Golgi apparatus hydrolases

Multivesicular bodies are usually found in association with the Golgi apparatus and are visualized by EM as small, single membrane-bound sacs approximately 0.5 Jim in diameter. They contain several minute, spherical profiles, sometimes arranged about the periphery. They are believed to belong to the lysosome series prior to secondary lysosomes because they contain acid hydrolases and apparently are derived from primary lysosomes. 

Modified LDL taken up by the cell are delivered to the endolysosome pathway, where enzymes hydrolyze cholesteryl esters to free cholesterol and fatty acids. The levels of free cholesterol and cholesteryl esters are regulated with the help of neutral cholesteryl ester hydrolase, which converts cholesteryl ester to free cholesterol. After cholesterol leaves the lysosome, it is transported to the ER and to the plasma membrane by means of an intermediate step through the Golgi apparatus. Recycling compartments, especially multivesicular endosomes, harbor most of the cholesterol in the endocytic pathway. The intra-endosomal membranes of multivesicular late endosomes that are enriched in the phospholipids lyso-bisphosphatidic acid/bismonoacylglycerophosphate serve as important regulators of cholesterol transport (Kobayashi et al. 1999 Ikonen 2008). Lysobisphosphatidic acid is structurally... 

The Golgi apparatus is also believed to concentrate lysosomal enzymes and to be at the source of small vacuoles filled with acid hydrolases, which constitute the primary lysosomes. The evidence invoked in support of this hypothesis is based primarily on histochemical and electron microscopic observations that have demonstrated the presence of such lysosomal enzymes as acid phosphatase, j8-glucuronidase, sulfa-tase, and esterases in the Golgi apparatus. 

Glycoside hydrolases (GHs) are widespread carbohydrate-specific enzymes involved in many processes in biological systems. In mammals, GHs are mostly extracellular, such as in saliva or digestive tract, and act to the degradation of glycans for nutritive or anti-bacterial needs. However, some of GHs are located in the Endoplasmic Reticulum or Golgi apparatus participating to post-translational processes. ... 

It is likewise of equal interest that the Golgi apparatus is the site of many hydrolase activities such as alkaline and acid phosphatases, j8-glu-curonidase, nucleoside diphosphatase, and thiamine pyrophosphatase. 

Stoolmiller et al. (1972) emphasize the fact that glycosyltransferases and their macromolecular acceptors are membrane-associated, frequently in both rough and smooth microsomal fractions, quite apart from the Golgi apparatus. The requirement that the product of one glycosyl transfer reaction becomes a specific substrate for the next transfer reaction and this sequence is composed of at least seven transfers including that of sulfate makes it seem reasonable that macromolecular substrates, transferases, and hydrolases along with lipid molecules all enter into membrane structure... 

Relatively little is known concerning the mode of formation of these particles this problem is discussed extensively in a review article We may summarize by saying that the acid hydrolases appear to be synthesized in the rough endoplasmic reticulum and thence conveyed to the Golgi apparatus through the smooth endoplasmic reticulum there, they are released within vesicles which may be regarded as the primary lysosomes. As we shall see later, these granules represent only a fraction of the lysosomal system. 

Both nonsecretory and secretory tissues undoubtedly employ the same protein biosynthetic pathway involving ribosomes, RNA, etc. The transfer of the synthesized enzyme proceeds through the endoplasmic reticulum to the Golgi apparatus where the hydrolase is enveloped in a vesicle. The distinction of the secretoiy type of cells is that the enzyme produced this way travels to the exterior of the cell into an acinus, possibly by reverse pinocytosis. 

Fig. 1. Model schema of the lysosome-vacuolar apparatus in mammalian cells. The Integrated components of the membrane structured apparatus are presented as existing in dynamic equilibrium. Acid hydrolases, produced (phase 1) in the endoplasmic reticulum, are channeled and packaged in the Golgi system (phase of vesiculatlon, phase 2). The subsequent translocation of acid hydrolases or secretory product is dependent upon the metabolic state of the cell, need for segregated catabolic activity of macromolecules (phase 3), or endocytic-exocytic activity.

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