Inhibition by amine bases or by sodium ions and protection by divalent cations in the hydrolysis of phosphoenzyme of (Na,K)-ATPase.

Abstract

In the absence of K+, the hydrolysis of low-energy phosphoenzyme (E2P) of (Na,K)-ATPase, but not Ca-ATPase of sarcoplasmic reticulum, was inhibited by Na+ or by amine compounds, such as Tris, imidazole, arginine, or lysine, with half-maximum inhibition concentrations of millimolar order at pH 7.4 and 0 degrees C. Histidine was slightly inhibitory. Divalent cations alone were also inhibitors. However, divalent cations, especially Ca2+ or Mn2+, were apparently activators in the presence of another inhibitor, Na+, or an amine compound, probably because the inhibitory action of the divalent cations was less powerful than that of Na+ or amines, which appear to compete for the same binding site on E2P. In most reported experiments on the hydrolysis of the phosphoenzyme, Tris or imidazole (as a buffer component) and Na+ (to obtain a sufficient amount of phosphoenzyme) have been present in the reaction mixture in amounts that are now seen to be inhibitory. Consequently, the reactivity of the phosphoenzyme with water would have been substantially underestimated. Under these inhibitory conditions, however, reactivity with K+ or ADP was little influenced.