Lipids sodium pump

A full consideration of the mechanism of the sodium pump requires an account of the role of the lipid, the binding sites for Na+, K+, Mg2+ and ATP, the mechanism of hydrolysis of ATP and the way in which this is coupled to the transport of the cation. In addition it should be noted that the enzyme also functions as a K+-dependent phosphatase, a reaction usually studied with p-nitrophenyl phosphate as substrate. Studies with inhibitors have been informative, notably with ouabain and with vanadate. Ouabain binds at one site per pump and so has been of value in quantitatively defining the enzyme in various preparations. 

Lipid peroxidation is one of the major sources of free-radical mediated injury that directly damages membranes and generates a number of secondary products. In particular, markers of lipid peroxidation have been found to be elevated in brain tissues and body fluids in several neurodegenerative diseases, and the role of lipid peroxidation has been extensively discussed in the context of their pathogenesis. Peroxidation of membrane lipids can have numerous effects, including increased membrane rigidity, decreased activity of membrane-bound enzymes (e.g., sodium pumps), altered activity of membrane receptors, and altered permeability [Anzai et al., 1999 Yehuda et al., 2002], In addition to effects on phospholipids, lipid-initiated radicals can also directly attack membrane proteins and induce lipid-lipid, lipid-protein, and protein-protein cross-linking, all of which obviously have effects on membrane function. 

Perhaps the main peroxide-induced alterations, within cells and tissues, are those that affect calcium and sodium homeostasis. Na, K-ATPase, which is considered as the core of the sodium pump , is strongly affected by peroxides, and especially by lipid hydroperoxides [137-139]. This implies that oxidative stress will usually be associated with cellular edema . Alternatively, activation of the Na, K-ATPase of vascular endothelia, such as the blood-brain barrier, will result in extracellular edema on the antiluminal side of the endothelium, due to massive influx of sodium ions [119]. 

The activity of a number of membrane proteins has been linked to the thickness of the membrane, including channels, ion pumps, and sugar transporters. For example, the Na" K "ATPase (sodium pump) is an integral membrane protein that shows maximal activity in synthetic bilayers with longer chain saturated lipids. In the presence of cholesterol, the maximum shifts to shorter chain saturated lipids, consistent with the fact that cholesterol thickens a bilayer by encouraging the extended chain conformation. 

The Na, ld -ATPase (EC 3.6.1.4). This key element of the sodium pump is localized on the cytoplasmic surface of the plasma membrane, and that is where we should expect to find it for teleologic reasons. Much structural and functional information about this enzyme is available and there is no need to elaborate here. It may suffice to recall that the lipid environment of purified (and presumably native) Na , ld -ATPase is crucial for its proper functioning and that much evidence points to phosphatidyl-serine as a necessary complement for catalytic activity. There is also some evidence that the phospholipid bi-layer vicinal to the ATPase is more ordered In situ than the overall phospholipid bi-layer of the membrane (GRISHAM 6e BARNETT, 1972). 

Since phosphatidylinositol is still present in this enzyme preparation, it might be that this acidic phospholipid takes over the place of phosphatidylserine in this system. However, quite recently Hilden and Hokin (1976) have replaced all phospholipids by phosphatidylcholine with a lipid replacement method (Warren et al., 1974) and in this system the enz3nne is still functional. Moreover, Packer and Fisher (1975) have reported a reconstitution of the sodium pump with liposomes of phosphatidylcholine and phosphatidylethanblamine and mixtures of these phospholipids (a 1 4 ratio being the most effective) but they do not give an analysis of the residual phospholipids in the enz3nne preparation. 

The mechanism of action of U ions remains to be fully elucidated. Oiemi-cally, lithium is the lightest of the alkaU metals, which include such biologically important elements as sodium and potassium. Apart from interference with transmembrane cation fluxes (via ion channels and pumps), a lithium effect of major significance appears to be membrane depletion of phosphatidylinositol bisphosphates, the principal lipid substrate used by various receptors in transmembrane signalling (p. 66). 

Acid hydrolysis of starch is conducted with hydrochloric acid or sulfuric acid, mainly in a continuous process, yielding syrups with 20 to 68 DE. The process consists of the acidification of a starch slurry with hydrochloric acid to a pH of about 1.8 to 1.9, and pumping the suspension into a converter (autoclave) where live steam is gradually admitted to a pressure of 30 to 45 psi. Converted liquids are neutralized with sodium carbonate to a pH of 5 to 7. Proteins, lipids, and colloidal matter are separated as sludge. Pigments are eliminated with activated carbon and minerals with ion exchangers. The raw juice, thus obtained, is evaporated under a vacuum (falling-film evaporator) up to a solids content of 70 to 85 percent. 

See also Passive Transport Mechanisms, Sodium-Potassium Pump, Lipid Bilayer... 

The cell membrane, In Its Interior, is like a hydrocarbon because it consists in this region primarily of the hydrocarbon portions of lipids (Chapter 23). Normally, cells must maintain a gradient between the concentrations of sodium and potassium ions inside and outside the cell membrane. Potassium ions are pumped in, and sodium ions are pumped out. This gradient is essential to the functions of nerves, transport of nutrients into the cell, and maintenance of proper cell volume. The biochemical transport of sodium and potassium ions... 

---