Kinetic studies on androstenedione production in ovarian microsomes from immature rats

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

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.

Kinetic processivity of the two-step oxidations of progesterone and pregnenolone to androgens by human cytochrome P450 17A1

Cytochrome P450 (P450, CYP) 17A1 plays a critical role in steroid metabolism, catalyzing both the 17α-hydroxylation of pregnenolone and progesterone and the subsequent 17α,20-lyase reactions to form dehydroepiandrosterone (DHEA) and androstenedione (Andro), respectively, critical for generating glucocorticoids and androgens. Human P450 17A1 reaction rates examined are enhanced by the accessory protein cytochrome b₅ (b₅), but the exact role of b₅ in P450 17A1-catalyzed reactions is unclear as are several details of these reactions. Here, we examined in detail the processivity of the 17α-hydroxylation and lyase steps. b₅ did not enhance reaction rates by decreasing the k_off rates of any of the steroids. Steroid binding to P450 17A1 was more complex than a simple two-state system. Pre-steady-state experiments indicated lag phases for Andro production from progesterone and for DHEA from pregnenolone, indicating a distributive character of the enzyme. However, we observed processivity in pregnenolone/DHEA pulse-chase experiments. (S)-Orteronel was three times more inhibitory toward the conversion of 17α-hydroxypregnenolone to DHEA than toward the 17α-hydroxylation of pregnenolone. IC₅₀ values for (S)-orteronel were identical for blocking DHEA formation from pregnenolone and for 17α-hydroxylation, suggestive of processivity. Global kinetic modeling helped assign sets of rate constants for individual or groups of reactions, indicating that human P450 17A1 is an inherently distributive enzyme but that some processivity is present, i.e. some of the 17α-OH pregnenolone formed from pregnenolone did not dissociate from P450 17A1 before conversion to DHEA. Our results also suggest multiple conformations of P450 17A1, as previously proposed on the basis of NMR spectroscopy and X-ray crystallography.

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Structural and kinetic basis of steroid 17α,20-lyase activity in teleost fish cytochrome P450 17A1 and its absence in cytochrome P450 17A2

Cytochrome P450 (P450) 17A enzymes play a critical role in the oxidation of the steroids progesterone (Prog) and pregnenolone (Preg) to glucocorticoids and androgens. In mammals, a single enzyme, P450 17A1, catalyzes both 17α-hydroxylation and a subsequent 17α,20-lyase reaction with both Prog and Preg. Teleost fish contain two 17A P450s; zebrafish P450 17A1 catalyzes both 17α-hydroxylation and lyase reactions with Prog and Preg, and P450 17A2 is more efficient in pregnenolone 17α-hydroxylation but does not catalyze the lyase reaction, even in the presence of cytochrome b5. P450 17A2 binds all substrates and products, although more loosely than P450 17A1. Pulse-chase and kinetic spectral experiments and modeling established that the two-step P450 17A1 Prog oxidation is more distributive than the Preg reaction, i.e. 17α-OH product dissociates more prior to the lyase step. The drug orteronel selectively blocked the lyase reaction of P450 17A1 but only in the case of Prog. X-ray crystal structures of zebrafish P450 17A1 and 17A2 were obtained with the ligand abiraterone and with Prog for P450 17A2. Comparison of the two fish P450 17A-abiraterone structures with human P450 17A1 (DeVore, N. M., and Scott, E. E. (2013) Nature 482, 116-119) showed only a few differences near the active site, despite only ∼50% identity among the three proteins. The P450 17A2 structure differed in four residues near the heme periphery. These residues may allow the proposed alternative ferric peroxide mechanism for the lyase reaction, or residues removed from the active site may allow conformations that lead to the lyase activity.

Kinetic analysis of duodenal and testicular cytochrome P450c17 in the rat

In the adult rat, the duodenal tissue of both sexes can convert progesterone to 17alpha-hydroxyprogesterone, androstenedione and testosterone. The transition from C21 to C19 steroids is apparently controlled by the same cytochrome P450c17 expressed in the testis, which catalyzes both 17alpha-hydroxylation and C-17,20 bond scission at a single bifunctional active site. The kinetic parameters of this enzyme were measured at the steady state for both reactions using [1,2-3H]progesterone and [1,2-3H]17alpha-hydroxyprogesterone as substrates. In the testis and male and female duodena, the Km values for progesterone 17alpha-hydroxylation were 14.2, 23.8 and 23.2 nM, whereas the Vmax values were 105, 3.5 and 3.1 pmol/mg protein/min, respectively. With respect to C-17,20 lyase activity, the Km values for exogenous 17alpha-hydroxyprogesterone were 525, 675 and 637 nM, whereas the Vmax values were 283, 7.8 and 7.8 pmol/mg protein/min, respectively. However, when the Km values were calculated with respect to intermediate 17alpha-hydroxyprogesterone formed from progesterone, they were similar to the Km values for 17alpha-hydroxylase, being 15, 31.4 and 24.8 nM, whereas the Vmax values were 26.3, 2 and 1.8 pmol/mg protein/min, respectively. The similarity of Km values is due to the fact that the relative androgen formation efficiency (bond scission events/total 17alpha-hydroxylation events ratio) was remarkably constant in both testicular and duodenal incubates, irrespective of progesterone concentration. Efficiency values were 2-fold higher in duodenal tissue (0.54) than in testis (0.25). Estradiol-17beta inhibited 17alpha-hydroxylation but not bond scission on intermediate 17alpha-hydroxyprogesterone, because it did not affect the efficiency value. Rat duodenal P450c17 has the same substrate affinity, a lower specific activity and a higher androgen formation efficiency than testicular P450c17.

Regulation mechanism of the catalytic activity of bovine adrenal cytochrome P-450(11)beta

In our previous paper (Ikushiro et al. (1992) J. Biol. Chem. 267, 1464), two catalytic states were proposed for bovine adrenocortical P-450(11)beta at 37 degrees C: one in liposome membranes and the other in liposome membranes containing P-450scc. Similar reaction characteristics were observed at 5 degrees C and all the experiments in this study were performed at 5 degrees C. P-450(11)beta-proteoliposomes had relatively low 11 beta-hydroxylase activity and could catalyze aldosterone formation from 11-deoxycorticosterone. Relatively high 11 beta-hydroxylase activity was observed in P450(11)beta-proteoliposomes containing P-450scc and in Tween-20 solubilized P-450(11)beta, in which no aldosterone formation could be detected. Optical titration indicated binding of corticosterone to P-450(11)beta to be much weaker in the Tween-20 solubilized state than in proteoliposomes. Corticosterone competitively inhibited 11 beta-hydroxylation reaction of P-450(11)beta-proteoliposomes, but neither in P-450(11)beta-proteoliposomes containing P-450scc nor in the Tween-20 solubilized system. The binding of corticosterone to P-450(11)beta was concluded quite weak in proteoliposomes in the presence of P-450scc and in the Tween-20 solubilized state. Aldosterone formation thus was not possible in these systems. Inability of the bovine adrenocortical zonae fasciculata and reticularis to produce aldosterone may be due to the weak binding of corticosterone to P-450(11)beta in these zones.