Analysis of the Major RNA Adduct Derived from Aminophenylnorharman, a Novel Endogenous Mutagen and Carcinogen

Abstract

9-(4′-Aminophenyl)-9H-pyrido[3,4-b]indole (aminophenylnorharman, APNH), a novel endogenous mutagenic/ carcinogenic heterocyclic amine, is known to be a reaction product of 9H-pyrido[3,4-b]indole (norharman) and aniline. The major APNH-DNA adduct has been reported to be 2′-deoxyguanosin-8-yl-aminophenylnorharman (dGuo-C8-APNH). However, RNA adducts may also be important. We here demonstrated formation of APNH-RNA adducts and conducted a structural analysis using various spectrometric approaches. When a reaction mixture of an ultimate mutagenic form of APNH, N-acetoxy-APNH, and guanosine (Guo) was subjected to LC-ESI/MS analysis, one peak, with a similar UV spectrum to dGuo-C8-APNH, exhibited a molecular ion peak at m/z 541 along with a fragment peak at m/z 409, consistent with loss of a ribose moiety. From ¹H-NMR analysis, its chemical structure was concluded to be N⁴-(guanosin-8-yl)-9-(4′-aminophenyl)-9H-pyrido[3,4-b]indole (Guo-C8-APNH). The same adduct was yielded in yeast tRNA incubated with N-acetoxy-APNH. Digestion of tRNA treated with N-acetoxy-APNH resulted in the appearance of one adduct spot visualized by the ³²P-postlabeling method, corresponding to Guo-C8-APNH. No spot was seen with tRNA alone. Additional analysis of in vivo adduct formation in the livers of rats administered APNH at a concentration of 100 mg/kg revealed that several adduct spots, including one corresponded to Guo-C8-APNH, were observed. The total adduct levels of APNH-RNA were 28±13.3 (mean±SD) adducts per 10⁶ nucleotides. Comparisons demonstrated six times higher levels of total APNH-RNA than total APNH-DNA adducts in the same rat liver samples. These results indicate that APNH-RNA might be a useful biomarker for exposure to APNH.