Ascorbic acid and dehydroascorbic acid interconversion without net oxidation or reduction.

The interconversion of ascorbic acid and dehydroascorbic acid was examined in aqueous solution using unlabeled dehydroascorbic acid and nonlabile, stable isotope-labeled ascorbic acid by gas chromatographic/mass spectrometric analysis. Although the formation of unlabeled ascorbic acid from unlabeled dehydroascorbic acid or labeled dehydroascorbic acid from labeled ascorbic acid did not occur to any significant extent when either solutions of unlabeled dehydroascorbic acid or labeled ascorbic acid were incubated alone, significant amounts of both labeled dehydroascorbic acid and unlabeled ascorbic acid formed when unlabeled dehydroascorbic acid was incubated with labeled ascorbic acid at acid pH. At alkaline pH, interconversion did not occur to any appreciable extent. Likewise, interconversion did not appear to occur in plasma at physiologic concentrations of ascorbic acid, but did occur with pharmacologic concentrations. These data show that ascorbic acid and dehydroascorbic acid interconvert in acidic solution, suggesting the reducing hydrogen atoms are delocalized when ascorbic acid is paired with dehydroascorbic acid under these circumstances. Alkaline pH and plasma inhibit the interconversion.