Biphasic response for hepatic microsomal enzyme induction by 2,3,7,8-tetrachlorodibenzo-p-dioxin in C57BL/6J and DBA/2J mice

Importance of Asparagine 202 in Manipulating Active Site Structure and Substrate Preference for Human CYP17A1

The multifunctional cytochrome P450 17A1 (CYP17A1) plays a crucial role in human steroid hormone synthesis (UniProtKB─P05093). It first carries out standard monooxygenase chemistry, converting pregnenolone (PREG) and progesterone (PROG) into 17OH-PREG and 17OH-PROG, utilizing a "Compound I" to initiate hydrogen abstraction and radical recombination in the classic "oxygen rebound" mechanism. Additionally, these hydroxylated products also serve as substrates in a second oxidative cycle which cleaves the 17-20 carbon-carbon bond to form dehydroepiandrosterone and androstenedione, which are key precursors in the generation of powerful androgens and estrogens. Interestingly, in humans, with 17OH-PREG, this so-called lyase reaction is more efficient than with 17OH-PROG, based on Kcat/Km values. In the present work, the asparagine residue at 202 position was replaced by serine, an alteration which can affect substrate orientation and control substrate preference for the lyase reaction. First, we report studies of solvent isotope effects for the N202S CYP17A1 mutant in the presence of 17OH-PREG and 17OH-PROG, which suggest that the ferric peroxo species is the predominant catalytically active intermediate in the lyase step. This conclusion is further supported by employing a combination of cryoradiolysis and resonance Raman techniques to successfully trap and structurally characterize the key reaction intermediates, including the peroxo, the hydroperoxo, and the crucial peroxo-hemiketal intermediate. Collectively, these studies show that the mutation causes active site structural changes that alter the H-bonding interactions with the key Fe-O-O fragment and the degree of protonation of the reactive ferric peroxo intermediate, thereby impacting lyase efficiency.

Active site proton delivery and the lyase activity of human CYP17A1

Cytochrome P450 CYP17A1 catalyzes a series of reactions that lie at the intersection of corticoid and androgen biosynthesis and thus occupies an essential role in steroid hormone metabolism. This multifunctional enzyme catalyzes the 17α-hydroxylation of Δ4- and Δ5-steroids progesterone and pregnenolone to form the corresponding 17α-hydroxy products through its hydroxylase activity, and a subsequent 17,20-carbon-carbon scission of pregnene-side chain produce the androgens androstenedione (AD) and dehydroepiandrosterone (DHEA). While the former hydroxylation reaction is believed to proceed through a conventional "Compound I" rebound mechanism, it has been suggested that the latter carbon cleavage is initiated by an iron-peroxy intermediate. We report on the role of Thr306 in CYP17 catalysis. Thr306 is a member of the conserved acid/alcohol pair thought to be essential for the efficient delivery of protons required for hydroperoxoanion heterolysis and formation of Compound I in the cytochromes P450. Wild type and T306A CYP17A1 self-assembled in Nanodiscs were used to quantitate turnover and coupling efficiencies of CYP17's physiological Δ4- and Δ5-substrates. We observed that T306A co-incorporated in Nanodiscs with its redox partner cytochrome P450 oxidoreductase, coupled NADPH only by 0.9% and 0.7% compared to the wild type (97% and 22%) during the conversion of pregnenolone and progesterone, respectively, to the corresponding 17-OH products. Despite increased oxidation of pyridine nucleotide, hydroxylase activity was drastically diminished in the T306A mutant, suggesting a high degree of uncoupling in which reducing equivalents and protons are funneled into non-productive pathways. This is similar to previous work with other P450 catalyzed hydroxylation. However, catalysis of carbon-carbon bond scission by the T306A mutant was largely unimpeded by disruption of the CYP17A1 acid-alcohol pair. The unique response of CYP17A1 lyase activity to mutation of Thr306 is consistent with a reactive intermediate formed independently of proton delivery in the active site, and supports involvement of a nucleophilic peroxo-anion rather than the traditional Compound I in catalysis.

Cooperativity in cytochrome P450 3A4: linkages in substrate binding, spin state, uncoupling, and product formation

Understanding the detailed metabolic mechanisms of membrane-associated cytochromes P450 is often hampered by heterogeneity, ill-defined oligomeric state of the enzyme, and variation in the stoichiometry of the functional P450.reductase complexes in various reconstituted systems. Here, we describe the detailed characterization of a functionally homogeneous 1:1 complex of cytochrome P450 3A4 (CYP3A4) and cytochrome P450 reductase solubilized via self-assembly in a nanoscale phospholipid bilayer. CYP3A4 in this complex showed a nearly complete conversion from the low- to high-spin state when saturated with testosterone (TS) and no noticeable modulation due to the presence of cytochrome P450 reductase. Global analysis of equilibrium substrate binding and steady-state NADPH consumption kinetics provided precise resolution of the fractional contributions to turnover of CYP3A4 intermediates with one, two, or three TS molecules bound. The first binding event accelerates NADPH consumption but does not result in significant product formation due to essentially complete uncoupling. Binding of the second substrate molecule is critically important for catalysis, as the product formation rate reaches a maximum value with two TS molecules bound, whereas the third binding event significantly improves the coupling efficiency of redox equivalent usage with no further increase in product formation rate. The resolution of the fractional contributions of binding intermediates of CYP3A4 into experimentally observed overall spin shift and the rates of steady-state NADPH oxidation and product formation provide new detailed insight into the mechanisms of cooperativity and allosteric regulation in this human cytochrome P450.

The murine Cyp1a1 gene is expressed in a restricted spatial and temporal pattern during embryonic development

In adult mice the cytochrome P450 Cyp1a1 gene is not constitutively expressed but is highly inducible by foreign compounds acting through the aryl hydrocarbon (Ah) receptor. However, the expression profile of the Cyp1a1 gene in the developing embryo is not well under-stood. Using established transgenic mouse lines where 8.5 kb of the rat CYP1A1 promoter is cloned upstream of the lacZ reporter gene (1), we describe the expression of the CYP1A1-driven reporter gene in all tissues through-out stages E7-E14 of embryonic development. In contrast to the absence of constitutive Cyp1a1 and lacZ transgene expression in tissues of the adult mouse, a constitutive cell-specific and time-dependent pattern of CYP1A1 promoter activity was observed in the embryo. This expression pattern was confirmed as reflecting the endogenous gene by measuring Cyp1a1 mRNA levels and protein expression by immunohistochemistry. The number of cells displaying endogenous CYP1A1 activity could be increased in the embryo upon xenobiotic challenge, but only within areas where the CYP1A1 promotor was already active. When reporter mice were bred onto a genetic background expressing a lower affinity form of the Ah receptor (DBA allele), transgene and murine Cyp1a1 protein expression were both attenuated in the adult mouse liver upon xenobiotic challenge. By comparison, constitutive CYP1A1 promoter activity in the embryo was identical in the presence of either the high or low affinity Ah receptor. These novel data suggest that the Cyp1a1 protein may play a role in murine development and that regulation of the Cyp1a1 gene during this period is either through the action of a high affinity Ah receptor ligand or by an alternative regulatory pathway.

A teratologic suppressor role for p53 in benzo[a]pyrene-treated transgenic p53-deficient mice

DNA damage may mediate birth defects caused by many drugs and environmental chemicals, therefore p53, a tumour suppressor gene that facilitates DNA repair, may be critically embryoprotective. We have studied the effects of the environmental teratogen, benzo[a]pyrene, on pregnant heterozygous p53-deficient mice. Such mice exhibited between 2- to 4-fold higher embryotoxicity and teratogenicity than normal p53-controls. Fetal resorptions reflecting in utero death were genotyped using the polymerase chain reaction and found to be increased 2.6-fold and 3.6-fold respectively with heterozygous and homozygous p53-deficient embryos. These results provide the first direct evidence that p53 may be an important teratological suppressor gene which protects the embryo from DNA-damaging chemicals and developmental oxidative stress.

Differential susceptibility to seizures induced by systemic kainic acid treatment in mature DBA/2J and C57BL/6J mice

Mature DBA/2J (D2) and C57BL/6J (B6) mice aged 9-10 weeks were studied to determine susceptibility to behavioral seizures induced by kainic acid (KA) and the possible influence exerted by differences in metabolism and blood-brain barrier (BBB) transport. Mice were observed for 4 h after subcutaneous (s.c.) KA injection. Behavioral seizure parameters included latency to first seizure (clonus), latency to tonic/clonic seizure, and latency to status epilepticus (SE). At a KA dose of 25 mg/kg, 80% of D2 mice exhibited tonic/clonic seizures, whereas all B6 mice remained seizure-free. At 30 mg/kg, tonic/clonic seizures were observed in 100% of D2 mice and 25% of B6 mice. Of D2 mice exhibiting at least one clonic seizure in response to KA at a dose of 25 mg/kg, 50% entered SE and eventually died. Administration of [3H]KA (6.6 x 10(6) dpm) at doses of 25 mg/kg (convulsive) or 11.1 micrograms (nonconvulsive) to mice of both strains resulted in similar levels of radioactivity in cortex, hippocampus, and cerebellum 30 and 60 min after injection. Bioconversion of [3H]KA to a radiolabeled brain metabolite in vivo could not be documented in mice from either strain. Results confirm previously reported differences between D2 and B6 mice in their relative susceptibility to seizures induced by systemic KA administration and suggest that these differences are not related to strain-specific variation in metabolism or BBB transport of KA. Further studies of these two strains of mice may be useful for investigating genetic influences upon seizure susceptibility.

Effect of 2,3,7,8-tetrachlorodibenzoy-p-dioxin on the hepatic 7-ethoxyresorufin O-deethylase activity in four rodent species

Temporal and dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on hepatic 7-ethoxyresorufin O-deethylase (EROD) activity were investigated in young male Hartley guinea pigs, Sprague-Dawley rats, C57Bl/6 mice, DBA/2 mice and Golden Syrian hamsters. Animals were terminated 1, 7, 14, 28, 56 and 112 days after the administration of a single i.p. dose of TCDD. The maximal induction of EROD activity, at doses producing a similar toxic and limited lethal effect in all species/strains, was 42-, 18-, 7- and 3-fold in rats, DBA/2 mice, C57Bl/6 mice and guinea pigs, respectively. No treatment-related induction of EROD activity was observed in hamsters. Generally, maximal induction occurred 1-4 weeks after injection in all species. The guinea pig alone maintained the same magnitude of induction of EROD activity throughout the study. Observed toxic effects, i.e., lethality, loss of body weight gain, liver enlargement and thymic atrophy, did not correlate with the TCDD-induced hepatic EROD activity. The obtained results suggest that the fold induction of cytochrome P450 1A1 activity is not a critical event for the expression of the lethal effect of TCDD in rodents.

Comparison of 2,3,7,8-tetrachlorodibenzo-p-dioxin-mediated hepatotoxicity in C57BL/6J and DBA/2J mice

The toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was examined by clinical chemistry and liver histopathology in Ah-responsive C57BL/6J (C57) and Ah-nonresponsive DBA/2J (DBA) mice. Hepatotoxicity was assessed at 1, 3, and 7 d following a single ip injection of TCDD at doses that maximally induce hepatic aryl hydrocarbon hydroxylase (AHH) activity (3 micrograms/kg for C57 and 30 micrograms/kg for DBA mice) and at doses approaching the LD50 (150 micrograms/kg for C57 and 600 micrograms/kg for DBA mice). Histological examination of liver sections was found to be a more sensitive detection method for TCDD-induced hepatic changes than clinical chemistry analyses. Dramatic differences in the development and type of liver injury were observed between TCDD-treated C57 and DBA mice. C57 mice given 3 micrograms TCDD/kg developed mild to moderate hepatic lipid accumulation in the absence of both inflammation and necrosis. Severe fatty change and mild inflammation and necrosis occurred in C57 mice that received 150 micrograms TCDD/kg. In contrast, DBA mice exposed to 30 micrograms TCDD/kg developed hepatocellular necrosis and inflammation without any fatty change. Only slight hepatic lipid accumulation occurred with some necrosis and inflammation in DBA mice given 600 micrograms TCDD/kg. The Ah locus may play a role in determining the sensitivity of C57 mice to the steatotic effects of TCDD.