Activation and detoxification of dinitropyrenes by cytosol and microsomes from Aroclor-pretreated rats in the Ames and umu assays.

Abstract

1,3-, 1,6-, and 1,8-Dinitropyrene (1,3-, 1,6-, and 1,8-DNP) are direct-acting mutagens in that they do not require an exogenous source of enzymes for activation to mutagens in the Ames assay. However, the addition of mammalian S9 preparations, or the microsomal and cytosolic compartments comprising S9, modulate the mutagenic response of these DNPs. In this study, we compared the mutagenic response of these DNPs in the presence of cytosol and microsomal fractions from the liver of Aroclor-pretreated (AR) and control rats, in the Ames mutagenicity assay and umu gene induction assay. 1,3- and 1,8-DNP were deactivated to a greater extent by microsomes from AR-induced and control rats than was 1,6-DNP, in both the umu and Ames assays. In the Ames assay, S9 was more potent in deactivating the DNP than an equivalent concentration of microsomes from the same S9 preparation. Also, S9 from AR-pretreated rats deactivated the isomers to a greater extent than S9 from control rats. In contrast to the constant deactivation of all the isomers in the two assays catalyzed by microsomes and S9, the response with cytosol from AR-pretreated rats differed with respect to the three isomers in the Ames and umu assays. When cytosol from AR-treated rats was added, the mutagenicity of 1,3- and 1,6-DNP, but not 1,8-DNP, was significantly (P < 0.05) increased in the Ames assay while the mutagenicity of the three DNPs was increased in the umu assay. Also, a biphasic response was observed in the umu assay with 1,6- and 1,8-DNP, in that AR-cytosol enhanced the mutagenicity at low protein concentrations (5-50 micrograms protein/reaction) but abrogated the response at higher protein concentrations. The effect of cytosol from control rats depended on the isomer tested; 1,3-DNP was activated above the background level in both assays (nearly towfold) while 1,6-DNP and 1,8-DNP were only activated at low protein concentrations in the umu assay. In the Ames assay, cytosol from AR-pretreated rats did not alter the mutagenic response with 1,8-DNP, while control cytosol significantly (P < 0.05) deactivated 1,8-DNP at all substrate concentrations tested. In summary, this study showed that the mutagenicity of 1,3-DNP was similar in the two assays but the responses with 1,6- and 1,8-DNP differed in the two assays. These isomeric differences could be due to the varying metabolic pathways of the three DNPs as well as the detectable end points of the two assays.