The kinetic and physicochemical properties of the calcium-pumping protein, (Ca2+ + Mg2+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) were studied in ghost membranes isolated from porcine erythrocytes. 2. The membrane-bound enzyme in situ has a specific activity of 3.12 +/- 0.08 micron/mg protein/hr and a Vmax of 3.47 +/- 0.21 mumol/mg protein/hr in the absence of calmodulin. 3. Its activity was stimulated by calmodulin about 5-fold. The enzyme is also highly sensitive to inhibition by vanadate (Ki = 1.6 +/- 0.2 microM). 4. Calmodulin also affects the pH- and Ca2+-sensitivity of the enzyme. The optimum pH, in the presence of calmodulin, is 7.5 and the optimum temperature is 38 degrees C with an activation energy of 11.9 kcal/mol.
