Cholesterol 7α-Hydroxylase and 12α-Hydroxylase

Three major cytochrome P450 enzymes are involved in the conversion of cholesterol into bile acids, i.e., cholesterol 7α-hydroxylase (EC 1.14.13.17), 12α-hydroxylase and 5β-cholestane-3α,7α,12α-triol 27-hydroxylase (EC 1.14.13.15) of which cholesterol 7α-hydroxylase catalyzes a rate-limiting step for overall conversion, whereas 12α-hydroxylase seems to regulate the relative amount of cholic acid over chenodeoxycholic acid (Bjorkhem 1985). 27-Hydroxylase catalyzes the hydroxylation of one of the terminal methyl groups of 5β-cholestane-3α,7α,12α-triol and 5β-cholestane-3α,7α-diol. In this chapter cholesterol 7α-hydroxylase and 12α-hydroxylase are reviewed. These two hydroxylases modify the steroid nucleus (Fig. 1). The cytochrome P450 species involved in side chain oxidation is reviewed elsewhere in this book by Wikvall (see Chap. 46).