D-threo-isocitrate + NADP(+) = 2-oxoglutarate + CO2 + NADPH Binds 1 Mg(2+) or Mn(2+) ion per subunit. Optimum pH is 6.5-7.7. Detected in both the small cell variant (SCV) and in the large cell variant (LCV) stage (at protein level). LCVs are more metabolically active than SCVs. Belongs to the isocitrate and isopropylmalate dehydrogenases family.