Cholesterol systems

While the thermodynamic treatment is applicable to the mixed cholesterol systems we listed, we are not reporting the excess thermodynamic quantities for these systems at this time because of difficulties in allowing for differences in the initial physical states of the pure component mono-layers. The conclusions drawn by van Deenen et al. (9) for the dimyris-toyl lecithin-cholesterol system must be regarded as invalid since they did not take this factor into account. 

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. 

A remarkable effect of lecithin is seen in the discontinuities of the All curves for both the lecithin-] actoside and the lecithin-cholesterol systems (Figures 2 and 3) when these are compared with the linear increase of An in the cholesterol-lactoside system (Figure 1). 

On the one hand, the cholesterol-lactoside system did not show film contraction (unpublished data). This was expected since the monolayers of dihydroceramide lactosides (7) and of cholesterol are not compressible. On the other hand, lecithin-lactoside and lecithin-cholesterol systems did show contraction, which could have been predicted since the lecithin monolayer is of the expanded type and is very compressible. (The area per molecule of lecithin at 2 dynes per cm. is large, 110 sq. A., as opposed to 52 sq. A. for Ci -dihydroceramide lactoside and 40 sq. A. for cholesterol.)... 

The work on the cholesterol system stimulated investigation of other examples of homoallylic participation. Roberts found that exo-5- and endo-5-bicyclo[2.2.1]heptenyl (i.e., exo-5- and enrfo-5-norbornenyl) halides (47 and 48) both solvolyze in aqueous ethanol to give the same product (49) the exo compound (47) solvolyzes about ten times more rapidly than the endo compound (48). Roberts pointed out that backside homoallylic participation in ionization was possible in 47 but not in 48 (see Figure 6.6). Once 48 has ionized it can, in a second... 

In the simulation of Tu et al. [72] with 12.5 mol% cholesterol, the starting configuration was obtained from a simulation of a pure DPPC bilayer composed of 64 DPPC molecules. Four DPPC molecules per leaflet were replaced by cholesterol to generate the DPPC-cholesterol system. The system was simulated for 1.4 ns the area per phosholipid decreased during the first 700 ps of the simulation but then remained stable for the rest of the simulation. As pointed out by the authors, the decrease in the surface area was due to the replacement of the phospholipids by the thinner cholesterol and not to an ordering effect of cholesterol. Accordingly, little effect of cholesterol on the order of the alkyl chains was observed in this simulation, in contradiction to the results of a cholesterol-DPPC simulation performed by Smondyrev and Berkowitz that is described below. 

Vibrational spectroscopy also shows interactions of polyene antibiotic ion channels nystatin and amphotericin B with phospholipid bilayers (Bunow and Lewin, 1977a Iqbal and Weidekamm, 1979 Van de Ven et al., 1984). In particular, Fourier Transform Raman spectroscopy demonstrates that at high temperature, the amphotericin A complex of DPPC/cholesterol is more ordered, whereas the amphotericin B complex is as ordered as the pure lipid/cholesterol system. In the low temperature phase and in the presence of the sterol-antibiotic complex, the bilayers were suggested to be in the interdigitated state (Levin and Neil Lewis, 1990). 

Ipsen JH, Karlstrbm G, Mouritsen OG, Wennerstrbm H, Zucker- 57. mann Ml. Phase equilibria in the phosphatidylcholine-cholesterol system. Biochim. Biophys. Acta 1987 905 162-172. 

Lewis RNAH, McElhaney RN. The mesomorphic phase behaviour of lipid bilayers. In The Structure of Biological Membranes. Yeagle PE, ed. 1991. CRC Press, Boca Raton, FL. pp. 73-155. Ipsen JH, Karlstrom G, Mouritsen OG, Wennerstrom H, Zuk-ermann MJ. Phase equilibria in phosphatidylcholine-cholesterol system. Biochim. Biophys. Acta 1987 905 162-172. 

Control Chem-Chex C for use with the Reflotron cholesterol system. Clin. Chem. 36, 1008-1009, Abstr. 267. 

Clark, L.W. and Phillips, R.N. (1985). Clinical laboratory evaluation of the Seralyzer cholesterol system. Clin. Chem. 31, 918, Abstr. 78. 

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...

DPL-Cholesterol System. The increase of film pressure with the quantitative addition of a DPL solution in chloroform-methanol (85 15) on the surface of 0.15M NaCl shows a typical surface saturation curve (Figure 9). Surface viscosity increased with each addition and with film... 

Joos data on distearoyl lecithin (DSL)-cholesterol mixed films (35) coincide with data from our DPL-cholesterol system in the sense that cholesterol reduced the viscosity of the lecithin film, and the surface viscosity decreased with increasing cholesterol concentrations. However, the comparison and interpretation of surface viscosity data require caution (2,6). For example, in Joos experiments the lipid was distearoyl lecithin (DSL), the subphase was distilled water, phospholipid and cholesterol were premixed, and viscosity was measured by the rotational surface Couette method. By the torsion oscillation method, at all film pressures... 

In the lumen of the small intestine, dietary fat does not only meet bile salt but the much more complex bile in which bile salts are about half saturated with lecithin in a mixed micellar system of bile salt-lecithin-cholesterol. On dilution in the intestinal content, the micelles grow in size as the phase limit is approached and large disk-like micelles form which fold into vesicles [49]. These changes are due to the phase transition that occurs when the bile salt concentration is decreased and the solubility limit for lecithin in the mixed micelles is exceeded. The information is mostly derived from in vitro studies with model systems but most probably is applicable to the in vivo situation. What in fact takes place when the bile-derived lamellar bile salt-lecithin-cholesterol system meets the partly digested dietary fat can only be pictured. Most probably it involves an exchange of surface components, a continuous lipolysis at the interphase by pancreatic enzymes and the formation of amphiphilic products which go into different lamellar systems for further uptake by the enterocyte. Due to the relatively low bile salt concentration and the potentially high concentration of product phases in intestinal content early in fat digestion, the micellar and monomeric concentration of bile salt can be expected to be low but to increase towards the end of absorption. 

Kircher, H. W. In "Cholesterol Systems in Insects and Animals" Dupont, J., Ed., CRC Press Boca Raton, Florida, 1982 pp. 

Similar DSC studies have been carried out with a related binary mixture, namely sphingomyelin/cholesterol. Different sphingomyelins (SM) vary in melting temperature, but all of them appear to have a gel-fluid transition. AT-palmitoyl SM, the main component of egg SM has a transition at 41 °C. Its mixtures with cholesterol behave similarly to the DPPC/cholesterol system, in that (a) cholesterol causes the endotherm to broaden and its enthalpy to decrease without marked changes in the transition temperature (b) the transition is abolished at or above 50 mol percent cholesterol, and (c) mixtures containing up to 20 mol per cent sterol can be deconvolved into a sharp... 

Marinov, R. and Dufourc E.J. (1996). Thermotropism and hydration properties of POPE and POPE-cholesterol systems as revealed by solid state and P-NMR. Euro. Biophys. J. 24 423-431. 

Sreenivasan also made qualitative investigations into the extent of hydrogen bonding associated with the template-monomer binding. Carbonyl infrared absorption shifts were measured in the absence and presence of cholesterol. There is a 15 cm shift upon cholesterol binding to acrylic acid [1721-1706 cm ], whereas a more pronounced 26 cm shift is observed for the binding of cholesterol to copper acrylate [1729-1703 cm ]. The larger shift is attributed to an enhanced interaction in the Cu acrylate-cholesterol system. 

Literature data support the use of rice bran oil and constituents as antioxidants in food systems. Tocols and oryzanols appear to have different degrees of activity depending on the Upid system evaluated. Oryzanols are more effective in cholesterol systems and tocols the most effective in linoleic acid model systems. The combination of these components greatly enhances the activity of rice bran oil as an antioxidant. Further study is recommended to assess other lipid systems as well as synergistic activities between the oryzanols and tocols. 

Moments can in some cases provide a means of calculating the relative amounts of coexisting phases (14), which is otherwise difficult. An example of the analysis is the bilayer-hexagonal transition of a phosphatidylethanolamine-cholesterol system (14). The phase fractions were calculated according to eq. 16,... 

STRUCTURAL PROPERTIES OF A LECITHIN-CHOLESTEROL SYSTEM RIPPLE STRUCTURE AND PHASE DIAGRAM... 

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]. 

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