Na+-Coupled Alanine Transport in LLC-PK1 Cells: The Relationship Between the K m for Na+ at Low [Alanine] and Potential Dependence for the System

Na+-Coupled Alanine Transport in LLC-PK1 Cells: The Relationship Between the K m for Na+ at Low... Analysis of the mechanistic basis by which sodium-coupled transport systems respond to changes in membrane potential is inherently complex. Algebraic expressions for the primary kinetic parameters (K m and V max ) consist of multiple terms that encompass most rate constants in the transport cycle. Even for a relatively simple cotransport system such as the Na+/alanine cotransporter in LLC-PK1 cells (1:1 Na+ to substrate coupling, and an ordered binding sequence), the algebraic expressions for K m for either substrate includes ten of the twelve rate constants necessary for modeling the full transport cycle. We show here that the expression of K m of the first-bound substrate (Na+) simplifies markedly if the second-bound substrate (alanine) is held at a low concentration so that its' binding becomes the rate limiting step. Under these conditions, the expression for the K Na m includes rate constants for only two steps in the full cycle: (i) binding/dissociation of Na+, and (ii) conformational `translocation' of the substrate-free protein. The influence of imposed changes in membrane potential on the apparent K Na m for the LLC-PK1 alanine cotransporter at low alanine thus provides insight to potential dependence at these sites. The data show no potential dependence for K Na m at 5 μm alanine, despite marked potential dependence at 2 mm alanine when the full algebraic expression applies. The results suggest that neither translocation of the substrate-free form of the transporter nor binding/dissociation of extracellular sodium are potential dependent events for this transport system. The Journal of Membrane Biology Springer Journals

Na+-Coupled Alanine Transport in LLC-PK1 Cells: The Relationship Between the K m for Na+ at Low [Alanine] and Potential Dependence for the System

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Copyright © Inc. by 1998 Springer-Verlag New York
Life Sciences; Biochemistry, general; Human Physiology
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