The interaction of inhibitors with adenosine metabolising enzymes in intact isolated cells

A small-molecule inhibitor of Haspin alters the kinetochore functions of Aurora B

A chemical biology study characterizes the role of Haspin kinase in centromere recruitment of the chromosome passenger complex and in spindle assembly checkpoint function.

By phosphorylating Thr3 of histone H3, Haspin promotes centromeric recruitment of the chromosome passenger complex (CPC) during mitosis. Aurora B kinase, a CPC subunit, sustains chromosome bi-orientation and the spindle assembly checkpoint (SAC). Here, we characterize the small molecule 5-iodotubercidin (5-ITu) as a potent Haspin inhibitor. In vitro, 5-ITu potently inhibited Haspin but not Aurora B. Consistently, 5-ITu counteracted the centromeric localization of the CPC without affecting the bulk of Aurora B activity in HeLa cells. Mislocalization of Aurora B correlated with dephosphorylation of CENP-A and Hec1 and SAC override at high nocodazole concentrations. 5-ITu also impaired kinetochore recruitment of Bub1 and BubR1 kinases, and this effect was reversed by concomitant inhibition of phosphatase activity. Forcing localization of Aurora B to centromeres in 5-ITu also restored Bub1 and BubR1 localization but failed to rescue the SAC override. This result suggests that a target of 5-ITu, possibly Haspin itself, may further contribute to SAC signaling downstream of Aurora B.

Structure-activity relationship for adenosine kinase from Mycobacterium tuberculosis II. Modifications to the ribofuranosyl moiety

Adenosine kinase (Ado kinase) from Mycobacterium tuberculosis is structurally and biochemically unique from other known Ado kinases. This purine salvage enzyme catalyzes the first step in the conversion of the adenosine analog, 2-methyl-Ado (methyl-Ado), into a metabolite with antitubercular activity. Methyl-Ado has provided proof of concept that the purine salvage pathway from M. tuberculosis may be utilized for the development of antitubercular compounds with novel mechanisms of action. In order to utilize this enzyme, it is necessary to understand the topography of the active site to rationally design compounds that are more potent and selective substrates for Ado kinase. A previous structure-activity relationship identified modifications to the base moiety of adenosine (Ado) that result in substrate and inhibitor activity. In an extension of that work, 62 Ado analogs with modifications to the ribofuranosyl moiety, modifications to the base and ribofuranosyl moiety, or modifications to the glycosidic bond position have been analyzed as substrates and inhibitors of M. tuberculosis Ado kinase. A subset of these compounds was further analyzed in human Ado kinase for the sake of comparison. Although no modifications to the ribose moiety resulted in compounds as active as Ado, the best substrates identified were carbocyclic-Ado, 8-aza-carbocyclic-Ado, and 9-[alpha-l-lyxofuranosyl]-adenine with 38%, 4.3%, and 3.8% of the activity of Ado, respectively. The most potent inhibitor identified, 5'-amino-5'-deoxy-Ado, had a K(i)=0.8muM and a competitive mode of inhibition. MIC studies demonstrated that poor substrates could still have potent antitubercular activity.

Structure-activity relationship for nucleoside analogs as inhibitors or substrates of adenosine kinase from Mycobacterium tuberculosis. I. Modifications to the adenine moiety

Adenosine kinase (Ado kinase, EC 2.7.1.20) is a purine salvage enzyme that phosphorylates adenosine (Ado) to AMP. Ado kinase from Mycobacterium tuberculosis also catalyzes an essential step in the conversion of 2-methyl-Ado to a compound with selective antimycobacterial activity. In order to aid in the design of more potent and selective Ado analogs, eighty nucleoside analogs with modifications to the adenine (Ade) moiety of Ado were evaluated as both substrates and inhibitors of Ado kinase from M. tuberculosis, and a subset was further tested with human Ado kinase for the sake of comparison. The best substrates were 2-aza-Ado, 8-aza-9-deaza-Ado, and 2-fluoro-Ado and the most potent inhibitors were N1-benzyl-Ado (Ki=0.19 microM), 2-fluoro-Ado (Ki=0.5 microM), 6-cyclopentyloxy-purine riboside (Ki=0.15 microM), and 7-iodo-7-deaza-Ado (Ki=0.21 microM). These studies revealed the presence of a hydrophobic pocket near the N6- and N1-positions that can accommodate substitutions at least as large as a benzyl group. The ability to fit into this pocket increased the likelihood that a compound would be an inhibitor and not a substrate. The 2-position was able to accommodate exocyclic substitutions as large as a methoxy group, although substrate activity was low. Similarly, the 7-position could bind an exocyclic group as large as a carboxamido moiety. However, all of the compounds tested with modifications at the 7-position were much better inhibitors than substrates. MIC studies performed with selected compounds have yielded several Ado analogs with promising antitubercular activity. Future studies will utilize this information for the design of new analogs that may be selective antitubercular agents.

Identification and characterization of a unique adenosine kinase from Mycobacterium tuberculosis

Adenosine kinase (AK) is a purine salvage enzyme that catalyzes the phosphorylation of adenosine to AMP. In Mycobacterium tuberculosis, AK can also catalyze the phosphorylation of the adenosine analog 2-methyladenosine (methyl-Ado), the first step in the metabolism of this compound to an active form. Purification of AK from M. tuberculosis yielded a 35-kDa protein that existed as a dimer in its native form. Adenosine (Ado) was preferred as a substrate at least 30-fold (Km = 0.8 +/- 0.08 microM) over other natural nucleosides, and substrate inhibition was observed when Ado concentrations exceeded 5 micro M. M. tuberculosis and human AKs exhibited different affinities for methyl-Ado, with Km values of 79 and 960 microM, respectively, indicating that differences exist between the substrate binding sites of these enzymes. ATP was a good phosphate donor (Km = 1100 +/- 140 microM); however, the activity levels observed with dGTP and GTP were 4.7 and 2.5 times the levels observed with ATP, respectively. M. tuberculosis AK activity was dependent on Mg2+, and activity was stimulated by potassium, as reflected by a decrease in the Km and an increase in Vmax for both Ado and methyl-Ado. The N-terminal amino acid sequence of the purified enzyme revealed complete identity with Rv2202c, a protein currently classified as a hypothetical sugar kinase. When an AK-deficient strain of M. tuberculosis (SRICK1) was transformed with this gene, it exhibited a 5,000-fold increase in AK activity compared to extracts from the original mutants. These results verified that the protein that we identified as AK was coded for by Rv2202c. AK is not commonly found in bacteria, and to the best of our knowledge, M. tuberculosis AK is the first bacterial AK to be characterized. The enzyme shows greater sequence homology with ribokinase and fructokinase than it does with other AKs. The multiple differences that exist between M. tuberculosis and human AKs may provide the molecular basis for the development of nucleoside analog compounds with selective activity against M. tuberculosis.

Metabolic and functional consequences of cytosolic 5'-nucleotidase-IA overexpression in neonatal rat cardiomyocytes

Adenosine exerts a spectrum of energy-preserving actions on the heart negative chronotropic effects. The pathways leading to adenosine formation have remained controversial. In particular, although cytosolic 5'-nucleotidases can catalyze adenosine formation in cardiomyocytes, their contribution to the actions of adenosine has not been documented previously. We recently cloned two closely related AMP-preferring cytosolic 5'-nucleotidases (cN-IA and -IB); the A form predominates in the heart. In this study, we overexpressed pigeon cN-IA in neonatal rat cardiomyocytes using an adenovirus. cN-IA overexpression increased adenosine formation and release into the medium caused by simulated hypoxia and by isoproterenol in the absence and presence of inhibitors of adenosine metabolism. Adenosine release was not affected by an ecto-5'-nucleotidase inhibitor, alpha,beta-methylene-ADP, but was affected by a nucleoside transporter, dipyridamole. The positive chronotropic effect of isoproterenol (130 +/-3 vs. 100 +/-4 beats/min) was inhibited (107 +/-3 vs. 94 +/-3 beats/min) in cells overexpressing cN-IA, and this was reversed by the addition of the adenosine receptor antagonist 8-(p-sulfophenyl)theophilline (120 +/- 3 vs. 90 +/- 4 beats/min). Our results demonstrate that overexpressed cN-IA can be sufficiently active in cardiomyocytes to generate physiologically effective concentrations of adenosine at its receptors.

Cloning of a mouse cytosolic 5'-nucleotidase-I identifies a new gene related to human autoimmune infertility-related protein

Adenosine production catalysed by cytosolic 5'-nucleotidase (cN-I) regulates diverse physiological processes. We report here a mouse cN-I (mcN-I) cloned from heart and testis. The open reading frame contains several potential translation initiation sites, which yield similarly active 5'-nucleotidases. Using overexpression in COS-7 cells we showed that mcN-I, like the previously cloned pigeon cN-I, is activated by ADP and catalyses adenosine formation during ATP breakdown. The N- and C-termini of mcN-I and pcN-I are divergent. Deletion of the 12 C-terminal amino acids or the first 19 N-terminal amino acids of pcN-I does not diminish activity, although deletion of the first 31 N-terminal amino acids reduces activity by 70%. Overall mcN-I is only 66% identical to pcN-I or the recently cloned human cN-I (hcN-I), while hcN-I and pcN-I are 85% identical. We report here a partial hcN-I sequence that is only 70% identical with the published hcN-I amino acid sequence but is 87% identical with mcN-I. Both hcN-I sequences have perfect matches to distinct human genome sequences. Our data imply the existence of at least two genes for cN-I, cN-I(A), previously cloned from pigeon and human, and cN-I(B) that we report here from mouse and partially from human.

Intracellular, nonreceptor-mediated signaling by adenosine: induction and prevention of neuronal apoptosis

Inhibitory effect of adenosine on the isolated heart muscle and vascular system were first described in 1929. Since then, numerous reviews have been published on the diverse actions of this nucleoside on a wide variety of cell types. Essentially all effects of adenosine in neurons and non-neuronal cells are mediated by activation of nucleoside membrane receptors coupled to specific intracellular second messenger pathways. This brief review describes two novel actions of adenosine in peripheral sympathetic neurons, which are not mediated by adenosine receptors. First is described how adenosine and related nucleosides are able to induce apoptosis during the initial stages of neuronal growth and development in vitro and in vivo. Second is discussed how adenosine is able to prevent or delay apoptosis in more mature sympathetic neurons subjected to nerve growth factor deprivation in culture. Both the induction and prevention of apoptosis are independent of receptor activation, and totally dependent on the intracellular accumulation and subsequent phosphorylation of adenosine. The physiological significance and mechanisms by which adenosine can induce apoptosis in one situation, and rescue from apoptosis in another, are described in this article.

Distinct roles for recombinant cytosolic 5'-nucleotidase-I and -II in AMP and IMP catabolism in COS-7 and H9c2 rat myoblast cell lines

Catabolism of AMP during ATP breakdown produces adenosine, which restores energy balance. Catabolism of IMP may be a key step regulating purine nucleotide pools. Two, cloned cytosolic 5'-nucleotidases (cN-I and cN-II) have been implicated in AMP and IMP breakdown. To evaluate their roles directly, we expressed recombinant pigeon cN-I or human cN-II at similar activities in COS-7 or H9c2 cells. During rapid (more than 90% in 10 min) or slower (30-40% in 10 min) ATP catabolism, cN-I-transfected COS-7 and H9c2 cells produced significantly more adenosine than cN-II-transfected cells, which were similar to control-transfected cells. Inosine and hypoxanthine concentrations increased only during slower ATP catabolism. In COS-7 cells, 5'-nucleotidase activity was not rate-limiting for inosine and hypoxanthine production, which was therefore unaffected by cN-II- and actually reduced by cN-I- overexpression. In H9c2 cells, in which 5'-nucleotidase activity was rate-limiting, only cN-II overexpression accelerated inosine and hypoxanthine formation. Guanosine formation from GMP was also increased by cN-II. Our results imply distinct roles for cN-I and cN-II. Under the conditions tested in these cells, only cN-I plays a significant role in AMP breakdown to adenosine, whereas only cN-II breaks down IMP to inosine and GMP to guanosine.

The mechanism of adenosine formation in cells. Cloning of cytosolic 5'-nucleotidase-I

Adenosine increases blood flow and decreases excitatory nerve firing. In the heart, it reduces rate and force of contraction and preconditions the heart against injury by prolonged ischemia. Based on indirect kinetic arguments, an AMP-selective cytosolic 5'-nucleotidase designated cN-I has been implicated in adenosine formation during ATP breakdown. The molecular identity of cN-I is unknown, although an IMP/GMP-selective cytosolic 5'-nucleotidase (cN-II) and an ecto-5'-nucleotidase (e-N) have been cloned. We utilized the high abundance of cN-I in pigeon heart to purify a 40-kDa subunit for partial protein sequencing and subsequent cDNA cloning. We obtained a full-length clone encoding a novel 40-kDa peptide, unrelated to cN-II or e-N, that was most abundant in heart, brain, and breast muscle. Immunolocalization in heart showed a striated cytoplasmic location, suggesting association with contractile elements. Transient expression in COS-7 cells, generated a 5'-nucleotidase that catalyzed adenosine formation from AMP, which was increased during ATP catabolism. In conclusion, the cloning and expression of cN-I provides definitive evidence of its ability to produce adenosine during ATP breakdown.

Uptake and metabolism of adenosine mediate a meiosis-arresting action on mouse oocytes

This study was carried out to evaluate the possible role of adenosine uptake and metabolism in mediating the inhibitory actions of this nucleoside on spontaneous mouse oocyte maturation. Uridine blocked 3H-adenosine uptake by oocyte-cumulus cell complexes (OCCs) and cumulus cell-enclosed oocytes (CEOs) by 82-85%, whereas uptake by denuded oocytes (DOs) was suppressed by 97%. Uridine had no effect on germinal vesicle breakdown (GVB) in CEOs when meiotic arrest was maintained with hypoxanthine or hypoxanthine plus adenosine but reversed the combined inhibitory action of these purines in DOs. Five of six adenosine analogs that bind to purinoceptors demonstrated meiosis-arresting activity but not in relation to their relative affinities for inhibitory or stimulatory adenosine receptors and only at high concentrations. Moreover, in DOs, uridine reversed the inhibitory effect of 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine, two receptor agonists that are poor substrates for adenosine-metabolizing enzymes. Results of experiments with adenosine kinase inhibitors showed that methylmercaptopurine riboside (MMPR) and tubercidin, but not 5'-amino-5'-deoxyadenosine, reversed meiotic arrest maintained by hypoxanthine +/- adenosine, but this required an additional inhibitory action on de novo purine synthesis. Inhibition of de novo purine synthesis alone was not sufficient because azaserine failed to reverse meiotic arrest. MMPR was a very potent meiosis-inducing agent, completely reversing meiotic arrest in CEOs and DOs in the presence of a variety of meiotic inhibitors. The adenosine deaminase inhibitor deoxycoformycin had opposite effects on oocyte maturation depending on the presence or absence of adenosine: the inhibitory action of hypoxanthine alone was bolstered, but the meiosis-arresting action of adenosine was reversed. These data therefore indicate that at low adenosine concentrations phosphorylation predominates, but at higher adenosine concentrations deaminated products contribute to the meiotic inhibition. This idea was borne out by the ability of inosine to mimic the synergistic interaction of adenosine with hypoxanthine. The action of adenosine is not due to deamination to inosine and conversion to nucleotides through the hypoxanthine salvage pathway because adenosine-mediated inhibition was not compromised in oocytes from mutant mice unable to salvage hypoxanthine.

Modulation of the chemotactic peptide- and immunoglobulin G-triggered respiratory burst in human neutrophils by exogenous and endogenous adenosine

The effects of exogenous and endogenous adenosine on the production of oxygen metabolites in neutrophils triggered by the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) or immunoglobulin G (IgG)-opsonized yeast particles, were investigated. By using luminol-enhanced chemiluminescence, we found that adenosine A1 receptor activation did not affect, whereas adenosine A receptor activation, through a mechanism involving the cyclic AMP (cAMP)-protein kinase A signalling pathway, both inhibited the fMLP- and IgG-triggered respiratory burst. The adenosine-induced inhibition was however more pronounced after exposure to fMLP than to IgG-yeast. Stimulation with fMLP caused an extracellular accumulation of endogenous adenosine, which indicates that this event is a negative-feedback mechanism preventing an uncontrolled activation of chemoattractant-stimulated neutrophils. On the contrary, exposure of neutrophils to IgG-yeast did not appear to accumulate extracellular adenosine, probably due to increased adenosine deaminase activity during phagocytosis. In conclusion, this work accentuates the importance of adenosine, both exogenously applied and endogenously formed, as an inflammatory agent modulating the respiratory burst during the different phases in neutrophil activation.

Adenosine inhibits actin dynamics in human neutrophils: evidence for the involvement of cAMP

The mechanisms by which adenosine regulates the inflammatory reaction are poorly characterized. In this study, we investigated the effects of adenosine on neutrophil actin polymerization elicited by the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (fMLP) or IgG-opsonized yeast particles. We used bodipy-phallacidin staining in combination with flow cytometry and found that adenosine markedly reduced actin polymerization triggered by IgG-yeast, whereas the effect on the fMLP-response was less pronounced. Similar or even more pronounced effects were obtained with the adenosine A2 receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), suggesting an A2 receptor-mediated mechanism. The following observations indicate that the A2 receptor-induced effects involve the cAMP-protein kinase A (PKA) signaling pathway: (1) a combination of NECA and the cAMP-specific phosphodiesterase (PDE) inhibitor Ro 20-1724 raised the cAMP content in both unstimulated and stimulated neutrophils and also further inhibited the actin dynamics; (2) the PKA inhibitor H89 reversed the inhibitory effects of NECA on the actin dynamics; (3) Ro 20-1724, isoproterenol and dibutyryl cAMP (DBcAMP) reduced actin polymerization in almost the same way as NECA did. NECA together with Ro 20-1724 impaired the fMLP-induced shape changes and cortical accumulation of actin filaments. In contrast, H89 potentiated the fMLP-induced formation of a submembranous ring of actin filaments. Neutrophils phagocytosing yeast particles in the presence of NECA and Ro 20-1724 were predominantly round in shape, and their ability to extend actin-rich pseudopods around the prey was reduced. These effects were partly antagonized by H89. In correlation with the effects on actin polymerization, NECA more effectively diminished IgG-induced upregulation of the beta2 integrin CD11b/CD18 than such upregulation induced by fMLP. The inhibitory effects of A2-receptor activation on actin dynamics and beta2 integrin expression in neutrophils exposed to IgG-yeast were also associated with a cAMP-dependent reduction of the phagocytic capacity. In conclusion, we show that adenosine inhibits actin dynamics and shape changes in neutrophils via a cAMP-dependent pathway. This finding further characterizes the mechanisms by which adenosine functions as an important modulator of the inflammatory response.

Downregulation of 5'-nucleotidase in rabbit heart during coronary underperfusion

The hydrolysis of AMP to adenosine during acute coronary underperfusion is temporarily beneficial to myocardial survival yet may cause tissue injury during sustained underperfusion because of depletion of adenine nucleotides. We hypothesized that the enzyme mediating AMP hydrolysis, 5'-nucleotidase (5'-NT), is downregulated during sustained coronary underperfusion to prevent excessive loss of nucleotides. Langendorff-perfused rabbit hearts were subjected to two successive, identical 45-min periods of underperfusion (4-5% of baseline flow) separated by 20 min of reperfusion. Although coronary venous lactate efflux was comparable in the two periods, total coronary purine efflux during the second period of underperfusion was attenuated by 75%. Phosphorus nuclear magnetic resonance data showed that ATP fell 46% in the first period but fell only another 10% in the second period. Phosphocreatine levels fell comparably (75-78%) during both periods of underperfusion. Analysis using a mathematical model describing the kinetics of myocardial energetics revealed that the combined data set was best described by a lower activity of 5'-NT (52% decrease in maximal reaction velocity) during the second period of under-perfusion. Additional time course experiments showed that the decrease in 5'-NT activity was slow in onset, requiring approximately 20 min of underperfusion. The decrease in 5'-NT activity during sustained underperfusion may benefit tissue survival by limiting the depletion of myocardial adenine nucleotides. In conclusion, at the onset of coronary underperfusion, there is a high activity of 5'-NT, but later during sustained under-perfusion, 5'-NT is downregulated, resulting in decreased AMP hydrolysis to adenosine.

Modulation of atrioventricular nodal function by metabolic and allosteric regulators of endogenous adenosine in guinea pig heart

BACKGROUND

There has been increasing interest in the development of agents that utilize endogenous adenosine to exert their actions. We tested the hypothesis that substances that either potentiate the activity (allosteric enhancers) or increase the interstitial concentration (inhibitors of metabolism) of endogenous adenosine may cause event (tachycardia)-specific depression of AV nodal conduction.

METHODS AND RESULTS

The frequency-dependent effects of iodotubercidin (ITU, an inhibitor of adenosine kinase), erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, an inhibitor of adenosine deaminase), draflazine (a nucleoside transport blocker), and PD81,723 (an allosteric enhancer of the A1 adenosine receptor binding) on the stimulus-to-His bundle (SH) interval, a measure of AV nodal conduction, were determined in guinea pig hearts and compared with those of adenosine and diltiazem. All drugs depressed AV nodal conduction in a frequency-dependent manner. The ratios of SH interval prolongations at fast to slow pacing rates for draflazine, ITU + EHNA, PD81,723, adenosine, and diltiazem were 17.5 +/- 3.4, 11.1 +/- 5.0, 3.5 +/- 0.9, 10.1 +/- 2.8, and 8.3 +/- 3.5, respectively. Coincident with the prolongation of the SH interval at rapid pacing rates, draflazine and ITU + EHNA increased the epicardial fluid adenosine concentrations by 2.2- and 2.6-fold, respectively. In contrast, epicardial transudate levels of adenosine do not change in the presence of PD81,723. The AV nodal effects of draflazine, ITU, EHNA, and PD81,723 were reversed by the A1 adenosine receptor antagonist 8-cyclopentyltheophylline and adenosine deaminase, implicating endogenous adenosine acting at the A1 adenosine receptor.

CONCLUSIONS

Adenosine-regulating agents that act in an event- and site-specific manner represent a novel drug design strategy that may potentially be valuable for the long-term treatment of supraventricular arrhythmias and control of ventricular rate during atrial fibrillation or flutter.

Adenosine kinase inhibitors augment release of adenosine from spinal cord slices

Inhibitors of adenosine kinase, but not adenosine deaminase, produce antinociception when administered spinally. In this study, we evaluated the relative contribution of adenosine kinase and adenosine deaminase to the regulation of adenosine release into the extracellular space within the spinal cord by determining the effects of the adenosine kinase inhibitors 5'-amino-5'-deoxyadenosine and 5-iodotubercidin, and the adenosine deaminase inhibitor 2'-deoxycoformycin on adenosine release from spinal cord slices in an in vitro perfusion system. Both 5'-amino-5'-deoxyadenosine (5-50 microM) and 5-iodotubercidin (5-50 microM), but not 2'-deoxycoformycin (50 microM), augmented adenosine release. 5-Iodotubercidin was slightly more potent and effective than 5'-amino-5'-deoxyadenosine in augmenting release except at the highest concentration, where it was considerably more effective. Combinations of 2'-deoxycoformycin (50 microM) and minimally active concentrations of 5'-amino-5'-deoxyadenosine and 5-iodotubercidin (5 microM each) produced a synergistic enhancement of release. These results support a predominant involvement of adenosine kinase in regulating extracellular adenosine levels in the spinal cord, but adenosine deaminase also can play a significant role.

Correlation of retention of tumor methylmercaptopurine riboside-5'-phosphate with effectiveness in CD8F1 murine mammary tumor regression

Treatment with a combination (PMA) of (N-phosphonacetyl)-L-aspartic acid (PALA), methylmercaptopurine riboside (MMPR), and 6-aminonicotinamide (6AN) induced partial regressions of CD8F1 murine mammary tumors and provided for tumor growth inhibition without regression of Colon 38 tumors. HPLC-nucleotide pool analysis of CD8 mammary tumors obtained at various times after treatment with PMA revealed that MMPR-5'-phosphate, which inhibits de novo purine nucleotide biosynthesis, was constant at levels of approximately 2.5 nmol/mg protein for 72 hr after treatment. In contrast, the MMPR-5'-phosphate levels of C38 tumors decreased from 24-hr levels at 1.5 nmol/mg protein with a half-time of about 24 hr. Treatment of CD8 tumor-bearing mice with iodotubercidin, a potent inhibitor of adenosine/MMPR kinase, at various times after PMA, reversed both the accumulation of high levels of MMPR-5'-phosphate and the number of partial tumor regressions. These data demonstrate that a cycle of MMPR rephosphorylation is active in the CD8 mammary tumor and suggest that this recycling of MMPR is important for the optimal effect of PMA treatment.

Adenosine kinase and adenosine deaminase inhibition modulate spinal adenosine- and opioid agonist-induced antinociception in mice

Endogenous purinergic systems mediating antinociception, and their interactions with opioids, were characterized following intrathecal (i.t.) administration of inhibitors of adenosine clearance in mice. 5'-Amino,5'-deoxyadenosine (5'-NH2dAdo), an inhibitor of adenosine kinase, induced significant antinociception after i.t. injection and enhanced antinociception induced by selected opioids (i.t.). Isobolographic analysis of antinociception following coadministration (i.t.) of 5'-NH2dAdo with opioids revealed additive interactions with mu-, and synergistic interactions with delta-, opioid receptor-selective agonists. Inhibitors of adenosine deaminase, deoxycoformycin and erythro-9-(2-hydroxy-3nonyl) adenine (EHNA), generally failed to induce antinociception when administered (i.t.) alone or to enhance opioid (i.t.)-induced antinociception, however, was significantly enhanced by either 5'-NH2dAdo or deoxycoformycin. These results confirm different physiologic roles for adenosine kinase and adenosine deaminase in spinal purinergic systems. 5'-NH2dAdo interactions with opioid receptor-selective agonists demonstrate significant, but heterogeneous interactions between endogenous adenosine and opioid spinal systems mediating antinociception.

Inhibition of adenosine kinase increases endogenous adenosine and depresses neuronal activity in hippocampal slices

Endogenous adenosine in the extracellular space inhibits neuronal activity. The roles of adenosine kinase, S-adenosylhomocysteine-hydrolase and adenosine deaminase activities in the regulation of the adenosine levels were investigated in rat hippocampal slices. Iodotubercidin, an inhibitor of adenosine kinase, added to the perfusion fluid at 5 microM increased the release of adenosine from the slices more than 2-fold. Iodotubercidin treatment caused inhibition of population spike discharges and hyperpolarization of pyramidal cells, mimicking the effects of exogenously applied adenosine. Adenosine dialdehyde, an inhibitor of S-adenosylhomocysteine hydrolase, and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), an inhibitor of adenosine deaminase had little or no effect on the parameters tested. The action of iodotubercidin was greater during deaminase inhibition. The A1-receptor antagonist DPCPX had actions opposite to those of adenosine and blocked the electrophysiological effects of exogenous adenosine and of iodotubercidin. Thus adenosine kinase activity is a significant factor in the regulation of adenosine levels in the hippocampus.

2'-Deoxycoformycin inhibition of adenosine deaminase in rat brain: in vivo and in vitro analysis of specificity, potency, and enzyme recovery

2'-Deoxycoformycin (DCF), a potent inhibitor of adenosine deaminase (ADA), is increasingly used as a tool to investigate adenosine metabolism and neuromodulation. To advance further the usefulness of DCF for studies of purines in the CNS, we determined the inhibitory potency of this compound against ADA and adenylate deaminase (AMPDA) in brain, the rate of ADA recovery in various brain regions after single or repeated intraperitoneal DCF administrations, and the effect of DCF on several neurotransmitter synthetic enzymes. In vitro, the Ki values for inhibition of ADA and AMPDA were found to be 23 pM and 233 microM, respectively. In vivo, DCF inhibited ADA with ED50 values ranging from 155 to 280 micrograms/kg at 2 h posttreatment, and 98% inhibition was achieved with 1 mg/kg. AMPDA activity was not affected by doses up to 5.0 mg/kg. In contrast to the greater than 95% inhibition of ADA seen 1 day after DCF at 5 mg/kg, the effectiveness of a second similar DCF treatment on the activity that had recovered by 14 days was dramatically reduced. Eight days after DCF treatment with doses of 5-50 mg/kg, the degree of ADA activity recovery in 10 brain regions examined was similar; it averaged 35% of control values at the low dose but showed some heterogeneity, ranging from 15 to 54% of control values, at the higher doses. Forty days after treatment with a single dose of 5 mg/kg, ADA activity recovered by 68-78% of control values in brain regions with normally high levels of activity and by 44-59% of control values in other regions. The activities of choline acetyltransferase, glutamic acid decarboxylase, and histidine decarboxylase (an enzyme colocalized with ADA in hypothalamic neurons) were unaffected by DCF treatment, a result suggesting the lack of a generalized neurotoxic effect. The very low doses of DCF required for ADA inhibition in vivo are consistent with the high potency of this drug against ADA in vitro, and any physiological effects observed at low doses might therefore be ascribed to inhibition of ADA.

Effects of deoxycoformycin on adenosine, inosine, hypoxanthine, xanthine, and uric acid release from the hypoxemic rat cerebral cortex

The effects of the adenosine deaminase inhibitor, deoxycoformycin, on purine release from the rat cerebral cortex were studied with the cortical cup technique. Deoxycoformycin (5 and 500 micrograms/kg i.v.) enhanced the hypoxia/ischemia-evoked release of adenosine from the cerebral cortex, indicating a marked rise in the adenosine content of interstitial fluid in the cerebral cortex. Inosine and hypoxanthine release were attenuated at the higher dose of deoxycoformycin. Uric acid release into the cortical perfusates was enhanced at the higher dose level. These results demonstrate that low doses of deoxycoformycin can be used to elevate interstitial levels of adenosine in the brain during hypoxia, and to depress the formation of some of its metabolites. The elevation of hypoxia/ischemia-evoked adenosine levels can account for the previously reported potentiation of hypoxia-evoked increases in rat cerebral blood flow after deoxycoformycin administration. The potential therapeutic utility of these findings is discussed.

Selective purine release from P388D1 macrophages injured by silica

Silica particles are toxic to primary cultures of macrophages or the P388D1 cell line in vitro. Loss of viability in these model systems is accompanied by depletion of ATP content within 3 to 6 hours. The mechanisms responsible for ATP depletion will be explored in this paper. After prelabeling for 1 hour with 3H-adenine, silica-treated cells released 60-80% of their labeled acid-soluble pool into the culture medium. This release did not occur after phagocytosis of nontoxic titanium dioxide particles and was specific for purines. ATP depletion was accompanied by purine catabolism: inosine, hypoxanthine, xanthine, and uric acid were detected in the culture medium using thin layer or high-performance liquid chromatography. The final xanthine oxidase step in purine catabolism generates reactive oxygen metabolites. Silica toxicity was not prevented by the xanthine oxidase inhibitor allopurinol nor exogenous purines. It is concluded that adenine nucleotide depletion and purine catabolism are not solely responsible for irreversible injury in silica toxicity. It is hypothesized that purine catabolism and release from injured macrophages may lead to generation of reactive oxygen species, injury to surrounding tissue, and fibrosis.

Absolute rates of adenosine formation during ischaemia in rat and pigeon hearts

1. The activities of ecto- and cytosolic 5'-nucleotidase (EC 3.1.3.5), adenosine kinase (EC 2.7.1.20), adenosine deaminase (EC 3.5.4.4) and AMP deaminase (EC 3.5.4.6) were compared in ventricular myocardium from man, rats, rabbits, guinea pigs, pigeons and turtles. The most striking variation was in the activity of the ecto-5'-nucleotidase, which was 20 times less active in rabbit heart and 300 times less active in pigeon heart than in rat heart. The cytochemical distribution of ecto-5'-nucleotidase was also highly variable between species. 2. Adenosine formation was quantified in pigeon and rat ventricular myocardium in the presence of inhibitors of adenosine kinase and adenosine deaminase. 3. Both adenosine formation rates and the proportion of ATP catabolized to adenosine were greatest during the first 2 min of total ischaemia at 37 degrees C. Adenosine formation rates were 410 +/- 40 nmol/min per g wet wt. in pigeon hearts and 470 +/- 60 nmol/min per g wet wt. in rat hearts. Formation of adenosine accounted for 46% of ATP plus ADP broken down in pigeon hearts and 88% in rat hearts. 4. The data show that, in both pigeon and rat hearts, adenosine is the major catabolite of ATP in the early stages of normothermic myocardial ischaemia. The activity of ecto-5'-nucleotidase in pigeon ventricle (16 +/- 4 nmol/min per g wet wt.) was insufficient to account for adenosine formation, indicating the existence of an alternative catabolic pathway.

Evidence for extracellular deamination of adenosine in the rat heart

1. In rat heart perfused with adenosine (10(-6) M), dilazep (10(-4) M) inhibited incorporation of adenosine into nucleotides (an index of nucleoside transport and phosphorylation) to a greater extent (70%) than metabolism to inosine and uric acid (40%) and actually increased the recovery of inosine to 30% of the adenosine infused. 2. Extrapolating for complete inhibition of transport suggested that 60% of adenosine metabolism was intracellular and 40% extracellular. 3. Static incubations of atria also gave an estimate for extracellular metabolism of 40%. 4. Adenosine deaminase was localised by immunocytochemistry to the extracellular surface of endothelial cells of small coronary arteries. 5. Extracellular deamination may explain the lack of effect of nucleoside transport inhibitors on responses to adenosine in rat heart.

Adenosine formation and release from neonatal-rat heart cells in culture

The incorporation of [3H]adenosine (10 microM) into neonatal-rat heart cell nucleotides was inhibited in a concentration-dependent manner, such that 50% inhibition was obtained with 0.75 microM-dipyridamole, 0.26 microM-hexobendine or 0.22 microM-dilazep. Adenosine formation was accelerated 2.5-fold to 2.1 +/- 0.3 nmol/10(7) cells in 10 min when cells were incubated with a combination of 30 mM-2-deoxyglucose and 2 micrograms of oligomycin/ml. Of the newly formed adenosine, 6 +/- 2% was in the cells. Dipyridamole, hexobendine or dilazep (10 microM) increased the amount of adenosine in the cells and decreased that in the medium such that 45-50% of the newly formed adenosine was in the cells. Antibodies which inhibited ecto-5'-nucleotidase by 98.7 +/- 0.3% did not alter the rate of adenosine formation or its distribution between cells and medium. We conclude that adenosine was formed in the cytoplasm during catabolism of cellular ATP and was released via the dipyridamole-sensitive symmetric nucleoside transporter.

The control of adenosine concentration in polymorphonuclear leucocytes, cultured heart cells and isolated perfused heart from the rat

Rat polymorphonuclear leucocytes or neonatal-rat heart cells in culture were treated with 2'-deoxycoformycin and 5-iodotubercidin at concentrations that inhibited adenosine deaminase (EC 3.5.4.4) and adenosine kinase (EC 2.7.1.20) inside the intact cells, and the rate of adenosine accumulation was determined. The basal rate of adenosine formation was 2% (polymorphonuclear leucocytes) or 9% (heart cells) of the maximal activity of adenosine kinase also measured in intact cells. Greatly increased rates of adenosine formation were observed during adenine nucleotide catabolism. This condition also led to a decrease in adenosine kinase activity. When isolated rat hearts were perfused with 5-iodotubercidin alone at a concentration which inhibited adenosine kinase, no increase in tissue or perfusate adenosine or inosine concentration was observed. However, perfusion with hypoxic buffer or infusion of adenosine into the coronary circulation at a rate (20 nmol/min) equivalent to 40% of the activity of adenosine kinase caused large increases in effluent perfusate adenosine and inosine concentrations. These data argue unanimously against the existence of a substrate cycle controlling adenosine concentration. They suggest instead that an increase in the rate of adenosine formation is the principal cause of elevations in adenosine concentration during ATP catabolism.

Metabolism of circulating adenosine by the porcine isolated perfused lung

Adenosine uptake was studied in the piglet isolated perfused lung by means of the single-circulation paired-tracer dilution technique. Adenosine was efficiently taken up from the pulmonary vascular bed, and the process was potently inhibited by dipyridamole. Following uptake, adenosine was incorporated into intracellular nucleotides, and at low perfusate concentrations, little or none of the incorporated radioactivity returned to the circulation. At higher concentrations, cellular uptake was saturable and products of intracellular catabolism (inosine and hypoxanthine) were returned to the circulation. Perfusion of low concentrations of adenosine after inhibition of pulmonary adenosine kinase led to a proportional decrease in the retention of nucleotides and to a release of inosine and hypoxanthine. A small proportion of adenosine was metabolised extracellularly by adenosine deaminase; this activity was not released from perfused lungs and is apparently an ecto-enzyme.

Effects of the chiral isomers of erythro- and threo-9-(2-hydroxy-3-nonyl)adenine on purine metabolism in sarcoma 180 cells

The effects of the chiral isomers of erythro- and threo-9-(2-hydroxy-3-nonyl)adenines (EHNA and THNA) on purine metabolism in Sarcoma 180 cells have been determined. At concentrations of 10-80 microM [10- to 1000-fold greater than their Ki values with adenosine deaminase (ADA)], all isomers inhibited purine salvage and biosynthesis de novo. Although (+)-EHNA, the most potent ADA inhibitor, exerted the greatest effects, there was no direct correlation between the potency of ADA inhibition and the secondary effects on purine metabolism, e.g. (+)-EHNA is about 2-fold more inhibitory than (-)-EHNA in blocking purine base incorporation but about 250-fold more potent as an inhibitor of ADA (Ki of (+)-EHNA = 2 nM; Ki of (-)-EHNA = 500 nM [Bessodes et al., Biochem. Pharmac. 31, 879 (1982)]). All the isomers inhibited the incorporation of radiolabeled purine bases (adenine, guanine and hypoxanthine) and nucleosides (guanosine and inosine) into acid-soluble nucleotides and of glycine into 5'-phosphoribosyl-formylglycineamide. Unlike the results of Henderson et al. [Biochem. Pharmac. 26, 1967 (1977)] with Ehrlich ascites cells, the incorporation of adenosine into nucleotides was only slightly inhibited in Sarcoma 180 cells. (+)-EHNA did not inhibit the activities of 5-phosphoribosyl-1-pyrophosphate (PRPP) synthetase, purine phosphoribosyltransferases or nucleotide kinases in cell extracts. Accumulation of PRPP was inhibited only under conditions that fostered rapid synthesis.

A new procedure for haptenizing adenosine leading to a more specific radioimmunoassay method

Adenosine was coupled to human serum albumin by two different procedures that preserved the purine and ribose rings. The conjugates were evaluated for antigenicity in rabbits and guinea pigs. A conjugate containing 2'(3')-O-succinoyladenosine failed to elicit antibodies, whereas one containing laevulinic acid (O2', 3'-adenosine-acetal) elicited antibodies in all animals injected. The affinity and specificity of binding of adenosine to three selected antisera were evaluated. Dissociation constants of 31-187 nM were observed. Displacement of adenosine binding to all antisera by adenine 5'-nucleotides, adenine, inosine and hypoxanthine required more than 1000-fold higher concentrations than of adenosine itself. Similar affinities for adenosine and 2'-deoxyadenosine were observed. By exploiting the high specificity of the antisera, a radioimmunoassay method was established that was capable of detecting down to 1 pmol of adenosine (20 nM) in unfractionated heart perfusates and cell extracts. The acetal-mediated coupling procedure is applicable to other biologically important cis-diols.

Inactivation of liver S-adenosylhomocysteine hydrolase in vitro of rats treated with erythro-9-(2-hydroxynon-3-yl)adenine

S-Adenosylhomocysteine hydrolase activity decreased in vitro time-dependently in liver homogenates obtained from rats treated in vivo with erythro-9-(2-hydroxynon-3-yl)adenine, a potent inhibitor of adenosine deaminase. The inhibitor in itself had no effect on the stability of the hydrolase. The inactivation of S-adenosylhomocysteine hydrolase was irreversible, proceeded fairly rapidly at a low temperature (0 degrees C) and showed first-order reaction kinetics. Adenosine was found to accumulate in these tissue homogenates during storage. Several lines of evidence suggest that adenosine caused the observed suicide-like inactivation post mortem. Pre-incubation of purified S-adenosylhomocysteine hydrolase at 0 degrees C with adenosine showed a half-maximal inactivation rate at 33 microM substrate concentration; the rate constant of inactivation was 0.01 min-1. Inactivation during tissue preparation and storage complicates the assay of S-adenosylhomocysteine hydrolase activity in samples that contain an inhibitor of adenosine deaminase. These results also suggest that the decrease of S-adenosylhomocysteine hydrolase activity reported to occur in several disturbances of purine metabolism should be re-examined to exclude the possibility of inactivation of the enzyme in vitro.

Nucleoside exchange catalysed by the cytoplasmic 5'-nucleotidase

The inhibition of the cytoplasmic 5'-nucleotidase (EC 3.1.3.5) by its product, inosine, was studied with a partially purified preparation of the enzyme from rat liver. Inhibition of Pi production was found to be due to exchange of the inosine moiety between inosine and IMP. Exchange was not catalysed by reversal of the hydrolytic reaction, suggesting, instead, the mediation of an enzyme-phosphate intermediate. Two models for the catalytic mechanism are proposed and rate equations for the dependence of Pi production on inosine concentration are derived. The experimentally determined dependence was consistent with a mechanism in which hydrolysis of the enzyme-phosphate intermediate occurred only when it was unoccupied by inosine. This conclusion suggests that inosine analogues that cannot participate in exchange should inhibit the enzyme. Such inhibitors might be useful in defining the enzyme's physiological role or as pharmacological agents to decrease breakdown of purine nucleotides. The possibility that nucleoside exchange provides an alternative route for the phosphorylation of mutagenic or cytotoxic nucleoside analogues should also be considered.

Uptake and utilization of 5'-methylthioadenosine by cultured baby-hamster kidney cells

5'-Methylthioadenosine was taken up and immediately metabolized further by cultured baby-hamster kidney cells during the exponential phase of growth. The adenine moiety supplied the purine-nucleotide pool via the salvage pathway and was efficiently incorporated into nucleic acids. Catabolites of methylthioadenosine excreted by the cells included adenine, purinic compounds and metabolites of the ribose portion. 5'-Methylthiotubercidin had no significant effect on the cellular metabolism of methyl-thioadenosine, but greatly inhibited its uptake. erythro-9-(2-Hydroxy-3-nonyl)adenine had no effect on the uptake, but markedly interfered with the further utilization of methylthioadenosine after cleavage in the cells.

Adenosine production inside rat polymorphonuclear leucocytes

Adenosine synthesis was studied during 2-deoxyglucose-induced ATP catabolism in intact rat polymorphonuclear leucocytes. When both adenosine kinase (EC 2.7.1.20) and adenosine deaminase (EC 3.5.4.4) were selectively inhibited, adenosine accumulated. Adenosine formation took place inside the intact cells by a metabolic pathway independent of the ecto-5'-nucleotidase (EC 3.1.3.5). Distinct metabolic pathways are proposed for adenosine production from intracellular or extracellular nucleotides.