Figure 1. Biosynthetic Pathways.

Biosynthesis and regeneration of biopterin and monoamines (A). Dopaminergic synapse (B). 3-OMD = 3-ortho-methyldopa; 5-HIAA = 5-hydroxyindoleacetic acid; 5-HT = serotonin; 5-HTP = 5-hydroxytryptophan; 6-PTP = 6-pyruvoyltetrahydropterin; AADC = aromatic l-amino acid decarboxylase; ADH = alcohol dehydrogenase; AR = aldose reductase; BH₄ = tetrahydrobiopterin; COMT = catechol-O-methyltransferase; D1R = dopamine receptor type 1; D2R = dopamine receptor type 2; DA = dopamine; DAT = dopamine active transporter; DBH = dopamine beta-hydroxylase; DHPR = dihydropteridine reductase; DOPAC = 3,4-dihydroxyphenylacetic acid; E = epinephrine; GCH-1 = GTP cyclohydrolase 1; GTP = guanosine triphosphate; HVA = homovanillic acid; l-dopa = levodopa; MAO = monoamine oxidase; MHPG = 3-methoxy-4-hydroxyphenylglycol; MTHF = methyltetrahydrofolate; MTHFR = methylene tetrahydrofolate reductase; NE = norepinephrine; NH2TP = d-erythro-7,8-dihydroneopterin triphosphate; PAH = phenylalanine hydroxylase; PC = pterin-4-alpha-carbinolamine; PCD = pterin-4-alpha-carbinolamine dehydratase; Phe = phenylalanine; PNMT = phenylethanolamine N-methyltransferase; PTPS = pyruvoyl-tetrahydropterin synthase; qBH₂ = quinonoid-dihydrobiopterin; SP = sepiapterin; SR = sepiapterin reductase; TH = tyrosine hydroxylase; TPH = tryptophan hydroxylase; Trp = tryptophan; Tyr = tyrosine; VMA = vanillylmandelic acid; VMAT = vesicular monoamine transporter.