INTERACTION OF L-CYSTEINE WITH A HUMAN EXCITATORY AMINO-ACID TRANSPORTER

1. The interaction of L-cysteine with three excitatory amino acid tran sporter subtypes cloned from human brain (EAAT1-3) was examined by mea suring transporter-mediated electrical currents and radiolabelled amin o acid flux in voltage-clamped Xenopus oocytes expressing the transpor ters. 2. L-Cysteine was transported by the neuronal subtype EAAT3 (EAA C1) with an affinity constant of 1.90 mu M and a maximal rate of flux similar to that of L-glutamate; the relative efficacies (V-max/K-m) of the EAAT1 and EAAT2 subtypes for transporting L-cysteine were 10- to 20-fold lower. 3. Changing the ionization state of L-cysteine by raisi ng the external pH did not significantly change the apparent affinity, transport rate, or magnitude of currents induced by L-cysteine, sugge sting that both the neutral zwitterionic and anionic forms of the amin o acid are transported with the same net charge stoichiometry. 4. In a ddition to competing with L-glutamate for uptake by the neuronal carri er, L-cysteine caused transporter-mediated release of transmitter by h eteroexchange; both actions would elevate extracellular glutamate conc entrations and may thus contribute to the known excitotoxic actions of L-cysteine in the brain.5. Because the EAAT3 transporter is also expr essed in tissues including kidney and intestine, the results suggest t he possibility of a heretofore unrecognized mechanism of L-cysteine up take in peripheral tissues as well as in brain.