Human Alanine-Glyoxylate Aminotransferase 2 Lowers Asymmetric Dimethylarginine and Protects from Inhibition of Nitric Oxide Production

Elevated blood concentrations of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric-oxide (NO) synthase, are found in association with diabetes, hypertension, congestive heart failure, and atherosclerosis. ADMA levels are controlled by dimethylarginine dimethylaminohydrolases (DDAHs), cytosolic enzymes that hydrolyze ADMA to citrulline and dimethylamine. ADMA also has been proposed to be regulated through an alternative pathway by alanine-glyoxylate aminotransferase 2 (AGXT2), a mitochondrial aminotransferase expressed primarily in the kidney. The goal of this study was to define the subcellular localization of human AGXT2 and test the hypothesis that overexpression of human AGXT2 protects from ADMA-induced inhibition in nitric oxide (NO) production. AGXT2 was cloned from human kidney cDNA and overexpressed in COS-7 cells and human umbilical vein endothelial cells with a C-terminal FLAG epitope tag. Mitochondrial localization of human AGXT2 was demonstrated by confocal microscopy and a 41-amino acid N-terminal mitochondrial cleavage sequence was delineated by N-terminal sequencing of the mature protein. Overexpression of human AGXT2 in the liver of C57BL/6 mice using an adenoviral expression vector produced significant decreases in ADMA levels in plasma and liver. Overexpression of human AGXT2 also protected endothelial cells from ADMA-mediated inhibition of NO production. We conclude that mitochondrially localized human AGXT2 is able to effectively metabolize ADMA in vivo resulting in decreased ADMA levels and improved endothelial NO production.