Na+/K(+)-ATPase regulation by neurotransmitters.

A long period of experimental work has led to the conclusion that Na+/K(+)-ATPase is the enzymatic version of the Na+/K+ pump. This enzymatic system is in charge of various important cell functions. Among them cationic equilibrium and recovering of resting membrane potential in neurons is relevant. A tetrameric ensemble of peptides conform the system known as alpha and beta subunits. The alpha subunit is subdivided in alpha 1, alpha 2 and alpha 3, according to different location and properties. Regulatory factors intrinsic to the Na+/K(+)-ATPase system are: ATP, Na+ and Mg2+ concentrations inside the cell, and K+ outside. The enzyme activity is also regulated by extrinsic factors like some hormones (insulin and thyroxine). Induction of gene expression or post-translational modifications of the preexisting pool of the enzyme are the basic mechanisms of regulation proposed. Other extrinsic factors that seem to regulate the enzyme activity are some neurotransmitters. Among them the most extensively studied are catecholamines, mainly norepinephrine (NE) and lately serotonin (5-HT). The mechanism suggested for NE activation of the enzyme seems to involve specific receptors or a non-specific chelating action related to the catechol group that would relieve the inhibition by divalent cations. Another possibility is that NE removes an endogenous inhibitory factor present in the cytoplasm. The Na+/K(+)-ATPase is activated also by 5-HT. In vivo pharmacological and nutriological manipulations of brain 5-HT are accompanied by parallel responses of Na+/K(+)-ATPase activity. Serotonin agonists do activate the enzyme and antagonists neutralize the activation. In vitro there is a different dose dependent activation, according to the brain region. The mechanism involved seems to implicate a specific receptor system. Serotonin-Na+/K(+)-ATPase interaction in the rat brain is probably of functional relevance because it disappears in amygdaloid kindling. Also it seems to influence the ionic regulation of the pigment transport mechanism in crayfish photoreceptors. In relation to other neurotransmitters, a weak response to histamine was observed with acetylcholine, GABA and glutamic acid, the results were negative.     

Effects of choline on Na(+)- and K(+)-interactions with the Na+/K(+)-ATPase.

Choline chloride, 100 mM, stimulates Na+/K(+)-ATPase activity of a purified dog kidney enzyme preparation when Na+ is suboptimal (9 mM Na+ and 10 mM K+) and inhibits when K+ is suboptimal (90 mM Na+ and 1 mM K+), but has a negligible effect at optimal concentrations of both (90 mM Na+ and 10 mM K+). Stimulation occurs at low Na+ to K+ ratios, but not at those same ratios when the actual Na+ concentration is high (90 mM). Stimulation decreases or disappears when incubation pH or temperature is increased or when Li+ is substituted for K+ or Rb+. Choline+ also reduces the Km for MgATP at the low ratio of Na+ to K+ but not at the optimal ratio. In the absence of K+, however, choline+ does not stimulate at low Na+ concentrations: either in the Na(+)-ATPase reaction or in the E1 to E2P conformational transition. Together, these observations indicate that choline+ accelerates the rate-limiting step in the Na+/K(+)-ATPase reaction cycle, K(+)-deocclusion; consequently, optimal Na+ concentrations reflect Na+ accelerating that step also. Thus, the observed K0.5 for Na+ includes high-affinity activation of enzyme phosphorylation and low-affinity acceleration of K(+)-deocclusion. Inhibition of Na+/K(+)-ATPase and K(+)-nitrophenylphosphatase reactions by choline+ increases as the K(+)-concentration is decreased; the competition between choline+ and K+ may represent a similar antagonism between conformations selected by choline+ and by K+.     

Trophectoderm development and function: the roles of Na+/K(+)-ATPase subunit isoforms

Preimplantation development is a period of cell division, cell shape change, and cell differentiation leading to the formation of an epithelium, the trophectoderm. The trophectoderm is the part of the conceptus that initiates uterine contact and, after transformation to become the trophoblast, uterine invasion. Thus, trophectoderm development during preimplantation stages is a necessary antecedent to the events of implantation. The preimplantation trophectoderm is a transporting epithelium with distinct apical and basolateral membrane domains that facilitate transepithelial Na+ and fluid transport for blastocoel formation. That transport is driven by Na+/K(+)-ATPase localized in basolateral membranes of the trophectoderm. Preimplantation embryos express multiple alpha and beta subunit isoforms of Na+/K(+)-ATPase, potentially constituting multiple isozymes, but the basolaterally located alpha1beta1, isozyme uniquely functions to drive fluid transport. They also express the gamma subunit, which is a modulator of Na+/K(+)-ATPase activity. In the mouse, two splice variants of the gamma subunit, gammaa and gammab, are expressed in the trophectoderm. Antisense knockdown of gamma subunit accumulation caused a delay of cavitation, implying an important role in trophectoderm function. The preimplantation trophectoderm offers a unique model for understanding the roles of Na+/K(+)-ATPase subunit isoforms in transepithelial transport.     

Evidence for the binding of beta-adrenoceptor blockers to microsomal Na+/K+-ATPase in guinea pig heart preparations

We reported in a previous study that beta-adrenoceptor blockers inhibit the Mg2+-dependent ATP-hydrolytic function of Na+/K+-ATPase. To determine if this action is a result of binding of beta-blockers to the receptor sites that bind the digitalis glycosides, we performed displacement binding assays of eight beta-blockers with [3H]-ouabain (OUA) in guinea pig myocardial microsomal preparations. In the first series of experiments, 10-200 microM of the beta-blockers were displaced with 250 nM OUA. In the second set of experiments, 10-500 nM of OUA was displaced using 200 microM of the beta-blockers. The drugs showed concentration-dependent receptor occupancy at the different OUA levels. Propranolol (PPN), metoprolol (MTP), and sotalol (STL) showed the strongest binding; nadolol (NDL), indenolol (IDN), and atenolol (ATN) had intermediate binding; carazolol (CRZ) and celiprolol (CLP) had the weakest binding properties. The results suggest that beta-blockers may compete for the same binding sites with ouabain in their inhibition of the Na+/K+-ATPase. These actions may contribute to the mechanism for some of their cardiac effects, especially their proarrhythmic and arrhythmogenic actions.     

Assembly of a hybrid from the alpha subunit of Na+/K(+)-ATPase and the beta subunit of H+/K(+)-ATPase.

Messenger RNA for the alpha subunit of Torpedo californica Na+/K(+)-ATPase was injected into Xenopus oocytes together with that of the beta subunit of rabbit H+/K(+)-ATPase. The Na+/K(+)-ATPase alpha subunit was assembled in the microsomal membranes with the H+/K(+)-ATPase beta subunit, and became resistant to trypsin. These results suggest that the H+/K(+)-ATPase beta subunit facilitates the stable assembly of the Na+/K(+)-ATPase alpha subunit in microsomes.     

Na+/K(+)-ATPase as an effector of synaptic transmission.

Evidence is presented in support of the hypothesis that transmitter monoamines can exert their post-synaptic effects by stimulation or inhibition of Na+/K(+)-ATPase in neuronal or glial cell plasma membranes. Stimulation of electrogenic sodium pumping, causing a hyperpolarization with an increase in membrane resistance, could account for the depression of neuronal spontaneous firing and the signal/noise enhancing actions of these amines. Conversely, inhibition of an electrogenic sodium pump in neuronal plasma membranes would lead to depolarization and enhanced excitability.     

Anomalies in the electrophoretic resolution of Na+/K(+)-ATPase catalytic subunit isoforms reveal unusual protein--detergent interactions

Three different isozymes of the Na+/K(+)-ATPase have slightly different different electrophoretic mobilities in sodium dodecyl sulfate (SDS). Certain procedures (reduction and alkylation, heating, and the use of sodium tetradecyl sulfate) have been reported either to improve the electrophoretic separation of isoforms or to reveal the presence of new isoforms. The variables affecting gel electrophoretic mobility were investigated here. Reduction and alkylation decreased the mobility of all three isozymes, and slightly improved the separation of alpha 1 from alpha 2 and alpha 3 without causing a qualitative change in the alpha isoforms detected. Heating the enzyme in SDS caused splitting into two bands. Both bands were intact polypeptides but migrated differently in 5% and 15% polyacrylamide, disclosing an anomalous conformation in detergent. The use of sodium tetradecyl or decyl sulfate instead of dodecyl sulfate altered the relative mobilities of the isozymes, revealing differences in detergent affinity, but no new isoforms were found. In conclusion, Na+/K(+)-ATPase alpha-subunit mobility reflects complex detergent-protein interaction that can be affected by experimental conditions. The existence of more than one band on gels may reflect different conformations in detergent, but should not be accepted alone as evidence for subunit structural heterogeneity.     

Kinetics of oligomycin inhibition and activation of Na+/K(+)-ATPase.

Oligomycin inhibition of the maximal hydrolysis activity of ox brain Na+/K(+)-ATPase was studied at varying NaCl concentrations and it was found that for a given amount of live enzyme, the observed inhibition of a particular total oligomycin concentration decreased as the amount of added, (heat-) denatured enzyme increased. In the present article we derive a scale factor for the oligomycin concentration, i.e., the fraction of the total concentration of oligomycin which is free in solution, as a function of the enzyme concentration used. This fraction decreased linearly with the protein concentration and may attain quite small values. We also study the Na(+)-dependence of the hydrolysis rate at saturating substrate concentrations ([Mg2+] = [ATP] = 3 mM), in the presence as well as the absence of KCl, at various concentrations of oligomycin. These data may be explained if it is assumed that the sole effect of oligomycin is to confer upon the enzyme an increased affinity for Na+, i.e., oligomycin merely enhances the inhibitory effect of Na+ on the (maximal) activity seen at high Na(+)-concentrations. The increased Na(+)-affinity in the presence of oligomycin should result in activation of the hydrolysis rate measured under conditions where Na(+)-activation is predominant, i.e., at low Na(+)-concentration and sub-saturating substrate concentrations. This prediction is verified for both Na(+)-ATPase and for Na+/K(+)-ATPase. This proposed action of oligomycin seems to be corroborated also by other evidence discussed in the text.     

Effect of low calcium on high-energy phosphates and sarcolemmal Na+/K(+)-ATPase in the infarcted-reperfused heart.

This study was undertaken to compare the effect of low to normal serum calcium on biochemical parameters in the myocardium of dogs subjected to 90 min of coronary artery ligation followed by 30 min reperfusion. The accumulation of calcium, the decrease of adenosine triphosphate (ATP) and creatine phosphate (CP) and the inhibition of sarcolemmal ouabain-sensitive Na+/K(+)-ATPase which are prominent findings in the ischemic-reperfused myocardium, were studied under normal and low serum Ca produced by normal and modified hemodialysis (HD). The results showed a lower accumulation of Ca (P less than 0.002) in the ligated-reperfused myocardium of dogs subjected to low-calcium HD. In the same group of animals ATP was protected to some extent while CP was completely preserved. This may indicate that during reperfusion with low Ca, restored ATP is further utilized for CP regeneration. The activity of Na+/K(+)-ATPase was within normal values in the ligated-reperfused myocardium of the low-calcium group. The significantly (P less than 0.001) negative correlation between tissue calcium concentration and Na+/K(+)-ATPase activity under various conditions examined, provided additional evidence that low calcium is a protective factor of the enzyme activity during ischemia and reperfusion.     

The effect of cytoplasmic K+ on the activity of the Na+/K(+)-ATPase.

Experiments with the reconstituted (Na+ + K+)-ATPase show that besides the ATP-dependent cytoplasmic Na(+)-K+ competition for Na+ activation there is a high affinity inhibitory effect of cytoplasmic K+. In contrast to the high affinity K+ inhibition seen with the unsided preparation at a low ATP especially at a low temperature, the high affinity inhibition by cytoplasmic K+ does not disappear when the ATP concentration an-or the temperature is increased. The high affinity inhibition by cytoplasmic K+ is also observed with Cs+, Li+ or K+ as the extracellular cation, but the fractional inhibition is much less pronounced than with Na+ as the extracellular cation. The results suggest that either there are two populations of enzyme, one with the normal ATP dependent cytoplasmic Na(+)-K+ competition, and another which due to the preparative procedure has lost this ATP sensitivity. Or that the normal enzyme has two pathways for the transition from E2-P to E1ATP. One on which the enzyme with the translocated ion binds cytoplasmic K+ with a high affinity but not ATP, and another on which ATP is bound but not K+. A kinetic model which can accommodate this is suggested.     

Binding of ethylenediamine to phosphatidylserine is inhibitory to Na+/K(+)-ATPase.

Covalent linkage of ethylenediamine with the Na+/K(+)-ATPase complex from rabbit kidney outer medulla by the use of the water-soluble carbodiimide, N-ethyl,N'-(3-dimethylaminopropyl)carbodiimide, resulted in a 73% reaction with phosphatidylserine and only 27% with carboxylic groups in the proteic component of the enzyme. Condensation products from the reaction between phosphatidylserine and ethylenediamine, N-(O-phosphatidylseryl)ethylenediamine, N,N'-bis(O-phosphatidylseryl)ethylenediamine and its intermediary product O-phosphatidyl-[N,N'-bis(seryl)]ethylenediamine, were synthesised. Symmetrically substituted ethylenediamine was the most likely condensation product of ethylenediamine with endogenous phosphatidylserine. The synthesised lipids were incorporated in proteoliposomes containing Na+/K(+)-ATPase and only the addition of the phospholipid phosphatidylcholine. The ratio of phospholipid to protein was 52 (w/w). These proteoliposomes were perforated by the addition of 0.5% cholate and both the Na(+)-dependent phosphorylation level and its dependence on Na+, Mg2+ and ATP were measured. Phosphatidylcholine alone increased the half-maximal activation concentration for Na+ ([Na+]0.5) from 0.2 to 1-2 mM, for Mg2+ from 0.1 to 0.8 microM and for ATP from 0.02 to 0.3 microM. The Ki for K+ (in the absence of Na+) was unaffected: 12.8 microM vs. 12.5 microM in the non-reconstituted system. Replacing 10 mol% of phosphatidylcholine by phosphatidylethanolamine: or phosphatidylserine had no significant effect on [Na+]0.5: 1.1 and 0.7 mM, respectively. Replacing 5 mol% phosphatidylcholine by the bis(phosphatidylseryl) substituent of ethylenediamine further increased [Na+]0.5 to 13.7 mM, while half-maximal activation concentrations for Mg2+ and ATP were unaltered. The mono-phosphatidylseryl derivatives of ethylenediamine, each 5 mol%, also increased [Na+]0.5, but to a lesser extent (3.2-3.8 mM). In addition to their competitive effects, the phosphatidylseryl-substituted ethylenediamine compounds exerted a slowly-increasing non-competitive inhibition, not only in phosphorylation, but also in overall ATPase activity, which was reduced, although not abolished, by exogenous protein (bovine serum albumin). A detergent-like action in the usual sense is unlikely since liposomes containing these lipids remained intact. These studies prove that phospholipids are not only required for optimal activity of this transport enzyme, but in excess or in compositions deviating from the normal, may also be inhibitory.     

Mechanism of Na+/K(+)-ATPase activation by trypsin and kallikrein

The mechanism of the Na+/K(+)-ATPase activation by trypsin (from bovine pancreas) and kallikrein (from human plasma) was investigated on enzyme preparations from different sources (beef heart and dog kidney) and at different degrees of purification (beef heart). Kallikrein was effective on both beef and dog enzymes, whereas trypsin stimulated only the beef-heart Na+/K(+)-ATPase. The extent of activation by the proteinases was inversely related to the degree of purification (maximal enzyme activation about 60 and 20% on the partially purified and the more purified enzymes, respectively). Enzyme activation was observed up to 0.5-0.6 microgram/ml of proteinase. At higher concentrations the activation decreased and was converted into inhibition at proteinase concentrations above 1.0 micrograms/ml. Na+/K(+)-ATPase stimulation was due to an increase in the Vmax of the enzyme reaction. Km for ATP remained unaffected. The activating effect was favoured by sodium and counteracted by potassium. Accordingly, Na(+)-ATPase activity was stimulated to a greater extent (up to 350%), whereas K(+)-dependent p-nitrophenylphosphatase activity proved to be insensitive to the actions of the proteinases. The Na+/K(+)-ATPase stimulation by both proteinases was antagonized by either ouabain or canrenone, two drugs that bind on the extracellular side of the Na+/K(+)-ATPase molecule. On the contrary, the enzyme inactivation observed at high proteinase concentrations was not counteracted by these two drugs. The stimulation of either Na+/K(+)- or Na(+)-ATPase activity was shown to be an irreversible effect without any significant protein degradation detectable by SDS gel electrophoresis. The results obtained suggest that proteinases exert their stimulatory effects by interacting preferentially with the E2 conformation of Na+/K(+)-ATPase at site(s) located on the extracellular moiety of the enzyme.     

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.     

钠钾ATP酶的信号转导功能新进展

钠钾ATP酶不仅是主动跨膜转运钠钾离子的载体蛋白,而且可能作为内源性洋地黄物质的受体参与信号转导。它通过在质膜上与小窝蛋白、Src激酶之间的相互作用,并在细胞内借助Src激酶反式激活上皮生长因子受体,组装信号转导的复合物激活信号转导的级联反应,从而介导内源性洋地黄物质增加心脏和血管的收缩性、促进正常细胞肥大或增殖、促进肿瘤细胞的凋亡等作用。研究内源性洋地黄物质与钠钾ATP酶结合后激活的信号转导通路将有助于明确它在病理生理条件下的重要性,有可能为治疗高血压和肿瘤提供新思路,并深化对细胞内信号转导机制的认识。     

Polarized distribution of Na+, K(+)-ATPase alpha-subunit isoforms in electrocyte membranes

We have previously demonstrated that Na+, K(+)-ATPase activity is present in both differentiated plasma membranes from Electrophorus electricus (L.) electrocyte. Considering that the alpha subunit is responsible for the catalytic properties of the enzyme, the aim of this work was to study the presence and localization of alpha isoforms (alpha1 and alpha2) in the electrocyte. Dose-response curves showed that non-innervated membranes present a Na+, K(+)-ATPase activity 2.6-fold more sensitive to ouabain (I50=1.0+/-0.1 microM) than the activity of innervated membranes (I50=2.6+/-0.2 microM). As depicted in [3H]ouabain binding experiments, when the [3H]ouabain-enzyme complex was incubated in a medium containing unlabeled ouabain, reversal of binding occurred differently: the bound inhibitor dissociated 32% from Na+, K(+)-ATPase in non-innervated membrane fractions within 1 h, while about 50% of the ouabain bound to the enzyme in innervated membrane fractions was released in the same time. These data are consistent with the distribution of alpha1 and alpha2 isoforms, restricted to the innervated and non-innervated membrane faces, respectively, as demonstrated by Western blotting from membrane fractions and immunohistochemical analysis of the main electric organ. The results provide direct evidence for a distinct distribution of Na+, K(+)-ATPase alpha-subunit isoforms in the differentiated membrane faces of the electrocyte, a characteristic not yet described for any polarized cell.     

Modulatory effect of two cardioglycosides on reconstituted Na+/K(+)-ATPase in proteoliposomes.

1. Na,K-ATPase was extracted from Cavia cobaya kidneys, solubilized with nonionic detergent C12E8 (octaethyleneglycol dodecyl monoether) in mixed lipid-detergent-protein micelles. The Na,K-ATPase specific activity was 30-35 IU/mg protein. 2. The enzyme was reconstituted in vesicles, made of phosphatidylethanolamine and cholesterol: an enhancement of +60% in specific activity was obtained. 3. Two different vesicle-types were carried out: open liposomes (partially organized membranes) and closed liposomes. 4. Proteoliposomes were employed for measuring the modulatory effect of two cardioglycosides: ouabain and digoxin. 5. Inhibition of the Na,K-ATPase activity revealed apparent Ki of 1.25 microM for ouabain and 0.25 microM for digoxin in open liposomes, and apparent Ki of 0.75 microM for ouabain and of 1.75 microM for digoxin in closed liposomes. 6. Maximum enhancement of enzymatic activity was found at concentrations of 5-0.5 nM for ouabain and 5-1 nM for digoxin in open liposomes, and 25-1 nM for both digoxin and ouabain in closed liposomes.     

Eosin fluorescence changes during Rb+ occlusion in the Na+/K(+)-ATPase

We used suspensions of partially purified Na(+)/K(+)-ATPase from pig kidney to compare the effects of Rb(+), as a K(+) congener, on the time course and on the equilibrium values of eosin fluorescence and of Rb(+) occlusion. Both sets of data were collected under identical conditions in the same enzyme preparations. The incubation media lacked ATP so that all changes led to an equilibrium distribution between enzyme conformers with and without bound eosin and with and without bound or occluded Rb(+). Results showed that as Rb(+) concentration was increased, the equilibrium value of fluorescence decreased and occlusion increased along rectangular hyperbolas with similar half-maximal values. The time courses of attainment of equilibrium showed an initial phase which was so quick as to fall below the time resolution of our rapid-mixing apparatus. This phase was followed by the sum of at least two exponential functions of time. In the case of fluorescence the fast exponential term accounted for a larger fraction of the time course than in the case of occlusion. Comparison between experimental and simulated results suggests that fluorescence changes express a process that is coupled to Rb(+) occlusion but that is completed before occlusion reaches equilibrium.     

Chimeric domain analysis of the compatibility between H(+), K(+)-ATPase and Na(+),K(+)-ATPase beta-subunits for the functional expression of gastric H(+),K(+)-ATPase.

Gastric H(+),K(+)-ATPase consists of alpha-subunit with 10 transmembrane domains and beta-subunit with a single transmembrane domain. We constructed cDNAs encoding chimeric beta-subunits between the gastric H(+),K(+)-ATPase and Na(+),K(+)-ATPase beta-subunits and co-transfected them with the H(+),K(+)-ATPase alpha-subunit cDNA in HEK-293 cells. A chimeric beta-subunit that consists of the cytoplasmic plus transmembrane domains of Na(+),K(+)-ATPase beta-subunit and the ectodomain of H(+),K(+)-ATPase beta-subunit assembled with the H(+),K(+)-ATPase alpha-subunit and expressed the K(+)-ATPase activity. Therefore, the whole cytoplasmic and transmembrane domains of H(+),K(+)-ATPase beta-subunit were replaced by those of Na(+),K(+)-ATPase beta-subunit without losing the enzyme activity. However, most parts of the ectodomain of H(+),K(+)-ATPase beta-subunit were not replaced by the corresponding domains of Na(+), K(+)-ATPase beta-subunit. Interestingly, the extracellular segment between Cys(152) and Cys(178), which contains the second disulfide bond, was exchangeable between H(+),K(+)-ATPase and Na(+), K(+)-ATPase, preserving the K(+)-ATPase activity intact. Furthermore, the K(+)-ATPase activity was preserved when the N-terminal first 4 amino acids ((67)DPYT(70)) in the ectodomain of H(+),K(+)-ATPase beta-subunit were replaced by the corresponding amino acids ((63)SDFE(66)) of Na(+),K(+)-ATPase beta-subunit. The ATPase activity was abolished, however, when 4 amino acids ((76)QLKS(79)) in the ectodomain of H(+),K(+)-ATPase beta-subunit were replaced by the counterpart ((72)RVAP(75)) of Na(+),K(+)-ATPase beta-subunit, indicating that this region is the most N-terminal one that discriminates the H(+),K(+)-ATPase beta-subunit from that of Na(+), K(+)-ATPase.