EXTRACELLULAR GLUTATHIONE IS A SOURCE OF CYSTEINE FOR CELLS THAT EXPRESS GAMMA-GLUTAMYL-TRANSPEPTIDASE

We show that gamma-glutamyl transpeptidase (GGT) is a glutathionase th at enables cells to use extracellular glutathione as a source of cyste ine. We transfected NIH/3T3 mouse fibroblasts with a plasmid containin g cDNA for human GGT, and obtained stably transformed cell lines that expressed GGT in its proper orientation on the outer surface of the ce ll. NIH/3T3 fibroblasts require cysteine for growth and are unable to use extracellular glutathione as a source of cysteine. We demonstrate GGT-positive fibroblasts are able to grow in cysteine-free medium supp lemented with glutathione. Cysteine derived from the cleavage of extra cellular glutathione can be used to maintain intracellular levels of g lutathione. GGT-positive NIH/3T3 cells were able to replenish intracel lular glutathione when incubated in cysteine-free medium containing gl utathione. GGT-negative cells could not. Therefore, GGT is a glutathio nase that provides the cell with access to a secondary source of cyste ine.