(Na+ + K+)-ATPase: confirmation of the three-pool model for the phosphointermediates of Na+-ATPase activity. Estimation of the enzyme-ATP dissociation rate constant

Na,K-ATPase structure/function relationships probed by the denaturant urea

Urea interacts with the Na,K-ATPase, leading to reversible as well as irreversible inhibition of the hydrolytic activity. The enzyme purified from shark rectal glands is more sensitive to urea than Na,K-ATPase purified from pig kidney. An immediate and reversible inhibition under steady-state conditions of hydrolytic activity at 37°C is demonstrated for the three reactions studied: the overall Na,K-ATPase activity, the Na-ATPase activity observed in the absence of K+ as well as the K+-dependent phosphatase reaction (K-pNPPase) seen in the absence of Na+. Half-maximal inhibition is seen with about 1M urea for shark enzyme and about 2M urea for pig enzyme. In the presence of substrates there is also an irreversible inhibition in addition to the reversible process, and we show that ATP protects against the irreversible inhibition for both the Na,K-ATPase and Na-ATPase reaction, whereas the substrate paranitrophenylphosphate leads to a slight increase in the rate of irreversible inhibition of the K-pNPPase. The rate of the irreversible inactivation in the absence of substrates is much more rapid for shark enzyme than for pig enzyme. The larger number of potentially urea-sensitive hydrogen bonds in shark enzyme compared to pig enzyme suggests that interference with the extensive hydrogen bonding network might account for the higher urea sensitivity of shark enzyme. The reversible inactivation is interpreted in terms of domain interactions and domain accessibilities using as templates the available crystal structures of Na,K-ATPase. It is suggested that a few interdomain hydrogen bonds are those mainly affected by urea during reversible inactivation.

Effect of ADP on Na(+)-Na(+) exchange reaction kinetics of Na,K-ATPase

The whole-cell voltage-clamp technique was used in rat cardiac myocytes to investigate the kinetics of ADP binding to phosphorylated states of Na,K-ATPase and its effects on presteady-state Na(+)-dependent charge movements by this enzyme. Ouabain-sensitive transient currents generated by Na,K-ATPase functioning in electroneutral Na(+)-Na(+) exchange mode were measured at 23 degrees C with pipette ADP concentrations ([ADP]) of up to 4.3 mM and extracellular Na(+) concentrations ([Na](o)) between 36 and 145 mM at membrane potentials (V(M)) from -160 to +80 mV. Analysis of charge-V(M) curves showed that the midpoint potential of charge distribution was shifted toward more positive V(M) both by increasing [ADP] at constant Na(+)(o) and by increasing [Na](o) at constant ADP. The total quantity of mobile charge, on the other hand, was found to be independent of changes in [ADP] or [Na](o). The presence of ADP increased the apparent rate constant for current relaxation at hyperpolarizing V(M) but decreased it at depolarizing V(M) as compared to control (no added ADP), an indication that ADP binding facilitates backward reaction steps during Na(+)-Na(+) exchange while slowing forward reactions. Data analysis using a pseudo three-state model yielded an apparent K(d) of approximately 6 mM for ADP binding to and release from the Na,K-ATPase phosphoenzyme; a value of 130 s(-1) for k(2), a rate constant that groups Na(+) deocclusion/release and the enzyme conformational transition E(1) approximately P --> E(2)-P; a value of 162 s(-1)M(-1) for k(-2), a lumped second-order V(M)-independent rate constant describing the reverse reactions; and a Hill coefficient of approximately 1 for Na(+)(o) binding to E(2)-P. The results are consistent with electroneutral release of ADP before Na(+) is deoccluded and released through an ion well. The same approach can be used to study additional charge-moving reactions and associated electrically silent steps of the Na,K-pump and other transporters.

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).

Equilibrium of phosphointermediates of sodium and potassium ion transport adenosine triphosphatase: action of sodium ion and Hofmeister effect

Sodium and potassium ion transport adenosine triphosphatase accepts and donates a phosphate group in the course of its reaction sequence. The phosphorylated enzyme has two principal reactive states, E1P and E2P. E1P is formed reversibly from ATP in the presence of Na+ and is precursor to E2P, which equilibrates with P(i) in the presence of K+. We studied equilibrium between these states at 4 degrees C and the effect of Na+ on it. To optimize the reaction system we used a Hofmeister effect, replacing the usual anion, chloride, with a chaotropic anion, usually nitrate. We phosphorylated enzyme from canine kidney with [32P]ATP. We estimated interconversion rate constants for the reaction E1P <--> E2P and their ratio. To estimate rate constants we terminated phosphorylation and observed decay kinetics. We observed E1P or E2P selectively by adding K+ or ADP respectively. K+ dephosphorylates E2P leaving E1P as observable species; ADP dephosphorylates E1P leaving E2P as observable species. We fitted a 2-pool model comprising two reactive species or a twin 2-pool model, comprising a pair of independent 2-pool models, to the data and obtained interconversion and hydrolysis rate constants for each state. Replacing Na+ with Tris+ or lysine+ did not change the ratio of interconversion rate constants between E1P and E2P. Thus Na+ binds about equally strongly to E1P and E2P. This conclusion is consistent with a model of Pedemonte (1988. J. Theor. Biol. 134:165-182.). We found that Na+ affected another equilibrium, that of transphosphorylation between ATP x dephosphoenzyme and ADP x E1P. We used the reactions and model of Pickart and Jencks (1982. J. Biol. Chem. 257:5319-5322.) to generate and fit data. Decreasing the concentration of Na+ 10-fold shifted the equilibrium constant 10-fold favoring ADP x E1P over ATP x dephosphoenzyme. Thus Na+ can dissociate from E1P x Na3. Furthermore, we found two characteristics of Hofmeister effects on this enzyme.

Interpretation of extraordinary kinetics of Na(+)-K(+)-ATPase by a phase change

We interpret at a molecular level an extraordinary response in the transient kinetics of the phosphointermediate of Na(+)-K(+)-ATPase (I. Klodos, R. L. Post, and B. Forbush III. J. Biol. Chem. 269: 1734-1743, 1994). The phosphointermediate comprises two principal states. The partition between these states varies with salt concentration. A jump in salt concentration changes the partition of some of the molecules more rapidly than they interconvert in a steady state at constant salt concentration. We propose that interconversion is limited by free volume in the lipid of the surrounding membrane. This lipid is partitioned into phases that vary with salt concentration. Free volume is larger at the interface between these phases than within the phases themselves. Na(+)-K(+)-ATPase molecules are distributed at random in the membrane. When the phase boundary moves in response to a jump in salt concentration, it crosses some Na+ -K+ -ATPase molecules, which transiently experience an increase in free volume of the surrounding lipid. Thus their phosphointermediate states equilibrate more rapidly than at a constant salt concentration. Functional and structural heterogeneity of Na(+)-K(+)-ATPase molecules is discussed.

Influence of intramembrane electric charge on Na,K-ATPase

Effects of lipophilic ions, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-), on interactions of Na+ and K+ with Na,K-ATPase were studied with membrane-bound enzyme from bovine brain, pig kidney, and shark rectal gland. Na+ and K+ interactions with the inward-facing binding sites, monitored by eosin fluorescence and phosphorylation, were not influenced by lipophilic ions. Phosphoenzyme interactions with extracellular cations were evaluated through K(+)-, ADP-, and Na(+)-dependent dephosphorylation. TPP+ decreased: 1) the rate of transition of ADP-insensitive to ADP-sensitive phosphoenzyme, 2) the K+ affinity and the rate coefficient for dephosphorylation of the K-sensitive phosphoenzyme, 3) the Na+ affinity and the rate coefficient for Na(+)-dependent dephosphorylation. Pre-steady state phosphorylation experiments indicate that the subsequent occlusion of extracellular cations was prevented by TPP+. TPB- had opposite effects. Effects of lipophilic ions on the transition between phosphoenzymes were significantly diminished when Na+ was replaced by N-methyl-D-glucamine or Tris+, but were unaffected by the replacement of Cl- by other anions. Lipophilic ions affected Na-ATPase, Na,K-ATPase, and p-nitrophenylphosphatase activities in accordance with their effects on the partial reactions. Effects of lipophilic ions appear to be due to their charge indicating that Na+ and K+ access to their extracellular binding sites is modified by the intramembrane electric field.

An unexpected effect of ATP on the ratio between activity and phosphoenzyme level of Na+/K(+)-ATPase in steady state

According to the Albers-Post model the hydrolysis of ATP catalyzed by the Na+/K(+)-ATPase requires the sequential formation of at least two conformers of a phosphoenzyme (E1P and E2P), followed by the K(+)-stimulated hydrolysis of E2P. In this paper we show that this model is a particular case of a more general class of models in all of which the ratio between ATPase activity (v) and total phosphoenzyme level (EP) in steady state is determined solely by the rate constants of interconversion between phosphoconformers and of dephosphorylation. Since these are thought to be unaffected by ATP, the substrate curves for ATPase activity and EP should be identical in shape so that the ratio v/EP ought to be independent of the concentration of ATP. We tested this prediction by parallel measurements of v and EP as a function of [ATP] in the absence or presence of non-limiting concentrations of K+, Rb+ or NH+4. In the absence of K+ or its congeners, both curves followed Michaelis-Menten kinetics, with almost identical Km values (0.16 microM) so that v/EP remained independent of [ATP]. In the presence of either K+, Rb+ or NH+4, v and EP increased with [ATP] along the sum of two Michaelis-Menten equations. The biphasic response of v is well known but, to the best of our knowledge, our results are the first demonstration that the response of EP to [ATP] is also biphasic. Under these conditions, the ratio v/EP increased with [ATP] from 19.8 to 40.1 s-1 along a hyperbola that was half-maximal at 9.5 microM. To preserve the validity of the current model it seems necessary to assume that ATP acts on the E1P <--> E2P transition and/or on the rate of hydrolysis of E2P. The latter possibility was ruled out. We also found that to fit the Albers-Post model to our data, the rate constant for K+ deocclussion from E2 has to be about 10-times higher than that reported from measurements of partial reactions. The results indicate that the Albers-Post model quantitatively predicts the experimental behavior of the Na(+)-ATPase activity but is unable to do this for the Na+/K(+)-ATPase activity, unless additional and yet unproved hypothesis are included.

Determination of rate constants for nucleotide dissociation from Na,K-ATPase

A method for determining individual rate constants for nucleotide binding to and dissociation from membrane bound pig kidney Na,K-ATPase is presented. The method involves determination of the rate of relaxation when Na,K-ATPase in the presence of eosin is mixed with ADP or ATP in a stopped-flow fluorescence apparatus. It is shown that the nucleotide dependence of this rate of relaxation--taken together with measured equilibrium binding values for eosin and ADP--makes possible a reasonably reliable determination of the rate constant for dissociation of nucleotide, i.e., determination of the rate constant k-1 in the following model (where E denotes Na,K-ATPase): [formula: see text] All experiments are carried out at about 4 degrees C in a buffer containing 200 mM sucrose, 10 mM EDTA, 25 mM Tris and 73 mM NaCl (pH 7.4). Values obtained for the rate constants for dissociation are about 6 s-1 for ADP and 2-3 s-1 for ATP.

Effects of magnesium and ATP on pre-steady-state phosphorylation kinetics of the Na+,K(+)-ATPase

The aim of the present work was to elucidate the role played by ATP and Mg2+ ions in the early steps of the Na+,K(+)-ATPase cycle. The approach was to follow pre-steady-state phosphorylation kinetics in Na(+)-containing K(+)-free solutions under variable ATP and MgCl2 concentrations. The experiments were performed with a rapid mixing apparatus at 20 +/- 2 degrees C. The concentrations of free and complexes species of Mg2+ and ATP were calculated on the basis of a dissociation constant of 0.091 +/- 0.004 mM, estimated with Arsenazo III under identical conditions. A simplified scheme were ATP binds to the ENa enzyme, which is phosphorylated to MgEPNa and consequently dephosphorylated returning to the ENa form, was used. In the absence of ADP and phosphate four rate constants are relevant: k1 and k-1, the on and off rate constants for ATP binding; k2, the transphosphorylation rate constant and k3, the constant that governs the dephosphorylation rate. The values obtained were: k1 = 0.025 +/- 0.003 microM-1 ms-1 for both free ATP and ATPMg; k-1 = 0.038 +/- 0.004 ms-1 for free ATP and 0.009 +/- 0.002 ms-1 for ATPMg; k2 = 0.199 +/- 0.005 ms-1; k3 = 0.0019 +/- 0.0002 ms-1. The model that seems best to explain the data is one where (i) the role of true substrate can be played equally well by free ATP or ATPMg, and (ii) free Mg2+, an essential activator, acts by binding to a specific Mg2+ site on the enzyme molecule.

The role of Mg2+ and K+ in the phosphorylation of Na+,K(+)-ATPase by ATP in the presence of dimethylsulfoxide but in the absence of Na+

We have previously demonstrated that Na+,K(+)-ATPase can be phosphorylated by 100 microM ATP and 5 mM Mg2+ and in the absence of Na+, provided that 40% dimethylsulfoxide (Me2SO) is present. Phosphorylation was stimulated by K+ up to a steady-state level of about 50% of Etot (Barrabin et al. (1990) Biochim. Biophys. Acta 1023, 266-273). Here we describe the time-course of phosphointermediate (EP) formation and of dephosphorylation of EP at concentrations of Mg2+ from 0.1 to 5000 microM and of K+ from 0.01 to 100 mM. The results were simulated by a simplified version of the commonly accepted Albers-Post model, i.e. a 3-step reaction scheme with a phosphorylation, a dephosphorylation and an isomerization/deocclusion step. Furthermore it was necessary to include an a priori, Mg(2+)- and K(+)-independent, equilibration between two enzyme forms, only one of which (constituting 14% of Etot) reacted directly with ATP. The role of Mg(2+) was two-fold: At low Mg2+, phosphorylation was stimulated by Mg2+ due to formation of the substrate MgATP, whereas at higher concentrations it acted as an inhibitor at all three steps. The affinity for the inhibitory Mg(2+)-binding was increased several-fold, relative to that in aqueous media, by dimethylsulfoxide. K+ stimulated dephosphorylation at all Mg(2+)-concentrations, but at high, inhibitory [Mg2+], K+ also stimulated the phosphorylation reaction, increasing both the rate coefficient and the steady-state level of EP. Generally, the presence of Me2SO seems to inhibit the dephosphorylation step, the isomerization/deocclusion step, and to a lesser extent (if at all) the phosphorylation reaction, and we discuss whether this reflects that Me2SO stabilizes occluded conformations of the enzyme even in the absence of monovalent cations. The results confirm and elucidate the stimulating effect of K+ on EP formation from ATP in the absence of Na+, but they leave open the question of the molecular mechanism by which Me2SO, inhibitory Mg2+ and stimulating K+ interact with the Na+,K(+)-ATPase.

Role of protein conformation changes and transphosphorylations in the function of Na+/K(+)-transporting adenosine triphosphatase: an attempt at an integration into the Na+/K+ pump mechanism

The particular aim of the review on some basic facets of the mechanism of Na+/K(+)-transporting ATPase (Na/K-ATPase) has been to integrate the experimental findings concerning the Na(+)- and K(+)-elicited protein conformation changes and transphosphorylations into the perspective of an allosterically regulated, phosphoryl energy transferring enzyme. This has led the authors to the following summarizing evaluations. 1. The currently dominating hypothesis on a link between protein conformation changes ('E1 in equilibrium with E2') and Na+/K+ transport (the 'Albers-Post scheme') has been constructed from a variety of partial reactions and elementary steps, which, however, do not all unequivocally support the hypothesis. 2. The Na(+)- and K(+)-elicited protein conformation changes are inducible by a variety of other ligands and modulatory factors and therefore cannot be accepted as evidence for their direct participation in effecting cation translocation. 3. There is no evidence that the 'E1 in equilibrium with E2' protein conformation changes are moving Na+ and K+ across the plasma membrane. 4. The allosterically caused ER in equilibrium with ET ('E1 in equilibrium with E2') conformer transitions and the associated cation 'occlusion' in equilibrium with 'de-occlusion' processes regulate the actual catalytic power of an enzyme ensemble. 5. A host of experimental variables determines the proportion of functionally competent ER enzyme conformers and incompetent ET conformers so that any enzyme population, even at the start of a reaction, consists of an unknown mixture of these conformers. These circumstances account for the occurrence of contradictory observations and apparent failures in their comparability. 6. The modelling of the mechanism of the Na/K-ATPase and Na+/K+ pump from the results of reductionistically designed experiments requires the careful consideration of the physiological boundary conditions. 7. Na+ and K+ ligandation of Na/K-ATPase controls the geometry and chemical reactivity of the catalytic centre in the cycle of E1 in equilibrium with E2 state conversions. This is possibly effected by hinge-bending, concerted motions of three adjacent, intracellularly exposed peptide sequences, which shape open and closed forms of the catalytic centre in lock-and-key responses. 8. The Na(+)-dependent enzyme phosphorylation with ATP and the K(+)-dependent hydrolysis of the phosphoenzyme formed are integral steps in the transport mechanism of Na/K-ATPase, but the translocations of Na+ and K+ do not occur via a phosphate-cation symport mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

The plasma membrane H(+)-ATPase from yeast. Effects of pH, vanadate and erythrosine B on ATP hydrolysis and ATP binding

The H(+)-ATPase from the plasma membrane of Saccharomyces cerevisiae was isolated and purified. The rate of ATP hydrolysis and ATP binding was measured as a function of pH and the effect of the vanadate and erythrosine B inhibitors was investigated. The pH dependence of the rate of ATP hydrolysis forms a bell-shaped curve with a maximum at pH 6 and half-maximal rates at pH 5.0 and 7.4. Only the pH dependence between pH 6 and pH 7.6 is reversible. Above pH 7.6 and below pH 5.5, denaturation of the isolated enzyme is observed. The rate of ATP binding shows the same pH dependency as that of ATP hydrolysis. Both pH dependencies can be described by the dissociation of a monovalent acidic group with a pK of 7.4. It is concluded that the enzyme must be protonated before ATP binding. Vanadate does not inhibit ATP binding, ADP release or Pi release at concentrations where complete inhibition of ATP hydrolysis is observed. It is concluded that vanadate inhibits a step of the reaction cycle which occurs after Pi release. In contrast, erythrosine B inhibits ATP binding and thus affects the first step of the reaction cycle.

Dimethyl sulfoxide: a possible effect on the interconversion of phosphorylated forms of Na+,K(+)-ATPase

Purified Na+,K(+)-ATPase from kidney outer medulla was phosphorylated by Pi in a reaction synergistically stimulated by Mg2+, when 40% (v/v) dimethyl sulfoxide was added to the assay medium. The phosphoenzyme formed at this solvent concentration was able to synthesize ATP even in the presence of Mg2+, because hydrolysis was impaired. ATP in equilibrium [32P]Pi exchange was also inhibited, indicating that partial reactions in the forward direction were blocked by the solvent. In 40% (v/v) dimethyl sulfoxide the enzyme's affinity for ADP decreased, in comparison with the values observed in purely aqueous medium. Addition of K+, which accelerated dephosphorylation of Na+,K(+)-ATPase in a totally water medium, partially reversed the inhibition of hydrolysis that was observed in the presence of dimethyl sulfoxide.

Na,K-ATPase in artificial lipid vesicles. Comparison of Na,K and Na-only pumping mode

Na,K-ATPase from rabbit kidney outer medulla was reconstituted in large unilamellar lipid vesicles by detergent dialysis. Vesicles prepared in the presence or absence of potassium allowed to study two different transport modes: the (physiological) Na,K-mode in buffers containing Na+ and K+ and the Na-only mode in buffers containing Na+ but no K+. The ATP hydrolysis activity was obtained by determination of the liberated inorganic phosphate, Pi, and the inward directed Na+ flux was measured by 22Na-tracer flux. Electrogenic transport properties were studied using the membrane potential sensitive fluorescence-dye oxonol VI. The ratio upsilon(Na,K)/upsilon(Na) of the turnover rates in the Na,K-mode and in the Na-only mode is 6.6 +/- 2.0 under otherwise identical conditions and nonlimiting Na+ concentrations. Strong evidence is found that the Na-only mode exhibits a stoichiometry of 3Na+cyt/2Na+ext/1ATP, i.e. the extracellular (= intravesicular) Na+ has a potassium-like effect. In the Na-only mode one high-affinity binding side for ATP (KM congruent to 50 nM) was found, in the Na,K-mode a high- and low-affinity binding side with equilibrium dissociation constants, KM, of 60 nM and 13 microM, respectively. The sensitivity against the noncompetitively inhibiting ADP (KI = 6 microM) is higher by a factor of 20 in the Na-only mode compared to the Na,K-mode. From the temperature dependence of the pumping activity in both transport modes, activation energies of 160 kJ/mol for the Na,K-mode and 110 kJ/mol for the Na-only mode were determined.

Cation sidedness in the phosphorylation step of Na+/K+-ATPase

Na+/K+ -ATPase, reconstituted into phospholipid vesicles, has been used to study the localisation of binding sites of ligands involved in the phosphorylation reaction. Inside-out oriented Na+/K+ -ATPase molecules are the only population in this system, which can be phosphorylated, as the rightside-out oriented as well as the non-incorporated enzyme molecules are inhibited by ouabain. In addition, the right-side-out oriented Na+/K+ -ATPase molecules have their ATP binding site intravesicularly and are thus not accessible to substrate added to the extravesicular medium. Functional binding sites for the following ligands have been demonstrated: (i) Potassium, acting at the extracellular side with high affinity (stimulating the dephosphorylation rate of the E2P conformation) and low affinity (inducing the non-phosphorylating E2K complex). (ii) Potassium, acting at the cytoplasmic side with both high and low affinity. The latter sites are also responsible for the formation of an E2K complex and complete with Na+ for its binding sites. (iii) Sodium at the cytoplasmic side responsible for stimulation of the phosphorylation reaction. (iv) Sodium (and amine buffers) at the extracellular side enhancing the phosphorylation level of Na+/K+ -ATPase where choline chloride has no effect. (v) Magnesium at the cytoplasmic side, stimulating the phosphorylation reaction and inhibiting it above optimal concentrations.

Principles of selectivity of sodium and potassium binding sites of the Na+/K+-ATPase. A corollary hypothesis

The mechanisms whereby the sodium and potassium binding sites of heart sacrolemmal Na+/K+-ATPase (EC 3.6.1.3) distinguished between monovalent cations were investigated using methods of enzyme kinetics. The properties of the sodium binding sites were studied in the presence of 2,4,6-trinitrobenzenesulfonic acid in concentrations completely inhibiting the action of potassium on the enzyme. To test the selectivity of potassium binding sites, K+-p-nitrophenylphosphatase activity was employed as a model. The results suggest that the selectivity of Na+- and K+-binding sites of Na+/K+-ATPase may be due to two independent mechanisms: (i) The principle of key and lock (formation of coordination bounds); (ii) Optimal difference between solvatation energy (in the specific binding site) and hydration enthalpy of the respective cation.

Transient behaviour of the Na+/K+-pump: microscopic analysis of nonstationary ion-translocation

In recent years fast perturbation techniques have been applied for investigating the mechanism of the Na+/K+-pump. Experiments in which nonstationary pump-currents and ion fluxes are measured after a voltage or ATP-concentration jump yield kinetic information which cannot be obtained from ordinary steady-state experiments. In this paper a theoretical treatment is described by which transient pump-currents and ion fluxes can be analyzed in a unified way. The method is based on the assumption that the operation of the pump involves a sequence of conformational transitions and ion-binding and -release steps. The charge displacements associated with the individual reaction steps are described by a set of dielectric coefficients. The nonstationary behaviour of the Na+/K+-pump is analyzed on the basis of the Albers-Post reaction cycle. It is shown that the different studies of transient pump-currents and ion fluxes carried out so far lead to internally consistent conclusions with respect to the nature of the electrogenic steps of the transport cycle.

Distinction between the intermediates in Na+-ATPase and Na+,K+-ATPase reactions. II. Exchange and hydrolysis kinetics at micromolar nucleotide concentrations

The ATP hydrolysis rate and the ADP-ATP exchange rate of (Na+ + K+)-ATPase from ox brain were measured at 10 microM Mg2+free and at micromolar concentrations of free ATP and ADP. (1) In the absence of K+, substrate inhibition of the hydrolysis rate was observed. It disappeared at low Na+ and diminished at increasing concentrations of ADP. This was interpreted in terms of free ATP binding to E1P. In support of this interpretation, free ATP was found to competitively inhibit ADP-ATP exchange. (2) In the presence of K+, substrate activation of the hydrolysis rate was observed. Increasing (microM) concentrations of ADP did not give rise to competitive inhibition in contrast to the situation in the absence of K+ (cf. 1, above). This was interpreted to show that at micromolar substrate, some low-affinity, high-turnover Na+ + K+ activity is possible, provided the Mg2+ concentration is low. (3) While small concentrations of K+ increased the hydrolysis rate (cf. 2) they decreased the rate of ADP-ATP exchange. To elucidate this phenomenon, parallel measurements of exchange and hydrolysis rates were performed over a wide range of ATP and ADP concentrations, with and without K+. If, in the presence and absence of K+, ADP (and ATP competing) are binding to the same phosphorylated intermediate for the backward reaction, it places quantitative restrictions on the ratio of rate constants with and without K+. The results did not conform to these restrictions, and the discrepancy is taken as evidence for the necessity for a bicyclic scheme for the action of the (Na+ + K+)-ATPase. (4) An earlier statement concerning the nature of the phosphoenzyme obtained in the presence of Na+ and K+ is amended.