Quasielastic light scattering studies of aqueous biliary lipid systems and native bile

During the past 15 yr, the technique of quasielastic light scattering has been used by a number of laboratories to systematically investigate the aggregative behavior of model bile systems and more recently to characterize particles present in native bile. Quasielastic light scattering investigations of aqueous bile salt solutions have indicated important similarities and differences between the various bile salt species and have led to a quantitative model for the formation of globular and rodlike micelles, based on Small and Ekwall's primary-secondary micelle hypothesis. Studies of aqueous bile salt-lecithin systems have indicated three important aggregation regimens dependent on the lecithin/bile salt molar ratio and total solute concentration. Region I of the phase diagram, which includes the lecithin/bile salt and total solute concentration values found in most mammalian biles, corresponds to a population of "simple" bile salt micelles coexisting in equilibrium with a population of mixed bile salt-lecithin micelles. Region II contains only mixed micelles, whose apparent size and shape vary with lecithin/bile salt and total solute concentration in a manner consistent with a "mixed disc" model. In this model, bile salts not only coat the perimeter of the disc (as proposed in Small's original model) but are also incorporated within the lecithin bilayer, possibly as hydrogen-bonded dimers. Finally, in region III, where total solute concentration values are typically less than the critical micelle concentration of the pure bile salt, the systems contain mixed vesicles (spherical bilayer shells) whose size (approximately 130 to 500A) depends on lecithin/bile salt and total solute concentration in accordance with a simple partition equilibrium that determines the composition of the mixed vesicle bilayer.(ABSTRACT TRUNCATED AT 250 WORDS)