Analysis of Methylarginine Metabolism in the Cardiovascular System Identifies the Lung As a Major Source of ADMA

Protein arginine methylation is catalyzed by a family of enzymes called protein arginine methyltransferases (PRMTs). Three forms of methylarginine have been identified in eukaryotes: monomethylarginine (l-NMMA), asymmetric dimethylarginine (ADMA), and symmetric dimethylarginine (SDMA), all characterized by methylation of one or both guanidine nitrogen atoms of arginine. l-NMMA and ADMA, but not SDMA, are competitive inhibitors of all nitric oxide synthase isoforms. SDMA is eliminated almost entirely by renal excretion, whereas l-NMMA and ADMA are further metabolized by dimethylarginine dimethylaminohydrolase (DDAH). To explore the interplay between methylarginine synthesis and degradation in vivo, we determined PRMT expression and DDAH activity in mouse lung, heart, liver, and kidney homogenates. In addition, we employed HPLC-based quantification of protein-incorporated and free methylarginine, combined with immunoblotting for the assessment of tissue-specific patterns of arginine methylation. The salient findings of the present investigation can be summarized as follows: 1) pulmonary expression of type I PRMTs was correlated with enhanced protein arginine methylation; 2) pulmonary ADMA degradation was undertaken by DDAH1; 3) bronchoalveolar lavage fluid and serum exhibited almost identical ADMA/SDMA ratios, and 4) kidney and liver provide complementary routes for clearance and metabolic conversion of circulating ADMA. Together, these observations suggest that methylarginine metabolism by the pulmonary system significantly contributes to circulating ADMA and SDMA levels.