RT Journal Article SR Electronic T1 Structure-Activity Relationships of Cardiotonic Steroids for the Inhibition of Sodium- and Potassium-Dependent Adenosine Triphosphatase JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 766 OP 773 VO 9 IS 6 A1 ATSUNOBU YODA A1 SHIZUKO YODA A1 AWNI M. SARRIF YR 1973 UL http://molpharm.aspetjournals.org/content/9/6/766.abstract AB The association rate constants (ka) of cardiac glycoside-(Na+ + K+)-ATPase (EC 3.6.1.3) complexes at 25° in presence of magnesium and phosphate and of sodium, magnesium, and ATP were determined by enzymatic assay after dilution. Among the various cardiac monoglycosides, ka was dependent on the nature of the steroid moiety (aglycone) but not on the sugar moiety in both ligand systems, in contrast to the dissociation rate constants of the complexes formed in the system containing magnesium and phosphate. The order of ka values was the same as the order of I50 values for different steroids in the assay system. Digitoxigenin competed with ouabain in both ligand systems. These results suggest that association reverses the sequence of the dissociation reaction, as follows: association of the steroid moiety of the cardiac glycoside with the steroid-specific site of the enzyme, conformational change of the sugar-specific site, and association of the sugar moiety with the sugar-specific site. In the case of cardiac glycosides, the number of sugars in the oligosaccharide moiety produced a corresponding bulk effect on the association rate constants in both ligand systems. For a gie the association rate constants decreasven aglyconed in the following order: monodigitoxide > bisdigitoxide > tridigitoxide > tetraidigitoxide. ACKNOWLEDGMENT We thank Dr. Lowell E. Hokin for his interest and thoughtful help with the manuscript.