Concentration-dependent inactivation of superoxide dismutase.

The inactivation yield of superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) from bovine erythrocytes, when Co60-gamma-irradiated in air, N2 or N2O-saturated solutions, increases exponentially with the initial enzyme concentration. In aerated solutions at less than or equal to 10 micrometers, the inactivation process continues in a concentration-dependent manner in the subsequent 72 h. This post-irradiation effect is inhibited by catalase. Above 10 micrometers, radiation-induced inactivation of the enzyme is partially reversed in a concentration-dependent manner and is not affected by catalase. In aerated and N2O-saturated solutions, competitive scavenging of radiation chemical species by catalase and EDTA in combination reduces the inactivation yield by 80%; the residual yield remains dependent on enzyme concentration. Radiation-induced loss of copper and zinc initially exhibits a linear dose-response relationship and is less severe than the drop in enzyme activity.