Purification and properties of adenosine kinase from rat brain

Adenosine and Ketogenic Treatments

It is well known that the neuromodulator adenosine, acting through the adenosine A₁ receptor subtype, can limit or stop seizures. In 2008, adenosine was proposed as a key component of the anticonvulsant mechanism of the ketogenic diet (KD), a very low carbohydrate diet that can be highly effective in drug-refractory epilepsy. In this study, we review the accumulated data on the intersection among adenosine, ketosis, and anticonvulsant/antiepileptogenic effects. In several rodent models of epilepsy and seizures, antiseizure effects of ketogenic treatments (the KD itself, exogenous ketone bodies, medium-chain triglycerides or fatty acids) are reversed by administration of an adenosine A₁ receptor antagonist. In addition, KD treatment elevates extracellular adenosine and tissue adenosine content in brain. Efforts to maintain or mimic a ketogenic milieu in brain slices reveal a state of reduced excitability produced by pre- and postsynaptic adenosine A₁ receptor-based effects. Long-lasting seizure reduction may be due to adenosine-based epigenetic effects. In conclusion, there is accumulating evidence for an adenosinergic anticonvulsant action in the ketogenic state. In some cases, the main trigger is mildly but consistently lowered glucose in the brain. More research is needed to investigate the importance of adenosine in the antiepileptogenic and neuroprotective effects of these treatments. Future research may begin to investigate alternative adenosine-promoting strategies to enhance the KD or to find use as treatments themselves.

Activity- and pH-dependent adenosine shifts at the end of a focal seizure in the entorhinal cortex

Adenosine (ADO) is an endogenous modulator of neuronal excitability, with anticonvulsant and neuroprotective effects. It has been proposed that the activity-dependent release of ADO promoted by the extracellular acidification occurring during seizures contributes to seizure termination. To verify this hypothesis, we recorded field potentials, pH and ADO changes measured with enzymatic biosensors during acute focal seizures in the medial entorhinal cortex (mEC) of the isolated guinea-pig brain maintained in vitro. The effect of ADO on seizure-like events (SLEs) induced by GABAa receptor antagonism with bicuculline methiodide (BMI; 50 μM) was assessed by arterial applications of 1 mM ADO. ADO either reduced or prevented epileptiform activity. The A1 receptor antagonist DPCPX (100-500 μM) prolonged BMI-induced seizures and was able to precipitate SLEs in the absence of proconvulsant. Simultaneous recordings of brain activity, extracellular ADO and pH shifts demonstrated that ADO decreases at the onset and progressively rises toward the end of SLEs induced by either BMI or 4-aminopyridine (4AP; 50 μM), reaching maximal values 1-5 min after SLE termination. ADO changes were preceded by a SLE-dependent extracellular acid shift. Both pH acidification and ADO changes were abolished by 22 mM HEPES in the arterial perfusate. In these conditions, SLE duration was prolonged. Our data confirm that ADO plays a role in regulating brain excitability. Its increase depends on seizure-induced acid pH shift and it is maximal after the end of the SLE. These findings strongly suggest that ADO contributes to termination of focal seizures and to the establishment of the postictal depression.

S-Adenosyl-l-Methionine Salvage Impacts Psilocybin Formation in "Magic" Mushrooms

Psychotropic Psilocybe mushrooms biosynthesize their principal natural product psilocybin in five steps, among them a phosphotransfer and two methyltransfer reactions, which consume one equivalent of 5'-adenosine triphosphate (ATP) and two equivalents of S-adenosyl-l-methionine (SAM). This short but co-substrate-intensive pathway requires nucleoside cofactor salvage to maintain high psilocybin production rates. We characterized the adenosine kinase (AdoK) and S-adenosyl-l-homocysteine (SAH) hydrolase (SahH) of Psilocybe cubensis. Both enzymes are directly or indirectly involved in regenerating SAM. qRT-PCR expression analysis revealed an induced expression of the genes in the fungal primordia and carpophores. A one-pot in vitro reaction with the N-methyltransferase PsiM of the psilocybin pathway demonstrates a concerted action with SahH to facilitate biosynthesis by removal of accumulating SAH.

Neuromodulation and metamodulation by adenosine: Impact and subtleties upon synaptic plasticity regulation

Synaptic plasticity mechanisms, i.e. the sequence of events that underlies persistent changes in synaptic strength as a consequence of transient alteration in neuronal firing, are greatly influenced by the 'chemical atmosphere' of the synapses, that is to say by the presence of molecules at the synaptic cleft able to fine-tune the activity of other molecules more directly related to plasticity. One of those fine tuners is adenosine, known for a long time as an ubiquitous neuromodulator and metamodulator and recognized early as influencing synaptic plasticity. In this review we will refer to the mechanisms that adenosine can use to affect plasticity, emphasizing aspects of the neurobiology of adenosine relevant to its ability to control synaptic functioning. This article is part of a Special Issue entitled Brain and Memory.

Adenosine, glutamate and pH: interactions and implications

Adenosine's role in the nervous system is multifaceted. As the core molecule of adenosine triphosphate (ATP), adenosine exists in equilibrium with the adenine nucleotide pool and contributes to cellular energy charge, a quantification of relative amounts of available ATP, ADP, AMP and adenosine. Beyond participating in overall energy balance and thus in maintaining cellular homeostasis, adenosine critically influences dynamic signaling in the nervous system. In particular, adenosine has an effect on and is affected by excitatory synaptic transmission. This report describes the ubiquitous nature of adenosine's influence, outlines specific scenarios of clinical import and highlights emerging knowledge about the regulation of adenosine.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

Novel metabolic aspects related to adenosine deaminase inhibition in a human astrocytoma cell line

Adenosine deaminase, which catalyzes the deamination of adenosine and deoxyadenosine, plays a central role in purine metabolism. Indeed, its deficiency is associated with severe immunodeficiency and abnormalities in the functioning of many organs, including nervous system. We have mimicked an adenosine deaminase-deficient situation by incubating a human astrocytoma cell line in the presence of deoxycoformycin, a strong adenosine deaminase inhibitor, and deoxyadenosine, which accumulates in vivo when the enzyme is deficient, and have monitored the effect of the combination on cell viability, mitochondrial functions, and other metabolic features. Astrocytomas are the most common neoplastic transformations occurring in glial cell types, often characterized by a poor prognosis. Our experimental approach may provide evidence both for the response to a treatment affecting purine metabolism of a tumor reported to be particularly resistant to chemotherapeutic approaches and for the understanding of the molecular basis of neurological manifestations related to errors in purine metabolism. Cells incubated in the presence of the combination, but not those incubated with deoxyadenosine or deoxycoformycin alone, underwent apoptotic death, which appears to proceed through a mitochondrial pathway, since release of cytochrome c has been observed. The inhibition of adenosine deaminase increases both mitochondrial reactive oxygen species level and mitochondrial mass. A surprising effect of the combination is the significant reduction in lactate production, suggestive of a reduced glycolytic capacity, not ascribable to alterations in NAD⁺/NADH ratio, nor to a consumption of inorganic phosphate. This is a hitherto unknown effect presenting early during the incubation with deoxyadenosine and deoxycoformycin, which precedes their effect on cell viability.

Ischaemic preconditioning is related to decreasing levels of extracellular adenosine that may be metabolically useful in the at-risk myocardium: an experimental study in the pig

AIM

'Pre-treatment' with short repetitive periods of ischaemia (ischaemic preconditioning) has proved to be a powerful mechanism for modification of the extent of myocardial damage following acute coronary artery occlusion. The exact mechanism of protection induced by ischaemic preconditioning is not known. We herewith put forward a contributing component for protection with preconditioning involving a shift in the adenylate kinase (AK) equilibrium reaction in favour of adenosine triphosphate (ATP) formation.

METHODS

A coronary artery was occluded in anaesthetized thoracotomized pigs to induce ischaemic preconditioning as well as a longer period of ischaemia. Microdialysis probes were inserted in ischaemic and control myocardium and were infused with (14)C- adenosine with two different specific activities. (14)C-lactate was identified and measured in the effluent.

RESULTS

(14)C-adenosine was taken up by non-preconditioned and preconditioned myocardium during ischaemia. Significantly increased levels of (14)C-lactate were recovered in preconditioned myocardium. (14)C-adenosine with high specific activity resulted in a specific activity of lactate that was 2.7 times higher than that of lactate after administration of (14)C-adenosine with low specific activity. Mass spectrography verified the identity of (14)C-lactate.

CONCLUSIONS

Preconditioning up-regulates a new metabolic pathway (starting with 5'-nucleotidase and ending up with lactate) resulting in ATP formation in the micromolar range on top of another effect terminating in a useful shift in the AK equilibrium reaction in favour of ATP generation in the millimolar range. Although the up-regulation of the purine nucleoside phosphorylase pathway is clearly demonstrated, its biological relevance remains to be proved.

Adenosine kinase from rat liver: new biochemical properties

Adenosine kinase is a well-known enzyme which catalyzes the phosphorylation of adenosine to AMP: Its metabolic and kinetic properties are well studied. Here, we report new properties of rat liver enzyme, demonstrating a new reaction: ADP can be a phosphate donor instead ATP, according to the reaction: adenosine + ADP --> 2AMP) demonstrating the efficiency of AdK to phosphorylate adenosine, also starting from ADP. Cells could exploited this property in situations in which ATP levels are strongly decreased and ADP decreases slowly.

Involvement of adenosine in depression of synaptic transmission during hypercapnia in isolated spinal cord of neonatal rats

Adenosine is one of the most important neuromodulators in the CNS, both under physiological and pathological conditions. In the isolated spinal cord of the neonatal rat in vitro, acute hypercapnic acidosis (20% CO2, pH 6.7) reversibly depressed electrically evoked spinal reflex potentials. This depression was partially reversed by 8-cyclopentlyl-1,3-dimethylxanthine (CPT), a selective A1 adenosine receptor antagonist. Isohydric hypercapnia (20% CO2, pH 7.3), but not isocapnic acidosis (5% CO2, pH 6.7), depressed the reflex potentials, which were also reversed by CPT. An ecto-5'-nucleotidase inhibitor did not affect the hypercapnic acidosis-evoked depression. An inhibitor of adenosine kinase, but not deaminase, mimicked the inhibitory effect of hypercapnic acidosis on the spinal reflex potentials. Accumulation of extracellular adenosine and inhibition of adenosine kinase activity were caused by hypercapnic acidosis and isohydric hypercapnia, but not isohydric acidosis. These results indicate that the activation of adenosine A1 receptors is involved in the hypercapnia-evoked depression of reflex potentials in the isolated spinal cord of the neonatal rat. The inhibition of adenosine kinase activity is suggested to cause the accumulation of extracellular adenosine during hypercapnia.

Evidence for the involvement of cytosolic 5'-nucleotidase (cN-II) in the synthesis of guanine nucleotides from xanthosine

In this paper, we show that in vitro xanthosine does not enter any of the pathways known to salvage the other three main natural purine nucleosides: guanosine; inosine; and adenosine. In rat brain extracts and in intact LoVo cells, xanthosine is salvaged to XMP via the phosphotransferase activity of cytosolic 5'-nucleotidase. IMP is the preferred phosphate donor (IMP + xanthosine --> XMP + inosine). XMP is not further phosphorylated. However, in the presence of glutamine, it is readily converted to guanyl compounds. Thus, phosphorylation of xanthosine by cytosolic 5'-nucleotidase circumvents the activity of IMP dehydrogenase, a rate-limiting enzyme, catalyzing the NAD(+)-dependent conversion of IMP to XMP at the branch point of de novo nucleotide synthesis, thus leading to the generation of guanine nucleotides. Mycophenolic acid, an inhibitor of IMP dehydrogenase, inhibits the guanyl compound synthesis via the IMP dehydrogenase pathway but has no effect on the cytosolic 5'-nucleotidase pathway of guanine nucleotides synthesis. We propose that the latter pathway might contribute to the reversal of the in vitro antiproliferative effect exerted by IMP dehydrogenase inhibitors routinely seen with repletion of the guanine nucleotide pools.

Metabolism of adenosine in human colorectal tumour

The aim of this work is to analyse the activities of the enzymes metabolising adenosine in fragments of neoplastic and normal-appearing mucosa, surrounding the tumour in 20 patients affected by colorectal cancer. The results show that the activities of the enzymes are markedly higher in tumour in comparison to normal mucosa to coope with the accelerated purine metabolism in cancerous tissues.

Enzyme activities controlling adenosine levels in normal and neoplastic tissues

Adenosine is known to be associated with effects such as inhibition of immune response, coronary vasodilation, stimulation of angiogenesis, and inhibition of inflammatory reactions. Some authors suggest that adenosine may also have similar functions in tumor tissues. Tissue levels of adenosine are under close regulation by different enzymes acting at different levels. Adenosine is produced from AMP by the action of 5'-nucleotidase (5'-NT) and is converted back into AMP by adenosine kinase (AK) or into inosine by adenosine deaminase (ADA). Inosine is converted into purine catabolites by purine nucleoside phosphorylase (PNP), whereas AMP is converted into ADP and ATP by adenylate kinase (MK). The aim of this study was to analyze the activities of the above enzymes in fragments of neoplastic and apparently normal mucosa, obtained less than 5 cm and at least 10 cm from tumors, in 40 patients with colorectal cancer. The results showed much higher activities of ADA, AK, 5'-NT, and PNP in tumor tissue than in neighboring mucosa (p > 0.01 for ADA, AK, and PNP; p > 0.05 for 5'-NT), suggesting that the activities of purine metabolizing enzymes increase to cope with accelerated purine metabolism in cancerous tissue. The simultaneous increase in ADA and 5'-NT activities might be a physiological attempt by cancer cells to provide more substrate to accelerate salvage pathway activity.

Discovery and optimization of 2-aryl oxazolo-pyrimidines as adenosine kinase inhibitors using liquid phase parallel synthesis

Adenosine kinase inhibition is an attractive therapeutic approach for several conditions for example, neurodegeneration, seizures, ischemia, inflammation and pain. Several nucleosidic and non-nucleosidic inhibitors are available. Using a virtual screening approach, we have discovered that 2-aryl oxazolo-pyrimidines are adenosine kinase inhibitors. Subsequent high throughput derivatization enabled the optimization of this new inhibitor chemotype resulting in highly potent derivatives. A variety of analogues were produced by applying liquid phase parallel synthesis to vary the 7-amino residues as well as the 2-aryl moiety.

Adenosine kinase and 5'-nucleotidase activity after prolonged wakefulness in the cortex and the basal forebrain of rat

The effect of prolonged wakefulness on adenosine kinase (AK), ecto-5'-nucleotidase and endo-5'-nucleotidase activity was assessed in the present study. Rats were sleep deprived for 3 or 6h, and one group was allowed to sleep 2h of recovery sleep after the 6h deprivation. The cortex and the basal forebrain were dissected, and frozen rapidly on dry ice. The enzyme activity of adenosine kinase was measured by monitoring the conversion of [2-3H]-adenosine into [3H]-adenosine monophosphate (AMP) and the ecto-5'-nucleotidase and endo-5'-nucleotidase activities by monitoring the conversion of [2-3H]-AMP into [3H]-adenosine. The enzyme activities did not change during deprivation or recovery sleep in either cortex or basal forebrain when compared to unhandled controls. Significant diurnal variation in enzyme activities was noted in both brain areas. In the basal forebrain adenosine kinase and both nucleotidases showed their lowest activity in the middle of the rest phase, 6h after lights on, suggesting a low level of adenosine metabolism, both production and degradation at this time point. In the cortex adenosine kinase had a diurnal activity pattern similar to the basal forebrain and the ecto-5'-nucleotidase activity was low already early in the rest phase, 3h after lights on, and remained low until the end part of the rest phase, 8h after lights on. Endo-5'-nucleotidase lacked diurnal variation. These activity patterns may be associated with the lower level of energy metabolism during sleep compared to wakefulness.

Intracellular phosphorylation of carbocyclic 3-deazaadenosine, an anti-Ebola virus agent

Carbocyclic 3-deazaadenosine (C-c3Ado) is a potent inhibitor of Ebola virus in mice by infrequent dosing, even though its half life in plasma is only 23-28 min. This prompted studies to determine whether C-c3Ado undergoes intracellular metabolism to derivatives that may promote in vivo activity. In cells, radiolabelled compound readily underwent metabolism to monophosphate, diphosphate and triphosphate (C-c3ATP) forms, with C-c3ATP being the major metabolite detected. A non-polar metabolite was also detected both inside and outside treated cells. The retention time of C-c3ATP was similar but not identical to ATP on a strong anion exchange high performance liquid chromatography (HPLC) column or on a DEAE-Sephadex open column. C-c3ATP and ATP were susceptible to degradation to their respective nucleosides by bovine alkaline phosphatase. Intracellular formation of C-c3ATP reached a plateau by about 4 h after treatment of monkey (Vero 76) and mouse (Balb/3T3 clone A31) cells with 10 or 100 microM extracellular compound. Phosphorylation was linearly dose responsive at 1, 3 and 10 microM. However, the extent of phosphorylation decreased with increasingly higher concentrations (30, 100 and 300 microM). When compound was removed from the medium, the nucleoside cleared the cells within 1 min, whereas C-c3ATP had a half life of decay of 2-3 h in five cell lines. Phosphorylation of C-c3Ado to C-c3ATP was not inhibited by cotreatment of cells (at a 20:1 ratio) with adenosine, guanosine, inosine, xanthosine, cytidine or uridine. There was no evidence of incorporation of C-c3Ado (10 microM) into macromolecules of cells over 72 h, whereas adenosine was readily incorporated. C-c3ATP may represent a form of C-c3Ado that might contribute to extending its intracellular half life or otherwise exhibit antiviral activity and/or toxicity.

Metabolic fate of AMP, IMP, GMP and XMP in the cytosol of rat brain: an experimental and theoretical analysis

A systematic study of the metabolic fate of AMP, IMP, GMP and XMP (NMP) in the presence of cytosol from rat brain is here presented; the kinetics of both disappearance of NMP, and appearance of their degradation products was followed by HPLC. In the absence of ATP, AMP was preferentially degraded to adenosine with concomitant appearance of inosine and hypoxanthine. In the presence of ATP, AMP was preferentially degraded via IMP. The nucleosides generated in the course of the reactions are further degraded, almost exclusively, via nucleoside phosphorylase using as cofactor the P(i) generated in the reaction mixture. In order to quantify the effect of each one of the enzymes involved in the degradation of NMP, two complementary approaches were followed: (i) the V:(max) and K:(m) values of the enzymes acting in the intermediate steps of the reactions were determined; (ii) these data were introduced into differential equations describing the concentration of the nucleotides and their degradation products as a function of the time of incubation. Factors affecting kinetic parameters of the equation velocity as a function of ATP concentration were introduced when required. The differential equations were solved with the help of Mathematica 3.0. The theoretical method can be used to simulate situations not feasible to be carried out, such as to measure the influence of nM-microM concentrations of ATP on the metabolism of AMP.

Crystal structure of adenosine kinase from Toxoplasma gondii at 1.8 A resolution

Human infection with Toxoplasma gondii is an important cause of morbidity and mortality. Protozoan parasites such as T. gondii are incapable of de novo purine biosynthesis and must acquire purines from their host, so the purine salvage pathway offers a number of potential targets for antiparasitic chemotherapy. In T. gondii tachyzoites, adenosine is the predominantly salvaged purine nucleoside, and thus adenosine kinase is a key enzyme in the purine salvage pathway of this parasite. The structure of T. gondii adenosine kinase was solved using molecular replacement and refined by simulated annealing at 1.8 A resolution to an R-factor of 0.214. The overall structure and the active site geometry are similar to human adenosine kinase, although there are significant differences. The T. gondii adenosine kinase has several unique features compared to the human sequence, including a five-residue deletion in one of the four linking segments between the two domains, which is probably responsible for a major change in the orientation of the two domains with respect to each other. These structural differences suggest the possibility of developing specific inhibitors of the parasitic enzyme.

Inhibition of adenosine kinase during oxygen-glucose deprivation in rat cortical neuronal cultures

Adenosine kinase (AK) inhibitors potentiate the actions of endogenous adenosine (ADO) and ameliorate cerebral ischemic damage in animal models. The present study examined the effects of the AK inhibitor, 5-iodotubercidin (5-IT) in an in vitro model of neuronal ischemia, specifically, combined oxygen-glucose deprivation of rat cortical mixed neuronal-glial cultures. Oxygen-glucose deprivation caused extensive neuronal loss which was accompanied by a marked increase in ADO release into the extracellular medium, was ameliorated by exogenous ADO (10 microM(-1) mM), and was exacerbated by a high concentration of the selective A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT; 10 microM). 5-IT (1 microM) had no effect on extracellular ADO levels nor on neuronal loss. However, AK activity in these cultures was markedly suppressed during oxygen-glucose deprivation. Taken together, these data demonstrate a marked down-regulation of AK activity during oxygen-glucose deprivation in this in vitro model, providing an endogenous mechanism contributing to the accumulation of extracellular ADO, which exerts neuroprotective effects by activating the ADO A1 receptor.

Isolation, cloning and characterization of a low-molecular-mass purine nucleoside- and nucleotide-binding protein

A purine nucleoside- and nucleotide-binding protein has been isolated from extracts of rat and rabbit heart, calf aortic smooth muscle and rat liver using an affinity column containing adenosine bound through the N6-position. The protein, which was eluted by adenosine, was cloned and expressed in Escherichia coli. The deduced amino acid sequence has a calculated Mr of 13693 (p13.7). The expressed protein has properties identical with the protein isolated from heart and liver, including an anomalous, apparent Mr of 15300, observed on gel electrophoresis. Gel filtration shows it to be a dimer. p13.7 differs by only three amino acids out of 125 from protein kinase C inhibitor 1 [Pearson, DeWald, Mathews, Mozier, Zürcher-Neely, Heinrikson, Morris, McCubbin, McDonald, Fraser et al. (1990) J. Biol. Chem. 265, 4583-4591]. However, we have not been able to demonstrate inhibition of protein kinase C by physiological concentrations of p13.7, regardless of whether it was isolated from tissue extracts or expressed in E. coli. p13.7 is a member of the histidine triad motif family of proteins [Séraphin (1992) J. DNA Sequencing Mapping 3, 177-179]. The affinity of p13.7 for a number of different purine nucleosides and nucleotides, as measured by fluorescence titration and gel filtration, falls within the range 5-50 microM. On the basis of these properties and its crystal structure [Brenner, Garrison, Gilmour, Peisach, Ringe, Petsko and Lowenstein (1997) Nature Struct. Biol. 4, 231-238], we have coined the acronym HINT (histidine triad nucleotide-binding motif) to describe the family of proteins of which p13.7 is a member. Other proteins that bind to the affinity column have been identified as malate and lactate dehydrogenases, cAMP-binding proteins, adenosine kinase and S-adenosylhomocysteine hydrolase.

Diadenosine polyphosphates inhibit adenosine kinase activity but decrease levels of endogenous adenosine in rat brain

Findings in peripheral tissues that diadenosine polyphosphates (Ap(n)As) activate 5'-nucleotidase activity and inhibit adenosine kinase activity in vitro led us to test the hypothesis that Ap(n)As and analogues thereof, through such actions on purine enzymes, increase brain levels of endogenous adenosine in vivo. Accordingly, we tested Ap(n)As for their effects on the in vitro activities of adenosine kinase, adenosine deaminase, AMP deaminase and 5'-nucleotidase and, following unilateral microinjections in rat striatum, on in vivo levels of endogenous adenosine. Adenosine kinase activity was not affected significantly by 5',5'''-P1,P2-diadenosine pyrophosphate (Ap2A) or by 5',5'''-P1,P3-diadenosine triphosphate (Ap3A), but was inhibited by 5',5'''-P1,P4-diadenosine tetraphosphate (Ap4A), 5',5'''-P1,P5-diadenosine pentaphosphate (Ap5A) and 5',5'''-P1,P6-diadenosine hexaphosphate (Ap6A); apparent IC50 values were 5.0, 3.3 and 500 microM, respectively. Inhibition of adenosine kinase activity by Ap4A and the four metabolically stable analogues of Ap4A tested was uncompetitive. Following unilateral intrastriatal injections, adenosine levels, relative to uninjected contralateral striatum, were decreased significantly (P < 0.05) by 48% with Ap4A and by 37% with AppCH2ppA, a metabolically stable analogue of Ap4A. Striatal levels of adenosine were not affected significantly by Ap5A or Ap6A. Cytosolic, but not particulate 5'-nucleotidase activity was inhibited and AMP deaminase activity was increased by some Ap(n)As. Although adenosine kinase inhibitors increase levels of endogenous adenosine and we showed here that Ap(n)As were potent inhibitors of this enzyme, these particular actions of Ap(n)As were not consistent with their effects on levels of endogenous adenosine.

Cloning and expression of the adenosine kinase gene from rat and human tissues

Adenosine kinase is ubiquitous in eukaryotes and is a key enzyme in the regulation of the intracellular levels of adenosine, an important physiological effector of many cells and tissues. In this paper we report the cloning of cDNAs encoding adenosine kinase from both rat and human tissues. Two distinct forms of adenosine kinase mRNA were identified in human tissues. Sequence variation between the two forms is restricted to the extreme 5'-end of the adenosine kinase mRNA, including a portion of the coding region, and is consistent with differential splicing of a single transcriptional product. We have expressed both forms in E. coli and produced soluble active enzyme which catalyzes the phosphorylation of adenosine with high specific activity in vitro and is susceptible to known adenosine kinase inhibitors.

Production of adenosine and nucleoside analogs by the exchange reaction catalyzed by rat liver adenosine kinase

We have previously shown [8] that rat liver adenosine kinase can produce [14C]AMP from [14C]adenosine (Ado) and unlabelled adenosine monophosphate (AMP), in the absence of ATP, by an exchange reaction. In this study, we investigated whether Ado or AMP could be replaced in this exchange reaction by other nucleosides or nucleoside monophosphates (NMP), respectively. In the presence of 1 mM of the unlabelled NMP analogs 7-deazaadenosine (tubercidin) 5'-monophosphate, 6-chloropurine riboside 5'-monophosphate, or N6-methyl-AMP, [14C]AMP was formed from 20 microM [14C]Ado at up to 50% of the rate recorded with 1 mM unlabelled AMP. In the presence of 0.2 mM of the unlabelled analog nucleosides tubercidin, N6-methyladenosine, or 6-methylmercaptopurine riboside, [14C]Ado was generated from 1 mM [14C]AMP at up to 60% of the rate recorded with 0.2 mM unlabeled Ado. Small amounts of [14C]Ado were also formed from the natural nucleosides 5-amino-4-imidazolecarboxamide (AICA) riboside or 2'-deoxyadenosine. Administration of therapeutic anticancer and antiviral nucleosides that can serve as substrates for the exchange reaction catalyzed by adenosine kinase might, thus, result in a net production of Ado, a potent autacoid with physiological effects in numerous tissues.

Intrinsic tryptophan fluorescence of bovine liver adenosine kinase, characterization of ligand binding sites and conformational changes

Bovine liver adenosine kinase is a 45-kDa monomeric protein which exhibits a characteristic intrinsic tryptophan fluorescence with a maximal excitation at 284 nm and an emission peak centered at 335 nm. A total of three tryptophan residues/molecule has been estimated by using a fluorescence titration method. Low values of Stern-Volmer quenching constants in the presence of either acrylamide or iodide (4.2 M-1 or 1.5 M-1, respectively) indicated that the tryptophan residues are relatively buried in the native molecule. Tryptophan residues also showed a high heterogeneity, with a fractional accessible fluorescence value for iodide of 0.65. The enzyme fluorescence was very sensitive to substrate binding, which induced a marked fluorescence quenching, a lower tryptophan accessibility to acrylamide and iodide, and an increase in the tryptophan heterogeneity. ADP or ATP showed a monophasic saturation curve consistent with the existence of one binding site. In contrast, adenosine and AMP gave biphasic saturation curves, suggesting the existence of at least two binding sites, with a high and a low affinity. The presence of MgCl2 increased the affinity of ATP or ADP, whereas the binding of adenosine or AMP was not affected.

2'-Deoxyadenosine metabolism in human and opossum Didelphis virginiana erythrocytes in vitro

1. In erythrocytes of both species, deamination and phosphorylation of dADO was completely inhibited by 5 microM 2'deoxycoformycin and 10 microM 5-ITU respectively. 2. Under physiologic conditions, provided with nanomolar concentrations of dADO there was complete deamination of dADO in human red cells. In opossum erythrocytes deamination and phosphorylation of the deoxynucleoside were nearly; additionally, 1-2% of the substrate was metabolized to AXP. 3. With ADA inhibited in intact red cells, apparent Km and Vmax for dADO were 0.5 vs 0.1 mM and 0.8 vs 6.25 mumol/g hg/hr in human and opossum cells, respectively. 4. In opossum red cell lysates, GTP was superior to ATP and to dATP as a phosphate donor in the dADO kinase reaction.

Phosphorylation of adenosine in anoxic hepatocytes by an exchange reaction catalysed by adenosine kinase

The elevation of adenosine levels induced by anoxia in isolated rat hepatocytes has been shown to result mainly from an arrest of the recycling of the nucleoside by adenosine kinase [Bontemps, Vincent and Van den Berghe (1993) Biochem. J. 290, 671-677]. To assess the activity of the latter enzyme in intact hepatocytes, incorporation of radioactive adenosine into the cells' adenine nucleotides was measured. Unexpectedly, despite the near-absence of ATP in anoxic cells, 40% of 50 microM [8-14C]adenosine was still incorporated into adenylates over 5 min. Moreover, whereas unlabelled and labelled adenosine were utilized in parallel in normoxic cells, uptake of [8-14C]adenosine did not correspond to a net disappearance of adenosine in anoxic cells. Addition of 1 mM unlabelled adenosine to anoxic hepatocytes in which the adenine nucleotides had been prelabelled with [U-14C]adenine induced an immediate loss of their radioactivity. The latter was recovered in the form of adenosine, but the size of the adenylate pool was not modified. Taken together, these results suggest the occurrence of an exchange reaction between AMP and adenosine. Incubation of Sephadex G-25-filtered high-speed supernatants of rat liver with 20 microM [8-14C]adenosine, 10 mM MgCl2 and 1 mM AMP resulted in the labelling of AMP in the total absence of ATP. This labelling was influenced by effectors of both adenosine kinase and cytosolic IMP-GMP 5'-nucleotidase; the latter is known to catalyse an exchange reaction [Worku and Newby (1982) Biochem. J. 205, 503-510]. Chromatography of cytosolic fractions of rat liver on DEAE-Sepharose, followed by Sephacryl S-200 and AMP-Sepharose, demonstrated that the exchange reaction between adenosine and AMP co-purified with adenosine kinase. It is concluded that incorporation of labelled adenosine into adenine nucleotides should not be considered to be proof of adenosine kinase activity in anoxia.

Increases in interstitial adenosine and cerebral blood flow with inhibition of adenosine kinase and adenosine deaminase

The purpose of this study was to determine the changes in interstitial fluid (ISF) adenosine and cerebral blood flow (CBF) during inhibition of adenosine kinase or adenosine deaminase. Brain microdialysis was used to (a) measure CBF (H2 clearance), (b) sample cerebral ISF, and (c) deliver drugs locally to the brain. Microdialysis probes were implanted bilaterally in the caudate nucleus of halothane-anesthetized rats (n = 11). One probe was perfused with artificial cerebrospinal fluid (CSF) containing iodotubercidin (IODO), an adenosine kinase inhibitor, while the other probe was perfused with erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor. Both probes were subsequently perfused with EHNA+IODO. Finally, 8-(p-sulfophenyl)theophylline (SPT), an adenosine receptor antagonist, was added to EHNA + IODO in one probe, while the other probe continued to receive only EHNA + IODO. CBF and dialysate adenosine levels increased with either EHNA or IODO; however, the increases were greater with IODO. EHNA + IODO further increased CBF and dialysate adenosine. The hyperemia observed with EHNA + IODO was abolished by adenosine receptor blockade. These data suggest that basal adenosine levels are influenced to a greater extent by adenosine kinase than by adenosine deaminase. In addition, the increased CBF observed with inhibition of adenosine metabolism and the attenuation of this vasodilatory response with adenosine receptor blockade support a role for adenosine in CBF regulation.

Mathematical modelling of the purine metabolism of the rat liver

The regulation of the purine metabolism of the rat liver is studied on the basis of a mathematical model which comprises rate laws and kinetic constants of all physiologically relevant reactions. The computed stationary and time-dependent concentrations are in good accordance with experimental data obtained in the ischaemic rat liver and in isolated hepatocytes. In particular, model-based simulations of the adenine nucleotide metabolism have been performed for situations where ATP-deficient states of the cell (hypoxia, anoxia or ischaemia) of various length are followed by onset of ATP production (reoxygenation). These simulations confirm the experimentally observed incomplete recovery of ATP and of the total pool of adenine nucleotides within a few hours of reoxygenation after long-term ATP depletion. Therefore, it can be concluded that this phenomenon is an intrinsic regulatory property of the purine metabolism and not necessarily due to some irreversible changes in the activity of the enzymes involved.

Pharmacological characterization of rapidly accumulated adenosine by dissociated brain cells from adult rat

Mechanically dissociated brain cells from adult rats were used to study biochemically and pharmacologically their capacity to accumulate rapidly [3H]adenosine. The assay, which used an inhibitor-stop method to prevent further uptake into cells, was characterized with respect to protein and optimal substrate concentrations, and incubation times that ranged from 5 to 180 s. The accumulation of [3H]adenosine using 15-s incubation periods, conditions under which less than 10% of accumulated [3H]adenosine was metabolized, was best described kinetically by a two-component system with Km and Vmax values for the high-affinity component of 0.8 microM and 6.2 pmol/mg protein/15 s and for the low-affinity component 259 microM and 2,217 pmol/mg protein/15 s, respectively. The potencies with which nucleosides, adenosine deaminase resistant adenosine receptor agonists, and nucleoside uptake inhibitors competed for these uptake components were determined. Of the nucleosides examined, adenosine was the "preferred" substrate for the uptake site. The Ki value of adenosine for the high-affinity component was 10.7 microM. Inosine and uridine competed for a single lower affinity uptake system: Ki values were 142 and 696 microM, respectively. Nucleoside uptake inhibitors--nitrobenzylthioinosine, dipyridamole, and dilazep--were the most potent inhibitors of [3H]adenosine accumulation tested: the Ki values for the high-affinity system were 0.11, 1.3, and 570 nM, respectively. The adenosine analogs S-phenylisopropyladenosine, R-phenylisopropyladenosine, and cyclohexyladenosine inhibited the high-affinity component with Ki values of 2.3, 9.3, and 14.5 microM, respectively. N-Ethylcarboxamidoadenosine competed for a single lower affinity uptake system: Ki, 292 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine transporters in chromaffin cells: subcellular distribution and characterization

Adenosine transporters in freshly isolated and cultured chromaffin cells were quantified by the [3H]dipyridamole binding technique, showing a maximal bound capacity of 0.4 +/- 0.05 pmol/10(6) cells (240,000 +/- 20,000 transporters by cell). Scatchard analysis showed a similar affinity for [3H]dipyridamole in isolated cells and subcellular fractions (Kd = 5 +/- 0.6 nM). For enriched plasma membrane preparations and chromaffin granule membranes, the maximal binding capacities were also very similar, 2.3 +/- 0.3 and 1.8 +/- 0.4 pmol/mg protein, respectively. When [3H]nitrobenzylthioinosine was employed as a radioligand, the maximal bound capacity in cultured chromaffin cells was 0.053 +/- 0.004 pmol/10(6) cells (32,000 +/- 3000 transporters per cell) with a high affinity constant (Kd = 0.25 +/- 0.03 nM); similar values were obtained in all subcellular fractions (Kd = 0.1 +/- 0.01). Also, plasma and chromaffin granule membranes showed similar maximal binding values (0.4 +/- 0.06 pmol/mg protein). Photoincorporation studies with [3H]nitrobenzylthioinosine into plasma membrane polypeptides showed the presence of three molecular species of 115 +/- 10; 58 +/- 6 and 42 +/- 5 kDa. Chromaffin granule membranes showed only the 105 +/- 9 and 51 +/- 4 molecular species.

Effects of reserpine and tetrabenazine on catecholamine and ATP storage in cultured bovine adrenal medullary chromaffin cells

The in vivo storage relationship between catecholamines and ATP in chromaffin vesicles of cultured bovine adrenal medulla cells was investigated using drugs that block vesicular catecholamine uptake. Three-day treatments with reserpine and tetrabenazine causing 85-90% depletion of catecholamines resulted in 41-46% reductions in cellular ATP content. Subcellular fractionation of reserpine-treated cells indicated that the ATP is lost from the chromaffin vesicle pool. This was confirmed in experiments using metabolic inhibitors to differentiate the vesicular and extravesicular ATP pools. The vesicular ATP loss was not proportional to that of catecholamines, resulting in a reduction by 50% in the chromaffin vesicle mole ratio of catecholamines to ATP after 48 h of treatment. In metabolic labeling studies, it was found that reserpine treatment reduced the incorporation of [3H]adenosine into vesicular ATP selectively, but it reduced the incorporation of 32Pi into both the vesicular and extravesicular pools. The reduction of the [3H]adenosine incorporation was not due to diminished vesicular nucleotide uptake resulting from low catecholamine levels, because when the catecholamines were depleted by tetrabenazine pretreatment followed by removal of the drug before labeling, no reduction in [3H]adenosine incorporation was observed. When present during the labeling, tetrabenazine was found to be a reversible inhibitor of plasma membrane adenosine uptake. The observed loss of adenine nucleotides from catecholamine-depleted chromaffin vesicles in vivo provides evidence that interactions between ATP and catecholamines are important in the vesicular storage of high concentration of these compounds.

Metabolism of ribavirin in respiratory syncytial virus-infected and uninfected cells

The metabolism of ribavirin to its mono-, di-, and triphosphate derivatives was examined in uninfected and respiratory syncytial virus-infected cells. The degree of phosphorylation was dose dependent upon extracellular ribavirin concentration. The major species formed was the triphosphate, with mono- and diphosphates being approximately 12 and 4% of the triphosphate, respectively. Amounts of triphosphate formed in infected cells were up to 2.6-fold greater than those in uninfected cells. Upon drug removal, ribavirin triphosphate degradation was very rapid, with decay half-lives of 70 to 100 min. Actinomycin D inhibited triphosphate production and also neutralized the antiviral effect of ribavirin.

Regulation of deoxyadenosine and nucleoside analog phosphorylation by human placental adenosine kinase

The enzymes responsible for phosphorylation of adenosine and nucleoside analogs are important in the pathogenesis of adenosine deaminase deficiency and for the activation of specific anticancer and antiviral drugs. We examined the role of adenosine kinase in catalyzing these reactions using an enzyme purified 4000-fold (2.1 umol/min/mg) from human placenta. The Km values of adenosine and ATP are 135 uM and 4 uM, respectively. Adenosine kinase phosphorylates adenine arabinoside with an apparent Km value of 1 mM using adenosine kinase assay conditions. The Km values for 6-methylmercaptopurine riboside and 5-iodotubercidin, substrates for adenosine kinase, are estimated to be 4.5 uM and 2.6 nM, respectively. These data indicate that dadenosine phosphorylation by adenosine kinase is primarily regulated by its Km, and the concentrations of Mg2+, ADP and AMP. The high Km values for phosphorylation of dadenosine and adenine arabinoside suggest that adenosine kinase may be less likely to phosphorylate these nucleosides in vivo than other enzymes with lower Km values. Adenosine kinase appears to be important for adenosine analog phosphorylation where the Michaelis constant is in the low micromolar range.

Adenosine kinase from bovine adrenal medulla

Adenosine kinase from bovine adrenal medulla was purified 1600-fold by using ammonium sulfate precipitation, gel filtration and affinity chromatography. Gel filtration yielded a relative molecular mass around 42000 and Michaelis constants were 0.2 microM for adenosine and 20 microM for MgATP. The enzyme showed a broad specificity for purine nucleoside triphosphate as phosphate donors. Both free Mg2+ and ATP were inhibitors. AMP was a competitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP, while ADP was a uncompetitive inhibitor with regard to adenosine and a non-competitive inhibitor versus MgATP. Adenosine kinase was strongly inhibited by the bis(adenylyl) polyphosphates Ap4A and Ap5A. These compounds inhibited the enzyme competitively versus MgATP (Ki = 0.06 microM for Ap4A and 0.4 microM for Ap5A) and uncompetitively with regard to adenosine. The results of the kinetic analysis suggest an ordered bi-bi mechanism, adenosine being the first substrate. The phosphorylation of adenosine was unaffected in the presence of vanadate ions.

An analysis of multiple mechanisms of adenosine toxicity in baby hamster kidney cells

Analysis of the response of baby hamster kidney cells to adenosine in the presence of the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl) adenine has revealed two distinct mechanisms of toxicity. The first is apparent at low concentrations of adenosine (less than 5 microM) and is dependent upon the presence of a functional adenosine kinase. The initial toxicity is abolished by uridine, is unrelated to the inhibition of ribonucleotide reductase, and is accompanied by a decrease in the size of the pyrimidine nucleotide pool. Toxicity at higher concentrations of adenosine is adenosine kinase independent and is potentiated by homocysteine thiolactone. An elevation in the intracellular level of S-adenosylhomocysteine, which was observed following treatment with higher concentrations of adenosine (greater than 10 microM), is believed to mediate toxicity at these levels. Interestingly, BHK cells were resistant to intermediate levels of adenosine. The mechanism of resistance is currently unknown, but appears unrelated to a lack of inhibition of adenosine deaminase. It is proposed that substrate inhibition of adenosine kinase may be a determinant of this property.

Purine nucleotide synthesis in adrenal chromaffin cells

The synthesis of purine nucleotides from the salvage precursors adenine and adenosine, and from the de novo precursors formate and glycine, was studied in isolated adrenal chromaffin cells. Both [8-14C]adenine and [8-14C]adenosine from extracellular medium are effectively incorporated into intracellular nucleotides. [14C]Formate and [U-14C]glycine are also incorporated, but de novo synthesis is clearly lower than synthesis from salvage precursors, although similar to de novo synthesis in liver. The enzymes responsible for adenine and adenosine salvage, adenine phosphoribosyltransferase and adenosine kinase, were purified about 1,500-fold. Both enzymes are mainly cytosolic and exhibit a similar molecular weight of around 42,000. The results suggest that chromaffin cells can replenish their intracellular nucleotides lost during the secretory event by an active synthesis from salvage and de novo precursors.

An adenosine kinase mutation in baby hamster kidney cells causing increased sensitivity to adenosine

A class of arabinosyladenine (araA)-resistant mutants of baby hamster kidney (BHK 21/C13) cells exhibits multiple phenotypes: resistance to araA and deoxyadenosine, extreme sensitivity to adenosine (Ado) and varying degrees of deficiency in adenosine kinase (AK) activity. One of these Ados/araAr strains, ara-S10d, was isolated without mutagenesis and was shown to possess about 59% level of the wild-type AK activity. The AK from ara-S10d had an altered Km and pH optimum and was stimulated by K+ cations. A number of Ados to Ador revertants were isolated from ara-S10d, and in all of the 7 examined, the AK activity was reduced to a nondetectable level. The altered kinetic parameters of the AK enzyme in ara-S10d cells suggest a mutation of the AK gene that leads to the synthesis of an altered enzyme. The loss of AK activity in the Ador revertants suggests an association of the enhanced Ado sensitivity to the AK mutation.

Properties of rat heart adenosine kinase

Adenosine kinase was purified 870-fold from rat heart by a combination of gel filtration and affinity chromatography. The preparation was free of purine-metabolizing enzymes that could interfere in the assay of the kinase. A study of the properties of the purified enzyme showed that it is activated by Na+ and K+, it possesses a broad pH optimum between 6 and 8, MgATP is the nucleotide substrate, free Mg2+ is an inhibitor with respect to both MgATP and adenosine, and the enzyme is subject to substrate inhibition by adenosine. The severity of this inhibition increases as the concentration of free Mg2+ increase. The Km for MgATP was calculated to be 0.8 mM and that for adenosine, at likely physiological concentrations of MgATP and free MgCl2, was about 0.2 microM. In vivo the enzyme is likely to be saturated with both MgATP and adenosine. Indeed, the adenosine concentration in rat heart in vivo is probably sufficient to cause substrate inhibition, and this would be increased by an increase in free Mg2+ concentration. Changes in the concentrations of adenosine and free Mg2+ may play a role in modifying the activity of the enzyme in vivo.

Measurement of nucleoside kinases in crude tissue extracts

The measurements of deoxyadenosine kinase, adenosine kinase, and deoxycytidine kinase were examined in human placental cytosol to achieve a valid and reliable assay linear with time and protein. Our studies confirm the need to inhibit deaminase enzymes, since deoxyadenosine and deoxycytidine undergo extensive deamination and phosphorolysis. The use of a uniformly labeled nucleoside substrate introduced an artifact because the chromatographic behavior of the deoxyribose-1-phosphate, formed during the assay, was difficult to distinguish from the deoxynucleoside phosphate product. Accurate product identification was also essential. Finally, the substitution of GTP in place of ATP as the phosphate donor, the addition of a sulfhydryl reducing agent and a monovalent cation need to be considered when an assay is optimized. The use of these methods have lead to valid assays in placental cytosol that are linear with time and protein. Consideration of these important principles are necessary when establishing a valid and reliable nucleoside kinase assay in a crude tissue preparation.

Differences of adenosine kinases from various mammalian tissues

1. Adenosine kinases purified to homogeneity from various mammalian tissues have a monomeric structure, and their molecular weights were estimated around 40,000. 2. The enzyme activity per wet weight of tissue appears to vary from source to source, but the specific activities of the final enzyme preparations were alike, which were 6.3--7.8 mumol/min/mg protein. 3. No or small difference was observed in the kinetic properties among all seven adenosine kinases. 4. Some differences in structure were observed among five liver enzymes from human, rabbit, rat, mouse and Mongolian gerbil, but no difference was observed between the enzymes from human placenta and liver, or the enzymes from rat liver and brain.

Adenosine kinase from human liver

Adenosine kinase (ATP: adenosine 5'-phosphotransferase, EC 2.7.1.20) has been purified to homogeneity from human liver. The yield was 55% of the initial activity with a final specific activity of 6.3 mumol/min per mg protein. The molecular weight was estimated as about 40 000 by Sephadex G-100 gel filtration and polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (SDS). The enzyme catalyzed the phosphorylation of adenosine, deoxyadenosine, arabinoadenosine, inosine and ribavirin. The activity of deoxyadenosine phosphorylation was 18% of that of adenosine. The pH optimum profile was biphasic; a sharp pH optimum at pH 5.5 and a broad optimum at pH 7.5--8.5. The Km value for adenosine was 0.15 micrometer, and the activity was strongly inhibited at higher concentrations than 0.5 micrometer. ATP, dATP, GTP and dGTP were proved to be effective phosphate donors. Co2+ was more effective than Mg2+, and Ca2+, Mn2+, Fe2+ and Ni2+ showed about 50% of the activity for Mg2+. Some difference in structure between the adenosine kinase from human liver and that from rabbit or rat tissue, was observed by amino acid analysis and peptide mapping analysis.