Activation of (Na+ + K+)-dependent ATPase by lipid vesicles of negative phospholipids

The activity of the intrinsically water-soluble enzyme ADAMTS13 correlates with the membrane state when bound to a phospholipid bilayer

Membrane-associated enzymes have been found to behave differently qualitatively and quantitatively in terms of activity. These findings were highly debated in the 1970s and many general correlations and reaction specific models have been proposed, reviewed, and discarded. However, new biological applications brought up the need for clarification and elucidation. To address literature shortcomings, we chose the intrinsically water-soluble enzyme a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) and large unilamellar vesicles with a relative broad phase transition. We here present activity measurements of ADAMTS13 in the freely dissolved state and the membrane associated state for phosphocholine lipids with different acyl-chain lengths (13:0, 14:0 and 15:0) and thus main phase transition temperatures. While the freely dissolved enzyme shows a simple Arrhenius behavior, the activity of membrane associated ADAMTS13 in addition shows a peak. This peak temperature correlates with the main phase transition temperature of the used lipids. These findings support an alternative theory of catalysis. This theory predicts a correlation of the membrane associated activity and the heat capacity, as both are susceptibilities of the same surface Gibb's free energy, since the enzyme is attached to the membrane.

Ethanol potentiates the uptake of [14C]serine into phosphatidylserine by base-exchange reaction in NG 108-15 cells

Phospholipid base-exchange enzymes catalyze the incorporation of nitrogenous bases into phosphoglycerides by a calcium-dependent mechanism. In this study, we describe the effect of ethanol on the incorporation of radioactive serine, choline and ethanolamine into their respective phospholipids in a neuroblastoma x glioma hybrid cell line (NG 108-15). Long term ethanol exposure induced a potentiation of the incorporation of [14C]serine into phosphatidylserine. Moreover, the phosphorus content of PS was found to be increased after long-term ethanol exposure. No concomitant changes in the phosphorus content of other phospholipids were observed. The results indicate that in NG 108-15 cells, the incorporation of radiolabelled serine into PS is potentiated during chronic ethanol exposure.

In utero ethanol exposure decreases the biosynthesis of phosphatidylserine in rat pup cerebrum

Phosphatidylserine is enriched in the brain and has been implicated to play a role in regulating neuronal membrane functions. In this study, three experimental protocols were used to examine the effects of in utero ethanol exposure on phosphatidylserine biosynthesis in rat pup brain, namely, (1) assay of the serine base-exchange enzyme activity in brain microsomes, (2) incubation of brain slices with [3H] serine, and (3) incorporation in vivo of [3H]serine into phosphatidylserine as well as serine-related phospholipids in brain. Results from all three protocols point to a decrease in phosphatidylserine biosynthesis in newborn rat pup cerebrum on exposure to ethanol in utero compared with the pair-fed controls. When in utero ethanol-exposed pups were nursed by mothers given a chow diet, the differences gradually returned to control levels by 17 days of age. The decrease in phosphatidylserine biosynthesis may be important in explaining some of the neuronal deficits associated with in utero ethanol exposure.

Phospholipid base exchange enzyme activity in sarcolemmal membranes from the heart of cardiomyopathic hamsters

The activity of phospholipid base exchange enzymes has been evaluated in cardiac sarcolemmal membranes from Syrian Golden hamsters and from a hamster strain (UM-X7.1) characterized by a genetic form of hypertrophic cardiomyopathy. No choline base exchange activity and only a little serine base exchange activity were detected, whereas the ethanolamine base exchange enzyme was found highly active in membranes from both strains. For this reason, the present study is focussed on the ethanolamine base exchange enzyme. The apparent Km for ethanolamine of ethanolamine base exchange enzyme from Syrian Golden membranes and from UM-X7.1 strain membranes are 18 and 32 microM, respectively. The specific activity of the sarcolemmal ethanolamine base exchange enzyme is lower in the UM-X7.1 strain than in Syrian Golden hamsters. The calcium-dependence of the enzyme appears different when the membranes from the two strains are compared. Indeed, after removal of the membrane-bound divalent cations, comparable activities are found in both membrane preparations, whereas, upon addition of Ca2+ to the incubation mixtures, the activity of the enzyme is enhanced in the membranes from Syrian Golden strain more than in those from UM-X7.1 strain. The cholesterol content of sarcolemmal membranes is higher in the cardiomyopathic strain than in the Syrian Golden hamsters. A possible relation between changes of the membrane lipid composition and of the ethanolamine base exchange activity is discussed.

Neurite-promoting activities of phosphatidylinositol and other lipids on fetal rat septal neurons in culture

Neurite-promoting activities of lipids were assessed using serum-free cultures of fetal rat septal neurons. The most potent one was phosphatidylinositol (PI), followed by PI 4-phosphate, phosphatidylserine, sphingomyelin, and phosphatidylcholine. The EC50 value for PI was 1.5 micrograms/ml (1.8 microM), and activity was maximal at 4 micrograms/ml (56% of total cells had neurites after 24 h). Cerebroside, sulfatide, and di- and triacylglycerols showed relatively low activities. Synthetic dipalmitoyl phosphatidylcholine was also active, with a maximal activity (47%) at 100 micrograms/ml, a finding implying that the unsaturated fatty acid moiety is not released and further used as substrate for the arachidonic acid cascade. Lysophospholipids, phosphatidylglycerol, and cardiolipin were rather cytotoxic and lacked activity, an observation suggesting that membrane perturbation is not responsible for the neurite-promoting activity. Treatment with a protein kinase C inhibitor, H-7, or an Na+,K(+)-ATPase inhibitor, ouabain, inhibited the PI-induced neurite outgrowth, but the cyclic AMP- and cyclic GMP-dependent protein kinase inhibitor HA1004 did not inhibit this activity, a result indicating that multiple elements (protein kinase C and Na+,K(+)-ATPase) are involved in the induction of neurites. Because phospholipids can be provided either as lipid vesicles or as lipoproteins produced by macrophages at regeneration sites, they may play an important role in the regeneration of certain populations of neurons.

24-hour preservation of isolated rat hearts perfused with pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) solution at low temperature

We examined the effects of long-term perfusion with pyridoxalated hemoglobin polyoxyethylene conjugate (PHP) solution on cardiac function of isolated rat hearts. Hearts were perfused with oxygenated Krebs-Henseleit (K-H) solution containing 3% PHP or hydroxyethyl starch (HES) at a constant pressure of 13 mm Hg for 24 h at 15 degrees C. After 24 h preservation, hearts were rewarmed with K-H solution. Heart rate (HR) in PHP-preserved hearts was almost the same as control and cardiac contractility was maintained at 70% of control, but coronary outflow decreased to about 50% of control. No edema developed in the PHP group. Addition of a Ca2+ antagonist, diltiazem, inhibited the elevation of the end-diastolic pressure significantly. In HES-preserved hearts, HR did not recover to control levels, but there was no significant difference in cardiac contractility between PHP- and HES-perfused hearts. The results demonstrate that isolated hearts can be preserved by perfusion with PHP solution under hypothermic conditions for 24 h and suggest that PHP solution would be useful for a perfusate of isolated organs and tissues for preservation.

Partial purification and characterization of the human erythrocyte Mg2(+)-ATPase. A candidate aminophospholipid translocase

A Mg2(+)-ATPase-enriched fraction was obtained from solubilized human erythrocyte membranes by ammonium sulphate precipitation and anion-exchange chromatography. The solubilized enzyme, of apparent molecular weight 120 kDa, requires phosphatidylserine to be fully active. Phosphatidylethanolamine but not other anionic phospholipids can only partially restore the activity. The Mg-ATPase has a low affinity for Mg2(+)-ATP and is inhibited by fluoride, vanadate, vanadyl and calcium ions. From these characteristics, we infer that this Mg2(+)-ATPase is the same protein as the aminophospholipid translocase which regulates the membrane phospholipid transverse distribution in human erythrocytes by actively transporting aminophospholipids from the outer to the inner monolayer.

Spin label studies on the selectivity of lipid-protein interaction of cardiolipin analogues with the Na+/K+-ATPase

The selectivity of lipid-protein interaction for various spin-labelled cardiolipin analogues in Na+/K+-ATPase membranes from Squalus acanthias has been investigated by ESR spectroscopy. Cardiolipin derivatives with different numbers of acyl chains, or in which the headgroup charge has been removed by methylation of the phosphate groups, all show a pronounced selectivity relative to phosphatidylcholine. Maximally three times more of the cardiolipin analogue is associated with the protein, than is phosphatidylcholine. The selectivity pattern in the absence of salt is in the order: cardiolipin approximately monolysocardiolipin greater than or equal to acylcardiolipin greater than dimethylcardiolipin much greater than phosphatidylcholine, where acylcardiolipin has the spin label chain attached to the centre-OH group of the headgroup. The degree of association of the negatively charged cardiolipins with the protein is reduced by salt, corresponding to the lower selectivity for dimethylcardiolipin. It is concluded that the selectivity of the Na+/K+-ATPase for cardiolipin is not solely of electrostatic origin, nor is it likely to originate in the larger number of fatty acid chains relative to diacyl phospholipids.

Phospholipid environment alters hormone-sensitivity of the purified insulin receptor kinase

Insulin receptor kinase, affinity-purified by adsorption and elution from immobilized insulin, is stimulated 2-3-fold by insulin in detergent solution. Reconstitution of the receptor kinase into leaky vesicles containing phosphatidylcholine and phosphatidylethanolamine (1:1, w/w) by detergent removal on Sephadex G-50 results in the complete loss of receptor kinase sensitivity to activation by insulin. Insulin receptors in these vesicles also exhibit an increase in their apparent affinity for 125I-insulin (Kd = 0.12 nM versus 0.76 nM). Inclusion of 8.3-16.7% phosphatidylserine into the reconstituted vesicles restores 40-50% of the insulin-sensitivity to the receptor kinase. An elevated apparent affinity for 125I-insulin of insulin receptors in vesicles containing phosphatidylcholine and phosphatidylethanolamine is also restored to the value observed in detergent solution by the inclusion of phosphatidylserine in the reconstituted system. The effect of phosphatidylserine on insulin receptor kinase appears specific, because cholesterol, phosphatidylinositol and phosphatidic acid are all unable to restore insulin-sensitivity to the receptor kinase. Autophosphorylation sites on the insulin receptor as analysed by h.p.l.c. of tryptic 32P-labelled receptor phosphopeptides are not different for insulin receptors autophosphorylated in detergent solution or for the reconstituted vesicles in the presence or absence of phosphatidylserine. These data indicate that the phospholipid environment of insulin receptors can modulate its binding and kinase activity, and phosphatidylserine acts to restore insulin-sensitivity to the receptor kinase incorporated into phosphatidylcholine/phosphatidylethanolamine vesicles.

Selectivity of lipid-protein interactions

The spin label ESR and intrinsic fluorescence quenching methods of determining the selectivity of interactions of lipids with integral membrane proteins are summarized. The selectivity patterns of phospholipids, fatty acids, and steroids are reviewed for a variety of integral proteins. Where appropriate, correlations are established with biochemical assays of the effects of specific lipids on enzymatic activity and transport function.

Spin-label studies of lipid-protein interactions in (Na+,K+)-ATPase membranes from rectal glands of Squalus acanthias

Lipid-protein interactions in (Na+,K+)-ATPase-rich membranes from the rectal gland of Squalus acanthias have been studied by using spin-labeled lipids in conjunction with electron spin resonance (ESR) spectroscopy. Lipid-protein associations are revealed by the presence of a second component in the ESR spectra of the membranes in addition to a component which corresponds very closely to the ESR spectra obtained from dispersions of the extracted membrane lipids. This second component corresponds to spin-labeled lipids whose motion is very significantly restricted relative to that of the fluid lipids in the membrane or the lipid extract. A stoichiometry of approximately 66 lipids per 265 000-dalton protein is found for the motionally restricted component of those spin-labeled lipids (e.g., phosphatidylcholine) which show least specificity for the protein. This corresponds approximately to the number of lipids which may be accommodated within the first shell around the alpha 2 beta 2 protein dimer. A selectivity of the various spin-labeled lipids for the motionally restricted component associated with the protein is found in the following order: cardiolipin greater than phosphatidylserine approximately stearic acid greater than or equal to phosphatidic acid greater than phosphatidylglycerol approximately phosphatidylcholine approximately phosphatidylethanolamine approximately androstanol.

Ethanol and membrane lipids

Although ethanol is known to exert its primary mode of action on the central nervous system, the exact molecular interaction underlying the behavioral and physiological manifestations of alcohol intoxication has not been elucidated. Chronic ethanol administration results in changes in organ functions. These changes are reflective of the adaptive mechanisms in response to the acute effects of ethanol. Biophysical studies have shown that ethanol in vitro disorders the membrane and perturbs the fine structural arrangement of the membrane lipids. In the chronic state, these membranes develop resistance to the disordering effects. Tolerance development is also accompanied by biochemical changes. Although ethanol-induced changes in membrane lipids have been implicated in both biophysical and biochemical studies, measurements of membrane lipids, such as cholesterol content, fatty acid unsaturation, phospholipid distribution, and ganglioside profiles, have not produced conclusive evidence that any of these parameters are directly involved in the action of ethanol. On the other hand, there is increasing evidence indicating that although ethanol in vitro produces a membrane-fluidizing effect, the chronic response to this effect is not to change the membrane bulk lipid composition. Instead, changes in membrane lipids may pertain to small metabolically active pools located in certain subcellular fractions. Most likely, these lipids are involved in important membrane functions. For example, the increase in PS in brain plasma membranes may provide an explanation for the adaptive increase in synaptic membrane ion transport activity, especially (Na,K)-ATPase. There is also evidence that the lipid pool involved in the deacylation-reacylation mechanism (i.e., PI and PC with 20:4 groups) is altered after ethanol administration. An increase in metabolic turnover of these phospholipid pools may have important implications for the membrane functional changes. Obviously, there are other lipid-metabolizing enzyme systems that may exert similar effects but have not yet been investigated in detail. From the results of these studies, it is concluded that the multiple actions of ethanol are associated with changes in enzymic systems important in the functional expression of the membranes.

Response of rat heart membranes and associated ion-transporting ATPases to dietary lipid

The effects of different dietary fat intake on the lipid composition and enzyme behaviour of sarcolemmal (Na+ + K+)ATPase and sarcoplasmic reticulum Ca2+-ATPase from rat heart were investigated. Rat diets were supplemented with either sunflower seed oil (unsatd./satd. 5.6) or sheep kidney fat (unsatd./satd. 0.8). Significant changes in the phospholipid fatty acid composition were observed in both membranes after 9 weeks dietary lipid treatment. For both membranes, the total saturated/unsaturated fatty acid levels were unaffected by the dietary lipid treatment, however the proportions of the major unsaturated fatty acids were altered. Animals fed the sunflower seed oil diet exhibited an increase in n-6 fatty acids, including linoleic (18:2(n-6] and arachidonic (20:4(n-6] while the sheep kidney fat dietary rats were higher in n-3 fatty acids, principally docosahexaenoic (22:6), with the net result being a higher n-6/n-3 ratio in the sunflower seed oil group compared to sheep kidney fat dietary animals. Fluorescence polarization indicated that the fluidity of sarcoplasmic reticular membrane was greater than that of sarcolemmal membrane, with a dietary lipid-induced decrease in fluidity being observed in the sarcoplasmic reticular membrane from sheep kidney fat dietary animals. Despite these significant changes in membrane composition and physical properties, neither the specific activity nor the temperature-activity relationship (Arrhenius profile) of the associated ATPases were altered. These results suggest that with regard to the parameters measured in this study, the two ion-transporting ATPases are not modulated by changes which occur in the membrane lipid composition as a result of the diet.

Ischemia-induced changes in sarcolemmal (Na+, K+)-ATPase, K+-pNPPase, sialic acid, and phospholipid in the dog and effects of the nisoldipine and chlorpromazine treatment

This work was undertaken to study functional and structural changes of the cardiac sarcolemmal membrane which was isolated from the ischemic lesion in the dog. The sarcolemmal fraction was prepared, by adopting the method devised by Reeves and Sutko , from the right ventricle and the subendocardial and subepicardial layers of the left ventricle. Ischemic lesion was produced by occlusion of a branch of the left anterior descending coronary artery for a period of 1.5 hr in the thoracotomized dog, followed by release of the occlusion for 3 hr. Nisoldipine, 5 micrograms/kg, was given twice intravenously, and chlorpromazine was infused at a rate of 10 micrograms/kg X min, in addition to the administration of twice bolus doses of 400 micrograms/kg each. Nisoldipine significantly decreased the incidence of premature ventricular contractions and microvascular hemorrhage. Sarcolemmal purity was monitored by using enzyme and chemical markers; the results indicated that the membrane preparation was tenfold purified over the homogenate. Although the activities of ouabain-sensitive (Na+, K+)-ATPase and ouabain-sensitive K+-p-nitrophenylphosphatase ( pNPPase ) of the sarcolemmal preparation isolated from the subendocardial layer were similar to those from the subepicardial layer in the nonischemic left ventricle, a significant decrease in these activities was observed only when the sarcolemmal fraction isolated from the subendocardial layer of ischemic area was compared with that from the subendocardial layer of nonischemic area. In contrast, the sialic acid content of the sarcolemma from the ischemic subendocardial layer was significantly increased compared to that of the nonischemic subendocardial layer. No such changes occurred in sarcolemma prepared from the ischemic subepicardial layer. The total phospholipid content as well as phosphatidylcholine and -ethanolamine contents of the sarcolemmal membrane prepared from the subendocardial layer of ischemic area were significantly decreased compared to nonischemic area. Nisoldipine prevented the ischemia-induced alterations in sarcolemmal (Na+, K+)-ATPase, pNPPase , sialic acid and phospholipids of the subendocardial layer. Chlorpromazine showed a less consistent effect than did Nisoldipine under our experimental conditions. Our study thus demonstrates that the lipid component and function of cardiac sarcolemmal membrane are altered in the early ischemic lesion and that these alterations are nonuniform in distribution and are alleviated by some pharmacological intervention.

Phospholipid composition and metabolism of crustacean gills as related to changes in environmental salinities: relationship between Na+-K+-ATPase activity and phospholipids

The phospholipid composition and metabolism are studied in crustacean gills. It is reported that branchiae are rich in PC, PE and DPG and abundant in long chain polyunsaturated fatty acids (20:4 omega 6 and 20:5 omega 3 acids). The pathways of phospholipids synthesis appear similar to those described for vertebrates. It is demonstrated that there exist significant differences in the level of phosphatides between the anterior and posterior gills of Eriocheir sinensis. No matter what the salinity, the three more posterior located gills of Chinese crabs are shown to contain much more unsaturated phospholipids (PE and DPG). This is particularly true when animals are acclimated to dilute media. Moreover, lipids of posterior gills appear more fluid than the anterior ones as reported by the values of the degree of fluorescence polarization and the index of unsaturation of fatty acids. It is suggested that these lipid changes may indicate the existence of a functional difference between the various branchiae of euryhaline Eriocheir sinensis with respect to their ability to transport salt. It is shown that the renewal of DPG and PS is increased in posterior gills isolated from freshwater Chinese crabs. It is postulated that the enhanced formation of DPG in posterior gills is an indicator of an increased synthesis of mitochondria having as principal function to produce the necessary energy for the Na+ uptake. An attempt is made to correlate the PS metabolism and the Na+-K+-ATPase activity which is particularly located in the mitochondrial fractions of the three pairs of posterior gills.

Chronic ethanol administration induced an increase in phosphatidylserine in guinea pig synaptic plasma membranes

Chronic ethanol administration to guinea pigs via intragastric intubation elicited a specific increase (50%) in phosphatidylserine in the synaptic plasma membrane. The ethanol-treated group also showed a 53% increase in synaptosomal (Na,K)-ATPase activity. Analysis of the acyl group composition of individual phospholipids in the same membrane fraction revealed only small changes which varied depending on the type of phospholipids. Since the (Na,K)-ATPase is known to be specifically activated by phosphatidylserine, the adaptive increase in enzyme activity during chronic ethanol treatment may be related to the increase in this type of negatively charged phospholipid.

Modulation of cardiac glycoside inhibition of (Na+ + K+)-ATPase by membrane lipids. Difference between species

The role of lipids in the modulation of the ouabain-sensitivity of membrane (Na+ + K+)-ATPase from different species has been studied using a reconstitution procedure which promotes lipid exchange during detergent depletion by Sephadex chromatography. Hybrid reconstitution of delipidated (Na+ + K+)-ATPase preparations from bovine brain into the lipids obtained from crab nerve enzyme preparations significantly reduces the sensitivity of the brain enzyme to inhibition by ouabain. Conversely, reconstitution of crab nerve enzyme into the lipids from bovine brain enzyme preparations increases the sensitivity of the crab enzyme to ouabain inhibition. These opposing effects demonstrate the role of membrane lipids in modulating the enzyme-inhibition relationship in preparations from these different species.

Lipid-protein interactions of reconstituted membrane-associated adenosinetriphosphatases. Use of a gel-filtration procedure to examine phospholipid-activity relationships

A gel-filtration procedure is described for the reconstitution of partially delipidated membrane adenosinetriphosphatases (Mg2+-ATPase and (Na+ + K+)-ATPase) into liposomes of defined composition. After detergent solubilization of membrane enzyme preparations, reconstitution of these ATPases was achieved by the rapid removal of deoxycholate by Sephadex G-50 chromatography. Proteoliposomes were separated from unincorporated enzyme by chromatography on Sepharose CL-4B. Sedimentation characteristics in sucrose density gradients and electron microscopy confirmed that both Mg2+-ATPase and (Na+ + K+)-ATPase were reconstituted into liposomes of phosphatidylcholine and yielded preparations having high recoveries of enzyme activity by comparison with the control membrane preparations. Reconstitution of (Na+ + K+)-ATPase into synthetic phosphatidylcholines of defined fatty acid composition reveals an inverse relationship between enzyme activity and the chain length of the saturated fatty acids DMPC, DPPC and DSPC. Higher recoveries were obtained when one or more fatty acid chains was unsaturated. Full reactivation occurred with DOPC (18:1/18:1). There was a positive correlation between the specific activity of reconstituted (Na+ + K+)-ATPase and the temperature of the thermal phase transition of the synthetic phosphatidyl cholines studied. This was not seen with Mg2+-ATPase. It is suggested that 'membrane fluidity' influences the catalytic activity of (Na+ + K+)-ATPase but not that of Mg2+-ATPase.

Lipid-dependent membrane enzymes. A kinetic model for cooperative activation in the absence of cooperativity in lipid binding

The dependence of integral membrane enzymes on lipid activators in analyzed in terms of multiple binding site kinetics. Rate equations for an enzyme with n independent and indentical lipid binding sites are derived for the case that enzyme activity is proportional to the total amount of lipid bound, or that only fully substituted enzyme is active. A third equation applies to the case that lipids bind with infinite cooperativity to give fully substituted and active enzyme. None of the three models was entirely consistent with existing experimental data. The following kinetic model is shown to accommodate the degree of cooperativity observed in lipid activation experiments as well as the number of independent lipid-binding sites determined by electron-spin resonance measurements. The membrane enzyme is assumed to have n non-interacting and identical lipid-binding sites. Only fully substituted enzyme (ELn) and the next most highly substituted forms such as ELn-1 and ELn-2 may possess enzyme activity. These assumptions lead to cooperativity in activation. Cooperativity reaches a maximum when enzyme activity starts to appear with about 80% of the full lipid substitution. The increase in cooperativity is accompanied by a decrease in the lipid concentration required for half-maximal activation. Further kinetic aspects of a dynamic boundary lipid layer around integral membrane enzymes are discussed.

Regulation of GABA receptor binding to synaptic plasma membrane of rat cerebral cortex: the role of endogenous phospholipids

Triton X-100 treatments produced an extensive depletion of proteins and phospholipids and a marked increase of [3H] GABA binding on synaptic plasma membranes (SPM). Maximal [3H]GABA binding was obtained with three Triton X-100 treatments (+ 174% with respect to control). Phospholipase C, which removes only the phospholipid polar head, induces a 40% increase of [3H]GABA binding only after treatments resulting in extensive protein depletion. In reconstitution experiments phosphatidylethanolamine, the largest phospholipid removed, induced a 30-35% inhibition of [3H]GABA binding in Triton X-100 treated membranes; in contrast phosphatidylserine and phosphatidylcholine did not produce significant changes. The reconstitution of phospholipase C-treated SPM preparations with exogenous phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine or phosphoethanolamine and 1,2-dipalmitoylglycerol, products of phospholipase C activity, did not yield significant changes. This evidence, which argues against a direct role of phospholipids on the regulation of GABA binding, should, however, suggest that the GABA binding component of the receptor site is a lipoprotein or a lipid-depending protein.

The fraction of phosphatidylinositol that activates the (Na+ + K+)-ATPase in rabbit kidney microsomes is clearly associated with the enzyme protein

1. Extensive treatment of rabbit kidney microsomes with phosphatidylinositol-specific phospholipase C under various conditions never resulted in more than 75% hydrolysis of the substrate. 2. The non-degraded fraction of the phosphatidylinositol (10-12 nmol per mg microsomal protein) could be recovered only by an acidic extraction procedure. 3. The (Na+ + K+)-ATPase activity found in those membranes was not affected by this treatment. 4. Complete degradation of phosphatidylinositol could be easily achieved when the phospholipase was applied to rat liver microsomes which do not contain any detectable (Na+ + K+)-ATPase activity. 5. It is concluded that in rabbit kidney microsomes a close association exists between the (Na+ + K+)-ATPase and that fraction of the phosphatidylinositol that is directly involved in the maintenance of its activity.

The role of phospholipids in the modulation of enzyme activities in the chromaffin granule membrane

(1) 93% of protein of chromaffin granule membranes can be solubilized by 1.3% (w/v) sodium cholate. The solubilized material can be substantially delipidated by ammonium sulphate precipitation. After three such cycles less than 2% of the endogenous phospholipids remain. (2) The chromaffin granule membrane Mg2+-ATPase depends on the presence of phospholipids for retention of its full activity. Soybean and extracted chromaffin granule phospholipids fully reactivate the delipidated enzyme provided only one delipidation step is used. (3) Successive ammonium sulphate precipitation steps result in a delipidated, and deactivated ATPase preparation which can be only partially reactivated on re-addition of phospholipids. (4) The phospholipid specificity for reactivation of the Mg2+-ATPase is broad. Although acidic phospholipids allow higher activities than neutral phospholipids, the main requirement appears to be the hydrophobic environment provided by the phospholipid hydrocarbon chains. (5) Correlations between changes in slope in the Arrhenius plot of the Mg2+-ATPase, and phase transitions in the phospholipid used for reactivation suggest that the 'fluidity' of the hydrocarbon chains modulates the activity of the enzyme.

Electrostatic control by lipids upon the membrane-bound (Na+ + K+)-ATPase

In this paper, the membrane-bound (Na+ + K+)-ATPase from bovine brain is shown to be controlled by electrostatic alterations of the charged lipids surrounding the enzyme. The properties under investigation are the enzymatic activity, activation energy and the response of the enzymatic system to temperature. Arrhenius plots of the ATPase activity are biphasic with a break at temperature Ti. The temperature Ti, the activation energies at temperatures above and below Ti, and the enzymatic activity at any constant temperature have been shown to depend upon the concentrations of alkali and alkaline-earth metal ions in the solution. These electrolyte dependencies are ascribed to changes of electrostatic conditions at the lipids surrounding the ATPase. If the higher electrostatic screening ability of divalent ions is taken into account, the results in the presence of mono- and divalent ions become virtually the same. As a result of this work, it is concluded that electrostatic alterations are transmitted to the ATPase from the lipids of the membrane in which the enzyme is embedded. Inhibition and activation of the enzyme by mono-and divalent metal ions may thus be explained without any auxiliary hypothesis, particularly without postulating specific binding sites for the different ionic species at the protein. In addition, the specific lipid requirement of the ATPase may be understood better in the light of this interpretation.

Diphosphatidylglycerol is required for optimal activity of beef heart cytochrome c oxidase

Isolated beef heart cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) contains four or five molecules of tightly bound diphosphatidylglycerol per monomer (2-heme complex). This lipid could be removed in part, or wholly, by mixing the enzyme with high concentrations of Triton X-100 and then centrifuging the mixture through a glycerol gradient equilibrated in the same detergent. Cytochrome c oxidase retaining three or more diphosphatidylglycerol molecules per monomer was fully active when assayed in 1-oleoyl lysophosphatidylcholine. Upon removal of one or more of these diphosphatidylglycerols, enzymic activity was lost. Full activation could be obtained by adding diphosphatidylglycerol to the assay mixture along with lysophosphatidylcholine but not by adding phosphatidylcholine or phosphatidylethanolamine. Direct binding experiments, kinetic studies, and previous work using arylazidocytochrome c derivatives [Bisson, R., Jacobs, B. & Capaldi, R. A. (1980) Biochemistry 10, 4173-4178], indicate that diphosphatidylglycerol is involved in binding of substrate cytochrome c to cytochrome c oxidase.

Charge selectivity at the lipid-protein interface of membranous Na,K-ATPase

Lipid interactions with the integral membrane protein Na,K-ATPase (ATP phosphohydrolase, EC 3.6.1.3) purified from the electric organ of Electrophorus electricus were studied by spin labeling. A protein-associated component (boundary layer) in equilibrium with the fluid bilayer is clearly evident in the electron spin resonance spectra. The influence of charge on this equilibrium was determined by varying the head group of the lipid while maintaining the chain length and the position of the label constant. The lipid spin labels were 14-proxylstearylmethyl phosphate and the corresponding dimethylphosphate, alcohol, and quaternary amine. By using a pairwise spectral analysis, as well as a conventional spectral analysis, the binding affinity was found to decrease in the order of negative greater than neutral greater than positive charges. The fraction bound decreased from about 0.57 for the negatively charged phosphate to 0.25 for the positively charged quaternary amine. The amount of each bound lipid was nearly constant over the temperature range investigated (5-35 degrees C). High salt concentrations reversibly abolished the selectivity between the labels, confirming the role of charge in the binding equilibria.

Lipid environment of gastric potassium ion-stimulated adenosine triphosphatase

The K+-stimulated ATPase associated with the purified gastric microsomal fraction can be completely inactivated by treatment with 15% (v/v) ethanol for 60s at 37 degrees C, but not at 25 degrees C. Sequential exposure of the microsomal fraction to 15% ethanol at 25 degrees C and 37 degrees C caused release of 2.5% and 2.9% of the total membrane phospholipids respectively. Restoration of the enzyme activity was achieved by sonication with phosphatidylcholine in the presence of Mg2+, K+ and ATP, which were essential for the reconstitution. Our data suggest that the phospholipids extracted by 15% ethanol at 37 degrees C are derived primarily from the immediate lipid environment of the enzyme, and ATP, together with the metal ions, helps the partially delipidated enzyme to retain the appropriate configuration for the subsequent reconstitution.

Erythrocyte lipid composition and sodium transport in human liver disease

In patients with liver disease there are usually increases in erythrocyte cholesterol and phosphatidylcholine concentrations. This increase in membrane lipid changes the shape of the erythrocyte and "spur" or "target" cells may be present. Sodium fluxes were measured in erythrocytes from 17 patients with a variety of liver diseases and from 17 normal subjects and the values related to the lipid content of the membrane. Ouabain-insensitive and ouabain-sensitive effluxes were lower in patients than in normal subjects and the reduction in ouabain-insensitive efflux was more marked. Sodium influx was also significantly lower in erythrocytes from patients than controls. Ouabain-sensitive and ouabain-insensitive effluxes and sodium influx did not correlate with the cholesterol content of erythrocytes from patients. Significant negative correlations were noted between ouabain-insensitive sodium efflux (r=--0.63, P less than 0.01), sodium influx (r=--0.61, P less than 0.01) and intracellular sodium concentration (r=--0.66, P less than 0.01) and the cholesterol : phospholipid molar ratio of the cell but there was no significant correlation between this ratio and the ouabain-sensitive sodium efflux (r=0.41, P less than 0.05). These results support the hypothesis that an altered lipid composition may affect the permeability of the erythrocyte membrane in patients with liver disease.

Phosphatidylinositol as the endogenous activator of the (Na+ + K+)-ATPase in microsomes of rabbit kidney

Incubation of rabbit kidney microsomes with pig pancreatic phospholipase A2 produced residual membrane preparations with very low (Na+ + K+)-ATPase activity. The activity could be restored by recombination with lipid vesicles of negatively-charged glycerophospholipids. Vesicles of pure phosphatidylcholine and phosphatidylethanolamine were virtually inactive in this respect, but could reactivate in the presence of cholate. Incubation of the microsomes with a combination of phospholipase C (Bacillus cereus) and spingomyelinase C (Staphylococcus aureus) resulted in 90--95% release of the phospholipids. The residual membrane contained only phosphatidylinositol and still showed 50--100% of the (Na+ + K+)-ATPase activity.

Role of negatively charged phospholipids in highly purified (Na+ + K+)-ATPase from rabbit kidney outer medulla studies on (Na+ + K+)-activated ATPase, XXXIX

1. The requirement for specific polar head groups of phospholipids for activity of purified (Na+ + K+)ATPase from rabbit kidney outer medulla has been investigated. 2. Comparison of content and composition of phospholipids in microsomes and the purified enzyme indicates that purification leads to an increase in the phospholipid/protein ratio and in phosphatidylserine content. 3. The purified preparation contains 267 molecules phospholipid per molecule (Na+ + K+)-ATPase, viz. 95 phosphatidylcholine, 74 phosphatidylethanolamine, 48 spingomyelin, 35 phosphatidylserine and 15 phosphatidylinositol. 4. Complete conversion of phosphatidylserine into phosphatidylethanolamine by the enzyme phosphatidylserine decarboxylase has no effect on the (Na+ + K+)-ATPase activity of the purified preparation. 5. Complete hydrolysis of phosphatidylinositol by a phospholipase C from Staphylococcus aureus, which is specific for this phospholipid, has no effect on the (Na+ + K+)-ATPase activity. 6. Hydrolysis of 95% of the phosphatidylcholine and 60--70% of the spingomyelin and phosphatidylethanolamine by another phospholipase C (Clostridium welchii) lowers the (Na+ + K+)-ATPase activity by about 20%. 7. Combination of the phospholipid-converting enzymes has the same effect as can be calculated from the effects of the enzymes separately. Only complete conversion of both phosphatidylserine and phosphatidylinositol results in a loss of 44% of the (NA+ + K+)-ATPase activity and 36% of the potassium 4-nitrophenylphosphatase activity. 8. These experiments indicate that there is no absolute requirement for one of the polar head groups, although in the absence of negative charges the activity is lower than in their presence.