Membrane metabolism

Crowe, L.M. Crowe, J.H. (1986). Hydration-dependent phase transitions and permeability properties of biological membranes. In Membranes, Metabolism and Dry Organisms, ed. A.C. Leopold, pp. 210-30. Ithaca, N.Y. Comstock Publishing Associates. 

Leopold A.C. 9 6) Membranes, Metabolism and Dry Organisms. Idiaca Cornell University Press. 

The coupling of solute transport in the GI lumen with solute lumenal metabolism (homogeneous reaction) and membrane metabolism (heterogeneous reaction) has been discussed by Sinko et al. [54] and is more generally treated in Cussler s text [55], At the cellular level, solute metabolism can occur at the mucosal membrane, in the enterocyte cytosol, and in the endoplasmic reticulum (or microsomal compartment). For peptide drugs, the extent of hydrolysis by lumenal and membrane-bound peptidases reduces drug availability for intestinal absorption [56], Preferential hydrolysis (metabolic specificity) has been targeted for reconversion... 

GL Amidon, GD Lessman, RL Elliot. Improving intestinal absorption of water-insoluble compounds A membrane metabolism strategy. J Pharm Sci 69 1363, 1980. 

From the above, it is clear that the gut wall represents more than just a physical barrier to oral drug absorption. In addition to the requirement to permeate the membrane of the enterocyte, the drug must avoid metabolism by the enzymes present in the gut wall cell as well as counter-absorptive efflux by transport proteins in the gut wall cell membrane. Metabolic enzymes expressed by the enterocyte include the cytochrome P450, glucuronyltransferases, sulfotransferases and esterases. The levels of expression of these enzymes in the small intestine can approach that of the liver. The most well-studied efflux transporter expressed by the enterocyte is P-gp. 

Fluoro-choline (p F]-FCH) Cell membrane metabolism Oncology... 

Amidon, G. L., G. D. Leesman, and R. L. Elliott. 1980. Improving intestinal absorption of water-insoluble compounds a membrane metabolism stratd.c. harm. Sci69 1363-1368. 

Lastly, eicosanoids are important pro-inflammatory mediators derived from membrane metabolism. PLA2 plays a key role in the production of eicosanoids, derived from arachi-donic acid of the phospholipids contained in the cell membrane (40,41). As mentioned earlier, arachidonic acid is liberated from the membrane-bound phospholipids by several forms of PLA2 and is the substrate for COX-1, COX-2, and 12-lipoxygenases (LOX) involved in vascular inflammation. 

Several aspects of Schwann cell metabolism emerge as potential points of vulnerability to toxicants. The Schwann cell perikaryon (cell body) supports an enormous peripheral structure, the myelin sheath, which, if unwrapped, would dwarf the body of the Schwann cell (see Figure 30.4). Thus, as in the case of axonal transport in neurons, there may be specialized processes involved in supporting the topologically distant myelin. Furthermore, myelin has a specialized lipid and protein composition and a relatively rigid and ordered structure as compared to other membranes. Metabolic perturbations that potentially cause alterations in the composition of lipids and proteins assembled to form this membrane may cause destabilization and collapse of the myelin membrane. In this context, myelin might be much more vulnerable than plasma membranes of other cells. 

Leopold, C., ed. (1986). Membranes Metabolism and Dry Organisms. Cornell Univ. Press, Ithaca, New York. 

Luminal and Membrane Metabolism of Peptides and Proteins. In meaningful studies on peptide and protein drug absorption in the small intestine, it is prerequisite to distinguish among cavital, membrane contact, and intracellular drug metabolism.Cavital metabolism takes place in the lumen of the small intestine by enzymes such as trypsin, chymotrypsin, carboxypepti-dase, and elastase, which are secreted by the pancreas. Membrane contact metabolism is carried out by aminopeptidases lo-calized on the brush border membrane. Intracellular metabolism occurs inside of the cells. The known intra-celluar enzymes are cytoplasmic peptidases, prolidase, dipeptidase, and tripeptidase.A more detailed dis-cussion of this topic is presented in section Intestinal Absorption Barriers, later. 

Conversely, membranes composed of phospholipids with highly unsaturated acyl chains are less rigid, because of interactions between such bulky acyl chains. This state of membrane disorder produces a physical fluidity and it is proposed that such membranes offer minimal hindrance lo the function of proteins (e.g., receptors or channels) that lie within the membrane. Metabolically active membranes, such as those found in neuron cell bodies, are comprised of phospholipids containing unsaturated sn-2 chains. Work by Brown (1994), Dratz (Dratz Holte, 1993), and Litman (Littman Mitchell, 1996) support this notion, as recombinant membranes that comprise higher levels of unsaturated PUFA, particularly the long-chain n-3, DHA, mimic, more closely, biological activity when measured in vitro. 

Clegg, J. S. (1986). The physical properties and metabolic status of Anemia cysts at low water contents The water replacement hypothesis. In Membrane, Metabolism and Dry Organisms (Leopold, A. C ed.), H>. 169-187, Cornell University Press, Ithaca, NY. 

Caffrey M (1986) In Leopold AC (ed) Membranes, metabolism and dry organisms. Comstock, Ithaca, New York p 242... 

Leopold AC (1986) Membranes, metabolism and dry organism, Comstock, Ithaka, NY, p 374... 

Pentoxifylline improves deformability of erythrocytes by increasing cellular ATP concentration via a membrane-metabolizing action, which in tnm rednces the aggregation of erythrocytes and local hyperviscosity. It stimulates prostacyclin formation and release, and inhibits phosphodiesterase degradation of platelet cAMP. The increase of cAMP levels decreases the synthesis of thromboxane A2, and the net resnlt is rednced platelet aggregation. It... 

Lipid and Membrane Metabolism of the Malaria Parasite and the African Trypanosome... 

Comparison of the lipids and lipid and membrane metabolism of the malaria parasite and the African typanosome with that of their environments emphasizes the unique ways in which parasites are able to access the nutrients available, and modify them to their advantage. 

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