Elsevier Inc. L-DOPA dopamine tyrosine BH2 BH4 Ddc Th Qdpr dopamine signaling pathway dopamine --+> dopamine signaling pathway Slc18a2 ---> dopamine Slc18a2 Ddc ---- Slc18a2 Slc18a2 ---- Th diet L-Phe hydroxylation BH4 ---> BH2 L-DOPA ---> dopamine tyrosine ---> L-DOPA diet ---> tyrosine L-Phe hydroxylation ---> tyrosine BH2 ---> BH4