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Environmental Science and Pollution Research

, Volume 23, Issue 3, pp 2138–2147 | Cite as

Breaking the dogma: PCB-derived semiquinone free radicals do not form covalent adducts with DNA, GSH, and amino acids

  • Orarat Wangpradit
  • Asif Rahaman
  • S. V. Santhana Mariappan
  • Garry R. Buettner
  • Larry W. Robertson
  • Gregor Luthe
PCBs: Exposures, Effects, Remediation and Regulation with special reference to PCBs in Schools

Abstract

Covalent bond formations of free radical metabolites with biomolecules like DNA and proteins are thought to constitute a major mechanism of toxicity and carcinogenesis. Glutathione (GSH) is generally accepted as a radical scavenger protecting the cell. In the present study, we investigated a semiquinone radical (SQ●-) metabolite of the semivolatile 4-chlorobiphenyl, using electron paramagnetic resonance spectroscopy, and oxygen consumption. Proton nuclear magnetic resonance (1H NMR) and liquid chromatography–mass spectrometry (LC-MS) were also employed to elucidate the radical interaction with DNA, amino acids, and GSH. We found that DNA and oligonucleotides stabilized SQ●- by electron delocalization in the π-stacking system, resulting in persistent radical intercalated, rather than forming a covalent bond with SQ●-. This finding was strongly supported by the semiempirical calculation of the semioccupied molecular orbital and the linear combination of the atomic orbitals, indicating 9.8 kcal mol−1 energy gain. The insertion of SQ●- into the DNA strand may result in DNA strand breaks and interruption of DNA replication process or even activate radical mediated secondary reactions. The presence of amino acids resulted in a decrease of the electron paramagnetic resonance (EPR) signal of SQ●- and correlated with their isoelectric points. The pH shifts the equilibrium of the dianions of hydroquinone and influenced indirectly the formation of SQ●-. Similar findings were observed with GSH and Cys. GSH and Cys functioned as indirect radical scavengers; their activities depend on their chemical equilibria with the corresponding quinones, and their further reaction via Michael addition. The generally accepted role of GSH as radical scavenger in biological systems should be reconsidered based upon these findings, questioning the generally accepted view of radical interaction of semiquinones with biologically active compounds, like DNA, amino acids, proteins, and radical scavengers like GSH.

Keywords

Electron paramagnetic resonance Glutathione Prostaglandin H synthase-2 Polychlorinated biphenyls Reactive oxygen species Semiquinone free radicals Carcinogenesis DNA 

Abbreviations

4-CB-2’,5’-Q

4-Chlorobiphenyl-2’,5’-benzoquinone

4-CB-2’,5’-H2Q

4-Chlorobiphenyl-2’,5’-hydroquinone

4-CB-2’,5’-SQ•–

4-Chlorobiphenyl-2’,5’-semiquinone radical

AA

Arachidonic acid

DMSO

Dimethyl sulfoxide

EPR

Electron paramagnetic resonance

GSH

Glutathione, reduced form

H2Q

Hydroquinones

hPGHS-2

Human recombinant prostaglandin H synthase-2

KAA

Potassium arachidonate

LCAO

Linear combination of the atomic orbitals

PCBs

Polychlorinated biphenyls

Q

Quinones

ROS

Reactive oxygen species

SOMO

Semioccupied molecular orbital

SQ•–

Semiquinone free radicals

Notes

Acknowledgments

We would like to thank Brett A. Wagner and Joost van’t Erve, the University of Iowa, for their assistance with EPR and oxygraph measurements. The University of Iowa EPR Facility provided invaluable support. We thank Dr. Lynn M. Teesch, and the high-resolution mass spectrometry facility for HPLC-MS support. This publication was made possible by NIH grant P42 ES 013661 and its training core from the National Institute of Environmental Health Sciences (NIEHS), and by The University of Iowa Environmental Health Sciences Research Center, P30 ES 05605. The project was supported by the Tech For Future fund, an initiative of the Saxion and Windesheim Universities of Applied Sciences and the regional government Overijsel, The Netherlands. The contents of this article are solely the responsibility of the authors and do not necessarily represent the official views of the funding agencies.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • Orarat Wangpradit
    • 1
    • 2
    • 7
  • Asif Rahaman
    • 3
  • S. V. Santhana Mariappan
    • 4
  • Garry R. Buettner
    • 1
    • 5
  • Larry W. Robertson
    • 1
    • 2
  • Gregor Luthe
    • 1
    • 6
    • 8
  1. 1.Interdisciplinary Graduate Program in Human ToxicologyThe University of IowaIowaUSA
  2. 2.Department of Occupational and Environmental HealthThe University of IowaIowaUSA
  3. 3.Department of ChemistryCity College of New YorkNew YorkUSA
  4. 4.Central High-Field NMR Research Facility, Department of ChemistryThe University of IowaIowaUSA
  5. 5.Free Radical and Radiation Biology ProgramThe University of IowaIowaUSA
  6. 6.School of Life Science, Engineering & DesignSaxion University of Applied SciencesEnschedeNetherlands
  7. 7.Sirindhorn College of Public HealthChonburiThailand
  8. 8.Luthe-PharmaFabrikstrasse 3Gronau

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