Effect of pH on the sensitivity of mitochondrial ATPase to free ATP, ADP and anions.

The pH activity curves of the hydrolyzing activity of ATPase obtained with rat liver mitochondria or with the isolated F1-ATPase at three different substrate concentrations exhibited different optimum pH values. At 0.06 mM ATPMg2+ maximal activity was reached at pH 7; at 0.67 mM ATPMg2+ the optimum value was around 8; and at 3 mM ATPMg2+ between pH 8.2 and 9. These results suggest the presence of different catalytic sites in the enzyme with different affinities for the substrate and different optimum pH values. The sensitivity to the activating anions dinitrophenol and bicarbonate decreased with increasing pH values; the decrease in the activating effect was sharper when approaching the optimum pH value at any of the three substrate concentrations tested. These results might indicate that either OH-, dinitrophenol, or bicarbonate could compete for a regulatory site or sites in ATPase. The activating effect of free ATP on the hydrolyzing activity of isolated F1-ATPase was found to be dependent on the pH of the medium. The activating effect was more pronounced above the optimum corresponding to each of the three ATPMg2+ concentrations used, whereas the inhibitory effect of ADP was more manifest at pH values below that optimum point.