Inactivation and degradation of CuZn-SOD by active oxygen species in wheat chloroplasts exposed to photooxidative stress.

Changes in CuZn-SOD activity and content in isolated wheat chloroplasts under the light, and the involvement of protease(s) and/or active oxygen species in this process were studied. Both SOD activity and content decayed with exposure time to photooxidative stress. Ascorbate, a H2O2 scavenger, prevented photooxidation-associated inactivation of SOD, while benzoate, a .OH scavenger, prevented SOD degradation. Wheat chloroplasts incubated in the dark did not hydrolyze exogenous or endogenous SOD, either H2O2-pretreated or not. Protease inhibitors did not prevent SOD degradation under photooxidative treatment, suggesting that plastid protease(s) did not participate in this process. Purified chloroplast CuZn-SOD was exposed to H2O2 and O2- or .OH-generating systems. O2- had no effect on either SOD activity or stability (estimated by native PAGE). H2O2 up to 700 microM inhibited SOD in a dose-dependent manner and induced charge/mass changes as seen by native PAGE. .OH also reduced SOD activity by inducing its fragmentation. High levels of active oxygen, as can be generated under strong stress conditions, could directly inactivate and degrade chloroplastic SOD.