PEROXISOMAL STRUCTURE AND FUNCTIONS

General Aspects of Peroxisomes

Peroxisomes have the distinction of being the last true organelle discovered. They were first identified in renal proximal tubule cells by a Swedish graduate student in 1954. Initially called microbodies, these organelles were studied intensively by de Duve and coworkers. Because they contained enzymes that both produced (e.g., amino acid and urate oxidases) and degraded (e.g., catalase) hydrogen peroxide, de Duve and Baudhuin [1] proposed the name peroxisomes. Microbodies found in some lower organisms and plants were named for the specialized functions that they carry out. For example, glyoxysomes of fungi and plants contain the five enzymes of the glyoxylate cycle and glycosomes house the enzymes of glycolysis in trypanosomes [2, 3]. Peroxisomes have been found in essentially all plant and animal cells with the exception of mature erythrocytes, and they range in size from about 0.1 μm (microperoxisomes of intestine and brain) up to 1.0 μm (characteristic of hepatic and renal peroxisomes; range: 0.2–1.0 μm) [4] (Figure 35.1).

A single lipid bilayer comprises the peroxisomal membrane. The organelle's matrix is finely granular, but microcrystalline cores of urate oxidase are present in the hepatic peroxisomes of some species (e.g., rats). No cores are found in human peroxisomes because humans lack urate oxidase. Unlike chloroplasts and mitochondria, peroxisomes contain no DNA, although it has been speculated that all three organelles evolved from endosymbionts. Since discovery of peroxisomes, numerous membrane proteins and matrix enzymes have been identified.