Acyl-CoA synthetase catalyzes the synthesis of diadenosine hexaphosphate (Ap6A).

The synthesis of diadenosine hexaphosphate (Ap6A), a potent vasoconstrictor, is catalyzed by acyl-CoA synthetase from Pseudomonas fragi. In a first step AMP is transferred from ATP to tetrapolyphosphate (P4) originating adenosine pentaphosphate (p5A) which, subsequently, is the acceptor of another AMP moiety from ATP generating diadenosine hexaphosphate (Ap6A). Diadenosine pentaphosphate (Ap5A) and diadenosine tetraphosphate (Ap4A) were also synthesized in the course of the reaction. In view of the variety of biological effects described for these compounds the potential capacity of synthesis of diadenosine polyphosphates by the mammalian acyl-CoA synthetases may be relevant.     

Ca<Superscript>2+</Superscript> Signalling in Brain Synaptosomes Activated by Dinucleotides

Diadenosine polyphosphates are a family of dinucleotides formed by two adenosines joined by a variable number of phosphates. Diadenosine tetraphosphate, Ap₄A, diadenosine pentaphosphate Ap₅A, and diadenosine hexaphosphate, Ap₆A, are stored in synaptic vesicles and are released upon nerve terminal depolarization. At the extracellular level, diadenosine polyphosphates can stimulate presynaptic dinucleotide receptors. Responses to diadenosine polyphosphates have been described in isolated synaptic terminals (synaptosomes) from several brain areas in different animal species, including man. Dinucleotide receptors are ligand-operated ion channels that allow the influx of cations into the terminals. These cations reach a threshold for N- and P/Q-type voltage-dependent calcium channels, which become activated. The activation of the dinucleotide receptor together with the activation of these calcium channels triggers the release of neurotransmitters. The ability of Ap₅A to promote glutamate, GABA or acetylcholine release has been recently described by the present authors in rat midbrain synaptosomes.     

Fhit-nucleotide specificity probed with novel fluorescent and fluorogenic substrates

Fhit, a member of the histidine triad superfamily of nucleotide-binding proteins, binds and cleaves diadenosine polyphosphates and functions as a tumor suppressor in human epithelial cancers. Function of Fhit in tumor suppression does not require diadenosine polyphosphate cleavage but correlates with the ability to form substrate complexes. As diadenosine polyphosphates are at lower cellular concentrations than mononucleotides, we sought to quantify interactions between Fhit and competitive inhibitors with the use of diadenosine polyphosphate analogs containing fluorophores in place of one nucleoside. Appp-S-(7-diethylamino-4-methyl-3-(4-succinimidylphenyl)) coumarin (ApppAMC), Appp-S-(4-4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacine-3-yl) methylaminoacetyl (ApppBODIPY), and GpppBODIPY, synthesized in high yield, are effective Fhit substrates, producing AMP or GMP plus fluorophore diphosphates. GpppBODIPY cleavage is accompanied by a 5.4-fold increase in fluorescence because BODIPY fluorescence is quenched by stacking with guanine. Titration of unlabeled diadenosine polyphosphates, inorganic pyrophosphate, mononucleotides, and inorganic phosphate into fluorescent assays provided values of K(m) and K(I) as competitive inhibitors. The data indicate that Fhit discriminates between good substrates via k(cat) and against cellular competitors in equilibrium binding terms. Surprisingly, pyrophosphate competes better than purine mononucleotides.     

Isothermal titration calorimetry reveals a zinc ion as an atomic switch in the diadenosine polyphosphates

Diadenosine polyphosphates (diadenosine 5',5'-P(1),P(n)-polyphosphate (Ap(n)A)) are 5'-5'-phosphate-bridged dinucleosides that have been proposed to act as signaling molecules in a variety of biological systems. Isothermal titration calorimetry was used to measure the affinities of a variety of metal cations for ATP, diadenosine 5',5'-P(1),P(3)-triphosphate (Ap(3)A), diadenosine 5',5'-P(1),P(4)-tetraphosphate (Ap(4)A), and diadenosine 5',5'-P(1),P(5)-pentaphosphate (Ap(5)A). The binding of Mg(2+), Ca(2+), and Mn(2+) to ATP is shown to take place with the beta,gamma-phosphates (primary site) and be endothermic in character. The binding of Ni(2+), Cd(2+), and Zn(2+) to ATP is found to take place at both the primary site and at a secondary site identified as N-7 of the adenine ring. Binding to this second site is exothermic in character. Generally, the binding of metal cations to diadenosine polyphosphates involves a similar primary site to ATP. No exothermic binding events are identified. Critically, the binding of Zn(2+) to diadenosine polyphosphates proves to be exceptional. This appears to involve a very high affinity association involving the N-7 atoms of both adenine rings in each Ap(n)A, as well as the more usual endothermic association with the phosphate chain. The high affinity association is also endothermic in character. A combination of NMR and CD evidence is provided in support of the calorimetry data demonstrating chemical shift changes and base stacking disruptions entirely consistent with N-7 bridging interactions. N-7 bridging interactions are entirely reversible, as demonstrated by EDTA titration. Considering the effects of Zn(2+) on a wide variety of dinucleoside polyphosphate-metabolizing enzymes, we examine the possibility of Zn(2+) acting as an atomic switch to control the biological function of the diadenosine polyphosphates.     

Modulation by diadenosine polyphosphates of synaptic transmission in the hippocampus

The influence of diadenosine polyphosphates (diadenosine tetraphosphate and diadenosine pentaphosphate) on synaptic transmission in theCA3-CA1 region was studied in rat hippocampal slices. We used combined recording of excitatory postsynaptic current, EPSC (byin situ whole-cell voltage clamp), and population action potential, PAP (in the hippocampalCA1 region). Diadenosine polyphosphates were shown to suppress both EPSC and PAP. This effect could be blocked by A₁ adenosine receptor antagonist, and differed qualitatively from that produced by adenosine itself. As distinct from adenosine, prolonged application of diadenosine polyphosphates caused fas inhibition of PAP followed, by its slow partial recovery which could be removed by preincubation with protein kinase C inhibitor (staurosporine or sphingosine).Neirofiziologiya/Neurophysiology, Vol. 26, No. 6, pp. 423–426, November–December, 1994.     

Presence of diadenosine polyphosphates in microdialysis samples from rat cerebellum in vivo: effect of mild hyperammonemia on their receptors

Diadenosine triphosphate (Ap₃A), diadenosine tetraphosphate (Ap₄A), and diadenosine pentaphosphate (Ap₅A) have been identified in microdialysis samples from the cerebellum of conscious freely moving rats, under basal conditions, by means of a high-performance liquid chromatography method. The occurrence of Ap₃A in the cerebellar microdyalisates is noteworthy, as the presence of this compound in the interstitial medium in neural tissues has not been previously described. The concentrations measured for the diadenosine polyphosphates in the cerebellar dialysate were (in nanomolar) 10.5 ± 2.9, 5.4 ± 1.2, and 5.8 ± 1.3 for Ap₃A, Ap₄A, and Ap₅A, respectively. These concentrations are in the range that allows the activation of the presynaptic dinucleotide receptor in nerve terminals. However, a possible interaction of these dinucleotides with other purinergic receptors cannot be ruled out, as rat cerebellum expresses a variety of P2X or P2Y receptors susceptible to be activated by diadenosine polyphosphates, such as the P2X1-4, P2Y₁, P2Y₂, P2Y₄, and P2Y₁₂ receptors, as demonstrated by quantitative real-time PCR. Also, the ecto-nucleotide pyrophosphatases/phosphodiesterases NPP1 and NPP3, able to hydrolyze the diadenosine polyphosphates and terminate their extracellular actions, are expressed in the rat cerebellum. All these evidences contribute to reinforce the role of diadenosine polyphosphates as signaling molecules in the central nervous system. Finally, we have analyzed the possible differences in the concentration of diadenosine polyphosphates in the cerebellar extracellular medium and changes in the expression levels of their receptors and hydrolyzing enzymes in an animal model of moderate hyperammonemia.

Electronic supplementary material

The online version of this article (doi:10.1007/s11302-013-9382-3) contains supplementary material, which is available to authorized users.     

Identification of dinucleoside polyphosphates in adrenal glands

Dinucleoside polyphosphates have been characterised as extracellular mediators controlling numerous physiological functions like vascular tone or cell proliferation. Here we describe the isolation and identification of dinucleoside polyphosphates Ap(n)A (with n=2-3), Ap(n)G (with n=2-6) as well as Gp(n)G (with n=2-6) from adrenal glands. These dinucleoside polyphosphates are localised in granules of the adrenal glands. The dinucleoside polyphosphates diadenosine diphosphate (Ap(2)A), diadenosine triphosphate (Ap(3)A), adenosine guanosine polyphosphates (Ap(n)G) and diguanosine polyphosphates (Gp(n)G), both with phosphate group (p) numbers (n) ranging from 2 to 6, were identified by fractionating them to homogeneity by preparative size-exclusion- and affinity-chromatography as well as analytical anion-exchange and reversed-phase-chromatography from deproteinised adrenal glands and by analysis of the homogeneous dinucleoside polyphosphates containing fractions with post-source-decay (PSD) matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS). The identity of the dinucleoside polyphosphates was confirmed by retention time comparison with authentic dinucleoside polyphosphates. Enzymatic analysis demonstrated an interconnection of the phosphate groups with the adenosines in the 5(')-positions of the riboses in all dinucleoside polyphosphates purified from adrenal glands. In conclusion, the identification of these dinucleoside polyphosphates in adrenal gland granules emphasises that these dinucleoside polyphosphates can be released from the adrenal glands upon stimulation into the circulation.     

Diadenosine polyphosphates and the control of cyclic AMP concentrations in isolated rat liver cells.

Extracellular diadenosine polyphosphates (Ap(n)A), through their interactions with appropriate P(2) receptors, influence a diverse range of intracellular activities. In particular, Ap(4)A stimulates alterations in intracellular calcium homeostasis and subsequent activation of glycogen breakdown in isolated liver cells. Here we show that, like ATP, Ap(4)A and other naturally occurring diadenosine polyphosphates attenuate glucagon-stimulated accumulation of cyclic AMP in isolated rat liver cells. The characteristics of Ap(4)A- and ATP-dependent modulation of glucagon-stimulated cyclic AMP accumulation are similar. These results are discussed in the context of the repertoire of intracellular signalling processes modulated by extracellular nucleotides.     

Hydrolysis of diadenosine polyphosphates by nucleotide pyrophosphatases/phosphodiesterases.

Diadenosine polyphosphates (ApnAs) act as extracellular signaling molecules in a broad variety of tissues. They were shown to be hydrolyzed by surface-located enzymes in an asymmetric manner, generating AMP and Apn-1 from ApnA. The molecular identity of the enzymes responsible remains unclear. We analyzed the potential of NPP1, NPP2, and NPP3, the three members of the ecto-nucleotide pyrophosphatase/phosphodiesterase family, to hydrolyze the diadenosine polyphosphates diadenosine 5',5"'-P1,P3-triphosphate (Ap3A), diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A), and diadenosine 5',5"'-P1,P5-pentaphosphate, (Ap5A), and the diguanosine polyphosphate, diguanosine 5',5"'-P1,P4-tetraphosphate (Gp4G). Each of the three enzymes hydrolyzed Ap3A, Ap4A, and Ap5A at comparable rates. Gp4G was hydrolyzed by NPP1 and NPP2 at rates similar to Ap4A, but only at half this rate by NPP3. Hydrolysis was asymmetric, involving the alpha,beta-pyrophosphate bond. ApnA hydrolysis had a very alkaline pH optimum and was inhibited by EDTA. Michaelis constant (Km) values for Ap3A were 5.1 micro m, 8.0 micro m, and 49.5 micro m for NPP1, NPP2, and NPP3, respectively. Our results suggest that NPP1, NPP2, and NPP3 are major enzyme candidates for the hydrolysis of extracellular diadenosine polyphosphates in vertebrate tissues.     

Alteration of hemoglobin function by diadenosine 5',5'-P1,P4-tetraphosphate and other alarmones.

Heat-shocked organisms are known to produce not only "heat shock proteins" but also diadenosine tetraphosphate (Ap4A) and related compounds that may act as "alarmones" that alert the cell to the onset of metabolic stress. We found that Ap4A is synthesized in chicken erythrocytes and that the Ap4A level in the whole blood of heat-stressed birds increases about 10-fold. In searching for alarmone receptors, we found that the diadenosine polyphosphates bind preferentially with high affinity to the deoxy conformation of hemoglobin in a ratio of one/tetramer. The binding affinity of this new class of effectors of hemoglobin function is directly related to the number of phosphates which bridge the nucleotide moieties, with the most dramatic in vitro effect on oxygen affinity being shown by Ap6A. Decreasing effects are brought about by diadenosine penta-, tetra-, tri-, di-, and monophosphates. The association constant for Ap4A binding to deoxygenated human hemoglobin at pH 7.25 is 26 microM-1, close to that for 2,3-diphosphoglycerate. At 100-fold excess over heme, Ap4A increases the P50 of stripped Hb A in 0.05 M HEPES buffer at pH 7.25, 20 degrees C, from 0.85 to 6.03 mm Hg. The binding, which markedly enhances the Bohr effect, involves the beta chain anion-binding site. The kinetics of both ligand binding and dissociation are affected, with a greater quantitative effect on the oxygen dissociation process. Although the low concentration of the diadenosine polyphosphates in red cells precludes a physiologically significant modulation of oxygen delivery, competition with the ATP- and NAD(P)H-binding sites on hemoglobin or regulatory enzymes may prove to be of adaptive significance.     

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.     

In human and rat lung membranes [35S]GTPgammaS binding is a tool for pharmacological characterization of G protein-coupled dinucleotide receptors.

The P2Y receptor family is activated by extracellular nucleotides such as ATP and UTP. P2Y receptors regulate physiological functions in numerous cell types. In lung, the P2Y2 receptor subtype plays a role in controlling Cl- and fluid transport. Besides ATP or UTP, also diadenosine tetraphosphate (Ap4A), a stable nucleotide, seems to be of physiological importance. In membrane preparations from human and rat lung we applied several diadenosine polyphosphates to investigate whether they act as agonists for G protein-coupled receptors. We assessed this by determining the stimulation of [35S]GTPgammaS binding. Stimulation of [35S]GTPgammaS binding to G proteins has already been successfully applied to elucidate agonist binding to various G protein-coupled receptors. Ap(n)A (n = 2 to 6) enhanced [35S]GTPgammaS binding similarly in human and rat lung membranes, an indication of the existence of G protein-coupled receptor binding sites specific for diadenosine polyphosphates. Moreover, in both human and rat lung membranes comparable pharmacological properties were found for a diadenosine polyphosphate ([3H]Ap4A) binding site. The affinity for Ap2A, Ap3A, Ap4A, Ap5A, and Ap6A was also comparable. 8-Diazido-Ap4A and ATP were less potent, whereas the pyrimidine nucleotide UTP showed hardly any affinity. Thus, we present evidence that different diadenosine polyphosphates bind to a common G protein-coupled receptor binding site in membranes derived either from human or rat lung.     

Isolation and quantification of dinucleoside polyphosphates by using monolithic reversed phase chromatography columns

In former studies, dinucleoside polyphosphates were quantified using ion-pair reversed-phase perfusion chromatography columns, which allows a detection limit in the micromolar range. The aim of this study was both to describe a chromatographic assay with an increased efficiency of the dinucleoside separation, which enables the reduction of analytical run times, and to establish a chromatographic assay using conditions, which allow MALDI-mass spectrometric analysis of the resulting fractions. We compared the performance of conventional silica reversed phase chromatography columns, a perfusion chromatography column and a monolithic reversed-phase C18 chromatography column. The effects of different ion-pair reagents, flow-rates and gradients on the separation of synthetic diadenosine polyphosphates as well as of diadenosine polyphosphates isolated from human platelets were analysed. Sensitivity and resolution of the monolithic reversed-phase chromatography column were both higher than that of the perfusion chromatography and the conventional reversed phase chromatography columns. Using a monolithic reversed-phase C18 chromatography column, diadenosine polyphosphates were separable baseline not only in the presence of tetrabutylammonium hydrogensulfate (TBA) but also in the presence of triethylammonium acetate (TEAA) as ion-pair reagent. The later reagent is useful because, in contrast to TBA, it is compatible with MALDI mass-spectrometric methods. This makes TEAA particularly suitable for identification of unknown nucleoside polyphosphates. Furthermore, because of the lower backpressure of monolithic reversed-phase chromatography columns, we were able to significantly increase the flow rate, decreasing the amount of time for the analysis close to 50%, especially using TBA as ion-pair reagent. In summary, monolithic reversed phase C18 columns markedly increase the sensitivity and resolution of dinucleoside polyphosphate analysis in a time-efficient manner compared to reversed-phase perfusion chromatography columns or conventional reversed-phase columns. Therefore, further dinucleoside polyphosphate analytic assays should be based on monolithic silica C18 columns instead of perfusion chromatography or conventional silica reversed phase chromatography columns. In conclusion, the use of monolithic silica C18 columns will lead to isolation and quantification of up to now unknown dinucleoside polyphosphates. These chromatography columns may facilitate further research on the biological roles of dinucleoside polyphosphates.     

Selective degradation of 2'-adenylated diadenosine tri- and tetraphosphates, Ap(3)A and Ap(4)A, by two specific human dinucleoside polyphosphate hydrolases

It is known that the interferon-inducible 2',5'-oligoadenylate synthetase can catalyze the 2'-adenylation of various diadenosine polyphosphates. However, catabolism of those 2'-adenylated compounds has not been investigated so far. This study shows that the mono- and bis-adenylated (or mono- and bis-deoxyadenylated) diadenosine triphosphates are not substrates of the human Fhit (fragile histidine triad) protein, which acts as a typical dinucleoside triphosphate hydrolase (EC 3.6.1.29). In contrast, the diadenosine tetraphosphate counterparts are substrates for the human (asymmetrical) Ap(4)A hydrolase (EC 3.6.1.17). The relative rates of the hydrolysis of 0.15 mM AppppA, (2'-pdA)AppppA, and (2'-pdA)AppppA(2"'-pdA) catalyzed by the latter enzyme were determined as 100:232:38, respectively. The asymmetrical substrate was hydrolyzed to ATP + (2'-pdA)AMP (80%) and to (2'-pdA)ATP + AMP (20%). The human Fhit protein, for which Ap(4)A is a poor substrate, did not degrade the 2'-adenylated diadenosine tetraphosphates either. The preference of the interferon-inducible 2'-5' oligoadenylate synthetase to use Ap(3)A over Ap(4)A as a primer for 2'-adenylation and the difference in the recognition of the 2'-adenylated diadenosine triphosphates versus the 2'-adenylated diadenosine tetraphosphates by the dinucleoside polyphosphate hydrolases described here provide a mechanism by which the ratio of the 2'-adenylated forms of the signalling molecules, Ap(3)A and Ap(4)A, could be regulated in vivo.     

The diadenosine hexaphosphate hydrolases from Schizosaccharomyces pombe and Saccharomyces cerevisiae are homologues of the human diphosphoinositol polyphosphate phosphohydrolase. Overlapping substrate specificities in a MutT-type protein.

Aps1 from Schizosaccharomyces pombe (Ingram, S. W., Stratemann, S. A. , and Barnes, L. D. (1999) Biochemistry 38, 3649-3655) and YOR163w from Saccharomyces cerevisiae (Cartwright, J. L., and McLennan, A. G. (1999) 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.     

Identification and characterization of diadenosine 5',5"'-P1,P2 -diphosphate and diadenosine 5',5"'-P1,P3-triphosphate in human myocardial tissue.

We examined whether human cardiac tissue contains diadenosine polyphosphates and investigated their physiological role. Extracts from human cardiac tissue from transplant recipients were fractionated by size exclusion-, affinity-, anion exchange- and reversed-phase chromatography. MALDI-MS analysis of two absorbing fractions revealed molecular masses of 676.2 Da and 756.0 Da. The UV spectra of both fractions were identical to that of adenosine. Postsource decay MALDI mass spectrometry indicated that the molecules with a mass of 676.2 Da and 757.0 Da contained AMP and ATP, respectively. As shown by enzymatic cleavage, both molecules consist of two adenosines interconnected by either two or three phosphates in 5'-positions of the riboses. Two substances can be identified as 5',5"'-P1,P2-diphosphate (Ap2A) and 5',5"'-P1, P3-triphosphate (Ap3A). Ap2A and Ap3A, together with ATP and ADP, are stored in myocardial-specific granules in biologically active concentrations. In the isolated perfused rat heart, Ap2A and Ap3A caused dose-dependent coronary vasodilations. In myocardial preparations, Ap2A and Ap3A attenuated the effect of isoproterenol, exerting a negative inotropic effect. The calcium current of guinea pig ventricular myocytes, stimulated by isoproterenol, was also attenuated by Ap2A and Ap3A. The presence of Ap2A and Ap3A in cardiac-specific granules and the actions of these substances on the myocardium and coronary vessels indicate a role for these substances as endogenous modulators of myocardial functions and coronary perfusion.     

Dinucleotide Receptor Modulation by Protein Kinases (Protein Kinases A and C) and Protein Phosphatases in Rat Brain Synaptic Terminals

Abstract: The diadenosine polyphosphates, diadenosine tetraphosphate and diadenosine pentaphosphate (Ap₅A), can activate an ionotropic dinucleotide receptor that induces Ca²⁺ transients into synaptosomes prepared from rat brain. This receptor, also termed the P₄ purinoceptor, is sensitive only to adenine dinucleotides and is insensitive to ATP. Studies on the modulatory role of protein kinase A (PKA), protein kinase C (PKC), and protein phosphatases on the response of diadenosine polyphosphate receptors were performed by measuring the changes in the intracellular Ca²⁺ levels with fura-2. Activation and inhibition of PKA were carried out by means of forskolin and the PKA inhibitory peptide (PKA-IP), respectively. The Ap₅A response was inhibited by forksolin to 35% of control values, but PKA-IP induced an increase of 37%. The effect of PKC activation was similar to that observed for PKA. PKC stimulation with phorbol 12,13-dibutyrate produced an inhibition of 67%, whereas the PKC inhibitors staurosporine and PKC inhibitory peptide enhanced the responses elicited by Ap₅A to 40% in both cases. Protein phosphatase inhibitors diminished the responses elicited by Ap₅A to 17% in the case of okadaic acid, to 50% for microcystin, and to 45% in the case of cyclosporin A. Thus, the activity of dinucleotide receptors in rat brain synaptosomes appears to be modulated by phosphorylation/dephosphorylation. These processes could be of physiological significance in the control of transmitter release from neurons that are postsynaptic to nerves that release diadenosine polyphosphates.     

Identification and characterization of P(1), P(7)-Di(adenosine-5')-heptaphosphate from human platelets.

Diadenosine pentaphosphate and diadenosine hexaphosphate have been isolated in human platelets and have been postulated to play an important role in the control of vascular tone. Here we describe the isolation and identification of diadenosine heptaphosphate from human platelets. Dinucleoside polyphosphates were concentrated by affinity chromatography from a nucleotide-containing fraction from deproteinated human platelets. Dinucleoside polyphosphates were purified by anion-exchange and reversed phase high performance liquid chromatography to homogeneity. Analysis of one of these fractions with matrix-assisted laser desorption/ionization mass spectrometry revealed a molecular mass of 1076.4 (1077.4 = [M + H](+)) Da. UV spectroscopic analysis of this fraction showed the spectrum of an adenosine derivative. Comparison of the postsource decay matrix-assisted laser desorption/ionization mass spectrum of the fraction minus that of diadenosine heptaphosphate (Ap(7)A) demonstrated that the isolated substance was identical to Ap(7)A. The identity of the retention times of the authentic and the isolated compound confirmed this result. Enzymatic analysis demonstrated an interconnection of the phosphate groups with the adenosines in the 5'-positions of the riboses. With thrombin-induced platelet aggregation, Ap(7)A is released from the platelets into the extracellular space. The vasoconstrictive action of Ap(7)A on the vasculature of the isolated perfused rat kidney Ap(7)A was slightly less than that of Ap(6)A. The threshold of the vasoconstrictive action of Ap(7)A was 10(-5) mol/liter. The vasoconstrictive effect was abolished by suramin and pyridoxal phosphate 6-azophenyl-2', 4'-disulfonic acid, suggesting an activation of P(2x) receptors. Furthermore, Ap(7)A inhibits ADP-induced platelet aggregation. Thus, the potent vasoconstrictor Ap(7)A derived from human platelets, like other diadenosine polyphosphates, may play a role in the regulation of vascular tone and hemostasis.     

Location of dinucleoside triphosphatase in the matrix space of rat liver mitochondria.

The submitochondrial location of dinucleoside triphosphatase (EC 3.6.1.29), previously shown to be in part associated with mitochondria, has been studied in rat liver. The precipitability and latency of activity in organelle suspensions, and the profile of solubilization by digitonin, were like those of the matrix space marker glutamate dehydrogenase, and differed from those of other submitochondrial fractions. This, and the synthesis of diadenosine polyphosphates by mitochondrial aminoacyl-tRNA synthetases, suggest the occurrence of a pathway for the intramitochondrial turnover of diadenosine 5',5'-P1,P3-triphosphate (Ap3A).     

Mitochondrial location of rat liver dinucleoside triphosphatase

Rat liver dinucleoside triphosphatase (EC 3.6.1.29) is associated with sucrose-gradient purified mitochondria and can be extracted by freeze and thaw treatment. The proportion of mitochondrial dinucleoside triphosphatase approaches 50% of total liver enzyme. Evidence is also presented that 10% of total liver bis(5'-guanosyl)tetraphosphatase (EC 3.6.1.17) might be equally linked to mitochondria. Those data suggest that diadenosine 5',5'-P1,P3-triphosphate, diadenosine 5',5'-P1,P4-tetraphosphate, or other substrates of those enzymes, might be somehow related to mitochondria or mitochondrial function(s), although the occurrence of dinucleoside polyphosphates has not been reported in that organelle.