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 mumol/min/l. A number of compounds at low concentrations were found to inhibit this oxidation rate. These were in order of effectiveness: EDTA (10(-6) M) greater than Mercaptoethanol greater than aminoethyl cysteine, oxidized glutathione greater than glycylglycylhistidine greater than glycylhistidyllysine greater than 3-methyl histidine approximately histidine greater than histamine greater than hypertensin greater than cysteic acid greater than imidazole greater than glutamine greater than 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.