Higher export rate of homocysteine in a human endothelial cell line than in other human cell lines.

Even mild hyperhomocysteinemia is associated with premature vascular disease. Despite the growing evidence that plasma homocysteine is a cardiovascular risk factor, the mechanism behind the vascular injuries is still unknown. Information about the metabolism of homocysteine is, therefore, essential for an understanding of its role in atherogenesis. In the present study we have, therefore, investigated the export mechanism of homocysteine. In HeLa cell lines the release of homocysteine was found to be a continuous process, which was increased in the presence of copper ions. High cell density led to a lowered release of homocysteine, probably due to a more extensive metabolism of the intracellular homocysteine. It was also found that HeLa cells were able to take up extracellularly released homocysteine and use it in the cellular metabolism. The ratio between intracellular homocysteine and the total amount of homocysteine is a measure of the ability of the cell to export the intracellularly produced homocysteine. The ratio also reflects the reuse of extracellular homocysteine. Under basal conditions, endothelial cells exported most of the intracellularly produced homocysteine and exhibited a very low concentration of homocysteine intracellularly, low reusage of exported homocysteine and consequently a low ratio in comparison with HeLa and hepatoma cell lines. After addition of homocysteine, all cell lines exhibited similar ratios. Thus, the intracellular homocysteine concentration in endothelial cells is more influenced by the extracellular concentration of homocysteine than is the intracellular concentration in HeLa and hepatoma cells. It may be speculated that this phenomenon could be associated with an increased sensitivity of endothelial cells to homocysteine and explain the association between hyperhomocysteinemia and vascular disease.