Anion carriers

The conditional association constant for [18]aneN4 ( cyclic spermine ) with ATP - at pH 7.5 is calculated from the (3L value (Table 5) and protonation constants (Table 1) to be 2.4x 105 M-1, which is larger than the association constants for the linear spermidine (9 xi 02 M ) and spermine (9.5 x 103 M-1)23). It is also of interest that cyclic spermine is selective for ATP over AMP (ratio association constants is 700), while linear spermine prefers ATP to AMP only by a ratio of 26 to 1 43). The selective complexation of biologically important anions is of particular interest, especially if the ligands are converted into selective anion carriers by attachment of lipophilic hydrocarbon chains. 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

In the case of co-extraction, a selective anion-carrier (ionophore) extracts the analyte anion into the lipophilic sensor membrane. In order to maintain electroneutrality, a proton is co-extracted into the membrane where it protonates a pH indicator dye contained in the polymer membrane. Due to protonation, the dye undergoes a change in either absorption or fluorescence. (Figure 6 and Tables 13 and 14). 

The statins, lovastatin (L), simvastatin (S), pravastatin (P), fluvastatin (F), cerivastatin, and atorvastatin, inhibit HMG CoA reductase. The active group of L, S, P, and F (or their metabolites) resembles that of the physiological substrate of the enzyme (A). L and S are lactones that are rapidly absorbed by the enteral route, subjected to extensive first-pass extraction in the liver, and there hydrolyzed into active metabolites. P and F represent the active form and, as acids, are actively transported by a specific anion carrier that moves bile acids from blood into liver and also mediates the selective hepatic uptake of the mycotoxin, amanitin (A), Atorvastatin has the longest duration of action. 

L-phenyl-jJanine and various other amino acids Aliquat-336 KCland KOH Coimter-trans- ELM port of CL Concentration as anion carrier specificity for various aminoacids depends on hydrophobicity scale [45,95 ... 

Although one might think of neutral anion carriers for use in membrane electrodes, I cannot offer good suggestions. It is possible, however, to use charged ligands (classical ion exchangers) as membrane components. This makes accessible the measurement of, for example, Cl- and HCO in blood serum, without serious mutual interference. 

Does Professor Williams have good suggestions for highly polarizable groups that might be incorporated in ligands to obtain anion carriers ... 

Tire uncoupling protein resembles the ATP/ADP and phosphate anion carriers (Table 18-8), 1 which all have similar sizes and function as homodimers. Each monomeric subunit has a triply repeated 100-residue sequence, each repeat forming two transmembrane helices. Most mitochondrial transporters carry anions, and UCP1 will transport Cl. h/1 However, the relationship of chloride transport to its real function is unclear. Does the protein transport H+... 

Cation-anion cotransport was effected by an optically active macrotricyclic cryp-tand that carried simultaneously an alkali cation and a mandelate anion and displayed weak chiroselectivity [4.23a], as did also the transport of mandelate by an optically active acyclic ammonium cation [6.39]. Employing together a cation and an anion carrier should give rise to synergetic transport with double selection by facilitating the flow of both components of a salt (see the electron-cation symport below). Selective transport of amino acids is effected by a convergent dicarboxylic acid receptor [4.24b]. 

As earlier reported for electrochemical sensing, often the active chromo-phore will be dispersed in a polymeric matrix. For example, Mohr and Wolfbeis reported a nitrate sensor [121] where the active chromophore is a rhodamine B dye which had been modified with an octadecyl side chain to render it hydrophobic and prevent leaching. The dye was dispersed in a plasticised PVC membrane containing a hydrophobic anion carrier (tridodecylmethylammo-nium chloride). On exposure to nitrate, the fluorescence of the dye increased. This membrane, however, only displayed Hofmeister-type selectivity and was also affected by pH. Replacing the quaternary ammonium anion carrier with a palladium phospine chloride carrier led to selectivity for nitrite [ 122], probably due to a preferential interaction between Pd and nitrite ion. 

Meier PJ, St. Meier-Abt A, Barrett C, et al. Mechanisms of taurocholate transport in canalicular and basolateral rat liver plasma membrane vesicles. Evidence for an electrogenic canalicular organic anion carrier. J Biol Chem 1984 259 10614—10622. 

Among the anion carriers, calixpyrrole receptors have found applications as components in anion-selective membrane electrodes. The potentiometric selectivity for membranes ISEs based on calix[4]pyrrole, 1 towards a range of anions, namely fluoride, chloride, bromide and dihydrogen phosphate was found to be pH-dependent. 

McNally BA et al (2008) Structure-activity relationships in cholapod anion carriers enhanced transmembrane chloride transport through substituent tuning. Chem Eur J 14 9599-9606... 

In the case of expanded porphyrins, the transport-based approach, which is relevant to potential applications involving the regulation of anion flux (e.g., the development of chloride anion carriers for treatment of cystic fibrosis or adjuvants to allow improved uptake into cells of antiviral agents), has for the most part involved the use of a U-tube aqueous I-dichloromethane-... 

Figure 10.5.30 Schematic Representation of Standard U-Tube-type Model Membrane System Used to Test the Anion Carrier Capability of a Given Expanded Porphyrin System (Illustrated with a Generalized Nucleotide Serving as the Putative Substrate).

Safavi A, Peiravian F, and Shams E. A selective uphill transport of copper through bulk liquid membrane using Janus Green as an anion carrier. Sep Purif Technol, 2002 26(2-3) 221-226. 

Safavi A and Shams E. Selective and efficient liquid membrane transport of gold as gold cyanide using an anion carrier. J Mem Sci, 1999 157(2) 171-176. 

In rats, ACE inhibitors reduced renal cefdinir excretion, probably by competition at the tubular anionic carrier (201). 

Terlouw SA,Tanriseven O, Russel FGM, Masereeuw R. Metabolite anion carriers mediate the uptake ofthe anion drug fluorescein in renal cortical mitochondria. J Pharmacol Exp Ther 2000 293(3) 968-973. 

ArSX), and among these the most used anionic carrier of the sulfenylium ions is the chloride anion . However, sulfenyl bromides have been also used whereas iodides and fluorides are unstable although they can be prepared in. Other sulfenylating... 

Cytosolic generation of acetyl-CoA ( citrate shuttle ) This pathway is shown in Figure 18-13. Citrate synthesized from oxaloacetate and acetyl-CoA is transported to the cytosol via the tricarboxylate anion carrier system and cleaved to yield acetyl-CoA and oxaloacetate. 

Significant research on LM enzyme encapsulation systems has also been conducted at the University of Hannover, West Germany. Scheper et al. (19) proposed the use of LMs to resolve racemic D,L-phenylalanine methyl ester with encapsulated chymotrypsin. This enzyme cleaves the ester bond of the L-isomer only. The process employed Adogen 464 (TOMAC) as an anion carrier, but the pHs used were such that any L-phenylalanine formed would be zwitterionic LM transport of zwitterions would be expected to be poor. Further work has included development of an LM enzyme reactor for detoxification of blood (33), reductive amination of a-ketoisocaproate by L-leucine dehydrogenase with a coencapsulated... 

It has been shown that carrier-facilitated transport of amino acid ions plays a central role of development of LM enzyme systems. Better anion carriers and new cation carriers are needed to exploit enzymatic processes where amino acids (or derivatives) and their products must transport readily through the LM. Facile transport with little or no enzyme deactivation is required. Design of new carriers tailored specifically for LM processes will help pave the way for the industrial development of liquid membrane enzyme reactors. 

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