Journal of Lipid Research, Vol 29, 1289-1296, Copyright © 1988 by Lipid Research, Inc.
Interactions of unconjugated bilirubin with bile salts
RV Rege, CC Webster and JD Ostrow
Department of Surgery, Northwestern University, Chicago, IL.
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 degrees 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-3
alpha,7 alpha-dihydroxy-12-oxo-5 beta- 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.
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