Objective-To determine how hypochlorous acid (HOCl), the principal product of myeloperoxidase, modulates vascular function. Methods and Results-Rabbit arterial rings exposed to HOCl (0 to 500 mol/L) exhibited dose-and time-dependent impairment of endothelium-dependent arterial relaxation to acetylcholine and A23187, but not the NO donor, diethylamine NONOate, suggesting that HOCl targets the endothelium. This effect was not because of cytotoxicity, as HOCl treatment produced no significant change in endothelial cell morphology or lactate dehydrogenase release. We observed HOCl-mediated endothelial cell protein oxidation by immunoreactivity to HOP-1, a monoclonal antibody specific for HOCl-oxidized protein.In support of this notion, known HOCl scavengers, such as methionine and N-acetylcysteine, partially preserved endothelium-derived NO bioactivity in response to HOCl. In an unanticipated observation, HOCl-mediated impairment of NO bioactivity was prevented by manganese superoxide dismutase in a manner dependent on its enzymatic activity. Finally, we found that HOCl reduced endothelial nitric oxide synthase dimer stability, an effect that was also inhibited by superoxide dismutase. Conclusions-Taken together, these data indicate that HOCl imparts a defect in endothelial NO production due to a superoxide-dependent reduction in endothelial nitric oxide synthase dimer stability. These data provide another mechanism whereby myeloperoxidase-derived oxidants can contribute to the impairment of NO bioactivity that is characteristic of atherosclerosis. Key Words: oxidant stress Ⅲ nitric oxide Ⅲ endothelium Ⅲ superoxide A therosclerosis and its associated pathological conditions, such as hypercholesterolemia 1 and diabetes mellitus, 2 are characterized by abnormal endothelium-derived nitric oxide (EDNO) bioactivity. In atherosclerosis, vasodilatation to agonists for EDNO release, such as acetylcholine 3 or aggregating platelets, 4 is impaired or replaced by constriction. Endothelial dysfunction has been directly demonstrated in atherosclerotic human coronary arteries, 3 and there is strong evidence that it contributes to plaque activation and the pathogenesis of myocardial ischemia. 5 In fact, the presence of vascular dysfunction predicts the occurrence of clinical events in patients with atherosclerosis. 6 Available evidence suggests that vascular oxidative events play an important role in endothelial dysfunction. In animal models of atherosclerosis and hypercholesterolemia, the vascular production of superoxide anion radical is increased, 7 and this abnormality can decrease NO bioactivity through peroxynitrite formation. 8 Indeed, limiting superoxide production with oxypurinol, a xanthine oxidase inhibitor, partially restores EDNO bioactivity. 7 Similarly, in atherosclerotic rabbits, chronic treatment with polyethylene glycolatedsuperoxide dismutase (SOD) increases vascular SOD activity and partially restores NO-mediated relaxation to acetylcholine. 9 Later stages of atherosclerosis, however, are more complex. In Watan...