Editing Citrulline (section)

==Biosynthesis==
Citrulline can be derived from:
* from arginine via [[nitric oxide synthase]], as a byproduct of the production of nitric oxide for [[Second messenger system|signaling]] purposes
* from [[ornithine]] through the breakdown of [[proline]] or [[glutamine]]/[[glutamate]]
* from [[asymmetric dimethylarginine]] via DDAH

Citrulline is made from [[ornithine]] and [[carbamoyl phosphate]] in one of the central reactions in the urea cycle. It is also produced from [[arginine]] as a byproduct of the reaction catalyzed by [[nitric oxide synthase|NOS]] family (NOS; [https://enzyme.expasy.org/EC/1.14.13.39 EC 1.14.13.39]).<ref>{{cite book |vauthors=Cox M, Lehninger AL, Nelson DR |title=Lehninger principles of biochemistry |publisher=Worth Publishers |location=New York |year=2000 |page= [https://archive.org/details/lehningerprincip01lehn/page/449 449] |isbn=978-1-57259-153-0 |edition=3rd |access-date= 13 March 2020 |url=https://archive.org/details/lehningerprincip01lehn }}</ref> It is also prevalent in [[TCHH|trichohyalin]] at the inner root [[Root sheath (hair)|sheath]] and medulla of hair follicles, where it is synthesized from arginine.<ref>{{Cite journal 
| last1 = Rogers | first1 = G. E. 
| last2 = Rothnagel | first2 = J. A. 
| title = A sensitive assay for the enzyme activity in hair follicles and epidermis that catalyses the peptidyl-arginine-citrulline post-translational modification 
| journal = Current Problems in Dermatology 
| volume = 11 
| pages = 171–184 
| year = 1983 
| pmid = 6653155
| doi = 10.1159/000408673 
| isbn = 978-3-8055-3752-0 
}}</ref> [[Arginine]] is first oxidized into ''N''-hydroxyl-arginine, which is then further oxidized to citrulline concomitant with release of nitric oxide.

Citrulline is also made by [[enterocytes]] of the small intestine.<ref name=":0" /><ref>{{Citation|last=DeLegge|first=Mark H.|title=Chapter 7 - Enteral Access and Enteral Nutrition in Patients With Short Bowel Syndrome|date=2019-01-01|url=http://www.sciencedirect.com/science/article/pii/B978012814330800007X|work=Adult Short Bowel Syndrome|pages=81–96|editor-last=Corrigan|editor-first=Mandy L.|publisher=Academic Press|language=en|doi=10.1016/b978-0-12-814330-8.00007-x|isbn=978-0-12-814330-8|s2cid=81295089|access-date=2020-11-10|editor2-last=Roberts|editor2-first=Kristen|editor3-last=Steiger|editor3-first=Ezra}}</ref>