Mechanism of 17?,20-Lyase and New Hydroxylation Reactions of Human Cytochrome P450 17A1: 18O LABELING AND OXYGEN SURROGATE EVIDENCE FOR A ROLE OF A PERFERRYL OXYGEN.

ABSTRACT: Cytochrome P450 (P450) reactions can involve C-C bond cleavage, and several of these are critical in steroid and sterol biosynthesis. The mechanisms of P450s 11A1, 17A1, 19A1, and 51A1 have been controversial, in the context of the role of ferric peroxide (FeO2 (-)) versus perferryl (FeO(3+), compound I) chemistry. We reinvestigated the 17?-hydroxyprogesterone and 17?-hydroxypregnenolone 17?,20-lyase reactions of human P450 17A1 and found incorporation of one (18)O atom (from (18)O2) into acetic acid, consonant with proposals for a ferric peroxide mechanism (Akhtar, M., Lee-Robichaud, P., Akhtar, M. E., and Wright, J. N. (1997) J. Steroid Biochem. Mol. Biol. 61, 127-132; Akhtar, M., Wright, J. N., and Lee-Robichaud, P. (2011) J. Steroid Biochem. Mol. Biol. 125, 2-12). However, the reactions were supported by iodosylbenzene (a precursor of the FeO(3+) species) but not by H2O2 We propose three mechanisms that can involve the FeO(3+) entity and that explain the (18)O label in the acetic acid, two involving the intermediacy of an acetyl radical and one a steroid 17,20-dioxetane. P450 17A1 was found to perform 16-hydroxylation reactions on its 17?-hydroxylated products to yield 16,17?-dihydroxypregnenolone and progesterone, suggesting the presence of an active perferryloxo active species of P450 17A1 when its lyase substrate is bound. The 6?-hydroxylation of 16?,17?-dihydroxyprogesterone and the oxidation of both 16?,17?-dihydroxyprogesterone and 16?,17?-dihydroxypregnenolone to 16-hydroxy lyase products were also observed. We provide evidence for the contribution of a compound I mechanism, although contribution of a ferric peroxide pathway in the 17?,20-lyase reaction cannot be excluded.

SUBMITTER: Yoshimoto FK 

PROVIDER: S-EPMC5016118 | biostudies-literature | 2016 Aug

REPOSITORIES: biostudies-literature