Structural basis for the growth factor activity of human adenosine deaminase ADA2

Two distinct adenosine deaminases, ADA1 and ADA2, are found in humans. ADA1 has an important role in lymphocyte function and inherited mutations in ADA1 result in severe combined immunodeficiency. The recently isolated ADA2 belongs to the novel family of adenosine deaminase growth factors (ADGFs), which play an important role in tissue development. The crystal structures of ADA2 and ADA2 bound to a transition state analogue presented here reveal the structural basis of the catalytic/signaling activity of ADGF/ADA2 proteins. In addition to the catalytic domain, the structures discovered two ADGF/ADA2-specific domains of novel folds that mediate the protein dimerization and binding to the cell surface receptors. This complex architecture is in sharp contrast with that of monomeric single domain ADA1. An extensive glycosylation and the presence of a conserved disulfide bond and a signal peptide in ADA2 strongly suggest that ADA2, in contrast to ADA1, is specifically designed to act in the extracellular environment. The comparison of catalytic sites of ADA2 and ADA1 demonstrates large differences in the arrangement of the substrate-binding pockets. These structural differences explain the substrate and inhibitor specificity of adenosine deaminases and provide the basis for a rational design of ADA2-targeting drugs to modulate the immune system responses in pathophysiological conditions.

Adenosine in the tuberomammillary nucleus inhibits the histaminergic system via A1 receptors and promotes non-rapid eye movement sleep

Adenosine has been proposed to promote sleep through A(1) receptors (A(1)R's) and/or A(2A) receptors in the brain. We previously reported that A(2A) receptors mediate the sleep-promoting effect of prostaglandin D(2), an endogenous sleep-inducing substance, and that activation of these receptors induces sleep and blockade of them by caffeine results in wakefulness. On the other hand, A(1)R has been suggested to increase sleep by inhibition of the cholinergic region of the basal forebrain. However, the role and target sites of A(1)R in sleep-wake regulation remained controversial. In this study, immunohistochemistry revealed that A(1)R was expressed in histaminergic neurons of the rat tuberomammillary nucleus (TMN). In vivo microdialysis showed that the histamine release in the frontal cortex was decreased by microinjection into the TMN of N(6)-cyclopentyladenosine (CPA), an A(1)R agonist, adenosine or coformycin, an inhibitor of adenosine deaminase, which catabolizes adenosine to inosine. Bilateral injection of CPA into the rat TMN significantly increased the amount and the delta power density of non-rapid eye movement (non-REM; NREM) sleep but did not affect REM sleep. CPA-promoted sleep was observed in WT mice but not in KO mice for A(1)R or histamine H(1) receptor, indicating that the NREM sleep promoted by A(1)R-specific agonist depended on the histaminergic system. Furthermore, the bilateral injection of adenosine or coformycin into the rat TMN increased NREM sleep, which was completely abolished by coadministration of 1,3-dimethyl-8-cyclopenthylxanthine, a selective A(1)R antagonist. These results indicate that endogenous adenosine in the TMN suppresses the histaminergic system via A(1)R to promote NREM sleep.

Synthesis of 5'-methylthio coformycins: specific inhibitors for malarial adenosine deaminase

Transition state theory suggests that enzymatic rate acceleration (kcat/knon) is related to the stabilization of the transition state for a given reaction. Chemically stable analogues of a transition state complex are predicted to convert catalytic energy into binding energy. Because transition state stabilization is a function of catalytic efficiency, differences in substrate specificity can be exploited in the design of tight-binding transition state analogue inhibitors. Coformycin and 2'-deoxycoformycin are natural product transition state analogue inhibitors of adenosine deaminases (ADAs). These compounds mimic the tetrahedral geometry of the ADA transition state and bind with picomolar dissociation constants to enzymes from bovine, human, and protozoan sources. The purine salvage pathway in malaria parasites is unique in that Plasmodium falciparum ADA (PfADA) catalyzes the deamination of both adenosine and 5'-methylthioadenosine. In contrast, neither human adenosine deaminase (HsADA) nor the bovine enzyme (BtADA) can deaminate 5'-methylthioadenosine. 5'-Methylthiocoformycin and 5'-methylthio-2'-deoxycoformycin were synthesized to be specific transition state mimics of the P. falciparum enzyme. These analogues inhibited PfADA with dissociation constants of 430 and 790 pM, respectively. Remarkably, they gave no detectable inhibition of the human and bovine enzymes. Adenosine deamination is involved in the essential pathway of purine salvage in P. falciparum, and prior studies have shown that inhibition of purine salvage results in parasite death. Inhibitors of HsADA are known to be toxic to humans, and the availability of parasite-specific ADA inhibitors may prevent this side-effect. The potent and P. falciparum-specific inhibitors described here have potential for development as antimalarials without inhibition of host ADA.

Relatively small increases in the steady-state levels of nucleobase deamination products in DNA from human TK6 cells exposed to toxic levels of nitric oxide

Nitric oxide (NO) is a physiologically important molecule that has been implicated in the pathophysiology of diseases associated with chronic inflammation, such as cancer. While the complicated chemistry of NO-mediated genotoxicity has been extensively study in vitro, neither the spectrum of DNA lesions nor their consequences in vivo have been rigorously defined. We have approached this problem by exposing human TK6 lymphoblastoid cells to controlled steady-state concentrations of 1.75 or 0.65 microM NO along with 186 microM O2 in a recently developed reactor that avoids the anomalous gas-phase chemistry of NO and approximates the conditions at sites of inflammation in tissues. The resulting spectrum of nucleobase deamination products was defined using a recently developed liquid chromatography/mass spectrometry (LC/MS) method, and the results were correlated with cytotoxicity and apoptosis. A series of control experiments revealed the necessity of using dC and dA deaminase inhibitors to avoid adventitious formation of 2'-deoxyuridine (dU) and 2'-deoxyinosine (dI), respectively, during DNA isolation and processing. Exposure of TK6 cells to 1.75 microM NO and 186 microM O2 for 12 h (1260 microM x min dose) resulted in 32% loss of cell viability measured immediately after exposure and 87% cytotoxicity after a 24 h recovery period. The same exposure resulted in 3.5-, 3.8-, and 4.1-fold increases in dX, dI, and dU, respectively, to reach the following levels: dX, 7 (+/- 1) per 10(6) nt; dI, 25 (+/- 2.1) per 10(6) nt; and dU, 40 (+/- 3.8) per 10(6) nt. dO was not detected above the limit of detection of 6 lesions per 10(7) nt in 50 microg of DNA. A 12 h exposure to 0.65 microM NO and 190 microM O2 (468 microM x min dose) caused 1.7-, 1.8-, and 2.0-fold increases in dX, dI, and dU, respectively, accompanied by a approximately 15% (+/- 3.6) reduction in cell viability immediately after exposure. Again, dO was not detected. These results reveal modest increases in the steady-state levels of DNA deamination products in cells exposed to relatively cytotoxic levels of NO. This could result from limited nitrosative chemistry in nuclear DNA in cells exposed to NO or high levels of formation balanced by rapid repair of nucleobase deamination lesions in DNA.

Dominant localization of adenosine deaminase in leptomeninges and involvement of the enzyme in sleep

Adenosine is an endogenous hypnotic molecule. However, the mechanism by which the level of extracellular adenosine is regulated remains to be elucidated. We found by Northern hybridization and enzyme assay that ecto-5(')-nucleotidase and adenosine deaminase (ADA), major enzymes responsible for the production and degradation of adenosine, respectively, were localized most abundantly in the leptomeninges within the rat brain. Immunohistochemical study showed that ADA was dominantly localized in arachnoid barrier and trabecular cells of the leptomeninges. In vivo microdialysis demonstrated that externally applied adenosine was rapidly metabolized by ADA to inosine in the subarachnoid space. Perfusion of an ADA inhibitor, coformycin, increased the extracellular adenosine level in the subarachnoid space under the rostral basal forebrain. When coformycin was continuously infused into the subarachnoid space, non-rapid eye movement sleep was increased with prolonged duration of the sleep episode. These results demonstrate that the leptomeninges control the extracellular level of adenosine in the subarachnoid space by their high 5(')-nucleotidase and ADA activities and regulate non-rapid eye movement sleep.

Extracellular ATP and adenosine induce cell apoptosis of human hepatoma Li-7A cells via the A3 adenosine receptor

1. Extracellular ATP is a potent signaling molecule that modulates a myriad of cellular functions through the activation of P2 purinergic receptors and is cytotoxic to a variety of cells at higher concentrations. The mechanism of ATP-elicited cytotoxicity is not fully understood. In this study, we investigated the effect of extracellular ATP on the human hepatoma Li-7A cells. 2. We observed a time- and dose-dependent growth inhibition of Li-7A cells by ATP, which is accompanied by an increase in the active form of caspase-3 as well as increased cleavage of its substrate, poly (ADP-ribose) polymerase. The cytotoxic effect of extracellular ATP was not mediated by the P2X7 receptor, since (1).the effect was not abolished by the P2X7 receptor antagonists oxidized ATP and KN-62, and (2).extracellular ADP, AMP, and adenosine were also cytotoxic. 3. We found that ATP and ADP were degraded to adenosine by Li-7A cells and that treatment of Li-7A cells by adenosine resulted in growth inhibition and caspase-3 activation, indicating that adenosine is the apoptotic agent. Using adenosine receptor agonists and antagonists, as well as inhibitors of adenosine transport and deamination, we showed that the cytotoxic effect of adenosine is specifically mediated by the A3 receptor even though transcripts of A1, A2A, A2B, and a splice variant of the P2X7 receptors were detected in Li-7A cells by RT-PCR. 4. Cytotoxicity caused by exogenous ATP and adenosine was completely abolished by the caspase-3 inhibitor Z-DEVD-FMK, demonstrating the central role of caspase-3 in apoptosis of Li-7A cells.

Enhancement of cellular adenosine triphosphate levels in PC12 cells by extracellular adenosine

To elucidate the biological significance of extracellular adenine compounds, the effects of adenosine (Ado) on cellular levels of adenine compounds, especially adenosine triphosphate (ATP), in PC12 cells were studied. Ado and inosine but not adenosine 5'-monophosphate, adenosine 5'-diphosphate, ATP, guanosine, cytosine, thymidine, and uridine, significantly enhanced cellular ATP levels in PC12 cells in time- and dose-dependent manners. Various P1 receptor agonists of Ado did not enhance the ATP level. In addition, theophylline, an antagonist of P1 receptors, did not inhibit the Ado-evoked ATP enhancement. These results suggest that the Ado receptor is not involved in the augmentation of the cellular ATP level induced by Ado in PC12 cells. The ATP-enhancing effect of Ado was potentiated by dipyridamole, an inhibitor of Ado uptake, or coformycin, an inhibitor of Ado deaminase. The effect of Ado on the ATP level was also observed when PC12 cells were incubated in glucose-free medium. Together these results suggest that enhancement of cellular ATP levels in PC12 cells by extracellular Ado might be acceleration of ATP synthesis through the Ado salvage system using hypoxanthine-guanine phosphoribosyltransferase rather than Ado kinase since 5'-iodotubercidin, an inhibitor of Ado kinase, had no effect on the enhancement elicited by Ado.

Direct measurement of adenosine release during hypoxia in the CA1 region of the rat hippocampal slice

We have used an enzyme-based, twin-barrelled sensor to measure adenosine release during hypoxia in the CA1 region of rat hippocampal slices in conjunction with simultaneous extracellular field recordings of excitatory synaptic transmission. When loaded with a combination of adenosine deaminase, nucleoside phosphorylase and xanthine oxidase, the sensor responded linearly to exogenous adenosine over the concentration range 10 nM to 20 microM. Without enzymes, the sensor when placed on the surface of hippocampal slices recorded a very small net signal during hypoxia of 40 +/- 43 pA (mean +/- s.e.m.; n = 7). Only when one barrel was loaded with the complete sequence of enzymes and the other with the last two in the cascade did the sensor record a large net difference signal during hypoxia (1226 +/- 423 pA; n = 7). This signal increased progressively during the hypoxic episode, scaled with the hypoxic depression of the simultaneously recorded field excitatory postsynaptic potential and was greatly reduced (67 +/- 6.5 %; n = 9) by coformycin (0.5-2 microM), a selective inhibitor of adenosine deaminase, the first enzyme in the enzymic cascade within the sensor. For 5 min hypoxic episodes, the sensor recorded a peak concentration of adenosine of 5.6 +/- 1.2 microM (n = 16) with an IC(50) for the depression of transmission of approximately 3 microM. In slices pre-incubated for 3-6 h in nominally Ca(2+)-free artificial cerebrospinal fluid, 5 min of hypoxia resulted in an approximately 9-fold greater release of adenosine (48.9 +/- 17.7 microM; n = 6). High extracellular Ca(2+) (4 mM) both reduced the adenosine signal recorded by the sensor during hypoxia (3.5 +/- 0.6 microM; n = 4) and delayed the hypoxic depression of excitatory synaptic transmission.

Purine analogues kill resting lymphocytes by p53-dependent and -independent mechanisms

To resolve the controversy concerning the role of p53 in the killing of resting lymphocytes by purine nucleoside analogues, we examined the cytotoxic effects of chlorodeoxyadenosine, fludarabine and deoxycoformycin (plus deoxyadenosine) on unstimulated spleen cells from p53-knockout versus wild-type mice. p53-knockout cells were more resistant to all three nucleosides than were wild-type cells. However, substantial killing still occurred in the absence of p53, indicating that purine analogues can kill resting lymphocytes by both p53-dependent and -independent mechanisms. We suggest that these results are relevant to chronic lymphoid malignancies, and that characterization of the p53-independent component of nucleoside action may indicate potential ways of overcoming therapeutic resistance.

Adenine nucleotide synthesis in human erythrocytes depends on the mode of supplementation of cell suspension with adenosine

In suspensions of washed human erythrocytes, adenosine added in a single dose to concentrations of 0.1-10.0 mmol/l suspension was deaminated at rates ranging from 10 to 50 mmol/l cells h. The sum of adenosine, inosine, and hypoxanthine concentrations in the suspension, as well as the intracellular concentration of ATP, remained constant. In the presence of 25-50 mmol/l orthophosphate, addition of a single dose of adenosine into erythrocyte suspension increased the ATP concentration by up to 280% of the initial level. If the initial adenosine concentrations were greater than 5 mmol/l suspension, ATP increased independently of adenosine concentration to the level determined only by the concentration of orthophosphate. After orthophosphate was returned to its initial level, ATP in erythrocytes began to decrease. In the presence of coformycin, erythrocytes utilised adenosine at a rate of 0.2-0.3 mmol/l cells h. Their adenylate pool increased at a rate of 0.10-0.16 mmol/l cells h for several hours, but intracellular ATP increased only slightly. The energy charge of cells decreased significantly from 0.86 +/- 0.05 (control) to 0.82 +/- 0.06. Adenosine continuously pumped into erythrocyte suspensions at rates of 0.02-5.0 mmol/l cells h for several hours caused the adenylate pool of erythrocytes and intracellular ATP to increase synchronously at a rate of 0.02-0.35 mmol/l cells h. The energy charge of these erythrocytes increased significantly up to 0.91 +/- 0.03. After pumping of adenosine was stopped, the intracellular ATP and the adenylate pool began to decrease, returning sometimes to the initial level in 2-3 h.

5'-Nucleotidase as a marker of both general and local inflammation in rheumatoid arthritis patients

OBJECTIVES

To evaluate measurements of serum and synovial fluid 5'-nucleotidase (5'N) activity as a marker of general and local inflammation in arthritis, and to resolve a contradiction in the literature as to whether or not the activity of 5'N in the synovial fluids of rheumatoid arthritis (RA) patients is raised in comparison with that in the synovial fluids of other arthritis patients.

METHODS

Assays for 5'N were carried out in the presence of inhibitors of other phosphatases, AMP deaminase and of 5'N itself.

RESULTS

The 5'N activity in the synovial fluid of RA patients was both significantly higher (mean 1.7-fold) and had a greater variance than that in the synovial fluids of other arthritis patients, and the contradiction in the literature was resolved. There was a strong correlation between the 5'N activity in the sera of RA patients and their erythrocyte sedimentation rate. There was no significant correlation between the 5'N in the serum and synovial fluid for the RA patients, in marked contrast to the strong correlation between the two 5'N activities shown by the osteoarthritis patients. The 5'N activity was greater in the synovial fluid than in the serum for virtually all the patients, showing that it was being made locally.

CONCLUSIONS

The 5'N activity in the serum (which came mostly from the liver) could be used as a marker of general inflammation, whereas the 5'N in the synovial fluid was mostly produced locally, and could be used as a marker of joint inflammation, particularly for the RA patients.

Delayed production of adenosine underlies temporal modulation of swimming in frog embryo

1. To investigate the dynamics of adenosine production in the spinal cord during motor activity, and its possible contribution to the temporal modulation of motor patterns, a sensor sensitive to adenosine at concentrations as low as 10 nM was devised. 2. When pressed against the outside of the spinal cord, the sensor detected slow changes in the levels of adenosine during fictive swimming that ranged from 10 to 650 nM. In four embryos where particularly large signals were recorded due to favourable probe placement, the adenosine levels continued to rise for up to a minute following cessation of activity before slowly returning to baseline. In the remaining thirteen embryos, levels of adenosine started to return slowly to baseline almost immediately after activity had stopped. 3. Inhibitors of adenosine uptake increased the magnitude of the signal recorded and slowed the recovery following cessation of activity. 4. A realistic computational model of the spinal circuitry was combined with models of extracellular breakdown of ATP to adenosine. ATP and adenosine inhibited, as in the real embryo, the voltage-gated K+ and Ca2+ currents, respectively. The model reproduced the temporal run-down of motor activity seen in the real embryo suggesting that synaptic release of ATP together with its extracellular breakdown to adenosine is sufficient to exert time-dependent control over motor pattern generation. 5. The computational analysis also suggested that the delay in the rise of adenosine levels is likely to result from feed-forward inhibition of the 5'-ectonucleotidase in the spinal cord. This inhibition is a key determinant of the rate of run-down.

Adenosine, inosine, and guanosine protect glial cells during glucose deprivation and mitochondrial inhibition: correlation between protection and ATP preservation

The purpose of this study was to determine the mechanism by which adenosine, inosine, and guanosine delay cell death in glial cells (ROC-1) that are subjected to glucose deprivation and mitochondrial respiratory chain inhibition with amobarbital (GDMI). ROC-1 cells are hybrid cells formed by fusion of a rat oligodendrocyte and a rat C6 glioma cell. Under GDMI, ATP was depleted rapidly from ROC-1 cells, followed on a much larger time scale by a loss of cell viability. Restoration of ATP synthesis during this interlude between ATP depletion and cell death prevented further loss of viability. Moreover, the addition of adenosine, inosine, or guanosine immediately before the amobarbital retarded the decline in ATP and preserved cell viability. The protective effects on ATP and viability were dependent on nucleoside concentration between 50 and 1,500 microM. Furthermore, protection required nucleoside transport into the cell and the continued presence of nucleoside during GDMI. A significant positive correlation between ATP content at 16 min and cell viability at 350 min after the onset of GDMI was established (r = 0.98). Modest increases in cellular lactate levels were observed during GDMI (1.2 nmol/mg/min lactate produced); however, incubation with 1,500 microM inosine or guanosine increased lactate accumulation sixfold. The protective effects of inosine and guanosine on cell viability and ATP were >90% blocked after treatment with 50 microM BCX-34, a nucleoside phosphorylase inhibitor. Accordingly, lactate levels also were lower in BCX-34-treated cells incubated with inosine or guanosine. We conclude that under GDMI, the ribose moiety of inosine and guanosine is converted to phosphorylated glycolytic intermediates via the pentose phosphate pathway, and its subsequent catabolism in glycolysis provides the ATP necessary for maintaining plasmalemmal integrity.

Evidence for a low temperature transition state binding preference in bovine adenosine deaminase

Arrhenius plots of the interactions of bovine adenosine deaminase (ADA) and of coformycin-inhibited ADA with adenosine are non-linear and reveal that coformycin significantly increases the activation energy for reaction only at temperatures well below the normal operating temperature of the enzyme (38.3 degrees C). This apparent enhanced affinity of the enzyme for the transition state analog at low temperature is confirmed from determinations of coformycin binding at 38.3 degrees C (KI = 5.3 x 10(-11) M) and at 21 degrees C (KI = 1.1 x 10(-11) M). It is suggested that these data are inconsistent with a model for general enzyme catalysis that requires an initial transition state complementary active site. Instead, it is suggested that an initial active site transition state complementarity is undesirable and the tendency of the enzyme to exist in this conformer at low temperatures is responsible for its inefficient interaction with adenosine substrate.

The extracellular fluid of solid carcinomas contains immunosuppressive concentrations of adenosine

The purine nucleoside adenosine (9-beta-D-ribofuranosyladenine) inhibits a number of lymphocyte functions in vitro, including the ability of activated T lymphocytes and natural killer cells to adhere to and kill tumor targets. Solid tumors, such as adenocarcinomas of the lung and colon, are frequently hypoxic and are, therefore, likely to exhibit increased adenine nucleotide breakdown through the 5'-nucleotidase pathway, yielding adenosine. We examined whether the concentration of adenosine in the extracellular fluid of such tumors is adequate to cause immunosuppression. Murine tumors grown in syngeneic hosts or human tumors grown in immunodeficient nu/nu mice were subjected to microdialysis, and adenosine levels in the microdialysate were measured by high-performance liquid chromatography. Treatment of the tumor microdialysates with adenosine deaminase eliminated the adenosine peak. Recovery of adenosine ranged from 15 to 29%, depending on the microdialysis probe, and concentrations of adenosine in tumors ranged from 0.2 to 2.4 microM with a mean of 0.5 microM. In contrast, the adenosine concentration measured s.c. at the same location was 30 +/- 5 nM (mean +/- SE). Inclusion of the adenosine deaminase inhibitor coformycin (10 microM) and the adenosine kinase inhibitor 5'-iodotubercidin (0.1 microM) in the microdialysis perfusion buffer increased extracellular adenosine concentration in tumors to as high as 13 microM. These data show that extracellular adenosine levels in solid tumors are sufficient to suppress the local antitumor immune response and that interference with pathways of adenosine metabolism causes marked increases in tumor extracellular adenosine concentration.

Adenosine-5'-phosphate deaminase. A novel herbicide target

The isolation of carbocyclic coformycin as the herbicidally active component from a fermentation of Saccharothrix species was described previously (B.D. Bush, G.V. Fitchett, D.A. Gates, D. Langley [1993] Phytochemistry 32: 737-739). Here we report that the primary mode of action of carbocyclic coformycin has been identified as inhibition of the enzyme AMP deaminase (EC 3.5.4.6) following phosphorylation at the 5' hydroxyl on the carbocyclic ring in vivo. When pea (Pisum sativum L. var Onward) seedlings are treated with carbocyclic coformycin, there is a very rapid and dramatic increase in ATP levels, indicating a perturbation in purine metabolism. Investigation of the enzymes of purine metabolism showed a decrease in the extractable activity of AMP deaminase that correlates with a strong, noncovalent association of the phosphorylated natural product with the protein. The 5'-phosphate analog of the carbocyclic coformycin was synthesized and shown to be a potent, tight binding inhibitor of AMP deaminase isolated from pea seedlings. Through the use of a synthetic radiolabeled marker, rapid conversion of carbocyclic coformycin to the 5'-phosphate analog could be demonstrated in vivo. It is proposed that inhibition of AMP deaminase leads to the death of the plant through perturbation of the intracellular ATP pool.

Expression of fragile sites triggers intrachromosomal mammalian gene amplification and sets boundaries to early amplicons

Drug-selected intrachromosomal gene amplification by breakage-fusion-bridge (BFB) cycles is well documented in mammalian cells, but factors governing this mechanism are not clear. Here, we show that only some clastogenic drugs induce drug resistance through intrachromosomal amplification. We strictly correlate triggering of BFB cycles to induction of fragile site expression. We demonstrate a dual role for fragile sites in intrachromosomal amplification: a site telomeric to the selected gene is involved in initiation, while a centromeric site defines the size and organization of early amplified units. The positions of fragile sites relative to boundaries of amplicons found in human cancers support the hypothesis that fragile sites play a key role in the amplification of at least some oncogenes during tumor progression.

Purine nucleotide cycle in rat renal cortex and medulla under conditions that mimic normal and low oxygen supply

The distinctive feature of the renal function and metabolism implicate a possibility of excessive ATP degradation during insufficient oxygen supply. Protection of the purine ring against degradation is one among other functions of the purine nucleotide cycle (PNC). The purpose of this study was to estimate the activity of PNC in cytosol of rat renal cortex and medulla under conditions that mimic normal and low oxygen supply in vivo. In normoxic-like condition the rate of AMP deamination was 1.7 and 2.0 nmol/mg protein/min in the cytosol of cortex and medulla, respectively. Under this condition, the rate of IMP reamination was similar to that of AMP deamination. In a hypoxia-like condition the rate of AMP deamination increased by 41% in cytosol from both parts of the kidney, while the rate of IMP reamination remained unchanged in the cytosol of medulla and decreased by 46% in the cortex cytosol. Distribution of the other enzymes of the PNC, that is, adenylosuccinate synthetase and adenylosuccinate lyase, in the cytosol of cortex and medulla correlated with that observed for AMP deamination and IMP reamination potentials. At 150 microM IMP, the activity of adenylosuccinate synthetase in the cortex and medulla was 0.34 and 1.24 nmol/mg protein/min, respectively. Activity of the adenylosuccinate lyase was severalfold greater than the respective activity of the adenylosuccinate synthetase. These results show that the efficiency of PNC is about twice as high in the medulla cytosol as in the cortex cytosol, and that the activity of PNC in kidney is mainly limited by the activity of adenylosuccinate synthetase and supply of AMP.

Induction of apoptosis by cordycepin in ADA-inhibited TdT-positive leukemia cells

The nucleoside analogue cordycepin (3'-deoxyadenosine), when protected against ADA deamination, is specifically cytotoxic for TdT-positive leukemia cells. Cordycepin-treated, ADA-inhibited, TdT-positive cells undergo the classic changes associated with drug-induced apoptosis: reduction in cell volume, chromatin clumping, membrane blebbing, and 180-bp multimer DNA laddering on agarose gels. In common with the apoptosis seen in normal TdT-positive thymocytes, following exposure to various agents, apoptosis induced by cordycepin in TdT-positive leukemia cells was associated with increased protein kinase A (PK-A) activity. Unlike thymocyte apoptosis however, no elevation in cAMP levels was seen preceding the rise in PK-A activity. Ex vivo we show that cordycepin monophosphate can activate PK-A as efficiently as cAMP. On this basis we speculate that cordycepin monophosphate in TdT-positive cells may be able to activate PK-A in place of cAMP, and that PK-A may phosphorylate TdT, augmenting its activity as an endonuclease. In cell-free experiments, the activity of recombinant TdT as an endonuclease digesting supercoiled plasmid DNA into linear fragments was dramatically increased following phosphorylation of TdT by PK-A. A role for TdT as an apoptotic endonuclease in TdT-positive leukemia cells following cordycepin exposure is now the subject of on-going work.

Theoretical study of inhibition of adenosine deaminase by (8R)-coformycin and (8R)-deoxycoformycin

Molecular dynamics and free energy simulations were performed to examine the binding of (8R)-deoxycoformycin and (8R)-coformycin to adenosine deaminase. The two inhibitors differ only at the 2' position of the sugar ring; the sugar moiety of conformycin is ribose, while it is deoxyribose for deoxycoformycin. The 100 ps molecular dynamics trajectories reveal that Asp 19 and His 17 interact strongly with the 5' hydroxyl group of the sugar moiety of both inhibitors and appear to play an important role in binding the sugar. The 2' and 3' groups of the sugars are near the protein-water interface and can be stabilized by either protein residues or water. The flexibility of the residues at the opening of the active site helps to explain the modest difference in binding of the two inhibitors and how substrates/inhibitors can enter an otherwise inaccessible binding site.

2',3'-Dideoxyadenosine killing of TdT-positive cells is due to a trace contaminant

We have previously reported that the chain-terminating nucleoside analogue 2',3'-dideoxyadenosine (ddA) is specifically cytotoxic for TdT-positive cells in the co-presence of the adenosine deaminase (ADA) inhibitor coformycin (CF). Further studies with ddA/CF revealed that cytotoxicity occurs only if ddA is supplied from the Calbiochem or Fluka companies. ddA supplied from other commercial sources (Pharmacia, Sigma) or the NCI Pharmaceutical Resources Branch is non-cytotoxic. To explore the basis for this difference, ddA from various sources was subjected to reverse-phase high-pressure liquid chromatography (HPLC) analysis. The Calbiochem and Fluka ddA had a unique HPLC peak, with a retention time of 12.8 min, representing a contaminant of less than 0.1% of the bulk material applied to the C-18 HPLC column. Study of all HPLC peaks resolved from the bulk material showed cytotoxic activity in only the 12.8 min peak. To identify the nature of the unknown compound, we compared HPLC characteristics of the synthetic intermediates and byproducts of ddA synthesis to the peak eluting at 12.8 min. Of these, only 3'-deoxyadenosine (cordycepin) had similar HPLC characteristics. In addition, in the co-presence of CF, cordycepin was specifically cytotoxic (IC50 < 0.5 microM) for all TdT-positive cell lines tested. Cytotoxicity was seen in TdT-negative cells only at concentrations 10-100-fold higher. We conclude that our previous report on ddA/CF as a TdT-specific cytotoxic combination was due to contamination of the ddA supplied by Calbiochem by cordycepin. ddA itself is non-cytotoxic for TdT-positive cells. Cordycepin in the co-presence of an ADA inhibitor may be effective in the treatment of TdT-positive hematological malignancies.

Apoptosis induced by adenosine in human leukemia HL-60 cells

Among several nucleosides and nucleotides, which showed strong inhibition of growth of HL-60 cells, only adenosine (Ado) specifically induced typical apoptotic death of the cells, accompanying double-strand cleavage of DNA into nucleosomal size fragments, and subsequent apoptotic body formation. A marked enhancement of endogenous poly(ADP-ribosyl)ation activity in the cell was detected at a relatively early stage of cell death, whereas other nucleosides and nucleotides tested were ineffective on poly(ADP-ribosyl)ation activity, suggesting that the enzyme activation is closely related to apoptosis. The observed Ado effect was not mediated by Ado receptors, in contrast to the Ado-induced apoptotic death of thymocytes, judging from the facts that all of the receptor agonists tested did not substitute for Ado and that a receptor antagonist did not inhibit the effect of Ado. Ado transport into the cell seemed to be essential for the induction of apoptosis, since an inhibitor of Ado transport (dipyridamole) strongly suppressed apoptosis. Cytochalasin B blocked Ado-induced apoptotic body formation without affecting activation of endogenous poly(ADP-ribosyl)ation activity in the cell. Thus, the process of apoptosis in HL-60 cells induced by Ado seems to be separated into at least two steps, an initial step of DNA degradation and a following morphological change. While the adenine moiety of Ado was essential for its apoptosis-inducing activity, the sugar was replaceable, and various analogs with modified sugar were inducers of apoptosis, although they were less efficient than Ado.

[Gene amplification and chromosomal rearrangements during acquisition of cellular resistance to the antimetabolite coformycin]

We studied the early stages of gene amplification in a Chinese hamster cell line and we show that two distinct mechanisms can operate at a single locus. Both of them rely on an unequal segregation of gene copies at mitosis. We conclude that cycles of chromatid breakage, followed by fusion of sister chromatids devoid of a telomere that lead to further breaks in mitosis, have a key role in the coupling of gene amplification and genome remodeling. Rearrangements are first limited to a single chromosome but can then potentially spread to any additional chromosome. Occasionally, a sequence containing the selected gene can be looped out, generating a "double minute" and thus initiating an independent process of extrachromosomal amplification.

Adenosine deaminase inhibitors attenuate ischemic injury and preserve energy balance in isolated guinea pig heart

We investigated the effect of the adenosine deaminase inhibitors erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and coformycin on high-energy phosphate metabolism, tissue nucleotides and nucleosides, and recovery of contractile function in isolated, perfused guinea pig hearts. EHNA and coformycin (10 microM) improved postischemic recovery of contractile function approximately 85% and enhanced coronary flow rate in reperfused tissue approximately 40%. The protective effect of EHNA on recovery of contractile function was concentration dependent. Although adenosine (10 microM) increased coronary flow rate on reperfusion approximately twofold over vehicle, it failed to improve postischemic recovery of contractile function. EHNA and coformycin preserved cardiac ATP levels and increased endogenous tissue adenosine during ischemia. During reperfusion, these agents enhanced recovery of high-energy phosphates approximately twofold and potentiated adenosine release into the perfusate with concentration dependency. Furthermore, EHNA and coformycin reduced the extent of myocardial ischemia-reperfusion injury, as indicated by the approximately 55% reduction in creatine phosphokinase release. We conclude that inhibitors of adenosine deaminase attenuate myocardial ischemic injury and improve postischemic recovery of contractile function and metabolism through endogenous myocardial adenosine enhancement and ATP preservation.

The evolution of the amplified adenylate deaminase 2 domains in Chinese hamster cells suggests the sequential operation of different mechanisms of DNA amplification

Fluorescent in situ hybridization was used to localize the adenylate deaminase 2 (AMPD2) genes and flanking sequences on the chromosomes of the Chinese hamster line GMA32 and to study the distribution of additional copies of these genetic sequences in amplified mutants selected at several early stages of the amplification process. The synteny of AMPD2 genes and MDR1 genes, located on chromosomes 1, was demonstrated; in GMA32 the existence of a rearrangement positioning the two AMPD2 genes at different distances from the telomeres was disclosed. Using this structural marker, we showed that the amplified copies distribute along only one of the chromosomes 1. Their organization in different cells of clonal mutant populations at a very early stage of amplification was extremely heterogeneous; classes of organization could be recognized however. Their quantitative distribution at this stage and in cells which went through 10 more division cycles suggests an evolution pathway common to the mutant clones under study: as a rule, tandems of few units of identical and very large size (47 Mb) appear to be the first detected product of amplification; this organization is progressively overtaken by structures with more units of reduced and irregular size, while, in a growing number of cells, clusters of much shorter units can be observed. The nature of segregative amplification mechanisms operating in these processes and the possible involvement of replicative ones are discussed.

The mechanism of adenosine to inosine conversion by the double-stranded RNA unwinding/modifying activity: a high-performance liquid chromatography-mass spectrometry analysis

We have used directly combined high-performance liquid chromatography-mass spectrometry (LC/MS) to examine the mechanism of the reaction catalyzed by the double-stranded RNA unwinding/modifying activity [Bass & Weintraub (1988) Cell 55, 1089-1098]. A double-stranded RNA substrate in which all adenosines were uniformly labeled with 13C was synthesized. An LC/MS analysis of the nucleoside products from the modified, labeled substrate confirmed that adenosine is modified to inosine during the unwinding/modifying reaction. Most importantly, we found that no carbons are exchanged during the reaction. By including H2(18)O in the reaction, we showed that water serves efficiently as the oxygen donor in vitro. These results are consistent with a hydrolytic deamination mechanism and rule out a base replacement mechanism. Although the double-stranded RNA unwinding/modifying activity appears to utilize a catalytic mechanism similar to that of adenosine deaminase, coformycin, a transition-state analogue, will not inhibit the unwinding/modifying activity.

Structure-activity relationship of ligands of human plasma adenosine deaminase2

Diethylaminoethyl-cellulose chromatography was used to separate the two isoenzymes of adenosine deaminase (EC3.5.4.4), adenosine deaminase1 (ADA1) and adenosine deaminase2 (ADA2), in human plasma. One hundred and fifteen purine base, nucleoside, and nucleotide analogs were tested as inhibitors of this partially purified preparation of ADA2. Coformycin and 2'-deoxycoformycin were by far the most potent inhibitors of this isoenzyme (apparent Ki values 20 and 19 nM, respectively). ADA2 was also inhibited by nebularine (apparent Ki 1.5 mM) but was resistant to the potent ADA1 inhibitor (+)-erythro-9(2-S-hydroxy-3-R-nonyl)adenine. alpha-D-Adenosine also inhibited ADA2, as did several halogenated purine and adenine base analogs. Structural requirements for the binding of purine analogs to ADA2 are presented which provide a general basis for the design of specific inhibitors of ADA2. Such inhibitors may be useful in studies designed to provide an understanding of the physiological role of ADA2 both in the normal state and in diseases such as human immunodeficiency virus-1 infection where levels in plasma are increased markedly.

Extracellular metabolism of adenine nucleotides and adenosine in the innervated skeletal muscle of the frog

The effects of coformycin, alpha,beta-methylene ADP, dipyridamole in the absence and presence of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), nitrobenzylthioinosine (NBTI), mioflazine and ouabain on the metabolic pathways of exogenously applied ATP and its metabolites in the frog innervated sartorius muscle were investigated. ATP catabolism yielded ADP, AMP, IMP, adenosine and inosine; the ecto-ATPase in situ was shown to be Ca(2+)- or Mg(2+)-activated with a Kmapp for ATP of 767 +/- 48 microM. AMP catabolism yielded IMP, adenosine and inosine; inosine was formed from either exogenous IMP or exogenous adenosine. Catabolism of AMP into IMP was blocked by coformycin, which enhanced adenosine and inosine formation from AMP. alpha,beta-Methylene ADP blocked adenosine formation from AMP and inosine formation from IMP; formation of IMP from AMP was enhanced by alpha,beta-methylene ADP. Complete blockade of AMP degradation was achieved with the simultaneous use of coformycin and alpha,beta-methylene ADP. Dipyridamole attenuated but did not completely block extracellular adenosine removal and inosine appearance in the bath. EHNA, applied in the presence of dipyridamole, did not cause any further attenuation of extracellular adenosine removal. Mioflazine, NBTI and ouabain did not affect adenosine disappearance from the bath. The results suggest that, in the frog innervated sartorius muscle, ATP can be sequentially catabolized into AMP which is then catabolized either into IMP or into adenosine. This extracellular degradation of AMP into IMP might then constitute a shunt-like mechanism to control the levels of adenosine formed from adenine nucleotides.

Effects of deoxyadenosine on ribonucleotide reductase in adenosine deaminase-deficient lymphocytes

To explore the relationship between ribonucleotide reductase and immunodysfunction in adenosine deaminase deficiency, the effects of deoxyadenosine on ribonucleotide reductase in ADA-deficient lymphocytes was investigated. An assay system for ribonucleotide reductase in intact permeabilized lymphocytes was developed to approximate physiological conditions. The activity of cytidine diphosphate (CDP) reductase in resting but not in proliferating lymphocytes in culture was inhibited by 1 to 10 mumol/L deoxyadenosine. The resting cells were protected from the toxicity of 1 mumol/L deoxyadenosine by 5 mmol/L nicotinamide or 30 mumol/L deoxycytidine and from that of 10 mumol/L deoxyadenosine by 30 mumol/L deoxycytidine. These findings suggest that depletion of nicotinamide adenine dinucleotide might be the principal cause of death in resting lymphocytes with ADA deficiency. It is concluded that the mechanism of deoxyadenosine toxicity on non-replicating lymphocytes, which may not be mediated by ribonucleotide reductase inhibition, is closely related to the mechanism of immunodysfunction in patients with ADA deficiency.

The rate constant describing slow-onset inhibition of yeast AMP deaminase by coformycin analogues is independent of inhibitor structure

(R)- and (S)-2'-deoxycoformycin, (R)-coformycin, and the corresponding 5'-monophosphates were compared as inhibitors of yeast AMP deaminase. The overall inhibition constants ranged from 4.2 mM for (S)-2'-deoxycoformycin to 10 pM for (R)-coformycin 5'-monophosphate, a difference of 3.8 x 10(8) in affinities. (R)-Coformycin, (R)-2'-deoxycoformycin 5'-monophosphate, and (R)-coformycin 5'-monophosphate exhibited both rapid and slow-onset inhibition. The S inhibitors and (R)-2'-deoxycoformycin exhibited classical competitive inhibition but no time-dependent onset of inhibition. The results indicate that the presence of the 2'-hydroxyl and 5'-phosphate and the R stereochemistry at the C-8 position of the diazepine ring are necessary for the optimum interaction of inhibitors with yeast AMP deaminase. This differs from the results for rabbit muscle AMP deaminase [Frieden C., Kurz, L. C., & Gilbert, H. R. (1980) Biochemistry 19, 5303-5309] and calf intestinal adenosine deaminase [Schramm, V. L., & Baker, D. C. (1985) Biochemistry 24, 641-646], in which a tetrahedral hydroxyl at C-8 in the R stereochemistry is sufficient for slow-onset inhibition with the coformycins. The results suggest that the transition state contains a tetrahedral carbon with the R configuration as a result of the direct attack of an oxygen nucleophile at C-6 of AMP. Slow-onset inhibition of yeast AMP deaminase is consistent with the mechanism [formula: see text] in which the combination of E and I is rapidly reversible. For these inhibitors, Ki varied by a factor of 3 x 10(3), and the overall inhibition constant (Ki*) varied by a factor of 2 x 10(5).(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of adenosine triphosphate catabolism induced by deoxyadenosine and by nucleoside analogues in adenosine deaminase-inhibited human erythrocytes

The mechanism of the depletion of ATP, recorded in the erythrocytes of adenosine deaminase-deficient children and of leukemia patients treated with deoxycoformycin, was investigated in normal human erythrocytes treated with this inhibitor of adenosine deaminase. Deoxyadenosine, which accumulates in both clinical conditions, provoked a dose-dependent accumulation of dATP, depletion of ATP, and increases in the production of inosine plus hypoxanthine. Concomitantly, there was an increase of AMP and IMP, but not of adenosine, indicating that catabolism proceeded by way of AMP deaminase. A series of nucleoside analogues (9-beta-D-arabinofuranosyladenine, N6-methyladenosine, 6-methylmercaptopurine ribonucleoside, tubercidin, ribavirin, and N-1-ribosyl-5-aminoimidazole-4-carboxamide riboside) also stimulated adenine nucleotide catabolism and increased AMP and IMP to various extents. The effects of deoxyadenosine and of the nucleoside analogues were prevented by 5'-iodotubercidin, an inhibitor of adenosine kinase. Strikingly, they were reversed if the inhibitor was added after the accumulation of nucleotide analogues and initiation of adenine nucleotide catabolism. Further analyses revealed linear relationships between the rate of phosphorylation of deoxyadenosine and nucleoside analogues and the increase in AMP and between the elevation of the latter above a threshold concentration of 10 microM and the rate of adenine nucleotide catabolism. Kinetic studies with purified erythrocytic AMP deaminase, at physiological concentrations of its effectors, showed that the enzyme is nearly inactive up to 10 microM AMP and increases in activity above this threshold. We conclude that the main mechanism whereby deoxyadenosine and nucleoside analogues stimulate catabolism of adenine nucleotides by way of AMP deaminase in erythrocytes is elevation of AMP, secondary to the phosphorylation of the nucleosides.

IMP production by ATP-depleted adult rat heart cells. Effects of glycolysis and alpha 1-adrenergic stimulation

A rapid deenergization procedure was used to probe the regulation of in situ adenylate deaminase and 5'-nucleotidase in isolated adult rat heart cells. In cells depleted of ATP, the rate of ionosine monophosphate (IMP) production was fourfold greater in cells that had been respiring prior to deenergization than in cells that had been maintaining ATP stores through anaerobic glycolysis. This effect of respiratory inhibition was fully reversed by reaeration. When phenylephrine was present during preincubation, IMP production during a subsequent 5-minute rapid deenergization was increased by 70% in respiring cells and by 88% in those that had not been respiring. These effects of phenylephrine were abolished by prazosin. Adenosine production by cells without ATP was inversely related to that of IMP, whereas it was positively correlated with the amount of AMP remaining in cells after 5 minutes. We conclude from these data that rat heart adenylate deaminase is regulated by a product(s) of anaerobic glycolysis and by alpha 1-adrenergic stimulation. The production of intracellular adenosine in cells without ATP, on the other hand, is governed primarily by the concentration of AMP and appears to be catalyzed by the cytosolic type I 5'-nucleotidase.

The erythrocyte as instigator of inflammation. Generation of amidated C3 by erythrocyte adenosine deaminase

Myocardial ischemia is characterized by the liberation of adenosine and by complement-mediated inflammation. We have reported that amidated C3, formed when ammonia (NH3) disrupts the thiolester bond of C3, serves as an alternative pathway convertase, generates C5b-9, and stimulates phagocytic oxidative metabolism. We investigated whether the deamination of adenosine by adenosine deaminase in hematopoietic cells might liberate sufficient ammonia to form amidated C3 and thereby trigger complement-mediated inflammation at ischemic sites. In the presence of 4 mM adenosine, NH3 production per erythrocyte (RBC) was equal to that per neutrophil (PMN) (3.3 X 10(-15) mol/cell per h). Because RBC outnumber PMN in normal blood by a thousandfold, RBC are the major source of NH3 production in the presence of adenosine. NH3 production derived only from the deamination of adenosine by the enzyme adenosine deaminase and was abolished by 0.4 microM 2'-deoxycoformycin, a specific inhibitor of adenosine deaminase. When purified human C3 was incubated with 5 X 10(8) human RBC in the presence of adenosine, disruption of the C3 thiolester increased more than twofold over that measured in C3 incubated with buffer, or in C3 incubated with RBC (P less than 0.05). The formation of amidated C3 was abolished by the preincubation of RBC with 2'-deoxycoformycin (P less than 0.001). Amidated C3 elicited statistically significant release of superoxide, myeloperoxidase, and lactoferrin from PMN. Thus, the formation of amidated C3 by RBC deamination of adenosine triggers a cascade of complement-mediated inflammatory reactions.

Inhibition of the zymosan-induced chemiluminescence of human phagocytes by adenosine, polyadenylic acid and agents influencing adenosine metabolism

The zymosan-induced chemiluminescence of human phagocytes (neutrophil granulocytes) could be inhibited by adenosine, polyadenylic acid, dipyridamole (an agent blocking the cellular adenosine uptake) and coformycin (a highly potent inhibitor of enzyme adenosine deaminase). It is suggested that in estimating the antianginal and antithrombotic action of dipyridamole, its action on the phagocytes should be taken into account. The same consideration would be true for 2'-deoxycoformycin (a coformycin analogue) used as a potent anticancer and immunosuppressive drug.

Permeation and salvage of dideoxyadenosine in mammalian cells

Transmembrane equilibration of 2',3'-dideoxyadenosine (ddAdo) was measured by rapid kinetic techniques in deoxycoformycintreated P388 and L1210 mouse leukemia cells and human erythrocytes, at 25 degrees. It was only about 10% as rapid as that of other purine nucleosides that are known substrates for the nucleoside transporters of these cells. ddAdo entry was nonsaturable up to a concentration of 1 mM and was not inhibited by other nucleosides or two nucleoside transport inhibitors, dipyridamole and nitrobenzylthioinosine. Thus, ddAdo permeation was mainly nonmediated. It was relatively rapid because of the high lipid solubility of ddAdo. ddAdo entered the cells at least 100 times more rapidly than dideoxycytidine but less rapidly than trideoxythymidine, with an even greater lipophilicity than ddAdo. ddAdo was not phosphorylated in human erythrocytes, but there was some phosphorylation in deoxycoformycin-treated P388 and L1210 cells. In situ conversion of 10 microM ddAdo to ddATP, however, was slow and ceased after 5-10 min at 25 degrees or 37 degrees. Cessation of net uptake was not due to turnover of dideoxy-ATP or deamination of dideoxy-AMP. The results suggest that ddAdo salvage in the absence of deamination is limited by feedback inhibition of its phosphorylation, perhaps by deoxycytidine kinase. Permeation into the cells was not rate limiting to ddAdo salvage. In P388 and L1210 cells that had not been treated with deoxycoformycin, ddAdo was salvaged at least 100 times more efficiently than in deoxycoformycin-treated cells and converted to nucleoside triphosphates, but the end-products and pathways of salvage have not been resolved entirely. Salvage of ddAdo required deamination but was not primarily via dideoxyinosine----hypoxanthine----IMP, as is the case for 2'-deoxyadenosine salvage, because [3H]ddAdo salvage was only little inhibited by unlabeled hypoxanthine, whereas it was strongly inhibited by 2'-deoxyadenosine, adenosine, and adenine.

Inhibition by adenosine of histamine and leukotriene release from human basophils

Adenosine inhibited the release of histamine and leukotriene C4 (LTC4) from immunologically-activated basophils in a dose-dependent manner. Structural congeners of adenosine also attenuated the elaboration of these two mediators from stimulated basophils and a rank order of potency for the inhibition was observed following the sequence 2-chloroadenosine greater than or equal to N-ethylcarboxamidoadenosine (NECA) greater than adenosine greater than or equal to R-phenylisopropyladenosine (R-PIA) greater than or equal to S-PIA. These same nucleosides modulated the generation of LTC4 more potently than the release of histamine. A number of methylxanthines, which are antagonists of cell surface adenosine receptors, reversed the inhibition by adenosine and its congeners of the release of both histamine and LTC4 to varying extents. Dipyridamole and nitrobenzylthioinosine (NBTI), agents that block the intracellular uptake of adenosine, antagonized the inhibition of histamine release by adenosine (and 2-chloroadenosine) but failed to reverse the attenuation of LTC4 generation by the nucleoside. These same uptake blockers were unable to antagonize the inhibitory effects of NECA on either histamine or LTC4 release. In purified basophils, NECA and R-PIA, and in that order of decreasing reactivity, increased total cell cyclic adenosine monophosphate (cAMP) levels and inhibited the stimulated release of mediators. In total, these results suggest that the basophil possesses a cell surface adenosine receptor which, on the basis of both pharmacological and biochemical criteria, most closely conforms to an A2/Ra-like receptor. However, in addition to an interaction at the cell surface, studies with agents that block the intracellular uptake of adenosine suggest that the nucleoside may also exert intracellular effects when countering the release of histamine (but not LTC4).

Expression of a multidrug resistance-adenosine deaminase fusion gene

A novel fusion gene has been created in which the expression of a dominant selectable marker, the human multidrug resistance gene, is directly linked to the expression of human adenosine deaminase cDNA. The chimeric gene was inserted between the long terminal repeats of a Harvey murine sarcoma virus expression vector and used to transfect drug-sensitive human KB carcinoma cells. Transfectants were selected in increasing concentrations of colchicine and found to contain multiple copies of the intact fusion gene, which is stably and efficiently expressed. A membrane-associated 210-kDa human P-glycoprotein-adenosine deaminase fusion protein is synthesized which retains function of the multidrug transporter and also exhibits adenosine deaminase activity. The data indicate that the human multidrug resistance gene may be used as a dominant selectable marker to introduce other genes in the form of gene fusions into cultured cells.

Potentiation of growth-inhibitory activity of 9-beta-D-arabinofuranosyladenine by 2'-deoxycoformycin in human cultured cell lines derived from leukemias and lymphomas

Growth-inhibitory activity of 2'-deoxycoformycin (DCF) and 9-beta-D-arabinofuranosyladenine (Ara-A) used either singly or in combination was assessed in 30 human cultured cell lines (seven T-cell, nine B-cell, five non-T,non-B and nine myeloid cell lines) derived from leukemias and lymphomas. DCF had little activity even at 100 microM on any of the cell lines, while Ara-A had an obvious inhibitory effect on them, especially on non-T,non-B cell lines at 10 microM or less. Lymphoid cell lines were apparently more sensitive to the combined use of Ara-A and DCF than myeloid cell lines. DCF potentiated the antiproliferative activity of Ara-A not only in T-cell lines with high adenosine deaminase (ADA) activity, but also in some other cell lines with low ADA activity. DCF was stable in the culture medium, but Ara-A in the medium containing cultured cells was rapidly inactivated. DCF completely inhibited the inactivation of Ara-A in the medium containing P12/ICH or NALM-6, but not in the medium containing Daudi. This suggests that there is some unknown mechanism(s) of inactivation of Ara-A other than ADA in Daudi, which was insensitive to Ara-A in the presence of 1 microM DCF. The capacity of DCF to inhibit degradation of Ara-A in the medium containing these cultured cells correlated with the level of Ara-A sensitivity potentiated by DCF. In all seven T-cell lines, seven of the nine B-cell lines, all five non-T,non-B cell lines, and only three of nine myeloid cell lines, the IC50 value for Ara-A decreased to 5 microM or less in the presence of 1 microM DCF. These results suggest that the combination of DCF and Ara-A may be effective against various types of lymphoid malignancies and some myeloid leukemias.

dATP-mediated inhibition of DNA ligase by 2'-deoxycoformycin in T and B cell leukemia

2'-Deoxycoformycin (dCF), a potent adenosine deaminase inhibitor, has been reported to display greater toxicity for T than for B lymphoblasts. Since this compound can block DNA replication and since this effect is mediated by the intracellular ATP/dATP balance, its possible effect on DNA ligase was investigated. dCF at relatively low concentrations (1 microM), in association with dATP (100 microM), is a strong inhibitor of DNA ligase in T blasts, whereas it has no significant effect in B blasts at this concentration. The AMP-ligase complex is the target of the observed inhibition because the combined presence of the inhibitor and dATP results in a more stable dAMP-ligase complex. Because of this observation and of the greater adenosine deaminase activity observed in T cells, the dATP mediated dCF inhibition of ligase might be the crucial replication target of T cell toxicity. These observations are discussed in terms of T immunodeficiencies including Graft Versus Host Disease and related syndromes.

Peripheral blood and bone marrow changes following 2'-deoxycoformycin therapy in hairy cell leukemia. Results of 200 weeks' follow-up

Peripheral blood (PB) and bone marrow (BM) changes during 200 weeks' follow-up of 15 patients with hairy cell leukemia (HCL) undergoing low-dose 2'-deoxycoformycin (dCF) therapy are reported. Thirteen patients rapidly achieved complete remissions (CR) (median, 16 weeks). Previous splenectomy (two patients), or chemotherapy (two patients) had no effect on dCF response. Twelve patients have remained in CR. Patients with marked BM infiltration (hairy cell index [HCI] greater than 0.5; n = 5) had more pronounced pancytopenia and showed a slower hematologic recovery than those with a lesser degrees of infiltration. Additionally, patients with cytologic type II HCL (n = 5) had more pronounced pancytopenia with a greater tumor load in the BM, and exhibited slower hematologic recovery than those with type I (n = 5) HCL. There was a gradual decline in BM cellularity from 65% to 25% during year 1, a level which remained stable thereafter. Reticulin in the BM regressed in all nine patients in whom it was increased before dCF therapy. The authors have not seen any dysplastic changes in the hematopoietic cells during the period of follow-up.

Enzyme activities of leukemic cells and biochemical changes induced by deoxycoformycin in vitro--lack of correlation with clinical response

Deoxycoformycin (DCF) is a specific inhibitor of adenosine deaminase (ADA) and has been shown to be active in lymphoid neoplasms. Cytotoxicity is thought to be mediated by the accumulation of deoxyadenosine (AdR) and deoxyadenosine triphosphate (dATP) which inhibits ribonucleotide reductase and DNA synthesis in rapidly proliferating cells. Others suggested mechanisms leading to cell death particularly in non-dividing cells include depletion of ATP and NAD pools, inhibition of S-adenosylhomocysteine (SAH) hydrolase and induction of DNA strand breaks. In patients with high leukemic counts who were subsequently treated with DCF, we have studied (a) the levels of ADA, ecto-5'-nucleotidase (5NT), deoxyadenosine kinase (AdR-kinase) and SAH-hydrolase in the leukemic cells; [b) the in-vitro effects of DCF on dATP, ATP, NAD, SAH-hydrolase levels and on DNA strand breaks; and (c) the correlation between these parameters with clinical response to DCF. No significant difference in ADA, 5NT, AdR-kinase and SAH-hydrolase activities could be found between responders and non-responders. Incubation of the leukemic cells in vitro with DCF caused an inhibition of ADA, an accumulation of dATP, a moderate reduction in ATP and NAD levels, a suppression of SAH-hydrolase activity and an increase in DNA strand breaks in practically all the leukemic samples, irrespective of clinical response. Our results show that neither measurement of these enzymes nor studies of these biochemical sequelae of ADA inhibition in vitro predicts clinical responsiveness to DCF therapy.

Pentostatin (2'deoxycoformycin) for the treatment of lymphoid neoplasms

Knowledge of the vital role of adenosine deaminase in lymphatic tissues has led to the development of enzyme inhibitors for treatment of lymphoid neoplasms. Deoxycoformycin is a potent ADA inhibitor and has been shown to be active in acute lymphoblastic leukemia at high doses but associated with unpredictable toxicity. In indolent lymphocytic leukemia or lymphoma with low ADA concentrations, this drug is effective at low doses with mild toxicity. The on-going EORTC trial shows that pentostatin is highly effective in hairy cell leukemia and can achieve durable complete remissions even if interferon alpha has failed. It will probably play an important role in the treatment of prolymphocytic leukemia, T- and B-cell chronic lymphocytic leukemia and Sézary syndrome.

Adenosine induction of rapid catabolism of adenine ribonucleotides and independent elevation of the ATP content in quiescent mouse fibroblasts

The influence of adenosine on the ribonucleotide metabolism in quiescent BALB/c 3T3 cells was studied. The cellular adenine ribonucleotides were labelled by pretreating the cells with [2-3H]-adenine. After addition of adenosine to the cell cultures, the amount and radioactivity of the cellular purine ribonucleotides and the radioactivity of the purine compounds in the medium were determined. It appeared that adenosine gave rise both to rapid catabolism of adenine ribonucleotides with inosine 5'-monophosphate (IMP) as an intermediate and to expansion of the cellular adenosine 5'-triphosphate (ATP) pool. The maximal rates and the apparent activation constants for the two processes have been determined. Experiments with varying concentrations of coformycin (an inhibitor of adenosine 5'-monophosphate [AMP] deaminase and adenosine deaminase) and of 5'-amino-5'-deoxyadenosine (an inhibitor of adenosine kinase), respectively, showed that each compound may almost completely inhibit the adenosine-induced catabolism. This effect can be obtained under conditions where there was little or no effect by the two inhibitors on the rate of expansion of the cellular ATP pool. These results may best be explained by assuming that the process of expansion of the ATP pool is independent of the induced catabolism of adenine ribonucleotides, even though both processes seem to depend on the phosphorylation of adenosine to AMP. The total increase in the pool size of ATP and of guanosine 5'-triphosphate (GTP), both caused by adenosine, seems not to have regulatory effect on adenine ribonucleotide catabolism.

Inhibition of adenosine-metabolizing enzymes modulates mouse sperm fertilizing ability: a changing role for endogenously generated adenosine during capacitation

The effect of inhibiting adenosine-metabolizing enzymes on sperm fertilizing ability was studied to investigate a possible role for endogenously generated adenosine in the regulation of capacitation. The compounds used have been shown to be effective inhibitors of the relevant enzymes in similarly incubated mouse sperm suspensions. Inhibition of 5'-nucleotidase activity with alpha, beta-methylene adenosine 5'-diphosphate (AMPCP), to reduce available endogenous adenosine, caused a dose-dependent inhibition of the fertilizing ability of partially capacitated spermatozoa, which was significant with 100 and 250 microM AMPCP. Conversely, inhibition of adenosine deaminase with 100 nM coformycin, to increase available endogenous adenosine, promoted the fertilizing ability of partially capacitated spermatozoa when the fertilization rate of control suspensions was low. However, coformycin had no effect on sperm suspensions with moderate fertilizing ability, and it inhibited fertilizing ability when added to capacitated spermatozoa. These data are consistent with a promotion of the early stages of capacitation by endogenously generated adenosine and suggest that sensitivity to adenosine changes as capacitation proceeds. Because the majority of adenosine-metabolizing enzyme activity residues in or is directed toward the extracellular compartment in such suspensions, these effects of adenosine may be mediated at the outer surface of the cell. By interacting with receptors on adenylate cyclase, externally produced adenosine could modulate intracellular levels of cyclic adenosine monophosphate (cAMP), thereby influencing fertilizing ability.

Cultures of glial cells release purines under field electrical stimulation: the possible ionic mechanisms

Dissociated primary cultures of glial cells released a remarkable amount of purines, at rest and during field electrical stimulation. The HPLC identification of labelled compounds derived from 3H-Adenosine (3H-Ado) (employed to preload the cultures) indicated that nucleotides and nucleosides were represented in the superfusate in equivalent proportions (43.86% and 56.14% respectively). Very much higher amounts of unlabelled purines prevalently constituted by nucleotides compounds (91.10%) were also released and detectable in the superfusate. In all the experimental conditions their evoked release did not result frequency-dependent. Since: a linear increase related to the stimulation frequencies was found for the released labelled compounds; no labelled purines were assayed in 5 x 10-5M Dipyridamole-treated cultures; any significant presence of labelled nucleotides, inosine and hypoxantine was not found in cultures simultaneously treated with 1 x 10-5M 2'-deoxycoformycin and 1 x 10-4M 1-(-5-isoquinolinsulfonyl)-2-methylpiperizine (H7) (3H-Ado amounts resulted more than doubled in these experimental conditions); labelled compounds have been assumed as tracers of a glial purine rate whose release can be connected to electrically-evoked action potentials. Purine outflow from glial cells is not sodium dependent, in fact TTX (5 x 10-7M) did not affect their basal or electrically-evoked release. A remarkable calcium-dependence was also evidentiated by the 1 x 10-4M Verapamil-induced inhibition of basal and evoked release. TEA (1 x 10-2M), a specific inhibitor of potassium efflux throughout calcium-mediated specific channels, strongly reduced the evoked purine outflow and any additive effect of its was not detectable when administered simultaneously to the calcium antagonist. These findings indicate that the frequency-dependent purine release from cultured glial cells is linked to ionic mechanisms, which calcium and potassium are mainly involved in.

Metabolic and functional consequences of inhibiting adenosine deaminase during renal ischemia in rats

The concentrations of renal ATP have been measured by 31P-nuclear magnetic resonance (NMR) before, during, and after bilateral renal artery occlusion. Using in vivo NMR, the initial postischemic recovery of ATP increased with the magnitude of the residual nucleotide pool at the end of ischemia. ATP levels after 120 min of reflow correlated with functional recovery at 24 h. In the present study the effect of blocking the degradation of ATP during ischemia upon the postischemic restoration of ATP was investigated. Inhibition of adenosine deaminase by 80% with the tight-binding inhibitor 2'-deoxycoformycin led to a 20% increase in the residual adenine nucleotide pool. This increased the ATP initial recovery after 45 min of ischemia from 52% (in controls) to 62% (in the treated animals), as compared to the basal levels. The inhibition also caused an accelerated postischemic restoration of cellular ATP so that at 120 min it was 83% in treated rats vs. 63% in untreated animals. There was a corresponding improvement in the functional recovery from the insult (increase of 33% in inulin clearance 24 h after the injury). Inhibition of adenosine deaminase during ischemia results in a injury similar to that seen after a shorter period of insult.

Adenosine potentiates mediator release from human lung mast cells

Micromolar concentrations of adenosine were found to potentiate the release of histamine and leukotriene C4 (LTC4) from immunologically activated human lung mast cells (HLMC). Structurally modified congeners of adenosine including 5'-N-ethylcarboxamideadenosine (NECA) and R-phenylisopropyladenosine (R-PIA) also potentiated mediator release. A rank order of potency was established where NECA greater than R-PIA for the potentiation of both LTC4 production and histamine secretion. Mast cells isolated by either enzymatic or mechanical means from human lung parenchyma were both similarly responsive to the modulatory effects of adenosine and analogues, and the potency series of NECA greater than R-PIA also applied. Moreover, histamine release induced by the calcium ionophore A23187 was augmented by NECA, R-PIA, and adenosine and in that potency order. Dipyridamole, an agent thought to impede the intracellular uptake of adenosine, failed to reverse the nucleoside's enhancement of IgE-mediated secretion. The irreversible inhibitor of adenosine deaminase, deoxycoformycin, did not modify the adenosine enhancement of stimulated secretion. Low concentrations of methylxanthines, which antagonize responses mediated at cell surface adenosine receptors, were inconsistent in their effects. Theophylline modestly reversed the adenosine-induced potentiation of IgE-mediated LTC4 generation but not histamine release. Studies employing 8-phenyltheophylline were complicated by the methylxanthine possessing inhibitory properties of its own at concentrations expected to antagonize a nucleoside-mediated effect. In total, these results suggest that the response of HLMC to adenosine describes properties most consistent with an A2/Ra-like process, although an interaction via an, as yet, uncharacterized cell surface receptor cannot be excluded.

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.