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Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston, Texas
By catalyzing the first steps in different pathways of cholesterol
degradation, cytochromes P450 (P450s) 7A1, 27A1, 11A1, and 46A1 play key roles
in cholesterol homeostasis. CYP7A1 is a microsomal liver-specific enzyme that
converts cholesterol to 7
-hydroxycholesterol. CYP27A1 is a ubiquitously
expressed mitochondrial P450 that metabolizes cholesterol to
27-hydroxycholesterol. CYP11A1 also resides in mitochondria but is expressed
mainly in steroidogenic tissues, where it catalyzes the conversion of
cholesterol to pregnenolone. Finally, CYP46A1 is a brain-selective microsomal
monooxygenase producing 24S-hydroxycholesterol from cholesterol.
Catalytic efficiencies of cholesterol-metabolizing P450s vary significantly
and probably reflect physiological requirements of different organs for the
rate of cholesterol turnover. P450s 7A1, 27A1, 11A1, and 46A1 represent a
unique system for elucidation of how different enzymes have adapted to fit
their specific roles in cholesterol elimination. Studies of
cholesterol-metabolizing P450s suggest that their activities could be
modulated post-translationally and that they should also be considered as
targets for regulation of cholesterol homeostasis.
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