Oxidation-Reduction Potential-Dependent Adenosine Triphosphatase Activity of Chromatophores from Rhodospirillum rubrum

Abstract

1. Chromatophores from Rhodospirillum rubrum hydrolyze ATP into ADP and P₁ either in the light or in darkness; this activity is called ATPase activity. 2. The ATPase activity in some, though not all, cases is inhibited by light. The light appears to be a competitive inhibitor against ATP. The light inhibition is neutralized by adding antimycin A; the maximum extent of neutralization is approximately 70%. 3. In the presence of phenazine methosulfate (PMS), the ATPase activity is stimulated in the light, but hardly affected in darkness. The light appears to be a non-competitive “stimulant” against ATP. 4. Oligomycin inhibits the ATPase activities in the light and in darkness; both activities are approximately 90% inhibited by 3.3 μg/ml of the inhibitor, which can inhibit the photosynthetic ATP formation completely. 5. The ATPase activities in the light and in darkness are accelerated with increasing concentrations of 2, 6-dichlorophenol indophenol in the oxidized form (DCPI) up to 6.7 ×10⁻⁵ M and retarded at higher concentrations. At 6.7×10⁻⁵M DCPI, both activities are almost the same in rate. The activities are significantly stimulated by 2×10⁻⁵ M 2, 4-dinitrophenol (DNP) in the range of DCPI concentration lower than 6.7×10⁻⁵M but not above this concentration. In the presence of 6.7×10⁻⁴M DCPI, the ATPase activities in the light and in darkness are accelerated with increasing concentrations of ascorbate up to 5.0×10⁻⁴M and retarded at higher concentrations. The E_h value of the reaction mixture containing 6.7×10⁻⁴M DCPI and 5.0×10⁻⁴M ascorbate is approximately +0.15 volt. 6. From these findings, it is concluded that the ATPase activity by chromatophores is to a great part exhibited by one of the coupling systems leading to ATP formation, and brought about depending upon the E_h value of the reaction mixture.