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

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Abstract
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-ATPasc, strongly suggest the existence of a low-affinity binding site which exhibits a large positive cooperativity, Kd = 2.8 × 10−5 ± 0.4 × 10−5 M and Hill coefficient of 2.9 ± 0.2. The calcium concentration (1.9 × 10−5 M sufficient to produce significant (24%) inhibition of the Na-K-ATPase is approximately equal to the Kd observed for calcium binding.