Dehydro-digitoxosides of digitoxigenin and digoxigenin: binding to beef heart (Na+ + K+)-ATPase in relation to unchanged digitoxosides

Binding of dihydrodigitoxin to beef and human cardiac (Na+ + K+)-ATPase: evidence for two binding sites in cell membranes

The specific binding of three cardiac glycosides, 3H-ouabain, 3H-digitoxin and 3H-dihydrodigitoxin, to beef cardiac (Na+ + K+)-ATPase was compared. Non-specific binding was defined as that in the presence of 0.1 mM unlabelled compound, or in the absence of ligands. The dissociation constants (KD-values) calculated from the inhibition of 3H-ouabain binding were: ouabain, 2.9 X 10(-9)M; digitoxin, 1.1 X 10(-9)M; and dihydrodigitoxin 2.7 X 10(-8)M. The concentrations which inhibited beef cardiac (Na+ + K+)-ATPase by 50% were: ouabain, 5.9 X 10(-9)M; digitoxin, 1.6 X 10(-9)M; and dihydrodigitoxin, 2.5 X 10(-8)M. Ouabain and digitoxin showed straight Scatchard plots for one site of high affinity (ouabain, KD = 2.6 X 10(-9)M; digitoxin, KD = 1.7 X 10(-9)M). However, dihydrodigitoxin gave a curved Scatchard plot. Analysis of this binding by the methods of M. J. Weidemann, H. Erdelt and M. Klingenberger (Eur. J. Biochem. 16, 313 (1970) for two binding sites gave the following results: for Mg2+,Pi-supported binding, the KD of the high affinity site was 1.6 X 10(-8)M with a capacity similar to that for ouabain of about 30 pmole/mg protein. For binding supported by Na+,ATP,Mg2+, the KD-value of the high affinity site was 5.3 X 10(-8)M of similar capacity. The low affinity binding site (KD = 4.0 X 10(-6)M for Mg2+,Pi; KD = 5.5 X 10(-6)M for Na+,ATP,Mg2+) bound about 350 pmole/mg protein. The low affinity site but not the high affinity site was also present in heat-denatured enzyme. Binding supported by Mg2+,Pi showed one low affinity site only for ouabain and dihydrodigitoxin in the presence of 200 mM Na+. The high affinity sites for these three cardiac glycosides were further characterized by measurement of the association and dissociation rate constants. The specific binding of 3H-ouabain and 3H-dihydrodigitoxin to human cardiac (Na+ + K+)-ATPase was measured. 3H-Ouabain showed a straight Scatchard plot for one high affinity site only (KD = 4.5 X 10(-9) M, capacity about 15 pmole/mg protein). 3H-Dihydrodigitoxin gave two binding sites: a high affinity site (KD = 1.8 X 10(-8) M) of similar capacity to ouabain, and a low affinity site (KD = 2.0 X 10(-6) M) of about 10-fold greater capacity.(ABSTRACT TRUNCATED AT 400 WORDS)

Digitalis structure-activity relationship analyses. Conclusions from indirect binding studies with cardiac (Na+ + K+)-ATPase

We have performed direct and indirect binding studies with [3H]ouabain or [3H]digitoxin on beef or guinea pig cardiac (Na+ + K+)-ATPase to measure the potencies of a broad range of cardiotonic steroids for structure-activity relationship (SAR) studies for comparison with previously determined positive inotropic potencies. The positive inotropic potencies of twelve compounds on contracting guinea pig left atria correlated well with the equilibrium dissociation constants (KD values) from the inhibition of [3H]ouabain binding to guinea pig cardiac (Na+ + K+)-ATPase (r = 0.98 for seven 5 beta-compounds, r = 0.95 for five 5 alpha-compounds). Further we calculated KD values from the inhibition of [3H]ouabain binding data for a total of 33 digitalis derivatives on the digitalis-sensitive beef cardiac (Na+ + K+)-ATPase. For the 27 compounds tested on both beef cardiac (Na+ + K+)-ATPase and guinea pig left atria, the potencies showed a significant correlation (r = 0.92 for 22 5 beta-compounds, r = 0.96 for five 5 alpha-compounds. For seven compounds, KD values were measured on beef cardiac (Na+ + K+)-ATPase using inhibition of binding of [3H]digitoxin. These values correlated well (r = 0.99) with the KD values from the [3H]ouabain studies. These results show that: (1) The significant correlation observed between KD values on guinea pig cardiac (Na+ + K+)-ATPase and positive inotropic potency in guinea pig left atria is further evidence that the pharmacological receptor for inotropy is part of the enzyme, (2) Inhibition of the binding of [3H]ouabain or [3H]digitoxin can be used to determine the relative potencies of unlabelled digitalis derivatives. The similar relative potencies on beef and guinea pig cardiac (Na+ + K+)-ATPase of a broad range of digitalis derivatives indicate that the binding site is similar for both species; and (3) SAR studies indicate that functional groups on these steroids have the same influence on potency on either the positive inotropy or cardiac (Na+ + K+)-ATPase studies.