Conversion of biomembrane-produced energy into electric form. III. Chromatophores of Rhodospirillum rubrum

Abstract

The mechanisms of energy coupling and ion transport in Rhodospirillum rubrum chromatophores have been studied. Photoreduction of NAD + and photophosphorylation have been measured under anaerobic conditions in the presence of N,N,N′,N′-tetramethyl- p-phenylenediamine (TMPD), ascorbate and antimycin A. The uncoupler p-trifluoromethoxycarbonyl cyanide phenylhydrazone (FCCP), as well as ADP + phosphate, has been found to inhibit the photoreduction of NAD + in this system. Addition of an electron acceptor, such as methylviologen, fumarate or O 2, to antimycin-treated chromatophores initiates the process of photophosphorylation. Chromatophores of R. rubrum accumulate the penetrating anions, phenyl dicarboundecaborane and tetraphenyl boron, as well as iodide, if the I − carrier, di-(pentafluorophenyl) mercury, is added. The anion accumulation can be supported by light-induced cyclic electron flow (NADH → O 2, succinate → ferricyanide), by hydrolysis of ATP or inorganic pyrophosphate, as well as by reversal of the energy-requiring transhydrogenase reaction (NADPH → NAD +). The type of energy source influences only the extent of the anion accumulation process. Cessation of the energy supply ( e.g. by exhaustion of the energy source or poisoning of the system by specific inhibitors or an uncoupler) brings about an efflux of the accumulated anions. Uptake of anions is accompanied by alkalinization of the outer solution; release of anions is accompanied by acidification. It is concluded that there is an energy-dependent charge-specific mechanism for anion accumulation in the chromatophore membrane resembling that found in the membrane of submitochondrial particles. It is stated that the electric field (the “ plus” inside the chromatophore) is the motive force for ion transfer through the chromatophore membrane against a concentration gradient. The data on NAD + photoreduction, noncyclic photophosphorylation and energy-dependent anion transport are summarized as the concept of four sites of energy coupling in the chromatophore redox chain localized at the same steps as in animal mitochondria (NADPH → NAD +, NADH → cytochrome b, cytochrome b → cytochrome c, and the region after cytochrome c). Each of these coupling sites can provide energy for generation of a membrane potential.