Hepatic microsomal metabolism and covalent binding of 2,4-dinitrotoluene

The effects of 2,4-dinitrotoluene (2,4-DNT) on xenobiotic metabolizing enzymes and the hepatic metabolism and covalent binding of this compound to microsomal proteins in vitro were studied. Male Fischer-344 rats received po doses of DNT daily for 5 days at 14, 35, and 70 mg/kg/day. Hepatic oxygen-insensitive cytosolic azoreductase activity was increased and microsomal nitroreductase was decreased by DNT treatments. A small but significant increase in liver/body weight ratio and in hepatic cytochromes P-450 and b₅ occurred in the absence of changes in microsomal biphenyl hydroxylase or aryl hydrocarbon hydroxylase activities. The patterns of in vitro microsomal metabolism of DNT were dependent on oxygen tension: under aerobic conditions, 2,4-dinitrobenzyl alcohol (DNBAlc) was the major metabolite whereas under anaerobic conditions no DNBAlc was detected; 2-amino-4-nitrotoluene (2A4NT) and 4-amino-2-nitrotoluene (4A2NT) were the major metabolites. Pretreatment of rats with phenobarbital or Aroclor 1254 increased the metabolism of 2,4-DNT to DNBAlc by six- to sevenfold. Metabolism to the alcohol was inhibited by SKF-525A. These data suggested that oxidative metabolism of 2,4-DNT to DNBAlc was mediated by cytochrome P-450-dependent mixed-function oxidases. Covalent binding studies showed that a maximum of only 7 pmol of 2,4-DNT-derived radioactivity was bound per milligram of microsomal protein per hour; this binding was increased to 1.0 nmol bound/mg protein/hr in microsomes from phenobarbital of Aroclor 1254-pretreated rats. It is concluded that 2,4-DNT treatment had little effect on the activity of some hepatic xenobiotic metabolizing enzymes and was readily metabolized by liver preparations in vitro. The pathways of in vitro metabolism were dependent on oxygen tension. This in vitro metabolism produced mostly polar metabolites which did not bind appreciably to microsomal macromolecules.