D-Arginine

D-Arginine (D-Arg) is the D (R) enantiomer of arginine, a basic amino acid with a guanidino group in its side chain. Cells use L-arginine, not D-arginine, to make proteins and nitric oxide.

Control in nitric oxide research. Nitric oxide synthases (NOS) oxidise L-arginine to L-citrulline and nitric oxide (NO), using oxygen and the cofactor NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate). D-Arginine is not a substrate of these enzymes. In isolated blood vessels, L-arginine caused a relaxation that depends on the endothelium, the cell layer that lines the vessel, and it reversed the effect of an inhibitor of NOS; D-arginine did neither. D-Arginine is therefore used as the inactive enantiomer control in studies with L-arginine.

D-Amino acid research. D-Arginine occurs naturally in the mouse brain, where it has been measured together with other D-amino acids by chiral liquid chromatography–mass spectrometry. Its level changes in mice that lack D-amino acid oxidase, an enzyme that breaks down several D-amino acids.

Neuroprotection (claim). It has been proposed, in review articles, that D-arginine protects the central nervous system against damage caused by long-term high levels of glucocorticoids, the stress hormones.