Evidence for formation of an S-[2-(N7-guanyl)ethyl]glutathione adduct in glutathione-mediated binding of the carcinogen 1,2-dibromoethane to DNA.

The carcinogen 1,2-dibromoethane and reduced glutathione (GSH) were irreversibly bound to calf thymus DNA in equimolar amounts when in vitro incubations were carried out in the presence of GSH S-transferase. In studies carried out with isolated hepatocytes, equimolar amounts of 1,2-dibromoethane and endogenous GSH were also bound to intracellular DNA and RNA and extracellular DNA. These findings support the hypothesis that the major interaction of 1,2-dibromoethane with DNA involves covalent modification by a preformed complex of the carcinogen and GSH--i.e., S-(2-bromoethyl)GSH or the resulting episulfonium ion. Enzymatic hydrolysis of calf thymus DNA labeled with 1,2-dibromoethane in the presence of GSH and GSH S-transferase and subsequent high-performance liquid chromatography of the residues yielded a major fraction, which also was found to contain radiolabel derived from GSH. The fraction thus isolated was reductively desulfurized to yield N7-ethylguanine, which was isolated and identified by comparison with authentic material in two other high-performance liquid chromatography systems and by UV and mass spectrometry. Therefore, the structure of the undesulfurized adduct is assigned as S-[2-(N7-guanyl)ethyl]GSH. This adduct is unusual in that it is involved in a situation in which GSH plays a role in the bioactivation of a chemical carcinogen, as opposed to the more typical detoxication reactions. Further, a chemical carcinogen has been shown to cross-link DNA with a small physiological peptide.