Loss of transforming activity of plasmid DNA (pBR322) in E. coli caused by singlet molecular oxygen

A New Perspective on the Origin of DNA Double-Strand Breaks and Its Implications for Ageing

It is estimated that 10-50 DNA double-strand breaks (DSBs) occur in a nucleated human cell per cell cycle. We reviewed the present state of knowledge and hypothesized that the currently accepted mechanisms cannot explain such high frequency of DSBs occurring daily under normal physiological conditions. We propose an alternative model that implicates illegitimate genomic integration into healthy cells of cell-free chromatin (cfCh) particles released from the billions of cells that die in the body every day. Repeated genomic integration of cfCh may have catastrophic consequences for the cell, such as DSBs, their faulty repair by nonhomologous end joining (NHEJ) followed by apoptosis with release of more cfCh which would integrate into genomes of surrounding cells. This can creates a vicious cycle of cfCh integration, DSBs, NHEJ, and more apoptosis, thereby providing a potential explanation as to why so many billions of cells die in the body on a daily basis. We also recount the recent observation that cfCh integration and the resulting DSBs activate inflammatory cytokines. This leads us to propose that concurrent DSBs and induction of inflammation occurring throughout life may be the underlying cause of ageing, degenerative disorders, and cancer. Finally, we discuss the prospect that agents that can inactivate/degrade cfCh may hold the key to making healthy ageing a realizable goal.

DNA damage is a late event in resveratrol-mediated inhibition of Escherichia coli

Resveratrol is an important phytoalexin notable for a wide variety of beneficial activities. Resveratrol has been reported to be active against various pathogenic bacteria. However, it is not clear at the molecular level how this important activity is manifested. Resveratrol has been reported to bind to cupric ions and reduce it. In the process, it generates copper-peroxide complex and reactive oxygen species (ROS). Due to this ability, resveratrol has been shown to cleave plasmid DNA in several studies. To this end, we envisaged DNA damage to play a role in resveratrol mediated inhibition in Escherichia coli. We employed DNA damage repair deficient mutants from keio collection to demonstrate the hypersensitive phenotype upon resveratrol treatment. Analysis of integrity and PCR efficiency of plasmid DNA from resveratrol-treated cells revealed significant DNA damage after 6 h or more compared to DNA from vehicle-treated cells. RAPD-PCR was performed to demonstrate the damage in genomic DNA from resveratrol-treated cells. In addition, in situ DNA damage was observed under fluorescence microscopy after resveratrol treatment. Further resveratrol treatment resulted in cell cycle arrest of significant fraction of population revealed by flow cytometry. However, a robust induction was not observed in phage induction assay and induction of DNA damage response genes quantified by promoter fused fluorescent tracker protein. These observations along with our previous observation that resveratrol induces membrane damage in E. coli at early time point reveal, DNA damage is a late event, occurring after a few hours of treatment.

Scirpusin A, a hydroxystilbene dimer from Xinjiang wine grape, acts as an effective singlet oxygen quencher and DNA damage protector

BACKGROUND

Grapes and red wines are rich sources of phenolic compounds such as anthocyanins, catechins, flavonols and stilbenes, most of which are potent antioxidants showing cardioprotective properties. We first isolated scirpusin A, a hydroxystilbene dimer, from a wine grape of Xinjiang, and studied its antioxidant activity.

RESULTS

Reactive oxygen species scavenging effects and the protection against reactive singlet oxygen-induced DNA damage of scirpusin A have been investigated in our experiments. The concentration of scirpusin A required to inhibit 50% of (1)O(2) generation was 17 micromol L(-1), while addition of scirpusin A at 140 micromol L(-1) caused complete inhibition. Further kinetic study revealed that the reaction of Scirpusin A with singlet oxygen has an extremely high rate constant (k(a) = 4.68 x 10(9) L mol(-1) s(-1)). Scirpusin A (140 micromol L(-1)) exhibited significant inhibition effects on pBR322 DNA breakage. However, scavenging effects of scirpusin A on superoxide anion O(2) (*-) and hydroxyl radical .OH were not potent as the inhibitor rates at a concentration of 1400 micromol L(-1) were 28.83% and 19.5%, respectively.

CONCLUSION

The present study shows that scirpusin A is a selective quencher of singlet oxygen and a protector against reactive singlet oxygen-induced pBR322 DNA damage at very low concentrations.

Oxygen free radical modified DNA: Implications in the etiopathogenesis of Systemic lupus erythematosus

The present study was designed to probe the possible role of singlet oxygen and superoxide anion radical modified DNA in the etiopathogenesis of Systemic lupus erythematosus. These species were generated by the exposure of riboflavin to 365 nm UV light. Modified DNA showed single strand breaks, hyperchromicity at 260nm and decrease in Tm. The modified DNA induced high titer antibodies in experimental animals. The antibodies showed reactivity with various nucleic acid polymers, a property commonly associated with Systemic lupus erythematosus anti-DNA autoantibodies. Systemic lupus erythematosus sera showed preferential binding of modified DNA over native DNA in direct binding and competitive binding solid phase immunoassays and band shift assays. The results suggest for the possible involvement of the singlet- superoxide modified DNA as a potential trigger for anti- DNA autoantibody production in SLE and thus in the etiopathogenesis of the disease.

Characterization of chromatin modified with reactive oxygen species: recognition by autoantibodies in cancer

OBJECTIVES

To study the binding of chromatin modified with reactive oxygen species (ROS) with circulating autoantibodies present in cancer patients.

DESIGN AND METHODS

Chromatin isolated from goat liver was modified by superoxide radical plus singlet oxygen and hydroxyl radicals. Sera from 47 patients with various types of cancers were tested for binding to native and modified chromatin by direct binding and competition ELISA.

RESULTS

Maximum modification of thymine (54%) was found in case of chromatin modified with hydroxyl radical whereas by the combined action of singlet oxygen and superoxide anion radical, guanine was modified most (72%). Autoantibodies in cancer sera recognized modified chromatin in preference to native chromatin. Band shift assay with affinity-purified IgG from sera of cancer patients reiterated the results obtained with serum samples.

CONCLUSION

Greater recognition of ROS-modified chromatin with the autoantibodies in cancer sera is indicative of reactive-oxygen-species-induced chromatin damage in cancer patients.

Adenosine production and its interaction with protection of ischemic and reperfusion injury of the myocardium

Adenosine exerts cardioprotective effects on the ischemic myocardium. A flexibly mounted microdialysis probe was used to measure the concentration of interstitial adenosine and to assess the activity of ecto-5'-nucleotidase (a key enzyme responsible for adenosine production) in in vivo rat hearts. The level of adenosine during perfusion of adenosine 5'-adenosine monophosphate (AMP) was given as an index of the activity of ecto-5'-nucleotidase in the tissue. Endogenous norepinephrine (NE) activates both alpha(1)-adrenoceptors and protein kinase C (PKC), which, in turn, activates ecto-5'-nucleotidase via phosphorylation thereby enhancing the production of interstitial adenosine. Histamine-release NE activates PKC, which increased ecto-5'-nucleotidase activity and augmented release of adenosine. Opening of cardiac ATP sensitive K(+) (K(ATP)) channels may cause hydroxyl radical (.OH) generation through NE release. Lysophosphatidylcholine (LPC), an endogenous amphiphiphilic lipid metabolite, also increases the concentration of interstitial adenosine in rat hearts, through the PKC-mediated activation of endogenous ecto-5'-nucleotidase. Nitric oxide (NO) facilitates the production of interstitial adenosine, via guanosine 3',5'-cyclic monophosphate (cGMP)-mediated activation of ecto-5'-nucleotidase as another pathway. These mechanisms play an important role in high sensitivity of the cardiac adenosine system. Adenosine plays an important role as a modulator of ischemic reperfusion injury, and that the production and mechanism of action of adenosine are linked with NE release.

Environmental estrogen-like chemicals and hydroxyl radicals induced by MPTP in the striatum: a review

Oxygen free radical formation has been implicated in lesions caused by the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and iron. Although MPTP produces a parkinsonian syndrome after its conversion to 1-methyl-4-phenylpyridine (MPP+) by type B monoamine oxidase (MAO) in the brain, the etiology of this disease remains obscure. This review focuses on the role of an environmental neurotoxin chemically related to MPP+-induced free radical generation in the pathogenesis of Parkinson's disease. Environmental-like chemicals, such as para-nonylphenol or bisphenol A, significantly stimulated hydroxyl radical (*OH) formation in the striatum. Allopurinol, a xanthine oxidase inhibitor, prevents para-nonylphenol and MPP+-induced *OH generation. Tamoxifen, a synthetic nonsteroidal antiestrogen, suppressed the *OH generation via dopamine efflux induced by MPP+. These results confirm that free radical production might make a major contribution at certain stages in the progression of the injury. Such findings may be useful in elucidating the actual mechanism of free radical formation in the pathogenesis of neurodegenerative brain disorders, including Parkinson's disease and traumatic brain injuries.

Protective effect of histidine on para-nonylphenol-enhanced hydroxyl free radical generation induced by 1-methyl-4-phenylpyridinium ion (MPP+) in rat striatum

The present study examined the antioxidant effect of histidine, a singlet oxygen ((1)O(2)) scavenger, on para-nonylphenol (an environmental estrogen-like chemical)-enhanced hydroxyl radical (.OH) generation induced by 1-methyl-4-phenylpyridinium ion (MPP+) in extracellular fluid of rat striatum. Rats were anesthetized, and sodium salicylate in Ringer's solution (0.5 nmol/microl/min) was infused through a microdialysis probe to detect the generation of.OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. Introduction of para-nonylphenol (10 microM) significantly enhanced MPP+ -induced.OH generation. Histidine (25 mM) decreased the para-nonylphenol-enhanced.OH formation. Although the level of MPP+ -induced.OH formation trapped as DHBA after para-nonylphenol treatment increased, para-nonylphenol failed to increase either the level of dopamine and DHBA formation in the reserpinized animals. These results indicate that para-nonylphenol and MPP+ -enhanced.OH generation was based on 1O(2) production, and histidine may have a preventive effect on para-nonylphenol and MPP+ -induced.OH generation in rat striatum.

Quenching of singlet oxygen by carotenoids produced in escherichia coli - attenuation of singlet oxygen-mediated bacterial killing by carotenoids

We examined the viability of Escherichia coli transformants harboring various carotenoids synthesizing genes in a medium containing an enzymatic singlet oxygen generating system, which contained myeloperoxidase, hydrogen peroxide and Br(-) at pH 4.5. Singlet oxygen quenching activities of various carotenoids in phosphatidyl choline micelles in aqueous media were also studied using the same enzymatic singlet oxygen generating system. Viability of the transformants producing carotenoids was higher than that of the wild type E. coli in the singlet oxygen generation mixture. Of the transformants tested, the viability of zeaxanthin-diglucoside producing transformant was the highest. Carotenoids in increasing order of k(q) values were beta-carotene, a cyclic carotene<zeaxanthin with hydroxy groups < or =lycopene, an acyclic carotene<canthaxanthin and astaxanthin with keto groups <<zeaxanthin-diglucoside. The k(q) value of zeaxanthin-diglucoside was 3.5 times higher than that of beta-carotene. These results suggest that orientation of the carotenoids in lipid layers of micelles and also in phospholipid membrane of bacteria is important for quenching of singlet oxygen. Furthermore, the viability of transformants producing lycopene and phytoene was almost as high as that of the transformant producing zeaxanthin-glucoside.

Protective effect of histidine on hydroxyl radical generation induced by potassium-depolarization in rat myocardium

We investigated the efficacy of histidine on potassium-depolarization induced hydroxyl radical (*OH) generation in the extracellular fluid of rat myocardium by a flexibly mounted microdialysis technique (O system). After the rat was anesthetized, a microdialysis probe was implanted in the left ventricular myocardium, and then sodium salicylate in Ringer's solution (0.5 nmol/microl per minute) was infused to detect the generation of *OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA). Infusion of KCl (70 mM) clearly produced an increase in *OH formation. However, when KCl in the presence of a high concentration of histidine (25 mM) was infused through the microdialysis probe, KCl failed to increase the 2,3-DHBA formation. To examine the effect of histidine on ischemia-reperfusion of the myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, a marked elevation of the levels of 2,3-DHBA was observed in the heart dialysate. However, when corresponding experiments were performed with histidine (25 mM)-pretreated animals, histidine prevented the ischemia-reperfusion induced *OH formation trapped as 2,3-DHBA. These results indicate that histidine may protect against K+-depolarization-evoked *OH generation in rat myocardium.

Protective effect of histidine on iron (II)-induced hydroxyl radical generation in rat hearts

We investigated the efficacy of histidine on iron (II)-induced hydroxyl radical (.OH) generation in extracellular fluid of the rat myocardium using a flexibly mounted microdialysis technique (O system). Rats were anesthetized and a microdialysis probe was implanted in the left ventricular, followed by infusion of sodium salicylate in Ringer's solution (0.5 nmol/microL/min) to detect the generation .OH as reflected by the non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA). Iron (II) clearly produced a concentration-dependent increase in .OH formation. A positive linear correlation between iron (II) and the formation of 2,3-DHBA (R2 = 0.987) was observed. However, histidine (25 mM) was infused through a microdialysis probe; iron (II) failed to increase the 2,3-DHBA formation obtained. To examine the effect of histidine on ischemia-reperfusion of the myocardium, the heart was subjected to myocardial ischemia for 15 min by occlusion of the left anterior descending coronary artery (LAD). When the heart was reperfused, a marked elevation of the levels of 2,3-DHBA was observed in the heart dialysate. When corresponding experiments were performed with histidine (25 mM)-pretreated animals, histidine prevented the ischemia-reperfusion induced .OH generation trapped as 2,3-DHBA. These results indicate that histidine protects the myocardium against ischemia-reperfusion damage by .OH generation.

Protective effect of histidine on MPP+-induced hydroxyl radical generation in rat striatum

We investigated the efficacy of histidine on MPP+-induced hydroxyl radical (.OH) formation in extracellular fluid of rat striatum. Rats were anesthetized and sodium salicylate in Ringer's solution (0.5 nmol microl-1 min-1) was infused through a microdialysis probe to detect the generation of.OH as reflected by the nonenzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) in the striatum. MPP+ (5 mM) clearly produced an increase in.OH formation. However, histidine (25 mM) reduced the.OH formation by the action of MPP+. These results indicate that histidine protects MPP+-induced.OH formation in rat striatum.

Adjacent guanines as preferred sites for strand breaks in plasmid DNA irradiated with 193 nm and 248 nm UV laser light

193 nm UV laser light induces single strand breaks as well as double strand breaks in plasmid DNA. The frequency of strand breaks is increased at sites where at least two guanine or, less frequently, a guanine and an adenine are adjacent to each other. 248 nm UV laser light induces predominantly single strand breaks with a less pronounced preference for guanine clusters. At both wavelengths, the presence of oxygen does not change the pattern of strand breaks, but in the presence of nitrous oxide, selectivity is lost; this is attributable to the production of the hydroxyl radical. These findings can be explained by a model in which the propagation of a radical or an electron hole along the DNA helix competes kinetically with the strand cleavage reaction. The difference in selectivity at the two different wavelengths is ascribed to the preferential light absorption by the purine bases at 193 nm.

Protection of plasmid pBR322 DNA by flavonoids against single-stranded breaks induced by singlet molecular oxygen

Flavonoids, the dominant colouring pigments of plants, as well as the related polyphenol tannic acid significantly inhibit single-strand breaks in plasmid pBR322 DNA induced by singlet molecular oxygen ((1)O2). This reactive species of oxygen was generated in an aqueous buffer system by the thermal dissociation of the endoperoxide of 3,3'-(1,4-naphthylene)dipropionate. Among the antioxidants examined, myricetin showed the highest protective ability, followed by tannic acid, (+)catechin, rutin, fisetin, luteolin and apigenin, when the inhibitory abilities were compared at 90 min after incubation. The protective abilities of these compounds were both time and concentration dependent. At equimolar concentrations (100 microM) the antioxidant effect of myricetin was better than that of other known antioxidants such as lipoate, alpha-tocopherol and beta-carotene. Data, when analysed in relation to the structures of various compounds, showed a rough correlation with protective abilities. Owing to the abundance of these compounds in our normal diet, they may play significant roles in preventing oxidative damage resulting from potentially deleterious (1)O2.

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.

Possible occurrence of new mutagens with the DNA breaking activity in coffee

Brewed and instant coffee emitted strong chemiluminescence due to singlet oxygen and excited carbonyls, which may be originated by the Maillard reaction of sugars and amino acids but not by the reaction of polyphenolics. Instant coffee cleaved DNA giving single-strand breaks only after it was purified by high performance liquid chromatography (HPLC) or by gel filtration. Retention times of component(s) with strong DNA breaking activity in HPLC were different from those of chemiluminescence emitters, although they were coeluted on a gel filtration. The major DNA breaking component(s) must be different from chemiluminescence emitters. Active oxygen radicals participated little in DNA breaking because active oxygen radical scavengers had only a marginal effect on DNA breaking of the active gel fraction. DNA breaking by the active gel fraction was inhibited by high concentrations of inorganic salts probably because the salts stabilized the DNA double strands. The active gel fraction was mutagenic to Salmonella typhimurium TA98 without metabolic activation. The number of His+ revertant colonies/g of instant coffee powder was estimated to be 4000.

Damage to plasmid DNA by singlet oxygen and its protection

Singlet oxygen, generated by photoexcitation or by chemiexcitation, selectively reacts with the deoxyguanosine moiety in DNA (kq + kr about 5 x 10(6) M-1s-1). The oxidation products include 8-oxo-7,8-dihydroeoxyguanosine (8-oxodG; also called 8-hydroxydeoxyguanosine) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua). Singlet oxygen also causes strand breaks in DNA, studied in plasmids and bacteriophages. The biological consequences include a loss of transforming activity as well as mutagenicity and genotoxicity. Employing shuttle vectors, it was shown that double-stranded vectors carrying singlet-oxygen-induced lesions seem to be processed in mammalian cells by DNA repair mechanisms efficient in preserving the biological activity of the plasmid but highly mutagenic in mammalian cells. Biological protection against singlet oxygen is afforded by quenchers, notably carotenoids (kq = 10(9) - 10(10) M-1s-1) and tocopherols. Whether this activity explains the protective effect of carotenoids on neoplastic transformation is still unknown.

Generation of DNA-breaking activity in the Maillard reaction of glucose-amino acid mixtures in a solid system

Maillard products of 1:1 solid mixtures of glucose-amino acid heated at 200 degrees C for 5 min induced single-strand breaks of DNA after incubation at 37 degrees C and pH 7.4 overnight. The products of Gly, His, Trp, Tyr, Phe and CySH transformed a plasmid supercoiled DNA (form I) into an open circular relaxed form (form II) and a linear form (form III). The breaking activity of the product of CySH was the highest. The product of Tyr caused fragmentation of single-stranded DNA, but did not cleave double-stranded linear DNA. The breaking activity of the product of Tyr was little or not at all inhibited by scavengers of singlet oxygen, hydroxyl radical, superoxide and hydrogen peroxide. Aqueous solutions of the products of all amino acids except CySH emitted chemiluminescence, which was partly due to singlet oxygen. High-performance liquid chromatography of the product of Tyr showed that the substances responsible for the DNA-breaking activity were eluted in a broad range, whereas those for chemiluminescence were eluted in an overlapped sharp peak. The Maillard reaction of glucose-amino acid in a solid state produced substances having DNA-breaking activity together with those generating single oxygen and emitting chemiluminescence.

Prevention of singlet oxygen-induced DNA damage by lipoate

Among the several biologically and pharmacologically active sulfur compounds examined, only lipoic acid and dihydrolipoic acid provided protection to plasmid DNA against singlet molecular oxygen (1O2). 1O2 was generated in phosphate buffer by the thermal dissociation of the endoperoxide of 3,3'-(1,4-naphthylidene) dipropionate (NDPO2). The protecting effect of lipoic acid was time- and pH-dependent and significant protection was seen even at 50 microM. The antioxidant effect was adversely affected by temperatures above 45 degrees C. Superoxide dismutase and catalase marginally enhanced this effect. Metal chelation with EDTA decreased the protection by lipoate, indicating that metal ions may be involved. The protective effect was diminished when the disulfide was added after single-strand breaks were induced by 1O2. The formation of 8-oxoguanosine from guanosine upon exposure to NDPO2 was not altered by lipoate.

DNA breaking activity of the carbon-centered radical generated from 2,2'-azobis(2-amidinopropane) hydrochloride (AAPH)

When supercoiled plasmid DNA was incubated with 2,2'-azobis (2-amidinopropane)hydrochloride (AAPH) at pH 7.4 in the presence and absence of oxygen, the DNA single strands were effectively cleaved. The breaking in the presence of oxygen was not inhibited by superoxide dismutase and catalase, but inhibited by mannitol, ethanol, butyl hydroxyanisole, thiol compounds, tertiary amines and spin trapping agents N-tert-butyl-alpha-phenylnitrone (PBN) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The breaking in the absence of oxygen was inhibited by ethanol, a tertiary amine and PBN. By electron spin resonance spin-trapping with PBN, the carbon-centered radical was detected both in the presence and the absence of oxygen. Hydroxyl radical was detected by use of DMPO only in the presence of oxygen. The DNA breaking activity of AAPH was found to be due primarily to the aliphatic carbon-centered radical. While the reactivity of carbon-centered radicals have received little attention, the aliphatic carbon-centered radical generated from AAPH was found to be highly reactive to break the DNA strands.

Contrasting effects of SH-compounds on oxidative DNA damage: repair and increase of damage

The non-radical singlet oxygen (1O2) and the OH radical (.OH) are the major damaging oxidative species that can be generated inside cells during normal aerobic metabolism and by processes such as photosensitization. Both reactive oxygen species fulfill essential prerequisites to be a genotoxic agent. Due to their continuous production they represent an ever-present threat to all vital cellular molecules, especially DNA. As might be anticipated from the difference in character between these reactive species (non-radical versus radical) the pattern of DNA modifications caused by singlet oxygen is different from that produced by OH radicals. All cells possess an elaborate defense system against oxidative damage. This paper focuses mainly on the effect of thiols such as glutathione, which are thought to play a role as antioxidants. Under certain conditions thiols can repair chemically, probably by H-donation, some of the DNA damage caused by .OH; for instance breaks can be rather easily prevented in this way. This process will compete with fixation of damage by oxygen. However, there is ample evidence that H-atom donation does not always lead to 'correct' repair. Moreover under aerobic conditions thiyl peroxy radicals might increase DNA damage. Although the repair/fixation process could not be examined in the case of 1O2 yet, it could be demonstrated that reactive species can be formed out of the reaction of thiols with 1O2 capable of enhancing the number of DNA modifications such as 8-oxoguanine and single-strand breaks, probably arising from different pathways. Although it is quite clear that thiols are to some extent excellent antioxidants they possess unexpected properties which, depending on the conditions, can have genotoxic consequences.

Singlet oxygen induced DNA damage

Singlet oxygen generated by photoexcitation and by chemiexcitation selectively reacts with the guanine moiety in nucleosides (kq + kr about 5 x 10(6) M-1s-1) and in DNA. The oxidation products include 8-oxo-7-hydro-deoxyguanosine (8-oxodG; also called 8-hydroxydeoxyguanosine) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (FapyGua). Singlet oxygen also causes alkali-labile sites and single-strand breaks in DNA. The biological consequences include a loss of transforming activity as studied with plasmids and bacteriophage DNA, and mutagenicity and genotoxicity. Employing shuttle vectors, it was shown that double-stranded vectors carrying singlet oxygen induced lesions seem to be processed in mammalian cells by DNA repair mechanisms efficient in preserving the biological activity of the plasmid but highly mutagenic in mammalian cells. Biological protection against singlet oxygen is afforded by quenchers, notably carotenoids and tocopherols. Major repair occurs by excision of the oxidized deoxyguanosine moieties by the Fpg protein, preventing mismatch of 8-oxodG with dA, which would generate G:C to T:A transversions.

Synthetic carotenoids, novel polyene polyketones and new capsorubin isomers as efficient quenchers of singlet molecular oxygen

Novel synthetic polyene polyketones and new synthetic capsorubin isomers were examined for their ability to quench singlet molecular oxygen (1O2) generated by the thermodissociation of the endoperoxide of 3,3'-(1,4-naphthylene) dipropionate (NDPO2). C28-polyene-tetrone (1) exhibits the highest physical quenching rate constant with 1O2 (kq = 16 x 10(9) M-1 s-1). For comparison, the rate constant for the most efficient biological carotenoid, lycopene (3) is kq = 9 x 10(9) M-1 s-1 and that of beta-carotene (5) kq = 5 x 10(9) M-1 s-1. The presence of two oxalyl chromophores at the ends of the polyene chain seems to enhance the 1O2 quenching ability in the C28-polyene-tetrone (1). C28-polyene-tetrone-diacetal (2) (kq = 9 x 10(9) M-1 s-1) and C40-epiisocapsorubin (4) (kq = 8 x 10(9) M-1 s-1) also have high 1O2 quenching abilities. Two carotenoids from plants, phytoene and phytofluene, were much less efficient, kq values being below 10(7) M-1 s-1. Due to the very high singlet oxygen quenching abilities, C28-polyene-tetrone (1), C28-polyene-tetrone-diacetal (2) and C40-epiisocapsorubin (4) may have potential use in preventing 1O2-induced damage in biological and non-biological systems.

Singlet oxygen induced DNA damage and mutagenicity in a single-stranded SV40-based shuttle vector

The effects of singlet oxygen (1O2), generated by the thermal decomposition of water soluble NDPO2 (endoperoxide of the disodium 3,3'-(1,4-naphthylidene) dipropionate), on a single-stranded shuttle vector were analysed. 1O2 induces a much higher level of breaks in the phosphodiester backbone of single-stranded than double-stranded DNA. This may be due to a higher accessibility of guanine residue, primarily damaged by 1O2. The damaged vector was transfected into monkey COS7 cells where single-stranded DNA was converted to the double-stranded replicative form DNA. After 3 days, extrachromosomal DNA was extracted and the plasmids rescued in E. coli to study mutagenesis. There is a significant increase in mutation frequency of damaged single-stranded DNA in comparison to untreated DNA. It is concluded that 1O2 induces breaks in the backbone of single-stranded DNA and that the 1O2-damaged molecules are mutated after passage through mammalian cells.

Biological consequences associated with DNA oxidation mediated by singlet oxygen

Singlet oxygen is a major oxidative species that can be generated by numerous biological processes such as photosensitization. This oxidant can react with deoxyguanosine and with guanine in deoxyribonucleic acid (DNA) leading to the induction of at least four different reaction products such as 4,8-dihydro-4-hydroxy-8-oxodeoxyguanosine and 7,8-dihydro-8-oxodeoxyguanosine. The induction of true single-stranded breaks in the oxidated DNA is still a matter of controversy and is not yet clearly established. This paper focuses mainly on several biological consequences which can be associated with the induction of DNA lesions by singlet oxygen. Oxidated DNA loses its transformation efficiency probably because unrepaired lesions can partially inhibit DNA replication. Mutagenesis is one of the main effects induced by guanine oxidation products. Molecular analysis of mutated genes reveals that G to T transversions are the most frequent mutations; these are probably introduced in DNA by misincorporation of deoxyadenosine monophosphate (dAMP) opposite to the lesion. Efficient repair of these oxidated guanine residues can take place via specific glycosylase, endonuclease or the SOS network. However, the data concerning the toxicity of singlet oxygen for eukaryotic cells are not frequent enough in the literature to draw a clear picture of the effects of this activated species in several biologically revelant phenomena.

Formation of 8-hydroxy(deoxy)guanosine and generation of strand breaks at guanine residues in DNA by singlet oxygen

Singlet molecular oxygen (1O2) was generated in aqueous solution (H2O or D2O) at 37 degrees C by the thermal dissociation of the endoperoxide of 3,3'-(1,4-naphthylidene) dipropionate (NDPO2). Guanosine and deoxyguanosine quench 1O2 with overall quenching rate constants of 6.2 X 10(6) M-1 s-1 and 5.2 X 10(6) M-1 s-1, respectively. Reaction with 1O2 results in the formation of 8-hydroxyguanosine (8-OH-Guo) and 8-hydroxydeoxyguanosine (8-OH-dGuo), respectively, with a yield of 1.5% at 1 mM substrate with an NDPO2 concentration of 40 mM; a corresponding 8-hydroxy derivative is not formed from deoxyadenosine. In D2O the yield of 8-OH-Guo is 1.5-fold that in H2O. Sodium azide suppresses 8-OH-Guo and 8-OH-dGuo production. In contrast, the hydroxyl radical scavengers, tert-butanol, 2-propanol, or sodium formate, do not decrease the production of the 8-OH derivatives. The formation of 8-OH derivatives is significantly increased (2-5-fold) by thiols such as dithiothreitol, glutathione, cysteine, and cysteamine. With use of a plasmid containing a fragment of the mouse metallothionein I promoter (pMTP3') and a novel end-labeling technique, the position of 1O2-induced single-strand breaks in DNA was examined. Strand breaks occur selectively at dGuo; no major differences (hot spots) were observed between individual guanines.

Activity of thiols as singlet molecular oxygen quenchers

Singlet molecular oxygen O2(1 delta g) arising from the thermodissociation of the endoperoxide of 3,3'-(1,4-naphthylidene) dipropionate (NDPO2) was used to assess the quenching ability of various thiols and related compounds in sodium phosphate buffer in D2O at 37 degrees C. The overall quenching ability decreases in the sequence ergothioneine, methionine, cysteine, beta,beta-dimethyl cysteine (penicillamine), mercaptopropionylglycine, mesna, glutathione (GSH), dithiothreitol, N-acetyl cysteine and captopril. Cystine, glutathione disulphide, dimesna, methionine sulphone and methionine sulphoxide have no quenching effect. Comparison of the rate constants for physical (kq) with chemical (kr) quenching by thiols indicates that chemical reactivity accounts fully for their ability to quench O2(1 delta g), and pD dependence indicates that the thiolate anion reacts with O2(1 delta g). Loss of thiol groups, as exemplified by GSH, is not affected by the free radical scavengers superoxide dismutase and mannitol. However, sodium azide, a scavenger of O2(1 delta g), completely prevents NDPO2-induced thiol depletion. Depletion of GSH by NDPO2 is accompanied by the formation of its disulphide, sulphinate, sulphonate, sulphoxide and other products.

Singlet oxygen induced single-strand breaks in plasmid pBR322 DNA: the enhancing effect of thiols

The biologically occurring thiols, glutathione, cysteamine and cysteine, significantly enhance the single-strand breaks in plasmid pBR322 DNA induced by singlet molecular oxygen (1O2) generated by the thermodissociation of the endoperoxide of 3,3'-(1,4-naphthylidene)dipropionate. The enhancing effect was also observed with chemically related sulfhydryl compounds but not by disulfides. In contrast, dihydrolipoate and its disulfide lipoate protected the plasmid DNA. Metal chelators as well as superoxide dismutase or catalase had no effect, whereas mannitol or sodium azide, decreased the thiol-1O2-induced strand breaks. It is concluded that the observed effects are mediated by reactive oxidation products arising from the 1O2-oxidation of thiols.

A new fluorescence method to detect singlet oxygen inside phospholipid model membranes

A fluorescence method for detecting singlet oxygen (1O2) in model membranes is proposed. 1O2 was generated by hydrogen peroxide/sodium hypochlorite system. 1,3-Diphenylisobenzofuran (DPBF), a specific 1O2 trap, dissolved in organic solvents gives a strong fluorescence spectrum when excited at 410 nm. A similar spectrum, with a maximum at 455 nm, is obtained when DPBF is incorporated in unilamellar dipalmitoylphosphatidylcholine liposomes. The intensity of fluorescence spectrum decreases when DPBF-labeled liposomes are exposed to singlet oxygen. This decrease is sensitive to 1O2 traps and quenchers like tryptophan and sodium azide, to lipid membrane fluidity and to the concentration of sodium hypochlorite and hydrogen peroxide.

Genotoxicity of singlet oxygen

Singlet oxygen, 1O2 (1 delta g), fulfills essential prerequisites for a genotoxic substance, like hydroxyl radicals and other oxygen radicals: it can react efficiently with DNA and it can be generated inside cells, e.g. by photosensitization and enzymatic oxidation. As might be anticipated from the non-radical character of singlet oxygen, the pattern of DNA modifications it produces is very different from that caused by hydroxyl radicals. While hydroxyl radicals produce DNA strand breaks and sites of base loss (AP sites) in high yield and react with all four bases of DNA, singlet oxygen generates predominantly modified guanine residues and few strand breaks and AP sites. There is now convincing evidence that a major product of base modification caused by singlet oxygen is 8-hydroxyguanine (7,8-dihydro-8-oxoguanine). Indeed, the recently reported miscoding properties of 8-hydroxyguanine can explain the predominant type of mutations observed when DNA modified by singlet oxygen is replicated in cells. There are also strong indications that singlet oxygen generated by photosensitization can act as an ultimate DNA modifying species inside cells. However, indirect genotoxic mechanisms involving other reactive oxygen species produced from singlet oxygen are also possible and appear to predominate in some cases. The cellular defense system against oxidants consists of effective singlet oxygen scavengers such as carotenoids. The observation that carotenoids can inhibit neoplastic cell transformation when administered not only together with but also after the application of chemical or physical carcinogens might indicate a role of singlet oxygen in tumor promotion that could be independent of the direct or indirect DNA damaging properties.

Singlet molecular oxygen induced mutagenicity in a mammalian SV40-based shuttle vector

We have determined the deleterious effects of singlet oxygen (1O2), generated by thermal decomposition of the water-soluble endoperoxide 3,3'-(1,4-naphthylidene)dipropionate (NDPO2), on plasmid DNA. By following the electrophoretic mobility of DNA on agarose gels, we detected single and double strand breaks induced by treatment with NDPO2. The vector employed was a mammalian shuttle vector and the mutagenic consequences of these damages were investigated, using as mutation target the supF suppressor tRNA gene. A high increase of the mutation frequency, over the background, was observed in plasmids transfected in bacteria or after passage through mammalian cells. Trapping agents and quencher effects and other controls confirm the involvement of 1O2 in DNA damage and mutagenicity. These findings indicate that 1O2 can induce DNA lesions which are repaired by an error-prone process in prokaryotic and eukaryotic cells.

Physical and chemical scavenging of singlet molecular oxygen by tocopherols

Singlet molecular oxygen (1O2) arising from the thermal decomposition of the endoperoxide of 3,3'-(1,4-naphthylidene) dipropionate was used to assess the effectiveness of alpha-, beta-, gamma-, and delta-tocopherol in the physical quenching as well as the chemical reaction of 1O2. The relative physical quenching efficiencies of the tocopherol homologs were found to decrease in the order of alpha greater than or equal to beta greater than gamma greater than delta-tocopherol. The ability of physical quenching depends on a free hydroxyl group in position 6 of the chromane ring. Chemical reactivity of the tocopherol homologs with 1O2 was low, accounting for 0.1-1.5% of physical quenching with beta-tocopherol showing particularly low reactivity, resulting in the sequence alpha greater than gamma greater than delta greater than beta-tocopherol. Tocopheryl quinones were products of all tocopherol homologs, and in addition a quinone epoxide was a major product from gamma-tocopherol. This quinone epoxide was not cleaved by rat liver microsomal epoxide hydrolase; however, it reacted further with 1O2. It is concluded that methylation in position 5 of the chromane ring enhances physical quenching of 1O2, whereas chemical reactivity is favored by a methylated position 7. In view of the fact that beta-tocopherol is as effective as alpha-tocopherol in physical quenching of 1O2 but shows very low chemical reactivity, this tocopherol homolog might be particularly suitable for biological conditions in which an accumulation of oxidation products might weaken the antioxidant defense.

Singlet molecular oxygen causes loss of biological activity in plasmid and bacteriophage DNA and induces single-strand breaks

Damage of plasmid and bacteriophage DNA inflicted by singlet molecular oxygen (1O2) includes loss of the biological activity measured as transforming capacity in E. coli and single-strand break formation. Three different sources of 1O2 were employed: (i) photosensitization with Rose bengal immobilized on a glass plate physically separated from the solution; (ii) thermal decomposition of the water-soluble endoperoxide 3,3'-(1,4-naphthylidene) dipropionate (NDPO2); and (iii) microwave discharge. Loss of transforming activity was documented after exposing bacteriophage M13 DNA to 1O2 generated by photosensitization employing immobilized Rose bengal, and with bacteriophage luminal diameter X174 DNA, using the thermodissociable endoperoxide (NDPO2) as a source of 1O2. These findings are in agreement with experiments in which plasmid DNA pBR322 was exposed to a gas stream of 1O2 generated by microwave discharge. The effects of 1O2 quenchers and of 2H2O indicate 1O2 to be the species responsible. Strand-break formation in pBR322 and luminal diameter X174, measured as an increase of the open circular form at the expense of the closed circular supercoiled form, was observed without alkaline treatment after exposing the DNA to 1O2, using either agarose gel electrophoresis or sucrose gradient separation. The effect of quenchers and 2H2O indicate the involvement of 1O2 in DNA damage. We conclude that singlet oxygen can cause loss of biological activity and DNA strand breakage.

Singlet oxygen production by biological systems

Singlet oxygen (1 delta g) is a highly reactive, short-lived intermediate which readily oxidizes a variety of biological molecules. The biochemical production of singlet oxygen has been proposed to contribute to the destructive effects seen in a number of biological processes. Several model biochemical systems have been shown to produce singlet oxygen. These systems include the peroxidase-catalyzed oxidations of halide ions, the peroxidase-catalyzed oxidations of indole-3-acetic acid, the lipoxygenase-catalyzed oxidation of unsaturated long chain fatty acids and the bleomycin-catalyzed decomposition of hydroperoxides. Results from these model systems should not be uncritically extrapolated to living systems. Recently, however, an intact cell, the human eosinophil, was shown to generate detectable amounts of singlet oxygen. This result suggests that singlet oxygen may be shown to be a significant biochemical intermediate in a few biological processes.

Pure exogenous singlet oxygen: nonmutagenicity in bacteria

Singlet oxygen (1 delta gO2) is the lowest energy-excited state of molecular oxygen, and more reactive than the triplet ground-state molecule. Although singlet oxygen has been implicated in a variety of biological effects, including reactions with DNA or some of its components, evidence for mutagenesis by singlet oxygen has remained unclear. We have previously described a system for bacterial exposure to pure exogenous singlet oxygen that eliminates ambiguity regarding the identity of the reactive species responsible for observed results. Despite the potent toxicity of pure singlet oxygen for several different strains of bacteria, we have found no evidence for mutagenicity of singlet oxygen in 26 Salmonella typhimurium histidine-auxotrophic strains killed to 35% survival. These strains included a variety of base-pair substitution or frameshift target sequences for reversion, including targets responsive to oxidative damage and targets rich in GC base pairs. Some strains combined histidine mutations with one or more mutations affecting DNA-repair capacity. 4 strains possessing the hisG46 mutation also were not mutated when exposed to dose ranges killing less than 28% and up to 99% of the bacteria. The relative frequency of small inphase deletions was assayed in hisG428 bacteria exposed to single oxygen and found to be the same as the spontaneous level. In addition to lack of induction of mutation in these strains, the 8-azaguanine forward mutation assay yielded no evidence of mutagenesis by singlet oxygen in strains killed to 15% survival. No induction of genetic changes by singlet oxygen was seen in an assay for duplication of approximately 1/3 of the bacterial chromosome. Tests for the ability of singlet oxygen to induce lambda prophage in E. coli K12 also proved negative. These studies collectively indicate that pure singlet oxygen generated outside the bacterial cell does not react significantly with the bacterial chromosome in ways leading to base-pair substitutions, frameshift mutations, small or large deletions, large duplications, or damage that interferes with DNA replication and induces the SOS system.