Lipid polymorphism and hydrocarbon order

The use of 2H nuclear magnetic resonance for the characterization of the polymorphic behavior of lipids is illustrated. Different lipid phase preferences may be expected to influence the orientational order and its variation along the acyl chains. Several results are presented to support that view. An increase of motional freedom and a redistribution of the order along the acyl chains are observed during the lamellar-to-hexagonal phase transition, showing that the order profile is sensitive to the lipid phase symmetry. In addition, if the preferences for nonlamellar phases are not expressed explicitly, the presence of "nonbilayer" lipids constrained in bilayer environment induces increased hydrocarbon order. This suggests that order parameters of the acyl chains and lipid polymorphic tendencies are intimately related.

Protection of liposomes during dehydration or freezing

When liposomes are subjected to dehydration or freeze-thawing, vesicle fusion and/or leakage of vesicle contents can occur. The disaccharide, trehalose and the cryoprotectant, glycerol, are known to protect vesicle integrity during dehydration and freezing respectively. Here we examine their protective abilities as a function of vesicle size and lipid composition. It is shown that fatty acyl composition, cholesterol content and, with the exception of phosphatidylglycerol, acidic lipid content do not significantly alter the retention of aqueous contents by vesicles dehydrated and rehydrated in the presence of trehalose. The susceptibility to leakage induced by both dehydration and freezing is, however, critically dependent upon vesicle size with the smallest systems (70-100 nm diameter) being most stable. The mechanism whereby trehalose protects against vesicle fusion and leakage is also discussed.

Studies on the myelosuppressive activity of doxorubicin entrapped in liposomes

The myelosuppressive activity of doxorubicin encapsulated in liposomes of differing lipid composition and size was quantified in mice by measurement of changes in spleen weight, peripheral white blood cells (WBC), and bone marrow nucleated cells. Following i.v. administration of free doxorubicin at a dose of 20 mg/kg, a 90% reduction in marrow cellularity was observed on day 3. The marrow nucleated cell count was similar to control values by day 7. Administration of an equivalent dose of doxorubicin that was encapsulated in large (diameter, approximately 1.0 microns) egg phosphatidylcholine/cholesterol (EPC/Chol)(molar ratio, 55:45) liposomes induced an 80% reduction in bone marrow cellularity that lasted for periods of greater than 7 days. Similar results were obtained following administration of large (1.0 microns) liposomal doxorubicin systems formulated with distearoylphosphatidylcholine/cholesterol (DSPC/Chol) (molar ratio 55:45). In contrast, liposomal doxorubicin prepared using small (diameter, approximately 0.1 micron) DSPC/Chol liposomes induced only a 40% reduction (day 3) in bone marrow cellularity, which returned to control values by day 7. Other indicators of doxorubicin-mediated myelosuppressive activity (spleen weight loss and peripheral leukopenia) correlated well with changes observed in marrow cellularity. An exception to this, however, was observed in animals treated with small (0.1 -micron) DSPC/Chol Liposomal doxorubicin, which displayed peripheral leukopenia for periods of greater than 14 days. This extended leukopenia was not observed following administration of small (0.1 -micron) EPC/Chol liposomal doxorubicin. Marrow-associated liposomal lipid and doxorubicin were quantified to determine if the extent of doxorubicin-mediated myeloid toxicity could be correlated to changes in biodistribution of the entrapped drug. It was demonstrated that 10-20 times more doxorubicin is delivered to the bone marrow when the drug is given encapsulated in large liposomes than when it is associated with small liposomes. These data are useful in defining characteristics of liposomal preparations that modulate the myelosuppressive behaviour of entrapped antineoplastic agents.

Freeze-fracture of lipids and model membrane systems

Model membrane systems are used extensively in all aspects of membrane research, and freeze-fracture is the preeminent procedure for directly visualizing local structure in these lipid dispersions. Here we describe in detail the formation of liposomes and how freeze-fracture is routinely employed as a complementary technique to biophysical and biochemical procedures in the characterization of multilamellar vesicles (most commonly known as liposomes) and unilamellar vesicles. Many preparative procedures exist for the formation of multi- and unilamellar vesicles. Examples of each system are given and their properties as well as freeze-fracture morphology are discussed. The detection of lipid-phase transitions is considered, in particular, with emphasis on the application of freeze-fracture to the study of lipid polymorphism. We briefly discuss the fracturing of apolar lipids which do not adopt bilayer structures but which can be stabilized into microemulsions by a phospholipid monolayer. Finally, a critical assessment is made of filipin as a morphological marker for cholesterol domains in the plane of the bilayer.

Influence of vesicle size, lipid composition, and drug-to-lipid ratio on the biological activity of liposomal doxorubicin in mice

The effects of vesicle size, lipid composition, and drug-to-lipid ratio on the biological activity of liposomal doxorubicin in mice have been investigated using a versatile procedure for encapsulating doxorubicin inside liposomes. In this procedure, vesicles exhibiting transmembrane pH gradients (acidic inside) were employed to achieve drug trapping efficiencies in excess of 98%. Drug-to-lipid ratios as high as 0.3:1 (wt:wt) could be obtained in a manner that is relatively independent of lipid composition and vesicle size. Egg phosphatidylcholine (EPC)/cholesterol (55:45; mol/mol) vesicles sized through filters with a 200-nm pore size and loaded employing transmembrane pH gradients to achieve a doxorubicin-to-lipid ratio of 0.3:1 (wt/wt) increased the LD50 of free drug by approximately twofold. Removing cholesterol or decreasing the drug-to-lipid ratio in EPC/cholesterol preparations led to significant decreases in the LD50 of liposomal doxorubicin whereas, the LD50 increased 4- to 6-fold when distearoylphosphatidylcholine was substituted for EPC. The results suggest that the stability of liposomally entrapped doxorubicin in the circulation is an important factor in the toxicity of this drug in liposomal form. In contrast, the antitumor activity of liposomal doxorubicin is not influenced dramatically by alterations in lipid composition. Liposomal doxorubicin preparations of EPC, EPC/cholesterol (55:45; mol:mol), EPC/egg phosphatidylglycerol (EPG)/cholesterol (27.5:27.5:45; mol:mol), and distearoylphosphatidylcholine/cholesterol (55:45; mol:mol) all demonstrated similar efficacy to that of free drug when given at doses of 20 mg/kg and below. Higher dose levels of the less toxic formulations could be administered, leading to enhanced increases in life span (ILS) values. Variations in vesicle size, however, strongly influenced the antitumor activity of liposomal doxorubicin. At a dose of 20 mg/kg, large EPC/cholesterol systems are significantly less effective than free drug (with ILS values of 65% and 145%, respectively). In contrast, small systems sized through filters with a 100-nm pore size are more effective than free drug, resulting in an ILS of 375% and a 30% long term (greater than 60 days) survival rate when administered at a dose of 20 mg/kg. Similar size-dependent effects are observed for distearoylphosphatidylcholine/cholesterol systems.

Smoothed orientational order profile of lipid bilayers by 2H-nuclear magnetic resonance

A new method has been developed to determine the complete orientational order profile of lipid bilayers using 2H-NMR. The profile is obtained from a single powder spectrum of a lipid which has a saturated chain fully deuteriated. The smoothed order profile is determined directly from the normalized dePaked spectrum assuming a monotonic decrease of the order along the acyl chain. The oscillatory variations of the order at the beginning of the chain are not described by this method. However the smoothed order profile reveals in a straightforward way the crucial features of the anisotropic order of the bilayer.

Use of liposomes as injectable-drug delivery systems

The formation of liposomes and their application as delivery systems for injectable drugs are described. Liposomes are microscopic vesicles composed of one or more lipid membranes surrounding discrete aqueous compartments. These vesicles can encapsulate water-soluble drugs in their aqueous spaces and lipid-soluble drugs within the membrane itself. Liposomes release their contents by interacting with cells in one of four ways: adsorption, endocytosis, lipid exchange, or fusion. Liposome-entrapped drugs are distributed within the body much differently than free drugs; when administered intravenously to healthy animals and humans, most of the injected vesicles accumulate in the liver, spleen, lungs, bone marrow, and lymph nodes. Liposomes also accumulate preferentially at the sites of inflammation and infection and in some solid tumors; however, the reason for this accumulation is not clear. Four major factors influence liposomes' in vivo behavior and biodistribution: (1) liposomes tend to leak if cholesterol is not included in the vesicle membrane, (2) small liposomes are cleared more slowly than large liposomes, (3) the half-life of a liposome increases as the lipid dose increases, and (4) charged liposomal systems are cleared more rapidly than uncharged systems. The most advanced application of liposome-based therapy is in the treatment of systemic fungal infections, especially with amphotericin B. Liposomes are also under investigation for treatment of neoplastic disorders. Liposomes' uses in cancer therapy include encapsulation of known antineoplastic agents such as doxorubicin and methotrexate, delivery of immune modulators such as N-acetylmuramyl-L-alanine-D-isoglutamine, and encapsulation of new chemical entities that are synthesized with lipophilic segments tailored for insertion into lipid bilayers. Liposomal formulations of injectable antimicrobial agents and antineoplastic agents already are undergoing clinical testing, and most probably will receive approval for marketing in the early 1990s. Liposomal encapsulation of drugs represents a new drug delivery system that appears to offer important therapeutic advantages over existing methods of drug delivery.

Proton flux in large unilamellar vesicles in response to membrane potentials and pH gradients

The transport of protons across liposomes composed of phosphatidylcholine in response to electrical potentials or pH gradients has been investigated. The results support three major conclusions. The first of these concerns the need for reliable measurements of electrical potentials and pH gradients. It is shown that the potential probe tetraphenylphosphonium and the pH probe methylamine provide accurate and self consistent measures of electrical potentials and pH gradients respectively in these systems. Second, it is shown by two independent techniques that the pH gradients induced in response to valinomycin and potassium dependent electrical potentials are significantly smaller than would be expected for electrochemical equilibrium. The pH gradients observed are stable over an 8 h time course and are sensitive to the ionic composition of the buffers employed, where the presence of external sodium results in the smallest induced pH gradients. These results are discussed in terms of current models of proton conductance across membranes. In a final area of investigation, it is shown that valinomycin and carbonyl cyanide m-chlorophenyl hydrazone (CCCP) can transport sodium ions in a synergistic manner.

Platelet distribution in rabbits following infusion of liposomes

This investigation determined the organ distribution of liposomes containing egg phosphatidylcholine and cholesterol with egg phosphatidylglycerol (PG liposomes) or without (PC liposomes) and the effect of each liposome on platelet distribution in rabbits. Eight minutes after 51chromium-labelled platelets were given intravenously, either saline (n = 7), iodinated PG liposomes (n = 5) or iodinated PC liposomes (n = 5) were infused. Two minutes later the organ distribution of 51Cr-platelets and 125I-liposomes were compared. The PG liposomes produced a 41 +/- 5% reduction in circulating platelet counts while PC liposomes did not. The PG liposomes decreased circulating 51Cr-platelets by a factor of 2 and increased platelet recoveries in the liver and lungs. The increased platelet recovery in the liver was associated with a greater PG liposome recovery. When animals receiving PG liposomes were studied over 60 minutes, both the labelled and unlabelled platelet counts returned to control values by 30 minutes and the 51Cr-platelet distribution between organs was similar to control values. These data indicate that platelets and PG liposomes initially sequester together and that this platelet-liposome interaction is specific for PG liposomes. However, the platelet sequestration is transient and by 60 minutes the platelets were released and circulating.

Intervesicular exchange of lipids with weak acid and weak base characteristics: influence of transmembrane pH gradients

Transmembrane pH gradients have previously been shown to induce an asymmetric transmembrane distribution of simple lipids that exhibit weak acid or basic characteristics (Hope, M.J. and Cullis, P.R. (1987) J. Biol. Chem. 262, 4360-4366). In the present study we have examined the influence of proton gradients on the inter-vesicular exchange of stearylamine and oleic acid. We show that vesicles containing stearylamine immediately aggregate with vesicles containing phosphatidylserine and that disaggregation occurs subsequently as stearylamine equilibrates between the two vesicle populations. Despite visible flocculation during the aggregation phase, vesicle integrity is maintained. Stearylamine is the only lipid to exchange, fusion does not occur and vesicles are able to maintain a proton gradient. When stearylamine is sequestered to the inner monolayer in response to a transmembrane pH gradient (inside acidic) aggregation is not observed and diffusion of stearylamine to acceptor vesicles is greatly reduced. The ability of delta pH-dependent lipid asymmetry to modulate lipid exchange is also demonstrated for fatty acids. Oleic acid can be induced to transfer from one population of vesicles to another by maintaining a basic interior pH in the acceptor vesicles. Moreover, it is shown that the same acceptor vesicles are capable of depleting serum albumin of bound fatty acid. These results are discussed with respect to the mechanism and modulation of lipid flow between membranes both in vitro and in vivo.

Phospholipid asymmetry in large unilamellar vesicles induced by transmembrane pH gradients

The influence of membrane pH gradients on the transbilayer distribution of some common phospholipids has been investigated. We demonstrate that the transbilayer equilibrium of the acidic phospholipids egg phosphatidylglycerol (EPG) and egg phosphatidic acid (EPA) can be manipulated by membrane proton gradients, whereas phosphatidylethanolamine, a zwitterionic phospholipid, remains equally distributed between the inner and outer monolayers of large unilamellar vesicles (LUVs). Asymmetry of EPG is examined in detail and demonstrated by employing three independent techniques: ion-exchange chromatography, 13C NMR, and periodic acid oxidation of the (exterior) EPG headgroup. In the absence of a transmembrane pH gradient (delta pH) EPG is equally distributed between the outer and inner monolayers of LUVs. When vesicles composed of either egg phosphatidylcholine (EPC) or DOPC together with 5 mol % EPG are prepared with a transmembrane delta pH (inside basic, outside acidic), EPG equilibrates across the bilayer until 80-90% of the EPG is located in the inner monolayer. Reversing the pH gradient (inside acidic, outside basic) results in the opposite asymmetry. The rate at which EPG equilibrates across the membrane is temperature dependent. These observations are consistent with a mechanism in which the protonated (neutral) species of EPG is able to traverse the bilayer. Under these circumstances EPG would be expected to equilibrate across the bilayer in a manner that reflects the transmembrane proton gradient. A similar mechanism has been demonstrated to apply to simple lipids that exhibit weak acid or base characteristics [Hope, M. J., & Cullis, P. R. (1987) J. Biol. Chem 262, 4360-4366]

Detection of vesicular lipoproteins in lecithin:cholesterol acyltransferase-deficient plasma by 1H-NMR spectroscopy

The proton NMR spectra of the N-methyl choline region of normal and lecithin:cholesterol acyltransferase (LCAT)-deficient lipoproteins and of egg yolk phosphatidylcholine-cholesterol 55:45 (mol %) vesicle mixtures have been examined in the presence and absence of manganous sulfate as a line-broadening reagent. Manganous ions quenched all of the signal arising from normal lipoproteins and only part of the vesicle signal corresponding to the outer monolayer. There was no net loss of vesicular phospholipid when vesicles were added to normal lipoproteins and as little as 5% (or 100 micrograms) of the vesicular phospholipid could be detected and quantitated in the mixture of lipoproteins. Similar experiments performed on plasma lipoproteins from an LCAT-deficient patient indicated that 42% of the phospholipid was associated with vesicular lipoproteins. These experiments demonstrate that this technique can be used to detect and quantify small amounts of vesicular structures directly in a mixture of micellar lipoproteins.

Analysis of the effect of liposome encapsulation on the vesicant properties, acute and cardiac toxicities, and antitumor efficacy of doxorubicin

Numerous studies have demonstrated that liposomal encapsulation decreases the life-threatening chronic and acute toxicities of doxorubicin in the face of unaltered or improved antitumor activity. Minimal attention has been paid to the encapsulation effect on the lesser toxicities of the drug, specifically the vesicant properties. In this report we assess the effect of the encapsulation of doxorubicin in an egg-yolk phosphatidylcholine (EPC) cholesterol liposome on the drug's topical toxicity. In addition, to ensure acceptable activity and reduction in toxicity comparable with those of previously assessed formulations, the cardiac and acute toxicities and antitumor activity of the liposomal doxorubicin complex were also investigated. Antitumor efficacy was assessed using the metastatic murine P815 mastocytoma model. Equivalent doses of free and encapsulated doxorubicin possessed the same antitumor activity in the prolongation of animal survival in 14-day survival studies conducted to assess the effect of liposomal encapsulation on the acute toxicity of this drug. The LD50 of liposomal doxorubicin was found to be 40 mg/kg, 53% higher than that of free doxorubicin (26 mg/kg). Histologic examination of cardiac sections taken from DBA/2J mice 7 days after a single i.v. injection of free or liposomal doxorubicin (25 mg/kg) revealed that the liposomal preparation was much less cardiotoxic. In animals receiving the free drug, edema, monocytic infiltration, and cell necrosis were evident. In contrast, those receiving the liposomal preparation demonstrated slight cellular edema but showed no evidence of cellular necrosis. To assess vesicant properties, DBA/2J mice were given a single s.c. injection (0.2 ml) of free or liposomal doxorubicin (2 mg/ml). Those receiving the free drug immediately developed erythema and edema at the injection site, which progressed to ulceration. Those receiving the liposomal complex developed slight erythema and edema but did not ulcerate at any time. All signs of irritation in this group had subsided 3 weeks postinjection. In summary, the liposomal complex used eliminated the vesicant properties of doxorubicin as well as significantly decreasing its cardiac and acute toxicities in the face of unaltered antitumor activity.

Interactions of liposomes and platelets

Rats were injected intravenously with liposomes of various compositions and sizes and blood platelet count measured. It was found that negatively-charged liposomal systems produced a transient reduction in platelet count in the first 5 minutes after injection which recovered by 60 minutes post-injection. This effect was most striking for multilamellar vesicles (MLV's) containing phosphatidylglycerol (PG). Dose levels of 25 mg/kg of MLV's containing 10 mole% PG caused the platelet count to drop from a control value of 1,086 +/- 21 X 10(9)/l to 193 +/- 14 X 10(9)/l by 2 minutes post-injection, an 82% decline. This thrombocytopenic effect was observed to diminish as vesicle size or vesicle dose was decreased. Positively-charged liposomes produced a less pronounced transient reduction in platelet count while neutral liposomes caused only a mild, transient platelet decline. This transient thrombocytopenic effect was not blocked by common anticoagulants and fibrinolytic agents but was prevented by liposomal pretreatment. Radiolabeled platelet studies revealed that transient sequestration of platelets occurs in the liver and spleen 2 minutes after PG:EPC:CHOL MLV injection with a normalization of platelet distribution by 60 minutes post-injection. In vitro studies, using an automated blood counter, suggest a transient association of liposomes and platelets occurring following injection. Liposomally-induced transient thrombocytopenia suggests a role for platelets in the biodistribution of liposomes.

Spontaneous vesiculation of large multilamellar vesicles composed of saturated phosphatidylcholine and phosphatidylglycerol mixtures

The influence of temperature and ionic strength on the vesiculation properties of large multilamellar vesicles containing various proportions of dimyristoylphosphatidylglycerol has been investigated. It is shown that at low ionic strengths preformed large multilamellar vesicles composed of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol (7:3) on incubation at the gel to liquid-crystalline transition temperature (Tc approximately 23 degrees C) spontaneously vesiculate to form predominantly unilamellar systems with a mean diameter of 120 nm. Such vesiculation is not observed for incubations at temperatures appreciably above or below Tc, and is also inhibited by higher ionic strengths. Stable large multilamellar vesicles are formed, however, in systems containing the dioleoyl species of phosphatidylcholine or phosphatidylglycerol and also for dimyristoylphosphatidylcholine/dimyristoylphosphatidylserine mixtures. The vesiculation properties of dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol mixtures, therefore, appear to reflect an instability in the region of the Tc driven by surface potential effects which are specific for the glycerol headgroup.

Acyl chain orientational order in the hexagonal HII phase of phospholipid-water dispersions

The deuterium nuclear magnetic resonance (2H NMR) spectrum of perdeuterated tetradecanol in a mixture of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE) and water was used to compare the variation of the acyl chain orientational order parameter, S(n), with carbon position, n, in the liquid crystalline lamellar (L alpha) and hexagonal (HII) phases. The characteristics independence of S(n) with n (plateau) normally observed in the L alpha phase is replaced by a more rapid decrease of S(n) with n in the HII phase. It is suggested that as a consequence of the geometrical characteristics of the HII phase, there is an increase in conformational freedom available to different parts of the acyl chain.

Unusual lipid structures selectively reduce the toxicity of amphotericin B

Ribbon-like structures result when amphotericin B interacts with lipid in an aqueous environment. At high ratios of amphotericin to lipid these structures, which are lipid-stabilized amphotericin aggregates, become prevalent resulting in a dramatic attenuation of amphotericin-mediated mammalian cell, but not fungal cell, toxicity. Studies utilizing freeze-etch electron microscopy, differential scanning calorimetry, 31P NMR, x-ray diffraction, and optical spectroscopy revealed that this toxicity attenuation is related to the macromolecular structure of the complexes in a definable fashion. It is likely that amphotericin in this specific form will have a much improved therapeutic utility.

Characterization of cholesterol hemisuccinate and alpha-tocopherol hemisuccinate vesicles

Cholesterol hemisuccinate (CHS) and alpha-tocopherol hemisuccinate (alpha-THS) were found to be capable of forming liposomes of multi- or single lamellar character. Such vesicles formed spontaneously, did not require the use of organic solvents and yielded high trapping efficiencies and captured volumes. Both CHS and alpha-THS systems greatly restricted the motion of intercalated spin labelled fatty acids, yet were more osmotically responsive than similar vesicle types comprised of phosphatidylcholine. Small angle X-ray diffraction measurements were consistent with vesicles possessing extremely weak interlamellar forces. CHS vesicles were found to remain intact in vivo, yet followed a pattern of distribution dissimilar to phosphatidylcholine vesicles.

Dopamine accumulation in large unilamellar vesicle systems induced by transmembrane ion gradients

Transmembrane movement of dopamine in response to K+ or H+ ion gradients has been investigated. It is shown that dopamine can accumulate rapidly into large unilamellar vesicles (LUVs) composed of egg phosphatidylcholine exhibiting either a K+ diffusion potential (delta psi; negative inside) or a pH gradient (inside acidic). This can result in entrapped dopamine concentrations of 30-40 mM and inside-outside concentration gradients of nearly 300-fold. The transmembrane dopamine gradients formed in LUV systems exhibiting delta pH (inside acidic) indicate that the transport process can be dictated by movement of the neutral form of dopamine which redistributes according to a simple Henderson-Hasselbach equilibrium. The mechanism of dopamine transport in response to a valinomycin-induced K+ potential is more complex. Although generation of a K+ diffusion potential results in acidification of the vesicle interior, the magnitude of the induced delta pH (approx. 1 pH unit) is insufficient to account for the dopamine concentration gradient achieved (greater than 200-fold). Further, data presented here suggest that higher uptake levels of dopamine can be achieved when certain anions (ATP and citrate) are entrapped within the LUV system. These anions may complex with the protonated form of dopamine creating a non-equilibrium trapping phenomena resulting in interior concentrations of dopamine in excess of that predicted by a simple Henderson-Hasselbach equilibrium.

X-ray diffraction study of the polymorphic behavior of N-methylated dioleoylphosphatidylethanolamine

The polymorphic phase behavior of aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and its N-methylated analogues, DOPE-Me, DOPE-Me2, and DOPC, has been investigated by X-ray diffraction. In the fully hydrated lamellar (L alpha) phase at 2 degrees C, the major structural difference is a large increase in the interlamellar water width from DOPE to DOPE-Me, with minor increases with successive methylation. Consistent with earlier reports, inverted hexagonal (HII) phases are observed upon heating at 5-10 degrees C in DOPE and at 65-75 degrees C in DOPE-Me and are not observed to at least 85 degrees C in DOPE-Me2 or DOPC. In DOPE, the L alpha-HII transition is facile and is characterized by a relatively narrow temperature range of coexistence of L alpha and HII domains, each with long-range order. DOPE-Me exhibits complex nonequilibrium behavior below the occurrence of the HII phase: Upon heating, the L alpha lattice spontaneously disorders on a time scale of days; on cooling from the HII phase, the disorder rises on a time scale of minutes. It is shown that, in copious water, the disordered state transforms very slowly into phases with cubic symmetry. This process is assisted by the generation of small amounts of lipid degradation products. The relative magnitudes of the monolayer spontaneous radius of curvature, R0 [Kirk, G. L., Gruner, S. M., & Stein, D. L. (1984) Biochemistry 23, 1093; Gruner, S. M. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 3665], are inferred from the HII lattice spacings vs temperature and are shown to increase with increasing methylation. The relative magnitudes of R0 are categorized as small for DOPE, intermediate for DOPE-Me, and large for DOPC. It is suggested, and examples are used to illustrate, that small R0 lipid systems exhibit facile, low-temperature L alpha-HII transitions, intermediate R0 systems exhibit complex nonequilibrium transition behavior and are likely to form cubic phases, and large R0 systems are stable as L alpha phases. The relationship between the cubic phases and minimal periodic surfaces is discussed. It is suggested that minimal periodic surfaces represent geometries in which near constant, intermediate R0 values can be obtained concomitantly with monolayers of near constant thickness, thereby leading to equilibrium cubic phases. Thus, the relative magnitude of the spontaneous radius of curvature may be used to predict mesomorphic behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of ion gradients on the transbilayer distribution of dibucaine in large unilamellar vesicles

The uptake of dibucaine into large unilamellar vesicles in response to proton gradients (delta pH; inside acidic) or membrane potentials (delta psi; inside negative) has been investigated. Dibucaine uptake in response to delta pH proceeds rapidly in a manner consistent with permeation of the neutral (deprotonated) form of the drug, reaching a Henderson-Hasselbach equilibrium where [dibucaine]in/[dibucaine]out = [H+]in/[H+]out and where the absolute amount of drug accumulated is sensitive to the buffering capacity of the interior environment. Under appropriate conditions, high absolute interior concentrations of the drug can be achieved (approximately 120 mM) in combination with high trapping efficiencies (in excess of 90%). Dibucaine uptake in response to delta psi proceeds more than an order of magnitude more slowly and cannot be directly attributed to uptake in response to the delta pH induced by delta psi. This induced delta pH is too small (less than or equal to 1.5 pH units) to account for the transmembrane dibucaine concentration gradients achieved and does not come to electrochemical equilibrium with delta psi. Results supporting the possibility that the charged (protonated) form of dibucaine can be accumulated in response to delta psi were obtained by employing a permanently positively charged dibucaine analogue (N-methyldibucaine). Further, the results suggest that delta psi-dependent uptake may depend on formation of a precipitate of the drug in the vesicle interior. The uptake of dibucaine into vesicles in response to ion gradients is of direct utility in drug delivery and controlled release applications and is related to processes of drug sequestration by cells and organelles in vivo.

Calcium-induced phase separation phenomena in multicomponent unsaturated lipid mixtures

The ability of calcium to induce phase separation in multicomponent lipid mixtures containing various unsaturated species of acidic and neutral phospholipids has been investigated by 31P NMR, 3H NMR, and small-angle X-ray diffraction techniques. It is shown that, in unsaturated (dioleoyl-) phosphatidylglycerol (PG)/phosphatidylethanolamine (PE) (1:1) and phosphatidic acid (PA)/phosphatidylcholine (PC) (1:1) mixtures, calcium is unable to induce lateral phase separation of the acidic and neutral lipids and that all the lipids adopt a hexagonal (HII) phase in the presence of calcium. In multicomponent mixtures containing one or more acidic species the presence of cholesterol either facilitates calcium-induced lamellar to hexagonal (HII) transitions for all the lipid components or, in systems already in a hexagonal (HII) phase, mitigates against calcium-induced lateral phase separations. Further, cholesterol is shown to exhibit no preferential interaction on the NMR time scale with either PC, PE, or phosphatidylserine (PS) when the lipids are in the liquid-crystal state. The ability of cholesterol to directly induce HII phase formation in PC/PE mixtures is also shown to be common to various other sterols including ergosterol, stigmasterol, coprostanol, epicoprostanol, and androstanol.

N-succinyldioleoylphosphatidylethanolamine: structural preferences in pure and mixed model membranes

The structural preferences of the pH-sensitive phospholipid, N-succinyldioleoylphosphatidylethanolamine (N-succinyl-DOPE), have been examined alone and in mixtures with DOPE by 31P-NMR, fluorescence energy transfer, and freeze-fracture techniques. The basic polymorphic behavior of pure N-succinyl-DOPE and DOPE/N-succinyl-DOPE lipid systems and the influence of calcium and pH were investigated. It is shown that, similar to other negatively charged acidic phospholipids, N-succinyl-DOPE adopts the bilayer organization upon hydration. This structure is maintained at both pH 7.4 and 4.0 in the presence or absence of calcium. In the mixed lipid system, N-succinyl-DOPE can stabilize the non-bilayer lipid, DOPE, into a bilayer structure at both pH 7.4 and 4.0 at more than 10 mol% N-succinyl-DOPE, although a narrow 31P-NMR lineshape is observed at acidic pH values. This corresponds to the presence of smaller vesicles as shown by quasi-elastic light scattering measurements. Addition of equimolar calcium (with respect to N-succinyl-DOPE) to the DOPE/N-succinyl-DOPE systems induces the hexagonal HII phase at both pH values. In unilamellar systems with similar lipid composition the addition of Ca2+ results in membrane fusion as indicated by fluorescence energy-transfer experiments. These findings are discussed with regard to the molecular mechanism of the bilayer to hexagonal HII phase transition and membrane fusion and the utility of N-succinyl-DOPE containing pH-sensitive vesicles as drug-delivery vehicles.