Arginine CitrullineArginine and citrulline are two amino acids that are integral to protein metabolism and utilisation, as well as to the maintenance of muscle tissue. Both are key intermediates in the urea cycle, where they participate in the detoxification of ammonia via the production of urea. In addition, any arginine not converted to urea enters general circulation where it is distributed to various tissues and metabolised for other uses such as protein synthesis.