Reduction of dehydroascorbic acid by homocysteine

To determine the reductive process of extracellular dehydroascorbic acid (DHA), molecules (homocysteine, homocysteine thiolactone, methionine, cysteine, and homoserine) were tested to identify those with the potential to reduce DHA to ascorbic acid (AA). Homocysteine (Hcy) was the most potent of the molecules tested. The efficacy of Hey was compared with that of other molecules able to reduce DHA (reduced glutathione (GSH) and cysteine (Cy)). Although all three molecules were able to reduce DHA, GSH and Cy were not to reduce DHA to AA at concentrations lower than 100 mu mol/l, and only less than 5% DHA was reduced to AA at concentrations of 200-300 mu mol/l. In contrast, Hey reduced DHA to AA stoichiometrically at concentrations as low as 10 mu mol/l. In Jurkat and U937 cells, the increasing concentrations of extracellular Hey suppressed intracellular dehydroascorbic acid uptake, indicating that extracellular reduction of DHA by Hey leads to decreasing extracellular DHA available for its intracellular uptake. Simultaneous oxidation and reduction of Hey and DHA were accelerated extracellularly in the presence of quercetin, an inhibitor of DHA uptake, suggesting that extracellular ascorbic acid concentration increased via blocking DHA uptake by quercetin and reducing extracellular DHA by Hey. The effect of homocysteine on DHA reduction and uptake was confirmed with human umbilical vein endothelial cells. The oxidation of Hey also prevented the decrease in DNA synthesis in human umbilical vein endothelial cells, which would occur following exposure to Hey. (C) 2001 Elsevier Science B.V. All rights reserved.