ACYLPHOSPHATASE: A POTENTIAL MODULATOR OF HEART SARCOLEMMA NA+,K+ PUMP

Acylphosphatase, purified from cardiac muscle, catalyzes the hydrolysis of the phosphorylated intermediate of heart sarcolemmal Na+,K+-ATPase. This effect was remarkable even using acylphosphatase amounts (100-300 unitslmg of membrane protein) near the lower limit of the physiological range; besides the low value of the apparent Kmr on the order of lo-' M, indicates that the enzyme has a high affinity for this special substrate. The results of a dot-immunobinding assay suggest the possibility of an interaction between acylphosphatase and native, undenaturated Na+,K+-ATPase. Moreover, when added to sarcolemmal vesicles, acylphosphatase was found to affect the functional properties of the Na+,K+ pump with regard to the rate of both ATP hydrolysis and cation transport. However, while ATPase activity and Na+ uptake were stimulated, the last at a greater extent, the active Kf transport was inhibited, so that the Na+/K+ ratio, which was calculated as 1.50 without acylphosphatase, rose to 6.68 in the presence of 300 unitslmg of vesicle protein of this enzyme. Taken together, the reported results indicate that acylphosphatase, because of its hydrolytic activity on the phosphoenzyme intermediate, induces a sort of "uncoupling" effect on the heart sarcolemmal membrane Na+,K+ pump. Possible mechanisms for such an effect, which suggests a potential role of acylphosphatase in the control of this active transport system, are discussed.