Particulate diadenosine 5',5"'-P1,P3-triphosphate hydrolases in rat brain: two specific dinucleoside triphosphatases and two phosphodiesterase I-like hydrolases

Biochemical analysis of ecto-nucleotide pyrophosphatase phosphodiesterase activity in brain membranes indicates involvement of NPP1 isoenzyme in extracellular hydrolysis of diadenosine polyphosphates in central nervous system

Synaptosomes and plasma membranes obtained from rat brain display ectoenzymatic hydrolytic activity responsible for hydrolysis of the neurotransmitter/neuroregulatory nucleotides diadenosine polyphosphates. Intact synaptosomes and plasma and synaptic membranes isolated by sucrose-gradient ultracentrifugation from several brain regions (hypothalamus, hippocampus, temporal cortex, frontal cortex striatum and cerebellum) degraded the fluorogenic substrates diethenoadenosine polyphosphates up to ethenoadenosine as by-product. Purified ectoenzyme cleaved substrates always releasing the mononucleotide moieties ethenoadenosine 5'-monophosphate and the corresponding ethenoadenosine (n-1) 5'-phosphate. Ectoenzymatic hydrolysis reached maximal activity at pH 9.0 (pH range 6.5-9.0) and was activated by Ca(2+) and Mg(2+) ions, with maximal effects around 2.0 mM cation. EDTA drastically reduced activity and Zn(2+) was required for enzyme reactivation. Hydrolysis of substrates followed hyperbolic kinetics with K(m) values in the 3-10 microM range. Diadenosine polyphosphates and heparin behaved as competitive inhibitors in the enzymatic hydrolysis of diethenoadenosine polyphosphates and AMP, ATP, alpha,beta-methyleneADP, ADPbetaS ATPgammaS, beta,gamma-methyleneATP, suramin and diethyl pyrocarbonate were also inhibitors. Ectoenzymatic activity shared the typical characteristics of members of the ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family and inhibition data suggest that NPP1 ectoenzyme is involved in the cleavage of extracellular diadenosine polyphosphates in brain. Synaptic membranes from cerebellum, hypothalamus and hippocampus presented the highest activities and no activity differences were observed between young and aged animals. However, plasma membranes showed a more homogeneous distribution of ectoenzymatic activity but a general increase was detected in aged animals. Enhancement of ectoenzymatic diadenosine polyphosphate cleaving activity found in plasma membranes from old animals could play a deleterious role in aged brain by limiting neuroprotective effects reported for extracellular diadenosine tetraphosphate.

Biochemical and immunochemical characterisation of human diadenosine triphosphatase provides evidence for its identification with the tumour suppressor Fhit protein

We describe here the purification and characterisation of the human enzyme diadenosine triphosphatase isolated from human platelets and leukocytes, offering biochemical and immunochemical evidence to identify this enzyme with the novel tumour suppressor Fhit protein, a homodimer composed of approximately 17 kDa monomers. It catalyses the Mg(2+)-dependent hydrolysis of diadenosine triphosphate, Ap(3)A, to AMP+ADP. The fluorogenic substrate di-ethenoadenosine triphosphate, epsilon-(Ap(3)A), and Fhit antibodies were used for enzymatic and immunochemical characterisations, respectively. Human Ap(3)Aase presents a native molecular mass of approximately 32 kDa and no significant differences were found in K(m) values (2 microM), activating effects by Mg(2+), Ca(2+), and Mn(2+), optimum pH (7.0-7.2) or inhibition by Zn(2+) and diethyl pyrocarbonate between the human enzyme and the recombinant Fhit protein. Suramin is a very potent competitive inhibitor of both human Ap(3)Aase and Fhit protein with K(i) values in the range 20-30 nM. Both human and rat Ap(3)Aase activity co-purifies with Fhit immunoreactivity under gel filtration, ion-exchange and affinity chromatography. Homogeneous human Ap(3)Aase preparations analysed by SDS-PAGE and Western blot analysis with Fhit antibodies elicit immunochemical responses corresponding to a approximately 17 kDa polypeptide, indicating a dimeric structure for the enzyme Ap(3)Aase. The strong inhibition of Fhit enzyme by the drug suramin, supports the need to investigate the therapeutic potential of Fhit-Ap(3)Aase mediated by its interaction with suramin or related drugs.

Distribution of Fhit protein in rat tissues and its intracellular localization

The FHIT (fragile histidine triad) gene located at chromosome 3p14.2 has been proposed as a candidate tumor suppressor gene in human cancers. Fhit protein with the diadenosine 5',5'''-P1,P3-triphosphate (Ap3A) hydrolase activity is the protein product of FHIT gene. The way in which Fhit exerts its tumor suppressor activity and the relationship of the Ap3A hydrolase activity to tumor suppression are not known. As a step toward understanding of the Fhit function in the cell we have explored its intracellular localization and distribution in the rat tissues. Data obtained from immunoblot analysis showed that Fhit protein was most abundant in spleen and brain. Moderate amount of Fhit was detected in kidney and liver, whereas the level of Fhit protein in heart, skeletal muscle and kidney glomeruli was undetectable. RT-PCR performed on RNA isolated from these tissues showed no product, whereas the level of Fhit mRNA in spleen, brain, kidney, liver and lung correlated with the Fhit protein level. The immunoblot analysis performed on subcellular fractions of various rat tissues obtained by differential and density-gradient centrifugation showed that Fhit protein was localized exclusively in nucleus and at the plasma membrane. Presented data showing nuclear and plasma membrane localization of Fhit may support the hypothesis concerning Fhit as a signaling molecule.

Dinucleoside polyphosphates-friend or foe?

Despite being known for over 30 years, the functions of the dinucleoside polyphosphates, such as diadenosine 5',5"'-P(1), P(4)-tetraphosphate (Ap(4)A) and diadenosine 5',5"'-P(1), P(3)-triphosphate (Ap(3)A), are still unclear. On the one hand, they may have important signalling functions, both inside and outside the cell (friend), while on the other hand, they may simply be the unavoidable by-products of certain biochemical reactions, which, if allowed to accumulate, would be potentially toxic through their structural similarity to ATP and other essential mononucleotides (foe). Here, the occurrence, synthesis, degradation, and proposed functions of these compounds are briefly reviewed, along with some new data and recent evidence supporting roles for Ap(3)A and Ap(4)A in the cellular decision making processes leading to proliferation, quiescence, differentiation, and apoptosis. Hypotheses are forwarded for the involvement of Ap(4)A in the intra-S phase DNA damage checkpoint and for Ap(3)A and the pFhit (fragile histidine triad gene product) protein in tumour suppression. It is concluded that the roles of friend and foe are not incompatible, but are distinguished by the concentration range of nucleotide achieved under different circumstances.

Specific and nonspecific enzymes involved in the catabolism of mononucleoside and dinucleoside polyphosphates

This review concerns enzymes that can degrade nucleoside 5'-tetra- and pentaphosphates (p(4)N and p(5)N) and those that can degrade various dinucleoside polyphosphates (Np(3-6)N'). Most of these enzymes are hydrolases, and they occur in all types of organisms. Certain fungi and protozoa also possess specific Np(n)N' phosphorylases. Specific p(4)N hydrolases have been demonstrated in mammals and in plants. In yeast, p(4)N and p(5)N are hydrolyzed by exopolyphosphatases. Among other hydrolases that can degrade these minor mononucleotides are phosphatases, apyrase, and (asymmetrical) Np(4)N' hydrolase, as well as the nonspecific adenylate deaminase. Np(n)N's are good substrates for Type I phosphodiesterases and nucleotide pyrophosphatases, and diadenosine polyphosphates are easily deaminated to diinosine polyphosphates by nonspecific adenylate deaminases. Specific Np(3)N' hydrolases occur in both prokaryotes and eukaryotes. Interestingly, the human fragile histidine triad (Fhit) tumor suppressor protein appears to be a typical Np(3)N' hydrolase. Among the specific Np(4)N' hydrolases are asymmetrically cleaving ones, which are typical of higher eukaryotes, and symmetrically cleaving enzymes found in Physarum polycephalum and in many bacteria. An enzyme that hydrolyzes both diadenosine tetraphosphate and diadenosine triphosphate has been found in the fission yeast Schizosaccharomyces pombe. Its amino acid sequence is similar to that of the human Fhit/Np(3)N' hydrolase. Very recently, a typical (asymmetrical) Np(4)N' hydrolase has been demonstrated for the first time in a bacterium-the pathogenic Bartonella bacilliformis. Another novelty is the discovery of diadenosine 5', 5"'-P(1),P 6-hexaphosphate hydrolases in budding and fission yeasts and in mammalian cells. These enzymes and the (asymmetrical) Np(4)N' hydrolases have the amino acid motif typical of the MutT (or Nudix hydrolase) family. In contrast, the Schizosaccharomyces pombe Ap(4)A/Ap(3)A hydrolase, the human Fhit protein, and the yeast Np(n)N' phosphorylases belong to a superfamily GAFH, which includes the histidine triad proteins.

Potent inhibition of specific diadenosine polyphosphate hydrolases by suramin

The cytosolic enzymes asymmetrical diadenosine tetraphosphate hydrolase (EC 3.6.1.17, Ap4Aase) and diadenosine triphosphate hydrolase (EC 3.6.1.29, Ap3Aase) are inhibited competitively by suramin. Ap4Aase and Ap3Aase were assayed in cytosolic rat brain extracts using fluorogenic analogues of the respective substrates diadenosine tetraphosphate (Ap4A) and diadenosine triphosphate (Ap3A). Ki values for suramin as inhibitor of Ap4Aase and Ap3Aase were 5 x 10(-6) M and 3 x 10(-7) M, respectively. Results indicate that suramin or suramin-like derivatives may be useful tools to investigate diadenosine polyphosphate cleaving enzymes and that the intracellular diadenosine polyphosphate metabolism may be a pharmacological target of suramin with biological and clinical implications.

Human diadenosine 5',5"'-P1,P4-tetraphosphate pyrophosphohydrolase is a member of the MutT family of nucleotide pyrophosphatases

The cDNA and derived amino acid sequence of human diadenosine 5',5"'-P1,P4-tetraphosphate pyrophosphohydrolase have been determined with the aid of the GenBank Expressed Sequence Tag database. This enzyme possesses a modification of the MutT sequence motif found in certain nucleotide pyrophosphatases. It is unrelated to the enzymes of diadenosine tetraphosphate catabolism found in prokaryotes and fungi.

Characterisation of a bis(5'-nucleosidyl) triphosphate pyrophosphohydrolase from encysted embryos of the brine shrimp Artemia

1. A P1,P3-bis(5'-nucleosidyl)triphosphate pyrophosphohydrolase (Np3 Nase) has been partially purified from Artemia embryos. 2. The Np3 Nase has a native Mr of 115,000 and preferentially hydrolyses substrates of the form Np3 N. Relative rates of hydrolysis are Ap3A (Vrel = 1.0), Gp3G (Vrel = 0.71), Ap4A (Vrel = 0.08), Ap5A (Vrel = 0.09), Gp4G (Vrel = 0.3) and Gp5G (Vrel = 0.33). An NMP is always one of the products. 3. The Km values for Ap3A and Gp3G are 15 and 10 microM respectively. 4. Mg2+, Mn2+ and Ca2+ ions all stimulate the activity, while Zn2+, Co2+ and Ni2+ ions are inhibitory. 5. The activity of the Np3 Nase remains constant during pre-emergence development of encysted embryos but decreases slightly after hatching.