Stereospeific 1H-NMR analysis of trimethylsilyl derivatives of glycerides with a chiral shift reagent

A proton magnetic resonance procedure with tri(3-heptafluorobutyryl-d-camphorato)praseodymium (III) as a chiral shift eagent has been developed to determine the enantimeric purity of monoglycerides 1,2-diglycerides and triglycerides with one mono-unsaturated fatty acid at position $\text{sn-1}$ or $\text{sn-3}$ and two saturated fatty acids at the two other glycerol positions. A model compound, 1-oleoyl-2,3-dipalmitoyl-sn-glycerol, was converted ito the trimethylsilyl either of 2,3-dipalmitoyl-an-glycerol by epoxidation of the double bond, followed by pancreatic hydrolysis and separation and trimethylsilylation of the resulting $\text{sn-1,2}$, and $\text{sn-2,3-}\text{diglycerides}$. This separation becomes feasible by the contribution of the epoxy group to the polarity of the diglyceride. The protons of the trimethysilyl ether group were used for determining the enantiomeric ratio. The addition of a chira shift reagent induces a useful enantiomeric splitting which allows the accurate determination of the ratio of both enantiomers. The trimethylsilyl emers of 1,2-diglycerides are better suited for this purpose than the acetyl compounds. For monoglycetides, the earlier published method with the diaceltates gives a better line separation in ¹H-NMR spectra.     

Characterization of the transacylase activity of rat liver 60-kDa lysophospholipase-transacylase. Acyl transfer from the sn-2 to the sn-1 position

Rat liver 60-kDa lysophospholipase-transacylase catalyzes not only the hydrolysis of 1-acyl-sn-glycero-3-phosphocholine, but also the transfer of its acyl chain to a second molecule of 1-acyl-sn-glycero-3-phosphocholine to form phosphatidylcholine (H. Sugimoto, S. Yamashita, J. Biol. Chem. 269 (1994) 6252–6258). Here we report the detailed characterization of the transacylase activity of the enzyme. The enzyme mediated three types of acyl transfer between donor and acceptor lipids, transferring acyl residues from: (1) the sn-1 to -1(3); (2) sn-1 to -2; and (3) sn-2 to -1 positions. In the sn-1 to -1(3) transfer, the sn-1 acyl residue of 1-acyl-sn-glycero-3-phosphocholine was transferred to the sn-1(3) positions of glycerol and 2-acyl-sn-glycerol, producing 1(3)-acyl-sn-glycerol and 1,2-diacyl-sn-glycerol, respectively. In the sn-1 to -2 transfer, the sn-1 acyl residue of 1-acyl-sn-glycero-3-phosphocholine was transferred to not only the sn-2 positions of 1-acyl-sn-glycero-3-phosphocholine, but also 1-acyl-sn-glycero-3-phosphoethanolamine, producing phosphatidylcholine and phosphatidylethanolamine, respectively. 1-Acyl-sn-glycero-3-phospho-myo-inositol and 1-acyl-sn-glycero-3-phosphoserine were much less effectively transacylated by the enzyme. In the sn-2 to -1 transfer, the sn-2 acyl residue of 2-acyl-sn-glycero-3-phosphocholine was transferred to the sn-1 position of 2-acyl-sn-glycero-3-phosphocholine and 2-acyl-sn-glycero-3-phosphoethanolamine, producing phosphatidylcholine and phosphatidylethanolamine, respectively. Consistently, the enzyme hydrolyzed the sn-2 acyl residue from 2-acyl-sn-glycero-3-phosphocholine. By the sn-2 to -1 transfer activity, arachidonic acid was transferred from the sn-2 position of donor lipids to the sn-1 position of acceptor lipids, thus producing 1-arachidonoyl phosphatidylcholine. When 2-arachidonoyl-sn-glycero-3-phosphocholine was used as the sole substrate, diarachidonoyl phosphatidylcholine was synthesized at a rate of 0.23 μmol/min/mg protein. Thus, 60-kDa lysophospholipase-transacylase may play a role in the synthesis of 1-arachidonoyl phosphatidylcholine needed for important cell functions, such as anandamide synthesis.     

The interaction of cholesterol with bilayers of phosphatidylethanolamine

The interaction of cholesterol with the glycerol backbone segments of phospholipids was studied in bilayers of phosphatidylethanolamine containing equimolar amounts of cholesterol. Glycerol selectively deuterated at various positions was supplied to the growth medium of Escherichia coli strain 131 GP which is defective in endogeneous glycerol synthesis. The procedure enables the stereospecific labeling of the three glycerol backbone segments of the membrane phospholipids. Phosphatidylethanolamine with wild-type fatty acid composition was purified from E. coli cells and deuterium magnetic resonance spectra were obtained either from dispersions of pure phosphatidylethanolamine or from equimolar mixtures of phosphatidylethanolamine with cholesterol. For comparative purposes 1,2-di[9,10-²H₂]elaidoyl-sn-glycero-3-phosphoethanolamine and [3-α-²H]cholesterol were synthesized in order to monitor the behavior of the fatty acyl chains and of the cholesterol molecule itself. For all deuterated segments the deuterium quadrupole splittings as well as the deuterium spin-lattice (T₁) relaxation times were measured as a function of temperature. The glycerol backbone was found to be a remarkably stable structural element of the phospholipid molecule. The quadrupole splittings of the backbone segments changed only by at most 2 kHz upon incorporation of 50 mol % cholesterol. This was in contrast to the fatty acyl chains where the same amount of cholesterol increased the quadrupole splitting by more than 20 kHz. The glycerol segments exhibited the shortest T₁ relaxation times of all CH₂ segments indicating that the glycerol backbone is the slowest motional moiety of the lipid molecule. Addition of cholesterol has no effect on the backbone motion but the fast reorientation rate of the trans-double bonds in 1,2-dielaidoyl-sn-glycero-3-phosphoethanolamine is increased dramatically.     

Analysis of Acyl Fluxes through Multiple Pathways of Triacylglycerol Synthesis in Developing Soybean Embryos

The reactions leading to triacylglycerol (TAG) synthesis in oilseeds have been well characterized. However, quantitative analyses of acyl group and glycerol backbone fluxes that comprise extraplastidic phospholipid and TAG synthesis, including acyl editing and phosphatidylcholine-diacylglycerol interconversion, are lacking. To investigate these fluxes, we rapidly labeled developing soybean (Glycine max) embryos with [¹⁴C]acetate and [¹⁴C]glycerol. Cultured intact embryos that mimic in planta growth were used. The initial kinetics of newly synthesized acyl chain and glycerol backbone incorporation into phosphatidylcholine (PC), 1,2-sn-diacylglycerol (DAG), and TAG were analyzed along with their initial labeled molecular species and positional distributions. Almost 60% of the newly synthesized fatty acids first enter glycerolipids through PC acyl editing, largely at the sn-2 position. This flux, mostly of oleate, was over three times the flux of nascent [¹⁴C]fatty acids incorporated into the sn-1 and sn-2 positions of DAG through glycerol-3-phosphate acylation. Furthermore, the total flux for PC acyl editing, which includes both nascent and preexisting fatty acids, was estimated to be 1.5 to 5 times the flux of fatty acid synthesis. Thus, recycled acyl groups (16:0, 18:1, 18:2, and 18:3) in the acyl-coenzyme A pool provide most of the acyl chains for de novo glycerol-3-phosphate acylation. Our results also show kinetically distinct DAG pools. DAG used for TAG synthesis is mostly derived from PC, whereas de novo synthesized DAG is mostly used for PC synthesis. In addition, two kinetically distinct sn-3 acylations of DAG were observed, providing TAG molecular species enriched in saturated or polyunsaturated fatty acids.     

Chemical and physicochemical studies of lysylphosphatidylglycerol derivatives. Occurrence of a2' yields 3' lysyl migration

Syntheses of 1,2-didodecanoyl-sn-glycero-3-phosphoryl-1′-(3′-O-L-lysyl)-sn-glycerol (IV) and 1,2-didodecanoyl-sn-glycero-3-phosphoryl-1′-(2′-O-L-lysyl)-sn-glycerol (VIII) as well as 1,2-didodecanoyl-sn-glycerol-3-phosphoryl-1′-sn-glycerol (XII) are described. 2′- and 3′-lysylphosphatidylglycerol are obtained as pure isomers and can be distinguished spectroscopically (infrared, 100 and 300 MHZ NMR). By these criteria a migration of the lysyl group from the 2′ to the 3′ position of the glycerol occurs in the presence of a strong acid catalyst such as HCl. On the other hand, a weak acid such as acetic acid appears ineffective in inducing lysyl migration, even at very high concentrations.Spectroscopic analysis furthermore demonstrated that lysylphosphatidylglycerol extracted from the Staphylococcus aureus membrane, is a 3′-isomer.     

Some properties of diacylglycerol acyltransferase in a particulate fraction from maturing safflower seeds

The activity of diacylglycerol acyltransferase of a subcellular particulate fraction from maturing safflower seeds was remarkably stimulated by the addition of 1, 2-diacylglycerols which were previously emulsified in a gelatin solution by sonication. Metal ions were inhibitory to the reaction. Deoxycholate and diisopropyl fluorophosphate were the most effective inhibitors. Sulfhydryl groups seemed to be of limited significance in the enzyme. Both 1, 2-dioleoyl-sn-glycerol and 2, 3-dioleoyl-sn-glycerol were good substrates of diacylglycerol acyltransferase, but the 1, 3-isomer did not serve as an acyl acceptor. The enzyme showed broad specificity for synthetic rac-1, 2-diacylglycerols containing various fatty acids. However, rac-1, 2-diacetylglycerol and rac-1, 2-dibutyrylglycerol, which are soluble in water, were ineffective. The enzyme exhibited no significant specificity for saturated and unsaturated fatty acyl-CoA thioesters as acyl donors. This suggests that the fatty acid composition at the 3-position of the glycerol molecule of safflower triacylglycerols may depend on the composition of the endogenous acyl-CoA pool.     

Effect of temperature on the NMR spectra of sonicated dispersions of isomers of dipalmitoylphosphatidylcholine

Sonicated dispersions of 1,2-dipalmitoylsn-glycero-3-phosphorylcholine and of 1,3-dipalmitoylglycero-2-phosphorylcholine were examined by proton nuclear magnetic resonance (NMR) as a function of temperature. The —(CH₂)_n)— peak in the spectrum of the sn-3-isomer of dipalmitoylphosphatidylcholine showed the characteristic dramatic changes in the peak intensity and width associated with the phase transition between the liquid crystalline and gel states of the phospholipid. This occurred over a 2–3°C temperature range with the midpoint of the transition at 38.5°C. With the 2-isomer the change in phase took place over a similar temperature range but the midpoint was at 33.8°C. This lower phase transition temperature is presumably the result of increased acyl chain mobility caused by the increased separation of the two acyl chains by the centre carbon of the glycerol backbone. The effect of sonication of the broadening of the range and lowering of the midpoint temperature of the phase transition from that of the corresponding unsonicated dispersions was similar with each isomer. This suggests that the overall geometry of the sonicated vesicles of the isomers is similar.     

Chiroptical properties of S-alkyl derivatives of 1-thioglycerol

Several racemic and optically active S-alkyl derivatives of 1-thioglycerol have been synthesized. From 3-S-tetradecyl-3-thio-sn-glycerol the dioleoyl and diacetyl esters were prepared. 1,2-Dioleoyl-3-S-tetradecyl-3-thio-sn-glycerol was converted into the corresponding sulphoxide and sulphone. The chiroptical properties of these compounds were investigated by ORD and CD. Such measurements can be used in studies on the metabolism of enantiomeric acylglycerols.     

Conformational differences between sn-3-phospholipids and sn-2-phospholipids. A neutron and x-ray diffraction investigation

Extensive work has been reported on the conformation in membranes of sn-3-phosphatidylcholines, -ethanolamines, -glycerols and -serines (sn-3-phospholipids), where the headgroup is linked to the third carbon atom in the glycerol backbone. One important feature common to all these naturally occurring phospholipids is that the glycerol moiety is oriented almost perpendicular to the membrane surface, with the sn-1 chain continuing in this direction, whereas the sn-2 chain starts first in a direction parallel to the layer and then bends sharply at the second carbon atom. We present here neutron diffraction results on 1,3-dipalmitoyl-glycero-2-phosphocholine (1,3-DPPC) in which the headgroup is attached to the second carbon atom in the middle of the glycerol part and the two other adjacent carbon atoms are linked to the paraffin chains. Two 1,3-DPPC samples. ²H-labelled at different positions, were studied. One sample was deuterated at the first methylene segment in each fatty acyl chain, and the other at the first segment in one chain and at the second segment in the other chain. The Patterson maps as well as the neutron density profiles show that both fatty acyl chains in 1,3-DPPC have the same conformation introduced by this symmetric chemical bond pattern. It is concluded from this that the C₍₁₎C₍₃₎ vector of the glycerol backbone part is oriented parallel to the bilayer surface and the ²H nuclear magnetic resonance signals indicate that both chains have a conformation similar to that observed for the bent chain in sn-3-phospholipids. These findings indirectly confirm the idea that an intramolecular energy minimum, together with the packing geometry of the lipids in the membrane, produce the characteristic conformation around the glycerol backbone as is found for all naturally occurring phospholipids that have been studied so far.     

Carbohydrate Conformation and Lipid Condensation in Monolayers Containing Glycosphingolipid Gb3: Influence of Acyl Chain Structure

Globotriaosylceramide (Gb3), a glycosphingolipid found in the plasma membrane of animal cells, is the endocytic receptor of the bacterial Shiga toxin. Using x-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), lipid monolayers containing Gb3 were investigated at the air-water interface. XR probed Gb3 carbohydrate conformation normal to the interface, whereas GIXD precisely characterized Gb3’s influence on acyl chain in-plane packing and area per molecule (APM). Two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), were used to study Gb3 packing in different lipid environments. Furthermore, the impact on monolayer structure of a naturally extracted Gb3 mixture was compared to synthetic Gb3 species with uniquely defined acyl chain structures. XR results showed that lipid environment and Gb3 acyl chain structure impact carbohydrate conformation with greater solvent accessibility observed for smaller phospholipid headgroups and long Gb3 acyl chains. In general, GIXD showed that Gb3 condensed phospholipid packing resulting in smaller APM than predicted by ideal mixing. Gb3’s capacity to condense APM was larger for DSPC monolayers and exhibited different dependencies on acyl chain structure depending on the lipid environment. The interplay between Gb3-induced changes in lipid packing and the lipid environment’s impact on carbohydrate conformation has broad implications for glycosphingolipid macromolecule recognition and ligand binding.     

Carbohydrate Conformation and Lipid Condensation in Monolayers Containing Glycosphingolipid Gb3: Influence of Acyl Chain Structure

Globotriaosylceramide (Gb3), a glycosphingolipid found in the plasma membrane of animal cells, is the endocytic receptor of the bacterial Shiga toxin. Using x-ray reflectivity (XR) and grazing incidence x-ray diffraction (GIXD), lipid monolayers containing Gb3 were investigated at the air-water interface. XR probed Gb3 carbohydrate conformation normal to the interface, whereas GIXD precisely characterized Gb3’s influence on acyl chain in-plane packing and area per molecule (APM). Two phospholipids, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), were used to study Gb3 packing in different lipid environments. Furthermore, the impact on monolayer structure of a naturally extracted Gb3 mixture was compared to synthetic Gb3 species with uniquely defined acyl chain structures. XR results showed that lipid environment and Gb3 acyl chain structure impact carbohydrate conformation with greater solvent accessibility observed for smaller phospholipid headgroups and long Gb3 acyl chains. In general, GIXD showed that Gb3 condensed phospholipid packing resulting in smaller APM than predicted by ideal mixing. Gb3’s capacity to condense APM was larger for DSPC monolayers and exhibited different dependencies on acyl chain structure depending on the lipid environment. The interplay between Gb3-induced changes in lipid packing and the lipid environment’s impact on carbohydrate conformation has broad implications for glycosphingolipid macromolecule recognition and ligand binding.     

Metabolism of hepatic haem and `green pigments'' in rats given 2-allyl-2-isopropylacetamide and ferric citrate. A new model for hepatic haem turnover

The acyl specificities of several acyltransferases located in the microsomal fraction of lactating rat mammary gland have been investigated using palmitate and oleate as substrates along with CoA, ATP and Mg²⁺, bovine serum albumin and NaF. With either sn-glycerol 3-phosphate or dihydroxyacetone phosphate (plus NADPH) as acyl acceptor, phosphatidic acid containing palmitate preferentially esterified at position-2 and oleate at position-1 was the major product. Dihydroxyacetone phosphate and sn-glycerol 3-phosphate competitively inhibited each other's acylations, suggesting that a single enzyme might be responsible for both esterifications and oleate was the preferred substrate for the formation of acyldihydroxyacetone phosphate. The specificities of the acyl-CoA–1-monoacyl-sn-glycerol 3-phosphate and the acyl-CoA–2-monoacyl-sn-glycerol 3-phosphate acyltransferases were also studied. The specificities observed combined with the relative velocities of these reactions suggest that phosphatidic acid is formed in the mammary gland with the first acylation occurring at position-1 favouring oleate followed by the second acylation at position-2 favouring palmitate. This is consistent with the unusual structure found in the triacylglycerols of rat milk. When a mouse liver microsomal fraction was used the opposite specificities were observed consistent with the structure of the triacylglycerols of mouse liver. The microsomal acylation of the monoacyl-sn-glycerol 3-phosphocholines was also investigated. Although no marked acyl specificity could be detected when the 2-monoacyl-sn-glycerol 3-phosphocholine was used as the acyl acceptor, both oleate and linoleate were esterified in preference to palmitate to the 1-monoacyl-sn-glycerol 3-phosphocholine.     

Labelling of glycerolipids in the cotyledons of developing oilseeds by [1-14C]acetate and [2-3H]glycerol

1. 3-sn-Phosphatidylcholine was identified as the major lipid in cotyledons from the developing seeds of soya bean, linseed and safflower when tissue was steamed before lipid extraction. The proportion of oleate in this lipid decreased markedly and that of the polyunsaturated C₁₈ fatty acids increased when detached developing cotyledons were incubated for up to 3h. Similar but less pronounced changes occurred in diacylglycerol, which had a fatty acid composition resembling that of the 3-sn-phosphatidylcholine from cotyledons of the same species. 2. [1-¹⁴C]Acetate supplied to detached cotyledons was incorporated into the acyl moieties of mainly 3-sn-phosphatidylcholine, 1,2-diacylglycerol and triacylglycerol. Initially label was predominantly in oleate, but subsequently entered at accelerating rates the linoleoyl moieties of the above lipids in soya-bean and safflower cotyledons and the linoleoyl and linolenyl moieties of these lipids in linseed cotyledons. In pulse–chase experiments label was rapidly lost from the oleate of 3-sn-phosphatidylcholine and accumulated in the linoleoyl and linolenoyl moieties of this phospholipid and of the di- and tri-acylglycerols. 3. [2-³H]Glycerol was incorporated into the glycerol moieties of mainly 3-sn-phosphatidylcholine and di- and tri-acylglycerols of developing linseed and soya-bean cotyledons. The label entered the phospholipid and diacylglycerol at rates essentially linear with time from the moment the substrate was supplied, and entered the triacylglycerol at an accelerating rate. With linseed cotyledons the labelled glycerol was incorporated initially mainly into species of 3-sn-phosphatidylcholine and diacylglycerol that contained oleate, but accumulated with time in more highly unsaturated species. In pulse–chase experiments with linseed cotyledons, label was lost from both 3-sn-phosphatidylcholine and diacylglycerol, preferentially from the dioleoyl species, and accumulated in triacylglycerol, mainly in species containing two molecules of linolenate. 4. The results suggest a rapid turnover of 3-sn-phosphatidylcholine during triacylglycerol accumulation in developing oilseeds, and are consistent with the operation of a biosynthetic route whereby oleate initially esterified to the phospholipid is first desaturated, then polyunsaturated fatty acids transferred to triacylglycerol, via diacylglycerol. The possible role of oleoyl phosphatidylcholine as a substrate for oleate desaturation is discussed.     

Digestion and absorption of a sulphoxide analogue of triacylglycerol in the rat

The stereochemistry of fat digestion and absorption was studied by feeding a triacylglycerol analogue to rats with a thoracic duct cannula. The analogue, rac-1,2-dioleoyl-3-S-tetradecyl-3-thioglycerol-S-oxide was chosen since its enantiomers exhibited high rotation in optical rotatory dispersion (ORD) and circular dichroism (CD). In the chyle, triacylglycerol was the major lipid but X-1,2-diacyl-3-S-tetradecyl-3-thioglycerol-S-oxide constituted 8% of lipid weight. It was resolved by thin-layer chromatography (TLC) into two diastereomers. Each of the diastereomers were analyzed for the proportions of 1-thio-sn-glycerol/3-thio-sn-glycerol isomers by ORD and CD. The 1-thio-sn-glycerol isomers dominated for both compounds indicating that they were enriched during the absorption processes, since a racemic compound was fed. The stereospecificities are probably exerted by acyltransferase(s) during chyle lipid synthesis. The methods used will be valuable tools in studies on the metabolism of enantiomeric glycerides, and also for characterization of naturally occurring sulphur-containing lipids.     

An improved method for the preparation of 1,2-isopropylidene-SN-glycerol

A simple and fast route for the preparation of 1,2-isopropylidene-sn-glycerol from D-mannitol in 45% yield is described. The value of optical rotation, [α]_D²⁰ + 15.2°, is higher than usual indicating considerable racemization for other procedures. Since 1,2-isopropylidene-sn-glycerol serves as general intermediate for the synthesis of glycerides and of phosphoglycerides these lipids contain substantial amounts of the isomer, for instance 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine may consist of up to 15% of 2,3-dipalmitoyl-sn-glycerol-1-phosphocholine in earlier preparations.     

Barotropic and thermotropic bilayer phase behavior of positional isomers of unsaturated mixed-chain phosphatidylcholines

The bilayer phase transitions of six kinds of mixed-chain phosphatidylcholines (PCs) with an unsaturated acyl chain in the sn-1 or sn-2 position, 1-oleoyl-2-stearoyl- (OSPC), 1-stearoyl-2-oleoyl- (SOPC), 1-oleoyl-2-palmitoyl- (OPPC), 1-palmitoyl-2-oleoyl- (POPC), 1-oleoyl-2-myristoyl- (OMPC) and 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light transmittance measurements. Bilayer membranes of SOPC, POPC and MOPC with an unsaturated acyl chain in the sn-2 position exhibited only one phase transition, which was identified as the main transition between the lamellar gel (L_β) and liquid crystalline (L_α) phases. On the other hand, the bilayer membranes of OSPC, OPPC and OMPC with an unsaturated acyl chain in the sn-1 position exhibited not only the main transition but also a transition from the lamellar crystal (L_c) to the L_β (or L_α) phase. The stability of their gel phases was markedly affected by pressure and chain length of the saturated acyl chain in the sn-2 position. Considering the effective chain lengths of unsaturated mixed-chain PCs, the difference in the effective chain length between the sn-1 and sn-2 acyl chains was proven to be closely related to the temperature difference of the main transition. That is, a mismatch of the effective chain length promotes a temperature difference of the main transition between the positional isomers. Anomalously small volume changes of the L_c/L_α transition for the OPPC and OMPC bilayers were found despite their large enthalpy changes. This behavior is attributable to the existence of a cis double bond and to significant inequivalence between the sn-1 and sn-2 acyl chains, which brings about a small volume change for chain melting due to loose chain packing, corresponding to a large partial molar volume, even in the L_c phase. Further, the bilayer behavior of unsaturated mixed-chain PCs containing an unsaturated acyl chain in the sn-1 or sn-2 position was well explained by the chemical-potential diagram of a lipid in each phase.     

Oxidatively modified fatty acyl chain determines physicochemical properties of aggregates of oxidized phospholipids

In vivo oxidation of glycerophospholipid generates a variety of products including truncated oxidized phospholipids (tOx-PLs). The fatty acyl chains at the sn-2 position of tOx-PLs are shorter in length than the parent non-oxidized phospholipids and contain a polar functional group(s) at the end. The effect of oxidatively modified sn-2 fatty acyl chain on the physicochemical properties of tOx-PLs aggregates has not been addressed in detail, although there are few reports that modified fatty acyl chain primarily determines the biological activities of tOx-PLs. In this study we have compared the properties of four closely related tOx-PLs which differ only in the type of modified fatty acyl chain present at the sn-2 position: 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PazePC), 1-palmitoyl-2-(9′-oxo-nonanoyl)-sn-glycero-3-phosphocholine (PoxnoPC), 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC), and 1-palmitoyl-2-(5′-oxo-valeroyl)-sn-glycero-3-phosphocholine (POVPC). Aggregates of individual tOx-PL in aqueous solution were characterized by fluorescence spectroscopy, size exclusion chromatography, native polyacrylamide and agarose gel electrophoresis. The data suggest that aggregates of four closely related tOx-PLs form micelle-like particles of considerably different properties. Our result provides first direct evidence that because of the specific chemical composition of the sn-2 fatty acyl chain aggregates of particular tOx-PL possess a distinctive set of physicochemical properties.     

Glycerolipid biosynthesis in rat adipose tissue. Effect of polyamines on triglyceride synthesis.

Triacylglycerol formation from sn-glycerol 3-phosphate and 1,2-diacyl-sn-glycerol was markedly elevated in the presence of spermine and spermidine. This was attributed to the activation of microsomal sn-glycerol 3-phosphate acyltransferase and 1,2-diacyl-sn-glycerol acyltransferase and to the inhibition of palmitoyl-CoA hydrolase. Spermine was more effective than spermidine, and putrescine did not stimulate triacylglycerol formation. The stimulatory effect of spermine on triacylglycerol-forming enzymes was observed in the presence of Mg²⁺ and was apparent in the presence or absence of bovine serum albumin. The activation of 1,2-diacyl-sn-glycerol acyltransferase by spermine was specific, and other diacylglycerol-utilizing enzymes were not affected under these conditions. These studies demonstrate that polyamines may be important regulators of triacylglycerol formation in adipose tissue.     

Nanosized bilayer disks: Attractive model membranes for drug partition studies

Stable nanosized bilayer disks were prepared from either 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol, or lipid mixtures with a composition reflecting that of the porcine brush border membrane. Two different polyethylene glycol (PEG)-grafted lipids, the negatively charged 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy (polyethylene glycol)-5000] (DSPE-PEG₅₀₀₀) and the neutral N-palmitoyl-sphingosine-1-[succinyl (methoxy (polyethylene glycol) 5000] (Ceramide-PEG₅₀₀₀), were used to stabilize the disks. The disks were employed as model membranes in drug partition studies based on a fast chromatography method. Results show that the lipid composition, as well as the choice of PEG-lipid, have an important influence on the partition behavior of charged drugs. Comparative studies using multilamellar liposomes indicate that bilayer disks have the potential to generate more accurate partition data than do liposomes. Further, initial investigations using bacteriorhodopsin suggest that membrane proteins can be reconstituted into the bilayer disks. This fact further strengthens the potential of the bilayer disk as an attractive model membrane.     

Specific volumes of unsaturated phosphatidylcholines in the liquid crystalline lamellar phase

The specific volumes of seven 1,2-diacyl-sn-glycero-3-phosphocholines with symmetric, unbranched acyl chains containing one, four, or six cis double bonds per chain, or with a saturated sn-1 chain and one, four, or six cis double bonds in the sn-2 chain were determined by the neutral buoyancy method. Experiments were conducted in the liquid crystalline lamellar phase over the temperature range from 5 to 35 °C. It is demonstrated that the molecular volume of phosphatidylcholines can be well approximated as the sum of a constant volume of the polar lipid head region and the temperature-dependent volumes of hydrocarbon chain CH₂, CH, and terminal CH₃ groups. A linear dependence of chain segment volumes on temperature was observed. A self-consistent set of partially temperature-dependent volumes is obtained that allows prediction of phosphatidylcholine molecular volumes within very tight error margins.