Influence of Ca2+ and Mg2+ on the thermotropic behaviour and permeability properties of liposomes prepared from dimyristoyl phosphatidylglycerol and mixtures of dimyristoyl phosphatidylglycerol and dimyristoyl phosphatidylcholine

Study on the in situ aggregation of liposomes with negatively charged phospholipids for use as injectable depot formulation

Compared to conventional parenteral formulations injectable depot formulations, owing to a sustained drug release, offer several advantages, such as a reduced dosing frequency - and consequent improved compliance - or a predictable release profile. Additionally, fluctuations in the drug blood level may be smoothened and consequently side effects reduced. Because of their capability to encapsulate water soluble drugs and their very low toxicity profile liposomes comprising phospholipids, most certainly represent a vehicle of choice for subcutaneous (s.c.) or intramuscular (i.m.) administration typical for depot injections too. In the past, especially liposomes containing negatively charged phosphatidylserines were investigated regarding their aggregation and fusion behavior upon addition of calcium ions. Liposomes need to have a large size to prevent fast removal from the s.c. or i.m. injection site to make them useful as depot vehicle. In order to obtain such large liposomes, aggregation of smaller liposomes may be considered. Aim of the present study was to induce and investigate controlled aggregation of vesicles containing other negatively charged phospholipids besides phosphatidylserines upon mixing with a solution of divalent cations. L-α-phosphatidylcholine from egg (EPC) liposomes formulated with 25 mol% of 1,2-dipalmitoyl-sn-glycero-3-phosphate (DPPA) or 1,2-distearoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DSPG) proved to be possible lipid-based depot candidates due to their strong aggregation induced by calcium and magnesium cations. Different aggregation profiles with both cations could be observed. Morphological investigations of the aggregates showed that individual liposomes remain stable in the aggregates and no fusion occurred. A fluorescence-based fusion assay confirmed these results. Differential scanning calorimetry measurements supported the findings of the diverse aggregation profiles with calcium or magnesium owing to different binding sites of the cations to the lipid molecules.

Is phosphatidylglycerol essential for terrestrial life?

Lipids are of increasing importance in understanding biological systems. Lipids carrying an anionic charge are noted in particular for their electrostatic interactions with both proteins and divalent cations. However, the biological, analytical, chemical and biophysical data of such species are rarely considered together, limiting our ability to assess the true role of such lipids in vivo. In this review, evidence from a range of studies about the lipid phosphatidylglycerol is considered. This evidence supports the conclusions that this lipid is ubiquitous in living systems and generally of low abundance but probably fundamental for terrestrial life. Possible reasons for this are discussed and further questions posed.

Thermal phase behavior of DMPG bilayers in aqueous dispersions as revealed by 2H- and 31P-NMR

The synthetic lipid 1,2-dimyristoyl-sn-3-phosphoglycerol (DMPG), when dispersed in water/NaCl exhibits a complex phase behavior caused by its almost unlimited swelling in excess water. Using deuterium ((2)H)- and phosphorus ((31)P)-NMR we have studied the molecular properties of DMPG/water/NaCl dispersions as a function of lipid and NaCl concentration. We have measured the order profile of the hydrophobic part of the lipid bilayer with deuterated DMPG while the orientation of the phosphoglycerol headgroup was deduced from the (31)P NMR chemical shielding anisotropy. At temperatures >30 °C we observe well-resolved (2)H- and (31)P NMR spectra not much different from other liquid crystalline bilayers. From the order profiles it is possible to deduce the average length of the flexible fatty acyl chain. Unusual spectra are obtained in the temperature interval of 20-25 °C, indicating one or several phase transitions. The most dramatic changes are seen at low lipid concentration and low ionic strength. Under these conditions and at 25 °C, the phosphoglycerol headgroup rotates into the hydrocarbon layer and the hydrocarbon chains show larger flexing motions than at higher temperatures. The orientation of the phosphoglycerol headgroup depends on the bilayer surface charge and correlates with the degree of dissociation of DMPG-Na(+). The larger the negative surface charge, the more the headgroup rotates toward the nonpolar region.

Effect of Na(+) concentration on the subgel phases of negatively charged phosphatidylglycerol

The effect of Na(+) concentration on the subgel phase of dimyristoylphosphatidylglycerol (DMPG) was investigated by differential scanning calorimetry (DSC) and negative stain electron microscopy, and the results were compared with dipalmitoylphosphatidylglycerol (DPPG). The conversion mode of DMPG vesicle to the subgel phase by annealing at 5 degrees C was grouped into two types depending on whether Na(+) concentration is above or below 200-250 mM. For [Na(+)]>200-250 mM, the subgel phase of a crystalline superstructure of bilayers wrapped in a cylinder was attained during a 24-h period of annealing and transformed directly to the liquid crystal phase on heating. For [Na(+)]<200-250 mM, two subgel phases which transform to the gel phase on heating were observed after annealing up to 24 h. Both subgel phases showed belt-like structures composed of loosely and closely stacked lamellae, respectively, and their fractions were found to depend on Na(+) concentration. With a further annealing up to 30 days, only the closely stacked subgel phase converted subsequently into the cylindrical superstructure of a more ordered phase. Similar two subgel phases were detected for DPPG at [Na(+)]< or =100 mM. The difference in the relative enthalpy between the gel and subgel phases was investigated from the van der Waals interaction energy between the hydrocarbon chains.

Cholesterol sulfate and Ca(2+) modulate the mixing properties of lipids in stratum corneum model mixtures

The influence of cholesterol sulfate (CS) and calcium on the phase behavior of lipid mixtures mimicking the stratum corneum (SC) lipids was examined using vibrational spectroscopy. Raman microspectrocopy showed that equimolar mixtures of ceramide, palmitic acid, and cholesterol underwent a phase transition in which, at low temperatures, lipids formed mainly a mosaic of microcrystalline phase-separated domains, and above 45 degrees C, a more fluid and disordered phase in which the three lipid species were more miscible. In the presence of Ca(2+), there was the formation of fatty acid-Ca(2+) complexes that led to domains stable on heating. Consequently, these lipid mixtures remained heterogeneous, and the fatty acid molecules were not extensively involved in the formation of the fluid lipid phase, which included mainly ceramide and cholesterol. However, the presence of CS displaced the association site of Ca(2+) ions and inhibited the formation of domains formed by the fatty acid molecules complexed with Ca(2+) ions. This work reveals that CS and Ca(2+) modulate the lipid mixing properties and the lipid order in SC lipid models. The balance in the equilibria involving Ca(2+), CS, and fatty acids is proposed to have an impact on the organization and the function of the epidermis.

Ca2+-reversible inhibition of the mitochondrial megachannel by ubiquinone analogues

Ubiquinone 0 and decylubiquinone have been reported to inhibit the mitochondrial permeability transition pore (PTP) [Fontaine, E., Ichas, F. and Bernardi, P. (1998) J. Biol. Chem. 273, 25734-257401, offering a new clue to its molecular composition. In patch-clamp experiments on rat liver mitochondria we have observed that these compounds also inhibit the previously described mitochondrial megachannel (MMC), confirming its identification as the PTP. Inhibition can be reversed by increasing [Ca2+], in analogy to the behavior observed with several other disparate PTP/MMC inhibitors. To rationalize the ability of Ca2+ to overcome inhibition by various quite different compounds we propose that it acts via the phospholipid bilayer.

A Fourier transform infrared spectroscopic study of the interaction of alkaline earth cations with the negatively charged phospholipid 1, 2-dimyristoyl-sn-glycero-3-phosphoglycerol

The interaction of aqueous phospholipid dispersions of negatively charged 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol, sodium salt (DMPG) with the divalent cations Mg(2+), Ca(2+) and Sr(2+) at equimolar ratios in 100 mM NaCl at pH 7 was investigated by Fourier transform infrared spectroscopy. The binding of the three cations induces a crystalline-like gel phase with highly ordered and rigid all-trans acyl chains. These features are observed after storage below room temperature for 24 h. When the gel phase is heated after prolonged incubation at low temperature phase transitions into the liquid crystalline phase are observed at 58 degrees C for the DMPG:Sr(2+), 65 degrees C for the DMPG:Mg(2+), and 80 degrees C for the DMPG:Ca(2+) complex. By subsequent cooling from temperatures above T(m) these complexes retain the features of a liquid crystalline phase with disordered acyl chains until a metastable gel phase is formed at temperatures between 38 and 32 degrees C. This phase is characterized by predominantly all-trans acyl chains, arranged in a loosely packed hexagonal or distorted hexagonal subcell lattice. Reheating the DMPG:Sr(2+) samples after a storage time of 2 h at 4 degrees C results in the transition of the metastable gel to the liquid crystalline phase at 35 degrees C. This phase transition into the liquid crystalline state at 35 degrees C is also observed for the Mg(2+) complex. However, for DMPG:Mg(2+) at higher temperatures, a partial recrystallization of the acyl chains occurs and the high temperature phase transition at 65 degrees C is also detected. In contrast, DMPG:Ca(2+) exhibits only the phase transition at 80 degrees C from the crystalline gel into the fluid state upon reheating. Below 20 degrees C, the rate of conversion from the metastable gel to a thermodynamically stable, crystalline-like gel phase decreases in the order Ca(2+)&z. Gt;Mg(2+)>Sr(2+). This conversion into the crystalline gel phase is accompanied by a complete dehydration of the phosphate groups in DMPG:Mg(2+) and by a reorientation of the polar lipid head groups in DMPG:Ca(2+) and in DMPG:Sr(2+). The primary binding sites of the cations are the PO(2)(-) groups of the phosphodiester moiety. Our infrared spectroscopic results suggest a deep penetration of the divalent cations into the polar head group region of DMPG bilayers, whereby the ester carbonyl groups, located in the interfacial region of the bilayers, are indirectly affected by strong hydrogen bonding of immobilized water molecules. In the liquid crystalline phase, the interaction of all three cations with DMPG is weak, but still observable in the infrared spectra of the DMPG:Ca(2+) complex by a slight ordering effect induced in the acyl chains, when compared to pure DMPG liposomes.

Transport of the anti-cancer drug doxorubicin across cytoplasmic membranes and membranes composed of phospholipids derived from Escherichia coli occurs via a similar mechanism

An assay was developed to measure and directly compare transport of doxorubicin across right-side-out cytoplasmic membrane vesicles (ROV) and across model membranes (LUVET) composed of pure phospholipids, isolated from the corresponding cells. Escherichia coli was used as a model organism, since mutants are available which differ in phospholipid composition. Both in LUVET and ROV only passive diffusion across the bilayer is involved, because effects of drug concentration, pH, divalent cations, the phospholipid composition, and the active transport inhibitor verapamil were comparable. Permeability coefficients were about 2-3-times higher in ROV compared to LUVET. Furthermore, in LUVET an average activation energy of 87 kJ/mol and in ROV of 50 kJ/mol was observed. These differences are suggested to result from differences in membrane order between LUVET and ROV and differences in the temperature dependence of membrane order in LUVET and ROV, respectively. Because no background carrier-facilitated doxorubicin transport seems to be present, ROV are an excellent model system to study the effect of phospholipid composition on drug transport after expression of a multidrug resistance-conferring protein. Furthermore, data of passive diffusion of doxorubicin obtained with LUVET are representative for more complex, biologically relevant membrane systems.

The interaction of the anti-cancer drug cisplatin with phospholipids is specific for negatively charged phospholipids and takes place at low chloride ion concentration

The interaction of the anti-cancer drug cis-diamminedichloroplatinum(II) (cisPt) with model membranes was studied, with emphasis on the cisPt and phospholipid species involved. Binding studies using large unilamellar vesicles have revealed that: (i) Interaction involved negatively charged phospholipids only, and (ii) Interaction with negatively charged phospholipids was observed only in buffers with low Cl- concentration, indicating that aquated, positively charged cisPt is involved. Binding to all negatively charged phospholipids tested was highest at pH 6.0. At pH 7.4 a high and specific binding was observed with phosphatidic acid and phosphatidylserine. The consequences of cisPt binding on the organization of lipids was investigated with differential scanning calorimetry studies. These studies have indicated a higher ordering of dispersions of negatively charged phospholipids in the presence of divalent cationic cisPt. Summarizing, the interaction of positively charged cisPt species with negatively charged phospholipids is significant and should be considered in in vivo experiments.

Molecular parameters involved in aminoglycoside nephrotoxicity

Aminoglycoside antibiotics are hydrophilic molecules consisting of an animated cyclitol associated with amino sugar. They bind in vivo as well as in vitro to negatively charged membranes. Their use as chemotherapeutic agents is unfortunately accompanied by oto- and nephrotoxic reactions, and the purpose of this review is to examine the role of the molecular interactions between aminoglycosides and membranes in the development of nephrotoxicity. 31P Nuclear magnetic resonance (NMR) and fluorescence depolarization have been used to characterize the effect of aminoglycosides on phosphate heads and fatty acyl chains of phospholipids. 15N NMR has been used to obtain interesting information on regioselective interactions of amino groups of antibiotics with phospholipids. The binding of aminoglycosides with negatively charged membranes is associated with impairment of phospholipid catabolism, change in membrane permeability, and membrane aggregation. Biochemical analysis and 1H NMR spectroscopy have brought information on the molecular mechanism involved in the impairment of phospholipid catabolism. Nephrotoxic aminoglycosides could induce sequestration of phosphatidylinositol and therefore reduce the amount of negative charge available for optimal lysosomal phospholipase activity toward phosphatidylcholine included in liposomes that also contain cholesterol and sphingomyelin. Conformational analysis shows that aminoglycosides, which have a high potency to inhibit lysosomal phospholipase activity, adopt an orientation parallel to the lipid/water interface. This orientation of the aminoglycoside molecule at the interface is also critical to explain the marked increase of membrane permeability induced by less nephrotoxic aminoglycosides such as isepamicin and amikacin. This effect is indeed only observed with aminoglycosides oriented perpendicular to this interface, probably related to the creation of a local condition of disorder. The impairment of phospholipid catabolism, which is considered to be an early and significant step in the development of aminoglycoside toxicity, is therefore not related to the change in membrane permeability. However, the role of this latter phenomenon and of membrane aggregation for aminoglycoside nephrotoxicity could be further investigated.

Domain formation induced by lipid-ion and lipid-peptide interactions

High sensitivity titration calorimetry was performed for metal ions such as calcium and lanthanum and for different types of Alzheimer peptides. Ca2+ adsorbs to mixed phosphatidylcholine (PC)/phosphatidylglycerol (PG) membranes with an endothermic reaction enthalpy of delta H approximately +0.1 kcal/mol. La3+ binds to sonified PC vesicles with a reaction enthalpy of delta H approximately + 1.8 kcal/mol. The binding constants are of the order of 10 M-1 for Ca2+ and 4 x 10(3) M-1 for La3+. The role of lipids in the random coil<-->beta-sheet equilibrium of different types of Alzheimer model peptides was investigated with circular dichroism (CD) and high sensitivity titration calorimetry. Alzheimer peptide beta AP(1-40)OH and several fragments of this peptide undergo a concentration-dependent, co-operative random coil<-->beta-sheet transition in solution which can be described by a linear association model with a nucleation parameter sigma approximately 0.2-0.01 and a growth parameter s approximately 10(4) M-1. Addition of sonified lipid vesicles containing negatively charged lipids shifts the equilibrium towards the beta-sheet conformation. This can be explained by an aggregation phenomenon at the lipid/water interphase. The cationic peptides are attracted to the negatively charged membrane surface causing a local increase in peptide concentration. The high peptide concentration, together with the ordering of the peptide molecules on the membrane surface, facilitates beta-sheet formation, constituting the first experimental evidence for the induction of beta-sheet formation via the membrane surface. The binding of Alzheimer peptide fragments to the lipid membrane is accompanied by an exothermic heat of reaction with delta H in the range -2 - -8 kcal/mol.

Interactions of liposome bilayers composed of 1,2-diacyl-3-succinylglycerol with protons and divalent cations

Bilayer liposomes were prepared by using pure DOSG (1,2-dioleoyl-3-succinylglycerol) or DPSG (1,2-dipalmitoyl-3-succinylglycerol) at pH 7.4 or above. These liposomes undergo destabilization upon incubation with acid. When calcein was used as an entrapped aqueous marker, half maximal content leakage was observed between pH 5.8-6.3. Differential scanning calorimetry showed that at pH 7.4, the chain-melting temperature (Tm) of DPSG was 60.4 degrees C, and increased with decreasing pH (Tm = 57.0 degrees C and 62.7 degrees C at pH 8.9 and 6.7, respectively). Below pH 6.7, extensive phase separation occurred as the major chain melting peak split into three peaks. These three peaks coalesced into one peak below pH 5. Freeze fracture electron micrographs of DOSG liposomes at pH 4 showed the formation of non-bilayer as well as hexagonal phase structures. The effects of divalent cations, such as Ca2+ and Mg2+, on the destabilization of DASG bilayers have also been studied. Differential scanning calorimetry studies of bilayers composed of DPSG showed that both Ca2+ and Mg2+ could increase the Tm of DPSG with increasing concentrations. However, under identical conditions Mg2+ was more effective than Ca2+ in increasing the Tm of DPSG. X-ray diffraction indicated that both Ca2+ and Mg2+ could induce DPSG bilayers to undergo a complete lamellar to hexagonal phase transition. There was a size-dependency on the plasma stability of DOSG liposomes. DOSG liposomes that were smaller in size were more stable in plasma than the larger ones. After incubation with plasma, DOSG liposomes became less acid-sensitive. DOSG immunoliposomes entrapping diphtheria toxin A chain were used as a model for cytoplasmic delivery of the novel pH-sensitive liposomes. The delivery activity was comparable to that of the conventional pH-sensitive liposomes containing unsaturated phosphatidylethanolamine. Our data indicate that the mechanism of liposome destabilization involves extensive bilayer phase separation as well as the formation of non-bilayer structures.

Adsorption of Ca2+ and La3+ to bilayer membranes: measurement of the adsorption enthalpy and binding constant with titration calorimetry

The adsorption of Ca2+ and La3+ ions to the surface of lipid bilayer membranes was studied with high sensitivity titration calorimetry. Ca2+ adsorbs to mixed phosphatidylcholine/phosphatidylglycerol membranes with a reaction enthalpy of delta H approximately 0.1-0.2 kcal/mol. La3+ binds to sonified phosphatidylcholine vesicles with a reaction enthalpy of delta H approximately +1.8 kcal/mol. Adsorption of La3+ to phosphatidylcholine bilayers imparts a net positive charge to the membrane surface which makes the binding of further La3+ increasingly more difficult. From the decreasing amplitudes in the calorimetric titration experiment a La3+ adsorption constant of K approximately (4.1 +/- 1.1) x 10(3) M-1 was evaluated. Electrostatic effects were corrected for by means of the Gouy-Chapman theory. The adsorption constant of Ca2+ was determined previously as K approximately 10-20 M-1 using the same binding model. Since the reaction enthalpies of Ca2+ and La3+ adsorption are endothermic, the adsorption of both metal ions to the membrane surface is driven by a distinct change in entropy.

Phase transition of dimyristoylphosphatidylglycerol bilayers induced by electric field pulses

The phase transition of dimyristoylphosphatidylglycerol (DMPG) bilayers has been studied by measurements of light scattering under high electric field pulses. Midpoints of phase transitions have been identified by a clear discontinuity of field induced relaxation amplitudes. We show that the phase transition of DMPG suspensions in monovalent salt is virtually independent of the electric field strength up to approx. 35 kV/cm. A shift of the lipid phase by electric field pulses has been observed, however, for DMPG suspensions in the presence of Ca2+ ions. DMPG suspensions exhibit a jump of the phase transition temperature from 17 degrees C at Ca/DMPG molar ratios r less than 1/7 to 32 degrees C at r greater than 1/7. Field pulses of 60 to 100 microseconds applied to DMPG suspensions with Ca2+ at r greater than 1/7 induce discontinuities of relaxation amplitudes in the temperature range 15 to 22 degrees C in addition to the 'standard' one at 32 degrees C, when the electric field strength is above 15 kV/cm. These results indicate that electric field pulses induce a transition from the phase formed at 'high' Ca(2+)- to the one formed at 'low' Ca(2+)-ion concentrations. Our results are consistent with a dissociation field effect on Ca(2+)-lipid complexes which drives the phase transition.

Physical studies on membrane lipids of Bacillus stearothermophilus temperature and calcium effects

Bacillus stearothermophilus was grown at the optimal temperature range (center, 65 degrees C), below it (48 and 55 degrees C), and above it (68 degrees C), in a complex medium with or without 2.5 mM Ca2+. The Ca(2+)-supplement improves growth at sub- and supraoptimal temperatures and extends it to higher temperatures (Jurado et al. (1987) J. Gen. Microbiol. 133, 507-513). The phospholipid composition of cultures obtained in the different growth conditions was studied. Phosphatidylethanolamine was always the major phospholipid (40 to 50% of the total phospholipid). Diphosphatidylglycerol, phosphatidylglycerol, a phosphoglycolipid (pgl) and two minor phospholipids (not identified) were also found in the polar lipid extract. The pgl shows a threefold concentration increase as the growth temperature raises from 48 to 68 degrees C. The thermotropic behavior of membrane lipids was studied by differential scanning calorimetry (DSC) and by means of two fluorescent probes of fluidity, 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1,3-di(2-pyrenyl)propane (2Py(3)2Py). The results reveal similar features and clearly show a shift of the temperature range of the phase transition to higher values and an increased structural order of the bilayer, as the growth temperature rises from 55 to 68 degrees C, but an opposite effect was observed from 48 to 55 degrees C. Although the Ca(2+)-supplement to the growth medium has no detectable effect, the addition of Ca2+ to the buffer of liposomes (Ca(2+)-liposomes) has a significant ordering effect at all growth temperatures. These liposomes show a shift of the transition range to higher temperatures and the fluorescent parameters (DPH polarization and intramolecular excimerization of the 2Py(3)2Py) detected an order increase of the probes environment, along and above the main phase transition. Spectra of 31P-NMR and polarized light microscopy clearly show that the lipid extracts exhibit, in all the conditions, typical lamellar phase geometry. We concluded that B. stearothermophilus controls the membrane lipid composition to compensate for the destabilizing effect of high temperatures on the membrane organization or to provide an appropriate packing of phospholipid molecules in a stable bilayer. At high temperatures, Ca(2+)-stimulatory effect on growth is presumably due to a direct Ca2+ interaction with the membrane phospholipids, inducing an increased structural order on the bilayer. The increase of the phase transition temperature in the total lipid extracts as compared with the respective polar lipid fractions probably indicates a stabilizing effect of neutral lipids on membrane bilayers.

Composition of polar lipid acyl chains of Bacillus stearothermophilus as affected by temperature and calcium

Bacillus stearothermophilus was grown within the temperature range of 48 to 68 degrees C in a complex medium and in the range of 45 to 72 degrees C in the presence of 2.5 mM Ca2+. The main fatty acids of lipid extracts contain 15 to 17 carbon atoms, mostly branched-chain species. The most prevalent saturated straight-chain fatty acid is n-C16. The total amount of branched-chain species decreases with increasing temperature of growth from 48 to 68 degrees C, whereas the straight-chain species increase. Thus, n-C16 almost doubles while i-C16, i-C17 and a-C17 decrease by 41.2, 28.9 and 41.9%, respectively. In the presence of Ca2+, the lipid metabolism favours the biosynthesis of straight-chain fatty acids with depression of branched-chain species, especially at lower temperatures. At high temperatures, Ca2+ has a less pronounced effect in the lipid biosynthesis. However, above 68 degrees C, a significant decrease is observed among the branched-chain fatty acids i-C15, i-C17 and a-C17 with a consequent increase in n-C16. Furthermore, a remarkable increase is observed in oleic acid (from 2.7% at 68 degrees C to 11.5% at the extreme 72 degrees C.

Effect of propylgallate on a dipalmitoylphosphatidylglycerol and calcium mixture

Effect of propylgallate (PrG) on the thermotropic behavior of mixtures of dipalmitoylphosphatidylglycerol (DPPG) and Ca2+ was studied by means of differential scanning calorimetry (DSC). In the case of DPPG or DPPG/Ca (molar ratio, 15 : 1), the transition temperature (Tm) of the main transition and the subtransition decreased from 40 degrees C to 29 degrees C and from 29 degrees C to 20 degrees C, respectively, with an increase in the concentration of PrG. The addition of PrG to the DPPG/Ca mixture induced a shoulder on the high temperature side in the reheating scan. Neither PrG nor low concentrations of Ca2+ bind to the Lc phase of DPPG. When the molar ratio of DPPG to Ca was 1 : 1, the subtransition did not occur, that is, only the main transition (Tm = 90 degrees C) appeared. The Tm of the main transition was slightly affected by PrG. On the addition of PrG, another metastable endothermic transition peak (Tm = 78 degrees C) appeared. It is concluded that Ca2+ and PrG inhibit each other's binding.

Thermotropic properties of mixtures of negatively charged phospholipids with cholesterol in the presence and absence of Li+ or Ca2+ ions

Mixtures of cholesterol with dipalmitoylphosphatidylserine or phosphatidic acid were investigated by differential scanning calorimetry. As in mixtures of natural phosphatidylserine with cholesterol (Bach, D. (1984) Chem. Phys. Lipids 35, 385-392), also here phase separation of cholesterol at molar ratios of 2:1 (phospholipid:cholesterol) and below is observed. The limited solubility of cholesterol in negatively charged phospholipids is found to be independent of the nature of the acyl chain residues, and independent of whether the negative charge resides on both COO- and PO- groups (as in phosphatidylserine) or on PO- only (as in phosphatidic acid). The separate cholesterol phase is also seen by DSC in mixtures of natural phosphatidylserine or phosphatidic acid with cholesterol in the presence of Ca2+; and in phosphatidylserine/cholesterol mixtures in the presence of Li+, by DSC and X-ray diffraction.

Tris buffer causes acyl chain interdigitation in phosphatidylglycerol

The structure of the gel phase and the properties of the acyl chain disordering transition of dipalmitoyl phosphatidylglycerol (DPPG) have been studied using differential scanning calorimetry, differential scanning dilatometry, and X-ray diffraction. In the presence of small, monovalent cations, DPPG at 22 degrees C exists in a lamellar phase in which the hydrocarbon chains are tilted from the perpendicular to the bilayer surface. Around 34 degrees C, there is a small pretransition (delta H less than 1 kcal/mol) followed by the main transition at 40.4 degrees C (delta H = 8.3 kcal/mol; delta V = 0.0381 ml/g). If DPPG is suspended in Tris-HCl buffer in the absence of other monovalent cations, X-ray diffraction data show that at 22 degrees C, the gel phase consists of interdigitated acyl chains perpendicular to the plane of the bilayer. No pretransition is observed and the main transition occurs at 41.3 degrees C with delta H = 9.1 kcal/mol and delta V = 0.0514 ml/g. If sufficient Na+ or K+ ions are added to the Tris-buffered DPPG, the phase behavior reverts to what is observed in the absence of Tris. Analysis of the energetics of the main transition shows that the increase in van der Waals interaction energy resulting from the larger delta V in Tris can be compensated by the favorable energetics of removing terminal methyl groups from the bilayer surface. The amount of disordering, i.e. formation of gauche rotamers, is likely to be the same in Tris as it is in buffers without amphiphilic cations.

2H-NMR, 31P-NMR and DSC characterization of a novel lipid organization in calcium-dioleoylphosphatidate membranes. Implications for the mechanism of the phosphatidate calcium transmembrane shuttle

2H-NMR, 31P-NMR and DSC investigations are presented on the structure and dynamics of the Ca2+-dioleoylphosphatidate complex which is formed upon addition of calcium to dispersions of pure dioleoylphosphatidate or of dioleoylphosphatidate in mixtures with dioleoylphosphatidylcholine (DOPC). It is concluded that the phosphate region in the polar headgroup of dioleoylphosphatidate is immobilized, while the oleate chains remain liquid and have increased disorder. In mixtures of dioleoylphosphatidate and DOPC in the presence of calcium a dioleoylphosphatidate-rich phase is segregated, in which the molecular behaviour of phosphatidate is rather similar to that of the pure Ca2+-dioleoylphosphatidate complex. A hypothetical model is proposed for the structure of this complex and this is correlated with the dioleoylphosphatidate-mediated transmembrane transport of calcium (Smaal, E.B., Mandersloot, J.G., De Kruijff, B. and De Gier, J. (1986) Biochim. Biophys. Acta 860, 99-108). Data indicate that this transmembrane shuttle is an inverted organization of phosphatidate molecules enclosing calcium ions in an anhydrous core.

Changes in physicochemical properties of mitochondrial membranes during the formation process of megamitochondria induced by hydrazine

Changes in some biochemical and physico-chemical properties of rat liver mitochondrial membranes during the formation process of megamitochondria induced by hydrazine were analyzed. Hepatic mitochondria obtained from rats placed on a 1% hydrazine diet for 3 days became slightly enlarged and sometimes elongated, while they became gigantic after 7 days of hydrazine intoxication. Changes were observed in mitochondria from rats treated with hydrazine for 3 days. Total amounts of phospholipids extracted from mitochondria and submitochondrial fractions were increased. Among phospholipid species, relative amounts of acidic phospholipids were increased. Contents of Ca2+ in mitochondria were increased. Differential scanning calorimetric analysis of mitochondria, especially that of the outer membrane fraction, showed that the thermotropic lipid phase transition temperatures were elevated accompanying the broadening of thermograms and the increase in transition enthalpy. Contents of water in mitochondria were increased significantly with the ratio of freezable water to unfreezable water unchanged. Among the changes observed was that the total amount of phospholipids (except for that of the outer membrane fraction) and the contents of water and Ca2+ nearly returned to normal in megamitochondria after 7 days of hydrazine intoxication. Relative amounts of phospholipids and thermotropic lipid phase transition temperatures of megamitochondria did not return to normal levels and yet changes were smaller than those obtained from 3 days of hydrazine intoxication. The fluidity of mitochondrial membranes was not affected by hydrazine treatment. These data would suggest that hydrazine-induced megamitochondrial formation is not due simply to the swelling of mitochondria, but might be due to the fusion of adjacent mitochondria by Ca2+-acidic phospholipid interactions, and once megamitochondria are formed the mitochondrial membranes are stabilized.

Phase behavior of synthetic N-acylethanolamine phospholipids

Both saturated and unsaturated N-acylethanolamine phospholipids form lamellar structures when dispersed in buffer. The addition of excess Ca2+ (Ca2+/N-acylphosphatidylethanolamine greater than 0.5) results in precipitation. Freeze-fracture replicas indicate that the addition of Ca2+ to the unsaturated lipid results in a non-bilayer structure while the Ca2+-complex of the saturated lipid is lamellar. Since unsaturated phosphatidylethanolamine (PE) is a non-bilayer lipid, its N-acylation with a saturated fatty acid converts a non-bilayer lipid into an acidic bilayer lipid capable of interacting with Ca2+ to return to a non-bilayer structure. Ca2+ may thereby exert an influence on membrane phenomena by regulating phase behavior within certain membrane domains. Differential scanning calorimetry (DSC) indicates that N-acylation of unsaturated PE with a saturated fatty acid also results in changes in thermotropic phase behavior. Therefore, N-acylation may affect fluidity within certain membrane domains.

Direct measurement of the interaction between phosphatidylglycerol bilayers in aqueous electrolyte solutions

Results are presented of force measurements between deposited bilayers of dimyristoylphosphatidyl glycerol (DMPG) at T greater than Tm, and distearoylphosphatidyl glycerol (DSPG) at T less than Tm. Below a bilayer separation of 100 nm, a repulsive double-layer force is measured, which can be explained through the combined screening and binding effect of the counterions in electrolyte solutions of NaCl, HCl, CaCl2, or mixtures of these. The binding of cations to bilayers in the fluid phase (DMPG) appears to be greater than to bilayers in the gel phase (DSPG). At shorter range, below approximately 3 nm, an attractive interaction is measured in solutions containing CaCl2, which was found to be slightly stronger than the theoretically expected van der Waals interaction. No hydration force was observed to exist in solutions containing CaCl2. In NaCl solutions, the measured interbilayer force can completely be accounted for by the electrostatic repulsion, down to a bilayer separation of at least 2 nm, below which no accurate measurements were possible anymore. Parallel measurements on PG monolayers show that the contraction of a DMPG monolayer following addition of CaCl2 is significantly greater than what is predicted from the change in the double-layer free energy alone. This indicates that changes in the lateral interactions between the lipid headgroups probably involve Ca2+-bridge binding and/or a possible dehydration of the lipid headgroups through Ca2+ binding. The results shed new light on both the interbilayer and intrabilayer interactions of PG and identify the possible factors responsible for the morphological behavior of PG aggregates.

Consequences of the interaction of calcium with dioleoylphosphatidate-containing model membranes: changes in membrane permeability

The permeability behaviour of dioleoylphosphatidate/dioleoylphosphatidylcholine (20:80, mol%) large unilamellar vesicles at low millimolar calcium concentrations is different for various solutes. Between 0.5 mM and 2.5 mM of calcium a selective influx of calcium and efflux of enclosed calcium chelating anions is observed. At higher calcium concentrations the membrane loses its barrier function for a large variety of solutes. These permeability increases are a specific consequence of calcium phosphatidate interactions, because control experiments in which calcium was replaced by magnesium or in which dioleoylphosphatidate was replaced by dioleoylphosphatidylglycerol showed under the same conditions no permeability changes. These results are discussed on the basis of various putative mechanistic models for phosphatidate-mediated calcium translocation across membranes. Furthermore a kinetical model is presented by which the observed selective calcium and calcium-chelator translocation can be explained.

Effects of polyamines on the thermotropic behaviour of dipalmitoylphosphatidylglycerol

Using differential scanning calorimetry the thermotropic behaviour of the acidic phospholipid dipalmitoylphosphatidylglycerol (DPPG) was studied as a function of pH, concentration of NaCl and the concentration of organic polyvalent cations, polyamines. Although changes in hydration forces and hydrophobic effect may be involved the measured pH/[NaCl]/Tm phase diagram for DPPG suggests that, even at pH 8.5, DPPG is incompletely deprotonated. In terms of energetics however, the observed changes of about 1 degree C are relatively minor corresponding to a bilayer free energy change of 30 cal/mol. The effects of spermine and spermidine on the phase transition of DPPG were strongly concentration dependent even in the micromolar range. The observed effects correlated to the number of positive charges in these cations and thus the greatest effect was observed for spermine. Specific and nonspecific interactions between DPPG and spermine are suggested to exist which are both reversed by high concentrations of NaCl. These results indicate that under proper conditions changes in the intracellular concentrations of polyamines may be capable of triggering an isothermal phase change in membrane acidic phospholipids.

A lipid-phase separation model of low-temperature damage to biological membranes

An hypothesis is proposed to explain the damage caused to biological membranes exposed to low temperatures. The thesis rests on the general observation that the lipid components of most membranes are heterogeneous and undergo phase transitions from gel-phase lamellae to liquid-crystalline lamellae and some to a non-lamellar, hexagonal-II phase over a wide range of temperatures. As a consequence of these phase transitions the lateral distribution of the lipids characteristic of the growth temperature is disturbed and redistribution takes place on the basis of the temperature at which phase transitions occur. When membranes are cooled, first the non-lamellar forming lipids pass through a transition to a fluid lamellar phase and are miscible with bilayer-forming lipids into which they diffuse. On further cooling the high-melting-point lipids begin to crystallize and separate into a lamellar gel phase, in the process excluding the low-melting point lipids and intrinsic proteins. The lipids in these remaining regions form a gel phase at the lowest temperature. It is suggested that, because the non-lamellar lipids tend to undergo a liquid-crystalline to gel-phase transition at higher temperatures than lamellar-forming lipids, these will tend to phase separate into a gel phase domain rich in these lipids. Damage results when the membrane is reheated, whereupon the hexagonal-II-forming lipids give rise to non-lamellar structures. These probably take the form of inverted micelles sandwiched within the lipid bilayer and they completely destroy the permeability barrier properties of the membrane. The model is consistent with the phase behavior of membrane lipids and the action of cryoprotective agents in modifying lipid phase properties.

Lipid-apolipoprotein interactions in surfactant studied by reassembly

Recent evidence suggests that the structure of tubular myelin, an extracellular form of pulmonary surfactant, is dependent on the interaction of lipids with certain proteins specific for this material and with calcium ions. In order to investigate how protein and calcium may affect the surfactant complex, we studied the composition and properties of reassembly materials formed with a major surfactant apolipoprotein (35,000-38,000 molecular weight) and the principal lipids found in the natural material. We were interested in three questions: 1) Does this apolipoprotein preferentially associate with certain of the lipids in surfactant? 2) What forces are involved in the binding? 3) Does the interaction result in changes in the physical state of the lipid? We found that this apolipoprotein binds phosphatidylcholines that are in a gel phase with much greater affinity than it does phosphatidylcholines that are liquid-crystalline. However, maximum binding does not occur with the pure phosphatidylcholines but rather with mixtures of phosphatidylcholines and 15% phosphatidylglycerol. Calcium ions have no effect on the amount of apolipoprotein that is bound, but they do modify the physical state of the reassembly lipoprotein and the stoichiometry of lipid to protein. These results indicate that the binding of the apolipoprotein to the lipid does not primarily involve ionic bonds. However, apolar interactions, which are influenced by the state of the lamellar phospholipid, appear to be important. Small amounts of phosphatidylglycerol and other glycolipids, which probably disrupt the regularity of a gel-phase lamellar structure when mixed with saturated phosphatidylcholines, may provide binding sites favoring the interaction. Indirect evidence, based on thermodynamic analyses, suggests that phosphatidylcholines may be partially immobilized about the protein in the formation of the complex. This conclusions is reinforced by the preliminary findings obtained from the differential scanning calorimetry of the reassembly materials.

Correlation of 3,4-dihydroxybutyl 1-phosphonate resistance with a defect in cardiolipin synthesis in Escherichia coli

Escherichia coli treated for 1 h with 100 microM rac-3,4-dihydroxybutyl 1-phosphonate (DBP), a glycerol-3-phosphate analog, die when sorted at 5 degrees C, whereas the viability of untreated cells is relatively unaffected. This observation formed the basis of a selection procedure that was used to isolate mutants that are partially resistant to DBP. One such mutant, strain 6204, is constitutive for DBP transport, exhibits a particularly high degree of cold resistance, has the same doubling time as the parent, and is similar to the parent strain in terms of incorporation of DBP into the lipid fraction. Glycerol-3-phosphate and phosphatidylglycerol phosphate synthetases obtained from strain 6204 and its parent were identical in terms of DBP recognition. The parent strain is killed when incubated in the presence of a combination of 70 microM rac-DBP and 0.25% deoxycholate, whereas strain 6204 continues to grow, albeit more slowly, in the presence of this combination. Strain 6204 can be distinguished from the parent strain on agar plates (low phosphate minimal medium with glucuronate as the sole carbon source) containing 15 microM rac-DBP. The insertion of Tn10 near the 6204 mutation has facilitated genetic manipulations. All phenotypic effects attributed to strain 6204 appear to be due to a single mutation. Genetic analysis indicates that Tn10, inserted near the gene responsible for DBP resistance, maps in the vicinity of 27 min. Three-factor crosses reveal a gene order of hemA-Dbpr-Tn10(zch)-trp. The only gene for phosphoglyceride metabolism known to map in this region is the gene associated with cardiolipin synthetase, cls. Genetic results suggest that the mutation responsible for DBP resistance maps in or very near cls. Analysis of the lipids isolated from untreated strain 6204 (and from each of the transductants prepared by P1 vir-mediated transfer of DBP resistance of wild-type strains) reveals that cardiolipin synthesis is defective. These results strongly suggest that the mutation responsible for DBP resistance has its primary effect on cardiolipin synthesis. To further test this hypothesis, strains with an authentic cls mutation were constructed and examined for resistance to DBP. These strains had growth properties that were identical with those of strain 6204. Wild-type strains and mutants defective in cardiolipin synthesis were treated with DBP and 20 mM magnesium or calcium chloride. Simultaneous treatment of either cell type with DBP and divalent cation not only failed to stimulate growth but, quite the contrary, had a marked synergistic growth inhibitory effect.

Low-energy Mg2+-induced temperature transitions in liver microsomes

The differential scanning microcalorimetry and fluorescence methods, using probes ANS and pyrene, have been employed to study thermotropic behaviour of rat liver microsomes in the presence and absence of Mg2+. Addition of Mg2+ yields three partially reversible phase transitions at 18, 27 and 32 degrees C, respectively. A character of Mg2+-induced rearrangements in a membrane and their relation to a catalytic function of a cytochrome P-450-dependent enzymatic system is discussed.

Effects of calcium and neomycin on phase behavior of phospholipid bilayers

Calcium ion (Ca2+, 5 mM) caused a large upward shift in the transition temperature (Tc) of dipalmitoyl phosphatidylglycerol liposomes, apparently interacting with external and internal lipid in the bilayer. Neomycin (1 mM) caused only a small shift, apparently not penetrating the internal lipid domains. When liposomes were first incubated with Ca2+ followed by neomycin, the shift in Tc indicated displacement of Ca2+ by the drug. Liposomes of dipalmitoyl phosphatidylcholine or distearoyl phosphatidylcholine did not interact significantly with either cation. In mixed dipalmitoyl phosphatidylcholine-dipalmitoyl phosphatidylglycerol and distearoyl phosphatidylcholine-dipalmitoyl phosphatidylglycerol liposomes, addition of Ca2+ or neomycin resulted in phase separation.

Effect of pH and monovalent cations on the ionization state of phosphatidylglycerol in monolayers. An experimental (surface potential) and theoretical (Gouy-Chapman) approach

Ionization of the acidic phospholipid phosphatidylglycerol has been studied by measuring the surface potential of monomolecular films of the lipid as a function of the aqueous subphase pH and the concentration of monovalent cations (Li, Na, Cs). It is shown that the experimental data can be interpreted by means of the Gouy-Chapman theory in its simplest formulation, provided an adsorption of cations at the membrane surface is accounted for. This allows us to predict the ionization state of the lipid for given ionic conditions in the subphase. Above pH 4, for subphase ion concentration higher than 10 mM, or for ion concentrations above 0.1 mM at pH 5.6, phosphatidylglycerol is fully deprotonated. Within the limits of our theoretical approach, association constants of the cations to the lipid lie around 0.1-0.6 M-1.

Effect of pH, mono- and divalent cations on the mixing of phosphatidylglycerol with phosphatidylcholine. A monolayer (pi, delta V) and fluorescence study

The mixing of various molecular species of phosphatidylglycerol and phosphatidylcholine differing in their acyl chain lengths has been studied both in monolayers (pi, delta V), and in water dispersions (fluorescence polarization) with varying pH and ionic strength of the aqueous phase and in the presence of the divalent cations Mg2+ and Ca2+. In dilauroylphosphatidylglycerol/dipalmitoylphosphatidylcholine mixtures, both in monolayers and in water dispersions, no phase separation was detected at pH 2.9 where phosphatidylglycerol was protonated. With dipalmitoylphosphatidylglycerol/dipalmitoylphosphatidylcholine mixtures, in monolayers and at the same pH, no phase separation was detected for surface pressures below pi = 40 mN.m-1. In monolayers, and under ionic conditions such that phosphatidylglycerol was ionized (pH 5.6, 10 mM NaCl) miscibility was observed with dilauroylphosphatidylglycerol and dipalmitoylphosphatidylcholine and also with dipalmitoylphosphatidylglycerol and dilauroylphosphatidylcholine. Varying the ionic strength did not alter the miscibility of these lipids. The divalent cations Mg2+ and Ca2+ did not modify that of dilauroylphosphatidylglycerol with dilauroylphosphatidylcholine or with dipalmitoylphosphatidylcholine. Both in monolayers and in water dispersions, dipalmitoylphosphatidylglycerol and dilauroylphosphatidylcholine appeared to be at least partly miscible, in the presence of magnesium. Only in the presence of calcium and at high surface pressure might the monolayer data account for phase separation between these two lipids. The data presented demonstrate the existence of strong cohesive forces between phosphatidylcholine and phosphatidylglycerol with a marked influence of the former on the physical state of the latter. From an analysis of the delta V data, it is suggested that intrafacial hydrogen bonds may play a significant role in stabilizing phosphatidylcholine/phosphatidylglycerol mixtures.

The use of differential scanning calorimetry and differential thermal analysis in studies of model and biological membranes

Differential scanning calorimetry (DSC), and to a lesser extent differential thermal analysis (DTA), are powerful yet relatively rapid and inexpensive thermodynamic techniques for studying the thermotropic phase behavior of lipids in model and biological membranes, without the introduction of exogenous probe molecules. In this review the principles as well as the scope and limitations of DSC and DTA are discussed first. The application of these techniques to the study of the thermotropic phase behavior of aqueous dispersions of various single synthetic phospholipids are then summarized, and the effects of cholesterol, free fatty acids, lysophospholipids, drugs, anesthetics and proteins on the gel to liquid-crystalline phase transitions exhibited by these model systems are discussed. The phase mixing properties of model membranes consisting of mixtures of two or more synthetic or natural phospholipids are considered next. Finally, the thermotropic phase behavior of prokaryotic plasma membranes and of the plasma, microsomal and mitochondrial membranes of eukaryotic cells are reviewed, and the applications of DSC and DTA to study the thermal behavior of specific membrane proteins, as well as the physical properties of the membrane lipid phase, are summarized.

Effect of Ca2+ on thermotropic properties of saturated phosphatidylcholine liposomes

The effect of various concentrations of calcium ion (Ca2+) on dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC) and mixed DPPC/DSPC (1:1) liposomes was studied by differential scanning calorimetry. Ca2+ concentrations of 10 mM and above split the main transition peak of DPPC into two distinguishable components, and, at 30 mM and above, also resulted in the disappearance of a pre-transition peak. The effect of Ca2+ on DSPC liposomes was even more dramatic in that it induced a more definitive split in the main transition peak and caused the loss of the pretransition peak at only 10 mM concentration. The thermograms of the DPPC/DSPC mixed liposomes were unaltered in the presence of Ca2+, even at a concentration of 50 mM. Whether or not Ca2+ is able to alter thermograms of phosphatidylcholine liposomes appears to be dependent on the degree of molecular order of the bilayer prior to interaction with Ca2+.

Electro-osmosis at the surface of phospholipid bilayer membranes

The electro-osmotic velocity is the velocity of a fluid near an interface produced by an electric field parallel to a surface. The velocity adjacent to fixed phospholipid bilayer membranes was measured by observing the velocity of small vesicles suspended in the fluid. The charge densities of the bilayers ranged from 0 to 1 electronic charge per lipid and experiments were performed at temperatures above and below the transition temperature of the phospholipid bilayer in 1, 10 and 100 mM NaCl solutions. The Helmholtz-Smoluchowski equation correctly predicted the electro-osmotic velocity from the known value of zeta potential of the phospholipid bilayer.

The adsorption of divalent cations to phosphatidylglycerol bilayer membranes

The ability of Stern equation to describe the adsorption of divalent cations to phosphatidylglycerol membranes was tested by combining 31P-NMR and electrophoretic mobility measurements. In 0.1 M sodium chloride both the 31P-NMR and the zeta potential data are well described by the Stern equation. 31P-NMR and 13C-NMR results indicate that cobalt forms inner-sphere complexes only with the phosphate group of phosphatidylglycerol molecules and that a substantial fraction of the adsorbed cobalt ions form outer-sphere complexes. Evidence is presented that suggests the alkaline earth cations also bind to phospholipids mainly by forming outer sphere complexes. Electrophoretic mobility measurements were performed with several different divalent cations. In all cases the zeta potentials in 0.1 M sodium chloride were well described by the Stern equation. The intrinsic 1:1 association constants (M-1) for the phosphatidylglycerol complexes decreased in the sequence: Mn2+, 11.5; Ca2+, 8.5; Ni2+, 7.5; Co2+, 6.5; Mg2+, 6.0; Ba2+, 5.5 and Sr2+, 5.0.