Pyrrolizidine alkaloids are cytotoxic, carcinogenic, and anti-carcinogenic in vivo and in vitro, and they produce many hazardous effects in humans and animals. Pyrrolizidine alkaloids (PAs) also cross-link with DNA and/or protein. However, whether such cross-linking is important to the toxic action of PAs is not known. In addition, the exact mechanism underlying these DNA cross-links or cytotoxicity is also not clear. In three separate studies, I characterized the nature of PA-induced DNA cross-links and the relationships between PA structures and cross-linking potency. In the first study (Chapter II), I found that cross-linking potency of PA congeners coincided with their abilities to cause cytopathologic effects. Macrocyclic a,p-unsaturated diesters PAs and their pyrrolic metabolites were the most potent inhibitors of colony formation, and inducers of cytopathologic changes and megalocyte formation. The macrocyclic α, β-saturated diester PA and open diesters PAs slightly inhibited colony formation, and slightly changed cell morphology. Retronecine and indicine N-oxide were completely inactive. In the next study (Chapter Ill), I found that pyrrolic macrocyclic metabolites were more potent DNA cross-linkers than their parent compounds as determined by alkaline elution. The pyrroles of the macrocyclic diester PAs were potent DNADNA (inter- and/or intra) cross-linkers in BstEll-digested λ-phage DNA or pBR322 plasmid DNA but dehydroretronecine and indicine N-oxide were not. I also examined which DNA sequences were more susceptible to PA-induced cross-links by using a series of restriction endonucleases to determine sequence specificity. The most favorable cross-linking site for PAs appeared to be 5'd(GG) and 5'-d(GA) although other sites, 5'-d(CC) or 5'-d(CG), might be also preferable cross-linking targets. In the next study (Chapter IV), I characterized the nature of DNA-protein interactions induced by PAs, because I found in previous studies that PA-induced cross-links are largely protein associated. In PA or pyrrolic PA exposed cells, cross-linked proteins with molecular weights 40 - 60 kD were detected. Two-dimensional electrophoretic analysis revealed that these proteins were probably acidic in nature. In an in vitro system utilizing pBR322 or Bst Ell-digested λ-phage DNA. dehydrosenecionine induced DNAprotein cross-links with BSA, indicating that such interactions might be related to amino acid composition of protein. These results confirmed that PA-induced DNA cross-links (DNA-DNA, DNA-protein cross-links) are influenced by three structural features: the C1 ,2 unsaturation of pyrrolizidine ring, α, β-unsaturation, and size of the macrocyclic diester ring. The ability to form cross-links was closely related to the known toxic potencies of these PAs. From this research, I also conclude that DNA crosslinking is the most critical event leading to PA-related diseases and that crosslinking is due to pyrrolic metabolites of PAs, not via a common metabolite as was once thought.

L'ADN est en permanence exposé à un grand nombre d'événements dommageables déclenchées par des agents endogènes et exogènes. De nombreux travaux expérimentaux ont fourni des informations cruciales sur les propriétés structurelles et la réparation de certains des lésions de l'ADN. Cependant, il manque une vision mécanistique ou énergétique sur leur formation. La biochimie computationnelle a émergé comme un outil puissant pour comprendre les réactions biochimiques et les propriétés électroniques de systèmes complexes.Dans cette thèse, nous étudions la formation de lésions complexes intra-brin et inter-brin. Ces lésions tandem constituent une puissant menace à l'intégrité du génome, en raison de leur haute fréquence mutagenique. Tout d'abord, nous discutons l'attaque d'une liaison covalente entre un radical pyrimidinique. En comparant avec les bases isolees, nos simulations hybrides Car-Parrinello demontrent que la reactivité de la thymine et de la cytosine radicalaires sont inversees dans l'environnement B-helical. De plus, nos resultats montrent egalement une deformation plus importante pour la lesion G[8-5]C.Nous rationalisons également la plus grande réactivité des cytosines par rapport aux purines vers la formation multi-etapes de lésions complexes inter-brins par condensation avec un site C4' abasique. Ces résultats bases sur des simulations avec solvatation explicite et combines a la théorie de la fonctionnelle de la densité sont en accord avec les données expérimentales. / DNA is continuously exposed to a vast number of damaging events triggered by endogenous and exogenous agents. Numerous experimental studies have provided key information regarding structural properties of some of the DNA lesions and their repair. However, they lack in mechanistic or energetic information pertaining to their formation. Computational Biochemistry has emerged as a powerful tool to understand biochemical reactions and electronic properties of large systems.In this thesis we study the formation of inter- and intra-strand cross-links. These tandem lesions pose a potent threat to genome integrity, because of their high mutagenic frequency. First, we discuss the formation of complex defects which arise from the attack of a pyrimidine radical onto guanine. In comparison with the reactivity of isolated nucleobases, our hybrid Car-Parrinello Molecular Dynamics simulations reveal that the reactivity of hydrogen-abstracted thymine and cytosine is reversed within a B-helix environment. Further, our data also suggest a more severe distortion of the B-helix for G[8-5]C.Second, we rationalize the higher reactivity of cytosine vs. purines toward the multistep formation of inter-strand crosslinks with a C4' oxidized a basic site, which is in qualitative agreement with experiments on isolated nucleobases, using explicit solvent simulations combined to density functional theory.