This diagram shows the biosynthesis of bioactive compounds ( indole and certain other derivatives) from tryptophan by bacteria in the gut. ^[1] Indole is produced from tryptophan by bacteria that express tryptophanase. ^[1] Clostridium sporogenes metabolizes tryptophan into indole and subsequently 3-indolepropionic acid (IPA), ^[2] a highly potent neuroprotective antioxidant that scavenges hydroxyl radicals. ^{[1] [3] [4]} IPA binds to the pregnane X receptor (PXR) in intestinal cells, thereby facilitating mucosal homeostasis and barrier function. ^[1] Following absorption from the intestine and distribution to the brain, IPA confers a neuroprotective effect against cerebral ischemia and Alzheimer's disease. ^[1] Lactobacillaceae (Lactobacillus s.l.) species metabolize tryptophan into indole-3-aldehyde (I3A) which acts on the aryl hydrocarbon receptor (AhR) in intestinal immune cells, in turn increasing interleukin-22 (IL-22) production. ^[1] Indole itself triggers the secretion of glucagon-like peptide-1 (GLP-1) in intestinal L cells and acts as a ligand for AhR. ^[1] Indole can also be metabolized by the liver into indoxyl sulfate, a compound that is toxic in high concentrations and associated with vascular disease and renal dysfunction. ^[1] AST-120 ( activated charcoal), an intestinal sorbent that is taken by mouth, adsorbs indole, in turn decreasing the concentration of indoxyl sulfate in blood plasma. ^[1]