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Metabolism of polycyclic aza-aromatic carcinogens catalyzed by four expressed human cytochromes P450
S J Roberts-Thomson1, M E McManus, R H Tukey
1Department of Pharmacy, University of Sydney, New South Wales, Australia.
Abstract:
The role of human cytochromes P4501A1, -1A2, -3A4, and -3A5 in the metabolism of the polycyclic aza-aromatic hydrocarbons 7-methylbenz(c)acridine and dibenz(aj)acridine was investigated. The regioselectivity of the reactions was determined, as well as the associated stereoselectivity in the production of dihydrodiol metabolites and K-region oxides. Metabolite distributions were also examined in the presence of the epoxide hydrolase inhibitor 1,1,1-trichloropropylene-2,3-oxide and the P450 modulator alpha-naphthoflavone. P4501A2 was most regioselective for the production of the proximate carcinogen; the 3,4-dihydrodiol of 7-methylbenz(c)acridine and P4503A4 showed the highest regioselectivity for K-region oxidation. In contrast, the analogous putative proximate carcinogen of dibenz(aj)acridine was formed with the highest relative abundance by P4503A4, while P4501A2 was most regioselective for K-region oxidation. For both compounds the proximate carcinogens possessed predominantly the 3R,4R-absolute configuration, independent of the P450 catalyzing the reaction. The K-region dihydrodiols of 7-methylbenz(c)acridine were formed with no stereoselectivity, except with P4501A2 which favored production of the S,S isomer. In contrast the K-region dihydrodiol of dibenz(aj)acridine was formed by P4501A1 and P4501A2 as the R,R isomer with almost 100% optical purity. P4501A2 and 3A4 showed no stereoselectivity in the formation of the K-region oxide of 7-methylbenz(c)acridine, while P4501A1 produced the 5R,6S-oxide with low optical purity. For dibenz(aj)acridine 5,6-oxide, P4501A1 predominantly formed 5S,6R-oxide (80% pure). These results emphasize the importance of the composition and levels of expressed P450s of an individual in relation to the activation and detoxification of toxicants.
Insights
Human cytochromes P4501A2 and P4503A4 play key roles in metabolizing polycyclic aza-aromatic hydrocarbons, influencing toxicant activation and detoxification pathways. Understanding these P450 enzyme specificities is crucial for assessing individual risk from environmental pollutants.
Area of Science:
- Biochemistry
- Toxicology
- Pharmacology
Background:
- Polycyclic aza-aromatic hydrocarbons (PAHs) are environmental toxicants.
- Human cytochromes P450 (CYPs) are crucial for metabolizing xenobiotics.
- CYP1A and CYP3A subfamilies are involved in PAH metabolism.
Purpose of the Study:
- Investigate the role of human CYPs 1A1, 1A2, 3A4, and 3A5 in metabolizing 7-methylbenz(c)acridine and dibenz(aj)acridine.
- Determine regioselectivity and stereoselectivity of PAH metabolism by specific CYPs.
- Examine metabolite distributions with enzyme inhibitors and modulators.
Main Methods:
- Incubation of PAHs with human liver microsomes expressing specific CYPs.
- Analysis of metabolites using High-Performance Liquid Chromatography (HPLC) and Mass Spectrometry (MS).
- Use of epoxide hydrolase inhibitor and CYP modulator to study metabolic pathways.
Main Results:
- CYP1A2 showed high regioselectivity for proximate carcinogen formation from 7-methylbenz(c)acridine.
- CYP3A4 was most regioselective for K-region oxidation of 7-methylbenz(c)acridine.
- Dibenz(aj)acridine metabolism showed different regioselectivity patterns depending on the CYP involved.
- Stereochemical analysis revealed specific configurations for dihydrodiol and epoxide metabolites, with some CYP-dependent stereoselectivity.
Conclusions:
- CYP isoforms exhibit distinct regioselectivity and stereoselectivity in PAH metabolism.
- Metabolite profiles and stereochemistry are influenced by the specific CYP enzyme involved.
- Individual variations in CYP expression levels can significantly impact the activation and detoxification of PAHs, affecting toxicological outcomes.