Sod and catalase inactivation by singlet oxygen and peroxyl radicals.

Both superoxide dismutase and catalase are readily deactivated by singlet oxygen and by the radicals produced in the pyrolysis of 2,2'-azo-bis-(2-amidinpropano) under aerobic conditions. The rate constant for the loss of enzymatic activity induced by singlet oxygen are 3.9 x 10(7) and 2.5 x 10(7) M-1 sec-1 for SOD and catalase, respectively. The similarity between these values implies that in systems where SOD and catalase are exposed to similar singlet oxygen concentrations, it can be expected a parallel inactivation of both enzymes. The inactivation of both enzymes by the radicals produced by 2,2'-azo-bis-(2-amidinopropane) pyrolysis under aerobic conditions follows a first-order kinetics at low enzyme concentrations and a zero-order kinetics at higher concentrations. Although at low enzymatic concentrations the rate of inactivation of both enzymes is similar, this results from a compensation of effects because there are wide differences in the reactivity of both enzymes towards peroxyalkyl radicals. Catalase is considerably more reactive, but a large number of protein/radical reactive interactions are needed to inactivate one enzyme. On the other hand, the reactivity of SOD is smaller, but the average enzyme activity decreases by nearly 20% in each SOD/radical reactive interaction.