Association and dissociation rate constants of the complexes between various cardiac aglycones and sodium- and potassium-dependent adenosine triphosphatase formed in the presence of magnesium and phosphate
Yoda, A.; Yoda, S.
Molecular Pharmacology 13(2): 352-361
1977
ISSN/ISBN: 0026-895X PMID: 140301 Document Number: 123088
Association and dissociation rates of cardiac aglycone-(Na+ + K+)-ATPase [[EC 3.6.1.3] from beef brain] complexes formed in the presence of Mg and Pi were examined by assay of the phosphorylated protein formed in the presence of .gamma.-32P-ATP, which is influenced by the amount of bound aglycone. Association and dissociation followed pseudo-1st-order and 1st-order rate kinetics, respectively. The dissociation rate constants of 4 cardiac aglycones-digitoxigenin, digoxigenin, strophanthidin and ouabagenin-were all the same (0.28 min-1 at 25.degree. C and 0.63 min-1 at 30.degree. C), but their pseudo-1st-order association rate constants (K.alpha.') varied. Both Mg2+ and Pi gave linear relationships against k.alpha.' in double-reciprocal plots over a wide range of concentrations, and the effects of both ligands were identical. The 4 cardiac aglycones showed the same maximum association rates with increasing drug concentration. The binding of ligands (Mg and phosphate) appears to result in activation of the enzyme to bind the cardiac aglycone, but dissociation of ligands from the cardiac aglycone-enzyme complex appears to precede the release of cardiac aglycone.