The organic compound<unk>  is an α-amino acid. Its name is derived from citrullus, the Latin word for watermelo], from which it was first isolated in 1930.
It has the idealized formula H2NC(O)NH(CH2)3CH(NH2)CO2H. It is a key intermediate in the urea cycle, the pathway by which mammals excrete ammonia.<unk>  is a substituted urea.
Citrulline
Cysteamine 
Pyruvic acid
Xanthine
Arginine
Choline
Phenylalanine
Aminohippuric acid