Phosphorylation and dephosphorylation of the Ca2+ pump of human red cells in the presence of monovalent cations.

(1) In the presence of calcium ions, K+ increases the rate and the steady state level of phosphorylation of human red cell membranes by [gamma-32P)ATP. The effect of K+ is mimicked by Rb+, NH4+ and Cs+. Electrophoresis experiments suggest that the phosphorus taken up by the membranes in the presence of K+ is bound to the phosphoenzyme of the Ca2+-ATPase. (2) (Ca2+ + K+)-dependent phosphorylation requires Ca2+ and ATP with the same apparent affinity as the phosphorylation of the Ca2+ pump and the effect of K+ on phosphorylation is exerted with the same apparent affinity as that for the activation of the Ca2+-ATPase by K+. (3) The rate of hydrolysis of phosphoenzyme made in the presence of K+ is higher than that made in its absence and K+ increases the ratio Ca2+-ATPase activity/Ca2+-dependent phosphoenzyme concentration. (4) Results suggest that monovalent cations activate the Ca2+ pump because they increase the level and the turnover of the phosphoenzyme of the Ca2+-ATPase.