Degree Of Acyl Chain Unsaturation Research Articles

Differential interactions of essential and toxic metal ions with biologically relevant phosphatidic acid and phosphatidylserine membranes.

Metal pollutants are a growing concern due to increased use in mining and other industrial processes. Moreover, the use of metals in daily life is becoming increasingly prevalent. Metals such as manganese (Mn), cobalt (Co), and nickel (Ni) are toxic in high amounts whereas lead (Pb) and cadmium (Cd) are acutely toxic at low µM concentrations. These metals are associated with system dysfunction in humans including cancer, neurodegenerative diseases, Alzheimer's disease, Parkinson's disease, and other cellular process'. One known but lesser studied target of these metals are lipids that are key membrane building blocks or serve signalling functions. It was shown that Mn, Co, Ni, Pb, and Cd cause rigidification of liposomes and increase the phase transition in membranes composed of both saturated or partly unsaturated phosphatidic acid (PA) and phosphatidylserine (PS). The selected metals showed differential effects that were more pronounced on saturated lipids. In addition, more rigidity was induced in the biologically relevant liquid-crystalline phase. Moreover, metal affinity, induced rigidification and liposome size increases also varied with the headgroup architecture, whereby the carboxyl group of PS appeared to play an important role. Thus, it can be inferred that Mn, Co, Ni, Cd, and Pb may have preferred binding coordination with the lipid headgroup, degree of acyl chain unsaturation, and membrane phase.

Membrane Curvature Revisited—the Archetype of Rhodopsin Studied by Time-Resolved Electronic Spectroscopy

G-protein-coupled receptors (GPCRs) comprise the largest and most pharmacologically targeted membrane protein family. Here, we used the visual receptor rhodopsin as an archetype for understanding membrane lipid influences on conformational changes involved in GPCR activation. Visual rhodopsin was recombined with lipids varying in their degree of acyl chain unsaturation and polar headgroup size using 1-palmitoyl-2-oleoyl-sn-glycero- and 1,2-dioleoyl-sn-glycerophospholipids with phosphocholine (PC) or phosphoethanolamine (PE) substituents. The receptor activation profile after light excitation was measured using time-resolved ultraviolet-visible spectroscopy. We discovered that more saturated POPC lipids back shifted the equilibrium to the inactive state, whereas the small-headgroup, highly unsaturated DOPE lipids favored the active state. Increasing unsaturation and decreasing headgroup size have similar effects that combine to yield control of rhodopsin activation, and necessitate factors beyond proteolipid solvation energy and bilayer surface electrostatics. Hence, we consider a balance of curvature free energy with hydrophobic matching and demonstrate how our data support a flexible surface model (FSM) for the coupling between proteins and lipids. The FSM is based on the Helfrich formulation of membrane bending energy as we previously first applied to lipid-protein interactions. Membrane elasticity and curvature strain are induced by lateral pressure imbalances between the constituent lipids and drive key physiological processes at the membrane level. Spontaneous negative monolayer curvature toward water is mediated by unsaturated, small-headgroup lipids and couples directly to GPCR activation upon light absorption by rhodopsin. For the first time to our knowledge, we demonstrate this modulation in both the equilibrium and pre-equilibrium evolving states using a time-resolved approach.

Dependence of Cholesterol-Phospholipid Affinity on Degree of Acyl Chain Unsaturation as Determined by EPR

Cholesterol-lipid interactions are thought to play an intrinsic role in determining lateral organization within cellular membranes. Cholesterol's tendency to restrict acyl chain motion makes interaction with highly disordered polyunsaturated phospholipid less energetically favorable compared with its interactions with more saturated species. The high affinity that the sterol has for saturated chains is the “glue” that holds cholesterol and sphingomyelin rich lipid rafts together. Conversely, poor affinity for cholesterol is hypothesized to drive the formation of polyunsaturated fatty acid (PUFA)-containing phospholipid domains depleted in the sterol. Here we describe a novel method using electron paramagnetic resonance (EPR) to measure the affinity of cholesterol for phospholipids with varying degrees of acyl chain unsaturation. We monitor the partitioning of the spin labeled cholesterol analog, cholestane, between large unilamellar vesicles (LUV) and cyclodextrin (cyd) through analysis of EPR spectra. Because the EPR spectrum of cholestane is sensitive to the very different tumbling rates in the two environments, the ratio of the two populations can be determined by deconvolution of the spectra. Our data show that a decreased affinity of cholesterol accompanies increasing acyl chain unsaturation. The virtue of this EPR method is that it provides a qualitative measure of cholesterol binding that is quick and avoids the necessity for physical separation of LUV and cyd.

Cholesterol in unusual places

Cholesterol is an essential component of mammalian cells, and is required for building and maintaining cell membranes, regulating their fluidity, and possibly acting as an antioxidant. Cholesterol has also been implicated in cell signaling processes, where it has been suggested that it triggers the formation of lipid rafts in the plasma membrane. Aside from cholesterol's physiological roles, what is also becoming clear is its poor affinity for lipids with unsaturated fatty acids as opposed to saturated lipids, such as sphingomyelin with which it forms rafts. We previously reported the location of cholesterol in membranes with varying degrees of acyl chain unsaturation as determined by neutron diffraction studies (Harroun et al 2006 Biochemistry 45, 1227; Harroun et al 2008 Biochemistry 47, 7090). In bilayers composed of phosphatidylcholine (PC) molecules with a saturated acyl chain at the sn-1 position or a monounsaturated acyl chain at both sn-1 and sn-2 positions, cholesterol was found in its much-accepted "upright" position. However, in dipolyunsaturated 1,2-diarachidonyl phosphatidylcholine (20:4-20:4PC) membranes the molecule was found sequestered in the center of the bilayers. In further experiments, mixing l-palmitoyl-2-oleoyl phosphatidylcholine (16:0-18:1 PC) with 20:4-20:4PC resulted in cholesterol reverting to its upright orientation at approximately 40 mol% 16:0-18:1 PC. Interestingly, the same effect was achieved with only 5 mol% 1,2-dimyristoyl phosphatidylchoile (14:0-14:0PC).

Quantitative analysis of phosphatidylcholine in rat liver tissue by nanoflow liquid chromatography/tandem mass spectrometry

A quantitative method was developed for the determination of phosphatidylcholine (PC) species concentration using nanoflow LC-ESI-MS/MS. In this study, a calibration method is developed to determine the effect of PC carbon chain length on MS peak intensity. Using the multiple standard addition method, a relationship between the peak intensities of different PC species from nanoflow LC-MS and carbon chain length is established first using different injection amounts of PC standards. From this relationship, a calibration curve for each carbon chain length can be obtained for the concentration calculation. It was found that the MS peak area of PC species analyzed by nanoflow LC-MS linearly decreased with increased acyl carbon numbers, and that the effect of the degree of acyl chain unsaturation on MS peak intensity was minimized when the injection amount was maintained at less than 1 pmol. The method was applied for the quantitative calculation of 34 PC species from rat liver, which were identified from data-dependent MS/MS analysis during nanoflow LC separation.

Quantifying the effects of melittin on liposomes

Melittin, the soluble peptide of bee venom, has been demonstrated to induce lysis of phospholipid liposomes. We have investigated the dependence of the lytic activity of melittin on lipid composition. The lysis of liposomes, measured by following their mass and dimensions when immobilised on a solid substrate, was close to zero when the negatively charged lipids phosphatidyl glycerol or phosphatidyl serine were used as the phospholipid component of the liposome. Whilst there was significant binding of melittin to the liposomes, there was little net change in their diameter with melittin binding reversed upon salt injection. For the zwitterionic phosphatidyl choline the lytic ability of melittin is dependent on the degree of acyl chain unsaturation, with melittin able to induce lysis of liposomes in the liquid crystalline state, whilst those in the gel state showed strong resistance to lysis. By directly measuring the dimensions and mass changes of liposomes on exposure to melittin using Dual Polarisation Interferometry, rather than following the florescence of entrapped dyes we attained further information about the initial stages of melittin binding to liposomes.

Molecular Organization of Cholesterol in Unsaturated Phosphatidylethanolamines: X-ray Diffraction and Solid State2H NMR Reveal Differences with Phosphatidylcholines

The major mammalian plasma membrane lipids are phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and cholesterol. Whereas PC-cholesterol interactions are well studied, far less is known about those between PE and cholesterol. Here, we investigated the molecular organization of cholesterol in PEs that vary in their degree of acyl chain unsaturation. For heteroacid sn-1 saturated (palmitoyl), sn-2 unsaturated (various acyl chain) PEs, cholesterol solubility determined by X-ray diffraction was essentially identical with 1 (oleoyl, 51 +/- 3 mol %) and 2 (linoleoyl, 49 +/- 2 mol %) double bonds before decreasing progressively with 4 (arachidonyl, 41 +/- 3 mol %) and 6 (docosahexaenoyl, 31 +/- 3 mol %) double bonds. With 6 double bonds in each chain, cholesterol solubility was further reduced to 8.5 +/- 1 mol %. However, (2)H NMR experiments established that the orientation of cholesterol in the same heteroacid PE membranes was unaffected by the degree of acyl chain unsaturation. A tilt angle of 15 +/- 1 degrees was measured when equimolar [3alpha-(2)H(1)]cholesterol was added, regardless of the number of double bonds in the sn-2 chain. The finding that solubility of cholesterol in sn-1 saturated PEs depends on the amount of polyunsaturation in the sn-2 chain of PE differs from the equivalent PCs that universally incorporate approximately 50 mol % sterol. Unlike PCs, a differential in affinity for cholesterol and tendency to drive lateral segregation is inferred between polyunsaturated PEs. This distinction may have biological implications reflected by the health benefits of dietary polyunsaturated fatty acids that are often taken up into PE > PC.

ESI-MS quantitation of increased sphingomyelin in Niemann-Pick disease type B HDL

HDLs have been proposed to have antiatherogenic properties because of their role in reverse cholesterol transport as lipid acceptors. To elucidate the phospholipid profile of these particles, we used electrospray ionization mass spectrometry to examine the phosphatidylcholine (PC) and sphingomyelin (SM) composition of HDLs purified from plasma and nascently generated in vitro from fibroblasts. We also quantitatively compared the phospholipids present in these lipoproteins between normal and Niemann-Pick disease type B (NPD-B) subjects characterized by sphingomyelinase (SMase) deficiency. We demonstrated that plasma HDLs from NPD-B were significantly enriched in SM by an average of 28%, particularly the palmitoyl SM (with an increase of 95%), which accounted for approximately 25-44% of total SM molecular species. Similarly, we observed an increase of approximately 63% in total SM levels in nascent HDLs prepared from NPD-B fibroblasts. Although PC levels in nascent HDLs were comparable between control and NPD-B cells, there was a 95% increase in total PC levels similar to that of SM in plasma HDLs extracted from NPD-B subjects. These data provide insight into the structure of HDLs and identify potential new roles for SMase in lipoprotein metabolism.

Effects of Phospholipid Composition on MinD-Membrane Interactions in Vitro and in Vivo

The peripheral membrane ATPase MinD is a component of the Min system responsible for correct placement of the division site in Escherichia coli cells. By rapidly migrating from one cell pole to the other, MinD helps to block unwanted septation events at the poles. MinD is an amphitropic protein that is localized to the membrane in its ATP-bound form. A C-terminal domain essential for membrane localization is predicted to be an amphipathic alpha-helix with hydrophobic residues interacting with lipid acyl chains and cationic residues on the opposite face of the helix interacting with the head groups of anionic phospholipids (Szeto, T. H., Rowland, S. L., Rothfield, L. I., and King, G. F. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 15693-15698). To investigate whether E. coli MinD displays a preference for anionic phospholipids, we first examined the localization dynamics of a green fluorescent protein-tagged derivative of MinD expressed in a mutant of E. coli that lacks phosphatidylethanolamine. In these cells, which contain only anionic phospholipids (phosphatidylglycerol and cardiolipin), green fluorescent protein-MinD assembled into dynamic focal clusters instead of the broad zones typical of cells with normal phospholipid content. In experiments with liposomes composed of only zwitterionic, only anionic, or a mixture of anionic and zwitterionic phospholipids, purified MinD bound to these liposomes in the presence of ATP with positive cooperativity with respect to the protein concentration and exhibited Hill coefficients of about 2. Oligomerization of MinD on the liposome surface also was detected by fluorescence resonance energy transfer between MinD molecules labeled with different fluorescent probes. The affinity of MinD-ATP for anionic liposomes as well as liposomes composed of both anionic and zwitterionic phospholipids increased 9- and 2-fold, respectively, relative to zwitterionic liposomes. The degree of acyl chain unsaturation contributed positively to binding strength. These results suggest that MinD has a preference for anionic phospholipids and that MinD oscillation behavior, and therefore cell division site selection, may be regulated by membrane phospholipid composition.

Quantitative determination of phospholipid compositions by ESI-MS: effects of acyl chain length, unsaturation, and lipid concentration on instrument response

Electrospray ionization-mass spectrometry (ESI-MS) is a very promising tool for the analysis of phospholipid compositions, but is hampered by the fact that not all molecular species are detected with equal efficiency. We studied this and other issues that need to be taken into account to obtain truly quantitative compositional data. The key findings were as follows: First, the instrument response for both saturated and unsaturated phospholipid species decreased with increasing acyl chain length. This effect became increasingly prominent with increasing overall lipid concentration. Second, the degree of acyl chain unsaturation also had a significant effect on instrument response. At the highest concentration studied (10 pmol/microl), polyunsaturated species gave 40% higher intensity than the fully saturated ones. The effect of unsaturation diminished and nearly disappeared with progressive dilution. Third, the instrument response for the different head group classes varied markedly depending on the infusion solvent used. Notably, inclusion of ammonia in the infusion solvent eliminated sodium adduct formation in the positive ion mode, thus greatly simplifying the interpretation of the spectra. The fact that instrument response is dependent on many structural features, overall lipid concentration, solvent composition, and instrument settings makes it necessary to include several internal standards for each phospholipid class to obtain accurate data. Preferably, both unsaturated and saturated standards should be used. Finally, we quantified the major phospholipid classes of BHK cells using ESI-MS. The data agreed closely with those obtained with thin-layer chromatography and phosphorus analysis. This study indicates that quantitative compositional data can be obtained with ESI-MS, provided that proper attention is paid to experimental details, particularly the choice of internal standards.

Total Lipids with Short and Long Acyl Chains from Acholeplasma Form Nonlamellar Phases

The cell-wall-less bacterium Acholeplasma laidlawii A-EF22 synthesizes eight glycerolipids. Some of them form lamellar phases, whereas others are able to form normal or reversed nonlamellar phases. In this study we examined the phase properties of total lipid extracts with limiting average acyl chain lengths of 15 and 19 carbon atoms. The temperature at which these extracts formed reversed hexagonal (H II) phases differed by 5–10°C when the water contents were 20–30 wt%. Thus the cells adjust the ratio between lamellar-forming and nonlamellar-forming lipids to the acyl chain lengths. Because short acyl chains generally increase the potential of lipids to form bilayers, it was judged interesting to determine which of the A. laidlawii A lipids are able to form reversed nonlamellar phases with short acyl chains. The two candidates with this ability are monoacyldiglucosyldiacylglycerol (MADGlcDAG) and monoglucosyldiacylglycerol. The average acyl chain lengths were 14.7 and 15.1 carbon atoms, and the degrees of acyl chain unsaturation were 32 and 46 mol%, respectively. The only liquid crystalline phase formed by MADGlcDAG is an H II phase. Monoglucosyldiacylglycerol forms reversed cubic ( Ia3d) and H II phases at high temperatures. Thus, even when the organism is grown with short fatty acids, it synthesizes two lipids that have the capacity to maintain the nonlamellar tendency of the lipid bilayer. MADGlcDAG in particular contributes very powerfully to this tendency.

Lipid structure of cytotoxic granules in living human killer T lymphocytes studied by Raman microspectroscopy

The structures of cytotoxic granules in interleukin-2-activated human killer T lymphocytes have been investigated by Raman microspectroscopy at a single cell level. The Raman spectra of granules share a common feature that lipid Raman bands are much stronger than the Raman bands due to protein, indicating that one of the main components of the granule is lipid. To analyze the lipid structures of individual granules, relationships between Raman spectra and structures have been examined for a series of triacylgycerols with varied degrees of acyl chain unsaturation. Analysis based on the relationships shows that the granulous lipid is characterized by a high content of cis CC bond, which ranges from about 1.5 CC bonds per acyl chain in isolated minor granules and to about 2.2 CC bonds in clustering major granules. The highly unsaturated lipid of major cytotoxic granules is in sharp contrast to the moderately unsaturated (about one CC bond per acyl chain) plasma membrane lipid. The large difference in lipid unsaturation between the granule and plasma membrane may have relevance to the role of granulous lipid in packaging cytotoxic proteins inside the granule and preventing them from attacking the killer lymphocyte itself.

The effect of lipid molecular packing stress on cationic liposome-induced rabbit erythrocyte fusion

The effect of curvature stress on the efficiency of cationic liposome-induced fusion between rabbit erythrocytes was studied. Multilamellar cationic liposomes containing 1,2-dioleoyl-3-trimethylammoniumpropane (DOTAP) and different PEs (1,2-dilinoleoyl- sn-glycero-3-phosphoethanolamine (dilin-PE), 1,2-dioleoyl- sn-glycero-3-phosphoethanolamine (DOPE), 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphoethanolamine (POPE), and lysophosphatidylethanolamine, egg (lyso-PE)) were used to induce cell–cell fusion. It was found that high cell–cell fusion yield (FY) of about 50% could be achieved in sucrose solution by using cationic liposomes containing 50% DOTAP. Cell–cell fusion was assayed by shape criterion and was verified by fluorescence microscopy, using a membrane dye mixing method. The curvature stress, as a result of mixing unsaturated PEs in cationic liposomes, had a significant effect on cell–cell FY which increased with the degree of acyl chain unsaturation, in the order dilin-PE>DOPE>POPE>lyso-PE. Replacement of dilin-PE, DOPE, or POPE by lyso-PE gradually in cationic liposomes lowered the cell–cell FY from 50% to 1%. Furthermore, cationic liposome-induced cell lysis, and fusion between cationic liposomes and cells, as assayed by the N-(lissamine rhodamine B sulfonyl)-1,2-dihexadecanoyl- sn-glycero-3-phosphoethanolamine, triethylammonium salt and N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl- sn-glycero-3-phosphoethanolamine, triethylammonium salt (Rh-PE/NBD-PE) energy transfer method, followed the same order as that for cell–cell fusion. Fusion between the negatively charged PS liposomes and cationic liposomes also followed the same order. The electric double layer screening by electrolytes in NaCl-containing solution and phosphate buffered saline (PBS) was found to reduce the cell–cell FY and cell lysis. These findings suggest that liposome-induced cell–cell fusion was achieved by cationic liposomes serving as fusion-bridges among cells.

Simultaneous microanalysis of biliary cholesterol, bile acids and fatty acids in lecithin using capillary column gas chromatography: an advantage to assess bile lithogenecity

Simultaneous determination of biliary lipids was performed by alkaline hydrolysis, the formation of the methyl ester derivatives of fatty acids that are constituents of phospholipids and of the acetylated methyl ester derivatives of bile acids, and subsequent analysis by capillary column gas chromatography. Complete separation and satisfactory recovery of cholesterol, bile acids, and fatty acids were achieved. Also, the accuracy of the calculation of the bile cholesterol saturation index was enhanced by computation. Since the degree of acyl chain unsaturation affects the cholesterol-holding capacity in vesicles, this method provides a unique insight into bile metastability by the quantitative assessment of fatty acids in lecithin.

Interactions of long-chain fatty acids and albumin: determination of free fatty acid levels using the fluorescent probe ADIFAB.

Equilibrium binding of long-chain fatty acids (FA) with albumin from human serum (HSA), bovine serum (BSA), and murine serum (MSA) has been studied by measuring the equilibrium levels of free fatty acids (FFA). FFA levels were measured directly, using a new fluorescent probe composed of acrylodan-derivatized intestinal fatty acid binding protein (ADIFAB). Measurements of [FFA] were done as a function of the ratio of total FA to total albumin (v) for v values between 0 and 6, at pH 7.4 and 37 degrees C. Under conditions observed in normal human physiology (v < or = 2), [FFA] values of the most abundant serum FA (palmitate, stearate, oleate) in equilibrium with human or bovine albumin are less than 15 nM. These values are considerably smaller than the generally quoted values of [FFA] in equilibrium with albumin: more than 20-fold for palmitate and more than 50-fold for oleate. FFA levels were found to increase monotonically with for all three albumins and all FA. In most cases [FFA] increased, for the same chain length, with increasing degree of acyl chain unsaturation, suggesting that FA aqueous solubility may play a significant role in the equilibrium between FA association with albumin and the aqueous phase. [The highest FFA levels (approximately 3000 nM), for example, were observed for linoleate (18:3) at the maximum v value (6).] Although aqueous-phase solubility of the FA may be important in understanding the interaction between FA and albumin, protein structure, as reflected in differences among the three albumins, also significantly affects the equilibrium.(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium transport by an ionizable and a neutral mobile carrier: effects of membrane structure on the apparent activation energy

Temperature-jump relaxation experiments on Na + transport by (221)C 10-cryptand (ionizable mobile carrier) and nonactin (neutral mobile carrier) were carried out in order to study the effects of cholesterol and the degree of acyl chain unsaturation, and their temperature-dependence on ion transport through thin lipid membranes. The experiments were performed on large, negatively charged unilamellar vesicles (LUV) prepared from mixtures of phosphatidylcholine (egg phosphatidylcholine, dioleoylphosphatidylcholine and dilinoleolylphosphatidylcholine), phosphatidic acid and cholesterol (mole fraction 0–0.43), at various temperatures and carrier concentrations. The apparent rate constants of Na + translocation by (221)C 10 and nonactin increased with the carrier concentration, the degree of acyl chain unsaturation and the temperature. The incorporation of cholesterol into the membranes significantly reduced the carrier concentration-, acyl chain unsaturation- and temperature-dependence of this parameter. The apparent energy required to activate the transport decreased significantly with increasing (221)C 10 concentrations and remained constant with increasing those of nonactin at any given cholesterol molar fraction and degree of acyl chain unsaturation. It increased significantly with increasing the cholesterol molar fraction at any given carrier concentration to an extent depending on the degree of acyl chain unsaturation. Our interpretation of the action of cholesterol on these transport systems is based on the assumption that the adsorption plane of Na +-(221)C 10 and Na +-nonactin complexes is likely to be located towards the aqueous and the hydrocarbon side of the dipole layer, respectively. The results are discussed in terms of the structural, physico-chemical and electrical characteristics of carriers and complexes, and of the interactions occuring between an ionizable or a neutral mobile carrier and the membrane.

Effect of phospholipid unsaturation on protein kinase C activation.

To examine the hypothesis that physical features of the membrane contribute to protein kinase C activation, phosphatidylcholine/phosphatidylserine/diolein (70:25:5) vesicles of defined acyl chain composition were tested for their ability to activate the enzyme. Maximal activation was found to correlate with the mole percent unsaturation in the system. Unsaturation could be provided by either the phosphatidylserine or the phosphatidylcholine component. Vesicles containing 5 mol% diolein but lacking any unsaturation in the phospholipid did not support activity, indicating that acidic head groups alone are not sufficient for activity. The saturated lipid vesicles could be rendered effective but only at very high (25 mol%) concentrations of diolein. The degree of acyl chain unsaturation and the positioning of the double bond had little effect on the activity, suggesting that the effect of the unsaturation is due to some physical property of the lipid rather than to a specific lipid-protein interaction. Addition of cholesterol to both saturated and unsaturated systems indicated that fluidity, as assessed by fluorescence anisotropy, did not correlate with activity. These results suggest that a physical property of the membrane other than fluidity is important for the activation of protein kinase C. A model for protein kinase C activation involving phase separation and/or head group spacing is discussed.

Interaction of α-tocopherol with fatty acids in membranes and ethanol

The techniques of fluorescence polarization, ultraviolet light absorbance and fluorescence quenching by acrylamide are used to probe the structural role of α-tocopherol in phospholipid bilayers. Using 1,6-diphenyl-1,3,5-hexatriene (DPH) and a series of (anthroyloxy)stearic acid (AS) fluorescence probes, α-tocopherol is shown to increase fluidity and decrease order of gel state bilayers, and to decrease fluidity and increase order of bilayers in the liquid crystalline state. More complex behavior is noted for bilayers made from mixed acyl chain phosphatidylcholines (PCs) where the sn-1 position is saturated and the sn-2 position unsaturated compared to bilayers composed of PCs where both acyl chains are either saturated or unsaturated. Complexation between α-tocopherol and either free fatty acids or fatty acids esterified to the sn-2 position of PCs is indicated by ultraviolet light absorbance in both organic solution and in lipid bilayers. The strength of the complexes, expressed as interaction constants, are dependent upon the number of acyl chain unsaturations from 0 (stearic acid), to 6 (docosahexaenoic acid). Relation of the strength of these complexes to the degree of acyl chain unsaturation is confirmed by monitoring the fatty acid protection from acrylamide bleaching of α-tocopherol. These experiments suggest that the extent of acrylamide bleaching is related to the extent of association with the fatty acids.

Fatty acid composition of galactolipids and phospholipids in neoplasmatic plant tissues (cecidia) and normal leaf tissue

Insect‐elicited modifications of leaf cells that produce leaf galls on higher plants are concurrent with changes at the biochemical level. The neoplasmatic alterations affect the composition and concentration of membrane phospho and galactolipids. In vitro labeling of developing leaf galls from oak [induced by cympids (Hymenoptera) on L.] with [14C]‐acetate and [3H]‐oleate was used to study the label distribution in monogalactosyldiacylglyceride, digalactosyldiacylglyceride, phospha‐tidylglycerol and phosphatidylinositol in these tissues and in the normal host organs (leaf laminae). The incorporation of both labels was higher into the leaf galactolipids than into gall galactolipids, whereas the incorporation of oleate was slightly increased in the 2 phospholipids in cynipid galls. The degree of acyl chain unsaturation of individual endogenous phosphoglycerides (phosphatidylcholine, phosphatidyletha‐nolamine. phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol) was, in most cases, higher in the gall tissues. Differences in the endogenous fatty acid composition of plant polar lipids and the molar concentrations of labeled fatty‐acid precursors in galacto‐ and phospholipids are discussed with regard to modifications of the leaf phenotype and cecidogenesis on Quercus palustris L.

Reversed hexagonal phase formation in lecithin-alkane-water systems with different acyl chain unsaturation and alkane length

Investigations of lipid-alkane systems are important for an understanding of the interactions between lipids and hydrophobic/amphiphilic peptides or other hydrophobic biological molecules. A study of the formation of nonlamellar phases in several phosphatidylcholine (PC)-alkane-2H2O systems has been performed. The PC molecules chosen in this work are dipalmitoyl-PC (DPPC), 1-palmitoyl-2-oleoyl-PC (POPC), dioleoyl-PC (DOPC), and dilinoleoyl-PC (DLiPC), lipids that in excess water form just a lamellar liquid-crystalline phase up to at least 90 degrees C. The addition of n-alkanes (C8-C20) to these PC-2H2O systems induces the formation of reversed hexagonal (HII) and isotropic phases. The water and dodecane concentrations required to form these phases depend on the degree of acyl chain unsaturation of the PC molecules and increase in the order DLiPC approximately DOPC less than POPC less than DPPC. The most likely explanation to this result is that the diameter of the lipid-water cylinders in the HII phase grows gradually larger with increased acyl chain saturation and more water and dodecane are consequently needed to fill the water cylinders and the void volumes between the cylinders, respectively. The ability of the alkanes to promote the formation of an HII phase is strongly chain length dependent. Although the number of alkane carbon atoms added per DOPC molecule in the DOPC-n-alkane-2H2O mixtures was kept constant, this ability decreased on going from octane to eicosane. The thermal history of a DPPC-n-dodecane-2H2O sample was important for its phase behavior.(ABSTRACT TRUNCATED AT 250 WORDS)