Involvement of plastoquinone bound within the isolated cytochrome b6- f complex from chloroplasts in oxidant-induced reduction of cytochrome b6

Abstract

(1) Oxidant-induced reduction of cytochrome b 6 is completely dependent on a reduced component within the isolated cytochrome b 6- f complex. This component can be reduced by dithionite or by NADH/ N-methylphenazonium methosulfate. It is a 2H +/2e − carrier with a midpoint potential of 100 mV at pH 7.0, which is very similar to the midpoint potential of the plastoquinone pool in chloroplasts. (2) Oxidant-induced reduction of cytochrome b 6 is stimulated by plastoquinol-1 as well as by plastoquinol-9. The midpoint potential of the transient reduction of cytochrome b 6, however, was not shifted by added plastoquinol. (3) Quinone analysis of the purified cytochrome b 6- f complex revealed about one plastoquinone per cytochrome f. The endogenous quinone is heterogeneous, a form more polar than plastoquinone-A, probably plastoquinone-C, dominating, This is different from the thylakoid membrane where plastoquinone-A is the main quinone. (4) The endogenous quinone can be extracted from the lyophilized cytochrome b 6- f complex by acetone, but not by hydrocarbon solvents. Oxidant-induced reduction of cytochrome b 6 was observed in the lyophilized and hexane-extracted complex, but was lost in the acetone-extracted complex. Reconstitution was achieved either with plastoquinol-1 or plastoquinol-9, suggesting that a plastoquinol molecule is involved in oxidant-induced reduction of cytochrome b 6.