Ethyl carbamate metabolism: in vivo inhibitors and in vitro enzymatic systems

The metabolism of ethyl carbamate and the localization of its metabolites have been shown to be almost completely inhibited by ethanol in the mouse [Waddell, Marlowe, Pierce: Food Chem. Toxicol.25, 527 (1987); Yamamoto, Pierce, Hurst, Chen, Waddell: Drug Metab. Dispos. 16, 355 (1988)]. The enzyme system catalyzing this metabolism which is inhibited by ethanol now has been further investigated in both in vivo and in vitro studies. There is a direct, highly significant relationship between the extent of metabolism of ethyl carbamate and covalent binding of metabolites to liver protein. Paraoxon, carbaryl, CCl4 ethanol, methimazole, 4-methylpyrazole, diethyl maleate, ethyl N-hydroxycarbamate, and t-butyl carbamate inhibit, to different extents, the metabolism of ethyl carbamate in vivo; SKF-525A, CoCl2, Cacyanamide, chloral hydrate, 2-oxo-4-thiazolidine carboxylic acid, allopurinol, and methyl carbamate do not. Porcine liver esterase, yeast aldehyde dehydrogenase and mouse liver catalase catalyzed the metabolism in vitro; dog or bovine catalase, acid phosphatase, alcohol dehydrogenase, or carbonic anhydrase did not under the conditions tested. Paraoxon, 4-methylpyrazole, carbaryl, and NaF significantly inhibited the hydrolytic activity of mouse liver homogenates toward p-nitrophenyl acetate; ethanol or ethyl carbamate did not. However, each of these, except 4-methylpyrazole, inhibited the metabolism of ethyl carbamate by mouse liver homogenate or porcine liver esterase to about the same extent. Ion exchange chromatography of mouse liver cytosol revealed that the fraction with ability to metabolize ethyl carbamate co-chromatographed almost exactly with the ability to hydrolyze p-nitrophenyl acetate. It is proposed that ethyl carbamate is metabolized in the mouse, at least partially, by esterases; however, metabolism by other enzyme systems cannot be excluded.