Do mammals make all their own inositol hexakisphosphate?

A highly specific and sensitive mass assay for inositol hexakisphosphate (InsP(6)) was characterized. This centres around phosphorylating InsP(6) with [(32)P]ATP using a recombinant InsP(6) kinase from Giardia lambia, followed by HPLC of the (32)P-labelled products with an internal [(3)H]InsP(7) standard. This assay was used to quantify InsP(6) levels in a variety of biological samples. Concentrations of InsP(6) in rat tissues varied from 10–20 μM (assuming 64% of wet weight of tissue is cytosol water), whereas using the same assumption axenic Dictyostelium discoideum cells contained 352±11 μM InsP(6). HeLa cells were seeded at low density and grown to confluence, at which point they contained InsP(6) levels per mg of protein similar to rat tissues. This amounted to 1.952±0.117 nmol InsP(6) per culture dish, despite the cells being grown in serum shown to contain no detectable (less than 20 pmol per dish) InsP(6). These results demonstrate that mammalian cells synthesize all their own InsP(6). Human blood was analysed, and although the white cell fraction contained InsP(6) at a concentration comparable with other tissues, in serum and platelet-free plasma no InsP(6) was detected (<1 nM InsP(6)). Human urine was also examined, and also contained no detectable (<5 nM) InsP(6). These results suggest that dietary studies purporting to measure InsP(6) at micromolar concentrations in human plasma or urine may not have been quantifying this inositol phosphate. Therefore claims that administrating InsP(6) in the diet or applying it topically can produce health benefits by increasing extracellular InsP(6) levels may require reassessment.