Formation of DNA adducts by the co-mutagen norharman with aromatic amines

Norharman, widely distributed in our environment, is alone not mutagenic to Salmonella typhimurium TA98 and TA100 either with or without S9 mix, but becomes mutagenic to S.typhimurium TA98 with S9 mix when non-mutagenic aromatic amines like aniline or o- or m-toluidine are added. Thus norharman has been called a 'co-mutagen'. In the present study we examined whether or not DNA adducts are formed in DNA of S.typhimurium TA98 by treatment with norharman and aromatic amines using 32P-post-labeling analysis under modified adduct intensification conditions. When a sample of norharman (8 mg) and aniline (4 mg) was incubated with 4 ml of overnight culture of S.typhimurium TA98 in the presence of 20 ml S9 mix for 6 h at 37 degrees C, three adduct spots were detected at a total relative adduct labeling (RAL) of 10.8 +/- 2.27/10(8) nucleotides. Under the same conditions, a mixture of norharman (8 mg) and o-toluidine (4 mg) yielded three adduct spots at a RAL of 3.74 +/- 1.71/10(8) nucleotides. With a combination of norharman and m-toluidine, a single adduct spot was seen at a RAL of 0.04 +/- 0.01/10(8) nucleotides. In contrast, norharman with p-toluidine did not produce adduct spots. Furthermore, neither norharman nor the aromatic amines themselves gave any evidence of adducts. Thus DNA adduct formation by norharman with aromatic amines correlates with the co-mutagenic action of norharman in S.typhimurium TA98.