MODULATION OF THE ACTIVITY OF HUMAN 17-ALPHA-HYDROXYLASE-17,20-LYASE (CYP17) BY CYTOCHROME B(5) - ENDOCRINOLOGIC AND MECHANISTIC IMPLICATIONS

Using NADPH-cytochrome P-450 reductase as electron donor the homogeneous pig 17 alpha-hydroxylase-17,20-lyase (CYP17) was shown to catalyse the conversion of Delta(5), as well as Delta(4), steroids (pregnenolone and progesterone respectively) predominantly into the corresponding 17 alpha-hydroxylated products. The latter were then cleaved by the lyase (desmolase) activity of the enzyme into androgens. Cytochrome b(5) stimulated both these activities, but its most noticeable effect was on the formation of Delta(16)-steroids, which compulsorily required the presence of cytochrome b(5). These results on the pig enzyme confirm the original findings [Nakajin, Takahashi, Shinoda and Ha11 (1985) Biochem. Biophys. Res. Commun. 132, 708-713]. The human CYPI7 expressed in Escherichia coli [Imai, Globerman, Gertner, Kagawa and Waterman (1993) J. Biol. Chem. 268, 19681-19689] was also purified to homogeneity and was found to catalyse the hydroxylation of pregnenolone and progesterone without requiring cytochrome b(5). Like the pig CYP17, the human CYP17 also catalysed the cytochrome b(5)-dependent direct cleavage of pregnenolone into the Delta(5,16)-steroid, but unlike it the human enzyme did not cleave progesterone at all. 17 alpha-Hydroxypregnenolone was, however, cleaved into the corresponding androgen but only in the presence of cytochrome b(5). 17 alpha-Hydroxyprogesterone was a poor substrate for the human CYP17; although it was converted into androstenedione in the presence of cytochrome b(5) its K-m was 5 times higher and V-max. 2.6 times lower than those for the hydroxylation of progesterone. The endocrinological and mechanistic implications of these results are discussed.