Phospholipids, liquids crystals and cell membranes

Evaluation of Hydrogenated Soybean Phosphatidylcholine Matrices Prepared by Hot Melt Extrusion for Oral Controlled Delivery of Water-Soluble Drugs

The aims of this study were to prepare hydrogenated soybean phosphatidylcholine (HSPC) matrices by hot melt extrusion and to evaluate resulting matrix potential to extend drug release in regard to drug loading and solubility for oral drug delivery of water-soluble drugs. The liquid crystalline nature of HSPC powder allowed its extrusion at 120°C, which was below its capillary melting point. Model drugs with a wide range of water solubilities (8, 20 and 240 mg/mL) and melting temperatures (160-270°C) were used. Extrudates with up to 70% drug loading were prepared at temperatures below the drugs' melting points. The original crystalline state of the drugs remained unchanged through the process as confirmed by XRPD and hot-stage microscopy. The time to achieve 80% release (t80) from extrudates with 50% drug loading was 3, 8 and 18 h for diprophylline, caffeine and theophylline, respectively. The effect of matrix preparation method (extrusion vs. compression) on drug release was evaluated. For non-eroding formulations, the drug release retarding properties of the HSPC matrix were mostly not influenced by the preparation method. However, with increasing drug loadings, compressed tablets eroded significantly more than extruded matrices, resulting in 2 to 11 times faster drug release. There were no signs of erosion observed in extrudates with different drugs up to 70% loadings. The mechanical robustness of HSPC extrudates was attributed to the formation of a skin-core structure and was identified as the main reason for the drug release controlling potential of the HSPC matrices produced by hot melt extrusion.

Phosphatidylcholine-fatty acid membranes: effects of headgroup hydration on the phase behaviour and structural parameters of the gel and inverse hexagonal (H(II)) phases

The phase behaviour and structural parameters of a homologous series of saturated diacyl phosphatidylcholine/fatty acid 1:2 (mol/mol) mixtures having chain lengths from C12 to C20 were studied by X-ray diffraction and calorimetry, as a function of water content. The chain-melting transition temperatures of the 1:2 PC/FA mixtures are found to be largely independent of the degree of hydration. For all chain lengths, the tilted L(beta') and rippled P(beta') gel phases of the pure PC component are replaced by an untilted L(beta) gel phase in the 1:2 PC/FA mixtures. This gel phase swells considerably upon hydration, with a limiting water layer thickness in the range 18-24 A, depending on the chain length. However, unlike pure phospholipid systems, the lateral chain packing within the gel phase bilayers is essentially identical in both the dry and the fully hydrated states. The fluid bilayer L(alpha) phase is suppressed in the 1:2 mixtures, being replaced by inverse non-lamellar phases for all chain lengths greater than C12, and at all levels of hydration. For chain lengths of C16 and greater, the inverse hexagonal H(II) phase is formed directly upon chain melting, at all water contents. For the shorter chain length mixtures, the behaviour is more complex, with the H(II) phase forming at low hydration, but with bicontinuous cubic phases appearing at higher levels of hydration. The implications of these surprising results are explored, in terms of the effective hydrophilicity of the associated PC and FA headgroups and the packing within the interfacial region. We suggest that the presence of the fatty acids significantly alters the lateral stress profile across the lipid monolayer in the fluid state, compared to that of the corresponding pure PC system, such that inverse phases, where the interface bends towards the water, become strongly favoured. Furthermore, for short chain lengths, packing constraints favour the formation of phases with negative interfacial Gaussian curvature, such as the bicontinuous cubic phases, rather than the H(II) phase, which has more severe chain packing frustration.

Nonlamellar phases formed by membrane lipids

A brief review of membrane lipids forming cubic and reversed hexagonal phases is presented. An emphasis is made on anionic lipids and particular microbial lipids.

Effect of head-group structure and counterion condensation on phase equilibria in anionic phospholipid-water systems studied by 2H, 23Na, and 31P NMR and X-ray diffraction

The phase equilibria, hydration, and sodium counterion association for the systems DOPA-2H2O, DOPS-2H2O, DOPG-2H2O, and DPG-2H2O were investigated with 2H, 23Na, and 31P NMR and X-ray diffraction. The following one-phase regions were found in the DOPA-water system: a reversed hexagonal liquid-crystalline (HII) phase up to about 35 wt % water and a lamellar liquid-crystalline (L alpha) phase between about 55 and 98 wt % water. The area per DOPA molecule was 36-65 A2 in the HII phase (10-40 wt % water) and 69 A2 in the L alpha phase (60 wt % water). DOPS and DOPG with 10-98 wt % water, and DPG with 20-95 wt % water formed an L alpha phase at temperatures between 25 and 55 degrees C. At temperatures above 55 degrees C, DPG with 20 and 30 wt % water formed a mixture of L alpha, HII, and cubic liquid-crystalline phases, the mole percent of lipid forming nonlamellar phases being smaller at 30 wt % water than at 20 wt % water. DPG with 10 wt % water probably formed a mixture of an L alpha phase and at least one nonlamellar liquid-crystalline phase at 25 and 35 degrees C, and a pure HII phase at 45 degrees C and higher temperatures. At water concentrations above about 50 wt % the 23Na quadrupole splitting was constant for all four lipid-water systems studied, implying that the counterion association to the charged lipid aggregates did not change upon dilution. These experimental observations can be described with an ion condensation model but not with a simple equilibrium model. The fraction of counterions located close to the lipid-water interface was calculated to be greater than 95%. The 2H and 23Na NMR quadrupole splittings of 2H2O and sodium counterions, respectively, indicate that the molecular order in the polar head-group region decreases for the L alpha phase in the order DOPA approximately DPG greater than DOPS greater than DOPG.

Phospholipids at the oil/water interface: adsorption and interfacial phenomena in an electric field

Interfacial effects produced in an immiscible liquid system by the action of an external electric field have been considered. The addition of small amounts of neutral phospholipids to the nonaqueous phase has been shown to result in a marked increase in the sensitivity of the interfacial boundary to the voltage applied, which is manifested by: (i) an accelerated decrease of the interfacial tension after the two immiscible liquid phases have been brought into contact; (ii) reduced interfacial tension, by 20-30 mN/m, at the oil/water interface at field strengths of 1-10 kV/m (the interfacial tension drop in the absence of phospholipids does not exceed 5 mN/m); (iii) development of electrohydrodynamic instability at the planar dividing surface between phases; and (iv) dispersion of water into the nonaqueous phase at smaller field strengths by a factor of about 100 as compared to those normally required in the absence of phospholipids. In order to gain a deeper insight into the mechanisms of interfacial phenomena, mainly exemplified by the n-heptane/water system containing phosphatidylcholine, three major issues have been considered: (1) Kinetics of the adsorption of phospholipid at the oil/water interface from the nonaqueous phase, and effects produced by exposure to an external electric field; also, the adsorption under equilibrium conditions, and the structure of the adsorption layer formed. (2) Interactions between neutral phospholipid and inorganic or organic ions at the interfacial boundary under the voltage applied. (3) Conditions for the occurrence of electrohydrodynamic instability at the dividing surface between oil and water and the formation of a water-in-oil emulsion; also aggregation and gelation processes induced in the nonaqueous phospholipid solution bulk by the action of a weak external electric field. Throughout the present paper, an attempt has been made to relate the microscopic behaviour of phospholipids under an external electric field to macroscopically observable properties at the movable interfacial boundaries. The adsorption studies have shown that phosphatidylcholine is prone to self-organization into a liquid-crystalline state at an immiscible liquid interface. The disintegration of the interfacial lipid film thus formed by the action of a weak electric field has been explained as due to an enhanced electrohydrodynamic instability of liquid crystals. This results in the formation of either an emulsion, or a microemulsion in the nonaqueous solution bulk. The formation of a microemulsion is manifested by the appearance of an optically anisotropic gel, stable only under an external applied electric field, in the nonaqueous solution bulk.(ABSTRACT TRUNCATED AT 400 WORDS)

Apolipoprotein, low density lipoprotein subfraction, and insulin associations with familial combined hyperlipidemia. Study of Utah patients with familial dyslipidemic hypertension

Familial dyslipidemic hypertension (FDH) is a syndrome recently described from sibships selected for early familial hypertension and found to have one or more of three fasting lipid abnormalities [high triglycerides, low high density lipoprotein (HDL) cholesterol, high low density lipoprotein (LDL) cholesterol]. In further analyses of these same 131 hypertensive subjects, apolipoprotein A-I and B, fasting plasma insulin (adjusted for body mass index), and detailed anthropometrics were different in two subgroups of FDH. Of 63 FDH patients, 19 met the criteria for familial combined hyperlipidemia (FCHL); 44 did not, but still had high triglyceride and/or low HDL cholesterol levels. When compared to 20 normolipidemic hypertensive patients, the 19 hypertensive patients with FCHL had 196% higher very low density lipoprotein cholesterol (p = 0.0001), 33% higher apolipoprotein B (p = 0.0002), smaller LDL particles (p = 0.007), and 73% higher fasting insulin (p = 0.003), but no significant differences in body mass index or skinfold thicknesses. The other 44 FDH patients without FCHL had 33% lower HDL (p = 0.0001), with only 8% lower apolipoprotein A-I levels (p = 0.20); significantly higher subscapular skinfolds (p = 0.02), weights (p = 0.002), body mass index (p = 0.006), knee widths (p = 0.0007), and wrist circumferences (p = 0.0009); smaller, denser LDL subfractions (p = 0.001); and increased apolipoprotein B levels (p = 0.01) compared to the normolipidemic hypertensive group. Increased fasting insulin levels were similar to the normolipidemic group and significantly lower than the FCHL group after adjustment for body mass index, suggesting a relationship between obesity and fasting insulin levels only in the non-FCHL group. We conclude that FDH consists of at least two subgroups: 1) FCHL with high apolipoprotein B, small LDL particles, and increased fasting plasma insulin levels, and 2) a less well-defined residual having upper central obesity with low HDL cholesterol and high triglyceride levels. Elevated insulin levels found in both groups, but possibly originating through different physiological mechanisms, may provide the pathophysiological connections between dyslipidemia, obesity, and hypertension.

Co-solubility in binary phospholipid crystals

Crystalline binary solid solutions of phosphatidylethanolamines are obtained when various fractions of compounds with different chain lengths are dissolved in chloroform and allowed to evaporate to dryness. Phase diagrams and electron diffraction measurements on chain mixtures with a difference of two or four methylene groups indicate that solubility is continuous, although non-ideal. Average molecular volume appears to increase according to Vegard's rule although deviations are noted. These deviations are similar to those observed for binary paraffin solids. Substitution of ether-links for ester-links in one component does not alter solubility behavior. In general the rules of solid solution formation appear to conform to those originally proposed by Kitaigorodskii [1961) Organic Chemical Crystallography, pp. 231-240, Consultants Bureau, New York).

Direct imaging of paracrystalline phospholipid structure in the electron microscope

A long chain amphiphilic molecule--the phospholipid 1,2-dihexadecyl sn glycerophosphoethanolamine--has been crystallized epitaxially so that the interlamellar molecular periodicity is parallel to the substrate and hence normal to the electron beam in the electron microscope. This has permitted the direct resolution of the 55.6 A lamellae in unstained crystals at room temperature. The lattice images have shown the presence of line dislocations and lenticular cracks in the crystals. Of significance to their biological properties is that the lattice is undulating with a periodicity of 0.1-0.5 micron. This would also account for the difficulties encountered by X-ray and electron diffraction techniques when examining these crystals.

Lipid polymorphism

In this review the polymorphic phase behaviour of several of the major classes of lipids found in biological membranes, both in isolation and also in mixtures, is briefly described. Emphasis is given to the ability of many membrane lipids to adopt non-lamellar phases in response to a variety of factors such as temperature, the presence of divalent cations or changes in pH. The phase behaviour of mixed lipid systems and factors which can modulate the phase preferences of such systems are considered in some detail particularly with regard to the effect of cholesterol upon lipid polymorphism.

Associations of three erythrocyte cation transport systems with plasma lipids in Utah subjects

To investigate the pathophysiology of essential hypertension, detailed biochemical and clinical variables were collected and analyzed for 2091 Utah subjects aged 3 to 83 years. Three different measurements of erythrocyte cation transport were obtained: Na+-Li+ countertransport, Li+-K+ cotransport, and furosemide-insensitive Li+ efflux into MgCl2. Total plasma cholesterol, triglycerides, and high density lipoprotein cholesterol levels were obtained from fasting subjects. Levels of high density lipoprotein subfractions 2 and 3 were also obtained from 350 subjects. Standardized data collection also included blood pressure, height, weight, and presence or absence of a diagnosis or treatment of essential hypertension. In univariate analyses of all 1420 adults, each of the three transport systems showed the same significant correlations with triglyceride levels (r = 0.33-0.35, p less than 0.0001), high density lipoprotein concentration (r = -0.19 to -0.21, p less than 0.001), and weight (r = 0.22-0.28, p less than 0.0001). In multivariate regression analyses, values for each transport system were significantly higher in hypertensive subjects; values for triglycerides, high density lipoprotein, and usually, the high density lipoprotein subfractions continued to have strong significant independent associations with all three transport systems; and weight remained significantly related only to Na+-Li+ countertransport. In separate logistic regressions, plasma triglyceride levels (positively, p less than 0.001) and high density lipoprotein subfraction 3 levels (inversely, p less than 0.03) were associated with hypertension itself. In multivariate analyses among 671 children, high density lipoprotein and high density lipoprotein subfraction 3 levels showed significant (p less than 0.05) inverse correlations with Na+-Li+ countertransport and furosemide-insensitive Li+ efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of phospholipid monolayers with carbohydrates

Surface pressure studies of phospholipid monomolecular films of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) formed at an air/water interface have been made and the effects on the films studied when various carbohydrates are present in the subphase. The results obtained show that at a given temperature, the area per molecule of DPPC increases with increasing concentration of the carbohydrate in the subphase. The carbohydrate which has the greatest expanding effect on the phospholipid monolayer is glycerol, followed in turn by trehalose, sucrose, glucose, raffinose, and inositol. The mechanism of monolayer expansion by glycerol is different from that observed in other carbohydrates, as the following experiments demonstrate. Below the phase transition temperature of DPPC, the area per molecule of DPPC at a pressure of 12.5 dyn/cm is the same with and without glycerol in the subphase. However, when the monolayer is heated to a temperature above the phase transition temperature for DPPC, the area/molecule on glycerol is considerably greater than the area/molecule on water at the same surface pressure. Cooling the monolayer back to the lower temperature produces an area/molecule of DPPC which is identical on both water and glycerol subphases. Glycerol therefore has no effect on the low-temperature (condensed) monolayers but causes expansion of the high-temperature (expanded) monolayers. By contrast with glycerol, both trehalose and sucrose interact with the DPPC monolayer producing an increased area/molecule over that observed on water, both with low-temperature (condensed) monolayers and with the high-temperature (expanded) monolayers. The efficiency of these carbohydrates at expanding the monolayer films (with the exception of glycerol) shows a strong correlation with their ability to stabilize membrane structure and function at low water contents.

Epitaxial crystallization of alkane chain lipids for electron diffraction analysis

Thin microcrystals of a wide variety of polymethylene chain materials, including n-alkanes, linear waxes, glycerides, a detergent, phospholipids and phospholipid analogs based on cyclopentane-1,2,3-triol, are epitaxially grown on naphthalene to give an orientation with long chain axes parallel to the best developed crystal face. These crystals, which represent a different orientation than those grown from solution, facilitate ab initio quantitative crystal structure analysis from electron diffraction intensity data from the projection yielding the most crystallographic information.

A comparison of the headgroup conformation and dynamics in synthetic analogs of dipalmitoylphosphatidylcholine

14N-NMR spectra and relaxation times for dipalmitoylphosphatidylcholine and three analogs were obtained in both the liquid crystal and gel phases. The analogs either changed the PO-4 to N+ (CH3)3 distance (P-N) within the headgroup by increasing the number of CH2 groups from two in the phosphocholine headgroup (PN-2) to six in the phospho-(N',N',N'-trimethyl)hexanolamine headgroup (PN-6), or replaced the ester linkages to the hydrocarbon chains with either linkages. 31P-NMR spectra were obtained for the four samples in the liquid-crystal phase. (1) The 14N- and 31P-NMR spectra and 14N relaxation times all indicate that increasing the P-N distance within the headgroup causes changes in both the average orientation of the C-N bond and its dynamics. (2) The 14N-NMR spectra provide evidence for a change in orientational order of the headgroup as a result of changing the linkage to the acyl chains. On the other hand, the relaxation time measurements indicate that the molecular motion for the headgroup is independent of the type of linkage. (3) The thermal behaviour of the four samples is clearly reflected in the 14N-NMR spectra. The second moments of the spectra show distinct changes at each of the phase transitions. (4) The 14N-NMR spectra show that the average conformation of the headgroups is not significantly altered by the main phase transition. For the PN-2 samples, T2e, the decay of the quadrupolar echo, decreases discontinuously in the P beta, phase, which is evidence for a possible exchange process between two molecular states within this phase.

Effect of succinylphosphatidylcholine on phosphatidylcholine vesicles: structural studies by gel chromatography, electron microscopy and nuclear magnetic resonance

The effect of an aqueous dispersion of succinylphosphatidylcholine on an aqueous suspension of phosphatidylcholine vesicles was studied by gel chromatography, freeze-fracture electron microscopy and proton nuclear magnetic resonance with Mn2+ (broadening paramagnetic reagent). Total phospholipid concentrations were in the range 10--20 mM. Succinylphosphatidylcholine is in micellar form and behaves as a detergent. The structures obtained depend on the molar percentage of succinylphosphatidylcholine. Above a succinylphosphatidylcholine molar percentage of 60%, mixed micelles are formed, assumed to be essentially spherical. Below a succinylphosphatidylcholine molar percentage of 30%, principally mixed vesicles are observed, with an external diameter of 215--240 A, and an almost constant internal volume. Between 30 and 60% of succinlyphosphatidylcholine, a mixture of these structures is obtained; rod-shaped profiles are also observed in electron microscopy, which may correspond to sections of leaky vesicles or to a new kind of cylindrical micelle.

The dynamics of membrane structure

The membranes of living organisms are involved in many aspects of the life, growth and development of all cells. The predominant structural elements of these membranes are lipids and proteins and the basic strucvture of these molecules has been reviewed. The physical properties of the lipid constituents particularly their behavior in aqueous systems has led to the concepts of thermotropic and lyotropic mesomorphism; the interaction between different types of lipid molecules modulate this behavior. Interaction of phospholipids in aqueous systems with cholesterol, ions and drugs have been examined in this context. In addition a variety of model lipid-protein systems have been investigated and the implications of interactions between lipids and different proteins in biological membranes has been evaluated. This leads to a detailed consideration of the way lipids and proteins ae organized in cell membranes and contains an appraisal of the evidence supporting contemporary views of membrane structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Particular attention has been devoted to the question of how mobile the components are within the structure. Finally the biosynthesis, turnover and modulation of the properties of interacting membrane constituents is critically reviewed and possible ways of controlling the behavior of cells and organisms by altering the structural parameters of different membranes has been considered.

Lipid abnormalities in hereditary neuropathy. Part 2. Serum phospholipids

The mean percentage of linoleate to total fatty acids in phosphatidylcholine and lysophosphatidylcholine fractions of serum phospholipids from neuropathic patients with HMN (hereditary motor neuropathy, also called distal type of progressive muscular atrophy), HMSN-I and HMSN-II (two types of peroneal muscular atrophy), and FA (Friedreich's ataxia) was reduced by approximately 10--20% (P less than 0.001). On the other hand, the mean percentage of nervonic acid in sphingomyelin was elevated by 9--20%. No significant difference was observed in phosphatidylethanolamine between neuropathic patients and control subjects. The mean concentration of phosphatidylcholine and sphingomyelin was also significantly reduced in neuropathic patients (except in HMN and HMSN-III). A significant correlation between endogenous 2-linoleoyl-sn-glycerol-3-phosphocholine and cholesteryl linoleate synthesis in vitro suggests that the decreased activity of phosphatidylcholine acyltransferase (EC 2.3.1.43; LCAT) in neuropathic patients is influenced by the fatty acid composition of their lipoprotein substrate. Furthermore, the reduction of phosphatidylcholine and of cholesteryl linoleate synthesis in vitro in neuropathic patients was affected by age and sex. It is unlikely that the reduced linoleate level in serum phosphatidylcholine for most, possibly all, of the inherited neuropathies studied here reflects a specific biochemical disorder. Possibly it reflects a more generalized biochemical alteration common to inherited neuropathy. One possibility is that biosynthesis of new membrane in axonal regeneration, segmental remyelination and Schwann cell hyperplasia may reduce the serum linoleate pool.

Capacitance studied of syntheric phospholipid Langmuir films

Synthetic phosphatidylcholine Langmuir films have been incorporated into metal-insulator-metal (MIM) thin film junctions. The capacitance characteristics of these junctions have been studied as a function of temperature, the number of lipid layers in the insulating layer, and the length of the hydrocarbon chains of the lipid molecule. The thickness of the oxide layer on the base aluminum electrode has been determined to be larger than or equal to 11 A, and its effects on the capacitance characteristics have been considered in some detail. Indications of phase transitions in the temperature dependence of the capacitance imply that the basic lemellar arrangement of the lipid molecules is retained even after the samples are subjected to a dehydrating vacuum annealing process. An examination of the effects of varying the hydrocarbon chain length and salt content of the subphase during sample fabrication showed that capacitance characteristics of the MIM junction are very sensitive to small structural changes in the insulating layer.

The metabolism of the phosphonium analogue of choline in vitro and in vivo, and its detection in phospholipids by 31P-NMR

1. The phosphonium analogues of choline, phosphorylcholine, CDPcholine and phosphatidylcholine were synthesized chemically and characterized by 1H-NMR and 31P-NMR; in 1,2-distearoyl-DL-glycero-3-phosphorylphosphocholine, the 31P-NMR chemical shift of phosphonium relative to phosphate was--28.2 ppm. 2. A comparison was made of the rates of reaction of choline kinase, cholinephosphate cytidyltransferase, cholinephosphotransferase and phospholipase C on natural and phosphonium substrates. Enzyme reaction rates were similar for all but the cytidyltransferase, which exhibited a 3-fold preference for the normal substrate. 3. Weanling rats were maintained for 6 weeks on a diet in which choline was fully replaced by phospho[1,2-14C2]choline mixed with a trace of [Me-3H] choline. Incorporation of phosphocholine into liver lipids was detectable by 31P-NMR even in crude tissue homogenates. Choline-based phospholipids of liver, kidney, lung and brain were extracted, and phosphocholine incorporation calculated from 31P-NMR peak area ratios. The phosphatidylcholine analogues were separated by preparative thin-layer chromatography. Incorporation of phosphocholine ranged from 33% in lung phosphatidylcholine to 6% in kidney sphingomyelin. Variations in 14C/3H ratio between feed and phospholipid extracts indicated preferences for exogenous choline over phosphocholine varying from 1.3: 1 in brain to 3.2: 1 in liver. The results indicated that phosphocholine is a potentially useful 31P-NMR probe for the study of membrane lipids.