Editing L-DOPA (section)

==Biological role==
{{Phenylalanine biosynthesis|align=right}}
{{sm|l}}-DOPA is produced from the amino acid {{sm|l}}-[[tyrosine]] by the enzyme [[tyrosine hydroxylase]]. {{sm|l}}-DOPA can act as an {{sm|l}}-tyrosine mimetic and be incorporated into proteins by mammalian cells in place of {{sm|l}}-tyrosine, generating [[protease]]-resistant and [[protein aggregation|aggregate-prone proteins]] ''in vitro'' and may contribute to [[neurotoxicity]] with chronic {{sm|l}}-DOPA administration.<ref>{{cite journal | vauthors = Rodgers KJ | title = Non-protein amino acids and neurodegeneration: the enemy within | journal = Experimental Neurology | volume = 253 | pages = 192–196 | date = March 2014 | pmid = 24374297 | doi = 10.1016/j.expneurol.2013.12.010 | s2cid = 2288729 }}</ref>
It is also the precursor for the [[monoamine]] or [[catecholamine]] neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). Dopamine is formed by the decarboxylation of {{sm|l}}-DOPA by [[aromatic L-amino acid decarboxylase|aromatic {{sm|l}}-amino acid decarboxylase]] (AADC).

{{sm|l}}-DOPA can be directly metabolized by [[catechol-O-methyl transferase|catechol-''O''-methyl transferase]] to [[3-O-methyldopa|3-''O''-methyldopa]], and then further to [[vanillactic acid]]. This metabolic pathway is nonexistent in the healthy body, but becomes important after peripheral {{sm|l}}-DOPA administration in patients with Parkinson's disease or in the rare cases of patients with AADC enzyme deficiency.<ref name="pmid1281049">{{cite journal | vauthors = Hyland K, Clayton PT | title = Aromatic L-amino acid decarboxylase deficiency: diagnostic methodology | journal = Clinical Chemistry | volume = 38 | issue = 12 | pages = 2405–10 | date = December 1992 | pmid = 1281049 | doi =  10.1093/clinchem/38.12.2405| url = http://www.clinchem.org/cgi/reprint/38/12/2405.pdf | access-date = 16 October 2008 | archive-url = https://web.archive.org/web/20110607122144/http://www.clinchem.org/cgi/reprint/38/12/2405.pdf | archive-date = 7 June 2011 | url-status = dead | doi-access = free }}</ref>

{{sm|l}}-Phenylalanine, {{sm|l}}-tyrosine, and {{sm|l}}-DOPA are all precursors to the biological [[pigment]] [[melanin]]. The enzyme [[tyrosinase]] [[catalyst|catalyzes]] the [[oxidation]] of {{sm|l}}-DOPA to the reactive intermediate [[dopaquinone]], which reacts further, eventually leading to melanin [[oligomer]]s. In addition, [[tyrosinase]] can convert tyrosine directly to {{sm|l}}-DOPA in the presence of a reducing agent such as [[ascorbic acid]].<ref>{{cite journal | vauthors = Ito S, Kato T, Shinpo K, Fujita K | title = Oxidation of tyrosine residues in proteins by tyrosinase. Formation of protein-bonded 3,4-dihydroxyphenylalanine and 5-S-cysteinyl-3,4-dihydroxyphenylalanine | journal = The Biochemical Journal | volume = 222 | issue = 2 | pages = 407–11 | date = September 1984 | pmid = 6433900 | pmc = 1144193 | doi = 10.1042/bj2220407 }}</ref>