Sodium-coupled transport

RJ Alpern. (1985). Mechanism of basolateralmembrane II -OH /IICOL transport in the rat proximal convoluted tubule. A sodium-coupled electrogenic process. J Gen Physiol 86 613-636. 

Glutamate and sodium/lithium-induced conformational changes in the GLT-1 transporter have been detected by the altered accessibility of trypsin-sensitive sites to the protease (59). These experiments in GLT-1 shows that lithium can occupy at least one of the sodium ion binding sites, but lithium by itself cannot support coupled transport (59). Therefore, at least one of the sodium binding sites in GLT-1 discriminates between sodium and lithium. As described earlier, this contrasts with EAAC-1, where lithium is able to support uptake. It should therefore be possible to identify residues that are responsible for the sodium/lithium selectivity difference between EAAC-1 and GLT-1. 

The active Irunsport of chloride has also been demonstrated across ihe wall of the IVog stomach, rat ileum, dog ileum, and the human ileum. Experiments have produced a double exchange model in which bicarbonate secretion and chloride absorption are linked hy an isoelectric mechanism to hydrogen ion secretion and sodium absorption across the human ileum. In 1972. a group of researchers proposed a similar model of coupled transport... 

Schultz, S.G. and Curran, P.F. (1970) Coupled transport of sodium and organic solutes. 

Historically important as an example of flux coupling, and one that was investigated in detail becoming a paradigm for coupled transport, was the sodium coupled glucose transport system of the small intestine and kidney (see below). This was a symport (or co-transport) rather than an antiport, normally carrying glucose into the cell coupled to a flow of sodium ions in the same direction. 

Uglem, G. L. Prior, D. J. (1980). Hymenolepis diminuta chloride fluxes and membrane potentials associated with sodium-coupled glucose transport. Experimental Parasitology, 50 287-94. 

Recycling of choline Choline may be recaptured by a sodium-coupled high affinity uptake system that transports the molecule back into the neuron, where it is acetylated and stored until released by a subsequent action potential. 

The mechanism by which the exergonic methyl-transfer reaction is coupled with vectorial electrogenic Na translocation across the membrane is not known. An electron transport chain appears not to be involved in Na transport. Sodium ion transport... 

Many sodium-coupled systems located in the proximal tubule of the kidney exhibit an analogous mechanism (see Fig. 3). When the transport of both substances, the substrate going uphill and the coupled substrate going downhill, is flowing in the same direction, the transporter is termed... 

Zehkovic I, Chesney RW. Sodium coupled amino acid transport in renal tubules. Kidney Int 1989 36 351-9. 

ABC MDRs (as all other members of this superfamily) are primary active transporters that couple substrate translocation to binding and hydrolysis of ATP. M D Rs in all the other superfamilies are secondary transporters that use electrochemical gradients of ions (most frequently protons but sometime sodium) to transport their diverse substrates. Both primary and secondary transporters are ubiquitous in bacteria li owcvcr, their relative presence seems to correlate with energy generation fermentative bacteria tend to rely more on the primary transporters while aerobic bacteria contain somewhat more secondary transporters in their genomes [32, 33]. 

Mackenzie B, Erickson JD. 2004. Sodium-coupled neutral amino acid (system N/A) transporters of the SLC38 gene family, Pfiugers Arch 447 784-795. 

A FIGURE 18-1 Overview of synthesis of major membrane lipids and their movement into and out of cells. Membrane lipids (e.g., phospholipids, cholesterol) are synthesized through complex multienzyme pathways that begin with sets of water-soluble enzymes and intermediates in the cytosol (D) that are then converted by membrane-associated enzymes into water-insoluble products embedded in the membrane (B), usually at the interface between the cytosolic leaflet of the endoplasmic reticulum (ER) and the cytosol. Membrane lipids can move from the ER to other organelles (H), such as the Golgi apparatus or the mitochondrion, by either vesicle-mediated or other poorly defined mechanisms. Lipids can move into or out of cells by plasma-membrane transport proteins or by lipoproteins. Transport proteins similar to those described in Chapter 7 that move lipids (0) include sodium-coupled symporters that mediate import CD36 and SR-BI superfamily proteins that can mediate... 

SLC28 Solute carrier family 28 (sodium-coupled nucleoside transporter) ... 

Active transport, on the other hand, couples transport of compounds across the membrane to energetically favorable processes, such as hydrolysis of ATP. Because of the additional energy provided by the coupled process, active transport systems can "pump" molecules against a concentration gradient. Thus, with active transport provided by the sodium-potassium pump, cells can maintain a higher concentration of potassium ions inside of the cell than outside and a higher concentration of sodium ions outside than inside. 

Frizzell RA, Field M and Schultz SG (1979) Sodium coupled chloride transport by epithelial tissues. Am J Physiol 236 F1—F8. 

Thies, M, Clancy, S. F., and Paradies, H. H. (1996). Multicomponent diffusion of distearyldi-methylammonium polyelectrolyte solution in the presence of salt and different pH Coupled transport of sodium chloride. J. Phys. Chem. 100, 9881-9891. 

Jorgensen, W.L. Rives, T. (1988). The OPLS Potential Functions For Proteins - Energy Minimizations For Crystals Of Cydic-Peptides And Crambin. Journal of the American Chemical Society, Vol.llO, No.6, pp. 1657-1666 Kandt, C., Ash, W. Tieleman, D.P. (July 2009). InflateGRO, Available from http // www.csb.bit.uni-bonn.de/inflategro.html Kandt, C., Ash, W.L. Tieleman, D.P. (2007). Setting up and running molecular dynamics simulations of membrane proteins. Methods, Vol.41, No.4, pp. 475-488 Kanner, B.l. Zomot, E. (2008). Sodium-coupled neurotransmitter transporters. Chem Rev, Vol.108, No.5, pp. 1654-68... 

Limited evidence is available regarding the molecular mechanism of boron homeostasis. Based on analysis of plasma boron concentrations in seven sibships, Barr et al. (1996) observed smaller variances within than between families and hypothesized that boron levels in humans are under genetic control. The data presented by Barr et al. (1996) need to be interpreted with caution as the blood samples were collected from individuals living in a rural region of northern Chile, and their dietary intakes were not determined. Also, the study was retrospective aud utilized blood samples that had been stored for over 20 years. More recent evidence suggests the involvemeut of a sodium-coupled boron transporter in animal cells, expressed in the basolateral manbrane, which determines the steady-state concentration of borate in the cytoplasm and hence maintains borate homeostasis (Park et al., 2004). 

---