Membrane microbody

Microbodies (97-101) are spherical organelles (0.1-2.0 pm in diameter) bounded by a single membrane. They possess a granular interior and sometimes crystalline protein body. A specialized type of microbody is the glyoxysome (0.5-1.5 pm) containing enzymes ofthe glyoxy-late cycle. Glyoxysomes are found in the endosperm or cotyledons of oily or fatty seeds. 

A). The envelope was often observed as a single entity (Fig. 7, 8S) and on occasions was not present at all (Fig. 7, arrow). In these moderately damaged cells, electron-dense accumulations also were often observed in association with the bounding envelope of the mitochondria (Fig. U, arrows). Other membranes in these cells such as the tonoplast, microbody membrane, and plasmalemma did not appear damaged, although mild-to-severe plasmolysis was not uncommon. 

The geometry of cell construction provides another important aspect of metabolic control. In a bacterium, the periplasmic space (Fig. 8-28) provides a compartment that is separate from the cytosol. Some enzymes are localized in this space and do not mix with those within the cell. Other enzymes are fixed within or attached to the membrane. Eukaryotic cells have more compartments nuclei, mitochondria (containing both matrix and intermembrane spaces), lysosomes, microbodies, plastids, and vacuoles. Within the cytosol the tubules and vesicles of the endoplasmic reticulum (ER) separate off other membrane-bounded compartments. The rate of transport of metabolites through the membranes between compartments is limited and often is controlled tightly. 

Peroxisomes or microbodies are spherical organelles that are 0.3-1.5 pm in diameter. Each peroxisome is enveloped by a single external membrane, and its interior is full of proteins, frequently in crystalline form. Peroxisomes are characterized by the presence of various oxidative enzymes, which have variable functions dependent upon the origin of the peroxisome. These enzymes generate and utilize hydrogen peroxide (H2O2), hence the name peroxisome. This compound is very toxic for cells and is decomposed by the enzyme catalase to water and oxygen. 

Roise, D and Maduke, M. (1994) Import of a Mitochondrial Presequence into P. Denitrificans, FEBS Letters, 337, 9-13 Cavalier-Smith, T. (1987) The Simultaneous Symbiotic Origin of Mitochondria, Chloroplasts and Microbodies, Annals of the New York Academy of Science, 503, 55-71 Cavalier-Smith, T. (1992) The Number of Symbiotic Origins of Organelles, BioSystems, 28, 91-106 Hartl, F Ostermann, J., Guiard, B and Neupert, W. (1987) Successive Translocation into and out of the Mitochondrial Matrix Targeting of Proteins to the Inner Membrane Space by a Bipartite Signal Peptide, Cell, 51,1027-1037. 

Glycosomes are membrane-bound microbody like intracellular organelles, which contain all the enzymes necessary for glycolysis, glycerol metabolism and fixation of CO2. The glycosomes also possess some enzymes associated with pyrimidine synthesis, purine salvage and ether-lipid biosynthesis [1,2]. 

The soluble isozyme is generally considered to take part in the cytoplasmic side of the malate shuttle, providing a means of transporting NADH equivalents, in the form of malate, across the mitochondrial membrane. The mitochondrial enzyme, in addition to its role in the other half of the malate shuttle, is also a necessary component of the tricarboxylic acid cycle. The microbody malate dehydrogenase found in some plants appears to function in the glyoxylate cycle (5) or possibly in photorespiration ( ). 

The enzymes involved in penicillin biosynthesis are distributed at different sites of the cell ACV-activity was found to be bound to vacuole membranes, IPN-synthase occurs dissolved in the cytoplasm and lAT-activity is microbody associated. 

Other membraneous structures which are either associated with or develop from the endoplasmic reticulum are (/) the Golgi bodies, which may be associated with cell wall synthesis, (//) lipid droplets (sphaerosomes) and (Hi) microbodies containing catalase. 

Triacylglycerols are hydrolysed by lipolytic enzymes (lipases) in a step-wise fashion ultimately to yield glycerol and long chain fatty acids. In S. cerevisiae, lipase action is located in the plasma membrane [70]. Phospholipid hydrolysis is catalysed by phospholipases yielding glycerol and fatty acids, as well as products such as choline etc. Glycerol may be phosphorylated, oxidized and enter the EMP pathway as dihydroxyacetone phosphate. The long chain fatty acids are presumed to be converted to acetyl CoA by the p-oxidation pathway (Fig. 17.17). This pathway may be located either within the mitochondria or microbodies. 

Han liver and described as microbodies (Hruban and Rechcigl, 1969). Subsequently they were isolated on sucrose gradients and intensively studied by de Duve and his colleagues (de Duve and Baudhuin, 1966). Of particular interest was that the microbodies had been shown to be bound by a single membrane they were 0.5-1 /am in diameter, and in sucrose gradients they achieved an equilibrium density of 1.25 g/cm . 

The tridimensional structure of the endoplasmic reticulum remains one of the most intriguing problems of biology extensive serial section studies must be done for it to be solved. In discussing the tridimensional structure of the endoplasmic reticulum, we have pointed out its possible connection with the nuclear membrane and the cell membrane however, the problem of its relationship with other cell structures is not restricted to the membrane. Connections between the endoplasmic reticulum and the Golgi apparatus and microbodies have also been described. 

Microbodies cannot be seen with the light microscope on electron micrographs they appear like small round bodies (5 p in diameter in liver) with a single membrane surrounding a homogeneous electron-dense mass. 

These data suggest that P-enolpyruvate carboxylase may not be associated with chloroplasts, and that it is not localized in the same subcellular compartment with NADP-triose phosphate dehydrogenase. Linear, nonaqueous gradient (CCl4-hexane) purification of chloroplasts indicate a particulate, but not a chloroplast localization of PEP carboxylase (Fig. 4.4). We conclude from these data that P-enolpyruvate carboxylase may be membrane-associated in situ, but the majority of the activity is not associated with chloroplasts, mitochondria, or microbodies. This conclusion is completely consistent with the metabolic compartmentation data. 

Lysosomes and microbodies are single-membrane organelles each of which possess a discrete series of enzymes. Lysosomes (Figure 9.2) contain a wide variety of hydrolases which, at acidic pH, collectively are capable of degrading most classes of biological molecules. To protect the cell from autolysis, enzymic activation involves the metabolic acidification of the lysosomal milieu. Lysosomes feature significantly in endocytosis (Section 9.3). 

Mitochondria contain an outer membrane which has PC and PE as its main components while the inner membrane is the unique site of diphos-phatidylglycerol (DPG, cardiolipin) in plant cells. " Microbodies, such as those of potato or the glyoxysomes of castor bean, have a rather simple lipid pattern with PC, PE, and PG usually predominating. On the other hand, microsomal fractions, which are assumed to contain mainly endoplasmic reticulum, have a mixture of PC, PE, PG, and PI, with PC predominating. Some examples of the phospholipid compositions of different plant membranes are given in Table 3.3. 

Carnitine is not involved in the movement of fatty acids into microbodies. Either free fatty acids or acyl-CoAs appear to be able to cross the organelle s membrane. Moreover, because of the absence of an electron transport chain in microbodies there is no internal means of regenerating NAD". The reoxidation of NADH is thought to occur either by a glycerol phosphate shuttle or by movement of NADH to the cytosol and NAD back. 

---