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Cytochrome P450c17 (EC 1.14.99.9) catalyzes both 17 alpha-hydroxylase and 17,20-lyase activities in mammalian steroidogenesis and also has some 16 alpha-hydroxylase activity. The ratio of 17 alpha-hydroxylase to 17,20-lyase activity differs in the adrenal and testis and is developmentally regulated at adrenarche, but the nature of the enzyme's active site and the differential regulation of its two principal activities are unknown. The spontaneous human P450c17 mutation Ser106-->Pro eliminates all enzymatic activity. We used site-directed mutagenesis to construct expression vectors for the conservative P450c17 mutations Ser106-->Thr and Ser106-->Ala. When expressed in transfected COS-1 cells, these mutants retain only 20-30% of the 17 alpha-hydroxylase and 17,20-lyase activities, but retain 60% of the 16 alpha-hydroxylase activity of the Ser106 wild type. Thus, the amino acid occupying position 106 greatly affects enzymatic activity. Ser is found at position 106 in P450c17 in all mammals and birds studied, but the corresponding residue (position 112) in fish (trout) is Thr. Both the trout Thr112 wild type and a Thr112-->Ser trout mutant had equivalent 16 alpha-hydroxylase, 17 alpha-hydroxylase, and 17,20-lyase activities, although these were only 5%, 5%, and 10%, respectively, of human Ser106. To catalyze its activities, P450c17 must receive electrons from NADPH via a flavoprotein termed P450 reductase. We examined the influence of the ratio of P450c17 to P450 reductase on enzymatic activity by cotransfecting COS-1 cells with varying amounts of vectors expressing each protein. The endogenous P450 reductase of COS-1 cells was sufficient to confer maximal 17 alpha-hydroxylase activity. P450 reductase produced from the transfected expression vector did not increase the conversion of [14C]progesterone to 17 alpha- or 16 alpha-hydroxyprogesterone, indicating that the endogenous immunodetectable P450 reductase of COS-1 cells was sufficient to confer maximal 17 alpha-hydroxylase activity. By contrast, the additional P450 reductase produced by the expression vector increased 17,20-lyase activity about 3-fold. Thus, the availability of reducing equivalents is a crucial factor in regulating 17,20-lyase activity. P450 reductase also increased the 17,20-lyase activity of the Thr106 and Ala106 mutants. These data suggest that the essential role of Ser106 is in the active site, rather than in interacting with P450 reductase, and that electron transfer may play an important role in regulating the 17,20-lyase activity of P450c17.