Effect of Na+ and nucleotide on the stability of solubilized Ca(2+)-free Ca-ATPase from scallop sarcoplasmic reticulum.

In membranous scallop sarcoplasmic reticulum, the alkali metal cations Na+ and K+ and nucleotide together promote dimer formation by the Ca(2+)-free Ca-ATPase and stabilize the enzyme activity [Kalabokis, V. N., Bozzola, J. J., Castellani, L., & Hardwicke, P. M. D. (1991) J. Biol. Chem. 266, 22044-22050]. The dependence of stabilization of the Ca(2+)-free membranous scallop Ca-ATPase on Na+ concentration does not show saturation and may involve several superimposed effects. In order to assess the contribution of dimer toward stabilization, i.e., determine the relative importance of intra- and intermolecular effects on stabilization, the influence of varying Na+ concentration and nucleotide on the decay of enzyme activity of the Ca(2+)-free detergent-solubilized Ca-ATPase was studied. Loss of enzyme activity on removal of Ca2+ with EGTA was associated with loss of capacity for phosphorylation by ATP, a Ca(2+)-dependent function. Stabilization of the soluble Ca(2+)-free enzyme by Na+ showed major differences from that seen with the membranous enzyme. The extent of stabilization of the Ca(2+)-free soluble enzyme by Na+ showed clear saturation with increasing Na+ concentration. In contrast to the Ca(2+)-free membranous enzyme, which is inactivated at pH 7.0 with biphasic first-order kinetics, loss of enzymatic function by the solubilized Ca-ATPase at pH 6.92, 0 degrees C, followed monophasic first-order kinetics.(ABSTRACT TRUNCATED AT 250 WORDS)