Different contributions of cytochrome P450 2E1 and P450 2B1/2 to chloroform hepatotoxicity in rat.

The contribution of cytochrome P450 isozymes CYP2E1 and CYP2B1/2 to chloroform-induced hepatotoxicity taken at 18 hr after the treatment was investigated in rats treated with n-hexane as an inducer of CYP2E1, 2-hexanone as an inducer of CYP2E1 and CYP2B1/2, and phenobarbital (PB) as an inducer of CYP2B1/2. Hepatic damage was evaluated by gross measurement of plasma alanine aminotransferase activity and histopathological examination. All treatments potentiated chloroform-induced hepatic damage. In n-hexane-pretreated rats, the damage was maximal with the middle dose of chloroform (0.2 ml/kg), whereas the damage increased with dose in rats treated with 2-hexanone or PB. The degree of hepatic damage induced with the three pretreatments was in the following order: n-hexane > 2-hexanone = PB with the middle dose of chloroform and PB >> 2-hexanone > n-hexane with the high dose (0.5 ml/kg); little difference among the pretreatments was seen with the low dose (0.1 ml/kg). These findings suggest that CYP2E1 is a low Km isoform and CYP2B1/2 a high Km isoform for chloroform activation. CYP2E1-dependent hepatic damage was characterized by ballooned hepatocytes, which were restricted to the centrilobular area; with CYP2B1/2, more necrotic than ballooned hepatocytes were seen and the necrotic hepatocytes were found not only in the centrilobular but also in the midzonal and periportal areas. Chloroform treatment did not affect the activity of N-nitrosodimethylamine N-demethylase in pretreated rats; the high dose increased the activity in control rats. In contrast, the high dose of chloroform decreased the activity of 7-pentoxyresorufin O-depentylase in all induced rats but not in controls. Immunoinhibition and immunoblot analyses showed that the high dose of chloroform induced CYP2E1 in control rats but decreased CYP2B1/2 in all pretreated rats. These results suggest that although both CYP2E1 and CYP2B1/2 contribute to chloroform-induced hepatic damage, they do so quite differently.