Abstract

Urethane ([carbonyl-¹⁴C]ethyl carbamate) is a fermentation by-product in alcoholic beverages and foods and is classified as reasonably anticipated to be a human carcinogen. Early studies indicated that while CYP2E1 is involved, esterases are the primary enzymes responsible for urethane metabolism. Using CYP2E1-null (KO) mice, current studies were undertaken to elucidate CYP2E1's contribution to urethane metabolism. [Carbonyl-¹⁴C]urethane was administered by gavage to male CYP2E1-null and wild-type mice at 10 or 100 mg/kg and its metabolism and disposition were investigated. CO₂ was confirmed as the main metabolite of urethane. Significant inhibition of urethane metabolism to CO₂ occurred in CYP2E1-null versus wild-type mice. Pharmacokinetic modeling of¹⁴CO₂ exhalation data revealed that CYP2E1 is responsible for approximately 96% of urethane metabolism to CO₂ in wild-type mice. The contributions of other enzymes to urethane metabolism merely account for the remaining 4%. The half-life of urethane in wild-type and CYP2E1-null mice was estimated at 0.8 and 22 h, respectively. Additionally, the concentration of urethane-derived radioactivity in blood and tissues was dose-dependent and significantly higher in CYP2E1-null mice. High-performance liquid chromatography analysis showed only urethane in the plasma and liver extracts of CYP2E1-null mice. Because the lack of CYP2E1 did not completely inhibit urethane metabolism, the disposition of 10 mg/kg urethane was compared in mice pretreated with the P450 inhibitor, 1-aminobenzotriazole or the esterase inhibitor, paraoxon. Unlike paraoxon, 1-aminobenzotriazole resulted in significant inhibition of urethane metabolism to CO₂ in both genotypes. In conclusion, this work demonstrated that CYP2E1, not esterase, is the principal enzyme responsible for urethane metabolism.