Food Lipids: Chemistry, Nutrition, and Biotechnology

of cholesterol to bile acids (Scheme 8). The first stage is the hydroxylation of cholesterol
at the 7� position to form cholest-5-ene-3�,7�-diol. Elucidation of the role
of LXR receptors has furthered our knowledge of 7�-hydroxylase and the role of
oxysterols in sterol metabolism [48d]. In the second stage, cholest-5-ene-3�,7�-diol
is first oxidized to 7�-hydroxycholest-5-en-3-one, which is isomerized to 7�-hydroxycholest-4-en-3-one.
Further enzymatic transformation leads to 5�-cholesta-
3�,7�-diol and 5�-cholesta-3�,7�,12�-triol. The third stage is the degradation of
the hydrocarbon side chain, which is less well understood. However, in cholic acid
formation it is generally considered to commence when the steroid ring modifications
have been completed. The side chain oxidation begins at the C-26 position; 3�,
7�,12�-trihydroxy-5�-cholestan-26-oic acid is an important intermediate. The removal
of the three terminal atoms is believed to proceed by a �-oxidation mechanism
analogous to that occurring in fatty acid catabolism.
Scheme 8 Biosynthesis of cholic acid from cholesterol.
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