No abstract

Abstract-Hyperhomocysteinemia is associated with endothelial dysfunction, although its mechanism is unknown.Isometric tension recordings and lucigenin chemiluminescence were used to assess the effects of homocysteine exposure on endothelium-dependent and -independent relaxation in isolated rabbit aortic rings and superoxide anion (O 2 Ϫ ) production by cultured porcine aortic endothelial cells, respectively. Homocysteine (0.1 to 10 mmol/L) produced a significant (PϽ0.001) concentration-and time-dependent inhibition of endothelium-dependent relaxation in response to both acetylcholine and the calcium ionophore A23187. Only the intracellular O 2 Ϫ scavenger 4,5-dihydroxy-1,3-benzene disulfonic acid (Tiron, 10 mmol/L) significantly (PϽ0.001) inhibited the effect of homocysteine on acetylcholine-and A23187-induced relaxation. Incubation of porcine aortic endothelial cells with homocysteine (0.03 to 1 mmol/L for up to 72 hours) caused a significant (PϽ0.001) time-dependent increase in the O 2 Ϫ released by these cells on the addition of Triton X-100 (1% [vol/vol]), with levels returning to values comparable to those of control cells at the 72-hour time point. These changes in O 2 Ϫ levels were associated with a time-dependent increase in endothelial cell superoxide dismutase activity, becoming significant (PϽ0.001) after 72 hours. Furthermore, the homocysteine-induced increase in endothelial cell O 2 Ϫ levels was completely inhibited (PϽ0.001) by the concomitant incubation with either Tiron (10 mmol/L), vitamin C (10 mol/L), or vitamin E (10 mol/L). These data suggest that the inhibitory effect of homocysteine on endothelium-dependent relaxation is due to an increase in the endothelial cell intracellular levels of O 2 Ϫ and provide a possible mechanism for the endothelial dysfunction associated with hyperhomocysteinemia. Key Words: homocysteine Ⅲ endothelial function Ⅲ nitric oxide Ⅲ oxygen-derived free radicals D eficiencies in the enzymes cystathionine ␤ synthase and methylenetetrahydrofolate reductase lead to elevated plasma homocyst(e)ine levels (free and bound homocysteine plus homocystine and homocysteine-cysteine mixed disulfide). 1 Patients with these disorders have much higher plasma concentrations of homocysteine than do normal subjects (Ն80 mol/L compared with Յ10 mol/L, respectively) and are prone to premature cardiovascular disease. 2 Severe hyperhomocysteinemia, arising, for example, from cystathionine ␤ synthase deficiency, is rare among the general population, whereas mild hyperhomocysteinemia (15 to 30 mol/L) is common. A recent epidemiological study confirms that even such modestly elevated homocysteine levels are associated with an increased risk of atherosclerosis 3 and can theoretically be caused by reduced activity of the enzyme methylenetetrahydrofolate reductase or deficiencies in folic acid and vitamin B 12 .The mechanism of the vascular injury seen in hyperhomocysteinemia is not known, although growing evidence suggests that endothelial dysfunction plays a major role. 4,5 Indeed, homocystein...

A functional phagocyte-type NADPH oxidase is expressed in coronary microvascular endothelial cells, where it may contribute to the physiological and/or pathophysiological effects of reactive oxygen species. These data, together with reports of the presence of a similar oxidase in other non-phagocytic cell types, suggest that this enzyme complex is widely expressed in many tissues where it may subserve signaling and other functions.