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.