Metal-ion transport

Alkali metal ion transport and biochemical activity. P. B. Chock and E. Titus, Prog. Inorg. Chem., 1973,18, 287-382 (450). 

Finally, some authors have computed metal loading to the enviroranent from specific human activities, such as discharges of waste-water, and compared these with natural release rates. While the details of the computations and conclusions vary, the general observation for many metals is that anthropogenic contributions to metal ion transport rates and environmental burdens are approaching and in many cases have already exceeded natural contributions. A few such comparisons are provided in Tables 15-1-15-4. 

Mucosal cells can take up iron from the lumen across their brush border membranes by at least two separate pathways, both of which are thought to be receptor mediated. Non-haem dietary iron seems most likely to be taken across the brush border membrane after reduction by an apical, membrane-bound, ferrireductase and subsequent transport of the Fe2+ by a divalent metal-ion transporter protein, known both as DCT1 and Nramp2. 

The cyclohexane-containing system (203) was also prepared in an attempt to obtain metal complexes with clam type geometries (Owen, 1983). As for the previous system, it was considered that such complexes might show enhanced shielding of the cation, from both the solvent and the counter ion present, but still allow the bound metal to be readily released on demand. As is evident from our earlier discussion, both these are desirable properties for metal-ion transport systems. 

The compounds just discussed have all been implicated in alkali metal-ion transport and related phenomena in biological systems. Substances such as these, which are capable of carrying ions across a hydrophobic membrane, are called ionophores. 

Figure 9.4. Schematic representation of carrier-mediated metal-ion transport through a liquid membrane (A = anion).

Other factors influencing the rate of metal-ion transport across artificial membranes have been identified. As might be expected, such transport is dependent on the interplay of several factors. For example, as briefly mentioned already in Chapter 4, it is clear that the strength of complex-ation of the cation by the carrier must be neither too high nor too low if efficient transport is to be achieved. If the stability is low, then uptake of the metal ion from the source phase will be inhibited. Conversely, for those cases where highly stable complexes are formed, there will be a reluctance by the carrier to release the cation into the receiving phase. 

Chock, P. B. and Titus, E. O., Alkali Metal Ions Transport and Biochemical... 

Hamilton, R. T. and Kaler, E. W. (1990). Alkali metal ion transport through thin bilayers, J. Phys. Chem., 94, 2560-2566. 

Figure 7.10 Predicted membrane topologies for the ZIP/SLC39 and CDF/Znt/SLC30 families of metal ion transporters. (From Eide, 2006. Copyright 2006, with permission from Elsevier.)...

Since in mammalian species metals first need to be assimilated from dietary sources in the intestinal tract and subsequently transported to the cells of the different organs of the body through the bloodstream, we will restrict ourselves in this section to the transport of metal ions across the enterocytes of the upper part of the small intestine (essentially the duodenum), where essentially all of the uptake of dietary constituents, whether they be metal ions, carbohydrates, fats, amino acids, vitamins, etc., takes place. We will then briefly review the mechanisms by which metal ions are transported across the plasma membrane of mammalian cells and enter the cytoplasm, as we did for bacteria, fungi and plants. The specific molecules involved in extracellular metal ion transport in the circulation will be dealt with in Chapter 8. 

Eide, D.J. (2004) The SLC39 family of metal ion transporters, Pflugers Arch. Eur. J. Physiol., 447, 796-800. 

Hydroxamic acids are important bioligands and are involved in numerous biological processes including metal-ion transport and inhibition of metalloenzymes . 1 1 Metal binding to hydroxamic acids usually occurs in a bidentate fashion (Scheme 99) with... 

Specific (or selective) polyfunctionalization of cyclodextrin is, in principle, possible based upon stepwise regiospecific flamingo-type capping as is shown schematically in Scheme 5. Simple tetrasubstitution (or hexasubstitu-tion) is more easily attained through double (or triple) capping. A typical example is the successful preparation of tetrasubstituted /J-cyclodextrin (8), prepared from 7 by the authors as a channel-forming compound for metal ion transport (77). 

Looking at the literature in the field of biomineralization, one notices, that the majority of articles is descriptive in nature. On the basis of electron micrographs or thin section studies, the intricate relationships between mineral phase and organic matrix are investigated. Other papers deal with the chemical composition of the mineralized tissue and the minerals. Only a few authors address themselves to the question of metal ion transport mechanisms in cellular systems and the solid state principles involved in mineral deposition on organic substrates. All three sets of information, however, are essential to understand calcification processes. It appears, therefore, that information on the functionality of metal ions in living systems and their role in mineral deposition are particularly desired in this area of research. 

Gunshin H., Mackenzie B., Berger U. V., Gunshin Y., Romero M. F., Boron W. F., Nussberger S., Gollan J. L., and Hediger M. A. (1997). Cloning and characterization of a mammalian proton-coupled metal-ion transporter. Nature 388 482-488. 

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