Cholesterol versus alpha-tocopherol: effects on properties of bilayers made from heteroacid phosphatidylcholines

Cholesterol enhances the negative impact of vaping additives on lung surfactant model systems

Aims: Vaping has given rise to e-cigarette or vaping product use-associated lung injury. Model lung surfactant films were used to assess the impact of vape additives (vitamin E, vitamin E acetate, tetrahydrocannabinol, cannabidiol). This work builds upon our previous findings, by incorporating cholesterol, to understand the interplay between the additives and the sterol in surfactant function. Materials & methods: Compression-expansion cycles of lipid monofilm at the air-water interface and Brewster angle microscopy allowed elucidating the effects of vape additives. Results & conclusion: Vape additives at 5 mol% inhibited proper lipid packing and reduced film stability. Cholesterol enhanced the additive effects, resulting in significantly destabilized films and altered domains. The observed impact could signify dysfunctional lung surfactant and impaired lung function.

Effect of Methylation of the Hydrophilic Domain of Tocopheryl Ammonium-Based Lipids on their Nucleic Acid Delivery Properties

Lipid-enabled nucleic acid delivery has garnered tremendous attention in recent times. Tocopherol among the cationic lipids, 3b-[N-(N',N'-dimethylamino-ethane)carbamoyl]-cholesterol hydrochloride (DC-Chol) with a headgroup of dimethylammonium, and cholesterol as a hydrophobic moiety are found to be some of the most successful lipids and are being used in clinical trials. However, limited efficacy is a major limitation for their broader therapeutic application. In our prior studies, we demonstrated tocopherol to be a potential alternative hydrophobic moiety having additional antioxidant properties to develop efficient and safer liposomal formulations. Inspired by DC-Chol applications and taking cues from our own prior findings, herein, we report the design and synthesis of four alpha-tocopherol-based cationic derivatives with varying degrees of methylation, AC-Toc (no methylation), MC-Toc (monomethylation derivative), DC-Toc (dimethylation derivative), and TC-Toc (trimethylation derivative) and the evaluation of their gene delivery properties. The transfection studies showed that AC-Toc liposomes exhibited superior transfection compared to MC-Toc, DC-Toc, TC-Toc, and control DC-Chol, indicating that methylation in the hydrophilic moiety of Toc-lipids reduced their transfection properties. Cellular internalization studies in the presence of different endocytosis blockers revealed that all four tocopherol lipids were internalized through clathrin-mediated endocytosis, whereas control DC-Chol was found to be internalized through both macropinocytosis and clathrin-mediated endocytosis. These novel Toc-lipids exhibited higher antioxidant properties than DC-Chol by generating less reactive oxygen species, indicating lower cytotoxicity. Our present findings suggest that AC-Toc may be considered as an alternative to DC-Chol in liposomal transfections.

Vaping additives negatively impact the stability and lateral film organization of lung surfactant model systems

Aims: Inhalation of vaping additives has recently been shown to impair respiratory function, leading to e-cigarette or vaping product use associated with lung injuries. This work was designed to understand the impact of additives (vitamin E, vitamin E acetate, tetrahydrocannabinol and cannabidiol) on model lung surfactants. Materials & methods: Lipid monofilms at the air-water interface and Brewster angle microscopy were used to assess the impact of vaping additives on model lung surfactant films. Results & conclusion: The addition of 5 mol % of vaping additives, and even more so mixtures of vitamins and cannabinoids, negatively impacts lipid packing and film stability, induces material loss upon cycling and significantly reduces functionally relevant lipid domains. This range of detrimental effects could affect proper lung function.

Physical-Chemical Characterization of Bilayer Membranes Derived from (±) α-Tocopherol-Based Gemini Lipids and Their Interaction with Phosphatidylcholine Bilayers and Lipoplex Formation with Plasmid DNA

Membrane formation and aggregation properties of two series of (±) α-tocopherol-based cationic gemini lipids without and with hydroxyl functionalities at the headgroup region (TnS n = 3, 4, 5, 6, 8, and 12; THnS n = 4, 5, 6, 8, and 12) with varying polymethylene spacer lengths were investigated extensively while comparing with the corresponding properties of the monomeric counterparts (TM and THM). Liposomal suspensions of each cationic lipid were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), zeta potential measurements, and small-angle X-ray diffraction studies. The length of the spacer and the presence of hydroxyl functionalities at the headgroup region strongly contribute to the aggregation behavior of these gemini lipids in water. The interaction of each tocopherol lipid with a model phospholipid, 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC)-derived vesicles, was thoroughly examined by differential scanning calorimetry (DSC) and 1,6-diphenyl-1,3,5-hexatriene (DPH)-doped fluorescence anisotropy measurements. The binding efficiency of the cationic tocopherol liposomes with plasmid DNA (pDNA) was followed by an ethidium bromide (EB) exclusion assay and zeta potential measurements, whereas negatively charged micellar sodium dodecyl sulfate (SDS)-mediated release of the pDNA from various preformed pDNA-liposomal complexes (lipoplex) was studied by an ethidium bromide (EB) reintercalation assay. The structural transformation of pDNA upon complexation with liposome was characterized using circular dichroism (CD) spectroscopic measurements. Gemini lipid-pDNA interactions depend on both the presence of hydroxyl functionalities at the headgroups and the length of the spacer chain between the headgroups. Succinctly, we performed a detailed physical-chemical characterization of the membranes formed from cationic monomeric and gemini lipids bearing tocopherol as their hydrophobic backbone and describe the role of inserting the -OH group at the headgroup of such lipids.

Enhanced Prevention of Breast Tumor Metastasis by Nanoparticle-Delivered Vitamin E in Combination with Interferon-Gamma

Preventing cancer metastasis is one of the remaining challenges in cancer therapy. As an efficient natural product, alpha-tocopheryl succinate (α-TOS), the most effective form of vitamin E, holds great anticancer potential. To improve its efficacy and bioavailability, lipid-coated calcium carbonate/phosphate (LCCP) nanoparticles (NPs) with folic acid and PEG modification are synthesized for efficient delivery of α-TOS to 4T1 cancer cells. The optimized LCCP-FA NPs (NP-TOS15) show an α-TOS loading efficiency of around 60%, and enhanced uptake by 4T1 metastatic cancer cells. Consequently, NP-TOS15 significantly enhance the anticancer effect in combination with interferon-gamma (IFN-γ) in terms of apoptosis facilitation and migration inhibition. Importantly, NP-TOS15 upregulate the anticancer immunity via downregulating program death ligand 1 (PD-L1) expression that is initially induced by IFN-γ, and remarkably prevent the lung metastasis, particularly in combination with IFN-γ. Further investigation reveals that this combination therapy also modulates the cytotoxic lymphocyte infiltration into the tumor microenvironment for tumor elimination. Taken together, the NP delivery of α-TOS in combination with IFN-γ provides an applicable strategy for cancer therapy.

Beneficial and harmful effects of cyclodextrin-vitamin E complex on cryopreserved ram sperm

Vitamin E is a potent molecule, especially when loaded in cyclodextrin, in modulating oxidative stress during the freeze-thawing process. The present study aimed to investigate the effect of different concentrations of cyclodextrin-vitamin E complex (CD-Vit E) on cryopreserved ram sperm. Ejaculates collected from five adult rams were pooled and divided into four aliquots. All aliquots were treated in Tris-extender (Tris-glucose-citric acid) containing 2 mg cholesterol-loaded methyl-β- cyclodextrin/120 × 10⁶ spermatozoa and either 0 (Control), 2, 4 or 6 mg CD-Vit E/120 × 10⁶ spermatozoa, corresponding to 0, 0.5, 1 or 1.5 of pure vitamin E, respectively. After incubation at 22 °C for 15 min and the addition of Tris-extender containing glycerol and egg yolk (v/v), all aliquots were frozen in liquid nitrogen. After thawing, motility (computer aided sperm analysis), viability (eosin staining), membrane integrity (HOST), acrosome integrity (Coomassie G-250 staining) and lipid peroxidation (Thiobarbituric acid assay) were evaluated. Compared to control, 2 mg CD-Vit E had a significant positive effect on total motility, progressive motility, movement linearity (LIN%), viability and lipid peroxidation. At 4 mg, however, CD-Vit E had a significant negative effect on total motility, progressive motility, membrane functionality and acrosome integrity. At a greater concentration (6 mg), the negative effects were greater as compared with inclusion of 4 mg in the cryoprotectant and the percentage of rapidly and moderately motile gametes and viability were also altered. In conclusion, the effect of CD-Vit E on cryopreserved ram sperm was concentration-dependent with the 2 mg amount having a beneficial effect while greater concentrations (4 and 6 mg) had a harmful effect on sperm motility and gamete integrity but without affecting oxidative stress status.

Effects of Acyl Chain Unsaturation on Activation Energy of Water Permeability across Droplet Bilayers of Homologous Monoglycerides: Role of Cholesterol

Cholesterol is an important component of total lipid in mammalian cellular membranes; hence, the knowledge of its association with lipid bilayer membranes will be essential to understanding membrane structure and function. A droplet interface bilayer (DIB) provides a convenient and reliable platform through which values for permeability coefficient and activation energy of water transport across the membrane can be extracted. In this study, we investigated the effect of acyl chain structure in amphiphilic monoglycerides on the permeability of water across DIB membranes composed of cholesterol and these monoglycerides, where the acyl chain length, number of double bonds, and the position of double bond are varied systematically along the acyl chains. To elucidate the role of cholesterol in these membranes, we investigated its influence on water permeability and associated activation energies at two different cholesterol concentrations. Our systematic studies show dramatic sensitivity and selectivity of specific interaction of cholesterol with the monoglyceride bilayer having structural variations in acyl chain compositions. Our findings allow us to delineate the exquisite interplay between membrane properties and structural components and understand the balanced contribution of each.

Effects of Sterol-Like Additives on Phase Transition Behavior of Ion-Pair Amphiphile Bilayers

The incorporation of additive in lipid bilayers is one of the ordinary approaches for modulating their properties. Additive effect on phase transition of ion-pair amphiphile (IPA) bilayers, however, is not known. In this work, four double-chained IPAs with different hydrocarbon chain lengths and symmetry were designed and synthesized from single-chained cationic and anionic surfactants by the precipitation method. By using differential scanning calorimetry (DSC), the thermotropic transition behavior from gel phase (L_β) through rippled phase (P_β') if any to liquid-crystalline phase (L_α) was studied for bilayers of these lipid-like IPAs in excess water. The effects of three sterol-like additives (cholesterol, α-tocopherol, and α-tocopheryl acetate) in IPA bilayers on thermal phase behavior were then systematically investigated. The experimental results revealed that with increasing concentration of additive, the phase transition temperatures were unaffected on the one hand and the enthalpies of phase transition were decreased on the other hand. When the addition of additive exceeded a specific amount, the phase transition disappeared. More hasty disappearance of phase transition was found for IPAs with lower total number of carbon atoms in the hydrocarbon chains. For IPAs with the same total number of carbon atoms in the hydrocarbon chains, the disappearance of phase transition is more hasty for the asymmetric one than for the symmetric one. Similar effects on thermal phase behavior of four IPA bilayers were exhibited by the three additives with similar chemical structures. Possible mechanism of additive effects on phase transition of IPA bilayers was then proposed in line with that of lipid bilayers.

Influence of dietary canola oil and palm oil blend and refrigerated storage on fatty acids, myofibrillar proteins, chemical composition, antioxidant profile and quality attributes of semimembranosus muscle in goats

BACKGROUND

Improving the unsaturated fatty acid content of ruminant meat is essential due to the generally saturated nature of fatty acids in ruminant meat and the negative effects this can have on human health. Nonetheless, enhancing the unsaturated fatty acid content of ruminant meat can have adverse effects on the shelf life and quality attributes of the meat. This study assessed the effects of dietary 80 % canola oil and 20 % palm oil blend (CPOB) on fatty acid composition, antioxidants, oxidative spoilage, cholesterol and physicochemical properties of semimembranosus (SM) muscle from goats. Twenty four Boer bucks were randomly assigned to diets containing on dry matter basis 0, 4 and 8 % CPOB, fed for 100 d and slaughtered. The carcasses were subjected to a 7 d postmortem refrigerated storage. All analyses were conducted on the SM muscle.

RESULTS

Diet had no effect (P > 0.05) on the concentration of free thiol and carbonyl and the band intensity of myosin heavy chain, actin and troponin T. The muscle glycogen, pH, water holding capacity, tenderness, glutathione peroxidase (GPX) activity, total carotenoid, δ-tocopherol, cholesterol and proximate composition did not differ (P > 0.05) between diets. The SM muscle from goats fed 4 and 8 % CPOB had lower (P < 0.05) concentration of C14:0 and C16:0 and higher (P < 0.05) concentration of C18:1 trans-11, C18:1ω-9, C18:3ω-3, C20:5ω-3 and C22:5ω-3 than the SM muscle from the control goats. Dietary CPOB increased (P < 0.05) the concentration of α and γ tocopherol and meat redness (a*) on d 1 and 4 postmortem. Regardless of diet, antioxidant vitamins, and shear force decreased (P < 0.05) while drip loss, lipid and protein oxidation increased (P < 0.05) as postmortem storage progressed.

CONCLUSION

Results evince that dietary CPOB can be used as a management tool to enhance the beneficial fatty acids and antioxidant contents of chevon without deleterious effects on its physicochemical properties and shelf life.

Products of lipid peroxidation, but not membrane susceptibility to oxidative damage, are conserved in skeletal muscle following temperature acclimation

Changes in oxidative capacities and phospholipid remodeling accompany temperature acclimation in ectothermic animals. Both responses may alter redox status and membrane susceptibility to lipid peroxidation (LPO). We tested the hypothesis that phospholipid remodeling is sufficient to offset temperature-driven rates of LPO and, thus, membrane susceptibility to LPO is conserved. We also predicted that the content of LPO products is maintained over a range of physiological temperatures. To assess LPO susceptibility, rates of LPO were quantified with the fluorescent probe C11-BODIPY in mitochondria and sarcoplasmic reticulum from oxidative and glycolytic muscle of striped bass (Morone saxatilis) acclimated to 7°C and 25°C. We also measured phospholipid compositions, contents of LPO products [i.e., individual classes of phospholipid hydroperoxides (PLOOH)], and two membrane antioxidants. Despite phospholipid headgroup and acyl chain remodeling, these alterations do not counter the effect of temperature on LPO rates (i.e., LPO rates are generally not different among acclimation groups when normalized to phospholipid content and compared at a common temperature). Although absolute levels of PLOOH are higher in muscles from cold- than warm-acclimated fish, this difference is lost when PLOOH levels are normalized to total phospholipid. Contents of vitamin E and two homologs of ubiquinone are more than four times higher in mitochondria prepared from oxidative muscle of warm- than cold-acclimated fish. Collectively, our data demonstrate that although phospholipid remodeling does not provide a means for offsetting thermal effects on rates of LPO, differences in phospholipid quantity ensure a constant proportion of LPO products with temperature variation.

Seasonal changes in the composition of storage and membrane lipids in overwintering larvae of the codling moth, Cydia pomonella

The codling moth (Cydia pomonella) is a major insect pest of apples worldwide. It overwinters as a diapausing fifth instar larva. The overwintering is often a critical part of the insect life-cycle in temperate zone. This study brings detailed analysis of seasonal changes in lipid composition and fluidity in overwintering larvae sampled in the field. Fatty acid composition of triacylglycerol (TG) depots in the fat body and relative proportions of phospholipid (PL) molecular species in biological membranes were analyzed. In addition, temperature of melting (Tm) in TG depots was assessed by using differential scanning calorimetry and the conformational order (fluidity) of PL membranes was analyzed by measuring the anisotropy of fluorescence polarization of diphenylhexatriene probe in membrane vesicles. We observed a significant increase of relative proportion of linoleic acid (C18:2n6) at the expense of palmitic acid (C16:0) in TG depots during the larval transition to diapause accompanied with decreasing melting temperature of total lipids, which might increase the accessibility of depot fats for enzymatic breakdown during overwintering. The fluidity of membranes was maintained very high irrespective of developmental mode or seasonally changing acclimation status of larvae. The seasonal changes in PL composition were relatively small. We discuss these results in light of alternative survival strategies of codling moth larvae (supercooling vs. freezing), variability and low predictability of environmental conditions, and other cold tolerance mechanisms such as extending the supercooling capacity and massive accumulation of cryoprotective metabolites.

Role of callose synthases in transfer cell wall development in tocopherol deficient Arabidopsis mutants

Tocopherols (vitamin E) are lipid-soluble antioxidants produced by all plants and algae, and many cyanobacteria, yet their functions in these photosynthetic organisms are still not fully understood. We have previously reported that the vitamin E deficient 2 (vte2) mutant of Arabidopsis thaliana is sensitive to low temperature (LT) due to impaired transfer cell wall (TCW) development and photoassimilate export associated with massive callose deposition in transfer cells of the phloem. To further understand the roles of tocopherols in LT induced TCW development we compared the global transcript profiles of vte2 and wild-type leaves during LT treatment. Tocopherol deficiency had no significant impact on global gene expression in permissive conditions, but significantly affected expression of 77 genes after 48 h of LT treatment. In vte2 relative to wild type, genes associated with solute transport were repressed, while those involved in various pathogen responses and cell wall modifications, including two members of callose synthase gene family, GLUCAN SYNTHASE LIKE 4 (GSL4) and GSL11, were induced. However, introduction of gsl4 or gsl11 mutations individually into the vte2 background did not suppress callose deposition or the overall LT-induced phenotypes of vte2. Intriguingly, introduction of a mutation disrupting GSL5, the major GSL responsible for pathogen-induced callose deposition, into vte2 substantially reduced vascular callose deposition at LT, but again had no effect on the photoassimilate export phenotype of LT-treated vte2. These results suggest that GSL5 plays a major role in TCW callose deposition in LT-treated vte2 but that this GSL5-dependent callose deposition is not the primary cause of the impaired photoassimilate export phenotype.

Seasonal changes in minor membrane phospholipid classes, sterols and tocopherols in overwintering insect, Pyrrhocoris apterus

Ectotherm animals including insects are known to undergo seasonal restructuring of the cell membranes in order to keep their functionality and/or protect their structural integrity at low body temperatures. Studies on insects so far focused either on fatty acids or on composition of molecular species in major phospholipid classes. Here we extend the scope of analysis and bring results on seasonal changes in minor phospholipid classes, lysophospholipids (LPLs), free fatty acids, phytosterols and tocopherols in heteropteran insect, Pyrrhocoris apterus. We found that muscle tissue contains unusually high amounts of LPLs. Muscle and fat body tissues also contain high amounts of β-sitosterol and campesterol, two phytosterols derived from plant food, while only small amounts of cholesterol are present. In addition, two isomers (γ and δ) of tocopherol (vitamin E) are present in quantities comparable to, or even higher than phytosterols in both tissues. Distinct seasonal patterns of sterol and tocopherol concentrations were observed showing a minimum in reproductively active bugs in summer and a maximum in diapausing, cold-acclimated bugs in winter. Possible adaptive meanings of such changes are discussed including: preventing the unregulated transition of membrane lipids from functional liquid crystalline phase to non-functional gel phase; decreasing the rates of ion/solute leakage; silencing the activities of membrane bound enzymes and receptors; and counteracting the higher risk of oxidative damage to PUFA in winter membranes.

Role of lipid ordered/disordered phase coexistence in pulmonary surfactant function

The respiratory epithelium has evolved to produce a complicated network of extracellular membranes that are essential for breathing and, ultimately, survival. Surfactant membranes form a stable monolayer at the air-liquid interface with bilayer structures attached to it. By reducing the surface tension at the air-liquid interface, surfactant stabilizes the lung against collapse and facilitates inflation. The special composition of surfactant membranes results in the coexistence of two distinct micrometer-sized ordered/disordered phases maintained up to physiological temperatures. Phase coexistence might facilitate monolayer folding to form three-dimensional structures during exhalation and hence allow the film to attain minimal surface tension. These folded structures may act as a membrane reserve and attenuate the increase in membrane tension during inspiration. The present review summarizes what is known of ordered/disordered lipid phase coexistence in lung surfactant, paying attention to the possible role played by domain boundaries in the monolayer-to-multilayer transition, and the correlations of biophysical inactivation of pulmonary surfactant with alterations in phase coexistence.

Tuning membrane phase separation using nonlipid amphiphiles

Lipid phase separation may be a mechanism by which lipids participate in sorting membrane proteins and facilitate membrane-mediated biochemical signaling in cells. To provide new tools for membrane lipid phase manipulation that avoid direct effects on protein activity and lipid composition, we studied phase separation in binary and ternary lipid mixtures under the influence of three nonlipid amphiphiles, vitamin E (VE), Triton-X (TX)-100, and benzyl alcohol (BA). Mechanisms of additive-induced phase separation were elucidated using coarse-grained molecular dynamics simulations of these additives in a liquid bilayer made from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dilinoleoyl-sn-glycero-3-phosphocholine [corrected]. From simulations, the additive's partitioning preference, changes in membrane thickness, and alterations in lipid order were quantified. Simulations showed that VE favored the DPPC phase but partitioned predominantly to the domain boundaries and lowered the tendency for domain formation, and therefore acted as a linactant. This simulated behavior was consistent with experimental observations in which VE promoted lipid mixing and dispersed domains in both gel/liquid and liquid-ordered/liquid-disordered systems. From simulation, BA partitioned predominantly to the DUPC phase, decreased lipid order there, and thinned the membrane. These actions explain why, experimentally, BA promoted phase separation in both binary and ternary lipid mixtures. In contrast, TX, a popular detergent used to isolate raft membranes in cells, exhibited equal preference for both phases, as demonstrated by simulations, but nonetheless, was a strong domain promoter in all lipid mixtures. Further analysis showed that TX increased membrane thickness of the DPPC phase to a greater extent than the DUPC phase and thus increased hydrophobic mismatch, which may explain experimental observation of phase separation in the presence of TX. In summary, these nonlipid amphiphiles provide new tools to tune domain formation in model vesicle systems and could provide the means to form or disperse membrane lipid domains in cells, in addition to the well-known methods involving cholesterol enrichment and sequestration.

The effect of tocopherol on the structure and permeability of phosphatidylcholine liposomes

There are numerous phospholipid formulations that incorporate α-tocopherol as a stabilizing agent but there are few studies of the effect of α-tocopherol on phospholipid structure and bilayer permeability. This study uses synchrotron X-ray powder diffraction methods to investigate how α-tocopherol changes the structure of distearoylphosphatidylcholines bilayers. Increasing proportions of α-tocopherol up to 20 mol% induces ripple structures in the bilayers. Two types of ripple structure are produced which are seen in electron micrographs of freeze-fracture replicas with periodicities of 16 and 12 nm, respectively. The stoichiometry of phospholipid: α-tocopherol in the ripple structures at 37 °C is 8:1. The presence of α-tocopherol tends to reduce the angle of tilt of the hydrocarbon chains of the phospholipid in the gel phase from about 34° to the bilayer normal at 20 °C into a more vertical orientation. Increasing proportions of α-tocopherol progressively decrease the temperature of the gel to liquid-crystal phase transition of the phospholipid. The presence of up to 20 mol% α-tocopherol in 1-palmitoyl-2-oleoyl-phosphocholine inhibits leakage of phenol red dye from liposomes. The effect of 7 mol% α-tocopherol on leakage was compared with phospholipid liposomes containing 50 mol% cholesterol. The cholesterol-containing liposomes inhibited leakage to a greater extent than the vesicles incorporating α-tocopherol but the effect of α-tocopherol at equivalent molar proportions was comparable to cholesterol.

The location and behavior of alpha-tocopherol in membranes

Vitamin E (alpha-tocopherol) has long been recognized as the major antioxidant in biological membranes, and yet many structurally related questions persist of how the vitamin functions. For example, the very low levels of alpha-tocopherol reported for whole cell extracts question how this molecule can successfully protect the comparatively enormous quantities of PUFA-containing phospholipids found in membranes that are highly susceptible to oxidative attack. The contemporary realization that membranes laterally segregate into regions of distinct lipid composition (domains), we propose, provides the answer. We hypothesize alpha-tocopherol partitions into domains that are enriched in polyunsaturated phospholipids, amplifying the concentration of the vitamin in the place where it is most needed. These highly disordered domains depleted in cholesterol are analogous, but organizationally antithetical, to the well-studied lipid rafts. We review here the ideas that led to our hypothesis. Experimental evidence in support of the formation of PUFA-rich domains in model membranes is presented, focusing upon docosahexaenoic acid that is the most unsaturated fatty acid commonly found. Physical methodologies are then described to elucidate the nature of the interaction of alpha-tocopherol with PUFA and to establish that the vitamin and PUFA-containing phospholipids co-localize in non-raft domains.

Differential effects of conjugated linoleic acid isomers on the biophysical and biochemical properties of model membranes

Conjugated linoleic acids (CLA) are known to exert several isomer-specific biological effects, but their mechanisms of action are unclear. In order to determine whether the physicochemical effects of CLA on membranes play a role in their isomer-specific effects, we synthesized phosphatidylcholines (PCs) with 16:0 at sn-1 position and one of four CLA isomers (trans 10 cis 12 (A), trans 9 trans 11 (B), cis 9 trans 11 (C), and cis 9 cis 11 (D)) at sn-2, and determined their biophysical properties in monolayers and bilayers. The surface areas of the PCs with the two natural CLA (A and C) were similar at all pressures, but they differed significantly in the presence of cholesterol, with PC-A condensing more than PC-C. Liposomes of PC-A similarly showed increased binding of cholesterol compared to PC-C liposomes. PC-A liposomes were less permeable to carboxyfluorescein compared to PC-C liposomes. The PC with two trans double bonds (B) showed the highest affinity to cholesterol and lowest permeability. The two natural CLA-PCs (A and C) stimulated lecithin-cholesterol acyltransferase activity by 2-fold, whereas the unnatural CLA-PCs (B and D) were inhibitory. These results suggest that the differences in the biophysical properties of CLA isomers A and C may partly contribute to the known differences in their biological effects.

Cholesterol-mediated activation of P-glycoprotein: distinct effects on basal and drug-induced ATPase activities

Cholesterol promotes basal and verapamil-induced ATPase activity of P-glycoprotein (P-gp). We investigated whether these effects are related to each other and to the impact of the sterol on bilayer fluidity and verapamil membrane affinity. P-gp was reconstituted in egg-phosphatidylcholine (PhC) liposomes with or without cholesterol, 1,2-dipalmitoyl-phosphatidylcholine (DPPC), alpha-tocopherol (alpha-Toc) or 2,2,5,7,8-pentamethyl-6-chromanol (PMC). Basal and verapamil-induced ATPase activities were studied with an enzymatic assay. Membrane fluidity was characterized with diphenyl-hexatriene anisotropy measurements and membrane affinity by equilibrium dialysis. DPPC (70% mol/mol) decreased the fluidity of PhC bilayers to the same level as 20% cholesterol. PMC (20%) and alpha-Toc (20%) decreased the fluidity to lesser extents. alpha-Toc and PMC, but not DPPC increased the verapamil membrane affinity. While 20% cholesterol strikingly enhanced the basal ATPase activity, none of the other constituents had a similar effect. In contrast, verapamil stimulation of P-gp ATPase activity was not only enabled by cholesterol but also by alpha-Toc and DPPC. PMC had no effect. In conclusion, cholesterol exerts distinct effects on basal and verapamil-induced ATPase activity. The influence on basal ATPase activity is sterol-specific while its effect on verapamil-induced ATPase activity is unspecific and not related to its influence on membrane fluidity and on verapamil membrane affinity.

Effects of alpha-tocopherol (vitamin E) on the stability and lipid dynamics of model membranes mimicking the lipid composition of plant chloroplast membranes

Tocopherol (vitamin E) is widely recognized as a cellular antioxidant. It is essential for human and animal health, but only synthesized in photosynthetic organisms, where it is localized in chloroplast membranes. While many studies have investigated non-antioxidative effects of tocopherol on phospholipid membranes, nothing is known about its effects on membranes containing chloroplast glycolipids. Here, liposomes resembling plant chloroplast membranes were used to investigate the effects of alpha-tocopherol on vesicle stability during freezing and on lipid dynamics. alpha-Tocopherol had a pronounced influence on membrane dynamics and showed strong interactions in its effects on membrane stability during freezing with the cryoprotectant sucrose. alpha-Tocopherol showed maximal effects at low concentrations (around 2mol%), close to its contents in chloroplast membranes.

The cold but not hard fats in ectotherms: consequences of lipid restructuring on susceptibility of biological membranes to peroxidation, a review

The production of reactive oxygen species is a regular feature of life in the presence of oxygen. Some reactive oxygen species possess sufficient energy to initiate lipid peroxidation in biological membranes, self-propagating reactions with the potential to damage membranes by altering their physical properties and ultimately their function. Two of the most prominent patterns of lipid restructuring in membranes of ectotherms involve contents of polyunsaturated fatty acids and ratios of the abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine. Since polyunsaturated fatty acids and phosphatidylethanolamine are particularly vulnerable to oxidation, it is likely that higher contents of these lipids at low body temperature elevate the inherent susceptibility of membranes to lipid peroxidation. Although membranes from animals living at low body temperatures may be more prone to oxidation, the generation of reactive oxygen species and lipid peroxidation are sensitive to temperature. These scenarios raise the possibility that membrane susceptibility to lipid peroxidation is conserved at physiological temperatures. Reduced levels of polyunsaturated fatty acids and phosphatidylethanolamine may protect membranes at warm temperatures from deleterious oxidations when rates of reactive oxygen species production and lipid peroxidation are relatively high. At low temperatures, enhanced susceptibility may ensure sufficient lipid peroxidation for cellular processes that require lipid oxidation products.

Tocopherols and tocotrienols in membranes: a critical review

The familiar role of tocols (tocopherols and tocotrienols) as lipid-soluble chain-terminating inhibitors of lipid peroxidation is currently in the midst of a reinterpretation. New biological activities have been described for tocols that apparently are not dependent on their well-established antioxidant behaviour. These activities could well be real, but there remain large gaps in our understanding of the behaviour of tocols in membranes, especially when it comes to the alpha-, beta-, gamma-, delta-chroman methylation patterns and the seemingly special nature of tocotrienols. It is inappropriate to make conclusions and develop models based on in vivo (or cell culture) results with reference to in vitro measurements of antioxidant activity. When present in biological membranes, tocols will experience a large variation in the local composition of phospholipids and the presence of neutral lipids such as cholesterol, both of which would be expected to change the efficiency of antioxidant action. It is likely that tocols are not homogeneously dispersed in a membrane, but it is still not known whether any specific combination of lipid head group and acyl chains are conferred special protection from peroxidation, nor do we currently appreciate the structural role that tocols play in membranes. Tocols may enhance curvature stress or counteract similar stresses generated by other lipids such as lysolipids. This review will outline what is known about the location and behaviour of tocols in phospholipid bilayers. We will draw mainly from the biophysical literature, but will attempt to extend the discussion to biologically relevant phenomena when appropriate. We hope that it will assist researchers when designing new experiments and when critically assessing the results, in turn providing a more thorough understanding of the biochemistry of tocols.

Tocopherols modulate extraplastidic polyunsaturated fatty acid metabolism in Arabidopsis at low temperature

Tocopherols (vitamin E) are synthesized in plastids and have long been assumed to have essential functions restricted to these organelles. We previously reported that the vitamin e-deficient2 (vte2) mutant of Arabidopsis thaliana is defective in transfer cell wall development and photoassimilate transport at low temperature (LT). Here, we demonstrate that LT-treated vte2 has a distinct composition of polyunsaturated fatty acids (PUFAs): lower levels of linolenic acid (18:3) and higher levels of linoleic acid (18:2) compared with the wild type. Enhanced 18:3 oxidation was not involved, as indicated by the limited differences in oxidized lipid species between LT-treated vte2 and the wild type and by a lack of impact on the LT-induced vte2 phenotype in a vte2 fad3 fad7 fad8 quadruple mutant deficient in 18:3. PUFA changes in LT-treated vte2 occur primarily in phospholipids due to reduced conversion of dienoic to trienoic fatty acids in the endoplasmic reticulum (ER) pathway. Introduction of the ER fatty acid desaturase mutation, fad2, and to a lesser extent the plastidic fad6 mutation into the vte2 background suppressed the LT-induced vte2 phenotypes, including abnormal transfer cell wall development. These results provide biochemical and genetic evidence that plastid-synthesized tocopherols modulate ER PUFA metabolism early in the LT adaptation response of Arabidopsis.

Molecular associations of vitamin E

To understand how vitamin E fulfills its functions in membranes and lipoproteins, it is necessary to know how it associates with the lipid components of these structures and the effects its presence has on their structure and stability. Studies of model membrane systems containing vitamin E have proved to be an informative approach to address these questions. A review of the way vitamin E interacts with phospholipid bilayers, how it distributes within the structure, its motional diffusion characteristics, and orientation has been undertaken. The effect of vitamin E on membrane stability and permeability has been described. The tendency of vitamin E to form complexes with certain phospholipids is examined as is the way modulation of protein functions takes place. Finally, recent evidence relevant to the putative role of vitamin E in protecting membranes from free radical attack and the consequences of lipid oxidation in lipoproteins and membranes is examined.

Inverted hexagonal and cubic phases induced by alpha-tocopherol in fully hydrated dispersions of dilauroylphosphatidylethanolamine

The effect of alpha-tocopherol on the thermotropic phase behaviour and structure of aqueous dispersions of 1,2-di-lauryl-sn-glycero-3-phosphoethanolamine was examined by synchrotron X-ray diffraction. The pure phospholipid exhibited a lamellar gel to liquid-crystal phase transition at 30 degrees C on heating at 3 degrees C min(-1) between 10 degrees C and 90 degrees C. The transition was reversible with a temperature hysteresis of 0.3 degrees C on cooling. At temperatures less than 10 degrees C only lamellar gel phase of the pure phospholipid was seen in co-dispersions of up to 20 mol % alpha-tocopherol. The presence of 2.5 mol % alpha-tocopherol caused the appearance of inverted hexagonal phase at temperatures just below the main phase transition temperature that co-existed with the lamellar gel phase. The intensity of scattering from the hexagonal-II phase increased with increasing proportion of alpha-tocopherol in the mixture and in proportions greater than 10 mol % it persisted at temperatures above the main transition and co-existed with the lamellar liquid-crystal phase of the pure phospholipid. At higher temperatures all co-dispersions containing up to 15 mol % alpha-tocopherol showed the presence of cubic phases. These phases indexed a Pn3m or Pn3 space grouping. When the proportion of alpha-tocopherol was increased to 20 mol % the only non-lamellar phase observed was inverted hexagonal phase. This phase co-existed with lamellar gel and liquid-crystal phases of the pure phospholipid, but was the only phase present at temperatures >60 degrees C. The X-ray diffraction data were used to construct a partial phase diagram of the lipid mixture in excess water between 10 degrees and 90 degrees C and up to 20 mol % alpha-tocopherol in phospholipid.

The structure and phase behaviour of α-tocopherol-rich domains in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine dispersed in excess water was examined by synchrotron X-ray diffraction and differential scanning calorimetry. Small- and wide-angle X-ray scattering intensity profiles were recorded from mixed dispersions containing up to 20 mol% alpha-tocopherol during temperature scans over the range 10-75 degrees C. These showed that a domain enriched in alpha-tocopherol phase separated from pure phospholipid in the mixture. This domain tends to have inverted hexagonal structure which coexists with phospholipid bilayers depleted of alpha-tocopherol. The scattering intensity and dimensions of the phase are dependent on the temperature and proportion of alpha-tocopherol in the mixture. Phase separations were also manifest in calorimetric scans of the mixed dispersions evidenced from the appearance of multiple peaks at temperatures corresponding to transitions observed in the X-ray scattering experiments. The effect of alpha-tocopherol in the range 0-20 mol% on the phase behaviour and structure of the phospholipid as observed from the X-ray scattering and calorimetric results have been used to construct a partial phase diagram of the mixture in the temperature range 10-75 degrees C. This shows that alpha-tocopherol has a marked tendency to partition from bilayers of the phospholipid to form an enriched domain in which the phospholipid assumes a hexagonal-II structure.

Tocopherols play a crucial role in low-temperature adaptation and Phloem loading in Arabidopsis

To test whether tocopherols (vitamin E) are essential in the protection against oxidative stress in plants, a series of Arabidopsis thaliana vitamin E (vte) biosynthetic mutants that accumulate different types and levels of tocopherols and pathway intermediates were analyzed under abiotic stress. Surprisingly subtle differences were observed between the tocopherol-deficient vte2 mutant and the wild type during high-light, salinity, and drought stresses. However, vte2, and to a lesser extent vte1, exhibited dramatic phenotypes under low temperature (i.e., increased anthocyanin levels and reduced growth and seed production). That these changes were independent of light level and occurred in the absence of photoinhibition or lipid peroxidation suggests that the mechanisms involved are independent of tocopherol functions in photoprotection. Compared with the wild type, vte1 and vte2 had reduced rates of photoassimilate export as early as 6 h into low-temperature treatment, increased soluble sugar levels by 60 h, and increased starch and reduced photosynthetic electron transport rate by 14 d. The rapid reduction in photoassimilate export in vte2 coincides with callose deposition exclusively in phloem parenchyma transfer cell walls adjacent to the companion cell/sieve element complex. Together, these results indicate that tocopherols have a more limited role in photoprotection than previously assumed but play crucial roles in low-temperature adaptation and phloem loading.

A Cholesterol‐Enriched Diet Enhances Egg Production and Egg Viability without Altering Cholesterol Content of Biological Membranes in the CopepodAcartia hudsonica

Copepods may lack the capacity for de novo synthesis of cholesterol, while at the same time their dietary levels of sterol vary. We tested the hypothesis that copepods maintain the cholesterol contents of their biological membranes despite varying dietary levels of cholesterol. Acartia hudsonica were acclimated for 5 d to phytoplankton alone or phytoplankton supplemented with cholesterol, at a level sufficient to induce a maximal response on egg production rates. Biological membranes were prepared from the copepods and cholesterol contents assayed. Egg production and hatch rates were measured (the former to confirm that supplemented cholesterol was being assimilated). Analyses of marker enzymes indicate that the majority of membrane-associated cholesterol in the copepod resides in the plasma membrane. In membranes fractions, cholesterol normalized to protein or activity of Na+/K+-ATPase is not significantly different for supplemented and unsupplemented groups (29 and 33 mu g cholesterol mg(-1) protein, respectively; 0.24 and 0.25 mg cholesterol U(-1) Na+/K+-ATPase, respectively). At the same time, acclimating animals to a diet enriched with cholesterol enhances egg production by up to 1.8-fold and egg viability by 1.5-fold. We conclude that a cholesterol-enriched diet stimulates both egg production and hatching rates without altering cholesterol contents of plasma membranes in the copepod A. hudsonica.

Order from disorder, corralling cholesterol with chaotic lipids. The role of polyunsaturated lipids in membrane raft formation

A myriad of health benefits including the prevention of cancer and heart disease accompanies consumption of polyunsaturated fatty acids (PUFA). Of special importance is the omega-3-PUFA docosahexaenoic acid (DHA), with 22 carbons and six double bonds that constitute the most highly unsaturated fatty acid naturally occurring. Our experiments target the membrane as a likely site of action and focus upon the interaction of cholesterol with PUFA-containing phospholipids. They support the idea that steric incompatibility of the rigid steroid moiety for highly disordered PUFA chains promotes lateral segregation of lipids into PUFA-rich/sterol-poor and PUFA-poor/sterol-rich regions. Solid state 2H NMR and X-ray diffraction demonstrate that the solubility of cholesterol is low in polyunsaturated bilayers. In mixed membranes of phosphatidylethanolamine (PE) with the lipid raft-forming molecules sphingomyelin (SM) and cholesterol, diminished affinity of the sterol for 1-[2H31]palmitoyl-2-docosahexaenoylphosphatidylethanolamine ([2H31]16:0-22:6PE) relative to 1-[2H31]palmitoyl-2-oleoylphosphatidylethanolamine ([2H31]16:0-18:1PE) is identified by 2H NMR order parameters. Here, lies the origin of a potential biological advantage of the relatively modest increase in PUFA content of plasma membranes that would be conferred by dietary supplementation. We hypothesize that the enhanced propensity to form SM-/cholesterol-rich rafts as well as PUFA-rich/cholesterol-poor microdomains would modify the function of proteins for which these respective regions provide a platform.

Chemometric modeling to relate antioxidants, neutral lipid fatty acids, and flavor components in chicken breasts

Relationships among quality factors in retailed free-range, corn-fed, organic, and conventional chicken breasts (9) were modeled using chemometric approaches. Use of principal component analysis (PCA) to neutral lipid composition data explained the majority (93%) of variability (variance) in fatty acid contents in 2 significant multivariate factors. PCA explained 88 and 75% variance in 3 factors for, respectively, flame ionization detection (FID) and nitrogen phosphorus (NPD) components in chromatographic flavor data from cooked chicken after simultaneous distillation extraction. Relationships to tissue antioxidant contents were modeled. Partial least square regression (PLS2), interrelating total data matrices, provided no useful models. By using single antioxidants as Y variables in PLS (1), good models (r2 values > 0.9) were obtained for alpha-tocopherol, glutathione, catalase, glutathione peroxidase, and reductase and FID flavor components and among the variables total mono and polyunsaturated fatty acids and subsets of FID, and saturated fatty acid and NPD components. Alpha-tocopherol had a modest (r2 = 0.63) relationship with neutral lipid n-3 fatty acid content. Such factors thus relate to flavor development and quality in chicken breast meat.

Permeability behaviour of lipid vesicles prepared from plant plasma membranes--impact of compositional changes

Exposure of oat seedlings to repeated moderate water deficit stress causes a drought acclimation of the seedlings. This acclimation is associated with changes in the lipid composition of the plasma membrane of root cells. Here, plasma membranes from root cells of acclimated and control plants were isolated using the two-phase partitioning method. Membrane vesicles were prepared of total lipids extracted from the plasma membranes. In a series of tests the vesicle permeability for glucose and for protons were analysed and compared with the permeability of model vesicles. Further, the importance of critical components for the permeability properties was analysed by modifying the lipid composition of the vesicles from acclimated and from control plants. The purpose was to add specific lipids to vesicles from acclimated plants to mimic the composition of the vesicles from control plants and vice versa. The plasma membrane lipid vesicles from acclimated plants had a significantly increased permeability for glucose and decreased permeability for protons as compared to control vesicles. The results point to the importance of the ratio phosphatidylcholine (PC)/phosphatidylethanolamine (PE), the levels of cerebrosides and free sterols and the possible interaction of these components for the plasma membrane as a permeability barrier.

α-Tocopherol influences the lipid membrane affinity of desipramine in a pH-dependent manner

Phopholipidosis is a lipid storage disorder caused by cationic amphiphilic drugs (CADs) characterized by the lysosomal accumulation of phospholipids and drug. alpha-Tocopherol (alpha-Toc) has a reversible effect on phospholipidosis in rats and cell culture. We studied the influence of alpha-Toc on the partitioning of the CAD desipramine in a liposome/buffer system using equilibrium dialysis with the following lipid compositions: egg phosphatidylcholine (PhC) or wheat germ phosphatidylinositol (PhI) or a combination of PhC, PhI and cholesterol, containing between 1.5 and 20% (mol per mol total lipids) of alpha-Toc, alpha-tocopherol acetate (alpha-TocAc), 2,2,5,7,8-pentamethyl-6-chromanol (PMC) or cholesterol. alpha-Toc (1.5%) enhanced the partition coefficient of neutral desipramine by up to 1.1 log units while it had no influence on the partitioning of the ionized compound. In the PhC liposome system, at pH 7.4 logD increased with increasing alpha-Toc concentrations but was unchanged at pH 4.5. Similar effects were found with PMC while alpha-TocAc or cholesterol, between 1.5 and 20%, had no influence on the partitioning of desipramine. From these results we postulate that in vivo, alpha-Toc could mediate a redistribution of CADs from lysosomal membranes (pH approximately 4.5) to membranes and lipoproteins at physiological pH.

Rafts, little caves and large potholes: how lipid structure interacts with membrane proteins to create functionally diverse membrane environments

This chapter reviews how diverse lipid microdomains form in the membrane and partition proteins into different functional units that regulate cell trafficking, signalling and movement. We will concentrate upon five major issues: 1. the diversity of lipid structure that produces diverse microenvironments into which different subsets of proteins partition; 2. why ordered lipid domains exclude proteins, and the conditions required for select subsets of proteins to enter these domains; 3. the coupling of the inner and outer leaflets within ordered microdomains; 4. the effect of ordered lipid domains upon membrane properties including curvature and hydrophobicity that affect membrane fission, fusion and extension of filopodia; 5. the biological effects of these structural constraints; in particular how the properties of these domains combine to provide a very different signalling, trafficking and membrane fusion environment to that found in disordered (fluid mosaic) membrane. In addressing these problems, the review draws upon studies ranging from molecular dynamic modelling of lipid interactions, through physical studies of model membrane systems to structural and biological studies of whole cells, examining in the process problems inherent in visualising and purifying these microdomains. While the diversity of structure and function of ordered lipid microdomains is emphasised, some general roles emerge. In particular, the basis for having quite different, non-interacting ordered lipid domains on the same membrane is evident in the diversity of lipid structure and plays a key role in sorting signalling systems. The exclusion of ordered membrane from coated pits, and hence rapid endocytosis, is suggested to underlie the ability of highly ordered domains to establish stable secondary signalling systems required, for instance, in T cell receptor, insulin and neurotrophin signalling.

Docosahexaenoic acid: membrane properties of a unique fatty acid

Docosahexaenoic acid (DHA) with 22-carbons and 6 double bonds is the extreme example of an omega-3 polyunsaturated fatty acid (PUFA). DHA has strong medical implications since its dietary presence has been positively linked to the prevention of numerous human afflictions including cancer and heart disease. The PUFA, moreover, is essential to neurological function. It is remarkable that one simple molecule has been reported to affect so many seemingly unrelated biological processes. Although details of a molecular mode of action remain elusive, DHA must be acting at a fundamental level common to many tissues that is related to the high degree of conformational flexibility that the multiple double bonds have been identified to confer. One likely target for DHA action is at the cell membrane where the fatty acid is known to readily incorporate into membrane phospholipids. Once esterified into phospholipids DHA has been demonstrated to significantly alter many basic properties of membranes including acyl chain order and "fluidity", phase behavior, elastic compressibility, permeability, fusion, flip-flop and protein activity. It is concluded that DHA's interaction with other membrane lipids, particularly cholesterol, may play a prominent role in modulating the local structure and function of cell membranes.

Membrane microviscosity regulates endothelial cell motility

Endothelial cell (EC) movement is an initiating and rate-limiting event in the neogenesis and repair of blood vessels. Here, we explore the hypothesis that microviscosity of the plasma membrane (PM) is a key physiological regulator of cell movement. Aortic ECs treated with membrane-active agents, such as alpha-tocopherol, cholesterol and lysophospholipids, exhibited a biphasic dependency on membrane microviscosity, in which moderate increases enhanced EC migration, but increases beyond a threshold markedly inhibited migration. Surprisingly, angiogenic growth factors, that is, basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF), also increased membrane microviscosity, as measured in live cells by fluorescence recovery after photobleaching (FRAP). The localization of Rac to the PM was modified in cells treated with membrane-active agents or growth factors, suggesting a molecular mechanism for how membrane microviscosity influences cell movement. Our data show that angiogenic growth factors, as well as certain lipophilic molecules, regulate cell motility through alterations in membrane properties and the consequent relocalization of critical signalling molecules to membranes.

Phase separations of alpha-tocopherol in aqueous dispersions of distearoylphosphatidylethanolamine

The effect of alpha-tocopherol on the structure and thermotropic phase behaviour of distearoylphosphatidylethanolamine was examined by using synchrotron X-ray diffraction methods. There was evidence that alpha-tocopherol does not distribute randomly in the dispersed phospholipid but instead phospholipid phases enriched in alpha-tocopherol are formed. Heating codispersions from lamellar gel phase induced formation of hexagonal-II phase at temperatures below the main transition of the pure phospholipid and which were enriched in alpha-tocopherol. Codispersions containing 5 or 10 mol% alpha-tocopherol were induced to form a cubic phase at temperatures above the lamellar to hexagonal-II phase transition. Such phases were not observed in codispersions containing 2.5 or 20 mol% alpha-tocopherol in which only lamellar and hexagonal-II phases were formed. The space group of the cubic phases were tentatively assigned as Pn3m. Equilibration of codispersions at 4 degrees C results in the formation of lamellar crystalline phases enriched in alpha-tocopherol and phase separated domains of pure phospholipid. Two lamellar crystalline phases were characterized on the basis of their particular wide-angle X-ray scattering patterns. The lamellar crystalline phases were also distinguished from other lamellar phases of the pure phospholipid by the lamellar repeat. Partitioning of alpha-tocopherol into phosphatidylethanolamine domains in membranes may introduce instability into the structure.

Interfacial properties of phosphatidylcholine bilayers containing vitamin E derivatives

alpha-Tocopherol and alpha-tocopheryl succinate are biologically active lipids. The activity of these lipids may be related to how they affect membrane physical-chemical properties. Utilizing fluorescence methods, we have investigated the effect of alpha-tocopherol, alpha-tocopheryl succinate, and alpha-tocopheryl acetate on the properties of model membranes consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine. In liquid-crystalline phase phospholipid bilayers, alpha-tocopherol decreased acyl chain mobility and decreased the interfacial polarity, but had no effect on the interfacial surface charge. In contrast, alpha-tocopheryl succinate had little effect on acyl chain motion or interfacial hydration, but increased the interfacial surface charge. alpha-Tocopheryl acetate had very little effect on any of the measurements of these bilayer properties. In a gel phase bilayer, alpha-tocopherol decreased acyl chain order, whereas alpha-tocopheryl succinate and alpha-tocopheryl acetate did not. Each alpha-tocopheryl derivative had a different effect on interfacial polarity, however, only alpha-tocopheryl succinate increased the interfacial surface charge. The acylation of alpha-tocopherol abolishes its antioxidant activity and generates molecules with different membrane physical properties. The non-polar acetate group of alpha-tocopheryl acetate locates this compound in a region of the bilayer where it has little effect on bilayer interfacial properties. The free carboxyl group of alpha-tocopheryl succinate is located in the interfacial region of the bilayer where it increases the membrane surface charge.

The curvature and cholesterol content of phospholipid bilayers alter the transbilayer distribution of specific molecular species of phosphatidylethanolamine

The curvature, cholesterol content, and transbilayer distribution of phospholipids significantly influence the functional properties of cellular membranes, yet little is known of how these parameters interact. In this study, the transbilayer distribution of phosphatidylethanolamine (PE) is determined in vesicles with large (98 nm) and small (19 nm) radii of curvature and with different proportions of PE, phosphatidylcholine, and cholesterol. It was found that the mean diameters of both types of vesicles were not influenced by their lipid composition, and that the amino-reactive compound 2,4,6-trinitrobenzenesulphonic acid (TNBS) was unable to cross the bilayer of either type of vesicle. When large vesicles were treated with TNBS, approximately 40% of the total membrane PE was derivatized; in the small vesicles 55% reacted. These values are interpreted as representing the percentage of total membrane PE residing in the outer leaflet of the vesicle bilayer. The large vesicles likely contained approximately 20% of the total membrane lipid as internal membranes. Therefore, in both types of vesicles, PE as a phospholipid class was randomly distributed between the inner and outer leaflets of the bilayer. The proportion of total PE residing in the outer leaflet was unaffected by changes in either the cholesterol or PE content of the vesicles. However, the transbilayer distributions of individual molecular species of PE were not random, and were significantly influenced by radius of curvature, membrane cholesterol content, or both. For example, palmitate- and docosahexaenoate-containing species of PE were preferentially located in the outer leaflet of the bilayer. Membrane cholesterol content affected the transbilayer distributions of stearate-, oleate-, and linoleate-containing species. The transbilayer distributions of palmitate-, docosahexaenoate-, and stearate-containing species were significantly influenced by membrane curvature, but only in the presence of high levels of cholesterol. Thus, differences in membrane curvature and cholesterol content alter the array of PE molecules present on the surfaces of phospholipid bilayers. In cells and organelles, these differences could have profound effects on a number of critical membrane functions and processes.

Liquid crystalline/gel state phase separation in docosahexaenoic acid-containing bilayers and monolayers

The phase behavior of lipid mixtures containing 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0, 22:6 PC) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with bilayers using differential scanning calorimetry (DSC), and with monolayers monitoring pressure/area isotherms and surface elasticity, and lipid domain formation followed by epifluorescence microscopy. From DSC studies it is concluded that DPPC/18:0, 22:6 PC phase separates into DPPC-rich and 18:0, 22:6 PC-rich phases. In monolayers, phase separation is indicated by changes in pressure-area isotherms implying phase separation where 18:0, 22:6 PC is 'squeezed out' of the remaining DPPC monolayer. Phase separation into lipid domains in the mixed PC monolayer is quantified by epifluorescence microscopy using the fluorescently labeled phospholipid membrane probe, 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl). These results further describe the ability of docosahexaenoic acid to participate in lipid phase separations in membranes.

Detection of lipid domains in docasahexaenoic acid-rich bilayers by acyl chain-specific FRET probes

A major problem in defining biological membrane structure is deducing the nature and even existence of lipid microdomains. Lipid microdomains have been defined operationally as heterogeneities in the behavior of fluorescent membrane probes, particularly the fluorescence resonance energy transfer (FRET) probes 7-nitrobenz-2-oxa-1,3-diazol-4-yl-diacyl-sn-glycero-3-phosphoethan olamine (N-NBD-PE) and (N-lissamine rhodamine B sulfonyl)-diacyl-snglycero-3-phosphoethanolamine (N-Rh-PE). Here we test a variety of N-NBD-PEs and N-Rh-PEs containing: (a) undefined acyl chains, (b) liquid crystalline- and gel-state acyl chains, and (c) defined acyl chains matching those of phase separated membrane lipids. The phospholipid bilayer systems employed represent a liquid crystalline/gel phase separation and a cholesterol-driven fluid/fluid phase separation; phase separation is confirmed by differential scanning calorimetry. We tested the hypothesis that acyl chain affinities may dictate the phase into which N-NBD-PE and N-Rh-PE FRET probes partition. While these FRET probes were largely successful at tracking liquid crystalline/gel phase separations, they were less useful in following fluid/fluid separations and appeared to preferentially partition into the liquid-disordered phase. Additionally, partition measurements indicate that the rhodamine-containing probes are substantially less hydrophobic than the analogous NBD probes. These experiments indicate that acyl chain affinities may not be sufficient to employ acyl chain-specific N-NBD-PE/N-Rh-PE FRET probes to investigate phase separations into biologically relevant fluid/fluid lipid microdomains.

Docosahexaenoic acid (DHA) alters the phospholipid molecular species composition of membranous vesicles exfoliated from the surface of a murine leukemia cell line

Previously, we presented evidence that the vesicles routinely exfoliated from the surface of T27A tumor cells arise from vesicle-forming regions of the plasma membrane and possess a set of lateral microdomains distinct from those of the plasma membrane as a whole. We also showed that docosahexaenoic acid (DHA, or 22:6n-3), a fatty acyl chain known to alter microdomain structure in model membranes, also alters the structure and composition of exfoliated vesicles, implying a DHA-induced change in microdomain structure on the cell surface. In this report we show that enrichment of the cells with DHA reverses some of the characteristic differences in composition between the parent plasma membrane and shed microdomain vesicles, but does not alter their phospholipid class composition. In untreated cells, DHA-containing species were found to be a much greater proportion of the total phosphatidylethanolamine (PE) pool than the total phosphatidylcholine (PC) pool in both the plasma membrane and the shed vesicles. After DHA treatment, the proportion of DHA-containing species in the PE and PC pools of the plasma membrane were elevated, and unlike in untreated cells, their proportions were equal in the two pools. In the vesicles shed from DHA-loaded cells, the proportion of DHA-containing species of PE was the same as in the plasma membrane. However, the proportion of DHA-containing species of PC in the vesicles (0.089) was much lower than that found in the plasma membrane (0.194), and was relatively devoid of species with 16-carbon acyl components. These data suggested that DHA-containing species of PC, particularly those having a 16-carbon chain in the sn-1 position, were preferentially retained in the plasma membrane. The data can be interpreted as indicating that DHA induces a restructuring of lateral microdomains on the surface of living cells similar to that predicted by its behavior in model membranes.