Uncouplers and energy-transfer inhibitors

Non cyclic Photophosphorylation The non-cyclic photophosphory-lation coupled to electron transport was reduced drastically at flucrescamine concentration of O.56 umoles/500 ug Chi (Fig.l). However, we observed that beyond. 5 umoles flucrescamine/500 ug Chi, the rates were higher than at lower concentrations of fluores-camine. As usual, the uncouplers and energy transfer inhibitor decreased significantly the rate of non-cyclic photophosphorylation both in control and treated chloroplast (data not shown . 

Uncoupling and energy transfer. Representative oxygen consumption polarographic traces that compare the effects of standard Inhibitors with that of quercetin on the oxidation of malate are... 

Herbicides that inhibit the photochemical reactions of isolated chloroplasts have been called routinely inhibitors of the Hill reaction. This has been done primarily for convenience and because, for many years, their action was evaluated under nonphosphorylating conditions, frequently with ferricyanide as the electron acceptor. In the past few years, more sophisticated studies have been conducted with herbicides and more is known about their differential actions. Consequently, Moreland and Hilton (2) separated herbicidal inhibitors of the photochemically induced reactions into the following classes (a) electron transport inhibitors, (b) uncouplers, (c) energy transfer inhibitors,... 

Figure 1. Representative polarographic traces that depict inhibition by energy transfer inhibitors and allelochemicals of ADP-stimulated electron transport in isolated spinach thylakoids and circumvention of the inhibition by an uncoupler (FCCP, 2 pM). Trace A chlorotributyltin (TBT, 1 pM) trace B phlorizin (400 pM) trace C DCCD (20 pM) trace D quercetin (200 pM) trace E naringenin (1 mM). Water served as electron donor and methyl viologen as electron acceptor. Rates of oxygen utilization, that resulted from the autooxidation of methyl viologen, expressed as pmol 0 consumed/mg Chi h, are indicated parenthet ically.

Uncouplers. Uncouplers dissociate electron transport from photophosphorylation. Both noncyclic and cyclic phosphorylation are inhibited, but electron transport reactions are either unaffected or stimulated. Because uncouplers relieve the inhibition of electron transport imposed by energy transfer inhibitors, they are considered to act at a site closer to the electron transport chain than the site of phosphate uptake. In Figure 2, they are shown (site 2) as dissipating some form of conserved energy represented as on the noncyclic and cyclic ATP-gener-ating pathways. Perfluidone is the only herbicide identified to date that functions as a pure uncoupler at pH 8.0 (2). Compounds that uncouple photophosphorylation also uncouple mitochondrial oxidative phosphorylation. 

The effects of the electron inhibitor DCMU, the uncoupler S-13 and the energ transfer inhibitor DCCD on electron transfer and photophosphorylation in membrane vesicles of Syneohocoocus 6716 are shown in Fig. 1. DCMU behaves as expected, fully blocking electron transfer and the resulting phosphorylation at 1 to 10 yM. S-13 is a potent uncoupler full inhibition of ATP synthesis occurs at 1 yM however, electron transfer is not enhanced. In linear electron transfer DCCD seems to act as an electron transfer inhibitor at lower concentrations as was found before in chloroplasts, where DCCD bloc) the reduction site of plastoquinone (Sane et al. 1979). Only at higher concentrations (10 M) DCCD also inhibits energy transfer as can be seen in th< PMS-mediated cyclic photophosphorylation. 

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