ABSTRACT
Arginine (Arg) was fi rst isolated and named in 1886. Its presence in animal protein was documented in 1895. Arg was classifi ed as a semi-essential amino acid aft er a study by Rose (1937) observed that adult rats did not require dietary Arg for growth and maintenance of nitrogen balance, but that young growing rats demonstrated more rapid growth when receiving dietary Arg. Th is fi nding indicated that endogenous biosynthesis of Arg occurs but not at a suffi cient rate for maximal growth in the young. Th is was also found to be true for humans. Arg has been shown to possess numerous physiological properties. It functions in the body as a free amino acid, a component of most proteins, and as a substrate for several non-protein, nitrogen-containing compounds. As a free amino acid, Arg is an integral constituent of the urea cycle. Arg contains a guanidine group, which is essential for the synthesis of urea in most mammals. In the cytosol of hepatocytes, arginase removes the guanidine group from Arg to produce urea and ornithine. Urea is then transported from the hepatocyte into the blood, and ornithin is used to regenerate Arg within hepatocytes. Another function of Arg is protein synthesis. Arg can be converted into proline, glutamate, and glutamine, all of which are common amino acids found within most proteins. Arg metabolism also generates several essential non-protein and nitrogen-containing compounds, including creatine, polyamines, and nitric oxide (NO). Synthesis of creatine is dependent on the guanidine group of Arg. Creatine functions as a carrier for phosphate and is needed for the rapid regeneration of adenosine triphosphate in muscles. Polyamines function in membrane transport and in cell growth, proliferation, and diff erentiation. Arg and products of Arg metabolism are necessary for both the regulation and synthesis of polyamine. Arg not only provides the substrate for polyamine synthesis, it also indirectly stimulates the release of growth hormone, which in turn promotes polyamine synthesis by increasing ornithine decarboxylase activity. Arg is a precursor of NO. NO, a short-lived small molecule produced by most cell types, has a variety of well-defi ned pathophysiological roles. NO is an antimicrobial agent that is eff ective against pathogens, parasites, and bacteria. NO is also a neurotransmitter and vasodilator. Th e enzyme that produces NO is nitric oxide synthase (NOS). Th ere are three isoforms of NOS. NOS-1 (also known as nNOS) is constitutive and is predominantly located in neuronal tissue. NOS-2 (iNOS) is inducible and is located in a variety of tissues. NOS-3 (eNOS) is constitutive and is primarily localized in endothelial tissue.
