Molecular Pharmacology, Vol 15, 43-48, Copyright © 1979 by the American Society for Pharmacology and Experimental Therapeutics

Effects of Pb⁺⁺ and other Divalent Cations on Ouabain Binding to E. electricus Electroplax (Na⁺ + K⁺)-Adenosinetriphosphatase

¹ Neurology Research Laboratory, Neurology Department, University of Michigan Medical School, Ann Arbor, Michigan 48109

PbCl₂ reduces the level of [³H]ouabain binding to (Na⁺ + K⁺)-adenosinetriphosphatase in E. electricus electroplax microsomal preparations in the presence of Mg⁺⁺ and ATP with or without Na⁺. The inhibition is competitive with ATP, and Na⁺ is cooperative with Pb⁺⁺ in reducing the affinity for ATP at a site involved in ouabain binding. The ATP site involved in ouabain binding is not the substrate site since no such cooperative interactions among Pb⁺⁺, Na⁺, and ATP could be observed with regard to phosphorylation or inhibition of hydrolysis. ADP also stimulates ouabain binding, which is inhibited competitively by Pb⁺⁺. Cd⁺⁺, Cu⁺⁺, Hg⁺⁺, and Zn⁺⁺ also inhibit ouabain binding in the presence of Mg⁺⁺, ATP, with or without 50 mM Na⁺ but only the inhibition by Cd⁺⁺ is potentiated by 50 mM Na⁺. BaCl₂, FeCl₂, CaCl₂, MnC1₂, NiCl₂, CoC1₂, and SrCl₂ at concentrations of 60 µM did not inhibit drug binding.