Figure 2.

Tryptophan Metabolism and Transport. Dietary and microbial tryptophan (TPH) is absorbed in small intestinal epithelium. In enterochromaffin cells (ECs), TPH is released unaltered into the peripheral circulation, where it is degraded in the liver and other peripheral tissues to kynurenine and its metabolites. Or, it is converted to serotonin in ECs via tryptophan hydroxylase 1 (TPH1). TPH is transported across the blood-brain barrier (BBB) into astrocytes via Lat-1 large neutral amino acid transporter. In neurons and glia, it is converted to serotonin via TPH2, or catabolized by the kynurenine degradation pathway to kynurenic acid. Most of the brain quinolinic acid derives from TPH catabolism in microglia.