Studies on phosphatidylcholine vesicles. Formation and physical characteristics

Attachment of Water-Soluble Proteins to the Surface of (Magnetizable) Phospholipid Colloids via NeutrAvidin-Derivatized Phospholipids

The present work describes the incorporation of a functionalized phospholipid derivative into the phospholipid bilayer of both classical small unilamellar vesicles and recently developed magnetoliposomes, resulting in unique biocolloid structures onto which peripheral water-soluble enzymes can be immobilized on the surfaces. In the first part of this work, a synthesis protocol is outlined for a universal membrane anchor for water-soluble proteins. Dioleoylphosphatidylethanolamine-N-dodecanyl was used as the starting lipid molecule. After activation of the terminal -COOH group, alpha,omega-diamino-poly(ethylene glycol), used as a hydrophilic, flexible spacer arm, was coupled covalently. Subsequently, NeutrAvidin was bound, after blocking the free -NH(2) groups with citraconic anhydride. In the second part, the resulting lipid-NeutrAvidin derivative was incorporated into small unilamellar vesicles comprised of dimyristoylphosphatidylglycerol. FPLC with Superdex 200 as the column matrix clearly showed that biotinylated alkaline phosphatase, which served as a representative model of water-soluble proteins, was attached to the vesicles. Furthermore, magnetoliposomes, constructed of the same type of phospholipid molecules, were presented as interesting colloids to assess the degree of enzyme immobilization in a rapid and elegant manner. Potential applications that can emerge from this study are briefly discussed.

Copper-induced peroxidation of liposomal palmitoyllinoleoylphosphatidylcholine (PLPC), effect of antioxidants and its dependence on the oxidative stress

In an attempt to deepen our understanding of the mechanisms responsible for lipoprotein peroxidation, we have studied the kinetics of copper-induced peroxidation of the polyunsaturated fatty acid residues in model membranes (small, unilamellar liposomes) composed of palmitoyllinoleoylphosphatidylcholine (PLPC). Liposomes were prepared by sonication and exposed to CuCl(2) in the absence or presence of naturally occurring reductants (ascorbic acid (AA) and/or alpha-tocopherol (Toc)) and/or a Cu(I) chelator (bathocuproinedisulfonic acid (BC) or neocuproine (NC)). The resultant oxidation process was monitored by recording the time-dependence of the absorbance at several wavelengths. The observed results reveal that copper-induced peroxidation of PLPC is very slow even at relatively high copper concentrations, but occurs rapidly in the presence of ascorbate, even at sub-micromolar copper concentrations. When added from an ethanolic solution, tocopherol had similar pro-oxidative effects, whereas when introduced into the liposomes by co-sonication tocopherol exhibited a marked antioxidative effect. Under the latter conditions, ascorbate inhibited peroxidation of the tocopherol-containing bilayers possibly by regeneration of tocopherol. Similarly, both ascorbate and tocopherol exhibit antioxidative potency when the PLPC liposomes are exposed to the high oxidative stress imposed by chelated copper, which is more redox-active than free copper. The biological significance of these results has yet to be evaluated.

Behaviour of bovine phosphatidylethanolamine-binding protein with model membranes. Evidence of affinity for negatively charged membranes

The ability of phosphatidylethanolamine-binding protein (PEBP) to bind membranes was tested by using small and large unilamellar vesicles and monolayers composed of l-alpha-1,2-dimyristoylphosphatidylcholine, l-alpha-1,2-dimyristoylphosphatidylglycerol and l-alpha-1,2-dimyristoylphosphatidylethanolamine. PEBP only bound to model membranes containing l-alpha-1,2-dimyristoylphosphatidylglycerol; the interaction was primarily due to electrostatic forces between the basic protein and the acidic phospholipids. Further experiments indicated that the interaction was not dependent on the length and unsaturation of the phospholipid acyl chains and was not modified by the presence of cholesterol in the membrane. PEBP affinity for negatively charged membranes is puzzling considering the previous identification of the protein as a phosphatidylethanolamine-binding protein, and suggests that the association of PEBP with phospholipid membranes is driven by a mechanism other than its binding to solubilized phosphatidylethanolamine. An explanation was suggested by its three-dimensional structure: a small cavity at the protein surface has been reported to be the binding site of the polar head of phosphatidylethanolamine, while the N-terminal and C-terminal parts of PEBP, exposed at the protein surface, appear to be involved in the interaction with membranes. To test this hypothesis, we synthesized the two PEBP terminal regions and tested them with model membranes in parallel with the whole protein. Both peptides displayed the same behaviour as whole PEBP, indicating that they could participate in the binding of PEBP to membranes. Our results strongly suggest that PEBP directly interacts with negatively charged membrane microdomains in living cells.

Enzymes inside lipid vesicles: preparation, reactivity and applications

There are a number of methods that can be used for the preparation of enzyme-containing lipid vesicles (liposomes) which are lipid dispersions that contain water-soluble enzymes in the trapped aqueous space. This has been shown by many investigations carried out with a variety of enzymes. A review of these studies is given and some of the main results are summarized. With respect to the vesicle-forming amphiphiles used, most preparations are based on phosphatidylcholine, either the natural mixtures obtained from soybean or egg yolk, or chemically defined compounds, such as DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) or POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). Charged enzyme-containing lipid vesicles are often prepared by adding a certain amount of a negatively charged amphiphile (typically dicetylphosphate) or a positively charged lipid (usually stearylamine). The presence of charges in the vesicle membrane may lead to an adsorption of the enzyme onto the interior or exterior site of the vesicle bilayers. If (i) the high enzyme encapsulation efficiencies; (ii) avoidance of the use of organic solvents during the entrapment procedure; (iii) relatively monodisperse spherical vesicles of about 100 nm diameter; and (iv) a high degree of unilamellarity are required, then the use of the so-called 'dehydration-rehydration method', followed by the 'extrusion technique' has shown to be superior over other procedures. In addition to many investigations in the field of cheese production--there are several studies on the (potential) medical and biomedical applications of enzyme-containing lipid vesicles (e.g. in the enzyme-replacement therapy or for immunoassays)--including a few in vivo studies. In many cases, the enzyme molecules are expected to be released from the vesicles at the target site, and the vesicles in these cases serve as the carrier system. For (potential) medical applications as enzyme carriers in the blood circulation, the preparation of sterically stabilized lipid vesicles has proven to be advantageous. Regarding the use of enzyme-containing vesicles as submicrometer-sized nanoreactors, substrates are added to the bulk phase. Upon permeation across the vesicle bilayer(s), the trapped enzymes inside the vesicles catalyze the conversion of the substrate molecules into products. Using physical (e.g. microwave irradiation) or chemical methods (e.g. addition of micelle-forming amphiphiles at sublytic concentration), the bilayer permeability can be controlled to a certain extent. A detailed molecular understanding of these (usually) submicrometer-sized bioreactor systems is still not there. There are only a few approaches towards a deeper understanding and modeling of the catalytic activity of the entrapped enzyme molecules upon externally added substrates. Using micrometer-sized vesicles (so-called 'giant vesicles') as simple models for the lipidic matrix of biological cells, enzyme molecules can be microinjected inside individual target vesicles, and the corresponding enzymatic reaction can be monitored by fluorescence microscopy using appropriate fluorogenic substrate molecules.

Liposomes: applications in medicine

Liposomes are spherical lipid bilayers from 50 nm to 1000 nm in diameter that serve as convenient delivery vehicles for biologically active compounds. The field of liposome research has expanded considerably over the last 30 years. It is now possible to engineer a wide range of liposomes varying in size, phospholipid composition and surface characteristics to suit the specific application for which they are intended. This paper gives an overview of the main advances in liposome research from a point of view of their applications in medicine. Aqueous contrast enhancing agents entrapped in liposomal carriers can be targeted to the liver and spleen and distinctions can be made between normal and tumorous tissue using computed tomography. Topical application of liposomes has great potential in dermatology. Liposomes have been used to deliver anticancer agents in order to reduce the toxic effects of the drugs when given alone or to increase the circulation time and effectiveness of the drugs. From the original concept of encapsulating hemoglobin in an inert shell, liposome-encapsulated hemoglobin (LEH) has evolved into a fluid proven to carry oxygen, capable of surviving for reasonable periods in the circulation and amenable to large-scale production. Liposomes may be used to target specific cells by attaching amino acid fragments such as antibodies or proteins or appropriate fragments that target specific receptor sites. Liposomal DNA delivery vectors and further enhancements in the forms of LPDI and LPDII are some of the safest and potentially most versatile transfer vectors used to date. DNA vaccination and improved efficiency of gene therapy are just a few of the upcoming applications of liposomes.

Lupus antibody bivalency is required to enhance prothrombin binding to phospholipid

Lupus anticoagulants (LA) are a family of autoantibodies that are associated with in vitro anticoagulant activity but a strong predisposition to in vivo thrombosis. They are directed against plasma phospholipid-binding proteins including prothrombin. We have proposed that LA propagates coagulation in flowing blood by facilitating prothrombin interaction with the damaged blood vessel wall. A murine monoclonal anti-prothrombin Ab and three of three LA IgGs enhanced prothrombin binding to 75:25 phosphatidyl choline:phosphatidyl serine vesicles measured by either ultracentrifugation or right-angle light scattering. The assembly of prothrombin and LA IgG on phospholipid vesicles was estimated by surface plasmon resonance. The on rates for prothrombin and LA IgG were approximately the same as the on rate for prothrombin alone. In contrast, the off rates for prothrombin and LA IgG were 2- to 3-fold slower than the off rate for prothrombin. LA IgG bivalency was required for enhanced prothrombin binding to phospholipid vesicles, as Fab of the LA IgGs did not influence prothrombin binding at concentrations up to 40 microM. Modeling of the interactions of prothrombin, LA IgG and phospholipid vesicles indicated that augmentation of prothrombin binding to phospholipid vesicles by LA IgG could be accounted for by the bivalency of the LA IgG and the elevated microenvironmental concentration of prothrombin on the surface of phospholipid vesicles.

Molten-globule structure and membrane binding of the N-terminal protease-resistant domain (63-193) of the steroidogenic acute regulatory protein (StAR)

The first step in steroidogenesis is the movement of cholesterol from the outer to inner mitochondrial membrane; this movement is facilitated by the steroidogenic acute regulatory protein (StAR). StAR has molten-globule properties at low pH and a protease-resistant N-terminal domain at pH 4 and pH 8 comprising residues 63-193. To explore the mechanism of action of StAR we investigated the structural properties of the bacterially expressed N-terminal domain (63-193 StAR) using CD, limited proteolysis and NMR. Far- and near-UV CD showed that the amount of secondary structure was greater at acidic than at neutral pH, but there was little tertiary structure at any pH. Unlike 63-193 StAR liberated from N-62 StAR by proteolysis, biosynthetic 63-193 StAR was no longer resistant to trypsin or proteinase K at pH 7, or to pepsin at pH 4. Addition of trifluoroethanol and SDS increased secondary structure at pH 7, and dodecylphosphocholine and CHAPS increased secondary structure at pH 2, pH 4 and pH 7. However, none of these conditions induced tertiary structure, as monitored by near-UV CD or NMR. Liposomes of phosphatidylcholine, phosphatidylserine and their mixture increased secondary structure of 63-193 StAR at pH 7, as monitored by far-UV CD, and stable protein-liposome complexes were identified by gel-permeation chromatography. These results provide further evidence that the N-terminal domain of StAR is a molten globule, and provide evidence that this conformation facilitates the interaction of the N-terminal domain of StAR with membranes. We suggest that this interaction is the key to understanding the mechanism of StAR's action.

A novel fluorescent coronenyl-phospholipid analogue for investigations of submicrosecond lipid fluctuations

A fluorescent phospholipid derivative, the 2'-(4-coronenylbutyric) ester of lyso-egg phosphatidylcholine, has been synthesized for use in studies of submicrosecond lipid dynamics. Synthesis of the phospholipid derivative involves Friedel-Crafts acylation of free coronene, followed by a Huang-Minlon reduction to yield the fatty-acyl derivative, 4-coronenylbutyric acid. Esterification of the carboxylic acid with lyso-phosphatidylcholine is achieved through a mixed anhydride intermediate. The resultant coronenyl-phospholipid adduct (Cor-PC) has been incorporated into sonicated unilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) for dynamic lipid studies. Fluorescence quenching studies using potassium iodide, together with steady-state emission anisotropy (EA) measurements, confirm that the coronene moiety of the phospholipid adduct resides towards the head group interfacial region of the lipid bilayer. Unique properties of this new fluorescent phospholipid adduct are its long mean fluorescence lifetime (tau av approximately 112 ns at 14 degrees C), the planar symmetry of the fluorophore and its defined bilayer location. As a consequence, depolarizing motions of the coronene moiety target submicrosecond 'gel-fluid' lipid dynamics arising from a relatively narrow bilayer distribution. Our data suggest that the sensitivity of this new long-lived fluorescent phospholipid analogue to localized transverse submicrosecond lipid dynamics can provide important biological insights into varied processes including lipid-peptide interactions, bilayer fluidity gradients and passive ion transport.

How to measure and analyze tryptophan fluorescence in membranes properly, and why bother?

Tryptophan fluorescence is a powerful tool for studying protein structure and function, especially membrane-active proteins and peptides. It is arguably the most frequently used tool for examining the interactions of proteins and peptides with vesicular unilamellar model membranes. However, high light scattering associated with vesicular membrane systems presents special challenges. Because of their reduced light scattering compared to large unilamellar vesicles (LUV), small unilamellar vesicles (SUV) produced by sonication are widely used membrane models. Unfortunately, SUV, unlike LUV, are metastable and consequently unsuitable for equilibrium thermodynamic measurements. We present simple and easily implemented experimental procedures for the accurate determination of tryptophan (Trp) fluorescence in either LUV or SUV. Specifically, we show that Trp spectra can be obtained in the presence of up to 6 mM LUV that are virtually identical to spectra obtained in buffer alone, which obviates the use of SUV. We show how the widths and peak positions of such spectra can be used to evaluate the heterogeneity of the membrane conformation and penetration of peptides. Finally, we show how to use a reference fluorophore for the correction of intensity measurements so that the energetics of peptide partitioning into membranes can be accurately determined.

Inhibition of human platelet function by sulfatides

Sulfatides are glycolipid constituents of human platelet cell membranes and have been shown to interact with platelet-binding proteins involved in hemostasis. Because little is known about the physiological role of sulfatides in platelet function, the effect of sulfatide on platelet adhesion, aggregation, release, and ristocetin-induced platelet agglutination (RIPA) was studied. These processes are inhibited when exogenous sulfatide is present in vitro. Inhibition of aggregation induced by collagen, thrombin, and ristocetin by sulfatide was dose dependent. Adenosine diphosphate-mediated adhesion and aggregation were not significantly affected by sulfatide, nor was serotonin- and epinephrine-mediated aggregation. Collagen mediate release of serotonin was reduced sulfatide. RIPA demonstrated dose-dependent inhibition in response to sulfatide. These results suggest that sulfatide may play a role in modulating platelet activation.

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.

Formation of supported membranes from vesicles

Using a combination of the quartz crystal microbalance and surface plasmon resonance techniques, we have studied the spontaneous formation of supported lipid bilayers from small (approximately 25 nm) unilamellar vesicles. Together these experimental methods measure the amount of lipid adsorbed on the surface and the amount of water trapped by the lipid. With this approach, we have, for the first time, been able to observe in detail the progression from the adsorption of intact vesicles to rupture and bilayer formation. Monte Carlo simulations reproduce the data.

Single repeat deletion in ApoA-I blocks cholesterol esterification and results in rapid catabolism of delta6 and wild-type ApoA-I in transgenic mice

The deletion mutation Delta6 apolipoprotein A-I lacks residues 143-164 or repeat 6 in the mature apoA-I protein. In vitro studies show this mutation dramatically reduces the rate of lecithin:cholesterol acyltransferase (LCAT) catalyzed cholesterol esterification. The present study was initiated to investigate the effect of this mutation on in vivo high density lipoprotein (HDL) cholesterol esterification and metabolism. Transgenic mice expressing human Delta6 apoA-I (TgDelta6 +/+) were created and then crossed with apoA-I knockout mice (-/-) to generate mice expressing only human Delta6 apoA-I (TgDelta6 -/-). Human Delta6 apoA-I was associated with homogeneous sized alpha-HDL, when wild-type mouse apoA-I was present (in TgDelta6 +/+ and +/- mice). However, in the absence of endogenous mouse apoA-I, Delta6 apoA-I was found exclusively in cholesterol ester-poor HDL, and lipid-free HDL fractions. This observation coincides with the 6-fold lower cholesterol ester mass in TgDelta6 -/- mouse plasma compared with control. Structural studies show that despite the structural perturbation of a domain extending from repeat 5 to repeat 8 (137-178), Delta6 apoA-I binds to spherical unilamellar vesicles with only 2-fold less binding affinity. In summary, these data show a domain corresponding to apoA-I repeat 6 is responsible for providing an essential conformation for LCAT catalyzed generation of cholesterol esters. Deletion of apoA-I repeat 6 not only blocks normal levels of cholesterol esterification but also exerts a dominant inhibition on the ability of wild-type apoA-I to activate LCAT in vivo.

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.

DepoFoam technology: a vehicle for controlled delivery of protein and peptide drugs

A major challenge in the development of sustained-release formulations for protein and peptide drugs is to achieve high drug loading sufficient for prolonged therapeutic effect coupled with a high recovery of the protein/peptide. This challenge has been successfully met in the formulation of several peptide and protein drugs using the DepoFoam, multivesicular lipid-based drug delivery system. DepoFoam technology consists of novel multivesicular liposomes characterized by their unique structure of multiple non-concentric aqueous chambers surrounded by a network of lipid membranes. The objective of this paper is to demonstrate that DepoFoam technology can be used to develop sustained-release formulations of therapeutic proteins and peptides with high loading. DepoFoam formulations of a protein such as insulin, and peptides such as leuprolide, enkephalin and octreotide have been developed and characterized. The data show that these formulations have high drug loading, high encapsulation efficiency, low content of free drug in the suspension, little chemical change in the drug caused by the formulation process, narrow particle size distribution, and spherical particle morphology. Drug release assays conducted in vitro in biological suspending media such as human plasma indicate that these formulations provide sustained release of encapsulated drug over a period from a few days to several weeks, and that the rate of release can be modulated. In vivo pharmacodynamic studies in rats also show a sustained therapeutic effect over a prolonged period. These results demonstrate that the DepoFoam system is capable of efficiently encapsulating therapeutic proteins and peptides and effectively providing controlled delivery of these biologically active macromolecules.

Effect of pH on the interfacial tension of lipid bilayer membrane

The dependence of the interfacial tension of a lipid bilayer on the pH of the aqueous solution has been studied. A theoretical equation is derived to describe this dependence. Interfacial tension measurements of an egg phosphatidylcholine bilayer were carried out. The experimental results agreed with those derived from the theoretical equation obtained close to the isoelectric point within a range of three pH units. A maximum corresponding to the isoelectric point appears both in the theoretical equation and in the experimental data.

Engineering microsomal cytochrome P450 2C5 to be a soluble, monomeric enzyme. Mutations that alter aggregation, phospholipid dependence of catalysis, and membrane binding

Deletion of the N-terminal membrane-spanning domain from microsomal P450s 2C5 and 2C3 generates the enzymes, 2C5dH and 2C3dH, that exhibit a salt-dependent association with membranes indicating that they retain a monofacial membrane interaction domain. The two proteins are tetramers and dimers, respectively, in high salt buffers, and only 2C5dH requires phospholipids to reconstitute fully the catalytic activity of the enzyme. Amino acid residues derived from P450 2C3dH between residues 201 and 210 were substituted for the corresponding residues in P450 2C5 to identify those that would diminish the membrane interaction, the phospholipid dependence of catalysis, and aggregation of 2C5dH. Each of four substitutions, N202H, I207L, S209G, and S210T, diminished the aggregation of P450 2C5dH and produced a monomeric enzyme. The N202H and I207L mutations also diminished the stimulation of catalytic activity by phospholipid and reduced the binding of P450 2C5dH to phospholipid vesicles. The modified enzymes exhibit rates of progesterone 21-hydroxylation that are similar to that of P450 2C5dH. These conditionally membrane-bound P450s with improved solubility in high salt buffers are suitable for crystallization and structural determination by x-ray diffraction studies.

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.

Membrane-perturbing activity of Viperidae myotoxins: an electrostatic surface potential approach to a puzzling problem

Phospholipase-like myotoxins are a class of proteins present in Viperidae venom. Despite the high level of amino acid and structural homology with soluble phospholipases A(2), myotoxins are devoid of enzymatic activity and share cytolytic activity by means of a totally unknown mechanism involving the lipid bilayer perturbation. The distribution of electrostatic surface potentials of four myotoxins and seven phospholipases A(2) has been compared. The charge distribution is similar in all active non-cytolytic phospholipases with a strongly positive side corresponding to the domain interacting with the micellar substrate and with the opposite side negatively charged. In contrast, all myotoxins examined are positively charged on both sides. Myotoxin III, the only known example of a myotoxin sharing enzymatic activity, displays the same electrostatic surface potential as other related toxins. Using liposomes made with non-hydrolysable phospholipids, we demonstrate that myotoxin III perturbs the lipid bilayer like other myotoxins. Based on these results, a molecular model for myotoxin-membrane perturbing activity is proposed. In this model, potential double-face binding of myotoxic phospholipases A(2) to lipid surfaces could trigger a lipid bilayer destabilization and could generate a stable fusion pore, probably because of the presence of hydrophobic moieties that flank the cationic sites.

Long-term analysis of differentiation in human myoblasts repopulated with mitochondria harboring mtDNA mutations

Short-term analysis of myogenesis in respiration-deficient myoblasts demonstrated that respiratory chain dysfunction impairs muscle differentiation. To investigate long-term consequences of a deficiency in oxidative phosphorylation on myogenesis, we quantitated myoblast fusion and expression of sarcomeric myosin in respiration-deficient myogenic cybrids. We produced viable myoblasts harboring exclusively mtDNA with large-scale deletions by treating wild-type myoblasts with rhodamine 6G and fusing them with cytoplasts homoplasmic for two different mutated mtDNAs. Recovery of growth in transmitochondrial myoblasts demonstrated that respiratory chain function is not required for recovery of rhodamine 6G-treated cells. Both transmitochondrial respiration-deficient cultures exhibited impaired myoblast fusion. Expression of sarcomeric myosin was also delayed in deficient myoblasts. However, 4 weeks after induction of differentiation, one cell line was able to quantitatively recover its capacity to form postmitotic muscle cells. This indicates that while oxidative phosphorylation is an important source of ATP for muscle development, myoblast differentiation can be supported entirely by glycolysis.

Liposome-water and octanol-water partitioning of alcohol ethoxylates

Liposome-water partitioning coefficients, (K_lipw s), were determined for eight pure alcohol ethoxylates using equilibrium dialysis and ultracentrifugation. Both methods yielded statistically indistinguishable results. The experimentally determined log K_lipw s were compared with log K_ow values estimated with the fragment method using different literature sources for the fragment constants. Fragments of log K_lipw were calculated for the ethoxy group (EO) and the -CH₂ - units from the experimentally determined data. An additional -CH₂ - unit causes an average increase of log K_lipw by 0.45, and an additional EO causes an average decrease of log K_lipw by -0.12. With these fragments, the quality of log K_lipw estimations can be improved significantly as compared to simple linear regression of log K_lipw versus log K_ow . The K_lipw values calculated according to the new fragment method for pure compounds and for commercial mixtures are shown to be adequate descriptors for quantitative structure-activity relationships of bioaccumulation, toxicity, and sorption to natural organic material.

Evidence of a phospholipid binding species within human fibrinogen preparations

Fibrinogen has been reported to interact with phospholipid; however, the properties of this binding interaction have not been characterized. Purified preparations of human fibrinogen bound to small unilamellar vesicles containing phosphatidylserine (PS) as measured by light scattering and radioisotope filtration. Binding to 100% PS was saturable (apparent Kd=5 microM, Bmax=1.9 g protein/g lipid), reversible, and involved a minor subfraction of the fibrinogen preparation (3-6% of total protein). Fibrinogen interacted minimally with phosphatidylinositol, and not at all with pure phosphatidylcholine (PC) or PC vesicles containing 5% glycosphingolipid (lactosylceramide, ganglioside GM3, ganglioside GD3). Binding efficiency decreased as the PS content of vesicles was diluted with PC. Calcium chloride (2 mM) enhanced protein binding to PS, which was reversed by EDTA. Fibrin clot formation almost quantitatively precipitated the PS binding activity. PS, but not PC, increased the final turbidity of fibrin clots. Computerized sequence analysis of fibrinogen revealed three candidate acidic phospholipid binding motifs located at position 143-210 in the alpha chain, and positions 59-77 and 101-139 in the beta chain. Further study of the PS binding activity of fibrinogen may lead to new insights about fibrinogen function.

Phosphatidic acid is important to the translocation of Rab3A from the cytosol to phospholipid membranes

RAB3A, a Ras-related GTP-binding protein, is found in a rat neuronal cytosol in the form of complex with RabGDI or located on the synaptic vesicles and it cycles between the cytosol and the membranes. However, the regulatory mechanism of the translocation of Rab3A has not been clearly understood. To understand the mechanism of the translocation of Rab3A from the cytosol to the membranes, we examined which membranenous phospholipid is required for the translocation of Rab3A. Phosphatidic acid (PA) was found to be required for the translocation of Rab3A to the membranes and GTPgammaS stimulated the translocation of Rab3A.

Detection, synthesis, structure, and function of oligo(3-hydroxyalkanoates): contributions by synthetic organic chemists

Two types of the biological macromolecules poly(R-3-hydroxyalkanoates) have been identified: the high-molecular-weight microbial storage material (sPHA) and a short-chain variety, consisting of butyrate and valerate residues, complexed with other biomacromolecules such as calcium polyphosphate or proteins (cPHB/PHV). While sPHA has attracted, and still enjoys, a lot of attention from numerous scientists around the world, research on cPHB and the structurally and functionally related polymalate (PMA) is still in its infancy. In this article, we present a review on the chemical synthesis, structure, function and interactions of monodisperse cPHAs, the oligo(3-hydroxyalkanoates), with emphasis on the butyrates (OHB); we report hitherto unpublished results on the enzymatic degradation of cPHB and PMA, on a new analytical method for HB/HV detection in biological samples, and on OHB-mediated Ca2+ transport through phospholipid bilayers of artificial vesicles; finally, we discuss possible mechanisms of ion transport through cell membranes, as caused by cPHB. The speculative--and provocative--question is asked whether the structurally simple PHAs may have evolved as storage materials and amphiphilic macromolecules before poly-peptides, -saccharides, and -nucleic acids, in the history of life, or under prebiotic conditions.

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.

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.

Membrane interaction of an antitumor antibiotic, mithramycin, with anionic phospholipid vesicles

Small unilamellar vesicles (SUV) composed of zwitterionic phosphatidylcholine and two different anionic phospholipids, phosphatidic acid and phosphatidylserine, in different compositions, were employed to study the membrane interaction of an antitumor antibiotic, mithramycin (MTR). Binding of MTR to dimyristoylphosphatidylcholine (DMPC) liposomes containing the anionic phospholipid dimyristoylphosphatidic acid (DMPA) was estimated by measuring the increase in intensity of the intrinsic fluorescence of MTR with increasing concentrations of phospholipids. Membrane perturbations were observed in acidic SUV of DMPC/DMPA and DMPC/bovine brain phosphatidylserine by MTR and its magnesium complex as studied by monitoring the leakage of the entrapped fluorescent marker carboxyfluorescein and by electron microscopic measurements of the size of the liposomes. These results indicated a possible role of anionic phospholipids in mediating binding of MTR and its magnesium complex to the cell surface membranes before reaching the target DNA.

A novel strategy for the preparation of liposomes: rapid solvent exchange

During the preparation of multi-component model membranes, a primary consideration is that compositional homogeneity should prevail throughout the suspension. Some conventional sample preparation methods pass the lipid mixture through an intermediary, solvent-free state. This is an ordered, solid state and may favor the demixing of membrane components. A new preparative method has been developed which is specifically designed to avoid this intermediary state. This novel strategy is called rapid solvent exchange (RSE) and entails the direct transfer of lipid mixtures between organic solvent and aqueous buffer. RSE liposomes require no more than a minute to prepare and manifest considerable entrapment volumes with a high fraction of external surface area. In phospholipid/cholesterol mixtures of high cholesterol content, suspensions prepared by more conventional methods reveal evidence of artifactual demixing, whereas samples prepared by rapid solvent exchange do not. The principles which may lead to artifactual demixing during conventional sample preparation are discussed.

Chromatographic approaches to liposomes, proteoliposomes and biomembrane vesicles

Size-exclusion chromatography has been used for fractionation of liposomes, proteoliposomes and biomembrane vesicles of up to approximately 500 nm in size and for separation of these entities from smaller components. Liposome sizes, encapsulation stability, and solute affinities for membrane proteins have been determined. Counter-current distribution in aqueous two-phase systems has widened the range of applications to larger structures. Immobilized biomembrane vesicles and (proteo)liposomes provide stationary phases for chromatographic analysis of specific or nonspecific membrane-solute interactions.

Enhancement of human protein C function by site-directed mutagenesis of the gamma-carboxyglutamic acid domain

This study reports properties of site-directed mutants of human protein C that display enhanced calcium and/or membrane binding properties. Mutants containing the S11G modification all showed increased affinity for membranes at saturating calcium concentration. Ser-11 is unique to human protein C, whereas all other vitamin K-dependent proteins contain glycine. This site is located in a compact region of the protein, close to a suggested membrane contact site. Additional changes of H10Q or S12N resulted in proteins with lower calcium requirement for membrane contact but without further increase in membrane affinity at saturating calcium. Mutations Q32E and N33D did not, by themselves, alter membrane affinity to a significant degree. These mutations were included in other mutant proteins and may contribute somewhat to higher function in these mutants. This family of mutants helped discriminate events that are necessary for protein-membrane binding. These include calcium binding to the free protein and subsequent protein-membrane contact. Depending on conditions of the assay used, the mutants displayed increased activity of the corresponding activated protein C (APC) derivatives. The degree of enhanced activity (up to 10-fold) was dependent on the concentration of phospholipid and quality of phospholipid (+/- phosphatidylethanolamine) used in the assay. This was expected, because APC is active in its membrane-associated form, which can be regulated by changes in either the protein or phospholipid. As expected, the largest impact of the mutants occurred at low phospholipid concentration and in the absence of phosphatidylethanolamine. The anticoagulant activity of all proteins was stimulated by protein S, with the greatest impact on the enhanced mutants. Whereas plasma containing Factor V:R506Q was partially resistant to all forms of APC, the enhanced variants were more active than normal APC. Protein C variants with enhanced function present new reagents for study of coagulation and may offer improved materials for biomedical applications.

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.

Immobilized liposome chromatography for studies of protein-membrane interactions and refolding of denatured bovine carbonic anhydrase

Small unilamellar vesicles (SUVs) composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and 1 mol% phosphatidylethanolamine were covalently coupled to chromatographic gel beads. Interactions of liposomal lipid bilayers with several water-soluble proteins, which had been denatured or partially denatured by 0.1-5 M guanidinium hydrochloride (GuHCl), were studied on gel beads containing the immobilized SUVs. The partially-denatured proteins treated with 0.5-1.0 M GuHCl were significantly retarded on the immobilized liposome column, whereas little retardation of native or unfolded proteins treated by >2 M GuHCl was observed on the same liposome columns. The retardation on the immobilized liposome column was found to be well correlated with local hydrophobicity, which was determined by the aqueous two-phase partitioning method using 1 mM Triton X-405 as a hydrophobic probe. It implies that the partially-denatured proteins are likely in a molten-globule state and associated with liposomal lipid bilayers. Chromatographic refolding of denatured bovine carbonic anhydrase (CAB) was achieved on the immobilized liposome column. The enzymatic activity of an unfolded CAB treated by 5 M GuHCl was recovered up to 83% after passing it through immobilized liposome column, whereas only 58% of the enzymatic activity was recovered when the denatured CAB was run on a liposome-free column. The refolding process is probably involved in the interaction of molten-globule state of CAB with the liposomal lipid bilayers.