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Positional specificity for methyl-n-amylnitrosamine hydroxylation by cytochrome P-450 isozymes determined with
1Eppley Institute for Research in Cancer, University of Nebraska Medical Center, Omaha 68105.
Abstract:
Inhibitory monoclonal antibodies (MAbs) were used to determine the contribution of epitope-specific cytochrome P-450 isozymes in rat liver microsomes to hydroxylation of the esophageal carcinogen methyl-n-amylnitrosamine. These P-450-catalyzed reactions form 2-, 3-, 4-, and 5-hydroxymethyl-n-amylnitrosamine, formaldehyde (demethylation), and pentaldehyde (depentylation). With uninduced microsomes from male rats, MAb 1-68-11 inhibited 4-hydroxylation by 73% and demethylation by 46%. This indicated the major contribution of constitutive male-specific P-450 IIC11 to the metabolism. Inhibition studies with MAbs 2-66-3 and 1-91-3 indicated that P-450 IIB1 contributed 19% and IIE1 35% to demethylation. With uninduced microsomes from females, MAb 1-68-11 produced similar inhibitions to those in male rats, indicating that female-specific P-450 IIC12 (which is closely related to IIC11) also catalyzed 4-hydroxylation and demethylation. With microsomes from 3-methylcholanthrene-induced male rats, P-450 IA1 and/or IA2 were responsible for 60% of 3-hydroxylation and 40% of depentylation. With microsomes from phenobarbital-treated rats, P-450 IIB1 and IIB2 catalyzed all 6 reactions but especially 4-hydroxylation and depentylation, which were 50-75% inhibited by MAb 2-66-3. Microsomes from Aroclor-induced males behaved as if they were induced by both 3-methylcholanthrene and phenobarbital. After treatment with isoniazid (a P-450 IIE1 inducer), inhibition by MAb 1-91-3 indicated a 45% contribution of P-450 IIE1 to demethylation, and both P-450 IIE1 and IIB1 (or IIB2) appear to have been induced. A major finding with uninduced microsomes was the high specificity of MAb 1-68-11 for inhibiting 4-hydroxylation, indicating that P-450 IIC11 and IIC12 catalyzed most of this omega-1-hydroxylation. In microsomes from induced rats, the MAb inhibitions showed the role of the induced P-450 IA1 (or IA2), IIB1 (or IIB2), and IIE1 in methyl-n-amylnitrosamine hydroxylation at different positions, as well as the presence of P-450 IIC11. This study illustrates the usefulness of inhibitory MAbs for defining the contribution of individual P-450s to position-specific metabolism.
Insights
Monoclonal antibodies identified specific cytochrome P-450 isozymes responsible for metabolizing the carcinogen methyl-n-amylnitrosamine in rats. This research clarifies the roles of P-450 IIC11, IIC12, IIB1, IIE1, and IA1/IA2 in the hydroxylation and demethylation pathways.
Area of Science:
- Biochemistry
- Pharmacology
- Toxicology
Background:
- Cytochrome P-450 (P-450) enzymes are crucial for metabolizing xenobiotics, including carcinogens.
- Methyl-n-amylnitrosamine is an esophageal carcinogen whose metabolic pathways are not fully elucidated.
- Understanding the specific P-450 isozymes involved is key to assessing carcinogenic risk and developing targeted interventions.
Purpose of the Study:
- To determine the contribution of epitope-specific cytochrome P-450 isozymes to the metabolism of methyl-n-amylnitrosamine in rat liver microsomes.
- To elucidate the role of individual P-450s in the hydroxylation and demethylation of this carcinogen.
- To demonstrate the utility of inhibitory monoclonal antibodies (MAbs) in dissecting P-450-mediated reactions.
Main Methods:
- Utilized inhibitory monoclonal antibodies (MAbs) targeting specific cytochrome P-450 isozymes.
- Assessed the impact of MAbs on the hydroxylation and demethylation of methyl-n-amylnitrosamine in rat liver microsomes from both uninduced and induced rats.
- Quantified the percentage of inhibition for specific metabolic products (e.g., hydroxylated metabolites, formaldehyde, pentaldehyde).
Main Results:
- Constitutive P-450 IIC11 (male-specific) and P-450 IIC12 (female-specific) were major contributors to 4-hydroxylation and demethylation in uninduced rats.
- P-450 IIE1 and IIB1 were also significant contributors to demethylation, with P-450 IIE1 showing a 45% contribution after isoniazid induction.
- Induced P-450 IA1/IA2, IIB1/IIB2, and IIE1 played roles in various hydroxylation and depentylation reactions, depending on the inducer used (3-methylcholanthrene, phenobarbital, Aroclor).
Conclusions:
- Inhibitory MAbs are effective tools for delineating the specific roles of individual P-450 isozymes in the metabolism of methyl-n-amylnitrosamine.
- Multiple P-450 isozymes contribute to the carcinogen's metabolism in a sex- and induction-dependent manner.
- The study highlights the importance of P-450 IIC11/IIC12 in omega-1-hydroxylation and the broader involvement of other P-450s in detoxification or activation pathways.