[제 4 주제] NADPH의 항산화효과

NADPH is an important cofactor in many biosynthesis pathways and the regeneration of reduced glutathione, critically important in cellular defense against oxidative damage. It is mainly produced by glucose 6-phosphate dehydrogenase (G6PD), malic enzyme, and the cytosolic form of NADP'-dependent isocitrate dehydrogenase (IDPc). Little information is available about the role of lDPc in antioxidant defense. In this study we investigated the role of IDPc against cytotoxicity induced by oxidative stress by comparing the relative degree of cellular responses in three different NIH3T3 cells with stable transfection with the cDNA for mouse lDPc in sense and antisense orientations, where IDPc activities were 3-4 fold higher and 35% lower, respectively, than that in the parental cells carrying the vector alone. Although the activities of other antioxidant enzymes, such as superoxide dismutase, catalase, glutathione reductase, glutathione peroxidase, and G6PO, were comparable in all transformed cells, the ratio of GSSG to total glutathione was significantly higher in the cells expressing the lower level of IDPc. This finding indicates that IDPc is essential for the efficient glutathione recycling. Upon transient exposure to increasing doses of H2O2, menadione, an intracellular source of free radicals and reactive oxygen species, or UVB (312 nm), the cells with low levels of IDPc became more sensitive to oxidative damage by H2O2, menadione, or UVB. Lipid peroxidation, oxidative DNA damage, and intracellular peroxide generation were higher in the cell-line expressing the lower level of lDPc. However, the cells with the highly over-expressed lDPc exhibited enhanced resistance against oxidative stress, compared to the control cells. We also demonstrated that mitochondrial NADP'-dependent isocitrate dehydrogenase (IDPm) also plays a protective role against oxidative damage. This study provides direct evidence correlating the activities of IDPc and IDPm and the maintenance of the cellular redox state, suggesting that IDPc and IDPm play an important role in cellular defense against oxidative stress.