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Molecular Pharmacology, Vol 13, 352-361, Copyright © 1977 by the American Society for Pharmacology and Experimental Therapeutics

Association and Dissociation Rate Constants of the Complexes between Various Cardiac Aglycones and Sodium- and Potassium-Dependent Adenosine Triphosphatase Formed in the Presence of Magnesium and Phosphate

ATSUNOBU YODA 1 and SHIZUKO YODA 1

1 Department of Pharmacology, University of Wisconsin-Madison, Madison, Wisconsin 53706

Association and dissociation rates of cardiac aglycone-(Na+ + K+-ATPase complexes formed in the presence of magnesium and inorganic phosphate were examined by assay of the phosphorylated protein formed in the presence of [ggr-32P]ATP, which is influenced by the amount of bound aglycone. Association and dissociation followed pseudo-first-order and first-order rate kinetics, respectively. The dissociation rate constants of four cardiac aglycones-digitoxigenin, digoxigenin, strophanthidin, and ouabagenin-were all the same (0.28 min-1 at 25° and 0.63 min-1 at 30°), but their pseudo-first-order association rate constants (ka' varied. Both Mg2+ and P1 gave linear relationships against ka' in double-reciprocal plots over a wide range of concentrations, and the effects of both ligands were identical. The four cardiac aglycones showed the same maximum association rates with increasing drug concentration. These results suggest that the binding of ligands (magnesium and phosphate) results in activation of the enzyme to bind the cardiac aglycone, but that dissociation of ligands from the cardiac aglyconeenzyme complex precedes the release of cardiac aglycone.

Note:
ACKNOWLEDGMENTS We thank Dr. Lowell E. Hokin for his kind help with the manuscript, and Mrs. Mary Lochner, for preparation of beef microsomes.

Submitted on July 5, 1976
Accepted on November 9, 1976




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