Influence of cholesterol on bilayers of ester- and ether-linked phospholipids. Permeability and 13C-nuclear magnetic resonance measurements

Crucial Role of the Double Bond Isomerism in the Steroid B-Ring on the Membrane Properties of Sterols. Grazing Incidence X-Ray Diffraction and Brewster Angle Microscopy Studies

Three cholesterol precursors-desmosterol, zymosterol, and lanosterol-were comprehensively characterized in monolayers formed at the air/water interface. The studies were based on registration of the surface pressure (π)-area (A) isotherms complemented with in situ analysis performed with application of modern physicochemical techniques: grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy (BAM). In this approach we were interested in the correlation between molecular structures of the studied sterols found in the cholesterol biosynthetic pathway and their membrane properties. Our results revealed that only desmosterol behaves in Langmuir monolayers comparably to cholesterol, the molecules of which arrange in the monolayers into a hexagonal lattice, while the two remaining sterols possess extremely different properties. We found that molecules of both zymosterol and lanosterol are organized on the water surface in the two-dimensional oblique unit cells despite the fact that they are oriented perpendicular to the monolayer plane. The comparison of chemical structures of the investigated sterols leads to the conclusion that the only structural motive that can be responsible for such unusual behavior is the double bond in the B sterol ring, which is located in desmosterol in a different position from in the other two sterols. This issue, which was neglected in the scientific literature, seems to have crucial importance for sterol activity in biomembranes. We showed that this structural modification in sterol molecules is directly responsible for their adaptation to proper functioning in biomembranes.

Cholesterol-induced condensing and disordering effects on a rigid catanionic bilayer: a molecular dynamics study

Molecular dynamics simulation is applied to investigate the bilayer properties of a novel catanionic vesicle composed of an ion pair amphiphile, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), with cholesterol. Structural properties, such as molecular organization, orientation, and conformation, were analyzed from the resulting trajectory. Simulation results showed that cholesterol could induce both condensing and disordering effects on the rigid HTMA-DS bilayer. The condensing effect of cholesterol was ascribed to the maximizing contact between cholesterol ring and the neighboring hydrocarbon chains. Thus, the inserted cholesterol ring restrained the neighboring hydrocarbon chain segments from motion and increased the order of the neighboring hydrocarbon chains. However, the presence of cholesterol would increase the distance between head groups of HTMA-DS and induce a shift of DS(-) head groups toward the inside of the bilayer. This led to the protrusion of the HTMA(+) head groups and conformational disorder in the front segments of HTMA(+) hydrocarbon chains. In addition, the cholesterol-induced void in the hydrophobic core of the HTMA-DS bilayer increased the motion freedom of the terminal segments of the hydrocarbon chains. The cholesterol-induced space in the polar region and void in the nonpolar region of the bilayer led to a conformational disorder. With high cholesterol contents, the conformational disorder effect would overwhelm the condensing effect, resulting in the apparent disordering effect on the rigid HTMA-DS bilayer.

Influence of cholesterol on the collective dynamics of the phospholipid acyl chains in model membranes

We have studied the packing and collective dynamics of the phospholipid acyl chains in a model membrane composed of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and cholesterol in varied phase state. After a structural characterization of this two-component model bilayer using X-ray reflectivity, we have carried out coherent inelastic neutron scattering to investigate the chain dynamics. Both DMPC/cholesterol membranes exhibited much sharper and more pronounced low-energy inelastic excitations than a pure DMPC membrane. In the high-energy regime above 10 meV, the insertion of cholesterol into the membrane was found to shift the position of the inelastic excitation towards values otherwise found in the pure lipids gel phase. Thus, the dissipative collective short-range dynamics of the acyl chains is strongly influenced by the presence of cholesterol.

Understanding sterol-membrane interactions, part II: complete 1H and 13C assignments by solid-state NMR spectroscopy and determination of the hydrogen-bonding partners of cholesterol in a lipid bilayer

The complete assignment of cholesterol 1H and 13C NMR resonances in a lipid bilayer environment (Lalpha-dimyristoylphosphatidylcholine/cholesterol 2:1) has been obtained by a combination of 1D and 2D MAS NMR experiments: 13C spectral editing, ge-HSQC, dipolar HETCOR and J-based HETCOR. Specific chemical shift variations have been observed for the C1-C6 atoms of cholesterol measured in CCl4 solution and in the membrane. Based on previous work (F. Jolibois, O. Soubias, V. Reat, A. Milon, Chem. Eur. J. 2004, 10, preceding paper in this issue: DOI: 10.1002/chem.200400245) these variations were attributed to local changes around the cholesterol hydroxy group, such as the three major rotameric states of the C3-O3 bond and different hydrogen bonding partners (water molecules, carboxy and phosphodiester groups of phosphatidylcholine). Comparison of the experimental and theoretical chemical shifts obtained from quantum-chemistry calculations of various transient molecular complexes has allowed the distributions of hydrogen bonding partners and hydroxy rotameric states to be determined. This is the first time that the probability of hydrogen bonding occurring between cholesterol's hydroxy group and phosphatidylcholine's phosphodiester has been determined experimentally.

Cholesterol effects on a mixed-chain phosphatidylcholine bilayer: a molecular dynamics simulation study

A molecular dynamics simulation of a mono-cis-unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer containing approximately 22 mol% of cholesterol (POPC-Chol) was carried out for 15 ns. An 8-ns trajectory was analysed to determine the effects of Chol on the membrane properties and compare it with that on the fully saturated 1,2-dimyristoyl-phosphatidylcholine bilayer containing approximately 22 mol% of Chol (DMPC-Chol). The study suggests that the experimentally observed weaker effect of Chol on the POPC than DMPC bilayer might result from a different vertical localisation of the Chol hydroxyl group (OH-Chol) in both bilayers: in the POPC-Chol bilayer, OH-Chol is placed approximately 3 A higher in the bilayer interface than in the DMPC-Chol bilayer. Because of the rigid cis double bond in the beta-chain of POPC, Chol fits worse to the POPC-Chol membrane environment and is pushed up, in effect all Chol ring atoms are, on average, located above the double bond. Both in mono-cis-unsaturated and fully saturated PC bilayers, Chol induces stronger van der Waals interactions among the chains, whereas its interactions with the chains are weak. In contrast to DMPC, the smooth alpha-face of the Chol ring lowers the order of POPC chains, whereas the rough beta-face increases the order.

Impact of membrane cholesterol content on the resistance of vesicles to surfactant attack

Vesicle leakage experiments were carried out to establish how cholesterol content regulates membrane permeability as induced by surfactant exposure. Vesicles containing up to 50% cholesterol were examined. Four different surfactants were chosen as membrane perturbants, including nonoxynol-9 which is commonly used in spermicidal formulations. As part of this study, we establish that the extrusion procedure commonly used to fabricate unilamellar vesicles does not unintentionally alter the desired composition of these model membrane systems. The kinetics of the leakage process is well characterized by a single exponential rate of release, similar to the form seen in the absence of membrane cholesterol. Our leakage experiments show that membranes become more resistant toward surfactant attack, in direct proportion with cholesterol content. This rise in resistance is surfactant specific. Above 30%, all membranes show positive deviation from the linear increase in resistance with increasing cholesterol content. Two other sterols, dihydrocholesterol and coprostanol, were also found to increase membrane resistance and behaved similarly despite a key difference in molecular structure. A peculiar leakage response was observed when membranes were exposed to the surfactant sodium dodecyl sulfate (SDS) above its critical micelle concentration. Our findings support the hypothesis that SDS micelles solubilize phospholipid molecules, creating a membrane with higher cholesterol content that is extremely resistant to perturbation.

Dietary gangliosides increase the content and molecular percentage of ether phospholipids containing 20:4n-6 and 22:6n-3 in weanling rat intestine

This study was conducted to determine whether dietary ganglioside (GG) increases the content of ether phospholipids (EPL) in intestinal mucosa. Weanling Sprague-Dawley rats were fed a semipurified diet consisting of 20% fat as a control diet. Two experimental diets were formulated by adding either 0.1% (w/w fat) GGs (GG diet) or 1.0% (w/w fat) sphingomyelin (SM diet) to the control diet. Fatty acid methyl esters from the alkenylacyl, alkylacyl and diacyl subclasses of phospholipids were measured to determine total and molecular percentage of EPL comprising the choline phosphoglyceride (CPG) and ethanolamine phosphoglyceride (EPG) fraction. Animals fed the GG diet significantly increased total EPL content both in CPG (by 36%) and in EPG (by 66%), and the molecular percentage of EPL in CPG (by 76%) and in EPG (by 59%) compared to animals fed the control diet. Dietary GG-induced increase in EPL resulted in a higher level of polyunsaturated fatty acids (PUFA) specifically in 20:4n-6 and 22:6n-3 compared to control animals, leading to a decrease in the ratio of saturated fatty acids (SFA) to PUFA both in CPG and in EPG. Feeding animals the SM diet showed a higher level of EPL than control animals with a concomitant increase in 22:6n-3 in EPL. The present data demonstrate that dietary GG increases the content and composition of EPL containing PUFA in the weanling rat intestine.

Sphingomyelin modulates the transbilayer distribution of galactosylceramide in phospholipid membranes

The interrelationships among sphingolipid structure, membrane curvature, and glycosphingolipid transmembrane distribution remain poorly defined despite the emerging importance of sphingolipids in curved regions and vesicle buds of biomembranes. Here, we describe a novel approach to investigate the transmembrane distribution of galactosylceramide in phospholipid small unilamellar vesicles by (13)C NMR spectroscopy. Quantitation of the transbilayer distribution of [6-(13)C]galactosylceramide (99.8% isotopic enrichment) was achieved by exposure of vesicles to the paramagnetic ion, Mn(2+). The data show that [6-(13)C]galactosylceramide prefers (70%) the inner leaflet of phosphatidylcholine vesicles. Increasing the sphingomyelin content of the 1-palmitoyl-2-oleoyl-phosphatidylcholine vesicles shifted galactosylceramide from the inner to the outer leaflet. The amount of galactosylceramide localized in the inner leaflet decreased from 70% in pure 1-palmitoyl-2-oleoyl-phosphatidylcholine vesicles to only 40% in 1-palmitoyl-2-oleoyl-phosphatidylcholine/sphingomyelin (1:2) vesicles. The present study demonstrates that sphingomyelin can dramatically alter the transbilayer distribution of a monohexosylceramide, such as galactosylceramide, in 1-palmitoyl-2-oleoyl-phosphatidylcholine/sphingomyelin vesicles. The results suggest that sphingolipid-sphingolipid interactions that occur even in the absence of cholesterol play a role in controlling the transmembrane distributions of cerebrosides.

Cholesterol effects on the phosphatidylcholine bilayer nonpolar region: a molecular simulation study

A 15-ns molecular dynamics (MD) simulation of the fully hydrated dimyristoylphosphatidylcholine-cholesterol (DMPC-Chol) bilayer in the liquid-crystalline state was carried out to investigate the effect of Chol on the hydrocarbon chain region of the bilayer. The last 8-ns fragment of the generated trajectory was used for analyses. As a reference system, a pure DMPC bilayer (M. Pasenkiewicz-Gierula, Y. Takaoka, H. Miyagawa, K. Kitamura, and A. Kusumi, 1999, Biophys. J. 76:1228-1240) simulated for 14 ns was used. The study shows that a Chol-induced increase of the bulk molecular order parameter along both beta- and gamma-chain is mainly caused by a decrease of the average tilt of the chains, because the bulk average number of gauche rotamers/myristoyl chain is not significantly changed by Chol. Nevertheless, for DMPCs located near Chol molecules both the number of gauche rotamers/chain and the chain tilt are decreased. The magnitude of the Chol effect on the PC alkyl chains depends, in addition to the PC-Chol distance, on the side of the Chol molecule (alpha- or beta-face) that the chains are in contact with. This study provides some new insight into the properties of the coexistence region of the partial phase diagram for DMPC-Chol bilayers.

Interactions between cholesterol and lipids in bilayer membranes. Role of lipid headgroup and hydrocarbon chain-backbone linkage

We have employed four lipids in the present study, of which two are cationic and two bear phosphatidylcholine (PC) headgroups. Unlike dipalmitoylphosphatidylcholine, the other lipids employed herein do not have any ester linkage between the hydrocarbon chains and the respective lipid backbones. Small unilamellar vesicles formed from each of the PC and cationic lipids with or without varying amounts of cholesterol have been examined using the steady-state fluorescence anisotropy method as a function of temperature. The anisotropy data clearly indicate that the order in the lipid bilayer packing is strongly affected upon inclusion of cholesterol. This effect is similar irrespective of the electrostatic character of the lipid employed. The influence of cholesterol inclusion on multi-lamellar lipid dispersions has also been examined by 1H-nuclear magnetic resonance spectroscopy above the phase transition temperatures. With all the lipids, the line widths of (CH2)n protons of hydrocarbon chains in the NMR spectra respond to the addition of cholesterol to membranes. The influence on the bilayer widths of various lipids upon inclusion of cholesterol was determined from X-ray diffraction studies of the cast films of the lipid-cholesterol coaggregates in water. The effect of cholesterol on the efflux rates of entrapped carboxyfluorescein (CF) from the phospholipid vesicles was determined. Upon incremental incorporation of cholesterol into the phospholipid vesicles, the CF leakage rates were progressively reduced. Independent experiments measuring transmembrane OH- ion permeation rates from cholesterol-doped cationic lipid vesicles using entrapped dye riboflavin also demonstrated that the addition of cholesterol into the cationic lipid vesicles reduced the leakage rates irrespective of lipid molecular structure. It was found that the cholesterol induced changes on the membrane properties such as lipid order, linewidth broadening, efflux rates, bilayer widths, etc., did not depend on the ability of the lipids to participate in the hydrogen bonding interactions with the 3beta-OH of cholesterol. These findings emphasize the importance of hydrophobic interaction between lipid and cholesterol and demonstrate that it is not necessary to explain the observed cholesterol induced effects on the basis of the presence of hydrogen bonding between the 3beta-OH of cholesterol and the lipid chain-backbone linkage region or headgroup region.

Cholesterol effects on the phosphatidylcholine bilayer polar region: a molecular simulation study

A molecular dynamics (MD) simulation of a fully hydrated, liquid-crystalline dimyristoylphosphatidylcholine (DMPC)-Chol bilayer membrane containing approximately 22 mol% Chol was carried out for 4.3 ns. The bilayer reached thermal equilibrium after 2.3 ns of MD simulation. A 2.0-ns trajectory generated during 2.3-4.3 ns of MD simulation was used for analyses to determine the effects of Chol on the membrane/water interfacial region. In this region, 70% of Chol molecules are linked to DMPC molecules via short-distance interactions, where the Chol hydroxyl group (OH-Chol) is 1) charge paired to methyl groups of the DMPC choline moiety ( approximately 34%), via the hydroxyl oxygen atom (Och); 2) water bridged to carbonyl ( approximately 19%) and nonester phosphate ( approximately 14%) oxygen atoms, via both Och and the hydroxyl hydrogen atom (Hch); and 3) directly hydrogen (H) bonded to carbonyl ( approximately 11%) and nonester phosphate ( approximately 5%) oxygen atoms, via Hch ( approximately 17% of DMPC-Chol links are multiple). DMPC's gamma-chain carbonyl oxygen atom is involved in 44% of water bridges and 51% of direct H bonds formed between DMPC and Chol. On average, a Chol molecule forms 0.9 links with DMPC molecules, while a DMPC molecule forms 2.2 and 0.3 links with DMPC and Chol molecules, respectively. OH-Chol makes hydrogen bonds with 1.1 water molecules, preferentially via Hch. The average number of water molecules H bonded to the DMPC headgroup is increased by 7% in the presence of Chol. These results indicate that inclusion of Chol decreases interlipid links and increases hydration in the polar region of the membrane.

Membrane fusion in vesicles of oligomerizable lipids

Membrane fusion has been examined in a model system of small unilamellar vesicles of synthetic lipids that can be oligomerized through the lipid headgroups. The oligomerization can be induced either in both bilayer leaflets or in the inner leaflet exclusively. Oligomerization leads to denser lipid headgroup packing, with concomitant reduction of lipid lateral diffusion and membrane permeability. As evidenced by lipid mixing assays, electron microscopy, and light scattering, calcium-induced fusion of the bilayer vesicles is strongly retarded and inhibited by oligomerization. Remarkably, oligomerization of only the inner leaflet of the bilayer is already sufficient to affect fusion. The efficiency of inhibition and retardation of fusion critically depend on the relative amount of oligomeric lipid present, on the concentration of calcium ions, and on temperature. Implications for the mechanism of bilayer membrane fusion are discussed in terms of lipid lateral diffusion and membrane curvature effects.

The influence of cholesterol on phospholipid membrane curvature and bending elasticity

The behavior of dioleoylphosphatidylethanolamine (DOPE)/cholesterol/tetradecane and dioleoylphosphatidylcholine (DOPC)/cholesterol/tetradecane were examined using x-ray diffraction and the osmotic stress method. DOPE/tetradecane, with or without cholesterol, forms inverted hexagonal (HII) phases in excess water. DOPC/tetradecane forms lamellar phases without cholesterol at lower temperatures. With tetradecane, as little as 5 mol% cholesterol in DOPC induced the formation of HII phases of very large dimension. Increasing levels of cholesterol result in a systematic decrease in the HII lattice dimension for both DOPE and DOPC in excess water. Using osmotic pressure to control hydration, we applied a recent prescription to estimate the intrinsic curvature and bending modulus of the HII monolayers. The radii of the intrinsic curvature, RPO, at a pivotal plane of constant area within the monolayer were determined to be 29.4 A for DOPE/tetradecane at 22 degrees C, decreasing to 27 A at 30 mol% cholesterol. For DOPC/tetradecane at 32 degrees C, RPO decreased from 62.5 A to 40 A as its cholesterol content increased from 30 to 50 mol%. These data yielded an estimate of the intrinsic radius of curvature for pure DOPC of 87.3 A. The bending moduli kc of DOPE/tetradecane and DOPC/tetradecane, each with 30 mol% cholesterol, are 15 and 9 kT, respectively. Tetradecane itself was shown to have little effect on the bending modulus in the cases of DOPE and cholesterol/DOPE. Surprisingly, cholesterol effected only a modest increase in the kc of these monolayers, which is much smaller than estimated from its effect on the area compressibility modulus in bilayers. We discuss possible reasons for this difference.

Role of apolipoprotein A-I in cholesterol transfer between lipoproteins. Evidence for involvement of specific apoA-I domains

A series of monoclonal antibodies against epitopes spanning different domains of apoA-I have been tested for their effects on unesterified cholesterol transfer between low density lipoprotein (LDL) and well-defined homogenous lipoproteins reconstituted with phosphatidylcholine, cholesterol, and apoA-I (LpA-I). Antibodies 2G11 (reacting between residues 25 and 110), A05 (residues 25-82), A03 (residues 135-140), A44 and r5G9 (residues 149-186), and 4A12 (residues 173-205) significantly inhibit cholesterol transfer from LDL to Lp2A-I while they enhance transfer in the opposite direction, thus causing an increased net transfer to LDL. Most of these monoclonal antibodies (mAbs) also enhance phospholipid transfer to LDL but in a lesser and variable proportion relative to cholesterol. Their epitopes are mainly contained within domains that are predicted to be amphipathic alpha-helices. In contrast, mAbs 4H1 (residues 2-8), 3G10 (residues 96-121), and 5F6 (residues 116-141) have little or no effect on either cholesterol or phospholipid transfer, and the epitopes for these three mAbs have been shown in earlier studies to be structurally and functionally related. Their immunoreactivity responds similarly to variation in lipoprotein cholesterol content, and the antibodies binding to these sites compete with one another and have similar effects on the cholesterol esterification reaction. Thus, the current results are compatible with the hypothesis that they form an integrated domain with a common function in cholesterol metabolism, possibly as part of a hinge domain. Most mAbs were found to increase significantly the alpha-helicity of apoA-I in the Lp2A-I immunecomplexes, suggesting that they may increase the stability of the lipid-bound apoA-I. However, not unexpectedly, there is no correlation between the effects of mAbs on alpha-helicity and their effects on cholesterol or phospholipid transfer since each mAb has a discrete effect on these transfers. These studies demonstrate the specificity of LpA-I particles in cholesterol transport and document the existence of apoA-I domains with different functions in cholesterol transport.

Effect of LpA-I composition and structure on cholesterol transfer between lipoproteins

The effect of high density lipoprotein composition on the rates of unesterified cholesterol exchange between low density lipoproteins (LDL) and well-defined homogeneous discoidal lipoproteins (LpA-I) reconstituted with phosphatidylcholine, cholesterol, and apolipoprotein A-I (apoA-I) has been investigated. LpA-I containing cholesterol and 2, 3, and 4 apoA-I molecules per particle differed in their ability to accept or donate cholesterol. A significant cholesterol exchange occurs between LDL and Lp2A-I (7.8 and 9.6 nm), while there is little or no cholesterol exchange detectable between LDL and Lp3A-I (10.8 and 13.4 nm) and Lp4A-I (17.0 nm) complexes. The cholesterol transfer from LDL to the cholesterol-free Lp2A-I (9.6 nm), Lp3A-I (13.4 nm), and Lp4A-I (17.0 nm) particles also shows significant cholesterol transfer to Lp2A-I, while there is no detectable transfer to Lp3- and 4A-I particles. The rates of cholesterol transfer to cholesterol-free and cholesterol-containing Lp2A-I appear to differ significantly. Cholesterol transfer from LDL to cholesterol-free Lp2A-I is zero order with respect to acceptor concentrations when the Lp2A-I/LDL ratio is above 10. Transfer rates from LDL to cholesterol-free Lp2A-I are faster for the smaller Lp2A-I (8.5 nm) than to the larger Lp2A-I (9.7 nm) and exhibit half-times (t1/2) at 25 degrees C of 4.0 and 5.3 h, respectively. In contrast, cholesterol transfer from LDL to cholesterol-containing Lp2A-I remains dependent upon acceptor concentrations to an acceptor/donor particle ratio of 80. In addition, transfer from LDL to cholesterol-containing Lp2A-I is faster to the 9.6 nm than to 7.8 nm particles, with t1/2 of 1.4 and 2.3 h, respectively. The rates of cholesterol transfer from Lp2A-I to LDL are higher than in the opposite direction, in particular for the small Lp2A-I (7.8 nm), which has a t1/2 of approximately 50 min. The results show that changes in the composition and structure of apoA-I-containing particles have a significant effect on inter-lipoprotein exchange of cholesterol. This suggests that the kinetics of cholesterol transfer to and from reconstituted discoidal LpA-I particles cannot be fully explained by passive aqueous diffusion.

Ether lipids in biomembranes

Plasmalogens (1-O-1'-alkenyl-2-acylglycerophospholipids) and to a lesser extent the 1-O-alkyl analogs are ubiquitous and in some cases major constituents of mammalian cellular membranes and of anaerobic bacteria. In archaebacteria polar lipids of the cell envelope are either diphytanylglycerolipids or bipolar macrocyclic tetraether lipids capable of forming covalently linked 'bilayers'. Information on the possible role of ether lipids as membrane constituents has been obtained from studies on the biophysical properties of model membranes consisting of these lipids. In addition, effects of modified ether lipid content on properties of biological membranes have been investigated using microorganisms or mammalian cells which carry genetic defects in ether lipid biosynthesis. Differential utilization of ether glycerophospholipids by specific phospholipases might play a role in the generation of lipid mediators that are involved in signal transduction. A possible function of plasmalogens as antioxidants has been demonstrated with cultured cells and might play a role in serum lipoproteins. Synthetic ether lipid analogs exert cytostatic effects, most likely by interfering with membrane structure and by specific interaction with components of signal transmission pathways, such as phospholipase C and protein kinase C.

Synthesis of 1-palmitoyl-2-hexadecyl-sn-glycero-3-phosphocholine (PHPC)

A general method for the chirospecific synthesis of 1-acyl-2-alkyl-sn-glycero-3-phosphocholines is described. 1-Palmitoyl-2-hexadecyl-sn-glycero-3-phosphocholine (PHPC) was synthesized in 18% overall yield in ten steps via five new synthetic intermediates, and 1-acetyl-2-hexadecyl-sn-glycero-3- phosphocholine (AHPC) was also synthesized. 1-Acyl-2-alkyl-sn-glycero-3-phosphocholines, which have not been found to exist in nature, are ether lipid analogs of 1,2-diacyl-sn-glycero-3-phosphocholines, which are important components of cell membranes. Biophysical studies of hydrated bilayers of PHPC will be of interest in probing the critical importance of the central region of these amphiphilic molecules to the molecular assemblies that are formed.

13C-NMR determination of the molecular dynamics of cholesterol in dimyristoylphosphatidylcholine vesicles

Using 13C-NMR measurements of T1, T2 and the nuclear Overhauser enhancement factor at 50.32, 90.56 and 150.87 MHz, we have measured the dynamics of cholesterol in dimyristoylphosphatidylcholine (DMPC) vesicles from 28 to 50 degrees C. Using the model-free approach of Lipari and Szabo, we have found that at 37 degrees C the motion of the rigid steroid ring can be described by an equal contribution from two effective motions with correlation times of 63 and 0.85 ns. The C26 and C27 carbon atoms of cholesterol were found to have an effective correlation time of 8 +/- 2 ps and a value for the square of the generalised order parameter of 0.03 +/- 0.01. The corresponding values for the C25 carbon atom were 17 +/- 4 ps and 0.09 +/- 0.02, showing slower motion and greater order for this carbon atom, which is nearer to the rigid steroid ring. Apart from the effect of vesicle size on T2, no concentration dependence of the dynamics of cholesterol was detected over the cholesterol concentration range 2-30 mol%. The order parameters and correlation times from the present 13C-NMR experiments are shown to be compatible with those from 2H-NMR experiments. This establishes the validity of the present approach, which we are currently extending to low concentrations of cholesteryl oleate in DMPC vesicles.

Role of cell cholesterol in modulating vincristine uptake and resistance

The relationship between cell-membrane permeability to vincristine and cholesterol/phospholipid levels was studied in L5178Y murine leukemic lymphoblasts and in 2 multidrug-resistant cell sublines, VCR/P60 and VCR/P200, which expressed increasing levels of vincristine resistance. The uptake of 3H-vincristine was measured in all cell lines and in cholesterol-depleted and -reloaded L5178Y and VCR/P200 cells. The initial rate of drug entry in resistant cells was lower than that measured in the parental cell line and it decreased as the relative resistance increased. An increment of cholesterol content, characterized in resistant cells, was directly proportional to the relative resistance to vincristine. Cholesterol depletion in both sensitive and resistant cells resulted in an increase in the rate of vincristine uptake, which reverted to the respective basal levels when each cell line was cholesterol-reloaded. The rate of drug uptake was inversely correlated with the molar ratio of cholesterol to phospholipids. Although both VCR/P cell sublines, but not the sensitive parental cells, expressed the P-glycoprotein in their plasma membrane, there were no differences in drug efflux and retention between resistant and parental cells. These results indicate that cholesterol modulates the permeation of vincristine through the plasma membrane and strongly suggest that increased levels of cholesterol/phospholipid account for the lower drug accumulation and greater resistance in these multidrug-resistant cells.

Interaction of cholesterol with conformationally restricted phospholipids in vesicles

The interaction of cholesterol with conformationally restricted analogs of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in the liquid-crystalline phase has been studied in vesicles. These analogs contain one of three cyclopentane triols in place of the glycerol moiety found in natural phospholipids and make possible an analysis of whether a limitation of the conformational mobility in the glycerol backbone region affects the interaction with cholesterol. When cholesterol was incorporated into vesicles from cyclopentanoid phospholipids in which the acyl group vicinal to the head group is trans, the first-order rate constant for Cl- efflux is decreased similarly to that in vesicles from 'natural' DPPC or DPPG (about 50%). However, when the head group is in the unnatural 2 position, cholesterol has a much smaller effect on the rate of Cl- efflux (a decrease of about 20%). Cholesterol decreased the rate constants for valinomycin-mediated 86Rb+ efflux from vesicles of the cyclopentanoid PC analogs and of DPPC to a similar extent. The half-time values for spontaneous intervesicle cholesterol exchange were not markedly different using vesicles prepared with the natural glycerophospholipids and with the cyclopentano-phospholipids, suggesting that the geometrical orientation of the acyl chains or the head group has little influence on cholesterol desorption from the lipid/water interface.

Cholesterol transport between cells and high-density lipoproteins

Various types of studies in humans and animals suggest strongly that HDL is anti-atherogenic. The anti-atherogenic potential of HDL is thought to be due to its participation in reverse cholesterol transport, the process by which cholesterol is removed from non-hepatic cells and transported to the liver for elimination from the body. Extensive studies in cell culture systems have demonstrated that HDL is an important mediator of sterol transport between cells and the plasma compartment. The topic of this review is the mechanisms that account for sterol movement between HDL and cells. The most prominent and easily measured aspect of sterol movement between HDL and cells is the rapid bidirectional transfer of cholesterol between the lipoprotein and the plasma membrane. This movement occurs by unmediated diffusion, and in most situations its rate in each direction is limited by the rate of desorption of sterol molecules from the donor surface into the adjacent water phase. The net transfer of sterol mass out of cells occurs when there is either a relative enrichment of sterol within the plasma membrane or a depletion of sterol in HDL. Recent studies suggest that certain minor subfractions of HDL (with pre-beta mobility on agarose gel electrophoresis and containing apoprotein A-I but no apo A-II) are unusually efficient at promoting efflux of cell sterol. To what extent efflux to these HDL fractions is balanced by influx from the lipoprotein has not yet been established clearly. The prevention and reversal of atherosclerosis require the mobilization of cholesterol from internal (non-plasma membrane) cellular locations. To some extent, this may involve the retroendocytosis of HDL. However, most mobilization probably involves the transport of internal sterol to the plasma membrane, followed by desorption to extracellular HDL. Several laboratories are investigating the transport of sterol from intracellular locations to the plasma membrane. Studies on biosynthetic sterol (probably originating mostly in the smooth endoplasmic reticulum) suggest that there is rapid transport to the plasma membrane in lipid-rich vesicles. Important features of this transport are that it bypasses the Golgi apparatus and may be positively regulated by the specific binding of HDL to the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane electrostatics

In conclusion, charged membrane together with their adjacent electrolyte solution form a thermodynamic and physico-chemical entity. Their surfaces represent an exceptionally complicated interfacial system owing to intrinsic membrane complexity, as well as to the polarity and often large thickness of the interfacial region. Despite this, charged membranes can be described reasonably accurately within the framework of available theoretical models, provided that the latter are chosen on the basis of suitable criteria, which are briefly discussed in Section A. Interion correlations are likely to be important for the regular and/or rigid, thin membrane-solution interfaces. Lateral distribution of the structural membrane charge is seldom and charge distribution perpendicular to the membranes is nearly always electrostatically important. So is the interfacial hydration, which to a large extent determines the properties of the innermost part of the interfacial region, with a thickness of 2-3 nm. Fine structure of the ion double-layer and the interfacial smearing of the structural membrane charge decrease whilst the surface hydration increases the calculated value of the electrostatic membrane potential relative to the result of common Gouy-Chapman approximation. In some cases these effects partly cancel-out; simple electrostatic models are then fairly accurate. Notwithstanding this, it is at present difficult to draw detailed molecular conclusions from a large part of the published data, mainly owing to the lack of really stringent controls or calibrations. Ion binding to the membrane surface is a complicated process which involves charge-charge as well as charge-solvent interactions. Its efficiency normally increases with the ion valency and with the membrane charge density, but it is also strongly dependent on the physico-chemical and thermodynamic state of the membrane. Except in the case of the stereospecific ion binding to a membrane, the relatively easily accessible phosphate and carboxylic groups on lipids and integral membrane proteins are the main cation binding sites. Anions bind preferentially to the amine groups, even on zwitterionic molecules. Membrane structure is apt to change upon ion binding but not always in the same direction: membranes with bound ions can either expand or become more condensed, depending on the final hydrophilicity (polarity) of the membrane surface. The more polar membranes, as a rule, are less tightly packed and more fluid. Diffusive ion flow across a membrane depends on the transmembrane potential and concentration gradients, but also on the coulombic and hydration potentials at the membrane surface.(ABSTRACT TRUNCATED AT 400 WORDS)

Organization and interaction of cholesterol and phosphatidylcholine in model bilayer membranes

The molecular organization of sterols in liposomes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) at 37 degrees C is examined by utilizing the fluorescent analogue of cholesterol cholesta-5,7,9-trien-3 beta-ol (cholestatrienol). (1) Cholestatrienol is shown to be indistinguishable from native cholesterol in terms of its ability to condense POPC, as determined by (i) pressure/area studies of mixed-lipid monolayers and (ii) its ability to increase the order of POPC bilayers (determined by electron spin resonance studies) whether on its own or admixed with cholesterol at various ratios. (2) By analysis of the perturbation of the absorption spectra, cholestatrienol was found to be freely miscible in aggregates of cholesterol in buffer. In contrast, a lack of any detectable direct interaction of the sterol molecules in POPC bilayers was detected. (3) Fluorescence intensity and lifetime measurements of POPC/sterol (1:1 mol/mol) at various cholesterol/cholestratrienol molar ratios (0.5:1 up to 1:1 cholestatrienol/POPC) confirmed that sterol molecules in the membrane matrix were not associated to any great degree. (4) A quantitative estimate of how close sterol molecules approach each other in the membrane matrix was evaluated from the concentration dependence of the steady-state depolarization of fluorescence and was found to be 10.6 A. From geometrical considerations, the sterol/phospholipid phase at 1:1 mol/mol is depicted as each sterol having four POPC molecules as nearest neighbors. We term this arrangement of the lipid matrix an "ordered bimolecular mesomorphic lattice". (5) The concentration dependence of depolarization of fluorescence of cholestatrienol in POPC liposomes in the absence of cholesterol yielded results that were consistent with the cholestatrienol molecules being homogeneously dispersed throughout the phospholipid phase at sterol/POPC ratios of less than 1:1. (6) From qualitative calculations of the van der Walls' hydrophobic interactions of the lipid species, the phospholipid condensing effect of cholesterol is postulated to arise from increased interpenetration of the flexible methylene segments of the acyl chains, as a direct result of their greater mutual attraction compared to their attraction for neighboring sterol molecules. (7) The interdependence of the ordered bimolecular mesomorphic lattice and the acyl chain condensation is discussed in an effort to understand the ability of cholesterol to modulate the physical and mechanical properties of biological membranes.

Hydrogen bonding between anhydrous cholesterol and phosphatidylcholines: an infrared spectroscopic study

Fourier transform infrared spectroscopy performed with a high pressure diamond anvil cell was used to study hydrogen bonding between anhydrous phosphatidylcholines and cholesterol at the molar ratio 4:1. The hydroxyl group of cholesterol which acts as a proton donor, engages in strong hydrogen bonding to the sn-2 chain carbonyl group of DMPC, DPPC and HPPC and in weak hydrogen bonding to the phosphate group of all these phospholipids. No evidence of hydrogen bonding between cholesterol and the sn-1 chain carbonyl group of DMPC and DPPC was found. From a comparison of the relative hydrogen-bond strengths between cholesterol or water and the sn-2 chain carbonyl and phosphate groups of all these phospholipids, it is predicted that in aqueous dispersions of cholesterol containing phospholipids, the hydrogen bond of cholesterol to the phosphate group would be replaced by that of water, while the hydrogen bond of cholesterol to the sn-2 chain carbonyl group would remain intact.

Influence of PAF-acether on lecithin-oriented multibilayers monitored by ESR: interaction of PAF-acether with cholesterol

The influence of PAF-acether on natural ovolecithin oriented multibilayers is detected by ESR of intercalated 5 doxyl stearic acid. Simulation of lineshapes demonstrates an enlarged orientational distribution of the local director of the phospholipid phase and a small increase of the order parameter. The amount of PAF-acether required to destabilize the ovolecithin lamellar phase depends on the degree of hydration. By contrast, cholesterol displays an organizing effect on the PAF-acether phase. Simulation establishes a sharp orientational distribution of the local director when cholesterol reaches stoichiometric ratio relative to PAF-acether. At the same time, cholesterol increases the order parameter.

Permeability studies of lipid vesicles from alkalophilic Bacillus firmus showing opposing effects of membrane isoprenoid and diacylglycerol fractions and suggesting a possible basis for obligate alkalophily

Previous studies of the membrane lipids of extremely alkalophilic bacilli had indicated that both facultative and obligate alkalophiles contained a substantial fraction of isoprenoid lipid as well as high concentrations of cardiolipin. Facultative alkalophiles differed from obligate strains in having a phospholipid fatty acid composition that would be expected to result in a more ordered membrane structure. Current studies of ion permeability in vesicles prepared from lipids from obligately alkalophilic Bacillus firmus RAB and its facultatively alkalophilic strain, OF4, support the suggestion that membranes of the latter strain form a tighter barrier structure, with the difference especially pronounced at near neutral pH values. The water permeability of whole cells and the reflection coefficients for acetamide in vesicles were also consistent with a tighter membrane in the facultatively alkalophilic strain than in the obligately alkalophilic strain. The permeability properties of vesicles prepared from phospholipids from these organisms were studied as a function of the addition of either homologous membrane isoprenoid or diacylglycerol. For each permeability parameter that was assayed, in lipids from both strains, the isoprenoid fraction decreased the permeability, whereas the diacylglycerol fraction increased the permeability of the vesicles to solute.

Influence of molecular packing and phospholipid type on rates of cholesterol exchange

The rates of [14C]cholesterol transfer from small unilamellar vesicles containing cholesterol dissolved in bilayers of different phospholipids have been determined to examine the influence of phospholipid-cholesterol interactions on the rate of cholesterol desorption from the lipid-water interface. The phospholipids included unsaturated phosphatidylcholines (PC's) (egg PC, dioleoyl-PC, and soybean PC), saturated PC (dimyristoyl-PC and dipalmitoyl-PC), and sphingomyelins (SM's) (egg SM, bovine brain SM, and N-palmitoyl-SM). At 37 degrees C, for vesicles containing 10 mol% cholesterol, the half-times for exchange are about 1, 13, and 80 h, respectively, for unsaturated PC, saturated PC, and SM. In order to probe how differences in molecular packing in the bilayers cause the rate constants for cholesterol desorption to be in the order unsaturated PC greater than saturated PC greater than SM, nuclear magnetic resonance (NMR) and monolayer methods were used to evaluate the cholesterol physical state and interactions with phospholipid. The NMR relaxation parameters for [4-13C]cholesterol reveal no differences in molecular dynamics in the above bilayers. Surface pressure (pi)-molecular area isotherms for mixed monolayers of cholesterol and the above phospholipids reveal that SM lateral packing density is greater than that of the PC with the same acyl chain saturation and length (e.g., at pi = 5 mN/m, where both monolayers are in the same physical state, dipalmitoyl-PC and palmitoyl-SM occupy 87 and 81 A2/molecule, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Lipid intermolecular hydrogen bonding: influence on structural organization and membrane function

The great variety of different lipids in membranes, with modifications to the hydrocarbon chains, polar groups and backbone structure suggests that many of these lipids may have unique roles in membrane structure and function. Acidic groups on lipids are clearly important, since they allow interaction with basic groups on proteins and with divalent cations. Another important property of certain lipids is their ability to interact intermolecularly with other lipids via hydrogen bonds. This interaction occurs through acidic and basic moieties in the polar head groups of phospholipids, and the amide moiety and hydroxyl groups on the acyl chain, sphingosine base and sugar groups of sphingo- and glycolipids. The putative ability of different classes of lipids to interact by intermolecular hydrogen bonding, the molecular groups which may participate and the effect of these interactions on some of their physical properties are summarized in Table IX. It is frequently questioned whether intermolecular hydrogen bonding could occur between lipids in the presence of water. Correlations of their properties with their molecular structures, however, suggest that it can. Participation in intermolecular hydrogen bonding increases the lipid phase transition temperature by approx. 8-16 Cdeg relative to the electrostatically shielded state and by 20-30 Cdeg relative to the repulsively charged state, while having variable effects on the enthalpy. It increases the packing density in monolayers, possibly also in the liquid-crystalline phase in bilayers, and decreases the lipid hydration. These effects can probably be accounted for by transient, fluctuating hydrogen bonds involving only a small percentage of the lipid at any one time. Thus, rotational and lateral diffusion of the lipids may take place but at a slower rate, and the lateral expansion is limited. Intermolecular hydrogen bonding between lipids in bilayers may be significantly stabilized, despite the presence of water, by the fact that the lipids are already intermolecularly associated as a result of the hydrophobic effect and the Van der Waals' interactions between their chains. The tendency of certain lipids to self-associate, their asymmetric distribution in SUVs, their preferential association with cholesterol in non-cocrystallizing mixtures, their temperature-induced transitions to the hexagonal phase and their inhibitory effect on penetration of hydrophobic residues of proteins partway into the bilayer can all be explained by their participation in intermolecular hydrogen bonding interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of cholesterol on lipid dynamics and packing in diether phosphatidylcholine bilayers. X-ray diffraction and 2H-NMR study

In order to compare the lipid packing, conformation and dynamics of ether- and ester-linked phosphatidylcholines in the presence of equimolar concentrations of cholesterol, multilamellar dispersions of these lipid-sterol mixtures were investigated by X-ray diffraction and 2H-NMR. A comparison of the X-ray diffraction patterns at 22 degrees C of 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) dispersion, each containing 50 mol% cholesterol, demonstrate that the structural characteristics of DHPC and DPPC bilayers in the presence of cholesterol are essentially indistinguishable by X-ray diffraction. In contradistinction to the similar structural characteristics of DHPC and DPPC in the presence of cholesterol, the low-angle lamellar reflections of DHPC at 22 degrees C in the absence of cholesterol are indicative of an interdigitated phase, demonstrating that cholesterol facilitates the conversion from an interdigitated to a non-interdigitated phase. Above Tc in each lipid-sterol mixture, the quadrupolar splittings from the alpha-methylene segments of 1,2[1',1'-2H]DHPC and 1,2[2',2'-2H2]DPPC indicate that both chain inequivalence and magnetic inequivalence of any particular alpha-C2H2 deuteron pair are preserved and actually enhanced in the presence of cholesterol. Lowering of the temperature below Tc in 1,2[2',2'-2H2]DPPC/cholesterol dispersion leads to a progressive intensity loss of the sn-2 chain components, a thermotropic effect which is not observed in the corresponding components of the 1,2[1',1'-2H2]DHPC/cholesterol spectra.

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

Permeability behavior of liposomes prepared from fatty acids and fatty acid methyl esters

The permeability properties of liposomes prepared at pH 8.7 from a fatty acid and either methyl oleate or methyl elaidate, with or without cholesterol, were investigated. The fatty acids used were oleic acid, elaidic acid, and the selenium-containing fatty acids 9-selenaheptadecanoic acid and 13-selenaheneicosanoic acid. The liposomes trapped sucrose and carboxyfluorescein. Their volume change resulting from osmotic shock was directly proportional to the change in absorbance (light scattering). Liposomes prepared from oleic acid and either methyl oleate or methyl elaidate underwent osmotic swelling much more slowly than liposomes prepared from elaidic acid and either methyl oleate or methyl elaidate. Incorporation of cholesterol decreased the initial rate of erythritol permeation, especially in liposomes containing methyl oleate. The swelling rates of liposomes prepared with the selenium-containing fatty acids indicated that incorporation of methyl elaidate gave more tightly packed bilayers than did incorporation of methyl oleate. The effect of cholesterol on the initial rate of erythritol influx was greater in oleic acid and elaidic acid liposomes than in selenium-containing fatty acid liposomes, indicating that the large bulk of the selenium heteroatom suppresses the ability of cholesterol to interact with the hydrocarbon chain.

A simple physical model for fungicide induced hexagonal clustering of intramembrane particles in the plasmalemma of Ustilago avenae

Freeze fracture electron microscopy is used to study the influence of fungicide (triadimenol) treatment on the plasmalemma of sporidia of Ustilago avenae. The intramembrane particles (IMPs) randomly distributed in untreated samples form large hexagonal clusters after triadimenol treatment. A simple physical model based on length mismatch between lipid bilayer and IMPs is used to describe the interaction between the IMPs in terms of a lipid mediated potential. Using for a rough estimation also data on artificial membranes the fungicide induced ordering effect of the IMPs is discussed. A homogeneous change of lipid bilayer parameters due to the fungicide action is considered as well as a fluid-gel phase transition of the lipid matrix, which might be related to the cluster formation. Further implications of the different possibilities with respect to the mode of action of the fungicide are suggested.

The effect of cholesterol on glycerophosphono- and glycerophosphinocholines. Permeability measurements in lipid vesicles

The kinetics of spontaneous chloride ion efflux and valinomycin-mediated rubidium-86 efflux from vesicles prepared from synthetic phospholipids with carbon-phosphorus linkages were investigated at temperatures above the gel-to-liquid-crystalline phase transition. The rate constants for the movement of chloride and rubidium ions were reduced by incorporation of cholesterol into bilayers of phosphono- and phosphinocholines. Nonisosteric phosphonolipids in which the oxygen was removed from the glycerol side of phosphorus without substitution by a methylene group interacted less with cholesterol than the analogous isosteric derivatives, as judged from the magnitude of the decrease in the rate constants for chloride and rubidium ion efflux. The experiments reported in this study suggest that steric factors in the glycerol side of the phosphorus function are important in phosphatidylcholine-cholesterol interaction. However, the oxygen atom on the choline side of the phosphorus in the phosphatidylcholine molecule is not required for strong phosphatidylcholine-cholesterol interaction, since isosteric glycerophosphinocholines interacted as well as the corresponding isosteric glycerophosphonocholines. Furthermore, steric requirements on the choline side of phosphorus are not important in this interaction since phosphinates whose head-group structures are -P(O-)CH2CH2N+(CH3)3 and -P(O-)CH2CH2CH2N+(CH3)3 interacted equally well with cholesterol, as estimated by these permeability studies.

Cholesterol and the cell membrane

Recent studies concerning cholesterol, its behavior and its roles in cell growth provide important new clues to the role of this fascinating molecule in normal and pathological states.

Effect of cholesterol on vesicle bilayer geometry of choline plasmalogen and comparison with dialkyl-, alkylacyl- and diacyl-glycerophosphocholines

Small unilamellar vesicles containing alkenylacyl-, alkylacyl-, dialkyl- or diacyl-glycerophosphocholine were prepared by sonication. Their size was determined from the average internal volume after chromatography on Sepharose 2B and from 31P-NMR linewidths. Alkenylacyl glycerophosphocholine (choline plasmalogen) was found to form the largest vesicles. By addition of 30 mol% cholesterol, the size of plasmalogen vesicles, but not of those containing the alkyl and acyl analogue lipids, was significantly increased. The presence of 50 mol% sterol led to highly increased vesicle sizes of alkylacyl, dialkyl and diacyl-glycerophosphocholine. Mixtures of plasmalogens with 50 mol% cholesterol did not form unilamellar vesicles upon sonication. Bilayer thickness and surface area per phospholipid molecule were determined by small angle X-ray scattering and measurement of partial specific volumes. There is little difference between alkenylacyl glycerophosphocholine and the corresponding diacyl-analog, whereas bilayers consisting of dioleoyl glycerophosphocholine are significantly thinner. Correspondingly their molecular surface area is by about 8% larger than that of the mixed-chain diradyl glycerophosphocholine, since the partial molar volumes are similar for all vesicles tested.

A Raman spectroscopic study on the effect of cholesterol on lipid packing in diether phosphatidylcholine bilayer dispersions

For assessing lipid-sterol packing characteristics in model membrane systems, the vibrational Raman spectra of 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) multilamellar dispersions containing 18 mol% cholesterol were examined. The thermotropic behavior of the pure diether- and cholesterol-containing bilayers were studied in both the C-H stretching (2800-3100 cm-1) and C-C stretching (1000-1200 cm-1) mode regions, spectral intervals reflecting intermolecular and intramolecular order/disorder characteristics. Pure DHPC bilayers exhibit the pretransition and gel to liquid-crystalline phase transition temperatures at approx. 31 and 42.8 degrees C, respectively. In contrast to the temperature behavior of dipalmitoylphosphatidylcholine-cholesterol bilayers in which the primary phase transition Tm, is simply broadened, Tm is increased to approx. 49.6 degrees C in the DHPC (diether PC) liposomes containing 18 mol% cholesterol. As in the saturated chain diacyl-cholesterol bilayer systems, the sterol disorders the DHPC gel phase and eliminates the pretransition, while ordering the liquid-crystalline phase. Pure diether liposomes exhibit a headgroup dehydration effect as the multilamellar dispersions are cycled slowly over a temperature range including the gel and liquid-crystalline forms. That is, on multiple passages through the phase transitions, both the gel and liquid-crystalline bilayer forms assume an increased hydrocarbon chain order. In the cholesterol-containing diether bilayers, only the disordered intra- and interchain states are formed.

Distribution of filipin-sterol complexes in the myelinated nerve fiber

Using filipin as a cytochemical probe to reveal the distribution of cholesterol, myelinated peripheral nerve fibers were examined in freeze-fracture replicas. Filipin-sterol complexes were most abundant in the Schwann cell and axonal plasma membranes. In the Schwann cell plasma membrane there was no heterogeneity in complex distribution in relation to the subjacent cytoplasmic network. In myelin lamellae there was a decrease in complexes from outer to inner lamellae and some aggregation of complexes in individual lamellae. The density of complexes in cytoplasmic organelles varied from absent in mitochondria to high in lysosome-like bodies. The results are interpreted in terms of the related biochemical composition and biophysical properties of cell membranes, with particular reference to the myelinated nerve fiber. The influence of diffusion barriers and gradients on the formation of complexes by filipin is considered.

Ether glycerolipids: novel substrates for studying specificity of enzymes involved in glycerolipid biosynthesis in higher plants

Ether glycerolipids, predominantly alkylacylglycerols and alkylacylglycerophosphocholines, are synthesized in photomixotrophic rape (Brassica napus) suspension cells from various exogenous monoalkylglycerols. The stereospecific distribution of acyl moieties was studied in these ether glycerolipids with regard to chain-length and degree of unsaturation of alkyl moieties and compared with the distribution of acyl moieties in the corresponding endogenous acyl glycerolipids. The results show the following: (1) Alkylacylglycerophosphocholines replaced up to one-half of the corresponding physiological membrane lipids, i.e. diacylglycerophosphocholines, without changing the total amount of cholineglycerophospholipids as compared to untreated cells. (2) The composition of acyl moieties in total lipids of rape cells was practically unaltered by fatty acids derived via oxidative cleavage from the various alkyl moieties of either glycerolipids. (3) In 1-O-alkyl-2-acylglycerols derived from exogenous alkylglycerols and in endogenous 1,2-diacylglycerols compositions of acyl moieties were found to be different indicating that different pathways were operative in the biosynthesis of these two neutral glycerolipids. (4) Enzymes involved in synthesizing molecular species of 1-O-alkyl-2-acylglycerophosphocholines or 2-O-alkyl-1-acylglycerophosphocholines as well as 1,2-diacylglycerophosphocholines showed similar specificities with regard to chain-length and degree of unsaturation of both alkyl and corresponding acyl moieties. Thus, ether glycerolipids formed by plant cells from exogenous alkylglycerols are suitable metabolites for studying the specificity of enzymes involved in the biosynthesis of glyerolipids.