Ion selectivity of the cytoplasmic binding sites of the Na,K-ATPase: II. Competition of various cations.

In the E1 state of the Na,K-ATPase all cations present in the cytoplasm compete for the ion binding sites. The mutual effects of mono-, di- and trivalent cations were investigated by experiments with the electrochromic fluorescent dye RH421. Three sites with significantly different properties could be identified. The most unspecific binding site is able to bind all cations, independent of their valence and size. The large organic cation Br2-Titu3+ is bound with the highest affinity (<microM), among the tested divalent cations Ca2+ binds the strongest, and Na+ binds with about the same equilibrium dissociation constant as Mg2+ (approximately 0.8 mM). For alkali ions it exhibits binding affinities following the order of Rb+ approximately equals K+ > Na+ > Cs+ > Li+. The second type of binding site is specific for monovalent cations. its binding affinity is higher than that of the first type, for Na+ ions the equilibrium dissociation constant is < 0.01 mM. Since binding to that site is not electrogenic it has to be close to the cytoplasmic surface. The third site is specific for Na+, no other ions were found to bind, the binding is electrogenic and the equilibrium dissociation constant is 0.2 mM.