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PubMed1990

Mixing behavior of binary insoluble phospholipid monolayers. Analysis of the mixing properties of binary lecithin and cephalin systems by application of several surface and spreading techniques.

The detailed miscibility analysis of binary phospholipid monolayers requires the application of a variety of spreading and surface techniques which often yield complementary results. Testing the equilibrium state of the binary monolayer by long-time experiments is also of great importance. Studies of the compression and spreading behavior of binary monomolecular systems form a basis for the determination of binary monomolecular phase diagrams. Within these plots different phase regions occur which permit clear statements regarding the miscibility state. Additional knowledge of the miscibility properties (phase diagrams) of the binary bulk systems is required. From the analogy of the properties of the bulk systems, the miscibility state of the monolayers is also determined by the temperature, and we can classify the monolayers of binary lecithin and cephalin systems into systems of complete miscibility, partial miscibility and complete immiscibility. In addition to the differences in the chemical structure of the mixing components, the film states in the monolayer and the miscibility behavior of the bulk systems are also influencing factors. If one of the components does not produce a spreading pressure, miscibility gaps occur in the phase diagram of the phospholipid monolayer. The miscibility gap, expressed by a constant spreading pressure, indicates complete immiscibility within this concentration range. If both components produce spreading pressures, and condensed and liquid-expanded film states within the considered temperature range, partial miscibility of the components becomes probable. The most effective parameter is then the difference in the chemical structure of the components. When both components produce spreading pressures and condensed films, the chemical structure of the mixing phospholipid compounds within their hydrophilic and hydrophobic parts is of essential importance. Depending on the differences in the chemical structures of their chains and their head groups in the case of binary phospholipid monolayers, the following possibilities result: complete miscibility, partial miscibility and complete immiscibility of the lecithins and the cephalins. Complete miscibility within the binary phospholipid monolayer takes place in the case of identical head-group structure and where there are only small differences in the chain length of the fatty acid groups. With increasing hydrocarbon chain length differences, partial miscibility or even complete immiscibility can occur within the monolayer. Chemical differences in the head-group structure of the mixing components have a similar influence. In the case of binary lecithin/cephalin mixtures, the differences in the head-group structure affect the miscibility behavior more than the chain length differences do in the case of lecithin/lecithin and cephalin/cephalin mixtures.(ABSTRACT TRUNCATED AT 400 WORDS)

Liposomes↗
PubMed1988

Microviscosity in lecithin liposomes: effect of nicotinic acid.

We have studied the effect of nicotinic acid, a drug commonly used as a vasodilatory agent and also for the treatment of hypercholesterolemia, on the fluidity profile of liposomes of egg lecithin and dipalmitoyl lecithin, using a fluorescent polarization probe. In both cases the drug decreases the membrane fluidity and for cholesterol-probed liposomes, it disrupts the "intermediate fluid condition" induced by cholesterol. The drug also affects the activation energy for diffusion in the hydrophobic region of the liposomes.

Algorithms↗
PubMed1975

Fluorescence quenching in lecithin and lecithin/cholesterol liposomes by parmagenetic lipid analogues. Introduction of a new probe approach.

1. Perylene, whether incorporated into lecithin or lecithin/cholesterol (1:1) liposomes, exhibits identical fluorescence spectra, but fluorescence in the presence of cholesterol is enhanced by 30-50%. 2. The fluorescence of perylene in pure dipalmitoyllecithin vesicles increases sharply at the transition temperature (Tt equals 41 degrees C). No such fluorescence jump is observed in lecithin/cholesterol (1:1) micelles. 3. In lecithin liposomes maximal quenching of perylene fluorescence at 25 degrees C is effected by cholestane spin label (80%) followed by androstane spin label (70%), 5-nitroxide stearate (60%) and 16-nitroxide stearate (50%). 4. In liposomes containing 5 mol % cholesterol these differences are reduced; however, the sequence of quenching efficiencies is the same except for the nitroxide stearates, which interchange their positions. 5. 5. Paramagnetic quenching of perylene fluorescence is stable below 35 degrees C and above 45 degrees C, but decreases sharply about the phase-transition temperature of dipalmitoyllecithin. 6. In lecithin/cholesterol (1:1, molar ratio) lipsomes fluorescence quenching diminishes linearly, but only slightly, with increasing temperature. 7. Cholestane spin label and androstane spin label at concentrations of greater than 20 mol % themselves suppress the quenching discontinuity at Tt, indicating a cholesterol-like structural effect. 8. The quenching phenomena observed are attributed to a non-random accommodation of fluorophore and quencher molecules (co-clustering) below the phase transition and a statistical distribution of both impurities above Tt. 9. In the presence of cholesterol the clustering tendencies are reduced or even eliminated; this is compatible with the concept that cholesterol fluidizes the phosphatide acyl chains below the transtion temperature.

Androstanes↗
PubMed1975

The permeation of organic acids through lecithin bilayers. Resemblance to diffusion in polymers.

1. The fluxes of aliphatic acids and their derivatives through black lipid membranes made of egg lecithin in decane were measured by means of a proton titration method. 2. Permeability coefficients were calculated and these were divided by the partition coefficient of the diffusing solute in different solvent systems: n-decane, olive oil, ether and octanol. The logarithms of the diffusion coefficients thus obtained were plotted against the logarithm of the molecular weight. The data could not be fitted to a single regression line in any solvent system. 3. When the logarithm of the diffusion coefficients were correlated to the logarithm of the molecular volume (equals molecular weight/ specific gravity) all the diffusants could be fitted to the same regression line, indicating that the molecular volume is a better index of molecular size and shape than the molecular weight. 4. Analysis of the experimental results assuming a model of diffusion through soft polymers (Lieb, W.R. and Stein, W. D. (1971) Current Topics in Membranes and Transport, vol. 2, pp. 1-39, Academic Press, New York) showed that decane and olive oil are not adequate model solvents for planar lecithin membranes but ether and octanol are good models. 5. The differential mass selectivity coefficient was found to be similar to that for soft polymers and biological membranes, i.e. greater than 3.0. 6. Water could be fitted by the same regression line, thus emphasizing the generality of passive transfer and implying that water crosses lipid membranes as single molecules.

Acids↗
PubMed1977

Studies on spin-labelled egg lecithin dispersions.

ESR spectra of egg lecithin dispersions labelled with 5-nitroxide stearic acid are recorded with a 50 G field sweep, and also with a new technique which "expands" the spectrum by (1) recording pairs of adjoining peaks with a smaller field sweep and (2) superposing the common peaks. The expansion technique improves the precision of the order parameters determined from the hyperfine splitting measurements, and may prove useful in future spin label membrane studies. Approximate order parameters are derived to describe the fluidity of fatty acid spin-labelled membranes in those cases where either the inner or outer hyperfine extrema are not well defined. The ability of these expressions to measure the fluidity of labelled egg lecithin disperions for the temperature range 14-42 degrees C is examined.

Egg Yolk↗
PubMed1975

Effect of plasma lipoproteins and lecithin-cholesterol dispersions on the activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase of isolated rat hepatocytes.

Incubation of rat hepatocytes for 3 h in a medium containing amino acids, salts and albumin resulted in a 2-fold increase in the activity of 3-hydroxy-3-methylglutaryl-CoA reductase. Inclusion of 10% human plasma, rat serum or dialysed rat serum in the medium resulted in an approximate 7-fold increase in reductase activity. These increases were specific since there was little change in the rate of fatty acid synthesis or in the activity of tyrosine aminotransferase. Reductase levels were increased above control values when high density lipoproteins or leithin dispersions were added to the cells. Lecithin dispersions were also shown to increase the rate of efflux of cell cholesterol to the medium. In contrast, reductase levels were reduced when cells were incubated with low density lipoproteins or cholesterol added as an equimolar cholesterol-lecithin dispersion. This inhibition of the reductase by cholesterol dispersions was dependent on the continued de novo protein synthesis. Our data indicate that in normal rat hepatocytes the relative rates of efflux and influx of cholesterol may be critical to the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase activity and cholesterogenesis.

Acetates↗
PubMed1982

Action of lecithin:cholesterol acyltransferase on model lipoproteins. Preparation and characterization of model nascent high density lipoprotein.

Apolipoprotein A-I, the major protein of human plasma high density lipoprotein, is the primary activator of plasma lecithin:cholesterol acyltransferase. In vitro, the association of apolipoprotein A-I with physiological phosphatidylcholines can be catalyzed by mixing the protein and lipid with sodium cholate, which is removed by chromatography. The apolipoprotein A-I/phospholipid complex has the physical properties of an HDL, and when cholesterol is present the complex is a highly reactive substrate in the lecithin:cholesterol acyltransferase-catalyzed reaction. The relative reactivity of this complex compared with a number of other lipid-protein complexes is presented and discussed.

Apolipoprotein A-I↗
PubMed1982

Dependence of human plasma lecithin:cholesterol acyltransferase activity upon high-density lipoproteins.

The conclusion that HDL2 suppress lecithin:cholesterol acyltransferase activity has been re-examined using three assay methods. Removal by ultracentrifugation of lipoproteins of d 1.125 g/ml reduced activity to 53% (n = 15) of control plasma. Replacement of VLDL and LDL was without effect, but addition of HDL2 restored activity. Addition of HDL2 to d 1.125 g/ml infranate in quantities greater than had been present in original plasma had no further effect on activity, and addition of HDL2 to plasma had no effect on activity. It is concluded that HDL2 are required for full expression of plasma lecithin:cholesterol acyltransferase activity.

Dithionitrobenzoic Acid↗
PubMed1983

A method for isolating human plasma lecithin:cholesterol acyltransferase without using anti-apolipoprotein D, and its characterization.

A method for isolating human plasma lecithin:cholesterol acyltransferase (EC 2.3.1.43) purified more than 50 000-fold is described. The crude enzyme obtained by initial ammonium sulfate and citric acid treatment of 21 of human plasma is subjected to repeated DEAE-cellulose chromatography to yield a preparation purified more than 600-fold. Hydroxyapatite chromatography of concentrates from this fraction using 0.5 mM phosphate buffer, pH 6.8, yields enzyme preparations purified more than 50 000-fold. The enzyme isolated by this procedure was free of apolipoprotein D, as shown by the absence of an arc in immunodiffusion with anti-apolipoprotein D. The enzyme showed a single band by polyacrylamide gel electrophoresis in the presence and absence of SDS. Upon analytical isoelectrofocusing the enzyme separated into three iso forms with isoelectric points below that of egg albumin (pI 4.6). The enzyme was characterized by a high content of glutamic acid, leucine and glycine, and a lower content of tyrosine. The enzyme possessed both transferase and phospholipase A2 activities and both activities show absolute requirement for apolipoprotein A-I. The purified enzyme was injected into Balb/c mice and the antiserum reacted both with the purified enzyme and normal human serum in immunodiffusion, giving lines of complete identity. The antiserum gave no precipitation lines with albumin or apolipoprotein D, providing additional evidence for the absence of apolipoprotein D in the purified enzyme. The gamma-globulin isolated from the antiserum inhibited human lecithin:cholesterol acyltransferase activity.

Animals↗
PubMed1984

Reaction of lecithin: cholesterol acyltransferase with micellar substrates. Effect of particle sizes.

Micellar, discoidal complexes were prepared from L-alpha-dipalmitoylphosphatidylcholine (DPPC) or egg phosphatidylcholine (egg-PC), cholesterol, and human apolipoprotein A-I by the cholate dialysis method. Reaction mixtures containing from 70:7:1 to 500:50:1, PC/cholesterol/apolipoprotein A-I (mol/mol) were fractionated by gel-filtration into various complex fractions. The isolated DPPC complexes ranged in size from 103 to 380 A in diameter, and in composition from 70:7:1 to 470:45:1, PC/cholesterol/apolipoprotein A-I (mol/mol), respectively. In contrast, the isolated egg-PC complexes only ranged in size from 105 to 214 A in diameter, and in composition from 65:5:1 to 153:17:1, PC/cholesterol/apolipoprotein A-I (mol/mol), respectively. Measurements of fluorescence wavelength maxima and fluorescence polarization of tryptophan residues of apolipoprotein A-I, in both series of complexes, revealed uniform spectral properties for all the egg-PC containing complexes. The DPPC complexes, on the other hand, had maxima in the fluorescence parameters for complexes with diameters around 200 A. When reacted with purified human lecithin:cholesterol acyltransferase, either at constant apolipoprotein A-I or at constant lipid concentration, all egg-PC complexes had very similar reaction rates, but the DPPC complex series exhibited major differences in reactivity. Minima in reaction rates occurred for DPPC complexes around 200 A in diameter, and optimal rates were observed with the small discoidal complexes (110 A in diameter). These reaction rates correlate well with the apolipoprotein A-I fluorescence properties and indicate that the apolipoprotein structure, reflected at the interface with phosphatidylcholine, may be the most important factor in determining complex reactivity with lecithin:cholesterol acyltransferase.

Apolipoprotein A-I↗
PubMed1984

Reduction of lecithin-cholesterol acyltransferase, apolipoprotein D and the Lp(a) lipoprotein with the anabolic steroid stanozolol.

The effects of the anabolic steroid stanozolol (17-methyl-2H-5 alpha-androst-2-eno-(3,2-c)pyrazol-17 beta-ol) on lecithin-cholesterol acyltransferase, apolipoproteins B and D and the Lp(a) lipoprotein were determined in a prospective study of ten normolipidemic women with postmenopausal osteoporosis. Lecithin-cholesterol acyltransferase was reduced approx. 30% by 6 weeks of treatment with stanozolol (off treatment 5.1 +/- 1.2, on treatment 3.4 +/- 0.8 muml; P less than 0.02). The Lp(a) lipoprotein was reduced 65 +/- 23% by the steroid treatment (off treatment 5.5 +/- 5.5, on treatment 1.4 +/- 0.7 mg/dl; P less than 0.02). Apolipoprotein D was reduced 23 +/- 9% by the treatment (off treatment 5.9 +/- 0.9, on treatment 4.5 +/- 0.7 mg/dl; P less than 0.02). In contrast, apolipoprotein B increased slightly but insignificantly on steroid therapy (off treatment 90 +/- 21, on treatment 112 +/- 24 mg/dl). By 5 weeks after the drug was discontinued, all four of these proteins were near pretreatment levels. These significant changes in lipoprotein metabolism, combined with our previous report of reductions of HDL and particularly HDL2, suggest the need for caution in the long-term use of anabolic steroids.

Aged↗
PubMed1986

Increases in the particle size of high-density lipoproteins induced by purified lecithin: cholesterol acyltransferase: effect of low-density lipoproteins.

Homogeneous subpopulations of human high-density lipoproteins subfraction-3 (HDL3) have been incubated at 37 degrees C with purified lecithin: cholesterol acyltransferase, human serum albumin and varying concentrations of human low-density lipoproteins (LDL). Changes in HDL particle size and composition during these incubations were monitored. Incubation of HDL3a (particle radius 4.3 nm) in the absence of LDL resulted in an esterification of more than 70% of the HDL free cholesterol after 24 h of incubation. This, however, was sufficient to increase the HDL cholesteryl ester by less than 10% and was not accompanied by any change in particle size. When this mixture was incubated in the presence of progressively increasing concentrations of LDL, which donated free cholesterol to the HDL, the molar rate of production of cholesteryl ester was much greater; at the highest LDL concentration HDL cholesteryl ester content was almost doubled after 24 h and there was an increase in the HDL particle size up to the HDL2 range. In the case of HDL3b (radius 3.9 nm), there were again only minimal changes in particle size in incubations not containing LDL. In the presence of the highest concentration of LDL tested, however, the particles were again enlarged into the HDL2 size range after 24 h incubation. These HDL2-like particles were markedly enriched with cholesteryl ester but depleted of phospholipid and free cholesterol when compared with native HDL2. Furthermore, the ratio of apolipoprotein A-I to apolipoprotein A-II resembled that in the parent-HDL3 and was very much lower than that in native HDL2. It has been concluded that purified lecithin: cholesterol acyltransferase is capable of increasing the size of HDL3 towards that of HDL2 but that other factors must operate in vivo to modulate the chemical composition of the enlarged particles.

Adult↗
PubMed1988

Transformation of large discoidal complexes of apolipoprotein A-I and phosphatidylcholine by lecithin-cholesterol acyltransferase.

Using a cholate-dialysis recombination procedure, complexes of apolipoprotein A-I and synthetic phosphatidylcholine (1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or dioleoylphosphatidylcholine (DOPC] were prepared in mixtures at a relatively high molar ratio of 150:1 phosphatidylcholine/apolipoprotein A-I. Particle size distribution analysis by gradient gel electrophoresis of the recombinant mixtures indicated the presence of a series of discrete complexes that included species migrating at RF values observed for discoidal particles in nascent high-density lipoproteins (HDL) in plasma of lecithin-cholesterol acyltransferase-deficient subjects. One of these complex species, designated complex class 6, formed with either phosphatidylcholine, was isolated by gel filtration and characterized at follows: discoidal shape (mean diameter 20.8 nm (POPC) and 19.0 nm (DOPC]; molar ratio, phosphatidylcholine/apolipoprotein A-I, 155:1 (POPC) and 130:1 (DOPC); and both containing 4 molecules of apolipoprotein A-I per particle. Incubation of class 6 complexes with lecithin-cholesterol acyltransferase (EC 2.3.1.43) and a source of unesterified cholesterol (low-density lipoprotein (LDL] was shown by electron microscopy to result in a progressive transformation of the discoidal particles (0 h) to deformable (2.5 h) and to spherical particles (24 h). The spherical particles (diameter 13.6 nm (POPC) and 12.5 nm (DOPC) exhibit sizes at the upper boundary of the interval defining the human plasma (HDL2b)gge (12.9-9.8 nm). The spherical particles contain a cholesteryl ester core that reaches a limiting molar ratio of approx. 50-55:1 cholesteryl ester/apolipoprotein A-I. The deformable particles assume a rectangular shape under negative staining and, relative to the 24-h spherical product, are enriched in phosphatidylcholine. Chemical crosslinking (by dimethyl suberimidate) of the isolated transformation products shows the 24-h spherical particle to contain predominantly 4 apolipoprotein A-I molecules; products produced after intermediate periods of time appear to contain species with 3 and 4 apolipoproteins per particle. Our in vitro studies indicate a potential pathway in the origins of large, apolipoprotein A-I-containing plasma HDL particles. The deformable species observed during transformation were similar in size and shape to particles observed in interstitial fluid.

Apolipoprotein A-I↗
PubMed1991

Properties of phosphatidylethanolamine-containing phospholipid-apolipoprotein complexes modified by lecithin-cholesterol acyltransferase.

The effect of the inclusion of phosphatidylethanolamine (PE), a phospholipid with unusual packing properties, on the substrate properties of protein-lipid complexes toward lecithin-cholesterol acyltransferase (LCAT) has been studied. Recombinant particles of apolipoprotein A-I with dimyristoylphosphatidylcholine (DMPC), dilauroylphosphatidylethanolamine (DLPE) and cholesterol were prepared at a molar ratio of 1:140:14 (A-I/DMPC/cholesterol) or 1:70:70:14 (A-I/DMPC/DLPE/cholesterol); the efficiency of cholesterol incorporation into complexes containing phosphatidylethanolamine was found to be very pH-dependent, with enhanced cholesterol incorporation at elevated pH values. By incubating the complexes with either purified human LCAT or the d greater than 1.21 g/ml fraction of rat serum as a source of LCAT activity, it was found that a high degree of cholesterol esterification could be achieved with either complex; however, the DLPE-containing complex possessed a much smaller Stokes' diameter than the DMPC-only particle despite compositional similarities between these complexes. With respect to particle diameter the DLPE-containing particles behaved more like complexes prepared with egg yolk lecithin than did complexes prepared with DMPC alone. When human LDL was added to the incubations to provide a source of additional cholesterol, the products were markedly different. Concomitant with an increased cholesteryl ester core was an increase in the protein stoichiometry in both types of particles, from 2 to 3 or 4 apo A-I per particle. The proportion of DLPE to DMPC in the products was reduced from 1:1 to 0.3:1, reflecting a preferential hydrolysis of PE by LCAT, and the Stokes' diameters of the DMPC-only and the DLPE-containing complexes were closely similar. We conclude that the presence of elevated proportions of certain phospholipid species may significantly alter both the physical properties of the particles and their substrate properties with regard to reactions with enzymes of lipid metabolism.

Apolipoproteins↗
PubMed1996

Formation of pregnenolone- and dehydroepiandrosterone-fatty acid esters by lecithin-cholesterol acyltransferase in human plasma high density lipoproteins.

Pregnenolone- (PREG-), and dehydroepiandrosterone- (DHEA-) fatty acid esters (FA) are present in human plasma, where they are associated with lipoproteins. Because plasma has the ability to form PREG-FA and DHEA-FA in vitro from their unconjugated steroid counterparts, we postulated that the LCAT enzyme might be responsible for their formation. Here we show that lecithin-cholesterol acyltransferase (LCAT) has PREG and DHEA esterifying activities. First, VLDL, IDL, LDL, and HDL were isolated by the sequential ultracentrifugation micromethod from the plasma of fasting men and women and tested for their ability to form PREG-FA, DHEA-FA, and cholesteryl esters in vitro from their respective unconjugated counterparts. The results showed that the three steroids were esterified only in HDL subfractions. The rate of tritiated PREG esterification was clearly higher than that of tritiated cholesterol and DHEA, both in total plasma and isolated HDL, and no gender difference was observed. Second, human and guinea pig LCAT were purified and used in phosphatidylcholine-reconstituted vesicles containing human apoAI to show their ability to esterify tritiated cholesterol, PREG, and DHEA in the absence of unlabeled steroid. The amount of cholesteryl ester, PREG-FA, and DHEA-FA increased after incubation as a function of time and amount of purified LCAT, showing that PREG is preferentially acylated by LCAT compared to cholesterol and DHEA. The PREG and DHEA esterifying activities of LCAT were cofactor-dependent, as shown by the absence of acylation without apoAI. Finally, we determined by HPLC the fatty acid moiety of PREG-FA and DHEA-FA formed in human plasma and guinea pig and rat sera in vitro after incubation with unconjugated tritiated PREG and DHEA. We showed that the fatty acid moieties of newly formed tritiated PREG-FA and DHEA-FA were similar to that reported for cholesteryl esters in the plasma of the three species. We conclude that LCAT has a lecithin-steroid acyltransferase activity and that PREG is probably the preferential substrate of this enzyme. In addition, the fact that the differences in the fatty acid moieties of cholesteryl esters of human, guinea pig, and rat plasmas are also observed for PREG-FA and DHEA-FA suggests that the LCAT is the sole circulating enzyme that has PREG and DHEA esterifying activities.

Adult↗
PubMed1985

Interaction of morphine and naloxone with mixed monolayers of lecithin and gangliosides.

Monomolecular films of lecithin, gangliosides or lecithin/gangliosides mixtures were studied on a Langmuir through in order to examine the interactions between these lipids and opioid agonists or antagonists. Lecithin alone did not interact in a monolayer structure with opioids. However, gangliosides and lecithin/gangliosides mixtures were expanded by both morphine and naloxone. The expansion of ganglioside-containing monolayers was greater with morphine than with the antagonist, naloxone.

Chemical Phenomena↗
PubMed1990

Dipalmitoylphosphatidylcholine (L-alpha-lecithin) stimulates phospholipase A2 activity in human amnion.

In order to investigate the mechanism of dipalmitoylphosphatidylcholine (DPPC, L-alpha-lecithin) stimulation of the prostaglandin E (PGE) production of the amniotic membrane, effects of DPPC (50-800 micrograms/ml) on phospholipase A2 (PLA2), phospholipase C (PLC), PG endoperoxide synthase, and PGE synthase activities of human amniotic membrane were studied. Only PLA2 activity was increased by DPPC, suggesting that lecithin, the major surfactant component, increases the PGE production of the amniotic membrane by activating PLA2.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed1991

Lecithin organogels as matrix for the transdermal transport of drugs.

The stable, transparent, thermoreversible organogels which are prepared by adding a minute amount of water to a solution of lecithin in isopropylpalmitate are studied as matrices for promoting the transdermal transport of drugs throughout the skin. In this work we study this process by using two model drug substances, scopolamine (already well known as a transdermally active substance) and broxaterol, a new drug against asthma. It is found that the transport of scopolamine is much more effective in gel than in aqueous solution and that the transport of broxaterol is also proceeding well. Since the same can be observed with a variety of substances differing in the chemical structure, we speculate on the application of lecithin gels as general matrices for the transdermal transport.

Adenocarcinoma↗