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Arginine

|Section2={{Chembox Properties | C=6 | H=14 | N=4 | O=2 | Appearance = White crystals | Odor = Odourless | MeltingPtK = 533 | BoilingPtC = 368 | Solubility = 14.87 g/100 mL (20 °C) | SolubleOther = slightly soluble in ethanolinsoluble in ethyl ether | LogP = −1.652 | pKa = 12.488 | pKb = 1.509 }} |Section5={{Chembox Thermochemistry | DeltaHf = −624.9–−622.3 kJ mol−1 | DeltaHc = −3.7396–−3.7370 MJ mol−1 | Entropy = 250.6 J K−1 mol−1 | HeatCapacity = 232.8 J K−1 mol−1 (at 23.7 °C) }} |Section6={{Chembox Pharmacology | ATC_Supplemental = S }} |Section7={{Chembox Hazards | ExternalSDS = | GHSPictograms = | GHSSignalWord = WARNING | HPhrases = | PPhrases = | LD50 = 5110 mg/kg (rat, oral) }} |Section8={{Chembox Related | OtherFunction_label = alkanoic acids | OtherFunction = | OtherCompounds = }} |ImageFile1 = Arginine-3D-balls-by-AHRLS-2012.png}} Arginine (abbreviated as Arg or R) encoded by the codons CGU, CGC, CGA, CGG, AGA, and AGG is an α- amino acid that is used in the biosynthesis of proteins. Arginine is classified as a semiessential or conditionally essential amino acid, depending on the developmental stage and health status of the individual. Preterm infants are unable to synthesize or create arginine internally, making the amino acid nutritionally essential for them. Most healthy people do not need to supplement with arginine because it is a component of all protein-containing foods and their body produces sufficient amounts. Arginine was first isolated from a lupin seedling extract in 1886 by the German chemist Ernst Schultze. It contains an α-amino group (which is in the protonated −NH3+ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and a side chain of a 3-carbon aliphatic straight chain capped by a complex guanidinium, classifying it as a charged (at physiological pH), aliphatic amino acid.

Sources

Dietary sources

A conditionally essential amino acid is one that may be required depending on the health status or life cycle of the individual. Arginine is one such conditionally essential amino acid. Endogenous synthesis may suffice, if not that, then synthesis plus absorption from protein-containing foods is enough. In rare instances a healthcare professional may recommend an arginine dietary supplement. Almost all dietary protein contains arginine (exception: gelatin):
  • Animal sources: Meat, fish, fowl, eggs, dairy
  • Plant sources: Grains, beans, corn, nuts; not fruits, vegetables or leafy greens

Biosynthesis

Arginine is synthesized from citrulline in arginine and proline metabolism by the sequential action of the cytosolic enzymes argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL). In terms of energy, this is costly, as the synthesis of each molecule of argininosuccinate requires hydrolysis of adenosine triphosphate (ATP) to adenosine monophosphate (AMP), i.e., two ATP equivalents. In essence, taking an excess of arginine gives more energy by saving ATPs that can be used elsewhere. Citrulline can be derived from multiple sources: The pathways linking arginine, glutamine, and proline are bidirectional. Thus, the net utilization or production of these amino acids is highly dependent on cell type and developmental stage. On a whole-body basis, synthesis of arginine occurs principally via the intestinal–renal axis, wherein epithelial cells of the small intestine, which produce citrulline primarily from glutamine and glutamate, collaborate with the proximal tubule cells of the kidney, which extract citrulline from the circulation and convert it to arginine, which is returned to the circulation. As a consequence, impairment of small bowel or renal function can reduce endogenous arginine synthesis, thereby increasing the dietary requirement. Synthesis of arginine from citrulline also occurs at a low level in many other cells, and cellular capacity for arginine synthesis can be markedly increased under circumstances that also induce iNOS. Thus, citrulline, a coproduct of the NOS-catalyzed reaction, can be recycled to arginine in a pathway known as the citrulline-NO or arginine-citrulline pathway. This is demonstrated by the fact that, in many cell types, citrulline can substitute for arginine to some degree in supporting NO synthesis. However, recycling is not quantitative because citrulline accumulates along with nitrate and nitrite, the stable end-products of NO, in NO-producing cells.

Function

Arginine plays an important role in cell division, the healing of wounds, removing ammonia from the body, immune function, and the release of hormones. The roles of endogenous arginine include:

Proteins

The distributing basics of the moderate structure found in geometry, charge distribution, and ability to form multiple H-bonds make arginine ideal for binding negatively charged groups. For this reason, arginine prefers to be on the outside of the proteins, where it can interact with the polar environment. Incorporated in proteins, arginine can also be converted to citrulline by PAD enzymes. In addition, arginine can be methylated by protein methyltransferases.

Precursor

Arginine is the immediate precursor of nitric oxide (NO), urea, ornithine, and agmatine; is necessary for the synthesis of creatine; and can also be used for the synthesis of polyamines (mainly through ornithine and to a lesser degree through agmatine), citrulline, and glutamate. As a precursor of nitric oxide, arginine may have a role in the treatment of some conditions where vasodilation is required. The presence of asymmetric dimethylarginine (ADMA), a close relative, inhibits the nitric oxide reaction; therefore, ADMA is considered a marker for vascular disease, just as L-arginine is considered a sign of a healthy endothelium.

Safety

L-arginine is generally recognized as safe (GRAS-status) at intakes of up to 20 grams per day.

Structure

The amino acid side-chain of arginine consists of a 3-carbon aliphatic straight chain, the distal end of which is capped by a complex guanidinium group. With a pKa of 12.48, the guanidinium group is positively charged in neutral, acidic, and even most basic environments, and thus imparts basic chemical properties to arginine. Because of the conjugation between the double bond and the nitrogen lone pairs, the positive charge is delocalized, enabling the formation of multiple H-bonds.

Research

Growth hormone

Intravenously-administered arginine stimulates the secretion of growth hormone, and for this reason is used in growth hormone stimulation tests.U.S. National Library of Medicine (September 2009 Growth hormone stimulation test A review of clinical trials concluded that oral arginine increases growth hormone. However, a more recent trial did not report an increase in growth hormone despite being effective at increasing plasma levels of L-arginine.

MELAS syndrome

Several trials delved into effects of L-arginine in Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS syndrome), an inherited mitochondrial disease.

High blood pressure

A meta-analysis showed that L-arginine reduces blood pressure with pooled estimates of 5.4/2.7 mmHg for SBP/DBP. Supplementation with L-arginine reduces diastolic blood pressure and lengthens pregnancy for women with gestational hypertension, including women with high blood pressure as part of pre-eclampsia. It did not lower systolic blood pressure or improve weight at birth.

Hypersensitive teeth

A 2016 review of clinical trials of toothpastes marketed to help people with sensitive teeth and meta-analysis found that clinical trials were generally of poor quality, but based on the data at hand arginine-containing toothpastes appeared to be as effective in the long term as the other toothpastes tested.

See also

References

External links

"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/Arginine, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Arginine&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany