Abstract
The bioactivation of arylamines was investigated in hepatic microsomal and cytosolic fractions. The bioactivation of 6-aminochrysene to mutagenic intermediates in the Ames test was demonstrated for the first time in cytosol from Aroclor 1254-induced rat. Substantial evidence was obtained to demonstrate that this activity was not a consequence of microsomal contamination, including the inhibition of cytosol-mediated bioactivation by the addition of microsomal protein. Investigation of substrate specificity demonstrated that the cytosolic activation of promutagens requires an exocyclic amino group and consequently bioactivation probably involves N-hydroxylation. Extrahepatic cytosol cannot bioactivate either 6AC or 2AA to mutagens and therefore the major site for this activity is the liver. No constitutive hepatic bioactivating activity was demonstrated by rat, mouse, hamster, ferret or pig hepatic cytosol but, moderate bioactivation of 2-aminoanthracene was carried out by human hepatic cytosol. The cytosolic protein responsible for the bioactivation of arylamines was only induced by polychlorinated biphenyls and the best induction was accomplished by highly-substituted planar PCB congeners. However, Aroclor 1254 could not induce cytosol-mediated bioactivation of arylamines in Ah receptor non-responsive (DBA2) mice. Therefore induction of this enzyme system requires the cytosolic Ah receptor Purification of the cytosolic amine oxidase was undertaken with partial success and a protein with an approximate molecular weight of approximately 65. 5kDa was identified as the most likely candidate for the cytosolic enzyme. Microsomes isolated from Aroclor 1254-treated rat could bioactivate arylamines to mutagens in the presence of either NADH or NADPH. In these investigations NADH-dependent bioactivation of IQ and 6-aminochrysene was demonstrated for the first time. Investigation of arylamine bioactivation in the presence of these two cofactors demonstrated sufficient differences to suggest these activities are carried out by different microsomal enzymes. However, both NADPH- and NADH-dependent bioactivation probably involves N-hydroxylation. Two microsomal enzyme systems which may perform this metabolism are the cytochromes P450 and the flavin-monooxygenases. However, the complete oxidation of xenobiotics by cytochrome P450 in the presence of NADH had not been demonstrated previously. In these investigations the cytochrome P450-dependent dealkylation of ethoxy- and methoxy-resorafin were performed in the presence of NADH by Aroclor 1254-induced microsomes. However, there is insufficient evidence to determine whether a cytochrome P450 isoform is responsible for the bioactivation of arylamine but like CYP1 isoforms induction of the microsoe enzyme was regulated by the cytosolic Ah receptor.