Role of cytochrome P-450 2E1 in methacrylonitrile metabolism and disposition

Methacrylonitrile (MAN) is a widely used aliphatic nitrile and is structura lly similar to the known rat carcinogen and suspected human carcinogen acry lonitrile (AN). There is evidence that AN is metabolized via the cytochrome P-450 (CYP) 2E1. Recently, we identified two biliary conjugates originatin g from the interaction of MAN and its epoxide with glutathione. Mercapturic acids formed via the degradation of the two conjugates were also identifie d in rat and mouse urine. Additionally, a significant portion of MAN was el iminated in the expired air as CO2 (formed via the epoxide pathway) and unc hanged MAN. The objective of the present work was to determine whether CYP2 E1 is involved in the oxidative metabolism of MAN as was suggested for AN. 2-C-14-MAN was administered to CYP2E1-null or wild-type mice by gavage at 1 2 mg/kg. Although total urinary and fecal excretion of MAN-derived radioact ivity was slightly different in CYP2E1-null versus wild-type mice, the rati o of mercapturic acids originating from the epoxide-glutathione versus MAN- glutathione conjugates were lower in urine of CYP2E1-null mice than in that of wild-type animals. Exhalation of MAN-derived organic volatiles (primari ly parent MAN) was 12- and 42-fold greater in female and male CYP2E1-null m ice than in wild-type mice, respectively. Additionally, exhalation of CO, d erived from metabolism of MAN via the CYP2E1 pathway was 3- to 5-fold great er in wild-type than in CYP2E1-null animals. Although these data indicate t hat CYP2E1 is the principal enzyme responsible for the oxidative metabolism of MAN, other cytochrome P-450 enzymes may be involved. Assessment of MAN metabolism in CYP2E1-null mice pretreated with 1-aminobenzotriazole (CYP in hibitor) resulted in a further decrease in oxidative metabolites of MAN. Co mparison of the tissue concentrations of MAN-derived radioactivity in mouse tissues revealed that MAN-derived radioactivity is generally higher in wil d-type > CYP2E1-null mice > CYP2E1-null mice pretreated with 1-aminobenzotr iazole, suggesting a direct relationship between MAN oxidative metabolism a nd the half-life of MAN and/or its metabolites in various tissues. It is th erefore concluded that MAN oxidative metabolites such as the epoxide interm ediate have greater reactivity than parent MAN.