SIRT4 and neoplasm: Functionally, SIRT4 has been characterized in mitochondria as tumor suppressor and inhibitor of the metabolic gatekeeper enzymes pyruvate dehydrogenase (PDH; [25]) and glutamate dehydrogenase (GDH; [26, 27]) as well as based on its deacetylase activity as a regulator of leucine metabolism and insulin secretion [28].