Partial characterization of a nonmicellar system of cholesterol solubilization in bile

We have shown that there were two distinctly separate cholesterol-containing fractions in human hepatic and gallbladder bile. In addition to mixed micelles that were composed of bile salts, cholesterol, and phospholipids and measured at ~25 Å by quasi-elastic light scattering spectroscopy, there was a nonmicellar fraction made up of cholesterol and phospholipids with no, or only trace amount of bile salts. This fraction had a mean hydrodynamic radius of 600 Å. When studied with electron microscopy, the fraction consisted of particles spherical in shape that measured 900-1,300 Å in diameter and were monodisperse. This form of cholesterol had a low buoyant density of <1.05 g/ml by density gradient ultracentrifugation and eluted as a macromolecular aggregate (mol wt>200,000) employing Sephadex G-75 chromatography. The quantity of nonmicellar cholesterol in bile correlated positively with the cholesterol saturation index (r=0.649; P<0.001) and inversely with relative bile salt concentration (r=-0.572, P≤0.03) and total lipid concentration (r=-0.844, P<0.0001). In vitro and in vivo addition of bile salts resulted in a shift of nonmicellar cholesterol to micellar cholesterol. In hepatic bile, nonmicellar cholesterol was the predominant and sometimes the exclusive form of cholesterol transport. When nucleation experiments were performed on gallbladder bile samples, the cholesterol that had nucleated were almost exclusively derived from the nonmicellar fraction.