Antioxidant chemistry: Reactivity and oxidation of DL-cysteine by some common oxidants

The reactivity of DL-cysteine, a physiologically important aminothiol, was studied by reacting it with several well known oxidants. No activity was observed on the amino and carboxyl groups. The only reactivity of physiological significance was at the sulfur centre. Reactions of cysteine with hydrogen peroxide show that the thiol group is capable of mopping up free radicals by forming thyl radicals, as expected in its role as an antioxidant. A four-electron oxidation of cysteine gave reasonably stable cysteine sulfinic acid. Oxidants in the form of peracids do oxidize cysteine only as far as the sulfinic acid. Stronger oxidizing agents can oxidize cysteine as far as the cysteine sulfonic acid. No further oxidation can be detected as the C-S bond is not cleaved. The inertness of the amino group in cysteine makes it incapable of reversibly mopping up the dangerous oxyhalogens HOCl and HOBr which are produced by myeloperoxidase-catalysed oxidation of halides by hydrogen peroxide, as is the case with taurine. A detailed mechanism, together with a computer simulation study of the oxidation of cysteine by acidified bromate, is proposed.