Evidence of quinone metabolites of naphthalene covalently bound to sulfur nucleophiles of proteins of murine Clara cells after exposure to naphthalene

Naphthalene-induced Clara cell toxicity in the mouse is associated with the covalent binding of electrophilic metabolites to cellular proteins. Epoxide and quinone metabolites of naphthalene are proposed to be the reactive metabolites responsible for covalent binding to proteins. To identify the nature of reactive metabolites bound to proteins (cysteine residues), we alkaline-permethylated proteins obtained from mouse Clara cells incubated with 0.5 mM naphthalene in vitro. Alkaline permethylation of protein adducts produced (methylthio)naphthalene derivatives detected by GC-MS. 3,4-Dimethoxy(methylthio)naphthalene was observed to be a predominant (methylthio)naphthalene derivative formed in the alkaline-permethylated protein sample obtained from Clara cells after exposure to naphthalene. This indicates that 1,2-naphthoquinone is a major metabolite covalently bound to cysteine residues of the cellular proteins. We have developed an immunoblotting approach to detect 1,2-naphthoquinone covalently bound to cysteine residues of proteins [Zheng, J., and Hammock, B. D. (1996) Chem. Res. Toxicol. 9, 904-909]. To identify 1,2-naphthoquinone covalently bound to sulfur nucleophiles of proteins, homogenates obtained from naphthalene-exposed Clara cells were separated by SDS-PAGE followed by Western blotting and immunostaining with the antibodies. Two protein bands with 24 and 25 kDa were detected by the antibodies, further supporting the view that 1,2-naphthoquinone is a reactive metabolite of naphthalene which binds to Clara cell proteins in vitro.