Exchange transporters

THE RELATIVE IMPERMEABILITY OF THE INNER MITOCHONDRIAL MEMBRANE NECESSITATES EXCHANGE TRANSPORTERS... 

Because the inner mitochondrial membrane is impermeable to protons and other ions, special exchange transporters span the membrane to allow passage of ions such as OH, Pf, ATP , ADP, and metabo-htes, without discharging the electrochemical gradient across the membrane. 

Carnitine (p-hydroxy-y-trimethylammonium butyrate), (CHjljN"—CH2—CH(OH)—CH2—COO , is widely distributed and is particularly abundant in muscle. Long-chain acyl-CoA (or FFA) will not penetrate the inner membrane of mitochondria. However, carnitine palmitoyltransferase-I, present in the outer mitochondrial membrane, converts long-chain acyl-CoA to acylcarnitine, which is able to penetrate the inner membrane and gain access to the P-oxidation system of enzymes (Figure 22-1). Carnitine-acylcar-nitine translocase acts as an inner membrane exchange transporter. Acylcarnitine is transported in, coupled with the transport out of one molecule of carnitine. The acylcarnitine then reacts with CoA, cat-... 

Candidates for the renal brush border Na /H exchanger transport protein identified by covalent labeling, affinity chromatography, or other methods... 

Stimulation of basolateral Na /H exchanger transport activity in LLC-PK] cells during chronic metabolic acidosis is accompanied by a parallel increase in NHE-1 transcript abundance [80]. Chronic metabolic acidosis also increases the abundance of Na /H exchanger transcripts in rat renal cortices [81] and SV40-transformed mouse proximal tubule cells [82]. 

Ogihara, T., Tamai, I., Tsuji, A., Structural characterization of substrates for the anion exchange transporter in Caco-2 cells, J. Pharm. Sci. 1999, 88, 1217-1221. 

Moller, N., Greenberg, J. P. Weare, J. H. 1998. Computer modeling for geothermal systems predicting carbonate and silica scale formation, CO2 breakout, and H2S exchange. Transport in Porous Media, 33, 173-204. 

Antiporter or ion exchange transporters are also common. For example, E. coli uses a metal ion-tetracycline / H+ transporter to carry the antibiotic tetracycline out of cells. This protein, when present, provides a high level of antibiotic resistance to the bacteria.444... 

The dynamic behavior of processes (pipe-vessel combinations, heat exchangers, transport pipelines, furnaces, boilers, pumps, compressors, turbines, and distillation columns) can be described using simplified models composed of process gains, dead times, and process dynamics. 

Therefore, the generalized kinetic equations for exchange (transport) processes and chemical reactions are of similar structure. 

Figure 2.19. Active secretion of organic acids in the proximal tubule, a Secretion of p-Aminohippnrate. Two exchange transporters are involved. The first one is located in the basolateral membrane its operation is electrically driven. Transport by the second one, which is located in the apical (luminal) membrane, is drivenby concentrationgradients. This transporter is inhibited by probenecid, b Penicillin G, like p-aminohippurate, is a substrate for both transporters. Probenicid inhibits the apical transporter and therefore prevents the rapid elimination of penicillin.

The continuous flux of ions through leak channels (and exchange transporters) requires continuous operation of the ion pumps. Therefore, a sizeable fraction of our metabolic energy is expended just to keep up the ion gradients across our cell membranes. 

The exchange transport mechanism for Na+ and K can be briefly explained as follows. When three Na ions bind to the protein, phosphorylation occurs. 

Since continuous transfer of ion exchangers as dense beds in the face of solution counterflows at a high rate in cylindrical columns is not practicable, the problem of exchanger transport was solved by alternating the movement of solution through the fixed dense bed with the movement of the sorbent for the transfer of its exhausted portion and for the addition of its replacement. 

The and of exchange transport have been measured and the major laboratories obtain fairly consistent values [5,18]. Pyruvate exchanges for mono-carboxylic acids including acetoacetate and possibly the branched chain keto acids [24]. It has been demonstrated in perfused rat livers that exchange of internal acetoacetate for external pyruvate is important in the control of ketogenesis [114]. The measured of exchange appears to be independent of the nature of the metabolites exchanged. At 4°C and pH 7.0, the value is 16 nmol min mg and the for external pyruvate is 0.11 mM, whereas the of net uptake is 0.6 nmol min mg [18]. 

Other electroneutral exchange transporters. Although it may be tempting to discount the Sluse studies due to the kinetic complexity of the data obtained and obvious technical problems, other workers studying exchange transporters have concluded that substrate binding sites on either side of the membrane are independent of each other and that the ratio is not a constant. Variation of the... 

Ion [Exchange Transport Systems of the Tubuloveskles and Plasma Membrane... 

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