Inositol hexakisphosphate kinases (IP6Ks) phosphorylate inositol hexakisphosphate (InsP6) to yield 5-diphospho-inositol pentakisphosphate (5-[PP]-InsP5 or InsP7). In this study, we report the characterization of a selective inhibitor N2-(m-(trifluoromethy)lbenzyl) N6-(p-nitrobenzyl)purine, (TNP) for these enzymes. TNP dose-dependently and selectively inhibited the activity of IP6K in vitro and inhibited InsP7 and InsP8 synthesis in vivo without affecting levels of other inositol phosphates. TNP did not inhibit either human or yeast Vip/PPIP5K, a newly described InsP6/InsP7 1/3-kinase. Overexpression of IP6K1, 2 or 3 in cells rescued TNP inhibition of InsP7 synthesis. TNP had no effect on the activity of a large number of protein kinases suggesting that it is selective for IP6Ks. TNP reversibly reduced InsP7/InsP8 levels. TNP in combination with genetic studies was used to implicate the involvement of two pathways for synthesis of InsP8 in yeast. TNP induced a fragmented vacuole phenotype in yeast, consistent with inhibition of Kcs1, a Saccharomyces cerevisiae IP6K. In addition, it also inhibited insulin release from Min6 cells in a dose dependent manner further implicating InsP7 in this process. TNP thus provides a means of selectively and rapidly modulating cellular InsP7 levels providing a new and versatile tool to study the biological function and metabolic relationships of inositol pyrophosphates.
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