Studies on the protolytic reactions coupled with water cleavage in photosystem II membrane fragments from spinach.

The protolytic reactions of PSII membrane fragments were analyzed by measurements of absorption changes of the water soluble indicator dye bromocresol purple induced by a train of 10 μs flashes in dark-adapted samples. It was found that: a) in the first flash a rapid H(+)-release takes place followed by a slower H(+)-uptake. The deprotonation is insensitive to DCMU but is completely eliminated by linolenic acid treatment of the samples; b) the extent of the H(+)-uptake in the first flash depends on the redox potential of the suspension. In this time domain no H(+)-uptake is observed in the subsequent flashes; c) the extent of the H(+)-release as a function of the flash number in the sequence exhibits a characteristic oscillation pattern. Multiphasic release kinetics are observed. The oscillation pattern can be satisfactorily described by a 1, 0, 1, 2 stoichiometry for the redox transitions Si → Si+1 (i=0, 1, 2, 3) in the water oxidizing enzyme system Y. The H(+)-uptake after the first flash is assumed to be a consequence of the very fast reduction of oxidized Q400(Fe(3+)) formed due to dark incubation with K3[Fe(CN)6]. The possible participation of component Z in the deprotonation reactions at the PSII donor side is discussed.