ATPase and phosphatase activity of Na+,K+-ATPase: molar and specific activity, protein determination

Specific binding of L-triiodothyronine modulates Na(+)-K(+)-ATPase activity in adult rat cerebrocortical synaptosomes

Interactions of L-triiodothyronine (T3) in adult rat cerebrocortical synaptosomes were studied in vitro. Scatchard plot analysis revealed two sets of T3 binding sites. The degree of saturation of T3 binding sites (putative receptor) correlated well with the dose-dependent inhibition of Na(+)-K(+)-ATPase activity in synaptosomes. The relative binding affinities and relative inhibition of enzyme activities for different TH analogues were L-T3 > T3-amine > TRIAC = L-T4 > r-T3 > T2 and L-T3 > T3-amine > TRIAC > L-T4 > r-T3 > T2, respectively. The present study demonstrates the nature of inhibition of synaptosomal Na(+)-K(+)-ATPase activity may be as a function of T3 occupancy of synaptosomal receptor sites in adult mammalian brain.

Sodium, potassium-activated adenosine triphosphatase activity is impaired in the guinea pig pancreatic duct system in streptozotocin-induced diabetes

In patients with type I diabetes mellitus, clinical studies have demonstrated decreased secretion of pancreatic juice by the pancreatic excretory duct system. The cause of this decrease is unknown, but could involve changes in initial signal transduction pathways or one or more of the electrolyte transport components that subserve regulated fluid secretion. We have compared responsiveness to secretin in pancreatic ducts isolated from healthy and diabetic Hartley guinea pigs and also have compared the expression of CFTR and Na+, K(+)-ATPase in these two groups, as the activities of these two proteins are essential for secretion of pancreatic juice. The increases in cyclic AMP levels evoked by exposure to either 0.1 nM or 0.1 microM secretin were not significantly different in pancreatic ducts isolated from healthy and diabetic guinea pigs nor were levels of CFTR or Na+, K(+)-ATPase expression. By contrast, Na+, K(+)-ATPase activity in pancreatic ducts isolated from diabetic guinea pigs was decreased by 70%, suggesting a change in the enzyme's catalytic properties in the diabetic tissues. The observed decrease would be expected to seriously compromise the production of pancreatic juice.

Na(+)-K(+)-ATPase in frog esophagus mucociliary cell membranes: inhibition by protein kinase C activation

We examined protein kinase C (PKC)-dependent regulation of Na(+)-K(+)-ATPase in frog mucociliary cells. Activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) or 1,2-dioctanoyl-sn-glycerol (diC8) either in intact cells or isolated membranes resulted in a specific inhibition of Na(+)-K(+)-ATPase activity by approximately 25-45%. The inhibitory effects in membranes exhibited time dependence and dose dependence [half-maximal inhibition concentration (IC50) = 0.5 +/- 0.1 nM and 2.4 +/- 0.2 microM, respectively, for TPA and diC8] and were not influenced by Ca2+. Analysis of the ouabain inhibition pattern revealed the presence of two Na(+)-K(+)-ATPase isoforms with IC50 values for cardiac glycoside of 2.6 +/- 0.8 nM and 409 +/- 65 nM, respectively. Most importantly, the isoform possessing a higher affinity for ouabain was almost completely inhibited by TPA, whereas its counterpart was hardly sensitive to the PKC activator. The results suggest that, in frog mucociliary cells, PKC regulates Na(+)-K(+)-ATPase and that this action is related to the specific Na(+)-K(+)-ATPase isoform.

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.

Modifications induced by insulin-dependent diabetes mellitus on human placental Na+/K+-adenosine triphosphatase

The causes of the reduced activity of Na+/K+-adenosine triphosphatase (ATPase) in human diabetes are still the object of controversy. The aim of this work was to investigate the mechanisms of inhibition by means of the study of the Na+/K+-ATPase purified from human placenta. We purified Na+/K+-ATPase from term placentas of six healthy women and six age-matched women with insulin-dependent diabetes mellitus (IDDM) in good metabolic control. The enzymatic activity was reduced in both the microsomal fraction and the purified Na+/K+-ATPase obtained from diabetic women, whereas no difference was found in the number of active molecules determined by anthroyl ouabain binding. The Na+/K+-ATPase purified from women with IDDM did not show any modification in the ouabain affinity or changes in the physicochemical structure of the ouabain binding site investigated by dynamic fluorescence or alterations in lateral diffusion. The activation energy of the enzyme was increased, whereas the tryptophan accessibility of the enzyme was lower in women with IDDM. The fluidity of the lipid anulus of the enzyme was higher in women with IDDM than in control women, as suggested by fluorescence polarization of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5-hexatriene. The adenosine triphosphate-binding site, investigated by anisotropy decay studies of the fluorescent probe pyrene isothiocyanate, was modified in women with IDDM. It appears that the Na+/K+-ATPase of human placenta is altered in its disposition in IDDM.

Effect of high ionic strength and inhibitors of H,K-ATPase on the ouabain-sensitive K-p-nitrophenylphosphatase activity in the sea anemone Stichodactyla helianthus

The ouabain-sensitive, K-stimulated p-nitrophenyl phosphatase (K-pNPPase) activity, an associated activity of the Na,K-ATPase, was assayed in tentacles of the sea anemone Stichodactyla helianthus to investigate the possibility that the sea anemone Na,K-ATPase activity is an associated activity of an H,K-ATPase. Activity was maximal at pH 6.5-7.0, decreasing only slightly in acidic medium but falling abruptly in alkaline medium to 60% of maximum at pH 7.4. The pH of maximum activity was not remarkably altered in high ionic strength medium (560 mM choline chloride), but ouabain-sensitive K-pNPPase activity of both rat and sea anemone was strongly inhibited. Inhibitors of the gastric H,K-ATPase, 100 microM omeprazole and 10 microM SCH 28080, did not inhibit the ouabain-sensitive K-pNPPase activity. Activity of the sea anemone enzyme was inhibited by 10 microM ammonium vanadate, an inhibitor of P-type ATPase, and not by 2.5 mM sodium azide, an inhibitor of both F-type and V-type ATPase. Because the sea anemone K-pNPPase activity was previously found to be more sensitive to ouabain than the Na,K-ATPase activity, K(+)-ouabain antagonism was investigated and found to be relatively muted, whereas K(+)-Na+ competition was stronger than in the rat kidney.

The effect of ionic strength and specific anions on substrate binding and hydrolytic activities of Na,K-ATPase

The physiological ligands for Na,K-ATPase (the Na,K-pump) are ions, and electrostatic forces, that could be revealed by their ionic strength dependence, are therefore expected to be important for their reaction with the enzyme. We found that the affinities for ADP3-, eosine2-, p-nitrophenylphosphate, and V(max) for Na,K-ATPase and K+-activated p-nitrophenylphosphatase activity, were all decreased by increasing salt concentration and by specific anions. Equilibrium binding of ADP was measured at 0-0.5 M of NaCl, Na2SO4, and NaNO3 and in 0.1 M Na-acetate, NaSCN, and NaClO4. The apparent affinity for ADP decreased up to 30 times. At equal ionic strength, I, the ranking of the salt effect was NaCl approximately Na2SO4 approximately Na-acetate < NaNO3 < NaSCN < NaCl04. We treated the influence of NaCl and Na2SO4 on K(diss) for E x ADP as a "pure" ionic strength effect. It is quantitatively simulated by a model where the binding site and ADP are point charges, and where their activity coefficients are related to I by the limiting law of Debye and Hückel. The estimated net charge at the binding site of the enzyme was about +1. Eosin binding followed the same model. The NO3- effect was compatible with competitive binding of NO3- and ADP in addition to the general I-effect. K(diss) for E x NO3 was approximately 32 mM. Analysis of V(max)/K(m) for Na,K-ATPase and K+-p-nitrophenylphosphatase activity shows that electrostatic forces are important for the binding of p-nitrophenylphosphate but not for the catalytic effect of ATP on the low affinity site. The net charge at the p-nitrophenylphosphate-binding site was also about +1. The results reported here indicate that the reversible interactions between ions and Na,K-ATPase can be grouped according to either simple Debye-Hückel behavior or to specific anion or cation interactions with the enzyme.

Modifications induced by plasma of gestational hypertensive women on the Na+/K+-ATPase obtained from human placenta

In order to investigate the molecular mechanisms of the inhibition of Na+/K+-ATPase in Gestational Hypertension (GH), we incubated Na+/K+-ATPase purified from human placenta of 6 healthy normotensive women with plasma from 6 GH women and 6 healthy controls. We determined the enzyme activity by the method of Esman, and the anthroyl-ouabain-binding capacity, dissociation constant (Kd) and average lifetime values (tau) by the static and dynamic fluorescence of anthroyl-ouabain. The lipid annulus of the enzyme was studied by static and dynamic fluorescence of 1-(4-trimethylaminophenyl)-6-phenyl-1,3,5- hexatriene (TMA-DPH). The addition of total and protein-free GH plasma to normal Na+/K+-ATPase significantly inhibited the enzymatic activity even at the lowest concentration studied (1:100), as well as the ouabain-binding capacity, Kd and tau. GH plasma significantly decreased the fluorescence polarization and lifetime values of TMA-DPH. These observations indicate that the inhibition caused by GH plasma on Na+/K+-ATPase might be due to a reduction of the number of active molecules or a modification of the ouabain-binding site suggesting the existence of digitalis-like factor. A link between the modification of the lipid moiety of the enzyme and the Na+/K+-ATPase inhibition might be hypothesized.

Red blood cell ghosts are affected by adrenoleucodystrophy

X-linked adrenoleucodystrophy is a disorder occurring in different clinical forms, characterized by adrenal, gonadal and nervous system dysfunction. The basis of the illness is a derangement of the peroxisomal system necessary to oxidize very long-chain fatty acids that accumulate in various tissues. The diagnosis relies on clinical signs and symptoms and on biochemical findings. The six reported cases presented idiopathic adrenal insufficiency. We measured the lipid composition of red blood cell (RBC) ghosts of patients and control subjects. The distribution of phosphorus among phospholipid classes was unaffected; we could not demonstrate any differences between the fatty acid patterns of RBC membrane, either in total lipid extracts or in separated lipid classes. However, we found an increase in total lipid (both phospholipid and cholesterol), in membrane viscosity and in the Na+/K(+)-dependent ATPase. Therefore, we report four main findings on ghosts in adrenoleucodystrophy patients: (a) very long-chain fatty acids do not accumulate; (b) the lipid-protein ratio increases; (c) fluidity decreases; and (d) the activity of ATPase increases. The last finding is proposed as a possible biochemical marker of the illness. We conclude that adrenoleucodystrophy affects deeply RBC membranes.

Phosphorylation by protein kinase C of serine-23 of the alpha-1 subunit of rat Na+,K(+)-ATPase affects its conformational equilibrium

Phosphorylation of the alpha-1 subunit of rat Na+,K(+)-ATPase by protein kinase C has been shown previously to decrease the activity of the enzyme in vitro. We have now undertaken an investigation of the mechanism by which this inhibition occurs. Analysis of the phosphorylation of recombinant glutathione S-transferase fusion proteins containing putative cytoplasmic domains of the protein, site-directed mutagenesis, and two-dimensional peptide mapping indicated that protein kinase C phosphorylated the alpha-1 subunit of the rat Na+,K(+)-ATPase within the extreme NH2-terminal domain, on serine-23. The phosphorylation of this residue resulted in a shift in the equilibrium toward the E1 form, as measured by eosin fluorescence studies, and this was associated with a decrease in the apparent K+ affinity of the enzyme, as measured by ATPase activity assays. The rate of transition from E2 to E1 was apparently unaffected by phosphorylation by protein kinase C. These results, together with previous studies that examined the effects of tryptic digestion of Na+,K(+)-ATPase, suggest that the NH2-terminal domain of the alpha-1 subunit, including serine-23, is involved in regulating the activity of the enzyme.

Changes in Na,K-adenosine triphosphatase (ATPase) concentration and Na,K-ATPase-dependent adenosine triphosphate turnover in human erythrocytes in diabetes

The concentration of Na,K-adenosine triphosphatase (ATPase) and Na,K-ATPase-dependent adenosine triphosphate (ATP) turnover was measured in fasting blood samples of 20 subjects with insulin-dependent diabetes mellitus (IDDM), 22 subjects with non-insulin-dependent diabetes mellitus (NIDDM), and 20 nondiabetic subjects. [3H]ouabain binding was used to determine Na,K-ATPase concentration. There were 471 +/- 70 (mean +/- SD) ouabain binding sites per erythrocyte, normally distributed in the nondiabetic subjects. The number of ouabain sites per cell was lognormally distributed in the two populations of diabetic subjects. The mean of lognormal distributions of ouabain sites per cell was significantly lower in the IDDM group. The mean of the lognormal distribution for the NIDDM group was not significantly different from that of the nondiabetic subjects. Na,K-ATPase-dependent ATP turnover (molar activity) was 9,580 +/- 742 mol/mol minute (mean +/- SD) normally distributed in the nondiabetic population. A lognormal distribution was observed in the diabetic population. Means of the lognormal distributions were significantly different: 3.98 +/- 0.05 for the nondiabetic population and 3.13 +/- 0.48 for both diabetic populations. Changes in the concentration of Na,K-ATPase (ouabain sites per cell) and Na,K-ATPase-dependent ATP turnover did not correlate with hemoglobin A1C (HbA1C) or with blood glucose. This would suggest that elevated glucose concentrations do not directly cause decreased Na,K-ATPase function in the diabetic erythrocyte.

Evidence against a regulation of Na+/K(+)-ATPase by Gi proteins. Failure to detect an influence of G proteins on Na+/Ca(2+)-exchange in cardiac sarcolemmal membranes

In the myocardium the inhibitory guanine nucleotide-binding regulatory proteins (Gi proteins) mediate negative chronotropic and negative inotropic effects by activation of K+ channels and inhibition of adenylyl cyclase. The concept of a uniform inhibitory action of Gi proteins on myocardial cellular activity has been questioned by the recent observations of adenosine-induced activation of the Na+/Ca(2+) exchange and a carbachol-induced inhibition of the Na+/K(+)-ATPase activity in cardiac sarcolemmal membranes. The aim of the present study, therefore, was to reinvestigate the putative regulation of Na+/Ca(2+) exchange and Na+/K(+)-ATPase activity in purified canine sarcolemmal membranes. These membranes were enriched in adenosine A1 (Maximum number of receptors, Bmax 0.033 pmol/mg) and muscarinic M2 (Bmax 2.9 pmol/mg) receptors and contained Gi2 and Gi3, two Gi protein isoforms, and G0, another pertussis toxin-sensitive G protein, as detected with specific antibodies. The adenosine A1-selective agonist, (-)-N6-(2-phenylisopropyl)-adenosine, and the muscarinic agonist, carbachol, both inhibited isoprenaline-stimulated adenylyl cyclase activity by 25% and 35% respectively, and the stable GTP analogue 5'-guanylylimidodiphosphate inhibited forskolin-stimulated adenylyl cyclase activity by 35% in these membranes. The characteristics of Na+/Ca(2+) exchange and Na+/K(+)-ATPase activity as well as those of the ouabain-sensitive, K(+)-activated 4-nitrophenylphosphatase, an ATP-independent, partial reaction of the Na+/K(+)-ATPase, were in agreement with published data with regard to specific activity, time course of activity and substrate dependency. However, none of these activities were influenced by adenosine, (-)-N6-(2-phenylisopropyl)-adenosine, carbachol, or stable GTP analogs, suggesting that Na+/Ca(2+) exchange and Na+/K(+)-ATPase are not regulated by Gi proteins in canine cardiac sarcolemmal membranes.

Isocaloric glutamine-free diet and the morphology and function of rat small intestine

BACKGROUND

The importance of L-glutamine as metabolic fuel for enterocytes and its role in prevention of mucosal atrophy during total parenteral nutrition is well documented. No data are available to date that document whether a glutamine-free complete enteral diet, requiring full energy expenditure for hydrolysis and absorption, is associated with changes in the morphology and function of the small intestine. Our aim was to examine the effect of such a diet during a 4-week period on the morphology and function of the small intestine of rats.

METHODS

Three isocaloric solid rat food, containing 0%, 4%, and 8% of glutamate, respectively, were fed to three groups of rats. On the 7th and 28th days the morphology of the jejunum, the subcellular structure of enterocytes on transmission electron microscopy, enzyme activities, blood, and muscle glutamine were examined and compared in the three groups.

RESULTS

The rats on the glutamine-free diet had significantly lower mucosal wet weight, protein and DNA content, and number of intraepithelial lymphocytes on the 7th day, whereas the number of mitoses in the Lieberkuhn's crypts was significantly less on the 28th day. The height of the enterocytes and villi was 20% higher on average in the glutamine-free group. Electron microscopy revealed either early (swelling of cristae) or terminal (swelling of matrix) mitochondrial degenerative changes, homogenization of apical cytoplasm, and degeneration and fragmentation of microvilli with loss of their rootlets. The Na+, K(+)-ATPase activity was markedly decreased in the glutamine-free group compared with that of the other groups, most likely because of a diminished energy supply. Among brush border membrane enzymes, lactase activity decreased markedly (p < .05) in the first week. The glutamine-free diet resulted in an increase of the lung glutamine synthetase activity and decrease in muscle glutamine content by the 28th day of the diet.

CONCLUSIONS

Our study shows for the first time that a complete enteral diet, deficient only in glutamine, is associated with significant early morphologic and functional changes in the small intestine. The precise effect on intracellular events and the time of onset of these changes needs to be clarified in the future.

Purification and characterization of a plasma membrane ferricyanide-utilizing NADH dehydrogenase from Ehrlich tumour cells

A ferricyanide-utilizing NADH dehydrogenase (NADH-ferricyanide oxidoreductase) from the plasma membrane of Ehrlich ascites tumour cells has been purified about 1500-fold to apparent homogeneity. The method comprises the isolation of an enriched plasma membrane fraction, solubilization with Triton X-100, ion-exchange chromatography, ammonium sulphate precipitation, Cibacron Blue chromatography and fast-protein liquid chromatography with a Superose-6 gel filtration column. The specific activity of the final pool was more than 61 units/mg protein. The pure enzyme examined by SDS/PAGE displayed only one type of subunit with an apparent molecular mass of 32.0 kDa. The molecular mass of the native protein (117.0 kDa) was estimated by gel filtration; these results suggest a protein composed of four subunits of identical molecular mass. The enzyme was stable in the pH interval between 6 and 9, with maximum activity at pH values from 7.5 to 8.5. The purified enzyme showed Michaelis-Menten kinetics for the substrates, with apparent K(m) values of 4.3 X 10(-5) M and 6.7 X 10(-5) M for NADH and ferricyanide respectively. The isolated protein was strongly inhibited by Zn2+ and the thio-specific reagents mersalyl and p-chloromercuribenzenesulphonic acid.

Quantification of the Na+/K(+)-pump in solubilized tissue by the ouabain binding method coupled with high-performance gel chromatography

Membrane-bound Na+/K(+)-ATPase purified from dog kidney outer medulla was solubilized with octaethylene glycol n-dodecyl ether (C12E8) and incubated with [3H]ouabain in the presence of NaCl. ATP and MgCl2 for 10 min at 0 degrees C. The resulting enzyme was separated, by high-performance gel chromatography executed at 0.2 degrees C. Mainly into its (alpha beta)2-diprotomer and alpha beta-protomer, which both bound stoichiometrically to [3H]ouabain. The amounts of ouabain that bound to the tissue itself and its microsomes could be estimated in the same way, as [3H]ouabain was found to bind only to the diprotomer and protomer they possessed. The amounts of ouabain that bound to them in the solubilized state were at least 5-times higher than those that did so when they were non-solubilized, suggesting that the surfactant rendered the enzyme accessible to ouabain. When the solubilized tissue (138 mg ml-1 wet tissue) was reacted with ouabain in the presence of 0.1 M NaCl and 4.8 mM MgCl2 for 10 min at 0 degrees C, maximal ouabain binding was attained in the presence of 18.3 microM [3H]ouabain, 1.2 mM ATP and 3 to 5 mg ml-1 C12E8, which was common to the outer medulla and human colon cancer cells. The present method enabled the pump number in protein and tissue samples in the range 7.2 x 10(-9) (purified pump) to 1.5 x 10(-12) (cancer tissue) mol/mg protein to be estimated within 2 h.

Steroidal saponins from Allium chinense and their inhibitory activities on cyclic AMP phosphodiesterase and Na+/K+ ATPase

The saponin fraction prepared from the methanolic extract of Allium chinense bulbs exhibited inhibitory activities on cyclic AMP phosphodiesterase (cAMP PDE) (43.5%) and Na+/K+ ATPase (59.3%) at a sample concentration of 100 micrograms ml-1, respectively. Attempted purification of the active fraction through column chromatography on silica gel and ODS silica gel resulted in the isolation of six steroidal saponins, one of which appeared to be a new compound and one to be the first isolation from a natural source. (25R,S)-5 alpha-Spirostan-3 beta-ol tetrasaccharide showed inhibitory activities on both cAMP PDE and Na+/K+ ATPase, while (25R)-3 beta-hydroxy-5 alpha-spirostan-6-one di- and tri-saccharides inhibited only cAMP PDE.

Hormonal regulation of the Na(+)-K(+)-ATPase: mechanisms underlying rapid and sustained changes in pump activity

The sodium-potassium-activated adenosinetriphosphatase (Na(+)-K(+)-ATPase; Na(+)-K+ pump) is a ubiquitous plasma membrane enzyme that catalyzes the movement of K+ into cells in exchange for Na+. In addition, it provides the driving force for the transport of other solutes, notably amino acids, sugar, and phosphate. The regulation of Na(+)-K(+)-ATPase in various tissues is under the control of a number of circulating hormones that impart both short- and long-term control over its activity. The molecular mechanisms by which hormones alter Na(+)-K(+)-ATPase activity have only begun to be studied. In this review, we assess the acute and long-term actions of a number of hormones (aldosterone, thyroid hormone, catecholamines, insulin, carbachol) on the Na(+)-K+ pump. The long-term regulation exerted by thyroid hormone and aldosterone is mediated by changes in gene expression. The short-term regulation exerted by catecholamines is mediated by reversible phosphorylation of the pump catalytic subunit. Recent evidence supports regulation of the pump by phosphorylation in vitro and in intact cells. Finally, in some tissues the rapid action of insulin, aldosterone, and carbachol involves changes in the subcellular distribution of pump units.

Characterization of Syrian hamster gastric mucosal H+,K+-ATPase

Employing a simple one-step sucrose gradient fractionation method, gastric mucosal membrane of Syrian hamster was prepared and demonstrated to be specifically enriched in H+,K+-ATPase activity. The preparation is practically devoid of other ATP hydrolyzing activity and contains high K+-stimulated ATPase activity of at least 4-5 fold compared to basal ATPase activity. The H+,K+-ATPase showed hydroxylamine-sensitive phosphorylation and K+-dependent dephosphorylation of the phosphoenzyme, characteristic inhibition by vanadate, omeprazole and SCH 28080, and nigericin-reversible K+-dependent H+-transport--properties characteristic of gastric proton pump. One notable difference with H+,K+-ATPase of other species has been the observation of valinomycin-independent H+ transport in such membrane vesicles. It is proposed that such H+,K+-ATPase-rich hamster gastric mucosal membrane preparation might provide a unique model to study physiological aspects of H+,K+-ATPase function in relation to HCl secretion.

Glutamine transport by vesicles isolated from tumour-cell mitochondrial inner membrane

Mitochondrial-inner-membrane vesicles, isolated from Ehrlich ascites carcinoma cells by titration with detergents, accumulated L-glutamine by a very efficient transport system. The vesicles lack any phosphate-activated glutaminase activity, allowing measurement of transport rates without interference by L-glutamine metabolism. The time course of the transport was linear for the first 60 s, reaching a steady state after 120 min. L-Glutamine transport showed co-operativity, with a Hill coefficient of 2.2; the kinetic parameters S0.5 and Vmax had values of 5 mM and 26 nmol/30 s per mg of protein respectively. The pH-dependence curve showed a bell shape, with a pH optimum about 8.0. The uptake of L-glutamine was not affected by the presence of a 50-fold molar excess of D-glutamine, L-cysteine, L-histidine, L-alanine, L-serine and L-leucine, whereas L-glutamate behaved as a poor inhibitor. The structural analogue L-glutamate gamma-hydroxamate (5mM) inhibited the net uptake by 68%; interestingly, other analogues (6-diazo-5-oxo-L-norleucine, acivicin and L-glutamate gamma-hydrazide) were ineffective. The impermeant thiol reagent p-chloromercuriphenylsulphonic acid (0.5mM) completely abolished the mitochondrial L-glutamine uptake; in contrast, other thiol reagents (mersalyl and N-ethylmaleimide) did not significantly affect the transport. These data confirm the existence of a specific transport system with high capacity for L-glutamine in the mitochondrial inner membrane, a step preceding the highly operative glutaminolysis in tumour cells.

Effect of rejection on electrophysiologic function of canine intestinal grafts: correlation with histopathology and Na-K-ATPase activity

To investigate whether electrophysiologic changes can detect the early onset and progress of intestinal rejection, changes in in vitro electrophysiologic function, intestinal histopathology, and Na-K-ATPase activity were studied in dogs. Adult mongrel dogs of both sexes, weighing 18-24 kg, were used for auto and allo small bowel transplantation. The entire small bowels, except for short segments at the proximal and distal ends, were switched between a pair of dogs (allograft). Animals receiving intestinal autotransplantation were used as controls. Allograft recipients were sacrificed 3, 4, 5, 7, or 9 days after transplantation, and autograft recipients were sacrificed 3, 7, or 14 days after transplantation. Immunosuppression was not used. Electrophysiologic measurements were done with an Ussing chamber. Histological analysis was performed blindly using whole thickness sections. Na-K-ATPase activity in the mucosal tissue, which is said to regulate the potential difference, was also measured. Potential difference, resistance, and Na-K-ATPase activity of the allograft intestine decreased with time and were significantly lower 7 and 9 days after transplantation compared to host intestine, normal intestine, and graft intestine of controls (autograft). Potential difference, resistance, and Na-K-ATPase activity of the native intestinal tissue and the autografts did not decrease with time. Detection of histologically mild rejection of the intestine, which is important for appropriate immunosuppressive treatment in clinical cases, could not be achieved based on electrophysiology or Na-K-ATPase activity. Deterioration of electrophysiologic function during rejection correlated with the histological rejection process and Na-K-ATPase activity; however, electrophysiology may not be a reliable tool for monitoring grafts, since it cannot detect early intestinal rejection.

Subunit requirements for expression of functional sodium pumps in yeast cells

Na+/K(+)-ATPase from animal cell membranes is known to consist of an alpha-subunit and a beta-subunit. Amino acids within the alpha-subunit have been shown to participate in the catalytic functions of the enzyme and in the binding of cardioactive steroids. Although the function of the beta-subunit is not known, expression of both alpha- and beta-subunits is required for the functional enzyme. A putative third subunit, the gamma-subunit, has been suggested to be a part of the functional Na+/K(+)-ATPase complex, based on experiments showing that both the catalytic alpha-subunit and a small peptide of M(r) = 11,000 can be labeled by a photoreactive ouabain analog. Although the primary structure for the putative gamma-subunit from rat and sheep was recently deduced from cDNA clones, participation of this small protein in the catalytic activity of the Na+/K(+)-ATPase has not been demonstrated. In experiments described here, the heterologous expression of Na+/K(+)-ATPase in yeast cells was used to investigate whether the gamma-subunit is an essential component of the Na+/K(+)-ATPase. Yeast cells do not contain an endogenous Na+/K(+)-ATPase. The alpha- and beta-subunits or the alpha-, beta- and the putative gamma-subunits of Na+/K(+)-ATPase were expressed in the yeast Saccharomyces cerevisiae and ouabain-sensitive ATPase, p-nitrophenylphosphatase, and 86Rb uptake activities were measured either in membranes prepared from transformed yeast cells, or in intact yeast cells. Nontransformed yeast cells or yeast cells transformed with the gamma-subunit alone served as controls. Northern analysis and Western blots demonstrated that yeast cells do not contain an endogenous peptide with significant sequence homology to the putative gamma-subunit. Yeast samples containing only Na+/K(+)-ATPase alpha and beta subunits were capable of ouabain-inhibitable enzymatic activity and 86Rb transport. No gamma-subunit-dependent differences in the measured enzymatic activities or transport properties were detected in the different samples. These observations establish that the alpha beta-subunit complex is the minimum structural unit required for all the ouabain-sensitive reactions of Na+/K(+)-ATPase.

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.

Short-term effect of epidermal growth factor (EGF) on sodium and glucose cotransport of isolated jejunal epithelial cells

This study was undertaken to assess the short-term effects of EGF on sodium and glucose uptake, glucose metabolism and Na+/K(+)-ATPase activity in isolated enterocytes of rats. Jejunal cells exposed to EGF had a significantly greater total uptake of sodium compared to controls after 6 min. Kinetic analysis of glucose transport across BBMV's demonstrated similar Km values but a significant increase of the Vmax in vesicles prepared from cells first exposed to EGF as compared to controls. EGF was also associated with a significant increase in glucose metabolism of jejunal enterocytes after 15 min. The activity of Na+/K(+)-ATPase increased in jejunal enterocytes exposed to EGF. The increase in Na+/K(+)-ATPase activity of the cells following EGF exposure was not accompanied by an increase in immunodetectable total or assembled Na+/K(+)-ATPase protein. EGF's effect on enzyme activity was abolished by removing NaCl from the incubation solution, and by preincubating the enterocytes with phlorizin prior to addition of EGF. Preincubation with amiloride did not inhibit the effect of EGF on Na+/K(+)-ATPase. The results confirm that EGF promotes uptake of both sodium and glucose by the jejunal mucosal cells, and suggest the effect of EGF on glucose and sodium is mediated through the brush-border membrane glucose-sodium transporter. The increase in Na+/K(+)-ATPase activity that occurs with EGF appears to be secondary to a rise in intracellular Na+ concentration. The short-term effects of EGF on glucose and sodium transport by the small intestine may have potential therapeutic implications.

Cytoprotection against merocyanine 540-sensitized photoinactivation of the Na+,K(+)-adenosine triphosphatase in leukemia cells: glutathione and selenoperoxidase involvement

When irradiated with broad-band visible light in the presence of merocyanine 540 (MC540), murine leukemia L1210 cells grown under selenium-deficient conditions (Se(-) cells) accumulated lipid hydroperoxides and lost viability more rapidly than selenium-satisfied (Se(+) cells). These findings suggest that cytoprotection against photoperoxidation and photokilling is mediated at least in part by selenoperoxidase (SePX) action. Similar protection against photoinactivation of an intrinsic membrane enzyme, the Na+,K(+)-ATPase, has been observed. Thus, irradiation of MC540-sensitized Se(-) cells resulted in an immediate and progressive inactivation of ouabain-sensitive Na+,K(+)-ATPase; by contrast, activity loss in Se(+) cells was preceded by a prominent lag. Enzyme photo-inactivation in Se(-) cells was inhibited by ebselen, an SePX mimetic, confirming that SePX(s) is (are) involved in natural protection. Desferrioxamine treatment (iron sequestration/inactivation) resulted in higher hydroperoxide levels and slower Na+,K(+)-ATPase inactivation during MC540/light exposure, whereas ferric-8-hydroxyquinoline treatment (iron supplementation) had the opposite effect. Thus, iron appears to play an important role in both of these processes. In contrast, photoinactivation of another intrinsic enzyme in L1210 cells, acetylcholinesterase (AChE), was unaffected by selenium or iron manipulation. On the basis of these findings, we propose that lipid peroxidation plays an important role in the photoinactivation of Na+,K(+)-ATPase, but not AChE. This is consistent with the fact that Na+,K(+)-ATPase's active site lies within the membrane bilayer, whereas AChE's active site lies outside the bilayer.

Erythrocyte Na+,K(+)-ATPase properties and adenylate energy charge in normotensives and in essential hypertensives

The activity and some kinetic properties of RBC Na+,K(+)-ATPase (EC 3.6.1.37) were investigated in essential hypertensives (EH; 40 subjects) and normotensives (NT; 20 subjects). A decrease in ouabain-sensitive 86Rb uptake as well as ouabain-sensitive ATPase activity was found in EH. [Na+]i and [K+]i of EH did not show any statistical difference from NT. Na+,K(+)-ATPase showed a reduced Mg2+ activation and the apparent Km value for Mg2+ was 2-fold increased in the EH group. The influence of temperature on the Na+,K(+)-ATPase showed a reduced modulation and a minor activity peak at 37 degrees C in the patients, consequently the calculated activation energy of the enzyme was increased at temperatures lower than 40 degrees C. Increased RBC adenylate energy charge (EC) was observed in EH when compared with NT. A negative correlation between EC and total Na+,K(+)-ATPase activity was found when all subjects were compared and also in both groups, showing a possible pump involvement in the regulation of the RBC metabolic flux in EH. These data provide evidence about some modifications in active Na+,K+ transport and in EC in RBC which allows a further characterization of membrane cation fluxes in EH.

Subcellular distribution of carbonic anhydrase and Na+,K(+)-ATPase in the brain of the hyt/hyt hypothyroid mice

Activities of carbonic anhydrase and Na+,K(+)-ATPase in tissue homogenates and in subcellular fractions from different brain regions were studied in inherited primary hypothyroid (hyt/hyt) mice. The body weight, the weight of different brain regions, and the plasma thyroxine and triiodothyronine levels of hyt/hyt mice were significantly lower than those of the age-matched hyt/+ controls. In tissue homogenates of cerebral cortex, brain stem and cerebellum of hypothyroid mice, the activity of carbonic anhydrase (units/mg protein) was 59.2, 57.6, and 43.2%, and the activity of Na+,K(+)-ATPase (nmol Pi/mg protein/min) was 73.7, 74.4 and 68.7%, respectively, of that in corresponding regions of euthyroid littermates. The decrease in enzyme activity in tissue homogenates was also reflected in different subcellular fractions. In cerebral cortex and brain stem, carbonic anhydrase activity in cytosol, myelin and mitochondrial fractions of hypothyroid mice was about 45-50% of that in euthyroid mice, while in cerebellum the carbonic anhydrase activity in these subcellular fractions of hyt/hyt mice was only 33-38% of that in hyt/+ controls. Na+,K(+)-ATPase activity in myelin fraction of different brain regions of hyt/hyt mice was about 34-42% of that in hyt/+ mice, while in mitochondria, synaptosome and microsome fractions were about 44-52, 46-53, and 66-68%, respectively of controls. These data indicate that the activity of both carbonic anhydrase and Na+,K(+)-ATPase was affected more in the myelin than other subcellular fractions and more in the cerebellum than cerebral cortex and brain stem by deficiency of thyroid hormones.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dibutyryl cyclic AMP on Na+,K(+)-ATPase activity and intracellular Na+ and K+ in primary cultures of astrocytes from DBA and C57 mice

Effects of chronic treatment of dibutyryl cyclic AMP (db-cyclic AMP) on Na+,K(+)-ATPase activity in cell homogenates and intracellular Na+ and K+ contents [(Na+)i and (K+)i] were studied in primary cultures of astrocytes derived from cerebral cortex of neonatal audiogenic seizure-susceptible DBA and audiogenic seizure-resistant C57 mice. Na+,K(+)-ATPase activity in cell homogenates was greater and (Na+)i was less in DBA astrocytes than in C57 astrocytes. There was no difference in (K+)i between astrocytes from DBA and C57 mice. Addition of db-cyclic AMP to the medium from day 14 to day 21 in culture (final concentration 0.25 mM) increased Na+,K(+)-ATPase activity in cell homogenates and decreased (Na+)i, but had no significant effect on (K+)i in astrocytes from either DBA or C57 mice. Chronic treatment with db-cyclic AMP altered cell growth. Protein and DNA content of cultured astrocytes from both DBA and C57 mice was decreased. DNA was more affected than protein. Modifying K+ and Na+ concentration in medium altered Na+,K(+)-ATPase activity in cell homogenates as well as (Na+)i and (K+)i in cultured astrocytes of both DBA and C57 mice. Changes in (Na+)i and (K+)i at different K+ concentrations in medium paralleled those in Na+,K(+)-ATPase activity in cell homogenates. Results indicate that the ability to transport Na+ across the cell membrane and the response of Na+,K(+)-ATPase to db-cyclic AMP and to the changes in K+ in medium of cultured astrocytes from audiogenic seizure-susceptible DBA mice are sufficient.

Taurine regulation of Ca2+ uptake and (Ca(2+)+Mg2+)-ATPase in developing chick B-cells

1. The objective of the present study was to determine the effect of age and taurine on chick B cell calcium uptake and membrane (Ca(2+)+Mg2+)-ATPase activity in 1-4-week-old chicks. 2. The calcium uptake rate decreased with age (P < 0.05) and was further decreased by taurine (P < 0.05). 3. (Ca(2+)+Mg2+)-ATPase activity increased with age (P < 0.05) and was stimulated by taurine (P < 0.05). 4. The data demonstrate that the flux of calcium across the B-cell membrane changes during early post-hatch development, and that taurine regulates both the influx and efflux of calcium in chick B-cells.

Studies of the levamisole inhibitory effect on rat stromal-cell commitment to mineralization

The ability of Levamisole to decrease mineralization in skeletal tissue is usually related to its effect on alkaline phosphatase (ALP). However, Levamisole is also suspected to diminish mineralization by an additional mechanism which is unrelated to the ALP control of apatite crystal growth. To delineate the time in differentiation during which Levamisole inhibits mineralization, a tissue culture model system of bone marrow stromal cells was used. Secondary cultures of stromal cells were propagated in osteoprogenitor cell (OPC) induction medium for three weeks, followed by measurement of calcium precipitation. In situ ALP assays at pH 7.6 were also performed. When cells were cultured with 0.2 mM Levamisole for three weeks, Day 20 values of calcium precipitates were lower than in controls, but Day 20 ALP values were paradoxically higher. The correlation between calcium and ALP within each group was low. The correlation slightly improved, in uninhibited cultures, when Day 21 calcium values were matched with earlier Day 12 ALP values. This suggested the existence of a Levamisole-sensitive mechanism for mineralization inhibition effective prior to the culture's mineralization stage. To focus on this early effect on mineralization Levamisole was added to stromal cultures on different days and removed on Day 12. Levamisole decreased Day 21 mineralization when added on Days 0, 3, 5, and 7, but not when added on Day 9. The Levamisole-induced inhibition of mineralization was accompanied by an increase in Day 12 ALP specific activity, compared to controls, when added from Day 5 and thereafter. The results indicate that part of the ability of stromal cells to mineralize is determined during the first week of culture.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Energy transduction in the thermophilic anaerobic bacterium Clostridium fervidus is exclusively coupled to sodium ions

The thermophilic, peptidolytic, anaerobic bacterium Clostridium fervidus is unable to generate a pH gradient in the range of 5.5-8.0, which limits growth of the organism to a narrow pH range (6.3-7.7). A significant membrane potential (delta psi approximately -60 mV) and chemical gradient of Na+ (-Z delta pNa approximately -60 mV) are formed in the presence of metabolizable substrates. Energy-dependent Na+ efflux is inhibited by the Na+/H+ ionophore monensin but is stimulated by uncouplers, suggesting that the Na+ gradient is formed by a primary pumping mechanism rather than by secondary Na+/H+ antiport. This primary sodium pump was found to be an ATPase that has been characterized in inside-out membrane vesicles and in proteoliposomes in which solubilized ATPase was reconstituted. The enzyme is stimulated by Na+, resistant to vanadate, and sensitive to nitrate, which is indicative of an F/V-type Na(+)-ATPase. In the proteoliposomes Na+ accumulation depends on the presence of ATP, is inhibited by the ATPase inhibitor nitrate, and is completely prevented by the ionophore monensin but is stimulated by protonophores and valinomycin. These and previous observations, which indicated that secondary amino acid transport uses solely Na+ as coupling ion, demonstrate that energy transduction at the membrane in C. fervidus is exclusively dependent on a Na+ cycle.

Functional expression of the alpha 2 and alpha 3 isoforms of the Na,K-ATPase in baculovirus-infected insect cells

Multiple isoforms of both the alpha and beta subunits of Na,K-ATPase have been identified. Elucidating their roles has been complicated by the fact that most tissues express multiple isoforms and purification techniques specific for each isoform have not been achieved. The baculovirus expression system, which uses the baculovirus Autographica californica to infect insect cells, is an ideal system for studying the Na,K-ATPase isoforms since high amounts of foreign proteins can be produced and some insect cell lines have low levels of endogenous Na,K-ATPase. Recombinant baculoviruses containing the cDNAs for the alpha 2, alpha 3, and beta 1 isoforms of the rat Na,K-ATPase were prepared and used to infect Sf-9 cells, an insect cell line derived from the ovary of the fall armyworm Spodoptera frugiperda. By using this system, Na,K-ATPase alpha 2 and alpha 3 subunits that were antigenically and electrophoretically indistinguishable from the native subunits were produced. When each subunit is expressed independently in the Sf-9 cells, it is primarily delivered to the plasma membrane. Although the isolated expression of each Na,K-ATPase subunit did not render active Na,K-ATPase molecules, the coexpression of alpha 2 or alpha 3 with beta 1 resulted in catalytically active molecules. This activity could be measured as a ouabain-sensitive ATPase activity or directly demonstrated using either [gamma-32P]ATP or 32Pi to identify the phosphorylated intermediates of the alpha 2 and alpha 3 isoforms. [3H]Ouabain binding studies showed that both isoforms are capable of binding the cardiotonic steroid with high affinity, alpha 3 being more sensitive to ouabain. These results demonstrate that the baculovirus system is suitable for the expression of the Na,K-ATPase isoforms and should provide a useful method for the characterization of the enzymatic properties of each isoform.

Phosphatase and ATPase activities in isonuclear lines of cytoplasmic male-sterile and male-fertile petunia

Soluble and membrane-bound fractions of plant leaves, cell suspension cultures and seedlings of petunia were examined for phosphohydrolase activity on p-nitrophenyl phosphate (pNPPase) and adenosine triphosphate (ATPase). One cytoplasmic male-sterile (CMS) and one fertile (F) line was examined for each tissue. Both pNPPase and ATPase exhibited a broad optimal activity between pH 5.5-7.0 for the membrane-bound fraction and between 4.5-7.0 for the soluble fractions. The activity of both were inhibited by divalent ions including Mg(2+). At pH 7.2, the activities on various triphosphonucleotides were similar and they were hydrolyzed by a rate of 20-50% of that of ATP. Significant differences between CMS and F extracts were: (a) higher activities in CMS membranes; (b) lower Ea (energy of activation) values for activities in CMS membrane functions; (c) seedling and cell-culture CMS extracts exhibited a higher sensitivity to high temperature denaturation; (d) the hydrolase activity on monoand triphospho-cytosine compounds was significantly higher in CMS than in F membranes.

Cordil reversibly inhibits the Na,K-ATPase from outside of the cell membrane. Role of K-dependent dephosphorylation

Cordil-LND796 is a new cardiotonic glycoside under development. In rat brain microsomes where three isoforms of the Na,K-ATPase with differential affinities for cardiac glycosides have been identified, Cordil had higher affinity for the alpha 3 (IC50 = 0.02 microM) than for the alpha 2 (IC50 = 0.6 microM) and the alpha 1 (IC50 = 30 microM) isozymes. Cordil is potentially a selective inhibitor for both alpha 2 and alpha 3 Na,K-ATPase isoforms. Using inside out vesicles we have shown that Cordil binds to and inhibits Na,K-ATPase at an extracellular site. The dissociation kinetic rates (k-1) from the ATPase and the phosphatase activity (K-dependent dephosphorylation) of the Na,K-ATPase were similar for Cordil. Despite these similarities to ouabain comparison of the kinetics of the Na,K-ATPase inhibition by ouabain and Cordil revealed marked differences in their association rates (k+1 = 0.7 l mol-1 min-1 and k+1 = 6 x 10(-3) l mol-1 min-1 respectively) and their dissociation rates (k-1 = 1.3 +/- 0.2 x 10(-4) s-1 and k-1 = 69 +/- 7 x 10(-4) s-1 respectively). Both binding association and dissociation rates were enhanced for Cordil. These data are compatible with a stabilizing effect of Cordil on the E2P conformational state of Na,K-ATPase.

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.

Determination of rate constants for nucleotide dissociation from Na,K-ATPase

A method for determining individual rate constants for nucleotide binding to and dissociation from membrane bound pig kidney Na,K-ATPase is presented. The method involves determination of the rate of relaxation when Na,K-ATPase in the presence of eosin is mixed with ADP or ATP in a stopped-flow fluorescence apparatus. It is shown that the nucleotide dependence of this rate of relaxation--taken together with measured equilibrium binding values for eosin and ADP--makes possible a reasonably reliable determination of the rate constant for dissociation of nucleotide, i.e., determination of the rate constant k-1 in the following model (where E denotes Na,K-ATPase): [formula: see text] All experiments are carried out at about 4 degrees C in a buffer containing 200 mM sucrose, 10 mM EDTA, 25 mM Tris and 73 mM NaCl (pH 7.4). Values obtained for the rate constants for dissociation are about 6 s-1 for ADP and 2-3 s-1 for ATP.

Conventional and saturation transfer EPR spectroscopy of Na+/K(+)-ATPase modified with different maleimide-nitroxide derivatives

The membranous Na+/K(+)-ATPase from Squalus acanthias has been covalently modified on either Class I or Class II sulphydryl groups using derivatives of 3-(maleimidomethyl)-1-oxyl-2,2,5,5-tetramethylpyrrolidine with substituents of different charge and hydrophobicity attached at the remaining unsubstituted position of the pyrrolidine ring. The substituent groups used were a methyl and a hexyl ester, and di- and tri-methylammonium ethyl esters, as well as the parent underivatized compound. Additionally, another series of maleimide-nitroxides differing (by zero to seven intervening atoms) in the length of the linking group between the maleimide and the pyrrolidine moieties was used. The sites of attachment have been characterized in terms of the rotational mobility and environmental polarity by using conventional and saturation transfer EPR spectroscopy of these spin-labelled reagents. This provides a further sub-classification of the primary Class I and Class II SH-groups on the alpha-subunit of the enzyme, which differ both in their reactivity and influence on the Na+/K(+)-ATPase activity.

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.

Occlusion of Rb+ after extensive tryptic digestion of shark rectal gland Na,K-ATPase

Na,K-ATPase from rectal glands of Squalus acanthias has been subjected to proteolysis with trypsin. The E1- and E2-forms of the enzyme can be distinguished from the inactivation patterns at low trypsin concentrations, as previously seen with kidney enzyme. Extensive degradation by trypsin in the presence of 5 mM Rb+ yields membrane fragments with a 19 kDa peptide as the major proteolytic fragment of the alpha-subunit. The sequence of the N-terminal 40 residues of this peptide is almost identical to that of a similar proteolytic fragment isolated by Capasso et al. (Capasso, J.M., Hoving, S., Tal, D.M., Goldshleger, R. and Karlish, S.J.D. (1992) J. Biol. Chem. 267, 1150-1158) using kidney Na,K-ATPase. Rb+ occlusion can be fully retained under these circumstances, supporting the findings with kidney enzyme that only minor parts of the alpha-subunit are required to form a functional occlusion-site.

K(+)-site-directed pyridine derivative, AU-1421, activates hydrolysis of the K(+)-sensitive phosphoenzyme of sarcoplasmic reticulum Ca(2+)-ATPase and inactivates that of K(+)-transporting ATPases

(Z)-5-Methyl-2-[2-(1-naphthyl)ethenyl]-4-piperidinopyridine, AU-1421, interacted at 0 degree C with the K(+)-sensitive phosphoenzymes of three transport ATPases, Ca(2+)-, H+/K(+)- and Na+/K(+)-ATPase. In the case of Ca(2+)-ATPase, AU-1421 at about 80 microM stimulated 6-fold the rate of splitting of the phosphoenzyme, on which K+ simply functions as an accelerator from one side of the membrane. Probably AU-1421 also simply interacts with the K(+)-binding site of the phosphoenzyme that is easily accessible from the aqueous phase. In the cases of H(+)/K(+)- and Na(+)/K(+)-ATPases, AU-1421 stabilized the phosphoenzymes which accept K+ as the translocating ion. The rate constants of dephosphorylation for H(+)/K(+)-ATPase and Na(+)/K(+)-ATPase were decreased to half by AU-1421 at about 5 and 10 microM, respectively. Presumably after binding of AU-1421 to a K(+)-recognition site of the phosphoenzyme, local motion of the peptide region near the binding site that serves to move the bound ion into the ion-transport pathway (occlusion center) might be inhibited. Thus AU-1421 may be able to distinguish two modes of K+ action on the K(+)-sensitive phosphoenzymes.

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.

(Na+ + K+)-ATPase: inactivation and degradation induced by oxygen radicals

1. Purified (Na+ + K+)-ATPase was irreversibly inhibited upon exposure to hydrogen peroxide, the superoxide anion, and the hydroxyl radical. 2. Comparison of the SDS-gel electrophoretic patterns of the ATPase samples exposed to these oxidants revealed that inhibition occurred either without gross structural changes, or concomitant with fragmentation and cross-linking of the enzyme subunits. 3. The oxidant modified ATPase was also shown to be more susceptible to degradation by several proteolytic enzymes.

Levels of G-proteins in liver and brain of lean and obese (ob/ob) mice

G-protein levels were assessed in liver and brain membranes of lean and obese mice. ADP-ribosylation and immunodetection studies revealed a decrease in the abundance of Gs and Gi alpha-subunits in the liver membranes of obese mice compared with lean mice. In contrast, in brain membranes, the abundance of these proteins was not significantly different between lean and obese mice. Studies at the mRNA level in both liver and brain revealed no difference in gene expression between lean and obese mice. Protein and mRNA studies both showed that Gs, Gi alpha 1, Gi alpha 2, Go alpha and G beta subunits are present in brain membranes, and Gi alpha 3 is barely detectable. In liver, Ga alpha, Gi alpha 2 and G beta subunits are the major constituents, whereas Gi alpha 1, Gi alpha 3 and Go alpha are barely detectable. It is possible that the differences observed at the protein level are due to different rates of translation of the mRNA. Different rates of release of the alpha-subunits from the membrane and/or different rates of degradation would also explain these results.