Peroxisomes

Peroxisomes (microbodies) are cytoplasmic organelles involved in metab­olism of hydrogen peroxide, H₂O₂. The peroxisome is about 0.5 microns in diameter, and is surrounded by a membrane. They are widely distributed in most animal and plant cells. Although peroxisomes possess more than sixty proteins, their function requires at least one H₂O₂-generating en­zyme, flavinoxidase, and an H₂O₂-degrading enzyme, catalase, a peroxi­some. Peroxisomal proteins are synthesized on free polyribosomes and a peroxisomal targeting signal position enables them to be targeted to perox­isomes post-translationally.

Peroxisomes are major sites of oxygen consumption in the cell and par­ticipate in several metabolic functions that use oxygen. Oxygen consump­tion in the peroxisome leads to H₂O₂ production, which is then used to oxidize a variety of molecules. Important reactions in the peroxisome in­clude oxidation of long-chain and very long-chain fatty acids, metabolism of glyoxalate, degradation of uric acid, and synthesis of ether lipids and cho­lesterol, among others. Alcohol is detoxified to acetaldehyde in part by ac­tion of peroxisomes. In plants, peroxisomes perform photorespiration.

In humans, defects in peroxisome biogenesis lead to at least twelve per­oxisomal disorders, most of them lethal during childhood. In liver cells per­oxisomes are few in number, but several drugs that lower serum lipids, and many other chemicals designated as peroxisome proliferators, induce a pro­found increase in peroxisome number by activating a nuclear receptor called peroxisome proliferator-activated receptor (PPAR). Sustained activation of PPAR and induction of peroxisome proliferation in liver leads to the de­velopment of liver cancer in rats and mice.

References

Masters, Colin, and Denis Crane. The Peroxisome: A Vital Organelle. Cambridge: Cam­bridge University Press, 1995.