Huang W H, Askari A
Arch Biochem Biophys. 1984 Jun;231(2):287-92. doi: 10.1016/0003-9861(84)90390-4.
Ca2+ inhibited the Mg2+-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 Mg2+-dependent and Ca2+-stimulated phosphatase was observed, the maximal velocity of which, at pH 7.2, was about 20% of that of the K+-stimulated phosphatase. The Ca2+-stimulated phosphatase, like the K+-stimulated activity, was inhibited by either ouabain or Na+ or ATP. Ouabain sensitivity was decreased with increase in Ca2+, but the K0.5 values of the inhibitory effects of Na+ and ATP were independent of Ca2+ concentration. Optimal pH was 7.0 for Ca2+-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) Ca2+-stimulated phosphatase is catalyzed by (Na+ + K+)-ATPase; (b) there is a site of Ca2+ action different from the site at which Ca2+ inhibits in competition with Mg2+; and (c) Ca2+ stimulation can not be explained easily by the action of Ca2+ at either the Na+ site or the K+ site.
钙离子抑制了高度纯化的犬肾（钠钾）-ATP酶制剂中依赖镁离子且受钾离子刺激的对硝基苯磷酸酶活性。然而，在没有钾离子的情况下，观察到一种依赖镁离子且受钙离子刺激的磷酸酶，在pH 7.2时，其最大反应速度约为受钾离子刺激的磷酸酶的20%。与受钾离子刺激的活性一样，受钙离子刺激的磷酸酶也受到哇巴因、钠离子或ATP的抑制。随着钙离子浓度的增加，哇巴因敏感性降低，但钠离子和ATP抑制作用的半数抑制浓度值与钙离子浓度无关。受钙离子刺激的活性的最适pH为7.0，受钾离子刺激的活性的最适pH为7.8 - 8.2。在几种不同纯度状态的酶制剂中，这两种活性的比值相同。这些数据表明：（a）受钙离子刺激的磷酸酶由（钠钾）-ATP酶催化；（b）存在一个与钙离子在与镁离子竞争中起抑制作用的位点不同的钙离子作用位点；（c）钙离子的刺激作用不能简单地用钙离子在钠离子位点或钾离子位点的作用来解释。
