Arginine is a complex amino acid that is often found at the catalytic (active) site in proteins and enzymes due to its amine-containing side chain. Arginine plays an important role in cell division, the healing of wounds, stimulation of protein synthesis, immune function, and the release of hormones. Arginine is required for the generation of urea, which is necessary for the removal of toxic ammonia from the body, and is also required for the synthesis of creatine, which degrades to creatinine, a waste product that is cleared from the body by the kidney. Measurement of arginine is included in the Amino Acids, Plasma and Amino Acids, Urine tests along with 23 other amino acids.
Arginine (Arg/R) is a semi-essential amino acid that is particularly rich in certain foods such as meats and nuts (almonds, cashews, pumpkin seeds). Foods that are high in protein naturally contain l-arginine and also include fish (salmon, haddock), legumes (soybeans, chickpeas), whole grains (brown rice, oats), and dairy products (milk, yogurt, and cheese). Arginine is the substrate for the enzyme nitric oxide synthase (NOS), which is responsible for the production of nitric oxide. Nitric oxide produced in the vascular endothelium by endothelial NOS is responsible for smooth muscle cell relaxation and is essential for reducing blood pressure. Any improvement in endothelial function will help in the prevention of cardiovascular disease. Arginine is also used by the cells of the immune system where the enzyme inducible NOS produces nitric oxide for cell signaling or oxidative bactericidal actions. Therefore, arginine could help reduce infection rates, especially in situations that compromise immune function such as surgery or critical illness.
Aside from being a structural component of many proteins, arginine also serves several other roles within the body that make it vital to overall health. Arginine and its metabolism have been increasingly recognized as potential therapeutic targets in a number of disease states, most notably cardiovascular disease, wound healing as well as cancer.
Arginine is used for a number of biological processes, including being available to be broken down into chemical intermediates that replenish the Krebs Cycle. In addition to this important anaplerotic role through conversion to glutamate and subsequently alpha-keto-glutarate, arginine is a necessary substrate in humans as an intermediate of the urea cycle. The urea cycle, probably the most well-known metabolic pathway, involves arginine as a carrier of nitrogenous waste. The final step in that pathway is catalyzed by the enzyme arginase (ARG), converting arginine to ornithine and urea; this allows urea to be available for excretion and regenerates ornithine to re-enter the cycle.
Arginine plays a number of other vital biological functions. It has a role in acid/base balance, which is not well appreciated clinically, but the urea cycle is a significant source of bicarbonate consumption and critical for maintaining acid/base homeostasis. Additionally, arginine is critically important for T-cell proliferation and acts as a substrate for nitric oxide (NO) production which is key to host immune response and defenses. Arginine is also an important building block for collagen synthesis, which is a critical part of mammalian wound healing. Another notable function of arginine is its contribution to NO synthesis by vascular endothelial cells that regulate vascular tone and cardiovascular function.
Biologically available arginine comes from three sources: (1) recycling of amino acids from normal cellular protein turnover, (2) dietary intake, and (3) de novo synthesis from arginine precursor compounds. The human body expresses enzymes that are able to synthesize arginine endogenously, and therefore it is not an essential amino acid that needs to be obtained from the diet. However, during times of stress and rapid growth, arginine requirements can be markedly elevated, making exogenous arginine provision necessary. Thus, arginine is classified as a conditionally essential amino acid.
Most of the arginine for host metabolic requirements in non-stressed states is obtained endogenously, mostly, from protein turnover. Normal dietary arginine intake, in the absence of supplementation, contributes approximately 20–25% of total arginine requirements.
Arginine can also be synthesized from other amino acid precursors. This endogenous synthesis occurs from the conversion of proline, glutamate/glutamine, as well as the non-proteinogenic amino acid citrulline, all of which can be converted to arginine.