REDOX POTENTIALS OF CYTOCHROME-B-559 IN THE D1 D2 CYTOCHROME-B-559 REACTION-CENTER OF PHOTOSYSTEM-II

Redox titrations of a stable form of the Photosystem II (PS II) reaction centre, isolated from peas, have detected three redox forms of cytochrome b-559: a high-potential form (+430 mV), an intermediate-potential form (+180 mV) and a low-potential form (+25 mV) with relative amplitudes of 28%, 62% and 10%, respectively. These results contrast with the observations of Shuvalov et al. (Shuvalov, V.A., Heber, U. and Schreiber, U. (1989) FEBS Lett. 258, 27-31), who reported midpoint potentials much lower than these for cytochrome b-559 in isolated PS II reaction centres. Our data show that, if the reaction centre complex is handled appropriately, then cytochrome b-559 can have redox potentials quite close to those found in intact membrane systems.