Characterization of the phosphorylated intermediate of the isolated high-affinity (Ca2+ + Mg2+)-ATPase of human erythrocyte membranes

Phosphorylation of solubilized and purified high-affinity (Ca²⁺ + Mg²⁺)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) of human erythrocyte membranes shows no dependence on cyclic AMP concentration in the range 0.1–1000 μM.Ca²⁺-dependent phosphoprotein is sensitive to hydroxylamine and molybdate treatment. The phosphate linkage shows maximum stability at low pH values, which is progressively lost as the pH rises, with a shoulder around pH 6. SDS gel electrophoresis of the phosphorylated protein yields a peak which shows relative mobility corresponding to a molecular weight of 145 000 and sensitivity to MgATP-chase and hydroxylamine treatment. This indicates that the phosphoprotein represents the phosphorylated intermediate of the high-affinity (Ca²⁺ + Mg²⁺)-ATPase of human erythrocyte membranes.