Figure 2: γ-Glutamyl cycle. In the γ-glutamyl cycle, GSH is released from the cell and the ectoenzyme GGT transfers the γ-glutamyl moiety of GSH to an amino acid (aa, the best acceptor being cysteine), forming γ-glutamyl-aa and cysteinyl-glycine. The γ-glutamyl-amino acid can then be transported back into the cell and once inside can be further metabolized to release the aa and 5-oxoproline, which can be converted to glutamate and used for GSH synthesis. Cysteinyl-glycine is broken down by dipeptidase (DP) to generate cysteine and glycine. Once inside the cell, the majority of cysteine is incorporated into GSH, some being incorporated into protein, and some degraded into sulfate and taurine [18].