The critical role of superoxide anion radicals on delaying tetrachlorohydroquinone autooxidation by penicillamine.

We have recently found that penicillamine, a classic copper-chelating thiol-drug for Wilson's disease, can delay tetrachlorohydroquinone (TCHQ) autooxidation via a previously unrecognized redox-activity. However, its underlying molecular mechanism remains not fully understood. In this study, we found, interestingly and unexpectedly, that superoxide dismutase (SOD) can significantly shorten the delay of TCHQ autooxidation by penicillamine, but not by ascorbate; SOD can also markedly increase the yields of the oxidized form of penicillamine. Similar effects were observed with a recently-developed specific and sensitive superoxide anion radical (O2•-) probe CT-02H, which was also employed to successfully measure O2•- generated from both TCHQ and TCHQ/penicillamine systems for the first time. More importantly, addition of extra O2•- (KO2/18-crown-6) can further prolong the delaying effects by penicillamine and slow down penicillamine consumption. Taken together, an unexpected critical role of O2•- in TCHQ/penicillamine interaction was proposed: O2•- may regenerate penicillamine, thereby continuously reducing TCSQ•- to TCHQ and finally delaying TCHQ autooxidation; In contrast, if O2•- were eliminated, which can not only markedly change the reaction equilibrium, accelerate the rate of interaction, and ultimately shorten the delay of TCHQ autooxidation by penicillamine, but can also accelerate penicillamine oxidation to form its corresponding disulfide solely via redox reaction without any minor nucleophilic reaction. These findings not only further support our previously-proposed redox mechanism for the protection against TCHQ-induced cytotoxicity by penicillamine, but also reveal a new mode of action for O2•- in the inhibition of haloquinoids-induced toxicity by thiol antioxidants.