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.

see also Alcohol and Health; C4 and CAM Plants; Lipids; Metabolism, Cellular; Organelle

Janardan Reddy

Bibliography

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