|MadSci Network: Biochemistry|
Dear Shadi, Hydrogen peroxide is made by quite a few enzymes in the body. In particular, some enzymes breaking down certain amino acids and fatty acids (D-amino acid oxidase and acyl-CoA oxidase) make significant amounts of hydrogen peroxide. Since hydrogen peroxide can be damaging to normal tissue, these enzymes are kept inside specialized organelles inside cells called peroxisomes. The peroxisomes also contain large amounts of catalase to break down the hydrogen peroxide before it can escape. Other enzymes that make significant amounts of hydrogen peroxide are plasma amine oxidase and xanthine oxidase. In addition to enzymes that produce hydrogen peroxide as part of their normal catalytic cycle, many enzymes that undergo oxidation and reduction make hydrogen peroxide and other reactive oxygen species by autoxidation (a kind of side-reaction that is not part of their catalytic cycle). This happens quite a bit in the mitochondria. Many of these autoxidation reactions do not produce hydrogen peroxide directly, but rather superoxide (the product of adding one electron to an oxygen molecule). In order to get rid of superoxide (which is more toxic than hydrogen peroxide) the body also contains lots of superoxide dismutase, that converts the superoxide into water and hydrogen peroxide. One of the most interesting sources of superoxide in the body is that produced by white blood cells when they encounter harmful microorganisms. The white blood cells produce very large amounts of superoxide, hydrogen peroxide, and even hypochlorous acid (household bleach!) to kill the germs, just like in the TV ads.
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