The reduction of plastocyanin by plastoquinol-1 in the presence of chloroplasts. A dark electron transfer reaction involving components between the two photosystems.

The reduction of plastocyanin by plastoquinol-1 was efficiently catalysed by disrupted chloroplasts or etioplasts in the dark. The reaction was inhibited by 2,5-dibromomethylisopropyl-p-benzo-quinone which inhibits photosynthetic electron transport between plastoquinone and cytochrome f. Evidence is presented that the reduction took place via cytochrome f, and that plastoquinone-9 was not involved. Triton X-100 and organic solvents were inhibitory, but partial fractionation was achieved without loss of activity by density gradient centrifugation in the presence of high digitonin concentrations. All active material contained cytochromes b-559LP and b-563 in addition to cytochrome f, but these b-type cytochromes were not directly involved. Other 1-electron acceptors could be used in place of plastocyanin, for instance ferricyanide and Pseudomonas cytochrome c-551. The reaction can be applied to give a sensitive dark assay for active cytochrome f. It is suggested that cytochrome f possesses two sites for interaction with redox reagents: a hydrophilic site with which plastocyanin reacts by electron transfer and a hydrophobic site with which plastoquinol reacts by hydrogen atom transfer.