Ascorbic Acid Metabolism
In man ascorbic acid is partly excreted unchanged and partly as diketo-L-gulonic acid and oxalic acid. Hellman and Burns using labeled ascorbic acid, found 12 to 24 per cent of the urinary C14 activity in L-ascorbic acid, 12 to 18 per cent in diketogulbnic acid, and 24 to 63 per cent as oxalic acid in several experiments with three subjects. No C14 was detected in respiratory CO2, The vitamin exists in the body in an equilibrium between the reduced and the oxidized (dehydro) states, with only a small fraction in the latter form. The oxidized form can be either reduced to ascorbic acid reversibly, or metabolized to diketogulonioacid irreversibly.
Both the oxidized and reduced forms have vitamin activity, while the diketogulonic acid has none. Oxalate and ascorbic acid in urine accounted for about half the total ascorbic acid turnover in an adult man in the experiments of Atkins and others using C1sMagged ascorbic acid. Most of the oxalate was shown to arise from ascorbic acid or glycine. Respiratory CO2 contains carbon atoms from C-l of ascorbic acid. Oxalate arises from carbons 1 and 2 of ascorbic acid. Rats and guinea pigs metabolize the carbon chain of ascorbic acid producing principally CO2 and oxalic acid, although pathway differences in these species are established. Chan and others found that guinea pig liver preparations formed dehydroascorbic acid which was degraded to-oxalate, CO2, and L-xylose. No xylose was found from the catabolism of ascorbic acid by rat kidney in studies by Burns and co-workers. The half-life of ascorbic acid in the guinea pig is only a few days, while in man it has been estimated to be about 16 days. This may account for the long period required to produce a deficiency state in man compared to the time involved to produce symptoms in the guinea pig.
