Metabolic activation of the carcinogen 6-hydroxymethylbenzo[a]pyrene: formation of an electrophilic sulfuric acid ester and benzylic DNA adducts in rat liver in vivo and in reactions in vitro.

Abstract

Hydroxylation of meso-methyl groups with subsequent formation of reactive electrophilic esters has been proposed as a possible activation pathway in the metabolism, DNA binding and carcinogenicity of some methyl-substituted polycyclic aromatic hydrocarbons. Some data in vitro have been reported in support of this concept. In this study, sulfotransferase activity for 6-hydroxymethylbenzo[a]pyrene (HMBP) in rat and mouse liver cytosols was demonstrated to mediate formation of benzylic adducts from this hydrocarbon with guanosine and with deoxyguanosine and deoxyadenosine in DNA. These benzylic adducts were also obtained from reactions of synthetic 6-sulfooxymethylbenzo[a]pyrene (SMBP) with individual (deoxy)ribonucleosides or DNA. The structure of the major DNA adduct formed from HMBP and SMBP was determined from NMR spectroscopy to be N2-(benzo[a]pyrene-6-methylenyl)-deoxyguanosine. Low levels of a deoxycytidine adduct were also obtained from DNA reacted with SMBP. Covalent modification of DNA by acetyl-CoA- and ATP-dependent activation of HMBP also produced the identical benzylic adducts, but the amounts were smaller than those obtained in the sulfotransferase-mediated reaction. The i.p. administration of HMBP to rats resulted in the formation of a hepatic DNA adduct. After enzymatic hydrolysis to the nucleoside level, this DNA adduct was chromatographically identical with the deoxyguanosine adduct formed in the above in vitro reactions. This adduct accounted for approximately 20-30% of total HMBP residues bound to hepatic DNA and its formation was significantly inhibited by pretreatment of rats with dehydroepiandrosterone, an inhibitor of the sulfotransferase activity for HMBP. The i.p. administration of comparable doses of SMBP to rats led to the formation of much larger amounts of the adducts with the guanine, adenine, and cytosine bases in the liver DNA. The data indicate that the sulfotransferase activity in the rat liver for HMBP plays a major role in the benzylic DNA adduct formation from this hydrocarbon in rat liver in vivo.