Phosphatidylcholine: cholesterol

FIGURE 3 Effect of the amount of cholesterol on the particle size. Phosphatidylcholine/cholesterol liposomes were prepared by the octyl glucoside dilution technique. The begin concentration of the mixed micelles was 150 mM octyl glucoside and 10 mM phosphatidylcholine in 10 mM tris(hydroxymethyl)aminomethane and 0.9% NaCl, pH 7.4. Dilution was performed with an automatic titration unit at a dilution rate (= dilution factor, relative to the initial volume, per unit of time) of 0.026 sec"l ( a and ) or 0.69 sec l ( and o). Mean diameters after dilution and ) and after filtration ( L and q) are repi sented. (Adapted from Jiskoot et al, 1986a.)... 

T. E. (1978). Studies on anomalous thermotropic behavior of aqueous dispersions of dipalmitoyl phosphatidylcholine-cholesterol mixtures. Biochemistry. H, 1984-1989,... 

Solubilization of an active H,K-ATPase is also a prerequisite for reconstitution of the enzyme into liposomes. With these H,K-ATPase proteoliposomes it is then possible to study the transport characteristics of pure H,K-ATPase, without the interference of residual protein contamination that is usually present in native vesicular H,K-ATPase preparations. Rabon et al. [118] first reported the reconstitution of choleate or n-octylglucoside solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. The enzyme was reconstituted asymmetrically into the proteoliposomes with 70% of the pump molecules having the cytoplasmic side extravesicular. In the presence of intravesicular K, the proteoliposomes exhibited an Mg-ATP-dependent H transport, as monitored by acridine orange fluorescence quenching. Moreover, as seen with native H,K-ATPase vesicles, reconstituted H,K-... 

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

Kusumi, A., W. K. Subczynski, M. Pasenkiewicz-Gierula, J. S. Hyde, and H. Merkle. 1986. Spin-label studies on phosphatidylcholine-cholesterol membranes Effects of alkyl chain length and unsaturation in the fluid phase. Biochim. Biophys. Acta 854 307-317. 

Subczynski, W. K., J. S. Hyde, and A. Kusumi. 1989. Oxygen permeability of phosphatidylcholine-cholesterol membranes. Proc. Natl. Acad. Sci. USA 86 4474-4478. 

Cholesterol esters are produced by transferring an acyl moiety from acyl-CoA or from phosphatidylcholine onto the cholesterol hydroxyl group. The latter process is catalyzed by phosphatidylcholine cholesterol acyltransferase ... 

Langner, M., Gabrielska, J. and Przestalski, S. A. (2000). Adsorption of phenyltin compounds onto phosphatidylcholine/cholesterol bilayers, Appl. Organomet. Chem., 14, 25-33. 

ICAT (or PCAT, phosphatidylcholine-cholesterol acyltransferase) is an enzyme in the blood that is activated by apoA-1 on HDL. LCAT adds a fatty add to cholesterol, producing cholesterol esters, which dissolve in the core of the HDL, allowing HDL to transport cholesterol from the periphery to the liver. This process of reverse cholesterol transport is shown in Figure 1-15-7. 

Epididymis Carnitine Inositol Phosphatidylcholine Cholesterol Glycoproteins Facihtates acetyl-CoA oxidation by spermatozoa (Chapter 9) Precursor for formation of phosphatidyhnositol bisphosphate Buffer to maintain pH and a source of chohne Stabilises membranes They coat the surface of the sperm to protect against IgA... 

Phosphatidylcholine/cholesterol liposomal formulation with doxorubicin. 

Along with phosphatidylcholines, cholesterol is often included in the vesicle bilayers in as high as a 1 1 ratio because it reduces permeability of the bilayer to aqueous solutes. 

Many studies show changes in the physical and even biochemical properties of phosphatidylcholine cholesterol membranes, when the cholesterol concentration is increased above 20 to 40%. 

Egg yolk phosphatidylcholine. Cholesterol. Cholesteryl oleate. [3H]-cholesteryl oleate. Chloroform. Ethanol. Sodium phosphate. EDTA. NaN3. Sodium cholate. NaCl. 

Metabolism of HDL. PC = phosphatidylcholine lyso-PC = lysophosphatidylcholine. PC AT = Phosphatidylcholine cholesterol transferase. 

If more cholesterol enters the bile than can be solubilized by the available bile salts and phosphatidylcholine, cholesterol gallstone disease (cholelithiasis) can occur. This is generally caused by gross malabsorption of bile acids from the intestine, obstruction of the biliary tract, or severe hepatic dysfunction, leading to abnormalities in bile or bile salt production. 

Figure 8-12 (A) 31P NMR spectra of different phospholipid phases. Hydrated soya phosphatidylethanolamine adopts the hexagonal Hn phase at 30°C. In the presence of 50 mol% of egg phosphatidylcholine only the bilayer phase is observed. At intermediate (30%) phosphatidylcholine concentrations an isotropic component appears in the spectrum. (B) Inverted micelles proposed to explain "lipidic particles" seen in freeze fracture micrographs of bilayer mixture of phospholipids, e.g., of phosphatidylethanolanine + phosphatidylcholine + cholesterol. From de Kruijft et al.m Courtesy of B. de Kruijft.

Liposomes applied on the skin were also investigated for their delivery proprieties to the pilosebaceous units [15,23 28]. The in vitro skin penetration behavior of carboxyfluorescein incorporated in multilamellar liposomes (phosphatidylcholine cholesterol phosphatidylser-ine) and in another four nonliposomal systems (HEPES pH 7.4 buffer 5% propylene glycol 10% ethanol and 0.05% sodium lauryl sulfate) was studied by Lieb et al. [25]. Using two fluorescent techniques the authors found a higher accumulation of the probe within skin follicles when delivered from liposomes [25], Further, in an interesting setup of in vitro and in vivo experiments in mice, Hoffman s group observed liposomal delivery of the active Lac-Z gene and its expression mostly in the hair follicles [26,28]. 

Liu B. and Krieger M. (2002) Highly purified scavenger receptor class B, type I reconstituted into phosphatidylcholine/cholesterol liposomes mediates high affinity high density lipoprotein binding and selective lipid uptake. J. Biol. Chem. 277, 34125-34135. 

Fig. 11. Schematic representation of molecular recognition of phosphatidylcholine-cholesterol based liposomes bearing the complementary guanidinium and phosphate moieties...

Thewalt, J. and Bloom, M. (1992). Phosphatidylcholine cholesterol phase diagrams. Biophys. J. 65 1176. 

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. 

Heath et al. have proposed the use of immunoliposomes for the intracellular delivery of compounds that intrinsically do not enter diseased cells. These compounds are cytotoxic if they are transported intracellu-larly. Methotrexate-y-aspartate, a good example of this type of compound, has been encapsulated in liposomes composed of phosphatidylcholine, cholesterol, and 4-(p-maleimidophenyl)-butyryl-phosphatidylethanol-amine in a molar ratio of 10 10 1. The liposomes were... 

Straume M, Litman BJ. Equilibrium and dynamic bilayer structural properties of unsaturated acyl chain phosphatidylcholine-cholesterol-rhodopsin recombinant vesicles and rod outer segment disk membranes as determined from higher order analysis of fluorescence anisotropy decay. Biochemistry 1988 27 7723-7733. 

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