Detection of strand breaks in phiX 174 RFI and PM2 DNA reacted with ultimate and proximate carcinogens.

Supercoiled DNA duplexes of phages phiX 174 and PM2 were treated in aqueous solution at neutral pH with ultimate and proximate carcinogens. Subsequently, the carcinogen-treated phage DNAs were subjected to velocity sedimentation in neutral and alkaline sucrose to quantitative introduction of single strand breaks. Reaction of phage DNA with the ultimate carcinogens N-methyl-N-nitrosourea (MeNOUr), N-ethyl-N-nitrosourea (EtNOUr), 7-bromomethyl-benza[a]-anthracene, N-acetoxy-2-acetylaminofluorene [(Ac)2ONFln] and K-region oxides for short periods followed by sedimentation in neutral sucrose gradients led to very few breaks. Incubation with the proximate carcinogens N-hydroxy-2-acetylaminofluorene, 2-acetylaminofluorene, 7-methyl-, and 7,12-dimethyl-benza[a]anthracene did not result in breaks. However, when the phage DNAs were reacted with the ultimate carcinogens under the same conditions but subsequently alkali-denatured and sedimented in alkaline sucrose gradients, single strand breaks were readily introduced. Incubation with the proximate carcinogens followed by alkali denaturation and sedimentation in alkaline sucrose showed that only 7,12-dimethyl-benz[a]anthracene and, to a minor extent, 7-methyl-benz[]anthracene caused alkali-inducible breaks. The ability of N-methyl-N'-nitro-N-nitrosoguanidine to effect breakdown of superhelical phage DNA in alkali was found enhanced in the presence of N-acetyl-cysteine.