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.