Comparison of the effects of potassium on ouabain binding to native and site-directed mutants of Na,K-ATPase

K⁺ and Rb⁺ Affinities of the Na,K-ATPase α₁ and α₂ Isozymes: An Application of ICP-MS for Quantification of Na⁺ Pump Kinetics in Myofibers

The potassium affinities of Na,K-ATPase isozymes are important determinants of their physiological roles in skeletal muscle. This study measured the apparent K⁺ and Rb⁺ affinities of the Na,K-ATPase α₁ and α₂ isozymes in intact, dissociated myofibers obtained from WT and genetically altered mice (α₁^S/Sα₂^R/R and skα₂^−/−). It also validates a new method to quantify cations in intact, dissociated myofibers, using inductively coupled plasma mass spectrometry (ICP-MS). Our findings were that: (1) The extracellular substrate sites of Na,K-ATPase bind Rb⁺ and K⁺ with comparable apparent affinities; however; turnover rate is reduced when Rb⁺ is the transported ion; (2) The rate of Rb⁺ uptake by the Na,K-ATPase is not constant but declines with a half-time of approximately 1.5 min; (3) The apparent K⁺ affinity of the α₂ isozymes for K⁺ is significantly lower than α₁. When measured in intact fibers of WT and α₁^S/Sα₂^R/R mice in the presence of 10 µM ouabain; the K_1/2,K of α₁ and α₂ isozymes are 1.3 and 4 mM, respectively. Collectively, these results validate the single fiber model for studies of Na,K-ATPase transport and kinetic constants, and they imply the existence of mechanisms that dynamically limit pump activity during periods of active transport.

Isoform specificity of cardiac glycosides binding to human Na+,K+-ATPase alpha1beta1, alpha2beta1 and alpha3beta1

Cardiac glycosides inhibit the Na(+),K(+)-ATPase and are used for the treatment of symptomatic heart failure and atrial fibrillation. In human heart three isoforms of Na(+),K(+)-ATPase are expressed: alpha(1)beta(1), alpha(2)beta(1) and alpha(3)beta(1). It is unknown, if clinically used cardiac glycosides differ in isoform specific affinities, and if the isoforms have specific subcellular localization in human cardiac myocytes. Human Na(+),K(+)-ATPase isoforms alpha(1)beta(1), alpha(2)beta(1) and alpha(3)beta(1) were expressed in yeast which has no endogenous Na(+),K(+)-ATPase. Isoform specific affinities of digoxin, digitoxin, beta-acetyldigoxin, methyldigoxin and ouabain were assessed in [(3)H]-ouabain binding assays in the absence or presence of K(+) (each n=5). The subcellular localizations of the Na(+),K(+)-ATPase isoforms were investigated in isolated human atrial cardiomyocytes by immunohistochemistry. In the absence of K(+), methyldigoxin (alpha(1)>alpha(3)>alpha(2)) and ouabain (alpha(1)=alpha(3)>alpha(2)) showed distinct isoform specific affinities, while for digoxin, digitoxin and beta-acetyldigoxin no differences were found. In the presence of K(+), also digoxin (alpha(2)=alpha(3)>alpha(1)) and beta-acetyldigoxin (alpha(1)>alpha(3)) had isoform specificities. A comparison between the cardiac glycosides demonstrated highly different affinity profiles for the isoforms. Immunohistochemistry showed that all three isoforms are located in the plasma membrane and in intracellular membranes, but only alpha(1)beta(1) and alpha(2)beta(1) are located in the T-tubuli. Cardiac glycosides show distinct isoform specific affinities and different affinity profiles to Na(+),K(+)-ATPase isoforms which have different subcellular localizations in human cardiomyocytes. Thus, in contrast to current notion, different cardiac glycoside agents may significantly differ in their pharmacological profile which could be of hitherto unknown clinical relevance.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Site-directed mutagenesis of amino acids in the cytoplasmic loop 6/7 of Na,K-ATPase

The loop between transmembrane helices 6 and 7 (L6/7) of P-type ATPases has been suggested to be important for the functional linkage of ion binding and enzyme phosphorylation or to be a site of initial cation binding. To investigate the role of L6/7 in Na,K-ATPase, alanine substitutions were made for charged and conserved residues in L6/7 of the human alpha1 subunit and the proteins were expressed in yeast for analysis. All mutants except the triple mutant E825A/E828A/D830A bound ouabain. Although the equilibrium dissociation constant for ouabain binding by most mutants was similar to the wild-type value, the K(d) of R837A for ouabain binding was approximately 15-fold higher than the wild-type K(d). (18)O exchange measurements indicated that the apparent affinity of this mutant for Pi was reduced about 3-fold. The concentration dependence of KCl inhibition of ouabain binding or of NaCl inhibition of ouabain binding revealed 2-4-fold changes in the apparent affinity for cations in the E825A, E828A, and R837A mutants. The E825A and E828A mutants lost the ability to bind ouabain after extraction with 0.1% SDS or after brief heating, indicating that these mutations affected the stability of the enzyme. The ATPase activity of the other mutants was measured after extraction of crude yeast membranes with 0.1% SDS. For all mutants except R834A, R837A, and R848A, the activity was at least 50% of wild-type activity.

All human Na(+)-K(+)-ATPase alpha-subunit isoforms have a similar affinity for cardiac glycosides

Three alpha-subunit isoforms of the sodium pump, which is the receptor for cardiac glycosides, are expressed in human heart. The aim of this study was to determine whether these isoforms have distinct affinities for the cardiac glycoside ouabain. Equilibrium ouabain binding to membranes from a panel of different human tissues and cell lines derived from human tissues was compared by an F statistic to determine whether a single population of binding sites or two populations of sites with different affinities would better fit the data. For all tissues, the single-site model fit the data as well as the two-site model. The mean equilibrium dissociation constant (K(d)) for all samples calculated using the single-site model was 18 +/- 6 nM (mean +/- SD). No difference in K(d) was found between nonfailing and failing human heart samples, although the maximum number of binding sites in failing heart was only approximately 50% of the number of sites in nonfailing heart. Measurement of association rate constants and dissociation rate constants confirmed that the binding affinities of the different human alpha-isoforms are similar to each other, although calculated K(d) values were lower than those determined by equilibrium binding. These results indicate both that the affinity of all human alpha-subunit isoforms for ouabain is similar and that the increased sensitivity of failing human heart to cardiac glycosides is probably due to a reduction in the number of pumps in the heart rather than to a selective inhibition of a subset of pumps with different affinities for the drugs.

Ouabain and substrate affinities of human Na(+)-K(+)-ATPase alpha(1)beta(1), alpha(2)beta(1), and alpha(3)beta(1) when expressed separately in yeast cells

Human Na(+)-K(+)-ATPase alpha(1)beta(1), alpha(2)beta(1), and alpha(3)beta(1) heterodimers were expressed individually in yeast, and ouabain binding and ATP hydrolysis were measured in membrane fractions. The ouabain equilibrium dissociation constant was 13-17 nM for alpha(1)beta(1) and alpha(3)beta(1) at 37 degrees C and 32 nM for alpha(2)beta(1), indicating that the human alpha-subunit isoforms have a similar high affinity for cardiac glycosides. K(0.5) values for antagonism of ouabain binding by K(+) were ranked in order as follows: alpha(2) (6.3 +/- 2.4 mM) > alpha(3) (1.6 +/- 0.5 mM) approximately alpha(1) (0.9 +/- 0.6 mM), and K(0.5) values for Na(+) antagonism of ouabain binding to all heterodimers were 9.5-13.8 mM. The molecular turnover for ATP hydrolysis by alpha(1)beta(1) (6,652 min(-1)) was about twice as high as that by alpha(3)beta(1) (3,145 min(-1)). These properties of the human heterodimers expressed in yeast are in good agreement with properties of the human Na(+)-K(+)-ATPase expressed in Xenopus oocytes (G Crambert, U Hasler, AT Beggah, C Yu, NN Modyanov, J-D Horisberger, L Lelievie, and K Geering. J Biol Chem 275: 1976-1986, 2000). In contrast to Na(+) pumps expressed in Xenopus oocytes, the alpha(2)beta(1) complex in yeast membranes was significantly less stable than alpha(1)beta(1) or alpha(3)beta(1), resulting in a lower functional expression level. The alpha(2)beta(1) complex was also more easily denatured by SDS than was the alpha(1)beta(1) or the alpha(3)beta(1) complex.

Valine 904, tyrosine 898, and cysteine 908 in Na,K-ATPase alpha subunits are important for assembly with beta subunits

A 26-amino acid sequence in an extracellular loop of the Na,K-ATPase alpha subunit between membrane-spanning segments 7 and 8 has been shown to bind to the beta subunit of Na,K-ATPase and to promote alphabeta assembly (Lemas, M. V., Hamrick, M., Takeyasu, K., and Fambrough, D. M. (1994) J. Biol. Chem. 269, 8255-8259) When this 26-amino acid sequence of the rat Na,K-ATPase alpha3 subunit was replaced by the corresponding sequence of the rat gastric H,K-ATPase alpha subunit, the chimeric alpha subunit assembled preferentially with the rat gastric H,K-ATPase beta subunit (Wang, S.-G., Eakle, K. A., Levenson, R., and Farley, R. A. (1997) Am. J. Physiol. 272, C923-C930). In the present study, these 26 amino acids (Asn886-Ala911) of rat Na,K-ATPase alpha3 were replaced by the corresponding amino acids Asn908-Ala933 of rat distal colon H, K-ATPase. Site-directed mutagenesis of the chimeric alpha subunits and Na,K-ATPase alpha3 showed that Val904, Tyr898, and Cys908 in the Na,K-ATPase alpha3 subunit are key residues in alphabeta subunit interactions. The V904Q mutation in Na,K-ATPase alpha3 reduced the Bmax for ouabain binding and the ATPase activity of alpha3beta1 complexes by approximately 95%, and Y898R reduced the Bmax and ATPase activity by approximately 60%. The complementary mutations Q904V and R898Y increased the amount of ouabain bound by yeast membranes expressing the chimera with the colon H,K-ATPase sequence. The amount of ouabain bound by complexes assembled between Na, K-ATPase alpha3 containing the Y898R,C908G mutations and gastric H, K-ATPase beta was less than 10% of wild type Na,K-ATPase alpha3 expressed with the same beta subunit. The R898Y,G908C mutations in the chimeric alpha subunits also increased ouabain binding.

Mutational analysis of Glu-327 of Na(+)-K(+)-ATPase reveals stimulation of 86Rb+ uptake by external K+

A competition assay of 86Rb+ uptake in HeLa cells transfected with ouabain-resistant Na(+)-K(+)-ATPase mutants revealed a stimulation of 86Rb+ uptake at low external concentrations (1 mM) of competitor (K+). Of the models that were tested, those that require that two K+ be bound before transport occurs gave the worst fits. Random and ordered binding schemes described the data equally well. General models in which both binding and transport were allowed to be cooperative yielded parameter errors larger than the parameters themselves and could not be utilized. Models that assumed noncooperative transport always showed positive cooperativity in binding. E327Q and E327L mutated forms of rat alpha 2 had lower apparent affinities for the first K+ bound than did wild-type rat alpha 2 modified to be ouabain resistant. The mutations did not affect the apparent affinity of the second K+ bound. Models that assumed noncooperativity in binding always showed positively cooperative transport, i.e., enzymes with two K+ bound had a higher flux than those with one K+ bound. Increases in external Na+ decreased the apparent affinity for K+ for all models and decreased the ratio of the apparent influx rate constants for E327L.

The effects of elevated glucose on Na+/K(+)-ATPase of cultured bovine retinal pigment epithelial cells measured by a new nonradioactive rubidium uptake assay

The effects of stimulated hyperglycemia on the Na+/K(+)-ATPase activity of cultured bovine retinal pigment epithelial (RPE) cells were investigated. Total Rb+ uptake, measured by a chromatographic method, was decreased 20-30% by 55.5 mM glucose relative to 5.55 mM glucose for culture periods of 2 to 28 days. An acute hyperglycemic stress (< 1 week) had no effect on ouabain-inhibition of Rb+ uptake or ouabain binding to RPE cells (IC50 = 55 nM for both processes) and did not alter the IC50 value (near 10 nM) for binding of strophanthidin, another selective Na+/K(+)-ATPase inhibitor. A small increase in the apparent K(m) of Rb+ for Na+/K(+)-ATPase accompanied the decrease in maximal Rb+ uptake at 55.5 mM glucose. The continuous presence of AL-1576, an aldose reductase inhibitor (ARI), normalized the effect of severe hyperglycemia on Rb+ uptake in the chronic (28 days) but not the acute exposure protocols. Thus, decreased efficiency of Na+/K(+)-ATPase caused by chronic accumulation of intracellular sorbitol can account for previously reported functional and structural alterations in the RPE cell layer of diabetic rodents. The results of the present study suggest that hyperglycemia-induced loss of Na+/K(+)-ATPase function in RPE cells, which responds to aldose reductase inhibitor treatment, contributes to the pathogenesis of diabetic retinopathy.

Na+-K+-ATPase alpha-subunit containing Q905-V930 of gastric H+-K+-ATPase alpha preferentially assembles with H+-K+-ATPase beta

Amino acids N886-A911 of the rat Na+-K+-ATPase alpha3-subunit were replaced by the corresponding region (Q905-V930) of the rat gastric H+-K+-ATPase alpha-subunit. The chimera (NGH26) was expressed in yeast with the rat Na+-K+ -ATPase beta1-subunit (rbeta1), the rat H+-K+-ATPase beta-subunit (HKbeta), the chimeric beta-subunit NHbeta1 (containing the carboxy-terminal ectodomain of HKbeta), or the chimeric beta-subunit HNbeta1 (containing the carboxy-terminal ectodomain of rbeta1). Increased resistance to trypsin digestion indicated that NGH26 preferentially assembled with HKbeta and NHbeta1 rather than with rbeta1 or HNbeta1. Ouabain binding also indicated that more functional complexes were assembled when NGH26 was expressed with HKbeta or NHbeta1. These results suggest that the sequence Q905-V930 interacts with the HKbeta-subunit on the extracellular side of the cell membrane. The NGH26 + HKbeta complex is less stable than alpha3 + HKbeta when heated and also has a lower binding affinity for ouabain [dissociation constant (Kd) = 63 nM] compared with alpha3 + rbeta1 or alpha3 + HKbeta (K(d) = 5-10 nM). In contrast, the NGH26+NHbeta1 complex is thermally as stable as alpha3 + rbeta1 complexes, and its ouabain binding affinity (K(d) = 10 nM) is the same as the wild type. These results indicate that the amino acids Q905-V930 of the rat gastric H+-K+-ATPase alpha-subunit preferentially associate with the extracellular domain of H+-K+-ATPase beta-subunit to form functional pump complexes and that the cytoplasmic and/or transmembrane region of the beta-subunit influences the stability of the alpha beta complexes.

Asp804 and Asp808 in the transmembrane domain of the Na,K-ATPase alpha subunit are cation coordinating residues

The functional roles of Asp804 and Asp808, located in the sixth transmembrane segment of the Na,K-ATPase alpha subunit, were examined. Nonconservative replacement of these residues yielded enzymes unable to support cell viability. Only the conservative substitution, Ala808 --> Glu, was able to maintain the essential cation gradients (Van Huysse, J. W., Kuntzweiler, T. A., and Lingrel, J. B (1996) FEBS Lett. 389, 179-185). Asp804 and Asp808 were replaced by Ala, Asn, and Glu in the sheep alpha1 subunit and expressed in a mouse cell line where [3H]ouabain binding was utilized to probe the exogenous proteins. All of the heterologous proteins were targeted into the plasma membrane, bound ouabain and nucleotides, and adopted E1Na, E1ATP, and E2P conformations. K+ competition of ouabain binding to sheep alpha1 and Asp808 --> Glu enzymes displayed IC50 values of 4.11 mM (nHill = 1.4) and 23.8 mM (nHill = 1.6), respectively. All other substituted proteins lacked this K+-ouabain antagonism, e.g. 150 mM KCl did not inhibit ouabain binding. Na+ antagonized ouabain binding to all the expressed isoforms, however, the proteins carrying nonconservative substitutions displayed reduced Hill coefficients (nHill </= 2.0) compared to the control (nHill </= 2.8). Therefore, Asp804 and Asp808 of the Na,K-ATPase are required for normal Na+ and K+ transport, possibly coordinating these cations during transport.

Random mutagenesis of the sheep Na,K-ATPase alpha1 subunit generating the ouabain-resistant mutant L793P

The polymerase chain reaction was used to randomly mutagenize a cDNA cassette encoding amino acids 691-946 of the sheep Na,K-ATPase alpha subunit. The mutagenized cassettes were used to replace the wild-type region in the full-length cDNA, and pools of mutants were transfected into HeLa cells. After the generation of resistant cells via selection in 0.5 microM ouabain, polymerase chain reaction was used to amplify the mutagenized cassette from the genomic DNA of the stable transfectants. Sequence analysis of the polymerase chain reaction product revealed three amino acid substitutions: I729V, L793P, and K836R. Subsequent site-directed mutagenesis experiments showed that only L793P was important for resistance. To elucidate the role of L793 in ouabain inhibition, additional mutations at this position were prepared. L793A and L793I mutants were constructed and expressed in HeLa cells. Only L793A survived selection using ouabain, which suggested that resistance is not due to the specific substitution of leucine with proline. To explore the mechanism of resistance, apparent affinities of the L793P mutant for sodium and potassium were compared to the wild-type HeLa pump. Although the apparent affinities were comparable for sodium, the mutant had a 2-fold higher apparent affinity for potassium. This suggests that the mechanism of ouabain insensitivity of L793P is due to a perturbation in the region of the enzyme that may include the K+ binding site.

Modulation of pump function by mutations in the first transmembrane region of Na(+)-K(+)-ATPase alpha 1-subunit

The Cys in the first transmembrane region of the Na(+)-K(+)-adenosinetriphosphatase (ATPase) alpha 1-subunit has been shown to be a critical determinant of cardiac glycoside binding. To study the role of this Cys on ion transport activity, we measured pump currents in HeLa cells expressing wild-type or mutant alpha 1-subunit cDNAs. The endogenous ouabainsensitive Na(+)-K(+)-ATPase was selectively inhibited by growing the cells in 0.1 microM ouabain. A Cys-to-Tyr substituted mutant exhibited decreased sensitivity to digitoxin but not digoxin compared with wild type. The decreased affinity for digitoxin was due to a faster dissociation rate. In contrast, the Cys-to-Ala substitution did not significantly alter the sensitivity to digitoxin or digoxin. Both wild-type and mutant cells displayed marked external K(+)-dependent pump currents; however, the affinity for K+ was reduced by the mutations. The decrease in K+ affinity was due to a slower association rate. The results show that the Cys that interacts with cardiac glycosides also participates in the sensitivity of the pump to external K+.

Amino acid replacement of Asp369 in the sheep alpha 1 isoform eliminates ATP and phosphate stimulation of [3H]ouabain binding to the Na+, K(+)-ATPase without altering the cation binding properties of the enzyme

Modification of aspartic acid 369 in the sheep alpha 1 Na+,K(+)-ATPase to asparagine results in a membrane-associated form of Na+,K(+)-ATPase that can bind [3H]ouabain with high affinity in the presence of Mg2+ alone (KD = 20.4 +/- 2.6 nM). Ouabain binding to the D369N mutant is not stimulated by inorganic phosphate, confirming that Asp369 is both the catalytic phosphorylation site and the only Pi interaction site which stimulates ouabain binding. Cation inhibition of Mg(2+)-stimulated ouabain binding to the D369N mutant demonstrated that three Na+ and two K+ ions inhibit [3H]ouabain binding and suggests that this inhibition must occur via a cation-sensitive conformational change which does not directly involve dephosphorylation of the enzyme. In the presence of 10 mM Mg2+, ATP stimulates ouabain binding to the wild type protein, (AC50 = 21.4 +/- 2.7 microM) but inhibits the binding to the D369N mutant (IC50 = 2.52 +/- 0.17 microM) indicating that the mutation does not destroy the high affinity site for MgATP but does change the nature of the protein conformation normally induced by a nucleotide-Na+,K(+)-ATPase interaction. Increasing the Mg2+ from 1 to 10 mM did not alter the AC50 or IC50 values for ATP and reveals that the Mg2+ interaction which stimulates ouabain binding in the absence of nucleotide involves a distinct divalent cation site not associated with the binding of the magnesium-nucleotide complex. Thus, altering the catalytic phosphorylation site of Na+,K(+)-ATPase does not affect the expression of the ouabain-sensitive protein in the membrane fraction of NIH 3T3 cells and does not disrupt the binding of Na+, K+, Mg2+, ouabain, or ATP to the enzyme. However, the D369N substitution does inhibit the formation of a nucleotide-protein complex with high affinity for ouabain.

Glutamic acid 327 in the sheep alpha 1 isoform of Na+,K(+)-ATPase is a pivotal residue for cation-induced conformational changes

The cation binding characteristics of the mutant E327A formed in the sheep alpha 1 isoform of the Na+,K(+)-ATPase were examined using [3H]ouabain binding as a function of monovalent cation concentrations. Equilibrium competition binding assays in the presence of Mg2+, inorganic phosphate and various amounts of unlabelled ouabain indicated that both wild-type sheep alpha 1 protein and the E327A mutant expressed in 3T3 cells had similar affinities for ouabain (KD = 1.53 and 1.31 nM respectively). Sodium inhibition of ouabain binding appeared competitive in both enzymes. However, binding of three Na+ ions was required to explain the steep character of the Na+ inhibition curve for the wild-type Na+,K(+)-ATPase (Ki = 12.8 +/- 1.6 mM), whereas the binding of two Na+ ions was detected for the mutant E327A (Ki = 19.2 +/- 2.5 mM). Potassium binding of [3H]ouabain binding displayed a partially competitive nature with Hill coefficients of 2 for both wild-type sheep alpha 1 (Ki = 0.743 +/- 0.044 mM) and E327A (Ki = 0.875 +/- 0.067 mM). At concentrations of K+ above 10 mM, the sheep alpha 1 competition curve levelled off whereas the inhibition curve for E327A displayed a stimulation in ouabain binding. This stimulation in [3H]ouabain binding also occurred with Rb+, Cs+ and Li+, but was never observed with choline or Na+, suggesting that this effect was not due to ionic strength. From these [3H]ouabain-binding studies, it is obvious that the mutant enzyme E327A in the presence of Mg2+, Pi and ouabain, interacts with monovalent cations in a unique fashion. One interpretation of these data is that the glutamic acid residue at position 327 is involved in a conformational transition induced by the binding of monovalent cations to the Na+,K+-ATPase and that this transition is inhibited by the mutation of E327A.