Involvement of Thylakoid Membrane‐dependent Photosensitization in Photoinhibition of the Calvin Cycle Activity in Spinach Chloroplasts

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.