Cytochrome P450 2E1 (CYP2E1) is the principal enzyme responsible for urethane metabolism: comparative studies using CYP2E1-null and wild-type mice

Urethane ([carbonyl-(14)C]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-(14)C]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. CO(2) was confirmed as the main metabolite of urethane. Significant inhibition of urethane metabolism to CO(2) occurred in CYP2E1-null versus wild-type mice. Pharmacokinetic modeling of (14)CO(2) exhalation data revealed that CYP2E1 is responsible for approximately 96% of urethane metabolism to CO(2) 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 CO(2) in both genotypes. In conclusion, this work demonstrated that CYP2E1, not esterase, is the principal enzyme responsible for urethane metabolism.

Some aspects of the metabolism of urethane and N-hydroxyurethane in rodents

1. Urethane and N-hydroxyurethane are interconvertible in C(-) and C57 mice. 2. In newborn C57/DBA hybrid mice, prior treatment with 3-methylcholanthrene or urethane stimulated the N-hydroxylation of urethane; SKF 525A inhibited the N-hydroxylation at 24hr. but stimulated it at 48hr. after administration. 3. Liver homogenates of CBA and C3H mice, and of Chester Beatty and hooded rats, but not whole-body homogenates of 1-day-old C57/DBA mice or lung homogenate of 3-week-old Chester Beatty rats, metabolized urethane into N-hydroxyurethane in small but definite amounts. 4. Nitrite was detected in the bodies of newborn C57/DBA hybrid mice treated with lethal doses of urethane or N-hydroxyurethane; nitrite formation from N-hydroxyurethane was stimulated by pretreatment of the animals with 3-methylcholanthrene. 5. The rate of catabolism of N-hydroxyurethane by C57/DBA mice was faster in 8-day-old than in 1-day-old animals of the same sex, and faster in females than in males of the same age. 6. Liver slices of several species of rats and mice catabolized N-hydroxyurethane at rates that varied with the age and sex of animals of the same species; liver homogenates or microsomes were less effective than slices from the same liver. 7. The enzyme activity was destroyed by boiling or freezing the liver; it was inhibited by increasing substrate concentration and by urethane, n-butyl carbamate, cyanide, p-benzoquinone or 2,4-dinitrophenol, but not by p-chloromercuribenzoate or menadione. 8. The catabolism of N-hydroxyurethane by liver slices from adult H-strain rats was not oxygen-dependent. 9. Lung homogenates of 4-week-old female Chester Beatty rats catabolized N-hydroxyurethane at 40% of the rate of liver slices from the same source. 10. O-Acetyl- and O-ethoxycarbonyl-N-hydroxyurethane were rapidly deacylated by liver homogenates from adult hooded rats and adult C57 mice, and by human erythrocytes. 11. N-Hydroxyurethane reacted rapidly with pyridoxal phosphate at pH7.4 and 37 degrees . 12. The rate of decomposition of N-hydroxyurethane in 0.1 n-sodium hydroxide was increased by Ni(2+), Cu(2+), Mn(2+) and [Fe(CN)(6)](3-) and decreased by Cr(2+), Zn(2+), Co(2+), Mg(2+) and Fe(2+). 13. Attempts to synthesize sulphonates of N-hydroxyurethane gave ethyl hydrogen sulphate, probably via rearrangement of the unstable O-sulphonate.