MECHANISMS OF DNA-DAMAGE BY CHROMIUM(V) CARCINOGENS

Reactions of bis(2-ethyl-2-hydroxy-butanato)oxochromate(V) with pUC19 DNA, single-stranded calf thymus DNA (ss-ctDNA), a synthetic oligonucl eotide, 5'-GATCTATGGACTTACTTCAAGGCCGGGTAATGCTA-3' (35mer), deoxyguanos ine and guanine were carried out in Bis-Tris buffer at pH 7.0. The pla smid DNA was only nicked, whereas the single-stranded DNA suffered ext ensive damage due to oxidation of the ribose moiety. The primary oxida tion product was characterized as 5-methylene-2-furanone. Although all four bases (A, C, G and T) were released during the oxidation process , the concentration of guanine exceeds the other three, Orthophosphate and 3'-phosphates were also detected in this reaction, Likewise, the synthetic oliogomer exhibits cleavage at all bases with a higher frequ ecncy at G sites, This increased cleavage at G sites was more apparent after treating the primary oxidation products with piperidine, which may indicate base oxidation as well. DNA oxidation is shown to proceed through a Cr(V)-DNA intermediate in which chromium(V) is coordinated through the phosphodiester moiety. Two alternative mechanisms for DNA oxidation by oxochromate(V) are proposed to account for formation of 5 -methylene-2-furanone, based on hydrogen abstraction or hydride transf er from the C1' site of the ribose followed by hydration and two succe ssive beta-eliminations, It appears that phosphate coordination is a p rerequisite for DNA oxidation, since no reactions between chromium(V) and deoxyguanosine or guanine were observed. Two other additional path ways, hydrogen abstraction from C4' and guanine base oxidation, are al so discussed.