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.     

Polyunsaturated fatty acyl residues of galactolipids are involved in the control of bilayer/non-bilayer lipid transitions in higher plant chloroplasts

Total polar lipid extracts of chloroplasts isolated from broad beans (Vicia faba) tend to form non-bilayer structures when dispersed in dilute salt solutions. Monoglactosyldiacylglycerol is shown to play a dominant role in this process. The tendency of this lipid to form non-bilayer structures when dispersed alone in water was found to depend upon the degree of unsaturation of its associated fatty acyl chains. Highly unsaturated lipids (average number of double bonds per lipid molecule greater than about 5.0) form inverted hexagonal (Hex_II) structures in water at 20°C, whilst more saturated lipids (average number of double bonds per lipid molecule less than about 4.5) form lamellar sheets. Wide-angle X-ray diffraction and differential scanning calorimetry measurements indicate that these lamellae consist of gel-phase lipid that can adopt either of two structures depending on the thermal history of the sample. Freeze-fracture studies performed on total polar lipid extracts that have been hydrogenated using Adams' catalyst, and reconstituted extracts in which monogalactosyldiacylglycerol has been selectively hydrogenated, show that the degree of unsaturation of this lipid is a key factor in determining whether or not non-bilayer structures are formed in such extracts. Increasing the extent of saturation of the acyl residues of monogalactosyldiacylglycerol reduces the tendency to form non-bilayer structures. Similar effects are observed on lowering the temperature of the dispersions. Fluorescence polarisation measurements using 1,6-diphenyl-1,3,5-hexatriene indicate that the disappearance of non-bilayer structures is accompanied by a marked decrease in the fluidity of the lipid matrix. The possible significance of these observations is discussed in terms of the thermal adaptation and chilling sensitivity of plant membranes.     

Phase properties of binary mixtures of monogalactosyldiacylglycerols differing in hydrocarbon chain substituents dispersed in aqueous systems

Monogalactosyldiacylglycerols isolated from spinach leaves contain a high proportion of polyunsaturated fatty acyl substituents and form hexagonal-II structures when dispersed in excess water. Catalytic hydrogenation of the lipid in the presence of Adam's catalyst completely saturates the hydrocarbon chains and the lipid forms typical open sheet bilayer structures in water at 20°C. Binary mixtures of the native and hydrogenated lipid tend to phase separate at 20°C. Freeze-fracture electron microscopy reveals lamellar phase lipid indispersed with regions of hexagonal-II structure and the proportions of each reflect the composition of the mixture. X-ray diffraction in both wide- and low-angle regions show that the saturated lipid forms the typical stable gel-phase structure in mixtures that are allowed to equilibrate over three days at 20°C. The phase transition behaviour of binary mixtures of the two galactolipids was investigated by differential scanning calorimetry and fluorescence probe methods. Thermal studies indicate that the phase-separated gel structure undergoes an anomalous transition compared with the saturated pure lipid in that the transition temperature is reduced from about 57°C to 41°C and the enthalpy of the transition is also somewhat reduced. Furthermore, the transition appears to involve the conversion of the completely phase-separated system into bilayer coexisting with phases intermediate between bilayer and hexagonal-II. A homogeneous hexagonal-II phase is presumably formed at higher temperatures. The thermal and structural studies were consistent with fluorescence polarization measurements of 1,6-diphenyl-1,3,5-hexatriene interpolated into the hydrocarbon domain of the structure.     

Synthesis and physical properties of phosphatidylcholine labelled with 9-(2-anthryl)nonanoic acid,a new fluorescent probe

Synthesis and physical properties of a new anthracene fatty acid, 9-(2-anthryl)nonanoic acid, and the corresponding anthracene-phosphatidylcholines which were obtained by condensing the acid with sn-1-palmitoyl-lysophosphatidylcholine (PAPC) and with egg lysophosphatidylcholine (EAPC) are described. Differential scanning calorimetry experiments show that these lipids can undergo a liquid-crystal to gel phase transition at temperatures of 15°C and 18°C for EAPC and PAPC, respectively. In monolayers, PAPC exhibits a compression curve nearly superimposable to that of dipalmitoylphosphatidylcholine (DPPC), with a molecular area of 0.48 nm² at π = 30 mN m^?1. The data indicate that in these lipids, the anthracene group is only slightly more bulky than a normal acyl chain and that it does not significantly affect the regular phospholipid molecular packing. In ethanol solutions or when incorporated into egg phosphatidylcholine liposomes in a molar ratio of 1%, these lipids display UV absorption spectra and fluorescence emission spectra similar to those of 2-methyl anthracene. For EAPC liposomes, a broad and structureless fluorescence emission spectrum centered at around 450 nm, was recorded, suggesting the occurrence of anthracene excimers. As ascertained by UV spectrophotometry, differential scanning calorimetry, fluorescence polarization and anthracene photodimerization experiments, EAPC displays good miscibility properties with lipids in the liquid state (egg phosphatidylcholine) or in the gel state (distearoylphosphatidylcholine (DSPC)). The potential of these anthracene derivatives for studying the dynamics and the topological distribution of lipids in biomembranes is discussed.     

Calorimetric and molecular mechanics studies of the thermotropic phase behavior of membrane phospholipids

In this review, we summarize the results of recent studies on the main phase transition behavior of phospholipid bilayers using the combined approaches of molecular mechanics simulations and high-resolution differential scanning calorimetry. Following a brief overview of the phase transition phenomenon exhibited by the lipid bilayer, we begin with the review by showing how several structural parameters underlying various phospholipids including phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol are defined and determined. Specifically, these structural parameters are obtained with saturated lipids packed in the gel-state bilayer using computer-based molecular mechanics calculations. Then we proceed to present the calorimetric data obtained with the lipid bilayer composed of saturated phospholipids as it undergoes the gel-to-liquid-crystalline phase transition in excess water. The general equations that can correlate the gel-to-liquid-crystalline phase transition temperature (T_m) of the lipid bilayer with the structural parameters of the lipid molecule constituting the lipid bilayer are subsequently presented. From these equations, two tables of predicated T_m values for well over 400 molecular species of saturated phosphatidylcholine and saturated phosphatidylethanolamine are generated. We further review the structure and chain-melting behavior of a large number of sn-1 saturated/sn-2 unsaturated phospholipids. Two T_m-diagrams are shown, from which the effects of the number and the position of one to five cis carbon–carbon double bonds on T_m can be viewed simultaneously. Finally, in the last part of this review, simple molecular models that have been invoked to interpret the characteristic T_m trends exhibited by lipid bilayers composed of unsaturated lipids with different numbers and positions of cis carbon–carbon double bonds as seen in the T_m-diagram are presented.     

Miscibility of Sphingomyelins and Phosphatidylcholines in Unsaturated Phosphatidylcholine Bilayers

Polyunsaturated phospholipids are common in biological membranes and affect the lateral structure of bilayers. We have examined how saturated sphingomyelin (SM; palmitoyl and stearoyl SM (PSM and SSM, respectively)) and phosphatidylcholine (PC; dipalmitoyl PC and 1-palmitoyl-2-stearoyl PC (DPPC and PSPC, respectively)) segregate laterally to form ordered gel phases in increasingly unsaturated PC bilayers (sn-1: 16:0 and sn-2: 18:1...22:6; or sn-1 and sn-2: 18:1…22:6). The formation of gel phases was determined from the lifetime analysis of trans-parinaric acid. Using calorimetry, we also determined gel phase formation by PSM and DPPC in unsaturated PC mixed bilayers. Comparing PSM with DPPC, we observed that PSM formed a gel phase with less order than DPPC at comparable bilayer concentrations. The same was true when SSM was compared with PSPC. Furthermore, we observed that at equal saturated phospholipid concentration, the gel phases formed were less ordered in unsaturated PCs having 16:0 in sn-1, as compared to PCs having unsaturated acyl chains in both sn-1 and sn-2. The gel phases formed by the saturated phospholipids in unsaturated PC bilayers did not appear to achieve properties similar to pure saturated phospholipid bilayers, suggesting that complete lateral phase separation did not occur. Based on scanning calorimetry analysis, the melting of the gel phases formed by PSM and DPPC in unsaturated PC mixed bilayers (at 45 mol % saturated phospholipid) had low cooperativity and hence most likely were of mixed composition, in good agreement with trans-parinaric acid lifetime data. We conclude that both interfacial properties of the saturated phospholipids and their chain length, as well as the presence of 16:0 in sn-1 of the unsaturated PCs and the total number of cis unsaturations and acyl chain length (18 to 22) of the unsaturated PCs, all affected the formation of gel phases enriched in saturated phospholipids, under the conditions used.     

Phase behaviour of the membrane lipids of the thermophilic blue-green alga Anacystis nidulans

The phase behaviour of total membrane lipid extracts of the blue-green alga Anacystis nidulans is compared with that of the individual lipid classes present in such extracts using fluorescence probe, differential scanning calorimetry, wide-angle X-ray diffraction and freeze-fracture techniques. Marked differences are observed in the properties of the isolated lipids as compared to the total lipid extracts. In particular, purified samples of monogalactosyldiacylglycerol and phosphatidylglycerol form complex high melting-point gel phases on storage which are not found in the membrane extracts. Addition of Mg²⁺ ions to the extracts is also shown to lead to an extensive phase separation of monogalactosyldiacylglycerol from the extracts. The enthalpy changes associated with phase separations occurring in the lipid extracts are found to be approx. 30% higher than those for the corresponding membranes, suggesting that the presence of other components, such as membrane proteins, may influence the phase behaviour of the lipids. The significance of these observations is discussed in terms of the factors limiting the stability of membrane systems.     

Phase properties of the aqueous dispersions of n-octadecylphosphocholine

Properties of the aqueous dispersions of n-octadecylphosphocholine are examined by differential scanning calorimetry, fluorescence depolarization, light scattering, ³¹P-NMR, pig pancreatic phospholipase A₂ binding, and X-ray diffraction. On heating, these dispersions exhibit a sharp lamellar to micelle transition at 20.5°C. The lamellar phase consists of frozen (gel-state) alkyl chains which do not bind phospholipase A₂. The kinetics of the transition are asymmetric: the micelle to lamellar transition is very slow and the lamellar to micelle transition is fast. It is suggested that the lamellar phase is a frozen chain bilayer in which the chains interdigitate.     

Phase behavior of synthetic N-acylethanolamine phospholipids

Both saturated and unsaturated N-acylethanolamine phospholipids form lamellar structures when dispersed in buffer. The addition of excess Ca2+ (Ca2+/N-acylphosphatidylethanolamine greater than 0.5) results in precipitation. Freeze-fracture replicas indicate that the addition of Ca2+ to the unsaturated lipid results in a non-bilayer structure while the Ca2+-complex of the saturated lipid is lamellar. Since unsaturated phosphatidylethanolamine (PE) is a non-bilayer lipid, its N-acylation with a saturated fatty acid converts a non-bilayer lipid into an acidic bilayer lipid capable of interacting with Ca2+ to return to a non-bilayer structure. Ca2+ may thereby exert an influence on membrane phenomena by regulating phase behavior within certain membrane domains. Differential scanning calorimetry (DSC) indicates that N-acylation of unsaturated PE with a saturated fatty acid also results in changes in thermotropic phase behavior. Therefore, N-acylation may affect fluidity within certain membrane domains.     

Structure of molecular aggregates of 1-(3-sn-phosphatidyl)-l-myo-inositol 3,4-bis(phosphate) in water

The molecular organization of 1-(3-sn-phosphatidyl)-l-myo-inositol 3,4-bis-(phosphate)/water systems is investigated over a wide range of lipid concentrations using X-ray diffraction, calorimetry, analytical ultracentrifugation, densitometry and viscometry.At high lipid concentrations, the lipid molecules are found to form a lamellar phase. The repeat distance increases from 60 to 120 Å with increasing water content to 70 wt% and the surface area per lipid molecule increases from 41.7 Ų to a limiting value of 100 Ų.On the other hand, at very low lipid concentrations the molecules are found to form not vesicles but micelles, the total molecular weight of which takes a value of 93 000.This finding revises the prevalent view that lipids containing two (or more) hydrocarbon chains form extended bilayers or vesicles, whereas single chained lipids form micelles. (Tanford, C.(1972) J. Phys. Chem. 76, 3020–3024).     

Phase behavior of ethanolamine plasmalogen

In the present study the phase behavior of multilamellar dispersions of 1-O-(1′-alkenyl)-2-oleoyl-glycerophosphoethanolamine (ethanolamine plasmalogen), 1-O-alkyl-2-oleoyl-glycerophosphoethanolamine and 1-acyl-2-oleoyl-glycerophosphoethanolamine was compared using differential scanning calorimetry (DSC) and ³¹P-NMR. The three compounds differed only in the type of bonding (vinyl ether, alkyl ether or acyl ester) linking the aliphatic moiety to position 1 of sn-glycerol.The gel to liquid-crystalline phase transition temperature as determined by DSC was lowest for ethanolamine plasmalogen (26°C) and was similar for the alkylacyl and diacyl analogs (29.5° and 30°C, respectively). Enthalpies of the G → L phase transition were not significantly different for the three phospholipids tested.Ethanolamine plasmalogen undergoes the lamellar to hexagonal phase transition at 30°C, the analogous alkylacyl-glycerophosphoethanolamine(alkylacyl-GPE) and diacyl-GPE at 53°C and 69°C, respectively. Thus, an alkenyl ether bond in position 1 of sn-glycerol, the structural characteristic of plasmalogens, effectively stabilizes the hexagonal H_II arrangement of ethanolamine glycerophospholipids, while it has relatively little effect on destabilization of the lamellar gel state.     

Metabolism of Unsaturated Monogalactosyldiacylglycerol Molecular Species in Arabidopsis thaliana Reveals Different Sites and Substrates for Linolenic Acid Synthesis

Synthesis of unsaturated monogalactosyldiacylglycerol (MGDG) was examined in a mutant of Arabidopsis thaliana (L.) Heynh. containing reduced levels of hexadecatrienoic (16:3) and linolenic (18:3) acids in leaf lipids. Molecular species composition and labeling kinetics following the incorporation of exogenous [¹⁴C]fatty acids suggest that at least two pathways and multiple substrates are involved in desaturation of linoleic acid (18:2) to 18:3 for production of unsaturated galactolipids. A reduction in 18:3/16:3 MGDG and an increase in 18:2/16:2 MGDG, together with labeling kinetics of these molecular species following the incorporation of exogenous [¹⁴C]12:0 fatty acids, suggests that a chloroplastic pathway for production of 18:3 at the sn-1 position of MGDG utilizes 18:2/16:2 MGDG as a substrate. This chloroplastic (prokaryotic) pathway is deficient in the mutant. When exogenous [¹⁴C]18:1 was supplied, a eukaryotic (cytoplasmic) pathway involving the desaturation of 18:2 to 18:3 on phosphatidylcholine serves as the source of 18:3 for the sn-2 position of MGDG. This eucaryotic pathway predominates in the mutant.     

Phase equilibria of mixtures of plant galactolipids. The formation of a bicontinuous cubic phase

The chloroplast galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) were isolated from wheat leaves. The phase equilibria of galactolipid-water systems with MGDG / DGDG molar ratios equal to 0:1, 1:2, 1.2:1, 2:1 and 1:0 were investigated, using nuclear magnetic resonance (NMR) methods. MGDG and DGDG form reversed hexagonal and lamellar phases, respectively, at temperatures between 10 and 40°C at all water contents studied (up to about 14 mol ²H₂O per mol lipid). The galactolipid mixtures show a complex phase forming reversed hexagonal, lamellar and reversed cubic phases, depending on water content and temperature. It was found that the water hydration is similar for the lamellar and hexagonal phases formed by DGDG and MGDG, respectively. The non-lamellar phase areas increase with increasing content of MGDG. Small-angle X-ray measurements show that the cubic phase belongs to the Ia3d space group. From translational diffusion studies by NMR it is concluded that the structure of this cubic phase is bicontinuous.     

Differential scanning calorimetry on mixtures of lecithin,lysolecithin and cholesterol

The effect of increasing concentrations of lysolecithin (1-palmitoyl-sn-glycerol-3-phosphorylcholine) on the gel → liquid crystal thermal transition of lecithin (1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine) in the aqueous phase was studied by differential scanning calorimetry (DSC). Lysolecithin showed an endothermic transition at 3.4°C whereas the transition of the lecithin occurred at 42°C. No phase separation could be observed calorimetrically at lysolecithin concentrations up to 60 mol%. Freeze etch electron microscopy showed that mixtures containing as much as 50 mol% lysolecithin exist in a lamellar phase. The lysolecithin was found to cause an initial slight increase in the enthalpy of transition followed by a gradual decrease. The enthalpy increased again at very high lysolecithin concentrations. The lysolecithin also caused a non-linear decrease in the temperature at which the lecithin transition took place.Cholesterol was found to decrease the enthalpy of transition of the lysolecithin, eliminating it at a concentration of 50 mol%. Cholesterol caused an increase in the temperature at which the lysolecithin transition took place.     

Diffusion and Partitioning of Fluorescent Lipid Probes in Phospholipid Monolayers

The pressure-dependent diffusion and partitioning of single lipid fluorophores in DMPC and DPPC monolayers were investigated with the use of a custom-made monolayer trough mounted on a combined fluorescence correlation spectroscopy (FCS) and wide-field microscopy setup. It is shown that lipid diffusion, which is essential for the function of biological membranes, is heavily influenced by the lateral pressure and phase of the lipid structure. Both of these may change dynamically during, e.g., protein adsorption and desorption processes. Using FCS, we measured lipid diffusion coefficients over a wide range of lateral pressures in DMPC monolayers and fitted them to a free-area model as well as the direct experimental observable mean molecular area. FCS measurements on DPPC monolayers were also performed below the onset of the phase transition (Π < 5 mN/m). At higher pressures, FCS was not applicable for measuring diffusion coefficients in DPPC monolayers. Single-molecule fluorescence microscopy and differential scanning calorimetry clearly showed that this was due to heterogeneous partitioning of the lipid fluorophores in condensed phases. The results were compared with dye partitioning in giant lipid vesicles. These findings are significant in relation to the application of lipid fluorophores to study diffusion in both model systems and biological systems.     

Effect of free fatty acids on the permeability of 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayer at the main phase transition

We measured the influence of saturated and unsaturated free fatty acids on the permeability and partition of ions into 1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayers. The bilayer permeability was measured using the depletion of N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1, 2-dihexadecanoyl-sn-glycero-3-phosphatidylethanolamine (N-NBD-PE) fluorescence as a result of its reduction by dithionite. We observed a distinct increase of dithionite permeability at the main gel-fluid phase transition of DMPC. When vesicles were formed from a mixture of DMPC and oleic acid, the membrane permeability at the phase transition was reduced drastically. Stearic acid and methyl ester of oleic acid have little effect. Similar results in the quenching of pyrene-PC in DMPC vesicles by iodide were obtained. Again, the increase of iodide partition into the lipid phase at the main phase transition of DMPC was abolished by the addition of unsaturated free fatty acids. Free fatty acids, in concentrations up to 5 mol%, do not abolish DMPC phase transition when measured by differential scanning calorimetry. It seems that unsaturated, but not saturated, free fatty acids reduce the lipid bilayer permeability to dithionite and iodide ions at the main phase transition of DMPC, without altering the thermodynamic properties of the bilayer.     

Lamellar Gel (Lβ) Phases of Ternary Lipid Composition Containing Ceramide and Cholesterol

Lipid lateral segregation into specific domains in cellular membranes is associated with cell signaling and metabolic regulation. This phenomenon partially arises as a consequence of the very distinct bilayer-associated lipid physico-chemical properties that give rise to defined phase states at a given temperature. Until now lamellar gel (L_β) phases have been described in detail in single or two-lipid systems. Using x-ray scattering, differential scanning calorimetry, confocal fluorescence microscopy, and atomic force microscopy, we have characterized phases of ternary lipid compositions in the presence of saturated phospholipids, cholesterol, and palmitoyl ceramide mixtures. These phases stabilized by direct cholesterol-ceramide interaction can exist either with palmitoyl sphingomyelin or with dipalmitoyl phosphatidylcholine and present intermediate properties between raft-associated phospholipid-cholesterol liquid-ordered and phospholipid-ceramide L_β phases. The present data provide novel, to our knowledge, evidence of a chemically defined, multicomponent lipid system that could cooperate in building heterogeneous segregated platforms in cell membranes.     

An estimate of the minimum amount of fluid lipid required for the growth of escherichia coli

The lipid phase transition of Escherichia coli was studied by high sensitivity differential scanning calorimetry. A temperature sensitive unsaturated fatty acid auxotroph was used to obtain lipids with subnormal unsaturated fatty acid contents. From these studies it was concluded that E. coli can grow normally with as much as 20% of its membrane lipids in the ordered state but that if more than 55% of the lipids are ordered, growth ceases. Studies with wild-type cells show that the phase transition ends more than 10°C below the growth temperature when the growth temperature when the growth temperature is either 25°C or 37°C.     

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.     

The effect of cadmium on lipid and fatty acid biosynthesis in tomato leaves

This research aims to examine the effect of cadmium uptake on lipid composition and fatty acid biosynthesis, in young leaves of tomato treated seedlings (Lycopersicon esculentum cv. Ibiza F1). Results in membrane lipids investigations revealed that high cadmium concentrations affect the main lipid classes, leading to strong changes in their composition and fatty acid content. Thus, the exposure of tomato plants to cadmium caused a concentration-related decrease in the unsaturated fatty acid content, resulting in a lower degree of fatty acid unsaturation. The level of lipid peroxides was significantly enhanced at high Cd concentrations. Studies of the lipid metabolism using radioactive labelling with [1-¹⁴C]acetate as a major precursor of lipid biosynthesis, showed that levels of radioactivity incorporation in total lipids as well as in all lipid classes were lowered by Cd doses. In total lipid fatty acids, [1-¹⁴C]acetate incorporation was reduced in tri-unsaturated fatty acids (C16:3 and C18:3); While it was enhanced in the palmitic (C16:0), palmitoleic (C16:1), stearic (C18:0) and linoleic (C18:2) acids. [1-¹⁴C]acetate incorporation into C16:3 and C18:3 of galactolipids [monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG)] and some phospholipids [phosphatidylcholine (PC) and phosphatidylglycerol (PG)] was inhibited by Cd stress. Our results showed that in tomato plants, cadmium stress provoked an inhibition of polar lipid biosynthesis and reduced fatty acid desaturation process.