Cation-coupled transport systems

The demonstration by Crane (1960, 1965) that Na+ ions were essential for the translocation of monosaccharides by segments of the intestine brought in a new era of understanding of the central role of ion coupled transport, particularly in higher organisms. While Na+ is clearly the predominant cation involved in cation driven solute accumulation in mammalian systems, current work has provided examples of H+ driven solute transport in intestine and kidney (Jessen et al., 1989 Ganapathy and Leibach, 1986). Conversely, in yeast and bacteria, H+ driven mechanisms are in the majority (Seaston et al., 1973 Hirata et al., 1973), but examples of Na+-cou-pled fluxes exist, e.g., proline transport (Dibrov, 1991). 

Two hypotheses have been proposed (Eisenman, 1962 Mullins, 1975) to account for the high Na+/K+ selectivity in Na+ dependent transport systems. Mullins emphasizes a geometrical fit of the cation. Eisenman emphasizes the electrical field strength of the putative binding sites. Many of the predictions made by Eisenman have been borne out experimentally in the ion-coupled systems and are complemented by the construction of ion-specific glass electrodes (Eisenman, 1962). 

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. 

Studies on the uptake of Ni by M. bryantii have demonstrated the presence of a highly specific uptake system with = 3.1 ji-mol dm. This pathway was not afiected by high levels of other metal cations, including Mg but with the exception of 00 . Transport of Ni is coupled to movement of protons. Usually uptake of Ni into bacteria occurs by the transport process for magnesium, not surprisingly in view of their similar ionic radii. It is appropriate, therefore, that an organism such as M. bryantii which has a specific requirement for Ni should have a specific transport system for its uptake. 

Pyles TM, mcMalik-Diemer VA, McGavin CA, Whitfield DM, Membrane transport systems. III. A mechanistic study of cation-proton coupled countertransport. Can. j Chem. 1982 60, 2259. 

Such complexation-reaction coupling processes may be of particular interest for the study of cation transport since they contain the possibility of devising systems undergoing active transport. 

According to the second law, the dissipation function must be positive if not zero, which of course is to be expected here, since we are dealing with a spontaneously occurring passive process. The thermodynamic force A/x+, which contains both a concentration-dependent component and an electrical component, is the sole cause of the flow J+. In a system in which more than one process occurs, each process gives rise to a term in the dissipation function consisting of the product of an appropriate force and its conjugate flow. In the case of active transport of the cation, as found, for example, in certain epithelial tissues, the cation flux is coupled to a metabolic reaction. If we represent the flow or velocity of the reaction per unit area of membrane by Jr, the appropriate force driving the reaction is... 

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