Photoinhibition of Photosystem I electron transfer activity in isolated Photosystem I preparations with different chlorophyll contents.

Photoinhibition of the light-induced Photosystem I (PS I) electron transfer activity from the reduced dichlorophenol indophenol to methyl viologen was studied. PS I preparations with Chl/P700 ratios of about 180 (PS I-180), 100 (PS I-100) and 40 (PS I(HA)-40) were isolated from spinach thylakoid membranes by the treatments with Triton X-100, followed by sucrose density gradient centrifugation and hydroxylapatite column chromatography. White light irradiation (1.1 × 10(4)μE m(-2) s(-1)) of PS I-180 for 2 hours bleached 50% of the chlorophyll and caused a 58% decrease in the electron transfer activity with virtually no loss of the primary donor, P700. The flash-induced absorbance change showed the decay phase with a half time of about 10 μs that was attributed to the P700 triplet, suggesting that the photoinhibitory light treatment caused the destruction of the PS I acceptor(s), Fx and possibly A1. PS I-100 was similarly photobleached by the irradiation and the electron transfer activity decreased. There was, however, no apparent photoinhibition of the electron transport activity in PS I(HA)-40. Photoinhibition similar to that seen in PS I-180 also occurred in membrane fragments that were isolated without any detergent from a PS II-deficient mutant strain of the cyanobacterium Synechocystis sp. PCC 6803. PS I-180 was not photoinhibited under anaerobic conditions. The production of superoxide and fatty acid hydroperoxide during white light irradiation was significantly greater in PS I-180 than in PS I(HA)-40. The mechanism of photoinhibition in PS I preparations is discussed in relation to the formation of toxic oxygen molecules.