Valinomycin inhibition of chloroplast electron transport at or near plastoquinone

Valinomycin inhibits the coupled portion of whole-chain noncyclic electron transport and phosphorylation and also inhibits the anaerobic menadione-dependent cyclic phosphorylation. Both of these electron transport systems are thought to involve plastoquinone. Electron transport and phosphorylation in various photosystem I and II partial reactions not involving plastoquinone or phosphorylation using other cyclic cofactors that are not dependent on plastoquinone were relatively insensitive to valinomycin. Single-beam ultraviolet spectrophotometric measurements showed that valinomycin decreased the apparent first-order rate constant for plastohydroquinone oxidation by more than twofold and had no observable effect on the reduction rate constant. The valinomycin inhibitions did not require K⁺ addition to the media. However, it was shown that extensive washing or dialysis did not reduce the chloroplast endogenous K⁺ content below 0.1 μmol K⁺/mg of chloroplasts.