Arginine is a conditionally essential amino acid, first isolated in 1886 by E. Schulze and E. Steiger; its structure was determined by E. Schulze and E. Winterstein in 1897. The average daily consumption of L-arginine is 5.4 g. The physiological requirements of tissues and organs in most mammals of arginine are met by its endogenous synthesis and/or dietary intake; however, in the young and in adults in stress or disease this amino acid becomes essential. Arginine is an important precursor for the synthesis of proteins and many biologically active molecules, such as ornithine, proline, polyamines, creatine and agmatine However, the main function of arginine in human body is to be a substrate for the synthesis of nitric oxide (NO) (Visek W.J., 1986; Wu G., Morris S.M. Jr., 1998; Böger R.H., 2007).
Dietary L-arginine is absorbed in the small intestine with subsequent transport to the liver, where most of it is utilised in the ornithine cycle. The fraction of L-arginine not metabolised in the liver is used as a substrate for NO production. The main source of endogenous arginine in the body is protein metabolism; however, the endogenous synthesis of arginine does not play an important role in the regulation of homoeostasis in healthy adults (Wu G., Morris S.M. Jr., 1998; Böger R.H., 2007).
Under physiological conditions, the synthesis of NO from L-arginine occurs with the NO-synthase enzymes (NO-synthase — NOS), L-citrulline being the second product of the reaction. NOS is the only currently known enzyme to execute the above process with 5 cofactors / prosthetic groups simultaneously (flavin adenine dinucleotide, flavin mononucleotide, heme, tetrahydrobiopterin and calcium / calmodulin), being, therefore, one of the most regulated enzymes in the nature (Bryan N.S. et al., 2009).