Catalyzes the NAD(+)-dependent oxidative decarboxylation of homoisocitrate to 2-oxoadipate (alpha-ketoadipate), and of isocitrate to 2-oxoglutarate, at near equal efficiency. May thus play a dual role in glutamate and lysine biosynthesis in vivo. Preferentially uses NAD over NADP. D-threo-isocitrate + NAD(+) = 2-oxoglutarate + CO2 + NADH (2R,3S)-homoisocitrate + NAD(+) = 2-oxoadipate + CO2 + NADH kcat is 14.8 sec(-1) with isocitrate as substrate and 13.7 sec(-1) with homoisocitrate as substrate. Amino-acid biosynthesis; L-lysine biosynthesis via AAA pathway; L-alpha-aminoadipate from 2-oxoglutarate: step 4/5. Belongs to the isocitrate and isopropylmalate dehydrogenases family. Truncated N-terminus.