Fig. 1
From: Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (BH4) deficiencies
Biosynthesis and regeneration of tetrahydrobiopterin (BH4) and its functions as cofactor in the synthesis of serotonin, dopamine, and other catecholamines as well as the catabolism of phenylalanine. Simplified scheme of the biosynthesis and regeneration of tetrahydrobiopterin (BH4) in the presynaptic axonal end. BH4 serves as essential cofactor of the aromatic amino acid hydroxylases phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), and tryptophan hydroxylase (TPH) which catalyse key reactions in the synthesis of the monoamines dopamine, serotonin, norepinephrine, and epinephrine. Note that AGMO and NOSs are not depicted in this overview. 5-HIAA, 5-hydroxyindoleacetic acid; 5-HIAL, 5-hydroxyindoleacetaldehyde; 7,8-BH2, 7,8-dihydrobiopterin; BH4, tetrahydrobiopterin; DOPAC, 3,4-dihydroxyphenylacetic acid; DOPAL, 3,4-dihydroxyphenylacetaldehyde; DTDS, dopamine transport deficiency syndrome; GTP, guanosine-5′-triphosphate; HVA, homovanillic acid; Oxo-PH41, oxo-2-hydroxy-tetrahydropterin; PLP, pyridoxal 5′-phosphate; PTP, 6-pyruvoyltetrahydropterin; qBH2, quinonoid dihydrobiopterin; VLA, vanillyllactic acid; VMA, vanillylmandelic acid; VMAT 2, vesicular monoamine transporter