Antioxidant systems

Production of free radicals and hydrogen peroxide is a normal aspect of cellular metabolism, and intracellular enzymes usually eliminate the most toxic products. The prototypes of these antioxidant enzymes are metalloproteins termed superoxide dismutases that catabolize superoxide by extremely rapid (rate constant of 10 9 l/mol/s) conversion to hydrogen peroxide. Catalase (found in peroxisomes and mitochondria) and glutathione peroxidase (found in cytosol and mitochondria) sequentially degrade hydrogen peroxide to water. Glutathione peroxidase removes the products of lipid peroxidation and hydrogen peroxide. Two related cytoplasmic enzymes, glutathione reductase (NADPH dependent) and glucose-6-phosphate dehydrogenase (NADPH producing) participate in antioxidant defense by generating intracellular reduced glutathione. Glutathione is the primary non-protein sulfhydryl intracellular antioxidant. Other non-enzymatic antioxidants, including vitamin A, a-tocopherol, ascorbate, cysteine, ceruloplasmin, urate, and hemoglobin, are present within cells and extracellular fluid. -NO also functions as an antioxidant by quenching lipid peroxidation chain reactions.

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