It has been stated that elevated homocysteine level causes endothelial dysfunction and damage. By what mechanism does this happen? Why is it the endothel where elevated homocysteine causes problems?
In addition Alan Boyd's answer homocysteine interferes with Glutathione synthesis by reducing the catalytic efficiency of GCL (Glutamate–cysteine ligase), the enzyme which performs the rate-limiting step in GSH synthesis.
Results of the present study are the first to suggest that the relatively high levels of homocysteine that accumulate during aging can interfere with de novo GSH synthesis. Although the precise nature of the underlying mechanism is at present unclear, our results demonstrate that physiological concentrations of free homocysteine can inhibit the efficiency of cysteine utilization by GCL in a competitive manner. Homocysteine has been shown to bind GCLc at the active site in vitro  as well as in vivo , forming γ-glutamylhomocysteine , which is then rapidly degraded enzymatically. An additional mechanism by which an age-related increase in free homocysteine can inhibit the catalytic efficiency of GCL in a competitive manner is described in . It is possible that an aging-related increase in the competition between cysteine and homocysteine for the cysteine binding site of GCL might lead to a decrease in GC synthesis.
Low glutathione means a reduced ability to detoxify and deal with oxidative stress and a more oxidised redox state which has many implications
Inhibition of glutathione synthesis in brain endothelial cells lengthens S-phase transit time in the cell cycle: Implications for proliferation in recovery from oxidative stress and endothelial cell damage http://www.sciencedirect.com/science/article/pii/S2213231713000244
Glutathione reverses endothelial dysfunction and improves nitric oxide bioavailability http://content.onlinejacc.org/article.aspx?articleid=1125915
Short answer: homocysteine promotes elevated levels of reactive oxygen species which interfere with signalling via nitric oxide.
There is an extensive literature on this topic: a Web of Science search using [TITLE: endotheli* AND homocysteine] produces 567 hits.
The prevailing view seems to be that homocysteine causes oxidative stress in cells because autooxidation of the sulphydryl group of homocysteine promotes the production of reactive oxygen species (ROS): superoxide, hydrogen peroxide and hydroxide radical. There is also evidence for decreased levels of glutathione (GSH) and glutathione peroxidase (GPX) (McCully, 2009). The classical pathway for dealing with ROS is conversion of the primary species,superoxide, to hydrogen peroxide (by superoxide dismutase) followed by reduction of peroxide to water by GPX using GSH as a reductant. This pathway rapidly destroys ROS and importantly avoids the generation of the damaging hydroxide radical. Reduced glutathione is regenerated at the expense of NADPH.
Endothelial cells use nitric oxide (NO) to signal to surrounding smooth muscle, resulting in vasodilation. ROS react with NO, inactivating it. There is also evidence that NO production by epithial nitric oxide synthase (eNOS) is affected, and Leung et al., 2013 present data indicating that the phosphorylation level of eNOS is reduced by homocysteine. Phosphorylation is a key regulator of eNOS activity (Mount et al., 2007).
Leung SB et al. (2013) Salidroside Improves Homocysteine-Induced Endothelial Dysfunction by Reducing Oxidative Stress. Evidence-Based Complementary and Alternative Medicine 20: Article ID 679635
McCully, KS (2009) Chemical Pathology of Homocysteine. IV. Excitotoxicity, Oxidative Stress, Endothelial Dysfunction, and Inflammation. Annals Clin. Lab. Sci. 39:219-232
Mount, PF et al. (2007) Regulation of endothelial and myocardial NO synthesis by multi-site eNOS phosphorylation. J Molec. Cell. Cardiology 42: 271-279