Studies on photosynthetic inorganic pyrophosphate formation in Rhodospirillum rubrum chromatophores

Photosynthetic formation of inorganic pyrophosphate (PP(i)) in Rhodospirillum rubrum chromatophores has been studied utilizing a new and sensitive method for continuous monitoring of PP(i) synthesis. Studies of the reaction kinetics under a variety of conditions, e.g., at different substrate concentrations and different electron-transport rates, have been performed. At very low light intensities the rate of PP(i) synthesis is twice the rate of ATP synthesis. Antimycin A, at a concentration which strongly inhibited the photosynthetic ATP formation, inhibited the PP(i) synthesis much less. Even at low rates of electron transport a significant rate of PP(i) synthesis is obtained. The rate of photosynthetic ATP formation is stimulated up to 20% when PPI synthesis is inhibited. It is shown that PP(i) synthesis and ATP synthesis compete with each other. No inhibition of pyrophosphatase activity is observed at high carbonyl cyanide p-trifluoromethoxyhydrazone concentration while ATPase activity is strongly inhibited under the same conditions.