Geometric packing constraints in egg phosphatidylcholine vesicles

Formation of the dispersed particles composed of retinol and phosphatidylchiline

The purpose of this study was to investigate the dispersal mechanism of retinol (Vitamin A, VA) into phospholipid. VA was dispersed with soybean phosphatidylcholine (PC) using sonication and the dispersal mechanism was evaluated by characterizing the dispersed particles using dynamic light scattering, fluorescence spectroscopy and surface monolayer techniques. The dispersions in the VA mole fraction range of 0.1-0.7 were stable at room temperature for 3 days. A limited amount of VA was incorporated into PC bilayer membranes (approximately 3 mol%). The excess VA separated from the PC bilayers was stabilized as emulsion particles by the PC surface monolayer. When the PC content was less than the solubility in VA (mole fraction of VA: more than 0.8), the PC monolayer did not completely cover the hydrophobic VA particle surfaces. In the case, the particle size increased drastically and the separation into oil/water occurred. The miscibility between VA and PC and the lipid composition were critically important for the stability of the dispersed particles (coexistence of emulsion particles (surface monolayer of PC+core of VA) with vesicular particles (bilayer)) of the lipid mixtures.

Cytotoxic targeting of F9 teratocarcinoma tumours with anti-ED-B fibronectin scFv antibody modified liposomes

We prepared small unilamellar liposomes derivatised with single chain antibody fragments specific for the ED-B domain of B-fibronectin. This extracellular matrix associated protein is expressed around newly forming blood vessels in the vicinity of many types of tumours. The single chain antibody fragments were functionalised by introduction of C-terminal cysteines and linked to liposomes via maleimide groups located at the terminal ends of poly(ethylene glycol) modified phospholipids. The properties of these anti-ED-B single chain antibody fragments-liposomes were analysed in vitro on ED-B fibronectin expressing Caco-2 cells and in vivo by studying their biodistribution and their therapeutic potential in mice bearing subcutanous F9 teratocarcinoma tumours. Radioactively labelled ((114m)Indium) single chain antibody fragments-liposomes accumulated in the tumours at 2-3-fold higher concentrations during the first 2 h after i.v. injection compared to unmodified liposomes. After 6-24 h both liposome types were found in similar amounts (8-10% injected dose g(-1)) in the tumours. Animals treated i.v. with single chain antibody fragments-liposomes containing the new cytotoxic agent 2'-deoxy-5-fluorouridylyl-N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine (30 mg kg(-1) per dose, five times every 24 h) showed a reduction of tumour growth by 62-90% determined on days 5 and 8, respectively, compared to animals receiving control liposomes. Histological analysis revealed a marked reduction of F9 tumour cells and excessive deposition of fibronectin in the extracellular matrix after treatment with single chain antibody fragments-2-dioxy-5-fluorouridylyl-N(4)-octadecyl-1-beta-D-arabinofuranosylcytosine-liposomes. Single chain antibody fragments-liposomes targeted to ED-B fibronectin positive tumours therefore represent a promising and versatile novel drug delivery system for the treatment of tumours.

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.

Phospholipids chiral at phosphorus. Properties of small unilamellar vesicles of chiral thiophosphatidylcholine

The recent observation of the differences in the biophysical properties between 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and the Rp, Sp, and Rp + Sp isomers of 1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine (DPPsC) in the multilamellar phase [Tsai, M.-D., Jiang, R.-T., & Bruzik, K. (1983) J. Am. Chem. Soc. 105, 2478-2480] prompted us to investigate the biophysical properties of the small unilamellar vesicles (SUV) of the above phospholipids. It was found that DPPC and DPPsC isomers showed approximately the same critical micelle concentrations and formed spherical SUV upon injection of their ethanolic solutions into an aqueous solution. However, the average sizes of the SUV of DPPsC isomers were significantly greater than that of DPPC prepared under the same conditions, as shown by their electron micrographs. The results of both 31P NMR line widths and the ratios of the entrapped solute to the total phospholipids further supported the following order in the average radius of the SUV: (Sp)-DPPsC greater than (Rp + Sp)-DPPsC greater than (Rp)-DPPsC greater than DPPC. Complete lysis of the SUV by melittin was demonstrated in all four cases. The DPPsC isomers showed gel-liquid-crystal transition temperatures of 43.8 +/- 0.1 degrees C, which are considerably higher than that of DPPC (37.9 degrees C) under the same conditions. In the SUV of an equimolar mixture of DPPC and (Rp + Sp)-DPPsC, DPPsC preferred to stay in the inner layer on the basis of 31P NMR studies by use of a shift reagent PrCl3.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothesis: a phospholipid translocase couples lateral and transverse bilayer asymmetries in dividing bacteria

Cell division in bacteria such as Escherichia coli entails changes in the radii of curvature of the invaginating cytoplasmic membrane which culminate in rearrangements of its monolayers. Division therefore risks perturbing transverse and lateral asymmetries and compromising membrane integrity. This leads us to propose that a strong selective pressure exists for a phospholipid translocator that would transfer phospholipids across the cytoplasmic membrane so as to both demarcate the division site and mediate lipid composition during division. This translocase has an affinity for phospholipids with small headgroups and unsaturated acyl chains which it translocates so as to (1) generate changes in the radius of curvature, (2) facilitate septum formation, (3) minimise bilayer disruption during fusion and (4) prevent septum formation at old or inappropriate division sites. We discuss briefly possible candidates for this translocase including ABC transporters and proteins localised to the division site.

Four hydrophobic amino acids of the factor VIII C2 domain are constituents of both the membrane-binding and von Willebrand factor-binding motifs

Factor VIII binds to phospholipid membranes and to von Willebrand factor (vWf) via its second C domain, which has lectin homology. The crystal structure of the C2 domain has prompted a model in which membrane binding is mediated by two hydrophobic spikes, each composed of a pair of residues displayed on a beta-hairpin turn, and also by net positive charge and specific interactions with phospho-l-serine. To test this model, we prepared 16 factor VIII mutants in which single or multiple amino acids were changed to alanine. Mutants at Arg(2215), Arg(2220), Lys(2227), Lys(2249), Gln(2213), Asn(2217), and Phe(2196)/Thr(2197) had specific activities that were >70% of the wild type. Mutants at Arg(2209), Lys(2227), Trp(2313), and Arg(2320) were degraded within the cell. Hydrophobic spike mutants at Met(2199)/Phe(2200), Leu(2251)/Leu(2252), and Met(2199)/Phe(2200)/Leu(2251)/Leu(2252) (4-Ala) exhibited 43, 59, and 91% reduction in specific activity in the activated partial thromboplastin time assay. In a phospholipid-limiting factor Xa activation assay, these mutants had a 65, 85, and 96% reduction in specific activity. Equilibrium binding of fluorescent, sonicated phospholipid vesicles to mutants immobilized on Superose beads was measured by flow cytometry. The affinities for phospholipid were reduced approximately 20-, 30-, and >35-fold for 2199/2200, 2251/2252, and 4-Ala, respectively. A dimeric form of mature vWf bound to immobilized factor VIII and the same mutants, but the affinities of the mutants were reduced approximately 5-, 10-, and >20-fold, respectively. In a competition, solution phase enzyme-linked immunosorbent assay, plasma vWf bound factor VIII and the same mutants with the affinities for the mutants reduced >5-, >5-, and >50-fold, respectively. We conclude that the two hydrophobic spikes are constituents of both the phospholipid-binding and vWf-binding motifs. In plasma, vWf apparently binds the inherently sticky membrane-binding motif, preventing nonspecific interactions.

Thermotropic phase behavior of monoglyceride-dicetylphosphate dispersions and interactions with proteins: a (2)H and (31)P NMR study

The phase behavior of a 1-[(2)H(35)]-stearoyl-rac-glycerol ([(2)H(35)]-MSG)/dicetylphosphate (DCP) mixture and its interaction with beta-lactoglobulin and lysozyme were studied by (2)H and (31)P nuclear magnetic resonance (NMR). The behavior of the lipids was monitored by using deuterium-labeled [(2)H(35)]-MSG as a selective probe for (2)H NMR and DCP for (31)P NMR. Both (2)H and (31)P NMR spectra exhibit characteristic features representative of different phases. In the lamellar phases, (31)P NMR spectra of DCP are different from the spectra of natural phospholipids, which is attributable to differences in the intramolecular motions and the orientation of the shielding tensor of DCP compared with phospholipids. The presence of the negatively charged amphiphile DCP has a large effect on the phase behavior of [(2)H(35)]-MSG. At low temperature, the presence of DCP inhibits crystallization of the gel phase into the coagel. Upon increasing the temperature, the gel phase of [(2)H(35)]-MSG transforms in the liquid-crystalline lamellar phase. In the presence of DCP, the gel phase directly transforms into an isotropic phase. The negatively charged beta-lactoglobulin and the positively charged lysozyme completely neutralize the destabilizing effect of DCP on the monoglyceride liquid-crystalline phase and they even stabilize this phase. Without DCP the proteins do not seem to interact with the monoglyceride. These results suggest that interaction is facilitated by electrostatic interactions between the negatively charged DCP and positively charged residues in the proteins. In addition, the nonbilayer-forming DCP creates insertion sites for proteins in the bilayer.

Probing for preferential interactions among sphingolipids in bilayer vesicles using the glycolipid transfer protein

We have investigated the intervesicular transfer of galactosylceramide between unilamellar bilayer vesicles composed of differing sphingomyelin and phosphatidylcholine molar ratios. To monitor glycolipid transfer from donor to acceptor vesicles, we used a fluorescence resonance energy transfer assay involving anthrylvinyl-labeled galactosylceramide (AV-GalCer) and perylenoyl-labeled triglyceride. The transfer was mediated by glycolipid transfer protein (GLTP), purified from bovine brain and specific for glycolipids. The initial transfer rate and the total accessible pool of glycolipid in the donor vesicles were both measured. An increase in the sphingomyelin content of 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) vesicles decreased the transfer rate in a nonlinear fashion. Decreased transfer rates were clearly evident at sphingomyelin mole fractions of 0.22 or higher. The pool of AV-GalCer available for GLTP-mediated transfer also was smaller in vesicles containing high sphingomyelin content. In contrast, AV-GalCer was more readily transferred from vesicles composed of POPC and different disaturated phosphatidylcholines. Our results show that GLTP acts as a sensitive probe for detecting interactions of glycosphingolipids with neighboring lipids and that the lateral mixing of glycolipids is probably affected by the matrix lipid composition. The compositionally driven changes in lipid interactions, sensed by GLTP, occur in membranes that are either macroscopically fluid-phase or gel/fluid-phase mixtures. Gaining insights into how changes in membrane sphingolipid composition alter accessibility to soluble proteins with affinity for membrane glycolipids is likely to help increase our understanding of how sphingolipid-enriched microdomains (i.e., "rafts" and caveolae) are formed and maintained in cells.

Effects of the acyl chain composition of phosphatidylcholines on the stability of freeze-dried small liposomes in the presence of maltose

The effects of the acyl chain composition of phosphatidylcholines (PCs) on the stability of small unilamellar vesicles during freeze-drying and rehydration in the presence of maltose were studied by monitoring the retention of a trapped marker, calcein, in the internal liposome compartment. In dipalmitoyl PC, beta-oleoyl-gamma-palmitoyl-PC and egg yolk PC liposomes, good or fair retentions (>50%) were observed in the presence of maltose, but maltose was ineffective in preserving retention in the dioleoyl PC (DOPC) liposomes (<10%). The extremely low retention in the DOPC liposome was ascribed to neither a formation of the inverted hexagonal phase of the liposomal membrane nor the fusion/aggregation of the liposomes in the drying-rehydration process. Differential scanning calorimetry measurements suggested that interactions of maltose with PC headgroups were essential to stabilizing the dry liposomes. These interactions were significant in the saturated or mixed chain liposomes but were markedly reduced in the DOPC liposomes.

Cholesterol organization in membranes at low concentrations: effects of curvature stress and membrane thickness

Cholesterol is often found distributed nonrandomly in domains in biological and model membranes and has been reported to be distributed heterogeneously among various intracellular membranes. Although a large body of literature exists on the organization of cholesterol in plasma membranes or membranes with high cholesterol content, very little is known about organization of cholesterol in membranes containing low amounts of cholesterol. Using a fluorescent cholesterol analog (25-[N-[(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-methyl]amino]-27-norcholesterol, or NBD-cholesterol), we have previously shown that cholesterol may exhibit local organization even at very low concentrations in membranes, which could possibly be attributable to transbilayer tail-to-tail dimers. This is supported by similar observations reported by other groups using cholesterol or dehydroergosterol, a naturally occurring fluorescent cholesterol analog which closely mimics cholesterol. In this paper, we have tested the basic features of cholesterol organization in membranes at low concentrations using spectral features of dehydroergosterol. More importantly, we have investigated the role of membrane surface curvature and thickness on transbilayer dimer arrangement of cholesterol using NBD-cholesterol. We find that dimerization is not favored in membranes with high curvature. However, cholesterol dimers are observed again if the curvature stress is relieved. Further, we have monitored the effect of membrane thickness on the dimerization process. Our results show that the dimerization process is stringently controlled by a narrow window of membrane thickness. Interestingly, this type of local organization of NBD-cholesterol at low concentrations is also observed in sphingomyelin-containing membranes. These results could be significant in membranes that have very low cholesterol content, such as the endoplasmic reticulum and the inner mitochondrial membrane, and in trafficking and sorting of cellular cholesterol.

The rate-determining step in P450 C21-catalyzing reactions in a membrane-reconstituted system

Adrenal cytochrome P450 C21 in a membrane-reconstituted system catalyzed 21-hydroxylation of 17alpha-hydroxyprogesterone at a rate higher than that for progesterone in the steady state at 37 degrees C. The rate of product formation in the steady state increased with the concentration of the complex between P450 C21 and the reductase in the membranes. The complex formation was independent of the volume of the reaction, showing that the effective concentrations of the membrane proteins should be defined with the volume of the lipid phase. The rates of conversion of progesterone and 17alpha-hydroxyprogesterone to the product in a single cycle of the P450 C21 reaction were measured with a reaction rapid quenching device. The first-order rate constant for the conversion of progesterone by P450 C21 was 4.3 +/- 0.7 s(-)1, and that for 17alpha-hydroxyprogesterone was 1.8 +/- 0.5 s(-)1 at 37 degrees C. It was found from the analysis of kinetic data that the rate-determining step in 21-hydroxylation of progesterone in the steady state was the dissociation of product from P450 C21, whereas the conversion to deoxycortisol was the rate-determining step in the reaction of 17alpha-hydroxyprogesterone. The difference in the rate-determining steps in the reactions for the two substrates was clearly demonstrated in the pre-steady-state kinetics.

Effect of liposome type and membrane fluidity on drug–membrane partitioning analyzed by immobilized liposome chromatography

Immobilized liposome chromatography (ILC) has been proven to be a useful method for the study or rapid screening of drug-membrane interactions. To obtain an adequate liposomal membrane phase for ILC, unilamellar liposomes were immobilized in gel beads by avidin-biotin binding. The retardation of 15 basic drugs on the liposome column could be converted to membrane partitioning coefficients, K(LM). The effects of small or large unilamellar liposomes and multilamellar liposomes on the drug-membrane partitioning were compared. The K(LM) values for both small and large liposomes were similar, but higher than those for the multilamellar liposomes. The basic drugs showed stronger partitioning into negatively charged liposomes than into either neutral liposomes or positively charged liposomes. The membrane fluidity of the immobilized liposomes was modulated by incorporating cholesterol into the liposomal membranes, by changing the acyl chain length and degree of unsaturation of the phospholipids, and by changing the temperature for ILC runs. Our data show that K(LM) obtained using ILC correlated well with those reported by batch studies using free liposomes. It is concluded that negatively charged or cholesterol-containing large unilamellar liposomes are suitable models for the ILC analysis of drug-membrane interactions.

Adsorption of liposomes and emulsions studied with a quartz crystal microbalance

The adsorption from phospholipid liposome solutions (1.2%) and phospholipid stabilized oil-in-water emulsions (20% purified soybean oil) with the same phospholipid liposome concentration, has been followed by means of a quartz crystal microbalance allowing the simultaneous determination of changes in resonance frequency and energy dissipation. Both the fundamental resonance frequency and the third overtone were used for following the interfacial processes. The adsorption from the liposome solution resulted in formation of a phospholipid bilayer with an additional and incomplete outer layer of liposomes. The outer layer was removed by dilution leaving a bilayer of phospholipids on the surface. The adsorption process observed from the concentrated emulsion solution was considerably more complex. A slow spreading process that also resulted in some expulsion of material from the interface followed the rapid initial adsorption of emulsion droplets. After rinsing with water a phospholipid bilayer was retained on the surface.

Formation and characterization of phosphatidylethanolamine/lysophosphatidylcholine mixed vesicles

The lipid aggregates formed by adding lysophosphatidylcholine (lysoPC) solution to phosphatidylethanolamine (PE) dispersion at 4 degrees C followed by incubating it at 37 degrees C were proved to be a vesicle system judged from the negatively stained electron micrographs and the latency of calcein fluorescence. The results obtained are analogous to those described for phosphatidylcholine (PC) vesicles. The chromatography results showed that the incorporation of PE and lysoPC into the PE/lysoPC vesicles was in a molar ratio of 5 to 2. The PE/lysoPC membrane was found to have similar barrier potentials for Cl- or calcein efflux to the PC membrane. 1H Nuclear magnetic resonance measurement suggested that lysoPC dominated the external monolayer of the vesicles. Furthermore, it was found that PE/lysoPC vesicles and micelles could coexist when a large amount of lysoPC was added to the PE/lysoPC vesicle suspension. The formation of PE/lysoPC vesicles is discussed in combination with the inhibition of interlayer attachment by lysoPC from the PE membrane.

The curvature and cholesterol content of phospholipid bilayers alter the transbilayer distribution of specific molecular species of phosphatidylethanolamine

The curvature, cholesterol content, and transbilayer distribution of phospholipids significantly influence the functional properties of cellular membranes, yet little is known of how these parameters interact. In this study, the transbilayer distribution of phosphatidylethanolamine (PE) is determined in vesicles with large (98 nm) and small (19 nm) radii of curvature and with different proportions of PE, phosphatidylcholine, and cholesterol. It was found that the mean diameters of both types of vesicles were not influenced by their lipid composition, and that the amino-reactive compound 2,4,6-trinitrobenzenesulphonic acid (TNBS) was unable to cross the bilayer of either type of vesicle. When large vesicles were treated with TNBS, approximately 40% of the total membrane PE was derivatized; in the small vesicles 55% reacted. These values are interpreted as representing the percentage of total membrane PE residing in the outer leaflet of the vesicle bilayer. The large vesicles likely contained approximately 20% of the total membrane lipid as internal membranes. Therefore, in both types of vesicles, PE as a phospholipid class was randomly distributed between the inner and outer leaflets of the bilayer. The proportion of total PE residing in the outer leaflet was unaffected by changes in either the cholesterol or PE content of the vesicles. However, the transbilayer distributions of individual molecular species of PE were not random, and were significantly influenced by radius of curvature, membrane cholesterol content, or both. For example, palmitate- and docosahexaenoate-containing species of PE were preferentially located in the outer leaflet of the bilayer. Membrane cholesterol content affected the transbilayer distributions of stearate-, oleate-, and linoleate-containing species. The transbilayer distributions of palmitate-, docosahexaenoate-, and stearate-containing species were significantly influenced by membrane curvature, but only in the presence of high levels of cholesterol. Thus, differences in membrane curvature and cholesterol content alter the array of PE molecules present on the surfaces of phospholipid bilayers. In cells and organelles, these differences could have profound effects on a number of critical membrane functions and processes.

Lipid peroxidation in small and large phospholipid unilamellar vesicles induced by water-soluble free radical sources

The susceptibility of small and large egg yolk phosphatidylcholine unilamellar vesicles to Fe(2+)/histidine-Fe(3+)- and Fenton reagent (Fe(2+)-H(2)O(2))-induced lipid peroxidation was evaluated by measuring the formation of thiobarbituric acid reactive substances (TBARS). It has been found that surface curvature or phospholipid packing exerts significant effect on the oxidative susceptibility of the unsaturated lipid bilayers and the highly curved and loosely packed small unilamellar vesicles (SUVs) exhibit much less resistance to the oxidative stress induced by the water-soluble free radical sources. The presence of lipid hydroperoxides in sonicated vesicles was excluded as the cause for higher level of lipid peroxidation in the phospholipid SUVs. Instead, the experimental results can be explained by the difference in ability of the water-soluble oxidants to penetrate the two types of lipid membranes. This hypothesis is supported by data obtained from fluorescence lifetime and quenching studies.

Biophysical studies of lipopeptide-membrane interactions

There are a number of naturally occurring motifs for lipidation of peptides and proteins. In cases in which this involves adding a single hydrocarbon chain to the peptide, it is either a fatty acid or an isoprenyl group. Lipopeptides will partition between membrane and aqueous phases. When only one hydrocarbon chain is attached to the peptide, the affinity of the lipopeptide for the membrane is only marginally increased over that of the free peptide. The resulting partitioning is largely determined by the extent of the interaction of the peptide moiety with the membrane. In contrast, lipidation involving two hydrocarbon chains, either as two single chains attached at distinct locations of the peptide or a double-chain lipid anchor, firmly attaches the lipopeptide to the membrane. This can allow the placement of specific binding sites on a membrane surface. Such a strategy can be used, for example, to place specific antibodies on the surface of drug-carrying liposomes for the purpose of targeting drug delivery. In addition, lipopeptides will alter the physical properties of membranes. One of these effects is to increase the bilayer to hexagonal phase transition temperature. Substances having this property may also alter functional properties of membranes. While it is unlikely that these changes in the biophysical properties of the membranes. While it is unlikely that these changes in the biophysical properties of the membrane are responsible for specific functions of lipopeptides, such changes may be used to modulate certain properties of a membrane, such as the rate of viral fusion.

Formation and stability of the dispersed particles composed of retinyl palmitate and phosphatidylcholine

The purpose of this study was to develop an intravenous formulation composed of retinyl palmitate (RP) for the treatment of cancer. RP was dispersed with soybean phosphatidylcholine (PC) using sonication and the dispersal mechanism was evaluated by characterizing the dispersed particles using dynamic light-scattering, fluorescence spectroscopy, and surface monolayer techniques. The dispersions in the RP mole fraction range of 0.1-0.8 were stable at room temperature for 3 days. A limited amount of RP was incorporated into PC bilayer membranes (approximately 3 mol%). The excess RP separated from the PC bilayers was stabilized as emulsion particles by the PC surface monolayer. When the PC content was less than the solubility in RP, the PC monolayer did not completely cover the hydrophobic RP particle surfaces and separation into oil/water occurred. The miscibility between RP and PC and the lipid composition were critically important for the stability of the dispersed particles (coexistence of emulsion particles [surface monolayer of PC + core of RP] with vesicular particles [bilayer]) of the lipid mixtures.

Properties of lipid microdomains in a muscle cell membrane visualized by single molecule microscopy

The lateral motion of single fluorescence labeled lipid molecules was imaged in native cell membranes on a millisecond time scale and with positional accuracy of approximately 50 nm, using 'single dye tracing'. This first application of single molecule microscopy to living cells rendered possible the direct observation of lipid-specific membrane domains. These domains were sensed by a lipid probe with saturated acyl chains as small areas in a liquid-ordered phase: the probe showed confined but fast diffusion, with high partitioning (approximately 100-fold) and long residence time (approximately 13 s). The analogous probe with mono-unsaturated chains diffused predominantly unconfined within the membrane. With approximately 15 saturated probes per domain, the locations, sizes, shapes and motions of individual domains became clearly visible. Domains had a size of 0.7 micrometer (0.2-2 micrometer), covering approximately 13% of total membrane area. Both the liquid-ordered phase characteristics and the sizes of domains match properties of membrane fractions described as detergent-resistant membranes (DRMs), strongly suggesting that the domains seen are the in vivo correlate of DRMs and thus may be identified as lipid rafts.

In vitro characterization of a novel polymeric-based pH-sensitive liposome system

This study demonstrates rapid and pH-sensitive release of a highly water-soluble fluorescent aqueous content marker, pyranine, from egg phosphatidylcholine liposomes following incorporation of N-isopropylacrylamide (NIPA) copolymers in liposomal membranes. The pH-sensitivity of this system correlates with the precipitation of the copolymers at acidic pH. In vitro release can be significantly improved by increasing the percentage of anchor in the copolymer and thus favoring its binding to the liposomal bilayer. In the case of liposomes containing a poly(ethylene glycol)-phospholipid conjugate, the insertion of the pH-sensitive copolymer in the liposomal membrane appears to be sterically inhibited. Dye release from these formulations at acidic pH can still be achieved by varying the anchor molar ratio and/or molecular mass of the polymers or by including the latter during the liposome preparation procedure. Removal of unbound polymer results in decreased leakage only when the copolymer is inserted by incubation with preformed liposomes, but can be overcome by preparing liposomes in the presence of polymer. Aqueous content and lipid mixing assays suggest contents release can occur without membrane fusion. The results of this study indicate that the addition of pH-sensitive copolymers of NIPA represents promising strategy for improving liposomal drug delivery.

Formation and stability of the dispersed particles composed of retinoic acid, sesame oil and phosphatidylcholine

All trans-retinoic acid (RA) was dispersed by sonication with soybean phosphatidylcholine (PC). The particle size in the dispersion was increased to 240 nm up to the RA mol fraction range (X(RA)) of 0.4. At X(RA)=0.5, the RA/PC mixture was difficult to disperse and the macroscopic oil/water phase separation was observed. On the other hand, by the addition of sesame oil (SO) to RA (molar ratio of RA:SO=1:1), stable aqueous dispersions (diameter: 40-80 nm) were obtained in the mol fraction range RA and SO mixture (X(M)) of 0.1-0.8. In order to clarify these dispersal mechanism, the dispersed particles were characterized and the interaction among RA, SO and PC was investigated using several physicochemical techniques. The trapped aqueous volume inside the RA/PC particles was determined using the aqueous space marker, calcein and it was increased with the addition of RA into small unilamellar vesicles of PC. On the other hand, that of RA/SO/PC particles was decreased remarkably with increase in X(M) and the decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the PC membrane by the addition of the quencher CuSO(4). These results indicate that the interaction of RA with PC bilayers and the structure of RA/PC mixture will be changed by the addition of SO.

Interaction of ubiquinone-10 with dipalmitoylphosphatidylcholine and their formation of small dispersed particles

Stable aqueous dispersions of ubiquinone-10 (UQ) were obtained by cosonication with dipalmitoylphosphatidylcholine (DPPC) in the UQ mole fraction range 0.1-0.7. To clarify the dispersal mechanism, the dispersed particles were characterized, and the interaction between UQ and DPPC was investigated using several physicochemical techniques. Dynamic light scattering (DLS) measurements showed that the diameter of the dispersed particles was 50-70 nm. A limited amount of UQ was incorporated into DPPC bilayer membranes (approximately 5 mol%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein, and the volume in the UQ/DPPC particles decreased remarkably with the addition of UQ into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess UQ separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer.

Serum albumin-lipid membrane interaction influencing the uptake of porphyrins

It is frequently observed in pharmaceutical practice that entrapped substances are lost rapidly when liposomes are used as carriers to introduce substances into cells. The reason for the loss is the interaction of serum components with liposomes. To elucidate the mechanism of this phenomenon the partition of mesoporphyrin (MP) was systematically studied in model systems composed of various lipids and human serum albumin (HSA). As surface charge is an important factor in the interaction, neutral (1, 2-dimyristoyl-sn-glycero-3-phosphatidylcoline, DMPC) and negatively charged (1,2-dimyristoyl-sn-glycero-3-phosphatidylcoline/1, 2-dimyristoyl-sn-glycero-3-phosphatidylglycerol, DMPC/DMPG = 19/1 w/w) lipids were compared. The liposome/apomyoglobin system was the negative control. The size distribution of sonicated samples was carefully analyzed by dynamic light scattering. Constants of association of MP to the proteins and to the liposomes were determined: K(p,1) = (2.5 +/- 0.7) x 10(7) M(-1), K(p,2) = (1.0 +/- 0.7) x 10(8) M(-1), K(L,1) = (1.3 +/- 0.3) x 10(5) M(-1), and K(L,2) = (3.2 +/- 0.6) x 10(4) M(-1) for HSA, apomyoglobin, DMPC, and DMPC/DMPG liposomes, respectively. These data were used to evaluate the partition experiments. The transfer of MP from the liposomes to the proteins was followed by fluorescence spectroscopy. In the case of apomyoglobin, the experimental points could be interpreted by ruling out the protein-liposome interaction. In the case of HSA, the efflux of MP from the liposomes was strongly inhibited above a critical HSA concentration range for negatively charged vesicles. This effect was interpreted as the result of HSA coat formation on the liposome surface. This direct interaction is significant for small liposomes. The interpretation is fully supported by differential scanning calorimetry experiments.

Interaction of sesame oil with soybean phosphatidylcholine and their formation of small dispersed particles

Stable aqueous dispersions of sesame oil (SO) were obtained by co-sonication with soybean phosphatidylcholine (PC) in the SO mole fraction range of 0.1-0.8. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction of SO with PC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 40-60 nm. The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker, calcein. The trapped volume in the SO/PC particles decreased remarkably with the addition of SO into small unilamellar vesicles of PC. The decline in fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the PC membrane by the addition of the quencher CuSO4. These results indicate that the excess SO separated from the PC bilayers is stabilized as emulsion particles by the PC surface monolayer. Monolayer-bilayer equilibrium of SO/PC mixtures was estimated by measurements of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of PC + core of SO) with vesicular particles (bilayer) was critically important for the formation of stably dispersed particles of the lipid mixture.

Regulation of flavin dehydrogenase compartmentalization: requirements for PutA-membrane association in Salmonella typhimurium

PutA is a multifunctional, peripheral membrane protein which functions both as an autogenous transcriptional repressor and the enzyme which catalyzes the two-step conversion of proline to glutamate in Salmonella typhimurium and Escherichia coli. To understand how PutA associates with the membrane, we determined the role of FAD redox and membrane components in PutA-membrane association. Reduction of the tightly bound FAD is required for both derepression of the put operon and membrane association of PutA. FADH(2) alters the conformation of PutA, resulting in an increased hydrophobicity. Previous studies used enzymatic activity as an assay for membrane association and concluded that electron transfer from the reduced FAD in PutA to the membrane is required for the PutA-membrane interaction. However, direct physical assays of PutA association with membrane vesicles from quinone deficient mutants demonstrated that although electron transfer is essential for proline dehydrogenase activity, it is not required for PutA-membrane association per se. Furthermore, PutA efficiently associated with liposomes, indicating that PutA-membrane association does not require interactions with other membrane proteins. PutA enzymatic activity can be efficiently reconstituted with liposomes containing ubiquinone and cytochrome bo, confirming that proline dehydrogenase can pass electrons directly to the quinone pool. These results indicate that PutA-membrane association is due strictly to a protein-lipid interaction initiated by reduction of FAD.

Pore-forming action of mastoparan peptides on liposomes: a quantitative analysis

We have investigated the wasp venom peptides mastoparan X and polistes mastoparan regarding their apparent potential to induce pore-like defects in phosphatidylcholine unilamellar vesicles. Based on a fundamental theoretical model, the pore activation and deactivation kinetics have been evaluated from the observed efflux of liposome entrapped carboxyfluorescein in relation to the bound peptide to lipid ratio. We can quantitatively describe our experimental data very well in terms of a specific reaction scheme resulting in only a few short-lived pores. They evidently emerge rapidly from a prepore nucleus being produced by two rate-limiting monomeric states of bound peptide. These peculiar states would be favorably populated in an early stage of bilayer perturbation, but tend to die out in the course of a peptide/lipid restabilization process.

Distribution characteristics of entrapped recombinant human erythropoietin in liposomes and its intestinal absorption in rats

Recombinant human erythropoietin (Epo) is frequently administered by intravenous (i.v.) injection for the clinical treatment of renal anemia. Oral (per os; p.o.) administration is desired as an alternative route to i.v. administration, and liposomes have been chosen as a drug carrier. We found previously that after a p.o. administration to rats of Epo entrapped in liposomes before gel filtration, the Epo was absorbed, but variability in the number of days of appearance and in the levels of pharmacological effects, i.e. , the peak of circulating reticulocyte counts (RTC), was observed. The purpose of the present study was to examine the distribution characteristics of Epo in liposomes and intestinal absorption of liposomal Epo in rats by using purified Epo entrapped in liposomes after gel filtration (Epo/liposomes). The distribution characteristics of Epo/liposomes were determined by measuring the Epo in liposomes by a radioimmunoassay, high-performance liquid chromatography and zeta potential measurements. We observed that the protein part of Epo was mostly entrapped in liposomes, and was not adsorbed by the liposomal membrane at middle and high Epo p.o. doses, but the zeta potential of the Epo/liposomes increased negatively with the increase in the Epo p.o. doses. These results suggest that the sialic acid part of Epo entrapped in liposomes may project out from liposomes, depending on the entrapped Epo concentration. Little Epo was adsorbed or penetrated into liposomes when it was added to empty liposomes. After the p. o. administration of Epo/liposomes, the peak of RTC appeared at a 2-day delay on day 6, without variation and without dose dependency in comparison with that after i.v. administration. These results suggest that one of the reasons for the variability may be because the non-entrapped Epo and/or Epo/liposomes itself affected the intestinal absorption of Epo/liposomes. In conclusion, Epo/liposomes without nonentrapped Epo may be clinically useful for the oral administration of Epo.

Formation and structure of stably dispersed particles composed of retinal with dipalmitoylphosphatidylcholine: coexistence of emulsion particles with bilayer vesicles

In order to develop an intravenous formulation of all-trans-retinal (vitamin A aldehyde, VAA) for the treatment of night blindness, VAA and dipalmitoylphosphatidylcholine (DPPC) were sonicated and the dispersions in the VAA mole fraction range of 0.1-0.7 were stable at room temperature for 3 days. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between VAA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50-70 nm. A limited amount of VAA is incorporated into DPPC bilayer membranes (approximately 5 mole%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein and the volume in the VAA/DPPC particles was decreased remarkably with the addition of VAA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO(4). These results indicate that the excess VAA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer. The monolayer-bilayer equilibrium of VAA/DPPC mixtures was estimated by measurement of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of DPPC+core of VAA) with vesicular particles (bilayer) was critically important for the formation of the stably dispersed particles of the lipid mixture.

Interaction of alpha-Tocopherol and Soybean Oil with Phosphatidylcholine and Their Formation of Small Dispersed Particles

In this study, alpha-tocopherol (alpha-T) was dispersed by cosonication with soybean phosphatidylcholine (PC). The particle size in the dispersion was increased to 250 nm up to the alpha-T mole fraction (XT) 0.4. At XT = 0.5, the alpha-T/PC mixture was difficult to disperse and the macroscopic oil/water phase separation was observed. By the addition of soybean oil (SO) to alpha-T (molar ratio alpha-T:SO = 1:1), stable aqueous dispersions (diameter 50-70 nm) were obtained in the mole fraction range for the alpha-T and SO mixture (XM) 0.1-0.8. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction among alpha-T, SO, and PC was investigated using several physicochemical techniques. The trapped aqueous volume inside the alpha-T/PC particles was determined using the aqueous space marker calcein, and this volume was increased with the addition of alpha-T into small unilamellar vesicles of PC. The trapped aqueous volume of alpha-T/SO/PC particles was decreased remarkably with an increase in XM, and the decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the PC membrane by the addition of the quencher CuSO4. These results indicate that the interaction of alpha-T with PC bilayers and the structure of the alpha-T/PC mixture will be changed by the addition of SO. Copyright 1999 Academic Press.

Interaction of retinol with dipalmitoylphosphatidylcholine and their formation of small dispersed particles

Stable aqueous dispersions of all-trans-retinol (vitamin A, VA) were obtained by sonication with dipalmitoylphos-phatidylcholine (DPPC) in the VA mole fraction range 0.1-0.7. In order to clarify the dispersal mechanism, the dispersed particles were characterized and the interaction between VA and DPPC was investigated using several physicochemical techniques. Dynamic light scattering measurements showed that the diameter of the dispersed particles was 50-70 nm. A limited amount of VA was incorporated into DPPC bilayer membranes (approximately 5 mol%). The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein and the volume in the VA/DPPC particles was decreased markedly with the addition of VA into small unilamellar vesicles of DPPC. The decline in the fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess VA separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer.

Flow cytometric detection of transbilayer movement of fluorescent phospholipid analogues across the boar sperm plasma membrane: elimination of labeling artifacts

Reliable protocols were established for investigating asymmetric distributions of 6-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino-caproyl (C6NBD) phospholipids in the plasma membrane of boar sperm cells under physiological conditions. A method based on fluorescence resonance energy transfer was used to ensure that incorporation of the fluorescent phospholipids into the sperm proceeded via monomeric transfer. The total amount of incorporated phospholipid fluorescence and the proportion of translocated phospholipid fluorescence were determined by flow cytometric analysis before, and after, dithionite destruction of outer leaflet fluorescence. Catabolism of incorporated fluorescent phospholipids was blocked with phenylmethylsulfonyl fluoride. Membrane-damaged cells were detected with impermeant DNA stains, thereby enabling their exclusion from subsequent analyses of the flow cytometric data, whence it could be demonstrated that the labeled phospholipids were incorporated only via the outer plasma membrane leaflet in living sperm cells. Phospholipid uptake and internalization was followed at 38 degrees C. After 1 hr of labeling, about 96% of the incorporated C6NBD-phosphatidylserine, 80% of C6NBD-phosphatidylethanolamine, 18% of C6NBD-phosphatidylcholine, and 4% of C6NBD-sphingomyelin were found to have moved across the plasma membrane bilayer to the interior of the spermatozoa. These inward movements of fluorescent phospholipids were ATP-dependent and could be blocked with sulfhydryl reagents. Movements from the inner to the outer leaflet of the sperm plasma membrane were minimal for intact fluorescent phospholipids, but were rapid and ATP-independent for fluorescent lipid metabolites. The described method enables, for the first time, assessment of changes in lipid asymmetry under fertilizing conditions.

Interaction of soybean oil with phosphatidylcholine and their formation of small dispersed particles

Stable aqueous dispersions of soybean oil (SO) were obtained by cosonication with dipalmitoylphosphatidylcholine (DPPC) in the SO mole fraction range 0.1-0.8. To clarify the dispersal mechanism, the dispersed particles were characterized, and the interaction between SO and DPPC was investigated using several physicochemical techniques. Dynamic light scattering (DLS) measurements showed that the diameter of the dispersed particles was 40-60 nm. The trapped aqueous volume inside the particles was determined fluorometrically using the aqueous space marker calcein. The trapped volume in the SO/DPPC particles decreased remarkably with the addition of SO into small unilamellar vesicles of DPPC. The decline in fraction of vesicular particles was also confirmed by fluorescence quenching of N-dansylhexadecylamine in the DPPC membrane by the addition of the quencher CuSO4. These results indicate that the excess SO separated from the DPPC bilayers is stabilized as emulsion particles by the DPPC surface monolayer. Monolayer-bilayer equilibrium of SO/DPPC mixtures was estimated by measurement of spreading and collapse pressures. The results showed that the coexistence of emulsion particles (surface monolayer of DPPC + core of SO) with vesicular particles (bilayer) was critically important for the formation of stably dispersed particles of the lipid mixture.

Genetics of synaptic vesicle function: toward the complete functional anatomy of an organelle

Synaptic transmission starts with the release of neurotransmitters by exocytosis of synaptic vesicles. As a relatively simple organelle with a limited number of components, synaptic vesicles are in principle accessible to complete structural and functional genetic analysis. At present, the majority of synaptic vesicle proteins has been characterized, and many have been genetically analyzed in mice, Drosophila, and Caenorhabditis elegans. These studies have shown that synaptic vesicles contain proteins with diverse structures and functions. Although the genetic studies are as yet unfinished, they promise to lead to a full description of synaptic vesicles as macromolecular machines involved in all aspects of presynaptic neurotransmitter release.

A fluorescence resonance energy transfer approach for monitoring protein-mediated glycolipid transfer between vesicle membranes

A lipid transfer protein, purified from bovine brain (23.7 kDa, 208 amino acids) and specific for glycolipids, has been used to develop a fluorescence resonance energy transfer assay (anthrylvinyl-labeled lipids; energy donors and perylenoyl-labeled lipids; energy acceptors) for monitoring the transfer of lipids between membranes. Small unilamellar vesicles composed of 1 mol% anthrylvinyl-galactosylceramide, 1.5 mol% perylenoyl-triglyceride, and 97.5% 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) served as donor membranes. Acceptor membranes were 100% POPC vesicles. Addition of glycolipid transfer protein to mixtures of donor and acceptor vesicles resulted in increasing emission intensity of anthrylvinyl-galactosylceramide and decreasing emission intensity of the nontransferable perylenoyl-triglyceride as a function of time. The behavior was consistent with anthrylvinyl-galactosylceramide being transferred from donor to acceptor vesicles. The anthrylvinyl and perylenoyl energy transfer pair offers advantages over frequently used energy transfer pairs such as NBD and rhodamine. The anthrylvinyl emission overlaps effectively the perylenoyl excitation spectrum and the fluorescence parameters of the anthrylvinyl fluorophore are nearly independent of the medium polarity. The nonpolar fluorophores are localized in the hydrophobic region of the bilayer thus producing minimal disturbance of the bilayer polar region. Our results indicate that this method is suitable for assay of lipid transfer proteins including mechanistic studies of transfer protein function.

Remote loading of diclofenac, insulin and fluorescein isothiocyanate labeled insulin into liposomes by pH and acetate gradient methods

Remote loading of the model drugs diclofenac, insulin and fluorescein isothiocyanate labeled insulin (FITC-insulin) into liposomes by formation of transmembrane gradients were examined. A trapping efficiency of almost 100% was obtained for liposomal diclofenac, by the calcium acetate gradient method, whereas liposomes prepared by the conventional reverse-phase evaporation vesicle method had 1-8% trapping efficiencies. Soybean-derived sterol was a better stabilizer of the dipalmitoylphosphatidylcholine bilayer membrane than cholesterol, as shown from trapping efficiencies and drug release. The pH gradient method resulted in a 5-50% of FITC-insulin liposomal trapping efficiency, while insulin could not be loaded by this method. Liposomes released calcein in response to insulin, showing insulin interacts with the liposomal membrane in the presence of a transmembrane gradient. The present work has demonstrated a remote loading method for weak acids such as diclofenac into liposomes by the acetate gradient method. From the result of remote loading of FITC-insulin into liposomes by the pH gradient method, this method may be available for the preparation of liposomal peptides.

Partitioning of triphenylalkylphosphonium homologues in gel bead-immobilized liposomes: chromatographic measurement of their membrane partition coefficients

Unilamellar liposomes of small or large size, SUVs and LUVs, respectively, were stably immobilized in the highly hydrophilic Sepharose 4B or Sephacryl S-1000 gel beads as a membrane stationary phase for immobilized liposome chromatography (ILC). Lipophilic cations of triphenylmethylphosphonium and tetraphenylphosphonium (TPP+) have been used as probes of the membrane potential of cells. Interaction of TPP+ and triphenylalkylphosphonium homologues with the immobilized liposomal membranes was shown by their elution profiles on both zonal and frontal ILC. Retardation of the lipophilic cations on the liposome gel bed was increased as the hydrophobicity of the cations increased, indicating the partitioning of lipophilic cations into the hydrocarbon region of the membranes. The cations did not retard on the Sepharose or Sephacryl gel bed without liposomes, confirming that the cations only interact with the immobilized liposomes. Effects of the solute concentration, flow rate, and gel-matrix substance on the ILC were studied. The stationary phase volume of the liposomal membranes was calculated from the volume of a phospholipid molecule and the amount of the immobilized phospholipid, which allowed us to determine the membrane partition coefficient (KLM) for the lipophilic cations distributed between the aqueous mobile and membrane stationary phases. The values of KLM were generally increased with the hydrophobicity of the solutes increased, and were higher for the SUVs than for the LUVs. The ILC method described here can be applied to measure membrane partition coefficients for other lipophilic solutes (e.g., drugs).

Surface specific kinetics of lipid vesicle adsorption measured with a quartz crystal microbalance

We have measured the kinetics of adsorption of small (12.5-nm radius) unilamellar vesicles onto SiO2, oxidized gold, and a self-assembled monolayer of methyl-terminated thiols, using a quartz crystal microbalance (QCM). Simultaneous measurements of the shift in resonant frequency and the change in energy dissipation as a function of time provide a simple way of characterizing the adsorption process. The measured parameters correspond, respectively, to adsorbed mass and to the mechanical properties of the adsorbed layer as it is formed. The adsorption kinetics are surface specific; different surfaces cause monolayer, bilayer, and intact vesicle adsorption. The formation of a lipid bilayer on SiO2 is a two-phase process in which adsorption of a layer of intact vesicles precedes the formation of the bilayer. This is, to our knowledge, the first direct evidence of intact vesicles as a precursor to bilayer formation on a planar substrate. On an oxidized gold surface, the vesicles adsorb intact. The intact adsorption of such small vesicles has not previously been demonstrated. Based on these results, we discuss the capacity of QCM measurements to provide information about the kinetics of formation and the properties of adsorbed layers.

Rate constants for quenching singlet oxygen and activities for inhibiting lipid peroxidation of carotenoids and alpha-tocopherol in liposomes

The (1)O2 quenching rate constants (kQ) of alpha-tocopherol (alpha-Toc) and carotenoids such as beta-carotene, astaxanthin, canthaxanthin, and lycopene in liposomes were determined in light of the localization of their active sites in membranes and the micropolarity of the membrane regions, and compared with those in ethanol solution. The activities of alpha-Toc and carotenoids in inhibiting (1)O2-dependent lipid peroxidation (reciprocal of the concentration required for 50% inhibition of lipid peroxidation: [IC50](-1)) were also measured in liposomes and ethanol solution and compared with their kQ values. The kQ and [IC50](-1) values were also compared in two photosensitizing systems containing Rose bengal (RB) and pyrenedodecanoic acid (PDA), respectively, which generate (1)O2 at different sites in membranes. The kQ values of alpha-Toc were 2.9 x 10(8) M(-1) s(-1) in ethanol solution and 1.4 x 10(7) M(-1) s(-1) (RB system) or 2.5 x 10(6) M(-1) s(-1) (PDA system) in liposomes. The relative [IC50](-1) value of alpha-Toc in liposomes was also five times higher in the RB system than in the PDA-system. In consideration of the local concentration of the OH-group of alpha-Toc in membranes, the kQ value of alpha-Toc in liposomes was recalculated as 3.3 x 10(6) M(-1) s(-1) in both the RB and PDA systems. The kQ values of all the carotenoids tested in two photosensitizing systems were almost the same. The kQ value of alpha-Toc in liposomes was 88 times less than in ethanol solution, but those of carotenoids in liposomes were 600-1200 times less than those in ethanol solution. The [IC50](-1) value of alpha-Toc in liposomes was 19 times less than that in ethanol solution, whereas those of carotenoids in liposomes were 60-170 times less those in ethanol solution. There were no great differences (less than twice) in the kQ and [IC50](-1) values of any carotenoids. The kQ values of all carotenoids were 40-80 times higher than that of alpha-Toc in ethanol solution but only six times higher that of alpha-Toc in liposomes. The [IC50](-1) values of carotenoid were also higher than that of alpha-Toc in ethanol solution than in liposomes, and these correlated well with the kQ values.