468
a. B. COWEY AND J. R. SAROENT
Many animals are able to synthesize ascorbic acid from various
hexoses (glucose, galactose)-man, the primates and the guinea pig
being exceptions. Ikeda and Saito (1964) demonstrated that carp
possess this capacity, labelled L-ascorbic acid being formed from injected D-glucose-l-14C and D-glucuronolactone-6-1W by a pathway
similar to that occurring in rats. However, it now seems that many fish
may in fact require ascorbic acid in the diet for maximal growth.
No biochemical system in which ascorbic acid acts as a specific
coenzyme is yet known. It has non-specific activity in several areas of
metabolism, especially those involving hydroxylation reactions :
(i) Tyrosine metabolism. Tyrosine is transaminated to p-hydroxyphenyl pyruvic acid which is enzymically oxidized to homogentisic
acid. This latter reaction is inhibited by very large amounts of substrate and the inhibition can be overcome by reducing agents such as
ascorbic acid.
Collagen is
abundant in the bodies of animals but is a very inert protein. During
wound repair newly synthesized collagen is laid down in the fibrous
tissue formed around the wound. In ascorbic acid-deficient animals far
less collagen is present in this fibrous tissue than in normal animals.
Radioautographs of normal rainbow trout which had been fed (by
stomach tube) both tritium labelled ascorbic acid and 14C labelled
ascorbic acid showed that over 90% of the ingested dose was laid down
in areas of active collagen synthesis (J. E. Halver, personal communication and Johnson et al., 1971). A specXc reaction in collagen biosynthesis such as the conversion of proline to hydroxyproline may
require ascorbic acid or some stable metabolite of it.
(iii) High concentrations of ascorbic acid occur in the adrenal gland
(and in the head kidney of fish) ; this may influence the synthesis of
adrenocorticosteroid hormones.
The most striking consequence of ascorbic acid deficiency in
(ii) Collagen and mucopolysaccharide biosynthesis.
Fro. 11 (above opposite). Spinal deformities in coho salmon resdting from ascorbio acid
deficiency. Upper fish-spinal curvature typical of scoliosis. Middle fish-normal
coho fed complete test diet containing 100 mg of ascorbic acid/100 g dry diet.
Lower fish-spinal curvature typical of lordosis. Reproduced with permission from
Halver et al. (1969).
FIG. 12 (below opposite). Horizontal section through spinal column of ascorbic acid
deficient coho salmon in area of lordosis. The slipped vertebrae (hour-glass shaped
structures with internal spongy bone and peripheral compact bone) are clearly
shown. The lack of collagen i n the spinal ligaments is noteworthy. Reproduced
with permission from Halver et al, (1969).
a. B. COWEY AND J. R. SAROENT
Many animals are able to synthesize ascorbic acid from various
hexoses (glucose, galactose)-man, the primates and the guinea pig
being exceptions. Ikeda and Saito (1964) demonstrated that carp
possess this capacity, labelled L-ascorbic acid being formed from injected D-glucose-l-14C and D-glucuronolactone-6-1W by a pathway
similar to that occurring in rats. However, it now seems that many fish
may in fact require ascorbic acid in the diet for maximal growth.
No biochemical system in which ascorbic acid acts as a specific
coenzyme is yet known. It has non-specific activity in several areas of
metabolism, especially those involving hydroxylation reactions :
(i) Tyrosine metabolism. Tyrosine is transaminated to p-hydroxyphenyl pyruvic acid which is enzymically oxidized to homogentisic
acid. This latter reaction is inhibited by very large amounts of substrate and the inhibition can be overcome by reducing agents such as
ascorbic acid.
Collagen is
abundant in the bodies of animals but is a very inert protein. During
wound repair newly synthesized collagen is laid down in the fibrous
tissue formed around the wound. In ascorbic acid-deficient animals far
less collagen is present in this fibrous tissue than in normal animals.
Radioautographs of normal rainbow trout which had been fed (by
stomach tube) both tritium labelled ascorbic acid and 14C labelled
ascorbic acid showed that over 90% of the ingested dose was laid down
in areas of active collagen synthesis (J. E. Halver, personal communication and Johnson et al., 1971). A specXc reaction in collagen biosynthesis such as the conversion of proline to hydroxyproline may
require ascorbic acid or some stable metabolite of it.
(iii) High concentrations of ascorbic acid occur in the adrenal gland
(and in the head kidney of fish) ; this may influence the synthesis of
adrenocorticosteroid hormones.
The most striking consequence of ascorbic acid deficiency in
(ii) Collagen and mucopolysaccharide biosynthesis.
Fro. 11 (above opposite). Spinal deformities in coho salmon resdting from ascorbio acid
deficiency. Upper fish-spinal curvature typical of scoliosis. Middle fish-normal
coho fed complete test diet containing 100 mg of ascorbic acid/100 g dry diet.
Lower fish-spinal curvature typical of lordosis. Reproduced with permission from
Halver et al. (1969).
FIG. 12 (below opposite). Horizontal section through spinal column of ascorbic acid
deficient coho salmon in area of lordosis. The slipped vertebrae (hour-glass shaped
structures with internal spongy bone and peripheral compact bone) are clearly
shown. The lack of collagen i n the spinal ligaments is noteworthy. Reproduced
with permission from Halver et al, (1969).