2'-O-Dansyl analogs of ATP bind with high affinity to the low affinity ATP site of Na+/K+-ATPase and reveal the interaction of two ATP sites during catalysis

Proteoliposomes in nanobiotechnology

Proteoliposomes are systems that mimic lipid membranes (liposomes) to which a protein has been incorporated or inserted. During the last decade, these systems have gained prominence as tools for biophysical studies on lipid-protein interactions as well as for their biotechnological applications. Proteoliposomes have a major advantage when compared with natural membrane systems, since they can be obtained with a smaller number of lipidic (and protein) components, facilitating the design and interpretation of certain experiments. However, they have the disadvantage of requiring methodological standardization for incorporation of each specific protein, and the need to verify that the reconstitution procedure has yielded the correct orientation of the protein in the proteoliposome system with recovery of its functional activity. In this review, we chose two proteins under study in our laboratory to exemplify the steps necessary for the standardization of the reconstitution of membrane proteins in liposome systems: (1) alkaline phosphatase, a protein with a glycosylphosphatidylinositol anchor, and (2) Na,K-ATPase, an integral membrane protein. In these examples, we focus on the production of the specific proteoliposomes, as well as on their biochemical and biophysical characterization, with emphasis on studies of lipid-protein interactions. We conclude the chapter by highlighting current prospects of this technology for biotechnological applications, including the construction of nanosensors and of a multi-protein nanovesicular biomimetic to study the processes of initiation of skeletal mineralization.

Unraveling the Na,K-ATPase alpha(4) subunit assembling induced by large amounts of C(12)E(8) by means of small-angle X-ray scattering

In the current work, we studied the effect of the nonionic detergent dodecyloctaethyleneglycol, C(12)E(8), on the structure and oligomeric form of the Na,K-ATPase membrane enzyme (sodium-potassium pump) in aqueous suspension, by means of small-angle X-ray scattering (SAXS). Samples composed of 2 mg/mL of Na,K-ATPase, extracted from rabbit kidney medulla, in the presence of a small amount of C(12)E(8) (0.005 mg/mL) and in larger concentrations ranging from 2.7 to 27 mg/mL did not present catalytic activity. Under this condition, an oligomerization of the alpha subunits is expected. SAXS data were analyzed by means of a global fitting procedure supposing that the scattering is due to two independent contributions: one coming from the enzyme and the other one from C(12)E(8) micelles. In the small detergent content (0.005 mg/mL), the SAXS results evidenced that Na,K-ATPase is associated into aggregates larger than (alphabeta)(2) form. When 2.7 mg/mL of C(12)E(8) is added, the data analysis revealed the presence of alpha(4) aggregates in the solution and some free micelles. Increasing the detergent amount up to 27 mg/mL does not disturb the alpha(4) aggregate: just more micelles of the same size and shape are proportionally formed in solution. We believe that our results shed light on a better understanding of how nonionic detergents induce subunit dissociation and reassembling to minimize the exposure of hydrophobic residues to the aqueous solvent.

Mechanism of allosteric effects of ATP on the kinetics of P-type ATPases

The roles of allosteric effects of ATP and protein oligomerisation in the mechanisms of P-type ATPases belong to the most controversial and least well understood topics in the field. Recent crystal structural and kinetic data, however, now allow certain hypotheses to be definitely excluded and consistent hypotheses to be developed. The aim of this review is to critically discuss recent results and, in the light of them, to present a set of conclusions which could form the basis of future research. The major conclusions are: (1) at saturating ATP concentrations P-type ATPases function as monomeric enzymes, (2) the catalytic units of P-type ATPases only possess a single ATP binding site, (3) at non-saturating ATP concentrations P-type ATPases exist as diprotomeric (or higher oligomeric) complexes, (4) protein-protein interactions within a diprotomeric complex enhances the enzymes' ATP binding affinity, (5) ATP binding to both protomers within a diprotomeric complex causes it to dissociate into two separate monomers. The physiological role of protein-protein interactions within a diprotomer may be to enhance ATP binding affinity so as to scavenge ATP and maximize the ion pumping rate under hypoxic or anoxic conditions. For the first time a structural basis for the well-known ATP allosteric acceleration of the E2 --> E1 transition is presented. This is considered to be due to a minimization of steric hindrance between neighbouring protomers because of the ability of ATP to induce a compact conformation of the enzymes' cytoplasmic domains.

The association of Na,K-ATPase subunits studied by circular dichroism, surface tension and dilatational elasticity

Different stoichiometries are observed between alpha and beta subunits of Na,K-ATPase that depend on the method employed to solubilize and purify the enzyme. It is not known whether this variability is due to loss of protein-protein association, or is a result of the replacement of essential phospholipids by detergent molecules. With the aim of understanding the effect of enzyme/surfactant ratio on both the catalytic activity and the enzyme structure, we have investigated the bulk and surface properties of the enzyme. The circular dichroism (CD) spectra, surface tension and dilatational surface elasticity results were compared with the residual ATPase activity of the Na,K-ATPase in different surfactant and protein concentrations. Na,K-ATPase in the (alphabeta)(2) form dissociated to the alphabeta form on dilution, and associated to the (alphabeta)(4) form when concentrated. These different stoichiometries have similar ATPase activities and are in equilibrium at C(12)E(8) concentrations below the CMC (0.053 mg mL(-1)). At detergent concentrations above the CMC the ATPase activity of all forms was abolished, which is concomitant with the dissociation of the alpha and beta subunits.

Gill (Na+,K+)-ATPase in diadromous, freshwater palaemonid shrimps: Species-specific kinetic characteristics and α-subunit expression

To better comprehend physiological adaptation to dilute media and the molecular mechanisms underlying ammonia excretion in palaemonid shrimps, we characterized the (Na+,K+)-ATPase from Macrobrachium amazonicum gills, disclosing high- (K(0.5) = 4.2+/-0.2 micromol L(-1); V = 33.9+/-1.9 U mg(-1)) and low-affinity (K(0.5) = 0.144+/-0.010 mmol L(-1); V = 232.9+/-15.3 U mg(-1)) ATP hydrolyzing sites. Stimulation by Na+ (K(0.5) = 5.5+/-0.3 mmol L(-1); V = 275.1+/-15.1 U mg(-1)), Mg2+ (K(0.5) = 0.79+/-0.06 mmol L(-1); V = 261.9+/-18.3 U mg(-1)), K+ (K(M) = 0.88+/-0.04 mmol L(-1); V = 271.8+/-10.9 U mg(-1)) and NH4(+) (K(M) = 5.0+/-0.2 mmol L(-1); V = 385.9+/-15.8 U mg(-1)) obeys single saturation curves, activity being stimulated synergistically by NH4(+) and K+. There is a single K+ binding site, NH4(+) binding to a second, exclusive site, stimulating activity by 33%, modulating K+ affinity. (Na+,K+)-ATPase activity constitutes approximately 80% of total ATPase activity (K(Iouabain) = 147.5+/-8.9 micromol L(-1)); Na+-, K+-, Ca2+-, V- and F(o)F(1)-ATPases are also present. M. amazonicum microsomal fractions possess approximately 2-fold less (Na+,K+)-ATPase alpha-subunit than M. olfersi, consistent with a 2.6-fold lower specific activity. These differences in (Na+, K+)-ATPase stimulation by ATP and ions, and specific activities of other ATPases, suggest the presence of distinct biochemical adaptations to life in fresh water in these related species.

The phosphatase activity of the isolated H4-H5 loop of Na+/K+ ATPase resides outside its ATP binding site

The structural stability of the large cytoplasmic domain (H(4)-H(5) loop) of mouse alpha(1) subunit of Na(+)/K(+) ATPase (L354-I777), the number and the location of its binding sites for 2'-3'-O-(trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP) and p-nitrophenylphosphate (pNPP) were investigated. C- and N-terminal shortening revealed that neither part of the phosphorylation (P)-domain are necessary for TNP-ATP binding. There is no indication of a second ATP site on the P-domain of the isolated loop, even though others reported previously of its existence by TNP-N(3)ADP affinity labeling of the full enzyme. Fluorescein isothiocyanate (FITC)-anisotropy measurements reveal a considerable stability of the nucleotide (N)-domain suggesting that it may not undergo a substantial conformational change upon ATP binding. The FITC modified loop showed only slightly diminished phosphatase activity, most likely due to a pNPP site on the N-domain around N398 whose mutation to D reduced the phosphatase activity by 50%. The amino acids forming this pNPP site (M384, L414, W411, S400, S408) are conserved in the alpha(1-4) isoforms of Na(+)/K(+) ATPase, whereas N398 is only conserved in the vertebrates' alpha(1) subunit. The phosphatase activity of the isolated H(4)-H(5) loop was neither inhibited by ATP, nor affected by mutation of D369, which is phosphorylated in native Na(+)/K(+) ATPase.

FITC binding site and p-nitrophenyl phosphatase activity of the Kdp-ATPase of Escherichia coli

The KdpFABC complex of Escherichia coli, which belongs to the P-type ATPase family, has a unique structure, since catalytic activity (KdpB) and the capacity to transport potassium ions (KdpA) are located on different subunits. We found that fluorescein 5-isothiocyanate (FITC) inhibits ATPase activity, probably by covalently modifying lysine 395 in KdpB. In addition, we observed that the KdpFABC complex is able to hydrolyze p-nitrophenyl phosphate (pNPP) in a Mg(2+)-dependent reaction. The pNPPase activity is inhibited by FITC and o-vanadate. Low concentrations of ATP (1-30 microM) stimulate the pNPPase activity, while concentrations of >500 microM are inhibitory. This behavior can be explained either by a regulatory ATP binding site, where ATP hydrolysis is required, or by proposing an interactive dimer. The notion that FITC inhibits pNPPase and ATPase activity supports the idea that the catalytic domain of KdpB is much more compact than other P-type ATPases, like Na(+),K(+)-ATPase, H(+),K(+)-ATPase, and Ca(2+)-ATPase.

Quantitative analysis of the interaction between the fluorescent probe eosin and the Na+/K+-ATPase studied through Rb+ occlusion

We report a study on the effect of the fluorescent probe eosin on some of the reactions involved in the conformational transitions that lead to the occlusion of the K(+)-congener Rb(+) in the Na(+)/K(+)-ATPase. Eosin decreases the equilibrium levels of occluded Rb(+), this effect being fully attributable to a decrease in the apparent affinity of the enzyme for Rb(+) since the capacity for occlusion remains independent of eosin concentration. The results can be quantitatively described by a model that assumes that two molecules of eosin are able to bind to the Na(+)/K(+)-ATPase, both to the Rb(+)-free and to the Rb(+)-occluded enzyme regardless of the degree of cation occlusion. Concerning the effect on the affinity for Rb(+) occlusion, transient state experiments show that eosin reduces the initial velocity of occlusion, and that, like ATP, it increases the velocity of deocclusion of Rb(+). Interactions between eosin and ATP on Rb(+)-release experiments seem to indicate that eosin binds to the low-affinity site of ATP from which it exerts effects that are similar to those of the nucleotide.

The complex ATP-Fe(2+) serves as a specific affinity cleavage reagent in ATP-Mg(2+) sites of Na,K-ATPase: altered ligation of Fe(2+) (Mg(2+)) ions accompanies the E(1)-->E(2) conformational change

In the presence of ascorbate/H(2)O(2), ATP-Fe(2+) or AMP-PNP-Fe(2+) complexes act as affinity cleavage reagents, mediating selective cleavage of the alpha subunit of Na,K-ATPase at high affinity ATP-Mg(2+) sites. The cleavages reveal contact points of Fe(2+) or Mg(2+) ions. In E(1) and E(1)Na conformations, two major cleavages are detected within the conserved (708)TGDGVNDSPALKK sequence (at V712 and nearby), and one (E(1)Na) or two (E(1)) minor cleavages near V440. In media containing sodium and ATP, Fe(2+) substitutes for Mg(2+) in activating phosphorylation and ATP hydrolysis. In the E(1)P conformation, cleavages are the same as in E(1). Fe(2+) is not bound tightly. By contrast, in the E(2)P conformation, the pattern is different. A major cleavage occurs near the conserved sequence (212)TGES, whereas those in TGDGVNDSPALKK are less prominent. Fe(2+) is bound very tightly. On E(2)P hydrolysis, the Fe(2+) dissociates. The results are consistent with E(1)<-->E(2) conformation-dependent movements of cytoplasmic domains and sites for P(i) and Mg(2+) ions, inferred from previous Fe-cleavage experiments. Furthermore, these concepts fit well with the crystal structure of Ca-ATPase [Toyoshima, C., Nakasako, M., Nomura, H. & Ogawa, H. (2000) Nature (London) 405, 647-655]. Altered ligation of Mg(2+) ions in E(2)P may be crucial in facilitating nucleophilic attack of water on the OP bond. Mg(2+) ions may play a similar role in all P-type pumps. As affinity cleavage reagents, ATP-Fe(2+) or other nucleotide-Fe(2+) complexes could be widely used to investigate nucleotide binding proteins.

Effect of streptozotocin-induced diabetes on rat liver Na+/K+-ATPase

Na+/K+-ATPase during diabetes may be regulated by synthesis of its alpha and beta subunits and by changes in membrane fluidity and lipid composition. As these mechanisms were unknown in liver, we studied in rats the effect of streptozotocin-induced diabetes on liver Na+/K+-ATPase. We then evaluated whether fish oil treatment prevented the diabetes-induced changes. Diabetes mellitus induced an increased Na+/K+-ATPase activity and an enhanced expression of the beta1 subunit; there was no change in the amount of the alpha1 and beta3 isoenzymes. Biphasic ouabain inhibition curves were obtained for diabetic groups indicating the presence of low and high affinity sites. No alpha2 and alpha3 isoenzymes could be detected. Diabetes mellitus led to a decrease in membrane fluidity and a change in membrane lipid composition. The diabetes-induced changes are not prevented by fish oil treatment. The results suggest that the increase of Na+/K+-ATPase activity can be associated with the enhanced expression of the beta1 subunit in the diabetic state, but cannot be attributed to changes in membrane fluidity as typically this enzyme will increase in response to an enhancement of membrane fluidity. The presence of a high-affinity site for ouabain (IC50 = 10-7 M) could be explained by the presence of (alphabeta)2 diprotomeric structure of Na+/K+-ATPase or an as yet unknown alpha subunit isoform that may exist in diabetes mellitus. These stimulations might be related, in part, to the modification of fatty acid content during diabetes.

Effects of fluorescent pseudo-ATP and ATP-metal analogs on secondary structure of Na(+)/K(+)-ATPase

The secondary structure of Na(+)/K(+)-ATPase after modification of the ATP-binding sites was analyzed. Consistently with recent reports, we found in trypsin-treated Na(+)/K(+)-ATPase additionally to alpha-helix also beta-sheet structures in the transmembrane segments. However, binding of fluorescein 5'-isothiocyanate (FITC), the pseudo-ATP analog, to the ATP-binding site did not affect the secondary structure of undigested Na(+)/K(+)-ATPase. Consequently, fluorescence intensity changes of FITC-labeled Na(+)/K(+)-ATPase commonly used to observe conformational transitions of the enzyme reflect physiological changes of the native structure. The metal complex analogues of ATP, Cr(H(2)O)(4)ATP and Co(NH(3))(4)ATP, on the other hand, affected the secondary structure of Na(+)/K(+)-ATPase. We propose that these changes in the secondary structure are responsible for inhibition of backdoor phosphorylation.

Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase

To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase.

Tight binding of bulky fluorescent derivatives of adenosine to the low affinity E2ATP site leads to inhibition of Na+/K+-ATPase. Analysis of structural requirements of fluorescent ATP derivatives with a Koshland-Némethy-Filmer model of two interacting ATP sites

A Koshland-Némethy-Filmer model of two cooperating ATP sites has previously been shown to explain the kinetics of inhibition of Na+/K+-ATPase (EC 3.6.1.37) by dansylated ATP (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321). The present work demonstrates that this model adequately describes all types of interactions and kinetics of a number of ATP analogs that differ in their cooperativity of the high and low affinity ATP binding sites of the enzyme. 2',3'-O(2,4,6-trinitrophenyl)ATP binds in a negative cooperative way to the E1ATP site (Kd = 0.7 microM) and to the E2ATP site (Kd = 210 microM), but 3'(2')-O-methylanthraniloyl-ATP in a positive cooperative way with a lower affinity to the E1ATP binding site (Kd = 200 microM) than to the E2ATP binding site (Kd = 80 microM). 3'(2')-O(5-Fluor-2,4-dinitrophenyl)-ATP, however, binds in a noncooperative way, with equal affinities to both ATP binding sites (Kd = 10 microM). In a research for the structural parameters determining ATP site specificity and cooperativity, we became aware that structural flexibility of ribose is necessary for catalysis. Moreover, puckering of the ring atoms in the ribose is essential for the interaction between ATP sites in Na+/K+-ATPase. A number of derivatives of 2'(3')-O-adenosine with bulky fluorescent substitutes bind with high affinity to the E2ATP site and inhibit Na+/K+-ATPase activity. Evidently, an increased number of interactions of such a bulky adenosine with the enzyme protein tightens binding to the E2ATP site.

Microenvironment of the high affinity ATP-binding site of Na+/K+-ATPase is slightly acidic

Fluorescein-5'-isothiocyanate (FITC) was used to study the high-affinity ATP-binding site of Na+/K+-ATPase. The molar ratio of specifically bound FITC per alpha-subunit of Na+/K+-ATPase was found to be 0.5 as followed from pretreatment experiments with another specific E1ATP-inhibitor Cr(H2O)4AdoPP[CH2]P. This indicated an existence of one high affinity ATP-binding site (E1ATP-binding site) in the native (alphabeta)2-diprotomer of Na+/K+-ATPase. Fluorescence dual-excitation ratio of specifically bound FITC revealed that at external pH 7.5, the pH value inside the E1ATP-binding site is 6.95 +/- 0.18. In addition, FITC fluorescence quenching by anti-fluorescein and by iodide choline indicated the limited access of water into the small pocket of the E1ATP-binding site.

Affinity labeling of two nucleotide sites on Na,K-ATPase using 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-[alpha-32P]diphosphate (TNP-8N3-[alpha-32P]ADP) as a photoactivatable probe. Label incorporation before and after blocking the high affinity ATP site with fluorescein isothiocyanate

ATP and its analogues act on the minimal functional unit of Na, K-ATPase, the alpha beta protomer, with high and low affinity effects. Fluorescein isothiocyanate (FITC) irreversibly blocks the high affinity, or catalytic, ATP site, and yet the surviving K+-phosphatase activity of soluble FITC-modified alphabeta protomers can be photoinactivated by 2'(3')-O-trinitrophenyl (TNP)-8N3-ADP (Ward, D. G., and Cavieres, J. D. (1998) J. Biol. Chem. 273, 14277-14284). We have now used TNP-8N3-[alpha-32P]ADP as a photoaffinity label for Na,K-ATPase. The native enzyme can be photolabeled at 5 microM TNP-8N3-[alpha-32P]ADP, and ATP or FITC treatment prevents labeling of the alpha chain. At 25 microM, however, TNP-8N3-[alpha-32P]ADP can be incorporated in the FITC-modified alpha chain, concurrently with the inactivation of the K+-phosphatase activity, to an extrapolated level of 0.5-1.2 mol of 32P-probe per mol of alpha chain. Photoinactivation and labeling are prevented by TNP-ADP, vanadate, or strophanthidin and are promoted by Na+ or Mg2+, but not K+. The cation effects suggest that the fluorescein-modified enzyme incorporates the TNP-8N3-[alpha-32P]ADP. Mg complex preferentially, and the free probe when in the E1 enzyme form and after occupation of a low-affinity Na+ site. Partial trypsinolysis reveals that the point of TNP-8N3-[alpha-32P]ADP attachment is on the C-terminal 58-kDa fragment of the FITC-modified alpha chain. The affinity labeling of the fluorescein enzyme by TNP-8N3-[alpha-32P]ADP endorses the view that two nucleotide sites can be occupied simultaneously in each alpha subunit of Na,K-ATPase.

Erythrosin 5'-isothiocyanate labels Cys549 as part of the low-affinity ATP binding site of Na+/K+-ATPase

The high-affinity E1ATP site of Na+/K+-ATPase labeled with fluorescein 5'-isothiocyanate and its E2ATP site labeled with erythrosin 5'-isothiocyanate (ErITC), as was shown recently [Linnertz et al. (1998) J. Biol. Chem. 273, 28813-28821], reside on separate and adjacent catalytic alpha subunits. This paper provides evidence that specific labeling of the E2ATP binding site with ErITC resulted in a modification of the Cys549 residue in the tryptic fragment with the sequence Val545-Leu-Gly-Phe-Cys549-His550. Hence, Cys549 is part of or close to the low-affinity E2ATP binding site of Na+/K+-ATPase.

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).

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.

Does binding of ouabain to human alpha1-subunit of Na+, K+-ATPase affect the ATPase activity of adjacent rat alpha1-subunit?

To ascertain whether ouabain binding to human alpha1-subunit influences coexpression of rat alpha1-subunit, the ouabain-sensitive profiles of Na+,K+-ATPase activity and 86Rb+ uptake activity and ouabain binding capacity were measured in HeLa cells stably expressing rat alpha1-subunit. The ouabain-sensitive profile of ATPase and 86Rb+ uptake activity seemed to be the sum of two components, one with high and one with low apparent affinity to ouabain, which were similar to that observed in HeLa and NRK-52E cells derived from human and rat, respectively. The ATPase activity with low sensitivity to ouabain increased in simple proportion to the amount of the rat alpha1 mRNA derived from transfected cDNA, which was determined by the reverse transcription-polymerase chain reaction method. The turnover number of the human Na+,K+-ATPase activity obtained from the ratio of the Na+,K+-ATPase activity to the ouabain binding capacity is about 150/sec. The expression of the rat alpha1-subunit had no effect on the turnover numbers of the Na+,K+-ATPase activity with high affinity to ouabain estimated from the ouabain binding capacity as the active site concentration. These results suggested that the ouabain bound to human alpha1-subunit did not inhibit the ATPase activity of the coexpressing rat alpha1 in these cells.

Quenching of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole-modified Na+/K+-ATPase reveals a higher accessibility of the low-affinity ATP-binding site

7-Chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) labeled Na+/K+-ATPase covalently with two different inactivation constants (Ki = 2.5 microM; Ki' = 10 microM). It apparently modified the two different ATP-binding sites of the enzyme since it decreased the activity of the E2ATP site, i.e. the K+-activated para-nitrophenylphosphatase activity, in an enzyme whose high-affinity E1ATP site had been blocked by fluorescein 5'isothiocyanate (FITC). It also reduced the activity of the E1ATP site, i.e. the Na+-activated protein phosphorylation, in an enzyme whose low-affinity E2ATP site had been blocked by Co(NH3)4PO4. Fluorescence quenching experiments with KI, CsCl and MnCl2 of the NBD-Cl-labeled Na+/K+-ATPase revealed two differently accessible types of fluorophores depending on the ATP site: The E2ATP site apparently differs from the E1ATP site in that it is more open because the fluorophore labeling in the E2ATP site was sterically better accessible for quenchers.