Effects of taurocholate on the size of mixed lipid micelles and their associations with pigment and proteins in rat bile.

Mixed lipid micelles were isolated from rat bile on taurocholate-equilibrated Sephadex G100 and G200 columns (5-60 mM) to study relationships between lipids and other constituents of bile. Phospholipid, cholesterol, and a bile salt peak co-eluted as mixed micelles at all taurocholate concentrations. The micellar radius, derived from the elution profile, increased progressively from approximately 1.6 nm to approximately 3.5 nm when the column taurocholate concentration was reduced from 40-60 mM to 5 mM (the physiological range for rat bile). Biliary bile pigment and bromsulphthalein, added in vivo, eluted as self-aggregates that were smaller than the lipid micelles. In contrast, on bromsulphthalein-equilibrated columns, unconjugated bromsulphthalein associated weakly with lipid micelles but this association accounted for less than 10% of the unconjugated dye in bile. No associations were found between lipid and proteins when SDS-polyacrylamide gel electrophoretic polypeptide patterns of column fractions were compared with the lipid elution profiles at different taurocholate concentrations. Two high molecular weight protein aggregates were demonstrated in bile (greater than 222,000 Mr) by Sephadex G200 chromatography. These studies provide a reliable estimate of rat bile lipid micelle size and suggest that bile pigment, bromsulphthalein and proteins do not form strong associations with biliary mixed micelles but exist in bile predominantly as self-aggregates.