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PubMed1964

UREA: APPARENT CARRIER-MEDIATED TRANSPORT BY FACILITATED DIFFUSION IN DOGFISH ERYTHROCYTES.

The exposure of erythrocytes from the elasmobranch, Squalus acanthias, to solutions isosmotic with plasma (IM) but containing urea or hydroxyurea as the sole solute does not produce hemolysis. Exposure of these cells to IM methylurea, thiourea and acetamide does produce hemolysis. Low concentrations of urea, which are associated with hemolysis, protect dogfish red cells against hemolysis by methylurea and thiourea. Dogfish red cells exposed to mediums containing high concentrations of urea, or no urea, reach 95 percent of their equilibrium concentration in less than 5 minutes.

Acetamides↗
PubMed1976

Boundary layer resistance of steady-state oxygen diffusion facilitated by a four-step chemical reaction with hemoglobin in solution.

The steady-state diffusion of oxygen in layers of hemoglobin solutions takes place under conditions of chemical equilibrium everywhere except in two boundary layers where deviations occur. The transport resistance of a boundary layer is defined in this paper. It is independent of the solution in the region of equilibrium and therefore forms a suitable basis to compare facilitated transport with different reaction schemes. Results are presented for a four-step reaction scheme between oxygen and hemoglobin, and compared with those of a single-step reaction for various reaction rates. The sensitivity of the results to changes in the reaction rates of the four-step model is also studied. With a knowledge of the boundary layer resistance the results of equilibrium calculations can be corrected in a simple way, which allows dirrect evaluation of experimental results.

Diffusion↗
PubMed1985

Carrier facilitated diffusion.

The concept of a mobile carrier combining reversibly with a substrate is considered as a possible mechanism for facilitated transport across biological membranes. The mathematical model is a system of three reaction diffusion equations with certain boundary conditions. Two limiting cases are discussed in detail: The case of a "thin" membrane where the diffusion of bound and unbound carrier from one surface to the other may be simulated by a single jump. If the diffusion rate of the substance to be transported is small, then an approximate stationary solution is derived using singular perturbation theory. Finally, the results of numerical simulations are presented for a wide range of parameters.

Biological Transport↗
PubMed1977

The facilitated diffusion of oxygen by hemoglobin and myoglobin.

We have clarified the use of Wyman's differential equation for the facilitated oxygen flux through a slab of solution of myoglobin or hemoglobin by showing that there is a unique choice of boundary condition on the carrier concentration to be employed in conjunction with it. The singular perturbation solution of Wyman's equation, due to Murrayand Mitchell and Murray, has been extended. By means of it, the paradox of Wittenberg, that the facilitated oxygen flux per mole of heme is apparently independent of the protein carrier, has been resolved.

Diffusion↗
PubMed2002

Ammonium/methylammonium transport (Amt) proteins facilitate diffusion of NH3 bidirectionally.

The ammonium/methylammonium transport (Amt) proteins of enteric bacteria and their homologues, the methylammonium/ammonium permeases of Saccharomyces cerevisiae, are required for fast growth at very low concentrations of the uncharged species NH(3). For example, they are essential at low ammonium (NH(4)+ + NH(3)) concentrations under acidic conditions. Based on growth studies in batch culture, the Amt protein of Salmonella typhimurium (AmtB) cannot concentrate either NH(3) or NH(4)+ and this organism appears to have no means of doing so. We now show that S. typhimurium releases ammonium into the medium when grown on the alternative nitrogen source arginine and that outward diffusion of ammonium is enhanced by the activity of AmtB. The latter result indicates that AmtB acts bidirectionally. We also confirm a prediction that the AmtB protein would be required at pH 7.0 in ammonium-limited continuous culture, i.e., when the concentration of NH(3) is < or =50 nM. Together with our previous studies, current results are in accord with the view that Amt and methylammonium/ammonium permease proteins increase the rate of diffusion of the uncharged species NH(3) across the cytoplasmic membrane. These proteins are examples of protein facilitators for a gas.

Ammonia↗