Human steroid hormone biosynthesis is the result of a complex series of chemical transformations operating on cholesterol, with key steps mediated by members of the cytochrome P450 superfamily. In the formation of the male hormone dehydroepiandrosterone, pregnenolone is first hydroxylated by P450 CYP17A1 at the 17-carbon, followed a second round of catalysis by the same enzyme that cleaves the C17-C20 bond, releasing acetic acid and the 17-keto product. In order to explore the mechanism of this C-C "lyase" activity, we investigated the kinetic isotope effect on the steady-state turnover of Nanodisc-incorporated CYP17A1. Our experiments revealed the expected small positive (similar to 1.3) isotope effect for the hydroxylase chemistry. However, a surprising result was the large inverse isotope effect (similar to 0.39) observed for the C C bond cleavage activity. These results strongly suggest that the P450 reactive intermediate involved in this latter step is an iron-bound ferric peroxoanion.