Diadenosine polyphosphates: their biological and pharmacological significance

Deletion of the yqeK gene leads to the accumulation of Ap4A and reduced biofilm formation in Streptococcus mutans

Diadenosine-5',5'''-P1, P4-tetraphosphate (Ap4A) is a second messenger playing a crucial role in various life activities of bacteria. The increase of Ap4A expression is pleiotropic, resulting in an impairment in the formation of biofilm and other physiological functions in some bacteria. However, Ap4A function in Streptococcus mutans, an important pathogen related to dental caries, remains unknown. In this work, the Ap4A hydrolase, YqeK, was identified and characterized in S. mutans. Then, the effects of yqeK deletion on the growth, biofilm formation, and exopolysaccharide (EPS) quantification in S. mutans were determined by the assessment of the growth curve, crystal violet, and anthrone-sulfuric acid, respectively, and visualized by microscopy. The results showed that the in-frame deletion of the yqeK gene in S. mutans UA159 led to an increase in Ap4A levels, lag phase in the early growth, as well as decrease in biofilm formation and water-insoluble exopolysaccharide production. Global gene expression profile showed that the expression of 88 genes was changed in the yqeK mutant, and among these, 42 were upregulated and 46 were downregulated when compared with the wild-type S. mutans UA159. Upregulated genes were mainly involved in post-translational modification, protein turnover, and chaperones, while downregulated genes were mainly involved in carbohydrate transport and metabolism. Important virulence genes related to biofilms, such as gtfB, gtfC, and gbpC, were also significantly downregulated. In conclusion, these results indicated that YqeK affected the formation of biofilms and the expression of biofilm-related genes in S. mutans.

Diadenosine-Polyphosphate Analogue AppCH2ppA Suppresses Seizures by Enhancing Adenosine Signaling in the Cortex

Epilepsy is a multifactorial disorder associated with neuronal hyperexcitability that affects more than 1% of the human population. It has long been known that adenosine can reduce seizure generation in animal models of epilepsies. However, in addition to various side effects, the instability of adenosine has precluded its use as an anticonvulsant treatment. Here we report that a stable analogue of diadenosine-tetraphosphate: AppCH2ppA effectively suppresses spontaneous epileptiform activity in vitro and in vivo in a Tuberous Sclerosis Complex (TSC) mouse model (Tsc1+/-), and in postsurgery cortical samples from TSC human patients. These effects are mediated by enhanced adenosine signaling in the cortex post local neuronal adenosine release. The released adenosine induces A1 receptor-dependent activation of potassium channels thereby reducing neuronal excitability, temporal summation, and hypersynchronicity. AppCH2ppA does not cause any disturbances of the main vital autonomous functions of Tsc1+/- mice in vivo. Therefore, we propose this compound to be a potent new candidate for adenosine-related treatment strategies to suppress intractable epilepsies.

Research Note: The effect of selection for 16-week body weight on turkey serum metabolome

The phenotype of modern commercial turkeys is substantially different than that of unselected, heritage turkey lines. These phenotypic changes have arisen from alterations in the genome/transcriptome, as well as the influence of many external factors on growth performance including nutrition, environment, and management. To investigate the phenotypic changes resulting from genetic selection for increased body weight, The Ohio State University maintains 2 unique genetic turkey lines: the randombred control (RBC2) line, which is comprised of genetics from 1960 era commercial turkeys and has been maintained without conscious selection for any trait; and the F line, which was originally selected from the RBC2 line and has been selected for increased 16 wk body weight for over 50 generations. This study used broad-spectrum mass-spectrometry profiling techniques to identify and quantify differences in the metabolome of the serum of F and RBC2 turkey lines. Serum samples from both F and RBC2 turkeys were subject to quantitative time of flight liquid chromatography tandem mass spectrometry analyses. Principle component analyses showed distinct populations of metabolites in the F vs. RBC2 serum, suggesting that increased body weight is associated with the accumulation of several metabolites. Comparing the spectral features to online databases resulted in the selection of 104 features with potentially identifiable chemical structures. Of these 104 features, 25 were found at higher levels in the serum of the RBC2 line turkeys, while 79 were found at a greater abundance in the F line turkeys. A more detailed analysis of these 104 features allowed for the putative identification of 49 compounds, which were clustered into 6 functional groups: 1) energy metabolism; 2) vitamins; 3) hormones and signaling molecules; 4) lipid derivatives, fatty acid metabolites, and membrane components; 5) amino acid/protein metabolism; and 6) microbial metabolites. Further validation and experimentation is needed to confirm the identity of these metabolites and understand their biological relevance and association with selection for increased body weight.

Moonlighting matrix metalloproteinase substrates: Enhancement of proinflammatory functions of extracellular tyrosyl-tRNA synthetase upon cleavage

Tyrosyl-tRNA synthetase ligates tyrosine to its cognate tRNA in the cytoplasm, but it can also be secreted through a noncanonical pathway. We found that extracellular tyrosyl-tRNA synthetase (YRS) exhibited proinflammatory activities. In addition to acting as a monocyte/macrophage chemoattractant, YRS initiated signaling through Toll-like receptor 2 (TLR2) resulting in NF-κB activation and release of tumor necrosis factor α (TNFα) and multiple chemokines, including MIP-1α/β, CXCL8 (IL8), and CXCL1 (KC) from THP1 monocyte and peripheral blood mononuclear cell–derived macrophages. Furthermore, YRS up-regulated matrix metalloproteinase (MMP) activity in a TNFα-dependent manner in M0 macrophages. Because MMPs process a variety of intracellular proteins that also exhibit extracellular moonlighting functions, we profiled 10 MMPs for YRS cleavage and identified 55 cleavage sites by amino-terminal oriented mass spectrometry of substrates (ATOMS) positional proteomics and Edman degradation. Stable proteoforms resulted from cleavages near the start of the YRS C-terminal EMAPII domain. All of the MMPs tested cleaved at ADS³⁸⁶↓³⁸⁷LYV and VSG⁴⁰⁵↓⁴⁰⁶LVQ, generating 43- and 45-kDa fragments. The highest catalytic efficiency for YRS was demonstrated by MMP7, which is highly expressed by monocytes and macrophages, and by neutrophil-specific MMP8. MMP-cleaved YRS enhanced TLR2 signaling, increased TNFα secretion from macrophages, and amplified monocyte/macrophage chemotaxis compared with unprocessed YRS. The cleavage of YRS by MMP8, but not MMP7, was inhibited by tyrosine, a substrate of the YRS aminoacylation reaction. Overall, the proinflammatory activity of YRS is enhanced by MMP cleavage, which we suggest forms a feed-forward mechanism to promote inflammation.

Formation of the Alarmones Diadenosine Triphosphate and Tetraphosphate by Ubiquitin- and Ubiquitin-like-Activating Enzymes

Diadenosine polyphosphates (Ap_nAs) such as diadenosine tri- and tetraphosphates are formed in prokaryotic as well as eukaryotic cells. Since upon stress intracellular Ap_nA concentrations increase, it was postulated that Ap_nAs are alarmones triggering stress-adaptive processes. The major synthesis pathway of Ap_nAs is assumed to be a side reaction of amino acid activation. How this process is linked to stress adaptation remains enigmatic. The first step of one of the most prominent eukaryotic post-translational modification systems-the conjugation of ubiquitin (Ub) and ubiquitin-like proteins (Ubl) to target proteins-involves the formation of an adenylate as intermediate. Like Ap_nA formation, Ub and Ubl conjugation is significantly enhanced during stress conditions. Here, we demonstrate that diadenosine tri- and tetraphosphates are indeed synthesized during activation of Ub and Ubls. This links one of the most prevalent eukaryotic protein-modification systems to Ap_nA formation for the first time.

Discriminating cyclic from linear nucleotides - CRISPR/Cas-related cyclic hexaadenosine monophosphate as a case study

Nucleic acids exist in biological systems as linear and cyclic forms and in most cases the biology of the cyclic form is different from the linear form of exactly the same sequence. Case examples are cyclic nucleotides, second messengers in both prokaryotes and eukaryotes whereby the cyclic forms account for their interesting biological profiles and the actions of the cyclic nucleotides are terminated upon phosphodiesterase hydrolysis into linear forms. For mono and dinucleotides, it has been shown that vast conformational changes that accompany the hydrolysis of the cyclized form allow for discrimination between the cyclized and linear forms. As the ring size increases, it becomes difficult to use conformational or structural differences alone to discriminate between cyclic and linear nucleotides. Here we reveal that for the recently discovered CRISPR/Cas-related cyclic hexaadenosine monophosphate, it is possible to discriminate between the cyclized and linear forms. The structures of c-HexaAMP and linear form are different in acidic media and this structural difference facilitated a simple and practical detection platform for this interesting and new bacterial immunity-related molecule, and we also demonstrate that it is possible to distinguish between linear and cyclized polynucleotides using simple spectroscopic techniques, such as CD and fluorescence-based methods.

Diadenosine Polyphosphates Suppress the Effects of Sympathetic Nerve Stimulation in Rabbit Heart Pacemaker

The modulatory influence of diadenosine tetraphosphate (Ap4A) and diadenosine pentaphosphate (Ap5A) on the effect of intramural autonomic nerve stimulation in isolated rabbit sinoatrial node were examined. Electrical activity of the sinoatrial node was recorded intracellularly. Against the background of blockade of adrenergic effects with propranolol (3×10^-6 M) or in preparations isolated 2 h after injection of reserpine (2 mg/kg), nerve stimulation induced short-term membrane hyperpolarization and diminished the sinus node firing rate. These phenomena were not affected by Ap4A or Ap5A (10^-5 M). Under the action of atropine (3×10^-6 M) that completely eliminated the cholinergic influences, nerve stimulation enhanced the sinus node firing rate by 17.30±3.45% from the initial rate. Both Ap4A and Ap5A moderated the stimulation-induced elevation of firing rate to 9.9±2.8 and 10.5±2.9%, respectively. The data suggest that diadenosine polyphosphates significantly modulate the sympathetic influences on the heart rhythm, but have no effect on the parasympathetic control over activity of sinoatrial node.

Hydrolysis of diadenosine polyphosphates. Exploration of an additional role of Mycobacterium smegmatis MutT1

Diadenosine polyphosphates (Ap_nA, n=2-6), particularly Ap₄A, are involved in several important physiological processes. The substantial sequence identity of the Nudix hydrolase domain (domain 1) of Mycobacterium smegmatis MutT1 (MsMutT1) with a known Ap₄A hydrolase suggested that MsMutT1 could also hydrolyse diadenosine polyphosphates. Biochemical experiments yielded results in conformity with this suggestion, with Ap₄A as the best among the substrates. ATP is a product in all experiments; small amounts of ADP were also observed in the experiments involving Ap₄A and Ap₆A. Hydrolysis was inhibited by fluoride ions in all cases. The mechanism of action and its inhibition in relation to Ap_nA were explored through the X-ray analysis of the crystals of the MsMutT1 complexes with Ap₅A; Ap₅A and MnCl₂; Ap₄A; ATP; and ATP.NaF.MgCl₂. The aggregation pattern of molecules in the first four crystals is similar to that found in a majority of MsMutT1-NTP crystals. Substrate molecules occupy the primary binding site and ATP occupies a site at an intermolecular interface, in the first two. ATP occupies both the sites in the third and fourth crystal. The protein-ligand interactions observed in these crystal structures lead to an explanation of the molecular mechanism of hydrolysis of Ap_nA by MsMutT1. The fifth crystal exhibits a new packing arrangement. The structure of the complex provides an explanation for the fluoride inhibition of the activity of the enzyme. It would thus appear that MutT1 has a major role involving the hydrolysis of diadenosine polyphosphates, which could be elucidated at the molecular level.

Effects of Nicotinamide Adenine Dinucleotide (NAD(+)) and Diadenosine Tetraphosphate (Ap4A) on Electrical Activity of Working and Pacemaker Atrial Myocardium in Guinea Pigs

Effects of nucleotide polyphosphate compounds (nicotinamide adenine dinucleotide, NAD(+); diadenosine tetraphosphate, Ap4A) on the confi guration of action potentials were studied in isolated preparations of guinea pig sinoatrial node and right atrial appendage (auricle). In the working myocardium, NAD(+) and Ap4A in concentrations of 10(-5) and 10(-4) M had no effect on resting potential, but significantly reduced the duration of action potentials; the most pronounced decrease was found at 25% repolarization. In the primary pacemaker of the sinoatrial node, both concentrations of NAD(+) and Ap4A induced hyperpolarization and reduction in the rate of slow diastolic depolarization, but significant slowing of the sinus rhythm was produced by these substances only in the concentration of 10(-4) M. Moreover, AP shortening and marked acceleration of AP upstroke were observed in the pacemaker myocardium after application of polyphosphates. Comparative analysis of the effects of NAD(+) and Ap4A in the working and pacemaker myocardium drove us to a hypothesis on inhibitory effects of these substances on L-type calcium current accompanied by stimulation of one or several potassium currents, which induce enhancement of repolarization and hyperpolarization of membranes probably mediated by the activation of purine receptors.

Enzymatic characteristics of an ApaH-like phosphatase, PrpA, and a diadenosine tetraphosphate hydrolase, ApaH, from Myxococcus xanthus

We characterized the activities of the Myxococcus xanthus ApaH-like phosphatases PrpA and ApaH, which share homologies with both phosphoprotein phosphatases and diadenosine tetraphosphate (Ap4A) hydrolases. PrpA exhibited a phosphatase activity towards p-nitrophenyl phosphate (pNPP), tyrosine phosphopeptide and tyrosine-phosphorylated protein, and a weak hydrolase activity towards ApnA and ATP. In the presence of Mn(2+), PrpA hydrolyzed Ap4A into AMP and ATP, whereas in the presence of Co(2+) PrpA hydrolyzed Ap4A into two molecules of ADP. ApaH exhibited high phosphatase activity towards pNPP, and hydrolase activity towards ApnA and ATP. Mn(2+) was required for ApaH-mediated pNPP dephosphorylation and ATP hydrolysis, whereas Co(2+) was required for ApnA hydrolysis. Thus, PrpA and ApaH may function mainly as a tyrosine protein phosphatase and an ApnA hydrolase, respectively.

Highly sensitive, selective and rapid LC-MS method for simultaneous quantification of diadenosine polyphosphates in human plasma

BACKGROUND

Diadenosine polyphosphates (ApnAs) are endogenous mediators involved in large number of physiologic and pathophysiologic processes. The quantification of diadenosine polyphosphates in plasma and biological matrices is still challenging. Therefore, there is an urgent need for a simple and reliable quantification method suitable for clinical studies. The classical quantification of diadenosine polyphosphates is based on chromatographic separation and UV adsorption of the resulting fractions. These procedures are associated with low selectivity due to co-eluting plasma components. Therefore, we developed and validated a highly sensitive, selective and rapid LC-ESI-MS method for simultaneous quantification of ApnAs (with n=3-6) in human plasma within this study. The identities of the endogenous ApnAs (with n=3-6) were revealed by comparison of ESI-MS/MS fragment spectra of isolated endogenous compounds with those of authentic ApnAs.

METHODS

Diadenosine polyphosphates were extracted from 100μl human plasma using weak anion-exchange extraction cartridges. The separation of ApnAs was achieved using capillary C18 columns. ESI-HCT mass spectrometer (Bruker Daltonik, Germany) operated in negative ion mode was used for detection and quantification of ApnAs.

RESULTS

A calibration curve was established for diadenosine polyphosphate free plasma in the concentration range 1.9-125nM (r(2)>0.998) for all analytes. The intra- and inter-day accuracies were in the range of 91.4% and 110.9%. The intra- and inter-day precisions were determines as 0.1% and 11.4%, respectively. The mean plasma concentrations of ApnAs were quantified as 31.9±5.9nM for Ap3A, 40.4±6.6nM for Ap4A, 10.7±1.5nM for Ap5A and 10.0±18.9nM for Ap6A.

DISCUSSION

The developed and validated ESI MS-based method for quantification of diadenosine polyphosphates in human plasma was successfully evaluated within the study. Conclusion Since the quantification is based on a volume of 100μl plasma, this method is highly applicable for clinical applications aiming at the validation of the impact of highly physiological and pathophysiological active diadenosine polyphosphates.

Crystal structure of wild-type and mutant human Ap4A hydrolase

Ap₄A hydrolase (asymmetrical diadenosine tetraphosphate hydrolase, EC 3.6.1.17), an enzyme involved in a number of biological processes, is characterized as cleaving the polyphosphate chain at the fourth phosphate from the bound adenosine moiety. This paper presents the crystal structure of wild-type and E58A mutant human Ap₄A hydrolase. Similar to the canonical Nudix fold, human Ap₄A hydrolase shows the common αβα-sandwich architecture. Interestingly, two sulfate ions and one diphosphate coordinated with some conserved residues were observed in the active cleft, which affords a better understanding of a possible mode of substrate binding.

InvA protein is a Nudix hydrolase required for infection by pathogenic Leptospira in cell lines and animals

Leptospirosis caused by pathogenic species of the genus Leptospira is a re-emerging zoonotic disease, which affects a wide variety of host species and is transmitted by contaminated water. The genomes of several pathogenic Leptospira species contain a gene named invA, which contains a Nudix domain. However, the function of this gene has never been characterized. Here, we demonstrated that the invA gene was highly conserved in protein sequence and present in all tested pathogenic Leptospira species. The recombinant InvA protein of pathogenic L. interrogans strain Lai hydrolyzed several specific dinucleoside oligophosphate substrates, reflecting the enzymatic activity of Nudix in Leptospira species. Pathogenic leptospires did not express this protein in media but temporarily expressed it at early stages (within 60 min) of infection of macrophages and nephric epithelial cells. Comparing with the wild type, the invA-deficient mutant displayed much lower infectivity and a significantly reduced survival rate in macrophages and nephric epithelial cells. Moreover, the invA-deficient leptospires presented an attenuated virulence in hamsters, caused mild histopathological damage, and were transmitted in lower numbers in the urine, compared with the wild-type strain. The invA revertant, made by complementing the invA-deficient mutant with the invA gene, reacquired virulence similar to the wild type in vitro and in vivo. The LD(50) in hamsters was 1000-fold higher for the invA-deficient mutant than for the invA revertant and wild type. These results demonstrate that the InvA protein is a Nudix hydrolase, and the invA gene is essential for virulence in pathogenic Leptospira species.

Oxidatively modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Alzheimer's disease: many pathways to neurodegeneration

Recently, the oxidoreductase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), has become a subject of interest as more and more studies reveal a surfeit of diverse GAPDH functions, extending beyond traditional aerobic metabolism of glucose. As a result of multiple isoforms and cellular locales, GAPDH is able to come in contact with a variety of small molecules, proteins, membranes, etc., that play important roles in normal and pathologic cell function. Specifically, GAPDH has been shown to interact with neurodegenerative disease-associated proteins, including the amyloid-beta protein precursor (AbetaPP). Studies from our laboratory have shown significant inhibition of GAPDH dehydrogenase activity in Alzheimer's disease (AD) brain due to oxidative modification. Although oxidative stress and damage is a common phenomenon in the AD brain, it would seem that inhibition of glycolytic enzyme activity is merely one avenue in which AD pathology affects neuronal cell development and survival, as oxidative modification can also impart a toxic gain-of-function to many proteins, including GAPDH. In this review, we examine the many functions of GAPDH with respect to AD brain; in particular, the apparent role(s) of GAPDH in AD-related apoptotic cell death is emphasized.

Dinucleoside polyphosphates: newly detected uraemic compounds with an impact on leucocyte oxidative burst

BACKGROUND

Dinucleoside polyphosphates (Np(n)N) have pathophysiologic roles in cardiovascular disease and are newly detected uraemic retention solutes. They were retrieved in human plasma, tissues and cells. Although their impact on several cell systems involved in vascular damage (endothelium, smooth muscle cells and thrombocytes) has been evaluated, their effect on different types of leucocytes has never been studied.

METHODS

This study evaluates, for the first time, the impact of Np(n)N on monocyte, granulocyte and lymphocyte oxidative burst activity at baseline and after stimulation with N-formyl-methionine-leucine-phenylalanine (fMLP) and phorbol 12-myristate 13-acetate (PMA) in whole blood. Diadenosine triphosphate (Ap(3)A) to diadenosine hexaphosphate (Ap(6)A) were tested to investigate the effect of the number of phosphate groups on reactive oxygen species (ROS) production. The effect of the type of nucleoside was evaluated by comparing adenosine guanosine tetraphosphate, diguanosine tetraphosphate, uridine adenosine tetraphosphate (Up(4)A) and diadenosine tetraphosphate (Ap(4)A).

RESULTS

This study demonstrated that lymphocytes are especially susceptible to intracellular diadenosine polyphosphates. Depending on the phosphate chain length, different effects were observed. At baseline and with fMLP, Ap(4)A, Ap(5)A and Ap(6)A enhanced lymphocyted-free radical production. In addition, Ap(3)A, Ap(4)A and Ap(5)A increased PMA-stimulated ROS production in lymphocytes. Monocytes and granulocytes parallel the lymphocyte response albeit with an inhibition of Ap(6)A on granulocytes. Considering Np(n)N with four phosphate groups, Up(4)A showed the most important stimulatory effects on monocytes and Ap(4)A on lymphocytes.

CONCLUSIONS

Np(n)N mainly have a leucocyte-activating impact, most significant for Ap(4)A, considering phosphate chain length, and for Up(4)A, considering the type of nucleosides. These results suggest that the pro-inflammatory effects of Np(n)N can contribute to the development of atherosclerosis, probably in the early stages of chronic kidney disease, but their chemical composition affects their activity.

Inositol pyrophosphates: structure, enzymology and function

The stereochemistry of the inositol backbone provides a platform on which to generate a vast array of distinct molecular motifs that are used to convey information both in signal transduction and many other critical areas of cell biology. Diphosphoinositol phosphates, or inositol pyrophosphates, are the most recently characterized members of the inositide family. They represent a new frontier with both novel targets within the cell and novel modes of action. This includes the proposed pyrophosphorylation of a unique subset of proteins. We review recent insights into the structures of these molecules and the properties of the enzymes which regulate their concentration. These enzymes also act independently of their catalytic activity via protein-protein interactions. This unique combination of enzymes and products has an important role in diverse cellular processes including vesicle trafficking, endo- and exocytosis, apoptosis, telomere length regulation, chromatin hyperrecombination, the response to osmotic stress, and elements of nucleolar function.

Diadenosine pentaphosphate is a potent activator of cardiac ryanodine receptors revealing a novel high-affinity binding site for adenine nucleotides

BACKGROUND AND PURPOSE

Diadenosine polyphosphates are normally present in cells at low levels, but significant increases in concentrations can occur during cellular stress. The aim of this study was to investigate the effects of diadenosine pentaphosphate (Ap5A) and an oxidized analogue, oAp5A on the gating of sheep cardiac ryanodine receptors (RyR2).

EXPERIMENTAL APPROACH

RyR2 channel function was monitored after incorporation into planar bilayers under voltage-clamp conditions.

KEY RESULTS

With10 micromol.L(-1) cytosolic Ca2+, a significant 'hump' or plateau at the base of the dose-response relationship to Ap5A was revealed. Open probability (Po) was significantly increased to a plateau of approximately 0.2 in the concentration range 100 pmol x L(-1)-10 micromol x L(-1). High Po values were observed at >10 micromol x L(-1) Ap5A, and Po values close to 1 could be achieved. Nanomolar levels of ATP and adenosine also revealed a hump at the base of the dose-response relationships, although GTP did not activate at any concentration, indicating a common, high-affinity binding site on RyR2 for adenine-based compounds. The oxidized analogue, oAp5A, did not significantly activate RyR2 via the high-affinity binding site; however, it could fully open the channel with an EC(50) of 16 micromol.L(-1) (Ap5A EC(50) = 140 micromol x L(-1)). Perfusion experiments suggest that oAp5A and Ap5A dissociate slowly from their binding sites on RyR2.

CONCLUSIONS AND IMPLICATIONS

The ability of Ap5A compounds to increase Po even in the presence of ATP and their slow dissociation from the channel may enable these compounds to act as physiological regulators of RyR2, particularly under conditions of cellular stress.

Crystal structure of the 25 kDa subunit of human cleavage factor Im

Cleavage factor I_m is an essential component of the pre-messenger RNA 3′-end processing machinery in higher eukaryotes, participating in both the polyadenylation and cleavage steps. Cleavage factor I_m is an oligomer composed of a small 25 kDa subunit (CF I_m25) and a variable larger subunit of either 59, 68 or 72 kDa. The small subunit also interacts with RNA, poly(A) polymerase, and the nuclear poly(A)-binding protein. These protein–protein interactions are thought to be facilitated by the Nudix domain of CF I_m25, a hydrolase motif with a characteristic α/β/α fold and a conserved catalytic sequence or Nudix box. We present here the crystal structures of human CF I_m25 in its free and diadenosine tetraphosphate (Ap₄A) bound forms at 1.85 and 1.80 Å, respectively. CF I_m25 crystallizes as a dimer and presents the classical Nudix fold. Results from crystallographic and biochemical experiments suggest that CF I_m25 makes use of its Nudix fold to bind but not hydrolyze ATP and Ap₄A. The complex and apo protein structures provide insight into the active oligomeric state of CF I_m and suggest a possible role of nucleotide binding in either the polyadenylation and/or cleavage steps of pre-messenger RNA 3′-end processing.

Crystal structures of Salmonella typhimurium propionate kinase and its complex with Ap4A: evidence for a novel Ap4A synthetic activity

Propionate kinase catalyses the last step in the anaerobic breakdown of L-threonine to propionate in which propionyl phosphate and ADP are converted to propionate and ATP. Here we report the structures of propionate kinase (TdcD) in the native form as well as in complex with diadenosine 5',5'''-P1,P4-tetraphosphate (Ap4A) by X-ray crystallography. Structure of TdcD obtained after cocrystallization with ATP showed Ap4A bound to the active site pocket suggesting the presence of Ap4A synthetic activity in TdcD. Binding of Ap4A to the enzyme was confirmed by the structure determination of a TdcD-Ap4A complex obtained after cocrystallization of TdcD with commercially available Ap4A. Mass spectroscopic studies provided further evidence for the formation of Ap4A by propionate kinase in the presence of ATP. In the TdcD-Ap4A complex structure, Ap4A is present in an extended conformation with one adenosine moiety present in the nucleotide binding site and other in the proposed propionate binding site. These observations tend to support direct in-line transfer of phosphoryl group during the kinase reaction.

Diadenosine tetraphosphate stimulates atrial ANP release via A(1) receptor: involvement of K(ATP) channel and PKC

Diadenosine polyphosphates (APnAs) are endogenous compounds and exert diverse cardiovascular functions. However, the effects of APnAs on atrial ANP release and contractility have not been studied. In this study, the effects of diadenosine tetraphosphate (AP4A) on atrial ANP release and contractility, and their mechanisms were studied using isolated perfused rat atria. Treatment of atria with AP4A resulted in decreases in atrial contractility and extracellular fluid (ECF) translocation whereas ANP secretion and cAMP levels in perfusate were increased in a dose-dependent manner. These effects of AP4A were attenuated by A(1) receptor antagonist but not by A(2A) or A(3) receptor antagonist. Other purinoceptor antagonists also did not show any effects on AP4A-induced ANF release and contractility. The increment of ANP release and negative inotropy induced by AP4A was similar to those induced by AP3A, AP5A, and AP6A. Protein kinase A inhibitors accentuated AP4A-induced ANP secretion. In contrast, an inhibitor of phospholipase C, protein kinase C or sarcolemma K(ATP) channel completely blocked AP4A-induced ANP secretion. However, an inhibitor of adenylyl cyclase or mitochondria K(ATP) channel had no significant modification of AP4A effects. These results suggest that AP4A regulates atrial inotropy and ANP release mainly through A(1) receptor signaling involving phospholipase C-protein kinase C and sarcolemmal K(ATP) channel and that protein kinase A negatively modulates the effects of AP4A.

Positive inotropic and sustained anti-beta-adrenergic effect of diadenosine pentaphosphate in human and guinea pig hearts. Role of dinucleotide receptors and adenosine receptors

AIM

Diadenosine polyphosphates are present intracellularly and in extracellular fluid due to release from secretory vesicles in platelets, chromaffin cells and other cells. This study investigates effects of diadenosine pentaphosphate (AP5A) on heart muscle function.

METHODS

Contractile force amplitude and action potential duration at 90% repolarization (APD90) were measured after challenge with AP5A 50 microm or isoproterenol 50-70 nM in guinea pig papillary muscles. Isoproterenol was given immediately after AP5A-exposure or after 45 min washout. AP5A was combined with antagonists to the purinergic P2 receptor (suramin 100 microm), the dinucleotide receptor [diinosine pentaphosphate 30 microm (IP5I)] or adenosine receptors [8-(P-sulfophenyl) theophylline 50 microm (8-SPT)].

RESULTS

Results are %-change (mean +/- SEM) from value before exposure. AP5A increased contractile force by 22 +/- 3%* (*P <0.05), and IP5I abolished this. AP5A prolonged APD90 by 7 +/- 2%*. AP5A significantly reduced response to isoproterenol acutely from 31 +/- 4* (controls) to 9 +/- 4% and after 45 min washout from 61 +/- 14* (controls) to 16 +/- 5%. 8-SPT abolished the sustained effect. Increase in contractile force by AP5A was confirmed in human atria trabecula preparations.

CONCLUSION

AP5A increased contractile force and prolonged APD90. Contractile force increased by stimulation of the dinucleotide receptor in guinea pig myocardium. The sustained anti-beta-adrenergic effect of AP5A was due to adenosine receptor stimulation.

Interaction between dinucleotide and nicotinic receptors in individual cholinergic terminals

Functional ionotropic nucleotidic receptors responding to diadenosine pentaphospate and nicotinic receptors responding to epibatidine coexpress in 19% of the total rat midbrain cholinergic terminals, as determined by the combination of immunological and microfluorimetric techniques. Activation of each independent receptor induces the intrasynaptosomal [Ca2+]i and acetylcholine (ACh) release in a dose-dependent way. The responses are inhibited by antagonists of the dinucleotide receptor and nicotinic receptors, thus confirming the involvement of specific receptors in both functions. Stimulation of single cholinergic terminal with both agonists altogether results in a significant decrease of the [Ca2+]i signaling compared with responses of each independent agonist. Inhibitory interaction between both receptors is reverted when one of them is blocked by specific antagonists, both in [Ca2+]i, and subsequent ACh release. The receptor's inhibitory cross talk confirm the involvement of calcium/calmodulin-dependent protein kinase II, CaMKII, as the inhibitory effects are reverted in the presence of the specific inhibitors KN-62 (2-[N-(4'-methoxybenzenesulfonyl)]-amino-N-(4'-chlorophenyl)-2-propenyl-N-methylbenzylamine phosphate) and KN-93 (N-(2-[N-[4-chlorocinnamyl]-N-methylaminomethyl]phenyl)-N-(2-hydroxyethyl)-4-methoxybenzenesulphonamide). These results demonstrate the existence of an efficient interaction between these two channel populations, opening a new understanding of the functioning of the cholinergic synaptic terminals or terminals containing other neurotransmitters but exhibiting these receptor types or ones that are similar.

Thioredoxin, a regulator of gene expression

Cancer cells have high levels of thioredoxin (Trx) and of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Cells from patients with the cancer-prone disease Fanconi anemia (FA) exhibit reduced Trx levels. We found the activity of GAPDH to correlate directly with the endogenous Trx content and mRNA transcripts for GAPDH and TRx reduced in FA cells. The treatment of cells with reduced human Trx stimulated the synthesis of GAPDH mRNA. Similarly, the transfection of cells with an expression plasmid for Trx increased GAPDH mRNA synthesis. Trx treatment of cells and subsequent analysis of the differential gene expression by human cDNA arrays containing about 50 000 different PCR products resulted in more than 300 up- or downregulated genes. Two representative genes, GAPDH and IkappaBalpha/MAD-3, were further investigated to confirm their stimulation by Trx. Trx besides being the major carrier of redox potential of cells is also a regulator of gene expression on the transcriptional level. By regulation via Trx, cells are able to adapt to the prevailing redox conditions. These findings also enlighten the pathophysiology of FA in the respect that the characteristic diminution of Trx that results in the dysregulation of gene expression is a basis for the major symptoms of this disease.

Null mutations in a Nudix gene, ygdP, implicate an alarmone response in a novel suppression of hybrid jamming

Induction of the toxic LamB-LacZ protein fusion, Hyb42-1, leads to a lethal generalized protein export defect. The prlF1 suppressor causes hyperactivation of the cytoplasmic Lon protease and relieves the inducer sensitivity of Hyb42-1. Since prlF1 does not cause a detectable change in the stability or level of the hybrid protein, we conducted a suppressor screen, seeking factors genetically downstream of lon with prlF1-like phenotypes. Two independent insertions in the ygdP open reading frame relieve the toxicity of the fusion protein and share two additional properties with prlF1: cold sensitivity and the ability to suppress the temperature sensitivity of a degP null mutation. Despite these similarities, ygdP does not appear to act in the same genetic pathway as prlF1 and lon, suggesting a fundamental link between the phenotypes. We speculate that the common properties of the suppressors relate to secretion defects. The ygdP gene (also known as nudH) has been shown to encode a Nudix protein that acts as a dinucleotide oligophosphate (alarmone) hydrolase. Our results suggest that loss of ygdP function leads to the induction of an alarmone-mediated response that affects secretion. Using an epitope-tagged ygdP construct, we present evidence that this response is sensitive to secretion-related stress and is regulated by differential proteolysis of YgdP in a self-limiting manner.

Efficient incorporation of CoA, NAD and FAD into RNA by in vitro transcription

Protein enzymes frequently recruit small molecule coenzymes to perform a variety of biochemical reactions. While the catalytic activities of RNA have been expanding rapidly, a similar strategy for RNA to utilize coenzymes and to increase its functional capabilities has yet to be demonstrated. A general in vitro transcription procedure has been developed to efficiently prepare RNA with coenzymes CoA, NAD and FAD covalently attached to the 5' end. These adenosine-containing coenzymes initiate transcription under the T7 class II promoter by T7 RNA polymerase. In addition to the three coenzymes, other adenosine-containing molecules may be incorporated into the first nucleotide position of RNA as well. This method provides easy access to CoA-, NAD- and FAD-RNA, which may find broad applications in generating coenzyme- utilizing ribozymes. In addition, both oxidized FAD and reduced NADH are highly fluorescent. NADH-RNA and FAD-RNA can therefore be used as probes for DNA/RNA detection and for structural investigation of RNA function by fluorescence spectroscopy.

The crystal structure of diadenosine tetraphosphate hydrolase from Caenorhabditis elegans in free and binary complex forms

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.

Activity of diadenosine polyphosphates at P2Y receptors stably expressed in 1321N1 cells

The selectivities of the diadenosine polyphosphates (Ap(n)As, n=2-6) at the human P2Y(1), P2Y(2), P2Y(4), P2Y(6) and P2Y(11) receptors stably expressed in 1321N1 human astrocytoma cells was determined using a Fluorescence Imaging Plate Reader (FLIPR) to measure intracellular Ca(2+) mobilisation. The rank order of agonist potencies at P2Y(1) were: ADP>P(1),P(3)-diadenosine triphosphate (Ap(3)A)>P(1),P(3)-diadenosine hexaphosphate (Ap(6)A)=P(1),P(3)-diadenosine diphosphate (Ap(2)A)>>P(1),P(3)-diadenosine pentaphosphate (Ap(5)A). P(1),P(3)-diadenosine tetraphosphate (Ap(4)A) was inactive up to 1 mM. The rank order of agonist potencies at P2Y(2) were: UTP>Ap(4)A>>Ap(6)A>Ap(5)A>Ap(3)A>>Ap(2)A. The Ap(4)A concentration response curve appeared to be bi-phasic. At P2Y(4) all the Ap(n)As tested were inactive as agonists. At P2Y(6), only Ap(3)A and Ap(5)A showed significant agonist activity. At P2Y(11), only Ap(4)A showed significant agonist activity. Ap(n)As were inactive as antagonists of the P2Y(1), P2Y(2), P2Y(4), P2Y(6) and P2Y(11) receptors. At P2Y(4), however, the Ap(n)As potentiated the UTP response.

Poly(A) polymerase from Escherichia coli adenylylates the 3'-hydroxyl residue of nucleosides, nucleoside 5'-phosphates and nucleoside(5')oligophospho(5')nucleosides (NpnN)

The capacity of Escherichia coli poly(A) polymerase to adenylylate the 3'-OH residue of a variety of nucleosides, nucleoside 5'-phosphates and dinucleotides of the type nucleoside(5')oligophospho(5')nucleoside is described here for the first time. Using micromolar concentrations of [alpha-32P]ATP, the following nucleosides/nucleotides were found to be substrates of the reaction: guanosine, AMP, CMP, GMP, IMP, GDP, CTP, dGTP, GTP, XTP, adenosine(5')diphospho(5')adenosine (Ap2A), adenosine (5')triphospho(5')adenosine (Ap3A), adenosine(5')tetraphospho(5')adenosine (Ap4A), adenosine(5')pentaphospho(5')adenosine (Ap5A), guanosine(5')diphospho(5') guanosine (Gp2G), guanosine(5')triphospho(5')guanosine (Gp3G), guanosine(5')tetraphospho(5')guanosine (Gp4G), and guanosine(5')pentaphospho(5')guanosine (Gp5G). The synthesized products were analysed by TLC or HPLC and characterized by their UV spectra, and by treatment with alkaline phosphatase and snake venom phosphodiesterase. The presence of 1 mM GMP inhibited competitively the polyadenylylation of tRNA. We hypothesize that the type of methods used to measure polyadenylation of RNA is the reason why this novel property of E. coli poly(A) polymerase has not been observed previously.

Coronary vasomotor and cardiac electrophysiologic effects of diadenosine polyphosphates and nonhydrolyzable analogs in the guinea pig

Platelet activation in heart disease is important owing to the effects of platelet-derived compounds on myocardial perfusion and cardiac electrophysiology. Diadenosine polyphosphates are secreted from platelets and present in the myocardium, but their electrophysiologic and vasomotor effects are incompletely understood. We used isolated guinea-pig hearts to study the effects of diadenosine triphosphate (Ap3A), tetraphosphate (Ap4A), pentaphosphate (Ap5A), and hexaphosphate (Ap6A) (10 pM-0.1 mM), comparing their actions to those of adenosine, adenosine triphosphate, and non-hydrolyzable Ap4A and Ap5A analogs. Diadenosine polyphosphates (0.1 nM-0.1 microM) transiently reduced coronary perfusion pressure, which recovered during the continued presence of the compounds. At concentrations greater than 0.1 microM effects were maximal and sustained (perfusion pressure decreased from 36.5+/-3.4 to 18.6+/-2.5 mm Hg, p < 0.001, with 1 microM Ap4A). The changes in action potential duration and refractory period developed slowly but were maintained (0.1 nM-1 microM). With 1 nM Ap4A, action potential duration increased from 170.6+/-2.6 to 187.3+/-3.8 ms, p < 0.05, and refractory period increased from 138.5+/-1.6 to 147.9+/-2.0 ms, p < 0.05. Ap4A and its analog reduced QRS duration (from 24.7+/-1.1 to 13.9+/-1.6 ms with 1 microM Ap4A, p < 0.05). P2-purinergic (adenosine triphosphate) receptor antagonism (suramin) reduced perfusion pressure but was without electrophysiologic effect. Other changes in coronary perfusion pressure and electrophysiologic variables associated with Ap4A were not seen in the presence of suramin. P1-(adenosine) antagonism (8-[p-sulfophenyl]theophylline) attenuated the electrophysiologic effects only. Diadenosine polyphosphates have potent cardiac electrophysiologic and coronary vasomotor effects via purinergic receptors, suggesting an important role during platelet activation in acute coronary syndromes.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

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.

Inotropic effects of diadenosine monophosphate (AP1A) in isolated human cardiac preparations

Dependent on the number of phosphate residues, diadenosine polyphosphates (APnP) exert divergent inotropic effects in the human heart. We studied the inotropic effects of the smallest member of this family, diadenosine monophosphate (AP1A). Force of contraction was measured in an isometric setup in isolated electrically driven (0.5 Hz) preparations from human atria. AP1A exerted a concentration-dependent negative inotropic effect. The IC50 value was 20.2 microM and the IC50 value was 3.1 microM (n = 5-8). At 100 microM AP1A, force of contraction declined to 50% of the predrug value after 2.5 +/- 0.5 min of incubation (n = 8). AP1A antagonized the positive inotropic effect of the beta-adrenoceptor agonist isoprenaline (10 nM). For 100 microM AP1A, the time to 50% of the predrug force in the presence of isoprenaline amounted to 2.3 +/- 0.2 min (n = 5). The positive inotropic and lusitropic effects of isoprenaline were antagonized by AP1A. The direct (AP1A alone) and indirect (AP1A in the presence of isoprenaline) negative inotropic effects of AP1A were blocked by the A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX, 0.3 microM). The inotropic effect of AP1A was not blocked by adenosine deaminase. In conclusion, AP1A exerts indirect and direct negative inotropic effects in the human heart through A1-adenosine receptors. These effects might protect the heart against excessive beta-adrenergic stimulation.

Modulation of the dinucleotide receptor present in rat midbrain synaptosomes by adenosine and ATP

Diadenosine polyphosphates activate dinucleotide receptors in rat midbrain synaptic terminals. The agonist with highest affinity at this receptor, diadenosine pentaphosphate (Ap(5)A), elicits Ca(2+) transients at concentrations ranging from 10(-7) to 10(-3) M with a single-phase curve and an EC(50) value of 56.21+/-1.82 microM. Treatment of synaptosomal preparations with alkaline phosphatase (AP) changes the dose-response control curve into a biphasic one presenting two EC(50) values of 6.47+/-1.25 nM and 11.16+/-0.83 microM respectively. The adenosine A(1) antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) reversed the biphasic concentration-response for Ap(5)A curve in the presence of AP, to a monophasic one with an EC(50) value of 76.05+/-7.51 microM. The application of adenosine deaminase produced the same effect as DPCPX, the EC(50) value for Ap(5)A, in the presence of AP being 18.62+/-4.03 microM. Activation of the adenosine A(1) receptor by means of cyclohexyladenosine (CHA) shifted the dose response curve for Ap(5)A to the left, resulting in a monophasic curve with an EC(50) of 5. 01+/-0.02 pM. The destruction of extrasynaptosomal nucleotides by AP or the addition of pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), a broad P2 antagonist compound, enhance maximal effect of the Ap(5)A up to 55.6% on the dose response curve, thus suggesting a negative modulation by P2 receptors. In a summary, ATP and adenosine present at the extra-synaptosomal space, are relevant natural modulators of the dinucleotide receptor, via P2 and adenosine A(1) receptors respectively.

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.

T4 RNA ligase catalyzes the synthesis of dinucleoside polyphosphates

T4 RNA ligase has been shown to synthesize nucleoside and dinucleoside 5'-polyphosphates by displacement of the AMP from the E-AMP complex with polyphosphates and nucleoside diphosphates and triphosphates. Displacement of the AMP by tripolyphosphate (P3) was concentration dependent, as measured by SDS/PAGE. When the enzyme was incubated in the presence of 0.02 mm [alpha-32P] ATP, synthesis of labeled Ap4A was observed: ATP was acting as both donor (Km, microm) and acceptor (Km, mm) of AMP from the enzyme. Whereas, as previously known, ATP or dATP (but not other nucleotides) were able to form the E-AMP complex, the specificity of a compound to be acceptor of AMP from the E-AMP complex was very broad, and with Km values between 1 and 2 mm. In the presence of a low concentration (0.02 mm) of [alpha-32P] ATP (enough to form the E-AMP complex, but only marginally enough to form Ap4A) and 4 mm of the indicated nucleotides or P3, the relative rate of synthesis of the following radioactive (di)nucleotides was observed: Ap4X (from XTP, 100); Ap4dG (from dGTP, 74); Ap4G (from GTP, 49); Ap4dC (from dCTP, 23); Ap4C (from CTP, 9); Ap3A (from ADP, 5); Ap4ddA, (from ddATP, 1); p4A (from P3, 200). The enzyme also synthesized efficiently Ap3A in the presence of 1 mm ATP and 2 mm ADP. The following T4 RNA ligase donors were inhibitors of the synthesis of Ap4G: pCp > pAp > pA2'p.

The Saccharomyces cerevisiae YOR163w gene encodes a diadenosine 5', 5"'-P1,P6-hexaphosphate (Ap6A) hydrolase member of the MutT motif (Nudix hydrolase) family

The YOR163w open reading frame on chromosome XV of the Saccharomyces cerevisiae genome encodes a member of the MutT motif (nudix hydrolase) family of enzymes of Mr 21,443. By cloning and expressing this gene in Escherichia coli and S. cerevisiae, we have shown the product to be a (di)adenosine polyphosphate hydrolase with a previously undescribed substrate specificity. Diadenosine 5',5"'-P1, P6-hexaphosphate is the preferred substrate, and hydrolysis in H218O shows that ADP and adenosine 5'-tetraphosphate are produced by attack at Pbeta and AMP and adenosine 5'-pentaphosphate are produced by attack at Palpha with a Km of 56 microM and kcat of 0.4 s-1. Diadenosine 5',5"'-P1,P5-pentaphosphate, adenosine 5'-pentaphosphate, and adenosine 5'-tetraphosphate are also substrates, but not diadenosine 5',5"'-P1,P4-tetraphosphate or other dinucleotides, mononucleotides, nucleotide sugars, or nucleotide alcohols. The enzyme, which was shown to be expressed in log phase yeast cells by immunoblotting, displays optimal activity at pH 6.9, 50 degrees C, and 4-10 mM Mg2+ (or 200 microM Mn2+). It has an absolute requirement for a reducing agent, such as dithiothreitol (1 mM), and is inhibited by Ca2+ with an IC50 of 3.3 mM and F- (noncompetitively) with a Ki of 80 microM. Its function may be to eliminate potentially toxic dinucleoside polyphosphates during sporulation.

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.

Presence of dinucleotide and ATP receptors in human cerebrocortical synaptic terminals

Human cerebrocortical synaptic terminals elicited concentration-dependent Ca2+ transients after Ap5A (diadenosine pentaphosphate) and ATP stimulation, with EC50 values of 23.44 +/- 3.70 microM and 11.48 +/- 2.12 microM, respectively. The lack of cross-desensitisation and the selective antagonism by Ip5I (diinosine pentaphosphate), suggests the activation of a dinucleotide receptor by Ap5A, and a P2X receptor by ATP. Ap5A Ca2+ transients were partially abolished by omega-conotoxin GVI-A (53%), suggesting the participation of a N-type Ca2+ channel in the dinucleotide response. ATP effect on Ca2+ entry was abolished by nicardipine (44%) and by omega-conotoxin GVI-A (52%), suggesting the participation of L- and N-type Ca2+ channels. These data suggest that Ap5A and ATP activate dinucleotide and P2X receptors, respectively, in human brain synaptic terminals.

The gene, ialA, associated with the invasion of human erythrocytes by Bartonella bacilliformis, designates a nudix hydrolase active on dinucleoside 5'-polyphosphates

ialA, one of two genes associated with the invasion of human red blood cells by Bartonella bacilliformis, the causative agent of several diseases, has been cloned and expressed in Escherichia coli. The protein, IalA, contains an amino acid array characteristic of a family of enzymes, the Nudix hydrolases, active on a variety of nucleoside diphosphate derivatives. IalA has been purified, identified, and characterized as an enzyme catalyzing the hydrolysis of members of a class of signaling nucleotides, the dinucleoside polyphosphates, with its highest activity on adenosine 5'-tetraphospho-5'-adenosine (Ap4A), but also hydrolyzing Ap5A, Ap6A, Gp4G, and Gp5G. In each case, a pyrophosphate linkage is cleaved yielding a nucleoside triphosphate and the remaining nucleotide moiety.

Inotropic effects of diadenosine tetraphosphate in isolated canine cardiac preparations

We studied the effects of diadenosine tetraphosphate (AP4A) on the force of contraction in canine preparations. The force of contraction was measured in isolated electrically driven (1 Hz) atrial and ventricular cardiac trabeculae from adult dogs. AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced force of contraction in atrial preparations by approximately 24%. Moreover, AP4A (100 microM) alone and after prestimulation with 10 nM isoproterenol reduced the force of contraction in ventricular preparations by 29 and 29%, respectively. The negative inotropic effects of AP4A were abolished by the A1-adenosine receptor antagonist 1,3-dipropyl-cyclopentyl-xanthine (DPCPX). In summary, in canine myocardium, AP4A alone and after prestimulation with a beta-adrenoceptor agonist exerts negative inotropic effects, which are probably mediated via A1-adenosine receptors.

Unblocking of chain-terminated primer by HIV-1 reverse transcriptase through a nucleotide-dependent mechanism

HIV-1 replication is inhibited by the incorporation of chain-terminating nucleotides at the 3' end of the growing DNA chain. Here we show a nucleotide-dependent reaction catalyzed by HIV-1 reverse transcriptase that can efficiently remove the chain-terminating residue, yielding an extendible primer terminus. Radioactively labeled 3'-terminal residue from the primer can be transferred into a product that is resistant to calf intestinal alkaline phosphatase and sensitive to cleavage by snake venom phosphodiesterase. The products formed from different nucleotide substrates have unique electrophoretic migrations and have been identified as dinucleoside tri- or tetraphosphates. The reaction is inhibited by dNTPs that are complementary to the next position on the template (Ki approximately 5 microM), suggesting competition between dinucleoside polyphosphate synthesis and DNA polymerization. Dinucleoside polyphosphate synthesis was inhibited by an HIV-1 specific non-nucleoside inhibitor and was absent in mutant HIV-1 reverse transcriptase deficient in polymerase activity, indicating that this activity requires a functional polymerase active site. We suggest that dinucleoside polyphosphate synthesis occurs by transfer of the 3' nucleotide from the primer to the pyrophosphate moiety in the nucleoside di- or triphosphate substrate through a mechanism analogous to pyrophosphorolysis. Unlike pyrophosphorolysis, however, the reaction is nucleotide-dependent, is resistant to pyrophosphatase, and produces dinucleoside polyphosphates. Because it occurs at physiological concentrations of ribonucleoside triphosphates, this reaction may determine the in vivo activity of many nucleoside antiretroviral drugs.

T4 DNA ligase synthesizes dinucleoside polyphosphates

T4 DNA ligase (EC 6.5.1.1), one of the most widely used enzymes in genetic engineering, transfers AMP from the E-AMP complex to tripolyphosphate, ADP, ATP, GTP or dATP producing p4A, Ap3A, Ap4A, Ap4G and Ap4dA, respectively. Nicked DNA competes very effectively with GTP for the synthesis of Ap4G and, conversely, tripolyphosphate (or GTP) inhibits the ligation of DNA by the ligase. As T4 DNA ligase has similar requirements for ATP as the mammalian DNA ligase(s), the latter enzyme(s) could also synthesize dinucleoside polyphosphates. The present report may be related to the recent finding that human Fhit (fragile histidine triad) protein, encoded by the FHIT putative tumor suppressor gene, is a typical dinucleoside 5',5''-P1,P3-triphosphate (Ap3A) hydrolase (EC 3.6.1.29).

The effects of adenine dinucleotides on epileptiform activity in the CA3 region of rat hippocampal slices

Alpha, omega-adenine dinucleotides (Ap(n)A) consist of two adenosine molecules linked at the 5' position by phosphate groups, the number of which is denoted by n and can range from 2 to 6. The aim of this study was to investigate the effect of Ap4A and Ap5A on the rate of epileptiform activity. Hippocampal slices (450 microm), when perfused with a medium containing no added magnesium and 4-aminopyridine (50 microM), generate epileptiform activity of an interictal nature. Ap4A and Ap5A at 1 microM depressed the discharge rate to a significant extent. At this concentration adenosine (1 microM) did not produce any effect. However at 10 microM adenosine, Ap4A and Ap5A all decreased the burst frequency. Adenosine deaminase (0.2 U/ml) totally annulled the inhibition of epileptiform activity produced by 10 microM adenosine or 1 microM Ap4A and Ap5A. Adenosine deaminase did not significantly change the maximum depression of activity produced by 10 microM Ap4A and Ap5A. 8-cyclopentyl-1,3-dimethylxanthine, an A1, receptor antagonist, increased the basal rate of epileptiform activity and prevented the depression of burst discharges by Ap4A. 5'-adenylic acid deaminase converts AMP into IMP which is inactive. 5'-adenylic acid deaminase did not prevent the inhibitory effects of Ap4A. The results suggests that in the CA3 region of the hippocampus, Ap4A and Ap5A act partly by stimulating xanthine-sensitive receptors directly and partly through the formation of the metabolite, adenosine.

Diadenosine polyphosphate-mediated activation of phospholipase D in isolated rat liver cells

Diadenosine polyphosphates (ApnAs) can, through interaction with appropriate purinoceptors, affect a range of cellular activities. Ap4A, the most prominent naturally occurring diadenosine polyphosphate, stimulates alterations in intracellular calcium homeostasis and subsequent activation of glycogen breakdown in isolated liver cells. Here we show that Ap4A, and other naturally occurring diadenosine polyphosphates, also stimulates phospholipase D (PLD) activity in isolated rat liver cells. The characteristics of Ap4A-mediated activation of PLD are similar to those for the activation of PLD by extracellular ATP. These results are discussed in the context of the relation between diadenosine polyphosphate- and adenine mononucleotide-mediated cellular signalling processes.

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.

Diadenosine oligophosphates (Ap(n)A), a novel class of signalling molecules?

The diadenosine oligophosphates (Ap(n)A) were discovered in the mid-sixties in the course of studies on aminoacyl-tRNA synthetases (aaRS). Now, more than 30 years later, about 300 papers have been published around these substances in attempt to decipher their role in cells. Recently, Ap(n)A have emerged as intracellular and extracellular signalling molecules implicated in the maintenance and regulation of vital cellular functions and become considered as second messengers. Great variety of physiological and pathological effects in mammalian cells was found to be associated with alterations of Ap(n)A levels (n from 2 to 6) and Ap3A/Ap4A ratio. Cell differentiation and apoptosis have substantial and opposite effects on Ap3A/Ap4A ratio in cultured cells. A human Ap3A hydrolase, Fhit, appeared to be involved in protection of cells against tumourigenesis. Ap3A is synthesised by mammalian u synthetase (TrpRS) which in contrast to most other aaRS is unable to synthesise Ap4A and is an interferon-inducible protein. Moreover, Ap3A appeared to be a preferred substrate for 2-5A synthetase, also interferon-inducible, priming the synthesis of 2' adenylated derivatives of Ap3A, which in turn may serve as substrates of Fhit. Tumour suppressor activity of Fhit is assumed to be associated with involvement of the Fhit.Ap3A complex in cytokine signalling pathway(s) controlling cell proliferation. The Ap(n)A family is potentially a novel class of signal-transducing molecules whose functions are yet to be determined.

Control of 5',5'-dinucleoside triphosphate catabolism by APH1, a Saccharomyces cerevisiae analog of human FHIT

The putative human tumor suppressor gene FHIT (fragile histidine triad) (M. Ohta et al., Cell 84:587-597, 1996) encodes a protein behaving in vitro as a dinucleoside 5',5"'-P1,P3-triphosphate (Ap3A) hydrolase. In this report, we show that the Saccharomyces cerevisiae APH1 gene product, which resembles human Fhit protein, also hydrolyzes dinucleoside 5',5'-polyphosphates, with Ap3A being the preferred substrate. Accordingly, disruption of the APH1 gene produced viable S. cerevisiae cells containing reduced Ap3A-hydrolyzing activity and a 30-fold-elevated Ap3N concentration.

Orf186 represents a new member of the Nudix hydrolases, active on adenosine(5')triphospho(5')adenosine, ADP-ribose, and NADH

orf186, a new member of the Nudix hydrolase family of genes, has been cloned and expressed, and the protein has been purified and identified as an enzyme highly specific for compounds of ADP. Its three major substrates are adenosine(5')triphospho(5')adenosine, ADP-ribose, and NADH, all implicated in a variety of cellular regulatory processes, supporting the notion that the function of the Nudix hydrolases is to monitor the concentrations of reactive nucleoside diphosphate derivatives and to help modulate their accumulation during cellular metabolism.