Equilibrative transporter

Nucleoside analogues are widely used for the treatment of cancers and viral infections. Although there have been considerable advances in the development of new nucleoside analogs, little is known about the transport mechanisms involved in the intestinal absorption of these compounds. Nucleoside transporters have been subdivided into two major classes by Na+-independent equilibrative transporters (ENT family) and Na+-dependent concentrative transporters (CNT family) [77,100-103],... 

The brain needs the influx of nucleosides because the brain is deficient in de novo nucleotide synthesis (102). Purine and pyrimidine nucleosides are necessary for the synthesis of DNA and RNA, but nucleosides also influence many other biological processes. In addition, nucleosides play an important role in the treatment of diseases, such as cardiac diseases, brain cancers, and infections [parasitic and viral (103)]. Nucleosides are hydrophilic compounds, and the influx and efflux of these compounds is therefore mediated by a number of distinct transporters (104). Nucleoside transporters are membrane-fixed transporters and are classified by their transport mechanisms (e = equilibrative, c = concentrative), their sensitivity to the transport inhibitor nitrobenzylmercaptopurine riboside (NBMPR s = sensitive, i = insensitive), and their substrates. Presently, there are two equilibrative transporters (ENTs es and ei) and six concentrative nucleoside transporters [CNTs cif (concentrative, NBMPR insensitive, broad specificity Nl), cit (concentrative, NBMPR insensitive, common permeant thymidine N2), cib (concentrative, NBMPR insensitive, broad specificity N3), cib (concentrative, MBMPR insensitive, broad specificity N4), cs (concentrative, NBMPR sensitive N5), and csg (concentrative, NBMPR sensitive, accepts guanosine as permeant N6) (104)]. The equilibrative es and ei nucleoside transporters are widely expressed in mammalian cells and are present at cultured endothelial cells and brain capillaries (105). In these cells, the expression of concentrative transporter cit (N2) was demonstrated also. In other parts of the rat brain, ei and es nucleoside transport systems have... 

SLC2A2, glucose transporter 2 SLC28A1, concentrative nucleotide transporter 1 SLC29A1, equilibrative transporter 1 and SLC15A1, di/tripeptide transporter. 

In human B-cell lines, activators like phorbol esters (PMA) and bacterial lipopoly-saccharide (LPS) upregulatethe concentrative transporters, whereas the equilibrative transporter hENTl is downregulated. This effect can also be produced by TNF-a,... 

In the chemical model, equilibrium is assumed between protons and water with a hydronium ion. This equilibrium considers the tightly bound water in the membrane [13, 19, 44] and agrees with the vapor-equilibrated transport picture of a hydronium ion being the dominant proton-transfer species in the membrane. The equilibrium relates the electrochemical potentials of the species, and at the boundary the water in the membrane is in equilibrium... 

One comment should be made regarding the form of the transport equations. In the literature, two-phase flow has often been modeled using Schlogl s equation [50, 51]. This equation is similar in form to Eq. (5.9), but it is empirical and ignores the Onsager cross coefficients. Equations (5.8) and (5.9) stem from concentrated-solution theory and take into account all the relevant interactions. Furthermore, the equations for the liquid-equilibrated transport mode are almost identical to those for the vapor-equilibrated transport mode making it easier to compare the two with a single set of properties (i.e., it is not necessary to introduce another parameter, the elec-trokinetic permeability). 

As in the case for the vapor-equilibrated transport mode, the properties of the liquid-equilibrated transport mode depend on the water content and temperature of the membrane. For a fully liquid-equilibrated membrane, the properties are uniform at the given temperature. This is because the water content remains constant for the liquid-equilibrated mode unlike in the vapor-equilibrate one. From experimental data, the value of A, for liquid-equilibrated Nafion is around 22, assuming the membrane has been pretreated correctly [6, 7, 52]. In agreement with the physical model, the water content is only a very weak function of temperature for extended (E)-form membranes (as assumed in our analysis) and can be ignored [6]. For other membrane forms, this dependence is much stronger and cannot be ignored, as discussed in the Section 5.10.1. 

When the membrane is in contact with liquid water on one side and vapor on the other (i.e., it is neither fully liquid nor vapor equilibrated), as can often occur during fuel-cell operation, both the liquid- and vapor-equilibrated transport modes will occur. This results in a transition between modes that exists in the membrane. As discussed in the physical model, a continuous transition between the two transport modes is assumed. Thus, transport in the transition region is a superposition between the two transport modes they are treated as separate transport mechanisms occurring in parallel (i.e., the middle region in Figure 5.3). In this section, an approach to modeling the transition region is introduced followed by a discussion of its limitations, other approaches, and points to consider. 

In summary, when both the liquid- and vapor-equilibrated transport modes occur in the membrane they are assumed to occur in parallel. In other words, there are two separate contiguous pathways through the membrane, one with liquid-filled channels and another that is a one-phase-type region with collapsed channels. To determine how much of the overall water flux is distributed between the two transport modes, the fraction of expanded channels is used. As a final note, at the limits of S = 1 and S = 0, Eqs. (5.17) and (5.18) or their effective property analogs collapse to the respective equations for the single transport mode, as expected. 

Using the above equations, isotherms of the fraction of expanded channels versus liquid pressure can be generated as shown in Figure 5.5. From the curves, the temperature dependence of the saturation is not strong since the transition still occurs over a small liquid-pressure range. All of the curves show that, at a liquid pressure of 1 bar, the channels are completely expanded and filled with liquid in agreement with experimental observations. If the liquid pressure falls below about 0.15 bar, then the liquid water phase ceases to exist at all temperatures and the transport of water is solely by the vapor-equilibrated transport mode, which also agrees with the physical model. If the liquid pressure is above around 0.6 bar, then X is 22 (only the liquid-equilibrated transport mode). 

Figure 5.6. Arrhenius plot of the liquid- and vapor-equilibrated transport coefficients, ol and ay, respectively, as functions of temperature. a is evaluated at unit...

The use of radiolabeled nucleosides, taken up into proliferating cells and incorporated into DNA, is a well-known approach for the determination of cell proliferation, e.g., utilizing [ H] thymidine or fluorinated compounds (Shields et al. 1996). Nucleosides are taken up into cells by facilitated diffusion controlled by equilibrative and concentrative transporters. From the two groups of equilibrative transporters (es and is), the es-transporter is mainly expressed in human cell lines investigated so far. 

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