A correlation of Salmonella mutagenicity with DNA adducts induced by the cooked-food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine.

The correlation of bacterial mutagenicity with DNA adducts from the heterocyclic amine cooked-food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) was investigated in Salmonella typhimurium strains TA98 (uvrB deficient) and TA1978 (uvrB proficient). Bacterial cells were exposed to PhIP using a modification of the Ames/Salmonella microsuspension assay. Half of the cells, generated from a 90 min pre-incubation and washing, were plated for revertant formation while the remaining half was subjected to DNA adduct analysis via 32P-postlabeling. In TA98, DNA adducts were detected at an RAL (relative adduct labeling) of 10 x 10(-7) and 21 x 10(-7) at PhIP concentrations of 5.5 and 17 microM, respectively. This corresponded to 28.8 and 20.9 adducts/revertant, respectively. These values were based on the assumption that only four repeating GC bases within a 75 DNA base region is the gene target site for PhIP induced mutations. In TA1978, no revertants above background were detected at any concentration of PhIP tested. DNA adducts, however, were detected at 11 x 10(-7) and 21 x 10(-7) adducts per nucleotide at 223 and 1116 microM PhIP, respectively. The lack of detectable revertants, but the presence of DNA adducts, suggests pre-mutational lesions did occur during the 90 min pre-incubation. Presumably, when the S9 activating system and PhIP were removed (via washing with phosphate buffered saline) prior to plating, the cells containing an intact uvrB repair system repaired the lesions during the incubation time on the plates. In conclusion, the induction of revertants by adducts appears quite efficient, as approximately 25 adducts are required for one mutational event in the excision repair deficient bacteria.