Biochemical Characteristics of Micro-bodies: Peroxisomes and Glyoxysomes (With Diagram)

The term “microbody” was used by cell biologists and cytologists for many years to describe a variety of different small cellular components.

More recently, it has usu­ally been restricted to organelles possessing flavin ox­idases and catalase.

Organelles possessing these ac­tivities are typically spherical or ovoid structures having a single bounding membrane, a diameter of about 0.5-1.5 μm, and containing an amorphous gran­ular matrix, occasionally with crystalloid inclusions (Fig. 19-6).

The organelles vary somewhat in struc­ture, appearance, and function from one tissue to an­other and from species to species. Certain micro- bodies exhibit specific biochemical characteristics as well as specific distributions among animal, plant, and microbial cells. Included here are the peroxisomes and glyoxysomes.

Peroxisomes:

The modern usage of the term “microbody” dates back to 1954 and the work of J. Rhodin, who described the structure and properties of these organelles in mouse kidney tissue. Since then, organelles of similar organization have been reported in many other animal tissues and also in plants (Fig. 19-6).

In 1965, de Duve showed that microbodies of rat liver contained a num­ber of oxidases that transfer hydrogen atoms to mo­lecular oxygen, thereby forming hydrogen peroxide (Fig. 19-7). de Duve coined the term peroxisomes for these organelles, although a true peroxidatic activity is generally demonstrable only in vitro. In vivo, condi­tions favor the removal (or degradation) of hydrogen peroxide by catalase rather than by a peroxidase.

However, because hydrogen peroxide is an intermedi­ate in the reaction, the term “peroxisome” may be ap­propriate. The chemical and enzymatic relationships between an oxidase, peroxidase, and catalase are shown in Figure 19-7.

A number of enzymes are characteristically present in peroxisomes, including uric acid oxidase (also called uricase), D-amino acid oxidase, acyl-CoA oxi­dase, polyamine oxidase, β-hyroxyacid oxidase, NADH-glyoxylate reductase, NADP-isocitrate dehy­drogenase, and catalase. When uric acid oxidase is present in large amounts, it frequently takes the form of a paracrystalline “nucleoid” at the center of the or­ganelle. The functions of peroxisomes in animal cells are diverse.

Peroxisomal catalase is thought to be in­volved in the degradation of H₂O₂, which is extremely toxic, the source of the peroxide being other peroxiso­mal reactions (e.g., those catalyzed by the flavin oxi­dases). Uric acid oxidase is important in the catabolic pathway that degrades purines. The early observation that there is an abundance of peroxisomes in cells en­gaged in lipid metabolism suggested that these organ­elles may be involved in lipid metabolism. Recently; it has been shown that liver peroxisomes contain a ma­jor system for the β-oxidation of fatty acids however; the enzymes are different from those of mitochondria although they produce the same end product, acetyl-CoA.

Electron-microscopic studies of tissue sections of­ten reveal a close proximity between peroxisomes and mitochondria. This is not surprising as the products of peroxisomal activity may serve as substrates for mito­chondrial activity. For example, glyoxylate produced in peroxisomes may be converted there to glycine by transamination. After passage to neighboring mito­chondria, the glycine may be further metabolized in a variety of ways including conversion to other amino acids or incorporation into heme.

One of the characteristic fea­tures of lysosomal enzyme activity is its latency. No latency is exhibited by the peroxisomal enzymes, as relatively large molecules (including the peroxisomal enzyme substrates) readily permeate the peroxisome membrane.

Isolation of Peroxisomes:

The sedimentation coeffi­cients and densities of peroxisomes in sucrose gradi­ents are close to those of lysosomes and account for the fact that for some time peroxisomal enzyme ac­tivities were ascribed to lysosomes. Density gradient centrifugation of the so-called “light mitochondrial fraction” prepared by preliminary differential centrif­ugation is the method of choice for isolating peroxiso­mes.

The greatest success in peroxisome purification is obtained if the lysosomes are first allowed to accu­mulate Triton WR-1339. Triton-loaded lysosomes are considerably less dense than normal lysosomes and so they are easily “floated” away from the peroxisomes during sucrose (Fig. 19-8).

Formation of Peroxisomes:

For many years, it was be­lieved that peroxisomes of both plant and animal tis­sues arose as outgrowths of the endoplasmic reticu­lum and that the peroxisomal enzymes were dispatched into the cisternae of the endoplasmic retic­ulum by attached ribosomes. The enzymes made their way into the organelle prior to its physical separation from the ER.

However, the growing tide of recent evi­dence appears to argue in favor of an alternative model for the biogenesis of peroxisomes. In liver and in other tissues as well, many of the peroxisomal en­zymes are synthesized by unattached ribosomes (i.e., ribosomes that are not bound to the ER) and are re­leased into the cytosol.

From there, the enzymes are slowly taken up by preexisting peroxisomes. Studies with peroxisomal catalase indicate that the subunits of the enzyme and the heme enter the microbody and are then assembled to form the functional enzyme. The nature of the mechanism that translocates perox­isomal proteins into peroxisomes is uncertain, but it is speculated that the proteins are first bound to a mem­brane receptor and are then drawn through the mem­brane and trapped inside as multiprotein complexes. Indeed, dense crystalline aggregates are often ob­served inside peroxisomes.

Translocation of proteins through the peroxisome membrane is readily demon­strable in vitro. When newly synthesized peroxisomal proteins are tagged with a radioactive label and incu­bated with isolated peroxisomes, some of the protein may soon be recovered from within the organelles. Peroxisomes often appear in clusters when exam­ined by electron microscopy and occasional dumbbell- shaped images are observed. This suggests that the “tails” seen on peroxisomes are connections to other peroxisomes. Indeed, either permanent or transient connections between peroxisomes might form a “peroxisome reticulum.”

Such interconnections would explain the remarkable biochemical homogene­ity of peroxisomes and the synchronous turnover of peroxisomal proteins. New peroxisomes may be formed either by the fission of preexisting peroxi­somes or by budding from the peroxisome reticulum.

Glyoxysomes:

In 1967, R. W. Breidenbach and H. Beevers discov­ered that microbodies of the fat-storing cells of germi­nating fatty seeds contain enzymes of the glyoxylate cycle in addition to peroxisomal en­zymes. They coined the specific term glyoxysomes for these particles, although it is to be understood that glyoxysomes are a form of peroxisome.

Glyoxysomes contain not only enzymes specific to the glyoxylate cy­cle (i.e., isocitrate lyase and malate synthetase) but they also contain several of the essential enzymes of the Krebs cycle, which therefore function simultaneously in both mitochondria and gly­oxysomes. The name “glyoxysome” takes precedence over “peroxisome” whenever glyoxylate cycle en­zymes are identified in the microbody.

The relationship between the Krebs cycle and the glyoxylate cycle is shown in Figure 19-9. Both cycles employ the same reactions to produce isocitrate from acetyl-CoA and oxaloacetate, but beyond this point the pathways differ. In the Krebs cycle, isocitrate is successively decarboxylated, producing two molecules of carbon dioxide and succinate. In the glyoxylate cy­cle, isocitrate is converted to succinate and glyoxy­late. Instead of being lost as two molecules of CO₂, the two-carbon glyoxylate condenses with another acetyl- CoA to form the four-carbon dicarboxylic acid malate.

The four carbon atoms of the two acetyl-CoA mole­cules are thus conserved as one four-carbon compound that, after conversion to succinate and migration to the mitochondrion, may be converted to oxaloacetate. The oxaloacetate may then be utilized in gluconeogenesis.

Oxaloacetate formed in mitochondria from glyoxysomal succinate is presumed to serve as a direct pre­cursor of phosphoenol pyruvate (PEP). The conver­sion of PEP to glucose and other carbohydrates occurs essentially by the reversal of the steps of glyco­lysis. Glyoxysome-containing tissues are thus able to convert simple two-carbon sources such as acetate into carbohydrate. In some tissues, such as fat-storing cells in seeds, the acetate is ob­tained through the degradation of fatty acids; therefore glyoxysomes participate in the con­version of fat to carbohydrate.

Distribution and Origin of Glyoxysomes:

The glyoxy­late cycle is especially significant for cells growing ex­clusively on acetate or fatty acids (e.g., a number of microorganisms), where the cycle acts as a source of four-carbon dicarboxylic acids. Certain microorgan­isms including Euglena, Chlorella, Neurospora, and Polytomella contain “glyoxysomelike” particles, but the term glyoxysome is normally reserved for the or­ganelles of fat-storing endosperm or cotyledons of germinating fatty seeds.

Reports have appeared in the literature from time to time indicating the DNA is present in glyoxysomes, raising the possibility that these organelles possess some degree of autonomy. However, this notion is not generally accepted.

Glyoxysomes appear to be formed by a process similar to that which gives rise to peroxisomes (see above), namely, by the fission or budding of preexisting glyoxysomes. Most (if not all) glyoxysomal enzymes are synthesized by free (i.e., unattached) ribosomes in the cytosol and are then translocated into or through the membranes of glyoxysomes al­ready present in the cell.

In concluding this discussion of micro-bodies, it is important to note that some organelles fitting the general microscopic description of micro-bodies do not clearly fit into either the peroxisome or glyoxysome category when evaluated in terms of their enzymatic activities.

It is entirely possible that micro-bodies may be associated with varying activities depending on the specialization of the cell and that micro-bodies exist whose actions and cellular functions remain to be determined. It is clear that one cannot name particles solely on the basis of microscopic characteristics and expect that all function identically.