Elsevier Inc. L-Phe hydroxylation norepinephrine dopamine L-DOPA BH2 BH4 tyrosine Dbh Ddc Th Qdpr Slc18a2 ---> dopamine Slc18a2 Ddc ---- Slc18a2 Slc18a2 ---- Th diet BH4 ---> BH2 L-DOPA ---> dopamine tyrosine ---> L-DOPA diet ---> tyrosine L-Phe hydroxylation ---> tyrosine BH2 ---> BH4 dopamine ---> norepinephrine