Abstract
The studies in the author's laboratory on the synthesis and degradation of L-ascorbic acid in animal tissues are reviewed, together with the related studies done in other laboratories. The biosynthesis of L-ascorbic acid proceeds from D-glucuronic acid through L-gulonic acid (or D-glucuronolactone) and L-gulonolactone. The enzymes participating in these reactions are : TPN L-hexonate dehydrogenase, DPN L-gulonate dehydrogenase and lactonase in the soluble fraction of the tissues and L-gulonolactone dehydrogenase in microsomes. In case L-ascorbic acid is formed from D-glucuronolactone in the presence of cyanide, L-gulonolactone is also formed as intermediate, L-Gulonolactone dehydrogenase is lacking in men, monkeys and guinea pigs. The lactonase seems to be identical with D-gluconolactonase. It acts for the formation of lactone ring. It is inhibited by lycorine. Microsomes contain another lactonase which acts on D-glucuronolactone, and its activity increases in the animals treated with Chloretone, barbiturate and antipyrine. 2,3-Diketo-L-gulonic acid is formed from dehydro-L-ascorbic acid by the action of lactonase, and it is decomposed to CO_2,L-lyxonic acid and L-xylonic acid by 2,3-diketoaldonate decarboxylase. Oxalic acid and L-threonic acid are also formed from 2,3-diketo-L-gulonic acid in the tissues, and the reaction is accelerated by the presence of H_2O_2 and inhibited by catalase.
