Bile concentration promotes nucleation of cholesterol monohydrate crystals by increasing the cholesterol concentration in the vesicles

Abstract

Cholesterol in bile is solubilized in mixed micelles and cholesterol-phospholipid vesicles. Biliary cholesterol supersaturation and increased concentration of bile in the gallbladder promotes nucleation of cholesterol monohydrate crystals and gallstone formation, possibly by creating unstable vesicles with a high cholesterol/phospholipid ratio. In the present study super-saturated and unsaturated biles (cholesterol saturation index (CSI) 1.4 and 0.8 respectively) were prepared with concentrations typical of gallbladder and more dilute hepatic bile (total lipid concentration (TLCo) 10 and 2.5 g dl-1 respectively). The distribution of cholesterol between vesicles and micelles, and vesicular cholesterol/phospholipid ratio were studied using ultracentrifugation and gel-permeation chromatography. The nucleation time of cholesterol crystals was determined in whole model bile, and in the vesicular and micellar peak fractions. Increased CSI and bile dilution led to an increased proportion of cholesterol solubilized in vesicles. The concentration 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 the ratio was significantly higher in supersaturated than in unsaturated biles in ultracentrifugation studies. Nucleation of cholesterol crystals from whole model bile was more rapid at the higher bile concentration and higher cholesterol saturation. Nucleation time in whole model bile correlated significantly with nucleation time in the corresponding vesicular peak fraction obtained by gel-permeation chromatography (r = 0.58: P < 0.01) and with the cholesterol concentration in this vesicular peak (r = -0.77; P < 0.002) but not with vesicular peak cholesterol/phospholipid ratio.(ABSTRACT TRUNCATED AT 250 WORDS)