Adenylate kinase (AK; adenosine triphosphate-adenosine monophosphate [ATP-AMP] phosphotransferase) is a ubiquitous monomeric enzyme involved energy metabolism of prokaryotic and eukaryotic cells. Three isozymes (AK1, AK2 and AK3) are characterized in vertebrates. Expression of these isozymes is tissue-specific and developmentally regulated. AK2 is localized in the mitochondrial intermembrane space and may play a role in apoptosis.