The condensing effects of egg lecithin and cholesterol on triolein monolayers are inhibited by substitution of one saturated acyl chain in the triacylglycerol

Previous work showed that the clearance from plasma of chylomicron-like emulsions injected intravenously was affected by the acyl chains of the constituent triacylglycerols. Compared with emulsions containing triolein (OOO) as the only triacylglycerol, clearances were decreased by a single saturated chain in emulsions containing 1,3-dioleoyl-2-stearoyl-sn-glycerol (OSO), 1,2-dioleoyl-3-stearoyl-sn-glycerol (OOS) or 1-stearoyl-2,3-dioleoyl-sn-glycerol (SOO). The differences in clearance may reflect physical differences at the oil-water interface related to chain interactions of the triacylglycerol structures with other lipid components. In the present work lipid monomolecular films at the air-water interface were used to establish the capacity of OOO to interact with the pure synthetic triacylglycerols OOS and SOO, and the capacity of OOS and SOO to co-exist in monolayers of lecithin and of cholesterol was compared with OOO. Substituting one oleoyl chain by a stearoyl chain induced a 20% condensation in monomolecular films of the pure triacylglycerols. Mixtures of OOO with either pure egg yolk phosphatidylcholine or cholesterol also showed substantial condensing effects. In contrast substituting one oleoyl chain by a stearoyl chain substantially lessened the condensing effects. At surface pressures above the collapse pressure of the pure triacylglycerols, substantially more OOO than OOS or SOO was retained in mixed monolayers with phosphatidylcholine. These differences could underlie the effects on metabolism of saturated chains in emulsion triacylglycerols.

Multinuclear and magic-angle spinning NMR investigations of molecular organization in phospholipid-triglyceride aqueous dispersions

The supramolecular organization of multi-bilayers formed by aqueous egg phosphatidylcholine-triolein (PC-TO) mixtures has been investigated using 31P, 13C, and 1H nuclear magnetic resonance (NMR). For these emulsions, which mimic substrates in the early hydrolytic stages of fat digestion, the NMR spectra obtained with magic-angle spinning (MAS) exhibit resolution comparable to that of sonicated vesicles and integrated peak intensities consistent with their chemical composition. Both 31P line shapes and MAS sideband patterns from the phosphocholine group indicate that mixing with triolein produces a PC bilayer which remains predominantly liquid crystalline in its organization; nevertheless, anomalous spectral features in MAS spectra may be attributed to additional phases in which the headgroups adopt a different orientation with respect to the bilayer normal, and tight packing enhances phosphorus-phosphorus interactions. 13C and 1H line widths monitored as a function of PC mole fraction, spinning speed, and decoupling strength show that the choline headgroups and glycerol backbones are anchored preferentially in the phospholipid-triglyceride assemblies, whereas all acyl chains become very fluid. The average degree of chain order also decreases for the mixed dispersions, as judged from spinning-sideband intensities in 1H MAS NMR spectra. The absence of proton spin-diffusion effects in the PC-TO multilayers is demonstrated by examination of their spinning sidebands and relaxation times, making it possible to use MAS-assisted two-dimensional NMR to assign overlapped 1H resonances and to identify proximal interactions between the two constituents. The usefulness of these NMR strategies in mechanistic studies of gastric fat digestion is also discussed.

Stable self-sustained potential oscillations across a membrane filter impregnated with triolein

When applying constant electric current, periodic self-sustained potential oscillations with high stability are observed across a membrane filter impregnated with triolein, placed between KCl and NaCl aqueous solutions. Stability, reproducibility and controllability of the oscillation are much improved by the application of constant electric current compared with those obtained without application of electric current. Relations among value of electric current, base potential and period of the oscillations are studied, and it is concluded that the oscillation period can be controlled by base electric potential. Effects of temperature on the oscillations are investigated, and it is shown that Arrhenius plots for both base conductance and conductance amplitude of each oscillation fitted well to an individual straight line and that values of activation energies are similar to those of bulk salt solutions. From all data obtained it is suggested that the oscillations occur as a result of rhythmic repetition of opening and closing of hole(s) in the membrane, which is due to breakdown and restoration of a part of the membrane.

Eutectic mixed monolayers in equilibrium with phospholipid-bilayers and triolein-liquid phase

Triolein (TO) and phospholipids (egg yolk phosphatidylcholine, egg yolk phosphatidylethanolamine, and bovine brain phosphatidylserine) had low mutual solubilities and separated into the TO-liquid phase and phospholipid-bilayers. Spreading pressures of the TO-phospholipid mixture (i.e., surface pressures of the mixed monolayer in equilibrium with the phase-separating lipid mixture) at the air/saline interface were independent of the lipid composition. On the other hand, collapse pressures of the mixed monolayer of TO and phospholipid (i.e., surface pressures of the mixed monolayer in equilibrium with the TO-liquid phase) at the interface changed with the monolayer composition and were lower than the spreading pressure. The experimental data indicated the spreading and collapse pressures as offering a phase diagram for the presence of equilibrium between the mixed monolayer, the phospholipid-bilayers and the TO-liquid phase. The diagram showed that TO and the phospholipids were miscible in the mixed monolayer, forming an eutectic mixed monolayer. When the mixed monolayer initially had the eutectic composition, no collapse of the monolayer was detected until the surface pressure reached the value of the spreading pressure. No specific complex between TO and the phospholipid is required to explain the stability and collapse of the mixed monolayers. The bulk immiscibility of the lipids elucidated by the spreading pressure-measurements, immediately leads to the phase behaviors observed.

Effect of cholesteryl ester on the distribution of fluorescent cholesterol analogues in triacylglycerol-rich emulsions

Lipid emulsions consisting of a surface monolayer of phospholipid enclosing a core of neutral lipid (cholesteryl ester and/or triacylglycerol) are useful models of the lipid phase of lipoproteins. The physical state of the emulsion surface may determine the extent and nature of interaction of enzymes and lipid transfer proteins (e.g., lipoprotein lipase, cholesteryl ester transfer protein) with the particle. Unesterified cholesterol, which is a major determinant of the physical state of the surface phase, is able to partition between surface and core compartments. This report describes a fluorescence quenching method which determines the equilibrium distribution of a fluorescent cholesterol analogue (dehydroergosterol) between the surface and core compartments of triacylglycerol-rich emulsions. Quenching by iodide is used to distinguish a pool of unesterified cholesterol readily accessible to the aqueous phase. Quenching by 5-nitroxystearate identifies a pool of unesterified cholesterol in the phospholipid monolayer and the pool of unesterified cholesterol in the core compartment is found by difference. It is shown that the substitution of cholesteryl oleate for triolein in the core of the emulsion substantially increases the partition of unesterified cholesterol into the core compartment with a consequent depletion of unesterified cholesterol in the surface monolayer. The distribution of unesterified cholesterol between surface and core compartments is largely enthalpically driven.

Lateral interactions of pig apolipoprotein A-1 with egg yolk phosphatidylcholine and with cholesterol in mixed monolayers at the triolein-saline interface

Interfacial tensions of egg yolk phosphatidylcholine (PC) and cholesterol monolayers adsorbed at the triolein-saline interface were measured in the presence and absence of pig apolipoprotein A-1 (apoA-1) in the saline phase. In the absence of apoA-1, the adsorptions of PC and cholesterol at the interface from the triolein phase are cooperative, showing large lateral attractive interactions between the PC molecules and the cholesterol molecules in the monolayer. In the presence of apoA-1, the PC adsorption is anti-cooperative, indicating strong lateral attractive interactions between the PC and the apoA-1 molecules, i.e., apparently, repulsive lateral interactions between the PC molecules. On the other hand, lateral interactions of very low magnitude are observed between the cholesterol and apoA-1 molecules in the monolayer. Values of the lateral interaction energy are evaluated from the adsorption data by the Defay-Prigogine-Flory theory of monolayers. The large difference in lateral interaction energy with apoA-1 between PC and cholesterol in a mixed monolayer is discussed in connection with current problems in lipoprotein catabolism: reverse cholesterol transport, alterations in affinity of lipid particles to apoA-1, and formation of high-density lipoproteins and abnormal lipoproteins.

The use of liposomes for the preparation of protein-free lipid emulsions models of chylomicron remnants

Artificial chylomicron remnants were investigated as a new drug carrier system for the targeting of hepatic parenchymal cells. The emulsions presented here are similar in particle size and composition to natural lipoproteins. The preparations contained triolein, phospholipid, cholesterol and cholesteryl oleate. Egg yolk lecithin was either used to form multilamellar or unilamellar liposomes or it was incorporated into a lipid film prior to emulsification. Typically the lipid film contained triolein, cholesterol and cholesteryl oleate. When multilamellar liposomes were used however, cholesterol and cholesteryl oleate were incorporated into the vesicles. The emulsions were prepared by ultrasonication or by means of a microemulsifier. The unilamellar liposomes used with the microemulsifier yielded the best particle distribution, i.e. in the range of 40-60 as determined by quasi-elastic light scattering. The advantage of the method results from the complete emulsification of the components. The particle size remained unchanged during storage, although flocculation was observed. The results show that the synthesis of artificial chylomicron remnants in a microemulsifier is possible and reproducible.

Interactions of lecithin and pig apolipoproteins of high density lipoproteins at the surface monolayer of reconstituted very small particles

Cosonication of egg yolk lecithin and triolein with apolipoproteins isolated from pig high density lipoprotein (apoHDL) gave us reconstituted high density lipoprotein particles (r-HDLs) of 9 nm in average diameter. They were smaller than microemulsion particles (MEs) composed of the lipids (35 nm). The protein/egg yolk lecithin ratio in the fractionated r-HDLs was higher in the smaller particles. Binding of a hydrophobic probe, 2-p-toluidinylnaphthalene-6-sulfonate (TNS), to MEs, r-HDLs and apoHDL were evaluated on the basis of Halfman and Nishida's method. The reconstitution of apoHDL into MEs led to a 68% reduction in the binding of TNS and a small increase in the alpha-helix content as compared with free apoHDL. The binding experiments also showed the condensation of lecithin molecules at the r-HDL surface. The amphipathic helixes of apoHDL are located in the surface monolayer of egg yolk lecithin surrounding the triolein core. The intercalation of the hydrophobic residues of apoHDL between egg yolk lecithin molecules brings about a pronounced curvature of the surface and a decrease in the particle diameter.