Deconjugation of bilirubin accelerates coprecipitation of cholesterol, fatty acids, and mucin in human bile--in vitro study.

To examine the initial step of brown pigment gallstone formation, sterile human gallbladder bile samples were incubated with or without beta-glucuronidase in vitro. Enhanced bilirubin deconjugation achieved by adding beta-glucuronidase significantly accelerated the formation of a precipitate that contained bilirubin (28.2 +/- 3.8% of dry weight), cholesterol (14.3 +/- 5.2%), free fatty acids (12.0 +/- 1.3%), and glycoprotein (10.0 +/- 6.7%). Both the composition and scanning electron microscopic appearance of the precipitate were similar to these features in brown pigment gallstones. The cholesterol saturation index and nucleation time in the supernatant did not change with various incubation periods. The weight ratios of bilirubin to cholesterol in the precipitates correlated with those in bile (r = 0.76; P = 0.017). Gel chromatography of the precipitate showed high molecular weight glycoprotein to be the major constituent. Bilirubin, cholesterol, fatty acids, and mucin were found to coprecipitate in accordance with bilirubin deconjugation, which process may play an important role in an early stage of the formation of brown pigment gallstones.