Vanadate inhibits the red cell (Na⁺, K⁺) ATPase from the cytoplasmic side

Abstract

THE enzyme (Na⁺, K⁺)ATPase exists in the plasma membrane of animal cells and maintains a high intracellular K⁺-to-Na⁺ ratio by coupling movement of these ions to the hydrolysis of ATP. This enzyme spans the plasma membrane¹ and is asymmetrically positioned with the ATP hydrolysing site on the cytoplasmic side². Cardiac glycosides inhibit the (Na⁺, K⁺)-ATPase by binding to the part of the enzyme facing the extracellular fluid³. Although there is considerable evidence that hormonal regulation of (Na⁺, K⁺)ATPase occurs in certain tissues^4–8, no endogenous regulatory agents of the purified enzyme have been identified. Recently we discovered that vanadate (vanadium in the +5 oxidation state) inhibits purified (Na⁺, K⁺)ATPase from several tissues at concentrations of 10⁻⁷–10⁻⁸ M (refs 9,10). Vanadium has been established as a nutritional requirement in chickens and rats, and our estimates of the concentration of vanadium in muscle tissue (10⁻⁶–10⁻⁷ M) agree with concentrations measured in other tissues¹¹. It is known that vanadate has properties similar to those of the phosphate ion¹², and it therefore seemed likely that it might inhibit the enzyme by binding to the hydrolysis site which is on the cytoplasmic surface of the enzyme. We demonstrate here that vanadate is transported into the red cell and inhibits the (Na⁺, K ⁺)ATPase from the cytoplasmic side.