Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: II. Comparison with transport models

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Abstract
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 statesNC′S andNC″S 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 beK N ″ <-64mm andK S ″ <-18mm. Furthermore, the ratioK N ″ /K N S″ 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.