Effect of amino acids,peptides and related compounds on the autooxidation of ascorbic acid

The autooxidation rate of ascorbic acid (AA) at 10^-5 m under aerobic conditions at pH 7.4 was found to be 1.16 μmol/min/1. A number of compounds at low concentrations were found to inhibit this oxidation rate. These were in order of effectiveness: EDTA (10^-6 m ) > Mercaptoethanol > aminoethyl cysteine, oxidized glutathione > glycylglycylhistidine > glycylhistidyllysine > 3-methyl histidine ? histidine > histamine > hypertensin > cysteic acid > imidazole > glutamine > hydroxyproline, and lysine. All other amino acids and peptides examined had little or no effect on the autooxidation rate of ascorbic acid. Ascorbate solutions, treated with Chelex-100 (divalent chelating resin) or containing low concentrations of EDTA (10^-7 m ) did not show a significant reduction of the rate of autooxidation. Of particular interest was the finding that 3-methyl histidine had a significant inhibitory effect on ascorbic acid oxidation whereas 1-methyl histidine had no effect. These data suggest that ascorbic acid forms complexes with certain compounds and that this interaction stabilizes ascorbic acid against auto-oxidation.