Abstract
Kinetic data for the oxidation of glutathione (reduced, GSH), cysteine, glycine and glutamic acid by colloidal manganese dioxide, (MnO^sub 2^)^sub n^ are reported. Colloidal MnO^sub 2^, oxidized glutathione to disulphide (glutathione, oxidized), was reduced to manganese (II). Glycine and glutamic acid (structural units of glutathione) are not oxidized by colloidal MnO^sub 2^, but the other structural unit, cysteine, is also oxidized by the same oxidant under similar experimental conditions. This is interpreted in terms of the rate-determining colloidal MnO^sub 2^-S bonded intermediate. The reactivity of GSH towards colloidal MnO^sub 2^ is very much higher than cysteine. Kinetics of oxidation of GSH and cysteine by colloidal MnO^sub 2^ were performed spectrophotometrically as a function of [GSH], [cysteine], colloidal [(MnO^sub 2^)^sub n^], [HClO^sub 4^], temperature and trapping agents sodium fluoride and manganese (II) (reduction product of colloidal MnO^sub 2^). The purpose of this work was to study the role of -NH^sub 2^, -COOH, -SH groups present in the carbon chain of the above amino acids. It was found that the reactivity of -SH group is higher than -NH^sub 2^ and -COOH groups. The mechanisms, involving a colloidal MnO^sub 2^ complex with GSH and cysteine, are proposed. The complexes decompose in a rate-determining step, leading to the formation of free radical and manganese (III), which is also an intermediate. The dimerization of radicals takes place in a subsequent fast step to yield the products.[PUBLICATION ABSTRACT]