CYP17 catalyzes the cleavage of the C-17 side chain of progesterone to form androstenedione. The two-step reaction involves an initial 17 alpha-hydroxylation catalyzed by oxene chemistry followed by cleavage of the C-17 side chain. We have recently shown that C-17 side-chain cleavage may involve the rearrangement of a peroxy intermediate via a Baeyer-Villiger rearrangement [Mak, A. Y., & Swinney, D. C. (1992) J. Am. Chem. Soc. 114, 8309]. Accordingly, CYP17 is proposed to catalyze oxidations via both oxene and peroxide chemistry. This study was initiated to investigate the possibility that protons may play a determining role in differentiating between the oxene and peroxide chemistries associated with product formation. The pL dependence of the deuterium solvent isotope effects associated with progesterone oxidation to 17 alpha-hydroxyprogesterone and 17-O-acetyltestosterone and 17 alpha-hydroxyprogesterone oxidation to androstenedione was determined in microsomes from pig testes. The formation of 17 alpha-hydroxyprogesterone is assumed to occur via oxene chemistry and the formation of 17-O-acetyltestosterone and androstenedione by peroxide chemistry. The initial rate of progesterone oxidation to 17 alpha-hydroxyprogesterone was associated with a pL-independent inverse solvent isotope effect (Hk/Dk = 0.75-0.95, in 30% DOD), whereas the rate of oxidation to 17-O-acetyltestosterone was associated with a pL-independent positive solvent isotope effect in the presence of 30% DOD (Hk/Dk approximately 2). In contrast, DOD inhibited the formation of androstenedione from 17 alpha-hydroxyprogesterone in a noncompetitive, pL-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)