IDH isoforms. The isocitrate dehydrogenases (IDH1, IDH2 and IDH3) catalyze the oxidative decarboxylation of isocitrate to α-KG. IDH3 is localized to the mitochondria and serves as an intermediate within the Tricarboxylic Acid (TCA) cycle. IDH3 converts NAD+ to NADH, while IDH2 convert NADP+ to NADPH in the mitochondrion. IDH1 is localized to the cytosol convert NADP+ to NADPH. IDH1 = cytosolic isocitrate dehydrogenase 1, IDH2 = mitochondrial isocitrate dehydrogenase 2, IDH3 = mitochondrial isocitrate dehydrogenase 3, NAD+ = oxidized nicotinamide adenine dinucleotide, NADH = reduced nicotinamide adenine dinucleotide, NADP+ = oxidized nicotinamide adenine dinucleotide phosphate, NADPH = reduced nicotinamide adenine dinucleotide phosphate