TY - JOUR T1 - Calcium Ion and Sodium- and Potassium-Dependent Adenosine Triphosphatase: Its Mechanism of Inhibition and Identification of the <em>E</em><sub>1</sub>—P Intermediate JF - Molecular Pharmacology JO - Mol Pharmacol SP - 336 LP - 349 VL - 9 IS - 3 AU - T. TOBIN AU - T. AKERA AU - S. I. BASKIN AU - T. M. BRODY Y1 - 1973/05/01 UR - http://molpharm.aspetjournals.org/content/9/3/336.abstract N2 - Calcium ion inhibits (Na+ + K+)-ATPase (EC 3.6.1.3) primarily by reducing the rate of the phosphorylation step. Inhibition of this step is competitive with respect to Na+ and noncompetitive with respect to Mg++. Although Ca++ acts to reduce the rate of the phosphorylation step, high concentrations are required to reduce the steady-state levels of phospho-enzyme, and Ca++ alone stimulates a slow phosphorylation of the enzyme. In the presence of Ca++ and Mg++ the phospho-enzyme formed is sensitive to K+ and ouabain, and its steady-state levels are low when K+ is present. If the concentration of Mg++ is low, of the phosphoenzyme formed in the presence of Ca++ is insensitive to K+ and ouabain, and reacts readily with ADP. The reactivity of the Ca++-dependent phospho-enzyme with ADP is reduced by the addition of Mg++; conversely, the reactivity of the Mg++-dependent phospho-enzyme with ADP is increased by high concentrations of Na+. Ca++ also inhibits the Mg++- and P[unknown]-dependent pathway of [3H]ouabain binding. The identification of an ADP-sensitive intermediate in the reaction cycle of this enzyme is reported. Its transformation to a K+-sensitive intermediate is influenced by Na+, Mg++, and Ca++ in a manner consistent with proposed reaction mechanisms for this enzyme. ACKNOWLEDGMENTS The authors would like to thank Mrs. Annie Han for excellent technical assistance, and Mr. Ralph Evans, who assisted as a summer student. ER -