Three major classes of steroids include sterols, steroid hormones, and bile acids. Perhaps the most important sterol, cholesterol, serves as the precursor of both steroid hormones and bile acids. The synthesis of these compounds requires hydroxylation reactions. In addition, steroid-like substances such as vitamin D also undergo hydroxylation reactions. Many of these reactions are performed by the cytochrome P450 superfamily of enzymes. The many different forms of cytochrome P450 oxidize a wide variety of substrates. Moreover, in recent years, it has become apparent that many of these reactions exhibit remarkable regio- or stereoselectivity. For the most part, hydroxylation serves to increase the polarity of the rather hydrophobic steroids. By increasing their aqueous solubility, the …