Quinones as mediators of both artificial and cyclic phosphorylation in spinach chloroplasts

A new method to prereduce mediators catalyzing cyclic electron transfer in washed, spinach thylakoid membranes was developed. Hydrophilic and lipophilic quinones were tested for their ability to catalyze phosphorylation in both cyclic electron transfer and electron transfer in an artificial transmembrane redox reaction. Quinones varied widely in their ability to catalyze cyclic photophosphorylation, but cyclic phosphorylation in all cases was inhibited by the plastoquinone antagonist dibromothymoquinone. Many of the quinones also catalyzed transmembrane electron transfer to ferricyanide trapped internally within the thylakoid vesicles. In this system, phosphorylation catalyzed by hydrophilic quinones was inhibited by dibromothymoquinone, whereas phosphorylation catalyzed by lipophilic quinones was dibromothymoquinone-insensitive. This is taken as evidence that transmembrane electron transfer catalyzed by hydrophilic quinones is mediated by the endogenous plastoquinone pool within the thylakoid membrane.