Tryptophan catabolism by gut microbiota: a bridge connecting food science and human nutrition

Tryptophan is an essential amino acid that is supplied in the diet. Dietary tryptophan is primarily catabolized through endogenous metabolism included kynurenine pathway and serotonin pathway, and direct bacterial metabolism. However, recent studies have revealed that tryptophan bacterial metabolism can directly or indirectly influence the endogenous tryptophan metabolism. Tryptophan metabolites generated by gut microbiota are important contributors to intestinal homeostasis and they regulate the intestinal homeostasis by activating aryl hydrocarbon receptor (AhR), which is a cytosolic ligand-activated transcription factor that regulate the intestinal immunity and inflammation. In addition, some tryptophan bacterial metabolites may also promote intestinal epithelial barrier function by acting as ligands for pregnane X receptor (PXR). Several tryptophan bacterial metabolites, including indole, indole acetic acid, indole propionic acid, skatole, indole-3-aldehyde, and tryptamine, have been proven to be AhR and/or PXR ligands, and they are considered to play important roles in preventing inflammatory bowel diseases and gastrointestinal infections.