DNA-binding studies with 6BT and 5I: implications for DNA-binding/carcinogenicity and DNA-binding/mutagenicity correlations

The divergent activities of a reported carcinogen/noncarcinogen pair of monoazo dyes related to the hepatocarcinogen Butter Yellow (DAB) are currently under investigation in our laboratories. As part of these studies we have determined (a) target organ distribution after oral dosing to rats and (b) covalent binding of 14C-labelled compound to DNA. In DNA-binding studies, 3 rat liver-metabolising systems were employed: in vivo (whole liver), isolated intact hepatocytes, and liver subcellular fractions. Distribution studies revealed that comparable levels of both compounds were detected in the liver at similar times after dosing, and these in vivo tissue concentrations were used for in vitro DNA-binding studies. At this 'in vivo equivalent dose', the carcinogen was consistently bound to DNA more effectively, and the difference (ratio of DNA binding) between the 2 compounds was far greater in vivo. In subsequent studies, covalent DNA binding to bacterial (Salmonella) DNA was assessed at the in vivo equivalent dose. In contrast to the afore-mentioned findings in mammalian systems, the carcinogen was bound less effectively to DNA, and gave fewer revertant counts/plate when the 2 compounds were bound to an equivalent extent. These data are discussed in view of their implications for DNA-binding/carcinogenicity correlations, and with respect to the relationship between DNA binding and mutagenicity in the Salmonella assay.