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Interactions between Cardiac Glycosides and Sodium/Potassium-ATPase: Three-Dimensional Structure−Activity Relationship Models for Ligand Binding to the E2-Pi Form of the Enzyme versus Activity Inhibition†
journal contribution
posted on 2005-01-18, 00:00 authored by Stefan Paula, Michael R. Tabet, William James BallSodium/potassium-ATPase (Na/K-ATPase) is a transmembrane enzyme that utilizes energy
gained from ATP hydrolysis to transport sodium and potassium ions across cell membranes in opposite
directions against their chemical and electrical gradients. Its transport activity is effectively inhibited by
cardiac glycosides, which bind to the extracellular side of the enzyme and are of significant therapeutic
value in the treatment of congestive heart failure. To determine the extent to which high-affinity binding
of cardiac glycosides correlates with their potency in inhibiting pump activity, we determined experimentally
both the binding affinities and inhibitory potencies of a series of 37 cardiac glycosides using radioligand
binding and ATPase activity assays. The observed variations in key structural elements of these compounds
correlating with binding and inhibition were analyzed by comparative molecular similarity index analysis
(CoMSIA), which allowed a molecular level characterization and comparison of drug−Na/K-ATPase
interactions that are important for ligand binding and activity inhibition. In agreement with our earlier
comparative molecular field analysis studies [Farr, C. D., et al. (2002) Biochemistry 41, 1137−1148], the
CoMSIA models predicted favorable inhibitor interactions primarily at the α-sugar and lactone ring moieties
of the cardiac glycosides. Unfavorable interactions were located about the γ-sugar group and at several
positions about the steroid ring system. Whereas for most compounds a correlation between binding affinity
and inhibitory potency was found, some notable exceptions were identified. Substitution of the five-membered lactone of cardenolides with the six-membered lactone of bufadienolides caused binding affinity
to decline but inhibitory potency to increase. Furthermore, while the removal of ouabain's rhamnose
moiety had little effect on inhibitory potency, it caused a dramatic decline in ligand binding affinity.