Protein body lysosomes

In addition, the proportions of individual proteins excreted in the urine depend on the extent of their reabsorption by the proximal renal tubules, which is also inversely proportional to molecular size. The reabsorption of low molecular weight proteins involves binding of the protein to specific receptors on the tubule epithelial cells, uptake into the cells by pinocytosis, fusion of uptake vacuoles with lysosomes, and hydrolysis of the protein by lysosomal enzymes, followed by return of the constituent amino acids to the body pool. Very little of the total excreted urinary protein normally consists of small proteins. Only a small amount of protein is excreted normally (20 to 150mg/dL), and most of it is albumin because of its concentration—it is not completely removed from the filtrate by the tubular cells. The remainder is almost entirely the Tamm-Horsfall protein uromucoid, a constituent of urinary casts, probably secreted by the distal tubules. 

Various hydrolases are in fact associated with the protein bodies, though we should realize that some of the enzymes may be contaminants from the cytoplasm. Acid proteinases have been found in protein bodies of Gossypium [47, 79], V faba [54], Hordeum [55] and Cannabis [66]. In the last, the proteinase is active against the crystalloid moiety (edestin) and has, therefore, been called edestinase [66]. The occurrence of these various enzymes for the hydrolysis of the native protein therefore suggests the possible lysosomal nature of protein bodies. 

Spatial separation of carbohydrases within the cell must also be considered if the contentions of Matile [95] are correct. Matile suggests that the pea protein bodies (aleurone vacuoles), which store protein and phytin in the dry seed, become lysosomal when their reserves are depleted, and contain hydrolases (including ribonucleases, phosphatases, proteinases). Furthermore, he contends that a-amylase (and presumably the amylopectin-l,6-glucosidase) act as free enzymes in the cytoplasm and that the products of a-amylolysis then pass into the vacuole/lysosome for completion of the digestion process. While such a suggestion is appealing, it is still far from proven. 

Green fluorescent protein-RhoB GFP-RhoB is localized in endocytic vesicles and has been shown to highlight early endosomes, recycling endosomes, and multivesicular bodies, but is absent from lysosomes (123). Because RhoB is toxic when applied for long periods of time, the cells should be analyzed within 24 hours of transient transfection. 

Based on the knowledge of the endocytotic pathway, it is reasonable to assume that uhiquitination is reversible until the endocytosed membrane proteins such as neurotransmitter receptors are routed to the multivesicular body for lysosomal degradation. 

Figure 8 Ubiquitin and endocytosis. Receptors on the plasma membrane undergo monoubiquitination as a result of ligand (e.g., neurotransmitter). Ubiquitinated receptors bind to proteins called epsins, which in turn interact with adaptor proteins (adaptin) bound to clathrin-coated pits. Ubiquitination also functions to sort the internalized membrane protein into early endosomes, which directs them to degradation by lysosome through the multivesicular body. If ubiquitin from the endocytosed receptors is removed by an UBP, the receptor recycles back to the membrane. Proteasome inhibitors block endocytotic degradation of some proteins such as glutamate receptor subunits indicating a possible role for the proteasome.

The answer is D. As this patient ages, a variety of skeletal defects and short stature that are consistent with a lysosomal storage disease (mucolipidosis), either I-cell disease or pseudo-Hurler polydystrophy, are developing. Both diseases arise from a deficiency of an enzyme involved in synthesis of the Man-6-P marker on lysosomal enzymes. Such misaddressed proteins are secreted rather than trafficked to the lysosomes. The degradative function of lysosomes is impaired as a result and the organelles tend to accumulate waste products (hence, the term storage disease ). It is these inclusion bodies or dense structures that would be visible by microscopic examination of the patient s cells in a biopsy specimen. 

Proteolytic enzymes such as proteases and peptidases are ubiquitous throughout the body. Sites capable of extensive peptide and protein metabolism are not only limited to the liver, kidneys, and gastrointestinal tissue, but also include the blood and vascular endothelium as well as other organs and tissues. As proteases and peptidases are also located within cells, intracellular uptake is per se more an elimination rather than a distribution process [13]. While peptidases and proteases in the gastrointestinal tract and in lysosomes are relatively unspecific, soluble peptidases in the interstitial space and exopeptidases on the cell surface have a higher selectivity and determine the specific metabolism pattern of an organ. The proteolytic activity of subcutaneous tissue, for example, results in a partial loss of activity of SC compared to IV administered interferon-y. 

The majority of body iron is not chelatable (iron from cytochromes and hemoglobin). There are two major pools of chelatable iron by DFO (19). The first is that delivered from the breakdown of red cells by macrophages. DFO competes with transferrin for iron released from macrophages. DFO will also compete with other plasma proteins for this iron, when transferrin becomes saturated in iron overload. The quantity of chelatable iron from this turnover is 20mg/day in healthy individuals and iron chelated from this pool is excreted in the urine (19). The second major pool of iron available to DFO is derived from the breakdown of ferritin and hemosiderin. The ferritin is catabolized every 72 hours in hepatocytes, predominantly within lysosomes (I). DFO can chelate iron that remains within lysosomes shortly after ferritin catabolism or once this iron reaches a dynamic, transiently chelatable, cytosolic low-molecular-weight iron pool (20). Cellular iron status, the rate of uptake of exogenous iron, and the rate of ferritin catabolism are influent on the level of a labile iron pool (21). Excess ferritin and... 

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