Polypeptide molecular weights of the (Na+,K+)-ATPase from porcine kidney medulla

REM sleep loss increases brain excitability: role of noradrenaline and its mechanism of action

Ever since the discovery of rapid eye movement sleep (REMS), studies have been undertaken to understand its necessity, function and mechanism of action on normal physiological processes as well as in pathological conditions. In this review, first, we briefly surveyed the literature which led us to hypothesise REMS maintains brain excitability. Thereafter, we present evidence from in vivo and in vitro studies tracing behavioural to cellular to molecular pathways showing REMS deprivation (REMSD) increases noradrenaline level in the brain, which stimulates neuronal Na-K ATPase, the key factor for maintaining neuronal excitability, the fundamental property of a neuron for executing brain functions; we also show for the first time the role of glia in maintaining ionic homeostasis in the brain. As REMSD exerts a global effect on most of the physiological processes regulated by the brain, we propose that REMS possibly serves a housekeeping function in the brain. Finally, subject to confirmation from clinical studies, based on the results reviewed here, it is being proposed that the subjects suffering from REMS loss may be effectively treated by reducing either noradrenaline level or Na-K ATPase activity in the brain.

An intrinsic membrane glycoprotein with cytosolically oriented n-linked sugars

We demonstrate that the Na(+)-pump alpha-subunit polypeptide is glycosylated by using bovine milk galactosyltransferase, a specific enzyme which attaches galactose to terminal N-acetylglucosamine residues. The galactose acceptor sites are available for glycosylation only after permeabilization of right-side-out vesicles prepared from kidney outer medulla; therefore, the oligosaccharide moieties are facing the cytoplasm of the cell. We further show that the oligosaccharides are bound to asparagine residues of the alpha-subunit polypeptide, since the protein-carbohydrate linkage is hydrolyzed by peptide-N glycosidase F (an enzyme specific for N-linked sugars). Thus, the Na(+)-pump alpha subunit is a glycoprotein with its N-linked oligosaccharide moieties located at the cytosolic face of the cell membrane. Intrinsic membrane glycoproteins with such an oligosaccharide-protein linkage and cell membrane orientation have not been previously reported, to our knowledge.

Configuration of subunits within crystals of Na, K-ATPase maintained in the frozen-hydrated state

Two-dimensional crystalline sheets of Na, K-ATPase were studied in the vitrified, frozen-hydrated state by electron microscopy and image processing. The technique of correlation averaging was used to determine the projected structure. The projection map shows asymmetry between the pair of "alpha beta" protomers comprising a dimer of Na, K-ATPase molecules. The two protomers differ in overall density as well as in shape. One protomer has an oblong shape, whereas the other with higher density has a head and a hook region. Such an asymmetry has not been reported by other laboratories. This asymmetry may either be due to the coexistence of two different conformations of the enzyme in the dimeric form or due to the simultaneous existence of two molecular species of Na, K-ATPase.

Identification of monoclonal antibody binding domains of Na+,K(+)-ATPase by immunoelectron microscopy

Treatment of purified preparations of porcine Na+,K(+)-ATPase with phospholipase A2, MgCl2 and NaVO3 leads to the formation of two-dimensional crystals exclusively in a dimeric configuration. Two-dimensional computer-averaged projections of the electron microscopy images of the crystalline enzyme with bound Fab fragments of monoclonal antibody M10-P5-C11 were accomplished using image enhancement software and showed that the antibody fragments caused only a modest increase in the unit cell size, while reducing the extent of asymmetry of the two promoters in each unit cell. The digital imaging also showed that the antibody's epitope on the alpha subunit resides on the 'lobe' or 'hook' region of the intracellular portion of the enzyme. Since functional studies indicate that M10-P5-C11 binds near or between the ATP binding site and the phosphorylation site, this visualized 'lobe' region of alpha may comprise the catalytic site. In addition, the binding of another inhibitory antibody, 9-A5, has been found to prevent crystal formation and the presence of the carbohydrate sugars on the enzyme's beta subunit shown to be required for crystal formation.

Directional immobilization of sodium- and potassium-activated ATPase to expose its cytoplasmic part to the liquid phase on microtiter plates by wheat germ agglutinin

A convenient method for highly efficient and directional immobilization of intact sodium- and potassium-activated ATPase (Na,K-ATPase) using wheat germ agglutinin linked on microtiter plates was developed. Wheat germ agglutinin, which bound tightly to the beta-subunit of Na,K-ATPase and had no effect on the Na,K-ATPase activity, the potassium-activated p-nitrophenylphosphatase activity, or the inhibitory action of ouabain, was covalently linked to microtiter plates and used as an immobilizer of the enzyme. The amount of Na,K-ATPase coupled to microtiter plates in this immobilizing system was more than 10-fold greater than that used in the direct immobilizing system (O. Urayama, M. Nakao, H. Nagamune, and H. Sugiyama, (1984) Anal. Biochem. 141, 194-198). Also in this system, the cytoplasmic domain of Na,K-ATPase was exposed to the liquid phase. This technique was useful for investigating the reactivities of monoclonal antibody specific for the cytoplasmic domain of the enzyme. Moreover, because this technique was used successfully in the immobilization of periodic acid--Schiff positive staining glycoprotein 1 prepared from human erythrocytes and human alpha 2-macroglobulin, the technique should also be useful for other membrane or secreted proteins that possess N-linked sugar chains containing bisecting N-acetylglucosamine or a high amount of sialic acid.

Digital image analysis of two-dimensional Na,K-ATPase crystals: dissimilarity between pump units

Two-dimensional crystals of purified Na,K-ATPase were induced by treatment with phospholipase-A2 and vanadate. The negatively stained crystals were imaged by electron microscopy and analysed by digital image processing. Two-dimensional averaged projections of the crystals were calculated by the technique of correlation analysis, utilizing SPIDER (System for Processing of Image Data in Electron microscopy and Related fields) image processing software. The calculated dimensions of the unit cell were found to be 13.3 X 4.59 nm with included angle of 98 degrees, comparable to those reported by others. However, the two protomers of the unit cell were found always to be dissimilar in shape and in orientation. All protomers of one side of the dimer ribbon had a triangular outline, and all protomers of the opposing side had a comma shape. This dissimilarity could be explained by two orientations of identical protomers: one orientation for one side of the dimer ribbon, and another orientation for the protomers of the opposing side of the ribbon. An alternative explanation is that the protomers of one side of the dimer ribbon are actually in a conformation different from that of the protomers of the opposing of the ribbon.

The molecular size required varies according to the reaction step round the sodium pump cycle

Progress along the path of the sodium pump cycle requires a stepwise recruitment of additional subunits for maximal activity. These results show that whereas a particle the size of the alpha beta protomer presents Na+,K+-ATPase activity at 10 microM ATP, an additional subunit, perhaps a second alpha-chain, is required to obtain the much greater Na+,K+-ATPase activity resulting from the occupation of low-affinity ATP sites at physiological ATP concentrations. A non-phosphorylating ATP analogue, however, will modestly stimulate the Na+,K+-ATPase activity acting at an alternative low-affinity site or step on the alpha beta protomer.

Cooperativity of the alpha beta-protomer structure in Na+,K+-ATPase functioning. A scanning microcalorimetry study

Heat denaturation of the free and ligand-bound forms of purified Na+,K+-ATPase from pig kidney is studied with the scanning microcalorimetry technique. A single two-state transition is observed during denaturation of the free enzyme, the molar concentration of the cooperatively melting units being equal to the concentration of alpha beta-protomers (Mr approximately equal to 140 000). Upon interaction of the enzyme with phosphate, Mg2+, and strophanthidin, but not with Na+, the cooperativity of the protomer unfolding is lost, and the protein stabilization enthalpy becomes approximately equal to 230 kJ/mol higher. The data suggest that in a functionally active enzyme form, the alpha beta-protomers possess a rigid structure with tight association of their subunits and domains, this structural rigidity is essential for the Na+,K+-ATPase functioning and there is a unique non-active conformation of the enzyme which may play an important role in its in vivo regulation.

Circular dichroism of the two major conformational states of mammalian (Na+ + K+)-ATPase

No alteration in the circular dichroic spectrum of fully active, membrane-bound (Na+ + K+)-ATPase is observed when the protein is cycled between the two major conformational states, E1 and E2. This finding is in agreement with the infrared study by Chetverin and Brazhnikov (J. Biol. Chem. 260 (1985) 7817) and demonstrates that any difference in secondary structure between the two conformers must be less than 2%.

Solubilization and further chromatographic purification of highly purified, membrane-bound Na,K-ATPase

Highly purified membrane-bound Na,K-ATPase from pig kidney outer medulla was dissolved in the non-ionic detergent C12E8. Chromatography of the dissolved material on a DEAE matrix yielded enzymatical material having a ouabain-binding capacity of 6.9 nmoles per mg protein (measured according to Lowry et al., with bovine serum albumin as standard). This material, which after addition of lipids had the same K+-phosphatase turnover as the membrane-bound enzyme, could consist entirely of live molecules with a molecular weight of 145 kDa, a value close to that expected for alpha beta-promoters of Na,K-ATPase.

Evidence for a diprotomeric structure of Na,K-ATPase. Accurate determination of protein concentration and quantitative end-group analysis

Three methods were used to assess protein concentration in membrane-bound Na,K-ATPase preparations: standard Lowry assay, Kjeldahl nitrogen determination and amino acid analysis. While the first two methods showed excellent agreement, the third one always gave a lower value which varied drastically depending on the condition of sample treatment before amino acid analysis. This result reinforces the Lowry method in assessing the true concentration of Na,K-ATPase protein and suggests 250 kDa to be a true estimate of the molecular mass of the smallest ligand-binding unit of the enzyme. The cyanate method reveals two NH2-terminal residues of the beta-subunit (NH2-Ala) and one such residue of the alpha-subunit (NH2-Gly) per ligand-binding unit. From the data on equimolarity of the alpha- and beta-subunits in Na,K-ATPase this suggests that the enzyme molecule is composed of two alpha beta-protomers, one possessing a modified (presumably an N-blocked) alpha-subunit.

Papain digestion of the subunits of (Na,K)-ATPase

Membrane-bound (Na,K)-ATPases were exposed to limited papain digestion. We could not find the active (Na,K)-ATPase lacking glycoprotein subunit for the enzymes from three different sources (outer medulla of dog kidney, electric organs of Narke japonica and larvae of Artemia salina). It seemed unlikely that the glycoprotein subunit was selectively removed from (Na,K)-ATPase by papain digestion.

Asymmetric orientation of amino groups in the alpha-subunit and the beta-subunit of (Na+ + K+)-ATPase in tight right-side-out vesicles of basolateral membranes from outer medulla

The orientation of amino groups in the membrane in the alpha- and beta-subunits of (Na+ + K+)-ATPase was examined by labeling with Boldon-Hunter reagent, N-succinimidyl 3-(4-hydroxy,5-[125I]iodophenyl)propionate), in right-side-out vesicles or in open membrane fragments from the thick ascending limbs of the Henles loop of pig kidney. Sealed right-side-out vesicles of basolateral membranes were separated from open membrane fragments by centrifugation in a linear metrizamide density gradient. After labeling, (Na+ + K+)-ATPase was purified using a micro-scale version of the ATP-SDS procedure. Distribution of label was analyzed after SDS-gel electrophoresis of alpha-subunit, beta-subunit and proteolytic fragments of alpha-subunit. Both the alpha- and the beta-subunit of (Na+ + K+)-ATPase are uniformly labeled, but the distribution of labeled residues on the two membrane surfaces differs markedly. All the labeled residues in the beta-subunit are located on the extracellular surface. In the alpha-subunit, 65-80% of modified groups are localized to the cytoplasmic surface and 20-35% to the extracellular membrane surface. Proteolytic cleavage provides evidence for the random distribution of 125I-labeling within the alpha-subunit. The preservation of (Na+ + K+)-ATPase activity and the observation of distinct proteolytic cleavage patterns of the E1- and E2-forms of the alpha-subunit show that the native enzyme structure is unaffected by labeling with Bolton-Hunter reagent. Bolton-Hunter reagent was shown not to permeate into sheep erythrocytes under the conditions of the labeling experiment. The data therefore allow the conclusion that the mass distribution is asymmetric, with all the labeled amino groups in the beta-subunit being on the extracellular surface, while the alpha-subunit exposes 2.6-fold more amino groups on the cytoplasmic than on the extracellular surface.

Minimal functional unit for transport and enzyme activities of (Na+ + K+)-ATPase as determined by radiation inactivation

Frozen aqueous suspensions of partially purified membrane-bound renal (Na+ + K+)-ATPase have been irradiated at -135 degrees C with high-energy electrons. (Na+ + K+)-ATPase and K+-phosphatase activities are inactivated exponentially with apparent target sizes of 184 +/- 4 kDa and 125 +/- 3 kDa, respectively. These values are significantly lower then found previously from irradiation of lyophilized membranes. After reconstitution of irradiated (Na+ + K+)-ATPase into phospholipid vesicles the following transport functions have been measured and target sizes calculated from the exponential inactivation curves: ATP-dependent Na+-K+ exchange, 201 +/- 4 kDa; (ATP + Pi)-activated Rb+-Rb+ exchange, 206 +/- 7 kDa and ATP-independent Rb+-Rb+ exchange, 117 +/- 4 kDa. The apparent size of the alpha-chain, judged by disappearance of Coomassie stain on SDS-gels, lies between 115 and 141 kDa. That for the beta-glycoprotein, though clearly smaller, could not be estimated. We draw the following conclusions: (1) The simplest interpretation of the results is that the minimal functional unit for (Na+ + K+)-ATPase is alpha beta. (2) The inactivation target size for (Na+ + K+)-dependent ATP hydrolysis is the same as for ATP-dependent pumping of Na+ and K+. (3) The target sizes, for K+-phosphatase (125 kDa) and ATP-independent Rb+-Rb+ exchange (117 kDa) are indistinguishable from that of the alpha-chain itself, suggesting that cation binding sites and transport pathways, and the p-nitrophenyl phosphate binding site are located exclusively on the alpha-chain. (4) ATP-dependent activities appear to depend on the integrity of an alpha beta complex.

Evidence for essential disulfide bonds in the beta-subunit of (Na+ + K+)-ATPase

(Na+ + K+)-ATPase from dog kidney lost its activity when heated at 55 degrees C in the presence of 0.3 M 2-mercaptoethanol. Either heat treatment alone or addition of reducing agent at around 25 degrees C caused little inactivation. One disulfide bond per protomer (mol. wt. 146 000) was reduced in the inactivated sample but in active samples no reduction occurred. Neither K+-dependent phosphatase activity nor phosphoenzyme formation in the presence of Na+ was detected in the inactivated sample, suggesting that the disulfide bond was essential for the catalytic cycle of (Na+ + K+)-ATPase. This essential disulfide bond belonged to the beta-subunit, the glycoprotein component of the enzyme, indicating that the beta-subunit may be an integral component of the (Na+ + K+)-ATPase system.

Molecular weights of alpha beta-protomeric and oligomeric units of soluble (Na+, K+)-ATPase determined by low-angle laser light scattering after high-performance gel chromatography

The (Na+, K+)-ATPase of canine renal outer medulla was solubilized with a nonionic surfactant, octaethylene glycol n-dodecyl ether (C12E8), in the presence of 0.2 M sodium ion. The solubilized ATPase retained 74% of the enzymatic activity expressed before solubilization. Molecular species of the solubilized ATPase were analyzed by high-performance chromatography through a TSK-GEL G3000SW column in the presence of 1 mg/ml C12E8 at 23 degrees C. The eluate was monitored by one or two monitors chosen from the following: an ultraviolet absorption monitor, a precision differential refractometer and a low-angle laser light scattering photometer. The three kinds of elution pattern thus obtained can best be interpreted by assuming the presence of at least four kinds of protein component with molecular weights 1 740 000 +/- 230 000, 836 000 +/- 82 000, 286 000 +/- 30 000 and 123 000 +/- 8 000, respectively. Among them, those with the last two molecular weight were the major components. The amounts of the first three components were found to increase with time during the incubation before application to the column at the expense of that of the last one. The amounts of the last two were 18 and 73%, respectively, when measured immediately after the solubilization. A stoichiometric composition of 1:1 molar ratio for the alpha and beta polypeptide chains was obtained for the two major components as well as for the intact ATPase by high-performance gel chromatography in the presence of sodium dodecyl sulfate using the same column as above. The (Na+, K+)-ATPase was, thus, indicated to be solubilized with C12E8 to give the alpha beta-protomer and its dimer as the main components.

The (Na+, K+)ATPase exhibits enzymic activity in the absence of the glycoprotein subunit

Membrane-bound (Na+, K+)ATPase from avian nasal salt glands was exposed to limited papain digestion. Such treatment results in the selective removal of the beta-subunit rendering the alpha-subunit still membrane-bound and expressing full enzymic activity. With further exposure to papain the alpha-chain becomes fragmented into two major polypeptide components. The fragmented membrane-bound catalytic chain is extremely sensitive to detergent treatment and cannot be solubilized in an active state.

Soluble and enzymatically stable (Na+ + K+)-ATPase from mammalian kidney consisting predominantly of protomer alpha beta-units. Preparation, assay and reconstitution of active Na+, K+ transport

Soluble (Na+ + K+)-ATPase consisting predominantly of alpha beta-units with Mr below 170 000 was prepared by incubating pure membrane-bound (Na+ + K+)-ATPase (35-48 mumol Pi/min per mg protein) from the outer renal medulla with the non-ionic detergent dodecyloctaethyleneglycol monoether (C12E8). (Na+ + K+)-ATPase and potassium phosphatase remained fully active in the detergent solution at C12E8/protein ratios of 2.5-3, at which 50-70% of the membrane protein was solubilized. The soluble protomeric (Na+ + K+)-ATPase was reconstituted to Na+, K+ pumps in phospholipid vesicles by the freeze-thaw sonication procedure. Protein solubilization was complete at C12E8/protein ratios of 5-6, at the expense of partial inactivation, but (Na+ + K+)-ATPase and potassium phosphatase could be reactivated after binding of C12E8 to Bio-Beads SM2. At C12E8/protein ratios higher than 6 the activities were irreversibly lost. Inactivation could be explained by delipidation. It was not due to subunit dissociation since only small changes in sedimentation velocities were seen when the C12E8/protein ratio was increased from 2.9 to 46. As determined immediately after solubilization, S20,w was 7.4 S for the fully active (Na+ + K+)-ATPase, 7.3 S for the partially active particle, and 6.5 S for the inactive particle at high C12E8/protein ratios. The maximum molecular masses determined by analytical ultracentrifugation were 141 000-170 000 dalton for these protein particles. Secondary aggregation occurred during column chromatography, with formation of enzymatically active (alpha beta)2-dimers or (alpha beta)3-trimers with S20,w = 10-12 S and apparent molecular masses in the range 273 000-386 000 daltons. This may reflect non-specific time-dependent aggregation of the detergent micelles.

ATP binding to solubilized (Na+ + K+)-ATPase. The abolition of subunit-subunit interaction and the maximum weight of the nucleotide-binding unit

Membrane-bound (Na+ + K+)-ATPase from pig kidney outer medulla shows apparent heterogeneity in its ATP-binding site population when assays are carried out in the presence of K+. This finding has been interpreted as being due to interaction between (at least) two subunits, each containing an ATP-binding site. Treating the membrane-bound enzyme with the detergent, C12E8, has been shown to solubilize enzymatically active alpha beta-protomers. We show that in the dissolved enzyme all ATP-binding sites in the population are identical both in the absence and in the presence of K+, which would be consistent with an abolition of identical both in the absence and in the presence of K+, which would be consistent with an abolition of subunit-subunit interaction. This supports previous suggestions that enzyme solubilized by C12E8 is monomeric and that the membrane-bound enzyme is not. Differential extraction of enzyme-containing membranes with C12E8 yielded preparations with an ATP-binding capacity of up to 5.8 nmol per mg protein, measured by the method of Lowry et al. (Lowry, O.H., Rosebrough, N.J., Farr, A.L. and Randall, R.J. (1951) J. Biol. Chem. 193, 265-275), with bovine serum albumin as standard. Evidence is presented that makes it likely that preparations with an ATP-binding capacity of 7.5 nmol per mg protein (as determined by the above-mentioned assay) will be obtainable. This corresponds to an alpha beta-protomer molecular weight of 133 000 which approximates closely to the minimum value found in the literature for an alpha beta-protomer (i.e., 126 000).

The K+-induced apparent heterogeneity of high-affinity nucleotide-binding sites in (Na+ + K+)-ATPase can only be due to the oligomeric structure of the enzyme

K+ induces an apparent heterogeneity among an otherwise homogeneous population of nucleotide-binding sites in (Na+ + K+)-ATPase preparations from pig kidney. With the help of ouabain we show that this heterogeneity cannot be due to a mixture of different and independent sites and conclude that each enzyme molecule must contain two nucleotide site-containing units that show interaction. Na+ induces an apparent heterogeneity among an otherwise homogeneous population of ouabain-binding sites. The argument is, therefore, extended to include one ouabain site on each of the structural units that bind nucleotide. All these structural units are shown to hydrolyse substrate at identical rates. Using the presently available molecular weight data, it is concluded that the enzyme is composed of two subunits each possessing one nucleotide-binding site, one ouabain-binding site, one alpha-peptide and the capacity for hydrolysing ATP and p-nitrophenyl phosphate.