H2O2 and free radicals are produced as a result of certain oxidative reactions involved in the break down of amino acids and fats. These are very reactive chemical species that could damage cellular machinery. To protect the cell from these destructive byproducts, such reactions are segregated within these small membrane-bounded peroxisomes. Peroxisomes contain four enzymes related to the metabolism of H2O2. The enzyme urate oxidase and two other enzymes present in amphibian and avian peroxisomes are related to the catabolism of purines.
Free radicals are molecules that, because they contain an unpaired electron, are extremely reactive.
H2O2 also reacts with the superoxide radical to produce even more reactive free radicals. Most Cytoplasmic H2O2 is produced by mitochondria and membranes of the endoplasmic reticulum, although there are also H2O2-producing enzymes in the cytoplasmic matrix. The production of superoxide anion in mitochondria and cytosol is regulated mailnly by the enzyme superoxide dismutase. All of these protective enzymes are present in high levels in aerobic tissues. Such radicals acting on DNA could produce mutations, altering the transcrition into mRNA and the translation into proteins.