C-6 oxidation of ascorbic acid: A major metabolic process in animals

Metabolic activity of the C-6 carbon of ascorbic acid has been examined using two experimental methods. 43% of the label from ascorbic-6- 3H acid is converted to water in the monkey after i.v. injection. The rate of formation of 3H 2O corresponds to the rate of excretion of organic ascorbate metabolites in the urine as measured using ascorbic-6- 3H or ascorbic-6- 14C acid. Periodate degradations of whole urine from rats or monkeys given ascorbic-6- 14C acid by i.v. injection show that 45% of the labeled metabolites do not give formaldehyde from the C-6 carbon, indicating the C-6 carbon is no longer a primary alcohol or that the C-5 and/or C-6 hydroxyl groups are derivitized. There is no significant excretion of 14CO 2 in these experiments indicating the metabolites do not enter general carbohydrate catabolism. The combination of these results show that about 45% of the C-6 carbon of ascorbic acid is oxidized in vivo. Because the monkeys had an adequate dietary intake of ascorbic acid and the rats synthesize ascorbic acid as needed, the results also indicate that the precursor of the oxidation is a slowly exchanging form of ascorbic acid in which the C-6 carbon is in the primary alcohol oxidation state.