Mechanisms of Lipoplex Formation: Dependence of the Biological Properties ofTransfection Complexes on Formulation Procedures

Phospholipid-DNA complexes were made of the cationic triester derivative of phosphatidylcholine, EDOPC (1,2-dioleoyl- sn-glycero-3-ethylphosphocholine), by varying conditions of complex formation, in particular, the rate and direction of mixing, as well as by changing the mode of dispersing the lipid (extrusion or vortexing). The biological effects of variations in the formulation procedure were assessed by measuring transfection activity and cell association in cultures of BHK cells. Formulation procedures generally had little effect on cell association, but had marked effects on transfection efficiency. Transfection varied from effectively nil to extremely efficient with what appeared to be modest changes in formulation procedure. Formulation procedures also had significant effects on average sizes and size distributions of lipoplexes as determined by dynamic light scattering. Among the four possibilities of rapid or slow mixing combined with the two possible directions of mixing, slow addition of DNA to lipid gave results that differed significantly from the other three modes. In the case of vortexed lipid, the latter procedure was much less satisfactory than the other three, whereas in the case of extruded lipid, it was the only mode that produced satisfactory transfection. The factors that determine the difference in lipoplex properties can be identified as both geometric and physical. The geometric factor has to do with the symmetries of the participating units. There are three physical factors that are critical: the difference in vesicle stability upon interaction with DNA, the time dependence of interdiffusion of the components relative to that of vesicle rupture, and difference in input concentrations. These factors determine lipoplex size and, as already also shown by others, lipoplex size influences transfection efficiency.

New strategy for transfection: mixtures of medium-chain and long-chain cationic lipids synergistically enhance transfection

To date, the primary approach to improve the transfection properties of cationic lipids has been the synthesis of new kinds of cationic amphipaths or the inclusion of noncationic helper lipids. Here, it is reported that an alternative approach can be unusually effective, namely, the combination of two cationic lipid derivatives having the same head group but tails of different chain lengths. Particularly efficient was the combination of dilauroyl (12 carbon chain) and dioleoyl (18 carbon chain) homologues of O-ethylphosphatidylcholine. This mixture transfected DNA into human umbilical artery endothelial cells (HUAEC) more than 30-fold more efficiently than either compound separately. A unique advantage of this kind of combination agent is that transfection can be optimized either in the presence or absence of serum by adjusting the component ratio.

Bilayer mixing, fusion, and lysis following the interaction of populations of cationic and anionic phospholipid bilayer vesicles

Cationic, O-alkylphosphatidylcholines, recently developed as DNA transfection agents, form bilayers indistinguishable from those of natural phospholipids and undergo fusion with anionic bilayers. Membrane merging (lipid mixing), contents release, and contents mixing between populations of positive vesicles containing O-ethylphosphatidylcholine (EDOPC) and negative vesicles containing dioleolylphosphatidylglycerol (DOPG) have been determined with standard fluorometric vesicle-population assays. Surface-charge densities were varied from zero to full charge. All interactions depended critically on surface-charge density, as expected from the adhesion-condensation mechanism. Membrane mixing ranged from zero to 100%, with significant mixing (>10 <70%) occurring between cationic vesicles that were fully charged and anionic vesicles that had fractional surface charges as low as 0.1. Such mixing with membranes as weakly charged as cell membranes should be relevant to transfection with cationic lipids. Unexpectedly, lipid mixing was higher at high than at low ionic strength when one lipid dispersion was prepared from EDOPC plus DOPG (in different proportions), especially when the other vesicles were of EDOPC; this may somehow be a consequence of the ability of the former mixture to assume non-lamellar phases. Leakage of aqueous contents was also a strong function of charge, with fully charged vesicles releasing essentially all of their contents less than 1 min after mixing. EDOPC was more active in this regard than was DOPG, which probably reflects stronger intermolecular interactions of DOPG. Fusion, as measured by contents mixing, exhibited maximal values of 10% at intermediate surface charge. Reduced fusion at higher charge is attributed to multiple vesicle interactions leading to rupture. The existence of previously published data on individual interactions of vesicles of the same composition made it possible for the first time to compare pairwise with population interactions, confirming the likelihood of population studies to overestimate rupture and hemifusion and underestimate true vesicle fusion.

Real-time observation of lipoplex formation and interaction with anionic bilayer vesicles

A novel development has allowed for the direct observation of single, pairwise interactions of linear DNA with cationic vesicles and of DNA-cationic lipid complexes with anionic vesicles. A new cationic phospholipid derivative, l,2-dioleoyl-sn-glycero-3-ethylphosphocholine, was used to prepare giant bilayer vesicles and to form DNA-cationic lipid complexes (lipoplexes). The cationic vesicles were electrophoretically maneuvered into contact with DNA, and similarly, complexes were brought into contact with anionic phospholipid vesicles composed of dioleoylphosphatidylglycerol (DOPG; 100%), DOPG/dioleoylphosphatidylethanolamine (DOPE; 1:1) or DOPG/dioleoylphosphatidylcholine (DOPC; 1:1). Video fluorescence microscopy revealed that upon contact with phospholipid anionic vesicles, lipoplexes exhibited four different types of behavior: adhesion, vesicle rupture, membrane perforation (manifested as vesicle shrinkage and/or content loss), and expansion of DNA (which was always concomitant with membrane perforation.) In one instance, the lipoplex was injected into the target vesicle just prior to DNA expansion. In all other instances, the DNA expanded over the outer surface of the vesicle, and expansion was faster, the larger the area of vesicle over which it expanded. Given the likelihood of incorporation of cellular anionic lipids into lipoplexes, the expansion of the DNA could be important in DNA release during cell transfection. Upon contact with naked DNA, giant cationic vesicles usually ruptured and condensed the DNA into a small particle. Contact of cationic vesicles that were partially coated with DNA usually caused the DNA to wrap around the vesicle, leading to vesicle rupture, vesicle fusion (with other attached vesicles or lipid aggregates), or simply cessation of movement. These behaviors clearly indicated that both DNA and vesicles could be partly or fully covered by the other, thus modifying surface charges, which, among others, allowed adhesion of DNA-coated vesicles with uncoated vesicles and of lipid-coated DNA with uncoated DNA.

Cholesterol stabilizes hemifused phospholipid bilayer vesicles

Cholesterol was found to inhibit full fusion of oppositely charged phospholipid bilayer vesicles by stabilizing the contacting membranes at the stage of the hemifused intermediate. Vesicles of opposite charge containing different amounts of cholesterol were prepared using cationic (1,2-dioleoyl-sn-glycero-3-ethylphosphocholine) and anionic (dioleoylphosphatidylglycerol) phospholipids. Pairwise interactions between such vesicles were observed by fluorescence video microscopy in real time after electrophoretically maneuvering the vesicles into contact. Hemifusion accounted for more than 80% of the observed events when the vesicles contained 33-50 mole% cholesterol. In contrast, vesicles containing only a small proportion of cholesterol (</=10 mole%), underwent full fusion in approx. 70% of the interactions monitored. The role of cholesterol is explained both as favoring the formation of the hemifused intermediate according to the adhesion-condensation mechanism of bilayer fusion and as disfavoring the transition from hemifusion to full fusion on the basis of reduced tension in the vesicle bilayers.

Diquaternary ammonium compounds as transfection agents

Diquaternary ammonium salts constitute a new class of reagent for mediating transfection of DNA in mammalian cell lines. N,N'-dioleyl-N,N,N',N'-tetramethyl-1,2-ethanediamine (TmedEce), N,N'-dioleyl-N,N,N',N'-tetramethyl-1,3-propanediamine (PropEce), N,N'-dioleyl-N,N,N',N'-tetramethyl-1,6-hexanediamine (HexEce), and their corresponding N,N'-dicetyl saturated analogues (TmedAce, PropAce and HexAce) have all been synthesized and characterized. They were prepared via a bis-Menshutkin reaction of the corresponding tetramethyldiamine with 2.2 M equiv of a long-chain alkyl halide (saturated or unsaturated). The reaction was run in anhydrous acetonitrile for ca. 3 days at 60 degrees C, which produced the diquaternary ammonium halides in good to nearly quantitative yields for most derivatives. DNA transfection comparable to commercially available reagents such as Lipofectin, Lipofectace, Lipofectamine, and O-ethyldioleoylphosphatidylcholinium triflate has been achieved in vitro with these new reagents. There was no need to use a colipid for effective transfection, but serum did significantly inhibit transfection. The saturated and the unsaturated derivatives differed with respect to hydration behavior. The saturated derivatives appeared to retain a lamellar-type crystalline array structure upon hydration, whereas the unsaturated versions formed micelles and/or liposomes, depending on the ionic strength: HexEce was micellar in both water and saline; PropEce was micellar in water but lamellar in saline; and TmedEce was lamellar in both. Despite these different hydration patterns, all of these unsaturated derivatives formed productive transfection complexes with DNA. Varying the distance between the quaternary sites affected transfection efficacy in the order HexAce > TmedAce = PropAce for the saturated derivatives and in the order PropEce = HexEce > TmedEce, with a smaller spread, for the unsaturated derivatives.

Electrostatic control of phospholipid polymorphism

A regular progression of polymorphic phase behavior was observed for mixtures of the anionic phospholipid, cardiolipin, and the cationic phospholipid derivative, 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine. As revealed by freeze-fracture electron microscopy and small-angle x-ray diffraction, whereas the two lipids separately assume only lamellar phases, their mixtures exhibit a symmetrical (depending on charge ratio and not polarity) sequence of nonlamellar phases. The inverted hexagonal phase, H(II,) formed from equimolar mixtures of the two lipids, i.e., at net charge neutrality (charge ratio (CR((+/-))) = 1:1). When one type of lipid was in significant excess (CR((+/-)) = 2:1 or CR((+/-)) = 1:2), a bicontinuous cubic structure was observed. These cubic phases were very similar to those sometimes present in cellular organelles that contain cardiolipin. Increasing the excess of cationic or anionic charge to CR((+/-)) = 4:1 or CR((+/-)) = 1:4 led to the appearance of membrane bilayers with numerous interlamellar contacts, i.e., sponge structures. It is evident that interactions between cationic and anionic moieties can influence the packing of polar heads and hence control polymorphic phase transitions. The facile isothermal, polymorphic interconversion of these lipids may have important biological and technical implications.

Development of echogenic, plasmid-incorporated, tissue-targeted cationic liposomes that can be used for directed gene delivery

RATIONALE AND OBJECTIVES

Echogenic antibody-conjugated anionic liposomes have been developed that allow directed tissue targeting and acoustic enhancement. These are not efficient for gene delivery. A cationic formulation that allows directed gene delivery while retaining acoustic properties may provide more efficient transfection.

METHODS

Cationic liposomes were prepared and acoustic reflectivity was determined. Anti-fibrinogen-conjugated liposomes were laid on fibrin-coated slides and adherence was quantified using fluorescence techniques. Liposomes were combined with a reporter gene and plated on cell cultures. Human umbilical vein endothelial cells were stimulated to upregulate intercellular adhesion molecule-1 (ICAM-1) and were treated with anti-ICAM-1-conjugated liposomes, and gene expression was quantified.

RESULTS

Cationic liposomes retained their acoustic reflectivity and demonstrated specific adherence to fibrin under flow conditions. Significant transfection of human umbilical vein endothelial cells was demonstrated, with higher gene expression seen with specific antibody-conjugated liposomes.

CONCLUSIONS

Novel acoustic cationic liposomes have been developed that can be antibody conjugated for site-specific adherence and directed cell modification. This presents exciting potential for a vector that allows tissue enhancement and targeted gene delivery.

An assessment of the validity of a computer system for probabilistic record linkage of birth and infant death records in Canada. The Fetal and Infant Health Study Group

Studies on the validity of probabilistic record linkage are sparse. We performed a probabilistic linkage to link the 1984-1994 birth records (obtained from the Canadian Birth Data Base) with 1984-1995 infant death records (from the Canadian Mortality Data Base) in Canada. We extracted the linked birth-death records for Nova Scotia and Alberta (from January 1990 to December 1991) obtained from Statistics Canada's vital registration data and compared them with corresponding records from provincial data (primarily hospital records). The results showed that over 99% of infant deaths (153/155) in the Nova Scotia provincial data were successfully located in the linked Statistics Canada file; the corresponding figure for Alberta neonatal deaths was also 99% (365/367). The distributions of gestational age and birth weight in matched cases demonstrated high agreement between the two data sources. We conclude that the computer system for probabilistic linkage developed by Statistics Canada using the available personal identifying variables in the Canadian Birth Data Base and the Canadian Mortality Data Base is valid.

O-Alkyl dioleoylphosphatidylcholinium compounds: the effect of varying alkyl chain length on their physical properties and in vitro DNA transfection activity

1,2-Dioleoyl-sn-3-ethylphosphocholine (EDOPC) has been previously shown be a highly effective DNA transfection reagent in vitro. To assess the effect of alkyl chain length on transfection efficiency, the O-methyl, O-propyl, O-hexyl, O-decyl, and O-octadecyl derivatives have been prepared from dioleoylphosphatidylcholine using the corresponding alkyl trifluoromethylsulfonate. The methyl, ethyl, and propyl derivatives formed liposomes which were very large and unilamellar. The ethyl and propyl derivatives were equally efficient at mediating transfection (even in the presence of serum) of BHK cells, but the chemically labile methyl derivative was a much weaker transfection agent. The O-decyl and O-octadecyl compounds, which assume the inverted hexagonal phase in excess water (as determined by X-ray diffraction), were almost inactive after manual agitation in both water and in saline; however, after sonication, these compounds exhibited good transfection activity. The O-hexyl derivative displayed novel behavior, assuming the lamellar phase at low and a cubic phase at high ionic strength. All compounds, whether lamellar or not, formed lamellar structures when complexed with DNA. In water, where the hexyl compound dispersed well, sonication diminished transfection activity, whereas at physiological ionic strength, which led to poor manual dispersion, sonication was essential for good transfection. These results emphasize the importance of optimal dispersion of a cationic lipid: too little, and interaction with DNA is handicapped, too much, and the resultant particle transfects poorly. Lipid dispersibility is thus an important variable in assessing lipid transfection agents, and caution is advised in attributing too much significance to chemical structure until interaction with DNA has been optimized.

The lymphocytic choriomeningitis virus RING protein Z associates with eukaryotic initiation factor 4E and selectively represses translation in a RING-dependent manner

Only a few host cell proteins that associate with arenaviruses have been identified. To date, the arenavirus Z protein associates with the promyelocytic leukemia protein PML and the ribosomal P proteins. The majority of PML is present in nuclear bodies which are translocated to the cytoplasm by infection with the arenavirus, lymphocytic choriomeningitis virus (LCMV). The Z protein is a small zinc-binding RING protein with an unknown function which is required for the viral life cycle. Here, we demonstrate an association between Z and the host cell translation factor, eukaryotic initiation factor 4E (eIF-4E) in infected and transfected cells. Z's association with both ribosomal proteins and this translation factor led us to investigate whether Z could modulate host cell translation. In cell culture, Z selectively represses protein production in an eIF-4E-dependent manner. Specifically, we see reduction in cyclin D1 protein production with no effect on glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in cells transfected with Z. Previous reports indicate that cyclin D1 is sensitive to eIF-4E levels, whereas GAPDH is not. Consistent with this, we observe preferential downregulation of cyclin D1 during infection and no effect on GAPDH. Further, no changes in RNA levels were observed for cyclin D1 or GAPDH transcripts. The interaction between eIF-4E and Z may provide a mechanism for slower growth observed in infected cells and a viral strategy for establishing chronic infection.

Factors governing the assembly of cationic phospholipid-DNA complexes

The interaction of DNA with a novel cationic phospholipid transfection reagent, 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine (EDOPC), was investigated by monitoring thermal effects, particle size, vesicle rupture, and lipid mixing. By isothermal titration calorimetry, the heat of interaction between large unilamellar EDOPC vesicles and plasmid DNA was endothermic at both physiological and low ionic strength, although the heat absorbed was slightly larger at the higher ionic strength. The energetic driving force for DNA-EDOPC association is thus an increase in entropy, presumably due to release of counterions and water. The estimated minimum entropy gain per released counterion was 1.4 cal/mole- degrees K (about 0.7 kT), consistent with previous theoretical predictions. All experimental approaches revealed significant differences in the DNA-lipid particle, depending upon whether complexes were formed by the addition of DNA to lipid or vice versa. When EDOPC vesicles were titrated with DNA at physiological ionic strength, particle size increased, vesicles ruptured, and membrane lipids became mixed as the amount of DNA was added up to a 1.6:1 (+:-) charge ratio. This charge ratio also corresponded to the calorimetric end point. In contrast, when lipid was added to DNA, vesicles remained separate and intact until a charge ratio of 1:1 (+:-) was exceeded. Under such conditions, the calorimetric end point was 3:1 (+:-). Thus it is clear that fundamental differences in DNA-cationic lipid complexes exist, depending upon their mode of formation. A model is proposed to explain the major differences between these two situations. Significant effects of ionic strength were observed; these are rationalized in terms of the model. The implications of the analysis are that considerable control can be exerted over the structure of the complex by exploiting vectorial preparation methods and manipulating ionic strength.

Physical and biological properties of cationic triesters of phosphatidylcholine

The properties of a new class of phospholipids, alkyl phosphocholine triesters, are described. These compounds were prepared from phosphatidylcholines through substitution of the phosphate oxygen by reaction with alkyl trifluoromethylsulfonates. Their unusual behavior is ascribed to their net positive charge and absence of intermolecular hydrogen bonding. The O-ethyl, unsaturated derivatives hydrated to generate large, unilamellar liposomes. The phase transition temperature of the saturated derivatives is very similar to that of the precursor phosphatidylcholine and quite insensitive to ionic strength. The dissociation of single molecules from bilayers is unusually facile, as revealed by the surface activity of aqueous liposome dispersions. Vesicles of cationic phospholipids fused with vesicles of anionic lipids. Liquid crystalline cationic phospholipids such as 1, 2-dioleoyl-sn-glycero-3-ethylphosphocholine triflate formed normal lipid bilayers in aqueous phases that interacted with short, linear DNA and supercoiled plasmid DNA to form a sandwich-structured complex in which bilayers were separated by strands of DNA. DNA in a 1:1 (mol) complex with cationic lipid was shielded from the aqueous phase, but was released by neutralizing the cationic charge with anionic lipid. DNA-lipid complexes transfected DNA into cells very effectively. Transfection efficiency depended upon the form of the lipid dispersion used to generate DNA-lipid complexes; in the case of the O-ethyl derivative described here, large vesicle preparations in the liquid crystalline phase were most effective.

O-ethylphosphatidylcholine: A metabolizable cationic phospholipid which is a serum-compatible DNA transfection agent

1,2-dioleoyl-sn-glycero-3-ethylphosphocholine was prepared in a one-step reaction from phosphatidylcholine by reaction with ethyl trifluoromethanesulfonate. This and related O-alkyl phosphatidylcholines constitute the first chemically stable triesters of biological lipid structures and the first cationic derivatives of phospholipids consisting entirely of biological metabolites linked with ester bonds. The complex of cationic phospholipid and plasmid DNA transfected cells with high efficiency. Maximum efficiency of transfection was obtained with complexes in which the positive charge was a few percent in excess over the negative charge. Modest stimulation of transfection of common cell lines was obtained by continuous culture in the presence of 10% serum. Incubation of the phospholipid complex for at least 2 h at 37 degrees C in nearly pure serum had no deleterious effects on transfection efficiency. The lipid has low toxicity; BHK cells tolerated amounts of 2 mg/2 x 10(6) cells at concentrations of 1 mg/mL. The lipid is biodegradable; it was hydrolyzed by phospholipase A(2) in vitro and was metabolized with a half-life of a few days in cells in culture. The synthetic route to cationic phospholipids is well suited to the preparation of derivatives that are tailor-made to have a wide variety of different properties.

Characterization of vascular gene transfer using a novel cationic lipid

INTRODUCTION

Cationic liposomes are an alternative non-viral vector for gene therapy, but several factors affect transfection efficiency. A novel cationic lipid, o-ethyldioleoylphosphatidylcholinium (EDOPC), was studied for characterization of the time course and effects of lipid composition, concentration, charge ratio, mixing techniques, passage number, and stimulated state on transfection of human vascular cells, represented by human umbilical vein endothelial cells (HUVEC).

METHODS

HUVEC cultures were seeded at a density of 45,000 cells/well in 24-well plates and incubated overnight. Triplicate wells were transfected with samples of EDOPC/reporter plasmid for 2 h, followed by a 24-h expression time, which was the peak expression time point in an initial time-course experiment. Measuring luciferase in cell lysates quantitated gene expression.

RESULTS

Transfection of HUVEC with EDOPC was optimal with a concentration of 100 microgram lipid/well, ratio of 3:1 EDOPC:plasmid, fractional mixing of lipid and plasmid, centrifugation, and incubation in serum-free media. Transfections in sequential passages showed striking decreases in gene expression and regression analysis revealed the relationship: RLU = 120,000 - (10, 400 x passage number), r(2) = 0.947. HUVEC activated by cytokine stimulation remain susceptible to gene transfer specifically with EDOPC.

SUMMARY

During transfection of HUVEC with cationic lipid species, an increase in passage number is associated with linear reduction in luciferase expression, and hence passage number must be controlled in comparative experiments. Characteristics of EDOPC may permit site-specific efficient transfection of activated human vascular cells that can be isolated from serum by mechanical methods.

Directly observed membrane fusion between oppositely charged phospholipid bilayers

A novel method was developed for the direct examination of pairwise encounters between positively and negatively charged phospholipid bilayer vesicles. Giant bilayer vesicles (unilamellar, 4-20 micron in diameter) prepared from 1,2-dioleoyl-sn-glycero-3-ethylphosphocholine, a new cationic phospholipid derivative, were electrophoretically maneuvered into contact with individual anionic phospholipid vesicles. Fluorescence video microscopy revealed that such vesicles commonly underwent fusion within milliseconds (1 video field) after contact, without leakage. Fusion occurred at constant volume and, since flaccid vesicles were rare, the excess membrane was not available after fusion. Hemifusion (the outer monolayers of each vesicle fused while the inner monolayers remained intact) was inferred from membrane-bound dye transfer and a change in the contact area. Hemifusion was observed as a final stable state and as an intermediate to fusion of vesicles composed of charged phospholipids plus zwitterionic phospholipids. Hemifusion occurred in one of three ways following adhesion: either delayed with an abrupt increase in area of contact, immediately with a gradual increase in area of contact, or with retraction during which adherent vesicles dissociated from a flat contact to a point contact. Phosphatidylethanolamine strongly promoted immediate hemifusion; the resultant hemifused state was stable and seldom underwent complete fusion. Although sometimes single contacts between vesicles led to rupture of both, in other cases, a single vesicle underwent multiple fusion events. Direct observation has unequivocally demonstrated the fusion of two, isolated bilayer-bounded bodies to yield a stable, non-leaky product, as occurs in cells, in the absence of proteins.

Cytoskeletal protein binding kinetics at planar phospholipid membranes

It has been hypothesized that nonspecific reversible binding of cytoskeletal proteins to lipids in cells may guide their binding to integral membrane anchor proteins. In a model system, we measured desorption rates k(off) (off-rates) of the erythrocyte cytoskeletal proteins spectrin and protein 4.1 labeled with carboxyfluorescein (CF), at two different compositions of planar phospholipid membranes (supported on glass), using the total internal reflection/fluorescence recovery after photobleaching (TIR/FRAP) technique. The lipid membranes consisted of either pure phosphatidylcholine (PC) or a 3:1 mixture of PC with phosphatidylserine (PS). In general, the off-rates were not single exponentials and were fit to a combination of fast, slow, and irreversible fractions, reported both separately and as a weighted average. By a variation of TIR/FRAP, we also measured equilibrium affinities (the ratio of surface-bound to bulk protein concentration) and thereby calculated on-rates, k(on). The average off-rate of CF-4.1 from PC/PS (approximately 0.008/s) is much slower than that from pure PC (approximately 1.7/s). Despite the consequent increase in equilibrium affinity at PC/PS, the on-rate at PC/PS is also substantially decreased (by a factor of 40) relative to that at pure PC. The simultaneous presence of (unlabeled) spectrin tends to substantially decrease the on-rate (and the affinity) of CF-4.1 at both membrane types. Similar experiments for CF-spectrin alone showed much less sensitivity to membrane type and generally faster off-rates than those exhibited by CF-4.1. However, when mixed with (unlabeled) 4.1, both the on-rate and off-rate of CF-spectrin decreased drastically at PC/PS (but not PC), leading to a somewhat increased affinity. Clearly, changes in affinity often involve countervailing changes in both on-rates and off-rates. In many of these studies, the effect of varying ionic strength and bulk concentrations was examined; it appears that the binding is an electrostatic attraction and is far from saturation at the concentrations employed. These results and the techniques implemented carry general implications for understanding the functional role of nonspecific protein binding to cellular lipid membranes.

Delivery of membrane-impermeant fluorescent probes into living neural cell populations by lipotransfer

Use of fluorescent probes to monitor f-actin in living cells typically relies on difficult microinjection procedures. The current work has developed cationic lipotransfer of membrane-impermeant probes as an alternative to microinjection. BODIPY FL-phallacidin, a fluorescent f-actin probe, was packaged into 40-50 nm cationic liposomes. Packaging, verified by gel filtration, enabled delivery of the probe into living nerve cells and provided an image of f-actin that was identical to that seen in fixed, permeabilized cells. Phallacidin alone did not enter living cells, nor was its uptake stimulated by the presence of empty liposomes. All predicted f-actin structures were fluorescent in living cells, indicating a high efficacy of delivery. Cationic lipotransfer of fluorescent probes was rapid, not disruptive to cells, and delivered a probe en masse to a large sample population. Lipotransfer appears to be a promising alternative to microinjection for introducing membrane-impermeant probes and reagents into living cells.

A tethered adhesive particle model of two-dimensional elasticity and its application to the erythrocyte membrane

A new model of two-dimensional elasticity with application to the erythrocyte membrane is proposed. The system consists of a planar array of self-adhesive particles attached to nearest neighbors with flexible tethers. Stretching from the equilibrium dimension is resisted because force is required to dissociate the particle clusters and to decrease the distribution entropy. Release of the external force is accompanied by a contraction as thermal diffusion randomizes the particles and allows interparticle attachments to form again. Analysis of membrane thermodynamics and mechanics under the two-state particle assumption results in a shear softening stress-strain relation. The shear modulus is found proportional to the square root of the surface density of particles, the interparticle adhesive energy, and is inversely proportional to the tether length. Applied to the erythrocyte membrane under the assumption that band 3 tetramer represents the particle and spectrin the tether, the shear modulus predicted corresponds to the measured value when the interparticle adhesive energy is approximately 4.0-5.9 kT, where kT is the Boltzmann constant multiplied by the temperature. This model suggests a mechanism wherein erythrocyte membrane deformability depends on integral protein homomultimeric interactions and can be modulated from the external surface.

Effects of chain unsaturation on the equation of state for lipid monolayers at the air-water interface

An equation of state for lipid monolayers at the air-water interface is presented, which takes into account the effects of the conformation and the number and position of double bonds of the hydrocarbon chains. The total Hamiltonian of the monolayer is assumed to consist of three terms. Two of them are calculated exactly within the limitations of the formulation. These are the two-dimensional entropy of mixing of the lipid and water molecules at the surface and the conformational entropy of the chains using a model available from the literature. These two terms give rise to positive surface pressure. The third term, which includes all energies that are not amenable to calculation, was obtained as the difference between the sum of the two calculated terms and experimental data and is found to represent an approximately area-independent tension. The effects of chain unsaturation on the equation of state were modeled in the present theory in two ways; the chain bend caused by cis double bonds increases the minimal molecular area, and the double bond linkage on a chain decreases the degrees of freedom of the chain. Calculations revealed that the former is highly significant whereas the latter is negligible. The deduced equation of state reproduces experimental data with appropriate values for three parameters, which represent the collapse area, the overlap of adjacent chains, and the combined effects of the intra- and intermolecular interactions other than the surface mixing entropy and the chain conformational energy.

Methanol Emission from Leaves (Enzymatic Detection of Gas-Phase Methanol and Relation of Methanol Fluxes to Stomatal Conductance and Leaf Development)

We recently reported the detection of methanol emissions from leaves (R. MacDonald, R. Fall [1993] Atmos Environ 27A: 1709-1713). This could represent a substantial flux of methanol to the atmosphere. Leaf methanol production and emission have not been investigated in detail, in part because of difficulties in sampling and analyzing methanol. In this study we used an enzymatic method to convert methanol to a fluorescent product and verified that leaves from several species emit methanol. Methanol was emitted almost exclusively from the abaxial surfaces of hypostomatous leaves but from both surfaces of amphistomatous leaves, suggesting that methanol exits leaves via stomates. The role of stomatal conductance was verified in experiments in which stomates were induced to close, resulting in reduced methanol. Free methanol was detected in bean leaf extracts, ranging from 26.8 [mu]g g-1 fresh weight in young leaves to 10.0 [mu]g g-1 fresh weight in older leaves. Methanol emission was related to leaf development, generally declining with increasing leaf age after leaf expansion; this is consistent with volatilization from a cellular pool that declines in older leaves. It is possible that leaf emission could be a major source of methanol found in the atmosphere of forests.

Neurotransmitter release evoked by nerve impulses without Ca2+ entry through Ca2+ channels in frog motor nerve endings

1. The requirement for extracellular Ca2+ in the process of evoked acetylcholine (ACh) release by nerve impulses was tested at endplates in frog skeletal muscle. Ca(2+)-containing lipid vesicles (Ca2+ liposomes) were used to elevate cytoplasmic Ca2+ concentrations under conditions in which Ca2+ entry from the extracellular fluid was prevented. 2. In an extracellular solution containing no added Ca2+ and 1 mM Mg2+ ('Ca(2+)-free' solution), Ca2+ liposomes promoted the synchronous release of ACh quanta, reflected electrophysiologically as endplate potentials (EPPs), in response to temporally isolated nerve impulses. 3. Motor nerve stimulation generated EPPs during superfusion with Ca2+ liposomes in Ca(2+)-free solutions containing the Ca2+ channel blocker Co2+ (1 mM), and the Ca2+ chelator EGTA (2 mM). As a physiological control for Ca2+ leakage from the liposomes to the extracellular fluid, the effect of Ca2+ liposomes on asynchronous evoked ACh release mediated by Ba2+ was examined. In contrast to the effects of 0.2-0.3 mM extracellular Ca2+, which generated EPPs but antagonized Ba(2+)-mediated asynchronous ACh release, Ca2+ liposomes generated EPPs but did not reduce asynchronous release mediated by Ba2+. The effects of Ca2+ liposomes were thus not due to leakage of Ca2+ from the liposome to the extracellular fluid. 4. Morphological studies using fluorescently labelled liposomes in conjunction with a confocal microscope demonstrate that lipid is transferred from the liposomes to nerve endings and liposomal contents are delivered to the nerve terminal cytoplasm. 5. The results suggest that when intracellular Ca2+ is elevated using liposomes as a vehicle, evoked ACh release can occur in the absence of Ca2+ entry via Ca2+ channels.

A metastable state of high surface activity produced by sonication of phospholipids

Sonication of phosphatidylcholine dispersions generates a metastable high energy assembly of molecules, the existence of which is revealed by its conspicuous surface activity. Freshly sonicated liposome dispersions release molecules to the air/water interface at rates sufficient to produce a close-packed monolayer within minutes. In contrast, monolayers at the surface of multilamellar and extruded vesicles take hours to form. The highly surface active species appears within the first few minutes of sonication, long before a major reduction in turbidity occurs, and accumulates over the course of a few hours of sonication. It disappears upon exhaustive sonication, extrusion, addition of extruded vesicles, or, more slowly, simply on standing. Tests for extraneous substances in the lipids before as well as after sonication revealed amounts of degradation products too small to represent the observed surfactant. Direct evidence that the metastable aggregate releases intact phospholipids was provided by a novel procedure to characterize monolayer composition by comparing surface tension with surface potential, both as a function of surface density. Centrifugation and gel filtration chromatography indicate that the surface activity is associated with a particle of diameter larger than a lysophosphatidylcholine micelle but not larger than limit sonicated vesicles. The metastable material appears to be lipid molecules in other than the normal stable vesicular state, perhaps an incompletely closed vesicle, one in which the inner and outer monolayers have not equilibrated, or possibly a micellar form.

Fragmentation into small vesicles of dioleoylphosphatidylcholine bilayers during freezing and thawing

Multilayered liposomes of some phosphatidylcholines progressively fragment into small vesicles when the electrolyte solution in which they are suspended is subjected to successive cycles of freezing and thawing. The fragmentation process, routinely monitored by absorbance measurements and verified by electron microscopy and dynamic light scattering, involves bilayer breakage and resealing. After 10 cycles of freezing and thawing in 0.1 M electrolyte solution, the result is a population of vesicles smaller than 200 nm diameter. Sucrose, a common cryoprotectant, completely inhibits fragmentation. Fragmentation is absolutely dependent upon the presence of an electrolyte. Those electrolytes most effective in promoting liposome fragmentation have large freezing point depressions and corresponding high solubilities at the freezing point. This, coupled with the observation that saturating concentrations of electrolyte are less effective than 0.1 M solutions indicates that an essential stage in the fragmentation process is osmotic extraction of water from the vesicles, i.e., ice formation in the external phase leads to a progressive increase in the electrolyte concentration of the residual external solution, which, in turn, dehydrates the vesicle. In addition, for maximal fragmentation, the minimum temperature must be at least as low as the solute eutectic temperature. Particular physical properties of the bilayer are also important, for dioleoyl and diphytanoyl derivatives are much more susceptible to fragmentation than are other phosphatidylcholines, and inclusion of 50 mol% cholesterol in dioleoylphosphatidylcholine completely inhibits membrane breakup. This system provides insight into mechanisms of freezing damage to membranes and may also offer a very simple and rapid assay for biological cryoprotectants.

Lipid unsaturation influences melittin-induced leakage of vesicles

Investigation of vesicles composed of different phosphatidylcholines revealed that the extent of leakage of internal contents induced by the lytic agent melittin can range from practically none to essentially complete, depending upon the fatty acyl chain composition of the phospholipid. The extent of leakage increases with the number of double bonds in the series dioleoylphosphatidylcholine < dilinoleoylphosphatidylcholine < dilinolenoylphosphatidylcholine. It depends on the length of the saturated chain with 1-myristoyl-2-arachidonoylphosphatidylcholine vesicles being more sensitive to melittin induced leakage than 1-palmi-toyl-2-arachidonoylphosphatidylcholine, 1-stearoyl-2-arachidonoylphosphatidylcholine or 1-palmitoyl-2- docosahexaenoylphosphatidylcholine vesicles. The extent of leakage induced by melittin from vesicles composed of 1-palmitoyl-2-oleoylphosphatidylcholine, dioleoylphosphatidylcholine, 1-palmitoyl-2-arachidonoylphosphatidylcholine and 1-palmitoyl-2-docosahexaenoyl-phosphatidylcholine increases with the free volume parameter of these lipids for 1,6-diphenylhexatriene (Straume, M. and Litman, B.J. (1987) Biochemistry 26, 5113-5120). Among the lipid examined here, diphytanoylphosphatidylcholine vesicles were least susceptible to melittin induced leakage. The results indicate that lipid fatty acyl structure may be important in lipid-protein interactions of the kind simulated by melittin.

Fluorescence studies of spectrin and its subunits

To better understand the solution structure of spectrin, the environment of its tryptophan residues have been examined by fluorescence spectroscopy. The spectra and the extent of quenching by several quenching agents have been determined for intact spectrin and its alpha and beta subunits. The arsenal of quenchers used in the study represented both hydrophilic and hydrophobic species including anionic, cationic and neutral compounds. Effects on spectrin fluorescence of ethanol and ionic strength, which extend and/or rigidify spectrin, and of glycerol, which is commonly used in electron microscopy of the protein, have also been assessed in the presence and absence of quenchers. Most of the tryptophans of spectrin are either internally quenched or are sequestered, hindering the approach of hydrophilic quenching agents. Both the spectral shape and the extent of quenching by acrylamide indicate that some tryptophans of the beta subunit are slightly more exposed in the isolated chain than in the dimer. Similar effects on spectra and on quenching of the intact dimer and of the isolated beta chain are seen when the ionic strength is reduced. Ethanol and glycerol reduce spectrin tryptophan accessibility to 2-p-toluidinyl napthalene-6-sulfonic acid (TNS). It therefore appears that low ionic strength, alpha-beta association and neutral solute (or lowered dielectric constant) all induce a similar, but modest conformational change in the domain structure. The extent of TNS binding is not increased by lowering the ionic strength, suggesting that the expansion and/or stiffening of the molecule in low electrolyte solution does not involve exposure of significant numbers of hydrophobic sites.

Experimental method to correct fluorescence intensities for the inner filter effect

An empirical procedure for the correction of measured fluorescence intensity for the inner filter effect is described. The procedure is more reliable than those described previously as fluorophores with the fluorescence properties of interest are sequestered from the external aqueous phase. The inner filter effect can therefore be assessed in the absence of chemical reaction or Forster energy transfer between the fluorophore and chromophore. A correction curve for the inner filter effect is obtained by measuring the fluorescence intensity of the sequestered fluorophore after adding a chromophore to increase the absorbance of the solution. The procedure was tested with liposomes containing calcein and two commercial polymer bead preparations containing embedded fluorophores. The former have the advantage of a wider choice of fluorophores and the latter of stability and convenience. The inner filter effect was varied using small aliquots of potassium chromate or pyridinium chloride solution. It was shown that the magnitude of the inner filter effect must be experimentally determined for each instrument and whenever the instrumental configuration is altered if an accurate correction for the inner filter effect is to be obtained. The magnitude of the correction depends on the wavelength range and the pathlength but not on slit-width or sample turbidity for the instrument employed for most of the experiments reported here. This empirical procedure makes possible studies of protein fluorescence quenching by agents previously unsuitable because of high absorbances.

Metabolism of Transpired Ethanol by Eastern Cottonwood (Populus deltoides Bartr.)

Ethanol has previously been shown to be present in the xylem sap of flooded and nonflooded trees. Because of the constitutive presence of alcohol dehydrogenase in the mature leaves of woody plants, we hypothesized that the leaves and shoots of trees had the ability to metabolize ethanol supplied by the transpiration stream. 1-[14C]Ethanol was supplied to excised leaves and shoots of eastern cottonwood (Populus deltoides Bartr.) in short- and long-term experiments. More than 99% of the radiolabel was incorporated into plant tissue in short-term experiments, with more than 95% of the label remaining in plant tissue after 24 h. In all experiments, less than 5% of the label was transpired as ethanol and less than 1% was emitted as CO2. In excised leaf experiments, less than 0.5% of the radiolabel escaped from the leaf. Fifty percent of the label was incorporated into the petioles of excised leaves; 56% was incorporated into the stems of excised shoots. Very little label reached the leaf mesophyll cells of excised shoots, as revealed by autoradiography. Radiolabel appeared primarily in the water- and chloroform-soluble fractions in short-term experiments, whereas in long-term experiments, label was also incorporated into protein. These results demonstrate that the leaves and stems of trees appear to have substantial ability to scavenge ethanol from the transpiration stream, allowing efficient recovery of ethanol produced elsewhere by hypoxic tissues. When labeled ethanol was supplied to excised petioles in a 5-min pulse, 41% of the label was incorporated into organic acids. Some label was also incorporated into amino acids, protein, and the chloroform-soluble fraction, with very little appearing in neutral sugars, starch, or the insoluble pellet. Labeled organic acids were separated by high performance liquid chromatography and were composed of acetate, isocitrate, [alpha]-ketoglutarate, and succinate. There was no apparent incorporation of label into phosphorylated compounds. We conclude that, in higher plants, ethanol is metabolized to acetaldehyde and then to acetate by alcohol and aldehyde dehydrogenases, and then into general metabolism.

Band 4.1 enhances spectrin binding to phosphatidylserine vesicles

Erythroid band 4.1 enhances the binding of erythroid spectrin to phosphatidylserine vesicles under conditions of ionic strength and at protein concentrations similar to those in the red cell. The extent of enhancement depends on the concentration of band 4.1; at 2 microM 4.1, spectrin binding increases approximately 10-fold (to 600 mumoles/mole lipid). The Kd is 0.5 microM, as measured by SDS-PAGE of protein-bound vesicles recovered by ficoll gradient centrifugation. The 4.1-enhanced binding of spectrin was also measured by a gel filtration assay. The electrostatic nature of the enhancement of spectrin binding is indicated by its dependence on the phosphatidylserine content of the vesicles.

Characteristics of spectrin-induced leakage of extruded, phosphatidylserine vesicles

At neutral pH spectrin induces modest leakage of trapped calcein from reverse-phase or extruded, but not sonicated, vesicles composed of phosphatidylserine, but not phosphatidylcholine. The extent of leakage from extruded vesicles is not or is only slightly affected by magnesium ions at a physiological concentration or calcium ions at a greater than physiological concentration, respectively. In addition to accounting for several previously discrepant observations on the lytic effects of spectrin, these findings indicate that some proteins like spectrin may destabilize vesicles with low curvature more readily than vesicles of high curvature, in contrast to certain amphiphilic peptides. 60% less leakage is induced from phosphatidylserine vesicles by heat-denatured than by native spectrin. In contrast, both trypsin- and subtilisin-treated spectrins, if sufficiently digested, induce several-fold more leakage than undigested spectrin. Since spectrin prepared either by 1 M Tris dissociation of Triton-extracted cytoskeletons or by low ionic strength extraction of ghosts released the same amounts of calcein from vesicles of various compositions, these effects are unlikely to reflect artifacts of spectrin preparation. Furthermore, spectrin is unlikely to promote leakage in vivo, since vesicles composed of phosphatidylserine, cholesterol and/or phosphatidylethanolamine, which constitute the lipid composition of the inner monolayer of the red cell membrane, did not leak on addition of spectrin, whereas vesicles composed of phosphatidylserine and phosphatidylcholine, did leak in the presence of spectrin.

Small-volume extrusion apparatus for preparation of large, unilamellar vesicles

The design and performance of a filter holder which enables convenient preparation of volumes of up to a milliliter of large, unilamellar vesicles formed by extrusion (LUVETs) from multilamellar vesicles (MLVs) are described. The filter holder provides for back-and-forth passage of the sample between two syringes, a design that minimizes filter blockage, eliminates the need to change filters during LUVET preparation and reduces preparation time to a few minutes. Replicas of slam-frozen LUVETs in the electron microscope are unilamellar and reasonably homogeneous with an average diameter close to the pore size of the filters used to extrude them. Extrusion per se does not destabilize the vesicles, which trapped a fluorescent dye only when they were disrupted on freeze-thawing and during the first extrusion when most of the MLVs were apparently converted to LUVETs.

Spontaneous rupture of liver due to cholangitis

Spontaneous rupture of the liver is rare in the Western world and most cases are associated with primary or metastatic tumours. Spontaneous rupture of the intrahepatic biliary tree is not well documented and only two reports have been found in the literature. The authors describe a further case due to ascending cholangitis but presenting with biliary peritonitis. The literature is reviewed and the diagnostic problems discussed.

Surface chemistry of phospholipid vesicles relevant to their aggregation and fusion

A general description of those forces that are important in the stability of the lipid bilayer is presented. The analysis focuses on those that may change under conditions in which aggregation or fusion of vesicles or exchange of their molecules may occur. It is that class of membrane forces that is most likely to play a role in the process(es) that leads to the formation of cholesterol crystals in bile, which eventually grow into gallstones. A specific, hypothetical mechanism for cholesterol crystal nucleation based on vesicle fusion is also given. This mechanism is consistent with the principles of surface chemistry outlined.

Interactions of liposomes with planar bilayer membranes

The adhesion to horizontal, planar lipid membranes of lipid vesicles containing calcein in the aqueous compartment or fluorescent phospholipids in the membranes has been examined by phase contrast, differential interference contrast and fluorescence microscopy. With water-immersion lenses, it was possible to study the interactions of vesicles with planar bilayers at magnifications up to the useful limit of light microscopy. In the presence of 15 mM calcium chloride, vesicles composed of phosphatidylserine and either phosphatidylethanolamine or soybean lipids adhere to the torus, bilayer and lenses of planar bilayers of the same composition. Lenses of solvent appear at the site where vesicles attach to decane-based bilayers and lipid fluorophores move from the vesicles to the lenses. Because the calcein contained in such vesicles is not released, we interpret this as indicating fusion of only the outer monolayer (hemifusion) of the vesicles with the decane lenses. In the case of squalene-based black lipid membranes (BLMs), in contrast, vesicles do not nucleate lenses but they apparently do fuse with the torus at the bilayer boundary. Interactions leading to hemifusions between vesicles and planar membranes thus occur predominantly in regions where hydrocarbon solvent is present. Osmotic water flow, induced by addition of urea to the compartment containing vesicles, causes coalescence of lenses in decane-based BLMs as well as coalescence of the aqueous spaces of the vesicles that have undergone hemifusion with the lenses. We did not observe transfer of the aqueous phase of vesicles to the trans side of either decane- or squalene-based planar membranes; however, we cannot rule out the possibility particularly in the latter case, that rupture of the planar membrane may have been an immediate result of vesicle fusion and thus precluded its detection.

A fluorometric assay for acetylcholine with picomole sensitivity

A sensitive fluorogenic assay for acetylcholine is described. The assay is based upon the reactions: (1) hydrolysis of acetylcholine to choline and acetate, catalyzed by acetylcholinesterase. (2) Oxidation of choline to betaine and hydrogen peroxide by choline oxidase. (3) Oxidation of p-hydroxyphenylacetic acid by hydrogen peroxide to a fluorescent product, catalyzed by peroxidase. With a sensitivity comparable to chemiluminescence procedures, the assay should find application to those situations requiring the persistence of a fluorescence signal or the necessity of assaying small volumes.

Fusion of synaptic vesicle membranes with planar bilayer membranes

The interaction of synaptic vesicles with horizontal bilayer lipid membranes (BLMs) was investigated as a model system for neurotransmitter release. High concentrations (200 mM) of the fluorescent dye, calcein, were trapped within synaptic vesicles by freezing and thawing. In the presence of divalent ions (usually 15 mM CaCl2), these frozen and thawed synaptic vesicles (FTSVs) adhere to squalene-based phosphatidylserine-phosphatidylethanolamine BLMs whereupon they spontaneously release their contents which is visible by fluorescence microscopy as bright flashes. The highest rate of release was obtained in KCl solutions. Release was virtually eliminated in isotonic glucose, but could be elicited by perfusion with KCl or by addition of urea. The fusion and lysis of adhering FTSVs appears to be the consequence of stress resulting from entry of permeable external solute (KCl, urea) and accompanying water. An analysis of flash diameters in experiments where Co+2, which quenches calcein fluorescence, was present on one or both sides of the BLM, indicates that more than half of the flashes represent fusion events, i.e., release of vesicle contents on the trans side of the BLM. A population of small, barely visible FTSVs bind to BLMs at calcium ion concentrations of 100 microM. Although fusion of these small FTSVs to BLMs could not be demonstrated, fusion with giant lipid vesicles was obvious and dramatic, albeit infrequent. Addition of FTSVs or synaptic vesicles to BLMs in the presence of 100 microM-15 mM Ca2+ produced large increases in BLM conductance. The results presented demonstrate that synaptic vesicles are capable of fusing with model lipid membranes in the presence of Ca+2 ion which, at the lower limit, may begin to approach physiological concentrations.

Aerobic ethanol production by leaves: Evidence for air pollution stress in trees of the Ohio River Valley, USA

We measured the frequency with which leaves of trees in the Ohio River Valley produced ethanol aerobically, to determine if aerobic ethanol production might provide a viable field assay for air pollution stress. Leaves were collected from trees during the summers of 1985 and 1986 and ethanol production was determined using headspace GC. Frequency of ethanol production was compared with environmental factors, including air pollution concentrations. We found frequent foliar ethanol production and elevated alcohol dehydrogenase activity in the leaves of several species of trees in the Ohio River Valley, USA. The ethanol concentrations measured were often equivalent to those produced by anaerobic leaves. Ethanol production was associated with hot, hazy weather and elevated NO(2) concentrations. Ethanol production was more frequent in urban and industrialized areas. Ethanol production was not associated with natural stresses such as flooding and herbivory. We propose that aerobic ethanol production is the result of cell acidification due to the accumulation of acidic gases in the cytoplasm. The use of ethanol production as a diagnostic tool for detecting stress imposed by acidic gases is discussed.

Membrane surface pressure can account for differential activities of membrane-penetrating molecules

It is a common observation that cis-unsaturated and branched chain fatty acids, which are usually liquid, affect membrane function differently from saturated and trans-unsaturated fatty acids, which are usually solid. We also found that the former are much more potent than the latter in inhibiting viral hemolytic activity. A search for the origin of this difference revealed a correlation between inhibition and equilibrium surface pressure (the surface pressure at the air/water interface of a solution of the substance in question). Using a simple but rigorous thermodynamic analysis, we show that penetration of a lipid bilayer is correlated with equilibrium surface pressure of the penetrating molecule. We therefore conclude that an important reason for the difference in effects of liquid and solid fatty acids on membranes is the greater penetrability of the former relative to the latter. We suggest that attributing such effects to fluidity changes in the membrane should await demonstration of actual intramonolayer residence of the fatty acid in the membrane. The thermodynamic analysis is readily generalized and, in the absence of specific interactions between penetration and bilayer molecules, provides a convenient method for predicting membrane penetration by virtually any type of exogenous molecule.

Determination of reflection coefficients of liposomes for some non-electrolytes by osmotic pressure measurement

The reflection coefficients of bilayer lipid vesicles (liposomes) of various compositions have been determined for a number of non-electrolytes. The solutes were the same and the method of measurement was essentially the same as those which have been used to estimate an equivalent pore radius for erythrocytes. The method involves matching the osmotic pressure of solutions of a permeant test solute with that of a known inpermeant solute. Reflection coefficients for cholesterol-containing liposomes and those of erythrocytes are, when account is taken of those solutes known to permeate the erythrocyte by specialized pathways, not greatly different. Lipid bilayers can thus account for most of the permeability characteristics of the cell originally interpreted as due to aqueous pores. Reflection coefficients are significantly higher for egg phosphatidylcholine membranes that contain cholesterol than those which do not. There is a strong correlation between relative permeabilities derived from reflection coefficients and oil-water partition coefficients. There is also good agreement between these permeabilities and permeabilities measured by others, which exhibit an inverse dependence on molecular size. It is suggested that this tendency of membranes to pass small molecules more readily than large molecules, other properties being equal, is a consequence of the surface pressure of the constituent monolayers of the membrane.

Lipid monolayer states and their relationships to bilayers

Uncommon methods of formation and analysis of lipid monolayers have enabled the recognition of several monolayer states and the identification of that in which molecular organization corresponds closely to that of the bilayer. Monolayers were formed by continuously adding a solution of phospholipid [dimyristoyl phosphatidylcholine in hexane/ethanol, 9:1 (vol/vol)] to the air/water interface of a constant-area trough. This procedure generates unconventional surface pressure (pi)-surface concentration (gamma) isotherms, which for liquid-crystalline monolayers consist of straight lines with three prominent intersections, two of which are not apparent in conventional pi-A isotherms. The regions of linear change of pi are explicable in terms of the area dependence of alkyl chain entropy. The two breaks at lower pi delimit states in which both chains lie parallel to the surface. The third occurs at collapse, which corresponds to a true equilibrium for unstressed liposomes. Mechanical and thermodynamic properties of bilayers, particularly phase-transition parameters, correspond closely to those of monolayers with which they are in equilibrium.

Inhibition of sendai virus-induced hemolysis by long chain fatty acids

A number of fatty acids were found to inhibit Sendai virus-induced hemolysis. cis-Unsaturated fatty acids such as oleate, as well as the methyl-branched isostearate, completely inhibited viral hemolysis at concentrations as low as 5-10 micrograms/ml, whereas the saturated, normal acids such as palmitate and stearate were comparably inhibitory only at 2-5 times those concentrations. trans-Unsaturated acids, as well as several other amphiphilic compounds, were either not or only weakly inhibitory. In contrast to their disparate effects on viral hemolysis, cis- and trans-unsaturated acids lysed erythrocytes in the same concentration range, which is several times higher than that at which the former compounds inhibited viral hemolysis. The mechanism of inhibition of viral hemolysis by isostearate involves the inactivation of viral hemolytic activity per se, since isostearate neither inhibited viral hemagglutination nor rendered erythrocytes significantly less susceptible to hemolysis. Furthermore, the concentration dependence of hemolysis inhibition by isostearate was biphasic, increasing sharply at the critical micelle concentration from a linear relationship below that concentration. Finally, an inhibitory concentration of isostearate was well below that at which amphiphiles dissolved membranes and did not dissolve Sendai virus, as shown by sucrose gradient centrifugation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was concluded that low concentrations of fatty acids--particularly cis-unsaturated or fluid-phase types--could block the fusion, as opposed to agglutination, step of viral hemolysis by perturbing hydrophobic regions of the Sendai virus membrane.

Lipid mixing during freeze-thawing of liposomal membranes as monitored by fluorescence energy transfer

A new pair of fluorescence-energy-transferring probes, dansylphosphatidylethanolamine and dioctadecylindocarbocyanine, were incorporated separately into phospholipid vesicles to monitor intervesicle lipid mixing under various conditions. The transfer efficiencies of mixtures of sonicated vesicles labeled with 2 wt% donor dansylphosphatidylethanolamine (DnsPE) or with 1 wt% acceptor dioctadecylindocarbocyanine (DiI-C18) were negligible, but increased to about 25% after the vesicles had been frozen in a solid CO2/ethanol bath, thawed and diluted. The freeze-thaw-induced mixing of lipids between vesicles, signified by energy transfer, was dependent on lipid concentration and was promoted by 0.5-1.5 M KCl, 0.5 M potassium trichloroacetate and 5 mM sodium acetate (pH 4) and inhibited by 0.5 M LiCl, 0.5 M glycerol, 0.5 M sucrose, 0.15 M KCl and 0.15-1.5 M NaCl. These results support and complement previously reported measurements of the trapped volumes, turbidities and population size distributions of similarly treated liposomes. Comparison of the responses of paucilamellar vesicles with those of multilamellar vesicles suggests that lipid mixing during freeze-thawing can occur either during interaction of the outermost bilayers of vesicles or during interaction of all bilayers, possibly as a result of breakdown and reformation of bilayer structure.

Characterization of a fluorescence assay to monitor changes in the aqueous volume of lipid vesicles

The fluorescent compound, 4',5'-bis[N,N-bis(carboxymethyl)aminomethyl] fluorescein (calcein) has been characterized for use in lipid vesicle studies. Particularly useful is its reaction with Co2+, which results in fluorescence quenching. This is accompanied by about a 10-nm blue shift in the uv absorbance bands and a small reduction in the visible absorbance band. For vesicle studies, Co2+ may be combined with citrate, which does not significantly hinder calcein quenching by Co2+. It does augment the absorbance of the metal ion. No significant interaction of citrate X Co2+ with phosphatidylserine vesicles was observed. Zn2+ is capable of displacing Co2+ and restoring calcein fluorescence. Fluorescence quenching due to formation of the calcein X Co2+ complex can also be reversed with EDTA. Thus, calcein is the basis of some simple reactions which can be used to assay changes in the aqueous volume of lipid vesicles.

Detergent effects on enzyme activity and solubilization of lipid bilayer membranes

Over 50 detergents were tested to establish which would be most effective in releasing proteins from membrane-bounded compartments without denaturing them. Various concentrations each of detergent were tested for two activities: (1) solubilization of egg phospholipid liposomes as measured by reduction of turbidity and (2) effect of detergent concentration on the activities of soluble, hydrolytic enzymes. Those detergents most effective in solubilizing 0.2% lipid and least detrimental to enzymes were five pure, synthetic compounds recently introduced: CHAPS, CHAPSO, Zwittergents 310 and 312, and octylglucoside. Industrial detergents were generally much inferior, insofar as they solubilized membranes inefficiently and/or inactivated certain hydrolytic enzymes readily. The five detergents were characterized by (a) an unusually high critical micelle concentration and (b) a preference for forming mixed micelles with lipids instead of forming pure micelles, as indicated by an ability to solubilize lipid at concentrations of detergent significantly below the critical micelle concentration. This characteristic permits solubilization of high concentrations of membrane below the critical micelle concentration of the detergent so that protein denaturation is minimized. A generally applicable guideline that emerged from this study is that detergents should be used at approximately their critical micelle concentration which should not be exceeded by the concentration of membrane. Similar considerations should apply to the use of detergents in purifying and reconstituting intrinsic membrane proteins.

Solubilization of phospholipids by chaotropic ion solutions

Phosphatidylcholine (PC) dissolves in solutions of concentrated neutral trichloroacetate and tribromoacetate, anions known to be extremely chaotropic. Solubilization of egg yolk PC occurs in Na-trichloroacetate between 2 and 3 M and in Na-tribromoacetate between 1.5 and 2 M. The resulting optically clear solutions were found by gel exclusion chromatography to consist of micelles containing of the order of 10(2) lipid molecules which, according to 31P NMR, are undergoing rapid, isotropic motion. Dipalmitoyl-PC is not solubilized below its normal phase transition temperature, although it remains in solution if first dissolved above that temperature; transition temperature and enthalpy change are both drastically depressed in 3 M Na-trichloroacetate. Na-trichloroacetate (3 M) also dissolves sphingomyelin but not phosphatidylethanolamine, acidic phospholipids, or egg PC-cholesterol mixtures of more than 33% cholesterol. Destabilization of the lamellar phase of egg PC relative to the micellar phase is promoted by trichloroacetate binding to and intercalating between the lipid molecules. Trichloroacetate was found to bind to egg PC liposomes with an affinity constant of about 1 M-1, so that nearly 1:1 association between egg PC and trichloroacetate is predicted at the concentration at which solubilization occurs. A surface area dilation by an amount (50+%) consistent with formation of micelles was confirmed by surface tension isotherms of egg PC on 3 M Na-trichloroacetate. The chaotropic effect may synergize binding by reducing the energy cost of the exposure of hydrophobic portions of lipid that is necessitated by the small radius of curvature of micelles. Several chaotropic agents less potent than trichloroacetate and tribromoacetate did not solubilize egg PC.