Single bilayer vesicles prepared without sonication. Physico-chemical properties

Enhanced biopotency of synthetic C3a analogues by membrane binding. A fluorescence anisotropy decay study

The biological activity of oligopeptide analogues of C3a is markedly increased by N-terminal attachment of a hydrophobic group as, for instance, 9-fluorenylmethoxycarbonyl (Fmoc), either direct or via a flexible 6-aminohexanoyl (Ahx) spacer. This study presents evidence from fluorescence anisotropy decay measurements that the hydrophobic appendix mediates non-specific binding of the synthetic peptide analogues to phospholipid vesicles. According to quantitative considerations no alternative or additional rate-enhancing mechanisms other than surface diffusion are required to account for the gain in biopotency.

Temperature-reversible eruptions of vesicles in model membranes studied by NMR

Deuterium (2H) and phosphorus (31P) nuclear magnetic resonance (NMR) and freeze-fracture electron microscopy were used to study spontaneous vesiculation in model membranes composed of POPC:POPS with or without cholesterol. The NMR spectra indicated the presence of a central isotropic line, the intensity of which is reversibly and linearly dependent upon temperature in the L alpha phase, with no hysteresis when cycling between higher and lower temperatures. Freeze-fracture microscopy showed small, apparently connected vesicles that were only present when the samples were frozen (for freeze-fracture) from an initial temperature of 40-60 degrees C, and absent when the samples are frozen from an initial temperature of 20 degrees C. Analysis of motional narrowing was consistent with the isotropic lines being due to lateral diffusion in (and tumbling of) small vesicles (diameters approximately 50 nm). These results were interpreted in terms of current theories of shape fluctuations in large unilamellar vesicles which predict that small daughter vesicles may spontaneously "erupt" from larger parent vesicles in order to expel the excess area created by thermal expansion of the bilayer surface at constant volume. Assuming that all the increased area due to increasing temperature is associated with the isotropic lines, the NMR results allowed a novel estimate of the coefficient of area expansion alpha A in multilamellar vesicles (MLVs) which is in good agreement with micromechanical measurements upon giant unilamellar vesicles of similar composition. Experiments performed on unilamellar vesicles, which had been placed upon glass beads, confirmed that alpha A determined in this way is unchanged compared with the MLV case. Addition of the highly positively charged (extrinsic) myelin basic protein (MBP) to a POPC:POPS system showed that membrane eruptions of the type described here occur in response to the presence of this protein.

The chemical synthesis of N-[1-(2-naphthol)]-phosphatidylethanolamine, a fluorescent phospholipid for excited-state proton transfer studies

A procedure for the preparation of N-[1-(2-naphthol)]-phosphatidylethanolamine (NAPH-PE) has been developed. The synthesis is based on the Schiff base formation between the NH2 of the phospholipid and the aldehyde moiety of 2-hydroxy-1-naphthaldehyde. Then selective reduction of the imine is used to obtain the stable secondary amine, NAPH-PE. Formation of the intermediate Schiff base and the final product is confirmed by 13C- and 1H-NMR. Similar to free 2-naphthol, the excited-state pKa (pKa*) of its phospholipid derivative appears to be significantly lower than the ground-state pKa. At pH 7.4, the excitation spectrum of NAPH-PE shows no deprotonated species in the ground-state, while the emission spectrum presents a significant contribution of this species. Thus the fluorescent phospholipid exhibits the typical behavior of excited-state proton-transfer probes. NAPH-PE is found to incorporate in dimyristoyllecithin (DML) vesicles. The emission spectrum of the probe inserted in the liposomes is affected by acetate used as a proton acceptor. These properties should also be manifest in other lipid bilayers (e.g., plasma membranes of cells) and used for excited-state proton transfer studies.

The size dependence of cholate-dialyzed vesicles on phosphatidylcholine concentration

The size dependence of vesicles prepared by dialysis of cholate from phosphatidylcholine (PC) dispersions has been investigated as a function of lipid concentration (at a constant applied lipid: detergent molar ratio of 0.7). Gel filtration of dialyzed samples produced a symmetrical profile shape, although quasielastic laser light scattering analysis of the fractions revealed an asymmetrical range of sizes about the peak for solutions containing elevated lipid concentrations. Vesicle diameters increased by approximately 20 nm for PC concentrations ranging from 10 to a maximum of 45 mg/ml. This was attributed to mixed micelle sizes being proportional to lipid concentration, since the diameters of vesicles produced from dialysis are determined by mixed micelle sizes. Before commencement of dialysis, mixed micelle sizes are proportional to lipid concentration and, although dialysis causes an increase in mixed micelle sizes, the phase ratios attained are larger for solutions containing elevated lipid concentrations.

Activation of reconstituted Escherichia coli outer-membrane phospholipase A by membrane-perturbing peptides results in an increased reactivity towards the affinity label hexadecanesulfonyl fluoride

The activity of the Escherichia coli outer-membrane phospholipase (OM PLA) is strictly regulated in its natural habitat, the E. coli outer membrane. OM PLA can be reconstituted in phospholipid bilayers, resulting in low specific activity of the enzyme compared to its activity on mixed lipid/detergent micelles. The enzyme can be activated by the addition to these vesicles of the membrane-perturbing peptides polymyxin B, melittin or cardiotoxin resulting in hydrolysis of mainly the sn-1 ester bond of the phospholipids as is also observed in vivo. We used the affinity label hexadecanesulfonyl fluoride to probe the influence of lipid environment on the activity of OM PLA. In detergent and substrate micelles, the rate constant for the sulfonylation of the active-center serine of the purified OM PLA by the affinity label hexadecanesulfonyl fluoride depends on amphiphile concentration. We have reported a similar influence of amphiphile concentration on the activity of the enzyme [Horrevoets, A. J. G. et al. (1989) Biochemistry 28, 1139-1147]. Analysis of the rates of inactivation of OM PLA by hexadecanesulfonyl fluoride in vesicles composed of various phospholipids indicated that activation of the enzyme by membrane-perturbing peptides can be accurately quantified with this affinity label. Our results show that the affinity label hexadecanesulfonyl fluoride can be used to monitor the state of activation of OM PLA in different lipid environments, including non-hydrolyzable substrate analogues. Implications for the in vivo situation are discussed.

Liposome chromatography: liposomes immobilized in gel beads as a stationary phase for aqueous column chromatography

Liposomes have been used as a stationary phase for column chromatography with an aqueous mobile phase. They were immobilized in the pores of carrier gel beads by two methods: (A) hydrophobic ligands were coupled to the matrix of gel beads, which then were packed into a column and liposomes were applied and became associated with the ligands by hydrophobic interaction; and (B) phospholipids and detergent were dialysed in the presence of gel beads; many of the liposomes that formed in the pores of the beads were sterically immobilized by the gel matrix. Proteoliposomes containing red cell glucose transport protein in the lipid bilayers were immobilized in a column by method A. This column retained D-glucose longer than L-glucose. In contrast to L-glucose, D-glucose was transported into and out of the immobilized liposomes, causing an increased retention. Liposomes with (stearylamine)+ or (phosphatidylserine)- in their lipid bilayers were immobilized by method B and the gel beads were packed into a column. A protein of opposite charge was applied in excess. Under suitable conditions, the protein molecules became close-packed on the liposome surfaces. Ion-exchange chromatographic experiments with proteins showed that these sterically immobilized liposomes were also stable enough to be used as a stationary phase. The loss of lipids was 5-23% in the first run at high protein load and with sodium chloride gradient elution but was lower in subsequent runs. It is proposed that water-soluble molecules can be separated and their interactions with liposome surfaces studied by chromatography on immobilized liposomes in detergent-free aqueous solution. Membrane proteins can be inserted and ligands can be anchored in the lipid bilayers for chromatographic purposes.

Pharmacological and biochemical properties of saxiphilin, a soluble saxitoxin-binding protein from the bullfrog (Rana catesbeiana)

Supernatant fractions of various tissues and plasma from the North American bullfrog, Rana catesbeiana, specifically bind saxitoxin with high affinity. Binding of [3H]saxitoxin to bullfrog plasma follows single-site behavior with an equilibrium dissociation constant of Kd = 0.16 +/- 0.03 nM at 0 degrees C and a maximum binding capacity of 380 +/- 60 pmole/ml plasma. High-affinity binding of [3H]saxitoxin is chemically specific since it is unaffected by tetrodotoxin and a variety of cationic peptides, amino acids and drugs. The structure-activity dependence of binding to this site was investigated with eight different natural and synthetic derivatives of saxitoxin. Substitution of the carbamoyl side chain or the C-12 beta-hydroxyl group of saxitoxin with a hydrogen atom had little effect on binding affinity, but addition of a hydroxyl group at the N-1 position decreased the binding affinity from 430- to 710-fold in three different molecular pairs. High performance size exclusion chromatography of supernatant from bullfrog skeletal muscle showed that the [3H]saxitoxin-binding component migrates with an apparent molecular weight of Mr = 74,000 +/- 8000 or a Stokes radius of 35 +/- 2A. The [3H]saxitoxin-binding protein in skeletal muscle extract or plasma is retained on a cation-exchange column at pH 6.0, suggesting that the protein contains a region of exposed basic residues. Column isoelectric focusing of a sample from plasma indicated that the protein has a basic isoelectric point near pH = 10.7.(ABSTRACT TRUNCATED AT 250 WORDS)

Chlorin e6-liposome interaction. Investigation by the methods of fluorescence spectroscopy and inductive resonance energy transfer

On the basis of spectral fluorescence and polarization measurements and results obtained on the luminescence quenching of the membrane fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) by incorporated chlorin e6 (chl e6) molecules, it is shown that the interaction of the water-soluble pigment with smaller unilamellar lipid vesicles occurs by a mechanism of partition between the aqueous and lipid phases (partition coefficient Kp = 6.7 x 10(3) and provides rigid fixing of chl e6 monomers at the boundary between the polar and non-polar parts of the lipid membrane. In terms of inductive resonance electronic excitation energy transfer between DPH and chl e6 (R0 = 36.2 A), we have analysed data on DPH fluorescence quenching under different conditions of chl e6 localization in the lipid bilayer and have concluded that the incorporation of the pigment molecules into the vesicles from the aqueous phase occurs mainly into the external monolayer.

Tumor cell growth inhibition by liposome-encapsulated aromatic polyamidines

Apart from its antiproteinase activity, the aromatic polyamidine TAPP-Br [the bromo derivative of 1,3-di-(p-amidinophenoxy)-2,2-bis-(p-amidinophenoxymethyl)propane (TAPP-H)] is able to inhibit the in vitro growth of a variety of tumor cell lines, including human melanoma, and breast and kidney carcinoma. We have now shown that TAPP-Br can efficiently be encapsulated into egg phosphatidylcholine vesicles. When incorporated into these liposomes, the inhibitory effect of TAPP-Br is significantly enhanced compared with that of the free drug. Based on these promising results, a proposal is made for the delivery of this antiproliferative agent to tumor cells by using liposomes as the vehicle.

Adenosine triphosphate liposomes: encapsulation and distribution studies

Four methods for encapsulating adenosine triphosphate (ATP) in liposomes were evaluated. Optimum entrapment required emulsifying ATP with the lipids used to form the liposome membrane in a high-speed homogenizer followed by evaporating the organic solvent with vigorous stirring. Under these optimum conditions ATP entrapment was 38.9%; i.e., the dosage form contained 38.9 g of ATP per 100 g of lipid. The distribution of positively charged liposomes loaded with ATP was studied in dogs with experimentally induced myocardial infarction. Intravenous injection of positively charged ATP liposomes caused accumulation of ATP in myocardial infarct tissue. Myocardial infarct tissue has reduced blood flow; since positively charged liposomes accumulated in infarct tissue, liposomes may be a drug delivery system for this disease state.

Differential effects of liposome-entrapped desferrioxamine on proliferation and erythroid differentiation of murine erythroleukemic Friend cells

It is known that iron chelators (such as desferrioxamine) are potent inhibitors of both cell proliferation and erythroid differentiation. We have shown with in vitro studies that in the case of tumor cells desferrioxamine is even more efficient in inhibiting cell proliferation when entrapped in liposomes consisting of egg yolk phosphatidylcholine. At the same time liposome-entrapped desferrioxamine retains only a slight effect on hexamethylenebisacetamide induction of erythroid differentiation and hemoglobin accumulation of murine erythroleukemic Friend cells. Based on these findings, we propose liposome-entrapped desferrioxamine as potential antineoplastic agent as well as a specific chemical for the study of both iron metabolism and distribution in normal and neoplastic cells. In addition, unlike free desferrioxamine, the liposome-entrapped drug could also be used in combination with inducers of differentiation. With respect to this issue, it is possible that liposome-entrapped desferrioxamine, might permit erythroid differentiation of both neoplastic cells as well as normal stem cells.

Identification of a sulfoglycolipid epitope shared by cells of neuroectodermal and hematopoietic origin

Monoclonal antibody (mAb) SNH.1 detects an epitope which is restricted to cells of neuroectodermal and hematopoietic origin. The mAb was obtained by immunization of a mouse with liposomes containing a crude extract of human melanoma acidic glycolipids. The SNH.1 antigen isolated from melanoma was identified as a sulfated glycolipid, closely related or identical to sulfogalactosyl-ceramide. When tested with different lipids, mAb SNH.1 reacted as well with other sulfoglycolipids. The staining of mAb SNH.1 is restricted to the cytoplasm and often localized to the perinuclear region. Therefore, the SNH.1 mAb epitope may be detectable only during the biosynthesis of sulfoglycolipids.

Effects of normalization errors on size distributions obtained from dynamic light scattering data

This paper presents a study of the influence of normalization errors on size distributions obtained from the analysis of intensity fluctuations by photon correlation spectroscopy. The effects of these errors are demonstrated by means of computer-generated autocorrelation functions simulating light scattered from a monomodal Schulz distribution of small, spherical, unilamellar lipid vesicles. The calculations show that even small errors in the baseline, modifying the data upon normalization systematically, will cause serious errors in the estimated size distribution. As it turns out this is due to the peculiar characteristics of normalization errors in data of the first order autocorrelation function. The errors introduced there are described in parts by functions of the delay time having positive exponents. Such components are not considered in the integral equations commonly used to analyze the measured data. The error's property to be a function of delay time in turn enables us to obtain the relative baseline error from the inversion of the data. The new method for its determination is described in some detail. Here, it has been realized with a modified version of the size distribution algorithm CONTIN.

Photoinduced electron transfer across lipid bilayers containing magnesium octaethylporphyrin

Both photoinitiated (thermodynamically downhill) and photodriven (thermodynamically uphill) electron transfer reactions across lipid bilayers are sensitized by magnesium octaethyl porphyrin (MgOEP). It is shown that the reaction mechanism is via reduction of photoexcited MgOEP at the reducing (ascorbate) side of the bilayer and the charge carrier is likely the neutral protonated MgOEP anion. The MgOEP cation (or its neutral form) does not contribute to charge passage across the bilayer even though it is readily formed at the acceptor (ferricyanide or methyl viologen) side of the membrane. Photoelectric measurements on planar bilayers show that the time constant for reduction of excited MgOEP is about 10 microseconds with 10 mM ascorbate. The membrane transport of the mediator appears to be rate limiting when the reaction is photoinitiated and the interfacial reaction appears to be limiting when the reaction is photodriven. The quantum yield of the process is about 0.1 in the latter case and about 0.02 in the former. The former yield is increased to about 0.15 in the presence of a redox mediator, duroquinone. In these systems, the magnesium porphyrin is both sensitizer and trans membrane redox mediator.

Solubilization and reconstitution of the sodium-and-chloride-coupled glycine transporter from rat spinal cord

Synaptic membranes from rat spinal cord were solubilized in the presence of 2% sodium cholate, phospholipids and 15% ammonium sulphate. The soluble extract was incorporated into liposomes consisting of asolectin and crude rat brain lipids. Reconstitution of the functional transporter protein was achieved by removal of detergent by gel filtration. Several parameters proved to be important for optimal reconstitution efficiency: (a) the lipid composition of the liposomes, (b) the type of detergent, and (c) the phospholipid/protein and detergent/protein ratio during reconstitution. In the reconstituted system, the transport of glycine showed a specific activity about twice that of native vesicles. The ionic dependence of the transport, the inhibitory effect of nigericin in the presence of external sodium and the stimulatory effect of valinomycin in the presence of internal potassium on glycine transport were preserved and more clearly observed in the reconstituted system. These results indicate that, in this preparation, the glycine transporter protein retains the same features displayed in the synaptic plasma membrane vesicles, namely dependence on sodium and chloride, electrogenicity and inhibitor sensitivity.

Electrostatic and hydrophobic interactions of synapsin I and synapsin I fragments with phospholipid bilayers

Synapsin I, a major neuron-specific phosphoprotein, is localized on the cytoplasmic surface of small synaptic vesicles to which it binds with high affinity. It contains a collagenase-resistant head domain and a collagenase-sensitive elongated tail domain. In the present study, the interaction between synapsin I and phospholipid vesicles has been characterized, and the protein domains involved in these interactions have been identified. When lipid vesicles were prepared from cholesterol and phospholipids using a lipid composition similar to that found in native synaptic vesicle membranes (40% phosphatidylcholine, 32% phosphatidylethanolamine, 12% phosphatidylserine, 5% phosphatidylinositol, 10% cholesterol, wt/wt), synapsin I bound with a dissociation constant of 14 nM and a maximal binding capacity of about 160 fmol of synapsin I/microgram of phospholipid. Increasing the ionic strength decreased the affinity without greatly affecting the maximal amount of synapsin I bound. When vesicles containing cholesterol and either phosphatidylcholine or phosphatidylcholine/phosphatidylethanolamine were tested, no significant binding was detected under any conditions examined. On the other hand, phosphatidylcholine vesicles containing either phosphatidylserine or phosphatidylinositol strongly interacted with synapsin I. The amount of synapsin I maximally bound was directly proportional to the percentage of acidic phospholipids present in the lipid bilayer, whereas the Kd value was not affected by varying the phospholipid composition. A study of synapsin I fragments obtained by cysteine-specific cleavage showed that the collagenase-resistant head domain actively bound to phospholipid vesicles; in contrast, the collagenase-sensitive tail domain, though strongly basic, did not significantly interact. Photolabeling of synapsin I was performed with the phosphatidylcholine analogue 1-palmitoyl-2-[11-[4-[3-(trifluoromethyl)diazirinyl]phenyl] [2-3H]undecanoyl]-sn-glycero-3-phosphocholine; this compound generates a highly reactive carbene that selectively interacts with membrane-embedded domains of membrane proteins. Synapsin I was significantly labeled upon photolysis when incubated with lipid vesicles containing acidic phospholipids and trace amounts of the photoactivatable phospholipid. Proteolytic cleavage of photolabeled synapsin I localized the label to the head domain of the molecule.(ABSTRACT TRUNCATED AT 400 WORDS)

Xanthene-dye-labelled phosphatidylethanolamines as probes of interfacial pH. Studies in phospholipid vesicles

We have been developing the use of plasma-membrane-bound fluorescent probes to measure the pH values at the surfaces of living chondrocytes. For this purpose, three lipophilic pH indicators were made by covalently binding the xanthene dyes fluorescein, eosin or dichlorofluorescein to the amino group of phosphatidylethanolamine. The probes were incorporated into phospholipid vesicles and the effect of pH on the fluorescence was characterized. Fluorescence was measured at a single emission wavelength during excitation at two wavelengths, and the ratio of the intensities was calculated. The experimentally observed pKobs. values were determined by fitting the fluorescence ratios to the Henderson-Hasselbalch equation. All three probes acted as pH indicators, and the eosinyl-, dichlorofluoresceinyl- and fluoresceinylphosphatidylethanolamines had pKobs. values of 3.5, 6.3 and 7.5 respectively. At physiological salt concentrations, changes in the composition of the vesicle membrane had little effect on these values. We concluded that these probes were promising candidates for the measurement of pH values at cell surfaces.

Reconstitution of the Rous sarcoma virus transforming protein pp60v-src into phospholipid vesicles

An artificial membrane system was developed to study the molecular basis for interaction of pp60v-src, the Rous sarcoma virus transforming protein, with lipid bilayers. pp60v-src was extracted from cell membranes by detergent solubilization and reincorporated into phospholipid vesicles. Reconstituted pp60v-src retained tyrosine kinase activity and was integrally associated with the liposome through a 10-kilodalton (kDa) amino-terminal domain. The same 10-kDa domain was shown to anchor pp60v-src to the plasma membrane of transformed cells. Reconstitution experiments performed with nonmyristylated pp60v-src proteins revealed that these polypeptides did not interact with phospholipid vesicles. In contrast, myristylated, soluble pp60v-src molecules (including a highly purified pp60v-src preparation) could be reconstituted into liposomes, but their interaction with the liposomal bilayer was not mediated by the 10-kDa amino-terminal domain. When membrane proteins were included during reconstitution of purified pp60v-src, binding through the 10-kDa anchor was restored. A model is presented to accommodate the different types of interactions of pp60v-src with liposomes; the model postulates the existence of an additional membrane component that anchors the pp60v-src polypeptide to the phospholipid bilayer.

Reconstitution of the Rous sarcoma virus transforming protein pp60v-src into phospholipid vesicles

An artificial membrane system was developed to study the molecular basis for interaction of pp60v-src, the Rous sarcoma virus transforming protein, with lipid bilayers. pp60v-src was extracted from cell membranes by detergent solubilization and reincorporated into phospholipid vesicles. Reconstituted pp60v-src retained tyrosine kinase activity and was integrally associated with the liposome through a 10-kilodalton (kDa) amino-terminal domain. The same 10-kDa domain was shown to anchor pp60v-src to the plasma membrane of transformed cells. Reconstitution experiments performed with nonmyristylated pp60v-src proteins revealed that these polypeptides did not interact with phospholipid vesicles. In contrast, myristylated, soluble pp60v-src molecules (including a highly purified pp60v-src preparation) could be reconstituted into liposomes, but their interaction with the liposomal bilayer was not mediated by the 10-kDa amino-terminal domain. When membrane proteins were included during reconstitution of purified pp60v-src, binding through the 10-kDa anchor was restored. A model is presented to accommodate the different types of interactions of pp60v-src with liposomes; the model postulates the existence of an additional membrane component that anchors the pp60v-src polypeptide to the phospholipid bilayer.

Liposomes as membrane model for study of lipid peroxidation

This article describes the properties, production and characterization of liposomes with special reference to their use as membrane model for the study of lipid peroxidation. It presents briefly the methods that can be used for the assay of liposomal lipid peroxidation and brings out the special advantages these liposomes provide in elucidating the mechanism of lipid peroxidation by different physical and chemical agents. Studies involving liposomal lipid peroxidation by different agents and the consequent changes in the structure and function of liposomal membrane have been reviewed briefly.

Phospholipid environment alters hormone-sensitivity of the purified insulin receptor kinase

Insulin receptor kinase, affinity-purified by adsorption and elution from immobilized insulin, is stimulated 2-3-fold by insulin in detergent solution. Reconstitution of the receptor kinase into leaky vesicles containing phosphatidylcholine and phosphatidylethanolamine (1:1, w/w) by detergent removal on Sephadex G-50 results in the complete loss of receptor kinase sensitivity to activation by insulin. Insulin receptors in these vesicles also exhibit an increase in their apparent affinity for 125I-insulin (Kd = 0.12 nM versus 0.76 nM). Inclusion of 8.3-16.7% phosphatidylserine into the reconstituted vesicles restores 40-50% of the insulin-sensitivity to the receptor kinase. An elevated apparent affinity for 125I-insulin of insulin receptors in vesicles containing phosphatidylcholine and phosphatidylethanolamine is also restored to the value observed in detergent solution by the inclusion of phosphatidylserine in the reconstituted system. The effect of phosphatidylserine on insulin receptor kinase appears specific, because cholesterol, phosphatidylinositol and phosphatidic acid are all unable to restore insulin-sensitivity to the receptor kinase. Autophosphorylation sites on the insulin receptor as analysed by h.p.l.c. of tryptic 32P-labelled receptor phosphopeptides are not different for insulin receptors autophosphorylated in detergent solution or for the reconstituted vesicles in the presence or absence of phosphatidylserine. These data indicate that the phospholipid environment of insulin receptors can modulate its binding and kinase activity, and phosphatidylserine acts to restore insulin-sensitivity to the receptor kinase incorporated into phosphatidylcholine/phosphatidylethanolamine vesicles.

Asymmetric disposition of detergents within vesicle bilayer and its effect on ion permeation through the membrane

Phospholipid vesicles were prepared by detergent removal using hydrophobic porous beads, Amberlite XAD-2, or dialysis from detergent-phospholipid mixed micelles. The liposomes formed were found to be mostly unilammellar vesicles. The vesicle diameter was estimated, by both quasi-elastic light-scattering and gel-exclusion chromatography on Sephacryl S-1000, to be 80 nm for the vesicles formed by removal of octaethylene glycol monododecyl ether by the bead method. The effect of detergents within a bilayer on ion permeation was demonstrated. When the content of octaethylene glycol monododecyl ether reached a molar ratio of 0.2, the intrinsic ion selectivity of the phospholipid membrane between anion and cation was diminished. The ion permeability measured for vesicles with detergent incorporated into initially detergent-free vesicles was about 10-times greater than that for vesicles with detergent remaining following the process of detergent removal. This observation was explained by the different disposition of the detergent in the bilayer, that is, when vesicles were formed by the removal of detergent from mixed micelles, the residual detergent became distributed in both the outer and inner leaflets, and when the detergent was incorporated into initially detergent-free vesicles, the detergent became distributed only in the outer leaflet within the experimental time limits. This idea was supported by the NMR studies. It was also found that, as a detergent, octaethylene glycol monododecyl ether has a stronger effect on ion permeation than octyl glucoside.

Dynamics of proteoliposome formation. Intermediate states during detergent dialysis

1. The intermediate structures formed during dialysis of mixtures of cholate, phospholipid and cytochrome c oxidase were analysed by gel chromatography and electron microscopy. Measurements of trapped phosphate and the degree of respiratory control were used to assess the integrity of the vesicular structures formed. Protein orientation in the bilayer was monitored by the accessibility of cytochrome c to cytochrome c oxidase. 2. The results indicate that proteoliposome formation by the detergent-dialysis procedure takes place in three distinct stages. In the first stage, cholate/phospholipid and cholate/phospholipid/protein micelles coexist in solution and grow in size as the detergent is slowly removed. At a detergent/phospholipid molar ratio of about 0.2, micelle fusion results in the formation of large bilayer aggregates permeable to both phosphate and cytochrome c. It is at this stage that cytochrome c oxidase is incorporated into the bilayer. In the final stage of dialysis the bilayer sheets fragment into small unilamellar vesicles. 3. The orientation of membrane protein in the final vesicles appears to be determined by the effect of protein conformation on the initial curvature of the bilayer sheets during the fragmentation process.

Asymmetric reassociation of calf spleen NAD+ glycohydrolase into liposomes

NAD+ glycohydrolase (NAD+ nucleosidase, EC 3.2.2.6) can be solubilized from calf spleen microsomes (microsomal fractions) by steapsin or by detergents to yield respectively a hydrophilic (i.e. water-soluble) and a hydrophobic form of the enzyme. The detergent-solubilized enzyme was successfully reassociated into phosphatidylcholine liposomes either by a cholate-dialysis or by a gel-filtration procedure. In both cases the incorporation of NAD+ glycohydrolase was found to be completely asymmetric, i.e. the active site of the enzyme was exposed only at the outer surface of the vesicles. By contrast, as judged by flotation experiments, the hydrophilic form of NAD+ glycohydrolase could not be reassociated into liposomes. These results are in agreement with the hypothesis that calf spleen NAD+ glycohydrolase is an amphipathic protein. When incorporated into large unilamellar vesicles composed of phosphatidylcholine, NAD+ glycohydrolase was not found to catalyse vectorial transfer of NAD+ by transglycosidation with nicotinamide as acceptor.

Mechanisms and consequences of cellular cholesterol exchange and transfer

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

Molecular interactions of the intrinsic activation complex of coagulation: binding of native and activated human factors IX and X to defined phospholipid vesicles

The assembly of proteins of the intrinsic activation complex has been partially elucidated. In the present study we examine the association of gamma-carboxylated serine proteinase zymogens factors IX and X, and their proteolytically activated counterparts factors IXa and Xa to unilamellar lipid vesicles of defined composition using three types of physical measurement. Utilizing relative light scatter to estimate the dissociation constants for binding in the presence of calcium ions, it appears that factor IXa (0.93 +/- 0.37 microM) may preferentially associate with phospholipids relative to factor IX (0.35 +/- 0.08 microM). In contrast, factor X (0.34 +/- 0.14 microM), the substrate for factor IXa, appears to bind to phospholipid with a higher affinity than factor Xa (0.58 +/- 0.13 microM). These observations are compatible with the hypothesized dynamics where the forward 'traffic' is facilitated by favoring the association of factor IXa with factor X. The dissociation constants were estimated by molecular exclusion chromatography (1.1 - 2.5 microM) and do not reflect these relative and ordered differences in association with lipid vesicles. Quasi-elastic light scatter analyses indicate that each protein appears to saturate the same vesicle surface, consistent with competition for similar surface lipids, although the molecular shell formed by factor Xa (36 A) is smaller, suggesting that it has a different packing on the phospholipid surface than the other proteins (64-79 A). The pattern of preferential affinities for phospholipid is consistent with a kinetically functional forward traffic through the reaction precursors to products, and suggests that these preferential affinities may assist in the ordering of the four proteins in the intrinsic activation complex.

Reconstitution of purified chicken gizzard 5'-nucleotidase in phospholipid vesicles. Evidence for its transmembraneous character and the existence of functional domains on both sides of the phospholipid bilayer

5'-Nucleotidase, purified to homogeneity from chicken gizzard using published procedures [Dieckhoff, J., Knebel, H., Heidemann, M. and Mannherz, H. G. (1985) Eur. J. Biochem. 151, 377-383] was incorporated into artificial phospholipid vesicles after prolonged dialysis against detergent-free buffer or by a gel filtration procedure. After dialysis the obtained liposomes exhibit a mean diameter of 80 nm and contain 5'-nucleotidase at random orientation, demonstrated by finding up to 50% of the total liposome-incorporated AMPase activity to be cryptic, i.e. could only be measured after their permeabilization by addition of detergent. By affinity chromatography a phospholipid vesicle fraction could be obtained containing almost exclusively cryptic AMPase activity, thus representing the inside-out orientation of 5'-nucleotidase. Comparative analysis of physiochemical and enzymatic properties of 5'-nucleotidase reveals differences between the detergent-solubilized and the liposome-incorporated 5'-nucleotidase including a changed accessibility of the enzyme to polyclonal and monoclonal antibodies. Binding and AMPase inhibition studies with different polyclonal antibodies strongly indicate to the existence of a cytoplasmic domain of chicken gizzard 5'-nucleotidase. F-actin appears preferentially to interact with the cytoplasmic domain of liposome-incorporated 5'-nucleotidase.

Fractionation and reconstitution of the sarcoplasmic reticulum Ca2+ pump solubilized and stabilized by CHAPS/lipid micelles

A procedure for solubilization, fractionation, and reconstitution of sarcoplasmic reticulum (SR) protein is presented. The SR protein is solubilized with the zwitterionic detergent CHAPS in the presence of added 5-mM phosphatidylcholine and 20% glycerol, which stabilize the reconstitutable Ca2+ transport activity. For reconstitution the solubilized SR protein is incorporated into preformed French-pressed unilamellar vesicles that had been treated with 10-mM sodium cholate. By passing the proteoliposomes through a centrifuged Sephadex G-50 column that had been equilibrated with potassium oxalate, the detergent is removed, and the proteoliposomes become sealed with potassium oxalate trapped inside. This procedure requires less than 2 h and results in Ca2+ uptake active of over 1 mumol/min/mg of protein. The solubilized SR protein was fractionated on a DEAE-Biogel A column. A fraction containing the Ca2+-ATPase but not the Mr 55,000 glycoprotein had reconstitutable Ca2+ uptake activity of 2.2 mumol/min/mg of protein. Inclusion of the Mr 55,000 glycoprotein during the reconstitution procedure did not increase the Ca2+ uptake activity of the reconstituted fraction containing the Ca2+-ATPase. This result demonstrates that the glycoprotein is not required for Ca2+ uptake.

Interaction of surfactants with vesicle membrane of dipalmitoylphosphatidylcholine. Effect on gel-to-liquid-crystalline phase transition of lipid bilayer

The gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was observed in the presence of various types of surfactants; sodium alkylsulfates, alkyltrimethylammonium bromides, alkanoyl-N-methylglucamides, and hexaethyleneglycol mono n-dodecyl ether. The phase transition was monitored by a change in scattered light intensity of the lipid suspension. For all the surfactants examined, the phase transition temperature was depressed linearly with the surfactant concentration in the measured concentration range, from which the partition coefficient, K, of the surfactant between bulk solution and lipid membrane was estimated. Except alkyltrimethylammonium bromides, log K and log CMC showed a linear relationship, which indicates that the driving force to transfer the surfactant from bulk solution to lipid membrane is a hydrophobic interaction. The addition of surfactants increased the transition width. The extent of widening the transition width was in the order of sodium alkylsulfate greater than alkyltrimethylammonium bromides greater than hexaethyleneglycol mono n-dodecyl ether; in the case of alkanoyl-N-methylglucamides, the transition width was not affected by the addition. These effects on the transition width was interpreted qualitatively in terms of the cooperativity of the transition.

Preparation of liposomes via detergent removal from mixed micelles by dilution. The effect of bilayer composition and process parameters on liposome characteristics

Liposomes were prepared from mixed micelles by a dilution method. Mixed micellar solutions, containing constant octyl glucoside and egg phosphatidylcholine concentrations and varying amounts of cholesterol and/or a charged compound, were diluted at defined rates. After dilution, the resulting liposome dispersions were sequentially concentrated, washed or dialysed, and filtered. The effect of lipid composition and experimental conditions on physicochemical characteristics was studied. Fairly homogeneous liposome dispersions with mean diameters ranging from 100 to over 200 nm could be obtained. The particle size was dependent on cholesterol content and surface charge, and could be reproducibly controlled by adjustment of the dilution rate. Liposomes with a mean diameter below 100 nm could also be obtained, but were heterodisperse and unstable. The incorporation of charged compounds was monitored by microelectrophoresis. 31P-NMR measurements indicated that the liposomes were unilamellar. Dialysis appeared to be more convenient than washing to remove octyl glucoside.

Preparation and characterization of unilamellar vesicles from cholate-phospholipid micelle treated with cholestyramine

Cholestyramine, a well-known bile-salt sequestrant, can be used effectively to remove cholate or deoxycholate from a solution of phosphatidylcholine-bile salt mixed micelle. Upon removal of the bile salt, unilamellar phospholipid vesicles form essentially instantaneously. Cholestyramine resin could be pelleted and removed from the vesicle solution after a low speed centrifugation. Based on phosphate analyses, the recovery of vesicles was approximately 60% of the starting material. The average diameter of these vesicles, as estimated by gel exclusion chromatography on sephacryl S-1000 beads and by trapped volume measurement using [3H]sucrose, ranged between 85 to 121 nm. Phosphatidylethanolamine, cholesterol, or n-alkane such as tetradecane can be incorporated into the vesicles without any selective loss; however, selective loss was experienced when negatively charged phospholipid species such as phosphatidylglycerol or phosphatidylserine was included in vesicle formation.

Vesicles of variable sizes produced by a rapid extrusion procedure

Previous studies from this laboratory have shown that large unilamellar vesicles can be efficiently produced by extrusion of multilamellar vesicles through polycarbonate filters with a pore size of 100 nm (Hope, M.J., Bally, M.B., Webb, G. and Cullis, P.R. (1985) Biochim. Biophys. Acta 812, 55-65). In this work it is shown that similar procedures can be employed for the production of homogeneously sized unilamellar or plurilamellar vesicles by utilizing filters with pore sizes ranging from 30 to 400 nm. The unilamellarity and trapping efficiencies of these vesicles can be significantly enhanced by freezing and thawing the multilamellar vesicles prior to extrusion. This procedure is particularly applicable when very high lipid concentrations (400 mg/ml) are used, where extrusion of the frozen and thawed multilamellar vesicles through 100 and 400 nm filters results in trapping efficiencies of 56 and 80%, respectively. Freeze-fracture electron microscopy revealed that vesicles produced at these lipid concentrations exhibit size distributions and extent of multilamellar character comparable to systems produced at lower lipid levels. These results indicate that the freeze-thaw and extrusion process is the technique of choice for the production of vesicles of variable sizes and high trapping efficiency.

Techniques for encapsulating bioactive agents into liposomes

As a prerequisite for the use of liposomes for delivery of biologically active agents, techniques are required for the efficient and rapid entrapment of such agents in liposomes. Here we review the variety of procedures available for trapping hydrophilic and hydrophobic compounds. Considerations which are addressed include factors influencing the choice of a particular liposomal system and techniques for the passive entrapment of drugs in multilamellar vesicles and unilamellar vesicles. Attention is also paid to active trapping procedures relying on the presence of (negatively) charged lipid or transmembrane ion gradients. Such gradients are particularly useful for concentrating lipophilic cationic drugs inside liposomes, allowing trapping efficiencies approaching 100%.

Effect of liposomes on lymphocytes: induction of proliferation of B lymphocytes and potentiation of the cytotoxic response of T lymphocytes to alloantigens

Liposomes of certain lipid composition prepared by the detergent removal method (Brunner, J. et al., Biochim. Biophys. Acta 1976. 455: 322) induced the proliferation of spleen cells from different mouse strains. Spleen cell populations enriched in B lymphocytes and those obtained from nude mice were induced to proliferate, whereas spleen cell fractions enriched in T lymphocytes and thymocytes were not. The mitogenic effect of liposomes resembled that of lipopolysaccharide (LPS) and it depended upon their lipid composition. Liposomes prepared from dimyristoyl lecithin (DML), 2:1 dimyristoyl lecithin:cholesterol (DML:C), 2:1 dioleoyl lecithin: cholesterol (DOL:C), and 2:1 egg yolk lecithin:cholesterol (EYL:C) were mitogenic, whereas liposomes prepared from egg yolk lecithin (EYL) alone were not mitogenic for spleen cells. The mitogenic effects of these liposome preparations were in the decreasing order DML greater than DOL:C greater than or equal to EYL:C greater than DML:C greater than EYL. The results suggest a correlation between the membrane fluidity of liposomes and their mitogenic effect. Although no proliferative response was induced on T lymphocytes, two of these liposomes, DML and EYL:C, had the ability to potentiate the cytotoxic response of T lymphocytes to alloantigens in mixed leukocyte culture. In responder-stimulator combinations which differed for the H-2K, H-2D or the entire H-2 region, these liposomes potentiated the cytotoxic response significantly. The results suggest that liposomes have an ability to modulate T lymphocyte response.