Transport of L-cystine across the cell membrane is essential for synthesis of 
the major cellular antioxidant, glutathione (gamma-glutamylcysteinylglycine). In 
this study, uptake of L-[14C]cystine by three of the high affinity 
sodium-dependent mammalian glutamate transporters (GLT1, GLAST and EAAC1) 
individually expressed in HEK cells has been determined. All three transporters 
display saturable uptake of L-[14C]cystine with Michaelis affinity (K(m)) 
constants in the range of 20-110 microM. L-glutamate and L-homocysteate are 
potent inhibitors of sodium-dependent L-[14C]cystine uptake in HEK(GLAST), 
HEK(GLT1) and HEK(EAAC1) cells. Reduction of L-[14C]cystine to L-[14C]cysteine 
in the presence of 1mM cysteinylglycine increases the uptake rate in HEK(GLT1), 
HEK(GLAST) and HEK(EAAC1) cells, but only a small proportion (<10%) of 
L-[14C]cysteine uptake in HEK(GLT1) and HEK(GLAST) cells occurs by the high 
affinity glutamate transporters. The majority (>90%) of L-[14C]cysteine 
transport in these cells is mediated by the ASC transport system. In HEK(EAAC1) 
cells, on the other hand, L-[14C]cysteine is transported equally by the ASC and 
EAAC1 transporters. L-homocysteine inhibits L-[14C]cysteine transport in both 
HEK(GLAST) and HEK(GLT1) cells, but not in HEK(EAAC1) cells. It is concluded 
that the quantity of L-[14C]cyst(e)ine taken up by individual high affinity 
sodium-dependent glutamate transporters is determined both by the extracellular 
concentration of amino acids, such as glutamate and homocysteine, and by the 
extracellular redox potential, which will control the oxidation state of 
L-cystine.