The organic compound citrulline is an α-amino acid. Its name is derived from citrullus, the Latin word for watermelon, from which it was first isolated in 1914 by Koga and Odake. It was finally identified by Wada in 1930. It has the formula 

H2NC(O)NH(CH2)3CH(NH2)CO2H. It is a key intermediate in the urea cycle, the pathway by which mammals excrete 

ammonia by converting it into urea. Citrulline is also produced as a byproduct of the enzymatic production of nitric oxidefrom the amino acid arginine, catalyzed by nitric oxide synthase.

Functions

Several proteins contain citrulline as a result of a posttranslational modification. These citrulline residues are generated by a family of enzymes called peptidylarginine deiminases (PADs), which convert arginine into citrulline in a process called citrullination or deimination. Proteins that normally contain citrulline residues include myelin basic protein (MBP), filaggrin, and several histone proteins, whereas other proteins, such as fibrin and vimentin are susceptible to citrullination during cell death and tissue inflammation. Circulating citrulline concentration is a biomarker of intestinal functionality.