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EVIDENCE FOR TWO DIFFERENT Na + ‐DEPENDENT [ 3 H]‐OUABAIN BINDING SITES OF A Na + ‐K + ‐ATPASE OF GUINEA‐PIG HEARTS
Author(s) -
FRICKE U.,
KLAUS W.
Publication year - 1977
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1977.tb08435.x
Subject(s) - ouabain , chemistry , atpase , binding site , triphosphatase , guinea pig , biophysics , membrane , stereochemistry , biochemistry , sodium , biology , enzyme , endocrinology , organic chemistry
1 The influence of various Na + concentrations on [ 3 H]‐ouabain binding was studied in experiments on a microsomal Na + ‐K + ‐adenosine triphosphatase (ATPase) from guinea‐pig hearts. 2 The ATP‐independent cardiac glycoside binding was not influenced by increasing Na + concentrations. However, a good correlation was found between the ATP‐dependent [ 3 H]‐ouabain binding and Na + concentration. 3 A more detailed analysis of these results according to Hofstee (1952) revealed two distinct processes involved in this interaction: one ouabain binding process was activated at rather low Na + concentrations, ( K 0.5 = 4.5 mM); this type of [ 3 H]‐ouabain binding was strongly correlated to the Na + concentration necessary for half maximum phosphorylation ( K 0.5 = 1 mM). The other ouabain binding process was predominant at high Na + concentrations (K 0.5 = 69 mM). 4 On the basis of the commonly accepted ATPase reaction cycle a model for the interaction of cardiac glycosides with the Na + ‐K + ‐ATPase is proposed, assuming two different binding sites for cardiac glycosides (E 2 ‐P and E 1 ‐P) and involving a translocation of these drugs from an outer to an inner compartment of the cell membrane.