Modulation of DNA breakage induced via the Fenton reaction

Mechanism of DNA damage by cadmium and interplay of antioxidant enzymes and agents

Cadmium is an environmental toxicant, which causes cancer in different organs. It was found that it damages DNA in the various tissues and cultured cell lines. To investigate the mechanism of DNA damage, we have studied the effect of cadmium-induced DNA damage in plasmid pBR322 DNA, and the possible ameliorative effects of antioxidative agents under in vitro conditions. It was observed that cadmium alone did not cause DNA damage. However, it caused DNA damage in the presence of hydrogen peroxide, in a dose dependent manner, because of production of hydroxyl radicals. Findings from this study show the conversion of covalently closed circular double-stranded pBR 322 DNA to the open circular and linear forms of DNA when treated with 10 muM cadmium and various concentrations of H(2)O(2). The conversion was due to nicking in DNA strands. The observed rate of DNA strand breakage was dependent on H(2)O(2) concentration, temperature, and time. Metallothionein I failed to prevent cadmium-induced DNA nicking in the presence of H(2)O(2). Of the two antioxidant enzymes (catalase and superoxide dismutase) studied, only catalase conferred significant (50-60%) protection. EDTA and DMSO exhibited protection similar to catalase, while mannitol showed only about 20% protection against DNA damage. Ethyl alcohol failed to ameliorate cadmium-induced DNA strands break. From this study, it is plausible to infer that cadmium in the presence of hydrogen peroxide causes DNA damage probably by the formation of hydroxyl ions. These results may indicate that cadmium in vivo could play a major role in the DNA damage induced by oxidative stress.

Fungal chitin-glucan derivatives exert protective or damaging activity on plasmid DNA

Water-soluble derivatives of the chitin-glucan (Ch-G) complex isolated from the fungal mycelium of the industrial strain of Aspergillus niger have been previously shown to possess potent antimutagenic protective activity in vivo. Their direct action on DNA has not been yet evaluated. Using carboxymethylation, sulfoethylation and subsequent ultrasonic treatment, lower molecular weight water-soluble derivatives were obtained from the crude fungal Ch-G. The biological effects of the prepared compounds were evaluated in direct interaction on plasmid DNA in vitro. Monitoring of electrophoretic mobility of different conformers of plasmid DNA implied that carboxymethyl chitin-glucan (CM-Ch-G) induced single- and double-strand breaks into supercoiled DNA in a concentration-dependent manner. On the other hand, sulfoethyl chitin-glucan (SE-Ch-G) alone did not induce any DNA breaks in plasmid DNA. However, process of DNA damaging induced by free-radical oxidation initiated with Fe(2+) was inhibited, while the process of DNA breakage induced by H(2)O(2) was increased in the presence of SE-Ch-G.

High levels of intracellular cysteine promote oxidative DNA damage by driving the fenton reaction

Escherichia coli is generally resistant to H(2)O(2), with >75% of cells surviving a 3-min challenge with 2.5 mM H(2)O(2). However, when cells were cultured with poor sulfur sources and then exposed to cystine, they transiently exhibited a greatly increased susceptibility to H(2)O(2), with <1% surviving the challenge. Cell death was due to an unusually rapid rate of DNA damage, as indicated by their filamentation, a high rate of mutation among the survivors, and DNA lesions by a direct assay. Cell-permeable iron chelators eliminated sensitivity, indicating that intracellular free iron mediated the conversion of H(2)O(2) into a hydroxyl radical, the direct effector of DNA damage. The cystine treatment caused a temporary loss of cysteine homeostasis, with intracellular pools increasing about eightfold. In vitro analysis demonstrated that cysteine reduces ferric iron with exceptional speed. This action permits free iron to redox cycle rapidly in the presence of H(2)O(2), thereby augmenting the rate at which hydroxyl radicals are formed. During routine growth, cells maintain small cysteine pools, and cysteine is not a major contributor to DNA damage. Thus, the homeostatic control of cysteine levels is important in conferring resistance to oxidants. More generally, this study provides a new example of a situation in which the vulnerability of cells to oxidative DNA damage is strongly affected by their physiological state.

Kinetics of phosphate-mediated oxidation of ferrous iron and formation of 8-oxo-2'-deoxyguanosine in solutions of free 2'-deoxyguanosine and calf thymus DNA

Formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) in solutions of free 2'-deoxyguanosine (dG) and calf thymus DNA (DNA) was compared for the diffusion-dependent and localised production of oxygen radicals from phosphate-mediated oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+). The oxidation of Fe2+ to Fe3+ was followed at 304 nm at pH 7.2 under aerobic conditions. Given that the concentration of Fe2+ >or=phosphate concentration, the rate of Fe2+ oxidation was significantly higher in DNA-phosphate as compared for the same concentration of inorganic phosphate. Phosphate catalysed oxidation of ferrous ions in solutions of dG or DNA led through the production of reactive oxygen species to the formation of 8-oxo-dG. The yield of 8-oxo-dG in solutions of dG or DNA correlated positively with the inorganic-/DNA-phosphate concentrations as well as with the concentrations of ferrous ions added. The yield of 8-oxo-dG per unit oxidised Fe2+ were similar for dG and DNA; thus, it differed markedly from radiation-induced 8-oxo-dG, where the yield in DNA was several fold higher. For DNA in solution, the localisation of the phosphate ferrous iron complex relative to the target is an important factor for the yield of 8-oxo-dG. This was supported from the observation that the yield of 8-oxo-dG in solutions of dG was significantly increased over that in DNA only when Fe2+ was oxidised in a high excess of inorganic phosphate (50 mM) and from the lower protection of DNA damage by the radical scavenger (hydroxymethyl)aminomethane (Tris)-HCl.

Effects of 1,10-phenanthroline and hydrogen peroxide in Escherichia coli: lethal interaction

It has been observed that when Escherichia coli cells are treated simultaneously with phenanthroline and H2O2, there is a lethal interaction. In order to analyze the mechanism of this lethal interaction, wild-type and xthA mutant cells of E. coli were treated with 2.5 mM H2O2 and 1 mM phenanthroline. This treatment was preceded by treatments with different metal chelators (dipyridyl for Fe2+, desferal for Fe3+ and neocuproine for Cu2+) or conducted simultaneously to other treatments with chelators and radical scavengers (thiourea, ethanol and sodium benzoate). The lethal interaction was observed in both the E. coli wild-type strain and xthA mutant strain, which is deficient in the exonuclease III repair enzyme. Nevertheless, the mutant strain was much more sensitive than the wild-type one. Dipyridyl pretreatment protected the cells against the lethal interaction, while desferal pretreament was unable to do so. This suggests that the lethal interaction requires Fe2+ and not Fe3+ ions. Ethanol and sodium benzoate were incapable of protecting bacterial cells against the lethal interaction. Even a 20-min pretreatment with benzoate did not confer protection. On the other hand, thiourea protected the cells completely. Based on our results, we propose that the lethal interaction may be caused not only by the reaction kinetics of phenanthroline and Fe, but also by the ability of phenanthroline to intercalate in DNA. After forming the mono and bis complexes, phenanthroline would serve as a shuttle and take the Fe2+ ions to the DNA. So, the Fenton reaction would take its course with the consequent generation of OH. radicals near DNA. This proximity to the DNA would protect the OH. radicals against the scavengers' action, thus optimizing the Fenton reaction.

Cloning, expression, and characterization of two manganese superoxide dismutases from Caenorhabditis elegans

Two genes encoding manganese superoxide dismutase (sod-2 and sod-3) have been identified in the nematode Caenorhabditis elegans. Each gene is composed of five exons, and intron positions are identical; however, intron sizes and sequences are not the same. The predicted protein sequences are 86.3% homologous (91.8% conservative), and the cDNAs are only 75.2% homologous. Both deduced protein sequences contain the expected N-terminal mitochondrial transit peptides. Reverse transcriptase polymerase chain reaction analysis shows that both genes are expressed under normal growth conditions and that their RNA transcripts are trans-spliced to the SL-1 leader sequence. The latter result together with Northern blot analysis indicate that both genes have mono-cistronic transcripts. The sod-3 gene was mapped to chromosome X, and the location of sod-2 was confirmed to be chromosome I. Polymerase chain reaction was used to amplify the cDNA regions encoding the predicted mature manganese superoxide dismutase proteins and each was cloned and expressed to high levels in Escherichia coli cells deficient in cytosolic superoxide dismutases. Both proteins were shown to be active in E. coli, providing similar protection against methyl viologen-induced oxidative stress. The expressed enzymes, which were not inhibited by hydrogen peroxide or cyanide, are dimeric, show quite different electrophoretic mobilities and isoelectric points, but exhibit comparable specific activities.

The role of DNA damage in the cytotoxic response to hydrogen peroxide/histidine

1. Histidine enhances the cytotoxic and clastogenic effects of hydrogen peroxide. In this review, we will focus on two lesions that are generated in the presence of histidine in oxidatively injured cells--namely, DNA single- and double-strand breaks (SSBs and DSBs). 2. Hydrogen peroxide is a potent inducer of DNA SSBs, and histidine modulates the formation of these lesions. This effect has been extensively characterized with the use of purified DNA, and the results obtained have demonstrated that, upon exposure to low or high concentrations of H2O2, histidine reduces or enhances the formation of DNA SSBs, respectively. The protective effect has been ascribed to iron chelation, whereas the enhancing effect is probably the consequence of the formation of a histidine/iron/DNA complex. 3. In cultured cells, histidine potentiates the formation of H2O2-induced DNA SSBs but these lesions are efficiently repaired and do not appear to mediate the cytotoxic response. 4. In the presence of micromolar levels of histidine, H2O2 also induces DNA DSBs, a type of lesion that is not generated by the oxidant alone. The experimental evidence that has been thus far collected would suggest that these DNA DSBs are toxic and are indeed the cause of cell death induced by the cocktail H2O2/histidine.

Nitric oxide synergistically enhances DNA strand breakage induced by polyhydroxyaromatic compounds, but inhibits that induced by the Fenton reaction

Reactive oxygen and nitrogen species play an important role in many human diseases including cancer. We have found that incubation of pBR322 plasmid DNA with a nitric oxide (NO)-releasing compound such as diethylamine NONOate and a polyhydroxyaromatic compound such as catechol, 1,4-hydroquinone, or pyrogallol caused synergistic induction of single-strand breakage, whereas either compound alone induced much less breakage. Phenol, resorcinol, or guaiacol (O-methylcatechol) did not exhibit this synergistic effect of DNA damage with NO. The strand breakage induced by NO with pyrogallol was prevented by excess superoxide dismutase, carboxy-PTIO (an NO-trapping agent), or anti-oxidants (urate, ascorbate). Possible mechanisms for the induction of this synergistic effect of NO and polyhydroxyaromatic compounds on the strand breakage are proposed, including involvement of peroxynitrite formed from NO and O2.- derived from autooxidation of polyhydroxyaromatics. This pathway for generation of reactive species from NO and catechol-type compounds (e.g., L-dopa, catechol-estrogen) may be important in many pathological conditions, because both compounds are concurrently formed or present in vivo. On the other hand, NO dose-dependently inhibited the strand breakage mediated by 1,4-hydroquinone plus Cu2+ or Fenton reaction (H2O2, iron or copper). This inhibition could be due to formation of a complex between NO and a metal ion, inhibiting generation of reactive species from H2O2. Our results can account for contrasting activities of NO reported in relation to tissue injury. NO can play both detrimental and beneficial roles in DNA damage, depending on the type and amounts of reactive oxygen species and metal ions concurrently present.

pH-dependent DNA cleavage in permeabilized human fibroblasts

In several cell types, apoptosis is associated with intracellular acidification and activation of a pH-dependent endonuclease. We have examined the effect of acidic pH on the DNA of permeabilized human fibroblasts, and observed cleavage of DNA into high-molecular-mass fragments. This pH-dependent DNA breakage was modulated by temperature, the presence of histones and diethyl pyrocarbonate. Superoxide dismutase and chelators with high affinity for Cu prevented DNA fragmentation, whereas catalase, DMSO and Desferal (desferrioxamine mesylate) offered no protection. Fragmentation of DNA into high-molecular-mass fragments, which is occasionally observed as an early phase of apoptosis, is thought to result from the activation of endonuclease(s). Our results suggest that such fragmentation also occurs through induction of copper-mediated site-specific DNA damage that is enhanced by intracellular acidification.

Melatonin reduces both basal and bacterial lipopolysaccharide-induced lipid peroxidation in vitro

The protective effect of melatonin against lipopolysaccharide (LPS)-induced oxidative damage was examined in vitro. Lung, liver, and brain malonaldehyde (MDA) plus 4-hydroxyalkenals (4-HDA) concentrations were measured as indices of induced membrane peroxidative damage. Homogenates of brain, lung, and liver were incubated with LPS at concentrations of either 1, 10, 50, 200, or 400 micrograms/ml for 1 h and, in another study, LPS at a concentration of 400 micrograms/ml for either 0, 15, 30, or 60 min. Melatonin at increasing concentrations from 0.01-3 mM either alone or together with LPS (400 micrograms/ml) was used. Liver, brain, and lung MDA + 4-HDA levels increased after LPS at concentrations of 10, 50, 200 or 400 micrograms/ml; this effect was concentration-dependent. The highest levels of lipid peroxidation products were observed after tissues were incubated with an LPS concentration of 400 micrograms/ml for 60 min; in liver and lung this effect was totally suppressed by melatonin and partially suppressed in brain in a concentration-dependent manner. In addition, melatonin alone was effective in brain at concentrations of 0.1 to 3 mM, in lung at 2 to 3 mM, and in liver at 0.1 to 3 mM; in all cases, the inhibitory effects of melatonin on lipid peroxidation were always directly correlated with the concentration of melatonin in the medium. The results show that the direct effect of LPS on the lipid peroxidation following endotoxin exposure is markedly reduced by melatonin.

DNA base modifications and membrane damage in cultured mammalian cells treated with iron ions

We investigated DNA base damage in mammalian cells exposed to exogenous iron ions in culture. Murine hybridoma cells were treated with Fe(II) ions at concentrations of 10 microM, 100 microM, and 1 mM. Chromatin was isolated from treated and control cells and analyzed by gas chromatography/mass spectrometry for DNA base damage. Ten modified DNA bases were identified in both Fe(II)-treated and control cells. The quantification of modified bases was achieved by isotope-dilution mass spectrometry. In Fe(II)-treated cells, the amounts of modified bases were increased significantly above the background levels found in control cells. Dimethyl sulfoxide at concentrations up to 1 M in the culture medium did not significantly inhibit the formation of modified DNA bases. A mathematical simulation used to evaluate the plausibility of DNA damage upon Fe(II) treatment predicted a dose-dependent response, which agreed with the experimental results. In addition, Fe(II) treatment of cells increased the cell membrane permeability and caused production of lipid peroxides. The nature of DNA base lesions suggests the involvement of the hydroxyl radical in their formation. The failure of dimethyl sulfoxide to inhibit their formation indicates a site-specific mechanism for DNA damage with involvement of DNA-bound metal ions. Fe(II) treatment of cells may increase the intracellular iron ion concentration and/or cause oxidative stress releasing metal ions from their storage sites with subsequent binding to DNA. Identified DNA base lesions may be promutagenic and play a role in pathologic processes associated with iron ions.

Metallothionein protects DNA from copper-induced but not iron-induced cleavage in vitro

Iron and copper ions mediate generation of reactive oxygen radicals from O2 and H2O2 by the Fenton reaction: these radicals are capable of damaging DNA. We studied (a) the ability of these metals to induce double-strand breaks in DNA in vitro in the presence of H2O2 and ascorbic acid as donors of reactive oxygen, and (b) the ability of the metal-binding protein metallothionein (MT) to protect DNA from damage. Strand cleavage was measured by loss of fluorescence after binding to ethidium bromide and by increased mobility of DNA in agarose. The results show that Cu(II), Fe(II) and Fe(III) all can induce damage to calf thymus DNA under our experimental conditions. Cu(II)-induced DNA damage was dose-dependent and the degree of damage was proportional to the concentration of H2O2. On the other hand, DNA fragmentation was significant only in the presence of high concentrations of Fe(II) or Fe(III). Addition of Zn-MT to the reaction mixture prior to addition of Cu(II) inhibited fragmentation of DNA in a dose-dependent manner but had little effect on iron induced damage. Other proteins (histone or albumin) were not effective in protecting DNA from Cu-induced damage, as compared to Zn-MT. The formation of Cu(I) from Cu(II) in the presence of hydrogen peroxide and ascorbate was also inhibited by addition of Zn-MT. Thus, MT may protect DNA from damage by free radicals by sequestering copper and preventing its participation in redox reactions.

Detection and quantification of melatonin in a dinoflagellate, Gonyaulax polyedra: solutions to the problem of methoxyindole destruction in non-vertebrate material

Preservative procedures are described for the extraction and quantification of melatonin in cell material from the dinoflagellate Gonyaulax polyedra, an organism in which this indoleamine is rapidly degraded due to interaction with free oxygen radicals and photooxidation. Cells were shock-frozen in liquid nitrogen and pulverized. Various extraction methods were applied to the powder, and rates of recovery were compared. For the determination of melatonin by high performance liquid chromatography (HPLC), extractions with acetone or perchloric acid were more suitable than the use of other solvents. For purposes of radioimmunoassay (RIA), extraction with acetone gave the best results. Several other inorganic solvents, which are often applied in melatonin research, such as chloroform, dichloromethane, and diethyl ether, led to considerable losses of the indoleamine. The procedures developed for HPLC with either electrochemical or fluorescence detection also allow the quantification of other indolic compounds, in particular, tryptophan and 5-methoxytryptamine. The methods described may be of value in the further search for melatonin and related indoleamines in non-vertebrate material, especially, from unicells, multicellular plants, and invertebrates.

tert.-butyl hydroperoxide-mediated DNA base damage in cultured mammalian cells

tert.-Butyl hydroperoxide has been utilized to study the effect of oxidative stress on living cells; however, its effect on DNA bases in cells has not been characterized. In the present work, we have investigated DNA base damage in mammalian cells exposed to this organic hydroperoxide. SP2/0 derived murine hybridoma cells were treated with 4 concentrations of tert.-butyl hydroperoxide for varying periods of time. Chromatin was isolated from treated and control cells and subsequently analyzed by gas chromatography-mass spectrometry with selected-ion monitoring for DNA base damage. Quantification of damaged DNA bases was achieved by isotope-dilution mass spectrometry. The amounts of 8 products were significantly higher than control levels in cells treated with tert.-butyl hydroperoxide at a concentration range of 0.01-0.1 mM. At concentrations from 1.0 to 10 mM, product formation was inhibited and the amounts of products were similar to those in control cells. The bimodal nature of the dose-response may be qualitatively analogous to previous reports of bimodal killing of E. coli bacteria by hydrogen peroxide. The nature of the identified DNA base lesions suggests the involvement of the hydroxyl radical in their formation. tert.-Butyl hydroperoxide is known to produce the tert.-butoxyl radical in reactions with metal ions. However, it is unlikely that the tert.-butoxyl radical produces these DNA lesions. It is suggested that DNA base damage arises from tert.-butyl hydroperoxide-mediated oxidative stress in cells, resulting in formation of hydroxyl radicals in close proximity to DNA. The inhibition of product formation at high concentrations of tert.-butyl hydroperoxide may be explained by the scavenging of tert.-butoxyl radical by tert.-butyl hydroperoxide resulting in inhibition of oxidative stress. The plausibility of the scavenging mechanism was evaluated with a mathematical simulation of the dose-response for DNA damage in solutions containing hydrogen peroxide. The simulation model predicted a bimodal dose-response which agreed qualitatively with the results in this study and with other in vivo and in vitro studies reported in the literature.

Protonation of the imidazole ring prevents the modulation by histidine of oxidative DNA degradation

When supercoiled DNA is incubated with Fe(II) at pH 7 in the presence of hydrogen peroxide, the rate of nicking first increases with increasing H2O2 concentration to reach a maximum, then decreases and eventually increases again. When 0.1 mM histidine is added at neutral pH at low H2O2 concentration (< 3 mM), it hinders the nicking of DNA; when it is added at high H2O2 concentrations (> 10 mM), it enhances the rate of nicking. When similar experiments are performed at slightly acidic pH (4.5) the biphasic behavior is maintained, independent of the presence of histidine. One can conclude that the protonation of imidazole (pK = 5.9) abolishes the capability of histidine to modulate the oxidative degradation of DNA. Results of electron spin resonance experiments suggest that at low H2O2 concentration, the protective effect of histidine could be the consequence of its capability to bind OH. radicals.

Differential effects of histidine on hydrogen peroxide-induced bacterial killing and DNA nicking in vitro

The hydrogen peroxide dose-response curves for Escherichia coli killing and DNA nicking in vitro display remarkably similar bimodal patterns. The concentrations of the oxidant resulting in maximum mode one killing, however, exceeds by two orders of magnitude those resulting in the mode one DNA nicking response. Addition of histidine differentially affects the experimental curves describing the dose-dependency for bacterial killing and DNA damage in vitro. Indeed, the lethal effect elicited by the oxidant in the presence of the amino acid is strictly concentration-dependent and thus the inactivation curve loses its bimodal character. In marked contrast, histidine abolishes DNA damage generated by low concentrations of hydrogen peroxide (< 100 microM) in the in vitro system (the mode one DNA nicking response) but greatly increases DNA damage produced by concentrations of the oxidant higher than 1 mM (the mode two DNA nicking response). Experimental results also suggest that treatment of covalently closed circular double-stranded super-coiled DNA with hydrogen peroxide, in the presence of both histidine and iron, may result in the formation of DNA double strand breakage, a type of lesion which is not efficiently produced by the oxidant in the absence of the amino acid. Taken together, the above results indicate that histidine differentially affects the in vitro DNA cleavage and E. coli lethality induced by hydrogen peroxide and suggest that different molecular events mediate mode one DNA nicking in vitro and mode one killing of bacterial cells.