Hydroxylation and lyase reactions of steroids catalyzed by mouse cytochrome P450 17A1 (Cyp17a1)

Abstract

Cytochrome P450 17A1 (CYP17A1) catalyzes 17α-hydroxylation and 17,20-lyase reactions with steroid hormones. Mice contain an orthologous Cyp17a1 enzyme in the genome, and its amino acid sequence has high similarity with human CYP17A1. We purified recombinant mouse Cyp17a1 and characterized its oxidation reactions with progesterone and pregnenolone. The open reading frame of the mouse Cyp17a1 gene was inserted and successfully expressed in Escherichia coli and then purified using Ni²⁺-nitrilotriacetic acid (NTA) affinity column chromatography. Purified mouse Cyp17a1 displayed typical Type I binding titration spectral changes upon the addition of progesterone, 17α-OH progesterone, pregnenolone, and 17α-OH pregnenolone, with similar binding affinities to those of human CYP17A1. Catalytic activities for 17α-hydroxylation and 17,20-lyase reactions were studied using ultra-performance liquid chromatography (UPLC)-mass spectrometry analysis. Mouse Cyp17a1 showed cytochrome b₅ stimulation in catalysis. In comparison to human enzyme, much higher specificity constants (k_cat/K_m) were observed with mouse Cyp17a1. In the reactions of Δ4-steroids (progesterone and 17α-OH progesterone), the specificity constants were 2100 times higher than the human enzyme. The addition of cytochrome b₅ produced significant stimulation of 17,20-lyase activities of mouse Cyp17a1. Two Arg mutants of mouse Cyp17a1 (R347H and R358Q) displayed a larger decrease in 17,20-lyase reaction (from 17α-OH pregnenolone to dehydroepiandrosterone, DHEA) than 17α-hydroxylation, indicating that -as in human CYP17A1-these basic residues in mouse Cyp17a1 are important in interacting with the cytochrome b₅ protein in the lyase reactions.

Keywords: CYP17A1; Cytochrome P450; Cytochrome b(5); Enzyme catalysis; Enzyme inhibitor; Enzyme kinetics; Enzyme mechanism; Pregnenolone; Progesterone.