Electrogenic properties of the epithelial and neuronal high affinity glutamate transporter

Active ion-coupled glutamate transport is of critical importance for excitatory synaptic transmission, normal cellular function, and epithelial amino acid metabolism. We previously reported the cloning of the rabbit intestinal high affinity glutamate transporter EAAC1 (Kanai, Y., and Hediger, M. A. (1992) Nature 360, 467-471), which is expressed in numerous tissues including intestine, kidney, liver, heart, and brain. Here, we report a detailed stoichiometric and kinetic analysis of EAAC1 expressed in Xenopus laevis oocytes. Uptake studies of ²²Na⁺ and [¹⁴C]glutamate, in combination with measurements of intracellular pH with pH microelectrodes gave a glutamate to charge ratio of 1:1, a glutamate to Na⁺ ratio of 1:2, and a OH^-/H⁺ to charge ratio of 1:1. Since transport is K⁺ dependent it can be concluded that EAAC1-mediated glutamate transport is coupled to the cotransport of 2 Na⁺ ions, the countertransport of one OH^- ion or the cotransport of 1 H⁺ ion. We further demonstrate that under conditions where the electrochemical gradients of these ions are disrupted, EAAC1 runs in reverse, a transport mode which is of pathologic importance. ²²Na⁺ uptake studies revealed that there is a low level of Na⁺ uptake in the absence of extracellular glutamate which appears to the analogous to the NA⁺ leak observed for the intestinal Na⁺/glucose cotransporter SGLT1. In voltage clamp studies, reducing extracellular Na⁺ from 100 to 10 mM strongly increased K_0.5/(L-glutamate) and decreased I(max). The data indicate that Na⁺ binding at the extracellular transporter surface becomes rate-limiting. Studies addressing the coperativity of the substratre-binding sites indicate that there are two distinct Na⁺-binding sites with different affinities and that Na⁺ binding is modulated by extracellular glutamate. A hypothetical ordered kinetic transport model for EAAC1 is discussed.