3-AMINO-1,4-DIMETHYL-5H-PYRIDO[4,3-B]INDOLE (TRP-P-1) INHIBITS THE BINDING-ACTIVITY OF T4 ENDONUCLEASE-V TO UV-DAMAGED DNA

3-Amino-1,4-dimethyl-5H-pyrido[4,3-b]ind (Trp-P-1) is a mutagen/carcin ogen derived from cooked foods which enhances the induction of mutatio ns and chromosome aberrations by UV without microsomal activation, The se co-mutagenic effects are considered to arise from inhibition of DNA excision repair at the incision step. However, the inhibition mechani sm has not been clarified. In this study we show, using agarose gel el ectrophoresis, that Trp-P-1 inhibits incision by T4 endonuclease V, wh ich cleaves DNA at the site of cyclobutane dimers, Trp-P-1 also inhibi ts the binding of this enzyme to UV-damaged DNA in a gel shift assay. In addition, the results of DNA unwinding assay with topoisomerase I s uggest that Trp-P-1 intercalates into DNA molecules. The known interca lators ethidium bromide and acriflavine demonstrate similar effects in these experiments, However, 2-amino-1-methyl-6-phenylimidazo [4,5-b]p yridine (PhIP), which showed no co-mutagenic effects in our previous s tudy, does not demonstrate such effects. These results suggest that Tr p-P-1 changes DNA conformation by intercalation, causing inhibition of binding of repair enzymes to UV-damaged DNA, and this in turn leads t o inhibition of DNA excision repair and to co-mutagenic effects.