Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: II. Comparison with transport models |
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Authors: | P. Jauch P. Läuger |
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Affiliation: | (1) Department of Biology, University of Konstanz, D-7750 Konstanz, Federal Republic of Germany |
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Abstract: | Summary In this paper, the results of the preceding electrophysiological study of sodium-alanine cotransport in pancreatic acinar cells are compared with kinetic models. Two different types of transport mechanisms are considered. In the simultaneous mechanism the cotransporterC forms a ternary complexNCS with Na+ and the substrateS; coupled transport of Na+ andS involves a conformational transition between statesNCS andNCS with inward- and outward-facing binding sites. In the consecutive (or ping-pong) mechanism, formation of a ternary complex is not required; coupled transport occurs by an alternating sequence of association-dissociation steps and conformational transitions. It is shown that the experimentally observed alanine- and sodium-concentration dependence of transport rates is consistent with the predictions of the simultaneous model, but incompatible with the consecutive mechanism. Assuming that the association-dissociation reactions are not rate-limiting, a number of kinetic parameters of the simultaneous model can be estimated from the experimental results. The equilibrium dissociation constants of Na+ and alanine at the extracellular side are determined to beKN<-64mm andKS<-18mm. Furthermore, the ratioKN/KNS of the dissociation constants of Na+ from the binary (NC) and the ternary complex (NCS) at the extracellular side is estimated to be <-6. This indicates that the binding sequence of Na+ andS to the transporter is not ordered. The current-voltage behavior of the transporter is analyzed in terms of charge translocations associated with the single-reaction steps. The observed voltage-dependence of the half-saturation concentration of sodium is consistent with the assumption that a Na+ ion that migrates from the extracellular medium to the binding site has to traverse part of the transmembrane voltage. |
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Keywords: | cotransport electrogenic transport sodiumcoupled amino-acid transport current-voltage characteristic |
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