Radical Cation DNA Adducts

Figure 6.6. Structures of B[a]P radical cation DNA adducts from Chen et al. (1996).

Depurinating adducts leave behind an abasic site on the DNA. Replicative DNA polymerase will insert an A opposite an abasic site so that depurination provides a straightforward route to either G — T or A —> T transversions when the daughter strand is replicated [34], As radical cation depurinating adducts result in N-glyco-sidic bond cleavage, repair of the abasic site becomes important. Apurinic/apyri-midinic endonuclease creates a strand break at the site of the lesion, and the gap is then repaired by phosphodiesterase, DNA polymerase, and DNA ligase [35], as discussed in Chapter 11. 

In mouse skin studies, a correlation between radical cation depurinating adducts and mutations in the H-ms gene was noted after treatment with B[o]P, DMBA, and DB[a,I]P. The mutations observed were G —> T transversions at codon 13 or A —> T transversions at codon 61, which could thus be explained by the formation of radical cation depurinating DNA adducts that were detected [36]. In a small cohort of seven women who were exposed to either household coal-smoke or smoked cigarettes, three women had modest amounts of the B[o]P-6-N7-Gua adduct that exceeded the B[a]P-6-N7-Ade adduct by 200- to 300-fold [37]. The levels of other PAH-derived DNA lesions were not reported. 

Heteroatom (O and N) attachment to the C8-site of dG to form 8-oxo-dG and C8-arylamine adducts lowers the oxidation potential relative to dG. The oxidation potential of 8-oxo-dG is 0.74 V versus NHE. Consequently, 8-oxo-dG can act as a deep radical cation trap within duplex DNA. Depending on the DNA sequence, an 8-oxo-dG lesion will be the preferential site of further oxidation and will protect isolated Gs and GG steps from oxidation the oxidation of 8-oxo-dG by G(—H) occurs with a rate of 4.6 x 10 /M/s. Thus, there is speculation that GC-rich domains outside the coding regions of genes serve to protect the genome from mutagenesis by oxidation. ... 

Ionization of DNA s solvation shell produces water radical cations (H20 ) and fast electrons. The fate of the hole is dictated by two competing reactions hole transfer to DNA and formation of HO via proton transfer. If the ionized water is in direct contact with the DNA (F < 10), hole transfer dominates. If the ionized water is in the next layer out (9 < r < 22), HO formation dominates [67,89,90]. The thermalized excess electrons attach preferentially to bases, regardless of their origin. Thus the yield of one-electron reduced bases per DNA mass increases in lockstep with increasing F, up to an F of 20-25. This means that when F exceeds 9, there will be an imbalance between holes and electrons trapped on DNA, the balance of the holes being trapped as HO . At F = 17, an example where the water and DNA masses are about equal, the solvation shell doubles the number of electron adducts, increasing the DNA-centered holes by a bit over 50% [91-93]. 

One-electron oxidation to form cation radicals is the major pathway of activation for the most potent carcinogenic PAHs, whereas oxygenation is generally a minor pathway. For benz[a]pyrene and 7,12-dimethyl benz[a]anthracene, 80% and 90%, respectively, of the DNA adducts formed by rat liver microsomes or in mouse skin arise via the cation radicals (Cavaleri Rogan 1992). 

Crich D, Huang W (2001) Dynamics of alkene radical cations/phosphate anion pair formation from nucleotide C4 radicals. The DNA/RNA paradox revisited. J Am Chem Soc 123 9239-9245 Das S, Deeble DJ, Schuchmann MN, von Sonntag C (1984) Pulse radiolytic studies on uracil and uracil derivatives. Protonation of their electron adducts at oxygen and carbon. Int J Radiat Biol 46 7-9... 

Steenken S (1988) Electron transfer between radicals and organic molecules via addition/elimina-tion. An inner-sphere path. In Rice-Evans C, Dormandy T (eds) Free radicals chemistry, pathology and medicine. Richelieu Press, London, pp 53-71 Steenken S (1989) Purine bases, nucleosides and nucleotides Aqueous solution redox chemistry and transformation reactions of their radical cations e and OH adducts. Chem Rev 89 503-520 Steenken S (1992) Electron-transfer-induced acidity/basicity and reactivity changes of purine and pyrimidine bases. Consequences of redox processes for DNA base pairs. Free Radical Res Commun 16 349-379... 

Prostaglandin synthetase, peroxidase or lipid peroxidation have been shown to oxidise arylamine xenobiotics to reactive species that bind extensively to DNA. The binding could be an initial event in the toxic or carcinogenic process. Evidence is presented that cation radicals are involved in the formation of the various oxidation products and DNA adduct formation with the carcinogen aminofluorene. Furthermore methylaminoazobenzene (butter yellow) was found to form the same major GSH adduct as is formed in vivo. 

The nature of the DNA adduct remains to be determined. However the spectrum of the isolated DNA is similar to that of the DNA adduct formed when 2-aminodifluorenylamine is oxidized by a peroxidase-H202 reaction mixture. Furthermore a similar DNA adduct is formed when DNA is added at a time when the blue imino derivative is maximal. Furthermore in the presence of DNA much less extractable azofluorene, aminodif luoreny lamine or polymer was formed suggesting that DNA reacts with the amino-fluorene cation radical and 2-iminodifluorenylamine. 

Scheme 6.4 Depurinating DNA adducts formed from radical cations.

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