Kinetics of phospholipid exchange between bilayer membranes.

In an accompanying publication by Duckwitz-Peterlein, Eilenberger and Overath ((1977) Biochim. Biophys. Acta 469,311--325) it is shown that the exchange of lipid molecules between negatively charged vesicles consisting of total phospholipid extracts from Escherichia coli occurs by the transfer of single lipid monomers or small micelles through the water. Here a kinetic interpretation is presented in terms of a rate constant, k--, for the escape of lipid molecules from the vesicle bilayer into the water. The evaluated rate constants are kP- = (0.86 +/- 0.05) - 10(-5) S-1 and ke- = (1.09 +/- 0.13) - 10(-6) s-1 for phospholipid molecules with trans-delta 9-hexadecenoate and trans-delta 9-octadecenoate, respectively, as the predominant acyl chain component. The rate constants are discussed in terms of the acyl chain and polar head group composition of the lipids.     

Non-enzymatically derived minor lipids found in Escherichia coli lipid extracts

Electrospray ionization mass spectrometry is a powerful technique to analyze lipid extracts especially for the identification of new lipid metabolites. A hurdle to lipid identification is the presence of solvent contaminants that hinder the identification of low abundance species or covalently modify abundant lipid species. We have identified several non-enzymatically derived minor lipid species in lipid extracts of Escherichia coli; phosphatidylmethanol, ethyl and methyl carbamates of PE and N-succinyl PE were identified in lipid extracts of E. coli. Phosphatidylmethanol (PM) was identified by exact mass measurement and collision induced dissociation tandem mass spectrometry (MS/MS). Extraction in the presence of deuterated methanol leads to a 3 atomic mass unit shift in the [M-H](-) ions of PM indicating its formation during extraction. Ethyl and methyl carbamates of PE, also identified by exact mass measurement and MS/MS, are likely to be formed by phosgene, a breakdown product of chloroform. Addition of phosgene to extractions containing synthetic PE significantly increases the levels of PE-MC detected in the lipid extracts by ESI-MS. Extraction in the presence of methylene chloride significantly reduced the levels of these lipid species. N-succinyl PE is formed from reaction of succinyl-CoA with PE during extraction. Interestingly N-succinyl PE can be formed in an aqueous reaction mixture in the absence of added E. coli proteins. This work highlights the reactivity of the amine of PE and emphasizes that careful extraction controls are required to ensure that new minor lipid species identified using mass spectrometry are indeed endogenous lipid metabolites.     

Quantitative measurement of sn-1,2-diacylglycerols present in platelets, hepatocytes, and ras- and sis-transformed normal rat kidney cells

A simple enzymatic method for the quantitation of the mass of sn-1,2-diacylglycerol (DAG) present in crude lipid extracts was developed to assess the function of DAGs as intracellular "second messengers" of extracellular agents and of oncogene products. The assay employed Escherichia coli DAG kinase which constituted approximately 15% of the membrane protein of a plasmid-bearing strain and defined mixed micellar conditions to solubilize the DAG present and allow its quantitative conversion to [32P]phosphatidic acid. The assay was proportional with the amount of DAG added over the range of 25 pmol to 25 nmol. The rapid rise of DAG in platelets stimulated with thrombin (210% over basal) and in hepatocytes stimulated with vasopressin (230% over basal) was quantitated and the values agreed with previous measurements. The amounts of DAG in normal rat kidney (NRK) cells grown at 34 and 38 degrees C, respectively, were 0.47 and 0.61 nmol/100 nmol of phospholipid. In K-ras-transformed NRK cells grown at 34 or 38 degrees C, DAG levels were elevated 168 or 138%, respectively. When a temperature-sensitive K-ras NRK cell line was investigated, the amount of DAG present was elevated at the permissive but not at the restrictive temperature. These data are consistent with the K-ras protein functioning in transmembrane signalling by activating phospholipase C. Protein kinase C (Ca2+/phospholipid-dependent enzyme) activation by DAG may play an important role in cellular transformation.     

Expression of the phospholipid-dependent Escherichia coli sn-1,2-diacylglycerol kinase in COS cells perturbs cellular lipid composition

The Escherichia coli sn-1,2-diacylglycerol (DAG) kinase has been successfully expressed in COS cells. The E. coli dgkA locus which contains the coding sequences for DAG kinase was subcloned into an eukaryotic expression vector, pMT2. COS cells transfected with the vector pMT2dgk expressed the DAG kinase as shown by Western analysis. Immunofluorescence studies revealed that the E. coli DAG kinase was prominently but not exclusively located in the endoplasmic reticulum. In addition, mixed micellar assays in beta-octyl glucoside revealed that membranes prepared from pMT2dgk-transfected COS cells contained over a 1500-fold increase in DAG kinase activity: 107 nmol/min/mg compared with only 0.067 nmol/min/mg for controls. DAG kinase activity from the E. coli enzyme was distinguished from endogenous COS cell activity based on differences in thermolability and the ability of the E. coli enzyme to use ceramide as a substrate. No ceramide kinase activity was detected in control COS cells, so the activity detected in pMT2dgk transfectants must have resulted from the expressed E. coli DAG kinase. The Km values for DAG kinase derived from E. coli and COS cells were nearly identical. Finally, transfected COS cells were labeled with [32P]Pi to investigate possible perturbations in lipid composition induced by the action of the E. coli DAG kinase. Ceramide (generated by the action of sphingomyelinase) was also used to clearly implicate the E. coli enzyme. Levels of ceramide phosphate increased more than 150-fold in pMT2dgk-transfected cells relative to controls. The results of these studies show that the E. coli enzyme expressed in COS cells is active and perturbs lipid composition in the intact cell system; the absolute lipid cofactor requirement of E. coli DAG kinase can be satisfied in COS cells.     

Phospholipid turnover and acyl chain remodeling in the yeast ER

The turnover of phospholipids plays an essential role in membrane lipid homeostasis by impacting both lipid head group and acyl chain composition. This review focusses on the degradation and acyl chain remodeling of the major phospholipid classes present in the ER membrane of the reference eukaryote Saccharomyces cerevisiae, i.e. phosphatidylcholine (PC), phosphatidylinositol (PI) and phosphatidylethanolamine (PE). Phospholipid turnover reactions are introduced, and the occurrence and important functions of phospholipid remodeling in higher eukaryotes are briefly summarized. After presenting an inventory of established mechanisms of phospholipid acyl chain exchange, current knowledge of phospholipid degradation and remodeling by phospholipases and acyltransferases localized to the yeast ER is summarized. PC is subject to the PC deacylation-reacylation remodeling pathway (PC-DRP) involving a phospholipase B, the recently identified glycerophosphocholine acyltransferase Gpc1p, and the broad specificity acyltransferase Ale1p. PI is post-synthetically enriched in C18:0 acyl chains by remodeling reactions involving Cst26p. PE may undergo turnover by the phospholipid: diacylglycerol acyltransferase Lro1p as first step in acyl chain remodeling. Clues as to the functions of phospholipid acyl chain remodeling are discussed.     

Insights into the behavior of unsaturated diacylglycerols in mixed lipid bilayers in relation to protein kinase C activation—A molecular dynamics simulation study

The lipid second messenger diacylglycerol (DAG) is known for its involvement in many types of cellular signaling, especially as an endogenous agonist for protein kinase C (PKC). Evidence has emerged that the degree of saturation of the DAG molecules can affect PKC activation. DAG molecules with different acyl chain saturation have not only been observed to induce varying extents of PKC activation, but also to express selectivity towards different PKC isozymes. Both qualities are important for precise therapeutic activation of PKC; understanding DAG behavior at the molecular level in different environments has much potential in the development of drugs to target PKC. We used molecular dynamics simulations to study the behavior of two different unsaturated DAG species in lipid environments with varying degrees of unsaturation. We focus on phosphatidylethanolamine (PE) instead of phosphatidylcholine (PC) to more accurately model the relevant biomembranes. The effect of cholesterol (CHOL) on these systems was also explored. We found that both high level of unsaturation in the acyl chains of the DAG species and presence of CHOL in the surrounding membrane increase DAG molecule availability at the lipid–water interface. This can partially explain the previously observed differences in PKC activation strength and specificity, the complete mechanism is, however, likely to be more complex. Our simulations coupled with the current understanding of lipids highlight the need for more simulations of biologically accurate lipid environments in order to determine the correct correlations between molecular mechanisms and biological behavior when studying PKC activation.     

Insect lipid transfer particle catalyzes bidirectional vectorial transfer of diacylglycerol from lipophorin to human low density lipoprotein

Insect plasma lipid transfer particle (LTP) catalyzes vectorial net transfer of diacylglycerol (DAG) from Manduca sexta larval high density lipophorin (HDLp-L) to human low density lipoprotein (LDL) producing an LDL of lower density and lipophorin subspecies of higher density. At equilibrium, a stable DAG-depleted very high density lipophorin species (density = 1.25 g/ml) is formed. Electrophoretic analysis of the substrate and product lipoproteins showed that apoprotein exchange or transfer between human LDL and lipophorin did not occur during the lipid transfer reaction. Facilitated net transfer of cholesteryl ester, free cholesterol, and phospholipid occurred to a much lower extent than DAG net transfer, indicating that under these conditions, LDL serves as a sink for lipophorin-associated DAG. This reaction, therefore, provides a method whereby the mass of lipid associated with human LDL can be modified in vitro without alteration of its apoprotein component. The DAG content of LDL increased in a linear manner with respect to LTP concentration and time during the initial phase of the reaction, demonstrating the utility of this system as a quantitative assay method for LTP-mediated net DAG transfer. When [3H]DAG-labeled LDL was prepared and employed in transfer experiments with unlabeled lipophorin, labeled DAG was recovered in the HDLp-L fraction. The amount of labeled DAG recovered in the HDLp-L fraction was dependent on the ratio of LDL to HDLp-L in the reaction. Thus, in this system, LTP-mediated DAG redistribution is bidirectional, suggesting that the final equilibrium distribution of lipid may be dictated by the properties of potential donor/acceptor lipoproteins rather than by an inherent particle substrate specificity of LTP.     

Increased diacylglycerols characterize hepatic lipid changes in progression of human nonalcoholic fatty liver disease; comparison to a murine model

Background and Aims

The spectrum of nonalcoholic fatty liver disease (NAFLD) includes steatosis, nonalcoholic steatohepatitis (NASH), and progression to cirrhosis. While differences in liver lipids between disease states have been reported, precise composition of phospholipids and diacylglycerols (DAG) at a lipid species level has not been previously described. The goal of this study was to characterize changes in lipid species through progression of human NAFLD using advanced lipidomic technology and compare this with a murine model of early and advanced NAFLD.

Methods

Utilizing mass spectrometry lipidomics, over 250 phospholipid and diacylglycerol species (DAGs) were identified in normal and diseased human and murine liver extracts.

Results

Significant differences between phospholipid composition of normal and diseased livers were demonstrated, notably among DAG species, consistent with previous reports that DAG transferases are involved in the progression of NAFLD and liver fibrosis. In addition, a novel phospholipid species (ether linked phosphatidylinositol) was identified in human cirrhotic liver extracts.

Conclusions

Using parallel lipidomics analysis of murine and human liver tissues it was determined that mice maintained on a high-fat diet provide a reproducible model of NAFLD in regards to specificity of lipid species in the liver. These studies demonstrated that novel lipid species may serve as markers of advanced liver disease and importantly, marked increases in DAG species are a hallmark of NAFLD. Elevated DAGs may contribute to altered triglyceride, phosphatidylcholine (PC), and phosphatidylethanolamine (PE) levels characteristic of the disease and specific DAG species might be important lipid signaling molecules in the progression of NAFLD.     

Partial purification of glycerophosphate acyltransferase from Escherichia coli.

Glycerophosphate acyltransferase, a membrane-bound enzyme catalyzing the initial step of phospholipid biosynthesis in Escherichia coli, has been extracted with Triton X-100, a nonionic detergent, and purified 20- to 40-fold. This preparation is free from lysophosphatidate acyltransferase. Glycerophosphate acyltransferase is inactive in detergent extracts, but can be reconstituted by the addition of phospholipid. Under such conditions, the enzyme is associated with phospholipid. The sole product of the reaction with acyl coenzyme A as substrate is 1-acyl-sn-glycero-3-phosphate. Furthermore, the enzyme shows a marked preference for saturated fatty acyl conenzyme A, implying that this enzyme is responsible for the predominance of saturated moieties in position 1 of E. coli phospholipids. Acyltransferase from two mutants, plsA and plsB, was partially purified and characterized. Results support the view that plsB is a structural gene for the acyltransferase, but suggest that the plsA gene product is not directly involved in phospholipid biosynthesis.     

Evidence for a new inositol phospholipid in rat brain mitochondria.

Phosphorylation of phosphatidylinositol (PI), phosphatidylinositol monophosphate (PIP) and diacylglycerol (DAG) was studied in rat brain cortex myelin, synaptosomal and mitochondrial fractions, with ATP as phosphate donor and endogenous phospholipids as substrate. All fractions had PI, PIP and DAG phosphorylating activity with their own characteristic subcellular distribution. However, in the mitochondrial fraction an unidentified lipid was phosphorylated, which had a slower Rf value than PIP2 on TLC. After hydrolysis of the polar head group of the lipid and separation on anion exchange columns, it appeared to be a phosphoinositide. The elution profile showed that it was not phosphatidylinositol trisphosphate, or a lyso-compound. The available evidence suggests that the unknown inositol phospholipid in rat brain mitochondria is a phosphatidylinositol 4,5-bisphosphate isomer, although the possibility of it being a glycosyl-phosphoinositide cannot be excluded.     

Facilitated diacylglycerol exchange between insect hemolymph lipophorins. Properties of Manduca sexta lipid transfer particle

Manduca sexta hemolymph lipid transfer particle (LTP) is a very high density lipoprotein (d = 1.23 g/ml) containing 14% lipid and 5% carbohydrate. Each of three apoprotein components, apoLTP-I (Mr approximately 320,000), apoLTP-II (Mr = 85,000), and apoLTP-III (Mr = 55,000), is glycosylated. Carbohydrate analysis revealed the presence of mannose and N-acetylglucosamine in a ratio of 4.5:1. A native Mr greater than 670,000 was determined by pore limiting gradient gel electrophoresis. Lipid analysis of LTP revealed the presence of phospholipid, diacylglycerol (DAG), free fatty acid, and triacylglycerol. Rabbit polyclonal antibodies directed against LTP were obtained. Anti-LTP serum was employed in experiments which indicated the presence of LTP in larval and adult animals and confirmed that LTP was unrelated to other M. sexta hemolymph proteins and lipoproteins. A quantitative lipid transfer assay measuring facilitated DAG exchange between isolated M. sexta lipoproteins was established. The level of LTP-catalyzed exchange of DAG increased linearly with increasing time and protein during the initial phase of the reaction. Inclusion of anti-LTP serum in the assay inhibited facilitated DAG exchange. Experiments designed to determine if the LTP holoprotein is required for transfer or if a component of LTP is the active principle were performed. Incubation of [3H]DAG labeled high density lipophorin with substrate amounts of LTP resulted in incorporation of labeled DAG into LTP. Subsequent incubation of [3H]DAG-labeled LTP with unlabeled lipophorin resulted in exchange of DAG and the appearance of labeled DAG in lipophorin. Nitrocellulose-bound LTP apoproteins did not facilitate DAG exchange, and pretreatment of LTP with detergents resulted in loss of transfer activity. Extraction of LTP lipids with ethanol/ether also resulted in loss of activity. The results suggest that the lipid component of LTP may be important in the transfer reaction.     

Phosphatidylethanolamine and phosphatidylcholine are sources of diacylglycerol in ras-transformed NIH 3T3 fibroblasts

Ras-transformation of cells is accompanied by an increase of the level of diacylglycerol (DAG), which participates in the signal transduction pathways. DAG could be generated from phospholipids either by activation of phospholipase C or by a more complex pathway involving phospholipase D and phosphatidate phosphohydrolase. To clarify which phospholipids produce DAG and which pathways are involved, we examined the DAG generating enzyme activities, using phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI) as substrates. The study showed that the breakdown of PC and more markedly of PE by phospholipases C and D was stimulated in membranes from ras-transformed cells. Phosphatidate phosphohydrolase activity was also elevated in oncogene-expressing cells. The increase in glycerol uptake was most pronounced in cells given PE, followed by PC. The fatty acid analysis revealed apparent similarities between the acyl chains of PE and DAG only in the transformed cells. These findings suggest that PE is a source of DAG in ras-fibroblasts but does not rule out the role of PC in DAG production, due to the activation of the PC-specific phospholipases C and D.     

Electrospray ionization mass spectrometry and exogenous heavy isotope-labeled lipid species provide detailed information on aminophospholipid acyl chain remodeling

Mammalian cells maintain the phospholipid compositions of their different membranes remarkably constant. Beside de novo synthesis, degradation, and intracellular trafficking, acyl chain remodeling plays an important role in phospholipid homeostasis. However, many key details of this process remain unresolved, largely because of limitations of existing methodologies. Here we describe a novel approach that allows one to study metabolism of individual phospholipid species in unprecedented detail. Forty different phosphatidylethanolamine (PE) or -serine (PS) species with a deuterium-labeled head group were synthesized and introduced to BHK21 or HeLa cells using cyclodextrin-mediated transfer. Their metabolism was then monitored in detail by electrospray ionization mass spectrometry. Atypical PE and PS species were rapidly remodeled at both sn1 and sn2 position, yielding a molecular species profile similar to that the endogenous PE and PS. In contrast, remodeling of exogenous species identical or similar to major endogenous ones was more limited and much slower. Major differences in remodeling pathways and kinetics were observed between species within a class, as well as between corresponding PE and PS species. These data along with those obtained with pharmacological inhibitors strongly suggest that multiple lipid class-specific A-type phospholipases and acyl transferases are involved in aminophospholipid remodeling. In conclusion, the approach described here provides highly detailed information on remodeling of exogenously added (amino)glycerophospholipids and should thus be very helpful when elucidating the proteins and processes maintaining molecular species homeostasis.     

Effect of membrane sterol content on the susceptibility of phospholipids to phospholipase A2

The effects of membrane sterol level on the susceptibility of LM cell plasma membranes to exogenous phospholipases A2 has been investigated. Isolated plasma membranes, containing normal or decreased sterol content, were prepared from mutant LM cell sterol auxotrophs. beta-Bungarotoxin-catalyzed hydrolysis of both endogenous phospholipids and phospholipids introduced into the membranes with beef liver phospholipid exchange proteins was monitored. In both cases, phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were degraded at similar rates in normal membranes, while PC hydrolysis was specifically accelerated in sterol-depleted membranes. Additional data suggest that this preferential hydrolysis of PC is not a consequence of the phospholipid head group specificity of the phospholipase, nor of a difference in the accessibility of PC versus PE to the enzyme. Analysis of the reaction products formed during treatment of isolated membranes with phospholipase A2 showed almost no accumulation of lysophospholipids. This was found to be due to highly active lysophospholipase(s), present in LM cell plasma membranes, acting on the lysophospholipids formed by phospholipase A2 action. A soluble phospholipase A2 was partially purified from LM cells and found to behave as beta-bungarotoxin with regard to membrane sterol content. These results demonstrate that the nature of phospholipid hydrolysis, catalyzed by phospholipase A2, can be significantly affected by membrane lipid composition.     

Effects of temperature variation and phenethyl alcohol addition on acyl chain order and lipid organization in Escherichia coli derived membrane systems. A 2H- and 31P-NMR study.

Using 2H- and 31P-NMR techniques the effects of temperature variation and phenethyl alcohol addition were investigated on lipid acyl chain order and on the macroscopic lipid organization of membrane systems derived from cells of the Escherichia coli fatty acid auxotrophic strain K1059, which was grown in the presence of [11,11-2H2]oleic acid. Membranes of intact cells showed a gel to liquid-crystalline phase transition in the range of 4-20 degrees C, which was similar to that observed for the total lipid extract and for the dominant lipid species phosphatidylethanolamine (PE). Phosphatidylglycerol (PG) remained in a fluid bilayer throughout the whole temperature range (4-70 degrees C). At 30 degrees C acyl chain order was highest in PE, followed by the total lipid extract, PG, intact cells, and isolated inner membrane vesicles. Acyl chain order in E. coli PE and PG was much higher than in the corresponding dioleoylphospholipids. E. coli PE was found to maintain a bilayer organization up to about 60 degrees C, whereas in the total lipid extract as well as in intact E. coli cells bilayer destabilization occurred already at about 42 degrees C. It is proposed that the regulation of temperature at which the bilayer-to-non-bilayer transition occurs may be important for membrane functioning in E. coli. Addition of phenethyl alcohol did not affect the macroscopic lipid organization in E. coli cells or in the total lipid extract, but caused a large reduction in chain order of about 70% at 1 mol% of the alcohol in both membrane systems. It is concluded that while both increasing temperature and addition of phenethyl alcohol can affect membrane integrity, in the former case this is due to the induction of non-bilayer lipid structures, whereas in the latter case this is caused by an increase in membrane fluidity.     

sn-Glycerol-3-phosphate acyltransferase activity in particulate preparations from anaerobic, light-grown cells of Rhodopseudomonas spheroides. Involvement of acyl thiolester derivatives of acyl carrier protein in the synthesis of complex lipids.

Crude particulate preparations obtained from anaerobic, light-grown cells of Rhodopseudomonas spheroides have been shown to possess a significant level of sn-glycerol-3-phosphate acyltransferase (EC 2.3.1.15) activity. In contrast to the enzyme from Escherichia coli, the R. spheroides glycerophosphate acyltransferase has a high specificity for acyl thiolester derivatives of acyl carrier protein (ACP) as acyl donors for the reaction. Only limited , nonlinear glycerophosphate incorporation into lipid occurs when acyl coenzyme A (CoA) derivatives are employed as acyl substrate. With oleyl-ACP as substrate, maximal enzyme activity was observed at 40 degrees, over a broad pH range (6.0 to 8.5) and did not require a divalent metal cation. The presence of dithiothreitol stimulated enzyme-activity 15 to 20%. When oleyl-ACP or palmityl-ACP was employed as sole acyl group donor, the major products recoverable from the reaction mixtures were lysophosphatidic acid, phosphatidic acid, and monoglyceride. Althouh oleyl-ACP and palmityl-ACP gave comparable maximal velocities in the initial acylation of glycerophosphate, the formation of phosphatidic acid occurred preferentially with the unsaturated acyl-ACP derivative.     

Neuronal membrane lipid asymmetry.

The outer surface of intact synaptosomes was covalently labelled with trinitrobenzenesulfonic acid prior to isolation of the synaptic plasma membrane. Analysis of the membrane lipid demonstrated an asymmetric distribution of phospholipids across the synaptosomal plasma membrane. In addition, the fatty acyl composition of phosphatidylethanolamine from this neuronal membrane fraction was also distributed asymmetrically. The data are consistent with a $\text{de novo}$ generation of phospholipid asymmetry independent of serum lipid exchange processes. This structural asymmetry may have important consequences for neurotransmission.     

Phosphatidylcholine:diacylglycerol cholinephosphotransferase’s unique regulation of castor bean oil quality

Castor bean (Ricinus communis) seed oil (triacylglycerol [TAG]) is composed of ∼90% of the industrially important ricinoleoyl (12-hydroxy-9-octadecenoyl) groups. Here, phosphatidylcholine (PC):diacylglycerol (DAG) cholinephosphotransferase (PDCT) from castor bean was biochemically characterized and compared with camelina (Camelina sativa) PDCT. DAGs with ricinoleoyl groups were poorly used by Camelina PDCT, and their presence inhibited the utilization of DAG with “common” acyl groups. In contrast, castor PDCT utilized DAG with ricinoleoyl groups similarly to DAG with common acyl groups and showed a 10-fold selectivity for DAG with one ricinoleoyl group over DAG with two ricinoleoyl groups. Castor DAG acyltransferase2 specificities and selectivities toward different DAG and acyl-CoA species were assessed and shown to not acylate DAG without ricinoleoyl groups in the presence of ricinoleoyl-containing DAG. Eighty-five percent of the DAG species in microsomal membranes prepared from developing castor endosperm lacked ricinoleoyl groups. Most of these species were predicted to be derived from PC, which had been formed by PDCT in exchange with DAG with one ricinoleoyl group. A scheme of the function of PDCT in castor endosperm is proposed where one ricinoleoyl group from de novo-synthesized DAG is selectivity transferred to PC. Nonricinoleate DAG is formed and ricinoleoyl groups entering PC are re-used either in de novo synthesis of DAG with two ricinoleoyl groups or in direct synthesis of triricinoleoyl TAG by PDAT. The PC-derived DAG is not used in TAG synthesis but is proposed to serve as a substrate in membrane lipid biosynthesis during oil deposition.

The enzyme phosphatidylcholine:diacylglycerol cholinephosphotransferase facilitates accumulation of seed oil with three hydroxylated acyl groups in Ricinus communis.     

Elongation of Fatty Acids in Mycobacterium tuberculosis

Cell-free extracts of the H37Ra strain of Mycobacterium tuberculosis contain a soluble enzyme system which catalyzes an elongation reaction of long-chain fatty acids. The predominant reaction involves the addition of a single C(2) unit to the acceptor fatty acid; the elongation takes place exclusively at the carboxyl end of the acceptor molecule. The endogenous acceptor lipid can be removed by solvent extraction of the enzyme system. The lipid-depleted enzyme can be fully reactivated with external acyl coenzyme A, after which elongation with acetyl coenzyme A takes place. The elongation reaction is avidin-insensitive and does not require adenosine triphosphate. Reduced nicotinamide adenine dinucleotide is the source of reducing equivalent, whereas reduced nicotinamide adenine dinucleotide phosphate is without effect.     

Partial purification and characterization of phospholipid N-methyltransferases from murine thymocyte microsomes

Significant amounts of phospholipid N-methyltransferase activity in murine thymocytes were found to be distributed on the plasma membrane. The enzyme activity had an optimum pH of 9. The presence of divalent cations, Mg2+ (10 mM) or Ca2+ (1 mM), and EGTA separately in the assay had only a small effect on the enzyme activity. However, addition of both 10 mM Mg2+ and 1 mM Ca2+ increased the enzyme activity. The presence of two enzymes for each conversion of phosphatidylethanolamine (PE) to phosphatidylmonomethylethanolamine (PME) and PME to phosphatidylcholine (PC) was suggested by the result of the determination of the incorporated radioactivity into PME, phosphatidyldimethylethanolamine (PDE) and PC; the apparent Km values for S-adenosyl-L-methionine were 20 and 400-500 microM for the conversion of PE to PME and for the conversion of PME to PC they were 5 microM and 40 microM. S-Adenosyl-L-homocysteine (AdoHcy), a known inhibitor of enzymatic methylation, competitively inhibited [14C]methyl incorporation into total lipid. The apparent Ki value for AdoHcy was 44.7 microM. Two phospholipid N-methyltransferases were partially purified by extraction with sodium deoxycholate, gel filtration on Sephadex G-75, and affinity column chromatography on AdoHcy-Sepharose. One enzyme, mainly catalyzing the formation of PME, was purified approximately 1548-fold and the other catalyzing the formation of PDE and PC, was purified approximately 629- to 703-fold. However, the former still contained a little activity for PDE and PC formation and the latter contained a little activity for PME formation. In these partially purified phospholipid N-methyltransferase preparations, little contaminating protein O-carboxylmethyltransferase activity was observed; however, significant PC-phospholipase A2 activity was detected. This result may suggest that phospholipid N-methyltransferases associate with phospholipase A2 in the thymocyte plasma membrane.