n e Axis Leydig cells (interstitial cells) are situated in the vascularized compartments outside the seminiferous tubules

SECTION34.1 Testes

in the testes. Leydig cells are steroidogenic cells, converting cholesterol to pregnenolone under regulation by LH. The major steroid product of the Leydig cells is testosterone, which accounts for most of the steroid output by the adult testes. The preferred pathway is the /k 4 pathway, although detectable amounts of A 5 steroids (dehydroepiandrosterone, androstenediol) are secreted (Figure 34-1). LH acts at the cholesterol side chain cleavage step. LH release is held in check by the level of non-TeBG-bound testosterone in the circulation, although the principal inhibitor is DHT. Because the anterior pituitary contains 5o~-reductase activity, it converts testosterone to DHT and responds to DHT by diminishing output of LH. LH release is also inhibited by testosterone via inhibition of GnRH release. In the preoptic hypothalamus, testosterone can be converted to DHT or estradiol, and may thereby inhibit the pulse frequency (DHT) or pulse height (estradiol) of GnRH released at the median eminence. Secretion of testosterone is fairly constant and consistent in adult men, although higher levels occur in the morning and lower levels in late evening. This circadian rhythm is not accompanied by changes in the levels of LH; thus, it appears to be an intrinsic testicular rhythm. Testosterone levels decline by 10-15% between the ages of 30 and 70 years, accompanied by reduction in tissue responsiveness to androgenic stimulation.

Metabolism of Testosterone Testosterone in plasma exists in two fractions: TeBGbound (44%) and non-TeBG-bound (56%). The plasma level of TeBG in adult males is --,25 nM, which approximates the normal testosterone level in adult men (~ 22 nM); thus, an increase in testosterone production or a decrease in TeBG level will result in a higher level of testosterone available for tissue uptake. In androgen target tissues that lack 5o~-reductase, testosterone activates the androgen receptor (AR)-androgen response element (ARE) mechanism directly and induces the expression of androgen-dependent genes (Chapter 30). After dissociating from the AR, testosterone may be metabolized by the target cell or make its way to the liver for inactivation. The liver modifies three parts of the testosterone structure that are important for hormonal activity: oxidation of the 17/3-OH group (to produce a 17-keto derivative), saturation of the A 4 in ring A (to produce an androstane derivative), and reduction of the 3-keto- group (to produce a 3ot-hydroxylated derivative (Figure 34-2). In androgen target tissues that contain 5o~-reductase, testosterone is converted to DHT, which activates the AR-ARE sequence leading to an androgenic response. DHT undergoes inactivation when it dissociates from

785

the AR; the major metabolites (except in the liver and kidney) are the reduced, hydroxylated metabolites, 3ot-androstenediol and 3/3-androstenediol, which are then conjugated to glucuronic acid for renal excretion (Figure 34-2). These and other polar compounds account for about 30-50% of the testosterone metabolites excreted daily. In tissues that contain aromatase (CYP 19), testosterone can be converted to estradiol, which may exert an effect in situ if the tissue is an estrogen-responsive one, and/or may return to plasma for distribution to estrogen target tissues. Unlike the androgenic steroids, the only major site of estrogen inactivation appears to be the liver; therefore, estrogen theoretically can be recycled until being transported to the liver, which itself is an estrogen-responsive tissue. Testosterone-derived estradiol accounts for a small percentage (0.3 %) of the testosterone metabolized. Testosterone is known to exert some important biological effects in the liver and kidney, which are the major testosterone-inactivating tissues in the body. About 50% of the testosterone is removed from plasma with each passage through the liver. The major metabolites of testosterone in the liver are etiocholanolone, androsterone, and epiandrosterone, collectively referred to as the 17-ketosteroids (17KS), all of which are released as glucuronide conjugates for excretion by the kidney (Figure 34-2). This route accounts for 50-70% of the total testosterone metabolized daily. An epimer of testosterone, epitestosterone (17o~-hydroxylated testosterone), is produced by the testes and excreted as such in the urine in amounts approximately equal to that of testosterone(T:epiT ~ 1:1). Epitestosterone is biologically inactive, but it is not a metabolite, and is believed to be produced only by the gonads; thus, it is used as a gonadal steroid marker. In women, the ratio of T to epiT is also normally 1:1. Urinary T:epiT is useful in monitoring abuse of anabolic steroids by athletes because the ratio increases when any exogenous testosterone derivative is used.

Biological Effects of Androgens Thebiological effects of androgenic hormones can be of two types: (a) reproductive (androgenic), i.e., promoting the primary and secondary sexual characteristics of a male; and (b) nonreproductive, which includes anabolic effects. Both types of effects are mediated by the same AR; therefore, testosterone and DHT are capable of exerting either types of effect; however, under physiologic conditions, a critical factor that determines what hormone is active in a given tissue is the presence or absence of 5o~-reductase. As underscored above, pharmacological treatment with

Liver

PH

m\

0

DHT-responsivetissues

Testostemne

(unbound in phsma)

Testosterone

Testostemne

DHT

H

Excreied in urine as mnjugated 17-keio stems

FIGURE 34-2 Metabolism of testosterone.

I

I Conjugatedand excreted in unne

SECTION34.1 Testes

any steroid that can activate the AR will evoke both androgenic and anabolic responses.

Reproductive (Androgenic) Effects Testes (Spermatogenesis) At puberty, locally produced testosterone promotes spermatogenesis in the seminiferous tubules, although it is not known with certainty whether or not the Sertoli cells contain 5c~-reductase. As mentioned above, testosterone alone promotes the two meiotic (maturational) divisions if present in high concentrations within the tubule, but it is not capable of initiating spermatogenesis in the absence of FSH. Thus, in cases of drug-induced azoospermia, FSH (not androgens) is required for reinitiation of spermatogenesis. Internal and External Genitalia In the fetus, testosterone causes differentiation of the Wolffian ducts into the male-type reproductive tract, and thereby rescues the tissue from pre-programmed destruction. The seminal vesicles, which differentiate from the Wolffian duct as a result of testosterone action, produce 5ot-reductase-2 from week 13-14 and becomes DHT-responsive after that; thus, development and function (but not appearance) may be regulated by DHT from the second trimester through adulthood. On the other hand, the urogenital sinus and external genital primordium in the fetus contain 5ot-reductase-2 before week 6 and require DHT for their differentiation into the male phenotype. These tissues do not respond to the very high levels of testosterone in fetal plasma during the critical period (weeks 8-12) unless they are able to convert it into DHT. This explains why hereditary 5c~-reductase deficiency in genotypic male fetuses results in the development of female-type external genitalia, despite the normal production of testosterone by the fetal testes. At puberty, however, the derivative of the genital tubercle (clitoris and penis) becomes responsive to testosterone, which enhances the male-type morphology by promoting growth of the existing structure. On the other hand, the growth and initiation of function of the prostate (urogenital sinus derivative) from puberty through adulthood requires DHT, and testosterone alone does not substitute. The importance of DHT in prostate growth and function is underscored by the success with which benign prostatic hyperplasia (BPH) in adults can be controlled by blocking 5ot-reductase activity. Skin The hair follicles and sebaceous glands (collectively called pilosebaceous units) in the androgensensitive areas of the skin and sebaceous glands throughout the body contain 5ot-reductase type 1 and respond to DHT. At puberty, DHT stimulates the appearance and growth of terminal hair in certain androgen-sensitive regions of the body (face, chest, upper pubic triangle, nostrils, ex-

787

ternal ear). Testicular testosterone in boys and adrenal androstenedione in girls stimulate the appearance and growth of terminal hair in the axillae and lower pubis, presumably after conversion to DHT in the hair follicles in these regions. Axillary and lower pubic hair growth requires less androgen than that in the face, chest, upper pubic area, nose, and ear; thus, hair in mons pubis is the most sensitive to androgen (i.e., responds to lower amounts), beard is least sensitive (i.e., requires higher amounts). DHT stimulates the growth and secretory function (sebum production) of all sebaceous glands, and thus predisposes the skin to acne formation, This DHT-promoted effect can be reversed or attenuated by treatment with estrogens, which antagonize the actions of DHT on the sebaceous glands and cause a depression in glandular activity. In adulthood, the changes at puberty are maintained by the same hormones, but in many men and few women varying degrees of hairline recession occur as a result of sex-linked genetic predisposition and a DHT effect. Hair on the frontal and vertex scalp is converted from terminal to vellus types in response to DHT (the opposite of what occurs in other androgen-sensitive areas), resulting in the "male pattern baldness"; in women, this is one of the signs of hirsutism. A reduction in circulating testosterone and/or inhibition of 5oe-reductase activity (e.g., with finasteride) will prevent or retard the terminal-to-vellus transition. Voice Pitch Testosterone promotes enlargement of the larynx and thickening of the vocal folds (vocal cords), causing a lowering of the voice pitch. The larynx and associated vocal fold mucosa do not contain 5ot-reductase; thus, deepening of the voice occurs at puberty in boys due to the increased production of testosterone, and this is seen even in hereditary 5o~-reductase deficiency. Once established at puberty, the laryngeal vocal structures do not require significant amounts of androgens for maintenance; in fact, bilateral testicular failure or loss in a previously normal adult male may not result in any change in voice pitch, possibly because of the adequacy of androgens from the adrenals. Nonreproductive Effects

SkeletalMuscle Skeletal muscle has no 5ot-reductase, and, although all skeletal muscles are believed to contain AR, some are known to contain a higher concentration than others. The most androgen-responsive skeletal muscle in man (presumably because of a higher concentration of AR) are those of the pectoral and shoulder regions. In other species (e.g., rodents), testosterone promotes skeletal muscle growth by stimulating both hypertrophy and mitosis of myofibrils and increases the fast-twitch isoform

788

of myosin heavy chain. The extent to which skeletal muscle mass can be increased is limited by the concentration of androgen receptors in the muscle that can be activated. In normal adult males, the androgen receptors in most tissues are nearly saturated, and further increases in plasma androgens do not produce significant increases in tissue responsiveness. This explains why androgen treatment of boys, girls, and women results in increased skeletal muscle mass, whereas similar treatment of men does not; however, pharmacologic doses of potent synthetic androgens reportedly promoted skeletal muscle mass by a mechanism that is unclear. Conversion of the exogenous testosterone to estrogen may result in a stimulation of AR synthesis in muscle, since AR concentration in skeletal muscle is increased by estrogen and decreased (downregulated) by androgen. Bone Androgens promote skeletal growth and maturation by a direct effect on bone tissue and by an indirect effect on growth hormone (GH) release. At puberty, the increasing levels of androgens stimulate the release of GH and result in accelerated endochondral growth of the epiphyses of long bones, which causes a doubling of height gain that is maximal at about mid- to late puberty. This peak height velocity (PHV) or pubertal growth spurt is dependent on the increased secretion of androgens (estrogen in girls) and GH at this time. By mechanisms that are not yet understood, androgens also increase bone mass and accelerate the ossification of the epiphyseal growth plate, and thereby reduce the rate of linear growth during late puberty. Ultimately, androgens bring about the fusion of the epiphysis with the diaphysis (i.e., complete endochondral ossification of the growth plate), resulting in the irreversible cessation of linear growth, at about age 19 in boys and age 17 in girls. Even after epiphyseal closure, androgens continue to promote an increase in cortical bone mass, a process that continues to age 30 years. Testicular failure (or any other cause of androgen deficiency through childhood) allows the epiphyseal growth plate to grow for several additional years, resulting in eunuchoidal proportions, as indicated by a reduction in the upper-to-lower body ratio due to longer legs, and an arm span that is greater than the height due to longer arms. In addition, the absence of pubertal androgen results in osteopenia due to inadequate cortical bone mass. Blood Volume Testosterone acts on the proximal tubule of the nephron to promote the reabsorption of K +, Na +, and CI-, which, along with stimulated erythropoiesis (see below), contributes to the androgen-associated increase in blood volume. Athletes treated with synthetic androgens experience a weight gain that can largely be explained by an increase in blood volume, and men who undergo androgen treatment as a means of contraception also exhibit blood volume expansion and weight gain.

CHAPTER 34


Erythropoiesis Androgens stimulate the production of erythropoietin (Chapter 28) by the kidney and, in part, cause an increase in hematocrit by this mechanism. This may explain why males have a higher hematocrit than females. Adipose Tissue Androgens promote truncal-abdominal fat deposition and favor development of upper body obesity. In contrast to gluteofemoral (lower body) fat, upper body fat accumulation, particularly visceral fat, is characterized by an increase in fat cell size, increased lipoprotein lipase (LPL) activity, enhanced lipolysis, and reduced response to the antilipolytic effect of insulin. This explains why androgen-dominated states favor insulin resistance. Liver Androgens cause a reduction in the plasma levels of testosterone-estradiol-binding globulin (TeBG), which results in an increased percentage of testosterone that is accessible for tissue uptake. This leads to a greater androgenic response by the tissues but also results in accelerated clearance of testosterone from circulation. Ultimately, the outcome of a reduced TeBG level will be a reduction in total plasma androgen concentration due to increased negative feedback suppression of LH release and a reduction of testosterone synthesis. Androgens also modify the production of other hepatic proteins such as fibrinogen (decreased), transferrin (decreased), haptoglobin (increased), ctl-antitrypsin (increased), and hepatic triglyceride lipase (increased). Synthetic steroids exhibiting enhanced anabolic activity and reduced androgenicity (anabolic steroids) have been developed (Figure 34-3). Athletes who use these steroids to increase their strength and durability often consume high doses, which can have adverse effects. The anabolic steroids exert their effects by binding to cytosolic androgen receptors, and the neuroendocrine system responds by reducing secretion of LH and FSH. This response leads to diminished Leydig and Sertoli cell function and to a reduction of endogenous testosterone production and of spermatogenesis. Testicular atrophy and reduced sperm counts are documented consequences of excessive anabolic steroid treatment. Most of the orally active androgenic steroids contain a 17ct-alkyl group (17c~-methyl or 17ot-ethynyl) that makes the steroid resistant to liver inactivation. This explains why these steroids are orally active; however, long-term usage of these steroids is associated with hepatic disorders with varying severity (from abnormal liver function tests to jaundice to hepatic carcinoma). The probable cause of hepatic damage is the chronic demand on the liver to continue increasing its microsomal P450 redox activity, which is incapable of oxidizing 17ot-alkyl-substituted steroids. Structure-Function Relation The 17-hydroxyl group of testosterone and DHT appears to be important for androgenic activity, since testosterone enanthate and

SECTION34.1 Testes

78g

9--OC(CH~),CH,

H . . . . CH3

Testosterone enanthate

Fluoxymesterone (a) Androgens

OH

OH

CH3

o~

OH

~-CH~ H

Oxymesterone

Oxandmlone (b) Anabolic steroids

H3 C=O OCOCH 3 H3C-...

CI Cyproterone acetate (c) Antiandrogen

FIGURE 34-3 Synthetic androgens, anabolic steroids, and an antiandrogen.

fluoxymesterone (Figure 34-3), which have protected 17hydroxyl residues, are potent androgenic drugs. Substitutions in the A ring while protecting the 17-hydroxyl group decrease the androgenic potency of the steroid while enhancing the anabolic potency.

Pharmacological Inhibition of Spermatogenesis or Fertility (Male Contraception) Attempts to devise a male contraceptive (antifertility) have involved the testing of a number of chemicals for their ability to interfere with spermatogenesis or spermatozoa viability. In order to be effective, a contraceptive would

have to produce azoospermia (complete absence of spermatozoa in ejaculate) because pregnancies have resulted with sperm counts as low as 1 x 106 per mL or less. Testosterone Synthetic derivatives of testosterone with long biological half-lives have been used to create azoospermia in most subjects after 42 weekly treatments. The mechanism involved is the suppression of both LH and FSH release by way of hypothalamic GnRH inhibition. Although exogenous testosterone treatment maintains the androgen supply to most tissues, the seminiferous tubule compartment is androgen-deficient because the Leydig cells are not functional in this LH-deficient condition. Thus, the requirement for high local concentration

790

of testosterone within the seminiferous tubules is not met, and spermatogenesis ceases as a result. Progestins A synthetic progestin, medroxyprogesterone acetate (Provera), has been used for male contraceptive trials alone or in combination with nandrolone (a synthetic androgen). When given once monthly for 2-3 months, the combined treatment caused azoospermia in about one-half of the subjects. Synthetic GnRH Agonists Synthetic derivatives of GnRH with more biological activity than the native hormone have been used for the suppression of spermatogenesis after 2 or more weeks of treatment that downregulated GnRH receptors in the anterior pituitary. However, in most trials there was no uniform induction of azoospermia, probably because the reduction of LH and testosterone secretion was incomplete. Gossypol Gossypol is a naphthaphenol present in crude cottonseed oil that causes infertility in those who consume it regularly, as was found to be the case during the 1950s. Oral consumption of this agent for one month caused a reduction in sperm count and immobilization of spermatozoa, the result of the drug on the seminiferous tubule and epididymis that affected both spermatogenesis and sperm maturation. Leydig cell function was not affected. Aside from symptomatic hypokalemia, the disadvantage of this contraceptive is that its effect is not completely reversible after cessation of treatment. Androgen Antagonists Cyproterone acetate and flutamide have been shown to be ineffective in reducing fertility in men, primarily because their effects on the neuroendocrine system override any intratesticular effects they may exert. Thus, the increase in LH and FSH caused by these antiandrogens stimulates increased testosterone production by the Leydig cells leading to a compensatory increase in intratesticular androgen. Both spironolactone (a mineralocorticoid receptor antagonist) and cimetidine (an H2 blocker) interfere with androgen effects in the testes, and may cause a reduction in sperm count in those who are treated with either drug on a chronic basis. However, the effect is mild and completely reversible.

34.2 Ovaries M e n s t r u a l Cycle The menstrual cycle consists of the follicular phase and the lutealphase, each lasting about 2 weeks. The follicular phase is a period of ovarian follicular growth (folliculogenesis) that results in ovulation (release of secondary oocyte). This phase is dominated by estrogen produced by the growing follicle itself. During the follicular phase, the

CHAPTER34 Endocrine Metabolism V: Reproductive System

uterine endometrium is stimulated by estrogen to proliferate and to synthesize cytosolic receptors for progesterone. Follicular production of estrogen depends on FSH and LH. The luteal phase is progesterone-dominated. The follicle that ruptures at ovulation becomes the corpus luteum, which produces progesterone and estradiol. During this period, the uterine endometrium becomes secretory under the influence of progesterone. About 5 days into the luteal phase, the endometrium is ready to accept a blastocyst for implantation; in the absence of fertilization, however, the corpus luteum degenerates after about 12 days, steroid production ends, and the endometrium deteriorates (menstruation). The first day of menstruation is the first day of the menstrual cycle. Endocrine Control of Folliculogenesis Two types of endocrine cells are associated with the ovarian follicle. One is the granulosa cell, which resides within the follicle and is encased by the basal lamina. Like the Sertoli cell in the testes, the granulosa cell is perfused by plasma transudate, not blood, and possesses FSH-sensitive aromatase activity; thus, FSH stimulates the formation of estrogen by the granulosa cell. Unlike Sertoli cells, however, granulosa cells proliferate in response to estrogen, and this proliferation is inhibited by androgens. The nonendocrine function of granulosa cells is to promote growth of oocytes by conditioning the follicular fluid. The other type of follicular cell with endocrine function is the theca interna cell. Such cells are positioned along the outer border of the basal lamina; thus, they are located outside the follicle and are perfused by blood. Theca interna cells, like Leydig cells of the testes, respond to LH with androgen production from cholesterol. Unlike Leydig cells, they produce mainly androstenedione, although some testosterone is also produced. Theca interna cells provide androgens for estrogen production by granulosa cells. Hormonal Control of Follicle Growth During the follicular phase, the ovarian follicle (preantral follicle) grows by pronounced proliferation of granulosa cells. During the second half of the follicular phase, the follicle accumulates fluid, which leads to formation of an antrum (antral follicle). In the preantral follicle, LH stimulates the interna cells to produce androgens (mainly androstenedione), which diffuse through the basal lamina into the granulosa cell compartment. The granulosa cells, under the influence of FSH, aromatize the androgens into estrogens (mainly estradiol), which act directly on the granulosa cells to stimulate proliferation. This causes the follicle to grow and

SECTION 34.2

791

Ovaries

60

~ 40 zo

~"

20

I I

368-

-r if) u.

._

I

0

,

i I I

2~'~o

i

,.6.

i -r

17-OH O G

P

R

" "

"" "

8

~

/gf

B

0

B

n "r P R O G E S T E R O N E O]I

. . . .

" . . . .

[

0

'

I

i

I

,

I

i

/q

E

I

IOO ,

c o ""

,,

2

, ii i,i, J

, , iiiii,

l qI

,

/h

1 1

\

--9

I

i

, I

LH

I

I

42

' ,

~.

26 ~

I

io 14

FSH -

0

;58

I

I

~o ~E . "l"V

o

i

~, 200

v

"10

%

0

I IS

IO

imi, S

iI O

1-1

S

,

* IO

i 15

6

-16

-12

-8

-4

0

+4

I

8

12

1~,1

Days from LH peak

Day of cycle

F I G U R E 34-4 Hormonal and physiological changes during the menstrual cycle. BBT, Basal body temperature.

to accumulate fluid and leads to formation of an antrum. Although the intrafollicular concentration of estradiol is sufficient to stimulate proliferation of granulosa, it is not high enough to enter the general circulation. For this reason, the level of circulating estradiol does not increase during the first half of the follicular phase (Figure 34-4), and thus the release of LH and FSH is not inhibited but remains at a fairly constant level. In the antral stage of follicular growth, production of estradiol by granulosa cells increases owing to acquisition of LH receptors by the granulosa cells. Induction of LH receptors is brought about by the combined effects of FSH and estradiol and enables the granulosa cells to begin producing estradiol from pregnenolone. Thus, the formation of estradiol by aromatization of androgens derived from the theca interna is augmented by estradiol synthesized de n o v o . The increased estradiol pool causes marked

acceleration in follicle growth and spillage of estradiol into the general circulation. Consequently, a relatively rapid increase in circulating estradiol level is seen during the last 5 or 6 days of the follicular phase, which exerts an initial negative feedback on release of FSH (Figure 34-4). This rise in estradiol level is a critical cue for the neuroendocrine system because it indicates that the ovarian follicle is ready for ovulation. This message is in the form of an approximately threefold increase in estradiol level, which stimulates the release of a large surge of LH and FSH. In contrast to the negative feedback effect that estradiol normally exerts on the release of gonadotropins, the very high levels of estradiol presented over 2-3 days exert a positive feedback effect. Two mechanisms may operate in this positive feedback: High levels of estradiol for 2 or 3 days may increase the sensitivity of the pituitary to GnRH or increase the release of GnRH from the hypothalamus (or both).

~9~

The latter mechanism is believed to involve the formation of catecholestrogen (2-hydroxyestradiol) after uptake by the median eminence, which, because of its abundance, competes with hypothalamic norepinephrine for inactivation by catechol-O-methyltransferase (COMT). This inactivation results in a higher norepinephrine content in the median eminence, which favors GnRH release. The large bolus of LH released (preovulatory LH surge) in response to positive feedback by estradiol induces ovulation in about 1 day, probably by stimulation of production of granulosa plasminogen activator, which leads to formation of plasmin, an enzyme that may be responsible for the digestion of the basal lamina and, consequently, for the rupture of the follicle (ovulation).

Hormonal Control of Luteai Function After ovulation, the granulosa cells proliferate in response to the preovulatory LH surge, and the theca interna cells and perifollicular blood vessels invade the cavity of the collapsed follicle. Under the influence of LH, the granulosa and invasive theca cells differentiate into luteal cells, which are characterized by their high lipid content. These luteal cells are steroidogenic and produce large amounts of progesterone and moderate amounts of estradiol. The ruptured follicle thus becomes the corpus luteum. Morphogenesis of the corpus luteum is not complete until about 1 or 4 days after ovulation, and luteal production of progesterone and estradiol gradually increases to a maximum about 6 or 7 days after ovulation. Thus, there is an interval of about 3 days after ovulation during which levels of circulating estradiol are reduced (Figure 34-4), and this interval is required for proper transport of the ovum through the fallopian tube into the uterus. Exposure to high levels of estrogen during this interval would lead to expulsion of the ovum or to blockage of ovum transport. The rise in levels of progesterone and of estradiol during the first week of the luteal phase is required for the endometrium to become secretory in preparation for implantation and pregnancy. The corpus luteum has a life span of about 12 days; it can synthesize steroids autonomously without extraovarian hormonal stimulation. Although the corpus luteum has receptors for LH, release of LH (and FSH) during the luteal phase is strongly inhibited by the potent negative feedback effect of progesterone and estradiol. Thus, if fertilization and implantation do not occur, the corpus luteum degenerates (luteolysis), and its production of progesterone and estradiol rapidly declines (Figure 34-4). Withdrawal of progesterone and estradiol during luteolysis results in deterioration of the endometrium and its shedding (menstruation). If fertilization and implantation occur, secretion of chorionic gonadotropin (hCG) (Chapter 31) by the

CHAPTER34


implanting blastocyst stimulates the corpus luteum to continue producing progesterone, and luteolysis is prevented. The regularity of the menstrual cycle in women of reproductive age can be affected by anatomical defects of the uterus or vagina, or by functional or structural defects in the hypothalamic-pituitary-ovarian axis that affect hormonal secretions. Complete cessation of menses (for more than 6 months) is known as amenorrhea and a reduction in the frequency is known as oligomenorrhea. Physiologic states of amenorrhea include prepuberty, pregnancy, lactation, and postmenopause. A common pathologic cause of amenorrhea can result from a reduction in the secretion of GnRH from the hypothalamic neurons into the hypothalamic-hypophyseal portal system with a corresponding reduction in the secretion of FSH and LH from the pituitary. Decreased GnRH can occur due to weight loss, anorexia nervosa, excessive exercise, debilitating diseases, or psychological trauma.

Pregnancy

Human Chorionic Gonadotropin (hCG) After fertilization of the ootid in the fallopian tube, the zygote develops into a blastocyst by the time it enters the uterine cavity (about 4 days after ovulation). By the seventh day after ovulation, the blastocyst is completely embedded in the endometrium, and its outer trophoblast has differentiated into an inner, mitotically active cytotrophoblast and an outer, nonmitotic syncytiotrophoblast. The syncytiotrophoblast begins producing hCG from about the eighth day after ovulation, and it continues producing increasing amounts of hCG until maximal levels are attained at about 6-8 weeks of pregnancy. hCG stimulates the corpus luteum to continue producing progesterone from about the eighth day after ovulation and thus protects the corpus luteum from luteolysis. As mentioned above, hCG promotes expression of the male genotype in the fetus by stimulating testosterone production by fetal Leydig cells from about the eighth week of pregnancy. Moreover, hCG may be the stimulus for the initial development of the fetal zone of the adrenal cortex, which appears at about the sixth week. Lastly, and perhaps most importantly, hCG stimulates steroidogenesis in syncytiotrophoblasts at 4-5 weeks of pregnancy by inducing C Y P I I A (side chain cleavage enzyme), 3/3-hydroxysteroid dehydrogenase (3/3-HSDH/iso), and CYP 19 (aromatase). These enzymes are critical for maintenance of pregnancy because the placenta assumes the role of the ovaries as the major generator of progesterone and estrogen after the sixth week of pregnancy. The implantation of the fertilized ovum at a site other than the endometrium is known as ectopic pregnancy.

793

SECTION34.2 Ovaries

Patients with ectopic pregnancy frequently exhibit abdominal pain with amenorrhea. By far the most common cause is an impaired fallopian tubal function. The tubal pathology can result from pelvic infection, endometriosis, or previous surgery. The implanted embryo commonly dies in the early weeks of pregnancy. Tubal rupture and the associated bleeding require emergency care. Thus, early diagnosis is essential and consists of serial measurements of serum levels of hCG using antibodies specific for the /3 subunits of hCG (/3-hCG) and serum progesterone, and pelvic ultrasonography. In ectopic pregnancy, the serum /3-hCG levels are lower than those observed at the same gestational age in a normal pregnancy. A serum progesterone level of 25 ng/mL (79.5 nM/L) or higher excludes ectopic pregnancy with 97.5% sensitivity. The treatment for ectopic pregnancy consists of either surgery to remove the embryonic tissue or medical treatment with methrotrexate, a folic acid antagonist. Methotrexate inhibits both de novo purine and pyrimidine nucleotide biosynthesis (Chapter 27), blocking DNA synthesis and cell multiplication of the actively proliferating trophoblasts. Placental Steroids

The absence of progesterone is incompatible with the gravid state. Progesterone acts in three ways to maintain pregnancy: 1. It maintains placental viability, thus ensuring adequate exchange of substances between maternal and fetal compartments; 2. It maintains perfusion of the decidua basalis (maternal placenta), presumably by inhibiting the formation of vasoconstrictive prostaglandins; and 3. It diminishes myometrial contractility, possibly by increasing the resting membrane potential or by inhibiting the formation of prostaglandins E1 and Fzot (Chapter 18). The placenta is the major site of conversion of cholesterol to progesterone after the sixth week; however, it is not capable of synthesizing cholesterol from acetate. Moreover, the placenta lacks CYP 17 and CYP 21, which explains why the major steroid it produces is progesterone (Figure 34-5). Although the placenta cannot process cholesterol beyond progesterone, it contains some important steroid-modifying enzymes which enable it to produce substantial amounts of estrogen. Small amounts of estrogen are required for maintenance of pregnancy because estrogen maintains tissue responsiveness to progesterone. The placenta forms estrogen from androgenic steroids. The major androgen used is

Maternal circulation Cholesterol

Placenta

.,_

-- Cholesterol ~,~

[

Fetal circulation ~.

,

Cholesterol

CYPllA

Pregnenolone 3 [3 -HSDH Progesterone - - - -

/

Progesterone

Pregnanediol (excreted in urine) F I G U R E 34-5 Placental synthesis of progesterone. The placenta contains cholesterol side-chain cleavage enzyme (CYP11A) and 3fl-hydroxysteroid dehydrogenase (3fl-HSDH), which enable it to form progesterone from cholesterol. However, it does not make cholesterol d e n o v o and relies mainly on the maternal cholesterol pool for substrate. Progesterone exerts its effects on the uterus and placental vasculature and enters the maternal circulation. Maternal tissues reduce progesterone predominantly to pregnanediol, which is the major metabolite of progesterone in maternal urine.

dehydroepiandrosterone (DHEA), derived mainly from the fetal compartment in the form of DHEAS (DHEA sulfate). The source of DHEAS is the fetal zone of the fetal adrenal gland, which, produces mainly DHEAS because it lacks 3fl-HSDH (Chapter 30). The placenta takes up DHEAS from the fetal circulation and, by the action of placental sulfatase, liberates DHEA, which is then aromatized into estrone (El) (Figure 34-6). Some E1 is converted to E2 by placental 17-HSDH, and both are released into the maternal circulation. A substantial fraction of DHEAS produced by the fetal zone is taken up by the fetal liver, which detoxifies it to 16 fl-hydroxy-DHEAS. The placenta also processes this metabolite through the aromatase system, and the product is estriol (E3), a weak estrogen that is not produced in significant amounts in the nongravid state. The circulating level of E3 is an important index of fetal-placental function because its production depends on normal functioning of the fetal adrenal, fetal liver, and the placenta. It is therefore used clinically as a diagnostic tool. Human Placental Lactogen (hPL)

Human placental lactogen (hPL) is produced by the syncytiotrophoblast from about the time when production of hCG begins to diminish. Production of hPL is proportional to placental growth, and its level reflects placental wellbeing, hPL exerts GH-like effects in both fetal and maternal compartments. In the fetus, it promotes formation of insulin-like growth factor and growth factors believed to promote growth of most, if not all, fetal tissues. In the mother, hPL promotes nitrogen retention and utilization of free fatty acids, and it creates a state of mild insulin


794

Maternal compartment Maternal,,

adrenals

: DHEAS

Placenta DHEAS

Fetal compartment --

DHEAS

Fetal zone (fetal ;adrenal)

l Sulfatase DHEA

Aromatase ,51 -~

Estrone (E,)

o,O9149 o,,'E 2 --

Estra~tdiol (E2)

9

9 9

] Fetal liier

,,o

L....

~ oo

Urine 16(z-OH-DHEAS $ulfatase

16a-OH-DHEA Aromatase E3 ~

Estriol (~)

FIGURE 34-6 Placental synthesisof estrogens. The placenta lacks the key enzymenecessaryfor formationof estrogensfrom cholesterol (CYP 17) and relies on androgenicprecursorsfrom the maternal and fetal compartments.The major androgen used comes from the fetal zone of the fetal adrenal; this is DHEAS,which is also taken up and metabolizedby fetal liver into 16ot-hydroxy-DHEAS.The placenta converts DHEASinto estrone (El) and estradiol (E2)and processes 16o~-hydroxy-DHEASinto estriol (E3). Estrogensenter the maternal circulation and appear in maternal urine as conjugated estrogens.

resistance that benefits the fetus because it increases the availability of maternal glucose for fetal consumption. In the mother, hPL also exerts a prolactin-like effect and, with estrogen and progesterone, promotes ductal and alveolar growth in the mammary gland during the third trimester of pregnancy. However, prolactin is believed to be more important than hPL in this effect.

Decidual Prolactin The maternal placenta (decidua basalis) is believed to produce a hormone that is biologically and antigenically similar to pituitary prolactin. If released into the amniotic fluid during the third trimester of pregnancy, it may maintain amniotic fluid volume and osmolality.

Relaxin The maternal placenta also produces relaxin, a hormone produced by the ovaries. Relaxin causes relaxation of the cervix during parturition.

Parturition

Precisely what initiates parturition in humans is not known. Unlike the situation in sheep, there is no induction

of placental CYP 17 by fetal cortisol to bring about a reduction in progesterone production and, hence, parturition. During late gestation, rising levels of estrogen are thought to increase the synthesis of oxytocin in hypothalamic magnocellular neurons to induce oxytocin receptors in the myometrium and to increase myometrial contractility by lowering the membrane potential. During this period, relaxin from the decidua softens the cervix for impending delivery. At term an unidentified triggering factor in fetal urine stimulates uterine production of PGE] and PGF2,~, which lead to myometrial contraction, mainly in the fundus, and encourages movement of the infant through the cervical canal. Dilation of the cervix by the infant stimulates the release of oxytocin by neuroendocrine reflex, further stimulating uterine contractions, which in turn causes more cervical stretching and a positive feedback to oxytocin release, until delivery is complete.

Lactation

During midpregnancy and late pregnancy, growth and differentiation of mammary glands are promoted by the combined effects of estrogen, progesterone, and prolactin (or hPL). In the presence of prolactin or hPL, estrogen stimulates branching of the mammary ducts and increases their

SECTION34.2 Ovaries

growth. Estrogen also stimulates production of lactogenic receptors in mammary ductal cells and acts on the anterior pituitary to stimulate prolactin secretion. In the presence of prolactin or hPL, progesterone acts on the estrogenconditioned mammary gland to promote differentiation of the terminal ductal buds into alveoli and, consequently, into lobules. Synthesis and secretion of milk occur in these lobules. Lactogenesis begins during the third trimester of pregnancy and involves synthesis of the milk-specific proteins casein, lactalbumin, and lactoglobulin. The primary regulator of lactogenesis is prolactin, although the participation of additional hormones is needed (i.e., insulin, cortisol, thyroid hormone). Lactation involves release of milk into the alveolar lumen. Lactation is inhibited by estrogen and, thus, is held in check by the high levels of estrogen during pregnancy; the withdrawal of estrogen after birth triggers lactation. When lactation is unwanted by the mother, a large amount of estrogen is administered at the time of labor. Lactation is maintained by prolactin, the release of which is triggered when the mother's nipple is stimulated (neuroendocrine reflex). Prolactin does not stimulate the actual release of milk from the nipple (milk let-down or milk ejection); this is brought about by oxytocin released in response to suckling. Oxytocin acts on myoepithelial cells, contractile cells that surround the alveoli and ducts, and causes them to contract and squeeze out the milk. Mothers who nurse their infants have frequent prolactin surges in response to suckling. During the initial 6 months of lactation, the response amplitude of prolactin is high, LH and FSH levels are suppressed, and ovarian steroidogenesis is reduced. These effects are due to the antigonadotropic action of elevated levels of prolactin. They have been explained by reduced GnRH secretion by the hypothalamus. This condition, which is common during the first 6 months of lactation and usually does not persist beyond 9 months, is referred to as lactational amenorrhea. It resembles the gonadal quiescence of hyperprolactinemia.

Biological Effects of Estrogens The biological effects of estrogen are many. Estrogen is the major determinant of female reproductive function, bone maintenance, and cardioprotection. Its effect on the brain includes reproductive behavior and function, learning, and memory. The mechanism of action of estrogen in the improvement of cognitive function is not understood, but it is thought that estrogen delays the onset of Alzheimer's disease. Estrogen is also a male hormone and has important actions in the male urogenital tract and skeleton. The key role of estrogen in the normal bone metabolism of both sexes is illustrated in two human syndromes, namely,

795

defects in aromatase and estrogen receptor (ER). The latter is required for the conversion of testosterone to estrogen. Men and women with aromatase deficiency suffer from osteoporosis, incomplete epiphyseal fusion and continued linear growth in adulthood, and lack of sexual development. Treatment with estrogen in the aromatase- deficient patient increases bone mineral density and the epiphyseal closure. A mutation in the ER-ot gene can cause severe osteoporosis with unclosed epiphyses and continued linear growth in males. An undesirable property of estrogen is its potential carcinogenicity. Estrogen exposure increases the risk of breast and endometrial cancer. Exposure to highpotency estrogens (e.g., diethylstilbestrol) during embryonic and neonatal development results in abnormalities of the reproductive tract and in increased incidence of reproductive tract tumors. The antiestrogenic action of selective estrogen response modifiers (e.g., tamoxifen) is used in the treatment of breast cancer (discussed later). The diverse actions of estrogen and its differential effects in tissues are mediated by ER-ot and ER-/3. The ERs perdominantly found in the cell nucleus are transcription factors whose action is similar to that of other steroid hormones (Chapter 30). In the target cells, ER is in the inactive state bound to heat-shock proteins. Estrogen binding to ER releases the associated proteins and facilitates formation of ligand-bound receptors. Receptor dimers bind to the cognate DNA response elements. The overall complex, with the recruitment of coactivators and other transcription factors, becomes an active transcriptional complex capable of gene expression. The dimerization of ER can involve either homo- or heterodimers--(ER-ol)2, (ER-/~)2, or ER-od ER-/~--thus expanding the physiological specificity and action of estrogen in the target tissue. Breast tumors that contain ER are treated with selective estrogen response modifiers (SERMs). Current assays for ER in breast tumors only identify ER-o~, but ER-ot- negative, ER-/~- positive tumors may also respond to SERM therapy. Ovary Estradiol and FSH stimulate proliferation of granulosa cells in recruited follicles and promote their secretory function. Estradiol, with FSH, transforms the basal subpopulation of granulosa cells (next to the basal lamina) into LH-responsive granulosa cells by inducing the synthesis of LH receptor sites. These intraovarian effects of estradiol require high levels of the hormone locally, which are normally achieved because of the autocrine action of these effects. Estradiol also exerts a paracrine effect in the ovary, inhibiting CYP 17 (both 17o~-hydroxylase and 17,20-1yase) activity in theca interna cells during their luteinization. In the corpus luteum, estradiol may be the cause of luteolysis in a nonfertile cycle, although the involvement of intraluteal PGF2~, is also implicated.

~96

Endometrium Estrogen stimulates proliferation of the epithelial and stromal cells of the endometrium and is essential for its repair (regeneration) following menstruation. Estrogen induces the synthesis of progesterone receptors in the endometrium and thereby permits the endometrium to respond to progesterone during the luteal phase. This permissive effect of estrogen decays rapidly and requires the continued presence of low levels of estrogen for maintenance; this explains why the production of estradiol during the luteal phase and during pregnancy is important. In pathological states, the endometrial tissue may grow at ectopic sites of the body, and has been found in the fallopian tube, abdominal cavity, ovary, and other sites, including the lung. This condition (endometriosis) is dependent on estrogenic stimulation but is not caused by estrogen; treatment with either an estrogen antagonist or with an androgen is effective. The stimulatory effect of estrogen on endometrial growth also explains why estrogen treatment of postmenopausal women carries with it an increased risk of developing endometrial hyperplasia and cancer; however, when combined with progesterone, the risk is markedly reduced. Myometrium Estrogen decreases the resting membrane potential of the myometrium and increases ciliary activity of the endometrial epithelium. Rising levels of estradiol prior to ovulation cause the smooth muscle of the fallopian tubes to become increasingly contractile at about the time of ovulation. This and the increased activity of cilia on the endometrial lining facilitate entry of the oocyte-cumulus complex into the lumen of the fallopian tube at the time of ovulation. In the pregnant uterus shortly before parturition, estradiol stimulates the formation of gap junctions and the production of myometrial receptors for oxytocin, PGF, and PGE, and thereby promotes the myometrial contractions that facilitate parturition. Mammary Gland Estrogen, in the presence of prolactin (PRL), stimulates ductal proliferation and thereby increases the area occupied by the branching ductules. This effect also occurs in males if the level of estrogen is sufficiently high to overcome the inhibitory effect of androgens. Estrogen induces synthesis of progesterone receptors in the ductal epithelium and probably induces oxytocin receptor sites in the myoepithelial cells that surround the alveoli and ductules. The stimulatory effect of estrogen on mammary gland growth continues in the presence of progesterone, such that mitotic growth of the mammary gland peaks during the luteal phase, in contrast to the endometrium, in which mitotic growth peaks in the follicular phase and is inhibited by luteal progesterone. This explains why the risk of breast cancer is approximately the same ('~32-41%) whether or not estrogen treatment is supplemented with progesterone. Estradiol inhibits the release of GnRH by reducing the amount of GnRH released


per pulse (reduces pulse height) without affecting the pulse frequency. This results in a corresponding reduction in the pulse height of LH and FSH that reduces their plasma concentrations. Estradiol also directly inhibits FSH release by reducing the production of FSH-/~ subunits but has no inhibitory effect on either LH-/3 or the ot subunit. In addition to its effect on GnRH and the pituitary gonadotropins, estrogen stimulates growth hormone (GH) synthesis and promotes an increase in the GH response to provocative stimuli. Estrogen decreases hypothalamic somatastatin content, increases somatotrope number, GH mRNA levels, and GH content. Bone At puberty, estrogen promotes height gain and accounts for the peak height velocity in girls. As in pubertal boys, the pubertal growth spurt in girls leads to cessation of linear growth due to ossification of the epiphyseal plate of long bone. In the adult, estrogen has an overall protective effect on bone tissue that results from its action on osteoblasts and osteoclasts in both cancellous (trabecular) and cortical bone. This explains why estrogen replacement therapy begun at the time of menopause effectively prevents the loss of bone mass (osteoporosis) associated with the postmenopausal state, if begun prior to the onset of osteoporotic changes. The molecular mechanism of estrogen action on bone mass may partially occur through the regulation of the gene for transforming growth factor (TGF-/~). Of the number of specific effects estrogen has been found to have on bone, the two most important are as follows: 1. Estrogen inhibits osteoclast-mediated resorption of cancellous and cortical bone by a direct effect on the osteoclasts; and 2. Estrogen inhibits the bone-mobilizing effect of parathyroid hormone (PTH) by a direct effect on the osteoblasts (Chapter 37).

Liver Estrogen has an overall trophic effect on the liver and opposes some of the hepatic effects of androgens, while promoting the hepatotropic actions of glucocorticoids. Thus, estrogen increases circulating levels of TeBG and thereby reduces the percentage of bioactive androgen in plasma. Estrogen stimulates the hepatic production of corticosteroid-binding globulin and angiotensinogen, and provides support for glucocorticoids in an additive fashion. Estrogen promotes blood clotting by stimulating hepatic production of some of the clotting factors (VII-X), while inhibiting formation of antithrombin (Chapter 36). These and possibly other effects are believed to contribute to the increased incidence of thromboembolism and thrombophlebitis in women on estrogen treatment alone or in combination with progestin. Skin Estrogen inhibits the stimulatory effect of androgen (DHT) on pilosebaceous activity, and thereby reduces

SECTION 34.2

Ovaries

the incidence of acne vulgaris. There is no evidence that estrogen can prevent the androgen-sensitive conversion of terminal to vellus hair in male pattern baldness. Adipose Tissue Estrogenic states favor subcutaneous fat deposition in the gluteofemoral region (lower body) and promote lower body obesity. Current evidence suggests that a lower body fat pattern, or a low upper-lower body circumference ratio (referred to as "gynoid"), is associated with a lower incidence of coronary heart disease and may be due to the effect of estrogen on lipoprotein metabolism. Cardiovascular System In women, estrogen protects against cardiovascular disease. The protective cardiovascular effects of estrogen include decreased serum LDL cholesterol and increased HDL cholesterol levels, vasodilatory effect, and antioxidation of LDL cholesterol (Chapter 20). Extensive clinical trials have shown that estrogen replacement therapy of postmenopausal women reduces the risk of heart disease. Brain Estrogen mediates learning and memory functions and also enhances cognitive processes. The brain can produce significant levels of estrogen, and brain tissues possess ER-fl.

797

are converted to their respective 2-hydroxylated metabolites. As the name suggests, the A ring of the catecholestrogens has a catechol structure; this makes for their rapid processing by COMT, which is present in many tissues, including the erythrocytes. Thus, catecholestrogens are rapidly O-methylated at position 2 and excreted as such. Catecholestrogen formation is increased in hyperthyroidism and decreased in hypothyroidism. A potentially mutagenic and carcinogenic metabolite of estradiol is 4-hydroxyestradiol, which is also a catecholestrogen. The hydroxylation of estradiol at the 4-position is mediated by a specific hydroxylase (CYP1B1). The activated 4-hydroxyestradiol gives rise to reactive, free-radical semiquinone / quinone intermediates that can damage DNA. Finally, estrone is converted to estrone sulfate by sulfotransferase, present in the liver and other tissues such as the uterus. Estrone sulfate is the most abundant estrogen in plasma, with peak concentrations about 2-10 times higher than these of unconjugated estrogens, probably due to its long half-life in circulation. Estrone sulfate is biologically inactive and can be metabolized to estriol and catecholestrone, but it is usually excreted as such by the kidney.

Metabolism of Estrogen The liver is the principal site of estrogen inactivation although target tissues are equipped with a means of controlling their estrogen microenvironment. The major route of estradiol metabolism is by way of estrone formation, which is essentially an inactivation process that converts an active estrogen (estradiol : 100% activity) into a less active one (estrone = 30% activity). Peripheral 17flHSD converts estradiol to estrone (15%) to a greater extent than the reverse reaction (5%). Estrone is metabolized by three independent pathways in the liver but is present in other tissues as well. The three pathways are as follows: 1. Estriol formation; 2. Catecholestrogen formation; and 3. Sulfotransferase reaction. The estriol route is the most significant, judging from the fact that estriol is the major urinary estrogen in humans. As its name suggests, estriol contains three hydroxyl groups, two of which are identical to those of estradiol (3/3, 17/3), but the third is unique to estriol, a 16a-hydroxyl group that is attached to either estrone or estradiol. Estriol formation is increased in obesity and hypothyroidism, and is decreased in hyperthyroidism. Catecholestrogen formation is catalyzed by a 2-hydroxylase that is present in the liver, nerve cells, and other tissues, and both estrone and estradiol

Selective Estrogen Receptor Modulators ( S E R M ) As discussed earlier, estrogens have tissue selectivity in part, based on the type of the estrogen receptor and DNAbound transcription complex present in the target cells. Drugs are being developed to mimic (agonist) or antagonize (antagonist) the effect of estrogen (Figure 34-7). An example of a synthetic estrogen is diethylstilbestrol. Tamoxifen and raloxifene are mixed agonists and antagonists of estrogen activity and are named SERMs (also known as designer estrogens). Tamoxifen antagonizes the action of estrogen in breast tissue and is used in the treatment of ER-positive breast cancer. However, tamoxifen is an estrogen agonist in the uterus and increases the risk for endometrial cancer. In bone and in the cardiovascular system, tamoxifen has beneficial effects similar to those of estrogen. Raloxifene is used in the treatment of osteoporosis as an antiresorptive agent in postmenopausal women but does not exhibit estrogen-related adverse effects on the endometrium. Raloxifene has a similar effect on the serum lipid profile compared to estrogen; however, neither tamoxifen or raloxifene increases the serum concentration of HDL-cholesterol. Many phytoestrogens contain nonsteroidal precursors of estrogenic substances (Figure 34-8), which may affect estrogen sensitive tissues if ingested in significant amounts. However, indications are that the intake must be very high for this to occur.

798

CHAPTER 34


SELECTIVE ESTROGEN RECEPTOR MODIFIERS

H3C~N~O'~h/-/~ ESTROGENS

OH

,3c

Lr#r

HO TAMOXIFEN

1713- ESTRADIOL

O OH

OH HO HO

RALOXIFENE DIETHYLSTILBESTROL (Synthetic)

FIGURE 34-7 Structures of estrogens and selectiveestrogen receptormodifiers.

Biological Effects of P r o g e s t e r o n e The human progesterone receptor (PR) gene on chromosome 11 encodes a single PR protein that is homologous to the other steroid hormone receptors but that binds proges-

EQUOL

OH

HO DAIDZEIN

OH

HO

GENISTEIN

OH

HO

FIGURE 34-8 Structures of phytoestrogens. These are diphenoliccompoundsfound in plants, such as soybeans, sprouts of cloves, and alfalfa.

terone preferentially. Like the ER, PR is predominantly found in the nucleus; however, it shuttles between the nucleus and the cytoplasm. PR exists in a stable form bound to heat-shock proteins (hsp), but is activated when progesterone (P) displaces the hsp and binds to the hormone binding domain. The P-PR complex dimerizes and binds to two half-sites of progesterone response elements (PREs) in chromatin; this promotes transcription of a progesteronesensitive gene. The PRE is identical to the glucocorticoid response element and androgen response element, but the significance of three hormones sharing the same response element in DNA is not known. PR production is stimulated by estrogen and in some tissues is downregulated by progesterone. Endometrium Following adequate priming of the endometrium by estradiol, progesterone increases glycogen deposition in the glandular epithelium and stimulates secretory function of the glands from about day 4 after ovulation. Progesterone also stimulates fluid accumulation in the endometrial stroma, with maximal edema occurring at about the time of implantation (day 7). Progesterone promotes the decidual reaction in the stroma in response to the implanting blastocyst and stimulates the production of decidual prolactin (dPRL) by the decidual cells. In the absence of conception, progesterone causes predecidualization of the stroma from about day 9 in a process of terminal (irreversible) differentiation that leads to degeneration of the tissue when progesterone levels decline.

SECTION 34.2

7s

Ovaries

Progesterone is required for pregnancy to continue, in part because it keeps the spiral arteries patent and promotes maternal blood flow to the placenta. Progesterone is also involved in the pathogenesis of endometriosis, a condition in which endometrial tissue grows in ectopic sites (most commonly in the peritoneal cavity); treatment with danazol, a synthetic androgen that antagonizes progesterone, reduces growth of the tissue. Myometrium Progesterone increases the resting membrane potential of the myometrium and thereby reduces its contractility. Progesterone antagonizes the effect of estrogen by inhibiting synthesis of ER and blocks PGFzainduced contraction of the myometrium. Accordingly, the administration of an antiprogesterone causes a rise in myometrial ER and increased responsiveness to PGFzainduced contractions. There is evidence that myometrial contractility is promoted by the formation of gap junctions between muscle cells; progesterone presumably inhibits gap junction formation and therefore inhibits myometrial contractions. Mammary Gland In the presence of PRL, progesterone stimulates lobuloalveolar development in the breast of pubertal girls, but only after the tissue has been stimulated by estrogen. As discussed above, estrogen promotes the growth of the ducts and induces the synthesis of progesterone receptors in the tubular epithelium. During the luteal phase of the menstrual cycle and during pregnancy, progesterone (in the presence of estrogen and PRL) stimulates maximal proliferative growth of the breast mainly by alveolar (glandular) growth. In breast cancer cells, progesterone reduces the formation of estrogen from androgenic precursors but increases the production of some of the autocrine growth factors (e.g., TGF-o~). Central Nervous System In addition to its inhibitory effect on GnRH secretion, progesterone causes thermogenesis during the menstrual cycle and pregnancy. In a normal menstrual cycle, the oral basal temperature in most women increases by about 0.5~ around the time of ovulation and remains elevated until shortly before the onset of menses. In pregnant women, the oral basal temperature remains elevated to term. An anesthesizing and EEG-altering effect of progesterone administered to laboratory rodents and the increase in ventilatory drive (hyperventilation) stimulated by progesterone may also reflect CNS actions of the steroid. Immunosuppression and Antiinflammatory Effects In addition to its ability to suppress myometrial and endometrial prostaglandin formation, progesterone suppresses T-lymphocyte proliferation, interleukin-8 synthesis, and increases prostaglandin dehydrogenase activity, all of which contribute to preventing maternal rejection of the implanting conceptus (an allograft).

Pharmacological Enhancement of Fertility Female infertility due to inadequate gonadotropin stimulation of the ovary has been successfully treated by two approaches: 1. Increasing gonadotropin levels by supplying exogenous gonadotropins, and 2. Increasing gonadotropin levels by stimulating their release from the pituitary.

Exogenous Gonadotropin Treatment Women who are infertile because of a deficiency in gonadotropins are given exogenous FSH for 7-12 days to promote folliculogenesis followed by a single large dose of hCG to induce ovulation. This has resulted in ovulation in a majority of patients (~ 90%) and pregnancy in about half. However, about 17-20% of the patients have multiple births due to the secretion of more than one follicle.

Stimulation of Endogenous Gonadotropin Secretion In women who are infertile because of a neuroendocrine system that is overly sensitive to negative estrogen feedback, an antiestrogen is used to reduce the intensity of the negative feedback. By reducing the intensity of estrogen feedback, normal gonadotropin secretion should resume and lead to ovulation. Clomiphene (50 mg) is taken daily for 5 days to block the estrogen effect in the neuroendocrine system; in a majority of patients (~ 80%), ovulation occurs about a week later, and about 30-40% of the women conceive. The incidence of multiple birth by this method (,~ 8%) is higher than in untreated patients (,~ 1%) but lower than with gonadotropin treatment. In some laboratories that conduct in vitro fertilization, the two protocols (exogenous gonadotropins and clomiphene) are combined for a greater yield of secondary oocytes.

Pharmacological Prevention of Pregnancy (Female Contraception) Currently, the most effective method to prevent conception is by oral steroid hormone treatment (Figure 34-9) to inhibit or modify the cyclic changes in endogenous reproductive hormones. Two effective types of oral contraceptives are used with proven success: one to prevent ovulation, the other to prevent implantation. The most effective means of preventing ovulation is by the oral administration of ovulation-inducing gonadal steroid derivatives at dosages that will prevent cyclic changes in LH and FSH secretion. This method has proven to be more than 99% effective over the past 40 years and is currently used by more than 50 million women worldwide. The treatment protocol is of four different types, all of which involve the use of synthetic steroid

800

CHAPTER34 Endocrine Metabolism V: Reproductive System 0 II OCCH3 C-CH

OH

Estrogens

C-CH

HaCCO

NO

Ethinylestradiol

Ethynodiol diacetate H

OH

C-O

C-CH

CH30

0

Mestranol

CN 3 Medroxyprogesterone acetate

Progestins

OH C - C H

OH --C-CH

HO O

Norethindrone

Norgestrol

(A)

H C=O ,~Ac ~H3 H3C

~

0

OH

O

--

=CH-

CH3

Medgestrol acetate (B)

RU486 (Mifepristone)

(c) F I G U R E 34-9 Structures of steroids used as oral contraceptives. (A) Combination pills containing an estrogen + progestin. (B) The progestin-only pill. (C) RU-486 or mifepristone, which can be utilized as an abortifacient. [Reproduced with permission from S. W. Norman and G. Litwack, Hormones, 2nd ed. San Diego: Academic Press, 1997].

derivatives that have long biological half-lives. The first three protocols involve estrogen + progestin combinations taken for 3 weeks, followed by no treatment for 1 week. 1. Fixed combined dose of estrogen + progestin daily ("combination pill");

2. Biphasic type: fixed dose estrogen + two different doses of progestin; 3. Triphasic type: fixed dose estrogen + three different doses of progestin (stepwise increase at 1-week intervals); and 4. "Mini" pill, containing a progestin only; taken daily without interval of withdrawal.

801


Among these, combination pill is the most widely used and is probably the most effective at > 99%, whereas the mini pill is the least effective at ~ 9 5 % because it does not always prevent ovulation. The rationale for varying the dose of progestin was that by reducing the total amount of steroid, it would reduce the risk of myocardial infarction, hypertension, and stroke, all of which were subsequently shown not to be altered by varying the dose of progestin. The rationale for the progestin only protocol was to eliminate the risk of endometrial cancer and breast cancer associated with estrogen; however, some of the health benefits of estrogen (e.g., increased HDL, osteoprotection) are eliminated and some side effects (e.g., breakthrough bleeding) are introduced. In women using the combination pill, the plasma levels of LH, FSH, estradiol, and progesterone were found to be greatly suppressed and unchanged throughout the 3 week period on the pill. Folliculogenesis did not occur during treatment but there was no evidence of increased atresia; when electing to bear a child, former pill users experienced normal menstrual cyclicity, pregnancy, and lactation. Long term observations indicated that pill users experienced menopause within the normal age range, and had no health problems that could be attributed to the long term use of the pill, although recent epidemiological studies suggest that combination pill users may be at increased risk of developing breast cancer prior to age 45. Post-coital pharmacologic prevention of implantation can be achieved by oral administration of a synthetic estrogen (25 mg diethylstilbestrol) twice daily for 5 days. This "morning after pill" treatment stimulates fallopian tube contractions during the period of conceptus travel toward the uterus, such that the conceptus is propelled into the uterus prematurely and is resorbed, or is trapped within the fallopian tube because of spastic contraction of the isthmus. Although effective, the high dose estrogen produces nausea, vomiting, and menstrual problems. Alternatively, oral administration of a combination pill (50 # g ethinylestradiol + 0.5 mg norgestrol), two tablets within 72 h post-coitum and two tablets 12 h later, is also effective and does not produce undesirable side effects. This treatment does not involve an alteration in fallopian tube activity but the treatment may serve to convert the endometrium into a less receptive tissue for blastocyst implantation. Another post-coital treatment is the use of a progesterone antagonist, mifepristone (RU-486), which apparently works by reversing the effects of progesterone on the endometrium, and thereby interferes with implantation. Administration of the drug two days after the midcycle LH surge prevents implantation by a direct effect on the endometrium, and has no effect on the circulating levels of gonadotropins or on luteal function.

Supplemental Readings and References C. J. Bagatell and W. J. Bremner: Drug therapy: Androgens in men: Uses and abuses. New England Journal of Medicine 334, 707 (1996). S. Bhasin and W. J. Bremner: Clinical review 85: Emerging issues in androgen replacement therapy. Journal of Clinical Endocrinology and Metabolism 82, 3(1997). S. Bhasin, T. W. Storer, N. Berman, and others: The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. New England Journal of Medicine 335, 1 (1996). E. G. Biglieri: A prismatic case: 17ot-Hydroxylase deficiency: 19631966. Journal of Clinical Endocrinology and Metabolism 82, 48 (1997). B. C. J. M. Fauser and A. M. Van Heusden: Manipulation of human ovarian function: physiological concepts and clinical consequences. Endocrine Review 18, 71 (1997). A. Gougeon: Regulation of ovarian follicular development in primates: facts and hypotheses. Endocrine Review 17, 121(1996). K. B. Horwitz: The molecular biology of RU486. Is there a role for antiprogestins in the treatment of breast cancer? Endocrine Review 13, 146 (1992). J. L. Jameson and A. N. Hollenberg: Regulation of chorionic gonadotropin (hCG) gene expression. Endocrine Review 14, 203 (1993). A. L. Kierszenbaum: Mammalian spermatogenesis in vivo and in vitro: a partnership of spermatogenic and somatic cell lineages. Endocrine Review 15, 116 (1994). C. S. Kovacs and H. M. Kronenberg: Maternal-fetal calcium and bone metabolism during pregnancy, puerperium, and lactation. Endocrine Review 18, 832 (1997). R. A. Wild: Metabolic and cardiovascular issues in women with androgen excess. Endocrinologist 6, 120 (1996). J. D. Wilson: Androgen abuse by athletes Endocrine Review 9, 181 (1988). J. D. Wilson, J. E. Griffin, and D. W. Russell: Steroid 5ot-reductase 2 deficiency. Endocrine Review 14, 577 (1993). M. D. Pisarska, S. A. Carson, and J. E. Buster: Ectopic Pregnancy. The Lancet 351, 1115 (1998). D. T. Baird: Amenorrhoea. The Lancet 350, 275 (1997). E Muscatelli, T. M. Strom, A. E Walker, and others: Mutations in the DAXC-1 gene give rise to both X-linked adrenal hypoplasia congenita and hypogonadtropic hypogonadism. Nature 372, 672 (1994). A. Swain, V. Narvaez, E Burgoyne, and others: Daxl antagonizes Sry in mammalian sex determination. Nature 391, 761 (1998). H. E. Maclean, G. L. Wame, and J. D. Zajact: Intersex disorders: shedding light on male sexual differentiation beyond SRY. Clinical Molecular Endocrinology 46, 101 (1997). A. Glasier: Emergency postcoital contraception. New England Journal of Medicine 337, 1058 (1997). E Muscatelli, T. M. Strom, A. E Walker, et al.: Mutations in the DAXC-1 gene give rise to both X-linked adrenal hypoplasia congenita and hypogonado, tropic hypogonadism. Nature 372, 672 (1994). A. Swain, V. Narvaez, E Burgoyne, et al.: Daxl antagonizes Sry in mammalian sex determination. Nature 391, 761 (1998). H. E. Maclean, G. L. Warne, and Zajact: Intersex disorders: shedding light on male sexual differentiation beyond SRY. Clinical Molecular Endocrinology 46, 101 (1997). G. E1-Hajj Fuleihan: Tissue-specific estrogens--the promise for the future. New England Journal of Medicine 337, 1686 (1997). E D. Delmas, N. H. Bjarnason, B. H. Mitlak, et al.: Effects of raloxifene on bone mineral density, serum cholesterol concentrations, and uterine endometrium in postmenopausal women. New England Journal of Medicine 337, 1641 (1997). W. Khovidhunkit and D. M. Shoback: Clinical effects of raloxifene hydrochloride in women. Annals of Internal Medicine 130, 431 (1999).


CHAPTER

35

Molecular Immunology

The ability of vertebrate animals to resist and overcome disease caused by infectious agents and toxic proteins is provided by the immune system. The immune system comprises specialized tissues, cells, highly specific proteins (e.g., antibodies and receptors), and unique genetic mechanisms for creating the many different defenses that the organism requires for combating infectious diseases. Table 35-1 provides a glossary and cross-references to tables and figures that relate these components and their functions to the molecules of the immune system. The cells and molecules composing the immune system are designed to attack and destroy infectious and foreign substances, but do not under normal circumstances attack the host organism's tissues, cells, or molecules. This ability to discriminate between self and nonself is an essential property of the immune recognition system. Immunity is obtained through reactions of cells and molecules that are compone.nts of two distinguishable imThe term immunity,which is derived from the Greek immunitas, means exemption from service or duty to state; when extended to the organism,it means exemptionfrom disease. Abbreviations:Ab, antibody;Ag, antigen; MAb, monoclonalantibody; TcR, T-cellreceptor; Th, T helper cell; Tc, cytotoxic T cell; CH, constantregion, heavychain; CL, constantregion, light chain; VH, variable region, heavy chain; VL, variable region, light chain; CDR, complementarity-determiningregion; CD, cluster of differentiation; IL, interleukin; MAC, membrane attack complex CX, complement component (numbered); Fab, antibody fragment, single arm; F(ab)', antibody fragment, two arms;Fc, antibodyfragment,heavychains; ADCCantibodydependent, cell-mediated cytoxicity.

mune response systems. The first, innate immunity, is the simpler of the two systems and remains essentially unchanged upon repeated exposure to an infectious agent or foreign substance. In contrast, the second system, acquired immunity, is enhanced upon repeated exposure to infectious agents. The "memory" capacity of acquired immunity enables the immune system to respond more rapidly and more extensively when an infectious agent or foreign substance is encountered a second time and is the basis for resistance to organisms previously encountered and explains the effectiveness of vaccination.

35.1 Innate Immunity The first line of defense against infectious agents is provided by the innate immunity of the organism. This defense is the result of physical barriers such as skin, the cells that line the gastrointestinal and genitourinary tracts, and the mucus that these cells secrete. Mucous secretions act to move infectious agents away from their sites of attachment and out of the body. Infections from bacteria are also opposed by the enzyme lysozyme, which is capable of degrading the carbohydrate structures of the bacterial cell walls. One particular type of antibody molecule, IgA (see below), is present in the mucous secretions and participates in this first line of defense against invading organisms.

803

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CHAPTER35 Molecular Immunology

TABLE 35-1

Components of the Immune System Cellular (Leukocyte)

Components Lymphoid Cells B-cells (B lymphocytes) Mature in bone marrow, reside in lymph nodes, spleen, alimentary and respiratory tract mucosa

T-cells (T lymphocytes) Mature in thymus, reside in thymus

Th (T helper cells)

Thl Th2

T Suppressor cells CTLs (Cytotoxic Tlymphocytes)

Natural killer cells (NK cells)

Principal Functions

Characteristic Receptors/Specific Recognition Molecules (CD's)

Antigen receptors are surface IgM (monomeric IgM), IgD. As APCs B-cells bind exogenous antigens and present processed antigen via MHC Class II to T cells. Express CD3, CD19, CD20, CD21 (complement receptor), CD32 (FCTReceptor II), CD35 (complement receptor), CD40 and CD45 on surface Recognize antigens on APCs Regulation of T and B-cell via TcRs (T-cell receptors) immune response antigen (epitope) specific All T-cells express CD3 on their surface Recognize antigens (epitopes) T-cell subclass that release on APCs via TcRs, specifically necessary cytokines for cell MHC Class H receptors stimulation and proliferation Express CD3 and CD4, on Thl - regulation of cellular surface, CD4 used to identify immunity via IL2 and IFN-7 T-helper cells in cell sorting Th2- B-cell stimulation via procedures. Also express CD2, IL's 4, 5, 6 and 10 CD28 and CD45R Suppress B and T cell responses TcR, CD3 Express CD8 on surface to antigen stimulation Possess TcR, IL-2R, (IL-2 Cytotoxic against tumor cells, receptor) recognize processed and virus infected host cells. antigens (epitopes) via Destroy infected cells through MHC Class I receptors perforin-dependent permeability on APCs. Express CD3 changes. Produce TNFcz and and CD8, (also CD2, CD28) TNF/3. Promote apoptosis. NO Ig, CD3 or TcRs Granular lymphocytes that Express CD16 (FcTIIR), i.e. recognize and destroy virussurface receptors that bind via infected and cancer cells and Fc portion of IgGs use ADCC. Exclude destruction Express CD56 on surface. of cells that express MHC-I. CD16 and CD56 used for Stimulated by IL-2, IFN-Z identification in cell sorting TNF-a. Create pores that permit transfer of proteins which promote apoptosis of infected cells Recognition of antigens, become antibody-forming plasma cells (synthesize Ig molecules), act as antigen presenting cells (APCs), some B cells defferentiate into memory cells for rapid recognition of previously encountered antigens

Related Figure(s) or Tables Figure 1 Figure 18

Figure 1

Figure 3

Figure 4

(continued)

TABLE 35-1

Components of the Immune System (continued) Cellular (Leukocyte) Components Myeloid Cells Neutrophils Mature in bone marrow

Basophils Mast cells Mature in bone marrow, resident in tissue

Eosinophil

Monocytes Mature in bone marrow, found in blood

Macrophages Mature in bone marrow as monocytes, monocytes mature further to tissue resident macrophages

Principal Functions First cells recruited to sites of inflammation, dominant phagocytic granulocyte, able to mediate ADCC (antibodydependent cell-mediated cytotoxicity), major defense against pyogenic bacteria, use respiratory burst to kill cells Blood "analog" of tissue mast cell, stimulated by complement C3 Tissue granulocytes that produce cytokines, release heparin, histamine Attack parasites too large for phagocytosis, possess granules that contain toxic proteins, adhere to foreign organisms via C3b. Release protins and enzymes that damage foreign organism membranes, kill with active oxygen burst. Engage in IgE-mediated ADCC Mononuclear phagocyte in blood, are precursors to tissue macrophage. Process protein antigens and present peptides to T-cells via MHC I and II. Engage in IgG-mediated ADCC Tissue phagocytes (APCs) that processes antigens for presentation to T cells, possess MHC receptors, principal attack is on bacteria, viruses and protozoa that enter cells. Engage in IGG-mediated ADCC

Characteristic Receptors/Specific APC Recognition Adhere to foreign cells via CDllb/CD18 No MHC proteins. Neutrophils are attracted to sites of complement activation by C5a, a chemotactic peptide from C5

Related Figure(s) or Tables Figure 4

Possess Fc receptors that bind IgE causing degranulation and histamine release Fc receptors that bind IgE causing degranulation and histamine release High-affinity receptors for IgE, bind via Fc region of antibody

Express MHC Class II surface proteins, do not possess antigen-specific receptors

Figure 2

Express MHC Class II surface proteins, do not possess antigen-specific receptors, but have receptors for C3b.

Figure 2

Dendritic cells Follicular

Interdigitating

Antigen-antibody and Ag-Ab Possess Fc receptors complexes with complement No MHC class II receptors are localized in these cells. Involved in maintenance of B cell memory Found in skin (Langerhans Do not have Fc receptors cells) in T cell areas of lymph Possess MHC class II receptors. nodes and spleen. Recognize Express CD80 and CD86 and endocytose foreign carbohydrate antigens that are not found in vertebrate animals. Act as APCs to CD 28-bearing T cells, stimulated by IFN t~, TNF a.

(continued)

806


TABLE 35-1

Components of the Immune System (continued)

Protein Components Immunoglobulins

Cytokines

CD3 Complex CD3D(delta) CD3E(epsilon) CD3G(gamma) CD3H(eta) CD3Z(zeta) CD4

CD8

Interleukins Perforin B cell receptors T cell receptors (TcR) (Many copies of each TcR on each cell, all directed for the same epitope (antigen))

Principal Functions

Structural Characteristics

Four chains, two heavy and two Ig's confer humoral immunity, light chains, charateristic fold react with epitopes on antigens between/3-sheet domains, when bound to antigens initiate "immunoglobulin" fold complement activation Proteins that can communicate with and stimulate immune and other cells to proliferate, synthesize specific proteins Heterodimer of either: a/13 or T-cell surface glycoprotein found on all T-cells 6/Tchains Forms a complex with TcR Mediates signal transduction across the cell membrane T-cell surface glycoprotein, Interacts with IL-16, coreceptor with TcR in recognizing MHC Class II presented peptides, identifier of T-helper cells T-cell surface glycoprotein, Interacts with MHC Class I Identifier of cytotoxic or suppressor T-cells See Table 35-4 Permeability induction for ADCC cell destruction Specific to individual antigen (epitope) T-cell surface glycoprotein, with CD4 or CD8 binds to MHC proteins on surface of antigen presenting cells

Structure: Single polypeptide chain, 3 domains, 1) extracellular, (-,-370 res); 2) transmembrane, (-25 res) 3) intracellular (-38 res) Heterodimer of a/~ chains, can form homodimers

Related Figure(s) or Tables Figure 7 Table 35-2

Table 35-4

Figure 4 Figure 16 Figure 17

Figure 3

Table 35-4

IgM molecules anchored to B-cell membrane Structure: 2 a/3-polypeptide chains with immunoglobulin folds; VaC a and V/3C~ OR 2 6),polypeptide chains (less prevalent form)

Figure 1 Table 35-2 Figures I and 3 Figure 14

(continued)

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SECTION35.1 Innate Immunity

TABLE 35-1

Components of the Immune System (continued)

Protein Components Complement

Classical pathway

Alternative pathway (Properdin pathway) Lectin pathway

C3 Convertase

Opsonins

Antigen Presenting Proteins (MHC proteins)

Principal Functions Plasma proteins that act sequentially to produce cell lysis, facilitate phagocytosis Creates the proteolytic enzyme complex, C3 convertase (C4b2b) Creates the proteolytic enzyme complex, C3 convertase (C3bBb) Recognizes mannose in oligosaccharide chains Principal agent responsible for reactions that lead to formation of the membrane attack complex and lysis of the invading cells C3b, C5b, the mannose-binding protein, CRP (C-reactive protein) are examples of opsonins Proteins that display peptide fragments of foreign (nonself) proteins on the surface of cells

Class I MHC HLA-A, HLA-B, HLA-C

Present on all nucleated cells, present peptides to CD8 cytotoxic T-lymphocytes (CTLs, Tc), presents endogenously synthesized peptides (e.g. virus coded peptides)

Class II MHC HLA-DP, HLA-DR, HLA-DQ

Present on macrophages and B-cells, presents peptides from processed exogenous or vacuolar antigens for recognition by CD4, Tffcells Not MHC gene coded, but is a part of the MHC class I antigen processing complex

/32-microglobulin

Comments

Related Figure(s) or Tables Table 35-3

Initiated by interaction with antigen-antibody complexes

Figure 21

Initiated by reaction of the C3 thioester with nucleophilic molecules on cell surfaces Initiated by binding of mannose binding protein to foreign cell carbohydrate residues NOTE: Two different C3 convertases are formed by the classical and alternative pathways

Figure 21

Proteins that adhere to microbe surfaces making them "attractive" for attack by complement and phagocytes Human leukocyte antigens Class I: HLA-A, HLA-B, HLA-C Class H: HLA-DP, HLA-DR, HLA-DQ Structure: a-polypeptide chain, 3 domains, O~1, a2, a 3 and J~2 microglobulin (Not a MHC protein) High degree of sequence variability in a l and a 2 domains Structure: ~/3-polypeptide chain, 2 domains, a 1,131 and a 2 ~ 2 Variability in both a and 13chains

Figure 21

Figure 21

Figures 1, 2, and 3

Figure 3 Figures 13 and 14

Figures 2 and 3 Figure 15

Figure 14 Figure 15

808

Cells and proteins that participate in the innate immune response recognize generic "marker" molecules (e.g., complex polysaccharides) on foreign microbes and other substances that are generally not found in the host. Attack on the "marked" invading microorganisms by phagocytes engulf and destroy the infectious organisms. These phagocytic cells kill infectious microbes by releasing reactive oxygen species and nitric oxide that are produced by the phagocyte and by releasing enzymes that initiate programmed cell death (apoptosis). Organisms that are too large for phagocytosis, such as large parasites or worms, are attacked by a specialized leukocyte, the eosinophil, which is capable of injecting toxic substances into the organism to kill it. The phagocytic cells, i.e., neutrophils, monocytes, macrophages, and dendritic cells, are myeloid cells that actively participate in innate immunity and are one lineage of stem cell differentiation (Table 35-1). Mutations in myeloid cells are one of the causes of the myelocytic

leukemias. The proteins of the complement system provide a second line of defense for the infected host. Complement proteins are so named because they complement the action of the phagocytes and antibodies. However, antibodies are not involved in the action of complement in innate immune response. Complement proteins, which are present in the circulating blood, attach covalently to bacterial cells and undergo a series of proteolytic reactions that activate the complement system and enhance the innate immune response. Binding of the complement activation products C3b and C4b and another plasma protein, CRP (C-reactive protein, one of a group of proteins called acute phase reactants) to the surface of foreign organisms is a process called opsonization.This process labels the organism as nonself and promotes adhesion of phagocytes to the invading microbes, resulting in phagocytosis of the opsonized organisms. The terminal product of complement activation, the membrane attack complex (MAC), creates a pore in the organism, making the microbe permeable to water and other solutes and ultimately causing the microbe to lyse. The MAC is composed of components C5b, C6, C7, C8, and C9 (see Table 35-3). Fragments produced during complement activation also act as chemotactic and inflammatory agents and attract phagocytes to the sites of invasion and injury and to the opsonized microbes. These potent products of complement activation cause blood vessel dilation and facilitate the movement of phagocytic cells from the bloodstream into the surrounding tissues where the infectious organisms may be located. The innate immune response involves only one of three pathways by which complement is activated, i.e., the "alternative pathways of complement activation" (see complement).


35.2 Acquired or Adaptive Immunity The general mechanisms responsible for innate immunity are inadequate to deal with all types of organisms, e.g., different strains of bacteria and viruses. These inadequacies are compensated for by the specific abilities of the acquired or adaptive immune system. Leukocytes, i.e., white blood cells (specifically B and T lymphocytes), are the agents of acquired immunity. B cells are so designated because they mature in the bone marrow; T cells because they mature in the thymus. B and T cells perform several biological processes that are uniquely responsive to repeated infection by foreign organisms. Adaptive immune responses that are responsible for these unique properties of the immune system occur in specialized regions (germinal centers) of the lymph nodes, the spleen, and mucosal lymphoid tissues, e.g., tonsils and adenoids. Adaptive immunity provides a defence against some of the pathogens that avoid the innate immune system and can mount an attack against the evolving and ever changing characteristics of disease-causing organisms, e.g., different strains of bacteria and viruses, such as those that cause influenza. Preexisting B-lymphocytes are present from birth and can recognize and bind antigens, more specifically epitopes (restricted portions of the antigen), on infectious agents or foreign macromolecules. The receptors on a Bcell surface that recognize these epitopes are antibody molecules that are synthesized by the B cell. After endocytosis and processing of the bound antigen by the B cell, the B cells are stimulated to divide and produce more B cells. This is a cooperative interaction between the B cells and specific T cells and signaling molecules called cytokines that result in cooperative interactions with T cells is B-cell proliferation and differentiation. Some of the stimulated and proliferating B cells undergo a change to a specialized variety of B cell, called a plasma cell. The plasma cells synthesize large amounts of a specific antibody that recognizes an epitope on the antigen to which the B cell was originally bound. The antibodies are secreted into the extracellular fluid where they provide a highly specific line of defense against current and future disease-causing agents. Other stimulated B cells differentiate to become very long-lived cells called

memory cells. The antibodies secreted by plasma cells are distributed to blood, lymph, and interstitial fluid where they are capable of binding to the infectious organism. Once these antibodies react with cell surface antigens on the infectious organism or foreign macromolecule, the cell or macromolecule has a "signal" that attracts circulating complement proteins. This antibody-based component of acquired immunity is commonly designated humoral

SECTION 35.2

immunity.

809

Acquired or Adaptive Immunity

The antigen-antibody complexes cause complement proteins to be activated so that they can attack and lyse invading microbes or signal phagocytic cells to destroy the invaders. Complement activation that occurs in response to antigen-antibody complexes occurs via the "classical pathway" of complement activation. Acquired immunity is dependent on the presence of low levels of antibodies that recognise antigens of the infecting organism, phagocytic cells that respond to antibody tagged antigens, and memory B cells that are poised to produce large quantities of antibodies to the infecting agents. Memory B cells live for long periods and respond quickly with enhanced antibody production upon reinfection with organisms or foreign agents that have been previously encountered. Some T cells also possess memory and are similarly poised for rapid response should a reencounter with the same antigen occur. Immunological memory, involving both B cells and T cells, is the mechanism that makes vaccination effective and explains why an individual does not become ill a second time when exposed to the same pathogens, e.g., the pathogens that cause measles or whooping cough. Infectious agents such as viruses and other microbes (e.g., mycobacteria), as well as parasites, enter the host cells or are engulfed by macrophages and become inaccessible to attack by B cells and antibodies. Lysosomal (endosomal) enzymes of the infected host cells degrade the components of the infecting organism. The resulting degradation products (e.g., peptides), bound to specialized host cell proteins, are transported to the external surface of the cell where they are presented to the extracellular milieu bound to specialized host cell membrane proteins. The proteins that bind these antigen degradation products are components of the major histocompatibility complex (MHC), or HLA system. Cells capable of presentation of peptides of the infectious organisms are described as "antigen-presenting cells" (APCs). The group of MHC proteins involved in presentation of antigen by macrophages and by B cells are members of "class II" MHC proteins. The antigen-presenting cells with digestion products from the invading agent now bound to their external surfaces attract CD4 + T lymphocytes (T-helper cells) capable of recognizing the degraded pieces of the antigen from the infectious organism. The T-helper cells produce protein messengers or effector molecules (cytokines). Cytokines provided by the T cells "direct" the differentiation of B cells to form plasma cells and memory cells and also initiate mechanisms that result in inactivation of the virus. Cytokines also signal other specific T lymphocytes, thus preparing them to respond to the infectious organism(s). Recognition occurs between the MHC II-presented molecules

on the APC and an antigen (epitope)-specific T-cell receptor (TcR). The TcR molecules are, like antibodies, specific for the antigen (e.g., epitopes on the peptides) and for the particular MHC protein. The recognition of molecules presented by MHC II proteins involves another T-cell membrane protein, CD4 (cluster differentiation protein 4). The immune response that involves B cells and MHC class II antigen presentation is schematically illustrated in Figure 35-1 and that which involves macrophages and MHC class II antigen presentation in shown in Figure 35-2. Nucleated cells other than those specific to the immune system process infectious agents such as viruses somewhat differently from the MHC II mechanism. For example, viral DNA or RNA replication within the host cells results in the production of new virus, including viral proteins. These newly synthesized viral proteins are degraded to peptides within the infected cell and are bound to an MHCrelated carrier protein. The bound peptides are transported to the endoplasmic reticulum and subsequently to the external surface of the cell membrane. Recognition of the degraded pieces of the infectious agent again occurs via proteins of the MHC system proteins; however, in this case it is with proteins of the "class I" MHC. The presentation of the viral protein-derived peptides on the surface of the cell labels the cell as infected and makes it a target for destruction. A second group of T cells, CD8 + cytotoxie T cells (frequently abbreviated CTLs or Tc cells), are similar to CD4 + T cells in that each type of T cell clone interacts only with a particular peptide epitope presented by MHC proteins. Cytotoxic T cells bind to and kill the marked, infected cells via specific TcRs. These T cells undergo clonal proliferation to create many additional T cells that are specific for the particular epitopes that the MHC I protein presents and only that T-cell clone recognizes. Among theses T cells are some that persist for a long time and thus become "memory" T cells, poised to prevent later infection by the same agent. These T cells release the cytokine, interferon-F, which prepares other cells to resist and destroy the invading virus. Cytotoxic T cells and their TcRs also act in conjunction with another membrane protein, CD8. Cellular immune response that involves almost any "ordinary" cell and MHC class I antigen presentation is shown schematically in Figure 35-3. The cells that are the principal agents of adaptive or acquired immunity are from the lymphoid lineage of stem cells. Similar to the situation with myeloid cells, mutations in lymphoid precursor cells give rise to the lymphocytic

leukemias. In summary, acquired immunity is a lymphocytedependent (B and T cell) process by which molecular properties of infectious agents are recognized, anti-infectious

810


F I G U R E 35-1 (Also see color figure.) Antigen recognition by B-cell surface receptors. Response by B cells begins with antigen binding to receptors on the B-cell surface (membrane-bound IgM and/or IgD molecules). Phagocytosis by the B cell transports the antigen into the cell where it is degraded to peptides by proteolytic enzymes. The antigen-derived peptides are next transported to the surface and bind to membrane proteins of the MHC. The peptides are presented by the MHC proteins (specifically MHC II) to T-helper (Th)cells. The Th cells bind to MHC II proteins via a specific T-cell receptor (TcR) and the peptides displayed on MHC II molecules. Interleukin-4 from the Th cells (marked by the surface protein CD4) stimulate the B cells to proliferate and to synthesize and secrete antibodies.

F I G U R E 35-2 (Also see color figure.) Antigen-antibody complex formation and recognition by macrophages. Antigens that react directly with antibodies in the circulation and in the extravascular fluid are recognized and removed by macrophages. Recognition occurs with the Fc region of the antibody in the Ag-Ab complex. The antigen-antibody complex is bound to Fc receptors on the external surface of the macrophage. The antigen is taken into the cell by endocytosis and degraded proteolytically. The antigen-derived peptides are transported and bound to the surface MHC II proteins. The MHC II peptide complexes are presented to the antigen-specific T cells.

F I G U R E 35-3 (Also see color figure.) Antigen uptake and processing by nonimmune system cells. Microbes and viruses can invade and infect any nucleated cell, not only those involved in the immune response. The invading organism is digested by lysosomal enzymes. Peptides encoded by infecting DNA or RNA from the microbe are synthesized in the cell and then transported to and displayed on MHC I molecules on the surface. The MHC I displayed peptides are recognized by a T-cell receptor (TcR) on the surface of cytotoxic T lymphocytes (CTLs). The infected cells are made permeable by the protein perforin and destroyed by apoptosis.

811

SECTION35.5 Molecules and Chemical Processes of the Immune System

agent antibodies synthesized, and the infectious agents destroyed. Because the selection of B cells and T cells by an antigen results in proliferation of only clones that recognize a specific epitopes of the antigen, an amplified, specific response occurs. Persistence of some of the specific B and T lymphocytes provides memory of the first encounter with the infectious organism so that subsequent infections by the same agent are opposed by a faster and more effective immune response.

35.3 Antibody-Dependent Cell-Mediated Cytotoxicity Another process involving antigen-antibody complexes exists for killing infected cells. In this process, circulating antibodies bind to epitopes of the viral proteins or glycoproteins exposed on the host cell membrane surface. These "antibody-tagged" cells are then recognized by granular lymphocytes called natural killer (NK) cells. Recognition of the "tagged," infected cells by NK cells occurs through a structurally similar region of all antibody molecules (the Fc region), not through the specific antigen-recognizing regions of the antibody. This mechanism is called antibody-dependent, cell-mediated cytotoxicity (ADCC). Immune response involving neutrophils and NK cells is schematically illustrated in Figure 35-4. (Interaction of a bacterial protein with the Fc region of an antibody is shown in Figure 35-10). NK cells, monocytes, neutrophils, and macrophages are involved in IgG-mediated ADCC (see below); eosinophils are involved in IgE-mediated ADCC.

F I G U R E 35-4 (Also see color figure.) Microbe destruction by neutrophils and antibody-dependent, cell-mediated cytotoxicity. (A) Neutrophils recognize and attack antibody-tagged microorganisms by a mechanism called antibody-dependent, cell-mediated cytotoxicity (ADCC). In this defense mechanism, microbes "tagged" by antibodies are destroyed by a respiratory burst of Of, H202, and NO produced by the attacking neutrophils. (B) In a similar way, natural killer (NK) cells recognize antibody-tagged virus-infected cells and tumor cells.

35.4 Distinguishing Self from Nonself Distinguishing self from nonself is critical to homeostatic immune system function. If self is "perceived" as foreign, autoimmunity and autoimmune disease are the consequence. The ability to recognize self constitutes one form of immune tolerance. Conversely, if pathogenic organisms or toxic substances are unrecognized or unopposed, the consequential infection and organ damage is debilitating and frequently fatal. Tolerance or unresponsiveness to self-antigens is actively achieved. Exclusion of T cells that recognize epitopes on molecules that are "self" occurs in the thymus during T-cell differentiation and proliferation. Selfreactive T cells are "instructed" to die, a process generally described as negative selection. B cells that recognize epitopes on molecules that are "self" are distinguished from B cells that recognize foreign antigens by a mechanism generally described as positive selection. Positive selection occurs in lymphoid tissues. In this process, B cells stimulated by T-helper cells proliferate. B cells that are not stimulated to produce antibodies or are precluded from responding by the actions of T-suppressor cells die by apoptosis. Apoptosis, programmed cell death, occurs by mechanisms that include proteolytic processes catalyzed by caspases (a group of enzymes that are responsible for the destruction of intracellular proteins). Failed apoptosis can be a cause of autoimmunity. Autoimmune diabetes mellitus, caused by destruction of/3 cells in islets of Langerhans in the pancreas, is an example of failed apoptosis. Similarly, chronic lymphocytic leukemia is an expression of B cells surviving beyond their normal, apoptosis-determined lifetimes. B-cell lymphomas are cancerous growths that are caused by delayed apoptosis, not by stimulation of cell division. One approach to the development of therapeutic drugs for leukemia includes designing molecules that regulate apoptosis. Autoimmune disease can also arise from a foreign substance possessing an epitope very similar to an epitope that is found on a host molecule. This situation, termed "molecular mimicry," is the cause of post-streptococcal rheumatic fever because an epitope on the streptococcal M protein elicits formation antibody that reacts with an epitope on myosin.

35.5 Molecules and Chemical Processes of the Immune System The highly coordinated and regulated responses of the cells of the immune system directly reflect properties of

812 unique proteins of the immune system. Although new components and new functions for known components of the immune system continue to be discovered, the principal groups of molecules of immunity are immunoglobulins, T-cell receptors and coreceptors, cytokines, and the proteins of the complement system. Discrimination between self and nonself, which is the result of the elimination of B and T cells that recognize self-molecules, reflects the actions of substances that regulate programmed cell death. Recognition of epitopes on foreign substances and foreign cells by immunoglobulin molecules and T-cell receptors occurs because of complementarity between specialized "lock-like surface" regions of an immunoglobulin or T-cell receptor molecule and the "key-like" epitopes of an antigen molecule. Cell proliferation in response to foreign antigens involves receptors, coreceptors, and intracellular processing of the antigens by proteolytic enzymes. The transport and binding of the processed antigens is directed by proteins that are encoded by genes of the major histocompatibility complex. Finally, T-cell receptor recognition of the processed, antigen-derived peptides results in the synthesis and release of cytokines. Cytokines are pleiotropic protein molecules with functions for each cytokine molecule determined both by the structure of the cytokine molecule and the cells and cell receptors with which the cytokine interacts. Cytokines act as the messengers through which communication and coordination among the cells of the immune system are obtained. Complement proteins act on the cell membrane of foreign organism by assembly of the MAC, which causes permeability changes that destroy the cell. In a highly coordinated manner, the cells and molecules of the immune system protect the organism from pathogenic organisms and bacterial toxins. In the newborn infant, resistance to infectious agents is first achieved by maternal antibodies that cross the placenta during fetal development or antibodies from mammary gland secretions transferred to a nursing infant. Subsequent antibody-mediated protection is provided by antibodies synthesized by the B cells of the individual as described above. Following this natural sequence, the focus on molecules and how molecular structures account for biological function begins with antibody structure.

35.6 Immunoglobulin Structure and Function Antibodies, i.e., immunoglobulin molecules, account for approximately 15-20% of the mass of proteins in human serum, the protein solution that remains after blood or


F I G U R E 35-5 (Also see color figure.) Structural motif of the immunoglobulin family. Each domain of the immunoglobulin or other molecule of the immunoglobulin family is composed of a collection of/4 strands that form a globular structure. There are seven strands in the constant-region domains that form two/4 sheets; one sheet contains three antiparallel strands and the other four antiparallel strands. The orientation illustrates the two sheets that together create the/4 sandwich or immunoglobulin fold.

plasma has clotted. The immunoglobulins can be divided into five classes--IgG, IgM, IgA, IgE, and IgDmbased on their polypeptide chain sequences. The immunoglobulin classes are divided as follows: ~80% IgG, 5-10% IgM, 15% IgA, and 100 #g/g); however, bone zinc is not readily mobilized. Hair and nails also have high concentrations of the metal.

898

CHAPTER37 Mineral Metabolism

TABLE 37-6

Some M a m m a l i a n Zinc Proteins

Source

Molecular Weight

Alcohol dehydrogenase

Human liver

87,000

Alkaline phosphatase

Human placenta

125,000

Leucine aminopeptidase

Porcine kidney

300,000

5'-AMP aminohydrolase Carbonic anhydrase B

Rat muscle Human erythrocyte

290,000 26,600

Carboxypeptidase A

Bovine pancreas

Glutamate dehydrogenase a2-Macroglobulin Malate dehydrogenase Metallothionein

Bovine Human Bovine Human

Rhodanese (sulfur transferase)

Bovine liver

Enzyme

liver serum heart renal cortex

Zn Atoms/ Protein Molecule

--3

Requires 2NAD+; two zinc atoms used for catalysis; function of other two not known but may stabilize enzyme. Unknown function; role of Z n 2+ unknown but needed for activity. N-terminal exopeptidase; Zn binds at two sites, one for catalysis, the other for regulation. Catalyzes reversible hydration of CO2; needed for CO 2 transport, buffeting of ECE gastric and renal acid secretion; Zn is catalytic. C-terminal exopeptidase; involved in digestion of proteins; Zn functions in catalysis as a Lewis acid.

34,500

1,000,000 840,000 40,000 10,500

2-4 3-8 1 3

37,000

2

The zinc content of human blood is 8.8 #g/mL, of which 80-90% is within erythrocytes, mostly in carbonic anhydrase. Zinc binds to hemoglobin, increasing its oxygen affinity. By binding to erythrocyte membranes, it increases the flexibility of irreversibly sickled cells in vitro but plasma levels high enough to be therapeutic cannot be achieved. Leukocytes and platelets also contain an appreciable amount of zinc. Plasma zinc levels vary with sex, age, time of day, geographic location, and time elapsed since the last meal prior to phlebotomy. The normal range for an adult is probably 70-95 #g/dL of plasma. Approximately 60% of zinc in plasma is bound to albumin, 30-40% to an ot2macroglobulin of unknown function, and a small amount to transferrin (Chapter 29) and amino acids, particularly cysteine and histidine. Copper does not compete with zinc for binding sites on albumin. Zinc newly absorbed from

Functions/Comments

i

Usually also contains 4-5 Cd; may serve as source of Zn. Metabolizes SCN-.

the intestine is albumin-bound. This zinc is the fraction that changes most in disease states and that correlates best with total serum zinc. The zinc bound to amino acids accounts for urinary excretion of zinc, and it may be utilized by some tissues. The RDA for zinc depends on age. For infants younger than 6 months, it is 3 mg; from 6 months to 1 year, 5 mg; for ages 1-10 years, 10 mg; and more than 10 years, 15 mg. For women, it increases during pregnancy (20 mg) and lactation (25 mg). A typical daily diet in the United States supplies 12-15 mg (about 200 #mob, of which 30-40% is absorbed. Dietary fiber binds and decreases the absorption of zinc. Human zinc deficiency was first reported in boys from Iran and Egypt who were small in stature, hypogonadal, and mildly anemic. The zinc content of their hair, plasma, and erythrocytes was below normal. Etiological factors included high dietary phytate content, loss

899


of zinc through sweating, and geophagia (the practice of eating earth and clay, which bind zinc and other metals). All showed clinical improvement following oral supplementation with zinc. The jejunum is the principal absorptive region, although zinc absorption occurs through out the small intestine. Uptake occurs both by an active transport process, the activity of which varies inversely with zinc status, and by a passive process. The increase in serum zinc following a single oral or parenteral bolus of zinc decreases absorption within about 6 hours. In contrast, with slow intravenous infusion, significant decrease in zinc absorption may not occur for 4 days. Intestinal metallothionein is induced by and binds zinc. It increases by about 6 hours following a bolus of zinc and may block absorption by sequestering the metal in the mucosal cells. The evidence is largely circumstantial. Uptake by the liver is by an active process. Fecal zinc is derived from unabsorbed dietary zinc and zinc in desquamated epithelial cells and gastrointestinal secretions. Active intestinal secretion of zinc decreases in zinc-depleted animals and may be a regulated step in zinc homeostasis. Excretion of zinc in urine (0.3-0.6 mg/d) is independent of dietary intake. It is increased in renal disease, in alcoholic cirrhosis, in hepatic porphyria, following major operations, in severely burned patients, and in response to treatment with ethylenediaminetetraacetic acid. The normal concentration of zinc in sweat is 1.15 #g/mL, decreasing to about 0.6 #g/mL in zinc deficiency. In tropical climates, zinc loss from this source may reach 4 mg/d. Menstrual zinc loss is about 15/~g/d. The zinc content of human milk is highest in colostrum. The average zinc concentration in human milk is approximately 1.5 #g/mL. Fresh cow's milk averages 4 #g/mL, but human milk is a better source of zinc for infant nutrition because of lower bioavailability of zinc in cow's milk. Zinc is relatively nontoxic but, if present in the diet at high concentration and in readily available form, it can interfere with the absorption of calcium, copper, iron, and cadmium and can produce anemia. These elements, in turn, can decrease zinc absorption if present in sufficient quantities. Use of zinc salt lozenges has been claimed to reduce the duration of the common cold. However, many randomized, double-bind, placebo-controlled trials have failed to show any beneficial effects of zinc lozenges in reducing the duration of upper respiratory tract viral infections. Acrodermatitis enteropathica (AE) is an autosomal recessive disorder characterized by defective absorption of zinc, thickened, ulcerated skin on the extremities and around body orifices, alopecia, diarrhea, growth failure, and abnormalities in immune function. The concentration

of zinc in serum and hair is reduced. AE, and a similar disease of cattle known as adema disease or lethal trait A-46, respond to large amounts of oral zinc. Malabsorption of zinc occurs in both the human and animal diseases. Human milk seems to alleviate AE, presumably because it provides zinc in a form that can be readily absorbed.

Supplemental Readings and References J. S. Adams and G. Lee: Gains in bone mineral density with resolution of vitamin D intoxication. Annals of Internal Medicine 127, 203 (1997). D. G. Barceloux: Manganese. Clinical Toxicology 37, 293 91999). D. G. Barceloux: Zinc. Clinical Toxicology 37, 279 (1999). D. Beshgetoor and M. Hambidge: Clinical conditions altering copper metabolism in humans. Clinical Nutrition 67, 1017 S (1998). M. J. Bingham, T-J. Ong, K. H. Summer, et al.: Physiologic function of the Wilson disease gene product, ATP7B. Clinical Nutrition 67, 982S (1998). A. L. Boskey: Biomineralization: conflicts, challenges, and opportunities. Journal of Cellular Biochemistry Supplements 30, 83 (1998). E Bronner: Calcium absorption--a paradigm for mineral absorption. Journal of Nutrition 128, 917 (1998). F. Bronner and D. Pansu: Nutritional aspects of calcium absorption. Journal of Nutrition 129, 9 (1999). D. A. Bushinsky and R. D. Monk: Calcium. Lancet 352, 306 (1998). S. Chan, B. Gerson, and S. Subramaniam: The role of copper molybdenum, selenium, and zinc in nutrition and health. Toxicology 18, 673 (1998). T. W. Clarkson: Mercury--an element of mystery. New England Journal of Medicine 323, 1137 (1990). A. Cordano: Clinical manifestations of nutritional copper deficiency in infants and children. Clinical Nutrition 67, 1012S (1998). H. E DeLuca and C. Zierold: Mechanisms and functions of vitamin D. Nutrition Reviews 56, $4 (1998). J. S. Duthie, H. E Solanki, M. Knshnamurthy, et al.: Milk-alkali syndrome with metastatic calcification. American Journal of Medicine 99, 102 (1995). R. Eastell: Treatment of postmenopausal osteoporosis. New England Journal of Medicine 338, 736 (1998). K. E. Ensrud, D. M. Black, L. Palermo, et al.: Treatment with alendronate prevents fractures in women at highest risk. Archives of Internal Medicine 157, 2617 (1997). E. Evron, S. Goland, J. von der Walde, et al.: Idiopathic calcitriol-induced hypercalcemia. Archives of Internal Medicine 157, 2142 (1997). D. Feldman: Vitamin D, parathyroid hormone, and calcium: a complex regulatory network. American Journal of Medicine 107, 637 (1999). E Garnero, W. J. Shih, E. Gineyts, et al.: Comparison of new biochemical markers of bone turnover late postmenopausal osteoporotic women in response to alendronate treatment. Journal of Clinical Endocrinology and Metabolism 79, 1693 (1994). E H. Gloneux, N. J. Bishop, H. Plotkin, et al.: Cyclic administration of pamidronate in children with severe osteogenesis imperfecta. New England Journal of Medicine 339, 986 (1998). G. A. Greendale, N. E Lee, and E. R. Arriola: The Menopause. Lancet 353, 571 (1999). J. Guardiola, X. Xiol, R. Sallie, et al.: Influence of the vitamin D receptor gene polymorphism on bone loss in men after liver transplantation. Annals oflnternal Medicine 131, 752 (1999). J. G. Hall: A bone is not a bone is not a bone. Journal of Pediatrics 133, 5 (1998). J. R. Hansford and L. M. Mulligan: Multiple endocrine neoplasia type 2 and RET: from neoplasia to neurogenesis. Journal of Medical Genetics 37, 817 (2000).

900 Z. L. Harris, L. W. J. Klomp, and J. D. Gitlin: Aceruloplasminemia: an inherited neurodegenerative disease with impairment of iron homeostasis. Clinical Nutrition 67, 972S (1998). R. E Heaney: Bone mass, bone loss and osteoporosis prophylaxis. Annals oflnternal Medicine 128, 313 (1998). J. L. Jackson, C. Peterson, and E. Lesho: A meta-analysis of zinc salts lozenges and the common cold. Archives of Internal Medicine 157, 2373 (1997). H. M. Kronenberg, B. Lanske, C. S. Kovacs, et al.: Functional analysis of the PTH/PTHrP network of ligands and receptors. Recent Progress in Hormone Research 53, 283 (1998). L. Kuritzky, G. P. N. Samraj, and D. M. Quillen: Improving management of type 2 diabetes mellitus: 6. Chromium. Hospital Practice 35, 113 (2000). M. C. Linder, L. Wooten, E Cerveza, et al.: Copper transport. Clinical Nutrition 67, 9655 (1998). E J. Malloy and D. Feldman: Vitamin D resistance. American Journal of Medicine 106, 355 (1999). R. Marcus: Exercise and the regulation of bone mass. Archives oflnternal Medicine 149, 2170 (1989). E. Mariani, G. Ravaglia, A. Meneghetti, et al.: Natural immunity and bone remodelling hormones in the elderly. Mechanisms of Ageing and Development 102, 279 (1998). M. M. Maricic: Early prevention vs. late treatment for osteoporosis. Archives oflnternal Medicine 157, 2545 (1997). J. C. Marini: Osteogenesis imperfecta--managing brittle bones. New England Journal of Medicine 339, 986 (1998). L. A. Martini: Magnesium supplementation and bone turnover. Nutrition Reviews 57, 227. M. McClung, B. Clemmesen, and A. Daifotis: Alendronate prevents postmenopausal bone loss in women without osteoporosis. Annals oflnternal Medicine 128, 253 (1998). C. R. Meier, R. G. Schlienger, M. E. Kraenzlin, et al.: HMG-CoA reductase inhibitors and the risk of fractures. Journal of the American Medical Association 283, 3205 (2000). W. G. Meijer, E. Van Der Veer, and P. H. B. Willemse: Biochemical parameters of bone metabolism in bone metastases of solid tumors. Oncology Reports 5, 5 (1998). J. E B. Mercer: Menkes syndrome and animal models. Clinical Nutrition 67, 1022S (1998). B. A. Michel, D. A. Bloch, and J. E Fries: Weight-bearing exercise, overexercise, and lumbar bone density over age 50 years. Archives of Internal Medicine 149, 2325 (1989). W. L. Miller and A. A. Portale: Genetic disorders of vitamin D biosynthesis. Pediatric Endocrinology 28, 825 (1999). E D. Miller, N. B. Watts, A. A. Licata, et al.: Cyclical etidronate in the treatment of postmenopausal osteoporosis: efficacy and safety after seven years. American Journal of Medicine 103, 469 (1997). G. Mundy, R. Garrett, and S. Harris: Stimulation of bone formation in vitro and in rodents by statins. Science 286, 1946 (1999). G. R. Mundy and T. A. Guise: Hypercalcemia of malignancy. American Journal of Medicine 103, 134 (1997). G. R. Mundy and T. A. Guise: Hormonal control of calcium homeostasis. Clinical Chemistry 45, 1347 (1999). K. M. Prestwood, C. C. Pilbeam, J. A. Burleson, et al.: The short term effects of conjugated estrogen on bone turnover in older women. Journal of Clinical Endocrinology and Metabolism 79, 366 (1994). L. G. Raisz: The osteoporosis revolution. Annals oflnternal Medicine 126, 459 (1997).

CHAPTER37 Mineral Metabolism L. G. Raisz: Physiology and pathophysiology of bone remodeling. Clinical Chemistry 45, 1353 (1999). S. H. Ralston: Osteoporosis. British Medical Journal 315, 469 (1997). J. K. S. Rao, C. D. Katsetos, M. M. Herman, et al.: Experimental aluminum encephalomyelopathy. Toxicology 18, 687 (1998). J. Sainz, J. M. Van Tornout, M. L. Loro, et al.: Vitamin D-receptor gene polymorphisms and bone density in prepubertal American girls of Mexican descent. New England Journal of Medicine 337, 77 (1997). D. L. Schneider and E. L. Barrett-Connor: Urinary N-telopeptide levels discriminate normal osteopenic, and osteoporotic bone mineral density. Archives of Internal Medicine 157, 1241 (1997). E. Shane, D. Mancini, K. Aaronson, et al.: bone mass vitamin D deficiency, and hyperparathyroidism in congestive heart failure. American Journal of Medicine 103, 197 (1997). S. J. Silverberg, E. Shane, D. W. Dempster, and J. E Bilezikian: The effects of vitamin D insufficiency in patients with primary hyperparathyroidism. American Journal of Medicine 107, 561 (1999). M. Sowers, D. Eyre, B. W. Hollis, et al.: Biochemical markers of bone turnover in lactating and nonlactating postpartum women. Journal of Clinical Endocrinology and Metabolism 80, 2210 (1995). E Steindl, E Ferenci, and H. E Dienes: Wilson's disease in patients presenting with liver disease: a diagnostic challenge. Gastroenterology 113, 212 (1997). G. J. Strewler: The physiology of parathyroid hormone-related protein. New England Journal of Medicine 342, 177 (2000). R. Subramanian and R. Khardori: Severe hypophosphatemia: pathological implications, clinical presentations and treatment. Medicine 79, 1 (2000). T. D. Thacher, P. R. Fischer, J. M. Pettifor, et al.: A comparison of calcium, vitamin D, or both for nutritional rickets in Nigerian children. New England Journal of Medicine 341,563 (1999). R. V. Thakker: Editorial: Multiple endocrine neoplasia-syndromes of the twentieth century. Journal of Clinical Endocrinology and Metabolism 83, 2617 (1998). J. R. Turnlund: Human whole-body copper metabolism. Clinical Nutrition 67, 960S (1998). R. Uauy, M. Oiivares, and M. Gonzalez: Essentiality of copper in humans. Clinical Nutrition 67, 952S (1998). P. S. Wang, D. H. Solomon, H. Mogun, and J. Avorn: HMG-CoA reductase inhibitors and the risk of hip fractures in elderly patients. Journal of the American Medical Association 283, 3205 (2000). N. B. Watts: Clinical utility of biochemical markers of bone remodeling. Clinical Chemistry 45, 1359 (1999). R. S. Weinstein and S. C. Manolagas: Apoptosis and osteoporosis. American Journal of Medicine 108, 153 (2000). R. S. Weinstein and S. C. Manolagas: Bone cell apoptosis: implications for the pathogenesis and treatment of osteoporosis. Physiology in Medicine 108, 153 (2000). J. R. Weisinger and E. Beilorin-Font: Magnesium and phosphorous. Lancet 352, 391 (1998). P. Whittaker: Iron and zinc interactions in humans. American Journal of Clinical Nutrition 68, 442S (1998). G. Worley, S. J. Claerhout, and S. P. Combs: Hypophosphatemia in malnourished children during refeeding. Clinical Pediatrics 6, 347 (1998). Y. Kureishi, Z. Luo, I. Shiojima, et al: The HMG-CoA reductase inhibitor simvastatin activates the protein kinase Akt and promotes angiogenesis in normocholesterolemic, animals. Nature Medicine 6, 1004 (2000).

CHAPTER

38

Vitamin Metabolism

The word vitamin is used to describe any of a heterogeneous group of organic molecules that are needed in small quantities for normal growth, reproduction, and homeostasis but that the human body is unable to synthesize in adequate amounts. The group includes the fat-soluble vitamins (A, D, E, and K) and the water-soluble vitamins (B-complex and C). Vitamins are generally needed in catalytic quantities and do not function as structural elements in the cell. Other organic compounds are not synthesized in the body but are required for maintenance of normal metabolism, such as essential fatty acids (sometimes inaccurately called "vitamin F"; Chapter 18) and essential amino acids (Chapter 2). These substances are needed in relatively large quantities, serve as nonregenerated substrates in metabolic reactions, and are used primarily as structural components in lipids and proteins, respectively. A number of other substances are essential food factors in various nonhuman species, e.g., biopterin, inositol, ubiquinone, lipoic acid, phosphatidylcholine, and paraaminobenzoic acid. They are often classified as "vitamin-like." Vitamins discussed in other chapters include vitamin D (Chapter 37) and vitamin K (Chapter 36). All the B vitamins function as cofactors or precursors for cofactors in enzyme-catalyzed reactions and are discussed in appropriate chapters. Less well-defined actions are reviewed here.

General Considerations Classification of vitamins into fat-soluble and watersoluble groups reflects the history of their discovery. This grouping is still useful, despite the lack of chemical relatedness within each class, because it mirrors other underlying similarities. Vitamers are chemically similar substances that have a qualitatively similar vitamin activity. Thus, "vitamin D" refers to ergocalciferol (D2) and cholecalciferol (D3) and sometimes to their 25-hydroxy- and 1,25-dihydroxy derivatives (Chapter 37). Similarly, pyridoxine (pyridoxol), pyridoxal, and pyridoxamine are vitamin B6 vitamers, riboflavin is the active form of vitamin B2 and cobalamin is vitamin B I:. The members of a particular vitamin "family" are functionally interchangeable and protect against deficiency symptoms for that vitamin. A vitamin and its corresponding deficiency disease are related as follows: 1. The putative vitamin is a normal dietary constituent of healthy individuals. 2. The diet consumed by persons exhibiting symptoms of the putative deficiency disease is lacking in the vitamin. 3. Symptoms of such persons can be alleviated by addition of the vitamin to their diet.

901

902

CHAPTER38 Vitamin Metabolism

TABLE 38-1

Genetic Disorders That Respond Favorably to Treatment with Pharmacological Doses of Vitamins* Therapeutic Vitamin

Daily Dose

Vitamin D (adult RDA = 200 IU = 50 ~g of cholecalciferol)

> 100,000 units

Ascorbic acid (vitamin C; adult RDA = 60 mg) Biotin (RDA has not been established)

Cobalamin (vitamin B l 2; adult RDA = 3.0/sg)

Unknown

4g

Ehlers-Danlos syndrome, type VI (ocular type)

Procollagen lysyl hydroxylase

10g

Multiple carboxylase deficiency w

10mg

Propionic acidemia; ketoacidosis

Holocarboxylase synthetase, or defect in intestinal or cellular uptake, or intracellular transport Propionyl-CoA carboxylase

25,000 units

>500/~g

Niacin (adult RDA = 13 mg = 13 niacin equiv.) Pyridoxine [vitamin B6; adult RDA = 2.2 mg (men), 2.0 mg (women)]

Megaloblastic anemia Methylmalonic aciduria; ketoacidosis Methylmalonic aciduria; homocystinuria

Defect in 25-(OH)D- 1-a-hydroxylase

Decreased intestinal absorption due to abnormality of intrinsic factor or ileal mucosal cells, or absence of intrinsic factor Transcobalamin II Adenosyl cobalamin formation Formation of both adenosyl and methyl cobalamins

Megaloblastic anemia and mental retardation w Homocystinuria and hypomethioninemia Formiminoglutamic aciduria and mental retardation

Defect in intestinal and perhaps brain transport of folate NS, N 10_Methylenetetrahydrofolate reductase Probably formiminotransferase

Lactic and pyruvic acidemia ~

Pyruvate carboxylase

Hartnup disease

Intestinal and renal transport of tryptophan (high protein diet needed in addition to niacin to relieve symptoms)

10-50 mg

Infantile convulsions

>10mg >25 mg 5-10 mg > 100mg

B6-responsive anemia~ Cystathioninuria Homocystinuria Primary hyperoxaluria, type I Xanthurenic aciduriaw Gyrate atrophy of choroid and retina; hyperornithinemia

Perhaps glutamate decarboxylase Unknown u Cystathionine synthetase Soluble glyoxalate, a-ketoglutarate carboligase Kynureninase Ornithine: 2-oxoacid aminotransferase

>10mg >10mg >5 mg

Lipoic acid

Biochemical/Biological Defect

Familial X-linked hypophosphatemic rickets~ Vitamin D-dependent rickets, type I

> 100 ~g >250/~g

Folic acid (folacin; adult RDA = 400/~g)

Disorder t

3-10 mg 50-250 mg

5-10 mg 18-30 mg

(continued)

903


T A B L E 38-1

(continued)

Therapeutic Vitamin

Daily Dose

Disorder t

Thiamine (vitamin B 1; adult RDA = 1 mg/2000 kcal/d)

20 mg 5-20 mg

Megaloblastic anemia Branched-chain ketoaciduria (maple syrup urine disease) Pyruvic acidemia Lactic and pyruvic acidemia; Leigh's encephalomyelopathy ~

5-20 mg > 10 mg

Biochemical/Biological Defect Unknown Branched chain a-ketoacid decarboxylase Pyruvate decarboxylase Pyruvate decarboxylase (low K m form)

*These disorders can be thought of as relative vitamin deficiency states. Not all patients will respond to vitamins owing to the existence of phenocopies. Data from L. E. Rosenberg: Vitamin-responsive inherited metabolic disorders. In Advances in Human Genetics, Vol. 6, H. Harris and K. Hirschhorn, Eds. (Plenum Press, 1976); and S. H. Mudd: Vitamin-responsive genetic abnormalities, In Advances in Nutritional Research, Vol. 4 (Plenum Press, 1982). tAll disorders have an autosomal recessive pattern of inheritance except as follows: X-linked dominant inheritance; w of inheritance unknown; llAutosomal, probably recessive, inheritance; ~X-linked recessive inheritance.

4. The biochemical function of the vitamin should be related directly to the observed deficiency symptoms, although this is not always possible to demonstrate. The classic example is scurvy (scorbutus), which used to be common among sailors who ate no fresh fruit or vegetables during long sea voyages. Scurvy could be prevented by consumption of fresh citrus fruits, now known to be high in vitamin C (ascorbic acid). The deficiency arose because vitamin C was destroyed by the methods used to preserve food while at sea. Other examples of a single vitamin deficiency in human populations are rare, however, except where the dietary supply is adequate but utilization of the vitamin is impaired. Frequently, if one vitamin is lacking, others are as well, and the intake of protein, trace elements, and other nutrients is probably insufficient. This is particularly true of B vitamins. In addition to nutritional inadequacy, vitamin deficiency may result from malabsorption, effects of pharmacological agents, and abnormalities of vitamin metabolism or utilization. Thus, in biliary obstruction or pancreatic disease, the fat-soluble vitamins are poorly absorbed despite adequate dietary intake because of steatorrhea. Absorption, transport, activation, and utilization of vitamins require the participation of enzymes or other proteins whose synthesis is under genetic control. Dysfunction or absence of one of these proteins can produce a disease that is clinically indistinguishable from one caused by dietary deficiency. In vitamin-dependent or vitamin-responsive

disorders, use of pharmacological doses of the vitamin can sufficiently overcome the blockage for normal function to occur (Table 38-1; discussed at the end of the chapter). Vitamin deficiency can result from treatment with certain drugs. Thus, destruction of intestinal microorganisms by antibiotic therapy can produce symptoms of vitamin K deficiency. Isoniazid, used to treat tuberculosis, is a competitive inhibitor of pyridoxal kinase, which is needed to produce pyridoxal phosphate. Isoniazid can produce symptoms of pyridoxine deficiency. To prevent this, pyridoxine is often incorporated into isoniazid tablets. Methotrexate and related folate antagonists act by competitively inhibiting dihydrofolate reductase (Chapter 27). Two standards have been established to plan diets that contain adequate supplies of the vitamins and to aid in diagnosing vitamin deficiency diseases. 1. The minimum daily requirement (MDR) is the smallest amount of a substance needed by a person to prevent a deficiency syndrome. It is considered to represent the body's basic physiological requirement of the material. MDR values, which are established by the U.S. Food and Drug Administration (FDA), are not available for all vitamins. 2. The recommended daily allowance (RDA) is the amount of a compound needed daily to maintain good nutrition in most healthy people. RDA values are intended to serve as nutritional goals, not as dietary requirements. They are defined by the Food and Nutrition Board of the National Academy of Sciences

904


of the USA. RDA values have been established for mineral elements, energy, protein, electrolytes, and water. Those for the vitamins are given in Appendix IV. Overdosage of vitamins A and D produces hypervitaminosis. In recognition of this, the FDA has ruled that vitamin A in doses greater than 10,000 IU and vitamin D in doses greater than 400 IU should be available on a prescription-only basis and that all products containing quantities of a vitamin in excess of 150% of its RDA be classified as a drug. Hypervitaminosis D was discussed in Chapter 37; vitamin A toxicosis is discussed below. The toxicity of high doses of vitamin B6 is also covered later in the chapter. Certain vitamins can be synthesized by humans in limited quantities. Niacin can be formed from tryptophan (Chapter 17). This pathway is not active enough to satisfy all the body's needs; however, in calculating the RDA for niacin, 60 mg of dietary tryptophan is considered equivalent to 1 mg of dietary niacin. In Hartnup's disease (see Table 38-1 and Chapter 17), a rare hereditary disorder in the transport of monoaminomonocarboxylic acids (e.g., tryptophan), a pellagra-like rash may appear, suggesting that over a long period of time dietary intake of niacin is insufficient for metabolic needs. This pattern also occurs in carcinoid syndrome in which much tryptophan is shunted into the synthesis of 5-hydroxytryptamine. Vitamin D is synthesized in the skin, provided radiant energy is available for the conversion (Chapter 37): 7-Dehydrocholesterol

photons

> cholecalciferol (vitamin D3) (38.1)

This pathway is adequate to supply the body's need for vitamin D, provided exposure to sunlight is adequate. Vitamin D becomes a vitamin when environmental conditions prevent synthesis of an adequate supply by the skin. Physiological age-related changes in the elderly can affect the nutritional status. Decreased active intestinal transport and atrophic gastritis impair the absorption of vitamins and other nutrients. Reduced exposure to sunlight can lead to decreased vitamin D synthesis. Many drugs may impair both appetite and absorption of nutrients. Some examples of unfavorable drug-nutrient interactions are drugs that inhibit stomach acid production (e.g., omeprazole); drugs that reduce vitamin B12 absorption; anticonvulsant drugs (e.g., barbiturates, phenytoin, primidone) that act by inducing hepatic microsomal enzymes which accelerate inactivation of vitamin D metabolites and aggravate osteoporosis(Chapter 37); interference with folate metabolism by antifolate drugs (methotrexate) used in the treatment of some neoplastic diseases; and vitamin B6 metabolism affected by isoniazid, hydralazine, and D-penicillamine. Examples of negative impacts of

vitamins on drug action are vitamin B6-dependent action of peripheral conversion of L-3,4-dihydroxyphenylalanine (L-dopa) to L-dopamine that is mediated by aromatic L-amino acid decarboxylase and prevents L-dopa's transport across the blood-brain barrier; also ingestion of large amounts of vitamin K-rich foods or supplements, and action of warfarin on anticoagulation (Chapter 36). L-Dopa is the metabolic precursor of L-dopamine and is used in the treatment of Parkinson's disease (Chapter 32). Thus, L-dopa is administered along with a peripherally acting inhibitor of aromatic L-amino acid decarboxylase (e.g., carbidopa). Vitamins A (and retinoids), C, and E are antioxidants. Randomized trials of dietary supplementation of antioxidant vitamins to determine their favorable and/or unfavorable physiological effects on cardiovascular diseases, cancer, and other chronic diseases are in progress.

38.1 Fat-Soluble Vitamins The fat-soluble vitamins share many properties despite their limited chemical similarity. They are absorbed into the intestinal lymphatics, along with other dietary lipids, after emulsification by bile salts. Lipid malabsorption accompanied by steatorrhea usually results in poor uptake of all the fat-soluble vitamins. Deficiency disease (except in the case of vitamin K) is difficult to produce in adults because large amounts of most fat-soluble vitamins are stored in the liver and in adipose tissue. The fat-soluble vitamins are assembled from isoprenoid units; this fact is apparent from examination of the structures of vitamins A, E, and K; cholesterol, the precursor of vitamin D, is derived from six isoprenoid units (Chapter 18). Specific biochemical functions for vitamins A, D, and K are known, but a role for vitamin E, other than as a relatively nonspecific antioxidant, remains elusive. Vitamin A

Nutrition and Chemistry The role of vitamin A in vision is fairly well understood, but the other functions of vitamin A are only beginning to be elucidated (e.g., mucus secretion, maintenance of the integrity of differentiated epithelia and of the immune system, growth, and reproduction). Loss of night vision (nyctalopia) is an early sign of vitamin A deficiency, and clinical features of well-developed deficiency include epidermal lesions, ocular changes, growth retardation, glandular degeneration, increased susceptibility to infection, and sterility. Natural and synthetic compounds with vitamin A activity and inactive synthetic analogues of vitamin A are collectively termed retinoids. The most biologically active,

SECTION 38.1

Fat-SolubleVitamins

905 CH,

CH,

H

H=OH

:L.L'

,o

,,

13-1onone (trimethylcyclohexenyl) ring FIGURE 38-1 Chemical structureof all-transretinol (vitaminA1), the most activeform of vitaminA. Oxidationof C15 to an aldehydeor an acid produces, respectively,retinaldehyde(retinal) and retinoic acid. The cis-trans isomerizationof the doublebondbetweenCll and C12 occursduring functioningof retinaldehydein vision. naturally occurring retinoid is all-trans retinol, also called vitamin A (Figure 38-1). The/3-ionone ring is required for biological activity. Vitamin A1 exists free or as retinyl esters of fatty acids (primarily palmitic acid) in foods of animal origin, including eggs, butter, cod liver oil, and the livers of other vertebrates. In most of the dietary retinol, the four double bonds of the side chain are in the trans configuration. They are readily oxidized by atmospheric oxygen, inactivating the vitamin. They can be protected by antioxidants such as vitamin E. Several provitamins are present in yellow and dark green leafy vegetables and fruits, such as carrots, mangoes, apricots, collard greens, and broccoli. They are collectively known as the carotenoid pigments or carotenes. The most widely occurring and biologically active is /~-carotene (Figure 38-2). Other nutritionally important carotenoid pigments are cryptoxanthine, a yellow pigment found in corn, and o~- and v-carotenes. Other carotenoids, such as lycopene the red pigment of tomatoes, and xanthophyll, lack the/~-ionone ring essential for vitamin A activity.

Absorption, Transport, and Metabolism Retinyl esters are hydrolyzed in the intestinal lumen by pancreatic carboxylic ester hydrolase, which also hydrolyzes cholesteryl esters. In mucosal cells, retinol is reesterified, mostly with long-chain fatty acids, by

/

38-2 Structure of all-trans/~-carotene.

FIGURE

acyl-CoA: retinol acyltransferase. The retinyl esters are incorporated into chylomicron particles and secreted into the lacteals. In humans and rats, 50% of the retinol is esterified with palmitic acid, 25% with stearic acid, and smaller amounts with linoleic and oleic acids. These esters are eventually taken up by the liver in chylomicron remnants./~-Carotene is cleaved in the intestinal mucosa to two molecules of retinaldehyde by/3-carotene-15,15'dioxygenase, a soluble enzyme. Other provitamins are also activated upon cleavage by this enzyme. Retinaldehyde is then reduced to retinol by retinaldehyde reductase, using either NADH or NADPH. In the liver, retinyl esters are hydrolyzed and reesterifled. More than 95 % of hepatic retinol is present as esters of long-chain fatty acids, primarily palmitate. In an adult receiving the RDA of vitamin A, a year's supply or more may be stored in the liver. More than 90% of the body's supply of vitamin A is stored in the liver. The hepatic parenchymal cells are involved in its uptake, storage, and metabolism. Retinyl esters are transferred to hepatic fat-storing cells (also called Ito cells or lipocytes) from the parenchymal cells. The capacity of these fat-storing cells may determine when vitamin A toxicosis becomes symptomatic. During the development of hepatic fibrosis (e.g., in alcoholic liver disease), vitamin A stores in Ito cells disappear and the cells differentiate to myofibroblasts. These cells appear to be the ones responsible for the increased collagen synthesis seen in fibrotic and cirrhotic livers. Retinol is released from the liver and transported in plasma bound to retinol-binding protein (RBP), which is synthesized by hepatic parenchymal cells. Less than 5% circulates as retinyl esters. Retinol-binding protein from human plasma is a monomeric polypeptide (M.W. 21,000), which has a single binding site. Transfer of retinol into cells may be mediated by cell surface receptors that specifically recognize RBE After binding and releasing its vitamin A, RBP appears to have decreased affinity for prealbumin (see below) and is rapidly filtered by the kidney and degraded or excreted. The retinol-RBP complex circulates as a 1:1 complex with prealbumin (transthyretin; M.W. 55,000), which

[3"Carotene is cleaved here to produce.two

.

~

A

906 also functions in transport of thyroid hormones. Interaction between transthyretin and RBP is quite specific (dissociation constant 10-6-10 -7 mol/L). Retinol-RBPtransthyretin complex (RTC) (M.W. 76,000) migrates electrophoretically as an o~-globulin. Normal concentrations of RBP and transthyretin in human plasma are 40-50 and 200-300 #g/mL, respectively. Transthyretin, a glycoprotein with a high tryptophan content, is synthesized in the liver and migrates electrophoretically ahead of albumin. It has a half-life of 2 days and has a small pool size. These two properties make it a sensitive indicator of nutritional status (Chapter 17). Its plasma level decreases in protein-calorie malnutrition, liver disease, and acute inflammatory diseases. The serum concentration of retinol is held remarkably constant despite wide variation in dietary intake and hepatic stores of the vitamin by tight control over the concentration of RBE In vitamin A-deficient rats, hepatic secretion of RBP is specifically blocked, and its level in serum falls whereas that in the liver increases. Administration of vitamin A causes a rapid proportional release of hepatic RBE The rapidity indicates the release of pooled protein rather than new synthesis. Patients suffering from parenchymal liver disease or protein-calorie malnutrition have reduced amounts of serum RBP and vitamin A. Provision of an adequate diet to malnourished children increases plasma concentrations of RBP and retinol, even without vitamin A supplementation, suggesting the presence of stores of retinol that could not be mobilized on the inadequate diet. The plasma concentration of RBP increases in renal disease, indicating that the kidney is a major catabolic site for this protein. Complex formation with transthyretin reduces glomerular filtration and, consequently, renal catabolism of RBE In rat hepatocyte cultures, glucocorticoids stimulate net synthesis of RBE Oxidation of retinol produces retinoic acid (tretinoin). The reaction is irreversible. Retinoic acid enters the portal blood, is transported bound to albumin, and is not stored to any great extent. The concentration of retinoic acid in plasma is normally 3-4 ng/mL. A biologically active metabolite, 5,6-epoxyretinoic acid, has been isolated from the intestinal mucosa of vitamin A-deficient rats following administration of 3H-retinoic acid. Several tissues have specific cellular retinoic acid-binding proteins (CRABPs).

Function

Vitamin A and its derivatives retinal and retinoic acid have many essential biological roles in such processes as vision, regulation of cell proliferation and differentiation, and as morphogenetic agents during embryonic


development and differentiation. Both natural and synthetic analogues (known as retinoids) possess varying degrees of biological activity ascribed to vitamin A. Other than the role of retinal in vision, retinoids exert their biological actions by binding to specific nuclear receptors, such as transcription factors that modulate gene expression. The retinoid receptors belong to two subfamilies: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). Both the RARs and RXRs contain three isotypes, c~, fl, and V encoded by separate genes. Thus, the retinoid receptor family includes RARol, RARfl, RARv, RXRot, RXRfl, and RXRv, which are members of the steroid and thyroid hormone superfamily of receptors (Chapter 30). All of these receptors possess at least two domains: a ligand binding domain and a DNA binding domain. The DNA binding domain of the receptor recognizes retinoic acid response elements within the target gene via the two zinc finger binding motifs. The receptors form dimers of various combinations and permutations within the superfamily, and each dimer may perform a specific biological function by binding to specific DNA response elements. Activation of retinoid receptors can lead to inhibition of cell proliferation, induction of differentiation, and induction of apoptosis during normal cell development. The importance of RARs in cell proliferation and differentiation is illustrated by acute promyeloeyrie leukemia (PML), which is a subtype of acute myeloid leukemia. PML is characterized by abnormal hypergranular promyelocytes, a cytogenetic translocation, and a bleeding disorder secondary to consumptive coagulopathy and fibrinolysis. The bleeding problems are presumably initiated by procoagulant phospholipids present in the leukemic cells (Chapter 36). The cytogenetic hallmark of PML consists of a balanced reciprocal translocation between the long arms of chromosomes 15 and 17, designated as t(15;17) (Figure 38-3). The translocation results in two recombinant chromosomes, one abnormally long 15q+ and one shortened 17q-. This translocation fuses the PML gene located on chromosome 15 with the RARo~ gene located on chromosome 17. The two chimeric genes formed are PML-RAR~ and RARo~PML. Of these, the PML-RAR~ gene is transcriptionally active and produces a PML-RARc~ protein. This protein, which is an oncoprotein, is responsible for the pathogenesis of PML by interfering with the normal functions of PML and RARot genes in differentiation of myeloid precursors. The normal PML gene product has growth suppressor activity. Treatment with all-trans retinoic acid has produced complete remissions in PML by promoting the conversion of leukemic blast cells into mature cells. This is because the PML-RARo~ oncoprotein maintains responsiveness to retinoic acid's biological actions. Retinoic acid

907

SECTION38.1 Fat-Soluble Vitamins

F I G U R E 38-3 Karyotype of a promyelocytic leukemic cell, with balanced reciprocal translocation between chromosomes 15 and 17, t(15; 17). The rearranged chromosomes are identified by arrows. (Courtesy of Dr. Mark H. Bogart.)

therapy may also augment the degradation of the PMLRARo~ fusion protein. Transgenic mice with a PML-RARot fusion gene develop PML, which supports the pathogenic role of the fusion protein in human PML. Retinyl phosphate, acting as a sugar carrier in a manner similar to that of dolichol phosphate (Chapter 16), may be necessary for normal glycosylation of a subgroup of essential glycoproteins. In vitamin A deficiency, synthesis of some glycoproteins is decreased. Retinyl phosphate and mannosyl phosphoryl retinol have been identified in vivo and have been synthesized in vitro using biological preparations. In rat liver, GDP-mannose: retinyl phosphate mannosyltransferase has its highest specific activity in the endoplasmic reticulum. However, retinoic acid can fully support growth in the whole animal even though it is not converted to retinol and does not participate in sugar transfer reactions. More than 1500 retinoids have been synthesized. Two in particular, 13-cis-retinoic acid (isotretinoin) and etretihate (Figure 38-4), have generated considerable interest as agents for the treatment of dermatological disorders. 13-cis retinoic acid inhibits sebum production, is the drug of choice for treatment of severe cystic acne, and is useful in the treatment of other forms of acne. Etretinate is used in Europe for treatment of psoriasis and related disorders.

Hypo- and Hypervitaminosis A Serious vitamin A deficiency is not a major public health problem in North America; however, where poverty and poor nutrition are common many persons suffer from vitamin A deficiency with active corneal involvement.

This condition leads to permanent blindness in about half of cases, and many of those affected are preschool children. Night blindness occurs early in vitamin A deficiency. There is a reduction in rhodopsin concentration, followed by retinal degeneration and loss of photoreceptor cells. Degenerative changes may be due to instability of free opsin or may indicate an additional nutritive function for retinaldehyde (or retinoic acid) in retinal cells. The degenerative changes of retinitis pigmentosa, a relatively common, inherited cause of blindness, closely resemble those of vitamin A deficiency. Ingestion of vitamin A in large excess of the RDA can cause toxicity. Daily intake of more than 7500 retinol equivalents (25,000 IU) is not recommended, and doses in excess of 3000 retinol equivalents (10,000 IU) should only be used with medical supervision. Acute toxicity after

OOH

(a) 0

. 0o2

II

(b)

FIGURE 38-4 Structures of 13-cis-retinoic acid (isotretinoin) (a) and etretinate (b), synthetic retinoids used as pharmacological agents.

908

CHAPTER38 Vitamin Metabolism ROD CELL

CONE CELL

Disks

Outer

segment

Invaginations of plasma membrane (equivalent to disks)

Plasma membrane 3ytoplasm

ConneOn0 structure Mitochondrion _ _ ugh endoplasmic

reticulum

. , ."

~'. ~

~

i~

.%~..'jI

Light path in vivo

Inner

segment - -

Synaptic body

FIGURE 38-5 Schematic representations of mammalian retinal rod and cone cells. There are about 500-1500 disks in the outer segment of each rod cell. The deep membrane invaginationsin the plasma membrane of the cone cell outer segment are thought to be functionally equivalent to the rod cell disks.

a single large dose may last several days. Chronic toxicity occurs in adults after daily doses of 7500-15,000 retinol equivalents (25,000-50,000 IU) taken for several months. Ingestion of carotenes alone does not cause vitamin A toxicity, probably because of markedly decreased absorption at high doses and feedback inhibition of carotene conversion to retinaldehyde, but can cause harmless carotenemia with yellowing of the skin, particularly on the palmar and plantar surfaces. Carotenemia can cause falsely elevated values for the icterus index or for other direct reading methods for estimating serum bilirubin (Chapter 29). In cases of toxicity, total serum vitamin A concentration is elevated two- to eightfold, mostly in the retinyl esters, which are associated with plasma lipoproteins, while the RBP level is normal or only slightly increased. Unesterifled retinol is generally increased less than twofold. Vitamin A is a surfactant, and its toxicity may be due to labilization and disruption of biological membranes. Excessive amounts of retinol also increase synthesis and release oflysosomal hydrolases. In in vitro systems, retinol bound to RBP is not toxic, and RBP alone protects against

toxicity of previously added vitamin A. Thus, clinical toxicity occurs only when the binding capacity of RBP is exceeded and retinol and retinyl esters are present either free or in lipoproteins in the circulation.

Vision and Vitamin A

Photosensitivity in various organisms is based on photoisomerization of retinaldehyde. In humans, l l - c i s retinaldehyde complexes with opsin to produce rhodopsin (visual purple). Absorption of a photon by the :relectron system of retinaldehyde changes it to the alltrans isomer; this reaction--the only light-catalyzed step in vision--is transduced to an action potential transmitted in the optic nerve. The human visual system functions over a range of light intensity. If properly dark adapted, the eye can detect a single photon. Vitamin A deficiency reduces the amount of rhodopsin in the retina, thereby increasing the minimum amount of illumination that can be detected (the visual threshold) and causing night blindness.

909

SECTION38.1 Fat-Soluble Vitamins

The retina consists of a thin layer of photoreceptor cells connected to the optic nerve via axons, synapses, and the bodies of several intermediate cells. Other structures of the eye provide the optical system for focusing light onto the retina. Visual pigments are present in the approximately 100 million rod and 5 million cone cells (Figure 38-5). The cones are less sensitive than the rods but are required for color vision. Three types of cone cells differ with respect to the wavelengths of light to which they are maximally sensitive. Comparison and integration of inputs from these three cell types by the visual cortex produce color vision. At low light levels, only the rods are active and color vision is lost. A higher pigment content and greater molar absorptivity of the pigment contribute to the higher sensitivity of rods. Rhodopsin has a broad absorption spectrum with a maximum at 500 nm, making the rods most sensitive to green light. In the human retina, cone cell absorption maxima are 445 nm (blue), 535 nm (green), and 570 nm (red). Opsins in the three types of cone cell have different primary structures and are coded by three separate genes. Those for red- and green-sensitive opsins are on the X chromosome, while that for blue-sensitive opsin is on an autosome. Congenital color blindness, which affects about 9% of the male population, results from the absence of one or more cone cell types or from a decrease in amount of one of the pigments. Absence of red or green cones occurs in 2.5% of males. Absence of blue cones occurs in only 0.001% of males. Decrease in red cone pigment (protanomaly) or green cone pigment (deuteranomaly) occurs in 1.3% and 5.0%, respectively, of males. The decrease in pigment type presumably is due to mutation in a structural gene. Absence of two or three types of cone cell is extremely rare. Light reception and energy transduction take place in the rod outer segments, while cellular metabolism occurs in the rod inner segment, which is rich in glycogen, mitochondria, and rough endoplasmic reticulum. The outer segment contains 500-1500 flattened membrane sacs (disks), each about 16 nm thick, which are electrically isolated from the surrounding plasma membrane and which contain rhodopsin as an integral, transmembrane protein. The disks are continually replaced from the outer segment nearest the nucleus. Phagocytosis of old disks by cells of the retinal pigment epithelium occurs at the top of the outer segment. In the rhesus monkey, the lifetime of a disk is 9-13 days. Disks are formed by the sealing off of invaginations of plasma membrane, and the composition of their contents differs from that of cytoplasm. Blindness due to hereditary defects in regeneration of disks has been observed in rats. Cone cells are similar in many ways to rod cells (Figure 38-5). Because cone ceils lack distinct disks, their visual pigments are located on deep invaginations

of the plasma membrane that resemble disks and probably are formed by a similar mechanism. Disk membranes are composed primarily of phospholipid and rhodopsin at a molecular ratio of about 70:1. The phospholipids are 40% phosphatidylcholine, 40% phosphatidylethanolamine, and 13% phosphatidylserine. The membrane has high fluidity because of extensive unsaturation of the lipids, which gives rhodopsin considerable rotational and translational mobility. Average spacing of rhodopsin molecules is 5.6 nm. Bovine rhodopsin (M.W. 39,000) has two asparaginelinked oligosaccharides with the structure (mannose)3 (N-acetylglucosamine). Seven hydrophobic o~-helical segments, each 21-28 amino acids long, are embedded in the disk membrane and are connected by hydrophilic sequences that are exposed to either side of the membrane. The retinaldehyde moiety is within the membrane, halfway down the first helical segment, attached by an aldimine linkage to the e-amino group of lysine 53 residues from the carboxyl terminus by aldimine linkage (Figure 38-6).

2H

Additional hydrophilic and hydrophobic residues

(

)

Disk interior

Oligosaccharide chains FIGURE 38-6

Diagrammatic representation of bovinerhodopsinembeddedin the disk membrane. Onlythree of the seveno~-helicalsegmentsare shown.The only point of attachmentbetween retinaldehydeand opsin is the indicated aldimine bond. [Modifiedand reproduced, with permission, from D. E O'Brien, The chemistryof vision. Science 218, 961 (1982), (~) 1982 by the AmericanAssociationfor the Advancementof Science.]

910


In the dark, rhodopsin has a red color, which changes to pale yellow upon exposure to light. This bleaching takes place within a few milliseconds after absorption of a photon and occurs because all-trans retinaldehyde bound to opsin is unstable. At low temperatures, bleaching proceeds more slowly, and seven intermediates have been identified (Figure 38-7). The structures and order of appearance of these intermediates have yet to be clarified. The final

(53")-opsin Rhodopsin

H

hv

Hypsoriodopsin Bathorhodopsin

I Lumirhodopsin [ Metarhodopsin I 1 Metarhodopsin II l Pararhodopsin l

N'RetiHn~dene~ s -Retinaldehyde + H,N-Lys (53")-opsin F I G U R E 38-7 Bleaching of rhodopsin in bovine retina. Absorption of light by 11-cis-retinaldehyde initiates the configurational change to all-trans retinaldehyde, culminating in hydrolysis of the bond between retinaldehyde and Lys 53' of opsin. The order of formation of the intermediates is tentative. The symbol h v represents a photon of visible light. [Modified and reproduced, with permission, from C. D. B. Bridges, Retinoids in photosensitive systems. In: The Retinoids, Vol. 2, M. B. Sporn, A. B. Roberts, and D. S. Goodman, eds. Academic Press, San Diego, 1984).]

F I G U R E 38-8 Regeneration of 1 l-cis-retinaldehyde (1 l-cisRal). Following light exposure, rhodopsin (opsin -t- I l-cisRal) breaks down to opsin and all-trans retinaldehyde (atRal) initiating a neural signal. The restoration of I l-cisRal begins with the atRal combining with phosphatidylethanolamine (PE) to form the protonatal Schiff base N-retinylidine-PE complex. This complex is transported across the disk membrane by an ATP-binding cassette protein, exclusively found in the retinal outer segment (ABCR). Defects in ABCR cause early-onset maculopathy (Stargardt's disease). After reduction of atRal to all trans-retinol (atRol) by atRol dehydrogenase in the retinal outer segment, atRol is converted to 1 l-cisRal in a series of reactions occuring in the retinal pigment epithelial cell (RPE). Also shown in the bottom is the phagocytosis of disk by RPE.

compound (perhaps N-retinylideneopsin) is unstable and hydrolyzes to all-trans retinaldehyde plus opsin. The regeneration of rhodopsin following exposure to bright light requires the following steps. The all-trans retinaldehyde combines with phosphotidylethanolamine (PE) to form a protonated Schiff base N-retinylidene-PE. This complex is then exported out of the disk by an ATP-binding cassette (ABC) transporter exclusively located in the retinol disk membrane (ABCR). Outside the disk the all-trans retinaldehyde is reduced to all-trans retinol by a dehydrogenase and transported to retinal

911

SECTION 38.1 Fat-Soluble Vitamins

pigment epithelial (RPE) cells. In the RPE cells involving a series of reactions all-trans retinol is converted to 11-cis retinaldehyde which is transported to rod cells. In the rod cells, 11-cis retinaldehyde reassociates to form rhodopsin. The importance of ABCR becomes evident in early onset macular degeneration, due to mutations in the ABCR gene. This disorder is known as Stargardt's disease, an inherited autosomal recessive disease, which affects one person in 10,000. Age-related macular degeneration in susceptible individuals with risk factors (e.g., high lipid diet, smoking) may also be due in part to defects in ABCR gene. ABC transporters are involved in the transport of a wide variety of molecules. For example, cystic fibrosis is caused by a defective ABC transporter for chloride ion (cystic fibrosis transmembrane regulator, Chapter 12) and in Tangier's disease, the abnormality of cholesterol efflux is due to defects in an ABC protein (Chapter 20). In the dark, the cation (Na +, CaZ+)-specific cGMPgated channels located in the rod outer segment (ROS) are open, thus promoting the influx of Na + and Ca 2+. The steady state of cations is maintained by outward pumping of Ca 2+ by the Na+/Ca 2+ exchanger and Na +, K+-ATPase pumps located in the inner segment. Exposure to light blocks cGMP-gated cation channels, causing the inside of the plasma membrane to become more negative and resulting in hyperpolarization. The signal of hyperpolarization is transmitted to the synaptic body and eventually to the brain. In the dark, because of the influx of Na + and Ca 2+, the rod cells are depolarized and neurotransmitters (possibly aspartate or glutamate) are released from the presynaptic membrane of the rod cell at relatively high rates. In the hyperpolarized state the neurotransmitter release is inhibited. Thus, the postsynaptic bipolar cells are either inhibited (hyperpolarized) or excited (depolarized). Coupling of rhodopsin present in the disk membrane to cation-specific cGMP-gated channels of the plasma membrane involves signal amplification after photon absorption. The coupling mechanism (Figure 38-9) begins with the absorption of light by l l-cis-retinal-rhodopsin and its conversion to all-trans retinal and the photoactivated rhodopsin (R*). In the next step, activated rhodopsin initiates an amplification process that consists of activating transducin, a signal-coupling G-protein. The active form of transducin, T~.GTp, activates cGMP-phosphodiesterase (PDE). In the second amplification process, PDE hydrolyzes cGMP with the closure of cation channels. The opening and closing of these cation channels that is mediated by cGMP is similar to Ca 2+ ion channel gating by cAMP in olfaction. PDE belongs to a superfamily of enzymes that regulate cell function by maintaining cyclic nucleotide levels. There are cAMP-specific PDEs (PDE4, PDE7, and PDE8),

Light

Rhodopsin-11-cis retinal (present in the disc membrane) -~ AII-trans retinal Photoactivated Rhodopsin (R*)

I Amplificationprocess GTP Transducin (T)c~.GDP.13~,

GDP )

~

Inactive Phosphodiesterase (PDE-I)

) T .G-rP+ T~_

9Active PDE + T.GT P - I ' -"

H20 cGMP

\

), 5" -GMP

Plasma membrane cation channels closure; Decreased Ca2+ and Na § concentration; hyperpolarization

FIGURE 38-9 Mechanism for stimulus-response coupling of photon absorption to the closure of plasma membrane cation channels. I, Inhibitory subunit of phosphodiesterase.

cGMP-specific PDEs (PDE5, PDE6, and PDE9), and ones specific for both cAMP and cGMP PDEs (PDE1, PDE2, and PDE10). Light induced-activated transducin activates a cGMP-specific PDE, namely, PDE6. Another previously discussed cGMP-specific PDE is PDE5, which is abundant in vascular smooth muscle (Chapter 17). PDE5 is involved in the nitric oxide and cGMP signaling pathway. A specific inhibitor of PDE5, sildenafil, causes increased blood flow into the sinusoidal spaces of the corpus cavernosum and corpus spongiosum, resulting in erection of the penis (Chapter 17). Photoactivation of rhodopsin produces a conformational change that allows it to interact with and activate transducin, resulting in the replacement of GDP by GTP at the o~subunit. Metarhodopsin II (Figure 38-7) is the form that interacts with transducin at the disk membrane surface. Depending on the lifetime of the activated rhodopsin molecule, each molecule can activate up to 500 transducin molecules. The o~-subunit GTP complex of transducin disinhibits PDE by removing its inhibitor subunit (I), causing it to hydrolyze cGMP to 5'-GME The decrease in cGMP concentration closes the plasma membrane cation channels, decreasing Na + and Ca 2+ concentrations. Some cases of retinitis pigmentosa are caused by mutations in the transducin gene that prevent it from disinhibiting the PDE, leading to an accumulation of cGMP that is toxic to the retina. The transducin-GTP complex remains active as long as the GTP bound to G~ is not hydrolyzed. The recovery and adaptation of the rod and the cone cells to the dark state begins shortly after illumination

912

CHAPTER 38 Vitamin M e t a b o l i s m

(a)

Photoactivated Rhodopsin (R*) ,ow

;,

ATP

> [ R h o d o p s i n kinase

Recoverin J ~

~

ADP

Phosphorylated - R (P- R)

Arrestin

P,
medulla). Most signs of scurvy can be related to inadequate or abnormal collagen synthesis. Ascorbate enhances prolyl and lysyl hydroxylase activities (Chapter 25). Collagen formed in scorbutic patents is low in hydroxyproline and poorly cross-linked, resulting in skin lesions, bone fractures, and rupture of capillaries and other blood vessels. The absolute amount of collagen made in scorbutic animals may also decrease independently of the hydroxylation defect. The anemia of scurvy may result from a defect in iron absorption or folate metabolism. Ascorbate increases the activity of hydroxylases needed for the conversion of p-hydroxyphenylpyruvate to homogentisate (Chapter 17), synthesis of norepinephrine from dopamine (Chapter 32), and two reactions in carnitine synthesis (Chapter 18). It is not known whether decreased activity of these enzymes contributes to the clinical characteristics of scurvy. Although ascorbic acid is needed for maximal activity of these enzymes in vivo and in vitro, most show some activity when other reducing agents are used.

38.3 Vitamin-Responsive Inherited Metabolic Disorders The vitamin requirements indicated by the RDA 1 represent the amounts needed for normal health by most individuals in the population. Following absorption, 1(See Table 38-1.)

927

SECTION38.4 Vitamin-like Substances

transport, and metabolism, vitamins must interact with many biomolecules before they reach the location where they function. The active cellular form also must interact with one or more proteins in carrying out its biological function. This gives many potential sites for genetic disruption of vitamin metabolism and function. For example, because of mutation in an intestinal transport protein, a vitamin may be inadequately absorbed. Mutation of an enzyme may increase the Km for a cofactor derived from a vitamin and result in relative deficiency of activity when the vitamin is present at normal concentration. Although such diseases cannot yet be cured, their clinical signs and symptoms can sometimes be relieved by administration of a very large oral or parenteral dose of the appropriate vitamin (Table 38-1). Megadose vitamin therapy in these patients has a basis in human biochemistry and differs from indiscriminate self-medication for diseases (real or imagined) that have no established relationship to the vitamins used. Some inherited disorders respond to megadoses of a vitamin that is not directly related to the defective protein. Pyruvate carboxylase deficiency leads to encephalomyelopathy in which there is lactic and pyruvic acidemia. Pyruvate carboxylase requires biotin and converts pyruvate to oxaloacetate for the tricarboxylic acid cycle or for gluconeogenesis. A major metabolic pathway for pyruvate is oxidative decarboxylation to acetylCoA, catalyzed by the pyruvate dehydrogenase complex. Some patients have shown clinical improvement when treated with lipoic acid. Others have responded to thiamine (Table 38-1). Thiamine and lipoic acid participate in the pyruvate dehydrogenase complex. They presumably reduce the concentrations of pyruvate and lactate by increasing the flux through the pyruvate dehydrogenase complex. Other examples include the use of vitamin E to reduce hemolysis in some patients with deficiency of glutathione synthetase or glucose-6-phosphate dehydrogenase; folic acid, choline, or betaine in some cases of homocystinuria due to cystathionine fi-synthase deficiency; and pyridoxine in some cases of primary hyperoxaluria due to deficiency of soluble glyoxalate-o~-ketoglutarate carboligase. As in the case of pyruvate carboxylase deficiency, the effect is probably due to enhancement of alternative metabolic pathways that bypass the defective enzyme. Not all patients who present the same clinical picture respond to vitamin therapy. Thus, if the structural gene for an apoenzyme or transport molecule is completely absent because of a gene deletion, no amount of vitamin or cofactor will correct the defect. If the mutation affects substrate rather than cofactor binding, the pathway is blocked just as effectively and cannot be relieved by increased concentration of cofactor. Thus, six mutations have been identified that cause methylmalonic aciduria,

but not all respond to large doses of vitamin B12. Similarly, only some patients with homocystinuria due to NS,N l~ methylenetetrahydrofolate reductase deficiency respond to treatment with folic acid. About two dozen inherited diseases respond to pharmacological doses of a vitamin (Table 38-1). Many have been mentioned elsewhere in this book in conjunction with the affected metabolic pathways. Although most are very rare, their study has contributed much to acknowledge of metabolism in the human body. Some are heterogeneous in symptoms and in responsiveness to therapy, suggesting genetic heterogeneity, as discussed above.

38.4 Vitamin-like Substances Several compounds, e.g., vitamin D and niacin, are apparently required in the diet even though pathways for their synthesis occur in the body. Such a situation may arise if a pathway does not provide an adequate supply for the body's needs or if the material cannot be readily transported from the site of synthesis to the place of action. The compounds discussed below are essential dietary nutrients in one or more nonhuman species, but such a status in humans is not supported by evidence. Choline (N,N,N-trimethyl-fi-hydroxyethylamine) is an important constituent of phospholipids (lecithin is phosphatidylcholine) and of acetylcholine. It can be completely synthesized from serine (Chapter 19), but only in the form of phosphatidylserine and then only when the dietary supply of amino acids is adequate. Betaine (N,N,N-trimethylglycine) readily replaces dietary choline for all species. Choline is conserved by a salvage pathway. In the lung, this salvage route is the principal route for the synthesis of the phosphatidylcholine needed as a surfactant (see Chapter 19). Inositol (hexahydroxycyclohexane) occurs in several isomeric forms. Myo-inositol (or meso-inositol) is an important constituent of phospholipids and is the only isomer with biological activity. Inositol hexaphosphate (phytic acid) is found in avian erythrocytes, where it binds to hemoglobin, thereby regulating the oxygen capacity of the blood. It is also important as an intracellular messenger in a number of pathways. Lipoic acid (thioctic acid) functions as an intermediate in the oxidation-reduction reaction of the oxidative dccarboxylation of certain ketoacids. Ubiquinone (coenzyme Q) is involved in the mitochondrial electron transport. Although coenzyme Q supplements have been recommended as preventative measures for coronary artery disease and cancer, scientific studies do not confirm their effectiveness in preventing these diseases.

928 Supplemental Readings and References A. C. Antony: The biological chemistry of folate receptors. Blood 79, 2807 (1992). L. M. Ausman: Criteria and recommendation for vitamin C intake. Nutrition Reviews 57, 222 (1999). R. J. Berry, Z. Li, J. D. Erickson, et al.: Prevention of neural-tube defects with folic acid in China. New England journal of Medicine 341, 1485 (1999). A. BjCrneboe, G-E. Bjcrnboe, and C. A. Drevon: Absorption, transport and distribution of vitamin E. Journal of Nutrition 120, 233 (1990). J. Blanchard, T. N. Tozer, and M. Rowland: Pharmacokinetic perspectives on megadoses of ascorbic acid. American Journal of Clinical Nutrition 66, 1165 (1997). L. D. Botto, C. A. Moore, and M. J. Khoury: Neural-tube defects. New England Journal of Medicine 341, 1509 (1999). B. B. Bowman, D. B. McCormick, and I. H. Rosenberg: Epithelial transport of water-soluble vitamins. Annu. Rev. Nutr. 9, 187 (1989). R. Brigelius-Floh6 and M. G. Traber: Vitamin E: function and metabolism. FASEB Journal 13, 1145 (1999). R. Carmel: Cobalamin, the stomach, and aging. American Journal of Clinical Nutrition 66, 750 (1997). A. C. Carr and B. Frei: Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans. American Journal of Clinical Nutrition 69, 1086 (1999). Centers for Disease Control and Prevention: Knowledge and use of folic acid by women of childbearing age--United States. Morbidity and Mortality Weekly Report 46, 721 (1997). Centers for Disease Control and Prevention: Recommendations for the use of folic acid to reduce the number of cases of spina bifida and other neural tube defects. Morbidity and Mortality Weekly Report 41, RRI4 (1992). E Christian and K. E West Jr.: Interactions between zinc and vitamin A: an update. American Society for Clinical Nutrition 68, 435S (1998). A. C. Chan: Vitamin E and atherosclerosis. Journal of Nutrition 128, 1593 (1998). Committee on Genetics: Newborn screening fact sheet. Pediatrics 98, 473 (1996). E Di Mascio, M. E. Murphy, and H. Sies: Antioxidant defense systems: the role of carotenoids, tocopherols, and thiols. Am. J. Clin. Nutr. 53(1 Suppl), 194S (1991). M. N. Diaz, B. Frei, J. A. Vita, et al.: Antioxidants and atherosclerotic heart disease. New England Journal of Medicine 337, 408 (1997). A. T. Diplock: Safety of antioxidant vitamins and /3-carotene. American Journal of Clinical Nutrition 62, 1510S (1995). J. W. Eikelboom, E. Lonn, J. Genest Jr., et al.: Homocyst(e)ine and cardiovascular disease: a critical review of the epidemiologic evidence. Annals of lntemal Medicine 131, 363 (1999). T. R. J. Evans and S. B. Kaye: Retinoids: present role and future potential. British Journal of Cancer 80, 1 (1997). P. Fenaux and L. Degos: Differentiation therapy for acute promyelocytic leukemia. New England Journal of Medicine 337, 1076 (1997). S. C. Guba, L. M. Fink, and V. Fonseca: Hyperhomocysteinemia. American Journal of Clinical Pathology 105, 709 (1996). J. Haller: The vitamin status and its adequacy in the elderly: an international overview. Intemational Journal of Vitamin Nutrition Research 69, 160 (1999). E. B. Healton, D. G. Savage, J. C. M. Brust, et al.: Neurologic aspects of cobalamin deficiency. Medicine 70, 229 (1991). A. Kamal-Eldin and L-A. Appelqvist: The chemistry and antioxidant properties of tocopherols and tocotrienols. Lipids 31, 671 (1996).


H. Kasper: Vitamin absorption in the elderly. International Journal of Vitamin Nutrition Research 69, 169 (1999). M. Levine, S. C. Rumsey, R. Daruwala, et al.: Criteria and recommendations for vitamin C intake. Journal of American Medical Association 281, 1415 (1999). J. Lindenbaum, E. B. Healton, D. G. Savage, et al.: Neuropsychiatric disorders caused by cobalarnin deficiency in the absence of anemia or macrocytosis. New England Journal of Medicine 318, 1720 (1988). J. G. Locksmith and E Duff: Preventing neural tube defects: the importance of periconceptional folic acid supplements. Obstetrics and Gynecology 91, 1027 (1998). J. C. Mattson: Acute promyelocytic leukemia: from morphology to molecular lesions. Clinics in Laboratory Medicine 20, 83 (2000). M. Meydani: Vitamin E. Lancet 345, 170 (1995). A. Molloy, et al.: Thermolabile variant of 5.10-methylenetetrahydrofolate reductase associated with low red-cell folate: implications for folate intake recommendations. Lancet 349, 1591 (1997). National Research Council: Recommended Dietary Allowances, 10th ed. National Academy of Sciences, Washington, D.C., 1989. C. L. Rock, R. A. Jacob, and E E. Bowen: Update on the biological characteristics of the antioxidant micronutrients: vitamin C, vitamin E, and the carotenoids. Journal of the American Dietetic Association 96, 693 (1996). K. J. Rothman, L. L. Moore, M. R. Singer, et al.: Teragenicity of high vitamin A intake. New England Journal of Medicine 333, 1369 (1995). K. Schtimann: Interactions between drugs and vitamins at advanced age. Intemational Journal of Vitamin Nutrition Research 69, 173 (1999). R. H. Schwarz and R. B. Johnston: Folic acid supplementation--when and how. Obstetrics and Gynecology 88, 886 (1996). C. E Snow: Laboratory diagnosis of vitamin BI2 and folate deficiency. Archives of lnternal Medicine 159, 1289 (1999). S. E Stabler, R. H. Allen, D. G. Savage, et al.: Clinical spectrum and diagnosis of cobalamin deficiency. Blood 76, 871 (1990). S. P. Stabler, J. Lindenbaum, and R. H. Allen: Vitamin B~2 deficiency in the elderly: current dilemmas. American Journal of Clinical Nutrition 66, 741 (1997). N. G. Stephens, A. Parsons, and P. M. Schofield: Randomised controlled trial of vitamin E in patients with coronary disease: Cambridge Heart Antioxidant Study (CHAOS). Lancet 347, 781 (1996). M. S. Tallman, J. W. Andersen, C. A. Schiffer, et al.: New England Journal of Medicine 337, 1021 (1997). B. Termanini, E Gibril, V. Sutliff, et al.: Effect of long-term gastric acid suppressive therapy on serum vitamin B 12 levels in patients with ZollingerEllison syndrome. American Journal of Medicine 104, 422 (1998). A. Theriault, J-T. Chao, Q. Wang, et al.: Tocotrienol: a review of its therapeutic potential. Clinical Biochemistry 32, 309 (1999). M. G. Traber and H. Sies: Vitamin E in humans: demand and delivery. Annual Review of Nutrition 16, 321 (1996). J. M. Upston, A. C. Terentis, and R. Stocker: Tocopherol-mediated peroxidation of lipoproteins: implications for vitamin E as a potential antiatherogenic supplement. FASEB Journal 13, 977 (1999). J. Virtamo, J. M. Rapola, S. Ripatti, et al.: Effect of vitamin E and beta carotene on the incidence of primary nonfatal myocardial infarction and fatal coronary heart disease. Archives of Intemal Medicine 158, 668 (1998). U-W. Weigand, S. Hartmann, and H. Hummler: Safety of vitamin A: recent results. Intemational Journal of Vitamin Nutritional Resources 68, 411 (1998). N. J. Wald, J. W. Densem, D. Smith, et al.: Four-marker serum screening for Down's syndrome. Prenatal Diagnosis IL4, 707 (1994).

CHAPTER

39

Water, Electrolytes, and Acid-Base Balance

39.1 Water Metabolism Water is the most abundant body constituent; it is 45-60% of total body weight (Figure 39-1). In a lean person, it accounts for a larger fraction of the body mass than in a fat person. Since most biochemical reactions take place in an aqueous environment, control of water balance is an important requirement for homeostasis. Although water permeates freely across cell membranes, other solutes are less mobile because of barriers imposed by membrane systems. These barriers give rise to fluid pools or compartments of different but rather constant composition (Figure 39-2). Intracellular fluid makes up 30-40% of body weight, or about two-thirds of total body water. Potassium and magnesium are the predominant cations. The anions are mainly proteins and organic phosphates, with chloride and bicarbonate at low concentrations. Extracellular fluid contains sodium as the predominant cation and accounts for 20-25% of body weight, or onethird of total body water. It makes up vascular, interstitial, transcellular, and dense connective tissue fluid pools. Vascular fluid is the circulating portion, is rich in protein, and does not readily cross endothelial membranes. Interstitial fluid surrounds cells and accounts for 18-20% of total body water. It exchanges with vascular fluid via the lymph system. Transcellular fluid is present in digestive juices, intraocular fluid, cerebrospinal fluid (CSF),

and synovial (joint) fluid. These fluids are secretions of specialized cells. Their composition differs considerably from that of the rest of the extracellular fluid, with which they rapidly exchange contents under normal conditions. Dense connective tissue (bone, cartilage) fluid exchanges slowly with the rest of the extracellular fluid and accounts for 15% of total body water. Movements of water are due mainly to osmosis and filtration. In osmosis, water moves to the area of highest solute concentration. Thus, active movement of salts into an area creates a concentration gradient down which water flows passively. In filtration, hydrostatie pressure in arterial blood moves water and nonprotein solutes through specialized membranes to produce an almost protein-free filtrate: This process occurs in formation of the renal glomerular filtrate. Filtration also accounts for movement of water from the vascular space into the interstitial compartment, which is opposed by the osmotic (oncotic) pressure of plasma proteins. Cells move ions (especially Na + and K +) against a concentration gradient by a "sodium pump" that actively transports sodium across the plasma membranes (Chapter 12). The kidneys are the major organ to regulate extracellular fluid composition and volume. Three main processes occur in nephrons: 1. Formation of a virtually protein-free ultrafiltrate at the glomerulus;

929

CHAPTER 39 Water, Electrolytes, and Acid-Base Balance

930

by the secretion of Na + from the ascending loop and their uptake by the descending loop. As a result, the proximal end of the loop is hyperosmotic (1200 mosm/kg) and the distal end hypoosmotic with respect to blood. The collecting ducts run through the hyperosmotic region. In the absence of antidiuretic hormone (ADH; see Chapter 31), the cells of the ducts are relatively impermeable to water. They become permeable to water in the presence of ADH, however, and the urine becomes hyperosmotic with respect to blood.

Fat Nonfat components

Interstitial fluid

Ext race Ilu lar fluid

Cellular fluid

Intraceilu lar fluid

,

Solid

Water

I

39.2 Homeostatic Controls

F I G U R E 39-1 Proportional distribution of solids and water in a healthy adult.

2. Active reabsorption (principally in the proximal tubule) of solutes from the glomerular filtrate; and 3. Active excretion of substances such as hydrogen ions into the tubular lumen, usually in the distal portion of the tubule (Figure 39-3). The normal glomerular filtration rate (GFR) is 100-120 mL/min; about 150 L of fluid passes through the renal tubules each day. Since the average daily urine volume is 1-1.5 L, 99% of the glomerular filtrate is reabsorbed. Approximately 80% of the water is reabsorbed in the proximal tubule, a consequence of active absorption of solutes. Reabsorption in the rest of the tubule varies according to the individual's water balance, in contrast to the obligatory reabsorption that occurs in the proximal tubule. The facultative absorption of water depends on the establishment in the loop of Henle of an osmotic gradient

200--

Extracellular fluid

Proteinate Chloride Phosphate and sulfate Bicarbonate Sodium Potassium ca'*andM~'+ Intracellular fluid

o.

-1~6 150-. t,,.

~100-"

i

"~50E ~

m

0

Plasma

F I G U R E 39-2 Composition of body fluids.

Interstitial fluid

Transcellular fluid

(ileal secretion)

Cellular fluid

The composition and volume of extracellular fluid are regulated by complex hormonal and nervous mechanisms that interact to control its osmolality, volume, and pH. The osmolality of extracellular fluid is due mainly to Na + and accompanying anions. It is kept within narrow limits (285-295 mosm/kg) by regulation of water intake (via a thirst center) and water excretion by the kidney through the action of ADH. The volume is kept relatively constant, provided the individual's weight remains constant to within 4-1 kg. Volume receptors sense the effective circulating blood volume, which when decreased, stimulates the renin-angiotensin-aldosterone system and results in retention of Na + (Chapter 32). The increased Na + level leads to a rise in osmolality and secretion of ADH, with a resultant increase in water retention. Antagonistic systems exist that result in an increased Na + excretion. Atrial natriuretic peptide (ANP), also called atrial natriuetic factor or hormone, is released by the cardiocytes of the cardiac atria in response to mechanical stretch caused by the plasma volume expansion. ANF induces diuresis and natriuresis. These effects result from renal hemodynamic changes associated with increases in GFR and inhibition of Na + reabsorption from inner medullary collecting ducts. ANP is a 28-amino-acid peptide and has a single disulfide linkage (Figure 39-4). The precursor and the storage form of ANP in cardiocytes is a 126-amino-acid polypeptide. Some of the stimuli other than the blood volume which function as secretogosues of ANP include high blood pressure, elevated serum osmolality, increased heart rate, and elevated levels of plasma catecholamines. Activation of the ANP gene in cardiocytes by glucocorticoids leads to increased synthesis of ANE ANP also regulates Na + and water homeostasis by different mechanisms that include inhibition of steps in the renin-angiotensin-aldosterone pathway and inhibition of ADH secretion from posterior pituitary cells. The mechanism of action of ANP on target cells involves the formation of cGMP via the activation of plasma

SECTION39.3 Water and 0smolality Controls

931

Glomerulus

Na HCO3, Glucose, Amino'Acids, Water (isosmotic)

I I (/ / ~ ~/

~Hyposmotic

H+

K+

..

T NaCI (aldosterone-sensitive) I

! I

/

I k / /

/ ~ \

I I!

I15 I ]* ~ )

NaCI, Water

/

/

/|

Low Osmolality

I

/ ,BI~NaCI / |

(isosmotic)

| NaCI411" Water

(requires 4=iBB ADH)

Isosmotic

[ I

/

*

+ H + m~ K , =P

WaterNaci(=l=tl

1

Urea ==1

Ure

\

J

Hyperosmotic

F I G U R E 39-3 Principal transport processes in the renal nephron. ADH, Antidiuretic hormone. [Reproduced with permission from M. B. Burg, The nephron in transport of sodium, amino acids, and glucose. Hosp. Pract. 13(10), 99 (1978). A. Iselin, illustrator.]

membrane receptor. The ANP receptor itself is a guanylyl cyclase with its ligand binding domain located in the extracellular space and its catalytic domain in the cytsolic

+H3N~/I

S

t

domain. The receptor has only one membrane spanning domain. This ANP receptor-activated cGMP complex is unique and does not involve any G-proteins. A soluble cytosolic guanylyl cyclase that is activated by nitric oxide binding to the heme group of the enzyme causes vascular relaxation (Chapter 17). The intracellular formation of cGMP causes activation cGMP-dependent protein kinases which mediate the actions of ANE An ANP of 32 amino acid residues (known as B type-ANP) found in the ventricles of the heart (and in the brain) is secreted in response to ventricular expansion and pressure. B type-ANP has similar physiologic function compared to atrial ANE The pH of extracellular fluid is kept within very narrow limits (7.35-7.45) by buffering mechanisms (see also Chapter 1), the lungs, and the kidneys. These three systems do not act independently. For example, in acute blood loss release of ADH and aldosterone restores the blood volume and renal regulation of the pH leads to shifts in K + and Na + levels.

39.3 Water and Osmolality Controls COOF I G U R E 39-4 Amino acid sequence of atrial natriuretic peptide. It has a one-disulfide linkage.

Despite considerable variation in fluid intake, an individual maintains water balance and a constant composition of body fluids. The homeostatic regulation of water is

CHAPTER 39 Water, Electrolytes, and Acid-Base Balance

932

Diet. /Metabolic water

Psych~,,N~

Inpu~,~

Thirst

#

I Solutes-I (NaCI, |

Obligatorylosses (skinand lungs)

.oo,

water

glucose)J

I Bodyfluid

V

osmolarity I

Centralnervous systemeffects~ A ~ Drugs

l

m ppVre solvent- esolution

D

H

Kidney

appreciable fraction of the solution is not water). Thus, with urine the approximation is acceptable, whereas with serum it is not because of the large amount of protein present. Although osmolarity is more readily measured, it is temperature dependent, unlike osmolality. Osmolality is commonly measured by freezing point or vapor pressure depression. In terms of vapor pressure (pv), the osmotic pressure (zr) is defined as

] = Urine water (amount excreted controlledby ADH)

FIGURE 39-5 Regulation of osmolality in the body.

summarized in Figure 39-5. Body water is derived from 2-4 L of water consumed daily in food and drink and 300 mL of metabolic water formed daily by oxidation of lipids and carbohydrates. Water loss occurs by perspiration and expiration of air ( " 1 L/d), in feces (--200 mL) (Chapter 12), and in urine (1-2 L/d). Water balance is regulated to maintain constant osmolality of body fluids. This osmolality is directly related to the number of particles present per unit weight of solvent. A solution that contains 1 tool of particles in 22.4 kg of water (22.4 L at 4~ exerts an osmotic pressure of 1 atm and has an osmolality of 0.0446. Conversely, the osmotic pressure of an osmolal solution (1 mol of particles/kg of water) is 22.4 atm. In this sense, "number of particles" is roughly defined as the number of noninteracting molecular or ionic groups present. Since glucose does not readily dissociate, 1 mol dissolved 1 kg of water (a molal solution) produces 1 mol of "particles" and has an osmolality of 1. Sodium chloride dissociates completely in water to form two particles from each molecule of NaC1 so that a molal solution of NaC1 is a 2-osmolal solution. Similarly, a molal solution of NazSO4 or (NH4)2SO4 is a 3-osmolal solution. In practice, the milliosmole (mosm) is the unit used. With aqueous solutions, osmolarity is sometimes used interchangeably with osmolality. Although this practice is not strictly correct (moles of particles per liter of solution versus moles of particles per kilogram of solvent), in water at temperatures of biological interest the error is fairly small unless solute concentrations are high (i.e., when an

As defined above, osmolality -- Jr/22.4, where Jr is measured in atmospheres. In one instrument, solution and solvent vapor pressures are measured by use of sensitive thermistors to detect the difference in temperature decrease caused by evaporation of solvent from a drop of pure solvent and from a drop of solution. Because the rate of evaporation (vapor pressure) of the solution is lower, the temperature change will be less and the vapor pressure difference can be calculated. The freezing point of a solution is always lower than that of the solvent. The exact difference depends on the solvent and the osmolality of the solution. For water, Osmolality

AT 1.86

where AT is the freezing point depression in degrees Celsius. Instruments that measure the freezing point of a sample are used in clinical laboratories to determine serum and urine osmolality. Since water passes freely through most biological membranes, all body fluids are in osmotic equilibrium so that the osmolality of plasma is representative of the osmolality of other body fluids. The osmotic pressure of extracellular fluid is due primarily to its principal cation Na + and the anions C1and HCO 3. Taking twice the Na + concentration gives a good estimate of serum osmolality. Thus, normal plasma contains 135-145 mEq of Na+/L (3.1-3.3g/L) and normal plasma osmolality is about 270-290 mosmNg (this corresponds to an osmotic pressure of 6.8-7.3 atm and a freezing point depression of 0.50-0.54~ Glucose provides only 5-6 mosm/kg (or 0.1 atm to the osmotic pressure). Plasma protein contributes about 10.8 mosm/kg. Because of their size and general inability to pass through biological membranes, proteins are important determinants of fluid balance between intravascular and extravascular spaces. That portion of the osmotic pressure which is due to proteins is often referred to as the oncotic pressure. Since many molecules in plasma interact, the measured osmolality of a sample is an effective osmolality and is lower than the value calculated from the concentrations

933

SECTION39.4 Electrolyte Balance

of all the ions and molecules it contains. A solution that has the same effective osmolality as plasma is said to be isotonic, e.g., 0.9% saline, 5% glucose, and Ringer's and Locke's solutions. If a solute can permeate a membrane freely, then a solution of that solute will behave like pure water with respect to the membrane. Thus, a solution of urea will cause red cells to swell and burst as does pure water because urea moves freely across erythrocyte membranes. The osmolality of urine can differ markedly from that of plasma because of active concentrative processes in the renal tubules. The membranes of renal collecting ducts show varying degrees of water permeability and permit removal of certain solutes without simultaneous uptake of water. Plasma osmolality can be calculated from the concentrations of plasma Na +, glucose, and serum urea nitrogen: Osmolality = 1.86(Na + mEq/L) +

glucose (mg/dL)

18 serum urea nitrogen (mg/dL)

+

2.8

The numerical denominators for glucose and urea nitrogen convert the concentrations to moles per liter. Such an estimated osmolality is usually 6-9 mosm less than the value determined by freezing point or vapor pressure measurements. If the latter value is much greater than the estimated value, molecules other than Na +, glucose, and urea must account for the difference. Such "osmolal gaps" occur in individuals suffering from drug toxicity (alcohol, barbiturates, salicylates), acute poisoning due to unknown substances, and acidosis (keto-, lactic, or renal). Determination of osmolality is helpful in management of patients with fluid and electrolyte disorders, e.g., chronic renal disease, nonketotic diabetic coma, hypo- and hypernatremia, hyperglycemia, hyperlipemia, burns, sequelae to major surgery or severe trauma (particularly serious head injuries), hemodialysis, or diabetes insipidus. Changes of about 2% or more are detected by the hypothalamic osmoreceptors (Chapter 31) and elicit a sensation of thirst and production of hypertonic urine. Under conditions of fluid restriction, urine osmolality can reach 8001200 mosrrdkg (normal is 390-1090 mosm/kg), or three to four times the plasma levels. Decrease in plasma osmolarity (as in excessive water intake) produces urine with decreased osmolality. Water losses from skin and lungs are not subject to controls of this type. ADH acts at the renal tubules and collecting ducts to raise cAMP levels. Urinary levels of cAMP are increased by ADH. Factors other than plasma hypertonicity may stimulate ADH secretion. Thus, in acute hemorrhage

extracellular fluid volume drops abruptly, and ADH is secreted to increase the volume at the expense of a drop in osmolarity. Inappropriate ADH secretion can occur in the presence of water overload and a decline in plasma Na + concentration and osmolality. Fear, pain, and certain hormonesecreting tumors can cause inappropriate ADH secretion. It leads to hyponatremia and water retention. Morphine and barbiturates increase, and ethanol decreases, secretion of ADH. In diabetes insipidus due to defective ADH receptors or to diminished ADH secretion, renal tubules fail to recover the water from the glomerular filtrate. In cases of deficiency of ADH, hormone replacement by 8-1ysine vasopressin or 1-deamino-8-D-arginine vasopressin (administered as a nasal spray or subcutaneously) is effective. In osmotic diuresis, e.g., in diabetes mellitus with severe glycosuria, the solute load increases the osmolality of the glomerular filtrate and impairs the ability of the kidney to concentrate the urine. Extracellular fluid volume in a normal adult is kept constant; body weight does not vary by more than a pound per day despite fluctuations in food and fluid intake. A decrease in extracellular fluid volume lowers the effective blood volume and compromises the circulatory system. An increase may lead to hypertension, edema, or both. Volume control centers on renal regulation of Na + balance. When the extracellular fluid volume decreases, less Na + is excreted; when it increases, more Na + is lost. Na + retention leads to expansion of extracellular fluid volume, since Na + is confined to this region and causes increased water retention. Renal Na + flux is controlled by the aldosteroneangiotensin-renin system (Chapter 32) and atrial natriuretic peptide (discussed earlier).

39.4 Electrolyte Balance The major electrolytes are Na +, K +, CI-, and HCO~(HCOy is discussed below, under "Acid-Base Balance.")

Sodium The average Na + content of the human body is 60 m E @ g , of which 50% is in extracellular fluid, 40% is in bone, and 10% is intracellular. The chief dietary source of sodium is salt added in cooking. Excess sodium is largely excreted in the urine, although some is lost in perspiration. Gastrointestinal losses are small except in diarrhea. Sodium balance is integrated with regulation of extracellular fluid volume. Depletional hyponatremia (sodium loss greater than water loss) may result from

934

inadequate Na + intake, excessive fluid loss from vomiting or diarrhea, diuretic abuse, and adrenal insufficiency. Hyponatremia can decrease extracellular fluid volume, as occurs in congestive heart failure, uncontrolled diabetes, cirrhosis, nephrosis, and inappropriate ADH secretion. Hypernatremia results from loss of hypoosmotic fluid (e.g., in burns, fevers, high environmental temperature, exercise, kidney disease, diabetes insipidus) or increased Na + intake (e.g., administration of hypertonic NaC1 solutions, ingestion of NaHCO3). Potassium The average K + content of the human body is 40 mEq/kg. K + occurs mainly in intracellular space. It is required for carbohydrate metabolism, and increased cellular uptake of K + occurs during glucose catabolism. K + is widely distributed in plant and animal foods, the human requirement being about 4 g/day. Insulin and catecholamines promote a shift of K + into the cells. Excess K + is excreted in the urine, a process regulated by aldosterone. Plasma K + plays a role in the irritability of excitable tissue. A high concentration of plasma K + leads to electrocardiographic (ECG) abnormalities and possibly to cardiac arrhythmia, which may be due to lowering of the membrane potential. Low concentration of plasma K + increases the membrane potential, decreases irritability, and produces other ECG abnormalities and muscle paralysis. Hyperkalemia may occur in renal disease and adrenal insufficiency owing to impairment of normal secretory mechanisms. Metabolic acidosis, in particular diabetic acidosis, and catabolism of cellular protein in starvation or fever cause K + release from cells. Treatment consists of correction of the acidosis and promotion of cellular uptake of K + by administration of insulin, which enhances glucose intake. In severe cases, ion exchange resins given orally bind K + in intestinal secretions. Hypokalemia may occur from loss of gastrointestinal secretions (which contain significant amounts of K +) and from excessive loss in the urine because of increased aldosterone secretion or diuretic therapy. Hypokalemia is usually associated with alkalosis.

CHAPTER 39


39.5 Acid-Base Balance Normal blood pH is 7.35-7.45 (corresponding to 3545 nmol of H + per liter). Values below 6.80 (160 nmol of H + per liter) or above 7.70 (20 nmol of H+per liter) are seldom compatible with life. A large amount of acid produced is a byproduct of metabolism. The lungs remove 14,000 mEq of CO2 per day. From a diet that supplies 1-2 g of protein per kilogram per day, the kidneys remove 40-70 mEq of acid per day as sulfate (from oxidation of sulfur-containing amino acids), phosphate (from phospholipid, phosphoprotein, and nucleic acid catabolism), and organic acids (e.g., lactic,/3-hydroxybutyric, and acetoacetic). These organic acids are produced by incomplete oxidation of carbohydrate and fats, and in some conditions (e.g., ketosis; see Chapter 18) considerable amounts may be produced. The most important extracellular buffer is the carbonic acid-bicarbonate system:

C02 + H20 ~ H2CO3 ~ HCO3 + H + As discussed in Chapter l, at a blood pH of 7.4, the ratio of [HCO 3] to [H2CO3] is 20:1 and the system's buffering capacity can neutralize a large amount of acid. The system is independently regulated by the kidneys, which control the plasma HCO 3 level, and by the respiratory rate, which regulates the Pco2. Protein and phosphate buffer systems also operate in plasma and erythrocytes. Proteins are especially important buffers in the intracellular fluid. The hydroxyapatite of bone also acts as a buffer. The medullary respiratory center senses and responds to the pH of blood perfusing it and is the source of pulmonary control. Pco2 and perhaps Po2 also influence the center, together with nervous impulses from higher centers of the brain. A decrease in pH results in an increased respiratory rate and deeper breathing with a consequent increase in the respiratory exchange of gases and lowering of Pco2 which elevates the pH. Similarly, a decrease in respiratory rate leads to accumulation of CO2, increase in Pco2, and decrease in pH. Pulmonary responses to fluctuations in blood pH are rapid, while renal compensatory mechanisms are relatively slower. The kidneys actively secrete H + ions through three mechanisms:

Chloride Chloride is the major extracellular anion. About 70% is in the extracellular fluid. The average C 1- content of the human body is 35 mEq/kg. Chloride in food is almost completely absorbed. Plasma levels of Na + and C1- in general undergo parallel alterations. However, in metabolic alkalosis, chloride concentration increases.

1. Na+/H + exchange, 2. Reclamation of bicarbonate, and 3. Production of ammonia and excretion of NH +. The proximal tubule is responsible for reclamation of most of the 4500 mEq of HCO 3 filtered through the glomeruli each day. H + secreted into the tubules

935

SECTION39.5 Acid-Base Balance

Distal tubular lumen

Proximal tubular lumen Proximaltubular cell / Lumencapillary (Na+) (HCO3-) ] (from glomerular filtrate) [ H C O 3 - ~ /

Hoe3-+ H,+~ Na +

"

~ "(~I ~I H CO I I 2 ~ 0

(-)

Lumen capillary

(fr~ ~O~(HIP~Jfi-k)rate) I I-I=CO3~H.O + CO=-

~'~3H003-~./'~" 3HCO3CAll ~

Distal tubularcell

COz

.--~Na*

_+ CO2

in exchange for Na + from the tubular fluid (an energydependent process mediated by Na +, H+-ATPase) combines with HCO 3 to form CO2 and water. The CO2 diffuses into the tubular cells, where it is rehydrated to H2CO3 by carbonic anhydrase and dissociates to bicarbonate and H +. The HCO 3 diffuses into the bloodstream, resulting in reclamation of bicarbonate (Figure 39-6). The formation of H2CO3, from H + and HCO 3 in the tubular lumen iscatalyzed by the membrane-bound isoenzyme of carbonic anhydrase (CAIV). CAIV is located on the brush border lining the lumen of the proximal tubules of the kidney. The CO2 diffuses into the tubular cells where it is rehydrated to H2CO3 by a different isoenzyme of carbonic anhydrase, namely, carbonic anhydrase II (CAII). HCO 3 is transported from the cytosol of the proximal tubular cell to peritubular capillary blood by a basolateral cotransporter, which transports three molecules of HCO 3 and one of Na +. An inherited CAII deficiency is an autosomal recessive disorder causing renal tubular acidosis and osteopetrosis. The latter is due to lack of H + production required for bone remodeling processes (Chapter 37). The proximal tubular mechanism for reclamation of HCO 3 becomes saturated at approximately 26 mEq HCOf/L. At higher levels, HCO 3 appears in the distal tubule and may be excreted in urine. Under conditions of elevated Pco2, H + secretion is more active, possibly owing to intracellular acidosis. In proximal renal tubular acidosis, HCO 3 reabsorption is impaired and saturation occurs at a lower concentration (about 16-18 mEq/L), so that plasma [HCO 3 ] is low while urine pH may be high because of the presence of riCO 3. Na+/H + exchange may also be coupled to formation of HzPO 4 from HPO 2- in the lumen (Figure 39-7). This coupling is of particular importance in distal tubules and acidifies the urine to a maximum pH of about 4.4. The

HC[O~

(N~I')(I-I=PO,- )

HCO39

]"... Na+

ExcretiOa~.Oftitratable

H20 + C02" t F I G U R E 39-6 Reclamation of bicarbonate. The filtered Na + is reabsorbed by the proximal tubular cell in exchange for H +. The filtered HCO 3 is converted to H20 and CO2 catalyzed by the luminal carbonic anhydrase IV (CAW). CO2 diffuses in the tubular cell where it is hydrated to H2CO3 by carbonic anhydrase II and dissociated to H + and HCO 3. Three molecules of HCO3 and one of Na + are transported to the peritubular capillary by the basolateral cotransporter.

H

.Na+

t

F I G U R E 39-7 Hydrogen ion secretion and Na+/H + exchange coupled to the conversion of HPO]- to H2PO 4 in the distal tubule lumen. CA, carbonic anhydrase.

proximal tubule cannot maintain an H + gradient of more than one pH unit between the lumen and the intracellular fluid. The HzPO 4 present (termed the "titratable acidity") can be measured by titrating the urine to pH 7 (pK2 for H3PO4 = 6.2). Normal values for titratable acidity are 16-60 mEq/24 h, depending on the phosphate load. Generation of ammonia from glutamine and its excretion as NH + is an important mechanism for elimination of protons, particularly during severe metabolic acidosis, when it becomes a significant mode of nitrogen excretion. Most renal glutamine is derived from muscle (Chapters 17 and 22). Glutamine provides two molecules of NH3: glutaminase

Glutamine + H20

> glutamate- + NH3 + H + ( ~ -- NH +)

and glutamate dehydrogenase GlutamateNADH

NADH+ H+

(z- ketoglutarate2- + NH 3 + H + (,(

9NH~)

The ratio of [NH3]/[NH4+] depends on intracellular pH; however, the NH3 readily diffuses into the tubular lumen and there forms an ammonium ion that is no longer able to pass freely through the membranes and remains "trapped" in the urine, where it is associated with the dominant counteflon (Figure 39-8). Protons produced in these reactions are consumed when o~-ketoglutarate is either completely oxidized or converted to glucose. Thus, they do not add to the existing "proton burden" due to severe acidosis (see also Chapters 13 and 15).

Disorders of Acid-Base Balance These disorders are classified according to their cause and the direction of the pH change into respiratory acidosis, metabolic acidosis, respiratory alkalosis, or metabolic alkalosis. Any derangement of acid-base balance elicits

936

CHAPTER39

Tubular lumen

Tubular cell

Lumen

capillary

Glutamine

IGlutaminase

acid [Glutamate I dehydrogenase

Glutamic

o~-Ketoilutarate =_

9

CO2+ 1-120or glucose H+=_ Na +

NH +

el-

a+ .Na

[

( FIGURE 39-8 Formation of ammonia in the renal tubule cells from glutamine and secretion of ammonium ion in the urine.

compensatory changes in an attempt to restore homeostasis (Table 39-1). Acidosis due to respiratory failure leads to compensatory renal changes, which lead to increased reclamation of HCO~-. In assessment of acid-base disorders, commonly measured electrolytes are serum Na +, K +, H + (as pH), CI-, and HCO~. Other anions (e.g., sulfates, phosphates, proteins) and cations (e.g., calcium, magnesium, proteins) are not measured routinely but can be estimated indirectly, since (to maintain electrical neutrality) the sum of the cations must equal that of the anions. Serum Na + and K + content accounts for 95% of cations, and C1- and HCO3for about 85 % of anions. The concentration of phosphate, sulfate, and proteins can be calculated from the formula: Unmeasured anions - [Na +] + [K +] - [C1-] - [HCOy] The unmeasured anion is commonly known as the anion gap, which is normally 12 -4- 4 mEq/L. This value is useful in assessing the acid-base status of a patient and in diagnosing metabolic acidosis. Disorders that cause a high anion gap are metabolic acidosis, dehydration, therapy with sodium salts of strong acids, therapy with certain antibiotics (e.g., carbenicillin), and alkalosis. A decrease in the normal anion gap occurs in various plasma dilution states, hypercalcemia, hypermagnesemia, hypernatremia, hypoalbuminemia, disorders associated with hyperviscosity, some paraproteinemias, and bromide toxicity. Respiratory acidosis is characterized by accumulation of CO2, rise in Pc02 (hypercapnia or hypercarbia), decrease in [HCO3-]/[Pc02], and decrease in pH (see


Henderson-Hasselbalch equation, Chapter 1). It may result from central depression of respiration (e.g., narcotic or barbiturate overdose, trauma, infection, cerebrovascular accident) or from pulmonary disease (e.g., asthma, obstructive lung disease, infection). Increased [H +] is in part buffered by cellular uptake of H + with corresponding loss of intracellular K +. In acute hypercapnia, the primary compensatory mechanism is tissue buffering. In chronic hypercapnia, the kidneys respond to elevated plasma Pco2 increasing the amount of HCO~- formed by carbonic anhydrase in the tubules and by excreting more H +. The primary goal of treatment is to remove the cause of the disturbed ventilation. Immediate intubation and assisted ventilation also aid in improving the gas exchange. Metabolic acidosis with increased anion gap occurs in diabetic or alcoholic ketoacidosis; lactic acidosis from hypoxia, shock, severe anemia, alcoholism, cancer; toxicity from ingestion of salicylates, methanol, paraldehhyde, and ethylene glycol; and renal failure. Lactic acid acidosis caused by deprivation of tissue oxygenation, inhibition of gluconeogensis, and some drugs and toxins is due to accumulation of L-lactate, which is the end product of glycolysis (Chapter 13). Frequently, L-lactate (simply referred to as lactate), is the metabolite measured in assessing metabolic acidosis. However, D-lactate may be produced under certain clinical conditions, such as diminished colonic motility, short bowel syndrome, jejunoileal bypass, due to overgrowth of D-lactate producing gram-positive organisms (e.g., Lactobacillus species, Streptococcus bovis). Carbohydrate malabsorption and ingestion of large amounts of carbohydrate may also exasperate the development of D-lactate acidosis. In addition, an impairment of D-lactate metabolism may also contribute to D-lactic acidosis. D-Lactate is converted to pyruvate by D-2-hydroxy acid dehydrogenese, a mitochondrial enzyme present in liver, kidney, and other tissues. The clinical manifestations of D-lactic acidosis include episodes of encephalopathy and metabolic acidosis. Since serum D-lactate is not a normally measured clinical parameter, the critical indices of suspicion of D-lactic acidosis in the clinical conditions mentioned above include increased anion gap metabolic acidosis with negative tests for L-lactate and ketoacidosis. Metabolic acidosis with normal anion gap occurs in renal tubular acidosis, carbonic anhydrase inhibition, diarrhea, ammonium chloride administration, chronic pyelonephritis, and obstructive uropathy. In both groups, plasma HCO~ levels decrease and tissue buffering occurs by exchange of extracellular H + for intracellular K +. Thus, plasma K + levels may increase. Metabolic acidosis produces prompt stimulation of respiratory rate and decrease in Pco2. This effect cannot be sustained, however, because of tiring of respiratory

SECTION39.5 Acid-Base Balance

937

TABLE 39-1 Classification and Characteristics o f Simple Acid-Base Disorders*

Primary Change

Compensatory Response

Expected Compensation

Metabolic acidosis

$$$HCO 3-

,~,~Pco 2

Pc02 = 1.5 (HCO3-) + 8 + 2. Pco 2 falls by 1-1.3 mm Hg for each mEq/L fall in HCO3-. Last 2 digits of pH = Pco2 (thus, if Pc02 = 28, pH = 7.28). HCO 3- + 15 = last 2 digits of pH (HCO3- = 15, pH = 7.30).

Alkalosis

1"1"1"HCO3-

1"1"Pc02

PC02 increases 6 mm Hg for each 10 mEq/L rise in HCO3-. HCO 3- + 15 = last 2 digits of pH (HCO3- = 35, pH = 7.50).

Respiratory acidosis Acute

1"1"1"Pco2

I"HCO 3-

1"1"1"Pco2

I"I"HCO 3-

HCO 3- increases by 1 mEq/L for each 10 mm Hg rise in Pco2. HCO 3- increases by 3.5 mEq/L for each 10 mm Hg rise in PCO2.

,~,~,~Pco2

,~HCO3-

,~,~,~Pco 2

$$HCO 3-

Chronic Alkalosis Acute Chronic

HCO 3- falls by 2 mEq/L for each 10 mm Hg fall in PCO2. HCO 3- falls by 5 mEq/L for each 10 mm Hg fall in Pco2.

1"= Increase; $ = decrease. *Reproduced, with permission, from R. G. Narins and L. B. Gardner: Simple acid-base disturbances. Med. Clin. North Am. 65, 321 (1981).

muscles. Renal compensation is slower but can be maintained for an extended period because of induction of glutaminase. Treatment is by correction of the cause of the acidosis (e.g., insulin administration in diabetic ketoacidosis) and neutralization of the acid with NaHCO3, sodium lactate, or TRIS [tris(hydroxymethyl)aminomethane] buffer. Problems that may occur following alkali replacement therapy include development of respiratory alkalosis, particularly if the low CO2 tension persists, and further decline in the pH of CSF, which may decrease consciousness. The alkaline "overshoot" results from resumption of oxidation of organic anions (e.g., lactate, acetoacetate) with resultant production of bicarbonate from CO2. Severe acidosis should be corrected slowly over several hours. Potassium replacement therapy frequently is needed because of the shift of intracellular K + to extracellular fluid and loss of K + in the urine. Respiratory alkalosis occurs when the respiratory rate increases abnormally (hyperventilation), leading to decrease in Pco2 and rise in blood pH. Hyperventilation

occurs in hysteria, pulmonary irritation (pulmonary embolus), and head injury with damage to the respiratory center. The increase in blood pH is buffered by plasma HCO 3 and, to some extent, by exchange of plasma K + for intracellular H +. Renal compensation seldom occurs because this type of alkalosis is usually transitory. Metabolic alkalosis is characterized by elevated plasma HCOy level. It may result from administration of excessive amounts of alkali (e.g., during NaHCO3 treatment of peptic ulcer) or of acetate, citrate, lactate, and other substrates that are oxidized to HCO 3, and from vomiting, which causes loss of H + and C1-. In excessive loss of extracellular K + from the kidneys, cellular K + diffuses out and is replaced by Na + and H + from the extracellular fluid. Since K + and H + are normally secreted by the distal tubule cells to balance Na + uptake during Na + reabsorption (see Figure 39-3), if extracellular K + is depleted, more H + is lost to permit reabsorption of the same amount of Na +. Loss of H + by both routes causes hypokalemic alkalosis. Excessive amounts

938 T A B L E 39-2

Important Causes of Mixed Acid-Base Disturbances* Respiratory acidosis with metabolic acidosis Example: Cardiopulmonary arrest Severe pulmonary edema Drug ingestion with central nervous system depression Respiratory alkalosis and metabolic alkalosis Example: Hepatic failure and diuretics Patients on ventilation given nasogastric suction Respiratory alkalosis and metabolic acidosis Example: Septic shock Renal failure with sepsis Salicylate overdose Respiratory acidosis and metabolic alkalosis Example: Chronic lung disease and diuretic use Mixed acute and chronic respiratory acidosis Example: Chronic lung disease and superimposed infection Metabolic acidosis and metabolic alkalosis Example: Renal failure and vomiting Vomiting and hypotension (lactic acidosis) *Reproduced, with permission, from M. Bia and S. O. Thier: Mixed acid-based disturbances: A clinical approach. Med. Clin. North Am. 65, 347 (1981).

of some diuretics and increased aldosterone production can cause the hypokalemia that initiates this type of alkalosis. In compensation, the respiratory rate decreases, raising Pco2 and lowering the pH of blood. This mechanism is limited because if the respiratory rate falls too low, Po2 decreases to the point where respiration is again stimulated.

CHAPTER39 Water, Electrolytes, and Acid-Base Balance

Renal compensation involves decreased reabsorption of bicarbonate and formation of alkaline urine. Because the urinary bicarbonate is accompanied by Na + and K +, if the alkalosis is accompanied by extracellular fluid depletion, renal compensation by this mechanism may not be possible. Treatment consists of fluid and electrolyte replacement and NH4C1 to counteract the alkalosis. Acid-base disturbances frequently coexist with two or more simple disorders (Table 39-2). In these settings, blood pH is either severely depressed (e.g., a patient with metabolic acidosis and respiratory acidosis) or normal. Both plasma HCO 3 and pH may be within normal limits when metabolic alkalosis and metabolic ketoacidosis coexist, as in a patient with diabetic ketoacidosis who is vomiting. In this situation, an elevated anion gap may be the initial abnormality that can be detected in the underlying mixed acid-base disturbance.

Supplemental Readings and References H. J. Adrogue and N. E. Madias: Management of life-threatening acid-base disorders. New England Journal of Medicine 338, 26 (1998). D. G. Bichet: Nephrogenic diabetes insipidus. American Journal of Medicine 105, 431 (1998). S. L. Gluck: Acid-base. Lancet 352, 474 (1998). M. L. Halperin and K. S. Kamel: Potassium. Lancet 352, 135 (1998). V. L. Hood and R. L. Tannen: Protection of acid-base balance by pH regulation of acid production. New England Journal of Medicine 339, 819 (1998). S. Klahr and S. B. Miller: Acute oliguria. New England Journal of Medicine 338, 671 (1998). S. J. Scheinman, L. M. Guay-Woodford, R. V. Thakker, et al.: Genetic disorders of renal electrolyte transport. New England Journal of Medicine 340, 1177 (1999). J. Uribarri, M. S. Oh, and H. J. Carroll: D-Lactic acidosis. Medicine 77, 73 (1998). K. D. Wrenn, C. M. Slovis, G. E. Minion, et al.: The syndrome of alcoholic ketoacidosis. American Journal of Medicine 91, 119 ( 1991 ). H. J. Androgu6 and N. E. Madias: Hyponatremia New England Journal of Medicine 342, 1581 (2000).

APPENDIX

TABLE I-1

Median Heights and Weights and Recommended Energy Intake*

Average Energy Allowances (kcal) ~ Weight Category Infants Children

Males

Females

Pregnant

Lactating

Age (yr) or Condition 0.0-0.5 0.5-1.0 1-3 4-6 7-10 11-14 15-18 19-24 25-50 51+ 11-14 15-18 19-24 25-50 51+ 1st trimester 2nd trimester 3rd trimester 1st 5 months 2nd 6 months

Height

(kg)

(lb)

(cm)

(in)

6 9 13 20 28 45 66 72 79 77 46 55 58 63 65

13 20 29 44 62 99 145 160 174 170 101 120 128 138 143

60 71 90 112 132 157 176 177 176 173 157 163 164 163 160

24 28 35 44 52 62 69 70 70 68 62 64 65 64 63

REE t (kcal/day) 320 500 740 950 1,130 1,440 1,760 1,780 1,800 1,530 1,310 1,370 1,350 1,380 1,280

Multiples of REE

Per kg

1.70 1.67 1.67 1.60 1.50 1.67 1.60 1.60 1.55 1.50

108 98 102 90 70 55 45 40 37 30 47 40 38 36 30

Per day w 650 850 1,300 1,800 2,000 2,500 3,000 2,900 2,900 2,300 2,200 2,200 2,200 2,200 1,900 +0 +300 +300 +500 +500

*From: RecommendedDietary Allowances, 10th ed., Food and Nutrition Board, National Research Council--National Academy of Sciences, 1989. tREE = Resting energy expenditure. ;In the range of light to moderate activity, the coefficient of variation is _+20%. w is rounded.

939


II

APPENDIX

TABLE II-1

Weights for Heights of Adults in the United States** Weight, kg (lb) Females, by Percentile

Males, by Percentile Height cm (in) 147 152 157 163 168 173 178 183

(58) (60) (62) (64) (66) (68) (70) (72)

15th

57 58 61 65 67 73

(125) (129) (134) (143) (149) (161)

50th

64 67 71 76 79 83

(142) (148) (158) (167) (173) (183)

188 (74)

77 (171)

88 (194)

193 (76)

85 (187)

103 (227)

85th

76 79 83 88 93 99 99 106

(168) (174) (183) (195) (206) (218)

15th 45 49 51 54 55 59 61

(99) (107) (112) (118) (122) (130) (133)

50th 55 60 60 63 64 67 69

(122) (132) (132) (139) (141) (148) (152)

85th 72 75 77 79 81 83 78

(159) (164) (170) (175) (179) (184) (171)

(217) (234)

*From: Recommended Dietary Allowances, 10th ed. Food and Nutrition Board, National Research Council--National Academy of Sciences, 1989. *Unpublished data from NHANES II (1976-1980) provided by the National Center for Health Statistics. Values rounded to nearest whole number. Subjects were ages 18 to 74 years. Height determined without shoes. Weight includes clothing weight, ranging from an estimated 0.09 to 0.28 kg (0.20 to 0.62 lb).

941


APPENDIX

III

TABLE III-1 Weight and Height of Males and Females up to 18 Years in the United States*

Males, by Percentile Weight, kg (lb) Age

5th

Females, by Percentile Height, cm (in)

50th

95th

5th

50th

4.29 (9.4) 5.98 (13.2) 7.85 (17.3) 9.18 (20.2) 10.15 (22.3) 11.47 (25.2)

5.38 7.37 9.46 10.93 11.99 13.44

50.4 56.7 63.4 68.0 71.7 77.5

54.6 61.1 67.8 72.3 76.1 82.4

12.34 14.62 16.69 18.67 20.69 22.85 25.30 28.13 31.44 35.30 39.78 44.95 50.77 56.71 62.10 66.31 68.88

15.50 17.77 20.27 23.09 26.34 30.12 34.51 39.58 45.27 51.47 58.09 65.02 72.13 79.12 85.62 91.31 95.76

82.5 89.0 95.8 102.0 107.7 113.0 118.1 122.9 127.7 132.6 137.6 142.9 148.8 155.2 161.1 164.9 165.7

86.8 89.9 102.9 109.9 116.1 121.7 127.0 132.2 137.5 143.3 149.7 156.5 163.1 169.0 173.5 176.2 176.8

Weight, kg (lb) 95th

5th

50th

(21.5) (24.1) (26.7) (28.5) (30.0) (32.4)

58.6 65.4 72.3 77.1 81.2 88.1

2.97 4.18 5.79 7.00 7.84 8.92

3.98 5.40 7.21 8.56 9.53 10.82

(34.2) (37.4) (40.5) (43.3) (45.7) (47.9) (50.0) (52.0) (54.1) (56.4) (58.9) (61.6) (64.2) (66.5) (68.3) (69.4) (69.6)

94.4 102.0 109.9 117.0 123.5 129.7 135.7 141.8 148.1 154.9 162.3 169.8 176.7 181.9 185.4 187.3 187.6

9.95 11.61 13.11 14.55 16.05 17.71 19.62 21.82 24.36 27.24 30.52 34.14 37.76 40.99 43.41 44.74 45.26

11.80 14.10 15.96 17.66 19.52 21.84 24.84 28.46 32.55 36.95 41.53 46.10 50.28 53.68 55.89 56.69 56.62

Height, cm (in) 95th

5th

50th

95th

(8.8) (11.9) (15.9) (18.8) (21.0) (23.8)

4.92 6.74 8.73 10.17 11.24 12.76

49.2 55.4 61.8 66.1 69.8 76.0

53.5 59.5 65.9 70.4 74.3 80.9

(21.1) (23.4) (25.9) (27.7) (29.3) (31.9)

56.9 63.4 70.2 75.0 79.1 86.1

(26.0) (31.0) (35.1) (38.9) (42.9) (48.0) (54.6) (62.6) (71.6) (81.3) (91.4) (101.4) (110.6) (118.1) (123.0) (124.7) (124.6)

14.15 17.22 19.91 22.62 25.75 29.68 34.71 40.64 47.17 54.00 60.81 67.30 73.08 77.78 80.99 82.46 82.47

81.6 88.3 95.0 101.1 106.6 111.8 116.9 122.1 127.5 133.5 139.8 145.2 148.7 150.5 151.6 152.7 153.6

86.8 94.1 101.6 108.4 114.6 120.6 126.4 132.2 138.3 144.8 151.5 157.1 160.4 161.8 162.4 163.1 163.7

(34.2) (37.0) (40.0) (42.7) (45.1) (47.5) (49.8) (52.0) (54.4) (57.0) (59.6) (61.9) (63.1) (63.7) (63.9) (64.2) (64.4)

93.6 100.6 108.3 115.6 122.7 129.5 136.2 142.9 149.5 156.2 162.7 168.1 171.3 172.8 173.3 173.5 173.6

Months 1 3 6 9 12 18

3.16 4.43 6.20 7.52 8.43 9.59

Years 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

10.49 12.05 13.64 15.27 16.93 18.64 20.40 22.25 24.33 26.80 29.85 33.64 38.22 43.11 47.74 51.50 53.97

(27.1) (32.2) (36.7) (41.1) (45.5) (50.3) (55.7) (61.9) (69.2) (77.7) (87.5) (98.9) (111.7) (124.8) (136.6) (145.9) (151.5)

*From: Recommended DietaryAllowances, 10th ed. Food and Nutrition Board, National Research Council--National Academyof Sciences, 1989. Source: Hamill et al., 1979.

943


APPENBIX

IV

TABLE IV-1

Recommended Daily Dietary Allowances *t

Fat-Soluble Vitamins

Category Infants Children

Males

Females

Age (yr) or Condition 0.0-0.5 0.5-1.0 1-3 4-6 7-10 11-14 15-18 19-24 25-50 51+ 11-14 15-18 19-24 25-50 51+

Pregnant Lactating 1st 6 m o n t h s 2nd 6 m o n t h s

Weight S

Height S

(kg)

(lb)

(cm)

(in)

6 9 13 20 28 45 66 72 79 77 46 55 58 63 65

13 20 29 44 62 99 145 160 174 170 101 120 128 138 143

60 71 90 112 132 157 176 177 176 173 157 163 164 163 160

24 28 35 44 52 62 69 70 70 68 62 64 65 64 63

Protein (g)

Vitamin A

13 14 16 24 28 45 59 58 63 63 46 44 46 50 50 60 65 62

375 375 400 500 700 1,000 1,000 1,000 1,000 1,000 800 800 800 800 800 800 1,300 1,200

(/.tg R E ) w

Vitamin D (~tg) li 7.5 10 10 10 10 10 10 10 5 5 10 10 10 5 5 10 10 10

Vitamin E

Vitamin K

(mg a-TE)I[

(/_tg) #

3 4 6 7 7 10 10 10 10 10 8 8 8 8 8 10 12 11

5 10 15 20 30 45 65 70 80 80 45 55 60 65 65 65 65 65

*From: Recommended Dietary Allowances, 10th ed. Food and Nutrition Board, National Research Council--National Academy of Sciences, 1989. tThe allowances, expressed as average daily intakes over time, are intended to provide for individual variations among most normal persons as they live in the United States under usual environmental stresses. Diets should be based on a variety of common foods in order to provide other nutrients for which human requirements have been less well defined. ~Weights and heights of Reference Adults are actual medians for the U.S. population of the designated age, as reported by NHANES II. The median weights and heights of those under 19 years of age were taken from Hamill et al., 1979. The use of these figures does not imply that the height-toweight ratios are ideal. w equivalents. 1 retinol equivalent = 1 /ag of retinal or 6/ag of ]3-carotene. IIAs cholecalciferol. 10 ~tg of cholecalciferol = 400 IU of vitamin D. ~[~-Tocophenol equivalents. 1 mg of d-c~-tocopherol = 1 a-TE. # 1 NE (niacin equivalent) is equal to 1 mg of niacin or 60 mg of dietary tryptophan.

945

Water-Soluble Vitamins

Minerals

Vitamin C Thiamine Riboflavin Niacin Vitamin B6 Folate Vitamin B,, (mg) (mg) (mg) (mg NE)# (mg) ( Mg) (kg)

Calcium Phosphorus Magnesium Iron Zinc Iodine Selenium (mg (mg) (mg) (mg) (mg) (kg) (kg)

30 35 40 45 45 50 60 60 60 60 50 60 60 60 60 70 95 90

0.3 0.4 0.7 0.9 1.O 1.3 1.5 1.5 1.5 1.2 1.1 1.1 1.1 1.1 1.O 1.5 1.6 1.6

0.4 0.5 0.8 1.1 1.2 1.5 1.8 1.7 1.7 1.4 1.3 1.3 1.3 1.3 1.2 1.6 1.8 1.7

5 6 9 12 13 17 20 19 19 15 15 15 15 15 13 17 20 20

0.3 0.6 1.O 1.1 1.4 1.7 2.0 2.0 2.0 2.0 1.4 1.5 1.6 1.6 1.6 2.2 2.1 2.1

25 35 50 75 100 150 200 200 200 200 150 180 180 180 180 400 280 260

0.3 0.5 0.7 1 .O 1.4 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.2 2.6 2.6

APPENDIX

V

TABLE V- 1

Classification of Selected Enzymes with Clinical Importance According to the Enzyme Commission (EC) and Their Common Names EC Code 1 1.1 1.1.1 1.1.1.1 1.1.1.14 1.1.1.27 1.1.1.37 1.1.1.42 1.1.1.44 1.1.1.49 1.2 1.2.1 1.2.1.12 1.4 1.4.1 1.4.1.3 1.6 1.6.4 1.6.4.2 2 2.1 2.1.3 2.1.3.3 2.3 2.3.2.2 2.6 2.6.1 2.6.1.1 2.6.1.2 2.7 2.7.1 2.7.1.1 2.7.1.40 2.7.3 2.7.3.2

Systematic Name Oxidoreductases Acting on CHOH groups With NAD + or NADP + as hydrogen acceptor alcohol:NAD + oxidoreductase L-iditol:NAD + oxidoreductase L-lactate:NAD + oxidoreductase L-malate:NAD + oxidoreductase threo-Ds-isocitrate-NAD+ oxidoreductase (decarboxylating) 6-phospho-D-gluconate: NADP + 2-oxidoreductase (decarboxylating) D-glucose 6-phosphate: NADP + 1-oxidoreductase Acting on aldehyde or keto groups With NAD + or NADP + as acceptor D-glyceraldehyde 3-phosphate:NAD + oxidoreductase (phosphorylating) Acting on CH.NH 2 groups With NAD + or NADP + as acceptor L-glutamate:NAD(P) dehydrogenase (deaminating) Acting on NADH or NADPH With a disulfide as acceptor NAD(P)H:glutathione oxidoreductase Transferases Transferring one-carbon groups Carboxyl- and carbamoyltransferases Carbamoylphosphate:L-ornithine carbamoyltransferase Amino acid transferases y-glutamyl-peptide: amino acid 7'-glutamyl transferase Transferring nitrogenous groups Aminotransferases (transaminases) L-aspartate:2-oxoglutarate aminotransferase L-alanine:2-oxoglutarate aminotransferase Transferring phosphorus-containing groups Phosphotransferases with alcohol group as acceptor ATP:D-hexose 6-phosphotransferase ATP:pyruvate 2-0-phosphotransferase Phosphotransferases with nitrogenous group as acceptor ATP:creatine N-phosphotransferase /

2.7.4 2.7.4.3 2.7.5

Phosphotransferases with phospho-group as acceptor ATP:AMP phosphotransferase Phosphotransferases catalyzing intramolecular transfers

Common Name (with Some Abbreviations)

Alcohol dehydrogenase (AD) Sorbitol dehydrogenase (SD or ID) Lactate dehydrogenase (LD or LDH) Malate dehydrogenase (MD) Isocitrate dehydrogenase (ICD) Phosphogluconate dehydrogenase Glucose-6-phosphate dehydrogenase (GDP or G6PD)

Glyceraldehydephosphate (triosephosphate) dehydrogenase

Glutamate dehydrogenase

Glutathione reductase

Ornithine carbamoyltransferase (OCT)

~,-Glutamyltransferase (GGT or 7-GT)

Aspartate aminotransferase (AST) (also known as glutamate oxaloacetate transaminase, GOT) Alanine aminotransferase (ALT) (also known as glutamate alanine transaminase, GPT)

Hexokinase (HK) Pyruvate kinase (PK)

Creatine kinase (CK) (also known as creatine phosphokinase, CPK)

Adenylate kinase

(continued)

947

948

APPENDIXV

TABLE V-1 (continued) EC Code

2.7.5,1 2.7.7 2,7,7.i2 3 3,I 3.I.1 3,1.1.3 3.1.I.7 3.I,t,8 3.1.3 3.1.3.t 3.1.3.2 3.1.3.5 3.1.3.9 3.2 3.2.1 3,2.t ,1 3.2.1.3I 3.4 3.4.I 3.4.11.1 3.4,4 3,4.23.t 3.4.2t.4 3.4.21.1 3.5 .~.5.3 3,5.3.1 4 4.1 4.1.1 4.1.2 4,2 4.2.1 4.3 5 5.1 5.1.3 5.2 5.3 5.3.I 5.4 6 6.1 6.1.1 6.3 6:3.1 6.3.2 6.4 6.4.t

Systematic Name

a-D.-giucose 1,6-diphosphatexz-D-glucose J-phosphate phosphotransferase Nt~cteotidyltransferases UDP-glucose:tx-v- gafactoso t-phosphate uridylyltmnsferase Hydmlases Acting on esters Carboxylic estvr hydmlases triaeylgtyceroI acyt-hydrotase acetylcholine hydrol,x~e acylchotine acyl-hydrolase Phosphoric monoester hydrolase.s orthophosphoric monoezter phosphohydrolase or|hophosphoric monoester phosphohydrolase 5"--ribonucteotide phosphohydrol ase D-glacose 6-phosphate phosphohydrolase Acting on g]ycosyt compounds Glucoside hydro]ases 1,4-r ~uc.atd'tydrolase ~6-D-gtucuronide glycuronosohydrolase Acting on .peptide bonds ~-Aminopepfide amino acid hyda'olases ~-Aminoacyl-pepl;ide hydrolaso Peptide peptidohydrolasc.s no systematic name no systematic name no systematic name Acting on C-N bonds, other than p~tide bonds In linear amidine8 L-arginine amidinohydrolase Lyases C--C lyases Carboxy lyases Aldehyde lyases C-O lyases Hydrotyases C-N lyases lsomerases Racemases and epimerases Acting on carbohydrates cis- trans Jsonlerases Intramotecular oxidoreductases Interconvcrtiag aldose m~d ketoses Intramotecnlar transferases Ligases Forming C-O bonds A mine acid-RNA !!gasea Forming C.-.N bonds Acid-ammonia ligases Acid--amino acid ligases Forming C--C bonds Carboxytase~

Common Name (with Some Abbreviations) Phosphog|ucomutase Hexose- 1-phosphate uridylyItrat~sferase

Lipase Acetylcholinesterase Cholinesterase Alkaline phosphatase (ALP) Acid ptmsphatase 5"-Nucteotidase (5"-NT) Glucoso--6-phosph atase a-Amylasc /~Glucm~nidase l,eucine aminopeptidase Chymotrypsin Trypsin Pepsin Arginase Pymvate deeaarboxylase Aldolase Fumarate hydratase (= furnarase) Histidine-ammonia lyase (= histidase) Ribulose-5-phosphate epimer~se Mateylacetoacetate isomerase Glucosephosphate isomerasv Methyhnalonyt--CoA mutase Amino acid-activating enzymes Glutamine synthetase Peptide synthetase, gtutattdone synthetase Acetyl-CoA carboxytase

APPENDIX

VI.I Serum Protein Electrophoresis and Its Diagnostic Significance Serum contains more than 100 different proteins, each under separate genetic control. They are transport proteins for hormones, vitamins, lipids, metals, pigments, and drugs; enzymes; enzyme inhibitors (proteinase inhibitors); hormones; antibodies; clotting factors; complement components; and kinin precursors. Quantitation (by radial immunodiffusion, electroimmunoassay, nephelometric methods, enzyme-linked immunological methods, and radioimmunoassay) of the various constituents of serum is of value in diagnosing and following the course of certain diseases. Several of these proteins are discussed elsewhere in the text. A simple and useful technique involves separation of serum proteins by an electric field at pH 8.6, using cellulose acetate as a support medium (electrophoresis). Agarose gel electrophoresis provides a higher resolution in the separation of proteins than cellulose acetate electrophoresis. The former gives about 12 protein bands and the latter provides the basic five-band pattern. Separation of these proteins is possible because each carries different charges and hence migrates at a differing rate when subjected to an electric potential. Serum is generally used instead of plasma because the fibrinogen found in plasma appears as a narrow band in the/3 region, which may be

VI

mistaken for the sign of monoclonal paraproteinemia (see below). The support medium, cellulose acetate, possesses several advantages over paper: the time required to separate the major proteins is short, albumin trailing is absent, and rapid quantitative determination of protein fractions by photoelectric scanning (after a suitable staining procedure) is possible. The factors that affect electrophoresis are ionic strength of the buffer, voltage, temperature, application width, and staining. Figure VI-1 shows normal and some abnormal patterns of serum protein electrophoresis. The electrophoretic patterns obtained are n o t indicative of any one disease or class of disease. Furthermore, a characteristic pattern may be obscured or not found in a disease entity where normally such a pattern is expected. Serum electrophoretic patterns provide only a general impression of the disorder and require confirmation by other procedures. An alteration (depression or elevation) in a given fraction should be quantitated by more sensitive and specific methods. Five major fractions seen in cellulose acetate serum protein electrophoresis are albumin, o~1-globulin, o~z-globulin, /3-globulin, and y-globulin. Adult reference ranges for these five fractions, expressed as grams per 100 mL, are 3.2-5.6 for albumin, 0.1-0.4 for o~l-globulin, 0.4-0.9 for ol2-globulin, 0.5-1.1 for fl-globulin, and 0.5-1.6 for y-globulin. With the exception of y-globulin, adult values are attained by 3 months of age for all fractions. Cord blood

949

950

APPENDIXVl

~"~,;.s'~ ..4",.r.,p"#

/,

#

///7 -IALBUMIN

ALPHA I

ALPHA 2

BETA

(a) Normal pattern

(c) Monoclonal gammopathy

GAMMA

(b) 13-~ Bridge (active cirrhosis)

(d) Nephrotic syndrome: low albumin, high r 2 and 13and low Y

._J (e) Acute inflammation: high ~1 and ~2

(f) Polyclonal gammopathy: high y, polyclonal elevation of immunoglobulin

F I G U R E VI- 1 Serum protein electrophoretic patterns. [Pattern (a) is reproduced, with permission, from Helena Laboratories, Beaumont, Texas.]

y-globulin is largely of maternal origin and undergoes catabolism, reaching its lowest value at about 3 months. Adult levels for y-globulin are not reached until about 2-7 years. After age 40, the level of albumin gradually declines and the fl-globulin fraction increases.

Albumin Albumin is synthesized in the liver. Since albumin accounts for about 75% of the osmotic pressure in blood,

it is responsible for the stabilization of blood volume and regulation of vascular fluid exchange. Therefore, hypoalbuminemia can give rise to edema. Hyperalbuminemia is uncommon, but many types of abnormalities lead to hypoalbuminemia. Some hypoalbuminemic conditions include nephrotic syndrome, proteinlosing enteropathies, cystic fibrosis (hypoalbuminemia with edema may be an early abnormality in infants with this disease), glomerulonephritis, cirrhosis, carcinomatosis, bacterial infections, viral hepatitis, congestive heart failure, rheumatoid

SECTION Vl.1

951

Serum Protein Electrophoresis

arthritis, uncontrolled diabetes, intravenous feeding (the hypoalbuminemia is due to a deficiency of amino acids in portal blood), and dietary deficiency of proteins containing essential amino acids. Hypoalbuminemia also occurs in the acute stress reaction. The sharp drop of albumin is essentially due to adrenocortical stimulation, which gives rise to enhanced catabolism of albumin and sodium retention. The latter is responsible for hemodilution and expansion of extracellular fluid. The 0/l- and 0/z-globulin fractions (see below) are also increased in the acute stress reaction. The immunoglobulins (v-fraction) do not undergo significant alterations during the acute phase of a stress reaction but are elevated during the chronic phase. Some examples of situations in which acute stress patterns may be seen are acute infections in early stages, tissue necrosis (myocardium, renal, tumor), severe burns, rheumatoid disease of acute onset, surgery, and collagen disorders. Analbuminemia is a rare autosomal recessive disorder. Affected individuals do not exhibit serious clinical symptoms, not even edema. The lack of clinical edema is presumably due to osmotic compensation by the mildly elevated globulins. Osteoporosis in analbuminemia has been corrected by the administeration of human serum albumin. Affected females exhibit minimal pretibial edema, mild anemia, normal liver function tests, absence of proteinuria, lowered blood pressure, elevated serum cholesterol levels, and lipodystrophy. Despite elevated plasma cholesterol levels, severe atherosclerosis was not present. Bisalbuminemia is due to albumin polymorphism. Upon serum electrophoresis, a double albumin band is seen. These double peaks are due either to differences in electrical charge (11 electrophoretically distinct forms have been reported) or to albumin dimers. Both forms are expressed as autosomal dominant traits and apparently present no significant clinical abnormalities. However, acquired bisalbuminemia may be associated with diabetes mellitus, the nephrotic syndrome, hyperamylasemia, and penicillin therapy. In these instances, the bisalbuminemia disappears after correction of the underlying disorder. C~l-Globulins

The proteins that migrate in this region are 0/1antitrypsin (which accounts for 70-90% of the 0/1-peak), 0/1-acid glycoprotein, or orosomucoid (which accounts for 10-20% of the 0/1-peak), 0/i-lipoprotein, prothrombin, transcortin, and thyroxine-binding globulin. In acute phase reactions, elevation of the 0/1-peak is due to increases in 0/1-antitrypsin and 0/1-acid glycoprotein, the two principal constituents of the peak. 0/1-Peak elevations are also seen in chronic inflammatory and degenerative diseases and are highly elevated in some cancers. When the

0/1-peak is depressed, 0/1-antitrypsin deficiency (Chapter 6) must be ruled out by direct measurements using sensitive quantitative methods. The presence of 0/1fetoprotein (AFP) in nonfetal serum is of clinical significance because of its close association with primary carcinoma of the liver. However, during pregnancy, AFP synthesized by the fetus gains access in small but measurable amounts to maternal circulation. Measurement of maternal serum AFP (MSAFP) levels during the second trimester is widely used to detect fetal abnormalities. In neural tube defects, MSAFP levels are elevated, and in Down's syndrome, they are decreased (Chapter 38). Routine serum electrophoresis seldom gives such sensitive information, and specific immunochemical methods should be employed to detect 0/i-fetoprotein in the serum. c~2-Globulin

The 0/2 fraction includes haptoglobin, 0/2-macroglobulin, 0/2-microglobulin, ceruloplasmin, erythropoietin, and cholinesterase. Haptoglobin is nonspecifically increased in the acute stress reaction in the presence of inflammation, tissue necrosis, or destruction. A function of haptoglobin is to combine with hemoglobin to remove it from the circulation. Thus, during an episode of intravascular hemolysis, haptoglobin is depleted and may require a week or more to return to normal serum levels. Haptoglobin levels in these instances should be quantitated by immunochemical procedures. 0/2-Macroglobulin functions as a protease inhibitor (Chapter 6). In the nephrotic syndrome, there is a characteristic elevation of both 0/2 and 13 peaks with hypoalbuminemia (Figure VI-ld). The elevation in 0/2 and fl fractions is due to higher very-low-density lipoprotein (VLDL) and low-density lipoprotein (LDL) levels. Thus, characteristics of nephritic syndrome include proteinuria (> 3.5 g per 24 hours), hypoalbuminemia (< 3 g/dL), hyperlipidemia with elevated triglyceride and cholesterol levels, and lipiduria and generalized edema. The exact mechanism for the higher VLDL and LDL levels is not understood. In part, these elevations may be due to increased synthesis of VLDLs in the liver and then conversion to LDLs in the peripheral circulation (Chapter 20), as well as their decreased catabolism. It is thought that the decreased plasma oncotic pressure due to hypoalbuminemia may stimulate increased hepatic synthesis of VLDLs. Increased glomerular permeability for protein due to derangement in capillary walls is an early event in the nephrotic syndrome that has several causes, as listed below with examples: 1. Primary glomerular disease: membranous glomerulonephritis, lipid nephrosis

952 2. Systemic disease: diabetes mellitus, amyloidosis, systemic lupus erythematosus 3. Neprotoxins and drugs: gold, mercury, penicillamine 4. Infections: poststreptococcal glomerulonephritis 5. Allergens, venoms, and vaccines 6. Miscellaneous: toxemia of pregnancy, malignant hypertension The most common cause of primary nephrotic syndrome in adults is membranous nephropathy. It is characterized by deposition of immune complex in the subepithelial portion of glomerular capillary walls.

O-Globulins Quantitatively significant /3-globulins are composed of transferrin, /3-1ipoproteins, complement C3, and hemopexin. Transferrin, synthesized in the liver, has a half-life of about 8.8 days and is an iron transport protein in serum that accounts for about 60% of the/~ peak. It is increased in iron deficiency anemia and pregnancy (Chapter 29). Transferrin levels are decreased in metabolic (e.g., liver, kidney) and neoplastic diseases. Transferrin and transthyretin (Chapter 38) are both decreased in protein-calorie malnutrition. A pattern often observed in advanced cirrhosis is lack of separation of the fl peak with the y region, described as /3-y bridging (Figure VI-1). The pattern is presumably due to elevated levels of IgA. Hypoalbuminemia is also found. Complement levels (C3 and C4) are decreased (owing to their consumption) in diseases associated with the formation of immune complexes, e.g., glomerulonephritis (acute and membranoproliferative), systemic lupus erythematosus.

APPENDIX Vl

2. Polyclonal gammopathy: A diffuse polyclonal increase in ),-globulin, primarily in the IgG fraction (Figure VI-If). Recall that in chronic stress v-globulins are elevated. The major causes of polyclonal gammopathy are listed below. a. Chronic liver disease" An increase in polyclonal IgG and IgA with a decrease in albumin levels is a characteristic finding regardless of the cause. The /3-)/bridging observed in cirrhosis is presumably due to elevated levels of IgA that migrates to a position between the/3 and y region, increasing the trough between these bands. b. Sarcoidosis: The electrophoretic pattern shows a stepwise descent of globulin fractions starting from the y-globulins. c. Autoimmune disease: This group includes rheumatoid arthritis (increased IgA levels), systemic lupus erythematosus (increased IgG and IgM levels), and others. d. Chronic infectious disease: This group includes bronchiectasis, chronic pyelonephritis, malaria, chronic osteomyelitis, kala-azar, leprosy, and others. 3. Monoclonal gammopathy: Presence of narrow protein band in the/3-V region may indicate the

",/-Globulins The five major classes of immunoglobulins in descending order of quantity are IgG, IgA, IgM, IgD, and IgE (see also Chapter 35). C-reactive protein migrates with the y-globulins and is increased in trauma and acute inflammatory processes (discussed later). During an intense acute inflammatory process, its concentration may be highly elevated, giving rise to a sharp protein band that may be mistaken for monoclonal gammopathy. Variations in electrophoretic pattern due to v-globulins can be categorized into the following three groups. 1. Agammaglobulinemia and hypogammaglobulinemia: May be primary or secondary. Secondary forms may be found in chronic lymphocytic leukemia, lymphosarcoma, multiple myeloma, the nephrotic syndrome, long-term steroid treatment, and occasionally overwhelming infection.

FIGURE VI-2 Serum electrophoretic protein patterns consisting of monoclonal immunoglobulins identified by arrows. Normal serum pattern is identified as 1.

SECTIONVl.1 Serum Protein Electrophoresis

953

F I G U R E VI-3 Serum immunofixation electrophoresis of two patients with monoclonal gammopathies. Pattern A represents IgG(tc) monoclonal gammopathy and pattern B represents IgA(tc) monoclonal gammopathy, as indicated by arrows. The procedure consists of serum protein electrophoresis (SPE) separation, reaction of each track with the exception of SPE with specific respective antiserum, followed by protein staining to make visible the respective bands.

presence of a homogeneous class of protein (Figures VI-lc and VI-2). Such a pattern should be further investigated using other laboratory and clinical findings to rule out multiple myeloma. Means of investigation include quantitation of immunoglobins, immunofixation electrophoresis to identify the class and type of monoclonal immunoglobulin (Figure VI-3), bone marrow aspiration and biopsy to determine plasmacytosis, and radiological examination to identify osteolytic lesions. Multiple myeloma is a disorder of neoplastic proliferation of a single clone of plasma cells in the bone marrow that leads to overproduction of any one of the five immunoglobulins (IgG, IgA, IgM, IgD, or IgE) or light chains (to or )~). Less commonly, biclonal or triclonal gammopathy also occurs. If the light chains are overproduced, they pass through the glomeruli and appear in the urine; this is known as Bence Jones proteinuria. In a given individual, Bence Jones proteinuria refers to either tc or )~

light chains but not both. Persons over age forty may be affected and the initial findings may include spontaneous fractures, bone pain, anemia, or infections. Hypercalcemia may be found after bone destruction. Multiple myeloma is a progressive disorder and is usually fatal due to renal damage and/or recurrent infections. Cytotoxic measures (e.g., melphalan, a nitrogen mustard) are used in the treatment of multiple myeloma, and serum and/or urine monoclonal immunoglobulin or light chain levels serve in monitoring therapy. Although the monoclonal spike is usually observed in the/~-y region, it is occasionally seen in the O/2 region. In pregnancy, a monoclonal spike in the/3 region may be due to elevated transferrin levels. A serum electrophoretic pattern of hypogammaglobulinemia requires further studies of serum and urine (e.g., immunoglobulin measurement and immunofixation electrophoresis) to rule out multiple myeloma. The presence of monoclonal immunoglobulin in the serum is a characteristic feature of a majority

954

of multiple myeloma cases; however, 1-5% of patients do not show any detectable monoclonal protein in serum or urine. These disorders are known as nonsecretory myeloma. They fall into two classes: (a) producers of a paraprotein that remains in the cytosol of the malignant plasma cells due to secretory defects or (b) nonproducers of a paraprotein. Nonsecretory myelomas are characterized by immunoperoxidase staining methods using antibodies directed against light chain-specific immunoglobulins. Monoclonal immunoglobulins in serum without significant clinical illness have also been found, and the incidence increases with age. As many as 3 % of patients older than 70 show the presence of monoclonal immunoglobulin (benign monoclonal proteinemia, also known as monoclonal gammopathy of undermined significance). These individuals require periodic serum and urine studies. In some rare instances, monoclonal gammopathies of undetermined significance are associated with neuropathies and the monoclonal immunoglobulin possesses an antinerve activity. A lymphoproliferative disorder characterized by monoclonal lymphocytes that produce monoclonal IgM is known as WaldenstrOm's macroglobulinemia. In this disorder, the elevated serum viscosity is palliated by plasmapheresis and cytotoxic treatment (e.g., chlorambucil and nucleoside analogs, respectively) due to the presence of large quantities of IgM and the tumor burden. It should be noted that pseudomonoclonal bands appearing in the/3-?, region may be misidentified as authentic monoclonal gammopathy. Pseudomonoclonal bands occur in those serum specimens due to hemolysis or to the presence of fibrinogen due to inappropriate blood coagulation techniques. Serum immunofixation studies should clarify these problems. Secondary paraproteinemias may be seen in association with hematopoietic cancers (e.g., lymphomas and leukemias), other neoplasms (e.g., colon carcinoma), long-standing chronic urinary or biliary tract infection, rheumatoid factor related to IgM monoclonal protein, and amyloidosis. Changes in Serum Proteins during Acute Phase of Disorders of Tissue Injury The alterations in serum proteins (acute phase response) that occur within a few hours to a few days after tissue injury due to infection, trauma, burns, surgery, or infarction and inflammatory condition are divided into two categories. The first category includes those proteins that are increased by at least 25% (positive acute phase proteins), and the second category includes proteins that are decreased by at least 25% (negative acute phase proteins).

APPENDIXVI

During acute stress, macrophages and monocytes are activated and produce several intercellular signaling polypeptides, known as cytokines (Chapter 35). Some examples of the released cytokines are interleukin-6, interleukin1fl, tumor necrosis factor o~, interferon-F, and transforming growth factor/3. One of the functions of these cytokines is to alter the synthesis of acute phase proteins in hepatocytes by regulating expression of acute phase protein genes by both transcriptional and posttranscriptional mechanisms. Glucocorticoids stimulate the action of cytokines by promoting the production of some acute phase proteins. The postulated function of the acute phase response is to protect the body from the injurious processes. One action is activation of the complement system to fight infection or antagonize the activity of proteolytic enzymes. Other actions initiate or sustain inflammation, whereas others reflect antiinflammatory and antioxidant properties. Examples of positive and negative acute phase proteins are given in Table VI- 1. C-reactive protein and serum amyloid A are elevated in serum by as much as 1000-fold from their basal values. Serum amyloid A is an apolipoprotein; it is synthesized in hepatocytes in response to inflammatory stimuli and associated with HDL. The function of serum amyloid A is not clear and it is not commonly measured as an acute phase reactant. However, serum C-reactive protein (so named because it reacts with pneumococcal C-polysaccharide) is measured. It binds with phosphocholine of pathogens, phospholipid constituents of damaged blood cells, and phagocytic cells, and it activates the complement system. All of the functions of C-reactive protein modulate inflammatory conditions of the body. These TABI~E VI-I

Examples of Positive and Negative Acute Phase Proteins Positive 1. Several members of complement system (e.g., C3, C4) 2. Proteins of the coagulation and fibrinolytic systems (e.g., fibrinogen, plasminogen) 3. Antiproteases (e.g., al-antitrypsin) 4. Transport proteins (e.g., haptoglobin, ceruloplasmin, hemopexin) 5. Inflammatory response modulators and others (e.g., phospholipase A 2, C-reactive protein, amyloid A, fibronectin, ferritin)

Negative Albumin Transferrin Transthyretin Thyroxine-binding globulin

955


include induction of inflammatory cytokines, prevention of adhesion of neutrophils to endothelial cells by inhibiting the surface expression of L-selectin, and inhibition of superoxide production by neutrophils. Measurement of serum C-reactive protein is useful in differentiating an acute inflammatory condition from a noninflammatory one, as well as in the assessment of the severity of inflammation and its prognosis. Another widely used test in the assessment of acute phase disorders is erythrocyte sedimentation rate (ESR). ESR determines the rate at which erythrocytes fall through the plasma to the bottom (sediment) of the test tube, and it depends largely on the plasma concentration of fibrinogen, an acute phase reactant (Table VI-1). Thus, ESR is decreased during acute phase disorder. Compared to ESR measurement, serum C-reactive protein measurement has several advantages. ESR changes occur relatively slowly and increase with age, whereas C-reactive protein concentrations change rapidly and levels do not change with age. Since inflammation may play a role in cardiovascular disorders, measurement of a serum inflammation marker, such as C-reactive protein, is useful as a predictor of subsequent coronary events (Chapter 20). In one prospective study, healthy subjects with higher baseline serum C-reactive protein levels had increased risk of myocardial infarction and ischemic stroke. The use of aspirin, an antiinflammatory agent (Chapter 18), also was associated with reduction in the risk of myocardial infarction. Another inflammatory marker, lipoprotein-associated phospholipase A2, is an independent predictor of risk for abnormal cardiovascular events.

Acute phase phenomena also include a variety of metabolic and neuroendocrine changes. For example, fever is a neuroendocrine change that occurs as an acute phase response.

Supplemental Readings and References G. C. Blobe, W. E Schiemann, and H. E Lodish: Role of transforming growth factor/~ in human disease. New England Journal of Medicine 342, 1350

(2000). C. Gabay and I. Kushner: Acute-phase proteins and other systemic responses to inflammation. New England Journal of Medicine 340, 448 (1999). J-M. Halimi, J Ribstein, G. Du Cailar, et al.: Nephrotic-range proteinuria in patients with renovascular disease. American Journal of Medicine 108, 120 (2000). E. Kallee: Bennhold's analbuminemia: a follow up study of the first two cases (1953-1992). Journal of Laboratory and Clinical Medicine 127, 470 (1996). R. A. Kyle and M. A. Gertz: Monoclonal gammopathies and related disorders. Hematology/Oncology Clinics of North America 13 (1999). This complete volume is dedicated to topics on monoclonal gammopathies. D. A. Morrow and E M. Ridker: High sensitivity c-reactive protein (hs-CRP): a novel risk marker in cardiovascular disease. Preventive Cardiology 1, 13 (1999). C. J. Packard, D. S. J. O'Reilly, M. Caslake, et al.: Lipoprotein-associated phospholipase A2 as an independent predictor of coronary heart disease. New England Journal of Medicine 343, 1148 (2000). D. J. Rader: Inflammatory markers of coronary risk. New England Journal of Medicine 343, 1179 (2000). E M. Ridker and E Haughie: Prospective studies of c-reactive protein as a risk factor for cardiovascular disease. Journal of Investigative Medicine 46, 391 (1998). E M. Ridker, C. H. Hennekens, J. E. Buring, et al.: C-reactive protein and other markers of inflammation in the prediction of cardiovascular disease in women. New England Journal of Medicine 342, 836 (2000).


APPENDIX

VII.I Laboratory Evaluation of Hemoglobin Disorders The differential diagnosis of hemoglobinopathies and some thalassemias is made in the laboratory. Some of the laboratory methods are described below.

Electrophoretic Procedures Most of the c o m m o n hemoglobin mutants can be identified by electrophoretically separating the hemoglobins on cellulose acetate at pH 8.4-8.6 and on citrate agar at pH 6.0-6.2. At pH 8.4, the hemoglobins are negatively charged and migrate toward the anode (positive electrode) in an electric field at a rate dependent on their net charge (see Figure VII-1 and Table VII-l). Separation of the hemoglobins with citrate agar as the support medium is based on net charge and adsorption of the hemoglobin to the supporting gel. The degree of adsorption may depend on hemoglobin solubility and may be due to an impurity in the agar rather than to the agar itself. Patterns obtained on citrate agar with various hemoglobins are shown in Figures VII-2 through VII-4. The value of using these two electrophoretic systems can be seen in the following examples. (1) Hbs S, D, and G migrate together about halfway between Hbs A and A2 on cellulose acetate

VII

electrophoresis. With citrate agar gels, Hbs D and G migrate cathodally with HbA, while HbS continues to migrate anodally separating it from the other hemoglobins. (2) Similarly, HbC comigrates with Hbs A2, E, O, and C-Harlem on cellulose acetate but is readily separated from them on citrate agar. (3) On cellulose acetate at pH 8.4, HbC-Harlem (two mutations, each changing the net charge on the tetramer by +2) migrates with HbC (one mutation, change of +4). These two hemoglobins are well separated from each other on citrate agar. Two modifications of classical hemoglobin electrophoresis are also useful for studying these proteins: isoelectric focusing and electrophoresis of individual globin chains, with or without the heme. These techniques can be used to identify some hemoglobins that are not separable by electrophoresis of the complete hemoglobin tetramers.

Nonelectrophoretic Procedures It is apparent from the above discussion that many hemoglobins with different amino acid substitutions demonstrate identical electrophoretic mobilities. Methods that rely on differences other than net charge are needed to establish the identity of these hemoglobins. Definitive

957

958

APPENDIX VII

ORIGIN --I

I

I I I I

IA

I A2 C(Harlem)

I

I I! I I I I

I I

~l

+

!

c

ORIRGINIA Ki

Is

IF

I

I MONTERY

K(Woolwich) l(JE~ltimore)

SG(Hybrid) 1 (NseQttIe)

Ilo

I Bart's

I ! B,

|,

sphokid

I MOST FAST

I. ~VARIABLE BETWEEN A AND S

1A-.~.~ hybrid

FIGURE VII- 1 Relative mobilities of some hemoglobins on cellulose acetate (pH 8.4).

molecular biological techniques employed in the evaluation of hemoglobin disorders are discussed in Chapters 23 and 28. HbS can be differentiated from most other hemoglobins by its insolubility upon deoxygenation. The test is carried out in the presence of a reducing agent (sodium dithionite or sodium metabisulfite, which consumes dissolved

TABLE VII-I

Amino Acid Substitutions and Net Charge Alterations in Hemoglobins Whose Relative Mobilities Are Shown in Figure VII-1

Hemoglobin C E O-Arab C-Harlem S G - S a n Jose D - L o s Angeles Sydney I-Philadelphia

Amino Acid Change*

Charge Alteration in Tetramer*

Glu--->Lys Glu--->Lys Glu-->Lys

+4 +4 + 4 +4

Glu-->Val Glu--->Gly Glu--->Gln Val--->Ala Lys--->Glu

+2 +2 +2 0 - 4

*As compared to HbA. Hemoglobins A 2, F, Bart's and H have multiple amino acid differences from HbA, and their electrophoretic mobilities depend on the net effect of all these differences on their net charge at pH 8.4-8.6.

(WOOLWlCH)

FIGURE VII-2 Relative mobilities of some hemoglobins on citrate agar (pH 6.0). The differences between this gel and cellulose acetate are the differential interactions of the hemoglobins with the gel and the changes in net charge caused by the lower pH.

oxygen) in an appropriate buffer. When HbS is added, the solution becomes turbid owing to precipitation of deoxyHbS. The turbidity can be quantitated spectrophotometrically and related to the amount of HbS added. By using the same reagents with intact erythrocytes, HbS can be precipitated within the cells, causing them to sickle. The extent of sickling can be assessed by microscopic examination. Because H b C - H a r l e m and Hb-Bart's behave like HbS in the solubility test, they may give false-positive results in tests for HbS. HbF is identified and quantitated by its resistance to denaturation by s t r o n g alkali. The hemolysate is mixed with an alkaline buffer (pH 12.8) and incubated to denature the nonfetal hemoglobins. The denatured hemoglobin is

Alpha Chain Mutants Hemoglobin

Structure

M o b i l i t y in Citrate Agar

+C

~Glu6-->Val L -7 } Asp -- Asn

l

J

J Paris J Oxford

Fort Worth Hasharon (Sealy) iMontgomery Russ Shimono~k0 G -Philadelphia

Winnipeg Inkster Broussais Titusville G-Georgia Rampa .,

(~ 12 15 (~ 16 C~27 o 47 48 C~51 C~54 C~68 (:It 75 O~85 (~ 90 O~94 0t 95 0c 95

S

A

pH

6.0 ..... F

-

Ala -" Asp Gly --'Asp Lys * GIu GIu -* G l y Asp -~ His Leu -" Arg G l y --* Arg Gin -* Arg Ash -* Lys Asp --" Tyr Asp -" Val Lys --* Asn Asp -* Asn Pro -~ Leu Pro --~ Ser

FIGURE VII-3 Mobilities of or-chain mutants on citrate agar electrophoresis relative to hemoglobins C, S, A, and F. (Courtesy of Dr. Rose Schnieder. Reproduced with permission from Helena Laboratories, Beaumont, Texas.)

SECTIONVII.1 Laboratory Evaluation of Hemoglobin Disorders Beta Chain Mutants

Hemoglobin

Structure Mobility in Citrate Agar pH 6.0 S

A

F

S C C Harlem

j~ 6 Glu --* Val /~ 6 Glu -~ Lys j~ 6 Olu -* Val j~ 73 Asp -* Asn G-San Jose 7 Glu -* G l y J-Baltimore 16 G l y --* Asp G-Coushatta 22 Glu --' Ala E 26 Glu -* Lys Alabama j~ 39 Gin -* Lys G.Galveston 43 Glu --* Ala 51 Pro -§ Arg Williamette Osu Christiansborg 52 Asp --* Ash

~ ~

N-Seattle

~61 Ly$ -~ GIu

j~ 73 Asp -+ Asn 73 Asp -* Val 87 Thr --* Lys 91-95 deleted j~ 95 Lys -+ Glu /~ 97 His --* Gin j~ 98 Val -+ Met Koln 99 Asp -* Asn Kempsey 102 Ash --* Lys Richmond j~ 107 G l y -* Arg Burke 117 His-* Arg P ~121 Glu -* Gin D L.A. (Punjab) /~ 121 Glu -+ Lys O-Arab 131 Gin - * G I u Camden 131 Gin -~ 0 Deaconess 132 Ly$ -+ G In K-Woolwich 136 GIy -'* Asp Hope j3145 T y r -+ His Bethesda Cochin Port Royal j~ 146 His --* Arg Korle Bu

Mobile D-Ibadan Gun Hill N-Baltimore Malmo

~

FIGURE VII-4 Mobilities of/3-chain mutants on citrate agar electrophoresis relative to hemoglobins C, S, A, and F. (Courtesy of Dr. Rose Schnieder. Reproduced with permission from Helena Laboratories, Beaumont, Texas.)

then precipitated by addition of ammonium sulfate, and the undenatured HbF in the clear supernatant is measured by spectrophotometry. HbA2 can be separated from most other common hemoglobins by anion exchange chromatography using diethylaminoethylcellulose. Separation results from differences in the interactions of the charged groups of the various hemoglobins with the positively charged groups on the anion exchange resin. Following separation, the HbA2 can be quantitated by spectrophotometry. Although several other hemoglobins (including C, E, O, and D) are eluted with HbA2, it is unlikely that one of these hemoglobins would be present in a person being tested for elevated HbA2. HbA2 measurement is used in the diagnosis of fl-thalassemia trait, in which the HbA2 is elevated to about twice the normal level (the upper normal limit is about 3-3.5%). HbH and many unstable hemoglobins spontaneously precipitate within the red cells, forming Heinz bodies, which can be detected in splenectomized patients by staining with methylene blue. Alternatively, precipitation of these hemoglobins can be induced and the precipitates visualized by incubation of the erythrocytes with a redox dye such as brilliant cresyl blue (Chapter 28). Many abnormal hemoglobins are also much more readily denatured by heat than normal hemoglobins. Heating an unstable hemoglobin for 30 minutes at 60~ usually causes complete denaturation, whereas HbA is

959 hardly precipitated at all under the same conditions. The sulfhydryl groups of the mutant globins are generally more exposed than those in normal hemoglobin, making them more reactive toward parachloromercuribenzoate (PCMB). Treatment with PCMB for several hours precipitates many mutant hemoglobins, whereas it only causes dissociation of HbA. A sensitive method for identifying changes in the primary structure of hemoglobin is peptide mapping or fingerprinting (Chapter 3). High-performance liquid chromatographic techniques have been used as a sensitive method to detect various hemoglobins, including fetal hemoglobins yA and y c. Molecular biology techniques, such as Southern blot analysis and polymerase chain reaction (PCR), have been used in the diagnosis of hemoglobinopathies (see the following discussion). Thalassemia Syndromes Thalassemia syndromes (see also Chapter 28) are a group of heterogeneous inherited disorders. They are relatively common in persons of Mediterranean, African, and Southeast Asian ancestry and are characterized by an unbalanced and defective rate of (absent or reduced) synthesis of one or more globin chains of hemoglobin. Defects in ot-globin chain synthesis are designated as ot-thalassemia and defects in fi-globin chain synthesis are designated as fi-thalassemia. The clinical manifestations in thalassemia syndromes occur due to inadequate hemoglobin synthesis and the accumulation of unused globin subunits due to unbalanced synthesis. The former leads to hypochromic microcytic anemia and the latter leads to inaffective erythropoiesis and hemolytic anemia. The clinical severity of these disorders depends on the nature of mutation. They range from asymptomatic mild hypochromic microcytosis (ot-thalassemia silent trait) to early childhood mortality (homozygous fi-thalassemia) to in utero death (Hb-Bart's hydrops fetalis). fl-Thalassemia: The /3 and fl-like genes are located in chromosome 11 and their organization is discussed in Chapter 28. In homozygous fl-thalassemia syndrome the fl-globin chain synthesis is either absent (/3~ or severely reduced (/3+). The molecular defects are numerous and include gene deletion, as well as defects in transcription, processing, transport, and translation of mRNA. The clinical symptoms manifest after 6 months of age, when the switch of synthesis from HbF to HbA occurs. Numerous clinical problems follow and require transfusion therapy with its consequent complications (e.g., iron overload; see Chapter 29). Heterozygous fl-thalassemia (fl-thalassemia trait) is accompanied by mild hypochromic microcytic anemia without any significant clinical abnormalities, fl-Thalassemia trait can be identified by elevated

960

FIGURE

APPENDIXVII

VII-5

Restriction enzyme map of ct-globin gene cluster and some of the deletions (indicated as dark lines).

HbA2 (>3.5%) and, in some mutations, by HbF and normal serum iron studies. a-Thalassemia: The a-globin gene cluster is found in chromosome 16 and each chromosome contains two ot genes (ct2 and ctl). Thus each individual has four copies of ot gene. The genes are present in the order: 5'-(27r ~"1-Trot2-~pot 1-otz-ot 1-01-3' (Figure VII-5). The 7r genes (pseudogenes) and 01 gene are nonfunctional. The genes are thought to be relics of the past evolutionary gene mutations. The 01 gene does not yield a viable globin protein product and its function is not known, ct-Thalassemia is probably the most common genetic defect, and its prevalence correlates with geographic areas of malaria incidence. The clinical manifestations of ot-thalassemia depends on the number of genes affected. Gene deletions are the most common defect, and point mutations account only for about 5% of ct-thalassemia. a-Thalassemia silent carrier (ota/o~-)" This genotype is caused by a single otz-globin gene defect, primarily due to -or 37 o r - o r 4"2 deletion. The superscript indicates the length of deletion in kilobases of DNA. Both deletions are caused by unequal crossover points occurring at different locations. The reciprocal product of unequal crossover yields a triplicated ot-globin gene. Individuals who have inherited single o~-gene defect do not exhibit any hematological or clinical manifestations and

the relative proportion of hemoglobins A, A2, and F are normal. t~-Thalassemia-2 (ct-/ot-): Inheritance of two singlegene defects gives rise to clinically significant microcytosis, and the relative proportions of hemoglobins A, A2, and F are normal. c~-Thalassemia-1 (--/otot): Two-gene deletions on the same chromosome are characterized by microcytic anemia. Different deletions have been observed. Southeast Asian (__SEA)and Filipino (__Fil) are examples of two-gene deletions occurring in the designated geographic areas. Deletion of a regulatory element (HS-40, see Chapter 28) also results in thalassemia syndrome. Hemoglobin H disease: Compound heterozygosity for ct-thalassemia-1 and ot-thalassemia-2 deletions give rise to hemoglobin H (HbH) disease. These individuals exhibit hemolytic, microcytic anemia, which is not life threatening. HbH(/34) inclusions are identified by supravital staining with brillian cresyl blue (discussed earlier) and as fastmoving hemoglobin in cellulose acetate, alkaline pH 8.4 electrophoresis technique (Figure VII-l). Hemoglobin Bart's hydrops fetails: This occurs due to inheritance of two ot-thalassemia-1 allels and the offspring does not have any functional o~ genes. This condition is incompatible with life and results in stillborn or critically ill fetus with hydrops fetalis. Hemoglobin

961


FIGURE VII-6 Restriction enzyme map of Filipino-type deletion in o~-globin gene cluster.

Bart's is g-tetramer and does not function as a normal hemoglobin. Thalassemia syndromes also occur along with other mutations in the oe-globin gene. One example is hemoglobin constant spring which is a result of mutation in the stop codon and gives rise to an abnormally large oe-globin chain (Chapter 28). In rare cases, oe-thalassemia is associated with mental retardation. An X-linked form of mental retardation, known as ATR-X syndrome, is a result of mutations in XH2. gene. The XH2 gene is a member of a subgroup of the family of genes that codes for proteins within many diverse functions (e.g., DNA helicase, DNA repair enzymes, putative global transcriptional regulatory proteins). It is thought that XH2 mutations result in downregulation of oe-globin gene expression.

Laboratory Diagnosis of ot-Thalassemia Syndromes: DNA-based analysis is utilized for definitive testing in prospective parents for assessing the risk of transmission of oe-thalassemia-1 genes and in prenatal diagnosis to identify Hb-Bart's hydrops fetalis. The methods may include Southern blot analysis and PCR methodology using appropriate o~-globin gene cluster probes and primers, respectively. In Southern blot analysis, DNA obtained from leukocytes or fetal cells is subjected to digestion with specific restriction endonucleases, and the resulting fragments are separated by electrophoresis. Blotting is performed by alkaline transfer to nylon membranes and exposed to labeled (e.g., radioactive, fluorescent) DNA probes. The genotypes are established by using previously

defined restriction fragment length patterns. Figure VII5 shows the map of ot-globin gene locus with restriction sites useful for diagnosis. In the Filipino oe-thalassemia-1 (lyly/--Fil), due to large deletion involving the entire oe-globin gene locus, digestion with restriction endonuclease, Bam H1, and BglII is not useful in identifying the deletion. Thus, Filipino o~-thalassemia-1 is identified by using a probe ("LO Probe") derived from sequences of about 4 kb 5' to the (-globin gene and digestion with the resctriction endonuclease SacI or SstI (Figure VII-6).

Supplemental Readings and References B. J. Bain, R. J. Amos, D. Bareford, et al.: The laboratory diagnosis of haemoglobinopathies. British Journal of Haematology 101, 783 (1998). D. K. Bowden, M. A. Vickers, and D. R. Higgs: A PCR-based strategy to detect the common severe determinates of o~-thalassaemia. British Journal of Haematology 81, 104 (1992). D. H. K. Chui, R. Hardison, C. Riemer, et al.: An electronic database of human hemoglobin variants on the world wide web. Blood 91, 2643 (1998). N. Fischel-Ghodsian, M. A. Vickers, M. Seip, et al.: Characterization of two deletions that remove the entire human (-ol globin gene complex (__Thai and __Fil). British Journal of Haematology 70, 238 (1988). R. J. Gibbons, D. J. Picketts, L. Villard, et al.: Mutations in a putative global transcriptional regulator cause X-linked mental retardation with oe-thalassemia (ATR-X syndrome). Cell 80, 845 (1995). C. H. Joiner: Universal newborn screening for hemoglobinopathies. Journal of Pediatrics 136, 145 (2000). K. J. Skogerboe, S. E West, C. Smith, et al.: Screening for oe-thalassemia. Archives of Pathology and Laboratory Medicine 116, 1012 (1992).


APPENDIX

VIII

TABLE VIII-1 Acronyms and Abbreviations Found in This Text ABP ACAT ACTH ADH ADH ADP AFP AIP Ala ALA AMP Arg Asn Asp AST ATP BDGF BMP BMR BPG CaBP cAMP CBG CCK cDNA CDP cGMP CLIP CMP CoASH COMT CPK CRBP CRH CT CTP Cys D D-

DA DCC DES DHEA DHEAS DHT DIT DNA

Androgen-binding protein Acyl-CoA:cholesterol acyltransferase Adrenocorticotropic hormone, corticotropin Alcohol dehydrogenase Antidiuretic hormone, vasopressin Adenosine diphosphate a 1-Fetoprotein Aldosterone-induced protein Alanine Aminolevulinic acid Adenosine monophosphate Arginine Asparagine Aspartic acid Aspartate aminotransferase Adenosine triphosphate Bone-derived growth factor Bone morphogenic protein Basal metabolic rate Bisphosphoglycerate Calcium-binding protein 3',5'-Cyclic adenosine monophosphate, cyclic AMP Corticosteroid-binding globulin Cholecystokinin Complementary DNA Cytidine diphosphate 3',5'-Cyclic guanosine monophosphate, cyclic GMP Corticotropin-like intermediate lobe polypeptide Cytidine monophosphate Free (uncombined) coenzyme A Catechol-O-methyltransferase Creatine phosphokinase Cellular retinol-binding protein Corticotropin-releasing hormone Calcitonin Cytidine triphosphate Cysteine Dopamine receptor Geometric isomer of n form of chemical compound Dopamine Dicyclohexylcarbodiimide Diethylstilbestrol Dehydroepiandrosterone Dehydroepiandrosterone sulfate Dihydrotestosterone Diiodotyrosine Deoxyribonucleic acid

DOC Dopa DPF DPG DPN + DPNH dsRNA E EC EDTA EFA EGF ELISA emf Enz FAD FADH 2 FFA FH 4 Figlu FMN FMNH 2 FSH FST GABA GDP GFR GH GIP Gla Gln Glu Gly GMP GnRH GRH GTP HAL Hb hCG HDL 5-HETE 5-HIAA His HLA HMG-CoA HMM HMP

Deoxycorticosterone 3,4-Dihydroxyphenylalanine Diisopropylphosphofluoridate Diphosphoglycerate (bisphosphoglycerate) Diphosphopyridine nucleotide (oxidized) (now replaced by NAD) Diphosphopyridine nucleotide (reduced) (now replaced by NADH) Double-stranded RNA Epinephrine Enzyme Commission number (International Union of Biochemistry) system Ethylenediaminetetraacetic acid Essential fatty acid Epidermal growth factor Enzyme-linked immunosorbent assay Electromotive force Enzyme (also E) Flavin adenine dinucleotide (oxidized) Flavin adenine dinucleotide (reduced) Free fatty acid Tetrahydrofolic acid Formiminoglutamatic acid Flavin mononucleotide (oxidized) Flavin mononucleotide (reduced) Follicle-stimulating hormone Foam stability test 7-Aminobutyric acid Guanosine diphosphate Glomerular filtration rate Growth hormone Gastric inhibitory polypeptide y-Carboxyglutamic acid Glutamine Glutamic acid Glycine Guanosine monophosphate Gonadotropin-releasing hormone Growth hormone-releasing hormone Guanosine triphosphate Hepatic acylglycerol lipase Hemoglobin Human chorionic gonadotropin High-density lipoprotein Monohydroxyeicosatetraenoic acid 5-Hydroxyindoleacetic acid Histidine Major histocompatibility (MHC) locus /3-Hydroxy-/3-methylglutaryl-CoA Heavy meromyosin Hexose monophosphate

(continued)

963

964

APPENDIXVIII

TABLE VIII-1 (continued) hnRNA hNT 5-HPETE hPL HSL I IDL IF IFN Ig IGF IL Ile IMP ITP kat kcal kJ Km LLATS LCAT LDH LDL Leu LH LHRH LMM LPH LPL LT Lys M MAC MAO Mb MCR MDR Met MIF MIF MIT MJ MLCK MLCP mol MRF MRI mRNA

Heterogeneous nuclear RNA Human N-terminal fragment 5-Hydroperoxyeicosatetraenoic acid Human placental lactogen Hormone-sensitive lipase Isoproterenol Intermediate-density lipoprotein Intrinsic factor Interferon Immunoglobulin Insulin-like growth factor Interleukin Isoleucine Inosine monophosphate Inosine triphosphate Katal Kilocalorie (calorie) Kilojoule Michaelis constant Geometric isomer of o form of chemical compound Long-acting thyroid stimulator Lecithin-cholesterol acyltransferase Lactate dehydrogenase Low-density lipoprotein Leucine Luteinizing hormone Luteinizing hormone-releasing hormone Light meromyosin Lipotropic hormone Lipoprotein lipase Leukotriene Lysine Molar Membrane attack complex Monoamine oxidase Myoglobin Metabolic clearance rate Minimum daily requirement Methionine MSH release-inhibiting factor Mtillerian inhibiting factor Monoiodotyrosine Megajoule (1000 kJ) . Myosin light chain kinase Myosin light-chain phosphatase Mole(s) MSH-releasing factor Magnetic resonance image Messenger RNA

MSAFP MSH M.W. NAD + NADH NADP + NADPH NE NGF NMR NSN OHSDH PABA PAF PAPS PBPs PCR PDGF PE PEPCK PG PGE PGI Phe Pi pI PIF PIH PITC PKU PNMT POMC PP PRF PRL Pro PRPP PTH PUFA RBP RDA rDNA RFLP RNA

nQ

Maternal serum AFP Melanocyte- stimulating hormone Molecular weight Nicotinamide adenine dinucleotide (oxidized); same as DPN Nicotinamide adenine dinucleotide (reduced); same as DPNH Nicotinamide adenine dinucleotide phosphate (oxidized); same as TPN Nicotinamide adenine dinucleotide phosphate (reduced); same as TPNH Norepinephrine Nerve growth factor Nuclear magnetic resonance Nicotine-sensitive neurophysin Hydroxysteroid dehydrogenase p-Aminobenzoic acid Platelet-activating factor Phosphoadenosine-phosphosulfate Penicillin-binding protein Polymerase chain reaction Platelet-derived growth factor Phenylephrine Phosphoenolpyruvate carboxykinase Prostaglandin Prostaglandin E Prostacyclin Phenylalanine Inorganic phosphate Isoelectric point Prolactin-inhibiting factor Prolactin-inhibiting hormone Phenylisothiocyanate (Edman's reagent) Phenylketonuria Phenylethanolamin-N-methyltransferase Pro-opiomelanocortin Pancreatic polypeptide Prolactin-releasing factor Prolactin Proline 5-Phosphoribosyl- 1-pyrophosphate Parathyroid hormone Polyunsaturated fatty acid Retinol-binding protein Recommended daily (dietary) allowance Recombinant DNA Restriction fragment length polymorphism Ribonucleic acid Respiratory quotient

(continued)

APPENDIXVIII

965

TABLE VIII-1 (continued) rRNA SAM SC SDA SDS Ser SH SMP snRNA SRS-A T3 T4 TBG t-BOC TBPA TCA TCII TeBG TeBP TETRAC TG Thr TMP TMP TPA

Ribosomal RNA S-Adenosylmethionine Secretory component Specific dynamic action Sodium dodecyl sulfate Serine Sulfhydryl Submitochondrial particle Small nuclear RNA Slow-reacting substance of anaphylaxis Triiodothyronine, thyroid hormone Tetraiodothyronine, thyroxine Thyroxine-binding globulin Tertiary butyloxycarbonyl Thyroxine-binding prealbumin Tricarboxylic acid Transcobalamin Testosterone-estrogen-binding globulin, sex hormone-binding globulin Testosterone-estradiol-binding globulin 3,3', 5,5'-Tetraiodothyroacetic acid Thyroglobulin Threonine Thiamine monophosphate Thymidine monophosphate Tissue plasminogen activator

TPN + TPNH TPP TRH TRIAC TRIS tRNA Trp TSH TSI TTP TX Tyr U UDP UMP UTP Vmax Val VIP VLDL VMA XMP

Triphosphopyridine nucleotide (oxidized) (now replaced by NADP +) Triphosphopyridine nucleotide (reduced) (now replaced by NADPH) Thiamine pyrophosphate Thyrotropin-releasing hormone 3,3', 5-Triiodothyroacetic acid Tris(hydroxymethyl)aminomethane Transfer RNA Tryptophan Thyroid-stimulating hormone Thyroid-stimulating immunoglobulin Thymidine triphosphate Thromboxane Tyrosine International Unit(s) Urine diphosphate Uridine monophosphate Uridine triphosphate Maximal velocity Valine Vasoactive intestinal polypeptide Very low density lipoprotein 3-Methoxy-4-hydroxymandelic acid, vanillylmandelic acid Xanthosine monophosphate


APPENDIX

IX

TABLE IX-1

Reference Intervals for Clinical Laboratory Parameters*

Component

Present Reference Interval (Examples)

Present Unit

3.5-5.0 4.3-5.9 0

106/mm 3 106/mm 3 mm -3

Clinical Hematology Erythrocyte count (B) female male Erythrocyte count (Sf) Erythrocyte sedimentation rate (ESR) (BErc) female male Hamatocrit (BErcs) volume fraction female male Hemoglobin (B) mass concentration female male substance conc. Hb (Fe) female male Leukocyte count (B) number fraction ("differential") Leukocyte count (Sf) Mean corpuscular hemoglobin (MCH) (BErc) mass amount of substance Hb (Fe) Mean corpuscular hemoglobin concentration (MCHC) (BErc) mass concentration substance conc. Hb (Fe) Mean corpuscular volume (MCV) (BErc) erythrocyte volume Platelet count (B) Reticulocyte count (B)--adults number fraction

Clinical Chemistry Acetaminophen (P)--toxic Acetoacetate (S) Acetone (B, S) Acid phosphatase (S) Adrenocorticotropin (ACTH) (P) Alanine aminotransferase (ALT) (S) Albumin (S) Aldolase (S) Aldosterone (S) normal salt diet restricted salt diet Aldosterone (U)--sodium excretion = 25 mmol/d = 75-125 mmol/d = 200 mmol/d Alkaline phosphatase (S) Alpha 1-antitrypsin (S) Alpha-fetoprotein (S)

SI Reference Interval

SI Unit Symbol

Significant Digits

Suggested Minimum Increment

1 1 1

3.5-5.0 4.3-5.9 0

lO12]L 1012/[, 106/I_,

X.X X.X XX

0.1 1012/I_, 0.1 lO12]L 1 106/L

mm/h mm/h

1 1

0-30 0-20

mm/h mm/h

XX XX

1 mm/h 1 mm/h

% %

0.01 0.01

0.33-0.43 0.39-0.49

1 1

O.XX O.XX

0.01 0.01

12.0-15.0 13.6-17.2

g/dL g/dL

10 10

120-150 136-172

g/L g/L

XXX XXX

1 g/L 1 g/L

12.0-15.0 13.6-17.2 3200-9800

g/dL g/dL mm -3 %

0.6206 0.6206 0.001 0.01

mmol/L mmol/L 109/L 1

XX.XX XX.XX XX.X O.XX

0.05 mmol/L 0.05 mmol/L 0.1 109/L 0.01

0-5

mm -3

1

106]L

XX

1 106/L

27-33 27-33

pg pg

1 0.06206

27-33 1.70-2.05

Pg fmol

XX X.XX

1 pg 0.05 fmol

33-37 33-37

g/dL g/dL

10 0.6206

330-370 20-23

g/L mmol/L

XXO XX

10 g/L 1 mmol/L

1 1 0.001 0.001

76-100 130-400 10-75 0.001-0.024

fL 109/L 109/L 1

XXX XXX XX O.XXX

lfL 5 109/L 1 109/L 0.001

0.1-2.4

~m 3 103/mm 3 mm -3 0/00 (number per 1000 erythrocytes) %

0.01

0.001-0.024

1

O.XXX

0.001

>5.0 0.3-3.0 0 0-5.5 20-100 0-35 4.0-6.0 0-6

mg/dL mg/dL mg/dL U/L pg/ml U/L g/dl U/L

66.16 97.95 172.2 16.67 0.2202 0.01667 10.00 16.67

>330 30-300 0 0-90 4-22 0-0.58 40-60 0-100

pmol/L /.tmol/L /,tmol/L nkat/L pmol/L /_tkat/L g/L nkat/L

XXO XXO XXO XX XX X.XX XX XXO

10pmol/L 10 pmol/L 10 ~mol/L 2nkat/L 1 pmol/L 0.02/_tkat/L 1 g/L 20 nkat/L

8.1-15.5 20.8-44.4

ng/dL ng/dL

27.74 27.74

220-430 580-1240

pmol/L pmol/L

XXO XXO

10 pmol/L 10 pmol/L

pg/24 h /.tg/24 h /_tg/24 h U/L mg/dL ng/mL

2.774 2.774 2.774 0.01667 0.01 1.00

50-235 15-70 5-35 0.5-2.0 1.5-3.5 0-20

nmol/d nmol/d nmol/d /.tkat/L g/L pg/L

XXX XXX XXX X.X X.X XX

5 nmol/d 5 nmol/d 5 nmol/d O.1/akat/L 0.1 g/L 1 pg/L

0-30 0-20

33-43 39-49

76-100 130-400 10000-75000 1-24

18-85 5-26 1.5-12.5 30-120 150-350 0-20

Conversion Factor

7.45-9.30 8.45-10.65 3.2-9.8

0-5

967

968

T A B L E IX-1

APPENDIXIX

(continued)

Component

Alpha-fetoprotein (Amf) Alpha2-macroglobulin (S) Aluminum (S) Amino acid fractionation (P) Alanine Alpha-aminobutyfic acid Arginine Asparagine Aspartic acid Citrulline Cysfine Glutamic acid Glutamine Glycine Hisfidine Hydroxyproline Isoleucine Leucine Lysine Methionine Ornithine Phenylalanine Proline Sefine Taufine Threonine Tryptophan Tyrosine Valine Amino acid nitrogen (P) Amino acid nitrogen (U) Delta-aminolevulinate [as levulinic acid] (U) Amitfiptyline (P, S)-therapeutic Ammonia (vP) as ammonia (NH 3) as ammonium ion (NH4+) as nitrogen (N) Amylase (S) Androstenedione (S) male >18 years female > 18 years Angiotensin converting enzyme (S) Arsenic (H) (as As) Arsenic (U) (as As) (as As203 ) Ascorbate (P) (as ascorbic acid) Aspartate aminotransferase (AST) (S) Barbiturate (S)--overdose total expressed as: phenobarbital sodium phenobarbital barbitone Barbiturate (S)I therapeutic see phenobarbital see pentobarbital see thiopental

Present Reference Interval (Examples)

Depends on gestation 145-410 0-15

Present Unit

Conversion Factor

mg/dL

10

mg/dL /zg/L

0.01 37.06

SI Reference Interval

Depends on gestation 1.5-4.1 0-560

Suggested Minimum Increment

SI Unit Symbol

Significant Digits

mg/L

XX

1 mg/L

g/L nmol/L

X.X XXO

0.1 g/L 10 nmol/L

2.2-4.5 0.1-0.2 0.5-2.5 0.5-0.6 0.0-O.3 0.2-1.0 0.2-2.2 0.2-2.8 6.1-10.2 0.9-4.2 0.5-1.7 0-trace 0.5-1.3 1.0-2.3 1.2-3.5 0.1-0.6 0.4-1.4 0.6-1.5 1.2-3.9 0.8-1.8 0.3-2.1 0.9-2.5 0.5-2.5 0.4-1.6 1.7-3.7 4.0-6.O 50-200 1.0-7.0

mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL rag/alL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/dL mg/24 h mg/24 h

112.2 96.97 57.40 75.69 75.13 57.08 41.61 67.97 68.42 133.2 64.45 76.26 76.24 76.24 68.40 67.02 75.67 60.54 86.86 95.16 79.91 83.95 48.97 55.19 85.36 0.7139 0.07139 7.626

245-500 10-2 30-145 35-45 0-20 15-55 10-90 15-190 420-700 120-560 30-110 0-trace 40-100 75-175 80-240 5-40 30-400 35-90 105-340 75-170 25-170 75-210 25-125 20-90 145-315 2.9-4.3 3.6-14.3 8-53

/maol/L /jmol/L ~aol/L /irnol/L /vnol/L /irnol~ ~nol/L ~jmol/L /~mol/L /drnol/L /~mol/L /amol/L /jmol/L /maol/L ~jmol/L /maol/L /maol/L ~nol/L //rnol/L /./mol/L ,t/mol/L /jmol/L ~mol/L /2rnol/L /./mol/L mmol/L mmol/d ~tmol/d

XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX XXX X.X X.X XX

5/3mol/L 5/~aaol/L 5/Jmol/L 5/arnol/L 5/~nol/L 5/~rnol/L 5/xmol/L 5 ~anol/L 5/3mol/L 5/~nol/L 5 ~anol/L 5/lmol/L 5 ~mol/L 5 #mol/L 5/jmol/L 5 ~nol/L 5/~mol/L 5/~nol/L 5/xmol/L 5 ~mol/L 5 ~nol/L 5//mol/L 5/.trnol/L 5/jmol/L 5/Jmol/L 0.1 mmol/L 0.1 mmol/d 1//rnol/d

50-200

ng/mL

3.605

180-720

nmol/L

XX0

10 nmol/L

10-80 10-85 10-65 0-130

~g/dL ~g/dL gtg/dL U/L

0.5872 0.5543 0.7139 0.01667

5-50 5-50 5-50 0-2.17

/.onol/L /maol/L /maol/L ~atlL

XXX XXX XXX XXX

5 ~mol/L 5 #mol/L 5//rnol/L 0.01/.tkat/L

0.2-3.0 0.8-3.0

~g/L /~g/L

3.492 3.492

0.5-10.5 3.0-10.5

nmol/L nmol/L

XX.X XX.X

0.5 nmol/L 0.5 nmol/L

10.0

mg/L mg/L

4.582 4.582

25-46 >46

/xmol/L ~nol/L

XX XX

1 gmol/L 1 ~oaaol/L

4.0-8.0 >12.0 4.0-8.0

mg/L mg/L mg/L

4.249 4.249 3.606

17-34 >50 14-29

~mol/L ~mol/L /.tmol/L

XX XX XX

1/~nol/L 1/~nol/L 1/ffnol/L

10

Prolactin (P) Propoxyphene (p)itoxic Propranolol (P) (Inderal)I therapeutic Protein, total (S) Protein, total (Sf) Protein, total (U) Protryptyline (P) Pyruvate (B) (as pyruvic acid) Quinidine (P)-therapeutic toxic Renin (P) normal sodium diet restricted sodium diet

Present Unit

fmol XX progesterone bound/mg protein fmol XX progesterone bound/mg cytosol protein fmol XX progesterone bound/mg cytosol protein /.tg/L XX ~mol/L X.X nmol/L XX0

1 fmol/mg protein

1 fmol/mg protein

1 fmol/mg protein

1/3g/L 0.1/~aaol/L 10 nmol/L

2.0 50-200

ng/mL mg/L ng/mL

1.00 2.946 3.856

6.0-8.0

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