Heterologous expression of xenobiotic mammalian-metabolizing enzymes in mutagenicity tester bacteria: An update and practical considerations

There is an increasing need for metabolic competent cell systems for the me chanistic studies of biotransformation of xenobiotics in toxicology in gene ral and in genotoxicology in particular. These cell systems combine the het erologous expression of a particular mammalian biotransformation enzyme wit h a specific target/end point by which a functional analysis of the express ed gene product in the (geno)toxicity of chemicals can be performed, cDNAs of an increasing number of mammalian biotransformation enzymes is being clo ned. The construction of specific expression vectors permits their heterolo gous expression in laboratory bacteria, such as Escherichia coli strains. T his development does not only allow biochemical and enzymatic studies of (p ure) enzyme preparations but also facilitates the engineering of metabolica lly competent mutagenicity tester bacteria, thereby providing new tools for genotoxicity testing and for studying of the roles of biotransformation in chemical carcinogenesis. In this review, we describe an update as well as an evaluation of enzymes expressed in mutagenicity tester bacteria. Four ty pes of biotransformation enzymes are now expressed in these bacteria, namel y, GSTs, CYPs, NATs, and STs. The expression of these enzymes in the tester bacteria and their subsequent application in mutagenicity assays demonstra tes that heterologous expression in this type of bacteria has a number impl ications for the functionality of the biotransformation enzymes as well as for the functioning of the tester bacteria in mutagenicity detection. We al so describe here a number of practical considerations in this regard.