Biologic and pharmacologic regulation of mammalian glutathione synthesis

Glutathione (L-γ-glutamyl-L-cysteinylglycine, GSH) is synthesized from its constituent amino acids by the sequential action of γ-glutamylcysteine synthetase (γ-GCS) and GSH synthetase. The intracellular GSH concentration, typically 1–8 mM, reflects a dynamic balance between the rate of GSH synthesis and the combined rate of GSH consumption within the cell and loss through efflux. The γ-GCS reaction is rate limiting for GSH synthesis, and regulation of γ-GCS expression and activity is critical for GSH homeostasis. Transcription of the γ-GCS subunit genes is controlled by a variety of factors through mechanisms that are not yet fully elucidated. Glutathione synthesis is also modulated by the availability of γ-GCS substrates, primarily L-cysteine, by feedback inhibition of γ-GCS by GSH, and by covalent inhibition of γ-GCS by phosphorylation or nitrosation. Because GSH plays a critical role in cellular defenses against electrophiles, oxidative stress and nitrosating species, pharmacologic manipulation of GSH synthesis has received much attention. Administration of L-cysteine precursors and other strategies allow GSH levels to be maintained under conditions that would otherwise result in GSH depletion and cytotoxicity. Conversely, inhibitors of γ-GCS have been used to deplete GSH as a strategy for increasing the sensitivity of tumors and parasites to certain therapeutic interventions.