Tryptophan oxidation occurs via both extra-hepatic and hepatic pathways. Although these pathways share many enzymes, the first and rate-limiting step in each pathway is carried out by two different enzymes: Indoleamine-Pyrrole 2,3 Dioxygenase (INDO) and Tryptophan 2,3-Dioxygenase (TDO2). Over the course of the last forty years extensive and detailed research by many groups have led to an understanding of some of the important biologic functions of these pathways and their metabolic products. One of the tasks that now lie ahead is linking variations in these genes with variable human responses in different disease states. This short review will focus on known aspects of the INDO and TDO2 gene structure and variability. In addition to INDO and TDO2 a third related gene, the Indoleamine- Pyrrole 2,3 Dioxygenase-like 1 (INDOL1) gene will be discussed. INDOL1 is a gene of unknown function that lies adjacent to INDO on chromosome 8.
Keywords: Indoleamine 2,3-Dioxygenase (INDO), Tryptophan 2,3-Dioxygenase (TDO2), Indoleamine-Pyrrole 2,3 Dioxygenase-like 1 (INDOL1), tryptophan oxidation, gene, single nucleotide polymorphism
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