A monoclonal antibody against horse kidney (Na+ + K+)-ATPase inhibits sodium pump and E2K to E1 conversion of (Na+ + K+)-ATPase from outside of the cell membrane

Reconstruction of the Complete Ouabain-binding Pocket of Na,K-ATPase in Gastric H,K-ATPase by Substitution of Only Seven Amino Acids

Although cardiac glycosides have been used as drugs for more than 2 centuries and their primary target, the sodium pump (Na,K-ATPase), has already been known for 4 decades, their exact binding site is still elusive. In our efforts to define the molecular basis of digitalis glycosides binding we started from the fact that a closely related enzyme, the gastric H,K-ATPase, does not bind glycosides like ouabain. Previously, we showed that a chimera of these two enzymes, in which only the M3-M4 and M5-M6 hairpins were of Na,K-ATPase, bound ouabain with high affinity (Koenderink, J. B., Hermsen, H. P. H., Swarts, H. G. P., Willems, P. H. G. M., and De Pont, J. J. H. H. M. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 11209-11214). We also demonstrated that only three amino acids (Phe(783), Thr(797), and Asp(804)) present in the M5-M6 hairpin of Na,K-ATPase were sufficient to confer high affinity ouabain binding to a chimera which contained in addition the M3-M4 hairpin of Na,K-ATPase (Qiu, L. Y., Koenderink, J. B., Swarts, H. G., Willems, P. H., and De Pont, J. J. H. H. M. (2003) J. Biol. Chem. 278, 47240-47244). To further pinpoint the ouabain-binding site here we used a chimera-based loss-of-function strategy and identified four amino acids (Glu(312), Val(314), Ile(315), Gly(319)), all present in M4, as being important for ouabain binding. In a final gain-of-function study we showed that a gastric H,K-ATPase that contained Glu(312), Val(314), Ile(315), Gly(319), Phe(783), Thr(797), and Asp(804) of Na,K-ATPase bound ouabain with the same affinity as the native enzyme. Based on the E(2)P crystal structure of Ca(2+)-ATPase we constructed a homology model for the ouabain-binding site of Na,K-ATPase involving all seven amino acids as well as several earlier postulated amino acids.

Structural similarities of Na,K-ATPase and SERCA, the Ca(2+)-ATPase of the sarcoplasmic reticulum

The crystal structure of SERCA1a (skeletal-muscle sarcoplasmic-reticulum/endoplasmic-reticulum Ca(2+)-ATPase) has recently been determined at 2.6 A (note 1 A = 0.1 nm) resolution [Toyoshima, Nakasako, Nomura and Ogawa (2000) Nature (London) 405, 647-655]. Other P-type ATPases are thought to share key features of the ATP hydrolysis site and a central core of transmembrane helices. Outside of these most-conserved segments, structural similarities are less certain, and predicted transmembrane topology differs between subclasses. In the present review the homologous regions of several representative P-type ATPases are aligned with the SERCA sequence and mapped on to the SERCA structure for comparison. Homology between SERCA and the Na,K-ATPase is more extensive than with any other ATPase, even PMCA, the Ca(2+)-ATPase of plasma membrane. Structural features of the Na,K-ATPase are projected on to the Ca(2+)-ATPase crystal structure to assess the likelihood that they share the same fold. Homology extends through all ten transmembrane spans, and most insertions and deletions are predicted to be at the surface. The locations of specific residues are examined, such as proteolytic cleavage sites, intramolecular cross-linking sites, and the binding sites of certain other proteins. On the whole, the similarity supports a shared fold, with some particular exceptions.

Inhibition of H+,K+ -ATPase by hinesol, a major component of So-jutsu, by interaction with enzyme in the E1 state

Hinesol, a major component of the crude drug "So-jutsu" (Atractylodis Lanceae Rhizoma), strongly inhibited H+,K+-ATPase activity with a IC50 value of 5.8x10(-5) M. It also inhibited Na+,K+-ATPase, Mg2+-ATPase, Ca2+-ATPase, and H+-ATPase activities, although the inhibition rate was lower. No effects on alkaline or acid phosphatase activities were observed. The mechanism by which hinesol inhibited H+,K+-ATPase activity was studied in detail. The inhibition was uncompetitive with respect to ATP, and it increased as the Mg2+ concentration was raised, whereas it was not affected by the K+ concentration. The activity of K+-dependent p-nitrophenyl phosphatase (K+-pNPPase), a partial reaction of H+,K+-ATPase, was inhibited by hinesol noncompetitively with respect to pNPP (IC50 value of 1.6x10(-4) M), and competitively with respect to K+, whereas it was not affected by the Mg2+ concentration. These results suggest that hinesol is a relatively specific inhibitor of H+,K+-ATPase. It appears that hinesol reacts with enzyme in the E1 state in the presence of ATP and Mg2+ and forms the complex hinesol-H+ E1-ATP or hinesol x E1-P, blocking the conformational change to the E2 state. Furthermore, hinesol enhanced the inhibitory effect of omeprazole on H+,K+-ATPase, and the inhibitory site of hinesol was different from that of omeprazole. The effect of So-jutsu as an anti-gastric ulcer agent may be ascribed to the inhibitory effect of hinesol on H+,K+-ATPase activity.

Specific inhibition of Na+,K(+)-ATPase activity by atractylon, a major component of byaku-jutsu, by interaction with enzyme in the E2 state

Atractylon, a major component of the crude drug "Byaku-jutsu" (rhizomes of Atractylodes japonica), strongly inhibited Na+,K(+)-ATPase activity with an I50 value of 8.9 x 10(-6) M. It also inhibited Mg(2+)-ATPase, H+,K(+)-ATPase, H(+)-ATPase and Ca(2+)-ATPase activities, but less potently. No effects on alkaline and acid phosphatase activities were observed. The inhibition of Na+,K(+)-ATPase activity by atractylon was noncompetitive with respect to ATP and was greater with increasing K+ concentration, whereas it was not affected by Na+ concentration. The activity of K(+)-dependent p-nitrophenyl phosphatase, a partial reaction of Na+,K(+)-ATPase, was inhibited noncompetitively with respect to substrate (I50 value of 1.8 x 10(-5) M), and the inhibition rate was independent of the K+ concentration. Furthermore, atractylon increased the Ki value for Na+ from 130 to 190 mM, but did not alter the Ki value for ATP. Inhibition of the phosphoenzyme formation by atractylon was greater at 0.1 M than at 1 M NaCl. K(+)-dependent dephosphorylation (E2-P to K.E2) was inhibited by atractylon, whereas ADP-sensitive (Na.E1-P to Na.E1) and non-specific dephosphorylation steps were not affected. These results suggest that atractylon, a specific inhibitor of Na+,K(+)-ATPase, interacts with enzyme in the E2 state and inhibits the reaction step from E2-P to K.E2.

The epitope for the inhibitory antibody M7-PB-E9 contains Ser-646 and Asp-652 of the sheep Na+,K(+)-ATPase alpha-subunit

The binding of monoclonal antibody M7-PB-E9 to the alpha-subunit of Na+,K(+)-ATPase partially inhibits enzyme activity (35%) in competition with ATP, while in the presence of magnesium it stimulates the rate of ouabain binding severalfold [Ball, W. J. (1984) Biochemistry 23, 2275-2281]. These effects have been shown to result from an antibody-induced shifting of the enzyme's E1 <==> E2 conformational equilibrium to the right that affects all enzyme-ligand interactions except that with Mg2+ [Abbott, A.J., & Ball, W.J. (1992) Biochemistry 31, 11236-11243]. In order to identify the location of the M7-PB-E9 epitope, proteolytic fragments of the lamb kidney enzyme were generated and the immunoreactive alpha fragments were identified by Western blot analyses. These studies revealed a 47-kDa tryptic fragment, which bound both M7-PB-E9 and a -COOH terminus specific antisera and NH2-terminal sequencing showed to originate at Ala-590. Digestion with Staphylococcus aureus V8 protease produced a 36-kDa -COOH-terminus fragment which originated at Gly-697 and did not contain the antibody epitope. Thus the intracellular sequence region Ala-590 to Gly-697 was shown to contain the antibody epitope. When M7-PB-E9's ability to recognize the alpha subunits from various species and tissues was determined and correlated with available sequencing data, only Ser-646 was present in the highly reactive lamb, pig, and avian kidney alpha 1 proteins and altered (Asn) in the poorly recognized Xenopus and rat kidney and Torpedo electroplax organ enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of beta-eudesmol with Na+,K(+)-ATPase: inhibition of K(+)-pNPPase activity

The effect of beta-eudesmol, one of the major components in So-jutsu (Atractylodis Lanceae Rhizoma), on K(+)-dependent p-nitrophenyl phosphatase (K(+)-pNPPase) activity was studied. It inhibited K(+)-pNPPase activity with an I50 value of 4.1 x 10(-4) M. The inhibition rate decreased as the K+ concentration was increased, whereas greater inhibition was observed with high concentrations of either Na+ or ATP. The Ki values for Na+ in the presence of 0, 0.1 and 1 mM ATP were 140, 260 and 310 mM, respectively, but with the addition of beta-eudesmol, these values decreased to 90 mM regardless of the ATP concentration. This study on K(+)-pNPPase activity supports the conclusion obtained from the study on Na+,K(+)-ATPase activity (Satoh K et al., Biochem Pharmacol 44: 373-378, 1992) that is, beta-eudesmol interacts with enzyme in the Na.E1 form and inhibits the reaction step Na.E1----Na.E1-P. Furthermore, in the study of the effects of K+ and beta-eudesmol on K(+)-pNPPase activity, it was confirmed that beta-eudesmol prevents the conformational change of Na.E1----K.E2.

Inhibition of Na+,K(+)-ATPase activity by beta-eudesmol, a major component of atractylodis lanceae rhizoma, due to the interaction with enzyme in the Na.E1 state

beta-Eudesmol, a major component of the crude drug "So-jutsu" (Atractylodis Lanceae Rhizoma), inhibited Na+, K(+)-ATPase activity most strongly among the various kinds of phosphatases examined. It also inhibited Ca(2+)-ATPase and H+, K(+)-ATPase, but to a lesser extent. Its effect on Mg(2+)-ATPase was minute. No effects on H(+)-ATPase or alkaline and acid phosphatase activities were observed. The effects of beta-eudesmol on horse kidney Na+, K(+)-ATPase were studied in detail, and the following results were obtained: (1) beta-eudesmol inhibited the Na+, K(+)-ATPase activity with an I50 value of 1.6 x 10(-4) M. The mode of its inhibition was uncompetitive with respect to ATP; (2) it prevented the stimulation of enzyme activity by Na+. The inhibition gradually increased in accord with the increase of Na+ concentration, and it was constant when Na+ was higher than 6.3 mM; (3) it did not alter the K+ concentration necessary for half-maximal activation (K0.5 for K+); and (4) it inhibited the enzyme activity with a mode of action different from ouabain. Phosphorylation of enzyme with [gamma-32P]ATP was inhibited by beta-eudesmol with an I50 of 1.4 x 10(-4) M. The inhibition was greater in 1 M NaCl than in 0.1 M NaCl. It had no effects on dephosphorylation steps, i.e. none of the non-specific, the ADP-sensitive (Na.E1-P----Na.E1) and the K(+)-dependent (E2-P----K.E2) dephosphorylation processes were affected. These results suggest that beta-eudesmol, a relatively specific inhibitor of Na+, K(+)-ATPase, interacts with the enzyme in the Na.E1 form and inhibits the reaction step Na.E1----Na.E1-P.

A monoclonal antibody against a native conformation of the porcine renal Na+/K(+)-ATPase alpha-subunit protein

A monoclonal antibody (mAb50c) against the native porcine renal Na+/K(+)-transporting adenosinetriphosphatase (EC 3.6.1.37, ATP phosphohydrolase) (Na+/K(+)-ATPase) was characterized. The antibody could be classified as a conformation-dependent antibody, since it did not bind to Na+/K(+)-ATPase denatured by detergent and its binding was affected by the normal conformational changes of the enzyme induced by ligands. The binding was the greatest in the presence of Na+, ATP or Mg2+ (E1 form), slightly less in the presence of K+ (E2K form) and the least when the enzyme was phosphorylated, especially in the actively hydrolyzing form in the presence of Na+, Mg2+ and ATP. The antibody inhibited both the Na+,K(+)-ATPase activity and the K(+)-dependent p-nitrophenylphosphatase activity by 25%, but it had no effect on Na(+)-dependent ATPase activity. The antibody partially inhibited the fluorescence changes of the enzyme labeled with 5'-isothiocyanatofluorescein after the addition of orthophosphate and Mg2+, and after the addition of ouabain. Proteolytic studies suggest that a part of the epitope is located on the cytoplasmic surface of the N-terminal half of the alpha-subunit.

The binding of monoclonal and polyclonal antibodies to the Ca2(+)-ATPase of sarcoplasmic reticulum: effects on interactions between ATPase molecules

We analyzed the interaction of 14 monoclonal and 5 polyclonal anti-ATPase antibodies with the Ca2(+)-ATPase of rabbit sarcoplasmic reticulum and correlated the location of their epitopes with their effects on ATPase-ATPase interactions and Ca2+ transport activity. All antibodies were found to bind with high affinity to the denatured Ca2(+)-ATPase, but the binding to the native enzyme showed significant differences, depending on the location of antigenic sites within the ATPase molecule. Of the seven monoclonal antibodies directed against epitopes on the B tryptic fragment of the Ca2(+)-ATPase, all except one (VIE8) reacted with the enzyme in native sarcoplasmic reticulum vesicles in both the E1 and E2V conformations. Therefore these regions of the Ca2(+)-ATPase molecule are freely accessible in the native enzyme. The monoclonal antibody VIE8 bound with high affinity to the Ca2(+)-ATPase only in the E1 conformation stabilized by 0.5 mM Ca2+ but not in the E2V conformation stabilized by 0.5 mM EGTA and 5 mM vanadate. Several antibodies that reacted with the B fragment interfered with the crystallization of Ca2(+)-ATPase in the presence of EGTA and vanadate and at least two of them destabilized preformed Ca2(+)-ATPase crystals, suggesting inhibition of interactions between ATPase molecules. Of five monoclonal antibodies with epitopes on the A1 tryptic fragment of the Ca2(+)-ATPase only one gave strong reaction with the native enzyme, and none interfered with ATPase-ATPase interactions as measured by the polarization of fluorescence of FITC-labeled Ca2(+)-ATPase. Therefore the regions of the molecule containing these epitopes are relatively inaccessible in the native structure. Partial tryptic cleavage of the Ca2(+)-ATPase into the A1, A2 and B fragments did not promote the reaction of anti-A1 antibodies with sarcoplasmic reticulum vesicles, but solubilization of the membrane with C12E8 rendered the antigenic site fully accessible to several of them, suggesting that their epitopes are located in areas of contacts between ATPase molecules. Two monoclonal anti-B antibodies that interfered with ATPase-ATPase interactions, produced close to 50% inhibition of the rate of ATP-dependent Ca2+ transport, with significant inhibition of ATPase; this may suggest a role for ATPase oligomers in the regulation of Ca2+ transport. The other antibodies that interact with the native Ca2(+)-ATPase produced no significant inhibition of ATPase activity even at saturating concentrations; therefore their antigenic sites do not undergo major movements during Ca2+ transport.

Structure of the alpha 1 subunit of horse Na,K-ATPase gene

Genomic DNA for Na,K-ATPase alpha 1 subunit was obtained from libraries of horse kidney genomic DNA in Charon 4A and in EMBL3 bacteriophages by screening with the full sized cDNA probe of the alpha 1 subunit of rat Na,K-ATPase as probe. The gene spans 30 kb and consists of 23 exons and 22 intervening sequences. Intron-exon boundaries were analyzed. The protein-coding nucleotide sequence encodes 1016 amino acids with an Mr of 112,264. The putative amino acid sequence of horse alpha 1 is 96-97% homologous to those of other mammalian species.