This pathway describes the metabolism of tryptophan, an essential amino acid.
Originally adapted from: [KEGG](https://www.genome.jp/kegg-bin/show_pathway?hsa00380), and expanded with information from [Agus 2018](https://doi.org/10.1016/j.chom.2018.05.003), [Dehhaghi 2019](https://doi.org/10.1177/1178646919852996), and [Gao 2020](https://doi.org/10.1093/advances/nmz127).
"The tryptophan (Trp) metabolism follows three major pathways in the gastrointestinal tract: 1. direct transformation of Trp, including ligands of the aryl hydrocarbon receptor (AhR), by the gut microbiota; 2. through the kynurenine pathway in both immune and epithelial cells via indoleamine 2,3-dioxygenase (IDO) 1; 3. the serotonin (5-hydroxytryptamine [5-HT]) production pathway in enterochromaffin cells via Trp hydroxylase 1 (TpH1)." [Agus 2018](https://doi.org/10.1016/j.chom.2018.05.003).
Proteins on this pathway have targeted assays available via the [CPTAC Assay Portal](https://assays.cancer.gov/available_assays?wp_id=WP465).