The effects of an antiserum to Na+, K+-ATPase on the ion-transporting and hydrolytic activities of the enzyme

ATP hydrolysis associated with an uncoupled sodium flux through the sodium pump: evidence for allosteric effects of intracellular ATP and extracellular sodium

1. A method has been developed for regenerating [gamma(32)P]ATP of constant specific activity within resealed red cell ghosts, and for measuring its hydrolysis. The method may be used to follow the hydrolysis of ATP at concentrations down to 1 muM, and for periods long enough for the ATP at these very low concentrations to turn over several hundred times.2. Using this method we have been able to show that the ;uncoupled' efflux of Na caused by the Na pump when resealed red cell ghosts are incubated in (Na + K)-free media is associated with a hydrolysis of ATP. The stoicheiometry is roughly 2-3 Na ions expelled per molecule of ATP hydrolysed.3. Measurements of ATP hydrolysis and Na efflux as functions of intracellular ATP concentration have shown that uncoupled Na efflux, and its associated ATP hydrolysis, are saturated at intracellular ATP concentrations in the region of 1 muM.4. Measurement of ATP hydrolysis as a function of ATP concentration in resealed ghosts incubated in a K-containing medium gave a complicated activation curve suggesting the involvement of high-affinity (K(m)ca. 1 muM) and low-affinity (K(m)ca. 100 muM) sites.5. When resealed ghosts containing about 1 muM-ATP were incubated in a Na-free or in a high-Na medium, the addition of K to the medium reduced the rate of ouabain-sensitive ATP hydrolysis.6. Ouabain-sensitive ATP hydrolysis in resealed ghosts incubated in K-free choline media was inhibited by external Na at low concentrations (K(i) < 1 mM), but this inhibition was reversed as the external Na concentration was further increased.7. The results show that uncoupled Na efflux may be thought of as the transport mode associated with Na-ATPase activity, just as Na-K exchange is the transport mode associated with (Na + K)-ATPase activity. The significance of the differences between uncoupled Na efflux and Na-ATPase activity, on the one hand, and Na-K exchange and (Na + K)-ATPase activity, on the other, is discussed.

Different approaches to the mechanism of the sodium pump

The way in which the sodium pump uses energy from the hydrolysis of ATP to perform osmotic and electrical work is not yet understood. We attempt to bring together the results of a number of different approaches to this problem. One approach has been to correlate biochemical changes and ionic fluxes, both when the pump operates normally and when it operates in various abnormal 'modes' in particular unphysiological conditions. A second approach has been to expose fragments of cell membrane to (gamma-32P)ATP and to study the properties of components of the membrane that become labelled. It is now clear that 32P can be transferred to the beta-carboxy group of an aspartyl residue in a pump polypeptide, but there is controversy about the interrelations of different forms of this polypeptide and its role, if any, in the normal functioning of the pump. A third approach has been to attempt to purify the pump and to determine the properties of the pure enzyme. It seems that the pump contains a polypeptide (molecular weight about 100,000), which bears the phosphorylation site, and a smaller glycopeptide, but there is disagreement about the molecular ratios. The results of these and other approaches cannot yet be fitted into a satisfactory model for the sodium pump, but we shall consider some of the problems involved in this task.

Decoupling the Na+–K+–ATPase in vivo: A possible new role in the gills of freshwater fishes

The literature suggests that when Na(+)-K(+)-ATPase has reduced access to its glycosphingolipid cofactor sulfogalactosyl ceramide (SGC), it is converted to a Na(+) uniporter. We recently showed that such segregation can occur within a single membrane when Na(+)-K(+)-ATPase is excluded from membrane microdomains or 'lipid rafts' enriched in SGC (D. Lingwood, G. Harauz, J.S. Ballantyne, J. Biol. Chem. 280, 36545-36550). Specifically we demonstrated that Na(+)-K(+)-ATPase localizes to SGC-enriched rafts in the gill basolateral membrane (BLM) of rainbow trout exposed to seawater (SW) but not freshwater (FW). We therefore proposed that since the freshwater gill Na(+)-K(+)-ATPase was separated from BLM SGC it should also transport Na(+) only, suggesting a new role for the pump in this epithelium. In this paper we discuss the biochemical evidence for SGC-based modulation of transport stoichiometry and highlight how a unique asparagine-lysine substitution in the FW pump isoform and FW gill transport energetics gear the Na(+)-K(+)-ATPase to perform Na(+) uniport.

Cis-allosteric effects of cytoplasmic Na+/K+ discrimination at varying pH. Low-affinity multisite inhibition of cytoplasmic K+ in reconstituted Na+/K(+)-ATPase engaged in uncoupled Na(+)-efflux

In liposomes with reconstituted shark Na+/K(+)-ATPase the effect of cytoplasmic K+ was investigated in the absence of extracellular alkali ions. During such conditions the Na+/K(+)-ATPase is engaged in the so called uncoupled Na+ efflux mode in which cytoplasmic Na+ activates and binds to the enzyme and becomes translocated without countertransport of K+ as in the physiological Na+/K+ exchange mode. In this uncoupled flux mode only low-affinity inhibition by K+cyt is found to be present. The inhibition pattern is consistent with a model in which cytoplasmic K+ exhibit mixed inhibition of Na+ activation, probably by binding at the three cytoplasmic loading sites on E1ATP (E1A). With determined intrinsic binding constants for cytoplasmic Na+ to this form of KS1, KS2, KS3 = 40 mM, 2 mM, 2 mM the inhibition pattern can be simulated assuming three K+cyt sites with equal affinity for Ki = 40 mM, similar to KS1 for the first Na+cyt site. The discrimination between cytoplasmic Na+ and K+ is therefore enhanced by allosteric interaction initiated from the cis-side due to binding of the first Na+, as opposed to K+, which induces the positive cooperatively in the successive Na+ bindings. pH is found to influence the pattern of K+cyt inhibition: A lowering of the pH potentiates the K+cyt inhibition, whereas at increased pH the inhibition is decreased and transformed into a pure competitive competition.

Variable stoichiometry in reconstituted shark Na,K-ATPase engaged in uncoupled efflux

In liposomes with reconstituted shark Na,K-ATPase produced to contain no internal K+ or Na+ addition of external Na+ and ATP induce an uncoupled Na+ efflux on inside-out oriented pumps which is electrogenic and accompanied by hydrolysis of ATP (Cornelius, F. (1989) Biochem. Biophys. Res. Commun. 160, 801-807). At saturating cytoplasmic Na+ the net-charge translocated per ATP molecule split is compatible with a coupling ratio of Nacyt transported per ATP split of 3:1 at pH greater than or equal to 7.0. However, this ratio decreases to 1.5:1 below pH 7.0. At non-saturating cytoplasmic Na+ the 3:1 stoichiometry is attained at pH 7.0-7.5, whereas outside this range of pH the net-charge translocated per ATP molecule split decreases.

Uncoupled Na+-efflux on reconstituted shark Na,K-ATPase is electrogenic

In liposomes with reconstituted shark Na,K-ATPase produced to contain sucrose addition of external Na+ and ATP induce an uncoupled Na+-efflux on inside-out oriented pumps which can be inhibited by digitoxigenin. This flux mode is found to be electrogenic and accompanied by hydrolysis of ATP. The coupling ratio of Nacyt transported per ATP split is 3:1 measured as the initial rate of rise in transmembrane potential and initial rate of liberated Pi.

Immunochemical evidence for a transmembrane orientation of both the (Na+, K+)-ATPase subunits

Antibodies were raised against the large catalytic subunit (apparent Mr 96000) and the glycoprotein (apparent Mr 60000) of the sodium- and potassium-dependent adenosine triphosphatase [(Na+, K+)-ATPase] from Bufo marinus. The specificity of each antiserum was assessed by two-dimensional immunoelectrophoresis using toad kidney microsomes or the purified holoenzyme as a source of antigen and by indirect immunoprecipitation of detergent-solubilized (Na+, K+)-ATPase subunits from radioiodinated or biosynthetically labeled kidney holoenzyme, microsomes, or postnuclear supernatant. The anticatalytic subunit serum reacted exclusively with a 96000-dalton protein. The antiserum to the glycoprotein was rendered specific to this subunit by absorption with purified catalytic subunit. The two antisera were agglutinating and lytic in the presence of complement when toad erythrocytes were used as targets, indicating that antigenic determinants of both subunits were exposed on the cell surface. The specific reactivities with surface-exposed antigenic determinants of both subunits could be absorbed with toad red blood cells. Such absorbed antisera still reacted with detergent-treated or untreated kidney microsomes, revealing the presence of cytoplasmic and/or intramembranous antigenic sites. Our immunochemical data demonstrate that the glycoprotein subunit of (Na+, K+)-ATPase spans the lipid bilayer and confirm the transmembrane orientation of the catalytic subunit postulated from functional studies.

K+ influx components in ascites cells: the effects of agents interacting with the (Na+ + K+)-pump

Several agents known to interact with the (Na+ + K+)-pump were tested for their effects on the components of steady-state K+ flux in ascites cells. 86Rb+ was used as a tracer for K+, and influx was differentiated into a ouabain-inhibitable "pump" component, a Cl--dependent and furosemide-sensitive "exchange" component, and a residual "leak" flux. All agents tested (ouabain, quercetin, oligomycin, phosphate) affected both the "pump" flux and the Cl--linked flux. These findings suggest a linkage between the activity of the Na/K ATPase and the Cl--dependent K+ exchange flux. In the discussion we point out that the mechanism of this linkage could be direct; e.g., Cl--dependent exchange may represent a mode of operation of the Na/K ATPase. However, data from this and other systems tend to suggest an indirect linkage between the Na+ pump and a KCl symporter, perhaps via a change in the level of intracellular ATP.

Red cell sodium fluxes catalysed by the sodium pump in the absence of K+ and ADP

In the absence of extracellular Na+ or K+, the sodium pump catalyses an ouabain-sensitive "uncoupled" Na+ efflux1-4. With red cell ghosts Glynn and Karlish5 showed that this Na+ efflux is accompanied by ATP hydrolysis and that extracellular sodium ions, at low concentrations, inhibit this efflux as well as the associated ATP hydrolysis. At higher concentrations, extracellular sodium ions restore the hydrolysis of ATP3,6 but it is not known whether there is an associated increase in Na+ efflux and, perhaps, an influx. To answer this question we have used inside-out red cell membrane vesicles which are specially suitable for controlling the composition of the medium at the two membrane surfaces while measuring 22Na+ fluxes in both directions. We report here that the sodium pump can operate in a mode in which influx and efflux of sodium are associated with ATP hydrolysis. This mode is different from the Na-Na exchange described by Garrahan and Glynn7, and Glynn and Hoffman8, which requires ADP as well as ATP9 and is probably associated with ADP-ATP exchage rather than ATP hydrolysis10,11.

ATPase and phosphatase activities from human red cell membranes: I. The effects of N-ethylmaleimide

In human red cell membranes the sensitivity to N-ethylmaleimide of Ca2+-dependent ATPase and phosphatase activities is at least ten times larger than the sensitivity to N-ethylmaleimide of (Na+ + K+)-ATPase and K+-activated phosphatase activities. All activities are partially protected against N-ethylmaleimide by ATP but not by inorganic phosphate or by p-nitrophenylphosphate. (ii) Protection by ATP of (Na+ + K+)-ATPase is impeded by either Na+ or K+ whereas only K+ impedes protection by ATP of K+-activated phosphatase. On the other hand, Na+ or K+ slightly protects Ca2+-dependent activities against N-ethylmaleimide, this effect being independent of ATP. (iii) The sensitivity to N-ethylmaleimide of Ca2+-dependent ATPase and phosphatase activities is markedly enhanced by low concentrations of Ca2+. This effect is half-maximal at less than 1 micron Ca2+ and does not require ATP, which suggests that sites with high affinity for Ca2+ exist in the Ca2+-ATPase in the absence of ATP. (IV) Under all conditions tested the response to N-ethylmaleimide of the ATPase and phosphatase activities stimulated by K+ or Na+ in the presence of Ca2+ parallels that of the Ca2+-dependent activities, suggesting that the Ca2+-ATPase system possesses sites at which monovalent cations bind to increase its activity.

Immunochemical studies on the large polypeptide of electrophorus electroplax (Na+ + K+)-ATPase

Antibodies against Lubrol-solubilized Electrophorus electroplax (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) and its 96 000-dalton polypeptide (P96) were raised in rabbits. The P96 antibody does not cross react with the (Na+ + K+)-ATPase from mammalian species and tissues, but it cross reacts with the (Na+ + K+)-ATPase from both Electrophorus electroplax and brain. The combination of enzyme with anti-P96 is found to inhibit phosphoryl enzyme formation to the same extent that it inhibits enzyme activity. The rate of K+-sensitive dephosphorylation of phosphoryl enzyme appears to be unchanged. These are also found to be true with the antibody against the whole enzyme. Upon tryptic digestion of the enzyme-anti-P96 complex only the large polypeptide of the enzyme is protected. In the case of enzyme-anti-Lubrol-solubilized enzyme complex, both the large and small polypeptides are protected, whereas preimmune sera are without any protecting effect. The data indicate that the phosphoryl acceptor polypeptide and the Lubrol-solubilized electroplax (Na+ + K+)-ATPase from which the polypeptide is derived are phylogenetically distinct from those of the mammalian (Na+ + K+)-ATPases. The selective tryptic resistance of the enzyme-anti-P96 complex indicates that the two polypeptides are spatially well separated, possibly on opposite sides of the membrane.

Effects of (Na+ + K+)-ATPase-specific antibodies of enzymatic activity and monovalent cation transport

Antibodies have been obtained that specifically interact with the transport enzyme (Na+ + K+)-activated ATPase. The antigen used was purified (Na+ + Ka+)-ATPase from canine renal medulla. Purified gamma globulin from immunized animals, but not from control animals or preimmune serum, inhibited (Na+ + Ka+)-ATPase from canine renal medullar with reduction of activity to 33 +/- 4 (SD)% in concentration-dependent manner. Maximum inhibition occurred in less than 5 minutes at 37 degrees C. The Mg++ -dependent, nonouabain inhibited component of activity (Mg++ -ATPase) was unaffected. Fab fragments obtained by papain cleavage of the gamma globulin fraction had similar inhibitory activity and specificity. These antibodies also produced varying degrees of concentration-related inhibition of canine myocardial, calf brain, and human red cell ghost (Na++ + Ka+)-ATPase, but not Mg++-ATPase activity.

The (Na++K+) activated enzyme system and its relationship to transport of sodium and potassium

It seems to be the membrane bound (Na++K+)-activated enzyme system which transforms the energy from a hydrolysis of ATP into a vectorial movement of sodium out and potassium into the cell against electrochemical gradients, i.e. this systems seems to be the transport system for sodium and potassium (see, for example, review by Skou, 1972; Hokin & Dahl, 1972).

Isolation and characterization of the components of the sodium pump

A satisfactory understanding of the functions of the sodium pump, the system responsible for the active transport of sodium and potassium, require the isolation and characterization of its protein and lipid components which are integrated in the structure of the cell membrane. The enzyme system (Na++ K+)-ATPase, is located in membrane fragments and behaves in the test tube like the transport system in the intact cell membrane (Skou,1957) Purified preparations of this enzyme will contain some, if not all, of the components of the sodium pump.