LIGHT-INDUCED REDUCTION AND OXIDATION OF PLASTOCYANIN BY CHLOROPLAST PREPARATIONS

In the light the intact chloroplasts of spinach reduce plastocyaninwith a reaction rate comparable to that of the usual HILL reaction,while no reduction nor oxidation of the copper protein is inducedby the chloroplasts in the dark. The dependency of the rateof photoreduction upon light intensity, pH and presence of variousreagents was similar to that of the HILL reaction with the usualHILL oxidants. The photoreduction of plastocyanin was acceleratedby the addition of complete phosphorylating system or ammoniumsulfate to the chloroplasts. Digitonin-treated chloroplast wasfound to be inactive in photoreducing plastocyanin but highlyactive in photo-oxidizing reduced plastocyanin. The rate ofphotooxidation was saturated at about 5, 000 lux, and showeda rather broad pH optimum around pH 8.0—8.5. The effectsof various poisons on the reaction rate were studied. When thedigitonin-treated chloroplasts were fractionated with ethanolinto a chlorophyll-bearing participate fraction and a solublefrac tion, the former was active in catalyzing the photooxidationof reduced plastocyanin, but not in photooxidizing reduced cytochromec.An ap preciable photooxidation of reduced cytochrome cwiththe ethanol-precipi tated fraction was obtained on additionof the soluble fraction, which was effectively replaced by plastocyanin.The properties of the reaction systems responsible for photooxidationof plastocyanin and cytochrome C were compared, and a possiblerole of plastocyanin in the photooxidatory process of the chloroplastswas suggested. ¹A part of the present investigation was supported by a researchgrant (GAMN 6208) from ROCKEFELLER Foundation. ²Present address; C.F. KETTERING Research Laboratory, YellowSprings, Ohio, U.S.A.