Unlike green plants, the most photosynthetic bacteria are known to utilize light energy for the generation of ATP through photophosphorylation as the major photochemical reaction, thus NADH should be supplied by reversal electron transport pushed by photochemically generated high energy state. Recently, however, we have demonstrated from a series of experiments with Rs.rubrum that they have alternative path leading to direct photoreduction of $NAD^+$ via noncyclic electron transfer system. In this paper, we wish to report our finding of direct photoreduction of $NAD^+$ by Rs.rubrum chromatophores. The photoreduction activity was demonstrated with electron donor DCPIP plus ascorbate in the absence of phosphate pool. The chromatophore preparation of Rs.rubrum seemed to have both activities of $NAD^+$ reduction driven by ATP and light energy, respectively. However, inhibition study with several electron transfer inhibitors and uncouplers suggested that there may exist a control point between photoreduction and photophosphorylation in photosynthetic bacterial system. Exogenecus ferredoxin effect on the photoreduction of $NAD^+$ was also studied.
Chromatophores that contain ubiquinone, iron, and iron-sulfur proteins, catalyze $NAD^+$ photoreduction directly. Respiratory inhibitor enhance the photoreduction activities. Uncouplers could dissipate high energy, but did not change the photoreduction role of chromatophores. $ADP/P_i$ enhance the photophosphorylation, thus slow down the photoreduction rate. The $NAD^+$ photoreduction activity can be enhanced by increasing exogenous ferredoxin. And the $NAD^+$ photoreduction shows electron donor dependency, further the reduction can be enhanced by ferredoxin.
It seems that the role of the direct photoreduction in bacterial chromatophores is to produce reducing power before cytochromes, thus branch out the photon excited electrons from cycling.