Molecular considerations relevant to the mechanism of active transport

Decrease of lymphocyte (Na+,K+)ATP-ase activity in aged people

Activity of (Na+,K+)ATP-ase measured by means of influx of 86Rb was compared in lymphocytes from young, middle-age and old people. It was found that the (sensitive to ouabain) activity of (Na+,K+)ATPase was significantly diminished in the lymphocytes from aged subjects.

Multiple ion-dependent and substrate-dependent Na+/K+-ATPase conformational states. Transient and steady-state kinetic studies

The hydrolysis of beta-(2-furyl)acryloyl phosphate (FAP), catalyzed by the Na+/K+-ATPase, is faster than the catalyzed hydrolysis of ATP. This is due to catalyzed hydrolysis of the pseudosubstrate by K+-dependent states of the enzyme, thus bypassing the Na+-dependent enzyme states that are required and are rate limiting in ATP hydrolysis. Unlike ATP, FAP is a positive effector of the E2 state. A study of FAP hydrolysis permits a detailed analysis of later steps in the overall ion translocation-ATP hydrolysis pathway. During the steady state of FAP hydrolysis in the presence of K+, substantial phosphoryl-enzyme is formed, as is indicated by the covalent incorporation of 32P from [32P]FAP. A comparison of the phosphoryl-enzyme yield with the rate of overall hydrolysis reveals that at 25 degrees C the phosphoryl-enzyme formed is all kinetically competent. Both the yield of phosphoryl-enzyme and the rate of overall hydrolysis of FAP are [K+] dependent. The transition E1 in equilibrium E2 is also [K+] dependent, but the rate of transition is differently affected by [K+] than are the above-mentioned two processes. Two distinct roles for K+ are indicated, as an effector of the E1-E2 equilibrium and as a "catalyst" in the hydrolysis of the E2-P. In contrast to the results at 25 degrees C, a virtually stoichiometric yield of phosphoryl-enzyme occurs at 0 degree C in the presence of Na+ and the absence of K+. At lower concentrations of K+ and in the presence of Na+, the hydrolysis of FAP at 0 degree C proceeds substantially through the E1-E2 pathway characteristic of ATP hydrolysis. The selectivity of FAP for the E2-K+-dependent pathway is due to the thermal inactivation of E1 at 25 degrees C in the absence of ATP or ATP analogues, even at high concentrations of Na+. These results emphasize the existence of multiple functional "E1" and "E2" states in the overall ATPase-ion translocation pathway.

Potassium-induced reverse transformation of cells infected with a temperature-sensitive transformation mutant virus

High potassium concentrations altered the morphology and the ability to grow in soft agar in 6m2 cells, a clone of rat kidney cells infected with a temperature-sensitive mutant of Moloney sarcoma virus. Approximately 60% of cells exhibited normal morphology in the presence of 94.8 mM potassium in isotonic medium at the temperature permissive for transformation, whereas 100% were normal at 72 mM potassium in hypertonic media. A significant reduction of growth in soft agar was also induced with these conditions. However, the synthesis ratio of virus-specified transforming protein to marker viral protein was not altered. Na+K+-ATPase might play a role in this reverse-transformation process.

Equilibrium between monomers and oligomers of soluble Ca2+-ATPase during the functional cycle

Molecular sieve HPLC shows that soluble sarcoplasmic reticulum Ca2+-ATPase at low concentrations of the non-ionic detergent octaethylene glycol monododecyl ether exists as monomers in equilibrium with dimers and higher oligomers. Binding of vanadate or ATP as well as phosphoenzyme turnover shifts the equilibrium towards the monomer. This suggests that the Ca2+-pump cycle can occur without transient self-association of Ca2+-ATPase peptides.

Coupling of Ca2+ transport to ATP hydrolysis by the Ca2+-ATPase of sarcoplasmic reticulum: potential role of the 53-kilodalton glycoprotein

An essential feature of the function of the Ca2+-ATPase of sarcoplasmic reticulum (SR) is the close coupling between the hydrolysis of ATP and the active transport of Ca2+. The purpose of this study is to investigate the role of other components of the SR membrane in regulating the coupling of Ca2+-ATPase in SR isolated from rabbit skeletal muscle, reconstituted SR, and purified Ca2+-ATPase/phospholipid complexes. Our results suggest that (1) it is possible to systematically alter the degree of coupling obtained in reconstituted SR preparations by varying the [KC1] present during cholate solubilization, (2) the variation in coupling is not due to differences in the permeability of the reconstituted SR vesicles to Ca2+, and (3) vesicles reconstituted with purified Ca2+-ATPase are extensively uncoupled under our experimental conditions regardless of the lipid/protein ratio or phospholipid composition. In reconstituted SR preparations prepared by varying the [KC1] present during cholate treatment, we find a direct correlation between the relative degree of coupling between ATP hydrolysis and Ca2+ transport and the level of the 53-kilodalton (53-kDa) glycoprotein of the SR membrane. These results suggest that the 53-kDa glycoprotein may be involved in regulating the coupling between ATP hydrolysis and Ca2+ transport in the SR.

Amino-acid sequence of the catalytic subunit of the (Na+ + K+)ATPase deduced from a complementary DNA

We have isolated and characterized a complementary DNA for the catalytic subunit of the sheep kidney sodium/potassium-dependent ATPase. The 1,016-amino-acid protein seems to have eight transmembrane domains. The apparent ouabain binding site is located at the extracellular junction of two transmembrane domains and is linked to the phosphorylation site by a 60-amino-acid conserved sequence that may be a major channel for energy transduction.

Primary structure of the alpha-subunit of Torpedo californica (Na+ + K+)ATPase deduced from cDNA sequence

Sodium- and potassium-dependent ATPase [(Na+ + K+)ATPase], which is responsible for the active transport of Na+ and K+, is distributed universally among animal cell membranes and consists of two types of subunits, alpha and beta (refs 1-4). The larger alpha-subunit with a relative molecular mass (Mr) of 84,000-120,000 is thought to have the catalytic role. We have now cloned and sequenced DNA complementary to the Torpedo californica electroplax messenger RNA encoding the alpha-subunit of (Na+ + K+)ATPase and have deduced the complete amino-acid sequence of the polypeptide. Some structural features of the alpha-subunit molecule related to the function of this active-transport protein are discussed.

Fluorescence polarization analysis, lipid composition, and Na+, K+-ATPase kinetics of synaptosomal membranes in feline GM1 and GM2 gangliosidosis

Neurochemical studies were performed on synaptosomal membranes from cats with GM1 or GM2 gangliosidosis to examine possible mechanisms of neuronal dysfunction in these disorders. The basic hypothesis tested was that deficient ganglioside catabolism causes increased ganglioside content of synaptosomal plasma membrane which in turn disrupts normal function. Fluidity characteristics of synaptosomal membranes were examined using fluorescence polarization. Results showed markedly reduced membrane fluidity in both GM1 and GM2 gangliosidosis. These results were supported by a second study which revealed that isolated synaptosomal membranes of GM1 gangliosidosis cats had a 24-fold increase in total ganglioside content caused predominantly by excess GM1, a 2.3-fold increased cholesterol content, and a 1.4-fold increased phospholipid content. Finally, kinetic analysis of synaptosomal plasma membrane Na+,K+-ATPase from cats with GM1 gangliosidosis showed negligible differences in kinetic parameters compared with controls. Thus, the enzyme appeared protected from the global membrane changes in fluidity and composition. These observations provide evidence for a pathogenetic mechanism of neuronal dysfunction in the gangliosidoses while demonstrating protection of certain vital functional components, such as Na+,K+-ATPase.

Characterization of transverse tubule membrane proteins: tentative identification of the Mg-ATPase

Vesiculated fragments of chicken skeletal muscle transverse tubule (TT) membranes were analyzed for their content of loosely associated and integral membrane proteins. Of particular interest was the identification of the magnesium-stimulated ATPase (Mg-ATPase), which is characteristically located in native isolated TT vesicles of chicken skeletal muscle [R. A. Sabbadini and V. R. Okamoto (1983) Arch. Biochem. Biophys. 223, 107-119]. A number of the proteins found in vesicular TT preparations were found to be extractable by a mild Triton-X100 treatment and were identified as aldolase, enolase, creatine kinase, glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, and pyruvate kinase. Approximately 60% of TT-associated protein was extracted with Triton, resulting in a twofold enrichment of the Mg-ATPase. Concommitantly, one core integral membrane protein possessing a Mr of 102,000 was enriched, suggesting that it is responsible for the Mg-ATPase activity present in chicken skeletal muscle TT membranes.

Labeling of the glycoprotein subunit of (Na,K)ATPase with fluorescent probes

Sodium plus potassium activated adenosinetriphosphatase [(Na,K)ATPase] is composed of a catalytic subunit (alpha) and a glycoprotein subunit (beta) of unknown function. A method has been developed to label the beta subunit of purified dog kidney (Na,K)ATPase with fluorescent probes. The method consists of oxidation of beta-subunit oligosaccharides, reaction of the resulting aldehydes with fluorescent hydrazides, and reduction of the hydrazones and unreacted aldehydes with NaBH4. Two oxidation methods were compared. Simultaneous treatment with neuraminidase and galactose oxidase did not inhibit significantly (Na,K)ATPase activity and allowed insertion of up to 11 mol of probe per mol of beta. In contrast, oxidation of (Na,K)ATPase oligosaccharides with periodate resulted in 50-80% inhibition of the (Na,K)ATPase activity with low or undetectable labeling. Eleven commercial probes and two novel hydrazides were tested for labeling of (Na,K)ATPase treated with galactose oxidase and neuraminidase. Eight probes did not label (Na,-K)ATPase but labeled red cell ghosts oxidized with periodate. Four probes labeled beta specifically but either adsorbed to the membrane tightly, or cross-linked the beta subunits, or formed unstable adducts. Lucifer yellow CH labeled beta specifically without membrane adsorption. Labeling stoichiometries from 1 to 11 mol of lucifer yellow CH per mol of beta were obtained without inhibition of (Na,K)ATPase activity and without significant alteration of the anthroylouabain binding capacity or its association and dissociation kinetics. Anthroylouabain specifically bound to the lucifer-labeled (Na,K)ATPase had a decreased quantum yield, probably due to resonance energy transfer. This suggests that the sites of lucifer attachment on beta are within energy transfer distance from the cardiac glycoside site on alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for acid-induced transformation of omeprazole into an active inhibitor of (H+ + K+)-ATPase within the parietal cell

The chemical reactions of omeprazole, leading to inhibition of gastric acid secretion, were investigated. In acid buffer solutions, omeprazole was found to be labile, whereas at physiological pH it was stable (t1/2 greater than 17 h at pH 7.4). The stability of omeprazole was also studied in isolated, acid producing, gastric glands under conditions where acid formation was either stimulated or inhibited. The rate of transformation of omeprazole was high (t1/2 approximately 3 min) under stimulation. Inhibition of acid formation in the gland greatly retarded the decomposition of omeprazole (t1/2 approximately 73 min). The time-course for inhibition of acid formation by omeprazole was parallel to that for decomposition. The major product formed from omeprazole was the reduced form, H 168/22. The inhibitory action of omeprazole was shown to depend on acid-induced transformation, since no inhibition was obtained when omeprazole was incubated under neutral conditions, both in the isolated gastric mucosal- and the (H+ + K+)-ATPase preparations. Despite the fact that H 168/22 was the major product formed in the glandular preparation, it was found to be virtually inactive in both the glandular- and (H+ + K+)-ATPase preparations. Therefore, a model is proposed in which the inhibition of acid formation by omeprazole is mediated by a compound formed during the reduction of omeprazole to H 168/22 within the acid compartments of the parietal cell. Furthermore, mercaptanes, such as beta-mercaptoethanol, were found to prevent as well as reverse inhibition by omeprazole in both the glandular- and (H+ + K+)-ATPase preparations. This indicates that -SH groups are most likely involved in the chemical reactions leading to inhibition of acid secretion.

Minimal functional unit for transport and enzyme activities of (Na+ + K+)-ATPase as determined by radiation inactivation

Frozen aqueous suspensions of partially purified membrane-bound renal (Na+ + K+)-ATPase have been irradiated at -135 degrees C with high-energy electrons. (Na+ + K+)-ATPase and K+-phosphatase activities are inactivated exponentially with apparent target sizes of 184 +/- 4 kDa and 125 +/- 3 kDa, respectively. These values are significantly lower then found previously from irradiation of lyophilized membranes. After reconstitution of irradiated (Na+ + K+)-ATPase into phospholipid vesicles the following transport functions have been measured and target sizes calculated from the exponential inactivation curves: ATP-dependent Na+-K+ exchange, 201 +/- 4 kDa; (ATP + Pi)-activated Rb+-Rb+ exchange, 206 +/- 7 kDa and ATP-independent Rb+-Rb+ exchange, 117 +/- 4 kDa. The apparent size of the alpha-chain, judged by disappearance of Coomassie stain on SDS-gels, lies between 115 and 141 kDa. That for the beta-glycoprotein, though clearly smaller, could not be estimated. We draw the following conclusions: (1) The simplest interpretation of the results is that the minimal functional unit for (Na+ + K+)-ATPase is alpha beta. (2) The inactivation target size for (Na+ + K+)-dependent ATP hydrolysis is the same as for ATP-dependent pumping of Na+ and K+. (3) The target sizes, for K+-phosphatase (125 kDa) and ATP-independent Rb+-Rb+ exchange (117 kDa) are indistinguishable from that of the alpha-chain itself, suggesting that cation binding sites and transport pathways, and the p-nitrophenyl phosphate binding site are located exclusively on the alpha-chain. (4) ATP-dependent activities appear to depend on the integrity of an alpha beta complex.

Amplification of sodium- and potassium-activated adenosinetriphosphatase in HeLa cells by ouabain step selection

A multistep selection for ouabain resistance was used to isolate a clone of HeLa S3 cells that overproduces the plasma membrane sodium, potassium activated adenosinetriphosphatase (Na+,K+-ATPase). Measurements of specific [3H]ouabain-binding to the resistant clone, C+, and parental HeLa cells indicated that C+ cells contain 8-10 X 10(6) ouabain binding sites per cell compared with 8 X 10(5) per HeLa cell. Plasma membranes isolated from C+ cells by a vesiculation procedure and analyzed for ouabain-dependent incorporation of [32P]phosphate into a 100,000-mol-wt peptide demonstrated a ten- to twelvefold increase in Na+,K+-ATPase catalytic subunit. The affinity of the enzyme for ouabain on the C+ cells was reduced and the time for half maximal ouabain binding was increased compared with the values for the parental cells. The population doubling time for cultures of C+ cells grown in dishes was increased and C+ cells were unable to grow in suspension. Growth of C+ cells in ouabain-free medium resulted in revertant cells, C-, with biochemical and growth properties identical with HeLa. Karyotype analysis revealed that the ouabain-resistant phenotype of the C+ cells was associated with the presence of minute chromosomes which are absent in HeLa and C- cells. This suggests that a gene amplification event is responsible for the Na+,K+-ATPase increase in C+ cells.

Characterization of (Na+ + K+)-ATPase liposomes. I. Effect of enzyme concentration and modification on liposome size, intramembrane particle formation and Na+,K+-transport

Rabbit renal (Na+ + K+)-ATPase (EC 3.6.1.3) was purified and incorporated into phosphatidylcholine liposomes. Freeze-fracture analysis of the reconstituted system reveals intramembrane particles formed by (Na+ + K+)-ATPase molecules which are randomly distributed on concave and convex fracture faces. The reconstituted (Na+ + K+)-ATPase performs active Na+,K+-transport. The distribution of particles as well as the rate of active transport are directly proportional to the (Na+ + K+)-ATPase protein concentration used for reconstitution, while the total amount of sodium and potassium ions exchanged by ATP per volume vesicle suspension reaches maximum when each vesicle contains on the average more than two particles. (Na+ + K+)-ATPase pretreated with ouabain or vanadate yields the same particle density and vesicle size as control enzyme. However, detergent-denatured enzyme loses its ability to form intramembrane particles or to increase the vesicle size indicating that the lipids surrounding the protein part of the molecule are essential for the reconstitution process. The vesicle diameter increases as a function of the number of particles per vesicle. Histograms of the size distribution become wider with increasing intramembrane particle density and tend to show more than one maximum.

Lack of immunological cross reactivity between the transport enzymes (Na+ + K+)-ATPase and (K+ + H+)-ATPase

Goat antisera against (Na+ + K+)-ATPase and its isolated subunits and against (K+ + H+)-ATPase have been prepared in order to test for immune cross-reactivity between the two enzymes, whose catalytic subunits show great chemical similarity. None of the (Na+ + K+)-ATPase antisera cross-reacted with (K+ + H+)-ATPase or inhibited its enzyme activity. The same was true for the (K+ + H+)-ATPase antiserum with regard to (Na+ + K+)-ATPase and its subunits and its enzyme activity. So notwithstanding the chemical similarity of their subunits, there is no immunological cross-reactivity between these two plasma membrane ATPases.

Dimeric structure of single chloride channels from Torpedo electroplax

The inhibition by 4,4'-diisothiocyano-2,2'-stilbenedisulfonate (DIDS) of Cl- channels from Torpedo electroplax incorporated in planar phospholipid bilayer membranes is studied. DIDS irreversibly and rapidly inhibits the macroscopic conductance of membranes containing many channels. At the single-channel level, the effect of DIDS is more complicated. The uninhibited single channel displays three "substates" of conductances 20, 10, and 0 pS. Short exposure (5-30 s) to 10 microM DIDS converts this three-level active channel into a "conventional" channel of 10-pS conductance. Longer exposure eliminates all channel fluctuations. The results are taken as strong evidence that the Cl- channel is constructed as a functional dimer of identical protein subunits.

Effects of ouabain and potassium on rabbit arterial smooth muscle cells in culture

The membrane-bound Na/K-ATPase system is an important energy regulating system for all primate cells and is suspected to be either primary or secondary affected in different but important metabolic disorders as essential hypertension, diabetes II (mature onset diabetes) or severe overweight. Rabbit smooth muscle cells grown in culture have been incubated with different concentrations of ouabain (10(-7)-10(-4) mol/l) and potassium (4 and 6 mmol/l). In controlled series, the incorporation of 3H-thymidine (and collagen secretion) during incubation with ouabain was found to be diminished by at maximum 73% (P less than 0.01) and this was found to be reversible by changing to ouabain-free medium or partly by the addition of extra potassium. The intracellular ATP level and lactate production was diminished together with the fall in 3H-thymidine incorporation. These effects were probably not due to a non-specific toxic effect of ouabain because no difference in leakage of prelabelled 3H-thymidine from cells compared to control series was seen. We suggest that an optimal function of the membrane-bound Na/K-ATPase system is of great importance not only for intracellular energy production but also for cell proliferation and protein synthesis.

The molecular weight of the calcium-transport-ATPase of the human red blood cell determined by radiation inactivation

Radiation inactivation was applied to analyze the molecular weight of the functional unit of (Ca2+ + Mg2+)-ATPase in human erythrocyte membranes. The enzyme activity was stable for at least 7 days at room temperature in membranes lyophilized in the presence of sucrose (150-300 mM). The enzyme activity in the lyophilized membranes and remaining after irradiation from a 60Co source was activated by calmodulin. A Mr of 290,000 +/- 15,000 was determined for (Ca2+ + Mg2+)-ATPase activity. Since the Mr by SDS-polyacrylamide gel electrophoresis is approximately 138,000 (Niggli et al. (1979) J. Biol. Chem. 254, 9955-9958), our results suggest that the Ca2+ pump ATPase functions as a dimer in the native human erythrocyte membrane.

pH regulation of divalent/monovalent Ca/K cation transport selectivity by a macrocyclic carrier molecule

The lipophilic dicarboxylic acid-dicarboxamide macrocycle 1 is an efficient carrier for calcium and potassium transport through a liquid membrane. The process involves competitive Ca2+/K+ symport coupled to proton antiport in a pH gradient. It presents a very pronounced phenomenon of pH regulation of transport selectivity from preferential K+ transport to preferential Ca2+ transport as the pH increases from 2 to 9 in the starting aqueous phase containing the metal ions. The results demonstrate how carrier design allows control of the rate and selectivity of divalent/monovalent M2+/M+ cation transport.

Soluble and enzymatically stable (Na+ + K+)-ATPase from mammalian kidney consisting predominantly of protomer alpha beta-units. Preparation, assay and reconstitution of active Na+, K+ transport

Soluble (Na+ + K+)-ATPase consisting predominantly of alpha beta-units with Mr below 170 000 was prepared by incubating pure membrane-bound (Na+ + K+)-ATPase (35-48 mumol Pi/min per mg protein) from the outer renal medulla with the non-ionic detergent dodecyloctaethyleneglycol monoether (C12E8). (Na+ + K+)-ATPase and potassium phosphatase remained fully active in the detergent solution at C12E8/protein ratios of 2.5-3, at which 50-70% of the membrane protein was solubilized. The soluble protomeric (Na+ + K+)-ATPase was reconstituted to Na+, K+ pumps in phospholipid vesicles by the freeze-thaw sonication procedure. Protein solubilization was complete at C12E8/protein ratios of 5-6, at the expense of partial inactivation, but (Na+ + K+)-ATPase and potassium phosphatase could be reactivated after binding of C12E8 to Bio-Beads SM2. At C12E8/protein ratios higher than 6 the activities were irreversibly lost. Inactivation could be explained by delipidation. It was not due to subunit dissociation since only small changes in sedimentation velocities were seen when the C12E8/protein ratio was increased from 2.9 to 46. As determined immediately after solubilization, S20,w was 7.4 S for the fully active (Na+ + K+)-ATPase, 7.3 S for the partially active particle, and 6.5 S for the inactive particle at high C12E8/protein ratios. The maximum molecular masses determined by analytical ultracentrifugation were 141 000-170 000 dalton for these protein particles. Secondary aggregation occurred during column chromatography, with formation of enzymatically active (alpha beta)2-dimers or (alpha beta)3-trimers with S20,w = 10-12 S and apparent molecular masses in the range 273 000-386 000 daltons. This may reflect non-specific time-dependent aggregation of the detergent micelles.