Performs an essential role in the oxidative function of the citric acid cycle. Also binds RNA; specifically to the 5'-untranslated leaders of mitochondrial mRNAs. D-threo-isocitrate + NAD(+) = 2-oxoglutarate + CO2 + NADH Binds 1 Mg(2+) or Mn(2+) ion per subunit. Allosterically regulated by several compounds including AMP, NAD(+), and citrate. Octamer of two non-identical subunits IDH1 and IDH2. Present with 10500 molecules/cell in log phase SD medium. Belongs to the isocitrate and isopropylmalate dehydrogenases family.