Extracellular glutathione is a source of cysteine for cells that express .gamma.-glutamyl transpeptidase

Abstract

We show that gamma-glutamyl transpeptidase (GGT) is a glutathionase that enables cells to use extracellular glutathione as a source of cysteine. We transfected NIH/3T3 mouse fibroblasts with a plasmid containing cDNA for human GGT, and obtained stably transformed cell lines that expressed GGT in its proper orientation on the outer surface of the cell. 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 supplemented with glutathione. Cysteine derived from the cleavage of extracellular glutathione can be used to maintain intracellular levels of glutathione. GGT-positive NIH/3T3 cells were able to replenish intracellular glutathione when incubated in cysteine-free medium containing glutathione. GGT-negative cells could not. Therefore, GGT is a glutathionase that provides the cell with access to a secondary source of cysteine.

γ-Glutamyl transpeptidase is induced by 4-hydroxynonenal via EpRE/Nrf2 signaling in rat epithelial type II cells

“…γ-Glutamyl transpeptidase (γ-glutamyl transferase, GGT), an enzyme located on the outer surface of the plasma membrane, catalyzes the transfer of the γ-glutamyl moiety from glutathione (GSH), glutathione S-conjugates, or other γ-glutamyl compounds to γ-glutamyl acceptors such as amino acids, dipeptides, or H 2 O. GGT plays key roles in the maintenance of glutathione homeostasis [1][2][3][4][5][6][7] and metabolism of biomolecules such as leukotriene C4 and xenobiotics following their conjugation with GSH [8].…”

Human GGT2 Does Not Autocleave into a Functional Enzyme: A Cautionary Tale for Interpretation of Microarray Data on Redox Signaling

“…1) (17,19,32,45). In humans, GGT1 (hGGT1, P19440) is expressed on the apical surfaces of glands and ducts throughout the body (14).…”

“…The metabolism of glutathione by GGT1 releases free glutamate and the dipeptide, cysteinyl-glycine, which is hydrolyzed to cysteine and glycine by dipeptidases. The three constituent amino acids are then transported into the cell by amino-acid transporters (17,29). The highest level of GGT1 activity is on the apical surface of the proximal tubules in the kidney (14).…”

Up-regulation of γ-glutamyl transpeptidase activity following glutathione depletion has a compensatory rather than an inhibitory effect on mitochondrial complex I activity: implications for Parkinson's disease

“…Increased GGT activity in the presence of extracellular transpeptidases leads to the enhanced metabolism of extracellular GSH into cys and glu-cys (5) which can be taken up by glutamate-cyst(e)ine transporters into the cell (6). Within dopaminergic cells, either they can hypothetically conjugate with dopamine (DA) (7) to form cys-DA-Q whose metabolites have been proposed to inhibit mitochondrial complex I (CI) activity (8) or they can act to provide increased substrates for de novo (1) GSH synthesis via GCL (cys) or scavenger synthesis (9) via GS (glu-cys).…”

“…GGT can also transfer the γ-glutamyl moiety of GSH to extracellular cystine to form γ-glutamylcystine which can also be taken up into cells, reduced to γ-glutamylcysteine, and used, for scavenger glutathione synthesis via GS. GGT has, therefore, a role in cellular GSH homeostasis and may be of significant importance when the level of cyst(e)ine is reduced [9]. Increased GGT levels have been hypothesized to contribute to dopaminergic neurodegeneration associated with PD by increasing the amount of cysteine available for transport into dopaminergic neurons where it can interact with dopaminergic quinones formed during dopamine oxidation.…”