Measurement of bilirubin partition coefficients in bile salt micelle/aqueous buffer solutions by micellar electrokinetic chromatography

The partition coefficients for the distribution of bilirubin between aqueou s phosphate-borate buffer and cholic, taurocholic, taurodeoxycholic, and ta urochenodeoxycholic micelles have been measured by micellar electrokinetic chromatography at pH 8.5. Determination of the partition coefficients requi red that the critical micelle concentration and partial specific volumes be determined for each bile salt. Critical micelle concentrations were slight ly higher for the trihydroxy bile salts. Partial specific volumes of the bi le salt micelles differed very little from each other, and for each bile sa lt they were constant over the concentration range studied, which was typic ally from slightly above the critical micelle concentration to 35 mM. Capac ity factors were corrected for the effects of applied voltage by extrapolat ion of the capacity factor to zero applied volts. The free solution mobilit y of bilirubin, determined in the absence of bile salt, was also corrected for the effects of applied voltage. Plots of extrapolated capacity factor v ersus phase ratio yield the partition coefficient as the slope of a linear fit to the data. Partition coefficients for bilirubin were significantly hi gher for dihydroxy bile salts than for trihydroxy bile salts.