Molecular Pharmacology, Vol 10, 810-819, Copyright © 1974 by the American Society for Pharmacology and Experimental Therapeutics

Influence of Certain Ligands on the Dissociation Rate Constants of Cardiac Glycoside Complexes with Sodium- and Potassium-Dependent Adenosine Triphosphatase

¹ Department of Pharmacology, University of Wisconsin Medical School, Madison, Wisconsin 53706

The influence of certain ligands on the dissociation rate constants (k_d) of various cardiac monoglycoside-(Na⁺ + K⁺)-ATPase complexes was examined. If the complex was formed in the presence of sodium, magnesium, and ATP (type I complex), potassium ion reduced the k_d to the value of the complex formed in the presence of magnesium and phosphate (type II complex). Potassium also changed other dissociation characteristics of the type I complex to those of the type II complex: in the presence of potassium the k_d value became exclusively dependent on the nature of the sugar moiety of the cardiac glycoside. This potassium effect was not reversed by dilution. Sodium could reverse the potassium effect and increase the k_d value of the type I complex. Dissociation of the type II complex was only slightly influenced by potassium, sodium, or ATP alone. Sodium plus ATP increased the k_d value of the type II complex, and the dissociation characteristics of the type II complex were changed to those of the type I complex. This effect of sodium plus ATP was just opposite to the potassium effect, but it was reversible. Since 1,2-cyclohexylenedinitrilotetraacetic acid did not alter the effects of potassium on sodium plus ATP, such changes in the types of cardiac glycoside-enzyme complexes might not involve magnesium. The similarity of these ligand effects on the cardiac glycoside-(Na⁺ + K⁺)-ATPase complex to their effects on the native enzyme suggests that the inhibited enzyme complex still retains some portion of the enzyme activity.

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ACKNOWLEDGMENTS We thank Dr. Lowell E. Hokin for his kind help with the manuscript.