cit·rul·line 
(sĭtrə-lēn′)
(sĭtrə-lēn′)n.
An amino acid, C6H13N3O3, originally isolated from watermelon, that is produced as an intermediate in the conversion of ornithine to arginine during urea formation in the liver.
[New Latin Citrullus, watermelon genus (from Medieval Latin citrullus, watermelon, from Italian dialectal citrulo, from Late Latin citrium, from Latin citrus, citron) + -INE2.]