Definition: "Arginine" refers to a naturally occurring amino acid in plants that plays an essential role in protein synthesis, specifically within the urea cycle. Defining "arginine": It is a building block of ammonia and is required for cellular metabolism. In cells, arginine synthase converts arginine into ammonia via an enzyme called arginine deaminase, which also produces uric acid. The word "arginines" comes from Latin, meaning "salty ashes," indicating their content as ammonium salts.