Carbon Lysosomes

An important additional pathway is indicated in reactions I and II of Figure 47-9. This involves enzymes destined for lysosomes. Such enzymes are targeted to the lysosomes by a specific chemical marker. In reaction I, a residue of GIcNAc-1-P is added to carbon 6 of one or more specific Man residues of these enzymes. The reaction is catalyzed by a GIcNAc phosphotransferase, which uses UDP-GlcNAc as the donor and generates UMP as the other product ... 

Figure 48-12. Schematic illustration of some aspects of the role of the osteoclast in bone resorption. Lysosomal enzymes and hydrogen ions are released into the confined microenvironment created by the attachment between bone matrix and the peripheral clear zone of the osteoclast. The acidification of this confined space facilitates the dissolution of calcium phosphate from bone and is the optimal pH for the activity of lysosomal hydrolases. Bone matrix is thus removed, and the products of bone resorption are taken up into the cytoplasm of the osteoclast, probably digested further, and transferred into capillaries. The chemical equation shown in the figure refers to the action of carbonic anhydrase II, described in the text. (Reproduced, with permission, from Jun-queira LC, Carneiro J BasicHistology. Text Atlas, 10th ed. McGraw-Hill, 2003.)...

Radicals such as CCI3, produced during the oxidation of carbon tetrachloride, may induce lipid peroxidation and subsequent destruction of lipid membranes (Figure 8.3). Because of the critical nature of various cellular membranes (nuclear, mitochondrial, lysosomal, etc.), lipid peroxidation can be a pivotal event in cellular necrosis. 

D-Amino acid oxidase occurs in peroxisomes containing other enzymes that produce H2O2 (e.g., L-a-hydroxy acid oxidase, citrate dehydrogenase, and L-amino acid oxidase) and catalase and peroxidase, which destroy H2O2. In leukocytes, killing of bacteria involves hydrolases of lysosomes and production of H2O2 by NADPH oxidase (Chapter 15). Conversion of D-amino acids to the corresponding a-keto acids removes the asymmetry at the a-carbon atom. The keto acids may be aminated to L-amino acids. By this conversion from D- to L-amino acids, the body utilizes D-amino acids derived from the diet ... 

Phospholipases located in cell membranes or in lysosomes degrade glycerophospholipids. Phospholipase A1 removes the fatty acyl group on carbon 1 of the glycerol moiety, and phospholipase A2 removes the fatty acid on carbon 2 (Fig. 33.31). The C2 fatty acid in cell membrane phospholipids is usually an unsaturated fatty acid. 

In liver cells, methylmercury forms soluble complexes with cysteine and glutathione, which are secreted in bile and reabsorbed from the G1 tract. In general, however, organomercurials undergo cleavage of the carbon-mercury bond releasing ionic inorganic mercury. Mercuric mercury induces synthesis of metallothioneins, mainly in kidney cells. Mercury within renal cells becomes localized in lysosomes. 

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