Polychlorinated dibenzofurans--potent inducers of rat hepatic drug-metabolizing enzymes.

The effects of polychlorinated dibenzofurans (PCDFs), trace toxic contaminants of commercial polychlorinated biphenyl preparations (PCBs), on the induction of hepatic drug-metabolizing enzymes were studied in the rat. PCDFs were about a thousand times more potent than PCBs (Kanechlor-500) as inducers of cytochrome P-450. Rats given 10 microgram/kg of PCDFs intraperitoneally for 3 days showed significantly increased hepatic cytochrome P-450 levels. At the highest dose tested, 1000 microgram/kg, a two-fold increase of cytochrome P-450 and a three-fold increase of p-nitroanisole demethylase activity were observed. PCDFs and 3-methylcholanthrene had quite similar effects on microsomal drug-metabolizing enzymes. Both drugs increased p-nitroanisole demethylase activity strikingly and aniline hydroxylase activity moderately, but produced little change in aminopyrine demethylase activity. alpha-Naphthoflavone, which is known to be a specific inhibitor of aryl hydrocarbon hydroxylase induced by polycyclic aromatic hydrocarbons, inhibited at low concentrations p-nitroanisole demethylase activity of rats previously treated with both drugs. Further, both drugs increased the 455 nm to 430 nm peak ratios of ethyl isocyanide difference spectra. Following three daily doses of PCDFs (100 microgram/kg), cytochrome P-450 level and p-nitroanisole demethylase activity remained elevated for over 15 days, with a decrease to control levels after 30 days. Such indicates the slow excretion of PCDFs.