The association between hyperhomocysteinemia and cardiovascular disease is well established. The principal cause of morbidity and mortality in autosomal recessive inheritable homocystinuria is premature atherosclerosis with arterial and venous thrombosis. Several recent clinical studies involving over 3,000 patients have shown a relationship between slight to mild hyperhomocysteinemia and the development of coronary artery disease, cerebrovascular disease and peripheral vascular occlusive disease, independent of traditional risk factors such as hypertension, smoking, hyperlipidemia and diabetes. Early work, guided by the endothelial response-to-injury hypothesis, showed that high supraphysiological levels of homocysteine and homocysteine thiolactone were injurious and cytotoxic to vascular endothelial cells. However, little is known about the mechanism of homocysteine-induced cytotoxicity, or even how homocysteine is metabolized in endothelial cells. Furthermore, practically nothing is known about the chemical interactions between homocysteine and plasma proteins. Our work will focus on the biochemistry and metabolism of homocysteine in the vascular system and the effect that elevated levels of homocysteine might have on proatherogenic and prothrombogenic functions of endothelial cells. We hypothesize that aortic and coronary artery endothelial cells are sensitive to elevated homocysteine because the transsulfuration pathway is inactive in these cells. Furthermore elevated homocysteine induces the expression of adhesion molecules, thereby promoting monocyte recruitment.
The specific aims of this study are: 1) to investigate the interactions between homocysteine and specific plasma proteins using in vitro models that will allow us to determine their binding capacity for homocysteine and to analyze the exchange kinetics between homocysteine and protein-bound cysteine. Isolate, characterize and identify the endogenous carriers of homocysteine in normal and hyperhomocysteinemic plasma; 2) to determine the pathways of homocysteine metabolism in cultured human aorta endothelial cells by measuring cobalamin (B 12)-dependent methionine synthase, pyridoxal-dependent cystathionine (3-synthase and methylenetetrahydrofolate reductase and to compare these measurements with enzyme activities in fresh preparations of coronary arteries from human hearts 3) to establish the effect of physiological concentrations of homocysteine on endothelial cell function, especially the promotion of monocytic adhesion to cultured aortic endothelial cells, and to gain a mechanistic understanding of this phenomenon. These studies will provide valuable new insights on the vascular biochemistry and metabolism of homocysteine and may suggest a rationale for the treatment of hyperhomocysteinemia.
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