Quantitative assessment of comparative potencies of cholesterol-crystal-promoting factors: relation to mechanistic...

The crystallization of cholesterol is affected by various factors in bile. The present study evaluated the relative importance of cholesterol-nucleation-promoting factors and partially characterized the mechanisms of their action. Model biles with an identical relative composition of cholesterol, egg-yolk phosphatidylcholine and taurocholate, except for replacing phosphatidylcholine (5-20%) with dilinoleoyl-phosphatidylcholine or taurocholate (10-30%) with taurodeoxycholate. Cholesterol crystallization was quantitatively assessed spectrophotometrically and morphologically estimated by the laser-scattering diffraction analyser and video-enhanced microscopy in the absence and presence of concanavalin A-binding glycoprotein isolated from human bile. In a series of experiments, lipid distribution among particulate species was determined after isolation by FPLC. In all experiments, cholesterol crystallization was dose-dependently enhanced with a rank order of: concanavalin A-binding glycoprotein > dilinoleoyl - phosphatidyl choline> taurodeoxycholate. No morphological alteration was evident for vesicles and crystals, but the cholesterol/phospholipid ratio in vesicles was increased significantly by replacement with dilinoleoyl-phosphatidylcholine and excess cholesterol. A high proportion of relatively hydrophilic phosphatidylcholine species such as dilinoleoyl-phosphatidylcholine and excess cholesterol in bile cause a redistribution of cholesterol to increase a vesicular cholesterol/phospholipid ratio, eventually promoting cholesterol crystallization, whereas concanavalin A-binding glycoprotein acts via differing mechanisms.