Carcinogenic
polycyclic aromatic hydrocarbons (PAH), such as
benzo[a]pyrene (B[a]P), 7,12-dimethylbenz[a]
anthracene (DMBA), and
dibenzo[a,l]pyrene (DB[a,l]P), are metabolically activated to electrophilically reactive bay or fjord region diol
epoxides that bind to the exocyclic amino groups of
purine bases in
DNA to form stable adducts. In addition, it has been reported that these PAH can be enzymatically oxidized to yield radical
cations that form apurinic (AP) sites in
DNA via depurinating adducts. The formation of stable adducts and AP sites in
DNA of human cells exposed to PAH was examined in
cytochrome P450 (P450)-expressing mammary
carcinoma MCF-7 cells and in
leukemia HL-60 cells, which display a high
peroxidase but no P450-mediated activity, after exposure to these PAH. Stable
DNA adducts were assessed by (33)P-postlabeling/HPLC analysis, and the induction of AP sites in
DNA was analyzed by an
aldehyde reactive probe (ARP) and a slot blot method. After exposure for 4 h, the levels of stable
DNA adducts were comparable in MCF-7 cells treated with B[a]P and DMBA, but significantly lower than those observed in MCF-7 cells treated with the stronger
carcinogen DB[a,l]P. While the levels of stable adducts increased more than 10-fold (B[a]P and DMBA) or 100-fold (DB[a,l]P) after exposure for 24 h, the levels of AP sites remained low after both treatment periods. Thus, the levels of stable adducts were approximately 5-fold higher than the levels of AP sites
after treatment with B[a]P or DMBA and more than 100-fold higher in cells exposed to DB[a,l]P for 24 h. None of these carcinogenic PAH formed detectable levels of stable
DNA adducts or AP sites in HL-60 cells. The results demonstrate that metabolic activation of B[a]P, DMBA, and DB[a,l]P is catalyzed by
P450 enzymes leading to diol
epoxides that form predominantly stable
DNA adducts but only low levels of AP sites.