Intracellular glutathione cycling by gamma-glutamyl transpeptidase in tumorigenic and nontumorigenic cultured rat liver cells

Abstract

The nontumorigenic ARL-15C1 and tumorigenic gamma-glutamyl transpeptidase-containing ARL-16T2 cell lines were found to contain approximately equal amounts of glutathione, cysteine, and cystine, 65.6, 3.5, and 5 nmol/mg of protein for ARL-16C1, and 61.5, 3, and 3 nmol/mg of protein for ARL-16T2, respectively. The half-life for glutathione in these cell lines was 3.2 and 3.8 h in the ARL-15C1 and 16T2, respectively. In ARL-15C1 cells, the cysteine half-life was 0.2 h and that of cystine 2.0 h compared to 2.0 h and 0.5 h, respectively, in the ARL-16T2. The turnover of glutathione in the ARL-15C1 could be accounted for by efflux into the medium whereas only 10% of the glutathione expected from ARL-16T2 cells appeared in the medium. The ARL-16T2 cells appear to support glutathione synthesis by conservation and recycling of cysteine residues. Inhibition of gamma-glutamyl transpeptidase by AT-125 (acivicin) caused extensive loss of intracellular glutathione from ARL-16T2 cells but produced no effect on GSH levels in ARL-15C1 cells. No metabolism of medium glutathione by gamma-glutamyl transpeptidase was detected, independent of AT-125 treatment. AT-125 treatment caused a transient increase in intracellular GSH in the ARL-16T2 but not the ARL-15C1, further suggesting that the enzyme catalyzes intracellular GSH recycling to supply cysteine for cellular functions in the tumorigenic ARL-16T2 cell line. Transport of cysteine, cystine, and methionine was not altered by AT-125 treatment. These data are consistent with an intracellular orientation of gamma-glutamyl transpeptidase in this cell line and not participation in extracellular processes.