Kinetics of the Copper- and Iron-Catalysed Oxidation of Cysteine by Dioxygen.

Abstract

For the purpose of controlling levels of thiols and perioxide in biological experiments, the kinetics of ''autoxidation'' of cysteine (RSH) catalysed by copper or iron ions, added to the reaction medium as Cu(II) and Fe(II) or Fe(III) salts, has been studied. The reaction may be described as proceeding in two steps, only the second of which is spontaneous: 2 RSH + 02 .fwdarw. RSSR + H2O2 (i) 2 RSH + H2O2 .fwdarw. RSSR + 2 H2O (ii) The Cu(II)-catalysed reaction (i) follows Michaelis-Menten kinetics with respect to RSH, and probably O2, and is partly at least, second order with respect to Cu. The rate of (ii) is first-order with respect to RSH and H2O2 and is enhanced by Fe(II) and Fe(III) but practically not at all by Cu(II). With Cu(II) as the catalyst of the overall reaction, H2O2 is one of the final products. With Fe(II) or Fe(III) as the catalyst, the overall reaction, which proceeds without formation of H2O2, is first-order with respect to RS- and Fe(II)/Fe(III) (at least at high [Fe]). When catalysed by an FeO(OH) sol the kinetics change to zeroth-order with respect to RSH. The Cu- and Fe-catalysed rates are not additive; at certain concentrations Fe slows down the overall Cu-catalysed reaction and changes the kinetics towards zeroth order, reducing the levels of H2O2. The mechanisms of the catalysis by Cu of (i) and by Fe of (i) and (ii) have not yet been fully explained. The present study indicates a possible role for binuclear complexes (Cu + Cu or Cu + Fe) and shows further the usefulness of fitting data to numerical solutions of differential equations for models describing reaction rates, in work aimed at the elucidation of mechanisms.