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Metabolism and covalent binding to DNA of 7-methylbenzo(a)pyrene
Abstract:
The ultimate carcinogenic form of benzo(a)pyrene (BP) is thought to result from metabolic activation at the 7 to 10 positions. Substitution by a methyl group at these positions would be expected to inhibit strongly their metabolism even though 7-methylbenzo(a)pyrene (7-MeBP) has been reported to be carcinogenic in some tumor models. The metabolism of 7-MeBP was, therefore, studied using both microsomal preparations and whole cells, the products being analyzed by high-pressure liquid chromatography, fluorescence spectrophotometry, and mass spectrometry. These studies revealed that many of the expected metabolites were formed by microsomes, but in addition 7-MeBP yielded a compound which was isolated and identified as trans-7,8-dihydro-7,8-dihydroxy-7-methylbenzo(a)pyrene. These results indicate that, despite the presence of a methyl group at the 7 position, a substituted BP can undergo the same initial metabolic activation as BP itself. However, in contrast to BP, the 7,8-dihydrodiol formed from 7-MeBP was almost racemic, and neither enantiomer was very active in the Ames bacterial mutagenesis assay when compared with trans-7,8-dihydro-7,8-dihydroxybenzo(a)pyrene. The metabolism of 7-MeBP was also studied in 10T1/2 cells. The hydrocarbon was metabolized readily and bound to DNA of the cells to about one-eighth of the level found for BP. However, no 7,8-dihydro-7,8-dihydroxy-7-methylbenzo(a)pyrene could be detected in the culture medium.
Insights
Methyl substitution on benzo(a)pyrene (BP) does not fully inhibit its metabolic activation. However, the resulting 7,8-dihydrodiol metabolite of 7-methylbenzo(a)pyrene (7-MeBP) is less mutagenic than that of BP.
Area of Science:
- Environmental Chemistry
- Toxicology
- Biochemistry
Background:
- Benzo(a)pyrene (BP) is a potent carcinogen whose ultimate carcinogenic form arises from metabolic activation.
- Methyl substitution at critical positions (7-10) was hypothesized to inhibit BP metabolism.
- 7-methylbenzo(a)pyrene (7-MeBP) has shown carcinogenicity in some models, necessitating further metabolic investigation.
Purpose of the Study:
- To investigate the metabolic activation of 7-methylbenzo(a)pyrene (7-MeBP).
- To compare the metabolic products and mutagenicity of 7-MeBP with benzo(a)pyrene (BP).
- To understand the role of methyl substitution in BP metabolism and carcinogenicity.
Main Methods:
- Metabolism studies using isolated microsomes and whole 10T1/2 cells.
- Analysis of metabolites via high-pressure liquid chromatography (HPLC), fluorescence spectrophotometry, and mass spectrometry.
- Assessment of mutagenicity using the Ames bacterial mutagenesis assay.
Main Results:
- 7-MeBP undergoes metabolic activation, forming trans-7,8-dihydro-7,8-dihydroxy-7-methylbenzo(a)pyrene, similar to BP.
- The 7,8-dihydrodiol metabolite of 7-MeBP was found to be nearly racemic and significantly less mutagenic than the BP analog.
- In whole cells, 7-MeBP bound to DNA at one-eighth the level of BP, with no detectable 7,8-dihydrodiol in the culture medium.
Conclusions:
- Methyl substitution at the 7-position of BP does not prevent initial metabolic activation to the dihydrodiol form.
- The reduced mutagenicity of the 7-MeBP dihydrodiol suggests steric or electronic effects of the methyl group influence its biological activity.
- Despite metabolic activation, 7-MeBP exhibits lower DNA binding and mutagenicity compared to BP, indicating a modified carcinogenic potential.