<prompt> The following are major free radical scavengers except:Superoxide dismutaseCatalaseGlutathioneGlutamine  <response>: Ans. D. Glutamine. Ref. Harper Biochemistry 28th pg. 483; Robbins 8thpg. 20, 21; Table 1-3; Fig. 1-20A series of enzymes acts as free radical-scavenging systems and breaks down H202 include the following: Superoxide dismutases SODs --- Option A--- are found in many cell types and convert oxygen free radical to H2O2. Catalase, --- Option B--- present in peroxisomes, decomposes H2O2 2H2O2 ? 02 2H2O. Glutathione peroxidase--- Option C--- also protects against injury by catalyzing free radical breakdownH2O2 2GSH? GSSG 2H2O, or 2OH 2GSH ? GSSG 2H2O. The intracellular ratio of oxidized glutathione GSSG to reduced glutathione GSH is a reflection of the oxidative state of the cell and is an important indicator of the cells ability to detoxify ROS.REMOVAL OF FREE RADICALS:The principle free radicals reactive Oxygen species involved in cell injury are: Superoxide dismutase SOD:- in mitochondria- Converts O2 superoxide a H2O2. Catalase- In peroxisomes- Converts H2O2 a H2O O2. Glutathione Peroxidase- in mitochondria.- Converts OH hydroxyl radical a H2O2 a H2O O2. Peroxiredoxins- Cause conversion to HNO2.- In cytosol and mitochondria.Antioxidants either block the initiation of free radical formation or inactivate e.g., scavenge free radicals. Examples: Lipid-soluble vitamins E and A as well as Vit C and glutathione in the cytosol. Iron and copper can catalyze the formation of ROS. The levels of these reactive metals are minimized by binding of the ions to storage and transport proteins e.g., transferrin, ferritin, lactoferrin, and ceruloplasmin, thereby minimizing the formation of ROS.GENERATION OF FREE RADICALS:Inflammation, radiation, O2 toxicity, chemicals and reperfusion injury results in free radical formation, wherein following are mainly involved: NADPH oxidase, Oxidases in peroxisomes Superoxide dismutase SOD, NO synthase and Generation from H2O by hydrolysis .Free radicals may be generated within cells in several ways The reduction-oxidation reactions that occur during normal metabolic processes. Absorption of radiant energy e.g., ultraviolet light, x-rays. Rapid bursts of ROS are produced in activated leukocytes during inflammation. This occurs by a precisely controlled reaction in a plasma membrane multiprotein complex that uses NADPH oxidase for the redox reaction i.e. oxidative burst in phagocytes. Enzymatic metabolism of exogenous chemicals or drugs Transition metals such as iron and copper Nitric oxide NO, an important chemical mediator generated by endothelial cells, macrophages, neurons, and other cell types, can act as a free radical and can also be converted to highly reactive peroxynitrite anion ONOO- as well as NO2 and NO3. 