Figure 2.

Depiction of metabolic pathways of tryptophan. Serotonin, and kynurenine pathways (in yellow boxes) are processes occurring endogenously within host metabolism. Indole pyruvate pathway occurs intestinally as a result of enzymatic degradations performed by the microbiota population in three separate pathways; tryptophan dehydrogenase pathway, tryptophanase pathway, and the aromatic amino acid transaminase pathway. All catabolites are produced intestinally with the exception of indoxyl sulfate (in the grey box), which is produced hepatically [34].