Conformational change accompanying transition of ADP-sensitive phosphoenzyme to potassium-sensitive phosphoenzyme of (Na+,K+)-ATPase modified with N-[p-(2-benzimidazolyl)phenyl]maleimide

Oleic and linoleic acids are active principles in Nigella sativa and stabilize an E(2)P conformation of the Na,K-ATPase. Fatty acids differentially regulate cardiac glycoside interaction with the pump

Nigella sativa seed oil was found to contain a modulator of Na,K-ATPase. Separation analyses combined with (1)H NMR and GCMS identified the inhibitory fraction as a mixture of oleic and linoleic acids. These two fatty acids are specifically concentrated in several medicinal plant oils, and have particularly been implicated in decreasing high blood pressure. The ouabain binding site on Na,K-ATPase has also been implicated in blood pressure regulation. Thus, we aimed to determine how these two molecules modify pig kidney Na,K-ATPase. Oleic and linoleic acids did not modify reactions involving the E(1) (Na(+)) conformations of the Na,K-ATPase. In contrast, K(+) dependent reactions were strongly modified after treatment. Oleic and linoleic acids were found to stabilize a pump conformation that binds ouabain with high affinity, i.e., an ion free E(2)P form. Time-resolved binding assays using anthroylouabain, a fluorescent ouabain analog, revealed that the increased ouabain affinity is unique to oleic and linoleic acids, as compared with γ-linolenic acid, which decreased pump-mediated ATP hydrolysis but did not equally increase ouabain interaction with the pump. Thus, the dynamic changes in plasma levels of oleic and linoleic acids are important in the modulation of the sensitivity of the sodium pump to cardiac glycosides. Given the possible involvement of the cardiac glycoside binding site on Na,K-ATPase in the regulation of hypertension, we suggest oleic acid to be a specific chaperon that modulates interaction of cardiac glycosides with the sodium pump.

Regulation of human and pig renal Na(+),K (+)-ATPase activity by tyrosine phosphorylation of their alpha(1)-subunits

Modulation of the physiologically influential Na(+),K(+)-ATPase is a complex process involving a wide variety of factors. To determine the possible effects of the protein tyrosine phosphatase (PTP) inhibitors dephostatin and Et-3,4-dephostatin on human and pig, renal cells and enzymatic extracts, we treated our samples (15 min-24 h) with those PTP inhibitors (0-100 microM). PTP inhibitors were found to possess a concentration-dependent inhibition of Na(+),K(+)-ATPase activity in both human and pig samples. The inhibition was similarly demonstrated on all cellular, microsomal fraction and purified Na(+),K(+)-ATPase levels. Despite rigorous activity recovery attempts, the PTP inhibitors' effects were sustained on Na(+),K(+)-ATPase activity. Western blotting experiments revealed the expression of both alpha(1)- and beta(1)-subunits in both human and pig tissues. alpha(1)-Subunits possessed higher tyrosine phosphorylation levels with higher concentrations of PTP inhibitors. Meanwhile, serine/threonine residues of both alpha(1)- and beta(1)-subunits demonstrated diminished phosphorylation levels upon dephostatin treatment. Accordingly, we provide evidence that Na(+),K(+)-ATPase can be regulated through tyrosine phosphorylation of primarily their alpha(1)-subunits, using PTP inhibitors.

Structural analysis of 2D crystals of gastric H+,K+-ATPase in different states of the transport cycle

The H+,K+-ATPase uses ATP to pump protons across the gastric membrane. We used electron crystallography and limited trypsin proteolysis to study conformational changes in the H+,K+-ATPase. Well-ordered 2D crystals were obtained with detergent-solubilized H+,K+-ATPase at low pH in the absence of nucleotides, E1 state, and in the presence of fluoroaluminate and ADP, mimicking the E1PADP state. Projection maps obtained with frozen-hydrated two-dimensional crystals of the H+,K+-ATPase in these two states looked very similar, suggesting only small conformational changes during the transition from the E1 to the E1P x ADP state. This result differs from the X-ray crystal structures of the related ATPase SERCA, which revealed substantially different conformations in the E1 and E1P x ADP states. To further characterize the conformational changes in the H+,K+-ATPase during its transport cycle, we performed limited proteolysis with trypsin. All examined states of the H+,K+-ATPase, including the E1 and E1P x ADP states present in the 2D crystals,showed characteristic differences in the digestion patterns. While the results from the limited proteolysis experiments thus show that the H+,K+-ATPase adopts distinct conformations during different stages of the transport cycle, the projection maps indicate that the structural rearrangements in the H+,K+-ATPase are much smaller than those observed in the related SERCA ATPase.

Large-scale preparation of sodium-potassium ATPase from kidney outer medulla

BACKGROUND

Large amounts of Na,K-ATPase are needed for studies involving protein chemistry. Preparation of Na,K-ATPase from kidney by the widely used, rapid procedure of Jørgensen (Biochim Biophys Acta 356:36-52, 1974; Methods Enzymol 156:29-43, 1988) includes labor-intensive dissection of tissue from the outer medulla and centrifugation into a step gradient of sucrose solution.

METHODS

In a large-scale modification presented here, tissue was dissected with a surgical instrument, a rongeur, and centrifugation was simply a five times repeated differential centrifugation. The procedure took seven days and 68 person-hours of work.

RESULTS

The yield of activity from 26 kg of whole kidneys was 6600 units (micromol Pi/min) in one preparation. The amount of protein was 240 mg and the specific activity was 28 micromol Pi/min per mg protein.

CONCLUSIONS

There is a significant saving of labor to obtain a product with a specific activity similar to that commonly obtained. The microsomal fraction may be useful for preparing other membrane proteins from the outer medulla.

Acid-labile ATP and/or ADP/P(i) binding to the tetraprotomeric form of Na/K-ATPase accompanying catalytic phosphorylation-dephosphorylation cycle

The Na/K-ATPase has been shown to bind 1 and 0.5 mol of (32)P/mol of alpha-chain in the presence [gamma-(32)P]ATP and [alpha-(32)P]ATP, respectively, accompanied by a maximum accumulation of 0.5 mol of ADP-sensitive phosphoenzyme (NaE1P) and potassium-sensitive phosphoenzyme (E2P). The former accumulation was followed by the slow constant liberation of P(i), but the latter was accompanied with a rapid approximately 0.25 mol of acid-labile P(i) burst. The rubidium (potassium congener)-occluded enzyme (approximately 1.7 mol of rubidium/mol of alpha-chain) completely lost rubidium on the addition of sodium + magnesium. Further addition of approximately 100 microM [gamma-(32)P]ATP and [alpha-(32)P]ATP, both induced 0.5 mol of (32)P-ATP binding to the enzyme and caused accumulation of approximately 1 mol of rubidium/mol of alpha-chain, accompanied by a rapid approximately 0.5 mol of P(i) burst with no detectable phosphoenzyme under steady state conditions. Electron microscopy of rotary-shadowed soluble and membrane-bound Na/K-ATPases and an antibody-Na/K-ATPase complex, indicated the presence of tetraprotomeric structures (alphabeta)(4). These and other data suggest that Na/K-ATP hydrolysis occurs via four parallel paths, the sequential appearance of (NaE1P:E.ATP)(2), (E2P:E.ATP:E2P:E. ADP/P(i)), and (KE2:E.ADP/P(i))(2), each of which has been previously referred to as NaE1P, E2P, and KE2, respectively.

Half-site modification of Lys-480 of the Na+,K+-ATPase alpha-chain with pyridoxal 5'-diphospho-5'-adenosine reduces ATP-dependent phosphorylation stoichiometry from half to a quarter

Pig and dog kidney Na+,K+-ATPase preparations, irrespective of specific activity, showed approximately 0.5 mol of maximum phosphorylation/mol alpha-chain for ATP or acetyl phosphate (AcP) at steady state conditions. Pyridoxal 5'-diphospho-5'-adenosine (AP2PL)-treated pig kidney enzymes containing approximately 0.5 mol of AP2PL probe at Lys-480/mol (Tsuda, T., Kaya, S., Funatsu, H., Hayashi, Y., and Taniguchi, K. (1998) J. Biochem. (Tokyo) 123, 169-174) showed a quarter-site phosphorylation by ATP and half-site phosphorylation from AcP. The addition of 10 microM ATP to the Mg2+-Na+-bound AP2PL enzyme induced rapid quarter-site phosphorylation (47/s), followed by two different AP2PL fluorescence changes, a rapid decrease (29/s) and a slow increase (1.1/s). The addition of 1 mM AcP to the Mg2+-Na+-bound AP2PL enzyme induced a slow half-site phosphorylation (3/s), followed by a monophasic AP2PL fluorescence increase (1.2/s). After treatment of the AP2PL enzyme with fluorescein 5'-isothiocyanate to modify Lys-501 fully, the Mg2+-Na+-dependent phosphorylation capacity from ATP of the resulting AP2PL-fluorescein 5'-isothiocyanate enzyme was reduced to approximately 6% without significant changes in half-site phosphorylation capacity with respect to AcP, dynamic AP2PL fluorescence change by ATP and change by AcP. These data and others support the hypothesis that the functional membrane-bound Na+, K+-ATPase has tetrameric properties.

ATP and acetyl phosphate induces molecular events near the ATP binding site and the membrane domain of Na+,K+-ATPase. The tetrameric nature of the enzyme

The addition of ATP to Mg2+-Na+-bound-probe labeled Na+,K+-ATPase preparations containing approximately 0.5 mol of pyridoxal 5'-diphospho-5'-adenosine (AP2PL) probe at Lys-480 and approximately 0.9 mol of fluorescein 5'-isothiocyanate (FITC) probe at Lys-501 showed a decrease and an increase in the AP2PL fluorescence intensity with neither significant ATP-dependent phosphorylation nor FITC fluorescence change. The rate constants for the fluorescence change increased nearly linearly with increasing ATP concentrations. The substitution of AcP for ATP decreased the FITC fluorescence rather monophasically, 8.5/s, which was followed by the half-site phosphorylation with same amount of components with different rate constant, 7.2 and 4.6/s, followed by a much slower increase in the two components of AP2PL fluorescence, 1.4 and 0.2/s. The addition of Na+ with increasing concentrations of ATP to the K+-bound AP2PL-FITC enzymes induced accelerations in the decrease and an increase in the AP2PL fluorescence intensity with two different increases in the FITC fluorescence intensity, showing that the same concentration of ATP is capable of inducing four different fluorescence changes. The addition of ATP to the Mg2+-Na+-bound enzymes modified with N-[p-(2-benzimidazolyl)phenyl]-maleimide (BIPM) at Cys-964 and retaining full Na+,K+-ATPase activity induced two different increases in BIPM fluorescence intensity. Each rate constant for the BIPM fluorescence change versus concentrations of ATP gave two intersecting straight lines. These data and the stoichiometries of fluorescence probe bindings and ATP- and AcP-dependent phosphorylation provide strong support for the conclusion that the functional membrane-bound Na+,K+-ATPase is a tetramer.

Transient kinetics of substrate binding to Na+/K(+)-ATPase measured by fluorescence quenching

This paper examines the transient kinetics of substrate binding to the Na+/K(+)-ATPase labelled with iodoacetamidofluorescein (IAF) using fluorescence quenching by trinitrophenyl-ATP (TNP-ATP). Earlier work (E.H. Hellen, P.R. Pratap, 1996, Fluorescence quenching of IAF-Na+/K(+)-ATPase via energy transfer to TNP-labelled nucleotide, Proceedings of the VIIIth International Conference on the Na+/K(+)-ATPase, in press) has shown that TNP-nucleotide binds to specific sites (from which unlabelled nucleotide can displace it) and nonspecific sites (from which unlabelled nucleotide cannot displace it). Under stopped-flow conditions, quenching of IAF-enzyme fluorescence was well described by a stretched exponential (F(t) = F infinity + delta F exp[-Bt alpha]). Physically, this function may be interpreted in terms of its inverse Laplace transform phi (k), which describes a distribution of rate-constants; alpha reflects the width of this distribution. As TNP-ATP concentration increased, alpha decreased, reflecting TNP-ATP binding to sites with higher energy barriers. alpha decreased by about the same amount with increasing [TNP-ATP] in the presence of saturating ATP, indicating that the distribution of rate-constants is largely associated with the nonspecific binding sites. However, alpha was significantly less than 1 for ATP-induced fluorescence recovery in the presence of TNP-ATP, indicating that rate-constants associated with specific binding site are also distributed. The distribution of rate-constants for binding to the specific site indicates a distribution in the energy of the transition state for substrate binding. These results suggest that the specific binding site (in either the empty or the full state) may exist in a series of conformations separated by small energy barriers. However, the energy barriers for binding associated with these conformations are significantly distributed.

Identification of two conformationally sensitive cysteine residues at the extracellular surface of the Na,K-ATPase alpha-subunit

Na,K-ATPase in right-side-out oriented vesicles was stabilized in different conformations, and the location of intramembrane Cys residues of the alpha-subunit was assessed with membrane-permeable and membrane-impermeable Cys-directed reagents. In the presence of Mg2+ and Pi, Cys964 was the most accessible for both membrane-impermeable 4-acetamido-4'-maleimidylstilbene-2, 2'disulfonic acid (or stilbene disulfonate maleimide, SDSM) and membrane-permeable 7-diethylamino-3-(4'-maleimidyl)-4-methylcoumarin (CPM). In the presence of K+, Cys964 was modified only by hydrophobic CPM, indicating that the environment around Cys964 was different in these two conformations. Cys964 seems to mark the extracellular border of transmembrane segment M9. Cys911 in transmembrane segment M8 showed similar behavior; however, it was not so readily modified. Complete modification of Cys964 and Cys911 causes only partial (about 50%) inactivation of both ATPase activity and Rb+ (or K+) occlusion, indicating that the effect on cation occlusion is indirect and not within the occlusion cavity. The ATP binding capacity remains unaltered by the modifications. Treatment of the K+-stabilized post-tryptic preparation of purified Na, K-ATPase revealed labeling of several cysteines by CPM, none of which were labeled with SDSM. Removal of K+ ions from the preparation, which we have previously shown is accompanied by release of the M5M6 hairpin to the supernatant (), causes changes in the organization of the C-terminal 21-kDa fragment. In particular Cys983 in M10 became labeled by both CPM and SDSM, pointing to a tight association between the C terminus and the M5M6 hairpin of the alpha-subunit.

Inhibition of Na+/K(+)-ATPase by phenoxyl radicals of etoposide (VP-16): role of sulfhydryls oxidation

In the present work, we studied the effects of phenoxyl radicals, generated by tyrosinase-catalyzed oxidation of a phenolic antitumor drug, Etoposide (VP-16), on a purified dog kidney Na+/K(+)-ATPase by characterizing interactions of VP-16 phenoxyl radicals with the enzyme's SH-groups by ESR and correlating the loss of the enzymatic activity with the oxidation of its SH-groups, and oxidation of VP-16. VP-16/tyrosinase caused inhibition of Na+/K(+)-ATPase which was dependent on the incubation time and concentration of tyrosinase. The inhibition of Na+/K(+)-ATPase was accompanied by a decrease of DTNB (5,5'-dithiobis-(2-nitrobenzoic acid)-titratable SH-groups. In the presence of Na+/K(+)-ATPase, a typical ESR signal of the VP-16 phenoxyl radical could be observed only following a lag period the duration of which was proportional to the concentration of the Na+/K(+)-ATPase added. Our HPLC measurements demonstrated that Na+/K(+)-ATPase protected VP-16 against tyrosinase-catalyzed oxidation. Combined these results suggest that redox-cycling of VP-16/VP-16 phenoxyl radical by SH-groups of Na+/K(+)-ATPase occurred. Ascorbate which is known to reduce the VP-16 phenoxyl radicals, protected the enzyme against inactivation, prevented oxidation of the enzyme's SH-groups. Reduction of VP-16 phenoxyl radicals by ascorbate was directly observed by the semidehydroascorbyl radical signal in the ESR spectra. VP-16 phenoxyl radical-induced oxidation of sulfhydryls and inhibition of the Na+/K(+)-ATPase may be responsible for at least some of its clinical side effects (e.g., cardiotoxicity) which can be prevented by ascorbate.

Rapid kinetic analyses of the Na+/K(+)-ATPase distinguish among different criteria for conformational change

The Na+/K(+)-ATPase couples the hydrolysis of ATP to the transport of Na+ and K+ via a phosphorylated intermediate and conformational changes. In order to identify these conformational changes, we have probed the sequence of steps from EP(3Na+ in) to EP + 3Na+ out with three fluorescent probes (IAF: 5-iodoacetamidofluorescein; BIPM: N-[p-(2-benzimidazolyl)phenyl]maleimide; and RH421) and the sensitivity of their fluorescence change to oligomycin and divalent cations (Ca2+ and Mn2+). The magnitude (% delta F) and rate constant (k(obs)) of ATP-induced fluorescence changes were measured on a fluorescence stopped-flow apparatus, and yielded the following results. (a) With RH421, k(obs) and % delta F varied with [Na+] (maximal k(obs) = 100 s-1, K1/2 = 6 mM; % delta Fmax = 6%, K1/2 = 1 mM); these values are comparable to those previously reported using IAF-labeled enzyme (Pratap, P.R., Robinson, J.D. and Steinberg, M.I. (1991) Biochim. Biophys. Acta 1069, 288-298). (b) With BIPM-labeled enzyme k(obs) did not vary with [Na+] over the range tested, and was twice as high as the maximum k(obs) for RH421. (c) Treatment with oligomycin reduced k(obs) for all three probes to about the same level (approximately 1-2 s-1) while % delta Fmax was largely unaffected. (d) Replacing Mg2+ with Ca2+ had similar effects to treatment with oligomycin. (e) RH421 fluorescence change was completely abolished in the presence of oligomycin and Ca2+. (f) Replacing Mg2+ with Mn2+ decreased IAF fluorescence, i.e., put the enzyme in an E2-like form, reduced k(obs), and rendered oligomycin less effective in reducing k(obs). From these results, we conclude: (a) the release of the second/third Na+ is the rate-limiting step for the conformational change measured by IAF and charge transfer measured with RH421; (b) BIPM indicates an earlier step, either the deocclusion of Na+ and/or the release of the first Na+; (c) oligomycin blocks Na+ deocclusion, and this step is sensitive to the divalent cation used to activate enzyme phosphorylation; and (d) Ca2+ slows the step reported by IAF as well. These experiments indicate that a simple model with two conformations (E1 and E2) is insufficient to explain transient kinetic data.

Chemical modification as an approach to elucidation of sodium pump structure-function relations

Chemical modification of specific residues in enzymes, with the characterization of the type of inhibition and properties of the modified activity, is an established approach in structure-function studies of proteins. This strategy has become more productive in recent years with the advances made in obtaining primary sequence information from gene-cloning technologies. This article discusses the application of chemical modification procedures to the study of the Na(+)-K(+)-ATPase protein. A wide array of information has become available about the kinetics, enzyme structure, and various conformational states as a result of the combined use of inhibitors, ligands, modifiers, and proteolytic enzymes. We will review a variety of reagents and approaches that have been employed to arrive at structure-function correlates and discuss critically the limits and ambiguities in the type of information obtained from these methodologies. Chemical modification of the Na(+)-pump protein has already provided a body of data and will, we anticipate, guide the efforts of mutagenesis studies in the future when suitable expression systems become available.

Minimum enzyme unit for Na+/K+-ATPase is the alpha beta-protomer. Determination by low-angle laser light scattering photometry coupled with high-performance gel chromatography for substantially simultaneous measurement of ATPase activity and molecular weight

The oligomeric state of canine renal NA+/K+ -ATPase solubilized by octaethylene glycol n-dodecyl ether (C12E8) was studied by means of low-angle laser light scattering photometry coupled with high-performance gel chromatography (HPGC). At around 0 degree C the solubilized enzyme was separated into the (alpha beta)2-diprotomeric and alpha beta-protomeric protein components with Mr values of 302,000 +/- 10,000 and 156,000 +/- 4,000, respectively, in approximately equal quantities. As the temperature of chromatography was increased toward 20 degrees C, the two protein components converged into a single major component. The Mr of this component depended on the monovalent cation included in the elution buffer, and was 255,000 or 300,000 in the presence of 0.1 M NaCl or 0.1 M KCl, respectively. A computer simulation technique showed that the solubilized enzyme was in a dissociation-association equilibrium of 2 protomers = diprotomer at 20 degrees C, and the difference in apparent Mr of the solubilized enzyme between the two species of monovalent cation was interpreted by an association constant (Ka) in the presence of 0.1 M KCl that was about 50-fold larger than in the presence of 0.1 M NaCl. In order to measure ATPase activity and Mr of the solubilized enzyme simultaneously, a TSKgel G3000SW column had been equilibrated and was eluted with an elution buffer containing 0.30 mg/ml C12E8 and 60 microgram/ml phosphatidylserine (bovine brain) as well as the ligands necessary for the enzyme to exhibit the activity at pH 7.0 and 20 degrees C. The solubilized enzyme was always eluted as a single protein component irrespective of the the amount of the protein applied to the column, ranging between 240 and 10 microgram. The Mr of the protein component, however, decreased from 214,000 and 158,000 with the decrease of the protein amount. The specific ATPase activity, however, remained constant at a level of 64 +/- 4% of that of the membrane-bound enzyme even in the range of protein concentration sufficiently low as to allow the enzyme to exist only in the protomeric form. Thus, the alpha beta-protomer is concluded to be the minimum functional unit for the ATPase activity. The value of Ka obtained from the concentration-dependent dissociation curve was 5 . 10(5) M-1 for the enzyme turning over, and 1.1 . 10(7) M-1 for the enzyme inhibited with ouabain. It was discussed, based on the values of Ka obtained, that the enzyme would exist as the diprotomer or the higher oligomer in the membrane.

Uncoupling of ATP binding to Na+,K+-ATPase from its stimulation of ouabain binding: studies of the inhibition of Na+,K+-ATPase by a monoclonal antibody

The effects of a monoclonal antibody, prepared against the purified lamb kidney Na+,K+-ATPase, on the enzyme's Na+,K+-dependent ATPase activity were analyzed. This antibody, designated M10-P5-C11, is directed against the catalytic subunit of the "native" holoenzyme. It inhibits greater than 90% of the ATPase activity and acts as a noncompetitive or mixed inhibitor with respect to the ATP, Na+, and K+ dependence of enzyme activity. It inhibits the Na+- and Mg2+ATP-dependent phosphoenzyme intermediate formation. In contrast, it has no effect on K+-dependent p-nitrophenylphosphatase (pNPPase) activity, the interconversion of the phosphoenzyme intermediates, and ADP-sensitive or K+-dependent dephosphorylation. It does not alter ATP binding to the enzyme nor the covalent labeling of the enzyme at the presumed ATP site by fluorescein 5'-isothiocyanate (FITC), but it prevents the ATP-induced stimulation in the rate of cardiac glycoside [3H]ouabain binding to the Na+,K+-ATPase. M10-P5-C11 binding appears to inhibit enzyme function by blocking the transfer of the gamma-phosphoryl of ATP to the phosphorylation site after ATP binding to the enzyme has occurred. In the presence of Mg2+ATP, it also prevents the ATP-induced transmembrane conformational change that enhances cardiac glycoside binding. This uncoupling of ATP binding from its stimulation of ouabain binding and enzyme phosphorylation demonstrates the existence of an enzyme-Mg2+ATP transitional intermediate preceding the formation of the Na+-dependent ADP-sensitive phosphoenzyme intermediate. These results are also consistent with a model of the Na+,K+-ATPase active site being composed of two distinct but interacting regions, the ATP binding site and the phosphorylation site.

Model of anion and monovalent cation transport as neutral ion pairs through lipophilic water channels of the Na,K ATPase complex

A model of anion and monovalent cation transport through a lipophilic water channel of the Na,K ATPase complex is presented. Literature data for the Na,K ATPase cation binding sites are combined with data for the anion binding sites of Band 3 to obtain adjacent cation and anion combining sites at the inner and outer channel mouths. Cations and anions form neutral ion pairs or undissociated acids at these sites and then partition much more favorably into lipophilic channel water, passing through the channel in diffusive fashion. Cation movements in an "uphill" direction occur without an enzyme translocating moiety and its specific energetic requirement. The pertinent factors are the exclusion of unpaired cations by the tight channel and the site selectivity or pickup ratios for Na/K at each side which dominate over bulk and transmembrane concentration ratios. ATP hydrolysis provides phosphate for ion pairing.