Human gallbladder mucin binds biliary lipids and promotes cholesterol crystal nucleation in model bile

The binding of phosphatidylcholine and cholesterol in model bile to human gallbladder mucin was studied by means of a rapid filtration binding assay and sucrose density gradient ultracentrifugation. Numerous low affinity binding sites for phosphatidylcholine and cholesterol were present on gallbladder mucin. Binding of phosphatidylcholine and cholesterol to mucin increased as a function of cholesterol saturation index. Proteolytic digestion of mucin disaggregated the native mucin polymer and removed hydrophobic domains on the mucin peptide core that bind l-anilino-8-naphthalenesulfonic acid. Proteolytic digestion also resulted in a 91% and 78% decrease, respectively, in the binding of phosphatidylcholine and cholesterol to mucin. The ability of trypsin-treated and native mucin to promote the nucleation of cholesterol monohydrate crystals was compared in a model bile. The incidence of cholesterol monohydrate crystal nucleation with native mucin was significantly greater at 3 days than with trypsin-treated mucin or controls (P less than 0.001). After 3, 6, and 9 days of incubation, samples containing native mucin contained significantly more crystals than controls or trypsin-digested mucin samples (P less than 0.0001 for each). These data indicate that highly purified human gallbladder mucin binds phosphatidylcholine and cholesterol in model bile. Furthermore, this study demonstrates that structural integrity of the native mucin polymer and hydrophobic domains on the peptide core are essential for the nucleation of cholesterol monohydrate crystals by mucin in model bile.