Stimulating effects of monovalent cations on activator-dissociated and activator-associated states of Ca 2+-ATPase in human erythrocytes

Abstract

The additional activation by monovalent cations of the (Ca 2+ + Mg 2+)-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied. The Ca 2+-ATPase occurs in two different states. In the A-state the enzyme is virtually free of protein activator and the kinetics of Ca 2+ activation is characterized by low apparent Ca 2+ affinity and low maximum activity. In the B-state the enzyme is associated with activator and the kinetics is characterized by high Ca 2+ affinity and high maximum activity. At optimum concentrations of Ca 2+ the additional activation of the B-state by K +, NH 4 +, Na + and Rb + exceeded the corresponding activations of the A-state, and half-maximum activations by K +, NH 4 +, and Na + were achieved at lower concentrations in the B-state than in the A-state. Li + and Cs + activated the two states almost equally but maximum activation was obtained at lower cation concentrations in the B-state than in the A-state. The activation of the B-state by the various cations decreased in the order K + > NH 4 + > Na + = Rb + > Li + = Cs + . The A-state was activated almost equally by K +, Na +, NH 4 +, and Rb + and to a smaller extent by Li + and Cs +. At sub-optimum concentrations of Ca 2+ high concentrations of monovalent cations (100 mM) activated the Ca 2+-ATPase equally in the A-state and the B-state. In the absence of Ca 2+ the monovalent cations inhibited the Mg 2+-dependent ATPase in both types of membranes. This dependence on Ca 2+ indicates that the monovalent cations interact with the Ca 2+ sites in the B-state. The results suggest that K + or Na +, or both, contribute to the regulation of the Ca 2+ pump in erythrocytes.