Tryptophan Metabolism- Indoleamine 2,3-Dioxygenase- Friend and Foe

Tryptophan Metabolism- Indoleamine 2,3-Dioxygenase- Friend and Foe

Indoleamine 2,3-dioxygenase (IDO) (EC 1.13.11.42) is a cytoplasmic, heme-containing enzyme that mediates the initial and rate-limiting step in the oxidative catabolism of the essential amino acid L-tryptophan (L-Trp). Recently, an additional IDO molecule, termed IDO2 has been identified. The gene encoding IDO2 is adjacent to the IDO gene. The IDO2 protein has a different expression profile to IDO and while it is able to metabolize L-Trp, IDO2 has a much higher Km for this substrate. In addition both enzymes differ in their selectivity for some inhibitors. Degradation of L-Trp via IDO leads to the production of several metabolites, including N-formyl-kynurenine and Kynurenine (Kyn).