Endogenous Metabolite; Microbial Metabolite; eNOS

L-Arginine (Arg) is classified as an essential amino acid for birds, carnivores and young mammals and a conditionally essential amino acid for adults. It is converted by arginase to L-ornithine, a precursor of polyamines and urea, which is important in the urea cycle. Arg serves as a precursor for creatine, which plays an essential role in the energy metabolism of muscle, nerve and testis and accounts for Arg catabolism and for the synthesis of agmatine and proteins. Via its ability to increase growth hormone secretion it influences immune function. Depending on nutritional status and developmental stage, normal plasma Arg concentrations in humans and animals range from 95 to 250 micromol/l. Systemic or oral Arg administration has been shown to improve cardiovascular function and reduce myocardial ischemia in coronary artery disease patients. It reduces blood pressure and renal vascular resistance in essential hypertensive patients with normal or insufficient renal function. Although Arg plasma concentrations are not altered in hypercholesterolemic individuals, oral or intravenous Arg administration can reverse endothelial dysfunction in hypercholesterolemic patients and in cigarette smokers. The main importance of Arg is attributed to its role as a precursor for the synthesis of nitric oxide (NO), a free radical molecule that is synthesized in all mammalian cells from L-Arg by NO synthase (NOS). NO appears to be a major form of the endothelium-derived relaxing factor (EDRF). NO and EDRF share similar chemical and pharmacological properties and are derived from the oxidation of a terminal guanidine group of L-Arg. Various mechanisms have been implicated in the defect in vascular relaxation. These include, increased diffusional barrier for NO, L-Arg depletion, altered levels of reactive oxygen, inactivation of NO by superoxide anions (O2-). The independent reactions of O2-, NO and their reaction yielding peroxynitrite are critical in the initiation and maintenance of the atherosclerotic state and contribute to the defect in vasorelaxation. NO also plays a role as a neurotransmitter, mediator of immune response and as signaling molecule. The NO synthesized by iNOS in macrophages contributes to their cytotoxic activity against tumor cells, bacteria and protozoa. Our aim here is to review on some amino acids with high functional priority such as Arg and to define their effective activity in human health and pathologies.[1]

L-arginine can be utilized in animal modeling to build animal pancreatitis models. L-Arginine is a NO-producing substrate of endothelial nitric oxide synthase (eNOS) and can be metabolized into nitric oxide (NO), polyamines or L-proline to stimulate inflammation. L-Arginine can also selectively destroy pancreatic acinar cells, resulting in acute necrotizing pancreatitis.

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When male rats were given single i.p. injection of 500 mg of L-arginine/100 g body weight, the pancreatic acinar cells were destroyed selectively, without any morphological change of Langerhans' islets. As early as 24 hours after the injection, loss of basophilia, zymogen degranulation, and vacuolar and necrotic changes of the acinar cells were noted. After 3 days, fibroblastic activity and atrophy of pancreatic lobuli were evident. Early electron microscopic findings were changes of the endoplasmic reticulum, such as partial dilatation or vacuolation of the cisternae, usually with loss of ribosomes attached to the membrane. The effect of arginine excess may be ascribed to imbalance of amino acids and subsequent to decrease of protein synthesis in the acinar cells. In the course of this study, fat necrosis with marked infiltration of leucocytes was observed in adipose tissues in peripancreatic, epididymal, omental and retroperitoneal areas. This change correlated closely with the marked necrosis of the pancreas. An increase in the level of lipase in the blood was also demonstrated.[4]

Nitric oxide, a product of nitric oxide synthase activity, relaxes vascular smooth muscle and elevates brain blood flow. We evaluated the importance of eNOS to cerebral blood flow augmentation after L-arginine infusion and increases in flow after eNOS upregulation in SV-129 mice. Blood flow was measured by laser-Doppler flowmetry before and after L-arginine infusion (450 mg/kg during a 15-minute period) or measured by 14C-iodoamphetamine indicator fractionation or 14C-iodoantipyrine tissue equilibration techniques. rCBF increased by 26% (laser Doppler flowmetry) after L-arginine infusion but did not change in mutant mice deficient in eNOS expression. After eNOS upregulation by chronic simvastatin treatment (2 mg/kg subcutaneously, daily for 14 days), L-arginine amplified and sustained the hyperemia (38%) and increased absolute brain blood flow from 86 +/- 7 to 119 +/- 10 mL/100 g per minute. Furthermore, pretreatment with simvastatin enhanced blood flow within ischemic brain tissue after middle cerebral artery occlusion. Together, these findings suggest that eNOS activity is critical for blood flow augmentation during acute L-arginine infusion, and chronic eNOS upregulation combined with L-arginine administration provides a novel strategy to elevate cerebral blood flow in the normal and ischemic brain.[3]

Absorption, Distribution and Excretion
L-arginine is absorbed into intestinal cells from the small intestinal lumen. Absorption is highly efficient, occurring via active transport mechanisms. Metabolism/Metabolites Partial metabolism of L-arginine occurs within intestinal cells. L-arginine not metabolized within intestinal cells enters the portal circulation and is then transported to the liver, where some amino acids are further metabolized. The product of L-arginine oxidation, deamination, or transamination is α-keto-γ-guanidinovalerate; the product of decarboxylation is guanidinobutylamine. Metabolic pathways and products: Arginine to ornithine + urea; Arginine to citrulline + ammonia; Arginine + glycine to guanidinoacetic acid + ornithine /Excerpt from table/ Partial L-arginine is metabolized within intestinal cells. L-arginine not metabolized within intestinal cells enters the portal circulation and is then transported to the liver, where some amino acids are further metabolized.

Toxicity Summary
Many of the activities associated with L-arginine supplementation, including its potential anti-atherosclerotic effects, may be related to its role as a precursor to nitric oxide (NO). NO is produced by all tissues in the body and plays a vital role in the cardiovascular, immune, and nervous systems. NO is generated from L-arginine by NO synthase (NOS), whose action is primarily mediated by 3,5'-cyclic guanosine monophosphate (cGMP). NO activates guanylate cyclase, which catalyzes the synthesis of cGMP from guanosine triphosphate (GTP). cGMP is then converted to guanosine monophosphate by cGMP phosphodiesterase. NOS is a heme-containing enzyme whose partial sequence is similar to cytochrome P-450 reductase. Nitric oxide synthase (NOS) has several isoenzymes, two of which are constitutively expressed, and one is induced by immune stimulation. The constitutive NOS present in vascular endothelial cells is called eNOS, and the constitutive NOS present in the brain, spinal cord, and peripheral nervous system is called nNOS. NOS induced by immune or inflammatory stimuli is called iNOS. iNOS can be constitutively expressed in certain tissues, such as lung epithelial cells. All nitric oxide synthases use NADPH (reduced nicotinamide adenine dinucleotide phosphate) and oxygen (O₂) as cosubstrates, with cofactors including FAD (flavin adenine dinucleotide), FMN (flavin mononucleotide), tetrahydrobiopterin, and heme. Notably, ascorbic acid appears to enhance NOS activity by increasing intracellular tetrahydrobiopterin levels. eNOS and nNOS synthesize NO in response to elevated calcium ion concentrations or, under certain conditions, in response to calcium-independent stimuli (e.g., shear stress). In vitro studies have shown that the Km value of NOS for L-arginine is in the micromolar range. The concentration of L-arginine in endothelial cells, other cells, and plasma is in the millimolar range. This implies that under physiological conditions, NOS is saturated with its substrate L-arginine. In other words, L-arginine is not expected to be the rate-limiting step of this enzyme, and oral supplementation of this amino acid may lead to L-arginine concentrations exceeding physiological levels, but this does not appear to have any effect on NO production. The reaction appears to have reached its maximum activity. However, in vivo studies have shown that under certain conditions, such as hypercholesterolemia, L-arginine supplementation can enhance endothelium-dependent vasodilation and NO production.

[1]. I. Arginine. Biomed Pharmacother. 2002 Nov;56(9):439-45.

[2]. Administration of the nitric oxide synthase inhibitor NG-methyl-L-arginine hydrochloride (546C88) by intravenous infusion for up to 72 hours can promote the resolution of shock in patients with severe sepsis: results of a randomized, double-blind, placebo-controlled multicenter study (study no. 144-002). Crit Care Med. 2004 Jan;32(1):1-12.

[3]. Endothelial nitric oxide synthase-dependent cerebral blood flow augmentation by L-arginine after chronic statin treatment. J Cereb Blood Flow Metab. 2000 Apr;20(4):709-17.

[4]. Effects of injecting excess arginine on rat pancreas. J Nutr. 1984 Mar;114(3):467-71.

[5]. Effects of curcumin on oxidative stress, inflammation and apoptosis in L-arginine induced acute pancreatitis in mice. Heliyon. 2019 Aug 27;5(8):e02222.

Therapeutic Uses
Experimental Uses: In mice, L-arginine hydrochloride has an inhibitory effect on mouse sarcoma virus-Moroni strain and the C3H mammary adenocarcinoma tumor system. Experimental Uses: After 10 days of inoculation of Walker 256 carcinosarcoma cells into rats, administration of an experimental diet reduced tumor weight. Experimental Uses: L-arginine hydrochloride improved the motility of low-motility human semen samples in vitro. The effect was dose-dependent. Experimental Uses: Feeding rats 1% arginine increased thymus volume and prevented thymus atrophy caused by injury. Arginine promoted wound healing in rats. For more complete data on the therapeutic uses of (L)-arginine (6 types), please visit the HSDB record page. Pharmacodynamics Studies have shown that arginine can enhance immune responses against bacteria, viruses, and tumor cells; promote wound healing and liver regeneration; induce growth hormone release; and is considered essential for optimal muscle growth and tissue repair.

C6H14N4O2

174.2010

174.111

C, 41.37; H, 8.10; N, 32.16; O, 18.37

74-79-3

DL-Arginine;7200-25-1;L-Arginine (L-glutamate);4320-30-3;L-Arginine butanoate;80407-72-3; 74-79-3; 2485-55-4 (caprate); 4320-30-3 (glutamate); 1119-34-2 (HCl)

6322

White to off-white solid powder

1.5±0.1 g/cm3

367.6±52.0 °C at 760 mmHg

222 °C (dec.)(lit.)

176.1±30.7 °C

0.0±1.8 mmHg at 25°C

1.601

-1.79

4

4

5

12

176

1

C(C[C@@H](C(=O)O)N)CN=C(N)N

ODKSFYDXXFIFQN-BYPYZUCNSA-N

InChI=1S/C6H14N4O2/c7-4(5(11)12)2-1-3-10-6(8)9/h4H,1-3,7H2,(H,11,12)(H4,8,9,10)/t4-/m0/s1

(2S)-2-amino-5-(diaminomethylideneamino)pentanoic acid

L-arginine; arginine; 74-79-3; L-(+)-Arginine; (S)-2-Amino-5-guanidinopentanoic acid; L-Arg; H-Arg-OH; L(+)-Arginine;

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

H2O : ~50 mg/mL (~287.03 mM)

Solubility in Formulation 1: 100 mg/mL (574.05 mM) in PBS (add these co-solvents sequentially from left to right, and one by one), clear solution; with sonication.

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 (Please use freshly prepared in vivo formulations for optimal results.)