Potential Mechanism for Pentachlorophenol-Induced Carcinogenicity: A Novel Mechanism for Metal-Independent Production of Hydroxyl Radicals

Abstract

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 H₂O₂ 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 H₂O₂ was identified to be trichloro-hydroxy-1,4-benzoquinone (TrCBQ-OH), and H₂O₂ 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 H₂O₂ is not through a semiquinone-dependent organic Fenton reaction but rather through the following novel mechanism: a nucleophilic attack of H₂O₂ 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.