ATPase inhibitor, mitochondrial is an enzyme that in humans is encoded by the ATPIF1 gene. This gene encodes a mitochondrial ATPase inhibitor. Alternative splicing occurs at this locus and three transcript variants encoding distinct isoforms have been identified. It prevents ATPase from switching to ATP hydrolysis during collapse of the electrochemical gradient, for example during oxygen deprivation ATP synthase inhibitor forms a one to one complex with the F1 ATPase, possibly by binding at the alpha-beta interface. It is thought to inhibit ATP synthesis by preventing the release of ATP. The inhibitor has two oligomeric states, dimer (the active state) and tetramer.