Nutrient Metabolism (Food Science and Technology International)

NUTRIENT MITABOUSM Food Science and Technology International Series Series Editor Steve L Taylor Univmity of Nebmsh ...

Author: Martin Kohlmeier

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NUTRIENT MITABOUSM

Food Science and Technology International Series Series Editor

Steve L Taylor Univmity of Nebmsh

Advisory Board Ken Buckle Uniusrxity of New %uih Was, Alcstmlicr Bruce Chassy

Uniwrsiiy of Illinois, US4 Patrick Fox Uniwsiy College Cork, Republic of Ireland

Dennis Gordon

Nortk Dakota Stats Universiy, USA Rob& Huckins Unknip Ofrcebmsh, USA

RonaldJackson Q

M Gnada ~

Daryl B. Lund

6mll Uniwtsi&, USA Connie Weaver hrdue Universip, USA

Louise Wicker Universrfy of Centgi~,USA Howard Zhang Ohio Stars U n M & USA

A complete list of booh in this wies w's at the end of this dwms.

Nutrient Metabolism

Martin Kohlrneier Research Professor, School o f Public Health and School of Medicine, University o f North Carolina, USA

ACADEMIC PRESS An imprint of Elsevier

- -

€luxan * Heidtlbrrg - London NouYork Oxford Patis - h n Okgo 5an Francisco * S i n g a p Sydney Tokyo

Amnerdam

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-

Academic Press is an imprint of Elsevier 84 Theobalds Road, London WClX SRR,UK Radanveg 29,PO Box 21 1, 1000 AE Amsterdam, The Netherlands 30 Corporate Drive, Suite 400,Burlington, MA 01803,USA 525 B Street, Suite 1900,San Diego, C A 92101-4495,USA First edition 2003 Reprinted 2006 Copyright 02003 Elsevier Ltd. All rights reserved

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier's Science & Technology Rights Department in Oxford, UK: phone: (+44)(0) 1865 843830;fax: (+44)(0)1865 853333; email: [email protected]. Alternatively you can submit your request online by and selecting visiting the Elsevier web site at http://elsevier.com/locate/permissions, Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data Kohlmeier, Martin Nutrient metabolism. - (Food science and technology. International series) 1. Metabolism 2.Nutrition 1. Title 612.3'9 Library of Congress Catalog Number: 20021 15408 ISBN-13: 978-0-12-417762-8 ISBN-10: 0-12-417762-X For information on all Academic Press publications visit our website at books.elsevier.com Printed and bound in Spain

06 07 08 09 10 10 9 8 7 6 5 4 3 2

Working together to grow libraries in developing countries www.elsevier.com

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Contents

Foreword by Steven H Zeisel

IX

Acknowledgments

X

1

Introduction Nutrients

2

Chemical senses .. . . Molecular basis of smell . Taste ..•.....................•.............. Chemestesis .

7 7 10 21

3

Intake regulation . Appetite .................................................••. Thirst ..............•........•.......•..................

25 25 31

4

Absorption, transport, and retention . Digestion and absorption . . ........•................ Renal processi ng •.•....•..•..•.••..•....•........... The blood-brain barrier .. . . Materna-fetal nutrient transport .

37 37 56 70 77

5

Xenobiotics Heterocyclic amines Flavonoids and isoflavones Garlic compounds

85 85

6

Fatty acids Structure and function of fatty acids Overfeeding Acetate Myristic acid " ' ! " - " - -_.-

92 106 111

111

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

- _. - -

143 147 153

vi Contents

........................................ i57 ......... ........... . ............ . . 164 ............................................... . 175

Conjugated linoleic acid Docosahexaenoic acid T-antfatcy acids

i 1 4i

iL.,

.*... ...... . ... ...

.,

Chlorophyll/phytol/phytanic acid . . y i 7

i. L

i

~~1

*r

.

Cahohydrates. alcohols. agd organic acids Carbohydrates ..... i w. -.

s*.-

f;

.,.am

L

Ldwi

179

.

.... ............. ..*.. F-4

187

r.r.

..-.... ...... . .............;..................... 187 Glucose ..+ .................... .-.......................... .... . 193 Fructose ......-.+.* ..................................... ..*.*...#..... 210 Galactose ................. ..... ........ ...... . ..-.. ........ ... 216 Xylitol .................. ....-................. ............ ... 223 .

i.

.

. .*+

111

.i i.

A

i.

..i.r.~...

..... .......... . . .......* . ........, .........227 .......,..........'+. .-.. ..... .. .............. .. 232 ?.

Qruvate Oxalate . L * Ethanol . ,+. 8

at'.

4

t

Y

C.

. . . . 235 and ............ ....................... 243 244 a d s .. ..*.. . *. . .................. .................................................. 268 .......,.... :f.-. ,........... ............... ... 272 ...* ........ ... . . .**. .* .... 280 .... ..... ... .';*....... ....... .. ... ....... .. .. . . ....... . ... 295 . ................................... .. 300 .... ... .. ........................ - ..................... 308 .............- ................. ................. ......... ..... ......... .......... . . .............................. ........ ...:......... 328 .... ...,.*-. .. ................*.*... *. 338 ................ ................ ...................... 348 356 ... .., . . ..*.. ... ....... ., ........................ ...................... 363 .... ..-. ...?..-. 370 . ............. .. t i .... ....+......... ....................... .....-. ........ .. 377 ................................ ......... .......... 383 .......................................-............... 389 ..... .......................................... .........395 . . . ....:..,*..*.. . ......+.. ...........* ............... 404 ....... ....... ...... .. . ........ .. .......... ..................... .:.......... . . . ................... 421 .....-.. .-.................. ............ .,................. ........ .... ............*...-..............-....432 ...................................... .. .......'............,............. .. ......,.... 439 .-................. and . ...... ..;. *... ......... ....... . 457 +-.

..-

. .. .. .

. I . pTs

fl

i.$

.-

25 g), calcium (>8 g), or vitamin D (> 1000 ~g). The proximal tubule extends from the glomerulus to the loop of Henle. Characteristics of its epithelial cells change from the early convoluted segment (SI ceils), to the

58 Absorption,Transport,and Retention macula densa efferent ~ arteriole~ afferent ~....~ arterJole'"~./~,t

connecting__ segment tdistal ubule

glomerulum(~l

,..> collecting duct

thick ascending limb

tubule

descending limb

thin ascending limb

loop of Henle Figure 4.3

Theanatomicalorganizationofthenephrons

late convoluted segment and early straight (pars recta) segment ($2 cells), and late straight segment ($3 cells). The apical membrane facing the tubular lumen is folded (microvilli) and covered by protrusions (brush border). The basolateral membrane is adjacent to the interstitium and peritubular capillaries. Tight junctions, strand-like structures, seal the space between the epithelial ceils near the luminal side. The tight junctions of the proximal tubular epithelial cells are more permeable for water and electrolytes than the tight junctions downstream (Gumbiner, 1987). About 100 l/d of water (two-thirds of the amount filtered by the glomeruli) is reabsorbed from the proximal tubule, along with electrolytes, minerals and trace elements, glucose, amino acids, vitamins, and other filtered plasma constituents. The loop of Henle is a hairpin arrangement that extends from the renal cortex into the medulla and consists of the narrow-bore descending and thin ascending limbs followed by the thick ascending limb. Adjacent to the loops are the vasa recta, which start from glomeruli near the cortex-medulla interface, extending into the medulla, and returning into the cortex along with the ascending limb of the loop. The capillaries are fully permeable to small and medium-sized molecules. This is the site where further sodium chloride and water is reabsorbed by countercurrent exchange, lntraluminal osmolality greatly increases towards the tip of the loop, reaching as much as 1200 mosmol/l. High intracellular concentrations of osmolytes, including sorbitoi,

Renal Processing 59

myo-inositol, taurine and betaine, protect the Henle loop epithelia from the potentially disastrous effects of an excessive osmolar gradient. The import of myo-inositol and betaine is mediated by their respective transporters, sodium-dependent myo-inositol transporter (SLC5A3) and betaine transporter BGTI (SLC6AI2, sodium chloride dependent); taurine is taken up via the taurine transporter (TAUT, SLC6A6); sorbitol is produced locally (Bitoun et al., 2001 ). The distal tubule extends from the macula densa (an important site for blood volume regulation) to the connecting segment. The tight junctions between its epithelial cells, as those of the following segments, prevent most uncontrolled paracellular movement of electrolytes from the highly concentrated luminal fluid into blood. Another 5% of the originally fltered sodium chloride is reabsorbed here. Residual amino acids and other complex organic compounds can still be recovered from this nephron segment. The connecting segment contributes to calcium recovery (driven by a calciumtransporting ATPase at the basolateral membrane), absorbs some sodium, and reabsorbs calcium. About two-thirds of cells forming the cortical collecting tubule are principal cells with the ability to further modify the electrolyte content of the luminal contents. Intercalated cells, the remainder of the cells in the cortical collecting tubule, contribute to sodium-independent acid-base balance. Intercalated cells differ from more proximal tubular cells in that their ATPases (proton and potassium-transporting) are located at the luminal rather than the peritubular membrane. The medullary collecting tubule modifies urine composition further by adjusting its water, electrolyte and proton content as needed. Water recovery from the inner medullar segment is under the control of antidiuretic hormone.

Salvage of"complex nutrients Complex nutrients are recovered very efficiently from ultrafiltrate in the proximal renal lumen as long as intake levels are modest and renal function is normal. Some of this reabsorptive activity continues in parts of the distal tubule. The luminal side of the tubular epithelial cells has a brush border membrane with numerous specific transport systems that mediate the uptake of carbohydrates, proteins and amino acids, vitamins and most other essential nutrients. In many instances these are the same systems that also mediate nutrient uptake across the small intestinal brush border membrane. The major driving force for uptake from the lumen into the tubular epithelial cells is the low intracellular sodium concentration that is maintained by sodium/potassium ATPase at the basolatera] membrane of both proximal and distal tubular cells. Additional gradients involved in tubular reuptake include protons, formate, and an electric potential difference which favors inflow of cations. Receptormediated pinocytosis is another important mechanism of concentrative transport. In most cases a distinct set of transporters and channels then mediates the transport out of the epithelial cell across the basolateral membrane, this time mainly driven by the concentration gradient of the transported molecules, by antiport mechanisms, or by active transport. Once they have reached the basolateral intercellular space the

60

Absorption, Transport, and Retention

molecules can then move into the luminal space of peritubular blood capillaries by simple diffusion. Neither the basement membrane adjacent to the tubular cell layer nor the (fenestrated) epithelium of the capillaries constitutes a significant barrier to this last step of solute transfer from tubular lumen to capillary lumen. Carbohydrates: The sugar content of the ultrafiltrate reflects the composition of plasma, since these small molecules are readily filtered. Recovery of D-glucose and D-galactose from the lumen via sodium/glucose cotransporters proceeds with high capacity and low affinity in segments S I and $2 of the proximal tubule, and with low capacity but high affinity in segment $3. Glucose salvage becomes noticeably incomplete (i.e. glucose appears in urine) when the concentration in blood exceeds about 180 mg/dl; this threshold rises as GFR decreases (Rose, 1989, 102-3). Fructose crosses the brush border membrane via its own transporter, GLUT5 (Mate et al., 2001). D-mannose uptake across the brush border membrane proceeds via a sodium-dependent transporter distinct from the sodium-glucose transporters. Its renal recovery is a critical element for the regulation of D-mannose homeostasis (Blasco et al., 2000). All major sugars are transferred across the basolateral membrane by the glucose transporter 2 (GLUT2). Citrate: The sodium/dicarboxylate cotransporter (NaDC-I, SLCI3A2) in the proximal tubule mediates citrate recovery. The efficiency of this process is determined by acid-base balance, increasing with acidosis. Since citrate competes with phosphate and oxalate for binding to calcium, its residual concentration in urine contributes to protection against calcium oxalate and calcium phosphate stone formation (Coe and Parks, 1988). Daily citrate excretion typically is several hundred milligrams (Schwille et al., 1979). Proteins and amino acids: Several specific proteins, including retinol-binding protein, vitamin D-binding protein, transcobalamin-ll, insulin, and lysozyme, are taken up intact by megalin-mediated endocytosis as described in more detail below. Most of the smaller proteins are hydrolyzed by various brush border exoenzymes, including membrane Pro-X carboxypeptidase (EC3.4.17.16) and angiotensin lconverting enzyme (ACE; EC3.4.15.1 ). Two distinct sodium/peptide cotransporters then mediate the uptake of di- and tripeptides, but not of free amino acids. Sodium/peptide cotransporter 1 (PepTl, SLCI5AI) in the S I segment of the proximal tubule has lower affinity for the oligopeptides than sodium/peptide cotransporter 2 (PepT2, SLCI5A2) in the $3 segment (Shen et al., 1999). Neutral amino acids enter epithelial cells mainly via the sodium-dependent neutral amino acid transporters B ~ (Avissar et al., 2001 ), ASCT2, and B ~ Glutamate and aspartate use the EAACI/X Amtransport system. The sodium-dependent transporters GAT- 1 and GAT-3, which are better known for their role in neurotransmitter recovery in brain, ferry gamma-amino butyric acid (GABA), hypotaurine, and beta-alanine across the proximal tubular brush border membrane (Muth et al., 1998). Proline, hydroxyproline, taurine, and beta-alanine are taken up by the sodium-dependent imino transporter (Urdaneta et al., 1998), and betaine enters via the sodium- and chloridedependent betaine transporter (SLC6AI2). Taurine uptake via the taurine transporter (TAUT, SLC6A6) is sodium- and chloride-dependent (Chesney et al., 1990). High

Renal Processing 61

CAJ[TO,

CI ~ f o r m a t e peptidesl~

anion

H-

\ Na*~ A ~ sugars, ~ amino [ }

acids, \ v,amins

Ca2t~Na" ,~--sugars, ~ amino acids, vitamins sugars,y ~ . amino [ acids, \

amino acids-

A,

amino

acids

/

~

I

,.~

.il~amino

\

~

amino acids

proteins, lipids,"k vitamins

)

vitamins~l- ~

" " - - acids

I 3 Na'

Intesti lumen nal (} ~ [Na"] high (

Brushborder membrane

K"

Enterocyte [Na-] low

9j Basolateral membrane

r

1

Capillary lumen [Na' ] high

Capillary endothelium

receptorATP-driven antiport, ion-driven facilitated mediated transport exchanger cotransport transport Figure4.4 Diversemechanismsmediatethe recoveryof nutrientsfrom the proximaltubular lumen concentrations of osmolytes, such as taurine and betaine, protect epithelial cells against the high osmotic pressure in the medulla. Specificity and capacity of the sodium-dependent transporters is expanded considerably by the rBAT (SLC3AI)-linked transporter BATI (SLC7Ag). This transporter,

62 Absorption, Transport, and Retention

which accounts for most, if not all, activity of system b ~ shuttles small and large neutral amino acids across the brush border membrane in exchange for other neutral amino acids. Carnitine enters the cell via the organic cation transporter OCTN2 in exchange for tetraethylammonium or other organic cations (Ohashi et al., 2001 ). Amino acids are utilized to some extent in tubular epithelial cells for protein synthesis, energy production and other metabolic pathways. The case of hydroxyproline is somewhat special, because the kidneys are the main sites of its metabolism, mainly to serine and glycine (Lowry et al., 1985). Hydroxyproline is derived from dietary collagen and from endogenous muscle, connective tissue, and bone turnover. It reaches the mitochondria of the tubular epithelial cells through a translocator that is distinct from that for proline (Atlante et al., 1994). Hydroxyproline is then oxidized by 4-oxoproline reductase (hydroxyproline oxidase; EC1.1.1.104) to 4-oxoproline (Kim et al., 1997). 4-Hydroxy-2-oxoglutarate aldolase (EC4.1.3.16) generates pyruvate and glyoxylate. Glycine is produced when the pyridoxal-phosphate-dependent alanine-glyoxylate aminotransferase (EC2.6.1.44) uses alanine for the amination of glyoxylate.

Table 4.4

Amino acid transporters in the human kidney

Transporters

Apical

In

pepT1 and pepT2 ASC

Na + Na +

B~ BO, +

Na + Na +

TAUT BGT-1 GAT-1 and GAT-3

2Na+CI 3 Na § NaCI

IMINO EAAC1/XAG OCTN 2 y+ CAT

3 Na + Na + (Na § )

BAT1/b ~

ASCT1 TAUT BGT-1/GAT-2 system T (TAT1) asc

y( + )LATI (SLC7A7) +4F2 LAT2 + 4 F2

Betaine

K+

Neutral amino acids

Na +

Na + NaCI NaCI ? Amino acids Neutral amino acids

In

Di- and tripeptides G,A, S,C,T V, I, L,T, F, W, [A, S, C] H, C, R, taurine, beta-alanine, carnitine Taurine, beta-alanine

Na +

+ rBAT

Basolateral A

Out

GABA, hypotaurine, beta-alanine P, OH-P, taurine, beta-alanine D, E Carnitine R, K, ornithine, choline, polyamines K, H, R, E, D, S,T, F, W, G, A, C, V, I, L, P, M, cystine, ornithine A,S,Q G, A, S, C, T Taurine, beta-alanine

Betaine, hypotaurine,

beta-alanine F,Y,W G, A, S, C, T? K, R, H, Q, N, ornithine, choline, orotate Y, F, W,T, N, I, C, S, L, V, Q, [H, A, M, G]

Renal Processing 63

Transport: The main sodium-dependent amino acid transporters of the basolateral membrane are system A (preferentially transports alanine, serine, glutamine) and ASCTI (alanine, serine, cysteine, threonine). Net transfer of individual amino acids importantly depends on their own concentration gradient. As on the luminal side, some transporters operate in exchange mode. Small neutral amino acids are the main counter molecules, because their concentration is the highest. Functional studies have characterized transport system asc for small neutral amino acids, but no corresponding gene or protein has been identified, yet. Glycoprotein 4F2 anchors the amino acid exchangers typical for this side to the basolateral membrane (Verrey et al., 1999). The L-type transporter LAT2 (SLC7A8) accepts most neutral amino acids for transport in either direction. Arginine and other cationic amino acids can pass through related heterodimers; one of these is 4F2 in combination with y(+)LAT1 (SLCA7), another one consists of 4F2 and y(+)LAT2 (SLC7A6). These transporters can exchange a cationic amino acid for a neutral amino acid plus a sodium ion. GAT-2 mediates betaine, beta-alanine, and some taurine transport. The same compounds may also leave via the sodium chloride-dependent taurine transporter (SLC6A6). Urea: One of the major functions of the kidneys is to eliminate the potentially toxic end products of amino acid utilization. As the tubular fluid is concentrated, a urea concentration gradient builds up that drive the passive diffusion of urea across the tubular epithelium into the peritubular blood capillary. This diffusion is little hindered by cell membranes, because these are readily permeable to lipid soluble urea. Due to this reabsorption only about half of the filtered urea (>50g/day) is excreted with urine. A much smaller amount of protein-derived nitrogen is excreted as ammonia. The ammonia can be generated from glutamine by glutaminase and is secreted into the distal ($3) part of the proximal tubulus via the sodium-hydrogen ion antiporter (SLC9A1) in a sodium/ammonium ion exchange mode. Vitamins: The most significant effect of the kidney may be on vitamin D. After it is synthesized in the skin or absorbed from food, vitamin D is converted rapidly to 25-hydroxy-vitamin D (25-OH-D) in the liver. 25-OH-D is secreted into blood where it circulates in association with vitamin D-binding protein (VBP). Owing to its relatively small size, a significant percentage of the complex gets into renal ultrafiltrate. Megalin, a member of the lipoprotein-receptor family, binds VBP and mediates its uptake into the epithelial cells of the proximal tubule. 25-OH-D can then be hydroxylated by mitochondrial vitamin D- 1alpha-hydroxylase (P450c I alpha, CYP27B 1) to 1,25-dihydroxy-vitamin D (1,25-(OH)2-D). Parathyroid hormone (PTH), calcitonin (Shinki et al., 1999), phosphate concentration in blood (Prince et al., 1988), and other factors tightly control the rate of 1,25-(OH)2-D synthesis. In situations of limited vitamin D availability, however, the supply of precursor taken up from the proximal lumen is important. Diminished filtration in patients with end-stage renal disease severely limits vitamin D activation with all the attendant consequences of 1,25-dihydroxy-vitamin D deficiency. Cobalamin uptake from the proximal lumen is mediated by megalin as well. In blood, and hence in the filtrate, transcobalamin II is the cobalamin carrier protein. Additional transcobalamin II appears to be secreted into the proximal tubular lumen, which would ensure maximal recovery. Contrast this with the mechanism for


intestinal absorption, where cobalamin is bound to intrinsic factor and taken up via cubilin. Retinol, which circulates in blood bound to retinol-binding protein (RBP), is another vitamin relying on megalin for salvage from ultrafiltrate. Clara cell secretory protein (CCSP) is a blood transport protein for lipophilic (xenobiotic) substances, including polychlorinated biphenyl metabolites. This versatile carrier, with any ligands that might be attached to it, is extracted from primary filtrate by cubilin. The complex is then targeted towards lysosomes by its coreceptor megalin (Burmeister et al., 2001; Christensen and Birn, 2001 ). Thiamin pyrophosphate is dephosphorylated and the free thiamin taken up from the tubular lumen by a thiamin/H+antiporter with a 1:1 stoichiometric ratio (Gastaldi et al., 2000). Transport across the basolateral membrane uses an as yet uncharacterized ATP-driven thiamin carrier. Similarly, a nucleotide pyrophosphatase (EC3.6.1.9) cleaves several vitamin-derived nucleotides, including NAD, NADP, FAD, and coenzyme A. While this enzyme is certainly expressed in the distal tubule, its presence in proximal tubules has not been reported. The free vitamers (riboflavin, niacin, pantothenate) can be taken up via their respective transport systems. Pantothenate, like biotin and lipoate are taken up from the proximal tubular lumen via the sodium-dependent multivitamin transporter (SLC5A6). Folate is recovered from the proximal tubular lumen by folate receptors; the reduced folate carrier I(SLCI9AI) then completes transport across the basolateral membrane in exchange for organic phosphate (Sikka and McMartin, 1998; Wang et al., 2001 ).

Water, electrolytes, minerals, and trace elements Water: About two-thirds of the filtered water is reabsorbed from the proximal tubular lumen. Much of this water movement is by transcellular cotransport following the sodium-dependent uptake of organic compounds such as glucose. Water can also pass directly between the lumen and the basolateral space by passive diffusion. The tight junctions that seal the intercellular gaps near the luminal side permit more water movement in either direction than in the more distal segments of the nephron (Gumbiner, 1987). The countercurrent principle is responsible for further concentration of the luminal fluid in the descending limb of the loop of Henle. Water from the descending limb follows the sodium and potassium gradient built up by ATPase-driven electrolyte transport on the adjacent ascending limb. Final urine concentration is adjusted by water diffusion out of the distal and collecting tubules. Aquaporins (AQP) are well-defined water channels on the luminal as well as the basolateral side of the tubular epithelium (Nielsen et al., 2002). The kidney expresses at least seven different aquaporins. Their presence and regulated permeability determines whether water can follow a concentration gradient from the tubular lumen into the epithelial cells, and from there into the pericapillary space. Proximal tubules and the descending limbs of Henle's loop contain AQP7 at the luminal side and AQPI at the basolateral side. The ascending limb of the loop is largely devoid of aquaporins and thus not well water permeable. The principal cells of the connecting and the collecting tubules contain AQP2 at the luminal side, regulated by antidiuretic hormone

Renal Processing 65

(arginine vasopressine, ADH), and AQP3 plus AQP4 at the basolateral side. The roles of AQP6 and AQP8 are less well defined. Sodium, potassium, and chloride: The handling of the three main electrolytes in the kidney is intrinsically linked. The sodium-potassium ATPase at the basolateral membrane of most nephron segments pumps three sodium ions from the cell into the intercellular space in exchange for two potassium ions. This pumping action maintains the intracellular sodium concentration much lower than in the tubular lumen. The resulting sodium concentration gradient drives the cotransport for the recovery of glucose, amino acids, and other nutrients in the proximal tubule. At the same time, a large percentage of the filtered sodium is taken up. The dissociation of carbonic acid by carbonate dehydratase (carbonic acid anhydrase; EC4.2.1.1, zinc-dependent) provides the protons, which are exchanged for luminal sodium by the sodium-hydrogen ion antiporter (SLC9AI). The bicarbonate ions, in turn, drive sodium transport across the basolateral side membrane via the sodium/bicarbonate cotransporter (SLC4A4). Chloride is taken up from the proximal tubular lumen in exchange for formate by the formate/chloride exchanger and carried across the basolateral membrane together with a potassium ion by the potassium chloride cotransporter. A smaller proportion of the filtered chloride reaches the basolateral intercellular space directly by diffusion across the tight junctions. As pointed out previously, the tight junctions of the proximal nephron segments are more permeable than those of the more distal segments and allow some movement of electrolytes in either direction, depending on the direction of the prevailing concentration gradient. Sodium chloride flows into the interstitium around the thin ascending limb of Henle's loop by passive diffusion. In the segments distal to the thin ascending limb additional electrolytes are actively recovered. A passive carrier on the luminal side with a sodium-potassium-chloride stoichiometry of 1:1:2 and the sodium-potassium ATPase, in conjunction with potassium channels on the luminal side and chloride channels on the basolateral side, provide for the mechanisms of sodium chloride reabsorption in the thick ascending limb of Henle's loop. Sodium chloride reabsorption in the distal tubule depends on sodium chloride cotransporters on both sides of the epithelial cell and the sodium-potassium ATPase on the basolateral side. Final adjustments to electrolyte composition occur in the collecting tubules. In most situations this final nephron segment provides for net secretion of potassium into urine. Aldosterone in the principal cells of the cortical collecting tubules modulates the permeability of the tight junctions (for the paracellular diffusion of chloride) and of potassium and sodium channels. The intercalated cells of cortical collecting tubules contain a potassium ATPase, which allows for net potassium reabsorption in response to depletion. Calcium: A calcium-binding protein mediates calcium recovery from the lumen of the distal nephron segments between thick ascending limb and the connecting segment. The caicium-ATPase (EC3.6.3.8) at the basolateral side of the tubular epithelium establishes the sodium-independent calcium gradient that drives calcium transfer from the lumen back into the bloodstream. Typically, most of the filtered calcium (>8g/day) is recovered through this mechanism, but the efficiency is largely regulated by hormones and other agents. Most importantly, calcium-ATPase is stimulated


when parathyroid hormone (PTH) binds to the PTH receptor on the basolaterai side of the epithelial cell and triggers a cAMP-mediated signaling cascade. Magnesium: Free ionized magnesium and magnesium complexed with small anions, which constitutes the bulk of circulating magnesium, is freely filtered by the renal glomerulus. Normally, less than 5% of the filtered magnesium is lost with urine. Some of the luminal magnesium is recovered from the proximal tubule, a much greater proportion from the thick ascending limb of Henle's loop and from the distal tubule. Phosphate: The type IIa sodium/phosphate cotransporter of the proximal tubular brush border membrane extracts most (80-95%) phosphate from the ultrafiltrate (Murer et al., 2001 ). PTH modulates fractional phosphate reabsorption by decreasing transporter expression. Much less is known about the mechanisms underlying exit across the basolateral membrane. Several anion exchange pathways are likely to contribute. These pathways also can supply phosphate from blood, if uptake from the tubular lumen becomes insufficient for the cell's own needs. Iodide: The chloride/iodide transporter pendrin (SLC26A4) mediates iodide uptake across the luminal membranes of proximal tubular cells (Soleimani et al., 2001) and intercalated cells of the cortical collecting ducts (Royaux et al., 2001). Iodide may then be pumped by the sodium/iodide symporter across the basolateral membrane into the perivascular space (Spitzweg et al., 2001 ) and re-enter blood circulation.

Active secretion of food compounds Many ingested and absorbed food constituents are actively secreted into urine. The organic anion transporters I (OAT1) and 3 (OAT3) take up acidic xenobiotics from the pericapillary space in exchange for alpha-ketoglutarate and other dicarboxylic acids (Burckhardt and Wolff, 2000). Similarly, the organic cation transporters I (OCT1), 2 (OCT2), and 3 (OCT3) are present in the basolateral membrane of the proximal and distal tubules. The niacin catabolite N-methyl-nicotinamide (NMN) is one of the transported cations. Several active transport systems on the luminal side complete secretion. Multidrug resistance protein 2 (MRP2; ABCC2) transports creatinine, urate, hippurate, ketoacids, and salicylates (Berkhin and Humphreys, 2001). Xenobiotic-cysteine conjugates that have been secreted into the lumen can be cleaved by the PLP-dependent enzyme cysteine conjugate beta-lyase (EC4.4. I. 13). The reaction releases ammonia and pyruvate, which can be reabsorbed. The thio-derivatives of the xenobiotics may then react further, e.g. oxidize, or they are excreted unchanged (van Bladeren, 2000).

Hormones affecting renal function Several key hormones, including ADH, aldosterone, angiotensin II, atrial natriuretic peptide (ANP), prostaglandins, and parathyroid hormone (PTH), control nutrient homeostasis by acting on renal cells. In s o m e instances hormones act by modulating tubular reabsorption or secretion of nutrients or nutrient metabolites, in others by

Renal Processing 67

determining the rate of metabolic transformation of nutrients. A wide variety of additional hormones and hormone-like factors that are not mentioned in the following also may affect renal disposition of nutrients directly or indirectly. ADH is formed in supraoptic and ventricular nuclei and secreted into cerebrospinal fluid, the portal capillaries of the median eminence (which supply the anterior pituitary via the long portal vein), and the posterior pituitary gland. ADH release is governed through osmoreceptors. Through the venous system of the pituitary gland ADH can then reach systemic circulation and act on kidney, blood vessels, and other tissues. ADH activates cAMP-mediated signaling in epithelial cells of collecting tubules, thereby increasing water permeability of the apical membranes. ADH acts mainly by inducing the insertion of AQ2 into the luminal membrane of the ADH-sensitive collecting tubules. The increase in the number of these water-channels facilitates osmotic equilibration and water moves from the lumen into the more concentrated interstitium. With an inadequate level of ADH, urine, for instance in old people, cannot be concentrated properly. Aldosterone is synthesized from cholesterol in the zona glomerulosa of the adrenal cortex. Secretion of the hormone is promoted via the renin-angiotensin cascade in response to low renal perfusion and sympathetic activation. Renin is produced by the iuxtaglomerular cells located in the afferent arterioles of glomeruli near the macula densa. Renin (EC3.4.23.15) is a peptidase that converts angiotensinogen into angiotensin I (comprised often amino acids). Another peptidase, angiotensin-converting enzyme (ACE; EC3.4.15.1 ) generates angiotensin II by removing two additional amino acids. Angiotensin II then promotes aldosterone production in the adrenal gland. Angiotensin II also acts directly on the sodium-proton antiporter in the proximal tubule, which increases sodium reabsorption. Aldosterone acts mainly on connecting segments and collecting tubules. After entering the epithelial cells of those nephron segments aldosterone combines with a cytosolic receptor and binds to DNA in the nucleus. Gene activation by the aldosterone-receptor complex promotes sodiumpotassium ATPase pumping at the basolateral membrane and increases the number of sodium and potassium channels in the luminal membrane. An important consequence of aldosterone action is the increase in potassium content of urine. ANP is produced by myocardial cells in the atria and portions of the ventricles of the heart in response to volume expansion. ANP acts on a specific membrane receptor, which triggers a signaling cascade via the formation ofcGMP. ANP increases glomerular filtration and decreases sodium reabsorption from collecting tubules. Prostaglandins are produced within and outside the kidney by cyclooxygenase from long-chain polyunsaturated fatty acid precursors. PGE2 is a major form synthesized in the tubular epithelium. PGE2 and prostacyclin are produced in glomeruli and vascular epithelium. Amount and type of synthesized prostaglandin is influenced not only by medical drugs (non-steroidal anti-inflammatory agents such as acetylsalicylic acid), but also by intake levels of omega-3 fatty acids (e.g. eicosapentaenic acid in fish oils and flax seed), omega-6 fatty acids (e.g. linoleic acid in many seed oils), and salicylates (in many fruits and vegetables). Prostaglandins limit the activity of ADH and ensure adequate blood flow by easing vasoconstriction of the renal arterioles.

6 8 Absorption, Transport, and Retention

PTH is secreted by the four parathyroid glands in response to low ionized calcium concentration in blood. The stimulating influence of low blood calcium concentration is attenuated by the interaction of 1,25-(OH)2-D with specific receptors in the parathyroid glands (Russell et al., 1986). Binding of PTH to its receptors on tubular epithelial cells triggers a cAMP-initiated signaling cascade. PTH increases calcium reabsorption through activation of calcium-ATPase at the basolateral membrane and it promotes the synthesis of the active form of vitamin D, 1,25-(OH)2-D. At the same time PTH slows phosphate uptake from the proximal tubular lumen via the sodium/phosphate cotransporter. The capillary endothelial cells adjacent to the tubular epithelium in response to low oxygen saturation produce more than 90% of the body's erythropoietin. The main action of erythropoietin is on the proliferation and differentiation of red blood cell precursors. The attempt to overcome anemia resulting from inadequate erythropoietin production (e.g. in renal disease) with increased iron intakes is likely to cause more harm than benefit.

References

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Gumbiner B. Structure, biochemistry, and assembly of tight junctions. Am J Phvsiol 1987;253:C749 Kim SZ, Varvogli L, Waisbren SE, Levy HL. Hydroxyprolinemia: comparison of a patient and her unaffected twin sister. J Pediat 1997; 130:437-41 Lowry M, Hall DE, Brosnan JT. Hydroxyproline metabolism by the rat kidney: distribution of renal enzymes of hydroxyproline catabolism and renal conversion ofhydroxyproline to glycine and serine. Metab Clin Exp 1985;34:955-61 Mate A, de la Hermosa MA, Barfull A, Planas JM, Vazquez CM. Characterization of D-fructose transport by rat kidney brush border membrane vesicles: changes in hypertensive rats. Cell Mol L([e Sci 2001 ;58:196 I-7 Muter H, Hernando N, Forster I, Biber J. Molecular aspects in the regulation of renal inorganic phosphate reabsorption: the type Ila sodium/inorganic phosphate co-transporter as the key player. Curr Opin Nephrol Hypertens 2001 ; 10:555-61 Muth TR, Ahn J, Caplan MJ. Identification of sorting determinants in the C-terminal cytoplasmic tails of the gamma-aminobutyric acid transporters GAT-2 and GAT-3. J Biol Chem 1998;273:25616-27 Nielsen S, Froki~er J, Marples D, Kwon TH, Agre P, Knepper MA. Aquaporins in the kidney: from molecules to medicine. Phvs Rev 2002;82:205-44 Ohashi R, Tamai I, Nezu Ji J, Nikaido H, Hashimoto N, Oku A, Sai Y, Shimane M, Tsuji A. Molecular and physiological evidence for multifunctionality of carnitine/organic cation transporter OCTN2. Mol Pharmaeo12001 ;59:358-66 Prince RL, Hutchison BG, Kent JC. Calcitriol deficiency with retained synthetic reserve in chronic renal failure. Kidney hTt 1988;33:722-8 Rose BD. Clinical Physiolo~,:v of Acid-Base and Electrolyte Disorders. McGraw-Hill, New York, 1989 Royaux IE, Wall SM, Karniski LP, Everett LA, Suzuki K, Knepper MA, Green ED. Pendrin, encoded by the Pendred syndrome gene, resides in the apical region of renal intercalated cells and mediates bicarbonate secretion. P~vc NatlAead Sci USA 2001 ;98:4221~ Russell J, Lettieri D, Sherwood LM. Suppression by 1,25(OH)2D3 of transcription of pre-proparathyroid hormone gene. Endocrinology 1986; I 19:2864-6 Schwille PO, Scholz D, Paulus M, Engelhardt W, Sigel A. Citrate in daily and fasting urine: results of controls, patients with recurrent idiopathic calcium urolithiasis, and primary hyperparathyroidism, hn'est Urol 1979;16:457-62 Shen H, Smith DE, Yang T, Huang YG, Schnermann JB, Brosius FC, 3rd. Localization of PEPT1 and PEPT2 proton-coupled oligopeptide transporter mRNA and protein in rat kidney. Am J Phvsiol 1999;276:F658-665 Shinki T, Ueno Y, DeLuca HE Suda T. Calcitonin is a major regulator for the expression of renal 25-hydroxyvitamin D3-1alpha-hydroxylase gene in normocalcemic rats. Pivc' Natl Aead Sci USA 1999;96:8253-8 Sikka PK, McMartin KE. Determination of folate transport pathways in cultured rat proximal tubule cells. Chemico-Biol hlteraet 1998; 114:15-31 Smoyer WE, Mundel P. Regulation of podocyte structure during the development of nephrotic syndrome. J Mol Med 1998;76:172-83 Soleimani M, Greeley 1", Petrovic S, Wang Z, Amlal H, Kopp P, Burnham CE. Pendrin: an apical CI-/OH-/HCO3- exchanger in the kidney cortex. Am J Ph~wiol - Renal Fluid Eleetlvh,te Phvsiol 2001;280:F356-F364


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The blood-brain barrier Abbreviations BBB

CSF DMT1 GLUT1 Me-I-1 OCT2 SMVT

blood-brain barrier cerebrospinal fluid divalent metal ion transporter I (SLCI IA2) glucose transporter I (SLC2AI) monocarboxylic acid transporter I (SLCl 6AI ) organic cation transporter 2 sodium/multivitamin cotransporter (SLCSA6)

Anatomical background A web of arteries, capillaries, and veins permeates the brain, like every other large organ, supplies it with a constant stream of oxygen and nutrients, and carries away the waste products. However, a blood-brain barrier (BBB) restricts the exchange between blood and the intercellular space in brain much more than in other organs (Pardridge, 1998). Unusually effective tight junctions seal the narrow spaces between the endothelial cells of brain capillaries. These tight junction seals are formed by multiple threaded strands of specialized proteins, including claudin-l, claudin-2, occludin, 7H6, ZO-1, ZO-2, and ZO-3 (Kniesel and Wolburg, 2000; Prat et al., 2001). The foot processes (podocytes) ofastrocytes (microglia) extend to the basal side of the capillary endothelial cells and cover a significant portion of its abluminal surface. The narrow space adjacent to the endothelial cells is occupied by a basement membrane with collagen type IV, laminin, fibronectin, and proteoglycans (Prat et al., 2001 ). A blood-nerve barrier with properties similar to the BBB also separates the major nerves from their blood supply (Alit and Lawrenson, 2000). Another type of interface between bloodstream and brain space is seen in the choroid plexus. These highly convoluted structures in the ventricles produce the

The Blood-Brain Barrier 71

cerebrospinal fluid (CSF), which bathes and internally suspends the brain and the spinal cord and conveys nutrients and waste products around the intracerebral space. The choroidal epithelial cells are polarized cells with a specialized complement of carriers and channels on the luminal and abluminal sides. The spaces between adjacent choroidal cells are sealed much less tightly than those between typical brain capillaries and allow significant movement of water, electrolytes and small molecules through them. Transport from blood into brain can thus follow the more circuitous route of secretion into CSF and later uptake into brain cells. A few other specialized regions (including circumventricular organs, pituitary gland) are exempt from the strict separation of the blood and brain spaces. One particular example is the vascular organ of the lamina terminalis in the hypothalamus. The endothelial cells of the blood capillaries in this region do not maintain tight seals and allow rapid and unimpeded diffusion of water and small solutes (Johnson et al., 1996) to presumed osmosensors on the abluminal side (Stricker and Sved, 2000).

Nutrient transport across the BBB For all practical purposes, nutrients and their derivatives that are to reach the brain have

to be taken up from the capillary lumen into the endothelial cells, traffic to the other side, and exit across the abluminal (basal) membrane. With very few exceptions (gases, some lipophilic compounds), efficient transfer therefore requires the mediation by specific carriers, receptors or channels. Many of those have been identified and extensively characterized, but additional ones are likely to contribute. In some instances, especially at high blood concentrations, a carrier or channel is an important conduit of transfer for a nutrient for which it has much lower affinity than for its dominant substrate. Such 'unspecific' transfer is likely to account for significant residual transport capacity even when the designated dominant transporter of a nutrient is absent (e.g., GLUT I deficiency does not completely abolish glucose transport into brain).

Carbohydrates The glucose transporter 1 (GLUTI) mediates transfer of glucose, which is the predominant energy source in brain. Some cells lining the third ventricle have GLUT2 (SLC2A2). The insulin-inducible carrier GLUT4 (SLC2A4) is only expressed, along with GLUT1, at the BBB of the ventromedial hypothalamus where it may link the bloodstream to glucose-sensing neurons (Ngarmukos et al., 2001 ). In light of the near exclusive dependence on GLUTI for glucose transport into the brain, the consequences of inborn absence of this transporter, which include seizures and developmental delay, appear less catastrophic than one might expect (Boles et al., 1999).

Lipids Lipoproteins, cholesterol, and most fatty acids are effectively excluded from the brain. Long-chain polyunsaturated fatty acids are selectively transported by an unknown


mechanism. It has been theorized that lipoproteins are taken up through particular lipoprotein receptors and essential fatty acids transferred across the abluminal membrane by specific transporters (Edmond, 2001). Fatty-acid derived metabolites are an important energy fuel for brain, particularly with prolonged starvation. The monocarboxylic acid transporter I (MCT 1, SLC 16A 1) carries ketone bodies (and other carboxylic acids including acetate, lactate, pyruvate, propionate, and butyrate) across both sides of brain endothelial cells. About 0.5 nmol/ml/min are transported (Blomqvist et al., 1995). Ketosis strongly induces both MCTI and GLUT! (Leino et al., 2001). Fasting for a few days, which induces ketosis, increases the efficiency of both glucose and beta-hydroxybutyrate transfer across the BBB (Hasselbalch et al., 1995).

Amino acids A specific set of amino acid transporters mediates the transfer of amino acids. While most amino acids are preferentially carried into the brain, transport out of the brain is equally important to remove potentially excitotoxic amino acids, such as glutamate (Hosoya et al., 1999). A few specific proteins (e.g. leptin, insulin) can cross the bloodbrain barrier to a limited extent by endocytosis (Tamai et al., 1997). The L-type transporter 1 (LAT1) is the main conduit for neutral amino acids across both sides of the endothelial cells (Duelli et al., 2000; Killian and Chikhale, 2001). The closely related transporter LAT2 is also expressed at the BBB (Wagner et al., 2001 ). The sodium-dependent system A appears to contribute to some degree to the transfer of neutral amino acids from blood into brain (Kitazawa et al., 2001 ). Arginine and other cationic amino acids can cross via the 4F2-anchored exchange complex y+ LAT2 (Broer et al., 2000). Sodium-linked system N mediates the transfer of glutamate and aspartate into brain, but also the elimination into circulation when there is an excess in brain (Ennis et al., 1998; Hosoya et al., 1999; Smith, 2000). Choline, an amine, is an essential precursor for phospholipid structures of the brain and for the neurotransmitter acetylcholine. The organic cation transporter 2 (OCT2, SLC22A2), which is located at the luminal side of the brain capillary endothelial cell, drives choline transport into brain (Sweet et al., 2001). The mechanism of transport across the abluminal membrane remains unclear. At the same time, the concentration of choline in CSF is kept much lower than in the rest of the interstitial fluid of the brain. This is related to the uptake of choline from CSF via OCT2 and subsequent extrusion into blood circulation by an unknown mechanism.

Vitamins The brain is dependent on adequate supplies of essential nutrients like every other tissue, and vitamins are no exception. Several transporters have been identified that mediate the selective transfer of specific vitamers into brain, but often their exact location is not known. Vitamin C: Oxidized vitamin C (dehydroascorbate, DHA) enters the brain capillary endothelium via GLUTI and is trapped inside by reduction to ascorbate. The


sodium/ascorbate cotransporter 2 (SLC23A 1) then completes transfer into brain (Liang et al., 2001 ). This concentrative transport mechanism sustains a roughly ten-fold higher ascorbate concentration in brain compared to blood. Thiamin: Free thiamin and some TMP crosses the blood-brain barrier (Patrini et al., 1988). The thiamin transporter i (SLCI9A2) is expressed in brain, but its role at the BBB remains to be elucidated. A choline transport system has been described in brain that also appears to mediate thiamin uptake (Kang et al., 1990). Riboflavin: The free form of riboflavin rapidly crosses from blood circulation into brain or in the reverse direction. Riboflavin is trapped inside the endothelial cell as FMN and FAD (Spector, 1980). No specific carriers for riboflavin across either side of the endothelial barrier have been identified, yet. Vitamin B6: While it is clear that the pyridoxal in blood eventually reaches the brain (Sakurai et al., 1991), little is known about the mechanisms involved in this transfer. Biotin, pantothenate, and lipoate: A high-affinity sodium/multivitamin cotransporter (SMVT, SLC5A6) helps to move biotin, pantothenate and lipoate from blood into the brain (Prasad et al., 1998). So far, however, it is not known whether this carrier suffices for transfer across the BBB, or whether additional mechanisms complement its action. Niacin: PET scans have demonstrated the rapid transfer of both nicotinamide and nicotinate into brain (Hankes et al., 1991), but the underlying mechanisms are not well understood. Vitamin B 12: Megalin, which mediates endocytosis of the vitamin B 12/transcobalaminII complex, is present at the luminal side of the BBB (Zlokovic et al., 1996). It seems likely that this member of the LDL-receptor family provides the main route of entry for cobalamin. The actual contribution of this pathway to vitamin B 12 uptake and the mechanism of export across the abluminal membrane remain to be learned. Fat-soluble vitamins: Megalin also is likely to be important for the transfer of vitamins A and D, because it binds retinol-binding protein, vitamin D-binding protein (Christensen and Birn, 2001 ). The concentration of vitamin E in cerebrospinal fluid is one hundredfold lower than in plasma (Pappert et al., 1996) and there is uncertainty how even the small amounts get into brain. High-density lipoprotein (HDL), which is an important carrier of vitamin E in blood, enters brain capillary endothelium via an HDL-binding receptor (Goti et al., 2000). It is not known how the vitamin components are separated from other lipid constituents (which are not transferred into brain) and how they cross the abluminal membrane. HDL-mediated transfer delivers to brain all alpha-tocopherol isomers with similar efficiency. One may assume that vitamin K, which is also carried by HDL (Kohlmeier et al., 1995), also enters brain through this pathway, but this has not been explored, yet. Vitamin E, carotenoids, flavonoids, and similar antioxidants also are important for the protection of the BBB against the damaging effects of oxygen free radicals.

Minerals and trace elements /ron: Brain is critically dependent on adequate iron supplies, but threatened at the same time by the potential toxicity of excessive concentrations. Brain dysfunction in


older people (e.g. Parkinson's disease) is often accompanied by iron accumulation in some brain regions, which may add to the damage. The BBB effectively separates brain iron metabolism from whole body iron metabolism and all proteins involved in iron metabolism are produced within the brain (Rouault, 2001). A tentative model of iron transfer across the BBB might include the following mechanistic components: Iron-carrying transferrin binds to the transferrin receptor 1, the complex enters the endothelial cell through the endocytotic pathway, and the proton-coupled divalent metal ion transporter 1 (DMT1, SLC1 IA2) then extracts iron from the endosomes (Burdo et al., 2001 ). A significant amount of iron may be directly handed to DMT 1 in the adjacent astrocyte foot processes. Lactoferrin may be another vehicle to convey iron across brain capillary endothelial cells, possibly mediated by low-density lipoprotein receptor-related protein. A much smaller amount of iron (and other metals) may reach the intracerebral space through non-barrier sites (Gross et al., 1987). Redistribution of iron within the brain may use transport within axons as well as with CSE The choroid plexus recovers iron from CSF through the transferrin-transferrin receptor pathway. Iron export from brain into blood also involves the membrane protein ferroportin 1 (metal transport protein 1, MTP 1, SLC 11A3), but the precise mechanism remains to be resolved (Burdo et al., 2001). It has been further proposed that melanotransferrin at the luminal side of the BBB, which binds ferric iron, functions as an iron sensor (Rouault, 2001). Excessive iron concentration in blood, particularly of the portion that is not bound to transferrin, can increase oxidative damage to the BBB and increase its permeability. Other metals: Knowledge on the mechanism of copper transfer into and from brain is still rudimentary. Delivery is by ceruloplasmin and albumin and involves DMT1. Active transport by Cu-ATPase is likely to promote removal of copper from the brain (Qian et al., 1998). As with iron, an excess of copper can damage the BBB (Stuerenburg, 2000). Copper as a cofactor of copper/zinc-dependent superoxide dismutase (EC 1.15.1.1 ), on the other hand, helps to maintain BBB integrity (Kim et al., 2001 ). Zinc is an essential cofactor for numerous enzymes in brain, coordinates to regulators of gene transcription (zinc finger proteins), and modulates neurotransmission directly (ionic zinc in vesicles) and indirectly (interaction with GABA and NMDA receptors). The mechanism for zinc uptake into epithelial cells of the BBB is unresolved. It is likely to involve mediation of uptake from blood by a zinc-histidinyi complex and DMT1 (Takeda, 2000). Transport from blood into and out of CSF via the choroid plexus is a major pathway for the maintenance of brain zinc homeostasis, but the molecular mechanisms at that site are not any better resolved than for brain capillary epithelium. Manganese rapidly traverses from blood across the BBB (Rabin et al., 1993), presumably using transferrin, the transferrin receptor 1, DMTI and other elements of the iron transport system.

Xenobiotics Many non-nutrient compounds in foods are absorbed and circulate with blood. Common examples include flavonoids (e.g. naringenin in grapefruit), phenolic compounds


(e.g. catechins in black tea), and indoles (e.g. indolcarbinol in cabbage). The BBB actively prevents the transfer from blood into brain of many of these compounds (Strazielle and Ghersi-Egea, 1999; Miller et al., 2002). Like in other polarized epithelia, xenobiotics are preferentially conjugated to glutathione or glucuronate and then actively pumped across the membrane into the brain capillary lumen. The ATP-driven pumps for xenobiotics extrusion across the luminal membrane may include P-glycoprotein (ABCB1) and Mrp2 (ABCC2).

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Prat A, Biernacki K, Wosik K, Antel JP. Glial cell influence on the human blood-brain barrier. GLIA 2001;36:145-5 Qian Y, Tiffany-Castiglioni E, Welsh J, Harris ED. Copper efflux from murine microvascular cells requires expression of the menkes disease Cu-ATPase. J Nutr 1998;128:1276-82 Rabin O, Hegedus L, Bourre JM, Smith QR. Rapid brain uptake of manganese (II) across the blood-brain barrier. J Neurochem 1993;61:509-17 Rouault TA. Systemic iron metabolism: a review and implications for brain iron metabolism. Ped Neuro12001 ;25:130-7 Sakurai T, Asakura T, Mizuno A, Matsuda M. Absorption and metabolism of pyridoxamine in mice. I. Pyridoxal as the only form of transport in blood. J Nutr Sci Vitaminol 1991 ;37:341-8 Smith QR. Transport of glutamate and other amino acids at the blood-brain barrier. J Nutr 2000; 130:1016S- 1022S Spector R. Riboflavin homeostasis in the central nervous system. J Neurochem 1980; 35:202-9 Strazielle N, Ghersi-Egea JE Demonstration of a coupled metabolism of brain protection toward xenobiotics. J Neurosci 1999; 19:6275-89 Stricker EM, Sved AE Thirst. Nutr 2000; 16:821-6 Stuerenburg HJ. CSF copper concentrations, blood-brain barrier function, and coeruloplasmin synthesis during the treatment of Wilson's disease. JNeural Transm General Section 2000; 107:321-9 Sweet DH, Miller DS, Pritchard JB. Ventricular choline transport: a role for organic cation transporter 2 expressed in choroid plexus. J Biol Chem 2001;276:41611-19 Takeda A. Movement of zinc and its functional significance in the brain. Brain Res Rev 2000;34:137-48 Tamai I, Sai Y, Kobayashi H, Kamata M, Wakamiya T, Tsuji A. Structure-internalization relationship for adsorptive-mediated endocytosis of basic peptides at the blood-brain barrier. J Parmacol Exp Ther 1997;280:410-15 Wagner CA, Lang F, Broer S. Function and structure of heterodimeric amino acid transporters. Am J Physiol Cell Physio12001 ;281 :C 1077-93 Zlokovic BV, Martel CL, Matsubara E, McComb JG, Zheng G, McCluskey RT, Frangione B, Ghiso J. Glycoprotein 330/megalin: probable role in receptor-mediated transport of apolipoprotein J alone and in a complex with Alzheimer disease amyloid beta at the blood-brain and blood-cerebrospinal fluid barriers. Proc Natl Acad Sci USA 1996;93:4229-34

Materno-fetal nutrient transport Abbreviations

BM LATI LAT2 MVM

basal membrane (fetal side) L-type amino acid transporter I (SLC7A.S) L-type amino acid transporter 2 (SLC7A8) microvillous membrane (maternal side)


Nutriture of the early embryo During the first few days of embryonic development nutrients are directly transferred from the mother to the embryo without an intervening structure. Nutrient transfer to the embryo initially proceeds via the amnion fluid of the yolk sac until the placenta forms. The eight-week old embryo already has a well-developed placenta, which by then is the dominant organ for nutrient transfer from maternal blood to fetal circulation.

The mature placenta The placenta, which weighs about 500 g at term, provides the interface between maternal and fetal blood circulation. On the maternal side the placenta is firmly attached to the endometrium and derives its blood supply from the spiral arteries of the uterine wall. The maternal blood flows from arterial openings into the intervillous space of the placenta and returns into the exit openings of maternal veins. A system of finely branched finger-like tissue (villi) is suspended in the intervillous space, bathed in the pool of maternal blood. The villi consist of fetal capillaries in the core covered by a contiguous layer of trophoblast cells. The surface of the placental villi covers about 1 m 2 at the end of the first trimester and as much as 11 m 2 at term (Mayhew, 1996). The capillaries are the end-branches of an arteriovenous system that comes together in two arteries, and a single vein embedded in the umbilical cord. These blood vessels connect directly to the fetal vascular system.

The materno-Fetal barrier Compounds that move from maternal into fetal circulation encounter a layer of syncytiotrophoblast (syntrophoblast), a layer ofcytotrophoblasts, and the endothelial cell layer of the fetal capillaries. During the later months of pregnancy the cytotrophoblast layer becomes incomplete and eventually disappears. The endothelium of the fetal capillaries constitutes no significant barrier. Pores in the interendothelial clefts provide for the relatively unrestricted diffusion of small and large molecules (Michel and Curry, 1999). The syntrophoblast, on the other hand, is fused into a contiguous cell layer without gaps and blocks the transit of any compound that is not transported either through a carrier or channel or by endocytosis. The discussion of materno-fetal nutrient transfer in this section will focus primarily on the syntrophoblast, since this cell layer constitutes the main materno-fetal barrier and is the main site of placental nutrient metabolism later in pregnancy. The maternally facing side (microvillous membrane, MVM) of the syntrophoblast has numerous cell protrusions (microvilli) while the fetal side (basal membrane, BM) is relatively smooth. Due to this structural difference the ratio of microvillous membrane to basal membrane surface areas is typically about 6:1 (Teasdale and Jean-Jacques, 1988).

Materno-fetalNutrientTransport79

Ll/~/kl

J

1114 Maternal intervillous~ . ) space ~

Syntrophoblast

Microvillous membrane

Cytotrophoblasts

Fetal capillary i Capillary epithelium lumen

Basal f membrane

Figure4.5 Structuralorganizationofthehumanplacenta Carbohydrates The glucose transporter 1 (GLUT1) is active at both sides of the syntrophoblast cell layer (Illsley, 2000). This transporter mediates the transfer of both glucose and galactose, but not of other sugars to any significant extent. Expression of GLUT 1 appears to be down-regulated in response to high maternal glucose concentrations only during the first trimester and then stay constant for the remainder of the pregnancy (Jansson et al., 2001). GLUT3 and other additional glucose transporters are also active in placenta, but may be more important for placenta nutriture than for transfer to the fetus.

Lipids The placenta preferentially transports essential polyunsaturated fatty acids (Dutta-Roy, 2000), in particular arachidonic acid (omega-6 fatty acid) and docosahexaenoic acid (omega-3 fatty acid). A wide range of fatty acids is taken up from maternal circulation with iipoproteins. Some metabolic processing of essential fatty acids occurs in the syntrophoblast, in particular the conversion of linoleic acid to arachidonic acid and of eicosapentaenic acid to docosahexaenic acid by chain elongation and introduction of additional double bonds. A placenta-specific fatty acid binding protein preferentially


binds these long-chain polyunsaturated fatty acids (Dutta-Roy, 2000) and passes them on to other binding proteins and eventually to an unknown transporter. The placenta takes up cholesterol from maternal blood with HDL and other lipoproreins (Christiansen-Weber et al., 2000). Endocytotic uptake of maternal HDL is mediated by the intrinsic factor receptor (cubilin), a member of the LDL-receptor protein family (Kozyraki et al., 1999). The placenta also contains the LDL receptor (LDL-R), the acetylated-LDL receptor, the apoE receptor, the VLDL receptor, the scavenger receptor class B type I, and megalin. Significant additional amounts of cholesterol may enter the syntrophoblast without the mediation of receptors (Wyne and Woollett, 1998). Steroid hormone synthesis in the placenta is the main use for cholesterol taken up from maternal blood. The ATP-driven transporter ABCA 1 carries some cholesterol across the BM to the fetus (Christiansen-Weber et al., 2000).

Amino acids Net transfer: Because of the very rapid growth of the fetus transfer of amino acids is of greatest importance. Amino acids are used extensively by the fetus, and by the placenta itself, as building blocks for protein and peptide synthesis and as an important energy source. Leucine, isoleucine, valine, serine, and glutamine constitute the bulk of net amino acid uptake by the placenta from maternal circulation (Cetin, 2001). Other amino acids are taken up at more modest rates. At the other end of this spectrum is glycine, which plays a major role in placental metabolism, but of which there is little net transfer. Placental metabolism: Amino acid metabolism in the placenta is very active, providing energy and protein precursors for the growth and sustenance of this crucial organ. The placenta is also the major site for synthesis ofglycine from serine, both from maternal and fetal circulation (Cetin, 2001 ). Transporters: The interior of the syntrophoblast cell is electronegative in relation to the outside. This electric potential difference and the sodium gradient are the driving forces for concentrative amino acid transport from the maternal to the fetal side of the placenta. Sodium-dependent transport systems A (for alanine, serine, cysteine, methionine, praline, asparagine, and glutamine), B ~ (for valine, isoleucine, leucine, threonine, phenylalanine, and tryptophan), and to a slightly lesser extent ASC (for glycine, alanine, serine, cysteine, threonine) transport neutral amino acids across both MVM and BM (Jansson, 2001 ). Two distinct system A transporters have been found in placenta, ATAI and ATA2, but their localization is still uncertain. Indeed, ATA2 expression is higher in placenta than in all other tissues tested (Hatanaka et al., 2000), pointing to the special importance of this transporter as the driving force for all materno-fetal amino acid transport. Branched-chain (valine, leucine, and isoleucine) and aromatic amino acids (tryptophan, phenylalanine and tyrosine, also the thyroid hormone triiodothyrosine), which these bulk transporters do not accept, cross the MVM via LATI (Ritchie and Taylor, 2001 ), and the BM via LAT2 (Kudo and Boyd, 2001 ). LATI and LAT2 transport their substrates in exchange for other neutral amino acids and are not dependent on sodium

Materno-fetal Nutrient Transport 81

or proton gradients. The main driving force is thus the concentration gradient of the neutral bulk amino acids that is established by the sodium-dependent transporters. Transport system beta imports mainly beta-alanine and taurine. The cationic amino acids arginine and lysine, as well as choline and polyamines, are carried across the MVM by several members of the system y§ (CAT-1/SLC7AI, CAT-4/ SLC7A4, and CAT-2B/SLC7A2). The heterodimer formed by y+LAT1 (SLC7A7) and 4F2 (SLC3A2) carries them across the BM (Kudo and Boyd, 2001 ). Several members of transport system XA~, including EAAT 1 (SLC 1A3), EAAT2 (SLC 1A2), EAAT3 (SLC I A 1), and EAAT4 (SLC 1A6), mediate glutamate and aspartate uptake from maternal blood. Because these acidic amino acids are metabolized extensively within the placenta, there is no net significant transfer to the fetus (Jansson, 2001 ) The placenta also contains TAT1, a transporter with system T characteristics, which mediates concentration-driven transfer of phenylalanine, tyrosine, and tryptophan (Kim et al., 2001).

Vitamins Several of the water-soluble vitamins have their own specific transporters that ensure the transfer of these essential nutrients to the fetus, possibly even at the mother's expense. Information on others is still lacking. The fat-soluble vitamins tend to be taken up from maternal blood with lipoproteins and are exported specifically to the fetal side. Vitamin C: GLUT1 accepts the predominant form of vitamin C in blood, dehydroascorbate, as a substrate. The reduced form, ascorbate, can enter via sodiumdependent ascorbate transporter 1 (SVCT1). Most dehydroascorbate is reduced inside the syntrophoblast layer and ascorbate is then exported to the fetal side via SVCT2. Thiamin: Two specific transporters contribute to concentrative thiamin transfer to the fetus, but the exact locations and mechanisms remain to be resolved (Dutta et al., 1999; Rajgopal et al., 2001). Both the thiamin transporter 1 (SLC19A2) and ThTr2 (SLC19A3) are abundantly expressed in placenta. Riboflavin: Alkaline phosphatase has to cleave the ravin-adenine dinucleotide (FAD) in maternal blood before the free riboflavin can be taken up into the syntrophoblasts via an as yet unknown carrier. The riboflavin carrier protein (RCP) is an intracellular protein that is essential for riboflavin transport across the syntrophoblast (Schneider, ! 996). Niacin: The information on transfer of niacin to the fetus is quite limited. Since little maternal nicotinate reaches the fetus (Baker et al., 1981 ), the transfer of other metabolites must occur. Vitamin B6: Pyridoxal enters the syntrophoblast by an unknown mechanism. The free pyridoxal is phosphorylated to pyridoxal 5'-phosphate and then exported to the fetal side by another unknown mechanism (Schenker et al., 1992). Folate: Folate receptors in the intervillous blood space bind 5-methyltetrahydrofolate and concentrate it on the maternally facing chorionic surface. The resulting concentration gradient drives folates across the placental barrier (Henderson et al., 1995). The folate receptors FR 1 and FR2 as well as the reduced folate carrier (SLC 19A 1) are expressed in placenta.


Biotin, pantothenate, lipoate: The sodium/multivitamin cotransporter (SMVT, SLC5A6) operates at the MVM as the exclusive carrier for biotin, pantothenate, and lipoate (Prasad et al., 1998). There is still uncertainty about the mechanism responsible for extrusion across the BM.

Minerals and trace metals /ron: Over the course of a healthy pregnancy, about 500 mg of iron is shuttled from the mother to the fetus. Transferrin receptors at the MVM bind iron-carrying transferrin from maternal blood and enter the syntrophoblast with endosomes. The divalent metal ion transporter 1 (DMT 1) pumps iron out of these endosomes and across the BM (Georgieff et al., 2000). Another pathway is likely to use the integrin ferroportin-1 at the MVM (Donovan et al., 2000), iron oxidation by an incompletely characterized placental copper oxidase (not identical with ceruloplasmin), and ferric iron transfer to fetal transferrin. Maternal iron deficiency increases the efficiency of iron transfer to the fetus by increasing expression of copper oxidase, transferrin receptor (Gambling et al., 2001), and other elements of the iron transfer apparatus. The role of another placental irontransport protein, uteroferrin (a lysosomal glycoprotein with tartrate-resistant alkaline phosphatase type 5 properties), is uncertain (Laurenz et al., 1997). Calcium: Late in pregnancy large amounts of calcium are transported across the placenta. Most (80%) of total fetal accretion occurs during the last trimester. Maternal ionized calcium enters the syntrophoblast through calcium channels and binds to intracellular proteins, including 9 kDa calcium binding protein (calbindin, 9CBP). The net flow of calcium is driven by calcium ATPase at the BM. Several components of the calcium transport machinery, including MVM calcium channels, calbindin, and the calcium-ATPase, are induced by 1,25-dihydroxyvitamin D (Hoenderop et al., 1999). Xenobiotics: The ATP-binding cassette transporter ABCP (MXR, BCRP, ABCG2) is a xenobiotic transporter which is highly expressed in the placenta (Allikmets et al., 1998).

References Allikmets R, Schriml LM, Hutchinson A, Romano-Spica V, Dean M. A human placentaspecific ATP-binding cassette gene (ABCP) on chromosome 4q22 that is involved in multidrug resistance. Cancer Res 1998;58:5337-9 Baker H, Frank O, Deangelis B, Feingold S, Kaminetzky HA. Role of placenta in maternalfetal vitamin transfer in humans. Am J Obstet Gynecol 1981; 141:792-6 Battaglia FC, Regnault TR. Placental transport and metabolism of amino acids. Placenta 2001 ;22:145-61 Cetin I. Amino acid interconversions in the fetal-placental unit: the animal model and human studies in vivo. Pediatr Res 2001 ;49:148-53 Christiansen-Weber TA, Voland JR, Wu Y, Ngo K, Roland BL, Nguyen S, Peterson PA, Fung-Leung WE Functional loss ofABCA1 in mice causes severe placental malformation, aberrant lipid distribution, and kidney glomerulonephritis as well as highdensity lipoprotein cholesterol deficiency. Am J Patho12000; 157: l 017-29

Materno-fetal Nutrient Transport 83

Donovan A, Brownlie A, Zhou Y, Shepard J, Pratt SJ, Moynihan J, Paw BH, Drejer A, Barut B, Zapata Z, Law TC, Brugnara C, Lux SE, Pinkus GS, Pinkus JL, Kingsley PD, Palis J, Fleming MD, Andrews NC, Zon LI. Positional cloning of zebrafish ferroportinl identifies a conserved vertebrate iron exporter. Nature 2000;403:776-81 Dutta B, Huang W, Molero M, Kekuda R, Leibach FH, Devoe LD, Ganapathy V, Prasad PD. Cloning of the human thiamine transporter, a member of the folate transporter family. J Biol Chem 1999;274:31925-9 Dutta-Roy AK. Transport mechanisms for long-chain polyunsaturated fatty acids in the human placenta. Am J Clin Nutr 2000;71:315S-22S Gambling L, Danzeisen R, Gair S, Lea RG, Charania Z, Solanky N, Joory KD, Srai SK, McArdle HJ. Effect of iron deficiency on placental transfer of iron and expression of iron transport proteins in vivo and in vitro. Biochem J 2001;356:883-9 Georgieff MK, Wobken JK, Weile J, Burdo JR, Connor JR. Identification and localization of divalent metal transporter-I (DMT-I) in term human placenta. Placenta 2000; 21:799-804 Hatanaka T, Huang W, Wang H, Sugawara M, Prasad PD, Leibach FH, Ganapathy V. Primary structure, functional characteristics and tissue expression pattern of human ATA2, a subtype of amino acid transport system A. Biochim BiophysActa 2000; 1467:1~5 Henderson GI, Perez T, Schenker S, Mackins J, Antony AC. Maternal-to-fetal transfer of 5-methyltetrahydrofolate by the perfused human placental cotyledon: evidence for a concentrative role by placental folate receptors in fetal folate delivery. J Lab Clin Med 1995; 126:184-203 Hoenderop JG, van der Kemp AW, Hartog A, van de Graaf SK van Os CH, Willems PH, Bindeis RJ. Molecular identification of the apical Ca2+ channel in 1,25-dihydroxyvitamin D3-responsive epithelia. J Biol Chem 1999;274:8375-8 Illsley NP. Glucose transporters in the human placenta. Placenta 2000;21:14-22 Jansson T. Amino acid transporters in the human placenta. Ped Res 2001 ;49:141-7 Jansson T, Ekstrand Y, Wennergren M, Powell TL. Placental glucose transport in gestational diabetes mellitus. Am J Obstet Gyneco12001;184 :111-16 Kim DK, Kanai Y, Chairoungdua A, Matsuo, Cha SH, Endou H. Expression cloning of a Na+-independent aromatic amino acid transporter with structural similarity to H +/monocarboxylate transporters. J Biol Chem 2001 ;276:17221-8 Kozyraki R, Fyfe J, Kristiansen M, Gerdes C, Jacobsen C, Cui S, Christensen El, Aminoff M, de la Chapelle A, Krahe R, Verroust PJ, Moestrup SK. The intrinsic factorvitamin B l2 receptor, cubilin, is a high-affinity apolipoprotein A-I receptor facilitating endocytosis of high-density lipoprotein. Nature Med 1999;5:656-6 l Kudo Y, Boyd CA. Characterisation of L-tryptophan transporters in human placenta: a comparison of brush border and basal membrane vesicles. J Physiol 2001;531: 405-16 Laurenz JC, Hadjisavas M, Schuster D, Bazer FW. The effect of uteroferrin and recombinant GM-CSF on hematopoietic parameters in normal female pigs (Sus scrofa). Comp Biochem Physiol Part B 1997; l 18:579-86 Mayhew TM. Patterns of villous and intervilious space growth in human placentas from normal and abnormal pregnancies. Eur J Obstet Gynecol Reprod Biol 1996; 68:75-82

I]4 Absorption, Transport, and Retention

Michel CC, Curry FE. Microvascular permeability. Physiol Rev 1999;79:703-61 Prasad PD, Wang H, Kekuda R, Fujita T, Fei YJ, Devoe LD, Leibach FH, Ganapathy V. Cloning and functional expression of a cDNA encoding a mammalian sodiumdependent vitamin transporter mediating the uptake of pantothenate, biotin, and lipoate. J Biol Chem 1998;273:7501-6 Rajgopal A, Edmondson A, Goldman ID, Zhao R. SLCI9A3 encodes a second thiamine transporter ThTr2. Biochim Biophys Acta 200 ! ; 1537:175-8 Ritchie JW, Taylor PM. Role of the System L permease LATI in amino acid and iodothyronine transport in placenta. Biochem J 2001 ;356:719-25 Schenker S, Johnson RE Mahuren JD, Henderson GI, Coburn SP. Human placental vitamin B6 (pyridoxai) transport: normal characteristics and effects of ethanol. Am J Physiol 1992;262:R966-74 Schneider WJ. Vitellogenin receptors: oocyte-specific members of the low-density lipoprotein receptor supergene family. Int Rev Cytol 1996; 166: i 03-37 Teasdale E Jean-Jacques G. Intrauterine growth retardation: morphometry of the microvillous membrane of the human placenta. Placenta 1988;9:47-55 Wyne KL, Woollett LA. Transport of maternal LDL and LDL to the fetal membranes and placenta of the golden Syrian hamster is mediated by receptor-dependent and receptor-independent processes. J Lipid Res i 998;39:518-30

Xenobiotics

Heterocyclicamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flavonoids and isoflavones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Garlic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

85 92 106

Heterocyclic amines Heterocyclic amines (HAs), also heterocyclic aromatic amines (HAAs), include many carcinogenic compounds produced during cooking. Abbreviations

NAT CYP DiMelQx HA IQ MDR MelQx MRP PEITC PhlP Trp-P-1 Trp-P-2 UGT

arylamine N-acetyltransferase cytochrome P450

2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline

heterocyclic amine 2-amino-3-methylimidazo[4,5-f]quinoline multidrug resistancegene 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline multiclrug resistanceprotein beta-phenylethyl isothiocyanate 2-amino-1-methyl-6-phenylimidazo[4,5-/3] pyridine 3-amino-l,4-dimethyl-SH-pyrido[4,3-/3]indole 3-amino-l-methyl-5H-pyrido[4,3-~]indole UDP-glucuro nosyltransferase

Nutritional summary Function: Heterocyclic amines (HA) are potent mutagens and potential carcinogens. Food sources: Cooking o f flesh foods, including beef, pork, poultry, and fish, produces microgram amounts o f HAs per serving from the pyrolytic rearrangement ofcreatine and amino acids. Requirements: Beneficial effects o f intakes at any level are not likely. Handbook of'Nutrient Metabolism ISBN: O-12-417762-X

Copyright c' 2003 Elsevier Ltd All rights of reproduction ill any tbrm reserved

86

Xenobiotics

/CH3

H3C~N L,,~N

H3 N ~ N ~ II~.~NJ,

DiMelQx

CH3

NH2

.. MelQx

CH3 NaC

/CH~

~ ~ I Q N d ~NH2 H Trp-P-1 Figure

5.1

Heterocyclic amines in cooked meat

=L..~N

?H,

NH2 HN ~

OH"r

NH2

L-phenylalanine + Creatine

?H OH,

~O==~..~N HO... ..)L,,.

//

O//~,/" "NH2 HN

L-Tyrosine + Creatine

LN L NH PhlP Figure

5.2

4'-hydroxyPhlP

Cooking of meat generates PhlP

People with even modest HA intake from cooked meat and fish have more DNA adducts in tissues and a greater risk to develop cancers at various sites. Excessive intake:

Dietary sources Frying, broiling, and other forms of heating meats generates a wide variety of creatinederived carcinogens (Schut and Snyderwine, 1999; Wyss and Kaddurah-Daouk, 2000). One of these is 2-amino-1-methyi-6-phenylimidazo[4,5-/3] pyridine (PhlP), generated by the pyrolysis of creatine in the presence of phenylalanine, threonine or tyrosine; another is 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MelQx) derived from

Heterocyclic Amines 87

creatine, glycine and glucose (Oguri et al., 1998). Cooked chicken was found to contain 18-21 i~g PhlP per 200 g serving (Kulp et al., 2000). Intakes of PhlP have been estimated to range from nanograms to tens of micrograms per day, depending on flesh food consumption and preparation methods (Layton et al., 1995).

Digestion and absorption The site, mechanism, and effectiveness of HA uptake from the intestinal lumen is incompletely understood. More is known about several transporters that pump HAs back into the lumen, some of it from circulating blood. At equal concentrations the basolateral to apical efflux is several-fold greater than the apical to basolateral influx (Walle and Walle, 1999). The ABC transporter MRPI (multidrug resistance protein 1) pumps its substrates across the basolateral lumen into the enterocyte (Walle and Waile, 1999). P-glycoprotein, the product of the MDR-I gene (Waile and Walle, 1999; Hoffmeyer et al., 2000), MRP2 (Walgren et al., 2000), and possibly MRP3, pump HAs (and many other xenobiotic compounds) across the apical enterocyte membrane into the intestinal lumen.

Metabolism Metabolism of xenobiotics often involves two types of metabolic processes, mainly in the liver, to a lesser extent in kidney and other organs. The first, activating reactions, are referred to as phase I reactions. Phase I reactions most often involve hydroxylation or oxidation of the compound; a variety of other reactions are also possible. The second type, the phase II reactions, include conjugating and other modifying reactions. These molecular modifications tend to make the original compound more polar and enhance their renal excretion. Phase I reactions often increase the reactivity of xenobiotics towards DNA and proteins. Phase II enzymes, on the other hand, tend to decrease the potential for harm by speeding up their elimination. However, this is not universally the case. Since phase 1 enzymes activities are a prerequisite for phase II-dependent modification the concerted and balanced action of members from both classes minimize risk. What is more, some phase ll-catalyzed reactions (e.g. sulfotransferase-catalyzed generation of highly reactive N-sulfonyloxy HA derivatives) actually increase the potential for harm. Many of the enzymes that catalyze either phase I or phase lI enzymes vary greatly in their activity due to differences in genetic makeup, gender, age, or exposure to these or other compounds (Williams, 2001 ). Phase ! reactions: The initial step of HA metabolism, as of most xenobiotics, usually occurs in the liver and is mainly hydroxylation by microsomal cytochrome P450 isoenzyme 1A2 (CYP 1A2; EC 1.14.14.1 ). In peripheral tissues the isoenzymes CYP 1A 1 and CYP! B i catalyze the same reaction (Gooderham et al., 2001 ). Reactions catalyzed by epoxide hydrolases (EC3.3.2.3, microsomal and cytosolic isoenzymes), numerous peroxidases, and other enzymes may contribute to a lesser degree in liver, small intestines

88 Xenobiodcs

Readily excreted metabolites

l i ~ ~ N

~

Especially genotoxic metabolites

CH3

/

H

.C[OOH /O'I~TL~OH H

HO

, ,>- H HoH'/ oH "~-'--N" 0 H

N2-hydroxyPhlPN2-glucuronide ?H3

"~>N/

~"~..~T1A1

N--OH

~NxI-O, H

SULT1A2.SULT1A3

..................U..GT.I.A.9 .......

N--o.

N2-hydroxyPhlPN2-glucuronideH~/X~.. ,., H OHOH /

N

CH^ a

S

H H .CI3OH

~ I ~ o H

'

~N

I'

, H PhlP N3-glucuronideH~ --~/k H ,.~1;:1,,, H O t ~ \ / ~ ~/~un H~ctH H

um~'

HOO!C"x Hn HO..~'~"Y'r~

"~

UGT

h? "h

............. ~

IH" OH~'I L ~ - - ~ ~ - - d v T/ y

.'-hydroxyPh,P0,ucu.on,.e l i ~

/

~..

o~,~

I

extrahepatic

IlL ~ ~ ......

-

CYPIA2 hepatic [ / 1

y

A

/

NAT

N/

.... CYPIA1 (1B1)I

%A../--.~

,CH3

_

"~N /

II

O. ~

,/'""

.I

H O ~

H

r~

HO /~ "~/S/~'/~.,

o"

N

"1

O.

~"

....~ 13-Hydroxyocta9,11-dienyl

Figure 6.15 OxygenFreeradicalsreactwith unsaturatedFattyacids

(

P h ' L~in~~

" OH

~

~

-

....

~

Linoleyl radical

Structure and Function of Fatty Acids 133

O II

OH

H

o,,

C--O--CH O

CH 3

II

HO--CH-O--P--C--C--N--CH I

H2

OH 4-Hydroxynonenalcontaining phospholipid

H2

I

3

OH3

OH

Paraoxonase (calcium) H

4-Hydroxynonenal

O II

HO--CH O OH3 II HO--CH-O-- p--C--C--N--CH 3 I

OH

H2

H2

I

CH 3

Lysophospholipid

Figure 6.16

Metabolism of oxidized fatty acids

oxaloacetate. The citrate from this reaction can then be metabolized further providing FADH, NADH, and succinate for oxidative phosphorylation and ATP or GTP from succinyI-CoA. Ketogenesis: The production rate ofacetyI-CoA from fatty acid beta-oxidation in the liver with prolonged fasting often exceeds the capacity of the Krebs cycle. The coenzyme A for continued beta-oxidation and other functions can be released through the production of acetoacetate in three steps. Enzymic reactions convert excess mitochondrial acetyl-CoA either to acetoacetate or 3-hydroxybutyrate (the main product); acetoacetate can spontaneously decarboxylate to acetone. The term 'ketone bodies' for all three products is misleading, but continues to be widely used. The acetyl-CoA condensation sequence flees up CoA for continued breakdown of fatty acids, mainly in the liver during extended periods of fasting or in situations of abnormally high lipolysis (as in diabetes due to low insulin concentration). Since the reactions occur in mitochondria, ketogenesis regulation is independent of the same reactions for cholesterol synthesis in cytosol. The typical odor of a fasting individual is partially related to exhaled acetone formed from acetoacetate. The conversion of acetoacetate into beta-hydroxybutyrate taxes the body's acid-buffering capacity and may cause a drop

134 Fatty Acids

O II

CoA-S--C--CH 3 AcetyI-CoA AcetyI-CoA C-

acetyltransferase

~ CoA

O O II II CoA--S--C--C--C--CH 3

H2

AcetoacetyI-CoA

synthase

k

0

OH

II

I

H2

CoA - - S - - C - - C - - C - - C - - C \ H2

J OH 3

//

0

/

OH

3-Hydroxy-3-methylglutaryI-CoA

~

HMG-CoA

lyase

NAD

HO

, ,,

H3C --C - - C - - C \ H OH

I~-hydroxybutyrate

Figure 6.17

NADH

L J_

3-Hydroxybutyrate

dehydrogenase

C~

AcetyI-CoA

H§

O

,, ,,

H3C --C - - C - - C \

Acetoacetate

O II

OH

CO 2

\ 2_

non-enzymic-

O

"

H3C --C --CH 3

Acetone

Metabolism of 'ketone bodies'

in blood pH (acidosis) in diabetics and similarly susceptible patients. None of these events is related to dietary intake of acetate in typical quantities. AcetyI-CoA C-acetyltransferase (thiolase, EC2.3.1.9)joins two acetyl-CoA molecules, and hydroxymethylglutaryl-CoA synthase (HMG-CoA synthase; EC4.1.3.5) adds another one. The mitochondrial isoform of HMG-CoA synthase is genetically distinct from the cytosolic one, which generates the precursor for cholesterol synthesis. Hydroxymethylglutaryl-CoA lyase (HMG-CoA lyase, EC4.1.3.4)finally generates acetoacetate by cleaving offacetyI-CoA from the HMG-CoA intermediate. Spontaneous decarboxylation of acetoacetate generates the dead-end product acetone. Acetoacetate can also be reduced to beta-hydroxybutyrate by NADH-dependent 3-hydroxybutyrate dehydrogenase (ECI. 1.1.30). This enzyme is allosterically activated by phosphatidyl choline. The reaction is fully reversible. Net flux depends on substrate


CoA- S - -

0 II

C --

~H3 CH 2

PropionyI-CoA /

I f " ATP + H2CO 3 PropionyI-CoA carboxylase (biotin, magnesium) i,,,,~ADP + Pi

L

O CoA-S --

II

(~H3

C--C--COOH H D-MethylmalonyI-CoA

~

acetyl-

Oxalo-

acetate

Malate

racemase

CoA-S - -

Citrate

/

maM:t;YlcoA i

~A

\

Fumarate

1

Isocitrate

Succinate

0 II

~

C--CH --COOH

I

CH 3 L-MethylmalonyI-CoA

MethylmalonyI-CoA mutase (adenosylcobalamin)

o,

CoA-S --C--C

~-Ketoglutarate

/ --C--COOH H2 H2

SuccinyI-CoA

Figure 6.18 The metabolismof propionyI-CoAfrom odd-chain fatty acids concentrations. Acetoacetate and beta-hydroxybutyrate (but not acetone) can become a significant energy fuel for brain after several days of adaptation to starvation conditions. PropionyI-CoA metabolism: For continuation of its breakdown, propionyl-CoA has to be ferried from peroxisomes into mitochondria where the biotin-containing enzyme propionyi-CoA-carboxylase (EC6.4.1.3) adds a carbon, methylmalonyl-CoA converts D-methylmalonyl-CoA into the L-form, and methylmalonyl-CoA mutase (EC5.4.99.2, contains 5'-deoxyadenosylcobalamin) finally produces the Krebs cycle metabolite succinyI-CoA.

Excretion Normally neither free fatty acids nor fatty acid-containing compounds are excreted via urine. Excretion with bile is minimal, mainly as a component of phospholipids that are readily reabsorbed from the small intestinal lumen. Thus, almost no fatty acid is lost from the body once it has been absorbed.

Storage Fat is well suited for storage. It has by far the highest energy density of all nutrients and requires the smallest possible space. The lack of electric charges and insolubility

136

FattyAcids H2i--O HO--CH

o

H2C--o--P--OH I OH Glycerol-3-phosphate Glycerol3-phosphate I f O-acyl- [ transferase

C~

O AcyI-CoA

b CoA

O II .~i-~

NO--CN

O

o

H2C--O--P--OH I OH 1-Acyl-sn-glycerolphosphate II

1-Acylglycerol3-phosphate acyltransferase

II

C~

AcyI-CoA

CoA O II

,2c-o-cvvvVVVkA I

i

9,

"-o-co,A / V V ~ V V ~ A / II

H2C--O--P--OH OH phosphate

1,2-Diacyl-sn-glycerol

Phosphatide phosphatase

~ H20 Pi

O II H2(~--O--C, A A A A A A A

I

o,V V

v v v v v,

i H-O- c ~ V V V ~ V ~ A / H2C--OH 1,2-Diacyl-sn-glycerol Figure 6.19 Fat storage

AcyI-CoA

\ )

CoA

Diacylglycerol O-acyltransferase

O II H2C--O--qA A A A A A A

I

,o/VVVVVV,

CH--O--C A A A / ~ k A A A / ioiV V V V V V V V

H2C--o-- C ~ A / ~ v ~ A A Triacyl-sn-glycerol

/


in water means that it does not exert osmotic pressure and is chemically quite inert. Adipocytes are specialized cells that are able to store large amounts of fatty acids as triglyceride. Fatty acids in adipocytes and other tissues are converted into acyI-CoA by several ligases specific for short-chain, medium-chain, and long-chain fatty acids as outlined above. Triglyceride synthesis occurs at the cytosolic face of the microsomal membrane in most tissues, most extensively in adipose tissue, liver, and muscle. The initial steps can also lead to phospholipid synthesis. Only the last step, that adds a third fatty acid, commits the fatty acids to triglyceride synthesis. Glycerol-3-phosphate O-acyltransferase (EC2.3.1.15) links the first acyI-CoA to glycerol-3-phosphate. An alternative peroxisomai pathway with glycerone-phosphate O-acyltransferase (dihydroxyacetone phosphate acyltransferase, EC2.3.1.42), which supports mainly the synthesis ofetherlipids and plasmalogens, uses dihydroxyacetone phosphate as the initial fatty acid acceptor. In this case the product is converted to l-acylglycerol-3-phosphate by acylglycerone-phosphate reductase (acyldihydroxyacetone phosphate reductase, EC 1.1.1.101 ). 1-Acylglycerol-3-phosphate acyltransferase (EC2.3.1.51) then adds a second activated fatty acid. At least five different genes code for this enzyme, and additional isoforms arise from alternative splicing. Phosphatidate phosphatase (EC3.1.3.4), the regulatory enzyme of triglyceride synthesis, removes the phosphate group. Triglyceride synthesis is completed when diacylglycerol O-acyltransferase (EC2.3.1.20) adds a third fatty acid. A wide range of fatty acids can be incorporated at positions 1 and 3, but palmitate is preferred. The second fatty acid is often unsaturated. Overall, the fatty acid composition reflects long-term fatty acid intake patterns (Kohimeier and Kohlmeier, 1995). Stored triglycerides can be released again into blood circulation by the combined activity of hormone-sensitive lipase and monoglyceride lipase. Activity of hormonesensitive lipase is under hormonal and neuronal control through the cAMP-mediated phosphorylation of serine 563; the enzyme is activated by adrenaline (epinephrine) and inactivated by insulin.

Regulation Intake regulation: Adipose tissue produces several hormone-like factors and cytokines, some of which signal fat content to the brain and other tissues. Leptin crosses the blood-brain barrier and stimulates neuropeptide Y (NPY) secretion in brain. NPY induces satiety and thereby slows food-seeking behavior. Leptin also affects numerous metabolic processes in other tissues. The rate of fat utilization by muscle depends to a considerable extent on the rate of uptake, which is decreased by leptin (Steinberg et al., 2002). Fat storage: Energy metabolism depends to a large extent on fat stores to buffer variation in the availability of fuel energy from dietary sources. Storage and mobilization are largely under the control of insulin, glucagon, and adrenaline. Insulin slows the mobilization of fatty acids from adipocytes by inhibiting hormone-sensitive lipase (through cAMP-mediated phosphorylation by protein kinase A). Insulin also promotes fatty acid synthesis, though net production seems to be minor, as pointed out above. Glucagon and adrenaline slow fatty acid synthesis. Sterol regulatory element-binding proteins

138 Fatty Acids

(SREBPs) mediate some of the effects by modulating the expression of genes involved in fatty acid synthesis. The more important adrenaline effect is, however, its strong promotion of lipolysis. Peroxisomal metabolism: Fatty acid oxidation in peroxisomes, which may help to cope with excess, is regulated by the nuclear peroxisome proliferator-activated receptors (PPAR). These receptors are inducible by a wide range of compounds including lipidlowering (clofibrate and related fibrates) and glucose-lowering drugs (thiazolidinediones), phthalate plasticizers, leukotriene antagonists, and herbicides. At least three genetically distinct forms exist (alpha, beta, and delta) with different patterns of tissue expression and inducibility by specific compounds. PPAR delta is induced by unsaturated fatty acids. Fatty acid composition: The ratio of saturated to unsaturated fatty acids in membrane lipids is maintained within a narrow range, since this determines membrane fluidity. Transforming growth factor/3 and other cytokines contribute to the control of membrane fluidity by increasing stearyl-CoA desaturase (ECI. 14.99.5) expression.

Function Fuel energy: Its high energy content makes fat (triglycerides) a central player in fuel metabolism. On average, fats provide about 9 kcal/g. Their full oxidation depends on adequate supplies of thiamin, riboflavin, niacin, vitamin B I2, biotin, pantothenate, carnitine, ubiquinone, iron, and magnesium. Additional nutrients are needed for the metabolism of some fatty acids, such as vitamin B 12 for odd-chain fatty acids, or thiamin for phytanic acid. Since fat contains much less oxygen than carbohydrate, the ratio of carbon dioxide production to oxygen consumption (respiratory quotient) is much lower (0.7). At the same time, slightly more oxygen is needed to produce the same amount of energy from a fatty acid than from sugar. Complex lipid synthesis: The fatty acid-derived acyl chains in phospholipids and cholesterol esters provide more than half of the lipid mass in membranes. Phosphatidylcholine-sterol O-acyltransferase (lecithin-cholesterol acyltransferase (LCAT); EC2.3. 1.43) transfers the middle (sn-2) fatty acid from phosphatidylcholine to cholesterol and other sterols. Since the sn-2 position of these phospholipids contains predominantly unsaturated fatty acids, more than half of the fatty acids in cholesterol esters are linoleate, and more than 10% are other highly unsaturated fatty acids (Smedman et al., 1999). Phospholipids share the triglyceride synthesis pathway to the penultimate or the final step. Phosphatidate cytidylyltransferase (EC2.7.7.41, magnesium-dependent) replaces the phosphate group of phosphatidate with cytidyl diphosphate (CDP). CDP-diacylglycerol-inositol 3-phosphatidyltransferase (EC2.7.8.1 l) then completes the synthesis of phosphatidyl inositol. Phosphatidylglycerol is produced from by sequential action of CDP-diacylglycerol-glycerol-3-phosphate 3-phosphatidyltransferase (glycerophosphate phosphatidyltransferase; EC2.7.8.5) and phosphatidylglycerophosphatase (EC3.1.3.27). The synthesis of other phospholipids starts from 1,2-diacylglycerol. Cholinephosphate cytidylyltransferase (EC2.7.7.15) generates phosphatidylcholine,

Structure and Function o f Fatty Acids 139

~

OH

H

Cholesterol

2

~

o II

-

I

-

O

-

-

C

,I 0

CH3

HO--CH-O-- P--C--C--N--CH3 i "2 "2 i OH OH3

Phosphatidylcholinesterol O-acyltransferase

"2i-~

~

Phosphatidylcholine

0

H

CH--OH 0

OH3 I HO--CH-O--P--C--C--N--CH 3 [ H2 H2 [ OH OH3

II

1-Acylglycerophosphocholine

O

~176 Cholesteryllinoleate Figure 6.20

Synthesisofcholesterolesters

and ethanolamine-phosphate cytidylyltransferase (EC2.7.7.14) produces phosphatidylethanolamine. Two CDP-diacylglycerol-serine O-phosphatidyltransferase (phosphatidylserine synthase (PSS), base exchange enzyme; EC2.7.8.8) genes, PSS-1 and PSS-2, encode enzymes that can replace ethanolamine with serine to generate phosphatidylserine. PSS-I can also replace choline with serine. Cardiolipid and other more complex phospholipids are generated from these basic phospholipids. Protein acylafion: Myristic acid or palmitic acid can be attached to specific sites of numerous proteins. The hydrophobic side chain often is important for anchoring of receptors, transporters and enzymes to membranes. Examples for myristoylated proteins are alpha (KAPA) and beta (KAPB) catalytic subunits of cAMP-dependent protein kinase (EC2.7.1.37). A cysteine residue in tumor necrosis factor (TNF) is linked to a palmityl residue (Utsumi et al., 2001 ). 6icosanoid synthesis: Polyunsaturated fatty acids are essential precursors for a multimembered group of signaling compounds that affect platelet aggregation, uterine contractions, inflammation, pain sensation, blood flow, bone repair, and numerous other effects. Many of these effects are initiated when an eicosanoid binds to one of many specific receptors and starts an intracellular signaling cascade. The term eicosanoids is derived from the Greek term for the number of carbons in arachidonic acid, one

140 Fatty Acids

of the fatty acid precursors. Prostaglandin-endoperoxide synthase (prostaglandin H synthase, Cox-l/Cox-2; ECI. 14.99.1) is responsible for the first step in the synthesis of prostaglandins, prostacyclin, and thromboxanes. Arachidonate 5-1ipoxygenase (ECI. 13.11.34, iron-dependent) catalyzes the first step of leukotriene synthesis. Both enzymes accept arachidonic acid, linolenic acid, adrenic acid, gamma-linolenic acid, eicosapentaenoic acid, and other long-chain polyunsaturated fatty acids. The structure of the reaction products depends on the substrate (Larsen et al., 1996). Each precursor fatty acid generates eicosanoids with characteristic activity profiles. The difference in activities of compounds derived from omega-6 fatty acids versus those derived from omega-3 fatty acids is considerable and has been investigated extensively.

References

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142 Fatty Acids

Ng C J, Wadleigh DJ, Gangopadhyay A, Hama S, Grijalva VR, Navab M, Fogelman AM, Reddy ST. Paraoxonase-2 is a ubiquitously expressed protein with antioxidant properties and is capable of preventing cell-mediated oxidative modification of low density lipoprotein. J Biol Chem 2001;276:44444-9 Oliw EH. Oxygenation of polyunsaturated fatty acids by cytochrome P450 monooxygenases. Progr Lipid Res 1994;33:329-54 Qureshy A, Kubota K, Ono S, Sato T, Fukuda H. Thoracic duct scintigraphy by orally administered 1-123 BMIPP: normal findings and a case report. Clin Nucl Med 2001; 26:847-55 Rognstad R. On the estimation of alternative pathways of fatty acid oxidation in the liver in vivo. Bull Math Biol 1995;57:191-203 Sandhir R, Khan M, Singh I. Identification of the pathway of alpha-oxidation of cerebronic acid in peroxisomes. Lipids 2000;35:1127-33 Shachter NS. Apolipoproteins C-I and C-Ill as important modulators of lipoprotein metabolism. Curr Opin Lipid 2001 ; 12:297-304 Smedman AE, Gustafsson IB, Berglund LG, Vessby BO. Pentadecanoic acid in serum as a marker for intake of milk fat: relations between intake of milk fat and metabolic risk factors. Am J Clin Ntttr 1999;69:22-9 Staprans I, Hardman DA, Pan XM, Feingold KR. Effect of oxidized lipids in the diet on oxidized lipid levels in postprandial serum chylomicrons of diabetic patients. Diabetes" Care 1999;22:300-6 Steinberg GR, Dyck DJ, Calles-Escandon J, Tandon NN, Luiken J J, Glatz JF, Bonen A. Chronic leptin administration decreases fatty acid uptake and fatty acid transporters in rat skeletal muscle. J Biol Chem 2002;277:8854-60 Steward ME, Downing DT. Unusual cholesterol esters in the sebum of young children. J Invest Dermatol 1990;95:603-6 Storch J, Thumser AE. The fatty acid transport function of fatty acid-binding proteins. Biochim Biophys Acta 2000; 1486:28-44 Tacken PJ, Hofker MH, Havekes LM, van Dijk KW. Living up to a name: the role of the VLDL receptor in lipid metabolism. Curt Opin Lipid 2001 ; 12:275 9 Tanaga K, Bujo H, Inoue M, Mikami K, Kotani K, Takahashi K, Kanno T, Saito Y. Increased circulating malondialdehyde-modified LDL levels in patients with coronary artery diseases and their association with peak sizes of LDL particles. Art Thromb Vase Bio12002;22:662-6 Utsumi T, Takeshige T, Tanaka K, Takami K, Kira Y, Klostergaard J, lshisaka R. Transmembrane TNF (pro-TNF) is palmitoylated. FEBS Lett 2001 ;500:1-6 Verhoeven NM, Wanders RJA, Poll-The BT, Saudubray JM, Jacobs C. The metabolism of phytanic acid and pristanic acid in man: a review. J hlher Metab Dis" 1998; 21:697-728 Wilson R, Lyall K, Smyth L, Fernie CE, Riemersma RA. Dietary hydroxy fatty acids are absorbed in humans: implications for the measurement of 'oxidative stress' in vivo. Free Rad Biol &led 2002;32:162-8 Wolk A, Furuheim M, Vessby B. Fatty acid composition of adipose tissue and serum lipids are valid biological markers of dairy fat intake in men. J Ntttr 2001;131:828-33

Overfeeding 143

Overfeeding Abbreviations CoA cAMP LDL VLDL

coenzyme A

3',5'-cyclic AMP

low-density lipoprotein very-low-density lipoprotein

The nature of overfeeding Unhealthful expansion of fat stores due to persistent overfeeding is fast becoming the norm in the US and many other affluent countries. The major killers of older adults (type 2 diabetes and cardiovascular disease) hold their ground despite large advances in medical technology and now reach into adolescent and even childhood populations. Common diseases associated with persistent overfeeding include insulin resistance and type 2 diabetes, hyper- and dyslipidemia, hypertension, hyperuricemia and gout, heart disease, stroke, sleep apnea, cancer, cholesterol gallstone disease, and osteoarthritis (Pi-Sunyer, 1999). Even high levels of energy intake without severe body fatness may be undesirable. Evidence is mounting, for instance, that suggests a lower cancer risk with energy restriction (Thompson et al., 2002). The principal consequence of overfeeding is the expansion of fat stores. Subcutaneous and intraabdominal adipose tissue are the main sites of triglyceride storage, but fat accumulations also occur in other tissues, especially in diabetics and obese people (Ravussin and Smith, 2002). Large intraabdominal deposits (signaled by great abdominal girth) are associated with greater cardiovascular and other health risks than subcutaneous fat (Stevens, 1995). Preferential distribution of excess fat, resulting more or less in apple-like shapes for men and pear-shapes for women, may explain some of the differences in health consequences between the genders (Bertrais et al., 1999).

The fate of excess carbohydrate There has been a spirited debate about the question whether humans convert excess glucose into fat (Hellerstein, 2001). The more recent evidence indicates that such de novo fat synthesis is very limited, accounting for less than 10g per day under all but the most extraordinary circumstances. This finding should not detract from the obvious fact that de novo synthesis from carbohydrate is not needed to explain the storage of fat in overfed humans. The carbohydrate is utilized preferentially, and the fat is left over to be stored. As long as people consume more than they expend, there will be enough fat to put into storage and expand the fat mass further. A separate issue is the impact of overfeeding on lipoproteins in blood. The production rate of triglyceride-containing very-low-density lipoprotein (VLDL) is very low in healthy lean subjects. This is important, because the main atherogenic

144 Fatty Acids

(atherosclerosis-promoting) lipoprotein fraction, low-density lipoprotein (LDL), derives exclusively from VLDL. Even modest overfeeding increases VLDL production and thereby raises harmful LDL concentration in blood. High sugar consumption is a particularly potent recipe for raising VLDL production. Conversion of carbohydrate to saturated fat is partially responsible for the additional VLDL triglyceride synthesis (Hudgins et al., 2000).

Insulin resistance and hyperlipidemia One of the most persistent effects of overfeeding and obesity is the declining ability of tissues to increase glucose uptake upon stimulation with insulin. Insulin resistance is the conventional term for this phenomenon. The effect is particularly severe in muscle cells, because they import glucose largely via the insulin-stimulated transporter GLUT4 (SLC2A4). Glucose transfer into brain is little affected, because it proceeds mainly via GLUT 1 (SLC2A 1). Several explanations for the high prevalence of increased insulin resistance in overfed and obese people have been put forward. One of these sees the problem in the typically elevated free fatty acid concentrations in blood (Kraegen et al., 2001 ). The rise in free fatty acid concentration, which is typical after a fat-rich meal, rapidly decreases insulin effectiveness in skeletal muscle. The same is seen with chronic elevation of free fatty acid concentration due to obesity. Another mechanism may involve the hexosamine nutrient-sensing pathway mentioned below. A third scenario focuses on the depressed production ofadiponectin by expanded and overfed adipose tissue (Tsao et al., 2002). Low circulating levels of this peptide hormone, which is produced only in adipose tissue, induce insulin resistance (Maeda et al., 2002). Adiponectin blunts the typical rise of plasma free fatty acid after a fatty meal, which should relieve the free fatty acid-related resistance. A direct adiponectin effect promoting insulin action is also possible. Insulin serves in adipocytes to slow the release of fatty acids. The binding of circulating insulin to specific receptors activates adenylate cyclase (EC4.6.1.1) and the ensuing rise of intracellular 3',5'-cyclic AMP (cAMP) concentration inhibits the activity of hormone-sensitive lipase (EC3.1.1.3). The bottom line is that insulin resistance is associated with accelerated turnover of fat stores. The release of fatty acids from adipose tissue closely correlates with fat mass. The age-typical doubling of fat stores from about 9 kg in a lean young man of average weight (70 kg, BMI 21 ) to 18 kg thirty years later (77 kg, BMI 25)is likely to increase VLDL production several-fold. Insulin resistance then just adds to the quandary of the obese person by further ballooning VLDL output (Couillard et al., 1998). On top of all this, an increase of fat intake adds to the fat load of the liver and cranks up VLDL synthesis all on its own. Since VLDL is the obligate precursor of LDL, both overfeeding and obesity raise the concentration ofatherosclerosis-promoting lipoproteins in blood. Hyperlipidemia and cardiovascular disease are very common in obese people, therefore. An illustration of the role of expanded t:at stores in the genesis of hyperlipidemia is the close association between body fatness and LDL cholesterol concentration observed in patients with LDL receptor defects (Gaudet et al., 1998).

Overfeeding 145

Nutrient sensing Abundance alters the utilization of energy-rich nutrients in characteristic ways and modifies intake behavior. One of the metabolites that serve as fuel sensors is malonylCoA. Since the acetyl-CoA for its synthesis comes from carbohydrate, fat, and protein breakdown, malonyl-CoA is well suited to signal fuel nutrient availability. A high malonyl-CoA concentration slows the rate of fat oxidation by inhibiting carnitine palmitoyl transferase-mediated fatty acid import into mitochondria (Chien et al., 2000). At the same time, fat storage increases with high levels of malonyl-CoA (Abu-Elheiga et al., 2001 ). The hexosamine pathway provides another nutrient-sensing mechanism. A high level of hexosamine metabolites (particularly UDP-N-acetylglucosamine) increases the glycosylation of regulatory proteins and decreases the expression of mitochondrial genes for oxidative phosphorylation (Obici et al., 2002). The hexosamine metabolites also down-regulate the insulin-dependent uptake of glucose and trigger the release of leptin and other satiety signals. The high free fatty acid concentrations in blood that are typical for overfeeding and obesity might cause insulin resistance through stimulation of the hexosamine pathway.

Blood coagulation Large fat stores increase the risk of myocardial infarction and stroke partially due to an increased tendency to form thrombi (blood clots). This is not surprising, since adipose tissue, especially within the abdomen, plays an active role in fibrinolysis. Obese people have higher blood concentrations of coagulation factors VII! and VII, fibrinogen, von Willebrand factor, and plasminogen activator inhibitor (Mertens and Van Gaal, 2002).

Hyperuricemia Elevated blood concentration of uric acid is a cardiovascular risk indicator that is closely related to overfeeding and obesity (Nakanishi et al., 2001 ). The hyperuricemia of most obese people is due to increased uric acid synthesis and not due to impaired renal elimination. Increased production from dietary nucleic acid precursors contributes to the uric acid burden. Many obese people with severe hyperuricemia suffer from gout. This condition is characterized by the deposition of uric acid crystals in .joints and connective tissue at other sites and the ensuing inflammation and pain.

References Abu-Elhciga L, Matzuk MM, Abo-Hasllema KA, Wakil SJ. Continuous fatty acid oxidation and reduced lilt storage in mice lacking acetyI-CoA carboxylase 2. Science 2001: 291:2613 16 Bertrais S, Balkau B, Vol S, Forhan A, Calvet C, Marre M, Eschwege E. Relationships between abdominal body fat distribution and cardiovascular risk thctors: an explanation

146 Fatty Acids

for women's healthier cardiovascular risk profile. The DESIR Study. Int J Obes Rel Metab Dis 1999;23:1085-94

Chien D, Dean D, Saha AK, Flatt JE Ruderman NB. Malonyl-CoA content and fatty acid oxidation in rat muscle and liver in vivo. Am JPhysiol Endoclqn Metab 2000;279:E259~5 Couillard C, Bergeron N, Prud'homme D, Bergeron J, Tremblay A, Bouchard C, Mauriege E Despres JP. Postprandial triglyceride response in visceral obesity in men. Diabetes

1998;47(6):953-60 Gaudet D, Vohl MC, Perron P, Tremblay G, Gagne C, Lesiege D, Bergeron J, Moorjani S, Despres JP. Relationships of abdominal obesity and hyperinsulinemia to angiographically assessed coronary artery disease in men with known mutations in the LDL receptor gene. Circulation 1998;97:871-7 Hellerstein MK. No common energy currency: de novo lipogenesis as the road less traveled. Am J Clin Nutr 2001 ;74:707-8 Hudgins LC, Hellerstein MK, Seidman CE, Neese RA, Tremaroli JD, Hirsch J. Relationship between carbohydrate-induced hypertriglyceridemia and fatty acid synthesis in lean and obese subjects. J Lip Res 2000;41:595-604 Kraegen EW, Cooney GJ, Ye JM, Thompson AL, Furler SM. The role of lipids in the pathogenesis of muscle insulin resistance and beta cell failure in type II diabetes and obesity. Exp Clin Endoc~qnol Diab 2001 ; 109:S 189-$201 Maeda N, Shimomura !, Kishida K, Nishizawa H, Matsuda M, Nagaretani H, Furuyama N, Kondo H, Takahashi M, Arita Y, Komuro R, Ouchi N, Kihara S, Tochino Y, Okutomi K, Horie M, Takeda S, Aoyama 1", Funahashi T, Matsuzawa Y. Diet-induced insulin resistance in mice lacking adiponectin/ACRP30. Nature Med 2002;8:731-7 Mertens l, Van Gaal LE Obesity, haemostasis and the fibrinolytic system. Obesi O, Rev 2002;3:85-101 Nakanishi N, Yoshida H, Nakamura K, Suzuki K, Tatara K. Predictors for development of hyperuricemia: an 8-year longitudinal study in middle-aged Japanese men. Metab Clin Exp 2001 ;50:621-6 Obici S, Wang J, Chowdury R, Feng Z, Siddhanta U, Morgan K, Rossetti L. Identification of a biochemical link between energy intake and energy expenditure. J Clin Invest 2002; 109:1599-605 Pi-Sunyer FX. Comorbidities of overweight and obesity: current evidence and research issues. Med Sci Sports Exercise 1999;31 :$602-8 Ravussin E, Smith SR. Increased fat intake, impaired fat oxidation, and failure of fat cell proliferation result in ectopic fat storage, insulin resistance, and type 2 diabetes mellitus. Ann New York Acad Sci 2002;967:363-78 Stevens J. Obesity, fat patterning and cardiovascular risk. Adv EAp Med Biol 1995; 369:21-7 Thompson HL Zhu Z, Jiang W. Protection against cancer by energy restriction: all experimental approaches are not equal. J Nutr 2002;132:1047-9 Tsao TS, Lodish HE Fruebis J. ACRP30, a new hormone controlling fat and glucose metabolism. Eur J Pharmaco12002;440:213-21

Acetate

Acetate The term acetate (acetic acid; molecular weight 60) refers to both the carbonic acid with a pungent odor and its salts.

Abbreviations CoA MC

coenzymeA monocarboxylate transporter

Nutritional summary Function: Acetate and particularly its conjugate with coenzyme A (acetyl-CoA) is a critical intermediary metabolite for the utilization of carbohydrates, some amino acids (lysine, leucine, isoleucine, phenylalanine, tyrosine, tryptophan), fatty acids, and alcohol. It can be used as a precursor for fatty acid and cholesterol synthesis. Acetate can also be utilized as an energy fuel; its complete oxidation requires thiamin, riboflavin, niacin, pantothenate, lipoate, ubiquinone, iron and magnesium. Food sources: Only very small amounts are consumed with foods, mainly with vinegar, fruits, and vegetables. Alcohol is converted completely into acetate. Several hundred grams of acetyl-CoA are generated daily from the breakdown of carbohydrates, fat, and protein. Requirements: No dietary acetate intake is necessary. A beneficial effect of moderate vinegar intake on blood sugar control and chronic inflammatory polyarthritis has been claimed. Excessive intake: High intakes of acetic acid (more than 10-20 g/day) may induce gastric discomfort, alter pH balance (metabolic acidosis), cause the loss of bone minerals, and increase the risk of dental erosion.

Endogenous production The metabolism of carbohydrates, amino acids, and fatty acids generates several hundred grams of acetate per day, mainly as acetyl-CoA. Depending on intakes, significant amounts of free actetate may also be generated from ethanol. Most is utilized within the cells or tissues where the acetate or acetyl-CoA is generated, some is transported to other tissues and utilized there. Carbohydrates: The amount of acetate generated from glucose depends on the proportion used for glycolysis (as opposed to the smaller fraction metabolized via the pentose phosphate pathway) and the proportion used for the generation of oxaloacetate from pyruvate. Typically, about half a gram of acetate (as acetyl-CoA) is generated per gram of absorbed carbohydrate. Amino acids: Acetyl-CoA is generated during the catabolism of isoleucine, leucine, and threonine. Lysine and tryptophan each generate two acetyl-CoA molecules. Metabolism of cysteine, alanine, and tryptophan generates pyruvate, which may be

147

0%/0--OH3 HO Figure 6.21

Acetate

148 Fatty Acids

0% S/C --OH3 i

CH2 C,H2 NN C=O I C,H2 CH2 NH I

C--O i CN ON I HaC--C' --CH3 ON2 I

O I

O-P=O I

NH

o I

O-P=O I

0 I

OH2

o Figure 6.22

ON

Acetyl-CoA is a critical intermediate of f'uel metabolism

converted into acetyl-CoA. Acetoacetate is generated by the catabolism of phenylalanine, tyrosine, and leucine (for the latter in addition to one mole of acetyl-CoA). The acetoacetate can be activated by 3-oxoacid CoA-transferase (succinyl-CoA transferase, EC2.8.3.5) and then cleaved by acetyl-CoA C-acetyltransferase (thiolase, EC2.3.1.9) to generate two moles of acetyl-CoA. A minor pathway of threonine breakdown generates free acetate. Fatty acids: One mole of acetyl-CoA is released with each cycle of fatty acid beta-oxidation. Alcohol: Ethanol is oxidized by various alcohol dehydrogenases (ECI.I.I.I) or the microsomal ethanol oxidizing system (MEOS, unspecific monooxygenases of the cytochrome P-450 family, ECI. 14.14.1) in conjunction with several types of aldehyde dehydrogenases (EC1.2.1.3, EC1.2.1.4, and ECI.2.1.5) or acetaldehyde oxidase (EC1.2.3.1). Ethanol metabolism occurs mainly in the liver, and most of the resulting acetate is released into circulation (S ilet et al., 1999). One grarn of ethanol generates about 1.3 g of acetate. Fiber: Normal intestinal bacteria break down non-digestible carbohydrates and release significant amounts of short-chain fatty acids including acetate.

Acetate

Glucose, Fructose, Galactose

; l

Cysteine, Tryptophan, Alanine "~

0 II

0 II

OH--C - - C --CH 3 Tryptophan (2), Lysine (2), Leucine, Isoleucine 0 II H2 //O H3C - - C - - C - - C % OH Acetoacetate

OH - - C - - C H 3 H2 Ethanol

Pyruvate (Threonine)

P

CoA--S--C--CH

O II

0 II

3

9

OH - - C --CH 3 Acetate

Acetyl CoA

Dietary fiber (bacterial fermentation) Figure 6.23

Endogenous sources ofacetate and its metabolites

Dietary sources Acetate is ingested rnostly as vinegar (content typically 5-6%) and with pickle& marinated or fermented foods. Typical intake is likely to be less than 1 g/day corresponding to about one tablespoon ( 15 ml) of vinegar. Much smaller amounts are present in a wide range of plant- and animal-derived foods as acetyI-CoA.

Intestinal absorption Absorption of acetate from the srnail intestine (Watson et al., 1991; Tarnai eta/., 1995), especially the jejunum, appears to proceed mainly via the proton/monocarboxylic acid cotransporter (MCT 1, SLC 16A 1), which is possibly present in the apical and certainly in the basolateral enterocyte mernbrane (Garcia et al., 1994; Orsenigo et al., 1999). Acetate can also be absorbed from colon and rectum, which is an irnportant site of bacterial production from dietary fiber (Wolever et al., 1995). MCTI and possibly the SCFA /HCO3 antiporter contribute to this uptake (Stein et al., 2000). The flow of protons across the lurninal rnembrane of the proximal colon via the sodium/hydrogen exchanger also promotes the protonation of the acetate anion and its subsequent passage into the enterocyte by non-ionic diffusion (von Engelhardt et al., 1993).

149

15l)

Fatty Acids

Transport and cellular uptake The proton/monocarboxylic acid cotransporter (MCT 1, SLC 16A 1) is the main carrier for uptake of acetate, acetoacetate, and beta-hydroxybutyrate by the liver and other critical tissues. The related carriers MCT2, MCT3, and MCT4 have much more limited distribution. Blood-brain barrier: Limited transport of acetate occurs across the epithelial cells of the blood-brain barrier (Terasaki et al., 1991) via MCTI on both sides of the brain capillary endothelial cell (Halestrap and Price, 1999). Interestingly, the foot processes ofastroglial cell, which form part of the blood-brain barrier, express MCT2. This carrier has much higher affinity for monocarboxylates than MCT 1 (Halestrap and Price, 1999). Permeability of the blood-brain barrier increases greatly after several days of starvation and in diabetes mellitus. Blood drculation:

Metabolism Acetate can be utilized by muscle and other peripheral tissues (Pouteau et al., 1996). Complete oxidation of acetate requires thiamin, riboflavin, niacin, pantothenate, lipoate, ubiquinone, iron, and magnesium. First, free acetate must be conjugated to coenzyme A by acetate-CoA ligase (thiokinase; EC6.2.1.1 ). Most acetyI-CoA is utilized in mitochondria via the tricarboxylic acid (Krebs) cycle. Citrate synthase (EC4.1.3.7)joins acetyl CoA to oxaloacetate. The citrate from this reaction can then be metabolized further providing FADH, NADH, and succinate for oxidative phosphorylation and ATP or GTP from succinyl CoA. The production rate ofacetyl-CoA from fatty acid beta-oxidation in the liver with prolonged fasting usually exceeds the capacity of the Krebs cycle. The coenzyme A for continued beta-oxidation and other functions can be released through the production of acetoacetate in three steps. The typical odor of a fasting individual is partially related to exhaled acetone formed from acetoacetate. The conversion of acetoacetate into beta-hydroxybutyrate taxes the body's acid-buffering capacity and may cause a drop in blood pH (acidosis) in diabetics and similarly susceptible patients. None of these events is related to dietary intake of acetate. Ketogenesis takes place in the mitochondria where fatty acid catabolism generates acetyl-CoA. AcetyI-CoA C-acetyltransferase (thiolase; EC2.3.1.9)joins two acetylCoA molecules, and hydroxymethylglutaryl-CoA synthase (HMG-CoA synthase; EC4.1.3.5) adds another one. The mitochondrial isoform of HMG-CoA synthase is genetically distinct from the cytosolic one, which generates the precursor for cholesterol

0%

c-cH3

HO Acetate

ATP+CoA

/32) Fru which is a pervasive sweetener. High fructose corn syrup is a major source in the US, where it is added to many industrial food products including ketchup and bread. The Fru content in this syrup is increased by conversion of its Glc during industrial processing using glucose isomerase/D-xylulose ketol-isomerase (EC5.3.1.5).

212 Carbohydrates, Alcohols, and Organic Acids

Mixtures with equal amounts ofmonomeric Fru and Glc are i.3 times sweeter than the same amount of sucrose (Stone and Oliver, 1969). Fruits and vegetables also contain significant amounts of monomeric Fru and sucrose. About half of the dry weight of peaches is sucrose. Daily Fru intakes may be as high as 100 g, especially in populations with high intakes of sucrose and high-fructose corn syrup (Ruxton et al., 1999). Per capita disappearance of fructose was 81 g/day in the US for the year 1997 (Elliott et al., 2002).

Digestion and absorption The disaccharide sucrose (Glc o~-( i >/32) Fru) is hydrolyzed by sucrose ot-glucosidase (EC3.2.1.48), a component of the brush border enzyme complex sucrase-isomaltase. The facilitative transporter GLUT5 (SLC2A5), and to a lesser extent GLUT2 (SLC2A2), mediate Fru uptake from the small-intestinal lumen, mainly the jejunum (Helliwell et al., 2000). Both of the facilitative transporters GLUT5 (SLC2A5) and GLUT2 (SLC2A2) mediate Fru uptake from the small-intestinal lumen (Helliwell et al., 2000). Diffusion and paracellular passage via glucose-activated solute drag also may contribute to absorption. Large quantities (25 g) are poorly absorbed and will cause malabsorption symptoms in as many as one-third of healthy subjects (Born et al., 1994). GLUT2 (SLC2A2) facilitates the transport of Fru out ofenterocytes into interstitial fluid from where it joins the portal blood. The mixed oligosaccharides raffinose (Gal or-( 1>6) GIc a-( 1>/32) Fru), stachyose (Gal a - ( l > 6 ) Gal ot-(l>6) GIc o~-(1>/32) Fru), and verbascose (Gal cr-(l>6) Gal a - ( l > 6 ) Gal ce-(l>6) GIc a-(i>/32) Fru) in beans, peas, and other plant-derived foods are not digestible in humans and may cause abdominal distention and flatulence due to bacterial metabolism. Melezitose (GIc a - ( l > 3 ) Fru o~-Glc) and turanose

sucrose sucrase fructo

fructose

fructose~

Intestinal lumen

/

I

IRA

Enterocyte

Brushborder membrane

Figure7.9 Intestinalabsorptionof'fructose

] Basolateral

membrane

Capillary lumen Capillary endothelium

Fructose 213

(Glc a-( I >3) Fru), minor constituents of conifer-derived honey, are additional exaples of Fru-containing oligosaccharides to which humans may be exposed. Use of oral o~-galactosidase (EC3.2.1.22) along with a meal can initiate the digestion of these oligosaccharides (Ganiats et al., 1994). The indigestible carbohydrates (dietary fiber) inulin and various smaller oligofructose species are composed of/3(2-1 )fructose-fructosyl chains with a minor contribution of glucose residues (Flamm et al., 2001 ). Inulin from chicory roots consists of about 10-20 fructose residues, oligofructoses contain fewer than 10 fructose residues. Microflora, especially bifidobacteria, of the distal ileum and colon thrive on these indigestible carbohydrates. The bacterial breakdown generates short-chain fatty acids that are utilized by local enterocytes (mainly butyrate) or transported to the liver (mainly acetate and propionate).

Transport and cellular uptake Blood drculatiom Fru is transported in blood as a serum solute. The concentration in plasma of healthy people is around 0.13 mmol/l, and increases in response to very high Fru intakes (Hallfrisch eta/., 1986). Fru is taken up into cells via the facilitative transporters GLUT2 (Colville eta/., 1993) and GLUT5. Uptake of Fru into spermatozoa depends on GLUT5. Blood-brain barrier: There is no evidence that significant amounts of Fru cross from blood into brain. Materno-fetal transfer: The net transfer of Fru to the fetus is unknown, but is likely to be small.

Metabolism Most Fru is metabolized in the liver, which explains some of the metabolic differences between Fru and glucose. The dominant metabolic pathway proceeds via fructose lphosphate and joins the glycolysis pathway at the level of the trioses glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. A smaller proportion joins the glycolysis pathway immediately through the phosphorylation to the glycolysis intermediate fructose 6-phosphate by magnesium-dependent hexokinase (EC2.7.1.1). Another significant proportion of ingested fructose can be directly converted into glucose via sorbitol (Kawaguchi et al., 1996). Catabolism via fructose 7-phosphate: Ketohexokinase (hepatic fructokinase, EC 2.7.1.3) in liver and in pancreatic islet cells phosphorylates Fru to fructose 1-phosphate. Fructosebisphosphate aldolase (aldolase, EC4.1.2.13) cleaves both fructose l-phosphate and the glycolysis intermediate fructose 1,6-bisphosphate. There are three genetically distinct isoforms of this crucial enzyme: aldolase A predominantly in muscle, aldolase B in liver, and aldolase C in brain. People with a lack ofaldolase B cannot metabolize Fru properly (hereditary fructose intolerance). Glyceraldehyde is phosphorylated by triokinase (EC2.7.1.28). Triosephosphate isomerase (EC5.3. I. I) converts dihydroxyacetone phosphate into glyccraldehyde 3-phosphate, which can then continue along the glycolytic pathway or contributes to gluconeogenesis depending on prevailing conditions.


H2C--OH I

CH-OH

,

HC//O

2

\

HO--CH I CH-OH I CH-OH

Aldehyde reductase (sulfate)

-

I

HC//O

I

NA(P)D NAD(P)H

CH-OH

ATP ADP + Pi


"~ ~" : Hexokinase or Glucokinase (magnesium)

l

I

CH-OH

~,'

i

HO--CH I CH-OH I CH-OH O I II H2C--O--P--OH I OH Glucose 6-phosphate

I

H2C--OH

H2C--OH

Sorbitol

Glucose

/

L-Iditol ~ NAD 2-dehydr. [ (zinc) I ~ NADH H2C--OH I C= O

I


1

H2C--OH I C=O

ATP ADP +P,

_k,..~_

HO--CH I CH-OH I CH-OH O

I

I

H2C--OH

I

/

O II H2C--O--P--OH I I C=O OH

CH-OH

I H2C--OH Fructose 1-phosphate

I

HO--CH I CH-OH I CH-OH O I II H2C--O--P--OH

II

I

OH Fructose 6-phosphate

I F ATP Ketohexo-y kinase k ~"~ ADP + P,

i

\ 2

Phosph0fructo# kinasel (magnesium)

H2C--O--P--OH

Fructose

HO--CH CH-OH

ATP ADP +Pi

I

-"~ Hexokinase (magnesiun')

O II H2C--O~P--OH I I C=O OH

~

/

/

,,,4

HC//O

~ Fructose-

bisphosphate aldolase

0 II H2C--O--P--OH I I C=O OH I HO--CH 2 Dihydroxyacetone phosphate

.

i

CH-OH

I H2C--OH Glyceraldehyde

OH Fructose 1,6-bisphosphate _

~ Fructosebisphosphate aldolase

i

ATP ADP + P~

\2

Triokinase (magnesium)

~

HC~"O

,

CH-OH

I H2C--OH Glyceraldehyde 3-phosphate

Figure 7.1L~ Fructose metabolism

The sorbitolpathway: The zinc-containing L-iditol 2-dehydrogenase (sorbitol dehydrogenase, ECI. 1.1.14) converts small amounts of Fru to sorbitol, which can then be oxidized to Glc by NADP-dependent aldehyde reductase (aldose reductase, EC 1.1.1.21 ).

Storage There is no significant specific accumulation of Fru that could be mobilized in times of need. Excretion

Very little net loss of Fru occurs in healthy people even when consumption is 100 g or more. Significant amounts of the plasma solute Fru are filtered in the renal glomerulus.

Fructose 21S

Most of the Fru in renal ultrafiltrate is recovered from the proximal renal tubular lumen through the facilitative transporter GLUT5 (Sugawara-Yokoo et al., 1999) and returns into the bloodstream via GLUT2.

Regulation Absorption via GLUT2 at the luminal side of the enterocyte is rapidly and strongly upregulated in response to feeding and humoral factors (Helliwell et al., 2000). Stress and possibly hyperglycemia increase GLUT2 trafficking to the brush border membrane within minutes through activation of p38 MAP kinase signaling. Growth factors, insulin and other factors also influence the Fru absorption. Insulin enhances transcription of the aldolase B gene, and glucagon suppresses its transcription. The latter exerts its action by binding to a cAMP-responsive element in the promoter region (Takano et al., 2000). The metabolism of Fru to pyruvate (glycolysis) is not subject to the regulatory factors that act on phosphofructokinase-1 and that play a great role in regulation of Glc metabolism. Small amounts of Fru have been suggested to improve control of Glc metabolism (Hawkins et al., 2002). On the other hand, high Fru levels can promote hexosamine synthesis and thereby slow insulin-dependent Glc utilization in muscle and adipose tissue (Wu et al., 2001 ).

Function The complete oxidation of Fru yields about 4 kcal/g and requires adequate supplies of thiamin, riboflavin, niacin, lipoate, ubiquinone, iron, and magnesium. Unspecific precursor: All metabolizable sugars can provide carbons for numerous endogenously generated compounds such as amino acids (e.g., glutamate from the Krebs cycle intermediate alpha-ketoglutarate), cholesterol (from acetyl-coenzyme A), or the glycerol in triglycerides. Hexosamines: Fru is a precursor for glucosamine 6-phosphate synthesis by glutamine:fructose-6-phosphate transaminase/isomerizing (GFAT; EC2.6.1.16). N-acetyl glucosamine and other hexosamines are formed after the initial rate-limiting GFAT reaction. The addition of O-linked N-acetylglucosamine to proteins can modify their signaling function and give them roles in nutrient sensing (Hanover, 2001). Fru- (and Glc)-derived hexosamines are critical constituents of glycans (chondroitins, keratans, dermatans, hyaluronan, heparans, and heparin) in the extracellular matrix of all tissues. Energ),fuel:

References

Born P, Zech J, Stark M, Classen M, Lorenz R. Zuckeraustauschstoffe: Vergleichende Untersuchung zur intestinalen Resorption von Fructose, Sorbit und Xylit. Med Klin 1994:89:575-8 Colville CA, Seatter MJ, Jess TJ, Gould GW, Thomas HM. Kinetic analysis of the liver-type (GLUT2) and brain-type (GLUT3) glucose transporters in Xenopus oocytes: substrate specificities and effects of transport inhibitors. Biochem J 1993:290:701-6 Elliott SS, Keim NL, Stern JS, Teff K, Havel PJ. Fructose, weight gain, and the insulin resistance syndrome. Ant J Clin Nutr 2002,76:911-22


Flamm G, Olinsmann W, Kritchevsky 19, Prosky L, Roberfroid M. Inulin and oligofructose as dietary fiber: a review of the evidence. Crit Rev Food Sci Nutr 2001 ;41:353-62 Ganiats TG, Norcross WA, Halverson AL, Burford PA, Palinkas LA. Does Beano prevent gas? A double-blind crossover study of oral alpha-galactosidase to treat dietary oligosaccharide intolerance. J Fam Pratt 1994;39:441-5 Hallfrisch J, Ellwood K, Michaelis OE 4th, Reiser S, Prather ES. Plasma fructose, uric acid, and inorganic phosphorus responses of hyperinsulinemic men fed fructose. J A m Coil Nutr 1986;5:61 8 Hanover JA. Glycan-dependent signaling: O-linked N-acetylglucosamine. FASEB J 2001 ; 15:1865 76 Hawkins M, Gabriely I, Wozniak R, Vilcu C, Shamoon H, Rossetti L. Diabetes 2002;51: 606-14 Helliwell PA, Richardson M, Aflleck J, Kellett GL. Regulation of GLUT5, GLUT2 and intestinal brush border fructose absorption by the extracellular signal-regulated kinase, p38 mitogen-activated kinase and phosphatidylinositol 3-kinase intracellular signalling pathways: implications for adaptation to diabetes. Biochem J 2000;350:163 9 Kawaguchi M, Fujii T, Kamiya Y, lto J, Okada M, Sakuma N, Fujinami T. Effects of fructose ingestion on sorbitol and fructose 3-phosphate contents oferythrocytes from healthy men. Acta Diabetol 1996;33:100-2 Ruxton CH, Garceau FJ, Cottrell RC. Guidelines for sugar consumption in Europe: is a quantitative approach justified? Eur J Clin Nutr 1999;53:503-13 Stone H, Oliver SM. Measurement of the relative sweetness of selected sweeteners and sweetener mixtures. J Food Sci 1969;34:215-22 Sugawara-Yokoo M, Suzuki T, Matsuzaki T, Naruse T, Takata K. Presence of fructose transporter GLUT5 in the $3 proximal tubules in the rat kidney. KMnev lnt 1999;56:1022-8 Takano Y, luchi Y, Ito J, Otsu K, Kuzumaki T, Ishikawa K. Characterization of the responsive elements to hormones in the rat aldolase B gene. Aiz'h Biochem Biophys 2000; 377:58-64 Wu G, Haynes TE, Yan W, Meininger CJ. Presence of glutamine:fructose-6-phosphate amidotransferase for glucosamine-6-phosphate synthesis in endothelial cells: effects of hyperglycemia and glutamine. Diabetologia 2001 ;44:196-202

Galactose OH

Galactose (D-a-galactose, cerebrose; molecular weight 180) is a six-carbon aldose (aldohexose). Abbreviations

OH Figure 7.11 D-~-galactose

Gal GIc GalNAc GLUT1 SGLT1

D-galactose D-glucose Gal N-acetylglucosamine glucose transporter 1 (SLC2A1) sodium/glucose cotransporter 1 (SLC5A1)

Galactose 217

Nutritional summary Function: Galactose (Gal) is used as an energy fuel and for the synthesis of glycoproteins and glycolipids. Food sources: Most Gal is consumed with dairy products. Requirements: No dietary Gal is needed, since the required amounts are modest and can easily be produced endogenously from glucose. Deficiency: There are no health effects associated with low intake. Excessive intake: Lactose intolerance is the most common complaint related to higher than minimal intakes. Much higher Gal intake than likely to ever occur in humans induces lens deposits (cataracts) in animal models, most likely due to oxidative damage.

O H

OH Glucose

IF

Hexokinase [

ATP

~"-- ADP O

II _yo--,.-OH

,/~0

OH

OH

Glucose 6-phosphate

II

Phosphogluco-ll mutase

0 HO v

0 II

r u~P--OH OH I OH Glucose l-phosphate

UDP-glucose I/,~" UTP pyrophos- ~ phorylase ~,.. pp,

0 HOV

t~ ~ P - - U M P OH I

UDP-glucose OH Figure 7.12

Ho o o

0 II

Galactose is synthesized from glucose

UDP-glucose 4'-epimerase

I \~

l/ o- _UMP

OH

UDP-galactose

I OH


Endogenous sources Enough Gal for all functional requirements is endogenously produced from D-glucose (Glc). First, GIc is activated by conjugation with UDP (UTP-glucose-l-phosphate uridylyltransferase; EC2.7.7.9; there are two genetically distinct isoforms). UDPglucose-4'-epimerase (GALE; EC5.1.3.2) can then generate UDP-galactose in a reversible reaction. UDP-galactose is the direct precursor for lactose synthesis and for galactosyl-transfer for glycoprotein and glycolipid synthesis.

Dietary sources Lactose, the a-D-galactopyranosyl-( 1>4) D-glucose dimer, provides most of the carbohydrate in milk (including human milk), infant formula, and dairy products. Lactose provides about 40% of the energy for infants, and about 2% in a mixed diet of American adults (Perisse et al., 1969). Small amounts of Gal are also present in a wide variety of foods including legumes and meats (Acosta and Gross, 1995). The Gal content of peas (boiled, ready to eat) can be as high as 35 mg/g (Peterbauer et al., 2002). Lactose is used as an extender (bulking agent) in some medications.

Digestion and absorption The hydrolysis of lactose by brush border lactase (EC3.2.1.108) generates Glc and Gal. Intestinal lactase expression persists to a considerable degree beyond childhood in most humans. Persistent expression is least prevalent in Asian populations, and most common in Northern Europeans (Harvey et al., 1995). One particular haplotype from a set of 7 polymorphisms is associated with persistence and is more common in Northern Europeans than in Asians (Harvey et al., 1998). An additional locus more than 2 Mb from the lactase gene is responsible for some severe forms of congenital lactase deficiency, indicating that lactase expression and/or activity is under the control of an additional factor (Jarvela et al., 1998). Pyridoxine-beta-D-glucoside hydrolase (no EC number assigned) in jejunum enterocyte cytosol also cleaves lactose (McMahon et al., 1997). Gal is taken up into enterocytes by active transport via sodium/glucose cotransporter 1 (SGLT 1, SLC5A I ), and to a much lesser extent by SGLT2 (SLC5A2; Helliwell et al., 2OOO). Some of the absorbed Gal is used to provide energy or precursors for the enterocyte's own needs, or is lost into the intestinal lumen when the enterocyte is shed. Most Gal leaves the enterocyte via GLUT2 (SLC5A2) and diffuses into portal blood. The plant oligosaccharides raffinose (Gal a - ( l > 6 ) Glc o~-(1>f12) Fru), stachyose (Gala-( 1>6) Gala-( I >6) Glc a-( 1>/32) Fru), and verbascose (Gala-( 1>6) Gal a( 1>6) Gala-( 1>6) Glc a-( 1>/32) Fru) in beans, peas, and other plant foods are not well absorbed, because the Gal in alpha-position blocks digestion. Alpha-galactosidase is active in the lysosomes of most cells, but not in digestive secretions or the luminal side of the human intestine. It is not known to what extent Gal is released by

Galactose 219

lactose

)

Lactase galactose _,

galactose ~ JT~

}"

/

Capillary Intestinal Enterocyte lumen lumen Brush border Basolateral Capillary membrane membrane endothelium Figure 7.13 Intestinal absorption ofgalactose

unspecific digestive or bacterial enzyme action and how much of the released Gal is absorbed (particularly from the terminal ileum and colon). Ingestion of manufactured alpha-galactosidase (EC3.2.1.22, Beano) along with legumes promotes digestion and may decrease oligosaccharide utilization by gas-forming intestinal bacteria (Ganiats et al., 1994).

Transport and cellular uptake Blood drculation: Gal is transported in blood as a serum solute. The concentration in blood of healthy people is under 0.22 mmol/l. Gal is taken up into cells via facilitative

transporters, including GLUTI (brain), GLUT2 (SLC5A2, liver, kidney), GLUT3 (SLC5A3, many tissues), and several related transporters. Blood-brain barrier: GLUT l, which is present on both sides of the brain capillary epithelial cells, transports galactose as readily as glucose. Materno-fetal transfer: GLUTI mediates facilitative Gal transport across both sides of the syntrophoblast cell layer (lllsley, 2000). The net transfer of Gal to the fetus is unknown.

Metabolism Gal is mainly converted to glucose l-phosphate and then to glucose 6-phosphate in the liver. A minor alternate pathway exists, but rernains to be characterized (Berry et al., 2001 ). The initial critical step is phosphorylation by galactokinase (EC2.7.1.6). There are two genetically distinct isoforms of the enzyme with different tissue distribution.


Galactitol accumulation in the lenses of individuals with defective galactokinase ! can cause cataracts in childhood or early adulthood. The next step of Gal metabolism is the transfer of uridine diphosphate (UDP) by UDP-glucose-hexose-i-phosphate uridylyltransferase (EC2.7.7.12). UDP-glucose-4'-epimerase (EC5.1.3.2) epimerizes UDPGal to UDP-glucose. Since UDP-glucose provides the UDP again for the next Gal l-phosphate molecule, this works like an autocatalytic mechanism with a net conversion ofGal 1-phosphate to glucose l-phosphate. Magnesium-dependent phosphoglucomutase (EC5.4.2.2, two isoforms PGMI and PGM2) converts glucose l-phosphate into the readily metabolizable intermediate glucose 6-phosphate. Gal can alternatively be reduced to galactitol by NADPH-dependent aldehyde reductase (aldose reductase; ECI. 1.1.21), especially in the presence of Gal excess.

H O ' - ~ OH OH

(to glycolysis)

OH

Galactitol

Aldehyde ~NADP

HOk

reductase, ~ NADPH i OH

O

II _yo-P-O.

[/J .ol\~

' ~ 0 OH

OH

Galactose

Glucose 6-phosphate

Galacto-~" ATP kinase b A D P

.~

Phospho- 1 gluco- //

021

mutase /~

o

O

I\~ ~o-~-o. OH

I OH Galactose 1 - p h o s p h a t e ~

uridyltransferase

II [ v~P--OH OH I OH Glucose 1-phosphate UDP" I UTP glucose [/,1. pyrophos-I~ phorylase~ "'" PP, 0

OH

HOV

I

uoo-0a,ac,os~

0

t' v ~ P - - U M P OH I

uo - ,ucose~ UDP-glucose 4'-epimerase

Metabolism ofgalactose

O

H0 v

H

Figure 7.14

o~.

OH

Galactose 221

Storage There is no significant specific accumulation of Gal that could be mobilized in times of need.

Excretion Gal passes into renal primary filtrate owing to its small molecular size and complete water solubility. SGLTI actively transports Gal into the epithelium of proximal renal tubules from where it is exported into blood via the high-capacity transporter GLUT2.

Regulation The regulation of tissue and whole-body Gal homeostasis is complex and tightly integrated into the regulation of carbohydrate metabolism.

Function lactose synthesis: Nursing infants depend on the high lactose content of milk as the main energy fuel. Women produce, after a normal pregnancy, large amounts of lactose in their mammary glands. Human milk contains 60-80g/1 lactose and Gal-containing oligosaccharides. Gal and GIc are actively taken up from maternal blood across the basolateral membrane into the mammary epithelial cell by SGLTI (Obermeier et al., 2000). Transport of Gal into the Golgi system may require a transporter, possibly GLUT I (Nemeth et al., 2000). The presence ofGLUTI in human mammary glands is, however, in question (Obermeier et al., 2000). UDP-galactose and glucose are linked in the Golgi complex by lactase synthase (EC2.4.1.22), a heterodimer consisting of the enzymatically active A-protein (the shortened version of beta-l,4-galactosyltransferase 1, which is transcribed from an alternative initiation site) and alpha-lactalbumin. Energy fuel: The complete oxidation of Gal yields about 4 kcal/g and requires adequate supplies of thiamin, riboflavin, niacin, lipoate, ubiquinone, iron, and magnesium. Unspecific precursor: Gal can provide carbons for numerous endogenously generated compounds such as amino acids (e.g., glutamate from the Krebs cycle intermediate alpha-ketoglutarate), cholesterol (from acetyl-coenzyme A), or the glycerol in triglycerides. Glycoprotein synthesis." Gal and galactosamine are attached to numerous proteins. Examples of the presence of Gal in O-glycans are the Gal and Glc-o~-2-Gal side chains beta-linked to 5-hydroxylysine in collagens. Gal N-acetylglucosamine (GalNAc) is attached to serine and threonine residues in mucins. Glucosaminoglycans (chondroitin sulfate, dermatan sulfates, keratan sulfates) contain Gal, GalNAc, or both. Glycolipidsynthesis: Gal is a structural component of both neutral and acidic cerebrosides and gangliosides. The brain and the myelin sheath of nerves contain particularly large amounts of Gal-linked glycolipids, typically in association with specific proteins. Neutral glycolipids are of the types Gal(fl 1- 1) ceramide and Gal(/31-4) Glc(fl I - 1)


ceramide. Acidic glycolipids are of the types Neu5Ac(a2-3)Gal(fll-41) Glc(fll-l) ceramide and Neu5Ac(a2-8)Neu5Ac(tr2-3)Gal(fll-4)GIc(fll-l) ceramide (Neu5Ac = N-acetylneuraminic acid). The ceramides are sphingolipids with long-chain fatty acids.

References Acosta PB, Gross KC. Hidden sources of galactose in the environment. Eur d Ped 1995; 154:$87-$92 Berry GT, Leslie N, Reynolds R, Yager CT, Segal S. Evidence for alternate galactose oxidation in a patient with deletion of the galactose-1-phosphate uridyltransferase gene. Mol Genet Metab 2001 ;72:316-2 I Ganiats TG, Norcross WA, Halverson AL, Burford PA, Palinkas LA. Does Beano prevent gas'? A double-blind crossover study of oral alpha-galactosidase to treat dietary oligosaccharide intolerance. J Family Pract 1994;39:441-5 Harvey CB, Pratt WS, Islam I, Whitehouse DB, Swallow DM. DNA polymorphisms in the lactase gene: linkage disequilibrium across the 70-kb region. E u r J Hum Genet 1995; 3:27-41 Harvey CB, Hollox EJ, Poulter M, Wang Y, Rossi M, Auricchio S, lqbal TH, Cooper BT, Barton R, Sarner M, Korpela R, Swallow DM. Lactase haplotype frequencies in Caucasians: association with the lactase persistence/non-persistence polymorphism. Ann Hum Genet 1998;62:215-23 Helliwell PA, Richardson M, Affleck J, Kellett GL. Regulation of GLUT5, GLUT2 and intestinal brush border fructose absorption by the extracellular signal-regulated kinase, p38 mitogen-activated kinase and phosphatidylinositol 3-kinase intracellular signalling pathways: implications for adaptation to diabetes. Biochem J 2000;350:163-9 lllsley NP. Glucose transporters in the human placenta. Placenta 2000;21:14-22 Jarvela I, Sabri Enattah N, Kokkonen J, Varilo T, Savilahti E, Peltonen L. Assignment of the locus for congenital lactase deficiency to 2q21, in the vicinity of but separate from the lactase-phlorizin hydrolase gene. Am J Hum Genet 1998;63:1078-85 McMahon LG, Nakano H, Levy MD, Gregory JF 3rd. Cytosolic pyridoxine-betaD-glucoside hydrolase from porcine jejunal mucosa. Purification, properties, and comparison with broad specificity beta-glucosidase. J Biol Chenl 1997;272: 32025-33 Nemeth BA, Tsang SW, Geske RS, Haney PM. Golgi targeting of the GLUTI glucose transporter in lactating mouse mammary gland. Ped Res 2000;47:444-50 Obermeier S, Huselweh B, Tinel H, Kinne RH, Kunz C. Expression of glucose transporters in lactating human mammary gland epithelial cells. Eur J Ntttr 2000;39: 194-200 Perisse J, Sizaret E Francois P. The effect of income on the structure of the diet. FAO Nutr Neuwlett 1969;7:1-9 Pcterbauer T, Mucha J, Mach L, Richter A. Chain elongation of raffinose in pea seeds. Isolation, characterization, and molecular cloning of multifunctional enzyme catalyzing the synthesis of stachyose and verbascose. J Biol Chem 2002:277: 194 200

Xylitol 223

Xylitol Xylitol (xyloopentane-l,2,3,4,5-pentol; molecular weight 152) is a water-soluble sugar alcohol that sometimes is used as a glucose substitute for diabetics and in chewing gums.

Abbreviations GAP gtyceraldehyde 3-phosphate F6P Fructose6-phosphate XSP xylulose5-phosphate

Nutritional summary Function: Xylitol is sometimes used as a sweetener in gums, because it is less likely to promote caries than other sugars, or in dietetic foods for diabetics, because its uptake into cells is not dependent on insulin. Requirements: Dietary intake ofxylitol is not necessary. Food sources: It is present in modest amounts in plums and a few other fruits. Excessive intake: Intakes of 50 g per day or more may cause some malabsorption and gas production from bacterial fermentation of residual xylitol in the colon. Long-term health risks of high intakes have not been thoroughly evaluated.

E n d o g e n o u s sources Daily endogenous production of xylitol is 1-4 g. Xylitol derives from the catabolism in liver and kidney of D-glucuronate released from connective tissue and from glucuronate-containing proteoglycans. The enzyme catalyzing the initial glucuronate reduction, glucuronate reductase (ECI.I.I.19), actually may be identical with aldehyde reductase (ECI.I.I.2). L-gulonate can then be oxidized by L-gulonate 3-dehydrogenase (EC1.1.1.45). The third step is catalyzed by dehydro-L-gulonate decarboxylase (EC4.1.1.34), which can use either magnesium or manganese as a cofactor. Xylitol synthesis is then completed by L-xylulose reductase (ECI. 1.1.10). Two distinct genes encode this enzyme. The major isoenzyme occurs both in cytosol and mitochondria, the minor isoenzyme is limited to cytosol. A genetic defect of the major isoenzyme causes pentosuria. This benign condition disrupts the normal metabolism of glucuronate and is characterized by excretion of 1-4 g/day L-xylulose with urine. D i e t a r y sources Small amounts of xylitol are naturally consumed with plums (9 mg/g), raspberries, spinach (1.1 rag/g), carrots (0.9 rag/g); some sugarless chewing gums also contain xylitol. Dietary intakes may be significant, especially in diabetics consuming xylitol as a sugar replacer.

/OH H-C-OH I

OH-C-H

I

H-C-OH

I

H2C~ OH Figure 7.15 Xylitol


.r

H:"

H-C-OH O H - CI - H

I

H-C-OH I H-C-OH I

:%

NADPH NADP

H-C-OH O H - CI - H

i

H,,C"OH NAD

k.)

NADH

I

I

o:'oH

O:'OH

L-Gulonate

3-keto-L-gulonate

OH

/OH

H-C-OH O H - CI- H

H21C H-C-OH O H - CI- H

H2: CO2

_.)

Dehydro-L-gulonate decarboxylase

I

O=C-H

I H2C OH

L-xylulose 7.16

i

~ H-C-OH ~-O"C-H Glucuronate reductase I L-Gulonate 3-dehydrogenase I H-C-OH H-C-OH

D-Glucuronate

Figure

H-C-OH O H - CI - H

NADPH

NADP ~

L-Xylulose reductase

i

H-C-OH

I H2C OH

Xylitol

Endogenous xylitol synthesis

Digestion and absorption Xylitol is absorbed by passive diffusion, more slowly than glucose, presumably through one of the glucose transporters. It is much less likely to cause malabsorption symptoms than sorbitol or fructose (Born et al., 1994).

Transport and cellular uptake After transport via portal blood in free form, xylitol is taken up into the liver and other tissues by an unknown mechanism. Xylitol is sometimes used as a sweetener for diabetics, because most tissues can take it up without stimulation by insulin. Blood circulation:

Metabolism Xylitol is an important intermediate of the pentitol pathway in the liver for the reutilization of glucuronide from connective tissue (proteoglycan) breakdown. The reactions of the pentose-phosphate pathway can partially convert xylitol into glucose 6-phosphate, which can then either provide glucose or reenter the pentose-phosphate pathway. The reactions require adequate availability of thiamin, niacin, magnesium, and manganese. Oxidation to D-xylulose by D-xylulose reductase (EC I. 1.1.9, cofactor manganese) uses NAD, in contrast to the NADPH-dependent reduction of L-xylulose to xylitol.

Xylitol 225

/OH

/OH

H2C

NAD NADH

H-C-OH I OH-C-H

I

D-xylulose reductase

H-C-OH

I

He~ OH

H2~

OH-C-H I H-C-OH

I O-C-H I

H2~ OH D-Xylulose • ~ATP (magnesium),~'~ADP

Xylitol

OH I

OH

/o- -OH

H/

H21G O

I

OH -C-H I H -C-OH

OH-C-H

v

I

I

I

H2~ OH D-Xylulose-

H2C OH D-Xylulose5-phosphate

5-phosphate

y

OH / HC-H

o//C,,H

H-C-OH

I H2C, O

I

HO-C-H

I

O=C-H

I

H2~ OH D-Er~hrulose

O-P-OH I OH

,~/

5-phosphate

(TPP) A OH J ~, i

/

I

I

H2C~ OH D-Ribulose-

TransketolaseV

Transketolase] (TPP) ]

H-C-OH

I

O-C-H

I

I

%C/H

HO-C-H HO-C-H

O=C-H

2 D-Glyceraldehyde3-phosphates

O

O

I

I

H2C" ~

/O- -OH

H-C-OH

O=C-H

OH O-P-OH

OH

o- -OH

H2~

O-P-OH II O

OH

H-C-OH

I

H-C-OH I H-C-OH

I

HO-C-H I

O=C-H

I

/O-P-OH

H27

O

H -C-OH

I

o//C

\H

I

H2~ OH D-sedoheptulose7-phosphate

D-Glyceraldehydeo 3-phosphate

Figure 7.17 Xylitolmetabolism

The availability of NADPH from the early steps of the pentose-phosphate pathway can thus promote xylitol generation while its metabolism generates NADH. Phosphorylation to D-xylulose 5-phosphate (X5P) uses magnesium-dependent xylulokinase (EC2.7.1.17). Transketolase (EC2.2.1.1) with covalently bound thiamin-pyrophosphate catalyzes two rearrangement reactions. One of these converts two pentose phosphates

226

Carbohydrates, Alcohols, and Organic Acids

(X5P and ribose 5-phosphate) into a set of compounds with 7 (D-sedoheptulose 7-phosphate) and 3 (glyceraldehyde 3-phosphate, GAP) carbons. The other one rearranges X5P plus erythrose 4-phosphate into glyceraldehyde 3-phosphate and fructose 6-phosphate (F6P). A third possibility is the rearrangement of two X5P molecules into two GAP molecules and one erythrulose (Bykova et al., 2001 ). The same reactions are catalyzed by transketolase 2 (Coy et al., 1996), with different isoforms in brain and heart generated by alternative splicing. Variants of the transketolase 2 gene may be implicated in the pathogenesis of Wernicke-Korsakoff syndrome. Transaldolase (EC2.2.1.2) complements the transketolase-catalyzed rearranging reactions by converting the compounds with 7 and 3 carbons into erythrose 4-phosphate (4 carbons) and F6P (6 carbons). Two additional steps, catalyzed by glucose-6-phosphate isomerase (EC5.3.1.9) and glucose 6-phosphatase (EC3.1.3.9), can then generate glucose. Alternatively, depending on feeding status, 6-phosphofructokinase (phosphofructokinase [; EC2.7.1.11 ) can initiate utilization via glycolysis.

Function Xylitol has both passive and active anti-caries properties (Levine, 1998; Makinen et al., 1998) and appears to be more effective in arrest of dental caries than sorbitol (Makinen et al., 1996). Xylitol was found to inhibit the growth o f Streptococcus p n e u m o n i a e and possibly thereby reduce the incidence ofotitis media in children (Uhari et al., 1998).

Oral intake may increase intestinal absorption of iron, copper, and calcium (Hamalainen and Makinen, 1985). Animal studies suggest that xylitol feeding may improve bone architecture and biomechanical stability of bone (Mattila et al., 2002). Use of xylitol-containing solutions for parenteral nutrition support may lead to the development of oxalosis with the risk of seizures and renal failure (Leidig et al., 2001); such use is thus banned in the US. ReFerences

Born P, Zech J, Stark M, Classen M, Lorenz R. Zuckeraustauschstoffe: Vergleichende Untersuchung zur intestinalen Resorption yon Fructose, Sorbit und Xylit. Med Klin 1994;89:575-8 Bykova IA, Solovjeva ON, Meshalkina LE, Kovina MV, Kochetov GA. One-substrate transketolase-catalyzed reaction. Biochem Biophys Res Comm 2001;280:845-7 Coy JE Dubel S, Kioschis E Thomas K, Micklem G, Delius H, Poustka A. Molecular cloning of tissue-specific transcripts ofa transketolase-related gene: implications for the evolution of new vertebrate genes. Genomics 1996;32:309-16 Hamalainen MM, Makinen KK. Duodenal xanthine oxidase (EC 1.2.3.2) and ferroxidase activities in the rat in relation to the increased iron absorption caused by peroral xylitol. Br J Nutr 1985;54:493-8 Leidig E Gerding W, Arns W, Ortmann M. Renal oxalosis with renal failure after infusion of xylitol. Deutsche Med Wochenschr 2001 ; 126:1357-60

Pyruvate 227

Levine RS. Briefing paper: xylitol, caries and plaque. Br Dent J 1998;185:520 Makinen KK, Makinen PL, Pape HR Jr, Peldyak L Hujoel E Isotupa KE Soderling E, Isokangas PJ, Allen E Bennett C. Conclusion and review of the Michigan Xylitol Programme (1986-1995) for the prevention of dental caries, lnt Dent J 1996; 46:22-34 Makinen KK, Chiego DJ Jr, Allen E Bennett C, isotupa KE Tiekso J, Makinen PL. Physical chemical, and histologic changes in dentin caries lesions of primary teeth induced by regular use ofpolyol chewing gums. Acta Odontol Scand 1998:56:148-56 Mattila PT, Svanberg MJ, Jamsa 1", Knuuttila ML. Improved bone biomechanical properties in xylitol-fed aged rats. Metab Clin Exp 2002;51:92-6 Uhari M, Kontiokari 1", Niemela M. A novel use of xylitol sugar in preventing acute otitis media. Pediatrics 1998; 102:879-84

Pyruvate Pyruvate (pyruvic acid, 2-oxopropanoic acid, alpha-ketopropionic acid, acetylformic acid, pyroracemic acid; molecular weight 88) is a keto-monocarboxylic acid.

Abbreviations CoA coenzymeA MCT1 proton/monocarbo~lic acicl cotransporcer 1 (SLC16A1) MCT2 proton/monocarboxylic acid cotransporter 2 (SLC16A7) PLP pyridoxal5'-phosphate

Nutritional

summary

Function: Pyruvate is the product of glucose, L-alanine, and L-serine breakdown. Food sources: Insignificant amounts are present in foods from both animal and plant

sources. In comparison, more than a hundred grams of pyruvate are generated daily from the breakdown of carbohydrates and protein. Requirements: No dietary intakes are needed. Pyruvate is commercially available as a single compound or in combination with other ingredients. Deficiency: A lack of intake has no harmful consequences. The efficacy of oral pyruvate supplements to promote weight loss in conjunction with exercise and to improve exercise performance still is uncertain. The intracoronary application for the salvage of ischemic myocardium has been described, but requires further evaluation. Excessive intake: The risks associated with use of supplemental pyruvate are not known.

Endogenous sources Carbohydrates: Several hundred grams ofpyruvate are generated during the metabolism of glucose via glycolysis. As a rule of thumb, about one gram of absorbed carbohydrate generates one gram of pyruvate. As a result of the shuttling of anaerobic glucose metabolites from muscle to liver for complete utilization (Cori cycle) large amounts


of pyruvate are generated from S-lactate by NADH-dependent L-lactate dehydrogenase (EC1.1.1.27). Amino acids: Almost all ingested L-alanine is eventually broken down by alanine aminotransferase (EC2.6.1.2) to pyruvate. The L-alanine-pyruvate pair is critical for the transfer of protein-derived carbons for gluconeogensis from muscles to the liver during fasting and severe illness (alanine-glucose cycle). A small proportion of L-serine is converted to pyruvate by the PLP-dependent enzymes serine dehydratase (EC4.2. !. 13) and threonine dehydratase (EC4.2.1.16).

Dietary sources While many foods contain some pyruvate from the metabolic processes occurring in the food source, the amounts tend to be very small. Dietary supplements with gram amounts of pyruvate are commercially available.

Digestion and absorption A proton/monocarboxylic acid cotransporter (MCT 1, SLC ! 6A 1) is possibly present in the apical, and certainly in the basolateral membrane (Garcia et al., 1994; Orsenigo et al., 1999; Tamai et al., 1999) of the entire intestine, especially the jejunum (Tamai et al., 1995). In addition to lactate, acetate, acetoacetate,/3-hydroxybutyrate, propionate, butyrate, and benzoic acid, this transporter mediates uptake of pyruvate both across the brush border membrane and the basolateral membrane. Additional transporters or mechanisms of entry might exist.

Transport and cellular uptake Blood circulation: Pyruvate is present in blood in free form. Tissues can take it up via several members of the proton/monocarboxylate cotransporter (MCT) family (Halestrap and Price, 1999). MCT1 is the predominant form responsible for pyruvate uptake from circulation in liver, muscle, and brain. MCT2 (SLCI6A7) is a high-affinity transporter with a preference for pyruvate in many tissues. Expression is especially high in testis and in some neoplastic cells (Lin et al., 1998). Once inside a cell, pyruvate can enter mitochondria via the mitochondrial tricarboxylate carrier, a six-transmembrane helix that is not related to the MCT family of genes (Halestrap and Price, 1999). Materno-fetal transfer: Several members of the MCT family contribute to placental transport, but their individual locations still need to be claritied. Blood-brain barrier: MCTi is present at both sides of the brain endothelium. Ketosis increases MCTI expression in these cells.

Metabolism Some of the energy content of carbohydrates can be utilized even in the absence of oxygen, especially in strenuously exercised muscle. In this case pyruvate is reduced to

Pyruvate 229 CoA

Acetyl- H3C-~ )/C~ dihydro- O --ts ~ 9 . hpoam,de /"-J ~

o Dihydrolipoamide e ~

E1

co~..\

Pyruvate dehydrogenase

COOH C=O I CH3 Pyruvate

AcetyI-CoA

~

/

(

-/~'SH . ~SH DLhydfrT~ ~ ,,v, ........ /

E3 Dihydrolipoamide

~ehydrogenase/ /

S[8) l

FAD

I

NAD

Lipoamide

Figure 7.18 A multi-subunitenzymecomplexoxidizespyruvate

L-lactate by L-lactate dehydrogenase (EC 1.1.1.27) providing a renewed supply of oxidized NAD for continued glycolysis. The net yield of anaerobic glycolysis is two ATP for each glucose molecule metabolized to L-lactate. The shuttling of L-lactate from muscle to liver, eventual regeneration of pyruvate, glucose synthesis from pyruvate, and return transport of glucose to muscles is referred to as the Cori cycle. If enough oxygen is available, on the other hand, pyruvate is fully metabolized in mitochondria by oxidative decarboxylation to acetyl-CoA, oxidation in the citric acid cycle, and use of the resulting reductants (NADH and FADH2) for oxidative phosphorylation. A smaller proportion of pyruvate is carboxylated to oxaloacetate in a biotindependent reaction. This latter reaction is called anaplerotic (Greek for 'refilling'), because it replenishes the citric acid cycle intermediates and thus sustains their ability to metabolize acetyI-CoA. The metabolic fate of pyruvate is closely regulated. During glycolysis most pyruvate is metabolized to acetyl-CoA. When the prevailing metabolic direction is towards gluconeogenesis, a large proportion of available pyruvate is converted to oxaloacetate, which is then used to resynthesize glucose. Oxidative decarboxylation: Pyruvate dehydrogenase (EC3.1.3.43) in the mitochondrial matrix comprises multiple copies of three distinct moieties: El, E2, and E3. Thiamin pyrophosphate is covalently bound to El. Each subunit E2 (dihydrolipoamide S-acetyltransferase; EC2.3.1.12) contains two lipoate molecules, which are covalently bound to lysines 99 and 226. These lipoamides serve as acceptors for the acetyl residues from pyruvate, transfer them to acetyl CoA, and reduce lipoamide to dihydrolipoamide in the process. Another component of the complex, dihydrolipoamide dehydrogenase


(E3; ECI.8.1.4) transfers the hydrogen via FAD to NAD. A single gene encodes the dihydrolipoamide dehydrogenase ofpyruvate dehydrogenase and the other two alphaketoacid dehydrogenases. Carboxylation: The biotin-containing enzyme pyruvate carboxylase (EC6.4.1.1) generates oxaloacetate, a pivotal precursor for glucose synthesis in gluconeogenic tissues (liver, kidney). Pyruvate carboxylation is the only anaplerotic (refilling) reaction that can replenish Krebs cycle intermediates without drawing on L-glutamate or other amino acids.

Excretion Pyruvate is recovered from primary filtrate both in proximal renal tubules and collecting ducts. It has been suggested that a sodium-linked carrier, possibly MCT6, is responsible for uptake across the brush border membrane (Halestrap and Price, 1999). MCTI mediates transport across basolateral membranes of proximal tubules. MCT2 performs this function in collecting ducts (Garcia et al., 1995).

Regulation Both synthesis from glycolytic precursors and breakdown to either acetyI-CoA or oxaloacetate are tightly regulated. Pyruvate dehydrogenase (EC3.1.3.43) is the enzyme that connects glycolysis to the citric acid cycle. This enzyme complex is inactivated by phosphorylation ([pyruvate dehydrogenase (lipoamide)] kinase; EC2.7.1.99) of three serines in the El subunit and reactivated by removal of these phosphates by [pyruvate dehydrogenase (iipoamide)]-phosphatase (EC3.1.3.43). Inactivation is strongly subject to substrate and product feedback: ADP and pyruvate decrease the rate of inactivation, NADH and acetyI-CoA increase it. Activation, on the other hand, is mainly under hormonal control, mediated by calcium. The activity of pyruvate carboxylase increases with rising acetyI-CoA concentration, which prevents further accumulation from pyruvate metabolism and makes more oxaloacetate available to form citrate condensation with acetyI-CoA. Unlike glucose, pyruvate does not stimulate insulin secretion. This dissociation of insulin secretion from mitochondrial substrate oxidation has been called the pyruvate paradox (lshihara et al., 1999).

Function Fuel metabolism: As described above, pyruvate is the linchpin between glucose metabolism and the citric acid cycle. Ingested pyruvate provides about 4 kcal/g. Its complete oxidation requires adequate supplies of thiamin, riboflavin, niacin, pantothenate, lipoate, ubiquinone, magnesium, and iron. Amino acid synthesis: L-Alanine aminotransferase (EC2.6.1.2) uses L-glutamate to transaminate the glycolysis metabolite pyruvate and produce L-alanine. Since the reaction operates near equilibrium, high availability of glucose (and consequently of

Pyruvate 231

pyruvate) increases L-alanine production. During fasting or severe illness the alanine-glucose cycle uses pyruvate and the amino groups from protein catabolism to shuttle the gluconeogenesis precursor from extrahepatic tissues to the liver. Enzyme cofactor: Pyruvate is an essential cofactor of several bacterial enzymes, but no human enzymes with this type of requirement are known. Performance enhancement: A beneficial effect of pyruvate on myocardial contractility in patients with heart failure has been suggested (Hermann et al., 1999) which might be mediated by an increase of ionized calcium in the sarcoplasmic reticulum (Hermann et al., 2000). It has also been suggested that supplemental pyruvate (6 g/day) in combination with moderate exercise promotes weight loss (Kalman et al., 1999). This type of regimen did not increase short-term strength, however (Stone et al., 1999).

References

Garcia CK, Goldstein JL, Pathak RK, Anderson RG, Brown MS. Molecular characterization of a membrane transporter for lactate, pyruvate, and other monocarboxylates: implications for the Cori cycle. Cell 1994;76:865-73 Garcia CK, Brown MS, Pathak RK, Goldstein JL. cDNA cloning of MCT2, a second monoearboxylate transporter expressed in different cells than MCTI. J Biol Chem 1995;270:18a3-9 Halestrap AP, Price NT. The proton-linked monocarboxylate transporter (MCT) family: structure, function and regulation. Biochem J 1999;343:281-99 Hermann HE Pieske B, Schwarzmfiller E, et al. Haemodynamic effects of intracoronary pyruvate in patients with congestive heart failure: an open study. Lancet 1999;353: 1321 3 Hermann HE Zeitz O, Keweloh B, et al. Pyruvate potentiates inotropic effects of isoproterenol and Ca2+ in rabbit cardiac muscle preparations. Am J Phvsiol 2000;279: H702-H708 Ishihara H, Wang H, Drewes LR, Wollheim CB. Overexpression of monocarboxylate transporter and lactate dehydrogenase alters insulin secretory responses to pyruvate and lactate in beta cells. J Clin h~vest 1999; 104:1621-9 Kalman D, Colker CM, Wilets I, Roufs JB, Antonio J. The effects of pyruvate supplementation on body composition in overweight individuals. Nutrition 1999; 15:337-40 Lin RY, Vera JC, Chaganti RSK, Golde DW. Human monocarboxylate transporter 2 (MCT2) is a high affinity pyruvate transporter. J Biol Chem 1998;273:28959-65 Orsenigo MN, Tosco M, Bazzini C, Laforenza U, Faelli A. A monocarboxylate transporter MCTI is located at the basolateral pole of rat jejunum. Exp Ph~wiol 1999;84: 1033-42 Stone MH, Sanborn K, Smith LL, O'Bryant HS, Hoke T, Utter AC, Johnson RL, Boros R, Hruby J, Pierce KC, Stone ME, Garner B. Effects of in-season (5 weeks) creatine and pyruvate supplementation on anaerobic performance and body composition in American football players, hit J Sport Nutr 1999;9:146-65 Tamai !, Takanaga H, Maeda H, Sai Y, Ogihara T, Higashida H, Tsuji A. Participation of a proton/cotransporter, MCTI, in the intestinal transport of monocarboxylic acids. Biochem Biophys Res Comm 1995;214:482-9


Tamai I, Sai Y, Ono A, Kido Y, Yabuuchi H, Takanaga H, Satoh E, Ogihara T, Amano O, Izeki S, Tsuji A. lmmunohistochemical and functional characterization of pHdependent intestinal absorption of weak organic acids by the monocarboxylic acid transporter MCT I. J Pharm Pharmacol 1999;51 : 1113-21

Oxalate oII C-OH

I

C-OH II o Figure

7.19

Oxalate

Oxalate (oxalic acid, ethanedioic acid; molecular weight 90) is a dicarboxylic acid generated in the metabolism of most organisms.

Abbreviations CLD DRA DTDST PDS

congenital diarrhea gene (SLC26A3) down-regulated in adenoma gene (SLC26A3) diastrophic dysplasia sulfate carrier (SLC26A2) Pendred syndrome gene (SLC26A4)

Nutritional summary Oxalate is a minor breakdown product ofdehydroascorbate, ethanoloamine, serine, and glycine that has no known biological role. Requirements: There is no dietary intake requirement. Food sources: Spinach and rhubarb are especially rich in oxalate. Other fruits, vegetables, and herbs contain smaller amounts. Deficiency: No adverse effects are known or expected from a lack of intake. Excessive intake: Dietary intakes increase oxalate concentration in urine and can thereby increase risk of renal stone formation (calcium oxalate calculi). Function:

Endogenous sources Oxalate is generated mainly from the breakdown of dehydroascorbic acid and glyoxylate. Dehydroascorbic acid spontaneously decomposes to 2,3-ketogulonate and then to threonic acid and oxalate. This irreversible reaction contributes about 40% of total oxalate at moderate ascorbate intake levels. Oxalate is also produced from the oxidation of excess glyoxylate by (S)-2-hydroxy-acid oxidase (ECI. !.3.15) in liver peroxisomes (Seargeant et al., 1991). L-serine, hydroxy-proline (Ichiyama et al., 2000) and ethanolamine contribute to glyoxylate production via glycoaldehyde and glycolate; glycolate itself is ingested with plant foods (Ichiyama et al., 2000). Another source of oxalate is glycine; a small percentage (0.1%) of the glycine pool ends up as oxalate. This might be due to a reversal of the alanine-glyoxylate pathway in liver peroxisomes whereby the pyridoxal-phosphate-dependent enzyme alanine-glyoxylate aminotransferase (EC2.6.1.44) would then convert pyruvate and glycine into glyoxylate plus alanine. A third potential glyoxylate precursor is ethylene glycol, though significant exposure other than from antifreeze ingestion is rare. Since the bifunctional enzyme D-glycerate dehydrogenase/glyoxylate reductase (EC1.1.1.26) can

Oxalate 233

0 Glycolate C-OH ,i G l y c o l I aldehyde" H2C-OH H202 I _ FMN - - I - - N A D 1~)-2-hydroxy-~-glycerate DH/ | acid oxidase | glyoxylate red.

i .Ec, 13 I )A\/Ecl

26)

IO2"FMNH2" ~' ~ NADH2 o

Glyoxylate C-OH I C-H . O H202 H20 [//- FMN ~

Ethanolamine 9

L-Serine

0

OH,

NH2-CH L-Alanine I OH3 o

II

C-OH I C=O Pyruvate I OH3 II

Alanine-glyoxylate ~ aminotransferase " ~ (PLP)

r(S'l~2hyd r~oxida

-

O C-OH NH2-+HH Glycine

I "FMNH2}

02

O Oxalate C-OH I C-OH O II

Figure 7.20 Endogenoussourcesofoxalate reduce glyoxylate to glycolate, decreased activity of this enzyme may contribute to glyoxylate accumulation. A rise in urinary excretion of oxalate suggests that glucose ingestion promotes oxalate production, but the mechanism is not clear (Nguyen et al., 1998).

Dietary sources Major sources of dietary oxalate are cassava ( 13 mg/g), spinach ( I 0 mg/g), carrots and radishes (5 mg/g), rhubarb, bamboo shoots, parsley (17 mg/g), chives (15 mg/g), and chard (USDA, 1984). Human milk and formula contain about 8 mg/I, with a severalfold range of concentrations; this oxalate load can contribute to the increased kidney stone risk of premature infants (Hoppe et al., 1998).

Digestion and absorption Intestinal absorption is about 5%, both from the small intestine (Freel et al., 1998) and, possibly to a greater extent, from cecum and colon. Increased calcium intake reduces urinary oxalate excretion and calculus formation (Takei et al., 1998), presumably because calcium oxalate is very insoluble and thus less bioavailable than other forms ofoxalate. Oxalate is taken up via a sodium-independent transporter (CLD/DRA, SLC26A3) whose main substrates are sulfate and chloride (Silberg et al., 1995). This or a parallel


high-affinity transporter uses an oxalate-hydroxylate exchange mechanism for highaffinity transport (Tyagi et al., 2001 ). Defects in this transporter have been linked to congenital diarrhea (CLD) and colon adenoma (DRA = down-regulated in adenoma). The transporter is highly expressed in cecum and colon, to a lesser extent in small intestine. Additional transporters may similarly facilitate oxalate transport as a minor activity. Basolateral transport also appears to use a sulfate carrier.

Transport and cellular uptake Blood circulation: Oxalate is present in blood in its free ionic form. Several anion transporters in various tissues are known to accept oxalate. One of these is the sulfate carrier related to congenital diarrhea and familial risk of colon adenoma CLD/DRA (SLC26A3). Another is the diastrophic dysplasia sulfate transporter (DTDST, SLC26A2), and a third is the transporter related to Pendred syndrome (PDS, SLC26A4). Another member of the same transporter family, SLC26A6, is strongly expressed in pancreas and kidneys (Lohi et al., 2000).

Metabolism Oxalate appears to be a dead-end product in humans which cannot be metabolized.

Excretion Oxalate in primary filtrate is reabsorbed partially from the proximal renal tubule and papillary surface (Chandhoke and Fan, 2000): the transport across the basolateral membrane might be coupled indirectly with the reabsorption of chloride (Brandle et al., 1998). Normal oxalate excretion with urine is about 50 rag/day, depending on dietary intakes and metabolic and hormonal factors (Nguyen et al., 1998). Note: It is remarkable that high vitamin B6 intake from food (RR 0.66 highest vs. lowest intakes: >40 vs. 3 g/d) may cause diarrhea and induce an offensive, fishy odor.

Endogenous sources Daily endogenous production has been estimated to be 0.2 mg/kg or less and depends on the adequate availability oflysine, S-adenosylmethionine (SAM), ascorbate, iron, alphaketoglutarate, tetrahydrofolate, vitamin B6, and niacin. L-carnitine is produced in liver, kidney, and some other tissues, but not in skeletal and heart muscle. Synthesis proceeds in six steps. First, the iysine residues of myosin, actin, histones or other proteins are methylated three-fold by histone-lysine N-methyltransferase (EC2.1.1.43). In this reaction SAM provides the methyl groups; the resulting S-adenosylhomocysteine is deadenylated to homocysteine which can then be regenerated in a methylfolate and vitamin B 12-dependent reaction to methionine. As the proteins containing trimethylated lysine residues eventually are broken down by intracellular proteases, trimethyllysine is released and then hydroxylated by trimethyllysine dioxygenase (EC 1.14.11.8). This ferroenzyme uses c~-ketoglutarate and oxygen; its divalent iron is prone to oxidation itself and has to be maintained in the reduced state by ascorbate. 3-Hydroxy trimethyllysine is split into glycine and 4-trimethyl ammoniobutanal by the pyridoxal 5'-phosphate-dependent serine hydroxymethyl transferase (EC2.1.2.1). In the penultimate step, the product is oxidized by NAD-dependent 4-trimethyl ammoniobutyraldehyde dehydrogenase (EC1.2.1.47), and, finally, gamma-butyrobetaine hydroxylase (EC1.14.11.1) hydroxylates 4-trimethylammoniobutanoate to generate carnitine. This ultimate step of carnitine synthesis, just like the third reaction in the sequence using alpha-ketoglutarate and oxygen, depends on ferrous iron coordinated to the enzyme and on ascorbate to maintain this iron in the reduced state.

Dietary sources Foods may contain carnitine in its free form and as acylcarnitine. The richest sources

are red meat (50-120 mg/kg), whole milk and cheese (3 rag/l); peanut butter, asparagus (0.2 mg/kg), and whole wheat bread contain relatively more than other plant foods. The carnitine precursor trimethyllysine occurs in a few dietary proteins (histones, myosin, cytochrome c, actin, calmodulin); it is not well utilized, possibly due to its low bioavailability and high renal loss (Broquist, 1994), and high dietary intakes may actually deplete carnitine stores (Melegh et al., 1999).

434 Amino Acids and Nitrogen Compounds

OH3 NH3

CH3

H3C--N--CH 3

I

3 SAM

OH2

I

3 SAH

OH2

H3C ~ N - - C H 3

OH2

OH2

CH2

Histone-lysine I OH2 N-methyltransferase OH2 I

OH2

CH2

Intracellular proteases

CH2

OH2

I C--C--N " H H 0 Peplidyl-

--C--(;--N-" H 0

lysine

OH2 H

Peptidyltrimethyllysine

OH-C--HC--NH 2 0 Trimethyllysine

OH3 (x-Ketoglutarate + 02 ~

H3C -- N - - CH3 succinate I THF ~1 CH2 I

Trimethyllysine dioxygenase (ascorbate, Fe+§

OH2 I ?H2 CH- OH I

OH-C--HC--NH 2 O Trimethyllysine CH3

I

NAD H20

OH3

I

I

H3C -- N - - OH3

I

5,10-MTHF

Serine hydroxy methyltransferase (PLP)

CH2 ,

CHe I ?H2

HC = O

OH-C--C--NH 2 6 H2 4- Trimethylammoniobutanal + Glycine CH3

I

H3C--N--CH 3 H3C--N--CH 3 I (x-KetoI NADH CH2 glutarate succinate CH2

4-trimethylammonium butyraldeheyde dehydrogenase

CH2 I CH2 I oH,C~o

4-Trimethylammoniobutanote

!

OH-CH I GammabutyroCH2 betaine hydroxylase I (ascorbate, Fe§ oH,C~o Carnitine

Endogenoussynthesisof L-carnitine

Typical daily carnitine consumption in the US ranges between 100 mg and 300 mg (Broquist, 1994). Intakes are lowest in people consuming little meat and dairy, the main dietary sources (Sachan et al., 1997). In this case most carnitine has to be provided by endogenous synthesis, which may be inadequate by itself in very young infants (especially prematurely born infants), in elderly people, and in severely malnourished individuals.

Digestion and absorption About 65-75% of ingested carnitine is absorbed from the small intestine. Acylcarnitine in the small intestinal lumen is cleaved by pancreatic carboxylester lipase

Carnitine 435

(EC3.1.1.13). Free carnitine (both L- and D-forms) can be taken up via the sodium cotransporter OCTN2 (SLC22A5); carnitine and at least some of its esters, including propionylcarnitine and acetylcarnitine, are taken up through the amino acid transporter B~ (Nakanishi et al., 2001). Since uptake through the latter transporter is driven by sodium and chloride ions at a 2:1 ratio, it has greater concentrative power than OCTN2, which may enable it to extract carnitine at low concentrations from bacterial sources in the colon. Passive diffusion becomes important when very large doses are ingested. Uptake is two-fold greater in jejunum than ileum. D-carnitine is absorbed but not further metabolized. It is not clear how carnitine is exported from enterocytes into portal blood.

Transport and cellular u p t a k e Blood drculation: Both free and acylated carnitine is present in blood circulation. Carnitine enters liver cells with the sodium-dependent, plasmalemmal carnitine transporter OCTN2 (SLC22A5). This transporter is also present in kidney, skeletal muscle, heart, and placenta. Choline deficiency has been found to decrease tissue carnitine concentrations; this effect may be related to impaired function of the carnitine carrier (Zeisel, 1994). Blood-brain barrier: Only free carnitine, not acylcarnitine, is transported across the blood-brain barrier; a transport protein distinct from OCTN2 has been suggested (Mroczkowska et al., 1997).

Metabolism An unknown proportion ofcarnitine in tissues is broken down by carnitine decarboxylase (EC4.1. i.42) to methylcholine. This decarboxy]ase requires ATP and magnesium as cofactors. The unphysiological isomer D-carnitine is converted to trimethylaminoacetone (Bremer, 1983).

Storage Most (95%) of the body's carnitine is stored as acylcarnitine in muscle, small amounts in the liver; these stores have a turnover rate of 8 days (Broquist, 1994). For an as yet poorly understood reason choline deficiency appears to decrease carnitine stores. It has been suggested that an involvement of choline in carrier-mediated carnitine export may be the reason (Zeisel, 1994). Non-specific carboxylesterases, acylcarnitine hydrolase (EC3.1.1.28), monoacylglycerol lipase (EC3. l. 1.23), and palmitoyl-CoA hydrolase (EC3.1.2.2) in rough and smooth endoplasmic reticulum can release carnitine again from storage.

Excretion Free carnitine and acylcarnitine are filtered freely in the kidney. Reabsorption of carnitine in proximal and distal tubules from the primary filtrate uses the luminal

436

Amino Acids and Nitrogen Compounds

transporter sOCTN2 (SLC22A5) and B ~ (Nakanishi et al., 2001). Urine contains both free carnitine and acylcarnitine.

Regulation Recovery of carnitine from ultrafiltrate in the kidney is nearly complete at normal concentrations, but less at higher concentrations. Homeostatic control may therefore involve renal clearance (Broquist, 1994).

Function Fatty acid transport into mitochondria: Medium and long-chain fatty acids by themselves cannot enter mitochondria for beta-oxidation. Their translocation into the matrix depends on a shuttle system with carnitine acyltransferases on both sides of the inner mitochondrial membrane and an acylcarnitine translocase anchored to it. PalmitoyI-CoA:L-carnitine O-palmitoyltransferase I is associated with the outer mitochondrial membrane; it links a medium-chain or long-chain fatty acid from fatty acyl-CoA to free carnitine in the intermembrane space. The acylcarnitine is then transported by carnitine-acylcarnitine translocase (CACT) across the inner mitochondrial membrane in exchange for a free carnitine. In the mitochondrial matrix, finally, the fatty acid is transferred by palmitoyI-CoA:L-carnitine O-palmitoyltransferase I! (EC2.3.1.21) to coenzyme A and carnitine is released and ready for shuttling back into the intermembrane space. Short-chain fatty acids (2-6 carbons) are linked to carnitine by corresponding carnitine O-acetyltransferases (EC2.3.1.7) on the outer and inner mitochondrial membranes; carnitine O-octanoyltransferase (EC2.3.1.137) can deal with a wide spectrum of fatty acids. Fatty acid transport into peroxisomes: While carnitine is not necessary for the translocation of long-chain fatty acids into peroxisomes, carnitine acetyltransferase, specific for acyl groups with 2-6 carbons, and carnitine medium chain acyltransferase facilitate the oxidation ofacetyl-CoA and shortened fatty acids generated in the peroxisomes. Amino acid and organic acid metabolism: Acetyl-CoA newly generated from pyruvate by pyruvate dehydrogenase is readily conjugated to carnitine by acetyl-CoA C-acetyltransferase (EC2.3.1.9) and exported to other tissues, if carnitine concentrations are high (Lysiak et al., 1986). Likewise, the alpha-keto acids from catabolism of lysine and the branched-chain amino acids valine, leucinc, and isoleucine in mitochondria can be conjugated to carnitine. These and various other organic acids are exported as short-chain acylcarnitines into circulation (Ji et al., 1987; Bremer and Hokland, 1987; Bhuiyan et al., 1995). This is particularly important for short-chain fatty acids, which result from partial beta-oxidation. Their transconjugation from coenzyme A to carnitine frees up CoA for further use in beta-oxidation and tricarboxylic acid (Krebs) cycle reactions. This may be particularly important in heart or skeletal muscle when short-chain beta-oxidation is less efficient than long-chain beta-oxidation or if faster turnover in the Krebs cycle is needed during short-term exertions.

Carnitine 4 3 7

Cytosol

HS-CoA Ill

C=O

Carnitine acyltransferase I

C=O

I

S

CoA

HO OH 9 I CH3 I ..c - - C - - C - - G i N - - O H 3 O" H2 H H2 I OH3 Acylcarnitine

OH

HO, OH3 ~C--C~C--C--N ICH 3 Oz H2 H H2 I L-Carnitine CH3

Inner mitochondrial membrane

Inner mitochondrial membrane

Translocase

/ L-Carnitine OH3 Ox H2 H2 I ~C--C -CH - C - - N - - C H 3 OH ' I HO OH3 CoA Si C=O

L

Acylcarnitine OH3 O,, H2 H2 I "~C--C -CH - C - - N - - O H 3 OH I I

HO HS-CoA

ella

I

C=O

J

Carnitine acyltransferase II

Mitochondrial matrix Figure 8.102

Carnitineferriesfattyacidsacrossthe innermitochondrialmembrane

Carnitine may also have a role which makes it essential for ketogenesis in liver (Arenas et al., 1998). Gene regulation: Carnitine deficiency appears to be associated with a reduced expression of urea cycle enzymes due to unknown mechanisms; large amounts of supplemental


carnitine promote urea formation (Chapa et al., 1998). Another example is the modulation by carnitine of the extent to which expression of malic enzyme (EC1.1.1.38) and of fatty acid synthase (EC2.3.1.85) responds to triiodothyronine. Conjugation ofxenobiotics: Decreased carnitine availability can induce fatty liver following exposure to aflatoxin or carbon tetrachloride. The reason appears to be a role in conjugation and elimination of these toxins. Consequently, fewer adducts with DNA, RNA, and protein are formed (Sachan and Yatim, 1992). Membrane stability: Carnitine appears to promote the replacement ofperoxidized fatty acids in membrane phospholipids altered by oxygen free radical attack (Arenas et al., 1998); the exact mechanism remains to be elucidated. Acylcarnitine also interacts specifically with the apical membranes of renal and intestinal epithelium and thereby increases the intracellular calcium concentration. Other effects: Carnitine esters of drugs can be used to enhance their absorption from the intestine and to improve their delivery into mitochondria. Large doses ofcarnitine (2-5 g/day) are used by many athletes with the expectation to boost their energy, and by hyperlipidemic patients to lower their blood lipid levels.

References

Arenas J, Rubio JC, Martin MA, Campos Y. Biological roles of L-carnitine in perinatal metabolism. Earl), Hum Devel 1998;53:$43-$50 Bhuiyan AK, Seccombe 13, Bartlett K. Production ofacyl-carnitines from the metabolism of [U-14C]3-methyl-2-oxopentanoate by rat liver and skeletal muscle mitochondria. Clin Invest Med 1995;I 8:144-51 Bremer J. Camitine - metabolism and functions. Phvsiol Rev 1983;63:1420-80 Bremer J, Hokland B. Role of carnitine-dependent metabolic pathways in heart disease without primary ischemia. Z Kardiol 1987;76:9-13 Broquist HP. Carnitine. In Shils ME, Olson JA, Shike M, eds. Modern Nutrition in Health and Disease, Lea and Febiger, Philadelphia, 1994, pp.459-65 Chapa AM, Fernandez JM, White TW, Bunting LD, Gentry LR, Ward TL, Blum SA. Influence of intravenous L-carnitine administration in sheep preceding an oral urea drench. J Anim Sci 1998;76:2930-7 Ji LL, Miller RH, Nagle FJ, Lardy HA, Stratman FW. Amino acid metabolism during exercise in trained rats: the potential role of carnitine in the metabolic fate of branchedchain amino acids. Metab Clin Exp 1987;36:748-52 Lysiak W, Toth PE Suelter CH, Bieber LL. Quantitation of the efflux of acylcarnitines from rat heart, brain, and liver mitochondria. J Biol Chem 1986;261 : 13698-703 Melegh B, Toth G, Adamovich K, Szekely G, Gage D A, Bieber L L. Labeled trimethyllysine load depletes unlabeled carnitine in premature infants without evidence of incorporation. Biol Neonate 1999;76:19-25 Mroczkowska JE, Galla HJ, Nalecz MJ, Nalecz KA. Evidence for an asymmetrical uptake of L-carnitine in the blood-brain barrier in vitro. Biochem Biophys Res Comm 1997;241 : 127-31

Melatonin 439

Nakanishi T, Hatanaka T, Huang W, Prasad PD, Leibach FH, Ganapathy ME, Ganapathy V. Na § and Cl-coupled active transport of carnitine by the amino acid transporter ATB(0,+) from mouse colon expressed in HRPE cells and Xenopus oocytes. J Physio12001 ;532:297-304 Sachan DS, Daily JW 1II, Munroe SG, Beauchene RE. Vegetarian elderly women may risk compromised carnitine status. Veg Nutr 1997;i:64-9 Sachan DS, Yatim AM. Suppression of aflatoxin B l-induced lipid abnormalities and macromolecule-adduct formation by L-carnitine.J Env Pathol Toxicol Oncol 1992; 1i :205-10 Zeisel SH. Choline. In Shils ME, Olson JA, Shike M, eds. Modern Nutrition in Health and Disease. Lea & Febiger, Philadelphia, 1994, pp.449-58

Melatonin Melatonin (N-[2-(5-methoxy-lH-indoi-3-yl)ethyl]acetamide, amine; molecular weight 232) contains 12% nitrogen.

N-acetyl-5-methoxytrypt-

Abbreviations 5-HT 5-hydroxytryptamine(serotonin) 5-HTOL 5-hydroxytryptophol EMS eosinophilia-myalgiasyndrome PAPS 3'-phosphoadenosine5'-phosphosulfate 6-SMT 6-sulfatoxymelatonin

Nutritional summary Function: Melatonin, a hormone produced by brain, retina, and pineal gland, participates in the coordination of sleep patterns, thermoregulation, and possibly reproductive cycles to daylight. Sources: The body's production is dependent on adequate availability of L-tryptophan, niacin, vitamin B6, endogenously generated SAM (methionine, folate, and vitamin B I2), biopterin, pantotherate, ascorbate, and iron. Requirements: Endogenoussynthesis is usually adequate. Deficiency: Inadequacies of nutrients needed for endogenous synthesis may sometimes limit production, possibly contributing to insomnia and sexual dysfunction. Excessive intake: Many commercial melatonin preparations have been found to contain potentially toxic contaminants similar to those thought to have caused CH3-O

)

N H ! igu~"-"8.10

Melatonin

o H2 II C - - C - - N - - C --OH3 H2 H


eosinophilia- myalgia syndrome in some users oftryptophan supplements. Other consequences of supplemental intakes of melatonin have not been adequately evaluated.

Endogenous sources Melatonin is synthesized in brain, retina, and pineal gland. This endogenous synthesis proceeds in steps catalyzed by four enzymes and requires L-tryptophan, biopterin, niacin, vitamin B6, methionine, folate, vitamin B I2, pantotherate, ascorbate, and iron. 5-HT is an intermediate of melatonin synthesis, produced by the first two steps. The synthetic sequence is initiated by tryptophan 5-monooxygenase (ECI. 14.16.4, requires iron and ascorbate) in pinealocytes and raphe neurons as well as tissues outside the brain including beta-cells of the islets of Langerhans, enterochromaffin cells of pancreas and small intestine, mast cells and mononuclear leukocytes. The hydroxylation of L-tryptophan requires tetrahydropteridine as a cofactor and is inhibited by high concentrations of L-phenylalanine. A calcium ion-triggered protein kinase phosphorylates and thereby activates tryptophan 5-monooxygenase; melatonin, the ultimate end product, inhibits it. In the next step, aromatic-L-amino-acid decarboxylase (EC4.1.1.28), generates 5-HT. This PEP-dependent enzyme is also present in basal ganglia, sympathetic nerves, and adrenal medulla, where it acts mainly on L-tryptophan and DOPA. When aromaticL-amino-acid decarboxylase generates tryptamine by acting on L-tryptophan, 5-HT synthesis can be completed subsequently by tryptophan 5-monooxygenase. Significant quantities oftryptamine may be ingested with certain types of cheese or with foods exposed to bacterial action. 5-HT can then be acetylated by araikylamine N-acetyltransferase (EC2.Y 1.87) and finally converted into N-acetyl-5-methoxytryptamine (melatonin) by acetyiserotonin O-methyltransferase (EC2.1.1.4).

Dietary sources Normally, very little melatonin is obtained from food. Commercial preparations are now widely available as dietary supplements. Foods containing significant amounts of 5-HT include bananas (0.02 mg/g), pineapples and walnuts (Helander and Some, 2000). A few foods contain significant amount of the melatonin precursor 5-hydroxytryptamine (5-HT, serotonin), including bananas (0.02 mg/g), pineapples and walnuts (Helander and Some, 2000). Note: Eosinophilia myalgia syndrome (EMS) with symptoms that include fatigue, pain, depression, sleep disturbance, and verbal memory impairment may be caused by contaminants in commercial preparations of 5-hydroxytryptophan (Klarskov et al., 1999) and melatonin (Eaforce et al.. 1999).

Digestion and absorption Melatonin absorption proceeds rapidly from the proximal small intestine via an incompletely understood process (Aldhous et al., 1985). Fractional absorption has been

Melatonin 441

C--CH --C,, H2 I OH NH2

L-Tryptophan N

Tryptophan 5-monoxygenase (Fe2§

Tetrahydrobiopterin

oH j

Dihydrobiopterin

5-OH Tryptophan

//o C --CH - - C \ H2 I OH NH2

N"

5-OH-tryptophan decarboxylase (PLP)

~H CO2 H2 C ~C ~NH 2 H2

5-OH Tryptamine (Serotonin) N

AcetyI-CoA

oH j

Aralkylamine N-acetyltransferase

~"~CoA

N-Acetylserotonin

O H2 II C - - C - - N --C --OH 3 H2 H

N

H Acetylserotonin O-methyltransferase

~~.~ S-adenosylmethionine

CHOj

N-Acetyl-5-methoxy serotonin (melatonin)

Figure 8.104 Melatoninsynthesisin the pinealgland

S-adenosylhomocysteine

N H

O H2 II C ~ C ~ N - - C --OH 3 H2 H


found to vary between individuals by more than an order of magnitude (Waldhauser et al., 1984).

Transport and cellular uptake B/ood circu/ation: Only very small amounts of melatonin, normally around 7ng/I (Ludemann et aL, 2001 ), are circulating with blood. The melatonin precursor 5-HT is taken up into pinealocytes by a specific sodium- and chloride-dependent transporter (SLC6A4). The importance of transporter-mediated uptake to provide the precursor for melatonin synthesis is suggested by the frequent occurrence of a less active form in patients with seasonal-affective disorder (Partonen and Lonnqvist, 1998).

Metabolism The main melatonin metabolite is 6-hydroxymelatonin; smaller amounts are broken down into the serotonin metabolites 5-OH-indole 3-acetate and N l-acetyI-N2-formyl5-methoxykynuramine. Melatonin is hydroxylated at position 6 by CYPlA2 (ECl.14.14.1) and other cytochromes in the liver (Facciola et al., 2001). Conjugation by 3'-phosphoadenosine 5'-phosphosulfate (PAPS)-dependent sulfotransferase ST1A3 (EC2.8.2.3) then generates 6-suifatoxymelatonin (6-SMT), the main melatonin metabolite (Honma et al., 2001 ). Ring opening of melatonin and generation of N l-acetyl-N2-formyl-5-methoxykynuramine occurs either through enzyme action or by reaction with oxygen free radicals. The hemoproteins indoleamine-pyrrole 2,3-dioxygenase (ECl. 13.11.42) and peroxidase (myeloperoxidase, ECl. l I. 1.7) generate N l-acetyI-N2-formyl-5-methoxykynuramine (Tan et al., 2001). The same intermediate appears to be formed nonenzymically when melatonin reacts with various oxygen free radicals. Melatonin can be metabolized to 5-OH-indole 3-acetate in the liver, presumably via serotonin. N-acetylserotonin may be generated (Beedham et al., 1987) by a reversal of the reaction catalyzed by the melatonin-synthesis enzyme acetylserotonin O-methyltransferase (EC2. i. 1.4). Aryl-acylamidase (EC3.5. I. 13) deacetylates melatonin (Rogawski et al., 1979). Most 5-HT is metabolized by amine oxidase (monoamine oxidase A, MAO-A; EC 1.4.3.4, FAD-containing) and one or more of the aldehyde dehydrogenases (AIDH: EC 1.2. 1.3/NAD-requiring, EC 1.2. 1.4/NADP-requiring, EC 1.2. 1.5/NAD or NADPrequiring) to 5-hydroxyindole-3-acetate. Smaller amounts of 5-HT enter the main tryptophan catabolic pathway through the action of tryptophan 2,3-dioxygenase (ECl.13.1 l.l 1). Alcohol dehydrogenase (ECl.l.l.l) converts some 5-HT into the dead-end product 5-hydroxytryptophol (5-HTOL). Ethanol acutely increases conversion of 5-HT into 5-HTOL (Helander and Some, 2000).

Storage The dense core granules of pinealocytes contain melatonin; storage and release is mediated by vesicular monoamine transporters.

Melatonin 443

OH3 "O , ~ ' ~

O II

H2

r

II

N1-AcetyI-N2-formyl-5omethoxykynuramine Indoleamin-pyrrole 2,3-dioxygenase (heine)

F

02

CH3 "

?/

O H'r~ f, ~

i-

Peroxidaset (heme)

0 ~i~/"

.2

3

~

\ 1 o2 ~ Cytochrome ~ H 2 0 P450 1A2. ~.

o H c"~o"T~f ~

' " r ; ~ ' - ; - i ~ C - - C - - N - C -- C 3

\

.~

~,~

Sulfo-~ PAPS

transferase STIA3 I ~

acetate

OH.~'.~O__~N.3

adenosine-3',5'bisphosphate

OH I OH --S-OH I O CH3.o ~ J ~

5-OH Tryptamine (Serotonin) Amine L " " H20 + 02 oxidase (FAD) ~',,~ H202 + NH3

I,

o,,~~_c;_c.o 5-Hydroxyindole aldehyde I

Aldehyde ~*"NAD(P)+ H20 dehydrogenase F NAD(P)H OH

~-~;-ooo.

5-Hydroxyindole acetate Figure 8.105

o

" l T - ' ~ : ' r - - C - - C - - N -- C -- CH3

6-Hydroxymelatonin

N-Acetylserotonin Arylacylamidase

or -OH

O II

~ H2 ~C--C--N--C--CH

/ , , o Melatonin E ....

/

F

H202

Melatonin catabolism

6-Sulfatoxymelatonin

H2

O


Excretion The main products of melatonin metabolism in urine are 6-SMT and 5-hydroxyindole 3-acetate. 6-SMT excretion is about 0.4 i~g/kg body weight, slightly higher in winter than in summer (Davis et al., 2001 ). Supplemental intake of 5 mg melatonin was found to double 6SMT excretion (Pierard et al., 2001 ).

Regulation The activity of the 5-HT transporter that ferries the melatonin precursor into pinealocytes is tightly controlled. 5-HT-uptake and melatonin synthesis increase in the absence of light exposure (Lima and Schmeer, 1994).

Function Receptor binding: Melatonin binds to and activates specific G-protein-coupled membrane receptors. Nearly 20 genes have been identified that code for putative melatonin receptors, including rot l, MT2, and Mellc. Expression of mtl is particularly high in neurons of the suprachiasmatic nucleus and the pars tuberalis of the pituitary. Circadian rhythm: During the dark periods of the daily cycle the suprachiasmatic nucleus stimulates expression of the arylalkylamine N gene and thereby increases production and secretion of melatonin at night. Ocular light exposure, but not extraocular illumination, shifts circadian rhythms and suppresses nocturnal melatonin production (Lockley et al., 1998; Hebert et al., 1999). The efficacy of oral melatonin supplements for the relief of jet-lag remains in doubt (Spitzer et al., 1999). Seasonal rhythm: The modulation of melatonin rhythm by light exposure appears to entrain seasonal temporal organization (Pevet, 2000). The investigation of seasonally recurring events influenced by melatonin cycles has focussed mainly on estrus of horses and other mammals, and on the behavior of songbirds. The relevance for human performance or health remains unknown. Influences on mood: Melatonin may have anxiolytic properties. Preoperative use of 0.05 mg/kg reduced anxiety and relaxation without impairing cognitive and psychomotor skills (Naguib and Samarkandi, 2000). Abnormal attenuation of the nocturnal rise in plasma melatonin may be related to night-eating syndrome (NES), which consists of morning anorexia, evening hyperphagia, and insomnia (Birketvedt et al., 1999). Skin pigmentation: Uptake of melatonin increases pigment production of melanocytes and stimulates their cell division (lyengar, 2000). Free radicalscavenging: Protective effects against oxidative DNA damage and thereby antitumorigenic properties have been observed in some animal studies (Karbownik et al., 2000). It has been suggested that at sub-micromolar concentrations melatonin is 50-70 times more effective as an antioxidant compared to ascorbate or alphatocopherol. This high potency might be related to its ability to scavenge hydroxyl radicals directly and thereby terminate Fenton-type reactions (Qi et al., 2000). The

Melatonin 445

metabolite N 1-acetyl-N2-formyl-5-methoxykynuraminemay be an even more potent antioxidant than melatonin itself(Tan et al., 2001 ). Immune function: Circulating melatonin may be an important element of neuroimmune communication that periodically stimulates immune response. Cytokine (e.g. IL-6) production in lymphocytic and monocytic cells is stimulated through binding of melatonin to nuclear receptors (Guerrero et al., 2000). The pineal gland may exert oncostatic actions on distant tissues, and melatonin has been suggested to aid in the suppresion of tumor proliferation (Cos and Sanchez-Barcelo, 2000). Hormonal(unction: There have been reports that growth hormone secretion is influenced by melatonin at a hypothalamic level (Meeking et al., 1999). Melatonin reduces the sensitivity of the thyroid to thyrotropin (TSH), inhibits thyroid cell proliferation, and lowers thyroxine secretion. Fertility: Sperm cell maturation is promoted through a stimulating effect ofmelatonin on epididymal cells.

References Aldhous M, Franey C, Wright J, Arendt J. Plasma concentrations of melatonin in man following oral absorption of different preparations. Br J Clin Pharmacol 1985; 19:517-21 Beedham C, Smith JA, Steele DL, Wright PA. Chlorpromazine inhibition of melatonin metabolism by normal and induced rat liver microsomes. Eur J Drug Metab Pharmacokin 1987; 12:299-302 Birketvedt GS, Florholmen J, Sundsfjord J, Osterud B, Dinges D, Bilker W, Stunkard A. Behavioral and neuroendocrine characteristics of the night-eating syndrome. JAMA 1999;282:657-63 Cos S, Sanchez-Barcelo EJ. Melatonin, experimental basis for a possible application in breast cancer prevention and treatment. Histol Histopatho12000;15:637~,7 Davis S, Kaune WT, Mirick DK, Chen C, Stevens RG. Residential magnetic fields, lightat-night, and nocturnal urinary 6-sulfatoxymelatonin concentration in women. Ant J Epidemio12001 ; 154:591-600 Facciola G, Hidestrand M, von Bahr C, Tybring G. Cytochrome P450 isoforms involved in melatonin metabolism in human liver microsomes. Eur J Clin Pharmacol 2001;56:881-8 Guerrero JM, Pozo D, Garcia-Maurino S, Carrillo A, Osuna C, Molinero E Calvo JR. Nuclear receptors are involved in the enhanced IL-6 production by melatonin in U937 cells. Biol Signals" Recepto~w 2000;9:197-202 Hebert M, Martin SK, Eastman CI. Nocturnal melatonin secretion is not suppressed by light exposure behind the knee in humans. Neurosci Lett 1999;274:127-30 Helander A, Some M. Dietary serotonin and alcohol combined may provoke adverse physiological symptoms due to 5-hydroxytryptophol. L([i, Sci 2000;67:799-806 Honma W, Kamiyama Y, Yoshinari K, Sasano H, Shimada M, Nagata K, Yamazoe Y. Enzymatic characterization and interspecies difference of phenol sulfotransferases, ST 1A forms. Drug Metab Disp 2001 ;29:274-81 lyengar B. Melatonin and melanocyte functions. Biol Signals" Receptopw 2000;9:260-6


Karbownik M, Tan DX, Reiter RJ. Melatonin reduces the oxidation of nuclear DNA and membrane lipids induced by the carcinogen delta-aminolevulinic acid. Int J Cancer 2000;88:7-11 Klarskov K, Johnson KL, Benson LM, Gleich GJ, Naylor S. Eosinophilia-myalgia syndrome case-associated contaminants in commercially available 5-hydroxytryptophan. Adv Exp Med Biol 1999;467:461-8 Laforce R, Rigozzi K, Paganetti M, Mossi W, Guainazzi P, Calderari G. Aspects of melatonin manufacturing and requirements for a reliable active component. Biol Signals Receptors 1999;8:143-6 Lima L, Schmeer C. Characterization of serotonin transporter in goldfish retina by the binding of [3H]paroxetine and the uptake of [3H]serotonin: modulation by light. J Neurochem 1994;62:528-35 Lockley SW, Skene DJ, Thapan K, English J, Ribeiro D, Haimov I, Hampton S, Middleton B, yon Schantz M, Arendt J. Extraocular light exposure does not suppress plasma melatonin in humans. J Clin Endocrinol Metab 1998;83:3369-72 Ludemann E Zwernemann S, Lerchl A. Clearance ofmelatonin and 6-sulfatoxymelatonin by hemodialysis in patients with end-stage renal disease. J Pineal Res 2001; 31:222-7 Meeking DR, Wallace JD, Cuneo RC, Forsling M, Russell-Jones DL. Exercise-induced GH secretion is enhanced by the oral ingestion of melatonin in healthy adult male subjects. Eur J Endocrinol 1999; 141:22-6 Naguib M, Samarkandi AH. The comparative dose-response effects of melatonin and midazolam for premedication of adult patients: a double-blinded, placebo-controlled study. Anesth Analg 2000;91:473-9 Partonen T, Lonnqvist J. Seasonal affective disorder. Lancet 1998;352:1369-74 Pevet P. Melatonin and biological rhythms. Biol Signals Receptors 2000;9:203-12 Pierard C, Beaumont M, Enslen M, Chauffard E Tan DX, Reiter RJ, Fontan A, French J, Coste O, Lagarde D. Resynchronization of hormonal rhythms after an eastbound flight in humans: effects of slow-release caffeine and melatonin. Eur JAppl Phvsiol 2001 ;85:144-50 Qi W, Reiter RJ, Tan DX, Garcia JJ, Manchester LC, Karbownik M, Calvo JR. Chromium(llI)-induced 8-hydroxydeoxyguanosine in DNA and its reduction by antioxidants: comparative effects of melatonin, ascorbate, and vitamin E. Env Health Persp 2000; 108:399-402 Rogawski MA, Roth RH, Aghajanian GK. Melatonin: deacetylation to 5-methoxytryptamine by liver but not brain aryl acylamidase. J Neurochem 1979;32:1219-26 Spitzer RL, Terman M, Williams JB, Terman JS, Malt UE Singer E Lewy AJ. Jet lag: clinical features, validation of a new syndrome-specific scale, and lack of response to melatonin in a randomized, double-blind trial. Am J P,tvchiat 1999; 156:1392-6 Tan DX, Manchester LC, Burkhardt S, Sainz RM, Mayo JC, Kohen R, Shohami E, Huo YS, Hardeland R, Reiter RJ. N I-acetyl-N2-formyl-5-methoxykynuramine, a biogenic amine and melatonin metabolite, functions as a potent antioxidant. FASEB J 2001; 15:2294-6 Waldhauser E Waldhauser M, Lieberman HR, Deng MH, Lynch HL Wurtman RJ. Bioavailability of oral melatonin in humans. Neuroendocrinol 1984;39:307-13

Choline 447

Choline Choline (2-hydroxy-N,N,N-trimethylethanaminium, (beta-hydroxyethyl)trimethylammonium, bilineurine; molecular weight 104) is a weak base.

PEMT

I

sodium chloride-dependent betaine transporter (SLC6A12) cationic amino acid transporter 2 (y+, SLC7A2) organic cation transporter 1 (SLC22A1) organic cation transporter 2 (SLC22A2)

phosphatidylethanolamine-N-methyltransFerase

Nutritional

summary

Function: Choline is essential for the synthesis of phospholipids, betaine, and the

neurotransmitter acetylcholine, and to regenerate methionine from homocysteine. Adequate daily intakes are 550 mg/d for men and 425 mg/d for women. Pregnancy and lactation increase needs slightly. Sources: Egg yolk, organ meats, peanuts, and legumes are good sources. Many other foods provide smaller, but significant quantities. Lecithin supplements provide choline. Deficiency: Inadequate intake causes liver damage and possibly increases the risk of cancer and atherosclerosis. Young infants may suffer growth retardation and suboptimal cognitive development. Excessive intake: Consumption of more than 3.5 g/day can cause sweating, salivation, diarrhea, induce a fishy body odor and cause mild liver damage. Requirements:

Endogenous

I

H3C - - N - - - C H 3 CH 2

Abbreviations BGT1 CAT-2 OCT1 OCT2

CH 3

sources

Liver, and to a lesser extent brain and mammary gland, produce choline from phosphatidylethanolamine. However, the capacity of this pathway appears to be limited. De novo choline synthesis requires adequate supplies of serine, methionine, niacin, folate, vitamin B12, pantothenate, and magnesium. The immediate choline precursor phosphatidylethanolamine is produced from serine and CDP-diacylglycerol via synthesis of O-sn-phosphatidyl-L-serine (CDPdiacylglycerol-serine O-phosphatidyltransferase, EC2.7.8.8) and decarboxylation of the resulting phospholipid by pyruvate-containing phosphatidylserine decarboxylase (EC4.1.1.65; Dowhan, 1997). Phosphatidylethanolamine can be methylated to phosphatidylcholine in three successive reactions catalyzed by magnesium-dependent phosphatidylethanolamine-N-methyltransferase (PEMT, EC2.1.1.17). PEMTI in the endoplasmic reticulum is responsible for most of the activity. The genetically distinct isoform PEMT2 is located on mitochondrial membranes. Choline can be released from phosphatidylcholine through successive cleavage by phospholipase A2 (EC3.1.1.4), lysophospholipase (EC3.1. !.5), and glycerylphosphocholine phosphodiesterase (EC3.1.4.2).

I

H2C - - O H

Figure 8.106 Choline

448

Amino Acids and Nitrogen Compounds H2C - - acyl /

acyI--CH

I O II H 2 C - - O - - P--O-- C I H2 OH

NH 2 i C- COOH

i

,o

H2C--O--P--O-C-C-NH 2 I H2 H2 OH

I I I/S-adenosyl

N-methyltransferase (Mg" ") H2~ - - acyl

~methionine ll'q~"

O

I

CO2

Phosphatidylethanolamine

Phosphatidylethanolamine-

acyl~CH

~

Phosphatidylserine decarboxylase (pyruvate)

H2(~ - - acyl a c y l ~ CH

Phosphatidylserine

H

OH3

ii

I

H 2 C - - O - - P - - O - C - C - - N %CH 3 I H2 H2 OH OH3

S-adenosyl homocysteine

Phosphatidylcholine Phospholipase A2

~

H2l~--acyl

fatty acid

CH2

I O CH3 II I H 2 C - - O - - P - - O - C - C - - N '-CH 3 I H2 H2 I OH OH3

Lysophosphatidylcholine Lysophospholipase

~

OH3

I

OH2 I

O

OH3

II

I

H2C-- O-- P-- O - C - C-- N "- OH3 i H2 H2 OH

Glycerophosphocholine

OH3 Glycerylphosphocholine phosphodiesterase

OH3 I HO-C-C--N-CH 3 H2 H2 I CH 3

Figure 8.107

fatty acid

phosphoglycerol

Choline

Choisline from phosphatidylethanolamine synthesized

Carnitine decarboxylase (EC4.1.1.42; Habibulla and Newburgh, 1969) may also generate some choline. This enzyme, which contains a prosthetic complex of ATP and magnesium, generates 2-methylcholine. Cholinesterase (EC3.1.1.8, magnesiumdependent), which cleaves a wide spectrum of organoesters, is likely to demethylate

Choline 449

OH3

I H3C-- N *--CH 3 I CH2

I CH --OH I OH2

CO2

.,~

I H3C ~ N ~-- CHa I

= Carnitine decarboxylase

I COOH Camitine Figure 8.108

CH3

CH3

H

OH3

I

CH2

CH2

I CH --OH I

I

H3C --N +--CH3

Cholinesterase (Mg" +)

2-Methylcholine

I

H2C --OH

Choline

Decarboxylation and demethylation ofcarnitine generates choline

2-methylcholine. Note that choline deficiency reduces carnitine concentrations in tissues and that choline administration immediately relieves carnitine depression. Carnitine may serve as an acute supply for choline in tissues.

Dietary

sources

Foods contain free choline, choline-containing phosphatidyl choline (lecithin), sphingomyelin, and minimal amounts of acetylcholine. Good choline sources are egg yolk (5.6mg/g), liver (5.3mg/g), kidney, and legumes (e.g., peanuts 1 mg/g). Significant amounts of free choline are consumed with liver, oatmeal, soybeans, kale, and cabbages. Smaller quantities are present in many foods. Milk provides only 40 mg/l. Average daily intake of adults in the US has been estimated at 600-1000 mg (Zeisel, 1994).

Digestion

and

absorption

Choline-containing phospholipids are cleaved by pancreatic phospholipase A2 (EC3.1.1.4), and the resulting lysophosphatidylcholine (which becomes part of mixed micelles) is taken up into the small intestinal enterocyte. Free choline is taken up both by mediated transport and diffusion all along the small intestine (Zeisel, 1994). The cationic amino acid transporter 2 (CAT-2, y+, SLC7A2) is expressed in the brush border of the small intestine and specifically mediates choline uptake into enterocytes. A large proportion of absorbed choline is incorporated into phospholipids and secreted with chylomicrons into lymph. Lysophospholipase (EC3.1.1.5) cleaves iysophosphatidylcholine and then glycerylphosphocholine phosphodiesterase (EC3.1.4.2) finally releases choline. Export of free choline across the basolateral membranes is not well understood, yet. Betaine uptake from the intestinal lumen also needs further investigation. In chicks it was found to involve sodium-dependent and sodium-independent components (Kettunen et al., 2001 ). Note: Intestinal bacteria degrade a significant proportion of ingested choline and choline phospholipids to betaine and to aminotrimethylamine (TMA). Both betaine and TMA are absorbed. Flavin-containing monooxygenase (FMO, EC1.14.13.8)


N-oxidizes TMA. The main isoform responsible for metabolism in the liver is FMO3 (Treacy et al., 1998), FMO2 contributes to a lesser degree. Nearly 4% of subjects with suspected body malodor were found to have severely impaired TMA N-oxidation (fish odor syndrome). Their parents had a less pronounced (but still noticeable) decrease in FMO activity (Ayesh et al., 1993).

Transport and cellular uptake Blood circulation: Plasma contains about 15 i,tmol/1 free choline, and 50-300 mg/l phosphatidylcholine in lipoproteins. The red blood cell membranes consist mostly of choline-containing phospholipids. Free choline from blood enters cells through specific transport systems that are as yet insufficiently characterized. Uptake into liver may be mediated in part by the organic cation transporter OCT 1 (SLC22A 1). Neurons contain a high-affinity sodium chloride-dependent choline transporter that provides these cells with the essential precursor for production of the neurotransmitter acetylcholine (Kobayashi et al., 2002). Choline transporter-like proteins 1 (CDw92 is a C-terminal variant), 2 and 4 are expressed at the plasma membrane of neurons, endothelial cells, and leukocytes and play incompletely understood roles in choline transport. Choline-containing phospholipids are taken up with lipoproteins through receptormediated endocytosis. Significant amounts ofphospholipid-rich membranes are cleared by phagocytosis in spleen and reticuloendothelial system. The choline exchanger couples the influx of choline chloride into erythrocytes to the efflux of magnesium or other cations (Ebel et al., 2002). The concentration gradient drives choline via a specific choline transporter from cytosol into mitochondria of liver and kidneys (Porter et al., 1992) where most oxidation takes places. Blood-brain barrier: Choline is transported into brain both by high- and low-affinity systems (Lockman et al., 2001 ). Choline from neuronal release (acetylcholine) can leak into the extracellular space and from there into cerebrospinal fluid. The organic cation transporter 2 (SLC7A2) helps to maintain cerebral choline balance by moving excess choline across the choroid plexus into blood (Sweet et al., 2001 ). The sodium chloride-dependent betaine transporter 1 (BGTI, SLC6AI2) carries betaine from blood into the brain (Takanaga et al., 2001 ). Materno-fetal transfer: Specific transporters on maternal and fetal sides of the syncytiotrophoblast mediate transfer of choline across the placenta. The placenta contains members of the y+ amino acid transport system, including the cationic amino acid transporters 1 (SLC7AI) and 4 (SLC7A4) at the maternal side (Ayuk et al., 2000), which contribute to choline transfer. A y§ transporter in conjunction with its 4F2 (CD98, SLC3A2) glycoprotein anchor is likely to move choline across the basal membrane into fetal circulation. Cross-placental transport of choline is inhibited by many common medical drugs including propranolol, quinine, imipramine, verapamil, flurazepam, amiloride, and ritodrin (Grassl, 1994).

Choline 451

Metabolism The breakdown of choline occurs mainly in liver and kidneys and depends on adequate supplies of riboflavin, niacin, and folate. Choline in liver can be transported from cytosol into mitochondria and oxidized to betaine aldehyde (by choline dehydrogenase, EC1.1.99.1), and then to betaine (by betaine aldehyde dehydrogenase, ECI.2.1.8). Betaine provides one methyl group for the remethylation of homocysteine (by betaine homocysteine methyltransferase, EC2.1.1.5, contains zinc). The oxidation of the choline-metabolite dimethylglycine by the flavoenzyme dimethylglycine dehydrogenase (EC1.5.99.2, FAD) releases formaldehyde. The next step of choline metabolism, catalyzed by FMN-containing sarcosine dehydrogenase (ECI.5.99.1), also releases formaldehyde. An alternative pathway in kidney uses H202-producing sarcosine oxidase (Reuber et al., 1997). Glycine may be used for one of its various metabolic functions or oxidized by the glycine cleavage system. Phospholipase C (EC3. !.4.3) is a specialized zinc-enzyme in seminal plasma that generates phosphocholine.

Storage Choline is a component of the majority of phospholipids in membranes and other functional structures, and thus present in all tissues in significant amounts. The successive action of phospholipase A2 (EC3.1.1.4), lysophospholipase (EC3.1.1.5), and glycerylphosphocholine phosphodiesterase (EC3.1.4.2) releases choline from phosphatidylcholine.

Excretion MDR2 (ABCB4) transports phospholipids actively into bile. Due to very effective intestinal absorption, however, healthy people lose little choline with feces. About 270mg choline is filtered daily in the kidney, and most is reabsorbed through incompletely understood mechanisms. The organic cation transporters OCT I (SLC22A 1) at the basolateral membrane of the proximal tubules and OCT2 (SLC22A2) in the distal tubules are likely to be involved in choline transport (Arndt et al., 2001 ), but their significance may be as conduits for uptake from the pericapillary space. The choline metabolite betaine is taken up from the proximal tubule and the descending limb of Henle's loop via a brush border membrane transporter with high affinity to L-proline. Export across the basolateral membrane (Pummer et al., 2000) uses the sodium- and chloride-dependent betaine transporter (BGT 1, SLC6A 12).

Regulation The rate of choline breakdown by oxidation depends mainly on the activity of the mitochondriai transporter (Kaplan et al., 1998). The mechanisms controlling this activity are not yet known.

452

Amino Acids and Nitrogen Compounds

CH3

I

H3C --N ~--CH 3

Choline

I

OH2 I

H2C --OH

Choline dehydrogenase

H§

CH3

I

H3C --N§ I

OH2 I

COH

Betaine aldehyde Betaine ~ aldehyde dehydrogenase

NAD NADH

CH3

I

H3C --N ~--CH3 I

OH2 I

COOH

Betaine Betaine ~ homocysteine methyltransferase

H I

H3C--N '--OH 3 I

CH2 I

COOH H I

H3C --N+-H I

OH2 I

COOH H I

H--N+-H I

CH2 I

COOH

Homocysteine Methionine

~.

Dimethylglycine Dimethylglycine dehydrogenase (FAD)

THF + FAD 5-methylTHF + FADH2

Sarcosine ~~.. THF + FAD Sarcosine dehydrogenase (FMN) 5-methylTHF + FADH2

Glycine Glycine cleavage I f system (FAD, l i p o a t e )

THF + NAD 5-methylTHF + NADH

CO2 + NH3 Figure 8.109

Cholinebreakdownusesoxidationand successivedemethylation

Function

Meth),lgroup transfer: Choline is a major source of one-carbon groups. Breakdown of its metabolite betaine is directly coupled with the remethylation of methionine and

Choline 453

CH3 I HO --C --C -N "--CH 3 H2 H2 I CH3

esterase acetate~

H3C --C - - O H2

Choline

Choline f O-acetyltransferase

AcetyI-CoA CoA

OH3 I --C --C -N +--OH3 H2 H2 I OH3 Acetylcholine

Figure 8.110 Acetylcholinesynthesisand breakdown

each of the three catabolic steps after that generates the one-carbon donor methylene tetrahydrofolate. This gives choline a central role in the homeostasis of hormone synthesis, DNA methylation, and other critical metabolic events. Neurotransmission: A small amount of the intracellular choline, probably from phosphatidylcholine, is used for the synthesis of acetylcholine in neurons of the brain and the parasympathetic nervous system. Choline O-acetyltransferase (choline acetyltransferase, EC2.3.1.6) is responsible for the synthesis. Acetylcholine is cleaved again by acetylcholine esterase (YT blood group antigen, EC3. I. 1.7). Complex lipids: Choline is an important constituent of structural lipids such as phosphatidylcholine (membranes, digestive micelles, lipoproteins, intracellular signaling), sphingomyelin (enhances neural conductivity), and platelet-activating factor (hormonelike action). The synthesis of phosphatidyl choline starts with choline phosphorylation by choline kinase (EC2.7.1.32). CTP phosphocholine cytidyltransferase (EC2.7.7.15) then generates the activated form and magnesium-dependent diacylglycerol cholinephosphotransferase (CDP-choline:diacyl glycerol choline phosphotransferase, EC2.7.8.2) adds the i,2-diacylglycerol moiety. Countercurrent hypertonia: The choline metabolite betaine is an important osmolyte in epithelial cells from the renal inner medulla that helps to concentrate urine. Expression of the betaine transporter (SLC6AI2) in renal medulla is induced by hypertonicity. A similar osmoprotective function may be important for intestine (Kidd et al., ! 997). Enzyme activation: The activity of some enzymes is increased by phosphatidylcholine. A pertinent example is the aliosteric activation of 3-hydroxybutyrate dehydrogenase (ECl. I. 1.30), an enzyme of ketone body metabolism.


OH 3 I HO-C-C--N ~-CH3 H2 H2 ~H3 Choline

~ ATP

Choline kinase r (Mg++)

ADP

O OH3 II I HO--P--O-C-C--N*--CH 3 I H2 H2 OH C;H3 Phosphocholine I f CTP CTP phosphocholine It/ cytidyltransferase k I '~ PPi

NH2

O~~o

H2C O i O i O I C" H CH --OH --OH - 2 2 C H } H3

~I/i"~' ~ ' OH OH

CDP-choline Diacylglycerol cholinephosphotransferase (Mg++) H2~;- - acyl

t

l ,2-diacylglycerol CMP

acyl~CH 0 CH3 I ii I+ H2C--O--P--O-C-C--N -CH 3 i H 2 H 2 OH Phosphatidylcholine

CH3

Figure 8.111 Phosphatidylcholinesynthesisfrom choline Cancer: Choline deficiency increases cancer risk in various rodent models. An important mechanism may be inadequate DNA methylation due to a diminished pool of one-carbon groups. Many cancers are characterized by increased choline kinase activity, which effectively traps choline in affected cells (Roivainen et al., 2000). This phenomenon can be employed for radiological identification of some cancer cells by positron emission tomography (PET) after administration of I IC-labeled choline. Cell cycle regulation: Choline deficiency increases the rate of apoptotic cell death in many tissues (Shin et al., 1997).

Choline 455

References Arndt P, Volk C, Gorboulev V, Budiman 1", Popp C, Ulzheimer-Teuber I, Akhoundova A, Koppatz S, Bamberg E, Nagel G, Koepsell H. Interaction of cations, anions, and weak base quinine with rat renal cation transporter rOCT2 compared with rOCTI. Am J Phvsiol Renal Fhdd Electrolyte Physio12001 ;281 :F454-68 Ayesh R, Mitchell SC, Zhang A, Smith RL. The fish odour syndrome: biochemical, familial, and clinical aspects. Br M e d J 1993;307:655-7 Ayuk PTY, Sibley CP, Donnai P, D'Souza S, Glazier JD. Development and polarization of cationic amino acid transporters and regulators in the human placenta. Am J Physiol Cell Physiol 2000;278:C 1162-C 1171 Dowhan W. Phosphatidylserine decarboxylases:pyruvoyl-dependent enzymes from bacteria to mammals. Methods" Enqvmol 1997;280:81-8 Ebel H, Hollstein M, Gunther T. Role of the choline exchanger in Na(+)-independent Mg(2 + ) efflux from rat erythrocytes. Biochim Biophys Acta 2002; 1559:135-44 Grassl SM. Choline transport in human placental brush border membrane vesicles. Biochim Biophys Acta 1994; 1194:203-13 Habibulla M, Newburgh RW. Carnitine decarboxylase and phosphokinase in Phormia regina. J Insect Physiol 1969; 15:2245-53 Kaplan CE Porter RK, Brand MD. The choline transporter is the major site of control of choline oxidation in isolated rat liver mitochondria. FEBS Lett 1998;321:24-6 Kettunen H, Peuranen S, Tiihonen K, Saarinen M. Intestinal uptake ofbetaine in vitro and the distribution of methyl groups from betaine, choline, and methionine in the body of broiler chicks. Comp Biochem Physiol A 2001 ; 128:269-78 Kidd MT, Ferket PR, Garlich JD. Nutritional and osmoregulatory functions of betaine. Poulto, Sci 1997;53:125-39 Kobayashi Y, Okuda T, Fujioka Y, Matsumura G, Nishimura Y, Haga T. Distribution of the high-affinity choline transporter in the human and macaque monkey spinal cord. Neurosci Lett 2002;317:25-8 Lockman PR, Roder KE, Allen DD. Inhibition of the rat blood-brain barrier choline transporter by manganese chloride. J Neurochem 2001 ;79:588-594 Porter RK, Scott JM, Brand MD. Choline transport into rat liver mitochondria. Characterization and kinetics of a specific transporter. J Biol Chem 1992;267: 14637-46 Pummer S, Dantzler WH, Lien YH, Moeckel GW, Volker K, Silbernagl S. Reabsorption ofbetaine in Henle's loops of rat kidney in vivo. Ant J Physiol Renal Fhdd Elecovlyte Physiol 2000;278:F434-F439 Reuber BE, Karl C, Reimann SA, Mihalik SJ, Dodt G. Cloning and functional expression of a mammalian gene for a peroxisomal sarcosine oxidase. J Biol Chem 1997; 272:6766-76 Roivainen A, Forsback S, Gr6nroos T, Lehikoiunen E K~ihk6nen M, Sutinene E, Minn H. Blood metabolism of [methyl-I IC]choline; implications for in vivo imaging with positron emission tomography. Eur J Nucl Med 2000;27:25-32 Shin OH, Mar MH, Albright CD, Citarella MT, da Costa KA, Zeisel SH. Methyl-group donors cannot prevent apoptotic death of rat hepatocytes induced by cholinedeficiency. J Cell Biochem 1997;64:196-208


Sweet DH, Miller DS, Pritchard JB. Ventricular choline transport: a role for organic cation transporter 2 expressed in choroid plexus. J Biol Chem 2001 ;276:41611-19 Takanaga H, Ohtsuki S, Hosoya K, Terasaki T. GAT2/BGT-I as a system responsible for the transport of gamma-aminobutyric acid at the mouse blood-brain barrier. J Cereb Blood Flow Metab 2001 ;21 : 1232-9 Treacy EP, Akerman BR, Chow LML, Youil R, Bibeau C, Lin J, Bruce AG, Knight M, Danks DM, Cashman JR, Forrest SM. Mutations of the flavin-containing monooxygenase gene (FMO3) cause trimethylaminuria, a defect in detoxication. Hum Mol Genet 1998;7:839-45 Zeisel SH. Choline. In Shils ME, Olson JA, Shike M, eds. Modern Nutrition in Health and Disease. Lea & Febiger, Philadelphia, 1994, pp.449-58

Fat-soluble vitamins and non-nutrients

Free radicals and antioxidants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cholesterol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lipoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ubiquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

457 464 478 490 501 511 526 532

Free radicals and antioxidants Abbrevladon ROS

reactive oxygen

species

Various normal reactions and functions generate reactive oxygen species (ROS) and other compounds that are characterized by their high potential for causing oxidative damage to the body's DNA, proteins, membranes, and other components. Several o f these compounds are called free radicals because they contain an unpaired electron. Free radicals have a strong propensity to donate their unpaired electron to another compound or to abstract an electron from elsewhere to complement their own unpaired one. Their high and unspecific reactivity gives them the power to modify most biological macromolecules and disrupt their structure. These relentless attacks are thought to be a major cause for progressive functional decline with aging (e.g. macular degeneration) and major chronic diseases o f adulthood, including cardiovascular disease, cancer, and rheumatoid arthritis. Nonetheless, some ROS are o f vital importance for signaling and immune defense, and their elimination would probably be harmful. Several enzymecatalyzed reactions detoxify ROS and various redox-active metabolites provide additional protection. Adequate availability o f diet-derived cofactors, such as vitamins C and E, selenium, zinc, and manganese, maintains the body's natural antioxidant protection. Foods contain a wide range of additional antioxidants that may have beneficial properties. The potential for inadvertent suppression of vital ROS functions, their Handbookof Nutrient Metabolism ISBN: 0-12-417762-X

Copyright ~' 2003 ElsevierLtd All rights of reproduction ill any forrn reserved

458 Fat-soluble Vitamins and Non-nutrients

conversion into free radical metabolites, or activities unrelated to their free-radical fighting properties makes intake of large amounts of exogenous antioxidants a doubleedged sword. There is unequivocal evidence that at least some antioxidants, such as beta-carotene and vitamin E, cause harm when taken as high-dosed supplements for a long time (Albanes et al., 1995). The same compounds protect against atherosclerosis, cancer and other diseases when consumed in modest quantities from a mixed diet rich in fruits and vegetables.

Types of oxygen Free radicals ROS are to a large extent the non-stoichiometric byproducts of oxidative phosphorylation. When an electron in the respiratory chain moves to oxygen instead of the next acceptor in line, superoxide anion forms (Cadenas and Davies, 2000). A healthy 70 kg man may be expected to generate about 190-380 mmol per day, based on the assumption that 1-2% of the oxygen consumption produces superoxide anion. An additional mechanism for the production of ROS during oxidative phosphorylation occurs with the transfer of less than the required four electrons to oxygen. The transfer of only one electron generates superoxide anion, two added electrons yield hydrogen peroxide, and three electrons give rise to hydroxyl radical (*OH). Partial disruption of oxidative phosphorylation by alcohol (Bailey and Cunningham, 2002), methamphetamine (Virmani et al., 2002), other compounds, illness or genetic variants increases the production of ROS byproducts. Another major source of ROS is the breakdown of purine nucleotides (adenosine and guanosine) in peroxisomes. The final conversion to uric acid by xanthine oxidase (ECI. 1.3.22, contains FAD, iron, and molybdenum) produces hydrogen peroxide. The typical daily production of about 400-800 mg uric acid generates 2-5 mmol H202. Many other peroxisomai reactions also generate hydrogen peroxide. Several NADH/ NADPH oxidases, lipoxygenases, cyclooxygenase, and P-450 monooxygenases in other cellular compartments also contribute to ROS production. Ozone (03) is an inhaled ROS that readily reaches any tissue along with normal oxygen. At moderate levels around 50 ppb about I p,mol is inhaled per day. This amount adds little to total oxidant burden, but local effects at the point of first contact, i.e. respiratory mucosa and lung alveoli, are much more significant. Sunlight and other types of radiation are potent inducers of ROS production. Ultraviolet light is particularly damaging when acting on skin with little protection from pigments or sunscreen or on the retina of the eye. One of the most harmful ROS, the hydroxyl radical, is mainly generated during metal-catalyzed secondary reactions (Fenton reactions). Superoxide anion transfers its excess electron to a metal ion, usually iron or copper (reaction equation 1). The reduced metal ion can then abstract an electron again from hydrogen peroxide and cleave it into a hydroxyl ion and a hydroxyl radical (reaction equation 2). It is this reaction that makes unbound iron and copper so toxic even at micromolar concentrations, particularly in the presence of hydrogen peroxide. 0 2 + Fe3+ ~ 02 4- Fe2+ H202 4- Fe2+ -~ "OH + O H - +

(reaction equation I) Fe3+

(reaction equation 2)

Free Radicals and Antioxidants 45

Table 9.1

Free radicals, compounds with an unpaired electron, are common

Superoxideanion (02)

Ozone (03) Hydrogen peroxide(H202)

Singlet oxygen(102) Hydroxylradical (*OH) Tocopheroxyl radical Semidehydroascorbate Nitric oxide (NO") Peroxinitrite(ONOO) Fatty acid hydroxyperoxides Tryptophan radical ('Trp) Tyrosine (TyrO')

The signaling compound nitric oxide (NO*), which is produced from arginine, can combine with a superoxide anion and form the highly reactive peroxynitrite (reaction equation 3). O2 + NO --~ ONOO-


Protonation ofperoxynitrite forms the unstable intermediate HONOO, which rapidly decomposes with the release of a hydroxyl radical. The reaction of primary ROS with additional susceptible targets can convert these into radicals. Important examples include tryptophan (*Trp), tyrosine (TyrO*), and bilirubin radicals. Polyunsaturated fatty acids are particularly susceptible targets. Their oxidation initiates a rapidly cascading chain reaction because each radical generates two new oxidized fatty acid radicals. The metabolites of oxidized fatty acid, including 4-hydroxy-2,3-trans-nonenal (HNE), crotonaldehyde, and maiondialdehyde, are highly reactive compounds that can crosslink proteins and engage in other harmful reactions. The major antioxidants also come out of each encounter with ROS as free radicals that have to be detoxified by auxiliary reactions as described below. This is the case with vitamin E (tocopheroxyl radical), ascorbate (semidehydroascorbate), and flavonoids. Some commonly consumed foods, including coffee (Ruiz-Laguna and Pueyo, 1999), fried foods (Wilson et al., 2002), and even wine (Rossetto et al., 2001 ), can be a source of exogenous ROS and other free radical species.

Physiological functions The notion that certain ROS play important roles in normal body function is gaining momentum. ROS provide signals that can trigger mitogen-activated protein kinases (Klotz et al., 2002), modulate the adhesion of neutrophils to target sites and initiate their activation (Guo and Ward, 2002), and stimulate hormonal responses (Hsieh et al., 2002). The production of ROS is clearly regulated in some instances. A key step of programmed cell death (apoptosis) is the inactivation of mitochondrial cytochrome c for the enhanced production of ROS (Moncada and Erusalimsky, 2002). The inactivation can be spontaneous (indicating defective cell function) or the result of a regulatory event (to initiate removal of a targeted cell). The ensuing high level of ROS then contributes to the fragmentation of the cell's DNA and its ultimate demise.

460

Fat-soluble Vitamins and Non-nutrients

ROS also enable immune cells to destroy and remove pathogens. Before macrophages and other immune cells engulf bacteria, they can disrupt them with a directed stream of corrosive reactants. This oxidative blast uses ROS in addition to hypochiorous acid and other chemicals (Vazquez-Torres et al., 2000). An NADPH oxidase (phagocyte oxidase, no EC number assigned) uses FAD and cytochrome b (558) to transfer a single electron to oxygen and generate superoxide anion (Seguchi and Kobayashi, 2002). The nitric oxide produced by nitric oxide synthase (ECI. 14.13.39; contains heme and uses tetrahydrobiopterin as a cofactor) can then be combined by the lysosomal hemeenzyme myeloperoxidase (EC1.11.1.7) with superoxide to generate peroxynitrite (Eiserich et al., 2002). Superoxide also drives the production of hydrogen peroxide by superoxide dismutase (EC 1.15.1.1 ).

ROS-induced damage The main characteristic of ROS is their high reactivity with low specificity. They oxidize and crosslink proteins, fragment DNA and alter its bases, and disrupt membranes by oxidizing their fatty acids. DNA and RNA: The molecular structures of more than seventy ROS-induced DNA modifications have been identified (Pouget et al., 2002). Among the most common lesions are 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dGuo), 5-formyl-2'-deoxyuridine (5-FordUrd), 5-(hydroxymethyl)-2'-deoxyuridine (5-HmdUrd), and 5,6dihydroxy-5,6-dihydrothymidine (dThdGly). Polyunsaturated #arty acids: The changes to cho]esteryllinoleate of LDL exposed to monocytes should illustrate oxidative damage occurring with exposure to ROS. In our scenario the oxidative blast of activated monocytes has released superoxide and hydrogen peroxide and a Fenton reaction in the presence of free ionic iron has generated hydroxyl radicals. Lipid oxidation starts with the abstraction of a proton from carbon 1! between the two double bonds, but without its electron. This converts the hydroxyl radical into water, but leaves the cholesteryllinoleate with a supernumerary electron at carbon ! 1. Shift of the 12,13 double bond and movement of the extra electron to carbon 13 forms an unstable conjugated diene. The shift can also occur in the other direction o ,

HO--P=O OI

N

H2CI

,

.

N

/

0

\

o

NH2

o

HO--P=O OI

N

H2CI

OH

Guanine 2'-deoxynucleotide

Figure 9.1 Polynucleotidesare susceptibleto oxidative damage

/

0

\

8-O•

OH

2'-deoxynucleotide (8-o•

NH2

Free Radicals and Antioxidants 461 (Folcik and Cathcart, 1994). The peroxyllinoleate generated by the addition of two oxygen molecules to carbon 13 can react with an adjacent polyunsaturated linoleate and thus spread the damage. The resulting 13-hydroperoxylinoleate, however, can react with Fe2+ and generate a 13-hydroxy radical. It is this second free-radical-generating reaction that causes the amplification of the initial ROS attack. o

"OH

~.

O--C

Cholesteryllinoleate

~

O

-

-

C

H20

~

radical

O--C

~ ~

1-.

L"" 02

1 o

~, ~ - .

\ ~oleste~locta-9,11I dieny113-peroxideradical

I 1 6

1-

i

I dieny113-hydroxiderfidical O

I octa-c9,tl 1-diene Figure 9.2

o

c-~ Cholesteryllinoleate ,,L._

! OH

,~

-

,o,

-

J -

C

~

o.

oxy-

Oxidative damage to polyunsaturated lipids tends to spread

c-~

Cholesteryllinoleate~k~


Mechanisms of antioxidant action Metal ion chelation: The formation of ROS is effectively reduced by maintaining iron and copper in tightly bound form that cannot participate in Fenton-type reactions. The metal-chelating capacity of some food-derived compounds, such as flavonoids, may provide additional protection. The relevance of this effect at the cellular level remains unclear, however. Enzyme-catalyzed reactions: The body has an elaborate system to protect against ROS. These systems tend to be most active at the sites of greatest ROS release. Catalase (ECl. 1I. 1.6), which contains both heme and manganese, dissipates hydrogen peroxide in peroxisomes to oxygen and water. This enzyme with its high capacity and low affinity is best suited to detoxify overflow quantities and sudden bursts of hydrogen peroxide. Other enzymes with peroxidase activity have lower capacity, but their high substrate affinity keeps hydrogen peroxide concentrations very low. This group of high affinity peroxidases includes the peroxiredoxins (Prx), which are closely related heme enzymes. Different superoxide dismutase (ECI. 15. I. l) isoenzymes in cytosol and the extracellular space convert superoxide radicals to hydrogen peroxide (reaction equation 4). All isoenzymes contain copper and a second transition metal. The isoenzyme in mitochondria contains manganese, the ones in cytosol and extracellular fluids contain zinc or iron.

20~- + 2 H + --~ 02 + H202


Another high-capacity free radical scavenger in extracellular fluid is the copperenzyme ferroxidase (ceruloplasmin, iron (II):oxygen oxidoreductase, ECI. 16.3. l). Thioredoxin reductase (EC 1.6.4.5) is a ubiquitous NADPH-dependent selenoenzyme in cytosol that reduces both dehydroascorbate and the semidehydroascorbate radical to ascorbate (May et al., 1998). A different protective strategy seeks to remedy the damage. Four different seleniumcontaining glutathione peroxidases (ECI. l 1.1.9) with distinct tissue distributions and activity profiles use glutathione (GSH) for the reduction of peroxides of free fatty acids and other lipids. Another example is the activity ofarylesterase (paraoxonase 1, PAN1, EC3. I. 1.2) in high-density lipoprotein (HDL). This enzyme cleaves the fatty aldehydes from damaged phospholipids and releases them from the lipoprotein particle for further metabolic treatment in the liver and other tissues (Ahmed et al., 2001 ). Antioxidants: The body uses both fat-soluble and water-soluble compounds to reach all cellular compartments.

Catalase (EC1.1 1.1.6, heme) Superoxidedismutase (EC1.1S.1.1, iron, manganese,zinc) Peroxidase (EC1.11.1.7, heme) Glutathione peroxidases(EC1.11.1.9, selenium) Thioredoxin reductase (EC1.6.4.S, selenium) Arylesterase (EC3.1.1.2)

Free Radicals and Antioxidants 463

Ascorbate Thioredoxin Lipoate Tetrahydrobiopterin Uric acid Phenols, flavonoids/isoflavones

Vitamin E Ubiquinone Carotenoids Conjugated linoleic acid Protein disulfides Melatonin

The essential nutrient ascorbate is a particularly versatile antioxidant, because it can quench radicals that have one or two excess electrons. The systems for the regeneration of the oxidized forms include NADH-dependent monodehydroascorbate reductase (EC1.6.5.4), thioredoxin reductase (EC1.6.4.5), and an NADH-dependent dehydroascorbate-reducing transporter in erythrocytes (May et al., 1998). Thioredoxin is a small peptide with two redox-active cysteines that potently quenches singlet oxygen and hydroxyl radicals. The oxidation of its cysteines reduces oxidants or oxidized compounds. It also detoxifies hydrogen peroxide in conjunction with a group of enzymes, the peroxiredoxins. Thioredoxin reductase (ECI.6.4.5) uses NADH to rapidly regenerate the oxidized thioredoxin. Lipoate, tetrahydrobiopterin, uric acid, phenols, flavonoids and isoflavones, additional protein disulfides, and possibly melatonin add to the mix of water-soluble antioxidants. Vitamin E is particularly important for antioxidant protection in lipoproteins, membranes, and other lipophilic environments. Since the interaction of ROS with vitamin E generates the tocopheroxyl radical, the net effectiveness depends on adequate availability of ascorbate and other co-antioxidants for regeneration (Terentis et al., 2002). Ubiquinone, and tetrahydrobiopterin, have considerable antioxidant potential unrelated to their function as enzyme cofactors. In addition to these endogenous metabolites, a wide range of food-derived compounds is known to provide additional protection. Hundreds of carotenoids from fruits and vegetables increase the resistance of tissues to the harmful effects of ROS. ReFerences Ahmed Z, RavandiA, Maguire GF, Emili A, Draganov D, La Du BN, Kuksis A, Connelly PW. Apolipoprotein A-I promotes the formation ofphosphatidylcholine core aldehydes that are hydrolyzed by paraoxonase (PON-i) during high intensity lipoprotein oxidation with a peroxynitrite donor. J Biol Chem 2001;276:24473-81 Albanes D, Heinonen OP, Huttunen JK, Taylor PR, Virtamo J, Edwards BK, Haapakoski J, Rautalahti M, Hartman AM, Palmgren J. Effects of alpha-tocopherol and betacarotene supplements on cancer incidence in the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study. Am J Clin Nutr 1995;62:1427S- 1430S Bailey SM, Cunningham CC. Contribution of mitochondria to oxidative stress associated with alcoholic liver disease. Free Rad Biol Med 2002;32:11-16 Cadenas E, Davies KJ. Mitochondrial free radical generation, oxidative stress, and aging. Free Rad Biol Med 2000;29:222-30


Eiserich JE Baldus S, Brennan ML, Ma W, Zhang C, Tousson A, Castro L, Lusis AJ, Nauseef WM, White CR, Freeman BA. Myeloperoxidase, a leukocyte-derived vascular NO oxidase. Science 2002;296:2391-4 Folcik VA, Cathcart MK. Predominance ofesterified hydroperoxy-linoleic acid in human monocyte-oxidized LDL. J Lip Res 1994;35:1570-82 Guo RE Ward PA. Mediators and regulation of neutrophil accumulation in inflammatory responses in lung: insights from the IgG immune complex system. Free Rad Biol Med 2002;33:303-10 Hsieh T J, Zhang SL, Filep JG, Tang SS, Ingelfinger JR, Chan JS. High glucose stimulates angiotensinogen gene expression via reactive oxygen species generation in rat kidney proximal tubular cells. Endocrinol 2002; 143:2975-85 Klotz LO, Schroeder P, Sies H. Peroxynitrite signaling: receptor tyrosine kinases and activation of stress-responsive pathways. Free Rad Biol Med 2002;33:737-43 May JM, Cobb CE, Mendiratta S, Hill KE, Burk RE Reduction of the ascorbyl free radical to ascorbate by thioredoxin reductase. J Biol Chem 1998;273:23039-45 Moncada S, Erusalimsky JD. Does nitric oxide modulate mitochondrial energy generation and apoptosis? Nature Rev Mol Cell Bio12002;3:214-20 Pouget JP, Frelon S, Ravanat JL, Testard I, Odin F, Cadet J. Formation of modified DNA bases in cells exposed either to gamma radiation or to high-LET particles. Rad Res 2002;157:589-95 Rossetto M, Vianello F, Rigo A, Vrhovsek U, Mattivi F, Scarpa M. Stable free radicals as ubiquitous components of red wines. Free Rud Res 2001;35:933-9 Ruiz-Laguna J, Pueyo C. Hydrogen peroxide and coffee induce G:C --* T:A transversions in the lacl gene of catalase-defective Escherichiu coli. Mutagenesis 1999:14:95-102 Seguchi H, Kobayashi T. Study of NADPH oxidase-activated sites in human neutrophils. J Electron Microsc 2002;51:87-91 Terentis AC, Thomas SR, Burr JA, Liebler DC, Stocker R. Vitamin E oxidation in human atherosclerotic lesions. Circ Res 2002;90:333-9 Vazquez-Torres A, Jones-Carson J, Mastroeni P, Ischiropoulos H, Fang FC. Antimicrobial actions of the NADPH phagocyte oxidase and inducible nitric oxide synthase in experimental salmonellosis. I. Effects on microbial killing by activated peritoneal macrophages in vitro. J Exp &led 2000; 192:227-36 Virmani A, Gaetani E Imam S, Binienda Z, Ali S. The protective role of L-carnitine against neurotoxicity evoked by drug of abuse, methamphetamine, could be related to mitochondrial dysfunction. Ann NYAcud Sci 2002;965:225-32 Wilson R, Lyall K, Smyth L, Fernie CE, Riemersma RA. Dietary hydroxy fatty acids are absorbed in humans: implications for the measurement of"oxidative stress" in vivo. Free Rad Biol &led 2002;32:162-8

Vitamin A The term vitamin A usually refers to retinol (vitamin A 1,3,7-dimethyl-9(2,6,6-trimethyll-cyclohexen-l-yl)-2,4,6,8-nonatetraen-l-ol, obsolete name ophthalamin; molecular weight 286), its esters, and the metabolically equivalent retinal (retinaldehyde, retinene, vitamin A aldehyde, axerophthal; molecular weight 284). Beta-carotene, alpha-carotene,

Vitamin A 465

AII-trans-retinol

AII-trans-retinal

Figure 9.3 Severalstructurally related compounds havevitamin A activity

cryptoxanthin, and a few other carotenoids, which can be metabolically converted into retinal and retinol, are referred to as provitamin A. The term vitamin A should not be used for retinoic acid (vitamin A acid, 3,7-dimethyl-9-(2,6,6-trimethyl- l-cyclohexen- l-yl)2,4,6,8-nonatetraenoic acid, tretinoin; molecular weight 300), a metabolite ofretinol with important biological functions, because it cannot be converted back into its precursor.

Abbreviations ADH CRBP2 LDH RAE RALDH1 RALDH2 RBP1 RBP2 RBP3 RBP4 vitA

alcohol dehydrogenase (EC1.1.1.1) cellular retinol binding protein 2, RBP2 lactate dehydrogenase isoform (EC1.1.1.27) retinol activity equivalents retinal dehydrogenase 1 (EC1.2.1.36) retinal dehydrogenase 2 (EC1.2.1.36) retinol-binding protein 1 (cellular RBP) retinol-binding protein 2 (cellular RBP ofenterocytes) retinol-binding protein 3 (interstitial RBP) retinol-binding protein 4 (in blood)

vitamin A (all forms)

Nutritional summary Function: Vitamin A (vitA) is essential for vision, immune function, and regulation of cell growth. Food sources: Only animal foods contain retinol. Particularly rich sources are liver, flesh foods, eggs, and fortified milk. Good sources of provitamin A carotenoids are carrots, spinach, broccoli, yellow melons, mangos, and many other dark-green or orange-yellow colored fruits and vegetables. The activation of vitA requires riboflavin, niacin, zinc, and iron. Requirements: To account for metabolic differences between vitA and its carotenoid precursors dietary amounts are expressed as equivalents of I p.g retinoi (retinol activity equivalent = RAE). 12 ~g beta-carotene and 24 t-tg alpha-carotene or beta-cryptoxanthin corresponds to 1 RAE. Adults should get between 700 (women) and 900 (men) RAE. Women's needs are higher during lactation.


Deficiency: A lack ofvitA (for which about one-third of the world population is at risk) initially causes reversible night blindness, later increasingly severe and irreversible loss of vision due to changes of the eye structure (xerophthalmia with drying of the conjunctiva and increasing opacity of the cornea). Hyperkeratosis and other skin lesions are further typical effects of inadequate intake. Another concern with even mild deficiency is impaired immune function, especially in children. Excessive intake: Retinol intake above 1000 I-tg/d increases bone fracture risk in older people. Moderately high intakes of retinol (3000 Ixg/d), but not of provitamin A carotenoids, during early pregnancy increase the risk of birth defects. Daily ingestion of 15 000 p.g retinol initially may lead to itching, scaling of skin, malaise, and loss of appetite. Cerebrospinal pressure may increase causing nausea, vomiting, headaches, and eventually seizures, coma, respiratory failure, and death.

Endogenoussynthesis Beta-carotene 15,15'-dioxygenase (EC I. 14.99.36) in the cytosol of mature jejunal enterocytes, liver, kidney, and testes splits carotenoids in the middle (Duszka et al., 1996; Barua and Olson, 2000). This enzyme, which is actually a monooxygenase, requires bile acids and iron for its three-step activity (epoxidation at the 15,15'-double bond,

15,15'-dioxygenase (bile acids, iron)

P 02

-trans-retinal Alcohol dehydrogenase

(Zn§247

~~0Hs.rei~0 Figure 9.4

1 2 NADH

2 NAD

~i

~

~

Beta-carotene and a few ocher carotenoids can be converted into retinol

Vitamin A 467

hydration to the diol, and oxidative cleavage). The provitamin A carotenoids yield two (beta-carotene) or one (alpha-carotene, gamma-carotene, beta-cryptoxanthin, betazeacarotene) retinal molecules (Rock, 1997). Some other carotenoids are also cleaved (e.g. lycopene, lutein, and zeaxanthin), but do not generate a fragment that can be converted into retinol. Excentric (asymmetric) cleavage at the 14', 13' double (Dmitrovskii et al., 1997) or the 9', 10' double bond (Kiefer et al., 2001 ) by as yet incompletely characterized enzymes appears to play a much smaller role. Excentric cleavage products include 8'-fl-apocarotenal (the largest metabolite), ! 0'-fl-apocarotenal, 12'-fl-apocarotenal, and 14'-flapocarotenal. Oxygen free radicals, such as fatty acid hydroperoxides, can also react with any of the conjugated polyene double bonds and initiate cleavage (Yeum et al., 1995). The antioxidant alpha-tocopherol can prevent such random cleavage and keep retinal yield near the theoretical optimum (Yeum et al., 2000). The side chain of longer fragments can be shortened by beta-carotene 15,15'-dioxygenase to generate retinal (Paik et al., 2001 ). Retinal resulting from carotenoid cleavage is rapidly metabolized, mainly to retinol. A specific lactate dehydrogenase isoform (LDH-C, ECl.I.I.27) in testes is closely associated with beta-carotene 15,15'-dioxygenase, and the reduction of newly generated retinal may be its main physiological function (Paik et al., 2001 ). In most other tissues class II1 alcohol dehydrogenase (ADH, ECl.i.l.l) may be more important 18' 17

16

IL Oll I-'a ~11 4~~ Y " ~

~ \

Figure 9.5

7

/ 8

18

19

11 10 R~t~

/ 12 ~

20

lS 14

14' 15'

12' I 20'

10' 11'

Beta-carotene(13,13-Carotene)

Cryptoxanthin

C a r o t e n o i d s t h a t can be converted i n t o r e t i n o l

8' I i_ 19 .

7' .

.

II;~'

;'1

/ 16' .

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468


(Molotkov et al., 2002). Retinoic acid is exported through an unknown pathway into portal blood and taken up by the liver. Dietary intake Foods contain retinol, retinyl esters, provitamin A carotenoids, and also compounds with a restricted spectrum of vitamin A activity, such as retinoic acid. Several isomeric forms of both retinol derivatives and carotenoids are possible. The most common form of retinol in natural foods is the all-trans isomer. Much smaller amounts of 9-cis-retinol and other vitA metabolites are ingested with some foods. Synthetic retinol may also contain isomers in addition to the biologically active all-trans-retinol. Compounds with vitA activity are very sensitive to oxidation, isornerization, and polymerization. To facilitate assessment, intakes of different forms o f v i t A are usually expressed as retinol activity equivalents (RAE) with I ~g all-trans-retinol as the basis (Food and Nutrition Board, Institute of Medicine, 2002). The conversion factors for non-retinol compounds take bioavailability and metabolic yield into account. Thus, 12 IJ-g of ingested beta-carotene corresponds to i RAE, as do 24 p~g of alpha-carotene and 24 p,g ofbeta-cryptoxanthin. International Units (IU) for retinol are divided by 3.33 to convert into RAE, those for 13-carotene are divided by 20. Retinol and directly related compounds are only present in animal-derived foods. The highest concentration is in organ meats, such as beef liver ( 106 i,cg RAE/g). Another group of retinol-rich foods are hard and cream cheeses such as Swiss cheese (2.5 IJ-g/g), cheddar (2.8 IJ,g/g), and regular cream cheese (3.8 I-cg/g). Eggs contain about 1.9 I~g/g, milk 0.6 I-cg/g. Good plant-derived sources of vitamin A precursor carotenoids are green leafy vegetables, including spinach (8.2 i~g RAE/g), kale (7.4 rag/g), chard (3.1 i~g/g), and broccoli ( 1.4 ~g/g). Other good sources are orange or red-colored vegetables and fruits, such as sweet potatoes (16.4 i,zg/g) and carrots (24.6 ~g/g). Median daily intakes ofvitA from all sources were estimated to be around 900 I-~gfor men, and around 700 ~g for women in the US (Food and Nutrition Board, Institute of Medicine, 2002: Appendix C). More than half of their total vitA intake comes from preformed retinol.

Digestion and intestinal absorption Both retinol derivatives and carotenoids are absorbed from the proximal small intestine in a process that requires the formation of mixed micelles and concurrent fat absorption. About 70-90% of ingested retinol is absorbed (Sivakumar and Reddy, 1972), but only 3% or less of carotenoids (Edwards et al., 2002). Retinol: Retinylesters are cleaved by lipase (EC3.1.1.3) and sterol esterase (EC3.1.1.13) from pancreas and at least one other brush border esterase. Retinol is only absorbed efficiently when it is incorporated into mixed micelles along with lipase-digested fat (fatty acids and monoglycerides), bile acids, and phospholipids (Harrison and Hussain, 2001). It has been suggested that a specific transporter may be involved in retinol uptake, but its identity remains elusive. Retinol-binding protein 2 (RBP2, cellular retinol

Vitamin A 469

Y retinylesters Sterol II

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retinol 9

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J

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41

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retinol |

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Enterocyte

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Intestinal absorption of vitamin A

binding protein 2, CRBP2) is needed for retinol metabolism and trafficking across the enterocyte. For example, phosphatidylcholine-retinol O-acyltransferase (EC2.3.1.135 ) esterifies only retinol that is bound to RBP2. Retinoi and retinylesters exit enterocytes as integral components ofchylomicrons. The details of their transfer into chylomicrons are not yet understood. A small amount of newly absorbed retinol is oxidized to retinoic acid by class Ill ADH (ECI.I. 1.1; Molotkov et al., 2002). Retinoic acid is exported through an unknown pathway into portal blood and taken up by the liver. Carotenoids: Digestion by the various proteases and peptidases releases carotenoids, if they are attached to proteins. Since carotenoids in plants are embedded in the thylakoid matrix within cells with fairly digestion-resistant walls, cooking or extensive mechanical grinding (chewing) is usually necessary (Rock et al., 1998). The highest reported betacarotene bioavailability from carrots, in a finely homogenized preparation, is still less than 3%, however (Edwards et al., 2002). Carotenoids have to be incorporated into mixed micelles before they can be absorbed (Yeum and Russell, 2002). The exact mechanism of carotenoid uptake into enterocytes is uncertain. It does not appear to be very selective. Carotenoids are incorporated by unknown means into chylomicrons and exported into lymph. Carotenoids take about eight hours after a challenge meal to appear in blood.

Transport and cellular uptake Blood drculation: The concentration of retinol (about one-tenth as retinyl ester) in plasma is homeostatically maintained around 2 I~mol/l in men, and closer to 1.7 Ixmol/l in women (Food and Nutrition Board, Institute of Medicine, 2002: Appendix G; Olmedilla et al., 1994). On its first pass from portal blood through the liver, retinol is


taken up into hepatocytes through an unknown mechanism. It can then be resecreted with retinol-binding protein 4 (RBP4). RBPs are lipocalins, which means that the protein engulfs a retinol molecule and shields it from the hydrophilic environment in circulation or inside cells. After its release from the liver RBP4 combines in blood with transthyretin (Episkopou et al., 1993). A surface receptor on many peripheral cells binds RBP4 and mediates the uptake of the bound retinol. Some epithelial cells, including those in epididymis, thyroid, parathyroid, and endometrium, express the facultive RBP receptor megalin. This member of the LDL-receptor family delivers the RBP-bound retinol to the cell by endocytosis. In vitA deficiency, the production and release of RBP4 by the liver increases. About 0.19 p~mol/l beta-carotene, 0.22 p~mol/l beta-cryptoxanthin, and 0.04 p,mol/l alpha-carotene circulate with plasma (Ruiz Rejon et al., 2002). Chylomicrons carry much of the recently ingested carotenoids. They rapidly lose triglycerides, but not carotenoids, and the depleted chylomicron remnants are taken up into hepatocytes and extrahepatic cells via diverse lipoprotein receptors. Low-density lipoproteins (LDL) and high-density lipoproteins (HDL) also carry some carotenoids and bring them into cells when they are taken up through their typical receptor-mediated endocytotic pathways. Blood-brain barrier: The pathways for transfer ofcarotenoids from blood into brain are not well understood. Materno-fetal transfer: Megalin, which is expressed in syntrophoblasts, binds and internalizes RBP (Christensen and Birn, 2001 ). How much this pathway contributes to vitA transfer across the placenta is not known. Another putative RBP receptor has been identified (Johansson et al., 1999). To some extent retinol can also be esterified and stored in villous mesenchymal fibroblasts of the placenta (Sapin et al., 2000).

Metabolism Retinol and its provitamin A precursors are converted into active metabolites in tissue-specific patterns. Oxidation of retinoic acid is the only known inactivating catabolic pathway. Among the metabolites with functional importance are retinol itself, all-trans-retinal and I I-cis-retinal (vision), all-trans-retinoic acid, 9-cis-retinoic acid, 13-cis-retinoic acid, and 14-hydroxy-retroretinol. It may be safely assumed that not all enzymes with significant vitA metabolic activity are known, yet. The picture is additionally complicated by the extensive overlapping activity spectra of the various enzymes involved. All-trans-retinoic acid synthesis: Conversion of retinoi via retinal to retinoic acid requires two successive oxidation reactions. Among the widely expressed dehydrogenases suitable for the first oxidation are alcohol dehydrogenases I (ADH I, EC I. 1. !. 1, contains zinc), 4 (ADH4), and 7 (ADH7, in the cytoplasma of epithelial cells of the stomach). Aldehyde dehydrogenase 1 (ALDHI) and retinal dehydrogenase 2 (RALDH2, ECI.2.1.36, contains FAD) usually complete retinoic acid synthesis (Duester, 2000). AIl-trans-retinoic acid is quantitatively the predominant isomer. 9-cis-retinoic add: Liver, kidneys, small intestine and other tissues produce the 9-cis isomer ofretinoic acid that is functionally and metabolically distinct from the all-trans isomer (Zhuang et al., 2002). The enzyme responsible for conversion of ali-transretinol to 9-cis-retinol is not known. The isoenzymes 1 and 3 of cis-retinol/androgen

Vitamin A 4 7 1

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OH Al,-trans-retinoicacid

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?

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~ 13-cis-retinoic acid

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I

1 O'~'OH I 02 +reduced Cytochrornel/f flavoprotein p450RAI [ (heine) I~H20 + oxidized ~, flavoprotein

O OH

I

OH

O

4-oxo-all-trans- retinoic acid

O

retinoic acid

%" OH

Figure 9.7

Retinol is the precursor of several retinoic acid isomers

dehydrogenase (no EC number assigned) and retinol dehydrogenase (RDH5) oxidize 9-cis-retinol to 9-cis-retinal. Enzymes that accept this isomer as a substrate for further oxidation to 9-cis-retinoic acid include RALDH I, RALDH2, and ALDH 12. A retinol dehydrogenase (cRDH, ECI. 1.1.105) specifically oxidizes 9-cis-retinol to 9-cis-retinal, particularly early in fetal development (Gamble et al., 1999). Beta-carotene may also be converted directly into retinoic acid. Retinoic acid breakdown: Cytochrome p450RAI- 1 (CYP26A 1) in liver and many other tissues and p450RAI-2 (CYP26BI) in brain oxidizes retinoic acid to 4-hydroxy retinoic acid, 4-oxo-retinoic acid, and 18-hydroxy retinoic acid, and thereby contributes to retinoic acid clearance (White et al., 2000). Microsomal NADPH-ferrihemoprotein reductase (NADPH-cytochrome P450 oxidoreductase, EC 1.6.2.4, contains FMN and FAD) is the flavoprotein that provides the electrons for this reaction. Both p450RAI-I and p450RAI-2 prefer all-trans-retinoic acid, but also catabolize 9-cisretinoic acid and 13-cis-retinoic acid, though with lower activity. The ethanol-inducible cytochrome CYP2EI also inactivates retinoic acid (Liu et al., 2001).


3,4-Didehydroretinol: This metabolite (vitamin A2) is produced in keratinocytes and other skin cells through a poorly understood mechanism (Andersson et al., 1994). The conversion appears to be irreversible and controlled by all-trans-retinoic acid (Randolph and Simon, ! 996). Further oxidation by one or several of the previously described retinal oxidizing enzymes can then generate the 3,4-didehydroretinoic acid, which is a potent metabolite (Sani et al., 1997). It may be lack of this derivative that causes the typical scaly skin lesions and the production of abnormally large keratins during vitA deficiency (Olson, 1994).

Figure 9.8

The potent retinol metabolite 3,4-didehydroretinoic acid is produced in skin

Hydroxylated retinoids: Most body tissues can add a hydroxyl group to retinol and generate metabolites with distinct biological and metabolic properties. Conversion of retinol to 14-hydroxy-4,14-retro-retinol (14-HRR), 13,14-dihydroxyretinol, anhydroretinol, and 4-hydro-5-hydroxy-anhydroretinol, but not to retinoic acid analogs, has been demonstrated in the liver (Mao et al., 2000). The enzymes and other specific proteins involved in most of these conversions remain to be identified, however. In 14-hydroxy-4,14-retro-retinol (14-HRR) and anhydroretinol the double bonds of the retinoid side-chain are shifted towards the ring system. A phosphoadenosine 5'-phospho-sulfate (PAPS)-requiring retinol dehydratase (no EC number assigned) generates anhydroretinol from retinol in insects and fish, but no corresponding human enzyme has yet been identified (Pakhomova et al., 2001 ).

14-Hydroxy-4,14-retroretinol

Anhydroretinol

OH 4-Hydro-5-hydroxy-anhydroretinol Figure

9.9

Hydroxylated retinol metabolites

Storage Most (80%) of the body's vitA reserves (around 450 mg) of healthy vitA-replete adults are stored in liver (500 I~g/g wet tissue; Leo and Lieber, 1999). This amount can cover

Vitamin A 473

requirements for several months. The vitA in liver resides mainly as retinyl palmitate in lipid globules of hepatocytes (10-20%) and of stellate (lto) cells (80-90%). Two enzymes esterify free retinol, retinol O-fatty-acyltransferase (acyl CoA:retinol acyltransferase, ARAT, EC2.3.1.76) and phosphatidylcholine-retinol O-acyltransferase (lecithin:retinol acyltransferase, LRAT, EC2.3.1.135). The stored esters are released through the action ofall-trans-retinyl-palmitate hydrolase (EC3.1.1.64) or 1 l-cis-retinyl-palmitate hydroiase (EC3. i. 1.63). High alcohol intakes can mobilize vitA and deplete stores (Leo and Lieber, 1999).

Excretion Retinoic acid and other metabolites can be conjugated to glucuronide and excreted with bile. The relatively high efficiency of intestinal absorption for most retinoids minimizes losses, however, and maintains extensive enterohepatic cycling. Several milligrams ofretinol are filtered in the kidney, despite the considerable size of the RBP4-transthyretin complex to which retinol is attached. Megalin, a particularly large member of the LDL-receptor family, specifically binds RBP4 and mediates its uptake by endocytosis. Recent evidence suggests that RBP4 and its retinol load can proceed through the epithelial cell and return intact into circulation (Marino et al., 2001 ).

Regulation The availability ofretinoic acid in particular tissues is largely controlled by induction of producing (ALDH 1 and RALDH2) and catabolic (cytochrome P450RAI) activities (White et al., 2000).

Function The vitamin A metabolite 1l-cis-retinal is a critical component of the lightdetecting complex in the photoreceptor cells of the eye. Interaction with a single photon triggers the conversion of the 1 l-cis to the all-trans isoform, which leads to the release of retinal from the associated pigment protein (opsin) and the start of a signaling cascade. Rapid reduction of the free ail-trans-retinal by retinol dehydrogenase (all-trans-retinol dehydrogenase, ECI. 1. !. 105, NAD-dependent) prevents the reversion into the visionactive form (Saari et al., 1998). Four distinct l l-cis-retinal-containing rhodopsins (vision pigments) with specific properties enable humans to detect light at particular wavelengths. Rhodopsin in combination with ! I-cis-retinal absorbs light with a maximum at 495 nm in retinal rods. This pigment gives dual-mode vision (black-and-white). Three pigments in the retinal cones, each consisting of a specific cone pigment and 1-cis retinal, provide color vision. Blue cone pigment absorbs best at 440 nm and gives blue vision. Green cone pigment detects green due to its absorption maximum at 535 nm, and red cone pigment detects red and yellow with a maximum at 560 nm. The retinal pigment epithelium forms a layer between the photoreceptors and the capillary blood supply. The active metabolite, 1 l-cis-retinal, is produced from retinol by Vision:


......

L . . . , ~ AII-trans-retinol

If NAD

Retinol

AII-trans-retinolr dehydrogenase[~ NADH

isomerase

/

I . . . . . ~ All-trans-retinal

deli~CiAgd;a;:e

NADH

Light

= rhodopsin Pigment epithelium cell Figure 9.10

Photoreceptor cell

Vitamin A metabolism in the retina

the successive action ofretinol isomerase (EC5.2.1.7) and 1l-cis-retinol dehydrogenase (RDH5, no EC number assigned, can use NAD as well as NADP). It has been suggested that cellular retinaldehyde binding protein (CRALBP) acts as an acceptor for the 11-cis-retinal intermediate (Saari et al., 2001 ). Inhibition of RDH5 might contribute to the transient blindness sometimes seen with excessive licorice consumption (Dobbins and Saul, 2000). All-trans-retinal can also be activated by retinal isomerase (EC5.2.1.3). The pigment epithelial cells also store limited amounts of vitA as retinylesters. For this purpose phosphatidylcholine-retinol O-acyltransferase (EC2.3.1.135) moves a fatty acid from phospholipid to the RBPl-bound retinol. The stored esters are released through the actions ofall-trans-retinyl-palmitate hydrolase (EC3. i. !.64), I l-cis-retinylpalmitate hydrolase (EC3.1.1.63), or all-trans-retinylester isomerohydrolase (no EC number assigned). RBP3 (interphotoreceptor retinoid-binding protein, IRBP) is a specific lipocalin in the interstitial space between pigment epithelium and photoreceptors. Its importance may extend more to the survival of photoreceptor cells than to their supply of active 1 l-cis-retinal (Palczewski et al., 1999). Nuclear actions: VitA is of vital importance for normal endodermal differentiation, morphogenesis, regulation of embryonic and childhood development as well as for sustaining balanced cell proliferation, differentiation, and apoptosis in adulthood. Many of the underlying events involve the binding ofvitA-containing receptors to specific binding elements in nuclear DNA and control of the expression of associated genes. Two groups of retinoid receptors have been identified. The first includes the retinoic acid receptors (RAR) alpha, beta, and gamma. Their ligands are all-trans-retinoic acid, 9-cis-retinoic acid,

Vitamin A 475

O Figure 9.11

4-Oxoretinol

4-oxoretinoic acid, 4-oxoretinoi, and a few other candidate retinoids. The second group is comprised of the retinoic X receptors alpha, beta, and gamma with 9-cis-retinoic acid as the main activating ligand. The RXR group is particularly interesting because association with them enables the actions of numerous other nuclear receptors. The list of RXR-dependent nuclear binding proteins includes RAR, thyroid receptors (TR), vitamin D receptor (VDR), peroxisome proliferation activating receptors (PPAR), pregnane X receptor (steroid and xenobiotic receptor, SXR/PXR), liver X receptors (LXR), farnesoid X-activated receptor (FXR), and benzoate X receptor (BXR). The diversity of functions becomes evident just from the designations of these receptors. A compilation of published research data indicates more than a hundred genes that are known or likely targets of retinoic acid-mediated action alone (Balmer and Blomhoff, 2002). Cell cycleand apoptosis: In contrast to the generally growth-promoting properties of the major forms of vitA, anhydroretinol triggers (nonclassical) programmed cell death (Korichneva and Hammerling, 1999), possibly by inducing oxygen free radical production (Chen et al., 1999). 4-Oxoretinol is able to induce growth arrest and promote differentiation of promyelocytes (Faria et al., 1998). Apocarotenoic acids, the metabolites ofexcentric beta-carotene cleavage products, inhibit tumor cell growth through mechanisms that are distinct from all-trans-retinoic acid (Tibaduiza et al., 2002). Cell signaling: Retinoi and some metabolites interact directly with phosphokinase C (PKC), a central element of the intracellular signaling cascade (Imam et al., 2001 ). They may do this through binding to specific sites and directing the functionally important oxidation of particular cysteines.

References

Andersson E, Bjorklind C, Torma H, Vahlquist A. The metabolism of vitamin A to 3,4didehydroretinol can be demonstrated in human keratinocytes, melanoma cells and HeLa cells, and is correlated to cellular retinoid-binding protein expression. Biochim Biophys Acta 1994; 1224:349-54 Balmer JE, Blomhoff R. Gene expression regulation by retinoic acid. J Lip Res 2002; 43:1773-808 Barua AB, Olson JA./3-carotene is converted primarily to retinoids in rats in vivo. J Nutr 2000; 130:1996-2001 Chen Y, Buck J, Derguini F. Anhydroretinol induces oxidative stress and cell death. Cancer Res 1999;59:3985-90 Christensen EI, Birn H. Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule. Am J Physiol Renal Fhdd Elecovlyte Physio12001 ;280:F562-573


Dmitrovskii AA, Gessler NN, Gomboeva SB, Ershov YV, Bykhovsky VY. Enzymatic oxidation of beta-apo-8'-carotenol to beta-apo-14'-carotenal by an enzyme different from beta-carotene- 15,15'-dioxygenase. Biochem (Russia) 1997;62:787-92 Dobbins KR, Saul RE Transient visual loss after licorice ingestion. ,I Neuro Ophthalmol 2000;20:38-41 Duester G. Families of retinoid dehydrogenases regulating vitamin A function: production of visual pigment and retinoic acid. Eur,1 Biochem 2000;267:4315-24 Duszka C, Grolier P, Azim EM, Alexandre-Gouabau MC, Borel P, Azais-Braesco V. Rat intestinal beta-carotene dioxygenase activity is located primarily in the cytosol of mature jejunal enterocytes. ,1 Nutr 1996; 126:2550-6 Edwards A J, Nguyen CH, You CS, Swanson JE, Emenhiser C, Parker RS. Alpha- and betacarotene from a commercial puree are more bioavailable to humans than from boiledmashed carrots, as determined using an extrinsic stable isotope reference method. ,1 Nu02002; 132:159-67 Episkopou V, Maeda S, Nishiguchi S, Shimada K, Gaitanaris GA, Gottesman ME, Robertson EJ. Disruption of the transthyretin gene results in mice with depressed levels of plasma retinol and thyroid hormone. Proc NatlAcad Sci 1993;90:2375-9 Faria TN, Rivi R, Derguini F, Pandolfi PP, Gudas LJ. 4-Oxoretinol, a metabolite of retinol in the human promyelocytic leukemia cell line NB4, induces cell growth arrest and granulocytic differentiation. Cancer Res ! 998;58:2007-13 Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. National Academy Press, Washington, DC, 2002 Gamble MV, Shang E, Zott RP, Mertz JR, Wolgemuth DJ, Blaner WS. Biochemical properties, tissue expression, and gene structure of a short chain dehydrogenase/reductase able to catalyze cis-retinol oxidation. J Lipid Res 1999;40:2279-92 Harrison EH, Hussain MM. Mechanisms involved in the intestinal digestion and absorption of dietary vitamin A. ,1 Nuo" 2001; 131 : 1405-8 Imam A, Hoyos B, Swenson C, Levi E, Chua R, Viriya E, Hammerling U. Retinoids as ligands and coactivators of protein kinase C alpha. FASEB J 2001;15:28-30 Johansson S, Gustafson AL, Donovan M, Eriksson U, Dencker L. Retinoid binding proteinsexpression patterns in the human placenta. Placenta 1999;20:459-65 Kiefer C, Hessel S, Lampert JM, Vogt K, Lederer MO, Breithaupt DE, von Lintig J. Identification and characterization of a mammalian enzyme catalyzing the asymmetric oxidative cleavage of provitamin A. ,1 Biol Chem 2001 ;276:14110-16 Korichneva I, Hammerling U. F-actin as a functional target for retro-retinoids: a potential role in anhydroretinol-triggered cell death. J Cell Sci 1999; 112:2521-8 Leo MA, Lieber CS. Alcohol, vitamin A, and beta-carotene: adverse interactions, including hepatotoxicity and carcinogenicity. Am J Clin Nutr 1999;69:1071-85 Liu C, Russell RM, Seitz HK, Wang XD. Ethanol enhances retinoic acid metabolism into polar metabolites in rat liver via induction of cytochrome P4502EI. Gastroenterol 2001 ; 120:179-89 Mao GE, Collins ME), Derguini E Teratogenicity, tissue distribution, and metabolism of the retro-retinoids, 14-hydroxy-4,14-retro-retinol and anhydroretinol, in the C57BL/6J mouse. Toxicol Appl Pharmaco12000;163 :38-49

Vitamin A 477

Marino M, Andrews [3, Brown D, McCluskey RT. Transcytosis of retinol-binding protein across renal proximal tubule cells after megalin (gp330)-mediated endocytosis. J A m Soc Nephro12001 ; 12:637-48 Molotkov A, Fan X, Deltour L, Foglio MH, Martras S, Farres J, Pares X, Duester G. Stimulation of retinoic acid production and growth by ubiquitously expressed alcohol dehydrogenase Adh3. PJvc Natl Acad Sci USA 2002;99:5337-42 Olmedilla B, Granado E Blanco I, Rojas-Hidalgo E. Seasonal and sex-related variations in six serum carotenoids, retinol, and alpha-tocopherol. Am J Clin Nutr 1994;60:106-10 Olson JA. Vitamin A, retinoids, and carotenoids. In: Shils ME, Olson JA, Shike M, eds. Modern nutrition in health and disease. Lea & Febiger, Philadelphia, 1994, pp.287-307 Paik J, During A, Harrison EH, Mendelsohn CL, Lai K, Blaner WS. Expression and characterization of a murine enzyme able to cleave beta-carotene. The formation of retinoids. J Biol Chem 2001 ;276:32160-8 Pakhomova S, Kobayashi M, Buck J, Newcomer ME. A helical lid converts a sulfotransferase to a dehydratase. Nature Struct Bio12001;8:447-5 I Palczewski K, Van Hooser JP, Garwin GG, Chen J, Liou GI, Saari JC. Kinetics of visual pigment regeneration in excised mouse eyes and in mice with a targeted disruption of the gene encoding interphotoreceptor retinoid-binding protein or arrestin. Biochem 1999;38:12012-19 Randolph RK, Simon M. All-trans-retinoic acid regulates retinol and 3,4-didehydroretinol metabolism in cultured human epidermal keratinocytes. J Invest Dermatol 1996; 106:168-75 Rock CL. Carotenoids: biology and treatment. Pharmacol Ther 1997;75:185-97 Rock CL, Lovalvo JL, Emenhiser C, Ruffin MT, Flatt SW, Schwartz SJ. Bioavailability of beta-carotene is lower in raw than in processed carrots and spinach in women. J Nutr 1998;128:913-16 Ruiz Rejon F, Martin-Pena G, Granado F, Ruiz-Galiana J, Blanco I, Olmedilla B. Plasma status of retinol, alpha- and gamma-tocopherols, and main carotenoids to first myocardial infarction: case control and follow-up study. Nutr 2002; 18:26-31 Saari JC, Garwin GG, Van Hooser JE Palczewski K. Reduction of ail-trans-retinal limits regeneration of visual pigment in mice. Vision Res 1998;38:1325-33 Saari JC, Nawrot M, Kennedy BN, Garwin GG, Hurley JB, Huang J, Possin DE, Crabb JW. Visual cycle impairment in cellular retinaldehyde binding protein (CRALBP) knockout mice results in delayed dark adaptation. Neuron 2001;29:739-48 Sani BE Venepally PR, Levin AA. Didehydroretinoic acid: retinoid receptor-mediated transcriptional activation and binding properties. Biochem Pharmacol 1997;53:1049-53 Sapin V, Chaib S, Blanchon L, Alexandre-Gouabau MC, Lemery D, Charbonne E Gallot D, Jacquetin B, Dastugue B, Azais-Braesco V. Esterification of vitamin A by the human placenta involves villous mesenchymal fibroblasts. Pediatr Res 2000;48:565-72 Sivakumar B, Reddy V. Absorption of labelled vitamin A in children during infection. Br J Nutr 1972;27:299-304 Tibaduiza EC, Fleet JC, Russell RM, Krinsky Nl. Excentric cleavage products of betacarotene inhibit estrogen receptor positive and negative breast tumor cell growth in vitro and inhibit activator protein-l-mediated transcriptional activation. J Nutr 2002; 132:1368-75

478


White JA, Ramshaw H, Taimi M, Stangle W, Zhang A, Everingham S, Creighton S, Tam SP, Jones G, Petkovich M. Identification of the human cytochrome P450, P450RAI-2, which is predominantly expressed in the adult cerebellum and is responsible for all-trans-retinoic acid metabolism. Proc Natl Acad Sci USA 2000;97: 6403-8 Yeum KJ, dos Anjos Ferreira AL, Smith D, Krinsky NI, Russell RM. The effect of alphatocopherol on the oxidative cleavage of beta-carotene. Free Rad Biol Med 2000; 29:105-14 Yeum KJ, Russell RM. Carotenoid bioavailability and bioconversion. Annu Rev Nutr 2002;22:483-504 Yeum KJ, Lee-Kim YC, Yoon S, Lee KY, Park IS, Lee KS, Kim BS, Tang G, Russell RM, Krinsky NI. Similar metabolites formed from beta-carotene by human gastric mucosal homogenates, lipoxygenase, or linoleic acid hydroperoxide. Arch Biochem Biophys 1995;321 : 167-74 Zhuang R, Lin M, Napoli JL. cis-Retinol/androgen dehydrogenase, isozyme 3 (CRAD3): a short-chain dehydrogenase active in a reconstituted path of 9-cis-retinoic acid biosynthesis in intact cells. Biochem 2002;41:3477-83

Vitamin D The most common form of vitamin D in foods is vitamin D3 (9,10-seco(5Z,7E)-cholesta5,7,10(19)-trien-3-ol, cholecalciferol, colecalciferol, oleovitamin D3; molecular weight 384). The less common form vitamin D2 (ergocalciferol) is slightly less effective.

Abbreviations D2 D3 DBP 25-D 1,25-D 24,25-D PTH UV-B

Nutritional

vitamin D 2 vitamin D 3 vitamin D-binding protein 25-hydroxyvitamin D 1e~,25-dihydroxy-vitamin D 24R,25-dihydroxy-vitamin D parathyroid hormone ultraviolet B light (290-315 nm)

summary

Function: Promotes intestinal absorption of calcium and its retention in the body. Through its role in gene regulation the active form, lo~,25-dihydroxy-vitamin D (1,25D), it influences growth of bone and connective tissues and may protect against some forms of cancer. Requirements: Adults should get at least 5 v,g/day, three times as much with advanced age.

Vitamin D 4 7 9

21

dCH OH" Figure 9.12

6 j7

22

Vitamin Da

OH"

Dietary compounds with vitamin D activity

5ources: Fatty fish and fortified milk are good dietary sources; eggs, fortified cereals, and fortified margarines contribute smaller amounts. A young person gets a full day's supplies from 10-15 minutes of face and arm exposure to summer sun (ultraviolet B, UV-B, 290-315 nm), an older person needs several times longer exposure. Deficiency: A severe lack during childhood causes rickets, characterized by bone deformities in lower limbs (bowlegs) and chest. Deficiency at a later age causes loss of bone minerals (osteoporosis), in the most severe cases of both minerals and connective tissue (osteomalacia). Tetany and severe bone pain are characteristic signs. People with suboptimal vitamin D status tend to have elevated parathyroid hormone levels and absorb dietary calcium less well. Indicative symptoms for mild vitamin D deficiency include fatigue, muscle ache, and diffuse bone pain (Nykjaer et al., 2001 ). Excessive intake: Prolonged consumption of several hundred micrograms per day may cause hypercalcemia and soft tissue calcification. Continued exposure to doses of thousands of micrograms daily may cause coma and death in extreme cases.

Endogenous sources Exposure of skin to ultraviolet light with wavelengths between 290 and 315 nm (UV-B) converts some of the cholesterol precursor 7-dehydrocholesterol to previtamin D3, which rearranges spontaneously to vitamin D3 (Holick et al., 1989). Suberythemal (a dose that does not cause sunburn) irradiation of skin with UV-B (0.5 J/cm:) was found to convert about one-third of endogenous 7-dehydrocholesterol (2.3 Ixg/cm2) into previtamin D3, and another third into the precursor lumisterol and the inactive metabolite tachysterol (Obi-Tabot et al., 2000). UV-B inactivates some of the newly generated vitamin D and its unstable precursors. Vitamin D synthesis rapidly becomes maximal upon continued exposure, because light-induced production and destruction of vitamin D reach an equilibrium. It has been estimated that exposure of the entire body to summer sun for less than 20 minutes is sufficient to generate vitamin D in skin equivalent to an oral dose of 250 ~g or more (Vieth, 1999). Skin pigmentation decreases the effective light dose and greatly decreases vitamin D production with less than maximal sun exposure (Kreiter et al., 2000).


The diminished vitamin D production in older people (75% decline by age 70) has been attributed in part to a lower concentration of unesterified 7-dehydrocholesterol in skin (Holick, 1999).

Dietary

sources

Most natural vitamin D is consumed in the form of vitamin D3 (D3, cholecalciferol). The content of foods is expressed in I~g or International Units (I I~g = 40 IU). The only

~

7-Dehydrocholesterol

:

UV-B

",,

PrevitaminD3

"~

-

~'~ ~inaaChYvSt ro13 e~

non.enzymici

l, Vitamin03

r ~ ~ uv.

O H ' ~

H~C~2...~

uv.o " ~

'--~Suprast~erol

~ ~,,nact,ve, ~

"OH

Figure 9.13 Light-inducedsynthesisof vitaminD3

\

~ HO

(inactive)

~J'~

(inactive) SuprasterOI .I

Vitamin D 481

natural foods that contain the structurally related vitamin D 2 (D2, ergocalciferol) are mushrooms. Ocean fish is the main dietary source of D3. Particularly rich sources are the fatty types of fish, such as salmon (0.1-0.3 Ixg/g), sardines (0.4 ~g/g), and mackerel (0.1 Ixg/g). Lean ocean fish, such as cod (0.01 ~g/g), and freshwater fish, contain only little vitamin D. Most milk in the US is fortified at a level of 5 i~g/I. Considerable variation of actual milk vitamin D content have been observed in the past, however (Holick et al., 1992). Other dairy products, such as yoghurt or cheese, are not usually fortified. D2, which is the compound originally used for fortification, has been replaced with D3 in the US and many other countries. Vitamin D2 can be produced relatively simply by UV light irradiation of lanosterol. D2 is biologically less active than D3 (Trang et al., 1998). Typical daily vitamin D intakes in North American women may be as low as 2.5 ~g (Krail et al., 1989), and usually insufficient to prevent suboptimal vitamin D status in regions with low sunlight exposure (Vieth, Cole et al., 2001 ). In Denmark, where milk does not contain added vitamin D, median daily intakes around 3 t~g in men, and around 2 ~g in women were recorded (Osler et al., 1998). Even lower median intakes ( 1.2 i~g/d) were reported for Australians (Pasco et al., 2001 ). Older people in particular, who need much more vitamin D (at least 10 i~g for ages 51-70, and 15 Ixg for people over 70) than younger people, commonly do not get enough (Kohlmeier et al., 1997).

Digestion and absorption Vitamin D is highly fat-soluble and becomes part of mixed micelles (consisting mainly of bile acids, phospholipids, fatty acids and monoglycerides) during fat digestion. Nearly all of the ingested vitamin D is absorbed. The vitamin enters the small intestinal

vitamin D

fatty acids

bileacids"

~

m m " ~ l ~ vitamin D

){ed I / n :elle

chylomicron

phospholipids j

Intestinal lumen

Figure 9.14

Enterocyte Brush border membrane

Intestinal absorption of'vitamin D


Lymph duct


cell along with fatty acids and other lipids in an incompletely understood process. Chylomicrons then carry vitamin D into lymph vessels and eventually into blood circulation. Little, if any, vitamin D is released while the chylomicrons circulate and rapidly lose most of their triglyceride load. The liver takes up about half of the triglyceridedepleted chylomicrons through a receptor-mediated process that involves apolipoprotein E and the LDL receptor. Bone marrow and bone take up about 20%, and other extrahepatic tissues clear the remainder. Vitamin D reaching the liver can be secreted again into circulation as a complex with vitamin D-binding protein (DBP, group-specific component, Gc).

Transport and cellular uptake Vitamin D and all its normal metabolites in blood are bound to DBP. Almost all of the vitamin D in circulation is 25-hydroxy-vitamin D (25-D); much smaller amounts are 1,25-dihydroxy-vitamin D (1,25-D). Typical 25-D concentrations in plasma of young adults living under sun-rich conditions are well in excess of 100 nmol/l (Vieth, ! 999). Nonetheless, the lower limit of reference ranges is commonly set to 50 nmol/I or lower. Average 25-D concentrations of people living at latitudes of 50 or higher may be as low as 40 nmol/l during the winter months (Trang et al., 1998). Typical 1,25-D concentrations in vitamin D-replete people tend to be around 100 pmol/l. 25-D concentrations are low in vitamin D-deficient people, intermediate with adequate status, and increase further with excessive dietary intakes (but not with very intense UV light exposure). 1,25-D concentrations also are low in vitamin D deficiency, but do not increase further after adequate vitamin D intakes are exceeded. Thus, 25-D concentration in plasma is a good marker to reflect both inadequate and excessive vitamin D supplies. It has been the traditional view that because of their high fat solubility vitamin metabolites can cross plasma membranes by simple diffusion. A more directed entry pathway may pertain in some tissues, however, such as endocytotic uptake mediated by cubilin and/or megalin (LDL-receptor related protein 2, LRP2). Blood-brain barrier: Transport of vitamin D metabolites from blood into brain is very limited (Pardridge et al., 1985). The underlying mechanisms are not well understood. Materno-fetal transfer: 25-D is the main metabolite supplied by the mother to the fetus through incompletely understood mechanisms (Salle et al., 2000). Fetal concentrations are lower than on the maternal side. Vitamin D is not only transferred to the fetus, but also has important functions in the placenta itself. It is no surprise, therefore, that placenta expresses the vitamin D activating enzyme 25-hydroxy-vitamin D(3)-I a-hydroxylase (Zehnder et al., 2001). Blood circulation:

Metabolism Vitamin D is metabolized extensively in liver, intestines, and kidneys. More recent evidence shows that keratinocytes in skin are fully autonomous in respect to vitamin D metabolism and are capable of all major activating and inactivating reactions (Schuessler et al., 2001 ). Similarly, osteoblasts, parathyroid cells, myelocytes and other cell types

Vitamin D 483

have relevant metabolic activity. Three best-recognized enzyme reactions produce two biologically active metabolites, l a,25-dihydroxy-vitamin D and 24R,25-dihydroxyvitamin D (Norman, 2001 ). Microsomal vitamin D(3) 25-hydroxylase (CYP2D25, identical with sterol-27 hydroxylase, CYP27A) in liver and intestines catalyzes the essential first step in the bioactivation of the prohormone vitamin D (Theodoropoulos et al., 2001 ). This reaction is so effective upon first pass of newly absorbed vitamin D through small intestine and liver, that blood contains very little unmetabolized vitamin D. Activation to l o~,25-dihydroxy-vitamin D (1,25-D) is completed in proximal tubular epithelial cells of the kidneys by calcidioi l-monooxygenase (25-hydroxy-vitamin D-lahydroxylase, CYP27B 1, P450C 1-a, EC 1.14.13.13), a mitochondrial cytochrome P450

02

9

If~/oxidized ~ f NADPH ferredoxin

9

.

vltam=n D(3) I~ l~ YNADPH-ferredoxm 2S-hydroxylase~ ~ . ~ reductase (FAD) (berne) I~ reouceo / k

]"~erredoxin-"~"NADP H20

25-Hydmxy-Vitamin D a

OH"J~ ~CH2

02

I/~//oxidized \ /.NADPH

calcidiol

IV ferredoxin \ /

[ y NADPH-ferredoxin 1-monoxygenase(heme) kreduced A reductase (FAD)

[ ~erredoxin-" H20

~"NADP

i~

" 1~,25-Dihydroxy-Vitamin D3

Figure 9.15

Vitamin D activation depends on renal filtration and reabsorption


red.ferredoxin ox.ferredoxin 1(-z.25-Dihydroxy- I ' Y vitamin D

3

II

~

+o2\ ,,, +.20

\--OH "

CYP ? "

~ ~ II I red.ferredoxin~. r / J ~ O H 2 CYP241~ + 02 ? ~ ] I P.,,,.px.ferredoxin OH" ~"V~A~OH ~ -'H20

lOC24R 25

"~1 ~

~

~

-/~'~., , \--OH 1(z,23S,25-Trinyaroxyvitamin D3 "~.,-~ I ~z ) L . ~

~

~

1(x.25-Dihydroxy- Y

I red.ferredoxin C Y P 2 4 F +O2 I_l",~cxi[leSedoxin 1c(.25-Dihydroxy24-oxo-vitamin D3

"

O

~

\~OH

I J HO"X"V~OH

/\

H

C-23 oxidation

C-24 oxidation

pathway

I red.ferredoxin

pathway

CYP24F + 02 ,9

~+

1c(.23S,25-Trihydroxy-

O

24-oxo-vitamin D3

I

ox.ferredoxin H20

oxidized NADPH ferredoxin"~/ y NADPH-ferredoxin reduced ]~ductase

H

ferredoxin

red.ferredoxin

(FAD)

NADP

CYP24F + 02 9 ' ~ C O H 1~.25-Trihydroxy-24-oxo- I /% vitamin D3

P~,,ox.ferredoxin ~ -+ H20 + C4 fragment red.ferredoxin ox.ferredoxin

o2 9 .

CYP ?

+H2O

9

~1

I

J,(

"t-?

II

Ii[

OH" v Figure 9.16

"COOH

,._ Calcitroic ......

acid

c"2 "OH

Catabolicpathwaysfor 1o~,24R,25-trihydroxy-vitamin D3

oxidase. Ferredoxin, which provides the reducing equivalents for all cytochrome P450 systems, is regenerated by ferredoxin-NADP reductase (EC 1.18.1,2, FAD-containing). Before this hydroxylation can take place, however, the 25-D precursor has to reach the tubular cell. The main, and possibly exclusive route is glomerular filtration of the 25-D/DBP complex and endocytosis mediated jointly by cubilin and megalin (Nykjaer et al., 2001 ). Decreased filtration rate (as with advancing age or in renal failure) or defective cubilin or megalin diminish the production of 1,25-D. Some 25-D hydroxylation also occurs in extrarenal cells including skin and white blood cells (Hewison et al., 2000; Schuessler et al., 2001 ). The glomerular filtration rate of a healthy man can be expected to fall from about 140 ml/minute at age 20 to about 90 mi/minute by age 70 (Lindeman, 1999). This age-typical decline in renal function raises the threshold plasma 25-D concentration that sustains adequate 1,25-D production by more than half. A several-fold

Vitamin D 4 8 5

25-Dihydroxy- ~ vitaminD3

O

~CH2

OH"-x~

H

02

I/~//oxi dized\ n\[/NADPH I~ ferredoxi '~ NADPH-ferredoxin kreduced .)~reductase(FAD) ] ~feH~~ ~"NADP

CYP24r

24R,25-DihydroxyvitaminD3

OH'~

;CH, \

\

C24oxidation pathway

~COOH

~

23-hydroxy-24,25,26,27-II tetranor-vitaminD3 . J

CH2

OH'~ Figure 9.17 Catabolism of'25-hydroxy-vitaminD

increase in dietary intakes or endogenous production is necessary to make up for this difference (Vieth, 1999). Current recommendations recognize that people over 70 years of age need three times more vitamin D than young adults (Institute of Medicine, 1997). Another mitochondrial cytochrome P450 oxidase, 25-hydroxy-vitamin D-24Rhydroxylase (CYP24) converts 1,25-D into l~x,24R,25-trihydroxy-vitamin D (24,25D). Alternatively, hydroxylation at carbon 23 may occur. Irreversible 3-epoxidation initiates a distinct metabolic pathway. Hydroxylations of the side chain, possibly with the involvement ofCYP24 (|nouye and Sakaki, 2001 ), generate the water-soluble metabolite calcitroic acid ( ! a-hydroxy-24,25,26,27-tetranor-23-carboxyl-vitamin D). The bulk of 25-D is catabolized through the C24 oxidation pathway with 24R, 25-dihydroxy-vitamin D 3 as an important intermediate metabolite (Henry, 2001 ), which may have its own specific biological activities. Numerous additional hydroxylated and otherwise modified metabolites are present in blood and tissues. While it has been held that calcitroic acid and other metabolites


-..

,..

leLhydroxy-3-epi mi~I~l~,hydroxy-3-epi-vitamin n 20,25-cyclether ic-vitaD3, 24,25-epoxideD 3 ~ HO"~H

Figure

9.18 Metabolites derived Fromvitamin D3-3-epi-intermediates

HO ~ V

"OH

are inactive, more recent investigations seem to indicate that they retain some typical vitamin D activity (Harant et al., 2000).

Storage Vitamin D is known to be stored extensively in the liver. These stores sustain normal vitamin D-dependent functions during the winter at high latitudes even in the absence of significant dietary intakes. However, quantitative data on amounts stored, alternative storage sites, or the precise mechanisms for deposition and release, are not available. Smaller amounts of vitamin D are stored in extrahepatic tissues. The cartilage oligomeric matrix protein may provide a local storage mechanism that supports rapid delivery to nearby target structures (Guo et al., 1998).

Excretion Calcitroic acid, the 3- and 24-glucuronides of 24,25-D, and additional vitamin D metabolites are excreted with bile. Since intestinal absorption of vitamin D is very efficient, losses of active vitamin D via this route are likely to be minor. Quantitative information in this regard is limited, however. 25-D in plasma is bound to DBP (group-specific component, Gc), a single peptide chain with molecular weight of 52 000 (Witke et al., 1993). A small percentage of this complex is filtered in the renal glomeruli. DBP binds with high affinity to cubilin at the brush border membrane of the proximal renal tubule, as described above. Megalin assists with the endocytosis and intracellular trafficking ofcubilin and all its captured iigands (which include retinol-binding protein and transferrin among others). Due to the high efficiency of the process very little of the filtered vitamin D escapes with urine. Calcitroic acid is a major catabolite of both vitamin D 2 and D 3 in urine (Zimmerman et al., 2001 ).

Regulation Feedback inhibition strongly limits 1,25-D production (Norman, 2001). The main activator of renal production of 1,25-D is PTH, which increases expression of l a-hydroxylase (Theodoropoulos et al., 2001). Calcitonin, estrogen, prolactin, insulin, growth hormone, and glucocorticoids also activate this key enzyme.

Vitamin D 487

calcitonin estrogen prolactin insulin ~ , growthhormone glucocorticoids 25-hydroxyvitaminD

Figure9.19

Regulation of 1r

PTx

~

| CYP2

.

~

~

|

1ot,25-dihydroxyvitamin D

D synthesis

Conversely, 1,25-D decreases PTH secretion both directly by acting on a VDRE in the promoter of the PTH gene and indirectly through its effects on calcium and phosphate concentrations in blood and extracellular fluid (Sela-Brown et al., 1999). PTH also promotes the conversion of 25-D to 24,25-D (Armbrecht et al., 1998).

Function Nuclear effects: A complex consisting of 1,25-D, the nuclear vitamin D receptor (VDR), and the retinoid X receptor (RXR) binds to specific vitamin D response elements (VDRE) in the nucleus and modifies the rate of expression of the associated genes. RXR must contain the vitamin A metabolite 9-cis-retinoic acid in order to form the active receptor complex. VDRE increase the expression of many genes including osteocalcin, osteopontin, alkaline phosphatase, calbindin-9K, and calcium-transporting ATPase. Downregulated genes include those for many collagens, and for cell cycle regulators such as c-myc, c-fos, c-sis, and ubiquitin-conjugating enzyme 2 variant 2 (UBE2V2). Intestinal calcium absorption: The overall consequence of improved vitamin D status in small intestinal cells is faster calcium influx from the lumen, more efficient transcellular calcium transfer, and accelerated calcium pumping towards the bloodstream. These effects all add up to considerably increased fractional calcium absorption. Phosphate absorption also increases slightly. Daily oral doses of 1,25-D between 0.5 and 3 ixg are sufficient to increase intestinal calcium absorption. The effect of 25-D on intestinal calcium absorption is much smaller. It has been estimated that as much as one eighth of the vitamin D-related absorption-enhancing action is due to 25-D (Heaney et al., 1997). Fractional calcium absorption from small intestine increases much more rapidly (within seconds, sometimes referred to as transcaltachia) than could be explained by effects on protein expression. It has been postulated, therefore, that these rapid changes are mediated by an as yet unidentified 1,25-D receptor at the plasma membrane (Norman, 2001 ). Effects on bone: 1,25-Dacts directly on hematopoietic stem cells and induces the generation of osteoclast cells. These migrate into bone, start breaking down bone matrix

4 8 8 Fat-soluble Vitamins and Non-nutrients

and minerals, and release calcium and phosphate. The various effects of 1,25-D on the osteoblast nucleus have been alluded to above, and are mainly mediated by the vitamin D-receptor/retinoic X receptor complex. The increased expression of alkaline phosphatase, osteocalcin, and osteopontin supports the control of minerals released by osteoclast action and orderly redepositing of any excesses. There is persuasive evidence that 24,25-D is needed for proper bone mineralization in addition to 1,25-D (Norman, 2001; van Leeuwen et al., 2001 ). Some of the actions of 24,25-D may be mediated through a plasma membrane receptor that is different from the nuclear vitamin D receptor. The significance of 24,25-D remains in dispute, however, since information on this metabolite is much more limited than on 1,25-D. Cell differentiation: An important effect of vitamin D is on growth and maturation of a wide range of cell types. For example, 1,25-D regulates the differentiation of keratinocytes (Bikle et al., 2001). 1,25-D also influences quiescence, modulates growth factors, promotes apoptosis of cancer cells and slows metastasis in model systems. Suboptimal vitamin D status is likely to increase the risk of cancer of prostate (Polek and Weigel, 2002), breast (Colston and Hansen, 2002), and other sites. There are ongoing efforts to develop vitamin D analogs that retain their antiproliferative potency while minimizing hypercalcemic effects. Other cellular effects: The plasma-membrane receptor described above (Norman, 2001 ) may mediate rapid effects of 1,25-D on various cells. 1,25-D stimulates, among others, the rapid calcium influx into osteoblasts and other cells through voltage-gated calcium channels, the release of calcium from stores in muscle cells, and the activation of mitogen-activated kinase. References Armbrecht HJ, Hodam TL, Boltz MA, Partridge NC, Brown AJ, Kumar VB. Induction of the vitamin D 24-hydroxylase (CYP24) by 1,25-dihydroxyvitamin D3 is regulated by parathyroid hormone in UMR 106 osteoblastic cells. Endocrinol 1998; 139:3375-81 Bikle DD, Ng D, Tu CL, Oda Y, Xie Z. Calcium- and vitamin D-regulated keratinocyte differentiation. Mol Cell Endocrino12001 ; 177:161-71 Colston KW, Hansen CM. Mechanisms implicated in the growth regulatory effects of vitamin D in breast cancer. Endocrin Rel Cancer 2002;9:45-59 Guo Y, Bozic D, Malashkevich VN, Kammerer RA, Schuithess T, Engel J. AIl-trans retinol, vitamin D and other hydrophobic compounds bind in the axial pore of the five-stranded coiled-coil domain of cartilage oligomeric matrix protein. EMBO J 1998; 17:5265-72 Harant H, Spinner D, Reddy GS, Lindley IJ. Natural metabolites of l alpha,25-dihydroxyvitamin D(3) retain biologic activity mediated through the vitamin D receptor. J Cell Biochem 2000;78:112-20 Heaney RP, Barger-Lux MJ, Dowell MS, Chen TC, Holick ME Calcium absorptive effects of vitamin D and its major metabolites. J Clin Endocrinol Metuh 1997;82:4111-16 Henry HL. The 25(OH)D3/la,25(OH):D3-24R-hydroxylase: a catabolic or biosynthetic enzyme'? Steroids 2001 ;66:391-8 Hewison M, Zehnder D, Bland R, Stewart PM. l alpha-Hydroxylase and the action of vitamin D. J Mol Endocrino12000;25:141-8

Vitamin D 489

Holick ME Vitamin D: Photobiology, metabolism, mechanism of action, and clinical applications. In Favus M J, ed. Primer on the Metabolic Bone Diseases and Disorders o[Mineral Metabolism, 4th edn. Lippincott Williams & Wilkins, Philadelphia, 1999, pp.92-8 Holick ME Matsuoka LY, Wortsman J. Age, vitamin D, and solar ultraviolet radiation. Lancet 1989;4: I 104-5 Holick ME Shao Q, Liu WW, Chen TC. The vitamin D content of fortified milk and infant formula. N Engl J Med 1992;326:1178-81 Inouye K, Sakaki T. Enzymatic studies on the key enzymes of vitamin D metabolism; I alpha-hydroxylase (CYP27BI) and 24-hydroxylase (CYP24). BiotechnolAnn Rev 2001 ;7:179-94 Institute of Medicine. Dietary reference intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride. National Academy Press, Washington, DC, 1997 Kohlmeier M, Garris S, Anderson JJB. Vitamin K: A vegetarian promoter of bone health. Veg Nutr 1997; 1:53-7 Krall EA, Sahyoun N, Tannenbaum S, Dallal GE, Dawson-Hughes B. Effect of vitamin D intake on seasonal variations in parathyroid hormone secretion in postmenopausal women. N Engl J Med 1989;321 : 1777-83 Kreiter SR, Schwartz RE Kirkman HN Jr, Charlton PA, Calikoglu AS, Davenport ML. Nutritional rickets in African American breast-fed infants. J Pediatr 2000; 137:153-7 van Leeuwen JR van den Bemd G J, van Driel M, Buurman C J, Pols HA. 24,25Dihydroxyvitamin D(3) and bone metabolism. Steroids 2001;66:375-80 Lindeman RD. The aging renal system. In Chernoff R, ed. Geriatric' Nutrition. Aspen Publishers, Maryland, 1999, pp.275-87 Norman AW. Vitamin D. In Bowman BA, Russell RM, eds. Present Knowledge in Nutrition, 8th edn. ILSI Press, Washington, DC, 2001, pp. 146-55 Nykjaer A, Fyfe JC, Kozyraki R, Leheste JR, Jacobsen C, Nielsen MS, Verroust PJ, Aminoff M, de la Chapelle A, Moestrup SK, Ray R, Gliemann J, Willnow TE, Christensen El. Cubilin dysfunction causes abnormal metabolism of the steroid hormone 25(OH) vitamin D(3). Proc NatlAcad Sci USA 2001 ;98:13895-900 Obi-Tabot ET, Tian XQ, Chen TC, Holick ME A human skin equivalent model that mimics the photoproduction of vitamin D3 in human skin. In Vitro Cell Develop Biol Anita 2000;36:201-4 Osier M, Heitmann BL. Food patterns, flour fortification, and intakes of calcium and vitamin D: a longitudinal study of Danish adults. J Epidemiol Comm Health 1998;52:161-5 Pardridge WM, Sakiyama R, Coty WA. Restricted transport of vitamin D and A derivatives through the rat blood-brain barrier. J Neurochem 1985;44:1138-4 1 Pasco JA, Henry MJ, Nicholson GC, Sanders KM, Kotowicz MA. Vitamin D status of women in the Geelong Osteoporosis Study: association with diet and casual exposure to sunlight. Med J Aust 2001 ; 175:401-5 Polek TC, Weigel NL. Vitamin D and prostate cancer. JAndro12002;23:9-17 Salle BL, Delvin EE, Lapillonne A, Bishop N J, Glorieux FH. Perinatal metabolism of vitamin D. Am J Clin Nutr 2000;71 : 1317S-24S Schuessler M, Astecker N, Herzig G, Vorisek G, Schuster I. Skin is an autonomous organ in synthesis, two-step activation and degradation of vitamin D(3): CYP27 in epidermis completes the set of essential vitamin D(3)-hydroxylases. Steroids" 2001;66:399-408


Sela-Brown A, Naveh-Many T, Silver J. Transcriptional and post-transcriptional regulation ofPTH gene expression by vitamin D, calcium and phosphate. Min Elecmdyte Metab 1999;25:342-4 Theodoropoulos C, Demers C, Mirshahi A, Gascon-Barre M. 1,25-Dihydroxyvitamin D(3) downregulates the rat intestinal vitamin D(3)-25-hydroxylase CYP27A. Am J Physiol Endocrinol Metab 2001 ;281 :E315-25 Trang HM, Cole DE, Rubin LA, Pierratos A, Siu S, Vieth R. Evidence that vitamin D3 increases serum 25-hydroxyvitamin D more efficiently than does vitamin D2. Am J Clin Nutr 1998;68:854-8 Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr 1999;69:842-56 Vieth R, Chan PCR, MacFarlane GD. Efficacy and safety of vitamin D3 intake exceeding the lowest observed adverse effect level. Am J Clin Nutr 2001;73:288-94 Vieth R, Cole DE, Hawker GA, Trang HM, Rubin LA. Wintertime vitamin D insufficiency is common in young Canadian women, and their vitamin D intake does not prevent it. Eur J Clin Nutr 2001 ;55:1091-7 Witke FW, Gibbs PEM, Zielinski R, Yang E Bowman BH, Dugaiczyk A. Complete structure of the human Gc gene: differences and similarities between members of the albumin gene family. Genomicw 1993; 16:75 I-4 Zehnder D, Bland R, Williams MC, McNinch RW, Howie AJ, Stewart PM, Hewison M. Extrarenal expression of 25-hydroxyvitamin d(3)-I alpha-hydroxylase. J Clin Endocrinol Metab 2001 ;86:888-94 Zimmerman DR, Reinhardt TA, Kremer R, Beitz DC, Reddy GS, Horst RL. Calcitroic acid is a major catabolic metabolite in the metabolism of 1 alpha-dihydroxyvitamin D(2). Arch Biochem Biophys 2001;392:14-22

Vitamin E Vitamin E (RRR-alpha-tocopherol, 3,4-dihydro-2,5,7,8-tetramethyl-2-(4,8,12-trimethyltridecyl)-2H- 1-benzopyran-6-ol, 2,5,7,8-tatramethyl-2-(4',8', 12'-trimethyltrdecyl) 6chromanol, 5,7,8-trimethyltocol; obsolete name antisterility vitamin; molecular weight 430); there are seven stereoisomers of alpha-tocopherol with lower activity. The related compounds beta-, gamma-, and delta-tocopherol, tocotrienols, and others have more limited activity. Abbreviations

vitE HDL LDL TAP TBP -I-I-P VLDL

vitamin E (all Forms) high-density lipoproteins low-density lipoproteins tocopherol-associated protein tocopherol-binding protein alpha-tocopherol transfer protein very-low-density lipoproteins

Vitamin E 491

CH3

OH

OH3 ~ " ~0 / ~ ~ CH3 CHa

V

V

V

V

V

\CH a

CH3 "~

CH3~

i'~'~

''" ]

' ~ OH3

2R, 4'R, 8'R-e~-Tocopherol

'

CH3

CHa O

~

CH3

C

H

CH~

3

2R-ct-Tocotrienol

CH3

CH3

"O" i ~ OH3

v

v

v

9 v

v

2

OH.[./ -CH3

CH~

2R, 4'R, 8'R-[3-Tocopherol O H ~ " ~

CH3~ ' ~ " ~

CH3

q~[..]

0 ~

~H 3

CH3

CH~

OH3~

' OH3

~ ~

O OH3

CH3

.

OH3

0

A

~ ,~...~,CH~ ~ ~CH~ ~ . .~ . . . ~

OH3

Marine-derived-Tocopherol Figure 9.20

CH3

OH3

C

H

CH3

J

CHa

OH3

CH3

~

"]

~J [ ~ . . ~ H ~

.

2R, 4'R, 8'R-5-Tocopherol

o ~ ~ C "T" H

CH3

~

2R-~/-Tocotrienol OHT'/ ~ ' ~ i~~

.

OH3

CH~

OHm/-'~"'~ ~..~

2R, 4'R, 8'R-~,-Tocopherol

.

CH3

2R-~-Tocotrienol

A ~ ' ~ / ~ C Ha * " ~ ' . ~ cH3

.

CH3

CH3

CH3

C

H3

CH3

2R-B-Tocotrienol OH3

CH~ -CH 2

OH~ CH3~

j~"

~

' ~ CH3

'

ab,~...,j~CH 3 4~,,,~ .j~CH 3 ~ CH3

-

O

~

C

2R, 4'R, 8'R-c~-Tocomonoenol

Naturally occurring forms of vitamin E

Nutritional summary Vitamin E is a fat-soluble antioxidant that inactivates oxygen free radicals in membranes, lipoproteins, and other lipid-rich compartments. A specific role in maintaining fertility may exist. Requirements: Adults should get at least 15 mg/d. Pregnancy, breast feeding, and high intake of polyunsaturated fatty acids slightly increase needs. Adequate amounts ofascorbate are essential for reactivation. Sources: Wheat germ and sunflower oils as well as nuts are good sources. Other plant oils, fruits and vegetables provide smaller amounts. Deficiency: There is limited evidence that inadequate intake promotes progression of atherosclerosis, Parkinson's and Alzheimer's disease, cancer, cataract, and impairs immune function. Excessive intake: High doses (more than 1000 mg of any supplement form) can interfere with blood clotting and thus increase risk of hemorrhagic stroke. This adds to anticlotting effects of coumadins and salicylates. Function:

D i e t a r y sources The most confusing aspect of vitamin E nutrition may come from the great diversity of compounds with some kind of related activity. So far, 12 compounds with

H

iCH3 3

492


CH3

CHa OH

%~.

OH3 ~

OH3

CH a

C H ~ C H a CH3

CH~

CH3

2S, 4'R, 8'R-(x-Tocopherol

2R, 4'R, 8'R-~-Tocopherol

CH3 CH a ~

CHa

~O ~ j ~ CH 3

2c,/ ,xc . 2' X/

x/

/x

k/

k/

V

OHm. CH3

/x.

V

3 V

V

V

V

/x.

3 V

"CH3

OH3

CH3 %

H3c

%

C H ~ C H 3 CH3

OH3

2R, 4'S, 8'S-~-Tocopherol

HaG

"eCH3

CH 3 H3c

~CH~

/-'~

2S, 4'R, 8'S-~-Tocopherol~ f

2R, 4'R, 8'S-~-Tocopherol OH

~

2S, 4,S, 8,R.~.Tocopherol~C~

CH3 ~O~ " CH~ CH3

CHa ~ t

"CH 3

2R, 4'S, 8'R-~-Tocopherol

CH3 ~

CH~

-"7/ CH 3 HaG CH3

CH a CH

CH3

2S, 4'S, 8 ' S - ~ - T o c 6 p h e r o l ~

H~C /

CH3 ~ C H 3 H3C Figure 9.21 Syntheticall-rac alpha-tocopherol contains about equal amounts of eight isomers

characteristic vitE activity have been identified: alpha-tocopherol, beta-tocopherol, gamma-tocopherol, delta-tocopherol, alpha-tocotrienol, beta-tocotrienol, gammatocotrienol, delta-tocotrienol, the tocotrienols d-P(21 )-T3 and d-P(25)-T3 (Qureshi et al., 2001 ), alpha-tocomonoenol, and marine-derived tocopherol (Yamamoto et al., 2001 ). The food constituent most closely identified with vitE properties is RRR-alphatocopherol with methyl groups in the side chain at positions 5, 7, and 8. The side chain of RRR-beta-tocopherol is methylated at positions 5 and 8, in RRR-gamma-tocopherol at positions 7 and 8. RRR-delta-tocopherol has only one methyl group in the side chain at position 8. The members of the analogous series of tocotrienols contain three double bonds in the side chain. All four members of the tocopherol series and the four members of the tocotrienol series are naturally present, though in varying amounts, in a wide range of foods. Synthetic production ofvitE usually yields about equal amounts of the eight possible isomers, RRR, RSS, RRS, RSR, SRR, SRS, SSR, and SSS. The first three in this list of isomers are often called the 2R isomers because they are R-isomeric at position 2. The metabolic fate of the various isoforms differs and needs to be determined in every case. The biological potency ofvitE doses is often expressed as USP vitamin E units or International Units (IU). One such unit corresponds to 1.0 mg racemic (synthetic, all-rac) alpha-tocopheryl acetate (this is the original reference standard), or I. 1 mg all-rac alphatocopherol, or i .36 mg RRR-alpha-tocopheryl acetate, or 1.49 mg RRR-alpha-tocopherol,

Vitamin E 493

or 0.89mg all-rac-alpha-tocopheryl succinate, or 1.21mg RRR-alpha-tocopheryl succinate. In the following vitE content will be expressed as alpha-tocopherol equivalents (ATE), which is the amount ofRRR alpha-tocopherol that is expected to have the same potency as all vitE forms in a food combined. There are only a few good sources that provide one serving with at least 2.5 mg (onesixth of the recommended intake). Since gamma-tocopherol may differ in its action profile from gamma-tocopherol, the exact composition (which is often not reliably known) may be as important as the ATE figure. Vegetable oils with high to moderate content include wheat germ oil (1.9 mg ATE/g, more than half as alpha-tocopherol) and sunflower oil (0.5 mg/g, most as alpha-tocopherol). Most other commonly consumed oils have a much lower content, such as corn and soybean oil (0.2 mg/g, most as gammatocopherol), canola oil (0.2 mg/g, most as alpha-tocopherol), or olive oil (0.1 mg/g, most as alpha-tocopherol). Sunflower seeds (0.5mg/g) are also a good source. Walnuts (0.03 mg ATE/g) contain nearly equal amounts of alpha-tocopherol, gammatocopherol, and deita-tocopherol. American men have a daily vitE intake of about 8 mg, women get close to 6 mg (Phillips et al., 2000). American food consumption data indicate that only about 10% of men and virtually none of the women reach the recommended intake level ( 15 mg/day) with food alone (Food and Nutrition Board, Institute of Medicine, 2000: Appendix D).

Digestion and absorption Most forms of vitE are absorbed with similarly high efficiency from the proximal small intestine. VitE esters are cleaved by esterases from pancreas. Uptake into enterocytes depends on the prior incorporation of free vitE into mixed micelles (Borel et al., 2001 ). This means that a small amount of fat has to be absorbed along with vitE. A very low-fat meal or poor fat digestion effectively minimize vitE absorption. The exact mechanism of transfer from miceile to enterocyte is not well understood, but is likely to involve at least some of the actors involved in fatty acid uptake. VitE is exported with chylomicrons into intestinal lymph and eventually blood circulation.

Transport and cellular uptake Blood circulation: About 27 ixmol/l alpha-tocopherol, 1.7 i~mol/l gamma-tocopherol (Ruiz Rejon et aL, 2002), and much smaller quantities of other vitE species are present in plasma. Virtually all plasma vitE is associated with low-density lipoprotein (LDL; about 6 molecules ofalpha-tocopherol and 0.5 gamma-tocopherol molecules per particle in vitE-replete people; Esterbauer et al., 1992) and high-density lipoprotein (HDL). Liver cells take up chylomicron remnants with recently absorbed vitE via receptormediated endocytosis. The highly lipophilic vitE then redistributes rapidly to the plasma membrane and intracellular membranes. VitE, like all other plasma membrane constituents, is internalized into endocytic compartments (sorting endosomes) several times per hour (Hao and Maxfield, 2000). It seems that at this point alpha-tocopherol transfer protein (TTP) plays a critical role by preferentially redirecting RRRalphatocopherol back towards newly forming plasma and other membranes (Blatt et al., 2001 ).


Two additional proteins are likely to be involved in directing the intracellular recycling of vitE, tocopherol-associated protein (TAP) and tocopherol-binding protein (TBP). Due to their much lower affinity to TTP a smaller percentage of 2R-alpha-tocopherols and RRR-beta-tocopherol is recycled to the membranes, and an even smaller percentage of other forms ofvitE. The portion left behind tends to be eliminated into bile. Newly secreted very-low-density lipoproteins (VLDL) preferentially carry RRR-alphatocopherol, with lower preference 2R-alpha-tocopherol and beta-tocopherol, and very little of the other forms. The vitE may come mostly from plasma and endosomal membrane, hence the preference for the RRR-alpha-form (Blatt et al., 2001). A typical nascent VLDL contains about six molecules ofvitE (Mertens and Holvoet, 2001 ), but only every other particle contains a gamma-tocopherol molecule (Esterbauer et al., 1992). Delivery of vitE to some tissues (lung, ovaries, testes) involves the scavenger receptor class B type I (SR-BI; Mardones et al., 2002). Some vitE from extrahepatic tissues appears to be mobilized by the ATP-dependent transporter ABCAI (Oram et al., 2001 ), shuttled by phospholipid transfer protein to HDL (Huuskonen et al., 2001) and returned with this vehicle of reverse cholesterol transport back to the liver (Mardones et al., 2002). Blood-brain barrier: Very little vitamin E reaches the brain, which keeps the concentration in cerebrospinal fluid to about one-hundredth of the concentration in plasma (Pappert et al., 1996). The mechanisms whereby even those low concentrations are maintained remain uncertain. High-density lipoprotein (HDL) is five times more effective than LDL in delivering tocopherol to the blood-brain barrier (Goti et al., 2001 ). The transfer ofvitE from HDL into the capillary epithelial cells uses the SR-BI (Mardones et al., 2002). T T P in glial cells helps to direct vitE into compartments that ultimately supply it to Purkinje cells and other neurons (Biatt et al., 2001 ). Materno-fetal transfer: Understanding of the exact mechanism of vitE transport across the syntrophoblast is still incomplete. HDL (Christiansen-Weber et al., 2000)

i,~l~sorling

Go g"

2s9 Figure 9.22 endosomes

naVL;LI

bile

Alpha-tocopherol transfer protein preferentially recovers RRR-alpha-tocopherol From

Vitamin E 495

and other lipoproteins are known to deliver lipophilic compounds through receptormediated endocytosis. TTP plays an important role, possibly through a sequence of events like in liver (redirecting of vitE in sorting endosomes).

Metabolism A combination of microsomal to-oxidation, peroxisomal betaoxidation and less well-characterized reactions generate a series of hydroxy-chroman

Side-chain breakdown:

CH3

~Y" "0~ i ~ " CH3 CH3

v

v

v

v

v

"CH3

2R, 4'R, 8'R-~,-Tocopherol red.flavoprotein+ 02 CYP3A L"/ (heme) ox.flavoprotein+ H20

CH3

~

~Of i ~ CH3

CH3

v

v

v

v

v

[putative intermediate]

"CH 2 OH

CYP3A #I/" red,flavoprotein+ 02 (heme) ik'..-ox.flavoprotein+ H20 OH

H

CH~ y

CH3

~O/~ CH 3

V

~/

~/

~/

V

"COOH

[putative intermediate] one round of ,B-oxidation

CH;

y

1 I#

1

z.zc:

~ 0 / ~ CH3 CH 3

V

V

"COOH

o~-2-[6'-carboxy-4'-methylhexyl]hydroxychroman (.-CMHHC)

1 one round of

/3-oxidation

O H ~ CH; ~

CH3

#/

1

/'.~'>X ~cH~

" - O / i ~ , " X/ CH3

"COOH

ot-2-[6'-carboxy-4'-methylbutyl]hydroxychroman (o(-CMBHC)

1

one round of #I /3-oxidation

1 C~~COOH CH3 ell3 a-2-[6'-carboxy-4'-ethyl]hydro•

Figure 9.23

(a-CEHC)

Metabolism of vitamin E side chains generates polar compounds

496


metabolites (Parker et al., 2000; Birringer et al., 2001). TTP in liver shields RRRalpha-tocopherol to a large extent from this type of breakdown. Cytochrome p450 3A4 and/or CYP4F2 initiates breakdown by oxidizing the methyl group at the terminal end of the side chain (w-oxidation). After another oxidation step the molecule now has a carboxyl group at the end of the side chain. Successive rounds of beta-oxidation can then shorten the side chain. Since the methyl groups of the side chain are in configuration relative to the carboxyl group they do not block the initial step of betaoxidation, catalyzed by long-chain acyl-CoA dehydrogenase (EC1.3.99.13). The final metabolite is the 2(2'-carboxyethyl)-6-hydroxychroman (CEHC) of the original vitE

CH3 s,~. CH

OH3

CH:~i~.

CH3

CH3 CH3

CH3

~/

2R, 4'R, 8'R-~-Tocopherol Two-electron One-electron radical

O

OH3

4t,~..

CH3~,

CH3

C H ~ C H 3

radical,(R.)i

-3

Tocopheroxylium cation H20. I

H+~ l

RH~

CH3

H+ H29/

~

/ / ~

~~'y

CH3

CH3

C

H

Tocopherone

"~ / ,,,..~c~,~c.~.

CH~ ~ T / " ~ O / i ~ /

V

V

V

CH3 - CH3 . -- . /ocopneroxyl raoical I

~

CH~ ~'R ~

J

One-electron o - ' ~ 3 A

? H3

3

CH3

i

I /

"v"

V

/~.H,

"CH 3

Lipidperoxide

/

.

radical

CH~

CH;b">T/l~o/~I/ CH3OOH CH3

V

~./ V

V

V

"CH 3

CH; ~ T ~ i l ~ o i ~ CH3OOH CH 3

V

V

V

V

V

V

V

V

V

V

;:

"OH 3

8(~-Hydroxyperoxy-5,6-epoxytocopherone 8o~-Hydroxyperoxy-8,9-epoxytocopherone

H20

H20 H+J l

H+-I 1 CH3

C

H CH3

CH3

~

C

H.*~'~

3 CH3 0%.

CH3

CH3

Tocopherylquinone-5,6-epoxide Figure 9.24

CH 3

CH; y

CH~

I~O

Ay CH3

Tocopherylquinone-8,9-epoxyide

Reaction with free radicals initiates metabolism of the vitamin E ring

\CH 3

Vitamin E 497

compound. Since the ring system is not affected, distinctive CEHCs result from sidechain metabolism. This means that all isomers of alpha-tocopherol and alphatocotrienol result in alpha-CEHC, the beta-tocopherols and beta-tocotrienols generate beta-CEHC, and so on (Lodge et al., 2001 ). Ring modification: Reaction with two-electron oxidants, such as hypochlorous acid (from the myeloperoxidase-generated oxidative burst of leukocytes) and peroxynitrite (a reactive nitrogen species formed from the reaction of superoxide with nitric oxide) generate tocopheryl quinone in one step without the chance of regeneration (Terentis et al., 2002). Reaction of vitE with one-electron oxidants, such as superoxide anion, produces a tocopheroxyl radical, which may be converted into tocopheryl quinone by another reaction with an oxidant. Reaction of a lipid peroxyl radical with the tocopheroxyl radical converts this to 5,6-epoxy-tocopherol or 2,3-epoxy-tocopherol. NAD(P)H:quinone oxidoreductase 1 (EC1.6.99.2) may be able to revert some of the tocopheroxyl radical to tocopherol (Ross et al., 2000). More important may be the reduction of tocopheryl quinone to tocopheryl hydroquinone by this enzyme, because the reduced metabolite has antioxidant activity. It should be noted that quercetin, the most abundant flavonoid in food, induces NAD(P)H:quinone oxidoreductase 1 (Valerio et al., 2001 ).

Excretion Intact vitE and some of its apolar metabolites are excreted with bile. Various polar metabolites appear in urine. The scavenger receptor class B type I (SR-BI) plays an important role in the transfer ofvitE from HDL through the liver cells into bile (Mardones et al., 2002). This pathway is in some ways analogous to the reverse cholesterol transport from peripheral tissues into bile. The final step of translocation into the bile canaliculi uses the multidrug resistance P-glycoprotein 2 (MDR2, ABCBi; Mustacich et al., 1998). Biliary excretion of unchanged alpha-tocopherol increases with the presence of excess. The four different 2(2'-carboxyethyl)-6-hydroxychromans (CEHCs) from alpha-, beta-, gamma-, and delta-tocopherols and tocotrienols are excreted with urine (Lodge et al., 2001 ). It may be of note that all isomers of alpha-tocopherol generate the same alpha-CEHC, and provide no indication whether RRR-alpha-tocopherol or all-rac-alphatocopherol has been ingested.

Regulation The limited capacity of alpha-tocopherol transfer protein may be the most important protection against vitE excess. A modulating effect ofalpha-tocopherol on the expression of alpha-tocopherol transfer protein is likely (Azzi et al., 2001). However, experience clearly shows that high consumption levels can at least partially overwhelm such limitations, presumably by using unspecific pathways for transport.

Function Antioxidantprotection: Molecules with vitE biological activity can abstract free electrons from an oxygen free radical and thereby render it much less reactive. It is presumed


ascorbate

dehydro ascorbate

semidehydro ascorbate

J

H3G~+~ oH3 H3C~ ",.. RH

CH3 N3C

Tocopheroxylium cation Figure 9.25

ascorbate

o.3

o

.~c-~ '__~c.~ One-electro~ ~Co.~C~ radical(R " ) H 3 C C ~ radicalC ' ~RH H3C.~

Two-electron

J

(r

zymosterol > 5-alpha-cholest-8-en-3-beta-ol > lathosteroi > 7 dehydrocholesterol > cholesterol. Desmosterol (deita-3,24-cholestadien-3-beta-ol)is one of several alternative intermediates. 7-dehydrocholesterol is important because it is the precursor for ultraviolet (UV-B) light-induced synthesis of vitamin D in skin (Obi-Tabot et al., 2000).

Dietary sources Membranes and fat deposits of animal-derived foods contain cholesterol, plantderived foods contain plant sterols instead. A large portion of cholesterol in animals is linked to fatty acids, predominantly the long-chain mono- and polyunsaturated ones. The single largest source of Chol in the American diet is eggs (4.3 mg/g) providing about 215 mg per serving (50 g). Other Chol-rich include organ meats, such as liver (3.9 mg/g), and animal fats, such as tallow ( 1.1 mg/g), lard ( 1.0 mg/g), and butter (2.2 mg/g) and all meats and fish. The fat-free part of meats still contains between 0.7-0.9 mg/g. Typical daily intakes in the US are around 300-500 mg, but are much lower in vegans and other groups with minimal intake of animal-derived products. Significant amounts of squalen and other Chol precursors are also be present in some foods. Plant-derived foods contain a wide range of neutral sterols that resemble Choi structurally, but cannot substitute for most of its functions and have different metabolic fates. Several hundred milligrams of such phytosterols are consumed daily with typical mixed diets.


Digestion and absorption Dietary Chol is absorbed best from mixed micelles that contain bile acid, phospholipids, and monoglycerides. Bile acids and phospholipids are from bile, the monoglycerides are generated by the action of gastric and pancreatic lipases (EC3. I. 1.3) on dietary triglycerides. Bile-salt-activated lipase from pancreas and mammary gland has carboxylester lipase (EC3.1.1.13) activity and cleaves Chol esters in concert with pancreatic lipase (EC3.1.1.3) and colipase. Most people absorb 40--45% of a moderate Chol dose ingested with triglycerides (Rajaratnam et al., 2001). Absorption occurs mainly in the proximal small intestine, is selective (high compared to closely related plant sterols), and responsive to whole-body Chol status (Lu et al., 2001 ). The mechanism whereby Chol moves from the mixed micelles across the enterocyte brush border membrane is still under dispute (Mardones et al., 2001 ). It has been suggested that the scavenger receptor CD36 is involved in uptake of free Chol, and that the scavenger receptor class B type 1 (SR-B i ) plays a parallel role in the uptake of Chol esters (Werder et al., 2001 ). The interaction takes place at cholesterol and sphingomyelinenriched domains of the plasma membrane called caveolae, which facilitate lipid exchange between cells and Lp (Grafet al., 1999). The caveolae contain caveolin, a protein with as yet incompletely understood function. Once in the cell, most Chol is esterified by at least two genetically distinct forms of sterol O-acyltransferase (EC2.3.1.26), ACAT, and ACAT2 (Buhman et al., 2000). Both forms specifically esterify neutral sterols with omega-9 fatty acids, such as oleic acid.

Chol

oo,0,

~__~holeslersJChol~~

monoglycerides

~gl~hr;des ~ l ~

fatty acids bile ~ acids phospholipids

ChoI

7 ~ 1/2

ACAT

Cholester

micelle

other sterols Intestinal lumen

Enterocyte Brush border membrane Figure 9.35

Intestinal absorption of'cholesterol

~ I


Chol Lymph duct

Cholesterol 517

Some of the Chol is incorporated into chylomicrons (Chylos) and leaves the enterocyte as Chylos are secreted into adjacent lymph ducts. Some of the intracellular Chol is actively pumped back into the intestinal lumen (Repa et al., 2000) by the ATP-binding cassette transporter A 1 (ABCA 1). Absorption efficiency is modulated by a heterocomplex containing ABCG5 and ABCG8 (Berge et al., 2000) in an as yet unclear manner. Chol is exported from enterocytes into adjacent lymph ducts with chylomicrons (Chylos), very triglyceride-rich lipoproteins. Each chylomicron contains a single copy of apolipoprotein B48 (apoB48). A highly specific cytidine deaminase (apolipoprotein B editing catalytic subunit 1, APOBEC-1 ) is part ofa multiprotein complex, which modifies cytidine 6666 of the apoB mRNA to uridine and thereby introduces a stop codon into the sequence. Due to this modification, the intestinal protein transcripts are shortened to about 48% of the full-length version (hence B48). In humans this enzyme is expressed only in small intestine (Chen et al., 200 i ), which means that all intestinal apoB is of the apoB48 variety. Rodents and many other mammals, in contrast, produce in their enterocytes both apoB48 and the full-length version, apoBl00.

Transport and cellular uptake Blood circulation: All cholesterol in blood is contained in lipoproteins (Lp), and all Lp contain Chol. Lipoproteins are classified based on their buoyant density and additional pathogenetic characteristics. The major lipoproteins in blood are low-density lipoprotein (LDL), high-density lipoprotein (HDL), very-low-density lipoprotein (VLDL), Lp(a), Chylos, VLDL remnants, and Chylo remnants. Chylos, which carry triglycerides from small intestinal absorption, release much of their triglycerides as they pass through small arterioles and arterial capillaries, due to triglyceride hydrolysis by lipoprotein lipase (EC3.1.1.3). The resulting Chylo remnants are taken up mainly into liver and bone marrow (Cooper, 1997). VLDL, which transport

/.--holz z

Chylomicron

VLDL

Lipoprotein P" C~oq lipase Chylomi remnant

Lipoprotein ~ lipase

~o~o c ;t~rE~L~'~ remnant

% LDL

Figure 9.36

J__3

Modification and uptake of cholesterol-carryinglipoproteins

Hepatocyte

j

S18 Fat-soluble Vitamins and Non-nutrients

excess triglycerides from the liver, release triglycerides in the same way. The liver and peripheral tissues take up some of the resulting VLDL remnants, others are convened into LDL. All LDL in blood are derived from VLDL. While circulating with HDL in blood Chol can be esterified by phosphatidylcholine-sterol O-acyitransferase (LCAT; EC2.3.1.43). Since oJ6-polyunsaturated fatty acids (mostly linoleic acid) are preferentially used, about two-thirds of the Chol esters in blood contain linoleic acid. In addition, lipids can be transferred from one Lp to another; cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP) facilitate such exchanges. Numerous membrane-linked receptors recognize one or several of the circulating iipoproteins, usually leading to the endocytotic uptake of the bound Lp. The LDLreceptor (LDL-R) is present at the surface ofhepatocytes and other cells, and mediates the uptake of LDL and remnants. Some VLDL can be taken up, mainly into adipose tissue, via the VLDL receptor, another member of the LDL receptor family (Tacken et al., 2001). The LDL receptor-related protein-1 (LRP-I) is also involved in lipoprotein uptake and is especially important for the hepatic uptake of Chylo remnants (Herz and Strickland, 2001). Other members of the LDL-R family (LRP-2, LRP-3, LRP-4, LRP-5, LRP-6, LRI I ) have much more diverse roles. The receptor-associated protein (RAP) chaperones LDL-R, LRP-i and related receptors and modulates their activity (Willnow et al., 1995). LDL-R clusters in coated pits, which are destined for endocytosis. As these coated pits are folded into endosomes, moved through the cell and transformed into lysosomes, the cholesterol esters inside are hydrolyzed and free Chol is delivered by the NPCI protein (defective in Niemann-Pick disease type C) to the endoplasmic reticulum (Ory, 2000). Most Chol in the ESR is esterified again by sterol O-acyltransferase (ACAT, EC2.3.1.26). An alternative and less explored pathway for the uptake of VLDL-derived particles in smooth muscle cells may be mediated directly by heparan sulfate proteoglycans on the cell surface of smooth muscle and other cells with low expression of LDL-R family members (Weaver et al., 1997).

acetylated LDL

oxidized LDL

PL

small LDL

LDL

Figure 9.37

Lipoprorein

modification

-a• changes cholesterol flux

Macrophages, endothelial cells, etc.

into tissues

Cholesterol $19

Apolipoprotein C-I (ApoCl) is a small peptide constituent of Chylo and VLDL remnants. ApoCl inhibits binding of lipoproteins to the LDL-R, LRP-I, and the VLDL receptor, slows lipid exchange mediated by the cholesteryl ester transfer protein and decreases cellular fatty acid uptake (Shachter, 2001). As Lp circulate in blood they age due to exposure to enzymatic and chemical modification. A relatively common type of Lp modification is the unspecific oxidation of the protein or lipid constituents upon reaction with oxygen free radicals. Modification of LDL diminishes binding to LDL-R, but opens up new epitopes for binding to scavenger receptors, including scavenger receptor class A, class B scavenger receptors, and lysyl-oxidase-I (Krieger, 2001; Linton and Fazio, 2001; Mertens and Holvoet, 2001 ). Chol entering cells via scavenger receptors is sequestered in lysosomes and is less available for reverse transport. Since liver contains mainly LDL-R and few scavenger receptors, hepatic uptake and excretion of modified LDL is blocked and redirected to macrophages and other extrahepatic tissues. Reverse Chol transport: Some tissues, especially endothelial cells and macrophages, can export excess Chol (Fielding and Fielding, 2001). Since Chol transport is directed predominantly from blood into tissues, this is commonly called reverse Chol transport. This type of transfer involves the active transport of Choi by several ATP-binding cassette (ABC) transporters, including ABCA I (Liao et al., 2002), ABCG 1 (Kennedy et al., 2001), and ABCG4 (Engel et al., 2001) to L D L and HDL or precursors of HDL (Fielding and Fielding, 2001). The precise mechanisms are still under intense investigation. LDL and ligands of the retinoid X receptor (RXR) and liver X receptor (LXR) increase ABCAI expression (Liao et al., 2002). The scavenger receptor-Bl (CD36) appears to be instrumental for the binding of HDL (Silver and Tall, 2001) and the internalization of their Chol load in the liver (Thuahnai et al., 2001). A polymorphic

f

J o

~

Chol

CYP27] 27-OH-ChoI

Arterial endothelium

Figure 9.38

@tecs~

HDL

LDL

~"

Liver

Reverse transport is crucial for the removal of excess tissue cholesterol


variant of this receptor may confer survival benefits in populations exposed to parasite infections. Another mode of reverse transport uses the conversion of Chol into 27hydroxycholesterol by the mitochondrial sterol 27-hydroxylase (CYP27, no EC number assigned) followed by export of this oxysterol into blood, its uptake into liver, and metabolism to bile acids (Russell, 2000). An analogous mechanism with the initial synthesis of 24S-hydroxycholesterol in brain is described below. Materno-fetal transfer: Maternal Chol may be more important as a precursor for steroid hormone synthesis in the placenta than for use by the fetus. Without the adequate delivery of maternal Chol by HDL the placenta does not develop normally and fetal death is possible (Christiansen-Weber et al., 2000). Endocytotic uptake of maternal HDL is mediated by the intrinsic factor receptor (cubilin), a member of the LDL-receptor protein family (Kozyraki et al., 1999). Significant amounts of Chol may also enter the syntrophoblast without the mediation of receptors (Wyne and Woollett, 1998). Less certain are the individual contributions of the numerous other lipoprotein receptors at the maternal face of the placenta, which include the LDL-, acetylated LDL-, apoE-, and VLDL-receptors, as well as the scavenger receptor class B type 1 (SR-BI) and megalin. Chol efflux across the basal membrane into fetal circulation is mediated by the ATP-driven transporter ABCAI (Christiansen-Weber et al., 2000). Blood-brain barrier: It has recently been suggested that excess Chol (about 3 rag/day) is transported out of the brain back into blood as 24S-hydroxycholesterol (Bjorkhem et al., 2001 ). The nature of this transport is not yet fully understood.

Metabolism Oxysterols: The hydroxylation of Chol by specific enzymes generates oxysterols, in particular 24(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol (Russell, 2000). These oxysterols are minor precursors for bile acid synthesis, which helps some tissues to export excess Chol as described above. Even more importantly, these oxysterols serve as ligands for a group of nuclear receptors (including LXRc~ and LXR/3) that regulate Chol synthesis, intestinal absorption, and movement into and out of tissues. Sterol 27-hydroxylase (CYP27, no EC number assigned) is a mitochondrial cytochrome P450 enzyme that generates 27-hydroxycholesterol in liver and other tissues. Cholesterol 24-hydroxylase (CYP46, no EC number assigned) generates 24(S)-hydroxycholesterol in brain, but not to any significant extent in other tissues. Cholesterol 25-hydroxylase (no EC number assigned) is expressed at very low levels in many tissues. Bile acids: About half of the newly synthesized and ingested Chol is metabolized, much of this directed towards bile acid synthesis in liver. Smaller amounts of bile acids and their precursors are also produced in extrahepatic tissues, possibly as a means of eliminating excess Chol (Bjorkhem et al., 1999; Russell, 2000). A membrane-bound heme enzyme of the endoplasmic reticulum, cytochrome P450 7A 1 (cholesterol 7-alpha-monooxygenase, EC 1.14.13.17), catalyzes the initial ratelimiting step of the classic (neutral) pathway for bile acid synthesis from Chol in the

Cholesterol $21

OH~

O

H

OH

Figure 9.39

OH~

O~sterolsarebothbileacidprecursorsandimportantsignalingcompounds

2A ADPH H20 +

~

I

I

2

~

\

OHm'OH 7-c~-Hydroxycholesterol

+ H20 + N02OCHjAD

02 +

CYP7A1 _ . . [

(heme~

L /

~'

CYP46 -- A . ,,L (heme) ~ / " ~ y

\

~

~/

O H ~

OH-V

~ OH

27-Hydroxycholesterol

Cholesterol

If

02+

27-OH-cholesterolI f NADPH 7-cemonooxygenase[ (heme) ~ HgO+ ~NADP

.

HO'~,'d'~'OH

H OholyI-CoA

Figure 9.40

.

~-CoA

The mainmetabolicfate of'cholesterolis conversioninto bileacids

.

.

.

OH~,,-~..OH

CH

7-c~.27-Dihydroxycholesterol


liver. The NADPH-dependent reaction generates 7-alpha-cholesterol which can then be converted by subsequent reactions into cholic acid or chenodesoxycholic acid. The alternative (acidic) pathway starts from the oxysterol 27-hydroxycholesterol, which is produced in the liver and other tissues. 27-hydroxycholesterol 7-alphamonooxygenase (EC1.14.13.60) is a NADP-dependent p450 cytochrome enzyme in liver microsomes.

Excretion Daily secretion of Chol into bile is about 18 mg/kg body weight, about 50-60% of this biliary Chol is usually lost with the feces (Rajaratnam et al., 2001). In addition, large amounts of Chol-derived bile acids are secreted with bile. Due to the very efficient recovery from the ileum via the sodium/bile acid cotransporter (SLCIOA2), which works in conjunction with the intestinal bile-acid-binding protein, the loss of bile acids amounts to less than 2% (about 6 mg/kg body weight) of the amount secreted with bile. Taken together, daily excretion of Chol and Chol-derived products is typically around 17 mg/kg. Much smaller amounts are lost with skin and the secretions of seborrheic glands.

Regulation Chol homeostasis depends mainly on the rate of endogenous synthesis, the effectiveness of intestinal absorption of Chol from diet and bile, and the rate of reverse cholesterol transport from tissues back to the liver for excretion. The key enzyme of Choi synthesis is HMG CoA reductase (EC1.1.1.34). Inactivation by phosphorylation (hydroxymethylglutaryl-CoA reductase [NADPH] kinase; EC2.7.1.109) reactivation by dephosphorylation (hydroxymethylglutarylCoA reductase [NADPH]-phosphatase; EC3.1.3.47) modulate HMG CoA reductase activity. Considerable uncertainty still exists about the precise manner in which kinase and phosphatase activities are controlled by various regulators, including various receptors (LXR, RXR). In addition, every individual step of the cholesterol synthetic pathway is activated by the sterol regulatory element-binding proteins (SREBP) l a and 2 (Sakakura et al., 2001 ). Whole-body Chol status is known to influence fractional Chol absorption from the small intestine (Lu et al., 2001 ). While the active sterol transporter complex ABCG5/ ABCG8 is known to play a critical role in this regulation, it is still unclear what it is. One might speculate that import of sterols from circulation via this pathway, perhaps from apoE-containing lipoproteins, provides the enterocyte with the necessary signal indicating Chol availability in the system. Uptake into tissues: Lipoprotein transport into cells is differentially regulated in tissues and influenced by an exceedingly complex web of numerous receptors and modulators affecting their cell-specific expression. Only two examples will be mentioned here. Expression of the LDL-receptor, the main conduit for Chol from circulation into liver, is down-regulated by high Chol intake (Jones et al., 1996). Phosphorylation of the LDL receptor by low-density lipoprotein receptor kinase (EC2.7.1.131 ) is another

Cholesterol 523

tool for the rapid fine-tuning of Chol uptake to the individual needs of target cells (Kishimoto et al., 1987). Reverse Choltransport: Heterodimers comprising retinoid X receptors in combination with the bile acid receptor and similar ones containing oxysterol receptors increase expression of the reverse Chol transporter ABCAI (Repa et al., 2000). However, considerable gaps still persist in the understanding of mechanisms that control reverse Chol transport.

Function Bile acid synthesis: Chol is the precursor for bile acid synthesis as detailed above. Bile acids in bile contribute to the formation of mixed micelles, a prerequisite for the digestion and absorption of all fat-soluble nutrients including triglycerides, sterols, and the vitamins A, D, E, and K. 5teroid hormone synthesis: Chol is the direct precursor of corticosteroid, mineralocorticoid and steroid sex hormones. Cytochrome P450 I IAI (cholesterol side-chain cleavage enzyme, P450scc; EC 1.14.15.6), a mitochondrial heme enzyme, catalyzes the initial and rate-limiting step of steroid hormone synthesis. This oxidative cleavage reaction uses reduced adrenal ferredoxin and generates pregnenolone. Hydroxylation at the 3-beta position generates progesterone, 17-alpha-hydroxylation and side-chain cleavage gives rise to the important precursor dehydroepiandrosterone (DHEA). The tissue-specific actions of various enzymes on these precursors produce androgens, gestagens, estrogens, and other gonadal steroid hormones. Similar reactions in the adrenal cortex convert pregnenolone to glucocorticosteroids (cortisol, corticosterone) and mineralocorticosteroids (aldosterone). The synthesis rate of all steroid hormones is tightly regulated and not influenced by variations in Chol supply. Vitamin D synthesis: Exposure of skin to ultraviolet light with wave lengths between 290 and 315 nm can convert the Chol precursor 7-dehydrocholesterol to previtamin D3, which rearranges spontaneously to vitamin D3 (Holick et al., 1989). The diminished vitamin D production in older people has been attributed in part to a lower concentration of unesterified 7-dehydrocholesterol in their skin (Holick, 1999). References

Berge KE, Tian H, GrafGA, Yu L, Grishin NV, Schultz J, Kwiterovich P, Shan B, Barnes R, Hobbs HH. Accumulation of dietary cholesterol in sitosterolemia caused by mutations in adjacent ABC transporters. Science 2000;290:1771-5 Bjorkhem I, Diczfalusy U, Lutjohann D. Removal of cholesterol from extrahepatic sources by oxidative mechanisms. Curr Opin Lipidol 1999:10:161-5 Bjorkhem I, Andersson U, Ellis E, Alvelius G, Ellegard L, Diczfalusy U, Sjovall J, Einarsson C. From brain to bile. Evidence that conjugation and omega-hydroxylation are important for elimination of 24S-hydroxycholesterol (cerebrosterol) in humans. J Biol Chem 2001 ;276:37004-10 Buhman KK, Accad M, Novak S, Choi RS, Wang JS, Hamilton RL, Turley S, Farese RV Jr. Resistance to diet-induced hypercholesterolemia and gallstone formation in ACAT2deficient mice. Nature Med 2000;6:134 I-7


Chen Z, Eggerman TL, Patterson AP. Phosphorylation is a regulatory mechanism in apolipoprotein B mRNA editing. Biochem J 2001 ;357:661-72 Christiansen-Weber TA, Voland JR, Wu Y, Ngo K, Roland BL, Nguyen S, Peterson PA, Fung-Leung WE Functional loss ofABCA I in mice causes severe placental malformation, aberrant lipid distribution, and kidney glomerulonephritis as well as highdensity lipoprotein cholesterol deficiency. Am J Pathol 2000; 157: I 017-29 Cooper AD. Hepatic uptake of chylomicron remnants. J Lipid Res 1997;38:2173-92 Engel 1", Lorkowski S, Lueken A, Rust S, Schluter B, Berger G, Cullen P, Assmann G. The human ABCG4 gene is regulated by oxysterols and retinoids in monocyte-derived macrophages. Biochem Biophys Res Comm 2001;288:483-8 Fielding C J, Fielding PE. Cellular cholesterol efflux. Biochim Biophys Acta 2001;1533: 175-89 Graf GA, Matveev SV, Smart EJ. Class B scavenger receptors, caveolae and cholesterol homeostasis. 7)'ends Cardiovasc Med 1999;9:221-5 Herz J, Strickland DK. LRP: a multifunctional scavenger and signaling receptor. J Clin hn,est 2001 ; 108:779-84 Holick ME Matsuoka LY, Wortsman J. Age, vitamin D, and solar ultraviolet radiation. Lancet 1989;4:1104-5 Holick ME Vitamin D: Photobiology, metabolism, mechanism of action, and clinical applications. In Favus M J, ed. Primer on the Metabolic Bone Diseases and Disorders o f Mineral Metabolism. 4th edition. Lippincott Williams & Wilkins, Philadelphia, 1999, pp.92 8 Jones PJ, Pappu AS, Hatcher L, Li ZC, Illingworth DR, Connor WE. Dietary cholesterol feeding suppresses human cholesterol synthesis measured by deuterium incorporation and urinary mevalonic acid levels. Arterioscl Thromb Vase Biol 1996; 16:1222-8 Kennedy MA, Venkateswaran A, Tarr PT, Xenarios I, Kudoh J, Shimizu N, Edwards PA. Characterization of the human ABCGI gene: liver X receptor activates an internal promoter that produces a novel transcript encoding an alternative form of the protein. J Biol Chem 2001 ;276:39438-47 Kishimoto A, Goldstein Jk, Brown MS. Purification of catalytic subunit of low density lipoprotein receptor kinase and identification of heat-stable activator protein. J Biol Chem 1987;262:9367-73 Kozyraki R, Fyfe J, Kristiansen M, Gerdes C, Jacobsen C, Cui S, Christensen El, Aminoff M, de la Chapelle A, Krahe R, Verroust PJ, Moestrup SK. The intrinsic factor-vitamin B I2 receptor, cubilin, is a high-affinity apolipoprotein A-I receptor facilitating endocytosis of high-density lipoprotein. Nature Med 1999;5:656-61 Krieger M. Scavenger receptor class B type I is a multiligand HDL receptor that influences diverse physiologic systems. J Clin hn,est 2001 ; 108:793-7 kiao H, Langmann 1", Schmitz G, Zhu Y. Native LDL upregulation of ATP-binding cassette transporter-1 in human vascular endothelial cells. Arterioscl Th~vmh Vasc Bio12002; 22:127-32 Linazasoro JM, Hill R, Chevallier E Chaikoff IL. Regulation of cholesterol synthesis in the liver: the influence of dietary fat. J Exp Med 1958; 107:813-20 Linton ME Fazio S. Class A scavenger receptors, macrophages, and atherosclerosis. Cttrr Opin Lipid 2001 ; 12(5 ):489-95

Cholesterol 525

Lu K, Lee MH, Patel SB. Dietary cholesterol absorption; more than just bile. Trends Emtocrinol Metah 2001 ; 12:314-20 Mardones P, Quinones V, Amigo L, Moreno M, Miquel JF, Schwarz M, Miettinen HE, Trigatti B, Krieger M, VanPatten S, Cohen DE, Rigotti A. Hepatic cholesterol and bile acid metabolism and intestinal cholesterol absorption in scavenger receptor class B type I-deficient mice. J Lipid Res 2001 ;42:170-80 Mertens A, Holvoet P. Oxidized LDL and HDL: antagonists in atherothrombosis. FASEB J 2001;15:2073-84 Obi-Tabot ET, Tian XQ, Chen TC, Holick ME A human skin equivalent model that mimics the photoproduction of vitamin D3 in human skin. hi Vitro Cell Develop Biol Anita 2000;36:201-4 Ory DS. Niemann-Pick type C: a disorder of cellular cholesterol trafficking. Biochim Biophys Acta 2000;I 529:331-9 Rajaratnam RA, Gylling H, Miettinen TA. Cholesterol absorption, synthesis, and fecal output in postmenopausal women with and without coronary artery disease. Arterioscl Thromb Vase Biol 2001:21 : 1650-5 Relas H, Gylling H, Miettinen TA. Dietary squalene increases cholesterol synthesis measured with serum non-cholesterol sterols after a single oral dose in humans. Atheroscl 2000;152:377-83 Repa J J, Turley SD, Lobaccaro JA, Medina J, Li L, Lustig K, Shan B, Heyman RA, Dietschy JM, Mangelsdorf DJ. Regulation of absorption and ABC I-mediated efflux of cholesterol by RXR heterodimers. Science 2000;289:1524-9 Russell DW. Oxysterol biosynthetic enzymes. Biochim Biophy.s" Acta 2000;1529: 126-35 Sakakura Y, Shimano H, Sone H, Takahashi A, Inoue N, Toyoshima H, Suzuki S, Yamada N, lnoue K. Sterol regulatory element-binding proteins induce an entire pathway of cholesterol synthesis. Biochem Biophrs Res Comm 2001 ;286:176-83 Shachter NS. Apolipoproteins C-I and C-Ill as important modulators of lipoprotein metabolism. Curt Opin Lipid 2001 ; 12:297 304 Silver DL, Tall AR. The cellular biology of scavenger receptor class B type I. Curt Opin Lipid 2001 ; 12(5):497-504 Tacken PJ, Hofker MH, Havekes LM, van Dijk KW. Living up to a name: the role of the VLDL receptor in lipid metabolism. Curt Opin Lipid 2001;I 2:275-9 Thuahnai ST, Lund-Katz S, Williams DL, Phillips MC. Scavenger receptor class B, type I-mediated uptake of various lipids into cells. Influence of the nature of the donor particle interaction with the receptor. J Biol Chem 200 I;276:4380 I-8 Weaver AM, Lysiak J J, Gonias SL. LDL receptor family-dependent and -independent pathways for the internalization and digestion of lipoprotein lipase-associated beta-VLDL by rat vascular smooth muscle cells. J Lipid Res 1997;38:1841-50 Werder M, Han CH, Wehrli E, Bimmler D, Schulthess G, Hauser H. Role of scavenger receptors SR-BI and CD36 in selective sterol uptake in the small intestine. Biochemistr), 2001 ;40:11643-50 Willnow TE, Armstrong SA, Hammer RE, Herz J. Functional expression of low density lipoprotein receptor-related protein is controlled by receptor-associated protein in vivo. Proc Natl Acad Sci USA 1995;92:4537-41


Wyne KL, Woollett LA. Transport of maternal LDL and LDL to the fetal membranes and placenta of the golden Syrian hamster is mediated by receptor-dependent and receptorindependent processes. J Lipid Res 1998;39:518-30

Lipoate Lipoate (alpha-lipoic acid, thioctic acid; obsolete names protogen, thiocytin, factor 11 or 11A, pyruvic oxidation factor; molecular weight 208); the oxidized form is water-soluble and the reduced form is fat-soluble.

Abbreviations CoA TPP

coenzyme A thiamin pyrophosphate

Nutritional summary Function: Lipoate is a cofactor of several enzymes essential for fuel metabolism. It is a potent antioxidant and protects the liver and other organs against some toxins. Sources: Endogenous synthesis and absorption of lipoate from bacterial production in the colon account for most of the body's lipoate. Yeast, liver, and kidney are the best dietary sources, many other foods provide much smaller amounts. Requirements: Most people get enough from endogenous synthesis and bacterial production alone. Dietary intakes may provide some additional benefits, especially in regard to antioxidant action. Deficiency: No symptomatic deficiency has been observed in humans. Excessive intake: The elimination of some potentially harmful substances, including mercury compounds and benzoic acid, may be compromised by very high intakes.

Endogenous sources Most lipoate is synthesized locally in mitochondria, which may explain the presence of related enzymes and proteins (including acyl carrier protein) in this compartment (Wada et al., 1997). The principal steps are synthesis ofcapric acid, an 8-carbon fatty acid, and the sulfhydrylation of the carbon at the acyl end. While some progress has been made in

H2 H2 H2 O II / C \ / C ~ / C \ /C-OH H2C OH C C / H2 H2 I S S Figure 9.41 R-(+ )-Iipoate

Lipoate 527

the elucidation of the responsible enzymes in unicellular organisms, little is known about the corresponding human enzymes. The end product of endogenous synthesis is the R-( + ) enantiomer. (See Figure 9.43.)

Intestinal flora A significant amount oflipoate may be derived from normal intestinal bacteria, but the contribution from this source remains unclear.

Dietary sources Most foods contain some lipoate. The highest amounts are in yeast, liver, heart, and kidney, much less in muscle. Average or typical dietary intakes have not been estimated. Dietary supplements containing synthetic products contain equal amounts of the R-( + )- and S-( - )-enantiomers.

Digestion and absorption Much of the lipoate in foods and intestinal bacteria is covalently bound to lysine in enzymes. Lipoamidase (lipoyl-X hydrolase, no EC number assigned), which is present in milk, can release free lipoate from its protein-bound lipoamide form (Yoshikawa et al., 1996). It is not clear whether there are enteric forms of this enzyme that could act on ingested food. A sodium-dependent carrier for lipoate (SLC5A6), which also transports pantothenate and biotin, is located in the apical membrane of enterocyte of both small and large intestine (Prasad et al., 1998). Two sodium ions enter the cell with each lipoate molecule. H2 H2 H2 O II / C \ / C ~ / C ~ /C--OH H3C 02 C C ~-H2 H2 ? Octanoic acid

?

~

H2 H2 H2 O II / C \ / C ~ / C \ /C-OH H2C CH C C I H2 H2 SH SH R-(+)-dihydrolipoicacid

Figure 9.42 EndogenousR-(+)-Iipoatesynthesis H2 H2 H2 O II / C \ / C ~ / C \ /C-OH H2C CH C C / I H2 H2 SH SH R-(+)-dihydrolipoicacid Figure

H2 H2 O II HS~ / C ~ / C \ /C-OH CH C C HS\ I H2 H2 H2C ~ C H 2 S-(-)-dihydrolipoic acid

9.43 Synthetic productscontainboth R-(+)- and S-(-) enantiomers


The mechanism whereby lipoate is transferred into portal blood may involve a stereospecific transport system that prefers the R-(+)-enantiomer (BreithauptGrogler et al., 1999).

Transport and cellular uptake Transport with blood occurs as free lipoate (Breithaupt-Grogler et al., 1999). The liver takes up nearly a third of newly absorbed lipoate during the first pass. Uptake appears to be mediated by an as yet unspecified carrier (Peinado et al., 1989), possibly identical with the sodium-multivitamin transporter. Uptake of lipoate into other tissues might use the same transporter. Materno-fetal transfer: The sodium-multivitamin transporter (SLC5A6) is expressed in placenta (Wang et al., 1999) and participates in the transfer of lipoate to the fetus. Blood drculation:

Metabolism Lipoyltransferase (no EC number assigned) uses lipoyI-AMP generated by lipoate ligase (no EC number assigned) to covalently bond lipoate to a lysine residue of nascent proteins (Fujiwara et al., 1999). Another pathway in mitochondria that does not require the ATP-dependent activation of lipoate appears to be through a lipoate transferase (no EC number assigned) using the lipoyl-acyl carrier protein (Jordan and Cronan, 1997). Protein-bound lipoamide can be recycled in tissues through the action of lipoamidase (no EC number assigned); lipoamide-releasing activity has been detected in grey matter of the brain, liver, serum, and milk (Yoshikawa et al., 1996). Oxidized lipoate can be reduced to dihydrolipoate through two distinct mechanisms: the mitochondrial NADH-dependent dihydrolipoamide dehydrogenase which exhibits a marked preference for R(+)-Iipoate, and NADPH-dependent glutathione reductase which shows slightly greater activity toward the S(-)-Iipoate stereoisomer. Mitochondrial NADH-dependent dihydrolipoamide dehydrogenase is most active in heart and skeletal muscle. Both enzymes have similar activity in erythrocytes (Haramaki et al., 1997).

Storage The mechanisms for storage and release of lipoate, its tissue distribution and total amounts stored are uncertain. H2 H2 H2 O II /C\ / C ~ / C \ /C--OH H2C CH C C I H2 H2 SH SH R-(+)-dihydrolipoicacid Figure 9.44 Lipoateis a potent redoxreagent

~

H2 H2 H2 O II /C\ /C~ /C\ /C-OH H2C CH C C I H2 H2 S S R-(+)-Iipoic acid (ox.)

Lipoate $29

Excretion A small percentage of hepatic lipoate is excreted with bile (Peinado et al., 1989), and recovered by absorption both from small and large intestine; it is not clear what typical fecal losses are. Renal excretion of the water-soluble oxidized form may occur, but the extent has not been determined.

Regulation The processes maintaining adequate lipoate supplies are incompletely understood.

Function Pyruvate dehydrogenase (ECI.2.4.1): Pyruvate is decarboxylated by a large enzyme complex composed of multiple copies of three distinct subunits. The reaction catalyzed by subunit El and the overall stability of the complex is dependent on thiamin pyrophosphate (TPP). Each subunit E2 (dihydrolipoamide S-acetyltransferase; EC2.3.1.12) contains two lipoate molecules, which are covalently bound to lysines 99 H2

H2

H2

O II

I C=O H2q/C ~CH/G~c/C~c/C~NsC'cIC~c ~CHI I H2 H2 H H2 H2 I S S NH Lipoamide I Pyruvate H2 H2

-~ Thiamin

002 4

pyrophosphate

H2 H2 H2 O H2 H2 CI =O /C\ / C ~ c / C \ c / C ~ N J C ' c / C ~ c ~(~H CH H2C I ! H2 H2 H H2 H2 I CH3~ C ~ S SH NH II o Acetylhydrolipoamide I HS-CoA Acetyl-CoA

~

/C\H/CH2 H2

H2C SH

C

H2 iO1 H2 H2 C=O I ~ C / C \C/C, N ,C "C,C "C - - I~H

I H2 H2 H SH Dihydrolipoamide

H2 H2 I NH I

Figure 9.45 Lipoamideis a covalentlybound prostheticgroup of'pyruvate dehydrogenase and four other enzymes


and 226. These lipoamides serve as acceptors for the acetyl residues from pyruvate, transfer them to acetyI-CoA, and reduce lipoamide to dihydrolipoamide in the process. Another component of the complex, dihydrolipoamide dehydrogenase (E3; EC1.8.1.4) transfers the hydrogens via FAD to NAD. The same gene encodes the dihydrolipoamide dehydrogenase of pyruvate dehydrogenase and the other two alphaketoacid dehydrogenases. The enzyme complex is inactivated by phosphorylation ([pyruvate dehydrogenase (lipoamide)] kinase; EC2.7.1.99) of three serines in the El subunit and reactivated by removal of these phosphates by [pyruvate dehydrogenase (lipoamide)]-phosphatase (EC3.1.3.43). 2-Oxoglutarate dehydrogenase (EC2.3.1.61): The TCA-cycle intermediate 2-oxoglutarate is metabolized to succinyl-CoA by a large TPP-dependent multienzyme complex containing 24 copies of the lipoamide-containing subunit E2 (dihydrolipoamide suecinyltransferase) with octahedral symmetry; these subunits contain a single lipoamide attached to lysine 110. Branched-chain alpha-keto acid dehydrogenase (EC1.2.4.4): The alpha-ketoacids 3-methyl2-oxobutanoate, 4-methyl-2-oxopentanoate, and (S)-3-methyl-2-oxopentanoate, generated by deamination of the branched-chain amino acids valine, leucine, and isoleucine, are decarboxylated by another very large TPP-dependent enzyme complex containing multiple iipoamide-containing subunits E2 (3-methyl-2-oxobutanoate dehydrogenase (lipoamide); EC1.2.4.4). The enzyme complex is inactivated by phosphorylation ([3-methyl-2-oxobutanoate dehydrogenase (lipoamide)] kinase; EC2.7.1.115), and reactivated by dephosphorylation ([3-methyl-2-oxobutanoate dehydrogenase (lipoamide)]phosphatase; EC3.1.3.52). Glycine dehydrogenase (EC1.4.4.2): Glycine is decarboxylated in mitochondria by a large pyridoxal phosphate-dependent enzyme complex composed of multiple subunits E T, L, and H; the H subunit contains lipoamide. In a fashion, similar to the three lipoate-dependent aipha-ketoacid dehydrogenases, the lipoamide arm acts as an acceptor for a methylene group from glycine, transfers it to folate, and is reduced in the process. The T subunit then transfers the hydrogen via FAD to NAD. Antioxidation: Dehydrolipoate reduces ubiquinone and semiubiquinone to ubiquinol thereby enhancing the antioxidant potential of ubiquinoi and preventing the potent oxidant-free-radical action of the semiquinone (Kozlov et al., 1999). Since the mitochondrial oxidation ofpyruvate, aipha-ketoglutarate, branched-chain alpha-keto acids, and g~ycine continuously regenerates oxidized ~ipoate, there is a cons~am supply of antioxidant dehydrolipoate. The high iron and copper-binding potential oflipoate also reduces the risk of oxygen-free radical producing Fenton reactions. Lipoate can function as an oxygen free radical scavenger and decrease LDL oxidation and the production of F2-isoprostanes (Marangon et al., 1999). The mitigation of neuroleptic action (haloperidol) may be due to protection of enzymes (mitochondrial complex I) from oxidation (Balijepalli et al., 1999). Another important mechanism whereby lipoate protects against toxic effects of cisplatin and other compounds may be the maintenance of reduced glutathione concentrations and inhibiting lipid peroxidation. As with other antioxidants, lipoate may become a pro-oxidant under some conditions (Mottley and Mason, 2001 ), and the potential risks of large supplemental doses remain to be evaluated.

Lipoate 531

Lipoate increased insulin sensitivity (Jacob et al., ! 999) and cellular glucose uptake. Improvement of glucose transport may be the mechanism underlying the prevention of polyneuropathy by lipoate administration in an animal model (Kishi et al., 1999). A protective effect against diabetic embryopathy (neural tube defect) and vascular placenta damage has been suggested (Wiznitzer et al., 1999). Liver protection: Lipoic acid has been used with some success in the mitigation of the effects ofamanita poisoning. It may also protect hepatocytes through the activation of uroporphyrinogen decarboxylase (EC4.1.1.7, Vilas et al., 1999). Acetylcholine: Dihydrolipoic acid is a powerful activator of choline O-acetyltransferase (EC2.3.1.6) and may have an important regulatory effect on the synthesis of acetylcholine. Other effects: Depletion of CoA may impair glycine conjugation of benzoic acid by LA possibly compromising the tubular secretion of benzoylglycine and causing acute renal failure in an animal model of benzoic acid exposure (Gregus et al., 1996). Glucose metabolism:

References

Balijepalli S, Boyd MR, Ravindranath V. Inhibition of mitochondrial complex I by haioperidol: the role ofthiol oxidation. Neuropharmacol 1999;38:567-77 Breithaupt-Grogler K, Niebch G, Schneider E, Erb K, Hermann R, Blume HH, Schug BS, Belz GG. Dose-proportionality of oral thioctic acid - coincidence of assessments via pooled plasma and individual data. Eur J Pharmaceut Sci 1999;8:57-65 Fujiwara K, Suzuki M, Okumachi Y, Okamura-lkeda K, Fujiwara T, Takahashi E, Motokawa Y. Molecular cloning, structural characterization and chromosomal localization of human lipoyltransferase gene. Eur J Biochem 1999;260:761-7 Gregus Z, Fekete T, Halaszi E, Klaassen CD. Lipoic acid impairs glycine conjugation of benzoic acid and renal excretion of benzoyiglycine. Drug Metab Disp 1996;24:682-8 Haramaki N, Han D, Handelman G J, Tritschler HJ, Packer L. Cytosolic and mitochondrial systems for NADH- and NADPH-dependent reduction ofalpha-lipoic acid. Free Rad Biol Med 1997;22:535-42 Jacob S, Ruus P, Hermann R, Tritschler HJ, Maerker E, Renn W, Augustin HJ, Dietze G J, Rett K. Oral administration of RAC-alpha-lipoic acid modulates insulin sensitivity in patients with type-2 diabetes mellitus: a placebo-controlled pilot trial. Free Rad Biol Med 1999;27:309-14 Jordan SW, Cronan JE Jr. A new metabolic link. The acyl carrier protein of lipid synthesis donates lipoic acid to the pyruvate dehydrogenase complex in Escherichia coli and mitochondria. J Biol Chem 1997;272:17903-6 Kishi Y, Schmelzer JD, Yao JK, Zollman PJ, Nickander KK, Tritschler HJ, Low PA. Alphalipoic acid: effect on glucose uptake, sorbitol pathway, and energy metabolism in experimental diabetic neuropathy. Diabetes 1999;48:2045-51 Kozlov AM Gille L, Staniek K, Nohl H. Dihydrolipoic acid maintains ubiquinone in the antioxidant active form by two-electron reduction of ubiquinone and one-electron reduction of ubisemiquinone. Arch Biochem Biophys 1999;363:148-54 Marangon K, Devaraj S, Tirosh O, Packer L, Jialal I. Comparison of the effect of alphalipoic acid and alpha-tocopherol supplementation on measures of oxidative stress. Free Rad Biol Med 1999;27:1114-21


Mottley C, Mason RP. Sulfur-centered radical formation from the antioxidant dihydrolipoic acid. J Biol Chem 2001 ;276:42677-83 Peinado J, Sies H, Akerboom TP. Hepatic lipoate uptake. Arch Biochem Biophys 1989; 273:389-95 Prasad PD, Wang H, Kekuda R, Fujita 1", Fei YJ, Devoe LD, Leibach FH, Ganapathy V. Cloning and functional expression of a cDNA encoding a mammalian sodiumdependent vitamin transporter mediating the uptake of pantothenate, biotin, and lipoate. J Biol Chenl 1998;273:750 I-6 Vilas GL, Aldonatti C, San Martin de Viale LC, Rios de Molina MC. Effect of alpha lipoic acid amide on hexachlorobenzene porphyria. Biochem Mol Biol lnt 1999;47:815-23 Wada H, Shintani D, Ohlrogge J. Why do mitochondria synthesize fatty acids'? Evidence for involvement in lipoic acid production. Pmc NatlAcadSci USA 1997;94:1591-6 Wang H, Huang W, Fei YL Xia H, Yang-Feng TL, Leibach FH, Devoe LD, Ganapathy V, Prasad PD. Human placental Na+-dependent multivitamin transporter. Cloning, functional expression, gene structure, and chromosomal localization. J Biol Chem 1999;274:14875-83 Wiznitzer A, Ayalon N, Hershkovitz R, Khamaisi M, Reece EA, Trischler H, Bashan N. Lipoic acid prevention of neural tube defects in offspring of rats with streptozocininduced diabetes. Am J Ohstet Gvnecol 1999;180:188-93 Yoshikawa K, Hayakawa K, Katsumata N, Tanaka 1", Kimura 1", Yamauchi K. Highperformance liquid chromatographic determination of lipoamidase (lipoyI-X hydrolase) activity with a novel substrate, lipoyl-6-aminoquinoline. J Chlvm BiomedApp 1996;679:4 I-7

Ubiquinone The term ubiquinone (coenzyme Q, mitoquinone, SA, Q-275, 272-substance) comprises fat-soluble benzoquinones differing in the length of the isoprenyl side chain. The functional compound in humans (ubiquinone-10, coenzyme Q I0, Q-199, ubidecarenone, 2,3-dimethoxy-5-methyl-6-decaprenyl- 1,4-benzoquinone; molecular weight 862) has 10 repeating isoprenyl units in the side chain. Abbreviations CoA Q10 SAM

coenzymeA ubiquinone-10 S-adenosyl mechionine

CH3,.

O

~

CH3''O

O

I

"

CH3

Figure 9.46 Ubiquinone-10

"

""

"

"

"

"

"

"

"

Ubiquinone 533

Nutritional summary Ubiquinone-10 (QI0) is needed for the electron transport of oxidative phosphorylation, is a cofactor of pyrimidine nucleotide synthesis, aids nitric oxide recycling, and acts as an intracellular antioxidant. Food sources: Most ubiquinone-10 is consumed with meat and poultry, and some germ oils. Requirements: Normally adequate amounts are synthesized endogenously requiring sufficient availability of tyrosine and unrestricted isoprenyl synthesis. Deficiency: Genetically low synthesis can cause muscle weakness, fatigability, mental impairment and seizures (Ogasahara et al., 1989; Boitier et al., 1998). In some circumstances, such as heart failure and during treatment with HMG-CoA reductase inhibitors (statins), endogenous production may not be adequate, and additional dietary intakes of up to 30 mg/d may be beneficial. Excessive intake: Supplementation with 100mg ubiquinone-10/day has been used for up to six years without adverse effects. Function:

Endogenous sources Most tissues synthesize Q 10 from farnesyl diphosphate and tyrosine (Nagata et al., 1990) via a multistep process that requires vitamin B6, S-adenosyl methionine (methionine, folate, BI2), iron, and magnesium. Daily production is about 12mg (Elmberger et al., 1987). Decreased availability of tyrosine in phenylketonuria lowers Q 10 concentrations (Artuch et al., 1999). The relevance of additional dietary deficiencies and metabolic factors remains unclear. Synthesis of the ring system appears to take place in mitochondria. The side chain is produced in the Golgi system (Appelkvist et al., 1994). Since peroxisomal inducers promote endogenous synthesis, at least some steps may also occur in peroxisomes (Turunen et al., 2000). The ring moiety, 4-hydroxybenzoate, is derived mainly from L-tyrosine (Artuch et al., 1999). The reactions responsible for the conversion of L-tyrosine to 4-hydroxybenzoate have not been characterized, yet. Synthesis of the side-chain moiety of coenzyme Q uses farnesyl diphosphate, which is extended in several as yet unresolved steps to octaprenyl diphosphate (solanyl diphosphate). Further extension of the chain to the nonaprenyl, and presumably then to the decaprenyl, is catalyzed by transoctaprenyltranstransferase (EC2.5.1.11 ). The side chain can then be joined to the ring by 4-hydroxybenzoate nonaprenyltransferase (EC2.5.1.39) or a similar magnesiumdependent enzyme. Only a few of the following steps have been elucidated. 4,5Dihydroxybenzoate is 5-methylated by an enzyme identical or similar to the yeast protein Coq3 (hexaprenyl dihydroxybenzoate methyltransferase, EC2. i. 1.114) in an S-adenosyl methionine (SAM)-requiring reaction which might be the rate-limiting step. Synthesis is completed by the ferroenzyme 3-demethylubiquinone-9 3-0methyltransferase (EC2.1.1.64), which again requires SAM.

$34 Fat-soluble Vitamins and Non-nutrients

(a)

O~ ,OH Ox~ ,O" "OH "P'OH Isopentenyl . O diphosphate

.o:C O~

)-OH

O~C~OH I

NH2--CH I

O nonaprenyl ~1~,OH diphosphate + Ox\ ,0" "OH ,O"P'OH ,

I

~

~ % .OH Trans-octaprenyt- O o'P'oH transtransferase ~"p" (magnesium) O" "OH ~ decaprenylta

OH

L-tyrosine ,=-keto glutarate"~ Tyrosine aminotransferase L-glutarnate ~ (PLP)

._

%0.

)

~-

o=lc

~._

H2C

/~ 4-Hydroxyphenylpyruvate decaprenyl 4-OH-benzoate

H~O/

4-Hydroxybenzoate nonaprenyltransferase OH (magnesium) 4-OH-benzoate

(b)

3-DesmethyI-Q10 Q10 OH3 /H3C OH3 /H3c OH 0 0 0 0 0 t " SAM SAH SAM SAH ~-o. ~...&o=~_ )-OH O=~i~ ) 3-Demethyl HoC" ) Hexaprenyl ~ ubiquinone-9 ~ (~\ dihydroxybenzoate 3-O-methyltransferase ~-methyl transferase /H3C

% J=

HosC ~ . i

o. j =

,>-o..o.C ~_ )

~- L.._ 2 )

~

Figure 9.47

(a)

Endogenousubiquinone-lO, part 1; (b) endogenousubiquinone-lO, part 2

p

Ubiquinone 535

CLK-I (homologue of yeast Coq7p/Cat5p) is another mitochondrial inner membrane protein directly involved in ubiquinone biosynthesis (Vajo et al., 1999), but its exact function is not yet understood. O-methyltransferase catalyzes the SAM-dependent methylation of 3,4-dihydroxy5-polyprenyl benzoic acid to 3-methoxy-4-hydroxy-5-polyprenyl benzoic acid and of 2-polyprenyl-3-methyi-5-hydroxy-6-methoxy-l,4 benzoquinol (demethyl ubiquinol) to ubiquinol (Jonassen and Clarke, 2000).

Dietary sources Additional amounts of QI0 are derived from food, typically about 3-5 mg per day (Weber eta/., 1997). Good sources are meat, poultry, cereals, and corn oil (Dupont, 1990; Weber et al., 1997).

Digestion and absorption Q I0 appears to be absorbed intact from the intestine (Weber et al., 1997), but neither the exact location nor the extent or mechanism is known.

Transport and cellular uptake B/ood circu/ation: Normal Q 10 concentrations in serum are around 0.6 ~mol/l, slightly higher in early childhood (Artuch et al., 1999). Circulating Q I0 is taken up rapidly from blood into liver, then re-secreted with very-low-density lipoprotein (Yuzuriha et al., ! 983). The transfer mechanisms into other tissues remains in doubt, however (Turunen et al., 1999). Both the oxidized and reduced forms of Q I0 are apolar and readily permeate mitochondrial and other membranes.

Catabolism and excretion The steps involved in Q10 breakdown and excretion are not well characterized, yet.

Regulation Little is known about the mechanisms that maintain a constant supply of Q I0.

Function Complex I of the oxidative phosphorylation system is the electrontransferring-flavoprotein dehydrogenase (EC 1.5.5.1 ) which catalyzes the electron transfer from primary flavoprotein dehydrogenases in the mitochondrial matrix to Q 10 in the inner membrane. Q I0 also participates in electron transport of oxidative phosphorylation from succinate (which is converted to fumarate) to oxygen at the matrix face of the inner mitochondrial membrane by complex II (succinate dehydrogenase/ubiquinone; EC 1.3.5. ! ); Electron transport:


as a result of this oxidation protons are pumped into the intermembrane space for the eventual capturing of this energy by ATP synthase during the flow of proton back across the intermembrane into the matrix. At the cytosolic face the reduced form, ubisemiquinone (QH2), is oxidized by complex 111 to its semiquinone by transferring an electron via an Fe-S cluster to cytochrome c 1, and then to Q 10 by transferring another electron via b566 to b560. At the matrix face the transfer of two electrons from b560 to Q I0 reduces this again via the semiquinone to QH2. This ubiquinone cycle is possible because Q I0 and QH2 are uncharged and diffuse freely from one face of the inner mitochondrial membrane to the other. The importance of Q I0 for ion transport and ATP production, especially during rapid growth, is underscored by the finding that the viability of embryos depends on adequate Q10 availability (Stojkovic et al., 1999). Q I0 is also a constituent of a lysosomal electron transport chain. The redox potentials carried through this electron transport chain drive the transport of protons across the lysosomal membrane and help to build up the acid environment of lysosomes (Gille and Nohl, 2000). Redox reactions: Ubiquinone is an electron acceptor for various mitochondrial enzymes such as dihydroorotate dehydrogenase (EC 1.3.99.11 ) for uridine synthesis. Another redox reaction involving Q I0 is the removal of nitrite, the end product of intracellular nitric oxide degradation. A mitochondrial nitrite reductase (no EC number assigned) uses ubisemiquinone associated with the bcl complex to convert the potentially toxic nitrite back to nitric oxide thus providing an alternative source for this signaling compound which is independent ofarginine (Kozlov, Staniek et al., 1999). Ubiquinol also acts as a general intracellular antioxidant (Ernster and Dallner, 1995). In these various reactions the reduced form ubiquinole is oxidized to ubiquinone or to ubisemiquinone which has pro-oxidant properties itself (Nohl et al., 1999). The oxidized forms of Q I0 can then be reactivated to ubiquinole by dihydrolipoic acid (Kozlov, Gille et al., 1999). Supplement use: Contrary to common expectations dietary supplements did not improve aerobic power in healthy people (Bonetti et al., 2000), nor was ejection fraction, peak oxygen consumption, or exercise duration increased in patients with congestive heart failure receiving standard medical therapy (Khatta et al., 2000).

References Appelkvist EL, Aberg F, Guan Z, Parmryd I, Dallner G. Regulation of coenzyme Q biosynthesis. Mol Asp Med 1994; 15:$37-46 Artuch R, Vilaseca MA, Moreno J, Lambruschini N, Cambra FJ, Campistol J. Decreased serum ubiquinone- I0 concentrations in phenylketonuria.Am J Clin Nutr 1999;70:892 5 Bonetti A, Solito E Carmosino G, Bargossi AM, Fiorella PL. Effect of ubidecarenone oral treatment on aerobic power in middle-aged trained subjects. J Sports Med Phvs Fitness 2000;40:5 I-7 Boitier E, Degoul F, Desguerre I, Charpentier C, Francois D, Ponsot G, Diry M, Rustin P, Marsac C. A case of mitochondrial encephalomyopathy associated with a muscle coenzyme QI0 deficiency. J Neurol Sci 1998;156:41-6

Ubiquinone 5 3 7

Dupont J, White PJ, Carpenter ME Schaefer EJ, Meydani SN, Elson CE, Woods M, Gorbach SL. Food uses and health effects of corn oil. J A m Coil Nutr 1990;9:438-70 Elmberger PG, Kalen A~ Appelkvist EL, Dallner G. In vitlv and in vivo synthesis ofdolichol and other main mevalonate products in various organs of the rat. Eur J Biochem 1987:168:1 11 Ernster L, Dallner G. Biochemical, physiological and medical aspects ofubiquinone function. Biochim Bioph3w Acta 1995; 1271 : 195-204 Gille L, Nohl H. The existence of a lysosomal redox chain and the role of ubiquinone. Arch Biochem Bioph3w 2000;375:347-54 Jonassen T, Clarke CE Isolation and functional expression of human COQ3, a gene encoding a methyltransferase required for ubiquinone biosynthesis. J Biol Chem 2000; 275:12381 7 Khatta M, Alexander BS, Krichten CM, Fisher ML, Freudenberger R, Robinson SW, Gottlieb SS. The effect of coenzyme Q I0 in patients with congestive heart failure. Ann hTt Med 2000;132:636-40 Kozlov AV, Gille L, Staniek K, Nohl H. Dihydrolipoic acid maintains ubiquinone in the antioxidant active form by two-electron reduction of ubiquinone and one-electron reduction of ubisemiquinone. Arch Biochem Biophys 1999;363:148-54 Kozlov AV, Staniek K, Nohl H. Nitrite reductase activity is a novel function of mammalian mitochondria. FEBS Lett 1999;454:127 30 Nagata Y, H idaka Y, Ishida E Kamei T. Effects of simvastatin (MK-733) on branched pathway of mevalonate. Japn J Pharmacol 1990;54:315-24 Nohl H, Gille L, Kozlov AV. Critical aspects of the antioxidant function ofcoenzyme Q in biomembranes. Biqliwtors 1999;9:155-61 Ogasahara S, Engel AG, Frens D, Mack D. Muscle coenzyme Q deficiency in familial mitochondrial encephalomyopathy. Proc Natl Acad Sci USA 1989;86:2379-82 Stojkovic M, Westesen K, Zakhartchenko V, Stojkovic E Boxhammer K, WolfE. Coenzyme Q(10) in submicron-sized dispersion improves development, hatching, cell proliferation, and adenosine triphosphate content of in vitro-produced bovine embryos. Biol RepJvd 1999;61:541-7 Turunen M, Appelkvist EL, Sindelar E Dallner G. Blood concentration ofcoenzyme Q(I0) increases in rats when esterified forms are administered. J Nutr 1999; 129:2113-18 Turunen M, Peters JM, Gonzalez FJ, Schedin S, Dallner G. Influence of peroxisome proliferator-activated receptor alpha on ubiquinone biosynthesis. J Mol Biol 2000:297:607-14 Vajo Z, King LM, Jonassen T, Wilkin DJ, Ho N, Munnich A, Clarke CE Francomano CA. Conservation of the Caenorhabditis elegans timing gene clk-1 from yeast to human: a gene required for ubiquinone biosynthesis with potential implications for aging. Mamm Genome 1999; I 0:1000-4 Weber C, Bysted A, Holmer G. Coenzyme QI0 in the diet - daily intake and relative bioavailability. MolA,v~ Med 1997;I 8:$251-$254 Yuzuriha T, Takada M, Katayama K. Transport of [ 14C]coenzyme Q 10 from the liver to other tissues after intravenous administration to guinea pigs. Biochim Bioph3w Acta 1983; 759:286 91

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Water-soluble vitamins and non-nutrients

Methylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thiamin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Riboflavin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Niacin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin 136 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Folate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vitamin 1312 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biotin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pantothenate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Queuine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13iopterin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inositol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

539 542 551 561 570 581 591 603 613 619 625 628 634

Methylation Abbreviations H cys Met MTHFR MTR SAM

homocysteine L-methionine methylenetetrahydrofolate reductase (EC1.5.1.20) 5-rnethyltetrahydrofolate-homocysteine S-methyltransferase (EC2.1.1.1 3) S-adenosylmethionine

Metabolism is an integrated system o f reactions that cannot be viewed from the aspect o f just one component such as a particular nutrient. Silencing o f gene expression by D N A methylation, which depends on an adequate supply o f S - a d e n o s y l m e t h i o n e (SAM), provides an illustration o f the interdependence o f m e t h i o n i n e , folate, riboflavin, vitamin B6, vitamin B I2, choline, and other critical nutrients within a network o f transporters, binding proteins, and enzymes. W h e n intake o f one nutrient is at the low end, the outcome often depends on adequacy o f the other nutrients as well as on individual metabolic disposition. Lack o f a nutrient also can redirect the use o f another nutrient and have indirect consequences in a seemingly distant metabolic network. Thus, weaknesses Handbook of Nutrient Metabolism ISBN: 0-12-417762-X

Copyright c' 2003 Elsevier Ltd All rights of reproduction ill any forrn reserved

540 Water-soluble Vitamins and Non-nutrients

~

Glycine \ \

~ S - A d e n o s y l ~ ~ methionine ~ , /

/~

Betai/ne

#

Methionine

INAD Choline

S-Aden~ homoc'rstelne

/THF.

F~~5mTH F

dihydro

folate~ M P /

dUMP

5,10-THF

-"--- Homo( ste ne

1

\ hos0.a,,0y.J choline

Pyridoxal 5'phosphate

Cysetnie

Figure 10.1 DNA methylation dependson a web of nutrients and metabolic events

in one-carbon metabolism have broad consequences. They can cause epigenetic disorders even before conception, birth defects during early pregnancy, slowed brain growth and cognitive development, anemia, cancer, diabetes, atherosclerosis, thrombosis, and other diseases. Obviously, optimal nutrition is a complex goal that requires consideration of nutrient intake and health outcomes in a broad context.

SAM-dependentmethylation creatine synthesis uses about 70% of the SAM available for methylation (Wyss and Kaddurah-Daouk, 2000). SAM is also the prerequisite precursor for the production of the polyamines spermine and spermidine. Much smaller amounts go to the synthesis of carnitine (0.3 mmol/d), choline, estrogen, and other compounds. As the example of DNA methylation emphasizes, the quantity of the product does not indicate the importance of the reaction. Specific DNA target sequences are methylated by several DNA (cytosine-5-)methyltransferase (EC2.1.1.37, zinc-binding) isoenzymes. The presence of a 5-methyl group on cytosine usually blocks transcription directly or through attached binding proteins. Current evidence indicates that silencing by selective methylation regulates developmentally appropriate expression of genes and suppresses parasitic insertions. Hypomethylation of DNA due to suboptimal nutrient intake and metabolic disposition has been linked to an increased risk of cancer (Ehrlich, 2002). Cancer of colon, cervix, breast, stomach, esophagus, and other sites occurs with increased frequency in people with low habitual intake of folate. It is likely that additional nutrients involved in one-carbon metabolism influence cancer risk, but this issue needs more De novo

Methylation 541

investigation (Ames, 2001 ). Hypomethylation may also contribute to the deterioration of brain function with aging and other age-typical afflictions (Selhub, 2002). Methylation of parental DNA imprints some genomic regions. Faulty imprinting is linked to severe developmental defects in Willi-Prader and Angelman syndromes (Nicholls and Knepper, 2001 ) and possibly much more common adulthood conditions such as obesity and diabetes (Cooney et al., 2002).

Sources of methyl groups While methyl groups abound in nutrients and their metabolites, only a few of them can be used for SAM-dependent methylation. The major sources are methionine, choline, and 5-methyltetrahydrofolate (5 mTHF). Met intakes depend to a significant degree on dietary habits. Compared with meat eaters ovo-lacto-vegetarians have much lower (770 vs. 1450mg/d) average intakes (Sachan et al., 1997). The second step of choline breakdown, catalyzed by the zinc-enzyme betaine homocysteine methyltransferase (EC2.1.1.5), remethylates homocysteine (Hcys) to Met and thus provides a methyl group for SAM synthesis. Since the de novo synthesis of choline requires the SAM-mediated transfer of three methyl groups, only dietary choline can contribute to net SAM synthesis. Daily choline intakes have been estimated at 600-1000 mg (Zeisel, 1994). A much more significant pathway of Hcys remethylation is catalyzed by 5methyltetrahydrofolate-homocysteine S-methyltransferase (MTR, EC2.1.1.13). Very little of the necessary cosubstrate 5 mTHF is generated by direct methylation of THE such as in the betaine homocysteine methyltransferase-catalyzed reaction, but comes from the reduction of 5,10-methylene-THF by methylenetetrahydrofolate reductase (MTHFR; EC1.5.1.20). Large quantities of this metabolite arise from the metabolism of serine (40mmol/d), glycine (150mmol/d), histidine (17mmol/d), and formate. Both a low supply of folate and reduced MTHFR activity diminish the availability of the 5 mTHF cosubstrate for optimal Met recycling. The active cob(l)alamine form of MTR becomes slowly oxidized to the inactive cob(ll)alamine form. The FAD- and FMN-dependent methionine synthase reductase (EC2.1.1.135) can reactivate the cob(I)alamine-containing form again by reductive methylation (Leclerc et al., 1998). This reaction uses cytochrome b5, which in turn is regenerated by NADPH-dependent cytochrome P450 reductase (EC1.6.2.4), another enzyme with both FAD and FMN as prosthetic groups (Chen and Banerjee, 1998). Inhalation of nitrous oxide irreversibly inactivates MTR (Horne et al., 1989; Riedel et al., 1999).

References Ames BN. DNA damage from micronutrient deficiencies is likely to be a major cause of cancer. Mutation Res 2001;475:7-20 Chen Z, Banerjee R. Purification of soluble cytochrome b5 as a component of the reductive activation of porcine methionine synthase. J Biol Chem 1998;273:26248-55


Cooney CA, Dave AA, WolffGL. Maternal methyl supplements in mice affect epigenetic variation and DNA methylation of offspring. J Nutr 2002; 132:2393S-2400S Ehrlich M. DNA methylation, cancer, the immunodeficiency, centromeric region instability, facial anomalies syndrome and chromosomal rearrangements. J Nutr 2002; 132:2424S-2429S Horne DW, Patterson D, Cook RJ. Effect of nitrous oxide inactivation of vitamin BI2dependent methionine synthetase on the subcellular distribution of folate coenzymes in rat liver. Arz.h Biochem Biophys 1989;270:729-33 Leclerc D, Wilson A, Dumas R, Gafuik C, Song D, Watkins D, Heng HH, Rommens JM, Scherer SW, Rosenblatt DS, Gravel RA. Cloning and mapping of a cDNA for methionine synthase reductase, a flavoprotein defective in patients with homocystinuria. Proc Natl Acad Sci USA 1998;95:3059-64 Nicholls RD, Knepper JL. Genome organization, function, and imprinting in Prader-Willi and Angelman syndromes. Ann Rev Gen Human Gen 2001 ;2:153-75 Riedel B, Fiskerstrand T, Refsum H, Ueland PM. Co-ordinate variations in methylmalonyl-CoA mutase and methionine synthase, and the cobalaminee cofactors in human glioma cells during nitrous oxide exposure and the subsequent recovery phase. Biochem J 1999;341 : 133-8 Sachan DS, Daily JW III, Munroe SG, Beauchene RE. Vegetarian elderly women may risk compromised carnitine status. Veg Ntttr 1997;1:64-9 Selhub J. Folate, vitamin BI2 and vitamin B6 and one carbon metabolism. ,1 Nutr Hlth Aging 2002;6:39~,2 Wyss M, Kaddurah-Daouk R. Creatine and creatinine metabolism. Phvsiol Rev 2000; I 107-213 Zeisel SH. Choline. In Shils ME, Olson JA, Shike M, eds. Modern Natrition in Health and Disease. Lea & Febiger, Philadelphia, 1994, pp.449-58

Vitamin C The hexuronic lactone L-ascorbic acid is a water-soluble essential nutrient (ascorbate, L-xyloascorbic acid, antiscorbutic factor, antiscorbutic vitamin, L-3-ketothreohexuronic

.1-o.

.To.

H

H-C-OH

OH

Figure 10.2

H

H-C-OH

OH

L-Ascorbic acid

(ASC)

.-I-OH

H

OH/

H-C-OH

"~

Semidehydroascorbate Dehydroascorbate (SDA) (DHA)

Vitamin C has three different redox states

Vitamin C 543

acid, 3-oxo-L-gulofuranolactone[enoi form] molecular weight 176). Vitamin C occurs in three different redox states: Fully reduced as ascorbate (ASC), partially oxidized as semidehydroascorbate (SDA), and fully oxidized as dehydro-L-ascorbic acid (DHA). Several derivatives are also biologically active.

Abbreviadons ASC L-ascorbic acid (specifically to indicate the reduced form) A2S ascorbate-2-sulfate DHA dehydro-L-ascorbic acid GLUT1 glucose transporter 1 PAPS 3'-phosphoadenosine 5'-phosphosulfate SDA semidehydroascorbate SGLT1 sodium-glucose cotransporter SVCT1 sodium-ascorbate transporter 1 (SLC23A2) SVCT2 sodium-ascorbate transporter 2 (SLC23A1)

Nutritional

summary

Vitamin C is essential for gums, arteries, other soft tissues, and bone (collagen synthesis), for brain and nerve function (neurotransmitter and hormone synthesis), for nutrient metabolism (especially iron, protein, and fat), and for antioxidant defense (directly and by reactivating vitamin E) against free radicals (free radicals increase the risk of cancer and cardiovascular disease). Food sources: Many fruits and vegetables provide at least 20% of the recommended daily intake per serving; citrus fruits, berries, and tomatoes are especially rich sources. Prolonged storage, extensive processing, and overcooking greatly diminish vitamin C content of foods. Requirements: Adult women should get at least 75 mg/day, men at least 90 mg/day (Food and Nutrition Board, Institute of Medicine, 2000). Age over 50, smoking, strenuous exercise, heat, infections, and injuries each may increase needs. Deficiency: Scurvy (symptoms include painful swelling and bleeding into gums, joints, and extremities, poor wound healing, fatigue, and confusion) has become rare in most countries; 10mg/day prevent it. Lower than optimal intake may diminish immune function and wound healing, and increase the risk of heart disease and cancer, especially in susceptible individuals. If intakes are low, stores last only a few weeks. Excessive intake: Daily doses of 2000 mg or more may irritate stomach and bowels, cause kidney stones, and interfere with copper status. Function:

Endogenous sources Unlike most other vertebrates, humans (just like all primates and guinea pigs) lack the enzyme for the final step ofascorbate synthesis (L-gulonolactone oxidase; ECI. 1.3.8)


from hexose precursors due to multiple deletions and point mutations in the responsible gene. Thus, compounds with vitamin C activity have to be obtained from food.

Dietary sources Foods contain a mixture of compounds with vitamin C activity: ASC (the reduced form), SDA (the partially oxidized form), DHA (the oxidized form), and small amounts of ascorbate-2-sulfate (A2S). Acylated ASC, ascorbyl phosphate and other derivatives are sometimes used as food additives. (Note: food tables usually report only the ASC content of food, not total bioactive vitamin C). ASC and SDA are oxidized rapidly when exposed to air without losing their biological activity for humans. All forms of vitamin C decompose during prolonged heating or storage of foods. Typical intakes of vitamin C from food in the US are 77 mg/day in women and 109mg/day in men. The best food sources are fruits and vegetables, especially citrus, berries, peppers, tomatoes, broccoli, greens. On average, a serving of fruits or vegetables provides about 30 mg vitamin C (Lykkesfeldt et al., 2000). Consumption of nonfood sources is common, most often as multivitamin supplements, which usually provide 50-60 mg/day, or as megadose preparations which may contain 500 mg or more.

Digestion and absorption Small amounts of ingested ascorbate are absorbed nearly completely, but fractional absorption rapidly drops to less than 20% when daily intakes are above 200rag (Bianchard et al., 1997). ASC and SDA may be oxidized in the lumen of the intestine, partially through the activity of ceruloplasmin (EC I. 16.3. I ). DHA is taken up slightly more efficiently in the proximal than in the distal small intestine (Malo and Wilson, 2000) by facilitated transport via an unknown mechanism. The glucose transporters that facilitate DHA transfer at other sites (GLUTI, GLUT3, and GLUT4) are not sufficiently expressed at the luminal side to explain transport. GLUT2, GLUT5, and the sodium/glucose cotransporter 1 do not accept DHA (Liang et al., 200!). ASC uptake, in contrast, is more efficient in the distal small intestine. The sodium-dependent ascorbate transporter 1 (SVCTI, SLC23A2) mediates its uptake with high efficiency and specificity (only the L-form of ASC is transferred). Two sodium ions are needed for the transport of one ASC; one of these sodium ions may be actually coupled to the simultaneous transport of glucose via the sodium/glucose cotransporter (SGLT1, Malo and Wilson, 2000). Absorption is efficient, with a maximum around three hours after ingestion (Piotrovskij et al., 1993). Ascorbate-2-sulfate (A2S) is not effectively hydrolyzed in the intestine and not absorbed. Ascorbate-dependent glutathione reductase (EC 1.8.5.1 ), and reduced glutathione (non-enzymically) in the enterocyte reduce DHA to ASC; the oxidized glutathione, in turn, is regenerated by a system of thioredoxin and NADPH-dependent thioredoxin reductase (ECI.6.4.5). The high intraceilular concentration generates enough of a gradient to drive ASC via the sodium-ascorbate transporter 2 (SVCT2, SLC23AI) across the basolateral membrane (Rose and Wilson, 1997; Liang et al., 2001 ).

Vitamin C 545

L DHA

v

DHA GSH

~

GSSG 2N ASC ~

Intesti lumen nal

3 Na§

" ~ 2 K+

ASC

~

I

Enterocyte

Brushborder membrane Figure10.3

AT

Intestinal uptake of vitamin


Capillary lumen

Capillary endothelium

C

The colon might also have some absorptive capacity, since both sodium-ascorbate transporters are expressed there.

Transport and cellular uptake ASC, the main form in blood (95%), is transferred from blood to some tissues (chromaffin cells, osteoblasts, fibroblasts) predominantly by the two sodiumdependent ascorbate transporters (SVCTI and SVCT2) in an energy-dependent, concentrative process. SVCT1 is expressed in most tissues, while the expression of SVCT2 is not expressed in skeletal muscle and lung. On the basis of recent observations in erythrocytes the existence of an NADHdependent, reducing DHA transmembrane transporter has been proposed (Himmelreich et al., 1998). If the same transporter system were present in enterocytes this would provide another explanation for the near absence of DHA in blood. Blood-brain barrier: ASC in interstitial fluids is oxidized (by ceruloplasmin, iron compounds, or other agents) to DHA which is then transferred via glucose transporters (GLUT1, GLUT3, and GLUT4) into (and out of) cells (Liang et al., 2001). Reduction of DHA to ASC blocks reverse transport, since the GLUTs are impervious to ASC and SDA; the sodium/ascorbate cotransporter 2 (SLC23AI, distinct from the ubiquitous SVCTI ) completes the concentrative transport into brain. This mechanism may explain the ten-fold higher concentration of ASC in brain compared to blood. Blood circulation:

546

Water-soluble Vitamins

and Non-nutrients

Materno-fetal transfer: Similar to the arrangement in the small intestines, DHA transport into the syntrophoblast uses glucose transporters, while ASC is taken up via SVCTI. DHA can be reduced inside the syntrophoblast layer. ASC is then exported to the fetal side via SVCT2.

Metabolism DHA is reduced to ASC in cytosol by ascorbate-dependent glutathione reductase (EC1.8.5.1), NADH-dependent monodehydroascorbic acid reductase (ECI.6.5.4), the NADPH-dependent selenoenzyme thioredoxin reductase (EC1.6.4.5), or nonenzymatically by reduced glutathione (since this is a near-equilibrium reaction, ASC can also reduce oxidized glutathione). SDA, which is the unstable radical intermediate generated by some reactions, is reduced by NADH-dependent monodehydroascorbate reductase (EC1.6.5.4) and by thioredoxin reductase (ECI.6.4.5) (May etal., 1998); another important mechanism may be the NADH-dependent DHAreducing transporter found in erythrocytes. Additional DHA and SDA-reducing enzymes, including NADPH-dependent DHA-reductase, have been described, but are not fully characterized, yet.

NADPH

NADP

k_j

thioredoxin, oxidized

2 GSH

DHA

---

ASC

,- ASC

DHA

Figure 10.4 Intracellularmetabolism of'vitamin C

thioredoxin, reduced

GSSG

ASC

k

~

= ASC

Vitamin C 547

L-ascorbate-cytochrome-b5 reductase (EC1.10.2.1) catalyzes the shuttling of reducing equivalents with cytochrome b561 across the phospholipid bilayer enveloping microsomes, neurovesicles, and chromaffin granules, thereby coupling the oxidation of ASC to SDA inside to the reduction of SDA to ASC outside. A small amount of available ASC is converted by alcohol sulfotransferase (EC2.8.2.2) into A2S using 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as the cosubstrate; this enzyme is otherwise important for the conjugation of a wide range of steroids (e.g. dehydroepiandrosterone), bile acids, and drugs in the adrenals and liver.

Cytosol 1/2 NADH

1/2 NAD

ASC

DHA

?
LO" + OH- + Fe3+; or H202 + Fe2+ ~ "OH + OH + Fe3+); this iron recycling reaction (Fe~++ASC --+ FeZ++SDA) can perpetuate the generation of oxygen free radicals. Protein modification: Ascorbate maintains the reduced form of iron in two types of metalloenzymes that post-translationally hydroxylate lysine and proline residues in collagen (EC1.14.11.2 procollagen-proline, 2-oxoglutarate-dioxygenase and ECI.14,11.4 procollagen-lysine, 5-dioxygenase: the latter activity is exerted by two genetically distinct isoenzymes). Each reduction of Fe( Ill ) to Fe(II ) oxidizes one ASC to SDA. Another ascorbate-dependent copper enzyme (EC 1.14.17.3 peptidyl-glycine alpha-amidating monooxygenase) activates numerous proteohormones (including TRH, CRE GnRH, neuropeptide Y, endorphins, gastrin, pancreatic polypetide, atrial natriuretic factor, and arginine vasopressin) in neurosecretory vesicles; each modific reaction oxidizes one ASC molecule to DHA. Amino acid metabolism: ASC participates in the metabolism of phenylalanine and tyrosine, and the synthesis of carnitine. ASC is one of several alternative reductants for the synthesis ofhomogentisinate, an intermediary metabolite ofphenylalanine and tyrosine catabolism, by 4-hydroxyphenylpyruvate dioxygenase (ECI. 13.11.27). For the following hydroxylation ofhomogentisate, ASC is again necessary to maintain the ferrous state of enzyme-bound iron in homogentisate 1,2-dioxygenase (ECI. 13. I 1.5). In a similar way, ASC keeps the copper of dopamine-beta-monooxygenase (EC 1.14.17. I ) in its reduced state, thereby ensuring synthesis of noradrenaline (norepinephrine) from dopamine. At least two steps ofcarnitine synthesis from lysine are

5SO Water-soluble Vitamins and Non-nutrients

mediated by ASC-dependent ferroenzymes. Trimethyllysine is hydroxylated by trimethyllysine dioxygenase (ECI. 14.11.8), and the ultimate precursor, trimethylammoniobutanoate, is converted into carnitine by gamma-butyrobetaine hydroxylase (ECI.14.11.1). Steroids and lipids: ASC is important for the last three steps of aldosterone synthesis, 11 beta- and 18-hydroxylation (EC1.14.15.4 steroid l l-beta-monooxygenase and EC1.14.15.5 corticosterone 18-monooxygenase) and oxidation of 1 l-deoxycorticosterone, in the adrenal cortex. All three reactions are mediated by cytochrome P-450scc, the activity of which decreases with decreasing availability of ASC. The same is true for another cytochrome, CYPI I B2, which mediates cholesterol 7-alpha-hydroxylation in liver microsomes, a key step of bile acid synthesis. The mechanism and stoichiometry of ASC in these reactions is not well characterized. Cytochromes generate profuse amounts of oxygen free radicals, and it has been suggested that ASC is essential to protect the enzyme protein and the lipids in the surrounding microsomal membrane. In a reaction catalyzed by L-ascorbate cytochrome-b5 reductase (EC1.10.2.1)ASC generates the reducing equivalents (ferrocytochrome b5) for the desaturation of fatty acids in microsomes. ASC can provide, as an alternative to NADPH, reducing equivalents for the hydroxylation of N-acetylneuraminic acid (ECI. 14.99.18 CMP-N-acetylneuraminate monooxygenase), a critical step in the synthesis of GM3(NeuGc) gangliosides in many tissues. Iron metabolism: ASC enhances absorption of non-heme iron from the small intestine. Generally, it has been assumed this is due to a reduction of ferric iron by ASC, but this mechanism has been disputed. ASC is also necessary for the mobilization of ferritin iron deposits. It is able to penetrate through molecular pores of the ferritinFe(IlI) complex, where it reduces iron; the resulting ejection of Fe(ll) from the crystal lattice makes iron available for binding to transferrin and transport away from storage sites. For each mobilized iron one ASC is oxidized to DHA. 5ulfate transfer: A2S appears to be a reservoir of sulfate groups which become available when A2S is hydrolyzed by arylsulfatase A (EC3.1.6.8) or arylsulfatase B (EC3.1.6.12). This may explain how A2S promotes the sulfation ofcholesterok and of glucosaminoglycans such as chondroitin and dermatan sulfate. Since the activities of both arylsulfatase A or arylsulfatase B are inhibited by ASC, replete vitamin C status not only slows the release of ASC from A2S, but also the hydrolytic release of noradrenaline and a wide range of other sulfoconjugated compounds. Other functions: Dietary ASC in the stomach suppresses the reaction of ingested nitrites with food proteins to nitrosamines and thereby decreases carcinogen exposure (Helser et al., 1992). ASC also has been reported to directly inhibit the growth of some tumor cell lines in vitro. ASC can reduce various elements~ such as chromium, and thereby affect their bioavailability and biological activity. It has been proposed that ASC reduces selenite to elemental Se which then can form links with selenocysteine in proteins. The biological significance of such an effect is not known.

Thiamin SS1

ASC has also been suggested to act as an aldose reductase thereby diminishing the risk o f free radical production from sorbitol in diabetics (Crabbe and Goode, 1998). ASC is also a reducing agent for enzymes involved in prostaglandin synthesis (Horrobin, 1996), the mixed-function oxidases, and the cytochrome p450 electron transport system (Tsao, 1997) contributing to the metabolism o f xenobiotics. References Blanchard J, Tozer TN, Rowland M. Pharmacokinetic perspectives on megadoses ofascorbic acid. Am J Clin Nutr 1997;66:1165-71 Crabbe M J, Goode D. Aldose reductase: a window to the treatment of diabetic complications? Progr Ret Eve Res 1998; 17:313-83 Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for vitamin C, vitamin E, selenium, and carotenoids. National Academy Press, Washington, DC, 2000 Helser MA, Hotchkiss JH, Roe DA. Influence of fruit and vegetable juices on the endogenous formation of N-nitrosoproline and N-nitrosothiazolidine-4-carboxylic acid in humans on controlled diets. Carcinogenesis 1992; 13:2277-80 Himmelreich U, Drew KN, Serianni AS, Kuchel PW. 13C NMR studies of vitamin C transport and its redox cycling in human erythrocytes. Biochem 1998;37:7578-8 Horrobin DE Ascorbic acid and prostaglandin synthesis. Subeell Biochem 1996;25:109-15 Kallner A, Hartmann D, Hornig D. Steady-state turnover and body pool of ascorbic acid in man. Am J Clin Nutr 1979;32:530-9 Liang W J, Johnson D, Jarvis SM. Vitamin C transport systems of mammalian cells. Mol Membrane Bio12001 ; 18:87-95 Lykkesfeldt J, Christen S, Wallock LM, Chang HH, Jacob RA, Ames BN. Ascorbate is depleted by smoking and repleted by moderate supplementation: a study in male smokers and nonsmokers with matched dietary antioxidant intakes. Am J Clin Nutr 2000;71:530-6 Malo C, Wilson JX. Glucose modulates vitamin C transport in adult human small intestinal brush border membrane vesicles. J Nutr 2000; 130:63-9 May JM, Cobb CE, Mendiratta S, Hill KE, Burk RE Reduction of the ascorbyl free radical to ascorbate by thioredoxin reductase. J Biol Chem 1998;273:23039-45 Piotrovskij VK, Kallay Z, Gajdos M, Gerykova M, Trnovec T. The use of a nonlinear absorption model in the study of ascorbic acid bioavailability in man. Biopharm Drug Di.v~ 1993; 14:429-42 Rose RC, Wilson JX. Ascorbate membrane transport properties. In L Packer, J Fuchs, eds. Vitamin C in Health and Disease. Marcel Dekker, New York, NY, 1997, pp. 143-62 Tsao CS. An overview of ascorbic acid chemistry and biochemistry. In Packer L, Fuchs J, eds. Vitamin C in Health and Disease. Marcel Dekker, New York, NY, 1997, pp.25 58

Thiamin Thiamin (thiamine, vitamin B l, 3-[4-amino-2-methyl-5-pyrimidinylmethyl]-5-(2hydroxyethyl)-4-methylthiazolium, aneurine, molecular weight of the hydrochloride


CH3-~

~N~ /NH 2

f\

-

CH3

Figure10.8 Thiamin

337) is a water-soluble vitamin whose heterocyclic rings contain both nitrogen and sulfur.

Abbreviations RFC-1 TCP ThTr2 TMP TPP TYP AK

reduced folate carrier (SLC19A1 ) thiamin carrier protein thiamin transporter 2 (SLCl 9A3) thiamin monophosphate thiamin pyrophosphate thiamin triphosphate adenylatekinase

Nutritional

summary

Thiamin pyrophosphate is an essential cofactor of five enzymes involved in carbohydrate, amino acid, intermediary (tricarboxylic acid cycle), and phytol metabolism; thiamin triphosphate is important for brain function. Food sources: Good sources are yeast, legumes, enriched grain products, and pork. Requirements: Men should get 1.2 mg/d, women 1.1 mg/d; needs are slightly higher during pregnancy and lactation. Use of diuretics increases dietary needs. Deficiency: Early signs may include anorexia, weight loss, muscle weakness, apathy, confusion, and irritability. Later consequences include edema (wet Beriberi), muscle wasting (dry Beriberi), and psychosis (Wernicke-Korsakoffsyndrome). Onset of cardiac failure in young infants can be very sudden and rapidly lethal. Alcohol abuse is often associated with deficiency, possibly because alcohol interferes with thiamin uptake and metabolism; a rare genetic disturbance of transporter-mediated cellular uptake results in megaloblastic anemia, diabetes mellitus, and sensorineural deafness (Labay et al., 1999). Excessive intake: No adverse effects from high oral doses (as much as 10mg/d) have been reported. Anaphylactic reactions following parenteral administration have been observed. Function:

Dietary and other sources Foods may contain thiamin pyrophosphate (TPP), thiamin monophosphate (TMP), thiamin triphosphate (TTP), thiamin, thiamin sulfide, thiamin disulfide, thiamin

Thiamin

tetrahydrofurfuryl disulfide (added to wine), thiamin hydrochloride; thiamin is a monovalent cation at physiological pH. Median intakes of adult men in the US are about 1.9 mg/d (Food and Nutrition Board, Institute of Medicine, 1998). Good sources include yeast, legumes, enriched or whole grain products, bran, and pork. By law, bread in the US is enriched with 1.8 mg/lb, and flour with 2.9 mg/Ib. Significant amounts of thiamin are lost when foods are cooked in water and the vitamin leaches out. Thiamin also is unstable when exposed to high temperatures, irradiation, alkaline medium (bicarbonate treatment of peas and beans), or metabisulphite (preservatives in dried fruits and wine); oxidation generates thiochrome and other oxidation products. Antithiamin factors are present in betel, tea (caffeic acid, tannic acid, chiorogenic acid), and foods (possibly heme and hemin are involved); ascorbate and other organic acids may be protective. Thiaminase (EC3.5.99.2) is active in various microorganisms, including saccharomyces, cerevisiae and Staphylococcus atu'eus. Thiamin in foods contaminated with thiaminase is cleaved into the inactive breakdown products 4-amino-5-hydroxymethyl-2-methylpyrimidine and 5-(2-hydroxyethyl)-4-methylthiazole. Thiamin pyridinylase (EC2.5.1.2) from viscera of some fish and shellfish diminishes the vitamin content of exposed foods by convening thiamin to heteropyrithiamin. Large amounts of thiamin are produced in the colon by normal enteric bacteria and secreted in free form. Enterocytes of the colon express specific thiamin transport capacity and thus can take up thiamin synthesized by enteric bacteria (Said et al., 2001 ). The quantitative contribution from this source is not known, however.

Digestion and absorption Dietary thiamin is absorbed throughout the small intestine, maximally in the duodenum. Evidence is mounting that some thiamin from bacterial production can be absorbed in the colon (Said et al., 2001). Absorption of microgram amounts from the small intestine may be nearly complete; it increases with thiamin deficiency (Laforenza et al., 1997), and decreases with folate deficiency. Aging (Rindi and Laforenza, 2000) and alcohol intake (Breen et al., 1985) decrease fractional absorption; concomitant alcohol ingestion may decrease absorption from a single thiamin dose by nearly 30%. At doses around 5mg/d absorption becomes very ineffective (Hayes and Hegsted, 1973). Only about 5% o f a 50mg dose is absorbed (Tallaksen et al., 1993). The main forms of thiamin in unfortified foods, TPE TTP, and TME are hydrolyzed by nucleotide pyrophosphatase (EC3.6.1.9) from pancreatic exocrine secretions (Beaudoin et al., 1983) and brush border alkaline phosphatase (EC3.1.3.1, zinc- and magnesium-dependent). Unphosphorylated thiamin can then be taken up at low concentration via a thiamin-proton antiporter (Dudeja et al., 2001; Said et al., 2001) likely to be the high-affinity thiamin transporter SLCI9A2 (Fleming et al., 2001 ). This transporter is present both in the small and in the large intestine. In addition, a low-affinity, high-capacity system exists that can meet most of the body's needs in the absence of SLCI9A2 (Stagg et al., 1999). It may be this system that accounts for thiamin uptake by passive diffusion at high concentrations (Rindi and Laforenza, 2000).

553

554 Water-solubleVitamins and Non-nutrients

d

TPRTMPFrP (

Thiamin ~

J

j

TPP Thiamin~r AMP 1 PP-kinase ? Thiami (Mg) m"ATP TMP, TMP, H+ Thiamin T h i a m i ~ H+

Inlestinal~ lumen~

Enterocyte

Brushborder membrane Figure 10.9 Intestinalabsorptionofthiamin

I


I

] Capillary lumen Capillary endothelium

The reduced folate carrier (RFC-1, SLCI9AI ) may also play a role, since it can transport phosphorylated thiamin (Zhao, Gao, Wang et al., 2001 ). Once in the enterocyte, the free thiamin is phosphorylated to TPP by thiamin pyrophosphokinase (EC2.7.6.2) and thereby prevented from returning to the intestinal lumen. Transport across the basolateral membrane into portal blood uses an ATP-driven thiamin carrier (Laforenza et al., 1993) that is not yet well characterized. Some transport of phosphorylated thiamin, in particular TME may also be attributable to the reduced folate carrier (SLCI9AI; Zhao, Gao, Wang et al., 2001 ). Since most of the thiamin in blood is in free or monophosphorylated form, TPP must be cleaved prior to export. This step is not well characterized.

Transport and cellular uptake B/ood circulation: Transport in blood plasma occurs mostly as thiamin bound to albumin (about 10-20nmol/l); the concentrations of TMP are slightly lower. Concentrations in whole blood cells are an order of magnitude higher (around 200nmol/l). The predominant species in red blood cells is TPP; TMP and thiamin contribute much less (Tallaksen et al., 1997). Thiamin can be taken up into hepatocytes, erythrocytes, and other cells by active transport (Labay et aL, 1999) and from

Thiamin 555

there into mitochondria through the high-affinity thiamin transporter 1 (SLCi9A2). A second, closely related, thiamin transporter (ThTr2, SLCIgA3) in liver, heart and other tissues also mediates selective high-affinity uptake of free thiamin (Rajgopai et al., 2001). The reduced folate carrier 1 (SLCI9AI) provides an alternative minor access route for TMP (Zhao et al., 2002). TPP efflux via the reduced folate carrier 1 appears to help regulating cellular thiamin homeostasis (Zhao, Gao, Wang et al., 2001 ). Blood-brain barrier: Free thiamin and, to lesser extent, TMP are transported across the blood-brain barrier (Patrini et al., 1988). Thiamin may share a carrier-mediated transport system with choline that mediates uptake (Kang et al., 1990). The thiamin transporter 1 (SLCI9A2) is expressed in brain, but its exact location and role remains to be elucidated. Materno-fetal transfer: Transport across the placental membrane is known (Dutta et al., 1999) to involve both the thiamin transporter 1 (SLC19A2) and ThTr2 (SLCI9A3; Rajgopal et al., 2001), but is not yet completely characterized. In the process, thiamin is concentrated in the placenta, relative to both the maternal and the fetal side. Since transport in the materno-fetal direction is slower than in the fetomaternal direction, the net transfer is from mother to fetus (Schenker et al., 1990).

Metabolism Thiamin can be phosphorylated to TPP in most tissues (Zhao, Gao, and Goldman, 2001 ) by thiamin pyrophosphokinase (EC2.7.6.2). This enzyme converts free thiamin and TMP to TPP and TTP. In brain and other tissues a significant proportion of TPP is phosphorylated again to TTP by thiamin-diphosphate kinase (EC2.7.4.15) in an ATPdependent reaction. Some investigators (Shioda et al., 1993) reported that adenosylate kinase (EC2.7.4.3) facilitates transphosphorylation (TPP + ADP ~ AMP + TTP), but this finding has been disputed by others (Bettendorff et al., 1993). Both these enzymes are unusual in that they require creatine as cofactors (Shikata et al., 1986; Shikata et al., 1989). Thiamin-diphosphate kinase may also require glucose as an activating factor (Nishino et al., 1983). A broad array of enzymes in various tissues dephosphorylate the thiamin phosphates. Magnesium-dependent thiamin triphosphatase (EC3.6.1.28), which generates TPP from TTE is present in many tissues, both as cytosolic and membrane-bound form. TPP in mitochondria can be hydrolyzed by a heterodimeric isoenzyme of acid phosphatase (EC3.1.3.2). Thiamin pyrophosphatase, which converts TTP to TME could be a modified form of type B nucleoside diphosphatase (EC3.6.1.6) in the Golgi apparatus. Additional less specific phosphatases also act on thiamin phosphates. Thiamin can be metabolized to thiamin acetate, thiamin sulfide, pyrimidine carboxylic acid, thiazole acetate, 2-methyl-4-amino-5-formylaminomethylpyrimidine, thiochrome and other compounds (Pearson and Darby, 1967; White et al., 1970), but the exact nature or location of the involved metabolic processes in not well understood. It has been suggested that some of these compounds may be absorbed after intestinal bacteria have acted on thiamin or its derivates, but the exact nature and location of the involved reactions remains to be clarified.

$$6 Water-soluble Vitamins and Non-nutrients

AT

p,,x~X

Thiamin pyro- ~/ phosphokinase ] ( M g )

/

AM P

~

~\cH~~ "('~H~ ~---oH

Thiamin

\

~

//~AD

ATP . TPP kinase~ / (Mg, creatine, l glucose)///~ ADP

"

~

CH3.. /N%,/NH~ S "[I "T S ~ OH N~'-~-0~"~r ~-o-'-o. Thiamin' OH3 IOI monophosphate(TMP) CH3vN~

.

/

P Adenyla!e~ kinase (Mg, I creatine~J~k ' AMP-

NH2 ~S

~

/

/

\

III I t \ OH OH N/"'~c/N~o-Ip--o-Ip--OH H2

\CHo . 11 Thiamin , ,- o pyrophosphate(TPP)

Thiamin 1 II - ~ pyrophos-I o ~ phatase I ~ ] Thiamin tri- \ / phosphatase I (Mg)

] //

X CH~..~/N%,/NH~ S , 1 J \ Thiamin triphosphate (TTP)

Figure

~

Acid \ phos- / phatase I I

/ / OH OH O. / / ._o: , o o o

10.10 Thiamin metabolism

Storage Typically about 30 mg thiamin are stored in an adult, half of it in muscle, less in liver and kidney. The biological half-life of thiamin is 9-18 days (Ariaey-Nejad et al., 1970). About 80% of total body thiamin is TPP (mostly bound to pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase in mitochondria), 10% is TTP; smaller amounts are present as free thiamin and TMP (McCormick, 2000).

Excretion At high thiamin intakes most of the excess is rapidly excreted with urine (Davis et al., 1984), very little with bile. A mean creatinine/thiamin renal clearance ratio of 2.4 indicates that the thiamin excess is actively excreted. At low to moderate concentrations, on the other hand, recovery of intact thiamin from primary filtrate in the proximal tubule is very effective due to mechanisms very similar to those responsible for intestinal absorption. A thiamin/H+ antiporter with a 1:1 stoichiometric ratio in the brush border recovers filtered thiamin (Gastaldi et al., 2000), and an ATP-driven

Thiamin 557

thiamin carrier completes transport across the basolateral membrane. All diuretics appear to increase thiamin losses with urine (Suter and Vetter, 2000). Most thiamin losses with urine are in the form of pyrimidine carboxylic acid, thiazole acetic acid, or thiamin acetic acid, and a considerable number of additional minor metabolites (White et al., 1970).

Regulation Thiamin homeostasis is maintained both at the level of intestinal absorption and of renal tubular recovery, both of which are tightly limited. Expression of the thiamin transporter gene is induced by the p53 tumor suppressor (Lo et al., 2001) and regulated via intracellular calcium/calcmodulin signaling (Said et al., 2001). Export of both TMP and TPP from extraintestinal tissues by the reduced folate carrier (SLC 19A 1) is likely to limit concentrations and thereby contribute to the maintenance of homeostasis (Zhao, Gao, Wang et al., 2001).

Function Only five enzymes have been identified so far that have a strict thiamin requirement. All of them use TPP coordinated with magnesium as a cofactor. Additional actions of thiamin, especially as TTP in brain, appear to be similarly essential, but have not yet been completely characterized. Tramketolases: Transketolase (EC2.2.1.1) is needed for glucose metabolism via the pentose-phosphate pathway, the only pathway that generates significant amounts of NADPH. Two distinct genes are now known to encode proteins with transketolase activity. Alternative splicing of the more recently discovered one, transketolase 2 (Coy et al., 1996), gives rise to different isoforms in brain and heart. Decreased activity of this enzyme may contribute to the Wernicke-Korsakoffsyndrome observed in alcohol abusers (see below). The transketolase 2 gene locus is immediately adjacent to the protein-coding regions of the retina color pigment genes on the X chromosome (Hanna et al., 1997), which might suggest a particular importance for vision. Pyruvate dehydrogenase: This key enzyme (EC3.1.3.43) of glucose metabolism is embedded in the mitochondrial matrix and contains multiple copies of three distinct moieties: El, E2, and E3. TPP is associated with El. AIpha-ketoglutarate dehydrogenase: This enzyme (EC1.2.4.2) of the tricarboxylic acid cycle consists of three distinct moieties: El, E2, E3; TPP is associated with El. The enzyme probably also participates in the breakdown of tryptophan, lysine, and hydroxylysine. Branched-chain alpha-keto acid dehydrogenase: This enzyme with the systematic name 3-methyl-2-oxobutanoate dehydrogenase (EC1.2.4.4) comprises three distinct subunits, El (with TPP bound to His292), E2, and E3. The enzyme is needed for the catabolism of the branched-chain amino acids valine, isoleucine, and leucine. Branched-chain alpha-keto acid dehydrogenase and pyruvate dehydrogenase also cleave alpha-ketobutyrate (from L-threonine and homocysteine metabolism) into CO~ and priopionyl-CoA (Paxton et al., 1986: Pettit and Reed, 1988).


Phytanic acid metabolism: Alpha-oxidation of 3-methyl fatty acids such as phytanic acid (Foulon et al., 1999) involves as the third step a reaction catalyzed by the TPPdependent enzyme 2-hydroxyphytanoyl-CoA lyase (no EC number assigned). Brain function: TTP appears to be important for brain function, possibly by participating in the function of maxi-Cl- channels (chloride channels of large unitary conductance). Deficiency due to genetic causes during early fetal development or infancy may cause progressive degeneration of the cerebral cortex (Laurence and Cavanagh, 1968). Leigh syndrome is characterized by the degeneration and focal necrosis of gray matter, and capillary proliferation in the brain stem. Reduced production of TTP, possibly through inhibition of adenosine triphosphate-thiamin diphosphate phosphoryltransferase has been suggested as a causative factor, pointing to the critical importance of this thiamin metabolite. Severe confusion and agitation characterizes Wernicke-Korsakoff syndrome. It is seen most often in chronic alcohol abusers and usually responds well to thiamin administration. Working memory of alcohol abusers in a detoxification program appeared to improve in a dose-dependent manner with intramuscular injection of thiamin (Ambrose et al., 2001). Mitochondria: A facilitating role of thiamin for mitochondrial function has been suggested (Sato et al., 2000). References

Ambrose ML, Bowden SC, Whelan G. Thiamin treatment and working memory function of alcohol-dependent people: preliminary findings. AIc Clin Ext~ Res 2001;25: 112-16 Ariaey-Nejad MR, Balaghi M, Baker EM, Sauberlich HE. Thiamin metabolism in man. Am J Clin Nutr 1970;23:764-78 Beaudoin AR, Grondin G, Lord A, Roberge M, St-Jean P. The origin of the zymogen granule membrane of the pancreatic acinar cell as examined by ultrastructural cytochemistry of acid phosphatase, thiamine pyrophosphatase, and ATP-diphosphohydrolase activities. Eur J Cell Biol 1983;29:218-25 Bettendorff L, Peeters M, Wins E Schoffeniels E. Metabolism of thiamine triphosphate in rat brain: correlation with chloride permeability. J Netovchem 1993;60:423-34 Breen KJ, Buttigieg R, iossifidis S, Lourensz C, Wood B. Jejunal uptake of thiamin hydrochloride in man: influence of alcoholism and alcohol. Am J Clin Nutr 1985,42:121-6 Coy JF, Dubel S, Kioschis P, Thomas K, Micklem G, Delius H, Poustka A. Molecular cloning of tissue-specific transcripts ofa transketolase-related gene: implications for the evolution of new vertebrate genes. Genomics 1996:32:309 16 Davis RE, Icke GC, Thorn L Riley WJ. Intestinal absorption of thiamin in man compared with folate and pyridoxal and its subsequent urinary excretion. J Nutr Sci Vitaminol 1984;30:475-82 Dudeja PK, Tyagi S, Kavilaveettil RJ, Gill R, Said HM. Mechanism of thiamine uptake by human jejunal brush border membrane vesicles. Am J Phvsiol Cell Phvsiol 2001 ;281 :C786-C792

Thiamin 559

Dutta B, Huang W, Molero M, Kekuda R, Leibach FH, Devoe LD, Ganapathy V, Prasad PD. Cloning of the human thiamine transporter, a member of the folate transporter family. J Biol Chem 1999;274:31925-9 Fleming JC, Steinkamp ME Kawatsuji R, Tartaglini E, Pinkus JL, Pinkus GS, Fleming MD, Neufeld EJ. Characterization of a murine high-affinity thiamine transporter, Sic19a2. Mol Genet Metab 2001;74:273-80 Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B I2, pantothenic acid, biotin, and choline. National Academy Press, Washington, DC, 1998, pp.58-86; 480-1 Foulon V, Antonenkov VD, Croes K, Waelkens E, Mannaerts GE Van Veldhoven PE Casteels M. Purification, molecular cloning, and expression of 2-hydroxyphytanoylCoA lyase, a peroxisomal thiamine pyrophosphate-dependent enzyme that catalyzes the carbon-carbon bond cleavage during alpha-oxidation of 3-methyl-branched fatty acids. Pivc Natl Acad o['Sci USA 1999:96:10039-44 Gastaldi G, Cova E, Verri A, Laforenza U, Faelli A, Rindi G. Transport of thiamin in rat renal brush border membrane vesicles. Kidney Int 2000;57:2043-54 Gregory JF 3rd. Bioavailability of thiamin. Eur J Clin Nutr 1997;51 :$34-$37 Hanna MC, Platts JT, Kirkness EE Identification ofa gene within the tandem array of red and green color pigment genes. Genomicw 1997;43:384-6 Hayes KC, Hegsted DM. Toxicity of the vitamins. In Toxicants Occutv'ing Naturalh, in Foods'. Food and Nutrition Board, National Research Council, National Academy Press, Washington, DC, 1973, pp.235-53 Kang YS, Terasaki T, Ohnishi T, Tsuji A. In vivo and in vitro evidence for a common carrier mediated transport of choline and basic drugs through the blood-brain barrier. J Pharmacobio-Dvnamics 1990; 13:353-60 Labay V, Raz T, Baron D, Mandel H, Williams H, Barrett T, Szargel R, McDonald L, Shalata A, Nosaka K, Gregory S, Cohen N. Mutations in SLCI9A2 cause thiamineresponsive megaloblastic anaemia associated with diabetes mellitus and deafness. Nature Genet 1999;22:300-4 Laforenza U, Gastaldi G, Rindi G. Thiamine outflow from the enterocyte: a study using basolateral membrane vesicles from rat intestine. J Plnwiol (Lond) 1993; 468:401-12 Laforenza U, Patrinin C, Alvisi C, Faelli A, Licandro A, Rindi G. Thiamine uptake in human intestinal biopsy specimens, including observations from a patient with acute thiamine deficiency. Am J Clin Ntttr 1997:66:320-6 Laurence KM, Cavanagh JB. Progressive degeneration of the cerebral cortex in infancy. Brain 1968;91:261-80 Lo PK, Chen JY, Tang PP, I_in J, Lin CH, Su LT, Wu CH, Chen TL, Yang Y, Wang FF. Identification of a mouse thiamine transporter gene as a direct transcriptional target for p53. J Biol Chem 2001 ;276:37186-93 McCormick DB. Niacin, riboflavin, and thiamin. In Stipanuk MH, ed. Biochemical aml Ph.w'iological Aspects of Human Nutrition. W.B. Saunders, Philadelphia, 2000, pp.458-82 Nishino K, Itokawa Y, Nishino N, Piros K, Cooper JR. Enzyme system involved in the synthesis of thiamin triphosphate. I. Purification and characterization of protein-bound thiamin diphosphate: ATP phosphoryltransferase. J Biol Chem 1983;258:11871-8

5 6 0 Water-soluble Vitamins and Non-nutrients

Patrini C, Reggiani C, Laforenza U, Rindi G. Blood-brain transport of thiamine monophosphate in the rat: a kinetic study in vivo. J Neurochem 1988;50:90-3 Paxton R, Scislowski PWD, Davis EJ, Harris RA. Role of branched-chain 2-oxo acid dehydrogenase and pyruvate dehydrogenase in 2-oxobutyrate metabolism. Biochem J 1986;234:295-303 Pearson WN, Darby WJ Jr. Catabolism of 14C-labeled thiamine by the rat as influenced by dietary intake and body thiamine stores. J Nutr 1967;93:491-8 Pettit FH, Reed LJ. Branched-chain alpha-keto acid dehydrogenase complex from bovine kidney. Methods Enzvmol 1988; 166:309-12 Rajgopal A, Edmondnson A, Goldman ID, Zhao R. SLCI9A3 encodes a second thiamine transporter ThTr2. Biochim Biophys Acta 2001 ; 1537:175-8 Rindi G, Laforenza U. Thiamine intestinal transport and related issues: recent aspects. PJvc Soc Exp Biol Med 2000;224:246-55 Said HM, Ortiz A, Subramanian VS, Neufeld EJ, Moyer ME Dudeja PK. Mechanism of thiamine uptake by human colonocytes: studies with cultured colonic epithelial cell line NCM460. Am J Phvsiol - Gasowint Liver Phvsio12001;28 I:G 144-G 150 Sato Y, Nakagawa M, Higuchi I, Osame M, Naito E, Oizumi K. Mitochondrial myopathy and familial thiamine deficiency. Muscle Nerve 2000;23:1069-75 Schenker S, Johnson RE Hoyumpa AM, Henderson GI. Thiamine-transfer by human placenta: normal transport and effects of ethanol. J Lab Clin Med 1990; 116:106-15 Shikata H, Egi Y, Koyama S, Yamada K, Kawasaki T. Properties of the thiamin triphosphate-synthesizing activity catalyzed by adenylate kinase (isoenzyme I). Biochem lnt 1989; 18:943-9 Shikata H, Koyama S, Egi Y, Yamada K, Kawasaki T. Identification of creatine as a cofactor of thiamin-diphosphate kinase. FEBS Lett 1986;201 : 101-4 Shioda 1",Yasuda S, Yamada K, Yamada M, Nakazawa A, Kawasaki T. Thiamin-triphosphatesynthesizing activity of mutant cytosolic adenylate kinases: significance ofArg-128 for substrate specificity. Biochim Biophys Acta 1993; 1161:230-4 Stagg AR, Fleming JC, Baker MA, Sakamoto M, Cohen N, Neufeld EJ. Defective highaffinity thiamine transporter leads to cell death in thiamine-responsive megaloblastic anemia syndrome fibroblasts. J Clin Invest 1999; 103:723-9 Suter PM, Vetter W. Diuretics and vitamin B 1: are diuretics a risk factor for thiamin malnutrition? Nutr Rev 2000;58:319-23 Tallaksen CM, Bohmer T, Karlsen J, Bell H. Determination of thiamin and its phosphate esters in human blood, plasma, and urine. Meth Enzvmol 1997;279:67-74 Tallaksen CM, Sande A, Bohmer T, Bell H, Karlsen J. Kinetics of thiamin and thiamin phosphate esters in human blood, plasma and urine after 50mg intravenously or orally. Eur J Clin Pharmacol 1993;44:73-8 White WW 3rd, Amos WH jr, Neal RA. Isolation and identification of the pyrimidine moiety of thiamin in rat urine using gas chromatography-mass spectrometry. J Ntttr 1970; 100:1053-6 Zhao R, Gao E Goldman ID. Molecular cloning of human thiamin pyrophosphokinase. Biochim Biophys Acta 2001 ; 1517:320 2 Zhao R, Gao E Goldman ID. Reduced folate carrier transports thiamine monophosphate: an alternative route for thiamine delivery into mammalian cells. Am J PInwiol Cell Ph.vsiol 2002;282:C 1512 17

Riboflavin 561

Zhao R, Gao E Wang Y, Diaz GA, Gelb BD, Goldman ID. Impact of the reduced folate carrier on the accumulation of active thiamin metabolites in murine leukemia cells. J B i o l Chem 2001;276:1 ! 14-18

Riboflavin Riboflavin (7,8-dimethyl-10-(D-ribo-2,3,4,5-tetrahydroxypentyl)isoalloxazine, vitamin B2; obsolete names vitamin G, lyochrome, lactoflavin, hepatoflavin, ooflavin, uroflavin; molecular weight 376) is a water-soluble, heat-stable, and light- and alkalisensitive vitamin. Abbreviations AMP ETF FAD FMN RCP RDA

adenosinernonophosphate electron-transfer flavoprotein flavin adenine dinucleotide flavin rnononucleotide riboflavin carrier protein recommended dietary allowance

Nutritional summary Function: Riboflavin is the precursor of flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which serve both as prosthetic groups and as cofactors for a wide range of enzymes important for beta-oxidation and oxidative phosphorylation, antioxidant defense, vitamin metabolism (folate, niacin, vitamins A, C, B6, and B I2), amino acid utilization, hormone synthesis, and many other functions. Food sources: Milk and dairy products, meat, poultry and fish, cereals and bread, and green vegetables each can provide at least one-sixth of recommended intakes per serving.

H2C--OH CH-OH I CH-OH CjH-OH CH2

o Figure 10.11 Riboflavin

$62 Water-soluble Vitamins and Non-nutrients

Requirements: Current RDAs for women are I.I mg/d, and for men, 1.3mg/d. Pregnancy, lactation, and increased energy intake and expenditure all increase requirements. Since only 1-2 weeks' requirements are stored, regular adequate intakes are important. Deficiency: Prolonged low intakes cause cracking and swelling of the lips (cheilosis), cracking and inflammation of the angles of the mouth (angular stomatitis), dark-red colored tongue (glossitis), skin changes at other sites (seborrheic dermatitis), and normocytic anemia. Deficiency during infancy and childhood impairs growth. Excessive intake: There is little danger even when intakes exceed recommendation many times. Since excess riboflavin is lost rapidly, there is no additional benefit over recommended intake levels and stores will not increase.

Dietary sources Foods contain free and beta-glucosylated riboflavin (Gregory, 1998) as well as FMN and FAD, some of the latter covalently bound to proteins. Riboflavin is heat-stable, but light- and alkali-sensitive; the inactive form lumiflavin (7,8,10-trimethylisoalloxazine) is a product of photodecomposition (Chastain and McCormick, 1987). Best sources of riboflavin are milk and dairy products, meat, poultry and fish, green vegetables, cereals, and bread. Grain products in the US have to be fortified with 4 mg/kg. Median intake in the US is about 2 mg/d, much of this from fortified foods and dietary supplements.

H

H3cT N N' ~ O

Figure 10.12 Lumiflavin is a light-inactivated product of riboflavin

Digestion and absorption Riboflavin is present in foods mostly (80-90%) as FAD and FMN cofactors of proteins. Hydrochloric acid from the stomach readily releases the flavins that are only loosely bound to their proteins. A small percentage of food flavin is bound to a histidyl-nitrogen or cysteinyl-sulfur and proteolysis results in the release of amino acid-linked 8-alpha-FAD which is biologically inactive. FMN is dephosphorylated to riboflavin by alkaline phosphatase (EC3.1.3. I) in the small intestine. FAD is broken up by nucleotide pyrophosphatase (EC3.6.1.9) at the brush border of villous tip cells into AMP and FMN from which riboflavin can then be released (Byrd et al., 1985). Some of the riboflavin in plant-derived foods is present as beta-glucoside, which has to be cleaved by a beta-glucosidase (possibly lactase) prior to absorption.

Riboflavin 563

Fractional intestinal absorption of riboflavin and related compounds is high over a large range of intakes (75% of a 20 mg dose) and declines with intakes beyond that (Zempleni et al., i 996). Riboflavin is absorbed mainly from the jejunum, and only to a much lesser degree from the large intestine (Said et al., 2000). Uptake proceeds by a rapid process dependent on energy, but not on sodium or proton flux. At higher concentrations passive diffusion into the enterocyte becomes increasingly relevant. Retention in the enterocyte does not entail metabolic modification of free riboflavin (Said and Ma, 1994). The maximal amount that can be absorbed from a single dose appears to be about 27 mg (Zempleni et al., 1996). Phosphorylation of free riboflavin by riboflavin kinase (flavokinase, EC2.7.1.26; zinc) to FMN is critical for retaining riboflavin in the enterocyte (Gastaldi et al., 2000). FMN can then be convened to FAD by ATP:FMN adenylyltransferase (FAD synthetase; EC2.7.7.2). About 60% of absorbed riboflavin is exported as FMN or FAD (Gastaldi et al., 2000). It is not clear whether riboflavin and its metabolites leave the enterocyte by simple diffusion or by another process.

Transport and cellular uptake Blood circulation: FAD, riboflavin, and FMN are (in descending concentration order) the main forms in plasma; in severe malnutrition FAD concentration may be lower than FMN concentration (Capo-chichi et al., 2000). Neither the plasma concentration of flavo-coenzymes (around 79nmol/l) nor of riboflavin concentration in plasma (around 13 nmol/I) varies much in response to different levels of intake (Zempleni et al., 1996). Most riboflavin metabolites in plasma are associated with immunoglobulins, albumin, and other proteins (lnnis et al., 1985). Riboflavin is taken up into liver by an energy-dependent process (Said et al., 1998). Uptake into liver and other tissues first requires the hydrolysis of FAD and FMN to riboflavin (Lee and Ford, 1988). Some inactive metabolites such as 2'-hydroxyethylflavin impede cellular uptake of riboflavin by competition (Aw et al., 1983). The riboflavin carrier protein (RCP) mediates intracellular riboflavin transport (Schneider, 1996). RCP is expressed in placenta and in Sertoli and Leydig cells of the testes and in spermatozoa. The chicken vitellogenin receptor imports very-lowdensity lipoprotein, riboflavin-binding protein, and aipha-2-macroglobulin into growing oocytes. The similarity of vitellogenin receptor and VLDL receptor raises the question whether the latter may contribute to the uptake of riboflavin-binding protein in humans (Schneider, 1996). A4aterno-fetal transfer: Riboflavin-containing nucleotides have to be cleaved prior to uptake into syntrophoblasts via an as yet unknown carrier. RCP is critical for riboflavin transfer across the placental membrane and is inducible by estrogen. Antibodies against RCP reduce fertility (Adiga et al., 1997). Blood-brain barrier: Riboflavin rapidly crosses from blood circulation into brain and is converted into FMN and FAD (Spector, 1980). Reverse transport is also readily possible. The carrier mechanisms involved in this transfer and their regulation are not yet well understood.


Metabolism Flavin cofactor synthesis occurs in liver and most other tissues. In the initial, rate limiting step the zinc-dependent riboflavin kinase (EC2.7.1.26) phosphorylates riboflavin. From FMN the more commonly used cofactor FAD is produced by FMN adenylyltransferase (EC2.7.7.2). This magnesium-dependent reaction links the phosphate group of the AMP moiety to the phosphate group of FMN. There is no information about the mechanism(s) whereby FMN or FAD becomes covalently bound to specific histidyl or cysteinyl residues of just a few of the numerous flavoproteins.

Activation:

. ATP FIavoKinase

H2C--OH CH-OH

(zinc, magnesium)~,,~

I

CH-OH I CH-OH

CH-OH

q v/"- -Alkaline phosphatase Pi (zinc, magnesium)

CH-OH CH-OH CH 2

Riboflavin

OH

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CH2

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70 mg per serving). Plant foods are much poorer sources.

Digestion and absorption More than 70% of a small zinc dose (less than 3 rag) is absorbed from the small intestine when there is no interference from other meal constituents (L6nnerdal, 2000). Protein hydrolysates and some amino acids, particularly histidine and cysteine, increase fractional zinc absorption. Higher than minimal zinc doses, replete zinc status, and the presence ofphytate in the same meal greatly decrease absorption efficiency. Concurrent high intakes of iron, calcium or other divalent metal cations do not appear to impact zinc absorption significantly (Sandstr6m, 2001 ). Significant amounts of zinc enter the intestinal lumen with secretions from the pancreas, lntraluminal digestion by proteases, DNAses and RNAses releases zinc from the food matrix. Zinc forms complexes with histidine, cysteine, and nucleotides that are absorbed better than zinc alone. Absorption is reduced by phytate (L6nnerdal, 2000).

Zinc 687

The main mechanism for zinc uptake from the intestinal lumen is with ZIP4 (SLC39A4), a member of the ZIP family (Wang et al., 2002). This transporter is expressed at the luminal side of enterocytes throughout the small and large intestine. A related protein, provisionally designated hORFI, may contribute to zinc uptake from the colon (Wang et al., 2002). Genetic variants of this gene are associated with the rare familial condition acrodermatitis enteropathica. The proton-coupled divalent cation transporter I (DMTI, S LCI I A2) appears to provide a minor uptake route. DMTI also transports iron, copper, cadmium, and other divalent metal ions that may compete with zinc (McMahon and Cousins, 1998). This may be the basis for diminished zinc uptake in the presence of high amounts of calcium. Enterocyte intracellular iron concentration is the main determinant of DMT 1 expression and translocation to the apical membrane; low iron status upregulates DMTI expression, and more so if the subject is pregnant or has a familial hemochromatosis phenotype. A possible alternative route for zinc uptake is via the hydrogen ion/peptide cotransporter (SLCI5AI, PepTl) when complexed to small peptides (Tacnet et al., 1993). Uptake as a complex with individual amino acids may explain why histidine and cysteine improve intestinal zinc absorption (L6nnerdal, 2000). Metallothioneins are a ubiquitous family of cysteine-rich small peptides that bind zinc and some other heavy metals with high capacity (up to 12 atoms per peptide).

10 mg/day) include gastrointestinal discomfort and green coloring of the tongue. Slowed growth, renal failure, respiratory and cardiovascular distress might occur at very high doses.

Dietary sources Some foods contain vanadium, especially shellfish, mushrooms, parsley, dill, and foods exposed to vanadium-containing steels during processing (dried milk) or storage (canned foods). Human milk has been reported to contain 0.18 I.zg/l, infant formula an order of magnitude more than that (Krachler et al., 2000). The estimated daily vanadium intake of the US population ranges from 10 (Uthus and Seaborn, 1996) to 60 I.zg/d (Barceloux, 1999).

Digestion and absorption Only a small percentage (possibly I-5%) of ingested vanadium is absorbed. Vanadate appears to be 3-5 times more bioavailable than the vanadyl form. Since vanadylate is reduced in the stomach to vanadate, the efficiency of uptake can be expected to depend both on stomach conditions and residence time of an ingested dose. Vanadylate is known to bind to transferrin and ferritin, possibly also to other iron-binding proteins; this raises the possibility that vanadium uptake proceeds at least partially via the non-heme iron absorption pathways.

Transport and cellular uptake The vanadyl cation binds well to hemoglobin and transferrin. The vanadate anion, on the other hand, may be associated with phosphate-binding proteins. Thus, vanadium is likely to be transported in blood both as part of macromolecular complexes and within erythrocytes. Blood circulation:

Metabolism Vanaclate can be reduced non-enzymically to vanaclyl by ascorbic acid, NADH, or glutathione.

Storage Vanadate predominates in extracellular body fluids whereas vanadyl is the most common intracellular form (Barceloux, 1999). The total body content of vanadium is between

764 Minerals and Trace Elements

0.1 and i mg, much of that in bone. Other organs with higher than average vanadium concentration are kidney, spleen, liver, testes, and lung. Vanadium readily forms complexes with a wide range of compounds. Vanadate competes with other transition metals for binding to metalloproteins and ATE and complexes with cis-diois and other compounds. The vanadyl cation binds to hemoglobin and transferrin.

Excretion Most ingested vanadium appears to be excreted with urine; neither the forms excreted nor the mechanisms underlying excretion have been elucidated.

Regulation It is not known whether and how vanadium homeostasis might be maintained.

Function Vanadium, often in conjunction with iron or other transition metals, is an important cofactor in various bacterial, fungal, and vanadatedependent enzymes in algae, including chloro-, iodo-, and bromoperoxidases and nitrogenases. A highly specialized type ofcoelomic cell in Ascidiidae which accumulates vanadium from sea water, is distinguished from other cells of these organisms by the functioning of the pentose-phosphate pathway (Ueki et al., 2000); whether this might indicate a special role of a vanadium species in glucose metabolism remains to be seen. Humans: A variety of mammalian species require trace amounts of vanadium for survival and reproductive health, but a vanadium-deficiency disease has not been identified in humans (Barceloux, 1999). Vanadium has been reported to compete with phosphate for the active sites ofphosphatases, tyrosine phosphorylases, and phosphate transport proteins. Vanadium compounds (vanadyi sulfate, sodium metavanadate, peroxovanadate-nicotinic acid) in higher than physiological doses lower blood glucose levels independent of insulin action, possibly through the inhibition of protein phosphotyrosine phosphatases and activation of non-receptor protein tyrosine kinases (Terziyski et al., 1999; Goldwaser et al., 2000). Vanadium chelated to organic compounds may be less toxic than the free forms. Possibly through similar mechanisms, pharmacologic vanadium doses also may lower blood-lipid levels (Harland and Harden-Williams, 1994). Vanadium also may modulate thyroid metabolism. Vanadium deprivation was found to increase thyroid weight and decreased thyroid peroxidase activity in rats (Uthus and Nielsen, 1990). Vanadyl-diascorbate appears to be a physiologically important inhibitor of NaK-ATPase in a ouabain-like action. This complex may thus participate in the regulation of body fluid volume and blood pressure (Meyer-Lehnert et al., 2000). Non-mammalian organisms:

Vanadium 765

Another effect, observed in cell culture, is the promotion of NO synthesis by enhancing both endothelial (eNOS) and inducible (iNOS) isoform activity (Cortizo et al., 2000). A chemopreventive effect has been suggested on the basis of very limited animal hepatocarcinogenesis experiments (Basak and Chatterjee, 2000). Industrial exposure (oil-fired electrical generating plants, petrochemical, steel, and mining industries) to toxic doses of vanadate can induce activation of the tumor suppressor gene p53 and cell apoptosis through generation of H202 (Huang et al., 2000). The relevance of this effect for induction or prevention of cancer upon exposure to much lower dietary doses is unclear. Vanadyl sulfate is sometimes used as a supplement by weight lifters. The efficacy of such use as an ergogenic aid has not been investigated rigorously. References

Barceloux DG. Vanadium. J Toxicol Clin Toxicol 1999;37:265-78 Basak R, Chatterjee M. Combined supplementation of vanadium and l-alpha,25dihydroxyvitamin D3 inhibit placental glutathione S-transferase positive foci in rat liver carcinogenesis. L(I~~ Sci 2000;68:217-3 I Cortizo AM, Caporossi M, Lettieri G, Etcheverry SB. Vanadate-induced nitric oxide production: role in osteoblast growth and differentiation. Eur J Pharmaeol 2000; 400:279-85 Goldwaser 1, Qian S, Gershonov E, Fridkin M, Shechter Y. Organic vanadium chelators potentiate vanadium-evoked glucose metabolism in vitro and in vivo: establishing criteria for optimal chelators. Mol Pharmaco12000;58:738-46 Harland BE Harden-Williams BA. Is vanadium of human nutritional importance yet'? J A m Diet Assoc" 1994;94:891-4. Huang C, Zhang Z, Ding M, Li J, Ye J, Leonard SS, Shen HM, Butterworth L, Lu Y, Costa M, Rojanasakul Y, Castranova V, Vallyathan V, Shi X. Vanadate induces p53 transactivation through hydrogen peroxide and causes apoptosis. J Biol Chem 2000; 275: 32516-22 Krachler M~ Prohaska T, Koellensperger G, Rossipal E, Stingeder G. Concentrations of selected trace elements in human milk and in infant formulas determined by magnetic sector field inductively coupled plasma-mass spectrometry. Biol Trace Elem Res 2000;76:97 112 Meyer-Lehnert H, Backer A, Kramer HJ. Inhibitors of Na-K-ATPase in human urine: effects ofouabain-like factors and of vanadium-diascorbate on calcium mobilization in rat vascular smooth muscle cells: comparison with the effects ofouabain, angiotensin II, and arginine-vasopressin. Am J H3pertens 2000; 13:364-9 Terziyski K, Tzenova R, Milieva E, Vladeva S. Possible mechanism of action of vanadium ions as an antidiabetic agent. Folia Medica (Plovdiv) 1999;41:34-7 Ueki T, Uyama T, Yamamoto K, Kanamori K, Michibata H. Exclusive expression oftransketolase in the vanadocytes of the vanadium-rich ascidian, Ascidia sydneiensis samea. Bioehim Biophjw Acre 2000:1494:83 90 Uthus EO, Nielsen FH. Effect of vanadium, iodine and their interaction on growth, blood variables, liver trace elements and thyroid status indices in rats. Magnesium Trace Elements 1990;9:219-26


Uthus EO, Seaborn CD. Deliberations and evaluations of the approaches, endpoints and paradigms for dietary recommendations of the other trace elements. J Nutr 1996; 126:2452S-2459S

Nickel Nickel (Ni, atomic weight 58.69) is an essential transition metal with potential valences 1, 2, 3, and 4.

Abbreviation DMT1 divalent metal transporter

Nutritional

summary

Cell membrane stability and production of some hormones may be influenced by nickel availability. Nickel also is a cofactor of various microbial enzymes. Nickel intake might, therefore, influence microbial action in human intestines. Food sources: Grains, chocolate, nuts and dried legumes are good sources of nickel. Americans typically get about 300 Ixg nickel per day from water and food. Requirements: Less than 100 p,g/d appears to be needed by healthy adults. Deficiency: Low availability impairs reproduction and growth in some mammals and may impair iron, copper, and zinc metabolism. Excessive intake: Chronic intakes of several milligrams per day may cause oxidative damage to DNA and cell structures, interfere with hormonal function, promote excessive zinc and iron storage, and lead to magnesium deficiency. Function:

Dietary sources Significant amounts of nickel are consumed with chocolate, nuts, oatmeal and other grains, dried beans and peas. Americans take in between 100 (Uthus and Seaborn, 1996) and 300 (Barceloux, 1999) micrograms of nickel per day with food and drinking water.

Digestion and absorption Only 1% of dietary nickel is absorbed from foods, significantly more from water (Barceloux, 1999; Amich et al., 2000). Uptake from the enteral lumen in the small intestine, particularly the jejunum, proceeds via the divalent metal transporter (DMT 1), the expression of which is regulated by iron status. Nickel absorption into iron-loaded cells is diminished (Tallkvist and Tjalve, 1998). Phytate, tannins, and calcium appear to decrease fractional absorption.

Nickel 767

Transport and cellular uptake Albumin binds nickel and may be the main vehicle for transport with blood (Brennan et al., 1990). Nickel uptake into liver, heart, and other tissues proceeds via the sodium/calcium exchanger (Egger et al., 1999). DMTI in the cell membranes as well as endosome membranes of various tissues is known to accept nickel and thus probably contributes to its transport into erythroid cells, across the blood-brain barrier and placenta. Nickel sulfate is selectively concentrated within lysosomes with an involvement of arylsulfatases (Berry, 1996). The biological significance of increased lysosomal nickel concentrations is unknown. Blood circulation:

Excretion Colonic epithelial cells may have the ability to secrete excess nickel (Tallkvist and Tjalve, 1998). DMTI is present in the luminal membrane of the distal nephron (Ferguson et al., 2001 ). Whatever nickel is filtered in the kidneys may be recovered by the mechanisms best investigated for iron.

Regulation It is not known whether and how the body's content of nickel is regulated.

Function A specific function of nickel in humans is not known. Nickel stimulates erythropoietin production (Goldberg et al., 1988), possibly through a heme-containing sensor in which iron can be substituted by nickel or cobalt (Bunn et al., 1998). Bunn et al. suggested that the sensor is a multi-subunit assembly containing an NAD(P)H oxidase capable of generating peroxide and reactive oxygen intermediates, which serve as signaling molecules. Nickel is a cofactor for a few bacterial enzymes, mainly those involved in the utilization of hydrogen. Nickel intake may thus influence human health through its importance for intestinal flora. Nickel-dependent bacterial enzymes include carbon-monoxide dehydrogenase (EC 1.2.99.2), hydrogen dehydrogenase (EC 1.12.1.2), coenzyme F420 hydrogenase (EC 1.12.99.1 ), hydrogenase (EC 1.18.99.1 ), urease (EC3.5.1.5), methylcoenzyme M reductase (no EC number assigned), nickel-superoxide dismutase (no EC number assigned), lactoylglutathione lyase (glyoxalase I, EC4.4.1.5), and cis-trans isomerase (no EC number assigned).

Other effects Patients with severe nickel poisoning develop intense pulmonary and gastrointestinal toxicity. Diffuse interstitial pneumonitis and cerebral edema are the main cause of death (Barceloux, 1999).


The poorly water-soluble forms o f nickel can be carcinogenic causing lung and nasal cancer in humans (Hayes, 1997). References Arnich N, Lanhers MC, Cunat L, Joyeux M, Burnel D. Nickel absorption and distribution from rat small intestine in situ. Biol Trace Elem Res 2000;74:141-51 Barceloux DG. Nickel. J Toxicol Clin Toxicol 1999;37:239-58 Berry JP. The role of lysosomes in the selective concentration of mineral elements. A microanalytical study. Cell Mol Biol 1996;42:395-411 Brennan SO, Myles T, Peach RJ, Donaldson D, George PM. Albumin Redhill ( - 1 arg, ala320-to-thr): a glycoprotein variant of human serum albumin whose precursor has an aberrant signal peptidase cleavage site. Proc Natl Acad Sci 1990;87:26-30 Bunn HE Gu J, Huang LE, Park JW, Zhu H. Erythropoietin: a model system for studying oxygen-dependent gene regulation. J Exp Biol 1998;201 : I 197-201 Egger M, Ruknudin A, Niggli E, Lederer WJ, Schulze DH. Ni 2+ transport by the human Na+/Ca2 + exchanger expressed in Sf9 cells. Am J Phvsiol 1999;276:C 1184-92 Ferguson C J, Wareing M, Ward DT, Green R, Smith CP, Riccardi D. Cellular localization of divalent metal transporter DMT- 1 in rat kidney. Am J Ph.vsiol Renal Fhdd Electrolyte Phvsio12001 ;280: F803-14 Goldberg MA, Dunning SP, Bunn HE Regulation of the erythropoietin gene: evidence that the oxygen sensor is a heme protein. Science 1988;242:1412-15 Hayes RB. The carcinogenicity of metals in humans. Cuncer Causes Control 1997;8: 371-85 Tallkvist J, Tjalve H. Transport of nickel across monolayers of human intestinal Caco-2 cells. ToxicolAppl Pharmacol 1998;151:117 22 Uthus EO, Seaborn CD. Deliberations and evaluations of the approaches, endpoints and paradigms for dietary recommendations of the other trace elements. J Nutr 1996; 126:2452S-2459S

Applications

Genetic variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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N u t r i e n t a d e q u a c y and s u p p l e m e n t a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

775

Nutrient interactions .................................................... Using m o l e c u l a r databases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

779 782

Genetic variation Abbreviations FOLR1 HFE MTHFR MTR

Folatereceptor 1 symbol for the hemochromatosis gene 5,10-methylenetetrahydrofolate reductase (EC1.7.99.5)

S-methyltetrahydrofolate-homocysteineS-methyltransferase (methionine synthase, EC2.1.1.13)

Humans are defined by their shared genome. It has been known for a long time, however, that people have distinguishing traits that are clearly inherited. Knowledge about genetic variation has now been extended to nutrient metabolism. Indeed, the relatively high degree of variation in this respect may reflect the ability of human populations to thrive in extremely diverse environments, in tropical rainforests, deserts and icy northern climates. For example, variants of the HFE (hemochromatosis) gene may allow populations to do well on a diet with relatively low iron intake while the now most common form provides some protection against the dangers of an iron-rich diet. The fact that human populations have been moving rapidly (in a long-term historical context) between very diverse regions probably prevented the equilibrium of variants from reaching a steady state appropriate to their current habitat. While much of this historic evolution may remain speculation, there cannot be much doubt about the considerable impact of genetic variance on individual nutritional needs. Many examples are well studied by now, and it is likely that some genetic variation affects availability and disposition of each nutrient (see a selection in Table 12.1 ). Relatively common genetic variants with impact on the individual needs of folate and iron will be Handbook of Nutrient Metabolism ISBN: O-12-417762-X

Copyright c' 2003 Elsevier Ltd All rights of reproduction ill any tbrrn reserved

770 Applications

Protein

Variant

Affected nutrient Consequence

Agouti-related protein

A67"1-

Energy balance

Lactase

C/T(-1391 O)

Lactose

Hepatic lipase

C/T(-514)

Fat

Aldehyde dehydrogenase 2

ALDH2*2

Ethanol

Haptoglobin

Hp 2-2

Ascorbate

HFE

C282Y

Iron

Alpha-adducin

G460W

Sodium

Arylamine N-acetyltransferase 2"12A MDR-1

K268R

Caffeine

3435-1"

Phytoestrogens

Increased risk of late-onset obesity Two cups of milk typically induce abdominal discomfort Decreased HDL concentration at moderate to high fat intake A single alcoholic drink is likely to induce facial flushing Accelerated ascorbate oxidation increases requirements Moderate to high habitual iron intake increases susceptibility to systemic Vibrio vulnificans infection Added table salt increases blood pressure Rapid elimination of caffeine after coffee ingestion Decreased bioavailabiliW ofgenistein

described to emphasize how human health depends on the combined action of nature (genes) and nurture (nutrient intake).

Folate adequacy and gene variants Folate provides a good example for the interaction of genetic disposition and nutrient intake. A review of 14 enzymes, receptors, and transporters directly involved in folate absorption, utilization, and regeneration demonstrates genetic variation in each. Relatively common variants in at least five of the genes have been firmly linked to increased risk of important health consequences. Detailed studies may reveal risks related to polymorphisms in some of the other genes, also. A lack of active folate metabolites during the first weeks of pregnancy greatly increases the risk of neural tube malformations and other birth defects. Low folate intake in conjunction with variant genes of folate metabolism is most often the underlying cause. High folate intake (0.4-4mg/day) greatly reduces the risk associated with several, if not all, predisposing variants. It is essential that such high intakes start before conception. Increased concentration of the toxic metabolite homocysteine in people with low folate status is also associated with increased risk of cardiovascular disease, cancer, and cognitive decline. Pteroylpoly-gamma-glutamate carboxypeptidase (GCPII, EC3.4.17.21) is a membrane-bound, zinc-dependent exopeptidase of the jejunal brush border that shortens the polyglutamate tail of folate from foods. People with the relatively common variant H475Y, which has lower than normal activity, absorb a smaller percentage of food folate while the bioavailability of synthetic folate, however, is not diminished (Devlin et al., 2000). A follow-up examination of the Framingham

Genetic Variation 771

Table 12.2

All genes involved in folate absorption and metabolism contain common variants

Gene name Pteroylpoly-g-glutamate carboxypeptidase (GCPII, EC3.4.17.21 ) Folate carrier 1 (RFC1, SLC19A1 )

Variant H475Y

Effect Decreased bioavailability of food folate

A80G

Multidrug resistance protein 2 (MRP2, ABCC2) Folylpolyglutamate synthase (EC6.3.2.17) MDRI (P-glycoprotein I, ABCBI ) Dihydrofolate reductase (EC1.5.1.3) 5,1 O-Met hylenetetrahydrofolate reductase (EC1.7.99.5)

Several

Increased risk of birth defects at low maternal folate intakes ?

Several

P

C3435T

P

rs408626 A--~G

P

A222V

Decreased metabolic response to folate intake; increased cardiovascular risk and risk of birth defects Increased risk of birth defects at low maternal folate intakes

5-Methyltetrahyd rofolatehomocysteine S-methyltransferase (MTR, methionine synthase (EC2.1.1.13) Methionine synthase reductase (EC2.1 .I .I 35) Cytochrome b5 and cytochrome P450 reductase (EC1.6.2.4) Folate receptor I (FOLRI)

D919G

Several

Folate receptor 2 (FOLR2) Folate receptor 3 (FOLR3) Thymidylate synthase (EC2.1 .I .45)

254A-->G rs533207 A-~G TYMS3

Skin pigmentation

Various

122M

Numerous

Increased risk of birth defects at low maternal folate intakes ?

Increased risk of birth defects at low maternal folate intakes ? ? ? Decreased metabolic response to folate intake; increased colon cancer risk Increased folate degradation upon skin exposure to sun light

Offspring cohort, however, found no significant effect on blood folate concentrations (Vargas-Martinez et al., 2002). The high-affinity folate carrier 1 (RFC 1, SLC 19A 1) mediates uptake of monoglutamyl folate from the small intestine and colon. Allelic variation ofRFC 1 (A80G), with a slightly higher prevalence of the glycine (G) form in the United States, affects absorption. Among children of mothers with low folate intake, those with homozygosity for the G form are at highest risk for spina bifida (Shaw et al., 2002). The multidrug resistance protein 2 (MRP2; ABCC2) contributes to the export of folate across the basolateral enterocyte membrane. Five functionally relevant variants of the MRP2 gene have been observed and some of them may be common (Itoda et al., 2002). Their impact on folate availability has not yet been investigated, however. The addition of multiple glutamyl residues by folylpolyglutamate synthase (EC6.3.2.17) keeps folate in the cell. Several variants of the gene are listed in the single nucleotide polymorphism database of the National Center for Biotechnology Information (dbSNP, http://www.ncbi.nlm.nih.gov/SNP). Allele frequencies and functional significance remain to be established. Monoglutamyl folate and similar metabolites are also actively pumped out of the cell by ABC transporters, including MDRI

772

Applications

(ABCBI). Allelic variation of MDRI (codon C3435T) impacts the activity of this transporter (Kerb et al., 2001 ). Dihydrofolate reductase (EC1.5.1.3) catalyzes the NADP-dependent reduction of folate to 7,8-dihydrofolate and then to 5,6,7,8-tetrahydrofolate. The Genome Database (GDB, http://www.gdb.org/) indicates four polymorphisms with maximal allele frequencies between 0.38 and 0.65. Their impact on folate metabolism is not known. The metabolism ofserine and glycine generates large quantities of 5,10-methylenetetrahydro folate. FA DH-dependent 5,10-methylenetetrahydrofolate reductase (MTHFR; EC1.7.99.5) is essential for the recycling of folate for further reactions. The MTHFR variant A222V (codon 677C---~T), which occurs in populations worldwide, is metabolically less active than the main form due to its decreased stability. The human gene mutation database Cardiff(HGMD, www.hgmd.org) lists another 25 mutations, some of which are common. Carriers of the A222V variant (both homozygotes and heterozygotes) have distinctly lower plasma folate and homocysteine concentrations on moderately low folate regimes (250 i.tg or less) than non-carriers. Daily use of additional 400 I.tg of synthetic folate abolishes the differences between heterozygotes and non-carriers, but only narrows the gap between homozygotes and non-carriers (Ashfield-Watt et al., 2002). High folate consumption of the mothers during the first weeks of pregnancy moves their risk closer to the normal range. The cobalamin-containing enzyme 5-methyltetrahydrofolate-homocysteine Smethyltransferase (MTR, methionine synthase; EC2.1.1.13 ) catalyzes the main reaction that removes the methyl group from 5-methyltetrahydrofolate and thus frees up folate again. At least 12 genetic variants have been found. The polymorphic variant D919G (codon 2756A---~G) may increase cardiovascular risk (Wang et al., 1998). Other variants disrupt enzyme activity in homozygote carriers, but the impact on heterozygotes remains to be investigated. The cobalamin in MTR is susceptible to oxidation. The FADand FMN-containing methionine synthase reductase (EC2.1.1.135) revert oxidized MTR to its active cob(I)alamin form. The polymorphism 122M (codon 66A---~G) was found to increase the risk for birth defects several-fold in women with suboptimal vitamin BI2 status (Wilson et al., 1999). The reaction uses S-adenosylmethionine as a methyl donor. Cytochrome b5, which is regenerated by NADPH-dependent cytochrome P450 reductase (EC1.6.2.4), is the reductant. Genetic variants of both cytochrome b5 and its reductase are known, though the consequences for folate metabolism are not. Folate must efficiently cross the placenta to ensure adequate supplies to the fetus. Folate receptor 1 (FOLRI) binds 5-methyltetrahydrofolate at the maternally facing chorionic surface and builds up a concentration gradient that drives placental transfer. Polymorphisms at nucleotides 762, 631, and 610 in the promoter region of FOLR 1 might affect transfer efficiency and therefore birth defect risk (Barber et al., 2000). The closely related fetal folate receptor, FOLR2, is also expressed in placenta. At least one variant of this gene (254A---,G) has been observed. The activity of folate-using reactions also impacts folate homeostasis. An important example is genetic variation in thymidylate synthase (EC2.1.1.45), which provides the crucial dTMP precursor for DNA synthesis and is a major consumer of 5,10-methylenetetrahydrofolate. A common polymorphic variant, characterized by an additional tandem repeat (TYMS 3) is associated with decreased blood folate concentration and impaired homocysteine metabolism (Trinh et al., 2002).

Genetic Variation 773

Genetic variation of a very different kind may also influence folate status. Light with very short wavelength (particularly ultraviolet A) can penetrate deep into fair skin and might inactivate folate in capillaries (Jablonski, 1999). Skin pigments block ultraviolet rays and minimize folate losses. A mismatch between the exposure to ultraviolet light and genetically determined pigmentation might cause enough folate inactivation to affect folate status.

The iron-regulatory gene HFE Many people of North European ancestry have one or two copies of the C282Y of the HFE gene on the small arm of chromosome 6 (6p21.3), while this variant is rare in people of Asian descent (Beckman et al., 1997; Merryweather-Clarke et al., 1999). in Ireland, nearly one in five newborns are heterozygous for the C282Y (codon 845 G -~A) variant and 1% are homozygous (Byrnes et al., 2001 ). The variant HFE gene product loses its ability to limit iron uptake when stores are filled, because it is not effectively processed and moved from the Golgi compartment to the cell surface (Waheed et al., 1997). Since the HFE gene product favours iron uptake, the C282Y variant might confer a selective survival advantage to the offspring of iron-deficient women by enhanced iron transfer across the placenta (Parkkila et al., 1997). The downside of the C282Y variant, particularly in homozygotes, relates to the increased concentration of reactive (unbound) iron and excessive iron accumulation. Affected individuals appear to lose the ability to downregulate iron absorption when iron stores are sufficient, but retain the capacity to upregulate in response to deficiency (Ajioka et al., 2002). Several other common variants, particularly H63D and $65C, also compromise HFE function. The concentration of unbound iron is normally below 10s tool/I, which limits oxygen free radical generation and protects against the spread of iron-dependent bacteria in blood and tissues. In more than 1% of the US population the concentration of unbound iron is elevated, as indicated by their very high (>60%) transferrin saturation (Looker and Johnson, 1998). C282Y homozygotes can lower their concentration of unbound iron and prevent excessive iron storage by tightly limiting their iron intake or increasing iron losses (e.g. by blood donation). Chronic excessive iron intake, on the other hand, greatly increases risk (Bell et al., 2000). The C282Y variant also increases transferrin saturation and the tendency to accumulate iron in heterozygotes (Distante et al., 1999). The health consequences of increased unbound iron in blood can be dramatic. A single serving of raw oysters, which are very commonly contaminated with naturally occurring marine bacteria (Vihrio vulnificus), has infected and killed young homozygotic carriers of the C282Y variant in good health within a few days. Long-term health risks of excessive iron storage include diabetes, cancer, dementia, and premature heart disease. While the risk of heterozygotes tends to be lower compared to homozygotes, excessive iron intake compounds their problems. Fatal septicemia following the consumption of infected seafood has occurred in heterozygotes with iron accumulation (Gerhard el al., 2001 ). Expanded iron stores also increase the risk of colorectal cancer (Nelson, 2001 ), viral hepatitis (Fargion et al., 2001 ), and accelerated cognitive decline (Sampietro et al., 2001 ). The complexity of the issues is underscored, however, by the increased prevalence of C282Y heterozygotes among very old Sicilians (Lio et al., 2002).

774

Applications

References Ajioka RS, Levy JE, Andrews NC, Kushner JP. Regulation of iron absorption in Hfe mutant mice. Blood 2002; 100:1465-9 Ashfield-Watt PA, Pullin CH, Whiting JM, Clark ZE, Moat SJ, Newcombe RG, Burr ML, Lewis M J, Powers HJ, McDowell IE Methylenetetrahydrofolate reductase 677C---~T genotype modulates homocysteine responses to a folate-rich diet or a low-dose folic acid supplement: a randomized controlled trial. Am J Clin Nutr 2002;76:180-6 Barber R, Shalat S, Hendricks K, Joggerst B, Larsen R, Suarez L, Finnell R. Investigation of folate pathway gene polymorphisms and the incidence of neural tube defects in a Texas hispanic population. Mol Genet Metab 2000;70:45-52 Beckman LE, Saha N, Spitsyn V, Van Landeghem G, Beckman L. Ethnic differences in the HFE codon 282 (Cys/Tyr) polymorphism. Hum Hered 1997;47:263-7 Bell H, Berg JP, Undlien DE, Distante S, Raknerud N, Heier HE, Try K, Thomassen Y, Haug E, Raha-Chowdhury R, Thorsby E. The clinical expression of hemochromatosis in Oslo, Norway. Excessive oral iron intake may lead to secondary hemochromatosis even in HFE C282Y mutation negative subjects. Stand J Gastroentero12000;35:1301-7 Byrnes V, Ryan E, Barrett S, Kenny P, Mayne E Crowe J. Genetic hemochromatosis, a Celtic disease: is it now time for population screening'? Genetic' Testing 2001 ;5:127-30 Devlin AM, Ling EH, Peerson JM, Fernando S, Clarke R, Smith AD, Halsted CH. Glutamate carboxypeptidase 11: a polymorphism associated with lower levels of serum folate and hyperhomocysteinemia. Httm Mol Gen 2000;9:2837-44 Distante S, Berg JP. Lande K, Haug E, Bell H. High prevalence of the hemochromatosisassociated Cys282Tyr HFE gene mutation in a healthy Norwegian population in the city of Oslo, and its phenotypic expression. ScamtJ Gastroenterol 1999;34:529-34 Fargion S, Stazi MA, Fracanzani AL, Mattioli M, Sampietro M, Tavazzi D, Bertelli C, Patriarca V, Mariani C, Fiorelli G. Mutations in the HFE gene and their interaction with exogenous risk factors in hepatocellular carcinoma. Blood Cells Mol Dis 2001;27:505 11 Gerhard GS, Levin KA, Price Goldstein J, Wojnar MM, Chorney MJ, Belchis DA. Vibrio vulnificus septicemia in a patient with the hemochromatosis HFE C282Y mutation. Aiz.h Pathol Lab Meal 2001 ; 125:1107-9 Itoda M, Saito Y, Soyama A, Saeki M, Murayama N, Ishida S, Sai K, Nagano M, Suzuki H, Sugiyama Y, Ozawa S, Sawada Ji J. Polymorphisms in the ABCC2 (cMOAT/MRP2) gene found in 72 established cell lines derived from Japanese individuals: an association between single nucleotide polymorphisms in the 5'-untranslated region and exon 28. Drug Metab Di~sp 2002;30:363-4 Jablonski NG. A possible link between neural tube defects and ultraviolet light exposure. Med H)7~otheses 1999;52:58 I-2 Kerb R, Aynacioglu AS, Brockmoller J, Schlagenhaufer R, Bauer S, Szekeres T, Hamwi A, Fritzer-Szekeres M, Baumgartner C, Ongen HZ, Guzelbey E Roots I, Brinkmann U. The predictive value of MDRI, CYP2C9, and CYP2CI9 polymorphisms for phenytoin plasma levels. Pharmacogenomics J 2001;1:204-10 Lio D, Balistreri CR, Colonna-Romano G, Motta M, Franceschi C, Malaguarnera M, Candore G, Caruso C, Association between the MHC class I gene HFE polymorphisms and longevity: a study in Sicilian population. Genes hmmm 2002:3:20-4

Nutrient Adequacy and Supplementation 775

Looker AC, Johnson, CJ. Prevalence of elevated serum transferrin saturation in adults in the United States. Ann hTtern Med 1998;129:940-5 Merryweather-Clarke AT, Simonsen H, Shearman JD, Pointon J J, Norgaard-Pedersen B, Robson KJH. A retrospective anonymous pilot study in screening newborns for HFE mutations in Scandinavian populations. Hum Murat 1999;13:154-9 Nelson RL. Iron and colorectal cancer risk: human studies. Nutr Rev 2001 ;59:140-8 Parkkila S, Waheed A, Britton RS, Bacon BR, Zhou XY, Tomatsu S, Fleming RE, Sly WS. Association of the transferrin receptor in human placenta with HFE, the protein defective in hereditary hemochromatosis. Proc Natl Acad Sci USA 1997;94:13198-202 Sampietro M, Caputo L, Casatta A, Meregalli M, Pellagatti A, Tagliabue J, Annoni G, Vergani C. The hemochromatosis gene affects the age of onset of sporadic Alzheimer's disease. NeurobiolAging 2001;22:563 8 Shaw GM, Lammer EJ, Zhu H, Baker MW, Neri E, Finnell RH. Maternal periconceptional vitamin use, genetic variation of infant reduced folate carrier (A80G), and risk of spina bifida. Am d Med Genet 2002; 108:I-6 Trinh BN, Ong CN, Coetzee GA, Yu MC, Laird PW. Thymidylate synthase: a novel genetic determinant of plasma homocysteien and folate levels. Hunt Genet 2002; 111:299-302 Vargas-Martinez C, Ordovas JM, Wilson PW, Selhub J. The glutamate carboxypeptidase gene II (C>T) polymorphism does not affect folate status in the Framingham Offspring cohort, d Ntttr 2002;I 32:1176-9 Waheed A, Parkkila S, Zhou XY, Tomatsu S, Tsuchihashi Z, Feder JN, Schatzman RC, Britton RS, Bacon BR, Sly WS. Hereditary hemochromatosis: effects of C282Y and H63D mutations on association with beta-2-microglobulin, intracellular processing, and cell surface expression of the HFE protein in COS-7 cells. Proc NatlAcadSci USA 1997;94:12384-9 Wang XL, Cai H, Cranney G, Wilcken DE. The frequency of a common mutation of the methionine synthase gene in the Australian population and its relation to smoking and coronary artery disease. J Calztiovasc Risk 1998;5:289-95 Wilson A, Platt R, Wu Q, Leclerc D, Christensen B, Yang H, Gravel RA, Rozen R. A common variant in methionine synthase reductase combined with low cobalamin (vitamin B I2) increases risk for spina bifida. Mol Genet Metah 1999;67:317-23

Nutrient adequacy and supplementation Abbreviations AI DRI EAR IOM RDA UL

adequateintake Dietary ReFerence Intakes estimatedaverage requirement Institute of Medicine recommended dietary allowance tolerable upper intake level

776 Applications

Dietary Reference Intakes Nutrients are coming to be seen more and more like medications, where the 'dose makes the poison'. The Dietary Reference Intakes (DRI) published by the Food and Nutrition Board now take into consideration, at least in principle, both lower and upper desirable limits. How to determine appropriate limits for individuals or groups remains the unresolved and much debated question. The Food and Nutrition Board has established a basic framework for tackling this question. Needs are considered separately for each of 22 groups defined by age, gender, as well as pregnancy and lactation status. The Food and Nutrition Board is a subdivision of the Institute of Medicine at the National Academies of the United States and comprises panels that set guidelines for the US and Canada. Similar institutions exist in several other countries to provide guidance on optimal nutrient intake levels.

Minimal nutrient requirements The lower limits for nutrient intakes are based on the observed consumption level of healthy populations, if data from functional investigations are lacking. This intake level is called adequate intake (AI) and is assumed to cover the needs of healthy people. Because of the current limitations of scientific evidence, the Food and Nutrition Board established AIs applicable to adults for the following nutrients: total fat, omega-6 fatty acids, omega-3 fatty acids, vitamin D, vitamin K, pantothenic acid, biotin, choline, calcium, chromium, fluoride, and manganese. Because information about the requirements of infants (under one year old) is even more limited, only AIs were set for most nutrients. The exceptions are more definite lower limits for protein, iron, and zinc intakes of7-12month-old infants. Where functional information is found to be reasonably reliable, the Food and Nutrition Board sets recommended dietary allowances (RDA). This amount is thought to cover the needs of most healthy people (97-98%) in the designated group. According to this model the RDA is determined in a three-step process. First, the intake level is sought at which the risk of inadequacy of the healthy target population (e.g., 19- to 50-yearoold men) is 50%. This is called the estimated average requirement (EAR, an oxymoronic expression, since it relates to the median and not the average). The second step estimates the variance of requirements. For most nutrient requirements a normal distribution is assumed. Unless evidence to the contrary is available, the variation coefficient (standard deviation divided by the mean) is set at 10% (because this is thought to correspond to the variance of basal metabolic rate). The final step then either adds two standard deviations (usually 20%) to the EAR or determines the 97.5th percentile of requirements by a Monte Carlo simulation procedure. So far, variation coefficients of 10% were set for thiamin, riboflavin, vitamin B6, folate, vitamin B I2, phosphate, magnesium, and selenium because the actual variance was thought to be unknown. The decision to set the variation coefficient for niacin requirements at 15% was based on four separate studies on a total of 29 adults with an average variation coefficient of 34%. Similarly, the variation coefficient for vitamin A requirements was set at 20%, based on a single study of the vitamin A half-life in the

Nutrient Adequacy and Supplementation 777

livers of adults, that gave a 21% variation coefficient of the results. Based on a single study of adults, which gave a variation coefficient of 40%, the variation coefficient of iodine requirements was set to 20%. In each case a judgment was made about the relative contributions of measurement error versus intrinsic interindividual variation. The variation coefficients for copper and molybdenum requirements were set at 15%. The panel commented that data supporting the EARs are limited, but provided no explanation why they did not use a 10% value as for other nutrients. In all instances, the variation coefficients set for young adults were applied to children, pregnant and lactating women, and older people without the benefit of additional supporting evidence. A significant weakness of the current recommendations relates to the extremely narrow basis of supporting data for several nutrients. In the vast majority of instances data are completely lacking for specific age and gender groups and the recommendations are based on extrapolations from other groups. Information in children and old people is particularly sparse. When levels are set on the basis of observations in a few subjects, as is the case for most of the covered nutrients, there is little opportunity to differentiate needs by genetic disposition. Only rarely is the existence of genetic diversity acknowledged. A typical and important example pertains to niacin requirements. It is likely that many people can cover their niacin requirements through endogenous synthesis from tryptophan while others need significant intakes of preformed niacin. An even better documented example is the greater than average susceptibility to folate deficiency (Ashfield-Watt et al., 2002) in people with variant (thermolabile) 5,10-methylenetetrahydrofolate reductase (MTHFR; EC 1.7.99.5). The current intake guidelines take little note of such differences. So far, the functional assessment of adequate nutrient intakes has been limited on long-known properties. Reliable knowledge about more recently recognized functions has been disregarded without good reason. Metabolic and health consequences of suboptimal or nutrient status are most likely to be observed when they are monitored by focused observation. A major shortcoming of the current framework is the deliberate exclusion of any long-term effects, in particular chronic degenerative disease. This ignores that in affluent societies nutrition influences the main causes of morbidity and death such as atherosclerosis, cancer, and osteoporosis. It is with respect to these chronic degenerative diseases that genetic variation of nutrient metabolism is most significant today. Polymorphisms relating to metabolism of energy, glucose, lipids, folate and iron, to name just a few, are known to be important determinants of disease risk and outcome.

Excessive intakes There is little doubt that too much of any nutrient can do harm. For some nutrients the amounts that might cause concerns are so high that they are not likely to be used. The DRI framework formally explores the potential for harm with high intakes and relates them to the tolerable upper intake level (UL). The aim is to find the highest level at which no adverse effect has been observed (NOAEL) or, alternatively, the lowest level at which an adverse effect has been observed (LOAEL). In either case a judgment has to be made about how much lower the UL should be.

778 Applications

Controversies have arisen about some specific nutrients. An important example is the UL for vitamin D, which was set at 50 I-tg per day based on poorly documented selective evidence (Food and Nutrition Board, 1997; Vieth, 1999). This is much less than the approximate 250 p,g dose generated in a young person lying in the summer sun for just 20 minutes (Vieth, 1999). In the meantime, a well-designed study of the effects of 100 i,tg vitamin D in healthy adults did not find any unfavorable effects (Vieth et al., 2001 ). It is certain that discussions will continue as the current guidelines evolve.

Supplementation The practice of using concentrated sources of specific nutrients for the prevention of disease has a long history. The discovery by James Lind in 1753 (Rajakumar, 2001) that scurvy could be prevented by judicious use of citrus fruits enabled the British Navy to greatly extend the duration of their missions and build up their domination of the seas. The early ridicule (the sailors were called 'limeys' because they had to eat limes) notwithstanding, the use of supplements has exploded since and now sustains a major industry. Food fortification, which might be seen as a special case of supplement use, has been well established for a long time in the United States and other countries. The US currently requires the addition of vitamin D to milk, and thiamin, riboflavin, folic acid, and iron to grain products. Some jurisdictions have policies of adding iodine to salt or fluoride to water. Individual supplement use certainly makes sense, if it balances a nutrient deficit that would be left unattended because dietary changes alone would not be sufficient or feasible. Supplementation may also be needed to cover increased needs in times of illness (Zeisel, 2000). Genetic predisposition may be another reason for supplement use - for instance the intake of additional tyrosine by people with phenylketonuria. Many times, however, minimal required intakes are exceeded, 'to be on the safe side', and undesirable effects may occur. High nutrient intake is also often intended to achieve pharmacological effects and in this case extensive experimental data from human studies should be as much a prerequisite as with medical drugs (Zeisel, 1999), taking into account all known nutritional and metabolic aspects of the supplemented compound.

References Ashfield-Watt PA, Pullin CH, Whiting JM, Clark ZE, Moat SJ, Newcombe RG, Burr ML, Lewis MJ, Powers HJ, McDowell IE Methylenetetrahydrofolate reductase 677C--,T genotype modulates homocysteine responses to a folate-rich diet or a low-dose folic acid supplement: a randomized controlled trial. Am J Clin Nutr 2002;76:180-6 Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride. National Academy Press, Washington, DC, 1997 Food and Nutrition Board, Institute of Medicine. Dietary Reference Intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin BI2, pantothenic acid, biotin, and choline. National Academic Press, Washington, DC, 1998 Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for vitamin C, vitamin E, selenium, and carotenoids. National Academy Press, Washington, DC, 2000

Nutrient Interactions 779

Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. National Academy Press, Washington, DC, 2001 Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for energy, carbohydrates, fiber, fat, protein and amino acids (macronutrient). National Academy Press, Washington, DC, 2002 Rajakumar K. Infantile scurvy: a historical perspective. Pediatrics 2001;108:E76 Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am `1 C/in Nutr 1999;69:842-56 Vieth R, Chan PCR, MacFarlane GD. Efficacy and safety of vitamin D3 intake exceeding the lowest observed adverse effect level. Am ,1 Clin Nutr 2001;73:288-94 Zeisel SH. Regulation of 'nutraceuticals'. Science 1999;285:1853-5 Zeisel SH. Is there a metabolic basis for dietary supplementation'? Am ,1 Clin Ntttr 2000;72:507S-511S

Nutrient interactions Abbreviations DMT1 PLP

divalent metal ion transporter 1 (SLC11A2) pyridoxal 5'-phosphate

Deficiencies Some nutrients affect the metabolism of other nutrients or of medications because they are essential cofactors. A characteristic example is the disruption of amino acid, omega3 fatty acid, carbohydrate, niacin and selenium metabolism in vitamin B6 deficiency, since pyridoxal 5'-phosphate (PLP) is such a ubiquitous cofactor. The link to endogenous amino acid synthesis (and catabolism) is obvious, since all aminotransferases require PLP. Vitamin B6 is also necessary as a cofactor of 6-desaturase (EC 1.14.99.25) for the endogenous synthesis of docosahexaenoic acid (DHA) from eicosapentaenoic acid (EPA) and tx-linolenic acid (Tsuge and Hotta, 2000). The utilization of stored carbohydrate (glycogen) depends on the availability of PLP for the glycogen phosphorylases (EC2.4.1.1). Niacin synthesis from tryptophan is diminished in people with poor vitamin B6 status (Bender et al., 1979) because the key enzyme kynureninase (EC3.7.1.3) is PLP-dependent. Vitamin B6 interfaces with selenium metabolism, because it is a cofactor both for the insertion of selenium into serine by L-seryl-tRNA sec selenium transferase (EC2.9.1.1) and the release of hydrogen selenide by selenocysteine lyase (EC4.4.1.16) for reuse after the catabolism of proteins. Riboflavin similarly illustrates how a single nutrient affects many others, because this vitamin participates in the activation or endogenous synthesis of niacin, vitamins A, K, C, B6, and B 12, folate, and other nutrients, metabolites and medical drugs. The

780 Applications

availability of information on quantitative aspects in humans is very iimite& however. A significant proportion of niacin requirements can be covered by the catabolism of tryptophan. Activity of the FAD-containing enzyme kynurenine 3-monoxygenase (EC1.14.13.9) is decreased in riboflavin deficiency. Vitamin A use can be affected because the enzyme for retinoic acid synthesis from retinal by retinal dehydrogenase (EC 1.2.1.36) requires FAD. Vitamin K reactivation proceeds by two reducing steps catalyzed by several flavoenzymes including NADPH dehydrogenase (EC1.6.99.6) and two forms of NAD( P)H dehydrogenase ( EC 1.6.99.2). The reduction of oxidized vitamin C (dehydroascorbate) depends on glutathione, which needs to be maintained in the reduced state by FAD-containing thioredoxin reductase (EC1.6.4.5). Reduced glutathione is also critically important to protect sulfhydryl groups in enzymes for the metabolism of numerous nutrients. Riboflavin deficiency impacts PLP availability due to impaired activity of the FMN-containing enzyme pyridoxamine-phosphate oxidase (ECI.4.3.5), which converts pyridoxine into pyridoxal, and pyridoxine 5'phosphate into pyridoxal 5'-phosphate (McCormick, 1989). Vitamin B I2 requires three flavoenzymes for its metabolism: cob( II )alamin reductase ( EC 1.6.99.9), aquacobalamin reductase/NADPH (EC 1.6.99.11 ), and aquacobalamin reductase/NADH (EC !.6.99.8). The effect on folate metabolism is through FAD-dependent methylene tetrahydrofolate reductase (EC1.5.1.20), which ensures reutilization of the large amounts of folate metabolites generated by serine and glycine metabolism. Such interdependencies greatly broaden the consequences of severe deficiency of a single nutrient and mimic deficiency symptoms of the affected nutrients, especially when their intake is already marginal. What needs to be investigated is the extent to which mild deficiency of many individual nutrients affects the availability of others. Meaningful human studies of such relationships have been conducted for only very few nutrients.

High intakes There are many ways in which high intake of a particular nutrient or group of nutrients may affect the availability of other nutrients. The most obvious way is when the intake of a precursor sustains the synthesis of a nutrient, as is the case with alphacarotene, which is a precursor for retinol. Consumption of some nutrients furthers the bioavailability of others, illustrated by ascorbate, which enhances the absorption of non-heme iron eaten with the same meal. Of course, the opposite effect is illustrated by phytate, which strongly inhibits iron absorption. When nutrients use the same rate-limiting transporters or metabolic pathways, competition may occur. A large dose of iron, for instance, decreases the percentage of zinc that is taken up with the same meal (Chung el a]., 2002). A large dose of zinc similarly decreases iron absorption (Donangelo el al., 2002). The reason is that divalent metal cations use the same transporter, DMTI (SLC 1 ! A2), for entry into the intestinal cell and a large number of one kind of cation can crowd out the other. Similarly, high betacarotene intake decreases lutein bioavailability, because these fat-soluble polyisoprenoid molecules compete for the same uptake mechanism (van den Berg and van Vliet, 1998). Even a seemingly modest nutrient dose may tip the balance and cause health problems when it coincides with marginal intake of another nutrient needed for its metabolism.

Nutrient Interactions 781

An example may be the competition between branched-chain amino acid metabolism and the niacin synthesis pathway. Moderately high intake ofleucin with sorghum or corn (these grains contain about 50% more leucin than other protein sources) can induce the increased expression of branched-chain amino acid transaminase (EC2.6.1.42), which will then strongly attract much of the available vitamin B6. This competition may be enough in vulnerable individuals to lower the critical activity of kynureninase (EC3.7.1.3) in the niacin synthesis pathway and induce niacin deficiency with pellagra symptoms (Krishnaswamy et al., 1976). Regulatory effects of one nutrient can influence the absorption, metabolism or disposition of another. Increased expression of DMTI in response to iron deficiency will increase fractional absorption of a whole range of other metals, including manganese (Finley, 1999). Even moderately excessive zinc supplementation (20mg/day), on the other hand, can deplete copper stores (Boukaiba et al., 1993), in part by decreasing expression of the shared storage proteins, the metallothioneins, in liver and kidneys (Santon et al., 2002). Still another mechanism of nutrient interactions is competitive interference or the mimicking of one nutrient's action by another nutrient. Typically, such effects become significant only when a large excess of the mimicking compound is consumed, because the body is clearly adapted to any crosstaik that might occur at physiological consumption levels. An important example is the occurrence of bleeding with higher than normal vitamin E consumption. In a long-term intervention trial the consumption of 50 mg all-rac-et-tocopherol was found to increase mortality from hemorrhagic stroke (ATBC Cancer Prevention Study Group, 1994), presumably due to competition of the tocoquinone metabolite with vitamin K reactivation. References

ATBC Cancer Prevention Study Group. The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. N Engl J Med 1994; 330:1029-35 Bender DA, Earl CJ, Lees AJ. Niacin depletion in Parkinsonian patients treated with L-dopa, benserazide and carbidopa. Clin Sci 1979;56:89-93 Boukaiba N, Flament C, Acher S, Chappuis E Piau A, Fusselier M, Dardenne M, Lemonnier D. A physiological amount of zinc supplementation: effects on nutritional, lipid, and thymic status in an elderly population. Am J Clin Nutr 1993;57:566-72 Chung CS, Nagey DA, Veillon C, Patterson KY, Jackson RT, Moser-Veillon PB. A single 60 mg iron dose decreases zinc absorption in lactating women. J Nutr 2002; 132:1903-5 Donangelo CM, Woodhouse LR, King SM, Viteri FE, King JC. Supplemental zinc lowers measures of iron status in young women with low iron reserves. J Nutr 2002; 132:1860-4 Finley JW. Manganese absorption and retention by young women is associated with serum ferritin concentration. Am J Clin Nutr 1999;70:37-43 Krishnaswamy K, Rao SB, Raghuram TC, Srikantia SG. Effect of vitamin B6 on leucineinduced changes in human subjects. Am J Clin Nutr 1976;29:177-81 McCormick DB. Two interconnected B vitamins: riboflavin and pyridoxine. Phvsiol Rev 1989;69:1170-98

782 Applications

Santon A, Giannetto S, Sturniolo GC, Medici V, D'lnca R, lrato P, Albergoni V. Interactions between Zn and Cu in LEC rats, an animal model of Wilson's disease. Histochem Cell Bio12002; 117:275-81 Tsuge H, Hotta NT. Effects of vitamin B6 on (n-3) polyunsaturated fatty acid metabolism. J Nutr 2000; 130:333S-334S van den Berg H, van Vliet T. Effect of simultaneous, single oral doses of beta-carotene with lutein or lycopene on the beta-carotene and retinyl ester responses in the triacylglycerolrich lipoprotein fraction of men. An1J Clin Nutr 1998;68:82-9

Using molecular databases The vast amount o f data on biological systems can make it hard to find a particular piece o f information. The power o f current Web-based systems helps to access data with great speed. The following describes some typical scenarios and provides directions to potential resources.

How to get basic nutrient information Usually, a standard textbook (such as this one) will be the best starting point. The American Society o f Nutritional Sciences provides a website with brief outlines describing function, sources, and health significance o f essential and other nutrients (http://www.nutrition.org/nutinfo/). Teams o f established experts write the information on each individual nutrient. A free online literature search system (Pubmed) is provided by the National Institutes of Health (http://www.pubmedcentral.nih.gov/). Those with access to the electronic editions of major journals will be able to find many excellent review articles on nutrient metabolism. Publications that often carry pertinent reviews include the Annual Review ~?['Nutrition (http://nutr.AnnualReviews.org/), the American Journal of Physiology (http://ajpcon.physiology.org/), and Anlerican Journal of Clinical Nutrition (http://www.ajcn.org/).

How to get information on enzymes The Nomenclature Committee of the International Union of Biochemistry and Molecular Biology (IUBMB) has recommended a system to categorize enzymes. A four-component EC (Enzyme Commission) numbering system facilitates the reference to a specific group of enzymes. Note that the size of each of the four numeric components of the EC number is not limited. To take a particular example, alcohol dehydrogenase (ADH) has been designated the EC number 1.1.1.1. It can be informative to review neighboring entries if searching for enzymes with similar characteristics. EC 1.1.1.2, for instance, designates alcohol dehydrogenases, which use NADP as a cosubstrate. In many instances several human proteins correspond to the same EC number, sometimes with considerably divergent substrate specificities and other characteristics. Humans have genes for at least seven distinct ADHs, which vary greatly in catalytic

Using Molecular Databases 783

properties. While most oxidize ethanol well, ADH5 prefers long-chain primary alcohols and is actually a formaldehyde-oxidizing enzyme. The Swiss Institute of Bioinformatics maintains extensive databases on their ExPASy (Expert Protein Analysis System) proteomics server (http://www.expasy.ch/enzyme/). This is an important, comprehensive source of information on most enzymes, including those involved in nutrient metabolism. The enzyme database can be searched by EC number, enzyme name, reactant, cofactor or by viewing a chart of metabolic pathways. Brenda, the Enzyme Database (lnstitut for Biochemie, Universit~it zu K61n), is another informative enzyme database (http://www.brenda.uni-koeln.de). Use of the database requires registration and is free for non-commercial users.

How to get information on genes One of the best sources of information on genes and genetic syndromes is OMIM, the online version of McKusick's textbook Mendelian Inheritance in Man (http://www3.ncbi.nlm.nih.gov), currently hosted by the National Center for Biotechnology Information (NCBI) at the US National Library of Medicine (NLM). The National Library of Medicine also provides detailed sequence information and search engines such as BLAST (http://www.ncbi.nlm.nih.gov/BLAST/). Sequence data can be retrieved from the Genome Database (GDB), a collaboration related to the Human Genome Project (http://www.gdb.org/). The database also provides information on probes, primers for sequence amplification, and known polymorphisms. TRANSFAC (http://www.gene-regulation.com/) is a compilation of eucaryotic transcription factors. The Nutrition, Metabolism and Genomics Group, Wageningen University, The Netherlands has compiled information on all known human ABC transporters (http://nutrigene.4t.com/humanabc.htm). Specialized Web-based databases with information on genetic variants are becoming increasingly available. The National Center for Biotechnology Information has a database (dbSNR http://www.ncbi.nlm.nih.gov/SNP) with nearly 5 million polymorphisms due to single base nucleotide substitutions, short deletion, and insertions. The variants are listed with the information on each gene or protein and can be found by searching for gene name, sequence or a number of other characteristics. The Institute of Medical Genetics at the University of Wales College of Medicine at Cardiff, UK, houses the Human Gene Mutation Database (HGMD, www.hgmd.org). Information on numerous gene variants can be found in this database (Krawczak and Cooper, 1997). The Frequency of Inherited Disorders Database (FIDD, http://www.uwcm.ac, uk/ uwcm/mg/hgmd0.html) at the same institution provides information on nearly 300 conditions caused by genetic variation. An example for a narrowly specialized collection is the mutation database for the human glucose-6-phosphate dehydrogenase (G6PD, EC1.1.1.49) gene (http://www. rubic.rdg.ac.uk/g6pd/). References

Krawczak M, Cooper DN. The Human Gene Mutation Database. Trends Genet 1997; 13:121-2


Index

A cells 38 A-protein 22 I ABC transporters 45 ABCAI 45, 80, 494, 517, 519, 520, 523 ABCBI (P-glycoprotein) 75, 87, 97, 99. 102, 497 ABCB4 97, 102, 451 ABCC I (multidrug-resistance protein 1; MRPI) 54, 87, 97, 102, 759, 771-772 ABCC2 (multidrug-resistance protein 2; MRP2; cMOAT) 45, 49, 54, 66, 75, 87, 97, 594, 664, 771 ABCC3 (MRP3) 87, 98 ABCC5 (MRP5) 98 ABCC6 (MRP6) 98 ABCC7 (cystic fibrosis transmembrane regulator; CFTR) 45, 51, 662-663 ABCDI (ALDP; "ALD" protein) 128, 165 ABCGI 519 ABCG2 (ABCP; MXR; BCRP) 82 ABCG4 519 ABCG5/G8 45, 517. 522 ABCP (MXR; BCRP; ABCG2) 82 Abetalipoproteinemia 498 Absorption 37 54 acetate 149 amino acids 47, 251-253,274, 281 282. 289 290, 296, 302 303, 309, 31 I, 316, 330-331, 340 341,349-350, 357-358, 364 365, 371 372, 378, 384. 396, 406-407, 413-4 14 arsenic 53,758 ascorbate 49, 544 545 biopterin 630 biotin 50, 614-615 boron 747 bromine 53,754 calcium 51 52, 191,487, 693,694~95 carbohydrate 47-48, 51

carnitine 434-435 chloride 51. 661-663 cholesterol 48, 516-517 choline 449 chromium 53. 550, 743 cobalt 739 740 conjugated linoleic acid (CLA) 159 copper 52-53. 191,641. 679 creatine 429 docosahexaenoic acid 167-168 electrolytes 50-51 ethanol 237 fatty acids 48. 123-124 flavonoids 97 99 fluoride 53, 719 folate 49, 278, 593-594 fructose 46, 48, 212 213, 215 galactose 48, 218-219 glucose 48. 198 heterocyclic amines 87 inositol 636 iodine 53. 714 large intestine 43-44 lipids 48 lipoate 50, 527 528 macronutrients 47-48 magnesium 52, 709 manganese 53,729 minerals/trace elements 51 52 molecular transport mechanisms 45-47 molybdenum 53. 734 myristic acid 154 niacin 49, 574 nickel 53,766 pantothenate 50, 620 peptides 47 phosphate 52, 487, 694. 702 703 phytanic acid 180-18 I phytochcmicals 53-54 potassium 656-657 pristanic acid 180-181 proteins 47 queuine 626

riboflavin 49, 562-563 selenium 53, 723 silicon 75 I small intestine 40-42 sodium 5 I, 649 651 stomach 39 thiamin 49. 553-554 ubiquinone 48. 535 vanadium 53, 763 vitamin A 40. 48,468-469 vitamin B6 47, 49, 583 584 vitamin B I2 49 50. 606 607 vitamin D 48.50, 481-482 vitamin E 48, 50, 493 vitamin K 48, 50, 504 vitamins fat-soluble 50 water-soluble 49-50 water 43.44, 50 51,644-645 zinc 53, 191,420. 641,686 688 Acacetine 101 Acamprosate 240 Acelp 684 Acesulfame potassium 14 Acetaldehyde 237. 238 Acetaldehyde oxidase 238 Acetaminophen 240 Acetate 16, 22, 147 152, 236, 623 absorption 149 dietary sources 147. 149 endogenous synthcsis 147 148, 149 ethanol metabolism 237-238, 239 excretion 152 function 147. 152 metabolism 150 15 I nutritional summary 147 regulation 152 storage 151 transport/cellular uptake 150 intestinal proton cotransport 45 Acetate-CoA ligase (thiokinase) 150, 239

786

Index

Acetoacetate 133. 134. 135. 623 acetate generation 151 beta-hydroxybutyrate production 150, 151 ethanol metabolism 239 synthesis from amino acids 257, 317. 318-319, 326. 365 transport 150 utilization during starvation 269 s e e also Ketogenesis Acetone 133, 134 ethanol metabolism 239 s e e also Ketogenesis N-Acetyl aspartate 389 AcetyI-CoA 124, 128, 201. 208, 229. 230. 236. 624 cholesterol synthesis 512 endogenous synthesis 147-148 ethanol metabolism 236, 239 fatty acid synthesis 119 ketogenesis 133 135 leucine metabolism 369 oxidation 131, 133 regulation 152 synthesis from amino acids 257, 258. 297, 299, 319, 331. 333,359, 365. 379. 380 transport by carnitine 432. 436 Acetyl-CoA acctyltransferase 436 Acetyl-CoA acyltransferase 380 Acetyl-CoA C-acetyltransferase (thiolase) 128, 134, 150, 155, 162. 167, 170, 178, 239, 319, 326, 334. 513 AcetyI-CoA carboxylase 119, 165, 615, 617 [Acetyl-CoA carboxylase] kinase 119 [AcetyI-CoA carboxylase] phosphatase 119 Acetylaldehyde oxidase 148 Acetylanserine 419 Acetylcarnosine 419 Acctylcholine 447. 450, 453, 53 I. 690, 706 Acetylcholine esterase (YT antigenl 453 N-Acetylgalactosaminyltransferase 698. 731 N-AcetylmuramyI-L-alanineamidase 310 N-Acetylneuraminylgalactosylglucosylceramide beta- 1.4-Nacetylgalactosaminyltransferase 732 N-Acetylnuraminate monooxygenase 550 N-Acetylnuraminylgalactosylglucosylceramide beta- 1,4acetylgalactosyltransferase 731

Acetylserotonin O-methyltransferasc 336. 440, 442 Acid phosphatase 555 Aconitase (aconitate hydratase) 275, 674, 675 Acrodermatitis cnteropathica 687 Actin 249. 362.417. 433 Active transport 45.59 Acyl-carrier protein 120. 624 Acyl-carrier protein S-acctyltransferase 121 Acyl-carrier protein S-malonyltransfcrase 121 Acyl-CoA 127. 128 Acyl-CoA dehydrogenasc 128, 373. 380. 567 AcyI-CoA oxidase 128. 155, 165. 178. 568. 589 AcyI-CoA synthase 126, 160, 168 AcyI-CoA:retinol acyltransferase (ARAT; retinol O- fatty-acyltransferase} 473 Acylcarnitine hydrolase 435 I -Acylglycerol-3-phosphate acyltransferase 137 Acylglycerone-phosphate reductase 137 Adenine nucleotide translocators 704 Adenosylate kinase 555 Adenosylcobalamine 607 S-Adenosylhomocysteinase 341. 346, 611 S-Adenosylhomocysteine hydrolase 681, 682 S-Adenosylmethionine (SAM) 339, 344, 345. 400. 599, 610. 759. 760 carnitine synthesis 362. 433 creatine synthesis 428 DNA methylation 539. 540-541 melatonin synthesis 439, 440 nicotinamide metabolism 575-576 SAM cycle 341-342 ubiquinone synthesis 533 S-Adenosylmethionine decarboxylase 341. 345 Adenylate cyclase 144. 579 Adenylylsulfate kinase 355 Adequacy of intake 776 777 Adiponectin 144 Adipose tissue agouti protein release 29 conjugated linoleic acid (CLAI storage 162 docosahexaenoic acid storage 168. 170 fat storage 137, 143 fatty acid metabolism 137 fatty acid release regulation 144. 206-207 hormone/cytokine production 137

insulin actions 144 leptin release 29 mesentery 40 myristic acid storage 155 phospholipid metabolism 137 trans-fatty acids 175, 178 ADP-ribose 575. 577. 579 DNA repair 579 ADP-ribosylation factor 6 669 Adrenaline 269. 339. 400. 653 fat metabolism regulation 137. 138 glucose regulation 205. 206 hormone-sensitive lipase activation 137 synthesis 632 Adrenocorticotropic hormone (ACTH) 566 Advanced glycation end products 250 Aflatoxin 438 Aging appetite impairment 27 free radical-mediated mechanisms 457 glomerular filtration rate IGFR} decline 57 muscle mass reduction (sarcopenia) 259 olfaction sensitivity decline 8, 9 sensitivity to irritants in food 21 taste sensation variation 18 thirst perception impairment 32, 644 vitamin D metabolism renal tubule 484~.85 skin 480. 523 Aglycones 93 Agmatine 264. 398, 402 Agouti-related protein (AgRP) adipose tissue release 29 genetic variation 770 hypothalamic appetite regulation 26 Alanine 244, 245. 249, 253. 308-313 dietary sources 308, 310 digestion/absorption 309. 310-31 I endogenous synthesis 309 310, 351. 385. 588 glutamate 230-231. 309-310 serine 258 tryptophan 257. 331,333 excretion 312-313 function 308.313 gluconeogenesis 194, 269. 368 heme synthesis 313 metabolism 257. 312 glyoxylate pathway 232-233. 234. 313 nitrogen compounds synthesis 313 nutritional summary 308 protein synthesis 313 pyruvate synthesis 228

Index 7 8 7

regulation 313 storage 312 transport/cellular uptake 46, 254, 311, 662 D-Alanine 310, 311,312 Alanine aminopeptidase (aminopeptidase N) 251,311,588,690 Alanine aminotransferase 228, 230, 245, 273,309, 312, 400, 589 Alanine-glucose cycle 228, 231,263, 269, 313, 368 Alanine-glyoxylateaminotransferase 62, 158, 232, 291,312, 313, 588 Alanine glyoxylate pathway 232, 234, 313 Alanine-oxomalonate aminotransferase 312, 383-384 Alanine-tRNA ligase 313 Albendazole monooxygenase 569 Albumin calcium binding 695 copper binding 679, 680 fatty acids binding 125, 126, 168 nickel binding 767 thiamin binding 554 vitamin B6 binding 584 zinc binding 688, 689 Alcohol dehydrogenase 148, 238, 239, 240, 334, 442,467, 469, 578, 621,638, 675,690, 782 Alcohol dehydrogenase 1 (ADH 1) 470 Alcohol dehydrogenase 4 (ADH4) 470 Alcohol dehydrogenase 7 (ADH7) 470 Alcohol sulfotransferase 547 Alcoholic beverages 97, 236 Alcoholic liver cirrhosis 240 Alcoholism 240, 552, 557, 558, 708 treatment 240 Aldehyde dehydrogenase (AIDH) 148, 238, 240, 334, 442, 585,621 Aldehyde dehydrogenase 1 (ALDH 1) 470, 473 Aldehyde dehydrogenase 2 (ALDH2) 238 genetic variation 770 Aldehyde dehydrogenase 4 (l-pyrroline 5-carboxylic acid dehydrogenase) 407, 408 Aldehyde dehydrogenase 12 (ALDH 12) 471 Aldehyde dehydrogenase, fatty (ALDH3A2) 130 Aldehyde oxidase 576, 585, 675,737 Aldehyde oxidase homolog 1 (AOHI) 737 Aldehyde oxidase homolog 2 (AOH2) 737 Aldehyde reductase (aldose reductase) 208, 21 I, 214, 223, 297, 630

Aldohexoses 188 Aldolase 200, 213 Aldolase A 213 Aldolase B 213,215 Aldolase C 213 Aldosc reductase (aldehyde reductase) 208, 21 I, 214, 223. 297, 630 Aldoses 188-189 Aldosterone 13, 523,566 renal function regulation 66, 67, 653, 654, 659, 666 synthesis 67. 550 ALDP ('ALD' protein; ABCDI) 128, 165 Alfalfa sprouts 93 Algae 123, 191,273 Alginate 191 Alkaline phosphatase 49, 52,429, 487, 488, 553, 562, 564, 583, 620, 690, 702, 704 Alkaline sphingomyelinase 42 AIl-trans retinoic acid 470, 472 All-trans-retinyl-palmitatehydrolase 474 AII-trans-retinylesterisomerhydrolase 474 Allicin (diallyl thiosulfinate; DADSO) 107, 108 Alliin 106, 107 Alliinase 107 Allium genus sulfur compounds 106-108 Allose 188 S-Allylcysteine (SAC) 106 Alpha-adducin 770 Alpha-amylase 197 pancreatic 47, 197 salivary 47, 197 small intestinal 42 Alpha-carotene 50, 464. 467, 470, 780 Alpha-chymotrypsin 41, 47, 250 Alpha-crystallin 624 Alpha-fetoprotein 126 Alpha-galactosidase 190, 198, 213,218, 219 1,4-Alpha-glucanbranching enzyme 204 Alpha-glucosidase 47 Alpha-gustducin 15, 16 Alpha-ketoglutarate 194, 208, 244, 263, 557, 623 amino acid metabolism 256, 258, 273, 275, 284, 285, 319, 359, 365, 373,379, 386, 398, 400 carnitine synthesis 433 synthesis during starvation 269 AIpha-ketoglutarate dehydrogenase complex 275, 333,366, 373,380, 557 Alpha-ketoisocaproate dioxygenase 366 Alpha-lactalbumin 221

Alpha-limit dextrin 189 Alpha-linolenic acid dietary sources 167 docosahexaenoic acid synthesis 165, 166 Alpha-2-macroglobulin 688, 689, 734 2-(AIpha-mannopyranosyl)-I-tryptophan 334 AIpha-melanocyte stimulating hormone (MSH) 653 AIpha-melanotropin 683 Alpha-methylacyI-CoAracemase 183 Alpha-oxidation 183 Alphal-proteinase inhibitor 250 Alpha-tocomonoenol 492 Alpha-tocopherol 492-493, 549 dietary sources 493 nonantioxidant functions 498-499 s e e also Vitamin E Alpha-tocopherol transfer protein (TTP) 493,496, 497 defects 498 Alpha-tocotrienol 492 Altrose 188 Alzheimer's dementia 8, 491. 628, 632, 676 Amadori products 250 Amanita poisoning 531 Amanitin 249 Amidophosphoribosyltransferase 286 Amiloride 450 Amine oxidase (monoamine oxidase A) 442, 565, 567, 569 Amine oxidases 683 Amino acid N-acetyltransferase 259, 277, 386 D-Amino acid oxidase 256, 291,569 L-Amino acid oxidase 568 Amino acid transporter SLCIA4 53, 723 Amino acid transporters 47 Amino acids 243-454 absorption 47, 251-253. 274, 296, 309, 311,316, 33(~331, 340-341,349-350, 357-358. 378, 384, 406-407, 413-414 acetate production 147-148 bitter taste 16 blood-brain barrier transport 72 composition 244 conditionally essential 395 cooking effects 249-250, 270, 310. 329, 339, 357, 358 dietary sources 249-250 digestion 25~25 I enzymes 252 endogenous synthesis 244-245, 247-249

788

Index

Amino acids (cont.) amino acid precursors 247-248 post-translational modification 248-249 precursors from intermediary metabolism 244-245, 247 starvation 269 energy fuel metabolism 263 enterohepatic circulation 251 essential 3, 249, 270-271,295, 315, 322, 328. 339, 357, 363, 371,377 excretion with urine 261-262 fecal losses 262 function 262-264 glucogenic 256-257, 263,373, 388 gluconeogenesis 194, 262-263 ketogenesis 257 mediator synthesis 263-264 metabolism 256-259 pyruvate synthesis 228 urea synthesis 258-259, 260 water generation 644 nucleotide synthesis 264 placenta materno-fetal transport 80 metabolism 80 transporters 80-81 plasma concentrations 253, 255, 274, 282, 290, 297, 303, 311,316, 324, 331,341,358, 391,398 protein synthesis 262, 278, 285, 293, 299, 306, 313, 320, 327, 334, 345, 354, 361,369, 376, 382, 388, 393-394, 401,410, 418 regulation 262 renal processing 58, 59, 60-62, 63, 259-262 storage 259 structure 244, 245, 246, 247 sweet tasting 13, 14 transport/cellular uptake 63, 251-252, 253-255 chloride transport 662 2-Amino- 1-methyl-6-phenylimidazo[4.5beta]pyridine (PhlP) 86, 87 DNA adduct formation 90 excretion 89 metabolism 87.88 R-3-Amino-2-methylpropionate-pyruvate aminotransferase 310 2-Amino-3.8-dimethylimidazo[4,5-] quinoxaline (MelQx) 86 2-Amino-3-ketobutyratecoenzyme A ligasc 289. 297. 588, 589 Aminoacyl-tRNA synthetase 354 2-Aminoadipate aminotransferase 359. 589

Aminoadipate-scmialdehyde dehydrogenase 359 4-Aminobutyrate aminotransferase (GABA-transaminase) 278, 286, 588, 589 Aminocarboxymuconate-semialdehyde decarboxylase 333 Aminolevulinate aminotransferase 313 5-Aminolevulinic acid synthase 293, 313 Aminomethyltransferase 596 Aminomuconate-semialdehyde dehydrogenase 333 Aminopeptidase A (glutamyl aminopeptidase) 690 Aminopeptidase N (alanine aminopeptidase) 251, 311,588, 690 Aminopeptidases 47, 330, 396 DL( + )-2-Aminophosphono-butyricacid (DL-AP4) 17 Aminotrimethylamine (TMA) 449 Ammonia 2 amino acids synthesis 247 metabolism during starvation 270 pH regulation 287 synthesis from glutamine 284 urea synthesis 259 Ammoniated glycyrrhizin 14 Amphotericin 708 AMPL 731 Amylin 206 appetite regulation 26, 28 Amylo-1,6-glucosidase 204 Amyloid 206 Amylopectin 190, 196 Amylose 190, 196, 197 Anabolic hormones 262 Anandamide 22, 27 Androstenone odor sensitivity 9 Anemia 593, 668 Aneurine 551 Angelman syndrome 541 Angiogenesis 628 Angiotensin 1 29, 67. 653. 666 Angiotensin l-converting enzyme (ACE; peptidyl-dipeptidase A) 60, 67. 259. 653. 666. 690 Angiotensin I1 67. 653,666 renal function regulation 67 thirst regulation 33 Angular stomatitis 562, 614 Anhydroretinol 472. 475 Anion exchanger AEI 663 Anion exchanger AE2 49. 666 Anorexia 25 Anserine 244. 249. 258, 413. 417, 419

Anteiso-fatty acids 119, 123 Anthocyanidins 93, 97 Anthocyanins 92.97 Antidiuretic hormone (ADH; vasopressin) 13, 59. 64-65. 647, 683, 710 renal function regulation 66, 67, 658 synthesis 67 thirst regulation 32, 33 Antimetabolites 619 Antioxidant 1 (ATOXI) 681 Antioxidant activity biopterin 628, 633 enzyme-catalyzed reactions 462 flavonoids 92, 103 free radical-mediated tissue damage protection 457-458 garlic compounds 108 lipoate 463,530 manganese 457, 730 mechanisms 462-463 superoxide dismutases 683~84 melatonin 444 metal ion chelation 462 paraoxonases 131 selenium 457, 725 ubiquinol 463, 530, 536 vitamin E 457. 463,497-498, 499 Antioxidant vitamins 457-458, 462-463 free radical reactions 459 Anxiolytic activity 444 AP3A 726 AP4A 726 Apigenin 93, 96, 99 gestagen-like activity 103 Aplasmomycin 748 Apocarotenals 467 Apocarotenoic acids 475 Apochromodulin 743 Apolipoprotein AI 131,240 Apolipoprotein AIV 28, 29 Apolipoprotein B 691 Apolipoprotein B editing catalytic subunit I (APOBEC-1) 124, 517 Apolipoprotein B48 (apoB48) 124. 125, 159, 167, 517 Apolipoprotein BI00 (apoBl00) 125, 517 Apolipoprotein CI (apoCl) 125, 519 Apolipoprotein CII (apoCll) 125 Apolipoprotein CIII (apoClll) 125 Apolipoprotein E 482, 504, 522 Apolipoprotein E receptor (low-density lipoprotein-receptor related protein 1; LRP) 80, 504, 518. 519, 520, 671 Apoptosis 163. 184, 346.411,431. 454. 459-460. 498, 508, 691. 726, 730 731

Index 7 8 9

vitamin A regulation 474. 475 vitamin D regulation 488 Appendix 43 Appetite 25-30, 172 adipose tissue regulation 137, 155 calcium-containing foods in pregnancy 29 central regulation 26-27 cephalic phase response 27 conditioned taste aversion 25.28 enteral input 27-28 falling blood glucose response 28, 206 fat intake relationship 29 mineral deficiency responses 29-30 physical exercise influence 30 pica behaviour 29-30 salty foods 29 sensory input 27 s e e also Hunger sensation Apyrase 698~99 Aquacobalamine 603. 604. 605 s e e also Vitamin B 12 Aquacobalamine adenosyltransferase 608-609 Aquacobalamine reductase 567, 568, 780 Aquaporin 1 (AQPI) 64 Aquaporin 2 (AQP2) 64. 67, 646. 647 Aquaporin 3 (AQP3) 51.65, 645, 646 Aquaporin 4 (AQP4) 51, 65, 645,646 Aquaporin 6 (AQP6) 65 Aquaporin 7 (AQP7) 64 Aquaporin 8 (AQP8) 65, 646 Aquaporin 9 (AQP9) 646 Aquaporins 64-65 Arabinose 189 Arachidonate 5-1ipoxygenase 140 Arachidonic acid 640, 676 placental transport 79 Aralar 1 391 Aralkylamine N-acetyltransferase 336, 440 Arcuate nucleus 26. 27 Arginase 259, 386, 392, 399. 73 I Arginine 3. 244, 395-402 creatine synthesis 395. 396, 402, 428 dietary sources 395, 396 digestion/absorption 396-397 endogenous synthesis 247. 248. 395-396 aspartatc 388 citrulline 396 glutamate 395,396. 397 prolinc 257, 395, 396. 397. 410 excretion 4(1(I function 395, 401-402 hormone release stirnulation 402 metabolism 258, 398-4110, 589 nitric oxide synthesis 395. 396, 401-402

nutritional summary 395 phosphoarginine synthesis 395, 396, 402 protein ADP-ribose linkage 579 protein synthesis 401 regulation 400-401 storage 400 taste sensation 17 transport/cellular uptake 251. 254, 398 Arginine decarboxylase 402 Arginine deiminase 402 Arginine-tRNA ligase 401 Argininosuccinate lyase 259, 386, 388, 392, 396 Argininosuccinate synthase 259, 386, 388, 392, 396 Aromatic-amino acid transferase 351-352 Aromatic-amino acid-glyoxylate aminotransferase 289, 319, 326, 333, 417. 588, 589 Aromatic-L-amino acid decarboxylase 336, 440 Arsenate reductase 758 Arsenic 757-761 absorption 53,758 dietary sources 757-758 excretion 759 function 757, 760-761 metabolism 758-759, 760 nutritional summary 757 regulation 759 storage 759 toxicity 757, 761 transport/cellular uptake 758 Arsenic trioxide 761 Arsenite methyltransferase 759 Arsenobetaine 757, 758, 759 Arsenocholine 758, 759 Arsine gas 758, 759 Arterial baroreceptors, thirst regulation 32 Artery calcification 502. 507 Arthritis 102 Aryl-acylamidase 442 Arylamine N-acetyhransferases (NAT) 598 genetic variation 770 heterocyclic amines metabolism 89 Arylcsterase (paraoxonase 1: PON I ) 131,462 Arylformamidase 333 Arylsul fatase 5(18 Arylsulfatase A 548. 550 Arylsulfatase B 548, 550 ASC transporter 190, 251,255. 260. 261,274. 290, 291,295, 296, 298, 303,305.31 I. 350, 35 I, 353, 365. 379, 391. 407

Asc-I 253, 255, 311 Asc-2 255 Ascorbate 3,542-551 absorption 49, 544-545 amino acid metabolism 257, 315, 317, 320, 322, 327, 549-550 phenylalanine 549 tyrosine catabolism 319, 326, 549 antioxidant activity 457, 463, 549 semidehydroascorbate formation 459 carnitine synthesis 362, 433, 549-550 collagen metabolism 361,549 deficiency 543, 548 dietary sources 543,544 endogenous synthesis 543-544 folate interactions 549, 593 function 543, 549-551 iron metabolism 549, 550, 669. 780 lipid metabolism 550 metabolism 546-548 mineral absorption effects 52, 53,550 chromium 743 copper 679 iron 550 nutritional summary 543 phytanic acid oxidation 183 protein modification 549 regulation 548 steroids synthesis 550 storage 548 sulfate transfer 550 transport/cellular uptake 72-73.81, 545-546 Ascorbate-2-sulfate 544 L-Ascorbate-cytochrome-b5 reductase 547, 550 ASCT1 transporter 63, 261,285, 290, 291. 296, 303,305, 311,313. 351,353, 391 ASCT2 transporter 286, 350, 351,365 s e e also System BSos Asialoglycoprotein receptor 617, 671, 680, 715 Asparaginase 392 antitumor treatment 390, 394 Asparagine 244, 389-394 dietary sources 390 digestion/absorption 390-391 endogenous synthesis 247, 390 aspartate 388 excretion 392 function 390. 393-394 metabolism 258. 384, 392, 393, 589 nutritional summary 390 protein synthesis 393 394 regulation 393 storage 392 transport/cellular uptake 391 392

7 9 0 Index

Asparagine synthase 388, 390, 393 Asparagine-oxo-acid aminotransferase 392 Asparagine-tRNA ligase 393, 627 Asparagus odor sensitivity 9 Aspartame 14, 315 Aspartate 244, 245, 249, 383-389 arginine synthesis 388 asparagine synthesis 388 D-aspartate 388 dietary sources 384 digestion/absorption 384 endogenous synthesis 383-384. 390 oxaloacetate 383-384 excretion 386-387 function 383, 388-389 metabolism 258, 310, 385-386, 589 during starvation 270 protein modification 386 transamination 386 urea cycle 385, 386, 387 mitochondrial translocation (malate-aspartate shuttle) 385 nucleotides synthesis 264, 388 nutritional summary 383 protein synthesis 388 pyridoxamine synthesis 388 regulation 387 storage 386 transport/cellular uptake 25 I, 254, 385 into brain 72 renal tubule 60, 261,386-387 urea cycle 385, 386, 387, 392 Aspartate N-acetyltransferase 386 Aspanate aminotransferase 245, 273, 319, 325, 351,352, 383, 386, 390, 392, 400, 408, 424, 589 Aspartate 4-decarboxylase 310, 352, 385, 424 D-Aspartate oxidase 275, 388, 569 Aspanate racemase 588 Aspartate-ammonia ligase 388, 390 Aspartate-tRNA ligase 388, 627 Aspartate/glutamate transporter 1 (AGT1) 277, 387 Aspartoacylase 386 ATAI 80, 254. 283, 286, 311,340, 351, 392. 415 ATA2 80, 254, 255,282, 283, 285, 286, 290. 291,296, 303, 305.311, 313, 341,351,391,392, 415 ATA3 254, 283,311,341,351. 391,415 Ataxia telangiectasia 731 Atherosclerosis I, 5, 92, 112, 188, 491, 593, 675, 722, 755 hyperlipidemia 144 protective activity of antioxidants 458, 498

ATM protein kinase 730, 731 ATP7A (Menkes protein) 45, 53 ATP:FMN adenylyltransferase (ravin adenin dinucleotide synthetase) 563 ATP:L-methionine-S-adenosyltransferase (methionine adenosyltransferase; MAT) 341,345 ATP:RF-5'-phosphoryltransferase (flavokinase) 563 ATP-citrate (pro-S-)-lyase (citrate cleavage enzyme) 119, 513 Atrial natriuretic peptide (ANP) 683 renal function regulation 66, 653 secretion 67 Autism 628, 632 Autonomic signaling, central gustatory pathways 12 Avidin 615 Bacteroides 44 Balenine 419 Band 3 of red cell membrane (chloride/bicarbonate-exchanger; SLC4AI) 199, 681,703, 735 Base exchange enzyme (phosphatidylserine synthase; CDP-diacylglycerol-serine O-phosphatidyltransferase) 139, 306, 447 BAT1/bSo, +s (SLC7A9) 61,253, 261, 277, 281,290, 296, 303, 305, 311,316, 319, 323, 326, 330, 334, 340, 344, 350, 353, 354, 358, 361,365, 368, 371,375, 378, 380, 387, 390, 396, 407, 413,417 BCRP (ABCP; MXR; ABCG2) 82 Beano 219 Benzoate 45, 531,624 Benzoate X receptor (BXR) 475 Beriberi 552 Beta transporter 251,255 Beta-adrenergic agonists 26 Beta-alanine balenine (ophidine) 419 carnosine 417 placental transfer 81 renal tubule processing 60, 61,261 Beta-alanine-histidine dipeptidase 417 Beta-aminoalanine 310 S-Beta-aminobutyrate (BAIB) 373 Beta-carotene 50, 458, 464, 466, 467, 468. 470, 675. 780 Beta-carotene 15,15'-dioxygenase 50, 466. 467, 675 Beta-cryptoxanthin 50, 467, 468, 470 Beta-endorphin 653

7-Beta-estradiol 748 Beta-galactosyl-glucosylceramidebeta1,6-N-acetylglucosaminyltransferase 732 Beta-glucans 191 Beta-glucosidase 97, 99 Beta-hydroxy beta-methylbutyrate (HMB) 369 leucine metabolism 366, 367, 368 Beta-hydroxyacyI-CoAdehydrogenase 380 Beta-hydroxybutyrate 134, 135 ethanol metabolism 239 production from acetoacetate 150, 151,319 transport 150 utilization during starvation 269 11Beta-hydroxysteroid dehydrogenase type 2 659 Beta3-integrin-mobilferrin pathway 669 L-Beta-leucine aminomutase 368, 611, 730 Beta-tocopherol 492 Beta-tocotrienol 492 Beta-zeacarotene 467 Betaine choline metabolism 447, 449, 451 ergothioneine (2-mercaptohistidine betaine) 418-419 metabolism to glycine 289 renal tubule osmoprotective function 59. 61, 261,453 transport 59, 60, 261,451 Betaine aldehyde dehydrogenase 451 Betaine transporter BGT 1 (SLC6A 12) 59, 60, 261,450, 451,666, 759 Betaine-homocysteine S-methyltransferase 342, 451, 541,597-598 Betel 553 Bicarbonate/chloride exchanger 2 (SLC4A2) 39 Bifidobacteria 44, 190, 213 Bile 42 alpha tocopherol excretion 497 cholesterol excretion 522 composition 42 copper excretion 682 enterohepatic circulation 42-43 folate excretion 598 manganese excretion 730 mixed micelles generation 123,516 taurine conjugation 426 excretion 424 vitamin B12 excretion 610 vitamin K excretion 507

Index 791

Bile acid-CoA:amino acid N-acyltransferase (BAT; glycine N-choloyltransferase) 293-294, 426 Bile acids synthesis 520, 522. 523. 550. 624 Bile-salt activated lipase s e e Pancreatic lipase Bilirubin 568. 672 free radicals generation 459 Bilitranslocase 49 Biliverdin 568. 672 Biliverdin reductase 672 Biochanin A 93 estrogenic potency 103 Biopterin 628-633 absorption 630 O-alkylated glycerolipids regulation 633 amino acid metabolism 257, 315, 317 antioxidant activity 628, 633 catecholamine synthesis 628. 632 deficiency 628 dietary sources 628, 630 endogenous synthesis 628-629 excretion 632 function 628, 632-633 immune function 628, 633 melatonin synthesis 439, 628, 632 metabolism 630 reactivation 631 neuronal function 632-633 nitric oxide synthesis 628, 632 nutritional summary 628 regulation 632 serotonin synthesis 336, 628, 632 transport/cellular uptake 630 tyrosine synthesis 632 urinary metabolites 632 Biotin 43, 613-618, 776 absorption 50, 614-615 amino acid metabolism 272, 277, 280, 285. 288. 293, 295, 299. 306. 308, 313, 320, 322, 327, 339, 345, 350, 354. 357, 361,363, 365, 366, 371,374. 377. 379. 383, 388, 390, 394. 395, 401, 404, 410, 412, 418, 617 carboxylase prosthetic group 617 deficiency 614 dietary sources 614 excretion 64, 61 6-617 fatty acid oxidation 138, 617 function 614, 617-618 intestinal microflora production 44, 614 metabolism 615, 616 mRNA translation enhancement 617-618 nutritional summary 614

pyruvate metabolism 229, 313.617 regulation 617 storage 616 transport/cellular uptake 73.82, 615 tyrosinase regulation 633 Biotin-[acetyl-CoA-carboxylase] ligase 119 Biotin-[propionyl]-CoA-carboxylase 618 Biotin-[propionyl]-CoA-carboxylase ligase 361,615 Biotinidase 614, 615 Biotinyl-acetyl CoA carboxylase 616 Bitter taste 15-16 BLAST 783 Blood coagulation 502, 507 calcium dependence 699 Blood coagulation factors 278 copper interactions 684 obesity 145 vitamin K-dependent 507 Blood-brain barrier transport 70-75 acetate 150 alanine 311 amino acids 72, 254-255 anatomical aspects 33, 70-71 arginine 398 ascorbate 72-73, 545 asparagine 392 biotin 73,615 calcium 696 carbohydrates 71 carnitine 435 cholesterol 520 choline 450 conjugated linoleic acid (CLA) 160 copper 74, 68 I cysteine 351 docosahexaenoic acid 168 ethanol 237 fatty acids 71-72. 126 trans-fatty acids 176 flavonoids 99 folate 595 galactose 219 glucose 71. 199 glutamate 72, 274 glutamine 283 glycine 291 histidine 415 iron 73-74. 671 isoleucine 379 leucine 365 lipids 71-72 lipoate 73 lysine 359 magnesium 709 manganese 729 methionine 341

minerals/trace elements 73-74 myristic acid 154 niacin 575 pantothenate 73, 621 phenylalanine 316-317 phosphate 704 potassium 658 proline 407 pyruvate 228 riboflavin 73, 563 serine 303 sodium 651 taurine 423-424 thiamin 73, 555 threonine 297 tryptophan 33 I tyrosine 324 valine 372 vitamin B6 585 vitamin BI2 73, 608 vitamin E 73,494 vitamins 72-73 water 646 xenobiotics 74-75 zinc 688 Blood-nerve barrier 70 Bohr effect 675 Bone metabolism 226, 684 boron 748 calcium 693, 696, 699 fluoride 720-721 magnesium 709, 711 phosphate 704 silicon 752 vitamin D actions 487-488, 693 vitamin K actions 502, 508 Boracic acid (boric acid) 746 Boromycin 748 Boron 746-749 absorption 747 bone health 748 deficiency 746 dietary sources 746-747 excretion 747 function 746. 748-749 hormone action influences 748 neutron capture therapy 749 nutritional summary 746 regulation 747 storage 747 transport/cellular uptake 747 Boronic acid 746 Bowman-Birk inhibitors 270 Bowman's capsule 57 Brain docosahexaenoic acid 17 I. 172 energy metabolism 207 during starvation 269

792

Index

Brain (cont.) hemorrhage 499, 502, 781 inositol metabolism 641 thiamin actions 558 see also Blood brain barrier transport Branched-chain acyI-CoA oxidase (2-methylacyI-CoA dehydrogenase) 129, 183,370 Branched-chain alpha-ketoacid dehydrogenase 297, 366, 368, 373, 375,379, 380, 382, 530, 557 Branched-chain amino acid aminotransferase 365,372, 374, 378, 588. 781 Branched-chain amino acid aminotransferase 2 373, 379 Branched-chain amino acids conditioned taste aversion with intake imbalance 25, 28, 368. 375, 382 muscle protein synthesis 262, 369. 376 neurotransmitter metabolism (BCAA shuttle) 369, 376 Branched-chain fatty acids 112, 113, 116, 117, 119, 123 mitochondrial beta-oxidation 128 peroxisomal beta-oxidation 129-130 Brassicas 89, 714. 715,717 Brazzein 14 Brenda 783 Bromine 753 756 absorption 53,754 dietary sources 753,754 excretion 754 function 753, 755-756 immune defence 755 nutritional summary 753 sleep physiology 756 storage 754 thyroid function 756 transport/cellular uptake 754 Bromotaurine 755 Brunner's glands 40 Brush border alkaline phosphatase 49. 52 Brush border membrane 40 Buffers 706-707 Bulimia 336 Butylmercaptan odor sensitivity 9 Butyric acid 123 ButyryI-CoA dehydrogenase 128. 170. 567 C cells 697 c-fos, thirst regulation 33 Cadavcrinc 362 Caffcine 15, 18, 32 Calbindin (9CBP) 52.82. 695,696, 697. 710

Calcidiol l-monooxygenasc (25-hydroxy vitamin D- I alpha-hydroxylasc; CYP27BI; P450CI-alpha) 483 Calcitonin 63,696, 697, 710 Calcitroic acid 485, 486 Calcium 693-699. 776 absorption 51 52, 191. 694-695 phosphate complexes 694 vitamin D 51,487, 693, 694 blood clotting 699 bone metabolism 693,696, 699 cell growth 699 dietary sources 693,694 enzyme cofactor activity 698-699 excretion 696-697 function 693, 698-699 magnesium balance 711 muscle contraction 698 neurotransmitter release 698 nutritional summary 693 pica behaviour 29 regulation 697 renal processing 57.59, 65-66, 696-697 signaling 579. 698 storage 696 transport/cellular uptake 45, 82, 695-696 vestibular system 699 Calcium channels 694-695, 698 Calcium transporter protein (CaTI) 52 Calcium-transporting ATPase (PMCA I 52, 695 Calcium-transporting ATPase lb (PMCIb) 45, 697 Calcium-transporting ATPases 65, 695, 696, 698 muscle contraction 698 Calcium/calmodulin-dependent protein kinase 698 Calcium/magnesium ion sensing receptor (Casr) 697, 699, 710 Caldesmon kinase 698 Calmodulin 205, 557, 617. 623, 698 Calpain 698 Calsequestrin 698 cAMP-dependent protein kinase 139, 157. 513 Canavan disease 389 Cancer 92, 102. 143. 158. 163, 165, 240, 345. 491,543. 593. 635. 675. 722. 755, 757, 761,765 choline deficiency-related risk 454 DNA hypomethylation 540 free radical-mediated damage 457 melatonin antitumor activity 444, 445 prevention 1, 5. 184 antioxidants 458

omega-3 fatty acids 172 phytates 64 I vitamin D 478. 488 Candida alhicans 236 Cannabidiol 27 Cannabinoids 27 Cannabis 27 Capric acid 526 Capsaicin 22 Capsaicin receptor (vanilloid receptor 1: VR1) 22 Carageenan 191 Carbamoyl phosphate synthase I 258, 277. 286 Carbamoyl synthase II 284 Carbidopa 668 Carbohydrates 187 192 absorption 47-48 sodium cotransport 51 blood-brain barrier transport mechanisms 71 dietary sources 188 fate of excess 143-144 fatty acid synthesis 119. 143 very-low-density lipoprotein (VLDL) synthesis 144 function 187 gustatory perception 191-198 low-intake diets 188, 194 materno-fetal transport 79 metabolism acetate production 147 pyruvate synthesis 227-228 vitamin B6 587 water generation 644 nutritional summary 187-188 renal tubular transport mechanisms 60 small intestinal digestion 42 sweet-tasting 13. 14, 191-198 Carbon 2 Carbon dioxide 22 sour taste 16 Carbon tetrachloride 438 Carbon-monoxide dehydrogenase 767 Carbonic anhydrase 16. 39. 65, 666, 690. 704 Carbonic anhydrasc Vl (gustin) I I, 690 Carbonyl reductase 630 2(2'-Carboxyethyl)-6-hydroxychromans (CEHCs) 496, 497 Carboxylcster lipase 41.48, 123. 434. 516 Carboxypcptidasc A 250 Carboxypcptidase A I 41, 47, 311,690 Carboxypeptidase A2 41, 47, 31 I. 69(1 Carboxypcptidasc B 250 Carcinogens chromium exposure 744

Index 7 9 3

cooked amino acids generation 250 cooked meat-derived heterocyclic amines 86, 428 DNA adduct formation 90 nickel 768 nitrosamines 550 Cardiac arrhythmia 656, 657 Cardiac failure 231 Cardiolipid 139 Cardiovascular disease 165. 194, 512, 543 free radical-mediated damage 457 omega-3 fatty acid effects 172 overfeeding-related risk 143, 145 Carnicine 418 Carnitine 3, 127, 244, 249, 339, 345. 432-438.449 amino acid/organic acid metabolism 436-437 catabolism 435 dietary sources 432,433-434 digestion/absorption 434-435 endogenous synthesis 247, 433, 434, 540, 589, 593, 675 lysine 357. 361-362.433,549-550 fatty acids transport 138. 154, 155. 157. 158, 160, 162, 164. 170, 176, 184, 436. 437 function 432. 436-438 gene regulation 437-438 membrane stability 438 nutritional summary 432-433 regulation 436 renal tubule processing 62, 435-436 storage 435 transport/cellular uptake 251,435. 662 xenobiotics conjugation 438 D-Carnitine 435 Carnitine decarboxylase 435,448 Carnitine O-acetyltransferases 436 Carnitine O-octanoyltransferase 184, 436 Carnitine-acylcarnitine translocase (CACT; SLC25A20) 128, 160, 170. 184, 436 Carnosinase 417 Carnosine 244, 413. 417. 418. 420 Carnosine synthase 417.418 Carotenoids 465, 466 dietary sources 468 digestion/absorption 50. 469 vitamin A synthesis 466-467 Carotid sinus baroreceptors 32 CART (cocaine and amphetamineregulated transcript) peptide 27 Cartilage metabolism 731 732 Casr (calcium/magnesium ion sensing receptor) 697. 699, 710 Cassava 233,354

CAT-I (SLC7AI) 81,251,357. 359. 361,398, 400, 429, 450 CAT-2 (SLC7A2) 357, 361,401. 429, 449, 450 CAT-2A 398, 400, 401 CAT-2B 81,359, 398. 400 CAT-3 398, 400 CAT-4 (SLC7A4) 81,359, 398, 450 Catalase 462, 675 Cataract 211, 217, 220, 491 Catechins 16, 75, 92 dietary sources 97 metabolism 101 structure 93 Catechol oxidase 323 Catechol-O-methyltransferase (COMT) 99, 101 Catecholamines 262, 263, 569, 659, 683 adaptation to starvation 268 olfactory epithelium signaling cascade 8 synthesis 317, 320, 327. 345, 549, 588, 675 biopterin 628, 632 Caveolae 516 Caveolin 516 CBI receptor 27 CB2 receptor 27 cbl C 608 CD36 (fatty acid transport protein 1" FATP-I SLC27A1) 126. 160, 168, 516 CDP-choline:diacyl glycerol choline phosphotransferase (diacylglycerol choline phosphotransferase) 453 CDP-diacylglycerol-glycerol-3-phosphate 3-phosphatidyltransferase (glycerophosphate phosphatidyltransferase) 138 CDP-diacylglycerol-inositol 3-phosphatidyltransferase 138, 637 C DP-diacylglycerol-serine O-phosphatidyltransferase (phosphatidylserine synthase: base exchange enzyme) 139, 306. 447 Cecum 43 Cell cycle 454 Cell growth biopterin 632 calcium 699 quenine 627 selenium 726 ubiquinonc 536 vitamin A 465,474 vitamin D 488 zinc 690. 691

Cellobiose 197 Cellular retinaldehyde binding protein (CRALBP) 474 Cellular retinol-binding protein 2 (CRBP2; retinol-binding protein 2; RBP2) 468-469 Cellulose 188, 190, 197 Cephalic phase response 27 Ceramide 221. 222, 306, 732 Cereals s e e Grains Cerebronic acid 126, 130 Cerebrose 216 Cerebrosides 221 Cerebrospinal fluid (CSF) 71,646 Ceruloplasmin 49, 74, 462, 544, 545, 669, 670. 672, 679, 683 copper binding 680 Cesium chloride 13 Cesium salts 16 CFTR (cystic fibrosis transmembrane regulator: ABCC7) 45, 5 I, 662-663 Chamomile tea 96 Cheilosis 562 Chemestesis 21-22 mechanoreceptors 21-22 nociceptors 21-22 physical sensing 21 Chemical senses 7-9 cbemestesis 21-22 olfaction 7-9 taste 10-18 Chenodeoxycholoyltaurine hydrolase 422 Chenodesoxycholate 43, 522 Chewing gum 223 Chief cells 38. 39 Chives 106 Chk2 730 Chlorhexidine 18 Chloride 660-666 absorption 51,661-663 dietary sources 661 electrolyte balance 666 enzyme activation 666 excretion 664-665 perspiration 664-665 renal processing 57, 59, 65, 664 function 661,666 gastric acid production 666 immune defences 666 nutritional summary 661 regulation 665-666 storage 663 transport/cellular uptake 663 cotransport 46, 666 Chloride channel CIC2 663. 666 Chloride channel 5 (CCL5) 666 Chloride channel 7 (CCL7) 666

794

Index

Chloride channels 662, 663,666 Chloride-dependent taurine transporter (TAUT; SLC6A6) 42, 46. 59, 60, 63, 251. 261,422, 423,424. 425, 666 Chloride/bicarbonate-exchanger (band 3 of red cell membrane; SLC4AI) 199. 681,703, 735 Chloride/bicarbonate-exchanger 2 (SLC4A2) 39 Chloride/iodine transporter (pendrin; SLC26A4) 66, 664, 714. 716 Chlorophyll 119. 179-184 dietary sources 180 digestion/absorption 180-181 function 179, 184 nutritional summary 180 Chlorotaurine 755 Chocolate 92, 766 Cholecalciferol s e e Vitamin D3 Cholecystokinin (CCK) 653 appetite regulation 28, 29 gastric acid secretion regulation 29. 38 olfactory epithelium signaling 8 small intestine secretion 41 Cholecystokinin CCK-A receptors 28 Cholecystokinin CCK-B receptors 28 Cholera 645 Cholesterol 119, 366. 511-523 absorption 48, 516-517 bile acids synthesis 520, 522, 523, 550 dietary sources 512, 515 endogenous synthesis 512-515, 568 excretion 522 function 512, 523 metabolism 520-522. 624 nutritional summary 512 oxysterols synthesis 520 regulation 522-523 steroid hormone synthesis 523 transport/cellular uptake 517-521 blood-brain barrier 520 placenta 80, 520 reverse transport 519-520. 523 vitamin D synthesis 523 Cholesterol 7-alpha-monooxygenase (CYP7AI) 522 Cholesterol gallstones 143 Cholesterol 24-hydroxylase (CYP46) 520 Cholesterol 25-hydroxylase 522 Cholesterol side-chain cleavage enzyme (P450scc; CYPI IAI) 523. 550 Cholesteryl ester transfer protein (CETP) 498. 518. 519 Cholesteryl esters 122. 123 synthesis 138 Cholic acid 522

Choline 447-454. 776 acetylcholine synthesis 447. 450. 453 cell cycle regulation 454 complex lipid synthesis 453 deficiency 435, 447. 449 cancer risk 454 dietary sources 447, 449 digestion/absorption 449 endogenous synthesis 447-449, 540 serine 258 excretion 451 function 452-453 metabolism 451,569. 676 formaldehyde generation 599 glycine synthesis 289, 451 methyl group transfer 452-453,541 nutritional summary 447 regulation 45 I storage 451 transport/cellular uptake 450 into brain 72. 450 Choline O-acetyltransferase 453. 531 Choline dehydrogenase 451 Choline kinase 453,454 Choline monooxygenase 711 Choline transport-like protein 1 450 Choline transport-like protein 2 450 Choline transport-like protein 4 450 Cholinephosphate cytidylyltransferase 138 Cholinesterase 448 Chondroitin sulfate 208, 221,307, 731 Chondroitins 207, 208. 215,287, 550, 731 Choroid plexus 70-71.74 Chromium 742-744. 776 absorption 53, 550, 743 carcinogenic potential 744 dietary sources 742 DNA transcription 744 excretion 744 function 742. 744 insulin effects/insulin receptor activity 743. 744 nutritional summary 742 regulation 744 storage 743 transport/cellular uptake 743 Chromium picolinate 742. 743. 744 Chromodulin 206. 743. 744 Chrysin 93.98 phase II enzyme effects 103 Chylomicrons 48. 124. 125. 167. 168. 449. 517 cholesterol transport 517 conjugated linoleic acid 159. 160 fat-soluble vitamin transport 50. 469. 470. 482. 493. 504 phospholipids transport 703

processing in circulation 125, 517 remnants 517 Chymotrypsin 41.47, 250 legume inhibitors 270 Cinnamaldehyde 23 Ciprofloxacin 668 Circadian rhythms 444, 593,601 Circumvallate papillae 1 I, 12 Cis-retinol/androgen dehydrogenase 470-471 I l-Cis-retinyl-palmitate hydrolase 474 Cis-trans isomerase 767 Cisplatin 530, 708 Citrate 16. 18, 194 renal tubular transport 60 Citrate cleavage enzyme (ATP-citrate (pro-S-)-lyase) 119. 513 Citrate synthase 119. 150, 275 Citric acid cycle s e e Krebs (tricarboxylic acid) cycle Citrin 391 Citrovorum factor 591 Citrulline 244 arginine synthesis 396 endogenous synthesis 247, 248. 278. 396 proline 257 enterocyte metabolism 47 urea synthesis 259 Citrullinemia 401 Clara cell secretory protein (CCSP) 64 Claudin-1 70 Claudin-2 70 Claudin-16 (paracellin-1) 710 CLD/DRA (SLC26A3) 52. 233, 663. 665 Cleft palate 601 Clofibrate 138 Clostridia 44 Clover sprouts 93 Cobalt 739-741 absorption 739-740 dietary sources 739 enzyme cofactor 741 excretion 740 function 739, 741 gene translation 741 nutritional summary 739 oxygenation sensing 741 transport/cellular uptake 740 Cob(ll)alamin adenosyltransferase 730 Cob(ll)alamin reductase 568. 780 Cocaethylene 239 Cocaine/ethanol compound drug formation 239 Coenzyme A (CoA) 126. 127. 128. 345. 354, 619. 620 function 623 synthesis 621

Index 7 9 5

Coenzyme A (CoA) synthase complex 621 Coenzyme A (CoA)-glutathione reductase 567 Coenzyme F420 hydrogenase 767 Coenzyme QI0 532 Coenzyme Q 532 Coenzyme R 613 Coffee 38, 180, 571,572 Cognitive decline and iron stores 773 Cold perception 23 Cold-sensing sodium/calcium channel (TRPM8) 23 Colipase 123, 159, 167, 516 Collagen 221. 249, 361,404, 406. 549, 684 Collagenase (matrix metalloproteinase 8) 698 Colon chloride transport 663 folate absorption 594 microflora 44 fatty acid metabolism 48 thiamin production 553 potassium absorption 51. 657 queuine uptake 626 sodium absorption 649 water transport 51,645 zinc absorption 687 Colon adenoma 234 Colon cancer 240, 631,693 Colostrum 190 Complex lipids 118 synthesis 138-140, 453 Conditionally essential nutrients 3, 5 Conditioned taste aversion 25, 28. 368, 375, 382 Congenital diarrhea s e e CLD/DRA (SLC26A3) Congenital heart disease 601 Conjugated linoleic acid (CLA) 122. 157-163 dietary sources 158 digestion/absorption 159-160 endogenous synthesis 158, 178 function 158, 162-163 metabolism 160-162 nutritional summary 158 storage 162 transport/cellular uptake 160 Connective tissue 684, 731,748. 752 Connexin 43 (Cx43) 575 Conon 43 Contraceptive hormones 103 Cooking amino acids/protein availability impact 249 250, 270. 302, 310. 329. 339. 357, 358

ascorbate losses 544 carcinogen production 86, 250, 428 folate losses 593 pantothenate losses 619 thiamin losses 553 vitamin B6 losses 583 Copper 678-684, 777. 781 absorption 52-53, 191,679 phytate/inositol pentaphosphate inhibition 641,679 blood coagulation 684 dietary sources 679 energy metabolism 683 excretion 682 function 678, 683-684 hormone metabolism 683 iron metabolism 683 metabolism 681 molybdenum complexes 737 nutritional summary 678-679 reactive oxygen species (ROS) generation (Fenton reactions) 458, 462, 684 regulation 682~i83 storage 681-682 transcription factors 584 transport/cellular uptake 45, 74, 679-681 Copper chaperone for superoxide dismutase (CCS) 681 Copper oxidase 671,672 Copper transporter 1 (CTR 1; SLC31A I ) 680, 681 Copper transporter 2 (CTR2; SLC3 IA2) 680 Copper-transporting ATPase 7A (ATP7A; Menkes protein) 45.53, 679, 681,682, 683 Copper-transporting ATPase 7B (ATP7B; Wilson protein) 681,682 Cori cycle 200, 207, 227. 229 Coriander 122 Corrinoids 38, 608 Corticosteroids 523 Corticosterone 18-monooxygenase 550 Corticotrophin-releasing hormone (CRF) 27, 653, 683 Cortisol 262, 269, 400, 523. 659 Cortisone 659 Coumestrol 93 Cow milk protein, intestinal transcytosis 47 COXI7 681 Coxsackie virus B3 499 Craniofacial malformation 508 Creatine 427-431,555 cooked meat-derived carcinogens (heterocyclic amines) 87, 428

dietary sources 428 digestion/absorption 429 endogenous synthesis 293,339, 345, 395,396, 402. 428, 429, 540, 593 excretion 430-431 renal processing 57, 66, 261,430 function 428, 431 metabolism 429-430 nutritional summary 428 regulation 431 storage 430 transport/cellular uptake 429 Creatine kinase 429, 431 Creatine phosphate 428 digestion/absorption 429 function 431,705 metabolism 429 Creatinine 430 excretion in urine 261,430-431 Crotonaldehydc 131 Crotonase (3-hydroxybutyryI-CoA dehydratase) 374. 380 Cryptidins (defensins) 40 Cryptochrome I 601 Cryptochrome 2 601 Cryptoxanthin 50, 467. 468, 470 CTP phosphocholine cytidyltransferase 453 CTR 1 (copper transporter 1; SLC31A I ) 680, 681 Cubilin (intrinsic factor receptor) 50, 64, 80, 482, 484. 486, 520, 606, 607, 608, 610, 674 Curculin 14 Cyanide 9 detoxification 354-355.61 I, 714 Cyanidin 97 Cyanocobalamine 605 Cyclamate 14

1,2-Cyclic-inositol-phosphate phosphodiesterase 638 Cyclohexamide 16 Cyclooxygenase 67 Cyclo(prolyl-histidine) 406. 413 Cyclosporin 708 Cystathione beta-synthase 306, 343,349. 589 Cystathione gamma-lyase 306, 343. 349. 352. 353,589 Cysteamine 351 Cystcamine dioxygcnase 353 Cysteine 3, 244. 249. 253. 350 357 copper transport 687 cyanide detoxification 354-355 dietary sources 349 digestion/absorption 349-350 endogenous synthesis 247. 349

796

Index

Cysteine (cont.) methionine 248, 339. 341. 343, 344, 345, 349 serine 258. 306 excretion 353 function 348-349, 354-355 glutathione metabolism 353. 354 metabolism 258, 343,351-353 conversion to cystine 352 conversion to mercaptopyruvate 352-353 cysteinesulfinate generation 351-352 pyruvate synthesis 258, 351,352. 353, 354 taurine synthesis 248. 343, 35 I, 353, 354, 422 transsulfuration pathway 349. 589 nutritional summary 348-349 pantothenate sulfuration 354 protein synthesis 354 regulation 353-354 storage 353 sulfate/reduced sulfur supply 355 transport/cellular uptake 350-351 tRNA sulfuration 354 zinc binding to DNA 690 Cysteine aminotransferase 352, 354-355. 589 Cysteine conjugate beta-lyase 66, 590 Cysteine desulfurase 355 Cysteine dioxygenase 351,353,422 Cysteine-tRNA sulfurtransferase 354 Cysteinesulfinate 351-352 Cystic fibrosis transmembrane regulator (CFTR: ABCC7) 45, 51, 662-663 Cystine 349. 350. 354 synthesis from cysteine 352 transport mechanisms 46, 351 Cystine reductase 351,352 Cystinosin 351,353 Cystinuria 353 Cytochrome b5 reductase 122. 158. 165, 342.611 Cytochrome b(558) 460 Cytochrome c 459 Cytochrome c oxidase 675. 681,683 Cytochrome P450 CYP 1A I. heterocyclic amines metabolism 87, 89 CYPIA2 ethanol metabolism 237 hetcrocyclic amines metabolism 87. 89 mclatonin metabolism 442 CYP 1B 1. heterocyclic anaines metabolism 87.89

CYP2D25 (vitamin D(3) 25hydroxylase) 483 CYP2EI 108, 471 ethanol metabolism 237. 239. 240 CYP3A4 496 ethanol metabolism 237 naringenin inhibition 103 CYP4F2 496 CYP7A1 (cholesterol 7-alphamonooxygenase) 520 CYPI 1A 1 (cholesterol side-chain cleavage enzyme; P450scc) 523. 550 CYPIIB 550 CYP24 (25-hydroxy vitamin D-24Rhydroxylase) 485 CYP27 (sterol 27-hydrolase) 520 CYP27B1 (25-hydroxy vitamin D1alpha-hydroxylase) 483 CYP46 (cholesterol 24-hydroxylase) 520 Cytochrome P450 reductase 541 Cytochrome P450RAI-1 (CYP26A) 471. 473 Cytochrome P450RAI-2 (CYP26B 1) 471 Cytochromes 675 Cytokines 262 Cytosine-5-methyltransferase 540 Cytosol aminopeptidase 731 D cells 38, 41 Daidzein 92. 93, 98, 99-100. 102 metabolism to equol 99-100 Dairy products 122, 123. 154. 158, 175, 189, 217, 218, 273. 323, 329. 364, 378,422. 433. 468, 481. 561,562, 571,635,636, 693. 694. 701,702. 734. 742 Deamination 257 Decorin 699 Defensins (cryptidins) 40 Dehydration 644 7-Dehydro-cholesterol reductase 513 7-Dehydro-L-gulonate decarboxylase 639 Dehydroalanine 310 Dehydroascorbate 462.463 7-Dehydrocholestcrol 479-480 Dehydroepiandrosterone (DHEA) 523 Deiodinase DII 715 Delphinidin 97 Delta-aminolevulinate synthase 587 Delta-5 desaturasc 165 Delta-6 desaturase (linolcoyI-CoA desaturase} 122. 165. 676 Delta-9 dcsaturasc (steryI-CoA desaturase} 122

Delta I-pyrroline 5-carboxylate reductase 278, 405 Delta-9-tetrahydrocannabinol 27 Delta-tocopherol 492, 493 Delta-tocotrienol 492 3-Demethylubiquinone-9 3-0methyltransferase 533. 535 Denatonium 16 Dental caries 188, 194, 226, 718 Dental erosion 152 Dental re-mineralization 721 5'-Deoxyadenosylcobalamine 605, 608, 609, 610. 611 Dephospho-CoA kinase 621 Depression 172. 628. 632, 641 Dermatan sulfate 208, 221,307, 550 Dermatans 207, 208. 215. 287 6-Desaturase 589, 779 Desmethylxanthohumol 97 Detoxification cyanide 354-355. 714 glutathione metabolism 354 glycine metabolism 294 pantothenate metabolism 624 Dexfenfluramine 26 DHEA (dehydroepiandrosterone) 524 Diabetes mellitus 143. 188. 194. 268, 531. 635. 640. 708, 743, 744 Diabetic acidosis 150 Diacylglycerol 640 Diacylglycerol O-acyl-transferase 137 Diacylglycerol choline phosphotransferase (CDPcholine:diacyl glycerol choline phosphotransferase) 453 Diallyl disulfide (DADS) 106, 107, 108 Diallyl sulfide 106. 107, 108 Diallyl thiosulfinate (DADSO; allicin) 107. 108 Diallyl trisulfide (DATS) 106, 107. 108 Diarrhea 645. 650, 658 Dicarboxylate translocase 385 3,4-Didehydroretinol 472 Diego blood group antigens 735 2.4-DienoyI-CoA reductase 128. 170 Dietary fiber indigestible polysaccharides 190-191. 197 intestinal microflora processing 44, 191, 197, 213 acetate production 148. 149, 191 metal ion absorption interference 191 probiotic effects 191. 197 vitamin B6 bioavailability 584 Dietary Reference Intakes (DRI) 776 Dietrich pathway 575

Index 7 9 7

Diffusion intestinal transport 45.47 renal tubule 60, 65 Digestion 37-54 esophagus 37 large intestine 43-44 oral cavity 37-38 proteins 250-251,274, 281-282, 310-311,315-316, 340, 349. 390 cooking effects 270 small intestine 40-42 stomach 38-40 Digitalis 102 Diglycerides 119, 122 digestion/absorption 48, 123 Digoxin 103 Dihydrofolate 592, 595, 596 Dihydrofolate reductase 596 genetic variation 772 Dihydrolipoamide S-acetyltransferase 201,229, 529 Dihydrolipoamidedehydrogenase 201, 229, 333, 528, 530. 568 Dihydroorotate dehydrogenase 536 Dihydroorotate oxidase 568 3.4 Dihydrophenylalanine(DOPA) 320, 327 3.4 Dihydrophenylalanine(DOPA) decarboxylase 588 Dihydropteridine reductase 317, 322, 630. 631 Dihydroxyacetone 189 Dihydroxyphenylalanine aminotransferase 588 13,14-Dihydroxyretinol 472 1,25-Dihydroxy-vitaminD 63,478, 748 bone metabolism 487-488, 696 calbindin synthesis stimulation 695 calcium absorption 52.487, 694 cellular effects 488 metabolism 483, 697 phosphate absorption 52 regulation 486-487, 697 transport 482 Diiodotyrosine aminotransferase 590 Dimethylallyltransferase 513 Dimethylarginase (dimethylarginine dimethylaminohydrolase) 400 Dimethylglycinedchydrogcnase 451,569 4,8-Dimethylnonaoyl-CoAoxidation 184 Dipeptidase 690 2,3-Diphosphoglycerate 194 Diphosphomevalonatedccarboxylase 513 Dipropyl disulfide (DPDS) 106 Disulfiram 240 Diterpcnoids 14 Diuretics 18. 656

Divalent metal ion transporter (DMT I ; SLCI IA2) 52, 53, 74, 82. 669, 671,674, 675, 679, 687, 688, 729, 730, 740, 767, 780, 781 Divicine 201 DNA adduct formation 250 heterocyclic amines 90 oxidized fatty acids 131 bromination reactions 755 methylation 345, 539, 540, 726 reactive oxygen species (ROS) damage 460 repair 579 synthesis 675, 690, 699 transcription 690-691. 744, 760 DNA polymerase 690 Docosahexaenoic acid 3, 123, 126, 164-172, 779 dietary sources 165, 167 digestion/absorption 167-168 endogenous synthesis 165-167 function 164, 172 metabolism 168-169 eicosapentaenoic acid 169, 170, 171. 172 nutritional summary 164-165 storage 170-171 transport/cellular uptake 79, 168 Dodecanoyl-CoAdelta-isomerase 128, 162. 170. 177, 178 Dolichol 512 Dopamine 653, 732 hypothalamic appetite regulation 26 synthesis 632, 683 Dopamine DI receptors 26 Dopamine D2 receptors 26, 28 Dopamine-beta-monooxygenase (dopamine hydroxylase) 549, 683 Down syndrome (trisomy 21) 641 Down-regulated in adenoma see CLD/DRA (SLC26A3) Drug absorption 103 DTDST (SLC26A2) 52 Duodenase 41-42, 250 legume inhibitors 270 Duodenum 40 Dyslipidemia 143 4E binding protein (4E-BP1) 262, 369 EAACI/XS-sUAGu 60, 2 5 1 , 2 7 4 EAAT1 (GLASTI: SLC1A3) 81. 274. 303, 385 EAAT2 (GLTI: SLC1A2) 81,274, 303 EAAT3 (SLCIAI) 81,274, 384, 385, 666 EAAT4 (SLCIA6) 81,385 EAAT5 (SLCIA7) 274

EC cells 38 ECL cells 38 Eggs 295, 302. 315. 323,339, 349, 378, 396, 447, 449, 468, 479, 513, 605, 614, 619 Eicosanoids 112, 139-140, 640 Eicosapentadecaenoic acid 123 Eicosapentaenoic acid 165, 169, 170, 171, 172 Elastase (matrix metalloproteinase 12) 698 Elastase 11 251 Elastase IIA 41.47 Elastase lib 41, 47 Elastin 249, 684 Electrolytes absorption 50-51 large intestinal 43, 44 renal processing 57, 58, 59, 65-66 transport mechanisms 45 Electron transport 535-536 Electron-transferring-flavoprotein dehydrogenase 128, 160, 170, 177, 366, 373. 380. 535 Electron-transferringflavoprotein:ubiquinone oxidoreductase 568 Elements 2 Embryo, nutrient transfer 78 Endopeptidase E 251, 311 Endopeptidase EA 47 Endopeptidase EB 47 Endorphins 683 Energy metabolism acetate 152 amino acid oxidation 263, 277, 280. 285,293,299, 305-306. 313, 320, 326-327, 335, 345, 354, 361,369. 376, 382, 388, 394, 401,410, 418 glycine 293 leucine 368 creatine phosphate 428, 43 I. 705 ethanol 236. 239 fatty acids 112. 126-133, 138. 567, 624 conjugated linoleic acid (CLA) 162 docosahexaenoic acid 172 trans-fatty acids 178 fructose 21 O, 21 I, 215 galactose 221 glucose 207 exercise 207 glycolysis 199 200 pentose phosphate cycle (hcxose monophosphate shunt) 199, 201-202 intake bchaviour relationship 25

7 9 8 Index

Energy metabolism (cont.) myristic acid 156-162 niacin 138. 154, 157. 158, 162, 164, 207, 221,224, 230, 236, 313, 706 nutrient sensing 145, 215 pantothenate 138. 154. 157, 158. 162. 164, 207, 230, 236, 624, 706 phosphate esters 705 pyruvate 230, 313 riboflavin 138, 154, 157, 158, 162, 164, 207, 221,230, 236. 567-568, 706 thiamin 138, 201,202, 207, 215, 221, 224, 229, 230, 275, 529. 530, 557, 558, 706 vitamin B6 247, 256. 257, 258, 277. 280, 284, 285, 288, 289, 291, 293, 295, 297, 299, 303, 306, 308, 312. 313, 315, 317, 319, 320, 322, 327, 329, 333, 336, 339, 343, 345, 349, 351,352, 354, 357, 359, 361,362, 363, 365, 371,373,377, 379, 383, 386, 388, 390, 392, 394. 395, 396, 397, 398,400, 401,404, 410, 412, 416, 418, 588-589 Energy restriction 143 Enolase (phosphopyruvate hydratase) 196, 200, 305, 710 Enoyl-acyl-carrier protein reductase 121 EnoyI-CoA hydratase 129, 155, 165, 176, 183,333 EnoyI-CoA isomerase (ECI) 162 Enterocytes 40, 47 amino acid transport 251,274, 316 calcium uptake 694-695 cholesterol transport 516-517 fatty acids absorption 48, 123 galactose metabolism 218 glucose metabolism 198 iron absorption 668-670 molecular transport mechanisms 45-47. 48, 51.52, 198 phosphate absorption 702 protein transcytosis 251 sodium transport 649. 650-651 tight junctions 45 vitamin BI2 uptake 606. 607 water absorption 644-645 Enteroendocrine cells small intestine 40. 41 stomach 38 Enterohepatic circulation 42-43 amino acids 251 copper 682 flavonoids 98 folate 598 molybdenum 735

vitamin A 473 vitamin B12 43, 610 Enterokinase (enteropeptidase) 47, 250 activation 42 Enterostatin 28, 29, 653 ENVZ 426 Enzyme databases 783 Enzyme nomenclature 782 Eosinophil peroxidase 755 Eosinophilia-myalgia syndrome 329-330, 357, 440 Epicatechin 97 Epidermal growth factor (EGF) 197 Epigallocatechin 97 Epiglottis, taste buds 11, 12 Epithelial calcium channel I (ECaC1; TRPV5) 694, 697 Epithelial calcium channel 2 (ECaC2) 694 Epithelial sodium channel (ENaC) 653 Epoxide hydrolases 87 N-epsilon-carboxymethyllysine 358 Equol 99-100 Ergocalciferol (vitamin D2) 481 Ergothioneine 418-419 Eriodictyol 93 Erucic acid 122 Erythropoietin 68, 411,741,767 Erythrose 188, 189 Erythrulose 189, 226 D-Erythrulose reductase 189 Escherichia coli 44, 626 Esophagus 37 Essential elements 2 Essential amino acids 3, 249, 270-271, 315. 322, 328, 339, 357, 363, 371,377 Essential fatty acids 3, 115 Essential nutrients 1, 3, 4 Estimated average requirement (EAR) 776-777 Estrogen 540, 653 Estrogen-regulated genes 102 Ethanol 22, 235-240, 623 absorption 237 compound drugs 239 dietary sources 236 endogenous synthesis 236 excessive intake 236 iron metabolism 671 function 236. 240 habituation/alcoholism 240 metabolism 237 239 5-hydroxytryptamine conversion to 5-hydroxytryptophol 334. 442 acetaldehyde 238 acetate 147. 148. 237 238, 239 free radicals 239. 458

oxidation 237 water generation 644 nutritional summary 236 toxicity 240 transport/cellular uptake 237 Ethanol-induced flushing 239 Ethanolamine-phosphate cytidylyltransferase 139 Ethylene glycol 232 4-Ethyloctanoate 123 Excessive intake 777-778 Exercise appetite stimulation 30 glucose metabolism 207 glutamate metabolism 277 Exhaled air water losses 646-647 Exocarboxypeptidase II 595 ExPASy 783 4F2 (SLC3A2) 63, 72, 81,252, 253, 254, 261,274, 282, 290, 296, 303, 305, 311,316, 319, 324, 331,341,344, 350, 353, 358, 359, 361. 365, 368, 371,372, 375, 378, 380, 391,397, 398, 413,415,450 Facial nerve (cranial nerve VII) 12 Facilitated diffusion 46-47 Factor V 684 FactorVIl 145,699 Factor VIII 145, 684 Factor IX 699 Factor X 507, 699 Famciclovir 737 Farnesoid X-activatedreceptor (FXR) 475 Farnesyl diphosphate 533 Farnesyl-diphosphate farnesyltransferase (squalene synthase) 513 Farnesyl pyrophosphate 512 Farnesyl pyrophosphate synthetase (geranyltransferase) 513. 730 Farnesyltransferase 513 Fat 2 appearance at room temperature 119 emulsification 42 gluconeogenesis 194 intake 112. 776 appetite regulation 29 excessive 112, 143 regulation 137 mobilization during starvation 269 oxidation regulation 145 water generation 644 small intestinal digestion/absorption 42 storage 135-138. 143 distribution 143 gender differences 143

Index 7 9 9

taste sensation 17, 18 Lipids Fatty acid binding protein I 124, 159-160, 167 Fatty acid binding protein 2 124, 159-160, 167 Fatty acid binding protein, liver type (L-FABP) 184, 702 Fatty acid synthase 120, 121,438, 624 Fatty acid translocase (FAT) 126, 155, 160, 168, 176 Fatty acid transport protein I (FATP-I; CD36: SLC27AI) 126, 160, 168, 516 Fatty acid transport protein 4 (FATP4) 160, 168 Fatty acids 111-180 absorption 48, 123-124 branched-chain 112, 113, 116, 117, 119, 123, 128, 129-130 complex lipids 118 synthesis 138-140 deficiency I 12 dietary sources 112, 122-123 digestion 123 eicosanoids synthesis 139-140 endogenous synthesis 119-122 chain elongation 121, 122 desaturation 122 essential 3, 115 excretion 135 fuel metabolism 138 function 112, 138-140 glycerol binding 119 ketogenesis 133-135 long-chain 112, 113, 114 medium-chain 113, 114 membrane composition ratio maintenance 138 microflora production 44, 48, 191,213 monounsaturated 112, 114, 122 nomenclature 112-113, 115 nutritional summary 112 odor 123 overfeeding 143-145 oxidative metabolism 126133. 567. 624 acetate/acetyI-CoA production 148 microsomal omega oxidation 126. 130 131 mitochondrial beta-oxidation 126, 127 128 peroxisomal alpha-oxidation 130 peroxisomal beta-oxidation 126, 128 129, 138 peroxidation/free radical damage 115, 117, 130, 131, 132. 133, 163, 459. 460-461 see also

polyunsaturated 112, 115, 116, 122, 123 propionyl-CoA metabolism 135 protein acylation 139 regulation 137-138 saturated 112, 122 short-chain 44,48, 112, 113, 114,213 storage 135-137 structure 112- I 19 transport/cellular uptake 125-126 blood circulation 125 blood-brain barrier 71-72, 126 carnitine function 436 materno-fetal 79-80, 126 Fatty aldehyde dehydrogenase (ALDH3A2) 130. 183 Feeding behaviour/meal patterns 28 Fenton reactions 458, 460, 530, 549, 675 Ferredoxidase 670, 672 Ferredoxin 483 Ferredoxin-NADP reductase 483, 569 Ferritin 52, 598, 669, 672, 674 Ferroportin I (metal transport protein 1; MTPI; SLCI 1A3) 74, 82, 670, 671,672, 675 Ferroxidase s e e Ceruloplasmin Fetal alcohol syndrome 240 Fetal development 601 Fetuine 731 Fibrates 138 Fibrinogen 145, 699 Fish 123, 180, 339, 396, 479, 481,515, 561,571,572, 605, 635, 636, 656, 668, 712, 713 Fish odor syndrome 450 Fish oils 165, 167 FKBP52 184 Flavanols 92 Flavanones 92 absorption 99 dietary sources 96--97 metabolism 101 structure 93 Flavin adenine dinucleotide (FAD) 561, 562, 563 flavoproteins 566-567 lipid metabolism 568 metabolism 564-565 Flavin adenine dinucleotide synthetase (ATP:FMN adenylyltransferase) 563 Flavin mononucleotide (FMN) 561,562, 563 flavoproteins 567 metabolism 564-565 regulation of synthesis 566 Flavin mononucleotide adenylyltransferase 564

Flavin-containing monooxygenase 449-450 Flavokinase (ATP:RF-5'phosphoryltransferase) 563 Flavones 92 absorption 99 dietary sources 96 metabolism 101 Flavonoids 5, 92-103 absorption 53-54, 97-99 aglycones 93 bitter taste 16 dietary sources 92, 93, 95, 96-97 effects 102-103 antioxidant activity 103,459, 462, 463 drug absorption 103 phase I enzymes 103 phase II enzymes 103 sex-hormone-like actions 102-103 excretion 102 metabolism 99-101 nutritional functions 92 safety of high-dose food extracts 93 storage 101 structures 93, 94-95 sugar-linked 93, 96 sweet tasting 14 transport/cellular uptake 99 blood-brain barrier 74 Flavonols 92 absorption 99 dietary sources 96 metabolism 101 structure 93 Flaxseed (linseed) 165, 167 Fluoride 2, 718-721,776, 778 absorption 53,719 bone metabolism 720-721 dietary sources 719 excessive intake 719, 721 excretion 720 function 718, 720-72 I nutritional summary 718-719 regulation 720 tooth re-mineralization 721 transport/cellular uptake 720 Flurazepam 450 Folate 251,591-601,776, 778 absorption 49. 278, 593-594 activation 596 amino acid metabolism 599 ascorbate interactions 549. 593 bacterial synthesis 601 intestinal microflora 593 carnitine synthesis 362, 433,593 choline metabolism 447, 451 circadian rhythms 593, 601

800

Index

creatine metabolism 428, 593 deficiency 553,593, 777 dietary sources 593 excretion 598 renal tubule processing 64, 598, 599 fetal development 601 formate metabolism 599-601 function 593, 599-601 glycine metabolism 288, 293 histidine metabolism 412, 418 metabolism 568, 595-598, 690 catabolism 598 10-formyl-dihydrofolate 598 5-formyl-tetrahydrofolate 598 10-formyl-tetrahydrofolate 598 genetic variation 770-773,777 5,10-methenyl-tetrahydrofolate 597 5-methyl-tetrahydrofolate 597-598 5,10-methylene-tetrahydrofolate 596 polyglutamyl side-chains 595 nutritional summary 593 purine synthesis 599 reactivation 596 regulation 599 serine metabolism 306 storage 598 thymidylate synthesis 599 transport/cellular uptake 594-595 materno-fetal transport 81 vitamers 592 Folate receptor 1 (FRI) 81,595 genetic variation 772 Folate receptor 2 (FR2) 81,595 Folate receptor 3 (FR3) 595 Folate receptors 595, 599 Foliate papillae 11, 12 Folinic acid 591 Follicle-stimulating hormone (FSH) 548 Folylpoly-gamma-glutamate carboxypeptidase 251 Folylpolyglutamate synthase 595 genetic variation 771 Formaldehyde 599 Formaldehyde dehydrogenase 599 Formate metabolism 599-601 Formate-chloride exchanger 65 Formate-dihydrofolate ligasc 598, 601 Formatc-tetrahydrofolate ligase 183, 596, 601 Formiminotetrahydrofolatc cyclodeaminase 597 Formonctin 93 Formononetin 93 10-Formyl-dihydrofolatc 595,598 5-Formyl-tctrahydrofolatc 592, 595,598 10-Formyl-tctrahydrofolatc 592, 595. 598 5-Formyltetrahydrotblatc cyclo-ligase 598

Formyltetrahydrofolate dehydrogenase 598, 600 Fortification of foods 778 Free radicals 457-463 ascorbate scavenging system 549 breakdown of purine nucleotides 458 carnosine scavenging 418 ethanol metabolism 239, 458 fatty acids damage 131, 132, 133 melatonin scavenging 442, 444 vitamin E metabolism 497 zinc functions 691 s e e a l s o Reactive oxygen species (ROS) Frequency of Inherited Disorders Database 783 Fructose 14, 188, 189, 192. 194, 210-215 absorption 46, 48, 212-213 regulation 215 catabolism via fructose l-phosphate 213 dietary sources 211-212 digestion 212-213 endogenous synthesis 211 glucose 207-208, 211 function 210, 215 intolerance 211,213 metabolism 213, 214 nutritional summary 210-211 renal tubular transport 60, 214-215 sorbitol pathway 214 transport/cellular uptake 213 Fructose 1,6-bisphosphatase 196, 206 Fructose-bisphosphate aldolase 196. 200. 213 Fructose-2,6-bisphosphate 2-phosphatase 196, 206 Fructoselysine 358 Fruit 96, 189, 191, 212, 223, 329, 440, 463,465, 544. 582, 636, 656, 746 flavonoids/isoflavones 92.96, 97 Fruit sugar 210 Fuel metabolism s e e Energy metabolism Fumarase (fumarate hydratase) 275 Fumarate 16. 194. 317. 318. 319. 326. 388, 396 Fumarate hydratase (fumarase) 275 Fumarylacetoacetasc 319. 326 Fungiform papillae 11. 12. 18 Furosemide 653 G cells 38 G-protein-linked receptors 27. 647, 697 taste buds 1I, 14-15 Galactitol 220 Galactoccrebrosidc sulfotransferasc 508 Galactokinase 219

Galactokinase I defect 220 Galactomannans 191 Galactosamine 221 Galactose 14, 188. 192, 194, 216 222 dietary sources 217. 218 digestion/absorption 48, 218-219 endogenous synthesis from glucose 217,218 excretion 60, 221 function 217, 221-222 glycolipid synthesis 221-222 glycoprotein synthesis 221 lactose synthesis 221 metabolism 219-220 nutritional summary 217 transport/cellular uptake 79, 219 Galactosylgalactosylxylosylprotein 3beta-glucuronosyltransferase 731 Galactosylxylosyl 3-betagalactosyltransferase 731 Galanin 653 appetite stimulation 26-27 Gamma-amino butyric acid (GABA) 264, 278 glutamine cycling 286 renal tubule processing 60, 261 synthesis 278. 418. 588 Gamma-amino butyric acid (GABA) transaminase (4-aminobutyrate aminotransferase) 278. 286, 588. 589 Gamma-amino butyric acid (GABA) transporter I 422 Gamma-butyrobetaine hydroxylase 433, 550 Gamma-butyrobetaine 2-oxoglutarate dioxygenase 676 Gamma-carboxyaspartate 249 Gamma-carboxyglutamate 249 Gamma-carotene 467 Gamma-glutamate 5-kinase 259 Gamma-glutamyl hydrolase 595 Gamma-glutamyltransferase (transglutaminase) 278, 698 Gamma-glutamyltranspeptidase 351. 353 Gamma-tocopherol 492. 493 Gamma-tocotrienol 492 Gangliosides 22 I Garlic 106. 107 breath odor 108 Garlic compounds 106-108 dietary sources 106 107 effects 108 metabolism 108 Gases intestinal transcellular diffusion 47 microflora generation from plant foods 190. 197. 198. 212. 219. 223

Index 801

Gastric acid production 38, 39, 666 regulation 38-39 small intestinal neutralization 41, 50-51,661 Gastric lipase 39, 48, 123 Gastricsin (pepsinogen C) 39, 47, 250, 281,390 Gastrin 683 G cells 38, 39 gastric acid secretion regulation 38, 39, 690 Gastrin inhibitory peptide 28 Gastrin receptors 38 Gastrin releasing peptide (GRP) 28 Gastrointestinal tract bacterial microflora s e e Microflora digestive/absorptive processes 37-44 molecular transport mechanisms 45--47 movement of food 44 stretch/chemoreceptors 27 GAT-I 60, 261 GAT-2 63,261,423.424 GAT-3 60, 261 Gelatinase A (matrix metalloproteinase 2) 698 Gelatinase B 698 Genetic databases 783 Genetic variation 769-773 Genistein 92, 93, 98, 100, 102 estrogenic potency 103 sugar-linked (glycosides) 93 Genistin 93, 98 Gentamycin 708 Gephyrin 735 Y-Geranylchalconafingenin 97 Geranyltransferase (farnesyl pyrophosphate synthetase) 513,730 Germanium 2 Ghrelin 26. 28 Gla proteins 507, 508, 699 Glomerular filtration rate (GFR) 57, 431 Glomerulus 57 Glossitis 562 Glossopharyngeal nerve (cranial nerve IX) 12, 21 Glucagon 41,262,402, 691,710 appetite regulation 28 fat metabolism regulation 137 fructose metabolism regulation 215 gastric A cells 38 glucose regulation 205. 206 protein turnover regulation 269, 285, 293.400 sodium regulation 654 Glucagon-like peptide 1 (GLP-I) 28.41 Glucagon-like peptide 2 (GLP-2) 28.41

Glucocorticoid receptor 737 Glucocorticoids 285. 523. 653, 682 Glucokinase 200. 206, 617 Gluconeogenesis 194. 195, 205,206, 262. 269, 731 amino acids utilization 262-263,277. 285, 313 fructose metabolism 213 pyruvate metabolism 228, 229. 313 starvation 207 Gluconolactonase 202 Glucosamine 6-phosphate 215 Glucosamine-fructose-6-phosphate aminotransferase 587 Glucosaminoglycans 221,287 Glucose 14. 188, 192, 193-208 absorption 48, 198 dietary sources 193, 196 digestion 197-198 endogenous synthesis 194-196 amino acids 257 gluconeogenesis 194, 195 excretion 58, 60, 205 renal threshold 60, 205 fructose synthesis 207, 211 function 193, 207-208 carbon source 208 galactose synthesis 217, 218 glycolysis 199-200, 207 anaerobic steps 200 metabolism 199-203, 207 exercise 207 postprandial 206 starvation 207 nutritional summary 193-194 pentose-phosphate cycle (hexose monophosphate shunt) 199, 201-202, 207 regulation 205-207 appetite/satiety 206 hormonal 205-206 storage 204-205 transport/cellular uptake 71, 79, 198-199 UDP-galactose synthesis 208 Glucose alpha-(l>4) oligomers 189 Glucose isomerase/D-xylulose ketolisomerase 211 Glucose transporter 1 (GLUT1; SLC2A1) 49.71,72,79,81, 144, 198, 199, 205, 219, 221,544, 545 inborn absence 71, 199 Glucose transporter 2 (GLUT2; SLC2A2) 46. 48, 71. 198, 199, 205,212, 213,215, 218, 219, 544 regulation 215 renal tubular transport 60. 205, 215, 221

Glucose transporter 3 (GLUT3) 79, 199. 219, 544, 545 Glucose transporter 4 (GLUT4; SLC2A4) 71. 144, 199, 206, 207. 544. 545 Glucose transporter 5 (GLUT5; SLC2A5) 46, 48.60. 212, 213. 215, 544 Glucose transporter 6 (GLUT6) 199 Glucose transporter 7 (GLUT7) 199 Glucose transporter 8 (GLUT8) 199 Glucose transporter 10 (GLUTI0) 199 Glucose transporter 11 (GLUTI 1) 199 Glucose transporters 199 Glucose-alanine cycle 228, 231,263, 269, 313, 368 Glucose-dependent insulotropic peptide 41 Glucose-6-phosphatase 196, 200, 203, 226 Glucose-6-phosphate 196 Glucose-6-phosphate dehydrogenase (G6PD) 202, 783 deficiency 201-202 Glucose-6-phosphate isomerase 196, 200, 203, 226 Glucose-sensing neurons 28, 71, 199 Glucuronate reductase 223, 638 Glucuronylgalactosylproteoglycan beta- 1,4-N-acetylgalactosaminyltransferase 732 Glutamate 244, 245, 249, 254, 272-278 amino acids synthesis 247, 272, 278 alanine 230-23 I, 309-310 arginine 395, 396, 397 aspartate 383 ornithine 259 proline 404, 405 creatine synthesis 428 dietary sources 272, 273 digestion/absorption 274 endogenous synthesis 273, 312 excretion 277 renal tubule processing 60-61,261, 277 function 272, 277-278 gluconeogenesis 263 glutamylation 278 glutathione constituent 277 metabolic effects 278 metabolism 256. 258, 275, 278. 284. 568, 589 neurone excitation modulation/neurotransmitter function 264, 278-275. 307, 369, 376. 588 nutritional summary 272-273 protein synthesis 277 278

8 0 2 Index

Glutamate (cont.) regulation 277 storage 276-277 transport/cellular uptake 80, 25 l, 254, 261,274-275,277 brain 72 umami (meaty taste) enhancement 17, 272, 273 Glutamate ammonia ligase (glutamine synthase) 247, 281. 285, 286 Glutamate decarboxylase 278, 287, 588, 589 Glutamate dehydrogenase 247, 273, 275, 277 Glutamate formimidoyltransferase 597 Glutamate formiminotransferase 416, 598, 599 Glutamate gamma-semialdehyde dehydrogenase 259, 278, 405 Glutamate 5-kinase 278, 396, 405 Glutamate receptors 278 Glutamate-aspartate translocase 274, 385 Glutamate-hydroxide translocase 274 Glutamate-5-semialdehyde dehydrogenase 273, 396, 400 Glutamate-tRNA ligase 278 Glutaminase 63,277, 284, 285, 286, 287 Glutamine 244, 253, 254, 280-287 dietary sources 280, 281 digestion/absorption 281-282 endogenous synthesis 247, 281, 312 enterocyte metabolism 47 function 280. 285-287 gluconeogenesis 194, 269 hexosamine synthesis 287 metabolism 258, 284, 589 neurotransmitter cycling 286 nucleotide synthesis 286 nutritional summary 280-281 pH regulation 287 regulation 285 renal tubule processing 285, 287 storage 285 transport/cellular uptake 254, 282-284 Glutamine N-phenylacetyltransferase 319 Glutamine synthase (glutamate ammonia ligase) 247, 281,285, 286 Glutamine-phenylpyruvate aminotransferase 319, 588 Glutamine-pyruvate aminotransferase 284, 312, 341,589 Glutamine-tRNA ligase 277 Glutamine: fructose-6-phosphate transaminase/isomerizing (GFAT) 208, 215. 287 Glutamyl aminopeptidase (aminopeptidase A) 690

Glutaryl-CoA dehydrogenase 333, 359, 568 Glutathione 277, 345,349, 351,353, 723, 727, 758 ascorbate metabolism 546, 568, 780 copper metabolism 681 flavoprotein-dependent metabolism 546 heterocyclic amines conjugation 89 metabolism 353, 354 oxygen free radicals quenching 354, 462 synthesis cysteine 353 glycine 293. 353 threonine metabolism 297 xenobiotic compounds detoxification 354 Glutathione oxidase 567 Glutathione peroxidase 354, 462, 725 Glutathione reductase 354, 528, 544, 546, 567 Glutathione synthase 293, 353 Glutathione transferase omega I 759 Glutathione S-transferases 319, 326, 354 heterocyclic amines metabolism 89 Glutathione-cystine transhydrogenase 351,352 Glutaurine 426 Gluten, intestinal transcytosis 47 Glvr-I (gibbon ape leukemia virus receptor; Pit-l; SLC20AI) 702, 703, 704, 705 Glyceraldehyde 188 Glyceraldehyde-3-phosphate 194 Glyceraldehyde-3-phosphate dehydrogenase 196, 200 D-Glycerate dehydrogenase/glyoxylate reductase 232, 304-305 D-Glycerate kinase 305 Glycerol 194, 196, 208 fatty acids binding 119 utilization during starvation 269 Glycerol kinase 196 Glycerol 3-phosphate 196 Glycerol 3-phosphate O-acyltransferase 137 Glycerol 3-phosphate dehydrogenase 196 Glycerol-ester hydrolase 48 Glycerone kinase 189 Glycerone-phosphate O-acyltransferase 137 Glycerophosphate phosphatidyl-transferase (CDP-diacylglycerol-glycerol3-phosphate 3-phosphatidyltransferase) 138

Glycerophosphoinositol inositolphosphodiesterase 638 Glyceryl-ether monooxygenase 633 Glycerylphosphocholine phosphodiesterase 447, 449, 451 Glycinamide ribonucleotide (GAR) transformylase 599 Glycine 244, 253, 288-294 bile acid conjugation 293-294 cleavage system 291,366, 373,380, 596 creatine synthesis 293,428 detoxification 294 dietary sources 288, 289 digestion/absorption 289-290 endogenous synthesis 288-289, 312, 588 choline 289, 451 serine 258, 289, 300, 302, 306 threonine 258, 289, 297, 299 function 288, 293-294 glutathione synthesis 293 metabolism 257, 291,292, 530, 568 methionine metabolism 293 neurone excitation modulation/neurotransmitter function 264, 294, 588 nicotinate conjugation 576 nutritional summary 288 porphyrins synthesis 293 protein synthesis 293 purines synthesis 264, 293 regulation 292-293 renal tubule processing 62, 291-292 serine synthesis 257, 291,293 storage 291 transport/cellular uptake 290-291 materno-fetal 80, 255, 291 urinary 5-hydroxyproline biomarker 293 Glycine N-acylase 366 Glycine N-acyltransferase 294, 576, 624 Glycine amidinotransferase 259, 289, 293,428, 431 Glycine aminotransferase 291 Glycine N-benzoyltransferase 294, 576, 624 Glycine N-choloyltransferase (bile acid-CoA:amino acid N-acyltransferase; BAT) 293,426 Glycine dehydrogenase 291,293, 530 Glycine hydroxymethyltransferase 257, 289, 291,293, 297, 302, 306, 312, 588, 589, 596, 599 Glycine N-methyltransferase 293,341. 345 Glycine-cleavage system 292, 293, 366, 373, 380, 596

Index 8 0 3

Glycine-tRNA ligase 293 Glycitcin 93 Glycitin 93 Glycogen 188, 190, 193, 196, 204 depleted stores reconstitution 207 mobilization 204, 207 storage/mobilization regulation 204-205 insulin actions 206 synthesis 204, 278 Glycogen phosphorylase 205, 587, 779 regulation 205 Glycogen phosphorylase phosphatase 205 Glycogen synthase 205 regulation 205, 706 Glycogen-synthase a kinase 205 Glycogen-synthase-D-phosphatase 205 Glycogen-synthetase kinase 3 205 Glycogenin-1 204 Glycogenin-2 204 Glycolipids 221-222 Glycolysis 188, 199, 200, 201,205,206, 207, 731 Cori cycle 200, 207, 227, 229 fructose metabolism 213, 215 pyruvate synthesis 227-228, 229, 230 reducing equivalents 207 serine synthesis from intermediates 301-302 Glycoproteins 221,287 Glycorrhizic acid 659 Glycosaminoglycans 307 Glycosyl phosphatidylinositol (GPI) anchors 640 Glycyrrhizin 14 Glyoxylase I (lactoylglutathione lyase) 297, 767 Glyoxylase II (hydroxyacylglutathione hydrolase) 297 Glyoxylate 232, 244-245, 257, 261,289, 291,297, 304, 313,410 GLYT1 290 GM3 gangliosides 550 GMP (guanosine 5'-monophosphate) 17 Goiter 712, 717 Goiterogens 715 Goitrin 715 Gonadotrophin-releasing hormone (GnRH) 683 Gout 143 Grains/cereals 189, 190, 191,273, 295, 315,323, 349, 357, 390, 396, 413,535, 553. 561,562, 571, 572. 582, 593,636, 693, 708. 709, 728. 734. 742. 754, 766 Grapefruit 92.96 Graves' disease 715

Group-specific component (Gc; vitamin D-binding protein: DBP) 482 Growth arrest specific protein 6 (gas6) 508 Growth hormone 262,402, 431,445, 610 Growth hormone releasing hormone 26 GTP cyclohydrolase 630, 632 Guanidinoacetate N-methyltransferase 293,428 Guanine inserion enzyme (queuine t RNA-ribosyltransferase; RNAguanine transglycosylase; TGT) 626 Guanosine 5'-monophosphate (GMP) 17 Guar gum 191 L-Gulonolactone 3-dehydrogenase 223, 638 L-Gulonolactone oxidase 3, 543 Gulose 188 Gurmarin 14 Gustin (carbonic anhydrase VI) 1I, 690 Gymnemic acid 14 HS +s-ATPase 704 7H6 70 Hallervorden-Spatz syndrome 623 Haloperidol 530 Haptoglobin 770 Hartnup disease 326, 329, 330, 334, 335, 571 HasAh 669 Heat shock proteins 268 Heme 208, 422, 424 flavoproteins in metabolism 568 synthesis 313, 587-588 tryptophan metabolism requirement 329 Heme iron absorption 668, 669 salvage 672, 673 Heme oxygenase 669, 672 Hemicellulose 191 Hemochromatosis 671,674. 687 Hemoglobin 417, 584, 668. 670, 672, 673, 674, 763 oxygen transport 675 Hemorrhagic stroke 499, 781 Hemosiderin 672 Heparan sulfate 307, 518 Heparans 207. 208. 215, 287 Heparin 207, 208, 215, 287, 307 Hepatic fructokinase (ketohexokinase) 213 Hepatic lipase 770 Hepatocyte nuclear factor I 631 Hepcidin 674

Hephaestin 669, 670, 683 Hesperitin 92, 93, 97 Hesperitin-7-rutinoside 97 Heterocyclic amines 85-90 absorption 87 carcinogenicity 86, 90 dietary sources 86-87 digestion 87 DNA adduct formation 90 excretion 89-90 metabolism 87-89 Heterocyclic sweet tasting compounds 13, 14 Hexadeca-4,7,10,13-tetraenoic acid 123 Hexokinase 199-200, 202 magnesium dependent 213 Hexokinase 4 (glucokinase) 200, 206 Hexosamine nutrient-sensing pathway 144, 145 Hexosamines 207, 208, 215, 587 synthesis 287 HFE 671,674, 769, 770, 771 C282Y variant 773 H63D variant 676, 773 HIF-I (hypoxia-inducible factor I) 411, 741 HIFalpha prolyl hydroxylase 411 High-density lipoprotein (HDL) 517 blood-brain barrier transport docosahexaenoic acid 168 fatty acids 126 carotenoids transport 470 oxidative damage 131 placental transport 80, 520 reverse cholesterol transport 519 vitamin E lowering effect 497 vitamin E transport 73,493, 494-495 Hippurate 66 Histamine 264, 412,419, 588, 683 gastric acid secretion regulation 38, 39, 690 gastric ECL cells 38, 39 Histamine HI receptors 206, 419, 420 Histamine H2 receptor inhibitors 50 Histamine H2 receptors 38, 419. 420, 606 Histidase (histidine ammonia-lyase) 415 Histidine 244. 412-420 biomarker of dietary intake 420 copper transport 680 dietary sources 412, 413 digestion/absorption 413-414 excretion 417 function 412. 418-420 metabolism 258, 273,415-417, 589 nutritional summary 412-421 protein histidyl methylation 417 protein synthesis 418

804

Index

Histidine (cont.) storage 417 transport/cellular uptake 414-415 zinc binding 687. 688. 690 Histidine ammonia-lyase (histidase) 415 Histidine decarboxylases 419. 588 Histidine (phenylalanine) aminotransferase 3(14.312.319. 341. 416. 588 Histidine triad/dinucleoside 5'.5'"PI .P3-triphosphate (Ap3A) hydrolase 726 Histidine-tRNA ligase 418. 627 Histone-lysine N-methyltransferase 249. 433 Histones 249. 362 post-translational modification 306 HMB s e e Beta-hydroxy betamethylbutyrate Holo-acyl-carrier protein synthase 120. 624 Homocarnosin 278. 413.417.418 Homocysteine 253. 258. 343. 344. 345 choline metabolism 447.451. 541 cysteine synthesis 349 plasma level 345 remethylation to methionine 599. 610-611 Homogentisate 319. 326. 549 Homogentisate 1.2-dioxygenase 319. 326. 549 Homovanillic acid 319. 326 hORF1 687 Hormone-sensitive lipase 137. 144 Hot spice sensation 22-23 5-HTOI_ (5-hydroxytryptophol) 334. 442 Human Gene Mutation Database 783 Human milk 221. 233. 349. 364. 422. 636. 679. 735. 742. 763 fatty acids 123 transferred iron 673 zinc bioavailability 688 Hunger sensation enteral input 27-28 falling blood glucose response 28. 206 s e e also Appetite Hyaluronan 207. 208. 215. 287 Hydrochloric acid functions 39 secretion 38. 661. 690 regulation 38 39 synthesis 39 s e e also Gastric acid Hydrogen 3 Hydrogen dehydrogenase 767 Hydrogen ion/peptide cotransporter I (PepTl, SLCI5A1) 46. 47.53. 60. 251. 259-260. 274. 277. 282.

285. 290. 291. 296. 298. 303. 305. 311. 312. 316. 330. 340. 349. 357. 364. 371. 375. 378. 384. 390. 392. 396. 406. 413. 687. 723 Hydrogen ion/peptide cotransporter 2 (PepT2; SLCI5A2) 47.60. 251. 260. 274. 277. 282. 285. 290. 291. 296. 298. 303. 305. 313. 316. 340. 353. 357. 364. 371. 375. 378. 384. 390. 392. 396. 406 Hydrogen ions (protons). intestinal cotransport 45-46 Hydrogen peroxide 354. 458. 460. 675 enzymatic dissipation 462. 683 ethanol metabolism generation 239 proline catabolism generation 407 serine catabolism generation 304 Hydrogen/potassium-exchangingATPase 39. 657. 666 Hydrogenase 767 Hydrogenated vegetable oils 175 D-2Hydroxy-acid dehydrogenase 567 (S)-2-Hydroxy-acid oxidase 232. 304. 567. 568 14-Hydroxy-retroretinol 470. 472 3-Hydroxyacyl-CoA dehydrogenase 129. 155. 167. 176. 183. 333-334 Hydroxyacylglutathionehydrolase (glyoxylase II) 297 3-Hydroxyanthranilate 3.4-dioxygenase 333 4-Hydroxybenzoate nonaprenyltransferase 533 3-Hydroxybutyrate 133. 623 s e e also Ketogenesis 3-Hydroxybutyrate dehydrogenase 134. 151. 239. 319. 326. 374. 453 3-Hydroxybutyryl-CoA dehydratase (crotonase) 374. 380 27-Hydroxycholesterol 7-alphamonooxygenase 522 Hydroxycobalamine 607. 608 cyanide antidote 611 Hydroxyethyl radical 239 5-Hydroxyindoleacetic acid 334. 442. 444 3-Hydroxykynurenine glycoside 336 Hydroxyl radical 458. 459. 460. 549. 675 Hydroxylysine 244. 249. 557 Hydroxymethylglutaryl-CoA 366. 513. 624 Hydroxymethylglutaryl-CoAlyase (HMG-CoA lyase) 134. 151. 239. 366 Hydroxymethylglutaryl-CoAreductase (HMG-CoA reductase) 108. 366. 513. 522

HydroxymethylglutaryI-CoAreductase (NADPH) kinase 513. 522 HydroxymethylglutaryI-CoAreductase (NADPH) phosphatase 513. 522 Hydroxymethylglutaryl-CoAsynthase (HMG-CoA synthase) 134. 150. 239. 513 4-Hydroxy-2-nonenal 130 4-Hydroxy-2-oxoglutarate aldolase 62. 261. 410 3-Hydroxypalmitoyl-acyl-carrierprotein dehydratase 121 4-Hydroxyphenylpyruvatedioxygenase 319. 326. 366. 549 2-HydroxyphytanoyI-CoAlyase 130. 183. 558 Hydroxyproline 244. 249. 253. 304. 404 dietary sources 406 digestion/absorption 406 function 410 glycine 294 metabolism 257. 301. 407-410 renal tubule metabolism 62-63. 261. 407-408 synthesis 405 transport/cellular uptake 407. 662 Hydroxyproline oxidase (4-oxoproline reductase) 62. 261. 407 Hydroxypyruvate reductase 305 4-Hydroxy-2.3-trans-nonenal (HNE) 131 5-Hydroxytryptamine s e e Serotonin 5-Hydroxytryptophan 329. 330 5-Hydroxytryptophol (5-HTOL) 334. 442 25-Hydroxyvitamin D 482 25-Hydroxyvitamin D-lalphahydroxylase (CYP27B 1: P450Clalpha) 482.483 25-Hydroxyvitamin D-24R-hydroxylase (CYP24) 485 Hyperkalemia 656. 657. 659 Hyperkeratosis 466 Hyperlipidemia 112. 143. 144. 157. 194. 438 nicotinic acid treatment 574. 579 Hyperoside 93 Hyperphosphatcmia 705 Hypertension 112. 143. 194. 649. 661. 693 Hyperuricemia 112. 143. 145 Hypobromous acid 755 Hypochlorous acid 755 Hypochlorous ion 660. 666 Hypocretin 1 (orexin A) 27 Hypocretin 2 (orcxin B) 27 Hypodipsic hypernatremia 33 Hypokalemia 656. 657. 659

Index 8 0 5

Hypotaurine 422 antioxidant activity 426 metabolism 424 renal tubule processing 60. 261. 424 transport/cellular uptake 423 Hypotaurine dehydrogenase 353,422. 424, 676, 737 Hypothalamus appetite 25, 26 blood glucose regulation (glucostatic theory) 206 osmoreceptors 651 thirst regulation 32-33. 647, 654 sodium regulation 653 vascular organ of lamina terminalis 32, 71,647, 651,654 Hypoxia-inducible factor 1 (HIF-1) 411,741 I cells 41 Ibotenate 17 L-Iditol 2-dehydrogenase (sorbitol dehydrogenase) 208, 211,214 Idose 188 Ileal brake 38 Ileum 40 microflora 44 Imidazolonepropionase 416 IMINO transporter 51,251,261,406, 662 lmipramine 450 Immune function ascorbate 543 biopterin 628. 633 bromine 755 hypochlorous ion 666 iron 668 neuroimmune communication 445 reactive oxygen species (ROS) 460 vitamin A 465, 466 vitamin E 491 zinc 691 lmmunoglobulin heavy chain-binding protein 715 IMP (inosine-5'-monophosphate) 17 lndocarbinol 75 Indoleamine-pyrrole 2,3-dioxygenase 331,334, 442, 676 Indoles 75 Inhibitor 2 205 Innate 'nutritional wisdom' 25 Inorganic pyrophosphatase 52, 127 lnosinc-5'-monophosphate (IMP) 17 lnositol 634 641. 701. 702 absorption 636 brain metabolism 641 dietary sources 635. 636 eicosanoids synthesis 640

endogenous synthesis 635 excretion 640 function 635, 640-641 glycosyl phosphatidylinositol (GPI) anchors 640 intestinal cation absorption inhibition 641 intracellular signaling 638, 640 metabolism 637-639 catabolism 638-639 inositol phosphate interconversions 637 phospholipids 637-638 nutritional summary 635 osmoregulation 640-641 regulation 640 transport/cellular uptake 637 Inositol hexaphosphate (IP6) s e e Phytate Inositol-1,4,bisphosphate l-phosphatase 637, 641 Inositol-1,3,bisphosphate 3-phosphatase 637 Inositol-3,4,bisphosphate 4-phosphatase 637 lnositol- 1,3,4,5-tetrakisphosphate 3-phosphatase 637 Inositol-l,4,5-triphosphate 638, 640 Inositol- 1,4,5-trisphosphate Iphosphatase 637 Inositol- 1,4,5-trisphosphate 5phosphatase 637 Insulin 41. 198, 402, 691,726, 748 appetite regulation 25-26, 27, 28, 29 beta islet cell production 205 blood-brain barrier transport 72 chromium effects 743, 744 electrolyes regulation 653, 659 fat metabolism regulation 137, 206 fructose regulation 215 glucose regulation 205 growth stimulation 369 hormone-sensitive lipase regulation 137 lipoate actions 531 protein synthesis regulation 262 renal tubular transport 60 Insulin receptors 206. 744 Insulin resistance 143, 144, 206 Insulin-like growth factor 1 (tGF-I) 262, 369, 548 Insulysin 206 Integrin 669 Interactions 779-781 deficiencies 779-780 high intake 780-781 Interferon-gamma 691

lnterleukin 1 (IL-I) 262, 691 lnterleukin 2 (IL-2) 684, 691 Interleukin 6 (IL-6) 262, 269, 445, 508, 509, 682, 696 lnterphotoreceptor retinoid-binding protein (IRBP; retinol-binding protein 3) 474 lntracellular transformation 47 Intrinsic factor 38, 39, 50, 606, 610 Intrinsic factor receptor s e e Cubilin Inulin 213 Iodide peroxidase 676, 715 Iodine 712-717, 777, 778 absorption 53, 714 deficiency 712 dietary sources 712, 713 excretion 66, 716 function 712, 716-717 metabolism 714-715 nutritional summary 712 regulation 716 salvage 715 storage 715-716 thyroid hormone synthesis 713, 714-715, 716-717 transport/cellular uptake 714 IREG1 (ferroportin 1) 670 Iregl gene product 52 Iron 667%76, 778 absorption 52, 191,668-670 ascorbate 550, 669, 780 ferric iron (FeS3 +s) 669 ferrous iron (FeS2 +s) 669 heme iron 668, 669 phytate/polyphenols inhibition 641, 668, 669, 780 zinc interactions 687, 780 amino acid metabolism 256, 257, 258, 272, 277, 280, 285, 288, 293, 295, 299, 306, 312, 313, 317, 322, 327, 331,339, 345, 349, 354, 357, 361,383, 388, 390, 394, 395, 401,404, 410, 412, 418, 675%76 carnitine synthesis 362, 433 cholesterol synthesis 512 deficiency 268, 668, 673, 781 pica behaviour 29 in pregnancy 672 dietary sources 668 DNA synthesis 675 ethanol metabolism 236, 675 excessive intake 668 accidental poisoning 676 excretion 673-674 fatty acid metabolism 138, 154, 157, 158, 162, 164, 676 fructose metabolism 215

8 0 6 Index

Iron ( c o n t . ) function 668. 675-676 galactosc metabolism 221 glucose metabolism 207 heme iron salvage 672. 673 intracellular disposition 669 menstrual losses 673 nutritional summary 668 oxidative phosphorylation 675 oxygen transport 675 phytanic acid oxidation 183 protein modification 676 pyruvate metabolism 230 reactive oxygen species (ROS) generation (Fenton reactions) 458, 460, 462, 675 ascorbate metabolism 549 regulation 674-675,683 storage 672 excessive 674, 676 HFE genetic variants 773 sulfur metabolism 676 taurine synthesis 422 thyroid hormones 676 transport/cellular uptake 670-672 blood-brain barrier 73-74 export into blood 670 materno-fetal 82, 671~72 ubiquinone synthesis 533 vitamin A metabolism 465, 675 Iron (II):oxygen oxidoreductase s e e Ceruloplasmin Iron regulatory proteins (IRP) 674 Iron-responsive elements (IRE) 52, 674 Iron-sulfur proteins 348, 355 Isocitrate 194 Isocitrate dehydrogenase 275 Iso-fatty acids 119. 123 Isoflavans 92 lsoflavones 92-103 absorption 53-54, 98-99 antioxidant activity 463 dietary sources 93, 95 excretion 102 food sources 92 metabolism 99-101 nutritional summary 92-93 Isoleucine 244, 247, 249, 377-382 dietary sources 377. 378 digestion/absorption 378 excretion 380-381 function 377. 382 metabolism 257, 379-380, 530, 557. 568. 588 carnitine 432.436 nutritional summary 377-378 protein synthesis 382 regulation 382

storage 380 transport/cellular uptake 378-379 materno-fetal transfer 80. 254. 378-379 lsomaltose 189, 193, 196 Isopentenyl-diphosphatedelta-isomerase 513 lsoprenoids 512 lsoquercitrin 93 lsorhamnetin 101 lsovaleryl-CoA dehydrogenase 128. 366 lsoxanthopterin 632 Jacobson's organ (vomeronasal organ) 8 Jejunum 40 absorptive processes 48 K cells 41 Kaempferol 92, 93, 96 Kallikrein 250 Kallikrein 2 310 Kallistatin 250 Keratan sulfate 208, 221 Keratans 207, 208, 215, 287 Keshan disease 722, 726 3-Ketoacyl-CoA thiolase 129 Ketogenesis 133-135, 150-151,437 brain metabolism 72 Ketohexokinase (hepatic fructokinase) 213 Ketohexoses 189 Ketone bodies ethanol metabolism 239 monocarboxylic acid transporter I at the BBB 72 synthesis from amino acids 257 leucine 369 Ketoses 72, 188, 189 Ketosteroid monooxygenase 569 Kidneys 57-59 Krebs (tricarboxylic acid) cycle 131, 133, 150, 152, 162, 170, 194, 199, 201,202, 207, 229, 230, 239. 256, 257, 258, 275, 284, 312, 319, 326, 386, 436, 530-53 I. 557. 731 anaplerotic reactions 229, 230, 277, 278. 617 generation of intermediates during starvation 269 Kynurenic acid 333. 335 Kynureninase 310. 333. 589. 781 Kynurenine 333 Kynurenine 3-monooxygenase 333. 568. 780 Kynurenine-glyoxylateaminotransferase 289. 319, 333. 588

Kynurenine-oxoglutarate aminotransferase 333,589 KATI 333 KATII 333 L cells 41 L-type transporter (LATI) 72 Lactase 42, 97. 152, 197. 218 deficiency 218 variation in expression 197-198, 218, 770 Lactase synthase 221 Lactase/phlorhizin hydrolase 47 Lactate 194 appetite regulation 28 glycolytic pathway 199, 200, 207 Cori cycle 200, 207 intestinal proton cotransport 45 proline metabolism regulation 410 D-Lactate 297 L-Lactate dehydrogenase 194, 200, 228, 229, 467 Lactitol 14 Lactobacilli 44 Lactoferrin 74, 671 1,4-Lactonase 698 Lactose 14, 189, 192, 193, 196, 208, 218 digestion 197 intolerance 217 synthesis 221 Lactoylglutathione lyase (glyoxylase I) 297, 767 Lamina terminalis vascular organ 32, 71, 647, 651,654 Lanosterol demethylase 513 Lanosterol synthase 513 Large intestine ascorbate absorption 545 electrolyte absorption 51 microflora s e e Microflora water absorption 43, 44, 51,645 LATI 72, 80, 254, 255, 283, 291,297, 303, 311,316, 317, 324, 331, 341,351,354, 365, 372, 378. 379, 392, 407. 415 LAT2 (SLC7A8) 63, 72, 80, 253, 254, 255, 261,282. 283. 290. 297, 303. 305, 311. 316, 317, 319, 324, 331,341,344, 350, 351, 353, 354, 365, 368. 371. 372, 375,378. 379, 380, 391,392. 407, 413 Lathosterol oxidase 513 Lead 575 Lecithin 449 Lecithin-cholesterolacyltransferase (LCAT; phosphatidyl-cholinesterol O-acyltransferase) 138. 518

Index 8 0 7

Lecithin-retinol acyltransferase (LRAT; phosphatidyl-choline-retinol Oacyltransferase) 473,474 Lectins 270 Legumes 93, 122, 190, 212, 218. 302. 315, 323, 384, 396. 413, 447. 449, 553,571,593, 614, 668, 708, 709, 734, 746, 766 lectins 270 protease inhibitors 270 Leigh syndrome 558 Leptin 26, 72. 137, 145, 155, 172. 653 adipocyte release 29 Leucine 244, 245, 249, 254, 363-369 beta cell proliferation 369 dietary sources 363, 364 digestion/absorption 364-365 excretion 368 function 363,369 metabolism 257, 365-368, 568, 588, 611,781 beta-keto pathway 368 carnitine 432, 436 catabolism 365-366, 367, 368, 530, 557 conversion to beta-hydroxy betamethylbutyrate (HMB) 366, 367, 368 neurotransmitter metabolism 369 nutritional summary 363 protein synthesis 369 muscle 262, 369 regulation 368 storage 368 toxicity 369 transport/cellular uptake 365 materno-fetal 80, 254, 365 Leucine aminopeptidase 690 Leucine transporter 251 L-Leucine-aminotransferase (LAT) 365 Leucrose 190 Leucyl aminopeptidase 731 Leukemia 394. 402 Leukotrienes 140, 172, 640 Leukovorin 591 Levodopa 582, 668 Levulose 210 Licorice 14, 659 Lignin 191 Lingual lipase 48, 123 Linoleic acid 123 Linolenic acid 123 Linoleoyl-CoA desaturase (delta-6 desaturase) 122, 165,676 Lipase-related protein I 41 Lipase-related protein 2 41 Lipases 123 fat digestion 48

Lipids absorption 48 blood-brain barrier transport 71-72 hydroperoxide radicals 354 materno-fetal nutrient transport 79-80 see also Fat Lipoamidase (lipoyl-X hydrolase) 527, 528 Lipoamide dehydrogenase 366, 380, 530 Lipoamide kinase (pyruvate dehydrogenase kinase) 230, 530 [Lipoamide (3-methyl-2-oxobutanoate dehydrogenase)] kinase 366, 368, 373, 375, 380, 382, 530 [Lipoamide (3-methyl-2-oxobutanoate dehydrogenase)] phosphatase 366, 368, 373,375, 380, 382, 530 Lipoamide phosphatase (pyruvate dehydrogenase-phosphatase) 230, 530 Lipoate 526-531 absorption 50, 527-528 acetate metabolism 150 acetylcholine regulation 531 amino acid metabolism 256, 257, 258, 272, 277, 280, 285, 288, 293. 295, 297, 299, 306, 312, 313, 315, 320, 322, 327, 329, 331, 333, 339, 345, 349, 354, 357, 361,363,365, 366, 371,373, 377, 380, 383, 388, 390, 394, 395, 399, 401,404, 410, 412, 418, 530 antioxidant activity 463, 530 dietary sources 527 endogenous synthesis 526-527 energy metabolism 201,275, 530 ethanol metabolism 236 excretion 64, 529 fructose metabolism 215 function 526, 529-531 galactose metabolism 221 glucose metabolism 531 intestinal microflora synthesis 527 liver protection 531 metabolism 528 nutritional summary 526 2-oxoglutarate metabolism 530 531 pyruvate metabolism 230 oxidative decarboxylation 229. 529 storage 528 transport/cellular uptake 528 blood brain barrier 73 materno-fetal 82, 528 Lipoate ligase 528 Lipoate transfcrase 528 Lipocalin 1 13 Lipocalins 470

Lipolysis 278 Lipoprotein lipase 125. 160, 168, 504, 517 Lipoproteins 517-518, 706 docosahexaenoic acid 168 endocytic uptake 518, 522 garlic compound effects 108 impact of overfeeding 143 processing in circulation 125, 517-518 scavenger receptor binding 519 trans-fatty acids 175 Lipoyltransferase 528 Lipoyl-X hydrolase (lipoamidase) 527. 528 Lithium 2, 641 Lithium chloride 13 Lithocholate 43 Liver 190 folate storage 598 glycogen 204 vitamin A storage 472-473 vitamin D storage 486 vitamin K storage 507 Liver fatty acid binding protein {LFABP) 184, 702 Liver X receptors (LXR) 475, 520, 522 Locust bean gum 19 I Long-chain acyl-CoA dehydrogenase 128, 155. 160, 170, 176, 177, 496, 567 Long-chain fatty acids 112, 113, 114 Long-chain-fatty acid transport protein 127 Long-chain-fattyacyl elongase 122, 165 Long-chain-fatty-acid-CoA ligase 122, 124, 127, 155, 159, 165, 167, 170, 176, 178, 183 Long-chain-3-hydroxyacyl-CoA dehydrogenase 128, 162, 170 Loop of Henle 57, 58-59, 64 potassium reabsorption 658 sodium chloride reabsorption 65, 652, 664 Low-density lipoprotein (LDL) 517 carotenoids transport 470 docosahexaenoic acid 168 myristate metabolism 157 oxidative damage 131,460 production from very-low-density lipoprotein (VLDL) 143-144, 518 reverse cholesterol transport 519 scavenging receptor binding 519 vitamin E transport 493,494 Low-density lipoprotein (LDL) receptor 80, 518, 520 cholesterol uptake 518, 522

808

Index

Low-density lipoprotein (LDL) receptor (cont.) defects 144 post-translational modification 306 vitamin D uptake 482 vitamin K uptake 504 Low-density lipoprotein (LDL) receptor kinase 306, 522-523 Low-density lipoprotein (LDL)-receptor related protein 1 (LRP: apolipoprotein E receptor) 80. 504. 518. 520 lactoferrin-bound iron transport 671 lipoprotein uptake 518 Low-density lipoprotein (LDL)-receptor related protein 2 (LRP2) s e e Megalin Lp(a) 517 Lumiflavin 562 Lumisterol 479-480 Lung cancer 640 Lutein 504, 780 Luteinizing hormone (LH) 548 Luteolin 92, 93, 96 estrogenic potency 103 Lymphoblastic leukemia, acute 394 Lysine 244. 249. 254, 356-362 carnitine metabolism 357, 361-362. 432. 433,436, 549-550 dietary sources 357 digestion/absorption 357-358 excretion 361 function 357, 361-362 metabolism 258, 359, 360, 432, 436, 557. 589 nutritional summary 357 polyamines synthesis 362 protein/peptide synthesis 361 pyridoxal 5'-phosphate binding 587 regulation 361 storage 359 transport/cellular uptake 251, 358-359 Lysine ketoglutarate reductase 359 L-Lysine oxidase 417 Lysinoalanine 310, 359 Lysophospholipase 447, 449.451,703 Lysozyme 14. 60 Lysyl oxidase (protein-lysine 6-oxidase) 361,684 Lysyl oxidase-I 519 Lysyl oxidase-likc proteins (LOXLs) 684 Lyxose 189 Mabinlin 14 Maclp 684 Macroglycogen 204

Macrophages oxidative burst 460, 497, 666 reverse cholesterol transport 519 Macular degeneration 690 Magnesium 708-711,776 absorption 52, 709 amino acid metabolism 256, 257. 258, 272, 277, 280, 285, 288, 293, 295, 299, 306, 312, 313, 322, 327, 339, 345, 349, 354, 357, 361,383,388, 390, 394, 395, 399, 401,404, 410, 412, 418 bone metabolism 709, 711 calcium balance 711 cholesterol synthesis 512 choline synthesis 447 deficiency 30, 708 dietary sources 708, 709 enzyme cofactor 71 0-711 ethanol metabolism 236 excretion 57, 66, 710 fatty acid oxidation 138, 154, 157, 158, 162, 164 fructose metabolism 215 function 708, 71 0-711 galactose metabolism 221 glucose metabolism 207 nucleotide complexes 710 nutritional summary 708 pyruvate metabolism 230, 313 regulation 710 storage 709 transport/cellular uptake 709 ubiquinone synthesis 533 xylitol metabolism 224 Magnesium salts 16 Magnesium trisilicate 750, 751 Maillard products 250, 357, 358 Malate 16, 194 Malate dehydrogenase 275, 385, 578 Malate-aspartate shuttle 385 Maleylacetoacetate isomerase 319, 326 Malic enzyme 119, 438 Malitol 192 Malnutrition 268 health consequences 268 Malondialdehyde 131 Malonyl CoA 145 Maltase 152 Maltitol 14. 192 Maltodextrin 193, 196 Maltose 189. 193. 196 Maltotriose 189 Malvidin 97 Manganese 728-732. 776. 781 absorption 53, 729 antioxidant activity 457, 730 apoptosis 730 731

blood-brain barrier transport 74 cartilage formation 731-732 deficiency 728 dietary sources 728 729 enzyme cofactor 730 excretion 730 function 728, 730~732 glycolysis/gluconeogenesis 731 nitrogen metabolism 731 nutritional summary 728 peptide hydrolysis 731 regulation 730 storage 729 sulfate metabolism 732 toxic effects 732 transport/cellular uptake 729 vitamin B12 activation 730 xylitol metabolism 224 Manic depression 641 Mannose 188, 194 renal tubular transport 60 Maple syrup disease 366, 373, 380 Margarine 175 Marine-derived tocopherol 492 Materno-fetal nutrient transport 77-82 alanine 311 amino acids 80-81,254-255 anatomical aspects 78 arginine 398 ascorbate 81,546 asparagine 392 biotin 82, 615 calcium 82, 696 carbohydrates 79 chloride 663 cholesterol 80, 520 choline 450 conjugated linoleic acid (CLA) 160 copper 681 cysteine 351 docosahexaenoic acid 79, 168 ethanol 237 fatty acids 79-80. 126, 176 flavonoids 99 fluoride 720 folate 81,595 galactose 219 glucose 199 glutamate 80, 274 glutamine 283-284 glycine 80, 255, 291 histidine 415 iodide 714 iron 82, 671-672 isoleucine 80, 254, 378-379 leucine 80, 254. 365 lipids 79-80 lipoate 82, 528

Index 8 0 9

lysine 359 magnesium 709 manganese 729 methionine 255. 341 minerals/trace metals 82 molybdenum 735 myristic acid 154 niacin 575 pantothenate 82. 621 phenylalanine 255, 317 phosphate 704 potassium 658 proline 407 pyruvate 228 riboflavin 81,563 serine 254, 303 sodium 65 I syncytiotrophoblast layer 78-79 taurine 423 thiamin 81,555 threonine 297 tryptophan 331 tyrosine 324 valine 80, 254, 372 vitamin A 470 vitamin B6 81,585 vitamin BI2 608 vitamin D 482 vitamin E 494-495 vitamin K 504 vitamins 81-82 water 646 zinc 688 s e e also Placenta Matrilysin (matrix metalloproteinase 7) 40. 698 Matrix metalloproteinase 2 (gelatinase A) 698 Matrix metalloproteinase 3 (stromelysin 1) 698 Matrix metalloproteinase 7 (matrilysin) 40, 698 Matrix metalloproteinase 8 (collagenase) 698 Matrix metalloprotemase 10 (stromelysin 2) 698 Matrix metalloprotemase 12 (elastase) 698 Matrix metalloprotemases 40, 704 Maxi-CIS-s channels 558 MDR1 54. 770 MDR2 90, 451 MDR3 (ABCB4) 97, 102 Meat 273,315. 323, 339, 357, 378, 396.413, 422, 428. 433, 447, 468, 515. 527. 535,561,562, 571. 572, 583,605, 619, 635, 636, 668

cooking-related heterocyclic amines generation 86-87 Meaty taste (umami) 17, 270, 272 Mechanoreceptors 21 22 Medium-chain fatty acids 113, 114 absorption 48 Medullary vasomotor centre 32 Megalin 63.64, 73, 80, 470, 473, 520, 606-607, 608, 610, 672, 697 endocytosis 60, 470, 473,482, 484, 486 Mei-Tei-Sho 236 Mellc 444 Melanin 317, 320, 327, 628, 675 Melanin-concentrating hormone (MCH) 27, 653 Melanocortin 26, 29 Melanocortin receptor 1 (Mclr) 26 Melanocortin receptor 3 (Mc3r) 26 Melanocortin receptor 4 (Mc4r) 26, 29 Melanoma 631 Melanotransferrin 74 Melatonin 258. 263,439-445 anxiolytic properties 444 circadian rhythms 444 dietary sources 440 digestion/absorption 440, 442 endogenous synthesis 329. 331,336, 339, 345,440, 441,628, 632 excretion 444 free radical scavenging 442, 444, 463 function 336, 439, 444-445 hormone regulation 445 metabolism 442, 443. 571,676 neuroimmune communication 445 nutritional summary 439-440 receptor binding 444 regulation 444 seasonal rhythm 444 skin pigmentation 444 sperm maturation stimulation 445 storage 442 transport/cellular uptake 442 Melezitose 212 Membrane Pro-X carboxypeptidase 60, 259 Membranes docosahexaenoic acid 164 fatty acid composition ratio maintenance 138 myristate anchor for proteins 157 stability 438 Menaquinone-4 505-506 Menaquinoncs 502. 503 s e e also Vitamin K Menkes protein (copper-transporting ATPase 7A; ATP7A) 45, 53. 679. 681,682. 683

Menthol 23 l-Menthone 23 Meprin A 690 Mercaptopyruvate 352-353. 355 Mercaptopyruvate sulfurtransferase 353, 355 Mesentery 40 Metaboric acid 746 Metabotropic glutamate receptor 4 17 Metal ion chelation 462 Metal transport protein 1 (MTPI; ferroportin; SLCI IA3) 74, 82, 670, 671,672, 675 Metal-response element-binding transcription factor 1 (MTF-I) 682, 689 Metallic taste sensation 17 Metallochaperones 681 Metallothionine 53,781 copper transport/regulation 679, 680, 681,682 zinc transport/regulation 687-688, 689 Methamphetamine 458 Methanol 22 5,10-Methenyl-tetrahydrofolate 597 Methionine 3,244, 249, 338-346 acidity generation 346 choline synthesis 447 cysteine synthesis 248, 339, 341,343, 344, 345,447 dietary sources 339 digestion/absorption 340-341 excretion 343-344 function 339. 345-346 metabolism 258, 341-342, 624 glycine 293 oxidative damage repair 343 SAM (S-adenosylmethionine) cycle 341-342, 345, 599 transsulfuration 341. 343, 344, 589 methyl group donation 345, 541 nutritional summary 339 polyamines synthesis 345-346 protein synthesis 345 regulation 344-345 storage 343 transport/cellular uptake 255. 341 Methionine adenosyltransferase (MAT: ATP:L-methionine-Sadenosyltransferase) 341,345 Methionine aminopeptidase 741 Methionine synthase (5-methyltetrahydrofolatehomocysteine methyltransferase: MTR) 70, 341,342. 541,597, 599. 611,730, 772 Methionine synthase reductase 342. 541. 568, 608.611

810 Index

Methionine-tRNA ligase 345 Methotrexate 594, 596, 632 N-Methyl-L-amino acid oxidase 568 N-Methyl-D-aspartate (NMDA) 264, 588 synthesis 383,388 N-MethyI-D-aspartate (NMDA) receptor 294, 307, 331,333, 388, 588 2-Methyl-CoA dehydrogenase 128 Methyl-coenzyme M reductase 767 2-Methyl-3-hydroxybutyryl-CoA dehydrogenase (MHBD) 128 N-Methyl-nicotinamide (NMN) 66 [3-Methyl-2-oxobutanoatedehydrogenase (lipoamide)] kinase 366, 368, 373, 375. 380, 382. 530 [3-Methyl-2-oxobutanoatedehydrogenase (lipoamide)] phosphatase 366, 368. 373,375,380, 382, 530 5-Methyl-tetrahydrofolate 592, 594, 595 metabolism 597-598 2-Methylacyl-CoA dehydrogenase (branched chain acyl-CoA oxidase) 129, 183, 568 Methylation 539-541 S-adenosylmethionine (SAM)dependent 345, 539, 540-541 DNA 345, 539, 540-541,726 methyl group sources 452-453, 541 Methylcholine 435, 448--449 Methylcobalamine 605, 607, 608 Methylcrotonyl-CoA carboxylase 366. 615,617 Methyldehydroalanine 305 Methyldopa 668 5,10-Methylene tetrahydrofolate 592, 595, 780 metabolism 596 Methylene tetrahydrofolate cyclohydrolase 596 Methylene tetrahydrofolate dehydrogenase 596, 601 Methylene tetrahydrofolate reductase 568. 596, 601 genetic variation 772, 777 Methylglutaconyl-CoAhydratase 366 Methylhistidine 249, 417, 420 Methylmalonic semialdehyde dehydrogenase 373. 374 Methylmalonyl-CoAmutase 135, 183,611 Methylnaringin 16 Methylphenyltetrahydropyridine N-monooxygenase 569 5-Methyltetrahydrofolate 541,599 5-Methyltetrahydrofolate-homocysteine methyltransferase (MTR: methionine synthase) 70, 341, 342. 541. 597, 599. 611,730 genetic variation 772

5' Methylthioadenosine(MTA) 346 5'Methylthioadenosinephosphorylase 346 Mevalonate kinase 513. 741 Microflora 44 acetate production 148, 149 biotin synthesis 44, 614 breast-fed infants 190 choline metabolism 450 ethanol synthesis 236 fatty acid metabolism 48 folate production 593 gas-inducing plant foods 190, 197, 198, 212, 219, 223 ileum 44 'indigestible' polysaccharides metabolism 191 large intestine 44 lipoate synthesis 527 nickel metabolism 767 thiamin production 553 vitamin K production 44, 502, 503 Microsomal ethanol oxidizing system (MEOS) 148, 238-239, 240 Microsomal fatty acid omega oxidation 126, 130-131 Microsomal triglyceride transfer protein (MTP) 124, 160, 167 Microvilli 40 Milk s e e Dairy products Mineralocorticosteroid receptor (NR2C2) 659 Mineralocorticosteroids 523, 646 Minerals 643-767 absorption 51-53 dietary fiber interference 191 blood-brain barrier transport 73-74 materno-fetal nutrient transport 82 renal processing 58 Minimal nutrient requirements 776--777 Miraculin 14 Mitochondria acetyl-CoA oxidation 13 I aspartate translocation (malate-aspartate shuttle) 385 beta oxidation 126. 127-128, 567-568 carnitine functions 436, 437 conjugated linoleic acid (CLA) 160-162 docosahexaenoic acid 168 phytanic acid 183 trans-fatty acids 176-178 glycolysis 200 ketogenesis (ketone body metabolism) 133-135 myristic acid metabolism 155 oxidative decarboxylation 229-230 propionyI-CoA metabolism 135 thiamin actions 558

urea synthesis 259 vitamin K oxidation 509 Mitochondrial capsule selenoprotein (MCSP) 726 Mitochondrial enoyl-CoA isomerase (MECI) 162 Mitochondrial phosphate carrier (SLC25A3) 704 Mitogen-activatedkinases 459, 488 Mitoquinone 532 Mixed micelles 123, 154, 159, 167, 168, 180, 449 bile acids 523 cholesterol transport 516, 523 fat-soluble vitamin transport 50, 468, 469, 481,493, 504, 523 phospholipids transport 702 Mobilferrin 671,740 Molecular databases 782-783 Molecular transport mechanisms 45-47, 48 ATP-drivenactive transport 45 chloride cotransport 46 exchangers 46 facilitated diffusion 46-47 intracellular transformation 47 paracellular diffusion 45 proton cotransport 45-46 sodium cotransport 45 transcytosis 47 unmediated transcellular diffusion 47 Molybdenum 733-737, 777 absorption 53, 734 cofactor 735, 736, 737 copper complexes 737 cysteine metabolism 349, 354, 737 dietary sources 734 ethanol metabolism 236 excretion 735-736 function 734, 737 hormone-like effects 737 metabolism 735 methionine metabolism 339, 345 nutritional summary 734 regulation 736 storage 735 taurine metabolism 422, 424, 737 transport/cellular uptake 734--735 Molybdopterin 424, 735 Molybdopterin synthase 1 735 Molybdopterin synthase 2 735 Molybdopterin synthase sulfurylase 735 Molybdopterin synthases 735 Monellin 14 Monoacylglycerol lipase 435 Monoamine oxidase A (amine oxidase) 442. 565, 567, 569 Monoamine oxidase B 567, 569

Index 811

Monocarboxylic acid transporter I (MCTI; SI.CI6AI) 45.48.49. 72. 149. 150, 152, 200, 207, 228 Monocarboxylic acid transporter 2 (MCT2; SLCI6A2) 150. 152. 228. 230 Monocarboxylic acid transporter 3 (MCT3) 150 Monocarboxylic acid transporter 4 (MCT4) 150 Monocarboxylic acid transporter 6 (MCT6) 230 Monodehydroascorbate reductase 463, 546 Monoglyceride lipase 137 Monoglycerides 119, 122 Monomethyl arsonic acid methyltransferase 759 Monophenol monooxygenase 320, 327, 333, 681,683 Monosaccharides 48 Monosodium glutamate (MSG) 17, 18, 273 Monoterpenoids 14 Monounsaturated fatty acids 112, 114, 122 Motilin-related peptide 38 Mottled appearance of teeth 721 mtl 444 MT2 444 mTOR 369 Mucins 221,287, 669. 731 Multidrug-resistance protein 1 (MRPI; ABCCI) 54, 87. 97. 102. 595. 759 genetic variation 771-772 Multidrug-resistance protein 2 (MRP2; ABCC2; cMOAT) 45.49. 54. 66, 75, 87, 97, 593, 594, 664 genetic variation 771 Multidrug-resistance protein 3 (MRP3; ABCC3) 87, 98 Multidrug-resistance protein 5 (MRP5; ABCC5) 98 Multidrug-resistance protein 6 (MRP6; ABCC6) 98 Multifunctional protein 2 (MFP2) 129. 130. 155. 165. 178. 183 Multiple inositol phosphate phosphatase 637 Muscle calcium in contraction 698 docosahexaenoic acid uptake 168 energy metabolism 207 fatty acids uptake 125-126 trans-fatty acids 175 gluconeogenesis 263 alanine cycle 263

glucose metabolism 199, 200 glycogen 204 glycogenin 204 glycolysis 207 protein synthesis 262. 369, 376 Muscle mass 259 Mushrooms 189. 197. 481. 614. 763 Musk-like odor sensitivity 9 MXR (ABCP; BCRP; ABCG2) 82 Myelin 221,306, 706 docosahexaenoic acid 172 Myelin basic protein 386, 400 Myeloperoxidase 442.460, 666 Myo-inositol 634, 635, 637 renal tubule 59 Myo-inositol I-kinase 637 Myo-inositol oxygenase 638 Myo-inositol-1(or 4)-monophosphatase 637, 641 Myo-inositol-l-phosphate synthase 579, 635 1 D-Myo-inositol-tetrakisphosphate 1-kinase 637 1 D-Myo-inositol-tetrakisphosphate 5-kinase 637 1 D-Myo-inositol-triphosphate3-kinase 637. 698 1D-Myo-inositol-triphosphate 5-kinase 637 1D-Myo-inositol-triphosphate 6-kinase 637 Myoglobin 668, 672 oxygen transport 675 Myosin 249, 362,417, 433 Myosin light-chain kinase 698 Myricetin 93, 96 Myristic acid 123, 153-156 absorption 154 dietary sources 154 digestion 154 endogenous synthesis 154 excretion 155 function 154, 156-157 metabolism 155 nutritional summary 154 protein acylation 139 regulation 155-156 storage 155 transport/cellular uptake 154 NADH dehydrogenase (respiratory chain complex I) 675 NADH reductase (respiratory chain complex 1) 567 NADH-ubiquinone oxidoreductase 624 NAD(P)-arginine ADP-ribosyltransferase 579

NAD(P)H:quinone oxidoreductase 1 497, 499, 505 NAD(P)H:quinone oxidoreductase 2 505 NADPH dehydrogenase 568 NAD(P)H dehydrogenase 568 NADPH dehydrogenase 780 NAD(P)H dehydrogenase 780 NADPH-cytochrome c2 reductase 567 NADPH-ferrihemoprotein reductase (NADPH-cytochrome P450 oxidoreduetase) 237, 471,567, 672 Nalmefene 240 Naltrexone 240 Naringenin 74, 92, 93, 96, 97, 99, 101 cytochrome P450 3A4 (CYP3A4) inhibition 103 estrogenic potency 103 sugar-linked (glycosides) 93 Naringin 15, 16. 93, 96, 97 Narirutin 93 NATI 691 NCX 1 (sodium/calcium exchanger; SLC8A 1) 695 Neopterin 630, 632, 633 Nephron 57 Neprilysin 690 Neural tube defects 598, 601,640, 770 Neuroimmune communication 445 Neuronal function biopterin 632-633 D-serine 307 NeuropeptideY 137, 155. 653, 683 hypothalamic appetite regulation 26, 28, 29 Neuroprostanes 172 Neurotransmitters amino acids 263-264 calcium metabolism 698 glutamate 278 branched-chain amino acid shuttle 369, 376 glutamine cycling 286 glycine 294 hypothalamic appetite regulation 26-27 pyridoxal 5'-phosphate (PLP)dependent synthesis 588 serotonin 336 Neurotrophic activity 108 Niacin 3, 570-579, 776, 777 absorption 49, 574 acetate metabolism 150 ADP-ribose synthesis 575 DNA repair 579 protein modification 579

812 Index

Niacin ( c o n t . ) amino acid metabolism 256, 257. 258. 272, 277, 280. 285, 288, 293, 295. 299, 306, 308, 313, 315, 320. 322, 327, 329, 331,339, 345, 349, 354, 357, 361,363, 365, 371,373,377, 379, 383, 388, 390, 394, 395,396, 398, 401,404, 410, 412, 418 carnitine synthesis 362, 433 cholesterol synthesis 512 choline metabolism 447, 451 deficiency 335,571,781 dietary sources 571. 572, 574 endogenous synthesis 335-336, 571-572, 573,676, 781 energy metabolism 706 ethanol metabolism 236 excessive intake 571 excretion 64, 578 fatty acid oxidation 138, 154, 157, 158, 162, 164 fructose metabolism 215 function 571,578-579 galactose metabolism 221 glucose metabolism 207 glucose tolerance factor 579 hyperlipidemia 574, 579 intestinal proton cotransport 45 melatonin synthesis 439, 440 metabolism 575-577 catabolism 575, 576 salvage pathways 577 nicotinamide adenine dinucleotide (NAD) synthesis 571-572, 575 nucleotide synthesis 575 nutritional summary 571 pyruvate metabolism 230, 313 regulation 578 storage 577 synthesis 329, 333 tryptophan 335 336, 568, 571,572, 575,779 taurine synthesis 422 transport/cellular uptake 81. 574-575 vitamin A activation 465 xylitol metabolism 224 Niacinogens 572 Niacytins 572 Nickel 766-767 absorption 53. 766 dietary sources 766 excretion 767 function 766, 767 nutritional summary 766 toxicity 767 768 transport/cellular uptake 767 Nicotinamidase 575

Nicotinamide s e e Niacin Nicotinamide adenine dinucleotide (NAD) calcium signaling 579 function 578, 579 salvage pathways 577 synthesis 335-336, 571-572, 575 Nicotinamide adenine dinucleotide (NAD) glycohydrolasc (NAD nucleosidase) 575, 577, 579 Nicotinamide adenine dinucleotide (NAD) kinase 575 Nicotinamide adenine dinucleotide (NAD) nucleosidase (NAD glycohydrolase) 575, 577, 579 Nicotinamide adenine dinucleotide (NAD) synthase 335, 572, 575 Nicotinamide adenine dinucleotide phosphate (NADP) 575, 577 calcium signaling 579 function 578 Nicotinamide N-methyltransferase 575 Nicotinamide mononucleotide (NMN) 575, 576, 579 Nicotinate phosphoribosyltransferase 575 Nicotinate-nucleotide adenylyltransferase 335,572, 575 Nicotinate-nucleotide pyrophosphorylase 335, 572 Nicotine 15, 16 Nicotinic acid s e e Niacin Nieman-Pick disease type C 518 Night blindness 466 Night-eating syndrome 444 Nitrate reductase 536 Nitric oxide 264, 569, 731 degradation 536 peroxynitrite generation 459 synthesis 395, 396, 401~,02, 628, 632 Nitric oxide synthase 401,460, 569, 632, 691,765 Nitrites 550 Nitrogen 2 balance 262 excretion 261 262 Nitrosamines 550 Nitrous oxide 342. 541,610, 611 Nobiletin 92, 93, 96 Nociceptors 21-22 Noradrenaline 653. 683 NPCI 518 Nucleoside diphosphatase type B 555 5'-Nuclcotide phosphodiesterase 49 Nucleotide pyrophosphatase 49.64, 553. 562, 574. 577. 623 Nuclcotide synthesis 264, 286, 388 Nucleus of solitary tract 12

Nutrient sensing hexosamines 215 overfeeding 145 Nutrient-sensing response element 1 (NSRE-I) 393 Nutrient-sensing response element 2 (NSRE-2) 393 Nutrients conditionally essential 3, 5 definition 1 essential 1, 3, 4 non-essential confering health benefits 5 Nutrophil function 459 Nuts 180, 329, 440, 447, 493. 614, 636, 641,679, 708, 728, 739. 742, 754, 766 Obesity 143, 188, 194, 268, 269, 649, 661 abdominal 40 blood coagulation factors 145 hyperuricemia 145 insulin resistance 144, 206 nutrient sensing 145 Obsessive-compulsive disorder 641 Occludin 70 Odor/odorants central appetite stimulation (cephalic phase response) 27 nasal irritation 8 olfactory epithelium signaling cascade 7-8 salivation stimulation 37 Odorant-binding proteins 8, 9 Oleoyl-acyl-carrier protein hydrolase 121 Olfaction 7-9 age-related impairment 8, 9 anatomical structures 7-8 molecular mechanisms 8-9 variation in sensitivity 9 Olfactory epithelium 7 manganese uptake 729 odorant signaling cascade 7-8 Olfactory nerve (cranial nerve 1) 7 trauma 8 Olfactory receptors 7, 8 genetic regulation 8 Oligo( 1.4-1,4)-glucanotransferase 204 Oligosaccharidases 47 Oligosaccharides 189 indigestible 190 Olives 96 Omega oxidation 130, 183.495 Omega-3 fatty acids 3, 67, 79, 112. 140. 776 cancer effects 172 cardiovascular disease effects 172 deficiency 165 dietary sources 167

Index 8 1 3

docosahexaenoic acid synthesis 165 excessive intake 165 mental health effects 172 nomenclature 116 recommended daily consumption 165 structure I 15, 116 Omega-6 fatty acids 3.67.79, 112. 14(I, 776 nomenclature 116 structure I 15, 117 Omega-amidase 392 OMIM 783 Oncopterin 632 Onions 92, 96, 99, 106 Ophidine (balenine) 419 Opioid peptides 26 Oral cavity, digestive processes 37-38. 47.48 Oranges 92, 96 Orexigenic mechanisms 25 Orexin 653 Orexin A (hypocretin 1) 27 Orexin B (hypocretin 2) 27 Organic anion carrier (OATKI) 599 Organic anion carrier (OATK2) 599 Organic anion transporter (OATP8: SLC21A8) 97 Organic anion transporter 1 (OATI) 66 Organic anion transporter 3 (OAT3) 66 Organic cation transporter (OCTN2) 62, 251. 261,435,436 Organic cation transporter 1 (OCTI) 66, 450, 451 Organic cation transporter 2 (OCT2; SLC22A2) 66. 72, 451 Organic cation transporter 3 (OCT3) 66 Organophosphates 706 Ornithine 244. 249 endogenous synthesis 247, 248 enterocyte metabolism 47 glutamate metabolism 259, 273 synthesis from proline 257 transport 251 urea cycle 259, 273 Ornithine aminotransferase (OAT) 396 Ornithine carbamoyltransferasc 259. 396 Ornithine decarboxylase 362, 588. 589 Ornithine transcarbamylase 617 Ornithine transporter 1 (SLC25AI5) 259 Ornithine-dclta-aminotransferasc 259 Ornithine-oxo-acid aminotransferase 273,396, 400, 407, 588. 589 Ornithine/citrulline carrier (ORNT 1: SLC25AI5} 358, 396. 399. 400 Orocccal transit time 44 Oropharyngcal mechanoreceptors 32 Osmolytcs 58 59, 426. 64(1

Osmoprotection 59, 61. 261,453. 640 641 Osmoreccptors 651,654 Osmoregulation 647, 653 thirst sensation 32 Osteoarthritis 143 Osteoblasts 696, 707 Osteocalcin 487, 488, 508, 699 Osteoclasts 696, 697, 704 Osteomalacia 479 Osteonectin 699 Osteopontin 487, 488. 699 Osteoporosis 92, 273, 281,288, 295, 308. 322, 329, 339, 349, 357, 364, 371,378, 383,390, 395, 405, 413. 479, 502, 693, 701 Otoconia 699 Otolithic membrane 699 Ouabain-like factor (OLF) 653 Overfeeding fatty acids 143-145 insulin resistance 144 nutrient sensing 145 Oxalate 232-234 calcium absorption inhibition 694 dietary sources 232, 233 digestion/absorption 233-234 endogenous synthesis 232-233. 304 excretion 234 nutritional summary 232 transport/cellular uptake 234 Oxaloacetate 194, 208. 229, 230, 244, 258, 313,731 aspartate metabolism 383-384. 385, 386, 390, 392, 394 synthesis during starvation 269 Oxidative blast 460. 497, 666 Oxidative phosphorylation 567, 578, 675, 683,704. 730 3-Oxoacid CoA-transferase (succinyI-CoA transferase) 319. 326 3-Oxoacyl-aeyl-carrier protein reductase 121 3-Oxoacyl-acyl-carrier protein synthasc 121 2-Oxoadipate dehydrogenase 333 Oxoglutarate dehydrogenase 333. 359, 375, 530 4-Oxoproline reductasc (hydroxyproline oxidase) 62. 261,407 4-Oxoretinoic acid 475 4-Oxoretinol 475 Oxygen sensing 411,741 Oxygen transport 675 Oxyntic cells 38 Oxystcrols 520 Ozone 458

p21WAF/CIPI 641 p53 tumor suppressor 557, 730, 73 I. 765 P-glycoprotein (ABCB1) 75, 87, 97. 99, 102, 497 PABA (para-amino benzoic acid) 598, 601 Palatinose 190 Palmitoyl-CoA hydrolase 435 Palmitoyl-CoA:L-carnitine Opalmitoyltransferase I 128, 155, 160. 170, 176.436 PalmitoyI-CoA: L-carnitine Opalmitoyltransferase II 128, 155, 160. 170, 176, 184, 436 Pancreas 41 endocrine function 41 alpha cells 206 beta cells 205. 369, 440 exocrine secretions 41, 47, 48 protein digestion 250, 311,330 Pancreatic alpha-amylase 47 Pancreatic ducts 41 Pancreatic lipase 41, 48, 50, 123, 159, 167, 180, 468, 516 Pancreatic lipase-related protein 1 48 Pancreatic lipase-related protein 2 48 Pancreatic polypeptide 28. 683 Pancreozymin 41 s e e also Cholecystokinin (CCK) Paneth cells 40 Panic disorder 641 Pantetheine hydrolase 620, 621,622, 623 Pantetheine phosphate adenylyltransferase 621 Pantothenate 43. 348, 619-624, 776 absorption 50, 620 acetate metabolism 150 acyl carrier protein 120, 624 amino acid metabolism 256, 257, 258, 272, 277, 280, 285. 288, 293, 295,299, 306, 308, 313, 315, 320, 322, 327, 329, 331,339, 345,348, 354, 357, 361,363, 365,371,373, 377, 379, 383, 388, 390, 394. 395, 398, 401. 404, 410, 412.418 cholesterol synthesis 512 choline synthesis 447 coenzyme A (CoA) synthesis 621,623 deficiency 619 dietary sources 619-620 energy metabolism 706 ethanol metabolism 236 excretion 64, 623 fatty acid oxidation 138, 154, 157, 158, 162, 164, 624 function 619, 623 624 glucose metabolism 207 intermediary metabolism 623

8 1 4 Index

Pantothenate (cont.) metabolism 621-622 breakdown 622 microflora production 44 nutritional summary 619 protein acylation 624 pyruvate metabolism 230 regulation 623 storage 622 sulfuration 354 transport/cellular uptake 73, 82, 621 xenobiotics detoxification 624 Pantothenate kinase 621 Pantothenate kinase 2 (PANK2) 621,623 Pantothenate-cysteine ligase 354 Pantothenoylcysteinedecarboxylase 621 Para-amino benzoic acid (PABA) 598, 601 Paracellin-I (claudin-16) 710 Paraferritin 669 Paraoxonase (PON) 131 Paraoxonase 1 (arylesterase; PON 1) 131, 462 Paraoxonase 2 (PON2) 131 Paraoxonase 3 (PON3) 131 Parathyroid glands 697, 705 Parathyroid hormone (PTH) calcium regulation 697 bone metabolism 696, 697 renal mechanisms 66, 68 magnesium regulation 710 phosphate regulation 66, 68, 705 renal function regulation 66, 68, 653, 705 secretion 68 vitamin D regulation 63, 68, 486, 487, 697 Parathyroid hormone-related protein (PTHrP) 697 Paraventricular nuclei 32 Parietal cells 38, 39, 661 Parkinson's disease 491,628, 632 Parkinson's disease-like manganese toxicity 732 Parsley 96 PDS (SLC26A4) 52 Pectins 191 Pelargonidin 97 Pellagra 571,781 Penciclovir 737 Pendrin (chloride/iodine transporter; SLC26A4) 66, 664, 714, 716 Penicillamine 668 Pentadin 14 Pentitol pathway 224 Pentose-phosphate cycle (hexose monophosphate shunt) 188, 189, 199, 201-202, 207, 224. 225, 557, 639, 640

Pentosuria 223 Peonidin 97 Pepsin 39, 47, 250, 281,315, 330, 390 Pepsinogen A 39 Pepsinogen C (gastricsin) 39, 47, 250, 281,390 Peptide YY 38, 41 Peptide-aspartate beta-dioxygenase 386, 676 Peptides absorption 47 sweet-tasting 13, 14 Peptidyl-dipeptidaseA (angiotensin l-converting enzyme; ACE) 60, 67, 261,653, 666, 690 Peptidyl-glycine alpha-amidating monooxygenase 549, 683 Perchlorate ion 660 Performance enhancement carnitine 438 pyruvate 231 Peripheral sensory neuropathy 582 Peristalsis 39, 44 Peroxidase (myeloperoxidase) 442 Peroxidation of fatty acids 115, 117, 130, 131,132, 133, 163,460-461 Peroxiredoxins (PRX) 463 Peroxisome proliferator-activated receptors (PPAR) 138, 184, 475 Peroxisomes docosahexaenoic acid metabolism 166, 167, 168 fatty acid alpha-oxidation 130 phytanic acid 183 fatty acid beta-oxidation 126, 128-129, 568 branched-chain fatty acids 129-130 carnitine transport 436 conjugated linoleic acid (CLA) 160-162 phytanic acid 183 pristanic acid 183 trans-fatty acids 178 fatty acid metabolism regulation 138 myristic acid metabolism 155 Peroxynitrite 459 Petunidin 97 Peyer's patches 40 pH regulation 287, 690 sulfur-containing amino acids 346 Phagocyte oxidase 460 PHAS-I 730 Phase i reactions 87 flavonoid effects 103 heterocyclic amines metabolism 87, 89 piperine actions 22 Phase II reactions 87 flavonoid effects 103

heterocyclic amines metabolism 87, 89 piperine actions 22 Phenobarbital 582 Phenolic compounds 74-75 Phenols 16 antioxidant activity 463 Phenylacetate~CoA ligase 319 Phenylalanine 244, 249, 314-320 catcholamines synthesis 317, 320 dietary sources 315 digestion/absorption 315-316 endogenous synthesis 312 excretion 319 function 315,320 melanin synthesis 317, 320 metabolism 257, 317-319, 549, 588, 675 biopterin 628, 632 conversion to tyrosine 248, 317-318, 322, 632 direct transamination 319 tyrosine catabolism 318-319 nutritional summary 315 protein synthesis 320 storage 319 thyroid hormone synthesis 320 transport/cellular uptake 255, 316-317 ubiquinone synthesis 317, 320 Phenylalanine (histidine) aminotransferase 304, 312, 319, 341,416, 588 Phenylalanine hydroxylase 317, 322, 632, 675 Phenylalanine-tRNA ligase 320 Phenylketonuria 318, 322, 331,533, 628, 778 Phenylthiocarbamide (PTC) 16 Phenytoin 582 Pheophorbide 180 Pheophytin 180 Pheromones 8 PHEX 705 Phosphate 3, 700-707, 776 absorption 52, 487, 702-703 calcium complexes 694 buffers 706-707 dietary sources 701-702 bioavailability 701 esters high-energy 705 nutrient metabolism 706 excretion 57, 66, 704-705 function 700, 705-707 magnesium absorption inhibition 709 nutritional summary 700-701 organophosphates 706 polyphosphates 707 protein phosphorylation 706

Index 815

regulation 705 storage 704 transport/cellular uptake 703-704 vitamin D regulation 487, 702, 705 Phosphate-activated glutaminase 286 Phosphatidate cytidylyltransferase 138 Phosphatidate phosphatase 137 Phosphatidyl-choline-retinol Oacyltransferase (lecithin-retinol acyltransferase; LRAT) 473,474 Phosphatidyl-choline-sterol O-acyltransferase (lecithincholesterol acyltransferase; LCAT) 138, 518 Phosphatidylcholine 306, 339, 345,449, 451 enzyme activation 453 synthesis 453,454 Phosphatidylcholine synthase 71 I, 730 Phosphatidylcholine-retinol Oacyltransferase 469 Phosphatidylethanolamine 306, 447 Phosphatidylethanolamine-Nmethyltransferase (PEMT) 447 Phosphatidylinositol 636 eicosanoids synthesis 640 Phosphatidylinositol deacylase (phosphatidylinositol phospholipase A2) 640, 698 Phosphatidylinositol 3-hydroxy kinase 508 Phosphatidylinositol 3-kinase 638 l-Phosphatidylinositol 4-kinase 637, 638 1-Phosphatidylinositol phosphodiesterase 638 Phosphatidylinositol phospholipase A2 (phosphatidylinositol deacylase) 640, 698 Phosphatidylinositol-4,5-bisphosphate 638 Phosphatidylserine 306 Phosphatidylserine decarboxylase 447 Phosphatidylserine synthase (PSS; base exchange enzyme; CDPdiacylglycerol-serineOphosphatidyltrans ferase) 139, 306, 447 Phosphatonin 705 Phosphoadenosyl sulfate (PAPS) 355, 442, 547, 732 Phosphoamidase 429 Phosphoarginine 395, 396, 402, 705 Phosphoenolpyruvate carboxykinase (PEPCK) 194, 206, 731 cytosol isoenzyme (PEPCKI) 194 mitochondrial isoenzyme (PEPCK2) 194

Phosphoenolpyruvate carboxylase 313 Phosphofructokinase I (6phosphofructokinase) 196, 200, 203,206, 226 Phosphoglucomutase 204, 220, 711 6-Phosphogluconate dehydrogenase 202 3-Phosphoglycerate 194, 208, 301 Phosphoglycerate dehydrogenase 301 Phosphoglycerate kinase 196, 200 Phosphoglycerate mutase 196, 200 Phosphokinase C (PKC) 475,498 Phospholipase A2 41, 48, 52, 123, 167, 180, 447, 449, 451,636, 698, 702 Phospholipase B 180 Phospholipase C 640 Phospholipid transfer protein (PLTP) 518 Phospholipid-hydroperoxide glutathione peroxidase 725 Phospholipids 119, 122, 123,451,701, 706 absorption 48, 702-703 adipocyte synthesis 137 chylomicrons 124 docosahexaenoic acid 168 inositol metabolism 637--638 mixed micelles generation 516 phosphate absorption 52 synthesis 138, 306, 447, 568 transport/cellular uptake 703 Phosphomevalonate kinase 513 Phosphopantothenate-cysteine ligase 621 Phosphopyruvate hydratase (enolase) 196, 200, 305,710 Phosphoribosylamine-glycine ligase (GAR synthetase) 293 Phosphoribosylaminoimidazolecarboxam ide formyltransferase (AICAR transformylase) 598, 599, 601 Phosphoribosyl formylglycinamide formyl transferase 598 Phosphoribosyl formylglycinamidine synthase (FGAM synthase) 286 Phosphoric acid 16 Phosphorylase b kinase kinase 205 Phosphorylase kinase 698 Phosphorylation 47 Phosphoserine aminotransferase 302 Phosphoserine phosphatase 302 Photoreceptors 172 Phylloquinone 502, 503 absorption 50 see also Vitamin K Phylloquinone monooxygenase 505 Phytanic acid 119, 126, 129, 130, 179-184 dietary sources 180 digestion/absorption 180-181 function 180

metabolism 181, 183-184, 558 genetic defects 180 nutritional summary 180 storage 184 transport processes 181 PhytanoyI-CoA dioxygenase (phytanoylCoA hydroxylase) 130, 184 Phytanoyl-CoA ligase 183 Phytase 637 Phytate (inositol hexaphosphate; IP6) 3, 636, 701,702 absorption 635 cellular uptake 637 intracellular effects 641 mineral absorption inhibition 52, 53, 191,635, 641 calcium 694 chromium 743 copper 641,679 iron 641,668, 669, 780 magnesium 709 manganese 729 zinc 641,686 Phytochemicals 92 absorption 53-54 Phytoestrogens 102-103 Phytol 119, 179-184 dietary sources 180 digestion/absorption 180-181 function 180, 184 metabolism 183 nutritional summary 180 Phytosterols 5, 515 Pica behaviour 29, 30 Pinealocytes 440, 442 L-Pipecolate oxidase 359 Piperine 22 Pituitary 444 Placenta anatomical aspects 78 folate requirement 601 materno-fetal barrier 78 nutrient transfer see Materno-fetal nutrient transport Plant glycosides 14 Plant sterols 119 Plasma membrane fatty acid-binding protein (FABPpm) 126, 160, 168 Plasminogen activator inhibitor 145 Plasmodium falciparum 62 I Platelet-activating factor 453 PMCAI (calcium pumping ATPase) 52 PMC I b (calcium-transporting ATPase l b; plasma membrane calciumpumpingATPase lb) 45 Podocytes 57, 70 Poly(ADPR) polymerase 579 Polyamine oxidase 567, 569

8 1 6 Index

Polyamine transporter 398 Polyamines catabolism 569 synthesis 345 346, 362, 540 Polyarthritis 152 Polyphenols 16 Polyphosphates 707 Polysaccharides digestible 189-190 indigestible 190-191 Polyunsaturated fatty acids 112. 115. 116. 122. 123 Porphyrins 293, 587 Potassium 655-660 absorption 51,656-657 dietary sources 656 electrical excitation 659 enzyme activation 660 excretion 57, 65. 658-659 function 656, 659-660 nutritional summary 656 radioactive isotope 660 regulation 659 storage 658 transport/cellular uptake 45, 658 Potassium channels 658. 659, 698 Potassium chloride 13 Potassium/chloride cotransporter I (KCCI- SLCI2A4) 664 Potassium/chloride cotransponer 3 (KCC3, SLC12A6) 664 Potatoes, protease inhibitors 270 Pregnancy calcium intake regulation 29 immune tolerance of fetal tissue 334 iron deficiency 672 malnutrition 268 pica behaviour 29 Pregnane X receptor (PXR: steroid and xenobiotic receptor: SXR) 475 Preiss-Handler pathway 575 Primapterin 632 Prion protein 640 Pristane 180 Pristanic acid 119. 129. 130, 180. 181, 182 metabolism 183 storage 184 Proanthocyanidins 14. 92 Procollagen-lysine 5-dioxygenase 361, 549, 676 Procollagen-proline 3-dioxygenase 676 Procollagen-proline 2-oxoglutaratedioxygenase 549. 676 Progesterone 523 Proglycogen 204 Prolactin 402 Prolidase (Xaa-Pro dipeptidase) 731

Proline 254, 404-411 arginine synthesis 257, 395. 396, 397, 410 dietary sources 404, 406 digestion/absorption 406-407 endogenous synthesis 247, 248, 273. 278. 405. 588 glutamate 404, 405 function 404, 410-41 I brain 410-411 metabolism 257, 407-410 neurone excitation modulation/neurotransmitter function 264. 588 nutritional summary 404-405 oxygen sensing 411 protein synthesis 410 redox shuttle 411 regulation 410 renal tubule processing 60, 261 storage 410 transport/cellular uptake 407. 662 urea cycle 410 Proline dehydrogenase (proline oxidase) 407, 408, 411 Proline-tRNA ligase 410 Prolyl aminopeptidase 731 Prolyl 4-hydroxylase 405, 410 Proopiomelanocortin 26 Propionyl-CoA 135. 183,345, 373,374, 379, 380 PropionyI-CoA C(2)-trimethyltridecanoyltransferase 183 Propionyl-CoA-carboxylase 135, 183, 615.617,618 Propranolol 450 6-n-Propyl-2-thiouracil (PROP) 15. 16, 18 Prostacyclin renal function regulation 67 synthesis 140 Prostaglandin E2 renal function regulation 67 vitamin K regulation 509 Prostaglandin F2alpha 548 Prostaglandin H synthase (prostaglandinendoperoxide synthase) 140. 163, 508 Prostaglandins docosahexaenoic acid metabolism 172 renal function regulation 66. 67-68 synthesis 140. 551,640 Prostate cancer 595 Prostate-specific membrane antigen (PSM) 595 PROT 359, 407. 410-411 Protamine kinase 306 Protcases 252

digestive 47, 252 inhibitors in legumes 270 Protein C 507 Protein disulfide isomerase 715 Protein kinase A 204-205, 426 Protein kinase C 422, 425. 617, 623, 698 Protein phosphatase 1 205 Protein S 507 Protein Z 507 Protein-arginine N-methyltransferases 400 Protein-energy malnutrition 268 Protein-histidyl N-methyltransferase 417 Protein-L-isoaspartate (D-aspanate) O-methyltransferase 386 Protein-lysine 6-oxidase (lysyl oxidase) 361,684 Protein-methionine-S-oxide reductase 343 Proteins 2 absorption 47 acylation 139 aspartyl residues modification 386 cooking effects 249-250, 270, 302, 310, 329, 339, 357, 358 digestion 250-251. 270, 274, 281-282, 310-311,315-316. 340, 349, 390 glutamylation 278 histidyl methylation 417 metabolism in starvation 269, 271 phosphorylation 706 post-translational modification 676 aspanate 388 lysine 361 serine 306 tyrosine 327 renal tubular transport 60 selenocysteine incorporation 724 sweet-tasting 13, 14 synthesis 262, 277-278, 285. 293. 299, 306. 313, 320, 327, 334, 345, 354. 361,369. 376. 382, 388, 393-394. 401,410, 418 Proton-thiamin antiporter (SLC 19A2) 49, 73, 81,553. 555, 556 Protons cotransport 45-46 renal tubular reuptake 59 Protoporphyrin-lX 568 Protoporphyrinogen oxidase 568 Protoporphyrinogen-IX 568 Provitamin A s e e Carotenoids Prunin 93 Psicose 189 PSS-I 139 PSS-2 139 Pterin-4a-carbinolamine dehydratase 631

Index 8 1 7

Pterin-4a-carbinolamine dehydrogenase 630 Pteroylpoly-gamma-glutamate carboxypeptidase 49, 594. 601. 690 genetic variation 770 771 Pubmed 782 Purines catabolism 568 free radicals production 458 synthesis 264, 278, 286, 293,383. 388, 593. 598, 599 Putative anion transporter (PAT l: SLC26A6) 41,46, 51. 662 Putrescine 346, 683 Pyridoxal kinase 49, 583. 585,706 Pyridoxal 5-phosphate (PEP) s e e Vitamin B6 Pyridoxamine phosphate oxidase 567, 585, 780 Pyridoxamine-oxaloacetate aminotransferase 388, 585 Pyridoxamine-phosphate oxidase 568, 585 Pyridoxine-beta-D-glucoside hydrolase 49, 218, 584 Pyridoxine-5'-phosphate oxidase 585 Pyrimidines synthesis 264, 286, 383, 388, 568 Pyroglutamyl-peptidase II 413 Pyropheophytin 180 Pyrophosphatase 341,564, 702, 707 Pyrroline 5-carboxylate synthase 396 l-Pyrroline 5-carboxylic acid dehydrogenase (aldehyde dehydrogenase 4) 407, 408 Pyruvate 194, 208. 227-231,244. 623 aerobic metabolism 200-201 amino acid synthesis 230-231 appetite regulation 28 carboxylation 230 dietary sources 227, 228 digestion/absorption 228 endogenous synthesis alanine 257, 312, 313 amino acids 228. 312. 313 carbohydrate 227-228 cysteine 258, 351. 352, 353. 354 leucine 368 serine 228, 304. 305 enzyme cofactor activity 231 excretion 230 function 227. 230-231 gluconcogcnesis 263.313 intestinal proton cotransport 45 metabolism 228-230 nutritional summary 227 oxidative decarboxylation 229-230, 529 performance enhancement 231

regulation 230 transport/cellular uptake 228 Pyruvate carboxylase 152, 194. 230. 313, 615,617, 618. 660, 731 Pyruvate dehydrogenase 201,230, 297, 312, 366. 373,380, 529, 557 Pyruvate dehydrogenase kinase (lipoamide kinase) 230, 530 Pyruvate dehydrogenase-phosphatase (lipoamide phosphatase) 230, 530 Pyruvate kinase 305, 660 Pyruvate translocase 200 6-Pyruvoyltetrahydropterin synthase 630 PYYU3-36u 26 Quercetin 92, 96, 99, 101 antioxidant activity 103 phase II enzyme effects 103 sugar-linked (glycosides) 93 Quereitin 93 Quercitrin 93 Queuine 3. 625-627 absorption 626 function 626-627 intestinal microflora production 44, 626 nutritional summary 626 Queuine tRNA-ribosyltransferase (RNAguanine transglycosylase: TGT: guanine inserion enzyme) 626 Quinine 15. 18, 450 Quinolinic acid 33 I, 333, 335 R proteins 39 Raffinose 190. 197, 198, 212, 218 Ram-I 702, 703, 704, 705 Rapeseed oil 122 rBAT (SLC3AI) 53, 252, 261,277, 281, 290, 296, 303. 305, 311,316. 319, 323. 326, 330. 334, 340, 344. 350. 353, 358, 361,364. 368, 371,378, 380, 387. 390, 396, 407. 413, 723 Reactive oxygen species (ROS) 457. 458-459 free iron reactions 675 glutathione metabolism 354 physiological functions 459-460. 498 tissue damage 457, 460-461 DNA 460 polyunsaturated fatty acids 460-461 vital functions 457 s e e a l s o Free radicals Receptor-associated protein (RAP) 519, 607 Receptor-mediated endocytosis choline 450 docosahexaenoic acid uptake 168 megalin 60. 470. 473, 482. 484. 486

renal tubule 59 trans-fatty acids 176 transferrin-bound iron 671 vitamin B 12 606 Recommended dietary allowances (RDA) 776 Rectal cancer 240 Red blood cell energy metabolism 200, 201 Red wine 38, 97 Reduced folate carrier 1 (RFC-I: SLCI9AI) 49, 64, 82, 554, 555. 557, 594. 595, 599, 601 genetic variation 771 Refsum's disease 180. 184 Rehydration solutions 650 Relaxin 33 Renal anatomy 57-59 Renal function active secretion of food compounds 66 complex nutrients salvage 59-64 distal tubule 59 hormonal regulation 66 68 osmoregulation 640-641 proximal tubule 57-58 ultrafiltration 57 urine concentration regulation 64, 67, 645 see a/so Renal tubule processing Renal glomerular sclerosis 273,281. 288, 295. 308, 322, 329. 339, 349, 357, 364, 371. 378, 383, 390. 395. 405.413 Renal stones 232. 233. 234, 288, 304. 353. 410, 543,721,734, 737 Renal tubule processing 56-68 acetate 152 amino acids 60. 261-262. 277, 285, 287. 291-292. 298-299. 305, 312-313. 319, 326. 334, 344, 353, 361. 368. 375, 380. 386-387. 392. 400, 417 arsenic 759 biotin 64, 616-617 boron 747 calcium 57, 59. 65 66. 696-697 earnitine 62. 435-436 chloride 57, 59, 65,664 choline 451 creatine 430 flavonoids 102 fluoride 720 folate 64, 598, 599 fructose 214-215 galactose 221 glucose 205 inositol 64(} iodine 66. 716

B18 Index

Renal tubule processing (cont.) iron 674 magnesium 57, 66, 710 myo-inositol synthesis 635 niacin 64. 578 nickel 767 oxalate 234 pantothenate 64, 623 phosphate 57, 66, 704-705 potassium 57, 65, 658-659 pyruvate 230 riboflavin 64, 565 sodium 57, 58, 59, 65, 652-653 taurine 60, 261,424 thiamin 556-557 vitamin A 473 vitamin B6 587 vitamin BI2 610 vitamin D 57, 63,484-485,486--487, 697 synthetic activity 484-485 water 646 reabsorption 58, 59, 64-65, 646 Renin 653, 666 renal function regulation 67 Renin-angiotensinsystem 653, 659, 666 Respiratory alkalosis 695 Respiratory quotient 138 Retina 440, 508 Retinal 1 l-cis retinal 473,474 endogenous synthesis 467, 675, 690 visual physiology 473 s e e also Vitamin A Retinal dehydrogenase 568, 675, 780 Retinal dehydrogenase 1 (RALDH 1) 471 Retinal dehydrogenase 2 (RALDH2) 470, 471,473 Retinoic acid 465, 467, 469, 568, 675, 737, 780 absorption 50 all-trans 472 synthesis 470 dietary sources 468 excretion 473 metabolism 9-cis-retinoic acid synthesis 470-471 oxidative breakdown 470, 471 regulation 473 Retinoic acid receptor (RAR) alpha 474 Retinoic acid receptor (RAR) beta 474 Retinoic acid receptor (RAR) gamma 474 Retinoic acid receptors (RARs) 474, 475 Retinoic X receptor (RXR) alpha 475 Retinoic X receptor (RXR) beta 475 Retinoic X receptor (RXR} gamma 475

Retinoic X receptors (RXRs) 475,487, 488. 519, 522 s e e also RXR-like receptors Retinol 240, 690, 780 absorption 50, 468-469 cell signaling 475 dietary sources 468 endogenous synthesis 466--467 metabolism 470-472 all-trans retinoic acid synthesis 470 9-cis-retinoic acid synthesis 470-47 I hydroxylation 472 plasma concentration 469 renal processing 64, 473 transport/cellular uptake 470 s e e also Vitamin A Retinol dehydratase 472 Retinol dehydrogenase (RDH5) 471,474 Retinol O-fatty-acyltransferase (acyl CoA:retinol acyltransferase; ARAT) 473 Retinol isomerase 474 Retinol-binding protein (RBP) 60, 64, 73 Retinol-binding protein 1 (RBPI) 474 Retinol-binding protein 2 (RBP2; cellular retinol-binding protein 2) 468-469 Retinol-bindingprotein 3 (RBP3; interphotoreceptor retinoidbinding protein; IRBP) 474 Retinol-bindingprotein 4 (RBP4) 470, 473 Review articles 782 Rheumatoid arthritis 457 Rhodanase 9 Rhodopsin 473 Rhubarb 232, 233,694 Riboflavin 561-569, 776, 778 absorption 49, 562-563 acetate metabolism 150 activation 564-565 amine metabolism 568-569 amino acid metabolism 256, 257, 258, 272, 277, 280, 285, 288, 293, 295,299,306,308,313,315, 320,322,327,329,331,339, 345,349,354,357,361,363, 365,371,373,377,379,383, 388,390,394,395,399,401, 404, 410, 412, 418, 568-569 biomarker of intake 569 catabolism 564-565 cholesterol synthesis 512 choline catabolism 451 deficiency 562, 779-780 dietary sources 561. 562 energy metabolism 706

ethanol metabolism 236 excretion 64. 565 fatty acid oxidation 138, 154, 157, 158, 162, 164, 567-568 flavoproteins 566-567 fructose metabolism 215 function 561,566-569 galactose metabolism 221 glucose metabolism 207 glutathione-linked reactions 567 heme metabolism 568 hormonal/cell signaling 569 intermediary metabolism 567 lipid metabolism 568 metabolism 564-565 nucleotide metabolism 568 nutritional summary 561-562 oxidative phosphorylation 567 pyruvate metabolism 230 redox reactions 567 regulation 566 storage 565 transport/cellular uptake 73, 8 I, 563 vitamin A activation 465 vitamin metabolism 568 xenobiotic metabolism 569 Riboflavin carrier protein (RCP) 81,563 Riboflavin kinase 564-565, 566 Ribonucleoside-diphosphate reductase 675 Ribose 188, 200, 201 Rickets 479 Ritodrin 450 RNA editing 691 reactive oxygen species (ROS) damage 460 RNA polymerase 691 RNA-guanine transglycosylase (TGT; guanine inserion enzyme; queuine t RNA-ribosyltransferase ) 626 RPA32 730 Rubidium 2 Rumenic acid 161 s e e also Conjugated linoleic acid (CLA) Ruminant fat 154, 158 Ruminant meat 180 Rutin 93, 96 RXR-like receptors 180. 184 S cells 41 $6 kinase (S6KI) 262, 369 Saccharine 14, 15, 17 Saccharopine dehydrogenase 359 Salicylates 67, 294 renal transport 66 Saliva 37 composition 13, 38

Index 8 1 9

flow rate 38 neural control of production 37 tannins interaction 38 taste sensation enhancement 13 Salivary acid phosphatase 38 Salivary alpha-amylase 47 Salivary glands 37 Salty taste 13,654 Sapid molecules 1I, 13 Saponins 5, 14 Sarcoplasmic reticulum 698 Sarcosine dehydrogenase 451,567, 569 Sarcosine oxidase 407, 45 I Saturated fatty acids 112, 122 Scavenger receptor class A 519 Scavenger receptor class B 519 type I (SR-B1) 494, 497, 516, 519, 520 Schilling test 606 Schizophrenia 33, 307 Scurvy 543,548, 778 Seafood 679, 686, 712, 713, 722, 723, 754, 757, 763 Seasonal rhythm 444 Seasonal-affectivedisorder 442 Seaweed 712, 713, 758 Seborrheic dermatitis 562, 614 Sebum 119 Secretin 41 Seeds 191 Selenium 244, 722-727, 776, 779 absorption 53, 723 antioxidant activity 457, 725 cell replication 726 deficiency 722, 724, 726 dietary sources 722, 723 DNA methylation 726 excretion 725 function 722, 725-727 metabolism 724-725 nutritional summary 722-723 storage 725 transport/cellular uptake 723 viral mutation effects 726 Selenium transferase 249, 590 Selenocysteine 53, 244, 249, 310, 722, 723 antioxidant activity 725 incorporation into proteins 724 metabolism 724 synthesis from serine 258, 306, 724 thyroid hormone metabolism 715, 726 Selenocysteine lyase 590, 725 L-Selenocysteineselenide-lyase 310. 724 Selenomethionine 53, 722, 723 Selenophosphate synthase 711,724 Selenoprotein P 723,726 Selenoprotein W 727 Semialdehyde synthase 359

Semicarbazide-sensitiveamine oxidase 297 Semidehydroascorbate 459, 462 Sepiapterin reductase 630 Septicemia and hemochromatosis 773 Serine 244, 245, 249, 254, 300-307 choline synthesis 447 cysteine synthesis 258, 306 dietary sources 300, 302 digestion/absorption 302-303 endogenous synthesis 301-302, 312 from glycine 257, 291,301,302, 589 glycolysis intermediates 301-302 excretion 305 function 300 glycine synthesis 258, 289, 300, 302, 306 glycosaminoglycans 307 metabolism 258, 289, 303-305, 589 catabolic pathways 304-305 L-serine-sulfate 305 nutritional summary 300 phospholipid synthesis 306 post-translational protein modification 306 protein synthesis 306 pyruvate synthesis 228, 304, 305 selenocysteine synthesis 258, 306, 724 D-serine 588 neuronal function 307 synthesis in brain 302 storage 305 transport/cellular uptake 80, 254, 303 Serine dehydratase 228, 305, 589 Serine hydroxymethyltransferase 433, 589 Serine C-palmitoyltransferase 306 Serine racemase 302, 307, 588 Serine-glyoxylate aminotransferase 304 Serine-pyruvate aminotransferase 304, 312 Serine-sulfate ammonia-lyase 305 Serine-tRNA ligase 249, 306, 724 Serotonin (5-hydroxytryptamine) 258, 264, 329, 588 catabolism 334, 442, 571,676 metabolites in urine 334 gastric EC cells 38 hypothalamic appetite regulation 26, 30 melatonin synthesis 336, 440, 441,444 neurotransmitter/laormone-likeactions 336 small intestine secretion 41 synthesis from tryptophan 329, 331, 336, 675 biopterin 628, 632 taste bud receptor signaling I I, 15

L-Seryl-tRNASSecs selenium transferase 306, 724, 779 Shigellaflexneri 362, 627 Short-chain fatty acids 44, 48, 112, 113, 114,213 absorption 48 Sialyllacto-N-neotetraose c 190 Sialyllacto-N-tetraose a 190 Sialyllacto-N-tetraose b 190 Y-Sialyllactose 190 6'-Sialyllactose 190 Sibutramine 26 Sickle cell anaemia 30 Signaling calcium 579, 698 central gustatory pathways 12 inositol phospholipids 638, 640 olfactory epithelium 7-8 phosphate esters 705 retinol 475 riboflavin 569 taste bud receptor 11, 15 Silicon 750-752 absorption 751 dietary sources 750, 751 excretion 752 function 750, 752 nutritional summary 750 storage 751-752 transport/cellular uptake 751 SjSgren-Larson syndrome 180 Skin pigmentation 444 Sleep apnea 143 Sleep physiology 756 Small intestine absorption 40-42 brush border membrane enzymes 43, 197 digestive processes 41-42, 250 gastric acid neutralization 41, 50-51, 661 gross anatomy 40 microanatomy 40-4 I pancreatic secretions 41,250 Smell-blindness 9 Smoking 548 cessation 17 SN1 282, 286, 391,392 SN2 282, 291,391 Sodium 649-654 absorption 51,649-651 central regulation 33, 653 dietary sources 649 enzyme cofactor 654 excretion 651-653 renal handling 57, 58, 59, 65, 652-653 sweat 653

820

Index

Sodium (cont.) function 649. 654 nutritional summary 649 osmoregulation 653 thirst sensation 32 regulation 653-654 salty foods 29, 649 intake control 654 storage 651 transport/cellular uptake 45, 651 Sodium channels 651. 654 taste cells 13 Sodium chloride 13. 18 Sodium cotransport (sodium-driven bulk transport) 649, 65 I. 654 gastrointestinal tract 45, 51, 198, 251, 645,650 renal tubule 65 Sodium-dependent amino acid transporters 63 Sodium-dependent ascorbate transporter 1 (SVCTI; SLC23A2) 81. 544, 545. 546 Sodium-dependent ascorbate transporter 2 (SVCT2: SLC23AI) 73, 81, 544, 545. 546 Sodium-dependent multivitamin transporter (SLC5A6) 50. 64, 73, 82. 527, 615. 616. 617, 620, 623 Sodium-dependent myo-inositol transporter (SLC5A3) 59, 636. 637, 640 Sodium/bicarbonate cotransporter (NBCI; SLC4A4) 65, 651 Sodium/bile acid cotransporter (SLCIOA2) 522 Sodium/calcium exchanger (NCX I ; SLC8AI ) 695 Sodium/calcium exchanger (NCX2; SLC8A2) 696 Sodium/chloride cotransporter (SLCI2A3) 664, 710 Sodium/dicarboxylate cotransporter (NaDC-I; SLCI3A2) 60 Sodium/glucose cotransporter 1 (SGLTI: SLC5A I ) 42, 45.48.49, 51, 97, 198. 205. 218. 221. 544. 583, 645. 649. 654 Sodium/glucose cotransporter 2 (SGLT2" SLC5A2) 218, 654 Sodium/glucose cotransporters 60 Sodium/hydrogen exchanger 1 (NHEI: SLCgAI) 63, 65,651 Sodium/hydrogen exchanger 2 (NHE2: SLC9A2) 41.45.51. 650. 653 Sodium/hydrogen exchanger 3 (NHE3: SLC9A3) 41.45.51. 650, 651. 652. 664

Sodium/hydrogen cxchangers 41.45.46. 651 Sodium/iodide symporter (SLC5A5) 53, 714, 716 Sodium/phosphate cotransporter type I (Nptl; SLC17AI) 52, 702, 705 Sodium/phosphate cotransporter type lla (NaPi3; SLC4AI) 66, 705 Sodium/phosphate cotransporter type lib (NaPi3B; SLC34A2) 52, 702. 705 Sodium/phosphate cotransporters type 111 702, 703, 704, 705 Sodium/potassium-exchanging ATPase 650, 651. 654 hormonal regulation 653 intestinal transport mechanisms 45. 51,251 renal tubule basolateral membrane 59, 65. 652, 658. 659, 664 Sodium/potassium/chloride cotransporter (NKCCI) 39, 651,666 Sodium/potassium/chloride cotransporter (NKCC2) 664 Sodium/potassium/chloride transporter 1 (SLCI2A2) 651,663 Sodium/potassium/chloride transporter 2 (SLC12AI) 652, 658 Soft palate, taste buds 11, 12 Solute load 646 Somatostatin 38. 690 Sorbitol 14, 189, 192, 551 conversion to fructose 214 renal tubule 58 synthesis from glucose 211 Sorbitol dehydrogenase (L-iditol 2dehydrogenase) 208, 211,214 Sorbose 189 Sour taste 16-17 Soy bean curd (tofu) 693, 694 Soy beans 92, 93,273. 315,323. 378, 384. 614, 734 flavonoids 92, 93.95 Soya oil 180 Spermatozoa 445,640, 726 creatine phosphate 431 fructose metabolism 208, 211,213 Spermidine 341,346. 540, 683 Spcrmidine synthase 345-346 Spermine 341,346, 540, 683 Spermine synthase 346 Sphinganine oxidasc 568 Sphingolipids 306 Sphingomyelin 453 Sphingosine 568, 589 Spinach 180. 223,232. 233, 468. 502, 504, 694. 708. 709 Squalene 512, 513

Squalcnc monooxygenase (squalene epoxidase) 108, 513. 568 Squalene synthase (farnesyl-diphosphate farnesyltransferase) 513 Stachyose 190, 197, 198. 212, 218 Staphylococcus ato'eus 553 Starches 188, 189-190, 193, 196 digestion 47. 197 resistant 190 Starvation 196, 262, 268-270 acetate metabolism 150 alanine-glucose cycle 228. 231, 313 biopterin response 632 brain metabolism 151 glucose metabolism 207 health consequences 268 physical/metabolic adaptations 268 protein availability 270 protein quality 271 protein turnover 269 amino acid metabolism/transport 282, 285, 299, 303, 311, 313, 368, 375, 380, 382, 393,410 enzyme changes 269 Statins 103 Stearidonic acid 123 Stearyl-CoA desaturase 138, 158, 676 Steroid 1 l-beta-monooxygenase 550 Steroid hormones 523,550. 569 Steroid and xenobiotic receptor (SXR: pregnane X receptor: PXR) 475 Steroidal plant compounds, sweet tasting 14 Sterol O-acyltransferase (ACAT/ACAT2) 516.518 Sterol carrier protein X (SCPx) 130, 167, 183 Sterol esterase 50, 468 Sterol 27-hydrolase(CYP27) 520 Sterol regulatory element-binding proteins (SREBPs) 137-138. 522 Sterols 516 see also Phytosterols Steryl-CoA desaturase (delta-9 desaturase) 122 Stomach absorption 39 anatomical structure 38 digestive processes 38-40. 47. 250 motility 39-40. 152 secretory function 38-39 water absorption 51 Streptococcus pneumoniae 226 Stress alanine-glucosc cycle 313 anaino acid metabolism/mobilization 285. 313 Stroke 143, 499. 781

Index 821

Stromelysin 1 (matrix metalloproteinase 3) 698 Stromelysin 2 (matrix metalloproteinase 10) 698 Succinate 194 Succinate dehydrogenase complex 275, 535, 567 Succinate semialdehyde dehydrogenase (ALDH5A1) 278, 286. 590 Succinate-CoA ligase 275 Succinyl-CoA 208, 257. 258. 373, 374, 623 Succinyl-CoA synthase 275 Succinyl-CoA transferase (3-oxoacid CoA-transferase) 319. 326 Sucralose 14 Sucrase 152 Sucrase-isomaltase 197. 198. 212 Sucrose 14. 15, 18, 189, 192. 193, 196, 211 digestion 212 Sucrose alpha-glucosidase 47.212 Sugar substitutes s e e Sweeteners. artificial Sugars 188-189 structure 188 sweet taste 191-192 Suicide 172 Sulfate 3. 550 generation from sulfite 676, 737 metabolism 732 Sulfate adenylyltransferase 355 6-Sulfatoxymelatonin (6-SMT) 442,444 Sulfinoalanine decarboxylase 353,422, 424. 426, 588, 589 Sulfinpyrazone 571 Sulfite 676, 737 Sulfite oxidase 352, 424, 676, 737 Sulfotransferases heterocyclic amines metabolism 89 melatonin metabolism 442 Sulfur 3, 244 absorption 52 Sulfur amino acids 340 s e e a l s o Cysteine: Methionine Sulfurotransferase STIA3 442 SULT (phenol sulfotransferase) 97 SULT 1A 1 (phenol sulfotransfcrase A 1) 103 SULTIA2 89 SULT 1A3 (phenol-sulfating phenol sulfotransferase 1) 89 SULT1EI 89 Superoxide anions 458. 462. 675,683 Superoxidc dismutase 74. 460. 462. 675, 681. 683, 691,730, 767 Superoxide dismutasc 1 (SODI) 691 Superoxide dismutase 3 (SOD3) 691

Supertasters 18 Supplementation 778 Suprasterol 480 Sweating 647, 649 calcium losses 696 chloride losses 661,664 665 sodium losses 653 Sweet taste 13 15 Sweeteners. artificial 14, 189, 190 taste receptor signaling 14-15 xylitol 223 Syncytiotrophoblast (syntrophoblast) layer 78 nutrient transfer s e e Materno-fetal nutrient transport System A transporters 72, 251,254, 283. 290. 296, 303, 305, 31 I, 340, 341,344, 350, 351,365, 391 placenta 80, 255. 297, 379. 392, 407 renal tubule 63, 261,291 System BSos transporter (SLC 1A5) 45, 46, 60, 80. 251,255. 260, 281, 283, 285,290, 291,296, 298, 303. 305, 311. 312. 316, 319, 326. 330. 334, 340. 344, 350, 368, 371,372, 375. 378, 380, 390. 391. 392 System BSo, + s transporter (BAT I ; SLC7A9) 51, 60, 251,260, 351, 357. 358, 361. 364, 365, 368, 371. 375, 378, 380, 390, 396, 400. 407, 413,417, 435. 662 System L transporters 297. 303,365. 372, 378, 379. 407, 415.450 System N transporters 251,254. 282, 286, 290. 291. 297. 303, 311,391 System T transporter 316, 330-331 System yS + s transporters 357, 359, 361. 396, 398. 400, 415, 450 T I R taste receptors 15, 191 Tachysterol 480 Tagatose 189 Talosc 188 Tangeretin 92, 93.96 drug absorption effects 103 Tannins 38 Tartaric acid 16 Tartrate-resistant alkaline phosphatasc (TRAP) 696, 704 Taste buds I 0-11 innervation 12, 13 receptors 11. 15. 16 Taste receptors 1 I, 15. 16 polymorphisms 18 Taste sensation 10-18 additional taste qualities 17 bitter taste 15-16

central pathways 12 appetite stimulation 27 cold perception 23 conditioned aversion 25, 28, 368, 375, 382 cranial nerves 12. 13 fat 17. 18 hot spicy taste 22-23 irritant sensitivity 21-22 metallic taste 17 salivation stimulation 37 salty taste 13, 654 sour taste 16-17 supertasters 18 sweet taste 13 15. 191-192 taste-enhancing secretions 13 umami (meaty taste) 17, 272, 273 variation in sensitivity 18 T A T I 81. 252, 255, 316, 317, 324, 330, 331 Tau protein 306 Tau protein kinase 306 Taurine 3, 42, 244, 249. 345,349, 421-426, 588 bile acid conjugation 424, 426 bromination to N-bromotaurine 755 dietary sources 422 digestion/absorption 422 endogenous synthesis 247, 343. 351, 353,354. 422, 589, 676 cysteine 351. 353,354. 737 excretion 424 function 354. 421,426 metabolism 424, 425 nutritional summary 421-422 regulation 425 renal tubule osmoprotective function 59, 61, 261,426 processing 60, 261,424 storage 424 transport/cellular uptake 81.251, 422-424, 662 Tea 38, 92, 96, 97, 553,636, 641,669. 728 Telosmoside AI5 14 Temperature regulation 647 Terpenoids 13, 14 Testosterone 262 Tetracyclines 668 Tetrahydrobiopterin 317, 322 antioxidant activity 463. 633 Tetrahydrocannabinol 27 Tetrahydrofolate 592, 595,596 Tetrahydrofolate synthase 596 Thalamus 12 Thaumatin 14 Thcobromine 15

8 2 2 Index

Thermogenesis, garlic compound effects 108 Thiamin 551-558, 776, 778 absorption 49, 553-554 acetate metabolism 150 amino acid metabolism 256, 257, 258, 272. 277, 280, 285, 288, 293, 295, 297, 299, 305, 308, 313, 315, 320, 322, 327, 329. 331, 333, 339, 345, 349, 354, 357, 361,363, 365, 366, 371,373, 377, 379, 380, 383. 388, 390, 394, 395, 399, 401,404, 410, 412, 418, 530, 557 brain function 558 deficiency 552, 553, 558 dietary sources 552-553 energy metabolism 201,202, 275, 706 excretion 64, 556-557 fatty acid oxidation 138, 558 fructose metabolism 215 function 552, 557-558 galactose metabolism 221 glucose metabolism 207, 557 metabolism 555,556 mitochondrial function 558 nutritional summary 552 2-oxoglutarate metabolism 530 phytanic acid oxidation 183, 558 pyruvate metabolism 229, 230, 529 regulation 557 storage 556 transportJcellular uptake 73, 81, 554-555 xylitol metabolism 224 Thiamin pyridinylase 553 Thiamin pyrophosphokinase 554, 555 Thiamin transporter (ThTr2; SLC 19A3) 81,555 Thiamin transporter 1 s e e Protonthiamin antiporter (SLC 19A2) Thiamin triphosphatase 555 Thiamin-diphosphate kinase 555 Thiaminase 553 Thiazolidinediones 138 Thiochrome 553 Thiocyanate 38, 714, 715, 717 Thiokinase (acetate-CoA ligase) 150, 239 Thiolase (acetyl-CoA Cacetyltransferase) 128, 134, 150, 155, 162, 167, 170, 178, 239, 319, 326, 333,513 Thiophane 615 Thioredoxin 463, 544 Thioredoxin reductase 462, 463. 505, 544, 546, 568, 715, 725, 726, 780 Thirst 31-33. 644, 647

brain sensors 32-33 complex central nervous system input 33 peripheral sensors 32 sensation 32 sodium regulation 654 Threonine 244, 249, 254, 295-299 dietary sources 295 digestion/absorption 296 excretion 298-299 function 295-296, 299 metabolism 258, 289, 297-298, 299, 624 glycine synthesis 258, 289, 297, 299 nutritional summary 295-296 protein synthesis 299 storage 298 transport/cellular uptake 297 Threonine aldolase 297, 588, 589 Threonine dehydratase 228, 297, 305, 588, 589 L-Threonine dehydrogenase 289 Threonine-tRNA ligase 299 Threose 189 Thrombin 507, 508 Thromboxanes 140, 640 Thymidylate synthase 596, 599 genetic variation 772 Thymine 310 Thymulin 691 Thyroglobulin 715, 726 Thyroid hormone aminotransferase 590 Thyroid hormones 262, 431,445, 566, 590, 676, 712, 726 adaptation to starvation 268 function 716-171 synthesis 320, 327, 713, 714-715 Wolff~haikoff effect 716 Thyroid metabolism bromine 756 vanadium 764 Thyroid receptors 475 Thyroid-releasing hormone (TRH) 413, 683 Thyroid-stimulating hormone (thyrotropin; TSH) 445, 716 Thyroxine 668, 715, 726 synthesis 715 Thyroxine deiodinase 1 716, 726 Thyroxine deiodinase 2 716, 726 Thyroxine deiodinase 3 716, 726 Thyroxine deiodinases 715, 726 Tight junctions 45.51 brain capillary endothelial cells 70, 651 distal renal tubule 59 glomerulus 57 proximal renal tubule 58. 64, 65,710 Tin 2

Tmp (tubular maximum for phosphate) 704, 705 Tocomonoenol 492 Tocopherol transfer protein (TTP) 493, 494, 495,496, 497 Tocopherol-associated protein (TAP) 494 Tocopherol-bindingprotein (TBP) 494 Tocotrienols 492 Tomatoes 96 Tongue, taste buds 1I, 12 Tooth re-mineralization 721 Topaquinone 683 Trace elements 643-767 absorption 51-53 blood-brain barrier transport 73-74 materno-fetal transport 82 renal processing 58 Trans-elaidic acid 175 2-Trans-enoyl-CoA hydratase 128, 162, 170 3,2-Trans-enoyI-CoAisomerase 129, 165 Trans-fatty acids 175-178 dietary sources 175 function 178 metabolism 176-178 storage 178 transport/cellular uptake 175-176 Trans-linoelaidic acid 175 Trans-octaprenyltranstransferase 533 Trans-vaccenic acid 175 conjugated linoleic acid synthesis 158, 178 metabolism 176 Transaldolase 202, 226 Transamination 247, 256-257 Transcobalamin 1 606, 608 Transcobalamin II 60, 63, 607~508, 610 Transcobalamin Ill 606, 608 Transcuprein 680 Transcytosis 47 TRANSFAC 783 Transferrin 52, 74, 82, 550, 669, 670, 671,674, 730, 740, 743,763 Transferrin receptor 1 (TfR1) 74, 669, 671 Transferrin receptor 2 (TfR2) 671 Transferrin receptors 52, 82, 671,743 receptor-mediated endocytosis 671 Transglutaminase (gammaglutamyltransferase) 278, 698 Transit time, orocecal 44 Transketolase 202, 225, 557 Transketolase 2 202, 226, 557 Translation 741 Transport mechanisms alanine 46, 254, 311,662 amino acids 63, 251-252, 253 255, 261,274-275

Index 8 2 3

arginine 398 arsenic 758 ascorbate 72-73, 81,545-546 asparagine 391-392 aspartate 60, 72, 251,254, 261,385 biopterin 630 biotin 73, 82, 615 blood-brain barrier 71-75 boron 747 bromine 754 calcium 45, 695-696 carnitine 251,435 chloride 663, 666 cholesterol 80, 517-521 choline 72, 450 chromium 743 cobalt 740 copper 45, 74, 679~581 creatine 429 cysteine 350-351 docosahexaenoic acid 168 fatty acids 125-126, 154, 160 trans-fatty acids 175-176 flavonoids 98, 99 fluoride 720 fructose 213 galactose 219 glucose 199 glutamate 274-275 glutamine 282-284 glycine 290-291 histidine 414--415 inositol 637 iodine 714 isoleucine 378-379 leucine 365 lipoate 73, 82, 528 lysine 251,358-359 manganese 729 materno-fetal transport 77-82 molecular mechanisms 45-47, 48 molybdenum 734-735 niacin 574-575 oxalate 234 pantothenate 73, 82, 621 phenylalanine 316-317 phosphate 703-704 phytanic acid 18I potassium 45,658 proline 407 pyruvate 228 renal tubule 59-64, 261-262 selenium 723 serine 303 silicon 751 taurine 251,422-424 thiamin 73, 81. 554-555 threonine 297

tryptophan 331 tyrosine 324 ubiquinone 535 valine 372 vanadium 763 vitamin A 73,469--470 vitamin B6 584-585 vitamin B12 606--607, 608 vitamin D 482 vitamin E 73,493-495 water 45, 646 zinc 688 Transthyretin 470 Trehalase 47, 152, 197 Trehalose 189, 193, 196-197 digestion 197 Tretinoin 465 Tricarboxylate transporter (SLC25A 1) 119 Tricarboxylic acid cycle s e e Krebs cycle Tricetin 93 Trigeminus nerve (cranial nerve V) 8, 21, 23 Triglyceride-rich lipoproteins (TRL) 125, 168 Triglycerides 119, 122 adipocyte synthesis/release 137 chylomicrons 517 digestion/absorption 48, 123 energy metabolism 138-140 Trigonelline 572, 573 Triiodothyronine 566, 716, 726 synthesis 715 Trimethoprim 628, 630 4-Trimethyl ammoniobutyraldehyde dehydrogenase 433 Trimethyllysine dioxygenase 433, 550, 675 Triokinase 188, 213 Triose isomerase 196, 201 Triosephosphate isomerase 213 Trisomy 21 (Down syndrome) 641 Triterpenoids 14 tRNA-methyltransferase 354 tRNAs 348 sulfuration 354 Tropomyosin 306 Tropomyosin kinase 306 Trypsin 41, 42, 47, 250 legume inhibitors 270 pancreatic coenzyme activation 48 vitamin BI2 metabolism 606 Tryptamine 329, 334, 440, 571,588 serotonin synthesis 336 Tryptophan 3, 244, 249, 328-336 dietary sources 329 toxic contaminants 330 digestion/absorption 330-331

excretion 334 free radicals generation 459 function 329, 334-336 melatonin synthesis 329, 331,336, 439. 440 metabolism 256, 257-258, 331-334, 589, 628, 676 catabolism 331-334, 557 nicotinamide synthesis 335-336, 568, 571,572, 575, 779 nutritional summary 329 photoprotection 336 protein synthesis 334 regulation 334 serotonin synthesis 329, 331,336 storage 334 transport/cellular uptake 33 I Tryptophan aminotransferase 333, 589 Tryptophan 2,3-dioxygenase 331,334, 442, 676 Tryptophan hydroxylase 317, 322, 632 Tryptophan 5-monooxygenase 336, 440, 675 Tryptophan-tRNA ligase 334 Tubers 189 Tubular maximum for phosphate (Tmp) 704, 705 Tubulin 278 Tumor cachexia 262 Tumor necrosis factor (TNF) 139, 262, 313, 691 Tungstate 735 Turanose 212 Tyramine 323 Tyrosinase 633, 681,683 Tyrosine 244, 321-327 catecholamine synthesis 320, 327 dietary sources 323 digestion/absorption 323-324 endogenous synthesis 247, 322 excretion 326 free radicals generation 459 function 320, 322, 326-327 melanin synthesis 320, 327 metabolism 257 catabolism 318-319, 325-326, 549, 588 nutritional summary 322 protein synthesis 327 regulation 326 storage 326 synthesis from phenylalanine 248, 315, 317-318, 322 biopterin 628, 632 thyroid hormone synthesis 320, 327, 715 transport/cellular uptake 324 ubiquinonesynthesis 317, 320, 327, 533

8 2 4 Index

Tyrosine aminotransferase 318, 325, 588 Tyrosine decarboxylase 323 Tyrosine 3-monooxygenase 317, 320, 322. 327. 632, 675 Tyrosine phenol-lyase 588 Tyrosine phosphatase 590 Tyrosine-tRNA ligase 327. 627 Yyrosinemia 319, 326 Ubidecarenone 532 Ubiquinone 532-536 absorption 48, 535 acetate metabolism 150 amino acid metabolism 256, 257, 258, 272,277,280,285,288,293,

295.299,306,312,313,315, 320,322.327,329,331,339,

345.349.354.357,361,363, 365,371.377,379,383,388, 390,394,395,399.401,404,

410.412,418 antioxidant activity 463. 530, 536 electron transport 535 536 endogenous synthesis 533,534. 535 cholesterol 512 tyrosine 317, 320, 327, 533 ethanol metabolism 236 fatty acid oxidation 138, 154, 157, 158. 162, 164 fructose metabolism 215 function 533, 535 536 galactose metabolism 221 glucose metabolism 207 nutritional summary 533 pyruvate metabolism 230 redox reactions 536 transport/cellular uptake 535 UDP-glucose pyrophosphorylase (UTP-glucose- 1-phosphate uridyltransferase) 204, 208, 218 U DP-glucose-4'-epimerase 220 UDP-glucose-hexose- 1-phosphate uridyltransferase 220 UDP-glucuronosyltransferase 97, 103 heterocyclic amines metabolism 89 UGTIAI 89 UGTIA9 89 Ultraviolet light folate photolysis in skin 598. 773 reactive oxygen species (ROS) generation 458 vitamin D3 synthesis in skin 5,479. 515, 523 Uh'a pertusa 123 Umami (meaty taste) 17, 270, 272, 273 Umhelli/i, rae 122 Un&tria pinn([ida 123 Upper intake level 777

Upstream stimulatory factor (USF) 689 Urea 16 excretion in urine 261 fasting-related levels 269 intestinal transcellular diffusion 47 renal tubule processing 63 synthesis 258-259, 260, 263,277. 284. 313,731 Urea cycle 270, 273,437, 588 aspartate metabolism 385, 386, 387. 392 disorders 294 proline metabolism 410 Urease 767 Uric acid 261,292, 458, 737 antioxidant activity 463 renal processing 66 Urine concentration regulation 64, 67, 645 5-hydroxyproline biomarker 294 3-methylhistidine biomarker 420 pH 346 Urocanase (urocanate hydratase) 415, 579 Uroporphyrinogen decarboxylase 531 Urothione 736 Uteroferrin 82 UTP-glucose- 1-phosphate uridyltransferase (UDP-glucose pyrophosphorylase) 204, 208. 218 Vagus nerve (cranial nerve X) 12.21, 27 Valine 244, 245. 249, 253, 370-376 dietary sources 371 digestion/absorption 371-372 excretion 375 function 371,376 metabolism 256, 257, 373-375. 379, 530, 557, 568, 588. 624 carnitine 432, 436 neurotransmitter metabolism 376 nutritional summary 371 protein synthesis 376 regulation 375 storage 375 transport/cellular uptake 80. 254, 372 Valine transaminase 588 Valine-3-methyl-2-oxovalerate aminotransferase 373. 379 Valine-tRNA ligase 376 Vanadium 762 765 absorption 53. 763 dietary sources 763 excretion 764 function 762. 764-765 metabolism 763 nutritional summary 762 763 storage 763-764 transport/cellular uptake 763

Vanilloid receptor I (VRI; capsaicin receptor) 22 Vanillylmandelic acid 319, 326 Variant Creutzfeld Jacob disease 640 Vascular organ of lamina terminalis 32, 71,647, 651,654 Vasoactive intestinal peptide (VIP) 653 Vasopressin see Antidiuretic hormone Vegetable oils 493. 503 Vegetables 96, 106, 189, 190. 212, 233, 463,465, 468, 502, 544, 561, 562, 571,593,619, 636, 656, 694, 734. 746 flavonoids/isoflavones 92, 93, 95.96, 97 Ventromedial hypothalamus glucose transporters 199 glucose-sensing neurons 28, 71, 199 Ventromedial nucleus 26 Verapamil 450 Verbascose 190. 197, 198. 212. 218 Very-long-chain acyl-CoA synthetase 128, 129 Very-low-density lipoprotein (VLDL) 125. 160, 517-518 docosahexaenoie acid 168 impact of overfeeding 143. 144 low-density lipoprotein (LDL) production 143-144. 518 processing in circulation 125 remnants 517 uptake 518 vitamin E transport 494 Very-low-density lipoprotein receptor (VLDL-R) 80. 520, 563 very-low-density lipoprotein (VLDL) uptake 518 Vestibular system 699 Vinegar 147. 149, 152 Viral hepatitis and iron stores 773 Viral mutation 726 Visual input central appetite stimulation (cephalic phase response) 27 gastric secretions stimulation 38 salivation stimulation 37 Visual physiology 473-474. 557, 690 Vitamin A 464-475, 776 absorption 48.50. 468-469 cell cycle regulation 475 deficiency 268, 466, 470, 472 dietary sources 465, 468 endogenous synthesis 466-468 excessive intake 466 excretion 473 function 465.473-475

Index 8 2 5

metabolism 470-472 iron 465, 675 retina 473-474 nuclear actions 474-475 nutritional summary 465-466 storage 472-473 transport/cellular uptake 469-470 blood-brain barrier 73 visual physiology 473-474 Vitamin A2 (3,4-didehydroretinol} 472 Vitamin B1 see Thiamin Vitamin B3 570 Vitamin B5 619 Vitamin B6 3. 581-590. 776 absorption 47, 49, 583-584, 706 amino acid metabolism 247, 256, 257, 258. 277, 280. 284, 285. 288, 289. 291,293,295,297,299.303,306, 308,312,313,315,317,319,320, 322,327,329,333,336,339,343,

345,349,351,352,354,357,359, 361,362,363.365,371,373,377,

379,383,386.388,390,392,394, 395,396,397,398,400,401,404,

410, 412,416, 418, 588-589 carbohydrate metabolism 587 carnitine synthesis 362. 433 deficiency 582, 589, 779 dietary sources 582-583 excretion 587 function 582, 587-590 hormone metabolism 590 lipid metabolism 589 melatonin synthesis 439, 440 metabolic regulation of enzymes 590 metabolism 585, 586, 706 flavoproteins 568, 585 neurotransmitter synthesis 588 nutritional summary 582 regulation 587 renal stone risk reduction 234 selenium metabolism 590 serotonin synthesis 336 storage 585-586 taurine synthesis 422 transport/cellular uptake 73, 81, 584 585 ubiquinone synthesis 533 urea cycle 588 xenobiotic metabolism 590 Vitamin B8 591 Vitamin B9 591 Vitamin BI2 603-611,776 absorption 49-50, 606-607 amino acid metabolism 288, 293,295, 299, 306. 339. 345, 371. 373, 374. 377, 379

choline synthesis 447 creatine metabolism 428 cyanide antidote 611 deficiency 604 dietary sources 604. 605-606 digestion 606 endogenous binding proteins 606 excretion 63-64, 610 fatty acid oxidation 138 function 604, 610-611 hepatobiliary circulation 43, 610 homocysteine remethylation 610-611 leucine metabolism 611 metabolism 568, 608-610 manganese-dependent activation 730 nutritional summary 604 propionyl-CoA metabolism 61 I regulation 610 storage 610 transport/cellular uptake 608 blood-brain barrier 73, 608 ileal enterocytes 606 transcellular transport 606-607 transcobalamine-ll 607-608 Vitamin Bc 591 Vitamin BUTu 432 Vitamin C see Ascorbate Vitamin D 3, 5, 478-489, 776, 778 absorption 48, 50, 481-482 bone metabolism 487-488 calcium absorption 51,487, 693, 694 cell differentiation 488 deficiency 479 dietary sources 479, 480-481 endogenous synthesis 5,479-480 excretion 486-487 function 478, 487-488 magnesium regulation 710 metabolism 482-486 nuclear effects 487 nutritional summary 478-479 phosphate regulation 487, 702, 705 regulation 486-487 renal processing 57, 63,486-487, 697 1.25-dihydroxy-vitamin D synthesis 484-485 storage 486 transport/cellular uptake 73, 482 see also 1,25-Dihydroxy-vitamin D Vitamin D2 (ergocalciferol) 481 Vitamin D3 (cholecalciferol) 480 dietary sources 480-481 skin synthesis following sunlight exposure 5,479, 515,523 Vitamin D(3) 25-hydroxylase (CYP2D25) 483 Vitamin D receptor (VDR) 475,487. 488

Vitamin D response elements (VDRE) 487 Vitamin D-I alpha-hydroxylase (P450clalpha; CYP27BI) 63 Vitamin D-binding protein (DBP: groupspecific component; Gc} 60, 63, 73,482, 484, 486, 697 Vitamin E 458, 490-499, 781 absorption 48, 50, 493 antioxidant activity 457,463, 497-498, 499 reactions with free radicals 497 tocopheroxyl radical formation 459, 498, 549 ascorbate interactions 549 deficiency 491,498 dietary sources 491-493 excretion 497 function 491,497-499 host-pathogen interactions 499 metabolism 495-497 ring modification 497 side-chain breakdown 495-496 nutritional summary 491 regulation 497 reproductive function 498 transport/cellular uptake 73,493-495 vitamin K interactions 499. 504 Vitamin G 561 Vitamin H 613 Vitamin K 43,501-510, 776, 781 absorption 48, 50, 504 blood coagulation 502, 507 bone mineralization 502. 508 deficiency 502 dietary sources 502 excretion 507 function 502, 507-509 Gla proteins regulation 507, 508 intestinal microflora production 44. 502, 503 metabolism 505-507, 568 catabolism 507 glutamyl residues gamma carboxylation (vitamin K cycle) 505, 506 side chain resynthesis 505-506 mitochondrial metabolism 508 nutritional summary 502 prostaglandin metabolism 508 signaling 508 storage 507 sulfur metabolism 508 tissue calcification regulation 507 transport/cellular uptake 73-74, 504 vitamin E interactions 499, 504 Vitamin K-dependent carboxylase 505 Vitamin M 591

826

Index

Vitamin PP 570 Vitamin-K-epoxide reductase 505 Vitamins 3 absorption 49-50 blood-brain barrier transport 72-73 fat-soluble 50, 457-538 materno-fetal transport 81-82 renal processing 58, 63-64 water-soluble 49-50, 539-641 Vitellogenin receptor 563 Vitiligo 633 Volume depletion 32 Vomeronasal organ (Jacobson's organ) 8 Von Ebner glands 13, 37 von Willebrand factor 145

Xaa-Pro dipeptidase (prolidase) 73 I Xanthan 191 Xanthine dehydrogenase 568, 737 Xanthine oxidase 458 Xanthinuria 737 Xanthohumol 97 xCT 274, 351 Xenobiotic detoxification carnitine 438 glutathione 354 pantothenate/eoenzyme A 624 pregnane X receptor 475 riboflavin 569 vitamin B6 590 Xenobiotic/medium-chain fatty acid:CoA ligases 624 Xenobiotics 85-108, 438, 551,569, 590, 624 blood-brain barrier transport 74-75 materno-fetal transport 82 Xerophthalmia 466 Xylitol 14, 189, 192, 223-226 dietary sources 223 digestion/absorption 224 endogenous synthesis 223, 224 function 223, 226 metabolism 224-226 nutritional summary 223 transport/cellular uptake 224 Xylose 189 D-Xylose test 189 Xylosylprotein 4-betagalactosyltransferase 731 Xylulokinase 189, 225, 639 Xylulose 189 D-Xylulose reductase 224, 639 L-Xylulose reductase 223, 639

Warfarin 505. 508 Water 3,643-648 absorption 43, 44, 50-51,644-645 dietary sources 644 endogenous synthesis 644 excretion 646~:r47 exhalation 646~47 renal reabsorption 58, 59, 64-65, 646 renal ultrafiltrate 58 sweating 647 urine 646 function 644, 647-648 chemical reactions 648 solvent 647 temperature regulation 647 intake central regulation 33 excessive 32 inappropriate 33 regulation s e e Thirst thirst alleviation 31-32, 33 nutritional summary 644 regulation 647 transport/cellular uptake 45, 646 Wernicke-Korsakoff syndrome 202, 226, 552, 557, 558 Willi-Prader syndrome 541 Wilson disease 737 Wilson protein (copper-transporting ATPase 7B; ATP7B) 681,682 Wolff-Chaikoff effect 716 Wound healing 543

y(+)LATI (SLC7A7) 63, 81,253, 255, 261,282, 296, 358, 359, 361, 391,396, 398, 400, 413,415,417 y(+)LAT2 (SLC7A6) 63, 72, 253, 254, 261,282, 296, 358. 361,391, 396, 400. 415,417 Y I receptor 27 Y2 receptor 26 Yeast 189, 197. 527, 553, 571,619, 742 Yolk sac amnion fluid 78 YT antigen (acetylcholine esterase) 453

XS-sUAGu system 60, 81,255. 261, 274. 277. 384, 387 Xaa-Pro aminopeptidase 731

Zellweger syndrome 180 Zinc 685-691. 781 absorption 53. 191. 420. 686-688

iron interactions 687, 780 phytate inhibition 641. 686 amino acid metabolism 349. 354 antioxidant activity 457 blood-brain barrier transport 74 cofactor activity 690 deficiency 686, 691 dietary sources 686 DNA replication/transcription 690-69 I energy metabolism 691 ethanol metabolism 236, 690 excretion 689 folate metabolism 690 free radical metabolism 691 function 686, 689-690 immune system 691 nutritional summary 686 pH regulation 690 regulation 689 RNA editing 691 storage 589 taste bud growth 11 taste sensation 11 transport/cellular uptake 688 visual physiology 690 vitamin A activation 465 Zinc fingers 690-691 Zinc transporter 1 (ZnT- 1; SLC30A I ) 53,688, 689 Zinc transporter 2 (ZnT-2; SLC30A2) 53, 688 Zinc transporter 3 (ZnT-3; SLC30A3) 688 Zinc transporter 4 (ZnT-4; SLC30A4) 688 Zinc-dependent carbonic anhydrase 39 Zinc-regulated transporter 1 (SLC39AI) 53,688 ZIP2 688 ZIP4 (SLC39A4) 687 Ziziphin 14 ZO-1 70 ZO-2 70 ZO-3 70

Food Science and Technology Intemational Series

Maynard A. h e r h e , Rose Marie Pangborn, and Edward B. Roessler, Principles of Sensory Evaluation of Food. 1965. Martin Glicksman, Gum Technology in the Food Industry. 1970. Maynard A. kslyn, Methods in FoodAnulysis, second edition. 1970. C . R Stumbo, Thermobacteriolo~in Food Ptvcessing, second edition. 1973. Aaron M.Altschul (d), New Protein Foods..Volume 1, Technology. part A-1974. Volume 2, Technology,Purt B-1976. Volume 3, Animal Protein Suppliat Part A1978. Volume 4, Animal Protein Supplies, Part B-198 I. Volume 5 , Seed Stornge hieins-1 985. S. A. Goldblith, L. Rey, and W.W. Rothmayr, Freeze Drying and Advanced Food Technology. 1975. R B. Duckworth (ed),Water Relations of Food. 1975. John A. TroIler and J. H.B. Christian, Wuferlicclvity und F d ,1978. A. E.Bender, F d Processing and Nutrition. 1978. D. R. Osborne and l? Voogt, The Analysis oflvutrients iH Foods. 1978,

Mmel Loncin and R. L. M m n , Food Engineering: Principles and Selected Applications. 1979. J. G.Vaughan Id), Food Mictvscow. 1979. J. R A. Pollock (ed),B m h g Science,Volume 1-1979.

Volume 2-1980.

Volume

3-1987.

J. Christopher Bauemfeind (d), Camtenoids m Colonants and yitamin A Precursors: Technologicaland NumWonal Applications. 1981. Pericles Markakh Id), Anthmyunins CIS Foud Colors. 1982. George E Stewart and Maynard A. Amerine (eds.), I n d u c t i o n to F d Science and Technology,second edition. 1982. Malcolm C . Bourne, Food Textuw and fixcmity: Concept and Meusummenf.1982. Hector A. Iglesias and Jorge Chirife, Handbook of Food Isotherms: Water Sorption Pametersfor Food and Food Componennts. 1982. Colin Dennis (ed.), Post-Hanwt Pathology of Fmib and Yqgetdles. 1983. F? L Barnes (ed.),Lipids in Cemd Techmlogy. 1983. David Pimenkl and Carl W.Hall (eds.), Food and Energv Resources. 1984.

828 Food Science and Technology: Intemationd Series

Jw M. Regenstein and

Carrie E. Regenstein, Food Pmieirs Chemistry: An I n d u c t i o n for Food Scientists. 1984. Maxim0 C. Gacula, Jr., and Jagbir Singh, Statistical Methods in Food and Consumer Raeurch. 1984. F e w M.Clydesdale and KathrynL.Wiemer (eds,),Zmn Fortificution of Firoals.1985. Robert V. Nareau, Miemwaves in the F d Pracapsing Indwby. 1985. S. M. Herschdoerfer (ed.), Quality Cotatml in the FboJ Indusw, second edition, Volume 1-1985. Volume 2 4 9 8 5 . Volume 3 4 9 8 6 . Volume 4-1987. E E. Cunningham and N.A. Cox [eds.), Micmbiol'ogvof Poultty M a t Pmduch. 1987. Walter M. Urbain, Fbod irmdiation. 1986. Peter J. Bechtel, MuscIe us F d . 1986. H.W.-S. Chan, Aatoxidutiun ofUnsa$umted Lipids. 1986. Chester 0.McCorkle, lr., Economics of Foud Pmessing in the United States. 1987. Jethro Japtiani. Harvey T.Chan, Jr., and William S. Sakai, Tmpicul Fruit Processing.

1987.

J. Solms, D.A. Booth, R. M. Dangbctrn, and 0. Raunhardt, Food Acceplance and Nub-ition. 1987. R. Macme, HPLC in Food Analysis, second edition. 1988. A. M.Pearson and R. B. Young,M a d e und Mear Biochemishy. 1989. Marjorie F? Penfield and Ada Marie Campbell. €xperimental Foad Science, third

edition 1990. Leroy C.Blankenship, Colonization C o n t d of H w n m Bucterial Enlemplhogem in poulty. 199 1. Yeshajahu Pomeranz, Fwctiond Pmperfiesu f F d Components,second edition. I99 I . Reginald H.Walter, The Chemistry and Ethnology ofpectin. 1991. Herbert Stone and Joel L.Sidel, Senmy Evahufionhcnkes, second edition. 1993. Robert L. Shewfelt and Stanley E. Prussia, Posthawest Hudling: A Systems A p p m h . 1993. R. Paul Singh and Dennis R. Heldman, Infmducrion fu Food Engineering, second edition. 1993. Tilak Nagcdawithana and Gerald Reed, Enzymes in Food Pmessmg, third edition. 1993.

Dallas G.Hoover and Larry R. Steenson, l3acteriocin.s. 1993. Takayaki Shibamota and L s o Bjeldanes, ~ in~mduccion10 F w d Taricology. 1993. John A. Trotler, SnRitution in F d Pmcessirtg, second edition. 1993. Ronald S.Jackson, Wne Science: Principles and Applicatiom. 1994. Harold D.Hafs and Robert G.Zimbelman, Low-fd Meats. 1994. Lance G. Phillips, Dam M. Whitehead, and John Kinsella, Smrcturr-Fsmction Properliar of F d Proteins. 1994. Robwt G.Jmsen, Handbook of Milk Camposition. t 995. Y j 6 H.Roos, Phase Tmnsitions in Foods. 1995. Reginald H.Walter, Polysaccharide Dispersions, 1997. Gustavo V Barbosa-CgnoVas, M.Marcela Gbngora-Nieto, Usha R. Pothalcarnury, and Barry G. Swanson, Preservation of F d with Puised Eiecrric f i e k 1999. Ronald S.Jackson, Wine Scknce: Principltv, Fmctice, Perreption,m n d edition. 2000.

Food Science and Technology: internationalSeries 829

,R.Pad Sin& and Dennis R. Heldman, Inmduction

to

Food Engineering, third

edition. 2001* ]Ronald S.Jackson, mne Tasting:A Prafessioonal Handbook.2002. iMalcofm C.Bourne, Food Taturn and Hscmiq: Concept and Memumment, second edition. 2002.

]Benjamin Caballem and Barry M. Popkin (eds), The Nutrition knsiiion: Diet and Diseuse in the Developing World. 2002. ]Dean0.Cliver and Hans !F Riemann (eds),F d h n c Dhmsa, secondedition. 2002.


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