Synthesis of ATP by Soluble Mitochondrial F1 ATPase and F1-inhibitor-protein Complex in the Presence of Organic Solvents

The F1 and F1-inhibitor-protein complex synthesized tightly bound ATP from ADP and Pi when the organic solvents dimethylsulfoxide (20-50% v/v), ethylene glycol (20-60% v/v) or poly(ethylene glycol) 4000 and 8000 (30-50% w/v) were included in the assay media. There was no synthesis of tightly bound ATP in the absence of organic solvents. In the presence of 50% dimethylsulfoxide, maximal synthesis of ATP was obtained at pH values between 6.5 and 7.7. In both F1 and F1-inhibitor-protein there was no synthesis of ATP in the absence of MgCl2. The rate of ATP synthesis became faster as the MgCl2 concentration in the medium was raised from 0.1-10 mM. The Km for Pi of F1 was in the range of 0.8-1.5 mM. The Km for Pi of the F1-inhibitor-protein was much higher than that of F1 and could not be measured. In the presence of 10 mM MgCl2 and 2 mM Pi, the rate constants of ATP synthesis by F1 and F1-inhibitor-protein were 5.2-10.4 h-1 and 3.5-5.9 h-1 respectively. For both enzymes the rate constant of ATP hydrolysis was 0.69 h-1. The tightly bound ATP of F1 and F1-inhibitor-protein were hydrolyzed at a much slower rate when either the Pi concentration or the MgCl2 concentration was suddenly decreased. Both in presence and absence of Mg2+, 40-60% of the radioactive tightly bound ATP synthesized by F1 was hydrolyzed when non-radioactive ATP was added to the assay medium. This was not observed when F1-inhibitor-protein was used.