The thermodynamic essence of the reversible inactivation of Na+/K+-transporting ATPase by various digitalis derivatives is relaxation of enzyme conformational energy.

This paper reports on the kinetic and thermodynamic parameters describing the interaction of selected digitalis derivatives with hog and guinea-pig cardiac (Na+ + K+)-ATPase (Na+/K+-transporting ATPase EC 3.6.1.37). 32 digitalis derivatives were characterized as to the values of the delta G0', delta G----not equal to, and delta G----not equal to quantities in their interaction with (Na+ + K+)-ATPase from hog cardiac muscle in the presence of ATP, Mg2+, Na+ and K+. Nine derivatives were additionally characterized as to the values of the delta H0', delta S0', delta H----not equal to, delta S----not equal to, delta H not equal to, and delta S not equal to quantities in their interaction with the hog enzyme promoted by ATP, Mg2+ and Na+ in the presence or absence of K+. The formation of the inhibitory complexes is in any case an endothermic, entropically driven process. The Gibbs energy barriers in the formation and dissociation of the complexes, delta G----not equal to and delta G----not equal to, are imposed by large, unfavourable delta H not equal to values. K+ decreases the delta G0' value by increasing the delta G----not equal to value more than the delta G----not equal to value. In comparison with hog (Na+ + K+)-ATPase, the interaction of three derivatives with guinea-pig cardiac enzyme in the presence of ATP, Mg2+, Na+ and K+ is characterized by lower delta G0' values caused by lower favourable delta S0' values, and is accompanied by lower delta G----not equal to values. The magnitude of the kinetic parameters and the characteristic of the thermodynamic quantities describing the interaction between various digitalis derivatives and (Na+ + K+)-ATPase, indicate the induction of substantial conformational changes in the enzyme protein. A large entropy gain in the enzyme protein, observed irrespective of enzyme origin and ligation, appears to be the common denominator of the inhibitory action of all digitalis derivatives studied, suggesting that the digitalis-elicited relaxation of high conformational energy (negentropy strain) of the enzyme protein is the thermodynamic essence of the reversible inactivation of (Na+ + K+)-ATPase.

Interaction of cardiac glycosides and Na,K-ATPase is regulated by effector-controlled equilibrium between two limit enzyme conformers.

The paper describes the dissociation parameters of the complexes between [3H]-digitoxin and Na,K-ATPase (Na+ + K+-activated, Mg2+-dependent ATP phosphohydrolase, E.C. 3.6.1.3) from pig cardiac muscle and brain cortex formed and dissociated in the presence of different combinations and concentrations of the enzyme effectors ATP, Mg2+, Na+ and K+. Systematic variation of effector-ligation of Na,K-ATPase allowed production of glycoside complexes with two enzyme conformers only, which showed either rapid or slow dissociation kinetics. Appropriate changes of enzyme ligation allowed the interconversion of the two conformer types. Biphasic, rapid and slow glycoside release was not bound with the presence of two Na,K-ATPase isozymes, but caused by the enzyme ligation-determined coexistence of the two conformers of Na,K-ATPase. The rate constants for the rapid and slow glycoside release were within the complexes of each dissociation type much alike indicating uniform isomerization kinetics of the two conformers even when differently liganded. Taken together, the observations indicated the effector-controlled isomerizations of two conformers of Na,K-ATPase possessing different geometries of the glycoside binding domain. Present findings and relevant literature data were integrated in a circular, consecutive and simultaneous model for induced conformation changes that accounted for the regulation of the interaction of cardiac glycosides and Na,K-ATPase through an effector-controlled equilibrium between two limit enzyme conformers.

Penta-acetyl-gitoxin: the prototype of a prodrug in the cardiac glycoside series.

Penta-acetyl-gitoxin is a suitable prodrug of gitoxin since it shows--side-effect latentiation, due to its inactive application form;--bioavailability, due to its improved solubility and thus good enteral absorption and--bioactivation, due to rapid de-acetylation in the body after absorption.