Purification and partial characterization of brain adenosine deaminase: inhibition by purine compounds and by drugs

Inhibition of ecto-5'-nucleotidase and adenosine deaminase is able to reverse long-term behavioural effects of early ethanol exposure in zebrafish (Danio rerio)

The behavioural impacts of prenatal exposure to ethanol include a lower IQ, learning problems, anxiety and conduct disorders. Several components of the neurochemical network could contribute to the long-lasting effects of ethanol embryonic exposure. Adenosine is an important neuromodulator, that has been indicated to be affected by acute and chronic exposure to ethanol. Here, embryos of zebrafish exposed to 1% ethanol during the developmental stages of gastrula/segmentation or pharyngula exhibited anxiolytic effect, increased aggressiveness, and decreased social interaction. The exposure during pharyngula stage was able to affect all behavioural parameters analysed at 3 months-post fertilization (mpf), while the treatment during gastrula stage affected the anxiety and social interaction parameters. The aggressiveness was the only behavioural effect of early ethanol exposure that lasted to 12 mpf. The use of a specific inhibitor of adenosine production, the inhibitor of ecto-5′-nucleotidase (AMPCP/150 mg/kg), and the specific inhibitor of adenosine degradation, the inhibitor of adenosine deaminase, EHNA (100 mg/kg) did not affect the effects over anxiety. However, AMPCP at 3 mpf, but not EHNA, reversed aggressive parameters. AMPCP also recovered the social interaction parameter at 3 mpf in animals treated in both stages, while EHNA recovered this parameter just in those animals treated with ethanol during the gastrula stage. These results suggest that long-lasting behavioural effects of ethanol can be modulated by intervention on ecto-5′-nucleotidase and adenosine deaminase activities.

Therapeutic Perspectives of Adenosine Deaminase Inhibition in Cardiovascular Diseases

Adenosine deaminase (ADA) is an enzyme of purine metabolism that irreversibly converts adenosine to inosine or 2'deoxyadenosine to 2'deoxyinosine. ADA is active both inside the cell and on the cell surface where it was found to interact with membrane proteins, such as CD26 and adenosine receptors, forming ecto-ADA (eADA). In addition to adenosine uptake, the activity of eADA is an essential mechanism that terminates adenosine signaling. This is particularly important in cardiovascular system, where adenosine protects against endothelial dysfunction, vascular inflammation, or thrombosis. Besides enzymatic function, ADA protein mediates cell-to-cell interactions involved in lymphocyte co-stimulation or endothelial activation. Furthermore, alteration in ADA activity was demonstrated in many cardiovascular pathologies such as atherosclerosis, myocardial ischemia-reperfusion injury, hypertension, thrombosis, or diabetes. Modulation of ADA activity could be an important therapeutic target. This work provides a systematic review of ADA activity and anchoring inhibitors as well as summarizes the perspectives of their therapeutic use in cardiovascular pathologies associated with increased activity of ADA.

Probing inhibition mechanisms of adenosine deaminase by using molecular dynamics simulations

Adenosine deaminase (ADA) catalyzes the deamination of adenosine, which is important in purine metabolism. ADA is ubiquitous to almost all human tissues, and ADA abnormalities have been reported in various diseases, including rheumatoid arthritis. ADA can be divided into two conformations based on the inhibitor that it binds to: open and closed forms. Here, we chose three ligands, namely, FR117016 (FR0), FR221647 (FR2) (open form), and HDPR (PRH, closed form), to investigate the inhibition mechanism of ADA and its effect on ADA through molecular dynamics simulations. In open forms, Egap and electrostatic potential (ESP) indicated that electron transfer might occur more easily in FR0 than in FR2. Binding free energy and hydrogen bond occupation revealed that the ADA-FR0 complex had a more stable structure than ADA-FR2. The probability of residues Pro159 to Lys171 of ADA-FR0 and ADA-FR2 to form a helix moderately increased compared with that in nonligated ADA. In comparison with FR0 and FR2 PRH could maintain ADA in a closed form to inhibit the function of ADA. The α7 helix (residues Thr57 to Ala73) of ADA in the closed form was mostly unfastened because of the effect of PRH. The number of H bonds and the relative superiority of the binding free energy indicated that the binding strength of PRH to ADA was significantly lower than that of an open inhibitor, thereby supporting the comparison of the inhibitory activities of the three ligands. Alanine scanning results showed that His17, Gly184, Asp295, and Asp296 exerted the greatest effects on protein energy, suggesting that they played crucial roles in binding to inhibitors. This study served as a theoretical basis for the development of new ADA inhibitors.

Adenosine deaminase activity modulation by some street drug: molecular docking simulation and experimental investigation

BACKGROUND

Adenosine deaminase (ADA) is an enzyme that plays important roles in proliferation, maturation, function and development of the immune system. ADA activity may be altered by variety of substances including synthetic or natural products. Morphine, cocaine and their analogs exert immune suppressive activities by decreasing immune system function. The purpose of this study is to confirm that this possible effect may be modulated by interaction of these substances with ADA activity by experimental and computational method.

METHODS

The structural changes in ADA have been studied in presence of cocaine, ethylmorphine, homatropine, morphine and thebaine by determination of ADA hydrolytic activity, circular dichroism and fluorescence spectroscopy in different concentrations. Docking study was performed to evaluate interaction method of test compound with ADA active site using AutoDock4 software.

RESULTS

According to in-vitro studies all compounds inhibited ADA with different potencies, however thebaine activated it at concentration below 50 μM, ethylmorphine inhibited ADA at 35 μM. Moreover, fluorescence spectra patterns were differed from compounds based on structural resemblance which were very considerable for cocaine and homatropine.

CONCLUSION

The results of this study confirms that opioids and some other stimulant drugs such as cocaine can alter immune function in illegal drug abusers. These findings may lead other investigators to develop a new class of ADA activators or inhibitors in the near future.

Effect of cardiovascular drugs on adenosine deaminase activity

OBJECTIVE

Adenosine deaminase catalyzes the conversion of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. Because raising adenosine concentration can affect several physiological processes we studied the effect of a selection of cardiological drugs on adenosine deaminase activity in red blood cells and rabbit plasma after 21 days administration.

METHODS AND RESULTS

We determined the activity of adenosine deaminase isoenzymes (ADA(1) and ADA(2)). Simvastatin, aspirin, metoprolol, and isosorbide mononitrate significantly decreased plasma total adenosine activity (by 50%, 34%, 29%, and 19%, respectively; P < .05 to P < .001) mainly by decreasing the activity of ADA(2).

CONCLUSIONS

As a consequence of decreased ADA(2) activity , the half-life of adenosine will be lengthened. This may, at least in part, explain some of the beneficial effects of analyzed drugs. Our results might be clinically relevant in patients with coronary artery disease, acute coronary syndromes, heart failure, or stroke where the investigated drugs are commonly used. However, our results should be confirmed in large studies in humans.

Kinetic characterization of adenosine deaminase activity in zebrafish (Danio rerio) brain

Adenosine deaminase (ADA; EC 3.5.4.4) activity is responsible for cleaving adenosine to inosine. In this study we described the biochemical properties of adenosine deamination in soluble and membrane fractions of zebrafish (Danio rerio) brain. The optimum pH for ADA activity was in the range of 6.0-7.0 in soluble fraction and reached 5.0 in brain membranes. A decrease of 31.3% on adenosine deamination in membranes was observed in the presence of 5 mM Zn(2+), which was prevented by 5 mM EDTA. The apparent K(m) values for adenosine deamination were 0.22+/-0.03 and 0.19+/-0.04 mM for soluble and membrane fractions, respectively. The apparent V(max) value for soluble ADA activity was 12.3+/-0.73 nmol NH(3) min(-1) mg(-1) of protein whereas V(max) value in brain membranes was 17.5+/-0.51 nmol NH(3) min(-1) mg(-1) of protein. Adenosine and 2'-deoxyadenosine were deaminated in higher rates when compared to guanine nucleosides in both fractions. Furthermore, a significant inhibition on adenosine deamination in both soluble and membrane fractions was observed in the presence of 0.1 mM of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). The presence of ADA activity in zebrafish brain may be important to regulate the adenosine/inosine levels in the CNS of this species.

Transition-state variation in human, bovine, and Plasmodium falciparum adenosine deaminases

Adenosine deaminases (ADAs) from human, bovine, and Plasmodium falciparum sources were analyzed by kinetic isotope effects (KIEs) and shown to have distinct but related transition states. Human adenosine deaminase (HsADA) is present in most mammalian cells and is involved in B- and T-cell development. The ADA from Plasmodium falciparum (PfADA) is essential in this purine auxotroph, and its inhibition is expected to have therapeutic effects for malaria. Therefore, ADA is of continuing interest for inhibitor design. Stable structural mimics of ADA transition states are powerful inhibitors. Here we report the transition-state structures of PfADA, HsADA, and bovine ADA (BtADA) solved using competitive kinetic isotope effects (KIE) and density functional calculations. Adenines labeled at [6-13C], [6-15N], [6-13C, 6-15N], and [1-15N] were synthesized and enzymatically coupled with [1'-14C] ribose to give isotopically labeled adenosines as ADA substrates for KIE analysis. [6-13C], [6-15N], and [1-15N]adenosines reported intrinsic KIE values of (1.010, 1.011, 1.009), (1.005, 1.005, 1.002), and (1.004, 1.001, 0.995) for PfADA, HsADA, and BtADA, respectively. The differences in intrinsic KIEs reflect structural alterations in the transition states. The [1-15N] KIEs and computational modeling results indicate that PfADA, HsADA, and BtADA adopt early SNAr transition states, where N1 protonation is partial and the bond order to the attacking hydroxyl nucleophile is nearly complete. The key structural variation among PfADA, HsADA, and BtADA transition states lies in the degree of N1 protonation with the decreased bond lengths of 1.92, 1.55, and 1.28 A, respectively. Thus, PfADA has the earliest and BtADA has the most developed transition state. This conclusion is consistent with the 20-36-fold increase of kcat in comparing PfADA with HsADA and BtADA.

Molecular recognition of adenosine deaminase: 15N NMR studies

The elucidation of the molecular recognition of adenosine deaminase (ADA), the interpretation of the catalytic mechanism, and the design of novel inhibitors are based mostly on data obtained for the crystalline state of the enzyme. To obtain evidence for molecular recognition of the physiologically relevant soluble enzyme, we studied its interactions with the in situ formed inhibitor, 6-OH-purine riboside (HDPR), by 1D-15N- and 2D-(1H-15N)- NMR using the labeled primary inhibitor [15N4]-PR. We synthesized both [15N4]-PR and an [15N4]-HDPR model, from relatively inexpensive 15N sources. The [15N4]-HDPR model was used to simulate H-bonding and possible Zn2+-coordination of HDPR with ADA. We also explored possible ionic interactions between PR and ADA by 15N-NMR monitored pH-titrations of [15N4]-PR. Finally, we investigated the [15N4]-PR-ADA 1:1 complex by 2D-(1H-15N) NMR. We found that HDPR recognition determinants in ADA do not include any ionic-interactions. HDPR N1 H is an H-bond acceptor, and not an H-bond donor. Despite the proximity of N7 to the Zn2+-ion, no coordination occurs; instead, N7 is an H-bond acceptor. We found an overall agreement between the crystallographic data for the crystallized ADA:HDPR complex and the 15N-NMR signals for the corresponding soluble complex. This finding justifies the use of ADA's crystallographic data for the design of novel inhibitors.

Adenosine ecto-deaminase (ecto-ADA) from porcine cerebral cortex synaptic membrane

We have purified and investigated the role of adenosine ecto-deaminase (ecto-ADA) in porcine brain synaptic membranes and found a low activity of ecto-ADA in synaptic preparations from the cerebral cortex, hippocampus, striatum and medulla oblongata in the presence of purine transport inhibitors (NBTI, dipyridamole and papaverine). The purification procedure with affinity chromatography on epoxy-Toyopearl gel/purine riboside column as a crucial step of purification allowed a 214-fold purification of synaptic ecto-ADA with a yield of 30%. Gel filtration chromatography revealed a molecular mass estimated at 42.4+/-3.9 kDa. The enzyme had a broad optimum pH and was not affected by mono- and divalent cations. Ecto-ADA revealed a low affinity to adenosine (Ado) and 2'-deoxyadenosine (2'-dAdo) (K(M)=286.30+/-40.38 microM and 287.14+/-46.50 microM, respectively). We compared the affinity of ecto-ADA to the substrates with the physiological and pathological concentrations of the extracellular Ado in brains that do not exceed a low micromolar range even during ischemia and hypoxia, and with the affinity of adenosine receptors to Ado not exceeding a low nanomolar (A(1) and A(2A) receptors) or low micromolar (A(2B) and A(3)) range. Taken together, our data suggest that the role of synaptic ecto-ADA in the regulation of the ecto-Ado level in the brain and in the termination of adenosine receptor signaling is questionable. The porcine brain synapses must have other mechanisms for the ecto-Ado removal from the synaptic cleft and synaptic ecto-ADA may also play an extra-enzymatic role in cell adhesion and non-enzymatic regulation of adenosine receptor activity.

Adenosine deaminase from camel tick Hyalomma dromedarii: purification and characterization

Adenosine deaminase is involved in purine metabolism and is a key enzyme for the control of the cellular levels of adenosine. Adenosine deaminase activity showed significant changes during embryogenesis of the camel tick Hyalomma dromedarii. From the elution profile of chromatography on DEAE-sepharose, three forms of enzyme (ADAI, ADAII and ADAIII) were separated. ADAII was purified to homogeneity after chromatography on Sephacryl S-200. The molecular mass of adenosine deaminase ADAII was 42 kDa for the native enzyme and represented a monomer of 42 kDa by SDS-PAGE. The enzyme had a pH optimum at 7.5 and temperature optimum at 40 degrees C with heat stability up to 40 degrees C. ADAII had a K (m) of 0.5 mM adenosine with higher affinity toward deoxyadenosine and adenosine than other purines. Ni(2+), Ba(2+), Zn(2+), Li(2+), Hg(2+) and Mg(2+) partially inhibited the ADAII. Mg(2+) was the strongest inhibitor by 91% of the enzyme's activity.

A Product Inhibition Study on Adenosine Deaminase by Spectroscopy and Calorimetry

Kinetic and thermodynamic studies have been made on the effect of the inosine product on the activity of adenosine deaminase in a 50 mM sodium phosphate buffer, pH 7.5, at 27 degrees C using UV spectrophotometry and isothermal titration calorimetry (ITC). A competitive inhibition was observed for inosine as a product of the enzymatic reaction. A graphical-fitting method was used for determination of the binding constant and enthalpy of inhibitor binding by using isothermal titration microcalorimetry data. The dissociation-binding constant is equal to 140 microM by the microcalorimetry method, which agrees well with the value of 143 microM for the inhibition constant that was obtained from the spectroscopy method

Tryptophan environment in adenosine deaminase. I. Enzyme modification with N-bromosuccinimide in the presence of adenosine and EHNA analogues

Adenosine deaminase from bovine cerebral hemisphere (white and gray matter) and spleen was treated with N-bromosuccinimide, a reagent known to oxidize selectively tryptophan residues in proteins. Spectrally observable tryptophan modification was accompanied by enzyme inactivation. Tsow graphics revealed that two Trps are essential for the activity of enzyme from both tissues. Enzyme inhibitors and substrate analogues, derivatives of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and adenosine, were able to protect Trp against modification, and this effect correlated in general with the enzyme activity protection. In the presence of adenosine deaza analogues (the noninhibitor tubercidin among them) only two Trps were modified in the fully inactivated enzyme. In the presence of EHNA and its deaza analogues, full inactivation of the enzyme was accompanied by the modification of four Trps. The obtained data confirm the previous hypothesis about the presence on the enzyme of different binding sites for adenosine and EHNA derivatives that are responsible for the different effects on the enzyme conformation elicited by the corresponding derivatives. Moreover, these data allow us to suggest that Trp residues, still unidentified by X-ray analysis, are essential for the functioning of the enzyme.

Adenosine deaminase: functional implications and different classes of inhibitors

Adenosine deaminase (ADA) is an enzyme of the purine metabolism which catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. This ubiquitous enzyme has been found in a wide variety of microorganisms, plants, and invertebrates. In addition, it is present in all mammalian cells that play a central role in the differentiation and maturation of the lymphoid system. However, despite a number of studies performed to date, the physiological role played by ADA in the different tissues is not clear. Inherited ADA deficiency causes severe combined immunodeficiency disease (ADA-SCID), in which both B-cell and T-cell development is impaired. ADA-SCID has been the first disorder to be treated by gene therapy, using polyethylene glycol-modified bovine ADA (PEG-ADA). Conversely, there are several diseases in which the level of ADA is above normal. A number of ADA inhibitors have been designed and synthesized, classified as ground-state and transition-state inhibitors. They may be used to mimic the genetic deficiency of the enzyme, in lymphoproliferative disorders or immunosuppressive therapy (i.e., in graft rejection), to potentiate the effect of antileukemic or antiviral nucleosides, and, together with adenosine kinase, to reduce breakdown of adenosine in inflammation, hypertension, and ischemic injury.

Evidence for increased dorsal hippocampal adenosine release and metabolism during pharmacologically induced seizures in rats

There is growing pharmacological evidence from several animal models of seizure disorder that adenosine possesses endogenous anticonvulsant activity. In order to further evaluate the role of adenosine in seizure activity, we monitored adenosine and its major biochemical metabolites inosine, xanthine, and hypoxanthine in the dorsal hippocampus by in vivo microdialysis before and during the induction of generalized seizures. Seizures were induced pharmacologically in groups of urethane-anesthetized rats by the administration of bicuculline (0.5 mg/kg, i.v.), kainic acid (12.0 mg/kg, i.v.) or pentylenetetrazol (100-250 mg/kg, i.p). Seizure activity was monitored electrophysiologically from the dorsal hippocampus. Dialysate hippocampal purine levels increased during all three seizure types. The largest increases were for the adenosine metabolites hypoxanthine and inosine, with smaller increases observed for adenosine and xanthine. Intra-hippocampal perfusion with the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl-adenine, (EHNA, 300 microM), only slightly increased basal hippocampal adenosine. Guanosine levels in the hippocampus, a purine not directly related to adenosine metabolism, were unaffected by all treatments. These findings demonstrate that an increase in hippocampal adenosine release and metabolism is associated with seizure activity and support the hypothesis that the increased adenosine levels may attenuate hippocampal seizure activity, possibly by terminating ongoing seizures and altering the pattern of subsequent seizures.

Enthalpy and enzyme activity of modified histidine residues of adenosine deaminase and diethyl pyrocarbonate complexes

Kinetic and thermodynamic studies have been made on the effect of diethyl pyrocarbonate as a histidine modifier on the active site of adenosine deaminase in 50 mM sodium phosphate buffer pH 6.8, at 27 degrees C using UV spectrophotometry and isothermal titration calorimetry (ITC). Inactivation of adenosine deaminase by diethyl pyrocarbonate is correlated with modification of histidyl residues. The number of modified histidine residues complexed to active site of adenosine deaminase are equivalent to 4. The number and energy of histidine binding sets are determined by enthalpy curve, which represents triple stages. These stages are composed of 3,1 and 1 sites of histidyl modified residues at diethyl pyrocarbonate concentrations, 0.63, 1.8, 3.3 mM. The heat contents corresponding to the first, second and third sets are found to be 18000, 22000 and 21900 kJ mol(-1) respectively.

Purification and characterization of intestinal adenosine deaminase from mice

Adenosine deaminase (ADA) was isolated from small intestine of mice and purified to utmost homogeneity. SDS-PAGE of purified ADA gave a molecular weight of 41 kDa. Western blot analyses gave a single reactive band at 41 kDa and the other band was an associated ADA binding protein. The purified enzyme was more stable in the alkaline pH. The optimum pH and the pI values were about 7.0 and 4.96, respectively. Km values of the small intestinal ADA for adenosine and 2'-deoxyadenosine were 23 and 16 microM, respectively. Purine riboside was a competitive inhibitor with Ki of 5 microM, whereas 2'-3'-o-isopropylidene adenosine acted as an uncompetitive inhibitor (Ki 66 microM). Activity of ADA was inhibited by the presence of theophylline (-40%), caffeine (-30%), and L-cysteine (-50%). Significantly, Hg2+ (100 microM) inhibited 98% of the initial ADA activity. In addition, various purine analogs such as inosine, purine, alpha-adenosine and adenine showed variable inhibitions on the activity of ADA. Relative ADA activity towards 3'-deoxyadenosine and 6-chloropurine riboside was lower by 30% and 40%, respectively. However, the activity towards 2'-o-methyl adenosine was higher (30%) compared to the activity obtained using adenosine.

The levels of adenosine and its metabolites in the guinea pig and rat brain during complete ischemia-in vivo study

Changes in levels of adenosine, inosine, hypoxanthine and ATP during complete ischemia after decapitation were determined in various areas of the guinea pig and rat brain using an HPLC method. These results were compared with levels in brains fixed by microwave irradiation. The levels of adenosine during 60 min of complete ischemia were extremely high and unevenly distributed while levels in the microwaved brains were very low and evenly distributed. The ratios of inosine plus hypoxanthine levels to adenosine which indicate the rate of metabolic degradation from adenosine into inosine and hypoxanthine, were also unevenly distributed during complete ischemia in the cerebellum, superior colliculus, cerebral cortex and hippocampus of the guinea pig and rat, and the highest ratio was observed in the cerebellum of the guinea pig and the superior colliculus of the rat. The activities of adenosine deaminase (ADA), one of the enzymes involved in adenosine metabolism, were measured in the four regions of the guinea pig. The ADA activities were unevenly distributed and the highest ADA activity was found in the cerebellum. These regional differences in ADA activities are in good agreement with the regional differences in the ratio of inosine plus hypoxanthine levels to adenosine during complete ischemia. Furthermore, the administration of EHNA [erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride] (10 mg/kg, i.p.), an ADA inhibitor, caused a significant increase of adenosine and decrease of inosine formation in all four regions and a drastic effect on the cerebellum with high ADA activity compared with the other regions in the guinea pig brain. These results indicate that the changes in concentrations of adenosine and its metabolites (inosine and hypoxanthine) during complete ischemia depend on ADA activity in each brain region.

Enzymatic and extraenzymatic role of ecto-adenosine deaminase in lymphocytes

Adenosine deaminase (ADA, EC 3.5.4.4) is an enzyme of the purine metabolism which has been the object of considerable interest mainly because the congenital defect causes severe combined immunodeficiency (SCID). In the last 10 years, ADA, which was considered to be cytosolic, has been found on the cell surface of many cells and, therefore, it can be considered an ecto-enzyme. There is recent evidence about a specific role of ecto-ADA, which is different from that of intracellular ADA. Apart from degrading extracellular adenosine (Ado) or 2'-deoxyadenosine (dAdo), which are toxic for lymphocytes, ecto-ADA has an extraenzymatic function via its interaction with CD26. ADA/CD26 interaction results in co-stimulatory signals in T cells. This co-stimulation is blocked by HIV-1, thus evidencing a role for ecto-ADA in the pathophysiology of AIDS. The fact that, besides CD26, ADA can interact with different cell-surface proteins opens new perspectives in the research for a role of ecto-ADA in the function of the immune system and in the interactions that take place between different cells in the development of the immune system. The most interesting aspect is the possible participation of the ecto-enzyme in cell-to-cell contacts during ontogenesis and maturation of immunocompetent cells.

Cell surface adenosine deaminase: much more than an ectoenzyme

During the last 10 years, adenosine deaminase (ADA), an enzyme considered to be cytosolic, has been found on the cell surface of many cells, therefore it can be considered an ectoenzyme. EctoADA, which seems to be identical to intracellular ADA and has a globular structure, does not interact with membranes but with membrane proteins. Two of these cell surface receptors for ectoADA have been identified: CD26 and A1 adenosine receptors (A1R). Apart from degradation of extracellular adenosine another functional role of ectoADA has been assigned. EctoADA is able to transmit signals when interacting with either CD26 or A1R. In this way, it acts as a co-stimulatory molecule which facilitates a variety of specific signalling events in different cell types. The heterogeneous distribution of the enzyme in the nervous system indicates that ectoADA may be a neuroregulatory molecule. On the other hand, ectoADA might act as a bridge between two different cells thus raising the possibility that it may be important for the development of the nervous system.

Probing the functional role of two conserved active site aspartates in mouse adenosine deaminase

Two adjacent aspartates, Asp 295 and Asp 296, playing major roles in the reaction catalyzed by mouse adenosine deaminase (mADA) were altered using site-directed mutagenesis. These mutants were expressed and purified from an ADA-deficient bacterial strain and characterized. Circular dichroism spectroscopy shows the mutants to have unperturbed secondary structure. Their zinc content compares well to that of wild-type enzyme. Changing Asp 295 to a glutamate decreases the kcat but does not alter the Km for adenosine, confirming the importance of this residue in the catalytic process and its minimal role in substrate binding. The crystal structure of the D295E mutant reveals a displacement of the catalytic water from the active site due to the longer glutamate side chain, resulting in the mutant's inability to turn over the substrate. In contrast, Asp 296 mutants exhibit markedly increased Km values, establishing this residue's critical role in substrate binding. The Asp 296->Ala mutation causes a 70-fold increase in the Km for adenosine and retains 0.001% of the wild-type kcat/Km value, whereas the ASP 296->Asn mutant has a 10-fold higher Km and retains 1% of the wild-type kcat/Km value. The structure of the D296A mutant shows that the impaired binding of substrate is caused by the loss of a single hydrogen bond between a carboxylate oxygen and N7 of the purine ring. These results and others discussed below are in agreement with the postulated role of the adjacent aspartates in the catalytic mechanism for mADA.

Enzymatic activities affecting exogenous nicotinamide adenine dinucleotide in human skin fibroblasts

The fate of nicotinamide adenine dinucleotide (NAD), AMP, and ADP-ribose supplied to intact human skin fibroblasts was monitored, and the concentrations of intra- and extracellular pyridine and purine compounds were determined by HPLC analysis. Two enzymatic activities affecting extracellular NAD were detected on the plasma membrane, one hydrolyzing the pyrophosphoric bond and yielding nicotinamide mononucleotide (nucleotide pyrophosphatase) and the other cleaving the glycoside link and releasing nicotinamide (NAD-glycohydrolase). No AMP or ADP-ribose was found in the extracellular medium of cells incubated with NAD, the former being completely catabolized to hypoxanthine and the latter degraded to adenine and hypoxanthine.

A model for adenosine transport and metabolism

1. A model is presented for adenosine transport and metabolism in different steady states. The model considers steady-state equations for metabolic enzymes based on information from the literature on their kinetic behaviour. 2. Assuming that extracellular adenosine and inosine are translocated by three transporters, we have devised rate equations for these nucleoside transporters which are valid when both nucleosides are present. Since the Na(+)-independent transporter can either incorporate nucleosides into the cell or release them, various conditions have been simulated in which inosine was either incorporated or released. 3. Control analyses are reported which show that the fluxes towards intracellular adenine nucleosides are controlled by ecto-5'-nucleotidase in some circumstances and by the nucleoside transporters in others. The nucleoside transporter is responsible for five fluxes (two Na+ dependent adenosine transport mechanisms, a Na(+)-dependent inosine transport, a Na(+)-independent adenosine transport and a Na(+)-independent inosine influx or efflux) but the control is not always positive for all these fluxes. The control patterns of these five fluxes indicate that, in the presence of extracellular adenosine and inosine, the intracellular metabolism of adenine derivatives would be highly dependent on the extracellular and intracellular concentrations of both nucleosides, on the ectoenzymes (5'-nucleotidase and adenosine deaminase) and on the transporter. 4. Predictions of the model were examined. The results indicate that a change in one independent variable (extracellular AMP concentration) makes the system evolve towards a new steady state which is far from the initial one and has a different control pattern. In contrast, simulation of inhibition of the carriers produces only slight modification of the fluxes since the concentrations of the metabolites change to counteract the effect. Thus, for instance, a 50% inhibition of the three carriers does not affect the flux towards intracellular adenine nucleotides. Finally, our model has confirmed that the evolution of the concentration of extracellular adenosine, when an increase in extracellular AMP is produced, agrees with the behaviour expected for a neurohormone.

Purine nucleosides and nucleotides stimulate proliferation of a wide range of cell types

Presumptive astrocytes isolated from 10-day white Leghorn chick embryos, Factor VIII-positive human brain capillary endothelial cells, meningeal fibroblasts from 10-day chick embryos, Swiss mouse 3T3 cells, and human astrocytoma cell lines, SKMG-1 and U373, were rendered quiescent when placed in culture medium that contained 0 or 0.2% serum for 48 h; their proliferation was markedly reduced and they incorporated [3H]thymidine at a low rate. [3H]Thymidine incorporation and cell proliferation were induced in all types of cells by addition of guanosine, GMP, GDP, GTP, and to a lesser extent, adenosine, AMP, ADP or ATP to the culture medium. The stimulation of proliferation by adenosine and guanosine was abolished by 1,3-dipropyl-7-methylxanthine (DPMX), an adenosine A2 receptor antagonist, but not by 1,3-dipropyl-8-(2-amino-4-chorophenyl)xanthine (PACPX), an A1 antagonist. Stimulation of proliferation by the nucleotides was not abolished by either DPMX or PACPX. The P2 receptor agonists, alpha, beta-methyleneATP and 2-methylthioATP, also stimulated [3H]thymidine incorporation into the cells with peak activity at approximately 3.5 and 0.03 nM, respectively. These data imply that adenosine and guanosine stimulate proliferation of these cell types through activation of an adenosine A2 receptor, and the stimulation of cell proliferation by the nucleotides may be due to the activation of purinergic P2y receptors. As the primary cultures grew older their growth rate slowed. The capacity of the purine nucleosides and nucleotides to stimulate their growth diminished concomitantly. The 3T3 cells showed neither decreased growth with increased passages nor reduced response to the purines. In contrast, although the doubling time of the immortalized human astrocytoma cell lines SKMG-1 and U373 remained constant, the responsiveness to purinergic stimulation of the U373 cells decreased but that of the SKMG-1 did not. These data are compatible with a decrease in the number, or the ligand-binding affinity of the purinergic receptors, or a decreased coupling of purinergic receptors to intracellular mediators in primary cells aged in tissue culture.

Purinergic stimulation of cell division and differentiation: mechanisms and pharmacological implications

Extracellular purine nucleosides and nucleotides in micromolar concentrations stimulate proliferation of a variety of cell types in vitro and in vivo. As well they act synergistically with NGF to stimulate neurite outgrowth from PC12 cells. A variety of purine nucleosides and deoxyribonucleosides promote cell proliferation and increase intracellular cAMP. Their activities are inhibited by adenosine A2 receptor antagonists. Only adenosine interacts with the A2 receptor. We propose that the other nucleosides and deoxyribonucleosides inhibit extracellular adenosine deaminase, thereby increasing the extracellular concentration of adenosine. The nucleotides apparently act by stimulating P2y receptors coupled to inositol phosphate metabolism. We propose that the nucleosides and nucleotides act synergistically with other growth factors because each has distinct but complementary second messenger systems. If our hypotheses are correct, it should prove possible to modulate the growth and morphogenesis in several cell types using drugs that inhibit or stimulate adenosine A2 or purine P2y receptor agonists or the second messenger systems coupled to these receptors.

Extracellular levels of adenosine and its metabolites in the striatum of awake rats: inhibition of uptake and metabolism

The level of purines in the striatum of awake, freely moving rats was studied using microdialysis. The calculated extracellular concentration of adenosine and its metabolites inosine and hypoxanthine was very high immediately after implantation of the dialysis probe but decreased within 24 h to a level which remained stable for two days. Using in vitro calibration to determine the relative recovery of the dialysis probes we estimated resting levels in the striatal extracellular space to be 40, 110 and 580 nM, respectively. Inhibition of adenosine deaminase by deoxycoformycin produced a significant 1.4-fold increase in extracellular adenosine levels and a fall in inosine and hypoxanthine. A combination of three uptake blockers (dipyridamole, lidoflazine and nitrobenzylthioinosine), caused a 4.5-fold increase in extracellular adenosine levels without any change in inosine or hypoxanthine levels. After uptake inhibition deoxycoformycin did not have any significant effect. The present results show that the microdialysis technique can be used to determine levels of purines in the extracellular fluid of defined brain regions in awake animals. The high levels recorded during the first several hours after implantation may be artefactually high and reflect trauma. The results also show that adenosine levels can be altered in vivo by inhibitors of adenosine transport and adenosine deaminase. The present results indicate that the physiological adenosine level in striatal extracellular space is in the range 40-460 nM.

Adenosine deaminase in the adductor muscle of the scallop, Patinopecten yessoensis

1. The scallop enzyme was separated by DE52 ion-exchange chromatography into two forms with the same mol. wt of 38,000 and similar characteristics. 2. The enzyme was inactivated in the absence of dithiothreitol and complete reactivation was achieved by adding the agent within a critical storage period. 3. The apparent values of pKm and Vmax sensitively increased as ionic strength was raised to 250 mM and phosphate and sodium ions elevated the former value with a further increase of the ionic strength. 4. The apparent activation energies for the alpha (Vmax/Km) and beta (Vmax) parameters of both the forms were approximately 5 and 8 kcal/mol, respectively. 5. The enzyme deaminated 2'-, 3'-deoxyadenosine and 2',3'-isopropylidene adenosine but did not deaminate 5'-deoxyadenosine, alpha-adenosine and adenine nucleotides. 6. The affinity for inosine was much lowered with a high Ki value. Adenine and purine riboside inhibited the enzyme completely, and coformycin was a tight, slow binding inhibitor.

Slight differences between adenosine deaminases from different species an immunochemical study

IgGs against adenosine deaminase from rat brain, rat liver, mouse duodenum and human erythrocyte were purified from rabbit antisera with yields of 82-87%. The inhibition of adenosine deaminase by the antienzyme is studied, and it is demonstrated that rat and mouse antibodies are tight-binding inhibitors. These antibodies inhibit either the rat or the mouse enzymes and do not inhibit the human erythrocytes enzyme. The human antibody does not inhibit either the human or the rat or mouse enzyme. These results indicate that some differences in antigenic behaviour near the active site must be encountered among species. Comparing the sequenced of the two products corresponding to two adenosine deaminase genes recently sequenced (human and murine) a hypothesis concerning the localization of the adenosine deaminase active site is proposed.

Is adenosine deaminase involved in adenosine transport?

The hypothesis that adenosine metabolizing enzymes may have a key role in the transport of adenosine is discussed. The enhancement of adenosine transport by inhibitors of adenosine deaminase (the enzyme which deaminates adenosine to inosine) and the ecto-localization of adenosine deaminase suggest a contribution of the enzyme in taking up nucleosides. Two possible mechanisms are suggested: 1) transport and deamination of adenosine as a coupled process, or 2) uptake of inosine after cleavage of adenosine by ecto-adenosine deaminase. In both cases, the so-called adenosine deaminase binding protein which is a membrane protein could be the real nucleoside transporter. This behaviour of adenosine deaminase as an ectoenzyme anchored to a membrane protein remembers the behaviour of periplasmic binding proteins of bacteria. Thus, adenosine deaminase as well as, for instance, adenosine kinase would be a kind of 'periplasmic proteins' of eukaryotic cells. The function of adenosine deaminase and adenosine kinase would then be to take adenosine and give it to the true transporters.

A quantum chemical study of the enzymatic deamination of benzoadenine derivatives. A theoretical model of the interactions occurring between nucleosides and the active site of adenosine deaminase

A theoretical study is presented, where, by using both ab initio and semi-empirical methodologies, the properties of benzoadenine derivatives as substrates of adenosine deaminase are discussed. The results suggest that lin-benzoadenine and lin-benzoadenosine can be recognized with an affinity similar to that of adenosine, but only if they are introduced about 0.12 nm deeper inside the active site of the enzyme than the natural substrate adenosine. This fact implies the existence of non-linear hydrogen bonds inside the active site of adenosine deaminase. Ab initio molecular electrostatic potential values suggest that these hydrogen bonds can exist, and have stability similar to that of linear hydrogen bonds. Finally, the great rate of deamination of lin-benzoadenine, comparable with that of adenosine despite the absence of the ribose, is explained in the context of the hypothesis that the protonation at the N1 atom is the rate-determining step of the whole deamination reaction.

Quantum chemical study of the electronic and conformational characteristics of adenosine and 8-substituted derivatives: functional implications in the mechanism of reaction of adenosine deaminase

A quantum chemical study of 10 substrates of adenosine deaminase is performed. The conformational preference around the glycosidic bond of several 8-substituted derivatives of adenosine is studied using semiempirical modified neglect of diatomic overlap (MNDO) and Austin model 1 (AM1) methods. All the compounds studied show preference for the anti conformation; the syn - anti energetic differences calculated are small and in excellent agreement with experimental data. A relationship between the ab initio molecular electrostatic potential minimum energy of N3 and the syn - anti energetic difference is found. A highly significant relationship is also found between the ab initio net charge over the purine and pyrimidine rings and the logarithm of the maximum rate of deamination (log Vm) of the nucleosides by adenosine deaminase. In contrast, no significant relationship is found between the anti preference of 8-substituted derivatives of adenosine and their log Vm of deamination.