1-l-methyltryptophan is a more effective inhibitor of vertebrate IDO2 enzymes than 1-d-methyltryptophan

1-d-methyltryptophan (D-1MT) is an effective anti-cancer agent in mouse tumour models. It has been suggested to be a selective inhibitor of the recently described tryptophan-degrading enzyme indoleamine 2,3-dioxygenase 2 (IDO2) rather than the closely related enzyme IDO1. We found that mammalian (mouse, opossum and platypus), chicken, frog, and fish IDO2 could be functional tryptophan-catabolising enzymes. The characteristics of pH-dependent activity and inhibitor selectivity were conserved amongst the vertebrate IDO2 proteins tested. Like IDO1 enzymes, the enzymatic activity of all IDO2s was inhibited by L-1MT but not by D-1MT in a cell-free assay. When IDO2s were expressed in mammalian cells, L-1MT was also a better inhibitor than D-1MT.