Effect of alpha-tocopherol incorporation of glucose permeability and phase transition of lecithin liposomes

From fat to bilayers: Understanding where and how vitamin E works

Vitamin E was one of the last fat-soluble vitamins to be discovered. We provide here an historical review of the discovery and the increasingly more detailed understanding of the role of α-tocopherol both as an antioxidant and as a structural component of phospholipid bilayer membranes. Despite the detailed descriptions now available of the orientation, location, and dynamics of α-tocopherol in lipid bilayers, there are still gaps in our knowledge of the effect of α-tocopherol and its potential receptors than control gene transcription.

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.

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.

Controllable biomolecule release from self-assembled organic nanotubes with asymmetric surfaces: pH and temperature dependence

The release behavior of fluorescent dyes, oligo DNAs and spherical proteins from self-assembled organic nanotubes having 7-9 nm inner diameters has been studied in terms of novel nanocontainers with high-axial ratios. Both much smaller inner diameters and asymmetric inner and outer surfaces are characteristic of the nanotubes. The acid-dissociation constant (pKa) of the amino groups located at the inner surface and the thermal phase transition temperature (Tg-l) of the nanotube were evaluated based on the pH titration and variable-temperature circular dichroism (CD) spectroscopic experiments, respectively. Each guest was slowly released from both open ends of the nanotube under weak alkaline conditions (pH 8.5), as a result of the decrease in electrostatic attraction between the inner surface and the guests. Elevated temperatures above the obtained Tg-l converted the monolayer membrane of the nanotube from a solid state to a fluid one, promoting the remarkably fast release of the guests. The unique release properties of the nanotube as a nanocontainer with two terminal open ends were compared with those of liposomes that posses a closed hollow space covered with fluid bilayer membranes.

Effects of vitamin E supplementation on plasma membrane permeabilization and fluidization induced by chlorpromazine in the rat brain

Neurotransmitter receptors play a key role in most research on antipsychotic drugs, but little is known about the effects of these drugs on the plasma membrane in the central nervous system. Therefore, we investigated whether chlorpromazine (CPZ), a typical phenothiazine antipsychotic drug, affects the plasma membrane integrity in the rat brain, and if so, whether these membrane alterations can be prevented by dietary supplementation with vitamin E, which has been shown to be an antioxidant and also a membrane-stabilizer. Leakage of [(18)F]2-fluoro-2-deoxy-D-glucose ([(18)F]FDG)-6-phosphate from rat striatal slices and decrease in 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy were used as indexes for plasma membrane permeabilization and fluidization, respectively. CPZ induced leakage of [(18)F]FDG-6-phosphate from striatal slices, and the leakage was delayed in the vitamin E-supplemented group compared to that in the normal diet group. The decrease in plasma membrane anisotropy induced by CPZ was significantly attenuated by vitamin E supplementation. Chronic treatment with alpha-phenyl-N-tert-butyl nitrone, a free radical scavenger, had no effect on CPZ-induced plasma membrane permeabilization, and the treatment with CPZ did not induce lipid peroxidation. CPZ can reduce plasma membrane integrity in the brain, and this reduction can be prevented by vitamin E via its membrane-stabilizing properties, not via its antioxidant activity.

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.

Temporary membrane distortion of vascular smooth muscle cells is responsible for their apoptosis induced by platelet-activating factor-like oxidized phospholipids and their degradation product, lysophosphatidylcholine

To obtain information about the mechanism of apoptosis induced by oxidized low density lipoproteins (oxLDL) in atherosclerotic plaques, we examined the effects of lysophosphatidylcholine (LPC) and platelet-activating factor (PAF)-like lipids (PAF-LL), which can be derived from oxLDL, on rat vascular smooth muscle cells (VSMC). All the lipids with different structures examined induced apoptosis of VSMC, so we studied the mechanism of induction of apoptosis by LPC. LPC-induced apoptosis was inhibited by alpha-tocopherol (alpha-T) and cholesterol (Chol), but not by other antioxidants such as palmitoyl ascorbic acid and PAF receptor antagonist. The cells temporarily became spherical and highly permeable before induction of apoptosis, and their change in shape was prevented by alpha-T and Chol. From these results, we suggest that the apoptosis induced by oxLDL-derived phospholipids in VSMC is caused by temporary membrane distortion, not through specific receptors.

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.

Sediment-associated tri-n-butyltin chloride and its effects on osmoregulation of freshwater-adapted 0-group European flounder, Platichthys flesus (L.)

The disruption of osmoregulatory processes was examined in European flounders exposed to environmental concentrations (150 ng TBTCl g(-1) dry weight sediment) of sediment-associated tri-n-butyltin chloride (TBTCl), by using radiotracers to measure changes in hydromineral fluxes and water balance. The water permeabilities of TBTCl-exposed fish varied during the course of the experiment and were significantly lower than those of the corresponding controls that did not change significantly with time. It was found that the maximum decrease in water permeability of TBTCl-exposed fish occurred after 14 days; thereafter there was an increase towards control values. However, there was a differential reduction of the diffusional (P(d)) and osmotic (P(os)) permeability coefficients, where the former decreased more rapidly than the latter, reflecting the reduction of diffusional membrane permeability and the increasing importance of osmotic permeability. In fish exposed to TBTCl sodium efflux and drinking rates were significantly increased but Na(+)/K(+)-ATPase activities and urine production rates were not affected. The effects of TBTCl exposure are also manifested at the level of the whole organism by a reduction in the increase of the body length of exposed fish, when compared to controls. It was concluded that tributyltin-n-chloride in sediments is capable of significantly disrupting the osmoregulatory functions of a benthic estuarine fish, at concentrations found in the sediments of Southampton Water and the River Itchen.

Metastability of dimiristoylphosphatidylethanolamine as studied by FT-IR and the effect of alpha-tocopherol

The metastability of dimiristoylphosphatidylethanolamine (DMPE) has been studied by means of Fourier transform infrared spectroscopy (FT-IR), both in the absence and in the presence of alpha-tocopherol. Two different methods of hydration were used to prepare the samples, poorly hydrated and well hydrated, and the results have been compared with anhydrous DMPE. Poorly hydrated DMPE gave place to a high-melting phase formed upon melting from gel to L alpha at approx. 49 degrees C, with a new transition to L alpha at approx. 55 degrees C. However, well hydrated DMPE incubated at 4 degrees C for 49 days gave place to a subgel phase which was transformed by heating into a L beta phase at about 40 degrees C and this into a L alpha phase after further heating at 52 degrees C. The subgel phase was more hydrated and less rigid than the high-melting phase. On the other hand, alpha-tocopherol, when included in poorly hydrated DMPE, stabilized a high-melting phase, which was transformed by heating, directly into a L alpha. However, when a sample of DMPE containing alpha-tocopherol was incubated for 49 days at 4 degrees C a dehydrated solid phase different from the subgel and the high-melting phases was formed.

Characterisation of clusters of alpha-tocopherol in gel and fluid phases of dipalmitoylglycerophosphocholine

The effect of alpha-tocopherol on the phase behaviour of aqueous dispersions of dipalmitoylglycerophosphocholine has been examined by differential scanning calorimetry, freeze-fracture electron microscopy, and real-time X-ray diffraction methods. The presence of alpha-tocopherol in proportions 2.5, 5, 7.5, and 10 mol/100 mol results in a progressive decrease in the temperature of the gel to liquid-crystalline phase transition from 41.5 degrees C to 36 degrees C and a reduction in transition enthalpy from 35 kJ.mol-1 to 15 kJ.mol-1 phospholipid. The thermal data indicated that the pretransition of the phospholipid is eliminated even in mixtures containing 2.5 mol/100 mol alpha-tocopherol. Real-time X-ray diffraction measurements using synchrotron radiation performed under identical conditions to the thermal studies showed clear transition sequences of L beta-->P beta-->L alpha for all mixtures. The sequence was reversible with hysteresis of 2-3 degrees C on cooling. Low-angle X-ray scattering from mixtures in the gel phase showed three lamellar repeat spacings of 6.35, 7.5, and 8.4 nm. The spacing at 6.35 nm was assigned to pure phospholipid from which alpha-tocopherol has been phase separated into enriched domains giving lamellar repeat spacings of 7.5 nm and 8.4 nm. Low-angle diffraction patterns of mixtures in the fluid phase were characterised by two lamellar repeat spacings. The longer spacing of about 6.6 nm was assigned to pure phospholipid and the shorter spacing at about 6.1 nm to an alpha-tocopherol-enriched phase. Electron microscopy of freeze-fracture replicas of mixtures of phospholipid containing 10 mol/100 mol alpha-tocopherol thermally quenched from 10 degrees C and 60 degrees C, showed evidence of domain structures within the bilayer plane that appeared to be correlated between successive bilayers in multilamellar dispersions. Calculations of the stoichiometry of phospholipid: alpha-tocopherol in the alpha-tocopherol-enriched domains based on enthalpy data and integrated X-ray scattering intensity gave values of 9.6:1 for the fluid phase and 9.2:1 for the gel phase. This was consistent with a clustering of alpha-tocopherol molecules in both gel and liquid-crystal phases of dipalmitoyl-glycerophospholcholine in approximately the same stoichiometry.

Electron spin resonance study of the interaction of alpha-tocopherol with phospholipid model membranes

The effect of up to 20 mol% incorporation of alpha-tocopherol on acyl chain order and dynamics in liquid crystalline phosphatidylcholine (PC) membranes was studied as a function of acyl chain unsaturation by electron spin resonance (ESR) of 5-, 7-, 12- and 16-doxyl spin labelled stearic acids intercalated into the membrane. Order parameters S in the upper portion of the chain (positions 5 and 7) and correlation times tau C in the lower portion (positions 12 and 16) determined from the ESR spectra indicate that in general alpha-tocopherol restricts acyl chain motion within the membrane. The magnitude of the increases in order appears to be dependent upon phospholipid molecular area, being the greatest (up to 15%) in saturated dimyristoylphosphatidylcholine (14:0-14:0 PC) which possesses a relatively small area per molecule as opposed to much smaller increases (less than 3%) in unsaturated PC membranes of larger molecular area. This behavior is interpreted as incompatible with the hypothesis of Lucy and coworkers (A.T. Diplock and J.A. Lucy (1973) FEBS Lett. 29, 205-210), who proposed that membranes are structurally stabilized by interactions between the phytyl side chain of alpha-tocopherol and the polyunsaturated chains of phospholipids.

A spin label ESR and saturation transfer ESR study of alpha-tocopherol containing model membranes

An investigation into the effect of alpha-tocopherol on phospholipid model membranes has been carried out by electron spin resonance (ESR) and saturation transfer ESR. The use of stearic acid and of perdeutero -di-t-butyl nitroxide spin probes has allowed us to monitor, in particular, the effect of alpha-tocopherol on both the phospholipid chain order and the phospholipid chain mobility. The results obtained are mainly consistent with a differing action of alpha-tocopherol in the gel and in the liquid crystalline phases: in the former it induces a decrease of order and an increase in fluidity; while in the latter phase an indication of a slight increase in ordering and a clear decrease in fluidity are registered.

Influence of vitamin E on phosphatidylethanolamine lipid polymorphism

The effect of vitamin E, in its major form alpha-tocopherol and its synthetic analog alpha-tocopheryl acetate, on phosphatidylethanolamine lipid polymorphism has been studied by mean of differential scanning calorimetry and 31P-nuclear magnetic resonance techniques. From the interaction of these tocopherols with dielaidoylphosphatidylethanolamine it is concluded that both molecules promote the formation of the hexagonal HII phase at temperatures lower than those of the pure phospholipid. When the tocopherols were incorporated in the saturated dimiristoylphosphatidylethanolamine, which has been shown not to undergo bilayer to hexagonal HII phase transition, up to 90 degrees C, they induce the phospholipid to partially organize in hexagonal HII phase. From our experiments it is shown that alpha-tocopherol is more effective than its analog in promoting HII phase in these systems. It is also shown that, while alpha-tocopheryl acetate does not significantly perturb the gel to liquid-crystalline phase transition of dimirystoylphosphatidylethanolamine, alpha-tocopherol does so and more than one peak appears in the calorimetric profile, indicating that lateral phase separations are taking place.

Calcium and pH in anoxic and toxic injury

The critical events that lead to the transition from reversible to irreversible injury remain unclear. Studies are reviewed that have suggested that a rise in cytosolic free Ca2+ initiates plasma membrane bleb formation and a sequence of events that leads ultimately to cell death. In recent studies, we have measured changes in cytosolic free Ca2+, mitochondrial membrane potential, cytosolic pH, and cell surface blebbing in relation to the onset of irreversible injury and cell death following anoxic and toxic injury to single hepatocytes utilizing multiparameter digitized video microscopy (MDVM). MDVM is an emerging new technology that permits single living cells to be labeled with multiple probes whose fluorescence is responsive to specific cellular parameters of interest. Fluorescence images specific for each probe are collected over time, and then digitized and stored. Image analysis and processing then permits quantitation of the spatial distribution of the various parameters within the single living cells. Our results indicate the following: (1) formation of plasma membrane blebs accompanies all types of injury in hepatocytes; (2) cell death is a rapid event, initiated by rupture of a plasma membrane bleb, and is coincident with the onset of irreversible injury; (3) an increase of cytosolic free Ca2+ is not the stimulus for bleb formation or the final common pathway leading to cell death; (4) a decrease of mitochondrial membrane potential precedes loss of cell viability; (5) cytosolic pH falls by more than 1 pH unit during chemical hypoxia. This acidosis protects against the onset of cell death.

The interaction of alpha-tocopherol and homologues with shorter hydrocarbon chains with phospholipid bilayer dispersions. A fluorescence probe study

The intrinsic fluorescence of tocopherol homologues with hydrocarbon chains ranging from 1 carbon (C1) to 16 carbons (alpha-tocopherol, C16) and their ability to quench the fluorescence of 9-anthroyloxy derivatives of fatty acids with the fluorophore located at different positions in the hydrophobic domain of phospholipid bilayers has been used to model the interaction of tocopherol with lipid bilayer membranes. All the tocopherol homologues used, C1, C6, C11 and alpha-tocopherol, showed a similar fluorescence emission intensity at 325 nm in cyclohexane but were almost completely self-quenched by aggregation in water. Fluorescence was restored when dispersions of dimyristoylglycerophosphocholine were added but the maximum intensity was lower with the longer-chain homologues. Full intensity was observed in all homologues on addition of the detergent Triton X-100. Studies using 9-anthracenecarboxylic acid and 9-anthracenecarboxymethyl ester, 6-(9-anthroyloxy)stearic acid and 16-(9-anthroyloxy)palmitelaidic acid showed that the tocopherol homologues partitioned into the hydrophobic domain of phospholipid dispersions composed of dimyristoylglycerophosphocholine at 40 degrees C and dioleoylglycerophosphocholine at 40 degrees C. The 9-anthroyloxy fatty acids and 1,6-diphenyl-1,3,5-hexatriene were quenched by all the homologues and Stern-Volmer plots of the concentration dependence of the quenching indicated that this was predominantly via dynamic processes. No fluorescence energy transfer was observed between diphenylhexatriene and tocopherols but efficient transfer was recorded to the 9-anthroyloxy fatty acid probes. The results are consistent with a model in which the chromanol nucleus of tocopherol is oriented towards the lipid-water interface of the phospholipid bilayer. As the phytol chain length increases there is an increasing tendency for the chromanol nucleus to reside in the hydrophobic interior of the structure. alpha-Tocopherol appears to form clusters within the phospholipid dispersion which are not fluorescent and do not quench the fluorescence of the different fluorescent probes nor transfer fluorescence energy to them. It is suggested that the monomeric form is responsible for the vitamin effects of tocopherol and the aggregated form acts as a reservoir that does not markedly perturb bilayer stability.

Membrane stabilization of vitamin E; interactions of alpha-tocopherol with phospholipids in bilayer liposomes

13C Spin-lattice relaxation times (T1) of 13C-labeled alpha-tocopherol in three kinds of liposomes varying in their contents of arachidoyl residues have been measured by 13C-NMR spectroscopy. On the basis of T1 values, it is proved that the segmental motion of isoprenoid side chain of alpha-tocopherol tends to increase with an increase in the distance from the chromanol moiety, and that three methyl groups attached on the aromatic ring, have some affinity to unsaturated fatty acid residues rather than those of the isoprenoid side chain. These results are incompatible with the hypothesis of Diplock et al. (1) which 4'a- and 8'a-methyl groups of isoprenoid side chain are fitted in the Z-pockets of arachidoyl chain of polyunsaturated lipids in membrane.

A differential scanning calorimetry study of the interaction of alpha-tocopherol with mixtures of phospholipids

When alpha-tocopherol was included in multibilayer vesicles of dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine it induced a broadening of the main transition and a displacement of this transition to lower temperatures, as seen by differential scanning calorimetry. This effect was quantitatively more important in the samples of distearoylphosphatidylcholine than in those of the other phosphatidylcholines. Alpha-Tocopherol when present in equimolar mixtures of dimyristoylphosphatidylcholine and diastearoylphosphatidylcholine, which show monotectic behaviour, preferentially partitions in the most fluid phase. The effect of alpha-tocopherol on the phase transition of dilauroylphosphatidylethanolamine and dipalmitoylphosphatidylethanolamine is qualitatively different of that observed on phosphatidylcholines, and several peaks are observed in the calorimetric profile, probably indicating the formation of separated phases with different contents in alpha-tocopherol. The effect was more apparent in dipalmitoylphosphatidylethanolamine than in dilauroylphosphatidylethanolamine. The inclusion of alpha-tocopherol in equimolar mixtures of dilauroylphosphatidylethanolamine and dipalmitoylphosphatidylcholine, which show cocrystallization, only produced a broadening of the phase transition and a shift to lower temperatures. However, in the case of equimolar mixtures of dipalmitoylphosphatidylcholine which also show cocrystallization, the effect was to cause lateral phase separation with the formation of different mixtures of phospholipids and alpha-tocopherol. Alpha-Tocopherol was also included in equimolar mixtures of phosphatidylethanolamine and phosphatidylcholine showing monotectic behaviour, and in this case alpha-tocopherol preferentially partitioned in the most fluid phase, independently of whether this was composed mainly of phosphatidylcholine or of phosphatidylethanolamine.

Isotopic light scattering of lipid vesicles. Water permeation and effect of alpha-tocopherol

The influence of the index of refraction of the solvent on light scattering properties of lecithin bilayer vesicles is described. Large vesicles (diameter 300 nm) are considered where one lamella separates the intravesicular compartment from the external medium. Stationary and transient cases are distinguished with special emphasis on the isotopic substitution of the solvent, i.e. H2O vs. D2O. Theoretical calculations based on the Mie theory of light-scattering are in accord with results from experiments. The two stationary cases considered serve to calibrate the numerical calculations and illustrate the capability of the method. Transient experiments allow the determination of permeation rates; in particular the D2O/H2O permeability coefficients can be obtained. Single component vesicular lecithin bilayers and ones containing tocopherol are compared. In the crystalline state the incorporation of tocopherol increases the fluidity of the lipid bilayer in parallel with the water permeation rate.

Tocopherol-phospholipid liposomes: maximum content and stability to serum proteins

This study addresses two questions: 1) what is the maximum amount of tocopherol that can be contained in egg phosphatidylcholine liposomes, and 2) what is the stability of these vesicles in the presence of serum proteins? These liposomes, made with a French pressure cell, can contain no more than 33 mol% of tocopherol. Tocopherol changes liposomes in a manner similar to cholesterol, making them larger, less permeable to aqueous dyes and highly resistant to protein-induced disruption. The suppression of protein-induced disruption is more pronounced with tocopherol than with cholesterol, even at lower molar ratios. Thus, liposomes containing alpha tocopherol (15 to 30 mol%) may be useful for delivering physiological quantities of this vitamin to cells in culture or to tissue in vivo.

Effects of pressure on the phase transition of bilayers in liposomes. Influence of cholesterol and alpha-tocopherol

The effects of pressure on the gel-to-liquid crystalline phase transition temperature of dimyristoylphosphatidylcholine (DMPC) bilayers containing cholesterol, alpha-tocopherol, and alpha-tocopheryl acetate were studied by fluorescence depolarization. The transition temperature of cholesterol mixtures (greater than 7.5 mol%) was lower than that of 100% DMPC at atmospheric pressure, but it became higher than the latter on increase in pressure. The thermodynamic parameters of the transition (delta V, delta S, delta H) were estimated and the functions of cholesterol and alpha-tocopherols in the bilayers are discussed.

Transmembrane distribution of alpha-tocopherol in single-lamellar mixed lipid vesicles

A study of the molar ratio dependence of the incorporation of alpha-tocopherol into single-lamellar vesicles showed that the number of molecules which the bilayers can accommodate increased linearly with increasing alpha-tocopherol/phosphatidylcholine initial molar ratios till about 0.05, then approached a saturation limit. At 5 mol%, one alpha-tocopherol molecule per 60 phospholipids can be incorporated into the membranes. Up to this limit the distribution of alpha-tocopherol in the bilayers is uniform, while at initial molar ratios higher than 0.05 a disproportionation toward the inner monolayer of the vesicles is observed. The average outer/total ratio is found to be 0.27 +/- 0.03 at alpha-tocopherol/phosphatidylcholine molar ratios above 0.07 and is similar to asymmetrical distributions that have been reported in vesicles containing other one-chain amphiphiles (e.g., cholesterol). This large disproportionation is in contrast with the packing distribution of certain two-chain amphiphiles, and indicates that one of the driving forces for asymmetry formation in lipid bilayers might be dependent on the number of hydrocarbon chains per amphiphile molecule. A possible reason for the disproportionation effect observed in our experiments is the displacement of unsaturated phospholipids to the outer monolayer of the single-lamellar vesicles, by the more rigid isoprene units of alpha-tocopherol.

The involvement of the lipid phase transition in the plasma-induced dissolution of multilamellar phosphatidylcholine vesicles

Unsonicated liposomes prepared from dimyristoyl phosphatidylcholine were nearly completely dissolved during a 3 h incubation with rat plasma at or close to the phase-transition temperature of 24 degrees C. At 37 or 15 degrees C virtually no liposomal disintegration was observed even after 24 h of incubation. The liposomal solubilization, which was monitored by turbidity measurements or by determination of phospholipid sedimentability, was accompanied by the formation of a phospholipid-protein complex similar or identical to the one we previously reported to be formed from sonicated liposomes of egg phosphatidylcholine (Scherphof, G., Roerdink, F., Waite, M. and Parks, J. (1978) Biochim. Biophys. Acta 542, 296--307). Unsonicated multilamellar liposomes made of egg phosphatidylcholine were completely resistant to the dissolving potency of plasma when incubated at 37 degrees C. Liposomes from equimolar mixtures of dimyristoyl and dipalmitoyl phosphatidylcholine were only degraded by plasma in the temperature range between 30 and 35 degrees C at which temperature this cocrystallizing phospholipid mixture undergoes a phase transition. However, even at these temperatures the rate of dissolution of this mixture was significantly lower than of dimyristoyl phosphatidylcholine at 24 degrees C. In the dissolving process of this mixture a slight preference for the lower-melting component was observed. The ability of cholesterol to completely abolish the susceptibility of dimyristoyl phosphatidylcholine liposomes to plasma at a 1:2 molar ratio of cholesterol to phospholipid substantiates the essential role of the phase transition in the process of liposome solubilization. When liposomes of the monotectic mixtures dimyristoyl and distearoyl phosphatidylcholine or dilauroyl and distearoyl phosphatidylcholine were incubated with plasma at temperatures in between those at which the constituent lipids undergo a phase change in the mixture, the liposomes were slowly dissolved. Under those conditions a selective removal of the lipids in the liquid-crystalline phase was observed. It is concluded that for the plasma-induced dissolution of unsonicated liposomes, which is most probably achieved by interaction with (apo)lipoproteins, the presence of phase boundaries is required in much the same way as was first reported for phospholipases by Op den Kamp, J.A.F., de Gier, J. and Van Deenen, L.L.M. (1974) Biochim. Biophys. Acta 345, 253--256).