Modification of DNA by a direct-acting mutagen, 1-nitrosoindole-3-acetonitrile, which is formed from indole-3-acetonitrile upon nitrite treatment, was investigated. 32 P-Postlabeling analysis clearly demonstrated the formation of DNA adducts in the stomach of rats after intragastric administration of 1-nitrosoindole-3-acetonitrile. The level of DNA adducts in both the forestomach and glandular stomach 2 h after administraction of 100 mg/kg body weight of the compound was about one adduct per 10 7 nucleotides. The DNAs of the forestomach and glandular stomach gave six common spots on two-dimensional chromatography, three of which were also produced by in vitro reaction of this compound with DNA. Thus, 1-nitrosoindole-3-acetonitrile can form DNA adducts in vivo and in vitro . No DNA adducts were detected after treatment with the non-nitrosated compound indole-3-acetonitrile both in vivo and in vitro . These results suggest that 1-nitrosoindole-3-acetonitrile has the in vivo tumor initiating activity in the stomach.

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