Nicotinamide nucleotide transhydrogenase from Rhodobacter capsulatus; the [formula omitted] ratio and the activation state of the enzyme during reduction of acetyl pyridine adenine dinucleotide

Abstract

Chromatophores from Rhodobacter capsulatus were incubated in the dark with NADPH and acetylpyridineadenine dinucleotide (AcPdAD +) in the presence of different concentrations of myxothiazol. The transhydrogenase activity was monitored until an appropriate mass action ratio, [AcPdAD +][NADPH]/[AcPdADH][NADP +], was reached. The sample was then illuminated and the initial rate of either AcPdAD + reduction by NADPH or AcPdADH oxidation by NADP + was recorded. The ratio oof H + translocated per H + equivalent transferred by transhydrogenase was calculated from the value of the membrane potential ( ΔpH = 0) at which illumination caused no net reaction in either direction. The mean value for the H +/H ratio was 0.55. At greater values of [AcPdAD +][NADPH]/[AcPdADH][NADP +] than were employed in the above experiments and over a wider range of concentrations of myxothiazol, it was found that incremental increases in the membrane potential always gave rise to a decrease, never an increase in the rate of AcPdAD + reduction. In contrast to the H +-ATP synthase, there is no evidence of any activation/deactivation of H +-transhydrogenase by the protonmotive force.