Phospholipids in micelles

MICELLAR SUBSTRATES. Phospholipids in micelles are frequently found to be more active substrates in lipolysis than those phospholipids residing in a lipid bilayer". Dennis first described the use of Triton X-100 to manipulate the amount of phospholipid per unit surface area of a micelle in a systematic analysis of the interfacial interactions of lipases with lipid micelles. Verger and Jain et al have presented cogent accounts of the kinetics of interfacial catalysis by phospholipases. The complexity of the problem is illustrated in the diagram shown in Fig. 2 showing how the enzyme in the aqueous phase can bind to the interface (designated by the asterisk) and then become activated. Once this is achieved, E catalyzes conversion of S to release P. ... 

Table 9.12 H-NMR spectra of phospholipids in micelles (Dennis and Ribeiro, 1976)... 

The chemical shifts of various phospholipids in organic solvent and Triton X-100 micelles are shown in Tables 9.13 and 9.14. A detailed discussion on P-NMR of phospholipids in micelles, including applications to membrane solubilization by detergents and phospholipase activity may be found in the recent review of Dennis and Pliickthun (1984). 

Dennis, E. A. and Pliickthun, A. (1984) Phosphorus-31 NMR of phospholipids in micelles, in Phosphorus-31 NMR, Principles and Applications, Academic Press, New York, pp. 423-46. 

The chapters on - P NMR of phospholipids in micelles (Edward A. Dennis and Andreas Pliickthun) and membranes (Ian C. P. Smith and Irena H. Ekiel) provide details of the structure and dynamics of molecules in these systems. 

Phospholipids are found widely in both plant and animal tissues and make up approximately 50% to 60% of cell membranes. Because they are like soaps in having a long, nonpolar hydrocarbon tail bound to a polar ionic head, phospholipids in the cell membrane organize into a lipid bilayer about 5.0 nm (50 A) thick. As shown in Figure 27.2, the nonpolar tails aggregate in the center of the bilayer in much the same way that soap tails aggregate in the center of a micelle. This bilayer serves as an effective barrier to the passage of water, ions, and other components into and out of cells. 

Figure 22.1 The amphiphilic nature of phospholipids in solution drives the formation of complex structures. Spherical micelles may form in aqueous solution, wherein the hydrophilic head groups all point out toward the surrounding water environment and the hydrophobic tails point inward to the exclusion of water. Larger lipid bilayers may form by similar forces, creating sheets, spheres, and other highly complex morphologies. In non-aqueous solution, inverted micelles may form, wherein the tails all point toward the outer hydrophobic region and the heads point inward forming hexagonal shapes.

Figure 1. Various physical states of phospholipids in aqueous solution. Note the following features (a) phospholipids residing at the air/water interface are arranged such that their polar head groups maximize contact with the aqueous environment, whereas apolar side chains extend outward toward the air (b) solitary phospholipid molecules remain in equilibrium with various monolayer and bilayer structures (c) bilayer vesicles and micelles remain in equilibrium with solitary phospholipid molecules, provided that the total lipid content exceeds the critical micelle concentration.

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... 

Boicelli, C. A., Conti, F., Giomini, M., and Giuliani, A. M. (1982). Interactions of small molecules with phospholipids in inverted micelles. Chem. Phys. Lett., 89,490-6. 

Structures formed by (a) detergents and (b) phospholipids in aqueous solution. Each molecule is depicted schematically as a polar head-group ( ) attached to one or two long, nonpolar chains. Most detergents have one nonpolar chain phospholipids have two. At very low concentrations, detergents or phospholipids form monolayers at the air-water interface. At higher concentrations, when this interface is saturated, further molecules form micelles or bilayer vesicles (liposomes). 

PBS and gently blotted to remove blood and tissue fluids, then suspended over the lip of a small (250 pi) microcentrifuge tube and punctured with a needle to allow the bile to drain into the tube. Store frozen until assay. There is usually enough material to measure lipid composition (bile acids, cholesterol, phospholipids) with standard colorimetric kits (<1 pi needed for each assay). In addition to biliary cholesterol levels, it is important to take note of bile salt concentrations, since these are the detergents which suspend dietary lipids in micelles and deliver them to the intestinal epithelium for absorption by enterocytes. Differences in bile salt concentration alone could lead to differences in cholesterol absorption. 

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