Biochemical and biophysical properties of purified phospholipid vesicles containing bovine heart cytochrome c oxidase

Liposomes containing bovine heart cytochrome c oxidase (COV) prepared by the cholate dialysis technique were purified from those devoid of the enzyme using discontinuous sucrose density ultra centrifugation to eliminate interference in proton-pumping assays. This technique was also used to purify liposomes containing cytochrome c oxidase depleted in subunit III (COV-III), a COX enzyme preparation with altered subunit structure, to assess if the technique could be applied to COX enzymes in which structural and functional changes have occurred. Upon discontinuous sucrose density ultra gradient ultracentrifugation, either COV or COV-III were separated into two bands. Liposomes devoid of enzyme sedimented into the 12% sucrose layer, whereas enzyme-containing liposomes (pCOV or pCOV-III) were found in the 13% sucrose layer. The yield of both pCOV or pCOV-III was greater than 60% (based on heme aa(3) content), suggesting a similar distribution of cytochrome c oxidase (COX) and subunit III-depleted enzyme (COX-III) in the purified liposomes. The number of COX or COX-III molecules per phospholipid vesicle in purified fractions was estimated to be two. Removal of subunit III (M(r)=29,918) from COX resulted in a 30% decrease in electron transfer activity (either in COV-III or pCOV-III) when compared with COV and pCOV, respectively. Both pCOV and pCOV-III exhibited low endogenous proton permeability, as assessed by possessing high respiratory control ratios (14 and greater) and by having similar valinomycin concentration dependencies for stimulation of electron transfer activity in the presence of saturating amounts of CCCP. COV-III and pCOV-III exhibited a 39-44% decrease in proton-pumping activity when compared with COV and pCOV. These results showed that the separation of COX containing liposomes from those lacking enzyme by sucrose density gradient centrifugation can be used to characterize the biophysical properties of these liposomes.

Archaeosomes as adjuvants for combination vaccines

The present study evaluated the potential of archaesomes, prepared from the total polar lipids extracted from Methanobrevibacter smithii, as adjuvants for combination (multivalent) vaccines. Groups of Balb/c mice were immunized subcutaneously at day 0 and 21 with one of the following vaccines: trivalent vaccine formulated by the simultaneous co-encapsulation of bovine serum albumine (BSA), ovalbumin (OVA) and hen egg lysozyme (HEL) into archaeosomes (CEC vaccine); an univalent archaeosome vaccine (UVE vaccine) containing either BSA, OVA or HEL; or an admixture vaccine (AMC vaccine) consisting of the three UVE vaccines. Serum specific antibody (IgG + M) responses were determined at day 32, 112 and 203, and specific IgG1 and IgG2a responses were determined at day 112. Mice immunized with the CEC of AMC vaccine developed strong and sustained specific antibody responses to all three antigens at a magnitude similar to those seen in control mice immunized with UVE vaccines. Moreover, the serum BSA-, OVA-, and HEL-specific IgG1 and IgG2a levels in the CEC and AMC immunized mice were overall comparable to those of the UVE immunized control mice. Boosting CEC and AMC vaccinated mice with antigens alone at day 203 elicited strong antibody memory responses, comparable to those in the UVE vaccinated groups. These results show that archaeosomes could be used as adjuvants in developing combination vaccines.

Separation of liposomes from plasma components using fast protein liquid chromatography

We describe an efficient method for separating liposomes (large unilamellar vesicles, 120-150 nm diameter) from plasma lipoproteins employing fast protein liquid chromatography (FPLC). This method resolves very low density lipoprotein (VLDL), low-density lipoprotein, high-density lipoprotein, and other plasma components. Selective detection of liposomes (large unilamellar vesicles, 120-150 nm diameter) was achieved using either radiolabeled or fluorescent lipid probes. The liposomes were found to coelute with the earliest FPLC-eluting lipoprotein fraction, VLDL. The remaining plasma lipoprotein and protein components eluted at later times and were resolved from liposomes and VLDL. In order to separate VLDL from liposomes, we selectively precipitated the VLDL fraction from plasma using tungstophosphoric acid and magnesium chloride, prior to separation by FPLC. Furthermore, we demonstrate that this technique can be used to separate liposomes from lipoproteins in plasma samples collected after intravenous administration of liposomes to mice. This technique has wide application in studies of liposome stability in blood and, in particular, for the characterization of liposomal drug carrier systems.

Gel filtration of egg phosphatidylcholine vesicles

The abilities of Sepharose 2B (Pharmacia), Controlled Pore Glass (Electro-Nucleonics) and Bio-Gel A150m (Bio-Rad) to purify small unilamellar vesicles prepared by sonication and the ethanol-injection methods were compared. The Bio-Gel causes complete aggregation of the sonicated vesicles and partial aggregation of the ethanol-injection vesicles. Both Sepharose and Controlled Pore Glass are acceptable for purifying vesicles from multilamellar liposomes; however, neither will separate the vesicles from sonication by-products which might be formed.

Preparation of dexamethasone-based cationic liposome and its application to gene delivery in vitro

In this study, dexamethasone was conjugated to PAMAM dendrimer (generation 0) and its gene transfection efficiency was investigated. To make a liposomal solution for gene delivery, DOPE was used as a fusogenic helper lipid. In gel retardation assay, PAMAM-dexamethasone conjugate (PAM-Dex)/DOPE liposome/DNA complex was completely retarded at 8:1 N/P (nitrogen/phosphate) ratio. The physicochemical characteristics are studied by measuring the average size distribution and zeta-potential values of the complexes. In vitro transfection assay showed that the PAM-Dex/DOPE liposome/DNA complex displayed higher gene delivery efficiency compared to PAMAM/DNA complex. In addition, PAM-Dex/DOPE liposome showed the lowest toxicity compared to PAMAM, PEI 25 kD and Lipofectamine. These results indicate that PAM-Dex/DOPE liposome has a potential to be used as an efficient gene carrier for gene therapy.

Effects of wheat antioxidants on oxygen diffusion-concentration products in liposomes and mRNA levels of HMG-CoA reductase and cholesterol 7alpha-hydroxylase in primary rat hepatocytes

Three wheat antioxidant fractions were investigated for their potential effects on oxygen diffusion-concentration products in liposomes prepared with egg yolk phosphatidycholine (yolk PC) and rat liver PC (liver PC), using the electron spin resonance (ESR) oximetry method with 2,2'-azobis(2-aminopropane) dihydrochloride (AAPH) and 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) as radical generators. Both water-soluble wheat antioxidant (WWA) and the second lipophilic antioxidant (LWA2) fractions were able to inhibit oxygen diffusion-concentration product induced by either AAPH or AMVN. The first lipophilic wheat antioxidant (LWA1) fraction only showed antioxidant activity in yolk PC liposomes with AAPH as the radical initiator but had pro-oxidant activity under other testing conditions. Both liposome composition and radical initiator altered the antioxidative properties of WWA, LWA1, and LWA2. WWA also showed the strongest DPPH(*) scavenging capacity on a per grain weight basis. HPLC analysis showed that WWA had a much higher level of total phenolic acids, which may partially explain their antioxidant properties. In addition, wheat antioxidants significantly down-regulated the mRNA of HMG-CoA reductase, the key enzyme for cholesterol biosynthesis, and up-regulated the mRNA of cholesterol 7alpha-hydroxylase (CYP7A1), the key enzyme for cholesterol metabolism, in primary rat hepatocytes. These data indicated the potential of wheat antioxidants in reducing the risk of atherosclerosis through multimechanisms.

Preparation of liposomes that mimic the membrane of endoplasmic reticulum of rat hepatocytes

To examine the interaction between biomembranes and membrane-bound proteins, large unilamellar liposomes have been required. In the present study, we prepared liposomes from a mixture of phospholipids having a phospholipid composition similar to that in the endoplasmic reticula (microsomes) of rat hepatocytes by eight different methods. The resulting liposomes were examined by a combination of the freeze-fracture-replica procedure with biochemical methods. The freeze-thawing method of Pick (1981) gave the best results; large unilamellar liposomes that mimic the membrane of endoplasmic reticulum were obtained. Liposomes made by this method are thus suitable for analysis of the interaction between the endoplasmic reticulum membrane and membrane-bound proteins.

Renal cortical basolateral Na+/HCO3- cotransporter: II. Detection of conformational changes with fluorescein isothiocyanate labeling

Fluorescein isothiocyanate (FITC) fluorescently labels amino groups and has been useful in detecting conformational changes in transport proteins through quenching or enhancement of the fluorescence signal upon exposure of protein to substrates. Solubilized renal basolateral membrane proteins, enriched in Na+/HCO3- cotransporter activity, were reconstituted into liposomes and treated with FITC or its nonfluorescent analogue PITC (phenyl isothiocyanate). In the absence of Na+ and HCO3-, incubation of proteoliposomes with PITC or FITC significantly inhibited cotransporter activity. However, in the presence of Na+ and HCO3- during labeling both agents failed to inhibit cotransporter activity, indicating that these probes interact specifically with the cotransporter. In the presence of the substrates Na+ and HCO3-, PITC binds covalently to amino groups unprotected by substrates leaving the Na+/HCO3- cotransporter available for specific labeling with FITC. Addition of NaHCO3 to FITC-labeled proteoliposomes resulted in a concentration-dependent enhancement of the fluorescence signal which was inhibited by pretreatment with 4,4'-diisothiocyanostilbene 2',2-disulfonic acid (DIDS) prior to FITC labeling. SDS PAGE analysis of FITC-treated proteoliposomes showed the presence of two distinct fluorescent bands (approximate MW of 90 and 56 kD). In the presence of substrates, the fluorescence intensity of these bands was enhanced as confirmed by direct measurement of gel slice fluorescence. Thus, FITC detects conformational changes of the Na+/HCO3- cotransporter and labels proteins which may represent the cotransporter or components of this cotransporter.

Incorporation of Isoniazid into Liposomes with Different Lipid Composition

Liposomes containing isoniazid were obtained by extrusion of multilamellar phospholipid vesicles containing isoniazid, by ultrasound treatment, and from proliposomes. Optimal conditions for isolation of stable liposomes (200 nm in diameter) and the amount of incorporated isoniazid were determined by varying concentrations of phospholipids and isoniazid and by changing phospholipid composition.

The polymyxin sensitivity and the phosphatidyl ethanolamine content of the Vibrio cholerae membranes

The phospholipid composition of three different strains of Vibrio cholerae was determined by quantitative two-dimensional thin-layer chromatography. The polymyxin sensitivity of the whole organisms or of the liposomes derived from the total phospholipids of these organisms depended solely on the phosphatidyl ethanolamine content of the system concerned and could be quantitatively related by the equations Y1 = 1.074 X1 - 9.828 and Y2 = 1.22 X2 - 34.47 where Y represents the maximum lysis (%)/hr and X, the % phosphatidyl ethanolamine content of the system concerned, the suffix 1 and 2 corresponding to the liposomal and the whole organisms respectively. The analysis revealed the requirement of a threshold amount of phosphatidyl ethanolamine for the expression of polymyxin action on the system concerned.

Purification and functional properties of yeast Sec12 GEF

In order to reconstitute the generation of COPII vesicles from synthetic liposomes, the minimum requirements are the coat components, Sar1p GTPase, Sec23/24p, Sec13/31p, and a nonhydrolyzable GTP analog such as GMP-PNP. However, in the presence of GTP, nucleotide hydrolysis by Sar1p renders the coat insufficiently stable to sustain vesicle budding. Sar1p GTPase activity was activated by the Sec23/24p GTPase-activating protein (GAP), and further accelerated 10-fold by Sec13/31p. In order to study GTP-dependent budding, we introduced the Sar1p guanine nucleotide exchange factor (GEF), Sec12p. We evaluated Sar1p activation by Sec12p and the dynamics of coat assembly and disassembly in the presence of both Sec12p and Sec23/24p. The cytoplasmic domain of Sec12p activated Sar1p with a turnover 10-fold higher than the GAP activity of Sec23p in the presence of Sec13/31p. As a result, the entire COPII coat remains stable in the presence of GTP. Here, we describe methods to purify Sec12p, real-time fluorescence assays to evaluate COPII coat formation, and the relevant kinetic analyses.

Model Phospholipid Liposomes to Study the β-Amyloid-Peptide-Induced Membrane Disruption

Model phospholipid liposomes have been utilized widely to study the molecular interactions between peptides and membrane bilayers. In the mechanistic study of Alzheimer's disease (AD), disruption of neuronal cell membranes has been considered as a major contribution for the β-amyloid (Aβ) peptides' neurotoxicity. However, clear interpretation of the Aβ-induced cellular membrane at high-resolution level is challenging because of the co-existence of multiple pathways. Here we present the generation of simplified model liposome systems that will facilitate the in-depth mechanistic studies. Protocols for the preparation of model liposomes and the characterization of individual membrane disruption effects will be described.

Affinity purification of lipid vesicles

We present a novel column chromatography technique for recovery and purification of lipid vesicles, which can be extended to other macromolecular assemblies. This technique is based on reversible binding of biotinylated lipids to monomeric avidin. Unlike the very strong binding of biotin and biotin-functionalized molecules to streptavidin, the interaction between biotin-functionalized molecules and monomeric avidin can be disrupted effectively by ligand competition from free biotin. In this work, biotin-functionalized lipids (biotin-PEG-PE) were incorporated into synthetic lipid vesicles (DOPC), resulting in unilamellar biotinylated lipid vesicles. The vesicles were bound to immobilized monomeric avidin, washed extensively with buffer, and eluted with a buffer supplemented with free biotin. Increasing the biotinyl lipid molar ratio beyond 0.53% of all lipids did not increase the efficiency of vesicle recovery. A simple adsorption model suggests 1.1 x 10(13) active binding sites/mL of resin with an equilibrium binding constant of K = 1.0 x 10(8) M(-1). We also show that this method is very robust and reproducible and can accommodate vesicles of varying sizes with diverse contents. This method can be scaled up to larger columns and/or high throughput analysis, such as a 96-well plate format.

Lack of counteracting effect of liposomes on benserazide-induced hyperprolactinemia

Benserazide induces an increase of serum prolactin in man, possibly as the result of an impairment of the dopamine effect on the pituitary and/or on the outer median eminence caused by the inhibition on L-dopa decarboxylase. On the other hand, liposomes obtained from bovine brain cortex phospholipids reduced serum prolactin possibly through an effect of phosphatidylserine on dopamine biosynthesis at the level of tyrosine hydroxylase. Benserazide, given orally (125 mg) to 5 normal subjects, induced an increase of serum prolactin that did not change when 300 mg of phospholipid liposomes were given intravenously 60 min later. An increase of L-dopa synthesis does not seen to be capable to overcome the effects of the decarboxylase inhibition.

Inhibition of induced lymphocyte proliferation by lipid and protein components of the syncytiotrophoblast plasma membrane

PROBLEM

The aim of this work was to define the respective responsibilities of the lipid and protein components of syncytiotrophoblast plasma membranes on the inhibition of lymphocyte proliferation induced in vitro.

METHOD

A fractionation method using octyl-beta-D-glucopyranoside enabled lipoprotein, lipid, and protein fractions to be isolated from the membrane.

RESULTS

The lipid fraction was shown nonspecifically to inhibit lymphocyte proliferation, to a lower extent compared with the native membrane. Alternatively, the protein fraction used as a proteoliposome contained the totality of the cytostatic effect of the native fraction.

CONCLUSION

These results are discussed generally in the context of the immunoregulatory role of membrane lipids and proteins and in relation to the local properties of syncytiotrophoblast plasma membrane components in fetal graft tolerance.

Role of atheroma liposomes and malondialdehyde-modified low-density lipoproteins in complement activation

We investigated the ability of atheroma-associated liposomes and malondialdehyde (MDA)-modified low-density lipoproteins (MDA-LDL) to activate complement. Complement activation markers C3a, Bb, C4d and SC5b-9 were measured in both normal and complement-deficient sera. We found that MDA-LDL was able to generate C3a and SC5b-9, predominantly by the alternative pathway. High-density lipoproteins modified with MDA were also capable of C3a generation although to a lesser degree. The presence of atheroma-associated liposomes did not result in detectable levels of complement activation markers. We conclude that MDA-modified lipoproteins may represent a possible source for complement activation within atherosclerotic lesions.

SNARE-mediated fusion of liposomes

Lipid-mixing assay is now commonly used to study protein, temperature and ion-dependent membrane fusion events. This assay has been crucial to demonstrate the ability of neuronal and non-neuronal soluble NSF attachment receptor (SNARE) to promote spontaneous fusion of liposomes. This lipid-mixing assay is based on the fluorescence resonance energy transfer (FRET) capability between a donor fluorescent lipid and a quenching lipid. When fusion between donor fluorescent liposomes and nonfluorescent acceptor liposome occurred, FRET decreases. This assay allows a real-time reading of SNARE-mediated liposome fusion.

Large-scale blood substitute production using a microfluidizer

Microfluidization has been tested as a way to disperse phospholipids in aqueous hemoglobin solutions. Spherical and stable liposomes of 2 to 3 microns were obtained. Lipid incorporation (up to 85%) and hemoglobin encapsulation (up to 15%) in liposomes have been improved with respect to previous investigations. However, results show that a more efficient dispersion system using lower concentrations of lipid is required to obtain a high liposome hemoglobin concentration (limited actually to 150 g/l) and an economically and biologically suitable process for artificial blood production at large scale.

Purification and partial characterization of extracellular liposomes isolated from the hyperlipidemic rabbit aorta

Extracellular liposomes (EL) that accumulated in the aortic intima of rabbits on 2 weeks (prelesional stage) and 16 weeks (lesional stage) of diet-induced hyperlipidemia were isolated and purified by gel filtration, ultracentrifugation, and affinity chromatography on anti-apoB and anti-albumin Sepharose. The material obtained after each step was examined by negative staining electron microscopy, by protein analysis (SDS-PAGE, immunoblotting, autoradiography, uronic acid), and by lipid analysis for unesterified cholesterol (UC), cholesteryl esters (CE), phospholipids (PL), triglycerides (TG), thiobarbituric acid reactants (TBAR). EL represented the major constituent of intimal lipid deposits; their predominance on particulate beta-lipoproteins (LP) increased with the duration of hyperlipoproteinemia. As compared with serum low density lipoproteins (LDL) and beta-very low density lipoproteins (beta-VLDL), the crude EL fraction obtained after gel filtration and ultracentrifugation had a decrease in CE and TG, with augmentation of UC, PL, and apoB. After removal of apoB and some albumin by immunoadsorption, the purified EL fraction consisted only of UC, PL, and albumin. The albumin was resistant to proteolytic digestion with pronase, and reacted with anti-albumin antibody only after delipidation of EL. This indicated that albumin was trapped in the aqueous core of vesicles, presumably acting as a scavenger of oxygen-free radicals. TBAR was highly associated with intact or degraded beta-LP. The EL that accumulate in the aortic intima of hyperlipidemic rabbits represent the predominant form of lipid deposits, resulting from the transcytosed excess beta-LP, which is degraded and reassembled upon interaction with the extracellular matrix components.

Reconstitution of Synaptic SNAREs into Large Liposomes with Reduced Curvature Stress

Liposomes constitute a convenient biochemical model system to investigate mechanistic aspects of the membrane fusion of synaptic vesicles. The proteins responsible for mediating fusion are the SNAREs that belong to a highly conserved family of transmembrane proteins. Reconstituting SNAREs into liposomes using detergents has become a common approach not only to understand how SNAREs work, but also how fusion is regulated by the vast array of accessory proteins present at the presynapse. However, a concern has been that the high curvature stress of the small liposomes (diameters of ~40 nm) frequently used in many studies renders them prone to spontaneous fusion, bringing into question whether the measurements obtained faithfully represent SNARE-mediated fusion. By systematically varying the detergent concentration and characterizing the SNARE-liposome size distributions by light scattering, we describe a detailed procedure to reconstitute SNAREs into large liposomes with considerably reduced curvature stress.