Performs an essential role in the oxidative function of the tricarboxylic acid cycle and respiration (Probable). Catalyzes the decarboxylation of isocitrate to produce 2-oxoglutarate and generate NADH to provide electrons for energy production (Probable). D-threo-isocitrate + NAD(+) = 2-oxoglutarate + CO2 + NADH Binds 1 Mg(2+) or Mn(2+) ion per subunit. The homodimer exhibits allosteric regulation by isocitrate. Optimum pH is 8.0 with Mn(2+) as cofactor and 8.5 with Mg(2+) as cofactor. Optimum temperature is 45 degrees Celsius. Forms homodimers. Belongs to the isocitrate and isopropylmalate dehydrogenases family.