Generation of ROS by CYP. Cells generate ROS such as superoxide anion (O2.-) and H2O2 as a result of metabolism of xenobiotics by CYP. Both O2.- and H2O2 may be converted to the highly reactive hydroxyl radical (OH.-) by iron (Fe2+)-catalyzed Haber-Weiss and Fenton reactions. Many xenobiotics are converted to toxic quinones by CYP. These quinones are redox-sensitive agents and are reversibly reduced to semihydroquinones/hydroquinones, which generate O2.-.