Interactions of unconjugated bilirubin with bile salts

The rate of peroxidation of unbound, unconjugated bilirubin (UCB) was used to assess the interactions of UCB with four taurine-conjugated bile salts at pH 8.2, 37°C, and an ionic strength of 0.15. Each of the four structurally different bile salts markedly decreased the rate of peroxidation of UCB in the presence of horseradish peroxidase (HRP); 30% of UCB was bound even at low, premicellar bile salt concentrations (1 mM). At high bile salt concentrations (75 mM), taurocholate (TC) and tauro-3α,7α-dihydroxy-12-oxo-5β-cholan-24-oate (T12-OXO) exhibited the highest degree of inhibition of UCB peroxidation; only 0.6% and 1.1% of UCB were unbound respectively. Taurochenodeoxycholate (TCDC) yielded somewhat less inhibition with 2.0% of UCB unbound. Taurodehydrocholate (TDHC), a bile salt that does not form micelles but does form dimers, was comparable to TC and T12-OXO with unbound UCB of 1.0%. With TC and T12-OXO, apparent affinity for UCB was at least four times greater above the published critical micellar concentration (CMC) than in the premicellar range. TCDC was only studied above its CMC value and only one region of UCB binding was noted. Interaction of UCB with TDHC was similar to premicellar interactions with TC and T12-OXO below 25 mM, but increased to values intermediate between monomer and micelle above 40 mM TDHC, compatible with formation of TDHC dimers above 20 mM. These data show that there are differences in the ability of bile salts to bind UCB. Thus, alterations in bile salt profile in bile might lead to higher concentrations of free UCB in bile predisposing to pigment gallstones.