In metazoa, aromatic L-amino acid decarboxylase catalyzes the synthesis of biogenic amines, including the neurotransmitters serotonin and dopamine.
Plants contain usually several different proteins with AADC activity, involved in the production of secondary metabolites. Although plant AADCs are close sequence and functional homologs of animal AADCs, the enzymes display significant differences in their substrate specificities. AADCs from mammals and insects accept a broad range of aromatic L-amino acids, whereas plant enzymes exhibit exclusive substrate specificity for L-amino acids with either an indole side chain (Trp decarboxylases) or a phenol side chain (Tyr/Phe decarboxylases), but not both.

Phenylacetaldehyde synthases (EC 4.1.1.109) are specialized phenylalanine decarboxylases that stoichiometrically couple the decarboxylation of Phe with the further conversion of phenylethylamine to phenylacetaldehyde.