Gamma-glutamyl hydrolase (GH) is a lysosomal and secreted glycoprotein that hydrolyses the gamma-glutamyl tail of antifolate and folate polyglutamates. Tumour cells that have high levels of GH are inherently resistant to classical antifolates, and further resistance can be acquired by elevations in GH following exposure to this class of anti-tumour agents. The highest level of expression in normal tissues occurs in the liver and kidney in humans. GH is a low-affinity (micromolar), high-turnover enzyme that has a cysteine at the active site. GH is being evaluated as an intracellular target for inhibition in order to enhance the therapeutic activity of antifolates and fluorouracil [].The 3-dimensional structure of GH shows a central eight-stranded β-sheet, which is sandwiched by three and five α-helices on each side (see []. The fold resembles that of glutamine amidotransferases (GATase) of class I, which are characterised by a conserved Cys-His-Glu active site. The major differences consist of extensions in four loops and at the C terminus of GGH. The active site residues are well conserved and the catalytically essential cysteine, positioned at a nucleophile elbow, suggests that GGH is a cysteine peptidase.
