Modification of (Na+ + K+)-dependent ATPase by fluorescein isothiocyanate: evidence for the involvement of different amino groups at different PH values

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Purification, characterization and partial amino acid sequencing of a 70 kD inhibitor protein of Na+,K+-ATPase from goat testis cytosol

A protein isolated from goat testis cytosol is found to inhibit Na+,K+-ATPase from rat brain microsomes. The inhibitor has been purified by ammonium sulphate precipitation followed by hydroxyapatite column chromatography. The purified fraction appears as a single polypeptide band on 10% SDS-PAGE of approximate molecular mass of 70 kDa. The concentration at which 50% inhibition (I50) occurs is in the nanomolar range. The inhibitor seems to bind Na+,K+-ATPase reversibly at ATP binding site in a competitive manner with ATP, but away from ouabain binding site. It does not affect p-nitrophenyl-phosphatase activity. The inhibitor is found to inhibit the phosphorylation step of the Na+,K+-ATPase. The enhancement of tryptophan fluorescence and changes in CD pattern suggest conformational changes of Na+,K+-ATPase on binding to the inhibitor. Amino acid sequence of the trypsinised fragments show some homology with aldehyde reductase.

The effect of binding of chlorpromazine and chloroquine to ion transporting ATPases

The inhibition of ion transporting ATPases (Na+,K+-ATPase, Ca2+,Mg2+- and Ca2+-ATPase) by two amphiphilic drugs e.g. chlorpromazine (antipsychotic) and chloroquine (antimalarial) are found to be competitive in nature in vitro with respect to the substrate. Two binding sites - high and low affinity are found to exist on all the three ATPases toward these drugs as evident from the plot of F/F0 vs. different drug concentrations of tryptophan fluorescence of the enzymes. Circular dichroism analysis suggest that binding of drugs to the high affinity site does not involve any change in conformation of ATPase molecules which occur only when drug binds to the low affinity sites. The drug binding sites and possible effect on conformational change of ATPase molecules of these two drugs have been described in this report.

A 75-kDa Na+,K+-ATPase competitive inhibitor protein isolated from rat brain cytosol binds to a site different from the ouabain-binding site

A Na+,K+-ATPase inhibitor protein has been purified to homogeneity from rat brain cytosol by ammonium sulphate precipitation, DEAE anion-exchange chromatography and hydroxyapatite adsorption column chromatography. The purified protein migrates as a single polypeptide band of 75 kDa on 7.5% SDS/PAGE. Amino acid composition data shows the presence of a high number of acidic amino acids in the molecule in relation to the pI value of 4.6. The inhibitor binds Na+,K+-ATPase reversibly and blocks ATP binding sites at micromolar concentrations with an I50 of approximately 700 nm. As a result, formation of the phosphorylated intermediate of Na+,K+-ATPase is hindered in the presence of the inhibitor. It does not affect p-nitrophenylphosphatase activity. Tryptophan fluorescence studies and CD analysis suggest conformational changes of Na+,K+-ATPase on binding to the inhibitor.

Purification and functional characterization of a low-molecular-mass Ca2+,Mg2+- and Ca2+-ATPase modulator protein from rat brain cytosol

A low-molecular-mass modulator protein having a molecular mass of about 12 kDa has been purified from rat brain cytosol following gel filtration and FPLC/Mono Q anion-exchange chromatographic separation. A number of protein fractions were obtained from an FPLC column when eluted with a 0.1 M NaCl hold gradient. One fraction (peak no. 5) was found to stimulate Ca2+,Mg2+-ATPase but inhibit Ca2+-ATPase isolated from goat spermatozoa. The S50 (concentration producing 50% stimulation) and I50 were found to be in the nanomolar range. The modulator seems to bind to Ca2+, Mg2+- or Ca2+-ATPase at a site distal from the ATP binding site. The binding to both the ATPases is reversible and non-competitive in nature. The inhibitory activity is found to depend significantly on -SH or -NH2 group(s) of the modulator, whereas no appreciable dependency of the stimulatory effect was apparent. The study indicates that the modulator is not a glycoprotein. CD analysis suggests that the protein exists as an unordered secondary structure. An immuno-cross-reactivity study with specific antibody and inhibition by thapsigargin suggests that the Ca2+,Mg2+- and Ca2+-ATPases from goat testes microsomal membranes are two isoforms of the sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase (SERCA) family. The modulator does not contain any Trp molecules, as evident from Trp fluorescence analysis. Amino acid analysis shows that glycine, serine, derivatives of tyrosine and phenylalanine are the predominant amino acids. The data suggest that the modulator is a negatively charged protein and is a good tool for distinguishing the regulation of Ca2+,Mg2+- and Ca2+-ATPase activities.

Purification and functional characterization of a low-molecular-mass Na+, K+-ATPase inhibitor protein from rat brain cytosol

A number of low-molecular mass (12-13 kDa) Na+, K+-ATPase inhibitor proteins have been purified from rat brain cytosol by gel filtration followed by FPLC fractionation on a Mono Q anion-exchange column. Eight peaks were obtained using 0.1 M NaCl eluent of which one peak was found to be the most potent inhibitor of Na+, K+-ATPase. The molcular mass of the inhibitor was about 13 kDa on 16.5% SDS/PAGE. The concentration at which 50% inhibition (I50) was found was in the nanomolar range. The inhibitor seems to bind to Na+, K+-ATPase at a site distal from the ATP-binding site. The binding to the ATPase is non-competitive. The CD analysis suggests an unordered secondary structural element. It also inhibits p-nitrophenyl phosphatase activity from rat brain with comparable I50 value to that for Na+, K+-ATPase. The protein does not contain any Trp as evident from Trp fluorescence and amino acid analysis. Amino acid analysis shows that glycine and serine, derivatives of tyrosine and phenylalanine are the predominant amino acids. The data suggests that it is a negatively charged protein in which the contribution of the hydrophobic part is 27%.

Functional coupling of phosphorylation and nucleotide binding sites in the proteolytic fragments of Na+/K(+)-ATPase

Cleavage of the alpha-subunit of Na+/K(+)-ATPase by trypsin at Arg438-Ala439 causes enzyme inhibition which has been suggested to be due to altered alignment of phosphorylation site on the 48-kDa N-terminal fragment with nucleotide binding site on the 64-kDa C-terminal fragment. Our aims were to test this hypothesis and to assess the effect of the cleavage on the enzyme's two ATP sites. Na(+)-dependent phosphorylation of the partially cleaved enzyme by ATP showed that K0.5 values of ATP for phosphorylations of intact alpha and 48-kDa peptide were the same (0.4 microM). Unchanged interactions among the residues across the cleavage site were also indicated by data showing that reaction of fluorescein isothiocyanate with the 64-kDa peptide blocked phosphorylation of the 48-kDa peptide by ATP. ATP is known to block the reaction of fluorescein isothiocyanate with the enzyme. Experiments on the partially cleaved enzyme showed that K0.5 of ATP for protection of alpha was 30-60 microM, and the value for the protection of interacting 48-kDa and 64-kDa peptides was 1-3 mM. Evidently, while the cleavage does not affect the high affinity catalytic site, it disrupts the allosteric low affinity ATP site. Experiments on reconstituted preparations showed that the cleavage abolished ATP-dependent Na+/K+ exchange, Pi+ATP-dependent Rb+/Rb+ exchange, ATP-dependent Na+/Na+ exchange, and ADP+ATP-dependent Na+/Na+ exchange activities. Selective disruption of the low affinity ATP site accounts for the inhibitions of all functions involving K+(Rb+), based on the established role of this site in the control of K+ access channels. Cleavage-induced inhibitions of other activities, however, suggest additional roles of the low affinity ATP site in the reaction cycle.

Glutamic acid 327 in the sheep alpha 1 isoform of Na+,K(+)-ATPase stabilizes a K(+)-induced conformational change

By combining the tools of site-directed mutagenesis and [3H]ouabain binding, the functional role of glutamic acid 327 in the fourth transmembrane domain of the sheep alpha 1 isoform of Na+,K(+)-ATPase was examined with respect to its interactions with ouabain, Na+,K+,Mg2+, and inorganic phosphate. Using site-directed mutagenesis, this glutamic acid was substituted with alanine, aspartic acid, glutamine, and leucine. The mutant proteins were constructed in a sheep alpha 1 protein background such that [3H]ouabain binding could be utilized as a highly specific probe of the exogenous protein expressed in NIH 3T3 cells. Na+ competition of [3H]ouabain binding to the mutant forms of Na+,K(+)-ATPase revealed only slight alterations in their affinities for Na+ and in their abilities to undergo Na(+)-induced conformational changes which inhibit ouabain binding. In contrast, K+ competition of [3H]ouabain binding to all four mutant forms of Na+,K(+)-ATPase displayed severely altered interactions between these proteins and K+. Interestingly, [3H]ouabain binding to the mutant E327Q was not inhibited by the presence of K+. This mutant was previously reported to be functionally able to support cation transport with a 5-fold reduced K0.5 for K(+)-dependent ATPase activity (Jewell-Motz, E. A., and Lingrel, J.B. (1993) Biochemistry 32, 13523-13530; Vilsen, B. (1993) Biochemistry 32, 13340-13349). Thus, it appears that this glutamic acid in the fourth transmembrane domain may be important for stabilizing a K(+)-induced conformation within the catalytic cycle of Na+,K(+)-ATPase that is not rate-limiting in the overall ATPase cycle but that displays a greatly reduced affinity for ouabain.

Renal cortical basolateral Na+/HCO3- cotransporter: II. Detection of conformational changes with fluorescein isothiocyanate labeling

Fluorescein isothiocyanate (FITC) fluorescently labels amino groups and has been useful in detecting conformational changes in transport proteins through quenching or enhancement of the fluorescence signal upon exposure of protein to substrates. Solubilized renal basolateral membrane proteins, enriched in Na+/HCO3- cotransporter activity, were reconstituted into liposomes and treated with FITC or its nonfluorescent analogue PITC (phenyl isothiocyanate). In the absence of Na+ and HCO3-, incubation of proteoliposomes with PITC or FITC significantly inhibited cotransporter activity. However, in the presence of Na+ and HCO3- during labeling both agents failed to inhibit cotransporter activity, indicating that these probes interact specifically with the cotransporter. In the presence of the substrates Na+ and HCO3-, PITC binds covalently to amino groups unprotected by substrates leaving the Na+/HCO3- cotransporter available for specific labeling with FITC. Addition of NaHCO3 to FITC-labeled proteoliposomes resulted in a concentration-dependent enhancement of the fluorescence signal which was inhibited by pretreatment with 4,4'-diisothiocyanostilbene 2',2-disulfonic acid (DIDS) prior to FITC labeling. SDS PAGE analysis of FITC-treated proteoliposomes showed the presence of two distinct fluorescent bands (approximate MW of 90 and 56 kD). In the presence of substrates, the fluorescence intensity of these bands was enhanced as confirmed by direct measurement of gel slice fluorescence. Thus, FITC detects conformational changes of the Na+/HCO3- cotransporter and labels proteins which may represent the cotransporter or components of this cotransporter.

An Na+/K(+)-ATPase inhibitor protein from rat brain cytosol

A protein isolated from rat brain cytosol is found to inhibit Na+/K(+)-ATPase in rat brain and kidney and H+/K(+)-ATPase from toad gastric mucosa, but has no effect on Ca2+,Mg(2+)-ATPase and Ca(2+)-ATPase isolated either from rat testis or goat spermatozoa. The inhibitor has been partially purified by ammonium sulphate precipitation followed by gel-filtration through Sephadex G-100. The inhibitor seems to bind at or close to the ATP binding site of Na+/K(+)-ATPase, such that the binding of the inhibitor to ATPase is reversible and competitive in nature with respect to the substrate. Optimum inhibition is observed at around the phase transition temperature of brain Na+/K(+)-ATPase and the inhibitory activity is only partially dependent on -SH or -NH2 group(s) of the inhibitor protein.

The in vivo inhibition of transport enzyme activities by chloroquine in different organs of rat is reversible

The antimalarial drug chloroquine is found to inhibit Na+, K(+)-ATPase, Ca2+, Mg(2+)-ATPase, Ca(2+)-ATPase, pNPPase and acetylcholinesterase activities in different organs of rat in vivo when injected for a certain periods of time. The inhibition seems to be due to the changes in the level of phospholipid, cholesterol and the fatty acid of the lipid and the alteration of the fluidity of the microsomal membranes. However, the enzyme activities return to the normal level in about 2-3 weeks after the discontinuation of the drug suggesting that the drug effect is reversible.

N-acetylimidazole inactivates renal Na,K-ATPase by disrupting ATP binding to the catalytic site

Treatment of renal Na,K-ATPase with N-acetylimidazole (NAI) results in loss of Na,K-ATPase activity. The inactivation kinetics can be described by a model in which two classes of sites are acetylated by NAI. The class I sites are rapidly reacting, the acetylation is prevented by the presence of ATP (K0.5 congruent to 8 microM), and the inactivation is reversed by incubation with hydroxylamine. These data suggest that the class I sites are tyrosine residues at the ATP binding site. The second class of sites are more slowly reacting, not protected by ATP, nor reversed by hydroxylamine treatment. These are probably lysine residues elsewhere in the protein. The associated K-stimulated p-nitrophenylphosphatase activity is inactivated by acetylation of the class II sites only; thus the tyrosine residues associated with ATP binding to the catalytic center are not essential for phosphatase activity. Inactivated enzyme no longer has high-affinity ATP binding associated with the catalytic site, although low-affinity ATP effects (inhibition of phosphatase and deocclusion of Rb) are still present. The inactivated enzyme can still be phosphorylated by Pi, occlude Rb+ ions, and undergo the major conformational transitions between the E1 Na and E2 K forms of the enzyme. Thus acetylation of the Na,K-ATPase by NAI inhibits high-affinity ATP binding to the catalytic center and produces inactivation.

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.

Purification and characterization of a low molecular weight basic protein from marine turtle egg white

The egg white of marine turtle (Caretta caretta Linn.) and one species of tortoise (Geomyda trijuga trijuga Schariggar) contain a low molecular weight basic protein. It has been purified to homogeneity from the egg white of marine turtle and characterized in terms of its major physicochemical and chemical properties. The molecular weight of this protein calculated from gel filtration, sodium dodecyl sulfate-gel electrophoresis in the presence of urea, sedimentation-diffusion data, and amino acid composition is 4300. Its isoelectric point is at pH 11.1 and intrinsic viscosity is 0.038 dl g-1 in 0.2 M NaCl. It has a Stokes radius of 12.6 A and a diffusion coefficient of 16.50 x 10(-7) cm2 s-1. Analysis of the far-ultraviolet circular dichroic spectrum has shown that the basic protein contains 27% beta-pleated sheet and little or no alpha-helix. It possesses a single polypeptide chain of 40 amino acid residues with three disulfide bonds. It lacks serine, methionine, phenylalanine and carbohydrate moiety. It binds to DNA and stimulates ATPase activity due to its strong basicity. The complex of DNA-basis protein is partially resistant to the action of DNase.

Substrate sites of the (Na+ + K+)-ATPase: pertinence of the adenine and fluorescein binding sites

The (Na+ + K+)-activated ATPase catalyzes the K+-activated hydrolysis of 3-O-methylfluorescein phosphate (3OMFP) with a Km of 50 microM, nearly two orders of magnitude lower than the Km for nitrophenyl phosphate, 3 mM. Both ATP and nitrophenyl phosphate are competitors toward 3OMFP with Ki values corresponding to their Km values (for ATP that at the low-affinity sites of the E2 conformation). Enzyme treated with fluorescein isothiocyanate (FITC) such that 60% of the (Na+ + K+)-ATPase activity is lost still hydrolyzes both 3OMFP and nitrophenyl phosphate: the apparent Km values are increased less than 2-fold and the Vmax is unaffected. ATP still inhibits these K+-phosphatase reactions of the FITC-treated enzyme, and this inhibition can exceed the 40% of residual (Na+ + K+)-ATPase activity. Evaluation of a kinetic model indicates that the Ki for ATP is increased about an order of magnitude by FITC-binding. Similar results obtain with trinitrophenyl-ATP (TNP-ATP) as inhibitor, in this case with Ki values in the micromolar range. Finally, FITC treatment increases K+-activated ADPase activity. These observations are interpreted as the fluorescein ring of 3OMFP binding to the adenine pocket of the substrate site, thereby conferring high affinity, just as the fluorescein ring of FITC binding to the adenine pocket in the E1 conformation permits specific linkage of the isothiocyanate chain to a particular lysine, Lys-501. Then, coincident with the transition to the E2 conformation, which bears the low-affinity site for ATP and which catalyzes the K+-phosphatase reaction, the FITC molecule tethered to Lys-501 is pulled from the adenine pocket: allowing 3OMFP and ADP to bind as substrates and ATP and TNP-ATP as inhibitors, albeit in altered conformation. The E1 to E2 transition thus involves not only a change from high to low affinity for ATP, but also a distortion of the adenine pocket and the orientation between Lys-501 and Asp-369, the residue associated with catalysis.

Identification and characterization of a Mg2+-dependent and an independent Ca+2-ATPase in microsomal membranes of rat testis

Rat testicular microsomal membrane fraction contains both Mg+2-dependent and Mg+2-independent Ca+2-ATPase activity. The latter activity is about two times higher than the former. Calcium ion required for maximum activation of Mg+2-independent Ca+2-ATPase in 3.0 mM, whereas for the dependent one it is 2.5 mM. Both the enzymes are resistant to cold shock upto seven days. Histidine and imidazole buffers are found to be the most suitable for dependent and independent enzyme activities, respectively. The pH optima for dependent one is 7.5, whereas for the independent one it is 8.5. Temperature optima for the former is 37 degrees C and for latter one it is 40 degrees C. Among all the nucleotides tested, ATP is found to be the best substrate for both the enzymes. The optimum concentration of ATP for dependent and independent enzyme activities are 3.0 mM and 1.5 mM respectively. Divalent metal ions like Zn+2, Ba+2 and Mn+2 have been found to inhibit Mg+2-dependent Ca+2-ATPase activity whereas Mg+2-independent Ca+2-ATPase activity is inhibited by the divalent ions except zinc which is found to stimulate the enzyme activity. Both the enzymes are inhibited by vanadate, EDTA and EGTA. I50, for vanadate is 0.05 and 0.125 mM for dependent and independent activities, respectively. Sulfhydryl groups modifying agents e.g., NEM, DTNB and chlorpromazine are found to affect the enzyme activities in different ways. Thus NEM and chlorpromazine are found to inhibit and DTNB stimulate the enzyme activities in both the cases.

Monoclonal antibodies that bind the renal Na+/glucose symport system. 2. Stabilization of an active conformation

Conformation-dependent fluorescein isothiocyanate (FITC) labeling of the pig renal Na+/glucose symporter was investigated with specific monoclonal antibodies (MAb's). When renal brush border membranes were pretreated with phenyl isothiocyanate (PITC), washed, and then treated at neutral pH with FITC in the presence of transporter substrates Na+ and glucose, most of the incorporated fluorescence was associated with a single peak after resolution by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The apparent molecular mass of the FITC-labeled species ranged from 79 to 92 kDa. Labeling of this peak was specifically reduced by 70% if Na+ and glucose were omitted. Na+ could not be replaced by K+, Rb+, or Li+. FITC labeling of this peak was also stimulated after incubation of membranes with MAb's known to influence high-affinity phlorizin binding, and stimulation was synergistically increased when MAb's were added in the presence of Na+ and glucose. Substrate-induced or MAb-induced labeling correlated with inactivation of Na+-dependent phlorizin binding. MAb's recognized an antigen of 75 kDa in the native membranes whereas substrate-induced FITC labeling was accompanied by loss of antigen recognition and protection from proteolysis. These findings are consistent with a model in which MAb's stabilize a Na+-induced active conformer of the Na+/glucose symport system.

Persistent stimulation of lens fiber cell Na,K-ATPase by sodium thiocyanate

The Na,K-ATPase partially purified from porcine lens fiber cells (Sen and Pfeiffer, 1982) is stimulated fourfold (specific activity) by treatment with sodium thiocyanate. The optimum conditions are 1.5 M NaSCN, 2 mg protein ml-1 reaction mixture, pH 7.0, with incubation continued for 30 min at 23 degrees C. Sodium docecyl sulphate-gel electrophoresis and [3H]ouabain binding studies indicate that the extent of purity is not increased significantly by the procedure. The high-activity preparation has elevated phospholipid:protein and phosphatidylethanolamine:sphingomyelin ratios compared with the deoxycholate-extracted starting material. The cholesterol:phospholipid ratio and phospholipid acyl group composition are not significantly altered by SCN- treatment. Measurements of 1,6-diphenyl-1,3,5-hexatriene fluorescence polarization show that SNC- treatment produces approximately a 5 degrees C decrease in a membrane phase transition temperature. The phase transition also affects the activation energy of the Na,K-ATPase reaction and probably reflects the onset of the gel to liquid crystalline transition rather than the midpoint location of the transition per se. p-Nitrophenylphosphatase activity and Na,K-ATPase activity in the gel state membrane are also increased by SCN- treatment. Increased specific activity may result, in part, from a membrane fluidity-dependent enzyme activation but is also due, in part, to the expression of latent enzyme activity. Using ouabain-binding data and the specific activity of the activated preparation, it can be shown that the turnover number of the fiber cell enzyme is approximately 1% of that observed in most other tissues.

Reaction sequences for (Na+ + K+)-dependent ATPase hydrolytic activities: new quantitative kinetic models

To delineate better the reaction sequence of the (Na+ + K+)-ATPase and illuminate properties of the active site, kinetic data were fitted to specific quantitative models. For the (Na+ + K+)-ATPase reaction, double-reciprocal plots of velocity against ATP (in the millimolar range), with a series of fixed KCl concentrations, are nearly parallel, in accord with the ping pong kinetics of ATP binding at the low-affinity sites only after Pi release. However, contrary to requirements of usual formulations, Pi is not a competitor toward ATP. A new steady-state kinetic model accommodates these data quantitatively, requiring that under usual assay conditions most of the enzyme activity follows a sequence in which ATP adds after Pi release, but also requiring a minor alternative pathway with ATP adding after K+ binds but before Pi release. The fit to the data also reveals that Pi binds nearly as rapidly to E2 X K X ATP as to E2 X K, whereas ATP binds quite slowly to E2 X P X K: the site resembles a cul-de-sac with distal ATP and proximal Pi sites. For the K+-nitrophenyl phosphatase reaction also catalyzed by this enzyme, the apparent affinities for both substrate and Pi (as inhibitor) decrease with higher KCl concentrations, and both Pi and TNP-ATP appear to be competitive inhibitors toward substrate with 10 mM KCl but noncompetitive inhibitors with 1 mM KCl. These data are accommodated quantitatively by a steady-state model allowing cyclic hydrolytic activity without obligatory release of K+, and with exclusive binding of substrate vs. either Pi or TNP-ATP. The greater sensitivity of the phosphatase reaction to both Pi and arsenate is attributable to the weaker binding by the occluded-K+ enzyme form occurring in the (Na+ + K+)-ATPase reaction sequence. The steady-state models are consistent with cyclical interconversion of high- and low-affinity substrate sites accompanying E1/E2 transitions, with distortion to low-affinity sites altering not only affinity and route of access but also separating the adenine- and phosphate-binding regions, the latter serving in the E2 conformation as the active site for the phosphatase reaction.

Amino group modification of (Na+ + K+)-ATPase

The effects of three amino group reagents on the activity of (Na+ + K+)-ATPase and its component K+-stimulated p-nitrophenylphosphatase activity from rabbit kidney outer medulla have been studied. All three reagents cause inactivation of the enzyme. Modification of amino groups with trinitrobenzene sulfonic acid yields kinetics of inactivation of both activities, which depend on the type and concentration of the ligands present. In the absence of added ligands, or with either Na+ of Mg2+ present, the enzyme inactivation process follows complicated kinetics. In the presence of K+, Rb+, or Tl+, protection occurs due to a change of the kinetics of inactivation toward a first-order process. ATP protects against inactivation at a much lower concentration in the absence than in the presence of Mg2+ (P50 6 microM vs. 1.2 mM). Under certain conditions (100 microM reagent, 0.2 M triethanolamine buffer, pH 8.5) modification of only 2% of the amino groups is sufficient to obtain 50% inhibition of the ATPase activity. Modification of amino groups with ethylacetimidate causes a nonspecific type of inactivation of (Na+ + K+)-ATPase. Mg2+ and K+ have no effects, and ATP only a minor effect, on the degree of modification. The K+-stimulated p-nitrophenylphosphatase activity is less inhibited than the (Na+ + K+)-ATPase activity. Half-inhibition of the (Na+ + K+)-ATPase is obtained only after 25% modification of the amino groups. Modification of amino groups with acetic anhydride also causes nonspecific inactivation of (Na+ + K+)-ATPase. Mg2+ has no effect, and ATP has only a slight protecting effect. The K+-stimulated p-nitrophenylphosphatase activity is inhibited in parallel with the (Na+ + K+)-ATPase activity. Half-inactivation of the (Na+ + K+)-ATPase activity is obtained after 20% modification of the amino groups.

Effect of chlordecone on pH and temperature dependent substrate activation kinetics of rat brain synaptosomal ATPases

Chlordecone, a polycyclic chlorinated insecticide known as Kepone, inhibited the activities of (Na+-K+)ATPase and Mg2+-ATPase in rat brain synaptosomes. Altered pH and specific activity curves for both enzymes demonstrated significant inhibition by chlordecone in buffered acidic, neutral and alkaline pH ranges. Noncompetitive inhibition with respect to activation by ATP in the case of (Na+-K+)ATPase was indicated by altered Vmax values with no significant change in Km values at any pH studied, except at pH 9.5. Mg2+-ATPase was inhibited uncompetitively as evidenced by altered Vmax and Km values. The activities of both ATPase were decreased in the presence of chlordecone at higher temperatures. Activation energy (delta E) values were found to be decreased significantly in the presence of chlordecone at 37 degrees. Arrhenius plots of both ATPases preincubated with chlordecone were found to be nonlinear. In the presence of chlordecone, Vmax was decreased without significant change in Km values for (Na+-K+)ATPase at all temperatures, suggesting a noncompetitive type of inhibition. In the case of Mg2+-ATPase, similar noncompetitive type inhibition was obtained at 27 degrees but not at 32 and 37 degrees. The kinetic data in general suggest that the chlordecone inhibited (Na+-K+)ATPase noncompetitively and Mg2+-ATPase uncompetitively at all pHs and temperatures studied. The present data suggest that inhibition of (Na+-K+)ATPase and Mg2+-ATPase, the two membrane-bound enzymes in synaptosomes, by chlordecone is temperature dependent and pH independent.

Fluorescent labeling of (Na+ + K+)-ATPase by 5-iodoacetamidofluorescein

5-Iodoacetamidofluorescein (5-IAF) covalently labels dog kidney (Na+ + K+)-ATPase with approximately 2 moles incorporated per mole of enzyme. ATPase and K+-phosphatase activities are fully retained after reaction, and the kinetic parameters for Na+, K+, Mg2+, ATP and p-nitrophenyl phosphate are likewise not significantly affected. The fluorescence of the bound 5-IAF is increased by ATP, Na+, and Mg2+, and decreased by K+. These fluorescence changes likely reflect ligand-induced stabilization of the E1 or E2 states of the enzyme.

Characterization of partially purified (Na+ + K+)-ATPase from porcine lens

The partial purification of (Na+ + K+)-ATPase from pig lens has been achieved by treatment with deoxycholate followed by density gradient centrifugation. The specific activity of the final preparation, ranging from 300 to 500 nmol/h per mg protein, is increased approx. 100-fold compared to the homogenate. A parallel increase in rho-nitrophenylphosphatase activity is also observed. Sodium dodecyl sulfate (SDS) gel electrophoresis reveals six major protein bands, one of which is the 93 kDa alpha subunit of (Na+ + K+)-ATPase which can be phosphorylated by reaction with [gamma-32P]ATP. A second band contains a glycoprotein which displays an apparent molecular weight of 51000 and thus appears to be the beta subunit of the enzyme. The enzyme is sensitive to ouabain with the I50 for (Na+ + K+)-ATPase and rho-nitrophenylphosphatase inhibition being 1.2 and 1.3 microM, respectively. Several agents which inhibit (Na+ + K+)-ATPase from other tissues such as oligomycin, Ca2+, vanadate, N-ethylmaleimide, rho-chloromercuribenzenesulfonic acid (PCMBS) and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) also inhibit the lens enzyme. Monovalent cations other than K+ are partially effective in activating the (Na+ + K+)-ATPase and rho-nitrophenylphosphatase activities. The K+ congeners were relatively more effective in supporting (Na+ + K+)-ATPase compared to rho-nitrophenylphosphatase activity. Other kinetic properties of the lens enzyme are also comparable to those of the enzyme from other tissues. Utilizing the partially purified membrane bound enzyme, discontinuities in Arrhenius plots of (Na+ + K+)-ATPase activity, rho-nitrophenylphosphatase activity and fluorescence polarization of the fluidity probe, 1,6-diphenyl-1,3,5-hexatriene (DPH), are observed near the physiological temperature of lens. The possible significance of these observations for the mechanism of cataract formation are discussed.