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1. A simple mechanical procedure for the preparation of particle-bound Mg-ATPase is described. It represented at least 80°/, of total ATPase activity, both in the case of aerobically and of anaerobically grown yeast. When analyzed in a sucrose gradient, ATPase from aerobic yeast sedimented with NADH-oxidase. ATPase from aerobic or anaerobic yeast showed 2 pH optima, as had been described for ATPase localized in carefully prepared and purified yeast mitochondria. Particle-bound ATPase from aerobic or anaerobic yeast showed pronounced oligomycin sensitivity.2. Anaerobically grown yeast synthesized particle-bound Mg-ATPase a t the low basal rate of 7 -10 pmoles PI10 min/mg protein. During 0,-induction without growth, respiratory enzymes and ATPase were induced simultaneously. Both inductions were inhibited by acti-dione.3. Aerobically grown, glucose-repressed yeast synthesized ATPase at a rate similar to anaerobic yeast. Progressive derepression during growth produced a simultaneous increase of the rate of cell respiration (30-270) and of ATPase activity (10-60).4. Ethanol-grown cells had a constant high Qo, and synthesized ATPase a t the high constant rate of 60.5 . Regulation of ATPase synthesis was strictly identical when activity WDS measured a t the two pH optima, suggesting the presence of a single mitochondrial ATPase. The possibility of separate phosphorylating and hydrolytic sites on the enzyme is discussed. It appears therefore that both arguments in favour of an ATP-synthetase function of ATPase are indirect ; there is no direct correlation between ATPase activity itself and the physiological function (coupling factor, oligomycin sensivity) assigned to it.Unfortunately, a t the present time, ATP synthetase activity of ATPase cannot be measured directly, or indirectly, for the following reasons :Enzyme. ATPase or ATP phosphohydrolaso, activated by Mgz+ (EC 3.6.1.3). a) Direct measurement of this activity would imply isolating the reaction: X -P + ADP=ATP + X, X -P being an energy-rich phosphorylated intermediate. However, X -P is unknown, and the reaction hypothetical. b) Therefore, one would have to make an indirect determination of ATP synthetase, by measuring a whole series of complex reactions, from electron transport, through energy conservation, to phosphorylation of ADP. I n this case, ATP synthetase would be measured only if this activity were rate limiting. As the reactions of energy coupling and transfer are still unknown, the phosphorylating activity of a mitochondrial preparation cannot be taken as a measurement of ATP synthetase.Owing to the lack of information on the mechanism of oxidative phosphorylation, the physiological regulation of "ATP synthetase" cannot yet be studied. Mitochondrial ATPase activity is open to experimentation but it could be considered a priori as an experimental artifact without physiological meaning. We were tempted, nevertheless, to look for a quantitative correlation between the regulation of the synthesis of respiratory enzymes and mitochondrial ATPase, hoping to...

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