Mechanism of copper-enhanced photoinhibition in thylakoid membranes

The effect of copper on photoinhibition of photosystem II (PSII) in vitro was studied in bean (Phaseolus vulgaris L. cv. Dufrix) and pumpkin (Cucurbita pepo L.) thylakoids. The thylakoids were illuminated at 200-2000 mu mol photons m(-2) s(-1) in the presence of 70-1830 added Cu2+ ions per PSII. Three lines of evidence show that the irreversible damage of PSII caused by illumination of thylakoids in the presence of Cu2+ was mainly due to donor-side photoinhibition resulting from inhibition of the PSII donor side by Cu2+. First, addition of an artificial electron donor partially restored PSII activity of thylakoids that had been illuminated in the presence of Cu2+. Second, already moderate light was enough to cause rapid inhibition of PSII, and the inhibition could be saturated by light. Third, the extrinsic polypeptides of the oxygen-evolving complex were found to become oxidized by the combined effect of Cu2+ and light. The presence of oxygen was not necessary for the copper-induced enhancement of photoinhibition of PSII. When the illumination was prolonged, copper caused a gradual collapse of the thylakoid structure by increasing degradation of thylakoid proteins.