Interaction of L-cysteine with a human excitatory amino acid transporter.
1. The interaction of L-cysteine with three excitatory amino acid transporter subtypes cloned from human brain (EAAT1-3) was examined by measuring transporter-mediated electrical currents and radiolabelled amino acid flux in voltage-clamped Xenopus oocytes expressing the transporters. 2. L-Cysteine was transported by the neuronal subtype EAAT3 (EAAC1) with an affinity constant of 190 microM and a maximal rate of flux similar to that of L-glutamate; the relative efficacies (Vmax/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 raising the external pH did not significantly change the apparent affinity, transport rate, or magnitude of currents induced by L-cysteine, suggesting that both the neutral zwitterionic and anionic forms of the amino acid are transported with the same net charge stoichiometry. 4. In addition to competing with L-glutamate for uptake by the neuronal carrier, L-cysteine caused transporter-mediated release of transmitter by heteroexchange; both actions would elevate extracellular glutamate concentrations and may thus contribute to the known excitotoxic actions of L-cysteine in the brain. 5. Because the EAAT3 transporter is also expressed in tissues including kidney and intestine, the results suggest the possibility of a heretofore unrecognized mechanism of L-cysteine uptake in peripheral tissues as well as in brain.
Zerangue
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Vollum Institute, Oregon Health Sciences University, Portland 97201, USA.
Kavanaugh
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D000409:Alanine / Q000378:metabolism; D027322:Amino Acid Transport System X-AG; D000818:Animals; D001667:Binding, Competitive; D001693:Biological Transport, Active / Q000502:physiology; D001923:Brain Chemistry / Q000502:physiology*; D002352:Carrier Proteins / Q000737:chemistry / Q000378:metabolism*; D003545:Cysteine / Q000737:chemistry / Q000378:metabolism*; D004594:Electrophysiology; D027341:Excitatory Amino Acid Transporter 1; D027342:Excitatory Amino Acid Transporter 2; D050580:Excitatory Amino Acid Transporter 3; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011522:Protons; D017981:Receptors, Neurotransmitter / Q000737:chemistry / Q000378:metabolism*; D027981:Symporters; D014982:Xenopus laevis
D027322:Amino Acid Transport System X-AG; D002352:Carrier Proteins; D027341:Excitatory Amino Acid Transporter 1; D027342:Excitatory Amino Acid Transporter 2; D050580:Excitatory Amino Acid Transporter 3; D050491:Glutamate Plasma Membrane Transport Proteins; D011522:Protons; D017981:Receptors, Neurotransmitter; C493400:SLC1A1 protein, human; C493397:SLC1A2 protein, human; C493394:SLC1A3 protein, human; D027981:Symporters; D018698:Glutamic Acid; D003545:Cysteine; D000409:Alanine