Role of magnesium in the (Ca2+ + Mg2+)-stimulated membrane ATPase of human red blood cells.

(Ca2+ + Mg2+)-stimulated ATPase of human red cell membranes as a function of ATP concentration was measured at fixed Ca2+ concentration and at two different but constant Mg2+ concentrations. Under the assumption that free ATP rather than Mg-ATP is the substrate, a value for Km (for ATP) of 1-2 micron is found which is in good agreement with the value obtained in the phosphorylation reaction by A.F. Rega and P.J. Garrahan (1975. J. Membrane Biol. 22:313). Mg2+ increases both the maximal rate and the affinity for ATP, whereas Ca2+ increases the maximal rate without affecting Km for ATP. As a by-product of these experiments, it was shown that after thorough removal of intracellular proteins the adenylate kinase reaction at approximately 1 mM substrate concentration is several times faster than maximal rate of (Ca2+ + Mg2+)ATPase in red cell membranes.