Glyoxysomal membrane

There is a scarcity of data for the analyses of glyoxysomal membranes and plasma membranes of plant cells. The glyoxysomal membrane from castor bean (Ricinus communis L.) endosperm has PC as the major lipid together with PE, 80% of the phospholipid can be accounted for. Plasma membranes isolated from oat Avena saliva L.) roots showed relatively lai ge amounts of phosphatidic acid (PA), (1,2-diacyl-jn-glycerol 3-phosphate) and an unknown lipid (Keenan et al., 1973). It is possible that some lipid degradation took place in the preparation of this sample. Further analyses of the lipid composition would be most appropriate. The analyses of Keenan et al. (1973) indicated that only 28.6% of the plasma membrane lipid was phospho-... 

All membranes which have been examined so far show very different lipid (and protein) compositions for their two leaflets. A common feature of mammalian cell membranes is that the amino-phospholipids and phosphatidylinositol are concentrated in the cytosolic leaflet (Figure 6.13). In contrast, sphingolipids are highly concentrated in the external leaflet of the plasma membrane. In the bacterial example also shown in Figure 6.13 phosphatidylinositol is concentrated on the cytoplasmic face, but in the castor bean glyoxysomal membrane, whereas phosphatidylethanolamine was mainly in the outer (cytoplasmic) leaflet, the accessible phosphatidylinositol was mainly in the opposite leaflet. This latter example emphasizes two points - first that generalizations should not be made from system to system without firm evidence and, second, that the distribution calculated... 

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. 

In peroxisomes, membrane-enclosed organelles of animal and plant cells, the intermediates for /3 oxidation of fatty acids are coenzyme A derivatives, and the process consists of four steps, as in mitochondrial /3 oxidation (Fig. 17-13) (1) dehydrogenation, (2) addition of water to the resulting double bond, (3) oxidation of the /3-hydroxyacyl-CoA to a ketone, and (4) thiolytic cleavage by coenzyme A (The identical reactions also occur in glyoxysomes, as discussed below.)... 

Topic LI). Thus, animals cannot convert fatty acids into glucose. In contrast, plants have two additional enzymes, isocitrate lyase and malate synthase, that enable them to convert the carbon atoms of acetyl CoA into oxaloacetate. This is accomplished via the glyoxylate pathway, a route involving enzymes of both the mitochondrion and the glyoxysome, a specialized membranous plant organelle. 

Bunkelmann, J. R., and Trelease, R. N., 1996, Ascorbate peroxidase a prominent membrane protein in oilseed glyoxysomes. Plant Physiol. 110 5890598. 

Yamaguchi, K., Hori, H., and Nishimura, M., 1995, A novel isoenzyme of ascorbate peroxidase localized on glyoxysomal and leaf peroxisomal membranes in pumpkin. Plant Cell Physiol. 36 1157nll62. 

The lipolytic process in germinating oil seeds is still unclear. How seed oils, present in oil bodies, are converted to products which can then be handled by the fatty acid oxidation system of glyoxysomes, is not clear. It appears that membrane-bound lipases are involved in the process. A further area for clarification is the intracellular transport of insoluble triacylglycerols from oil bodies to the site of complete (or partial) hydrolysis and thence to the glyoxysomal enzymes of gluconeogenesis. Recent electron microscope studies with several oil seeds have provided evidence for a close association of ribosome-containing membranes with spherosomes these may be lipolytic membranes involved in triacylglycerol breakdown (Wanner and Theimer, 1978). 

Thus we are now in a position not only to delineate the pathway of conversion of fat to sucrose with confidence but to know where within the cells the various stages of the sequence occur (Fig. 4). The overall pathway depends on the cooperation among the spherosomes, where the stored fat is hydrolyzed by an acid lipase in the membrane, the glyoxysomes, the mitochondria, and enzymes in the cytosol. This cooperation implies specific movement of metabolites among different cellular compartments, but these transport processes are not yet understood. 

Fig. 6.11. Electron micrograph of a cucumber Cucumis sativus) cotyledonary cell three days after germination in the light. Glyoxysomes Mb) with their single limiting membrane are interspersed amongst the numerous oil bodies (L). Mitochondria (M) plastids (P) and dictyosomes (G) are also evident. From Trelease etal., 1971 [139]...

The listed phospholipids are localized in the lipid matrix of the cell membranes of the plasmalemma, of the tonoplast, of the endoplasmatic reticulum as well as in membranes of different cell organelles such as the nucleus, mitochondria, peroxisomes, glyoxysomes and oleosomes. In chloroplasts only relatively small amounts of phospholipids are present whereas the main membrane lipids of chloroplasts are monogalactolipids, digalactolipids, sulfolipids. These glycolipids do not contain methyl branched fatty acids. 

The monolayer of phospholipids of the lipid body membrane contains no unusual phospholipid components. In castor bean, the major components are phosphatidylcholine, phosphatldylethanolamlne, and phosphatldyllnosltol (12). Besides the monolayer of phospholipids, the membrane also contains proteins. In several seed species examined, the protein components of the lipid bodies are different from those in other subcellular organelles such as the microsomes, mitochondria, and glyoxysomes, as revealed by SDS polyacrylamide gel electrophoresis (8,12, 13). Also, as shown in identical electrophoretic analyses, lipid bodies Isolated from 11 taxonomically diverse species contain distinctly different protein patterns (14). 

Chapman KD, Trelease RN. Acquisition of membrane lipids by differentiating glyoxysomes -role of lipid bodies. J Cell Biol 1991 115 995-1007. 

Succinate leaves the glyoxysome and enters the mitochondrial matrix where it is converted to malate by enzymes of the tricarboxylate cycle. Malate may traverse the inner membrane to the cytosol where it is converted to glucose by gluco-neogenesis (Section 11.7). 

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. 

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