is a membrane-bound organelle that both generates and breaks down hydrogen peroxide. Peroxisomes exist in most eukaryotic cells, and are especially prominent in mammalian liver cells. Peroxisomes, about size of lysosomes (0.5-1.5 µm) contain oxidative enzymes, such as D-amino acid oxidase, ureate oxidase, & catalase. The enzymes & proteins destined for peroxisomes are synthesized in the cytosol. Each protein contains a peroxisomal targeting signal (PTS) that binds to a receptor molecule & takes the protein into the peroxisome. The membrane that surrounds a peroxisome is usually permeable, permitting many small molecules to enter easily.
Peroxisomes are distinguished by a crystalline structure inside a sac which also contains amorphous material. They are self replicating, like the mitochondria. Peroxisomes function to rid the body of toxic substances like hydrogen peroxide and are a major site of oxygen utilization. Peroxisomal enzymes remove hydrogen atoms from these small molecules and join the hydrogen to atoms of oxygen to form hydrogen peroxide. One of the peroxisomal enzymes, catalase, then neutralizes the hydrogen peroxide by catalyzing its breakdown into water and oxygen. This two-step process is the method that peroxisomes in the liver use to break down molecules of alcohol into substances that can be eliminated from the body. Some of the functions of the peroxisomes in the human liver include:
Breakdown (by oxidation) of excess fatty acids.
Breakdown of hydrogen peroxide (H2O2), a potentially dangerous product of fatty-acid
oxidation. It is catalysed by the enzyme catalase. [Link to further discussion]
Participates in the synthesis of cholesterol. One of the enzymes involved, HMG-CoA
reductase, is the target of the popular cholesterol-lowering "statins".
Participates in the synthesis of bile acids.
Participates in the synthesis of the lipids used to make myelin.
Breakdown of excess purines (AMP, GMP) to uric acid.
Peroxisomes are also present in plant cells where they participate is such functions as
symbiotic nitrogen fixation and photorespiration
Peroxisome & Lysosomes Knightley Images of the Peroxisome