Full Structure Assignments of Pyrrolizidine Alkaloid DNA Adducts and Mechanism of Tumor Initiation

Pyrrolizidine alkaloid-containing plants are widespread in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Pyrrolizidine alkaloids are among the first chemical carcinogens identified in plants. Previously, we determined that metabolism of pyrrolizidine alkaloids in vivo and in vitro generated a common set of DNA adducts that are responsible for tumor induction. Using LC-ESI/MS/MS analysis, we previously determined that four DNA adducts (DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4) were formed in rats dosed with riddelliine, a tumorigenic pyrrolizidine alkaloid. Because of the lack of an adequate amount of authentic standards, the structures of DHP-dA-3 and DHP-dA-4 were not elucidated, and the structural assignment for DHP-dG-4 warranted further validation. In this study, we developed an improved synthetic methodology for these DNA adducts, enabling their full structural elucidation by mass spectrometry and NMR spectroscopy. We determined that DHP-dA-3 and DHP-dA-4 are a pair of epimers of 7-hydroxy-9-(deoxyadenosin-N(6)-yl) dehydrosupinidine, while DHP-dG-4 is 7-hydroxy-9-(deoxyguanosin-N(2)-yl)dehydrosupinidine, an epimer of DHP-dG-3. With the structures of these DNA adducts unequivocally elucidated, we conclude that cellular DNA preferentially binds dehydropyrrolizidine alkaloid, for example, dehydroriddelliine, at the C9 position of the necine base, rather than at the C7 position. We also determined that DHP-dA-3 and DHP-dA-4, as well as DHP-dG-3 and DHP-dG-4, are interconvertible. This study represents the first report with detailed structural assignments of the DNA adducts that are responsible for pyrrolizidine alkaloid tumor induction on the molecular level. A mechanism of tumor initiation by pyrrolizidine alkaloids is consequently fully determined.