L-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. L-Citrulline is a very unusual amino acid. A variety of biological and chemical reactions define L-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.