The Diadenosine Hexaphosphate Hydrolases fromSchizosaccharomyces pombe and Saccharomyces cerevisiae Are Homologues of the Human Diphosphoinositol Polyphosphate Phosphohydrolase

Abstract

J. Biol. Chem. 274, 8604 -8610) have both previously been characterized as MutT family hydrolases with high specificity for diadenosine hexa-and pentaphosphates (Ap 6 A and Ap 5 A). Using purified recombinant preparations of these enzymes, we have now discovered that they have an important additional function, namely, the efficient hydrolysis of diphosphorylated inositol polyphosphates. This overlapping specificity of an enzyme for two completely different classes of substrate is not only of enzymological significance, but in addition, this finding provides important new information pertinent to the structure, function, and evolution of the MutT motif. Moreover, we report that the human protein previously characterized as a diphosphorylated inositol phosphate phosphohydrolase represents the first example, in any animal, of an enzyme that degrades Ap 6 A and Ap 5 A, in preference to other diadenosine polyphosphates. The emergence of Ap 6 A and Ap 5 A as extracellular effectors and intracellular ion-channel ligands points not only to diphosphorylated inositol phosphate phosphohydrolase as a candidate for regulating signaling by diadenosine polyphosphates, but also suggests that diphosphorylated inositol phosphates may competitively inhibit this process.Following the discovery of dinucleoside polyphosphates in biological systems over 30 years ago (1), these compounds have been studied extensively in prokaryotic and eukaryotic organisms. Several important intracellular and extracellular signaling functions have now been ascribed to the diadenosine compounds, Ap 3 A, 1 Ap 4 A, Ap 5 A, and Ap 6 A (2-4). Indeed, the ultimate fate of cell lineages and the very survival of an organism may depend upon the tight control of cellular diadenosine polyphosphate metabolism. For example, the intracellular level of Ap 4 A has long been known to be associated with cell proliferation (5). Moreover, it was recently proposed (6) that Ap 3 A has an antiproliferative role when complexed with the putative tumor suppressor Fhit protein, an Ap 3 A hydrolase (7,8). Thus, the Ap 3 A/Ap 4 A ratio may be an important factor in determining the alternative cellular fates of proliferation, differentiation, and apoptosis (3, 9). In higher eukaryotes, Ap n A appear also to be intracellular mediators of certain extracellular stimuli; they respond to glucose in pancreatic ␤-cells (10). Ap n A may also regulate ATP-sensitive K ϩ channels in ␤-cells and cardiac muscle (11-13) and intracellular ryanodine-binding Ca 2ϩ -release channels in cardiac and skeletal muscle, and in the brain (14, 15). Finally, extracellular Ap 5 A and Ap 6 A have also been identified as neurotransmitters (2) and vasomodulators (16,17).In addition to these physiological functions, Ap n A respond to heat shock and oxidative stress with an increase in concentration (18). If allowed to accumulate, they could prove toxic through their ability to inhibit nucleotide kinases (19,20), protein kinases (21,22), and other enzymes (23). Thus, there is considerable interest in the enzymes t...

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