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<p><strong>Introduction </strong></p> <p> <br>The concept that peroxisomes constitute a unique member of the organelle family took some time to mature. The first enzymes found in mammalian peroxisomes were H<sub>2</sub>O<sub>2</sub>-producing peroxidases, hence the name peroxisomes (De Duve, 1996<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF3"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>), and catalase. However, similar organelles from other organisms sometimes show remarkably different specializations and were classified under different names. The so-called microbody family also includes glyoxysomes in plants (which contain the glyoxylate cycle enzymes to convert lipid into carbohydrate) and glycosomes in trypanosomatids (which contain part of the glycolysis pathway). The variation in enzymatic content depends not only on the species but also on the cell type and environmental conditions. A feature common to members of the microbody family is the ability to degrade fatty acids (reviewed by Van den Bosch et al., 1992<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF45"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>; Purdue and Lazarow, 2001<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF35"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>; Eckert and Erdmann, 2003<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF4"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>). Moreover, enzymes imported into microbodies share similar trafficking signals: the peroxisomal targeting signals PTS1 and PTS2 (Subramani, 1998<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF41"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>). </p> <p>In the 1990s, studies began to examine peroxisome formation and maintenance by using genetic approaches. Model organisms including certain fungi and mammalian (Chinese hamster ovary) cells were screened for mutants that display defects in biogenesis, such as partial or complete loss of peroxisomes. Particularly in fungi, such screens are facilitated by the fact that the need for peroxisome function depends on the external conditions. For instance, <em>Saccharomyces cerevisiae</em> grown on glucose can dispense with peroxisomes; indeed, only when offered a fatty acid as the sole carbon source are peroxisomes required for growth, because they are the exclusive site for fatty acid degradation in this organism (Erdmann et al., 1989<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF6"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>). </p> <p>The combined efforts of several groups have identified some 32 <em>PEX</em> genes that contribute to biogenesis or maintenance of these organelles. Some of the proteins produced have a role in protein import. The functions of others can only be guessed at on the basis of their location within the membrane or matrix, their interactions with other Pex proteins, or the phenotypes that result from their deletion and/or overexpression. </p> <p>These studies provided a framework to understand an enigma presented by several peroxisome-related diseases: the peroxisome biogenesis disorders (PBDs) (Gould and Valle, 2000<a href="http://www.biology-online.org/articles/references-the_return_of_the_peroxisome.htm#REF10"><img alt="Go" src="http://jcs.biologists.org/icons/fig-down.gif"></a>). A defect in a gene can of course lead to a single enzyme deficiency, but in some extraordinary cases the complete peroxisome population disappears from the cell. We now understand the cause of such phenotypes: when the affected protein is involved in peroxisome biogenesis or maintenance, a severe pleiotropic effect is the result. Lack of protein import, for example, affects the targeting of up to 100 enzymes, which consequently remain in the cytosol, where they cannot function or are degraded. </p> <p><a></a><br></p>
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