The effects of Na+ and K+ on the conformational transitions of (Na+ + K+)-ATPase.

The equilibrium between the E1 form and the E2 form of the (Na+ + K+)-ATPase is poised towards E2 in the absence of cations when monitored by the fluorescence of eosin. Na+ converts the enzyme to the E1 form with a K0.5 of 1.9 mM (pH 7.4 and 2 microM eosin). The titration curves indicate that more than one Na+ is necessary. K0.5 for K+ for reversal of the Na+ effect is 0.8 mM with 10 mM Na+, 5.8 mM K+ with 30 mM Na+ and 37.7 mM with 100 mM Na+. In the absence of K+ the rate of transition from E2 to E1 is rapid when Na+ is added, t1/2 is about 50 ms at 6 degrees C (pH 7.4). K+ in low concentrations decreases the rate with a K0.5 for K+ of about 10 microM, and t1/2 is about 2 s at 1 mM K+. K+ in concentrations from 1 to 50 mM (the highest tested) increases the rate of transition from E2 to E1. E2 thus consists of a non-K+ form (E2), a K+ both with K+ bound with apparent high affinity, the 'occluded' form (KoccE2), and a form with K+ bound both at high and low affinity sites (KE2). K+ bound to the low-affinity sites facilitates the transition from KoocE2 to E1. So does an increase in the Na+ concentration, but Na+ is less effective than K+. The rate of transition from E1 to E2 is rapid when K+ is added to enzyme in 20 mM Na+. The rate increases with the K+ concentration, and the effect of K+ does not saturate at the concentrations of K+ sufficient to displace the equilibrium fully to E2. Na+ decreases the rate of transition from E1 to E2. A model is suggested for the effect of the cations on the transition between the different forms of the enzyme. The physiological implications of the effect of K+ at a low-affinity site are discussed.