[Different domain organization of two main conformational states of Na+/K+-ATPase]

Na+/K+-ATPase generates an electrochemical gradient of Na+ and K+, which is necessary for the functioning of animal cells. During the catalytic act, the enzyme passes through two ground conformational states, E1 and E2. To characterize the domain organization of the protein in these conformations, the thermal denaturation of Na+/K+-ATPase from duck salt glands and rabbit kidneys has been studied in the presence of Na+ and K+, which induce the transition of the enzyme to the conformation E1 or E2. The melting curves for the apoforms of Na+/K+-ATPases have different shapes: the curve for the enzyme from the rabbit shows one transition at 56.1 degrees C, whereas the denaturation of Na+/K+-ATPase from the duck is characterized by two transitions, at 49.8 and 56.9 degrees C. Sodium and potassium ions abolish the difference in the domain organization of Na+/K+-ATPases. The melting curves for Na+/K+-ATPases in conformation E2 in both cases exhibit a single peak of thermal absorption at about 63 degrees C. The melting curves for the enzymes in conformation E1 show three peaks of thermal absorption, indicating the denaturation of three domains. The difference in the domain organization of Na+/K+-ATPase in conformations E1 and E2 may be of importance in different sensitivity of these conformations of the enzyme to temperature, proteolytic enzymes, and oxidative stress.

Fluorescence measurements of nucleotide association with the Na(+)/K(+)-ATPase

The Na(+)/K(+)-ATPase, a membrane-associated ion pump, uses energy from the hydrolysis of ATP to pump 3 Na(+) ions out of and 2 K(+) into cells. The dependence of ATP hydrolysis on ATP concentration was measured using a fluorescence coupled-enzyme assay. The dependence on concentration of nucleotide association with the ATPase was examined using ADP and ATP-induced quenching of the fluorescence of ATPase labeled with Cy3-maleimide (Cy3-ATPase) or Alexa Fluor 546 carboxylic acid, succinimidyl ester (AF-ATPase). The kinetics of ATP hydrolysis in the presence of Na(+) and K(+) exhibited negative cooperativity with a Hill coefficient (n(H)) of 0.66 and a half-maximal concentration (K(0.5)) of 61 microM; in the absence of K(+), n(H) was 0.58 and K(0.5) was 13 microM. Nucleotide-induced fluorescence quenching exhibited negative cooperativity with an n(H) of 0.3-0.5. These results suggest that negative cooperativity observed in ATP hydrolysis is attributable to negative cooperativity in nucleotide association to the ATPase. Interaction between AF-ATPase and ATP labeled with Alexa Fluor 647 (AF-ATP) showed significant Förster resonance energy transfer (FRET). These results indicate that the ATPase exists as oligoprotomeric complexes in this preparation, and that this aggregation has significant effects on enzyme function.

Cation binding in Na,K-ATPase, investigated by 205Tl solid-state NMR spectroscopy

Cation binding to Na,K-ATPase is characterized in native membranes at room temperature by solid-state NMR spectroscopy using the K(+) congener (205)Tl. It has been demonstrated that the signals from occluded Tl(+) and nonspecifically bound Tl(+) can be detected and distinguished by NMR. Effects of dipole-dipole coupling between (1)H and (205)Tl in the occlusion sites show that the ions are rigidly bound, rather than just occluded. Furthermore, a low chemical shift suggests occlusion site geometries with a relatively small contribution from carboxylate and hydroxyl groups. Nonspecific binding of Tl(+) is characterized by rapid chemical exchange, in agreement with the observed low binding affinity.

Metabolic organization and effects of feeding on enzyme activities of the dogfish shark (Squalus acanthias) rectal gland

In order to investigate the metabolic poise of the elasmobranch rectal gland, we conducted two lines of experimentation. First, we examined the effects of feeding on plasma metabolites and enzyme activities from several metabolic pathways in several tissues of the dogfish shark, Squalus acanthias, after starvation and at 6, 20, 30 and 48 h post-feeding. We found a rapid and sustained ten-fold decrease in plasma beta-hydroxybutyrate at 6 h and beyond compared with starved dogfish, suggesting an upregulation in the use of this substrate, a decrease in production, or both. Plasma acetoacetate levels remain unchanged, whereas there was a slight and transient decrease in plasma glucose levels at 6 h. Several enzymes showed a large increase in activity post-feeding, including beta-hydroxybutyrate dehydrogenase in rectal gland and liver, and in rectal gland, isocitrate dehydrogenase, citrate synthase, lactate dehydrogenase, aspartate amino transferase, alanine amino transferase, glutamine synthetase and Na(+)/K(+) ATPase. Also notable in these enzyme measurements was the overall high level of activity in the rectal gland in general. For example, activity of the Krebs' TCA cycle enzyme citrate synthase (over 30 U g(-1)) was similar to activities in muscle from other species of highly active fish. Surprisingly, lactate dehydrogenase activity in the gland was also high (over 150 U g(-1)), suggesting either an ability to produce lactate anaerobically or use lactate as an aerobic fuel. Given these interesting observations, in the second aspect of the study we examined the ability of several metabolic substrates (alone and in combination) to support chloride secretion by the rectal gland. Among the substrates tested at physiological concentrations (glucose, beta-hydroxybutyrate, lactate, alanine, acetoacetate, and glutamate), only glucose could consistently maintain a viable preparation. Whereas beta-hydroxybutyrate could enhance gland activity when presented in combination with glucose, surprisingly it could not sustain chloride secretion when used as a lone substrate. Our results are discussed in the context of the in vivo role of the gland and mechanisms of possible upregulation of enzyme activities.

Structural Characterization of Na,K-ATPase from Shark Rectal Glands by Extensive Trypsinization

Extensive trypsinization of Na,K-ATPase from the salt gland of Squalus acanthias removes about half of the extramembranous protein mass of the alpha-subunit, while leaving the beta-subunit intact. Sequence analysis and epitope recognition of the remaining alpha-peptides show that transmembrane segments M1/M2 and M3/M4 are present when trypsinization is performed in either NaCl or RbCl. The M5/M6 segment and the intact 19-kDa peptide (M7-M10) are detected in Rb-trypsinized membranes but not in Na-trypsinized membranes. The L7/L8 loop is associated with Na-trypsinized membranes, indicating the presence of an M7/M8 or M8/M9 fragment. Freeze-fracture electron microscopy of both Rb- and Na-trypsinized membranes reveals intramembranous particles that indicate a retained cluster of peptides, even in the absence of an intact 19-kDa fragment. The rotational diffusion of covalently spin-labeled trypsinized complexes is studied in the presence of poly(ethylene glycol) or glycerol by using saturation transfer electron spin resonance. Rotational correlation times in aqueous poly(ethylene glycol) are longer than in glycerol solutions of the same viscosity and increase nonlinearly with the viscosity of the suspending medium, indicating that poly(ethylene glycol) induces aggregation of the tryptic peptides (and beta-subunit) within the membrane. The aggregates of enzyme trypsinized in the presence of NaCl are larger than those for enzyme trypsinized in RbCl, at both low and high aqueous viscosities. Similarities in mobility for native and Rb-trypsinized enzymes suggest either a change in average orientation of the spin-label upon trypsinization or that trypsinization leads to a reorganized protein structure that is more prone to aggregation.

Melittin induces both time-dependent aggregation and inhibition of Na,K-ATPase from duck salt glands however these two processes appear to occur independently

Using cupric phenanthroline as a cross-linking agent, we have shown that melittin induced time-dependent aggregations of Na,K-ATPase in microsomal fractions and in preparations of purified Na,K-ATPase from duck salt glands. Incubation of melittin with these preparations also led to the progressive loss of Na,K-ATPase activity. At melittin/protein molar ratio of 5:1, we did not observe inhibition of Na,K-ATPase in the microsomal fraction but the process of enzyme aggregation occurred. At higher melittin/protein molar ratios (10:1 and 30:1), the inhibition of the enzyme and its aggregation proceeded simultaneously but the rates of these processes and maximal values achieved were different. At a melittin/protein ratio of 30:1, Na,K-ATPase inhibition may be described as a biexponential curve with the values for pseudo-first order rate constants being 2.7 and 0.15 min(-1). However, the aggregation may be presented by a monoexponential curve with a pseudo-first order rate constant of 0.15 min(-1). In purified preparations of Na,K-ATPase, the maximal aggregation (about 90%) was achieved at a melittin/protein molar ratio of 2:1, and a further increase in the melittin/protein ratio increased the rate of aggregation but did not affect the value of maximal aggregation. The results show that melittin induced both aggregation and inhibition of Na,K-ATPase but these two processes proceeded independently.

Modulation of Na,K-ATPase by phospholipids and cholesterol. II. Steady-state and presteady-state kinetics

The effects of phospholipid acyl chain length (n(c)) and cholesterol on several partial reactions of Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. This regards the E(1)/E(2) equilibrium, the phosphoenzyme level, and the K(+)-deocclusion reaction. In addition, the lipid effects on some steady-state properties were investigated. Finally, the effects of cholesterol on the temperature sensitivity of the phosphorylation and spontaneous dephosphorylation reactions were investigated. The fatty acid and cholesterol composition of the native Na,K-ATPase membrane preparation showed a remarkable similarity to the lipid composition known to support maximum hydrolytic capacity as determined from in vitro experiments. The main rate-determining step of the Na,K-ATPase reaction, the E(2) --> E(1) reaction, as well as several other partial reactions were accelerated by cholesterol. This regards the phosphorylation by ATP as well as the E(1) - P --> E(2)-P reaction. Moreover, cholesterol shifted the E(1)/E(2) equilibrium toward the E(1) conformation and increased the K(+)-deocclusion rate. Finally, cholesterol significantly affected the temperature sensitivity of the spontaneous dephosphorylation reaction and the phosphorylation by ATP. The effects of cholesterol were not completely equivalent to those induced by increasing the phospholipid acyl chain length, indicating that the cholesterol effects are not entirely caused by increasing the hydrophobic bilayer thickness, which indicates an additional mechanism of action on the Na,K-ATPase.

The effects of dietary sodium loading on the activity and expression of Na, K-ATPase in the rectal gland of the European dogfish (Scyliorhinus canicula)

cDNA fragments of both the alpha- and beta-subunits of the Na, K-ATPase and a cDNA fragment of the secretory form of Na-K-Cl cotransporter from the European dogfish (Scyliorhinus canicula) were amplified and cloned using degenerate primers in RT-PCR. These clones were used along with a sCFTR cDNA from the related dogfish shark, Squalus acanthias to characterise the expression of mRNAs for these ion transporters in the dogfish rectal gland subsequent to an acute feeding episode. Following a single feeding event where starved dogfish were fed squid portions (20 g squid/kg fish), there was a delayed and transient 40-fold increase in the activity of Na, K-ATPase in crude rectal gland homogenates. Increases in enzyme activity were apparent 3 h after the feeding event and peaked at 9 h before returning to control values within 24 h. These increases in activity were accompanied by small and transient decreases in plasma sodium and chloride concentrations lasting up to 3 days. Significant increases in the expression of mRNAs for alpha- and beta-subunits of the Na, K-ATPase, the Na-K-Cl cotransporter and CFTR chloride channel were detected but not until 1-2 days after the feeding event. It is concluded that the transient increase in Na, K-ATPase activity is not attributable to increases in the abundance of alpha- and beta-subunit mRNAs but must be associated with some, as yet unknown, post-transcriptional activation mechanism.

Phosphorylation of the alpha-subunit of Na,K-ATPase from duck salt glands by cAMP-dependent protein kinase inhibits the enzyme activity

Although it was shown earlier that phosphorylation of Na,K-ATPase by cAMP-dependent protein kinase (PKA) occurs in intact cells, the purified enzyme in vitro is phosphorylated by PKA only after treatment by detergent. This is accompanied by an unfortunate side effect of the detergent that results in complete loss of Na,K-ATPase activity. To reveal the effect of Na,K-ATPase phosphorylation by PKA on the enzyme activity in vitro, the effects of different detergents and ligands on the stoichiometry of the phosphorylation and activity of Na,K-ATPase from duck salt glands (alpha1beta1-isoenzyme) were comparatively studied. Chaps was shown to cause the least inhibition of the enzyme. In the presence of 0.4% Chaps at 1 : 10 protein/detergent ratio in medium containing 100 mM KCl and 0.3 mM ATP, PKA phosphorylates serine residue(s) of the Na,K-ATPase with stoichiometry 0.6 mol Pi/mol of alpha-subunit. Phosphorylation of Na,K-ATPase by PKA in the presence of the detergent inhibits the Na,K-ATPase. A correlation was found between the inclusion of P(i) into the alpha-subunit and the loss of activity of the Na,K-ATPase.

Identification of a phospholemman-like protein from shark rectal glands. Evidence for indirect regulation of Na,K-ATPase by protein kinase c via a novel member of the FXYDY family

The Na,K-ATPase provides the driving force for many ion transport processes through control of Na(+) and K(+) concentration gradients across the plasma membranes of animal cells. It is composed of two subunits, alpha and beta. In many tissues, predominantly in kidney, it is associated with a small ancillary component, the gamma-subunit that plays a modulatory role. A novel 15-kDa protein, sharing considerable homology to the gamma-subunit and to phospholemman (PLM) was identified in purified Na,K-ATPase preparations from rectal glands of the shark Squalus acanthias, but was absent in pig kidney preparations. This PLM-like protein from shark (PLMS) was found to be a substrate for both PKA and PKC. Antibodies to the Na, K-ATPase alpha-subunit coimmunoprecipitated PLMS. Purified PLMS also coimmunoprecipitated with the alpha-subunit of pig kidney Na, K-ATPase, indicating specific association with different alpha-isoforms. Finally, PLMS and the alpha-subunit were expressed in stoichiometric amounts in rectal gland membrane preparations. Incubation of membrane bound Na,K-ATPase with non-solubilizing concentrations of C(12)E(8) resulted in functional dissociation of PLMS from Na,K-ATPase and increased the hydrolytic activity. The same effects were observed after PKC phosphorylation of Na,K-ATPase membrane preparations. Thus, PLMS may function as a modulator of shark Na,K-ATPase in a way resembling the phospholamban regulation of the Ca-ATPase.

Effects of environmental salinity on Na(+)/K(+)-ATPase in the gills and rectal gland of a euryhaline elasmobranch (Dasyatis sabina)

Changes in Na(+)/K(+)-ATPase activity and abundance associated with environmental salinity were investigated in the gills and rectal gland of the Atlantic stingray Dasyatis sabina. Using a ouabain-specific ATPase assay and western blotting, we found that stingrays from fresh water had the highest activity and highest relative abundance of Na(+)/K(+)-ATPase in the gills. Using immunohistochemistry, we also found that gills from freshwater stingrays had the greatest number of Na(+)/K(+)-ATPase-rich cells. When freshwater stingrays were acclimated to sea water for 1 week, the activity and abundance of Na(+)/K(+)-ATPase and the number of Na(+)/K(+)-ATPase-rich cells decreased in the gills. In seawater stingrays, the branchial activity and abundance of Na(+)/K(+)-ATPase and the number of Na(+)/K(+)-ATPase-rich cells were further reduced. In rectal glands, the activity and abundance of Na(+)/K(+)-ATPase were lower in freshwater animals than in seawater-acclimated and seawater stingrays, both of which had equivalent levels. These findings suggest that salinity-associated changes in gill and rectal gland Na(+)/K(+)-ATPase activity are due to changes in the abundance of Na(+)/K(+)-ATPase. We conclude that the gills may be important for active ion uptake in fresh water, while the rectal gland is important for active NaCl excretion in sea water. The results from this study are the first to demonstrate an effect of environmental salinity on Na(+)/K(+)-ATPase expression in the gills and rectal gland of an elasmobranch.

Protonation-dependent inactivation of Na,K-ATPase by hydrostatic pressure developed at high-speed centrifugation

Irreversible inactivation of membranous Na,K-ATPase by high-speed centrifugation in dilute aqueous solutions depends markedly on the protonation state of the protein. Pig kidney Na,K-ATPase is irreversibly inactivated at pH 5 but is fully protected at pH 7 and above. Shark rectal gland Na,K-ATPase is irreversibly inactivated at neutral or acidic pH and partially protected at an alkaline pH. The overall Na,K-ATPase activity and the K-dependent pNPPase activity were denatured in parallel. Cryoprotectants such as glycerol or sucrose at concentrations of 25-30% fully protect both enzymes against inactivation. The specific ligands NaCl and KCl protect the Na,K-ATPase activity partially and the pNPPase activity fully at concentrations of 0.2-0.3 M. Electron microscope analysis of the centrifuged Na,K-ATPase membranes revealed that the ultrastructure of the native membranes is preserved upon inactivation. It was also observed that the sarcoplasmic reticulum Ca-ATPase and hog gastric H, K-ATPase are susceptible to inactivation by high-speed centrifugation in a pH-dependent fashion. H,K-ATPase is protected at alkaline pH, whereas Ca-ATPase is protected only in the neutral pH range.

Ligands presumed to label high affinity and low affinity ATP binding sites do not interact in an (alpha beta)2 diprotomer in duck nasal gland Na+,K+-ATPase, nor Do the sites coexist in native enzyme

The interaction of ligands deemed to be ATP analogues with renal Na(+),K(+)-ATPase suggests that two ATP binding sites coexist on each functional unit. Previous studies in which fluorescein 5-isothiocyanate (FITC) was used to label the high affinity ATP site and 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP) was used to probe the low affinity site suggested that the two sites coexist on the same alphabeta protomer. Other studies in which FITC labeled the high affinity site and erythrosin-5-isothiocyanate (ErITC) labeled the low affinity site led to the conclusion that the high and low affinity sites exist on separate interacting protomers in a functional diprotomer. We report here that at 100% inhibition of ATPase activity by FITC, each alphabeta protomer of duck nasal gland enzyme has a single bound FITC. Both TNP-ADP and ErITC interact with FITC-bound protomers, which unambiguously demonstrates that putative high and low affinity ATP sites coexist on the same protomer. In unlabeled nasal gland enzyme, TNP-ADP and ErITC inhibit both ATPase activity and p-nitrophenyl phosphatase activity, functions attributed to the putative high and low affinity ATP site, respectively, by interacting with a single site with characteristics of the high affinity ATP binding site. In FITC-labeled enzyme, TNP-ADP and ErITC inhibit p- nitrophenyl phosphatase activity but at much higher concentrations than with the unmodified enzyme. Low affinity sites do not exist on the unmodified enzyme but can be detected only after the high affinity site is modified by FITC.

protomers of Na+,K+-ATPase from microsomes of duck salt gland are mostly monomeric: Formation of higher oligomers does not modify molecular activity

The distance that separates alphabeta protomers of the Na(+), K(+)-ATPase in microsomes and in purified membranes prepared from duck nasal salt glands was estimated by measuring fluorescence resonance energy transfer between anthroylouabain bound to a population of alphabeta protomers and either N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain or 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain bound to the rest. Energy transfer between probes bound in the microsomal preparation was less than in the purified membranes. The efficiency of energy transfer between anthroylouabain and N-[7-nitrobenz-2-oxa-1, 3-diazol-4-yl]-6-aminohexyl ouabain was 29.2% in the microsomes compared with 62.6% in the purified preparation. Similar results were obtained with 5-(and-6)-carboxyfluorescein-6-aminohexyl ouabain as acceptor. We calculate that either the protomer bound probes were on the average 13 A farther apart in the microsomes than in the purified membranes, or that 53% of the protomers are monomeric in the microsome preparation. Microsomes prepared in the presence of phalloidin (a toxin that binds to F actin and stabilizes the actin-based cytoskeleton) showed less quench than those prepared in its absence. The data support the hypothesis that protomers are kept apart by their association with the cytoskeleton. The turnover rate while hydrolyzing ATP is the same in the microsomal and purified preparations; higher oligomer formation has no significant effect on the enzyme reaction mechanism.

Hepatic toxicity and persistence of ser/thr protein phosphatase inhibition by microcystin in the little skate Raja erinacea

Microcystin-induced ser/thr protein phosphatase (PP) inhibition and toxicity were examined in the little skate (Raja erinacea), an evolutionarily primitive marine vertebrate. As in mammals, PP inhibition and toxicity were exclusively hepatocellular, but were much more persistent in the skate. A dose of 63 microg/kg given iv to adult male skates resulted in the near complete inhibition of hepatic PP activity at 24 h. PP activity was still 95% inhibited 7 days after dosing in skates given 125 microg/kg microcystin. Mortality occurred at doses of 500 microg/kg or more. Hepatic lesions were only seen in animals with fully inhibited PP activity in liver. The histological changes seen at 125 microg/kg were mild periportal inflammatory changes increasing in severity together with hepatocyte necrosis at higher doses of microcystin. Microcystin persisted and could be detected in plasma up to 7 days after dosing. This finding shows that, in the skate, as in mammals, the liver is the only organ capable of uptake of microcystin, since there was no significant inhibition of PP activity in the rectal gland and small decreases in PP activity of the kidney that were not time or dose dependent. In vitro microcystin caused dose-dependent inhibition of PP activity in isolated skate hepatocytes, while it was without effect in cultured rectal glands. Uptake of microcystin and the accompanying inhibition of PP activity in skate hepatocytes was prevented by the addition of a series of organic dyes and bile acids. The spectrum of inhibitors of microcystin uptake in skate is similar to that seen in the rat, indicating common features of the carrier(s) in these diverse species.

Preparation of Na+,K+-ATPase with near maximal specific activity and phosphorylation capacity: evidence that the reaction mechanism involves all of the sites

The phosphorylation capacity of Na+,K+-ATPase preparations in common use is much less than expected on the basis of the molecular weight of the enzyme deduced from cDNA sequences. This has led to the popularity of half-of-the-sites or flip-flop models for the enzyme reaction mechanism. We have prepared Na+,K+-ATPase from nasal salt glands of salt-adapted ducks which has a phosphorylation capacity and specific activity near the theoretical maxima. Preparations with specific activities of >60 micromol (mg of protein)-1 min-1 at 37 degrees C had phosphorylation capacities of >60 nmol/mg of protein, and the rate of turnover of the enzyme was 9690 min-1, within the range reported for the enzyme from other sources. The fraction of the maximal specific activity of the enzyme compared well with the fraction of the protein on SDS-PAGE which was alpha and beta chains, especially at the highest specific activity which indicates that all of the alphabeta protomers are active. The gels of the most reactive preparations contained only alpha and beta chains, but less active preparations contained a number of extraneous proteins. The major contaminant was actin. The preparation did not contain any protein which migrated in the molecular weight range of the gamma subunit. The subunit composition of the enzyme was alpha1 and beta1 only. This is the first report of a pure, homogeneous, fully active preparation of the protein. Reaction models which incorporate a half-of-the-sites or flip-flop mechanism do not apply to this enzyme.

Heterogeneity of Na+/K+-ATPase from rectal gland of Squalus acanthias is not due to alpha isoform diversity

Purified Na+/K+-ATPase (EC 3.6.1.37) isolated from the rectal gland of Squalus acanthias was characterized in ouabain-binding studies and with respect to isoform(s) of the alpha peptide. To avoid enzyme inactivation [3H]ouabain equilibrium binding was carried out at 20 degrees C. The heterogeneity of Na+/K+-ATPase isolated from shark rectal gland was similar in [3H]ouabain binding as previously seen in hydrolytic studies. The binding isotherms were compatible with the existence of a high-affinity (Kdis 0.69 nM) and a low-affinity (Kdis 42 nM) component of 1.46 and 0.79 nmol.(mg protein)-1, respectively. In Western blots the alpha peptide of the enzyme hybridized with an isoform-specific polyclonal antibody raised to an alpha3-specific region of the large intracellular domain of rat Na+/K+-ATPase, but not with the supposed alpha3-specific monoclonal antibody MA3-915 with its epitope near the N-terminus. Semi-quantitative analysis of the reaction of the alpha3-specific polyclonal antibody with the alpha peptide from the shark enzyme compared to the reaction with alpha peptide from rat brain enzyme indicated that this region is not exactly the same in the two species. The alpha peptide of shark enzyme was not recognized by alpha1- or alpha2-specific polyclonal antibodies, or by the alpha1-specific monoclonal antibodies 3B and F6. The large intracellular domain of Na+/K+-ATPase from shark rectal gland thus seems to be alpha3-like and no alpha isoform heterogeneity seems able to account for the heterogeneity seen in ouabain binding.

Cloning of sgk serine-threonine protein kinase from shark rectal gland – a gene induced by hypertonicity and secretagogues

Recently, the cell-volume-regulated serine-threonine protein kinase h-sgk was cloned from a human hepatoma cell line. The sgk gene was shown to be induced by cell shrinkage in many different mammalian cell lines. In this study, two highly conserved serine-threonine protein kinases, sgk-1 and sgk-2, were cloned from rectal gland tissue of the spiny dogfish (Squalus acanthias). Both kinases showed a distinct pattern of tissue specificity, with high expression levels in kidney, intestine, liver and heart. In rectal gland slices sgk-1 transcription was induced by exposure to hypertonic solution, reduction of the extracellular urea concentration, and addition of the secretagogues vasoactive intestinal polypeptide (VIP) and carbachol. The shark sgk-1 serine-threonine protein kinase may therefore provide a link between cell volume, Cl–secretion and protein phosphorylation state in shark rectal gland cells.

K+-dependence of electrogenic transport by the NaK-ATPase

Charge translocation by the NaK-ATPase from shark rectal gland was measured by adsorption of proteoliposomes to a planar lipid membrane. The proteoliposomes were prepared by reconstitution of purified NaK-ATPase into liposomes consisting of E. coli lipids. The protein was activated by applying an ATP concentration jump produced by photolysis of a protected derivative of ATP, caged ATP. K+ titrations were used to study the effect of K+ on the charge translocation kinetics of the protein. The time-dependent currents obtained after activation of the enzyme with caged ATP were analyzed with a simplified Albers-Post model (E1 (k1)-->E1ATP (k2)-->E2P (k3)-->E1) taking into account the capacitive coupling of the protein to the measuring system. The results of the K+ titrations show a strong dependence of the rate constant k3 on the K+ concentration at the extracellular side of the protein, indicating the K+ activated dephosphorylation reaction. In contrast, k1 and k2 remained constant. The K+ dependence of the rate k3 could be well described with a K+ binding model with two equivalent binding sites (E2P + 2K+ <==> E2P(K) + K+ <==> E2 P(2K)) followed by a rate limiting reaction (E2P(2K) --> E1(2K)). The half saturating K+ concentration K3,0.5 and the microscopic dissociation constant K3 for the K+ dependence of k3 were 4.5mM and 1.9mM respectively. At saturating K+ concentration the rate constant k3 was approximately 100 s(-1). The relative amount of net charge transported during the Na+ and the K+ dependent reactions could be determined from the experiments. Our results suggest electroneutral K+ translocation and do not support electrogenic K+ binding in an extracellular access channel. This is compatible with a model where 2 negative charges are cotransported with 3Na+ and 2K+ ions. Error analysis gives an upper limit of 20% charge transported during K+ translocation or during electrogenic K+ binding in a presumptive access channel compared to Na+ translocation.

Characterization of the secondary structure and assembly of the transmembrane domains of trypsinized Na,K-ATPase by Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy has been used to compare native Na,K-ATPase-containing membranes with those trypsinized in the presence of either Rb+ or Na+ ions to remove the extramembranous parts of the protein. The protein secondary structure content deduced from the amide I band is approximately 30-35% alpha-helix, 37-40% beta-structure, and 13-15% random coil for native membranes from shark rectal gland and from pig kidney, in both the Na- and K-forms. Trypsinization in either Rb+ (a K+ congener) or Na+ removes approximately 35% of the amide I band intensity of native membranes from shark rectal gland. The protein secondary structural content of the trypsinized membranes lies in the range of approximately 23-32% alpha-helix, 37-46% beta-structure, and 12-18% random coil for the shark and kidney enzymes. The distribution of intensity between the bands corresponding to protonated and deuterium-exchanged alpha-helices, and between the component bands attributed to beta-structure, changes considerably on trypsinization, in the direction of a greater proportion of protonated alpha-helix and a broader range of frequencies for beta-structure. The kinetics of deuteration of the slowly exchanging population of protein amide groups is also changed on trypsinization. The mean rate constant for deuteration of trypsinized membranes is approximately half that for native membranes, whereas the proportion of amides contributing to this population increases on trypsinization. The temperature dependence of the amide I band in the Fourier transform infrared spectra indicates that the onset of thermal denaturation occurs at 58 degrees C for native membranes (in either Na+ or K+) and for membranes trypsinized in Rb+, but the major denaturation event for membranes trypsinized in Na+ occurs at approximately 84 degrees C. These results correlate with the functional properties of the intramembranous section of the enzyme.

Mechanism of inhibition of E1-E2 ATPases by melittin

The inhibition of Na,K-ATPase from duck salt gland and Ca-ATPase from rabbit skeletal muscle sarcoplasmic reticulum by melittin, a 26-residue peptide from bee venom, was studied. Melittin irreversibly inhibits both enzymes. At melittin/ATPase molar ratio (30-50):1, the time dependence of the inhibition is described by the sum of two exponential curves. At pH 7.0, the fast phase of the inhibition provides for about 50% of total loss of activity with pseudo-first order rate constants of 1.52 +/- 0.17 and 1.20 +/- 0.21 min-1 for Na,K- and Ca-ATPase, respectively. The corresponding pseudo-first order rate constants for the slow phase were 0.12 +/- 0.02 and 0.09 +/- 0.02 min-1. The inhibition of both enzymes by melittin depends upon pH; the inhibition increases when the pH is increased from 6.0 to 8.5. The enhancement of the inhibition concomitant to increase in pH is mainly due to an increase in the rate constant of the fast phase. ATP protects both enzymes from the inhibition by melittin; however, the character of protection is different for Ca-versus Na,K-ATPase. The protection of Ca-ATPase activity by ATP is due to an increase in melittin-insensitive activity. The protective effect of ATP on Na,K-ATPase is due to a decrease in the rate constant of fast phase as well as an increase in melittin-insensitive activity. The data suggest that the inhibition of Ca- and Na,K-ATPases by melittin results from the interaction of the peptide with two different sites. One of the sites may be located on the catalytic subunit of the enzymes, the other can be related to the lipid bilayer of the membrane.

Mechanism of oxidative damage of dog kidney Na/K-ATPase

Oxidative modification of kidney Na/K-ATPase was found to be accompanied by a decrease in the amount of sulfhydryl groups accessible for Elmann reagent with subsequent transformation of kinetic behavior of the enzyme. Oxidation of Na/K-ATPase with 20 mM hydrogen peroxide during 20 min results in about 50% inhibition of its activity and subsequent transformation of complex substrate-velocity dependence into the simple hyperbolic curve. In terms of kinetic analysis the suggestion was made that partial oxidation of Na/K-ATPase by hydrogen peroxide results in disordering of interprotomer interaction in the oligomeric complex of Na/K-ATPase.

Functional regulation of reconstituted Na,K-ATPase by protein kinase A phosphorylation

Reconstituted Na+,K+-ATPase from either pig kidney or shark rectal glands was phosphorylated by cAMP dependent protein kinase, PKA. The stoichiometry was approximately 0.9 mol P(i)/mol alpha-subunit in the pig kidney enzyme and approximately 0.2 mol P(i)/mol alpha-subunit in the shark enzyme. In shark, Na+,K+-ATPase PKA phosphorylation increased the maximum hydrolytic activity for cytoplasmic Na+ activation and extracellular K+ activation without affecting the apparent K(m) values. In contrast, no significant functional effect after PKA phosphorylation was observed in pig kidney Na+,K+-ATPase.

Localization of NADPH-diaphorase activity in the salt gland of the saltwater-acclimated Pekin duck

Exocrine secretion of the avian salt gland is controlled by the autonomic nervous system. NADPH-diaphorase histochemistry was employed at the light and electron microscopic level to provide the morphological basis for a putative nitrergic regulation of salt gland function. NADPH-diaphorase staining was localized in two cell populations of the parasympathetic secretory ganglion at high cell density and equal distribution throughout the ganglionic mass. In addition, salt gland-intrinsic neurons, arranged in small clusters and associated with major nitrergic fiber bundles, proved to be NADPH-diaphorase positive. These postganglionic nerve fibers innervated the secretory parenchyma in close proximity to the basal membrane of single secretory tubules as well as arterioles. The findings suggest participation of the nitrergic pathway in the autonomic control of avian salt gland function.

Characterization of the subunit isoforms of duck salt gland Na/K adenosine triphosphatase

The N-terminal sequences of the alpha and beta subunits from the Na/K-ATPase of duck salt gland have been determined by automated Edman degradation chemistry. These sequences were compared to sequences previously reported for Na/K-ATPase subunits from other sources in order to determine the subunit isoform composition of the salt gland enzyme. The comparisons indicate that the duck salt gland enzyme is composed of an alpha-1 subunit and a beta-1 subunit. This subunit isoform composition is consistent with the involvement of this enzyme in sodium excretion as Na/K-ATPases in other tissues involved in sodium excretion also have this subunit isoform composition.

A comparative study of Na+/K(+)-ATPases of duck salt gland and canine kidney: implications for the enzyme's reaction mechanism

Highly purified preparations of duck salt gland and canine kidney Na+/K(+)-ATPases with comparable specific activities were used to clarify the causes of previously reported differences between the substrate-velocity curves of these enzymes. When assays were done under identical conditions (pH 7.4; 37 degrees C), and a wide range of closely spaced ATP concentrations were used, the curves of both enzymes exhibited intermediary plateaus, as noted before for the salt gland enzyme. The two enzymes also had the same numbers of phosphorylation and ouabain binding sites, and their catalytic subunits were of the alpha 1 isoform type as revealed by immunostaining with specific antibodies. The findings suggest that the substrate-velocity curves of all widely used Na+/K(+)-ATPases may contain an intermediary plateau which is diagnostic of reaction mechanisms that generate rate equations containing powers of substrate concentration greater than two, e.g., a mechanism involving an oligomer with more than two protomers.

[Effect of ligands on rotational mobility of Na,K-ATPase]

Phosphorescence anisotropy of eosin-5'-isothiocyanate labelled Na,K-ATPase purified from duck salt glands has been studied. The initial anisotropy value is 0.235 +/- 0.015 (room temperature) and does not depend on the enzyme conformation (sodium or potassium). The experimental curve is fitted into a two-exponential curve with residual term, the fast component corresponds to the rotational mobility of the functional unit of Na,K-ATPase (promoter), while the slow one--to that of larger associates. In the presence of ligands modifying the conformational state of Na,K-ATPase (sodium, potassium, ATP) the rotational mobility of the fast component does not change in contrast with the slow one. A comparison of the enzyme rotational mobility in the presence of ligands simulating different steps of hydrolytic cycle suggests that interprotomer interactions are changed in the course of hydrolytic cycle: the fraction of larger associates increases at the step of the enzyme interactions with potassium ions, whereas their mobility in the bilayer enhances sharply after interaction with ATP. In the presence of the 2% non-ionic detergent, C12E9, the initial anisotropy value decreases down to 0.1; the residual term disappears thereby, while the curve is still two-exponential. However, the difference in the rotational mobility of sodium and potassium conformers diminishes. At the same time, the ratios between protomers and oligomers in the presence of sodium and potassium become approximated. This indicates that in the presence of the detergent high molecular weight associates are solubilized, the mobility of the both protomers and oligomers of Na,K-ATPase increases, while the difference between the mobilities of sodium and potassium conformers is disappeared.

Influence of intramembrane electric charge on Na,K-ATPase

Effects of lipophilic ions, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-), on interactions of Na+ and K+ with Na,K-ATPase were studied with membrane-bound enzyme from bovine brain, pig kidney, and shark rectal gland. Na+ and K+ interactions with the inward-facing binding sites, monitored by eosin fluorescence and phosphorylation, were not influenced by lipophilic ions. Phosphoenzyme interactions with extracellular cations were evaluated through K(+)-, ADP-, and Na(+)-dependent dephosphorylation. TPP+ decreased: 1) the rate of transition of ADP-insensitive to ADP-sensitive phosphoenzyme, 2) the K+ affinity and the rate coefficient for dephosphorylation of the K-sensitive phosphoenzyme, 3) the Na+ affinity and the rate coefficient for Na(+)-dependent dephosphorylation. Pre-steady state phosphorylation experiments indicate that the subsequent occlusion of extracellular cations was prevented by TPP+. TPB- had opposite effects. Effects of lipophilic ions on the transition between phosphoenzymes were significantly diminished when Na+ was replaced by N-methyl-D-glucamine or Tris+, but were unaffected by the replacement of Cl- by other anions. Lipophilic ions affected Na-ATPase, Na,K-ATPase, and p-nitrophenylphosphatase activities in accordance with their effects on the partial reactions. Effects of lipophilic ions appear to be due to their charge indicating that Na+ and K+ access to their extracellular binding sites is modified by the intramembrane electric field.

Protein kinase C zeta is associated with the mitotic apparatus in primary cell cultures of the shark rectal gland

Protein kinase C zeta (PKC zeta) is an atypical PKC isoform that has recently been implicated in cell division and cell growth. However, there has been no morphologic evidence for the involvement of PKC zeta in mitogenic signal transduction. Here we use immunocytochemistry to demonstrate that PKC zeta co-localizes with microtubules in both interphase and metaphase cells of the shark rectal gland in primary culture. This co-localization is present after non-ionic detergent treatment and is disrupted by nocodazole. During mitosis, PKC zeta is associated with the mitotic apparatus and co-localizes with beta-tubulin in spindle microtubules, while entirely sparing astral microtubules. These findings provide the first evidence that PKC zeta is associated with the mitotic apparatus. The striking presence of PKC zeta in the central portion of the mitotic apparatus suggests a functional role for this kinase isoform in cell division.

[Rotational mobility of membrane-bound Na,K-ATPase]

The rotational mobility of E1 and E2 conformers of duck salt gland Na,K-ATPase labelled with eosine-5'-isothiocyanate (EITC) was studied using a time-resolved phosphorescence anisotropy approach. For each conformer, two types of the rotational mobility were found. The rotational correlation time of the faster component equal to about 15 microseconds at 20 degrees for the both conformers, was ascribed to the rotation of the (alpha beta) protomer with an apparent radius 2.4 nm. The slower component (100-500 microseconds depending on experimental conditions) was suggested to reflect the presence in the bilayer of associates between Na,K-ATPase molecules or those with other protein constituents of the membrane bilayer. A rise in temperature tends to decrease the fast component with a subsequent increase in the slow component of the experimental curve, apparently due to oligomerisation of the protomers into oligomers. The size of the oligomers depends on pH and temperature and under favourable conditions may come up to octamers.

Structural integrity of the membrane domains in extensively trypsinized Na,K-ATPase from shark rectal glands

Removal of extramembranous portions of the integral membrane protein Na,K-ATPase from shark salt glands by trypsin in the presence of Rb+ (a K+ congener) preserves the intramembranous association of the remaining membrane-spanning tryptic peptides. This is evidenced from comparison of the rotational mobility of native and trypsinized Na,K-ATPase using saturation transfer electron spin resonance spectroscopy (ESR) and from study of the lipid-protein interactions using conventional ESR spectroscopy. The interface between the lipids and the intramembranous domains is conserved on removal of the extramembranous parts of the protein, since the population of motionally restricted boundary lipids remains essentially the same in the native and trypsinized preparations. The ability to occlude Rb+ is also retained by the trypsinized membranes, as previously observed with pig kidney Na,K-ATPase. A 19-kDa fragment remaining when Na,K-ATPase is trypsinized in the presence of Rb+ is degraded further when the trypsinization is carried out in the presence of Na+ instead of Rb+. The rotational mobility of the tryptic fragments in the Na(+)-trypsinized membranes is lower than for the Rb(+)-trypsinized membranes, indicating rearrangement of the peptides. In addition, occlusion capacity is lost when trypsinization is carried out in Na+, suggesting a correlation between structure and function in the trypsinized membranes. The sequences of four membrane-spanning tryptic fragments of shark Na,K-ATPase are found to be almost identical to corresponding sequences in pig kidney Na,K-ATPase.

Influence of poly(ethylene glycol) and aqueous viscosity on the rotational diffusion of membranous Na,K-ATPase

The Na,K-ATPase [ATP phosphohydrolase (Na+/K(+)-transporting), E.C. 3.6.1.37] in native membranes from the salt gland of Squalus acanthias has been spin-labeled covalently with a chloromercuri nitroxide derivative, and the rotational diffusion of the protein has been studied, as a function of the concentration of glycerol or poly(ethylene glycol) in the suspending medium, by means of saturation-transfer electron spin resonance spectroscopy. The effective rotational correlation time of the protein increases linearly with the viscosity of the aqueous glycerol medium, with a gradient whose value indicates that ca. 50-70% of the volume of the Na,K-ATPase protein is external to the membrane. The effective rotational correlation times of the protein in poly(ethylene glycol) solutions are considerably greater than those in glycerol solutions of the same viscosity and increase nonlinearly with the viscosity of the suspending medium, indicating that increasing concentrations of poly(ethylene glycol) induce aggregation of the integral proteins within the membrane. The value reached at 50% poly(ethylene glycol) corresponds to a degree of aggregation of the proteins between 2 and 5 depending on whether the ethylene glycol polymer is excluded from the membrane surface region. The results are discussed with respect to hydration forces and poly(ethylene glycol)-induced cell fusion.

Ultrastructure of membrane-bound Na, K-ATPase after extensive tryptic digestion

Membrane-bound Na, K-ATPase was digested with trypsin in the presence of Rb+ to form the stable 19-kDa and smaller fragments of the alpha-chain known to preserve occlusion of Rb+ (K+) or Na+. The trypsinized membranes obtained from pig kidney and shark rectal gland were analyzed by electron microscopy. Tryptic digestion preserved general membrane structure but removed both the surface particles observed by negative staining and the protruding cytoplasmic portion of the alpha-subunit identified in thin sections. However, intramembrane particles defined by freeze-fracture were preserved after trypsinization suggesting that the remaining membrane spanning protein fragments retain the native structure within the lipid bilayer after proteolysis.

[The effect of temperature and pH on ATP hydrolysis of Na,K-ATPase in duck salt glands]

The shape of the substrate curve for duck salt gland Na,K-ATPase during ATP hydrolysis depends on incubation conditions. Under optimal conditions (pH 7.4, 37 degrees C) this curve has an intermediate plateau at 0.7-0.9 mM ATP. Both the acidification of the medium and the decrease in the incubation temperature below 20 degrees C transforms this dependence into a simple hyperbole which is characteristic of the hydrolysis of other substrates (GTP or UTP) under optimal conditions. Recently it has been suggested that the deviation from the Michaelis-Menten kinetics during Na,K-ATPase operation is due to the formation of short-living oligomers in the course of the ATP hydrolyzing cycle. The results obtained are interpreted in terms of possible effects of temperature and pH on the interprotomer interaction in the oligomeric complexes of Na,K-ATPase.

Inhibition of Na,K-ATPase by cadmium: different mechanisms in different species

The mechanism of action of Cd on Na,K-ATPase was investigated in two "classical" model systems, the shark rectal gland and rabbit kidney outer medulla. In lyophilized plasma membranes from dogfish rectal gland Cd inhibited Na,K-ATPase activity after 30 min of preincubation with an I50 of 1.3 x 10(-5) M. K-Dependent p-nitrophenylphosphatase (pNPPase) activity was inhibited 50% by Cd at 9.4 x 10(-6) M. Neither Na nor K altered the interaction of the enzyme with Cd. Decreasing the ATP concentration, however, lowered the apparent sensitivity of Na,K-ATPase to Cd. The inhibitory effect was also significantly reduced when the Mg concentration present during the preincubation was increased from 0.5 to 6.0 mM. The apparent Cd sensitivity of the K-dependent pNPPase is lower at 10 mM Mg than at 1 mM Mg. In initial rate experiments 4 x 10(-5) M Cd increased the apparent Km of the enzyme for Mg significantly from 0.88 +/- 0.29 mM to 1.73 +/- 0.3 mM whereas the Vmax (167 +/- 32 mumol/min x mg protein compared to 140 +/- 16 mumol/min x mg protein) remained essentially unchanged. In lyophilized rabbit kidney outer medulla, Cd was found to inhibit Na,K-ATPase activity with an I50 of 1.9 x 10(-5) M. K-Dependent pNPPase was inhibited 50% under identical conditions by Cd at a nominal concentration of 2.1 x 10(-4) M. Increasing K concentrations protected the enzyme from the inhibitory action of Cd as indicated by a 10-fold decrease in sensitivity of pNPPase when the K concentration was increased from 1 to 20 mM. K, 20 mM, delayed also the onset of inhibition by about 15 min at 37 degrees C. These studies suggest that the mode of action of Cd on Na,K-ATPase varies in different species. In rectal gland Cd competes with a Mg site (or sites) critically involved in ATP and pNPP hydrolysis, whereas in rabbit renal medulla Cd interacts with a potassium-binding site. Differences in the protein sequence, protein conformation, and/or in the kind of protein membrane-lipid interaction might contribute to this diversity observed in the inhibitory mechanisms.

Analysis of thiol-topography in Na,K-ATPase using labelling with different maleimide nitroxide derivatives

Spin-label EPR spectroscopy of shark rectal gland Na,K-ATPase modified at cysteine residues with a variety of maleimide-nitroxide derivatives is used to characterize the different classes of sulphydryl groups. The spin-labelled derivatives vary with respect to charge and lipophilicity, and the chemical reactivity towards modification and inactivation of the Na,K-ATPase is dependent on these properties. Ascorbate is used to reduce the spin-labels in situ, and the kinetics of reduction of the protein-bound spin-labels are found also to depend on the nature of the maleimide-nitroxide derivative. The Na,K-ATPase is labelled either at Class I groups (with retention of enzymatic activity) or at Class II groups (where the enzymatic activity is lost). Although Class I groups are labelled more readily than are Class II groups they are only slightly more susceptible to reduction by ascorbate than the Class II groups, indicating no major difference in environment. The spectral difference observed between immobilized and mobile spin-labels with both Class I and Class II groups labelling is not reflected in widely different reduction kinetics for these two spectral components. Solubilization of the enzyme in an active form does not change the protein structure in terms of increased accessibility of the SH-groups to reduction by ascorbate. The results are discussed in terms of the location of the different SH-groups and the origins of the differences in mobility evident in the EPR spectra of the spin-labelled SH-groups.

Phosphorylation of Na,K-ATPase alpha-subunits in microsomes and in homogenates of Xenopus oocytes resulting from the stimulation of protein kinase A and protein kinase C

The phosphorylation of the alpha-subunit of Na+/K(+)-transporting ATPase (Na,K-ATPase) by cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) was characterized in purified enzyme preparations of Bufo marinus kidney and duck salt gland and in microsomes of Xenopus oocytes. In addition, we have examined cAMP and phorbol esters, which are stimulators of PKA and PKC, respectively, for their ability to provoke the phosphorylation of alpha-subunits of Na,K-ATPase in homogenates of Xenopus oocytes. In the enzyme from the duct salt gland, phosphorylation by PKA and PKC occurs on serine and threonine residues, whereas in the enzyme from B. marinus kidney and Xenopus oocytes, phosphorylation by PKA occurs only on serine residues. Phosphopeptide analysis indicates that a site phosphorylated by PKA resides in a 12-kDa fragment comprising the C terminus of the polypeptide. Studies of phosphorylation performed on homogenates of Xenopus oocytes show that not only endogenous oocyte Na,K-ATPase but also exogenous Xenopus Na,K-ATPase expressed in the oocyte by microinjection of cRNA can be phosphorylated in response to stimulation of oocyte PKA and PKC. In conclusion, these data are consistent with the possibility that the alpha-subunit of Na,K-ATPase can serve as a substrate for PKA and PKC in vivo.

Local translational diffusion rates of membranous Na+,K(+)-ATPase measured by saturation transfer ESR spectroscopy

Diffusion-controlled Heisenberg spin exchange between spin-labeled Na+,K(+)-ATPase [ATP phosphohydrolase (Na+/K(+)-transporting), EC 3.6.1.37] proteins has been studied by saturation transfer ESR spectroscopy in reconstituted membranes. Na+,K(+)-ATPase from the salt gland of Squalus acanthias was solubilized in a polyoxyethylene ether detergent, octa(ethylene glycol) dodecyl monoether. Part of the solubilized enzyme was covalently spin-labeled with a nitroxide derivative of indanedione and recombined with various proportions of the unlabeled enzyme while the native lipid/protein ratio was maintained. Purified membranes were then reconstituted from the various samples by precipitation with divalent ions. The reciprocal integrated intensities of the saturation transfer ESR spectra were found to increase linearly with the fraction of protein that was spin-labeled, and the gradient of the concentration dependence increased with increasing temperature over the range 4 degrees-25 degrees C. Comparison with theoretical analyses of the effects of weak Heisenberg spin exchange [Marsh, D. & Horváth, L. I. (1992) J. Magn. Reson. 97, 13-26] suggests that the effects on the saturation transfer ESR intensity are attributable to short-range diffusional collisions between the spin-labeled protein molecules. The effective value of the local translational diffusion coefficient is 1.8-2.9 microns2.s-1 at 15 degrees C, depending on the diffusion model used, which is much larger than the values obtained for the long-range diffusion coefficient in cells by photobleaching techniques. The temperature dependence of the translational diffusion is larger than expected but correlates with the anomalous temperature dependence of the rotational diffusion observed in the same system.

Properties of oligomycin-induced occlusion of Na+ by detergent-solubilized Na,K-ATPase from pig kidney or shark rectal gland

Oligomycin induces occlusion of Na+ in membrane-bound Na,K-ATPase. Here it is shown that Na,K-ATPase from pig kidney or shark rectal gland solubilized in the nonionic detergent C12E8 is capable of occluding Na+ in the presence of oligomycin. The apparent affinity for Na+ is reduced for both enzymes upon solubilization, and there is an increase in the sigmoidicity of binding curves, which indicates a change in the cooperativity between the occluded ions. A high detergent/protein ratio leads to a decreased occlusion capacity. De-occlusion of Na+ by addition of K+ is slow for solubilized Na,K-ATPase, with a rate constant of about 0.1 s-1 at 6 degrees C. Stopped-flow fluorescence experiments with 6-carboxyeosin, which can be used to monitor the E1Na-form in detergent solution, show that the K(+)-induced de-occlusion of Na+ correlates well with the fluorescence decrease which follows the transition from the E1Na-form to the E2-form. There is a marked increase in the rate of fluorescence change at high detergent/protein ratios, indicating that the properties of solubilized enzyme are subject to modification by detergent in other respects than mere solubilization of the membrane-bound enzyme. The temperature dependence of the rate of de-occlusion in the range 2 degrees C to 12 degrees C is changed slightly upon solubilization, with activation energies in the range 20-23 kcal/mol for membrane-bound enzyme, increasing to 26-30 kcal/mol for solubilized enzyme. Titrations of the rate of transition from E1Na to E2K with oligomycin can be interpreted in a model with oligomycin having an apparent dissociation constant of about 2.5 microM for C12E8-solubilized shark Na,K-ATPase and 0.2 microM for solubilized pig kidney Na,K-ATPase.

Purification and localization of brain-type creatine kinase in sodium chloride transporting epithelia of the spiny dogfish, Squalus acanthias

The targeting of creatine kinase isoenzymes to specific sites within muscle cells provides a system for the regeneration of ATP in situ from ADP and creatine phosphate. We have recently reported the colocalization of brain-type (B) creatine kinase and the nonsarcomeric mitochondrial creatine kinase isoenzymes in the thick ascending limb of the loop of Henle in the rat kidney, suggesting that creatine kinase may regenerate ATP for sodium transport (Friedman, D.L., and Perryman, M.B. (1991) J. Biol. Chem. 266, 22404-22410). In order to test the hypothesis regarding the association of B creatine kinase with sodium transport, we examined the creatine kinase enzymes in the rectal (salt-secreting) gland of the dogfish shark which contains high levels of the Na+/K(+)-ATPase. The creatine kinase isoform composition was determined by non-denaturing electrophoresis, immunoblotting, protein purification, and amino acid sequence analysis. The results demonstrate both B creatine kinase and mitochondrial creatine kinase proteins are present in the rectal gland, an isoform composition which is the same as in the mammalian kidney. By using a combination of chromatographic techniques, shark B creatine kinase was purified to homogeneity and partial sequence data was obtained from two cyanogen bromide peptide fragments. One of these fragments contains the active site and is identical at all sequenced residues with the corresponding region from the echinoderm sperm flagellar creatine kinase, and is 96% homologous with both chicken and rat B creatine kinase subunits. The other fragment corresponds to a region near the N-terminal of mammalian creatine kinases and is 89% homologous with B creatine kinase from chicken. The localization of these isoforms was examined by immunocytochemistry using subunit specific antisera. Mitochondrial creatine kinase and B creatine kinase immunoreactivity are detected in all tubules, and is restricted to the basal region of the cells, which is the site of the Na+/K(+)-ATPase. The conservation of creatine kinase isoform expression in excretory tissue, and the localization of creatine kinase immunoreactivity in the basal region of the tubule cells, demonstrate that subcellular compartmentation of B creatine kinase may underly the functional coupling of creatine kinase activity with sodium transport.

Comparison of the kinetic properties of membrane-bound and solubilized Na,K-ATPase

Substrate-velocity curve for Na,K-ATPase under optimal conditions is described as a curve with intermediary plateau. C12E2 treatment of the enzyme changes its kinetic behaviour. The substrate-velocity curve transforms into hyperbolic one and the Km value for the solubilized enzyme approaches the Km value for the first phase of the complex curve. The experimental substrate-velocity curves obtained for Na,K-ATPase under different conditions were analyzed on the basis of the sum of Michaelis and Hill equations and the kinetic scheme for the enzyme was proposed. This model suggests that at the definite step of the reaction cycle the short-living oligomer is formed which can bind ATP with higher affinity thus accelerating E2----E1 transition. Several additional experimental facts that prove the hypothesis are presented.

Kinetics of phosphorylation of Na+/K(+)-ATPase by protein kinase C

The kinetics of phosphorylation of an integral membrane enzyme, Na+/K(+)-ATPase, by calcium- and phospholipid-dependent protein kinase C (PKC) were characterized in vitro. The phosphorylation by PKC occurred on the catalytic alpha-subunit of Na+/K(+)-ATPase in preparations of purified enzyme from dog kidney and duck salt-gland and in preparations of duck salt-gland microsomes. The phosphorylation required calcium (Ka approximately 1.0 microM) and was stimulated by tumor-promoting phorbol ester (12-O-tetradecanoylphorbol 13-acetate) in the presence of a low concentration of calcium (0.1 microM). PKC phosphorylation of Na+/K(+)-ATPase was rapid and plateaued within 30 min. The apparent Km of PKC for Na+/K(+)-ATPase as a substrate was 0.5 microM for dog kidney enzyme and 0.3 microM for duck salt-gland enzyme. Apparent substrate inhibition of PKC activity was observed at concentrations of purified salt-gland Na+/K(+)-ATPase greater than 1.0 microM. Phosphorylation of purified kidney and salt-gland Na+/K+ ATPases occurred at both serine and threonine residues. The 32P-phosphopeptide pattern on 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis after hydroxylamine cleavage of pure 32P-phosphorylated alpha subunit was the same for the two sources of enzyme, which suggests that the phosphorylation sites are similar. The results indicate that Na+/K(+)-ATPase may serve as a substrate for PKC phosphorylation in intact cells and that the Na+/K(+)-ATPase could be a useful in vitro model substrate for PKC interaction with integral membrane proteins.