Phase behavior of synthetic N-acylethanolamine phospholipids

Strategies for altering lipid self-assembly to trigger liposome cargo release

While liposomes have proven to be effective drug delivery nanocarriers, their therapeutic attributes could be improved through the development of clinically viable triggered release strategies in which encapsulated drug contents could be selectively released at the sites of diseased cells. As such, a significant amount of research has been reported involving the development of stimuli-responsive liposomes and a broad range of strategies have been explored for driving content release. These have included the introduction of trigger groups at either the lipid headgroup or within the acyl chains that alter lipid self-assembly properties of known lipids as well as the rational design of lipid analogs programed to undergo conformational changes induced by events such as binding interactions. This review article describes advances in the design of stimuli-responsive liposome strategies with an eye towards emerging trends in the field.

Bilayer phase transitions of N-methylated dioleoylphosphatidylethanolamines under high pressure

The bilayer phase transitions of four kinds of unsaturated phospholipids with different-sized polar head groups, dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidyl-N-methylethanolamine (DOMePE), dioleoylphosphatidyl-N,N-dimethylethanolamine (DOMe2PE) and dioleoylphosphatidylcholine (DOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light-transmittance. DSC thermogram and light-transmittance curve for each phospholipid vesicle solution exhibited only one phase transition under ambient pressure, respectively. The light-transmittance of DOPC solution at pressure higher than 234 MPa abruptly increased stepwise at two temperatures, which corresponds to the appearance of stable subgel and lamellar gel phases under high pressure in addition to the liquid crystal phase. The constructed temperature (T)-pressure (p) phase diagrams were compared among these phospholipids. The phase-transition temperatures of the phospholipids decreased stepwise by N-methylation of the head group. The slops of the T-p phase boundary (dT/dp) of DOPE, DOMePE and DOMe2PE bilayers (0.127-0.145 K MPa-1) were found to be close to that of the transition from the lamellar crystal (or subgel; Lc) phase to the liquid crystal (Lalpha) phase for DOPC bilayer (0.131 K MPa-1). On the other hand, the dT/dp value of the main transition from the lamellar gel (Lbeta) phase to the Lalpha phase for DOPC bilayer (0.233 K MPa-1) was significantly different from that of the Lc/Lalpha transition, hence both curves intersected with each other at 234 MPa. The thermodynamic quantities associated with the phase transition of DOPE, DOMePE and DOMe2PE bilayers had also similar values to those for the Lc/Lalpha transition of DOPC bilayer. Taking into account of the values of transition temperature, dT/dp and thermodynamic quantities compared with the corresponding results of saturated phospholipids, we identified the phase transitions observed in the DOPE, DOMePE and DOMe2PE bilayers as the transition from the Lc phase to the Lalpha phase although they have been regarded as the main transition in the previous studies. The Lbeta phase is probably unstable for DOPE, DOMePE and DOMe2PE bilayers at all pressures, it exists as a metastable phase at pressures below 234 MPa while as a stable phase at pressures above 234 MPa in DOPC bilayer. The difference in phase stability among the phospholipid bilayers is originated from that in hydration structure of the polar head groups.

N-acyl phosphatidylethanolamines affect the lateral distribution of cholesterol in membranes

N-Acyl phosphatidylethanolamines are negatively charged phospholipids, which are naturally occurring albeit at low abundance. In this study, we have examined how the amide-linked acyl chain affected the membrane behavior of the N-acyl-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (N-acyl-POPE) or N-acyl-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (N-acyl-DPPE), and how the molecules interacted with cholesterol. The gel-->liquid crystalline transition temperature of sonicated N-acyl phosphatidylethanolamine vesicles in water correlated positively with the number of palmitic acyl chains in the molecules. Based on diphenylhexatriene steady state anisotropy measurements, the presence of 33 mol% cholesterol in the membranes removed the phase transition from N-oleoyl-POPE bilayers, but failed to completely remove it from N-palmitoyl-DPPE and N-palmitoyl-POPE bilayers, suggesting rather weak interaction of cholesterol with the N-saturated NAPEs. The rate of cholesterol desorption from mixed monolayers containing N-palmitoyl-DPPE and cholesterol (1:1 molar ratio) was much higher compared to cholesterol/DPPE binary monolayers, suggesting a weak cholesterol interaction with N-palmitoyl-DPPE also in monolayers. In bilayer membranes, both N-palmitoyl-POPE and N-palmitoyl-DPPE failed to form sterol-rich domains, and in fact appeared to displace sterol from sterol/N-palmitoyl-sphingomyelin domains. The present data provide new information about the effects of saturated NAPEs on the lateral distribution of cholesterol in NAPE-containing membranes. These findings may be of relevance to neural cells which accumulate NAPEs during stress and cell injury.

Occurrence, metabolism, and prospective functions of N-acylethanolamines in plants

N-Acylethanolamines (NAEs) are fatty acid amides that are derived from an N-acylated phoshatidylethanolamine presursor, a minor membrane lipid constituent of plant and animal cells. Historically, the formation of N-acylethanolamines was associated with cellular stress and tissue damage in mammals, but more recently has been shown to be part of the endocannabinoid signaling system that regulates a variety of normal physiological functions, including neurotransmission, immune responses, vasodilation, embryo development and implantation, feeding behavior, cell proliferation, etc. The widespread regulation of vertebrate physiology by this class of lipid mediators and the conservation of the mechanisms for NAE formation, perception and degradation in higher plants raises the possibility that the metabolism of NAEs represents an evolutionarily conserved lipid signaling pathway that regulates an array of physiological processes in multicellular eukaryotes. Here the recent information on NAEs in plants is reviewed in the context of the occurrence, metabolism and functions of this bioactive class of lipid mediators.

Putative neuroprotective actions of N-acyl-ethanolamines

N-Acyl-ethanolamines (NAEs) and their precursors, N-acyl-ethanolamine phospholipids (NAPEs), are present in the mammalian brain at levels of a few hundred picomoles/gram tissue and a few nanomoles/gram tissue, respectively. NAE-containing arachidonic acid is called anandamide, and it has attracted particular attention since it is a partial agonist for the cannabinoid receptors, for which 2-arachidonoylglycerol is the full agonist. In addition, anandamide may also activate the vanilloid receptor. Anandamide usually amounts to 1-10% of NAEs, as the vast majority of N-acyl groups are saturated and monounsaturated fatty acids. Formation of NAPE and NAE is catalyzed by an N-acyltransferase and an NAPE-hydrolyzing phospholipase D, respectively, two enzymes that have been characterized only preliminary. Interestingly, NAPEs and NAEs accumulate in the brain in response to neurodegenerative insults at a time when other phospholipids are subjected to rapid degradation. This is an important biosynthetic aspect of NAPE and NAE, as NAEs may be neuroprotective by a number of different mechanisms involving both receptor activation and non-receptor-mediated effects, e.g. by binding to cannabinoid receptors and interfering with ceramide turnover, respectively.

Cannabinoid receptor-inactive N-acylethanolamines and other fatty acid amides: metabolism and function

Although it is now generally accepted that long-chain N-acylethanolamines and their precursors, N-acylethanolamine phospholipids, exist as trace constituents in virtually all vertebrate cells and tissues, their possible biological functions are just emerging. While anandamide (N-arachidonoylethanolamine) has received much attention due to its ability to bind to and activate cannabinoid receptors, the saturated and monounsaturated N-acylethanolamines, which usually represent the vast majority, are cannabinoid receptor-inactive but appear to interact with endocannabinoids and to have other signaling functions as well. Also, primary fatty acid amides, including the amide of oleic acid, which acts as a sleep-inducing agent, do not interact with cannabinoid receptors but are catabolically related to endocannabinoids. Here we review published information on the occurrence, metabolism, and possible signaling functions of the cannabinoid receptor-inactive N-acylethanolamines and primary fatty acid amides.

N-Myristoylated Phosphatidylethanolamine: Interfacial Behavior and Interaction with Cholesterol

The interfacial packing behavior of N-myristoyldimyristoylphosphatidylethanolamine (N-14:0 DMPE) and its interaction with cholesterol were characterized and compared to the behavior of dimyristoylphosphatidylethanolamine (DMPE) using an automated Langmuir type film balance. Surface pressure and surface potential were monitored as a function of lipid cross-sectional molecular area. N-14:0 DMPE exhibited two-dimensional (2D) phase transitions of a liquid-expanded to condensed nature at many temperatures in the 15-30 °C range, but isotherms showed only condensed behavior at 15 °C. The sharp decline in the surface compressional moduli upon entering the 2D-transition region is consistent with differences in the partial molar areas of coexisting liquid-expanded (chain-disordered) and condensed (chain-ordered) phases. Including Ca(2+) in the subphase beneath the negatively charged N-14:0 DMPE caused a downward shift in the 2D-transition onset pressure even in the presence of 100 mM NaCl. The average dipole moments perpendicular to the lipid-water interface for N-14:0 DMPE's liquid-expanded and condensed phases were higher than those of DMPE. At surface pressures sufficiently low (<10 mN/m) to produce liquid-expanded phase behavior in pure N-14:0 DMPE, mixing with cholesterol resulted in a classic "condensing effect". Maximal area condensation was observed near equimolar N-14:0 DMPE/cholesterol. Insights into mixing behavior at high surface pressures that mimic the lipid cross-sectional areas of biomembranes were provided by analyzing the surface compressional moduli as a function of cholesterol mole fraction. Complex mixing patterns were observed that deviated significantly from theoretical ideal mixing behavior suggesting the presence of lipid "complexes" and/or a liquid-ordered phase at high sterol mole fractions (>0.35) and low to intermediate surface pressures (<20 mN/m) as well as the possible coexistence of relatively immiscible solid phases at higher surface pressures (e.g., 35 mN/m).

N-Acylphosphatidylethanolamine accumulation in potato cells upon energy shortage caused by anoxia or respiratory inhibitors

A minor phospholipid was isolated from potato (Solanum tuberosum L. cv Bintje) cells, chromatographically purified, and identified by electrospray ionization mass spectrometry as N-acylphosphatidylethanolamine (NAPE). The NAPE level was low in unstressed cells (13 +/- 4 nmol g fresh weight(-1)). According to acyl chain length, only 16/18/18 species (group II) and 18/18/18 species (group III) were present. NAPE increased up to 13-fold in anoxia-stressed cells, but only when free fatty acids (FFAs) started being released, after about 10 h of treatment. The level of groups II and III was increased by unspecific N-acylation of phosphatidylethanolamine, and new 16/16/18 species (group I) appeared via N-palmitoylation. NAPE also accumulated in aerated cells treated with NaN(3) plus salicylhydroxamate. N-acyl patterns of NAPE were dominated by 18:1, 18:2, and 16:0, but never reflected the FFA composition. Moreover, they did not change greatly after the treatments, in contrast with O-acyl patterns. Anoxia-induced NAPE accumulation is rooted in the metabolic homeostasis failure due to energy deprivation, but not in the absence of O(2), and is part of an oncotic death process. The acyl composition of basal and stress-induced NAPE suggests the existence of spatially distinct FFA and phosphatidylethanolamine pools. It reflects the specificity of NAPE synthase, the acyl composition, localization and availability of substrates, which are intrinsic cell properties, but has no predictive value as to the type of stress imposed. Whether NAPE has a physiological role depends on the cell being still alive and its compartmentation maintained during the stress period.

Pathways and mechanisms of N-acylethanolamine biosynthesis: can anandamide be generated selectively?

Long-chain N-acylethanolamines (NAEs) and their precursors, N-acylethanolamine phospholipids, are ubiquitous trace constituents of animal and human cells, tissues and body fluids. Their cellular levels appear to be tightly regulated and they accumulate as the result of injury. Saturated and monounsaturated congeners which represent the vast majority of cellular NAEs can have cytoprotective effects while polyunsaturated NAEs, especially 20:4n-6 NAE (anandamide), elicit physiological effects by binding to and activating cannabinoid receptors. It is the purpose of this article to review published data on the pathways and mechanisms of NAE biosynthesis in mammals and to evaluate this information for its physiological significance. The generation and turnover of NAE via N-acyl PE through the transacylation-phosphodiesterase pathway may represent a novel cannabinoid receptor-independent signalling system, analogous to and possibly related to ceramide-mediated cell signalling.

N-Acylethanolamines and precursor phospholipids - relation to cell injury

The present review focuses on the relationship between formation of N-acylethanolamine phospholipids (NAPEs) and N-acyletransferase (NAEs) catalyzed by N-acyltranferase and NAPE-hydrolyzing phospholipase D, respectively, and cell injury in tissues like brain, heart, and testis. A number of mechanisms are proposed by which these two groups of lipids may have cytoprotective properties. The mechanisms may involve activation of cannabinoid receptors, as well as non-receptor-mediated effects such as stabilization of membrane bilayers, antioxidant mechanisms, inhibition of calcium leakage from mitochondria, and direct inhibition of ceramidase. Anandamide (20:4-NAE) is formed as a minor component along with other NAEs during cell injury. Whether 20:4-NAE has a separate physiological role is at present not known, but some data suggest that 20:4-NAE may be formed, e.g. in the uterus, by a more selective mechanism without being accompanied by a vast majority of saturated and monounsaturated NAEs.

Aggregation and fusion of vesicles composed of N-palmitoyl derivatives of membrane phospholipids

N-Acylphosphatidylethanolamines and N-acylphosphatidylserines have been isolated from mammalian cells and have been associated with some tissue degenerative changes, although the relationship between their synthesis and the uncontrolled sequence of events that ends in irreversible tissue damage is not completely established. Our results show that monovalent and divalent cations induce aggregation and fusion of liposomes constituted by N-palmitoylphosphatidylethanolamine (NPPE) and N-palmitoylphosphatidylserine (NPPS). The effectiveness of cations to induce the aggregation of NPPE and NPPS liposomes is Ca2+ > Mg2+ >> Na+. NPPS liposomes aggregate at lower concentrations of divalent cations than NPPE liposomes, but with sodium NPPE liposomes aggregate to a higher extent than NPPS liposomes. The reaction order for the aggregation processes depends on the lipid and the cation nature and range from 1.04 to 1.64. Dynamic light scattering shows an irreversible increase of the size of the aggregates in the presence of all cations tested. The irreversibility of the aggregation process and the intermixing of bilayer lipids, as studied by resonance energy transfer assay, suggest that fusion, rather than aggregation, occurs. The existence of a real fusion was demonstrated by the coalescence of the aqueous contents of both NPPS and NPPE liposomes in the presence of either monovalent or divalent cations. The different binding sensitivity of Ca2+ to NPPS and NPPE liposomes, determined by zeta potential measurements, agrees with the results obtained in the aggregation and fusion assays. Our results suggest that the synthesis in vivo of N-acylated phospholipids can introduce important changes in membrane-mediated processes.

Derivatised lipids in membranes. Physico-chemical aspects of N-biotinyl phosphatidylethanolamines, N-acyl phosphatidylethanolamines and N-acyl ethanolamines

The physical properties of N-biotinyl phosphatidylethanolamines, N-acyl phosphatidylethanolamines and of N-acyl ethanolamines, in aqueous dispersions, are reviewed. Emphasis is placed on the calorimetric (i.e. chain melting) properties, the thermotropic phase behaviour, certain aspects of the structure and dynamics, and the miscibility with other membrane lipids. In the case of N-biotinyl phosphatidylethanolamines, the specific binding of avidin, and in the case of N-acyl ethanolamines, the function of the third chain, is also considered. All of these properties are relevant to the role of these rather unusual lipids in membranes.

Formation of N-acyl-phosphatidylethanolamine and N-acylethanolamine (including anandamide) during glutamate-induced neurotoxicity

N-Acyl-phosphatidylethanolamine (NAPE) is present in very small amounts in mammalian tissues (less than 0.1% of total phospholipids). However, NAPE as well as its degradation product, N-acylethanolamine (NAE), can be formed in certain neuronal tissues in response to increased [Ca2+]i. A high [Ca2+]i will activate the NAPE-forming N-acyltransferase using the sn-1 acyl group of a donor phospholipid as substrate in the transfer reaction. This membrane-bound enzyme seems to have no substrate specificity with respect to transfer of acyl groups; thus the fatty acids in the N-acyl group of NAPE are mainly 16:0 and 18:1, corresponding to the fatty acids in the sn-1 acyl group of the donor phospholipids. The NAPE-hydrolyzing phospholipase D also seems not to be acyl-group specific. In mouse neocortical neurons in primary culture, formation of NAPE and NAE is stimulated by glutamate via activation of the N-methyl-D-aspartate-receptor. Both NAPE and, to a lesser extent, NAE accumulate in a linear fashion for many hours while at the same time the neurons are dying. Likewise, in neurons prelabeled with 14C-arachidonic acid, 14C-arachidonic acid-labeled NAPE, and anandamide (= N-arachidonoylethanolamine) are accumulating. The formation of NAPE and NAE may represent a cytoprotective response in relation to various forms of neurotoxicity.

Chain-melting transition temperatures of phospholipids with acylated or alkylated headgroups (N-acyl phosphatidylethanolamines and O-alkyl phosphatidic acids), or with alpha-branched chains

The biphasic dependence of the chain-melting transition temperature on chainlength, nN, of the headgroup-attached chain of N-acyl phosphatidylethanolamines and of O-alkyl phosphatidic acids is interpreted in terms of a different linear dependence of the transition enthalpy and entropy on nN for long and short chains, respectively. A consistent expression of the form [equation: see text], where DeltanN is the critical chainlength at which the packing mode of the headgroup-attached chains changes, is found to apply systematically to the transition temperatures for both sets of lipids, over the range nN=2-18. This thermodynamic analysis demonstrates that the headgroup-attached chain is located in an environment that differs for long and short chains. Similar considerations apply also to phosphatidylcholines with alpha-branched glycerol-attached chains.

Formation of N-acyl-phosphatidylethanolamines and N-acetylethanolamines: proposed role in neurotoxicity

The formation of N-acyl-phosphatidylethanolamine (NAPE) and N-acylethanolamine (NAE), including anandamide, in mammals in relation to neurotoxicity is discussed. Data on the characterization of the NAPE-forming N-acyltransferase, the NAPE-hydrolyzing phospholipase D, and the NAE-hydrolyzing amidase are reviewed. We suggest that NAPE and NAE, including anandamide, are formed in neurons in response to the high intracellular calcium concentrations that occur in injured neurons, e.g. due to glutamate excitotoxicity. NAPE may have functions of its own besides being a precursor for NAE. The formation of both of these lipids may serve as a cytoprotective response, whether mediated by physical interactions with membranes or enzymes, or mediated by activation of cannabinoid receptors. This suggestion implies that NAPE and NAE may have pathophysiological roles in the brain. Whether these lipids also have physiological roles is uncertain.

Cation-dependent fusogenicity of an N-acyl phosphatidylethanolamine

N-acyl phosphatidylethanolamines (NAPEs) are natural lipid components of many organisms. N-acylation of unsaturated phosphatidylethanolamines with a saturated fatty acid converts them from non-lamellar organizing lipids into lamellar organizing, acidic lipids which can interact with cations and potentially return to non-lamellar structures. These special properties make NAPEs candidates for fusogens. We tested the fusogenicity of one of the NAPEs, N-dodecanoyl-di-oleoylphosphatidylethanolamine (N-C12-DOPE) mixed with dioleoylphosphatidylcholine (DOPC) in liposomes. Binding and fusion to erythrocyte ghosts in the presence of 3 mM Ca2+ required at least 60 mol% of N-C12-DOPE. Fusion was not observed when phosphatidylglycerol or phosphatidylserine was substituted for N-C12-DOPE, indicating specificity for properties of this lipid. Binding of N-C12-DOPE/DOPC (70:30) liposomes required 1 mM Ca2+ while 1.25 mM Ca2+ and Mg2+ were sufficient for lipid mixing and delivery of encapsulated dextrans to erythrocyte ghosts. These liposomes also bound and possibly mixed lipid with nucleated U-937 cells in a Ca2+ -and endocytosis-dependent manner. Low pH-dependent fusion with ghosts was observed in the absence of any divalent cation, indicating that fusion with U-937 cells could result after endocytosis into the acidic endosomes. The possible mechanisms for N-C12-DOPE mediated binding and fusion and the potential application of these liposomes as delivery vehicles for therapeutic agents are discussed.

Differential scanning calorimetry of chain-melting phase transitions of N-acylphosphatidylethanolamines

Phosphatidylethanolamines in which the polar headgroup is N-acylated by a long-chain fatty acid (N-acyl PEs) are present in many plasma membranes under normal conditions, and their content increases dramatically in response to membrane stress in a variety of organisms. The thermotropic phase behavior of a homologous series of saturated N-acyl PEs, in which the length of the N-acyl chain is equal to that of the O-acyl chains attached at the glycerol backbone, has been investigated by differential scanning calorimetry (DSC). All fully hydrated N-acyl PEs with even chain lengths from C-12 to C-18 exhibit sharp endothermic chain-melting phase transitions in the absence of salt and in 1 M NaCl. Cooperative chain-melting is demonstrated directly by the temperature dependence of the electron spin resonance spectra from probe phospholipids bearing a spin label group in the acyl chain. The calorimetric transition enthalpy and the transition entropy obtained from DSC depend approximately linearly on the chain length with incremental values per CH2 group that exceed those of normal diacyl phosphatidylethanolamines, but to an extent that underrepresents the additional N-acyl chain. A thermodynamic model is constructed for the chain-length dependences and end effects of the calorimetric quantities, which includes a deficit proportional to the difference in O-acyl and N-acyl chain lengths for nonmatched chains, as is found and justified structurally for mixed-chain diacyl phospholipids. From data on the chain-length dependence of N-acyl diC16PEs, it is then deduced that the N-acyl chains are less well packed than the O-acyl chains and, from the data on the matched-chain N-acyl PEs, that the O-acyl chain packing is similar to that in normal diacyl PEs. The gel-to-fluid phase transition temperatures of the N-acyl PEs in the absence of salt are practically the same as those of the normal diacyl PEs of the corresponding chain lengths, although the transition enthalpies and entropies are appreciably greater, indicating entropy-enthalpy compensation. In 1 M NaCl, the transition temperatures are 3-4.5 degrees higher than in the absence of salt, representing the contribution of the electrostatic surface potential of the N-acyl PEs.

Differential scanning calorimetric studies on the thermotropic phase transitions of dry and hydrated forms of N-acylethanolamines of even chainlengths

N-acylethanolamines (NAEs) have attracted the attention of researchers in the last two decades due to their occurrence in biological membranes under conditions of stress as well as under normal conditions. Differential scanning calorimetric studies have been carried out on dry and hydrated samples of a homologous series of N-acylethanolamines containing saturated acyl chains of even number of carbon atoms (n = 8-20). In both cases a major sharp endothermic transition was observed which occurs at the melting point for the dry NAEs whereas for the hydrated samples it occurs at considerably lower temperatures. The enthalpies and entropies corresponding to this transition could be fitted, in each case, to a straight line suggesting that the transition enthalpy and transition entropy consist of a fixed component from the polar head group and the terminal methyl group, whereas the contribution of the methylene groups, (CH2)n, is linearly proportional to the number of carbon atoms in it. The contributions of each methylene unit to the transition enthalpy and transition entropy of NAEs were found to be deltaH(inc) = 0.82 (+/-0.02) and 0.96 (+/-0.06) kcal mol(-1), and deltaS(inc) = 2.01 (+/- 0.06) and 2.37 (+/-0.17) cal mol(-1) K(-1), respectively, for the dry and hydrated samples of NAEs, whereas the end contributions arising from the head group and the terminal methyl group were determined to be deltaH(o) = -0.10 (+/-0.26) and -0.52 (+/-0.82) kcal mol(-1) and deltaS(o) = 2.12 (+/-0.71) and 3.1 (+/-2.3) cal mol(-1) K(-1), respectively, for the dry and hydrated samples of NAEs. These results are relevant to an understanding of the thermodynamics of the phase properties of NAEs in membranes.

N-acylphosphatidylethanolamines: effect of the N-acyl chain length on its orientation

N-Acylphosphatidylethanolamines, or NAPEs, are found in tissues involved in degenerating processes, such as dehydrated endosperm of seeds, erythrocyte membranes, or cell injury. To determine the conformation and orientation of the acyl chains of these phospholipids, NAPEs with deuterated N-acyl chains of 6 and 16 carbon atoms were synthesized and studied by transmission and attenuated total reflectance (ATR) infrared spectroscopy. For N-C16d-DPPE, the ATR measurements show that the N-acyl chain has the same orientation as the two acyl chains attached to the glycerol moiety, while the N-acyl chain of N-C6d-DPPE is randomly oriented. These results demonstrate that for N-C16d-DPPE, the N-acyl chain is embedded into the hydrophobic core of the bilayer, while for the short chain derivative the N-acyl chain remains in the lipid headgroup region. The analysis of the carbonyl stretching band and of the amide I band suggests that, for the long N-acyl chain lipid, the ester C=O and the N-H groups are linked by intermolecular hydrogen bonds.

N-palmitoylphosphatidylethanolamine stabilizes liposomes in the presence of human serum: effect of lipidic composition and system characterization

Liposomes containing negatively-charged phospholipid, N-palmitoylphosphatidylethanolamine (NPPE) were examined for stability in the presence of human serum, using the release of the entrapped 5,6-carboxyfluorescein as an aqueous marker. Either small unilamellar vesicles (SUV) or large unilamellar vesicles (LUV) were used. Incorporation of NPPE into PC SUV decreases leakage in the presence of serum or phosphate-buffered saline, no strictly related to size increase observed and to the surface negative charge present. The stabilizing effect of NPPE and Chol were synergistic. Inhibition of destabilization induced by serum of PC/Chol liposomes was observed when NPPE concentrations were above 12 mol%. Change in the membrane fluidity or incorporation of a monosialoganglioside into liposomes do not significantly change the half-life of liposomes in the presence of a high NPPE concentration. Incorporation of NPPE into PC/Chol liposomes increases membrane rigidity which does not change after serum incubation. The presence of NPPE in liposomes decreases lipid transfer/exchange between liposomes and lipoproteins although the same amount of serum proteins were incorporated as in PC/Chol liposomes. As expected, these proteins are accessible to trypsin digestion. In accordance with these results, the liposome agglutination assay shows no steric barrier activity. As a whole, the results obtained in this paper suggest a complex mechanism for stabilization of NPPE containing liposomes in human serum.

Role of headgroup structure in the phase behaviour of N-acylethanolamine phospholipids: hydrogen-bonding ability and headgroup size

The physical properties of aqueous dispersions of N-acylphosphatidylethanolamine from natural origin with long N-acyl chain (NAPE) and headgroup modified analogues have been studied. N-Acylation of PE causes a significant increase in the gel-to-liquid crystalline lamellar phase transition temperature in contrast with saturated N-acyl(dipalmitoyl) PEs, and in addition it does not restrict the headgroup rotational mobility in gel phase. The results agree with the increase of hydration of the phosphate group compared with that in PE and suggest the formation of hydrogen bonds between amide groups. The modifications introduced modulate the headgroup size and their hydrogen bonding capability. An increasing number of methylene groups between the phosphate and amide groups does not modify the phase behaviour observed. N-methylation of the amide group, which prevents the possibility of intermolecular hydrogen bond formation, decreases the melting temperature and the cooperativity of the phase transition and does not change the phase behaviour, while the hydration at the ester carbonyl groups level is decreased. On the other hand, the addition of N-ethyl substituent to the amide group or substitution of an ester group for this group increases its tendency to form structures with inverted geometries. The behaviour of these compounds suggests that hydration forces must be more important than considerations of the lipid dynamic shape in predicting the relative stabilities of lamellar vs. non-lamellar phases for NAPEs with long saturated N-acyl chain.

Isozymes of cottonseed microsomal N-acylphosphatidylethanolamine synthase: detergent solubilization and electrophoretic separation of active enzymes with different properties

We recently reported that a novel acyltransferase activity (fatty acid: diacylphosphatidylethanolamine N-acyltransferase) synthesizes N-acylphosphatidylethanolamine (NAPE), an unusual derivative of phosphatidylethanolamine (PE), in microsomes of cotyledons of cotton seedlings by direct acylation of PE with free fatty acids (Chapman and Moore (1993) Plant Physiol. 102(3), 761-769). Here we report the solubilization of this membrane-bound NAPE synthase activity from cottonseed microsomes and the separation of three active isozymes with distinctly different characteristics. NAPE synthase activity was solubilized from NaCl-washed microsomal membranes by 0.2 mM dodecylmaltoside (DDM) at a 2:1 (w:w) detergent/protein ratio (assessed by enzyme activity after centrifugation at 150,000 x gmax, 1 h). Two sequential preparative isoelectric focussing separations of DDM-solubilized microsomal membrane proteins resulted in recovery of three distinct peaks of NAPE synthase activity--one at pH 6.3, one at pH 7.2, and one at pH 8.4 (designated A, B and C). These isozymes were purified 1148-fold (A), 269-fold (B), and 729-fold (C) from homogenates of cotton cotyledons. A 28 kDa subunit was enriched in all three isozyme fractions. Each of the isozymes exhibited unique kinetic properties with respect to palmitic acid and dioleoyl-PE. Each of the solubilized isozymes exhibited positive cooperativity toward palmitic acid (consistent with previous studies of NAPE synthase activity in intact microsomes) but not toward dioleyl-PE. Collectively, these results indicate that the synthesis of NAPE in cotton cotyledons is complex and has a potential for being a highly regulated process. The isolation of active NAPE synthase isozymes will provide the foundation for future studies into the physiological role of NAPE synthase (and NAPE) and the regulation of NAPE metabolism in membranes of plant cells.

Phases and phase transitions of the hydrated phosphatidylethanolamines

LIPIDAT is a computerized database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior. Here, a review of the LIPIDAT data subset referring to hydrated phosphatidylethanolamines (PE) is presented together with an analysis of these data. The PE subset represents 14% of all LIPIDAT records. It includes data collected over a 38-year period and consists of 1511 records obtained from 203 articles in 35 different journals. An analysis of the data in the subset has allowed us to identify trends in synthetic PE phase behavior reflecting changes in lipid chain length, chain unsaturation (number, isomeric type and position of double bonds), chain asymmetry and branching, type of chain-glycerol linkage (ether vs. ester) and headgroup modification. Also included is a summary of the data concerning the effect of pH, stereochemical purity, and different additives such as salts, saccharides, alcohols, amino adds and alkanes on PE phase behavior. Information on the phase behavior of biologically derived PE is also presented. This review includes 236 references.

Incorporation of N-acylethanolamine phospholipids into egg phosphatidylcholine vesicles: characterization and permeability properties of the binary systems

We have studied the effect of the N-acylphosphatidylethanolamine (N-acylPE) on the permeability properties of liposomes composed primarily of egg phosphatidylcholine using a fluorescent anionic dye, carboxyfluorescein, as model solute. Leakage from liposomes decreased and vesicle size increased with increasing N-acylPE content. In addition, measurement of the trapped aqueous space, using the same dye marker, showed a correlation between trapped volume and vesicle size determined by dynamic light scattering. Permeability parameters were calculated according to the pseudo-first-order analysis. It appears that N-acylPE stabilizes liposomes at least in part through its ability to impart surface negative charge, in accord with the results obtained with potassium chloride as encapsulated solute. These results agreed well with osmotic response of anionic lipid vesicles. Cholesterol stabilizes N-acylPE liposomes in a proportional manner to the molar fraction of the effector.

Thermotropic properties of phospholipid analogues

To understand the structural bases for the polymorphism of phospholipids, it is often essential to study the properties of "unnatural" phospholipid analogues with modified polar headgroups and or backbone structures. While the thermodynamic characteristics of the "classical" hydrated-gel-to-liquid-crystalline phase transition often appear surprisingly insensitive to these aspects of phospholipid structure, the rich and diverse solid-phase polymorphism of phospholipids is in fact exquisitely sensitive to the nature of both the polar headgroup and the backbone moieties. The tendencies of different phospholipids to form nonlamellar phases at higher temperatures also depend strongly (and in a sometimes surprising manner) on fine details of the headgroup and backbone structures. These points are illustrated by discussions of how the structures of headgroup- and backbone-modified phospholipid analogues influence their proclivities to form distinct types of hydrated solid phases, dehydrated "crystralline" phases and nonlamellar phases.