Free Radical Injury and Oxidative Stress

Key points

• • Free radicals are unstable molecules with an unpaired electron; they are very reactive and grab electrons from other molecules, damaging them in a chain reaction.
• • Reactive oxygen species (ROS) include both free radicals such as superoxide and hydroxyl and non-radical molecules such as hydrogen peroxide, forming normally during energy production and in much larger amounts during inflammation.
• • They damage cells in three main ways: attacking the fats of cell membranes (lipid peroxidation), breaking DNA, and altering proteins.
• • The body defends itself with antioxidant enzymes (such as superoxide dismutase and catalase) and with antioxidant molecules such as vitamins C and E and glutathione.
• • Oxidative stress is the state where free radicals overwhelm these defences, and it contributes to ageing, cancer and the injury that follows restoring blood flow to a starved tissue (reperfusion injury).
• • Free radicals also have a useful role: white cells deliberately make them to kill microbes they have engulfed.