Haptenic activity of galactosyl ceramide and its topographical distribution on liposomal membranes. I. Effect of cholesterol incorporation

The relation between the immune-reaction of phosphatidylcholine liposomes containing spin-labeled galactosyl ceramide with or without cholesterol and the topographical distribution of the glycolipid in membranes was studied. In egg yolk phosphatidylcholine liposomes, both immune agglutination and antibody binding occurred, irrespectively of the presence of cholesterol, though the motion of the fatty acyl chain of spin-labeled galactosyl ceramide was restricted by cholesterol. In dipalmitoyl phosphatidylcholine liposomes, unlike in egg yolk phosphatidylcholine liposomes, the immune-reaction depended on the cholesterol content. The electron spin resonance (ESR) spectra of spin-labeled galactosyl ceramide in dipalmitoyl phosphatidylcholine liposomes indicated that cholesterol affected the topographical distribution of spin-labeled galactosyl ceramide in the liposomes. Without cholesterol, most of the spin-labeled galactosyl ceramide was clustered on the dipalmitoyl phosphatidylcholine membrane, but with increase of cholesterol, random distribution of hapten on the membrane increased. The cholesterol-dependent change in the topographical distribution of hapten on the membranes was parallel with that of immune reactivity. 'Aggregates' composed solely of galactosyl ceramide did not show any binding activity with antibody. The findings suggest that the recognition of galactosyl ceramide by antibody depended on the topographical distribution of hapten molecules. Phosphatidylcholine and/or cholesterol may play roles as 'spacers' for the proper distribution of 'active' haptens on the membranes. The optimum density of haptens properly distributed on liposomal membranes is discussed.     

Phosphatidylcholine and cholesterol interactions in model membranes

Various phosphatidylcholines differing either in the stereochemistry around their chiral center or in the position of a cis double bond along the acyl chains were synthesized in order to study critical contact regions in the phospholipid molecule with adjacent cholesterol in model membranes. Microviscosities calculated from fluorescence depolarization of diphenylhexatriene and chain order from spin label studies were measured to monitor physical membrane properties. The enhancing effect of cholesterol on the microviscosity of membranes containing phosphatidylcholines with comparable acyl chain length was largest when the two acyl chains were saturated and smallest when both were unsaturated. Membranes prepared from phosphatidylcholines having a single cis double bond at different positions along the sn-2 acyl chain showed roughly the same changes of microviscosity or chain order upon incorporation of cholesterol. No discrimination was evident in the interaction between cholesterol and enantiomeric phosphatidylcholines or between the enantiomeric phosphatidylcholine molecules themselves. We conclude that the rigidifying effect of cholesterol in membranes does not depend on specific sites of interaction and that with respect to physical membrane properties phosphatidylcholine behaves as an achiral molecule.     

Exclusion of a cholesterol analog from the cholesterol-rich phase in model membranes

Vesicles of phosphatidylcholine/cholesterol mixtures show a wide composition range with coexistence of two fluid phases, the 'liquid disordered' (cholesterol-poor) and 'liquid ordered' (cholesterol-rich) phases. These systems have been widely used as models of membranes exhibiting lateral heterogeneity (membrane domains). The distributions of two fluorescent probes (a fluorescent cholesterol analog, NBD-cholesterol, and a lipophilic rhodamine probe, octadecylrhodamine B) in dimyristoylphosphatidylcholine/cholesterol vesicles were studied, at 30 degrees C and 40 degrees C. The steady-state fluorescence intensity of both probes decreases markedly with increasing cholesterol concentration, unlike the fluorescence lifetimes. The liquid ordered to liquid disordered phase partition coefficients K(p) were measured, and values much less than unity were obtained for both probes, pointing to preference for the cholesterol-poor phase. Globally analyzed time-resolved energy transfer results confirmed these findings. It is concluded that, in particular, NBD-cholesterol is not a suitable cholesterol analog and its distribution behavior in phosphatidylcholine/cholesterol bilayers is in fact opposite to that of cholesterol.     

Factors contributing to the distribution of cholesterol among phospholipid vesicles

The distribution of cholesterol between vesicles of different lipid composition at equilibrium has been determined. Small, sonicated unilamellar vesicles and large unilamellar vesicles were incubated at a defined temperature, and aliquots were then obtained at selected times for analysis. Inclusion of a small amount of phosphatidylserine or phosphatidylinositol in the membrane does not appreciably affect the distribution of cholesterol at equilibrium by these measurements. A membrane in the gel state is a poor acceptor of cholesterol. The length of the hydrocarbon chain on the phospholipid may also play a role. Bovine brain sphingomyelin dramatically slows the kinetics of cholesterol transfer, and the equilibrium distribution of cholesterol among vesicles containing sphingomyelin is therefore not observable in these experiments. Data obtained with vesicles containing phosphatidylethanolamine indicate a preference of cholesterol for vesicles composed of phosphatidylcholine compared to vesicles consisting primarily of phosphatidylethanolamine, at equilibrium. Experiments with a chaotropic agent indicate that the nature of the surface of the phosphatidylethanolamine bilayer, and its hydration, are important factors in the distribution of cholesterol among membranes in which phosphatidylethanolamine is present. These data suggest that membrane lipid content may play a role in the distribution of cholesterol among the membranes of a cell.     

Effect of cholesterol on the structure and dynamic properties of unsaturated phospholipid bilayers

Molecular dynamics (MD) computer simulations of five different hydrated unsaturated phosphatidylcholine lipid bilayers built up by 18:0/18:1(n-9)cis PC, 18:0/18:2(n-6)cis PC, 18:0/18:3(n-3)cis PC, 18:0/20:4(n-6)cis PC, and 18:0/22:6(n-3)cis PC molecules with 40 mol% cholesterol, and the same five pure phosphatidylcholine bilayers have been performed at 303 K. The simulation box of a lipid bilayer contained 96 phosphatidylcholines, 64 cholesterols, and 3840 water molecules (48 phosphatidylcholine molecules and 32 cholesterols per layer and 24 water molecules per phospholipid or cholesterol in each case). The lateral self-diffusion coefficients of the lipids in these systems and mass density profiles with respect to the bilayer normal have been analyzed. It has been found that the lateral diffusion coefficients of phosphatidylcholine molecules increase with increasing number of double bonds in one of the lipid chains, both in pure bilayers and in bilayers with cholesterol. It has been found as well that the lateral diffusion coefficient of phosphatidylcholine molecules of a lipid bilayer with 40 mol% cholesterol is smaller than that for the corresponding pure phosphatidylcholine bilayer.     

Interaction of cholesterol with sphingomyelin in mixed membranes containing phosphatidylcholine, studied by spin-label ESR and IR spectroscopies. A possible stabilization of gel-phase sphingolipid domains by cholesterol

The ESR spectra from different positional isomers of sphingomyelin and phosphatidylcholine spin-labeled in their acyl chain have been studied in sphingomyelin(cerebroside)-phosphatidylcholine mixed membranes that contain cholesterol. The aim was to investigate mechanisms by which cholesterol could stabilize possible domain formation in sphingolipid-glycerolipid membranes. The outer hyperfine splittings in the ESR spectra of sphingomyelin and phosphatidylcholine spin-labeled on the 5 C atom of the acyl chain were consistent with mixing of the components, but the perturbations on adding cholesterol were greater in the membranes containing sphingomyelin than in those containing phosphatidylcholine. Infrared spectra of the amide I band of egg sphingomyelin were shifted and broadened in the presence of cholesterol to a greater extent than the carbonyl band of phosphatidylcholine, which was affected very little by cholesterol. Two-component ESR spectra were observed from lipids spin-labeled on the 14 C atom of the acyl chain in cholesterol-containing membranes composed of sphingolipids, with or without glycerolipids (sphingomyelin/cerebroside and sphingomyelin/cerebroside/phosphatidylcholine mixtures). These results indicate the existence of gel-phase domains in otherwise liquid-ordered membranes that contain cholesterol. In the gel phase of egg sphingomyelin, the outer hyperfine splittings of sphingomyelin spin-labeled on the 14-C atom of the acyl chain are smaller than those for the corresponding spin-labeled phosphatidylcholine. In the presence of cholesterol, this situation is reversed; the outer splitting of 14-C spin-labeled sphingomyelin is then greater than that of 14-C spin-labeled phosphatidylcholine. This result provides some support for the suggestion that transbilayer interdigitation induced by cholesterol stabilizes the coexistence of gel-phase and "liquid-ordered" domains in membranes containing sphingolipids.     

EPR investigation of clomipramine interaction with phosphatidylcholine membranes in presence and absence of cholesterol

The effects of tricylic antidepressant clomipramine (CLO) on the membrane properties of saturated dimyristoyl phosphatidylcholine and dipalmitoyl phosphatidylcholine as well as on unsaturated egg yolk phosphatidylcholine liposomes were investigated by the electron paramagnetic resonance spin-labeling technique, in combination with the simulation of the spectra, taking into account that the membrane is heterogeneous and composed of the regions with different fluidity characteristics. Different spin labels, monitoring membrane properties in the upper and inner parts of the membrane, were used. In general, two spectral components, having different motional characteristics, were detected in all liposomes investigated. In liposomes with saturated chains, CLO decreased the phase-transition temperature, disordered the membrane, and increased polarity in the upper part of the membrane. However, less impact was observed in liposomes with unsaturated chains. In dipalmitoyl phosphatidylcholine liposomes, it also induced molecular rearrangements near the pretransition temperature. The presence of 30 mol% cholesterol increased the fluidizing effect of CLO and modified the lateral diffusion of nitroxide in the inner part of the membrane. A unique anomalous increase in diffusion of nitroxide, dependent on CLO concentration, was detected in the temperature region where the phosphatidylcholine membrane without cholesterol experiences the phase transitions. Since the changes in the central part of the membrane were even more pronounced than in the upper part of the membrane, it could be concluded that CLO incorporates into the membrane with its hydrophobic ring parallel to the phospholipid chains.     

Influence of cholesterol on electroporation of bilayer lipid membranes: chronopotentiometric studies

This paper presents the results of constant-current (chronopotentiometric) measurements of the egg yolk phosphatidylcholine (PC) bilayer membrane without and with cholesterol. The experiments were performed on planar bilayer lipid membrane (BLM) formed by the Mueller-Rudin method. It is demonstrated that the constant-intensity current flow through bilayer membranes generated fluctuating pores in their structure. The presence of cholesterol in the membrane caused an increase in the value of the breakdown potential. It is postulated that greater stability of the bilayer with cholesterol can result from an increased critical pore radius (at which the bilayer would undergo irreversible rupture). This confirms that cholesterol has a stabilizing effect on BLM. Besides, our results suggest that addition of cholesterol causes shift in the distribution of pore conductance towards a smaller value. It is suggested that this can be connected with the phenomenon of domain formation in the membranes containing high concentration of cholesterol. Moreover, it is shown that chronopotentiometry with programmable current intensity is a promising method for observation of the membrane recovery process.     

Free fatty acids modulate intermembrane trafficking of cholesterol by increasing lipid mobilities: novel 13C NMR analyses of free cholesterol partitioning

Cholesterol and free fatty acids in membranes modulate major biological processes, and their cellular metabolism and actions are often coordinately regulated. However, effects of free fatty acid on cholesterol-membrane interactions have proven difficult to monitor in real time in intact systems. We developed a novel (13)C NMR method to assess effects of free fatty acids on molecular interactions of cholesterol within--and transfer between--model membranes. An important advantage of this method is the ability to acquire kinetic data without separation of donor and acceptor membranes. Large unilamellar phospholipid vesicles (LUV) with phosphatidylcholine/cholesterol ratios of 4:1 served as cholesterol donors. Small unilamellar vesicles (SUV) made with phosphatidylcholine were acceptors. The (13)C(4)-cholesterol peak is narrow in SUV, but very broad in LUV, spectra; the increase in intensity of this peak over time monitored transfer. Oleic acid and other long chain free fatty acids [saturated (C12-18) and unsaturated (C18)] dose-dependently increased mobilities of lipids in LUV (phospholipid and cholesterol) and cholesterol transfer rates, whereas short (C8-10) and very long (C24) chain free fatty acids did not. Decreasing pH from 7.4 to 6.5 (+/-oleic acid) had no effect on cholesterol transfer, and 5 mol % fatty acyl-CoAs increased transfer rates, demonstrating greater importance of the fatty-acyl tail over the headgroup. In LUV containing sphingomyelin, transfer rates decreased, but the presence of oleic acid increased transfer 1.3-fold. These results demonstrate free fatty acid-facilitated cholesterol movement within and between membranes, which may contribute to their multiple biological effects.     

Influence of cholesterol on electroporation of bilayer lipid membranes: chronopotentiometric studies

This paper presents the results of constant-current (chronopotentiometric) measurements of the egg yolk phosphatidylcholine (PC) bilayer membrane without and with cholesterol. The experiments were performed on planar bilayer lipid membrane (BLM) formed by the Mueller-Rudin method. It is demonstrated that the constant-intensity current flow through bilayer membranes generated fluctuating pores in their structure. The presence of cholesterol in the membrane caused an increase in the value of the breakdown potential. It is postulated that greater stability of the bilayer with cholesterol can result from an increased critical pore radius (at which the bilayer would undergo irreversible rupture). This confirms that cholesterol has a stabilizing effect on BLM. Besides, our results suggest that addition of cholesterol causes shift in the distribution of pore conductance towards a smaller value. It is suggested that this can be connected with the phenomenon of domain formation in the membranes containing high concentration of cholesterol. Moreover, it is shown that chronopotentiometry with programmable current intensity is a promising method for observation of the membrane recovery process.     

Enzyme-catalyzed oxidation of cholesterol in mixed phospholipid monolayers reveals the stoichiometry at which free cholesterol clusters disappear.

In this study, we have used cholesterol oxidase as a probe to study cholesterol/phospholipid interactions in mixed monolayers at the air/water interface. Mixed monolayers, containing a single phospholipid class and cholesterol at differing cholesterol/phospholipid molar ratios, were exposed to cholesterol oxidase at a lateral surface pressure of 20 mN/m (at 22 degrees C). At equimolar ratios of cholesterol to phospholipid, the average rate of cholesterol oxidation was fastest in unsaturated phosphatidylcholine mixed monolayers (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and egg yolk phosphatidylcholine), intermediate in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, and slowest in sphingomyelin monolayers (egg yolk or bovine brain sphingomyelin). The average oxidation rate in mixed monolayers was not exclusively a function of monolayer packing density, since egg yolk and bovine brain sphingomyelin mixed monolayers occupied similar mean molecular areas even though the measured average oxidation rate was different with these two phospholipids. This suggests that the phospholipid acyl chain composition influenced the oxidation rate. The importance of the phospholipid acyl chain length on influencing the average oxidation rate was further examined in defined phosphatidylcholine mixed monolayers. The average oxidation rate decreased linearly with increasing acyl chain lengths (from di-8:0 to di-18:0). When the average oxidation rate was examined as a function of the cholesterol to phospholipid (C/PL) molar ratio in the monolayer, the otherwise linear function displayed a clear break at a 1:1 stoichiometry with phosphatidylcholine mixed monolayers, and at a 2:1 C/PL stoichiometry with sphingomyelin mixed monolayers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Membrane-rigidifying effects of anti-cancer dietary factors

Since several anti-cancer drugs interact with cell membrane lipids, the effects of anti-cancer dietary factors on liposomal membranes with different lipid composition were comparatively studied by measuring fluorescence polarization. Fluidity was imparted on both hydrophobic and hydrophilic regions of lipid bilayers by decreasing cholesterol and increasing unsaturated phosphatidylcholine in membranes. At 0.625-10 microM, (-)-epigallocatechin gallate, genistein, apigenin, resveratrol and a reference anti-cancer drug, doxorubicin, rigidified the tumor cell model membranes consisting of 20 mol% cholesterol and 80 mol% phosphatidylcholine with the acyl chain 18:1/16:0 ratio of 1.0, but not daidzein. They were more effective on the membrane core than the membrane surface. Quercetin showed a biphasic effect on the hydrophobic regions of membrane lipid bilayers to rigidify above 5 microM and fluidize below 2.5 microM. In contrast, anti-cancer dietary factors and doxorubicin were not or much less effective in rigidifying the normal cell model membranes consisting of 40 mol% cholesterol and 60 mol% phosphatidylcholine with the acyl chain 18:1/16:0 ratio of 0.5. The membrane-rigidifying effects were greater depending on a decrease of the cholesterol/phosphatidylcholine ratio and an increase of the phosphatidylcholine unsaturation degree. Membrane-active dietary factors and doxorubicin inhibited the growth of mouse myeloma cells at 10-100 microM, while the growth inhibition by membrane-inactive daidzein was relatively weak. Anti-cancer dietary factors appear to act on more fluid membranes like tumor cells as well as doxorubicin to induce rigidification, especially in the hydrocarbon core of membrane lipids, which is determined by the composition of cholesterol and unsaturated phospholipids.     

Melittin-lipid bilayer interactions and the role of cholesterol

The membrane-destabilizing effect of the peptide melittin on phosphatidylcholine membranes is modulated by the presence of cholesterol. This investigation shows that inclusion of 40 mol % cholesterol in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine or 1,2-dioleoyl-sn-glycero-3-phosphocholine liposomes reduces melittin's affinity for the membrane. It is significant that the presence of cholesterol does not increase the amount of membrane-associated melittin needed to cause maximum leakage from, or major structural rearrangements of, the liposomes. Furthermore, comparison of microscopy and leakage data suggests that melittin-induced leakage occurs via different mechanisms in the cholesterol-free and cholesterol-supplemented systems. In the absence of cholesterol, leakage of carboxyfluorescein takes place from intact liposomes in a manner compatible with the presence of small melittin-induced pores. In the presence of cholesterol, on the other hand, adsorption of the peptide causes complete membrane disruption and the formation of long-lived open-bilayer structures. Moreover, in the case of cholesterol-supplemented systems, melittin induces pronounced liposome aggregation. Cryotransmission electron microscopy was used, together with ellipsometry, circular dichroism, turbidity, and leakage measurements, to investigate the effects of melittin on phosphatidylcholine membranes in the absence and presence of cholesterol. The melittin partitioning behavior in the membrane systems was estimated by means of steady-state fluorescence spectroscopy measurements.     

Influence of the membrane environment on cholesterol transfer

Cholesterol, an essential component in biological membranes, is highly unevenly distributed within the cell, with most localized in the plasma membrane while only a small fraction is found in the endoplasmic reticulum, where it is synthesized. Cellular membranes differ in lipid composition and protein content, and these differences can exist across their leaflets too. This thermodynamic landscape that cellular membranes impose on cholesterol is expected to modulate its transport. To uncover the role the membrane environment has on cholesterol inter- and intra-membrane movement, we used time-resolved small angle neutron scattering to study the passive movement of cholesterol between and within membranes with varying degrees of saturation content. We found that cholesterol moves systematically slower as the degree of saturation in the membranes increases, from a palmitoyl oleyl phosphotidylcholine membrane, which is unsaturated, to a dipalmitoylphosphatidylcholine (DPPC) membrane, which is fully saturated. Additionally, we found that the energetic barrier to move cholesterol in these phosphatidylcholine membranes is independent of their relative lipid composition and remains constant for both flip-flop and exchange at ∼100 kJ/mol. Further, by replacing DPPC with the saturated lipid palmitoylsphingomyelin, an abundant saturated lipid of the outer leaflet of the plasma membrane, we found the rates decreased by a factor of two. This finding is in stark contrast with recent molecular dynamic simulations that predict a dramatic slow-down of seven orders of magnitude for cholesterol flipping in membranes with a similar phosphocholine and SM lipid composition.     

Influence of phospholipid unsaturation on the cholesterol distribution in membranes.

Over the last half decade, we have studied saturated and unsaturated phosphatidylcholine (PC)-cholesterol membranes, with special attention paid to fluid-phase immiscibility in cis-unsaturated PC-cholesterol membranes. The investigations were carried out with fatty acid and sterol analogue spin labels for which reorientational diffusion of the nitroxide was measured using conventional ESR technique. We also used saturation recovery ESR technique where dual probes were utilized. Bimolecular collision rates between a membrane-soluble square-planar copper complex,3-ethoxy-2-oxobutyraldehyde bis(N4,N4-dimethylthiosemicarbazonato)copper(II) (CuKTMS2) and one of several nitroxide radical lipid-type spin labels were determined by measuring the nitroxide spin-lattice relaxation time (T1). The results obtained in all these studies can be explained if the following model is assumed: 1) at physiological temperatures, fluid-phase micro-immiscibility takes place in cis-unsaturated PC-cholesterol membranes, which induces cholesterol-rich domains in the membrane due to the steric nonconformability between the rigid fused-ring structure of cholesterol and the 30 degrees bend at the cis double bond of the alkyl chains of unsaturated PC. 2) The cholesterol-rich domains are small and/or of short lifetime (10(-9) s to less than 10(-7) s). Our results also suggest that the extra space that is available for conformational disorder and accommodation of small molecules is created in the central part of the bilayer by intercalation of cholesterol in cis-unsaturated PC membrane due to the mismatch in the hydrophobic length and nonconformability between cis-unsaturated PC alkyl chains and the bulky tetracyclic ring of cholesterol.     

Effect of cholesterol on interaction of dibucaine with phospholipid vesicles: a fluorescence study

Interaction of the local anesthetic dibucaine with small unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) and dioleoyl phosphatidylcholine (DOPC) containing different mol percents of cholesterol has been studied by fluorescence spectroscopy. Fluorescence measurements on dibucaine in presence of phospholipid vesicles containing various amounts of cholesterol yielded a pattern of variation of wavelength at emission maximum and steady-state anisotropy which indicated that the microenvironment of dibucaine is more polar and flexible in membranes that contain cholesterol than in membranes without cholesterol. Experiments on quenching of fluorescence from membrane-associated dibucaine by potassium iodide showed a marked increase in quenching efficiency as the cholesterol content of the vesicles was increased, demonstrating increased accessibility of the iodide quenchers to dibucaine in the presence of cholesterol, when compared to that in its absence. Total emission intensity decay profiles of dibucaine yielded two lifetime components of approximately 1 ns and approximately 2.8--3.1 ns with mean relative contributions of approximately 25 and approximately 75%, respectively. The mean lifetime in vesicles was 20--30% smaller than in the aqueous medium and showed a moderate variation with cholesterol content. Fluorescence measurements at two different temperatures in DMPC SUVs, one at 33 degrees C, above the phase transition temperature and another at 25 degrees C, around the main phase transition, indicated two different mode of dibucaine localization. At 25 degrees C dibucaine partitioned differentially in presence and absence of cholesterol. However, at 33 degrees C the apparent partition coefficients remained unaltered indicating differences in the microenvironment of dibucaine in presence and absence of cholesterol in the phospholipid membranes.     

Transfer of cholesterol between liposomal membranes.

The transfer of cholesterol between liposomal membranes was examined. On incubation of liposomes compsoed of egg yolk phosphatidylcholine, phosphatidic acid and cholesterol (molar percentage, 65.8 : 1.3 : 32.9 or 65.5 : 6.3 : 31.2), almost complete equilibration of the cholesterol pools was achieved within 6 to 8 h at 37 degrees C. The rate of transfer of cholesterol from the liposomes, in which cholesterol was introduced by 'the exchange reaction', was not significantly different from that from liposomes prepared in the presence of cholesterol, in which the cholesterol was distributed homogenously. These findings indicate that half life for 'flip-flop' of cholesterol molecules in egg yolk phosphatidylcholine liposomes is less than 6 h at 37 degrees C. The transfer of cholesterol between liposomes was strongly dependent on temperature and was affected by the fatty acid composition of the phospholipid, suggesting that the 'fluidity' of the membranes strongly influences the transfer rate. A preferential distribution of cholesterol molecules was observed in heterogeneous liposomes with different classes of phospholipids. The 'affinity order' of cholesterol for phospholipid deduced from the present experiments is as follows: beef brain sphingomyelin greater than dipalmitoylglycerophosphocholine = dimyristoylglycerophosphocholine greater than egg yolk phosphatidylcholine.     

Effect of alkyl chain unsaturation and cholesterol intercalation on oxygen transport in membranes: a pulse ESR spin labeling study.

Transport and diffusion of molecular oxygen in phosphatidylcholine (PC)-cholesterol membranes and their molecular mechanism were investigated. A special attention was paid to the molecular interaction involving unsaturated alkyl chains and cholesterol. Oxygen transport was evaluated by monitoring the bimolecular collision rate of molecular oxygen and the lipid-type spin labels, tempocholine phosphatidic acid ester, 5-doxylstearic acid, and 16-doxylstearic acid. The collision rate was determined by measuring the spin-lattice relaxation times (T1's) in the presence and absence of molecular oxygen with long-pulse saturation-recovery ESR techniques. In the absence of cholesterol, incorporation of either a cis or trans double bond at the C9-C10 position of the alkyl chain decreases oxygen transport at all locations in the membrane. The activation energy for the translational diffusion of molecular oxygen in the absence of cholesterol is 3.7-6.5 kcal/mol, which is comparable to the activation energy theoretically estimated for kink migration or C-C bond rotation of alkyl chains [Tr?uble, H. (1971) J. Membr. Biol. 4, 193-208; Pace, R. J., & Chan, S. I. (1982) J. Chem. Phys. 76, 4241-4247]. Intercalation of cholesterol in saturated PC membranes reduces oxygen transport in the headgroup region and the hydrophobic region near the membrane surface but little affects the transport in the central part of the bilayer. In unsaturated PC membranes, intercalation of cholesterol also reduces oxygen transport in and near the headgroup regions. In contrast, it increases oxygen transport in the middle of the bilayer. On the basis of these observations, a model for the mechanism of oxygen transport in the membrane is proposed in which oxygen molecules reside in vacant pockets created by gauche-trans isomerization of alkyl chains and the structural nonconformability of neighboring lipids, unsaturated PC and cholesterol in particular, and oxygen molecules jump from one pocket to the adjacent one or move along with the movement of the pocket itself. The presence of cholesterol decreases oxygen permeability across the membrane in all membranes used in this work in spite of the increase in oxygen transport in the central part of unsaturated PC-cholesterol membranes because cholesterol decreases oxygen transport in and near the headgroup regions, where the major barriers for oxygen permeability are located. Oxygen gradients across the membranes of the cells and the mitochondria are evaluated. Arguments are advanced that oxygen permeation across the protein-rich mitochondrial membranes can be a rate-limiting step for oxygen consumption under hypoxic conditions in vivo.     

Cholesterol-phosphatidylcholine dispersions as donors of cholesterol to Mycoplasma membranes

Growing cells of sterol-requiring Mycoplasma hominis and sterol non-requiring Acholeplasma laidlawii were used to test the ability of cholesterol-dipalmitoyl phosphatidylcholine dispersions to serve as cholesterol donors to these organisms. Dispersions with high cholesterol to phosphatidylcholine ratios were more effective than dispersions with low cholesterol to phosphatidylcholine ratios in donating cholesterol to the membranes of both mycoplasmas and in promoting growth of the sterol-requiring species. M. hominis took up almost three times as much cholesterol as did A. laidlawii. In addition, significant quantities of the phosphatidylcholine component of the dispersions were found to be associated with M. hominis membranes as against none in the A. laidlawii membrane preparations. In all cases, the percentage of cholesterol taken up by M. hominis from the dispersions exceeded that of phosphatidylcholine by a factor of 3–5. These results were interpreted to suggest that all the cholesterol taken up by A. laidlawii is transferred from the dispersion to the membranes by a process which involves only a transient contact between the organisms and the lipid dispersions, whereas a certain amount of the cholesterol taken up by M. hominis may also be derived from lipid dispersions adhering to or fusing with the cell membranes.     

Effect of cholesterol on molecular order and dynamics in highly polyunsaturated phospholipid bilayers.

The effect of cholesterol on phospholipid acyl chain packing in bilayers consisting of highly unsaturated acyl chains in the liquid crystalline phase was examined for a series of symmetrically and asymmetrically substituted phosphatidylcholines (PCs). The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic structural properties of bilayers containing 30 mol % cholesterol. The bilayers were composed of symmetrically substituted PCs with acyl chains of 14:0, 18:1n9, 20:4n6, or 22:6n3, containing 0, 1, 4, or 6 double bonds, respectively, and mixed-chain PCs with a saturated 16:0 sn-1 chain and 1, 4, or 6 double bonds in the sn-2 chain. DPH excited-state lifetime was fit to a Lorentzian lifetime distribution, the center of which was increased 1-2 ns by 30 mol % cholesterol relative to the cholesterol-free bilayers. Lifetime distributions were dramatically narrowed by the addition of cholesterol in all bilayers except the two consisting of dipolyunsaturated PCs. DPH anisotropy decay was interpreted in terms of the Brownian rotational diffusion model. The effect of cholesterol on both the perpendicular diffusion coefficient D perpendicular and the orientational distribution function f(theta) varied with acyl chain unsaturation. In all bilayers, except the two dipolyunsaturated PCs, 30 mol % cholesterol dramatically slowed DPH rotational motion and restricted DPH orientational freedom. The effect of cholesterol was especially diminished in di-22:6n3 PC, suggesting that this phospholipid may be particularly effective at promoting lateral domains, which are cholesterol-rich and unsaturation-rich, respectively. The results are discussed in terms of a model for lipid packing in membranes containing cholesterol and PCs with highly unsaturated acyl chains.