Recognition of different pools of phosphatidylglycerol in intact cells and isolated membranes of Acholeplasma laidlawii by phospholipase A2

The nonbilayer/bilayer lipid balance in membranes. Regulatory enzyme in Acholeplasma laidlawii is stimulated by metabolic phosphates, activator phospholipids, and double-stranded DNA

In membranes of Acholeplasma laidlawii a single glucosyltransferase step between the major, nonbilayer-prone monoglucosyl-diacylglycerol (MGlcDAG) and the bilayer-forming diglucosyl-diacylglycerol (DGlcDAG) is important for maintenance of lipid phase equilibria and curvature packing stress. This DGlcDAG synthase is activated in a cooperative fashion by phosphatidylglycerol (PG), but in vivo PG amounts are not enough for efficient DGlcDAG synthesis. In vitro, phospholipids with an sn-glycero-3-phosphate backbone, and no positive head group charge, functioned as activators. Different metabolic, soluble phosphates could supplement PG for activation, depending on type, amount, and valency. Especially efficient were the glycolytic intermediates fructose 1,6-bisphosphate and ATP, active at cellular concentrations on the DGlcDAG but not on the preceding MGlcDAG synthase. Potencies of different phosphatidylinositol (foreign lipid) derivatives differed with numbers and positions of their phosphate moieties. A selective stimulation of the DGlcDAG, but not the MGlcDAG synthase, by minor amounts of double-stranded DNA was additive to the best phospholipid activators. These results support two types of activator sites on the enzyme: (i) lipid-phosphate ones close to the membrane interphase, and (ii) soluble (or particulate)-phosphate ones further out from the surface. Thereby, the nonbilayer (MGlcDAG) to bilayer (DGlcDAG) lipid balance may be integrated with the metabolic status of the cell and potentially also to membrane and cell division.

Topology and transport of membrane lipids in bacteria

The last two decades have witnessed a break-through in identifying and understanding the functions of both the proteins and lipids of bacterial membranes. This development was parallelled by increasing insights into the biogenesis, topology, transport and sorting of membrane proteins. However, progress in research on the membrane distribution and transport of lipids in bacteria has been slow in that period. The development of novel biochemical in vitro approaches and recent genetic studies have increased our understanding of these subjects. The aim of this review is to present an overview of the current knowledge of the distribution and transport of lipids in both Gram-positive and Gram-negative bacteria. Special attention is paid to recently obtained results, which are expected to inspire further research to finally unravel these poorly understood phenomena.

Role of membrane lipid asymmetry in aging

Recent advances in our understanding of the asymmetric distribution of lipids across nervous system membranes coupled with the application of biophysical techniques to examine transbilayer structure and function have led to the formulation of a new hypothesis. The author hopes that the insights presented herein will stimulate investigation into this developing new field. The theory provides an approach to correlation the accumulation of nervous tissue membrane peroxidative and cross-linking damage, the loss of transbilayer lipid asymmetry, and loss of transbilayer neuroendocrine, transport, secretory and immunoregulatory functions. Central to this scheme is the role of membrane lipid asymmetry in regulation to and/or coupling of transbilayer functions.

Biosynthesis of D-alanyl-lipoteichoic acid: role of diglyceride kinase in the synthesis of phosphatidylglycerol for chain elongation

Lipophilic and hydrophilic D-alanyl-lipoteichoic acids are elongated in Lactobacillus casei by the transfer of sn-glycerol 1-phosphate units from phosphatidylglycerol to the poly(glycerophosphate) moiety of the polymer. These sn-glycerol 1-phosphate units are added to the end of the poly(glycerophosphate) which is distal to the glycolipid anchor; 1,2-diglyceride results from this addition. The presence of a diglyceride kinase was suggested by the ATP-dependent phosphorylation of 1,2-diglyceride to phosphatidic acid. Inorganic phosphate was used to initiate the synthesis of lipophilic lipoteichoic acid (LTA) and the elongation of both lipophilic and hydrophilic LTA. Three observations suggest that phosphate and other anions play a role in the in vitro synthesis of LTA and its precursors. First, the conversion of 1,2-diglyceride to phosphatidic acid by diglyceride kinase was stimulated. Second, the synthesis of phosphatidylglycerol was increased. Third, the elongation of lipophilic and hydrophilic LTA was enhanced. These observations indicated that one effect of phosphate might be to enhance the utilization of 1,2-diglyceride for the synthesis of phosphatidic acid. This phospholipid is a precursor of phosphatidylglycerol, the donor of sn-glycerol 1-phosphate for elongation of LTA.

Lysophospholipase-catalyzed hydrolysis of lysophospholipids in Mycoplasma gallisepticum membranes

Mycoplasma gallisepticum strains have a membrane-bound lysophospholipase which hydrolyzes lysophospholipid generated in these membranes by treatment with an external phospholipase. This paper studies the hydrolysis of the membranous lysophospholipids by an enzyme residing in the same membrane (intramembrane utilization) or in adjacent membranes (intermembrane utilization). To study intermembrane hydrolysis, the phospholipids of M. gallisepticum were labeled with [3H]oleic acid. Membranes were prepared, heated at 65 degrees C, and subsequently treated with pancreatic phospholipase A2. This resulted in membranes whose enzyme was heat inactivated, but which contained lysophospholipid. When these membranes were mixed with M. gallisepticum cells or membranes, the lysophospholipid was hydrolyzed by the membranous lysophospholipase. To study intramembrane hydrolysis, [3H]oleyl-labeled membranes of M. gallisepticum were treated with pancreatic phospholipase A2 at pH 5.0. At this pH, lysophospholipid was generated but not hydrolyzed. Adjustment of the pH to 7.4 resulted in hydrolysis of the lysophospholipid by the membranous lysophospholipase. These procedures permitted measuring the initial rates of intramembrane and intermembrane hydrolysis of the lysophospholipid, showing that the time course and dependence on endogenous substrate concentration were different in the intramembrane and intermembrane modes of utilization. They also permitted calculation of the molar concentration of the lysophospholipid in the membrane and its rate of hydrolysis, expressed as moles per minute per cell or per square centimeter of cell surface.

A 2H-NMR study of Acholeplasma laidlawii membranes highly enriched in myristic acid

Myristic acid specifically deuterated at several positions along the acyl chain was biosynthetically incorporated into the membrane lipids of Acholeplasma laidlawii B to the level of greater than or equal to 90%. 2H-NMR was used to study the molecular order and lipid phase composition of the membranes as a function of temperature. Isolated membranes and intact cells give rise to similar 2H spectra. Below 25 degrees C the spectra exhibit a broad gel phase component which at 0 degrees C reaches the rigid limit value expected for an immobilized methylene group. Spectral moments were used to determine the relative amounts of gel and liquid crystalline phase lipids throughout the gel-liquid crystal phase transition. The results indicate that at the growth temperature (37 or 30 degrees C) the A. laidlawii B membrane lipids are approximately 85-90% in the gel state, and that protein has little effect on lipid order of the liquid crystalline lipid, but leads to an increase in the linewidth by approx. 20%.

Chronic exposure to inhaled anesthetics increases cholesterol content in Acholeplasma laidlawii

Acholeplasma laidlawii cells were grown in cholesterol-enriched medium and exposed continuously to either air (control), 4.0 vol.% halothane in air at 1 atm pressure (4% atm halothane), or 80% cyclopropane in oxygen for 24 h at 37 degrees C. Cells grown in the presence of 4% atm halothane or 80% cyclopropane had approximately twice as much membrane cholesterol content/mg protein as the control cells. Cells grown in an anesthetic environment also tended to have a higher membrane cholesterol/phospholipid molar ratio compared to control cells. Membranes isolated from halothane-exposed cells grown in a cholesterol-enriched medium were more ordered at 37 degrees C (measurements were made with no anesthetic present) than membranes from control cells grown in an identically enriched medium. This difference in membrane physical state between control and anesthetic-exposed cells decreased as the temperature decreased, and disappeared at approx. 23 degrees C. Continuous exposure of A. laidlawii to 4% atm halothane or 80% cyclopropane for 24 h did not markedly affect membrane fatty acid composition, either in cells grown on an unsupplemented medium or in cells grown in a medium enriched in myristic, palmitic or stearic acids. These results further support the hypothesis that an increased membrane cholesterol content may play a role in the tolerance or dependence that develops after chronic exposure to anesthetic agents.

Phospholipid topography of the photosynthetic membrane of Rhodopseudomonas sphaeroides

The topography of phospholipids in the photosynthetic membranes of Rhodopseudomonas sphaeroides was investigated by using purified chromatophores and spheroplast-derived vesicles (SDVs). Chromatophores are closed vesicles oriented inside out with respect to the cytoplasmic membrane (cytoplasmic side out) and obtained from French-pressed cell lysates. SDVs are oriented right side out (periplasmic side out) and are obtained after osmotic lysis of lysozyme-treated cells. Phosphatidylethanolamine (PE) comprised approximately 62% and phosphatidylglycerol (PG) comprised approximately 33% of the total phospholipid of both vesicle preparations. The relatively membrane impermeable reagent trinitrobenzenesulfonate (TNBS) at 3 mM concentration and 5 degrees C modified chromatophore and SDV PE with kinetics indicating the occurrence of fast- and slow-reacting pools of PE. The fast-reacting pools comprised 33% and 55% of the total PE of chromatophores and SDVs, respectively. The slow-reacting pools comprised 61% and 32% of the total PE of chromatophores and SDVs, respectively. Phospholipase A2 treatment of chromatophores (1 unit/mg of vesicle protein) for 1 h at 37 degrees C resulted in hydrolysis of 73% and 77% of the total PG and PE, respectively. Similar enzyme treatment of SDVs resulted in 14% and 60% hydrolysis of the total PG and PE, respectively. Phospholipase A2 treatment inhibited 60% of the succinate dehydrogenase activity of chromatophores but only 8% of the activity of SDVs, indicating the membrane impermeability of phospholipase A2. Incubation of chromatophores for 10 min with 3 mM TNBS at 5 degrees C and then treatment with phospholipase A2 for 10 min and 1 h resulted in the hydrolysis of 10% and 61%, respectively, of unmodified PE. The results indicate asymmetric distributions of PE polar head groups (32-33% cytoplasmic side, 55-61% periplasmic side) and PG (73% cytoplasmic side, 14% periplasmic side) across the membrane. Also, a rapid and unidirectional transbilayer movement of PE polar head groups from the periplasmic to cytoplasmic surfaces of the membrane appears to occur during phospholipase A2 hydrolysis on the chromatophore surfaces.

Biosynthesis of D-alanyl-lipoteichoic acid in Lactobacillus casei: D-alanyl-lipophilic compounds as intermediates

D-Alanyl-lipoteichoic acid (D-alanyl-LTA) from Lactobacillus casei contains a poly(glycerol phosphate) moiety that is selectively acylated with D-alanine ester residues. To characterize further the mechanism of D-alanine substitution, intermediates were sought that participate in the assembly of this LTA. From the incorporation system utilizing either toluene-treated cells or a combination of membrane fragments and supernatant fraction, a series of membrane-associated D-[14C]alanyl-lipophilic compounds was found. The assay of these compounds depended on their extractability into monophasic chloroform-methanol-water (0.8:3.2:1.0, vol/vol/vol) and subsequent partitioning into chloroform. Four lines of evidence suggested that the D-alanyl-lipophilic compounds are intermediates in the synthesis of D-alanyl-LTA. First, partial degradation of the poly(glycerol phosphate) moiety of D-alanyl-LTA by phosphodiesterase II/phosphatase from Aspergillus niger generated a series of D-alanyl-lipophilic compounds similar to those extracted from the toluene-treated cells during the incorporation of D-alanine. Second, enzymatic degradation of the D-alanyl-lipophilic compounds by the above procedure gave D-alanyl-glycerol, the same degradation product obtained from D-alanyl-LTA. Third, the incorporation of D-alanine into these compounds required the same components as the incorporation of D-alanine into membrane-associated D-alanyl-LTA. Fourth, the phosphate-induced loss of D-[14C]alanine-labeled lipophilic compounds could be correlated with the stimulation of phosphatidylglycerol synthesis in the presence of excess phosphate. We interpreted these experiments to indicate that the D-alanyl-lipophilic compounds are D-alanyl-LTA with short polymer chains and are most likely intermediates in the assembly of the completed polymer, D-alanyl-LTA.

Lipid and protein membrane components associated with cholesterol uptake by Mycoplasmas

Membranes of Mycoplasma species take up 2--4 times more exogenous cholesterol than membranes of Acholeplasma species. To test whether the lower cholesterol uptake capacity of Acholeplasma is due to the high glycolipid content of their membranes, the phospholipids of Acholeplasma laidlawii and Mycoplasma capricolum membranes were hydrolyzed by phospholipase A2. Digestion removed about 30% of the polar lipids of A. laidlawii, leaving the glycolipids and phospholglycolipids intact, and about 70% of the polar lipids of M. capricolum, the residue consisting mostly of sphingomyelin. Cholesterol uptake by the treated membranes from phosphatidylcholine/cholesterol vesicles decreased in rough proportion to the amount of polar lipid removed, indicating that the glycolipids in A. laidlawii membranes can participate in cholesterol uptake. Trypsin digestion of growing cells and isolated membranes of M. capricolum decreased cholesterol uptake by about one-half. Similar treatment of A. laidlawii cells and membranes had no effect on cholesterol uptake. These findings suggest the existence of protease-sensitive receptors on the cell surface of M. capricolum responsible for tighter contact with the cholesterol/phosphatidylcholine vesicles. It is proposed that the ability of Mycoplasma species to take up large quantities of exogenous cholesterol and phospholipids depends on the presence of protein receptors for cholesterol donors, receptors which are absent in Acholeplasma species.

Purification of two lipases with high phospholipase A1 activity from guinea-pig pancreas

1. Two cationic lipases (Ia and Ib) were purified from homogenates of fresh guinea-pig pancreas by ion-exchange chromatography on DEAE-Sepharose and CM-Sepharose (twice for the latter) followed by gel filtration on Sephadex G-100. 2. Both enzymes were homogeneous upon polyacrylamide gel electrophoresis. Their molecular weights are 37,000 and 42,000 for lipases Ia and Ib, respectively, as determined by gel filtration on Sephadex G-100. Very close values for isoelectric points were found in the pH range 9.3-9.4. 3. The cationic lipases are characterized by a high phospholipase A activity (500 IU/mg protein using a potentiometric assay with egg yolk lecithin as substrate), resulting in an unusual phospholipase/lipase activity ratio of 1. 4. Using doubly labelled phosphatidylcholine, a specificity, A1, was described for the two enzymes, which are unaffected by N-ethylmaleimide, diisopropylfluorophosphate and p-bromophenacylbromide. The enzymes are insensitive to EDTA and slightly inhibited by CaCl2 and MgCl2, whereas sodium deoxycholate is required for maximal activity.

Membrane asymmetry. A survey and critical appraisal of the methodology. II. Methods for assessing the unequal distribution of lipids

In the companion paper, I have reviewed the techniques employed for assessment of the asymmetric distribution and orientation of membrane proteins. This article deals with methods applicable to the investigation of the unequal distribution of lipids between the two membrane leaflets. Among the techniques I will discuss are the use of immunological techniques and lectins, chemical reagents, enzymatic isotopic labeling and degradation of membrane lipids, exchange proteins and physical techniques. Whenever appropriate, problems of crypticity and non-availability of lipids to interact with the appropriate ligands, reagents, modifying enzymes or exchange proteins have been envisaged. It appears that in many case, highly discordant results, sometimes with the same biological material, have been obtained. Some of the difficulties encountered presumably stem from the reported existence of non-bilayer arrangements and isotropic movement of lipids as evidenced by freeze-fracture and NMR studies. Other problems may be related to the induction of such arrangements, especially the inverted micellar arrangement, by the modifying agents, particularly degradation enzymes or exchange proteins when they cause severe unilateral modification of the lipids of the exposed leaflet. In addition, the situation is complicated by the role of the induced increase in the flip-flop rate under different experimental conditions and by modification of the rearrangement of lipid molecules as a result of the metabolic state of the cell or ghost preparation and of the reactivity of lipids as a consequence of temperature changes. Here, more so than with proteins, one must be cautious in interpreting experimental results. Moreover, it would appear that the use of different techniques in conjunction and the consequent comparison of results should be recommended. It has been emphasized that 'general rules' do not hold and that each new material should be assay again. To give one example, it is not pertinent to state that proteins enhance the flip-flop rate in lipid vesicles (and hence in membranes). This holds true for glycophorin from erythrocyte membrane, but could not be proved when mitochondrial cytochrome oxidase was used. There seems to be no rule for the distribution of lipids between the two leaflets of different membranes. For example, even for different strains of the same bacterial species, highly divergent results have been reported. It is generally (and probably under the influence of different studies with erythrocytes) believed that in mammalian plasma membranes, choline phospholipids are enriched in the outer leaflet and aminophospholipids in the inner leaflet. Though this contention may prove to be correct, different instances of contradictory results have been given in the text. This shows that if rules do exist, they remain to be discovered or established...

Localization of lysophosphatidylcholine in bovine chromaffin granules

One of the unique features of the chromaffin granule membrane is the presence of about 17 mol% lysophosphatidylcholine. Lysophosphatidylcholine isolated from the granules could be degraded by approx. 94% by lysophospholipase. This result is consistent with chemical analyses data showing that about 9% of this lysophospholipid is 1'-alkenyl glycerophosphocholine. The localization of the acylglycerophosphocholine in the chromaffin granule membrane was studied by using pure bovine liver lysophospholipases. In intact granules only about 10% of the total lysophosphatidylcholine was directly available for enzymic hydrolysis. In contrast, when granule membranes (ghosts) were treated with lysophospholipases approx. 60% of the lysophosphatidylcholine was deacylated. These values did not increase after pre-treatment of intact granules or ghosts with trypsin. Added 1-[1-14C]palmitoyl-sn-glycero-3-phosphocholine did not mix with the endogenous lysophosphatidylcholine pool(s) and remained completely accessible to added lysophospholipases.

Lipid distribution in Acholeplasma laidlawii membrane. A study using the lactoperoxidase-mediated iodination

The lactoperoxidase-mediated radioiodination has been applied to study the transbilayer distribution of phospho- and glycolipids in Acholeplasma laidlawii membranes. After radioiodination, about 5% of the 125I-iodine was found in membrane lipids. A comparison of the labeling intensities of the various lipid species between iodinated intact cells and isolated membranes revealed that the glycolipids monoglucosyldiglyceride and diglucosyldiglyceride are located almost exclusively in the outer half of the bilayer, whereas the phospholipids phosphatidylglycerol and diphosphatidylglycerol as well as the phosphoglycolipids glycerophosphoryl-diglucosyldiglyceride and glycerophosphoryl-monoglucosyldiglyceride are almost equally distributed in the outer and inner halves of A. laidlawii membranes.

The distribution of molecular classes of phosphatidylglycerol in the membrane of Acholeplasma laidlawii

A double-label technique has been applied to study the distribution of different molecular classes of phosphatidylglycerol in the membrane of Acholeplasma laidlawii. After growth on oleic acid, 16% of the total phosphatidylglycerol contains two oleic acid residues and 84% contains one oleic acid and one saturated fatty acid. The dioleoyl phosphatidylglycerol is present in equal amounts in the outer and the inner layer of the membrane. Phosphatidylglycerol which is associated with membrane proteins consists exclusively of the class containing only one oleic acid.

Evidence for the existence of different pools of microsomal phosphatidylinositol by the use of phosphatidylinositol-exchange protein

1. The phosphatidylinositol-exchange protein from bovine brain was used to determine to what extent phosphatidylinositol in rat liver microsomal membranes is available for transfer. 2. The microsomal membranes used in the transfer reaction contained either phosphatidyl[2-(3)H]inositol or (32)P-labelled phospholipid. The (32)P-labelled microsomal membranes were isolated from rat liver after an intraperitoneal injection of [(32)P]P(i). The (3)H-labelled microsomal membranes and rough- and smooth-endoplasmic-reticulum membranes were prepared in vitro by the incorporation of myo-[2-(3)H]inositol into phosphatidylinositol by either exchange in the presence of Mn(2+) or biosynthesis de novo in the presence of CTP and Mg(2+). 3. Tryptic or chymotryptic treatment of the microsomes impaired the biosynthesis de novo of phosphatidylinositol. It was therefore concluded that the biosynthesis of phosphatidylinositol and/or its immediate precursor CDP-diacylglycerol takes place on the cytoplasmic surface of the microsomal membrane. 4. Under the conditions of incubation 42% of the microsomal phosphatidyl[2-(3)H]inositol was transferred with an estimated half-life of 5min; 38% was transferred with an estimated half-life of about 1h; the remaining 20% was not transferable. Identical results were obtained irrespective of the method of myo-[2-(3)H]inositol incorporation. 5. Both measurement of phosphatidylinositol phosphorus in the microsomes after transfer and the transfer of microsomal [(32)P]phosphatidylinositol indicate that phosphatidyl[2-(3)H]-inositol formed by exchange or biosynthesis de novo was homogeneously distributed throughout the microsomal phosphatidylinositol. 6. We present evidence that the slowly transferable pool of phosphatidylinositol does not represent the luminal side of the microsomal membrane; hence we suggest that this phosphatidylinositol is bound to membrane proteins.

Phosphatidylcholine mobility in liver microsomal membranes

Purified phosphatidylcholine exchange protein from bovine liver was used to exchange rat liver microsomal phosphatidylcholine for egg phosphatidylcholine. It was found that at 25 and 37 degrees C rat liver microsomal phosphatidylcholine was completely and rapidly available for replacement by egg phosphatidylcholine. In contrast, phosphatidylcholine in vesicles prepared from total microsomal lipids could only be exchanged for about 60%. At 8 and 0 degrees C complex exchange kinetics were observed for phosphatidylcholine in rat liver microsomes. The exchange process had neither effect on the permeability of the microsomal membrane to mannose 6-phosphate, nor on the permeability of the phosphatidylcholine vesicles to neodymium (III) cations. Purified phospholipase A2 from Naja naja could hydrolyze some 55-60% of microsomal phosphatidylcholine at 0 degrees C, but 70-80% at 37 degrees C. Microsomal phosphatidylcholine, remaining after phospholipase treatment at 37 degrees C, could be exchanged for egg phosphatidylcholine at 37 degrees C, but at a slower rate than with intact microsomes. Microsomal phosphatidylcholine remaining after phospholipase treatment at 0 and 37 degrees C had a lower content of arachidonic acid than the original phosphatidylcholine. These results are discussed with respect to the localization and transmembrane movement of phosphatidylcholine in liver microsomes.