Abstract

Diphosphoinositol pentakisphosphate (PP-InsP₅ or 'InsP₇') and bisdiphosphoinositol tetrakisphosphate ([PP]₂-InsP₄ or 'InsP₈') are the most highly phosphorylated members of the inositol-based cell signaling family. We have purified a rat hepatic diphosphoinositol polyphosphate phosphohydrolase (DIPP) that cleaves a -phosphate from the diphosphate groups in PP-InsP₅ (K_m = 340 nM) and [PP]₂-InsP₄ (K_m = 34 nM). Inositol hexakisphophate (InsP₆) was not a substrate, but it inhibited metabolism of both [PP]₂-InsP₄ and PP-InsP₅ (IC₅₀ = 0.2 and 3 M, respectively). Microsequencing of DIPP revealed a 'MutT' domain, which in other contexts guards cellular integrity by dephosphorylating 8-oxo-dGTP, which causes AT to CG transversion mutations. The MutT domain also metabolizes some nucleoside phosphates that may play roles in signal transduction. The rat DIPP MutT domain is conserved in a novel recombinant human uterine DIPP. The nucleotide sequence of the human DIPP cDNA was aligned to chromosome 6; the candidate gene contains at least four exons. The dependence of DIPP's catalytic activity upon its MutT domain was confirmed by mutagenesis of a conserved glutamate residue. DIPP's low molecular size, Mg²⁺ dependency and catalytic preference for phosphoanhydride bonds are also features of other MutT-type proteins. Because overlapping substrate specificity is a feature of this class of proteins, our data provide new directions for future studies of higher inositol phosphates.