Reduction and inactivation of superoxide dismutase by hydrogen peroxide

Abstract

Reactions of H₂O₂ with superoxide dismutase were studied by e.p.r. (electron paramagnetic resonance) spectroscopy and other methods. In agreement with earlier work, the Cu²⁺ of the enzyme is reduced by H₂O₂, although the reaction does not go to completion and its kinetics are not simple. With dilute enzyme the time for half-reduction with 9mm-H₂O₂ is about 150ms. It is suggested that the reaction is a one-electron reduction, involving liberation of O₂⁻. On somewhat more prolonged exposure to H₂O₂, the enzyme is inactivated. For enzyme in dilute solution and over a limited range of H₂O₂ concentrations, inactivation is first-order with respect to enzyme and reagent, with k=3.1m⁻¹·s⁻¹ at 20–25°C. Inactivation is accompanied by marked changes in the e.p.r. and visible spectra and appears to be associated with destruction of one histidine residue per subunit. It is suggested that this histidine is close to the metal in the native enzyme and essential for its enzymic activity.