What L-Citrulline? Citrulline is a classified nonessential amino acid that is, oddly, neither a component of any major proteins or enzymes. It provides for processes related to the production of energy in the body, and is responsible for the proper functioning of the immune system. Because of the nonessential nature of citrulline, it may not need to be consistently ingested from any dietary sources. Citrulline is a very unusual amino acid. A variety of biological and chemical reactions define citrulline. This amino acid is formed in the urea cycle with the addition of carbon dioxide, forming ornithine. The process continues as ornithine is combined with aspartic acid to form arginosuccinic acid, which is then further metabolized to the most humanly bioavailable form, L-arginine. This critical transformation, of citrulline to arginine, is fundamental for maintaining the homeostasis of certain toxicities within the body. Arginine itself is a key component in the formation of urea in the liver. The process of removing nitrogen metabolites from the body are directly dependent upon citrulline's conversion to arginine. Citrulline, aspartic acid, citric acid, and magnesium work synergistically to remove excessive nitrogen build-up, and aid in the digestion, absorption, and metabolic processes of specific proteins. Watermelon has the highest concentration of citrulline. The rind of the melon contains the most citrulline but it is found throughout. Different species of watermelon have varying amounts. Cucumbers, pumpkins, some melons and certain varieties of gourds also have smaller amounts of citrulline.