DISPLACEMENT OF CHLOROPLAST PLASTOQUINONES BY PROTONOPHORIC UNCOUPLERS

Uncouplers of ADP phosphorylation carbonyl cyanide m-chlorophenylhydra zone (CCCP) and pentachlorophenol (PCP) inhibit O2 evolution in the Hi ll reaction with ferricyanide in pea chloroplasts incubated with grami cidin D. The IC50 values for CCCP and PCP are approximately 1.5 and 4. 0 muM, respectively. The CCCP and PCP inhibition is mitigated by quino nes added to the reaction mixture; the efficiency of quinones increase s in the order. 2,6-dichloro-p-benzoquinone, menadione, 2,5-dibromo-3- methyl-6-isopropyl-p-benzoquinone. Duroquinone and 3-methyl-p-benzoqui none were not efficient in relieving the effect of the inhibitors. The quinones did not influence the inhibitory action of diuron on the Hil l reaction when added at the same concentrations as in the experiments with uncouplers. From these data it is inferred that CCCP and PCP sup press photosynthetic electron transfer by displacing plastoquinone Q(C ) from the cytochrome-bf complex and/or the membrane pool quinone from the membrane. Addition of quinones with electron acceptor properties relieves the CCCP and PCP effect, because the quinones compete with th e inhibitors.