Exchange between inorganic phosphate and adenosine triphosphate in (Na+,K+)-ATPase

(Na+,K+)-ATPase is able to catalyze a continuous ATP in equilibrium Pi exchange in the presence of Na+ and in the absence of a transmembrane ionic gradient. At pH 7.6 the Na+ concentration required for half-maximal activity is 85 mM and at pH 5.1 it is 340 mM. In the presence of optimal Na+ concentration, the rate of exchange is maximal at pH 6.0 and varies with ADP and Pi concentration in the assay medium. ATP in equilibrium Pi exchange is inhibited by K+ and by ouabain.