Heterogeneous localization of some purine enzymes in subcellular fractions of rat brain and cerebellum

Microglial Adenosine Receptors: From Preconditioning to Modulating the M1/M2 Balance in Activated Cells

Neuronal survival depends on the glia, that is, on the astroglial and microglial support. Neurons die and microglia are activated not only in neurodegenerative diseases but also in physiological aging. Activated microglia, once considered harmful, express two main phenotypes: the pro-inflammatory or M1, and the neuroprotective or M2. When neuroinflammation, i.e., microglial activation occurs, it is important to achieve a good M1/M2 balance, i.e., at some point M1 microglia must be skewed into M2 cells to impede chronic inflammation and to afford neuronal survival. G protein-coupled receptors in general and adenosine receptors in particular are potential targets for increasing the number of M2 cells. This article describes the mechanisms underlying microglial activation and analyzes whether these cells exposed to a first damaging event may be ready to be preconditioned to better react to exposure to more damaging events. Adenosine receptors are relevant due to their participation in preconditioning. They can also be overexpressed in activated microglial cells. The potential of adenosine receptors and complexes formed by adenosine receptors and cannabinoids as therapeutic targets to provide microglia-mediated neuroprotection is here discussed.

Of adenosine and the blues: The adenosinergic system in the pathophysiology and treatment of major depressive disorder

Major depressive disorder (MDD) is the foremost cause of global disability, being responsible for enormous personal, societal, and economical costs. Importantly, existing pharmacological treatments for MDD are partially or totally ineffective in a large segment of patients. As such, the search for novel antidepressant drug targets, anchored on a clear understanding of the etiological and pathophysiological mechanisms underpinning MDD, becomes of the utmost importance. The adenosinergic system, a highly conserved neuromodulatory system, appears as a promising novel target, given both its regulatory actions over many MDD-affected systems and processes. With this goal in mind, we herein review the evidence concerning the role of adenosine as a potential player in pathophysiology and treatment of MDD, combining data from both human and animal studies. Altogether, evidence supports the assertions that the adenosinergic system is altered in both MDD patients and animal models, and that drugs targeting this system have considerable potential as putative antidepressants. Furthermore, evidence also suggests that modifications in adenosine signaling may have a key role in the effects of several pharmacological and non-pharmacological antidepressant treatments with demonstrated efficacy, such as electroconvulsive shock, sleep deprivation, and deep brain stimulation. Lastly, it becomes clear from the available literature that there is yet much to study regarding the role of the adenosinergic system in the pathophysiology and treatment of MDD, and we suggest several avenues of research that are likely to prove fruitful.

Adenosine Deaminase Activity in Chronic Lymphocytic Leukemia and Healthy Subjects

BACKGROUND

B cell chronic lymphocytic leukemia is one of the most frequent hematologic malignancies in the world. Cellular surface CD markers and serum Beta-2-microglobulin may be used as a prognostic tool in CLL patients.

OBJECTIVES

In the present study we introduce serum adenosine deaminase as a diagnostic marker in CLL.

MATERIALS AND METHODS

Blood samples were collected from B-CLL and healthy subjects. White blood cell, red blood cell and platelet count and blood Erythrocyte sedimentation rate was recorded and serum Beta-2-microglobulin, Lactate dehydrogenase and total ADA enzyme activity were determined.

RESULTS

Serum ADA activity was significantly higher in patients group than that of controls. ADA had a significant and direct correlation with B2M, WBC, LDH and ESR. However, there was not any relation between ADA and the stages of disease. Diagnostic cut-off, sensitivity and specificity of the serum ADA test were 27.97 U/L, 91% and 94%, respectively.

CONCLUSIONS

A higher ADA activity in patients group and its correlation with CLL markers were seen in our study. High diagnostic value of serum ADA in our study suggests that it might be considered as a useful screening tool among the other markers in CLL.

Adenosine deaminase regulates Treg expression in autologous T cell-dendritic cell cocultures from patients infected with HIV-1

Regulatory T cells have an important role in immune suppression during HIV-1 infection. As regulatory T cells produce the immunomodulatory molecule adenosine, our aim here was to assess the potential of adenosine removal to revert the suppression of anti-HIV responses exerted by regulatory T cells. The experimental setup consisted of ex vivo cocultures of T and dendritic cells, to which adenosine deaminase, an enzyme that hydrolyzes adenosine, was added. In cells from healthy individuals, adenosine hydrolysis decreased CD4(+)CD25(hi) regulatory T cells. Addition of 5'-N-ethylcarboxamidoadenosine, an adenosine receptor agonist, significantly decreased CD4(+)CD25(lo) cells, confirming a modulatory role of adenosine acting via adenosine receptors. In autologous cocultures of T cells with HIV-1-pulsed dendritic cells, addition of adenosine deaminase led to a significant decrease of HIV-1-induced CD4(+)CD25(hi) forkhead box p3(+) cells and to a significant enhancement of the HIV-1-specific CD4(+) responder T cells. An increase in the effector response was confirmed by the enhanced production of CD4(+) and CD8(+) CD25(-)CD45RO(+) memory cell generation and secretion of Th1 cytokines, including IFN-γ and IL-15 and chemokines MIP-1α/CCL3, MIP-1β/CCL4, and RANTES/CCL5. These ex vivo results show, in a physiologically relevant model, that adenosine deaminase is able to enhance HIV-1 effector responses markedly. The possibility to revert regulatory T cell-mediated inhibition of immune responses by use of adenosine deaminase, an enzyme that hydrolyzes adenosine, merits attention for restoring T lymphocyte function in HIV-1 infection.

Influence of infection by Toxoplasma gondii on purine levels and E-ADA activity in the brain of mice experimentally infected mice

The aim of this study was to assess the purine levels and E-ADA activity in the brain of mice (BALB/c) experimentally infected with Toxoplasma gondii. In experiment I (n=24) the mice were infected with RH strain of T. gondii, while in experiment II (n=36) they were infected with strain ME-49 of T. gondii. Our results showed that, for RH strain (acute phase), an increase in both periods in the levels of ATP, ADP, AMP, adenosine, hypoxanthine, xanthine (only on day 6 PI) and uric acid (only on day 6 PI). By the other hand, the RH strain led, on days 4 and 6 PI, to a reduction in the concentration of inosine. ME-49, a cystogenic strain, showed some differences in acute and chronic phase, since on day 6 PI the levels of ATP and ADP were increased, while on day 30 these same nucleotides were reduced. On day 60 PI, ME-49 induced a reduction in the levels of ATP, ADP, AMP, adenosine, inosine and xanthine, while uric acid was increased. A decrease of E-ADA activity was observed in brain on days 4 and 6 PI (RH), and 30 PI (ME-49); however on day 60 PI E-ADA activity was increased for infection by ME-49 strain. Therefore, it was possible to conclude that infection with T. gondii changes the purine levels and the activity of E-ADA in brain, which may be associated with neurological signs commonly observed in this disease.

Understanding the added value of g-protein-coupled receptor heteromers

G-protein-coupled receptors (GPCRs) constitute the most populated family of proteins within the human genome. Since the early sixties work on GPCRs and on GPCR-mediated signaling has led to a number of awards, the most recent being the Nobel Prize in Chemistry for 2012. The future of GPCRs research is surely based on their capacity for heteromerization. Receptor heteromers offer a series of challenges that will help in providing success in academic/basic research and translation into more effective and safer drugs.

Adenosine deaminase enhances the immunogenicity of human dendritic cells from healthy and HIV-infected individuals

ADA is an enzyme implicated in purine metabolism, and is critical to ensure normal immune function. Its congenital deficit leads to severe combined immunodeficiency (SCID). ADA binding to adenosine receptors on dendritic cell surface enables T-cell costimulation through CD26 crosslinking, which enhances T-cell activation and proliferation. Despite a large body of work on the actions of the ecto-enzyme ADA on T-cell activation, questions arise on whether ADA can also modulate dendritic cell maturation. To this end we investigated the effects of ADA on human monocyte derived dendritic cell biology. Our results show that both the enzymatic and non-enzymatic activities of ADA are implicated in the enhancement of CD80, CD83, CD86, CD40 and CCR7 expression on immature dendritic cells from healthy and HIV-infected individuals. These ADA-mediated increases in CD83 and costimulatory molecule expression is concomitant to an enhanced IL-12, IL-6, TNF-α, CXCL8(IL-8), CCL3(MIP1-α), CCL4(MIP-1β) and CCL5(RANTES) cytokine/chemokine secretion both in healthy and HIV-infected individuals and to an altered apoptotic death in cells from HIV-infected individuals. Consistently, ADA-mediated actions on iDCs are able to enhance allogeneic CD4 and CD8-T-cell proliferation, globally yielding increased iDC immunogenicity. Taken together, these findings suggest that ADA would promote enhanced and correctly polarized T-cell responses in strategies targeting asymptomatic HIV-infected individuals.

ATP and noradrenaline activate CREB in astrocytes via noncanonical Ca(2+) and cyclic AMP independent pathways

In neurons, it is well established that CREB contributes to learning and memory by orchestrating the translation of experience into the activity-dependent (i.e., driven by neurotransmitters) transcription of plasticity-related genes. The activity-dependent CREB-triggered transcription requires the concerted action of cyclic AMP/protein kinase A and Ca(2+) /calcineurin via the CREB-regulated transcription co-activator (CRTC). It is not known, however, whether a comparable molecular sequence occurs in astrocytes, despite the unquestionable contribution of these cells to brain plasticity. Here we sought to determine whether and how ATP and noradrenaline cause CREB-dependent transcription in rat cortical astrocyte cultures. Both transmitters induced CREB phosphorylation (Western Blots), CREB-dependent transcription (CRE-luciferase reporter assays), and the transcription of Bdnf, a canonical regulator of synaptic plasticity (quantitative RT-PCR). We indentified a Ca(2+) and diacylglycerol-independent protein kinase C at the uppermost position of the cascade leading to CREB-dependent transcription. Notably, CREB-dependent transcription was partially dependent on ERK1/2 and CRTC, but independent of cyclic AMP/protein kinase A or Ca(2+) /calcineurin. We conclude that ATP and noradrenaline activate CREB-dependent transcription in cortical astrocytes via an atypical protein kinase C. It is of relevance that the signaling involved be starkly different to the one described in neurons since there is no convergence of Ca(2+) and cyclic AMP-dependent pathways on CRTC, which, moreover, exerts a modulatory rather than a central role. Our data thus point to the existence of an alternative, non-neuronal, glia-based role of CREB in plasticity.

Adenosine deaminase potentiates the generation of effector, memory, and regulatory CD4+ T cells

By interacting with CD26 on the CD4+ T cell surface and with the AdoR A(₂B) on the DC surface, ADA triggers a costimulatory signal for human T cells. The aim of this study was to know whether ADA-mediated costimulation plays a role in the differentiation of T cells. The results show that irrespective of its enzymatic activity and dependent on TNF-α, IFN-γ, and IL-6 action, ADA enhanced the differentiation of CD4+CD45RA+CD45RO⁻ naïve T cells toward CD4+CD25+CD45RO+ Teffs and CD4+CD45RA⁻CD45RO+ memory T cells. Furthermore, ADA potentiated generation of CD4+CD25(high)Foxp3+ Tregs by a mechanism that seems to be mainly dependent on the enzymatic activity of ADA. Interestingly, an ADA-mediated increase on Teff, memory T cell, and Treg generation occurred, not only in cocultures from healthy individuals but also from HIV-infected patients. These data suggest that ADA is a relevant modulator of CD4+ T cell differentiation, even in cells from immunologically compromised individuals.

Influence of mercury chloride on adenosine deaminase activity and gene expression in zebrafish (Danio rerio) brain

Mercury is a widespread environmental contaminant that is neurotoxic even at very low concentrations. In this study we investigated the effects of mercury chloride on soluble and membrane adenosine deaminase (ADA) activity and gene expression in zebrafish brain. Inhibition of ADA activity was observed in the soluble fraction at 5-250 microM HgCl(2) (84.6-92.6%, respectively), whereas inhibition occurred at 50-250 microM in membrane fractions (20.9-26%, respectively). We performed in vitro experiments with chelants (EDTA and DTT) to test if these compounds prevented or reversed the inhibition caused by HgCl(2) and found that the inhibition was partially or fully abolished. The effect on ADA activity in soluble and membrane fractions was evaluated after acute (24h) and subchronic (96h) in vivo exposure of zebrafish to 20 microg/l HgCl(2). ADA activity in the soluble fraction was decreased after both acute (24.5%) and subchronic (40.8%) exposures, whereas in brain membranes the enzyme was inhibited only after subchronic exposure (21.9%). Semiquantitative RT-PCR analysis showed that HgCl(2) did not alter ADA gene expression. This study demonstrated that ADA activity was inhibited by mercury and this effect might be related to the neurotoxicity of this heavy metal.

Immunological dysfunction in HIV-1-infected individuals caused by impairment of adenosine deaminase-induced costimulation of T-cell activation

The cell surface association between CD26 and adenosine deaminase (ADA) has a costimulatory function during T-cell activation. Several studies have revealed correlations among CD4(+) CD26(+) T-cell depletion, increased serum levels of ADA, and the evolution of human immunodeficiency virus (HIV) infection, implicating CD26 and ADA in HIV disease progression. In this context, we aimed to determine whether ADA costimulation could be altered during HIV infection. ADA costimulation was investigated in cells from HIV-infected patients (n = 36) in terms of proliferation and cytokine secretion. An effect of ADA on T-cell proliferation was found in HIV-1-infected patients and correlated positively with the CD4(+) percentage and the nadir CD4 count and negatively with viral load, demonstrating that the response depends on the immunological status of the patient. The robust ADA-induced increase in cytokine production [interferon (IFN)-gamma, interleukin (IL)-6 and IL-10] was markedly reduced in T cells from HIV-1-infected subjects. To eliminate some of the variables associated with immunological defects in HIV-1-infected patients, anti-CD3 plus ADA assays with T cells from healthy volunteers were performed in the presence of recombinant glycoprotein 120 (gp120). It was found that gp120 was responsible for the impairment of the ADA-CD26 interaction and consequently of the ADA-induced effect on both costimulation and cytokine production. The gp120-mediated disruption of the CD26-ADA interaction is a novel mechanism that might explain, at least in part, the altered immunological features observed in HIV-1-infected patients and may have significant relevance in AIDS pathogenesis.

Adenosine deaminase enhances T-cell response elicited by dendritic cells loaded with inactivated HIV

As host immunological defenses are impaired during HIV infection, it is difficult to elicit good responses when attempting to develop therapeutic vaccines against HIV. To try to solve this situation, adjuvants, particularly cytokines, are currently under evaluation. Owing to the fact that adenosine deaminase (ADA) is a member of the family of growth factor with deaminase activity, we tested whether it could improve immune responses in the development of HIV dendritic-cell-based therapeutic vaccines. A co-culture model approach has been used to test the usefulness of ADA as adjuvant. Monocyte-derived dendritic cells from HIV-infected patients were pulsed with inactivated HIV, matured and co-cultured with autologous T cells. Addition of ADA to the co-cultures resulted in enhanced CD4(+) and CD8(+) T-cell proliferation and robust ADA-induced increase in cytokine production (IFN-gamma, TNF-alpha and IL-6). As IFN-gamma, TNF-alpha and IL-6 promote the Th1 versus Th2 phenotype and improve T helper proliferation responses and antigen-specific CTL responses ADA may be considered a promising candidate for therapeutic vaccine adjuvant.

Immunohistological Determination of Ecto-nucleoside Triphosphate Diphosphohydrolase1 (NTPDase1) and 5′-nucleotidase in Rat Hippocampus Reveals Overlapping Distribution

Distribution of two enzymes involved in the ectonucleotidase enzyme chain, ecto-nucleoside triphosphate diphosphohydrolase1 (NTPDase1) and ecto-5'-nucleotidase, was assessed by immunohistochemistry in the rat hippocampus. Obtained results have shown co-expression of the enzymes in the hippocampal region, as well as wide and strikingly similar cellular distribution. Both enzymes were expressed at the surface of pyramidal neurons in the CA1 and CA2 sections, while cells in the CA3 section were faintly stained. The granule cell layer of the dentate gyrus was moderately stained for NTPDase1, as well as for ecto-5'-nucleotidase. Glial association for ecto-5'-nucleotidase was also observed, and fiber tracts were intensively stained for both enzymes. This is the first comparative study of NTPDase1 and ecto-5'-nucleotidase distribution in the rat hippocampus. Obtained results suggest that the broad overlapping distribution of these enzymes in neurons and glial cells reflects the functional importance of ectonucleotidase actions in the nervous system.

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

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

CD26, adenosine deaminase, and adenosine receptors mediate costimulatory signals in the immunological synapse

Adenosine deaminase (ADA), a protein whose deficit leads to severe combined immunodeficiency, binds to the cell surface by means of either CD26, A(1) adenosine receptors, or A(2B) adenosine receptors. The physiological role of these interactions is not well understood. Our results show that by a 3-fold reduction in the EC(50) for the antigen, ADA potentiated T cell proliferation in autologous cocultures with antigen-pulsed immature or mature dendritic cells. Costimulation was not due to the enzymatic activity but to the interaction of ADA-CD26 complexes in T cells with an ADA-anchoring protein in dendritic cells. From colocalization studies, it is deduced that ADA colocalizing with adenosine receptors on dendritic cells interact with CD26 expressed on lymphocytes. This costimulatory signal in the immunological synapse leads to a marked increase (3- to 34-fold) in the production of the T helper 1 and proimmflamatory cytokines IFN-gamma, TNF-alpha, and IL-6.

Loss of ecto-5'nucleotidase from porcine endothelial cells after exposure to human blood: Implications for xenotransplantation

The endothelial cell surface expression of ecto-5'-nucleotidase (E5'N, CD73) is thought to be essential for the extracellular formation of cytoprotective, anti-thrombotic and immunosuppressive adenosine. Decreased E5'N activity may play a role in xenograft acute vascular rejection, preventing accommodation and tolerance mechanisms. We investigated the extent of changes in E5'N activity and other enzymes of purine metabolism in porcine hearts or endothelial cells when exposed to human blood or plasma and studied the role of humoral immunity in this context. Pig hearts, wild type (WT, n = 6) and transgenic (T, n = 5) for human decay accelerating factor (hDAF), were perfused ex vivo with fresh human blood for 4 h. Pig aortic endothelial cells (PAEC) were exposed for 3 h to autologous porcine plasma (PP), normal (NHP) or heat inactivated human plasma (HHP), with and without C1-inhibitor. Enzyme activities were measured in heart or endothelial cell homogenates with an HPLC based procedure. The baseline activity of E5'N in WT and T porcine hearts were 6.60 +/- 0.33 nmol/min/mg protein and 8.54 +/- 2.10 nmol/min/mg protein respectively (P < 0.01). Ex vivo perfusion of pig hearts with fresh human blood for 4 h resulted in a decrease in E5'N activity to 4.01 +/- 0.32 and 4.52 +/- 0.52 nmol/min/mg protein (P < 0.001) in WT and T hearts respectively, despite attenuation of hyperacute rejection in transgenic pigs. The initial PAEC activity of E5'N was 9.10 +/- 1.40 nmol/min/mg protein. Activity decreased to 6.76 +/- 0.57 and 4.58 +/- 0.47 nmol/min/mg protein (P < 0.01) after 3 h exposure of HHP and NHP respectively (P < 0.05), whereas it remained unchanged at 9.62 +/- 0.88 nmol/min/mg protein when incubated with PP controls. C1-inhibitor partially preserved E5'N activity, similar to the effect of HHP. Adenosine deaminase, adenosine kinase and AMP deaminase (other enzymes of purine metabolism) showed a downward trend in activity, but none were statistically significant. We demonstrate a specific decrease in E5'N activity in pig hearts following exposure to human blood which impairs adenosine production resulting in a loss of a cytoprotective phenotype, contributing to xenograft rejection. This effect is triggered by human humoral immune responses, and complement contributes but does not fully mediate E5'N depletion.

Distribution and expression of A1 adenosine receptors, adenosine deaminase and adenosine deaminase-binding protein (CD26) in goldfish brain

The expression patterns of adenosine A(1) receptors (A(1)Rs), adenosine deaminase (ADA) and ADA binding protein (CD26) were studied in goldfish brain using mammalian monoclonal antibody against A(1)R and polyclonal antibodies against ADA and CD26. Western blot analysis revealed the presence of a band of 35 kDa for A(1)R in membrane preparations and a band of 43 kDa for ADA in both cytosol and membranes. Immunohistochemistry on goldfish brain slices showed that A(1) receptors were present in several neuronal cell bodies diffused in the telencephalon, cerebellum, optic tectum. In the rhombencephalon, large and medium sized neurons of the raphe nucleus showed a strong immunopositivity. A(1)R immunoreactivity was also present in the glial cells of the rhombencephalon and optic tectum. An analogous distribution was observed for ADA immunoreactivity. Tests for the presence of CD26 gave positive labelling in several populations of neurons in the rhombencephalon as well as in the radial glia of optic tectum, where immunostaining for ADA and A(1)R was observed. In goldfish astrocyte cultures the immunohistochemical staining of A(1)R, ADA and CD26, performed on the same cell population, displayed a complete overlapping distribution of the three antibodies. The parallel immunopositivity, at least in some discrete neuronal areas, for A(1)Rs, ADA and CD26 led us to hypothesize that a co-localization among A(1)R, ecto-ADA and CD26 also exists in the neurons of goldfish since it has been established to exist in the neurons of mammals. Moreover, we have demonstrated for the first time, that A(1)R, ecto-ADA and CD26 co-localization is present on the astroglial component of the goldfish brain. This raises the possibility that a similar situation is also shown in the glia of the mammalian brain.

Ligand-induced caveolae-mediated internalization of A1 adenosine receptors: morphological evidence of endosomal sorting and receptor recycling

The involvement of caveolae in the internalization of A(1) adenosine receptors (A(1)R) and the receptor sorting and recycling was studied in the smooth muscle cell line DDT(1)MF-2, by binding assays, by confocal microscopy, and at the structural level. The use of cholera toxin-binding subunit adsorbed to gold as a specific probe for labeling the ganglioside GM(1) and immunoelectron microscopy techniques showed that agonist stimulation produced a clustering and sequestration of adenosine receptors in caveolae. Furthermore, pull-down experiments showed there to be a direct interaction between the C-terminal domain of A(1)R and caveolin-1. Addition of exogenous adenosine deaminase (ADA), a protein that binds to A(1)R and acts as a receptor activity modifying protein (RAMP) stimulated R-PIA-induced A(1) receptor internalization. Finally, the sorting and recycling of A(1)R/ADA complexes was analyzed. Detailed electron microscopy revealed that A(1)R/ADA complexes internalize together through caveolae, are differentially sorted in endosomes, and are recycled back to the cell surface by different groups of recycling endosomes. These results give insight into the spatiotemporal regulation and traffic of A(1)R and RAMPs.

Anti-adenosine deaminase antibodies in lupus erythematosus

Adenosine deaminase (ADA) is an enzyme involved in purine metabolism and has a major role in the development and function of lymphoid cells. Congenital deficiency of ADA results in severe immunodeficiency. Patients with congenital ADA deficiency treated with polyethylene glycol-conjugated bovine ADA develop antibodies to ADA. This leads us to investigate the role of anti-ADA antibodies in patients with systemic rheumatic diseases. Commercially available ADA was used in ELISA and immunoblots for detection of anti-ADA antibodies. Four out of 100 patients examined were positive for anti-ADA antibodies. Two of them had peripheral blood lymphopenia but the antibody levels did not appear to correlate with the lymphocyte counts. Immunoblotting revealed that the antibodies recognized a 40 kDa peptide of ADA, corresponding to ADA1, the major component of ADA. Affinity-purified antibodies were used to locate the distribution of ADA on Hep-2 cells and lymphocytes by indirect immunofluorescence. Anti-ADA antibodies gave a distinct nuclear speckled pattern on acetone-fixed cells. With viable cell immunofluorescence, anti-ADA antibodies also stained the cell surface of HEp-2 cells and lymphocytes, indicating surface expression of ADA. The anti-ADA antibodies failed to gain access into the cytoplasm or nuclei when added to the cultures of HEp-2 cells. In summary, this is the first report of detection of anti-ADA1 autoantibody which is a new type of ANA with discrete, speckled nuclear staining, but which may not be associated with lymphopenia.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

Involvement of astrocytes in purine‐mediated reparative processes in the brain

Astrocytes are involved in multiple brain functions in physiological conditions, participating in neuronal development, synaptic activity and homeostatic control of the extracellular environment. They also actively participate in the processes triggered by brain injuries, aimed at limiting and repairing brain damages. Purines may play a significant role in the pathophysiology of numerous acute and chronic disorders of the central nervous system (CNS). Astrocytes are the main source of cerebral purines. They release either adenine-based purines, e.g. adenosine and adenosine triphosphate, or guanine-based purines, e.g. guanosine and guanosine triphosphate, in physiological conditions and release even more of these purines in pathological conditions. Astrocytes express several receptor subtypes of P1 and P2 types for adenine-based purines. Receptors for guanine-based purines are being characterised. Specific ecto-enzymes such as nucleotidases, adenosine deaminase and, likely, purine nucleoside phosphorylase, metabolise both adenine- and guanine-based purines after release from astrocytes. This regulates the effects of nucleotides and nucleosides by reducing their interaction with specific membrane binding sites. Adenine-based nucleotides stimulate astrocyte proliferation by a P2-mediated increase in intracellular [Ca2+] and isoprenylated proteins. Adenosine also, via A2 receptors, may stimulate astrocyte proliferation, but mostly, via A1 and/or A3 receptors, inhibits astrocyte proliferation, thus controlling the excessive reactive astrogliosis triggered by P2 receptors. The activation of A1 receptors also stimulates astrocytes to produce trophic factors, such as nerve growth factor, S100beta protein and transforming growth factor beta, which contribute to protect neurons against injuries. Guanosine stimulates the output of adenine-based purines from astrocytes and in addition it directly triggers these cells to proliferate and to produce large amount of neuroprotective factors. These data indicate that adenine- and guanine-based purines released in large amounts from injured or dying cells of CNS may act as signals to initiate brain repair mechanisms widely involving astrocytes.

Adenosine A(1) receptor in cultured neurons from rat cerebral cortex: colocalization with adenosine deaminase

Adenosine A(1) receptors (A(1)Rs) have been characterized in primary cultures of neurons from cerebral cortex. The specific adenosine A(1) antagonist 8-cyclopentyl-1,3-[(3)H]dipropylxanthine bound to both membranes and intact cells. When saturation experiments were performed in membranes, a K(D) value of 0.76 nM and a B(max) of 57 fmol/mg of protein were obtained. Competition assays revealed a pharmacological profile characteristic of A(1)Rs. The presence of this receptor was further confirmed by RT-PCR analysis. The expression of the receptor showed no significant changes during the period of culture studied, up to 12 days in vitro. A(1)R agonist inhibited forskolin-stimulated adenylyl cyclase, showing the functional coupling of these receptors with the effector. alphaG(i1, 2) protein level, detected by immunoblot, presented an increase during the period of culture. This increase correlated with an increase in the mRNA level of alphaG(i1) but not alphaG(i2). By immunochemical assays, it is shown that these receptors are expressed in both the neuronal cell body and the proximal dendrites. Colocalization of A(1)Rs with microtubule-associated protein 2 and cell surface adenosine deaminase was shown by confocal microscopy. The high degree of colocalization observed between A(1)Rs and ectoadenosine deaminase in neurons could suggest an important role of the enzyme in adenosine-mediated neuromodulation.

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

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

Immunologically distinct isoforms of ecto-5'-nucleotidase in nerve terminals of different areas of the rat hippocampus

Ecto-5'-nucleotidase is regarded as being the key enzyme in the formation of the neuromodulator adenosine from released ATP. However, the association of ecto-5'-nucleotidase with nerve terminals is not consensual. Only enzyme histochemical and biochemical studies, but not immunocytochemical studies, agree on a general synaptic location of the enzyme. To clarify this issue further we tested the effect of an antibody against ecto-5'-nucleotidase, previously used in immunocytochemical studies, on the activity of ecto-5'-nucleotidase in fractions of nerve terminals isolated from different areas of rat hippocampus. The specific activity of extracellular AMP catabolism was higher in synaptosomes from the CA3 area (0.81+/-0.06 nmol/min/mg of protein) than from synaptosomes from the CA1 area or the dentate gyrus or from the whole hippocampus (0.49-0.68 nmol/ min/mg of protein). The catabolism of AMP (10 microM) was equally inhibited (85-92%) in synaptosomes from whole hippocampus, CA1, CA3, or dentate gyrus by alpha,beta-methylene-ADP (100 microM) and equally unaffected by p-nitrophenyl phosphate (0.5 mM) or rabbit IgGs (100 microg/ml). However, the antiserum against ecto-5'-nucleotidase (100 microg/ml) inhibited extracellular AMP catabolism by 44% in CA3 synaptosomes but had little or no effect in synaptosomes from CA1, dentate gyrus, or whole hippocampus. A similar difference in the inhibitory potential of the antibody was observed between fractions of isolated 5'-nucleotidase binding to concanavalin A-Sepharose (70%) and fractions not retained by the lectin column (18%). Taken together, these results suggest that immunological isoforms of ecto-5'-nucleotidase exist in the rat hippocampal nerve terminals, with predominance in the CA3 area.

Purine requirements for the expression of Cape buffalo serum trypanocidal activity

Cape buffalo serum contains xanthine oxidase which generates trypanocidal H(2)O(2) during the catabolism of hypoxanthine and xanthine. The present studies show that xanthine oxidase-dependent trypanocidal activity in Cape buffalo serum was also elicited by purine nucleotides, nucleosides, and bases even though xanthine oxidase did not catabolize those purines. The paradox was explained in part, by the presence in serum of purine nucleoside phosphorylase and adenosine deaminase, that, together with xanthine oxidase, catabolized adenosine, inosine, hypoxanthine, and xanthine to uric acid yielding trypanocidal H(2)O(2). In addition, purine catabolism by trypanosomes provided substrates for serum xanthine oxidase and was implicated in the triggering of xanthine oxidase-dependent trypanocidal activity by purines that were not directly catabolized to uric acid in Cape buffalo serum, namely guanosine, guanine, adenine monophosphate, guanosine diphosphate, adenosine 3':5-cyclic monophosphate, and 1-methylinosine. The concentrations of guanosine and guanine that elicited xanthine oxidase-dependent trypanocidal activity were 30-270-fold lower than those of other purines requiring trypanosome-processing which suggests differential processing by the parasites.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

Enzymatic and extraenzymatic role of ecto-adenosine deaminase in lymphocytes

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

Adenosine deaminase is a specific partner for the Grb2 isoform Grb3-3

Grb3-3 is an isoform of Grb2, thought to arise by alternative splicing, that lacks a functional SH2 domain but retains functional SH3 domains, which allow interaction with other proteins through binding to prolinerich sequences. Several evidences suggest that besides common partners for Grb2 and Grb3-3, specific targets could exist. In order to find specific partners for Grb3-3, we have screened a human cDNA library by the yeast two-hybrid system with Grb3-3 as a bait. We have identified adenosine deaminase, an enzyme involved in purine metabolism whose deficiency is associated with severe combined immunodeficiency, as a Grb3-3 binding protein that is not able to bind to Grb2. This interaction has been confirmed in vitro with GST fusion proteins and in vivo by coimmunoprecipitation experiments in NIH3T3 cells stably transfected with Grb3-3. The functional significance of this finding is discussed.

Molecular pathogenesis of alcohol withdrawal seizures: the modified lipid-protein interaction mechanism

The phrase alcohol withdrawal seizures (AWS) refers to seizures that result from the withdrawal of alcohol after a period of chronic alcohol administration. A mechanism of AWS is postulated, namely the modified lipid-protein interaction (MLPI) mechanism. This hypothesis is based upon an evaluation of the mechanisms of membrane fluidity, calcium channels, gamma-aminobutyric acid (GABA) and glutamate in the molecular pathogenesis of AWS. The mechanism hypothesizes that acute ethanol treatment alters the neuronal membrane lipids which then perturbs protein events, such as affecting the GABAA receptors, NMDA receptors and voltage-dependent Ca2+ channels synergistically or in combination. Subsequent adaptations in these systems occur after prolonged administration of ethanol. A sudden withdrawal of ethanol then leads to hyperexcitability which results in AWS.

Cell surface adenosine deaminase: much more than an ectoenzyme

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

Enzymatic activities affecting exogenous nicotinamide adenine dinucleotide in human skin fibroblasts

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

Adenosine deaminase affects ligand-induced signalling by interacting with cell surface adenosine receptors

Adenosine deaminase (ADA) is not only a cytosolic enzyme but can be found as an ecto-enzyme. At the plasma membrane, an adenosine deaminase binding protein (CD26, also known as dipeptidylpeptidase IV) has been identified but the functional role of this ADA/CD26 complex is unclear. Here by confocal microscopy, affinity chromatography and coprecipitation experiments we show that A1 adenosine receptor (A1R) is a second ecto-ADA binding protein. Binding of ADA to A1R increased its affinity for the ligand thus suggesting that ADA was needed for an effective coupling between A1R and heterotrimeric G proteins. This was confirmed by the fact that ASA, independently of its catalytic behaviour, enhanced the ligand-induced second messenger production via A1R. These findings demonstrate that, apart from the cleavage of adenosine, a further role of ecto-adenosine deaminase on the cell surface is to facilitate the signal transduction via A1R.

Surface adenosine deaminase. A novel B-cell marker in chronic lymphocytic leukemia

Previous studies found that ADA is present on the surface of mononuclear blood cells from healthy patients. Because the expression of this surface antigen depends upon the cell type, the presence of ADA on the plasma membrane of cells from patients with malignant hematologic diseases was studied by flow cytometry. The highest percentage of expression was found in CLL, whereas the lowest was found in T-cell-derived malignancies. The enzyme expression in immortalized cell lines showed a similar pattern, with the highest expression (95% +/- 5%) in the SKW64 B-derived cell line, the lowest (15% +/- 5%) in Jurkat T-lymphoma derived cells, and the intermediate (32% +/- 8%) in K562 cells derived from a chronic myelogenous leukemia. Double labeling ADA/CD5 and ADA/CD19, as well as the correlation of ADA expression with the expression of other surface markers, indicate that surface ADA might be considered a novel marker for CLL.

Surface expression of adenosine deaminase in mitogen-stimulated lymphocytes

Adenosine deaminase (ADA) expression on the surface of mitogen-stimulated lymphocytes was studied by flow cytometry. The gate for lymphocytes was located by cell size (forward scatter), cytoplasmic complexity (side scatter) and by expression of the markers CD2, CD4, CD8 and CD19. After mitogenic proliferation two populations appeared, one corresponding to non-stimulated cells, and the other consisting of larger cells which showed relatively high expression of adenosine deaminase on their surface. The increase was similar to that observed for CD71 expression, and paralleled the increase in 3H-thymidine incorporation. There was a correlation between ADA and CD71 expression (r = 0.92 for phytohaemagglutinin (PHA) and 0.97 for pokeweed mitogen (PWM)). These results suggest a role for ecto-adenosine deaminase in lymphocyte proliferation and/or triggering.

Changes in pial arteriolar diameter and CSF adenosine concentrations during hypoxia

We measured the changes in pial arteriolar diameter and CSF concentrations of adenosine, inosine, and hypoxanthine during hypoxia in the absence and presence of topically applied dipyridamole (10(-6) M) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA; 10(-5) M). Closed cranial windows were implanted in halothane-anesthetized adult male Sprague-Dawley rats for the observation of the pial circulation and collection of CSF. The mean resting arteriolar diameter in mock CSF was 31.2 +/- 5.9 microns. Topically applied dipyridamole and EHNA, in combination, caused a slight but significant (p < 0.05) increase in resting arteriolar diameter (33.8 +/- 4.3 microns). With mock CSF, moderate hypoxia caused a 22.1 +/- 9.7% increase in pial vessel diameter. Topically applied dipyridamole and EHNA significantly (p < 0.01) potentiated pial arteriolar vasodilation in response to hypoxia. Moreover, the potentiating effects of dipyridamole and EHNA during hypoxia were completely abolished by theophylline (0.20 mumol/g, i.p.; p < 0.05), an adenosine receptor antagonist. Resting concentrations of adenosine, inosine, and hypoxanthine in the subwindow CSF were 0.18 +/- 0.09, 0.35 +/- 0.21, and 0.62 +/- 0.12 microM, respectively. In the absence of dipyridamole and EHNA, these levels were not affected by sustained moderate hypoxia (PaO2 = 36 +/- 6 mm Hg). However, in the presence of dipyridamole and EHNA, the concentration of adenosine in the CSF during hypoxia was significantly (p < 0.05) increased. Our data indicate that dipyridamole and EHNA potentiate hypoxic vasodilation of pial arterioles while simultaneously increasing extracellular adenosine levels, thus supporting the hypothesis that adenosine is involved in the regulation of cerebral blood flow.

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

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

Role of phosphatidylethanol in membranes. Effects on membrane fluidity, tolerance to ethanol, and activity of membrane-bound enzymes

We investigated the effect of phosphatidylethanol (PEt) on fluidity and membrane tolerance to the fluidization induced by ethanol as well as on the activity of two membrane-bound enzymes, Na+/K+ ATPase and 5'-nucleotidase. PEt was synthesized from 1,2-dimyristoylphosphatidylcholine and phosphatidylcholine from bovine brain and studies were performed to determine the optimal experimental conditions for the insertion of PEt in natural bilayers. The effects of PEt, evaluated by differential scanning calorimetry or fluorescence polarization techniques, were studied in model membranes made of synthetic phospholipids or made of total lipids extracted from rat brain crude mitochondrial fraction (P2 fraction) and from natural membranes (P2 fraction). The presence of PEt increased the fluidity of artificial as well of natural membranes, but tolerance to the addition of ethanol, displayed by dimyristoylphosphatidylcholine vesicles and by natural membranes containing PEt, was lacking in vesicles made of dimyristoylphosphatidylethanolamine and in artificial bilayers reconstituted from total P2 lipid extracts, suggesting an involvement of PC on PEt-induced ethanol resistance. Na+/K+ ATPase activity was enhanced by the addition of small amounts of ethanol (up to 50 mM) and progressively inhibited at higher concentrations, while 5'-nucleotidase was not affected up to 400 mM ethanol. The presence of PEt in the bilayer exerted the opposite effects on the two enzymes, reducing the Na+/K+ ATPase activation induced by ethanol and enhancing 5'-nucleotidase activity. The mechanisms of the PEt-induced modifications are discussed.

Is adenosine deaminase involved in adenosine transport?

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