Reaction of bovine‐liver copper‐zinc superoxide dismutase with hydrogen peroxide

Abstract

The reaction of hydrogen peroxide with the copper-zinc bovine-liver superoxide dismutase at low molar ratios (0.2–20.0) of H₂O₂/active site between pH 7.3–10.0 leads to the loss of native enzyme as a distinct form monitored by electrophoresis. The pH dependence of the loss of native enzyme between 7.3 and 9.0 indicates the involvement of a conjugate base on the enzyme of pk_a of 8.7 ± 0.1. The rate of loss of the native enzyme is first order with respect to the concentration of both enzyme and hydrogen peroxide between pH 7.3 and 9.0 with no evidence for binding of peroxide. A second-order rate constant of 3.0 ± 1.0 M⁻¹ s⁻¹ is obtained from these data. At pH 10.0 the reaction is first order with respect to enzyme concentration but saturable in H₂O₂. All data are consistent with the interpretation that H₂O₂ reacts with the enzyme at the lower pH where the reaction is dependent upon the conjugate base of a functional group on the enzyme. At the higher pH, the data are consistent with the reaction of HO₂⁻ and H₂O₂ with the dismutase. The dissociation constant for HO₂⁻ calculated from the kinetic data at pH 10.0 is between 25–50 μM and the rate constant for the breakdown of the HO₂⁻· dismutase complex is 1.10 ± 0.05 × 10⁻² s⁻¹. The change in the electrophoretic pattern at all pH values is accompanied by the loss of the ability of the enzyme to bind copper. Weakly bound or free copper can be detected using bathocuproine disulfonate. Furthermore copper-defficient forms of the enzyme can be detected by staining gels of the peroxide-treated dismutase with diethyldithiocarbamate.