Proteolysis Lysosomal

A. Ilasilik, The Early and Late Processing of Lysosomal Enzymes Proteolysis and Compartimentation , Experientia 1992, 48, 130-151. 

Ubiquitin interacts avidly but not covalently with ABP and such complexes can be isolated from AD brain extracts (276). Ubiquitin and APP colocalize to endosome-lysosomes implicated in APP proteolysis (277). A clear link between defective APP processing and the ubiquitin-proteasome system was demonstrated by Chen et al. 

As discussed in detail by Dillard et al. and by Mittman et al. the possible relationship of lysosomal proteases to chronic lung disease has been inferred from the finding of an increased incidence of emphysema in subjects deficient in serum ai>antitrypsin factor, an -globulin that can inhibit lysosomal proteases. (No effect of ozone on serum aj-antitrypsin inhibitor was noted in rabbits chronically exposed to ozone. ) Thus, an ozone-induced increase in concentrations of such enzymes in the lung might produce excess proteolysis and result in eventual chronic lung disease. However, the available evidence is inadequate to support the belief that such a process occurs in humans intermittently exposed to ozone. Further studies of this potential hazard would be of value. 

Very little is known of the mechanism(s) by which the rate of degradation is controlled. The current view is that the concentrations of ubiquitin, together with changes in the activity of the proteasome complex control the rate of proteolysis by this system. Lysosomal degradation may be controlled by the number of particles transported into the cell. The calpains might be controlled by the ion concentration. 

Some intracellular protein degradation (proteolysis) takes place in the lysosomes (see p. 234). In addition, there are protein complexes in the cytoplasm, known as pro-teasomes, in which incorrectly folded or old proteins are degraded. These molecules are recognized by a special marking (see p. 176). The proteasome also plays an important part in the presentation of antigens by immune cells (see p. 296). 

ACh receptors are destroyed by endocytosis via coated pits and proteolysis in lysosomes. In myasthenia gravis, the receptors are crosslinked by antireceptor antibodies, which facilitate the rate-limiting endocytosis step receptor destmction occurs in less than half the normal time, resulting in net receptor loss. The chronic disease is characterized clinically by such muscular weakness and abnormal fatigue that patients cannot even keep their eyes open. Acetylcholinesterase inhibitors increase the ACh concentration and excitation of the neuromuscular junction, resulting in increased strength and endurance. As expected, AChE inhibitors are also potent curare antidotes because the increased ACh levels displace the blocker more readily. 

The liver eliminates proteins on first pass after oral administration and on each pass of hepatic blood flow. Hepatocytes, Kupffer cells, adipocytes, and endothelial cells can all be involved in proteolysis (Figure 5.6). Proteolysis can occur in lysosomes after endocytosis of a protein and lysosomal fusion. Endocytosis of a protein may be a nonspecific or receptor-mediated process. Proteolytic products are eliminated from the liver through biliary excretion, and subsequently digested further in the intestinal tract. 

Structural Organization of the Plasma Membrane. Although our purpose here is not to describe in detail the structure of cell membranes, a brief look at the structure of the plasma membrane will help us to understand the major problems and the role of specific proteolysis related to membrane assembly. All cells—those of bacteria (prokaryotes), higher plants, and animals (eukaryotes)—have plasma membranes, but other distinct internal membranes (88) are found in eukaryotic cells (nuclei, golgi bodies, mitochondria, endoplasmic reticula, and lysosomes). 

Like insulin, vanadate inhibits protein degradation in hepatocytes105. Proteolysis in liver homogenates at pH 5 is inhibited, suggesting that the lysosomal proteinases are affected. 

Within the ileal mucosal cell, the vitamin is released by lysosomal proteolysis of intrinsic factor, and is bound to transcobalamin II, a vitamin B12 binding protein synthesized in the enterocytes. Transcobalamin II is in vesicles destined for export from the enterocytes, and it is assumed that vitamin B12 binds to the apoprotein in these vesicles rather than in the lysosomes, because otherwise newly synthesized transcobalamin would be hydrolyzed by lysosomal proteases (Seetharam, 1999). 

Both avidin and the avidin-biotin complex are very stable to heat. To release biotin from avidin binding, autoclaving above 130°C is required, and free avidin is stable up to about 85°C. Avidin is also resistant to proteolysis and, as is obvious from the use of raw egg white diets to induce biotin deficiency, biotin cannot be released from avidin binding in the gastrointestinal tract. Lysosomal hydrolases do release biotin from avidin binding, and intravenously administered avidin-biotin can be a source of biotin. 

Serum albumin labeled with an iodine radionuclide was firstly used as a substrate for determining protease activity by Absolon This method was later on modified several times and applied for assaying various proteolytic activities in different materials. Mego et al. injected denaturated I-human %rum albumin into the tail vein of rats and measured the rate of intralysosomal proteolysis on isolated lysosomes containing endocytosed substrate. This method was also used for the determining the intralysosomal pH on the basis of differences found in the rate of I-albumin breakdown in intact and lysed lysosomes C-bovine serum albumin, I-casein or I-albumin have been alternatively used as substrate for measuring the activity of trypsin, chymotrypsin and papain - ). 

Cystatin refers to a diverse family of protein cysteine protease inhibitors. There are three general types of cystatins Type 1 (stefens), which are primarily found in the cytoplasm but can appear in extracellular fluids Type 2, which are secreted and found in most extracellular fluids and Type 3, which are multidomain protease inhibitors containing carbohydrates and that include the kininogens. Cystatin 3 is used to measure renal function in clinical chemistry. See Barrett, A.J., The cystatins a diverse superfamily of cysteine peptidase inhibitors, Biomed. Biochim. Acta 45,1363-1374,1986 Katunuma, N., Mechanisms and regulation of lysosomal proteolysis, Revis. Biol. Cellular 20, 35-61, 1989 Gauthier, F., Lalmanach, G., Moeau, T. et al., Cystatin mimicry by synthetic peptides, Biol Chem. Hoppe Seyler 373, 465-470, 1992 Bobek, L.A. and Levine,... 

Proteolysis is another way to control enzymatic activity, although unlike phosphorylation, the change in activity is irreversible. Many proteases are synthesized as inactive precursors (zymogens) that have to be cleaved by proteolysis to become active. These precursors include proteases that are secreted into digestive tracts or lysosomes, the catalytic active P subunits in the eukaryotic 20 S proteosome that are activated by... 

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