HOMOCYSTEINE--VASCULAR BIOCHEMISTRY AND METABOLISM

Cleveland Clinic Foundation, Cleveland OH

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Abstract

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease yet little is known about the vascular biochemistry and metabolism of homocysteine or the role it plays in atherogenesis. For the past three years our laboratory has focused on these problems. We have shown that vascular cells and tissues have a limited capacity to metabolize homocysteine. The catabolic transsulfuration pathway appears to be inoperative and the alternative remethylation pathway is not expressed. Thus, folate/B12-dependent remethylation may be the only pathway available in the vascular system for maintenance of low intracellular levels of homocysteine. We hypothesize that the overall efficiency of this pathway will be dependent upon cofactor availability and enzyme functionality. To test this hypothesis we have established a cellular model to explore gene-nutrient interactions. These studies will focus on methylenetetrahydrofolate reductase and methionine synthase, and enzymes that drive remethylation of homocysteine in vascular cells. Individuals who are homozygous for a common polymorphism (15 percent prevalence in many populations) of the reductase gene appear to have significantly higher total plasma homocysteine levels if coupled to low folate status. Because of their limited capacity to metabolize homocysteine, we hypothesize that endothelial cells are particularly sensitive to increases in plasma homocysteine, resulting in loss of function. Our efforts here are directed at the modulation of chemokine expression in endothelial cells by low concentrations of homocysteine. Our specific aims are: 1) to study the formation and stability of circulating forms of homocysteine. This aim is driven by the hypothesis that protein-bound homocysteine, the major form of homocysteine in circulation, may play a role in atherosclerosis; 2) to study gene-nutrient interactions in a vascular cells. This aim is driven by the hypothesis that, in vascular cells, folate/B12 dependent remethylation is the only pathway available for homocysteine metabolism and that the efficiency of this pathway is dependent upon cofactor availability and enzyme functionality; and 3) to study the effect of homocysteine on vascular cell function. This aim is driven by the hypothesis that, due to limited a capacity to metabolize homocysteine, vascular function can be altered by increases in intracellular homocysteine. These studies will provide a better understanding of the role of homocysteine in atherogenesis and perhaps more efficacious interventions to lower total plasma homocysteine.