Arginine is a complex amino acid often found in proteins and enzymes at the catalytic (active) site due to its amine-containing side chain. It is vital in cell division, wound healing, protein synthesis stimulation, immune function, and hormone release. Arginine is required to generate urea, which removes toxic ammonia from the body. It is also needed to synthesize creatine, which degrades to creatinine, a waste product cleared from the body by the kidney.
The measurement of arginine, along with 23 other amino acids, is included in the Amino Acids in Plasma and the Amino Acids in Urine tests.
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Arginine (Arg/R) is a semi-essential amino acid rich in 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 produces 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 immune system cells, 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 bodily roles that make it vital to overall health. Arginine and its metabolism have been increasingly recognized as potential therapeutic targets in several disease states, most notably cardiovascular disease, wound healing, and cancer.
Arginine is used for several biological processes, including being broken down into chemical intermediates that replenish the Krebs Cycle. In addition to this critical 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 several other vital biological functions. It has a role in acid/base balance, which is not clinically appreciated. Still, the urea cycle is a significant source of bicarbonate consumption and critical for maintaining acid/base homeostasis. Additionally, arginine is critically essential for T-cell proliferation and acts as a substrate for nitric oxide (NO) production, which is crucial to host immune response and defenses. Arginine is also an essential building block for collagen synthesis, 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 can synthesize arginine endogenously; therefore, it is not an essential amino acid that must be obtained from the diet. However, arginine requirements can be markedly elevated during stress and rapid growth, 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, primarily from protein turnover. Regular dietary arginine intake contributes approximately 20–25% of total arginine requirements without supplementation.
Arginine can also be synthesized from other amino acid precursors. This endogenous synthesis occurs from the conversion of proline, glutamate/glutamine, and the non-proteinogenic amino acid citrulline, all of which can be converted to arginine.
