Photoreduction of Pyridine Nucleotide by Subcellular Preparations from Rhodopseudomonas spheroides

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Abstract
Photoreduction of pyridine nucleotides by cell-free preparations of photosynthetically grown R. spheroides was investigated. Heavily pigmented supernatant materials obtained by centrifugation of broken cell suspensions at 37,000 x g support the photoreduction of nicotinamide adenine dinucleotide phosphate (NADP) but not NAD. This is in contrast to other photosynthetic bacteria where only NAD is photoreduced. Reduction of NAD is readily demonstrated, but there is no light requirement. High reduction rates were demonstrated with endogenous materials alone. The photoreduction of NADP depends upon NADH since photoreduction of NADP does not occur in the presence of pyruvate and lactic dehydrogenase. The photoreduction reaction can also be demonstrated with a chromatophore preparation (150,000 x g pellet) if an NADH generating system is present. There is no requirement for anaerobic conditions. Adenosine triphosphate (ATP) replaces light, and both the light-driven reaction and the ATP supported reaction are inhibited by antimycin-A. Oligomycin has no effect on the light reaction but inhibits the ATP-mediated reaction. The evidence may indicate a photoreduction mechanism based on an energy-linked transhydrogenase similar to that of mitochondria. In R. spheroides. the transhydrogenase reaction can be driven by a high energy intermediated) generated by light or ATP. The intermediate(s) may also be functional in ATP synthesis.