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Electrophorus electricus sodium potassium atpase ec evidence for simultaneous sodium and potassium binding in the presence of lead

, : Electrophorus electricus sodium potassium atpase ec evidence for simultaneous sodium and potassium binding in the presence of lead. Journal of Biological Chemistry 255(9): 3935-3943

The effects of Na+, K+ and MgCl2 on steady state levels of phosphorylation of E. electricus electroplax (Na+ + K+)-ATPase are studied with and without Pb2+. Pb2+ stimulates enzyme phosphorylation in the absence of Na+ while inhibiting hydrolysis activity (Siegel, G.J., and Fogt, S.K. Pb2+ may be a useful probe in studying the interaction of Na+ with other ligands by their effects on phosphoenzyme. In the presence of Pb2+ and 3 mM MgCl2, phosphorylation levels are reduced only by including Na+ and K+, and not either cation alone. The phosphorylation is in a steady state at all concentrations of Na+ and K+ used and is reduced by Na+ plus K+ to the same levels in the presence of Pb2+ as found when turnover is maximal in the absence of Pb2+. A model based on simultaneous binding of Na+ and K+ to Pb .cntdot. enzyme is the only compatible possibility considered. K+ increases [MgCl2]0.5 as measured by phosphorylation in the presence of Pb2+. The competitive inhibition is completely overcome by 3 mM MgCl2. Inclusion of 100 mM Na+ completely prevents the change in [MgCl2]0.5 produced by K+, but permits a noncompetitive type of inhibition produced by Na+ plus K+ that is only partially relieved by increasing MgCl2. Two kinetically distinguished loci for Na+-K+ interactions apparently determine the steady state level of phosphorylation: one at which K+ is antagonistic to Na+ and Mg2+ (or MgATP2+) and one at which K+ is synergistic with Na+ and possibly antagonistic to Mg2+ (or MgATP2+). NH4+, Rb+, Cs+, but not Li+ can substitute for K+, but not for Na+ in reducing phosphorylation levels in the presence of Pb2+ and 3 mM MgCl2. The effects of these cations on native (Na+ + K+)-ATPase were similar. In the absence of Pb2+, K+ reduces the affinity for Na+ as measured by Na+-phosphorylation, but Na+ increases the affinity for K+ to the same extent at high and low [MgCl2], as measured by K+ dephosphorylation. Na+ potentiated by MgCl2 reduces the maximal K+ effect. The 2 loci for Na+-K+ interactions (antagonistic and synergistic) are consistent with these properties of the native enzyme and suggest Na+ regulates a balance between K+-MgCl2 interaction strengths at both sites. A model incorporating consecutive and simultaneous reactions of Na+ and K+ at different portions of the enzyme cycle appears best.

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