Differential distribution of free and bound glutathione and cyst(e)ine in human blood.

The redox status of free and bound glutathione (GSH) and cyst(e)ine (Cys) is altered by oxidative stress, drugs, and disease. Most studies measure only their free forms and not the bound forms, which may have a crucial protective role. For this reason, we determined free and bound, reduced and oxidized GSH and Cys in whole blood, red cells, and plasma of human blood from healthy adults. Distinct compartments of GSH and Cys were found. In whole blood, > 99% GSH was in red cells, of which 16% was bound. GSH values were the same for red cells in whole blood or in cells isolated from the same samples. Only 0.5% of GSH was in plasma, all of which was bound. In contrast, 97% of Cys was in plasma and only 3% in red cells. This was a remarkable separation of these closely related metabolites in the same tissue. In plasma, 60% of Cys was bound. Also, strong correlations were shown of bound vs free Cys and also vs free plus bound Cys. The bound Cys was more constant and suggested that it is a metabolic reserve. Our findings demonstrate the occurrence of significant bound forms of GSH and Cys and have implications for future studies in disease and toxicology.