32P-postlabelling studies on the DNA adducts of the food mutagens/carcinogens IQ and PhIP--adduct formation in a chemical system, and by rat and human metabolism.

The DNA adducts of the food mutagens/carcinogens IQ and PhIP were studied by means of 32P-postlabelling techniques. Adducts were generated in vitro and in vivo by three techniques: by photolysis of azido-IQ and azido-PhIP in the presence of dGp, calf-thymus DNA or Salmonella; by administration of IQ and PhIP to rats; and by incubation of cultured COS-1 cells with IQ. These cells expressed the human cytochromes P450 1A1 or P450 1A2 and/or the human N-acetyltransferases NAT1 or NAT2. The data demonstrate that, in the photolytic, rat and human systems, a common IQ metabolite and a common PhIP metabolite are formed together with common sets of IQ adducts and PhIP adducts. The data obtained in the human system show that N-hydroxy-IQ, formed by cytochrome P450, binds poorly to DNA, whereas more efficient binding occurs in the presence of NAT1 and most efficient binding in presence of NAT2. This indicates an O-acetyltransferase activity of human NAT1 and NAT2 and formation of N-acetoxy-IQ as an intermediate and immediate precursor of the ultimate arylnitrenium ion. The effect of the polymorphic NAT2 suggests a critical role for the human acetylation polymorphism in the DNA-binding of IQ in humans and in its genotoxic implications.