Trans-membrane transport

IV. Ca2+ in lonophores, Channels, and Pumps A. Trans-Membrane Transport... 

EXAFS studies on tris-maltolatoiron(III) in the solid state and in solution, and on [Fe(Ll)3] hydrate, pave the way for detailed investigation of the hydration of complexes of this type in aqueous media.Solubilities and transfer chemical potentials have been determined for tris-maltolatoiron(III) in methanol-water, and for tris-etiwlmaltolatoiron(III) in alcohol-water mixtures and in isobutanol, 1-hexanol, and 1-octanol. Solubility maxima in mixed solvents, indicating synergic solvation, is relevant to trans-membrane transport of complexes of this type. Solubilities of tris-ethylmaltolatoiron(III) and of [Fe(Ll)3] have been determined in aqueous salt solutions (alkali halides NH4 and NR4 bromides). ... 

The relationship between Na-K ATPase activity and active trans-membrane transport of Na" " and K, discussed in detail in earlier reviews [6,127,128] rests on the following arguments. Both Na-K ATPase and Na - K transport are activated by the simultaneous presence of internal ATP, Mg and Na and external K and both are inhibited by externally present cardiac glycosides like ouabain. The half-maximal activating concentrations of Na and K, the values for ATP and the half-inhibitory concentrations of ouabain are nearly equal for the two activities in the systems where they have been determined. For a large variety of tissues there is a remarkably constant ratio of 3 Na transported per ATP hydrolyzed ([6] pp. 271-272 [128] p. 158). The 3Na /2K stoichiometry for the transport agrees with the ratio of Na released to K bound upon phosphorylation of the enzyme (Section 2). Definitive proof for the involvement of the enzyme in transmembrane transport of Na and K has come from reconstitution studies in which a purified... 

In the previous section, we showed that, because of their large surface areas, facial amphiphiles are active at hydrophobic/hydrophilic interfaces. This is not only the case for oil/water interfaces but also for the hydrophobic/hydrophilic interface of phospholipid bilayers. Because of this property, facial amphiphilic molecules have been found to carry out a variety of biological functions related to their interaction with cell membranes. One of the most prominent functions of facial amphiphiles is the formation of pores in bilayer membranes. In the case of peptide antibiotics and toxins, depending if the peptide is specific for bacterial or mammalian cells, respectively, the formation of pores leads to lysis of the cells and ultimately cell death. Many other facial amphiphiles form pores to regulate trans-membrane transport, such as the ion channels for selectively transport ions, or release osmotic pressure. 

The same velcrand has also been shown to engender cavitand-mediated endo-cytosis [188]. The authors demonstrated selective, controlled endocytosis of a trimethylammonium-tagged fluorescein to several types of living human cells with little observed cytotoxicity. This approach therefore represents a novel method of small-molecule trans-membrane transport controlled by host-guest complementarity. 

A different approach is the use of an ultrafiltration membrane with an immobilized chiral component [31]. The transport mechanism for the separation of d,l-phenylalanine by an enantioselective ultrafiltration membrane is shown schematically in Fig. 5-4a. Depending on the trans-membrane pressure, selectivities were found to be between 1.25 and 4.1, at permeabilities between 10 and 10 m s respectively (Fig. 5-4b). 

As shown in Table 11.1, there are various membrane transporters and isoforms. Many of these transporters are expressed in the GI tract. Among them, the peptide transport systems have received the most attention. Di-, tri-peptide trans-... 

Lee, S. S. Yoon, I. Park, K.-M. Jung, J. H. Lindoy, L. F. Nezhadali, A. Rounaghi, G. Competitive bulk membrane transport and solvent extraction of transition and post transition metal ions using mixed-donor acyclic ligands as ionophores. J. Chem. Soc.-Dalton Trans. 2002, 2180-2184. 

Taking the rate limiting step in the electron transport chain to be trans-membrane proton translocation, which occurs about five times per sulfate consumed (Rabus et al., 2006), the average stoichiometric number x (entered into REACT as to = 1/x) for Reaction 18.7 is five. Sulfate reducers conserve about 45 kJ mol-1 of sulfate consumed (Qusheng Jin, unpublished data), so we set AGp to this value and m to one. From equations 18.12 and 18.14, then, we can write... 

It should be mentioned that the placental villous fragments can be used to measure uptake into the syncytiotrophoblast layer but they cannot be used for trans-cellular transport studies. If the transporter is expressed in the microvillous border membrane, the effects of various factors on transporter function can be determined. Another disadvantage is that the villous fragments may be heterogenous in composition and hence, uptake experiments may not be reflective of syncytiotrophoblast uptake alone. 

The successful application of in vitro models of intestinal drug absorption depends on the ability of the in vitro model to mimic the relevant characteristics of the in vivo biological barrier. Most compounds are absorbed by passive transcellular diffusion. To undergo tran-scellular transport a molecule must cross the lipid bilayer of the apical and basolateral cell membranes. In recent years, there has been a widespread acceptance of a technique, artificial membrane permeation assay (PAMPA), to estimate intestinal permeability.117118 The principle of the PAMPA is that, diffusion across a lipid layer, mimics transepithelial permeation. Experiments are conducted by applying a drug solution on top of a lipid layer covering a filter that separates top (donor) and bottom (receiver) chambers. The rate of drug appearance in the bottom wells should reflect the diffusion across the lipid layer, and by extrapolation, across the epithelial cell layer. 

Just as there are cation channels, there are also trans-membrane channels involved in the transport of biologically important anions such as Cl-. The crystal structure of the CIC chloride channel from Salmonella typhimurium was reported in 2002.3 Along with the determination of the Streptomyces lividans potassium channel structure, this work won a share of the 2003 Nobel prize in chemistry for Roderick MacKinnon (Howard Hughes Medical Institute, New York, USA). Chloride channels catalyse the flow of chloride across cell membranes and play a significant role in functions such as... 

Cragg, P. J., Allen, M. C., Steed, J. W., A toothpaste tube model for ion transport through trans- membrane channels. 

---