Decrease of intracellular cystine content in cystinotic fibroblasts by inhibitors of gamma-glutamyl transpeptidase

Cystine content of skin fibroblasts derived from patients with cystinosis was decreased by inhibitors of gamma-glutamyl transpeptidase, the initial enzyme in glutathione catabolism. The addition of maleate or the gamma-glutamyl hydrazone of alpha-ketobutyric acid to culture medium (1-20 mM) resulted in dose-dependent decreases of up to 55% on intracellular cystine content of cystinotic cells in 24 h. L-Serine in sodium borate buffer (40 mM each) produced similar results and further decreased cystine levels to 14% of cystinotic control values after 10 days incubation. Analysis of intracellular amino acids showed that, in general, other amino acids remained unchanged following serine-borate treatment. These results suggest that cystine storage in cystinotic tissues may be related to metabolism of glutathione.