Endothelial cell injury due to copper-catalyzed hydrogen peroxide generation from homocysteine.

Abstract

We have examined whether the toxic effects of homocysteine on cultured endothelial cells could result from the formation and action of hydrogen peroxide. In initial experiments with a cellfree system, micromolar amounts ofcopper were found to catalyze an oxygen-dependent oxidation of homocysteine. The molar ratio of homocysteine oxidized to oxygen consumed was 4.0, which suggests that oxygen was reduced to water. The addition of catalase, however, decreased oxygen consumption by nearly onehalf, which suggests that H202 was formed during the reaction. Confirming this hypothesis, H202 formation was detected using the horseradish peroxidase-dependent oxidation of fluorescent scopoletin. Ceruloplasmin was also found to catalyze oxidation of homocysteine and generation of H202 in molar amounts equivalent to copper sulfate. Finally, homocysteine oxidation was catalyzed by normal human serum in a concentration-dependent manner.

Role of hyperhomocysteinemia in endothelial dysfunction and atherothrombotic disease

“…41 It is likely, therefore, that HHcy decreases nitric oxide bioavailability through alternative mechanisms, such as accelerated oxidative inactivation of nitric oxide. 40 Homocysteine-induced oxidative inactivation of nitric oxide has been observed in vitro in cultured endothelial cells, 42 and evidence for increased oxidative inactivation of nitric oxide during HHcy has been obtained in animals using both pharmacological 41,[43][44][45][46] and genetic 47,48 approaches. Several types of reactive oxygen species (ROS), including superoxide, hydrogen peroxide, and peroxynitrite, may contribute to the oxidative inactivation of endothelium-derived nitric oxide in HHcy.…”

“…The highly reactive thiol group of homocysteine is readily oxidized to form ROS, 42 suggesting that homocysteine induces cell injury/dysfunction through a mechanism involving auto-oxidation and oxidative damage. However, this hypothesis fails to explain why cysteine, which is present in plasma at much higher concentrations than homocysteine and is readily auto-oxidized, does not cause endothelial cell injury and is not considered a risk factor for cardiovascular disease.…”

Reduction in plasma total homocysteine through increasing folate intake in healthy individuals is not associated with changes in measures of antioxidant activity or oxidant damage

“…The pro-oxidant hypothesis for homocysteine is based largely on results from in vitro studies. When exposed to free homocysteine at high (supraphysiological) concentrations, endothelial cells in culture show a loss of structural and functional integrity that can be mitigated by the presence of additional antioxidants (Starkebaum & Harlan, 1986;Blundell et al, 1996). The deleterious effects of homocysteine on endothelial cells in culture may be mediated by the increased production of superoxide radicals resulting from the autooxidation of homocysteine (Lang et al, 2000).…”

“…One such mechanism invokes the potentially pro-oxidative potential of homocysteine, and argues that the increasing plasma concentrations of this amino acid are associated with increased oxidative stress and ensuing tissue damage (Bellamy & McDowell, 1997). Results from in vitro studies are generally supportive of a pro-oxidative role of homocysteine (Hirano et al, 1994;Starkebaum & Harlan, 1986), but the relevance of these observations to conditions in vivo are not known. Relatively few studies have explored this role for homocysteine in vivo and these have concentrated almost exclusively on events associated with greatly elevated plasma tHcy concentrations in patients with genetically determined homocystinuria, or with a transient acute elevation of plasma tHcy following an oral load of methionine Wilcken et al, 2000).…”

The effect of folic acid on the homocysteine metabolism in human umbilical vein endothelial cells (HUVECs)

“…In vitro high levels of homocysteine can generate HiO-> and the vasotoxic properties of homocysteine through lipid peroxidation have been proposed [15]. However, in plasma from patients with severe hyper homocysteinaemia no increase in lipid peroxidation could be found [16,17], Several in vitro studies with endothelial cells have shown cell dysfunction when homocysteine is added to the medium.…”