Molecular mechanisms of cystine transport

The transport of L-cystine into cells of the mammalian brain is an essential step in the supply of cysteine for synthesis of the antioxidant glutathione. Uptake of L-cystine in rat brain synaptosomes occurs by three mechanisms that are distinguishable on the basis of their ionic dependence, kinetics of transport and specificity of inhibitors. Almost 90% of L-cystine transport is by a low-affinity, sodium-dependent mechanism (K-m = 473 +/- 146 muM), that is mediated by the X-AG- family of glutamate transporters. Both L-glutamate (IC50 = 9.1 +/-0.4 muM) and L-cysteine sulphinate (IC50 = 16.4 +/-3.6 muM) are non-competitive inhibitors of sodium-dependent L-[C-11]cystine transport, whereas L-trans-pyrrolidine-2,4-dicarboxylic acid (IC50 = 5.6 +/-2.0 muM), L-serine-O-sulphate (IC50 = 13.2 +/-5.4 muM), kainate (IC50 =215 +/- 78 muM) and L-cysteine (IC50 = 363 +/- 63 muM) are competitive inhibitors. L-Cystine has no effect on the sodium-dependent uptake Of D-[H-3]aspartate. These results suggest that L-cystine binds to a site that is different from the L-glutamate recognition site on X-AG-glutamate transporters. In rat brain slices, sodium-dependent transport of both L-glutamate and L-cystine is necessary for maintaining glutathione levels. Uptake of L-Cystine is sensitive to inhibition by an increased extracellular concentration of L-glutamate, which has important implications for understanding conditions that may initiate oxidative stress.