Binding of Na+ ions to the Na,K-ATPase increases the reactivity of an essential residue in the ATP binding domain

Treatment of the canine renal Na,K-ATPase with N-(2-nitro-4-isothiocyanophenyl)-imidazole (NIPI), a new imidazole-based probe, results in irreversible loss of enzymatic activity. Inactivation of 95% of the Na,K-ATPase activity is achieved by the covalent binding of 1 molecule of [3H]NIPI to a single site on the alpha-subunit of the Na,K-ATPase. The reactivity of this site toward NIPI is about 10-fold greater when the enzyme is in the E1Na or sodium-bound form than when it is in the E2K or potassium-bound form. K+ ions prevent the enhanced reactivity associated with Na+ binding. Labeling and inactivation of the enzyme is prevented by the simultaneous presence of ATP or ADP (but not by AMP). The apparent affinity with which ATP prevents the inactivation by NIPI at pH 8.5 is increased from 30 to 3 microM by the presence of Na+ ions. This suggests that the affinity with which native enzyme binds ATP (or ADP) at this pH is enhanced by Na+ binding to the enzyme. Modification of the single sodium-responsive residue on the alpha-subunit of the Na,K-ATPase results in loss of high affinity ATP binding, without affecting phosphorylation from Pi. Modification with NIPI probably alters the adenosine binding region without affecting the region close to the phosphorylated carboxyl residue aspartate 369. Tightly bound (or occluded) Rb+ ions are not displaced by ATP (4 mM) in the inactivated enzyme. Thus modification of a single residue simultaneously blocks ATP acting with either high or low affinity on the Na,K-ATPase. These observations suggest that there is a single residue on the alpha-subunit (probably a lysine) which drastically alters its reactivity as Na+ binds to the enzyme. This lysine residue is essential for catalytic activity and is prevented from reacting with NIPI when ATP binds to the enzyme. Thus, the essential lysine residue involved may be part of the ATP binding domain of the Na,K-ATPase.     

Vanadate-sensitive phosphatidate phosphohydrolase activity in a purified rabbit kidney Na,K-ATPase preparation.

Reconstitution of purified rabbit kidney Na,K-ATPase in phosphatidylcholine/phosphatidic acid liposomes resulted in the absence of ATP in a time-, temperature- and protein-dependent formation of inorganic phosphate. This formation of inorganic phosphate could be attributed to a phosphatidate phosphohydrolase activity present in the Na,K-ATPase preparation. A close interaction of the enzyme with the substrate phosphatidic acid was important, since no or little Pi production was observed under any of the following conditions: without reconstitution, after reconstitution in the absence of phosphatidic acid, with low concentrations of detergent or at low lipid/protein ratios. The hydrolysis of phosphatidic acid was not influenced by the Na,K-ATPase inhibitor ouabain but was completely inhibited by the P-type ATPase inhibitor vanadate. Besides Pi diacylglycerol was also formed, confirming that a phosphatidate hydrolase activity was involved. Since the phosphatidate phosphohydrolase activity was rather heat- and N-ethylmaleimide-insensitive, we conclude that the phosphatidic acid hydrolysis was not due to Na,K-ATPase itself but to a membrane-bound phosphatidate phosphohydrolase, present as an impurity in the purified rabbit kidney Na,K-ATPase preparations.     

Inhibition and derivatization of the renal Na,K-ATPase by dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulfonate

Treatment of purified renal Na,K-ATPase with dihydro-4,4'-diisothiocyanatostilbene-2,2'-disulfonate (H2DIDS) produces both reversible and irreversible inhibition of the enzyme activity. The reversible inhibition is unaffected by the presence of saturating concentrations of the sodium pump ligands Na+,K+, Mg2+, and ATP, while the inactivation is prevented by either ATP or K+. The kinetics of protection against inactivation indicate that K+ binds to two sites on the enzyme with very different affinities. Na+ ions with high affinity facilitate the inactivation by H2DIDS and prevent the protective effect of K+ ions. The H2DIDS-inactivated enzyme no longer exhibits a high-affinity nucleotide binding site, and the covalent binding of fluorescein isothiocyanate is also greatly reduced, but phosphorylation by Pi is unaffected. The kinetics of inactivation by H2DIDS were first order with respect to time and H2DIDS concentration. The enzyme is completely inactivated by the covalent binding of one H2DIDS molecule at pH 9 per enzyme phosphorylation site, or two H2DIDS molecules at pH 7.2. H2DIDS binds exclusively to the alpha-subunit of the Na,K-ATPase, locking the enzyme in an E2-like conformation. The profile of radioactivity, following trypsinolysis and SDS-PAGE, showed H2DIDS attachment to a 52-kDa fragment which also contains the ATP binding site. These results suggest that H2DIDS treatment modifies a specific conformationally sensitive amino acid residue on the alpha-subunit of the Na,K-ATPase, resulting in the loss of nucleotide binding and enzymatic activity.     

Low affinity superphosphorylation of the Na,K-ATPase by ATP.

Pre-steady-state phosphorylation of purified Na,K-ATPase from red outer medulla of pig kidney was studied at 25 degrees C and an ample range of [tau-32P]ATP concentrations. At 10 microM ATP phosphorylation followed simple exponential kinetics reaching after 40 ms a steady level of 0.76 +/- 0.04 nmol of P/mg of protein with kapp = 73.0 +/- 6.5 s-1. At 500 microM ATP the time course of phosphorylation changed drastically, since the phosphoenzyme reached a level two to four times higher at a much higher rate (kapp greater than or equal to 370 s-1) and in about 40 ms dropped to the same steady level as with 10 microM ATP. This superphosphorylation was not observed in Na,K-ATPase undergoing turnover in a medium with Mg2+, Na+, and ATP, suggesting that it required the enzyme to be at rest. Superphosphorylation depended on Mg2+ and Na+ and was fully inhibited by ouabain and FITC. After denaturation the phosphoenzyme made by superphosphorylation had the electrophoretic mobility of the alpha-subunit of the Na,K-ATPase, and its hydrolysis was accelerated by hydroxylamine. On a molar basis, the stoichiometry of phosphate per ouabain bound was 2.40 +/- 0.60 after phosphorylation with 1000 microM ATP. The results are consistent with the idea that under proper conditions every functional Na,K-ATPase unit can accept two, or more, phosphates of rapid turnover from ATP.     

Catalytic function of nongastric H,K-ATPase expressed in Sf-21 insect cells

We previously demonstrated that the alpha-subunit of human nongastric H,K-ATPase (Atp1al1) can assemble with the gastric H,K-ATPase beta-subunit (betaHK) into an active ion pump upon coexpression in Xenopus oocytes. To gain insight into enzymatic functions, we have analyzed the Atp1al1-betaHK complex using a baculovirus expression system. The efficient formation of the functional Atp1al1-betaHK complex in membranes of Sf-21 insect cells was obtained upon co-infection with recombinant baculoviruses expressing Atp1al1 and betaHK. Expression of either protein alone did not produce active ATPase. The effects of K(+), Na(+), pH, and ATP and inhibitors on ATPase activity of the recombinant Atp1al1-betaHK complex were analyzed. The Atp1al1-betaHK complex was shown to exhibit significant ATPase activity in nominally K(+)-free medium. The addition of K(+) stimulated the ATP hydrolysis up to 3-fold with K(m) approximately 116 microM K(+). The ATPase activity was moderately sensitive to ouabain and to SCH 28080 with apparent K(i) values in K(+)-free medium of approximately 64 microM and approximately 93 microM, respectively. Potassium exhibited strong antagonism toward both inhibitors. Assays of the ouabain-sensitive ATPase activity revealed inhibitory effects of Na(+) with the apparent K(i) of approximately 24 mM in the absence of added K(+) and with K(i) within the range of 60-70 mM in the presence of > or = 1 mM K(+). Thus, the human nongastric H,K-ATPase represented by the recombinant Atp1al1-betaHK complex exhibits enzymatic properties of K(+)-dependent ATPase sensitive to ouabain, SCH 28080, and Na(+). It differs from Na,K-ATPase in cation dependence and differs from gastric H,K-ATPase and Na,K-ATPase in sensitivity to inhibitors.     

Effect of free fatty acids and detergents on H,K-ATPase. The steady-state ATP phosphorylation level and the orientation of the enzyme in membrane preparations.

The effects of detergents and free fatty acids on the K(+)-activated ATPase activity and on the steady-state phosphorylation level of pig gastric H,K-ATPase were studied. Unsaturated free fatty acids inhibited the K(+)-activated ATPase activity, due to inactivation of the enzyme (long-term effects) and to a decrease in the K(+)-sensitive dephosphorylation rate (short-term effects). The degree of inhibition depended on the reaction conditions: the protein concentration, the temperature and the ligands used. No effect was observed when saturated- or methylated unsaturated fatty acids were tested. Free fatty acids and the detergent C12E8 increased the steady-state ATP phosphorylation level, indicating the presence of vesicular structures in the H,K-ATPase preparations. At higher concentrations these compounds inactivated H,K-ATPase, which was measured as a decrease in phosphorylation capacity. By combining the data from the ATP phosphorylation level in the absence and presence of C12E8 (without inactivation) and the data from the K(+)-activated ATPase activity with and without ionophore the tightness of vesicular preparations and the orientation of H,K-ATPase was determined. A rather simple method for the isolation of H,K-ATPase is reported, which yields highly purified H,K-ATPase preparations with a ATP phosphorylation capacity of 3.9 nmol P per mg protein or 0.57 mol P per mol alpha beta protomer. This number suggests that each alpha-subunit H,K-ATPase can be phosphorylated at the same time.     

Localization of the phlorizin site on Na, K-ATPase in red cell membranes.

Phlorizin at 2 X 10(-4) M inhibited Na+ and Rb+-activated ATPase activities in human red cell membranes by 43%. It inhibited the 86Rb uptake activity of erythrocytes by only 15%. 86Rb uptake into resealed ghosts was inhibited strongly when phlorizin and ATP were preloaded in the ghosts before resealing. Na,K-ATPase activity in the resealed ghosts was also inhibited in the presence of phlorizin inside but not outside the ghosts. These findings suggested that the phlorizin site is located inside the cell.     

Proton transport catalyzed by the sodium pump. Ouabain-sensitive ATPase activity and the phosphorylation of Na,K-ATPase in the absence of sodium ions

The possibility that H+ might substitute for Na+ at Na+ sites of Na+,K+-ATPase was studied. Na+,K+-ATPase purified from pig kidney showed ouabain-sensitive K+-dependent ATPase activity in the absence of Na+ at acid pH (H+,K+-ATPase). The specific activity was 1.1 mumol Pi/mg/min at pH 5.7, whereas the specific activity of Na+,K+-ATPase was 14 mumol Pi/mg/min at pH 7.5. The enzyme was phosphorylated from ATP in the absence of Na+ at the acid pH. The initial rate of the phosphorylation was also accelerated at the acid pH in the absence of Na+, and the maximal rate obtained at pH 5.5 without Na+ was 9% of the rate at pH 7.0 with Na+. The phosphoenzyme was sensitive to K+ but almost insensitive to ADP. The phosphoenzyme was sensitive to hydroxylamine treatment and the alpha-subunit of the enzyme was found to be phosphorylated. H+,K+-ATPase was inhibited as effectively as Na+,K+-ATPase by N-ethylmaleimide but was less inhibited by oligomycin or dimethyl sulfoxide. These results indicate that protons have an Na+-like effect on the Na+ sites of Na+,K+-ATPase and suggest that protons can be transported by the sodium pump in place of Na+.     

The functional unit of Na,K-ATPase is a monomeric alphabeta protomer

The ouabain-resistant and ouabain-sensitive alpha-subunit cRNAs in various molar ratios were injected into Xenopus oocytes together with cRNA for the beta-subunit. The ouabain-resistant ATPase activity, as well as ouabain-resistant Rb+ uptake, of the injected oocytes increased linearly with increasing the amount of cRNA for the ouabain-resistant alpha-subunit. When a functionless mutant was used instead of the ouabain-sensitive alpha-subunit, similar results were obtained in ATPase activity and Rb+ uptake. These results indicate that a monomeric alphabeta protomer is a functional unit of membrane-bound Na,K-ATPase, even if the enzyme exists structurally as a diprotomer or higher oligomers in membranes.     

Lysine 480 is not an essential residue for ATP binding or hydrolysis by Na,K-ATPase.

Lysine 480 has been suggested to be essential for ATP binding and hydrolysis by Na,K-ATPase because it is labeled by reagents that are thought to react with the ATPase from within the ATP binding site. In order to test this hypothesis, Lys-480 was changed to Ala, Arg, or Glu by site-directed mutagenesis, and the resultant Na,K-ATPase molecules were expressed in yeast cells. The ATPase activity of each of the mutants was similar to the activity of the wild type enzyme indicating that Lys-480 is not essential for ATP hydrolysis. The binding of [3H]ouabain in both ATP-dependent and inorganic phosphate-dependent reactions was used to determine the apparent affinity of each mutant for ATP or Pi. The K0.5(ATP) for ouabain binding to phosphoenzyme formed from ATP was 1-3 microM for Lys-480, Arg-480, and Ala-480, whereas for Glu-480 the K0.5(ATP) was 18 microM. The K0.5(Pi) for ouabain binding to phosphoenzyme formed from inorganic phosphate was 16-28 microM for Lys-480, Arg-480, and Ala-480, but was 74 microM for Glu-480. The Kd for ouabain binding was similar for both the wild type and mutant Na,K-ATPase molecules (3-6 nM). These data indicate that the substitution of an acidic amino acid for lysine at position 480 appears to reduce the affinity of the Na,K-ATPase for both ATP and phosphate. It is concluded that Lys-480 is not essential for ATP binding or hydrolysis or for phosphate binding by Na,K-ATPase but is likely to be located within the ATP binding site of the Na,K-ATPase.     

The non-gastric H,K-ATPase is oligomycin-sensitive and can function as an H+,NH4(+)-ATPase

We used the baculovirus/Sf9 expression system to gain new information on the mechanistic properties of the rat non-gastric H,K-ATPase, an enzyme that is implicated in potassium homeostasis. The alpha2-subunit of this enzyme (HKalpha2) required a beta-subunit for ATPase activity thereby showing a clear preference for NaKbeta1 over NaKbeta3 and gastric HKbeta. NH4(+), K+, and Na+ maximally increased the activity of HKalpha2-NaKbeta1 to 24.0, 14.2, and 5.0 micromol P(i) x mg(-1) protein x h(-1), respectively. The enzyme was inhibited by relatively high concentrations of ouabain and SCH 28080, whereas it was potently inhibited by oligomycin. From the phosphorylation level in the presence of oligomycin and the maximal NH4(+)-stimulated ATPase activity, a turnover number of 20,000 min(-1) was determined. All three cations decreased the steady-state phosphorylation level and enhanced the dephosphorylation rate, disfavoring the hypothesis that Na+ can replace H+ as the activating cation. The potency with which vanadate inhibited the cation-activated enzyme decreased in the order K+ > NH4(+) > Na+, indicating that K+ is a stronger E2 promoter than NH4(+), whereas in the presence of Na+ the enzyme is in the E1 form. For K+ and NH4(+), the E2 to E1 conformational equilibrium correlated with their efficacy in the ATPase reaction, indicating that here the transition from E2 to E1 is rate-limiting. Conversely, the low maximal ATPase activity with Na+ is explained by a poor stimulatory effect on the dephosphorylation rate. These data show that NH4(+) can replace K+ with similar affinity but higher efficacy as an extracellular activating cation in rat nongastric H,K-ATPase.     

Interaction of Na,K-ATPase with modifying ATP analogs and chloromethylphosphonic acid

The interaction of synthetic ATP analogs, containing active groups in the triphosphate moiety and in the 8-position of the nucleotide molecule, with highly purified Na, K-ATPase from the medullar layer of porcine kidney was studied. It was found that 11 out of 17 ATP analogs studied irreversibly inhibit the ATPase activity of the enzyme. The pH optimum of the enzyme inactivation by adenosine-5'-(beta-chloroethylphosphate) and adenosine-5'-(p-fluorosulfonylphenylphosphate) beside the pronounced protective effect of ATP suggests possible covalent blocking of histidine and dicarboxylic amino acid residues in the enzyme active center. The irreversible inhibition of the enzyme by "oxo-ATP" containing aldehyde groups in the modified ribose residue in the presence of sodium borohydride suggests a possible presence of the lysine residue epsilon-amino group in the ATP binding site of the enzyme. Na, K-ATPase was found to possess an inorganic phosphate binding site, which is specifically blocked by chloromethylphosphonic acid. the accessibility of this site for modification depends on ATP, NA+ and K+.     

Functional characterization of a testes-specific alpha-subunit isoform of the sodium/potassium adenosinetriphosphatase.

Different isoforms of the sodium/potassium adenosinetriphosphatase (Na,K-ATPase) alpha and beta subunits have been identified in mammals. The association of the various alpha and beta polypeptides results in distinct Na,K-ATPase isozymes with unique enzymatic properties. We studied the function of the Na,K-ATPase alpha4 isoform in Sf-9 cells using recombinant baculoviruses. When alpha4 and the Na pump beta1 subunit are coexpressed in the cells, Na, K-ATPase activity is induced. This activity is reflected by a ouabain-sensitive hydrolysis of ATP, by a Na(+)-dependent, K(+)-sensitive, and ouabain-inhibitable phosphorylation from ATP, and by the ouabain-inhibitable transport of K(+). Furthermore, the activity of alpha4 is inhibited by the P-type ATPase blocker vanadate but not by compounds that inhibit the sarcoplasmic reticulum Ca-ATPase or the gastric H,K-ATPase. The Na,K-ATPase alpha4 isoform is specifically expressed in the testis of the rat. The gonad also expresses the beta1 and beta3 subunits. In insect cells, the alpha4 polypeptide is able to form active complexes with either of these subunits. Characterization of the enzymatic properties of the alpha4beta1 and alpha4beta3 isozymes indicates that both Na,K-ATPases have similar kinetics to Na(+), K(+), ATP, and ouabain. The enzymatic properties of alpha4beta1 and alpha4beta3 are, however, distinct from the other Na pump isozymes. A Na, K-ATPase activity with similar properties as the alpha4-containing enzymes was found in rat testis. This Na,K-ATPase activity represents approximately 55% of the total enzyme of the gonad. These results show that the alpha4 polypeptide is a functional isoform of the Na,K-ATPase both in vitro and in the native tissue.     

Structural relatedness of three ion-transport adenosine triphosphatases around their active sites of phosphorylation

Three membrane-bound adenosine triphosphatases were investigated for homology in the sequence of four amino acids about the active site of phosphorylation. The ATPases were as follows: sodium-potassium-dependent ATPase from dog kidney, Na,K-ATPase; hydrogen-potassium-dependent ATPase from hog gastric mucosa, H,K-ATPase, an ATPase similar to Na,K-ATPase; and an ATPase activity in the plasma membrane of corn, Zea mays, roots (CR-ATPase), a higher plant ATPase. A membrane preparation containing an ATPase of Acholeplasma laidlawii, a prokaryote, (AL) was also investigated. For most of the experiments, the preparations were phosphorylated from [gamma-32P]ATP, denatured in acid, and subjected to proteolytic digestion. Radioactive phosphopeptides were separated by high voltage paper electrophoresis and characterized by sensitivity to chemical reagents. In gastric H,K-ATPase, the aspartate residue at the active site was determined directly by labeling with [3H]borohydride. A common sequence around the active site was found for Na,K-ATPase, H,K-ATPase, and CR-ATPase. This sequence, -Cys-(Ser/Thr)-Asp(P)-Lys-, is similar to that in the calcium ion-transport ATPase of sarcoplasmic reticulum. The AL membrane preparation showed an acylphosphate that turned over rapidly after a chase of labeled membranes with unlabeled ATP. The corresponding sequence was different from that of the three ATPases. An acylphosphate was on two polypeptides with molecular weights of about 80,000 and 60,000; these appear not to correspond to subunits of a Na+-stimulated ATPase in this organism (Lewis, R. N. A. H., and McElhaney, R. N. (1983) Biochim. Biophys. Acta 735, 113-122).     

Characterization of membrane-bound adenosinetriphosphatase activity of sarcolemma-enriched fraction from vascular smooth muscle

Membrane-bound adenosinetriphosphatase (ATPase) activities of the sarcolemma-enriched fraction from bovine aorta were characterized. The membranes, isolated by a sucrose density gradient method, were enriched about 31-fold in sodium- and potassium-stimulated, magnesium-dependent ATPase (Na,K-ATPase) activity, and about 8-fold in 5'-nucleotidase activity compared to the homogenate, suggesting that the isolated membranes were substantially enriched with the sarcolemma. The membranes exhibited about 31, 33 and 42 mumol Pi/mg protein/h of Na,K-ATPase, magnesium-dependent ATPase and calcium-dependent ATPase activities, respectively, in the presence of 4 mmol/l ATP. The sarcolemma-enriched membranes required considerably high concentrations of well-known inhibitors for Na,K-ATPase such as vanadate (more than 1 mumol/l), lanthanum (more than 1 mmol/l) and calcium (10 mmol/l), to induce a significant inhibition in the Na,K-ATPase activity. Treatments of the membrane with physical disruptions and sodium dodecyl sulfate or deoxycholate reduced the total Na,K-ATPase activity, and did not expose fully the ouabain sensitivity of the Na,K-ATPase. These results indicate that there are marked differences in the properties of the ATPase between vascular smooth muscle sarcolemma and cardiac sarcolemma.     

Characterization of the ATP synthase of Propionigenium modestum as a primary sodium pump

The ATP synthase (F1F0) of Propionigenium modestum has been purified to a specific ATPase activity of 5.5 units/mg of protein, which is about 6 times higher than that of the bacterial membranes. Analysis by SDS gel electrophoresis indicated that in addition to the five subunits of the F1 ATPase, subunits of Mr 26,000 (a), 23,000 (b), and 7500 (c) have been purified. The ATPase activity of F1F0 was specifically activated about 10-fold by Na+ions. The enzyme was strongly inhibited by dicyclohexylcarbodiimide, venturicidin, tributyltin chloride, and azide. After incubation with [14C]dicyclohexylcarbodiimide, about 3-4 mol of the inhibitor was bound per 500,000 g of the enzyme. The radioactive label was specifically bound to submit c. These subunits form stable aggregates which resist dissociation by SDS at 100 degrees C. The monomer is formed upon heating with SDS to 121 degrees C or by extraction of the membranes with chloroform/methanol. The ATP synthase was incorporated into liposomes by a freeze-thaw-sonication procedure. The reconstituted proteoliposomes catalyzed the transport of Na+ions upon ATP hydrolysis. The transport was completely abolished by dicyclohexylcarbodiimide. Whereas monensin prevented the accumulation of Na+ions, the uptake rate was stimulated 4-5-fold in the presence of valinomycin or carbonyl cyanide m=chlorophenylhydrazone. These results indicate an electrogenic Na+ transport and also that it is a primary event and not accomplished by a H+-translocating ATP synthase in combination with a Na+/H+ antiporter.     

Characterization of the plasma membrane ATPase of Candida tropicalis

1) Plasma membrane vesicles from Candida tropicalis were isolated from protoplasts by differential centrifugation and purified in a continuous sucrose gradient. 2) The plasma membrane bound ATPase was characterized. It is highly specific for ATP and requires Mg2+. It is stimulated by K+, Na+ and NH4+. Lineweaver-Burk plots for ATPase activity are linear with a Vmax of 4.2 mumoles of ATP hydrolyzed min-1.mg-1 protein and a Km for ATP of 0.76 mM. The ATPase activity is inhibited competitively by ADP with a Ki of 1.7 mM and non competitively by vanadate with a Ki of 3 microM. The activity is unaffected by oligomycin or azide but is sensitive to DCCD.     

Hyposmotic stress causes ATP release and stimulates Na,K-ATPase activity in porcine lens

Purinergic receptors in lens epithelium suggest lens function can be altered by chemical signals from aqueous humor or the lens itself. Here we show release of ATP by intact porcine lenses exposed to hyposmotic solution (200 mOsm). 18α-glycyrrhetinic acid (AGA) added together with probenecid eliminated the ATP increase. N-ethylmaleimide (200 μM), an exocytotic inhibitor, had no significant effect on ATP increase. Lenses exposed to hyposmotic solution displayed a ~400% increase of propidium iodide (PI) entry into the epithelium. The increased ability of PI (MW 668) to enter the epithelium suggests possible opening of connexin and/or pannexin hemichannels. This is consistent with detection of connexin 43, connexin 50, and pannexin 1 in the epithelium and the ability of AGA + probenecid to prevent ATP release. Na,K-ATPase activity doubled in the epithelium of lenses exposed to hyposmotic solution. The increase of Na,K-ATPase activity did not occur when apyrase was used to prevent extracellular ATP accumulation or when AGA + probenecid prevented ATP release. The increase of Na,K-ATPase activity was inhibited by the purinergic P2 antagonist reactive blue-2 and pertussis toxin, a G-protein inhibitor, but not by the P2X antagonist PPADS. Hyposmotic solution activated Src family kinase (SFK) in the epithelium, judged by Western blot. The SFK inhibitor PP2 abolished both SFK activation and the Na,K-ATPase activity increase. In summary, hyposmotic shock-induced ATP release is sufficient to activate a purinergic receptor- and SFK-dependent mechanism that stimulates Na,K-ATPase activity. The responses might signify an autoregulatory loop initiated by mechanical stress or osmotic swelling.     

The use of triplet-sensitized photochromism phenomenon for the study of dynamic interaction of membrane proteins

Possibility of registration of protein interactions in the membranes was demonstrated. The membrane preparation of Na+, K+ ATPase was used in the investigations. The Na+, K+ ATPase was bound with 4-acetoamido-4'-isothiocyanatostilbene-2,2' disullfonic acid (SITS) and erythrosinisothiocyanate (ERITC). The label/Na+,K+ATPase (M/M) ratio was equal to 1:1 for SITS and changed from 1:1 to 1:5 for ERITC. The cis-trans isomerization of SITS was initiated by triplet-triplet energy transfer from light excited ERITC to SITS. The kinetics of isomerization was recorded by the SITS fluorescence measurements. The rate constant of triplet-triplet energy transfer (kT) from ERITC to cis isomer of SITS, (3 divided by 7) X 10(3) M-1 s-1 was determined at 25 degrees C. The kT value of the energy transfer between loose molecules of erythrosine and SITS in buffer solution equaled to 7 X 10(7) M-1 s-1. This drop of kT in the membrane at 10(4) reflected the decrease in the frequency of label collisions caused by the increase in the media viscosity and steric hindrances.     

Studies on (Na+ + K+)-activated ATPase. XLII. Evidence for two classes of essential sulfhydryl groups.

1. Preincubation of purified (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) preparations from rabbit kidney outer medulla with 5,5'-dithiobis-(2-nitrobenzoic acid) inhibits the (Na+ + 5+)-ATPase and K+-stimulated 4-nitro-phenylphosphatase activities. Phosphorylation of the enzyme by ATP and the Na+-stimulated ATPase activity are inhibited to the same extent as the (Na+ + K+)-ATPase activity, whereas the K+-stimulated 4-nitrophenylphosphatase activity is inhibited much less. 2. Titration with 5,5'-dithiobis-(2-nitrobenzoic acid) in sodium dodecyl sulphate shows the presence of 36 reactive sulfhydryl groups per molecule (Na+ + K+)-ATPase (Mr = 250 000). 3. Treatment with N-ethylmaleimide, resulting in complete inhibition of (Na+ + K+)-ATPase activity, leads to modification of 26 sulfhydryl groups, whereas treatment with 5,5'-dithiobis-(2-nitrobenzoic acid) results in modification of 12 sulfhydryl groups under the same conditions. 4. The reaction of N-ethylmaleimide with an essential SH-group is not prevented by previous blocking of sulfhydryl groups with 5,5'-dithiobis-(2-nitrobenzoic acid). 5. These findings indicate the existence of at least two classes of sulfhydryl groups on the enzyme, each containing at least one vital group. The difference between these classes consists in their different reactivity towards 5,5'-dithiobis-(2-nitrobenzoic acid) and N-ethylmaleimide.