Potassium Ion-activated Adenosine Triphosphatase Research Articles

Conformation-dependent phosphorylation of Na,K-ATPase by protein kinase A and protein kinase C.

Phosphorylation of sodium and potassium ion-activated adenosine triphosphatase (Na,K-ATPase) by protein kinase A (PKA) and protein kinase C (PKC) was investigated in vitro, where substrate conformation, kinase activity, and consequent effects on Na,K-ATPase activity could be controlled. With Na, K-ATPase from rat kidney, optimal stoichiometries were close to 1 mol 32P/mol Na,K-ATPase for both kinases. Addition of Na+, K+, P(i), or ouabain is known to stabilize the Na,K-ATPase in different states and was found to affect phosphorylation by the two kinases in a reciprocal way. This indicates that exposure of the phosphorylation sites varies with conformation and suggests a structural basis for the variable responses to kinase activation in intact cells. Further evidence for the importance of Na,K-ATPase conformation in its interaction with kinase came from the autophosphorylation of PKC, which varied in proportion to both the concentration and conformation of rat Na,K-ATPase. With pig and dog Na,K-ATPase, little phosphorylation by PKC was detected, and yet the PKC phosphorylated itself when the Na,K-ATPase was in the optimal conformation. The location of the PKA phosphorylation site was confirmed to be Ser-938 by sequence analysis of a tryptic peptide. Effects of PKA on Na,K-ATPase activity could not be measured because of inhibition by the Triton X-100 needed to obtain phosphorylation. Phosphorylation by PKC, even in optimal conditions, failed to result in inhibition of Na,K-ATPase activity. This suggests that any physiological role of phosphorylation either entails a subtle modulation of enzyme properties, or requires additional regulatory proteins.

Acid dissociation constant and apparent nucleophilicity of lysine-501 of the .alpha.-polypeptide of sodium and potassium ion-activated adenosinetriphosphatase

A combination of competitive labeling with [3H]acetic anhydride [Kaplan, H., Stevenson, K. J., & Hartley, B. S. (1971) Biochem. J. 124, 289-299] and immunoaffinity chromatography is described that permits the assignment of the acid dissociation constant and the absolute nucleophilicity of individual lysines in a native enzyme. The acid dissociation constant of lysine-501 of the alpha-polypeptide in native (Na+ + K+)-ATPase was determined. This lysine had a normal pKa of 10.4. The rate constant for the reaction of the free base of lysine-501 with acetic anhydride at 10 degrees C is 400 M-1 s-1. This value is only 30% that for a fully accessible lysine in a protein. The lower than normal apparent nucleophilicity suggests that lysine-501 is hindered from reacting with its intrinsic nucleophilicity by the tertiary structure of the enzyme and is consistent with its location within a pocket that forms the active site upon the surface of the native protein.

Nucleophilic behavior of lysine-501 of the .alpha.-polypeptide of sodium and potassium ion activated adenosine triphosphatase consistent with a role in binding adenosine triphosphate

An immunoadsorbent specific for the carboxy-terminal sequence -GAPER, which comprises residues 502-506 of the alpha-polypeptide of ovine sodium and potassium ion activated adenosinetriphosphatase [(Na+ + K+)-ATPase], was used to isolate the products of the reaction between the lysine immediately preceding this sequence in the intact protein and either [3H]acetic anhydride or fluorescein 5'-isothiocyanate. Changes in the apparent nucleophilicity of this lysine, Lys501, were observed with both reagents when ATP was bound by the intact, native enzyme poised in the E1 conformation or when the structure of the enzyme was changed from the E1 conformation into the E2-P conformation. With both reagents, a decrease of more than 4-fold in the yield of incorporation occurred during the former change, but a decrease of only 2-fold occurred during the latter. Because a much larger decrease occurred when ATP was bound in the absence of a conformational change than occurred when a major conformational change took place in the absence of the occupation of the active site, these changes in the incorporation of [3H]acetyl suggest that Lys501 from the alpha polypeptide is directly involved in binding ATP within the active site of (Na+ + K+)-ATPase. The immunochemical reactions between the specific polyclonal antibodies raised against the sequence-GAPER and denatured or enzymically active (Na+ + K+)-ATPase were also investigated. Western blots and the inhibition of enzymic activity caused by the antibody have shown that it can bind to both the denatured and the native form of the alpha-polypeptide, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration that lysine-501 of the alpha polypeptide of native sodium and potassium ion activated adenosinetriphosphatase is located on its cytoplasmic surface.

Evidence that the peptide HLLVMKGAPER, which can be released from intact sodium and potassium ion activated adenosinetriphosphatase by tryptic digestion, is located on the cytoplasmic surface of the native enzyme has been obtained. An immunoadsorbent directed against the carboxy-terminal sequence of this tryptic peptide has been constructed. The peptide KGAPER was synthesized by solid-phase techniques. Antibodies against the sequence -GAPER were purified by immunoadsorption, using the synthetic peptide attached to agarose beads. These antibodies, in turn, were coupled to agarose beads to produce an immunoadsorbent. Sealed, right-side-out vesicles, prepared from canine kidneys, were labeled with pyridoxal phosphate and sodium [3H]borohydride in the absence or presence of saponin, respectively. A tryptic digest of these labeled vesicles was passed over the immunoadsorbent. Large increases in the incorporation of radioactivity into the peptides bound by the immunoadsorbent were observed in the digests obtained from the vesicles exposed to saponin. From the results of several control experiments examining the labeling reaction as applied to these vesicles, it could be concluded that this increase in incorporation resulted only from the access that the reagents gained to the inside of the vesicles in the presence of saponin and that the increase in the extent of modification was due to the cytoplasmic disposition of this segment in the native enzyme.

Specific photoaffinity labeling of the digitalis binding site of the sodium and potassium ion activated adenosinetriphosphatase induced by energy transfer.

A ouabain p-aminobenzenediazonium derivative with a high specific radioactivity has been synthesized from ouabain and used as a photolabel for the (sodium plus potassium)-activated adenosinetriphosphatase from Electrophorus electricus electric organ and from dog kidney. In the dark it binds reversibly to the digitalis receptor site, with binding characteristics comparable to those of ouabain. The photoactivation of the ouabain derivative to produced covalent labeling of the receptor was obtained by energy transfer from a tryptophan residue in the (Na+,K+)ATPase to the ouabain p-aminobenzenediazonium molecule bound at the active site. The great advantage of this procedure compared to previous methods is that free molecules of the photoactivatable derivative are not photodecomposed. Analysis of the photolabeled polypeptides on sodium dodecyl sulfate gel electrophoresis showed that over 90% of the total radioactivity incorporated was found in the large molecular weight alpha-chain of the kidney enzyme (Mr 93 000). The same specific labeling of the alpha-subunit was obtained with a crude microsomal fraction from Electrophorus electricus. A mild tryptic fragmentation of the subunit into two peptide fragments of Mr 58 000 and 41 000, respectively, shows that the digitalis receptor is located in the N-terminal 41 000 fragment.

Topological localization of proteolytic sites of sodium and potassium ion stimulated adenosinetriphosphatase.

The (Na+ and K+)-stimulated adenosinetriphosphatase [(Na+,K+)-ATPase] consists of two different polypeptides, alpha and beta, both of which are embedded in the plasma membrane. The alpha chain from dog kidney (Na+,K+)-ATPase can be hydrolyzed at specific sites by trypsin and chymotrypsin [Castro, J., & Farley, R. A. (1979) J. Biol. Chem. 254, 2221-2228]. In order to position these sites with respect to the lipid bilayer, we have treated sealed, inside out vesicles from human red cells and unsealed kidney enzyme membranes with trypsin and chymotrypsin and have used ouabain-stimulated phosphorylation to identify the (Na+,K+)-ATPase and its fragments. All of the proteolytic sites observed in the kidney membranes are accessible in the inside out vesicles. The ouabain-inhibitable uptake of 86Rb+ in human red blood cells is resistant to externally added chymotrypsin. These results indicate that the proteolytic sites of the (Na+,K+)-ATPase are exposed on the cytoplasmic side of the membrane.

Structure of the sodium and potassium ion activated adenosine triphosphatase inhibitor L-681,110

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructure of the sodium and potassium ion activated adenosine triphosphatase inhibitor L-681,110Otto D. Hensens, Richard L. Monaghan, Leeyuan Huang, and George Albers-SchonbergCite this: J. Am. Chem. Soc. 1983, 105, 11, 3672–3679Publication Date (Print):June 1, 1983Publication History Published online1 May 2002Published inissue 1 June 1983https://doi.org/10.1021/ja00349a054RIGHTS & PERMISSIONSArticle Views101Altmetric-Citations28LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (995 KB) Get e-Alerts Get e-Alerts

Lactoperoxidase-catalyzed iodination of sodium and potassium ion-activated adenosine triphosphatase in the Madin-Darby canine kidney epithelial cell line and canine renal membranes

Experiments are described in which the large chain of ( Na + + K +)- ATPase is labeled by lactoperoxidase-catalyzed iodination either at its extracytoplasmic surface exclusively or at both its extracytoplasmic and its cytoplasmic surfaces simultaneously. The former was accomplished by labeling intact cells of the Madin-Darby canine kidney line, and the latter by labeling open membrane vesicles, also from canine kidney. A comparison of the specific radioactivities for the large chain from the open membranes and the large chain from the Madin-Darby canine kidney cells reveals that the former was labeled approximately 5-fold more extensively. This indicates that the large chain of ( Na + + K +)- ATPase is situated in the membrane such that more of its mass protrudes into the cytoplasm than into the extracytoplasmic environment.

Monomer of sodium and potassium ion activated adenosinetriphosphatase displays complete enzymatic function.

The distribution of sodium and potassium ion activated adenosinetriphosphatase [(Na+ + K+)-ATPase] among the various oligomeric forms present in a given solution is assessed unambiguously by cross-linking with glutaraldehyde. Purified enzyme dissolved in a solution of a nonionic detergent, octaethylene glycol dodecyl ether, remains dispersed and unaggregated after removal of the bulk of the detergent. Increases in the aggregation of the enzyme, which have been previously observed upon the addition of substrates to such a solution, are found to be due to changes in ionic strength rather than a consequence of the initiation of turnover. Furthermore, conditions are described that produce solutions containing stable, enzymatically active mixtures of the smaller oligomers of the asymmetric unit, alpha beta. Cross-linking by glutaraldehyde while the enzyme is turning over demonstrates that at least one of these oligomers is responsible for the observed enzymatic activity. A determination of which oligomers are present in each fraction from a glycerol gradient demonstrates that the profiles of the enzymatic activity and the concentration of monomer coincide. In addition, the monomer can form the sodium-dependent, phosphorylated intermediate of the mechanism for the enzyme. Finally, a preparation of (Na+ + K+)-ATPase, dissolved in solutions of the same nonionic detergent, can be prepared in which the predominant species (greater than 85%) is the monomer. The enzyme in this solution exhibits high specific activity, and its apparent Michaelis constants for the cationic substrates are very similar to those of the purified, membrane-bound enzyme. It is concluded from these results that a monomer of the alpha beta asymmetric unit is fully capable of catalyzing (Na+ + K+)-ATPase activity, and hence active transport, in the native enzyme. A reassessment of proposed molecular mechanisms for active transport is made in light of these discoveries.

Determination of the distribution of sodium and potassium ion activated adenosinetriphosphatase among the various oligomers formed in solutions of nonionic detergents.

Sodium and potassium ion activated adenosinetriphosphatase [(Na+ + K+)-ATPase] can be dispersed from the membrane-bound state, with the stable retention of the capacity to display (Na+ + K+)-ATPase activity, by treatment with solutions of a homogeneous, nonionic detergent, octaethylene glycol dodecyl ether. The dispersed enzyme is incapable of turnover, however, in solutions where the free detergent concentration is above the critical micelle concentration. Treatment of solutions of this enzyme with the crosslinking reagent glutaraldehyde results in the quantitative, covalent coupling of the alpha-and beta-polypeptides. The various covalent products formed, when visualized on sodium dodecyl sulfate-polyacrylamide gels, are integral oligomers of the asymmetric unit (alpha beta) of the enzyme. The noncovalent oligomers from which these products are derived can be separated on sucrose gradients based on differences in their respective sedimentation coefficients, but these sedimentation coefficients are highly dependent on the concentration of detergent in the gradient. Furthermore, the cross-linking assay reveals that changes in the aggregation state of the enzyme occur as detergent:protein ratios are varied or when the enzyme is added to the ATPase assay. These observations suggest that earlier conclusions about the oligomers of this enzyme present in detergent solution were significantly in error.

The reaction of sulfhydryl groups of sodium and potassium ion-activated adenosine triphosphatase with N-ethylmaleimide. The relationship between ligand-dependent alterations of nucleophilicity and enzymatic conformational states.

The reaction between N-ethylmaleimide and (Na+ + K+)-ATPase, performed under ligand conditions which produce each of the kinetic states of the enzyme and their associated conformational forms, was examined through an analysis of the inhibition of enzymatic activity and the incorporation of radiolabeled reagent into the enzyme. The inactivation reactions displayed pseudo-first order kinetics with respect to the concentration of active enzyme, indicating that the loss of activity is associated with the alkylation of a unique sulfhydryl group. In the absence of enzyme phosphorylation, the nucleophilicity of this sulfhydryl group is affected primarily by the nature of the monovalent cation present and does not correlate with the conformational state. A method for determining the actual concentration and specific radioactivity of radiolabeled N-ethylmaleimide during the reaction with (Na+ + K+)-ATPase was developed, allowing the measurement of the total reactive sulfhydryl groups of native (Na+ + K+)-ATPase under conditions identical with those of the inactivation studies. The labeling of the enzyme complex is associated almost exclusively with the large polypeptide, which contains four sulfhydryl groups which react with this reagent. One of these residues is presumably the sulfhydryl responsible for inactivation of the enzyme. Two react stoichiometrically and rapidly with N-ethylmaleimide under all conditions. The nucleophilicity of the fourth sulfhydryl group is governed by the conformational state of the enzyme, but the alkylation of this residue does not result in loss of enzymatic activity.

Stoichiometry and molecular weight of the minimum asymmetric unit of canine renal sodium and potassium ion-activated adenosine triphosphatase.

Sodium and potassium ion-activated adenosine triphosphatase is known to be composed of at least two different polypeptides, alpha and beta. When a detergent-treated supernatant preparation of the enzyme is reacted with the cross-linking reagent, cupric phenanthroline, a single, covalent heterodimer is formed. This product is formed from one of each of the two polypeptides. The remaining, unreacted alpha and beta chains maintain a constant ratio to each other throughout the reaction. The same heterodimer is formed in membrane-bound enzyme when reacted with several other cross-linking reagents. The protein mass ratio between the chains in the native enzyme, determined by two methods, is 2.15 +/- 0.16. Using this value and a value of 121,000 +/- 6,000 for the molecular weight of the larger polypeptide, a molecular weight of 56,000 +/- 7,000 can be calculated for the protein portion of the smaller polypeptide. Upon removal of a substantial portion of the carbohydrate from the smaller polypeptide, a change in its electrophoretic mobility is observed, while that of the larger polypeptide remains unaffected. The apparent length of this unglycosylated small chain is 450 residues, corresponding to a molecular weight of 51,000. Taken together, these results demonstrated that the two polypeptides of the (Na+ + K+)-ATPase exist in an equimolar, noncovalent association in the native enzyme, and that the protein molecular weight of the minimum asymmetric unit is 177,000 +/- 13,000, Previous results which address the question of the quaternary structure of the ATPase are re-examined in light of these determinations.

Effect of temperature on phosphorylation and ouabain binding to N-ethylmaleimide-treated (Na+, K+)-ATPase.

Treatment of purified sodium and potassium ion-activated adenosine triphosphatase from sheep kidney with N-ethylmaleimide in the presence of KC1 and ATP reduced hydrolytic activity by 83% while steady state phosphorylation was inhibited only 24%. At 3O”C, in the presence of magnesium plus inorganic phosphate, the ouabain-binding capacity of the N-ethylmaleimidetreated enzyme was only 15% less than the capacity of the native enzyme. The phosphorylated intermediate formed from the N-ethylmaleimide-treatedenzyme was less sensitive to decomposition by KC1 and more sensitive to ADP when measured at either 0” or 22”C, compared to a native enzyme. The ouabain-binding capacity of native enzyme at 0” and 30°C in the presence of either magnesium plus inorganic phosphate or magnesium plus ATP plus sodium was the same. The ouabain-binding capacity of the N-ethylmaleimidetreated enzyme in the presence of magnesium plus inorganic phosphate was the same at 0°C as it was at 30°C. However, in the presence of magnesium plus ATP plus sodium, the treated enzyme bound only half as much ouabain at 0°C as it did at 3O”C, even when the reaction was extended to several hours. The rates of ouabain binding were approximately equal for the native and N-ethylmaleimide-treated enzyme when measured in the presence of magnesium alone or magnesium plus ATP. N-Ethylmaleimide treatment, however, reduced the rates of ouabain binding in the presence of magnesium plus inorganic phosphate, magnesium plus ATP plus sodium, or magnesium plus ATP plus sodium plus potassium. The results indicate that ouabain does interact with N-ethylmaleimide-treated enzyme and that multiple conformers of (Na+,K+)-ATPase, as de-

Alteration of 86Rb+ influx and efflux following depletion of membrane sterol in L-cells.

Ouabain-sensitive uptake of 86Rb+ (an analogue of K+) was enhanced in L-cells that had been treated with 25-hydroxycholesterol or 7-ketocholesterol in order to deplete their sterol concentration. Ouabain-insensitive Rb+ efflux also increased in the sterol-depleted cells and the intracellular concentration of K+ diminished while the concentration of Na+ increased. All of these effects of 25-hydroxycholesterol were counteracted by the addition of mevalonate to the culture medium. Despite the evidence for increased active Rb+ transport in the 25-hydroxycholesterol-treated cells, the level of sodium and potassium ion-activated adenosine triphosphatase ((Na+ + K+)-activated ATPase) activity measured in homogenates and plasma membrane preparations from the treated cells was not significantly different from the control values. Rb+ uptake was more sensitive to ouabain inhibition in sterol-depleted cells than in control cells, although ATPase activity in plasma membrane fractions isolated from treated cells was not more sensitive to ouabain inhibition than was that from control cells. It is possible that the ability of the oxygenated sterols to inhibit DNA synthesis and cell division (Kandutsch, A. A., and Chen, H. W. (1977) J. Biol. Chem. 252, 409-415) is related to their effects upon cellular ion transport.

Active transport of sodium and potassium ions by the sodium and potassium ion-activated adenosine triphosphatase from renal medulla. Reconstitution of the purified enzyme into a well defined in vitro transport system.

Active transport of sodium and potassium ions by the sodium and potassium ion-activated adenosine triphosphatase from renal medulla. Reconstitution of the purified enzyme into a well defined in vitro transport system.

Specific sodium-22 binding to a purified sodium + potassium adenosine triphosphatase. Inhibition by ouabain.

Analysis of sodium-22 binding to purified sodium + potassium ion-activated adenosine triphosphatase (Na+, K+)-ATPase reveals the presence of two classes of binding sites. The higher affinity site (Kd = 0.2 mM) binds 6 to 7 nmol of sodium per mg of protein. Pretreatment of (Na+, K+)-ATPase with ouabain blocks the binding of sodium to this higher affinity site. Neither heat-denatured enzyme nor phospholipids extracted from the (Na+, K+)-ATPase contain a ouabain-inhibitable, higher affinity sodium binding site. The ouabain enzyme complex therefore appears to contain altered binding sites for cations.

Structural studies of sodium and potassium ion-activated adenosine triphosphatase. The relationship between molecular structure and the mechanism of active transport.

Sodium and potassium ion-activated adenosine triphosphatase is the enzyme responsible for the active transport of sodium and potassium across the plasma membrane. Strophanthidin, from the external surface of the membrane, and an antibody, from the cytoplasmic surface, bind simultaneously to the large polypeptide subunit of the enzyme. These results demonstrate that this polypeptide chain must span the plasma membrane, having different surfaces exposed on each side. When (Na+ + K+)-ATPase is incubated in the presence of cupric phenanthroline, a reagent which catalyzes the oxidation of cysteine residues to form intermolecular and intramolecular disulfide bonds, a covalent dimer of the larger chains is formed. Several characteristics of this dimerization reaction are consistent with the proposal that at least a noncovalent dimer of large chains exists in the native enzyme. These conclusions are discussed in the context of a specific description for the molecular mechanism of active transport.

Active potassium transport coupled to active sodium transport in vesicles reconstituted from purified sodium and potassium ion-activated adenosine triphosphatase from the rectal gland of Squalus acanthias

Vesicles containing a purified shark rectal gland (sodium + potassium)-activated adenosine triphosphatase-(NaK ATPase) were prepared by dialyzing for 2 days egg lecithin, cholate, and the NaK ATPase purified from the rectal gland of Squalus acanthias. These vesicles were capable of both Na+ and K+ transport. Studies of K+ transport were made by measuring the ATP-stimulated transport outward of 42K+ or 86Rb+. Vesicles were preloaded with isotope by equilibration at 4 degrees for 1 to 3 days. Transport of 42K+ or 86Rb+ was initiated by addition of MgATP to the vesicles. The ATP-dependent exit of either isotope was the same. Experiments are presented which show that this loss of isotope was not due to changes in ion binding but rather due to a loss in the amount of ion trapped in the vesicular volume. The transport of K+ was dependent on external Mg2+. CTP was almost as effective as ATP in stimulating K+ transport, while UTP was relatively ineffective. These effects of nucleotides parallel their effects on Na+ accumulation and their effectiveness as substrates for the enzyme. Potassium transport was inhibited by ouabain and required the presence of Na+. The following asymmetries were seen: (a) addition of external Mg2+ supported K+ transport; (b) ouabain inhibited K+ transport only if it was present inside the vesicles; (c) addition of external Na+ to the vesicles stimulated K+ transport. External Li+ was ineffective as a Na+ substitute. The specific requirement of external Na+ for K+ transport indicates that K+ exit is coupled to Na+ entry. Changes in the internal vesicular ion concentrations were studied with vesicles prepared in 20 mM NaCl and 50 mM KCl. After 1 hour of transport at 25 degrees, a typical Na+ concentration in the vesicles in the presence of ATP was 72 mM. A typical K+ concentration in the vesicles was 10 mM as measured with 42K+ or 6 mM as measured with 86Rb+. The following relationships have been calculated for Na+ transport, K+ transport and ATP hydrolysis: Na+/ATP = 1.42, K+/ATP =1.04, and Na+/K+ = 1.43. The ratio of 2.8 Na+ transported in to 2 K+ transported out is very close to the value reported for the red cell membrane. Potassium-potassium exchange similar to that observed in the red cell membrane and attributed to the Na+-K+ pump (stimulated by ATP and orthophosphate and inhibited by ouabain) was observed when vesicles were prepared in the absence of Na+. The results reported in this paper prove that the shark rectal gland NaK ATPase, which is 90 to 95% pure, is the isolated pump for the coupled transports of Na+ and K+.

Increased ouabain-sensitive 86Rb+ uptake and sodium and potassium ion-activated adenosine triphosphatase activity in transformed cell lines.

During the log phase of growth both the active, ouabain-sensitive K+ uptake, measured as 86Rb+, and the sodium and potassium ion-activated ATPase ((Na+ + K+)-ATPase) activity of SV40-transformed 3T3 cells were 2.5-and 5,5-fold higher, respectively, than in untransformed 3T3 cells. A similar higher active K+ uptake was found for Rous sarcoma virus and SV40-transformed baby hamster kidney cells compared with untransformed BHK cells. The active K+ uptake in SV403T3 and normal 3T3 cells decreased when the growth rate of both cell types diminished. Reduction in ouabain-sensitive ATP hydrolysis only occurred later, however, when appreciable decreases in cell viability were seen. Arrhenius plots of the (Na+ + K+)-ATPase activity of SV403T3 cells indicated a discontinuity at 24 degrees, whereas no similar discontinuity was indicated for 3T3 cells. The consequences of elevated K+ transport and (Na+ + K+)-ATPase activity in transformed cells and the possibility that the increased activity might be related to differences inphospholipid fatty acyl chain fluidity are discussed.

Sodium Transport by Phospholipid Vesicles Containing Purified Sodium and Potassium Ion-activated Adenosine Triphosphatase

Abstract When the purified sodium and potassium ion-activated ATPase (NaK-ATPase) from Squalus acanthias is added to a sonicated mixture of egg lecithin and cholate and dialyzed, vesicles are formed which trap either glucose or inulin. These vesicles are capable of ATP-dependent 22Na+ uptake. Optimum 22Na+ uptake occurs after 2 days dialysis at 4°. 22Na+ uptake was considerably higher at 25° than at 37°, presumably because of a higher lability of the vesicles at the higher temperature. Optimum incubation time for 22Na+ uptake was 1 to 2 hours. The pH profiles for NaK-ATPase activity and 22Na+ uptake were the same. 22Na+ uptake showed a dependence on the presence of Mg2+ but this dependence was not absolute, presumably because of some bound Mg2+ in the preparation. There was a good parallelism in the nucleotide specificity for the NaK-ATPase reaction and 22Na+ uptake, with ATP being the most effective substrate, UTP the poorest substrate, and CTP showing intermediate activity. Internal, but not external, ouabain inhibited 22Na+ uptake, as would be predicted from other observations in the literature which show that ouabain inhibits only on the side of the membrane opposite to the ATP substrate site. The ATP-dependent, ouabain-inhibitable uptake of 22Na+ was shown to be true active transport of Na+, by forming vesicles in the presence of 22Na+ so that the specific activity and concentration of Na+ were the same on both sides of the membrane. In the presence of external ATP there was an actual increase in the concentration of internal Na+ from 20 mm to 38.4 mm. The ratio of Na+ transported to ATP hydrolyzed was 0.3 to 0.4, which is only 10 to 15% of that reported in intact erythrocytes. In the absence of K+, an Na+-Na+ exchange which was dependent on ATP and inhibited by ouabain could be demonstrated; this is similar to Na+-Na+ exchange demonstrated in erythrocytes and muscle. Na+-Na+ exchange was a small percentage of active Na+ transport in the presence of K+. These observations provide strong evidence that vesicles formed from the purified NaK-ATPase of S. acanthias are capable of active sodium transport and that this transport shows the same properties as sodium transport observed in intact cells such as the erythrocyte.