Biomedical subjects

J M Smaby

Publications and source records attributed to J M Smaby.

At least 19 recordsLinked to original sources

PubMed1999

A fluorescence resonance energy transfer approach for monitoring protein-mediated glycolipid transfer between vesicle membranes.

A lipid transfer protein, purified from bovine brain (23.7 kDa, 208 amino acids) and specific for glycolipids, has been used to develop a fluorescence resonance energy transfer assay (anthrylvinyl-labeled lipids; energy donors and perylenoyl-labeled lipids; energy acceptors) for monitoring the transfer of lipids between membranes. Small unilamellar vesicles composed of 1 mol% anthrylvinyl-galactosylceramide, 1.5 mol% perylenoyl-triglyceride, and 97.5% 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC) served as donor membranes. Acceptor membranes were 100% POPC vesicles. Addition of glycolipid transfer protein to mixtures of donor and acceptor vesicles resulted in increasing emission intensity of anthrylvinyl-galactosylceramide and decreasing emission intensity of the nontransferable perylenoyl-triglyceride as a function of time. The behavior was consistent with anthrylvinyl-galactosylceramide being transferred from donor to acceptor vesicles. The anthrylvinyl and perylenoyl energy transfer pair offers advantages over frequently used energy transfer pairs such as NBD and rhodamine. The anthrylvinyl emission overlaps effectively the perylenoyl excitation spectrum and the fluorescence parameters of the anthrylvinyl fluorophore are nearly independent of the medium polarity. The nonpolar fluorophores are localized in the hydrophobic region of the bilayer thus producing minimal disturbance of the bilayer polar region. Our results indicate that this method is suitable for assay of lipid transfer proteins including mechanistic studies of transfer protein function.

Animals↗

PubMed1998

Acyl chain-length asymmetry alters the interfacial elastic interactions of phosphatidylcholines.

Phosphatidylcholines (PCs) with stearoyl (18:0) sn-1 chains and variable-length, saturated sn-2 acyl chains were synthesized and investigated using a Langmuir-type film balance. Surface pressure was monitored as a function of lipid molecular area at various constant temperatures between 10 degrees C and 30 degrees C. Over this temperature range, 18:0-10:0 PC displayed only liquid-expanded behavior. In contrast, di-14:0 PC displayed liquid-expanded behavior at 24 degrees C and 30 degrees C, but two-dimensional phase transitions were evident at 20 degrees C, 15 degrees C, and 10 degrees C. The average molecular area of 18:0-10:0 PC was larger than that of liquid-expanded di-14:0 PC at equivalent surface pressures, and the shapes of their liquid expanded isotherms were somewhat dissimilar. Analysis of the elastic moduli of area compressibility (Cs(-1)) as a function of molecular area revealed shallower slopes in the semilog plots of 18:0-10:0 PC compared to di-14:0 PC. At membrane-like surface pressures (e.g., 30 mN/m), 18:0-10:0 PC was 20-25% more elastic (in an in-plane sense) than di-14:0 PC. Other PCs with varying degrees of chain-length asymmetry (18:0-8:0 PC, 18:0-12:0 PC, 18:0-14:0 PC, 18:0-16:0 PC) were also investigated to determine whether the higher in-plane elasticity of fluid-phase 18:0-10:0 PC is a common feature of PCs with asymmetrical chain lengths. Two-dimensional phase transitions in 18:0-14:0 PC and 18:0-16:0 PC prevented meaningful comparison with other fluid-phase PCs at 30 mN/m. However, the Cs(-1) values for fluid-phase 18:0-8:0 PC and 18:0-12:0 PC were similar to that of 18:0-10:0 PC (85-90 mN/m). These values showed chain-length asymmetrical PCs to have 20-25% greater in-plane elasticity than fluid-phase PCs with mono- or diunsaturated acyl chains.

Elasticity↗

PubMed1997

Phosphatidylcholine acyl unsaturation modulates the decrease in interfacial elasticity induced by cholesterol.

The effect of cholesterol on the interfacial elastic packing interactions of various molecular species of phosphatidylcholines (PCs) has been investigated by using a Langmuir-type film balance and analyzing the elastic area compressibility moduli (Cs(-1)) as a function of average cross-sectional molecular area. Emphasis was on the high surface pressure regions (pi > or = 30 mN/m) which are thought to mimic biomembrane conditions. Increasing levels of cholesterol generally caused the in-plane elasticity of the mixed monolayers to decrease. Yet, the magnitude of the cholesterol-induced changes was markedly dependent upon PC hydrocarbon structure. Among PC species with a saturated sn-1 chain but different sn-2 chain cis unsaturation levels [e.g., myristate (14:0), oleate (18:1delta9(c), linoleate (18:2delta9,12(c), arachidonate (20:4delta5,8,11,14(c), or docosahexenoate (22:6delta4,7,10,13,16,19(c)], the in-plane elasticity moduli of PC species with higher sn-2 unsaturation levels were less affected by high cholesterol mol fractions (e.g., >30 mol %) than were the more saturated PC species. The largest cholesterol-induced decreases in the in-plane elasticity were observed when both chains of PC were saturated (e.g., di-14:0 PC). When both acyl chains were identically unsaturated, the resulting PCs were 20-25% more elastic in the presence of cholesterol than when their sn-1 chains were long and saturated (e.g., palmitate). The mixing of cholesterol with PC was found to diminish the in-plane elasticity of the films beyond what was predicted from the additive behavior of the individual lipid components apportioned by mole and area fraction. Deviations from additivity were greatest for di-14:0 PC and were least for diarachidonoyl PC and didocosahexenoyl PC. In contrast to Cs(-1) analyses, sterol-induced area condensations were relatively unresponsive to subtle structural differences in the PCs at high surface pressures. Cs(-1) versus average area plots also indicated the presence of cholesterol concentration-dependent, low-pressure (<14 mN/m) phase boundaries that became more prominent as PC acyl chain unsaturation increased. Hence, area condensations measured at low surface pressures often do not accurately portray which lipid structural features are important in the lipid-sterol interactions that occur at high membrane-like surface pressures.

Cholesterol↗

PubMed1996

Cholesterol-induced interfacial area condensations of galactosylceramides and sphingomyelins with identical acyl chains.

The interfacial interactions occurring between cholesterol and either galactosylceramides (GalCers) or sphingomyelins (SMs) with identical acyl chains have been investigated using Langmuir film balance techniques. Included among the synthesized GalCers and SMs were species containing palmitoyl (16:0), stearoyl (18:0), oleoyl [18:1 delta 9(c)], nervonoyl [24:1 delta 15(c)], or linoleoyl [18:2 delta 9,12(c)] acyl residues. The cholesterol-induced condensations in the average molecular areas of the monolayers were determined by classic mean molecular area vs composition plots as well as by expressing the changes in terms of sphingolipid cross-sectional area reduction over the surface pressure range from 1 to 40 mN/m (at 1 mN/m intervals). The results show that, at surface pressures approximating bilayer conditions (30 mN/m), acyl heterogeneity in naturally occurring SMs (bovine of egg SM) enhanced the area condensation induced by cholesterol compared with their predominant molecular species (e.g. 18:0 SM in bovine SM; 16:0 SM in egg SM). Nonetheless, cholesterol always had a greater condensing effect on SM compared to GalCer when these sphingolipids were acyl chain matched and in similar phase states (prior to mixing with cholesterol). Also, the cholesterol-induced area changes for a given sphingolipid type (e.g. SM or GalCer) were similar whether the acyl chains were saturated, cis-delta 9-monounsaturated, or cis-delta 9,12-diunsaturated if the sphingolipids were in similar phase states (prior to mixing with cholesterol) and compared at equivalent surface pressures. These results indicate that, under conditions were hydrocarbon structure is matched, the sphingolipid head group plays a dominant role in determining the extent to which cholesterol reduces sphingolipid cross-sectional area. Despite the larger cholesterol-induced area condensations observed in SMs compared to those in GalCers, the molecular-packing densities showed that equimolar GalCer-cholesterol films were generally packed as tight as or slightly tighter than those of the SM-cholesterol films. The results are discussed in terms of a molecular model for sphingolipid-cholesterol interactions. Our findings also do only raise questions as to whether cholesterol-induced condensation data provide a reliable measure of the affinity, i.e. interaction strength, between cholesterol and different lipids but also provide insight regarding the stability of sterol/sphingolipid 1-1 rich microdomains thought to exist in caveolae and other cell membrane regions.

Animals↗

PubMed1996

The interfacial elastic packing interactions of galactosylceramides, sphingomyelins, and phosphatidylcholines.

The interfacial elastic packing interactions of different galactosylceramides (GalCers), sphingomyelins (SMs), and phosphatidylcholines (PC) were compared by determining their elastic area compressibility moduli (Cs-1) as a function of lateral packing pressure (pi) in a Langmuir-type film balance. To assess the relative contributions of the lipid headgroups as well as those of the ceramide and diacylglycerol hydrocarbon regions, we synthesized various GalCer and SM species with identical, homogeneous acyl residues and compared their behavior to that of PCs possessing similar hydrocarbon structures. For PCs, this meant that the sn-1 acyl chain was long and saturated (e.g., palmitate) and the sn-2 chain composition was varied to match that of GalCer or SM. When at equivalent pi and in either the chain-disordered (liquid-expanded) or chain-ordered (liquid-condensed) state, GalCer films were less elastic than either SM or PC films. When lipid headgroups were identical (SM and PC), Cs-1 values (at equivalent pi) for chain-disordered SMs, but not chain-ordered SMs, were 25-30% higher than those of PCs. Typical values for fluid phase (liquid-expanded) GalCer at 30 mN/m and 24 degrees C were 158 (+/- 7) mN/m, whereas those of SM were 135 (+/- 7) mN/m and those of PC were 123 (+/- 2) mN/m. Pressure-induced transitions to chain-ordered states (liquid-condensed) resulted in significant increases (two- to fourfold) in the "in-plane" compressibility for all three lipid types. Typical Cs-1 values for chain-ordered GalCers at 30 mN/m and 24 degrees C were between 610 and 650 mN/m, whereas those of SM and of PC were very similar and were between 265 and 300 mN/m. Under fluid phase conditions, the pi-Cs-1 behavior for each lipid type was insensitive to whether the acyl chain was saturated or monounsaturated. Measurement of the Cs-1 values also provided an effective way to evaluate the two-dimensional phase transition region of SMs, GalCers, and PCs. Modest heterogeneity in the acyl composition led to transitional broadening. Our findings provide useful information regarding the in-plane elasticity of lipids that are difficult to investigate by alternative methods, i.e., micropipette aspiration technique. The results also provide insight into the stability of sphingolipid-enriched, membrane microdomains that are thought to play a role in the sorting and trafficking of proteins containing glycosylphosphatidylinositol anchors with cells.

Animals↗

PubMed1994

Cholesterol's interfacial interactions with sphingomyelins and phosphatidylcholines: hydrocarbon chain structure determines the magnitude of condensation.

Cholesterol's interfacial interaction with different sphingomyelins and phosphatidylcholines has been investigated using a Langmuir film balance. The average molecular area of cholesterol/sphingomyelin (SM) or cholesterol/phosphatidylcholine (PC) mixed monolayers was determined as a function of film composition from the force-area isotherms measured at 24 degrees C. In contrast to previous results [Lund-Katz, S., Laboda, H. M., McLean, L. R., & Phillips, M. C. (1988) Biochemistry 27, 3416-3423], little difference was observed in equimolar cholesterol's "condensing effect" of SMs compared to PCs when their phase state was similar and when their hydrocarbon structural differences were minimized. For PCs, this meant that one acyl chain had to be long and capable of assuming an extended conformation and thus configurationally similar to the long-chain base of SM. This condition facilitated strong van der Waals attractive interactions with cholesterol's planar steroid ring and was satisfied when the sn-1 acyl chain of PC was either myristate or palmitate. Under these conditions, the structural requirements of the sn-2 chain of PC were mitigated. For instance, at equimolar cholesterol, almost no difference was observed in the apparent molecular area condensations of 1-palmitoyl-2-oleoyl-PC and 1-palmitoyl-2-arachidonoyl-PC at surface pressures between 10 and 40 mN/m. In contrast, the apparent molecular area condensations of dioleoyl-PC and diarachidonoyl-PC were substantially reduced under identical experimental conditions. The results are discussed in terms of the relative importance of phospholipid/sphingolipid hydrocarbon and headgroup structure in determining the extent of interaction with cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol↗

PubMed1994

Cholesterol's interfacial interactions with galactosylceramides.

Recently, the influence of acyl structure on galactosylceramide's (GalCer) interfacial phase behavior was studied [Ali, S., Smaby, J. M., & Brown, R.E. (1993) Biochemistry 32, 11696-11703]. Here, we show that acyl structure is a key parameter controlling GalCer's ability to interact with cholesterol. Different chain-pure GalCer species containing saturated (24:0, 18:0, or 10:0), or unsaturated (24:1 delta 15, 22:1 delta 13, or 18:2 delta 9, 12) acyl chains were synthesized. After measurement of the force-area behavior of mixed cholesterol/GalCer films at 24 degrees C, the average molecular area and average compressibility were determined as a function of film composition. Cholesterol exerts only a slight condensing effect when the GalCer species are liquid-ordered [liquid-condensed], with maximum condensation occurring near 0.25 mole fraction. However, cholesterol exerts a marked condensing effect on liquid-disordered (liquid-expanded) GalCer species regardless of whether the acyl chain is saturated or unsaturated. Maximum condensation occurs at cholesterol mole fractions between 0.3 and 0.4. We also compared cholesterol's relative condensing effect on liquid-expanded GalCer versus sphingomyelin. Cholesterol's condensation of either bovine brain or egg sphingomyelin is 25-30% greater than that observed with different liquid-expanded GalCer species. Aside from average area behavior, we assessed cholesterol's interfacial interactions with the various sphingolipids by determining the average compressibility as a function of composition. The compressibility of condensed GalCer derivatives changes very little upon addition of cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Is lateral phase separation required for fatty acid to stimulate lipases in a phosphatidylcholine interface?

Lipase-catalyzed oxygen exchange between 13,16-cis,cis-docosadienoic acid and water in liquid-expanded monolayers with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine exhibits abrupt, lipid composition-dependent changes in extent and mechanism [e.g., Muderhwa, J. M. and Brockman, H. L. (1992) J. Biol. Chem. 267, 24184-24192]. The critical nature of this transition suggests possible lateral phase separation of the lipids. This has been addressed by substituting for either lipid species one which can exist in more condensed monolayer states. Analysis of phase transition surface pressures as a function of lipid composition shows that each set of fatty acid-phosphatidylcholine mixtures exhibits a finite range of miscibility in liquid-expanded monolayers. These results strongly suggest that 13,16-cis,cis-docosadienoic acid and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine are miscible under the conditions of the oxygen-exchange experiments. Furthermore, to address more directly the relation of lateral lipid phase separation to lipase regulation, oxygen exchange catalyzed by pancreatic carboxylester and triglyceride lipases was studied using mixed monolayers of [18O]2-docosadienoic acid and 1-myristoyl-2-palmitoyl-sn-glycero-3-phosphocholine. These lipids are miscible in the liquid-expanded state at all compositions. The lipid composition dependencies of both the extent and mechanism of lipase-catalyzed oxygen exchange were essentially identical to those obtained earlier. Thus, lateral lipid phase separation is not required for the critical transition in substrate accessibility to lipases. This finding supports a percolation-based model of lipase regulation within a single surface phase and suggests the "topo-temporal" regulation of lipid-mediated signaling in cells.

Chemical Phenomena↗

PubMed1993

Acyl structure regulates galactosylceramide's interfacial interactions.

Galactosylceramides (GalCer) with homogeneous acyl chains containing zero, one, or two cis double bonds have been synthesized and characterized at an argon-aqueous buffer interface using a Langmuir film balance. Both surface pressure and surface potential were measured as a function of molecular area at 24 degrees C. N-Lignoceroylgalactosylsphingosine (N-24:0-GalSph), N-stearoylgalactosylsphingosine (N-18:0-GalSph), and N-palmitoylgalactosylsphingosine (N-16:0-GalSph) form condensed films that are similar to that of bovine brain GalCer, which contains long saturated and mono-unsaturated acyl chains, almost half being hydroxylated. In contrast, a bovine brain GalCer subfraction (NFA-GalCer) that is devoid of the hydroxylated acyl chains displays an apparent two-dimensional phase transition near 9.0 mN/m at 54 A2/molecule. To determine the role of acyl unsaturation in regulating NFA-GalCer's surface behavior, GalCer derivatives containing different mono-unsaturated acyl residues were investigated. N-Nervonoyl-galactosylsphingosine (N-24:1 delta 15-GalSph) and N-docosenoylgalactosylsphingosine (N-22:1 delta 13-GalSph) show liquid-expanded to -condensed phase transitions in their force-area isotherms at 10 and 35 mN/m, respectively. Introduction of acyl chains that are short and saturated [e.g., N-decanoylgalactosylsphingosine (N-10:0-GalSph)] or that are long but contain two cis double bonds [e.g., N-linoleoylgalactosylsphingosine (N-18:2 delta 9,12-GalSph)] causes GalCer to display only liquid-expanded behavior at 24 degrees C. The surface potentials (delta V) of the condensed GalCer derivatives with long saturated acyl residues were quite similar and were over 100 mV higher than that of bovine brain GalCer.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

A simple method for estimating surfactant impurities in solvents and subphases used for monolayer studies.

It is important to assess levels of surface active impurities in solutions used for characterization of monomolecular films and for deposition of Langmuir-Blodgett multilayers. Traditional surface pressure-area measurements lack sufficient sensitivity because of the extremely low surface pressures surfactants exhibit below the formation of a coherent film. In contrast, surface potential measurements at the gas-liquid interface increase in a surfactant-dependent manner in the gaseous-liquid expanded transition region. Using this property of such films together with area reduction, levels of impurities representing less than or equal to 0.1% of a typical coherent monolayer can be quantitated. The measurement does not require ultrapure reference materials and can be performed on a solution immediately before spreading and compression of an experimental monolayer film.

Liposomes↗

PubMed1990

Surface dipole moments of lipids at the argon-water interface. Similarities among glycerol-ester-based lipids.

Surface potential-surface pressure-area isotherms at the argon-buffer interface have been determined for 38 lipid species comprising 19 chemical classes. These lipids all exhibited a finite range of liquid-expanded surface pressure-area behavior. For most species, the linearity of surface potential with reciprocal area was excellent, but nonzero intercepts were obtained. This suggests a lipid-induced reorganization of interfacial water molecules which is area independent. The linearity of the data permits calculation of the surface dipole moment, mu perpendicular, for each lipid. The values of mu perpendicular for a series of oleoyl-containing acylglycerols, dioleoyl phosphatidylcholine, and dioleoyl phosphatidylethanolamine exhibit acylglycerol ester group mu perpendicular's which are generally consistent with known conformational properties of such lipids. The values are 132 mD for the perpendicular oleoyl glycerol-ester group and 252 mD for that in the kinked-chain conformation. Comparison of mu perpendicular's calculated using these values with homologues confirms the approximate independence of mu perpendicular from aliphatic chain length and permits identification of exceptions with possible conformational or orientational differences. Notably, diphytanoyl phosphatidylcholine shows a 45% larger mu perpendicular than predicted. Differences in mu perpendicular among lipid classes allow estimation of the electrical consequences of lipid metabolism and exchange. Calculations show that reactions such as the generation of 1,2-diacylglycerol from diacyl glycerophosphocholine or diacyl glycerophosphoinositol should produce surface potential changes of -127 and +42 mV, respectively. Thus, the two phospholipids are not simply alternative sources of diacylglycerol with respect to processes dependent on surface potential.

Argon↗

PubMed1987

Regulation of carboxylester lipase adsorption to surfaces. 2. Physical state specificity.

The physical specificity of adsorption of porcine pancreatic carboxylester lipase to mixed-lipid surfaces was examined by using films at the argon-buffer interface. They were comprised of 1-palmitoyl-2-oleoylphosphatidylcholine and triolein, 1,3-diolein, methyl oleate, oleonitrile, oleyl alcohol, or 13,16-docosadienoic acid. Under conditions where the surfaces are thermodynamically well-defined, each of these binary systems exhibits the formation of a lipid-lipid complex that is completely miscible with uncomplexed non-phospholipid [Smaby, J. M., & Brockman, H. L. (1985) Biophys. J. 48, 701-707]. Initial rates of adsorption of enzyme to the complexes were less than or equal to 5% of those measured in the absence of phospholipid and comparable to its rate of adsorption to phospholipid alone. This occurred despite there being up to 46% of the surface area occupied by non-phospholipid in the complexes. Equilibrium binding measurements were made at a composition where phospholipid-fatty acid complex was the predominant species. These showed that the low rates were due to an absence of adsorption sites relative to surfaces of fatty acid alone. With diolein or fatty acid and phospholipid, equilibrium binding was also measured at compositions intermediate between that of the complex and pure non-phospholipid. In both systems surface concentrations of enzyme varied nonideally with respect to either the mole fraction or area fraction of complex and uncomplexed diolein or fatty acid in the film. At area fractions of uncomplexed lipid of 0.35 and 0.67, dissociation constants for enzyme adsorption were increased 10-20-fold relative to pure fatty acid or diolein.(ABSTRACT TRUNCATED AT 250 WORDS)

Adsorption↗

PubMed1987

Regulation of cholesteryl oleate and triolein miscibility in monolayers and bilayers.

The miscibility of triolein and cholesteryl oleate with 1-palmitoyl-2-oleoyl phosphatidylcholine was studied at the argon-buffer interface. The surface phase behavior of the system was analogous to that for cholesteryl ester-phospholipid mixtures in that both monolayer and double layer surface phases were formed. By considering the bulk properties of cholesteryl oleatetriolein mixtures and the two-dimensional phase rule, the entire system could be described. Double layer properties suggest that it consists of mostly triolein and phospholipid in the layer adjacent to the aqueous phase. The monolayer phase shows the formation of complexes between the neutral lipids and the phospholipid with stoichiometries nearly identical with those reported for bilayers (Hamilton, J. A., Miller, K. W., and Small, D. M. (1983) J. Biol. Chem. 258, 12821-12826). A second complex with a 3:1 stoichiometry is formed between triolein and cholesteryl oleate independently of interactions with phospholipid. Upon interaction with phospholipid, the triolein-cholesteryl oleate complex loses proportionately more area than either lipid alone. Because the area of complexes with phospholipid is constant, overall neutral lipid miscibility in such complexes is enhanced by the cholesteryl oleate-triolein interaction. Thus, our data explain the apparently nonideal mixing of cholesteryl oleate, triolein, and phospholipid in monolayers and in bilayers.

Cholesterol Esters↗

PubMed1987

Acyl unsaturation and cholesteryl ester miscibility in surfaces. Formation of lecithin-cholesteryl ester complexes.

The surface behavior of a series of cholesteryl esters was studied in mixtures with a model phospholipid, 1-palmitoyl-2-oleoyl phosphatidylcholine. The cholesteryl esters were representative of the predominant forms occurring naturally and qualitative similarities in their phase behavior permits generalization of their surface properties. Quantitative differences, however, show that the availability of cholesteryl esters in all surface states is dependent on the structure of the acyl moiety. All except cholesteryl stearate were surface-active and formed preferred packing arrays, i.e., complexes, with the lecithin at compositions grouped around cholesteryl ester mol fractions of 0.015. Exceptions were cholesteryl arachidonate and docosahexaenoate, with complex stoichiometries of 0.034 and 0.032, respectively. Lecithin had the same apparent area in all complexes, 56.5 +/- 1.04 A2, which was larger than that of uncomplexed lecithin, 53.3 +/- 0.7 A2. This implies that the conformation or orientation of the two polyunsaturated species in complexes is markedly different than the others studied. The areas and hydrations of all uncomplexed cholesteryl esters were similar. Because mixing of complex with uncomplexed cholesteryl ester deviated positively from ideality, the apparent molecular areas of the uncomplexed cholesteryl esters ranged from 161 +/- 22 (complex-rich) to 107 +/- 15 A2 (cholesteryl ester-rich). The similarity of the monolayer phase complex stoichiometries and the bilayer miscibilities of cholesteryl oleate suggests a correspondence between states. If so, the availability of cholesteryl arachidonate or docosahexaenoate in bilayers should be approximately twice that of other naturally occurring cholesteryl esters.

Acylation↗

PubMed1985

Miscibility, chain packing, and hydration of 1-palmitoyl-2-oleoyl phosphatidylcholine and other lipids in surface phases.

The miscibility of 1-palmitoyl-2-oleoyl phosphatidylcholine with triolein, 1,2-diolein, 1,3-diolein, 1(3)-monoolein, oleyl alcohol, methyl oleate, oleic acid, and oleyl cyanide (18:1 lipids) was studied at the argon-water interface. The isothermal phase diagrams for the mixtures at 24 degrees were characterized by two compositional regions. At the limit of miscibility with lower mol fractions of 18:1 lipid, the surface pressure was composition-independent, but above a mixture-specific stoichiometry, surface pressure at the limit of miscibility was composition-dependent. From the two-dimensional phase rule, it was determined that at low mol fractions of 18:1 lipids, the surface consisted of phospholipid and a preferred packing array or complex of phospholipid and 18:1 lipid, whereas, above the stoichiometry of the complex, the surface phase consisted of complex and excess 18:1 lipids. In both regions of the phase diagram, mixing along the phase boundary was apparently ideal allowing application of an equation of state described earlier (J. M. Smaby and H. L. Brockman, 1984, Biochemistry, 23:3312-3316). From such analysis, apparent partial molecular areas and hydrations for phospholipid, complex, and 18:1 lipid were obtained. Comparison of these calculated parameters for the complexed and uncomplexed states shows that the aliphatic moieties behave independently of polar head group. The transition of each 18:1 chain to the complexed state involves the loss of about one interfacial water molecule and its corresponding area. For 18:1 lipids with more than one chain another two water molecules per additional chain are present in both states but contribute little to molecular area. In contrast to 18:1 lipids, the phospholipid area and hydration change little upon complexation. The uniformity of chain packing and hydration behavior among 18:1 lipid species contrasts with complex stoichiometries that vary from 0.04 to 0.65. This suggests that the stoichiometry of the preferred packing array is determined by interactions involving the more polar moieties of the 18:1 lipids and the phospholipid.

Glycerides↗

PubMed1984

Thermodynamic equation of state for cholesteryl esters in surface phases.

Phase behavior at the air-water interface for binary mixtures of cholesteryl esters with several phospholipids was determined and combined with a large body of data from other similar studies. Analysis of areas along the phase boundary between the monolayer phase and either a double layer or a bulk phase revealed a simple, additive relationship between lipid components. This indicates that at its limits the monolayer phase behaves as an ideal mixture of components with fixed partial molecular areas. This concept is supported by the independence of the values of the partial molecular areas from the nature of the other lipid component. General agreement with the model was also found for data from the double-layer-bulk phase boundary. However, in contrast to the monolayer phase boundary, the double-layer phase behaves as if comprised of components with not only fixed but also identical partial molecular areas. The lack of a surface pressure dependency for partial molar areas apparently contradicts earlier thermodynamic descriptions of surfaces. However, satisfactory resolution with theory can be obtained if along the phase boundary each lipid species has a stoichiometric number of water molecules associated with it. This concept is supported by the consistency and values of the parameters calculated from the data by using this model. In both phases, the activity coefficient of interfacial water was constant, and the cholesteryl esters were relatively more hydrated than the other lipids. Overall, this thermodynamic equation of state for phase boundaries provides the basis for modeling the surfaces of natural, cholesteryl ester rich lipid inclusions like lipoproteins and arterial lipid deposits.

Cholesterol Esters↗

PubMed1981

Novel surface phase containing cholesteryl esters. 1. Structural characteristics determined from surface pressure--area measurements.

The behavior of cholesteryl myristoleate in mixtures with dioleoylphosphatidylcholine was investigated at the air--water interface. In addition to the previously described monolayer phase [Smaby, J. M., Baumann, W. J., & Brockman, H. L. (1979) J. Lipid Res. 20, 789--795], a second surface phase has been identified. Analysis of surface pressure and molecular area data as a function of composition shows that the molecules in the second phase can exist in two miscible, double-layer states or packing arrangements, only one of which contains lecithin. The mixed double-layer state is preferentially formed and has stoichiometry ranging between 2.0 and 9.5 molecules of cholesteryl ester for each lecithin molecule. The structure of this state resembles a mixed monolayer of pressure-dependent composition and area which is covered by a second layer of cholesteryl ester at 38.2 A2/molecule. The cholesteryl myristoleate/lecithin ratio of the layer in contact with the aqueous phase ranges from 0 to 2.8 between 39 and 0 mN/m. The second double-layer state is equivalent to a monolayer of cholesteryl ester at the lipid--water interface, covered by a layer of cholesteryl ester molecules at 38.2 A2. Overall, our data show that the presence of lecithin at a lipid--water inferface has a definite ordering effect on cholesteryl ester molecules at least 30--50 A from the interface.

Chemical Phenomena↗

PubMed1981

Novel surface phase containing cholesteryl esters. 2. Nonequivalence of cholesteryl arachidonate and those with 18-carbon, cis-unsaturated acyl groups.

Surface pressure--area isotherms for binary mixtures of cholesteryl octanoate, elaidate, stearate, oleate, linoleate, linolenate, and arachiodonate in mixtures with dioleoyllecithin, triolein, oleic acid, and oleoyl alcohol were measured at 24 degrees C. Analysis of the pressure and area characteristics as a function of composition showed that double-layer surface phase formation is primarily dependent on the structure of the acyl moiety of the cholesteryl ester. Cholesteryl esters with saturated or trans-unsaturated acyl chains apparently do not form double-layer surface phases. The esters of oleate, linoleate, and linolenate formed double-layer as well as monolayer phases and their properties in these phases were similar. In contrast to other cis-unsaturated esters, cholesteryl arachidonate formed a mixed monolayer phase with miscibility in all proportions and did not form a double-layer phase. Our results show that the polar lipid monolayer separating bulk cholesteryl ester from the aqueous milieu not only solubilizes finite amounts of cholesteryl esters but also can contribute to the organization of lipid adjacent to the monolayer. That such organization is observed with the predominant cholesteryl ester species of blood and aorta suggests a role for double-layer structure in regulating the transport and metabolism of cholesteryl esters in lipoproteins, arterial lipid deposits, and adrenal cortex. The absence of double-layer formation and high monolayer solubility of cholesteryl arachidonate suggest that it should be more abundant than other cholesteryl esters in bilayers and in monolayers surrounding bulk lipid phases.

Arachidonic Acids↗