Physiologica Effects o Food Carbohydrates
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Physiologica Effects o Food Carbohydrates
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Physiological Effects of Food Carbohydrates Allene Jeanes and John Hodge, Editors
A symposium co-sponsored by the Division of Carbohydrate Chemistry and the Division of Agricultural and Food Chemistry at the 168th Meeting of the American Chemical Society, Atlantic City, N . J., Sept. 11-12, 1974.
ACS SYMPOSIUM SERIES
AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C.
1975
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
15
Library of Congress
Data
Physiological effects of food carbohydrates (ACS symposium series; 15) Includes bibliographical references and index. 1. Carbohydrates—Physiological effect—Congresses. 2. Carbohydrate metabolism—Congresses. I. Jeanes, Allene Rosalind, 1906-. II. Hodge, John E., 1914. III. American Chemical Society. Division of Carbohydrate Chemistry. IV. American Chemical Society. Division of Agricultural and Food Chemistry. V. Series: American Chemical Society. ACS symposium series; 15. [DNLM: 1. Carbohydrates— Metabolism—Congresses. QU75 A512p 1974] QP701.P48 ISBN 0-8412-0246-X
612'.396 75-14071 ACSMC8 15 1-355 (1975)
Copyright © 1975 American Chemical Society All Rights Reserved PRINTED IN THE UNITED STATES OF AMERICA
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
ACS Symposium Series Robert F. Gould, Series Editor
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
FOREWORD The
ACS
SYMPOSIUM SERIES
a medium for publishing symposia quickly i n book form. The format of the SERIES parallels that of its predecessor, ADVANCES IN CHEMISTRY SERIES, except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors i n camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book. Papers published i n the A C S SYMPOSIUM SERIES are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
PREFACE Significant progress has been made in evaluating the physiological ^ effects that have been attributed to food carbohydrates. Because the findings are related to the structure, enzymology, and complexing interactions of carbohydrates, as well as to improved compositions of processed foods, this symposium was organized in the interests of the Division of Carbohydrate Chemistr and the Division of Agricultural and Food Chemistry. Some common disorders definitely associated with dietary carbohydrates are diabetes, stress responses resulting from hypoglycemia, dental caries, eye cataracts, flatulence, and fermentative diarrhea. The postulated role of sugar in the development of atherosclerosis and coronary heart disease has been impugned and debated, but, more importantly, it is being carefully investigated. Investigations center on underlying causes of carbohydrate-induced disorders—e.g., altered enzyme activity in the digestive tract and elevated insulin, cholesterol, and triglyceride levels in serum. Numerous studies show differences according to the type of carbohydrate ingested. Because refined sugars and starches have been referred to as "empty calories," one might wonder whether carbohydrate is needed at all in the diet. Some popular reducing diets contain little carbohydrate. The Food and Nutrition Board of the National Research Council, National Academy of Sciences, has stated in the 1974 edition of "Recommended Dietary Allowances": Carbohydrates can be made in the body from some amino acids and the glycerol moiety of fats; there is therefore no specific requirement for this nutrient i n the diet. However, it is desirable to include some preformed carbohydrate in the diet to avoid ketosis, excessive breakdown of body protein, loss of cations, especially sodium, and involuntary dehydration. Fifty to 100 g of digestible carbohydrate a day will offset the undesirable metabolic responses associated with high fat diets and fasting. The topics of this symposium are related to improved carbohydrate and mineral balance in foods of the future. As food supplies for expanding populations become more critical, the more abundant and economical carbohydrates should be used to maximum advantage to spare the less abundant proteins and fats. However, to establish the optimum ratio of carbohydrates to fats, the physiological effects of their combinations should be delineated more clearly. It now appears that indigestible, as ix In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
well as digestible, carbohydrate should be considered i n formulating processed foods. The postulated benefits of dietary fiber (largely indi gestible polysaccharides ) in aiding the elimination of toxins and in reduc ing serum cholesterol are discussed. Other advantages that might accrue from ingesting or infusing certain types of carbohydrate in place of other types are cited here. Disadvantages in refining high-carbohydrate cereals and sugar to the point of near depletion of essential mineral content are viewed from a physiological basis. During this century significant changes have taken place in the types of carbohydrate that are made available in our food supplies. U.S. De partment of Agriculture statistics show that we are being exposed more to refined sugars and less to starch and dietary fiber as the proportions of fat (mainly) and protei sugars, the proportion of D-glucose, and the maltose and maltooligosaccharides of starch hydrolyzates. The increasing incorporation of starch sirups, including those that contain D-fructose produced by glucose isomerase, is expected to accentu ate this trend. The level of lactose, ingested mainly from milk and dairy products rather than from added lactose, has remained about constant. Critics declare that consumers now have less control over their carbohydrate intake than their forebears had because the compositions of prepared convenience foods and beverages vary significantly from those of natural foods. Expanding urban populations dictate an increasing sup ply of stable processed foods; therefore, the benefits of adding sugars and modified polysaccharides to improve the stability and acceptance of pre pared foods should be weighed against adverse nutritional effects. A l though processing practices have been viewed with alarm in some sectors, we really cannot know whether the changes in carbohydrate composition are innocuous until the physiological effects of the additives have been defined under conditions of normal use. This symposium and others like it attest to the activity of scientists in different disciplines who are supply ing answers to the questions raised about the healthfulness of refined sugars and gums. Some of the subjects selected for this symposium were critically reviewed in the International Conference on Sugars in Nutrition held at the Vanderbilt University School of Medicine in 1972 ( "Sugars in Nutri tion," H. L. Sipple and K. W. McNutt, Eds., Academic Press, New York, 1974). However, more recent experimental results and additional infor mation on the physiology of dietary and infused sugars are presented here, and mostly by different authorities. Parts A and Β of this symposium might therefore be regarded as a supplement to "Sugars i n Nutrition." Part C covers physiological effects of polysaccharides, which were not cov ered i n the Vanderbilt conference, including some food additive gums χ In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
and dietary fiber. This information also can be expanded by reference to the Symposium on Fiber in Human Nutrition held by the Nutrition Society at the University of Edinburgh School of Medicine (Proc. Nutr. Soc. (1973) 32, 123-167). Another symposium publication, "Molecular Structure and Function of Food Carbohydrate" (G. G. Birch and L. F. Green, Eds., Wiley, New York, 1973), contains several papers that are related to the topics of this symposium. JOHN E. HODGE
Northern Regional Research Laboratory Agricultural Research Service, U S D A Peoria, Ill. December 1974
xi In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1 The Physiology of the Intestinal Absorption of Sugars ROBERT K. CRANE College of Medicine and Dentistry of New Jersey, Rutgers Medical School, Department of Physiology, Piscataway, N. J. 08854
This review has t physiology absorption of sugars and should properly begin with a brief discussion of the components of the physiological system which carries out this indispensable task. The small intestine where the absorption of sugars takes place is a tube connecting to the stomach at its upper end and to the large intestine at its lower. In the human adult the tube is about 280 cm (9 feet) in length and an average 4 cm (1-1/2 inches) in internal diameter. The area of the inner surface of the tube is much greater than implied by these two measurements because the inner surface is heavily folded and everywhere on the folds there are to be found numerous projec tions called v i l l i (1). Villi are readily seen under a micro scope of low power and there are perhaps as many as 25,000,000 v i l l i in all. As indicated in Figure 1, each villus is covered by a sheet of absorptive epithelial cells punctuated at intervals by the so-called goblet cells which supply protective mucous. Between the v i l l i are to be found crypts within which the cells are produced and from which they migrate outward along the sur face of a villus during a short 3-4 days of active life before being extruded into the lumen of the gut where they disintegrate and are digested. The v i l l u s i s the working u n i t o f the s m a l l i n t e s t i n e . I t i s on t h i s s t r u c t u r e t h a t the i n n e r ends of the a b s o r p t i v e c e l l s are brought i n t o c l o s e p r o x i m i t y t o the blood and lymph which must p i c k up absorbed n u t r i e n t s and c a r r y them t o the other p a r t s of the body. The outer ends of the a b s o r p t i v e c e l l s are i n contact w i t h the contents of the i n t e s t i n e and are s p e c i a l i z e d to perform t h e i r necessary work. The outer end of each c e l l i s a "brush border" made up o f c l o s e l y packed, p a r a l l e l c y l i n d r i c a l processes c a l l e d m i c r o v i l l i . The l i m i t i n g plasma membrane of t h e c e l l f o l l o w s the contours o f the m i c r o v i l l i . J u s t beneath the brush border along the s i d e s of the c e l l s are t o be found s p e c i a l i z e d j u n c t i o n a l s t r u c t u r e s by means of which the absorpt i v e c e l l s are h e l d together i n t o a more or l e s s continuous sheet. 2 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
CRÂNE
Intestinal Absorption
of Sugars
3
Expanding our h o r i z o n t o i n c l u d e the whole of the i n t e s t i n a l s u r f a c e w h i l e r e t a i n i n g our view o f i t s microscopic appearance i t i s c l e a r t h a t , so f a r as concerns d i g e s t i o n and a b s o r p t i o n , t h e c o l l e c t i v e brush borders of the e p i t h e l i a l sheet form a j u n c t i o n a l b a r r i e r between the o u t s i d e of the body and the i n s i d e through which n u t r i e n t s must pass i n order t o reach the c i r c u l a t i o n and enter metabolism. The c o l l e c t i v e brush borders separate t h e d i g e s t i v e f u n c t i o n s of t h e i n t e s t i n a l lumen c o n t r i b u t e d by the secreted enzymes of the pancreas from t h e metabolic f u n c t i o n s c o n t r i b u t e d by t h e c e l l s . The brush borders a l s o c o n t r i b u t e d i g e s t i v e f u n c t i o n s o f t h e i r own as w e l l as t h e s e l e c t i v i t y , energy t r a n s d u c t i o n and other p r o p e r t i e s of a b s o r p t i o n a n t i c i pated f o r a c e l l membrane occupying t h i s p a r t i c u l a r l o c a t i o n . The brush border membrane acts as a b i l a y e r l i p o i d a l m a t r i x composed of t h e f a t t y chains of p h o s p h o l i p i d s and glycosphingol i p i d s i n t e r s p e r s e d w i t h c h o l e s t e r o l ( 2 ) and p e r f o r a t e d here and there by aqueous channel may pass by d i f f u s i o n . L i p i d i f f u s e r e a d i l y across the m a t r i x of the membrane. However, t h e membrane i s a s u b s t a n t i a l b a r r i e r t o the r a p i d d i f f u s i o n o f large, h i g h l y water s o l u b l e molecules l i k e the hexoses because these do not enter the m a t r i x and the dimensional p r o p e r t i e s of the aqueous channels are too s m a l l , being e q u i v a l e n t only t o those o f pores o f 4-5 £ i n r a d i u s (3), ( 4 ) . There a r e a l s o aqueous channels between the c e l l s because t h e j u n c t i o n a l complexes o f the i n t e s t i n a l e p i t h e l i u m ' a r e n o t t i g h t {5). However, these channels are a l s o too s m a l l f o r the r a p i d passage of hexoses. Those hexoses which do g e t across the brush border membrane r a p i d l y and i n q u a n t i t y ; and t h i s group n a t u r a l l y i n c l u d e s the major d i e t a r y hexoses, glucose, g a l a c t o s e , and f r u c t o s e , do so because they f i t the s p e c i f i c i t y requirements and are able t o b i n d t o membrane t r a n s p o r t c a r r i e r s ( 6 ) . The a c t u a l mode of o p e r a t i o n o f c a r r i e r s i s c u r r e n t l y unknown. However, t h e i r apparent mode o f o p e r a t i o n , i n s o f a r as we can know i t from k i n e t i c s , i s most e a s i l y described as l i k e t h a t of a f e r r y b o a t , capable of s h u t t l i n g water s o l u b l e molecules across the l i p o i d a l matrix. C a r r i e r f u n c t i o n i s diagrammed i n F i g u r e 2 where t h e upper p a r t i s an o p e r a t i o n a l model and t h e lower p a r t i s a k i n e t i c model of a simple s o - c a l l e d f a c i l i t a t e d d i f f u s i o n type of c a r r i e r t o which constants may be assigned as i n d i c a t e d . The assumptions are few and simple. Substrate i n t e r a c t s w i t h the b i n d i n g s i t e o f a c a r r i e r on e i t h e r s i d e of the membrane and i s t r a n s l o c a t e d i n a s s o c i a t i o n w i t h the c a r r i e r . The b i n d i n g s i t e of the c a r r i e r can t r a n s l o c a t e whether or not i t c a r r i e s substrate. A l l i n t e r a c t i o n s are u s u a l l y assumed t o be symmetrical and e q u i l i b r i u m i s then achieved a t equal transmembrane concentrations or a c t i v i t i e s . F o r the most p a r t , f r u c t o s e crosses the brush border membrane by means of a c a r r i e r w i t h these p r o p e r t i e s ( 7 , 8, 9 ) .
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
4
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
BRUSH BORDER MEMBRANE
CELL CONTENTS
INTESTINAL CONTENTS
+ C ^ c s Figure 2.
^
C+ c s
Schematic of a facilitated diffusion (monofunctional) carrier. Ρ is the permeability coefficient.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
CRÂNE
Intestinal Absorption of Sugars
5
Glucose and g a l a c t o s e , however, use a c a r r i e r which though i t i s somewhat the same i s a l s o somewhat and i m p o r t a n t l y d i f f e r ent. The glucose-galactose c a r r i e r , depicted i n F i g u r e 3 both as an o p e r a t i o n a l model and as a k i n e t i c model, i s an e q u i l i b r a t i n g , symmetrical c a r r i e r l i k e the f r u c t o s e c a r r i e r except t h a t i t has two b i n d i n g s i t e s i n s t e a d o f one. The glucoseg a l a c t o s e c a r r i e r r e q u i r e s N a f o r i t s e f f i c i e n t o p e r a t i o n and cotransports N a i n a t e r n a r y complex along w i t h the sugar ( 6 ) . The p a r t i c u l a r v e r s i o n of the Na -dependent c a r r i e r shown i n F i g u r e 3 i n d i c a t e s t h a t the b i n d i n g s i t e can t r a n s l o c a t e e i t h e r empty o r i n a t e r n a r y complex w i t h both o f i t s s u b s t r a t e s , b u t not w i t h sugar alone. There are other v e r s i o n s w i t h other assumptions about the requirements f o r t r a n s l o c a t i o n but the e s s e n t i a l f e a t u r e s are very s i m i l a r ( 1 0 ) . In an i s o l a t e d system, a c a r r i e r w i t h two b i n d i n g s i t e s i s an e q u i l i b r a t i n g c a r r i e The c a r r i e r i t s e l f can s t a t i o n a r y s t a t e would f i n d equal concentrations o f sugar and equal concentrations of N a on the two s i d e s o f the membranes. In the c e l l u l a r system, however, the'Na -dependent glucoseg a l a c t o s e c a r r i e r i s able t o transduce metabolic energy and t o achieve " u p h i l l " o r a g a i n s t t h e g r a d i e n t t r a n s p o r t by coupling t o the t r a n s c e l l u l a r f l u x of Na . The system i n the i n t e s t i n e seems t o work as suggested by the diagram i n F i g u r e 4. M e t a b o l i c energy i n the form of ATP i s put i n t o a sodium pump i n the basol a t e r a l membranes of the a b s o r p t i v e c e l l s t o d r i v e a t r a n s c e l l u l a r f l u x of N a from the brush border end t o the basol a t e r a l end (12). The glucose-galactose c a r r i e r couples sugar entry to t h i s f l u x by being a route f o r the entry of sodium i o n at the brush border membrane and achieves an " u p h i l l " c e l l u l a r accumulation o f sugar a t the expense o f the " d o w n h i l l " f l u x of Na . I n t a c t d i - and higher saccharides do not get across the brush border membrane i n q u a n t i t y and we thus i n f e r t h a t the needed c a r r i e r s do not e x i s t (13). Tiny amounts o f d i e t a r y d i - and o l i g o s a c c h a r i d e s are sometimes found i n the u r i n e o f i n d i v i d u a l s under study but these t i n y amounts are a t t r i b u t a b l e t o d i f f u s i o n o f these l a r g e compounds through regions o f the i n t e s t i n e where the normal b a r r i e r has been broken down by i n j u r y or by disease. Recently our l a b o r a t o r y has i d e n t i f i e d a route of c e l l u l a r e n t r y of monosaccharides i n a d d i t i o n t o t h a t provided by the N a dependent c a r r i e r s , o f F i g u r e s 3 and 4 (14, 15.)· * ^ t o be b r i e f l y d e s c r i b e d l a t e r i s r e l a t e d t o the a c t i v i t y o f those hydrolases which are an i n t r i n s i c p a r t o f the brush border mem brane. As i s discussed a t f u r t h e r l e n g t h by Dr. Gary Cray i n t h i s Symposium and as i s shown i n Table I , there are imbedded i n the outer surface o f the brush border membrane a s u b s t a n t i a l l i s t of b o n d - s p e c i f i c h y d r o l y t i c o r t r a n s f e r a c t i v i t i e s . The +
+
+
+
+
+
+
+
+
T h
s
n
e
w
γ ο ι 1
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
θ
6
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
BRUSH BORDER MEMBRANE
INTESTINAL CONTENTS
Να pump
G
1+
C —
C +| G
k.Jfk-,
k|ik-
kJfk-
kjfik-s
4
4
Να 2
2
C-G-Na^C-G-Να Figure 3.
Schematic of a sodium-dependent bifunctional carrier
American Journal of Clinical Nutrition
Figure 4. Schematic of energy transduction between the baso-hteral sodium pump and brush border Να"-dependent carriers by means of the Na through flux (11) +
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
CRÂNE
7
Intestinal Absorption of Sugars
TABLE I BRUSH BORDER ENZYME ACTIVITIES* Oligopeptidase γ-glutamyl t r a n s p e p t i d a s e Enterokinase Glucoamylase ( o l i g o s a c c h a r i d a s e ) Maltase Sucrase Isomaltase (α-dextrinase) Lactase Trehalase P h l o r i z i n Hydrolase ( g l y c o s y l c e r a m i d a s e ) A l k a l i n e Phosphatase as o f 1974 accordin saccharidases among these enzymes; namely, glucoamylase (which i s h i g h l y a c t i v e a g a i n s t o l i g o s a c c h a r i d e s ) maltase, sucrase, i s o maltase, (which Gray would p r e f e r t o c a l l α-dextrinase a f t e r t h e n a t u r a l s u b s t r a t e found as a product o f p a n c r e a t i c amylase a c t i o n ) l a c t a s e , t r e h a l a s e , and p h l o r i z i n hydrolase share the work o f p o l y s a c c h a r i d e d i g e s t i o n w i t h p a n c r e a t i c amylase as suggested i n F i g u r e 5. Digestive-Sequence Polysaccharides P a n c r e a t i c Amylase
^ Oligosaccharides and D i s a c c h a r i d e s
Brush Border Saccharidases
^ Monosaccharides
Figure 5.
Sequential roles in carbohydrate digestion of pancreatic amylase and brush border saccharidases
In t h e a d u l t , p a n c r e a t i c amylase s p l i t s amylose o n l y as f a r as m a l t o t r i o s e and maltose (17) and amylopectin t o m a l t o t r i o s e , maltose and a - d e x t r i n s (18). The brush border saccharidases then take over t o cleave~Tree glucose from these products. The brush border enzymes a l s o c o n t r i b u t e d i r e c t l y the d i g e s t i v e
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
8
PHYSIOLOGICAL
EFFECTS OF
FOOD
CARBOHYDRATES
c a p a c i t y of t h e i n t e s t i n e f o r d i e t a r y d i s a c c h a r i d e s . A good d e a l o f work has made i t c l e a r t h a t t h e brush border membrane i s a d i g e s t i v e - a b s o r p t i v e surface on which the monosaccharide s u b s t r a t e s f o r t h e c a r r i e r s a r e formed by t h e a c t i o n of d i - and o l i g o s a c c h a r i d a s e s , i f they are not provided i n f r e e form i n t h e d i e t ( 1 9 ) · There i s a c l o s e p r o x i m i t y a t the brush border membrane between the s e q u e n t i a l processes of d i g e s t i o n and absorption and because of t h i s only a r e l a t i v e l y s m a l l amount of monosaccharide accumulates i n t h e lumen during t h e d i g e s t i o n of a d i s a c c h a r i d e . I n Figure 6, taken from Gray and Santiago ( 2 0 ) , i t i s seen t h a t only 10 percent of the glucose formed by brush border h y d r o l y s i s of sucrose over a 30 cm segment o f i n t e s t i n e was found i n the lumen; 90 percent having been absorbed. The experience w i t h l a c t o s e and maltose was s i m i l a r . Fructose was l e s s w e l l absorbed than glucose formed a t t h e same time from sucrose because i t s d i f f e r e n t t r a n s p o r t system i s l e s s e f f i c i e n t at equal concentrations glucose formed a t the compete w i t h t h a t glucose f o r the same t r a n s p o r t system and has a lower a f f i n i t y f o r i t . O v e r a l l , i t i s c l e a r t h a t the absorption of t h e monosaccharide products of d i s a c c h a r i d e d i g e s t i o n i s e f f i c i e n t . I n p a r t , as already mentioned, t h i s may be explained by the c l o s e f u n c t i o n a l p r o x i m i t y of the membrane d i g e s t i v e enzymes t o t h e membrane t r a n s p o r t systems; a p r o x i m i t y that we have l a b e l e d " k i n e t i c advantage" (19). A l s o i n p a r t t h i s may be explained by a f u n c t i o n of theïïTsaccharidasesas a route f o r the d i r e c t t r a n s l o c a t i o n of some of t h e i r products without the i n t e r v e n t i o n of t h e normal c a r r i e r mechanisms, as r e c e n t l y documented i n publ i c a t i o n s from our l a b o r a t o r y (14·, 15) and f u l l y corroborated by D i e d r i c h ( 2 1 ) . However, there i s no r e l i a b l e evidence t o support the i d e a t h a t the a b s o r p t i o n o f t h e monosaccharide products of d i s a c c h a r i d e s can be s u b s t a n t i a l l y more e f f i c i e n t than the a b s o r p t i o n o f the f r e e monosaccharides themselves. For t h e past 15 years, a concept has been f l o a t i n g about t o the e f f e c t t h a t there may be an advantage f o r absorption t o feed sugars i n the form of d i s a c c h a r i d e s r a t h e r than as f r e e monosaccharides. T h i s concept got i t s s t a r t w i t h some i n v i t r o experiments of Chain and h i s colleagues (22). Our s t u d i e s (19) d i d nothing t o d e t r a c t from the i d e a and d i r e c t i n v i v o experimental support f o r a s m a l l e f f e c t seemed t o be provided by human s t u d i e s c a r r i e d out by Ian MacDonald ( 2 3 ) . The most recent work on humans does n o t support the i d e a . I n f a c t , i t i s p o s s i b l e t h a t t h e i d e a has f i n a l l y been l a i d t o r e s t by the c a r e f u l s t u d i e s of Cook (24) who has found a b s o l u t e l y no d i f f e r e n c e i n the p o r t a l blood l e v e l s o f f r u c t o s e and glucose whether i t i s sucrose t h a t i s placed i n the lumen o f the i n t e s t i n e or whether i t i s an equimolar mixture of glucose and f r u c t o s e .
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
CRÂNE
Intestinal Absorption of Sugars
9
Gastroenterology
Figure 6. Role of monosaccharides released by the digestion of disaccharides over a 30-cm infusion-to-collection distance in human intestine (20)
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
The usefulness t o the organism o f the d i r e c t t r a n s l o c a t i o n of monosaccharides by brush border d i g e s t i v e hydrolases i s c u r r e n t l y a p h y s i o l o g i c a l p u z z l e and, f o r t h i s reason, the data base f o r t h i s new t r a n s p o r t pathway w i l l not be developed here t o any great extent. S u f f i c e i t t o say t h a t i n v i t r o s t u d i e s c a r r i e d out under c o n d i t i o n s where the normal c a r r i e r mechanisms f o r glucose t r a n s p o r t are e i t h e r s a t u r a t e d w i t h s u b s t r a t e and thus operating at a maximal r a t e or completely i n h i b i t e d by the omission of N a have demonstrated an a d d i t i o n a l component of glucose e n t r y i n t o the c e l l s when any d i s a c c h a r i d e substrate of a brush border enzyme i s provided. In the case of sucrose, f r u c t o s e as w e l l as glucose enters and accumulates i n the c e l l s . Moreover, the components of t r a n s l o c a t i o n c o n t r i b u t e d by the i n d i v i d u a l enzymes are a d d i t i v e when more than one d i s a c c h a r i d e i s used. C l e a r l y , these systems f o r d i r e c t t r a n s l o c a t i o n i n c r e a s e the t o t a l c a p a c i t y of the i n t e s t i n e f o r carbohydrate absorption s u b s t a n t i a l l monosaccharide c a r r i e r under which t h i s a d d i t i o n a l c a p a c i t y may f u l f i l l a need are f a r from obvious. The reason f o r t h i s , which i s probably not g e n e r a l l y appre c i a t e d , i s t h a t t h e c a p a c i t y of the i n t e s t i n e f o r a b s o r p t i o n of the monosaccharides, glucose, g a l a c t o s e and f r u c t o s e i s already t r u l y enormous. As shown i n Table I I , Holdsworth and Dawson ( 2 5 ) +
TABLE I I THE CAPACITY OF THE GUT TO ABSORB SUGARS Measured:
Glucose =
4
P' ^ min. χ 30 cm
Fructose = 0.9 x glucose C a l c u l a t e d : Glucose =
. °'
4 o n
x
mm. χ 30 cm
i d ^ £ i £ . 2 8 0 cm = 5374 g/day X
day
Fructose = 5374 χ 0.9 = 4#37 g/day THUS T o t a l D a i l y Capacity = 10,211 g > 22 l b . > 50,000 c a l . measured the a b s o r p t i v e c a p a c i t y over a 30 cm segment of i n t e s t i n e i n normal humans. At p e r f u s a t e sugar concentrations of 5 g/100 ml they obtained the measured values of 0.4 g/min/30 cm f o r glucose and 90 percent of t h a t value f o r f r u c t o s e . From these i t may be c a l c u l a t e d t h a t the t o t a l d a i l y c a p a c i t y i s 10,211 g of a mixture of glucose and f r u c t o s e ; an amount
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
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Intestinal Absorption of Sugars
11
e q u i v a l e n t t o over 22 pounds o f sugar and more than 50,000 c a l o r i e s . Although not a l l p a r t s o f the i n t e s t i n e have the same c a p a c i t y as the p a r t s t u d i e d by Holdsworth and Dawson, t h e e x t r a p o l a t e d maximal r a t e was n e a r l y twice t h a t of the value assumed i n Table I I ; namely 0.73 g/min/30 cm and the t o t a l capaci t y c a l c u l a t e d i s probably a reasonable compromise. Galactose, t e s t e d alone, was absorbed even more r a p i d l y than glucose. Such a c a p a c i t y f o r sugar absorption i s ten times more than would be needed t o provide f o r even the most unreasonable i n d i v i d u a l c a l o r i c requirements s i n c e foods i n a d d i t i o n t o sugars are g e n e r a l l y a l s o eaten and i t s great s i z e i n d i c a t e s t h a t cont r o l of sugar absorption i s not exerted a t the l e v e l of the processes of d i g e s t i o n and absorption a t the brush border membrane. C o n t r o l i s exerted by a negative feedback mechanism i n v o l v i n g receptors i n the upper i n t e s t i n e and the m o t i l i t y o f the stomach. The r e l a t i o n s h i p are diagrammed i n Figur stomach and o f the stomach i t s e l f are not i n c l u d e d because i t i s a matter o f f a c t t h a t the r e a l l y indispensable f u n c t i o n of the stomach i s t o serve as a r e s e r v o i r f o r f o o d s t u f f s and t o provide f o r t h e i r r e l e a s e i n s m a l l increments i n t o the s m a l l i n t e s t i n e through the i n t e r m i t t e n t opening o f the p y l o r i c v a l v e . The s m a l l i n t e s t i n e d i g e s t s and absorbs these increments as they are r e c e i v e d but i t s a b i l i t y t o do so depends upon c e r t a i n p h y s i o l o g i c a l l i m i t a t i o n s . Perhaps most important i s the f a c t t h a t the mucosal surface o f the s m a l l i n t e s t i n e i s osmoresponsive. That i s t o say, when the contents of the stomach are r e l e a s e d i n t o the upper s m a l l i n t e s t i n e water s h i f t s between the e x t r a c e l l u l a r f l u i d spaces o f the body and the lumen o f the i n t e s t i n e so as t o balance the osmotic a c t i v i t i e s across the mucosal membrane (26). Normally, the process i s g r o s s l y unremarkable and goes unnoticed. Under abnormal circumstances, however, such as f o l l o w i n g surgery of the stomach so extensive as t o e l i m i n a t e i t s r e s e r v o i r funct i o n , the simple a c t o f e a t i n g may l e a d t o sudden and excessive hyperosmolarity i n the upper i n t e s t i n e w i t h consequent water s h i f t s l a r g e enough t o r e s u l t i n the p h y s i o l o g i c a l response known as the "dumping dyndrome" wherein there can be s e r i o u s vasomotor disturbances i n c l u d i n g sweating, nausea, d i a r r h e a , a f a l l i n blood pressure and weakness (27). S i m i l a r l y , the l a r g e i n t e s t i n e i s a l s o osmoresponsive (28) but i t cannot absorb sugars. Thus when, due t o disease o r surgery, the s m a l l i n t e s t i n a l c a p a c i t y f o r sugar d i g e s t i o n o r a b s o r p t i o n i s so g r e a t l y reduced t h a t a s u b s t a n t i a l amount of unabsorbed sugar enters the large, i n t e s t i n e , d i a r r h e a w i l l ensue owing t o the osmotic p r o p e r t i e s o f the sugar i t s e l f as w e l l as t o any i n c r e a s e i n o s m o t i c a l l y a c t i v e molecules through b a c t e r i a l breakdown o f the sugar t o l a c t i c and other acids (2S). I t takes o n l y 54 grams of glucose t o produce one l i t e r of the osmotic equivalent of the e x t r a c e l l u l a r f l u i d s and, thus, a t l e a s t one
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
12
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
l i t e r o f excess e x c r e t i o n . Under normal circumstances the system i s under c o n t r o l and such untoward e f f e c t s do not happen. Foodstuffs i n general, f a t s , e s p e c i a l l y , but p r o t e i n s a l s o as w e l l as carbohydrates, when they enter the i n t e s t i n e through the p y l o r i c v a l v e e l i c i t responses which slow g a s t r i c emptying. The case of sugars i s shown i n F i g u r e 8 by a summation o f many s t u d i e s c a r r i e d out by J. N. Hunt and h i s colleagues. An i n i t i a l "meal" of 750 ml of a s o l u t i o n of c i t r a t e was placed by tube i n t o the stomach. Most of t h e "meal" was del i v e r e d t o the i n t e s t i n e over the next 20 minutes. The volume d e l i v e r e d i n the same span o f time was reduced by the a d d i t i o n of glucose and the degree t o which the d e l i v e r e d volume was r e duced i n c r e a s e d as the glucose c o n c e n t r a t i o n i n c r e a s e d . C l e a r l y , r e c e p t o r s i n the l i n i n g o f the s m a l l i n t e s t i n e respond t o t h e e n t e r i n g glucose i n p r o p o r t i o n t o i t s c o n c e n t r a t i o n and a c t t o reduce g a s t r i c m o t i l i t r e c e p t o r s f o r glucose ar specific. Fructose g e n e r a l l y d i d not e l i c i t an i n h i b i t o r y response at low c o n c e n t r a t i o n s . C l e a r responses t o f r u c t o s e r e q u i r e d concentrations o f about 300 m i l l i m o l e s / l i t e r and more. Sucrose, or a mixture of glucose and f r u c t o s e , as might be expected, were intermediate i n t h e i r e f f e c t s . Once t h e process of stomach emptying s t a r t s the stomach d e l i v e r s i t s contents a t a r a t e roughly p r o p o r t i o n a l t o t h e i r n u t r i t i v e d e n s i t y ( k c a l / m l ) ( J . N. Hunt, p e r s o n a l communication) and i n a p r e d i c t a b l e and e x p o n e n t i a l f a s h i o n u n t i l the stomach i s very n e a r l y empty. During t h i s process, monosaccharides, i n the d i e t or produced by d i g e s t i o n , are moving i n t o and down t h e i n t e s t i n e and are being absorbed by the c a r r i e r mechanisms e a r l i e r mentioned; f r u c t o s e by means of a f a c i l i t a t e d d i f f u s i o n c a r r i e r , glucose and g a l a c t o s e by means of a Na -dependent cotransport c a r r i e r . I t may be asked, why? What i s the value t o the economy o f the organism t h a t these p a r t i c u l a r mechanisms are used and t h a t d i f f e r e n t mechanisms are used f o r d i f f e r e n t kinds of sugar. An answer may be t h a t the needs are best matched i n t h i s way. The fundamental d i f f e r e n c e between the two c a r r i e r systems i s t h a t the one, the Na -dependent c a r r i e r , can be energized t o produce a c t i v e ( a g a i n s t t h e c o n c e n t r a t i o n g r a d i e n t ) t r a n s p o r t whereas the other, the f a c i l i t a t e d d i f f u s i o n c a r r i e r cannot. This d i f f e r e n c e would seem t o match the energy demands of t h e r e s p e c t i v e a b s o r p t i v e problems. In the case of f r u c t o s e , f r u c t o s e l e v e l s i n the blood during i t s absorption are low, being only one-tenth those of glucose during i t s a b s o r p t i o n , (10-15 mg % as against 150-200 mg %) and f r u c t o s e i s r a p i d l y metabolized reducing the l a t e - or p o s t a b s o r p t i v e blood f r u c t o s e t o very low l e v e l s . Consequently, there i s no l a r g e s t a b l e b l o o d - t o - i n t e s t i n a l lumen gradient of f r u c t o s e c o n c e n t r a t i o n and there may simply be no need f o r +
+
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
CRÂNE
13
Intestinal Absorption of Sugars
SMALL
STOMACH
LARGE INTESTINE
INTESTINE
J ' BACTERIAL
„
DIGESTION and ABSORPTION
Γ—ι
1
1
r
Ί
FERMENT ATION
Figure 7. Schematic of intestines
VOLUME PLACED IN STOMACH
100 Ο >
200
MILLIMOLES/LITER
300
400
500
OF MONOSACCHARIDE
Figure 8. Effect of carbohydrate con tent on the rate of stomach emptying of a test meal. Drawn from data published in graph ic form by Hunt and Knox (29).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
MINUTES Figure 9. Time course of glucose absorption from a loop of rabbit intestine, in vivo. Drawn from data published in graphic form by Barany and Sperber (30).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
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Intestinal Absorption of Sugars
15
f r u c t o s e t o be absorbed by an energized t r a n s p o r t system. Hence, the f a c i l i t a t e d d i f f u s i o n c a r r i e r s u f f i c e s . The case i s d i f f e r ent f o r glucose. The blood i n h e a l t h always contains a p p r e c i able (80-90 mg %) glucose and one may be c e r t a i n t h a t a q u a n t i t a t i v e l y important p a r t o f the absorption o f a load o f glucose w i l l r e q u i r e the p a r t i c i p a t i o n of an energized c a r r i e r because t h a t p a r t o f a b s o r p t i o n w i l l n e c e s s a r i l y be " u p h i l l " from the lumen t o the blood. What takes p l a c e i n the i n t e s t i n e f o l l o w i n g a l o a d of g l u cose i s w e l l i l l u s t r a t e d by the experiments o f Barany and Sperber (30) w i t h l i v e r a b b i t s as shown i n F i g u r e 9. These workers placed a c e r t a i n volume o f a concentrated glucose s o l u t i o n i n t o a c l o s e d loop of the r a b b i t s i n t e s t i n e and sampled the contents o f the loop a t the i n t e r v a l s t h e r e a f t e r . Initially the c o n c e n t r a t i o n o f glucose i n the i n t e s t i n e was higher than glucose i n the blood. Consequently a b s o r p t i o n during t h i s p e r i o d took place down f e r of sugar from the i n t e s t i n no energy input other than d i f f u s i o n a l . This i s the " d o w n h i l l " component. L a t e r , as a b s o r p t i o n progressed, the c o n c e n t r a t i o n of glucose i n the i n t e s t i n e became lower than i n the blood. Continued absorption consequently took p l a c e " u p h i l l " a g a i n s t t h e c o n c e n t r a t i o n g r a d i e n t and would r e q u i r e the i n p u t o f energy other than d i f f u s i o n a l . At t h e o u t s e t , one might suppose t h a t two d i f f e r e n t c a r r i e r s are used; one f o r the d o w n h i l l component and another f o r the uph i l l . However, t h i s i s not the case. The evidence says t h a t the same c a r r i e r s are used f o r both components. I f these c a r r i e r s were t o have the requirement f o r the consumption o f metabolic energy i n the u p h i l l mode b u i l t i n t o t h e biochemical mechanisms they would be w a s t e f u l when o p e r a t i n g i n the d o w n h i l l mode. In Table I I I are compared four types of membrane t r a n s p o r t which are e i t h e r known or have been proposed t o occur i n animal cells. These a r e : ( l ) F a c i l i t a t e d d i f f u s i o n which f o r present purposes i s viewed as having the c h a r a c t e r i s t i c s o f a symmetrical biochemical r e a c t i o n i n which the s t a t i o n a r y s t a t e achieved w i l l be a 1/1 e q u i l i b r i u m between f r u c t o s e i n s i d e and f r u c t o s e o u t s i d e the c e l L F a c i l i t a t e d d i f f u s i o n can operate only " d o w n h i l l " . (2) V e c t o r i a l biochemical r e a c t i o n s of t h e k i n d envisaged i n the phosphorylation-dephosphorylation hypothesis of the 1930 s - 1950*s ( 31) wherein i t was proposed t h a t the energy f o r accumulation was d e l i v e r e d t o the s u b s t r a t e , glucose, by the t r a n s f e r o f phosphate from ATP w i t h subsequent h y d r o l y s i s t o r e l e a s e f r e e sugar. Roseman and h i s colleagues and Kaback have s t u d i e d systems i n b a c t e r i a l membranes which are of t h i s g e n e r a l type except t h a t phosphorylated sugar and n o t f r e e sugar i s accumulated (32). The asymmetry of the biochemical r e a c t i o n s i n such systems i s obvious. ( 3 ) C o v a l e n t l y energized c a r r i e r s which a r e l i k e the 1
f
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
C + ATP C * Ρ + ADP C * Ρ + Go + HOH C + Gi + Ρ
C + Nao C-Na C-Na + Go C-Na-G C-Na-G «r+ C-Na + G i C'Na C + Nai t o pump
(3) C o v a l e n t l y Energized Reaction
(4) Cotransport Energized Carrier
[Gi] > [Go] [Nai] > [Nao]
no
yes
both
[Gi] > [Go]
both
yes
both
[ F i ] = [Fo]
[Gi] > [Go]
no
Downhill only
Stationary State
C = c a r r i e r , F = f r u c t o s e , G = glucose, Ρ = phosphate, ο = o u t s i d e , i = i n s i d e .
C-G6P + ADP C + Gi + Ρ
C + Go + ATP C-G6P + HOH
( 2 ) V e c t o r i a l Biochemical Reaction
C + Fi
C + Fo
C-F
Reaction Involved
(1) Symmetrical Biochemical Reaction ( F a c i l i t a t e d Diffusion)
D e s c r i p t i v e Name
I s Bond Energy Consumed i n Downhill Mode
Downhill or Uphill Transport Capability
Reactions Involved i n and Energy U t i l i z a t i o n by V a r i o u s H y p o t h e t i c a l Types o f Membrane Transport
TABLE I I I
1.
CRÂNE
Intestinal Absorption of Sugars
17
v e c t o r i a l b i o c h e m i c a l r e a c t i o n s i n t h a t they are fundamentally asymmetrical but which d i f f e r i n t h a t t h e energy f o r accumulation i s d e l i v e r e d t o t h e c a r r i e r r a t h e r than t o the s u b s t r a t e . Perhaps the best example of a c o v a l e n t l y energized c a r r i e r i s the c e l l membrane sodium pump which expresses i t s e l f as an Na^K " a c t i vated ATPase (33). (4) Cotransport energized c a r r i e r s o f the k i n d already d e s c r i b e d above. I n the absence o f a N a f l u x t h e r e a c t i o n s o f these c a r r i e r s a r e symmetrical. I n t h e presence o f a N a f l u x t h e r e a c t i o n s are, as i n d i c a t e d , asymmetrical. As a consequence of t h i s d u a l i t y the cotransport energized system i s the o n l y one o f t h e f o u r which i s not only capable o f both u p h i l l as w e l l as d o w n h i l l t r a n s p o r t but which a l s o does n o t have an absolute requirement t o u t i l i z e bond energy i n the d o w n h i l l mode. The c o t r a n s p o r t energized system i s capable o f a d j u s t i n g energy use t o energy need and i s thus c o n s e r v a t i v e . The a b i l i t y of the i n t e s t i n e t o absor quantitie as c a l c u l a t e d above i s evolved i t s f u n c t i o n s under c o n d i t i o n s o f l i m i t e d food supply where s t r e s s would be expected on developing a system w i t h the a b i l i t y t o capture every l a s t a v a i l a b l e molecule. Under t h e same c o n d i t i o n s there would seem t o be an advantage t o a t r a n s p o r t mechanism which d i d n o t waste t h i s precious food i n prov i d i n g energy merely t o s a t i s f y t h e needs o f the mechanism and not the needs of the work. 4
+
+
Summary The f o l l o w i n g p o i n t s can be r e i t e r a t e d i n summary. 1. Sugar i s absorbed i n t h e form o f monosaccharides by means o f s p e c i f i c brush border membrane c a r r i e r s which are i n c l o s e f u n c t i o n a l p r o x i m i t y t o the brush border d i g e s t i v e h y d r o l ases or by means o f an h y d r o l a s e - r e l a t e d d i r e c t t r a n s l o c a t i o n . 2. There i s normally no advantage f o r a b s o r p t i o n t o provide sugar i n t h e form o f d i s a c c h a r i d e . 3. The t o t a l c a p a c i t y of the s m a l l i n t e s t i n e f o r sugar a b s o r p t i o n i s enormous. 4. The r a t e o f carbohydrate a b s o r p t i o n i s c o n t r o l l e d by negative feed-back t o stomach emptying from s u g a r - s p e c i f i c osmor e c e p t o r s l o c a t e d i n t h e upper i n t e s t i n e . The r e c e p t o r s are l e s s responsive t o f r u c t o s e than t o glucose. 5. The a b s o r p t i o n o f glucose and g a l a c t o s e can take p l a c e both up as w e l l as down a c o n c e n t r a t i o n g r a d i e n t from i n t e s t i n e t o blood. The same c a r r i e r s are used i n both the d o w n h i l l and the u p h i l l modes. 6. The c a r r i e r s f o r glucose and g a l a c t o s e are energized by the cotransport of N a thus p r o v i d i n g a p o s s i b l e advantage f o r energy conservation i n t h a t bond energy need not be consumed i n the d o w n h i l l mode. +
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
18
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
Acknowledgments The work o f the author c i t e d i n t h i s review was supported bygrants from the N a t i o n a l Science Foundation and the N a t i o n a l I n s t i t u t e of A r t h r i t i s , Metabolism and D i g e s t i v e Diseases. Mr. R. M i l t o n prepared the i l l u s t r a t i o n s . Literature Cited
1. Bloom, W. and Fawcett, D. W. "A Textbook of Histology", 9th
Edition, pp. 560-568, W. B. Saunders Co., Philadelphia, 1968. 2. Forstner, G. and Wherrett, J. R. Biochim. Biophys. Acta. (1973) 306, 446-459. 3. Lindemann, B. and Solomon, A. K. J. Gen. Physiol. (1962) 45, 801-810. 4. Fordtran, J. S., Rector and Kinney, J. J. 5. Fromter, E. and Diamond, J. Nature New Biology (1972) 235, 9-13. 6. Crane, R. K. in Code, C. F. Editor "Handbook of Physiology, Section 6. Alimentary Canal, Volume 3. Intestinal Absorption," pp. 1323-1351, American Physiological Society, Washington, 1968. 7. Schultz, S. G. and Strecker, C. K. Biochim. Biophys. Acta. (1970) 211, 586-588. 8. Gracey, Μ., Burke, V. and Oshin, A. Biochim. Biophys. Acta. (1972) 266, 397-406. 9. Honegger, P. and Semenza, G. Biochim. Biophys. Acta. (1973) 318, 390-410. 10. Schultz, S. G. and Curran, P. F. Physiol. Revs. (1970) 50, 637-718. 11. Crane, R. K. Amer. J. Clinical Nutrition (1969) 22, 242-249. 12. Diamond, J. M. Federation Proc. (1971) 30, 6-13. 13. Crane, R. K. in M. Florkin and E. Stotz, Editors, "Compre hensive Biochemistry, Vol. 17, Carbohydrate Metabolism", pp. 1-14, Elsevier,Amsterdam, 1969. 14. Malathi, P., Ramaswamy, K., Caspary, W. F. and Crane, R. K. (1973) Biochim. Biophys. Acta. 307, 613-622. 15. Ramaswamy, Κ., Malathi, P., Caspary, W. F. and Crane, R. K. (1974) Biochim. Biophys. Acta. 345, 39-48. 16. Crane, R. K. in T. Z. Csaky, Editor, "Intestinal Absorption and Malabsorption", Raven, Press, New York, in press. 17. Messer, M. and Kerry, K. R. (1967) Biochim. Biophys. Acta. 132, 432-443. 18. Walker, G. J. and Whelan, W. J. (1960) Biochem. J. 76, 257-263. 19. Crane, R. K. in Κ. B. Warren, Editor, Symposia of the International Society for Cell Biology, Vol. 5, Intracellular Transport, pp. 71-102, Academic Press, New York, 1966.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
1.
CRANE
Intestinal
Absorption
of
Sugars
19
20. Gray, G. M. and Santiago, N. A. (1966) Gastroenterology 51, 489-498. 21. Diedrich, D. F. and Hanke, D. W. in T. Z. Csaky, Editor, Intestinal Absorption and Malabsorption", Raven Press, New York, in press. 22. Chain, Ε. B., Mansford, K. R. L. and Pocchiari, F. (1960) J. Physiol. 154, 39-51. 23. MacDonald, I. and Turner, L. J. (1968) The Lancet 1, 841-843. 24. Cook, G. C. (1970) Clinical Sci. 38, 687-697. 25. Holdsworth, C. D. and Dawson, A. M. (1964) Clinical Sci. 27, 371-379. 26. Code, C. F., Bass, P., McClary, G. B., Jr., Newnum, R. L. and Orvis, A. L.(1960)Amer. J. Physiol. 199, 281-288. 27. MacDonald, J. M. Webster M.M. Jr. Tennyson C H and Drapanas, T. (1969 28. Phillips, S. F. (1972) Gastroenterology , 29. Hunt, J. N. and Knox, M. T. in Code, C. F., Editor, Handbook of Physiology, Section 6: Alimentary Canal, Vol. 4, Motility, pp. 1917-1935, American Physiological Society, Washington, 1968. 30.Bárány,E. and Sperber, E. (1939) Skand. Arch. Physiol. 81, 290-299. 31. Crane, R. K.(1960)Physiol. Revs. 40, 789-825. 32. Kaback, H. R. in Tosteson, D. C., Editor, The Molecular Basis of Membrane Function, pp. 421-444, Prentice-Hall, Inc., Englewood Cliffs, 1969. 33. Caldwell, P. C. in Bittar, Ε. E., Editor, Membranes and Ion Transport, Vol. 1, pp. 433-461, Wiley-Interscience, New York, 1970.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2 Metabolic Effects of Dietary Carbohydrates—A Review SHELDON REISER U.S. Department of Agriculture, ARS, Nutrition Institute, Carbohydrate Nutrition Laboratory, Beltsville, Md. 20705
Many environmental changes have been characteristic of societies described as "Western", "urbanized" or "affluent." Among these changes are a decrease in physical activity, an increase in mental stress and changes in dietary patterns. The changes in dietary patterns of carbohydrate intake that have been developing in the United States are shown in Figure 1 (1). The share of total carbohydrate in the U.S. diet provided by sugars as compared to starch has risen from about 32% around the turn of the century until today sugars, predominantly sucrose, contribute more than 50% of the total carbohydrate. While the average per capita consumption of flours and cereals has dropped from 300 lbs/year in 1909 to 141 lbs/year in 1970, the consumption of refined sugar and other sweeteners has in creased from 87 to 126 lbs/year (1,2). These are average figures. Experts estimate that the sugar consumption by the young, that is between 6-20 years of age, probably ranges from 140 to 150 lbs/year (2). The metabolic implications of high sucrose intake in the young will be discussed later. These a f f l u e n t s o c i e t i e s are a l s o c h a r a c t e r i z e d by an i n crease i n the i n c i d e n c e o f heart disease and d i a b e t e s . Diabetes and heart disease represent two o f the most c r i t i c a l h e a l t h problems i n the U.S. today. Diabetes now a f f e c t s about 5 m i l l i o n Americans and r e s u l t s i n over 35,000 deaths a n n u a l l y . By 1980, i t i s estimated t h a t more than 10% o f a l l Americans w i l l have diabetes o r the i n h e r i t e d t r a i t o f diabetes ( 3 ) . One out o f every f o u r a d u l t s i n the U.S. between 18-79 years o f age has been diagnosed as having o r suspected o f having heart disease ( 4 ) . A c l o s e r e l a t i o n s h i p between heart disease and diabetes i s i n d i cated by the f i n d i n g s t h a t d i a b e t i c s have a much higher r i s k o f developing heart disease than the general p o p u l a t i o n (5,6) and t h a t 25% o r more o f p a t i e n t s w i t h v a s c u l a r disease are d i a b e t i c CDOne o f the most i n t r i g u i n g aspects o f t h i s changing p a t t e r n o f carbohydrate consumption i n urbanized c u l t u r e s i s the r e l a t i o n s h i p between the i n c r e a s e d i n t a k e o f r e f i n e d carbohydrates 20 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
2.
21
Metabolic Effects
REISER
683
1909-13
ι * 4ftf
1972
47.2 52.8
10 STARCH SUGAR
20
30
40
50
60
70
PERCENT
Figure I. Carbohydrate from starch and sugar. Carbohydrate consumption in the United States during the indicated years is based on food disappearing into con sumption channels. Carbohydrate in foods such as milk, fruit, and sweeteners was assumed to be present mainly as sugar, and carbohydrate in foods such as grain products and vegetables was assumed to be present mainly as starch. Adapted from Réf. 1.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
22
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
such as sucrose and the e t i o l o g y of v a r i o u s d i s e a s e s . In recent years s e v e r a l c o n t r o v e r s i a l hypotheses have i m p l i c a t e d * t h e i n creased i n g e s t i o n of sucrose, as compared to the more complex carbohydrates, as an important f a c t o r i n the e t i o l o g y o f coronary heart disease Ç8-10) and diabetes (10,11). I t i s recognized t h a t many environmental f a c t o r s , i n c l u d i n g d i e t a r y f a c t o r s , as w e l l as genetic f a c t o r s i n f l u e n c e these diseases and thus the unequivocal r o l e of one of these f a c t o r s independent of the others i s d i f f i c u l t to evaluate. However, i n view o f the controversy surroundi n g d i e t a r y carbohydrate, i t i s the purpose o f t h i s review to d e s c r i b e some of the metabolic e f f e c t s observed a f t e r f e e d i n g sucrose to experimental animals and man and to evaluate the r e l a t i o n s h i p between these metabolic changes and the disease s t a t e s . An area of c o n s i d e r a b l e controversy i s the i n t e r p r e t a t i o n of r e t r o s p e c t i v e or e p i d e m i o l o g i c a l s t u d i e s t h a t attempt t o prove the i n c r e a s e d i n c i d e n c e of a disea$e by c o r r e l a t i o n w i t h the increased i n t a k e o s p e c t i v e s t u d i e s do no as t o the causal s i g n i f i c a n c e o f the c o r r e l a t i o n s detected, they are u s e f u l i n i d e n t i f y i n g trends and i n suggesting s p e c i f i c problem areas worthy o f f u r t h e r study. Many r e t r o s p e c t i v e s t u d i e s have i n d i c a t e d a c o r r e l a t i o n between sucrose i n t a k e and heart disease (8,10,12-15), but Γ would l i k e t o concentrate p r i m a r i l y on the data gathered by an i n t e r n a t i o n a l cooperative study on the r e l a t i o n s h i p between d i e t a r y f a c t o r s and deaths from heart disease i n 37 c o u n t r i e s , p u b l i s h e d by M a s i r o n i i n 1970 (16). Table 1 summarizes these r e s u l t s . M o r t a l i t y data from heart disease ( a r t e r i o s c l e r o t i c and degenerative) were taken from "World Health S t a t i s t i c s Annuals" and v a r i o u s age and sex groups were d e f i n e d . D i e t a r y data were taken from the "Food balance sheets" p u b l i s h e d by the Food and A g r i c u l t u r e O r g a n i z a t i o n . From these c o r r e l a t i o n c o e f f i c i e n t s i t can be seen t h a t w h i l e sucrose i s somewhat l e s s s t r o n g l y c o r r e l a t e d w i t h death r a t e s than i s f a t , there i s nonetheless a s t r o n g p o s i t i v e c o r r e l a t i o n . Since the i n t a k e o f d i e t a r y c a l o r i e s , f a t and sucrose are s i m i l a r l y c o r r e l a t e d , these r e s u l t s are s t i l l open to v a r i o u s i n t e r p r e t a t i o n s . Although on the average d i e t a r y sucrose comprises w e l l over 50% of the simple sugars of the d i e t , i t appears to be much more s t r o n g l y c o r r e l a t e d w i t h heart disease deaths than the t o t a l of a l l the simple d i e t a r y sugars which a l s o i n c l u d e l a c t o s e , f r u c t o s e and glucose. This suggests t h a t sucrose has a s p e c i f i c a c t i o n on heart disease not shared by these other sugars. I t i s a l s o apparent t h a t a s t r o n g i n v e r s e r e l a t i o n s h i p e x i s t s between the amount of complex carbohydrate i n the d i e t and deaths from heart disease. This r e l a t i o n s h i p supports the contention of B u r k i t t (17) and Trowel1 (18) t h a t d i e t a r y f i b e r may exert an important p r e v e n t a t i v e a c t i o n against the i n c i d e n c e of heart disease. The question then a r i s e s as t o what metabolic p r o p e r t i e s o f d i e t a r y sucrose or i t s component monosaccharides can mediate
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. ?
-0.72 -0.63
0.32 0.24 0.52
0.64 0.56 0.75
0.70 0.84 0.70
Males, 45-54 years
Both sexes, a l l ages
Both sexes, a l l ages (1940 s-1950 s)
Death r a t e s per were taken from and the v a r i o u s balance sheets"
(16)
100,000 p o p u l a t i o n f o r a r t e r i o s c l e r o t i c and degenerative heart disease "World H e a l t h S t a t i s t i c s Annuals" (World H e a l t h O r g a n i z a t i o n , 1958, 1968) groups d e f i n e d by age and sex. D i e t a r y data were taken i n from "Food p u b l i s h e d by the Food and A g r i c u l t u r e O r g a n i z a t i o n (1949, 1966).
Adapted from
-0.71
0.33
0.64
0.55
Females, 55-64 years
,
-0.59
0.31
0.66
0.74
Males, 55-64 years
f
-0.74
Simple sugars
Sucrose
Age-sex
Total fat
Complex carbohydrates
C o r r e l a t i o n c o e f f i c i e n t s between death r a t e s from heart d i s e a s e and d i e t a r y f a c t o r s f o r 37 c o u n t r i e s d u r i n g the 1960 s
Table 1
24
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
an i n c r e a s e d i n c i d e n c e o f v a s c u l a r c o m p l i c a t i o n s , heart d i s e a s e and d i a b e t e s . Since a fundamental d i f f e r e n c e between dietarys t a r c h and sucrose r e s i d e s i n the nature o f the monosaccharide u n i t s comprising t h e r e s p e c t i v e molecules, much o f the i n t e r e s t i n d i e t a r y sucrose has focused on t h e metabolic e f f e c t s o f f r u c t o s e . S t u d i e s w i t h humans and experimental animals have e s t a b l i s h e d t h a t d i e t s c o n t a i n i n g sucrose o r f r u c t o s e produce l a r g e r i n c r e a s e s i n blood l i p i d s (19-23), e s p e c i a l l y the t r i g l y c e r i d e f r a c t i o n (24-28), and i n h e p a t i c l i p o g e n i c enzymes (29-35) than d i e t s c o n t a i n i n g an e q u i v a l e n t amount o f glucose or glucose polymers. The magnitude and d u r a t i o n o f the hyperl i p e m i a i s c o n t r o l l e d by f a c t o r s such as amount o f sugar f e d , age and sex o f t h e s u b j e c t , t h e nature o f the other d i e t a r y i n g r e d i e n t s and g e n e t i c p r e d i s p o s i t i o n . Younger s u b j e c t s and premenopausal females show less i n c r e a s e than o l d e r s u b j e c t s and males (25,26). An important f a c t o r i n t h i s sucrose e f f e c t appears t o be the amoun fat d i e t s , i . e . , l e s s tha s y n t h e s i s from carbohydrate i s a necessary and expected p h y s i o l o g i c a l process. Table 2, adapted from the work o f Macdonald (36), shows t h a t when young men were f e d a d i e t c o n t a i n i n g 60% carbohydrate, 30% f a t and 9% p r o t e i n f o r 5 days, the magnitude of the t r i g l y c e r i d e m i a was dependent on the nature o f both the carbohydrate and the f a t . Sucrose s i g n i f i c a n t l y i n c r e a s e d blood t r i g l y c e r i d e s o n l y when the d i e t a r y f a t was cream and not when i t was sunflower o i l . In c o n t r a s t , glucose d i d not i n c r e a s e the t r i g l y c e r i d e s w i t h e i t h e r f a t . The e f f e c t o f sunflower o i l may be due t o a c c e l e r a t i o n i n the removal o f endogenous t r i g l y c e r i d e s s i n c e N e s t e l and B a r t e r (37) have shown t h a t s u b j e c t s consuming d i e t s r i c h i n polyunsaturated fat have f a s t e r clearance r a t e s than s u b j e c t s f e d s a t u r a t e d fat. These r e s u l t s a l s o might e x p l a i n apparent c o n t r a d i c t i o n s found i n the l i t e r a t u r e as t o the e f f e c t o f d i e t a r y sucrose on serum l i p i d s i n t h a t unsaturated f a t t y a c i d s may mask t h i s e f f e c t . Work from Yudkin's l a b o r a t o r y (Table 3, (38)) shows t h a t sucrose produces a l a r g e r i n c r e a s e i n serum t r i g l y c e r i d e s and c h o l e s t e r o l than does s t a r c h i n r a t s f e d an atherogenic d i e t c o n t a i n i n g 16% hydrogenated coconut o i l and 1% c h o l e s t e r o l for 100 days. The carbohydrate comprised 47% o f the d i e t . By 180 days, t h e l e v e l s o f the blood l i p i d s had f a l l e n , but there was s t i l l the same r e l a t i v e i n c r e a s e i n c h o l e s t e r o l and t r i g l y c e r i d e l e v e l s produced by the sucrose as compared t o s t a r c h . These r e s u l t s again i n d i c a t e t h a t sucrose together w i t h other d i e t a r y f a c t o r s can produce a combination t h a t i s p o t e n t i a l l y more d e t r i m e n t a l t o the h e a l t h o f t h e consumer than e i t h e r o f the f a c t o r s alone. The major e f f e c t o f d i e t a r y sucrose on l i p i d metabolism i n v o l v e s the t r i g l y c e r i d e s . The r i s k f a c t o r i n v o l v e d i n e l e vated l e v e l s o f blood t r i g l y c e r i d e has r e c e n t l y been confirmed by a j o i n t statement on D i e t and Coronary Disease from the
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Ρ w i t h minimal u r i n a r y e x c r e t i o n of l ^ C . The recovery of l a b e l e d from maltose was comparable to that from i n j e c t i o n of l a b e l e d g l u cose or other monosaccharide mixtures (67). The extensive metabolism of i n j e c t e d maltose to ^C02 suggested a p o s s i b l e r e c i r c u l a t i o n through the i n t e s t i n a l mucosa w i t h subsequent o x i d a t i o n of maltose by i n t e s t i n a l maltases, o r , the p o s s i b i l i t y t h a t t i s s u e s other than s m a l l bowel mucosa might possess maltase a c t i v i t y . As seen i n Table IV, the h y d r o l y s i s and subsequent metabolism of i n t r a v e n o u s l y administered maltose was not s i g n i f i c a n t l y i n f l u e n c e d by s e l e c t i v e removal of the s m a l l bowel, kidneys, or 70% of the l i v e r (67). A n a l y s i s of maltase act i v i t y i n s e l e c t e d r a t organs (Table V) i n d i c a t e s the presence of maltase i n a v a r i e t y of t i s s u e s (67). Other estimates of r a t t i s sue maltase a c t i v i t y are comparable (70,71,72). I t t h e r e f o r e seems u n l i k e l y that c i r c u l a t i o n of i n j e c t e d maltose to s m a l l bowel mucosa p l a y s a s i g n i f i c a n t r o l e i n i t s o v e r - a l l metabolism, or
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
78
PHYSIOLOGICAL
TABLE 1
EFFECTS
4
No. animals
Glucose-1- C Glucose-U- C Glucose-1- C + galactose** Glucose-U- C + fructose-U- Ct Maltose-1- C Maltose-U- C Lactose-1- C l4
14
CARBOHYDRATES
III
Metabolism of C-labeled disaccharides iv administration in the rat*
Sugar
O F FOOD
1 4
C0
after
Urine
2
1
4
C
% dose/24 hours 5.3 ± 4.7 14.8 ± 10.3
5 5
62.0 ± 11.6 64.0 ± 12.0
4
52.0 ± 9.7
5 5 5 6 5
50.7 54.6 58.6 6.2 7.6
l4
9.8 ± 6.6
14
14
l4
14
14
Sucrose-U- C 14
± ± ± ± ±
19.3 4.8 3.2 62.1 68.4
7.9 7.0 5.8 2.7 2.4
± ± ± ± ±
4.6 3.9 3.0 13.5 10.8
* Animals received 5 mg of suger in 0.5 ml (1 μο per ml). * * M i x t u r e contained 2.5 mg of each sugar and 0.5 μο g l u c o s e - 1 - C . tMixture contained 2.5 mg and 0.25 μο of each sugar. 14
Journal of Clinical Investigation (67)
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
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Infused Maltose
AND WESER
T A B L E IV Oxidation
of
1
4
l 4
C-labeled sugars t o C 0 after iv injection in partially eviscerated rats 2
1 4
Organ removed
l4
l4
48.3 46.2 50.1 45.0
2
Glucose-1- C
Maltose-1- C
Sham Kidneys Liver (70%) Small bowel
C0
% dose/24 hours 55.3 ± 19.5 (3) 45.5 ± 17.6 (3) 43.9 ± 4.7 (3)
± 7.7(4)* ± 11.3 (4) ± 9.2 (5) ±
Lactose-1- C 14
2.9 ± 0.9 (3) 16.9 ± 5 . 9 (3) 2.4 (1)
* Number of rats is given in parenthesis. Journal of Clinical Investigation (67)
TABLE V Maltase activity in homogenates of rat organs
Maltase Organ
Intestinal mucosa Kidney Brain Liver Pancreas Spleen Muscle Serum Human serum
1
2
3
U*
U 390 61
205 73
485 17 14 2
U
1.6 4
1 0.1 9.1 0.3
0.1 0.3 12.5 0.1
4.0 1.7 5.6 0.2 0.3 8.9 0.2
*One U equals 1 μιτιοΐβ maltose hydrolyzed per minute per g protein. Journal of Clinical Investigation (67)
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
80
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
that a s i n g l e t i s s u e maltase was r e s p o n s i b l e f o r maltose o x i d a t i o n . I t i s more probable that c i r c u l a t i n g maltose, u n l i k e l a c tose o r sucrose, may be hydrolyzed by e x t r a i n t e s t i n a l maltases i n s e v e r a l t i s s u e s and subsequently metabolized. To explore whether m a l t o s y l o l i g o s a c c h a r i d e s were a l s o metab o l i z e d i n v i v o , the o x i d a t i o n of a t r a c e r dose of u n i f o r m l y l a beled - ^ C - m a l t o t r i o s e to -^CC^ a f t e r intravenous i n j e c t i o n i n the r a t was measured (73). As seen i n F i g u r e 1, t r a c e r doses of u n i formly l a b e l e d ^ C - m a l t o t r i o s e , as w e l l as u n i f o r m l y l a b e l e d 1*0maltose may be o x i d i z e d to xC>2 a f t e r intravenous i n j e c t i o n i n the r a t as e f f i c i e n t l y as U - C - g l u c o s e , w i t h 64.2 + 4.2%, 65.5 + 8.3% and 60.5 + 4.8% of the i n f u s e d dose recovered as 1*0)2, r e s p e c t i v e l y (73). When i n s u l i n i s s i m u l t a n e o u s l y administered w i t h g l u c o s e 1-14 maltose-l--^C to r a t s , the percentage of i n j e c t e d -^C ex p i r e d as ^CC>2 was the same as the recovery of 14C02 from adminis t r a t i o n of the sugar withou j e c t i o n of glucose-l-l^C. the e x p i r e d CO2 over a 6-hr p e r i o d . When i n s u l i n was added to the i n j e c t i o n s o l u t i o n , 47.5 + 6.6% of the l a b e l e d glucose was e x p i r e d as The f r a c t i o n of i n j e c t e d l^C recovered as 14(χ>2 f o l l o w i n g m a l t o s e - l - l ^ C i n j e c t i o n , w i t h and without i n s u l i n was a l s o the same, these values being 59.7 + 4.9 and 59.7 + 5.0%, r e s p e c t i v e l y . L i k e w i s e , when the two sugars are compared, there was no s i g n i f i cant d i f f e r e n c e i n the amount of glucose or maltose o x i d i z e d to CO2 w i t h or without i n s u l i n . As seen i n F i g u r e s 2 and 3, i n s u l i n d i d cause a more r a p i d o x i d a t i o n of both sugars to 1^C02« S p e c i f i c a c t i v i t y curves showed s i g n i f i c a n t l y e a r l i e r peaks when i n s u l i n was given w i t h e i t h e r glucose or maltose, than when these sugars were administered alone. When the peak e x c r e t i o n curves a f t e r maltose and glucose a d m i n i s t r a t i o n are compared, i t i s c o n s i s t e n t l y observed t h a t the peak o x i d a t i o n time a f t e r maltose i s delayed, suggesting a "precursor-product" r e l a t i o n s h i p which r e q u i r e s time f o r maltose to be hydrolyzed to glucose. I n s u l i n a l s o enhances the i n c o r p o r a t i o n of i n t r a v e n o u s l y i n j e c t e d glucose and maltose i n t o r a t epididymal l i p i d s (Figure 4 ) . The s p e c i f i c a c t i v i t y of e x t r a c t e d l i p i d s of r a t epididymal l i p i d s f o l l o w i n g intravenous i n f u s i o n of glucose p l u s i n s u l i n was 37% g r e a t e r than when glucose was i n f u s e d alone. Of p a r t i c u l a r i n t e r e s t i s the s i m i l a r response noted when i n s u l i n was i n j e c t e d w i t h maltose, r e p r e s e n t i n g a 29% i n c r e a s e over i n c o r p o r a t i o n observed when maltose was given alone. Rat epididymal t i s s u e was e q u a l l y i n s u l i n - s e n s i t i v e when e i t h e r glucose or maltose was the sugar donating i t s l a b e l e d carbon to the s y n t h e s i s of l i p i d s (74). These s t u d i e s i n d i c a t e t h a t glucose and maltose respond s i m i l a r l y to i n s u l i n stimulation. The o x i d a t i o n of i n t r a v e n o u s l y administered t r e h a l o s e has a l s o been s t u d i e d (75). This double sugar was s e l e c t e d f o r study because i t c l o s e l y resembles maltose, c o n s i s t i n g of two glucose molecules j o i n e d i n a n / - 1 , 1 l i n k a g e . Figure 5 p l o t s the 14
o r
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
YOUNG
AND WESER
Infused Maltose
ro
ο X
Minutes Biochimica et Biophysica Acta
Figure 1. Oxidation of uniformly labeled [ C] maltotriose, [ C] malt ose, and [ C] glucose to C0 after intravenous injection in the rat. Each point represents the mean value of five animate (73). 14
14
14
14
2
Time (Hours) Endocrinology 14
Figure 2. Effect of insulin on the oxidation of circulating maltose-l- C to C0 . Specific activity curve following maltose (50 mg) iv (O); maltose (50 mg) plus insulin (0.2 U)iv(m) (74).
14
2
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
Time (Hours) Endocrinology 14
14
Figure 3. Effect of insulin on the oxidation of circulating glucose-l- C to C0 L Specific activity curve following glucose (50 mg) iv (O); glucose (50 mg) plus insulin(0.2)iv{%) (74). 2
GLUCOSE
GLUCOSE INSULIN
MALTOSE
MALTOSE INSULIN Endocrinology
Figure 4. Effect of insulin on the incorporation of r e labeled glucose, glucose plus insulin, maltose and maltose plus insulin into rat epididymal tissue. Bars represent the means and S.D. for 6, 7, 6 and 6 animals, respectively (74).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
YOUNG
AND WESER
Infused Maltose
83
o x i d a t i o n o f u n i f o r m l y l a b e l e d glucose, maltose, sucrose and t r e halose f o l l o w i n g intravenous a d m i n i s t r a t i o n i n the r a t . A f t e r the i n j e c t i o n o f trehalose-U-l^C and sucrose-U-l^C only 5 . 2 6 + 0 . 8 8 % and 8.11 + 1.25% o f the i s o t o p e appeared i n the expired C02, r e s p e c t i v e l y . I n c o n t r a s t , a f t e r maltose o r glucose i n j e c t i o n , 59.60 + 2.30% and 49.30 + 3.50% r e s p e c t i v e l y , was excreted as 1*C02. Only 3 t o 5% o f the i n f u s e d maltose and glucose was excreted i n the u r i n e , w h i l e 40 to 47% o f the t r e h a l o s e and sucrose was excreted (75). The r e s u l t s o f t h i s study i n d i c a t e that w h i l e t r e halose and maltose both c o n s i s t o f two glucose molecules, t h e i r metabolic f a t e i s q u i t e d i f f e r e n t . U n l i k e maltose, t r e h a l o s e i s m i n i m a l l y o x i d i z e d t o CO2 and l a r g e l y excreted i n the u r i n e . I t has been suggested that the absence o f t r e h a l a s e and sucrase i n r a t serum and kidney (71,72,76,77) probably accounts f o r the m i n i mal o x i d a t i o n o f these sugars. I n c o n t r a s t , maltase i s r e l a t i v e l y high i n r a t kidney (67,77,78) and serum (67,77) which would exp l a i n the d i f f e r e n c e i given p a r e n t e r a l l y . I a c t i v i t y i n mammalian kidney may p l a y a r o l e i n t u b u l a r reabsorpt i o n o f t r e h a l o s e and maltose, as w e l l as glucose (79). Rat k i d ney s l i c e s have been shown to o x i d i z e l^C-maltose and l^C-maltot r i o s e t o ^C02 more than other t i s s u e s , w i t h the exception of s m a l l bowel mucosa (78). A f t e r the intravenous a d m i n i s t r a t i o n o f u n i f o r m l y l a b e l e d t r e h a l o s e and sucrose, the kidney t i s s u e accumulates the h i g h e s t cpm/g t i s s u e when compared to recovery o f the l a b e l i n other t i s s u e s (75). These s t u d i e s lend f u r t h e r support to the suggestion t h a t the presence of the d i s a c c h a r i d a s e i n the r e n a l t u b u l a r t i s s u e may be a major determinant o f the e f f i c i e n c y of d i s a c c h a r i d e metabolism subsequent t o intravenous i n f u s i o n . I n an animal such as the r a b b i t w i t h h i g h t r e h a l a s e a c t i v i t y i n the kidney (72,78), 64% o f i n t r a v e n o u s l y administered trehalose-U-l^C was o x i d i z e d t o ^*C02> w h i l e l e s s than 2% was excreted i n the u r i n e (75). The recovery o f 1*C as ^C02 a f t e r i n f u s i o n o f t r e halose i n the r a b b i t i s seen i n Figure 6. The e f f i c i e n t o x i d a t i o n o f i n t r a v e n o u s l y administered maltose poses a question concerning the entry o f t h i s sugar i n t o the c e l l p r i o r to i t s metabolism. A comparative study of the uptake of maltose and other sugars i n t o r a t diaphragm c e l l s i n d i c a t e d that at e q u i l i b r i u m a l l sugars entered c e l l s by d i f f u s i o n (80). A t a l l equimolar c o n c e n t r a t i o n s , d i s a c c h a r i d e t r a n s p o r t i n t o i n t r a c e l l u l a r water was e q u a l , but o n l y 50% that o f monosaccharides. When i n t r a c e l l u l a r sugar was c a l c u l a t e d on a weight b a s i s (mg/ml) t r a n s p o r t of a l l d i s a c c h a r i d e s and monosaccharides was equal (80). In c e l l s that possess i n t r a c e l l u l a r d i s a c c h a r i d a s e a c t i v i t y , h y d r o l y s i s o f d i s a c c h a r i d e s and monosaccharides would account f o r t h e i r subsequent metabolism. The slow 24-hr i n f u s i o n s o f n u t r i e n t s o l u t i o n c o n t a i n i n g 25% maltose hydrate, amino a c i d s , v i t a m i n s and e l e c t r o l y t e s i n r a t s over a 14 day p e r i o d r e s u l t e d i n severe weight l o s s e s and up t o 52% u r i n a r y e x c r e t i o n of the i n f u s e d maltose. S i m i l a r i n f u s i o n s
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
14
14
2
14
14
14
Comparative Biochemistry and Physiology
Figure 5. Oxidation of intravenously administered glucose-U- C, maltose-U- C, sucrose-U- C, and trehalose-U- C to C0 in rats over a 6-hour period. Each point represents the mean value for 6, 6, 4 and 7 animals respectively (75).
HOURS
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Figure 6. 2
Comparative Biochemistry and Physiology
Oxidation of circulating trehalose to C0 in the rabbit. Points represent mean ± S.D. for one animal (75).
14
HOURS
86
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
of glucose s o l u t i o n s r e s u l t e d i n weight maintenance and l i t t l e o r no u r i n a r y e x c r e t i o n (81). While t h i s study concludes that l o n g term p a r e n t e r a l maltose cannot serve as a t o t a l c a l o r i c s u b s t i t u t e f o r glucose i n complete p a r e n t e r a l n u t r i t i o n , r e s u l t s are l i m i t e d to observations i n only two animals. A d d i t i o n a l long-term parent e r a l s t u d i e s are needed. Human S t u d i e s . A f t e r l a c t o s e o r sucrose i n f u s i o n i n man, these d i s a c c h a r i d e s are l a r g e l y excreted i n the u r i n e (67,82). As seen i n Table V I , only s m a l l q u a n t i t i e s o f i n f u s e d maltose appear i n the u r i n e , suggesting that t h i s d i s a c c h a r i d e i s metabolized (67). O x i d a t i o n curves o f i n t r a v e n o u s l y administered maltose to h e a l t h y , normal s u b j e c t s are seen i n F i g u r e 7 (83). When 10 g maltose c o n t a i n i n g 5 u C i maltose-U-l^C given i n t r a v e n o u s l y , t h i s d i s a c c h a r i d e was r e a d i l y metabolized to CO2 w i t h 60% of the administered r a d i o a c t i v i t hr p e r i o d . Less than 8 i n the u r i n e e i t h e r as maltose or as glucose (83). The metabolic response o f intravenous i n f u s i o n of maltose and glucose t o normal s u b j e c t s i s compared i n F i g u r e 8. Blood glucose concentrations d i d not i n c r e a s e s i g n i f i c a n t l y a f t e r maltose i n f u s i o n , although a s i g n i f i c a n t r i s e i n t o t a l reducing substances was noted, i n d i c a t i n g the presence of t h i s d i s a c c h a r i d e i n the blood. This suggests t h a t e x t r a c e l l u l a r h y d r o l y s i s of maltose to glucose i s minimal. Since human serum c o n t a i n s almost no maltase a c t i v i t y (67,72), i t i s probable t h a t maltose enters t i s s u e c e l l s i n t a c t and i s subsequently metabolized. I n i t i a l l y , there was a f o u r f o l d i n c r e a s e i n serum i n s u l i n c o n c e n t r a t i o n a f t e r glucose and a t h r e e f o l d i n c r e a s e a f t e r maltose i n f u s i o n . T h e r e a f t e r , serum i n s u l i n c o n c e n t r a t i o n s g r a d u a l l y d e c l i n e d i n a s i m i l a r manner f o r both sugars. Data from t h i s study (83) demonstrates t h a t maltose i s r e a d i l y a v a i l a b l e as a metabolic s u b s t r a t e , and may provide the r e q u i r e d m e t a b o l i t e ( s ) necessary t o i n i t i a t e i n s u l i n s e c r e t i o n . The plasma f r e e f a t t y a c i d s a t 15 min decreased 371 u E q / l i t e r a f t e r glucose and 338 u E q / l i t e r a f t e r maltose i n f u s i o n . The r e s u l t s of t h i s study i n d i c a t e t h a t the u t i l i z a t i o n o f i n f u s e d maltose e l i c i t s s i m i l a r metabolic e f f e c t s as glucose i n the normal subject. Table V I I shows the s p e c i f i c a c t i v i t y (counts/minute per m i l l i g r a m ) of serum glucose and maltose a f t e r intravenous admini s t r a t i o n of 55 u C i maltose-U-^C i one s u b j e c t . Although there was no change i n serum glucose c o n c e n t r a t i o n , the s p e c i f i c a c t i v i t o f glucose s l o w l y i n c r e a s e d d u r i n g the 60 min p e r i o d , l i k e l y r e p r e s e n t i n g r e e n t r y o f l a b e l e d glucose from t i s s u e sources. On the other hand, the s p e c i f i c a c t i v i t y o f the i n j e c t e d maltose r e mained r e l a t i v e l y constant (83). The metabolism of maltose and glucose a f t e r intravenous i n j e c t i o n was compared i n normal and m i l d l y d i a b e t i c s u b j e c t s (84). The recovery o f as ^ 0 0 2 i normal s u b j e c t s was s i m i l a r a f t e r w
a
s
n
n
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
YOUNG
A N D WESER
Infused Maltose
T A B L E VI Disaccharide recovered in 24-hour urine sample, after iv administration of 10 g in adult humans
Disaccharide infused Subject
Lactose
Sucrose
Maltose
g
9
9
J.S. I.R. B.B. B.G.
10.5
7.2
0.09
4.8
0.12
6.8
0.15
7.1
0.08
8.6
Mean±SD
8.
Journal of Clinical Investigation ( 6 7 )
T A B L E VII Specific Activity of Serum Glucose and Maltose after Intravenous Administration of 10g Maltose-U- C* 14
Specific activity
Minutes
Serum glucose
Estimated maltose
mg/100 ml
mg/100 ml
Maltose
Glucose
cpm/mg
0
87
0
0
0
15
92
61
209
5773
30
90
50
487
6153
45
90
34
878
5933
60
94
31
1000
6975
* 5 5 MCI Maltose-U- C. 14
Journal of Clinical Investigation ( 8 3 )
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
PHYSIOLOGICAL
EFFECTS
OF FOOD
CARBOHYDRATES
180'
MINUTES Journal of Clinical Investigation 14
Figure 7. Fraction of injected C recovered as expired C0 per millimole C0 over a 6-hr period after the intravenous administration of 10 g C-hbeled maltose. Points are mean values for five subjects (83). 14
2
2
14
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
5.
YOUNG
89
Infused Maltose
A N D WESER
300 GLUCOSE INFUSION
250-
MALTOSE INFUSION 255200175·
GLUCOSE mg/100 ml
150 I25-| 100' 75
255 H 200-| TOTAL REDUCING |75SUBSTANCES mq/IOOml | 5 Q
125· 100-I
75-1 0
50-1 40-| SERUM INSULIN pU/ml
3020· I0H
900 700
PLASMA FREE * FATTY ACIDS μ E./liter 4
~i—ι—ι 0
20
40
1
60
1
80
1
100
Γ 120 140
Figure 8. Blood glucose, total reduc ing substances, insulin and free fatty acids for six subjects following the in travenous administration of 25 g malt ose or glucose. Points represent the means ± S.E.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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FOOD
CARBOHYDRATES
the a d m i n i s t r a t i o n of l a b e l e d glucose and maltose, with 33.3 + 1.6%, 37.7 + 3.4% and 3 6 . 5 + 1 . 9 % recovered a f t e r i n f u s i o n of maltose-U-l^C., glucose-U-l^C and glucose-l-^C., r e s p e c t i v e l y (Table V I I I ) . D i a b e t i c subjects excreted 25.4 + 1.3% of administered maltose-U- C as C 0 , while a s i m i l a r amount, 28.3 + 0.7% was excreted a f t e r g l u c o s e - l - ^ C i n f u s i o n . Both normal and d i a b e t i c subjects showed a delayed peak e x c r e t i o n of approximately 100 min a f t e r maltose i n f u s i o n as compared to glucose i n f u s i o n . The l^co2 e x c r e t i o n curves f o r the d i a b e t i c subjects are shown i n Figure 9. Our s t u d i e s i n d i c a t e that d i a b e t i c subjects have a decreased c a p a c i t y to shunt a loading dose of both glucose and maltose i n t o C02 pathways. The u r i n a r y e x c r e t i o n of was somewhat greater a f t e r maltose i n f u s i o n than a f t e r glucose i n f u s i o n but t h i s d i f f e r e n c e was not s i g n i f i c a n t except f o r lower u r i n a r y exc r e t i o n a f t e r g l u c o s e - l - l ^ C . The r e n a l threshold and clearance r a t e s f o r maltose i n man have not been determined and the a c t u a l amount of glucose and maltos upon the concentration an i s no maltase a c t i v i t y i n human serum, human kidney t i s s u e does have maltase a c t i v i t y (85). Chromatographic separation of the urinary i n d i c a t e s that some 50-55% of the excreted r a d i o a c t i v i t y f o l l o w i n g maltose i n f u s i o n i s excreted as l ^ C glucose (84). The metabolic response to the intravenous i n f u s i o n of maltose and glucose to normal and d i a b e t i c subjects i s shown i n F i g ures 10 and 11 and the s t a t i s t i c a l a n a l y s i s i s summarized i n Table IX. Serum glucose concentration a f t e r maltose i n f u s i o n r e mained l e s s than 95 mg/100 ml over the e n t i r e 2-hr p e r i o d , i n cont r a s t to the e l e v a t i o n of serum glucose f o l l o w i n g glucose administ r a t i o n . The increase i n t o t a l serum reducing substances was simi l a r a f t e r maltose and glucose a d m i n i s t r a t i o n . In normal and d i a b e t i c s u b j e c t s , there was a s i g n i f i c a n t r i s e i n serum i n s u l i n a f t e r maltose i n j e c t i o n , however t h i s increase was greater a f t e r glucose. D i a b e t i c subjects showed an abnormal i . v . glucose t o l erance t e s t and higher serum i n s u l i n l e v e l s at 60, 90, and 120 min a f t e r glucose i n f u s i o n as compared to normal s u b j e c t s . I n s u l i n disappearance curves i n both normal and d i a b e t i c subjects f o l l o w ing maltose i n f u s i o n i n d i c a t e a slow r a t e of removal of i n s u l i n from the serum. The r e l a t i o n s h i p between the delayed disappearance of serum i n s u l i n and the serum glucose concentrations (which remain below 95 mg/100 ml) i s not known. High l e v e l s of c i r c u l a t i n g maltose (as r e f l e c t e d i n the concentrations of t o t a l serum sugars) at t h i s same time i n t e r v a l may d i r e c t l y stimulate i n s u l i n r e l e a s e . I t i s not known i f i n s u l i n i s required f o r maltose entry i n t o mammalian c e l l s . Intravenously administered maltose appears to be as e f f i c i e n t l y u t i l i z e d as glucose i n m i l d l y d i a b e t i c and normal s u b j e c t s . Further s t u d i e s i n severely d i a b e t i c p a t i e n t s are needed to determine whether maltose may be e f f i c i e n t l y metabo l i z e d despite a reduced i n s u l i n response (84). Several studies i n Japan (86,87,88) have reported the use of maltose s o l u t i o n s i n a v a r i e t y of s u r g i c a l p a t i e n t s . In a study 14
1 4
2
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
IV. M a l t o s e - U - C (5) V . G l u c o s e - l - C (5)
Diabetic
1 4
11+ III III V IV V
14
14
14
11.0 ± 2 . 4 7.3 ± 2.0 NS ρ < 0.05 NS NS NS NS
240 ± 10 138 ± 15 ρ < 0.001 ρ < 0.001 NS ρ < 0.001 NS NS
NS NS NS NS ρ < 0.05 ρ < 0.05
Journal of Clinical Endocrinology and Metabolism (84)
t Mean ± S E . + Statistical comparisons are between groups as indicated.
1 4
25.4 ± 1.3 28.3 ± 0.7
Dose ± 1.6t ± 3.4 ± 1.9
2
% Dose 11.8 ± 1.9 6.9 ± 1.2 5.1 ± 0 . 6
% 33.3 37.7 36.5
C0
1 4
C-labeled maltose or glucose to
Minutes 235 ± 10 110 ± 13 120 ± 6
4
Urinary
1
Peak C
1 4
* Number in parenthesis indicates number of subjects,
1 vs I vs II vs IV vs 1 vs II vs
1. M a l t o s e - U - C (5)* II. G l u c o s e - U - C (5) III. G l u c o s e - l - C (4)
Normal
14
Carbohydrate
after the intravenous administration of 25 g normal and diabetic subjects
Subjects
l4
Recovery of C
T A B L E VIII
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
16.0-r
14.0-
"o
I2.0H
50
100
150
200
250
300
350
400
MINUTES Journal of Clinical Endocrinology and Metabolism 14
14
Figure 9. Fraction of injected C recovered as expired C0 over a 6-hr period after the intravenous administration of 25 g glucose-l- C or maltose-U- C to five mildly diabetic subjects (84) 2
14
14
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
YOUNG
AND WESER
Infused Maltose 240-
MINUTES
Figure 10. Metabolic response of 14 control and five mildly diabetic subjects follounng the intravenous administration of 25 g glucose-l- C. Each point represents mean value ± S.E. 14
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
94
PHYSIOLOGICAL
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MINUTES
Figure 11. Metabolic response of 14 control and five mildly diabetic subjects to the intravenous administration of 25 g maltose-U- C. Each point represents mean value ±S.E. 14
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. Journal of Clinical Endocrinology and Metabolism (84)
loading. III. Diabetic maltose loading. IV. Diabetic glucose loading.
I. Normal maltose loading. II. Normal glucose
NS NS NS NS ρ < 0.05 NS NS ρ < 0.01 NS ρ < 0.01 NS NS NS ρ < 0.01 NS NS
ρ < 0.05 ρ < 0.01 ρ < 0.05 NS
NS NS NS NS
I vs II III vs IV I vs IM II vs IV
"Statistical comparisons are between subject groups indicated:
Serum Insulin, μΙΙ/ml
NS NS NS
NS
NS
ρ < 0.01 NS NS ρ < 0.05 ρ < 0.01 NS NS ρ < 0.05
ρ < 0.01 ρ < 0.05 NS NS
NS NS NS NS
NS NS NS NS
NS NS ρ < 0.01 NS
NS
ρ < 0.05 NS NS ρ < 0.05
NS NS NS ρ < 0.05
NS NS NS ρ < 0.05
ρ < 0.05 ρ < 0.001 NS NS
120
90
ρ < 0.001 ρ < 0.001 NS NS
Minutes after infusion 60 30
NS* NS NS NS
0
15
of glucose and
I vs III
II IV III IV
I vs III vs I vs II vs
Total Serum Sugars. mg/100 ml
"Apparent Maltose/' mg/100 ml
II IV III IV
I vs III vs I vs II vs
Serum Glucose mg/100 ml
Subject groups
response to the intravenous administration
maltose to normal and diabetic subjects
Statistical comparisons of metabolic
T A B L E IX
96
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O F FOOD
CARBOHYDRATES
of 67 p a t i e n t s , no secondary e f f e c t s , c l i n i c a l a b n o r m a l i t i e s , or hyperosmolar problems were recognized (84), Conclusions To date, there i s no i d e a l carbohydrate source of c a l o r i e s for parenteral alimentation. Studies of i n t r a v e n o u s l y administer ed d i s a c c h a r i d e s i n d i c a t e that maltose and glucose are e f f i c i e n t l y and s i m i l a r l y metabolized i n man and i n the r a t . Maltose, be cause of i t s c a l o r i s d e n s i t y and a b i l i t y to be metabolized, may be of importance i n p a r e n t e r a l n u t r i t i o n , and warrants f u r t h e r study. Literature Cited
1. Annan, G. L., Bull. Ν. Y. Acad. Med. (1939) 15:622. 2. Foster, M. "Claude Bernard". Longmans Green Company, New York. 1899. 3. Biedl, A. and Kraus 4. Geyer, Robert P., Physiol. Rev. (1960) 40:150. 5. Law, David Η., Adv. Intern. Med. (1972) 18:389. 6. Shils, Maurice Ε., J. Amer. Med. Assoc. (1972) 220:14. 7. Wretlind, A. (Editor). "Colloquim on intravenous feeding". Colloquim held at The Royal Society of Medicine. London, May 1-2, 1962. Nutr dieta (1963) 5:295. 8. "Parenteral Nutrition Symposium". Symposium held at Kungalv, Sweden, Nov. 9-10, 1962. Acta Chir. Scand. (1964) Suppl. 325: 1. 9. Meng, H.C. and Law, David H. (Editors). "Parenteral Nutri tion". International Symposium on Parenteral Nutrition. Vanderbilt University School of Medicine, 1968. Charles C. Thomas, Springfield, 1970. 10. Cowan, G. S. M. and Scheetz, W. L. (Editors). "Intravenous Hyperalimentation". U. S. Army Institute of Surgical Research, San Antonio, Texas. 1970. Lea and Febiger, Philadelphia, 1972. 11. Wilkison, A. W. (Editor). "Parenteral Nutrition".. An Inter national Symposium in London, April 20, May 1, 1971. Williams and Wilkins Company, Baltimore, 1972. 12. American Medical Association. "Symposium on Total Parenteral Nutrition". Symposium held at Nashville, Tennessee, Jan. 1719, 1972. American Medical Association, Council on Food and Nutrition, Chicago, 1974. 13. Dudrick, S. J., Steiger, Ε., Long, J. M., Ruberg, R. L., Allen, T. R., Vars, H. M. and Rhoads, J. E. In "Intravenous Hyperalimentation", George Cowan, Jr. and Walter Scheetz, (Editors), p. 3. Lea and Febiger, Philadelphia, 1972. 14. Wyrick, W. J., Rea, W. S. and McClelland, R. Ν.,J.Amer. Med. Assoc. (1970) 211:1967. 15. Winters, R. W., Scaglione, P. R., Nahas, G. G. and Verosky, M. J. Clin. Invest. (1964) 43:647.
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16. Thoren, L., Nutr. Dieta, (1963) 5:305. 17. Froesch, E. R. and Keller, U. In "Parenteral Nutrition". A. W. Wilkinson, (Editor). An International Symposium, Lon don, April 30, May 1, 1971, Williams and Wilkins Company, Baltimore, 1972, p. 105. 18. Hayes, M. A. and Brandt, R. L., Surgery (1952) 32:819. 19. Elman, R. and Weichsclbaum, Τ. Ε., Arch. Surg. (1951) 62:683. 20. Mehnert, Η., Forster, Η., Geser, C., Α., Haslbeck, Μ., Dehmel, Κ. H., In "Parenteral Nutrition". H. C. Meng and D. H. Law (Editors). International Symposium on Parenteral Nutrition. Vanderbilt University School of Medicine, 1968. Charles C. Thomas, Springfield, 1970, p. 112. 21. Schaefer, H. F., Med. Welt. (1960) 32:1632. 22. Daughaday, W. H. and Weichselbaum. Τ. Ε., Metabolism (1953) 2:459. 23. Miller, M., Drucker W R. Owens J. E. Craig J. W and Woodward, Jr., Η. 24. Luke, R. G., Dinwoodie, J., , Kennedy, C., J. Lab. Clin. Med. (1964) 64:731. 25. Hinton, P., Littlejohn, S., Allison, S. P. and Lloyd, J., Lancet (1971) 1:767. 26. Dudrick, S. J., MacFadyen, Β. V., VanBuren, C. T., Ruberg, R. L., Ann. Surg. (1972) 176:259. 27. Bassler, Κ. H. In "Parenteral Nutrition". H. C. Meng and D. H. Law (Editors). Proceedings of an International Sympo sium, Vanderbilt University School of Medicine, Nashville, Tennessee, 1968. Charles C. Thomas, Springfield, 1970, p.96. 28. Weichselbaum, T. E., Elman, R., and Lune, R. H., Proc. Soc. Explt. Biol. (1950) 75:816. 29. Albanese, Α. Α., Felch, W. C., Higgons, R. Α., Vestal, B. L. and Stephanson, L., Metabolism (1952) 1:20. 30. Elman, R., Amer. J. Clin. Nutr. (1953) 1:287. 31. Thoren, L., Acta. Chir. Scand., (1964). Suppl. 325, p.75. 32. Wretlind, Α., Nutr. and Metabol. (1972) 14:Suppl. p.l. 33. Moncrief, J. Α., Coldwater, Κ. B. and Elman, R., Arch. Surg. (1953) 67:57. 34. Miller, M. J., Craig, W., Drucker, W. R. and Woodward, Η., Yale J. Biol. and Med. (1956) 29:335. 35. Froesch, E. R., Zapf, J., Keller, U. and Oelz, O., Europ. J. Clin. Invest. (1971) 2:8. 36. Heuckenkamp, P. U. and Zollner, Ν., Nutr. Metabol. (1972) 14:Suppl. p.58. 37. Aitkin, J. M. and Dunnigan, M. G., Brit. Med. J. (1969) 3:276. 38. Weinstein, J. J. and Roe, J. Η., J. Lab. Clin. Med. (1952) 40:47. 39. Pletscher, Α., Helv. Med. Acta. (1953) 20:100. 40. Mehnert, Η., Mahrhofer, Ε., and Forster, Η., Munchen Med. Wschr. (1964) 106:193. 41. Sunzel, Η., Acta Chir. Scand. (1958) 115:235. 42. Berry, Μ. Ν., Proc. Roy. Soc. Med. (1967) 60:1260.
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43. Woods, H. F., Oliva, P. Α., Amer. J. Med. (1970) 48:209. 44. Bergstrom, J., Hultman, E., and Roch-Norlund, A. E., Acta. Med. Scand. (1968) 184:359. 45. Craig, G. M. and Crane, C. W., Brit. Med. J. (1971) 4:211. 46. Sahebjami, H. and Scalettar, R., Lancet (1971) 1:366. 47. Cook, G. C. and Jacobson. J., Brit. J. Nutr. (1971) 26:187. 48. Woods, H. F., Eggleston, L. U. and Krebs, Η. Α., Biochem. J. (1970) 119:501. 49. Bode, C., Schumacher, Η., Goebell, Η., Zelder, D., and Pelzel, Η., Hormone Metab. Res. (1971) 3:289. 50. Fox, I. H. and Kelley, W. N., Metabolism (1972) 20:713. 51. Pearson, J. F. and Shuttleworth, R., Amer. J. Obstet. Gynec. (1971) 111:259. 52. Woods, H. F. and Alberti, K. G. Μ. Μ., Lancet (1972) 2:1354. 53. Harries, J. T. In "Parenteral Nutrition" A W Wilkinson (Editor). An Internationa May 1, 1971. William 54. Blackley, R. L., Biochem. J. (1951) 49:257. 55. Hers, H. G., J. Biol. Chem. (1955) 214:373. 56. Lee, Η. Α., Morgan, A. G., Waldrom, R., and Bennet, J. In "Parenteral Nutrition". An International Symposium in Lon don, April 30, May 1, 1971. A. W. Wilkinson (Editor). Williams and Wilkins, Baltimore, 1972, p.121. 57. Bye, P. Α., Brit. J. Surg. (1969) 56:653. 58. Seeberg, V. P., McQuarrie, Ε. B., and Secor, C. C ., Proc. Soc. Exper. Biol. (1955) 89:303. 59. Spitz, I. Μ., Rubenstein, Α. Η., Bersohn, I., and Bassler, Κ. H., Metabolism (1970) 19:24. 60. Thomas, D. W., Edwards, J. Β., and Edwards, R. G., New Eng. J. Med. (1970) 283:437. 61. Schumer, W., Metabolism (1971) 20:345. 62. Forster, Η., Meyer, Ε., and Ziege, Μ., Klin. Wschr. (1970) 48:878. 63. Mirsky, I. A. and Nelson, Ν., Amer. J. Physiol. (1939) 127:308. 64. Coats, D. A. In "Parenteral Nutrition". A. W. Wilkinson (Editor). An International Symposium in London, April 30, May 1, 1971. Williams and Wilkins, Baltimore, 1972, p.152. 65. Lieber, C. S., Gastroenterology (1973) 65:821. 66. Bassler, Κ. H. and Bickel, H. In "Parenteral Nutrition". A. W. Wilkinson (Editor). An International Symposium in London, April 30, May 1, 1971. Williams and Wilkins, Baltimore, 1972, p.99. 67. Weser, Elliot and Sleisenger, Marvin Η., J. Clin. Invest. (1967) 46:499. 68. Dahlqvist, A. and Thompson, D. L., Acta. Physiol. Scand. (1964) 61:20. 69. Dahlqvist, A. and Thompson, D. L., J. Physiol. (1963) 162:193.
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70. Semenza, G. In "Alimentary Canal". C. F. Code (Editor). Williams and Wilkins, Baltimore, 1968, Vol. 5, Section 6, p.2543. 71. Bittencourt, Η., Sleisenger, Μ. Η., and Weser, E. Gastro enterology (1969) 57:410. 72. Van Handel, F., Comp. Biochem. Physiol. (1968) 26:561. 73. Weser, Ε., Friedman, Μ., and Sleisenger, Μ. Η., Biochim et Biophys. Acta. (1967) 136:170. 74. Young, J. M. and Weser, E., Endocrinology (1970) 86:426. 75. Flores, M., Weser, Ε., and Young, Ε. Α., Comp. Biochem. Physiol. (1974) 50B: (In press) 76. Dahlqvist, A. and Brun, Α., J. Histochem. Cytochem. (1962) 10:294. 77. Courtois, J. E. and Demelier, J. E., Bull. Soc. Chim. Biol. (1966) 48:277. 78. Sacktor, Β., Proc Nat Acad Sci (1968) 60:1007 79. Silverman, Melvin. 80. Young, E. A. and Weser, , (1974) 81. Yoshimura, N. N., Ehrlich, H., Westman, T. L., and Deindoerfer, F. H., J. Nutr. (1973) 103:1256. 82. Deane, N., Schreiner, G. E. and Robertson, J. S., J. Clin. Invest. (1951) 30:1463. 83. Young, J. M. and Weser, E., J. Clin Invest. (1971) 50:986. 84. Young, E. A. and Weser, E., J. Clin. Endocrin. and Metab. (1974) 38:181. 85. Drayfus, J. C. and Alexandre, Υ., Biochim. Biophys. Res. Commun. (1972) 48:914. 86. Sunada, Tenitake, et al, Diag. and Treat. (1971) 59:2386. (Japanese) 87. Hayasaka, Α., et al, J. New Remedies and Clinics (1972) 21:3 (Japanese) 88. Tanaka, Takaya, et al, J. New Remedies and Clinics (1971) 20:125. (Japanese)
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
6 The Metabolism of Lactose and Galactose R. GAURTH
HANSEN
Utah State University, Logan, Utah 84322 RICHARD GITZELMANN Laboratory for Metabolic Research, Children's Hospital, Zurich, Switzerland
Lactose, which galactos glucose, primary carbohydrate source for developing mammals, and in humans it constitutes 40 percent of the energy consumed during the nursing period. Why lactose evolved as the unique carbohydrate of milk is un clear, especially since most individuals can meet their galactose need by biosynthesis from glucose. Whatever the rationale for lactose in milk, the occurrence of galactose in glyco-proteins, complex poly saccharides, and lipids, particularly in nervous tissue, has suggested specific functions. The organoleptic and physical properties of galac tose and, more specifically, the simultaneous occurrence of calcium and lactose in milk, may be significant evolutionary determinants. Lactose, in contrast to other saccharides, appears to enhance the absorption of calcium, as does vitamin D (1) (2). In man, calcium absorption is associated with the hydrolysis of lactose (3). The extensive literature concerned with galactose occurrence will not be reviewed, only some relevant examples will be cited. Col lagen contains glycosylated hydroxylysine, either as galactose or as glucosyl-galactose (4). Bone collagen mainly contains galactose mono saccharides, which could be important calcium binding centers, since many carbohydrates bind calcium in aqueous solution. In humans an excretion of three oligosaccharides containing galactose is enhanced by lactose ingestion (5). The three oligosaccharides appear to represent nonreducing terminals of the bloods antigenic determinants. I. Lactose Catabolism A. Microorganisms. The utilization of lactose f o r energy o r structural purposes i s preceded by hydrolysis to the hexoses, which can be absorbed. There i s considerable potential f o r improving the properties and acceptability of lactose i n foods by hydrolyzing i t to the monosaccharides. Toward that end, substantial progress has been 100 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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Galactose
101
made toward isolating and characterizing the relevant enzyme f r o m l a c tose-fermenting microorganisms (6) (7) (8). In microorganisms, galactosidases are induced by galactose and by other sugars that have configurations related to galactose. B. Mammalian Digestion. In mammals, the r a t e - l i m i t i n g step in lactose digestion appears to be the hydrolysis of the disaccharide into absorbable monosaccharides. Most mammalian disaccharidase activity is localized i n the brush-border fraction of the mucosal c e l l s of the s m a l l intestine (9)· 1. Lactase deficiencies. Whether lactase i s induced or constitutive i n man has not been c l e a r l y established. We do know, however, that i n populations that traditionally depend on m i l k as a significant source of energy, throughou lyze lactose. But when lactos adult diet, the capacity to hydrolyze lactose seems to decline over time (10). B y contrast, lactose intolerance i s common i n many non-milk consuming adult populations. Among these people, the ability to metabolize lactose, obviously present during infancy, disappears between the ages of one and four. Evidence that lactase levels respond to an altered lactose intake i s questionable (11). Neither exposure to extra lactose (12) (13) nor r e moval of lactose f r o m the diet for periods of 40 to 50 days altered l a c tase levels i n the intestinal tissues of adults. Perhaps of more s i g n i f i cance, nine i n a group of ten galactosemic children, ages 7 to 17 years, who had carefully avoided lactose-containing materials since e a r l y infancy, had normal blood glucose responses to o r a l lactose loads, suggesting that their lactase levels had not decreased during their long periods of lactose abstinence. The one exception was a 15-year old Negro who was determined by biopsy to be lactase deficient (14). Any potential for an adaptive response to lactose i n humans i s insignificant during the life span. A genetic basis f o r adult lactase deficiency i s indicated by the 70 percent of black A m e r i c a n s who are r e ported to be lactose intolerant, which duplicates the adult intolerance level of black A f r i c a n s , whose exposure to lactose i s probably much l e s s . Although the question has not been conclusively resolved i n humans, adult lactase deficiency may be p r i m a r i l y under genetic control. Intestinal lactase may have occurred i n i t i a l l y i n adult humans as a consequence of the domestication of milk-producing livestock. This concept i s substantiated by the lifelong presence of lactose i n most western European adults and in those ethnically derived f r o m Europe, These people can effectively digest the lactose i n d a i r y foods. A c c o r d -
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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ing to this hypothesis, adaptation to the presence of lactose in the diet required many generations and a period of several thousand years. Adult blacks in the United States have not developed intestinal lactase after exposure to lactose f o r 300 years. When milk-producing animals were domesticated it i s postulated that some adults with lactase deficiency became lactase producers through gene mutation (15). The specific selective advantage of the lactose-tolerant adult was associated with the lactose-induced enhancement of calcium absorption in an environment that provided a low dietary supply of vitamin D. M i l k consumption thus may be advantageous for lactose-tolerant individuals because m i l k supplies both calcium and a factor promoting its absorption. The r a r e l y o c c u r r i n g congenital lactose intolerance that i s due to a deficiency of lactase d b H o l z e l (16) "Alactasia " A limited number of infant the mode of inheritance i s not clear. Another congenital lactose intoler ance i s associated with the inability of infants to hydrolyze lactose and the subsequent appearance of lactose i n the urine. These phenomena generally seem to be secondary to mucosal damage associated with acute infectious diarrhea, and probably constitute a different and more complex syndrome than alactasia. A voluminous and important literature i s developing describing deficiencies of lysosomal beta- and alpha-galactosidases (17) (18). In F a b r y s Disease, there i s a deficiency of the ^-galactosidase which normally hydrolyzes ceramide trihexoside, resulting i n the accumulation of trihexosylceramide in various organs, p r i m a r i l y i n kidneys. Three other sphingolipidoses are attributed to the deficiency of an enzyme h y d r o l y z i n g ^ - g a l a c t o s i d i c bonds: Krabbe s Disease, Ceramidelactoside Lipidosis, and Generalized Gangliosidosis. Galactosidases obviously play an important role in the regulation of glycolipid levels in tissues. 1
T
II. Lactose Biosynthesis It was e a r l y concluded (19) that UDP-galactose was the donor and glucose-1-phosphate the acceptor during the synthesis of lactose. The product of the reaction, the phosphate ester of lactose, was postulated to have a role i n its excretion by the glandular tissue. By using C isotopes in the lactating cow and isolating the postulated i n t e r mediates and products, investigators concluded that a major pathway for lactose synthesis had UDP-galactose as the donor, and free glucose (not glucose-l-phosphate) as the acceptor (20). Tissue extracts subsequently were found to catalyze the synthesis of lactose f r o m UDPgalactose and glucose but, contrary to expectations, the yields were 1 4
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Lactose and Galactose
very low (21). This work confirmed the isotope studies in the whole animal. A. Lactose Synthetase. Our understanding of lactose synthesis phenomena was substantially advanced when the lactose synthetase was resolved into two components: a mixture of "A" protein fractionated from mammary glands, and of ^ f - l a c t a l b u m i n , a protein normally found i n m i l k (22) (23). In the absence of Pf-lactalbumin, the "A" protein w i l l catalyze galactosyl transfer to N-acetylglucosamine (24). Glucose i s not a good acceptor, however, having a high apparent M i c h a e l i s constant of 1 M. In the presence of c^f-lactalbumin, the A protein effectively catalyzes the synthesis of lactose, decreasing the K about 1,000 fold (25). Thus c^-lactalbumin i s a specifier protein for the synthesis of lactose The net effect of of-lactalbumin i s to con vert a glycosyl transferas plex polysaccharides to an efficient system f o r lactose synthesis (Figure 1). The mammary gland i s unique i n that it can produce ôf-lactalbumin, and in so doing it makes glucose an effective substrate f o r the galactosyl transferase enzyme. The p r i n c i p a l function of the galactosyl transferases that originate in tissues other than lactating mammary glands i s to transfer galactose to an appropriate carbohydrate side chain of glycoproteins and lipids. On this basis it i s expected that these transferases are widely distributed. Significantly, bovine • UDP-gal + P P i
*
UDP-glc
* UTP + glc-l-P
!
Sum d , c and d: gal-l-P —
±> g l c - l - P
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Due to its low specificity, the UDP-glc pyrophosphorylase w i l l catalyze both reactions d and d (although claims have been made that a separate enzyme (52) (53) exists i n human l i v e r for each of the two r e actions). f
G. Uridine Diphosphate Galactose C-4-epimerase. The epimerase, which i s required together with the pyrophosphorylase, seems to be ubiquitous and occurs e a r l y i n human development (54). The epimerase-catalyzed reaction i s the p r i m a r y site of glucose formation f r o m galactose when this sugar i s being metabolized for energy. Conversely, when galactose i s needed f o r structural purposes, it i s derived f r o m glucose at the nucleoside diphosphate hexose level, v i a the same r e action. The UDP-gal-C-4 epimerase reaction requires D P N as a cofactor. When the enzym D P N usually dissociate however, retain D P N more tenaciously. Reduced pyridine nucleotide strongly inhibits the animal enzymes. In hemolysates f r o m red cells f r o m newborn infants, two d i s tinct bands of epimerase are distinguishable on electrophoresis, while f r o m adult red cells only one distinctly different band i s found (55). When N A D i s added to the electrophoresis gel containing epimerase, the two-banded pattern characteristic of infant hemolysate i s seen. A n N A D dependent structural interchange must be responsible f o r this observation. The specific hydrogen atoms involved i n the hexose interconversion are removed f r o m one hexose substrate and replaced on the other i n the opposite steric mode at Carbon 4. This requires a substantial change i n the conformation of the hexoses during the reaction, with the carbon-bound hydrogens being retained. In the epimerase reaction, a keto intermediate at Carbon 4 was established by Nelsestuen (56) and confirmed by an isotope effect on the reaction rate (57) (56) (58) (59) (60). Ethanol oxidation generates an increase i n N A D H 2 , which apparently inhibits the UDP-galactose C-4-epimerase (61). This i n turn inhibits the clearance of galactose f r o m the blood (62). Adding N A D to a l i v e r preparation has partially prevented the enhancement effect of ethanol on N A D H 2 . In pregnant women the capacity of the l i v e r to remove galactose i s accelerated and the inhibitory effect of ethanol i s greatly reduced (63). +
+
+
+
+
+
H. Genetic Defects. Galactosemia i s an inborn e r r o r of metab o l i s m (64) caused by an inherited deficiency of g a l - l - P u r i d y t r a n s f e r ase, which normally catalyzes the second step i n the conversion of
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galactose to glucose. Disease manifestations are cataracts, l i v e r , and kidney dysfunction and disturbed mental development due to organic brain damage. Symptoms are apparent when galactose i s ingested and g a l - l - P (i.e., the product of the kinase reaction) accumulates. Defects in catalysis during the other two steps in the L e l o i r pathway have now been described in humans. Following the discovery of the transferase defect, a number of kinaseless f a m i l i e s were reported. In patients with a defect i n the kinase (65), galactose and galactitol, but not galactose-l-phosphate, accumulate i n the tissues and cataracts develop early i n infancy. The other toxic manifestations of transferaseless galactosemia do not occur in persons lacking the kinase. Since cataracts develop in patients with either defect, galactitol must be the causative agent for cataract formation in both the kinase and transferase defects. Galactokinase deficienc who had eye cataracts (66) develope g their f i r s t year of life. In humans, an epimerase deficit i n blood cells has been reported (67) (54). If the l i v e r and other organs are also epimerase deficient, the function of the epimerase reaction in metabolism would dictate a carefully controlled galactose intake. The dietary problem would be to provide enough galactose to meet the requirements for this sugar and its derivatives, but not so much as to produce toxic quantities of i n t e r mediates. In the absence of epimerase, presumably the kinase and pyrophosphorylase reactions would provide a biosynthetic route f o r UDP-gal. 1. Genetics of transferase deficiency. Since the absence of g a l a c t o s e - l - P - u r i d y l transferase activity i s the basis for diagnosing galactosemia, the development of quantitative methods f o r measuring the transferase in erythrocytes allowed precise definition of i n t e r r e l a tionships in families of patients with the disease (68) (69) (70). Direct measurements of the enzyme i n erythrocytes confirmed little o r no transferase activity i n a l l tested galactosemics. Further, the parents, some siblings, and some other relatives of the tested patients had, on the average, only half the normal level of enzyme. Both parents, as well as a paternal and a maternal grandparent, must therefore be c a r r i e r s , o r genetic hétérozygotes, before the disorder w i l l be expressed in offspring. This finding c l e a r l y establishes galactosemia as an autosomal-recessive disease. In tested galactosemic families, the offspring have demonstrated the expected mendelian ratios of one galactosemic:two hétérozygotes: one normal. At least one of the maternal and one of the paternal grandparents of the patient and other relatives have been hétérozygotes i n the
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frequency predicted f r o m the hypothesized mode of inheritance. 2. Genetic variants of transferase deficiency. A so-called Duarte variant of g a l - l - P - u r i d y l transferase has been defined by a r e finement of the chemical assay and by electrophoretic separation of blood c e l l proteins having transferase activity. F a m i l y relationships have been defined for the Duarte mutant. The hétérozygote has about three-fourths the normal transferase activity. The homozygote has one-half the normal value of the transferase. The g a l - l - P u r i d y l transferase i n the Duarte variant appears to be a structural mutation since starch gel electrophoresis reveals two faster moving proteins with catalytic activity (71) (72). The importance of having a detailed structural analysis of the transferase i s apparent. On the basis of electrophoresis and the u r i d y l transferase r e d c e l l value, individuals hav and the Duarte structura cal galactosemics i s revealed by immulogical procedures, but i s i n active according to chemical assays (74) (75). Structural alteration i s assumed to prevent the less active Duarte variant and the inactive galactosemia protein f r o m performing its normal catalytic function (76). Other transferase variants have been identified. It has been shown (77) that a so-called Negro variant could have a 10 percent norm a l transferase activity based on assays of uridyl transferase in l i v e r and intestinal tissues, even though the r e d cells showed no activity. Since this i s difficult to explain morphologically, a more rigorous characterization of this mutant must precede a tenable interpretation. The enzyme i n the Rennes mutant that was found in two galactosemic siblings migrates more slowly than normal transferase on electrophoresis (78). A further variation, which was revealed by an unstable transferase (Indiana variant), has been found i n a galactosemic f a m i l y (Z9).
A Los Angeles variant of galactosemia displays three t r a n s f e r ase bands during electrophoresis of erythrocyte hemolysates (80). In contrast to the Duarte variant, the total amount of transferase activity i n the hemolysate i s normal o r greater. I. Galactose-l-phosphate Toxicity. Galactose-1-phosphate i s the toxic agent causing most of the pathology i n c l a s s i c a l galactosemia. This can be i n f e r r e d f r o m the comparison of disease manifestations i n transferase deficiency with those i n galactokinase deficiency. Cataracts are the only common sign; they are caused by osmotic swelling and disruption of lens fibers due to the accumulation of galactitol (34). No b r a i n , l i v e r , and kidney pathology i s observed in kinase
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deficiency and, since g a l - l - P cannot be formed i n this disorder, one can conclude that it must play the deciding pathogenetic role in t r a n s f e r ase deficiency. Symptoms other than eye cataracts, which are observed i n patients with transferase deficiency, may be due to a high concentration of g a l - l - P within the c e l l . The blood glucose levels of some acutely i l l infants lacking transferase are subnormal, indicating a disturbed c a r bohydrate metabolism. Three key reactions involving glucose have been reported to be affected by galactose-l-phosphate: the mutase, dehydrogenase, and pyrophosphorylase reactions shown in Figure 5 (81) (82). In each case the evidence was based on kinetic studies of isolated enzymes, not f r o m human tissues; it i s therefore somewhat presumptive. About a 50-fold excess of galactose-l-phosphate over glucose phosphates i s required for significant inhibition of the isolated reactions. G l u c o s e - l - P , uridin side diphosphate-glucose inhibitor constants and calculated intracellular levels of these compounds it has been deduced that the uridine derivatives and glucose-1phosphate could have a physiologically regulatory function. J . Biosynthesis of Galactose-l-phosphate f r o m UDP-galactose. The idea that the pyrophosphorylase pathway i s a means of galactose metabolism i n the human erythrocyte stems f r o m the following observations: Epimerase and UDP-glc are normal red c e l l constituents. Hence UDP-gal i s also available as a substrate f o r other reactions. Incubation of hemolysates lacking transferase (84) with UDP-gal produced g a l - l - P in a reaction that was absolutely dependent on inorganic pyrophosphate and was stimulated by magnesium; the production of g a l - l - P was i n hibited by UDP-glc, by UDP and P i . Under identical conditions, the crystalline enzyme f r o m human l i v e r catalyzes the formation of g a l - l - P f r o m UDP-gal (85) (50). UDP-glucose pyrophosphorylase of both calf and rabbit have also been purified and c r y s t a l l i z e d (48) (85) (50). The biochemical evidence that one protein catalyzes reactions with both glucose and galactose derivatives i s convincing. Throughout purification and c r y s tallization, the ratio of activity of the enzyme towards the various substrates remains constant (86) (85) (50). UDP-gal i s bound to the p u r i fied enzyme as a function of the number of protomer subunits of pyrophosphorylase (49) (87). This bound UDP-gal may then be stoichiom e t r i c a l l y replaced by UDP-glc (87). UDP-glc also l i m i t s the synthesis of g a l - l - P f r o m UDP-gal by hemolysates (84). It i s concluded, therefore, that UDP-gal competes with UDP-glc (although l e s s effectively) for the same site on the enzyme. Immunological evidence supporting this conclusion has also been obtained (88).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
PHYSIOLOGICAL EFFECTS OF FOOD CARBOHYDRATES
.Glycogen,
Glucose
UTP Glc-6-P^
1
•Glc-l-P
r
UDP-GIc
^ PPi
NHIBITION by Gal-1-P 6-P-Gluconic
acid
Figure 5.
Toxicity of
galactose-l-phosphate
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The production of g a l - l - P f r o m g l c - l - P by the pyrophosphorylase pathway requires the enzymes pyrophosphorylase and epimerase and their substrates, the UDP-hexoses, U T P and P P i . A substantial concentration of U T P i s maintained i n t r a c e l l u l a r l y f o r nucleic acid synthesis and f o r UDP-glc formation. Besides being a p r e c u r s o r of UDPgal, UDP-glc i s a source of glycosyl donors f o r pentoses, uronic acids, and hexosamines; hence, there i s a constant and vital metabolic flux through UDP-glc. Metabolic interconversions of sugars and their d e r i vatives require epimerases, which appear to be generally distributed i n nature. Hence, the epimerase-catalyzed formation of UDP-gal f r o m UDP-glc i s a common intracellular reaction. Under most circumstances, when nucleotides are extracted f r o m the c e l l , UDP-gal and UDPglc are present i n a ratio of approximately 1/3 to 2/3, providing ample UDP-gal substrate f r o m which g a l - l - P may be enzymatically formed. Inorganic pyrophosphat g a l - l - P i s to be produce UDP-gal Pyrophosphat produc of most biosynthetic processes, including the formation of polysaccharides, proteins, and lipids (89). To permit the biosynthesis of g a l - l - P f r o m UDP-gal f r o m an equilibrium that favors reactants, P P i i s theor e t i c a l l y hydrolyzed by the intracellular pyrophosphatases. Quite aside f r o m the futile energy wastage, if this hydrolysis took place, substantial evidence indicates that not only do the c e l l s r e a l i z e the potential for productive use of the P P i , but P P i may have a regulatory function as well (90) (91). Furthermore, P P i accumulates i n man i n sufficient quantity to c r y s t a l l i z e with calcium and cause some joint disorders (92). The availability of P P i f o r g a l - l - P synthesis i s therefore most probable. The galactosemic has a special problem since the transferase block promotes an accumulation of g a l - l - P f r o m dietary galactose while, at the same time, a substantial need f o r UDP-gal obviously exists. In galactosemia, the pyrophosphorylase pathway appears to have adequate capacity to f o r m UDP-gal f r o m g l c - l - P but cannot readily convert g a l - l - P to UDP-gal. Hence, g a l - l - P accumulates when galactose i s ingested. The following observation indicates that the pyrophosphorylase pathway i s largely responsible f o r galactose metabolism i n red cells f r o m galactosemics. A n antibody to crystalline UDP-glucose pyrophosphorylase f r o m human l i v e r has been prepared (88). This antibody, when incubated with extracts of galactosemic red c e l l s , specifically precipitates the enzyme responsible f o r metabolism of galactose. Hence the galactosemic who lacks the L e l o i r pathway can convert some galactose to glucose in erythrocytes (and probably other tissues), v i a the pyrophosphorylase pathway.
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K. Uncontrolled Biosynthesis of Galactose-l-phosphate f r o m Glucose. The usual procedure for maintaining patients with galactosemia i s to exclude galactose sources f r o m their diets. When this i s carefully done, levels of g a l - l - P lower than 4 mg/100 m l are maintained in most patients. Any galactose ingestion i s followed by an immediate r i s e i n red c e l l g a l - l - P . Unfortunately, even when their intake of galactose sources i s carefully regulated, uncontrolled biosynthesis of g a l - l - P f r o m UDP-gal has been observed i n some galactosemic s (93). Much evidence, both c l i n i c a l and experimental, currently documents biosynthesis of galactose f r o m glucose i n man. Mayes and M i l l e r (94) have grown transferase-deficient skin fibroblasts i n a medium devoid of galactose and observed the formation of g a l - l - P f r o m glucose. The final concentrations (approx 2 mg/g protein) were s i m i l a r to those attained by re protein) (84). Cultured fibroblasts f r o m galactosemics without transferase activity have converted ( 1 - C ) galactose to C 0 2 (95), with an accumulation of g a l - l - P . Both C O 2 and g a l - l - P formation were inhibited by glucose, suggesting a common intermediate. Galactokinasedeficient cells s i m i l a r l y treated did not produce C O 2 f r o m ( l - l ^ C ) galactose, suggesting the pyrophosphorylase pathway as the alternate route of galactose metabolism i n galactosemics. Galactokinase-deficient children grow and develop normally when maintained on a diet free of galactose. Pregnant women who are hétérozygotes f o r transferase deficiency and have previously had t r a n s ferase-deficient offspring are routinely subjected to a galactose exclusion diet; despite galactose deprivation their fetuses develop normally. One completely transferase-deficient woman who followed a lactosefree regimen throughout pregnancy delivered a healthy infant and, presumably s t i l l on the diet, produced lactose (2.8 g/100 ml) i n her colost r u m on the second post partum day (96). In galactokinase-deficient twin infants on a suitable diet, s m a l l amounts of galactose remained detectable in plasma and urine (97). Both galactokinase- and t r a n s f e r ase-deficient infants, though on galactose exclusion diets, excreted some galactitol i n the urine (97) (98). High g a l - l - P i s commonly r e corded i n the cord blood of transferase-deficient newborns born of mothers on galactose-restricted diets (99). The need for galactose-containing polymers to assure functional and structural integrity of c e l l s and tissues i s satisfied by the biosynthetic reactions that have been detailed. Hence there i s a substantial capacity to synthesize galactose and its derivatives at a l l stages of human development. The g a l - l - P of a transferase-deficient newborn (at age 5 hrs) was 17 mg/100 m l of R B C . At the end of his f i r s t day, 14
1
4
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the g a l - l - P had actually r i s e n to 26 mg/100 m l , although he had been given intravenous glucose only and had taken no food (88). C l e a r l y his red blood cells had synthesized g a l - l - P , either f r o m endogenous galactose stores o r (more likely) f r o m glucose. We have observed what appears to be uncontrolled biosynthesis of g a l - l - P i n two galactosemic infants. A f t e r an initial exposure to m i l k and following diagnosis, both received a galactose-free formula. V e r y surprisingly, upon initiation of the diet at age 7 and 6 days, respectively, red c e l l g a l - l - P levels dropped rather slowly, and potentially toxic levels were maintained for months. At age 27 days, one infant was fed i s o c a l o r i c amounts of dextrimaltose i n water f o r 36 h r s ; however, the g a l - l - P did not drop but rose slightly. The female infant had breast swellings until her 25th day of age. A t age 19 days, her breast s e c r e tions, collected and compared to those of three other newborns contained what appeared to b synthesized f r o m glucose (93) Such biogenesis of galactose could take place i n tissues other than the red cells and the breast glands, e.g., the l i v e r and the central nervous system. It i s remarkable i n this connection that i n the few r e ported long-term follow-up studies (100) (101), some of the galactosemia patients who had been diagnosed at b i r t h and treated since, showed signs of organic b r a i n damage at school age, evidenced by difficulties in visual perception. Self-intoxication with g a l - l - P through a prolonged maintenance of high levels during infant life has been hypothesized as the cause of the damage. Evidently such biosynthesis of g a l - l - P has constituted a mechanism of potential se If-intoxication in well-treated galactosemic infants (102). Convincing evidence implicates the pyrophosphorylase pathway i n such cases (88). The regulation of this process to l i m i t the amount of UDP-gal formed to that required f o r biosynthetic reactions and thereby prevent the formation of toxic quantities of g a l - l - P i s the central problem of some transferase deficient children. T,
Abbreviations :
glc-l-P : gal-l-P: gal-6-P: UDP-glc: UDP-gal: UTP: ATP: ADP: Pi: PPi:
TT
glucose-l-phosphate galacto se -1 -pho sphate galactose-6-phosphate uridine diphosphate glucose uridine diphosphate galactose uridine triphosphate adenosine triphosphate adenosine diphosphate inorganic phosphate inorganic pyrophosphate
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49. Levine, S., T. A. Gillett, E. Hageman and R. G. Hansen, J. Biol. Chem. (1969) 244:5729-5734. 50. Turnquist, R. L., T. A. Gillett and R. G. Hansen, J. Biol. Chem. (1974) in press. 51. Isselbacher, K. J., J. Biol. Chem. (1958) 232:429-444. 52. Abraham, H. D. and R. R. Howell, J. Biol. Chem. (1969) 244:545-550. 53. Chacko, C. Μ., L. McCrone and H. L. Nadler, Biochim. Biophys. Acta (1972) 268:113-120. 54. Gitzelmann, R. and B. Steinmann, Helv. paed. Acta (1973) 28:497-510. 55. Bergren, W. R., W. G. Ng and G. Donnell, Biochem. Biophys. Acta (1973) 315:464. 56. Nelsestuen, Gary L. and Samuel Kirkwood, J. Biol. Chem. (1971) 246(24):7533-7543 57. Davis, L. and L. Glaser (1971) 43:1429-1435. 58. Wee, T. G., J. Davis and P. A. Frey, J. Biol. Chem. (1972) 247:1339-1342. 59. Wee, T. G. and P. A. Frey, J. Biol. Chem. (1973) 248:33-40. 60. Maitra, U. S., M. A. Gaunt and H. Ankel, J. Biol. Chem. (1974) 249:3075-3078. 61. Forsander, Olof A., Biochim. Biophys. Acta (1972) 268:253-256. 62. Isselbacher, K. J. and S. M. Krane, J. Biol. Chem. (1961) 236:2394. 63. Kesaniemi, Υ. Α., Κ. Ο. Kurppa and K. R. H. Husman, J. Obs. and Gyn. (1973) 80:344. 64. Segal, S., "The Metabolic Basis of Inherited Disease" (J. B. Stanbury, J. B. Wyngaarden and D. S. Fredrickson, eds), pp. 174-195, McGraw Hill, New York, (1972). 65. Gitzelmann, R. Ped. Res. (1967) 1:14. 66. Beutler, E., F. Matsumoto, W. Kuhl, A. Krill, N. Levy, R. Sparks and M. Degnan, N. Eng. J. Med. (1973) 288:1203. 67. Gitzelmann, R., Helv. Paediat. Acta (1972) 27:125-130. 68. Donnell, G. N., W. R. Bergren, R. K. Bretthauer and R. G. Hansen, Pediatrics (1960) 25:572-581. 69. Schwarz, V., A. R. Wells, A. Holzel, G. M. Komrower, Ann. Hum. Gen. (1961) 25:179. 70. Kirkman, H. N. and E. Bynum, Ann. Hum. Gen. (1958-59) 23:117-126. 71. Mathai, C. K. and E. Beutler, Science (1966) 154:1179-1180. 72. Ng, W. G., W. R. Bergren, M. Fields and G. N. Donnell, Biochem. Biophys. Res. Comm. (1969) 37:451.
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73. Gitzelmann, R., J. R. Poley and A. Prader, Helv. Paed. Acta (1967) 22:252-257. 74. Hansen, R. G., "Galactosemia" (D. Y. Hsia, ed) p. 55, Charles C. Thomas, Springfield, Ill., (1969). 75. Tedesco, T. A. and W. J. Mellman, Science (1971) 172:727-728. 76. Tedesco, Τ. A., J. Biol. Chem. (1972) 247:6631-6636. 77. Segal, S., A. Blair and H. Roth, Am. J. Med. (1965) 38:62. 78. Schapira, F. and J. C. Kaplan, Biochem. Biophys. Res. Comm. (1969) 35:451. 79. Chacko, C. Μ., J. C. Christian, and H. L. Nadler, J. Ped. (1971) 78:454. 80. Ng, W. G., W. R. Bergren and G. N. Donnell. Ann. Hum. Genet. (1973) 37:1. 81. Sidbury, J. B. Jr. "Galactosemia" (D Y Hsia ed) p 13 Charles C. Thomas 82. Oliver, I. Τ., Biochim. Biophys. Acta (1961) 52:75-81. 83. Segal, S. and S. Rogers, Biochim. Biophys. Acta (1971) 250: 351-360. 84. Gitzelmann, R., Pediat. Res. (1969) 3:279-286. 85. Knop, J. K. and R. G. Hansen, J. Biol. Chem. (1970) 245:24992504. 86. Ting, W. K. and R. G. Hansen, Proc. Soc. Exp. Biol. Med. (1968) 127:960-962. 87. Turnquist, R. L., M. M. Turnquist, R. C. Bachmann and R. G. Hansen, Biochim. Biophys. Acta (1974) 364:59-67. 88. Gitzelmann, R. and R. G. Hansen, Biochim. Biophys. Acta (1974) 372:374-378. 89. Lehninger, A. L., "Bioenergetics, " p. 183, Benjamin, New York (1965). 90. Schwenn, J. D., R. M. Lilley and D. A. Walker, Biochim. Biophys. Acta (1973) 325:586-595. 91. Levine, G. and J. A. Bassham, Biochim. Biophys. Acta (1974) 333:136-140. 92. Russell, R. G. G., S. Bisaz, H. Fleisch, H. L. F. Currey, H. M. Rubinstein, A. A. Dietz, J. Boussina, A. Micheli and G. Fallet, Lancet (1970) II:899-902. 93. Gitzelmann, R., B. Steinmann, R. G. Hansen, in press (1974). 94. Mayes, J. S. and L. R. Miller, Biochim. Biophys. Acta (1973) 313:9-16. 95. Petricciani, J. C., K. Binder, C. R. Merril and M. R. Geier, Science (1972) 175:1368. 96. Roe, T. F., J. G. Hallatt, G. N. Donnell and W. G. Ng, J. Pediat. (1971) 78:1026-1030.
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97. Olambiwonnu, N. O., R. McVie, W. G. Ng, S. D. Frasier and G. N. Donnell, Pediatrics (1974) 53:314-318. 98. Roe, T. F., W. G. Ng, W. R. Bergren and G. N. Donnell, Biochem. Med. (1973) 7:266-273. 99. Donnell, G. Ν., R. Koch and W. R. Bergren, "Galactosemia, " (D. Y. Y. Hsia, ed) p. 247, Charles C. Thomas, Springfield, Ill. (1969). 100. Komrower, G. M. and D. H. Lee, Arch. Dis. Child. (1970) 45:367-373. 101. Fishier, K., G. N. Donnell, W. R. Bergren and R. Koch, Pediatrics (1972) 50:412-419. 102. Gitzelmann, R. and R. G. Hansen, Abstr. Europ. Soc. Ped. Res., Ann. Meeting, Sevilla (1973); Pediat. Res. (1974) 8:137.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
7 Metabolism and Physiological Effects of the Polyols (Alditols) OSCAR TOUSTER Department of Molecular Biology, Vanderbilt University, Nashville, Tenn. 37235
I. Introduction Polyols, or alditols, are of course not strictly carbohydrates, but since they are produced chemically and biologically from sugars, we can consider them honorary carbohydrates. They are now established in mammalian metabolism and are of some importance in nutrition and pathology. In this paper I survey the roles of polyols and emphasize some of the most pertinent current uses and problems. II. Survey of the Occurrence and Enzymology of the Polyols The occurrence of polyols in animals is summarized in Table I. TABLE I. POLYOLS IN ANIMAL METABOLISM Occurrence in tissues Sorbitol—fetal blood, seminal vesicles and plasma, nerve, lens of alloxan-diabetic rats or rats given cataractogenic dose of D-xylose Xylitol—lens of rats given cataractogenic dose of D-xylose Dulcitol—various tissues after cataractogenic dose of D-galactose Occurrence in urine Erythritol D-arabitol, L-arabitol Sorbitol D-mannitol Utilization by mammals in vivo High--xylitol, ribitol, sorbitol Variable--D-mannitol (oral dose moderately utilized; parenteral--poorly) Poor--arabitol (D andL),dulcitol 123 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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S o r b i t o l i s most widely d i s t r i b u t e d , o c c u r r i n g in fetal blood and in seminal v e s i c l e s and plasma, where the p o l y o l is un doubtedly a normal metabolic intermediate. S o r b i t o l accumulates i n some t i s s u e s of d i a b e t i c animals, a matter which is discussed i n greater d e t a i l below. X y l i t o l and, to a smaller extent, sor bitol accumulate in the lens of r a t s given c a t a r a c t o g e n i c doses of Πτ-xylose (1). D u l c i t o l , or galactitol, accumulates a f t e r c a t aractogenic doses of D-galactose, and it is a l s o found i n the human genetic d i s e a s e , galactosemia. Several p o l y o l s have been i s o l a t e d from human u r i n e : e r y t h r i t o l (2), a r a b i t o l (3), mannitol and s o r b i t o l (4,5). The utilization of p o l y o l s by mammals is a l s o i n d i c a t e d i n Table I, with the wide range of utilization i n d i c a t e d . The most a p p r e c i a b l y used p o l y o l s are xylitol, ribitol, and s o r b i t o l . The main biochemical r e a c t i o n s of p o l y o l s a r e shown below:
Enzymati General
Polyol
Mammals a
L-Xylulose
D-Glucose ^
b
s
s
xylitol
sorbitol
a
a
s
D-xylulose
*D-fructose
P o l y o l s undergo o x i d a t i o n to the corresponding ketose (a) or to the corresponding aldose ( b ) , or phosphorylation to the p o l y o l 1-phosphate ( c ) . Many examples of r e a c t i o n s (a) and (b) a r e found i n mammals. The L - x y l u l o s e - x y l i t o l - D ^ x y l u l o s e i n t e r c o n v e r s i o n occurs i n the g l u c u r o n a t e - x y l u l o s e c y c l e . The glucoses o r b i t o l - f r u c t o s e i n t e r c o n v e r s i o n occurs i n male accessory organs and undoubtedly i n other t i s s u e s as w e l l . However, the phos p h o r y l a t i o n of p o l y o l s occurs i n microorganisms but not i n mam mals. A review on p o l y o l s w i l l supply the reader with a d d i t i o n a l information i n t h i s biochemical area ( 6 ) .
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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TOUSTER
X y l i t o l i n Metabolism and N u t r i t i o n
Many years ago, when we were i n v e s t i g a t i n g the metabolism of L r x y l u l o s e , the sugar excreted i n gram q u a n t i t i e s by humans w i t h the genetic metabolic abnormality known as e s s e n t i a l p e n t o s u r i a , we discovered that ^ - x y l u l o s e i s normally u t i l i z e d as shown i n the f o l l o w i n g equations:
CH.OH \
CH.OH I
2
0 0
1
HCOH I HOCH » CH OH
NADPH
ΝADP
1
HOCH I HCOH ο I HOCH CH OH
L-Xylulose
CH 0H I HCOH I HOCH I
NAD
NADH
HCOH I CH OH
D-Xylulose
Xylitol CH 0H I HCOH o 2
I
CH 0H ! C=0 I HOCH I HCOH I CH OH o 2
o 1
CH 0H I HCOH o 2
\
HCOH I
HOCH I
CH OH
HOCH I HOCH I CH OH
L-Arabitol
The J±-xylulose i s reduced t o x y l i t o l by an u n u s u a l l y s p e c i f i c NADPH-linked p o l y o l dehydrogenase (7,8) . The x y l i t o l i s then r e o x i d i z e d t o D-xylulose by an NAD-linked enzyme of r a t h e r broad but d e f i n i t e s p e c i f i c i t y . This enzyme i s most commonly known as s o r b i t o l dehydrogenase. P e n t o s u r i c i n d i v i d u a l s excrete smaller amounts o f L - a r a b i t o l , probably as a " d e t o x i c a t i o n " product of the accumulated L - x y l u l o s e . The s u b s t r a t e s p e c i f i c i t y of s o r b i t o l dehydrogenase, which i s b e t t e r expressed by the name, J v - i d i t o l dehydrogenase, i s shown below: S o r b i t o l dehydrogenase i s s p e c i f i c f o r erythro-1,2,4-polyol c o n f i g u r a t i o n .
CH OH 2
hOH hOH
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As can be seen from the formula on the r i g h t , s o r b i t o l , x y l i t o l , and r i b i t o l are good s u b s t r a t e s . The low l e v e l of o x i d a t i o n of L - a r a b i t o l by t h i s enzyme even when h i g h l y p u r i f i e d has not been explained. The work on L - x y l u l o s e metabolism i n our l a b o r a t o r y and of King, Burns and others on L-ascorbic a c i d b i o s y n t h e s i s l e d to the formulation of the glucuronate-xylulose c y c l e (9,10), which i s shown i n F i g u r e 1. This c y c l e occurs i n a l l mammals i n which i t has been studied, the r e a c t i o n s g e n e r a l l y o c c u r r i n g i n the l i v e r and i n the kidney. The f u n c t i o n of the c y c l e i s not completely understood. Since an i n a b i l i t y to c a r r y out the r e d u c t i o n of J±x y l u l o s e to x y l i t o l i s apparently not d e l e t e r i o u s , pentosuria i s an i n h e r i t e d metabolic d i s o r d e r (11,12), not a metabolic d i s e a s e . The e a r l y r e a c t i o n s i n the c y c l e are obviously important. I t appears that most higher animals b i o s y n t h e s i z e L r a s c o r b i c a c i d , although primates, the guinea p i g f l y i n g mammals and i n s e c t s (13) do not. I t has bee the guinea p i g l a c k th L-gulonolactone to the keto d e r i v a t i v e . I would a l s o point out the numerous o x i d a t i o n - r e d u c t i o n s using p y r i d i n e n u c l e o t i d e l i n k e d coenzymes which e f f e c t i v e l y move hydrogen from NADPH to NAD, thereby p o s s i b l y serving a transhydrogenase f u n c t i o n . I should a l s o mention that although i t i s accepted that UDP-glucose and UDP-glucuronate are e a r l y members of the c y c l e , and obviously serve as precursors of sugar moieties i n glyco-polymers, there i s some u n c e r t a i n t y as to how the f r e e glucuronate i s produced. Since, i n the conversion of glucuronate to I±-xylulose v i a L-gulonate, there occurs a decarboxylation of the C5 carbon of glucuronate, t h i s carbon being the same as of glucose, t h i s c y c l e can be considered a o x i d a t i o n pathway f o r glucose. It may then be asked whether much glucose i s o x i d i z e d normally through t h i s pathway. S p e c i f i c studies i n d i c a t e that only a small amount of glucose i s handled v i a t h i s route. From the f a c t that pentosuric i n d i v i d u a l s normally excrete s e v e r a l grams of Lr x y l u l o s e each day and from experiments on the extent of augmenta t i o n of t h i s e x c r e t i o n on feeding the precursor Ιλ-glucuronolactone, i t can be estimated that the carbohydrate f l u x through the c y c l e i s between 5 and 15 grams per day (14). I t was t h e r e f o r e of some i n t e r e s t that Winegrad and h i s a s s o c i a t e s reported s e v e r a l years ago that the o x i d a t i o n of glucose i s enhanced i n a l l o x a n - d i a b e t i c r a t s (15) and that L r x y l u l o s e l e v e l s i n the serum of d i a b e t i c humans are s e v e r a l times higher than normal (16). These f i n d i n g s are not w e l l understood. I t does appear, however, that i n diabetes the s o - c a l l e d i n s u l i n dependent pathways are more prominently employed and that more glucose appears to be o x i d i z e d v i a glucuronate and v i a s o r b i t o l , as w i l l be discussed i n more d e t a i l below. The u t i l i z a t i o n of p o l y o l s became of s p e c i a l i n t e r e s t when the metabolic importance of s o r b i t o l and x y l i t o l was discovered. The high conversion of an administered p o l y o l to l i v e r glycogen
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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Poly oh (Alditols)
L- Ascorbic Acid - Keto-L-gulonoloctone L- Gulonolactone
?
H0 :OH OH
ÇH OH 2
(SOT
I
HOCH I HCOH I
HOCH HCOH I
UDP- glucuronote
HCOH
0-Glucuronate
Ο
Ç0 H 2
HOÇH HOC H HCOH I
[NAD]
L- Gulonote
C0 H 2
HOCH I
?-°
HCOH
UOP-glucose
HOCH -Keto-LCH OHgulonote 2
Glycogen-
• C0
^Hexose phosphate ( pentose ^\ phosphate \V pathway) \ *
2
C-0 HCOH
CH 0H
HOCH CH OH
2
C-0 HOCH I HCOH I CH OP03H 2
2
CH OH
2
L-Xylulose 4L 1NADPH| 2
CH 0H
D-Xylulose \ ^ 5-phosphate
2
i-o
A O P ^ ^ HOCH A
T
P
HCOH
C H OH
the major route of u t i l i z a t i o n of administered sorbitoI^sHknown:
Route of U t i l i z a t i o n of S o r b i t o l and Fructose CH 0(f) 2
CH 0H
CH 0H
o
2
C=0 I
HCOH ι HOCH ι HCOH
HOC
-> F - l - P
HCOH
I
I
CHO I
HCOH
HCOH ι CH 0H
I
HCOH _ I
CH OH
2
CH 0H 2
Sorbitol
CHO I
CHOH C H O ®
D-Fructose
S o r b i t o l i s o x i d i z e d to f r u c t o s e , which i s then phosphorylated to fructose-l-phosphate followed by cleavage to dihydroxyacetone phosphate and glyceraldehyde. The l a t t e r i s then phosphorylated to the 3-phosphate. Both t r i o s e phosphates are members of the Embden-Meyerhof g l y c o l y t i c pathway. Gabbay (33) has r e c e n t l y observed another metabolic i n t e r r e l a t i o n s h i p between f r u c t o s e and h e x i t o l s . Dietary fructose i s converted, i n about 3% y i e l d , t o u r i n a r y ^-mannitol. He has presented evidence that the enzyme aldose reductase s u r p r i s i n g l y has the c a p a c i t y to c a t a l y z e t h i s r e d u c t i o n of f r u c t o s e to mannitol. S o r b i t o l i s i n f a c t an important metabolic intermediate as shown below:
n
n
1
NADPH. _ , , -. NAD Sorbitol
v
D-Glucose
v
(aldose reductase)
^-Fructose
(sorbitol dehydrogenase)
The s o r b i t o l pathway (34,35) i n v o l v e s the conversion of glucose to s o r b i t o l through the mediation of the enzyme aldose reductase,
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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131
with NADPH as coenzyme. S o r b i t o l i s o x i d i z e d to D-fructose by s o r b i t o l dehydrogenase. There i s some evidence that t h i s pathway of glucose u t i l i z a t i o n , as w e l l as the g l u c u r o n i c - x y l u l o s e c y c l e , are somewhat more e x t e n s i v e l y u t i l i z e d during i n s u l i n d e f i c i e n c y (35). Should glucose enter c e l l s i n higher amount than usual and be converted to s o r b i t o l and f r u c t o s e , which are p o o r l y permeable, osmotic changes may occur which are u n d e s i r a b l e . The evidence i s strong that t h i s sequence of events i s involved i n the production of d u l c i t o l from galactose i n the lens and the formation of c a t a r a c t i n i n d i v i d u a l s with galactosemia (1,36). Gabbay (37) has emphasized that p o l y o l accumulation i n nerve may be r e l a t e d to the occurrence of d i a b e t i c neuropathy. For example, galactosemia causes increases i n nerve d u l c i t o l and water content, and decreases the motor nerve conduction v e l o c i t y . If the animals are then placed on a normal d i e t , the d u l c i t o l and water content decrease d th conductio v e l o c i t i r e s t o r e d to normal. Moreover the r a t causes an increas glucose i n p e r i p h e r a l nerve. A decrease i n the nerve conduction v e l o c i t y occurs c o n c u r r e n t l y . In an attempt to modify these events, Gabbay, Dvornik, and t h e i r a s s o c i a t e s have been studying s y n t h e t i c i n h i b i t o r s of aldose reductase as p o s s i b l e agents f o r b l o c k i n g p o l y o l formation i n animals. A recent report demons t r a t e s that the i n h i b i t o r AY22284 reduces the accumulation of g a l a c t i t o l i n both the lens and s c i a t i c nerve of galactosemic r a t s , g r e a t l y reduces and delays the formation of d e t e c t a b l e c a t a r a c t s i n g a l a c t o s e - f e d r a t s , and reduces the accumulation of s o r b i t o l and of f r u c t o s e , but not of glucose, i n the s c i a t i c nerve of a r t i f i c i a l l y - d i a b e t i c r a t s (38) . Of course, l i t t l e work has been done on the human i n t h i s area, and there w i l l be continued d i f f i c u l t i e s i n doing such i n v e s t i g a t i o n s on human d i a b e t i c s . A cause and e f f e c t r e l a t i o n s h i p between p o l y o l accumulation i n nerve and decrease i n f u n c t i o n has not been d e f i n i t e l y establ i s h e d even i n experimental animals, and even i f t h i s were to be accomplished, i t would s t i l l remain to be determined whether d i a b e t i c neuropathy i n the human has the same cause. The moderate, and dose dependent, u t i l i z a t i o n of o r a l l y administered mannitol, and the e f f e c t s of t h i s h e x i t o l on i n t e s t i n a l f u n c t i o n , have been e x t e n s i v e l y studied (39) . The use of mannitol as a d i u r e t i c agent has r e c e n t l y been reviewed (40). V.
M a l t i t o l and
Isomaltitol
M a l t i t o l has been studied r e c e n t l y at Charles P f i z e r and Company because of r e p o r t s that t h i s p o l y o l i s low c a l o r i c and t h e r e f o r e might be a superior sweetening agent (41 ,42) . Dr. H. H. Rennard of P f i z e r has informed me of h i s work, some of which was done i n c o l l a b o r a t i o n with Dr. J. R. Bianchine of Johns Hopkins. The low recovery of administered l a b e l e d m a l t i t o l i n the u r i n e and feces of the r a t i n d i c a t e d that i t was e f f i c i e n t l y
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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CARBOHYDRATES
u t i l i z e d , and s i m i l a r l y encouraging r e s u l t s were obtained i n the human, i n c l u d i n g high recovery of -^C i n expired CO2. However, the f a c t that the expired 0 ^0 peak i n r a t s occurred a t 4 t o 5 hours a f t e r feeding suggested that the l a b e l might have been ab sorbed i n the lower gut. I t was indeed found that the m a l t i t o l was being u t i l i z e d by i n t e s t i n a l m i c r o f l o r a , which apparently were converting the l a b e l e d p o l y o l i n t o v o l a t i l e f a t t y a c i d s . In a d d i t i o n , i n t e s t i n a l mucosal preparations of r a t s have a low c a p a c i t y to hydrolyze m a l t i t o l . Therefore, although the u t i l i z a t i o n of the m a l t i t o l i s i n d i r e c t , i n v o l v i n g i t s p r e l i m i n a r y con v e r s i o n to f a t t y a c i d s , t h i s p o l y o l i s considered by Rennard t o be a w e l l u t i l i z e d substance. The u t i l i z a t i o n of i s o m a l t i t o l has a l s o been i n v e s t i g a t e d . From studies on r a t s i t was suggested that i t could be a u s e f u l i n g r e d i e n t i n c a l o r i e - r e d u c e d foods and beverages (43). 1Ζ
2
VI.
Concluding Remark
At the present time s o r b i t o l and x y l i t o l are the most impor tant a l d i t o l s i n that they are key metabolic intermediates and are being fed or administered t o humans i n considerable amounts. As f a r as the f u t u r e i s concerned, i t appears to me that the problems which w i l l most a c t i v e l y concern i n v e s t i g a t o r s are r e l a t e d t o the more general use of x y l i t o l , f u r t h e r i n v e s t i g a t i o n of the p o s s i b l e r o l e of s o r b i t o l i n diabetes, and the p o s s i b l e advantages of p o l y o l s i n decreasing d e n t a l c a r i e s . Literature Cited
1. van Heyningen, R., in "Proceedings of the International Sym posium on Metabolism, Physiology, and Clinical Use of Pentoses and Pentitols," Hakone, Japan, August 27-29, 1967 (B. L. Horecker, K. Lang, and Y. Takagi, eds.), pp. 109123, Springer-Verlag, Berlin, Heidelberg, New York (1969). 2. Touster, O., Fed. Proc. (1960) 19, 977-983. 3. Touster, O. and Harwell, S., J. Biol. Chem. (1958) 230, 1031-1041. 4. Pitkänen, E. and Pitkänen, Α., Ann. Med. Exp. Fenn. (1964) 42, 113-116. 5. Ingram, P., Applegarth, D. Α., Sturrock, S., and Whyte,J.N. C., Clin. Chim. Acta (1971) 35, 523-524. 6. Touster, O. and Shaw, D. R. D., Physiol. Rev. (1962) 42, 181225. 7. Hollmann, S. and Touster, O., J. Biol. Chem. (1957) 225, 87102. 8. Arsenis, C. and Touster, O., J. Biol. Chem. (1969) 244, 38953899. 9. McCormick, D. B. and Touster, O., J. Biol. Chem. (1957) 229, 451-461.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
7.
TOUSTER
Polyols
(Alditols)
133
10. Burns, J. J. and Kanfer, J., J. Am. Chem. Soc. (1957) 79, 3604-3605. 11. Touster, O., Fed. Proc. (1960) 19, 977-983. 12. Hiatt, H. H., in "The Metabolic Basis of Inherited Disease" (J. B. Stanbury, J. B. Wyngaarden, and D. S. Fredrickson, eds.), third ed., pp. 119-130, McGraw-Hill, New York (1972). 13. Gupta, S. D., Chaudhuri, C. R., and Chatterjee, I. B., Arch. Biochem. Biophys. (1972) 152, 889-890. 14. Hollmann, S. and Touster, O., "Non-glycolytic Pathways of the Metabolism of Glucose," p. 107, Academic Press, New York (1964). 15. Winegrad, A. I. and Shaw, W. Ν., Am. J. Physiol. (1964) 206, 165-168. 16. Winegrad, A. I. and Burden, C. L., New Engl. J. Med. (1966) 274, 298-305. 17. McCormick, D. B. and Touster O Biochim Biophys Act (1961) 54, 598-600 18. Horecker, B. L., Lang, Κ., and Takagi, Y., eds., "Proceed ings of the International Symposium on Metabolism, Physiology, and Clinical Use of Pentoses and Pentitols," Hakone, Japan, August 27-29, 1967, Springer-Verlag, Berlin, Heidelberg, New York (1969). 19. Lang, K. and Fekl, W., Z. Ernährungswissenschaft (1971) Suppl. 11. 20. Sipple, H. L. and McNutt, K. W., eds., "Sugars in Nutrition," Academic Press, New York, San Francisco, London (1974). 21. Förster, H., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 259-280, Academic Press, New York, San Francisco, London (1974). 22. Froesch, E. R. and Jakob, Α., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 241-258, Academic Press, New York, San Francisco, London (1974). 23. Meng, H. C., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 527-566, Academic Press, New York, San Francisco, London (1974). 24. Thomas, D. W., Edwards, J. B., and Edwards, R. G., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 567-590, Academic Press, New York, San Fran cisco, London (1974). 25. van Eys, J., Wang, Y. M., Chan, S., Tanphaichitr, V. S., and King, S. M., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 613-631, Academic Press, New York, San Francisco, London (1974). 26. Thomas, D. W., Edwards, J. B., Gilligan, J. E., Lawrence, J. R., and Edwards, R. G., Med. J. Australia (1972) 1, 12381246. 27. Brin, M. and Miller, O. N., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 591-606, Academic Press, New York, San Francisco, London (1974).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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PHYSIOLOGICAL EFFECTS OF FOOD CARBOHYDRATES
28. Grunberg, E., Beskid, G., and Brin, M., Int. J. Vitamin and Nutrition Res. (1973) 43, 227-232. 29. Mäkinen, Κ. Κ., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 645-687, Academic Press, New York, San Francisco, London (1974). 30. Bässler, Κ. H., in "Proceedings of the International Sympo sium on Metabolism, Physiology, and Clinical Use of Pentoses and Pentitols," Hakone, Japan, August 27-29, 1967 (B. L. Horecker, K. Lang, and Y. Takagi, eds.), pp. 190-196, Springer-Verlag, Berlin, Heidelberg, New York (1969). 31. Hers, H. G., Biochim. Biophys. Acta (1960) 37, 127-138. 32. Hue, L. and Hers, H.-G., Eur. J. Biochem. (1972) 29, 268275. 33. Gabbay, Κ. H., Clin. Research (1974) XXII (3), 468A. 34. Hers, H. G., Biochim. Biophys. Acta (1956) 22, 202-203. 35. Winegrad, A. I., Clements "Handbook of Physiology, ed.), Sect. 7, Vol. I, pp. 457-471, Williams & Wilkins, Baltimore, Maryland (1972). 36. Kinoshita, J. H., Invest. Ophthalmol. (1965) 4, 786-799. 37. Gabbay, Κ. H., New Engl. J. Med. (1973) 288, 831-836. 38. Dvornik, D., Simard-Duquesne, N., Krami, M., Sestanj, Κ., Gabbay, Κ. H., Kinoshita, J. H., Varma, S. D., and Merola, L. O., Science (1973) 182, 1146-1147. 39. Nasrallah, S. M. and Iber, F. L., Am. J. Med. Sci. (1969) 258, 80-88. 40. Ginn, Η. Ε., in "Sugars in Nutrition" (H. L. Sipple and K. W. McNutt, eds.), pp. 607-612, Academic Press, New York, San Francisco, London (1974). 41. Naito, F., New Food Industry (1971) 13, 1. 42. Hosoya, Ν., Ninth Internat. Congr. Nutrition (1972) Mexico. 43. Musch, K., Siebert, G., Schiweck, H., and Steinle, G., Z. Ernährungswissenschaft (1973), Suppl. 15, pp. 3-16.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
8 Metabolism and Physiological Effects of the Pentoses and Uronic Acids OSCAR TOUSTER Department of Molecular Biology, Vanderbilt University, Nashville, Tenn. 37235
I. Introduction The importance of the pentoses needs little comment, since they are so well known to be components of DNA, RNA, coenzymes, ATP, and proteoglycans. Moreover, pentose phosphates are impor tant metabolic intermediates in such processes as CO fixation in photosynthesis. Similarly, uronic acids are components of proteo glycans, or mucopolysaccharides, and of metabolic pathways, including those leading to the pentoses. These structural and metabolic aspects will be briefly reviewed, as well as the utili zation of these substances and relevant pathological considera tions. 2
II.
The Physiologically Important Pentoses
Table I lists pentoses which are important in mammalian metabolism. TABLE I. PHYSIOLOGICALLY-IMPORTANT PENTOSES D-Ribose - in nucleotides, pentose phosphate pathway D-2-Deoxyribose - in deoxyribonucleotides D-Ribulose - in pentose phosphate pathway; Ru-1,5-DP is CO accep tor in photosynthesis 2
D-Xylulose - in pentose phosphate pathway, in glucuronic acidxylulose cycle L-Xylulose - in glucuronic acid-xylulose cycle, excreted in gram quantities by humans with essential pentosuria D-Xylose - in gal-gal-xyl linkage of mucopolysaccharide to protein 135 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
136
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
D-Ribose of course is a key component of RNA and other r i b o n u c l e o t i d e s , and D-ribose 5-phosphate is a member of the pentose phosphate pathway of carbohydrate metabolism. D-2-Deoxyribose is a component of the n u c l e o t i d e s in DNA. D-Ribulose, as the 5phosphate, i s an intermediate in the pentose phosphate pathway and, as the 1,5-diphosphate, i s the CO acceptor in photosynthesis. D-Xylulose as the 5-phosphate d e r i v a t i v e is an intermediate i n the pentose phosphate pathway and occurs as the f r e e sugar in the g l u c u r o n a t e - x y l u l o s e c y c l e , as described in the preceding paper i n t h i s symposium. L - X y l u l o s e is a l s o a member of the c y c l e and is noteworthy because of its rather l a r g e e x c r e t i o n by humans with the genetic metabolic d i s o r d e r known as e s s e n t i a l p e n t o s u r i a . In proteoglycans (mucopolysaccharides) D-xylose is the sugar r e s i d u e attached to s e r i n e i n the bridge l i n k i n g the polymeric carbohydrate to the p r o t e i n core. The s t r u c t u r e of the g a l a c t o s e - g a l a c t o s e - x y l o s e bridge was mainly worked out through the work of Lennart Rodé occur i n nature and a r i n Table I are of the g r e a t e s t importance to mammals in t h e i r normal metabolism. 2
III.
U t i l i z a t i o n of Pentoses
14 Table I I summarizes some experiments i n which C-labeled pentoses were administered to animals and man and t h e i r u t i l i z a t i o n estimated. Examination of the l i v e r glycogen column shows that r i b o s e i s u t i l i z e d as e f f e c t i v e l y as glucose. I t should be borne i n mind, however, that s i n c e these a r e t r a c e r q u a n t i t i e s of pentose, i t need not be t r u e that equivalent but l a r g e amounts of pentose would be u t i l i z e d as w e l l as glucose. In man D-xylose, D-ribose and D-lyxose are o x i d i z e d t o C0 to a moderate extent, a f i n d i n g c o n s i s t e n t with the f a c t that most of the l a b e l which appears i n the u r i n e occurs i n a form that i s not the administered sugar. In these experiments the sugars were infused i n human subjects over a 15-minute p e r i o d . The C 0 and l i v e r g l y c o gen experiments i n d i c a t e that L-arabinose i s u t i l i z e d only to a n e g l i g i b l e extent. Although D-xylose may be isomerized to D-xylulose by plant and b a c t e r i a l enzymes, i n mammals t h i s aldopentose i s e i t h e r reduced to x y l i t o l , an intermediate i n the g l u c u r o n a t e - x y l u l o s e c y c l e (7), or o x i d i z e d to f^-xylonic a c i d by an NAD-linked Dxylose dehydrogenase detected i n c a l f lens by Van Heyningen (8). fJ-Ribose i s converted to r i b o s e 5-phosphate, an intermediate i n the pentose phosphate pathway. The route of u t i l i z a t i o n of D-lyxose has not been e s t a b l i s h e d ; perhaps i t i s isomerized to D-xylulose. 2
2
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Mouse Man Rat
Mouse Man Guinea p i g
Mouse Man
Mouse Man
Man
JL-Ribose
2-Xylose
D-Arabinose
L.-Arabinose
D-Lyxose
(Table adapted
-
-
-
-0.8 14
19
35 75
72
85
57
6-24
-
-
3 6-24
3 24 0
-
0.03
-
1.0
3 6-24 24
7.1
3 6-24 2
10.0
10
Time (Hrs.)
3
glycogen
8.3
Liver
-
16 15
In u r i n e
-
2
48 --
Oxid. to C 0
% of Administered Tracer Dose
UTILIZATION OF FREE PENTOSES BY MAMMALS
from Hollmann and Reinauer (6))
H i a t t (2) Segal and F o l e y (3) McCormick and Touster (4) McCormick and Touster (5)
Mouse
D-Glucose
a b c d
Species
Pentitose
TABLE I I .
b
a b
a b
a b d
a b c
a
Ref
138
IV.
PHYSIOLOGICAL
EFFECTS
Pentoses and D-Glucuronic A c i d as Metabolic
OF
FOOD
CARBOHYDRATES
Intermediates
The pentose phosphate pathway f o r glucose u t i l i z a t i o n i s shown i n F i g u r e 1. The main feature of t h i s pathway f o r glucose o x i d a t i o n i s that 3 molecules of glucose phosphate are o x i d i z e d , i n NADP-linked steps, to form 3 molecules of pentose phosphate. A s e r i e s of i s o m e r i z a t i o n s and group t r a n s f e r s occur which have the o v e r a l l e f f e c t of converting 3 molecules of hexose to 3 molec u l e s of f r u c t o s e phosphate and 1 molecule of glyceraldehyde phosphate plus 3 molecules of C0 . T h i s i s t h e r e f o r e the oxid a t i o n pathway of glucose metabolism. I t i s the main pathway f o r the production of NADPH, which i s the n u c l e o t i d e coenzyme commonly used i n r e d u c t i v e b i o s y n t h e t i c processes, and i t i s the route by which r i b o s e i s made f o r the production of n u c l e o t i d e s and f o r deoxyribose production as w e l l . I t should be emphasized that most of the r e a c t i o n s although not the decarboxylations are r e v e r s i b l e . Most o derived from t h i s pathwa l e f t to r i g h t , i n other words, from the non-oxidative p o r t i o n of the pathway. The phosphorylation of r i b u l o s e 5-phosphate to r i b u l o s e 1,5-diphosphate provides the C0 acceptor i n p l a n t s . We would a l s o point out that a l l pentoses i n t h i s pathway are phosphorylated. From the q u a n t i t a t i v e point of view, t h i s i s a minor pathway f o r carbohydrate o x i d a t i o n , i n comparison to the EmbdenMeyerhof g l y c o l y t i c route, but a greater amount of glucose i s d i r e c t e d through t h i s pathway when there i s need f o r NADPH. Another route by which pentoses are formed i s through the conversion of D-glucuronic a c i d to the x y l u l o s e s i n the g l u c u r o n i c - x y l u l o s e pathway,in which the pentose intermediates are not phosphorylated. A pentosuric i n d i v i d u a l excretes a rather constant amount of ^ - x y l u l o s e . The feeding of D-glucuronolactone elevates the u r i n a r y L - x y l u l o s e i n an amount i n d i c a t i n g that the conversion occurs i n rather high y i e l d (9). That t h i s i s a d i r e c t conversion has been demonstrated with l a b e l e d l a c t o n e . I t i s r e l e v a n t to mention that ^-glucuronate i s poorly converted to L - x y l u l o s e i n an experiment of t h i s type because, u n l i k e the lactone, the f r e e a c i d or s a l t i s poorly absorbed or impermeable to c e l l s . 2
2
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Pentoses and
TOUSTER
Uronic Acids
Λ u sa
QOX O-O-O X X
*2 χ κ χ ο ο ; X X Ί5Ε.
S - - Ο Χ Χ Ν X*u χ ο ό Χ ν>ο-υ-ο-ο-ο
" 3
- Χ Χ ο ο ο XXX
"Ν
ο
s i
μ
11
χ
7Γε 11
« Λ
J( -a ; ν
•Ο ι
ο ο χ
χ
ATP i r
CH OH HCOH
CH OH HOCH
2
2
tL)
HOCH H<JoH
H C 0 H
H0(|H
^ CH 0H 2
CHzOH j
Xylitol
Figure 2.
Glucuronic acid-xylulose cycle
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
142
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
UDP-glucuronic a c i d . There have been two suggestions as to D-glucuronate might be formed from UDP-glucuronic a c i d :
V
_ d
how
Glucuronic a c i d 1-phosphate \ ^
A
^
UDP-Glucuronic a c i d
Glucuronic a c i d Β
Ο
X-glucuronide
In pathway A, the enzyme UDP-glucuronic a c i d pyrophosphatase would cleave the n u c l e o t i d phosphate, which would f r e e glucuronate. An o b j e c t i o n to t h i s route i s that the pyro phosphatase has been l o c a l i z e d i n our l a b o r a t o r y as a c o n s t i t u e n t of the plasma membrane of l i v e r c e l l s (14). T h i s r e l a t i v e l y non s p e c i f i c enzyme, n u c l e o t i d e pyrophosphatase, i s the same enzyme as phosphodiesterase I. I t seems h i g h l y u n l i k e l y that an enzyme of such broad s p e c i f i c i t y and, i n p a r t i c u l a r , one l o c a t e d i n the plasma membrane plays a r o l e i n a metabolic pathway such as we are c o n s i d e r i n g here. A second o b j e c t i o n stems from the f a c t that the only phosphatases i n l i v e r which can hydrolyze g l u c u r o n i c acid-l-phosphate are l o c a t e d i n the lysosomes (15,16). These phosphatases a l s o are g e n e r a l l y not very s p e c i f i c . A second sug gested route (B) has UDPGlcUA donating a g l u c u r o n i c a c i d r e s i d u e to a h y p o t h e t i c a l acceptor, X, to form a glucuronide which e i t h e r hydrolyzes spontaneously because of i t s i n t r i n s i c i n s t a b i l i t y to y i e l d f r e e g l u c u r o n i c a c i d or i s hydrolyzed by the enzyme 3-glucuronidase. Although glucuronide formation i s a common r e a c t i o n i n l i v e r , there i s no b a s i s f o r s p e c u l a t i n g on the i d e n t i t y of X. More important, 3-glucuronidase i s l o c a t e d i n the endoplasmic r e t i c u l u m and, e s p e c i a l l y , i n the lysosomes. Although 3-glucuro nidase does produce g l u c u r o n i c a c i d from complex glucuronides, unpublished s t u d i e s i n our l a b o r a t o r y have f a i l e d to provide e v i dence that t h i s enzyme produces the g l u c u r o n i c a c i d that i s the precursor of L - a s c o r b i c a c i d . Whether we are naive about some aspect of the enzymology or c e l l biochemistry i n v o l v e d , or whether there i s another route not as yet apparent, i s u n c e r t a i n . For some years i t was b e l i e v e d that i n o s i t o l , which occurs r a t h e r commonly i n animals and i s obviously a food c o n s t i t u e n t , might be metabolized through more than one metabolic r o u t e . In a d d i t i o n , a kidney enzyme was discovered which converts i n o s i t o l to Drglucuronic a c i d . Consequently, a j o i n t study by four l a b o r a t o r i e s was undertaken i n which l a b e l e d i n o s i t o l was administered to normal and p e n t o s u r i c human beings (17). We found that the
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
8.
TOUSTER
Pentoses and
Uronic Acids
143
i n o s i t o l was o x i d i z e d to C0 to a considerable extent i n the normal but was much reduced i n pentosuric i n d i v i d u a l s , a r e s u l t cons i s t e n t with the formulation of a pathway from i n o s i t o l to D-glucuronic a c i d , which would then be metabolized through the glucuronate-xylulose c y c l e . Pentosuric subjects would accumulate l a b e l e d L - x y l u l o s e , whereas normal subjects would not have a block at t h i s p o i n t . The l a b e l i n g patterns i n i s o l a t e d metabolites conformed to the postulated metabolic pathways. M y o - i n o s i t o l l a b e l e d at p o s i t i o n 2 d i d i n f a c t y i e l d C-^-labeled u r i n a r y I±-xylulose i n the pentosuric i n d i v i d u a l s . There was a l s o a preponderance of radioactivity in of blood glucose, a f i n d i n g c o n s i s t e n t with the metabolism of the i n o s i t o l through the x y l u l o s e pathway and then on through the pentose phosphate pathway to glucose. 2
V.
P a t h o l o g i c a l Aspects of the Pentoses
Although e s s e n t i a order (9,18), not a metaboli e f f e c t the r e d u c t i o n of ^ - x y l u l o s e to x y l i t o l appears not to be d e l e t e r i o u s i n any way, the underlying b a s i s of t h i s d i s o r d e r w i l l be described at t h i s p o i n t . For some time t h i s enzymatic l o c a l i z a t i o n of the defect (at L - x y l u l o s e reduction) i n pentosuria was r a t h e r i n d i r e c t , pentosuric i n d i v i d u a l s being understandably r e l u c t a n t to donate a piece of l i v e r f o r enzymatic analyses. Not long ago Asakura i n Japan reported that the two x y l i t o l dehydrogenases are present i n red blood c e l l s , a f i n d i n g which made poss i b l e i n v e s t i g a t i o n of the normal and mutant enzymes i n human red c e l l s . When Wang and van Eys (19) c a r r i e d out such a study, i t was at f i r s t s u r p r i s i n g to f i n d i n pentosuric erythrocytes a moderate amount of a c t i v i t y of the NADP-linked x y l i t o l dehydrogenase c a t a l y z i n g the r e d u c t i o n of L - x y l u l o s e . However, f u r t h e r study showed that the enzyme was abnormal. I t s 1^ f o r NADP i s 10 to 20 times higher than that of the normal enzyme, that i s , i t has a much lower a f f i n i t y f o r t h i s substrate than does the normal enzyme. In other words, there i s a mutant enzyme present, and i t i s not very e f f e c t i v e . That the block i n pentosuria i s not t o t a l has i n f a c t been apparent from i n v i v o s t u d i e s . The feeding of xylose i n high amounts to r a t s causes x y l i t o l c a t a r a c t (20), but I should emphasize that x y l i t o l a d m i n i s t r a t i o n does not. The xylose i s transported i n t o the l e n s , where i t i s reduced to x y l i t o l and upsets the osmotic balance. In regard to the medical aspects of x y l o s e , I should mention that o r a l xylose doses are given to p a t i e n t s i n t e s t s of i n t e s t i n a l a b s o r p t i o n . Since the xylose i s poorly metabolized and i s t h e r e f o r e excreted to a considerable extent i n the u r i n e , the amount of u r i n a r y xylose i s a rough measure of the a b s o r p t i v e c a p a c i t y of the patients.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
144 VI.
PHYSIOLOGICAL
P h y s i o l o g i c a l l y Important Uronic
EFFECTS
OF
FOOD
CARBOHYDRATES
Acids
By f a r the most important u r o n i c a c i d i n mammals i s D-glucur o n i c a c i d . As shown i n Table I I I , i t i s important because i t i s a c o n s t i t u e n t of many mucopolysaccharides or proteoglycans and i s a c o n s t i t u e n t of glucuronide d e r i v a t i v e s of drugs and hormones. As already i n d i c a t e d above, i t i s a precursor of L - a s c o r b i c a c i d and the x y l u l o s e s and i s the d i r e c t product of the metabolic o x i d a t i o n of i n o s i t o l i n man. K a r l Meyer o r i g i n a l l y discovered that some of the uronic a c i d i n mucopolysaccharides i s L - i d u r o n i c a c i d , the 5 -epimer of ^ - g l u c u r o n i c a c i d . F i g u r e 3 shows a p o r t i o n of the heparan s u l f a t e molecule c o n t a i n i n g α-linked s u l f a t e d N-acetylglucosamine residues i n t e r s p e r s e d w i t h s u l f a t e d L - i d u r o n i c a c i d and 33-glucuronic a c i d . Some time ago the e p i m e r i z a t i o n of UDP-L-iduronic a c i d by r a b b i t t i s s u e e x t r a c t s was r e p o r t e d . How ever, t h i s work has not been confirmed i n any other l a b o r a t o r y and there i s now evidenc occurs at the macromolecula c o n c u r r e n t l y w i t h s u l f a t i o n of the polymer (21). ?
VII.
U t i l i z a t i o n and Production of the Uronic
Acids
The routes f o r the u t i l i z a t i o n of D-glucuronate i n v a r i o u s organisms are summarized below (7):
Microorganisms fi-Glucur ona t e
^ - f rue tur ona t e
>-D-mannona t e
>-2-keto-3-deoxygluconate (KDG)
*KDG 6-phosphate
^-pyruvate + t r i o s e phosphate Plants
^ g l u c u r o n i c a c i d 1-P D-Glucuronate
UTp
'>UDPGlcUA •pectins and h e m i c e l l u l o s e s
-glucaric acid Animals ^ x y l u l o s e pathway ^*L-gulonate ^ ΓΙ-Glucuronate -^L-ascorbic a c i d ^ ^ - g l u c a r i c acid
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TABLE I I I . PHYSIOLOGICALLY IMPORTANT URONIC ACIDS CHO I HCOH I HOCH I HCOH I HCOH I COOH
D-Glucuronic a c i d - i n h e p a r i n , h y a l u r o n i c a c i d , c h o n d r o i t i n s u l f a t e s , e t c . , and glucuronides (of drugs and hor mones) ; i n v o l v e d i n metabolism of i n o s i t o l , L-ascorbic a c i d , and the x y l u l o s e s
CHO I HCOH I HOCH
L-Iduronic a c i dermata
sulfate
i
HOCH I COOH Figure 3.
Ν S0
Portion of heparin and heparin sulfates
Ο 3
S0
Ν 3
S0
Ν 3
Biosynthesis : 1. 2. 3.
P o l y m e r i z a t i o n of GlcNAc and g l u c u r o n i c a c i d N-deacp.tylated N- and O-sulfated w i t h e p i m e r i z a t i o n of some D^-glucuronic acid to L-iduronic acid.
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The metabolism of glucuronate i n microorganisms i s q u i t e s p e c i a l , as shown by Ashwell and h i s c o l l a b o r a t o r s s e v e r a l years ago, the f i r s t step being e p i m e r i z a t i o n to D-fructuronate. In p l a n t s , Ιλglucuronate can be d i r e c t l y phosphorylated, a r e a c t i o n that does not occur i n animals. UDP-glucuronic a c i d i s a precursor of plant p o l y s a c c h a r i d e s . Glucuronate can a l s o be o x i d i z e d to the corresponding d i c a r b o x y l i c a c i d , g l u c a r i c a c i d . In animals the conversion to g l u c a r i c a c i d and the presence of the l a t t e r i n u r i n e have been demonstrated. We have already discussed the r e d u c t i o n of glucuronate to L-gulonate and i t s conversion to L ascorbate or i t s metabolism through the x y l u l o s e s . The u t i l i z a t i o n and production of g l u c u r o n i c a c i d i n v i v o are summarized below ( 7 ) :
A.
Utilization Utilization i the lactone because the a c i d probably does not enter c e l l s . High extent of u t i l i z a t i o n i s i n d i c a t e d by 14
B.
1)
high y i e l d of
CO^ from l a b e l e d lactone
2)
high y i e l d of u r i n a r y L - x y l u l o s e i n the pentosuric human
Production Glucuronic a c i d production i s increased by 1)
substances
excreted as glucuronides
2)
inducers of the microsomal P-450 system (e.g. s t e r o i d s , b a r b i t u r a t e s ) . These substances a l s o increase the production of a)
L-ascorbic acid
b)
glucaric acid
The production of g l u c u r o n i c a c i d , that i s , of UDP-glucuronate, i s responsive to some inducing agents which i n c r e a s e the produc t i o n of conjugated glucuronides, of L - a s c o r b i c a c i d , and of g l u c a r i c a c i d and, I might add, of L - x y l u l o s e i n the p e n t o s u r i c human. The mechanism of t h i s i n d u c t i o n has been studied f o r many years and i s probably s t i l l not very c l e a r l y understood. Various s t e r o i d s and b a r b i t u r a t e s and other drugs which induce the micro somal P450 system are s t i m u l a t o r s of the production of glucuronic acid derivatives.
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F i g u r e 4 shows that UDP-glucuronic intermediate i n metabolism:
a c i d i s somewhat of a key
Glucuronides
UDP-D-G lue ose»
>UDP-D-Glucuronic a c i d
UDP-^-Xylose
^ D-Glucuronic
acid
I
L-xylulose I
V xylitol
I D-xylulose Figure 4. Reactions of UDP-O-glucuronic acid UDP-Glucuronic a c i d i s used i n the b i o s y n t h e s i s of glucuronides and proteoglycans; i t i s the precursor of g l u c u r o n i c a c i d which goes to the x y l u l o s e s and to a s c o r b i c a c i d , and i t i s the precursor of the x y l o s e found i n the g a l - g a l - x y l bridge between mucopolysaccharide chains and polypeptide chains i n proteoglycans by v i r t u e of i t s d e c a r b o x y l a t i o n to UDP-xylose. VIII.
P a t h o l o g i c a l Aspects of D-Glucuronic and L-Iduronic A c i d s
There are now two h e r e d i t a r y lysosomal diseases s p e c i f i c a l l y a t t r i b u t a b l e to d e f i c i e n c i e s i n lysosomal uronidases. Gargoylism, or H u r l e r s d i s e a s e , a s s o c i a t e d with the accumulation of dermatan and heparan s u l f a t e s and with mental d e f i c i e n c y and a v a r i e t y of morphological changes, has r e c e n t l y been shown to be due to an L-iduronidase d e f i c i e n c y i n lysosomes (22). In the l a s t few years another type of mucopolysaccharidosis has been found that has some s i m i l a r i t y to H u r l e r ' s d i s e a s e . As a r e s u l t of the work of S l y et a l . (23) and of Neufeld and her a s s o c i a t e s (24), t h i s disease, a t y p i c a l mucopolysaccharidosis, can be a t t r i b u t e d to 3 - g l u c u r o n i dose d e f i c i e n c y i n the lysosomes. In connection with lysosomal d i s o r d e r s , i t i s r e l e v a n t to mention at a symposium such as the present one that severe lysosomal storage abnormalities have been caused by the a d m i n i s t r a t i o n of undegradable polymers, i n c l u d i n g dextrans and p o l y v i n y l p y r r o l i d o n e . Great c a u t i o n should be used i n i n j e c t i n g such m a t e r i a l s i n t o humans. 1
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Also known a r e abnormalities i n g l u c u r o n y l t r a n s f e r a s e , the enzyme system(s) c a t a l y z i n g the t r a n s f e r of g l u c u r o n i c a c i d from UDPGlcUA t o s u i t a b l e acceptors. In C r i g 1 e r - N a j j a r disease a d e f i ciency of g l u c u r o n y l t r a n s f e r a s e i s r e s p o n s i b l e f o r i n s u f f i c i e n t conversion of the heme degradation product b i l i r u b i n to b i l i r u b i n glucuronide, the r a p i d l y e x c r e t a b l e , more water-soluble a c i d i c form. In t h i s disease the unreacted b i l i r u b i n accumulates i n the nervous system, with d e l e t e r i o u s consequences t o the i n f a n t (25). There seems t o be an animal model of t h i s disease, the Gunn r a t , i n which s i m i l a r abnormal b i l i r u b i n metabolism i s observed as w e l l as low t r a n s f e r a s e l e v e l s . IX.
Summary and Prospects
In summary, i t i s evident that pentoses and uronic a c i d s are extremely important to mammalian b i o l o g y I have not commented on c e r t a i n aspects of u t i l i z a t i o c o n t a i n i n g polymers, suc general these seem t o be p o o r l y u t i l i z e d . One area that w i l l c e r t a i n l y witness an i n t e r e s t i n g f u t u r e concerns the lysosomal storage disease because so many of the accumulated proteoglycans are r i c h i n uronic a c i d s . Indeed, s i n c e pinocytosed chemotherap e u t i c agents a r e destined to enter lysosomes, where the metabolic abnormalities a r e found, these diseases are prime t a r g e t s f o r enzyme therapy attempts. C u r r e n t l y , 3-glucuronidase i s i n f a c t being studied as a chemotherapeutic agent i n a t y p i c a l mucopolysaccharidosis .
Literature Cited
1. Rodén, L., in "Metabolic Conjugation and Metabolic Hydrolysis" (W. H. Fishman, ed.), Vol. II, pp. 345-442, Academic Press, New York (1970). 2. Hiatt, H. H., J. Biol. Chem. (1957) 224, 851-859. 3. Segal, S. and Foley, J. B., J. Clin. Invest. (1959) 38, 407-413. 4. McCormick, D. B. and Touster, O., Biochim. Biophys. Acta (1961) 54, 598-600. 5. McCormick, D. B. and Touster, O., J. Biol. Chem. (1957) 229, 451-461. 6. Hollmann, S. and Reinauer, H., Z. Ernährungswissenschaft (1971) Suppl. 11, pp. 1-7. 7. Touster, O., in "Comprehensive Biochemistry" (M. Florkin and Ε. H. Stotz, eds.), Vol. 17, pp. 219-240, Elsevier Publishing Company, Amsterdam-London-New York (1969). 8. van Heyningen, R., Biochem. J. (1958) 69, 481-491. 9. Touster, O., Fed. Proc. (1960) 19, 977-983.
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10. Burns, J. J., in "Metabolic Pathways" (D. M. Greenberg, ed.), Vol. 1, pp. 341-356, Academic Press, New York (1960). 11. Burns, J. J. and Kanfer, J., J. Am. Chem. Soc. (1957) 79, 3604-3605. 12. Hollmann, S. and Touster, O., J. Am. Chem. Soc. (1956) 78, 3544-3545. 13. Hollmann, S. and Touster, O., J. Biol. Chem. (1957) 225, 87102. 14. Touster, O., Aronson, Ν. N., Jr., Dulaney, J. T., and Hendrickson, H., J. Cell Biol. (1970) 47, 604-618. 15. Arsenis, C., Hollmann, S., and Touster, O., Abstracts of the American Chemical Society Meeting, New York, September 1966, C-286. 16. Arsenis, C. and Touster, O., J. Biol. Chem. (1967) 242, 3400-3401. 17. Hankes, L. V., Politzer W M. Touster O., and Anderson L., Ann. Ν. Y. 18. Hiatt, H., in "Th (J. B. Stanbury, J. B. Wyngaarden, and D. S. Fredrickson, eds.), 3rd ed., pp. 119-130, McGraw Hill, New York, Toronto, London (1972). 19. Wang, Y. M. and van Eys, J., New Eng. J. Med. (1970) 282, 892-896. 20. van Heyningen, R., in "Proceedings of the International Sym posium on Metabolism, Physiology, and Clinical Use of Pentoses and Pentitols," Hakone, Japan, August 27-29, 1967 (B. L. Horecker, K. Lang, and Y. Takagi, eds.), pp. 109-123, Springer-Verlag, Berlin, Heidelberg, New York (1969). 21. Höök, M., Lindahl, U., Bäckström, G., Malmström, Α., and Fransson, L.-Å., J. Biol. Chem. (1974) 249, 3908-3915. 22. Matalon, R. and Dorfman, Α., Biochem. Biophys. Res. Commun. (1972) 47, 959-964. 23. Sly, W. S., Quinton, Β. Α., McAlister, W. H., and Rimoin, D. L., J. Pediat. (1973) 82, 249-257. 24. Hall, C. W., Cantz, M., and Neufeld, E. F., Arch. Biochem. Biophys. (1973) 155, 32-38. 25. Schmid, R. in "The Metabolic Basis of Inherited Disease" (J. B. Stanbury, J. B. Wyngaarden, and D. S. Fredrickson, eds.), 3rd ed., pp. 1141-1178, McGraw-Hill, New York, Toronto, London (1972).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
9 Role of Carbohydrates in Dental Caries WILLIAM H. BOWEN Caries Prevention and Research Branch, National Caries Program, National Institute of Dental Research, National Institutes of Health, Bethesda, Md. 20014
Dental caries result specifi which colonize the tooth surface and metabolize particular compo nents of the diet. The action results in the rapid and sometimes prolonged production of acid on the tooth surface resulting in the dissolution of the enamel. Since the time of Aristotle it has been considered that car bohydrates played an essential role in the pathogenesis of dental caries. There is now an abundance of evidence accumulated from epidemiological surveys (1) and animal experimentation (2) which clearly indicates that dental caries does not develop in the absence of dietary carbohydrate. In an elegant clinical study Gustafsson, et.al. (3) showed that the incidence of caries is related to the frequency of intake of carbohydrate and not to the total amount consumed. For example, patients who in 1 year con sumed 94 kg. of sugar with meals had fewer new carious lesions than patients who consumed 85 kg., 15 of which was taken between meals.
The restriction in carbohydrate intake which occurred during World War II was followed by a dramatic f a l l in the prevalence in dental caries in many European countries. (4)(5)(6) Carbohydrates and sugar in particular apparently can also affect the maturation of enamel, a process which leads to i n creased mineral uptake in enamel post-eruptively. It was observed (7) that the teeth of rats exposed to high sugar diets showed delayed maturation and were therefore presumably more susceptible to decay. Rats bred on a diet (8) conducive to the formation of severe protein-calorie imbalance have been shown to have enhanced susceptibility to caries. The effect was ascribed to altered tooth size (9) and to alterations in salivary composition. The physical form in which sugar is ingested will also influence its cariogenicity. Powdered sugar is more cariogenic than similar sugars given in an aqueous solution. (10) The size of particles and the adhesiveness of the diet also influence the cariogenicity of the diet. (11) In general the longer a poten150 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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t i a l l y cariogenic substance is retained in the mouth the greater is the likelihood that caries will develop. The formation of dental plaque is the earliest evidence that microorganisms and components of the diet are interacting. Dental plaque is the soft white tenacious material which occurs on tooth surfaces and is composed of microorganisms enmeshed in a matrix of carbohydrate and protein. Specific microorganisms are associated with the early formation of dental plaque and sucrose plays an important role in their establishment on the tooth surface. (12) Streptococcus mutans is a prime microbial agent in the pathogenesis of dental caries. (13)(14) It has several interesting properties; i t is found predominantly on the tooth surface and i t forms polyglucan and polyfructan from sucrose. (15)(16) Strep, mutans can become reversibly bound to the tooth surface in the absence of sucrose but following ingestion of this sugar extracellular polysaccharide is formed. (17) This substanc organism to acThere to the tooth surface and also leads to the aggregation of other microorganisms. (18) The available evidence indicates that the predominant polyglucan in plaque is composed of material possessing mainly 1-3 linkages; substantial material possessing 1-6 linkages is also present, (19) Up to 40% of the polyglucan formed in plaque is readily metaFolizable by plaque microorganisms. (20) Although polyfructan is also found i t is metabolized very rapidly(21) by substantial numbers of the microorganisms in plaque. (22T~ Apart from the contribution that extracellular polysaccharides make to the adherence of microorganisms their precise role in the pathogenesis of dental caries is unclear. Recent evidence has shown that phosphorus is tightly bound to the polysaccharide and that the carbohydrate is charged. (23) This indicates that the polysaccharide could limit the diffusion of charged substances into and out of plaque. Acid formed within plaque could not, therefore, be readily neutralized by the diffusion of such substances as bicarbonate. It is also likely that the extracellular polysaccharide protects the microorganisms from inimical influences. Whatever the precise role they have in the pathogeneses of caries there is l i t t l e doubt that the integrity of the tooth would be enhanced by either preventing their formation or their rapid removal. There is also clear evidence which indicates that microorganisms in plaque can synthesize an intracellular polysaccharide (IPS) of the amylo pectin type from a wide variety of carbohydrates. (24)(25) This material can be catabolized by plaque microorganTsms during periods when extraneous sources of carbohydrate are lacking. This catabolism is probably responsible for the comparatively low pH values observed in plaque around carious lesions even in patients who have been fasting. (26) There is some evidence to indicate that the number of intracellular polysaccharide forming organisms in plaque is positively correlated
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with the number of carious lesions.(27) The results of research carried out by Kanap and Hamilton(28) indicate that both the synthesis and catabolism of IPS is influenced by the presence of fluoride. They have shown that the low concentrations of fluoride inhibit enolase and glucose-6-p formation without penetrating the cell significantly indicating that fluoride may affect the transport of sugar into the bacterial c e l l . This is probably one of several mechanisms through which fluoride exercises its cariostatic effect. Dental plaque also forms in the absence of dietary carbohydrate, (29)(30) e.g., in patients or animals who receive their completedetTby gastric intubation, but i t lacks several properties found in plaque formed under conventional circumstances. Animals fed in this manner do not develop caries.(31)In addition the plaque so formed lacks the ability to lower the pH value of topically applied sugar solutions in contrast to that formed normally, which may lowe or less in a matter of minutes. (32) It is generally considered that rapid démineraiization of the tooth surface occurs below pH 5.5. The types and proportions of acids formed in dental plaque are attracting an increasing amount of attention because i t is conceivable that the difference in the cariogenicity of plaques may reside in some measure in the different types or proportions of acids present. Gilmour and Poole (33) found that a constant relationship between the concentration of lactic acid in plaque and pH decrease was lacking. In some plaques i t was found that the concentration of lactic, propionic and acetic acids accounted for less than 50% of the titratable acidity. In a study carried out by Geddes (34) i t was observed that 'fasting plaque contained 3 X 10- m moles of acid/mg wet weight and that five minutes after exposure to sugar the concentration had increased to 5 Χ 10" m moles. The major change was to a five-fold increase in D (-) lactate and an eight-fold increase in D (+) lactate. There is a positive correlation between the frequency of sugar intake and the incidence of caries. (35) Each ingestion of sugar is followed by a rapid f a l l in pH value on the tooth sur face. The pH returns to neutrality over a 20-30 minute period. The duration for which the plaque has been allowed to accumulate will influence both the magnitude of the f a l l in pH and the time required for its recovery. (36) In general the older the plaque the greater its pathogenic potential. An extreme example of the effects of prolonged exposure to sucrose can be seen in infants who have comforters dipped in a sucrose syrup or similar solu tion. (37) These children develop rampant caries on the palatal surfaces of the upper molars and incisors. It is occasionally argued that much caries could be elimina ted i f other sugars were substituted for sucrose in the diet. Some support for this concept can be found in patients who suffer from fructose intolerance. These patients must avoid sucrose and 1
5
5
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fructose. Fructose intolerant patients have substantially less caries than normal persons but they are not caries-free. (38) Clearly such patients must alter their dietary intake consTcTerably, other than merely avoiding sucrose. Sucrose has probably been blamed as the main dietary culprit in caries causation simply because i t is the sugar which is most frequently ingested. There is no evidence that its substitution by glucose or fructose would lead to a significant reduction in dental decay in humans. Results of many experiments carried in animals clearly indicate that glucose and fructose can induce significant levels of decay. (39)(40)(41) Primates which received their complete diet by gastric Intubation with the exception of glucose or fructose (i.e. plaque was formed in the presence of these sugars) formed plaque which contained significant levels of extracellular polysaccharide and in addition this plaque could lower the pH of sugar solutions rapidly. (42) The effects of polyol on plaque formation and the development of caries have been i n vestigated in animals and to a lesser extent in man. (43) It was observed in primates that the ingestion of sorbitol was followed i n i t i a l l y by the formation of plaque which had a syrupy consistency. It was also noted that the numbers of Strep, mutans in plaque declined markedly when sorbitol was substituted for sucrose even though Strep, mutans ferments sorbitol. Prolonged ingestion in man (44) or primates did not lead to the development of a plaque flora with an enhanced ability to metabolize sorbitol. All the available evidence indicates that sorbitol is substantially less cariogenic in animals than sugars. Xylitol was shown by Muhlemann, et. a l . (45) to be even less cariogenic than sorbitol. A possible explanation for the lower cariogenicity of sorbitol may be found in the manner in which i t is metabolized by microorganisms. The breakdown of sorbitol produces mainly formic acid and ethanol; in contrast, the metabolism of glucose results in the formation of primarily lactic acid. (46) It is possible that the extensive use ofpolyols would not be acceptable by the general public as in some persons even moderate doses have a cathartic effect. There is l i t t l e doubt that the incidence of caries would decline dramatically i f the general population would reduce the frequency of intake of fermentable carbohydrates. However the ingestion of candy is rarely associated in peoples minds with the formation of a carious lesion at some future time. Any disease which affects 95% of the population is unlikely to be controlled to a significant extent by individual effort. Effective control calls for public health measures and of these water fluoridation is the most effective.
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1. Read, T. and Knowles, E. Brit. Dent. J. (1938),64:185. 2. Frostell, G. and Baer, P.N. Acta Odont. Scand. (1971) 29:253. 3. Gustafson, B.E.; Quensel, C.E.; Swenander, L.; Lundquist, C.; Grahnen, H.; Bonow, B.; and Krasse, B. Acta Odont. Scand. (1954), 11: 232. 4. Toverud, K.V., Kjosnes, Ε., and Toverud, G. Odont. Tid. (1942), 50:529. 5. Sognnaes, R. and White, R. Amer. J. Dis. Child. (1940), 60: 283 6. Parfitt, G. J. Brit. Dent. J. (1954), 97:235. 7. Nikiforuk, G. J. Dent. Res. (1970), 49:1252. 8. Menaker, L. and Navia, J. J. Dent. Res. (1973), 52:680. 9. Holloway, P.J., Shaw, J.H. and Sweeney, E.A. Arch. oral Biol. (1961), 3:185. 10. Sognnaes, R.F. J. Nutr 11. Caldwell, R.C. J. Dent 12. Krasse, B. Arch. oral Biol. (1966), 11:429. 13. de Stoppelaar, J.D., van Houte, J.V. and de Moor, C.E. Arch. oral Biol. (1967), 12:1199. 14. Gibbons, R.J., De Paola, P.F., Spinell, D.M., and Skobe, Z. Infection and Immunity (1974), 9:481. 15. Gibbons, R.J. and Nygaard, M. Arch. oral Biol. (1968), 13: 1249. 16. Wood, J.M. and Critchley, P. Arch. oral Biol. (1966) 11:1039. 17. Gibbons, R.J. and van Houte, J. Infection and Immunity. (1971), 3:567. 18. Gibbons, R.J. and Fitzgerald, R.J. J. Bacteriol. (1969), 98: 341. 19. Baird, J.K., Longyear, V.M. and Ellwood, D.C. Microbios. (1973), 8:143. 20. Wood, J.M. Arch, oral Biol. (1969), 14:161. 21. van Houte, J. and Jansen, H.M. Arch, oral Biol. (1968), 13: 827. 22. da Costa, T. and Gibbons, R.J. Arch, oral Biol. (1968), 13: 609. 23. Melvaer, K.L., Helgeland, K. and Rolla, G. Arch, oral Biol. (1974), 19:589. 24. van Houte, J., Winkler, K.C. and Jansen, H.M. Arch. oral Biol. (1969), 14:45. 25. Gibbons, R.J. and Socransky, S.S. Arch, oral Biol. (1962), 7:73. 26. Stephan, R.M. J. Dent. Res. (1944), 23:257. 27. Loesche, W.J. and Henry, C.A. Arch, oral Biol. (1967), 12: 189. 28. Kanapka, J.A. and Hamilton, I.R. Arch. Biochem. and Biophys. (1971), 146:167. 29. Littleton, N.W., Carter, C.H. and Kelley, R.T. J. Amer. Dent. Assoc. (1967), 74:119.
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30. Bowen, W.H. and Cornick, D.E.R. Int. Dent. J. (1970),20:382. 31. Kite, O.W., Shaw, J.H. and Sognnaes, R.F. J. Nutr. (1950), 42:89. 32. Stephan, R.M. and Miller, B.F. J. Dent. Res. (1943), 22:45. 33. Gilmour, M.N. and Poole, A.E, Caries Res. (1967), 1:247. 34. Geddes, D. ORCA Abstracts (1973), Zurich. 35. Zita, Α., McDonald, R.E. and Andrews, A.L. J. Dent. Res. (1959), 38:860. 36. Kleinberg, I. J. Dent. Res. (1970), 49:1300. 37. Winter, G.B., Hamilton, M.C. and James, P.M.C. Arch. Dis. Child. (1966) 41:216. 38. Marthaler, T.M. and Froesch, E.R. Brit. Dent. J. (1967), 124:597. 39. Stephan, R.M. J. Dent. Res. (1966), 45:1551. 40. Campbell, R. and Zinner, D.D. J. Nutr. (1970), 100:11. 41. Green, R. and Harltes 42. Bowen, W.H. Arch. 43. Shaw, J.H. and Griffths, D. J. Dent. Res. (1960), 39:377. 44. Cornick, D. and Bowen, W.H. Arch, oral Biol. (1972), 17:1637. 45. Muhlemann, H., Regolati, B. and Marthaler, T.M. Helv. Odont. Acta (1970), 14:48. 46. Dallmeier, E., Bestmann, H.J. and Kroncke, A. Deutsch. Zahnaerztl. Z. (1970), 25:887.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10 The Role of Trace Elements in Human Nutrition and Metabolism R. W. TUMAN and R. J. DOISY Department of Biochemistry, State University of New York, Upstate Medical Center, Syracuse, Ν. Y. 13210
Introduction The ultimate goal of nutritionists, food chemists, and food technologists is to answer the question: What is adequate nutrition? The answer to this most important question requires know ledge of the nutrients required for good health, the amounts needed, and food sources which can supply the necessary nutrients. In the past, efforts to define human requirements have been preoccupied with discussions of the major essential nutrients, including: clarification of amino acid, vitamin and macro element requirements, recommending a proper balance of carbohy drate, fat and protein, and defining appropritate energy require ments (1). Very little attention has been given to the so-called trace elements. Only recently, however, have scientists begun to appreciate the full extent of the biochemical role of trace elements and their interactions with human health and nutrition. At the present time, the 14 trace elements listed in Table 1 have been identified as being essential for either human or animal nutrition (2). These include iron, iodine, copper, man ganese, zinc, cobalt, molybdenum, selenium, chromium, tin, vana dium, fluorine, silicon, and nickel. It i s i n t e r e s t i n g to note (Table l ) that during the f i r s t 100 p l u s years s i n c e i r o n was found to be e s s e n t i a l , only f i v e a d d i t i o n a l t r a c e elements were discovered to be r e q u i r e d f o r good h e a l t h , i n c l u d i n g i o d i n e , copper, manganese, z i n c , and c o balt. In c o n t r a s t w i t h t h i s r a t h e r slow e a r l y r e c o g n i t i o n of the importance of trace elements, during the 20 year p e r i o d from 1953-1973, a t o t a l o f e i g h t a d d i t i o n a l trace elements have been found to be e s s e n t i a l , i n c l u d i n g molybdenum, selenium, chromium, t i n , vanadium, f l u o r i n e , s i l i c o n , and n i c k e l . Furthermore, no l e s s than 5 of these 8, namely, n i c k e l , t i n , s i l i c o n , f l u o r i n e , and vanadium, have emerged as e s s e n t i a l n u t r i e n t s only i n the
156 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10.
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DOiSY
AND
157
Trace Elements in Nutrition TABLE 1
DISCOVERY OF ESSENTIAL TRACE ELEMENTS REQUIREMENTS 1. 2.
3.
Iodine . . . . Copper . . . .
4. 5.
Manganese.
. .
6.
Cobalt
7.
Molybdenum . .
8. 9. 10. 11.
Selenium . . ., Chromium . . ., , Tin Vanadium . . .,
12. 13.
Fluorine Silicon.
14.
N i c k e l . . . .,
. . . .
. . .. . . ..
Adapted from:
17th Century 1850 C h a t i n , A. 1928 Hart, Ε . Β . , H. Steenbock, J. Waddell, and C A. Elvehjem 1931 Kemmerer, A. R. and W. R. Todd 1934 Todd, W. R., C A. Elvehjem, and Ε . B. Hart 1935 Underwood, E. J. and J. F. F i l m e r ; Marston, H. R.; L i n e s , E. W. 1953 DeRenzo, E. C., E. K a l e i t a , P. H e y t l e r , J. J. Oleson, W i l l i a m s ; R i c h e r t , D. A. and W. W. Westerfeld Schwarz, K. and Schwarz, K. and Schwarz, K., D. Schwarz, K., D. and H. E. Mohr 1972 Schwarz, K. and 1972 Schwarz, K. and Ε . M. 1973 N i e l s e n , F. H.
1957 1959 1970 1971
Schwarz, K.,
C. W. B. B.
M. F o l t z Mertz M i l n e , and E. Vinyard M i l n e ; Hopkins, L. L.
D. B. Milne D. B. M i l n e ; C a r l i s l e ,
Federation Proceedings, ( £ ) .
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
158
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CARBOHYDRATES
l a s t 3-4 y e a r s . I n d i c a t i v e of the i n c r e a s e d r e c o g n i t i o n trace elements are r e c e i v i n g i n human, as w e l l as animal n u t r i t i o n , i s the r a p i d i t y with which the l i s t of e s s e n t i a l t r a c e elements i s growing. Table 2 describes the p e r i o d i c d i s t r i b u t i o n of those elements g e n e r a l l y considered to be m i c r o - n u t r i e n t elements. Examination of Table 2 r e v e a l s s e v e r a l i n t e r e s t i n g p o i n t s : l) With the recent a d d i t i o n of vanadium and n i c k e l to the l i s t of e s s e n t i a l t r a c e elements, a continuous set of 8 e s s e n t i a l e l e ments i s created i n the f i r s t t r a n s i t i o n s e r i e s , from vanadium (atomic number 23) through zinc (atomic number 30). Furthermore, when one i n c l u d e s molybdenum (atomic number 42), 9 of the Ine s s e n t i a l t r a c e elements are t r a n s i t i o n elements. Therefore, the t r a n s i t i o n s e r i e s elements, i n g e n e r a l , c o n s t i t u t e a r e g i o n of the p e r i o d i c t a b l e with s p e c i a l i n t e r e s t and importance (2). 2) With the exception of f l u o r i n e (atomic number 9) and s i l i c o n (atomic number are l o c a t e d above calcium (atomic number and none of the elements beyond i o d i n e (atomic number 53) have ever been shown to be e s s e n t i a l f o r animals or man (2). 3) More than 20 other t r a c e elements shown i n Table 2 are p o t e n t i a l l y important and are c u r r e n t l y under s p e c i a l c o n s i d e r a t i o n w i t h respect to e s s e n t i a l i t y (2). Progress i n d i s c o v e r i n g new e s s e n t i a l t r a c e elements has r e l i e d on improved research techniques. In t h i s r e s p e c t , e s t a b l i s h i n g the e s s e n t i a l i t y of the newer t r a c e elements ( t i n , vanadium, f l u o r i n e , s i l i c o n , and n i c k e l ) has r e l i e d on the development and i n t r o d u c t i o n of the u l t r a - c l e a n environment and p l a s t i c i s o l a t o r techniques, as w e l l as the use of pure c r y s t a l l i n e amino acids and vitamins i n p r e p a r i n g the d i e t s of l a b o r a t o r y animals. Recent developments i n trace element a n a l y s i s , p a r t i c u l a r l y s e n s i t i v e methods i n v o l v i n g atomic absorption spectroscopy and neutron a c t i v a t i o n , have f u r t h e r advanced the s t a t e of trace element n u t r i t i o n . At the present time, the l i s t i s composed of 14 e s s e n t i a l t r a c e elements, however, the absolute number o f r e q u i r e d trace elements i s s t i l l not known. I t i s probable that some or a l l o f the elements l i s t e d i n Table 2 as p o t e n t i a l l y important w i l l be found to p a r t i c i p a t e i n v i t a l processes as s t i l l newer experimental techniques are r e f i n e d and a p p l i e d . S e v e r a l trace elements and t h e i r d e r i v a t i v e s , f o r example, cadmium, mercury, a r s e n i c , and l e a d have been shown to be t o x i c . The d e t r i m e n t a l e f f e c t s o f l e a d and methyl-mercury on the c e n t r a l nervous system are w e l l known. B e r y l l i u m and a r s e n i c are h i g h l y carcinogenic i n l a b o r a t o r y animals, and n i c k e l carbonyls and c h r o m â t e s have been i m p l i c a t e d as a cause of lung cancer (_3)· However, i t i s obvious from past r e s u l t s , that t o x i c i t y cannot be used as a v a l i d argument a g a i n s t p o t e n t i a l e s s e n t i a l i t y . Most e s s e n t i a l elements become t o x i c at s u f f i c i e n t l y high l e v e l s and the margin between t o x i c and b e n e f i c i a l doses may be s m a l l . For example, the t o x i c i t y of selenium was demonstrated w e l l
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
C a
Mg
20
12
4jBe
IIA
IIIB
Adapted from:
55çs
37Rb
K
Na
19
n
Group IA
VIII Co
l Fe 2 7
1 2 6
^ Pd
Ni
1
6
2 8
C
I IB
7
9Au
S 0
HÊ
l 9 u l WZn 2
IB
31fe
Schwarz, K., F e d e r a t i o n Proceedings, (2^).
1 1 Elements w i t h known e s s e n t i a l i t y . — Elements p o t e n t i a l l y important. ( 3 Elements important i n normal carbohydrate metabolism.
2
h Mo|
ZhQ
23v l i ^ c r
22
T i
VI IB
VIB
VB
TVB
13M
IIIA
DISTRIBUTION OF TRACE ELEMENTS OF KNOWN AND POTENTIAL IMPORTANCE FOR HUMAN AND ANIMAL NUTRITION
PERIODIC TABLE
Table 2
8 2
51Sb Pb
S n
50
P
33AS
15
VA
3 Ge 2
^Si
IVA
34
S e
S
0 16
8
VIA
C1
F
53i
35_Br
1 7
9
VI IA
160
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before i t s e s s e n t i a l i t y , and i t i s now known that selenium possesses both t o x i c and b e n e f i c i a l p r o p e r t i e s , depending on the dose and chemical form ( O . A more recent example of t h i s concept i s the report by Schwarz (5) of a growth-promoting e f f e c t o f very low doses of l e a d i n r a t s maintained i n an i s o l a t o r environment. Thus, i t would not be s u r p r i s i n g i f other t r a c e elements, u s u a l l y regarded as t o x i c , w i l l a l s o be found to be b e n e f i c i a l or e s s e n t i a l . The b i o l o g i c a l p r o p e r t i e s of the 14 trace elements now r e cognized to be e s s e n t i a l are very diverse (Table 3)· I n d i v i d u a l Trace Elements Iron. I t i s w e l l recognized that i r o n i s e s s e n t i a l as an i n t e g r a l component of hemoglobin and as a component of some f l a v o p r o t e i n s and the o x i d a t i v e r e s p i r a t o r y c h a i n . Suboptimal d i e t a r y i r o n intake r e s u l t wide-spread incidence o of the United S t a t e s p o p u l a t i o n was r e c e n t l y documented i n the Ten-State N u t r i t i o n Survey ( 6 ) . The RDA f o r i r o n i n the 8th e d i t i o n of Recommended Dietary Allowances published by the Food and N u t r i t i o n Board of the National Research C o u n c i l has remained at 10 mg/day f o r a d u l t males and post-menopausal females and 18 mg/day f o r women of c h i l d b e a r i n g age (7). This l a t t e r amount i s d i f f i c u l t to o b t a i n by d i e t a r y means since i t has been estimated that a balanced, average American d i e t provides only about 6 mg i r o n per 1,000 k c a l (8). Therefore, women of c h i l d b e a r i n g age would be r e q u i r e d to eat a 3,000 k c a l d i e t to meet the recommenda t i o n of 18 mg i r o n , an undesirable c a l o r i c intake f o r most women. Thus, the major change i n the 1974· RDA f o r i r o n i s a recommendation f o r supplemental i r o n intake (30-60 mg) f o r c h i l d b e a r i n g women ( 7 ) . The use of i r o n enriched foods has been suggested as a means to meet the RDA f o r i r o n . 1
Iodine. The only known f u n c t i o n of i o d i n e i n human and animal physiology i s i t s e s s e n t i a l r o l e i n formation of the t h y r o i d hormones, t h y r o x i n , and t r i i o d o t h y r o n i n e . Iodine d e f i ciency r e s u l t s i n g o i t e r . The d i e t a r y allowance f o r i o d i n e reported i n the l a t e s t RDA s has not changed. For the prevention of g o i t e r , an i o d i n e intake of 1 ug/kg body weight i s recommended w i t h a d d i t i o n a l i o d i n e intake recommended f o r growing c h i l d r e n and pregnant women ( 7 ) . The adequacy of an i n d i v i d u a l s i o d i n e intake i s d i f f i c u l t to assess and intakes vary widely. Thus, i t i s s t i l l suggested that to prevent d e f i c i e n c i e s only i o d i z e d s a l t be used. f
Zinc. Zinc plays an e s s e n t i a l r o l e i n a number of important processes i n c l u d i n g : l ) enzymatic f u n c t i o n , 2) p r o t e i n synthesis and 3) carbohydrate metabolism ( 9 ) . There are at l e a s t 18 known z i n c - c o n t a i n i n g enzymes, i n c l u d i n g l a c t a t e dehydrogenase, carbon-
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10.
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Table 3 TRACE ELEMENTS WITH BENEFICIAL EFFECTS IN MAN AND ANIMALS
Iron
Element
Adult RDA
(Fe)
10-18 mg
Iodine
(I)
100-150 ]ig
(Cu)
Estimated 80 yg/kg
Copper
Manganese (Mh)
Zinc
(Zn)
Cobalt
^Molybdenum (Mo)
Selenium (Se )
Tin
(Cr)
(Sn)
Vanadium
(V)
Fluorine
(F)
Silicon
Nickel
i n t e g r a l component of Hb, cytochrome and enzymes. e s s e n t i a l f o r thyroxine formation and p r e v e n t i o n of g o i t e r , component of s e v e r a l enzymes, eg. cytochrome oxidase c; r o l e i n t i s s u e Fe m o b i l i z a t i o n and Hb s y n t h e s i s , r e q u i r e d by s e v e r a l enzymes, eg. glucokinase, phosphoglucomutase, a c e t y l CoA synthetase, a r g i n a s e .
15 mg
(Co)
Chromium
Estimated 2-3 mg
B i o l o g i c a l Function
(Si)
(Ni)
3 Ug as V i t . Bi2 Estimated 2 yg/kg Not e s tablished Not e s tablished Not e s tablished Not e s tablished Not e s tablished Not e s tablished
Not e s tablished
enzymes, eg. l a c t a t e dehydrogenase, carbonic anhydrase, peptidases i n t e g r a l component of V i t . B]_2 and RBC formation. i n t e g r a l component of s e v e r a l e n enzymes, eg. xanthine oxidase, a l d e hyde oxidase. i n t e g r a l component of g l u t a t h i o n e peroxidase. i n t e g r a l p a r t of Glucose Tolerance F a c t o r ; r e q u i r e d f o r normal i n s u l i n response and CHO metabolism, r e q u i r e d f o r optimal growth by r a t s (1-2 ppm of d i e t ) , r e q u i r e d f o r optimal growth by r a t s and chickens. p r e v e n t i o n of d e n t a l c a r i e s and maintenance of normal s k e l e t o n ; r e q u i r e d f o r optimal growth by r a t s . r e q u i r e d f o r optimal growth by r a t s and chickens; important r o l e i n normal bone c a l c i f i c a t i o n and s t r u c t u r a l connective t i s s u e , r e q u i r e d f o r normal l i v e r f u n c t i o n by r a t s and chickens.
C l i n i c a l evidence of d e f i c i e n c y i n ADULT man unknown.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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i c anhydrase, and s e v e r a l peptidases ( 1 0 ) . Zinc d e f i c i e n c y i n animals has r e s u l t e d i n decreased synthesis of b o t h DNA and RNA, and reduced p r o t e i n synthesis has been observed i n z i n c - d e f i c i e n t rats. The r o l e of z i n c i n carbohydrate metabolism i s s t i l l cont r o v e r s i a l ; however, impaired glucose t o l e r a n c e has been reported i n z i n c - d e f i c i e n t r a t s (11 ). More r e c e n t l y , p a t h o l o g i c a l conditions that appear to be a consequence of inadequate z i n c n u t r i t u r e have been i d e n t i f i e d i n man. Dietary d e f i c i e n c y of zinc i n man i s a s s o c i a t e d w i t h anorexia, hypogeusia (impaired t a s t e ) and hyposmia (impaired s m e l l ) , r e t a r d e d growth, delayed sexual maturation, and impaired wound h e a l i n g (12_, 13, 14_). Zinc d e f i c i e n c y was r e c e n t l y r e p o r t ed i n 8 percent of 150 c h i l d r e n from middle-income f a m i l i e s i n Denver, with another 36 percent of these c h i l d r e n probably marginal i n t h e i r z i n c i n t a k e . These d e f i c i e n t c h i l d r e n showed poor growth (below 10th p e r c e n t i l e ) , poor a p p e t i t e , impaired t a s t e a c u i t y , and low h a i vealed that the d i e t s o symptoms improved with z i n c supplementation (15_). In the Middle E a s t , zinc d e f i c i e n c y , a s s o c i a t e d w i t h hypogonadism and dwarfism, has been demonstrated i n man ( 1 6 ) . These studies suggest that marginal z i n c d e f i c i e n c y may be more widespread than p r e v i o u s l y thought and that d i e t a r y intake of z i n c i n the United States cannot be assumed to be o p t i m a l . I t i s estimated that a t y p i c a l American d i e t s u p p l i e s b e tween 10 and 15 mg z i n c per day to meet an estimated d a i l y requirement of 10 mg/day (12). Thus, the average d a i l y intake i s only s l i g h t l y more than the estimated d a i l y requirement, and does not provide a s u f f i c i e n t s a f e t y margin. Therefore, people who consume foods which are lower than average i n a v a i l a b l e z i n c , such as meat analogs made from vegetable p r o t e i n , may s u f f e r from marginal z i n c intakes ( 1 2 ) . In view of the above, the 1974 RDA's f o r the f i r s t time i n c l u d e a recommendation f o r z i n c , w i t h 15 mg b e i n g suggested f o r a d u l t men and women, and 20 to 25 mg during pregnancy and l a c t a t i o n ( 7 ) . Chromium. Chromium i s e s s e n t i a l f o r the maintenance of normal carbohydrate metabolism i n at l e a s t three species of experimental animals. T r i v a l e n t chromium i s an i n t e g r a l p a r t of Glucose Tolerance Factor (GTF), and f u n c t i o n s as a c o f a c t o r f o r the p e r i p h e r a l a c t i o n of i n s u l i n . In the r a t , the f i r s t observed consequence of m i l d chromium d e f i c i e n c y i s an impairment of glucose t o l e r a n c e , caused by a reduced s e n s i t i v i t y o f p e r i p h e r a l t i s s u e s to i n s u l i n . A more severe degree of chromium d e f i c i e n c y leads to f a s t i n g hyperglycemia, g l y c o s u r i a , and m i l d growth r e t a r d a t i o n (17, 1 8 ) . There i s considerable evidence that chromium i s a l s o e s s e n t i a l f o r man. Impaired glucose t o l e r a n c e i s the hallmark of chromium d e f i c i e n c y i n man. There has been s p e c u l a t i o n that chromium d e f i c i e n c y may c o n t r i b u t e to the development of
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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a t h e r o s c l e r o s i s ( 1 9 ) , and i t i s of i n t e r e s t that serum c h o l e s t e r o l l e v e l s increase w i t h age as t i s s u e chromium l e v e l s decrease w i t h age ( 1 9 ) . Evidence f o r the occurrence of chromium d e f i c i e n cy i n man and the e f f e c t of d i e t a r y chromium supplementation w i l l be discussed subsequently i n t h i s paper. Cobalt. Cobalt i s e s s e n t i a l to man only through i t s f u n c t i o n as an i n t e g r a l part of Vitamin B]_ and no other f u n c t i o n s f o r c o b a l t i n human n u t r i t i o n are known. Cobalt d e f i c i e n c y has never been produced i n a nonruminant animal and the r o l e of c o b a l t i n human n u t r i t i o n i s only a question of adequate d i e t a r y intake of Vitamin B , r a t h e r than o f c o b a l t i t s e l f ( ] J ) . RDA f o r Vitamin B i s 3 ug per day ( 7 ) . 2
1 2
T
n
e
1 2
Copper. The n u t r i t i o n a l e s s e n t i a l i t y of copper derives from i t s r o l e i n the s t r u c t u r e and f u n c t i o n of s e v e r a l cuproenzymes i n c l u d i n g cytochrom oxidase and a s c o r b i c a c i d oxidase. Copper-containing enzymes and copper-containing p r o t e i n s are r e q u i r e d f o r c e l l u l a r r e s p i r a t i o n , normal hemoglobin s y n t h e s i s , and normal bone formation. The copper-containing p r o t e i n ceruloplasmin i s reported to be i n t i m a t e l y i n v o l v e d i n t i s s u e i r o n m o b i l i z a t i o n (l_, 13). Absolute copper d e f i c i e n c y has never been observed i n human a d u l t s ; however, copper d e f i c i e n c y has been described i n p a t i e n t s s u f f e r i n g from general m a l n u t r i t i o n and i n i n f a n t s i n a d v e r t a n t l y f e d formulated d i e t s low i n copper (l). The normal d i e t a r y copper intake of between 2-5 mg per day i s s u f f i c i e n t to meet the recommended d i e t a r y allowance of 2 mg per day. Thus, copper d e f i c i e n c y does not appear to be a problem i n t h i s country ( 2 0 ) . Manganese. Manganese f u n c t i o n s as a c o f a c t o r f o r s e v e r a l metallo-enzymes i n c l u d i n g : glucokinase, phosphoglucomutase, a c e t y l CoA synthetase and a r g i n a s e . The existence of manganese d e f i c i e n c y has been demonstrated i n p i g s , p o u l t r y , r a t s , mice, c a t t l e , and sheep; however, evidence of human d e f i c i e n c y has never been obtained. Manganese d e f i c i e n c y was a c c i d e n t a l l y induced i n man through the feeding of a s y n t h e t i c r a t i o n (21 ). The manifestations of manganese d e f i c i e n c y i n animals i n c l u d e growth r e t a r d a t i o n , reduced f e r t i l i t y , s k e l e t a l abnormalities and d i s o r d e r s of the c e n t r a l nervous system ( a t a x i a of the newborn) (l_, l^). In a d d i t i o n , manganese plays an important r o l e i n normal carbohydrate metabolism, as suggested by the impaired glucose t o l e r a n c e and h y p o p l a s t i c p a n c r e a t i c i s l e t c e l l s observed i n manganese d e f i c i e n t guinea p i g s ( 2 2 ) . The d a i l y manganese requirements f o r man are unknown; however, from intake and balance s t u d i e s , i t i s estimated that a manganese intake of 2-3 mg/day i s adequate f o r a d u l t s ( 7 , 20). Molybdenum.
Molybdenum plays an e s s e n t i a l r o l e as an i n t e -
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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g r a l component of s e v e r a l enzymes i n c l u d i n g , xanthine oxidase, aldehyde oxidase and s u l f i t e oxidase. Molybdenum d e f i c i e n c y has been demonstrated i n lambs, c h i c k s , and turkey p o u l t s u s i n g h i g h l y p u r i f i e d d i e t s w i t h a low molybdenum content (approximate l y 20 yg/kg). Feeding t h i s low molybdenum d i e t r e s u l t e d i n depressed growth (23) and decreased a c t i v i t y of the enzyme xanthine oxidase ( 2 4 ) . Molybdenum d e f i c i e n c y has never been reported i n humans. An RDA f o r molybdenum has not been e s t a b l i s h e d ; however, balance s t u d i e s i n d i c a t e that p o s i t i v e balance i n man can be maintained w i t h a molybdenum intake of about 2 pg/kg body weight per day. Selenium (13, 2 3 ) . It has r e c e n t l y been demonstrated (29 ) that selenium i s an i n t e g r a l component of the enzyme g l u t a t h i o n e peroxidase. Glutathione peroxidase i s i n v o l v e d i n the r e o x i d a t i o n of reduced g l u t a t h i o n e . The enzyme has been i s o l a t e d from sheep r e d b l o o d c e l l s ; i 80,000; and i t contains one atom of selenium per p r o t e i n subunit of approximately 20,000 molecular weight (2). Selenium d e f i c i e n c y i s i n t i m a t e l y r e l a t e d to Vitamin Ε d e f i c i e n c y d i s e a s e s . For example, the simultaneous d e f i c i e n c y of selenium and Vitamin Ε causes a v a r i e t y of p a t h o l o g i e s i n animals i n c l u d i n g : f a t a l exudative d i a t h e s i s i n chicks and turkeys, l i v e r n e c r o s i s i n the r a t and p i g , m u l t i p l e n e c r o t i c degeneration of heart, l i v e r , muscle, and kidney i n the mouse, and muscular dystrophy i n lambs, c a l v e s , and chicks (white muscle d i s e a s e ) . Recent s t u d i e s i n chicks have demonstrated t h a t simple selenium d e f i c i e n c y , uncomplicated by inadequate Vitamin E, causes impaired growth, poor f e a t h e r i n g , and f i b r o t i c degenera t i o n of the pancreas (25). Selenium d e f i c i e n c y i n the r a t i s manifested by impaired growth, impaired h a i r coat development, and reproductive f a i l u r e ( 2 6 ) . Furthermore, feeding a seleniumd e f i c i e n t d i e t c o n t a i n i n g adequate Vitamin Ε to subhuman primates r e s u l t s i n h e p a t i c n e c r o s i s , nephrosis, degenerative changes i n c a r d i a c and s k e l e t a l muscle, weight l o s s and death ( 2 7 ) . As stated e a r l i e r , selenium possesses b o t h b e n e f i c i a l and toxic properties. N a t u r a l l y o c c u r r i n g selenium p o i s o n i n g i n animals r e s u l t s i n the acute t o x i c i t y syndrome, "blind staggers", and the chronic t o x i c i t y syndrome of " a l k a l i disease". In adult man, no evidence of e i t h e r selenium d e f i c i e n c y or t o x i c i t y has been demonstrated, however, selenium d e f i c i e n c y has been reported i n c h i l d r e n w i t h p r o t e i n - c a l o r i e m a l n u t r i t i o n ( 2 8 ) . Due to l a c k of s u f f i c i e n t information, i t has not been p o s s i b l e to e s t a b l i s h an RDA f o r selenium i n humans. T i n , Vanadium, F l u o r i n e ,
Silicon,
Nickel
The f i v e t r a c e elements discussed below have r e c e n t l y been found to be e s s e n t i a l f o r l a b o r a t o r y animals (2, 30, 31 ).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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T i n . T i n was r e c e n t l y found to be r e q u i r e d f o r o p t i m a l growth i n r a t s . T i n d e f i c i e n c y has been produced i n r a t s by housing i n an i s o l a t o r environment and be feeding h i g h l y p u r i f i e d amino a c i d d i e t s low i n t i n . S i g n i f i c a n t growth s t i m u l a t i o n was observed when the d i e t s were supplemented w i t h 1-2 mg t i n / k g (ppm) of d i e t (2, 30, 31, 32). Vanadium. Vanadium i s e s s e n t i a l i n at l e a s t two animal species, the chick and the r a t . Vanadium d e f i c i e n c y i n chickens ( d i e t s c o n t a i n i n g l e s s than 10 yg/kg) r e s u l t s i n poor f e a t h e r growth (33) and r e t a r d e d bone development (30, 31). In the r a t , d i e t s s u p p l y i n g l e s s than 100 ng vanadium/g of d i e t (100 ppb ) showed decreased body growth (3A_) and r e c e n t l y i t was shown that r a t s respond to 50-100 ng vanadium/g of d i e t w i t h s i g n i f i c a n t growth s t i m u l a t i o n (£). Fluorine. In additio as f l u o r i d e , i n the p r e v e n t i o tenance of a normal body s k e l e t o n (13), r e c e n t l y f l u o r i n e has been shown to be e s s e n t i a l f o r o p t i m a l growth i n the r a t . S i g n i f i c a n t growth e f f e c t s were produced w i t h 250 yg f l u o r i n e / 1 0 0 g of d i e t (2.5 ppm) ( 2 ) . S i l i c o n . S i l i c o n has been shown to be e s s e n t i a l f o r normal growth i n animals. S i l i c o n d e f i c i e n c y i n chicks and r a t s causes depressed growth, and abnormal bone c a l c i f i c a t i o n . S i l i c o n a l s o p l a y s an e s s e n t i a l r o l e i n mucopolysaccharide metabolism and normal, connective t i s s u e development. S i l i c o n was r e c e n t l y r e ported to have a growth-promoting e f f e c t i n r a t s (2_, 30). N i c k e l . N i c k e l appears to be e s s e n t i a l f o r animals, and pathology c o n s i s t e n t w i t h n i c k e l d e f i c i e n c y has been produced i n c h i c k s , r a t s , and swine. N i c k e l d e f i c i e n c y i n these animals causes metabolic a b n o r m a l i t i e s i n the l i v e r , i n c l u d i n g a decreas ed oxygen uptake by l i v e r homogenates i n the presence of α-gly cerophosphate, increased l i v e r l i p i d s , i n c r e a s e d p h o s p h o l i p i d and c h o l e s t e r o l f r a c t i o n , and hepatocyte u l t r a s t r u c t u r a l abnormali t i e s (30, 31). Furthermore, i n the r a t , n i c k e l d e f i c i e n c y r e s u l t e d i n abnormal r e p r o d u c t i o n as suggested by increased f e t a l m o r t a l i t y (30). At the present time, no evidence f o r human e s s e n t i a l i t y has been demonstrated f o r these f i v e newer t r a c e elements. No estimate of man* s requirements f o r these newer e s s e n t i a l t r a c e elements can be o f f e r e d i n view of the c u r r e n t i n s u f f i c i e n t evidence and knowledge of i n t a k e s i n humans. Future Considerations As brought out at a recent symposium (35.) l i t t l e i s known about the i n t e r a c t i o n s that occur between e s s e n t i a l t r a c e e l e -
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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ments. Knowledge of t r a c e element i n t e r a c t i o n s i s of utmost importance to any v a l i d recommendation o f d i e t a r y i n t a k e . That i s to say, a h i g h d i e t a r y intake of a given element may reduce a v a i l a b i l i t y o f some other element. Such known r e l a t i o n s h i p s as high calcium intake antagonizing (or reducing) copper a v a i l a b i l i t y i s but one example of such an i n t e r a c t i o n . Tungstate i s known to antagonize molybdate, and copper and zinc a l s o i n t e r a c t . For example, people who are r e c e i v i n g therapeutic doses of zinc f o r burns may have an increased copper requirement. Thus, knowing the d a i l y intake of any given element i s only part of the problem. The intake of other ( i n t e r f e r i n g ) elements and t h e i r balance i s of utmost importance. These i n t e r r e l a t i o n s are o n l y beginning to u n f o l d and much more work i s needed on t h i s aspect of t r a c e elements i n n u t r i t i o n . Role of Chromium i n Human and Animal
Nutrition
Schwarz and Mertz i d e n t i f i e d t r i v a l e n t chromium as an i n t e g r a l component of the b i o l o g i c a l l y a c t i v e Glucose Tolerance F a c t o r (GTF) i n 1959 ( 3 6 ) . Since t h a t time a l a r g e body of convincing experimental evidence has accumulated suggesting that GTF i s r e q u i r e d f o r the maintenance of normal carbohydrate metab o l i s m by both animals and man (18^). Chromium n u t r i t i o n i n man has r e c e n t l y been reviewed by Hambidge (18) and Doisy, et a l . ( 37) and w i l l not be d e a l t with i n d e t a i l i n t h i s paper. The c h a r a c t e r i s t i c s of GTF as known at t h i s time are summarized below (17): GTF i s a n a t u r a l l y o c c u r r i n g , d i a l y z a b l e , heat and a c i d s t a b l e , organic compound of low molecular weight (400-600 d a l tons). ï t can be extracted and concentrated from brewers y e a s t , while l i v e r and kidney are a l s o recognized as p o t e n t i a l l y r i c h sources. The p r e c i s e s t r u c t u r e of GTF i s not yet known; however, Mertz (39) r e c e n t l y suggested t h a t the complex contains two n i c o t i n i c a c i d molecules per chromium atom. Furthermore, g l y c i n e , c y s t e i n e , and p o s s i b l y glutamic a c i d may be r e q u i s i t e amino a c i d s . The amino a c i d s may only be r e q u i r e d to make the complex water s o l u b l e , and the b i o l o g i c a l a c t i v i t y may be due to the chromium and n i c o t i n i c a c i d s i n a unique c o o r d i n a t i o n complex. Recently, Mertz has prepared b i o l o g i c a l l y a c t i v i e s y n t h e t i c chromiumn i c o t i n i c a c i d complexes which seem to be s i m i l a r t o , but not i d e n t i c a l w i t h , the n a t u r a l l y o c c u r r i n g GTF complex ( 3 9 ) . GTF contains t r i v a l e n t chromium as the a c t i v e metal i o n . The b i o l o g i c a l e f f e c t of chromium "in v i t r o " and "in vivo", i s s o l e l y dependent on the valency s t a t e and o n l y t r i v a l e n t chromium exhibits b i o l o g i c a l a c t i v i t y . The b i o l o g i c a l e f f e c t s of GTFchromium are q u a l i t a t i v e l y s i m i l a r , but q u a n t i t a t i v e l y much g r e a t e r than simple i n o r g a n i c chromium complexes. For example, much s m a l l e r q u a n t i t i e s o f chromium are r e q u i r e d "in vivo" to r e s t o r e normal glucose t o l e r a n c e i n r a t s i f given i n the form of GTF. Furthermore, the b i o - a v a i l a b i l i t y of chromium to animals
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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and man i s dependent on the chemical form. Chromium i n the form of GTF, i s b i o l o g i c a l l y a v a i l a b l e and b e t t e r absorbed than simple i n o r g a n i c chromium s a l t s . For example, l e s s than 1 p e r cent of an o r a l dose of chromium i n the form of chromic c h l o r i d e i s absorbed as compared to 10-25 percent of the chromium i n an o r a l dose of GTF (38.)· The chemical form of chromium a l s o de termines i t s d i s t r i b u t i o n i n t i s s u e s . Only chromium i n the form of GTF i s concentrated i n the l i v e r and only GTF chromium i s a v a i l a b l e to the fetus through p l a c e n t a l t r a n s p o r t . "In v i t r o " s t u d i e s suggest that GTF f u n c t i o n s by p o t e n t i a t ing the a c t i o n of i n s u l i n and, as such, i t i s a r e q u i r e d c o f a c t o r f o r maximal i n s u l i n response i n a l l i n s u l i n - s e n s i t i v e t i s s u e s (17). As a r e s u l t of an uncorrected d e f i c i e n c y of chromium, a normal i n s u l i n response may only be achieved w i t h u n p h y s i o l o g i c a l l y high concentrations of i n s u l i n . Evidence f o r Chromium D e f i c i e n c It i s w e l l e s t a b l i s h e d that chromium d e f i c i e n c y can be i n duced (both a c c i d e n t a l l y and d e l i v e r a t e l y ) i n animals by feeding a d i e t that i s low i n a v a i l a b l e chromium ( 3 6 , 40, 41., 42 ) . Evidence has accumulated which suggests that chromium d e f i c i e n c y a l s o e x i s t s i n c e r t a i n segments o f the human p o p u l a t i o n . This evidence i s i n d i r e c t and i s based on the f o l l o w i n g observations: l ) t i s s u e chromium l e v e l s decrease w i t h i n c r e a s i n g age i n the United States ( 4 3 , 4 4 ) ; 2) absence of an acute r i s e of serum chromium f o l l o w i n g an i n s u l i n or glucose challenge i n d i a b e t i c subjects ( 4 5 ) , and i n pregnant woment w i t h impaired glucose t o l e r a n c e Γ46 ); 3) diabetes i s a s s o c i a t e d w i t h low chromium l e v e l s i n h a i r (47.) and l i v e r (AS) compared to n o n d i a b e t i c con t r o l s ; 4 ) insulin-dependent d i a b e t i c s metabolize chromium i n a manner that i s abnormal as compared to n o n - d i a b e t i c subjects (49 ) and 5) subjects w i t h impaired glucose t o l e r a n c e , i n c l u d i n g some maturity-onset d i a b e t i c s ( 5 0 ) , middle-aged subjects (51 ), c h i l d r e n i n Jordan s u f f e r i n g w i t h kwashiorkor (52_), c h i l d r e n i n Turkey s u f f e r i n g from marasmus (22)' l subjects (37, 49, 54 ) show improved glucose t o l e r a n c e a f t e r o r a l chromium supplementation of the d i e t . Thus i t appears l i k e l y that marginal or overt chromium d e f i c i e n c y occurs i n the United States and elsewhere i n the world. a
n
d
s
o
m
e
e
d
e
r
l
y
E f f e c t s of Chromium and GTF Supplementation of the Diet The f a c t that t i s s u e chromium l e v e l s decrease w i t h age i n the United States and are e x c e p t i o n a l l y low i n e l d e r l y s u b j e c t s , i s compatible w i t h , but not proof of chromium d e f i c i e n c y . How ever, these data suggest a r o l e f o r chromium i n e x p l a i n i n g the e t i o l o g i c a l b a s i s f o r impaired glucose t o l e r a n c e which i s ex h i b i t e d by the m a j o r i t y of e l d e r l y subjects over 70 years of age. Hence, a tempting c o n c l u s i o n i s that many e l d e r l y i n d i v i d u a l s
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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have impaired glucose tolerance on the b a s i s of n u t r i t i o n a l chromium d e f i c i e n c y . Therefore, i f a d e f i c i e n c y i s suspected, i n c r e a s i n g the d i e t a r y intake of chromium should r e l i e v e the d e f i c i e n c y and normalize the glucose t o l e r a n c e . This has been done with e l d e r l y subjects thought to be chromium d e f i c i e n t . In the o r i g i n a l study by Levine, et a l . (37, 54), 86 percent of the e l d e r l y p o p u l a t i o n l i v i n g i n the Onondaga County Home d i s p l a y e d abnormal glucose tolerance t e s t s . Table 4 summarizes the r e s u l t s obtained when ten e l d e r l y s u b j e c t s , with abnormal glucose t o l e r a n c e , were t r e a t e d with 150 y g d a i l y of supplemental i n o r g a n i c chromium f o r periods up to four months. In a d d i t i o n , two young subjects o r i g i n a l l y thought to be normal and one subj e c t with hemochromatosis are i n c l u d e d . Shown are the mean two hour glucose t o l e r a n c e t e s t s on seven s u b j e c t s , before and a f t e r d i e t a r y chromium supplementation. The c r i t e r i a used f o r abnorm a l i t y i n these and subsequent s t u d i e s are a peak plasma glucose l e v e l above 185 mg/dl and/o ( c r i t e r i a adapted from the mean plasma glucose l e v e l s while on the chromium supplement, p a r t i c u l a r l y at 60, 90, and 120 minutes, are lowered c o n s i d e r ably i n a l l seven subjects compared to the pre-chromium b a s e l i n e control tests. In a d d i t i o n , the mean peak plasma glucose l e v e l of the four "responding" e l d e r l y subjects as a group was lowered from 182 to 146 mg/dl and the mean plasma sugar l e v e l 2 hours a f t e r a glucose l o a d d e c l i n e d from 156 to 115 mg/dl. Thus, i n o r g a n i c chromium supplementation of the d i e t was e f f e c t i v e i n r e s t o r i n g the impaired glucose tolerance to normal i n these seven s u b j e c t s , and i t was concluded that these subjects were, i n f a c t , chromium d e f i c i e n t . It should be noted that of the ten e l d e r l y subjects t r e a t e d , only four (40%) responded f a v o r a b l y to chromium supplementation. However, i t should a l s o be pointed out that there are many e t i o l o g i e s f o r impaired glucose tolerance, i n c l u d i n g i n f e c t i o n , emotional s t r e s s , e t c . , and only those subjects w i t h a p r e - e x i s t i n g chromium d e f i c i e n c y would be expected to b e n e f i t from chromium supplementation. On the other hand, a f a i l u r e to respond to i n o r g a n i c chromium does not exclude the p o s s i b i l i t y of a GTF d e f i c i e n c y . It i s poss i b l e that the subjects that d i d not respond to i n o r g a n i c chromium may have l o s t the a b i l i t y to convert i n o r g a n i c chromium to GTF. These subjects may have a more favorable response to d i e t a r y supplementation w i t h GTF. More r e c e n t l y , the e f f e c t of d i e t a r y GTF supplementation on abnormal glucose tolerance i n e l d e r l y subjects was s t u d i e d ( 3 7 ) . In t h i s case, 45 percent (14/31) of the subjects over the age of 65 d i s p l a y e d impaired glucose t o l e r a n c e . Each of twelve s u b j e c t s , who volunteered to go on a commercial brewers yeast e x t r a c t c o n t a i n i n g GTF, r e c e i v e d a d a i l y supplement (4^g Yeastamin/day) f o r a p e r i o d o f one to two months. Yeastamin has been See
footnote f o l l o w i n g L i t e r a t u r e C i t e d .
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
10.
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AND
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Trace Elements in Nutrition Table 4
MEAN GLUCOSE TOLERANCE TESTS OF RESPONDERS - Cr PLASMA GLUCOSE CONCENTRATION mg/dl Elderly Subjects/
Age
Ο
30
60
90
120
No. of Tests
143
149
79
+
84 82
138
129
163 125
170 106
2 2
78
+
67 83
122 132
186 139
197 126
167 123
2 2
88
+
82 81
153 136
180 157
165 140
153 131
2 3
98
96
+ 101
148 142
181 152
152 111
135 100
2 3
GF
HH
Ech
FW
Young Subjects
Cr
Age
22
+
90 91
161 161
192 126
151 82
115 104
2 3
24
+
81 81
146 160
194 147
127 132
120 118
2 3
-ι-
111 97
209 155
225 170
166 150
125 139
2 2
AG
LS
Subjects w i t h Hemochromatosis
Age
49
Supplement: 50 yg of Cr three times d a i l y (CrCl3'6H20 ). ^Reprinted by permission of p u b l i s h e r . Doisy, et a l . , "Effects and Metabolism of Chromium i n Normals, E l d e r l y S u b j e c t s , and D i a b e t i c s " , In: Trace Substances i n Environmental H e a l t h - I I , D. D. Hemphill, E d . , U n i v e r s i t y o f M i s s o u r i , Columbia, pp. 75-82 (37).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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shown by Mertz to possess potent GTF a c t i v i t y . I t i s recognized that the observed responses may or may not be due to the GTF content of Yeastamin. That GTF i s the a c t i v e component must await the a v a i l a b i l i t y of pure GTF. The mean e f f e c t o f s u p p l e mentation of the d i e t on glucose t o l e r a n c e i n s i x "responding" e l d e r l y subjects i s shown i n Table 5. The subjects d i s p l a y s e v e r e l y impaired glucose t o l e r a n c e p r i o r to GTF supplementation, w i t h a peak plasma glucose l e v e l greater than 200 mg/dl and a two hour glucose value of 178 mg/dl. Following d i e t a r y GTF supplementation, however, s i x of the e l d e r l y subjects show an improved glucose t o l e r a n c e to values w i t h i n normal l i m i t s , with the one and two hour plasma glucose l e v e l s being s i g n i f i c a n t l y reduced to 162 and 132 mg/dl r e s p e c t i v e l y . In a d d i t i o n (Table 5 ) , the serum i n s u l i n l e v e l s measured during the t e s t s are decreased while on the supplement, p a r t i c u l a r l y at two hours. Thus, the work l o a d of the pancreas i s decreased and l e s s endogenou serum l e v e l s , i s neede when adequate amounts of GTF are a v a i l a b l e . This i s i n agreement w i t h the r o l e of GTF as a c o f a c t o r f o r normal i n s u l i n response. Furthermore, i n a d d i t i o n to the r e d u c t i o n i n plasma glucose and i n s u l i n l e v e l s , some subjects a l s o respond to GTF supplement a t i o n with a s i g n i f i c a n t r e d u c t i o n i n f a s t i n g serum c h o l e s t e r o l levels. C h o l e s t e r o l l e v e l s were s i g n i f i c a n t l y reduced i n these s i x subjects from a mean value of 245 to 205 mg/dl, a decrease of 40 mg/dl. In those subjects w i t h elevated t r i g l y c e r i d e l e v e l s there i s a l s o a r e d u c t i o n i n plasma t r i g l y c e r i d e . Another group of subjects d i s p l a y i n g an incidence of i m p a i r ed glucose t o l e r a n c e which i s c l e a r l y greater than that observed i n the general p o p u l a t i o n are the s i b l i n g s o f known d i a b e t i c s . The e f f e c t s of d i e t a r y GTF supplementation on the glucose t o l e r ance of a s i b l i n g o f an i n s u l i n - r e q u i r i n g d i a b e t i c are described i n Table 6 ( 3 7 ) . It i s apparent from the two i n i t i a l screening GTT s that t h i s subject d i s p l a y s " d i a b e t i c - l i k e " glucose t o l e r ance, with elevated plasma glucose l e v e l s g r e a t e r than 200 mg/dl and serum i n s u l i n l e v e l s g r e a t e r than 200 yU/ml, at two time p o i n t s during each t e s t . In a d d i t i o n , the subject i s h y p e r t r i glyceridemic. A f t e r approximately e i g h t months of d i e t a r y supplementation with 4 - 8 grams o f Yeastaiiin/day, glucose t o l e r ance i s normalized. In the l a s t t e s t (4/19/74) normal glucose l e v e l s are accompanied by a s i g n i f i c a n t r e d u c t i o n i n g l u c o s e induced plasma i n s u l i n l e v e l s . Furthermore, plasma t r i g l y c e r i d e s have been reduced to w i t h i n normal l i m i t s . As of t h i s w r i t i n g , approximately 80 other subjects with impaired glucose tolerance are on a d i e t a r y supplement c o n t a i n i n g GTF. Although not a l l subjects respond with improved glucose t o l e r a n c e , the r e s u l t s d e s c r i b e d here are more or l e s s t y p i c a l of the responses obtained i n the m a j o r i t y of s u b j e c t s . One major d i f f e r e n c e between subjects i s the v a r i a t i o n i n the length o f time on the supplement before improvement i s observed. This i s f
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
I
70 -
™ ;178 δ
δ 5
I 17
δ3 -
118
0.001
178 J 132 -
2
0.01
245 J 9t 205 - 10
Cholesterol mg/dl
NS
121 ΐ δ 112 - 12
(37).
Triglyceride mg/dl
IN
Number o f t e s t s i n parentheses. Mean - SE tUsing paired t t e s t , difference i s s i g n i f i c a n t . Supplement: 4 g of Yeastamin/day. GTT: 100 g o r a l l o a d . Reprinted by permission of p u b l i s h e r : N u t r i t i o n Foundation Monograph, Academic P r e s s , NY,
24 5 6 26 - 12
microunits/ml
Mean Serum I n s u l i n Levels
Before GTF A f t e r GTF
1 201 7 162 - 11 0.01
106 ΐ 4 9 9 - 4 NS
Before GTF ( l l ) * A f t e r GTF (9) Significance
Mean Plasma Glucose Levels mg/dl Time i n hours — 0
EFFECT OF GTF* SUPPLEMENTATION OF THE DIET ON GLUCOSE TOLERANCE TESTS ELDERLY SUBJECTS WITH IMPAIRED TOLERANCE
Table 5
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Table 6 EFFECT OF GTF SUPPLEMENT ON GLUCOSE TOLERANCE/SIBLING OF DIABETIC, AGE 30, MALE Time Glucose Insulin ChoiesTriglycerDate mg/dl yunits/ml t e r o l ides 117 251 267 150 96
35 >200 >200 95 27
194
265
108 176 220 206
38 200 >200 183
198
256
158 97
138 39
0' 15» 30' 45' 60' 90' 120' ISO 180'
112 162 212 204 185 156 85 73 76
5 52 110 125 166 130 25 12 12
150
68
0' 30' 60' 90' 120' 150' 180'
92 154 131 126 117 75 63
10 57 52 44 35 22 10
206
134
0 30 ' 60» 120' 180»
8/10/73
T
0'
8/23/73
15' 30 » 45 ' 60» 90' 120' GTF 8/24/73 1/4/74
1
4/19/74
Subject gained 8 l b s . between 1/4/74 to 4/19/74. Supplement: 4 g of Yeastamin per day 8/24/73 - 11/9/73. 8 g of Yeastamin per day 11/10/73 - 4/19/74. Reprinted by permission of p u b l i s h e r : N u t r i t i o n Foundation Monograph, Academic Press, NY (37).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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understandable s i n c e the degree of chromium and/or GTF d e f i c i e n c y might be expected to vary w i t h each s u b j e c t . For example, the e l d e r l y subjects responded i n 1-2 months time, whereas i n some s i b l i n g s of d i a b e t i c s 6 - 8 months are r e q u i r e d before n o r m a l i z a t i o n occurs. Thus, evidence i s accumulating suggesting that chromium d e f i c i e n c y does e x i s t i n c e r t a i n segments of our p o p u l a t i o n . The r i s k and incidence of chromium d e f i c i e n c y appears to be g r e a t e s t i n the f o l l o w i n g p o p u l a t i o n groups: l ) i n o l d e r age groups, 2 ) i n c h i l d r e n w i t h p r o t e i n - c a l o r i e m a l n u t r i t i o n , 3) i n i n s u l i n - r e q u i r i n g d i a b e t i c s , 4 ) i n pregnant women, p a r t i c u l a r l y multiparae ( g e s t a t i o n a l d i a b e t e s ) , and 5) subjects maintained on formulated d i e t s f o r long periods of time may be a high r i s k f o r chromium d e f i c i e n c y . I t i s proposed that chromium d e f i c i e n c y may be caused by inadequate d i e t a r y intake and/or poor a v a i l a b i l i t y o f chromium from f o o d s t u f f s . The average America chromium, with intakes i n the USA varying from 5 yg/day to over 100 yg/day ( l , 18). The average chromium intake f o r the e l d e r l y subjects (54T was 52 yg d a i l y . The d a i l y u r i n a r y l o s s of chromium i n normal a d u l t s ranged from 3-50 yg/day ( 4 6 , 55.) thus, t h i s i s the minimal amount that must be r e p l a c e d i n order to m a i n t a i n balance i n the a d u l t . As i n d i c a t e d e a r l i e r , absorption of chromium can vary from l e s s than 1 percent to 25 percent of a given dose, depending on the form i n which i t i s p r e s e n t . Theref o r e , the d i e t a r y intake r e q u i r e d to balance the u r i n a r y l o s s could vary from 50 yg to 500 yg. D i e t s e x c e p t i o n a l l y low i n chromium which do not adequately r e p l a c e l o s s e s could l e a d to chromium d e f i c i e n c y . The importance of e v a l u a t i n g f o o d s t u f f s on the b a s i s of b i o l o g i c a l l y meaningful chromium r a t h e r than t o t a l chromium content was r e c e n t l y d i s c u s s e d (56_). Due to inadequate knowledge of the forms and b i o l o g i c a l a v a i l a b i l i t y of chomium i n foods, an RDA cannot be e s t a b l i s h e d . However, i t has been suggested that a d a i l y intake of 10-30 yg of chromium i n the form of GTF would meet our d a i l y requirement ( 2 0 ) . I t has a l s o been suggested that inadequate d i e t a r y i n t a k e of chromium may occur because of l o s s e s of chromium i n food r e f i n i n g processes (57, 58). The chromium content of various wheat and sugar products and the marked l o s s o f chromium that occurs during the r e f i n i n g of these food s t a p l e s are described below. Whole g r a i n wheat contains 1.75 yg chromium/g Dry Wgt. compared to 0.23 yg/g Dry Wgt. f o r r e f i n e d white f l o u r and 0.14 yg/ g Dry Wgt. f o r white b r e a d . This represents an 87% l o s s of chromium i n the refinement of whole wheat to white f l o u r and a 92% l o s s of chromium i n going from n a t u r a l wheat to the consumer item of white b r e a d . Whole wheat bread r e t a i n s a l i t t l e more of the o r i g i n a l chromium, but there i s s t i l l a g r e a t e r than 70% l o s s of chromium i n the r e f i n i n g p r o c e s s . S i m i l a r l o s s e s of chromium occur i n the r e f i n i n g of sugar.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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Refined white t a b l e sugars r e t a i n very l i t t l e of the o r i g i n a l chromium contained i n the raw sugars. There i s a 77-94% l o s s of chromium on a dry weight b a s i s i n going from u n r e f i n e d raw sugar to the r e s u l t i n g consumer item which the American people use much of (106-120 lbs/person/annum). Raw sugar provides 6 - 8 . 8 yg Cr/100 k c a l compared to only 0 . 5 - 2 . 5 yg Cr/100 k c a l f o r r e f i n e d sugar. Most of the chromium i s removed during the r e f i n i n g p r o c e s s , i . e . , f i n a l molasses (47 yg Cr/100 k c a l ) . Therefore, i t i s obvious from data of t h i s k i n d that modern methods o f food p r o c e s s i n g remove a l a r g e percentage of chromium from two very important food items, wheat and sugar. Thus, d i e t s h i g h i n r e f i n e d sugar and r e f i n e d wheat products could c o n t r i b u t e to marginal chromium i n t a k e s . However, chromium i s not the only e s s e n t i a l t r a c e element that i s s i g n i f i c a n t l y decreased during the process of r e f i n i n g of wheat ( 5 9 ) . The l o s s e s of s i x other e s s e n t i a l t r a c e elements that occur during the r e f i n i n In a d d i t i o n to 76% of th e s s e n t i a l trace metals are removed during the m i l l i n g of wheat to white f l o u r . The l o s s e s i n f l o u r i n c l u d e : 76% of the o r i g i n a l i r o n content, 78% of the z i n c , 86% of the manganese, 68% of the copper, 48% of the molybdenum, and 89% of the c o b a l t . As with chromium, the h i g h e s t c o n c e n t r a t i o n of these trace elements i s found i n the l e s s r e f i n e d f r a c t i o n s l i k e germ and b r a n . S i m i l a r l o s s e s of e s s e n t i a l t r a c e elements occur when other important food items are d i v i d e d i n t o t h e i r component parts by e i t h e r refinement or e x t r a c t i o n (58_). As a r e s u l t of p a r t i t i o n ing r i c e , s i g n i f i c a n t amounts of f i v e e s s e n t i a l trace elements are l o s t , i n c l u d i n g : 75% o f the chromium, 46% of the manganese, 75% of the z i n c , 27% of the copper, and 38% of the c o b a l t . In a d d i t i o n , 83% of the magnesium i s removed i n the p o l i s h i n g p r o cess. In going from corn to corn meal, there i s a marked l o s s of three e s s e n t i a l trace elements i n c l u d i n g : a 56% l o s s of chromium, a 56% l o s s o f manganese, and a 51% l o s s of z i n c . In a d d i t i o n to the a l r e a d y noted h i g h l o s s of chromium i n r e f i n e d sugar, there i s a greater than 80% l o s s of f o u r other e s s e n t i a l t r a c e elements i n the p r o d u c t i o n of white t a b l e sugar. These l o s s e s i n c l u d e 90% of the manganese, 98% of the z i n c , 83% of the copper, 88% of the c o b a l t , as w e l l as 98% of the macro-element magnesium. From these d a t a , i t i s apparent that the t r a c e mineral cont e n t of many of our major f o o d s t u f f s , i n c l u d i n g r e f i n e d f l o u r , c e r e a l products, r e f i n e d sugar and r i c e , are markedly reduced during the r e f i n i n g p r o c e s s e s . Increased consumption of h i g h l y r e f i n e d foods, snack foods, and food analogs could l e a d to American d i e t s that may be marginal w i t h respect to adequate intakes of s e v e r a l t r a c e element e s s e n t i a l f o r good h e a l t h , and i n t e n t i o n a l excessive consumption of these low n u t r i e n t foods might l e a d to n u t r i t i o n a l d e f i c i e n c y diseases through replacement of conventional food n u t r i e n t s .
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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The N a t i o n a l Research C o u n c i l has r e c e n t l y recommended "enrichment" o f foods made from wheat, corn, and r i c e w i t h ten vitamins and m i n e r a l s , i n c l u d i n g i r o n , z i n c , calcium, and magnesium. It can be a n t i c i p a t e d that f u t u r e recommendations may i n clude a d d i t i o n a l e s s e n t i a l t r a c e elements not now i n c l u d e d i n t h i s suggested enrichment program. S c i e n t i f i c proof of marginal or overt d e f i c i e n c y must be obtained before any f u t u r e a d d i t i o n s would be considered. The evidence f o r chromium d e f i c i e n c y i s slowly a c c r u i n g , but f u r t h e r work remains to be done. Acknowledgement: This i n v e s t i g a t i o n was supported by part by USPHS Grants AM 15,100 and RR 229.
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In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
11 Oligosaccharidases of the Small Intestinal Brush Border GARY M. GRAY Division of Gastroenterology, Department of Medicine, Stanford University School of Medicine, Stanford, Calif. 94305
Carbohydrates represen of calories for man but must be digested to monosaccharides before absorption can occur in the small intestine. Until the last few years, it was commonly believed that all hydrolysis occurred within the intestinal lumen under the influence of secretions from the intestinal wall, the so-called succus entericus. However, the work of Crane and his colleagues (1-3) localizing disaccharidase activities to the brush border membrane of the intestine drew attention to the potential role of the small Table 1 intestinal cell in carbohydrate diges DIGESTION OF CARBOHYDRATE tion. Table 1 out LUMINAL INTESTINAL lines important FOOD SOURCE %OFCHO HYDROLYSIS HYDROLYSIS carbohydrates in the diet of man, STARCH 60 —•MALTOSE, —•GLUCOSE the amounts and (AMYLOPECTIN MALT0TRI0SE, ot-DEXTRINS proportion of each AMYLOSE) ingested (4), and the site of hydro L A C T O S E 10 NONE —GLUCOSE +GALACTOSE lysis. Notably only starch and glycogen are par SUCROSE —•GLUCOSE NONE 30 + FRUCTOSE tially hydrolyzed within the intestinal lumen. Hydrolysis of the residual oligo saccharide products of starch and of the disaccharides sucrose and lactose occurs under the influence of enzymes integral to the intestinal brush border membrane. Intraluminal Digestion of Polysaccharide Despite the common belief of 15-20 years ago that starches are hydrolyzed completely to glucose, Whelan and his colleagues 05,6) carried out extensive experiments demonstrating that the final products under physiological conditions are the oligo saccharides maltose, maltotriose and the α-limit dextrins 181
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
182
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
c o n t a i n i n g f i v e to nine glucose molecules and one or more α 1,6 branching l i n k s . This presumably r e f l e c t s a) the poor a f f i n i t y of α-amylase f o r α 1,6 branching l i n k s and f o r α 1,4 l i n k s a d j a cent to the 1,6 l i n k s (.5,6) and b) the preference of the a c t i v e s i t e of the enzyme f o r l i n e a r o l i g o s a c c h a r i d e s of f i v e or more glucose u n i t s by cleavage of the penultimate bond a t the reducing end of the molecule (7)· Despite claims that α-amylase secreted from the pancreas may b i n d to the i n t e s t i n a l s u r f a c e p r i o r to i t s a c t i o n on p o l y s a c charides ( 8 ) , i n t e s t i n a l f l u i d contains α-amylase at 10 times the c o n c e n t r a t i o n r e q u i r e d to e x p l a i n the i n v i v o s t a r c h h y d r o l y s i s i n man (9). Hence i t appears that d i g e s t i o n of s t a r c h and g l y cogen to o l i g o s a c c h a r i d e s i s p r i m a r i l y an i n t r a l u m i n a l process. Oligosaccharidases
of the I n t e s t i n a l
Surface
There i s only a t r a c l u m i n a l f l u i d s of the s m a l , hydrases are concentrated i n the brush border s u r f a c e membrane of the i n t e s t i n e . Table 2 l i s t s the enzymes from human s m a l l i n t e s t i n e that have been i d e n t i f i e d and c h a r a c t e r i z e d . A l l of these are l a r g e g l y c o p r o t e i n s . Notably there i s only a s i n g l e β-galactosidase (10) but s e v e r a l a-glucosidases (11,12) i n the brush border. There are other carbohydrases w i t h i n the i n t e r i o r of the i n t e s t i n a l c e l l that do not appear to have a d i g e s t i v e f u n c t i o n and these w i l l not be considered here. A l l of the a-glucosidases except t r e h a l a s e are capable of h y d r o l y z i n g maltose but i t seems p r e f e r a b l e to name them according to the s u b s t r a t e f o r which they are p e c u l i a r l y s p e c i f i c . The α 1,4 m a l t o - o l i g o s a c c h a r i d e and α-limit d e x t r i n products of amylase a c t i o n on s t a r c h are hydro l y z e d by glucoamylase and the α-dextrinase subunit of sucrase-ad e x t r i n a s e r e s p e c t i v e l y . The only i n t e s t i n a l carbohydrase that appears to have l i t t l e p h y s i o l o g i c a l r o l e i n d i g e s t i n g carbohy d r a t e i n the d i e t of modern man i s t r e h a l a s e s i n c e i t s appro p r i a t e s u b s t r a t e i s found only i n i n s e c t s and mushrooms. At l e a s t one of these enzymes, sucrase-a-dextrinase c o n s i s t s of a complex of two p r o t e i n s , each of which has i t s own, indepen d e n t l y a c t i n g enzyme s i t e (12). This p a r t i c u l a r h y b r i d enzyme has been cleaved i n t o a c t i v e , d i s t i n c t subunits of s l i g h t l y d i f f e r e n t molecular s i z e that r e t a i n the same biochemical c h a r a c t e r i s t i c s as found i n the n a t i v e h y b r i d (12). The α-dextrinase moiety i s commonly c a l l e d "isomaltase" because i t i s capable of h y d r o l y z i n g the 1,6 l i n k a g e s but the α 1,6 l i n k e d d i s a c c h a r i d e , isomaltose, i s not a saccharide product of amylase a c t i o n on s t a r c h and hence i s not a p h y s i o l o g i c a l s u b s t r a t e presented to the i n t e s t i n a l s u r f a c e . The reason f o r union of sucrase and α-dextrinase m o i e t i e s to form a h y b r i d molecule i s unknown s i n c e these enzymes, whether present i n h y b r i d or monomeric form, hydrolyze the a p p r o p r i a t e d i s a c c h a r i d e s u b s t r a t e i n an i d e n t i c a l
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975. 2
9
*~ 1.1 (G )
4.3 ( i s o maltose)
20
3.8 (G )
18
280,000
210,000
280,000
Mol. Wt.
tG i n d i c a t e s glucose and the s u b s c r i p t the number o f α 1,4 l i n k e d glucose u n i t s .
*Commonly c a l l e d sucrase-isomaltase although isomaltose i s not a p h y s i o l o g i c a l s u b s t r a t e i n the i n t e s t i n e .
Trehalose
50
a-Dextrins
Trehalase
25
25
Sucrose
9
Sucrase-adextrinase*
2
Malto-oligosaccharides (G t >- G )
Lactose
Glucoamylase
a-Glucosidase
Lactase
β-Galactosidase
Enzyme Type Name
HUMAN BRUSH BORDER PLIGOSACCHARIDASES % Total Principal Maltase Substrate Activity Km mM
TABLE 2
184
PHYSIOLOGICAL
EFFECTS
OF
FOOD
CARBOHYDRATES
manner. However, experiments u s i n g a p u r i f i e d α-limit d e x t r i n as s u b s t r a t e have not yet been accomplished and i t i s p o s s i b l e t h a t the sucrase a c t i v e s i t e hydrolyzes adjacent α 1,4 l i n k a g e s w h i l e the α-dextrinase s i t e simultaneously hydrolyzes the α 1,6 branching p o i n t of the branched saccharide. Such c o o p e r a t i v i t y might g r e a t l y f a c i l i t a t e h y d r o l y s i s of the o l i g o s a c c h a r i d e to f r e e glucose. There i s some suggestion that l a c t a s e may a l s o e x i s t as a h y b r i d w i t h an a-glucosidase (13) and that glucoamyl a s e may be complexed w i t h an o l i g o 1,6 glucosidase (14) but s u f f i c i e n t l y pure preparations of these enzymes are not yet a v a i l a b l e to e s t a b l i s h t h i s . Development of Brush Border O l i g o s a c c h a r i d a s e s The human i n t e s t i n a l carbohydrases develop at v a r i o u s stages of u t e r i n e l i f e (15) a o u t l i n e d i F i g u r 1 Th f o th d i f f e r e n t i a l times f o r Small i n t e s t i n a l c e l l s y spa appears to be r e g u l a t e d by the r a p i d maturation and m i g r a t i o n of c e l l s from c r y p t up along the v i l l u s f o r discharge of senescent c e l l s from the v i l l u s t i p . The o l i g o s a c c h a r i d a s e s , although not present i n the immature c e l l s of the i n t e s t i n a l c r y p t s , are acquired as c r y p t c e l l s develop m o r p h o l o g i c a l l y and migrate onto the v i l l u s , as shown s c h e m a t i c a l l y i n F i g u r e 2. R e g u l a t i o n of the Oligosaccharidases The feeding of sucrose (16), f r u c t o s e (16) or glucose (17) produces a doubling i n i n t e s t i n a l sucrase a c t i v i t y . Whether t h i s occurs by v i r t u e of an i n c r e a s e i n s y n t h e s i s of the enzyme or a decrease i n degradation, i . e . s t a b i l i z a t i o n , has not been c l e a r l y d e f i n e d but i t seems l i k e l y that feeding of carbohydrates r e t a r d s degradation of the enzyme (18,19). Although the synthe s i s and degradation of other o l i g o s a c c h a r i d a s e s have not been shown to be d i r e c t l y r e g u l a t e d by s u b s t r a t e or products, the mono saccharide products r e l e a s e d i n t o the i n t e s t i n a l lumen do compete f o r the a c t i v e h y d r o l y t i c s i t e , thereby r e t a r d i n g the r a t e s of h y d r o l y s i s (20). Role of Surface O l i g o s a c c h a r i d a s e s i n D i g e s t i o n The f i n a l o l i g o s a c c h a r i d e products from glycogen and s t a r c h d i g e s t i o n and the d i e t a r y d i s a c c h a r i d e s sucrose and l a c t o s e are hydrolyzed very e f f i c i e n t l y a t the brush border s u r f a c e of the i n t e s t i n e so t h a t the r e l e a s e d monosaccharides are produced i n great abundance. Hence, n e i t h e r h y d r o l y s i s i n the i n t e s t i n a l l u m i n a l contents by α-amylase nor s u r f a c e h y d r o l y s i s by o l i g o saccharidases i n t e g r a l to the i n t e s t i n e are r a t e - l i m i t i n g i n the o v e r a l l process of h y d r o l y s i s and t r a n s p o r t i n v i v o (21,22).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
11.
185
Intestinal Oligosaccharidases
GRAY
LACTASE OTHER ouGLUCOSIDASES
TREHALASE
SUCRASE ISOMALTASE
FERTILIZATION
i
0
TERM
ι 4
ι 8
ι 12
ι 16
ι 20
ι 24
ι 28
ι 32
ι • 36
FETAL AGE (WEEKS) Figure 1. Development of human lines locate the range of fetal age during
full activity
acquired.
132 HRS
96 HRS
VILLUS
C R Y P T
ON A SYNTHESIS
PROTEIN DISACCHARIDASE SYNTHESIS ACTIVITY New England Journal of Medicine
Figure 2. Functional localization of intestinal cells from the time of their birth at the crypt base until cells migrate up the villus and are shed from the villus tip 132 hours later. Width of shaded vertical bars indicates relative amounts of the particular ac tivity, and vertical position denotes location of the activity in the crypt-villus unit.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
186
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
Instead, r a t e - l i m i t i n g phenomena appear to be i n v o l v e d p r i n c i p a l l y i n the f i n a l t r a n s p o r t of the released monosaccharides. The s o l e exception to t h i s i s the h y d r o l y s i s of l a c t o s e which i s much slower than h y d r o l y s i s of other o l i g o s a c c h a r i d e s , as shown i n F i g u r e 3, so t h a t h y d r o l y s i s i s even slower than transport of r e l e a s e d monosaccharides (22). Thus normal man i s at a r e l a t i v e disadvantage f o r the d i g e s t i o n of l a c t o s e as compared to other d i e t a r y saccharides, and t h i s appears to become p a r t i c u l a r l y important when i n t e s t i n a l disease i s present. D e f i c i e n c y of I n t e s t i n a l
Oligosaccharidase
Since entry i n t o the i n t e r i o r of the i n t e s t i n a l c e l l i s reserved f o r monosaccharides, absence or marked r e d u c t i o n of an o l i g o s a c c h a r i d a s e r e s t r i c t s the o f f e n d i n g o l i g o s a c c h a r i d e to the i n t e s t i n a l lumen. This can have d i r e consequences as o u t l i n e d i n F i g u r e 4 s i n c e the o l i g o s a c c h a r i d of i t s osmotic f o r c e . A small i n t e s t i n e and c o l o n , b a c t e r i a metabolize i t to two and three carbon fragments that are poorly absorbed i n lower bowel. Hence, the osmotic e f f e c t i s increased s e v e r a l f o l d so that i n g e s t i o n of only 50 grams of a d i s a c c h a r i d e may produce d i a r r h e a of 2000-3000 ml of f l u i d on an osmotic b a s i s alone. Other f a c t o r s such as the low pH produced by the metabolized fragments and s t i mulation of i n t e s t i n a l motion because of d i s t e n t i o n of the w a l l s of the hollow gut may a l s o c o n t r i b u t e to the d i a r r h e a and may s e c o n d a r i l y produce malabsorption of other n u t r i e n t s (Figure 4 ) . Primary Oligosaccharidase
Deficiencies
L a c t a s e D e f i c i e n c y . Lactase i s the i n t e s t i n a l saccharidase t h a t i s most commonly d e f i c i e n t , but the enzyme i s u s u a l l y normally a c t i v e i n c h i l d r e n and only becomes reduced i n a d o l e s cence and adulthood. I n t e r e s t i n g l y enough, most of the world's p o p u l a t i o n has a d u l t l a c t a s e d e f i c i e n c y (23-36), as shown i n Table 3. Thus, i t i s comTable 3 monly b e l i e v e d that l a c P R E V A L E N C E O F L A C T A S E DEFICIENCY tase d e f i c i e n c y i s a genGROUP % LACTASE DEFICIENT e t i c c o n d i t i o n . However, many of the r a c i a l groups WHITE w i t h a high prevalence of SCANDINAVIAN 3 adult lactase deficiency NORTH AMERICAN 5-20 s u f f e r from poor n u t r i BLACK t i o n and have an appre70 AMERICAN c i a b l e i n c i d e n c e of 50 AFRICAN OTHER small i n t e s t i n a l 80-100 disease^ c o n d i t i o n s CHINESE 55 INDIAN which are known to 95 FILIPINO depress i n t e s t i n a l 85 ABORIGINE (AUSTRALIAN) l a c t a s e out of ISRAELIS
60
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
11.
187
Intestinal Oligosaccharidases
GRAY
60 τ
or ι 50J
if)
ο
ι Q CO
40
30
20
10
Gastroenterology
Figure 3. Hydrolysis rates of lactose (L), maltose (M), and sucrose (S) from perfusion of a 30-cm segment of human jejunum in vivo (22). Brackets indicate ± 2 SE. Lactose is hydrolyzed much more slowly than the other disaccharides (P < 0.01). INTESTINAL L U M E N
SMALL INTESTINE
NO
LACTASE
LACTIC ACID
Annual Review of Medicine
WATERY DIARRHEA
MALABSORBTION FATS, PROTEINS, DRUGS
Figure 4. Schematic of the effect of disaccharidase defi ciency on the fate and action of a dietary disaccharide (see text for elaboration). If intes tinal lactase were present in normal concentrations, hydrol ysis would occur on the surface of the intestine and the mono saccharide products would be assimilated (41).
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
188
PHYSIOLOGICAL
p r o p o r t i o n to that found f o r other
EFFECTS OF
FOOD CARBOHYDRATES
oligosaccharidases.
Sucrase-a-Dextrinase D e f i c i e n c y . This h y b r i d enzyme has been found t o be markedly depressed o r absent i n about 100 docu mented cases (37-40). The malady appears t o be i n h e r i t e d as an autosomal r e c e s s i v e . A l l p a t i e n t s w i t h sucrase d e f i c i e n c y appear to have markedly depressed l e v e l s of α-dextrinase when isomaltose i s used as the s u b s t r a t e , but some d e x t r i n a s e a c t i v i t y u s u a l l y p e r s i s t s . This f i n d i n g , coupled w i t h t h e f a c t that sucrase and α-dextrinase a r e d i s t i n c t p r o t e i n s (12), suggests that the p r i mary genetic defect c o n s t i t u t e s the a l e r a t i o n o r absence o f the sucrase subunit w i t h secondary r e d u c t i o n o f i t s α-dextrinase partner. Symptoms produced upon i n g e s t i o n of sucrase a r e i d e n t i c a l w i t h those discussed above f o r l a c t a s e d e f i c i e n c y . Amylopectin i s u s u a l l y w e l l t o l e r a t e d even though a p p r e c i a b l e q u a n t i t i e s o f α-dextrins a r e released the r e l a t i v e l y l a r g e molecula f a c t that a p p r o p r i a t e b a c t e r i a i n colon may not be present i n s u f f i c i e n t numbers to metabolize them to small o s m o t i c a l l y a c t i v e fragments; a l s o , some h y d r o l y s i s of these α-dextrins may occur by a c t i o n of the r e s i d u a l α-dextrinase subunits o r perhaps other surface α-glucosidases such as glucoamylase. Treatment o f Disaccharidase
Deficiencies
Although i t appears t o be p o s s i b l e t o administer enzymes along w i t h a d i e t a r y o l i g o s a c c h a r i d e t o promote d i g e s t i o n , t h e expenseusually makes t h i s i m p r a c t i c a l . By f a r t h e simplest form of therapy i s the e l i m i n a t i o n o f the o f f e n d i n g carbohydrate s i n c e no s i n g l e carbohydrate c o n s t i t u t e s an o b l i g a t e source o f c a l o r i e s . References
1. Miller, D. and Crane, R.K. Biochim. Biophys. Acta (1961) 52:281-293. 2. Eichholz, A. and Crane, R.K. J. Cell Biol. (1965) 26:687-691. 3. Maestracci, D., Schmitz, J., Preiser, H. and Crane, R.K. Biochim. Biophys. Acta (1973) 323:113-124. 4. Hollingsworth, D.F. and Greaves, J.P. Am. J. Clin. Nutr. (1967)20:65-72. 5. Roberts, P.J.P. and Whelan, W.J. Biochem. J. (1960) 76: 246-253. 6. Bines, B.J. and Whelan, W.J. Biochem. J. (1960) 76:253-263. 7. Robyt, J.F. and French, D. J. Biol. Chem. (1970) 245:39173927. 8. Ugolev, A.M. Physiol. Rev. (1965) 45:555-595. 9. Fogel, M.R. and Gray, G.M. J. Appl. Physiol. (1973) 35: 263-267.
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Oligosaccharidases
189
10. Gray, G.M. and Santiago, Ν.A. J. Clin. Invest. (1969) 48: 716-728. 11. Kelly, J.J. and Alpers, D.H. Biochim. Biophys. Acta (1973) 315:113-120. 12. Conklin, K.A., Yamashiro, K.M. and Gray, G.M. J. Biol. Chem. (1975) in press. 13. Lorenz-Meyer, Η., Blum, A.L., Haemmerli, H.P. and Semenza, G. Europ. J. clin. Invest. (1972) 2: 326-331. 14. Eggermont, E. and Hers, H.G. Eur. J. Biochem. (1969) 9:488496. 15. Dahlqvist, A. and Lindberg, T. Clin. Sci. (1966) 30:517. 16. Rosensweig, N.S. and Herman, R.H. J. Clin. Invest. (1968) 47:2253-2262. 17. Deren, J.J., Broitman, S.A., Zamcheck, N. J. Clin. Invest. (1967) 46:186-195 18. Das, B.C. and Gray 19. Das, B.C. and Gray, G.M. In preparation. 20. Alpers, D.H. and Cote, M.N. Amer. J. Phys. (1971) 221:865868. 21. Gray, G.M. and Ingelfinger, F.J. J. Clin. Invest. (1966) 45:388-398. 22. Gray, G.M. and Santiago, N.A. Gastro. (1966) 51-489-498. 23. Bayless, T.M. and Rosensweig, N.S. J. Am. Med. Assoc. (1966) 197:968. 24. Huang, S.-S. and Bayless, T.M. Science (1968) 160:83. 25. Newcomer, A.D. and McGill, D.B. Gastroenterology (1967) 53:881. 26. Littman, Α., Cady, A.B., Rhodes, J. Is. J. Med. Sci. (1968) 4:110. 27. Gudmand-Höyer, E., Dahlqvist, Α., Jarnum, S. Scand. J. Gastroenterol. (1969) 4:377. 28. Sheehy, T.W. and Anderson, P.R. Lancet (1965) 2:1. 29. Welsh, J.D., Rohrer, V., Knudsen, K.B., Paustian, F.F. Arch. Int. Med. (1967) 120:261. 30. Littman, Α., Cady, A.B., Rhodes, J. Is. J. Med. Sci.(1968) 4:110. 31. Cook, G.C., Kajubi, S.K. Lancet (1966) 1:725. 32. Chung, M.H. and McGill, D.B. Gastro. (1968) 54:225. 33. Elliott, R.B., Maxwell, G.M., Vawser, N. Med. J. Aust. (1967) 1:46. 34. Davis, A.E. and Bolin, T. Nature (1967) 216:1244. 35. Desai, H.G., Chitre, A.V., Parekh, D.V., Jeejeebhoy, K.N. Gastro. (1967) 53:375. 36. Gilat, T., Kuhn, R., Gelman, E. and Mizrahy, O. Dig. Dis. (1970) 15:895-904. 37. Burgess, E.A., Levin, B., Mahalanabis, D., Tonge, R.E. Arch. Dis. Childhood (1964) 39:431. 38. Prader, A. and Auricchio, S. Ann. Rev. Med. (1965) 16:345. 39. Sonntag, W.M. et al. Gastro. (1964) 47:18.
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40. Auricchio, S. et al. J. Pediat. (1965) 66:555. 41. Gray, G.M. Annual Review Medicine (1971) 22:391-404. *Research supported by USPHS Grant AM 11270 and Research Career Development Award AM 47443 from the N a t i o n a l I n s t i t u t e s o f Health.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
12 Lactose Intolerance and Lactose Hydrolyzed Milk DAVID M. PAIGE, THEODORE M. BAYLESS, SHI-SHUNG HUANG, and RICHARD WEXLER Johns Hopkins Medical Institutions, 615 North Wolfe St., Baltimore, Md. 21205
Prevalence
Low levels of intestinal lactase activity have been found in many otherwise healthy adults and children in populations of American Negroes (1-5), Asians (6-10), Bantu tribes (11-12), South American Indians (13-14), Thais (15-17), and other population groups (18-20). Current evidence would indicate these low levels to be the norm for most populations of the world with notable exceptions being Scandinavians and those of northern European extraction. Approximately 70% of the world's adult population is lactose intolerant (21). It appears that those who have genetically acquired low lactase levels as adults are able to drink milk as infants; but gradually become increasingly lactose-intolerant after infancy. The onset of acquired lactose intolerance depends on the population studied. In developing countries, as our data from Peru indicates, 50% of the population is intolerant by 3 years of age (13). In a more technologically developed country such as the United States, 40% of the black population is lactose intolerant by the end of the first decade (Figure I). The difference between the accelerated loss of enzyme activity in children in developing areas, contrasted with the slower decline in blacks in this country, may reflect a relatively better nutritional state influencing and retarding the genetic expression of this event. Normally, i n g e s t e d l a c t o s e i s h y d r o l y z e d by t h e enzyme l a c t a s e found w i t h i n t h e b r u s h b o r d e r o f m i c r o v i l l o u s a r e a o f t h e j e j u n a l mucosa, s p l i t t i n g t h e l a c t o s e i n t o g l u c o s e and g a l a c t o s e , which a r e then absorbed. I f enzyme a c t i v i t y i s low, t h e i n g e s t e d lactose i s not hydrolyzed. F l u i d then e n t e r s t h e i n t e s t i n a l lumen t o d i l u t e t h i s h y p e r t o n i c l o a d , c a u s i n g 191 In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
Figure I.
Normal lactose tolerance in U.S. white and black children and Peruvian mestizo children
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6)-linked bacterial glucan dextran. A dextrandegrading enzyme, which has been demonstrated frequently i n homogenates of the small i n t e s t i n a l mucosa, has not yet been i d e n t i f i e d and isolated. The r a t and the dog derive high c a l o r i c value from a predominantly 3- (1+3)-linked bacterial glucan. D e f i n i t i v e experimentation i s needed to establish the o r i gin, either mammalian or microfloral or both, of the small intest i n a l enzymes that make possible the high n u t r i t i v e u t i l i z a t i o n of dextran and 3- (1+3) -glucan.
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
344
PHYSIOLOGICAL
EFFECTS
O F FOOD
CARBOHYDRATES
The reported high c a l o r i c u t i l i z a t i o n of the heteropolysac charide gum arabic i n some rat-feeding tests and not i n others, indicates need for further experimentation. Dextran, 3-(l+3)-glucan and gum arabic are not important n u t r i t i o n a l l y to man. I t i s important, however, to obtain through use of these polysaccharides information now unknown on digestive processes that occur i n the small intestine of man, the rat and other laboratory test animals.
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In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
347
INDEX A Absorption bile acid 318 of calcium 100 of carrageenan 286 and metabolism, mannose 303 of nutrients, effect of pectin on the 321 radioactive strontium 275 of sugars, physiology of the intestinal 1 and toxicity of carrageenan Absorptive surface, digestive 8 Acid absorption, bile 318 ACTH 56 Activity of pepsin, proteolytic 287 Adipose tissue fatty acid synthesis 46 lipid metabolism 51 lipogenesis 49 response to food carbohydrates 46 Adrenal cortex, effects of steroid hormones of the 56 Adrenalin 56,60 Agar 342 Alactasia 102 Alcoholism 58 Alditols, metabolism and physiological effects of 123 Aldose reductase 130 Alginate, propylene glycol 272 Alginate, sodium 273 Alginates, toxicity studies of 269 Alimentary carcinoma 331 Amylaceous substances 338, 343 α-Amylase inhibitors 244 assay of 246 detection of 247 from phaseolus vulgaris 257 from plants 244 physiological effects of 259 properties of 249 from wheat 253 α-Amylases of saliva and pancreatic juice 338 Amylose starch, high339 Anaerobic fermentation of soybean products 213 Analysis, Bio-Rad 60 Anti-hypercholesterolemic action of pectin 316 Antihyperglycemic agent 263 Arabitol 123
L-Ascorbic acid from D-glucose Artichoke, Jerusalem Atherogenic diet ATP level, hepatic Atypical mucopolysaccharidosis
140 343 26 50 147
Β Bile acid absorption Bile acids
318 329
Biosynthesis of galactose-1phosphate 113, 116 Biosynthesis, lactose 102 Biosynthesis of uridine diphosphate glucose and uridine 109 Blood lipids in rats 26 Blood sugar 202 effect of mannose on 307 regulation 59 Blood triglycerides in man 25 Bone collagen 100 Bowel, water content of the 289 Breakdown of carrageenan 286 Brewers yeast extract 168 Brush-border enzyme activities 7 Brush-border membrane 3, 338 Brush-border oligosaccharidases, human 181,183 Brush-border saccharidases 7 Butanediol 47 C Calcium, absorption of 100 Calorie sources for intravenous feeding 74 Cancer, colonic 330 Carbohydrate in adipose tissue 46 diet, high-protein, low 60 digestion 7,181 substrate in intravenous feeding, maltose as a 74 Carbohydrates adipose tissue response to food 46 advantages of substituting fructose for other 69 in dental caries, role of 150 insulin-inducing 56 metabolic effects of dietary 20 Carbon dioxide 214
349
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
350
PHYSIOLOGICAL
Carcinoma, alimentary 331 Cardiovascular disease 328 Cariogenicity of sorbitol 153 Carrageenan 282, 299, 342 absorption of 286 breakdown of 284, 286 physiological effects of 287, 289 toxicity of 282 Carrier, Na -dependent 12 Catecholamine products in the urine 58 Cellobiase activity 337 Ceramine-lactoside lipidosis 102 Cereals 325 Cerebral hemorrhage 69 Cholesterol 59, 318, 328 levels 170 plasma 299 Chromatography 223 Chromium 157, 162 deficiency in man 16 in food refining, losses of 17 in nutrition, role of 166 trivalent 166 Chstridium perfringens 214 Cobalt 157,163 Collagen, bone 100 Colonic cancer 330 Copper 157,163 COMPT 56 Coronary damage 69 Crave-eater 58 Crigler-Najjar disease 148 Cyanogen bromide 255 Cyclic AMP 56, 238 +
D Deactivation of the catecholamine hormones, enzymatic Deficiency of intestinal oligosaccharidase Degraded carrageenan physiological effects of Dental caries role of carbohydrates in Dental plaque Deoxycholate D-2-Deoxyribose Depolymerases, origin of Detection of α-amylase inhibitors Dextran Dextranase activity Diabetes mellitus Diabetic humans fructose use by Diarrheal diseases Diet and cholesterol Diet, heart disease and Diet, high-protein, low carbohydrate Dietary fiber, physiological effects of
57 186 284 289 128 150 151 332 135 336 247 339 337 75 126 55 312 331 23 60 325
EFFECTS
O F FOOD
CARBOHYDRATES
Dietary fructose in alleviating the human stress response Dietetic agents, inhibitors as Digestibility of food polysaccharides by man Digestion of carbohydrate of mannans of polysaccharide, intraluminal Digestive-absorptive surface Disaccharidase déficiences, treatment of Disaccharides as a source of calories, intravenous Diverticular disease Dulcitol
54 263 336 336 181 298 181 8 186 77 327 123
Ε Eater crave
58
Effects of dietary carbohydrates, metabolic Effects of polyols on plaque formation Effects of steroid hormones of the adrenal cortex Electrophoresis Elements, micro-nutrient Elimination of flatulence Enzymatic deactivation of the catecholamine hormones Enzymatic reaction of polyols Enzyme activities Enzyme, human liver Enzyme inhibitors, hydrolytic Enzymes of glycogen metabolism Enzymology of the polyols Erythritol Essential trace elements lost during food refining Extracellular polysaccharide
20 153 56 224 158 220 57 124 7 223 244 235 123 123 174 151
F Fabry's disease 102 Facilitated diffusion carrier 12 Fatty acid synthesis, adipose tissue 46 role of carbohydrate in 46 Fermentation of soybean products, anaerobic 213 Fiber, physiological effects of dietary 325 Fibroblast sonicates, glycogen hydrolysis by 229 Fibroblasts 228 Film surface, molecular orientation of copolymer 324 Flatus problem, a-galactosidase activity and 207 Flatulence 211, 212, 220 Fluoridation, water 153 Fluoride 152,157,165
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
351
INDEX
Food carbohydrates, adipose tissue response to 46 Food legumes, oligosaccharides of 207 Food polysaccharides by man, digestibility of 336 Food refining, losses of essential trace elements during 174 Fructans 342 Fructose 3, 75, 153, 236 in alleviating the human stress response 54 infusion 75 intolerance 70, 153 metabolism 49, 52, 55 use of diabetics 55 utilization of sorbitol and 130 Fruits 325 Function of the stomach 11
Galactomannans 29 Galactose 5,236 biosynthesis in the mammary gland 104 metabolism 100,104 pyrophosphorylase pathway 109 -1-phosphate from glucose, uncon trolled biosynthesis of 116 -1-phosphate from UDP-galactose, biosynthesis of 113 -1-phosphate toxicity 112 Galactosemia 110, 131 α-Galactosidase activity and flatus 207 Galactosidases, lysosomal 102 Gangliosidosis, generalized 102 Gargoylism 147 Gastric secretion 288 Gels, isoelectric focusing 224 Genetics of transferase deficiency 111 Gliadins, wheat 256 £-(1^3)-Glucans 343 Glucoamylase 338 Glucocorticoids 240 Glucogen 240 Glucomannans 296 Glucose 5, 54, 74, 140, 153, 195, 235,238, 240 infusion 74 metabolism of 86 in parenteral nutrition, xylitol as a substitute for 128 -1-phosphate 236 tolerance factor 166,168 uncontrolled biosynthesis of galac tose-1-phosphate from 116 α-Glucosidases of mammalian tissues, lysosomal 223 Glucuronate-xylulose cycle 126,140 D-Glucuronic acid 144,147 as a metabolic intermediate 138 utilization and production of 146
Glycogen formation, effect of mannose on hydrolysis byfibroblastsonicates . in the liver metabolism, enzymes of storage disease (Pompe's disease) synthetase 236, Glycoproteins GTF supplementation of diet
235 307 229 55 235 223 242 342 171
H
Heart disease and diet Hepatic ATP level Hexitols Hexoses High-protein, low-carbohydrate diet Hormones of the adrenal cortex, effects of steroid Hormones enzymati deactivatio f Human brush border oligo saccharidases Humans, flatulence in Hurler's disease Hydrogen Hydrolysis by fibroblast sonicates, glycogen Hydrolytic enzyme inhibitors Hydrolyzed milk, lactoseHyperactive, human Hypocholesterolemic activity 288, Hypoglycemia Hypomanic activity Hypothermia Human liver enzyme
23 50 130 3 60 56
183 211 147 214 229 244 201 58 298 54 58 75 223
I L-Iduronic acid 145, 147 Impaired glucose tolerance 168 Indications for intravenous feeding 74 Infusion 75 fructose 75 glucose 74 maltose 73,77 sorbitol 76 xylitol 76 Inhibition, tumor 302 Inhibitor from phaseolus vulgaris, α-amylase 257 Inhibitors 250 of α-amylase 246, 247, 250 physiological effects of 259 from plants 244 from wheat 253 as dietetic agents 263 invertase 246 Injected maltose, metabolism of 77 Inorganic pyrophosphate 115 Inositol 142
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
352
PHYSIOLOGICAL
Insulin 52, 54, 240, 342 carbohydrates which induce 56 levels 170 secretion, effect of mannose on 307 Interferon stimulation 302 Intermediates, pentoses and D-glucuronic acid as metabolic 138 Intestinal absorption of sugars 1 brush border, oligosaccharidases of the small 181 mucosa, metabolism of oligosaccha rides in the 208 oligosaccharidase 182, 186 Intestine, role of microflora in the small 336 Intolerance, lactose 191 Intraluminal digestion of polysaccharide 181 Intracellular polysaccharide 15 Intravenous disaccharides as a of calories 77 Intravenous feeding 74, 80, 83 caloric sources for 74, 80, 83 indications for 74 maltose as a carbohydrate substrate in 74 Invertase inhibitors 246 Iodine 157,160 Iron 157,160 Isoelectric focusing gels 224 Isomaltitol 131
J Jerusalem artichoke
343 Κ
Kale, red Konjac mannan Krabbe's disease
342 299 102
L Lactase activity, low deficiencies Lactic acid in plaque Lactose biosynthesis catabolism hydrolyzed milk intolerance synthetase Legumes oligosaccharides of food Leloir pathway Lipid metabolism in adipose tissue Lipidosis, ceramide-lactoside
337 203 101,186 152 100 102 100 191, 201 101,191 103 325 207 107 49, 313 51 102
EFFECTS
O F FOOD
CARBOHYDRATES
Lipids 318 in rats, blood, serum, and liver 26, 28 Lipogenesis, adipose tissue 49 Liver enzyme, human 223 Liver, glycogen in the 55 Losses of essential trace elements during food refining 173, 174 Lysosomal α-glucosidases of mammalian tissues 223 Lysosomal galactosidases 102 Lysosomes 223
M Macrocystis pyrifera Maillard reaction Maltase Maltitol Maltose carbohydrat substrat
269 75, 128 337 131 224 i
infused 73,77 oxidation of 83 Maltosyloligosaccharides 80 Mammalian tissues, lysosomal α-glucosidases of 223 Man and animals, trace elements with beneficial effects in 161 Manganese 157, 163 Mannan, konjac 299 Mannans 296 digestion of 298 hypocholesterolemic activity of 298 D-Mannitol 123 Mannose, absorption and metabolism 303 Membrane, brush-border 3, 338 Membrane transport, types of 15 Metabolic effects of dietary carbohydrates 20 Metabolic intermediates, pentoses and D-glucuronic acid as 138 Metabolism in adipose tissue, lipid 51 enzymes of glycogen 235 fructose 49, 52 galactose 100,104 of infused maltose and other sugars 73 of injected maltose 77 of lactose and galactose 100 lipid 49,313 of maltose 86 mannose absorption and 303 and nutrition, xylitol in 125 of oligosaccharides in the intestinal mucosa 208 and physiological effects of pectins 312 and physiological effects of the pentoses and uronic acids 135 and physiological effects of the polyols (alditols) 123
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
353
INDEX
Metabolism (continued) pyrophosphorylase pathway of galactose 109 trace elements in human nutrition and 156 Methane 214 Microflora in the small intestine 336 Micro-nutrient elements 158 Microorganisms 100 Milk, lactose-hydrolyzed 191,201 Milk rejection 198 Molybdenum 157,163 Mucopolysaccharidosis, atypical 147 Mucosa, metabolism of oligosaccha rides in the intestinal 208 Ν Na -dependent carrier Navy beans Nickel 157 Nitrogen Noradrenalin Normetadrenalin Nutrients, effect of pectin on the absorption of Nutrition and metabolism, trace elements in human parenteral role of chromium in xylitol in 125,
12 24 21 56 60 321 156 73 166 128
Ο Oats 251 Oligosaccharides of food legumes 207 Oligosaccharides in the intestinal mucosa, metabolism of 208 Oligosaccharidase, deficiency of intestinal 186 Oligosaccharidases, human brush border 181, 183 Origin of depolymerases 336 Osmoreceptors, sugar-specific 12 Oxidation of intravenously administered maltose 83
Ρ Pancreatectomy Pancreatic amylase Pancreatic juice, α-amylases of Pancreatitis Parenteral nutrition xylitol as a substitute for glucose in Pathological aspects of the pentoses Pectin on the absorption of nutrients, effect of anti-hypercholesterolemic action of
75 7 338 75 73 128 143 299 321 316
Pectin (continued) metabolism and physiological effects of 312 Pentitols, utilization of free 129 Pentoses, pathological aspects of the 135,136,143 Pentosuria 126,143 Pepsin, proteolytic activity of 287 Peptic ulcers 282 Phaseolus vulgaris 247 α-amylase inhibitor from 257 Phosphatase system, regulation of the 240 Phospholipids 318 Phosphorylase 236 kinase 236 reaction, regulation of the 238 pH on the tooth surface 152 Physiological effects of α-amylase inhibitors 259 of dietary fiber 325 of pectins, metabolism and 312 of the pentoses and uronic acids, metabolism and 135 of the polyols (alditols), metabolism and 123 Physiologically important uronic acids 144 Physiology of the intestinal absorption of sugars 1 Plant polysaccharides 341 Plants, α-amylase inhibitors from 244 Plaque formation 152 effects of polyols on 153 Plaque, lactic acid in 152 Plasma cholesterol 299 Polyfructan 151 Polyglucan 151 Polyols (alditols) 123 in animal metabolism 123 enzymatic reactions of 124 on plaque formation, effects of 153 Polysaccharide extracellular 151 intracellular 151 intraluminal digestion of 181 Polysaccharides amylaceous 338 digestibility of food 336 plant 341 Potatoes 325 Potato tubers 249 Propylene glycol alginate 272 Protein, low-carbohydrate diet, high 60 Proteolysis 259 Proteolytic activity of pepsin 287 Pyrophosphate, inorganic 115 Pyrophosphorylase pathway of galactose metabolism 109
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
354
PHYSIOLOGICAL
Q Quar gum
299
R Radioactive strontium absorption 275 Raffinose 207 Rats, lipids in 26, 28 Reactions of UDP-D-glucuronic acid 147 Red kale 342 Refined white sugar 174 Refining, losses of chromium in food 173 Refining, losses of essential trace elements during food 174 Refining of wheat 174 Regulation of the phosphatase system 240 Regulation of the phosphorylase reaction 238 Ribitol 123 D-Ribose 13 D-Ribulose 13 Role of carbohydrates in dental caries 150 Role of chromium in nutrition 166 Role of dietary carbohydrate in adi pose tissue fatty acid synthesis 46 Role of microflora in the small intestine 336 Rye 251
S Saccharidases 7 Saliva and pancreatic juice 338 Seaweed 342 Secretion, effect of mannose on insulin 307 Secretion, gastric 288 Selenium 157,164 Silicon 157,165 Small intestine, function of 11 Small intestine, role of microflora in the 336 Sodium alginate 273 Sonicates, glycogen hydrolysis by fibroblast 229 Sorbitol 123, 124, 130 dehydrogenase 125 infusion 76 Sorghum 249 Source, properties, and uses of carrageenan 283 Soybean products 213 Soy protein products 212 Stachyose 208 Starch 249 digestion 259 high-amylose 339 Steroid hormones of the adrenal cortex 56 Stimulation, interferon 302 Stomach, function of 11 Streptococcus mutatis 151
EFFECTS
O F FOOD
CARBOHYDRATES
Stress response, benefits of dietary fructose in alleviating the human Strontium absorption, radioactive Sucrase Sugar, blood 59, Sugar, refined white Sugar-specific osmoreceptors Sugars, metabolism of infused maltose and other Sugars, physiology of the intestinal absorption of Sympathetic hormones Synthesis, adipose tissue fatty acid
54 275 337 202 174 12 73 1 56 46
Τ Tin Tooth surface, pH on the Toxicity f
157,165 152 301 282
Trace element analysis Trace elements with beneficial effects in man and animals Trace elements in human nutrition and metabolism Transferase deficiency, genetics of Transport, types of membrane Trehalase Trehalose, intravenously administered Triglycerides 25, 47, 59, Trivalent chromium Tryptophan Tumor inhibition
158 161 156 111 15 337 80 318 166 254 302
U Uncontrolled biosynthesis of galac tose-1-phosphate from glucose Uricemia Uridine diphosphate galactose C-4-epimerase Uridine diphosphate glucose and uridine, biosynthesis of Uremia Urine, catecholamine products in the Uronic acids, physiologically important 135, Utilization of free pentitols Utilization of pentoses Utilization and production of glucuronic acid Utilization of sorbitol and fructose ....
116 128 110 109 75 58 144 129 136 146 130
V Vanadium Vegetables Verbascose
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
157,165 325 208
355
INDEX
W Water content of the bowel Water fluoridation Wheat, α-amylase inhibitors from Wheat gliadins Wheat, refining of
289 153 253 256 174
Xylitol (continued) as a substitute for glucose in parenteral nutrition utilization Xylose Xylulose cycle, glucuronate
X Xanthan gum toxicity studies with Xylitol infusion in metabolism and nutrition
128 128 135,143 135 126,140
Y
269, 275 277 123 76 125
Yeast extract, brewers
168
Ζ Zinc
In Physiological Effects of Food Carbohydrates; Jeanes, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
157,160