Jie Shao

Molecular mechanism of metal-independent decomposition of lipid hydroperoxide 13-HPODE by halogenated quinoid carcinogens

Halogenated quinones are a class of carcinogenic intermediates and newly identified chlorination disinfection by-products in drinking water. 13-Hydroperoxy-9,11-octadecadienoic acid (13-HPODE) is the most extensively studied endogenous lipid hydroperoxide. Although it is well known that the decomposition of 13-HPODE can be catalyzed by transition metal ions, it is not clear whether halogenated quinones could enhance its decomposition independent of metal ions and, if so, what the unique characteristics and similarities are. Here we show that 2,5-dichloro-1,4-benzoquinone (DCBQ) could markedly enhance the decomposition of 13-HPODE and formation of reactive lipid alkyl radicals such as pentyl and 7-carboxyheptyl radicals, and the genotoxic 4-hydroxy-2-nonenal (HNE), through the complementary application of ESR spin trapping, HPLC-MS, and GC-MS methods. Interestingly, two chloroquinone-lipid alkoxyl conjugates were also detected and identified from the reaction between DCBQ and 13-HPODE. Analogous results were observed with other halogenated quinones. This represents the first report that halogenated quinoid carcinogens can enhance the decomposition of the endogenous lipid hydroperoxide 13-HPODE and formation of reactive lipid alkyl radicals and genotoxic HNE via a novel metal-independent nucleophilic substitution coupled with homolytic decomposition mechanism, which may partly explain their potential genotoxicity and carcinogenicity.

Mechanism of Intrinsic Chemiluminescence Production from the Degradation of Persistent Chlorinated Phenols by the Fenton System: A Structure–Activity Relationship Study and the Critical Role of Quinoid and Semiquinone Radical Intermediates

We found recently that intrinsic chemiluminescence (CL) could be produced by all 19 chlorophenolic persistent organic pollutants during environmentally friendly advanced oxidation processes. However, the underlying mechanism for the structure-activity relationship (SAR, i.e., the chemical structures and the CL generation) remains unclear. In this study, we found that, for all 19 chlorophenol congeners tested, the CL increased with an increasing number of chlorine atoms in general; and for chlorophenol isomers (such as the 6 trichlorophenols), the CL decreased in the order of meta- > ortho-/para-Cl-substituents with respect to the -OH group of chlorophenols. Further studies showed that not only chlorinated quinoid intermediates but also, more interestingly, chlorinated semiquinone radicals were produced during the degradation of trichlorophenols by the Fenton reagent; and the type and yield of which were determined by the directing effects, hydrogen bonding, and steric hindrance effect of the OH- and/or Cl-substitution groups. More importantly, a good correlation was observed between the formation of these quinoid intermediates and CL generation, which could fully explain the above SAR findings. This represents the first report on the structure-activity relationship study and the critical role of quinoid and semiquinone radical intermediates, which may have broad chemical and environmental implications for future studies on remediation of other halogenated persistent organic pollutants by advanced oxidation processes.

Mechanism of synergistic DNA damage induced by the hydroquinone metabolite of brominated phenolic environmental pollutants and Cu(II): Formation of DNA-Cu complex and site-specific production of hydroxyl radicals

ABSTRACT 2,6‐Dibromohydroquinone (2,6‐DBrHQ) has been identified as an reactive metabolite of many brominated phenolic environmental pollutants such as tetrabromobisphenol‐A (TBBPA), bromoxynil and 2,4,6‐tribromophenol, and was also found as one of disinfection byproducts in drinking water. In this study, we found that the combination of 2,6‐DBrHQ and Cu(II) together could induce synergistic DNA damage as measured by double strand breakage in plasmid DNA and 8‐oxo‐7,8‐dihydro‐2′‐deoxyguanosine (8‐oxodG) formation, while either of them alone has no effect. 2,6‐DBrHQ/Cu(II)‐induced DNA damage could be inhibited by the Cu(I)‐specific chelating agent bathocuproine disulfonate and catalase, but not by superoxide dismutase, nor by the typical hydroxyl radical (•OH) scavengers such as DMSO and mannitol. Interestingly, we found that Cu(II)/Cu(I) could be combined with DNA to form DNA‐Cu(II)/Cu(I) complex by complementary application of low temperature direct ESR, circular dichroism, cyclic voltammetry and oxygen consumption methods; and the highly reactive •OH were produced synergistically by DNA‐bound‐Cu(I) with H2O2 produced by the redox reactions between 2,6‐DBrHQ and Cu(II), which then immediately attack DNA in a site‐specific manner as demonstrated by both fluorescent method and by ESR spin‐trapping studies. Further DNA sequencing investigations provided more direct evidence that 2,6‐DBrHQ/Cu(II) caused preferential cleavage at guanine, thymine and cytosine residues. Based on these data, we proposed that the synergistic DNA damage induced by 2,6‐DBrHQ/Cu(II) might be due to the synergistic and site‐specific production of •OH near the binding site of copper and DNA. Our findings may have broad biological and environmental implications for future research on the carcinogenic polyhalogenated phenolic compounds. HIGHLIGHTSOH was found to be responsible for 2,6‐DBrHQ/Cu(II)‐induced DNA damage.First systematic investigation of the interactions between Cu(II)/Cu(I) and DNA.2,6‐DBrHQ/Cu(II) induce DNA damage via a site‐specific mechanism.Site‐specifically generated •OH attacks adjacent DNA sites, mainly at T, C and G.

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.

An Unusual Double Radical Homolysis Mechanism for the Unexpected Activation of the Aldoxime Nerve-Agent Antidotes by Polyhalogenated Quinoid Carcinogens under Normal Physiological Conditions

ABSTRACT We have recently shown that the pyridinium aldoximes, best‐known as therapeutic antidotes for chemical warfare nerve‐agents, could markedly detoxify the carcinogenic tetrachloro‐1,4‐benzoquinone (TCBQ) via an unusual double Beckmann fragmentation mechanism. However, it is still not clear why pralidoxime (2‐PAM) cannot provide full protection against TCBQ‐induced biological damages even when 2‐PAM was in excess. Here we show, unexpectedly, that TCBQ can also activate pralidoxime to generate a reactive iminyl radical intermediate in two‐consecutive steps, which was detected and unequivocally characterized by the complementary application of ESR spin‐trapping, HPLC/MS and nitrogen‐15 isotope‐labeling studies. The same iminyl radical was observed when TCBQ was substituted by other halogenated quinones. The end product of iminyl radical was isolated and identified as its corresponding reactive and toxic aldehyde. Based on these data, we proposed that the reaction of 2‐PAM and TCBQ might be through the following two competing pathways: a nucleophilic attack of 2‐PAM on TCBQ forms an unstable transient intermediate, which can decompose not only heterolytically to form 2‐CMP via double Beckmann fragmentation, but also homolytically leading to the formation of a reactive iminyl radical in double‐steps, which then via H abstraction and further hydrolyzation to form its corresponding more toxic aldehyde. Analogous radical homolysis mechanism was observed with other halogenated quinones and pyridinium aldoximes. This study represents the first detection and identification of reactive iminyl radical intermediates produced under normal physiological conditions, which provides direct experimental evidence to explain only the partial protection by 2‐PAM against TCBQ‐induced biological damages, and also the potential side‐toxic effects induced by 2‐PAM and other pyridinium aldoxime nerve‐agent antidotes. Graphic abstract Figure. No Caption available. HighlightsTCBQ activated 2‐PAM to generate reactive iminyl radicals in two‐consecutive steps.Analogous radical homolysis mechanism was observed with other haloquinones & aldoximes.This is the 1st detection of iminyl radical under normal physiological condition.The finding can explain partial protection by 2‐PAM against TCBQ‐induced damages.It may also explain some of the side‐toxic effects induced by pyridinium aldoximes.