Gamma-glutamyl transpeptidase (GGT) catalyses the transfer of a gamma-glutamyl moiety from a donor substrate to different acceptors, such as amino acids and water. GGT is known to display relatively low stereospecificity with respect to the alpha-stereocentre of its donor substrates. In this study we have studied its stereospecificity with respect to the stereocentre at the delta-position of different analogues of L-glutamic acid. Notably, L-methionine sulfoxide is well-recognised whereas L-methionine sulfone and L-methionine sulfoximine are not. Furthermore, when the synthetic gamma-diastereoisomers of L-methionine sulfoxide were separated and tested, it was discovered that GGT shows remarkable stereospecificity at the gamma-position, binding the S(C)S(S) diastereoisomer with a K(i) of 3.5 mM, whereas the S(C)R(S) diastereoisomer is not recognised. Finally, using a sulfoxide as a new pharmacophore for GGT, we have synthesized and tested an analogue of glutathione to obtain a very promising competitive inhibitor with a K(i) of (53 +/- 3) microM.