Binding of benzo[a]pyrene to DNA by cytochrome P 450 catalyzed one-electron oxidation in rat liver microsomes and nuclei

Abstract

To investigate whether cytochrome P-450 catalyzes the covalent binding of substrates to DNA by one-electron oxidation, the ability of both uninduced and 3-methylcholanthrene (MC) induced rat liver microsomes and nuclei to catalyze covalent binding of benzo[a]pyrene (BP) to DNA and formation of the labile adduct 7-(benzo[a]pyren-6-yl)guanine (BP-N7Gua) was investigated. This adduct arises from the reaction of the BP radical cation at C-6 with the nucleophilic N-7 of the guanine moiety. In the various systems studied, 1-9 times more BP-N7Gua adduct was isolated than the total amount of stable BP adducts in the DNA. The specific cytochrome P-450 inhibitor 2-[(4,6-dichloro-o-biphenyl)oxy]ethylamine hydrobromide (DPEA) reduced or eliminated BP metabolism, binding of BP to DNA, and formation of BP-N7Gua by cytochrome P-450 in both microsomes and nuclei. The effects of the antioxidants cysteine, glutathione, and p-methoxythiophenol were also investigated. Although cysteine had no effect on the microsome-catalyzed processes, glutathione and p-methoxythiophenol inhibited BP metabolism, binding of BP to DNA, and formation of BP-N7Gua by cytochrome P-450 in both microsomes and nuclei. The decreased levels of binding of BP to DNA in the presence of glutathione or p-methoxythiophenol are matched by decreased amounts of BP-N7Gua adduct and of stable BP-DNA adducts detected by the 32P-postlabeling technique. This study represents the first demonstration of cytochrome P-450 mediating covalent binding of substrates to DNA via one-electron oxidation and suggests that this enzyme can catalyze peroxidase-type electron-transfer reactions.

Polycyclic aromatic hydrocarbon-DNA adduct formation in prostate carcinogenesis

“…Alternately, unstable PAH-DNA adducts can be formed via p450 mediated one-electron oxidation, but these adducts quickly depurinate leading to apurinic sites in DNA [49]. While it is not clear which type of adducts, stable or unstable, have a greater role in carcinogenesis, both kinds likely have some carcinogenic potential [50,51].…”

The effect of doxorubicin on rats that received toxic and carcinogenic benzo(a)pyrene.

“…PAHs are substances causing significant environmental pollution and they have been investigated for their carcinogenic effects [1][2][3][4][5][6]. These chemical substances have been also proved to cause malignancies in experimental animals [6,7].…”

“…Furthermore, B(a)P has also mutagenic and teratogenic effects [2,6,[10][11][12]. It transforms into (+)-BP-7-8-oxide by the monooxigenase enzyme, into (-)-trans-BP-7,8-dihydrodiol by the epoxide hydrolase enzyme and into (+)-anti-BP-7,8-diol-9,10-epoxide by the monooxigenase enzyme [1,5].…”

Polycyclic Aromatic Hydrocarbon-DNA Adducts in Prostate Cancer

“…Classically for (ϩ)-antibenzo-pyrene diol epoxide, stable adduct formation occurs at the 2-amino group of deoxyguanosine (6). Alternately, unstable PAH-DNA adducts can be formed via P450-mediated, one-electron oxidation; these adducts quickly depurinate leading to apurinic sites in DNA (3,4,7). Both stable and unstable adducts are thought to contribute to carcinogenesis (2, 8 -11).…”