Inactivation and Degradation of CuZn-SOD by Active Oxygen Species in Wheat Chloroplasts Exposed to Photooxidative Stress

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Changes in CuZn-SOD actvity 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 H_2O_2 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 H_2O_2-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 H_2O_2 and O_2^- or ^<・>OH-generating systems. O_2^- had no effect on either SOD activity or stability (estimated by native PAGE). H_2O_2 up to 700 μM 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.
日本植物生理学会の論文