DNA damage in deoxynucleosides and oligonucleotides treated with peroxynitrite

A novel nitroimidazole compound formed during the reaction of peroxynitrite with 2',3',5'-tri-O-acetyl-guanosine

Peroxynitrite reacts with 2',3',5'-tri-O-acetyl-guanosine to yield a novel compound identified as 1-(2,3,5-tri-O-acetyl-beta-D-erythro-pentofuranosyl)-5-guanidino-4-nitroimidazole (6). This characterization was achieved using a combination of UV/vis spectroscopy and ESI-MS. Additionally, 1-(beta-D-erythro-pentofuranosyl)-5-guanidino-4-nitroimidazole (6a) was synthesized by an independent route, characterized by UV/vis spectroscopy, ESI-MS, and (1)H- and (13)C NMR, and shown to be identical to deacetylated 6. This product is extremely stable in aqueous solution at both pH extremes and is formed in significant yields. These characteristics suggest that this lesion may be useful as a specific biomarker of peroxynitrite-induced DNA damage. We also observed formation of 2',3',5'-tri-O-acetyl-8-nitroguanosine (2',3',5'-tri-O-acetyl-8-NO(2)()Guo), 2-amino-5-[(2,3,5-tri-O-acetyl-beta-D-erythro-pentofuranosyl)amino]-4H-imidazol-4-one (2',3',5'-tri-O-acetyl-Iz), and the peroxynitrite-induced oxidation products of 2',3',5'-tri-O-acetyl-8-oxoGuo. The formation of 6 and 2',3',5'-tri-O-acetyl-8-NO(2)()Guo was rationalized by a mechanism invoking formation of the guanine radical.

Ntg2 of Saccharomyces cerevisiae repairs the oxidation products of 8-hydroxyguanine

In Escherichia coli, endonuclease III (endo III) repairs the oxidation products of 8-OHGua. However, the corresponding repair enzymes in eukaryotes have not been identified. Here we report that 8-hydroxyguanine (8-OHGua) is highly sensitive to further oxidation. We also show that Ntg2, a functional homolog of endo III in Saccharomyces cerevisiae, is capable of nicking the irradiated duplex DNA containing 8-OHGua. Moreover, Ntg2 formed a stable complex with the DNA upon incubation with NaBH(4). In contrast, Ntg1, another functional homolog of endo III, showed no such activities. These findings indicate that Ntg2 is, at least in part, responsible for repairing the oxidation products of 8-OHGua in eukaryotic cells.

6-hydroxydopamine increases the hydroxylation and nitration of phenylalanine in vivo: implication of peroxynitrite formation

In the present study, we investigated the effect of the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA) on hydroxyl free radical and peroxynitrite formation in vivo using D-phenylalanine as a novel mechanistic probe. In vivo microdialysis was carried out in the striatum of freely moving male Wistar rats. The microdialysis probes were perfused with artificial cerebrospinal fluid containing 5 mM D-phenylalanine (flow rate 2 microL/min). After obtaining a stable baseline 6-OHDA was delivered into the striatum via reverse microdialysis for 60 min. HPLC measurements of the effluent were performed using photodiode array detection for determination of phenylalanine derived o-tyrosine and m-tyrosine (as hydroxylation markers) as well as of nitrotyrosine and nitrophenylalanine (as nitration markers). The basal levels of the hydroxylation derived products of phenylalanine were approximately 100-fold higher than those of the nitration derived products. 6-OHDA (0.1, 1, 10 mM) significantly increased o- and m-tyrosine up to nine- and 13-fold, respectively, whereas levels of 3-nitrotyrosine and 4-nitrophenylalanine were significantly increased up to 422- and 358-fold, respectively. The results demonstrate that phenylalanine is a sensitive in vivo marker for 6-OHDA-induced hydroxylation and nitration reactions which are clearly concentration dependent. We conclude that peroxynitrite formation is involved in 6-OHDA-induced neurochemical effects.

The carbonate radical is a site-selective oxidizing agent of guanine in double-stranded oligonucleotides

The carbonate radical anion (CO(3)) is believed to be an important intermediate oxidant derived from the oxidation of bicarbonate anions and nitrosoperoxocarboxylate anions (formed in the reaction of CO(2) with ONOO(-)) in cellular environments. Employing nanosecond laser flash photolysis methods, we show that the CO(3) anion can selectively oxidize guanines in the self-complementary oligonucleotide duplex d(AACGCGAATTCGCGTT) dissolved in air-equilibrated aqueous buffer solution (pH 7.5). In these time-resolved transient absorbance experiments, the CO(3) radicals are generated by one-electron oxidation of the bicarbonate anions (HCO(3)(-)) with sulfate radical anions (SO(4)) that, in turn, are derived from the photodissociation of persulfate anions (S(2)O(8)(2-)) initiated by 308-nm XeCl excimer laser pulse excitation. The kinetics of the CO(3) anion and neutral guanine radicals, G(-H)( small middle dot), arising from the rapid deprotonation of the guanine radical cation, are monitored via their transient absorption spectra (characteristic maxima at 600 and 315 nm, respectively) on time scales of microseconds to seconds. The bimolecular rate constant of oxidation of guanine in this oligonucleotide duplex by CO(3) is (1.9 +/- 0.2) x 10(7) m(-1) s(-1). The decay of the CO(3) anions and the formation of G(-H)( small middle dot) radicals are correlated with one another on the millisecond time scale, whereas the neutral guanine radicals decay on time scales of seconds. Alkali-labile guanine lesions are produced and are revealed by treatment of the irradiated oligonucleotides in hot piperidine solution. The DNA fragments thus formed are identified by a standard polyacrylamide gel electrophoresis assay, showing that strand cleavage occurs at the guanine sites only. The biological implications of these oxidative processes are discussed.

Oxidative DNA damage in cultured cells and rat lungs by carcinogenic nickel compounds

DNA damage in cultured cells and in lungs of rats induced by nickel compounds was investigated to clarify the mechanism of nickel carcinogenesis. DNA strand breaks in cultured cells exposed to nickel compounds were measured by using a pulsed field gel electrophoresis technique. Among nickel compounds (Ni(3)S(2), NiO (black), NiO (green), and NiSO(4)), only Ni(3)S(2), which is highly carcinogenic, induced lesions of both double- and single-stranded DNA in cultured human cells (Raji and HeLa cells). Treatment of cultured HeLa cells with Ni(3)S(2) (10 microg/ml) induced a 1.5-fold increase in 8-hydroxy-2'-deoxyguanosine (8-OH-dG) compared with control, whereas NiO (black), NiO (green), and NiSO(4) did not enhance the generation of 8-OH-dG. Intratracheal instillation of Ni(3)S(2), NiO(black), and NiO(green) to Wistar rats increased 8-OH-dG in the lungs significantly. NiSO(4) induced a smaller but significant increase in 8-OH-dG. Histological studies showed that all the nickel compounds used induced inflammation in lungs of the rats. Nitric oxide (NO) generation in phagocytic cells induced by Ni(3)S(2), NiO(black), and NiO(green) was examined using macrophage cell line RAW 264.7 cells. NO generation in RAW 264.7 cells stimulated with lipopolysaccharide was enhanced by all nickel particles. Two mechanisms for nickel-induced oxidative DNA damage have been proposed as follows: all the nickel compounds used induced indirect damage through inflammation, and Ni(3)S(2) also showed direct oxidative DNA damage through H(2)O(2) formation. This double action may explain relatively high carcinogenic risk of Ni(3)S(2).

Site-Selective Nitration of Tyrosine in Human Serum Albumin by Peroxynitrite

Peroxynitrite, which is formed in biological systems by the reaction of nitric oxide with superoxide anion, is a highly reactive molecule that can lead to cell injury or cell death. Reactions of peroxynitrite under physiological conditions include nitration of tyrosine-containing proteins or peptides, and we have been investigating the behavior of human serum albumin following exposure to peroxynitrite. Peroxynitrite, at relative concentrations ranging from 0.2 to 50 with respect to protein, was added to human serum albumin in buffer at pH 7.2. The resulting mixtures were dialyzed to remove small molecules, dried under vacuum, and then digested with trypsin. The digests were analyzed by high performance liquid chromatography with UV detection at 230 and 354 nm, the latter wavelength being selective for nitrotyrosine. At the higher relative concentrations of peroxynitrite, the 354-nm chromatograms contained a large number of peaks, including at least nine with molecular weights corresponding to nitration of nominal tryptic peptides. Following treatment with the lower relative concentrations of peroxynitrite, however, the 354-nm chromatograms were dominated by only two nitrated peptides; these were identified by comparison of LC retention times and collision-induced decomposition mass spectra as nitro-Y(411)TK(413) and nitro-Y(138)LYEIAR(144). Each of these tyrosines resides in a known reactive site within the protein, i.e., subdomains IIIA and IB, respectively.

Recombinational repair is critical for survival of Escherichia coli exposed to nitric oxide

Nitric oxide (NO(.)) is critical to numerous biological processes, including signal transduction and macrophage-mediated immunity. In this study, we have explored the biological effects of NO(.)-induced DNA damage on Escherichia coli. The relative importance of base excision repair, nucleotide excision repair (NER), and recombinational repair in preventing NO(.)-induced toxicity was determined. E. coli strains lacking either NER or DNA glycosylases (including those that repair alkylation damage [alkA tag strain], oxidative damage [fpg nei nth strain], and deaminated cytosine [ung strain]) showed essentially wild-type levels of NO(.) resistance. However, apyrimidinic/apurinic (AP) endonuclease-deficient cells (xth nfo strain) were very sensitive to killing by NO(.), which indicates that normal processing of abasic sites is critical for defense against NO(.). In addition, recA mutant cells were exquisitely sensitive to NO(.)-induced killing. Both SOS-deficient (lexA3) and Holliday junction resolvase-deficient (ruvC) cells were very sensitive to NO(.), indicating that both SOS and recombinational repair play important roles in defense against NO(.). Furthermore, strains specifically lacking double-strand end repair (recBCD strains) were very sensitive to NO(.), which suggests that NO(.) exposure leads to the formation of double-strand ends. One consequence of these double-strand ends is that NO(.) induces homologous recombination at a genetically engineered substrate. Taken together, it is now clear that, in addition to the known point mutagenic effects of NO(.), it is also important to consider recombination events among the spectrum of genetic changes that NO(. ) can induce. Furthermore, the importance of recombinational repair for cellular survival of NO(.) exposure reveals a potential susceptibility factor for invading microbes.

Mutations induced by reactive nitrogen oxide species in the supF forward mutation assay

Nitric oxide is an important bioregulatory molecule with a range of physiological functions. Nitric oxide can also react with oxygen species to produce a range of reactive nitrogen oxides that can damage DNA and lead to mutations of the DNA base sequence. The mutagenicity of a variety of reactive nitrogen oxide species and related DNA damaging agents in the supF assay are reviewed here, in the context of recent reports that relate to the nature of the DNA lesions responsible for the induced mutations. Mutations induced by nitric oxide in the supF assay are compared to those induced by N(2)O(3), nitrous acid, peroxynitrite and different reactive oxygen species. The effect of replication of the damaged pSP189 plasmid in human cells or Escherichia coli cells is also considered.

Peroxynitrite scavenging by metalloporphyrins and thiolates

The rate constant for the reaction of nitric oxide with superoxide virtually assures that peroxynitrite will be formed to some extent in any cell or tissue where both radicals exist simultaneously. The precise biological targets for peroxynitrite and the nature of the modification of those targets vary dramatically depending on their relative concentrations and the rates and duration of peroxynitrite formation. Thus, peroxynitrite may have physiological functions in addition to pathological ones. Peroxynitrite scavenger compounds may prove to be therapeutic by effectively intercepting higher levels of peroxynitrite and thereby preventing injurious oxidative modifications of cellular components. Thiols and thiolates comprise a class of sacrificial scavengers that react with peroxynitrite anion with rate constants ranging from 2 x 10(3) M(-1) s(-1) to 2 x 10(8) M(-1) s(-1), depending on the microenvironment of the thiol. Several Mn and Fe porphyrins have been shown to react quite rapidly with peroxynitrite (10(6) to 10(7) M(-1) s(-1)) and decompose it in a catalytic manner; Mn porphyrins require exogenous reductants for complete cycling whereas Fe porphyrins do not. Sacrificial thiol/thiolate scavengers effectively quench the total oxidative yield of peroxynitrite, whereas the catalytic porphyrins redirect it and can, under some conditions, enhance total nitration and oxidative yield.

Oxyradicals and DNA damage

A major development of carcinogenesis research in the past 20 years has been the discovery of significant levels of DNA damage arising from endogenous cellular sources. Dramatic improvements in analytical chemistry have provided sensitive and specific methodology for identification and quantitation of DNA adducts. Application of these techniques to the analysis of nuclear DNA from human tissues has debunked the notion that the human genome is pristine in the absence of exposure to environmental carcinogens. Much endogenous DNA damage arises from intermediates of oxygen reduction that either attack the bases or the deoxyribosyl backbone of DNA. Alternatively, oxygen radicals can attack other cellular components such as lipids to generate reactive intermediates that couple to DNA bases. Endogenous DNA lesions are genotoxic and induce mutations that are commonly observed in mutated oncogenes and tumor suppressor genes. Their mutagenicity is mitigated by repair via base excision and nucleotide excision pathways. The levels of oxidative DNA damage reported in many human tissues or in animal models of carcinogenesis exceed the levels of lesions induced by exposure to exogenous carcinogenic compounds. Thus, it seems likely that oxidative DNA damage is important in the etiology of many human cancers. This review highlights some of the major accomplishments in the study of oxidative DNA damage and its role in carcinogenesis. It also identifies controversies that need to be resolved. Unraveling the contributions to tumorigenesis of DNA damage from endogenous and exogenous sources represents a major challenge for the future.