Effects of alkali metal cations on phospho-enzyme levels and [ 3H]ouabain binding to (Na + + K +)-ATPase

Abstract

The effects of several alkali metal cations on the relationship between steady state phospho-enzyme levels and initial velocity and equilibrium levels of [ 3H]-ouabain binding to (Na + + K +)-ATPase (ATP phosphohydrolase EC 3.6.1.3.) were examined. Only Na + increased both phospho-enzyme and [ 3H] ouabain binding livels above those observed in the presence of Mg 2+ alone. While Na + stimulated phosphorylation with an apparent K m of about 1 mM, its stimulation of [ 3H] ouabain binding was biphasic, the lower K m for stimulation corresponding to the K m for formation of phospho-enzyme. Among the other alkali metal cations, potassium, rubidium and lithium were at least eight times more effective in reducing phospho-enzyme levels than in reducing [ 3H] ouabain binding. This discrepancy is not due to the stability of the enzyme-ouabain complex, nor to any action on the rates of formation or dissociation of the enzyme-ouabain complex. The data thus suggest that [ 3H] ouabain interacts with the K +, Rb + or Li +-enzyme complexes. For Li +, this hypothesis is further supported by the observation that Li + can directly increase the equilibrium level of [ 3H]-ouabain binding to this enzyme under certain conditions.