Nonlinear dose-response relationship for the binding of the carcinogen benzo(a)pyrene to rat liver DNA in vivo

With radioactive compounds of high specific activity, the binding of carcinogens to DNA can be measured with doses that are ineffective in long-term studies. The binding of tritiated benzo(a)pyrene to liver DNA of adult male rats has been determined 50 hr after a single i.p. injection of doses between 40 microgram/kg and 4 mg/kg. The dose-response relationship is linear up to 1 mg/kg, shows a step towards 2 mg/kg, and gives a shallow linear slope above that value. The observed binding ranges from 1.7 to 180 nmoles benzo(a)pyrene per mole DNA phosphate. The nonlinearity could be due to an induction of metabolizing enzymes. The microsomal aryl hydrocarbon hydroxylase activity increases significantly 24 hr after a single dose of 4 mg/kg and 48 hr after doses of 2 and 4 mg/kg, but no induction is found with 1 mg/kg. The binding from an equimolar dose is 35 times lower than the one found on mouse skin DNA and 300 times lower than that of N,N-dimethylnitrosamine in rat liver. A good correlation exists to the respective tumor formation in long-term studies.