Photosynthetic adenosine triphosphate formation and photo-reduction of diphosphopyridine nucleotide with chromatophores of Rhodospirillum rubrum.

Abstract

Abstract We have studied photosynthetic DPN reduction by succinate and ATP formation from ADP and P 1 in chromatophores of Rhodospirillum rubrum . Our findings are: 1. 1. Magnesium ion accelerates DPN photoreduction most effectively at 5·10 −3 M, but is inhibitory at higher concentrations. The stimulatory effect of the ion is similar to that on the photosynthetic ATP formation. 2. 2. Photoreduction of DPN is maximal in rate in the presence of 3·10 −4 M DPN, and is inhibited with increasing concentration. 3. 3. In response to variations in concentration of succinate or DPN, photoreduction of DPN reaches a steady state in which approximately half of the DPN present is reduced. 4. 4. DPNH/DPN (1:1) is oxidized by succinate/fumarate anaerobically in darkness, provided that the E h value of succinate/fumarate is more positive than −0.04 V. This accords with the suggestion that there is an enzyme system which catalyzes oxidation of DPNH by fumarate anaerobically in darkness. 5. 5. Photoreduction of DPN is inhibited in varying degree by various adenosine derivatives and pyrophosphate; i.e. strong inhibition with ATP, ADP and pyrophosphate; moderate with adenosine 3′-monophosphate, AMP and adenosine; none with orthophosphate. 6. 6. Photosynthetic ATP formation with “bound” ADP is inhibited by adenosine, adenosine 3′-monophosphate and DPN, but not by ADP or TPN. The presence of ATP or pyrophosphate inhibits ATP formation either with “bound” ADP or in the presence of exogenous ADP: The lower the concentration of ADP present, the greater is the inhibition. Kinetic data show that the inhibition by ATP is competitive with orthophosphate, but non-competitive with ADP, while pyrophosphate is competitive with both ATP and orthophosphate. Based on these findings, mechanisms for photosynthetic ATP formation and DPN reduction are suggested.