Vesicle aggregation in model systems of supersaturated bile: relation to crystal nucleation and lipid composition of the vesicular phase.

The presence of small vesicles composed of phospholipid and cholesterol has recently been demonstrated in super-saturated model and in dilute native human biles by several groups using differing methods. Among compositional factors shown to favor spontaneous vesicle formation and prolong the cholesterol monohydrate nucleation time in model bile systems are dilution, a raised cholesterol saturation index (CSI), and a low bile salt/phospholipid ratio. Time-lapse video-enhanced microscopy of a series of model bile systems representing systematically designed variations in the above factors revealed strong evidence for an essential linkage between antecedent vesicle aggregation and subsequent crystal nucleation. Stability of vesicles was inversely related to their degree of cholesterol saturation, i.e., the greater the degree of vesicular cholesterol saturation, the less their stability (metastability). Instability of vesicles was reflected by their early aggregation followed by rapid cholesterol crystal nucleation. The lowest degree of vesicular cholesterol saturation was found in dilute systems which also exhibited the greatest metastability despite a high degree of cholesterol solubility (raised CSI). Conversely, the more concentrated and least metastable systems exhibited both rapid vesicle aggregation and rapid onset of crystal nucleation. These systems, while influenced by the other compositional factors, were found to have a high degree of vesicular cholesterol saturation, i.e., cholesterol/phospholipid molar ratio = 2.0. An additional finding was the extreme variability in the proportionate distribution of total solution cholesterol distributed to the vesicular phase, i.e., from zero to as high as 37%. Higher solute concentration, raised bile salt/lecithin ratio, and raised CSI were interactive and almost equally capable of increasing the proportionate amount of cholesterol in the vesicular phase. In conclusion, lipid compositional differences in model bile systems drastically affect the cholesterol saturation of spontaneously formed phospholipid-cholesterol vesicles. This effect, in turn, exerts a potent influence upon the metastability of vesicles, subsequently affecting the cholesterol crystal nucleation time.