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Cytochrome P-448 and the activation of toxic chemicals and carcinogens
Abstract:
The metabolic activation of carcinogens and some toxic chemicals appears to involve oxygenation in conformationally hindered positions in the chemical molecules. Oxygenation of xenobiotics in hindered positions is effected by cytochrome P-448 (LM4) but not by cytochrome P-450 (LM2). Substrate-interaction spectra show that cytochrome P-448 has an active site with a conformation different from that of cytochrome P-450. Induction of cytochrome P-448, as specifically measured by ethoxyresorufin O-deethylase activity, occurs in rat liver, kidney and lung after administration of the carcinogens, 3-methylcholanthrene, Aroclor 1254, 2-anthramine, safrole, 7,12-dimethylbenz[a]anthracene, MNNG and 2-acetamidofluorene. The doubtful carcinogens, saccharin, DDT and aldrin, resulted in no significant induction. The drugs paracetamol, antipyrine, imipramine and rifampicin resulted in diminished enzyme activity, indicating the absence of any induction of cytochrome P-448. In studies with the matched pairs of carcinogens and non-carcinogens, benzo[a]pyrene and benzo[e]pyrene, and 1,2,5,6-dibenzanthracene and anthracene, only the carcinogenic analogue resulted in induction of cytochrome P-448. With alpha- and beta-naphthylamine, both resulted in marked induction of cytochrome P-448 in liver, kidney and lung, indicating that both isomers might be carcinogenic.
Insights
Certain carcinogens activate toxic chemicals through oxygenation, a process primarily handled by cytochrome P-448 (LM4), not cytochrome P-450 (LM2). This study investigates cytochrome P-448 induction by various compounds.
Area of Science:
- Biochemistry
- Toxicology
- Enzymology
Background:
- Metabolic activation of carcinogens and toxic chemicals often involves oxygenation at sterically hindered molecular positions.
- Cytochrome P-448 (LM4) uniquely catalyzes oxygenation in these hindered positions, distinguishing it from cytochrome P-450 (LM2).
- Cytochrome P-448 possesses a distinct active site conformation compared to cytochrome P-450.
Purpose of the Study:
- To investigate the induction of cytochrome P-448 (LM4) by various known and suspected carcinogens.
- To differentiate the inductive effects of carcinogenic compounds from non-carcinogenic analogues.
- To assess the potential carcinogenicity of alpha- and beta-naphthylamine based on cytochrome P-448 induction.
Main Methods:
- Measuring ethoxyresorufin O-deethylase activity as a specific indicator of cytochrome P-448 induction.
- Administering a range of chemicals (carcinogens, suspected carcinogens, drugs) to rats.
- Analyzing substrate-interaction spectra to understand active site conformations.
- Comparing induction patterns between carcinogenic and non-carcinogenic chemical pairs.
Main Results:
- Significant cytochrome P-448 induction was observed in rat liver, kidney, and lung following administration of potent carcinogens (e.g., 3-methylcholanthrene, safrole).
- Weak or no significant induction was noted for suspected carcinogens like saccharin, DDT, and aldrin.
- Drugs such as paracetamol and rifampicin diminished enzyme activity, confirming no cytochrome P-448 induction.
- Carcinogenic analogues (benzo[a]pyrene, 1,2,5,6-dibenzanthracene) induced cytochrome P-448, unlike their non-carcinogenic counterparts.
- Both alpha- and beta-naphthylamine induced significant cytochrome P-448, suggesting potential carcinogenicity for both isomers.
Conclusions:
- Cytochrome P-448 (LM4) is specifically induced by certain carcinogens, particularly those requiring oxygenation in hindered positions.
- The induction patterns of cytochrome P-448 can serve as a biomarker for potential carcinogenicity.
- Both alpha- and beta-naphthylamine warrant further investigation due to their potent cytochrome P-448 induction.