Activation of the moderate carcinogen 6-methylbenzo[a]pyrene (6-CH3BP) by one-electron oxidation to form DNA adducts was studied. Iodine oxidation of 6-CH3BP in the presence of dGuo produces BP-6-CH2-N2dGuo, BP-6-CH2- N7Gua and a mixture of 6-CH3BP-(1 and 3)-N7Gua, whereas in the presence of Ade the adducts BP-6-CH2-N1Ade, BP-6-CH2-N3Ade, BP-6-CH2-N7Ade and 6- CH3BP-(1 and 3)-N1Ade are obtained. Furthermore, for the first time an aromatic hydrocarbon radical cation afforded an adduct with dThd, the stable adduct BP-6-CH2-N3dThd. Formation of these adducts indicates that the 6- CH3BP radical cation has charge localized at the 6, 1 and 3 position. When 6-CH3BP was activated by horseradish peroxidase in the presence of DNA, two depurinating adducts were identified, BP-6-CH2-N7Gua (48%) and 6-CH3BP-(1 and 3)-N7Gua (23%), with 29% unidentified stable adducts. In the binding of 6-CH3BP catalyzed by rat liver microsomes, the same two depurinating adducts, BP-6-CH2-N7Gua (22%) and 6-CH3BP-(1 and 3)-N7Gua (10%), were identified, with 68% unidentified stable adducts. In 6-CH3BP-treated mouse skin, the two depurinating adducts, BP-6-CH2-N7Gua and 6-CH3BP-(1 and 3)- N7Gua, were identified. Although quantitation of these two adducts was not possible due to coelution of metabolites on HPLC, they appeared to be the major adducts found in mouse skin. These results show that 6-CH3BP forms depurinating adducts only with the guanine base of DNA, both in vitro and in mouse skin. The weaker reactivity of 6-CH3BP radical cation vs. BP radical cation could account for the weaker tumor-initiating activity of 6-CH3BP in comparison to that of BP. (C) 2000 Elsevier Science Ireland Ltd.
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