Ascorbic Acid: Chemistry, Metabolism, and Uses - American Chemical

aspects of various nutrient requirements of the monkey (1,2). .... tory effect on insulin secretion from mouse pancreatic islets (34). ... adrenal gla...
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15 Metabolism of L-Ascorbic A c i d in the Monkey STANLEY T. O M A Y E — U . S . Department of A g r i c u l t u r e - A R S , Western Regional Research Center, Berkeley, CA 94710 JERRY A. T I L L O T S O N — L e t t e r m a n Presidio of San Francisco, CA 94129

A r m y Institute of Research,

HOWERDE E. SAUBERLICH—U.S. Department of A g r i c u l t u r e - A R S , Western H u m a n Nutrition Research Center, Presidio of San Francisco, CA 94129

The functions and fate of L-ascorbic acid in humans and other primates are reviewed in this chapter. Topics included are use of subhuman primates for research in nutrition; evolution and subsequent loss of ascorbic acid biosynthesis; absorption, tissue transport, and distribution of ascorbic acid; and catabolism, functions, and requirements of ascor­ bic acid. In retrospect, the insight provided by this chapter suggests new work areas of emphasis for developing better understanding of the vitamin's role in human health.

Use of Subbuman Primates for Research in Nutrition The concept that studies of subhuman primates might provide an insight into the mechanisms of human health and disease led to the initiation of a wide range of investigations. Tremendous advances have occurred through studies of subhuman primates in the fields of infectious and degenerative disorders, toxicology, neurophysiology, space biology, organ transplantations, and behavioral sciences. However, studies of subhuman primates can often be influenced by the same genetic, health, and age-related considerations that made humans unsatisfactory candi­ dates for certain types of investigations. Therefore the value of data from studies of subhuman primates varied with the ability of the re­ searcher to define the health and condition of his experimental animal. In this regard, all investigations that utilize subhuman primates, whether biological or behavioral, must consider the nutritional status of the experiThis chapter not subject to U.S. copyright. Published 1982 American Chemical Society.

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ASCORBIC

ACID

m e n t a l a n i m a l p r i o r t o e v a l u a t i o n of a n y r e s u l t i n g d a t a . N u t r i t i o n is w e l l r e c o g n i z e d as affecting t h e rates of g r o w t h a n d m a t u r a t i o n , t h e course of infectious

disorders, a n d t h e efficiency

of h e a l i n g a n d r e p a i r

mecha­

nisms. Reviews

are available that consider

the taxonomy

of s u b h u m a n

p r i m a t e s , t h e d i e t of selected species i n t h e i r n a t u r a l h a b i t a t s , a n d some aspects of v a r i o u s n u t r i e n t r e q u i r e m e n t s of t h e m o n k e y (1,2).

Since the

1940s m a n y reports h a v e a p p e a r e d i n w h i c h m o n k e y s w e r e f e d s e m i p u r i fied

diets deficient i n i n d i v i d u a l v i t a m i n s .

S u c h reports h a v e

greatly

c l a r i f i e d t h e p a t h o p h y s i o l o g y a n d sequelae of v i t a m i n - d e f i c i e n c y states i n h u m a n s . H o w e v e r , most of the w o r k associated w i t h the s e a r c h f o r essen­ t i a l g r o w t h factors w a s d o n e w i t h other a n i m a l species a n d w i t h o u t t h e use of s u b h u m a n p r i m a t e s . C e r t a i n l y , r e l a t i v e l y f e w studies h a v e b e e n d o n e o n t h e effects of different levels of v a r i o u s n u t r i e n t s o n a n y species of s u b h u m a n p r i m a t e s . T h e r e a r e reasons to suspect t h a t t h e o p t i m u m diets a n d r e q u i r e m e n t s of t h e v a r i o u s p r i m a t e s m a y differ s u b s t a n t i a l l y (3). T y p e s of n a t u r a l f o o d eaten, size r a n g e , a n d g u t m o r p h o l o g y e m p h a ­ size t h e d i v e r s i t y of p r i m a t e order. A serious l i m i t a t i o n t o t h e e s t a b l i s h ­ m e n t of n u t r i e n t r e q u i r e m e n t s i n p r i m a t e s is t h e l a c k of a d e q u a t e on growth a n d development,

data

w i t h t h e Rhesus monkey a n d the c h i m ­

p a n z e e b e i n g t h e exceptions. An

important problem i n determining nutrient requirements a n d

o p t i m u m d i e t is t h e d e f i n i t i o n of t h e c r i t e r i a of h e a l t h t h a t m u s t b e m e t . O f t e n r e l a t i v e l y s h o r t - t e r m assays of r e q u i r e m e n t s b a s e d o n w e i g h t g a i n , m o r b i d i t y , a n d m o r t a l i t y , or t h e i n c i d e n c e s c h e m i c a l lesions a r e u s e d

of m o r p h o l o g i c a l

as i n d i c e s t o e s t a b l i s h o p t i m u m

or bio­ nutrition.

H o w e v e r , i f t h e s u b h u m a n p r i m a t e s a r e t o b e u s e d effectively as a m o d e l for h u m a n n u t r i t i o n , t h e n w e s h o u l d c o n s i d e r studies i n t o o p t i m u m n u t r i ­ t i o n f o r a l o n g a n d v i g o r o u s life. T h i s c h a p t e r considers o n e s u c h n u t r i e n t , a s c o r b i c a c i d ; h o w that n u t r i e n t is necessary to t h e s u b h u m a n p r i m a t e ; a n d h o w experiments of vitamin C nutrition i n the subhuman primate c a n be extrapolated to h u m a n s . W e i n t e n d to s u m m a r i z e t h e i n f o r m a t i o n a v a i l a b l e o n v i t a m i n C n u t r i t i o n i n t h e s u b h u m a n p r i m a t e so that n o w i n retrospect w e c a n establish where work should b e emphasized i n the future.

Evolution and Subsequent Loss of Ascorbic Acid Biosynthesis A s c o r b i c a c i d is b i o s y n t h e s i z e d f r o m c a r b o h y d r a t e precursors i n ­ c l u d i n g glucose a n d galactose b y a w i d e v a r i e t y of p l a n t a n d a n i m a l species. A f t e r s c u r v y w a s r e c o g n i z e d as a n u t r i t i o n a l deficiency disease, h u m a n s , other p r i m a t e s , a n d g u i n e a p i g s w e r e t h o u g h t t o b e t h e o n l y a n i m a l s t h a t a r e subject t o s c u r v y . I t is n o w r e c o g n i z e d t h a t t h e a b i l i t y to synthesize a s c o r b i c a c i d is absent i n insects, i n v e r t e b r a t e s , fishes, a n d c e r t a i n bats a n d b i r d s (4-6). Apparently the biosynthetic capacity

15.

O M A Y E

E T A L .

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Metabolism of L-Ascorbic Acid

s t a r t e d i n t h e k i d n e y of a m p h i b i a n s a n d r e p t i l e s , w a s t r a n s f e r r e d t o t h e l i v e r of m a m m a l s , a n d w a s t h e n lost i n g u i n e a p i g s , flying m a m m a l s , a n d primates ( 5 ) .

C h a t t e r j e e et a l . ( 5 ) suggested t h a t t h e e a r l y a m p h i b i a n s

s t a r t e d a s c o r b i c a c i d synthesis i n t h e k i d n e y b e c a u s e t h e v i t a m i n c o u l d be p r o d u c e d a t h i g h rates. T h e t r a n s i t i o n of a s c o r b i c a c i d synthesis f r o m the k i d n e y of reptiles t o t h e l i v e r of m a m m a l s corresponds t o w h e n t h e vertebrates w e r e e v o l v i n g t e m p e r a t u r e r e g u l a t o r y m e c h a n i s m s ; t h e n e w site w o u l d a c c o m m o d a t e t h e p r o b l e m s of l i f e o n d r y l a n d a n d t h e necessi­ ties of i o n r e g u l a t i o n ( 5 , 6 ) .

T w o other e x p l a n a t i o n s t h a t h a v e

been

offered f o r t h e t r a n s i t i o n of a s c o r b i c a c i d synthesis t o t h e l i v e r a r e t h a t t h e r e l a t i v e l y s m a l l k i d n e y s b e c a m e t o o c r o w d e d w i t h other d e m a n d s ( 7 ) , or that m a m m a l s n e e d e d m o r e a s c o r b i c a c i d o n a t o t a l b o d y w e i g h t basis t h a n d i d t h e reptiles (8) f o r t h e d e t o x i f i c a t i o n of h i s t a m i n e . A s i m i l a r t r a n s i t i o n i n t h e b i o s y n t h e t i c a b i l i t y of a s c o r b i c a c i d w a s s p e c u l a t e d f o r t h e e v o l u t i o n of b i r d s (5,6,9).

B i r d s a r e b e l i e v e d to h a v e

e v o l v e d f r o m a q u i t e different l i n e of reptiles (10).

Primitive birds

r e t a i n e d t h e b i o s y n t h e t i c c a p a c i t y i n t h e k i d n e y , b u t w i t h t h e progress of e v o l u t i o n , s y n t h e t i c c a p a c i t y is f o u n d i n t h e l i v e r of p a s s e r i f o r m b i r d s (4,11).

H i g h l y e v o l v e d Passeres b i r d s a r e i n c a p a b l e of p r o d u c i n g t h e

vitamin

(4).

N o r e q u i r e m e n t f o r a s c o r b i c a c i d is k n o w n f o r m i c r o b e s . F a i l u r e to synthesize a s c o r b i c a c i d is c a u s e d b y a c o m m o n

defect,

n a m e l y t h e absence of t h e e n z y m e L - g u l o n o oxidase ( E C 1.1.3.8)

(12).

T h i s m i c r o s o m a l e n z y m e is necessary f o r t h e t e r m i n a l step i n t h e c o n ­ v e r s i o n of glucose t o a s c o r b i c a c i d ( S c h e m e

1 ) . T h e absence of t h e

e n z y m e is c a u s e d b y a m u t a t i o n t h a t r e s u l t e d i n t h e loss of t h e gene responsible for synthesizing the enzyme.

F o r t u n a t e l y this m u t a t i o n w a s

n o t l e t h a l because a s c o r b i c a c i d w a s present i n f o o d of t h e affected species. Researchers

have

questioned

whether

the

one-enzyme-deficiency

t h e o r y a p p l i e s to s c u r v y - p r o n e a n i m a l s (13). S t u d i e s s h o w e d n o e v i d e n c e f o r m o r e t h a n a o n e e n z y m e d e f e c t i n s c u r v y - p r o n e a n i m a l s (14).

Based

o n i m m u n o l o g i c e v i d e n c e of p u r i f i e d L-gulono-A-lactone oxidase, s c u r v y prone animals do not contain i m m u n o l o g i c a l l y cross-reacting material to g u l o n o - A - l a c t o n e oxidase

(15,16).

A f e w p r o s i m i a n s a p p e a r a b l e t o synthesize v i t a m i n C f r o m L-1,4g u l o n o l a c t o n e b e c a u s e t h e i r n e e d f o r exogenous a s c o r b i c a c i d has n o t b e e n i d e n t i f i e d (17).

Absorption, Tissue Transport, and Distribution A b s o r p t i o n of a s c o r b i c a c i d i n t h e g u t is a passive process f o r t h e rat (18), w h i l e s c u r v y - p r o n e a n i m a l s r e q u i r e a n a c t i v e t r a n s p o r t system with

a Na -dependent, +

gradient-coupled

i n h i b i t e d b y o u a b a i n (19,20).

carrier mechanism

t h a t is

A t r a n s p o r t m o d e l is f a v o r e d t h a t f e a -

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ASCORBIC

ACID

D-Glucose,

D-Glucuronolactone^. PENTOSE PHOSPHATE PATHWAY

|

>

^

^

V D-Glucuronic A c i d

L-Gulonolactone D-Xylulose-5-P

L-Gulonic Acid

n 2-Keto-L-gulonolactone D-Xylulose

3-Keto-L-gulonic A c i d

L-Ascorbic A c i d Xylitol

Scheme 1.

*

L-Xylulose

Pathway for ascorbic acid biosynthesis in animals

tures a c a r r i e r - m e d i a t e d m e c h a n i s m f o r s i m u l t a n e o u s e n t r y of a s c o r b i c a c i d a n d N a across t h e b r u s h b o r d e r , s i m i l a r t o t h e N a - g r a d i e n t m e c h a ­ +

+

n i s m p o s t u l a t e d t o effect s u g a r a n d a m i n o a c i d t r a n s p o r t i n m a m m a l i a n m u c o s a (21).

T h i s seems r e a s o n a b l e b e c a u s e ascorbate resembles s u g a r

c o m p o u n d s i n s t r u c t u r e . S i m p l e sugars a r e r e a d i l y a b s o r b e d b y a c t i v e transport a n d diffusion i n the duodenum, jejunum, a n d i l e u m d e p e n d i n g u p o n t h e i r s t r u c t u r e , the a m o u n t s of N a a n d K present, a n d the p r e s e n c e +

of o t h e r sugars a n d a m i n o a c i d s . (22,23)

+

H o w e v e r , despite c o n t r a r y

evidence

t h e p r o p o s e d m i n i m u m r e q u i r e m e n t s f o r sugars a c t i v e l y t r a n s ­

p o r t e d across the g u t w a l l seem to e x c l u d e a s c o r b i c a c i d . A n alternate m o d e l of a s c o r b i c a c i d t r a n s p o r t b y d i f f u s i o n w o u l d b e t h a t

ascorbic

a c i d i n excess of tissue s a t u r a t i o n w o u l d n o t b e r e a d i l y a b s o r b e d .

Such a

m o d e l is i n l i n e w i t h t h e g e n e r a l h y p o t h e s i s (24,25) t h a t a s c o r b i c a c i d is r e a d i l y a b s o r b e d w h e n s m a l l q u a n t i t i e s a r e i n g e s t e d ; h o w e v e r , there i s a l i m i t e d i n t e s t i n a l a b s o r p t i o n w h e n excess a m o u n t s of t h e v i t a m i n a r e ingested.

T h e r e is l i t t l e i n f o r m a t i o n r e g a r d i n g t h e b i o a v a i l a b i l i t y o f

a s c o r b i c a c i d f r o m foods.

T h e w i d e occurrence

of t h e v i t a m i n w o u l d

15.

OMAYE

321

Metabolism of L-Ascorbic Acid

ET AL.

suggest i t w o u l d b e a n u t r i e n t w i t h l i t t l e b i o a v a i l a b i l i t y p r o b l e m ; ever, a recent s t u d y s h o w e d t h a t diets h i g h i n h e m i c e l l u l o s e

how­

enhanced

the u r i n a r y e x c r e t i o n of a s c o r b i c a c i d , a n d diets h i g h i n p e c t i n d e c r e a s e d u r i n a r y e x c r e t i o n of a s c o r b i c a c i d i n h u m a n s ( 2 6 ) . I n c r e a s e d u r i n a r y e x c r e t i o n of a s c o r b i c a c i d at constant levels of i n t a k e is i n d i c a t i v e of either enhanced absorption or decreased

need.

T h e o x i d a t i v e p r o d u c t of a s c o r b i c a c i d , d e h y d r o a s c o r b i c a c i d , is t h e p r e f e r r e d f o r m of t h e v i t a m i n f o r u p t a k e b y n e u t r o p h i l s , e r y t h r o c y t e s , and lymphocytes

( 2 7 ) . O n c e w i t h i n the erythrocyte,

dehydroascorbic

a c i d is r e d u c e d to ascorbic a c i d b y a g l u t a t h i o n e - d e p e n d e n t , ascorbic-acid-reducing enzyme (20,28).

dehydro-

H o w e v e r , t h e r e d u c e d f o r m of

a s c o r b i c a c i d is f o u n d i n most other tissues, t h a t is, l i v e r , l u n g s , k i d n e y s , s k i n , a n d p i t u i t a r y a n d a d r e n a l glands

(20,29).

F r o m these

studies,

a s c o r b i c a c i d is t a k e n u p b y s e v e r a l tissues b y a n e n e r g y - d e p e n d e n t a n d N a - s e n s i t i v e process, +

b u t t h e t r a n s p o r t of t h e o x i d i z e d v i t a m i n f o r m

f o l l o w s t h e p r i n c i p l e s of d i f f u s i o n . Adverse

reactions

sugars a n d ascorbate

might

occur

because

of a n t a g o n i s m

for transport mechanisms.

between

Hyperglycemia could

i m p a i r t h e i n t r a c e l l u l a r a v a i l a b i l i t y of v i t a m i n C ; therefore,

diabetics

c o u l d suffer v i t a m i n C deficiency w i t h a d e q u a t e v i t a m i n i n t a k e

(30-33).

A l s o , a s c o r b i c a c i d m a y i n h i b i t glucose u p t a k e b y tissues, r e s u l t i n g i n hyperglycemia a n d symptoms l a r g e doses of ascorbate.

of diabetes

following

t h e i n g e s t i o n of

R e l a t e d to this a n t a g o n i s m b e t w e e n sugars a n d

ascorbate are t h e findings t h a t d i a b e t i c s often h a v e e l e v a t e d s e r u m levels of d e h y d r o a s c o r b i c a c i d (4) a n d t h a t d e h y d r o a s c o r b i c a c i d has a n i n h i b i ­ t o r y effect o n i n s u l i n secretion f r o m m o u s e p a n c r e a t i c islets (34). T h e implication from

these

studies is t h a t t h e p r o b l e m s

associated

with

d i a b e t i c s m a y b e r e l a t e d t o a n i n a b i l i t y of t h e b o d y t o use d e h y d r o ­ a s c o r b i c a c i d o r t h a t excess d e h y d r o a s c o r b i c a c i d m a y i n h i b i t t h e release of i n s u l i n . A s c o r b i c a c i d is w i d e l y d i s t r i b u t e d t h r o u g h o u t t h e tissues of t h e b o d y , b o t h i n a n i m a l s i n w h i c h synthesis occurs a n d i n a n i m a l s of t h e s c u r v y - p r o n e g r o u p s p r o v i d e d a n a d e q u a t e a m o u n t of t h e v i t a m i n i n t h e diet.

T h e largest c o n c e n t r a t i o n of t h e v i t a m i n is i n t h e a d r e n a l s a n d

other g l a n d u l a r tissues ( 3 5 ) . H i g h levels are also f o u n d i n t h e l i v e r , spleen, a n d b r a i n .

M u s c l e content of ascorbate

other tissues i n t e r m e d i a t e (29).

is r e l a t i v e l y l o w w i t h

D a t a o n t h e a s c o r b i c a c i d content of

r o d e n t organs are a b u n d a n t , b u t v e r y l i t t l e d a t a o n t h e v i t a m i n content o f h u m a n o r s u b h u m a n p r i m a t e organs is r e p o r t e d . R e c e n t c o m p i l e d tables of a s c o r b i c a c i d c o n t e n t of h u m a n organs (29) i n d i c a t e d t h a t t h e c o n ­ c e n t r a t i o n of t h e v i t a m i n i n b r a i n a n d l i v e r is l o w c o m p a r e d w i t h g l a n d s a n d secretory organs, b u t t h e i r t o t a l c o m b i n e d o r g a n content of a s c o r b i c a c i d accounts

f o r t h e m a j o r a m o u n t of t h e t o t a l b o d y a s c o r b i c

acid.

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ASCORBIC

ACID

T h e r e f o r e , t h e b r a i n a n d t h e l i v e r a p p e a r t o act as storehouses t h a t t h e b o d y c o u l d c a l l u p o n i n deficiency states. Stress a n d v a r i o u s h o r m o n e s m a r k e d l y influence p l a s m a a n d tissue levels of a s c o r b i c a c i d . A decrease of a s c o r b i c a c i d c o n c e n t r a t i o n i n t h e a d r e n a l g l a n d , spleen, a n d b r a i n of g u i n e a p i g s w a s d e m o n s t r a t e d after s u b j e c t i n g t h e a n i m a l s t o p h y s i c a l stress c a u s e d

by swimming (36).

C i g a r e t t e smoke c a u s e d a significant r e d u c t i o n of a s c o r b i c a c i d i n g u i n e a p i g a d r e n a l glands ( 3 7 ) a n d h u m a n b l o o d ( 3 8 ) . C h a n g e s i n t h e a s c o r b i c a c i d concentrations

i n r a t tissues w e r e also o b s e r v e d

p h y s e c t o m y o r t h y r o i d t r e a t m e n t (20).

following

hypo-

T h e h i g h c o n c e n t r a t i o n of ascor­

b i c a c i d i n a d r e n a l g l a n d s w a s r e d u c e d b y f a t i g u e a n d stress-related changes.

Injections

of

the pituitary

hormone,

adrenocorticotropin

( A C T H ) , also deplete t h e a d r e n a l cortex of a s c o r b i c a c i d , w h i c h s u g ­ gests t h a t t h e v i t a m i n p l a y s a role i n t h e synthesis of a d r e n a l h o r m o n e s as a response to stress.

H o w e v e r , r e s e a r c h i n d i c a t e s t h a t ascorbate is

not necessary f o r either t h e synthesis of a d r e n a l h o r m o n e s , o r t h e m o b i l i ­ z a t i o n of g l u c o c o r t i c o i d s

o r m i n e r a l - c o r t i c o i d s (39).

t w e e n stress a n d ascorbate

Relationships be­

r e q u i r e m e n t s o r ascorbate

metabolism are

d i s c u s s e d elsewhere i n this c h a p t e r . M o r e w o r k is n e e d e d i n this area w i t h i n c r e a s e d emphasis

placed

o n t h e s u b h u m a n p r i m a t e . L i t t l e i n f o r m a t i o n is a v a i l a b l e o n t h e u p t a k e , transport, a n d d i s t r i b u t i o n of a s c o r b i c a c i d i n s u b h u m a n p r i m a t e s . Turnover—Catabolism I n h u m a n s t h e u r i n a r y tract is t h e p r i n c i p a l route f o r t h e e l i m i n a t i o n of m e t a b o l i c p r o d u c t s of a s c o r b i c a c i d .

A s c o r b i c a c i d is c o n v e r t e d t o

oxalate f r o m t h e C l a n d C 2 c a r b o n s ; some of t h e v i t a m i n is excreted unchanged.

A s i d e f r o m ascorbate-2-sulfate,

urinary ascorbic ascorbic

a c i d metabolites

(40).

l i t t l e is k n o w n a b o u t t h e

R e c e n t l y , authors

reviewing

a c i d m e t a b o l i s m g e n e r a l i z e d t h a t t h e c a t a b o l i s m of

ascorbic

a c i d t o c a r b o n d i o x i d e occurs i n t h e rat, g u i n e a p i g , a n d m o n k e y , b u t n o t i n h u m a n s (6,20,41).

S u c h g e n e r a l i z a t i o n s seem p a r a d o x i c a l b e c a u s e

the guinea p i g a n d the monkey, like humans, require ascorbic acid, b u t the r a t does n o t . T h e c u r r e n t b e l i e f is t h a t t h e g u i n e a p i g c a t a b o l i z e s ascorbic

a c i d extensively t o r e s p i r a t o r y c a r b o n d i o x i d e .

Others

have

s h o w n t h a t t h e o x i d a t i o n of ascorbate t o r e s p i r a t o r y c a r b o n d i o x i d e is d e p e n d e n t o n t h e d i e t a r y i n t a k e (42,43) a n d stress (43). is first o x i d i z e d t o d e h y d r o a s c o r b i c

Ascorbic acid

a c i d b y a v a r i e t y of

nonspecific

e n z y m i c a n d n o n e n z y m i c reactions. D e h y d r o a s c o r b i c a c i d is d e l a c t o n i z e d e n z y m a t i c a l l y to 2 3 - d i k e t o g u l o n a t e , w h i c h is s u b s e q u e n t l y

decarboxyl-

a t e d b y a specific d e c a r b o x y l a s e o r n o n e n z y m a t i c a l l y to c a r b o n d i o x i d e a n d p e n t o n i c a c i d ( S c h e m e 2 ) . T h e first r e a c t i o n is r e v e r s i b l e b u t t h e

Methylascorbic Acid

5-Ketoascorbic A c i d

Delactonized Ascorbic A c i d

Scheme 2.

Ascorbic acid metabolism in animals

324

ASCORBIC

o t h e r reactions a r e i r r e v e r s i b l e . H o w e v e r , this p a t h w a y of

ACID

ascorbate

c a t a b o l i s m i n t h e g u i n e a p i g seems to b e i n f l u e n c e d b y other factors. W h e n ( 1 - C ) - a s c o r b i c a c i d is g i v e n o r a l l y t o t h e m o n k e y , 2 0 - 9 0 % 1 4

of t h e l a b e l is e x c r e t e d as r e s p i r a t o r y l a b e l e d c a r b o n d i o x i d e (20, 44, 4 5 ) . Therefore, the monkey was thought to catabolize ascorbic a c i d to carbon dioxide i n a manner similar to the guinea p i g a n d that only humans h a d a n alternate p a t h w a y . W h e n t h e l a b e l is g i v e n to t h e m o n k e y b y i v i n j e c ­ t i o n , less t h a n 1 % is e x c r e t e d as r e s p i r a t o r y l a b e l e d c a r b o n d i o x i d e ( 2 0 , 44,45), s u g g e s t i n g t h a t w h e n t h e v i t a m i n is n o t s u b j e c t e d t o i n t e s t i n a l o x i d a t i o n , e i t h e r b y t h e g u t o r b y g u t flora, there is l i t t l e d e g r a d a t i o n t o carbon dioxide.

T h e results of these studies c o n t r a s t e d w i t h a n e a r l y

r e p o r t w h e r e l a b e l e d c a r b o n d i o x i d e c o u l d b e d e t e c t e d after t h e p a r e n ­ t e r a l ( i n t r a m u s c u l a r ) a d m i n i s t r a t i o n of l a b e l e d a s c o r b i c a c i d t o m o n k e y (46). O n e s t u d y s h o w e d t h a t o x i d a t i o n of ascorbate to c a r b o n d i o x i d e w a s to less t h a n 3 % w h e n 20 m g o r less of t h e v i t a m i n w a s f e d o r a l l y t o t h e " t r a i n e d " m o n k e y ( 4 7 ) . T r a i n e d w a s d e f i n e d as, " f a m i l i a r i z a t i o n of t h e m o n k e y to a l l c o n d i t i o n s , e x p e r i m e n t a l r o u t i n e , a n d p e r s o n n e l p r i o r t o the a c t u a l e x p e r i m e n t a l p e r i o d . "

T h e p u r p o s e of this t r a i n i n g w a s t o

m i n i m i z e a n y stress-related changes b e c a u s e of s u d d e n changes stress-related changes

that the monkey might

i n the animal's environment.

result i n decreased

undergo

I n general,

p l a s m a a n d tissue

concen­

trations of a s c o r b i c a c i d a n d a p p e a r t o increase t h e r e q u i r e m e n t f o r t h e v i t a m i n (48-52).

T h e m o n k e y does n o t synthesize a s c o r b i c a c i d ; t h e r e ­

fore, i t is l o g i c a l t o p r e s u m e t h e stress-related changes i n a s c o r b i c a c i d c o n c e n t r a t i o n s a r e c a u s e d at least i n p a r t b y m o d i f i c a t i o n i n t h e c a t a b o ­ l i s m of t h e v i t a m i n .

T h e r e d u c t i o n of a s c o r b i c d e g r a d a t i o n t o c a r b o n

d i o x i d e i n t h e t r a i n e d m o n k e y suggests t h a t ascorbic a c i d c a t a b o l i s m i s i n f l u e n c e d b y t h e a d r e n a l h y p o p h y s i s axis.

Among individual

organs,

a s c o r b i c a c i d is f o u n d i n t h e h i g h e s t c o n c e n t r a t i o n i n t h e a d r e n a l cortex. B o d i l y i n j u r y o r other stress results i n a d e p l e t i o n of a d r e n a l a s c o r b i c a c i d i n response to a n i n c r e a s e d secretion of A C T H b y t h e p i t u i t a r y g l a n d . T h e mechanism remains to be defined, b u t m a y be explained b y a spontaneous, r a p i d , h y d r o l y t i c d e c o m p o s i t i o n of d e h y d r o a s c o r b i c a c i d t o 2,3-diketogulonate Therefore,

followed

by rapid

t h e stress-related changes

catabolism to carbon

dioxide.

m a y influence e n d o g e n o u s

d e h y d r o a s c o r b i c a c i d tissue levels o r r e d u c i n g m e c h a n i s m s

free

(glutathione

p o o l s ) t h a t m a y b e i n v o l v e d i n t h e m a i n t e n a n c e of r e d u c e d a s c o r b i c a c i d levels. A s i m i l a r r e l a t i o n s h i p m a y also exist i n t h e c a t a b o l i s m of a s c o r b i c a c i d b y t h e g u i n e a p i g (43). S e v e r a l investigators h a v e

suggested

that the identification a n d

q u a n t i t a t i o n of a s c o r b i c a c i d m e t a b o l i t e s w i l l a i d i n o u r u n d e r s t a n d i n g t h e m e t a b o l i c r o l e o f a s c o r b i c a c i d (47,53,54).

Ascorbic acid probably

f u n c t i o n s i n m o r e w a y s t h a n as a h y d r o x y l a t i o n c o f a c t o r a n d as a r e d o x

15.

OMAYE

agent.

325

Metabolism of L-Ascorbic Acid

ET AL.

Information f r o m the vitamin's metabolites w i l l

e n a b l e us t o

u n d e r s t a n d w h a t those f u n c t i o n s m i g h t b e . I n h u m a n s , g u i n e a p i g s , a n d monkeys the ascorbic a c i d metabolites identified i n the urine are d e ­ hydroascorbic acid ( 4 8 , 5 5 , 5 6 ) , diketogulonic acid (55,56), sulfate (40,54),

oxalate

(55