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

The hydroxyl radical ((OH)-O-center dot) has been considered to be one of the most reactive oxygen species produced in biological systems. It has been shown that (OH)-O-center dot can cause DNA, protein, and lipid oxidation. One of the most widely accepted mechanisms for (OH)-O-center dot 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)-O-center dot 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(center dot)), is essential for (OH)-O-center dot 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)-O-center dot 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)-O-center dot. These findings represent a novel mechanism of (OH)-O-center dot 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.