Dependence of catalase photoinactivation in rye leaves on light intensity and quality and characterization of a chloroplast-mediated inactivation in red light

In green or etiolated rye leaves catalase was most efficiently inactivated by blue light absorbed by its prosthetic heme. Red light was ineffective at low intensity but induced marked inactivation in green leaves at higher photon flux, while far-red light was ineffective. At identical intensities of photosynthetically active radiation, Photosystem II (PS II) was equally inactivated by both blue and red light. Since catalase was insensitive to red light and no sensitizer for red light was detected in isolated peroxisomes, the inactivation of catalase observed in leaves in red light must result from photooxidative reactions initiated in the chloroplasts. In a simplified model system the inactivation of isolated catalase was induced by the presence of a suspension of either intact or broken chloroplasts in red light. This chloroplast-mediated inactivation of catalase in vitro was O₂-dependent. It was greatly retarded at low temperature, fully suppressed by the radic al scavenger Trolox, partially retarded by superoxide dismutase, but only little diminished by the singlet oxygen quencher histidine and not affected by dimethylsulfoxide, a hydroxyl radical scavenger. Chloroplast-mediated catalase inactivation in vitro was suppressed by suitable electron acceptors, in particular by methyl viologen. A comparison of the effects of inhibitors, donors, or acceptors for specific sites of the photosynthetic electron transport indicated that an overreduction of PS II and plastoquinone represented the major sources for the formation of O₂ and some unidentified radical that appeared to mediate the inactivation of catalase outside of the chloroplasts. Chloroplast-mediated catalase inactivation provides a means for the detection of a redox signalling system of chloroplasts that was postulated to indicate overreduction of plastoquinones. Similarly as in the in vitro system, catalase inactivation in red light was also in leaves temperature-dependent and stimulated by DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone). These results provide strong evidence that inactivation of catalase initiated by chloroplastic reactions in red light occurred also in leaves under identical conditions as in the model system in vitro.