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

Acylphosphatase, purified from cardiac muscle, catalyzes the hydrolysi s of the phosphorylated intermediate of heart sarcolemmal Na+,K+-ATPas e, This effect was remarkable even using acylphosphatase amounts (100- 300 units/mg of membrane protein) near the lower limit of the physiolo gical range; besides the low value of the apparent K-m, on the order o f 10(-7) M, indicates that the enzyme has a high affinity for this spe cial substrate. The results of a dot-immunobinding assay suggest the p ossibility of an interaction between acylphosphatase and native, unden aturated Na+,K+-ATPase. Moreover, when added to sarcolemmal vesicles, acylphosphatase was found to affect the functional properties of the N a+,K+ pump with regard to the rate of both ATP hydrolysis and cation t ransport. However, while ATPase activity and Na+ uptake were stimulate d, 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 acylp hosphatase, rose to 6.68 in the presence of 300 units/mg of vesicle pr otein of this enzyme. Taken together, the reported results indicate th at acylphosphatase, because of its hydrolytic activity on the phosphoe nzyme intermediate, induces a sort of ''uncoupling'' effect on the hea rt sarcolemmal membrane Na+,K+ pump. Possible mechanisms for such an e ffect, which suggests a potential role of acylphosphatase in the contr ol of this active transport system, are discussed.