Abstract
(1) In the presence of calcium ions, K + increases the rate and the steady state level of phosphorylation of human red cell membranes by [ γ- 32 P] ATP . The effect of K + is mimicked by Rb +, NH 4 + 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 Ca 2+-ATPase. (2) (Ca 2+ + K +)-dependent phosphorylation requires Ca 2+ and ATP with the same apparent affinity as the phosphorylation of the Ca 2+ pump and the effect of K + on phosphorylation is excerted with the same apparent affinity as that for the activation of the Ca 2+-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 Ca 2+-ATPase activity/Ca 2+-dependent phosphoenzyme concentration. (4) Results suggest that monovalent cations activate the Ca 2+ pump because they increase the level and the turnover of the phosphoenzyme of the Ca 2+-ATPase.
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