Biological significance of endogenous methylarginines that inhibit nitric oxide synthases.

The guanidino-methylated arginine analogue NG monomethyl-L-arginine (L-NMMA) has been the standard nitric oxide synthase inhibitor used to evaluate the role of the L-arginine:nitric oxide pathway. However, L-NMMA and other methylated arginine residues are also synthesised in vivo by the action of a family of enzymes known as protein arginine methyltransferases. Proteolysis of proteins containing methylated arginine residues releases free methylarginine residues into the cytosol from where they may pass out of the cell into plasma. Of the three known methylarginine residues produced in mammals only asymmetrically methylated forms (L-NMMA and asymmetric dimethylarginine (ADMA)) but not symmetrically methylated arginine (symmetric dimethylarginine (SDMA)) inhibit nitric oxide synthase (NOS). We and others have proposed that endogenously produced asymmetrically methylated arginines may modulate NO production and that the accumulation of these residues in disease states may contribute to pathology. The activity of the enzyme dimethylarginine dimethylaminohydrolase that metabolises asymmetric methylarginines may be of critical importance in affecting NO pathways in health or disease.