Human cytochrome P450 17a-hydroxylase (CYP17) cata-lyses not only the 17a-hydroxlation of pregnenolone and progesterone and the C17,20-side chain cleavage (lyase) of 17a-hydroxypregnenolone, necessary for the biosynthesis of C21-glucocorticoids and C19-androgens, but also catalyses the 16a-hydroxylation of progesterone. In efforts to under-stand the complex enzymology of CYP17, structure/func-tion relationships have been reported previously after expressingrecombinant DNAs, encodingCYP17 from various species, in nonsteroidogenic mammalian or yeast cells. .
