Maintains high levels of reduced glutathione (GSH) in the cytosol by reducing glutathione disulfide (oxidized glutathione or GSSG) (PubMed:16678813). The amount of GSH may affect the determination of cell fate (PubMed:16678813). 2 glutathione + NADP(+) = glutathione disulfide + H(+) + NADPH Binds 1 FAD per subunit. The active site is a redox-active disulfide bond. Belongs to the class-I pyridine nucleotide-disulfide oxidoreductase family.