Bovine gallbladder mucin binds bilirubin in vitro.

Gallbladder mucin glycoprotein and bilirubin are frequently identified at the center of human cholesterol gallstones, suggesting that mucin-bilirubin complexes might function as nucleating agents for cholesterol gallstones. This study was undertaken to determine if bovine gallbladder mucin was capable of binding bilirubin in vitro. Chromatography of dialyzed bovine gallbladder bile on Sepharose 4B revealed that bile pigment (OD420) eluted in a high-molecular weight fraction also containing mucin, as well as in an included (low-molecular weight) fraction. When bilirubin (98.8% as IX alpha by high-pressure liquid chromatography) and purified bovine gallbladder mucin were ultracentrifuged in a sucrose density gradient, the presence of bilirubin-mucin complexes was observed. Alteration of the polymeric structure of mucin with mercaptoethanol caused loss of its ability to bind bilirubin, suggesting that the native polymeric configuration is required. These results indicate that mucin can bind bilirubin in vitro, and might explain their occurrence in the nucleus of human cholesterol stones.