MicrobodiesIntroduction:

Microbodies were first reported at the ultra structural level in the proximal convoluted tubule of mouse kidney by Rhodin in1954.Microbodies are now recognized as ubiquitous subcellular respiratory organelles in eukaryotic cells . Microbodies from all tissues appear morphologically similar and have similar enzymatic properties, but the metabolic pathways within this subcellular compartment vary, depending upon the tissue. Microbodies (peroxisomes and glyoxysomes) were one of the last major subcellular compartments to be recognized, and it was not until the end of the 1960s that their significance was established by several reviews.A microbody is a type of organelle that is found in the cells of plants, protozoa, and animals. Organelles in the microbody family include peroxisomes, glyoxysomes, glycosomes and hydrogenosomes. In vertebrates, microbodies are especially prevalent in the liver and kidney organs.

History: .

Microbodies were first discovered and named in 1954 by Rhodin

.Two years later in 1956, Rouiller and Bernhard presented the first worldwide accepted images of microbodies in liver cells

Then in 1965, Christian de Duve and coworkers isolated microbodies from the liver of a rat. De Duve also believed that the name Microbody was too general and chose the name of Peroxisome because of its relationship with hydrogen peroxide .

In 1967, Breidenbach and Beevers were the first to isolate microbodies from plants, which they named Glyoxysomes because they were found to contain enzymes of the Glyoxylate cycle.

Morphology:

A microbody is usually a vesicle with a spherical shape, ranging from 0.2-1.5 micrometers in diameter. Microbodies are found in the cytoplasm of a cell, but they are only visible with the use of an electron microscope. They are surrounded by a single phospholipid bilayer membrane and they contain a matrix of intracellular material including enzymes and other proteins, but they do not seem to contain any genetic material to allow them to self-replicate.Microbodies are morphologically characterized as which are delimited by a single tripartite membrane and contain a finely granular matrix . In liver, an estimation of 1,000 microbodies per hepatocyte has been made.They show a close spatial relationship to the endoplasmic reticulum. Their catalase can be demonstrated cytochemically . Because microbodies without inner membranes must be pliable, they usually appear spherical or ovoid, but in the cell they may also be irregular in shape or show unusual projections .

Fig:Structure of Microbody

Types of Microbodies:There are two types of microbodies namely:

Peroxisomes

Glyoxisomes

Peroxisomes:

These (also called microbodies) are organelles found in virtually all eukaryotic cells Called Peroxisomes because of their ability to produce or utilize Hydrogen peroxide.They are small, oval or spherical in shape.They have a fine network of tubules in their matrix.About 50 enzymes have been identified.The number of enzymes fluctuates according to the function of the cells. These were identified as organelles by the Belgian cytologist Christian de Duve in 1967 after they had been first described by a Swedish doctoral student, J. Rhodin in 1954.

However, peroxisomes bud off from the endoplasmic reticulum, not the Golgi apparatus (that is the source of lysosomes). The enzymes and other proteins destined for peroxisomes are

synthesized in the cytosol. Each contains a peroxisomal targeting signal (PTS) that binds to a receptor molecule that takes the protein into the peroxisome and then returns for another load.

Peroxisomal diseases:It represent a class of medical conditions caused by defects in peroxisome functions.This may be due to defects in single enzymes important for peroxisome function or in peroxins, proteins encoded by PEX genes that are critical for normal peroxisome assembly and biogenesis. Peroxisomal diseases include:

ALD : adrenoleukodystrophy ZS: Zellweger syndrome

Adrenoleukodystrophy or ALD syndrome:

Adrenoleukodystrophy or ALD results in the over-accumulation of very long chain fatty acids or VLCFA and branched chain fatty acids, such as phytanic acid. In addition, PBD-ZSD patients show deficient levels of plasmalogens, ether-phospholipids necessary for normal brain and lung function.

Fig: Adrenoleukodystrophy or ALD Syndrome

Zellweger syndrome:also called cerebrohepatorenal syndrome, is a rare congenital disorder characterized by the reduction or absence of functional peroxisomes in the cells of an individual. It is one of a family of disorders called leukodystrophies. Zellweger syndrome is named after Hans Zellweger (1909–1990), a Swiss-American pediatrician. These are autosomal recessive developmental brain disorders that also result in skeletal and craniofacial dysmorphism, liver dysfunction, progressive sensory neural hearing loss, and retinopathy.

Fig:Zellweger syndrome

Functions: Peroxisomes perform many functions which include:

Breakdown (by oxidation) of excess fatty acids.

Breakdown of hydrogen peroxide (H2O2), a potentially dangerous product of fatty-acid oxidation. It is catalyzed by the enzyme catalase.

Participates in the synthesis of cholesterol. One of the enzymes involved, HMG-CoA reductase, is the target of the popular cholesterol-lowering "statins".

Participates in the synthesis of bile acids.

Participates in the synthesis of the lipids used to make myelin.

Breakdown of excess purines (AMP, GMP) to uric acid.

Glyoxysomes:These are specialized peroxisomes found in plants (particularly in the fat storage tissues of germinating seeds) and also in filamentous fungi. As in all peroxisomes, in glyoxysomes the fatty acids are hydrolyzed to acetyl-CoA by peroxisomal β-oxidation enzymes. Besides peroxisomal functions, glyoxysomes possess additionally the key enzymes of glyoxylate cycle (isocitrate lyase and malate synthase) which accomplish the glyoxylate cycle bypass.Thus, glyoxysomes (as all peroxisomes) contain enzymes that initiate the breakdown of fatty acids and additionally possess the enzymes to produce intermediate products for the synthesis of sugars by gluconeogenesis. The seedling uses these sugars synthesized from fats until it is mature enough to produce them by photosynthesis. Glyoxysomes also participate in photorespiration and nitrogen metabolism in root nodules.

Fig: Anatomy of Glyoxysomes

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