Benzo(a)pyrene metabolism: Role of bioalkylation

Previous studies of the metabolic activation of chemical carcinogens including benzo(a)pyrene have demonstrated that selected aromatic hydrocarbons can undergo a series of biological substitution reactions in tissue cytosol preparations when in the presence of the methyl donor S-adenosyl-L-methionine. Although identification of the methylated metabolites of carcinogens had been tentatively established using liquid chromatographic techniques, confirmation of the identity of these methylated derivatives has remained elusive. In the present study, the carcinogen benzo(a)pyrene was found to undergo methylation reactions in the presence of rat liver cytosol preparations to yield products that were identified as methylated, formyl and hydroxyalkyl derivatives of the parent carcinogen. Identification and confirmation of the identity of the 6-methylbenzo(a)pyrene metabolite was confirmed by both UV detection, gas chromatographic and mass spectral identification, and NMR analysis of metabolic fractions obtained from incubations of benzo(a)pyrene. The results conclusively demonstrate that the carcinogen benzo(a)pyrene undergoes biological substitution reactions to yield 6-methylbenzo(a)pyrene in the presence of cytosolic enzymes. The presence of 6-methylbenzo(a)pyrene as a metabolite of benzo(a)pyrene illustrates and confirms the presence of centers of reactivity in unsubstituted aromatic hydrocarbons. These centers of reactivity serve as sites for biochemical substitution reactions, yielding methyl substituted hydrocarbons. Furthermore, these methylated metabolites of polycyclic aromatic hydrocarbons are further metabolically activated to hydroxyalkyl and formyl derivatives which can be subsequently activated to electrophilic esters leading to DNA and protein reactive species