BILE CONCENTRATION PROMOTES NUCLEATION OF CHOLESTEROL MONOHYDRATE CRYSTALS BY INCREASING THE CHOLESTEROL CONCENTRATION IN THE VESICLES

Cholesterol in bile is solubilized in mixed micelles and cholesterol-p hospholipid vesicles. Biliary cholesterol supersaturation and increase d concentration of bile in the gallbladder promotes nucleation of chol esterol monohydrate crystals and gallstone formation, possibly by crea ting unstable vesicles with a high cholesterol/phospholipid ratio. In the present study super-saturated and unsaturated biles (cholesterol s aturation index (CSI) 1-4 and 0-8 respectively) were prepared with con centrations typical of gallbladder and more dilute hepatic bile (total lipid concentration (TLCo) 10 and 2.5 g dl-1 respectively). The distr ibution of cholesterol between vesicles and micelles, and vesicular ch olesterol/phospholipid ratio were studied using ultracentrifugation an d gel-permeation chromatography. The nucleation time of cholesterol cr ystals was determined in whole model bile, and in the vesicular and mi cellar peak fractions. Increased CSI and bile dilution led to an incre ased proportion of cholesterol solubilized in vesicles. The concentrat ion of bile did not influence vesicular cholesterol/phospholipid ratio . The vesicular cholesterol/phospholipid ratio found in gel-permeation chromatography experiments was similar at high and low CSI, whereas t he ratio was significantly higher in supersaturated than in unsaturate d biles in ultracentrifugation studies. Nucleation of cholesterol crys tals from whole model bile was more rapid at the higher bile concentra tion and higher cholesterol saturation. Nucleation time in whole model bile correlated significantly with nucleation time in the correspondi ng vesicular peak fraction obtained by gel-permeation chromatography ( r = 0.58: P < 0.01) and with the cholesterol concentration in this ves icular peak (r = -0.77; P < 0.002) but not with vesicular peak cholest erol/phospholipid ratio. Highest vesicular peak cholesterol concentrat ions and shortest nucleation times were found for concentrated supersa turated biles, whereas vesicular cholesterol/phospholipid ratio was no t different from dilute or unsaturated biles. The present study indica tes that increased concentration of bile promotes nucleation of choles terol crystals by increasing the concentration of cholesterol in the v esicular peak rather than by changing its cholesterol/phospholipid rat io.