Effects of vanadate on ouabain binding and inhibition of (Na+ + K+)-ATPase

Effects of vanadate on ouabain binding and inhibition of sodium and potassium adenosine triphosphatase (Na+ + K+)-ATPase) were investigated under various ionic conditions. 1. Vanadate facilitated ouabain binding to (Na+ + K+)-ATPase in the presence of Mg2+ and this facilitation was partially reversed by catechol. 2. Vanadate antagonized the ability of high concentrations of NaCl to inhibit ouabain binding in the presence of magnesium. 3. Ouabain binding to the vanadate-enzyme complex, formed from magnesium and vanadate, was more sensitive to depression by potassium than that to the phosphoenzyme formed from magnesium and inorganic phosphate. 4. Preincubation of (Na+ + K+)-ATPase with vanadate in the presence of magnesium initially formed a potassium-insensitive complex as shown by a rapid initial rate of ouabain binding. However, within 5 min potassium overcame the vanadate potentiation of ouabain binding regardless of the order in which it was added to the reaction mixture. 5. Under conditions of enzyme turnover, vanadate failed to antagonize the inhibitory power of ouabain despite the presence of a high concentration of potassium. This suggests a possible relationship between the sensitivity of the sodium pump in various tissues to the cardiac glycosides and intracellular vanadate concentrations.