Phospholipid-cholesterol mixtures

Bile is a mixture of electrolytes, bile acids, cholesterol, phospholipids and bilirubin. Adults produce between 400 and 800 ml of bile daily. Hepatocytes secrete bile into canaliculi, then into bile ducts, where it is modified by addition of a bicarbonate-rich secretion from ductal epithelial cells. Further modification occurs in the gall bladder, where it is concentrated up to fivefold, through absorption of water and electrolytes. Gallstones, most of which are composed... 

Huster D, Ar nold K, Gawrisch K. Influence of docosahexaenoic acid and cholesterol on lateral Upid organization in phospholipid mixtures. Biochemistry 1998 37 17,299-17,308. 

Synthetic phospholipids mixtures were also shown to form lyotropic liquid crystals with interesting properties. For example ternary lipid mixtures comprising of two lipids, dioleoylphosphatidylcholine (DOPC), dioleoylglycerol (DOG), and cholesterol, of molar ratios 1 2 1 and 1 2 2, in excess water induced formation of three dimensional (3D) hexagonal mesophases [12], Other synthetic such as polyoxyethylene-10-oleyl ether [13] also can form LLC but since these structures were less studied and have less potential to serve as delivery vehicles they will not be discussed in this chapter. 

Lipid classes obtained by chromatographic techniques in many cases are not pure, e.g. phospholipids obtained by thin-layer chromatography using unpolar solvents are often contaminated with monoglycerides. Furthermore all lipid fractions with the exception of free cholesterol are mixtures of substances which differ in their fatty acid moieties. Procedures are now available for their further differentiation. 

The narrowness of these signals from isolated erythrocyte membrane lipids (Chapman et al., 1968a) in contrast with their broadness observed with 1 1 mixtures of egg-yolk lecithin and cholesterol (Chapman and Penkett, 1966) is readily explained if an optimum cholesterol/phospholipid molecular ratio, 1 1, is required for maximum broadening effect. Red cell phospholipids and other membrane lipids in excess of the optimum ratio, i.e., lipids which could not form a complex with cholesterol (Vandenheuvel, 1963), would then be responsible for observed narrow signals. 

The quantitative determination of phospholipid acyl chain conformational disorder in hydrated systems of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamlne (DPPE), mixtures containing DPPC and cholesterol, and mixtures of DPPC and Gramicidin D has been made using FT-IR. The... 

The effect of cholesterol on other phospholipid mixtures is qualitatively similar. For PCs with saturated chains, it is observed that the mid point temperature of the broad transition is higher than that of the sharp transition when the PC carries shorter chains, such as in DMPC. For DAPC with its C20 chains, the broad transition is shifted to a lower temperature compared to the sharp transition [67,68],... 

There has been extensive activity in the study of lipid monolayers as discussed above in Section IV-4E. Coexisting fluid phases have been observed via fluorescence microscopy of mixtures of phospholipid and cholesterol where a critical point occurs near 30 mol% cholesterol [257]. 

The lipid content of the membranes can be varied, allowing systematic examination of the effects of varying lipid composition on certain functions. For instance, vesicles can be made that are composed solely of phosphatidylchohne or, alternatively, of known mixtures of different phospholipids, glycohpids, and cholesterol. The fatty acid moieties of the lipids used can also be varied by employing synthetic lipids of known... 

Sugano et al. [561,562] explored the lipid model containing several different phospholipids, closely resembling the mixture found in reconstituted brush border lipids [433,566] and demonstrated dramatically improved property predictions. The best-performing lipid composition consisted of a 3% wt/vol lipid solution in 1,7-octadiene (lipid consisting of 33% wt/wt cholesterol, 27% PC, 27% PE, 7% PS, 7% PI). The donor and acceptor compartments were adjusted in the pH interval between 5.0 and 7.4 [562]. With such a mixture, membrane retention is expected to be extensive when lipophilic drugs are assayed. The use of 1,7-octadiene in the assay was noted to require special safety precautions. 

FIGURE 10.12 The mole ratio of carotenoid/phospholipid and carotenoid/total lipid (phospholipid + cholesterol) in raft domain (detergent-resistant membrane, DRM) and bulk domain (detergent-soluble membrane, DSM) isolated from membranes made of raft-forming mixture (equimolar ternary mixture of dioleoyl-PC (DOPC)/sphingomyelin/cholesterol) with 1 mol% lutein (LUT), zeaxanthin (ZEA), P-cryptoxanthin (P-CXT), or P-carotene (P-CAR). 

Deuterium NMR studies of chain and headgroup deuterated phospholipid bilayers and phospholipid—cholesterol mixtures... 

Natural biological membranes consist of lipid bilayers, which typically comprise a complex mixture of phospholipids and sterol, along with embedded or surface associated proteins. The sterol cholesterol is an important component of animal cell membranes, which may consist of up to 50 mol% cholesterol. As cholesterol can significantly modify the bilayer physical properties, such as acyl-chain orientational order, model membranes containing cholesterol have been studied extensively. Spectroscopic and diffraction experiments reveal that cholesterol in a lipid-crystalline bilayer increases the orientational order of the lipid acyl-chains without substantially restricting the mobility of the lipid molecules. Cholesterol thickens a liquid-crystalline bilayer and increases the packing density of lipid acyl-chains in the plane of the bilayer in a way that has been referred to as a condensing effect. 

Results obtained in this way were helpful, but of limited value. The analyses told us whether or not the bile was supersaturated with cholesterol, but did not tell us whether the abnormality was due to too much cholesterol, too few bile acids, too few phospholipids or to some combined defect. The next step, therefore, was to measure the hour-by-hour bile lipid-secretion rates using marker-corrected perfusion techniques. These assume that, in response to the perfusion stimulus (such as an intra-duodenal amino acid mixture), the gallbladder remains tonically contracted throughout and steady-state conditions ensue. 

Our present ideas about the nature of biological membranes, which are so fundamental to all biochemical processes, are based on the Singer-Nicholson mosaic model. This model of the membrane is based on a phospholipid bilayer that is, however, asymmetrical. In the outside monolayer, phosphatidylcholine (lecithin) predominates, whereas the inner monolayer on the cytoplasmic side is rich in a mixture of phos-phatidylethanolamine, phosphatidylserine, and phosphatidylinositol. Cholesterol molecules are also inserted into the bilayer, with their 3-hydroxyl group pointed toward the aqueous side. The hydrophobic fatty acid tails and the steran skeleton of cholesterol... 

Liposomes were formed from 1,2-dipalmitoylphosphatidylcholine (DPPC) and cholesterol (Choi) and the effect of liposomal entrapment on pulmonary absorption of insulin was related to oligomerization of insulin (Liu et al. 1993). Instillation of both dimeric and hexameric insulin produced equivalent duration of hypoglycemic response. However, the initial response from the hexameric form was slightly slower than that from dimeric insulin, probably due to lower permeability across alveolar epithelium of the hexameric form caused by larger molecular size. The intratracheal administration of liposomal insulin enhanced pulmonary absorption and resulted in an absolute bioavailability of 30.3%. Nevertheless, a similar extent of absorption and hypoglycemic effects was obtained from a physical mixture of insulin and blank liposomes and from liposomal insulin. This suggests a specific interaction of the phospholipid with the surfactant layer or even with the alveolar membrane. 

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