Answer:
Free radicals are reactive oxygen intermediates that react with a variety of other molecules – inactivating proteins, damaging DNA, and most importantly, inducing lipid peroxidation in the membrane. Massive production of free radicals can occur after ischemia and subsequent reperfusion. In shock, the enzyme xanthine dehyrogenase is converted to xanthine oxidase which oxidizes purines with the formation of the highly toxic superoxide radical. When oxygen is reintroduced suddenly, large amounts of superoxide can be produced and the cell’s endogenous antioxidant defense systems can be overwhelmed.
Free radicals have been implicated in the aging process since the 1950’s. Superoxide and hydroxyl free radicals are thought to cause damage and eventual aging through reactions with nucleic acid, protein and lipid components of cells. Protective enzymes (superoxide dismutase, catalase) eliminate toxic oxygen intermediates.