INVOLVEMENT OF THYLAKOID MEMBRANE-DEPENDENT PHOTOSENSITIZATION IN PHOTOINHIBITION OF THE CALVIN CYCLE ACTIVITY IN SPINACH CHLOROPLASTS |
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Authors: | Jin Jung Yong Wook Kim |
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Institution: | Department of Agricultural Chemistry, Seoul National University, Suwon441–744, Korea |
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Abstract: | Photoinhibition of the light-regulated key enzymes of the photosynthetic carbon reduction (PCR) cycle was investigated using chloroplasts isolated from spinach leaves. Light quality dependence of the light-induced activity change (activation or inactivation) of key PCR enzymes in situ demonstrated that, while light activation is promoted mainly by red light (Λ.> 600 nm), inactivation takes place largely in the region of blue light (Λ < 500 nm). Inactivation was suppressed by a lipid soluble singlet oxygen (1O2,1Δg) quencher. When “stromal protein” was subjected to a severe photoinhibitory treatment, no significant loss of activity was observed for any PCR enzyme assayed. However, the inclusion of thylakoids in the photolysis system resulted in a substantial inactivation of the enzymes; this inactivation was significantly diminished in the presence of imidazole and enhanced to some extent by a partial deuteration of medium. In contrast, superoxide dismutase did not exert any effect. The blue light-induced inactivation of the enzymes was remarkably decreased in the presence of thylakoids whose Fe-S centers were destroyed. The results obtained in this study suggest that photoinactivation of the PCR enzymes in situ is mediated mainly by 1O2, which is photoproduced primarily by the Fe-S centers of thylakoids and diffuses into the stroma. |
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