Photoinhibition of Photosystem II from Higher Plants

Abstract

Strong illumination of Cu(II)-inhibited photosystem II membranes resulted in a faster loss of oxygen evolution activity compared with that of the intact samples. The phenomenon was oxygen- and temperature-dependent. However, D1 protein degradation rate was similar in both preparations and slower than that found in non-oxygen evolving PSII particles (i.e. Mn-depleted photosystem II). These results seem to indicate that during illumination Cu(II)-inhibited samples do not behave as a typical non-oxygen evolving photosystem II. Cytochrome b559 was functional in the presence of Cu(II). The effect of Cu(II) inhibition decreased the amount of photoreduced cytochrome b559 and slowed down the rate of its photoreduction. The presence of Cu(II) during illumination seems to protect P680 against photodamage as occurs in photosystem II reaction centers when the acceptor side is protected. The data were consistent with the finding that production of singlet oxygen was highly reduced in the preparations treated with Cu(II). EPR spin trapping experiments showed that inactivation of Cu(II)-treated samples was dominated by hydroxyl radical, and the loss of oxygen evolution activity was diminished by the presence of superoxide dismutase and catalase. These results indicate that the rapid loss of oxygen evolution activity in the presence of Cu(II) is mainly due to the formation of .OH radicals from superoxide ion via a Cu(II)-catalyzed Haber-Weiss mechanism. Considering that this inactivation process was oxygen-dependent, we propose that the formation of superoxide occurs in the acceptor side of photosystem II by interaction of molecular oxygen with reduced electron acceptor species, and thus, the primarily Cu(II)-inhibitory site in photosystem II is on the acceptor side.