Mechanism of the rate-determining step of the Na(+),K(+)-ATPase pump cycle

The kinetics of the E(2) --> E(1) conformational change of unphosphorylated Na(+),K(+)-ATPase from rabbit kidney and shark rectal gland were investigated via the stopped-flow technique using the fluorescent label RH421 (pH 7.4, 24 degrees C). The enzyme was pre-equilibrated in a solution containing 25 mM histidine and 0.1 mM EDTA to stabilize initially the E(2) conformation. When rabbit kidney enzyme was mixed with NaCl alone, tris ATP alone or NaCl, and tris ATP simultaneously, a fluorescence decrease was observed. The reciprocal relaxation time, 1/tau, of the fluorescent transient was found to increase with increasing NaCl concentration and reached a saturating value in the presence of 1 mM tris ATP of 54 +/- 3 s(-1) in the case of rabbit kidney enzyme. The experimental behavior could be described by a binding of Na(+) to the enzyme in the E(2) state with a dissociation constant of 31 +/- 7 mM, which induces a subsequent rate-limiting conformational change to the E(1) state. Similar behavior, but with a decreased saturating value of 1/tau, was found when NaCl was replaced by choline chloride. Analogous experiments performed with enzyme from shark rectal gland showed similar effects, but with a significantly lower amplitude of the fluorescence change and a higher saturating value of 1/tau for both the NaCl and choline chloride titrations. The results suggest that Na(+) ions or salt in general play a regulatory role, similar to that of ATP, in enhancing the rate of the rate-limiting E(2) --> E(1) conformational transition by interaction with the E(2) state.