The interrelationship of membrane and protein structure in the functioning of the (Na+ + K+)-activated ATPase

Nitric oxide and peroxynitrite anion modulate liver plasma membrane fluidity and Na(+)/K(+)-ATPase activity

Free radicals attack membranes and frequently alter their fluidity and function. The aim of the present work was to study the effect of nitric oxide (NO) radical and peroxynitrite anion on basolateral liver plasma membrane fluidity and on the activity of Na(+)/K(+)-ATPase. Basolateral membranes (BM) were isolated by ultracentrifugation in sucrose gradients and characterized enzymatically. BM were incubated with SNAP (a NO donor) or SIN-1 (a peroxynitrite donor). The release of NO or peroxynitrite was monitored by measuring NO(-)(2) + NO(-)(3). Relative fluidity was measured by polarization of fluorescence. NO increased membrane fluidity while peroxynitrite decreased it in a concentration-dependent manner. Na(+)/K(+)-ATPase activity was reduced by NO or peroxynitrite. Peroxynitrite anion inhibits ATPase activity in part by decreasing fluidity. However, it is very likely that both compounds inhibit ATPase activity by oxidation of the thiol groups of the enzyme. Our results suggest that NO may exert part of its biological effects by modulating membrane fluidity and function.

Alterations in the physicochemical properties of renal cortical membranes in rats with experimental cirrhosis of the liver

Changes in the major component of renal cortical membranes as well as membrane fluidity and Na+, K+, ATPase activity have been studied in membranes from the renal cortex of rats with experimental liver cirrhosis, which show renal sodium and water retention, and in normal animals. Rats with cirrhosis of the liver show a decrease in cholesterol, phospholipid and protein content, without changes in cholesterol/phospholipid molar ratio. In addition there is a small decrease in 14:0 and 18:2 and an increase in 20:4 content, without differences in unsaturation degree. Membrane fluidity was decreased in renal membranes from cirrhotic rats when compared with normal ones. Na+, K+, ATPase activity was higher in cirrhotic than in normal renal membranes could be related with the changes in renal water and electrolyte changes shown by cirrhotic rats.

The role of membrane composition in ATPase activities of cirrhotic rat liver: effect of silymarin

The activities of Ca2(+)- and Na+, K(+)-ATPases were studied in liver plasma membranes from CCl4-cirrhotic rats and from livers of rats treated with silymarin in addition to CCl4. CCl4 chronic treatment produced significant decreases in Na+, K(+)- and Ca2(+)-ATPase activities; however, the animals treated with silymarin along with CCl4 showed no differences in ATPase activities as compared to controls. The lipid analysis performed in plasma membranes revealed increases in the cholesterol/phospholipid (CH/PL) and sphingomyelin/phosphatidylcholine (SM/PC) ratios in the cirrhotic group. Again, the membranes isolated from rats receiving CCl4 + silymarin showed normal CH/PL and SM/PC values. Considering that CH/PL and SM/PC ratios are related to membrane microviscosity, this study suggests that a lower fluidity of the membrane may be responsible for the observed decreases in ATPase activities in the cirrhotic group. Additionally, the role of silymarin to improve liver function in CCl4-cirrhosis can be attributed partially to its action at membrane level by preventing the increases in CH/PL and SM/PC ratios.

Lipid permeability of the intestinal microvillus membrane may be modulated by membrane fluidity in the rat

Fatty acids and cholesterol permeate across the intestinal microvillus membrane at rates dictated by the hydrophobicity of the permeating lipid and the permeability properties of the microvillus membrane. A theory has evolved suggesting that the chemical composition and physical properties of the microvillus membrane are important in determining microvillus membrane lipid permeability in vivo. This communication reports a test of this hypothesis. To compare in vivo membrane lipid permeability within the same intestinal region, but under conditions were membrane physical properties were radically altered, rats were fed an inhibitor of cholesterol synthesis. This resulted in the replacement of 87-90% of membrane cholesterol with its' precursor, 7-dehydrocholesterol. Marked changes in membrane physical properties were observed, including a reduction in the static and dynamic component of membrane fluidity within the jejunal microvillus membrane. These changes were limited primarily to the outer regions of the bilayer. Associated with these alterations was a pronounced reduction in membrane lipid permeability. Therefore, microvillus membrane lipid permeability, in vivo, appears to be correlated with physical properties of the bilayer, especially those of the superficial regions.

Long-term intracoronary ethanol administration electrophysiologic and morphologic effects

The long-term intracoronary infusion of ethanol was used to evaluate the potential of ethanol to produce myocardial injury and cardiac rhythm disturbances. In 22 dogs, electrophysiologic testing was performed 48 hr after cessation of alcohol administration. Multiple premature ventricular beats occurred spontaneously in 3 dogs with spontaneous sustained monomorphic ventricular tachycardia observed in 1 dog. Provocative ventricular pacing produced ventricular tachycardia lasting 20 or more beats in 13 animals with sustained tachycardia observed in 3 animals. Provocative ventricular pacing in the presence of lidocaine or epinephrine produced sustained ventricular tachycardia in an additional 4 dogs. The electrophysiologic properties of Purkinje fibers from the zone receiving ethanol were altered when compared to the control zone. The resting membrane potential was decreased (-76 +/- 2 mV vs. -85 +/- mV, p less than 0.001) with a decrease in action potential amplitude (91 +/- 4 vs. 109 +/- 2 mV, p less than 0.001) and phase 0 upstroke (231 +/- 27 vs. 456 +/- 25 V/sec, p less than 0.02). Prolonged refractoriness was observed in the ethanol zone without a prolongation of action potential duration. Intramural lesions observed within the left circumflex distribution varied from focal acute myofibrillar degeneration and necrosis to severe local scarring. The data suggest that intracoronary ethanol administration at human abuse levels of blood alcohol concentrations produces histologic and electrophysiologic injury in the canine heart. The electrophysiologic ch changes provide a substrate sufficient for the induction and maintenance of ventricular arrhythmia.

Systems analysis of the culture physiology in acetone-butanol fermentation

The Pronounced differences in performance of a strain of Clostridium acetobutylicum ATCC 824 were analyzed by the method of systems analysis. The mechanism for cellular transport of substrate (glucose), solvents, and acids was studied and mathematically formulated. The systems analysis approach in the treatment of data from culture experiments pointed out the cell membrane malfunction indicated by its altered permeability and reflected in the altered number of active sugar transport sites. Experimental results obtained from the study of the cell uptake of 3-0-methyl glucose (0.7mM) by the "normal culture" and the "retarded culture" confirmed the theoretical predictions regarding a slower transport in the retarded culture. The initial uptake rate and the accumulation coefficient of the sugar in the normal culture were 15.0 and 4.1 times higher, respectively, than those for the retardedculture. Adjustment of the culture pH resulted in further increases in these parameters by factors of 3.0 and 3.5, respectively.

Lipid composition and fluidity of rat enterocyte basolateral membranes. Regional differences

The lipid composition and fluidity of basolateral membranes prepared from the mucosa of the proximal, middle and distal thirds of the rat small intestine were determined. Fluidity, as assessed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene and a series of anthroyloxy fatty acid derivatives, is decreased in the distal third as compared to the proximal segments. This pattern is similar to that described previously for microvillus membranes. The decrease in fluidity of the distal as compared to the proximal membranes results from an increase in cholesterol content, cholesterol/phospholipid molar ratio and degree of saturation of the fatty acid residues. In the middle and distal thirds of the gut, the degree of saturation of the fatty acid residues is higher in microvillus as compared to basolateral membranes, accounting in part for the characteristically lower fluidity of the luminal membranes. The specific activity of the basolateral membrane (Na+ + K+)-dependent adenosine triphosphatase is significantly lower in the distal as compared to the proximal and middle thirds of the intestinal mucosa. Studies of the binding of [3H]ouabain indicate that this pattern results from fewer enzyme sites in the distal membranes.

Effect of triethyl lead on the placental uptake and transfer of the non-metabolisable alpha-aminoisobutyric acid in guinea pigs

In order to study the effect of organolead on the placental transfer of amino acids, triethyl lead chloride was administered by i.p. injections to pregnant guinea-pigs. The following day, the fetal part of the placenta was perfused in situ during i.v. infusion of alpha-aminoisobutyric acid (AIB) to the dam. It was found that triethyl lead treatment (2.5 mg/kg body wt) decreased the AIB-concentration in perfusion media compared with that of the control group. Treatment with 1 mg/kg body wt has no effect. It was also shown that the placental uptake of AIB in animals receiving 2.5 mg/kg body wt is significantly reduced compared with that of the control group. No such effect was obtained in animals receiving 1 mg/kg body wt. It is suggested that triethyl lead may inhibit placental Na+-K+-adenosinetriphosphatase, an enzyme involved with placental amino acid transport. Another possible explanation is a reduction in maternal placental blood flow.

Lipid dynamics and protein-lipid interactions in rat colonic epithelial cell basolateral membranes

Lipid dynamics and lipid-protein interactions were examined in basolateral membranes prepared from rat proximal and distal colonic epithelial cells. The results demonstrate that: (1) these membranes have a high lipid fluidity, as assessed by steady-state fluorescence polarization studies using seven fluorescent probes; (2) lipid compositional differences exist between these membranes but their fluidity is similar; (3) fluorescence polarization studies, using diphenylhexatriene (DPH), detect a thermotropic transition at 22-23 degrees C in each membrane; (4) several membrane protein activities, including adenylate cyclase and sodium-potassium dependent adenosine triphosphatase ((Na+ + K+)-ATPase) appear to be functionally dependent on the physical state of the proximal basolateral membrane's lipid.

Ethanol-induced inhibition of glucose transport across the isolated brush-border membrane of hamster jejunum

Acute exposure of jejunal mucosa to ethanol has been reported to produce a depression of transmural glucose transport across this organ in vitro and in vivo. In an attempt to understand the mechanism of action of ethanol on intestinal transport, in the present study we have investigated the effect of ethanol on glucose uptake by purified brush-border membrane vesicles of hamster jejunum. Ethanol, in concentrations found in man after moderate drinking (1-5% w/v), was found to depress glucose uptake by the brush-border membrane in a dose-dependent and time-dependent manner. Mannose was used to measure nonspecific uptake, and we found that the ethanol-induced depression of glucose uptake was not related to an alteration of the nonspecific uptake of this sugar. The inhibition of glucose uptake of the ethanol-treated membranes completely disappeared after repeated washing of the membranes with ethanol-free buffer. Accordingly, the ethanol-induced depression of glucose uptake was not the result of irreversible damage to membrane proteins but was related to a direct effect of ethanol on the brush-border membrane. On the basis of these findings, it is concluded that a direct interference with glucose translocation across the brush border plays an important role in the ethanol-induced depression of transmural jejunal glucose absorption.

Na+-K+-ATPase inhibition stimulates PGE release in guinea pig taenia coli

Inhibition of the plasma membrane enzyme Na+-K+-ATPase by ouabain zero extracellular K+, or low extracellular Na+, markedly augmented prostaglandin E release from the guinea pig taenia coli. Data suggest this phenomenon may be linked directly to Na+-K+-ATPase or Na+ pump activities, or changes in intracellular K+ concentration. The augmented prostaglandin E release was not due to changes in intracellular Na+, Ca2+, pH, or membrane potential, resulting from Na+ pump inhibition. The characteristics of the plasma membrane may exert a control on prostaglandin E release in this smooth muscle.

Reversible inhibition of (Na+, K+) ATPase by Mg2+, adenosine triphosphate, and K+

Adenosine triphosphate (ATP) hydrolysis catalyzed by the plasma membrane (Na+,K+)ATPase isolated from several sources was inhibited by Mg+, provided that K+ and ATP were also present. Phosphorylation of the adenosine triphosphatase (ATPase) by ATP and by inorganic phosphate was also inhibited, as was p-nitrophenyl phosphatase activity. (Ethylenedinitrilo)tetraacetic acid (EDTA) and catecholamines protected from and reversed the inhibition of ATP hydrolysis by Mg2+, K+ and ATP. EDTA was protected by chelation of Mg2+ but catecholamines acted by some other mechanism. The specificities of various nucleotides as inhibitors (in conjunction with Mg2+ and K+) and as substrates for the (Na+, K+) ATPase were strikingly different. ATP, ADP, beta,gamma-CH2-ATP and alpha,beta-CH2-ADP were active as inhibitors, whereas inosine, cytidine, uridine, and guanosine triphosphates (ITP, CTP, UTP, and GTP) and adenosine monophosphate (AMP) were not. On the other hand, ATP and CTP were substrates and beta,gamma-NH-ATP was a competitive inhibitor of ATP hydrolysis, but not an inhibitor in conjunction with Mg2+ and K+. The Ca2+-ATPase from sarcoplasmic reticulum and F1, the Mg2+-ATPase from the inner mitochondrial membrane, were also inhibited by Mg2+. Catecholamines reversed inhibition of the Ca2+-ATPase, but not that of F1.

A possible molecular mechanism of the action of digitalis: ouabain action on calcium binding to sites associated with a purified sodium-potassium-activated adenosine triphosphatase from kidney

Calcium binding at 0 degrees C to a purified sheep kidney Na+,K+-ATPase was described by linear Scatchard plots. Binding at saturating free calcium was 65-80 nmol/mg of protein, or 30-40 mol of calcium/mol of enzyme. Aqueous emulsions of lipids extracted from Na+,K+-ATPase yielded dissociation constants and maximum calcium-binding values that were similar to those for native Na+,K+-ATPase. Phospholipase A treatment markedly reduced calcium binding. Pretreatment of native Na+,K+-ATPase with ouabain increased the dissociation constant for calcium binding from 131 +/- 7 to 192 +/- 7 muM without altering maximum calcium binding. Ouabain pretreatment did not affect calcium binding to extracted phospholipids, ouabain-insensitive ATPases, or heat denatured Na+,K+-ATPase, Na+ and K+ (5-20 mM) increased the dissociation constants for calcium, which suggests competition between the monovalent cations and calcium for the binding sites. At higher concentrations of monovalent cations, ouabain increased the apparent affinity of binding sites for calcium. Extrapolation to physiological cation concentrations revealed that the ouabain-induced increase in apparent affinity for calcium may be as much as 2- to 3-fold. These results suggest: (1) calcium binds to phospholipids associated with Na+,K+-ATPase; (2) ouabain interaction with Na+,K+-ATPase induces a perturbation that is transmitted to adjacent phospholipids, altering their affinity for calcium; and (3) at physiological concentrations of Na+ or K+, or both, ouabain interaction with Na+,K+-ATPase may lead to an increased pool of membrane-bound calcium.

Kinetic analysis of ATPase mechanisms

At even the simplest level we can expect an ATPase mechanism to comprise the following four steps: the binding of ATP, the reaction of ATP with water on the enzyme, and the release of the products ADP and P1. So at the outset techniques are needed to investigate these four processes. The range of techniques needed is soon extended once questions are asked about the role of protons and metal ions, the possibility of a multistep hydrolytic process, multistep substrate and product binding processes, and protein–lipid or protein–protein interactions. Since ATPases and ATP synthases are almost universally involved in some form of energy transduction there is a particular need in an ATPase or ATP synthase reaction to evaluate the equilibrium constants of the steps in the mechanism and to investigate the possibility of alternate reaction pathways. The nature of the coupling process by the protein of the chemical reactions of ATP to the other energetic process, be it muscle contraction, active transport, respiration or photosynthesis, is likewise of profound interest. Finally we would like to know as much as possible about the ATPase or ATP synthase mechanism during the period when the various forms of energy transduction are occurring.

Adaptation of membrane lipids to alcohols

The effects of alcohols of different chain lengths on the fatty acid composition of Escherichia coli K-12 have been examined. My results indicate that these cells radically change their fatty acid composition when grown in the presence of alcohols. These changes represent an adaptive membrane alteration compensating for the direct physicochemical interaction of alcohols with the membrane. Similar adaptive responses of membrane lipids are proposed as a possible biochemical basis for tolerance to alcohol and related drugs.

Polar head-group and acyl side-chain requirements for phospholipid-dependent (Na-+ plus K-+)-ATPase

The abilities of different phospholipids to reactivate a lipid-depleted (Na-+ plus K-+)-ATPase (EC 3.6.1.3) have been compared. The phospholipids contained either the same group of hydrocarbon chains withhh different polar groups, or different hydrocarbon groups wit the same polar group; they were prepared by enzymic modification of the polar group of phosphatidylserine isolated from bovine brain and of phosphatidylcholine extracted from egg. Only the acidic phospholipids reactivated the ATPase but the amount of reactivation depended on the nature of the hydrocarbon chains as well as the polar group. These findings are discussed in relations to the compositions of the two different groups of fatty acyl chains constituting the hydrocarbon portions of the phospholipids.

Phospholipid unsaturation and plasma membrane organization

A comparison has been made between the unsaturation of plasma-membrane phospholipids,present in the human erythrocyte, rat liver, mouse liver and a rapidly growing rat hepatoma. Of the double bonds present in the hydrocarbon chains of the membrane phospholipids,onethird is contributed by sphingomyelin plus phosphatidyl choline and the remainder by phosphatidyl serine, ethanolamine and inositol. Assuming that the phospholipids are asymmetrically distributed in the two leaflets of the bilayer in general, the consequences of this asymmetry in combination with cholesterol content and fatty acid distribution on plasma membrane organization and function are discussed. It is suggested, that the organizational disposition of plasma membrane components other than phospholipids is at least related if not dependent upon the latter's asymmetric distribution in the bilayer.

Magnetic resonance and kinetic studies of the mechanism of membrane-bound sodium and potassium ion- activated adenosine triphosphatase

EPR and water proton relaxation rate (1/T1) studies of partially (40%) and "fully" (90%) purified preparations of membrane-bound (Na+ + K+) activated ATPase from sheep kidney indicate one tight binding site for Mn2+ per enzyme dimer, with a dissociation constant (KD = 0.88 muM) in agreement with the kinetically determined activator constant, identifying this Mn2+-binding site as the active site of the ATPase. Competition studies indicate that Mg2+ binds at this site with a dissociation constant of 1 mM in agreement with its activator constant. Inorganic phosphate and methylphosphonate bind to the enzyme-Mn2+ complex with similar high affinities and decrease 1/T1 of water protons due to a decrease from four to three in the number of rapidly exchanging water protons in the coordination sphere of enzyme-bound Mn2+. The relative effectiveness of Na+ and K+ in facilitating ternary complex formation with HPO2-4 and CH3PO2-3 as a function of pH indicates that Na+ induces the phosphate monoanion to interact with enzyme-bound Mn2+. Thus protonation of an enzyme-bound phosphoryl group would convert a K+-binding site to a Na+-binding site. Dissociation constants for K+ and Na+, estimated from NMR titrations, agreed with kinetically determined activator constants of these ions consistent with binding to the active site. Parallel 32Pi-binding studies show negligible formation (less than 7%) of a covalent E-P complex under these conditions, indicating that the NMR method has detected an additional noncovalent intermediate in ion transport. Ouabain, which increases the extent of phosphorylation of the enzyme to 24% at pH 7.8 and to 106% at pH 6.1, produced further decreases in 1/T1 of water protons. Preliminary 31P- relaxation studies of CH3PO2-3 in the presence of ATPase and Mn2+ yield an Mn to P distance (6.9 +/- 0.5 A) suggesting a second sphere enzyme-Mn-ligand-CH3PO2-3 complex. Previous kinetic studies have shown that T1+ substitutes for K+ in the activation of the enzyme but competes with Na+ at higher levels. From the paramagnetic effect of Mn2+ at the active site on the enzyme on I/T1 of 205T1 bound at the Na+ site, a Mn2+ to T1+ distance of 4.0 +/- 0.1 A is calculated, suggesting the sharing of a common ligand atomy by Mn2+ and T1+ on the ATPase. Addition of Pi increases this distance to 5.4 A consistent with the insertion of P between Mn2+ and T1+. These results are consistent with a mechanism for the (Na+ + K+)-ATPase and for ion transport in which the ionization state of Pi at a single enzyme active site controls the binding and transport of Na+ and K+, and indicate that the transport site for monovalent cations is very near the catalytic site of the ATPase. Our mechanism also accounts for the order of magnitude weaker binding of Na+ compared to K+.

Isolation and characterization of the components of the sodium pump

A satisfactory understanding of the functions of the sodium pump, the system responsible for the active transport of sodium and potassium, require the isolation and characterization of its protein and lipid components which are integrated in the structure of the cell membrane. The enzyme system (Na++ K+)-ATPase, is located in membrane fragments and behaves in the test tube like the transport system in the intact cell membrane (Skou,1957) Purified preparations of this enzyme will contain some, if not all, of the components of the sodium pump.