Converts inositol hexakisphosphate (InsP6) to diphosphoinositol pentakisphosphate (InsP7/PP-InsP5) (By similarity). May play a role in the regulation of Na(+)-dependent phosphate cotransport, possibly via its role in diphosphoinositol pentakisphosphate (InsP7/PP-InsP5) biosynthesis (By similarity). 1D-myo-inositol hexakisphosphate + ATP = 5-diphospho-1D-myo-inositol 1,2,3,4,6-pentakisphosphate + ADP Phospholipid metabolism; phosphatidylinositol metabolism. Highly expressed in brain and lung, and at slightly lower levels in liver, kidney and testis. Belongs to the inositol phosphokinase (IPK) family.