Polymorphic Drug Metabolism by Human CYP1A2 and N-acetyltransferase: Studies with Recombinant Chinese Hamster Cells and Analyses in Japanese Populations

Abstract

Most promutagens and procarcinogens exert their genotoxicity after undergoing metabolic activation. Metabolism of chemicals is one of important factors limiting the extents of the action of chemicals. In this study, we established cell lines which carried cDNAs coding for human CYP1A2 and N-acetyltransferase (NAT); the latter functions as O-acetyltransferase for N-hydroxyarylamines formed by CYP1A2. A cell line which expresses CYP1A2 together with P450 reductase activated aflatoxin B₁, but not heterocyclic amines. A cell line which carries CYP1A2 and polymorphic NAT(NAT2) in addition to P450 reductase activated 2-amino-3, 8-dimethylimidazo[4, 5 f]quinoline (IQ) and some other heterocyclic amines efficiently. However, a cell line which carries CYP1A2 and monomorphic NAT (NAT1) showed only low activity toward the same heterocyclic amines. In order to determine the presence and the frequency of genetic polymorphisms of CYP1A2 and NAT2 in humans, we performed caffeine phenotyping test on 205 Japanese volunteers. Analyses of metabolic ratios of urinaly metabolites showed a bimodal distribution, indicating that about 86% and 91% of Japanese were extensive metabolizers (EM) of CYP1A2 and NAT2, respectively. The genotype of NAT2 determined by PCR-RFLP method agreed completely with the phenotype. To determine the mechanism of the differences in CYP1A2 activities, genomic DNA from peripheral lymphocytes of slow and rapid metabolizers was subjected to DNA sequencing. No differences in nucleotide sequence were observed between poor and extensive metabolizers in the exons, exon-intron junctions and 5'-flanking region of the CYP1A2 gene. Since CYP1A2 and NAT2 are enzymes activating heterocyclic amines efficiently, it is expected that extensive metabolizers of CYP1A2 and NAT2 are under high risk when exposed to these promutagens.