Structures and biological properties of DNA adducts derived from N-nitroso bile acid conjugates.

A kind of N-nitrosobile acid conjugate, N-nitrosotaurocholic acid (NO-TCA), was incubated with calf thymus DNA, and formation of an adduct was detected by the 32P-postlabeling method under nuclease P1 conditions. To examine the nucleotides containing the adduct from NO-TCA, each of 2'-deoxyribonucleotide 3'-monophosphates (3'-dAp, 3'-dGp, 3'-dCp, or 3'-Tp) was incubated with NO-TCA. The same adduct spot was detected in the reaction of NO-TCA with 3'-dCp. The structure of this adduct was determined to be 3-ethanesulfonic acid-dC by several spectrometry techniques. Moreover, bulky adducts containing bile acid moiety were also produced from the reaction of NO-TCA with 3'-dCp and 3'-dAp. From comparison with spectral data for authentic compounds, these adducts were concluded to be N4-cholyl-dC and N6-cholyl-dA. N4-Cholyl-dC and N6-cholyl-dA were also detected in calf thymus DNA treated with NO-TCA. In addition, 3-ethanesulfonic acid-dC and N4-deoxycholyl-dC were found to be produced from N-nitrosotaurodeoxycholic acid (NO-TDCA) with dC. NO-TCA and NO-TDCA induced mutations in Salmonella typhimurium TA100 but not in TA98. Mutational spectrum analysis revealed that NO-TCA induced G to A transitions predominantly. When NO-TCA (250 mg/kg) was singly administered to male Wistar rats by gavage, both ethanesulfonic acid-dC and N4-cholyl-dC could be detected in the glandular stomach and colon. The levels of ethanesulfonic acid-dC were 0.22-0.29 per 10(6) nucleotides, but values for N4-cholyl-dC were about 500-fold lower. These observations suggest that N-nitroso bile acid conjugates, NO-TCA and NO-TDCA, may induce G to A base substitutions in genes via DNA adduct formation, producing ethanesulfonic acid- and/or (deoxy)cholic acid-DNA and, therefore, may be related to human carcinogenesis as endogenous mutagens.