Cancer-associated Isocitrate Dehydrogenase Mutations Inactivate NADPH-dependent Reductive Carboxylation

Isocitrate dehydrogenase (IDH) is a reversible enzyme that catalyzes the NADP+-dependent oxidative decarboxylation of isocitrate (ICT) to α-ketoglutarate (αKG) and the NADPH/CO2-dependent reductive carboxylation of αKG to ICT. Reductive carboxylation by IDH1 was potently inhibited by NADP+ and, to a lesser extent, by ICT. IDH1 and IDH2 with cancer-associated mutations at the active site arginines were unable to carry out the reductive carboxylation of αKG. These mutants were also defective in ICT decarboxylation and converted αKG to 2-hydroxyglutarate using NADPH. These mutant proteins were thus defective in both of the normal reactions of IDH. Biochemical analysis of heterodimers between wild-type and mutant IDH1 subunits showed that the mutant subunit did not inactivate reductive carboxylation by the wild-type subunit. Cells expressing the mutant IDH are thus deficient in their capacity for reductive carboxylation and may be compromised in their ability to produce acetyl-CoA under hypoxia or when mitochondrial function is otherwise impaired. Background: Reductive carboxylation by isocitrate dehydrogenase (IDH) is required for hypoxic growth, and IDH mutations are associated with cancer. Results: Reductive carboxylation by IDH is inhibited by NADP+ and isocitrate and inactivated by cancer-associated mutations. Conclusion: Cancer-associated IDH mutations inactivate reductive carboxylation. Significance: IDH mutations may reduce the capacity of cells to produce acetyl-CoA via reductive carboxylation.