An approach to evaluate two-electron reduction of 9,10-phenanthraquinone and redox activity of the hydroquinone associated with oxidative stress

Quinones are widely used as medicines or redox agents. The chemical properties are based on the reactions against an electron donor. 9,10-Phenanthraquinone (PQ), which is a quinone contaminated in airborne particulate matters, forms redox cycling, not Michael addition, with electron donors. Redox cycling of PQ contributes to its toxicity, following generation of reactive oxygen species (ROS). Detoxification of quinones is generally thought to be two-electron reduction forming hydroquinones. However, a hydroquinone of PQ, 9,10-dihydroxyphenanthrene (PQH₂), has been never detected itself, because it is quite unstable. In this paper, we succeeded in detecting PQH₂ as its stable derivative, 9,10-diacetoxyphenanthrene (DAP). However, higher concentrations of PQ (> 4  μM) form disproportionately with PQH₂, producing the 9,10-phenanthraquinone radical (PQ^{{radical dot}-}) which is a one-electron reducing product of PQ. In cellular experiments using DAP as a precursor of PQH₂, it was shown that PQH₂ plays a critical role in the oxidative protein damage and cellular toxicity of PQ, showing that two-electron reduction of PQ can also initiate redox cycling to cause oxidative stress-dependent cytotoxicity.