Thiol redox status influences extracellular concentration of homocysteine in HeLa cell cultures

We have recently shown that low concentrations of copper ions increased the extracellular concentration of homocysteine in a HeLa cell culture system. This increase might be attributed to copper-induced oxidation of extracell ular homocysteine, since earlier reports suggest that the oxidized fraction of homocysteine seems to be less available for cellular uptake than the re duced fraction. To investigate further this mechanism we have therefore, in the present study, determined intra- and extracellular concentrations of h omocysteine and their relations to the extracellular thiol redox status. Th e results of this study show that the redox status of homocysteine is impor tant for its extracellular concentration. Increased oxidation of thiols cat alyzed by copper ions resulted in an increased extracellular concentration of homocysteine, whereas the addition of antioxidant agents (N-acetylcystei ne or dithiothreitol) or a specific copper(I) ion chelator (bathocuproinedi sulfonic acid) was accompanied by a decreased extracellular concentration o f homocysteine. These effects are probably dependent on differences in cell ular uptake between reduced and oxidized homocysteine.