Abstract

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 extracellular homocysteine, since earlier reports suggest that the oxidized fraction of homocysteine seems to be less available for cellular uptake than the reduced fraction. To investigate further this mechanism we have therefore, in the present study, determined intra- and extracellular concentrations of homocysteine and their relations to the extracellular thiol redox status. The results of this study show that the redox status of homocysteine is important for its extracellular concentration. Increased oxidation of thiols catalyzed by copper ions resulted in an increased extracellular concentration of homocysteine, whereas the addition of antioxidant agents (N-acetylcysteine or dithiothreitol) or a specific copper(I) ion chelator (bathocuproinedisulfonic acid) was accompanied by a decreased extracellular concentration of homocysteine. These effects are probably dependent on differences in cellular uptake between reduced and oxidized homocysteine.