Mutagenicity of nitrosamines in methyltransferase-deficient strains of Salmonella typhimurium coexpressing human cytochrome P450 2E1 and reductase

Although dialkylnitrosamines are environmentally significant carcinogens, t he use of short-term bioassays to assess the mutagenic potential of these c ompounds is problematic. The Ames test, a mutagenicity assay based on the r eversion of Salmonella typhimurium histidine auxotrophs, is the most widely used bioassay in genetic toxicology, but the traditional Ames tester strai ns are largely insensitive to dialkylnitrosamine mutagenicity. We have cons tructed two mutagenicity tester strains that co-express full-length human c ytochrome P450 2E1 and P450 reductase in S. typhimurium lacking ogt and ada methyltransferases (YG7104ER, ogt- and YG7108ER, ogt-, ada-). These new st rains are susceptible to dialkylnitrosamine mutagenicity in the absence of an exogenous metabolic activating system (S9 fraction). Mutagenicity is dep endent upon the coexpression of P450 2E1 with P450 reductase and is similar to or greater than that obtained with the parental strains in the presence of S9 fraction from ethanol-induced rat liver. These strains were also sen sitive to nitrosamines with longer alkyl side chains including diethylnitro samine, dipropylnitrosamine and dibutylnitrosamine. Mutagenicity decreased with alkyl chain length, consistent with the stringency of the ada-encoded enzyme for methyl and ethyl DNA adducts. These new strains may prove useful in the evaluation of nitrosamine contamination of food and environmental s amples. (C) 2000 Elsevier Science B.V. All rights reserved.