Abstract
Free methionine is oxidised in solutions containing ascorbic acid. For this oxidation to take place, molecular oxygen is required and trace amounts of metal ions catalyse the reaction. Our results indicate that ascorbic acid must autoxidise before reacting with methionine. Catalase, histidine and scavengers of hydroxyl radical (benzoic acid and ethanol) inhibit the reaction. The superoxide anion is not involved in the reaction as superoxide dismutase did not affect the oxidation rate. Dehydroascorbic acid also oxidises methionine, but at a much slower rate than ascorbic acid. In the presence of ascorbic acid, the main product in the reaction is methionine sulphoxide, but other products are also formed. Peptide-bound methionine is oxidised to a much lesser extent than free methionine.
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