Redox state of glutathione in human plasma

Thiol and disulfide forms of glutathione (GSH) and cysteine (Cys) were meas ured in plasma from 24 healthy individuals aged 25-35 and redox potential v alues (E-h) for thiol/disulfide couples were calculated using the Nernst eq uation. Although the concentration of GSH (2.8 +/- 0.9 mu M) was much great er than that of GSSG (0.14 +/- 0.04 mu M), the redox potential of the GSSG/ 2GSH pool (-137 +/- 9 mV) was considerably more oxidized than values for ti ssues and cultured cells (-185 to -258 mV). This indicates that a rapid oxi dation of GSH occurs upon release into plasma. The difference in values bet ween individuals was remarkably small, suggesting that the rates of reducti on and oxidation in the plasma are closely balanced to maintain this redox potential. The redox potential for the Cys and cystine (CySS) pool (-80 +/- 9 mV) was 57 mV more oxidized, showing that the GSSG/2GSH and the CySS/2Cy s pools are not in redox equilibrium in the plasma. Potentials for thiol/di sulfide couples involving CysGly were intermediate between the values for t hese couples. Regression analyses showed that the redox potentials for the different thiol/disulfide couples within individuals were correlated, with the E-h for CySS-mono-Gly/(Cys . CysGly) providing the best correlation wit h other low molecular weight pools as well as protein disulfides of GSH, Cy sGly and Cys. These results suggest that E-h values for GSSG/2GSH and CySS- mono-Gly/(Cys . CysGly) may provide useful means to quantitatively express the oxidant/antioxidant balance in clinical and epidemiologic studies. (C) 2000 Elsevier Science Inc.