PHOTOINHIBITION OF PHOTOSYSTEM-II FROM HIGHER-PLANTS - EFFECT OF COPPER INHIBITION

Strong illumination of Cu(II)-inhibited photosystem II membranes resul ted in a faster loss of oxygen evolution activity compared with that o f the intact samples. The phenomenon was oxygen- and temperature-depen dent, However, D1 protein degradation rate was similar in both prepara tions and slower than that found in non-oxygen evolving PSII particles (i.e., Mn-depleted photosystem II), These results seem to indicate th at during illumination Cu(II)-inhibited samples do not behave as a typ ical non-oxygen evolving photosystem II, Cytochrome b(559) was functio nal in the presence of Cu(II). The effect of Cu(II) inhibition decreas ed the amount of photoreduced cytochrome b(559) and slowed down the ra te of its photoreduction, The presence of Cu(II) during illumination s eems to protect P680 against photodamage as occurs in photosystem II r eaction centers when the acceptor side is protected, The data were con sistent with the finding that production of singlet oxygen was highly reduced in the preparations treated with Cu(II). EPR. spin trapping ex periments showed that inactivation of Cu(II)-treated samples was domin ated by hydroxyl radical, and the loss of oxygen evolution activity wa s diminished by the presence of superoxide dismutase and catalase, The se results indicate that the rapid loss of oxygen evolution activity i n the presence of Cu(II) is mainly due to the formation of (OH)-O-. ra dicals from superoxide ion via a Cu(II)-catalyzed Haber-Weiss mechanis m, 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 elec tron acceptor species, and thus, the primarily Cu(II)inhibitory site i n photosystem II is on the acceptor side.