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Polychlorinated biphenyls (PCBs) exert a broad range of toxicity via both parent compounds and their metabolites. Our previous study showed that hydroquinone (H2Q) metabolites of PCBs act as cosubstrates for prostaglandin H synthase (PGHS), and are oxidized by this enzyme to corresponding quinones (Q). The goal of this thesis is to illuminate the PGHS-mediated toxicity of lower chlorinated PCBs. It is hypothesized that PGHS catalyzes two sequential one-electron oxidations of PCB-H2Q to semiquinone (SQ), and Q that interact with biomolecules, such as amino acids, glutathione (GSH), protein, and DNA. In addition, the oxidation of H2Q by PGHS results in an elevation of downstream prostaglandin (PG) production in vivo.
Employing 4-chlorobiphenyl-2f,5f-hydroquinone (4-CB-2f,5f-H2Q) as a model compound, I found that PGHS-2 catalyzes the one-electron oxidation of 4-CB-2f,5f-H2Q to SQ. An unusual electronically distorted SQ spectrum was observed as a result of the mixture of two different SQ species, a quartet and a doublet.
Fate of 4-CB-2f,5f-SQ and/or Q in the presence of biomolecules was further investigated in the next study. 4-CB-2f,5f-SQ/Q reacts readily with the thiol-containing molecules, such as cysteine, and GSH. Oligonucleotides, and DNA did not form a covalent adduct with 4-CB-2f,5f-SQ but preferably stabilized 4-CB-2f,5f-SQ by pi-stacking interaction under the assay conditions.
The in vivo study of downstream PG production in rats treated with 4-CB-2f,5f-H2Q revealed that PGE2 was significantly elevated in ratsf kidneys at 24 h post intratracheal instillation. The increased PGE2 production was correlated with an elevation of alveolar macrophages. These findings suggest two possible mechanisms of enhanced PGE2 production: i) 4-CB-2f,5f-H2Q as a cosubstrate for PGHS in kidney, and 2) release of cytokines from macrophages, leading to stimulation of PGE2 production in other tissues but released and accumulated in kidney for excretion.
In summary, the toxicity of lower chlorinated PCBs metabolites is potentially mediated by PGHS. Quinones generated from the PGHS metabolic pathway covalently bind to GSH resulting in GSH depletion, and oxidative stress. The intercalation or pi-stacking of SQ in DNA may be implicated in genotoxicity as a result of the change in DNA structure.