H+-transhydrogenase in chromatophores from Rhodobacter capsulatus after periods of continuous illumination and short flash excitation

(1). H+-transhydrogenase was measured during trains of short flashes fired at high frequency. The yield of the transhydrogenase reaction per flash was half maximal when the time between the flashes was approx. 50 ms. The maximal yield was decreased when either electron transport or the transhydrogenase reaction was partially inhibited (by either myxothiazol or NAD+, respectively) but the flash frequency at which half-maximal yield was reached was not affected. The maximal yield per flash was substantially less than the theoretical maximum. The results are discussed with reference to the membrane proton flux and to the turnover time of the transhydrogenase enzyme. (2) In the presence but not in the absence of valinomycin the transhydrogenase reaction persisted for several seconds after cessation of either a flash train or a short period of continuous illumination. Inhibition by nigericin and correlation of post-illumination transhydrogenase with changes in the suspension pH measured with cresol indicated that the reaction was driven mainly by the slowly-decaying transmembrane pH gradient.