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.
318
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 .
319
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 -
320
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.
322
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
