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Micro Bodies

The cytoplasm contains other membrane bound vesicles called microbodies. Microbodies are granular bodies filled with an electron-dense substance (matrix) and bound by a single membrane.

Commonly two types of microbodies are distinguished, peroxisomes and glyoxysomes. The peroxisomes were so called because of their potential peroxidative activity. Glyoxysomes contain, in addition, all or part of the enzymes of the glycoxylate cycle.

The membrane is thinner than the lysosomes membrane and the plasma membrane and is similar to the endoplasmic reticulum membrane and the outer membrane of the mitochondrion. The liquid content of microbody membrane is very low as compared to mitochondrial and chloroplast membranes. Peroxisome membranes permit the passage of substrates like amino acid, α-hydroxy acids and uric acid. They have relatively unrestricted permeability as compared to lysosomal and mitochondrial membranes. The peroxisome membranes do not appear to be freely permeable to pyridine nucleotides.

The peroxisome membrane appears to arise from the endoplasmic reticulum membrane. Both membranes appears to arise from the endoplasmic reticulum membrane. Both membranes have similar lipid and enzymatic composition. Antimycin – A – insensitive NADH – cytochrome c reductase has been found in peroxisomal and glyoxysomal membranes, as well as the endoplasmic reticulum, the outer mitochondrial membrane and the nuclear membrane. The phospholipid composition of these membranes is also similar.

Enzymes. The microbodies contain catalases and oxidases. The peroxisomes oxidize a variety of substrates in a two-step reaction. In the first step substrates (AH2) like uric acid, amino acids and lactic acid are oxidize by molecular oxygen to form H2O2. The oxidation is catalysed by oxidases.
Catalase and flavin oxidases are consistently found in microbodies. Other catabolic enzymes differ with the organ or species. Different tissues probably have peroxisomes with different enzymes and functions. Urate oxidase is found in many, but not all, animal and plant tissues and in the protozoa. Allantoinase is found in the frog liver and plant peroxisomes. Xanthine dehydrogenase is found in chicken liver and kidney peroxisomes.

In glyoxysomes the glyxylate cycle converts two acetyl CoA units into C4 acids for gluconeogenesis. Glyoxysomes also contain the enzymes for β-oxidation of fatty acids. Glyoxylate cycle enzymes have been found in the Protozoa (e.g. Tetrahymena), some fungi and the Metazoa.

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