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J Biol Chem, Vol. 273, Issue 29, 18153-18160, July 17, 1998
From the The primary physiological significance of
cytochrome P450c27 (CYP27) has been associated with its role in the
degradation of the side chain of C27 steroids in the
hepatic bile acid biosynthesis pathway, which begins with
7
Activities of Recombinant Human Cytochrome P450c27 (CYP27)
Which Produce Intermediates of Alternative Bile Acid Biosynthetic
Pathways
,
, and
Department of Biochemistry, Vanderbilt
University School of Medicine, Nashville, Tennessee 37232-0146 and
the ¶ Department of Clinical Chemistry, Karolinska Institute,
Huddinge Hospital, S-141 88 Huddinge, Sweden
-hydroxylation of cholesterol in liver. However, recognition that
in humans P450c27 is a widely or ubiquitously expressed mitochondrial
P450, and that there are alternative pathways of bile acid synthesis
which begin with 27-hydroxylation of cholesterol catalyzed by P450c27,
suggests the need to reevaluate the role of this enzyme and its
catalytic properties. 27-Hydroxycholesterol was thought to be the only
product formed upon reaction of P450c27 with cholesterol. However, the
present study demonstrates that recombinant human P450c27 is also able
to further oxidize 27-hydroxycholesterol giving first an aldehyde and
then 3
-hydroxy-5-cholestenoic acid. Kinetic data indicate that in a
reconstituted system, after 27-hydroxycholesterol is formed from
cholesterol, it is released from the P450 and then competes with
cholesterol for reentry the enzyme active site for further oxidation.
Under subsaturating substrate concentrations, the efficiencies of
oxidation of 27-hydroxycholesterol and 3
-hydroxy-5-cholestenal to
the acid by human P450c27 are greater than the efficiency of hydroxylation of cholesterol to 27-hydroxycholesterol indicating that
the first hydroxylation step in the overall conversion of cholesterol
into 3
-hydroxy-5-cholestenoic acid is rate-limiting. Interestingly,
3
-hydroxy-5-cholestenoic acid was found to be further metabolized by
the recombinant human P450c27, giving two monohydroxylated products
with the hydroxyl group introduced at different positions on the
steroid nucleus.
Copyright © 1998 by The American Society for Biochemistry and Molecular Biology, Inc.
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