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

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.

Protein adducts and crosslinking by reactive metabolites of polychlorinated biphenyls (PCBs)

“…The derivatives contained one or two methylthioether groups indicating PCB3-pQ formed covalent bonds with one or two GSH through sulfhydryl groups (shown in Figure 2-1). The same results were also shown in previous research for incubation of GSH with PCB quinoid metabolites (Song et al 2009, Wangpradit et al 2015. Derivatives with two methylthiol groups were also found in microsomal incubation samples with HRP (Figure 2-3).…”

“…Protein or DNA Adducts from PCB Metabolites These arene oxide and quinoid metabolites of PCBs were considered as highly reactive (Ludewig et al 2008). Though semiquinone radicals in the quinone/semiquinone/hydroquinone triad were considered reactive to form covalent binding with DNA or proteins, recent research found the PCB-derived semiquinone free radicals do not form covalent adducts with DNA, GSH, and amino acids (Wangpradit et al 2015). The reactive metabolites, the arene oxide and quinoid metabolites of PCBs, can react towards nucleophiles on DNAs and proteins.…”

Free radical induced activity of an anthracycline analogue and its MnII complex on biological targets through in situ electrochemical generation of semiquinone

“…Interestingly, the data shown in Figure 8 has the same trend as that of interaction of species (generated electrochemically) with the four nucleobases. It was reported earlier DNA and oligonucleotides tend to stabilize semiquinone radical anions through delocalization of electrons in a π-stacking framework, that result in a radical intercalated situation rather than forming covalent bonds [ 43 ]. This was further supported by some semi-empirical calculations showing there is a gain in energy by ~ 9.8 kcal mol −1 [ 43 ].…”

“…With increase in percentage of water, two reduction peaks approach each other and a single step two electron reduction occurs [ 56 , 57 ]. Quinone-dianion and free quinone upon comproportionation form semiquinone radical anion that undergoes disproportionation as well [ 28 , 33 , 41 , 42 , 43 , 58 , 59 , 60 ]. This was again verified as a part of this study.…”

“…We made attempts to explore aspects related to free radical pathways of some drugs or their analogues when they are either on their own or complexed with metal ions [ 38 , 39 , 40 ]. For anthracyclines, accumulating evidence suggest there is in vivo formation of semiquinone either due to one-electron reduction or by comproportionation when they undergo two-electron reduction to form quinone-dianion [ 28 , 33 , 41 , 42 , 43 ]. The reactive intermediates that are formed, damage DNA, serving as important signal-transduction networks either promoting cell cycle arrest or causing cell death in order to repair DNA lesions [ 44 , 45 , 46 , 47 ].…”