The relative roles of the glutathione redox cycle and catalase in the detoxification of H 2O 2 by cultured rabbit lens epithelial cells

Abstract

The relative roles of the glutathione redox cycle and catalase in the detoxification of H 2O 2 were investigated in cultured rabbit lens epithelial cells. Exposure of cells to H 2O 2 was carried out following inhibition of either of the two antioxidant systems. Two different procedures were used to expose the cells to extracellular H 2O 2, one in which a low, steady state level of 0·025 m m H 2O 2 was maintained in the culture medium with the use of glucose oxidase and the other in which H 2O 2 was added to the medium as a single pulse at levels ranging from 0·03 to 0·5 m m. When lens cells were treated with a low, steady state level of H 2O 2, the glutathione redox cycle was the primary menas of defense against oxidative damage. Cells with fully active catalase but with inhibited glutathione reductase were not able to resist the cytotoxic effects of a 0·025 m m level of extracellular H 2O 2. Under these conditions the cells were nearly completely depleted of reduced glutathione within 15 min. The cellular damage observed after 1·5 hr of culture included loss of cell-to-cell contact, rounding up of the cells and formation of numerous blebs. In contrast, cells with completely inhibited catalase but with an unimpaired glutathione redox cycle suffered few damaging effects from a 3-hr exposure to 0·025 m m H 2O 2. When lens cells were pulsed with a single challenge of 0·5 m m H 2O 2, both the glutathione redox cycle and catalase were found to be essential for survival of the cells. While control cells were able to withstand the pulse of H 2O 2, cells with impaired activities of either the glutathione redox cycle or catalase were killed. Control cells treated with 0·5 m m H 2O 2 may have been protected from damage by the fact that the cellular level of GSH never dropped below 35% of normal. The cause of cell death following inhibition of catalase appeared to be related to an inability of the cells to remove peroxide from the culture medium, at a rapid rate, following the H 2O 2-pulse. Although cells with impaired glutathione reductase activity removed H 2O 2 from the medium at a rate comparable to that of control cells (due to uninhibited catalase activity), they did not survive the challenge. This indicated that although the glutathione redox cycle removed only a small percentage of the total H 2O 2 following a 0·5 m m pulse of the oxidant, the function of the redox cycle was essential for the cells' survival. This latter result supports the concept that H 2O 2-induced cytotoxicity may be caused by a lethal, secondary product of H 2O 2 which can be metabolized only by the glutathione redox cycle.