Inositol hexakisphosphate in urine: the relationship between oral intake and urinary excretion

Although myo-inositol hexakisphosphate (InsP6; phytate) is the most abundant inositol phosphate in nature and probably has a wide variety of functions, neither the route of its synthesis from myo-inositol nor its metabolic relationships with other inositol-containing compounds (such as the second messenger inositol 1,4,5-trisphosphate, Ins(1,4,5)P3) are known. Here we report that the pathway by which InsP6 is synthesized in the cellular slime mould Dictyostelium, and in cell-free preparations derived from them, is catalysed by a series of soluble ATP-dependent kinases independently of the metabolism of both phosphatidylinositol and Ins(1,4,5)P3. The intermediates between myo-inositol and InsP6 are Ins3P, Ins(3,6)P2, Ins(3,4,6)P3, Ins(1,3,4,6)P4 and Ins(1,3,4,5,6)P5. The 3- and 5-phosphates of InsP6 take part in futile cycles in which Ins(1,2,4,5,6)P5 and Ins(1,2,3,4,6)P5 are rapidly formed by dephosphorylation of InsP6, only to be rephosphorylated to yield their precursor.