Cation activation of the pig kidney sodium pump: transmembrane allosteric effects of sodium

Na+-K+-ATPase, a new class of plasma membrane receptors

The Na(+)-K(+)-ATPase (NKA) differs from most other ion transporters, not only in its capacity to maintain a steep electrochemical gradient across the plasma membrane, but also as a receptor for a family of cardiotonic steroids, to which ouabain belongs. Studies from many groups, performed during the last 15 years, have demonstrated that ouabain, a member of the cardiotonic steroid family, can activate a network of signaling molecules, and that NKA will also serve as a signal transducer that can provide a feedback loop between NKA and the mitochondria. This brief review summarizes the current knowledge and controversies with regard to the understanding of NKA signaling.

Extracellular allosteric Na(+) binding to the Na(+),K(+)-ATPase in cardiac myocytes

Whole-cell patch-clamp measurements of the current, Ip, produced by the Na(+),K(+)-ATPase across the plasma membrane of rabbit cardiac myocytes show an increase in Ip over the extracellular Na(+) concentration range 0-50 mM. This is not predicted by the classical Albers-Post scheme of the Na(+),K(+)-ATPase mechanism, where extracellular Na(+) should act as a competitive inhibitor of extracellular K(+) binding, which is necessary for the stimulation of enzyme dephosphorylation and the pumping of K(+) ions into the cytoplasm. The increase in Ip is consistent with Na(+) binding to an extracellular allosteric site, independent of the ion transport sites, and an increase in turnover via an acceleration of the rate-determining release of K(+) to the cytoplasm, E2(K(+))2 → E1 + 2K(+). At normal physiological concentrations of extracellular Na(+) of 140 mM, it is to be expected that binding of Na(+) to the allosteric site would be nearly saturated. Its purpose would seem to be simply to optimize the enzyme's ion pumping rate under its normal physiological conditions. Based on published crystal structures, a possible location of the allosteric site is within a cleft between the α- and β-subunits of the enzyme.

Kinetics behaviors of Na,K-ATPase: comparison of solubilized and DPPC:DPPE-liposome reconstituted enzyme

We describe and compare the main kinetic characteristics of rabbit kidney Na,K-ATPase incorporated inside-out in DPPC:DPPE-liposomes with the C(12)E(8) solubilized and purified form. In proteoliposomes, we observed that the ATP hydrolysis of the enzyme is favored and also its affinity for Na(+)-binding sites increases, keeping the negative cooperativity with two classes of hydrolysis sites: one of high affinity (K(0.5)=6 microM and 4 microM for reconstituted enzyme and purified form, respectively) and another of low affinity (K(0.5)=0.4 mM and 1.4 mM for reconstituted enzyme and purified form, respectively). Our data showed a biphasic curve for ATP hydrolysis, suggesting the presence of (alphabeta)(2) oligomer in reconstituted Na,K-ATPase similar to the solubilized enzyme. The Mg(2+) concentration dependence in the proteoliposomes stimulated the Na,K-ATPase activity up to 476 U/mg with a K(0.5) value of 0.4 mM. The Na(+) ions also presented a single saturation curve with V(M)=551 U/mg and K(0.5)=0.2 mM with cooperative effects. The activity was also stimulated by K(+) ions through a single curve of saturation sites (K(0.5)=2.8 mM), with cooperative effects and V(M)=641 U/mg. The lipid microenvironment close to the proteic structure and the K(+) internal to the liposome has a key role in enzyme regulation, affecting its kinetic parameters while it can also modulate the enzyme's affinity for substrate and ions.

Functional consequences of alterations to Ile279, Ile283, Glu284, His285, Phe286, and His288 in the NH2-terminal part of transmembrane helix M3 of the Na+,K(+)-ATPase

Mutations Ile279 --> Ala, Ile283 --> Ala, Glu284 --> Ala, His285 --> Ala, His285 --> Lys, His285 --> Glu, Phe286 --> Ala, and His288 --> Ala in transmembrane helix M3 of the Na+,K(+)-ATPase were studied. Except for His285 --> Ala, these mutations were compatible with cell viability, permitting analysis of their effects on the overall and partial reactions of the Na+,K(+)-transport cycle. In Ile279 --> Ala and Ile283 --> Ala, the E1 form accumulated, whereas in His285 --> Lys and His285 --> Glu, E1P accumulated. Phe286 --> Ala displaced the conformational equilibria of dephosphoenzyme and phosphoenzyme in parallel in favor of E2 and E2P, respectively, and showed a unique enhancement of the E1P --> E2P transition rate. These effects suggest that M3 undergoes significant rearrangements in relation to E1-E2 and E1P-E2P conformational changes. Because the E1-E2 and E1P-E2P conformational equilibria were differentially affected by some of the mutations, the phosphorylated conformations seem to differ significantly from the dephospho forms in the M3 region. Mutation of His285 furthermore increased the Na(+)-activated ATPase activity in the absence of K+ ("Na(+)-ATPase activity"). Ile279 --> Ala, Ile283 --> Ala, and His288 --> Ala showed reduced Na+ affinity of the E1 form. The rate of Na(+)-activated phosphorylation from ATP was reduced in Ile279 --> Ala and Ile283 --> Ala, and these mutants showed evidence similar to Glu329 --> Gln of destabilization of the Na(+)-occluded state.

Dependence of Na+-K+ pump current-voltage relationship on intracellular Na+, K+, and Cs+ in rabbit cardiac myocytes

To examine effects of cytosolic Na+, K+, and Cs+ on the voltage dependence of the Na+-K+ pump, we measured Na+-K+ pump current (Ip) of ventricular myocytes voltage-clamped at potentials (Vm) from 100 to +60 mV. Superfusates were designed to eliminate voltage dependence at extracellular pump sites. The cytosolic compartment of myocytes was perfused with patch pipette solutions with a Na+ concentration ([Na]pip) of 80 mM and a K+ concentration from 0 to 80 mM or with solutions containing Na+ in concentrations from 0.1 to 100 mM and K+ in a concentration of either 0 or 80 mM. When [Na]pip was 80 mM, K+ in pipette solutions had a voltage-dependent inhibitory effect on Ip and induced a negative slope of the Ip-Vm relationship. Cs+ in pipette solutions had an effect on Ip qualitatively similar to that of K+. Increases in Ip with increases in [Na]pip were voltage dependent. The dielectric coefficient derived from [Na]pip-Ip relationships at the different test potentials was 0.15 when pipette solutions included 80 mM K+ and 0.06 when pipette solutions were K+ free.

Breakdown of Na+/K+-exchanging ATPase phosphoenzymes formed from ATP and from inorganic phosphate during Na+-ATPase activity

The reactivity towards Na+ and K+ of Na+/K+-ATPase phosphoenzymes formed from ATP and Pi during Na+-ATPase turnover and that obtained from Pi in the absence of ATP, Na+ and K+ was studied. The phosphoenzyme formed from Pi in the absence of cycling and with no Na+ or K+ in the medium showed a biphasic time-dependent breakdown. The fast component, 96% of the total EP, had a decay rate of about 4 s(-1) in K+-free 130 mm Na+, and was 40% inhibited by 20 mm K+. The slow component, about 0.14 s(-1), was K+ insensitive. Values for the time-dependent breakdown of the phosphoenzymes obtained from ATP and from Pi during Na+-ATPase activity were indistinguishable from each other. In K+-free medium containing 130 mm Na+, the decays followed a single exponential with a rate constant of 0.45 s(-1). The addition of 20 mm K+ markedly increased the decays and made them biphasic. The fast components had a rate of approximately 220 s-1 and accounted for 92-93% of the total phosphoenzyme. The slow components decayed at a rate of about 47-53 s(-1). A second group of experiments examined the reactivity towards Na+ of the E2P forms obtained with ATP and Pi when the enzyme was cycling. In both cases, the rate of dephosphorylation was a biphasic function of [Na+]: inhibition at low [Na+], with a minimum at about 5 mm Na+, followed by recovery at higher [Na+]. Although qualitatively similar, the phosphoenzyme formed from Pi showed slightly less inhibition and more pronounced recovery. These results indicate that forward and backward phosphorylation during Na+-ATPase turnover share the same intermediates.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

Electrogenic sodium-sodium exchange carried out by Na,K-ATPase containing the amino acid substitution Glu779Ala

Na,K-ATPase containing the amino acid substitution glutamate to alanine at position 779 of the alpha subunit (Glu779Ala) supports a high level of Na-ATPase and electrogenic Na+-Na+ exchange activity in the absence of K+. In microsomal preparations of Glu779Ala enzyme, the Na+ concentration for half maximal activation of Na-ATPase activity was 161 +/- 14 mM (n = 3). Furthermore, enzyme activity with 800 mM Na+ was found to be similar in the presence and absence of 20 mM K+. These results showed that Na+, with low affinity, could stimulate enzyme turnover as effectively as K+. To gain further insight into the mechanism of this enzyme activity, HeLa cells expressing Glu779Ala enzyme were voltage clamped with patch electrodes containing 115 mM Na+ during superfusion in K+-free solutions. Electrogenic Na+-Na+ exchange was observed as an ouabain-inhibitable outward current whose amplitude was proportional to extracellular Na+ (Na+(o)) concentration. At all Na+(o) concentrations tested (3-148 mM), exchange current was maximal at negative membrane potentials (V(M)), but decreased as V(M) became more positive. Analyzing this current at each V(M) with a Hill equation showed that Na+-Na+ exchange had a high-affinity, low-capacity component with an apparent Na+(o) affinity at 0 mV (K0(0.5)) of 13.4 +/- 0.6 mM and a low-affinity, high-capacity component with a K0(0.5) of 120 +/- 13 mM (n = 17). Both high- and low-affinity exchange components were V(M) dependent, dissipating 30 +/- 3% and 82 +/- 6% (n = 17) of the membrane dielectric, respectively. The low-affinity, but not the high-affinity exchange component was inhibited with 2 mM free ADP in the patch electrode solution. These results suggest that the high-affinity component of electrogenic Na+-Na+ exchange could be explained by Na+(o) acting as a low-affinity K+ congener; however, the low-affinity component of electrogenic exchange appeared to be due to forward enzyme cycling activated by Na+(o) binding at a Na+-specific site deep in the membrane dielectric. A pseudo six-state model for the Na,K-ATPase was developed to simulate these data and the results of the accompanying paper (Peluffo, R.D., J.M. Argüello, and J.R. Berlin. 2000. J. Gen. Physiol. 116:47-59). This model showed that alterations in the kinetics of extracellular ion-dependent reactions alone could explain the effects of Glu779Ala substitution on the Na,K-ATPase.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Interaction of the fluorescent probe RH421 with ribulose-1,5-bisphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments

Fluorescence titrations have shown that the voltage-sensitive probe RH421 interacts with the water-soluble protein ribulose-1,5-bisphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments. The probe exhibits significantly different fluorescence excitation spectra in pure lipid and pure protein environments. Experiments with a range of polyamino acids showed interactions of the probe with tyrosine, lysine and arginine residues. At saturating RH421 concentrations (> or = microM) the probe quenches 60-75% of the total tryptophan fluorescence of the Na+,K(+)-ATPase preparation. Inhibition of the hydrolytic activity of the Na+,K(+)-ATPase occurs at RH421 concentrations in the micromolar range. This may be due to a probe-induced change in membrane fluidity. The sensitivity of the probe towards conformational changes of the Na+,K(+)-ATPase decreases hyperbolically as one increases the probe concentration. The decrease in sensitivity correlates well with association of the probe in the vicinity of membrane protein, as measured by tryptophan quenching. These results have important practical consequences for the application of RH421 as a voltage indicator in membrane preparations. Based on these and previously reported results, the fluorescent response of RH421 to the ATP-induced conformational change of the Na+,K+-ATPase is consistent with either a redistribution of dye from the liquid-crystalline lipid matrix into the vicinity of membrane protein or a reorganisation of the lipids surrounding the protein into a more rigid structure caused by the conformational change of the protein.

Novel aromatic isothiouronium derivatives which act as high affinity competitive antagonists of alkali metal cations on Na/K-ATPase

This paper describes properties of a novel family of aromatic isothiouronium derivatives, which act as Na(+)-like competitive antagonists on renal Na/K-ATPase. The derivatives are reversible competitors of Rb+ and Na+ occlusion. Ki values of the most potent compounds, 1-bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU) and 1,3-dibromo-2,4,6-tris(methylisothiouronium)benzene(Br2-TITU ), 0.65 and 0.32 microM, respectively, are 15-30-fold lower than Ki values of the bis-guanidinium derivatives described previously (David, P., Mayan, H., Cohen, H., Tal, D. M., and Karlish, S. J. D. (1992) J. Biol. Chem. 267, 1141-1149), and represent the lowest reported values for cation antagonists. Using fluorescein-labeled Na/K-ATPase, all derivatives have been shown to stabilize the E1 conformation when bound at high affinity sites (i.e. they are sodium-like). In addition, in one condition (10 mM Tris-HCl, pH 8.1), high concentrations of Br-TITU (KD approximately 10 microM) appear to stabilize an E2 conformation. We propose a model which allows for simultaneous binding of the antagonists to high affinity cytoplasmic sites and low affinity sites, which may be at the extracellular surface. Blockage of cation occlusion by the isothiouronium derivatives at the cytoplasmic surface probably occurs at the entrance to the occlusion sites, which is recognized both by Na+ antagonists and by Na+ or K+ ions. Unlike the alkali metal cations, the Na+ antagonists are not occluded or transported (see also Or, E., David, P., Shainskaya, A., Tal, D. M., and Karlish, S. J. D. (1993) J. Biol. Chem. 268, 16929-16937). The isothiouronium derivatives appear to be promising candidates for further development as affinity labels of cation binding domains, for kinetic analysis of isoforms or mutated Na/K pumps, or as probes of other cation transport proteins.

Phosphorylation/dephosphorylation of reconstituted shark Na+,K(+)-ATPase: one phosphorylation site per alpha beta protomer

In the present investigation reconstitution of Na+,K(+)-ATPase increases the number of phosphorylation sites (EP) of solubilized enzyme from 4.2 +/- 0.3 nmol/mg to 6.9 +/- 0.6 nmol/mg. The latter figure corresponds to one phosphorylation site per alpha beta-promoter. A cholesterol content > 10 mol% in the liposome bilayer and a high extracellular [Na+] are necessary to obtain this high value. Spontaneous dephosphorylation after maximum phosphorylation in Na+ is biphasic both in solubilized enzyme and after reconstitution. The rate of dephosphorylation compares with the specific hydrolytic Na(+)-ATPase activity measured at exactly identical conditions for all three preparations assuming parallel dephosphorylation of at least two phosphointermediates. The distribution of EP-species is found to vary among the three enzyme preparation used, i.e., membrane bound, solubilized, and reconstituted Na+,K(+)-ATPase, however in all the equilibrium is strongly poised away from the E1P-form.

Investigation of ion binding to the cytoplasmic binding sites of the Na,K-pump

A dual-wavelength fluorimeter was constructed, which used two light emitting diodes (LEDs) to excite the fluorescence dye RH 421 alternately with two different wavelengths. The ratio of the emissions at the two excitation wavelengths provided a drift-insensitive signal, which allowed detection of very small changes of the fluorescence intensity. Those small changes were induced by ion binding and release in conformation E1 of the Na,K-ATPase. Titration experiments were performed to determine equilibrium dissociation constants (+/- standard deviation) for each step in the complete binding and release sequence: 0.12 +/- 0.01 mM (E2(K2)<==>KE1), 0.08 +/- 0.01 mM (KE1<==>E1A), 3.0 +/- 0.2 mM (NaE1<==>E1), 5.2 +/- 0.4 mM (Na2E1<==>NaE1) and 6.5 +/- 0.4 mM (Na3E1<==>Na2E1) at pH 7.2 and T = 16 degrees C. These numbers show that the affinities of the binding sites exposed to the cytoplasm, are higher for K+ than for Na+ ions, similar to what was found on the extracellular side. The physiological requirement for extrusion of Na+ from the cytoplasm, and for import of K+ from the extracellular medium seems to be facilitated not by favorable binding affinities in state E1 but by the two ATP-driven reaction steps of the cycle, E2(K2) + ATP-->K2E1.ATP and Na3E1.ATP<==>(Na3) E1-P, which border the ion exchange reactions at the binding sites in conformation E1.

Triflocin, a novel inhibitor for the Na-HCO3 symport in the proximal tubule

1. Triflocin, applied at millimolar concentration hyperpolarizes the basolateral membrane of Necturus proximal convoluted tubular cells, in vivo. 2. Barium, 2.5 x 10(-3) M, ouabain, 10(-3) M, or amiloride 10(-4) M, fail to prevent this hyperpolarization. 3. Triflocin has no effect on the intracellular chloride activity. 4. In physiological acid base conditions, Triflocin increases intracellular pH. 5. Upon an acute isohydric hypercapnia, Triflocin depolarizes the basolateral membrane potential. 6. It is concluded that, Triflocin inhibits the basolateral electrogenic Na-(HCO3)n > 1 cotransport in proximal tubules.

Access channel model for the voltage dependence of the forward-running Na+/K+ pump

The voltage dependence of steady state current produced by the forward mode of operation of the endogenous electrogenic Na+/K+ pump in Na(+)-loaded Xenopus oocytes has been examined using a two-microelectrode voltage clamp technique. Four experimental cases (in a total of 18 different experimental conditions) were explored: variation of external [Na+] ([Na]o) at saturating (10 mM) external [K+] ([K]o), and activation of pump current by various [K]o at 0, 15, and 120 mM [Na]o (tetramethylammonium replacement). Ionic current through K+ channels was blocked by Ba2+ (5 mM) and tetraethylammonium (20 mM), thereby allowing pump-mediated current to be measured by addition or removal of external K+. Control measurements and corrections were made for pump current run-down and holding current drift. Additional controls were done to estimate the magnitude of the inwardly directed pump-mediated current that was present in K(+)-free solution and the residual K(+)-channel current. A pseudo two-state access channel model is described in the Appendix in which only the pseudo first-order rate coefficients for binding of external Na+ and K+ are assumed to be voltage dependent and all transitions between states in the Na+/K+ pump cycle are assumed to be voltage independent. Any three-state or higher order model with only two oppositely directed voltage-dependent rate coefficients can be reduced to an equivalent pseudo two-state model. The steady state current-voltage (I-V) equations derived from the model for each case were simultaneously fit to the I-V data for all four experimental cases and yielded least-squares estimates of the model parameters. The apparent fractional depth of the external access channel for Na+ is 0.486 +/- 0.010; for K+ it is 0.256 +/- 0.009. The Hill coefficient for Na+ is 2.18 +/- 0.06, and the Hill coefficient for K+ (which is dependent on [Na]o) ranges from 0.581 +/- 0.019 to 1.35 +/- 0.034 for 0 and 120 mM [Na]o, respectively. The model provides a reasonable fit to the data and supports the hypothesis that under conditions of saturating internal [Na+], the principal voltage dependence of the Na+/K+ pump cycle is a consequence of the existence of an external high-field access channel in the pump molecule through which Na+ and K+ ions must pass in order to reach their binding sites.

Effect of osmotic swelling and shrinkage on Na(+)-K+ pump activity in mammalian cardiac myocytes

The effect on the sarcolemmal Na(+)-K+ pump of exposure to anisosmolar solutions was examined using whole cell patch clamping and ion-selective microelectrodes. Na(+)-K+ pump currents were measured in single ventricular myocytes by using pipette Na+ concentrations ([Na]pip) of 0-70 mM. The relationship between [Na]pip and pump current was well described by the Hill equation. The [Na]pip for half-maximal pump current (K0.5) was 21.4 mM in isosmolar (310 mosM) solution. K0.5 was 12.8 mM during cell swelling in hyposmolar solution (240 mosM) and 39.0 mM during cell shrinkage in hyperosmolar solution (464 mosM). The maximal pump currents, derived from the best fit of the Hill equation, and the Hill coefficients were similar in isosmolar, hyposmolar, and hyperosmolar solutions. A sustained (> 20 min) decrease in the intracellular Na+ activity developed during exposure of intact papillary muscles to hyposmolar solutions, and a sustained increase developed during exposure to hyperosmolar solutions. We conclude that osmotic myocyte swelling stimulates the sarcolemmal Na(+)-K+ pump at near-physiological levels of intracellular Na+, whereas shrinkage inhibits the pump. These changes are due to increases and decreases, respectively, in the apparent affinity of the pump for Na+.

Functional consequences of alterations to Pro328 and Leu332 located in the 4th transmembrane segment of the alpha-subunit of the rat kidney Na+,K(+)-ATPase

Site-specific mutagenesis was used to analyse the functional roles of the residues Pro328 and Leu332 located in the conserved PEGLL motif of the predicted transmembrane helix M4 in the alpha 1-subunit of the ouabain resistant rat kidney Na+,K(+)-ATPase. cDNAs encoding either of the Na+,K(+)-ATPase mutants Pro328-->Ala and Leu332-->Ala, and wild type, were cloned into the expression vector pMT2 and transfected into COS-1 cells. Ouabain-resistant clones growing in the presence of 10 microM ouabain were isolated, and the Na+,K+, ATP and pH dependencies of the Na+,K(+)-ATPase activity measured in the presence of 10 microM ouabain were analysed. Under these conditions the exogenous expressed Na+,K(+)-ATPase contributed more than 95% of the Na+,K(+)-ATPase activity. The Pro328-->Ala mutant displayed a reduced apparent affinity for Na+ (K0.5 (Na+) 13.04 mM), relative to the wild type (K0.5 (Na+) 7.13 mM). By contrast, the apparent affinity for Na+ displayed by the Leu332-->Ala mutant was increased (K0.5 (Na+) 3.92 mM). Either of the mutants exhibited lower apparent affinity for K+ relative to the wild type (K0.5 (K+) 2.46 mM for Pro328-->Ala and 1.97 mM for Leu332-->Ala, compared with 0.78 mM for wild type). Both mutants exhibited higher apparent affinity for ATP than the wild type (K0.5 (ATP) 0.086 mM for Pro328-->Ala and 0.042 mM for Leu332-->Ala, compared with 0.287 mM for wild type). The influence of pH was in accordance with an acceleration of the E2 (K)-->E1 transition in the mutants relative to the wild type. These data are consistent with a role of Pro328 and Leu332 in the stabilization of the E2 form and of Pro328 in Na+ binding. The possible role of the mutated residues in K+ binding is discussed.

Spectroscopic investigations of the potential-sensitive membrane probe RH421

The absorbance spectra, fluorescence emission and excitation spectra, and fluorescence anisotropy of the potential-sensitive styryl dye RH421 have been investigated in aqueous solution and bound to the lipid membrane. The potential-sensitive response of the dye has been studied using a preparation of membrane fragments containing a high density of Na+, K(+)-ATPase molecules. In aqueous solution the dye is sensitive both to changes in pH and ionic strength. Evidence has been found that the dye readily aggregates in aqueous solution. Aggregation is enhanced by an increase in ionic strength. The aggregates formed display a low fluorescence intensity. At high pH values (above approx. 8) changes in the dye's fluorescence spectra are observed, which may be due to a reaction of the dye with hydroxide ions. When bound to the membrane the dye also exhibits concentration-dependent fluorescence changes. The potential-sensitive response of the dye in Na(+),K(+)-ATPase membrane fragments after addition of MgATP in the presence of Na+ ions cannot be explained by a purely electrochromic mechanism. The results are consistent with either a potential-dependent equilibrium between membrane-bound dye monomers and membrane-bound dimers, similar to that previously proposed for the dye merocyanine 540, or with a field-induced structural change of the membrane.

Cis-allosteric effects of cytoplasmic Na+/K+ discrimination at varying pH. Low-affinity multisite inhibition of cytoplasmic K+ in reconstituted Na+/K(+)-ATPase engaged in uncoupled Na(+)-efflux

In liposomes with reconstituted shark Na+/K(+)-ATPase the effect of cytoplasmic K+ was investigated in the absence of extracellular alkali ions. During such conditions the Na+/K(+)-ATPase is engaged in the so called uncoupled Na+ efflux mode in which cytoplasmic Na+ activates and binds to the enzyme and becomes translocated without countertransport of K+ as in the physiological Na+/K+ exchange mode. In this uncoupled flux mode only low-affinity inhibition by K+cyt is found to be present. The inhibition pattern is consistent with a model in which cytoplasmic K+ exhibit mixed inhibition of Na+ activation, probably by binding at the three cytoplasmic loading sites on E1ATP (E1A). With determined intrinsic binding constants for cytoplasmic Na+ to this form of KS1, KS2, KS3 = 40 mM, 2 mM, 2 mM the inhibition pattern can be simulated assuming three K+cyt sites with equal affinity for Ki = 40 mM, similar to KS1 for the first Na+cyt site. The discrimination between cytoplasmic Na+ and K+ is therefore enhanced by allosteric interaction initiated from the cis-side due to binding of the first Na+, as opposed to K+, which induces the positive cooperatively in the successive Na+ bindings. pH is found to influence the pattern of K+cyt inhibition: A lowering of the pH potentiates the K+cyt inhibition, whereas at increased pH the inhibition is decreased and transformed into a pure competitive competition.

The reaction sequence of the Na+/K(+)-ATPase: rapid kinetic measurements distinguish between alternative schemes

Conformational changes between E1 and E2 enzyme forms of a dog kidney Na+/K(+)-ATPase preparation labeled with 5-iodoacetamidofluorescein were followed with a stopped-flow fluorimeter, in terms of the rate constant, kobs, and the steady-state magnitude, % delta F of fluorescence change. On rapid mixing of enzyme plus Mg2+ plus Na+ with saturating (0.5 mM) ATP in the absence of K+, kobs varied with Na+ concentration in the range 0-155 mM, with a K1/2 of 10 mM, while % delta F was relatively insensitive to Na+, with a K1/2 of 0.5 mM. Oligomycin reduced kobs by 98-99% for Na+ greater than or equal to 10 mM, but only by 50% for Na+ = 1 mM; % delta F was reduced at most by 20%. At 155 mM Na+, both kobs and % delta F changed if K+ was present with the enzyme. kobs decreased by 50% when K+ was increased from 0 to 0.2 mM, but increased when K+ was varied in the range 0.2-5 mM. K+ increased % delta F by a factor of 3 with a K1/2 of 0.3-0.5 mM as measured in both stopped-flow and steady-state experiments. These data are considered in terms of the derived presteady-state equations for two alternate schemes for the enzyme, with the E1P to E2P conformational change either preceding (Albers-Post) or following (Nørby-Yoda-Skou) Na+ transport and release. The analysis indicates that: (i) Na+ must be released before the conformational transition, from an E1 form; (ii) the step in which the second and/or third Na+ is released is rate-limiting, but this release is accelerated by Na+; and (iii) the release is also accelerated by K+ acting with low affinity (possibly at extracellular sites).

A 19-kDa C-terminal tryptic fragment of the alpha chain of Na/K-ATPase is essential for occlusion and transport of cations

Tryptic digestion of pig renal Na/K-ATPase in the presence of Rb and absence of Ca ions removes about half of the protein but leaves a stable 19-kDa membrane-embedded fragment derived from the alpha chain, a largely intact beta chain, and essentially normal Rb- and Na-occlusion capacity. Subsequent digestion with trypsin in the presence of Ca or absence of Rb ions leads to rapid loss of the 19-kDa fragment and a parallel loss of Rb occlusion, demonstrating that the fragment is essential for occlusion. The N-terminal sequence of the 19-kDa fragment is Asn-Pro-Lys-Thr-Asp-Lys-Leu-Val-Asn-Glu-Arg-Leu-Ile-Ser-Met-Ala, beginning at residue 830 and extending toward the C terminus. Membranes containing the 19-kDa fragment have the following functional properties. (i) ATP-dependent functions are absent. (ii) The apparent affinity for occluding Rb is unchanged, the affinity for Na is lower than in the control enzyme, and activation is now strongly sigmoidal rather than hyperbolic. (iii) Membranes containing the 19-kDa fragment can be reconstituted into phospholipid vesicles and sustain slow Rb-Rb exchange. Thus the transport pathway is retained. We conclude that cation occlusion sites and the transport pathway within transmembrane segments are quite separate from the ATP binding site, located on the cytoplasmic domain of the alpha chain. Interactions between cation and ATP sites, the heart of active transport, must be indirect--mediated, presumably, by conformational changes of the protein.

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.

Electrogenic and electroneutral transport modes of renal Na/K ATPase reconstituted into proteoliposomes

This paper describes measurements of electrical potentials generated by renal Na/K-ATPase reconstituted into proteoliposomes, utilizing the anionic dye, oxonol VI. Calibration of absorption changes with imposed diffusion potentials allows estimation of absolute values of electrogenic potentials. ATP-dependent Nacyt/Kexc exchange in K-loaded vesicles generates large potentials, up to 250 mV. By comparing initial rates or steady-state potentials with ATP-dependent 22Na fluxes in different conditions, it is possible to infer whether coupling ratios are constant or variable. For concentrations of Nacyt (2-50 mM) and ATP (1-1000 microM) and pH's (6.5-8.5), the classical 3Nacyt/2Kexc coupling ratio is maintained. However, at low Nacyt concentrations (less than 0.8 mM), the coupling ratio is apparently less than 3Nacyt/2Kexc. ATP-dependent Nacyt/congenerexc exchange in vesicles loaded with Rb, Cs, Li and Na is electrogenic. In this mode congeners, including Naexc, act as Kexc surrogates in an electrogenic 3Nacyt/2congenerexc exchange. (ATP + Pi)-dependent Kcyt/Kexc exchange in K-loaded vesicles is electroneutral. ATP-dependent "uncoupled" Na flux into Na- and K-free vesicles is electroneutral at pH 6.5-7.0 but becomes progressively electrogenic as the pH is raised to 8.5. The 22Na flux shows no anion specificity. We propose that "uncoupled" Na flux is an electroneutral 3Nacyt/3Hexc exchange at pH 6.5-7.0 but at higher pH's the coupling ratio changes progressively, reaching 3Na/no ions at pH 8.5. Slow passive pump-mediated net K uptake into Na- and K-free vesicles is electroneutral, and may also involve Kcyt/Hexc exchange. We propose the general hypothesis that coupling ratios are fixed when cation transport sites are saturated, but at low concentrations of transported cations, e.g., Nacyt in Na/K exchange and Hexc in "uncoupled" Na flux, coupling ratios may change.

[Na] and [K] dependence of the Na/K pump current-voltage relationship in guinea pig ventricular myocytes

Na/K pump current was determined between -140 and +60 mV as steady-state, strophanthidin-sensitive, whole-cell current in guinea pig ventricular myocytes, voltage-clamped and internally dialyzed via wide-tipped pipettes. Solutions were designed to minimize all other components of membrane current. A device for exchanging the solution inside the pipette permitted investigation of Na/K pump current-voltage (I-V) relationships at several levels of pipette [Na] [( Na]pip) in a single cell; the effects of changes in external [Na] [( Na]o) or external [K] [( K]o) were also studied. At 50 mM [Na]pip, 5.4 mM [K]o, and approximately 150 mM [Na]o, Na/K pump current was steeply voltage dependent at negative potentials but was approximately constant at positive potentials. Under those conditions, reduction of [Na]o enhanced pump current at negative potentials but had little effect at positive potentials: at zero [Na]o, pump current was only weakly voltage dependent. At 5.4 mM [K]o and approximately 150 mM [Na]o, reduction of [Na]pip from 50 mM scaled down the sigmoid pump I-V relationship and shifted it slightly to the right (toward more positive potentials). Pump current at 0 mV was activated by [Na]pip according to the Hill equation with best-fit K0.5 approximately equal to 11 mM and Hill coefficient nH approximately equal to 1.4. At zero [Na]o, reduction of [Na]pip seemed to simply scale down the relatively flat pump I-V relationship: Hill fit parameters for pump activation by [Na]pip at 0 mV were K0.5 approximately equal to 10 mM, nH approximately equal to 1.4. At 50 mM [Na]pip and high [Na]o, reduction of [K]o from 5.4 mM scaled down the sigmoid I-V relationship and shifted it slightly to the right: at 0 mV, K0.5 approximately equal to 1.5 mM and nH approximately equal to 1.0. At zero [Na]o, lowering [K]o simply scaled down the flat pump I-V relationships yielding, at 0 mV, K0.5 approximately equal to 0.2 mM, nH approximately equal to 1.1. The voltage-independent activation of Na/K pump current by both intracellular Na ions and extracellular K ions, at zero [Na]o, suggests that neither ion binds within the membrane field. Extracellular Na ions, however, seem to have both a voltage-dependent and a voltage-independent influence on the Na/K pump: they inhibit outward Na/K pump current in a strongly voltage-dependent fashion, with higher apparent affinity at more negative potentials (K0.5 approximately equal to 90 mM at -120 mV, and approximately 170 mM at -80 mV), and they compete with extracellular K ions in a seemingly voltage-independent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Conformational transitions and change translocation by the Na,K pump: comparison of optical and electrical transients elicited by ATP-concentration jumps

The electrogenic properties of the Na,K-ATPase were studied by correlating transient electrical events in the pump molecule with conformational transitions elicited by an ATP-concentration jump. Flat membrane fragments containing a high density (approximately 8000 microm(-2)) of oriented Na,K-ATPase molecules were bound to a planar lipid bilayer acting as a capacitive electrode. ATP was released in the medium from a photolabile inactive ATP derivative ("caged" ATP) by a 40-microsec light flash. Electrical signals resulting from transient charge movements in the protein under single-turnover conditions were recorded in the external measuring circuit. In parallel experiments carried out under virtually identical conditions, the fluorescence of membrane fragments containing Na,K-ATPase with covalently-bound 5-iodoacetamido-fluorescein (5-IAF) was monitored after the ATP-concentration jump. When the medium contained Na+, but no K+, the fluorescence of the 5-IAF-labeled protein decreases monotonously after release of ATP. In the experiments with membrane fragments bound to a planar bilayer, a transient pump current was observed which exhibited virtually the same time behavior as the fluorescence decay. This indicates that optical and electrical transients are governed by the same rate-limiting reaction step. Experiments with chymotrypsin-modified Na,K-ATPase suggest that both the fluorescence change as well as the charge movement are associated with the deocclusion of Na+ and release to the extracellular side. In experiments with Na+-free K+ media, a large inverse fluorescence change is observed after the ATP-concentration jump, but no charge translocation can be detected. This indicates that deocclusion of K+ is an electrically silent process.

Voltage dependence of partial reactions of the Na+/K+ pump: predictions from microscopic models

A theoretical treatment of the voltage dependence of electroneutral Na+-Na+ and K+-K+ exchange mediated by the Na+/K+ pump is given. The analysis is based on the Post-Albers reaction scheme in which the overall transport process is described as a sequence of conformational transitions and ion-binding and ion-release steps. The voltage dependence of the exchange rate is determined by a set of 'dielectric coefficients' reflecting the magnitude of charge translocations associated with individual reaction steps. Charge movement may result from conformational changes of the transport protein and/or from migration of ions in an access channel connecting the binding sites with the aqueous medium. It is shown that valuable mechanistic information may be obtained by studying the voltage dependence of transport rates at different (saturating and nonsaturating) ion concentrations.

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.

The sided action of Na+ on reconstituted shark Na+/K+-ATPase engaged in Na+-Na+ exchange accompanied by ATP hydrolysis. II. Transmembrane allosteric effects on Na+ affinity

The objective of the present investigation was to characterize the ATP-dependent Na+-Na+ exchange, with respect to cation sensitivity on the two aspects of the Na+/K+-pump protein. In order to accomplish this, we used Na+/K+-ATPase reconstituted with known orientation in the proteoliposomes. Activation by cytoplasmic Na+ shows cooperative interaction between three sites. The apparent intrinsic site constants displayed transmembrane dependence on the extracellular Na+ concentration. However, the apparent K0.5 for cytoplasmic Na+ is independent of the extracellular Na+ concentration. The activation by extracellular Na+ at a fixed cytoplasmic Na+ concentration is biphasic with a component which saturates at a concentration of about 1-2 mM extracellular Na+, a plateau phase up to 20 mM, and another component which tends to saturate at about 80 mM followed by a slight deactivation at higher concentrations of Na+. The apparent K0.5 value for extracellular Na+ is also found to be independent of the Na+ concentration on the opposite side of the membrane. The activation by extracellular Na+ can be explained by the negative cooperativity in the binding of extracellular Na+, but positive cooperativity in the rate of dephosphorylation of enzyme species with one and three sodium ions bound extracellularly. Na+ bound to E2-PNa has a transmembrane effect on the cooperativity between binding of cytoplasmic Na+, and E2-PNa2 does not dephosphorylate. K0.5/Vm for cytoplasmic as well as for extracellular Na+ decreases with an increase in the trans Na+ concentration in the non-saturating concentration range. The experiments indicate that at a step in the reaction simultaneous binding of extracellular and cytoplasmic Na+ occurs.

Voltage dependence of the Na-K ATPase: measurements of ouabain-dependent membrane current and ouabain binding in oocytes of Xenopus laevis

Ouabain- or dihydroouabain(DHO)-sensitive membrane currents and binding of 3H-ouabain were investigated under voltage-clamp conditions in full-grown prophase-arrested oocytes of Xenopus laevis. (1) The ouabain-sensitive current is outwardly directed and usually exhibits a maximum at about +20 mV. The occurrence of the maximum is not affected by application of blockers for passive K+ currents. In the presence of Ba2+ as a K+ channel blocker, the KI value for inhibition of the Na-K ATPase by ouabain is reduced by an order of magnitude, the number of binding sites is not affected. In K+-free solution (which inhibits the normal reaction cycle of the Na-K ATPase), addition of DHO has no significant effect on the remaining currents. (2) The voltage-dependence of the ouabain-sensitive current can be modulated. Reduction of extracellular Na+ increases the pump current at the resting potential and reduces the positive slope of the I-V curve. Simultaneously, the number of binding sites for ouabain is reduced by about 25%. Seasonal variations of an unknown factor affect the negative slope. (3) Modulation of the voltage-clamp conditions has no effect on the number of binding sites for ouabain, on current inhibition by ouabain, or on ouabain binding at different concentrations of the inhibitor. It is concluded that the ouabain-sensitive current is not significantly affected by passive permeabilities and that its current-voltage dependence reflects the voltage dependence of current generated by the Na-K pump. Since ouabain binding, also at non-maximal binding concentrations, is not affected by membrane potential, steps that affect the probability of the E2P state should be voltage-insensitive.

Role of the sodium pump and the background K+ channel in passive K+(Rb+) uptake by isolated cardiac sarcolemmal vesicles

A simple procedure was developed for the isolation of a sarcolemma-enriched membrane preparation from homogenates of bullfrog (Rana catesbeiana) heart. Crude microsomes obtained by differential centrifugation were fractionated in Hypaque density gradients. The fraction enriched in surface membrane markers consisted of 87% tightly sealed vesicles. The uptake of 86Rb+ by the preparation was measured in the presence of an opposing K+ gradient using a rapid ion exchange technique. At low extravesicular Rb+ concentrations, at least 50% of the uptake was blocked by addition of 1 mM ouabain to the assay medium. Orthovanadate (50 microM), ADP (2.5 mM) or Mg (1 mM) were also partial inhibitors of Rb+ uptake under these conditions, and produced a complete block of Rb+ influx in the presence of 1 mM ouabain. When 86Rb+ was used as a tracer of extravesicular K+ (Rb+0 less than or equal to 40 microM, K+0 = 0.1-5 mM) a distinct uptake pathway emerged, as detected by its inhibition by 1 mM Ba2+ (K0.5 = 20 microM). At a constant internal K+ concentration (K+in = 50 mM), the magnitude of the Ba2+-sensitive K+ uptake was found to depend on K+0 in a manner that closely resembles the K+ concentration dependence of the background K+ conductance (IK1) observed electrophysiologically in intact cardiac cells. We conclude that K+ permeates passively this preparation through two distinct pathways, the sodium pump and a system identifiable as the background potassium channel.

Na,K pump stimulation by intracellular Na in isolated, intact sheep cardiac Purkinje fibers

Regulation of the Na,K pump in intact cells is strongly associated with the level of intracellular Na+. Experiments were carried out on intact, isolated sheep Purkinje strands at 37 degrees C. Membrane potential (Vm) was measured by an open-tipped glass electrode and intracellular Na+ activity (aNai) was calculated from the voltage difference between an Na+-selective microelectrode (ETH 227) and Vm. In some experiments, intracellular potassium (aiK) or chloride (aCli) was measured by a third separate microelectrode. Strands were loaded by Na,K pump inhibition produced by K+ removal and by increasing Na+ leak by removing Mg++ and lowering free Ca++ to 10(-8) M. Equilibrium with outside levels of Na+ was reached within 30-60 min. During sequential addition of 6 mM Mg++ and reduction of Na+ to 2.4 mM, the cells maintained a stable aNai ranging between 25 and 90 mM and Vm was -30.8 +/- 2.2 mV. The Na,K pump was reactivated with 30 mM Rb+ or K+. Vm increased over 50-60 s to -77.4 +/- 5.9 mV with Rb+ activation and to -66.0 +/- 7.7 mV with K+ activation. aiNa decreased in both cases to 0.5 +/- 0.2 mM in 5-15 min. The maximum rate of aiNa decline (maximum delta aNai/delta t) was the same with K+ and Rb+ at concentrations greater than 20 mM. The response was abolished by 10(-5) M acetylstrophantidin. Maximum delta aNai/delta t was independent of outside Na+, while aKi was negatively correlated with aNai (aKi = 88.4 - 0.86.aNai). aCli decreased by at most 3 mM during reactivation, which indicates that volume changes did not seriously affect aNai. This model provided a functional isolation of the Na,K pump, so that the relation between the pump rate (delta aNai/delta t) and aiNa could be examined. A Hill plot allowed calculation of Vmax ranging from 5.5 to 27 mM/min, which on average is equal to 25 pmol.cm-2.s-1.K 0.5 was 10.5 +/- 0.6 mM (the aNai that gives delta aNai/delta t = Vmax/2) and n equaled 1.94 +/- 0.13 (the Hill coefficient). These values were not different with K+ or Rb+ as an external activator. The number of ouabain-binding sites equaled 400 pmol.g-1, giving a maximum Na+ turnover of 300 s-1. The Na,K pump in intact Purkinje strands exhibited typical sigmoidal saturation kinetics with regard to aNai as described by the equation upsilon/Vmax = aNai(1.94)/(95.2 + aNai(1.94)). The maximum sensitivity of the Na,K pump to aiNa occurred at approximately 6 mM.

The sided action of Na+ and of K+ on reconstituted shark (Na+ + K+)-ATPase engaged in Na+-Na+ exchange accompanied by ATP hydrolysis. I. The ATP activation curve

The ATP hydrolysis dependent Na+-Na+ exchange of reconstituted shark (Na+ + K+)-ATPase is electrogenic with a transport stoichiometry as for the Na+-K+ exchange, suggesting that translocation of extracellular Na+ is taking place via the same route as extracellular K+. The preparation thus offers an opportunity to compare the sided action of Na+ and K+ on the affinity for ATP in a reaction in which the intermediary steps in the overall reaction seems to be the same without and with K+. With Na+ but no K+ on the two sides of the enzyme, the ATP-activation curve is hyperbolic and the affinity for ATP is high. Extracellular K+ in concentrations of 50 microM (the lowest tested) and up gives biphasic ATP activation curves, with both a high- and a low-affinity component for ATP. Cytoplasmic K+ also gives biphasic ATP-activation curves, however, only when the K+ concentration is 50 mM or higher (Na+ + K+ = 130 mM). The different ATP-activation curves are explained from the Albers-Post scheme, in which there is an ATP-dependent and an ATP-independent deocclusion of E2(Na2+) and E2(K2+), respectively, and in which the dephosphorylation of E2-P is rate limiting in the presence of Na+ (but no K+) extracellular, whereas in the presence of extracellular K+ it is the deocclusion of E2(K2+) which is rate limiting.

Rb+ occlusion in renal (Na+ + K+)-ATPase characterized with a simple manual assay

This paper describes properties of a simple manual assay for Rb+ occlusion on renal (Na+ + K+)-ATPase. Rb+ occlusion is measured by applying the enzyme plus Rb+ (86Rb) mixture to a Dowex-50 cation exchange column at 0 degree C, and eluting the enzyme with occluded Rb+ using an ice-cold sucrose solution. The enzyme-Rb+ complex is quite stable at 0 degree C. This method is useful for measuring Rb+ occlusion under equilibrium binding conditions and slow rates of dissociation of the enzyme-Rb+ complex. The stoichiometry of Rb+ occluded per phosphorylation site is 2. Rb+ saturation curves are strictly hyperbolic, suggesting that the two Rb+ sites have very different affinities, one in the micromolar range and one in the tens of millimolar range. ATP shifts the Rb+ saturation curves to the right (control K0.5 100-200 microM; plus ATP, K0.5 0.8-1.4 mM, in a 100 mM Tris-HCl medium, pH 7.0) and reduces the maximal level occluded (control approx. 4 nmol/mg; plus ATP approx. 3 nmol/mg protein). Thus, as expected, ATP shifts the E(1)2Rb+-E2(2Rb+)occ equilibrium towards E1. Sodium ions at concentrations of up to 30 mM compete with the rubidium ions, KNa = 1.86 mM in the Tris-HCl medium. Na+ at higher concentrations (30-100 mM) has an added non-competitive antagonistic effect. At room temperature, Rb+ dissociates slowly from the enzyme, kobs = 0.08 s-1, in the presence of either Rb+ (20 mM) or Na, (100 mM). As expected, dissociation is greatly accelerated by ATP, the rate being to fast to be measured by this technique. (Na+ + K+)-ATPase proteolyzed selectively by chymotrypsin in a Na+ medium, occludes Rb+. For control and proteolyzed (Na+ + K+)-ATPase the Rb+ saturation curves are similar and the rates of dissociation of the enzyme-Rb+ complex are identical. The chymotryptic split appears to disrupt antagonistic interactions between cation and ATP binding domains, while the E1-E2 conformational transition of the unphosphorylated protein probably remains.

The effect of membrane potential on the mammalian sodium-potassium pump reconstituted into phospholipid vesicles

1. We have studied effects of electrical diffusion potentials on active Na+-K+ exchange in phospholipid vesicles reconstituted with pig kidney Na+, K+-ATPase. 2. Diffusion potentials, negative inside, were established using outwardly directed K+ gradients plus valinomycin or Li+ gradients plus a Li+ ionophore, AS701. Measurement of fluorescence changes of the carbocyanine dye DiS-C3-(5) showed that the ionophores generated potentials of the expected orientation and of sufficient stability for their effects on active transport to be assessed. Measurement of rates of passive 22Na+ fluxes, over a wide range of diffusion potentials, were consistent with the quantitative predictions of the constant-field flux equation. This result demonstrates that values of diffusion potentials calculated from the Nernst or constant-field equation are accurate. 3. In some conditions, the inside-negative potential (-130 to -180 mV) accelerated the rate of ATP-dependent Na+-K+ exchange on inside-out-oriented pumps, compared to 'control' without the ionophores. Reduction in the size of the diffusion potentials by addition to the medium of Li+ with AS701 or Cs+ with the valinomycin progressively annulled the acceleratory effects, consistent with these being true effects of a change in membrane potentials. 4. At saturating cytoplasmic Na+ and ATP concentrations, the diffusion potential accelerated ATP-dependent Na+-K+ exchange by up to about 30% compared to control but this effect disappeared at rate-limiting ATP concentrations (approximately 1 microM). 5. Using prior knowledge of rate-limiting steps, we interpret this finding to mean that the conformational transition E2(2K)----E12K associated with transport of two K+ ions is voltage insensitive while E1P(3Na)----E2P3Na associated with transport of three Na+ ions is voltage sensitive. The simplest explanation is that the net charge in the transport domain of the protein when no ions, 2K+ or 3Na+ are bound is -2, 0 and +1 respectively. 6. The accelerating effect of the negative-inside diffusion potential on Na+-K+ exchange is greater at limitingly low cytoplasmic Na+ concentrations than at saturating cytoplasmic Na+ concentrations. Cytoplasmic Na+ activation curves show that the diffusion potential increases the apparent cytoplasmic Na+ affinity and reduces the sigmoidicity of cytoplasmic Na+ activation. 7. A kinetic analysis reveals that this effect on apparent affinity is due to an increase in intrinsic Na+ binding and occurs in addition to the effect on a transport rate constant.(ABSTRACT TRUNCATED AT 400 WORDS)