Mutagenic specificity of 2-amino-3-methylimidazo[4,5-f]quinoline and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in the supF shuttle vector system.

2-Amino-3-methylimidazo[4,5-f]quinoline (IQ) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) are heterocyclic amine mutagens/carcinogens formed from the cooking of meat. Here we used the pSP189 shuttle vector developed by Parris and Seidman (Gene, 117: 1-5, 1992) to study and compare the mutation spectra induced by these compounds. pSP189 was adducted by reaction with the N-acetoxy derivatives of IQ or PhIP. 32P-Postlabeling analysis was used to measure the C8-guanine adduct level and the total adduct levels formed in the plasmid. Plasmids were replicated and mutagenized in repair-proficient [GM0637(SV40)] or repair-deficient [XP12Be(SV40)] human fibroblasts. Resulting inactivation mutations in the supF gene were determined by the formation of white or light blue colonies on indicator bacteria (carrying a lacZ amber mutation) and cycle sequencing. With both compounds in either cell line, 85-93% of the mutations induced were base substitutions and the remainder of the mutations were base deletions. The majority of the substitution mutations involved a single base, and nearly all base substitution mutations (> 97%) were at guanine. This latter finding is consistent with the results from 32P-postlabeling showing that both compounds adduct to the guanine base with the major adduct being formed at the C8-guanine position. The predominant mutation found with IQ and PhIP in either cell line was G:C to T:A transversion, followed by G:C to A:T transition, and then G:C to C:G transversion; these mutations accounted for 59-72%, 19-27%, and 6-14% of total base substitution mutations, respectively. There was a preference seen with both compounds to induce mutations at a guanine base having a neighboring guanine or cytosine (i.e., GG and GC sites). However, despite the striking similarity in the kinds of base substitution mutations induced by IQ and PhIP, their mutation spectra were distinct. For example, in repair-proficient cells, 26% of the mutations induced with PhIP, but not with IQ, also involved a GA site, containing the 5-base pair sequence 5'-GCAGA-3'. Mutation spectra for IQ and PhIP were also different between repair-deficient and repair-proficient cells. The findings shown here may serve to be predictive of the kinds of mutations induced by the adducts of IQ and PhIP in oncogenes and tumor suppressor genes altered during heterocyclic amine-induced carcinogenesis.