There are four isoforms of the ␣ subunit (␣1-4) and three isoforms of the ␤ subunit (␤1-3) of Na,K-ATPase, with distinct tissue-specific distribution and physiological functions. ␣2 is thought to play a key role in cardiac and smooth muscle contraction and be an important target of cardiac glycosides. An ␣2-selective cardiac glycoside could provide important insights into physiological and pharmacological properties of ␣2. The isoform selectivity of a large number of cardiac glycosides has been assessed utilizing ␣1␤1, ␣2␤1, and ␣3␤1 isoforms of human Na,K-ATPase expressed in Pichia pastoris and the purified detergent-soluble isoform proteins. Binding affinities of the digitalis glycosides, digoxin, ␤-methyl digoxin, and digitoxin show moderate but highly significant selectivity (up to 4-fold) for ␣2/␣3 over ␣1 (K D ␣1 > ␣2 ‫؍‬ ␣3). By contrast, ouabain shows moderate selectivity (≈2.5-fold) for ␣1 over ␣2 (K D ␣1 < ␣3 < ␣2). Binding affinities for the three isoforms of digoxigenin, digitoxigenin, and all other aglycones tested are indistinguishable (K D ␣1 ‫؍‬ ␣3 ‫؍‬ ␣2), showing that the sugar determines isoform selectivity. Selectivity patterns for inhibition of Na,K-ATPase activity of the purified isoform proteins are consistent with binding selectivities, modified somewhat by different affinities of K ؉ ions for antagonizing cardiac glycoside binding on the three isoforms. The mechanistic insight on the role of the sugars is strongly supported by a recent structure of Na,K-ATPase with bound ouabain, which implies that aglycones of cardiac glycosides cannot discriminate between isoforms. In conclusion, several digitalis glycosides, but not ouabain, are moderately ␣2-selective. This supports a major role of ␣2 in cardiac contraction and cardiotonic effects of digitalis glycosides.

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Human alpha1 and alpha2 isoforms of Na,K-ATPase have been expressed with porcine 10*Histidine-tagged beta1 subunit in Pichia pastoris. Methanol-induced expression of alpha2 is optimal at 20 degrees C, whereas at 25 degrees C, which is optimal for expression of alpha1, alpha2 is not expressed. Detergent-soluble alpha2beta1 and alpha1beta1 complexes have been purified in a stable and functional state. alpha2beta1 shows a somewhat lower Na,K-ATPase activity and higher K0.5K compared to alpha1beta1, while values of K0.5Na and KmATP are similar. Ouabain inhibits both alpha1beta1 (K0.5 24.6 +/- 6 nM) and alpha2beta1 (K0.5 102 +/- 14 nM) with high affinity. A striking difference between the isoforms is that alpha2beta1 is unstable. Both alpha1beta1 and alpha2beta1 complexes, prepared in C12E8 with an added phosphatidyl serine, are active, but alpha2beta1 is rapidly inactivated at 0 degrees C. Addition of low concentrations of cholesterol with 1-stearoyl-2-oleoyl-sn-glycero-3-[phospho-l-serine] (SOPS) stabilizes strongly, maintaining alpha2beta1 active up to two weeks at 0 degrees C. By contrast, alpha1beta1 is stable at 0 degrees C without added cholesterol. Both alpha1beta1 and alpha2beta1 complexes are stabilized by cholesterol at 37 degrees C. Human FXYD1 spontaneously associates in vitro with either alpha1beta1 or alpha2beta1, to form alpha1beta1/FXYD1 and alpha2beta1/FXYD1 complexes. The reconstituted FXYD1 protects both alpha1beta1 and alpha2beta1 very strongly against thermal inactivation. Instability of alpha2 is attributable to suboptimal phophatidylserine-protein interactions. Residues within TM8, TM9 and TM10, near the alphabeta subunit interface, may play an important role in differential interactions of lipid with alpha1 and alpha2, and affect isoform stability. Possible physiological implications of isoform interactions with phospholipids and FXYD1 are discussed.

Na+,K+-ATPase (porcine alpha1/His10*beta1 or human alpha1/porcine His10*beta1) has been expressed in Pichia pastoris and purified by Co2+-chelate affinity resin chromatography, yielding about 80% pure, functional, and stable protein in a single step. The protein was eluted in nonionic detergents together with a phosphatidylserine. Size exclusion chromatography showed that the protein eluted in n-dodecyl beta-d-maltoside is an alpha1/beta1 protomer, whereas that in octaethylene glycol dodecyl monoether contains a mixture of alpha1/beta1 protomer and higher order oligomers. The Na+,K+-ATPase activity (8-16 (mumol/min)/mg of protein) is similar in both detergents. Thus, the minimal functional unit is the alpha1/beta1 protomer, and activity is unaffected by the presence of oligomeric forms. Screening of phospholipids for stabilization of the Na+,K+-ATPase activity shows that (a) acid phospholipids are required and phosphatidylserine is somewhat better than phosphatidylinositol and (b) optimal stabilization is achieved with asymmetric phosphatidylserines having saturated (18:0 >or= 16:0) and unsaturated (18:1 > 18:2) side chains at sn-1 an sn-2 positions, respectively. In the presence of phosphatidylserine, cholesterol stabilizes the protein at 37 degrees C, but not at 0 degrees C. Cholesterol also increases the "apparent affinity" of the phosphatidylserine and stabilizes optimally in the presence of phosphatidylserines with a saturated fatty acyl chain at the sn-1 position. Ergosterol is a poor stabilizer. We propose that phosphatidylserine and cholesterol interact specifically with each other near the alpha1/beta1 subunit interface, thus stabilizing the protein. These interactions do not seem to affect Na+,K+-ATPase activity.

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Topology of the alpha-subunit of Na,K-ATPase has been analyzed utilizing proteolytic digestion. Evidence is presented for a model with 10 transmembrane segments and lability of the C-terminal domain (M7-M10). Using reconstituted proteoliposomes, inside-out oriented pumps were digested with trypsin at the cytoplasmic surface. Evidence was obtained for the M7/M8 pair and cytoplasmic splits between M8 and M9 and between M9 and M10. Because an extracellular split between M9 and M10 was also observed, using right-side-out oriented renal microsomes, we propose that the M9/M10 pair either is destabilized by cytoplasmic digestion or is intrinsically mobile. Using renal microsomes, extracellular digestion of the alpha-subunit by trypsin, chymotrypsin, or an endogenous protease has been observed, after incubation at 55 or at 45 degrees C with beta-mercaptoethanol (beta-ME) and n-butanol. Both perturbations inactivate enzyme activity. Rb ions protect against inactivation and digestion. At 45 degrees C, with beta-ME and n-butanol, trypsin and chymotrypsin cut between M7 and M8 and between M9 and M10, consistent with the 10-segment model. At 55 degrees C, the topological organization is altered, the M8/M9 connecting loop is exposed at the extracellular surface, and an additional split between M8 and M9 is observed. Extracellular digestion of the alpha-subunit is associated with digestion of the beta-subunit near the first extracellular S-S bridge. Rb ions protect the beta-subunit. Exposure to proteases of extracellular domains of both subunits appears to be caused by disruption of subunit interactions.

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 COOHterminal, smaller (8-11 kDa) membrane-embedded fragments of the a chain, and a largely intact ( measured by examination of labeling of 19-kDa peptide purified from "19-kDa membranes" or of a 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 a 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 a chain. After cyanogen bromide digestion of purified, labeled a chain, or of 19-kDa peptide, a labeled fragment of apparent Mr -4 kDa was detected and was identified as that with NH2-terminal Lys-943.Rb+-protected []4CJDCCD incorporation was associated almost exclusively with Glu-953. We suggest that the cation occlusion "cage"l 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.Understanding energy transduction by cation pumps requires knowledge of structure of the cation and ATP sites and their interactions. The structure of the cation sites is largely unknown. By analogy with well-characterized cation-binding proteins, bound or occluded cations are likely to be largely dehydrated and ligated with six to eight oxygens of carboxyls and neutral groups. Some information on cation-binding residues in pumps comes from chemical modification, sitedirected mutagenesis, and charge-transfer studies (for review, see ref.

Background:The ␣2 isoform of Na,K-ATPase may play a major role in aqueous humor production. Results: We have chemically modified digoxin and obtained derivatives with enhanced selectivity for ␣2. When applied topically, they effectively reduce intraocular pressure in rabbits. Conclusion: ␣2 is crucial for aqueous humor production. Significance: Potentially, the derivatives may be useful for control of intraocular pressure.