Detoxification of Vinyl Carbamate Epoxide by Glutathione: Evidence for Participation of Glutathione S-Transferases in Metabolism of Ethyl Carbamate

Vinyl carbamate epoxide (VCO) is believed to be the metabolite of ethyl carbamate (EC) ultimately responsible for its carcinogenic effects. This study investigates the role of glutathione (GSH) in protection against VCO-mediated adduct formation, and the involvement of glutathione S-transferases (GSTs) in detoxification of VCO. Formation of 1, N6-ethenoadenosine from VCO and adenosine in vitro was employed as a measure of VCO toxicity. GSH inhibited formation of ethenoadenosine in a concentration-dependent manner at concentrations ranging from 1 to 8 mM. This effect was significantly enhanced by addition of rat liver GST. Mouse liver cytosol was also found to inhibit formation of ethenoadenosine in a concentration-dependent manner, and the inhibition was relieved by addition of S-octylglutathione, a competitive inhibitor of GST. Pretreatment of mice with 1% dietary 2(3)-tert-butyl-4-hydroxyanisole (BHA) caused parallel increases in cytosolic GST activity and cytosolic enhancement of detoxification of VCO by GSH. Furthermore, BHA increased hepatic steady-state concentrations of GSH greater than twofold. The effect of BHA on detoxification of EC in vivo was examined using formation of 2-oxoethylvaline (OEV) adducts of hemoglobin as a biomarker. Pretreatment with BHA decreased overall formation of OEV adducts 23%. The major conclusions of this study are (1) VCO can be detoxified by spontaneous conjugation with GSH, (2) conjugation of VCO with GST can be catalyzed by GST(s), (3) pretreatment with BHA protects against binding of active EC metabolites in vitro and in vivo, and (4) the protective effect of BHA against EC is mediated by increases in GST activity and GSH concentration.