Mechanism of 4-HNE mediated inhibition of hDDAH-1: implications in no regulation.

Nitric oxide synthase is inhibited by NG-methylated derivatives of arginine whose cellular levels are controlled by dimethylarginine dimethylamino-hydrolase (DDAH). DDAH-1 is a Zn(II)-containing enzyme that through hydrolysis of methylated l-arginines regulates the activity of NOS. Herein, we report the kinetic properties of hDDAH-1 and its redox-dependent regulation. Kinetic studies using recombinant enzyme demonstrated Km values of 68.7 and 53.6 microM and Vmax values of 356 and 154 nmols/mg/min for ADMA and L-NMMA, respectively. This enzymatic activity was selective for free ADMA and L-NMMA and was incapable of hydrolyzing peptide incorporated methylarginines. Subsequent studies performed to determine the effects of reactive oxygen and reactive nitrogen species on DDAH activity demonstrated that low level oxidant exposure had little effect on enzyme activity and that concentrations approaching >or=100 microM were needed to confer significant inhibition of DDAH activity. However, exposure of DDAH to the lipid oxidation product, 4-HNE, dose-dependently inhibited DDAH activity with 15% inhibition observed at 10 microM, 50% inhibition at 50 microM, and complete inhibition at 500 microM. Mass spectrometry analysis demonstrated that the mechanism of inhibition resulted from the formation of Michael adducts on His 173, which lies within the active site catalytic triad of hDDAH-1. These studies were performed with pathophysiologicaly relevant concentrations of this lipid peroxidation product and suggest that DDAH activity can be impaired under conditions of increased oxidative stress. Because DDAH is the primary enzyme involved in methylarginine metabolism, the loss of activity of this enzyme would result in impaired NOS activity and reduced NO bioavailability.

Role of DDAH-1 in lipid peroxidation product-mediated inhibition of endothelial NO generation.

Altered nitric oxide (NO) biosynthesis is thought to play a role in the initiation and progression of atherosclerosis and may contribute to increased risk seen in other cardiovascular diseases. It is hypothesized that altered NO bioavailability may result from an increase in endogenous NO synthase (NOS) inhibitors, asymmetric dimethly araginine (ADMA), and N(G)-monomethyl arginine, which are normally metabolized by dimethyarginine dimethylamine hydrolase (DDAH). Lipid hydroperoxides and their degradation products are generated during inflammation and oxidative stress and have been implicated in the pathogenesis of cardiovascular disorders. Here, we show that the lipid hydroperoxide degradation product 4-hydroxy-2-nonenal (4-HNE) causes a dose-dependent decrease in NO generation from bovine aortic endothelial cells, accompanied by a decrease in DDAH enzyme activity. The inhibitory effects of 4-HNE (50 microM) on endothelial NO production were partially reversed with L-Arg supplementation (1 mM). Overexpression of human DDAH-1 along with antioxidant supplementation completely restored endothelial NO production following exposure to 4-HNE (50 microM). These results demonstrate a critical role for the endogenous methylarginines in the pathogenesis of endothelial dysfunction. Because lipid hydroperoxides and their degradation products are known to be involved in atherosclerosis, modulation of DDAH and methylarginines may serve as a novel therapeutic target in the treatment of cardiovascular disorders associated with oxidative stress.