Phospholipids mixed micelles

Highly insoluble molecules are in part transported in the GIT by partitioning into the mixed micelles injected into the lumen from the biliary duct in the duodenum (Fig. 2.3). Mixed micelles consist of a 4 1 mixture of bile salts and phospholipids (Fig. 7.13). In contrast, at the point of absorption in the BBB, highly insoluble molecules are transported by serum proteins. This distinction is expected to be important in in vitro assay modeling. The use of simulated intestinal fluids is appealing. 

Phospholipids are digested and absorbed in a similar manner to that of triacylglycerol. Pancreatic lipase has some hydrolytic activity towards phospholipids and removes the fatty acid from the 1-position. The product is a lysophospholipid such as lysolecithin (Figure 4.12). It also acts as a detergent and contributes to the stability of the mixed micelles. 

Two types of micellar systems have been described, the first one includes Gd complexes capable of self-organization resulting in a supramolecular assembly 103), while the other class of micelles, also named mixed micelles is made of several components a lipophilic gadolinium chelate, one or several phospholipid(s) and a non-ionic surfactant containing a polyoxyethylene chain 104,105). 

Fig. 24. NMRD profiles of Gd-DTPA-bis-tetradecylamide (circles), Gd-DTPA-bis-hexadecylamide (triangles up), and DTPA-bis-octadecylamide (DTPA-BC18) (squares) incorporated into mixed micelles made of phospholipid (DPPC) and a surfactant (Tween 80) (T = 37°C) (112). The mean diameters of the micelles are ranging between 15 and 20 nm. The NMRD profile of Gd-DTPA (triangles down) has been added for comparison.

Triton X-100 has proved to be of great value in the surface dilution modeb for lipolytic enzyme action. In this experimental strategy, the surface concentration of phospholipid in mixed micelles is reduced by the addition of Triton as a neutral diluent, thereby increasing the average distance between phospholipids. This allows one to draw mechanistic inferences about the binding interactions of lipases and phospholipases with their lipid sub-stratesb... 

The fact that for c, no alteration of the electrophoretic properties of the drug was observed leads to the conclusion that the micelle constitution is constant. Any phospholipids or lipids, which form stable and consistent mixed micelles, may be used. 

Vahouny, G.V., Tombes, R., Cassidy, M.M., Kritchevsky, D., and Gallo, L.L. 1980. Dietary fibers. V. Binding of bile salts, phospholipids and cholesterol from mixed micelles by bile acid sequestrants and dietary fibers. Lipids 15, 1012-1018. 

Having shown that dibutyryl PC is monomeric under the enzyme assay conditions, we found that the phospholipase A2, which acts poorly on PE in mixed micelles, is activated by dibutyryl PC which is itself an even poorer substrate. 31p-NMR spectroscopy was employed to show that only PE is hydrolyzed in mixtures of various compositions of these two phospholipids. The fully activated enzyme hydrolyzes PE at a similar rate to its optimal substrate, PC containing long-chain fatty acid groups. Because dibutyryl PC is not incorporated into the micelles, these results are consistent with a mechanism of direct activation of the enzyme by phosphoryl-choline-containing lipids (either monomeric or micellar) rather than a change in the properties of the interface being responsible for the activation of phospholipase A2. Therefore, two functional sites on the enzyme have to be assumed an activator site and a catalytic site (6). 

As is obvious from the above discussion, the physical state of the phospholipid substrate is very important in any assay involving phospholipases. Besides the ether-methanol-water system, detergents have been used to prepare mixed micelles with the phospholipids. A widely used detergent is the neutral Triton X-100. A more detailed description of this approach can be gained from material presented in a monograph edited by Dennis (1991). 

The phospholipid and detergent form mixed micelles, dominated by the detergent with its single chain of hydrocarbon the micelles are therefore small. Dialysis lowers the concentration of the water-soluble detergent, so that the micelles become dominated by the phospholipid, which, having... 

Lopez, O., de la Maza, A., Coderch, L., Lopez-Iglesias, C., Wehrli E., and Parra, J. L. (1998), Direct formation of mixed micelles in the solubilization of phospholipid liposomes by Triton X-100, FEBS Lett., 426, 314-318. 

DeBony, J., and Dennis, E. A. (1981). Magnetic nonequivalence of two fatty acid chains in the phospholipids of small unilamellar vesicles and mixed micelles. Biochemistry 20, 5256-5260. 

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