One-electron Oxidation Of DNA Research Articles

UV and ionizing radiations induced DNA damage, differences and similarities

Both UV and ionizing radiations damage DNA. Two main mechanisms, so-called direct and indirect pathways, are involved in the degradation of DNA induced by ionizing radiations. The direct effect of radiation corresponds to direct ionization of DNA (one electron ejection) whereas indirect effects are produced by reactive oxygen species generated through water radiolysis, including the highly reactive hydroxyl radicals, which damage DNA. UV (and visible) light damages DNA by again two distinct mechanisms. UVC and to a lesser extend UVB photons are directly absorbed by DNA bases, generating their excited states that are at the origin of the formation of pyrimidine dimers. UVA (and visible) light by interaction with endogenous or exogenous photosensitizers induce the formation of DNA damage through photosensitization reactions. The excited photosensitizer is able to induce either a one-electron oxidation of DNA (type I) or to produce singlet oxygen (type II) that reacts with DNA. In addition, through an energy transfer from the excited photosensitizer to DNA bases (sometime called type III mechanism) formation of pyrimidine dimers could be produced. Interestingly it has been shown recently that pyrimidine dimers are also produced by direct absorption of UVA light by DNA, even if absorption of DNA bases at these wavelengths is very low. It should be stressed that some excited photosensitizers (such as psoralens) could add directly to DNA bases to generate adducts. The review will described the differences and similarities in terms of damage formation (structure and mechanisms) between these two physical genotoxic agents.

Oxidatively generated damage to DNA at 5-methylcytosine mispairs

Oxidatively generated damage to DNA has been implicated as causing mutations that lead to aging and disease. The one-electron oxidation of normal DNA leads to formation of a nucleobase radical cation that hops through the DNA until it is trapped irreversibly, primarily by reaction at guanine. It has been observed that 5-methylcytosine (C(m)) is a mutational "hot-spot". However, C(m) in a Watson-Crick base pair with G is not especially susceptible to oxidatively induced damage. Radical cation hopping is inhibited in duplexes that contain C-A or C-T mispairs, but no reaction is detected at cytosine. In contrast, we find that the one-electron oxidation of DNA that contains C(m)-A or C(m)-T mispairs results primarily in reaction at C(m) even in the presence of GG steps. The reaction at C(m) is attributed to proton coupled electron transfer, which provides a relatively low activation barrier path for reaction at 5-methylcytosine. This enhanced reactivity of C(m) in mispairs may contribute to the formation of mutational hot spots at C(m).

One-Electron Oxidation of DNA by Ionizing Radiation: Competition between Base-to-Base Hole-Transfer and Hole-Trapping

The distance of hole migration through DNA determines the degree to which radiation-induced lesions are clustered. It is the degree of clustering that confers to ionizing radiation its high toxicity. The migration distance is governed by a competition between hole transfer and irreversible trapping reactions. An important type of trapping is reactions that lead to the formation of deoxyribose radicals, which are precursors to free base release (fbr). Using HPLC, fbr was measured in X-irradiated films of d(CGCGCGCGCG)(2) and d(CGCGAATTCGCG)(2) as well as three genomic DNAs: M. luteus, calf thymus, and C. perfringens. The level of DNA hydration was varied from Gamma = 2.5 to 22 mol waters/mol nucleotide. The chemical yields of each base, G(base), were measured and used to calculate the modification factor, M(base). This factor compensates for differences in the GC/AT ratio, providing a measure of the degree to which a given base influences its own release. In the DNA oligomers, M(Gua) > M(Cyt), a result ascribed to the previously observed end effect in short oligomers. In the highly polymerized genomic DNA, we found that M(Cyt) > M(Gua) and that M(Thy) is consistently the smallest of the M factors. For these same DNA films, the yields of total DNA trapped radicals, G(tot)(fr), were measured using EPR spectroscopy. The yield of deoxyribose radicals was calculated using G(dRib)(fr) = approximately 0.11 x G(tot)(fr). Comparing G(dRib)(fr) with total fbr, we found that only about half of the fbr is accounted for by deoxyribose radical intermediates. We conclude that for a hole on cytosine, Cyt(*+), base-to-base hole transfer competes with irreversible trapping by the deoxyribose. In the case of a hole on thymine, Thy(*+), base-to-base hole transfer competes with irreversible trapping by methyl deprotonation. Close proximity of Gua protects the deoxyribose of Cyt but sensitizes the deoxyribose of Thy.

One-electron oxidation of DNA: reaction at thymine

The feature article is a review of the reaction of thymine in the one-electron oxidation of duplex DNA. Oxidation of DNA causes chemical reactions that result in remote damage (mutation) to a nucleobase. Normally this reaction occurs at guanine, but in oligonucleotides that lack guanines, or when the DNA contains a thymine-thymine mispair, reaction occurs primarily at thymine notwithstanding its high oxidation potential. Selective substitution of uracil for thymine in TT sequences indicates the operation of a tandem reaction mechanism at adjacent thymines. Analysis of the reaction products suggests that proton-coupled electron transfer generates the 5-thymidyl methyl radical, which is trapped by molecular oxygen to give eventually 5-formyl-2'-deoxyuridine and 5-(hydroxymethyl)-2'-deoxyuridine. In a second process, water adds to the 5,6-double bond of the oxidized thymine giving eventually the cis- and trans-diastereomers of 5,6-dihydroxy-5,6-dihydrothymidine.

One-electron oxidation of DNA: thymine versus guanine reactivity

One-electron oxidation of anthraquinone (AQ)-linked DNA oligonucleotides containing A/T base pairs with repeating TT steps results in the distance-dependent reaction of the resulting radical cation and base damage at the TT steps that is revealed by subsequent reaction as strand cleavage. However, the inclusion of a remote guanine or GG step inhibits the reaction at thymine and results in predominant reaction at the guanine bases. For the oligomers examined in this work, the results reveal that the specific sequence of nucleobases determines the distance dependence, location of reaction and the efficiency of radical cation migration. In particular, a sequence of A/T base pairs can behave either as a trap, shuttle or barrier, depending on the context of the entire oligomer. The A/T sequences act as a shuttle when reaction occurs at a remote G or GG step and the same sequence of A/T bases acts as a barrier when there is more than one GG step in the sequence. In contrast, the A/T steps act as a trap in sequences that lack guanines.

One-electron oxidation of DNA and inflammation processes

Nitrosoperoxycarbonate anion, a reactive species generated in inflammation processes, is able to specifically oxidize guanine bases with a sequence selectivity that is almost opposite from that usually observed for one-electron oxidants.

Preface

Preface

One-electron oxidation of DNA: the effect of replacement of cytosine with 5-methylcytosine on long-distance radical cation transport and reaction.

One-electron oxidation of duplex DNA generates a radical cation that migrates through the nucleobases until it is trapped by an irreversible reaction with water or oxygen. The trapping site is often a GG step, because this site has a relatively low ionization potential and this causes the radical cation to pause there momentarily. Modifications to guanine that lower its ionization potential convert it to a better trap for the radical cation. One such modification is the formation of the Watson-Crick base pair with cytosine, which is reported to very significantly decrease its ionization potential. Methylation of cytosine to form 5-methylcytosine (5-MeC) is a naturally occurring reaction in genomic DNA that may be associated with regions of enhanced oxidative damage. The G.5-MeC base pair is reported to be more rapidly oxidized than normal G.C base pairs. We examined the oxidation of DNA oligomers that were substituted in part with 5-MeC. Irradiation of a covalently linked anthraquinone group injects a radical cation into the DNA and results in strand cleavage after piperidine treatment. For the sequences examined, substitution of 5-MeC for C has no measurable effect on the reactions. Cytosine methylation is not a general cause of enhanced oxidative damage in DNA.

8-Methoxydeoxyguanosine as an Effective Precursor of 2-Aminoimidazolone, a Major Guanine Oxidation Product in One-Electron Oxidation of DNA

8-Methoxydeoxyguanosine as an Effective Precursor of 2-Aminoimidazolone, a Major Guanine Oxidation Product in One-Electron Oxidation of DNA