Cytotoxicity of Air Pollutant 9,10-Phenanthrenequinone: Role of Reactive Oxygen Species and Redox Signaling

Figure 2

Redox cycling of 9,10-phenanthrenequinone (9,10-PQ) for production of reactive oxygen species (ROS). First, 9,10-PQ quinone undergoes 1 electron reduction to produce semiquinone radical. Semiquinone is unstable and is very reactive to oxygen. It reoxidizes back to quinone and releases superoxide radical. Superoxide molecule is then reduced to hydrogen peroxide by the superoxide dismutase. Hydroxyl radicals are formed when hydrogen peroxide reacts with metals such as ferrous ions via Fenton reaction. Second, semiquinone can also undergo further 1 electron reduction to form 9,10-phenanthrene hydroquinone which can further redox cycles back to semiquinone by losing an electron and yielding superoxide radical. Lastly, 2-electron reduction of 9,10-PQ quinone by NADPH-quinone oxidoreductase-1 (NQO1) will directly form 9,10-phenanthrene hydroquinone. Thus, redox cycle of 9,10-PQ will generate a large amount of ROS.