Glutathione suppresses the autoxidation of 2',5'-dihydroxy-4-chlorobiphenyl

Abstract

Polychlorinated biphenyls (PCBs) are well known as classic environmental pollutants. Lower halogenated PCBs are mainly metabolized to mono- and di-hydroxyl compounds, and the latter further oxidized to quinones through a semiquinone intermediate. These quinone or semiquinone metabolites of PCBs can induce cytotoxicity by conjugating with functional protein and DNA as well as leading to oxidative stress in cells. Glutathione (GSH) is involved in lower chlorinated PCBs metabolites induced toxicities. Previous studies have shown that GSH reacts readily with PCB-quinones through Michael addition. The goal of this thesis was to investigate the effect of GSH on the reaction kinetics of PCB-hydroquinone autoxidation. Quinones produced during the autoxidation of hydroquinones serve as catalysts of the oxidation reaction. Therefore, it is hypothesized that GSH will suppress the autoxidation of PCB-hydroquinone and inhibit the formation of PCB-quinone.

One of the PCB-hydroquinones, 2',5'-dihydroxy-4-chlorobiphenyl (4-CB-2',5'-OH), was employed as a model compound. Methods were established with 1,4-hydroquinone (1,4-H₂Q). The reaction rate of 4-CB-2',5'-OH autoxidation showed pH dependency from my study. Superoxide dismutase (SOD) accelerated the reaction rate of 4-CB-2',5'-OH autoxidation dramatically at neutral pH.

I used SOD in the studies to explore the effect of GSH on the autoxidation of 4-CB-2',5'-OH at near-neutral pH. It was found that GSH suppressed the autoxidation of 4-CB-2',5'-OH by inhibiting the formation of corresponding 4-CB-2',5'-Q and 4-CB-2',5'-SQ*−. A mathematical model simulating the inhibiting effect of GSH on the autoxidation of 4-CB-2',5'-OH was constructed. The mathematical model reproduced my experimental data well, supporting my proposed mechanism for the autoxidation of the PCB-derived hydroquinones.

I also explored the reaction kinetics of 4-CB-2',5'-OH autoxidation in cell culture medium with fetal bovine serum (FBS). FBS showed a parallel inhibiting effect on 4-CB-2',5'-OH autoxidation as GSH.

In summary, GSH and other thiols present in FBS can suppress the autoxidation of lower halogenated PCB-hydroquinone and decrease the rate of PCB-quinone formation. This finding suggests an unrecognized role for GSH in the toxicity of lower chlorinated PCBs.