Endogenous nitric oxide synthase inhibitors in the biology of disease: markers, mediators, and regulators?

Abstract

The asymmetric methylarginines inhibit nitric oxide synthesis in vivo by competing with L-arginine at the active site of nitric oxide synthase. High circulating levels of asymmetric dimethylarginine predict adverse outcomes, specifically vascular events but there is now increasing experimental and epidemiological evidence that these molecules, and the enzymes that regulate this pathway, play a mechanistic role in cardiovascular diseases. Recent data have provided insight into the impact of altered levels of these amino acids in both humans and rodents, however these reports also suggest a simplistic approach based on measuring, and modulating circulating asymmetric dimethylarginine alone is inadequate. This review outlines the basic biochemistry and physiology of endogenous methylarginines, examines both the experimental and observational evidence for a role in disease pathogenesis, and examines the potential for therapeutic regulation of these molecules.

Figure 1. The structure and metabolism of the endogenous methylarginines

(A) The structure of L-arginine and the free endogenous methylarginines N^G, N^G-dimethyl-L-arginine (ADMA), N^G, N^G′-dimethyl-L-arginine (SDMA) and N^G-monomethyl-L-arginine (L-NMMA) which are formed following the liberation of methylated arginines from proteins; (B) Dimethylarginine dimethylaminohydrolase mediated hydrolysis of ADMA to form dimethylamine (DMA) and L- citrulline. L-NMMA also undergoes hydrolysis (not shown); (C) Alanine-glyoxylate aminotransferase-2 mediated deamination of ADMA to form (asymmetric) α-keto-δ-(N, N-dimethylguanidino) valeric acid (ADGV). Both pyruvate and glyoxylate can act as the amino-acceptor in this reaction. L-NMMA and SDMA are also thought to undergo deamination (not shown).

Figure 2. ADMA distribution, metabolism and competitive inhibition of NOS

L-arginine (L-Arg) is present in the circulation at >100 times the concentrations of the free endogenous methylarginines: asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA). ADMA but not SDMA inhibits all three isoforms of nitric oxide synthase (NOS), decreasing the production of nitric oxide. L-arginine and the free methylarginines are thought to enter the cell (shown on the left) through the y+ transporter. ADMA and SDMA are generated intracellularly following the methylation, by protein-arginine methyltransferases (PRMT), and subsequent proteolysis, of constituent protein arginine residues. ADMA can regulate protein expression through both NO-dependent and –independent pathways. ADMA but not SDMA is hydrolysed by DDAH to form dimethylamine (DMA) and L-citrulline (L-Cit), which can be reincorporated into proteins. The majority of ADMA is metabolised by DDAHs with the product DMA excreted in the urine whereas SDMA is excreted intact. Both SDMA and ADMA are substrates for AGXT2, which is expressed only in kidney (right) and liver (not shown), leading to the formation of, respectively, symmetrical and asymmetrical, dimethylguanidino valeric acid (DGV) that is also excreted in the urine. Dotted arrows show metabolic pathways for which limited in vivo evidence is available. Acetylation of ADMA has also been described but is not shown. Monomethylarginine (not shown for clarity) is thought to have a similar actions, distribution, and degradation pathway to ADMA.