Glutathione Conjugation of Electrophilic Metabolites of 1-Nitronaphthalene in Rat Tracheobronchial Airways and Liver: Identification by Mass Spectrometry and Proton Nuclear Magnetic Resonance Spectroscopy

1-Nitronaphthalene (I-NN) is a mutagenic nitroaromatic which has been detected in emissions from both heavy- and light-duty diesel engines, as well as in urban airborne particles. I-NN is a cytochrome P450-bioactivated) nonciliated bronchiolar epithelial (Clara) cell cytotoxicant. These studies examined the metabolism of 1-NN to electrophilic metabolites which were trapped as glutathione conjugates in highly susceptible (lung) and less susceptible (liver) tissues of the rat. Significant depletion of reduced glutathione was observed at all levels of tracheobronchial airways of rats treated with 200 mg/kg 1-NN, ip. This observation of depleted glutathione was consistent with the HPLC radioactivity profiles demonstrating six glutathione conjugates isolated from liver and lung microsomal incubations with I-NN, [^Hjglutathione, and glutathione S-transferase. Mass spectrometry of all six metabolites in electrospray positive ion mode yielded an ion of m/z 497 (M + H), and daughter ions of m/z 479 (loss of water), m/z 306 (glutathione), and m/z 177 (loss of the nitro group and formation of hydroxy naphthalene thiolate ion), demonstrating the formation of hydroxy-dihydroglutathionyl derivatives presumably via intermediate epoxide(s). Proton nuclear magnetic resonance spectroscopy identified four different regioisomeric conjugates from lung and liver microsomal incubations: l-nitro7-glutathionyl-8-hydroxy-7,8-dihydronaphthalene, l-nitro-7-hydroxy-8-glutathionyl-7,8-dihydronaphthalene, l-nitro-5-hydroxy-6-glutathionyl-5,6-dihydronaphthalene, and l-nitro-5glutathionyl-6-hydroxy-5,6-dihydronaphthalene. HPLC radioactivity profiles demonstrated that major conjugates generated in the lung were derived from the C7,C8-epoxide, whereas the most prominent metabolites in the liver were derived from the C5,C6-epoxide.