Converts inositol hexakisphosphate (InsP6) to diphosphoinositol pentakisphosphate (InsP7/PP-InsP5). 1D-myo-inositol hexakisphosphate + ATP = 5-diphospho-1D-myo-inositol 1,2,3,4,6-pentakisphosphate + ADP Phospholipid metabolism; phosphatidylinositol metabolism. Highly expressed in small intestine. Belongs to the inositol phosphokinase (IPK) family. Was first identified because of its ability to stimulate Na(+)-dependent phosphate cotransport. Chimeric cDNA.