ELECTROGENIC PROPERTIES OF THE EPITHELIAL AND NEURONAL HIGH-AFFINITY GLUTAMATE TRANSPORTER

Active ion coupled glutamate transport is of critical importance for e xcitatory synaptic transmission, normal cellular function, and epithel ial amino acid metabolism, We previously reported the cloning of the r abbit 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 re port a detailed stoichiometric and kinetic analysis of E AAC1 expressed in Xenopus laevis oocytes. Uptake studies of Na-22(+) a nd [C-14]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. Si nce transport is K+ dependent it can be concluded that EAAC1-mediated glutamate transport is coupled to the cotransport of 2 Na+ ions, the c ountertransport of one Kf ion and either the countertransport of one O H- ion or the cotransport of 1 H+ ion, We further demonstrate that und er conditions where the electro chemical gradients for these ions are disrupted, EAAC1 runs in reverse, a transport mode which is of patholo gic importance, Na-22(+) uptake studies revealed that there is a low l evel of Na+ uptake in the absence of extracellular glutamate which app ears to be analogous to the Na+ leak observed for the intestinal. Na+/ glucose cotransporter SGLT1. In voltage clamp studies, reducing extrac ellular Na+ from 100 to 10 mM strongly increased K-0.5(L-glutamate) an d decreased I-max. The data indicate that Na+ binding at the extracell ular transporter surface be comes rate-limiting. Studies addressing th e cooperativity of the substrate-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 order ed kinetic transport model for EAAC1 is discussed.