Nutrition and Supplements Amino acids Now Foods Arginine & Citrulline 500/250 mg 240 Vegan Capsules Supports protein metabolism. Helps maintain muscle tissue. Arginine and citrulline are two amino acids that are essential for metabolism and protein utilization, as well as for maintenance of muscle tissue. Both are key intermediaries in the urea cycle, where they participate in the detoxification of ammonia through the production of urea. In addition, any arginine not converted into urea enters general circulation where it is distributed to various tissues and metabolized for other uses such as protein synthesis.