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An intracellular organelle found in all eukaryotes except the archezoa (original lifeforms). In electron micrographs, peroxisomes appear round with a diameter of 0.1–1.0 micrometer, although there is evidence that in some mammalian tissues peroxisomes form an extensive reticulum (network). They contain more than 50 characterized enzymes and perform many biochemical functions, including detoxification. See Cell organization, Enzyme
Peroxisomes are important for lipid metabolism. In humans, the β-oxidation of fatty acids greater than 18 carbons in length occurs in peroxisomes. In yeast, all fatty acid β-oxidation occurs in peroxisomes. Peroxisomes contain the first two enzymes required for the synthesis of plasmalogens. Peroxisomes also play important roles in cholesterol and bile acid synthesis, purine and polyamine catabolism, and prostaglandin metabolism. In plants, peroxisomes are required for photorespiration. See Lipid metabolism, Photorespiration
A number of recessively inherited peroxisomal disorders have been described and grouped into three categories. Group I is the most severe and is characterized by a general loss of peroxisomal function. Many of the enzymes normally localized to the peroxisome are instead found in the cytosol. Among the diseases found in group I are Zellweger syndrome, neonatal adrenoleukodystrophy, and infantile Refsum disease. Patients with these disorders usually die within the first years after birth and exhibit neurological and hepatic (liver) dysfunction, along with craniofacial dysmorphism (malformation of the cranium and the face). Groups II and III peroxisomal disorders are characterized by a loss of peroxisomal function less severe than in group I.