Requres Mn2+ and K+ or NH4+ for activity. Unlike EC 1.1.1.41, isocitrate dehydrogenase (NAD+) and EC 1.1.1.87, homoisocitrate dehydrogenase, this enzyme, from Pyrococcus horikoshii, can use both isocitrate and homoisocitrate as substrates. The enzyme may play a role in both the lysine and glutamate biosynthesis pathways.