The kinetics of carrier-mediated transport: stationary-state approximations

Abstract

The fluxes predicted by various models of carrier-mediated active transport of amino acids have been worked out with use of the assumption of stationarity. the models considered include the various types of models in which there is a I:I carrier-amino acid binding ratio and the limiting cases of this model. The limiting case are: (I) equilibrium-reaction model: transfers of carrier and carrier-substrate complex across the cell membrane are rate limiting in comparison to the reaction between carrier and amino acid; (2) adsorption transport model: the reaction between carrier and amino acid is rate limiting. The stationary-state fluxes for an equilibrium carrier model in which carrier-amino acid stoichiometry was I:2 were also derived.The results of experiments on competition between amino acids could not be predicted by models of a I:I carrier-amino acid stoichiometry unless the assumption was also made that exchange reactions between free and carrier-bound amino acids could occur at the surfaces of the cell membrane. These exchange reactions proceed without prior dissociation of carrier-substrate complex. A model I:2 carrier-amino acid stoichiometry predicts the results of competition experiments without the necessity of assuming the existance of exchange reactions.