Evaluation of pyrazole and ethanol induced S9 fraction in bacterial mutagenicity testing.

A major constitutive enzyme in the liver of the uninduced rat is cytochrome P450-2E1. This isozyme has been shown to metabolize a number of carcinogens, including low molecular weight nitrosamines and a number of compounds normally regarded as non-mutagenic in the Ames test, e.g. aniline, urethane and benzene. Using the standard induction procedures [Aroclor 1254 or a combination of phenobarbitone (PB) and beta-naphthoflavone (beta-NF)] the level of CYP2E1 in rat liver is actually suppressed and it has been suggested that this may account for the negative findings with these compounds in the Ames test. S9 fractions were prepared from rats pre-treated with pyrazole or ethanol (inducers of CYP2E1) and then used in the Ames test (or pre-incubation modification) with urethane, acetaminophen, aniline, benzene, procarbazine and N-nitrosopyrrolidine. Both pyrazole and ethanol induced S9 were superior to PB/beta-NF-S9 and uninduced-S9 for the activation of N-nitrosopyrrolidine, a known CYP2E1 substrate. However, there was no evidence of mutagenic activity with urethane, aniline, benzene, procarbazine or acetaminophen. As these compounds have demonstrated genotoxicity in vivo, additional important metabolic pathways must be required which are not present in rat liver S9 fraction.