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

Strong pH dependence of coupling efficiency of the Na+ – translocating NADH:quinone oxidoreductase (Na+-NQR) of Vibrio cholerae

The Na+-translocating NADH:quinone oxidoreductase (NQR) is the entry site for electrons into the respiratory chain of Vibrio cholerae, the causative agent of cholera disease. NQR couples the electron transfer from NADH to ubiquinone to the translocation of sodium ions across the membrane. We investigated the pH dependence of electron transfer and generation of a transmembrane voltage (ΔΨ) by NQR reconstituted in liposomes with Na+ or Li+ as coupling cation. ΔΨ formation was followed with the voltage-sensitive dye oxonol. With Na+, ΔΨ was barely influenced by pH (6.5–8.5), while Q reduction activity exhibited a maximum at pH 7.5–8.0. With Li+, ΔΨ was generally lower, and the pH profile of electron transfer activity did not reveal a pronounced maximum. We conclude that the coupling efficiency of NQR is influenced by the nature of the transported cation, and by the concentration of protons. The 3D structure of NQR reveals a transmembrane channel in subunit NqrB. It is proposed that partial uncoupling of the NQR observed with the smaller Li+, or with Na+ at pH 7.5–8.0, is caused by the backflow of the coupling cation through the channel in NqrB.

Mechanisms of cell uptake and toxicity of the anticancer drug cisplatin

Two major issues which hamper the use of the anticancer drug cisplatin are the development of cancer cell resistance and its nephrotoxicity. One possible mechanism by which resistance is reported to develop is a reduction in drug uptake across the cell membrane. While the passive uptake of cisplatin has long been cited as an important contribution, far greater attention has been given to active modes of uptake, particularly in recent research. Using unilamellar lipid vesicles together with the stopped-flow kinetic method we show here that the permeability coefficient of cisplatin increases significantly with the chloride concentration of the medium. This supports the hypothesis that cisplatin can enter cells via passive permeation through the lipid phase of the membrane, but becomes trapped within the cytoplasm because dissociation of chloride ligands yields a membrane-impermeant positively-charged aqua derivative. This is important evidence for a major role of passive membrane diffusion in the uptake of cisplatin, and suggests that reduced cell uptake is unlikely to be a significant mechanism leading to the development of drug resistance. Studies of rubidium ion uptake into the cytoplasm of Xenopus oocytes via the Na(+),K(+)-ATPase show significant inhibition of this ion pump when cisplatin is present in the cytoplasm. Because Na(+),K(+)-ATPase activity is essential to the survival of all animal cells, e.g. via maintenance of cell volume, and the Na(+),K(+)-ATPase is expressed at particularly high levels within the membranes of kidney tubules where it plays a crucial role in nutrient reabsorption, these results suggest that cisplatin-induced inhibition of the Na(+),K(+)-ATPase is a likely contributing cause for the nephrotoxicity of cisplatin.

Identification of electric-field-dependent steps in the Na(+),K(+)-pump cycle

The charge-transporting activity of the Na(+),K(+)-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme's reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na(+),K(+)-ATPase's transport sites in competition with Na(+) and K(+), but is not occluded within the protein. We find that only the occludable ions Na(+), K(+), Rb(+), and Cs(+) cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na(+) or K(+) binding in a high field access channel is a major electrogenic reaction of the Na(+),K(+)-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.

Oleic and linoleic acids are active principles in Nigella sativa and stabilize an E(2)P conformation of the Na,K-ATPase. Fatty acids differentially regulate cardiac glycoside interaction with the pump

Nigella sativa seed oil was found to contain a modulator of Na,K-ATPase. Separation analyses combined with (1)H NMR and GCMS identified the inhibitory fraction as a mixture of oleic and linoleic acids. These two fatty acids are specifically concentrated in several medicinal plant oils, and have particularly been implicated in decreasing high blood pressure. The ouabain binding site on Na,K-ATPase has also been implicated in blood pressure regulation. Thus, we aimed to determine how these two molecules modify pig kidney Na,K-ATPase. Oleic and linoleic acids did not modify reactions involving the E(1) (Na(+)) conformations of the Na,K-ATPase. In contrast, K(+) dependent reactions were strongly modified after treatment. Oleic and linoleic acids were found to stabilize a pump conformation that binds ouabain with high affinity, i.e., an ion free E(2)P form. Time-resolved binding assays using anthroylouabain, a fluorescent ouabain analog, revealed that the increased ouabain affinity is unique to oleic and linoleic acids, as compared with γ-linolenic acid, which decreased pump-mediated ATP hydrolysis but did not equally increase ouabain interaction with the pump. Thus, the dynamic changes in plasma levels of oleic and linoleic acids are important in the modulation of the sensitivity of the sodium pump to cardiac glycosides. Given the possible involvement of the cardiac glycoside binding site on Na,K-ATPase in the regulation of hypertension, we suggest oleic acid to be a specific chaperon that modulates interaction of cardiac glycosides with the sodium pump.

Mechanism of action of AZD0865, a K+-competitive inhibitor of gastric H+,K+-ATPase

AZD0865 is a member of a drug class that inhibits gastric H(+),K(+)-ATPase by K(+)-competitive binding. The objective of these experiments was to characterize the mechanism of action, selectivity and inhibitory potency of AZD0865 in vitro. In porcine ion-leaky vesicles at pH 7.4, AZD0865 concentration-dependently inhibited K(+)-stimulated H(+),K(+)-ATPase activity (IC(50) 1.0+/-0.2 microM) but was more potent at pH 6.4 (IC(50) 0.13+/-0.01 microM). The IC(50) values for a permanent cation analogue, AR-H070091, were 11+/-1.2 microM at pH 7.4 and 16+/-1.8 microM at pH 6.4. These results suggest that the protonated form of AZD0865 inhibits H(+),K(+)-ATPase. In ion-tight vesicles, AZD0865 inhibited H(+),K(+)-ATPase more potently (IC(50) 6.9+/-0.4 nM) than in ion-leaky vesicles, suggesting a luminal site of action. AZD0865 inhibited acid formation in histamine- or dibutyryl-cAMP-stimulated rabbit gastric glands (IC(50) 0.28+/-0.01 and 0.26+/-0.003 microM, respectively). In ion-leaky vesicles at pH 7.4, AZD0865 (3 microM) immediately inhibited H(+),K(+)-ATPase activity by 88+/-1%. Immediately after a 10-fold dilution H(+),K(+)-ATPase inhibition was 41%, indicating reversible binding of AZD0865 to gastric H(+),K(+)-ATPase. In contrast to omeprazole, AZD0865 inhibited H(+),K(+)-ATPase activity in a K(+)-competitive manner (K(i) 46+/-3 nM). AZD0865 inhibited the process of cation occlusion concentration-dependently (IC(50) 1.7+/-0.06 microM). At 100 microM, AZD0865 reduced porcine renal Na(+),K(+)-ATPase activity by 9+/-2%, demonstrating a high selectivity for H(+),K(+)-ATPase. Thus, AZD0865 potently, K(+)-competitively, and selectively inhibits gastric H(+),K(+)-ATPase activity and acid formation in vitro, with a fast onset of effect.

Secondary active transport mediated by a prokaryotic homologue of ClC Cl- channels

ClC Cl- channels make up a large molecular family, ubiquitous with respect to both organisms and cell types. In eukaryotes, these channels fulfill numerous biological roles requiring gated anion conductance, from regulating skeletal muscle excitability to facilitating endosomal acidification by (H+)ATPases. In prokaryotes, ClC functions are unknown except in Escherichia coli, where the ClC-ec1 protein promotes H+ extrusion activated in the extreme acid-resistance response common to enteric bacteria. Recently, the high-resolution structure of ClC-ec1 was solved by X-ray crystallography. This primal prokaryotic ClC structure has productively guided understanding of gating and anion permeation in the extensively studied eukaryotic ClC channels. We now show that this bacterial homologue is not an ion channel, but rather a H+-Cl- exchange transporter. As the same molecular architecture can support two fundamentally different transport mechanisms, it seems that the structural boundary separating channels and transporters is not as clear cut as generally thought.

The Occlusion of Rb(+) in the Na(+)/K(+)-ATPase. II. The effects of Rb(+), Na(+), Mg2(+), or ATP on the equilibrium between free and occluded Rb(+)

We used the direct route of occlusion to study the equilibrium between free and occluded Rb(+) in the Na(+)/K(+)-ATPase, in media with different concentrations of ATP, Mg(2+), or Na(+). An empirical equation, with the restrictions imposed by the stoichiometry of ligand binding was fitted to the data. This allowed us to identify which states of the enzyme were present in each condition and to work out the schemes and equations that describe the equilibria between the ATPase, Rb(+), and ATP, Mg(2+), or Na(+). These equations were fitted to the corresponding experimental data to find out the values of the equilibrium constants of the reactions connecting the different enzyme states. The three ligands decreased the apparent affinity for Rb(+) occlusion without affecting the occlusion capacity. With [ATP] tending to infinity, enzyme species with one or two occluded Rb(+) seem to be present and full occlusion seems to occur in enzymes saturated with the nucleotide. In contrast, when either [Mg(2+)] or [Na(+)] tended to infinity no occlusion was detectable. Both Mg(2+) and Na(+) are displaced by Rb(+) through a process that seems to need the binding and occlusion of two Rb(+), which suggests that in these conditions Rb(+) occlusion regains the stoichiometry of the physiological operation of the Na(+) pump.

The Occlusion of Rb(+) in the Na(+)/K(+)-ATPase. I. The identity of occluded states formed by the physiological or the direct routes: occlusion/deocclusion kinetics through the direct route

Occlusion of K(+) or its congeners in the Na(+)/K(+)-ATPase occurs after K(+)-dependent dephosphorylation (physiological route) or in media lacking ATP and Na(+) (direct route). The effects of P(i) or ATP on the kinetics of deocclusion of the K(+)-congener Rb(+) formed by each of the above mentioned routes was independent of the route of occlusion, which suggests that both routes lead to the same enzyme intermediate. The time course of occlusion via the direct route can be described by the sum of two exponential functions plus a small component of very high velocity. At equilibrium, occluded Rb(+) is a hyperbolic function of free [Rb(+)] suggesting that the direct route results in enzyme states holding either one or two occluded Rb(+). Release of occluded Rb(+) follows the sum of two decreasing exponential functions of time, corresponding to two phases with similar sizes. These phases are not caused by independent physical compartments. The rate constant of one of the phases is reduced up to 30 times by free Rb(+). When Rb(+) is the only pump ligand, the kinetics of occlusion and deocclusion through the direct route are consistent with an ordered-sequential process with additional independent step(s) interposed between the uptake or the release of each occluded Rb(+).

Mechanistic basis for kinetic differences between the rat alpha 1, alpha 2, and alpha 3 isoforms of the Na,K-ATPase

Previous studies showed that the alpha 1, alpha 2, and alpha 3 isoforms of the catalytic subunit of the Na,K-ATPase differ in their apparent affinities for the ligands ATP, Na(+), and K(+). For the rat isoforms transfected into HeLa cells, K'(ATP) for ATP binding at its low affinity site is lower for alpha 2 and alpha 3 compared with alpha 1; relative to alpha 1 and alpha 2, alpha 3 has a higher K'(Na) and lower K'(K) (Jewell, E. A., and Lingrel, J. B (1991) J. Biol. Chem. 266, 16925--16930; Munzer, J. S., Daly, S. E., Jewell-Motz, E. A., Lingrel, J. B, and Blostein, R. (1994) J. Biol. Chem. 269, 16668--16676). The experiments described in the present study provide insight into the mechanistic basis for these differences. The results show that alpha 2 differs from alpha1 primarily by a shift in the E(1) E(2) equilibrium in favor of E(1) form(s) as evidenced by (i) a approximately 20-fold increase in IC(50) for vanadate, (ii) decreased catalytic turnover, and (iii) notable stability of Na,K-ATPase activity at acidic pH. In contrast, despite its lower K'(ATP) compared with alpha 1, the E(1) E(2) poise of alpha 3 is not shifted toward E(1). Distinct intrinsic interactions with Na(+) ions are underscored by the marked selectivity for Na(+) over Li(+) of alpha 3 compared with either alpha1 or alpha 2 and higher K'(Na) for cytoplasmic Na(+), which persists over a 100-fold range in proton concentration, independent of the presence of K(+). The kinetic analysis also suggests alpha 3-specific differences in relative rates of partial reactions, which impact this isoform's distinct apparent affinities for both Na(+) and K(+).

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.

Structure-function relationships of Na(+), K(+), ATP, or Mg(2+) binding and energy transduction in Na,K-ATPase

The focus of this article is on progress in establishing structure-function relationships through site-directed mutagenesis and direct binding assay of Tl(+), Rb(+), K(+), Na(+), Mg(2+) or free ATP at equilibrium in Na,K-ATPase. Direct binding may identify residues coordinating cations in the E(2)[2K] or E(1)P[3Na] forms of the ping-pong reaction sequence and allow estimates of their contributions to the change of Gibbs free energy of binding. This is required to understand the molecular basis for the pronounced Na/K selectivity at the cytoplasmic and extracellular surfaces. Intramembrane Glu(327) in transmembrane segment M4, Glu(779) in M5, Asp(804) and Asp(808) in M6 are essential for tight binding of K(+) and Na(+). Asn(324) and Glu(327) in M4, Thr(774), Asn(776), and Glu(779) in 771-YTLTSNIPEITP of M5 contribute to Na(+)/K(+) selectivity. Free ATP binding identifies Arg(544) as essential for high affinity binding of ATP or ADP. In the 708-TGDGVND segment, mutations of Asp(710) or Asn(713) do not interfere with free ATP binding. Asp(710) is essential and Asn(713) is important for coordination of Mg(2+) in the E(1)P[3Na] complex, but they do not contribute to Mg(2+) binding in the E(2)P-ouabain complex. Transition to the E(2)P form involves a shift of Mg(2+) coordination away from Asp(710) and Asn(713) and the two residues become more important for hydrolysis of the acyl phosphate bond at Asp(369).

Entrance port for Na(+) and K(+) ions on Na(+),K(+)-ATPase in the cytoplasmic loop between trans-membrane segments M6 and M7 of the alpha subunit. Proximity Of the cytoplasmic segment of the beta subunit

Based on the following observations we propose that the cytoplasmic loop between trans-membrane segments M6 and M7 (L6/7) of the alpha subunit of Na(+),K(+)-ATPase acts as an entrance port for Na(+) and K(+) ions. 1) In defined conditions chymotrypsin specifically cleaves L6/7 in the M5/M6 fragment of 19-kDa membranes, produced by extensive proteolysis of Na(+),K(+)-ATPase, and in parallel inactivates Rb(+) occlusion. 2) Dissociation of the M5/M6 fragment from 19-kDa membranes is prevented either by occluded cations or by competitive antagonists such as Ca(2+), Mg(2+), La(3+), p-xylylene bisguanidinium and m-xylylene bisguanidinium, or 1-bromo-2,4, 6-tris(methylisothiouronium)benzene and 1,3-dibromo-2,4,6-tris (methylisothiouronium)benzene (Br(2)-TITU(3+)). 3) Ca(2+) ions raise electrophoretic mobility of the M5/M6 fragment but not that of the other fragments of the alpha subunit. It appears that negatively charged residues in L6/7 recognize either Na(+) or K(+) ions or the competitive cation antagonists. Na(+) and K(+) ions are then occluded within trans-membrane segments and can be transported, whereas the cation antagonists are not occluded and block transport at the entrance port. The cytoplasmic segment of the beta subunit appears to be close to or contributes to the entrance port, as inferred from the following observations. 1) Specific chymotryptic cleavage of the 16-kDa fragment of the beta subunit to 15-kDa at 20 degrees C (Shainskaya, A., and Karlish, S. J. D. (1996) J. Biol. Chem. 271, 10309-10316) markedly reduces affinity for Br(2)-TITU(3+) and for Na(+) ions, detected by Na(+) occlusion assays or electrogenic Na(+) binding, whereas Rb(+) occlusion is unchanged. 2) Na(+) ions specifically protect the 16-kDa fragment against this chymotryptic cleavage.

Interactions between fragments of trypsinized Na,K-ATPase detected by thermal inactivation of Rb+ occlusion and dissociation of the M5/M6 fragment

This work provides evidence for interactions between fragments of "19-kDa membranes," a trypsinized preparation of Na,K-ATPase that retains cation occlusion and ouabain binding. Previously, we reported rapid thermal inactivation of Rb+ occlusion at 37 degreesC (Or, E., David, P., Shainskaya, A., Tal, D. M., and Karlish, S. J. D. (1993) J. Biol. Chem. 268, 16929-16937). We describe here the detailed kinetics of thermal inactivation. In the range 25-35 degreesC, a two-step model (N left and right arrow U --> I, where N is the native species, U is the reversibly unfolded intermediate, and I is the irreversibly denatured form) fits the data. Reversibility of inactivation has been observed at 25 degreesC, consistent with the model. At 37 degreesC and higher temperatures, the data can be fitted to the simple mechanism N --> I, i.e. U is not significant as an intermediate. Occluded cations (Na+, Rb+, K+, Tl+, NH4+, and Cs+) and ouabain protect strongly against thermal inactivation. Ca2+, Ba2+, and La3+ ions do not protect. Proteolysis experiments provide independent evidence that disorganization can occur in stages, first in transmembrane segments and then in extra-membrane segments of the fragments. Analysis of selective dissociation of the M5/M6 fragment at 37 degreesC (Lutsenko, S., and Kaplan, J. H. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 7936-7940), using a specific antibody, showed that inactivation of Rb+ occlusion precedes dissociation of the fragment, and only approximately 50% of the fragment is released when occlusion is fully inactivated. In the presence of Ca2+ ions, occlusion is inactivated, but the M5/M6 fragment is not released. The experiments demonstrate that occlusion is inactivated by disruption of interactions between fragments of 19-kDa membranes, and only then does the M5/M6 fragment dissociate. Interactions between the M5/M6 and M7/M10 fragments seem to be essential for maintenance of Rb+ occlusion.

Evidence for the existence of two ATP-sensitive Rb+ occlusion pockets within the transmembrane domains of Na+/K+-ATPase

A trypsin-digested Na+/K+-ATPase that has lost ATPase activity and about half of its protein content retains an essentially intact beta-subunit, the 10 transmembrane domains of the alpha-subunit, and the full capacity to occlude Na+ and Rb+ (a congener of K+). When this preparation was incubated at 37 degrees C in the absence of Rb+, it lost half of its Rb+ occluding capacity and two-thirds of its Na+ occluding capacity. Comparison of the Rb+ occlusion-deocclusion kinetics of the digested enzyme before and after partial inactivation indicated that (a) the affinities of the labile and the stable halves of occluded Rb+ were the same; (b) occlusion and deocclusion rates of the stable pool were lower than those of the labile pool; (c) ATP at a low affinity site (K0.5 = 25-300 microM) increased deocclusion rate in the stable pool and occlusion rate in the labile pool; (d) Na+ increased Rb+ deocclusion rate of the sum of the two pools but not that of the stable pool; and (e) occlusion and deocclusion rates of both pools were decreased by ouabain. These findings suggest that (a) the peptide complex of the digested enzyme contains two distinct but interacting cation occlusion pockets, one occluding two Na+ or one Rb+, and the other occluding one Na+ or one Rb+; (b) this peptide complex that is devoid of the catalytic ATP site retains an allosteric ATP site; and (c) the access channels of the two pockets are regulated differently by ATP but similarly by ouabain. Analyses of the gel electrophoretic patterns of the digested enzyme and the N termini of the appropriate bands showed that inactivation of the labile occlusion pocket was accompanied by 60-70% loss of two alpha-fragments containing H3-H4 and H5-H6 transmembrane domains. This and the previously established interactions among the transmembrane helices of alpha- and beta-subunits suggest that one occlusion pocket is associated with H3-H6 domains and that the other is located within a complex of beta-subunit and two alpha-fragments containing H1-H2 and H7-H10 transmembrane domains.

Functional consequences of a posttransfection mutation in the H2-H3 cytoplasmic loop of the alpha subunit of Na,K-ATPase

During kinetic studies of mutant rat Na,K-ATPases, we identified a spontaneous mutation in the first cytoplasmic loop between transmembrane helices 2 and 3 (H2-H3 loop) which results in a functional enzyme with distinct Na,K-ATPase kinetics. The mutant cDNA contained a single G950 to A substitution, which resulted in the replacement of glutamate at 233 with a lysine (E233K). E233K and alpha1 cDNAs were transfected into HeLa cells and their kinetic behavior was compared. Transport studies carried out under physiological conditions with intact cells indicate that the E233K mutant and alpha1 have similar apparent affinities for cytoplasmic Na+ and extracellular K+. In contrast, distinct kinetic properties are observed when ATPase activity is assayed under conditions (low ATP concentration) in which the K+ deocclusion pathway of the reaction is rate-limiting. At 1 microM ATP K+ inhibits Na+-ATPase of alpha1, but activates Na+-ATPase of E233K. This distinctive behavior of E233K is due to its faster rate of formation of dephosphoenzyme (E1) from K+-occluded enzyme (E2(K)), as well as 6-fold higher affinity for ATP at the low affinity ATP binding site. A lower ratio of Vmax to maximal level of phosphoenzyme indicates that E233K has a lower catalytic turnover than alpha1. These distinct kinetics of E233K suggest a shift in its E1/E2 conformational equilibrium toward E1. Furthermore, the importance of the H2-H3 loop in coupling conformational changes to ATP hydrolysis is underscored by a marked (2 orders of magnitude) reduction in vanadate sensitivity effected by this Glu233 --> Lys mutation.

Identification of two conformationally sensitive cysteine residues at the extracellular surface of the Na,K-ATPase alpha-subunit

Na,K-ATPase in right-side-out oriented vesicles was stabilized in different conformations, and the location of intramembrane Cys residues of the alpha-subunit was assessed with membrane-permeable and membrane-impermeable Cys-directed reagents. In the presence of Mg2+ and Pi, Cys964 was the most accessible for both membrane-impermeable 4-acetamido-4'-maleimidylstilbene-2, 2'disulfonic acid (or stilbene disulfonate maleimide, SDSM) and membrane-permeable 7-diethylamino-3-(4'-maleimidyl)-4-methylcoumarin (CPM). In the presence of K+, Cys964 was modified only by hydrophobic CPM, indicating that the environment around Cys964 was different in these two conformations. Cys964 seems to mark the extracellular border of transmembrane segment M9. Cys911 in transmembrane segment M8 showed similar behavior; however, it was not so readily modified. Complete modification of Cys964 and Cys911 causes only partial (about 50%) inactivation of both ATPase activity and Rb+ (or K+) occlusion, indicating that the effect on cation occlusion is indirect and not within the occlusion cavity. The ATP binding capacity remains unaltered by the modifications. Treatment of the K+-stabilized post-tryptic preparation of purified Na, K-ATPase revealed labeling of several cysteines by CPM, none of which were labeled with SDSM. Removal of K+ ions from the preparation, which we have previously shown is accompanied by release of the M5M6 hairpin to the supernatant (), causes changes in the organization of the C-terminal 21-kDa fragment. In particular Cys983 in M10 became labeled by both CPM and SDSM, pointing to a tight association between the C terminus and the M5M6 hairpin of the alpha-subunit.

Transmembrane carboxyl residues are essential for cation-dependent function in the gastric H,K-ATPase

The K+-dependent ATPase activity of the H,K-ATPase was irreversibly inhibited by the carboxyl activating reagent, dicyclohexylcarbodiimide (DCCD). The inhibition was first order and displayed a concentration dependence with the K0.5 (DCCD) = 0.65 +/- 0.04 mM. KCl protected 70% of the ATPase activity from DCCD-dependent inhibition in a concentration-dependent manner with a K0.5 (K+) = 0.58 +/- 0.1 mM KCl. DCCD modification selectively inhibited the K+-dependent rather than ATP-dependent partial reactions including eosin fluorescence responses and ligand-stabilized initial tryptic cleavage patterns of the membrane-associated enzyme. DCCD modification also inhibited the binding of 86Rb+ and the fluorescent responses of the K+-competitive, fluorescent inhibitor 1-(2-methylphenyl)-4-methylamino-6-methyl-2, 3-dihydropyrrolo[3,2-c]quinoline. [14C]DCCD was incorporated into the H,K-ATPase in a time course identical to that describing the inactivation of the K+-dependent ATPase activity of the H,K-ATPase. A component of the [14C]DCCD incorporated into the H,K-ATPase was K+-sensitive where K+ reduced the [14C]DCCD incorporated into the enzyme by 1.6 nmol of [14C]DCCD/mg of protein. Membrane-associated tryptic peptides resolved from the [14C]DCCD-modified H,K-ATPase exhibited various K+ sensitivities with peptides at 23, 9.6, 8.2, 7.1, and 6.1 kDa containing 10-78%, 23-52%, 24-36%, 2%, and 3-4% K+-sensitivity, respectively. The N-terminal sequence of the K+-sensitive, approximately 23- and 9.6-kDa peptides was LVNE857, a C-terminal fragment of the ATPase alpha-subunit. The mass of the smaller peptide limited the residue assignment to the transmembrane M7/M8 domains and an intervening extracytoplasmic loop. An N-terminal sequence, SD840IM, was obtained from a 3.3-kDa, [14C]DCCD-labeled peptide resolved from a V8 digest of the partially purified alpha-subunit. This mass was sufficient to include LVNE but would exclude M8 and the intervening loop between M7 and M8. Glu857 is a unique residue present in each of the proteolytic preparations of the H,K-ATPase modified by [14C]DCCD. These data provide functional evidence of the selective inactivation of the K+-dependent partial reactions of the H,K-ATPase and show that Glu857 located at the M7 boundary in the C terminus of the pump molecule is a significant site of DCCD modification. These data are interpreted to indicate that this carboxyl residue is important for cation binding function.

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

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

Renal Na,K-ATPase in genetic hypertension

Milan hypertensive rats (MHS) develop hypertension because of a primary renal alteration. Both apical and basolateral sodium transport are faster in membrane vesicles derived from renal tubules of MHS than in those of Milan normotensive control rats (MNS). These findings suggest that the increased renal sodium retention and concomitant development of hypertension in MHS may be linked to an altered transepithelial sodium transport. Since this transport is mainly under the control of the Na-K pump, we investigated whether an alteration of the enzymatic activity and/or protein expression of the renal Na,K-ATPase is detectable in prehypertensive MHS. We measured the Na,K-ATPase activity, Rb+ occlusion, turnover number, alpha 1- and beta 1-subunit protein abundance, and alpha 1 and beta 1 mRNA levels in microsomes from renal outer medulla of young (prehypertensive) and adult (hypertensive) MHS and in age-matched MNS. In both young and adult MHS, the Na,K-ATPase activity was significantly higher because of an enhanced number of active pump sites, as determined by Rb+ occlusion maximal binding. The higher number of pump sites was associated with a significant pretranslational increase of alpha 1 and beta 1 mRNA levels that preceded the development of hypertension in MHS. Since a molecular alteration of the cytoskeletal protein adducin is genetically associated with hypertension in MHS and is able to affect the actin-cytoskeleton and Na-K pump activity in transfected renal cells, we propose that the in vivo upregulation of Na-K pump in MHS is primary and linked to a genetic alteration of adducin.

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.

Structure/function analysis of the amino-terminal region of the 1 and 2 subunits of Na,K-ATPase

The alpha2 isoform of the Na,K-ATPase exhibits kinetic behavior distinct from that of the alpha1 isoform. The distinctive behavior is apparent when the reaction is carried out under conditions (micromolar ATP concentration) in which the K+ deocclusion pathway of the reaction cycle is rate-limiting; the alpha1 activity is inhibited by K+, whereas alpha2 is stimulated. When 32 NH2-terminal amino acid residues are removed from alpha1, the kinetic behavior of the mutant enzyme (alpha1M32) is similar to that of alpha2 (Daly, S. E., Lane, L. K., and Blostein, R. (1994) J. Biol. Chem. 269, 23944-23948). In the current study, the region of the alpha1 NH2 terminus involved in modulating this kinetic behavior has been localized to the highly charged sequence comprising residues 24-32. Within this nonapeptide, differences between alpha1 and alpha2 are conservative and are confined to residues 25-27. The behavior of two chimeric enzymes: (i) alpha1 with the first 32 residues identical to the alpha2 sequence, alpha1 (1-32alpha2), and (ii) alpha2 with the first 32 residues identical to the alpha1 sequence, alpha2(1-32alpha1), indicates that the distinctive kinetic behavior of alpha1 and alpha2 is not due to the 24-32 NH2-terminal domain, per se, but rather to its interaction with other, isoform-specific region(s) of the alpha1 protein. We also demonstrate that the distinct K+ activation profiles of either alpha2 or alpha1M32, compared to alpha1 is due to a faster release of K+ from the K+-occluded enzyme, and to a higher affinity for ATP. This was determined in studies using two approaches: (i) kinetic analysis of the reaction modeled according to a branched pathway of K+ deocclusion through low and high affinity ATP pathways and, (ii) measurements of the (rapid) phosphorylation of the enzyme (E1 conformation) by [gamma-32P]ATP following the rate-limiting formation of the K+-free enzyme from the K+-occluded state (E2(K) --> E1 + K+). The observed kinetic differences between alpha2 and alpha1 suggest that these Na,K-ATPase isoforms differ in the steady-state distribution of E1 and E2 conformational states.

Chymotryptic digestion of the cytoplasmic domain of the beta subunit of Na/K-ATPase alters kinetics of occlusion of Rb+ ions

This paper demonstrates that specific chymotryptic digestion of the cytoplasmic domain of the beta subunit of Na/K-ATPase leads to changes in the kinetics of occlusion of Rb+ ions. The experiments utilize extensively trypsinized Na/K-ATPase, "19-kDa membranes," which lack cytoplasmic loops of the alpha subunit, whereas membrane-embedded fragments (a COOH-terminal 19 kDa and three fragments of 8.1-11.7 kDa) containing transmembrane segments and extracellular loops are intact. The beta subunit is partially split into NH2- and COOH-terminal fragments of 16 and approximately 50 kDa, respectively. Cation occlusion and ouabain binding are preserved. The 19-kDa membranes were incubated, at 37 degrees C, with a selection of proteases, in the presence of Rb+ ions. In these conditions, only alpha-chymotrypsin destroyed the ability to occlude Rb+ ions. This process was associated with truncation of the 16-kDa fragment of the beta subunit in two stages. In the first stage, chymotrypsin removed 10 residues from the 16-kDa fragment to form a 15-kDa fragment (NH2-terminal Ile15) and 4 or 6 residues from the NH2 terminus of the alpha subunit fragment beginning at Asp68. In these membranes Rb+ occlusion was still intact at 37 degrees C. Strikingly, however, deocclusion of two Rb+ ions, which is characteristically biphasic in 19-kDa membranes, displayed deocclusion kinetic with mainly one fast phase. These membranes also showed a much lower affinity for Rb+ ions compared with 19-kDa membranes; and, consistent with the lower Rb+ affinity, Rb+ ions, at nonsaturating concentrations, protected less well against thermal inactivation of Rb+ occlusion. In the second stage, the 15-kDa fragment was truncated further to a 14-kDa fragment (NH2-terminal Leu24), followed by thermal destabilization of Rb+ occlusion even at high concentrations of Rb+ ions. Eventually, the thermally inactivated complex of fragments of alpha and beta subunits was digested to the limit peptides. The results suggest that the cytoplasmic domain of the beta subunit interacts with that of the alpha subunit, possibly with residues leading into the first transmembrane segment, and controls access of Rb+ ions into or out of the occlusion sites.

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.

Membrane disposition of the M5-M6 hairpin of Na+,K(+)-ATPase alpha subunit is ligand dependent

Extensive proteolytic digestion of Na+,K(+)-ATPase (EC 3.6.1.37) by trypsin produces a preparation where most of the extramembrane portions of the alpha subunit have been digested away and the beta subunit remains essentially intact. The fragment Gln-737-Arg-829 of the Na+,K(+)-ATPase alpha subunit, which includes the putative transmembrane hairpin M5-M6, is readily, selectively, and irreversibly released from the posttryptic membrane preparation after incubation at 37 degrees C for several minutes. Once released from the membrane, the fragment aggregates but remains water soluble. Occlusion of K+ or Rb+ specifically prevents release of the Gln-737-Arg-829 fragment into the supernatant. Labeling of the posttryptic membrane preparation with cysteine-directed reagents revealed that Cys-802 (which is thought to be located within the M6 segment) is protected against the modification by Rb+ while this fragment is in the membrane but can be readily modified upon release. Cation occlusion apparently alters the folding and/or disposition of the M5-M6 fragment in the membrane in a way that does not occur when the fragment migrates to the aqueous phase. The ligand-dependent disposition of the M5-M6 hairpin in the membrane along with recent labeling studies suggest a key role for this segment in cation pumping by Na+,K(+)-ATPase.

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.

Monoclonal antibody to phosphatidylserine inhibits Na+/K(+)-ATPase activity

A monoclonal IgG, directed to phosphatidylserine (PS1G3), partially (40-50%) inhibited Na+/K(+)-ATPase activity (forward running reaction cycle) without affecting the K0.5 values for Na+,K+ and MgATP. The Hill or interaction coefficients (nH) for Na+ and K+ for this reaction were reduced from 3.0 to 1.6 and from 1.6 to 0.8, respectively. The K(+)-stimulated p-nitrophenylphosphatase activity (p-NPPase), which is a partial reaction sequence of the Na+/K(+)-ATPase system (but in the backward running mode), was inhibited more strongly (about 70%) due to an increase in K+/substrate antagonism. In this system K0.5 and nH values for both p-nitrophenyl phosphate (p-NPP) and K+ were increased by the mAb. At the maximally inhibitory concentration of PS1G3 the Vmax of the p-NPPase was also reduced. Partial reactions, which were inhibited by PS1G3, are: (1) the Na(+)-activated phosphorylation (non-competitive vs. Na+), (2) the Rb+ occlusion (competitive vs. Rb+). Partial reactions not harmed by PS1G3 are: (3) the K(+)-dependent dephosphorylation, (4) the K(+)-dependent E1 + K+<-->E2K transition. We conclude that PtdSer is involved in cation occlusion, possibly by forming part of the access gate.

An essential role for the extracellular domain of the Na,K-ATPase beta-subunit in cation occlusion

The role of the Na,K-ATPase beta-subunit in stabilization of ion-binding sites has been investigated. Treatment of the purified renal Na,K-ATPase with 0.25 M DTT at 40 degrees C for 1 h resulted in 50% loss of Rb occlusion, which correlates with partial reduction of S-S bridges in the extracellular portion of the beta-subunit; both of these effects were prevented by the presence of 20 mM RbCl. To clarify the role of the extracellular portion of the beta-subunit, "19-kDa membranes" (Na,K-ATPase posttryptic residues, which have been shown to possess many of the cation-binding properties) were used. Incubation of the "19-kDa membranes" with 0.2 M DTT for 1 h at 37 degrees C abolished 70-80% of the 86Rb occlusion capacity. This was accompanied by accumulation of 16- and 17-kDa peptides (in SDS-PAGE of the membranes) and release of a 45-kDa band derived from the Na,K-ATPase beta-subunit to the supernatant. The appearance of the 45-kDa fragment of the beta-subunit in the supernatant confirms the existence of only one transmembrane fragment in this subunit. N-Terminal sequence analysis of the 16- and 17-kDa bands revealed the same structure, A-K-E-E-G-, which corresponds to the beta-subunit sequence beginning at Ala5. The simultaneous presence of 25 mM RbCl (but not 25 mM choline chloride) during DTT treatment prevents almost all (85%) of the loss of Rb occlusion, the appearance of 16- and 17-kDa bands, and reduction and release of the 45-kDa fragment.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of the E1ATP binding site of Na+/K(+)-ATPase by the chromium complex of adenosine 5'-[beta,gamma-methylene]triphosphate blocks the overall reaction but not the partial activities of the E2 conformation

The chromium complex of adenosine 5'-[beta,gamma-methylene]triphosphate, Cr(H2O)4AdoPP[CH2]P, inactivates Na+/K(+)-ATPase from pig kidney at 37 degrees C with an inactivation velocity constant of 7.1 x 10(-3) min-1 by binding to the high-affinity ATP site (E1ATP site). The dissociation constant (Kd) of the analogue at this site is 26 microM, and of ATP 0.8 microM. Inactivation of the overall reaction of Na+/K(+)-ATPase by Cr(H2O)4AdoPP[CH2]P did not alter the activities of the E2 conformational state such as K(+)-activated p-nitrophenylphosphatase, 86Rb+ occlusion and [3H]ouabain binding by the 'backdoor' phosphorylation. However, [3H]ouabain binding via the forwards reaction from E1ATP in the presence of Na+ + Mg2+ is inhibited. K(+)-activated p-nitrophenylphosphatase activity of the Cr(H2O)4AdoPP[CH2]P-inactivated enzyme decreases when an MgATP analogue, the tetraammine cobalt complex of ATP, Co(NH3)4ATP, is used additionally to inactivate the E2ATP site. The enzyme activity of K(+)-activated phosphatase is also lost if the beta,gamma-bidentate chromium(III) complex of ATP, Cr(H2O)4ATP, which may form a stable E1-chromo-phosphointermediate, is used for the inactivation of Na+/K(+)-ATPase. We conclude that the phenomenon of a blockade of the overall reaction of Na+/K(+)-ATPase by the formation of a stable E1.CrAdoPP[CH2]P complex, leading thereby to a loss of the partial activities of the E1 conformation, but not of the E2 conformation, is consistent with the postulate of an (alpha beta)2 diprotomeric nature of the sodium pump. The observation, moreover, that treatment of the sodium pump with Cr(H2O)4ATP but not with Cr(H2O)4AdoPP[CH2]P leads to an inactivation of K(+)-activated phosphatase seems to indicate that the formation of a E1-phosphointermediate affects the E2ATP site.

Structure-activity relations of amiloride derivatives, acting as antagonists of cation binding on Na+/K(+)-ATPase

In a search for an organic analogue of K+ or Na+ ions that binds to the cation binding sites of Na+/K(+)-ATPase with high affinity, the potency of the diuretic amiloride and its derivatives in blocking Rb+ occlusion has been tested. Although amiloride itself has a low affinity (> 200 microM), insertion of short alkyl chains in position 5 of the pyrazine ring of the molecule dramatically increased the affinity of the compound. For example, 5-(N-ethyl-N-isopropyl)amiloride (EIPA) competes with a Ki approximately 10 microM. In derivatives lacking a halogen in position 6 of the ring, a 6-fold decrease in affinity was found. Substitutions in the guanidinium moiety did not produce high affinity inhibitors of Rb+ occlusion. Several derivatives at positions 5 and 6 of the pyrazine ring were found to be strictly competitive inhibitors with respect to Rb+ ions. The highest affinity was observed around pH 8.0-8.2, and low temperature. EIPA and 5-(N-methyl-N-isobutyl)amiloride (MIBA) stabilized the E1 form of FITC1-labelled Na+/K(+)-ATPase, behaving as Na+ analogues. The present findings are similar to our previous results, showing that alkyl- and arylguanidinium derivatives are competitive Na(+)-like antagonists in cation sites. Conclusions concerning the structural features of amiloride derivatives which are necessary to produce the highest binding affinity, are being exploited in synthesis of competitive cation analogues. Derivatives with sufficiently high affinity (0.1-1 microM) will be converted to affinity and photoaffinity reagents.

The amino-terminal segment of the catalytic subunit of kidney Na,K-ATPase regulates the potassium deocclusion pathway of the reaction cycle

Tryptic cleavage of the catalytic subunit of kidney Na,K-ATPase in the E1 conformation effects a change in kinetic behavior apparent at low ATP concentration. Thus, at < or = 10 microM ATP, K+ inhibits Na(+)-dependent ATPase activity of the undigested enzyme but activates activity of the digested enzyme. With time of trypsinolysis, a transient increase followed by a decrease in activity is observed at low [ATP], whereas at high [ATP] (1 mM), activity is progressively reduced. At low [ATP], the trypsin-treated/control activity ratio was > or = 3-fold higher with K+ compared to the ratio observed with the K+ congener Li+. Also, the relative Na/K exchange activity (22Na+ influx into K(+)-loaded inside-out vesicles from erythrocytes) with either 0.01 mM ATP or 1 mM CTP compared to 1 mM ATP was greater for the trypsin-treated than for the control enzyme. The kinetic change is correlated with the initial rapid cleavage of the N-terminal tryptic fragment (< or = 30 residues) from the catalytic subunit. It is concluded that this segment regulates the K+ deocclusion pathway of the reaction; removal of this fragment produces a modified active species having an increased rate of K+ deocclusion.

Chemical modification of Glu-953 of the alpha chain of Na+,K(+)-ATPase associated with inactivation of cation occlusion

We have investigated the role, number, and identity of glutamate (or aspartate) residues involved in cation occlusion on Na+, K(+)-ATPase, using the carboxyl reagent N,N'-dicyclohexylcarbodiimide (DCCD). Extensive use is made of selectively trypsinized Na+,K(+)-ATPase--the so-called "19-kDa membranes"--containing a 19-kDa COOH-terminal, smaller (8-11 kDa) membrane-embedded fragments of the alpha chain, and a largely intact beta chain; these membranes have normal Rb+ and Na+ occlusion capacities. The 19-kDa peptide and a smaller (approximately 9 kDa) unidentified peptide(s) are labeled by [14C]DCCD in a Rb(+)-protectable fashion. Rb(+)-protected [14C]DCCD incorporation into the "19 kDa membranes" and into native Na+,K(+)-ATPase is linearly correlated with inactivation of Rb+ occlusion. Similar linear correlations are observed when Rb(+)-protected [14C]DCCD incorporation is measured by examination of labeling of 19-kDa peptide purified from "19-kDa membranes" or of alpha chain purified from native enzyme. Stoichiometries, estimated by extrapolation, are as follows: (for "19-kDa membranes") close to one DCCD per Rb+ site and one DCCD per 19-kDa peptide; and (for native enzyme) close to two DCCD per phosphoenzyme and two DCCD per alpha chain. We suggest that each of two K+ (or Na+) sites contains a carboxyl group, one located in the 19-kDa peptide and one elsewhere in the alpha chain. After cyanogen bromide digestion of purified, labeled alpha chain, or of 19-kDa peptide, a labeled fragment of apparent M(r) approximately 4 kDa was detected and was identified as that with NH2-terminal Lys-943. Rb(+)-protected [14C]DCCD incorporation was associated almost exclusively with Glu-953. We suggest that the cation occlusion "cage" consists of ligating groups donated by different trans-membrane segments and includes two carboxyl groups such as Glu-953 (and perhaps Glu-327) as well as neutral groups, in two K+ (or Na+) sites, but only neutral groups in the third Na+ site.

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+.

Phosphate binding and ATP-binding sites coexist in Na+/K(+)-transporting ATPase, as demonstrated by the inactivating MgPO4 complex analogue Co(NH3)4PO4

Tetrammine cobalt(III) phosphate [Co(NH3)4PO4] inactivates Na+/K(+)-ATPase in the E2 conformational state, dependent on time and concentration, according to Eqn (1): Co(NH3)4PO4 + E2 Kd in equilibrium E2.Co(NH3)4PO4k2----E'2.Co(NH3)4PO4. The inactivation rate constant k2 for the formation of a stable E'2.Co(NH3)4PO4 at 37 degrees C was 0.057 min-1; the dissociation constant, Kd = 300 microM. The activation energy for the inactivation process was 149 kJ/mol. ATP and the uncleavable adenosine 5'-[beta, gamma-methylene]triphosphate competed with Co(NH3)4PO4 for its binding site with Ks = 0.41 mM and 5 mM, respectively. MgPO4 competed with Co(NH3)4PO4 linearly, with Ks = 50 microM, as did phosphate (Ks = 16 mM) and Mg2+ (Ks = 160 microM). It is concluded that the MgPO4 analogue binds to the MgPO4-binding subsite of the low-affinity ATP-binding site (of the E2 conformation). Also, Na+ (Ks = 860 microM) protected the enzyme against inactivation in a competitive manner. From the intersecting (slope and intercept linear) noncompetitive effect of Na+ against the inactivation by Co(NH3)4PO4, apparent affinities of K+ for the free enzyme of 41 microM, and for the E.Co(NH3)4PO4 complex of 720 microM, were calculated. Binding of Co(NH3)4PO4 to the enzyme inactivated Na+/K(+)-ATPase and K(+)-activated phosphatase, and, moreover, prevented the occlusion of 86Rb+; however, the activity of the Na(+)-ATPase, the phosphorylation capacity of the high-affinity ATP-binding site and the ATP/ADP-exchange reaction remained unchanged. With Co(NH3)432PO4 a binding capacity of 135 pmol unit enzyme was found. Phosphorylation and complete inactivation of the enzyme with Co(NH3)432PO4 or the 32P-labelled tetramminecobalt ATP ([gamma-32P]Co(NH3)4ATP) at the low-affinity ATP-binding site, allowed (independent of the purity of the Na+/K(+)-ATPase preparation) a further incorporation of radioactivity from 32P-labelled tetraaquachromium(III) ATP ([gamma-32P]CrATP) to the high-affinity ATP-binding site with unchanged phosphorylation capacity. However, inactivation and phosphorylation of Na+/K(+)-ATPase by [gamma-32P]CrATP prevented the binding of Co(NH3)4 32PO4 or [gamma-32P]Co(NH3)4ATP to the enzyme. [gamma-32P]CO(NH3)4ATP and Co(NH3)432PO4 are mutually exclusive. The data are consistent with the assumption of a cooperation of catalytic subunits within an (alpha,beta)2-diprotomer, which change their interactions during the Na+/K(+)-pumping process. Our findings seem not to support a symmetrical Repke and Stein model of enzyme action.

Autoregulation of the phosphointermediate of Na+/K(+)-ATPase by the amino-terminal domain of the alpha-subunit

Chymotryptic cleavage of the alpha-subunit of the canine kidney Na+/K(+)-ATPase in the presence of Na+ abolishes ATPase activity and yields an 83 kDa peptide from Ala 267 to the COOH-terminus. To test the proposal that E1 to E2 conformational transition is blocked in this modified enzyme, we have made a detailed comparison of its phosphorylation with that of the native enzyme by ATP. While phosphorylation of alpha is dependent on Na+ and prevented by K+, that of the 83 kDa peptide is modestly stimulated by Na+; and only this stimulation, but not the Na(+)-independent phosphorylation is inhibited by K+. Ouabain, which inhibits alpha-phosphorylation by ATP, activates Na(+)-independent phosphorylation of the 83 kDa peptide by ATP, and inhibits the Na(+)-stimulation of this process. While there is a ouabain-stimulated phosphorylation of alpha by Pi, the 83 kDa peptide is not phosphorylated by Pi with or without ouabain. In its sensitivity to ADP, and insensitivity to K+, the phosphopeptide is similar to the E1P of the native enzyme; however, the spontaneous decomposition rate of the phosphopeptide is orders of magnitude lower than that of the native EP. Na+ has no effect on the spontaneous decomposition of the phosphopeptide; but at high Na+ concentrations (K0.5 = 350 mM) the ADP sensitivity of the phosphopeptide is reduced. The phosphopeptide, like the native EP, is acid-stable, alkaline-labile, and sensitive to hydroxylamine and molybdate. The chymotrypsin-treated enzyme catalyzes an ADP-ATP exchange activity that is stimulated by Na+. The Na(+)-independent part of this exchange, unlike that of the native enzyme, is activated by ouabain. Our findings establish that (a) the phosphorylation process and its control by Na+, K+ and ouabain are autoregulated by the NH2-terminal domain of the alpha-subunit; and (b) the often repeated assumption that the primary role of this domain is in the regulation of E1-E2 transitions is not valid.

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.

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.

Palytoxin acts through Na+,K+-ATPase

Palytoxin is the most potent animal toxin, with a unique structure. The author's group has searched for its mode of action with the following results: 1. Palytoxin (1 pM and less) causes a fast K+ outflow from erythrocytes; 2. Extracellular Ca2+ and borate, and intracellular ATP enhance, but ouabain potently inhibits the palytoxin effects; 3. Palytoxin increases the permeability for Na+ and K+ but not for Ca2+; 4. Palytoxin in comparatively high concentrations (100 nM and above) inhibits Na+,K+-ATPase; 5. Palytoxin can be radiolabeled with 125I. Its receptor is very similar to, but not identical to that of ouabain. A reaction scheme has been delineated which allows an explanation to be obtained for all the known actions of palytoxin. It centers on the hypothesis that palytoxin binds to Na+,K+-ATPase and converts the enzyme or its close vicinity into an open channel with the permselectivity measured on erythrocytes. Patch clamp data from myocytes were obtained in other laboratories. They prove the presence of the predicted palytoxin channel.