Peroxisomes are a heterogeneous group of small, spherical organelles with a single membrane and a diameter that ranges from approximately 0.15 to 0.5 |j.m (Figure 1-1-7).

Peroxisomes contain a number of enzymes diat transfer hydrogen atoms from organic substrates (urate, D-amino acids, and very long chain fatty acids) to molecular oxygen with the formation of hydrogen peroxide. Catalase, the major peroxisomal protein, degrades the hydrogen peroxide to water and oxygen.

Peroxisomal enzymes are synthesized on free polysomes. After translation, the enzymes are incorporated directly into peroxisomes.

Peroxisomes have several functions:

■ Synthesis and degradation of hydrogen peroxide

■ P-Oxidation of very long chain fatty acids (>C24) starts in the peroxisome and proceeds until the carbon chain has been reduced to a length of approximately 10 carbons. Oxidation of the residual 10 carbons is completed in the mitochondria.

■ Phospholipid exchange—-peroxisomes contain enzymes that convert phosphatidylser-ine and phosphatidylethanolamine.

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