Effect of calcium concentration on isolated hamster brain Na-K-, Mg-dependent ATPase

In these experiments the effect of different concentrations of calcium on the specific activity of isolated Na-K-ATPase was studied. The result of these investigations showed that calcium at 10(6) and 10(7) M stimulated the Na-K-ATPase activity. These studies also show that at higher calcium concentrations (10(5)-10(3)M), the activity of the enzyme is inhibited. The results from calcium binding to isolated membranes, rich in Na-K-ATPase, strongly suggest the existence of a low-affinity binding site which exhibits a large positive cooperativity. Kd = 2.8 x 10(5) +/- 0.4 x 10(5) M and Hill coefficient of 2.9 +/- 0.2. The calcium concentration (1.9 x 10(5) M sufficient to produce significant (24%) inhibition of the Na-K-ATPase is approximately equal to the Kd observed for calcium binding.