Experiments on the effects of varying concentrations of Ca
2+ on the Mg
2+ + Na
+-dependent ATPase activity of a highly purified preparation of dog kidney (Na
+ + K
+)-ATPase showed that Ca
2+ was a partial inhibitor of this activity. When Ca
2+ was added to the reaction mixture instead of Mg
2+, there was a ouabain-sensitive Ca
2+ + Na
+-dependent ATPase activity the maximal velocity of which was 30 to 50% of that of Mg
2+ + Na
+-dependent activity. The apparent affinities of the enzyme for Ca
2+ and CaATP seemed to be higher than those for Mg
2+ and MgATP. Addition of K
+, along with Ca
2+ and Na
+, increased the maximal velocity and the concentration of ATP required to obtain half-maximal velocity. The maximal velocity of the ouabain-sensitive Ca
2+ + Na
+ + K
+-dependent ATPase was about two orders of magnitude smaller than that of Mg
2+ + Na
+ + K
+-dependent activity. In agreement with previous observations, it was shown that in the presence of Ca
2+, Na
+, and ATP, an acid-stable phosphoenzyme was formed that was sensitive to either ADP or K
+. The enzyme also exhibited a Ca
2+ + Na
+-dependent ADP-ATP exchange activity. Neither the inhibitory effects of Ca
2+ on Mg
2+-dependent activities, nor the Ca
2+-dependent activities were influenced by the addition of calmodulin. Because of the presence of small quantities of endogenous Mg
2+ in all reaction mixtures, it could not be determined whether the apparent Ca
2+-dependent activities involved enzyme-substrate complexes containing Ca
2+ as the divalent cation or both Ca
2+ and Mg
2+.
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