US EPA

5145617 

Technical Report 

Microsomal Activation And Specific Enzyme Modification, Key Events For A Biotoxicological Study Of Chemical Carcinogenesis 

Roberfroid, M 

1978 

In Vitro Versus In Vivo Biotransformation and Toxicity 

The modification of kinetic parameters of microsomal enzymes by pretreatment with a chemical carcinogen was studied in rats. Male Wistar-rats received a single intraperitoneal (ip) injection of 40 milligrams per kilogram (mg/kg) 3-methylcholanthrene (56495) (MC), 5mg/kg 2-acetylaminofluorene (53963) (AAF), 10mg/kg 4-acetylaminofluorene (28322023) or 75mg/kg phenobarbital (50066) (PB) 24 hours before sacrifice. All animals dosed with PB had received a 75mg/kg ip injection 48 hours before sacrifice. Liver microsomes were prepared, and activities of benzo(a)pyrene-hydroxylase (9037529) and arylamide-N-hydroxylase were measured. The former was measured spectrofluorometrically, and the latter was measured by using a sensitive gas chromatographic method. Enzyme activities were used to calculate kinetic parameters. The Ames test for mutagenicity was performed using Salmonella-typhimurium. For both enzyme activities, the affinity of the enzyme for its substrate could be increased by pretreatment with a carcinogen. The effects, however, were specific. MC significantly increased the affinity of benzo(a)pyrene-hydroxylase for its substrate, but it had the opposite effect on arylamide-N-hydroxylase. Results from the Ames mutagenicity tests using the S9 mix to active benzpyrene (50328) correlated well with the kinetic data; however, results using S9 mix to activate AAF to a mutagen were poorly correlated with the kinetic data. The author concludes that pretreatment of rats with a chemical carcinogen is able to specifically modify kinetic parameters of the microsomal enzymes.