Mechanism of the reaction of bovine copper–zinc superoxide dismutase. An electron spin resonance study

Abstract

Exposure of the Cu^II form of bovine copper–zinc superoxide dismutase (SOD) in dilute aqueous glasses at 77 K to ⁶⁰Co γ-rays results in efficient reduction to the Cu^I form. In the presence of oxygen, superoxide ions are formed competitively. On annealing under these conditions, when the superoxide signal was lost there was little change in the residual concentration of Cu^II. However, when most of the Cu^II was reduced there was a clear gain in the Cu^II signal on loss of superoxide.

These results establish that reaction between O^–₂ and Cu^I gives Cu^II with high efficiency. Since there was no such gain in the experiments with Cu^II and Cu^I mixtures, we conclude that Cu^II also reacts with O^–₂ to give Cu^I or a Cu^II–O^–₂ complex with no detectable e.s.r. absorption.

These results are discussed in the light of proposed mechanisms for the overall disproportionation of superoxide ions by SOD.