Commercial ATP containing traces of vanadate alters the response of (Na⁺ + K⁺)ATPase to external potassium

Abstract

WE have reported that the puzzling inhibition of (Na⁺ + K⁺)ATPase preparations by K⁺ ions, at concentrations insufficient to displace Na⁺ ions from the intracellular Na⁺-loading sites, was observed only when Sigma ‘Sigma-grade’ ATP was used as a substrate¹. The effect was attributed to a contaminant of the ATP, which bound to the enzyme reversibly but with a high affinity; and because ‘Sigma-grade’ ATP is unique in being prepared from muscle, we suggested that the inhibitor might have a physiological role. Similar conclusions were reached independently by Cantley and Josephson^2,3, and they and their colleagues have now identified the contaminant as ortho-vanadate⁴. The inhibition of (Na⁺ + K⁺)ATPase by vanadate plus K⁺ ions is of interest both for its possible physiological significance—traces of vanadate occur widely in animal tissues⁵—and for its possible use in elucidating the mechanism of the sodium pump. From both points of view it is desirable to know whether the K⁺ ions act at the intracellular or extracellular surface of the membrane, and we report here experiments on resealed red cell ghosts showing that the action of K⁺ ions is at the extracellular surface. In the following paper, Cantley et al.⁶ describe experiments with intact red cells suggesting that the vanadate ions act at the intracellular surface.