Differentiation of substrate and non-substrate inhibitors of transport system xc(-): an obligate exchanger of L-glutamate and L-cystine.
In addition to the well-characterized sodium-dependent excitatory amino acid transporters (EAATs) present in the mammalian CNS, a chloride-dependent, sodium-independent transporter has also been identified that is capable of mediating the uptake of L-glutamate. Named system x(c)(-), this transporter is an obligate exchanger that normally couples the export of intracellular L-glutamate with the import of extracellular L-cystine. Two cell lines that express high levels of system x(c)(-) are used to delineate the pharmacology of the transporter and demonstrate that it is distinct from both the EAATs and EAA ionotropic receptors. Potent competitive inhibitors of system x(c)(-) include: L-homocysteate, ibotenate, L-serine-O-sulphate, (RS)-4-bromohomoibotenate, quisqualate, and (S)-4-carboxyphenylglycine. A fluorescent-based assay that allows system x(c)(-)-mediated exchange of L-glutamate and L-cystine to be followed in real time is used to assess substrate activity. Interestingly, those compounds that proved to be the most potent competitive inhibitors (e.g. L-quisqualate and 4-S-CPG) also proved to be the least active as substrates, suggesting that distinct structural features may control binding and translocation. Lastly, the finding that a number of system x(c)(-) inhibitors are also commonly used as probes of excitotoxic pathology (e.g., L-quisqualate, ibotenate and L-homocysteate) raises some interesting questions regarding the mechanisms through which these analogues produce CNS damage.
Patel
Sarjubhai A
SA
Department of Biomedical and Pharmaceutical Sciences, University of Montana, Missoula, MT 59812, USA.
Warren
Brady A
BA
Rhoderick
Joseph F
JF
Bridges
Richard J
RJ
D027182:Amino Acid Transport System y+; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D045744:Cell Line, Tumor; D003553:Cystine / Q000737:chemistry / Q000493:pharmacokinetics*; D018698:Glutamic Acid / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D051381:Rats; D013379:Substrate Specificity / Q000502:physiology
D027182:Amino Acid Transport System y+; D002352:Carrier Proteins; C118895:SLC7A11 protein, human; D018698:Glutamic Acid; D003553:Cystine