Counter transport

Uptake of noradrenaline into the vesicles depends on an electrochemical gradient driven by an excess of protons inside the vesicle core. This gradient is maintained by an ATP-dependent vesicular H+-triphosphatase. Uptake of one molecule of noradrenaline into the vesicle by the transporter is balanced by the counter-transport of two H+ ions (reviewed by Schuldiner 1998). It is thought that either binding or translocation of one H+ ion increases the affinity of the transporter for noradrenaline and that binding of the second H+ actually triggers its translocation. 

In an effort to address the poor membrane permeation of L-767,679, the benzyl ester pro-drug, L-775,318 was synthesized (Fig. 13.2) The latter compound exhibited significant lipophilicity (log P = 0.7) that was consistent with improved potential to cross the enterocyte membrane. However, this did not lead to a marked improvement in absorption potential (in the rat), as intestinal hydrolysis and counter-transport combined to prevent significant passage of the compound across Caco-2 cells and the rat gut. 

Antiporter, a secondary ion transporter that moves a solute against its electrochemical gradient by using energy derived from the movement of another solute in the opposite direction down its electrochemical gradient. Antiporters are also called exchangers, and the exchange process is sometimes referred to as counter transport. 

Secondary transport the transport of solutes against the electrochemical gradient using energy derived from the co- or counter-transport of another solute down its electrochemical gradient. The latter gradient ultimately being derived from other primary transporters. 

Figure 5.7 An example of counter-transport in the erythrocyte. The transport of CO from peripheral tissues to the lungs for excretion is more complex than simple solution of COj in the plasma and transport in the blood. The CO2 produced by the muscle (or any other tissue) enters the blood and then enters an erythrocyte where it reacts with water to produce hydrogen-carbonate, catalysed by the enzyme carbonate dehydratase ...

Co-transport, also known as symport, is the transport of one compound that is obligatorily linked to that of another. An example is the transport of glucose and Na ions (see below). In counter-transport, also known as antiport, the transporter transports one compound in one direction and... 

In order to apply this technique for quantitative characterization of counter-transport phenomena in zeolites it had to be checked first for the more simple case of single-component diffusion. As an example, benzene diffusion in H-ZSM-5 was chosen, because results for this case had been already reported by several authors. Their results, obtained from sorption kinetics as well as from NMR experiments, were in very good agreement and, thus, provided a reliable basis for comparison [13,14]. 

The other mechanism, known as hquid-hquid ion-exchange, involves water insoluble extractant such as Ahquat-336 (tri-caprylylmethyl ammonium chloride) and counter-transport of a second anion to provide electroneutrality. In a typical situation, the removal of cephalosporin anion, P" from the aqueous phase by an ion exchange with the anion. Cl" of the extractant (QCl) dissolved in the organic phase takes place according to... 

The mechanism of transport of GPG using SLM has been studied at the authors laboratory [56]. GPG could be permeated from alkaUne feed of carbonate buffer into an acidic stripping solution of acetate buffer across the membrane comprising Aliquat-336 in -butyl acetate immobiUzed in a polypropylene (Gelgard 2400) support. The transport mechanism is a case of counter transport exhibiting overall rate dependence on solute diffusion in the membrane phase as well as the mass transfer across the aqueous boundary films. 

Emulsion liquid membrane extraction of cephalosporins conform to the type II facilitated transport. Here the solute transport is either associated with a cotransport or counter-transport of an anionic species depending on whether ion-pair or ion-exchange extraction is exploited in the ELM system. 

The various proposed components of the permease system are based upon the response of the transport system to genetic or environmental changes. The complex nature postulated for the intact permease system is necessary to account for the various observed phenomena such as facilitated diffusion, active concentration, facilitated efflux, exchange diffusion, and counter transport of one compound driven by the downhill efflux of a second (2). 

Equations (11.1)—(11.10) provide a basis for rationalizing the principal features of coupled transport membranes. It follows from Equation (11.8) that coupled transport membranes can move metal ions from a dilute to a concentrated solution against the metal ion concentration gradient, provided the gradient in the second coupled ion concentration is sufficient. A typical experimental result demonstrating this unique feature of coupled transport is shown in Figure 11.7. The process is counter-transport of copper driven by hydrogen ions, as described in Equation (11.1). In this particular experiment, a pH difference of 1.5 units is... 

The drug permeates through the epithelial barrier of the gut into the enterocyte however, a P-glycoprotein transports it back into the intestinal lumen. As a result, the amount actually absorbed can be greatly diminished. This counter-transport can vary interindividually for an identical substance and moreover may be altered by other drugs. 

The carrier protein facilitating Pj and phosphate ester transport is of particular interest in leaves in connection with carbon processing - i.e., the synthesis, transport and degradation of carbohydrate, all of which occur in the cytosol [51]. This metabolite carrier, called the phosphate translocator, is a polypeptide with a molecular mass of 29 kDa and is a major component of the inner envelope membrane [52,53]. The phosphate translocator mediates the counter-transport of 3-PGA, DHAP and Pj. The rate of Pj transport alone is three orders of magnitude lower than with simultaneous DHAP or 3-PGA counter-transport [54]. Consequently operation of the phosphate translocator keeps the total amount of esterified phosphate and Pj constant inside the chloroplast. Significantly, the carrier is specific for the divalent anion of phosphate. 

Metal ions, amino acids Acidic, pH = 3 DEHPA Acidic, pH = 0 Complexes Counter transport of H+ [57]... 

As examples of coupled counter-transport (see Figure 13.2d) and coupled cotransport (see Figure 13.2e) the transport of titanium(lV) from low acidity (pH = 1) and high acidity ([H+] = 7 M) feed solutions, respectively using the HUM system [1,2] may be presented. The di-(2-ethyUiexyl) phosphoric acid (DEHPA) carrier reacts with Ti(IV) ion to form complexes on the feed side (see Equations 13.25 and 13.26) and reversible reactions take place on the strip side (see Equations 13.27 and 13.28). Energy for the titanium uphill transport is gained from the coupled transport of protons in the direction opposite to titanium transport from the strip to the feed solutions. In the second case (high-feed acidity), Cl anion cotransported with Ti(IV) cation in the same direction. In both cases fluxes of titanium, protons, and chlorine anion are stoichiometrically coupled. 

FIGURE 13.5 (a) Scheme of competitive counter-transport mediated by an ionic carrier CH (b) Compartmental model of... 

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