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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Oral l-arginine improves hemodynamic responses to stress and reduces plasma homocysteine in hypercholesterolemic men
From Alternative Medicine Review, 3/1/05 by S.G. West

West SG, Likos-Krick A, Brown P, Mariotti F. J Nutr 2005;135:212-217.

When administered intravenously, l-arginine substantially reduces blood pressure (BP) and peripheral vascular resistance in healthy adults and in patients with vascular disease. Oral l-arginine has been shown to improve endothelial function; however, it is not clear whether oral administration has significant effects on systemic hemodynamics. In a randomized, placebo-controlled, crossover study we tested whether oral l-arginine (12 g/d for 3 wk) affected hemodynamics, glucose, insulin, or C-reactive protein in 16 middle-age men with hypercholesterolemia. After each treatment, hemodynamic variables were measured at rest and during 2 standardized stressor tasks (a simulated public-speaking task and the cold pressor). Regardless of treatment, the stressor tasks increased BP and heart rate (P </= 0.02). Relative to placebo, 1-arginine changed cardiac output (0.4 L/m), diastolic BP (-1.9 mm Hg), pre-ejection period (+3.4 ms), and plasma homocysteine (-2.0 umol/L) (P </= 0.03). The change in plasma l-arginine was inversely correlated with the change in plasma homocysteine (r = -0.57, P = 0.03). Contrary to the results of previous studies of l-arginine administered intravenously, oral administration did not affect total peripheral resistance or plasma insulin. Oral l-arginine also did not affect plasma glucose, C-reactive protein, or lipids. This pattern of findings is consistent with the hypothesis that oral l-arginine reduces BE This study is the first to describe a hemodynamic mechanism for the hypotensive effect of oral l-arginine and the first to show substantial reductions in homocysteine with oral administration.

COPYRIGHT 2005 Thorne Research Inc.
COPYRIGHT 2005 Gale Group

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