Monitoring cholesterol crystallization from lithogenic model bile by time-lapse density gradient ultracentrifugation

Cholesterol crystallization in a dilute, bile salt-rich model bile is a multiphase process in which early filamentous crystals gradually transform to classical cholesterol monohydrate plates. The pertinence of similar transformations in more complex model systems or native bile is, however, unclear. The aim of the present study was to characterize and monitor cholesterol crystallization in a model bile of physiological relevance.

A supersaturated model bile was prepared with a lipid composition (18 mM cholesterol, 37 mM lecithin, 120 mM taurocholate) that was derived from analyzing 10 gallbladder biles from cholesterol gallstone patients. Cholesterol crystallization was followed by light and electron microscopy, and sequential density gradient analysis of cholesterol-containing precipitates.

During cholesterol crystallization a reproducible sequence of events was recorded. First (T<18 h), cholesterol-rich vesicular and multilamellar structures (density 1.005-1.015 g/ml) were observed. Later, (T>60 h) filamentous, helical, tubular (density 1.015-1.04 g/ml) and plate-like (density 1.04-1.06 g/ml) cholesterol crystals appeared. The concentration of crystals increased gradually, while bilayer structures became desaturated with cholesterol and disappeared, and early crystal forms were replaced by plates. Eventually (T>25 days) only classical plate-like cholesterol monohydrate crystals were present. Exposure of cholesterol-containing precipitates to micellar (100 mM) deoxycholate dissolved the bilayer structures but not the crystals.

These data demonstrate that cholesterol crystallization in a physiologically relevant model bile is a multiphase process consisting of a sequence of transitions from vesicular and multilamellar structures to early crystal forms and to classical plate-like cholesterol monohydrate crystals. These transitions are associated with increasing density and decreasing phospholipid content of cholesterol precipitates. We suggest that time-lapse density gradient ultracentrifugation is a useful method for investigating and quantitating the process of cholesterol crystallization and factors that influence this process in bile.