Miscibility of ternary mixtures of phospholipids and cholesterol in monolayers, and application to bilayer systems

We investigate miscibility transitions of two different ternary lipid mixtures, DOPC/DPPC/Chol and POPC/PSM/Chol. In vesicles, both of these mixtures of an unsaturated lipid, a saturated lipid, and cholesterol form micron-scale domains of immiscible liquid phases for only a limited range of compositions. In contrast, in monolayers, both of these mixtures produce two distinct regions of immiscible liquid phases that span all compositions studied, the alpha-region at low cholesterol and the beta-region at high cholesterol. In other words, we find only limited overlap in miscibility phase behavior of monolayers and bilayers for the lipids studied. For vesicles at 25 degrees C, the miscibility phase boundary spans portions of both the monolayer alpha-region and beta-region. Within the monolayer beta-region, domains persist to high pressures, yet within the alpha-region, miscibility phase transition pressures always fall below 15 mN/m, far below the bilayer equivalent pressure of 32 mN/m. Approximately equivalent phase behavior is observed for monolayers of DOPC/DPPC/Chol and for monolayers of POPC/PSM/Chol. As expected, pressure-area isotherms of our ternary lipid mixtures yield smaller molecular area and compressibility for monolayers containing more saturated acyl chains and cholesterol. All monolayer experiments were conducted under argon. We show that exposure of unsaturated lipids to air causes monolayer surface pressures to decrease rapidly and miscibility transition pressures to increase rapidly.     

Separation of liquid phases in giant vesicles of ternary mixtures of phospholipids and cholesterol

We use fluorescence microscopy to directly observe liquid phases in giant unilamellar vesicles. We find that a long list of ternary mixtures of high melting temperature (saturated) lipids, low melting temperature (usually unsaturated) lipids, and cholesterol produce liquid domains. For one model mixture in particular, DPPC/DOPC/Chol, we have mapped phase boundaries for the full ternary system. For this mixture we observe two coexisting liquid phases over a wide range of lipid composition and temperature, with one phase rich in the unsaturated lipid and the other rich in the saturated lipid and cholesterol. We find a simple relationship between chain melting temperature and miscibility transition temperature that holds for both phosphatidylcholine and sphingomyelin lipids. We experimentally cross miscibility boundaries both by changing temperature and by the depletion of cholesterol with beta-cyclodextrin. Liquid domains in vesicles exhibit interesting behavior: they collide and coalesce, can finger into stripes, and can bulge out of the vesicle. To date, we have not observed macroscopic separation of liquid phases in only binary lipid mixtures.     

Lowering line tension with high cholesterol content induces a transition from macroscopic to nanoscopic phase domains in model biomembranes

Chemically simplified lipid mixtures are used here as models of the cell plasma membrane exoplasmic leaflet. In such models, phase separation and morphology transitions controlled by line tension in the liquid-disordered (Ld)?+?liquid-ordered (Lo) coexistence regime have been described [1]. Here, we study two four-component lipid mixtures at different cholesterol fractions: brain sphingomyelin (BSM) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/cholesterol (Chol). On giant unilamellar vesicles (GUVs) display a nanoscopic-to-macroscopic transition of Ld?+?Lo phase domains as POPC is replaced by DOPC, and this transition also depends on the cholesterol fraction. Line tension decreases with increasing cholesterol mole fractions in both lipid mixtures. For the ternary BSM/DOPC/Chol mixture, the published phase diagram [19] requires a modification to show that when cholesterol mole fraction is >~0.33, coexisting phase domains become nanoscopic.     

Epifluorescence microscopic studies of monolayers containing mixtures of dioleoyl- and dipalmitoylphosphatidylcholines.

Monolayers of dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and some mixtures of these lipids were investigated using an epifluorescence microscopic surface balance. Monolayers were visualized at 23 +/- 1 degree C through the fluorescence of 1 mol% of two different fluorescent probes, 1-palmitoyl-2-(12-[(7-nitro-2-1,3-benzoxadizole-4- yl)amino]dodecanoyl)phosphatidylcholine (NBD-PC), which partitions into the liquid expanded (LE) or disordered lipid phase and 3,3'-dioctadecyloxacarbocyanine perchlorate (DiO-C18), which preferentially associates with the liquid condensed (LC) phase or lipid with ordered chains. LC domains were observed in pure DPPC monolayers at relatively low surface pressures (pi), and these domains grew with increasing surface pressure. Only liquid expanded phase was observed in pure DOPC monolayers up to the point of monolayer collapse. In monolayers containing 29:70:1, 49:50:1, and 69:30:1 (mol/mol/mol) of DPPC:DOPC:probe the domains of LC phase were smaller than those seen in DPPC monolayers at equivalent surface pressures. Quantitative analysis of the visual fields shown by the mixed monolayers showed a distribution of sizes of condensed domains at any given pi. At pi = 30 mN m-1, liquid-expanded, or fluid, regions occupied more than 70% of the total monolayer area in all three mixtures studied, whereas DPPC monolayers were more than 75% condensed or solid at that pressure. For monolayers of DPPC:DOPC:NBD-PC 49:50:1 and 69:30:1 the average domain size and the percentage of the total area covered with LC, or rigid, areas increased to a maximum at pi around 35 mN m-1 followed by a decrease at higher pi.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermodynamics of lipid interactions in complex bilayers

The mutual interactions between lipids in bilayers are reviewed, including mixtures of phospholipids, and mixtures of phospholipids and cholesterol (Chol). Binary mixtures and ternary mixtures are considered, with special emphasis on membranes containing Chol, an ordered phospholipid, and a disordered phospholipid. Typically the ordered phospholipid is a sphingomyelin (SM) or a long-chain saturated phosphatidylcholine (PC), both of which have high phase transitions temperatures; the disordered phospholipid is 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or dioleoylphosphatidylcholine (DOPC). The unlike nearest-neighbor interaction free energies (ω_AB) between lipids (including Chol), obtained by an variety of unrelated methods, are typically in the range of 0-400 cal/mol in absolute value. Most are positive, meaning that the interaction is unfavorable, but some are negative, meaning it is favorable. It is of special interest that favorable interactions occur mainly between ordered phospholipids and Chol. The interpretation of domain formation in complex mixtures of Chol and phospholipids in terms of phase separation or condensed complexes is discussed in the light of the values of lipid mutual interactions.     

Diffusion and Partitioning of Fluorescent Lipid Probes in Phospholipid Monolayers

The pressure-dependent diffusion and partitioning of single lipid fluorophores in DMPC and DPPC monolayers were investigated with the use of a custom-made monolayer trough mounted on a combined fluorescence correlation spectroscopy (FCS) and wide-field microscopy setup. It is shown that lipid diffusion, which is essential for the function of biological membranes, is heavily influenced by the lateral pressure and phase of the lipid structure. Both of these may change dynamically during, e.g., protein adsorption and desorption processes. Using FCS, we measured lipid diffusion coefficients over a wide range of lateral pressures in DMPC monolayers and fitted them to a free-area model as well as the direct experimental observable mean molecular area. FCS measurements on DPPC monolayers were also performed below the onset of the phase transition (Π < 5 mN/m). At higher pressures, FCS was not applicable for measuring diffusion coefficients in DPPC monolayers. Single-molecule fluorescence microscopy and differential scanning calorimetry clearly showed that this was due to heterogeneous partitioning of the lipid fluorophores in condensed phases. The results were compared with dye partitioning in giant lipid vesicles. These findings are significant in relation to the application of lipid fluorophores to study diffusion in both model systems and biological systems.     

Liquid-liquid phase transition temperatures increase when lipid bilayers are supported on glass

Membranes made from certain ternary mixtures of lipids can display coexisting liquid phases. In giant unilamellar vesicles, these phases appear as liquid domains which diffuse and coalesce after the vesicle is cooled below its miscibility transition temperature (T_m). Converting vesicles to supported lipid bilayers alters the mobility of the lipids and domains in the bilayer. At the same time, the miscibility transition temperature of the lipid mixture is altered. Here we compare T_m in vesicles and in supported bilayers formed by rupturing the same vesicles onto glass. We determine transition temperatures using fluorescence microscopy, and identify an increase in T_m when it is measured in identical membranes in solution and on a glass surface. We systematically alter the lipid composition of our membranes in order to observe the correlation between membrane composition and variation in T_m.     

Critical and off-critical miscibility transitions in model extracellular and cytoplasmic myelin lipid monolayers

Monolayers based on the composition of the cytoplasmic (CYT) or extracellular (EXT) sides of the myelin bilayer form coexisting immiscible liquid phases similar to the liquid-ordered/liquid-disordered phases in phospholipid/cholesterol monolayers. Increasing the temperature or surface pressure causes the two liquid phases to mix, although in significantly different fashion for the CYT and EXT monolayers. The cerebroside-rich EXT monolayer is near a critical composition and the domains undergo coalescence and a circle-to-stripe transition along with significant roughening of the domain boundaries before mixing. The phase transition in the cerebroside-free cytoplasmic side occurs abruptly without domain coalescence; hence, the cytoplasmic monolayer is not near a critical composition, although the domains exhibit shape instabilities within 1–2 mN/m of the transition. The change in mixing pressure decreases significantly with temperature for the EXT monolayer, with dΠ_crit/dT ∼ 1.5 mN/m/°C, but the mixing pressure of the CYT monolayer varies little with temperature. This is due to the differences in the nonideality of cholesterol interactions with cerebrosides (EXT) relative to phospholipids (CYT). EXT monolayers based on the composition of white matter from marmosets with experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, remain phase-separated at higher surface pressures than control, while EAE CYT monolayers are similar to control. Myelin basic protein, when added to the CYT monolayer, increases lipid miscibility in CYT monolayers; likely done by altering the dipole density difference between the two phases.     

Anaesthetic — phospholipid interaction

Binding of the positively charged drug chlorpromazine to phospholipid monolayers was investigated. A preferential uptake was observed near the phase transtion of the corresponding lipid. Cholesterol considerably diminishes the chlorpromazine uptake, again particularly near a lipid phase transition. The binding properties depend on the chlorpromazine concentration in the subphase. A critical concentration is 5·10^-5M, where higher uptake occurs in the liquid condensed than in the liquid expanded state of the monolayer at pressures of about 10 mN/m. Dipalmitoylphosphatidylcholine monolayers spread on a subphase containing chlorpromazine are comparable to monolayers at higher temperature but in the absence of chlorpromazine. These data are in agreement with previous fluorescence and electron paramagnetic resonance experiments on lipid bilayer membranes (Luxnat and Galla 1986).Abbreviations CPZ chlorpromazine - DPPC dipalmitoylphosphatidylcholine - DMPC dimyristoylphosphatidylcholine - LE liquid expanded - LC liquid condensed     

The use of monolayers for simple and quantitative analysis of lipid-drug interactions exemplified with dibucaine and substance P.

The interaction between lipids and water soluble amphiphiles was investigated by means of a monolayer technique, monitoring the area increase at constant surface pressure. The area increase could be quantitated and binding isotherms at different surface pressures were measured. A comparison of dibucaine binding to monolayers and bilayers showed that a surface pressure of 32 mN/m best represents the packing density in a lipid bilayer (Seelig, 1987). Binding isotherms measured for charged dibucaine and substance P (SP) were analyzed by means of two different models. If electrostatic effects were ignored the binding of dibucaine and SP showed biphasic Scatchard plots. If, however, electrostatic effects were taken into account by means of the Gouy-Chapman theory, the insertion of both amphiphiles was best described in terms of a partitioning into the monolayer lipids. The hydrophobic binding constant was Kp = 660 +/- 80 M-1 for charged dibucaine inserting into coarse liposomes or monolayers at 32 mN/m (Seelig et al., 1986) and 1-1.8 M-1 for SP inserting into monolayers at 32 mN/m (Seelig and Macdonald, 1989).     

Mitochondrial creatine kinase interaction with heterogeneous monolayers: Effect on lipid lateral organization

Our study highlights the tight relationship between protein binding to monolayers and the phase-state of the phospholipids. Interaction of mitochondrial creatine kinase with phospholipidic membranes was analysed using a two-phase monolayer system containing anionic phospholipids under chain mismatch conditions. Monolayers were made up of mixtures of DMPC/DPPG or DPPC/DMPG containing 40% negatively charged phospholipids which is approximately the negative charge content of the mitochondrial inner membrane. Langmuir isotherms of these monolayers showed that they underwent a phase transition from a liquid expanded state to a liquid-condensed phase at about 2 mN/m and 5 mN/m respectively. Interface morphology modifications caused by injection of mtCK under these monolayers at low or high surface pressure were monitored by Brewster angle microscopy. This work provides evidence that the presence at the air/water interface of discrete domains with increased charge density, may lead to difference in partition of soluble proteins such as mtCK, interacting with the lipid monolayer. Conversely these proteins may help to organize charged phospholipid domains in a membrane.     

Characterization of lipid insertion into monomolecular layers mediated by lung surfactant proteins SP-B and SP-C

Pulmonary surfactant proteins, SP-B and SP-C, if present in preformed monolayers can induce lipid insertion from lipid vesicles into the monolayer after the addition of (divalent) cations [Oosterlaken-Dijksterhuis, M. A., Haagsman, H. P., van Golde, L. M. G., & Demel, R. A. (1991) Biochemistry 30, 8276-8287]. This model system was used to study the mechanisms by which SP-B and SP-C induce monolayer formation from vesicles. Lipid insertion proceeds irrespectively of the molecular class, and PG is not required for this process. In addition to lipids that are immediately inserted from vesicles into the monolayer, large amounts of vesicles are bound to the monolayer and their lipids eventually inserted when the surface area is expanded. SP-B and SP-C are directly responsible for the binding of vesicles to the monolayer. By weight, the vesicle binding capacity of SP-B is approximately 4 times that of SP-C. For vesicle binding and insertion, the formation of close contacts between monolayer and vesicles is essential. SP-B and SP-C show very similar surface properties. Both proteins form extremely stable monolayers (collapse pressures 36-37 mN/m) of alpha-helical structures oriented parallel to the interface. In monolayers consisting of DPPC and SP-B or SP-C, an increase in mean molecular area is observed, which is mainly attributed to the phospholipid. This will greatly enhance the insertion of new lipid material into the monolayer. The results of this study suggest that the surface properties and the hydrophobic nature of SP-B and SP-C are important for the protein-mediated monolayer formation.     

Quantitative coherent anti-Stokes Raman scattering imaging of lipid distribution in coexisting domains

We demonstrate quantitative vibrational imaging of specific lipid molecules in single bilayers using laser-scanning coherent anti-Stokes Raman scattering (CARS) microscopy with a lateral resolution of 0.25 mum. A lipid is spectrally separated from other molecules by using deuterated acyl chains that provide a large CARS signal from the symmetric CD(2) stretch vibration around 2100 cm(-1). Our temperature control experiments show that d62-DPPC has similar bilayer phase segregation property as DPPC when mixing with DOPC. By using epi-detection and optimizing excitation and detection conditions, we are able to generate a clear vibrational contrast from d62-DPPC of 10% molar fraction in a single bilayer of DPPC/d62-DPPC mixture. We have developed and experimentally verified an image analysis model that can derive the relative molecular concentration from the difference of the two CARS intensities measured at the peak and dip frequencies of a CARS band. With the above strategies, we have measured the molar density of d62-DPPC in the coexisting domains inside the DOPC/d62-DPPC (1:1) supported bilayers incorporated with 0-40% cholesterol. The observed interesting changes of phospholipid organization upon addition of cholesterol to the bilayer are discussed.     

Compositional and structural characterization of monolayers and bilayers composed of native pulmonary surfactant from wild type mice

This work comprises a structural and dynamical study of monolayers and bilayers composed of native pulmonary surfactant from mice. Spatially resolved information was obtained using fluorescence (confocal, wide field and two photon excitation) and atomic force microscopy methods. Lipid mass spectrometry experiments were also performed in order to obtain relevant information on the lipid composition of this material. Bilayers composed of mice pulmonary surfactant showed coexistence of distinct domains at room temperature, with morphologies and lateral packing resembling the coexistence of liquid ordered (lo)/liquid disordered (ld)-like phases reported previously in porcine lung surfactant. Interestingly, the molar ratio of saturated (mostly DPPC)/non-saturated phospholipid species and cholesterol measured in the innate material corresponds with that of a DOPC/DPPC/cholesterol mixture showing lo/ld phase coexistence at a similar temperature. This suggests that at quasi-equilibrium conditions, key lipid classes in this complex biological material are still able to produce the same scaffold observed in relevant but simpler model lipid mixtures. Also, robust structural and dynamical similarities between mono- and bi-layers composed of mice pulmonary surfactant were observed when the monolayers reach a surface pressure of 30 mN/m. This value is in line with theoretically predicted and recently measured surface pressures, where the monolayer–bilayer equivalence occurs in samples composed of single phospholipids. Finally, squeezed out material attached to pulmonary surfactant monolayers was observed at surface pressures near the beginning of the monolayer reversible exclusion plateau (~ 40 mN/m). Under these conditions this material adopts elongated tubular shapes and displays ordered lateral packing as indicated by spatially resolved LAURDAN GP measurements.     

The effect of long-chain bases on polysialic acid-mediated membrane interactions

Negatively-charged polysialic acid (polySia) chains are usually membrane-bound and are often expressed on the surface of neuroinvasive bacterial cells, neural cells, and tumor cells. PolySia can mediate both repulsive and attractive cis interactions between membrane components, and trans interactions between membranes. Positively-charged long-chain bases are widely present in cells, are often localized in membranes and can function as bioactive lipids. Here we use Langmuir monolayer technique, fluorescence spectroscopy and electron microscopy of lipid vesicles to study the role of a simple long-chain base, octadecylamine (ODA), in both cis and trans interactions mediated by polySia in model membranes composed of ODA and dioleoylphospatidycholine (DOPC). When added free to an aqueous solution, polySia increases the collapse pressure of ODA/DOPC monolayers, reduces the effect of ODA on the limiting molecular area, inverses the values of excess area per molecule and of excess free energy of mixing from positive to negative, and induces fusion of ODA/DOPC vesicles. These results suggest that a polySia chain can act as a multi-bridge that mediates cis interactions between different components of a lipid membrane, disrupts membrane aggregates, and mediates trans interactions between lipids in apposing membranes. These observations imply that polySia in cellular systems can act in a similar way.     

Oxidized phosphatidylcholines promote phase separation of cholesterol-sphingomyelin domains

Lipid lateral segregation in the plasma membrane is believed to play an important role in cell physiology. Sphingomyelin (SM) and cholesterol (Chol)-enriched microdomains have been proposed as liquid-ordered phase platforms that serve to localize signaling complexes and modulate the intrinsic activities of the associated proteins. We modeled plasma membrane domain organization using Langmuir monolayers of ternary POPC/SM/Chol as well as DMPC/SM/Chol mixtures, which exhibit a surface-pressure-dependent miscibility transition of the coexisting liquid-ordered and -disordered phases. Using Brewster angle microscopy and Langmuir monolayer compression isotherms, we show that the presence of an oxidatively modified phosphatidylcholine, 1-palmitoyl-2-azelaoyl-sn-glydecero-3-phosphocholine, efficiently opposes the miscibility transition and stabilizes micron-sized domain separation at lipid lateral packing densities corresponding to the equilibrium lateral pressure of ～32 mN/m that is suggested to prevail in bilayer membranes. This effect is ascribed to augmented hydrophobic mismatch induced by the oxidatively truncated phosphatidylcholine. To our knowledge, our results represent the first quantitative estimate of the relevant level of phospholipid oxidation that can potentially induce changes in cell membrane organization and its associated functions.     

Transition from nanodomains to microdomains induced by exposure of lipid monolayers to air

The morphology of monolayers prepared from ternary lipid mixtures that have coexisting fluid phases has been examined by atomic force microscopy for samples transferred to mica before and after exposure to air. Mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine and cholesterol with either egg sphingomyelin or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine were studied at several surface pressures. Both lipid mixtures have a combination of small islands and large microdomains at low surface pressure (5-10 mN/m) for monolayers deposited in either air or nitrogen. By contrast, monolayers have small interconnected nanodomains when deposited under nitrogen at 30 mN/m but mixtures of large microdomains and small nanodomains when transferred after exposure to air. These results are consistent with an earlier report that concluded that the formation of large domains at high surface pressures (>30 mN/m) for monolayers exposed to air is caused by lipid oxidation. However, the higher spatial resolution available with atomic force microscopy indicates that exposure of the monolayers to air leads to an increase in the size of preexisting nanodomains, rather than a change in the miscibility pressure. Examination of changes in surface morphology as a function of surface pressure demonstrate a gradual evolution in size and surface coverage for both nano- and microdomains, before formation of a network of interconnected nanodomains. Similar studies for binary mixtures in the absence of cholesterol indicate that lipid oxidation results in analogous changes in domain size for monolayers with coexisting gel and fluid phases. These results illustrate the importance of using techniques capable of probing the nanoscale organization of membranes.     

SecA insertion into phospholipids is stimulated by negatively charged lipids and inhibited by ATP: a monolayer study.

SecA-lipid interactions are believed to be important for the translocation of precursor proteins across the inner membrane of Escherichia coli [Lill, R., Dowhan, W., & Wickner, W. (1990) Cell 60, 271-280]. SecA insertion into the phospholipid bilayer could a role in this process. We investigated this possibility by studying the interactions between SecA and different phospholipids using the monolayer technique. It was established that SecA is surface-active and can insert into lipid monolayers. This insertion was greatly enhanced by the negatively charged lipids DOPG and Escherichia coli cardiolipin. Insertion of SecA into these negatively charged lipids could be detected up to initial surface pressures of 34 mN/m for DOPG and 36 mN/m for Escherichia coli cardiolipin, implying a possible role for negatively charged lipids in the insertion of SecA in biological membranes. High salt concentrations did not inhibit the SecA insertion into DOPG monolayers, suggesting not only an electrostatic but also a hydrophobic interaction of SecA with the lipid monolayer. ATP decreased both the insertion (factor 2) and binding (factor 3) of SecA to DOPG monolayers. ADP and phosphate gave a decrease in the SecA insertion to the same extent as ATP, but the binding of SecA was only slightly reduced. AMP-PNP and ATP-gamma-S did not have large effects on the insertion or on the binding of SecA to DOPG monolayers. The physiological significance of these results in protein translocation is discussed.     

Physical Properties of Lipid Monolayers on Alkylated Planar Glass Surfaces

A method for transferring a lipid monolayer from an air-water interface to an alkylated glass slide is described. Specific antibodies bind tightly to lipid haptens contained in these monolayers on the glass slides. We conclude that the polar head groups of the lipids face the aqueous phase. A monolayer containing a fluorescent lipid was used to show that the monolayer is homogeneous as observed with an epifluorescence microscope. A periodic pattern photobleaching technique was used to measure the lateral diffusion of this fluorescent lipid probe in monolayers composed of dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine. Different regions of the pressure-area isotherms of the monolayers at the air-water interface can be correlated with the diffusion of the fluorescent probe molecules on the monolayer-coated glass slide. Monolayers derived from the so-called “solid-condensed” state of a monolayer at the air-water interface showed a very low probe diffusion coefficient in this monolayer when placed on a glass slide, D ≤ 10^-10 cm²/s. Monolayers derived from the “liquid condensed/liquid expanded” (LC/LE) region of the monolayer isotherms at the air-water interface showed rapid diffusion (D > 10^-8 cm²/s) when these same monolayers were observed on an alkylated glass slide. The monolayers attached to the glass slide appear to be homogeneous when derived from monolayers in the LC/LE region of monolayers at the air-water interface. There is no major variation of the diffusion coefficient of a fluorescent lipid probe when this diffusion is measured on a lipid monolayer on a glass slide, for monolayers derived from various regions of the LC/LE monolayers at the air-water interface. This is consistent with the view that the LC/LE region is most likely a single fluid phase. Monolayers supported on a planar glass substrate are of much potential interest for biophysical and biochemical studies of the interactions between model membranes and cellular membranes, and for physical chemical studies relating the properties of lipid monolayers to the properties of lipid bilayers.     

Surface-active properties of the antitumour ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (edelfosine)

The surface activity and interaction with lipid monolayers and bilayers of the antitumour ether lipid 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphocholine (edelfosine) have been studied. Edelfosine is a surface-active soluble amphiphile, with critical micellar concentrations at 3.5 microM and 19 microM in water. When the air-water interface is occupied by a phospholipid, edelfosine becomes inserted in the phospholipid monolayer, increasing surface pressure. This increase is dose-dependent, and reaches a plateau at ca. 2 microM edelfosine bulk concentration. The ether lipid can become inserted in phospholipid monolayers with initial surface pressures of up to 33 mN/m, which ensures its capacity to become inserted into cell membranes. Upon interaction with phospholipid vesicles, edelfosine exhibits a weak detergent activity, causing release of vesicle contents to a low extent (<5%), and a small proportion of lipid solubilization. The weak detergent properties of edelfosine can be related to its very low critical micellar concentrations. Its high affinity for lipid monolayers combined with low lytic properties support the use of edelfosine as a clinical drug. The surface-active properties of edelfosine are similar to those of other "single-chain" lipids, e.g. lysophosphatidylcholine, palmitoylcarnitine, or N-acetylsphingosine.