Protection by desferrioxamine and other hydroxamic acids against tetrachlorohydroquinone-induced cyto- and genotoxicity in human fibroblasts

Ultraviolet dechlorination of tetrachloro-p-benzoquinone by hydrogen sulfide: Theoretical confirmation of the significance of hydrosulfide radical

In this work, a systematical investigation on the role of hydrogen sulfide (H₂S) on the transformation of tetrachloro-p-benzoquinone (TCBQ) under ultraviolet (UV) irradiation (at 253.7 nm) in aqueous solution has been conducted through quantum chemical calculations. Under the UV irradiation, with the forward energy barrier (E_a,f, 11.7 kcal mol^-1) much lower than the reverse one (E_a,r, 22.3 kcal mol^-1), the first triplet state of TCBQ was kinetically feasible to react with bisulfide anion (HS^-) via the Michael addition, and the addition of HS^- could promote the release of Cl^- and the formation of primary dechlorination product (HS-TriCBQ). During the UV photolysis of the primary dechlorination products (HO-TriCBQ and HS-TriCBQ) in the presence of H₂O and H₂S, the addition of nucleophile (OH^- or HS^-) to the ortho-position of the hydroxyl or thiol group might be the most efficient pathway for the dechlorination, and their respective E_a,f were 9.2 kcal mol^-1 (for HS^--hydroxyl), 1.1 kcal mol^-1 (for OH^--thiol) and 8.9 kcal mol^-1 (for HS^--thiol). Moreover, the electron transfer from HS^- to the first triplet states could generate hydrosulfide radical for the dechlorination of TCBQ. The findings in the present study may provide some important theoretical foundation for the dehalogenation of TCBQ as well as other halobenzoquinones.

Ultraviolet photolysis of monochloro-p-benzoquinone (MCBQ) in aqueous solution: Theoretical investigation into the dechlorination

In this work, the UV-induced transformation of monochloro-p-benzoquinone (MCBQ) in aqueous solution has been systematically investigated through quantum chemical calculations. During the UV irradiation at 253.7 nm, the first triplet state of MCBQ (³MCBQ*) was from the intersystem crossing of its first excited singlet state (¹MCBQ*). In aqueous solution, the nucleophilic attack of OH^- on carbon atoms in ³MCBQ* was the central reaction. The addition of OH^- to olefinic carbon atoms was much more kinetically feasible than that to carbonyl carbon atoms, even though the carbonyl carbon atoms were more positively charged. Moreover, OH^- preferred to add to the ortho-position of C-Cl bond, where the unchlorinated atom was more negatively charged than the chlorinated one. The UV photolysis of the primary intermediate (HO-CBQ) was not the same as that of MCBQ. The attack of OH^- on the para-position of C-Cl bond was the most efficient pathway. The addition of OH^- to the chlorinated atom of ³HO-CBQ* was much more efficient than that in the case of ³MCBQ*, which reveals that more UV irradiation may promote the dechlorination. The findings in the present study may be helpful to enrich the understanding of the halobenzoquinones transformation in aqueous solution.

The critical role of superoxide anion radicals on delaying tetrachlorohydroquinone autooxidation by penicillamine

We have recently found that penicillamine, a classic copper-chelating thiol-drug for Wilson's disease, can delay tetrachlorohydroquinone (TCHQ) autooxidation via a previously unrecognized redox-activity. However, its underlying molecular mechanism remains not fully understood. In this study, we found, interestingly and unexpectedly, that superoxide dismutase (SOD) can significantly shorten the delay of TCHQ autooxidation by penicillamine, but not by ascorbate; SOD can also markedly increase the yields of the oxidized form of penicillamine. Similar effects were observed with a recently-developed specific and sensitive superoxide anion radical (O₂^•-) probe CT-02H, which was also employed to successfully measure O₂^•- generated from both TCHQ and TCHQ/penicillamine systems for the first time. More importantly, addition of extra O₂^•- (KO₂/18-crown-6) can further prolong the delaying effects by penicillamine and slow down penicillamine consumption. Taken together, an unexpected critical role of O₂^•- in TCHQ/penicillamine interaction was proposed: O₂^•- may regenerate penicillamine, thereby continuously reducing TCSQ^•- to TCHQ and finally delaying TCHQ autooxidation; In contrast, if O₂^•- were eliminated, which can not only markedly change the reaction equilibrium, accelerate the rate of interaction, and ultimately shorten the delay of TCHQ autooxidation by penicillamine, but can also accelerate penicillamine oxidation to form its corresponding disulfide solely via redox reaction without any minor nucleophilic reaction. These findings not only further support our previously-proposed redox mechanism for the protection against TCHQ-induced cytotoxicity by penicillamine, but also reveal a new mode of action for O₂^•- in the inhibition of haloquinoids-induced toxicity by thiol antioxidants.

An unexpected antioxidant and redox activity for the classic copper-chelating drug penicillamine

Penicillamine has been widely-used clinically as a copper-chelating drug for the treatment of copper-overload in Wilson's disease. In this study, we found that penicillamine provided marked protection against cytotoxicity induced by tetrachlorohydroquinone (TCHQ), a major toxic metabolite of the well-known wood preservative pentachlorophenol, while other classic copper-chelating agents do not. We found, unexpectedly, that both TCHQ autooxidation and tetrachlorosemiquinone radical (TCSQ^•-) formation were remarkably delayed by penicillamine. Further investigation showed that TCSQ^•- was reduced back to TCHQ by penicillamine, with the concurrent formation of its corresponding disulfide. These data demonstrated that the protection by penicillamine against TCHQ-induced toxicity was not due to its classic Cu-chelating property, but rather to its reduction of the reactive TCSQ^•- to the much less-reactive TCHQ. This is the first report of an unexpected antioxidant and redox activity for penicillamine, which might prove highly relevant to its biological activities.

An unexpected new pathway for nitroxide radical production via more reactve nitrogen-centered amidyl radical intermediate during detoxification of the carcinogenic halogenated quinones by N-alkyl hydroxamic acids

We found previously that nitroxide radical of desferrioxamine (DFO^•) could be produced from the interaction between the classic iron chelating agent desferrioxamine (DFO, an N-alkyl trihydroxamic acid) and tetrachlorohydroquinone (TCHQ), one of the carconogenic quinoind metabolites of the widely used wood preservative pentachlorophenol. However, the underlying molecular mechanism remains unclear. Here N-methylacetohydroxamic acid (N-MeAHA) was synthesized and used as a simple model compound of DFO for further mechanistic study. As expected, direct ESR studies showed that nitroxide radical of N-MeAHA (Ac-(CH₃)NO^•) can be produced from N-MeAHA/TCHQ. Interestingly and unexpectedly, when TCHQ was substituted by its oxidation product tetrachloro-1,4-benzoquinone (TCBQ), although Ac-(CH₃)NO^• could also be produced, no concurrent formation of tetrachlorosemiquinone radical (TCSQ^•) and TCHQ was detected, suggesting that Ac-(CH₃)NO^• did not result from direct oxidation of N-MeAHA by TCSQ^• or TCBQ as proposed previously. To our surprise, a new nitrogen-centered amidyl radical was found to be generated from N-MeAHA/TCBQ, which was observed by ESR with the spin-trapping agents and further unequivacally identified as Ac-(CH₃)N^• by HPLC-MS. The final product of amidyl radical was isolated and identified as its corresponding amine. Analogous radical homolysis mechanism was observed with other halogenated quinoid compounds and N-alkyl hydroxamic acids including DFO. Interestingly, amidyl radicals were found to induce both DNA strand breaks and DNA adduct formation, suggesting that N-alkyl hydroxamic acids may exert their potential side-toxic effects via forming the reactive amidyl radical species. This study represents the first report of an unexpected new pathway for nitroxide radical production via hydrogen abstration reaction of a more reactive amidyl radical intermediate during the detoxification of the carcinogenic polyhalogenated quinones by N-alkyl hydroxamic acids, which provides more direct experimental evidence to better explain not only our previous finding that excess DFO can provide effective but only partial protection against TCHQ (or TCBQ)-induced biological damage, and also the potential side-toxic effects induced by DFO and other N-alkyl hydroxamic acid drugs.

Unexpected activation of N-alkyl hydroxamic acids to produce reactive N-centered free radicals and DNA damage by carcinogenic chlorinated quinones under normal physiological conditions

We found recently that benzohydroxamic acid (BHA) could detoxify the chlorinated quinoid carcinogens via an unusual Lossen rearrangement reaction. However, it is not clear what would happen when the nitrogen hydrogen of BHA was substituted with methyl and other alkyl groups. Here we show that N-methyl benzohydroxamic acid (N-MeBHA, a simple model compound for the classic iron-chelator deferoxamine, which is a typical N-alkyl trihydroxamic acid) could react with 2,5-dichloro-1,4-benzoquinone (DCBQ) to form a relatively stable initial carbon-oxygen bonding conjugation intermediate CBQ-O-N-MeBHA. However, the major final product was identified, unexpectedly, as a carbon-nitrogen bonding conjugate CBQ(OH)-N(CH₃)-COAr, which is the rearranged isomer of CBQ-O-N-MeBHA. Interestingly, a new 18-line nitrogen-centered radical and a carbon-centered quinone ketoxy radical were observed by the ESR spin-trapping method, which was further confirmed by HPLC-MS and ¹⁵N-isotope labeling methods. We further found that both new DNA adducts and DNA strand breaks could be produced by the reactive nitrogen-centered radical. Taken together, we propose that the reaction between DCBQ and N-MeBHA was not via the Lossen rearrangement, but rather through a novel radical homolysis and recoupling pathway. Analogous results were observed for other chlorinated quinones and N-alkyl hydroxamic acids including the widely-used trihydroxamate iron-chelating drug deferoxamine. This represents the first report of unexpected radical pathway for the reaction between chlorinated quinones and N-alkyl hydroxamic acids under normal physiological conditions, which may have broad biological and environmental significance for future study of carcinogenic chloroquinones and hydroxamic acid drugs.

An Exceptionally Facile Two-Step Structural Isomerization and Detoxication via a Water-Assisted Double Lossen Rearrangement

N-hydroxyphthalimide (NHPI), which is best known as an organocatalyst for efficient C-H activation, has been found to be oxidized by quinoid compounds to its corresponding catalytically active nitroxide-radical. Here, we found that NHPI can be isomerized into isatoic anhydride by an unusually facile two-step method using tetrachloro-1,4-benzoquinone (TCBQ, p-chloranil), accompanied by a two-step hydrolytic dechlorination of highly toxic TCBQ into the much less toxic dihydroxylation product, 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid). Interestingly, through the complementary application of oxygen-18 isotope-labeling, HPLC combined with electrospray ionization quadrupole time-of-flight and high resolution Fourier transform ion cyclotron resonance mass spectrometric studies, we determined that water was the source and origin of oxygen for isatoic anhydride. Based on these data, we proposed that nucleophilic attack with a subsequent water-assisted Lossen rearrangement coupled with rapid intramolecular addition and cyclization in two consecutive steps was responsible for this unusual structural isomerization of NHPI and concurrent hydroxylation/detoxication of TCBQ. This is the first report of an exceptionally facile double-isomerization of NHPI via an unprecedented water-assisted double-Lossen rearrangement under normal physiological conditions. Our findings may have broad implications for future research on hydroxamic acids and polyhalogenated quinoid carcinogens, two important classes of compounds of major chemical and biological interest.

Copper Capture in a Thioether-Functionalized Porous Polymer Applied to the Detection of Wilson's Disease

Copper is an essential nutrient for life, but at the same time, hyperaccumulation of this redox-active metal in biological fluids and tissues is a hallmark of pathologies such as Wilson’s and Menkes diseases, various neurodegenerative diseases, and toxic environmental exposure. Diseases characterized by copper hyperaccumulation are currently challenging to identify due to costly diagnostic tools that involve extensive technical workup. Motivated to create simple yet highly selective and sensitive diagnostic tools, we have initiated a program to develop new materials that can enable monitoring of copper levels in biological fluid samples without complex and expensive instrumentation. Herein, we report the design, synthesis, and properties of PAF-1-SMe, a robust three-dimensional porous aromatic framework (PAF) densely functionalized with thioether groups for selective capture and concentration of copper from biofluids as well as aqueous samples. PAF-1-SMe exhibits a high selectivity for copper over other biologically relevant metals, with a saturation capacity reaching over 600 mg/g. Moreover, the combination of PAF-1-SMe as a material for capture and concentration of copper from biological samples with 8-hydroxyquinoline as a colorimetric indicator affords a method for identifying aberrant elevations of copper in urine samples from mice with Wilson’s disease and also tracing exogenously added copper in serum. This divide-and-conquer sensing strategy, where functional and robust porous materials serve as molecular recognition elements that can be used to capture and concentrate analytes in conjunction with molecular indicators for signal readouts, establishes a valuable starting point for the use of porous polymeric materials in noninvasive diagnostic applications.

Chloro-benzoquinones cause oxidative DNA damage through iron-mediated ROS production in Escherichia coli

Chloro-benzoquinones (CBQs) are a group of disinfection byproducts that are suspected to be potentially carcinogenic. Here, the mechanism of DNA damage caused by CBQs in the presence of ferrous ions was investigated in an Escherichia coli wild type M5 strain and a mutant L5 (ahpCF katEG mutant) strain that carried an enhanced green fluorescent protein reporter under the control of a SOS response gene (recA) promoter. All tested CBQs (including para-benzoquinone, 2-chloro-para-benzoquinone, and dichloro-para-benzoquinones with different substitutes) caused substantial oxidative DNA damage with EC50 values in the micromolar range. Moreover, 2,5-dichloro-para-benzoquinone (2,5-DCBQ), a typical CBQ, caused substantial ROS production in E. coli mutant cells. And ROS scavengers provided partial protective effects on genotoxicity of 2,5-DCBQ to E. coli mutant cells. The addition of Fe(2+) to the 2,5-DCBQ exposure system caused an increase in DNA oxidative damage; iron-chelating agents could partially prevent these cells from DNA damage. Finally, intracellular AhpCF, catalase E, and catalase G were all found to play an important role in the survival of E. coli cells exposed to CBQs, as indicated by an increased sensitivity of the ahpCF katEG mutant L5 strain to treatment compared with wild type M5 cells. Taken together, these results suggest that CBQs cause oxidative DNA damage in E. coli cells through the participation of iron-mediated ROS production.

Deferasirox-induced cytogenetic responses

Deferasirox (commercially formulated as Exjade(®)) is one of the effective iron chelators used in treatment of iron overload diseases. In this study the effect of this substance for chromosome aberration, sister chromatid exchange and mitotic index was studied by in vitro (by using human peripheral lymphocytes) and in vivo (by using rat) analysis. Deferasirox increased the sister chromatid exchange frequency in all tested concentrations and periods in vitro. Also, in the presence of metabolic activator, the substance led to a statistically significant increase in the sister chromatid exchange frequencies only at high concentration. While in in vitro analysis the substance significantly increased abnormal cell percentages in all concentrations, in in vivo study the substance increased chromosome aberrations only in two concentrations at 12h treatment. In the cultured lymphocytes, deferasirox showed cytotoxicity by significantly reducing proliferation index and mitotic index values. While in the presence of metabolic activation it did not affect the proliferation index frequency, it had a stimulant effect on the mitotic index frequency. Deferasirox reduced significantly the mitotic index value in the bone marrow cells especially in high concentration and short treatment period (12h).

A combined experimental and computational investigation on the unusual molecular mechanism of the Lossen rearrangement reaction activated by carcinogenic halogenated quinones

The classic Lossen rearrangement is a well-known reaction describing the transformation of an O-activated hydroxamic acid into the corresponding isocyanate. In this study, we found that chlorinated benzoquinones (CnBQ) serve as a new class of agents for the activation of benzohydroxamic acid (BHA), leading to Lossen rearrangement. Compared to the classic one, this new kind of CnBQ-activated Lossen rearrangement has the following unique characteristics: (1) The stability of CnBQ-activated BHA intermediates was found to depend not only on the degree but also on the position of Cl-substitution on CnBQs, which can be divided into two subgroups. (2) It is the relative energy of the anionic CnBQ-BHA intermediates that determine the rate of this CnBQ-activated rearrangement, which is the rate-limiting step, and the Cl or H ortho to the reaction site at CnBQ is crucial for the stability of the anionic intermediates. (3) A pKa-activation energy correlation was observed, which can explain why the correlation exists between the rate of the rearrangement and the acidity of the conjugate acid of the anionic leaving group, the hydroxlated quinones. These findings may have broad implications for future research on halogenated quinoid carcinogens and hydroxamate biomedical agents.

Tetrachlorobenzoquinone induces acute liver injury, up-regulates HO-1 and NQO1 expression in mice model: the protective role of chlorogenic acid

Tetrachlorobenzoquinone (TCBQ) is an active metabolite of pentachlorophenol (PCP). Although PCP has been investigated extensively, there are only a few reports describing the toxicity effect of TCBQ, and no report regarding TCBQ-induced liver injury in vivo. In the current study, we aimed to examine the acute hepatic toxicity of TCBQ in the mice model. Chlorogenic acid (CGA) exhibits promising antioxidant activity in the past studies, thus, the second aim of this study was to evaluate the protective effect of CGA on TCBQ-induced liver injury. Our results indicated TCBQ-intoxication caused marked liver cell necrosis and inflammation but not apoptosis, and this damage was alleviated by CGA treatment. Meantime, TCBQ-intoxication enhanced serum ALT, AST activities, TBIL content, hepatic oxidative stress and lipid peroxidation, decreased GSH content and inhibited the activities of antioxidant enzymes. Western blot and immunohistochemical analysis showed that TCBQ marked up-regulated HO-1 and NQO1 expression. On the other hand, pretreatment of CGA reduced TCBQ-induced liver damage remarkably. Taking together, these results revealed that TCBQ has strong hepatic toxic effect, and at least a part of this effect is initiated by free radical and relieved with CGA administration.

Signs of deferasirox genotoxicity

Iron overload is a major health problem for patients who have to have continuous blood transfusions. It brings some metabolic problems together. Various iron chelating agents are being used for treatment of hemochromatosis which arises from excess iron accumulation. This study was conducted with the aim of determining whether deferasirox used as an iron chelator in patients with hemochromatosis has genotoxic effects. Commercial form of deferasirox, Exjade was used as test material. Test material showed a general mutagen character in mutant strains of Salmonella typhimurium. Deferasirox has also led to an increase in mutagenity-related polymorphic band count in random amplification of polymorphic DNA test done with bone marrow cells of rats. Similarly, test material has increased micronucleus formation in cultured in vitro human peripheral lymphocytes particularly in 48 h period. Consistently with the abovementioned findings, deferasirox reduced nuclear division index (NDI) compared to controls and some part of these reductions are statistically significant. NDI reductions were found at positive control levels at high concentrations.

Characterization of TCHQ-induced genotoxicity and mutagenesis using the pSP189 shuttle vector in mammalian cells

Tetrachlorohydroquinone (TCHQ) is a major toxic metabolite of the widely used wood preservative, pentachlorophenol (PCP), and it has also been implicated in PCP genotoxicity. However, the underlying mechanisms of genotoxicity and mutagenesis induced by TCHQ remain unclear. In this study, we examined the genotoxicity of TCHQ by using comet assays to detect DNA breakage and formation of TCHQ-DNA adducts. Then, we further verified the levels of mutagenesis by using the pSP189 shuttle vector in A549 human lung carcinoma cells. We demonstrated that TCHQ causes significant genotoxicity by inducing DNA breakage and forming DNA adducts. Additionally, DNA sequence analysis of the TCHQ-induced mutations revealed that 85.36% were single base substitutions, 9.76% were single base insertions, and 4.88% were large fragment deletions. More than 80% of the base substitutions occurred at G:C base pairs, and the mutations were G:C to C:G, G:C to T:A or G:C to A:T transversions and transitions. The most common types of mutations in A549 cells were G:C to A:T (37.14%) and A:T to C:G transitions (14.29%) and G:C to C:G (34.29%) and G:C to T:A (11.43%) transversions. We identified hotspots at nucleotides 129, 141, and 155 in the supF gene of plasmid pSP189. These mutation hotspots accounted for 63% of all single base substitutions. We conclude that TCHQ induces sequence-specific DNA mutations at high frequencies. Therefore, the safety of using this product would be carefully examined.

Highly reactive oxygen species: detection, formation, and possible functions

The so-called reactive oxygen species (ROS) are defined as oxygen-containing species that are more reactive than O(2) itself, which include hydrogen peroxide and superoxide. Although these are quite stable, they may be converted in the presence of transition metal ions, such as Fe(II), to the highly reactive oxygen species (hROS). hROS may exist as free hydroxyl radicals (HO·), as bound ("crypto") radicals or as Fe(IV)-oxo (ferryl) species and the somewhat less reactive, non-radical species, singlet oxygen. This review outlines the processes by which hROS may be formed, their damaging potential, and the evidence that they might have signaling functions. Since our understanding of the formation and actions of hROS depends on reliable procedures for their detection, particular attention is given to procedures for hROS detection and quantitation and their applicability to in vivo studies.

Detoxifying carcinogenic polyhalogenated quinones by hydroxamic acids via an unusual double Lossen rearrangement mechanism

Hydroxamic acids, which are best-known for their metal-chelating properties in biomedical research, have been found to effectively detoxify the carcinogenic polyhalogenated quinoid metabolites of pentachlorophenol and other persistent organic pollutants. However, the chemical mechanism underlying such detoxication is unclear. Here we show that benzohydroxamic acid (BHA) could dramatically accelerate the conversion of the highly toxic tetrachloro-1, 4-benzoquinone (p-chloranil) to the much less toxic 2,5-dichloro-3, 6-dihydroxy-1, 4-benzoquonine (chloranilic acid), with rate accelerations of up to 150,000-fold. In contrast, no enhancing effect was observed with O-methyl BHA. The major reaction product of BHA was isolated and identified as O-phenylcarbamyl benzohydroxamate. On the basis of these data and oxygen-18 isotope-labeling studies, we proposed that suicidal nucleophilic attack coupled with an unexpected double Lossen rearrangement reaction was responsible for this remarkable acceleration of the detoxication reaction. This is the first report of an unusually mild and facile Lossen-type rearrangement, which could take place under normal physiological conditions in two consecutive steps. Our findings may have broad biological and environmental implications for future research on hydroxamic acids and polyhalogenated quinoid carcinogens, which are two important classes of compounds of major biomedical and environmental interest.

Potential mechanism for pentachlorophenol-induced carcinogenicity: a novel mechanism for metal-independent production of hydroxyl radicals

The hydroxyl radical ((*)OH) has been considered to be one of the most reactive oxygen species produced in biological systems. It has been shown that (*)OH can cause DNA, protein, and lipid oxidation. One of the most widely accepted mechanisms for (*)OH production is through the transition metal-catalyzed Fenton reaction. Pentachlorophenol (PCP) was one of the most widely used biocides, primarily for wood preservation. PCP is now ubiquitously present in our environment and even found in people who are not occupationally exposed to it. PCP has been listed as a priority pollutant by the U.S. Environmental Protection Agency (EPA) and classified as a group 2B environmental carcinogen by the International Association for Research on Cancer (IARC). The genotoxicity of PCP has been attributed to its two major quinoid metabolites: tetrachlorohydroquinone and tetrachloro-1,4-benzoquinone (TCBQ). Although the redox cycling of PCP quinoid metabolites to generate reactive oxygen species is believed to play an important role, the exact molecular mechanism underlying PCP genotoxicity is not clear. Using the salicylate hydroxylation assay and electron spin resonance (ESR) secondary spin-trapping methods, we found that (*)OH can be produced by TCBQ and H(2)O(2) independent of transition metal ions. Further studies showed that TCBQ, but not its corresponding semiquinone radical, the tetrachlorosemiquinone radical (TCSQ(*)), is essential for (*)OH production. The major reaction product between TCBQ and H(2)O(2) was identified to be trichloro-hydroxy-1,4-benzoquinone (TrCBQ-OH), and H(2)O(2) was found to be the source and origin of the oxygen atom inserted into this reaction product. On the basis of these data, we propose that (*)OH production by TCBQ and H(2)O(2) is not through a semiquinone-dependent organic Fenton reaction but rather through the following novel mechanism: a nucleophilic attack of H(2)O(2) to TCBQ, leading to the formation of an unstable trichloro-hydroperoxyl-1,4-benzoquinone (TrCBQ-OOH) intermediate, which decomposes homolytically to produce (*)OH. These findings represent a novel mechanism of (*)OH formation not requiring the involvement of redox-active transition metal ions and may partly explain the potential carcinogenicity of the widely used biocides such as PCP and other polyhalogenated aromatic compounds.

Prevention of the areca nut extract-induced unscheduled DNA synthesis of gingival keratinocytes by vitamin C and thiol compounds

There are about 600 million betel quid (BQ) chewers in the world. BQ chewing is the major risk factor of oral cancer in India, Taiwan, South Africa and numerous other countries. Areca nut (AN) extract, the main component of BQ, exerts cytotoxicity and genotoxicity to several types of cells. In the present study, AN extract induced the unscheduled DNA synthesis (UDS) of gingival keratinocytes (GK). Vitamin C, at concentration of 50 and 200 microg/ml prevented the AN-induced UDS by 41 and 56%, respectively. Glutathione (GSH, 1-3 mM) and N-acetyl-L-cysteine (NAC, 1-3 mM) also protected the AN-induced UDS by 89-100 and 76-90%. These preventive effects were not due to cytotoxicity as analyzed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. Deferoxamine (20 and 30 mM), an iron chelator and a free radical scavenger, also prevented AN extract induced UDS of GK by 30-55%. On the contrary, banthocuproine (50-200 microM, a copper chelator) and 1,10-phenanthroline (50, 100 microM, a lipid permeable iron chelator), lacked preventive effects. Specific reactive oxygen species scavengers such as dimethyl-sulfoxide (2%), mannitol (10-20 mM), dimethylthiourea (10-20 mM), pyruvate (10 mM), catalase (200 and 400 U/ml), and superoxide dismutase (50 and 200 U/ml) also lacked these preventive effects. Moreover, higher concentrations of H(2)O(2) (0.5-1 mM) inhibited the basal levels of UDS by 19-37%. Interestingly, NAC, GSH, Vitamin C and deferoxamine cannot prevent the AN-induced morphological changes of GK at similar concentrations. These results reveal that AN extract-induced UDS of GK is associated with free radical reactions. Possibly different ingredients of AN is responsible for genotoxicity and cytotoxicity. Vitamin C, GSH and NAC may be potentially used in the future for chemoprevention of BQ chewing related oral mucosal lesions.

Metal-independent production of hydroxyl radicals by halogenated quinones and hydrogen peroxide: an ESR spin trapping study

The metal-independent production of hydroxyl radicals (*OH) from H(2)O(2) and tetrachloro-1,4-benzoquinone (TCBQ), a carcinogenic metabolite of the widely used wood-preservative pentachlorophenol, was studied by electron spin resonance methods. When incubated with the spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO), TCBQ and H(2)O(2) produced the DMPO/*OH adduct. The formation of DMPO/*OH was markedly inhibited by the *OH scavenging agents dimethyl sulfoxide (DMSO), ethanol, formate, and azide, with the concomitant formation of the characteristic DMPO spin trapping adducts with *CH(3), *CH(CH(3))OH, *COO(-), and *N(3), respectively. The formation of DMPO/*OH and DMPO/*CH(3) from TCBQ and H(2)O(2) in the absence and presence, respectively, of DMSO was inhibited by the trihydroxamate compound desferrioxamine, accompanied by the formation of the desferrioxamine-nitroxide radical. In contrast, DMPO/*OH and DMPO/*CH(3) formation from TCBQ and H(2)O(2) was not affected by the nonhydroxamate iron chelators bathophenanthroline disulfonate, ferrozine, and ferene, as well as the copper-specific chelator bathocuproine disulfonate. A comparative study with ferrous iron and H(2)O(2), the classic Fenton system, strongly supports our conclusion that *OH is produced by TCBQ and H(2)O(2) through a metal-independent mechanism. Metal-independent production of *OH from H(2)O(2) was also observed with several other halogenated quinones.

Differences in genotoxicity of H(2)O(2) and tetrachlorohydroquinone in human fibroblasts

During autoxidation of the pentachlorophenol (PCP) metabolite tetrachlorohydroquinone (TCHQ) the semiquinone is formed as well as reactive oxygen species (ROS). It was examined if *OH or the semiquinone are the cause of TCHQ-induced genotoxicity by direct comparison of TCHQ- and H(2)O(2)-induced DNA damage in human cells. All endpoints tested (DNA damage, DNA repair, and mutagenicity) revealed a greater genotoxic potential for TCHQ than for H(2)O(2). In the comet assay, TCHQ induced DNA damage at lower concentrations than H(2)O(2). The damaging rate by TCHQ (tail moment (tm)/concentration) was 10-fold greater than by H(2)O(2). DNA repair was lower for TCHQ than for H(2)O(2) treatment. This was shown by measuring DNA repair in the unscheduled DNA synthesis (UDS) assay and the persistence of the DNA damage in the comet assay. In contrast to H(2)O(2), TCHQ in non-toxic concentrations was mutagenic in the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus of V79 cells. Finally, there were also differences observed in cytotoxicity (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay) of TCHQ and H(2)O(2). Whereas the TCHQ cytotoxicity was enhanced during a 21h recovery phase, the H(2)O(2) cytotoxicity did not change. The results demonstrated that the pronounced genotoxic properties of TCHQ in human cells were not caused by *OH radicals but more likely by the tetrachlorosemiquinone (TCSQ) radical.

Oxidative stress and liver toxicity in rats and human hepatoma cell line induced by pentachlorophenol and its major metabolite tetrachlorohydroquinone

Pentachlorophenol (PCP) is a pesticide used worldwide in industrial and domestic applications. It is used extensively as biocide and wood preservatives. Metabolic studies carried out in rodents and human liver homogenates have indicated that PCP undergoes oxidative dechlorination to form tetrachlorohydroquinone (TCHQ). Free radical catalyzed tissue injury is thought to play a fundamental role in human disease. In the present study, we examined the effects of PCP and TCHQ on the induction of lipid peroxidation and liver injury in rats. In addition, the cytotoxic dose, cell death mechanisms and related gene expressions induced by PCP and TCHQ were also determined for human hepatoma cell line (Hep G2). The results indicated that more toxic effects could be observed both in rats and human hepatoma cell line treated with TCHQ than its parent compound, PCP. Oxygen species may be involved in the mechanism of TCHQ intoxication since the urinary 8-epi-PGF2alpha and AST, ALT activities can be induced by TCHQ and attenuated by vitamin E treatment. Apoptosis features were found in cells treated with TCHQ but not PCP. TCHQ-induced cell damage may issue signals for the induction of HSPs, the decrease of the bcl/bax protein ratio and the decrease of CAS gene, whereas the PCP-induced damage may not.

Characterization of metabolic activation of pentachlorophenol to quinones and semiquinones in rodent liver

Pentachlorophenol (PCP), a widely used biocide, induces liver tumors in mice but not in rats. Metabolic activation of PCP to chlorinated quinones and semiquinones in liver cytosol from Sprague-Dawley rats and B6C3F1 mice was investigated in vitro (1) with microsomes in the presence of either beta-nicotinamide adenine dinucleotide phosphate (NADPH) or cumene hydroperoxide (CHP), (2) with CHP in the absence of microsomes, and (3) with horseradish peroxidase (HRP) and H2O2. Mono-S- and multi-S-substituted adducts of tetrachloro-1,4-benzoquinone (Cl4-1,4-BQ) and Cl4-1,2-BQ and their corresponding semiquinones [i.e. tetrachloro-1,4-benzosemiquinone (Cl4-1,4-SQ) and tetrachloro-1,2-benzosemiquinone (Cl4-1,2-SQ)] were measured by gas chromatography-mass spectrometry (GC-MS). Qualitatively, the metabolites of PCP were the same in both rats and mice for all activation systems. Induction of PCP metabolism by either 3MC or PB-treated microsomes was observed in NADPH- but not in CHP-supported systems. In rats, the amount of induction was comparable with either 3MC or PB. 3MC was a stronger inducer than PB in mice and also induced a greater amount of metabolism than in rats. This suggests that induction of specific P450 isozymes may play a role in the toxicity of PCP to mice. Both HRP/H2O2 and CHP led to production of the full spectrum of chlorinated quinones and semiquinones, confirming the direct oxidation of PCP. CHP (with or without microsomes) converted PCP into much greater quantities of quinones and semiquinones than did microsomal P450/NADPH or HRP/H2O2 in both species. This implies that, under conditions of oxidative stress, endogenous lipid hydroperoxides may increase PCP metabolism sufficiently to enhance the toxicity and carcinogenicity of PCP.

Mechanism of the synergistic cytotoxicity between pentachlorophenol and copper-1,10-phenanthroline complex: the formation of a lipophilic ternary complex

When non- or sub-toxic levels of pentachlorophenol (PCP) and bis-(1, 10-phenanthroline)cupric complex, Cu(II)(OP)(2), were combined, a remarkable synergistic toxicity was observed as indicated by growth inhibition and bacterial inactivation. Similar synergistic cytotoxic effects were observed with other polychlorinated phenols and other positively charged cupric complexes. The synergism observed for these chemicals and similar reactive pairs of chemicals was found to be due to the formation of lipophilic ternary complexes which facilitated copper transport into the bacterial cells. The formation of ternary complexes of similar lipophilic character could be of relevance as a general mechanism of toxicity.