Selectivity of digitalis glycosides for isoforms of human Na,K-ATPase.
There are four isoforms of the alpha subunit (alpha1-4) and three isoforms of the beta subunit (beta1-3) of Na,K-ATPase, with distinct tissue-specific distribution and physiological functions. alpha2 is thought to play a key role in cardiac and smooth muscle contraction and be an important target of cardiac glycosides. An alpha2-selective cardiac glycoside could provide important insights into physiological and pharmacological properties of alpha2. The isoform selectivity of a large number of cardiac glycosides has been assessed utilizing alpha1beta1, alpha2beta1, and alpha3beta1 isoforms of human Na,K-ATPase expressed in Pichia pastoris and the purified detergent-soluble isoform proteins. Binding affinities of the digitalis glycosides, digoxin, beta-methyl digoxin, and digitoxin show moderate but highly significant selectivity (up to 4-fold) for alpha2/alpha3 over alpha1 (K(D) alpha1 > alpha2 = alpha3). By contrast, ouabain shows moderate selectivity ( approximately 2.5-fold) for alpha1 over alpha2 (K(D) alpha1 <or= alpha3 < alpha2). Binding affinities for the three isoforms of digoxigenin, digitoxigenin, and all other aglycones tested are indistinguishable (K(D) alpha1 = alpha3 = alpha2), showing that the sugar determines isoform selectivity. Selectivity patterns for inhibition of Na,K-ATPase activity of the purified isoform proteins are consistent with binding selectivities, modified somewhat by different affinities of K(+) ions for antagonizing cardiac glycoside binding on the three isoforms. The mechanistic insight on the role of the sugars is strongly supported by a recent structure of Na,K-ATPase with bound ouabain, which implies that aglycones of cardiac glycosides cannot discriminate between isoforms. In conclusion, several digitalis glycosides, but not ouabain, are moderately alpha2-selective. This supports a major role of alpha2 in cardiac contraction and cardiotonic effects of digitalis glycosides.
Katz
Adriana
A
Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel.
Lifshitz
Yael
Y
Bab-Dinitz
Elizabeta
E
Kapri-Pardes
Einat
E
Goldshleger
Rivka
R
Tal
Daniel M
DM
Karlish
Steven J D
SJ
D002301:Cardiac Glycosides / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D004071:Digitalis Glycosides / Q000378:metabolism*; D006801:Humans; D007477:Ions; D007700:Kinetics; D008954:Models, Biological; D015233:Models, Statistical; D009206:Myocardium / Q000378:metabolism; D010843:Pichia / Q000378:metabolism; D011188:Potassium / Q000737:chemistry; D011485:Protein Binding; D020033:Protein Isoforms; D000254:Sodium-Potassium-Exchanging ATPase / Q000737:chemistry*; D013997:Time Factors
D002301:Cardiac Glycosides; D004071:Digitalis Glycosides; D007477:Ions; D020033:Protein Isoforms; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium
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