Review

Adenosine is a product of complete dephosphorylation of adenine nucleotides which takes place in various compartments of the cell. This nucleoside is a significant signal molecule engaged in regulation of physiology and modulation of the function of numerous cell types (i.e. neurons, platelets, neutrophils, mast cells and smooth muscle cells in bronchi and vasculature, myocytes etc.). As part a of purinergic signaling system, adenosine mediates neurotransmission, conduction, secretion, vasodilation, proliferation and cell death. Most of the effects of adenosine help to protect cells and tissues during stress conditions such as ischemia or anoxia. Adenosine receptors and nucleoside transporters are targets for potential drugs in many pathophysiological situations. The adenosine-producing system in vertebrates involves a cascade dephosphorylating ATP and ending with 5'-nucleotidase (EC 3.1.3.5) localized either on the membrane or inside the cell. In this paper the cytoplasmic variants of 5'-nucleotidase are broadly characterized as well as their clinical relevance. The role of AMP-selective 5'-nucleotidase (cN-I) in the heart, skeletal muscle and brain is highlighted. cN-I action is crucial during ischemia and important for the efficacy of some nucleoside-based drugs and in the regulation of the substrate pool for nucleic acids synthesis. Inhibitors used in studying the roles of cytoplasmic and membrane-bound 5'-nucleotidases are also described.


ADENOSINE -A REGULATORY NUCLEOSIDE
Adenosine (Ado) is a signaling molecule working in an autocrine or paracrine way.The potent effect of adenylic compounds in the heart and blood vessels was first described by Drury and Szent-Györgi in 1929.The large amounts of adenosine produced by a cascade hydrolysis of ATP can affect the homeostasis of high-energy compounds inside the cell in a feed-back way.This was the reason to name adenosine "a retaliatory metabolite" that is able to counteract the action of the enzymes raising cAMP level and triggering an energy burst.In such a way adenosine plays an important role in balancing energy supply with demand in the tissues where the energy output is high (Newby, 1984;Downey & Forman, 1993).
The metabolism of adenosine is illustrated in Fig. 1.This metabolite is generated mainly by two enzymatic systems: intra-(cN-I) or extracellularly localized 5'-nucleotidases (e5'-NT) and cytoplasmic S-adenosylhomocysteine hydrolase (SAHH; hydrolysis of SAH is associated with transmethylation reactions).The processes of adenosine elimination in the

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A. Borowiec and others cell involve reactions catalyzed by adenosine deaminase and adenosine kinase (Shryock & Belardinelli, 1997) yielding inosine or 5'-AMP, respectively.The reaction of phosphorylation predominates when adenosine occurs at a low physiological concentration (< 1 µM) whereas adenosine deaminase is activated at higher concentrations of the substrate (> 10 µM).Purine nucleoside phosphorylase has only a negligible activity towards adenosine and degrades mainly inosine and guanosine.Some cells may carry out purine conversion down to uric acid as the ultimate product, others release less oxidized purines.
5'-Nucleotidases (EC 3.1.3.6) have a regulatory impact on purine metabolism and can influence the cellular level of (deoxy)adenylates and other nucleotides.Dephosphorylation of different nucleoside monophosphates is performed by distinct isoforms although they are generally only regiospecific towards the 5'-position in ribose or deoxyribose.Seven mammalian 5'-nucleotidases have been cloned until now (for a review see Bianchi & Spychala, 2003).Four isoforms are ubiquitous (eN, cN-II, cdN, mdN) whereas the others display more tissue-specific expression (cN-IA, cN-IB, cN-III).
The physiological effect of adenosine is mediated via specific receptors.They are classified as purinergic P1-type and situated in the plasma membrane.These G protein-coupled receptors have seven trans-membrane protein domains.Up to now four types of adenosine receptors (A 1 , A 2A , A 2B and A 3 ) have been recognized and cloned from many mam-malian and non-mammalian species (for a recent review see Yaar et al., 2005).The subclassification of adenosine receptors is based on the effects which they cause over changing cAMP concentration in the cell and their affinity for the ligand (see Table 1).Within any particular subtype, the receptors from various mammalian tissues are highly similar to each other.Adenosine is their only endogenous agonist, however, A 3 receptor can be activated also by inosine (Fredholm et al., 2001).The distribution of the subtypes varies greatly.For example, those localized in the heart are classified as A 1 (present only on atrial cells), A 2A (of intermediate abundance) and A 3 (of low abundance) (Fredholm et al., 2000).The A 1 -, A 2A -and A 2B -type receptors are present on skeletal muscle filaments (Fredholm et al., 2000;Radegran & Hellsten, 2000).The genomic structure appears to be similar for all human adenosine receptor genes and all four types are asparagine-linked glycoproteins (Fredholm et al., 2000).The A 2A -and A 2B -type preferably interact with the G s protein and the A 1 -and A 3 -type act via the G i/o protein.
Adenosine exerts diverse physiological effects in various tissues (Imai & Nakazawa, 1991).The effects vary greatly depending on the population of the receptor subtype in the particular tissue (Table 2).Generally, this nucleoside is an inhibitory signaling molecule.It protects the myocardial and cerebrovascular systems against ischemia-induced damage (Ely & Berne, 1992;Obata, 2002) by slowing cardiac output.It also limits the degree of vascular Adenosine and cytoplasmic 5'-nucleotidases injury after ischemia and reperfusion by reducing reactive oxygen species release and inhibiting platelet aggregation (Ely & Berne, 1992).Adenosine is implicated in neuromodulatory (Dunwiddie & Haas, 1985;Lu et al., 2004) and anti-inflammatory mechanisms (Cronstein, 1994).It has also analgesic properties (Sawynok & Sweeney, 1989), may influence proliferation, survival or apoptosis of many different cells (Jacobson et al., 1999) and work as a potent suppressor in the immune system (Spychala et al., 1997).
Adenosine receptors are considered as putative targets for specific drug development.The adenosine receptors-based therapy concerns Parkinson's disease, hypoxia/ischemia, epilepsy, kidney disease and asthma (Fredholm, 2003).

ROLE OF ADENOSINE IN THE HEART
The physiological function of adenosine in the cardiovascular and central nervous systems is well recognized (Berne et al., 1983).When a stress situation occurs such as ischemia, hypoxia or increased workload, the myocardial metabolism is adapted to reverse the pathological consequences (Rubio et al., 1973).If the rate of energy consumption exceeds the rate of energy production, the net breakdown of ATP results in an increased production of adenosine.Elimination of the stress takes place by restoring the balance between ATP generation and utilization.The actions exerted then are: coronary vasodilation, inhibition of the sinoatrial and atrioventricular con-duction and antagonizing adrenergic stimulation.All of them bring the myocardial oxygen supplyto-demand ratio back to normal (Ballardinelli et al., 1989).
The adenosine production in the myocardium is dependent on both cytoplasmic and extracellular 5'-nucleotidases.However, only cN-I is the isoform that is sensitive to the level of energy compounds inside the cell.Decrease of ATP and increase of ADP is a primary stimulus for cN-I to work (Skladanowski & Newby, 1990;Darvish et al., 1996).Adenosine circulating in coronary vessels originates both from myocytes (Rubio et al., 1973) and endothelial cells (Deussen et al., 1986).This has led to a theory that not only myocardial but also endothelially-produced adenosine has a protective role.

ROLE OF ADENOSINE IN NERVOUS SYSTEM
As mentioned in Table 2 adenosine is a neuronal signaling substance which plays a significant role in the central and peripheral nervous systems (Zimmermann, 1994).In CNS adenosine targets are neurons, glia and cerebral blood vessels; in the periphery it regulates neurons, muscles and endothelial cells.Adenosine can modulate neural functions in many ways.It inhibits the release of most transmitters via A 1 receptors, presumably by inhibition of presynaptic Ca 2+ -channels.The neuroprotective role of adenosine concerns also the control of the release of ATP from purinergic neurons.Adenosine may play a specific role in the development and regener- Effects of G-protein coupling Explanation: â, decrease; and á, increase of concentration.ation of the nervous system (Zimmermann & Braun, 1996).In the salvage system, adenosine may serve as a substrate for restoration of adenylates after reuptake via nucleoside transporters (Thorn & Jarvis, 1996).

ROLE OF ADENOSINE IN THE SKELETAL MUSCLE
The role of adenosine in the skeletal muscle is less well elucidated.As a vasoactive substance, it is a candidate regulatory compound of blood flow (Radegran & Hellsten, 2000).A more important role probably concerns the connections between muscle and nerves.Adenosine-induced apoptosis may represent a pathogenic pathway in muscular dystrophy (Jacobson et al., 1999).Adenosine exerts effects in both neuromuscular junctions and contractile myofibrils.It is known that adenosine derived from newly released ATP by cascadic degradation is the cause of skeletal neuromuscular depression (Silinsky, 1975).In a negative feedback the secretion of acetylcholine and ATP is inhibited by extracellularly produced adenosine.
Adenosine takes part in regulation of energy metabolism of a working muscle.Two functional proteins: tropomyosin-and troponin-complex, present in thin filaments, regulate the process of contraction.The allosteric mechanism of this control is dependent on Ca 2+ concentration.A nervous impulse causes an increase of cytoplasmic calcium.Following that, Ca 2+ binds to troponin and induces conformational changes of the muscle proteins that allow contraction to occur.The release of adenosine has been found to parallel the cytoplasmic Ca 2+ pulse in the contracting animal muscle.Also adenosine formed outside the cell was elevated in electrically stimulated mammalian skeletal muscle cells (Hellsten, 1999).The role of adenosine in the skeletal muscle can be regarded from various aspects.The most important are the physiological regulation of blood flow in capillary vessels surrounding muscle bundles and the metabolic control of glucose utilization as a fuel (Dobson et al., 1971;Han et al., 1998).It has been found that adenosine may accumulate in the muscle tissue and unchanged can be released during contraction (Ballard, 1991;Radegran & Hellsten, 2000).Its sources might be the contracting myocytes, vascular endothelial cells or nerve endings (Radegran & Hellsten, 2000).
Adenosine is an autocrine or paracrine regulator in skeletal muscles as well as in the heart.In the myocytic plasma membrane, two types of adenosine receptors (A 2A and A 2B ) are present.Three receptors (A 1 , A 2A and A 2B ) are found on vascular endothelial and smooth muscle cells (Radegran & Hellsten, 2000).Adenosine is known to be released from endothelial cells during systemic hypoxia and then stimulates A 1 receptors (Bryan & Marshall, 1999a).Hypoxia-induced vasodilation is initiated by stimulation of A 1 receptor followed by K ATP -channel opening and evoking the synthesis of nitric oxide (Bryan & Marshall, 1999b).Hellsten and Frandsen (1997) confirmed that adenosine is released from the skeletal muscle mainly under conditions of an insufficient oxygen supply.The decrease of oxygen level in the working muscle however do not affect the rate of adenosine production (Ballard, 1991;Cheng et al., 2000).The adenosine efflux can be rather stimulated by decrease of intracellular pH, resulted from lactate accumulation during hypoxic exercise.This is driven mainly by increased extracellular AMP concentration and an increased affinity of membrane-bound e5'-NT for AMP (Cheng et al., 2000).It was observed in that paper, that putative cytoplasmic 5'-nucleotidase in the skeletal muscle has too low an activity for effective dephosphorylation of AMP at a micromolar concentration.
Despite that, it seems that the amount of adenosine in the contracting muscle cannot be supplied only by ecto-5'-nucleotidase (Rubio et al., 1973).A recent report confirmed the existence of a cytoplasmic AMP-specific form of 5'-nucleotidase (cN-I) closely resembling the heart isoform.The purified enzyme processed AMP at a submillimolar concentration and was allosterically activated by ADP.This observation gives more credibility to the role of intracellularly formed adenosine in physiological regulation of skeletal muscle function (Tkacz-Stachowska et al., 2005).

ROLE OF ADENOSINE IN TUMOR PROMOTION
Some of the adenosine functions have recently been implicated in the promotion of tumorigenesis.It acts as a very potent cytoprotective and immunosuppressive agent and also has an ability to inhibit cytokine synthesis and adhesion of cells to the endothelium as well as to influence the function of lymphocytes T, macrophages and natural killer cells.Moreover, adenosine takes part in cell induction and proliferation, also in relation to cancer cells (Spychala, 2000).However, more results are needed to verify this new aspect of adenosine as a signaling molecule.

THE FAMILY OF 5'-NUCLEOTIDASES
5'-Nucleotidases hydrolyze non-cyclic nucleoside monophosphates or deoxynucleoside monophosphates to (deoxy)nucleosides and inorganic Adenosine and cytoplasmic 5'-nucleotidases phosphate.Some variants can transfer inorganic phosphate from a nucleoside monophosphate onto another nucleoside.The group of 5'-nucleotidases together with nucleoside kinases regulate nucleotide levels in proliferating cells (Bianchi et al., 1986).The nomenclature of 5'-NTs is based mainly on their subcellular compartmentation.Five are localized in the cytosol, one is in the mitochondrial matrix and one is attached to the outer side of the plasma membrane.All cytoplasmic forms are Mg 2+ -dependent.They vary in substrate specificity, molecular and kinetic properties.Their common feature is subunit complexity and allosteric regulation.The enzymes are filogenetically distant and there are no significant similarities between the primary structures of these proteins.For details see the recent review by Bianchi and Spychala (2003).

cN-I
5'-Nucleotidase-I is highly expressed in skeletal and heart muscles (Sala- Newby et al., 1999;Sala-Newby et al., 2000).Until now cN-I has been purified and characterized from the heart of many species including rabbit (Yamazaki et al., 1991), pigeon (Sala- Newby et al., 2000), rat (Truong et al., 1988), dog (Darvish & Metting, 1993), and man (Skladanowski et al., 1996).cN-I has been cloned from pigeon (Sala- Newby et al., 2000), mouse (Sala-Newby & Newby, 2001) and human (Hunsucker et al., 2001).Heart is the richest source of cN-I in pigeon but this form is also abundant in the brain and breast muscle.In human tissues, there are two genes (NT5C1A and NT5C1B) giving closely related products -cN-IA and cN-IB, respectively.cN-IA mRNA expression is highest in skeletal and heart muscle, a moderate level of the enzyme appears in the pancreas and brain and a low level is present in the kidney, testes and uterus (Hunsucker et al., 2001).
Cytoplasmic 5'-nucleotidase-I has a high affinity for both nucleoside and deoxynucleoside monophosphates (see Table 3).The latter, however, are converted with a much lower rate.The preferred substrate for cN-I is AMP but also various pyrimidine deoxyribonucleoside monophosphates (Garvey et al., 1998;Hunsucker et al., 2001;Tkacz-Stachowska et al., 2005).The K m for AMP in partially purified human heart enzyme was 1.46 mM (Skladanowski et al., 1996), similarly to the purified recombinant human heart enzyme where it was 1.9 mM (Hunsucker et al., 2001).cN-I is strongly activated by ADP (Skladanowski & Newby, 1990).Depending on the substrate concentration and the source of enzyme, halfmaximal activation of cN-I appears to vary between 10 and 89 µM ADP (Skladanowski & Newby, 1990;Yamazaki et al., 1991;Garvey et al., 1998;Hunsucker et al., 2001).The impulse from ADP increase seems to be the main trigger to activate cN-I.To a lesser degree the enzyme is activated by GTP (Hunsucker et al., 2001).The activity of cN-I is influenced by pH (Newby, 1988;Yamazaki et al., 1991;Skladanowski et al., 1996) and different divalent cations: Mg 2+ , Mn 2+ and Co 2+ (Skladanowski & Newby, 1990;Yamazaki et al., 1991;Darvish & Metting, 1993;Skladanowski et al., 1996).A selective inhibitor of cN-I is 5-ethynyldideoxyuridine (5-EddU) (Garvey et al., 1998).An acknowledged role of the enzyme in humans is delivering adenosine during ischemic or hypoxic condition.This is crucial for prevention of heart damage (Cross et al., 2002).Also its role in modulation of the pharmacological effects of nucleoside-based drugs is considered (Hunsucker et al., 2005).
Murine cN-I has been cloned from the heart and testes (Sala- Newby & Newby, 2001).It appeared then that the enzyme was a product of a distinct gene.The sequence was closely related to human cN-IB but not to cN-IA.Among highly homologous genes one worth noting is a gene that encodes putative human autoimmune infertility protein (AIRP) but the exact relation of both genes calls further investigations.Human cN-IB is highly expressed in testes; the lowest level of expression estimated by RT-PCR analysis was observed in the brain and skeletal muscle.Full substrate specificity pattern of cN-IB is still unknown, but the initial enzyme characteristic appears similar to that of cN-IA.Overexpression of cloned murine cN-IB in COS-7 cells showed that the enzyme preferably hydrolyzes AMP and is allosterically activated by ADP, both characteristic also to cN-IA (Sala- Newby & Newby, 2001).

cN-II
Cytoplasmic 5'-nucleotidase-II (cN-II) is also known as high-K m 5'-NT, purine 5'-NT or IMP/GMP selective 5'-NT.It is a 6-hydroxypurine nucleoside monophosphate-specific enzyme (Itoh, 1993; see also Table 3).It is not involved in the regulation of the deoxyribonucleotide pool but affects IMP and GTP concentration (Gazziola et al., 2001).cN-II has a phosphotransferase activity and the formation of a covalently-bound phosphoenzyme intermediate has been found (Allegrini et al., 2001).The phosphate transfer onto a purine nucleoside acceptor is dependent on the local concentration of nucleosides.In its absence, the phosphate moiety is removed by hydrolysis.Under physiological conditions, the phos-

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A. Borowiec and others phate can be transferred onto inosine and guanosine and also onto nucleoside analogs.The enzyme is widely expressed and its sequence is remarkably conserved among various vertebrates.Human cN-II was cloned in 1994 by Oka and coworkers.Highly elevated activity of a soluble 5'-nucleotidase has been associated with developmental neurological disorders and Lesh-Nyhan syndrome (Page et al., 1997;Pesi et al., 2000).Judging by its substrate preferences the enzyme should be classed as cN-II.

cN-III
Cytoplasmic 5'-nucleotidase-III (cN-III) is also known as PN-I, P5'N-1 or UMPH-1 (uridine monophosphate hydrolase-1).Its enzymatic activity (see also Table 3) is inhibited by exposition to heavy metals and thiol-reactive compounds (Amici & Magni, 2002).The sequence of cN-III is identical with that of another protein termed p36, an interferon αinduced protein of unknown function (Amici et al., 2000).cN-III catalyzes dephosphorylation of pyrimidine nucleoside monophosphates to the corresponding nucleosides.It is much less active with purine nucleotides.The enzyme is highly expressed in red blood cells where it participates in the degradation of RNA during erythrocyte maturation.A cN-III deficiency is associated with inherited hemolytic anemia (Rees et al., 2003).

MOLECULAR GENETICS OF ISOFORM cN-I
The first successful cloning of a full length vertebrate 5'-nucleotidase-I was done by Sala-Newby and coworkers in 1999.They utilized PCRbased screening of pigeon ventricular library with a degenerated primer derived from the peptide sequence of the heart isoform.The longest among the clones obtained encoded an open reading frame of 358 amino acids with a predicted molecular mass of 39 472 Da.At that time, no homologs of the enzyme existed in the SWISS PROT database.The cloned cN-I molecule was not significantly similar to cN-II or to e5'-NT.Therefore it was classified as the first member of a new family of 5'-nucleotidases.A detailed sequence analysis, based on a motifs prediction software tool (PSORT program) revealed a putative nuclear localization sequence (NLS) at the N terminus.Expression studies, carried out by Northern blotting technique, found one major transcript of approx.2.3 kb in several pigeon tissues, including brain, skeletal muscle and heart.In spite of the NLS presence, when exogenously expressed in green monkey kidney cell line (COS-7), cN-I was localized to the cytoplasm, as judged by immunochemistry studies with enzyme-specific antibodies.This result was in line with the previous data obtained by electron microscopic immunogold labeling of AMPselective 5'-nucleotidase, as it was then named, in sections from dog heart (Darvish & Metting, 1993).This pointed to a lack of functional consequences of NLS presence.One could speculate on another possible explanation of the observed discrepancy between the predicted and actual localization of the enzyme, involving conditional targeting of cN-I to the nucleus upon an as yet unidentified stimulus.On the other hand, the PSORT program used in those studies distinguished three types of NLS sequence, and used the heuristic that nuclear proteins are usually enriched in basic residues which comprise more than 20% of the molecule, resulting in a prediction accuracy of 57% as estimated for yeast proteins.Until now there is no evidence in the literature that AMP-selective cytoplasmic 5'-nucleotidase of any studied species localizes to the nucleus.Other members of the cN-I family were cloned from mouse and human.Unlikely the pigeon, both organisms have two isoforms of cN-I.The human gene coding for cN-IA is located on chromosome 1, in the region 1p34.3-p33.It comprises six exons and gives rise to a protein of 369 amino acids.No alternative splicing products have been observed for this gene.The other gene coding for cN-IB is located on chromosome 2 and it maps to the region 2p24.2.The protein product of this gene is 550 amino acids long.A comparison of the sequences of those two genes reveals that cN-IB includes an additional exon, which adds 181 amino acids to the N-terminus of the protein.Both cN-Iencoding alleles contain the so-called "5-nucleotidase signature" composed of an ATP-binding motif and a Mg 2+ -binding site.All intracellular nucleotidases including also the 3',5'-specific variant cdN and the mitochondrial variant mdN (Rinaldo-Matthis et al., 2002) contain three common motifs: I, II, III, being conserved catalytic sequences and S motif involved in substrate recognition.

INHIBIToRS aND SUBSTRaTeS oF VaRIoUS 5'-NUCLEOTIDASES
It is almost canonical that high and stable ATP level ensures survival of the cell.On the other hand, adenosine coming from degradation of ATP has numerous positive effects on tissues.As cN-II has been found as a rate-limiting enzyme in ATP degradation, many studies used cN-II inhibitors to check how it influences the whole adenine nucleotide metabolism.Nucleoside analogs based on thioalkylation of the 5'-position in the ribose moiety in adenosine and inosine able to block the activity of cN-II and to some extent cN-I have been described earlier (Meghji et al., 1993;Skladanowski et al., 1998).Later, the cN-I selective compound -5-ethynyl-2',3'-dideoxyuridine inhibiting the hydrolysis of AMP to adenosine in a concentrationdependent manner with an IC 50 value of 20 µM was reported (Garvey & Prus, 1999).These studies were performed, however, only in vitro.ADP analog, AOPCP (α,β-methylene-adenosine diphosphate), has been frequently used to block ecto-5'nucleotidase when studying its role in organ physiology.When AOPCP was infused to the whole animal heart, the effect of a lowered adenosine level on ischemic preconditioning was visible (Kitakaze et al., 1994).Other e5'-NT inhibitors like pentoxifillin or 6-thiopurine enhanced the protective action of cardioplegic fluid during full ischemia (Bolling et al., 1992).The differences in the substrate binding site in cN-I and cN-II were studied and a cru-cial role of the dipole moment of the base in substrate analogs was reported (Skladanowski et al., 1996).These studies might have significance when the role of various cytoplasmic 5'-nucleotidases is considered in the metabolism of anti-cancer and anti-viral nucleoside analogs (for a review see Hunsucker et al., 2005).The nucleotide forms of those analogs were found to be deactivated back to the corresponding nucleoside by cytoplasmic 5'-nucleotidases.On the other hand, cN-II has been shown to introduce a phosphate moiety to nucleoside prodrugs like deoxycoformycin forming then an active drug (Pesi et al., 1994).

CLINICAL RELEVANCE OF CYTOPLASMIC 5'-NUCLEOTIDASES
The great variety of actions of adenosine emphasizes the importance of the enzymes involved in its production.The only described genetic disorder related to 5'-nucleotidase deficiency, hereditary hemolytic anemia, is caused by a mutation in erythrocyte-specific cN-III.Elevated levels of cN-II have been found in neurological defects and in patients with Lesh-Nyhan syndrome (Pesi et al., 2000;Allegrini et al., 2001).Several studies have shown increased levels of cN-II mRNA in some cancers such as acute myeloid leukemia that correlated with more aggressive leukemic phenotype and poor clinical outcome due to resistance to cytarabine therapy (Galmarini et al., 2001).
In the past decades, investigations were focused mostly on cN-IA and its role in hypoxia and ischemia.Nowadays the clinical potential of soluble 5'-nucleotidases has been linked to the resistance to anti-cancer and anti-viral drugs basing on their ability to metabolize several analogue monophosphates.Up to date, cN-II has been studied the most extensively in this aspect with the most promising results both in vivo and in vitro.The drug resistance was found to be closely related to the level of the enzyme in various cell lines and with different analogues.There is some evidence that the increased levels of cN-II are accompanied by a decrease in dCK and therefore the ratio of both enzymes might be an important determining factor whether the cell would respond positively to the therapy.Creating a compound that could modulate this ratio could be beneficial for the therapy (Hunsucker et al., 2005).
The family of soluble 5'-nucleotidases seems to have an increasing clinical potential, but there is still much investigation to be done before specific 5'-NT variants of relevance are identified.Development of their inhibitors or compounds that would either overcome the drug resistance or be poorer substrates could move the research forward.

TRANSPORT OF ADENOSINE VIA CELL MeMBRaNe
Adenosine can cross the cell membrane through influx or efflux.Like other purine or pyrimidine nucleosides, it is transported by systems classified into two classes: equilibrative and concentrative.Adenosine transport during the influx has been more thoroughly studied and now it is well established that targeting the transporter protein by inhibitors (also drugs) has a consequence in the elevation of the extracellular concentration of adenosine required for the activation of P1-type receptors (Paterson et al., 1977;Klabunde, 1983;Zhang & Lautt, 1991).This might be of pharmacological importance when adenosine is formed extracellularly.During intracellular adenosine formation and subsequent release from the cells, inhibition of the equilibrative transport system can elevate the cytosolic concentration of the metabolite.In such a case, counteracting nucleoside escape, for example during ischemia or washout during reperfusion, may be beneficial for regeneration of the nucleotide pool after a period of ATP breakdown.It has also been suggested that inhibitors of nucleoside transport may decrease the loss of chemotherapeutically active nucleoside-based drugs from the cells.For example, millimolar concentrations of dipyridamole prevented excretion of inosine from T cell mutants overproducing purine nucleosides (Ullman & Kaur, 1983).Regulation of adenosine transport in mammalian heart during periods of ischemia or hypoxia have been considered as an important modulator of the heart function strongly dependent on the level of this metabolite.Nucleoside transporters are also under tight regulation by various metabolic and physiological factors (for a review see Podgorska et al., 2005).

CONCLUDING REMARKS
The aim of this review was to highlight the signaling role of adenosine, a molecule formed by ATP metabolism.Several adenosine effects attract the attention of clinicians.In the heart, adenosine acts as a feed-back regulator of ATP metabolism and provides the fine tuning of the balance between ATP production and utilization.In addition it can convey also a cytoprotective effect against oxidative stress.Clinical studies proved that it might have some beneficial role during coronary reperfusion therapy and minimize the ischemia/reperfusion injury.Activated adenosine receptors in CNS and peripheral tissues have been found as part of a mechanism for modulation of pain transmission.The role in skeletal muscles has not been elaborated yet in details but there are some proofs that a substantial amount produced in this tissue may have an impact on other tissues in the body.
There is a variety of enzymes which can be responsible for adenosine production.Apart from SAHH of little activity and connected to the transmethylation pathway, there are AMP hydrolases or ADP apyrases linked to ATP degradation.They are ubiquitously localized inside and outside the cell.Both adenosine producing enzymes, nucleoside membrane transporters and adenosine receptors offer themselves as molecular targets for developing a whole range of pharmaceutical agents as receptor ligands, modulators or enzyme inhibitors.
The simplest conclusion is that after more than 50 years of studies on adenosine and its mechanisms of action we still have more questions than answers.This makes adenosine related research still exciting and promising to the clinical field.

Table 2 . Summary of physiological effects of the activation of adenosine receptors (according to Relavic & Burnstock, 1998) A
1 Inhibition of neurotransmission; negative chronotropy, dromotropy and inotropy; up-regulation of insulin secretion and control of lipolysis; ischemic preconditioning of heart; modulation of nociception A 2A Facilitation of neurotransmitter release; negative interaction with A 1 and A 3 types and other receptors; vasodilation of capillary blood vessels; stimulation of growth of endothelial cells and angiogenesis A 2B Degranulation of mastocytoma cells; possible role in allergic and inflammatory disorders; vasodilation of capillary blood vessels; possible tropic role in release of other signaling substances A 3 Possible role in inflammation; proapoptotic effects; cardioprotective effects (the same as A 1 ) 272 2006 A. Borowiec and others