Acylation of endogenous phospholipids and added lysoderivatives by rat liver plasma membranes

Vaccenic and elaidic acid equally esterify into triacylglycerols, but differently into phospholipids of fed rat liver cells

Elaidic acid (trans-9-C₁₈:₁ or trans-9) is assumed to exert atherogenic effects due to its double bond configuration. The possibility that trans-9 and vaccenic acid (trans-11-C₁₈:₁ or trans-11), its positional isomer, were biochemically equivalent and interchangeable compounds, was investigated by reference to their cis-isomers through esterification-related activities using rat liver cells and subcellular fractions. In hepatocytes, both trans-C₁₈:₁ were incorporated to the same extent in triacylglycerols, but trans-9 was more esterified than trans-11 into phospholipids (P < 0.05). Glycerol-3-phosphate acyltransferase activity in microsomes was lower with trans-11 than with trans-9, while this activity in mitochondria was ~40% greater with trans-11 than with trans-9 (P < 0.05). Activity of 2-lysophosphatidic acid acyltransferase in microsomes was of comparable extent with both trans isomers, but activity of 2-lysophosphatidylcholine acyltransferase was significantly greater with trans-9 than with trans-11 at P < 0.01. Lipoproteins secreted by hepatocytes reached equivalent levels in the presence of any isomers, but triacylglycerol production was more elevated with trans-11 than with trans-9 at P < 0.05. Cholesterol efflux from previously labelled hepatocytes was lower with trans-11 than with trans-9. When these cells were exposed to either trans-C₁₈:₁, the gene expression of proteins involved in fatty acid esterification and lipoprotein synthesis was unaffected, which indicates that the biochemical differences essentially depended on enzyme/substrate affinities. On the whole, vaccenic and elaidic acid were shown to incorporate cell phospholipids unequally, at least in vitro, which suggests they can differently affect lipid metabolic pathways in normal cells.

Lipidomics: practical aspects and applications

Lipidomics is the characterization of the molecular species of lipids in biological samples. The polar lipids that comprise the bilayer matrix of the constituent cell membranes of living tissues are highly complex and number many hundreds of distinct lipid species. These differ in the nature of the polar group representing the different classes of lipid. Each class consists of a range of molecular species depending on the length, position of attachment and number of unsaturated double bonds in the associated fatty acids. The origin of this complexity is described and the biochemical processes responsible for homeostasis of the lipid composition of each morphologically-distinct membrane is considered. The practical steps that have been developed for the isolation of membranes and the lipids there from, their storage, separation, detection and identification by liquid chromatography coupled to mass spectrometry are described. Application of lipidomic analyses and examples where clinical screening for lipidoses in collaboration with mass spectrometry facilities are considered from the user point of view.

Membrane Lipid Homeostasis

The lipid matrix of biological membranes is composed of a complex mixture of polar lipids. It has been estimated that more than 600 distinct molecular species of lipid are constituents of biological membranes. This rather remarkable feature raises the questions of why such complexity is required when barrier properties and many protein functions can be reconstituted with relatively simple lipid systems. Secondly, the molecular species composition of morphologically distinct membranes appears to be preserved within fairly narrow limits. The biochemical mechanism(s) responsible for this homeostasis are not fully understood. This review examines the origin of membrane lipid complexity, the methods that are currently employed to measure and detect lipid molecular species and the biochemical reactions associated with the turnover of membrane lipids in resting and stimulated cells.

Lysophosphatidylethanolamine acyltransferase activity is elevated during cardiac cell differentiation

We examined if elevation in lysophosphatidylethanolamine acyltransferase activity was associated with elevation in phosphatidylethanolamine content during differentiation of P19 teratocarcinoma cells into cardiac myocytes. P19 cells were induced to undergo differentiation into cardiac myocytes by the addition of 1% dimethylsulfoxide to the medium. Immunofluorescence microscopy revealed the presence of striated myosin at 8 days post-dimethylsulfoxide addition confirming differentiation into cardiac cells. The content of phosphatidylethanolamine was increased 2.1-fold (P<0.05) in differentiated cells compared to undifferentiated cells, whereas the content of phosphatidylcholine was reduced 29% (P<0.05). There were no alterations in the pool sizes of other phospholipids, including cardiolipin. The relative abundance of fatty acids in phospholipids of P19 cells was 18:1 > 18:0 > 16:1 = 18:2 > 16:0 = 14:0 > 20:4 and differentiation did not affect the relative amounts of these fatty acids within individual phospholipids. When cells were incubated with [1,3-(3)H]glycerol, radioactivity incorporated into phosphatidylethanolamine was elevated 5.8-fold, whereas radioactivity incorporated into phosphatidylcholine was unaltered. Ethanolaminephosphotransferase, cholinephosphotransferase and membrane CTP:phosphocholine cytidylyltransferase activities were elevated in differentiated cells compared to undifferentiated cells, whereas membrane and cytosolic phospholipase A2 activities were unaltered. Lysophosphatidylethanolamine acyltransferase activities were elevated 2.4-fold (P<0.05). Lysophosphatidylcholine acyltransferase, monolysocardiolipin acyltransferase, acyl-Coenzyme A synthetase and acyl-Coenzyme A hydrolase activities were unaltered in differentiated cells compared to undifferentiated cells. We postulate that during cardiac cell differentiation, the observed elevation in lysophosphatidylethanolamine acyltransferase activity accompanies the elevation in phosphatidylethanolamine mass, possibly to maintain the fatty acyl composition of this phospholipid within the membrane.

Substrate specificity of acyl-CoA:Lysophospholipid acyltransferase (LAT) from pig spleen

The present investigation was undertaken to gain insights into the nature of both substrate binding sites of acyl-CoA:lysophospholipid acyltransferase (LAT) which could be potentially useful for the identification and purification of this specific acyltransferase. Therefore, we have investigated the specificity of LAT from crude membranes of pig spleen toward various 1-palmitoyl-glycerophospholipids and 1-acyl-glycerophosphocholines (1-acyl-GPC). The enzyme showed the highest specificity toward 1-acyl-GPC and was able to distinguish between the acyl-chain length of the 1-acyl group within the 1-acyl-GPC molecule. We found preferential reactivity in the order C10:0 < C12:0 << C14:0, C18:0, C16:0 < C18:1 of 1-acyl-GPC. Lysophosphatidic acid or 1-O-alkyl-GPC were only poor substrates for the enzyme. In competition studies we could show that palmitic acid, oleic acid, arachidonic acid, and palmitoyl-CoA competitively inhibited LAT activity, whereas the coenzyme A failed to inhibit LAT enzyme activity in a concentration-dependent manner. We concluded that the ligand acyl-CoA is bound via its acyl chain. The finding that palmitoyl-CoA was a poor substrate as well as an inhibitor was the basis for protein purification. When palmitoyl-CoA-agarose was used as matrix for affinity chromatography, LAT enzyme activity was bound and eluted by high salt concentrations yielding an estimated 10-fold purification of the solubilized LAT enzyme.

Identification of two different lysophosphatidylcholine:acyl-CoA acyltransferases (LAT) in pig spleen with putative distinct topological localization

The lysophosphatidylcholine:acyl-CoA acyltransferase (LAT, EC 2.3.1.23) is an integral membrane protein participating in the membrane turnover and the T-cell activation process. Here, we present data that crude membranes of pig spleen contain two different LAT enzyme activities based on topological localization studies and the enzyme specificities towards various acyl-CoAs. When crude membranes are washed with solutions of high ionic strength the supernatant contains a distinct LAT activity that we refer to as peripheral LAT (pLAT). The majority of LAT activity is found in the membrane pellet also after treatment with CHAPS. The CHAPS-insoluble LAT activity is named integral LAT (iLAT) accordingly. While pLAT prefers arachidonoyl-CoA rather than oleoyl-CoA, iLAT shows no specificity towards both unsaturated acyl-CoAs. Further investigations reveal that the CHAPS-insoluble LAT activity in the membranes can be solubilized by n-octyl glucoside and restored to original activity by reconstitution with artificial membranes. The reconstituted iLAT prefers arachidonoyl-CoA rather than oleoyl-CoA. Despite a great deal of effort by several groups little progress has been made so far in LAT purification because of the enzyme instability. We establish experimental conditions that enhance the stability of both enzyme activities and, therefore, allow further protein purification.

Influence of carbicron (O-[(2-butenoic acid)-N,N-dimethylamide-3-yl] O,O-dimethylphosphate) on some biochemical and biophysical parameters of rat liver membranes

1. Treatment of rats with carbicron induced a reduction of the phospholipids in both microsomal and plasma membranes. 2. A decrease of the structural order parameter (SDPH) and an increase of the pyrene excimer-to-monomer fluorescence ratio (IE/IM) was also observed, indicating membrane fluidization. 3. The specific activity of membrane-bound phospholipase A2 and phospholipase C were decreased in both types of membranes, whereas acyl-CoA:lysophosphatidylcholine acyltransferase activity was augmented due to carbicron treatment.

Acyl-CoA: 1-acyl-glycero-3-phosphoethanolamine O-acyltransferase and liver plasma membrane fluidity

Investigations have been carried out on the influence of membrane lipid composition and physical state on acyl-CoA: 1-acyl-glycerol-3-phosphoethanolamine O-acyltransferase activity in rat liver plasma membranes. The lipid composition of the membranes was modified either by way of lipid transfer proteins or by partial delipidation with exogenous phospholipases and subsequent enrichment of the membranes with different phospholipids. The results indicated that membrane rigidification by enrichment of the membranes with DPPC or SM reduced the transfer of oleic and palmitic acid to lysophosphatidylethanolamine, whereas all phospholipids inducing membrane fluidization lead to acyltransferase activation. The eventual role of membrane fluidity in the deacylation-reacylation cycle is discussed.

Modulation of calcium-dependent neutral protease activity by fatty acids and lysophospholipids

The effect of fatty acids and lysophospholipids on calcium-activated neutral protease (CANP) was investigated. mu CANP, low calcium ion (microM concentration)-requiring CANP is more strongly inhibited by unsaturated fatty acids than is mCANP--the high calcium ion (mM concentration)-requiring form. Lysophospholipids in concentrations ranging from 10(-5) M to 10(-3) M inhibit mu CANP exclusively, whereas mCANP activity is unaffected or even slightly increased. Calpastatin decreases the activity of mCANP and, in the presence of polyunsaturated fatty acids such as docosahexaenoic acid, the inhibition is not increased. In the presence of lysophosphatidyl-ethanolamine, however, the inhibition of mCANP by calpastatin does not occur. The results indicate that fatty acids and lysocompounds liberated under different physiological and pathological conditions may modulate calcium-activated neutral protease.

Transfer of arachidonate from phosphatidylcholine to phosphatidylethanolamine and triacylglycerol in guinea pig alveolar macrophages

Guinea pig alveolar macrophages were labeled by incubation with either arachidonate or linoleate. Arachidonate labeled phosphatidylcholine (PC), phosphatidylethanolamine (PE) and triglycerides (TG) equally well, with each lipid containing about 30% of total cellular radioactivity. In comparison to arachidonate, linoleate was recovered significantly less in PE (7%) and more in TG (47%). To investigate whether redistributions of acyl chains among lipid classes took place, the macrophages were incubated with 1-acyl-2-[1-14C]arachidonoyl PC or 1-acyl-2-[1-14C]linoleoyl PC. After harvesting, the cells incubated with 1-acyl-2-[1-14C]linoleoyl PC contained 86% of the recovered cellular radioactivity in PC, with only small amounts of label being transferred to PE and TG (3 and 6%, respectively). More extensive redistributions were observed with arachidonate-labeled PC. In this case, only 60% of cellular radioactivity was still associated with PC, while 22 and 12%, respectively, had been transferred to PE and TG. Arachidonate transfer from PC to PE was unaffected by an excess of free arachidonate which inhibited this transfer to TG for over 90%, indicating that different mechanisms or arachidonoyl CoA pools were involved in the transfer of arachidonate from PC to PE and TG. Cells prelabeled with 1-acyl-2-[1-14C]arachidonoyl PC released 14C-label into the medium upon further incubation. This release was slightly stimulated by zymosan and threefold higher in the presence of the Ca2+-ionophore A23187. Labeling of macrophages with intact phospholipid molecules appears to be a suitable method for studying acyl chain redistribution and release reactions.

Enzymatic reconstitution of brain membrane and membrane opiate receptors

A new method using lysophosphatide and acyl-CoA as detergents has been used to solubilize the rat brain opiate receptor. After solubilization, lysophosphatide and acyl-CoA can be almost completely removed by an enzymatic reaction that uses an acyltransferase from rat liver microsomes and reconstitutes the solubilized receptor in membranous vesicles. Morphological studies performed with negative staining and freeze-fracture electron microscopy revealed that the general appearance and intramembrane particle distribution of fracture faces in the reconstituted membrane are similar to those of the native membrane; this indicates that hydrophobic protein components of the original membrane were incorporated during reconstitution. Reconstituted membrane, however, contained higher levels of phosphatidylcholine and lower levels of cholesterol. The activities of the membrane-bound enzymes Na+, K+-ATPase and Ca2+, Mg2+-ATPase in the reconstituted system were 24 and 3%, respectively, those of the native membrane. Although binding of opiate ligands to the reconstituted membrane was stereospecific and saturable, higher concentrations of some of the unlabeled ligands were required to inhibit binding of the radiolabeled ligands. These changes in receptor characteristics are likely due to changes in lipid composition, physical state, and/or distribution of the lipids in the reconstituted membrane bilayer. This conclusion is supported by an increase in the affinity of opiate ligands for reconstituted membrane after adjustment of the latter's lipid composition to match more closely that of the original membrane. This was accomplished by treatment with phospholipid exchange protein to remove the excess phosphatidylcholine and by incorporation of cholesterol into the reconstituted membrane.

Induction by lysophospholipids of CoA-dependent arachidonyl transfer between phospholipids in rat platelet homogenates

Rat platelet homogenates are able to catalyze CoA-mediated, ATP-independent transfer of arachidonic acid from platelet phospholipids to added lysophospholipids. Homogenates of platelets prelabelled with radioactive arachidonic or oleic acid were incubated in the presence of CoA and various lysophospholipids. Transfer observed with arachidonic acid-labelled platelets was dependent on the lysophospholipid added. When 1-alkenyl- or 1-acyllysophosphatidylethanolamine was used, there was a more efficient arachidonyl transfer from phosphatidylcholine than from phosphatidylinositol to the phosphatidylethanolamine fraction. Lysophosphatidylserine also accepted arachidonyl from phosphatidylcholine. Addition of lysophosphatidylcholine resulted in a decrease in the labelling of phosphatidylinositol and to a lesser extent of phosphatidylethanolamine with concomitant transfer to phosphatidylcholine. Lysophosphatidylinositol and lysophosphatic acid did not act as substrate for this transfer reaction. Free, non-radioactive arachidonic acid did not compete for the labelled arachidonic acid transfer. This pathway may play a major role in the synthesis of arachidonyl species of phosphatidylethanolamine and phosphatidylserine and for the arachidonyl transfer to the phosphatidylethanolamine plasmologen in stimulated platelets.

1-Acyl-lysolecithin acyltransferase and synthesis of biliary lecithins in rat liver

The role of lysolecithin acyltransferase activities in biliary lecithin formation was investigated, using livers perfused in the presence of labeled palmitoyl-lysolecithin and albumin, overloaded or not with linoleic acid. At the end of liver perfusion, the lecithins extracted from microsomes, mitochondria and plasma membranes displayed the same specific activity. Double-labeled lysolecithin was used to prove that labeled lecithins were synthesized by lysolecithin acylation. In the absence or presence of a linoleic acid overload, the level of lysolecithin incorporation into linoleyl and arachidonyl containing lecithin was identical. Hence fatty acids did not influence phosphatidylcholine synthesis by the acylation pathway. In vitro the rate of linoleyl lecithin synthesis was the same in plasma membranes, mitochondria and microsomes provided the linoleyl-CoA concentration was lower than 30 microM. Taurocholate was essential to the excretion of lecithin synthesized from lysolecithin and stimulated its synthesis. The specific activities of the two lecithin molecular species excreted in bile (linoleyl and arachidonyl) were not significantly different. These results enabled us to evaluate the contribution of the lysolecithin pathway to the synthesis of lecithin in liver and bile: this contribution in bile was less than 2% under the perfusion conditions used.

Linoleate incorporation into rat liver membranes phospholipids: effect on plasma membrane ATPase activities and physical properties

Plasma membrane phospholipids were modified by incubation in the presence of linoleyl-CoA with or without added lysolecithin (LPC) for various length of time. In the absence of LPC, a maximum of 10 nmoles linoleyl-phosphatidylcholine (PC) were synthesized and the ATPase specific activities were not affected whereas in the presence of LPC, when linoleyl-PC synthesis rose from 10 to 80 nmoles, the ATPase activities were decreased. The decrease was similar in the Na,K- or in the Mg-dependent-ATPase and reached maximally 30-40%. LPC by itself did not modify the ATPases. A concomitant decrease in DPH polarization was observed when linoleate was incorporated into phospholipids. We concluded that the decreased ATPase specific activities may be due to an increased fluidity of membranes produced by linoleyl- PC synthesis. We compare this modulation of ATPases by the membrane fluidity with the specific effect of linoleyl- PC species on adenylate cyclase.

Minireview. Roles of turnover and repair of macromolecules and supramolecular structural components

Macromolecules and supramolecular structural components that are incorrectly synthesized or are damaged by radiation or by reactive chemicals are either repaired or selectively degraded and resynthesized. In addition, turnover rates for macromolecules and supramolecular structures can be elevated by alternation of fasting and feeding periods and can be influenced by metabolic regulatory mechanisms which are governed by steady-state concentrations of labile macromolecules.

Plasma membrane: can its structure and function be modulated by dietary fat?

Compositional analysis of plasma membranes from rats fed nutritionally adequate diets different in fatty acid composition establishes that fundamentally different dietary fat intake results in alteration in structural lipid composition of plasma membranes in brain, liver and the intestinal mucosa. Dietary differences in fatty acid intake altered the fatty acyl tail composition of plasma membrane phospholipids in brain, liver and intestinal mucosa. Diet altered the phospholipid profile observed in brain synaptosomal and liver plasma membrane. Feeding high vs low polyunsaturated to saturated fat diets for 7 days altered the fatty acid composition of phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and monoglucosylceramide isolated from plasma membrane of the intestinal mucosa.

Purification of acyl CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase

Acyl coenzyme A:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase (EC 2.3.1.23) is capable of forming lipid bilayer vesicles from its soluble substrates lysophosphatidylcholine (LPC) and oleoyl CoA. This suggested a purification method in which rat liver microsomes are first washed with deoxycholate to increase specific activity of the endogenous acyltransferase approximately fivefold, then solubilized by the detergent effect of excess LPC and oleoyl CoA in 1:1 stoichiometric ratios. As the LPC is converted to phosphatidylcholine by acyl group transfer, the detergent effect is lost and lipid vesicles containing the enzyme activity are produced. Other microsomal proteins are excluded from the vesicles. The vesicles may be separated by density gradient flotation and are found to contain acyltransferase with a specific activity of 9-10 mu mol/mg/min. This reflects a purification of approximately 140-fold, about ten times greater than achieved in previous studies.

Incorporation of exogenous fatty acids into phospholipids by cultured hamster fibroblasts. Effect of SV40 transformation

In situ incorporation of two saturated (palmitic, 16:0; stearic, 18:0) and three unsaturated fatty acids (oleic, 18:1; linoleic, 18:2; arachidonic, 20:4) into the four major phospholipids, sphingomyelin, PC, PI and PE, was followed. Transformed cells incorporated unsaturated fatty acids more rapidly, whereas no significant differences were found concerning saturated fatty acids. In vitro determination of phospholipid acylation showed that incorporation of coenzyme A-activated forms of two saturated fatty acids (16:0 and 18:0) and one unsaturated fatty acid (18:1) into phospholipids was increased in transformed cells. Comparison of results obtained in situ and in vitro strongly suggests that incorporation of fatty acids into phospholipids in cultured cells is not limited by acyltransferase activities.

Animal cells dependent on exogenous phosphatidylcholine for membrane biogenesis

A Chinese hamster ovary cell (CHO) mutant (strain 58), defective in CDP-choline synthetase (cholinephosphate cytidylyltransferase; CTP:cholinephosphate cytidylyltransferase, EC 2.7.7.15), is temperature sensitive for growth and contains less than half of the normal amount of phosphatidylcholine under nonpermissive conditions [Esko, J. D. & Raetz, C. R. H. (1980) Proc. Natl. Acad. Sci. USA 77, 5192-5196]. We now report that the addition of 40 microM egg phosphatidylcholine or lysophosphatidylcholine to the medium suppresses the temperature sensitivity of mutant 58 and permits the growth of colonies at the restrictive temperature. Phospholipids with different polar headgroups, lipoprotein-bound phospholipids, sphingomyelin, and glycerophosphocholine do not support prolonged growth at 40 degrees C, whereas phosphatidylcholine analogs such as phosphatidyldimethylethanolamine, D-phosphatidylcholine, and beta-phosphatidylcholine are quite effective. A broad range of saturated phosphatidylcholines, especially those with fatty acids 12-18 carbons in length, suppresses the phenotype. Phospholipids containing ether-linked hydrocarbons are ineffective, whereas polyunsaturated phosphatidylcholines are toxic. Residual endogenous synthesis of phosphatidylcholine by the mutant is not stimulated under conditions of phenotypic bypass, but the uptake of exogenous lipid is enhanced considerably compared to the wild type. Our findings demonstrate that exogenous phospholipid can provide at least 50% of the phosphatidylcholine required for membrane biogenesis in animal cells and that uptake of exogenous phospholipids may be regulated.

Biosynthesis of platelet-activating factor. I. Evidence for an acetyl-transferase activity in murine macrophages

Platelet-activating factor (PAF-acether; 1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine) is released from murine peritoneal adherent cells by inflammatory and non-inflammatory stimuli. We have found, in extracts from these cells, an enzyme activity that synthesizes. PAF-acether from synthetic lyso-PAF-acether by transferring the acetyl moiety of acetyl-coenzyme A onto the lyso-PAF-acether molecule. The enzyme is stabilized by 1 mM dithiothreitol, is calcium-dependent, has an apparent Km of 172 microM for acetyl-CoA and is active in a 6-8 pH range. When the acetyl-CoA substrate is replaced by propionyl-CoA, an ether lipid is produced which turns out to be as potent an aggregating agent as PAF-acether. In all cases, the products of the reaction were characterized by their behaviour in platelet-aggregation tests and their high-pressure liquid chromatography (HPLC) elution profiles. The precise definition of this acetyl-transferase is of primary importance for the development of new pharmacological agents capable of moduling a potent platelet aggregating factor.

Phospholipid modifications and adenylate cyclase in rat liver plasma membranes

Rat liver plasma membrane phospholipid headgroups or fatty acids were modified by enzymatic transmethylation or transacylation. To evaluate the effect of phospholipid modification on adenylate cyclase (AC), one of the enzymes of phospholipid metabolism as well as AC were measured in the same incubation medium. Methyl transfer from S-adenosyl-L-methionine (SAM) to phosphatidylcholine (PC) was the highest at pH 9.2. At low concentrations of SAM, synthesis of PC as well as synthesis of the monomethyl derivative were considerably decreased and Mg2+ had no effect at pH 9.2 or at pH 6.5. When phospholipid methylation was increased in relation to SAM concentration, there was no change in basal, NaF- and glucagon-stimulated AC. Methylation was not modified when AC was stimulated. On the other hand, there was an increase in the basal, NaF- and glucagon-stimulated AC when linoleate was incorporated into the membrane phospholipids. Other unsaturated fatty acids had no effect. The synthesis of linoleoyl-PC from added lysophosphatidylcholine (LPC) also stimulated AC and this in turn partly prevented the inhibitory effect of LPC. Thus in isolated plasma membranes transmethylation has no direct effect on AC, whereas synthesis of linoleoyl molecular species can modulate AC in a way which does not depend on the membrane fluidity.