Ascorbic Acid's Biosynthesis
The rat supplies his own ascorbic acid requirement through synthesis, as do most species f animals. Plants also synthesize vitamin C. The rat has been studied extensively to establish _t..e synthetic pathway. Glucose or other hexoses convertible to glucose serve as the starting aterial. King and associates first established through isotope work that carbon 1 of D-glucose - comes carbon 6 of L-ascorbic acid, while carbon 6 of glucose turns up in the 1 position of corbic acid. This work indicated that during the biosynthesis the glucose carbon chain llilderwent inversion of configuration and that the intact glucose chain was converted into ascorbic acid. Specific enzymes mediating each step in the scheme of synthesis have been emonstrated in rat liver. Much ofthe work was done with labeled compounds.
A review by Wagner and Folkers gives details of the various studies involved in the deveolpment of the pathway. Preliminary to the inversion of the glucose chain the following reactions apparently occur: glucose
--->glucose-6-P04 ---> glucose-I-P04 ---> UDP-glucose (uridine diphosphate glucose) ---> UDP-glucuronic acid --->D-glucuronic acid-I-PO 4 ---> Dglucuronic acid. The next step yields L-gulonic acid and then L-gulonolactone, which is converted to L-ascorbic acid through 2-keto L-gulonolactone as an intermediate. The structures involved in the biosynthesis are not presented here, since they are given. Galactose is converted to ascorbic acid by rats through the following sequence of reactions: galactose
---> galactose-l-P04 ---> UDP galactose ---> UDP glucose ---> as above for glucose. It is further established that in man, monkeys, and guinea pigs there is a deficiency in the enzyme or enzymes responsible for the conversion of gulonolactone to ascorbic acid. Thus, Burns found no detectable conversion ofL-gulonolac-tone-l-C14 into labeled L-ascorbic acid in homogenates Of human, guinea pig, or monkey liver, while rat liver showed about 8 per cent conversion under similar conditions.
Two other animals in the avian species more recently have been found to be unable to synthesize ascorbic acid and must have a dietary supply of the vitamin. These are the Indian fruit bat and the red vented bulbul.
