Identification of a cysteinyl adduct of oxidized 3-methylindole from goat lung and human liver microsomal proteins.

3-Methylindole is a selective pneumotoxin that is oxidized by cytochrome P-450 enzymes to a reactive intermediate. 3-Methyleneidolenine, a methylene imine electrophile, is the postulated reactive intermediate, and it binds to proteins, a reaction that probably initiates the pneumotoxicity of 3-methylindole. Thioether adducts of this electrophile are formed with glutathione in vitro, but the identity of the adducted electrophile with amino acid residues of microsomal proteins had not previously been determined. 3-Methylindole was incubated with NADPH and goat lung or human liver microsomal proteins, and the proteins were hydrolyzed. 3-(Cystein-S-ylmethyl)indole was isolated and identified as the major amino acid adduct of 3-methyleneindolenine, demonstrating that cysteine thiols preferentially attack the exocyclic methylene position and result in a covalently (thioether) attached 3-methylindole residue to these pulmonary and hepatic proteins. These results demonstrate that the putative methylene imine intermediate is indeed the active electrophile that binds to proteins and presumably initiates the toxic events.