Kinetic studies on the (Na+ + K+)-dependent ATPase evidence for coexisting sites for Na+, K+ and Mg2+

Kinetic characterization of Na,K-ATPase from rabbit outer renal medulla: properties of the (alpha beta)(2) dimer

We describe and compare the main kinetic characteristics of the (alpha beta)(2) form of rabbit kidney Na,K-ATPase. The dependence of ATPase activity on ATP concentration revealed high (K(0.5)=4 microM) and low (K(0.5)=1.4 mM) affinity sites for ATP, exhibiting negative cooperativity and a specific activity of approximately 700 U/mg. For p-nitrophenylphosphate (PNPP) as substrate, a single saturation curve was found, with a smaller apparent affinity of the enzyme for this substrate (K(0.5)=0.5 mM) and a lower hydrolysis rate (V(M)=42 U/mg). Stimulation of ATPase activity by K(+) (K(0.5)=0.63 mM), Na(+) (K(0.5)=11 mM) and Mg(2+) (K(0.5)=0.60 mM) all showed V(M)'s of approximately 600 U/mg and negative cooperativity. K(+) (K(0.5)=0.69 mM) and Mg(2+) (K(0.5)=0.57 mM) also stimulated PNPPase activity of the (alpha beta)(2) form. Ouabain (K(0.5)=0.01 microM and K(0.5)=0.1 mM) and orthovanadate (K(0.5)=0.06 microM) completely inhibited the ATPase activity of the (alpha beta)(2) form. The kinetic characteristics obtained constitute reference values for diprotomeric (alpha beta)(2)-units of Na,K-ATPase, thus contributing to a better understanding of the biochemical mechanisms of the enzyme.

Kinetics of conformational changes associated with potassium binding to and release from Na+/K(+)-ATPase

The Na+/K(+)-ATPase functions in cells to couple energy from the hydrolysis of ATP to the transport Na+ out and K+ in. The fluorescent probe IAF (iodoacetamidofluorescein) covalently binds to this enzyme, reporting conformational changes without inhibiting enzyme activity. This paper describes experiments using dog kidney enzyme labeled with IAF to examine kinetics of conformational changes resulting from added Na+ and K+, measured in terms of steady-state and stopped-flow fluorescence changes. Kinetics of these fluorescence changes were examined as a function of temperature from two initial conditions: (a) enzyme in the high-fluorescence form (E(high)) was rapidly mixed with varying [K+]; and (b) enzyme in the low-fluorescence form (E(low)) was rapidly mixed with varying [ATP]. These experiments showed: (1) The rate constant for the fluorescence change from E(high) to E(low) was much larger than that for the opposite transition, E(low) to E(high); (2) the apparent free energy of activation (Ea(app)) for the two transitions were different (as estimated from Arrhenius plots); (3) under steady-state conditions, IAF fluorescence did not change when ATP was added to E(low)(K+) in the absence of Na+; (4) the apparent free energy of activation was independent of [K+] for the E(high) to E(low) transition (at 16.4 kcal/mol) but increased with [ATP] for the E(low) to E(high) transition; (5) Ea(app) for the E(low) to E(high) transition with 1 mM ATP was approximately the same as that in the absence of ATP (34 kcal/mol). These results can be interpreted as: (i) in the transition from E(low) to E(high), IAF reported a conformational change that occurred after K+ release to the intracellular side and which is involved in Na+ binding; (ii) Ea(app) increased with [ATP], while increasing the entropy of the transition state. Thus, ATP appeared to destabilize the enzyme during the transition from E(low) to E(high).