Interactions of cholesterol esters with phospholipids: cholesteryl myristate and dimyristoyl lecithin

A spectroscopic study of the composition and conformation of cholesteryl and wax esters purified from meibum

Elucidating wax ester (WE) and cholesteryl ester (CE) compositional, structural and functional relationships is key to our understanding of how these lipids are involved in natural and pathological processes. Little is known about how CE and WE interact with one another. The focus of the present study is to bridge this gap of knowledge. CE and WE were collected from human meibum as a source of esters with complex hydrocarbon chains. MgO column chromatography was used to separate WE and CE. The esters were characterized using ¹H-NMR and Fourier transform infrared spectroscopy. The complexity of the hydrocarbon chains of native WE and CE influenced how changes in the ratio of WE and CE ester influenced some lipid phase transitional parameters but not others. Changes in CE content of WE/CE mixtures undoubtedly modifies the hydrocarbon chain conformation and packing of the mixture. The nature of the change depends on the conformation of the WE and CE. Differences in the complexity of the hydrocarbon chains are likely not to be a major influence on alterations in the order or phase transition temperature when more ordered WE is added to less ordered CE.

Lipid conformational order and the etiology of cataract and dry eye

Lens and tear film lipids are as unique as the systems they reside in. The major lipid of the human lens is dihydrosphingomylein, found in quantity only in the lens. The lens contains a cholesterol to phospholipid molar ratio as high as 10:1, more than anywhere else in the body. Lens lipids contribute to maintaining lens clarity, and alterations in lens lipid composition due to age are likely to contribute to cataract. Lens lipid composition reflects adaptations to the unique characteristics of the lens: no turnover of lens lipids or proteins; the lowest amount of oxygen of any tissue; and contains almost no intracellular organelles. The tear film lipid layer (TFLL) is also unique. The TFLL is a thin (100 nm) layer of lipid on the surface of tears covering the cornea that contributes to tear film stability. The major lipids of the TFLL are wax esters and cholesterol esters that are not found in the lens. The hydrocarbon chains associated with the esters are longer than those found anywhere else in the body (as long as 32 carbons), and many are branched. Changes in the composition and structure of the 30,000 different moieties of TFLL contribute to the instability of tears. The focus of the current review is how spectroscopy has been used to elucidate the relationships between lipid composition, conformational order and function, and the etiology of cataract and dry eye.

Concentration dependent cholesteryl-ester and wax-ester structural relationships and meibomian gland dysfunction

Background

With dry eye, the ratio of cholesteryl ester (CE) to wax ester (WE) decreases substantially in meibum, but the functional and structural consequences of this change are speculative. The aim of this study is to confirm this finding and to bridge this gap in knowledge by investigating the effect of varying CE/WE ratios on lipid structure and thermodynamics.

Methods

Infrared spectroscopy was use to quantify CE and WE in human meibum and to measure hydrocarbon chain conformation and thermodynamics in a cholesteryl behenate, stearyl stearate model system.

Results

The CE/WE molar ratio was 36% lower for meibum from donors with dry eye due to meibomian gland dysfunction compared with meibum from donors without dry eye. CE (5 mol %) dramatically increased the phase transition temperature of pure WE from -0.12 °C to 63 °C in the mixture. Above 5 mol % CB, the phase transition temperature increased linearly, from 68.5 °C to 85 °C. In the ordered state, CE caused an increase in lipid order from about 72% trans rotamers to about 86% trans rotamers. Above 10% CE, the hydrocarbon chains were arranged in a monoclinic geometry.

Conclusions

The CE/WE is lower in meibum from donors with dry eye due to meibomian-gland dysfunction. Major conformational changes in the hydrocarbon chains of wax and cholesteryl ester mixtures begin to occur with just 5% CB and above.

General significance

CE-WE interactions may be important for in understanding lipid layer structure and functional relationships on the surface of tears, skin and plants.

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The CE/WE is lower in meibum from donors with meibomian-gland dysfunction.

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CE may be important for the tear film lipid layer structure and function.

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CE-WE interactions may be important on the surface of tears, skin and plants.

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Conformational changes in WE and CE mixtures begin to occur with just 5% CE.

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CE, cholesteryl ester; WE, wax ester.

Human Meibum Cholesteryl and Wax Ester Variability With Age, Sex, and Meibomian Gland Dysfunction

Purpose

Relationships between tear film lipid (TFL) layer composition, structure, and function could provide insight into the etiology of dry eye. The molar ratio of cholesteryl ester (CE)/wax ester (WE) was measured in meibum from normal donors (M_n) and compared with meibum from donors with meibomian gland dysfunction (M_MGD).

Methods

CE/WE was measured using nuclear magnetic resonance spectroscopy.

Results

CE/WE was distributed into two populations with 81% distributed near 0.55 and 19% near 0.3. CE/WE were higher in donors 13 to 19 years old compared with donors 1 to 12 years old and 20 to 88 years old. CE/WE for M_MGD was 30% lower, 0.34 ± 0.04, compared with M_n, 0.49 ± 0.04. There were no sex differences in CE/WE. There were no significant racial differences between the CE/WE ratios for Asians and Caucasians. The CE/WE ratio was higher for blacks and lower for Hispanics compared to Caucasians. Due to the small number sampled, confirmation of the later racial results is needed. The packing of CE and WE in the TFL layer was proposed.

Conclusions

Although M_MGD contains much less CE than M_n, factors other than the CE content, such as the levels of saturation and/or proteins, may be responsible for the higher order of M_MGD. In addition to saturation, CE could contribute to the increase in order of M_n between 0 and 20 years of age. Observed changes in the meibum content of CE alone is not likely to influence tear film stability.

Composition, structure and substrate properties of reconstituted discoidal HDL with apolipoprotein A-I and cholesteryl ester

To investigate the influence of lipid unsaturation and neutral lipid on the maturation of high density lipoproteins, the discoidal complexes of apoA-I, phosphatidylcholine and cholesteryl ester (CE) were prepared. Saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC), and fluorescent probe cholesteryl 1-pyrenedecanoate (CPD) that forms in a diffusion- and concentration-dependent manner short-lived dimer of unexcited and excited molecules (excimer) were used. The apoA-I/DPPC/CPD complexes were heterogeneous by size, composition and probe location. CPD molecules incorporated more efficiently into larger complexes and accumulated in a central part of the discs. The apoA-I/POPC(PLPC)/CPD were also heterogeneous, however, probe molecules distributed preferentially into smaller complexes and accumulated at disc periphery. The kinetics of CPD transfer by recombinant cholesteryl ester transfer protein (CETP) to human plasma LDL is well described by two-exponential decay, the fast component with a shorter transfer time being more populated in PLPC compared to DPPC complexes. The presence of CE molecules in discoidal HDL results in particle heterogeneity. ApoA-I influences the CETP activity modulating the properties of apolipoprotein-phospholipid interface. This may include CE molecules accumulation in the boundary lipid in unsaturated phosphatidylcholine and cluster formation in the bulk bilayer in saturated phosphatidylcholine.

Atomic Force Microscopy: Interaction Forces Measured in Phospholipid Monolayers, Bilayers and Cell Membranes

Atomic force microscopy (AFM) is a powerful technique which is commonly used to image surfaces at the nanoscale and single-molecule level, as well as to investigate physical properties of the sample surface using a technique known as force spectroscopy. In this chapter, we review our recent research where we used AFM to investigate physical properties of phospholipid monolayers, bilayers, and cell membranes. We describe the experimental procedures for AFM imaging, force measurements, and theoretical models to analyze force spectroscopy data. The data obtained allowed correlations between AFM topography and local adhesion and mechanoelastic properties of supported lipid bilayers in water, supported pulmonary surfactant films in air, and the plasma membrane of epithelial type II cells. Finally, AFM was applied to help elucidate the effect of anesthetics and cholesterol present in the lipid films.

Controlling the Passage of Light through Metal Microchannels by Nanocoatings of Phospholipids

The flow of polarized light through a metal film with an array of microchannels is controlled by the phase of an optically active, phospholipid nanocoating, even though the coating does not cover the open area of the microchannels. The molecular details of the assembly (DPPC phospholipid monolayer/bilayer on a hexadecanethiol monolayer on a copper- or nickel-coated microarray) were determined using the infrared, surface-plasmon-mediated, extraordinary transmission of the metal microarrays. Infrared absorption spectra with greatly enhanced absorptions by comparison to literature were recorded and used as a diagnostic for the phase, composition, and molecular geometry of these nanocoatings. This approach presents new tools for nanoscale construction in constricted microspaces, which may ultimately be useful with individual microchannels.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Effects of substrate composition on the esterification and hydrolysis activity of lysosomal acid sterol ester hydrolase

Lipid microemulsions with various core and surface lipid compositions were prepared by co-sonication of cholesteryl esters, triolein (TO), egg phosphatidylcholine (egg PC), and cholesterol. The heterogeneous emulsion particle mixture was purified by gel filtration and particles with the size and general organization of low density lipoproteins were obtained. These lipid microemulsion particles were used for studies of the cellular metabolism of lipoprotein-derived cholesterol and cholesteryl esters as catalyzed by the enzyme acid sterol ester hydrolase (EC 3.1.1.13). The hydrolysis of cholesteryl oleate (CO) was more than twice and that of cholesteryl linoleate (CL) more than three times faster than the hydrolysis of cholesteryl stearate (CS) over the temperature range 25-39.6 degrees C. Both the synthesis and hydrolysis of cholesteryl esters were insensitive to the physical state of the microemulsion cores. The synthesis of cholesteryl esters by this enzyme was also insensitive to the ratios of cholesterol and egg PC in the microemulsion surface layers. Incorporation of triolein into the microemulsion cholesteryl ester core slightly increased the rate of cholesteryl ester synthesis. A decreasing fatty acyl chain length (C18:0 to C14:0) and an increasing degree of unsaturation (C18:0 to C18:2) enhanced the synthesis rate. It is suggested that the hydrolysis and synthesis of cholesteryl esters in microemulsions (and lipoproteins) take place only in the particle surface layer and that the rate of catalysis is directly dependent on the amount of substrate in this surface layer.

The transfer of lipids from protein-free lipoprotein models to human fibroblasts in culture

Lipid microemulsions were prepared by sonication of mixtures of cholesteryl ester, triacylglycerol, phosphatidylcholine and cholesterol in aqueous dispersions and were purified by gel filtration. The resulting emulsion particles were characterized by differential scanning calorimetry, electron microscopy and analytical gel filtration and were shown to have the size and general organization of low-density lipoprotein. The lipid microemulsions were used as protein-free plasma lipoprotein models for studies of the receptor-independent transfer of lipids to human fibroblasts in culture. The transfer rate of [3H]cholesterol increased with the donor concentration and with the molar ratio between cholesterol and phosphatidylcholine in the donor particles. A maximal transfer value of 1 nmol per mg protein per h was obtained for cholesterol/phosphatidylcholine 1:1 particles. There was a profound temperature effect on the cholesterol transfer. The effect of altering the core lipid of the emulsion particles on the [3H]cholesterol transfer rate was small giving a somewhat higher rate with cholesteryl oleate and cholesteryl stearate than with cholesteryl linoleate. Addition of trioleoylglycerol to the cholesteryl ester core had no effect on the transfer rate. The transfer rate of palmitoyl[14C]oleoylphosphatidylcholine was found to be about 1/5 of that obtained for [3H]cholesterol. About 50% of the cell-associated [14C]cholesteryl oleate was found in the trypsin-releasable pool, while 25% was internalized by the cells at a rate of 0.06 nmol X mg-1 X h-1. Trioleoylglycerol was internalized at the same rate as the cholesteryl ester. Our data suggest that the lipoprotein lipid composition may play a role in the receptor-independent cellular uptake of cholesterol.

Effect of free cholesterol on incorporation of triolein in phospholipid bilayers

Triacylglycerols are the major substrates for lipolytic enzymes that act at the surface of emulsion-like particles such as triglyceride-rich lipoproteins, chylomicrons, and intracellular lipid droplets. This study examines the effect of cholesterol on the solubility of a triacylglycerol, triolein, in phospholipid surfaces. Solubilities of [carbonyl-13C]triolein in phospholipid bilayer vesicles containing between 0 and 50 mol % free cholesterol, prepared by cosonication, were measured by 13C NMR. The carbonyl resonances from bilayer-incorporated triglyceride were shifted downfield in the 13C NMR spectra from those corresponding to excess, nonincorporated material. This enabled solubilities to be determined directly from carbonyl peak intensities at most cholesterol concentrations. The bilayer solubility of triolein was inversely proportional to the cholesterol/phospholipid mole ratio. In pure phospholipid vesicles the triolein solubility was 2.2 mol %. The triglyceride incorporation decreased to 1.1 mol % at a cholesterol/phospholipid mole ratio of 0.5, and at a mole ratio of 1.0 for the bilayer lipids, the triolein solubility was reduced to just 0.15 mol %. The effects of free cholesterol were more pronounced and progressive than observed previously on the bilayer solubility of cholesteryl oleate (Spooner, P. J. R., Hamilton, J. A., Gantz, D. L., & Small, D. M. (1986) Biochim. Biophys. Acta 860, 345-353]. As with cholesteryl oleate, we suggest that cholesterol also displaces solubilized triglyceride to deeper regions of the bilayer.

The effect of free cholesterol on the solubilization of cholesteryl oleate in phosphatidylcholine bilayers: A 13C-NMR study

The solubilization of cholesteryl oleate in sonicated phosphatidylcholine vesicles containing between 0 and 50 mol% cholesterol was studied by 13C-NMR using isotopically enriched [carbonyl-13C]cholesteryl oleate. The carbonyl-13C chemical shift from cholesteryl oleate in the phospholipid/cholesterol bilayer was significantly downfield from that for cholesteryl oleate in an oil phase and the peak area, relative to that of the phospholipid carbonyl, was used to determine bilayer solubility of the ester. The solubility (with respect to phospholipid) in the phospholipid bilayer without cholesterol (2.9 mol%) was only moderately reduced (to 2.3 mol%) at cholesterol levels up to 33 mol% but showed a more marked reduction to 1.4 mol% at 40 mol% cholesterol or 1.2 mol% at 50 mol% cholesterol. Since the vesicles containing 50 mol% cholesterol were larger (520 +/- 152 A diameter) than those with no cholesterol (291 +/- 97 A diameter), we measured the solubility of cholesteryl oleate in large vesicles with no cholesterol, prepared by extrusion through polycarbonate membrane filters, and found it similar to that in small, sonicated vesicles with no cholesterol. Therefore, the larger size of vesicles was not the factor responsible for the decreased cholesteryl oleate solubility at high cholesterol contents. A more direct effect of cholesterol is envisioned where the ester becomes displaced to deeper regions of the bilayer.

Reaction of lecithin: cholesterol acyltransferase with micellar substrates. Effect of particle sizes

Micellar, discoidal complexes were prepared from L-alpha-dipalmitoylphosphatidylcholine (DPPC) or egg phosphatidylcholine (egg-PC), cholesterol, and human apolipoprotein A-I by the cholate dialysis method. Reaction mixtures containing from 70:7:1 to 500:50:1, PC/cholesterol/apolipoprotein A-I (mol/mol) were fractionated by gel-filtration into various complex fractions. The isolated DPPC complexes ranged in size from 103 to 380 A in diameter, and in composition from 70:7:1 to 470:45:1, PC/cholesterol/apolipoprotein A-I (mol/mol), respectively. In contrast, the isolated egg-PC complexes only ranged in size from 105 to 214 A in diameter, and in composition from 65:5:1 to 153:17:1, PC/cholesterol/apolipoprotein A-I (mol/mol), respectively. Measurements of fluorescence wavelength maxima and fluorescence polarization of tryptophan residues of apolipoprotein A-I, in both series of complexes, revealed uniform spectral properties for all the egg-PC containing complexes. The DPPC complexes, on the other hand, had maxima in the fluorescence parameters for complexes with diameters around 200 A. When reacted with purified human lecithin:cholesterol acyltransferase, either at constant apolipoprotein A-I or at constant lipid concentration, all egg-PC complexes had very similar reaction rates, but the DPPC complex series exhibited major differences in reactivity. Minima in reaction rates occurred for DPPC complexes around 200 A in diameter, and optimal rates were observed with the small discoidal complexes (110 A in diameter). These reaction rates correlate well with the apolipoprotein A-I fluorescence properties and indicate that the apolipoprotein structure, reflected at the interface with phosphatidylcholine, may be the most important factor in determining complex reactivity with lecithin:cholesterol acyltransferase.

Comparison of various methods for in vitro cholesteryl ester labeling of lipoproteins from hypercholesterolemic rabbits

Little or no information is available on biologically valid labeling of hypercholesterolemic plasma lipoproteins with cholesteryl ester. The esterification of labeled unesterified cholesterol in hypercholesterolemic rabbit plasma by the lecithin: cholesterol acyltransferase reaction is inefficient. The use of the d greater than 1.063 plasma fraction for this reaction greatly improves the efficiency, but some labeled unesterified cholesterol remains in the end products. The latter disadvantage can be avoided by the addition to whole plasma of labeled cholesteryl ester dissolved in DMSO or acetone. However, in hypercholesterolemic rabbit plasma only a small fraction of the added cholesteryl ester was associated with lipoproteins. When phosphatidylcholine/cholesteryl ester liposomes were incubated with hypercholesterolemic rabbit plasma for 18-24 h at 37 degrees C the labeled cholesteryl ester was quantitatively incorporated into lipoproteins. Chylomicron-like, cholesteryl ester-rich particles were removed by centrifugation (10(6) g X min) and the subsequently isolated d less than 1.019 and d = 1.019-1.063 (LDL) fractions were injected intravenously into normal and hypercholesterolemic rabbits. The disappearance of d less than 1.019 and LDL cholesteryl ester and the appearance of cholesteryl ester in other lipoprotein fractions was indistinguishable from that of in vivo-labeled lipoproteins. In vivo and in vitro cholesteryl ester-labeled lipoproteins were also compared by measuring the exchangeability of their cholesteryl ester with HDL cholesteryl ester in vitro. Equal exchangeability of the two labels was observed in the d less than 1.019 fraction from which the chylomicron-like particles had been removed. These findings demonstrate that when cholesteryl ester is incorporated by the liposome procedure, the distribution of labeled cholesteryl ester within the lipoprotein complex corresponds closely to that of the in vivo-incorporated labeled cholesteryl ester.

Transfer of cholesteryl linoleyl ether from phosphatidylcholine and phosphatidylethanolamine liposomes to cultured cells catalyzed by lipoprotein lipase

Unilamellar liposomes prepared from purified phospholipids (phosphatidylcholine, phosphatidylethanolamine or sphingomyelin) and labeled cholesteryl linoleyl ether were used to study lipoprotein lipase-catalyzed transfer of cholesteryl ester into cells in culture. In mesenchymal rat heart cell cultures, the transfer of cholesteryl linoleyl ether and cholesteryl linoleate was similar and related to the activity of endogenously produced lipoprotein lipase. In human skin fibroblasts transfer of labeled cholesteryl linoleyl ether was proportional to the concentration of milk lipoprotein lipase added to the incubation medium. Liposomes prepared from phosphatidylcholine or phosphatidylethanolamine were much better donors of cholesteryl ether to normal and apolipoprotein E-B receptor-negative fibroblasts and to endothelial cells than those prepared from sphingomyelin. Lysophosphatidylcholine was formed during incubation with milk lipoprotein lipase but was not considered to be directly responsible for the lipoprotein lipase-catalyzed transfer of cholesteryl ether. This conclusion was drawn because in the absence of lipoprotein lipase addition of lysophosphatidylcholine to liposomes, or almost complete phospholipolysis by phospholipase A2, did not result in the transfer of cholesteryl linoleyl ether from liposomes to cells. Attachment of lipoprotein lipase to the cell surface was mandatory for the transfer of cholesteryl ether and could be prevented by heparin. High density apolipoprotein reduced also the transfer of cholesteryl linoleyl ether, even though it did not interfere with the binding of labeled milk lipoprotein lipase to cultured fibroblasts. The present results provide evidence that lipoprotein lipase, and not the products of phospholipid hydrolysis, is the ligand for the non-apolipoprotein E-B receptor-mediated transfer of cholesteryl ester to cells.

Deuterium magnetic resonance of triacylglycerols in phospholipid bilayers

Aqueous dispersions of egg-phosphatidylcholine and egg-phosphatidylcholine/30 mol% cholesterol containing deuterated tripalmitin or triolein were studied at approx. 25 degrees C by 2H-NMR. Incorporation of tripalmitin into egg-phosphatidylcholine bilayers was found to be less than 0.1 mol%, while the incorporation of triolein is approx. 2.5 mol% in the absence and approx. 0.7 mol% in the presence of cholesterol. The profile of order parameter versus chain position for deuterated triolein suggests that the oleoyl chains of the triacylglycerol have an average orientation such that the C2 chain segments and the segments in the vicinity of the C9-C10 double bond are tilted with respect to the bilayer normal, while all other segments are parallel to the bilayer normal. Longitudinal relaxation times were also determined and indicate that the acyl chains of triolein have a motional behaviour similar to that of phospholipid acyl chains in the bilayer.

Interaction of a human plasma lipid transfer protein complex with lipid monolayers

The interaction of a purified human plasma lipid transfer complex with cholesteryl ester, triacylglycerol and phosphatidylcholine in binary and ternary lipid monolayers was investigated. The lipid transfer complex, designated LTC, catalyzes the removal of cholesteryl oleate and triacylglycerol from phosphatidylcholine monolayers. Preincubation of LTC with p-chloromercuriphenyl sulfonate inhibits LTC-catalyzed removal of triacylglycerol; cholesteryl ester removal is not affected. The rate of LTC-facilitated removal of cholesteryl oleate from a phosphatidylcholine monolayer depends on the amount of LTC added to the subphase up to 100 micrograms protein. In addition, the rate of the LTC-catalyzed transfer of cholesteryl oleate to the subphase increases linearly as the amount of cholesteryl oleate in the monolayer increases to 6 mol%. LTC also removes cholesterol from phosphatidylcholine-cholesterol monolayers, albeit at a rate which is 15% of that for removal of cholesteryl oleate. The ability of LTC to facilitate triacylglycerol and cholesteryl ester removal depends on the composition of the monolayer. Phosphatidylcholine supports cholesteryl ester transfer whereas sphingomyelin-cholesteryl ester monolayers are almost refractory to LTC. In contrast, LTC removes triacylglycerol from either a phosphatidylcholine or a sphingomyelin monolayer. The results suggest the existence of at least two lipid transfer proteins, one of which catalyzes the removal of cholesteryl ester and the other triacylglycerol. The role of these proteins as they relate to lipoprotein metabolism is discussed.

Optical detection of phase transitions in simple and mixed lipid-water phases

A simple method for the detection of phase transitions in lipid-water systems by recording the intensity of the transmitted light through a sample placed between crossed polarizers as a function of the temperature was developed. A very small amount of lipid material is sufficient for its effective application. Two zwitterionic lipids in water, dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE) as well as their mixtures were studied by means of this method. The results were compared with differential scanning calorimetry (DSC) data and a good correspondence was established. A phase diagram of the DPPC-DPPE mixture is constructed. This study also throws additional light on the nature of the 'pretransition' of DPPC.

2H- and 31P-NMR studies of cholesteryl palmitate in sphingomyelin dispersions

Dispersions (50 wt% in water) of sphingomyelin/cholesteryl palmitate (95 : 5 mol%) have been studied by 2H- and 31P-NMR spectroscopy between 25 and 60 degrees C. The deuterated esters, cholesteryl palmitate-d31 and cholesteryl palmitate-16, 16, 16-d3, were used for 2H-NMR studies. Of the 5 mol% cholesteryl palmitate added, 1.5 mol% was found to incorporate in the sphingomyelin and this amount remained constant throughout the temperature range studied. The remainder of the cholesteryl palmitate was present as regions of solid. At temperatures above the gel-to-liquid crystalline phase transition of sphingomyelin the NMR spectra indicate that a fraction of the cholesteryl palmitate and sphingomyelin is undergoing rapid isotropic motions. This fraction, which increases with increasing temperature above the phase transition, is probably composed of small bilayer structures. When 50 mol% cholesterol (relative to sphingomyelin) was added to the sphingomyelin/cholesteryl palmitate dispersion, the isotropic component in the NMR spectral was no longer present, the gel-to-liquid crystalline phase transition was removed, and the incorporation of ester in the membrane decreased by more than an order of magnitude.