Human erythrocyte membranes were preincubated with ethyleneglycolbis-(beta-aminoethyl)-N,N' tetraacetate (EGTA) and subsequently labelled for short periods with micromolar concentrations of [8-3-H, gamma-32-P]ATP. Under these conditions, and at temperatures smaller than or equal to 22 degrees C, both ATP hydrolysis and membrane phosphorylation were stimulated by Ca-2+. 2. The properties of the Ca-2+-stimulated ATP hydrolysis and associated phosphorylation of a 150 000 molecular weight protein component, previously described (Knauf, P. A., Proverbio, F. and Hoffman, J. F. (1974) J. Gen. Physiol. 63, 324-336), have been studied. The behavior of the phosphorylated component, ECaP, has properties consistent with its role as a phosphorylated intermediate of Ca-2+-ATPase activity, including: (1) similar dependence of the steady-state level of ECaP and Ca-2+-ATPase on ATP concentration; (2) rapid turnover apparent upon the addition of excess non-radioactive ATP; and (3) good correlation between the steady-state levels of Ca-2+-dependent phosphorylation and Ca-2+-ATPase activity in separate preparations possessing variable specific activity. Addition of excess EGTA to ECaP caused only partial dephosphorylation. Sensitivity of Ca-2+-stimulated ATP hydrolysis and associated phosphorylation to micromolar concentrations of Ca-2+ implicates this activity in the "high-affinity" Ca-2+-pump system of the human erythrocyte (Schatzmann, H. J. (1973) J. Physiol. London 235, 551-569).
