Interaction of Ca 2+ with (Na + + K +)-ATPase: Properties of the Ca 2+-stimulated phosphatase activity

Abstract

Ca 2+ inhibited the Mg 2+-dependent and K +-stimulated p-nitrophenylphosphatase activity of a highly purified preparation of dog kidney (Na + + K +)-ATPase. In the absence of K +, however, a Mg 2+-dependent and Ca 2+-stimulated phosphatase was observed, the maximal velocity of which, at pH 7.2, was about 20% of that of the K +-stimulated phosphatase. The Ca 2+-stimulated phosphatase, like the K +-stimulated activity, was inhibited by either ouabain or Na + or ATP. Ouabain sensitivity was decreased with increase in Ca 2+, but the K 0.5 values of the inhibitory effects of Na + and ATP were independent of Ca 2+ concentration. Optimal pH was 7.0 for Ca 2+-stimulated activity, and 7.8–8.2 for the K +-stimulated activity. The ratio of the two activities was the same in several enzyme preparations in different states of purity. The data indicate that (a) Ca 2+-stimulated phosphatase is catalyzed by (Na + + K +)-ATPase; (b) there is a site of Ca 2+ action different from the site at which Ca 2+ inhibits in competition with Mg 2+; and (c) Ca 2+ stimulation can not be explained easily by the action of Ca 2+ at either the Na + site or the K + site.