Enzymatic characterization of the polynuclear aromatic hydrocarbons activating rat-liver preparations used in the mutagenicity test of Ames

Stability studies were performed on the mono-oxygenase system involved, in particular, in the activation of polynuclear aromatic hydrocarbons (PAHs) present in rat-liver preparations used in the Ames mutagenicity test. The results indicated a good stability of the spectral response of the cytochrome-P-450 system, but a much lower stability of its enzymatic activities measured with various substrates, thus showing the inadequacy of the spectral response to characterize the PAH mono-oxygenase activity of the preparations. Epoxide hydrolase activity was found to be stable. Various mono-oxygenase activities were measured in preparations induced with phenobarbital, 3-methylcholanthrene or Aroclor 1254. The activities of two enzymes, benzo[a]pyrene hydroxylase and ethoxyresorufin-O-dealkylase, were found suitable to characterize the capacity of the preparations to metabolize PAH to mutagens. The efficiency of the same preparations to promote the mutagenicity of benzo[a]pyrene and aflatoxin B1 in the Ames test was determined. There was an excellent general correlation between the efficiencies for mutagenic activation of the preparations and the two enzymatic activities mentioned above. Determination of ethoxyresorufin-O-dealkylase (or benzo[a]pyrene hydroxylase) and benzo[a]pyrene 4,5-oxide hydrolase activities is proposed for characterizing the rat-liver preparations used in the Ames test.