Transport of conjugated bilirubin and other organic anions in bile: relation to biliary lipid structures.

Using gel-permeation chromatography, we studied associative relationships between conjugated bilirubin and various biliary lipid particle species, including lecithin/cholesterol vesicles, mixed-lipid micelles, and simple bile salt micelles. Five other organic anions were comparably studied: phenol red, Evans blue, sulfobromophthalein, rose bengal, and indocyanine green. For compounds of intermediate hydrophobicity, including conjugated bilirubin, the dominant association was with a bile salt/organic anion hybrid particle of dimensions larger than that of a simple pure bile salt micelle. Vesicular association was found to be dominant only for the most hydrophobic organic anions, indocyanine green and rose bengal; conversely, the most hydrophilic anion, phenol red, showed no vesicular association. Accordingly, a strong positive correlation (P less than 0.001) was found between percent vesicular association and degree of hydrophobicity of the organic anion. Alkaline conditions (eluant pH 9) decreased or prevented vesicular hydrophobic interaction with all anions. We conclude that two important particulate mechanisms for transport in bile of conjugated bilirubin and other water-soluble anions are bile salt/organic anion hybrid particles and vesicles. For most organic anions of intermediate hydrophobicity, including conjugated bilirubin, the bile salt/organic anion hybrid particle is the dominant transport vehicle.