Diadenosine 5',5"'-P1,P4-tetraphosphate hydrolase is present in human erythrocytes, leukocytes and platelets

Morphologically homogeneous red blood cells present a heterogeneous response to hormonal stimulation

Red blood cells (RBCs) are among the most intensively studied cells in natural history, elucidating numerous principles and ground-breaking knowledge in cell biology. Morphologically, RBCs are largely homogeneous, and most of the functional studies have been performed on large populations of cells, masking putative cellular variations. We studied human and mouse RBCs by live-cell video imaging, which allowed single cells to be followed over time. In particular we analysed functional responses to hormonal stimulation with lysophosphatidic acid (LPA), a signalling molecule occurring in blood plasma, with the Ca²⁺ sensor Fluo-4. Additionally, we developed an approach for analysing the Ca²⁺ responses of RBCs that allowed the quantitative characterization of single-cell signals. In RBCs, the LPA-induced Ca²⁺ influx showed substantial diversity in both kinetics and amplitude. Also the age-classification was determined for each particular RBC and consecutively analysed. While reticulocytes lack a Ca²⁺ response to LPA stimulation, old RBCs approaching clearance generated robust LPA-induced signals, which still displayed broad heterogeneity. Observing phospatidylserine exposure as an effector mechanism of intracellular Ca²⁺ revealed an even increased heterogeneity of RBC responses. The functional diversity of RBCs needs to be taken into account in future studies, which will increasingly require single-cell analysis approaches. The identified heterogeneity in RBC responses is important for the basic understanding of RBC signalling and their contribution to numerous diseases, especially with respect to Ca²⁺ influx and the associated pro-thrombotic activity.

Molecular characterization and mutational analysis of recombinant diadenosine 5',5″-P¹,P⁴-tetraphosphate hydrolase from Plasmodium falciparum

Asymmetrical diadenosine 5',5"-P¹,P⁴-tetraphosphate hydrolase (EC 3.6.1.17) from human malaria parasite Plasmodium falciparum was expressed in Escherichia coli, purified to homogeneity, and characterized for the first time as a biological target for chemotherapeutic agents against malaria. Plasmodium falciparum Ap₄A (PfAp₄A) hydrolase not only catalyzes diadenosine 5',5″-P¹,P⁴-tetraphosphate (Ap₄A) to ATP and AMP, but also diadenosine 5″-P¹,P⁵-pentaphosphate (Ap₅A) to ATP and ADP. Marked enzyme heat stability corresponding to the highest level of activity was observed at 60°C. The recombinant enzyme showed maximal activity in the presence of 5 mM Mg²⁺ ions. Kinetic analysis revealed the values of K(m) and K(cat) as 0.6 μM and 2.5 min⁻¹, respectively. Comparative protein modeling indicated an additional space in the substrate binding site of the parasitic enzyme compared with that of humans. Mutagenic analysis of the amino acid residue (Pro133) forming the additional space revealed a 5-fold increase in the wild-type Km value when replaced by a smaller (Ala) residue. Furthermore, catalytic activity was markedly affected by introducing a larger residue (Phe), thus creating the potential to develop a specific inhibitor of PfAp₄A hydrolase.

Dinucleoside polyphosphates: strong endogenous agonists of the purinergic system

The purinergic system is composed of mononucleosides, mononucleoside polyphosphates and dinucleoside polyphosphates as agonists, as well as the respective purinergic receptors. Interest in the role of the purinergic system in cardiovascular physiology and pathophysiology is on the rise. This review focuses on the overall impact of dinucleoside polyphosphates in the purinergic system. Platelets, adrenal glands, endothelial cells, cardiomyocytes and tubular cells release dinucleoside polyphosphates. Plasma concentrations of dinucleoside polyphosphates are sufficient to cause direct vasoregulatory effects and to induce proliferative effects on vascular smooth muscle cells and mesangial cells. In addition, increased plasma concentrations of a dinucleoside polyphosphate were recently demonstrated in juvenile hypertensive patients. In conclusion, the current literature accentuates the strong physiological and pathophysiological impact of dinucleoside polyphosphates on the cardiovascular system.

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

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

Cloning, characterisation and crystallisation of a diadenosine 5',5"'-P(1),P(4)-tetraphosphate pyrophosphohydrolase from Caenorhabditis elegans

Asymmetrically cleaving diadenosine 5',5"'-P(1),P(4)-tetraphosphate (Ap4A) hydrolase activity has been detected in extracts of adult Caenorhabditis elegans and the corresponding cDNA amplified and expressed in Escherichia coli. As expected, sequence analysis shows the enzyme to be a member of the Nudix hydrolase family. The purified recombinant enzyme behaves as a typical animal Ap4A hydrolase. It hydrolyses Ap4A with a K(m) of 7 microM and k(cat) of 27 s(-1) producing AMP and ATP as products. It is also active towards other adenosine and diadenosine polyphosphates with four or more phosphate groups, but not diadenosine triphosphate, always generating ATP as one of the products. It is inhibited non-competitively by fluoride (K(i)=25 microM) and competitively by adenosine 5'-tetraphosphate with Ap4A as substrate (K(i)=10 nM). Crystals of diffraction quality with the morphology of rectangular plates were readily obtained and preliminary data collected. These crystals diffract to a minimum d-spacing of 2 A and belong to either space group C222 or C222(1). Phylogenetic analysis of known and putative Ap4A hydrolases of the Nudix family suggests that they fall into two groups comprising plant and Proteobacterial enzymes on the one hand and animal and archaeal enzymes on the other. Complete structural determination of the C. elegans Ap4A hydrolase will help determine the basis of this grouping.

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

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

Cloning and expression of diadenosine 5',5'''-P1,P4-tetraphosphate hydrolase from Lupinus angustifolius L

The first isolation, cloning and expression of cDNA encoding an asymmetric diadenosine 5',5'''P1,P4-tetraphosphate pyrophosphohydrolase (Ap4A hydrolase) from a higher plant is described. Ap4A hydrolase protein was purified from seeds of both Lupinus luteus and Lupinus angustifolius and partially sequenced. The Ap4A hydrolase cDNA was cloned from L. angustifolius cotyledonary polyadenylated RNA using reverse transcription and PCR with primers based on the amino acid sequence. The cDNA encoded a protein of 199 amino acids, molecular mass 22982Da. When expressed in Escherichia coli fused to a maltose-binding protein, the enzyme catalysed asymmetric cleavage of Ap4A to AMP and ATP which was inhibited at concentrations of F- as low as 3 microM. These are properties characteristic of Ap4A hydrolase (asymmetrical) (EC 3.6.1. 17). Comparison of the Ap4A hydrolase sequences derived from the four known cDNAs from pig, human, lupin and fission yeast showed that, like the mammalian hydrolase, the lupin enzyme possesses a Mut T motif but no other significant similarities. No sequence similarity to the human fragile histidine triad protein, as found in the Ap4A hydrolase from Schizosaccharomyces pombe, was detected in the Ap4A hydrolase from lupin.

Molecular cloning of diadenosine tetraphosphatase from pig small intestinal mucosa and identification of sequence blocks common to diadenosine polyphosphate hydrolases and phosphorylases

Diadenosine 5',5"'-P1,P4-tetraphosphate (Ap4A) pyrophosphohydrolase is the enzyme responsible for reducing intracellular levels of the stress-responsive nucleotide diadenosine 5',5"'-P1,P4-tetraphosphate. In order to gain more information on the relationships between the enzymes hydrolysing diadenosine polyphosphates in different eukaryotes, the Ap4A hydrolase and a corresponding cDNA have been isolated from pig small intestinal mucosa by standard procedures. The enzyme is a typical mammalian Ap4A hydrolase (Km = 0.8 microM) being sensitive to inhibition by fluoride (Ki = 24 microM) and adenosine 5'-tetraphosphate (Ki = 10 nM) and yielding ATP and AMP as products. A low Km Ap4A hydrolase (Km = 0.3 microM) was also isolated from rabbit small intestinal mucosa. These enzymes differ from the rat intestinal mucosal hydrolase, which has much higher values of Km for Ap4A and Ki for adenosine 5'-tetraphosphate. A cDNA encoding the pig enzyme was isolated from a pig ileum cDNA library. The derived amino acid sequence of the 16.8 kDa gene product shows 88% identity and 96% similarity to that of the human enzyme. The sequence has the same modification of the MutT motif found in the human enzyme in which a threonine residue replaces a hydrophobic amino acid. Sequences comparisons among eukaryotic diadenosine polyphosphate hydrolases and phosphorylases reveal two blocks of amino acid similarity, including a motif, Z[AD]Gx[ED]AGQ, which may be involved in polyphosphate binding by the hydrolases, and an invariant histidine residue that may be involved in catalysis. These sequence similarities may have arisen by convergent evolution.

Adenine dinucleotides: a novel class of signalling molecules

1. Adenine dinucleotides (Ap3A, Ap4A, Ap5A, Ap6A) are stored in secretory granules of thrombocytes, chromaffin cells and neuronal cells. After release into the extracellular space, the dinucleotides exhibit divergent biological effects on a variety of target cells and organs. The dinucleotides are metabolized by soluble enzymes in the blood plasma as well as by membrane-bound ectoenzymes of endothelial cells, smooth muscle cells, and other cell types. 2. The enzymatic cleavage of the dinucleotides plays a dual role for their biological function: (a) termination of the signal; and (b) generation of purinergically active products such as ATP, ADP and finally adenosine. In contrast to ATP the dinucleotides are long-lived purine nucleotides in the blood. 3. The potential role of the dinucleotides as signalling molecules has been demonstrated in several systems. The adenosine polyphosphates have autocrine function for thrombocytes. Ap3A at low concentration reversibly activates isolated platelets. The mechanism of activation has been elucidated by showing a continuous cleavage of Ap3A, leading to the formation of ADP which is a known agonist of the P2T receptor on thrombocytes. Ap4A and other dinucleotides act as antagonists and inhibit platelet activation. 4. The vasotone of perfused isolated arteries as well as of resistance vessels in the beating heart is differentially influenced by adenine dinucleotides. While Ap3A and Ap4A exhibit relaxing effects at micromolar concentrations, Ap5A and Ap6A elicit vasoconstriction in these vessels. 5. In rat kidney mesangial cells adenine dinucleotides efficiently promote growth. Stimulation of DNA synthesis by various growth factors is enhanced synergistically. ApnA significantly increase the expression of the early growth response gene Egr-1. 6. The specificity and, in some tissues, the uniqueness of effects evoked by dinucleotides may be mediated by genuine dinucleotide receptors (P4) or by specialized P2 receptors (P2D).

Vascular actions of diadenosine phosphates

1. Diadenosine phosphates were isolated from platelets, adrenal gland and autonomic nerves. The presence of diadenosine phosphates in storage pools releasable into the circulation suggests an important role in the control of blood pressure, and potentially to a modulation of the actions of catecholamines. 2. Besides a role of the diadenosine phosphates in platelet aggregation, these agents have potent vasoactive properties. Vasoactive actions of the diadenosine phosphates were demonstrated in numerous vascular models including most of the physiologically important elements of blood pressure regulation. Mostly, the vasoactive action depends on the number of phosphates in the diadenosine phosphates. Vasodilation can be observed in intact vessels after administration of Ap2A, Ap3A and Ap4A whereas contraction is affected by Ap5A and Ap5A and Ap6A. Vasocontraction induced by the diadenosine phosphates in vascular smooth muscle cells is mediated by an increase in intracellular free Ca2+. 3. In vivo, intravenous injection of Ap4A lowers blood pressure whereas injections of Ap5A and Ap6A caused a prolonged increase in blood pressure. In blood, in contrast to ATP, diadenosine phosphates are relatively long-lived molecules, suggesting that the action of the latter is of intermediate time span. In a similar manner to the vasoconstrictor angiotensin II, diadenosine phosphates also act as mitogens. It can be assumed that diadenosine phosphates may be involved in pathophysiological events of circulation including hypertension and atherosclerosis.