Modification of the Rate of Ouabain Binding to (Na++ K+)ATPase by Lithium Ions

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Abstract
The interactions of Li+, a congener of K+ with the (Na+ + K+)-ATPase from Electrophorus electricus, as measured by their effects on the rate of [3H]-ouabain binding to this enzyme are reported. Like K+, Li+ slows ouabain binding under both Type I (Na+ + ATP) and Type II (Pi) conditions but with lower affinity. In contrast to K+, the Li+ inhibition curve is hyperbolic, suggesting interaction at an uncoupled site. Differing from the complete inhibition by high K+, a residual ouabain-binding rate persists at high Li+. The interactions of Li+ and K+ are synergistic; the apparent K+ affinity increases 3- to 4-fold in the presence of Li+. Li+ interacts with 1 of the 2 K+ sites and may be of interest in interpreting Li pharmacology [in treating human affective psychoses].