May. 23, 2022
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The starting substrate for the formation of catecholamines is tyrosine and for serotonin is tryptophan. Tyrosine is converted to L-dopa by tyrosine hydroxylase and tryptophan to 5-hydroxytryptophan by tryptophan hydroxylase. L-dopa and 5-hydroxytryptophan undergo decarboxylation through the action of aromatic amino acid decarboxylase, leading to the formation of dopamine and serotonin, respectively. Within noradrenergic neurons, dopamine is converted to norepinephrine, and within the adrenal medulla, norepinephrine is methylated to form epinephrine. Dopamine and serotonin are released and rapidly catabolized. Through the action of monoamine oxidase and catechol-O-methyltransferase, 5-hydroxyindolacetic acid (5-HIAA) is produced from serotonin, homovanillic acid from dopamine, and 3-methoxy-4-hydroxyphenylglycol from norepinephrine (Pons 2009). GTPCH: GTP cyclohydrolase; NH2 TP: Dihydroneopterin triphosphate; PTPS: 6-Pyruvoyltetrahydropterin synthase; 6PTP: 6-Pyruvoyltetrahydropterin; SR: Sepiapterin reductase; BH4: Tetrahydrobiopterin; qBH2: quinonoid dihydrobiopterin ; PCD: Pterin 4alpha-carbinolamine dehydratase; DHPR: Dihydropteridine reductase; 5-HTP: 5-hydroxytryptophan ; AADC: Aromatic L-amino acid decarboxylase; DBH: Dopamine β-hydroxylase; PNMT: Phenylethanolamine-N-methyltransferase; MAO: Monoamine oxidase; COMT: catechol-O-methyltransferase ; HVA: Homovanillic acid; 5-HIAA: 5-Hydroxyindolacetic acid; MHPG: 3-methoxy-4-hydroxyphenylglycol ; VMA: vanillylmandelic acid. (Contributed by Dr. Roser Pons.)