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 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 S″ N 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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References
Aronson, P.S. 1984. Electrochemical driving forces for secondary active transport: Energetics and kinetics of Na+−H+ exchange and Na+-glucose cotransport.In: Electrogenic Transport: Fundamental Principles and Physiological Implications. M.P. Blaustein and M. Liberman, editors. Raven Press, New York
Burckhardt, G., Kinne, R., Stange, G., Murer, H. 1980. The effects of potassium and membrane potential on sodium-dependent glutamic acid uptake.Biochim. Biophys. Acta 599:191–201
Carter-Su, C., Kimmich, G.A. 1980. Effects of membrane potential on Na-dependent sugar transport by ATP-depleted intestinal cells.Am. J. Physiol. 238:C73-C80
Ciani, S. 1984. Coupling between fluxes in one-particle pores with fluctuating energy profiles: A theoretical study.Biophys. J. 46:249–252
Crane, R.K., Dorando, F.C. 1980. The kinetics and mechanism of Na+-gradient-coupled glucose transport.In: Membranes and Transport. A.N. Martonosi, editor. Vol. 2, pp. 153–160. Plenum, New York
Eddy, A.A. 1980. Slip and leak models of gradient-coupled transport.Trans. Biochem. Soc. London 8:271–273
Ganapathy, V., Leibach, F.H. 1983. Electrogenic transport of 5-oxoproline in rabbit renal brush-border membrane vesicles: Effect of intravesicular potassium.Biochim. Biophys. Acta 732:32–40
Geck, P., Heinz, E. 1976. Coupling in secondary transport: Effect of electrical potentials on the kinetics of ion linked cotransport.Biochim. Biophys. Acta 443:49–53
Harrison, D.A., Rowe, G.W., Lumsden, C.F., Silverman, M. 1984. Computational analysis of models for cotransport.Biochim. Biophys. Acta 774:1–10
Hilden, H., Sacktor, B. 1982. Potential-dependentd-glucose uptake by renal brush border membrane vesicles in the absence of sodium.Am. J. Physiol. 242:F340-F345
Hopfer, U., Groseclose, R. 1980. The mechanism of Na+-dependentd-glucose transport.J. Biol. Chem. 255:4453–4462
Jauch, P., Petersen, O.H., Läuger, P. 1986. Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: I. Tight-seal whole-cell recordings.J. Membrane Biol. 94:99–115
Kaunitz, H.D., Wright, E.M. 1984. Kinetics of sodiumd-glucose cotransport in bovine intestinal brush border vesicles.J. Biol. Chem. 79:41–51
Kessler, M., Semenza, G. 1983. The small-intestinal Na+,d-glucose cotranporter: An asymmetric gated channel (or pore) responsive to Δψ.J. Membrane Biol. 76:27–56
Lapointe, J.-Y., Hudson, R.L., Schultz, S.G. 1986. Currentvoltage relations of sodium-coupled sugar transport acorss the apical membrane ofNecturus small intestine.J. Membrane Biol. 93:205–220
Läuger, P. 1984. Thermodynamic and kinetic properties of electrogenic ion pumps.Biochim. Biophys. Acta 779:307–341
Läuger, P. 1985. Ionic channels with conformational substates.Biophys. J. 47:581–591
Läuger, P., Jauch, P. 1986. Microscopic description of voltage effects on ion-driven cotransport systems.J. Membrane Biol. 91:275–284
Mitchell, P. 1969. Chemiosmotic coupling and energy transduction.Theor. Exp. Biophys. 2:159–216
Murer, H., Hopfer, U. 1974. Demonstration of electrogenic Na+-dependentd-glucose transport in intestinal brush border membranes.Proc. Natl. Acad. Sci. USA 71:484–488
Restrepo, D., Kimmich, G.A. 1985a. Kinetic analysis of the mechanism of intestinal Na+-dependent sugar transport.Am. J. Physiol. 248:C498-C509
Restrepo, D., Kimmich, G.A. 1985b. The mechanistic nature of the membrane potential dependence of sodium-sugar cotransport in small intestine.J. Biol. Chem. 87:159–172
Sanders, D. 1986. Generalized kinetic analysis of ion-driven cotransport systems: II. Random ligand binding as a simple explanation for non-Michaelian kinetics.J. Membrane Biol. 90:67–87
Sanders, D., Hansen, U.-P., Gradmann, D., Slayman, C.L. 1984. Generalized kinetic analysis of ion-driven cotransport systems: A unified interpretation of selective ionic effects on Michaelis parameters.J. Membrane Biol. 77:123–152
Schultz, S.G. 1986. Ion-coupled transport of organic solutes across biological membranes.In: Physiology of Membrane Disorders. T.E. Andreoli, J.F. Hoffman, D.D. Fanestil, and S.G. Schultz, editors, pp. 283–294. Plenum, New York
Schultz, S.G., Curran, P.F. 1970. Coupled transport of sodium and organic solutes.Physiol. Rev. 50:637–718
Segel, I.H. 1975. Enzyme Kinetics. John Wiley, New York
Semenza, G., Kessler, M., Hosang, M., Weber, J., Schmidt, U. 1984. Biochemistry of the Na+,d-glucose cotransporter of the small-intestinal brush-border membrane: The state of the art in 1984.Biochim. Biophys. Acta 779:343–379
Stein, W.D. 1976. An algorithm for writing down flux equations for carrier kinetics, and its application to cotransport.J. Theor. Biol. 62:467–478
Turner, R.J. 1981. Kinetic analysis of a family of cotransport models.Biochim. Biophys. Acta 649:269–280
Turner, R.J. 1983. Kinetic analysis of a family of cotransport Models and vesicles.J. Membrane Biol. 76:1–15
Turner, R.J., Silverman, M. 1980. Testing carrier models of cotransport using the binding kinetics of non-transported competitive inhibitors.Biochim. Biophys. Acta 596:272–291
Wright, J.K. 1986. Experimental analysis of ion/solute cotransport by substrate binding and facilitated diffusion.Biochim. Biophys. Acta 854:219–230
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Jauch, P., Läuger, P. Electrogenic properties of the sodium-alanine cotransporter in pancreatic acinar cells: II. Comparison with transport models. J. Membrain Biol. 94, 117–127 (1986). https://doi.org/10.1007/BF01871192
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DOI: https://doi.org/10.1007/BF01871192