Lipid-cholesterol mixtures

The work of membrane breakdown for membranes made from different lipids and lipid-cholesterol mixtures is shown in Figure 9.10. It is seen that, for membranes with small critical membrane tensions, i.e. largely single-component liquid-phase membranes, the work of membrane breakdown increases almost linearly with the critical membrane tension. For higher-strength membranes, i.e. saturated lipids with maximum cholesterol content, the work of membrane breakdown reaches a value on the order of 0.025 kT and remains almost constant regardless of the increase of the critical membrane tension. Thus, the question arises, as to whether tension creates new pores or acts on existing defects. 

Figure 9.10 Work of membrane breakdown versus the critical membrane tension for different lipids and lipid-cholesterol mixtures. (Data from references [82], [28] and [113]).

Cholesterol is abundant in many membranes of eukaryotic cells, the total percentage reaching up to 50% of the total lipid content. The behavior of lipid-cholesterol mixtures has therefore attracted much attention Despite the vast amount of publications on the thennotropic behavior of phospholipid-cholesterol mixtures, the macroscopic description of lipid-cholesterol systems using phase diagrams is still very much debated, because different analytical methods such as NMR-, ESR-, fluorescence- and FT-IR-spectroscopy monitor changes in physicochemical behavior of lipid-cholesterol mixtures as a function of composition and temperature which cannot easily reconciled with observations using DSC [61-72]. 

In conclusion, DSC-experiments on lipid-cholesterol mixtures are easily performed and have revealed important information. However, the interpretation of these DSC data in terms of a simple phase diagram is not easy. 

Liposomes can be prepared from pure lipids or mixtures of lipids. Cholesterol is known to serve as a "fluidity buffer" it enhances the fluidity of the gel state bilayer, while it decreases the fluidity of the fluid state bilayer. Increasing concentrations of cholesterol in bilayers cause a broadening and gradual disappearance of the phase transition (Demel and De Kruyff, 1976). 

Chromatographic Separation of Lipids A mixture of lipids is applied to a silica gel column, and the column is then washed with increasingly polar solvents. The mixture consists of phosphatidylserine, phosphatidylethanolamine, phosphatidylcholine, cholesteryl palmitate (a sterol ester), sphingomyelin, palmitate, -re-tetradecanol, triacylglycerol, and cholesterol. In what order do you expect the lipids to elute from the column Explain your reasoning. 

Discontinuities are seen in the relationship between increase in film pressure, An, and lipid composition following the injection of globulin under monolayers of lecithin-dihydro-ceramide lactoside and lecithin-cholesterol mixtures. The breaks occur at 80 mole % C 16-dihydrocaramide lactoside and 50 mole % cholesterol. Between 0 and 80 mole % lactoside and between 0 and 50 mole % cholesterol the mixed films behave as pure lecithin. Two possible explanations are the formation of complexes, having molar ratios of lecithin-lactoside 1 to 4 and lecithin-cholesterol 1 to 1 and/or the effect of monolayer configurations (surface micelles). In this model, lecithin is at the periphery of the surface micelle and shields the other lipid from interaction with globulin. 

Although glycosphingolipids are the specific lipid components in the antigen-antibody complex, their activity is markedly enhanced by other (auxiliary) lipids such as lecithin and lecithin-cholesterol mixtures (15). The present study deals with the effect of lipid composition on the penetration of lactoside—cholesterol and lactoside—lecithin monolayers by rabbit y-globulin. We also investigated the lecithin-cholesterol system. Furthemore, since criteria for the existence of lipid-lipid complexes in monolayers are still few (8, 17), we have used infrared spectroscopy to examine lipid mixtures for the presence of complexes. 

Film penetration studies show unequivocally that lecithin-cholesterol mixtures containing from 0 to 50 mole % cholesterol and lecithin—lactoside mixtures containing from 0 to 80 mole % Ci6-dihydroceramide lactoside have the same effect as pure lecithin. This suggests the presence of a lipid complex in which lecithin prevents the interaction of the cholesterol or ceramide lactoside with globulin. Over these ranges of composition the lipid film would consist of a mixture of the lecithin-cholesterol or the lecithin-lactoside complex with excess lecithin. One may picture two models in which the protein contact is restricted to molecules of lecithin. In one, individual polar groups of the protein interact with the excess lecithin molecules as well as with the lecithin portions of the complex. In the other model, the protein as a whole interacts with the lecithin sites of polymeric lipid structures. The latter, which could be referred to as surface micelles (I), are visualized also through the term "mono-... 

Epand RM, Epand RE. Non-raft forming sphingomyelin-cholesterol mixtures. Chem. Phys. Lipids 2004 132 37-46. 

Liposomes are prepared by sequential filter extrusion of the lipid/drug mixtures. The basic composition for the preparation of 5.0 mL liposomes is 1.0 g soy phophatidylcholine (SPC, L. Meyer GmbH, Hamburg, Germany), 125 mg cholesterol (Fluka, Buchs, Switzerland) (see Note 1), 6 mg D,L-a-tocopherol (Merck, Darmstadt, Germany) and the lipophilic drug at concentrations of 1-10 mg/mL. 

Figure 41.1 shows the gel-to-liquid crystalline phase transition temperatures (Tm) of DPPC-cholesterol mixtures as a function of the cholesterol-lipid molar ratio. The Tm of fully hydrated DPPC is 42°C (Crowe and Crowe, 1988 Vist and Davis, 1990 McMullen et al., 1993 Ohtake et al., 2004). Upon the addition of cholesterol, the transition enthalpy decreases continuously imtil it is no longer observable at 50 mol% cholesterol. The disappearance of the melting transition has been attributed to strong interactions between cholesterol and DPPC (McCoimell, 2003). Upon dehydration, the Tm for DPPC increases from 42 to 105°C (Crowe and Crowe, 1988 Ohtake et al., 2004). This Tm increase is caused by the reduction in the spacing between the phospholipids, which allows for increased van der Waals interactions between the lipid hydrocarbon chains (Koster et al., 1994). Between 10 and 70 mol% cholesterol, two endothermic transitions are observed, both lower than the Tm of the pure phospholipid (Figure 41.1). High-sensitivity DSC studies on fully hydrated DPPC-cholesterol systems reported endotherms consisting of two components, suggesting the existence of domains enriched/depleted in cholesterol (Vist and Davis, 1990 McMullen et al., 1993). The two peaks present in our freeze-dried systems also suggest the...

Hepatotoxic effects commonly induced in laboratory animals exposed to commercial PCB mixtures include increased serum levels of liver enzymes indicative of hepatocellular damage (e.g., AST and ALT), serum and tissue biochemical changes indicative of fiver dysfunction (e.g., altered levels of lipids, cholesterol, porphyrins, and vitamin A), and histopathologic changes (particularly fat deposition), fibrosis, and necrosis. Intermediate- and chronic-duration oral studies have shown hepatotoxic effects in monkeys that include fatty degeneration, hepatocellular necrosis, and hypertrophic and hyperplastic changes in the bile duct at oral doses of PCBs as low as 0.1-0.2 mg/kg/day (Aroclor 1254 or 1248). 

The dependence of the ttv values on the composition of the vapor and condensed states for DML-CHOL, DOL-CHOL, and DOL-DML mixtures is shown in Figure 6. The upper curve is the surface vapor pressure as a function of the mole fraction of the liquid-expanded film the lower curve is for the dependence of irv on the composition of the gaseous phase. Ideal mixing behavior is given by the linear dotted line which joins the 7ry° points for each of the pure compounds. In all cases there was complete miscibility of the components as represented by the continuous function of 7rv with x. In the cholesterol mixtures positive deviations from Raoult s law are observed for the mixture of lecithins, ideal mixing is observed. These results confirm those obtained with lipid mixtures—i.e., cholesterol mixed with liquid-expanded lipid films forms rion-ideal mixtures with positive deviations for mixtures of lipids which are in the same monolayer state, as in the case of the liquid-expanded DOL-DML mixtures, ideal mixing results (8). 

The positive heats of mixing for lecithin-cholesterol mixtures indicate that interactions between unlike molecules are smaller than the interactions between like molecules, i.e., the hydrocarbon chain interactions with cholesterol are smaller than in each of the pure phases. If the excess heats of mixing become large enough, phase separation will occur. It may occur when the surface pressure is increased (i.e., as the films are compressed). The point at which phase separation occurs is difficult to predict, measure, or detect however, evidence of phase separation can be deduced from the following experiment. If excess amounts of two lipids are placed in water, the equilibrium surface pressure should reflect whether the surface film is a mixture. According to the phase rule (11,12, 13,14), if two bulk lipid phases are present, only one surface phase can be present at the air—water surface. Thus the composition of the equi-... 

Liposomes are prepared in a pear-shaped flask in a rotating evaporator at 37° as previously described (Matsuoka et ah, 1998). Briefly, 100 p of a 3 M lipid stock mixture resembling the Golgi lipid composition (43 mol% phosphatidylcholine (PC), 19 mol% phosphatidylethanolamine (PE), 5 mol % phosphatidylserine (PS), 10 mol% phosphatidylinositol (PI), 7 mol% sphingomyelin (SM), 16 mol% cholesterol (CL)) are diluted in 2 ml chloroform and evaporated under an argon stream. The obtained lipid film is hydrated in 900 p of hydration buffer. Lipid suspensions are subjected to... 

Figure 15.1 Separation of yolk-saline medium (see text) following extraction in chloroform-methanol (2 1). Lipids were deveioped in petroleum ether-diethyl ether-acetic acid (80 20 1) and detected by spraying with PMA. Lane 1 contains neutral lipid standard mixture 18-4A, which consists of cholesterol (c), oleic acid (o), triolein (t), methyl oleate (m), and cholesteryl oleate (co). Lane 2 shows presence of triacylglycerols and free sterols that are the predominant neutral lipids in the yolk-saline medium. Lane 3 contains saline alone that is neutral lipid negative.

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