D-threo-isocitrate + NAD(+) = 2-oxoglutarate + CO2 + NADH Binds 1 Mg(2+) or Mn(2+) ion per subunit. Heterooligomer of subunits alpha, beta, and gamma in the apparent ratio of 2:1:1. Belongs to the isocitrate and isopropylmalate dehydrogenases family.