Ascorbic acid oxidation by hydrogen peroxide

The oxidative degradation of ascorbic acid by hydrogen peroxide was examined to determine routes of degradation and identify the initial products which form when ascorbic acid is oxidized. When reacted with hydrogen peroxide, solutions of ascorbic acid and dehydroascorbic acid are both ultimately oxidized to the same species, having a mass spectrum consistent with threonic acid. When the intermediate steps in the oxidation of ascorbic acid are examined in detail, ascorbic acid, dehydroascorbic acid, and solutions containing hydrolyzed dehydroascorbic acid are all oxidized through a six-carbon compound previously proposed to be tetrahydroxydiketohexanoic acid. Both dehydroascorbic acid and hydrolyzed dehydroascorbic acid (diketogulonic acid) are more susceptible to hydrogen peroxide oxidation than ascorbic acid. Based on mass spectral analysis, diketogulonic acid serves as an oxygen sink, implying that it may be a better reducing agent for toxic oxygen species than ascorbic acid. These data indicate that oxidation of ascorbic acid by hydrogen peroxide primarily proceeds through three major six-carbon intermediates, each with distinctive redox properties. The stable metabolite diketogulonic may be a critical antioxidant in ascorbic-acid-containing systems. (C) 1998 Academic Press.