Molecular Pharmacology, Vol 6, 680-690, Copyright © 1970 by the American Society for Pharmacology and Experimental Therapeutics

Inhibition of Sodium- and Potassium-Dependent Adenosine Triphosphatase by Ethacrynic Acid: Two Modes of Action

¹ Department of Pharmacology, Faculty of Medicine, University of Toronto, Toronto 181, Ontario, Canada

Ethacrynic acid inhibited sodium- and potassim-dependent adenosine triphosphatase (ATP phosphohydrolase, EC 3.6.1.3) by affecting two different steps of enzyme turnover. First, ethacrynic acid blocked phosphorylation of (Na⁺ + K⁺)-ATPase with insignificant effect upon the dephosphorylation step when the Na⁺:K⁺ ratio was 10. It also prevented the ADP-ATP exchange reaction. The reduction in specific activity of the enzyme by ethacrynic acid was closely correlated with the degree of inhibition of phosphorylation and ADP-ATP exchange. Second, ethacrynic acid stabilized the spontaneous disappearance of the phosphorylated intermediate and slightly decreased the apparent affinity for K⁺. The rate of decay of the phosphorylated intermediate in the presence of ADP was not significantly affected by ethacrynic acid treatment. The decrease in apparent affinity for K⁺ could not be observed in an assay medium with an Na⁺:K⁺ ratio of 4. When the concentrations of Na⁺ and K⁺ were changed to those found in extracellular fluid, ethacrynic acid-treated enzyme showed a 20-30% decrease in specific activity as compared to the usual assay system. The inhibition of phosphorylation by ethacrynic acid appears to be of significance for inhibition of enzyme activity in vitro, while the stabilization of the phosphorylated intermediate may be involved in drug-induced diuresis.

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ACKNOWLEDGMENTS We are grateful to Professors W. Kalow and H. Kalant for helpful criticism in the preparation of this manuscript. We wish to thank Mrs. S. M. E. Wong for valuable technical assistance. We are indebted to Dr. J. E. Baer for his generous supply of ethacrynic acid.