Chemical structure of Arginine
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Arginine

Arginine (Arg) is an α-amino acid. The L-form is one of the 20 most common natural amino acids. In mammals, arginine is classified as a semiessential or conditionally essential amino acid, depending on the developmental stage and health status of the individual. more...

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Structure

Arginine can be considered to be an amphipathic amino acid as the part of the side chain nearest to the backbone is long, carbon-containing and hydrophobic, whereas the end of the side chain is a complex guanidinium group. With pKa > 12, the guanidinium group is positively charged in neutral and acidic 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.

Synthesis

Arginine is synthesized from citrulline by the sequential action of the cytosolic enzymes ASS and ASL. This is energetically costly, as the synthesis of each molecule of argininosuccinate requires hydrolysis of ATP to AMP; i.e., two ATP equivalents.

Citrulline can be derived from multiple sources:

  • from arginine via nitric oxide synthase (NOS);
  • from ornithine via catabolism of proline or glutamine/glutamate;
  • from ADMA via DDAH.

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. Consequently, 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. (Morris SM Jr, 2004)

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.

In proteins

The 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.

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Treatment of erectile dysfunction with pycnogenol and L-arginine
From Alternative Medicine Review, 11/1/03 by R Stanislavov

Penile erection requires the relaxation of the cavernous smooth muscle, which is triggered by nitric oxide (NO). We investigated the possibility of overcoming erectile dysfunction (ED) by increasing the amounts of endogenous NO. For this purpose, we orally administered Pycnogenol, because it is known to increase production of NO by nitric oxide syntase together with L-arginine as substrate for this enzyme. The study included 40 men, aged 25-45 years, without confirmed organic erectile dysfunction. Throughout the 3-month trial period, patients received 3 ampoules Sargenor a day, a drinkable solution of the dipeptide arginyl aspartate (equivalent to 1.7 g L-arginine per day). During the second month, patients were additionally supplemented with 40 mg Pycnogenol two times per day; during the third month, the daily dosage was increased to three 40-mg Pycnogenol tablets. We obtained a sexual function questionnaire and a sexual activity diary from each patient. After 1 month of treatment with L-arginine, a statistically nonsignificant number of 2 patients (5%) experienced a normal erection. Treatment with a combination of L-arginine and Pycnogenol for the following month increased the number of men with restored sexual ability to 80%. Finally, after the third month of treatment, 92.5% of the men experienced a normal erection. We conclude that oral administration of L-arginine in combination with Pycnogenol causes a significant improvement in sexual function in men with ED without any side effects.

COPYRIGHT 2003 Thorne Research Inc.
COPYRIGHT 2003 Gale Group

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