Arginine (Arg), also known as L-argninine, belongs to the class of organic compounds known as L-alpha-amino acids. These are alpha amino acids which have the L-configuration of the alpha-carbon atom. Amino acids are organic compounds that contain amino (‚ÄìNH2) and carboxyl (‚ÄìCOOH) functional groups, along with a side chain (R group) specific to each amino acid. L-asparagine is one of 20 proteinogenic amino acids, i.e., the amino acids used in the biosynthesis of proteins. Arginine is found in all organisms ranging from bacteria to plants to animals. Arginine is an essential amino acid that is physiologically active in the L-form. It is classified as a charged, basic, aliphatic amino acid. Arginine is considered to be a basic amino acid as it has a strongly basic guanidinium group. With a pKa of 12.48, the guanidinium group is positively charged in neutral, acidic, and even most basic environments. Because of the conjugation between the double bond and the nitrogen lone pairs, the positive charge is delocalized. This group is able to form multiple H-bonds. In mammals, arginine is formally classified as a semi-essential or conditionally essential amino acid, depending on the developmental stage and health status of the individual. Infants are unable to effectively synthesize arginine, making it nutritionally essential for infants. Adults, however, are able to synthesize arginine in the urea cycle. L-Arginine is an amino acid that has numerous functions in the body. It helps dispose of ammonia, is used to make compounds such as nitric oxide, creatine, L-glutamate, and L-proline, and it can be converted into glucose and glycogen if needed. Arginine also plays an important role in cell division, immunity and wound healing. Arginine is the immediate precursor of nitric oxide (NO), an important signaling molecule which can act as a second messenger, as well as an intercellular messenger which regulates vasodilation, and also has functions in the immune system's reaction to infection. Nitric oxide is made via the enzyme nitric oxide synthase (PMID 10690324). Arginine is also a precursor for several important nitrogen-containing compounds including urea, ornithine, and agmatine. Arginine is necessary for the synthesis of creatine and can be used for the synthesis of polyamines (mainly through ornithine and to a lesser degree through agmatine, citrulline, and glutamate.) The presence of asymmetric dimethylarginine (ADMA) in serum or plasma, a close relative of argninine, inhibits the nitric oxide synthase reaction. ADMA is considered a marker for vascular disease, just as L-arginine is considered a sign of a healthy endothelium. In large doses, L-arginine also stimulates the release of the hormones growth hormone and prolactin. Arginine is a known inducer of mTOR (mammalian target of rapamycin) and is responsible for inducing protein synthesis through the mTOR pathway. mTOR inhibition by rapamycin partially reduces arginine-induced protein synthesis (PMID: 20841502). Catabolic disease states such as sepsis, injury, and cancer cause an increase in arginine utilization, which can exceed normal body production, leading to arginine depletion. Arginine also activates AMP kinase (AMPK) which then stimulates skeletal muscle fatty acid oxidation and muscle glucose uptake, thereby increasing insulin secretion by pancreatic beta-cells (PMID: 21311355). Arginine is found in plant and animal proteins, such as dairy products, meat, poultry, fish, and nuts. The ratio of L-arginine to lysine is also important: soy and other plant proteins have more L-arginine than animal sources of protein.