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3-Nitrobenzanthrone (3-NBA) was initially detected in diesel exhaust particulate matter and atmospheric particulate matter by the author, in Japan.This compound was highly mutagenic, and its activity was comparable to that of dinitropyrenes, the most mutagenic dinitroarenes.A dose of 2.5 mg 3-NBA could induce squamous cell carcinoma in the lungs of F344 rats.3-NBA is a widespread air pollutant, detected in airborne particulate matter in Tokyo,Germany, USA, and Denmark.The compound was also found in rainwater and soil surface samples in Japan.Because of its persistence in the environment as well as its high mutagenicity and carcinogenicity, the precise mechanism underlying 3-NBA carcinogenicity including the metabolic pathway for 3-NBA activation should be studied in detail.3-NBA is a nitro aromatic keto compound.In this class of compounds, the reduction of the nitro substituent is thought to proceed smoothly because of the electron-withdrawing effect of the keto moiety.Compared to the other monosubstituted derivatives of nitrobenzanthrone, 3-NBA has the lowest reduction pote ntial;therefore, the mutagenicity of these derivatives correlates well with the LUMO energy of these compounds.Moreover, the Ames test showed that the mutagenicity of 3-NBA was decreased by using strains that lack nitroreductase.These findings indicate that nitro reduction plays an important role in 3-NBA mutagenicity.Interestingly, polysubstituted nitro derivatives of nitrobenzanthrone exert mutagenicity to a lesser extent than 3-NBA, although these compounds readily accept electrons for nitro reduction.As generally observed for nitro compounds, mutagenicity resulted from the reduction of 3-NBA to 3-hydrcxyaminobenzanthrone (3-OHABA) followed by O-acetylation or O-sulfonylation.Several CYPs were involved in nitro reduction.We observed that O-acetylated 3-OHABA easily acts on DNA to form several DNA adducts.The chemical structures of several DNA adducts were deduced by independent chemical synthesis and NMR studies and 3 major DNA adducts, i.e., dG-C8, dG-N2, and dA adducts was formed in vivo.The mutagenicity of these adducts is not the same.The Escherichia coli TLS analysis system studies showed that the dG-N2 adduct has the most potent mutagenic properties, and this adduct is not easily repaired in vivo.In addition to these data, several recent findings regarding 3-NBA-induced mutagenicity will also be discussed during the presentation.