A PROTECTIVE ROLE FOR GLUTATHIONE-DEPENDENT REDUCTION OF DEHYDROASCORBIC ACID IN LENS EPITHELIUM

Purpose. In view of the antioxidant role of ascorbic acid and the glut athione redox cycle in the lens, the authors have studied the relation ship of the cycle to reduction of the oxidized product of ascorbic aci d, dehydroascorbic acid (DHA), in lens epithelium. Methods. Cultured d og lens epithelial cells and intact rabbit lenses were exposed to vari ous concentrations of DHA in experiments performed at 20 degrees C to minimize hydrolysis of the compound (t(1/2) of 5 minutes at 37 degrees C). Levels of glutathione (GSH) and oxidized glutathione (GSSG) were measured in lens cells and whole lens epithelial by electrochemical de tection. Results. Treatment of lens cells with 1 mM DHA for 0.5 to 3 h ours in the absence of glucose (glucose is required for the reduction of GSSG through the glutathione redox cycle) produced from 60% to comp lete oxidation of GSH (controls contained negligible GSSG) and distinc t morphologic changes (cell contraction and blebbing), as shown by sca nning electron microscopy. Glucose prevented these effects and allowed nearly immediate recovery of GSH after DHA exposure in the absence of glucose. A dose-dependent response was observed for the formation of GSSG in cultured cells from 0.05 to 0.5 mM DHA in the absence of gluco se. The results of experiments performed with DHA plus an inhibitor of glutathione reductase mimicked those obtained using DHA minus glucose . DHA produced a 3- to 10-fold stimulation of hexose monophosphate shu nt activity in cultured lens cells and whole lenses, which was prevent ed by the inhibition of glutathione reductase. Treatment of whole lens es with DHA minus glucose also produced oxidation of epithelial GSH an d was accompanied by the loss of lens transparency. No evidence was fo und for dehydroascorbate reductase activity in the lens epithelium. Co nclusions. The exposure of lenses and lens epithelial cells to DHA. un der conditions in which the glutathione redox cycle was compromised re sulted in the disappearance of GSH in the tissues and the appearance o f GSSG. The reduction of DHA was shown to be linked to the glutathione redox cycle by a nonenzymatic interaction between GSH and DHA. Reduct ion of DHA in the lens is important because of the potential toxicity of this oxidant and/or its degradation products.