The photoreduction of nicotinamide-adenine dinucleotide by chromatophore fractions from Rhodospirillum rubrum

The photoreduction of nicotinamide-adenine dinucleotide (NAD + ), catalyzed by chromatophore fractions from young (1 day) and old (4–5 days) cultures of Rhodospirillum rubrum, was measured in the presence of either succinate or 2,6-dichlorophenol indophenol (DPIP) and an excess of ascorbate. The time-course of photoreduction in the succinate system suggested a "reversed electron flow" from the donor to NAD + mediated by a high energy intermediate produced by a light-induced, cyclic electron transport in the chromatophore fractions. The effects of the uncoupler carbonyl cyanide [ p -(trifluoromethoxy)phenyl]hydrazone (FCCP) and of the inhibitors antimycin A and 2-heptyl-4-hydroxyquinoline- N -oxide (HQNO) were consistent with this interpretation. The time-course of NAD + photoreduction in the presence of DPIP and ascorbate suggested a direct, light-induced electron transport from the donor to the acceptor. We cannot yet distinguish between a model in which the same reaction center is utilized in the photoreduction by both donor systems (the reaction center component P-870 may relate to two primary acceptors at different redox potential levels) and a model in which each photoreducing system is driven by its own reaction center component.