Steady-state Kinetic Characterization of the Mouse B(0)AT1 Sodium-dependent Neutral Amino Acid Transporter

The members of the neurotransmitter transporter family SLC6A exhibit a high degree of structural homology; however differences arise in many aspects of their transport mechanisms. In this study we report that mouse B(0)AT1 (mouse Slc6a19) mediates the electrogenic transport of a broad range of neutral amino acids but not of the chemically similar substrates transported by other SLC6A family members. Cotransport of L: -Leu and Na(+) generates a saturable, reversible, inward current with Michaelis-Menten kinetics (Hill coefficient approximately 1) yielding a K(0.5) for L: -Leu of 1.16 mM and for Na(+) of 16 mM at a holding potential of -50 mV. Changing the membrane voltage influences both substrate binding and substrate translocation. Li(+) can substitute partially for Na(+) in the generation of L: -Leu-evoked inward currents, whereas both Cl(-) and H(+) concentrations influence its magnitude. The simultaneous measurement of charge translocation and L: -Leu uptake in the same cell indicates that B(0)AT1 transports one Na(+) per neutral amino acid. This appears to be accomplished by an ordered, simultaneous mechanism, with the amino acid binding prior to the Na(+), followed by the simultaneous translocation of both co-substrates across the plasma membrane. From this kinetic analysis, we conclude that the relatively constant [Na(+)] along the renal proximal tubule both drives the uptake of neutral amino acids via B(0)AT1 thermodynamically and ensures that, upon binding, these are translocated efficiently into the cell.