1 Introduction
Pentachlorophenol (PCP) is widely used as a major industrial and agricultural preservative for wood protection. In 2016, PCP was classified as a first-class environmental carcinogen. More than one-fifth of the sites identified by the EPA on the National Priority Monitoring List have been found to contain PCP
[1]. Tetrachloro-1,4-benzoquinone (TCBQ) and tetrachlorohydroquinone (TCHQ) are the two major genotoxic and carcinogenic quinone metabolites of PCP (Figure 1).
Haloquinones are a class of carcinogenic intermediates, which can lead to a variety of toxicities in vivo, including hepatotoxicity, nephrotoxicity and carcinogenicity, and are also often produced as a class of active intermediates in the oxidation and degradation processes of halogenated aromatic persistent organic pollutants in various chemical and enzymatic reactions
[2,3][4⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~17].
More recently, more than a dozen halogenated quinones with potential bladder carcinogenic effects have been identified as novel chlorinated disinfection by-products in drinking water
[6,8].
Although PCP does not react directly with DNA, both PCP and TCHQ have been shown to be potential carcinogens that promote tumorigenesis and development in CD-1 mice as a murine skin model
[18,19]. Overexpression of Bcl-2 in NIH3T3 cells inhibits TCHQ-induced apoptosis, which is considered to be a possible mechanism of cancer induction
[20]. TCHQ can also induce the production of reactive oxygen species (ROS) and upregulate the expression of p-ERK in splenocytes, resulting in the inhibition of apoptosis and cell necrosis
[21].
DNA damage induced by PCP and its quinone metabolites has been studied previously. DNA adducts were detected in the reaction system of calf thymus DNA (ctDNA) with TCBQ, the HeLa system of human tumor cells exposed to TCHQ, and the rat system exposed to PCP
[9][22][23]. The 2 '-deoxyguanosine (dG)/TCBQ adduct was identified as a dichlorobenzoquinone nucleoside
[24]. Since the redox potential of dG is the lowest among the four nucleobases, TCBQ reacts with dG more easily. In another mechanism of DNA adduct formation, PCP is oxidized to its corresponding phenoxy radical by the -H
2O
2 system of horseradish peroxidase, and then reacts with the nucleic acid bases to form C8-dG nucleoside adducts linked to oxygen atoms
[25]. In addition to covalent adducts, oxidative damage to DNA was also detected in ctDNA, tumor cells, and rats
[9,22,23]. In the process of PCP biotransformation catalyzed by enzymes, ROS is produced, which causes DNA damage. In the presence of Cu (Ⅱ) and NADPH, low concentration of TCBQ induced the formation of 8-hydroxydeoxyguanosine (8-OH-dG) and abasic sites
[9].
Hydroxyl radical (· OH) is an extremely reactive oxidative factor that is important in chemistry, biology, medicine, and environmental science
[26⇓~28]. In biology, hydroxyl radical is considered to be the most reactive and harmful reactive oxygen species, which can cause the oxidation of biological macromolecules such as DNA, protein and lipid
[26⇓~28]. Cancer, arthritis, and Parkinson's syndrome are just a few typical examples of diseases associated with · OH
[28]. The most widely accepted mechanism for · OH generation is the classical Fenton reaction catalyzed by transition metal ions
[26⇓~28].
Our research team found that halogenated quinones can promote the decomposition of peroxides to produce alkoxy/hydroxyl radicals through a new reaction mechanism of nucleophilic substitution and homolytic decomposition without relying on transition metal ions (Fig. 2)
[29⇓⇓⇓⇓⇓~35]. Interestingly, in addition to the common alkoxyl/hydroxyl radicals, we also detected novel carbon-centered quinonyl carbon radicals and oxygen-centered quinonyl oxygen radicals during halogenoquinone-mediated decomposition of organic hydroperoxides
[29,36].
Similar effects have been observed with several other halogenated quinones, including 2-chloro-1,4-benzoquinone (2-CBQ), 2,3-dichloro-1,4-benzoquinone (2,3-DCBQ), 2,5-dichloro-1,4- benzoquinone (2,5-DCBQ) and 2,6-dichloro-1,4- benzoquinone, 2,3,5-trichloro-1,4-benoquinone (TriCBQ.
However, it is not clear whether the hydroxyl radicals produced by halogenoquinone -H2O2 system can cause strong oxidative DNA damage, and if so, what is the difference between the oxidation of single nucleotides and double-stranded DNA. In addition, what are the unique characteristics of the haloquinone-mediated · OH production system compared with the classical iron-mediated · OH production system?
2 Halogenated Quinones and H2O2 Enhance Oxidative Damage by Insertion into DNA
To answer the above questions, the key is to know whether and how the haloquinone /H2O2 system can generate hydroxyl near DNA. Since halogenated quinones do not chelate to DNA as do transition metal ions, especially Fe (II), we speculate that these hydrophobic halogenated quinones may intercalate into DNA and thus enhance the ability to oxidize DNA.
To test the above hypothesis, we studied the oxidation of dG and dsDNA by halogenoquinone /H
2O
2 system, the insertion of halogenoquinone into DNA and its effect on DNA oxidation. 8-oxo-7,8-dihydro-2 '-deoxyguanosine (8-oxodG) is a well-established biomarker of oxidative DNA damage. We identified and accurately quantified 8-oxodG using ultra performance liquid chromatography-electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS) with high sensitivity and specificity
[37].
We found that · OH produced in a metal ion-independent manner could induce the formation of 8-oxodG from dG and dsDNA, and was more efficient than the classical iron-mediated Fenton system. Interestingly, the effect of TCBQ/H2O2 system on dsDNA oxidative damage was more significant than that of dG. This may be due to the strong hydrophobicity and planar structure of TCBQ, which can insert into dsDNA with high affinity. The intercalation of TCBQ into DNA was studied by using the classical DNA intercalation dye PO-PRO-3 and ethorphine bromide dimer (EthD-1). TCBQ inserted into dsDNA promotes the homolysis of H2O2 to produce hydroxyl radicals which are closer to dsDNA itself and do not need to diffuse, so it can cause oxidative damage to dsDNA more effectively. Similar effects were observed when TCBQ was replaced by low chlorine substituted benzoquinone and other HQ.
The efficient oxidation of dsDNA by halogenoquinone /H
2O
2 proposed here may partially explain the potential genotoxic effects of the ubiquitous PCP and low halogenated phenols. Although DNA adducts have been detected in PCP- or TCBQ-treated model animals and cultured mammalian cells, the concentration of DNA adducts is less than one-tenth of the concentration of oxidative DNA damage
[23,38,39]. Since H
2O
2 can be produced through the redox cycle of halogenated quinones and their reduced forms, the production of · OH through the halogenated quinone /H
2O
2 system may be closely related to the genotoxicity of halogenated quinones
[38]. Notably, the TCBQ/H
2O
2 system was able to oxidize dsDNA even at physiologically relevant concentrations of TCBQ (as low as 1 μmol/L).
In addition, as many as 12 haloquinones, including TCBQ and TriCBQ, were recently identified as new chlorinated disinfection by-products in drinking water and swimming pool water, indicating that haloquinones are commonly and chronically exposed to the public
[6,8]. Quantitative structure-toxicity relationship analysis indicated that haloquinones are highly toxic with a low LOAEL (μg/kg body weight per day), which is 1000-fold lower than most regulated DBPs
[6]. As shown in this study, the TriCBQ/H
2O
2 system can cause 3.7 times of DNA oxidative damage compared with the classical iron-mediated Fenton system. Therefore, the potential genotoxicity of chlorinated disinfection by-products in drinking water may also be related to the oxidation of DNA by halogenoquinone /H
2O
2.
3 Photoelectrochemical DNA Sensor for Detection of Haloquinone-mediated DNA Damage
As mentioned above, a variety of methods have been used to detect and identify DNA damage. In addition, HP LC separation combined with on-line electrochemical detection was also used to detect 8-oxodG, a classic marker of DNA oxidative damage. 32P-postlabeling experiments, combined with gel electrophoresis analysis, have also been used to detect DNA strand breaks and to identify and quantify DNA adducts, but this test procedure is very time consuming
[40]. A new Photoelectrochemical DNA sensor has been developed for rapid detection of DNA structural damage
[41⇓~43]. The surface of the sensor consists of a double-stranded DNA film covering the surface of SnO
2 semiconductor electrodes. The DNA intercalator Ru(bpy)
2(dppz)
2+ can bind to the double-stranded DNA membrane and generate photocurrent under illumination. When exposed to chemicals that can damage DNA, the double-stranded DNA membrane will be damaged, which will bind weakly to Ru(bpy)
2(dppz)
2+, thus further reducing the photocurrent. It has been reported that the biosensor can detect both oxidative DNA damage induced by Fe
2+/H
2O
2 system and DNA adducts mediated by styrene oxide
[42].
Therefore, we chose to employ a Photoelectrochemical DNA sensor to monitor tetrahaloquinone-induced DNA damage
[44]. It was found that the photocurrent decreased by 20% after exposing the DNA film to TFBQ, which may be due to the formation of TFBQ/DNA adducts. DNA intercalators Ru(bpy)
2(dppz)
2+stacked base pairs that can selectively intercalate into double-stranded DNA. The formation of adducts between some macromolecules such as TFBQ and DNA may cause large steric hindrance to DNA base pairing, and may also change the double helix structure of DNA, which will further lead to the reduction of binding signal reporters. In the TFBQ/H
2O
2 reaction system, the signal was reduced by 40%, indicating that Ru(bpy)
2(dppz)
2+ binds less to the double-stranded DNA whose structure has been damaged in the membrane. It is worth noting that the photocurrent reduction was also observed after exposure to TCBQ, but the degree of photocurrent reduction was lower than that of TFBQ system. In addition, the fluorescence analysis using coumarin-3-carboxylic acid as a probe showed that the TFBQ/H
2O
2 system produced more · OH than the TCBQ/H
2O
2 system. It was further confirmed by gel electrophoresis that both of the two halogenoquinone /H
2O
2 systems could induce DNA strand breaks, but halogenoquinone alone could not.
One of the characteristics of the Photoelectrochemical DNA sensor is that it is mainly based on the surface reaction, which allows the solute bound on the surface of the DNA sensor to be easily separated. Previous studies have used this surface reaction mode to explore the role of DNA-bound Fe (II) in DNA oxidative damage, from which it can be clearly observed that Fe (II) can indeed bind to double-stranded DNA in the sensor membrane, and also participate in Fenton reaction and mediate DNA damage
[45]. We also used this sensor approach to study the reactivity of covalently bound haloquinones. In the experiment, the conventionally assembled DNA film electrode was incubated with TCBQ (300 μmol/L), then rinsed with water and exposed to H
2O
2(2 mmol/L), and finally the electrode was exposed to Ru(bpy)
2(dppz)
2+ to start measuring the photocurrent. As a result, it was found that the value of the photocurrent was almost comparable to that of the TCBQ exposure system alone (no H
2O
2), indicating that no additional oxidative damage was caused even if the covalent binding of H
2O
2,TCBQ and DNA was present. Further studies under this experimental condition are needed to fully understand the underlying molecular mechanism.
Although the Photoelectrochemical sensor cannot distinguish whether DNA damage occurs in the form of adduct formation or oxidative damage, it has a series of advantages compared with traditional techniques: 1) high detection sensitivity, the concentration of TFBQ that can cause DNA damage detected by the sensor method is 30-100 μmol/L, while the concentration of TFBQ detected by 32P-postlabeled gel electrophoresis is mmol/L; 2) the amount of DNA used is small, and the amount of DNA immobilized on the membrane in the sensor method is 3.2 ng/mm2, while the 32P-postlabeling experimental method requires DNA at a concentration of μg level; 3) the detection is simple and rapid,The sensor method only needs to prepare a DNA film on the surface of the sensor, and the process of detecting DNA damage is very simple and rapid, while the experimental method of detecting DNA damage by 32 P-postlabeling is complex and time-consuming.
4 Genotoxicity and gene mutation induced by TCHQ in mammalian cells
As mentioned above, the reduction product of TCBQ, TCHQ, is also a toxic metabolite of PCP. However, the underlying genetic and mutational toxicity of TCHQ and its mechanisms are not fully understood. Therefore, we used comet assay to detect whether TCHQ could cause DNA strand breaks and form TCHQ-DNA adducts, so as to evaluate the genotoxicity of TCHQ. In addition, we further confirmed the variability by transfecting the human lung cancer cell line A549 with the plasmid pSP189
[46].
We found that TCHQ can significantly induce DNA strand breaks and adduct formation. DNA sequence analysis showed that TCHQ could cause three types of mutations: single base substitution (85.4%), single base insertion (9.8%) and large fragment deletion (4.9%). Single base substitution mutation mainly occurred in G: C base pair, and the base substitution rate reached 80%, including two types of substitution and transversion, such as G: C-C: G, G: C-T: A, G: C-A: T. The types of mutations in A549 cells were also mainly substitutions (37.1% G: C-A: T; 14.3% A: T-C: G) and transversions (34.3% G: C-C: G; 11.4% G; C-T: A). The mutational hotspots of plasmid pSP189 were located at nucleotides 129, 141 and 155 of supE gene. These mutational hotspots accounted for 63% of all single base substitution mutations. Based on the above results, we conclude that TCHQ can induce sequence-specific DNA mutations, and its safety needs to be monitored. As DNA mutation plays a key role in the process of cancer induction, the genotoxicity and mutagenicity of TCHQ in mammalian cells found in this study will provide new evidence for the carcinogenic effect of PCP.
5 Strong Methyl Oxidation of 5-Methylcytosine by Halogenated Quinone -H2O2 System
It is well known that, in addition to the four natural DNA nucleotides, 5-methylcytosine (5mC) is often considered to be the fifth base of a gene. DNA methylation can regulate gene expression and display cellular characteristics
[47,48]. DNA demethylation, which is the opposite of this process, is important in clearing the genomic whiteboard or rapidly activating silent genes during embryonic development
[49]. Recently, it has been shown that Tet protein can convert 5mC in vitro not only to 5-hydroxymethylcytosine (5HmC), but also to 5-formylcytosine (5fC) and 5-carboxycytosin (5caC)
[45,50,51]. In addition, there has been evidence that 5 fC and 5 caC have been detected in the genomic DNA of mouse ES cells
[50,52⇓~54]. New evidence suggests that derivatives of 5mC are involved in normal development and the pathogenesis of many diseases
[55⇓⇓~58].
Studies have shown that 5 mC can be converted to 5 HmC and 5 fC under Fenton reaction or UV exposure
[59⇓⇓~62]. However, it is not clear whether the interaction of halogenoquinone /H
2O
2 system can oxidize 5-methyl-2 '-deoxycytidine (5mdC) to methyl oxidation products, and what the underlying oxidation mechanism is
[63].
We found that TCBQ/H2O2 treatment of 5mdC could produce three oxidation products: 5-methylcytidine hydroperoxide (5HpmdC), 5-hydroxymethylcytidine (5HmdC), and 5-formylcytidine (5fdC). The addition of typical · OH scavengers (DMSO, ethanol and sodium formate) or anaerobic environment can significantly inhibit the formation of oxidation products. Similar phenomena can also be observed in other halogenated quinones and typical Fenton systems. The reactivity of halogenoquinones depends on the type, amount and position of halogen substitution. The reactivity of halogenoquinones from strong to weak is as follows: 2,3-DCBQ > TCBQ, TBrBQ > TriCBQ > TFBQ > 2,5-DCBQ > 2,6-DCBQ2-CBQ. Unexpectedly, 2,3-DCBQ was the most oxidatively potent of the nine HBQs examined, and its efficiency was almost more than twice that of TCBQ/H2O2. Based on the above results, we speculate that the process of converting 5mC into 5HpmC, 5HmC and 5fC in the TCBQ/H2O2 system may be a metal ion-independent mechanism process (Fig. 4).
To our knowledge, this is the first report that the interaction of haloquinones with H2O2 can produce strong methyl oxidation of 5mdC through a metal-ion-independent process. These results demonstrate that the interaction of TCBQ or other haloquinones with H2O2 can effectively oxidize 5mdC to methyl oxidation products through a metal-ion-independent process, and the oxidation efficiency of some haloquinones can even be comparable to that of the classical iron-mediated Fenton reaction.
6 Haloquinones and other redox-active quinones cause genome-wide DNA methylation changes at the cellular level
As described above, 5 HmC in mammalian cells is mainly produced by the oxidation of 5 HmC by three Tet proteases
[50⇓~52,64]. Further oxidation can reconvert the existing 5 HmC into 5 fC and 5 caC that can be eliminated, which provides a mechanism for active DNA demethylation
[50,52]. Interestingly, small molecules can alter epigenetic DNA methylation patterns and regulate life processes, and even cause diseases such as cancer
[64][65⇓⇓~68]. However, little is known about the effects of small molecules on Tet-mediated DNA demethylation.
Some cofactors, such as Fe (Ⅱ) and α-ketoglutarate, are required for Tet protein to catalyze 5 mC oxidation
[50⇓~52]. The intracellular environment, especially the redox state, can alter the catalytic activity of the Ter protein
[69⇓⇓~72]. Since halogenated quinones are ubiquitous and have redox activity, we speculate that halogenated quinones may change the equilibrium between Fe (II) and α-ketoglutarate, thus affecting the catalytic activity of Tet dioxygenase. Because Tet dioxygenase mediates the oxidation of 5 mC, causing either passive or active DNA demethylation, we hypothesized that haloquinones might alter DNA demethylation by altering the oxidative activity of Tet
[50,52].
Therefore, we examined whether haloquinones and other redox-active quinones cause a 5 mC to 5 HmC transition at the cellular level. If possible, we will explore the role of quinones in regulating DNA demethylation and explain possible mechanisms
[73].
In order to find out whether haloquinones with redox activity can affect the production of 5 HmC at the cellular level, MRC-5 cells exposed to TCBQ (20 μmol/L) were used to detect the content of 5 HmC by UHPLC-MRM-QQQ. The content of 5 HmC in TCBQ treated cells was about 1.9 times higher than that in DMSO treated control cells. 5 HmC can also be produced by treatment of cells with the reduced form of TCBQ, TCHQ (20 μmol/L). After the cells were exposed to TCBQ or TCHQ solution for more than 3 days, the content of 5 HmC still increased, indicating that the 5 HmC produced was stable and enriched in the cells. When the cells were incubated with fresh medium after 72 H of TCBQ treatment, the 5 HmC produced in the cells did not disappear within 96 H, which again proved the enrichment and persistence of 5 HmC produced in the cells after TCBQ treatment. Similar phenomena were found in other cell lines (A549 and HepG2 cells).
It has been reported that the content of PCP in body fluids (plasma or urine) of non-occupationally exposed people is 0~7.0μmol·L
-1, and the content of PCP in blood of occupationally exposed people is 22.5~170μmol·L
-1[9]. Since more than 20% of PCP can be converted to the reactive metabolites TCBQ and TCHQ, their overall concentrations are 0~1.5μmol·L
-1 in non-occupationally exposed populations and 4.5~34μmol·L
-1 in occupationally exposed populations
[74]. The content of 5 HmC was significantly increased after the cells were treated with TCHQ 1.0μmol·L
-1 (the concentration was equivalent to the non-occupational exposure concentration, but lower than the occupational exposure concentration). The results of TCBQ and TCHQ showed that their effects on intracellular 5 HmC were of physiological significance. Other low chlorinated and methylated quinones can also promote the formation of 5 HmC in cells, which indicates that quinones with redox activity can significantly promote the conversion of 5 HmC to 5 Hm C in mammalian cells.
Surprisingly, we further found that quinone treatment significantly increased the expression of free ferritin light chain at both mRNA and protein levels, suggesting a role for iron regulation in Tet-protein-mediated promotion of 5mC oxidation. At the same time, the specific chelator of intracellular free iron (1,10-phenanthroline) blocked the increase of 5HmC, and reduced the increase of 5mC by Tet1/Tet2 knockdown and inhibition of Tet induced by dimethoxyglycine, while the content of 5mC increased after the addition of free iron. These results indicate the mechanism of action of redox active quinones in iron-regulated Tet-mediated DNA demethylation.
It is likely that haloquinone-induced formation of aberrant 5 HmC alters the natural DNA methylation state, thereby further altering the epigenetic factors of gene expression. Genome-wide hypomethylation has been found in cancer cells, and tumorigenesis has been reported to be associated with oncogene activation mediated by DNA demethylation
[75][76]. In addition, it has been reported that the level of hypomethylation increases with genomic instability
[77]. Interestingly, polyhalophenols found in widely used fungicides are suspected carcinogens because they produce redox-active halogenated quinones during metabolism
[37]. We postulate that the ability to convert 5 mC to 5 HmC in DNA may be one of the reasons for the carcinogenic potential of these fungicides.
7 Halogenated quinones oxidize DNA to the more mutagenic midazolone-like product dIz via the quinoxyl radical
Organic hydroperoxides can be produced either by the nonenzymatic reaction of free radicals with polyunsaturated fatty acids or by lipoxygenase or cyclooxygenase catalyzed enzymatic oxidation of linoleic acid and arachidonic acid
[78]. As mention above, we have recently found that that reaction of haloquinones with H
2O
2 or organic hydroperoxide, without the involvement of transition metal ions, produces not only highly reactive hydroxyl/alkoxyl radicals, but also quinoxyl/quinocarbon radical generation
[29,30,32,33,35].
Our previous studies showed that 8-oxodG, DNA strand breaks, and three methyl oxidation products were produced when DNA was treated with TCBQ/H
2O
2, and these reactions were independent of transition metal ions
[37,44,63,73]. However, it is not clear whether the haloquinone and organic hydroperoxide system causes DNA damage resulting in other dG oxidation products, such as dIz: (2-amino-5- [ (2-deoxy-β-D-erythro-pentofuranosyl) amino] -4H-imidazol-4-one), dZ: (2,2-diamino-5- [2-deoxy-β-D-erythro-pentofuranosyl) amino] -5 (2H) -ox. If so, what are the possible reaction mechanisms
[79].
We found that the main oxidation product of dG after treatment with TCBQ and the classical hydroperoxide tert-butyl hydroperoxide (t-BuOOH) was dIz. The formation of dIz was significantly inhibited by the addition of classical radical spin trapping agents. It has been demonstrated that the decomposition of t-BuOOH mediated by Fe (II) or halogenoquinones not only produces alkoxyl radical t-BuO ·, but also produces oxygen-centered quinonyl oxygen radical and/or carbon-centered quinonyl carbon radical during the decomposition of t-BuOOH mediated by halogenoquinones
[29,33,36]. Interestingly, almost no dIz formation was detected in the Fe
2+/t-BuOOH system, suggesting that t-BuO · may not be involved in dIz formation. But dIz was detected in all haloquinone/t-BuOOH systems, and the more halogen atoms were substituted, the more dIz was produced.
These results suggest that the oxygen-centered quinone-oxygen radical and/or the carbon-centered quinone-carbon radical may be involved in the formation of dIz in the haloquinone/t-BuOOH system. In the 2,5-DCBQ/t-BuOOH system, the generated O-centered CBQ-O · can be isomerized into its corresponding more stable C-centered · CBQ = O. Using 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO) as spin trapping agent, the formation of BMPO/· CBQ = O was detected by ESR and HPLC-MS/MS methods, and the amount of BMPO/· CBQ = O decreased gradually with the addition of dG
[36]. These results indicate that the quinonyl oxygen radical and/or the quinonyl carbon radical in the haloquinone/t-BuOOH system are indeed involved in the formation of dIz. Our results also clearly show that haloquinones with more halogen atom substitutions induce more dG oxidation. In contrast, p-BQ and 2-CBQ caused much less oxidative damage to dG under the same experimental conditions.
Using
15N isotopically labeled 8-oxodG, it was also found that dIz could be produced not only directly from dG oxidation, but also from the resulting 8-oxodG. Together with other previous studies, we propose that TCBQ/t-BuOOH may oxidize dG into dIz through the following quinoxyl radical-mediated mechanism (Figure 5)
[79].
We further found that dIz was also produced when t-BuOOH was replaced by other hydroperoxides, such as the endogenous lipid hydroperoxide 13S-hydroperoxy-9Z, 11E-octadecadienoic acid (13-HPODE), suggesting that these results may have physiological implications. It is noteworthy that TCBQ/t-BuOOH caused oxidative damage to double-stranded DNA even at physiologically relevant concentrations (as low as 1 μmol/L).
Base modification has been demonstrated to be the most common DNA damage. 8-oxoG is more susceptible to one-electron oxidation or
1O
2 oxidation than G
[80]. Iz is a major and ubiquitous product of that in vitro oxidation of guanine
[81]. Oz, the hydrolysate of Iz, is very stable under physiological conditions. Both Iz and Oz have alkaline instability and mutagenicity. When replicated in vivo, they strongly inhibit DNA polymerase and are at least an order of magnitude more likely to induce mutations than 8-oxoG
[82,83]. If these base lesions persist in the cell, they may be a powerful source of mutation
[77⇓~79].
We demonstrated that the system of haloquinone and organic hydroperoxide could oxidize dG to mutagenic products such as dIz, dZ and 8-oxodG without depending on transition metal ions. Since the enzymatic oxidation of linoleic acid and arachidonic acid can produce organic hydroperoxides such as 13-HPODE, the haloquinone and organic hydroperoxide system may help to explain the genotoxicity of haloquinones
[84].
8 Conclusion.
In summary, our group found that 8-oxodG, DNA strand breaks and three methyl oxidation products could be produced when DNA was oxidized by TCBQ/H2O2 system, and these reactions were independent of transition metal ions. Similar phenomena have been observed on other halogenated quinones and are generally more efficient than the classical Fenton system. We have further extended our study from purified DNA to genomic DNA of living cells. We also found that the system of halogenated quinones and organic hydroperoxides can induce DNA oxidation to produce more mutagenic dIz through a unique mechanism mediated by quinone oxygen radicals. These findings provide new insights into the potential genotoxicity, mutagenicity, and carcinogenicity of the ubiquitous haloquinone carcinogenic intermediates and disinfection byproducts.
Strategic Priority Research Program Chinese Academy Sciences(XDB14030100)
from "0" to "1" Original Innovation Project of the Basic Frontier Science Research Program of the Chinese Academy of Sciences(ZDBS-LY-SLH027)
Natianal Natural Science Foundation of China(21976200)
Natianal Natural Science Foundation of China(21207150)
Natianal Natural Science Foundation of China(21836005)
Natianal Natural Science Foundation of China(21621064)
Natianal Natural Science Foundation of China(21321004)
Young Scientist Foundation of Research Center for Eco-Environmental Sciences(RCEES-QN-20130052F)
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