The dependence of the rate of transhydrogenase on the value of the protonmotive force in chromatophores from photosynthetic bacteria

Abstract

In conditions where the pH gradient is negligible, the rate of the pyridine nucleotide transhydrogenase in chromatophores of Rhodobacter eapsulatus has a threshold dependence on membrane potential. The relationship is similar when either antimycin or myxothiazal or carbonyl cyanide p-trifluoromethoxyphenylhydrazone is used to depress the membrane potential. The relationship is distorted when membrane potential is reduced by lowering the photosynthetic light intensity.

Energy coupling to ATP synthesis and pyridine nucleotide transhydrogenase in chromatophores from photosynthetic bacteria A ‘dual‐consumer’ test for localised interactions with electron transport components

“…This is consistent with the finding that J,, has a higher A+ threshold than has Jt [16,17]. It was established many years ago in Rhodospirillum rubrum chromatophores that ATP synthesis has a higher light-intensity requirement than transhydrogenation [20] and that in submitochondrial particles ATP synthesis is more sensitive to inhibition by FCCP than transhydrogenase [21].…”

“…The rates of ATP synthesis and pyridine nucleotide transhydrogenase were assayed aerobically in a medium containing 10% sucrose, 100 mM KCI, 10 mM MgCh, 1 mM K+-phosphate, 0.5 mM Na+-succinate, 100,cM cresol red, 1 mM ADP, 66gM thio-NADP, 133 ,LJM NADH, 1 /tM nigericin, 1 fig/ml rotenone and chromatophores to give a bacteriochforophyll concentration of 1OrM. The samples for the transhydrogenase assay contained, in addition, 50 mM Tricine, pH 7.6: the rate of transhydrogenase was estimated from the absorbance change due to the formation of thio-NADPH at 395 nm using an extinction coefficient of 11.3 mM_' [15,16]. The samples for the ATP synthesis assay were adjusted to pH 7.6 with dilute KOH: the rate of ATP synthesis was estimated from the absorbance change of cresol red at 572 nm resulting from the scalar uptake of H+ accompanying the phosphorylation reaction [17].…”

“…1B) and by antimycin ( fig.lC) was sigmoidal. Antimycin was the least potent inhibitor although sensitivity to antimycin of energy-consuming reactions is affected by the experimental conditions [16]. Valinomycin in the presence of nigericin and K+ was extremely potent, only 12.5 nM valinomycin being necessary to reduce JP by 50% ( fig.lD).…”