Abstract

Urethane is a fermentation by-product and a potent animal carcinogen. Human exposure to urethane occurs through consumption of alcoholic beverages and fermented foods. Recently, CYP2E1 was identified as the primary enzyme responsible for the metabolism of [¹⁴C]carbonyl-labeled urethane. Subsequently, attenuation of urethane-induced cell proliferation and genotoxicity in CYP2E1-/- mice was reported. The present work compares the metabolism of single versus multiple exposures of CYP2E1-/- and CYP2E1+/+ mice to ¹⁴C-ethyl-labeled urethane. Urethane was administered as a single 10 or 100 mg/kg gavage dose or at 100 mg/kg/day for 5 consecutive days. CYP2E1+/+ mice administered single or multiple doses exhaled 78 to 88% of dose as ¹⁴CO₂/day. CYP2E1-/- mice eliminated 30 to 38% of a single dose as ¹⁴CO₂ in 24 h and plateaued after day 3 at ≈52% of dose/day. The concentrations of urethane-derived radioactivity in plasma and tissues were dose-dependent, increased as a function of the number of doses administered, and were significantly higher in CYP2E1-/- versus CYP2E1+/+ mice. Whereas urethane was the main chemical found in the plasma and tissues of CYP2E1-/- mice, it was not detectable in CYP2E1+/+ mice. In conclusion, multiple dosing led to considerable bioaccumulation of urethane in mice of both genotypes; however, greater retention occurred in CYP2E1-/- versus CYP2E1+/+ mice. Furthermore, greater bioaccumulation of ¹⁴C-ethyl-labeled than [¹⁴C]carbonyl-labeled urethane was observed in mice. Comparison of the metabolism of ethyl-versus carbonyl-labeled urethane was necessary for tracing the source of CO₂ and led us to propose for the first time that C-hydroxylation is a likely pathway of urethane metabolism.