In vivo metabolism of isomeric naphthalene oxide glutathione conjugates

We have examined the fate of glutathione conjugates derived from naphthalene metabolism at various dose levels (5-80 mg/kg) in an effort to explore the potential use of urinary mercapturic acids as biomarkers of exposure to naphthalene and as indicators of the activity and stereoselectivity of cytochrome P-450-dependent naphthalene epoxidation. This approach extends previous studies which demonstrated a high degree of stereoselectivity in the formation of (+)-1R,2S-naphthalene oxide from naphthalene in target tissue microsomes (mouse lung), but not in microsomal preparations isolated from nontarget tissues such as mouse liver. To validate the use of mercapturic acids as indicators of epoxide formation in vivo, individual naphthalene oxide glutathione adducts isomers were administered iv to mice, and urinary metabolites were identified and quantified. Mercapturates accounted for 69-75% of the administered dose in the 8-hr urines of animals treated with trans-1-(S)-hydroxy-2-(S)-glutathionyl-1,2-dihydronaphthalene (adduct 1) and 76-84% for trans-1-(R)-hydroxy-2-(R)-glutathionyl-1,2-dihydronaphthalene (adduct 2). Only 39-57% of the dose of trans-1-(R)-glutathionyl-2-(R)-hydroxy-1,2-dihydronaphthalene (adduct 3) administered to mice was excreted as the mercapturic acid derivative; however, two additional metabolites were detected which were not present in the urine of animals trated with adducts 1 or 2. The first metabolite, accounting for 2-4% of the dose of adduct 3, was not identified. The second metabolite, isolated by HPLC and identified by mass spectrometry as (hydroxy-1,2-dihydronaphthalenylthio)pyruvic acid, accounted for 14-25% of the administered dose of adduct 3. Cysteine conjugate formation was both dose and isomer dependent, but was less than 7, 2, and 3% of the dose up to 40 mg/kg for adducts 1,2, and 3, respectively. Glutathione conjugate was not detected in any of the urines at doses up to 40 mg/kg. In mice treated with 80 mg/kg adduct, 2, 3.3% of the dose was eliminated unchanged as the glutathione conjugate. Administration of 10, 20, and 40 mg/kg racemic naphthalene oxide resulted in the excretion of all three mercapturic acid isomers in ratios which indicated no significant enantioselectivity in glutathione conjugation of the (±)-naphthalene oxide. The absence of enantioselectivity in the conjugation of naphthalene oxides with glutathione in combination with the similarity in the metabolic disposition of glutathione adducts 1 and 2 supports the conclusion that isomeric urinary mercapturic acids may be useful in monitoring the stereoselectivity of epoxide formation in vivo.