Isocitrate dehydrogenase (IDH) [PMID:2682654, PMID:1939242] is an important enzyme of carbohydrate metabolism which catalyzes the oxidative decarboxylation of isocitrate into alpha-ketoglutarate. IDH is either dependent on NAD⁺ (EC: 1.1.1.41) or on NADP⁺ (EC: 1.1.1.42). In eukaryotes there are at least three isozymes of IDH: two are located in the mitochondrial matrix (one NAD⁺-dependent, the other NADP⁺-dependent), while the third one (also NADP⁺-dependent) is cytoplasmic. In Escherichia coli the activity of a NADP⁺-dependent form of the enzyme is controlled by the phosphorylation of a serine residue; the phosphorylated form of IDH is completely inactivated.

The eukaryotic, NADP-dependent isocitrate dehydrogenases, are defined by this group that includes the cytosolic, mitochondrial, and chloroplast enzymes, but does also hit a small number of bacterial proteins.