Interaction of singlet oxygen with DNA and biological consequences

Singlet oxygen-mediated damage to cellular DNA determined by the comet assay associated with DNA repair enzymes

The damage profile produced by the reaction of singlet molecular oxygen with cellular DNA was determined using the comet assay associated with DNA repair enzymes. Singlet oxygen was produced intracellularly by thermal decomposition of a water-soluble endoperoxide of a naphthalene derivative which is able to penetrate through the membrane into mammalian cells. We found that the DNA modifications produced by singlet oxygen were almost exclusively oxidised purines recognised by the formamidopyrimidine DNA N-glycosylase. In contrast, significant amounts of direct strand breaks and alkali-labile sites or oxidised pyrimidines, detectable by the bacterial endonuclease III, were not produced.

Vitamin C matters: increased oxidative stress in cultured human aortic endothelial cells without supplemental ascorbic acid

Because standard culture media for human aortic endothelial cells (HAEC) do not contain vitamin C, we hypothesized that HAEC may be under significant oxidative insult compared with the situation in vivo. To assess parameters of oxidative stress, intracellular vitamin C, glutathione (GSH), GSH/GSSG, and NAD(P)H/NAD(P)+ ratios, as well as oxidant appearance and oxidative damage, were measured in HAEC with or without vitamin C addition. The effect of vitamin C on eNOS activity was also determined. Results showed that HAEC without vitamin C treatment were essentially scorbutic. On addition of 100 mM vitamin C to the culture media, intracellular vitamin C levels increased and peaked at 6 h. A concomitant increase in the total GSH and the GSH/GSSG ratio was also observed; the NAD(P)H/NAD(P)+ ratio increased more slowly over the 24-h time course. Significantly lower (P <0.05) oxidant appearance and steady-state oxidative damage were also observed following vitamin C repletion. Vitamin C treatment increased eNOS activity by 600%. Thus, HAEC are scorbutic under normal culture conditions and exhibit higher oxidative stress than vitamin C repleted cells. Vitamin C supplementation should be considered when using cultured cells, especially when experimental endpoints are related to cellular redox status and eNOS activity.

Inhibition of glyceraldehyde-3-phosphate dehydrogenase by peptide and protein peroxides generated by singlet oxygen attack

Reaction of certain peptides and proteins with singlet oxygen (generated by visible light in the presence of rose bengal dye) yields long-lived peptide and protein peroxides. Incubation of these peroxides with glyceraldehyde-3-phosphate dehydrogenase, in the absence of added metal ions, results in loss of enzymatic activity. Comparative studies with a range of peroxides have shown that this inhibition is concentration, peroxide, and time dependent, with H2O2 less efficient than some peptide peroxides. Enzyme inhibition correlates with loss of both the peroxide and enzyme thiol residues, with a stoichiometry of two thiols lost per peroxide consumed. Blocking the thiol residues prevents reaction with the peroxide. This stoichiometry, the lack of metal-ion dependence, and the absence of electron paramagnetic resonance (EPR)-detectable species, is consistent with a molecular (nonradical) reaction between the active-site thiol of the enzyme and the peroxide. A number of low-molecular-mass compounds including thiols and ascorbate, but not Trolox C, can prevent inhibition by removing the initial peroxide, or species derived from it. In contrast, glutathione reductase and lactate dehydrogenase are poorly inhibited by these peroxides in the absence of added Fe2+-EDTA. The presence of this metal-ion complex enhanced the inhibition observed with these enzymes consistent with the occurrence of radical-mediated reactions. Overall, these studies demonstrate that singlet oxygen-mediated damage to an initial target protein can result in selective subsequent damage to other proteins, as evidenced by loss of enzymatic activity, via the formation and subsequent reactions of protein peroxides. These reactions may be important in the development of cellular dysfunction as a result of photo-oxidation.

Effect of endogenous carotenoids on "adaptive" mutation in Escherichia coli FC40

The appearance over many days of Lac(+) frameshift mutations in Escherichia coli strain FC40 incubated on lactose selection plates is a classic example of apparent "adaptive" mutation in an episomal gene. We show that endogenously overproduced carotenoids reduce adaptive mutation under selective conditions by a factor of around two. Carotenoids are known to scavenge singlet oxygen suggesting that the accumulation of oxidative base damage may be an integral part of the adaptive mutation phenomenon. If so, the lesion cannot be 7,8-dihydro-8-oxoguanine since adaptive mutation in FC40 is unaffected by mutM and mutY mutations. If active oxygen species such as singlet oxygen are involved in adaptive mutation then they should also induce frameshift mutations in FC40 under non-selective conditions. We show that such mutations can be induced under non-selective conditions by protoporphyrin photosensitisation and that this photodynamic induction is reduced by a factor of just over two when endogenous carotenoids are present. We argue that the involvement of oxidative damage would in no way be inconsistent with current understanding of the mechanism of adaptive mutation and the role of DNA polymerases.

Singlet oxygen is the major species participating in the induction of DNA strand breakage and 8-hydroxydeoxyguanosine adduct by lead acetate

To investigate DNA damage induced by Pb2+ and its prevention by scavengers, we determined DNA strand breakage and the formation of 8-hydroxydeoxyguanosine (8-OHdG) in DNA using plasmid relaxation assay and HPLC with electrochemical detection, respectively. Lead acetate induced DNA strand breakage in 10 mM of Hepes buffer, pH 6.8, in a time- and dose-dependent manner. Compared with lead, zinc acetate did not significantly induce DNA breakage. The singlet oxygen scavengers NaN3 and 2,2,6,6-tetramethyl-4-piperidone (TEMP) inhibited lead-induced DNA breakage more efficiently than the hydroxyl radical scavengers mannitol and DMPO. Deuterium oxide (D2O), a singlet oxygen enhancer, potentiated lead-induced DNA breakage. At low ratios to Pb2+, NADPH, glutathione, and 2-mercaptoethanol enhanced lead-induced DNA breakage, whereas high ratios of these agents protected it. Catalase and superoxide dismutase (SOD) did not protect DNA breaks induced by Pb2+. Lead-induced DNA breakage was markedly enhanced by H2O2, and this induction was inhibited by NaN3, TEMP, EDTA, catalase, BSA, and glutathione. In contrast, mannitol and SOD potentiated Pb2+/H2O2-induced DNA breaks. The results indicate that singlet oxygen, lead, and H2O2 are all involved in the reaction system, whereas hydroxyl radical and superoxide did not. Lead could cause a small amount of 8-OHdG formation in calf thymus DNA and dose-dependently induced the formation of this adduct in the presence of H2O2. Singlet oxygen scavengers were more effective than hydroxyl radical scavengers in protection from lead/H2O2-induced 8-OHdG adducts. Taken together, these results suggest that lead may induce DNA damage through a Fenton-like reaction and that singlet oxygen is the principal species involved.

In vitro inhibition by N-acetylcysteine of oxidative DNA modifications detected by 32P postlabeling

Reactive oxygen species are involved in the pathogenesis of cancer and other chronic degenerative diseases through a variety of mechanisms, including DNA damage. We investigated by 32p and 33P postlabeling analyses the nucleotidic modifications induced in vitro by treating calf thymus DNA with H2O2 and CuSO4, interacting in a Fenton type reaction. Six different enrichment procedures and three chromatographic systems were comparatively assayed. The chromatographic system using phosphate/urea, which is more suitable for detecting bulky DNA adducts, was rather insensitive. In contrast, the system using acetic acid/ammonium formate revealed high levels of mononucleotidic modifications. In terms of ratio of adduct levels in treated and untreated DNA, the enrichment procedures ranked as follows: nuclease P1 (19.6), no enrichment (18.3), digestion to trinucleotides (17.6), digestion to monophosphate mononucleotides (8.4), digestion to dinucleotides (3.4), and extraction with butanol (<1.0). The system using formic acid/ammonium formate was quite efficient in detecting 8-hydroxy-2'-deoxyguanosine. Labeling with 33p further enhanced the sensitivity of the method. The oxidative damage was so intense to produce a strong DNA fragmentation detectable by agarose gel electrophoresis, and nucleotidic modifications were more intense when DNA fragmentation was greater. The DNA alterations produced by H2O2 alone were significantly lower than those produced following reaction of H2O2 with CuSO4. The thiol N-acetylcysteine (NAC) was quite efficient in inhibiting both nucleotidic modifications and DNA fragmentation produced in vitro by either H2O2 or the .OH generating system. These results support at a molecular level the findings of previous studies showing the ability of NAC to inhibit the genotoxicity of peroxides and of reactive oxygen species generated by electron transfer reactions.

ESR identification of free radicals formed from the oxidation of catechol estrogens by Cu2+

Catechol estrogens are genotoxic, indirectly through redox cycling mechanisms leading to oxidative DNA damage and directly by formation of quinone-DNA adducts. Previously, we demonstrated that Cu2+ can oxidize estradiol (E2) catechols, establishing a copper redox cycle leading to the formation of DNA strand breaks. The goal of this study was to use electron spin resonance techniques to identify the free radical intermediates formed. The 2- and 4-OH catechols of E2 and ethinyl estradiol (EE) were oxidized to semiquinone intermediates, stabilized by Mg2+, when incubated with Cu2+. The 4-OH-EE semiquinone decayed more slowly than the 2-OH-EE semiquinone. Using the spin trap alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone, 4-OH-E2 plus Cu2+ generated hydroxyl radicals at a greater rate than 2-OH-E2 plus Cu2+. Formation of hydroxyl and methyl radical adducts was detected, using 5,5-dimethyl-1-pyrroline-N-oxide as the spin trap, when 2-OH-E2 was incubated with Cu2+ and 1% dimethyl sulfoxide. This was inhibited by the Cu1+ chelator bathocuproinedisulfonic acid and catalase. These data demonstrate that the oxidation of estrogen catechols by Cu2+ leads to a Cu-dependent mechanism of hydroxyl radical production via a hydrogen peroxide intermediate and suggest a mechanism for estrogen-associated site-specific DNA damage and mutagenesis.

Changes in the respiratory chain complexes activities and in the mitochondrial DNA content during ageing in D. subobscura

The time course (age 0-8 weeks) of the enzyme activities of respiratory chain complexes I, III and IV and of citrate synthase, and the cell mitochondrial/nuclear DNA content ratio were studied in Drosophila subobscura. The activities of the three respiratory complexes decreased with age, but with different kinetics. The activities of complexes I and III remained nearly stable between weeks 0 and 3 (falling by 6% and 15%, respectively), and then gradually decreased; after 8 weeks residual activities were about 50% of the initial value for complexes I and III. The activity of complex IV fell in the first week, decreasing continually to week 8, where residual activity was 30% of the initial value. No significant age-related change in citrate synthase activity was observed. Mitochondrial DNA (measured by mitDNA/nucDNA) increased linearly up to week 5 (2.6-fold) and then dropped by 40% in week 6 though it remained higher than initial values.

Photooxidation of d(TpG) by riboflavin and methylene blue. Isolation and characterization of thymidylyl-(3',5')-2-amino-5-[(2-deoxy-beta-D- erythro-pentofuranosyl)amino]-4H-imidazol-4-one and its primary decomposition product thymidylyl-(3',5')-2,2-diamino-4-[(2-deoxy-beta-D- erythro-pentofuranosyl)amino]-5(2H)-oxazolone

The major initial product of riboflavin- and methylene blue-mediated photosensitization of 2'-deoxyguanosine (dG) in oxygen-saturated aqueous solution has previously been identified as 2-amino-5-[(2-deoxy-beta-D-erythro-pentofuranosyl)amino] 4H-imidazol-4-one (dlz). At room temperature in aqueous solution dlz decomposes quantitatively to 2,2-diamino-4-[(2-deoxy-beta-D-erythro- pentofuranosyl)amino]-5(2H)-oxazolone (dZ). The data presented here show that the same guanine photooxidation products are generated following riboflavin- and methylene blue-mediated photosensitization of thymidylyl-(3',5')-2'-deoxyguanosine [d(TpG)]. As observed for the monomers, the initial product, thymidylyl-(3',5')-2-amino-5-[(2-deoxy- beta-D-erythro-pentofuranosyl)amino]-4H-imidazol-4-one [d(Tplz)], decomposes in aqueous solution at room temperature to thymidylyl-(3',5')-2,2-diamino-4- [(2-deoxy-beta-D-erythro-pentofuranosyl)amino]-5(2H)-oxazolone [d(TpZ)]. Both modified dinucleoside monophosphates have been isolated by HPLC and characterized by proton NMR spectrometry, fast atom bombardment mass spectrometry, chemical analyses and enzymatic digestions. Among the chemical and enzymatic properties of these modified dinucleoside monophosphates are: (i) d(Tplz) and d(TpZ) are alkali-labile; (ii) d(Tplz) reacts with methoxyamine, while d(TpZ) is unreactive; (iii) d(Tplz) is digested by snake venom phosphodiesterase, while d(TpZ) is unaffected; (iv) relative to d(TpG), d(TpZ) and d(Tplz) are slowly digested by spleen phosphodiesterase; (v) d(Tplz) and d(TpZ) can be 5'-phosphorylated by T4 polynucleotide kinase. The first observation suggests that dlz and dZ may be responsible for some of the strand breaks detected following hot piperidine treatment of DNA exposed to photosensitizers.

Methylene blue-mediated photooxidation of 7,8-dihydro-8-oxo-2'-deoxyguanosine

One well known product of the methylene blue-mediated photosensitization of 2'-deoxyguanosine (dG) in oxygen saturated aqueous solution is 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxodG). We observed that the rate of 8-oxodG photodecomposition by methylene blue-mediated photosensitization is approx. 3-times faster than for dG. The primary products of the methylene blue-mediated photosensitization of 8-oxodG are 2-amino-5-((2-deoxy-beta-D-erythro-pentofuranosyl)amino)-4H-imidazol-4-o ne (dIz), 2,2-diamino-4-((2-deoxy-beta-D-erythro-pentofuranosyl)amino)-5(2H)-oxazo lone (dZ), the 4R* and 4S* diastereoisomers of 4,8-dihydro-4-hydroxy-8-oxo-2'-deoxyguanosine (dO), and an as yet unidentified product with a molecular weight of 287 (dX). Except for the latter product, these compounds have all been identified following the methylene blue-mediated photooxidation of dG. Methylene blue-mediated photooxidation of 8-oxodG in D2O instead of H2O leads to a 4-fold increase in the rate of 8-oxodG photodecomposition while the addition of sodium azide retards the reaction, observations which imply that the reaction occurs via a type II (singlet oxygen mediated) mechanism. Like 8-oxodG, dIz and dZ are sensitive to hot piperidine and likely contribute to strand breaks observed in double stranded DNA exposed to methylene blue plus light followed by hot piperidine. Because 8-oxodG generates predominately G-->T transversions, the photooxidation of 8-oxodG to dIz, dO, and dX may explain the predominance of G-->C transversions in single-stranded M13mp2 bacteriophage DNA exposed to methylene blue plus light and then transfected into SOS-induced Escherichia coli.

Induction of oxidative DNA damage and early lesions in rat gastro-intestinal epithelium in relation to prostaglandin H synthase-mediated metabolism of butylated hydroxyanisole

The effect of metabolic activation of the food additive 3-tert-butyl-4-hydroxyanisole (BHA) by prostaglandin H synthase on the gastro-intestinal cell proliferation was determined by studies of the nature and the time dependency of early lesions in the forestomach, glandular stomach and colon/rectum of rats given BHA with and without co-administration of acetylsalicyclic acid (ASA: an inhibitor of prostaglandin H synthase), in combination with the formation of oxidative DNA damage in the epithelial cells of glandular stomach and colon/rectum as well as in the liver. BHA appeared to be a strong inducer of oxidative DNA damage in the epithelial cells of the glandular stomach, increasing the level of 7-hydro-8-oxo-2'deoxyguanosine (8-oxodG) with increasing duration of BHA administration. Similar observations were made in colorectal DNA although levels of oxidative DNA damage tend to be smaller. In liver DNA, BHA appeared to be capable of increasing background 8-oxodG levels only after 14 days of treatment. This relatively slow response may be related to very low prostaglandin H synthase activity of liver cells. The severity of hyperplasia and inflammation in both forestomach and glandular stomach appeared to increase gradually with continued BHA administration. The hyperplasia induced by BHA was paralleled by inflammatory changes. In colorectal tissue, however, no tissue abnormalities were observed. This indicates that oxidative DNA damage induced by BHA is not a consequence of early lesions in gastro-intestinal epithelium, but might be the initial step in the stimulation of gastro-intestinal cell proliferation which, as shown previously, also occurs in colon epithelium. Co-administration of the prostaglandin H synthase inhibitor ASA resulted in a significant decrease of both epithelial oxidative DNA damage and the incidence of lesions, which indicates that this enzyme system is involved in the enhancement of cellular proliferation induced by BHA. Co-oxidation by prostaglandin H synthase of the BHA-metabolite tert-butylhydroquinone into tert-butylquinone, yielding active oxygen species, might therefore be responsible for the carcinogenic effects of this food antioxidant.

Measurement by 32P-postlabeling of (+/-)anti-benzo[a]pyrene-diolepoxide-N2-deoxyguanosine adduct persistence in unstimulated human peripheral blood lymphocytes

In order to study the relative importance of endogenous and environmental factors for the individual relation between DNA damage and DNA excision repair, a method was developed for measuring quantitatively the persistence of N2-deoxyguanosine adducts formed in non-stimulated isolated human peripheral blood lymphocytes after in vitro incubation with 0.2 microM (+/-)anti-BPDE, applying 32P-postlabeling. Total binding of radiolabeled (+/-)anti-BPDE to DNA and its removal has been studied previously in human peripheral blood lymphocytes, but the method presented here enables the direct investigation of repair of the main (+/-)anti-BPDE-DNA adduct, which is implicated in benzo[a]pyrene-induced mutagenesis. Using this method, it was found that in lymphocytes, obtained from 5 individuals, most (+/-)anti-BPDE-N2-dG adducts are removed within the first 24 h after treatment, while interindividual differences appear to exist in both adduct formation and rate and extent of removal.

The formation of one-G deletions as a consequence of single-oxygen-induced DNA damage

Single-stranded M13mp10 DNA containing a 144-bp mutational target sequence in the lacZ alpha gene was treated with singlet oxygen (1O2) generated by thermodissociation of the endoperoxide of 3,3'-(1,4-naphthalene-1,4-diyl)dipropionate (NDPO2). After transfection to non-SOS-induced E. coli cells, 32 mutants preselected for a mutation in the 144-bp target were collected and analyzed by DNA sequencing. One-G deletions represented the predominant type of mutation accounting for 50% of the mutations analyzed. The remaining part appeared to consist of base substitutions, i.e. G-->T transversions (34%), C-->T transitions (12.5%) and one T-->C transition (3%). Sixty percent of the mutations were found in two major mutational hotspots. We conclude that the predominant one-G deletions are due to a guanine reaction product which might be specific for 1O2.

Oxidative mutagens induce intrachromosomal recombination in yeast

Active oxygen species are thought to be involved in the causation of a number of diseases including cancers. We have investigated the effect of 5 oxidative mutagens, methyl viologen (paraquat), mitomycin C, phenylhydrazine, cumene hydroperoxide and hydrogen peroxide, on the frequency of both intrachromosomal recombination and interchromosomal recombination in the yeast Saccharomyces cerevisiae. All of the chemicals significantly increased the frequency of intrachromosomal recombination in a dose-dependent manner. Only hydrogen peroxide increased the frequency of interchromosomal recombination at the doses tested in this study. A role for hydroxyl radical (.OH) in the effect of H2O2 on recombination is indicated by the ability of the radical scavenger dimethyl sulfoxide (DMSO) to significantly inhibit the induction of both intrachromosomal and interchromosomal recombination by H2O2. The results presented here give further support for the suitability of intrachromosomal recombination measurements as a short-term test for the detection of mutagens and carcinogens.

The ambivalent role of glutathione in the protection of DNA against singlet oxygen

Glutathione (GSH) was examined with respect to its ability to protect DNA against 1O2 damage. We have found that GSH protected, at least partly, the DNA against inactivation by 1O2. Up to 10 mM the protection increased as a function of GSH concentration. Above 10 mM the protection remained constant and less than expected on the basis of scavenging/quenching of 1O2, in contrast to the protection offered by sodium-azide. Especially at the higher concentrations of GSH the protection against the biological inactivation is accompanied by an increase in single-strand breaks and also probably lethal base damage. However, all together the data suggest that at least in the physiologically important range (0.1-10 mM) GSH is able to protect efficiently against 1O2-induced inactivating DNA damage.

Single base mutations can be unequivocally and rapidly detected by analysis of DNA heteroduplexes, obtained with deletion-mutant instead of wild-type DNA

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Oxidative DNA damage in peripheral blood lymphocytes of coal workers

Reactive oxygen species are important mediators of both mineral dust-induced (malignant) lung disease and in vitro DNA damage. Therefore, we studied in vivo oxidative DNA damage in coal workers who had been chronically exposed to silica-containing dust. In peripheral blood lymphocytes of 38 retired coal miners (eight with coal workers pneumoconiosis, 30 references) and 24 age-matched, non-dust-exposed controls 7-hydro-8-oxo-2'-deoxyguanosine (8-oxodG) was determined by reversed phase high-performance liquid chromatography with electrochemical detection. The ratio of 8-oxodG residues to deoxyguanosine (dG) was related to individual cumulative dust exposure estimates and pneumoconiotic stage as established by chest radiography. The ratio of 8-oxodG to dG(x 10(-5)) in lymphocytes did not differ between miners with coal workers' pneumoconiosis (2.61 +/- 0.44) and miners without coal workers' pneumoconiosis (2.96 +/- 1.86). However, oxidative DNA damage in all miners was higher than in the non-dust-exposed controls (1.67 +/- 1.31). 8-oxodG/dG ratio was not related to individual cumulative coal dust exposure, age or smoking (pack years) when evaluated by multiple linear regression. We suggest that oxidative damage to the DNA of peripheral blood lymphocytes may be introduced by increased oxidative stress responses in subjects chronically exposed to mineral dusts. Whether this is an important pathway in the suggested carcinogenicity of silica is still an open question.