Nudix hydrolases are found in all classes of organism and hydrolyse a wide range of organic pyrophosphates, including nucleoside di- and triphosphates, dinucleoside and diphosphoinositol polyphosphates, nucleotide sugars and RNA caps, with varying degrees of substrate specificity. Some superfamily members, such as Escherichia coli MicrotT, have the ability to degrade potentially mutagenic, oxidised nucleotides while others control the levels of metabolic intermediates and signalling compounds. In prokaryotes and simple eukaryo tes, the number of Nudix genes varies from 0 to over 30, reflecting the metabolic complexity and adaptability of the organism. Mammals have around 24 Nudix genes, several of which encode more than one variant. This review integrates the sizeable recent literature on these proteins with information from global functional genomic studies to provide some insights into the possible roles of different superfamily members in cellular metabolism and homeostasis and to stimulate discussion and further research into this ubiquitous protein family.

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ITP and dITP exist in all cells. dITP is potentially mutagenic, and the levels of these nucleotides are controlled by inosine triphosphate pyrophosphatase (EC 3.6.1.19). Here we report the cloning, expression, and characterization of a 21.5-kDa human inosine triphosphate pyrophosphatase (hITPase), an enzyme whose activity has been reported in many animal tissues and studied in populations but whose protein sequence has not been determined before. At the optimal pH of 10.0, recombinant hITPase hydrolyzed ITP, dITP, and xanthosine 5-triphosphate to their respective monophosphates whereas activity with other nucleoside triphosphates was low. K m values for ITP, dITP, and xanthosine 5-triphosphate were 0.51, 0.31, and 0.57 mM, respectively, and k cat values were 580, 360, and 640 s ؊1 , respectively. A divalent cation was absolutely required for activity. The gene encoding the hITPase cDNA sequence was localized by radiation hybrid mapping to chromosome 20p in the interval D20S113-D20S97, the same interval in which the ITPA inosine triphosphatase gene was previously localized. A BLAST search revealed the existence of many similar sequences in organisms ranging from bacteria to mammals. The function of this ubiquitous protein family is proposed to be the elimination of minor potentially mutagenic or clastogenic purine nucleoside triphosphates from the cell.

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...

The crystal structure of C. elegans Ap(4)A hydrolase has been determined for the free enzyme and a binary complex at 2.0 A and 1.8 A, respectively. Ap(4)A hydrolase has a key role in regulating the intracellular Ap(4)A levels and hence potentially the cellular response to metabolic stress and/or differentiation and apoptosis via the Ap(3)A/Ap(4)A ratio. The structures reveal that the enzyme has the mixed alpha/beta fold of the Nudix family and also show how the enzyme binds and locates its substrate with respect to the catalytic machinery of the Nudix motif. These results suggest how the enzyme can catalyze the hydrolysis of a range of related dinucleoside tetraphosphate, but not triphosphate, compounds through precise orientation of key elements of the substrate.

The human NUDT12 Nudix hydrolase has been expressed in insect cells from a baculovirus vector as a His-tagged recombinant protein. In vitro, it efficiently hydrolyses NAD(P)H to NMNH and AMP (2',5'-ADP), and diadenosine diphosphate to AMP. It also has activity towards NAD(P)(+), ADP-ribose and diadenosine triphosphate. K (m) values for NADH, NADPH and NAD(+) are 11, 16 and 190 microM and k (cat) values are 11, 16 and 10.5 s(-1) respectively. Thus, like other NADH diphosphatases of the Nudix family, NUDT12 has a marked substrate preference for the reduced nicotinamide nucleotides. Optimal activity was supported by 50 microM Mn(2+) ions in vitro, with 3-fold lower activity at 0.4 mM Mg(2+). Expression of NUDT12 as a C-terminal fusion to green fluorescent protein revealed that it was targeted to peroxisomes by the C-terminal tripeptide PNL acting as a novel type 1 peroxisomal targeting signal. Deletion of PNL resulted in diffuse cellular fluorescence. In addition, C-terminal, but not N-terminal, fusions with or without the PNL signal accumulated in large, unidentified cytoplasmic structures. NUDT12 may act to regulate the concentration of peroxisomal nicotinamide nucleotide cofactors required for oxidative metabolism in this organelle.

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The ygdP and apaH genes of Salmonella enterica serovar Typhimurium (S. Typhimurium) encode two unrelated dinucleoside polyphosphate (Np n N) hydrolases.