Catalyzes the NADP(+)-dependent oxidative decarboxylation of isocitrate (D-threo-isocitrate) to 2-ketoglutarate (2-oxoglutarate), which is required by other enzymes such as the phytanoyl-CoA dioxygenase (By similarity). Plays a critical role in the generation of NADPH, an important cofactor in many biosynthesis pathways (By similarity). May act as a corneal epithelial crystallin and may be involved in maintaining corneal epithelial transparency (By similarity). D-threo-isocitrate + NADP(+) = 2-oxoglutarate + CO2 + NADPH Binds 1 Mg(2+) or Mn(2+) ion per subunit. Homodimer. Acetylation at Lys-374 dramatically reduces catalytic activity. Belongs to the isocitrate and isopropylmalate dehydrogenases family.