Title: The characteristics of mixed micellar solutions with particular reference to bile
Abstract: In higher animals lipid molecules are largely water-insoluble, but must be digested, absorbed, transported and secreted in an aqueous medium. One of the means by which these phenomena may occur is by the association of insoluble lipids with soluble lipids in small molecular aggregates called "mixed micelles." Insoluble lipids are thus rendered "soluble." Since bile and the aqueous phase of intestinal content are mixed micellar solutions containing large amounts of insoluble lipids, we thought it worthwhile to focus on the properties of micellar solutions and the mechanisms by which insoluble lipids, especially those of biologic interest, are brought into aqueous solution. We have, therefore, classified lipids into soluble and insoluble species and suggested, as far as is presently possible, how soluble lipids form mixed micelles with insoluble lipids. Mixed micelles are divided into two categories, depending on whether the micelle-forming component (soluble lipid) is an aliphatic detergent (soaps, etc.) or a complex aromatic detergent (bile salts, etc.). The amounts of insoluble lipid solubilized by either of the two types of detergent varies markedly with the chemical type of insoluble lipid (nonpolar, polar class I and polar class II). Bile salts have an extremely poor capacity to solubilize aliphatic or aromatic non-polar lipids. In fact, it is far from certain that these types of molecules are actually solubilized in the bile salt micelle. Aliphatic detergents, on the other hand, solubilize nonpolar detergents effectively within the core of the micelle. The ability of bile salt micelles to solubilize insoluble polar molecules depends, not only on the class of the insoluble molecule, but also on the molecular configuration and the nature of the polar and nonpolar parts of the molecules. If the chain length is only 8 to 12 carbons, the molecules are very well solubilized, regardless of the nature of the polar group. If the chain is long (C16−C22) the molecule is well solubilized only if the insoluble lipid possesses a strong enough polar moiety to permit water to swell the molecules and form liquid crystals (class II polar lipids). Long chain nonswelling lipids (class I polar lipids) are solubilized much less effectively. Polar molecules whose nonpolar part is a bulky aromatic structure (e.g., cholesterol) rather than an aliphatic chain are very poorly dispersed and the mechanism of solubilization is unknown. Aliphatic detergents solubilize both class I and class II insoluble lipids. In general, aliphatic detergents solubilize long chain and aromatic class I lipids far more efficiently than the bile salts, whereas the converse is true for class II lipids. The marked differences in solubilizing capacity between the mixed micelles formed by aliphatic detergents and by bile salts can be explained by their different micellar structure, a structure imposed by their very different molecular configuration. The very marked ability of the natural detergent bile salt to disperse and solubilize the class II lipids such as phospholipids, monoglycerides and "acid soaps" is physiologically important as these lipids are the main insoluble lipids found in bile and in intestinal contents during fat digestion. Further, a striking change in the solvent properties of bile salts occurs after class II lipid is added to their solutions. The swollen bile salt-lecithin or bile salt-monoglyceride micelle now contains a liquid hydrocarbon region similar to an aliphatic micelle, in which appreciable quantities of class I lipids such as cholesterol and long-chain fatty acids may be solubilized. The resulting complex micelle appears to have a distinctive molecular arrangement. The probable structure is that of a small bimolecular cylinder of class II lipid (e.g., lecithin or monoglyceride) containing interdigitated class I lipids and surrounded on its perimeter by a "hydrophobically bonded" layer of bile salt molecules. This structure permits the exterior of the micelle to be covered with the water-soluble polar groups and hides the hydrocarbon parts on the interior. Although this model has been primarily derived from in vitro studies on mixed micelles, especially the bile salt-lecithin micelle, little information is available concerning the properties of the complex micelles existing in bile and in intestinal content. Until the characteristics of these micelles are known, one can only extrapolate their properties from model systems.
Publication Year: 1970
Publication Date: 1970-11-01
Language: en
Type: review
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 387
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