Arginine is an essential amino acid in chickens. Citrulline, but not ornithine, can substitute for at least part of this nutritional requirement, and enzymes capable of converting cirulline to agininosuccinate, and the latter molecule to arginine and fumarate have been found in chicken kidney tissue, albeit not in liver. A mitochondrial enzyme that catalyzes the hydrolysis of arginine to ornithine and urea has also been found and biochemically characterized (Tamir and Ratner 1963a, b). It may play a role in polyamine biosynthesis (Grillo et al. 1983).
Biochemical studies and searches of the chicken genomic DNA sequence for open reading frames encoding proteins with homology to known human ones suggest that two additional reactions may occur in chickens that are related to arginine metabolism and the urea cycle in human. Their roles in normal chicken physiology are unknown. A chicken protein with ornithine transcarbamylase activity has been identified (Tsuji 1983), although conversion of ornithine to citrulline is undetectable in vivo (Tamir and Ratner 1963a). Transporters capable of exchanging ornithine and citrulline across the inner mitochondrial membrane have been inferred to exist from genome analysis.