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

Summary (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 forK _m (for ATP) of 1–2m 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 affectingK _m 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 1mm substrate concentration is several times faster than maximal rate of (Ca²⁺+Mg²⁺)-ATPase in red cell membranes.