The Influence of the Ionic Conductance on the Relation between Electron Transport and Proton-Motive Force in Intact Cells of Rhodopseudomonas capsulata

1 The dependence of membrane potential (ΔΦ) on the rate of respiration in darkened intact cell suspensions of Rhodopseudomonas capsulata was distinctly non-linear: severe inhibition of respiration with either rotenone or KCN led to only a small drop in ΔΦ. 2 In the presence of 0.3 μM carbonylcyanide p-trifluoromethoxyphenylhydrazone [CF3OPhzC(CN)2] the dependence of ΔΦ on respiratory rate became linear. Consequently, and particularly at lower concentrations of CF3OPhzC(CN)2, there was a pronounced, synergistic depression of the respiratory ΔΦ with CF3OPhzC(CN)2 and either rotenone or KCN. 3 Antimycin A, at a concentration which strongly inhibited the photosynthetic electron transport chain, only slightly lowered the light-induced ΔΦ in anaerobic cell suspensions. Antimycin and CF3OPhzC(CN)2 synergistically lowered ΔΦ generated by illumination. 4 The light-induced ΔΦ in anaerobic cells was only about 1.5-times larger than the respiratory-induced ΔΦ in darkened cells. Nevertheless it required approximately 16-times more CF3OPhzC(CN)2 to collapse the photosynthetic ΔΦ than the respiratory ΔΦ. 5 These results are discussed with reference to theionic current/ΔΦ relation described in [J. B. Jackson (1982) FEBS Lett. 139, 139–143]. The unifying feature is that the intrinsic conductance of the cell membrane is strongly dependent on ΔΦ but the conductance due to CF3OPhzC(CN)2 is independent of ΔΦ.