Chemical structure and biosynthesis pathways of trace amines (TAs). TAs, highlighted in red, include β-phenylethylamine (β-PEA), tyramine (TYRA), octopamine (OCT), synephrine (SYN), and tryptamine (TRY). They are synthesized from precursor aromatic amino acids (in blue), L-tyrosine (TYRO), L-phenylalanine (PHE) and L-tryptophan (TRP) by the aromatic-L-amino acid decarboxylase (AADC) together with catecholamines and metanephrine (MTN) (in green). Tyrosine hydroxylase (TH) converts PHE to TYRO and TYRO to L-DOPA, the precursor of dopamine (DA). TYRA and DA are metabolized by the dopamine β-hydroxylase (DBH) to OCT and norepinephrine (NE) respectively. The last two are converted by phenylethanolamine N-methyltransferase (PMNT) enzyme activity to SYN and epinephrine (E). TRP is the amino acid parent of serotonin (5-HT) and TRY, produced by the action of TRP hydroxylase (TPH) and decarboxylase (AADC) enzymes, respectively.