Editing Serotonin (section)

==Biochemical mechanisms==

===Biosynthesis===
[[File:Serotonin biosynthesis.svg|class=skin-invert-image|thumb|right|340px|alt= On top an L-tryptophan molecule with an arrow down to a 5-HTP molecule. [[Tryptophan hydroxylase]] catalyses this reaction with help of O<sub>2</sub> and [[tetrahydrobiopterin]], which becomes water and [[dihydrobiopterin]]. From the 5-HTP molecule goes an arrow down to a serotonin molecule. Aromatic L-amino acid decarboxylase or 5-Hydroxytryptophan decarboxylase catalyses this reaction with help of [[pyridoxal phosphate]]. From the serotonin molecule goes an arrow to a 5-HIAA molecule at the bottom of the image. Monoamine oxidase catalyses this reaction, in the process O<sub>2</sub> and water is consumed, and ammonia and hydrogen peroxide is produced.|The pathway for the synthesis of serotonin from tryptophan]]

In animals and humans, serotonin is [[Biosynthesis|synthesized]] from the [[amino acid]] <small>L</small>-[[tryptophan]] by a short [[metabolic pathway]] consisting of two [[enzyme]]s, [[tryptophan hydroxylase]] (TPH) and [[Aromatic L-amino acid decarboxylase|aromatic amino acid decarboxylase]] (DDC), and the coenzyme [[pyridoxal phosphate]]. The TPH-mediated reaction is the rate-limiting step in the pathway.

TPH has been shown to exist in two forms: [[TPH1]], found in several [[Biological tissue|tissues]], and [[TPH2]], which is a neuron-specific [[Protein isoform|isoform]].<ref name="pmid14597720">{{cite journal | vauthors = Côté F, Thévenot E, Fligny C, Fromes Y, Darmon M, Ripoche MA, Bayard E, Hanoun N, Saurini F, Lechat P, Dandolo L, Hamon M, Mallet J, Vodjdani G | title = Disruption of the nonneuronal tph1 gene demonstrates the importance of peripheral serotonin in cardiac function | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 23 | pages = 13525–13530 | date = November 2003 | pmid = 14597720 | pmc = 263847 | doi = 10.1073/pnas.2233056100 | first14 = G | doi-access = free | bibcode = 2003PNAS..10013525C }}</ref>

Serotonin can be synthesized from tryptophan in the lab using ''[[Aspergillus niger]]'' and ''[[Psilocybe coprophila]]'' as catalysts. The first phase to 5-hydroxytryptophan would require letting tryptophan sit in ethanol and water for 7 days, then mixing in enough HCl (or other acid) to bring the pH to 3, and then adding NaOH to make a pH of 13 for 1 hour. ''Aspergillus niger'' would be the catalyst for this first phase. The second phase to synthesizing tryptophan itself from the 5-hydroxytryptophan intermediate would require adding ethanol and water, and letting sit for 30 days this time. The next two steps would be the same as the first phase: adding HCl to make the pH = 3, and then adding NaOH to make the pH very basic at 13 for 1 hour. This phase uses the ''Psilocybe coprophila'' as the catalyst for the reaction.<ref>{{cite journal | vauthors = Alarcón J, Cid E, Lillo L, Céspedesa C, Aguila S, Alderete JB | title = Biotransformation of indole derivatives by mycelial cultures | journal = Zeitschrift für Naturforschung C | volume = 63 | issue = 1–2 | pages = 82–84 | year = 2008 | pmid = 18386493 | doi = 10.1515/znc-2008-1-215 | s2cid = 29472174 | doi-access = free }}</ref>

[[File:Biosynthesis and breakdown of serotonin and the catecholamines, and the metabolic block in AADC deficiency.png|class=skin-invert-image|thumb|Process]]

Serotonin taken orally does not pass into the serotonergic pathways of the central nervous system, because it does not cross the [[blood–brain barrier]].<ref name="pmid18043762" /> However, [[tryptophan]] and its [[metabolite]] [[5-hydroxytryptophan]] (5-HTP), from which serotonin is synthesized, do cross the blood–brain barrier. These agents are available as [[dietary supplement]]s and in various foods, and may be effective serotonergic agents.

One product of serotonin breakdown is [[5-hydroxyindoleacetic acid]] (5-HIAA), which is excreted in the [[urine]]. Serotonin and 5-HIAA are sometimes produced in excess amounts by certain [[tumor]]s or [[cancer]]s, and levels of these substances may be measured in the urine to test for these tumors.