CALCIUM AND THE ANIONIC POLYPEPTIDE FRACTION (APF) HAVE OPPOSING EFFECTS ON CHOLESTEROL CRYSTALLIZATION IN MODEL BILE

Background/Aims: Cholesterol gallstones contain both calcium and bilia ry proteins, but their respective roles in gallstone pathogenesis are unknown, We have studied the effects of calcium and a major biliary pr otein, anionic polypeptide fraction, on the process of cholesterol cry stallization in bile. Methods: Anionic polypeptide fraction was purifi ed from human bile, Model bile composed of cholesterol, egg yolk lecit hin and sodium taurocholate was prepared in a lipid concentration (18 mM, 37 mM, and 120 mM, respectively) simulating lithogenic human gallb ladder bile, The crystallization process was observed by phase contras t light microscopy, and sequential separation of precipitable choleste rol structures by sucrose density gradient ultracentrifugation. Result s: Addition of calcium, or anionic polypeptide fraction alone, or both together did not influence the crystal observation time of bile (the time which elapsed from initiation of supersaturation to the first app earance of crystals), However, the rate and quantity of cholesterol pr ecipitation and crystal formation were affected by both, Calcium incre ased in a dose-dependent manner the cholesterol monohydrate crystal ma ss before apparent equilibrium was reached, This effect was inhibited by anionic polypeptide fraction, which increased the amount of cholest erol within precipitable phospholipid vesicles, and decreased the rate of crystal formation, Fluorescence-labeled anionic polypeptide fracti on revealed that anionic polypeptide fraction (with and without calciu m) was primarily associated with vesicle aggregates. Conclusions: Our data demonstrate that calcium and anionic polypeptide fraction have op posing effects on the process of cholesterol crystallization and the r esultant crystal mass without influencing the crystal observation time of bile, These findings suggest that biliary proteins, in addition to being crystallization effecters by themselves, may further influence cholesterol crystallization and gallstone formation by interacting wit h calcium and possibly other elements that coexist in bile.