Journal of Lipid Research, Vol 28, 673-683, Copyright © 1987 by Lipid Research, Inc.
Enzymatic oxidation of unconjugated bilirubin to assess its interactions with taurocholate
RV Rege, CC Webster and JD Ostrow
The rate of peroxidation of unconjugated bilirubin (UCB), catalyzed by
horseradish peroxidase (HRP), has been employed by Jacobsen (1969. FEBS
Lett. 5: 112-114) to assess the fraction of unbound UCB in the presence of
serum albumin. We used this method to examine the interactions of UCB with
taurocholate (TC) at pH 8.2, assuming solubilization of UCB by TC is due to
pigment binding and/or to partitioning into the micelle, thus rendering UCB
unavailable for peroxidation. Inhibition of UCB peroxidation conformed with
predictions based on these assumptions and demonstrated significant
interaction of UCB with both monomeric and micellar TC. Although
significant inhibition of UCB peroxidation was seen with TC monomer,
inhibition was even greater with TC micelles. In contrast, pyrogallol,
another substrate of HRP, acted very differently in the presence of TC.
Inhibition of pyrogallol peroxidation by TC was much less than with UCB and
occurred primarily with monomeric TC, with little further inhibition in the
micellar range. The results of this study suggest that at taurocholate
concentrations above 50 mM, similar to the physiologic bile salt
concentrations in human bile, at least 99% of UCB is bound to bile salt,
dramatically decreasing the concentration of unbound UCB. Since bile salts
also bind Ca2+, they play a dual role in protection against the
precipitation of calcium bilirubinate from bile. Therefore, bile salts are
a major factor in the prevention of the formation and growth of pigment
gallstones.
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
