INHIBITION OF PHOTOSYNTHETIC OXYGEN EVOLUTION BY PROTONOPHORIC UNCOUPLERS

The protonophoric uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2,3,4,5,6-pentachlorophenol (PCP) and 7-tetrachloro-2-trifluor omethylbenzimidazole(TTFB) inhibited the Hill reaction with K-3 [Fe(CN )(6)] (but not with SiMo) in chloroplast and cyanobacterial membranes (the I-50 values were approx. 1-2, 4-6 and 0.04-0.10 mu M, respectivel y). The inhibition is due to oxidation of the uncouplers on the Photos ystem II donor side (ADRY effect) and their subsequent reduction on th e acceptor side, ie. to the formation of a cyclic electron transfer ch ain around Photosystem II involving the uncouplers as redox carriers. The relative amplitude of nanosecond chlorophyll fluorescence in chlor oplasts was increased by DCMU or HQNO and did not change upon addition of uncouplers, DBMIB or DNP-INT; the HQNO effect was not removed by t he uncouplers. The uncouplers did not inhibit the electron transfer fr om reduced TMPD or duroquinol to methylviologen which is driven by Pho tosystem I. These data show that CCCP, PCP and TTFB oxidized on the Ph otosystem II donor side are reduced by the membrane pool of plastoquin one (Q(p)) which is also the electron donor for K-3 [Fe(CN)(6)] in the Hill reaction as deduced from the data obtained in the presence of in hibitors. Inhibition of the Hill reaction by the uncouplers was maximu m at the pH values corresponding to the pK of these compounds. It is s uggested that the tested uncouplers serve as proton donors, and not me rely as electron donors on the oxidizing side of Photosystem II.