Correlation of Light-induced Absorbance Changes with Photophosphorylation in Rhodospirillum rubrum Extracts

Abstract The relationship between light-induced absorbance changes and photophosphorylation has been studied in extracts of the photosynthetic bacterium Rhodospirillum rubrum purified by partition in dextran-methyl cellulose mixtures. In these preparations infrared light induces absorbance changes in the visible region of the spectrum which are modified by the addition of reductants required for photophosphorylation. The kinetics of photophosphorylation has been compared with light-induced absorbance changes, by measuring ATP formation in flashing light. With submaximal flash duration, the ultimate yield of ATP per flash parallels the absorbance change induced during the flash. The light-off absorbance change is characterized by a rapid phase and slower subsequent phase, or phases. The dark stage of photophosphorylation coincides with the rapid phase of the light-off absorbance change (the first 0.1 sec). The effects of potent inhibitors of photophosphorylation upon absorbance change kinetics are limited to the rapid phase of the light-off reaction. This phase is almost eliminated by antimycin A and other inhibitors of similar characteristics. It is significantly slowed by phenylmercuric acetate and by oligomycin. The carbonyl cyanide phenylhydrazones stimulate the rapid phase of the absorbance change, but only in the absence of adenosine diphosphate or in the presence of oligomycin. The rapid phase of the light-off reaction is also stimulated by adenosine diphosphate. These results suggest that the rapid phase of the light-off absorbance change may represent electron transport associated with phosphorylation.