InsP₆ (inositol hexakisphosphate), the most abundant inositol phosphate in metazoa, is pyrophosphorylated to InsP₇ [5PP-InsP₅ (diphosphoinositol pentakisphosphate)] by cytosolic and nuclear IP6Ks (InsP₆ kinases) and to 1PP-InsP₅ by another InsP₆/InsP₇ kinase family. MINPP1 (multiple inositol-polyphosphate phosphatase 1), the only known InsP₆ phosphatase, is localized in the ER (endoplasmic reticulum) and lysosome lumina. A mechanism of cytosolic InsP₆ dephosphorylation has remained enigmatic so far. In the present study, we demonstrated that IP6Ks change their kinase activity towards InsP₆ at a decreasing ATP/ADP ratio to an ADP phosphotransferase activity and dephosphorylate InsP₆. Enantio-selective analysis revealed that Ins(2,3,4,5,6)P₅ is the main InsP₅ product of the IP6K reaction, whereas the exclusive product of MINPP1 activity is the enantiomer Ins(1,2,4,5,6)P₅. Whereas lentiviral RNAi-based depletion of MINPP1 at falling cellular ATP/ADP ratios had no significant impact on Ins(2,3,4,5,6)P₅ production, the use of the selective IP6K inhibitor TNP [N2-(m-trifluorobenzyl),N6-(p-nitrobenzyl)purine] abolished the production of this enatiomer in different types of cells. Furthermore, by analysis of rat tissue and human blood samples all (main and minor) dephosphorylation products of InsP₆ were detected in vivo. In summary, we identified IP6Ks as novel nuclear and cytosolic InsP₆- (and InsP₅-) dephosphorylating enzymes whose activity is sensitively driven by a decrease in the cellular ATP/ADP ratio, thus suggesting a role for IP6Ks as cellular adenylate energy ‘sensors’.