Transport and metabolism of adenosine in human erythrocytes: effect of transport inhibitors and regulation by phosphate

Adaptative mechanism of the equilibrative nucleoside transporter 1 (ENT-1) and blood adenosine levels in elite freedivers

PURPOSE

Long static or intense dynamic apnoea-like high-altitude exposure is inducing hypoxia. Adenosine is known to participate to the adaptive response to hypoxia leading to the control of heart rate, blood pressure and vasodilation. Extracellular adenosine level is controlled through the equilibrative nucleoside transporter 1 (ENT-1) and the enzyme adenosine deaminase (ADA). The aim of this study was to determine the control of adenosine blood level (ABL) via ENT-1 and ADA during apnoea-induced hypoxia in elite freedivers was similar to high-altitude adaptation.

METHODS

Ten freediver champions and ten controls were studied. Biological (e.g. ENT-1, ADA, ABL, PaO₂, PaCO₂ and pH) and cardiovascular (e.g. heart rate, arterial pressure) parameters were measured at rest and after a submaximal dry static apnoea.

RESULTS

In freedivers, ABL was higher than in control participants in basal condition and increased more in response to apnoea. Also, freedivers showed an ADA increased in response to apnoea. Finally, ENT-1 level and function were reduced for the free divers.

CONCLUSION

Our results suggest in freedivers the presence of an adaptive mechanism similar to the one observed in human exposed to chronic hypoxia induced by high-altitude environment.

Purine and pyrimidine transporters of pathogenic protozoa - conduits for therapeutic agents

Purines and pyrimidines are essential nutrients for any cell. Most organisms are able to synthesize their own purines and pyrimidines, but this ability was lost in protozoans that adapted to parasitism, leading to a great diversification in transporter activities in these organisms, especially for the acquisition of amino acids and nucleosides from their hosts throughout their life cycles. Many of these transporters have been shown to have sufficiently different substrate affinities from mammalian transporters, making them good carriers for therapeutic agents. In this review, we summarize the knowledge obtained on purine and pyrimidine activities identified in protozoan parasites to date and discuss their importance for the survival of these parasites and as drug carriers, as well as the perspectives of developments in the field.

Identification via a Parallel Hit Progression Strategy of Improved Small Molecule Inhibitors of the Malaria Purine Uptake Transporter that Inhibit Plasmodium falciparum Parasite Proliferation

Emerging resistance to current antimalarial medicines underscores the importance of identifying new drug targets and novel compounds. Malaria parasites are purine auxotrophic and import purines via the Plasmodium falciparum equilibrative nucleoside transporter type 1 (PfENT1). We previously showed that PfENT1 inhibitors block parasite proliferation in culture. Our goal was to identify additional, possibly more optimal chemical starting points for a drug discovery campaign. We performed a high throughput screen (HTS) of GlaxoSmithKline's 1.8 million compound library with a yeast-based assay to identify PfENT1 inhibitors. We used a parallel progression strategy for hit validation and expansion, with an emphasis on chemical properties in addition to potency. In one arm, the most active hits were tested for human cell toxicity; 201 had minimal toxicity. The second arm, hit expansion, used a scaffold-based substructure search with the HTS hits as templates to identify over 2000 compounds; 123 compounds had activity. Of these 324 compounds, 175 compounds inhibited proliferation of P. falciparum parasite strain 3D7 with IC₅₀ values between 0.8 and ∼180 μM. One hundred forty-two compounds inhibited PfENT1 knockout (pfent1Δ) parasite growth, indicating they also hit secondary targets. Thirty-two hits inhibited growth of 3D7 but not pfent1Δ parasites. Thus, PfENT1 inhibition was sufficient to block parasite proliferation. Therefore, PfENT1 may be a viable target for antimalarial drug development. Six compounds with novel chemical scaffolds were extensively characterized in yeast-, parasite-, and human-erythrocyte-based assays. The inhibitors showed similar potencies against drug sensitive and resistant P. falciparum strains. They represent attractive starting points for development of novel antimalarial drugs.

Ticagrelor: Effects Beyond the P2Y12 Receptor

Platelet P2Y₁₂ receptor inhibitors are crucial in the treatment of patients with acute coronary syndrome or undergoing percutaneous coronary intervention. Ticagrelor is a reversibly binding, potent oral P2Y₁₂ inhibitor that also is a weak inhibitor of the equilibrative nucleoside transporter-1 pathway for cellular adenosine uptake. It is hypothesized that ticagrelor has clinically relevant "off-target" effects, independent of its effect on platelet aggregation and thrombosis. This review considers the pleiotropic effects of ticagrelor and some of the possible mechanisms related to these effects.

Endogenous adenosine release is involved in the control of heart rate in rats

Intravenous (i.v.) injections of adenosine exert marked effects on heart rate (HR) and arterial blood pressure (BP), but the role of an endogenous adenosine release by vagal stimulation has not been evaluated. In anaesthetized rats, we examined HR and BP changes induced by 1 min electrical vagal stimulation in the control condition, and then after i.v. injections of (i) atropine, (ii) propranolol, (iii) caffeine, (iv) 8 cyclopentyl-1,3-dipropylxanthine (DPCPX), or (v) dipyridamole to increase the plasma concentration of adenosine (APC). APC was measured by chromatography in the arterial blood before and at the end of vagal stimulation. The decrease in HR in the controls during vagal stimulation was markedly attenuated, but persisted after i.v. injections of atropine and propranolol. When first administered, DPCPX modestly but significantly reduced the HR response to vagal stimulation, but this disappeared after i.v. caffeine administration. Both the HR and BP responses were significantly accentuated after i.v. injection of dipyridamole. Vagal stimulation induced a significant increase in APC, proportional to the magnitude of HR decrease. Our data suggest that the inhibitory effects of electrical vagal stimulations on HR and BP were partly mediated through the activation of A1 and A2 receptors by an endogenous adenosine release. Our experimental data could help to understand the effects of ischemic preconditioning, which are partially mediated by adenosine.

Purine import into malaria parasites as a target for antimalarial drug development

Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophs. In all life cycle stages, they require purines for RNA and DNA synthesis and other cellular metabolic processes. Purines are imported from the host erythrocyte by equilibrative nucleoside transporters (ENTs). They are processed via purine salvage pathway enzymes to form the required purine nucleotides. The Plasmodium falciparum genome encodes four putative ENTs (PfENT1-4). Genetic, biochemical, and physiologic evidence suggest that PfENT1 is the primary purine transporter supplying the purine salvage pathway. Protein mass spectrometry shows that PfENT1 is expressed in all parasite stages. PfENT1 knockout parasites are not viable in culture at purine concentrations found in human blood (<10 μM). Thus, PfENT1 is a potential target for novel antimalarial drugs, but no PfENT1 inhibitors have been identified to test the hypothesis. Identifying inhibitors of PfENT1 is an essential step to validate PfENT1 as a potential antimalarial drug target.

Association between Related Purine Metabolites and Diabetic Retinopathy in Type 2 Diabetic Patients

Aims. The purpose of the study was to investigate the differences of adenosine, adenine, inosine, xanthine, hypoxanthine, and uric acid concentrations in patients with type 2 diabetes mellitus and diabetic retinopathy and assess the relationship between purine metabolites and disease. Materials and Methods. The study group consisted of 114 subjects which were divided into three groups: control (n = 40), type 2 diabetes without retinopathy (n = 35), and type 2 diabetes with retinopathy (n = 39). Levels of metabolites were measured in plasma of all participants. Results. There is a significant increase of levels of adenosine (0.94 ± 0.17 mg/L versus 0.17 ± 0.01 mg/L, P < 0.001), inosine (0.297 ± 0.078 mg/L versus 0.086 ± 0.010 mg/L, P < 0.001), xanthine (1.01 ± 0.21 mg/L versus 0.54 ± 0.05 mg/L, P = 0.009), and uric acid (70.55 ± 3.97 mg/L versus 53.81 ± 2.36 mg/L, P < 0.001) with diabetic retinopathy compared to diabetes mellitus. The levels of adenine, hypoxanthine, and xanthine oxidase did not change. Uric acid, xanthine, inosine, and adenosine correlated positively with systolic blood pressure and urea nitrogen. Conclusions. The levels of adenosine, inosine, uric acid, and xanthine may be useful for monitoring the progression of diabetic retinopathy and evaluating the treatment.

Dipyridamole synergizes with nitric oxide to prolong inhibition of thrombin-induced platelet shape change

We and others have previously demonstrated that nitric oxide (NO)-induced inhibition of platelet shape change is important in regulating platelet adhesion and aggregation, and therapeutic intervention of this pathway is clinically relevant for secondary prevention of stroke with dipyridamole. In the present study, we investigated whether dipyridamole affected the shape change of aspirinated platelets. Platelet shape change was inhibited using both authentic NO and sodium nitroprusside, as monitored by light scattering and mean platelet volume measurements. Dipyridamole synergized with NO, even at supra-therapeutic levels, to inhibit thrombin-induced shape change and further potentiated cAMP dependent protein kinase (PKA) mediated phosphorylation of vasodilator stimulated phosphoprotein (VASP) Ser157, even without altered levels of platelet cAMP. The effect of dipyridamole on NO-inhibited shape change depended on cGMP synthesis as evaluated by inhibition of soluble guanylyl cyclase. Measured increases in cGMP levels by dipyridamole and NO was assessed by mathematical modeling and found to be consistent with inhibition of phosphodiesterase 5 (PDE5). The model could explain the unexpected efficiency of dipyridamole in inhibiting PDE5 at the measured cGMP levels, by the majority of cGMP being bound to cGMP-dependent protein kinase (PKG). Still, selective activators of PKG failed to extend NO-mediated inhibition of the thrombin-induced platelet shape change, suggesting that PKG was not responsible for the inhibitory effect of NO and dipyridamole on shape change. The effects of dipyridamole were independent of the prostanoid and ADP pathways. Thus, the effect of dipyridamole on NO-mediated inhibition of platelet shape change may be an important and additional beneficial therapeutic effect of dipyridamole, which we suggest, is acting though localized amplification of the NO/cGMP/Phosphodiesterase3/cAMP/PKA-pathway. Probably, the efficiency of dipyridamole could be amplified clinically with NO donors.

The purine metabolism of human erythrocytes

This review summarizes currently available information about a crucial part of erythrocyte metabolism, that is, purine nucleotide conversions and their relationships with other conversion pathways. We describe the cellular resynthesis, interconversion, and degradation of purine compounds, and also the regulatory mechanisms in the conversion pathways. We also mention purine metabolism disorders and their clinical consequences. The literature is fragmentary because studies have concentrated only on selected aspects of purine metabolism; hence the need for a synthetic approach.

Influence of the Dialysis Membrane on Markers of Tissue Ischemia

BACKGROUND

Hemodialysis (HD) is often accompanied by adverse effects, such as tissue ischemia. We have already observed an increase in plasma adenosine (ADO) levels during HD sessions, which may be the result of tissue ischemia. Here we evaluate the influence of the dialysis membrane on two sensitive and early markers of ischemia: ADO and ischemia-modified albumin (IMA).

METHODS

We included in the study 50 patients with end-stage renal failure, 39 hemodialyzed (mean age 61+/-24 years; 24 male; membranes: 23 synthetic and 16 cellulose based) and 11 undialyzed (mean age 55+/-12 years; 6 male), and 10 healthy subjects (mean age 47+/-11 years; 4 male). We compensated for hemoconcentration during HD by measuring either the IMA to albumin (Alb) or the ADO to Alb ratio.

RESULTS

Under basal conditions, the IMA to Alb ratio was not significantly different in patients and controls and HD did not significantly modify this ratio. Conversely, the ADO to Alb ratio (mean+/-SD in micromol/g) was higher in patients before HD compared with either undialyzed patients or controls (before HD: 0.077+/-0.02; undialyzed patients: 0.026+/-0.11; controls: 0.022+/-0.01). During HD, there was a significant increase in the ADO to Alb ratio (before HD: 0.077+/-0.02; after HD: 0.09+/-0.029; p<.01). We found no significant difference in the IMA to Alb or ADO to Alb ratio using either synthetic or cellulose-based membranes.

CONCLUSIONS

We concluded that ADO is a more sensitive marker of ischemia than IMA and that, under our HD conditions, the ischemia caused by HD was very weak, independent of the dialysis membrane.

Role of endogenous adenosine as a predictive marker of vasoplegia during cardiopulmonary bypass and postoperative severe systemic inflammatory response

OBJECTIVE

Systemic inflammatory response (SIRS) and severe SIRS (SIRS with organ dysfunction) occurring after cardiopulmonary bypass (CPB) are common causes of morbidity and mortality among cardiac surgical patients. These syndromes are often preceded by a profound vasodilation, characterized by vasoplegia occurring during surgery. Many substances have been implicated in their pathophysiology. Adenosine is a strong endogenous vasodilating agent released by endothelial cells and myocytes under metabolic stress and may be involved in blood pressure failure during CPB induced by severe SIRS.

DESIGN

A prospective comparative observational study.

SETTING

The operating room and intensive care unit of a tertiary care university hospital.

PATIENTS

Adenosine plasma levels (mean+/-sd; APLs) were measured before (baseline), during, and immediately after surgery in 35 patients who underwent aortic valve replacement involving CPB. APLs were correlated to operative and postoperative clinical courses.

MEASUREMENTS AND MAIN RESULTS

APLs were significantly higher in seven patients with vasoplegia and postoperative severe SIRS (1.6 micromol.L [0.2-2.6] vs. 0.4 micromol.L [0.1-1.0]) at baseline and during surgery. The duration of mechanical ventilation and stay in the intensive care unit were significantly longer for patients with higher APLs. Mean arterial pressure was inversely correlated with mean arterial APLs (Pearson's correlation coefficient: R=-0.66; p<.001).

CONCLUSIONS

High APLs were found in patients with operative vasoplegia and postoperative severe SIRS occurring after cardiopulmonary bypass. This suggests that adenosine release is involved in vasoplegia that occurs during the systemic inflammatory response to cardiac surgery. Further studies are needed to clarify the association between cytokine production and adenosine release in severe SIRS following cardiac surgery.

Kinetic study of adenosine concentrations and the expression of adenosine deaminase in mononuclear cells during hemodialysis

BACKGROUND

We previously showed that high intralymphocytic adenosine (Ado) concentrations are found in hemodialyzed patients due to the reduced activity of mononuclear cell adenosine deaminase (MCADA). These abnormalities contribute to the immune defect observed in HD patients. The kinetics of these abnormalities and the causes of the low MCADA activity, however, have not been investigated. Here, we addressed this question. Since interferon gamma (IFNgamma) partially modulates MCADA, we also evaluated the effect of IFNgamma on MCADA activity in vitro.

METHODS AND RESULTS

The study included 12 patients (eight men and four women) who were observed from the first to the 36th hemodialysis (HD) session, and eight healthy subjects (controls). MCADA activity and Ado concentrations were normal before the first HD session. Ado concentrations progressively increased from the first (10.5 +/- 3.1 pmol/10(7) cells) to the fourth session (26.7 +/- 3 pmol/10(7) cells), before stabilizing at a high level. MCADA activity increased transiently until the second session (2.2 +/- 0.5 IU/10(7) cells before HD vs. 2.8 +/- 0.6 IU/10(7)cells), and then decreased and stabilized at a low level (1.0 +/- 0.5 IU/10(7)cells). The amount of MCADA mRNA transiently increased until the third session (mRNA MCADA/mRNA beta-actin: 0.6 +/- 0.2 vs. 0.8 +/- 0.2), and then decreased to 0.3 +/- 0.1 at the 36th session. MCADA activity underwent a dose-dependent increase after IFNgamma stimulation.

CONCLUSION

HD affects the transcription of the gene encoding MCADA after just three HD sessions, leading to decreased MCADA activity and increased plasma concentration of Ado.

High adenosine and deoxyadenosine concentrations in mononuclear cells of hemodialyzed patients

Infections are one of the most important complications of hemodialysis (HD). The high concentrations of adenosine (Ado) and of its metabolites during HD may contribute to the dialysis-induced immune deficiency through their known ability to alter lymphocyte function. The influence of HD on Ado metabolism was assessed in mononuclear cells through the measurement of (1) the concentrations of nucleosides in mononuclear cells and (2) the activities of mononuclear cell Ado deaminase (MCADA) and Ado kinase, two enzymes involved in Ado concentration regulation. Nine end-stage renal failure hemodialyzed patients (five men and four women; mean age, 69 +/- 10 yr) and eight healthy volunteers (four men and four women; mean age, 53 +/- 19 yr) were included in the study. Before HD, Ado, deoxyadenosine, and inosine concentrations were respectively 2.9-, 2.5-, and 2.5-fold higher in mononuclear cells of patients than in healthy volunteers. During HD, Ado concentration decreased by 34%, whereas inosine concentration increased by 27%. Before HD, MCADA activity level was 2.1-fold lower in patients than in control subjects. After HD, MCADA activity increased by nearly 50% but remained lower than in control subjects. Ado kinase activity level of patients did not differ from that of control subjects and was unchanged by HD. The influence of Ado on in vitro mononuclear cell proliferation and interferon-gamma production also was evaluated. Ado inhibited cell proliferation and interferon-gamma production in a dose-dependent manner, and these inhibitions were stronger for patients than for healthy volunteers. The high concentrations of Ado and deoxyadenosine in mononuclear cells and the low MCADA activity level likely are involved in the immune defect of patients who are undergoing HD.

Adenine incorporation in human and rat endothelium

Adenine (ADE) reutilisation is an important pathway of adenylate pool regeneration. Data on the rate of this process in different types of cells, its regulation and the importance of species differences is limited. In this study we evaluated adenine incorporation rate and the effect of metabolic factors on this process in human and rat endothelium and compared it to adenine phosphoribosyltransferase (APRT) activity. Microvascular endothelial cells from human (HE) and rat (RE) hearts and a transformed human microvascular endothelial cell line (HMEC-1) were investigated. The rate of adenine incorporation into the adenine nucleotide pool under control conditions was 3.1+/-0.3, 82.8+/-11.1 and 115.1+/-11.2 pmol/min per mg protein for HE, RE and HMEC-1, respectively. In the presence of 2.5 mM ribose or elevated inorganic phosphate concentration in the medium (4.8 mM), few changes were observed in all types of cells. In the presence of both ribose and high inorganic phosphate, the rate of adenine incorporation for RE and HMEC-1 was not significantly different from control, while in HE the rate of adenine incorporation into adenine nucleotides was increased by 75%. Activities of APRT in RE and HMEC-1 were 237.7+/-23.2 and 262.0+/-30.6 pmol/min per mg protein respectively while the activity in HE was markedly lower 48.7+/-3.0 pmol/min per mg protein. In conclusion, nucleotide synthesis from adenine seems to be a slow process in human cardiac microvascular endothelium but it is fast and efficient in rat heart microvascular endothelial cells. Low APRT activity in normal human endothelial cells seems to be the most likely mechanism for this. However, adenine incorporation rate and APRT activity could be greatly enhanced in human endothelium, as demonstrated in transformed cells.

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.

19F NMR study of the uptake of 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil in erythrocytes: evidence of transport by facilitated and nonfacilitated pathways

The 19F NMR resonances of intra- and extracellular 2'-fluoro-5-methyl-beta-L-arabinofuranosyluracil (L-FMAU) in suspensions of human erythrocytes are well resolved. This phenomenon allows its transport behavior to be monitored in a 19F NMR time-course experiment. The rate of L-FMAU uptake at 25 degrees in a suspension containing L-FMAU at an initial extracellular concentration of 4 mM was 7.6 +/- 1.0 x 10(-7) pmol cell(-1) sec(-1) (N = 5). Concentration-dependent uptake studies of L-FMAU indicate the existence of both saturable and nonsaturable transport mechanisms, with a Km for the saturable uptake of approximately 1 mM. Although the transport of L-FMAU at 25 degrees was inhibited significantly (54-65%) by nitrobenzylthioinosine (NBTI) and dipyridamole, consistent with the participation of the nucleoside transporter, these inhibitors did not achieve complete blockage of L-FMAU uptake. The participation of the nucleobase transporter in L-FMAU uptake was ruled out by the absence of competition with uracil uptake, and by the lack of inhibition by papaverine. In addition, the NBTI-insensitive uptake of L-FMAU was not affected by pretreatment of the cells with the sulfhydryl reagent, p-chloromercuriphenylsulfonic acid (pCMBS). However, the NBTI- and dipyridamole-insensitive transport of L-FMAU was found to increase upon treatment of the erythrocytes with butanol, an agent that affects membrane fluidity. The partition coefficient of L-FMAU in octanol/phosphate-buffered saline determined by absorption spectrophotometry was 0.31. These data indicate that under the conditions of the studies, L-FMAU uptake by erythrocytes proceeds by both the nucleoside transporter and nonfacilitated membrane diffusion.

Contrast in adenine uptake by chicken and rabbit erythrocytes in vitro

1. Relative to rabbit erythrocytes, chicken red blood cells exhibit a much greater capacity to utilize [3H]adenine for nucleotide synthesis in vitro, even at 5 degrees C and in the absence of added inorganic phosphate. 2. This difference is largely due to a higher concentration of phosphoribosylpyrophosphate and greater activity of adenine phosphoribosyltransferase in the avian cells. 3. The capacity of avian erythrocytes for utilization of guanine and hypoxanthine is several fold less than that of adenine. 4. The data are consistent with lower activity for hypoxanthine/guanine phosphoribosyltransferase than for adenine phosphoribosyltransferase in intact chicken erythrocytes. 5. The results indicate that reutilization of adenine by chicken erythrocytes may be physiologically significant.

Is adenosine a second metabolic substrate for human red blood cells?

Adenosine is present in the micromolar range in human plasma. In this study, metabolism of adenosine, which was maintained between 0.62 +/- 0.03 and 2.92 +/- 0.43 microM by means of a continuous infusion using a Harvard infusion pump, was investigated in human red blood cells. It was found that lactate production increases linearly as the adenosine concentration was raised. Cells infused with an average adenosine concentration of 2 microM produced lactate comparable to that produced by 5 mM glucose. The extent to which ATP concentration is maintained by adenosine also depends on its concentration. After a 4 h infusion with an average adenosine concentration of 0.7 microM, ATP content amounts to 75% of the glucose control. Raising the adenosine infusion concentration to 1.5 microM results in a full maintenance of ATP levels and at concentrations higher than 1.5 microM, adenosine produces a net synthesis of ATP. A net synthesis of ATP also occurs with adenosine concentration below 1.5 microM, if supplemented with glucose. In contrast, inosine infusion provides only a partial support of ATP and fails to produce a net synthesis of ATP in the presence of glucose. In addition, the presence of purine nucleoside and glucose together influence the metabolism of each other, depending on inorganic phosphate content (Pi). At a Pi concentration of 1 mM, the glucose consumption rate is reduced by approx. 25% by purine nucleoside infusion and vice versa. In sharp contrast, glucose consumption at 16 mM Pi is potentiated by adenosine. These findings suggest that plasma adenosine contributes significantly to human red cell energetics, even though it is present at a concentration several orders of magnitude lower than glucose.

Mycoplasma contamination alters 2'-deoxyadenosine metabolism in deoxycoformycin-treated mouse leukemia cells

Deoxycoformycin-treated P388 and L1210 mouse leukemia cells salvage 2'-deoxyadenosine from the medium only inefficiently, because deoxyadenosine deamination is blocked and its phosphorylation is limited by feedback controls. Mycoplasma contamination at a level that had no significant effect on the growth of the cells increased the salvage of deoxyadenosine greater than 10 fold over a 90 min period of incubation at 37 degrees C, but in this case deoxyadenosine was mainly incorporated into ribonucleotides and RNA via adenine formed from deoxyadenosine by mycoplasma adenosine phosphorylase. Deoxyadenosine was an efficient substrate for this enzyme, in contrast to 2',3'-dideoxyadenosine which was not phosphorolyzed. Mycoplasma infection was confirmed by the presence of uracil phosphoribosyltransferase activity and by culture isolation. The contaminant has been identified as Mycoplasma orale. Mycoplasma infection had no effect on the deamination and phosphorylation of deoxyadenosine and adenosine, on the salvage of hypoxanthine and adenine, or on the degradation of dAMP and dATP by the cells or on their acid and alkaline phosphatase activities.

Adenosine metabolism in kidney slices under normoxic conditions

The effect of adenosine in rat kidney under normoxic conditions has been studied. It is demonstrated that adenosine modulates cell nucleotide levels. HPLC analysis of the purine compounds inside the cell indicates that adenosine improves the ATP/ADP ratio, whereas it diminishes the adenine content. This behaviour is not due to mediation by specific receptors, as agonists at P1 purinoceptors did not have any effect. Further evidence using inosine as well as dipyridamole and deoxycoformycin indicates that all effects are dependent on the previous uptake of adenosine. The origin of free adenine in the kidney has been investigated, and it appears to come from the phosphorolysis of 5'-methylthio-adenosine. This report is the first to describe the activity of methylthioadenosine phosphorylase (E.C. 2.4.2.28) in the kidney. It is concluded that 1) extracellular adenosine improves guinea pig renal function by increasing the ATP level and the ATP/ADP ratio; and 2) there exists a functional pathway in the kidney that produces adenine and AMP coming from methionine and ATP. This latter pathway probably produces spermine and spermidine, which are likely to be important for renal function.

Turnover of adenosine in plasma of human and dog blood

To determine half-life and turnover of plasma adenosine, heparinized blood from healthy volunteers was incubated with radiolabeled adenosine in the physiological concentration range of 0.1-1 microM. Plasma levels of adenosine in vitro were 82 +/- 14 nM and were similar to those determined immediately after blood collection with a "stopping solution." Dipyridamole (83 microM) and erythro-9(2-hydroxynon-3yl)-adenine (EHNA) (8 microM) did not measurably alter basal adenosine levels but completely blocked the uptake of added adenosine. Inhibition of ecto-5'-nucleotidase with 100 microM alpha, beta-methyleneadenosine 5'-diphosphate (AOPCP) reduced plasma adenosine to 22 +/- 6 nM. For the determination of adenosine turnover, the decrease in specific radioactivity of added [3H]adenosine was measured using a dipyridamole-containing stopping solution. Without altering basal adenosine levels, the half-life was estimated to be 0.6 s. Similar experiments were carried out with washed erythrocytes or in the presence of AOPCP, yielding half-lives of 0.7 and 0.9 s, respectively. When the initial adenosine concentration was 1 microM, its specific activity decreased by only 11% within 5 s, whereas total plasma adenosine exponentially decreased with a half-life of 1.5 s. Venous plasma concentrations were measured after relief of a 3-min forearm ischemia. Changes in plasma adenosine did not correlate well with changes in blood flow but were augmented in the presence of dipyridamole.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of formycin B as a general substrate for measuring facilitated nucleoside transport in mammalian cells

Formycin B, a C-nucleoside analog of inosine, is not catabolized by human erythrocytes and mouse P388 leukemia cells and is only very inefficiently phosphorylated in these cells. This relative inertness allows the measurement of its transport into and out of the cells uncomplicated by metabolic conversions. We have measured the zero-trans and equilibrium exchange flux of formycin B in these cells by rapid kinetic techniques. The Michaelis-Menten constants and maximum velocities for formycin B transport in both types of cell were similar to those previously reported for uridine and thymidine. Nevertheless, the differential mobility of the substrate-loaded and empty carrier of human erythrocytes was less for formycin B than uridine as substrate. Formycin B influx was inhibited by other nucleosides in accordance with their affinities for the carrier, but unaffected by purines. The inhibition of formycin B influx by nitrobenzylthioinosine and dipyridamole was also identical to that observed with uridine as substrate (IC50 = 10 and 30 nM, respectively). Formycin B accumulated in both types of cell to 30-40% higher concentrations than were present in the medium. This concentrative accumulation was not due to active transport, metabolism or partitioning into membrane lipids. It seems to reflect binding of formycin B to intracellular components, but does not interfere significantly with measurements of its transport.

A comparison of nucleoside transport and metabolism in hypertensive and normotensive rats

In a previous study, we discovered that spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) are dissimilar with respect to the depressor potency differentiated between intravenously and intra-arterially infused adenosine. To test the hypothesis that this dissimilarity may reflect a difference between the two strains in adenosine transport or metabolism, we compared the kinetics of nucleoside transport (i.e., [3H]uridine uptake) in erythrocytes and the pulmonary disposition of [3H]adenosine in SHR versus WKY. [3H]Uridine uptake in rat erythrocytes was linear for 4 minutes and inhibitable with dipyridamole. Kinetic analysis (i.e., Hofstee plots) of initial uptake velocity indicated no difference between the two strains with respect to apparent Km (196 +/- 40 vs 230 +/- 29 microM in WKY and SHR, respectively) and maximum velocity (7.5 +/- 0.4 vs 8.3 +/- 0.5 pmol/2 min/12% Hct in WKY and SHR, respectively). Approximately 50% of [3H]adenosine infused into the pulmonary artery of perfused rat lung was transported into the lung, and 85% of this material was incorporated into the nucleotide pool. Radioactivity in the lung perfusate consisted initially of equal amounts of adenosine and inosine; however, within 60 seconds after administration of [3H]adenosine most of the effluent radioactivity was inosine. No differences were detected in adenosine uptake, intracellular metabolism, or extracellular metabolism in lung from SHR versus WKY. Our data indicate that any difference between SHR and WKY with respect to the biological response to adenosine cannot be attributed to differences in adenosine disposition and, therefore, must be due to pharmacodynamic differences between the strains.

Adenosine metabolism in human whole blood. Effects of nucleoside transport inhibitors and phosphate concentration

Adenosine (Ado, 10 microM) was metabolized in whole blood within 1 min, primarily to hypoxanthine and ATP. The concentration of Ado, the activities of adenosine deaminase (ADA) and Ado kinase, the Km values for Ado with ADA and Ado kinase, and the substrate inhibition of Ado kinase are factors that govern the Ado metabolism between deamination and phosphorylation. If ADA activity was blocked by 2'-deoxycoformycin (dCF, 5 microM), a tight-binding inhibitor of ADA, most of the Ado (96%) was incorporated into adenine nucleotides, whereas if Ado kinase activity was blocked with 5-iodotubercidin (10 microM), Ado was mainly (95%) metabolized into hypoxanthine. A high phosphate concentration (25 mM) caused marked increases in the formation of IMP. The nucleoside transport inhibitors dilazep (1 microM), dipyridamole (10 microM) and nitrobenzylthioinosine (NBMPR, 1 microM) strongly blocked cellular Ado metabolism. In the presence of nucleoside transport inhibitors, Ado which slowly enters the cell was metabolized principally by Ado kinase rather than ADA. Dilazep, NBMPR and dipyridamole were more effective in blocking Ado uptake and metabolism by erythrocytes suspended in a protein-free medium than by cells suspended in plasma.

Pathways of adenine nucleotide catabolism in primary rat muscle cultures

The pathways of AMP degradation and the metabolic fate of adenosine were studied in cultured myotubes under physiological conditions and during artificially induced enhanced degradation of ATP. The metabolic pathways were gauged by tracing the flow of radioactivity from ATP, prelabelled by incubation of the cultures with [14C]adenine, into the various purine derivatives. The fractional flow from AMP to inosine through adenosine was estimated by the use of the adenosine deaminase (EC 3.5.4.4) inhibitors, coformycin and 2'-deoxycoformycin. The activities of the enzymes involved with AMP and adenosine metabolism were determined in cell extracts. The results demonstrate that under physiological conditions, there is a small but significant flow of label from ATP to diffusible bases and nucleosides, most of which are effluxed to the incubation medium. This catabolic flow is mediated almost exclusively by the activity of AMP deaminase (EC 3.5.4.6), rather than by AMP 5'-nucleotidase (EC 3.1.3.5), reflecting the markedly higher Vmax/Km ratio for the deaminase. Enhancement of ATP degradation by inhibition of glycolysis or by combined inhibition of glycolysis and of electron transport resulted in a markedly greater flux of label from adenine nucleotides to nucleosides and bases, but did not alter significantly the ratio between AMP deamination and AMP dephosphorylation, which remained around 19:1. Combined inhibition of glycolysis and of electron transport resulted, in addition, in accumulation of label in IMP, reaching about 20% of total AMP degraded. In the intact myotubes at low adenosine concentration, the anabolic activity of adenosine kinase was at least 4.9-fold the catabolic activity of adenosine deaminase, in accord with the markedly higher Vmax/Km ratio of the kinase for adenosine. The results indicate the operation in the myotube cultures, under various rates of ATP degradation, of the AMP to IMP limb of the purine nucleotide cycle. On the other hand, the formation of purine bases and nucleosides, representing the majority of degraded ATP, indicates inefficient activity of the IMP to AMP limb of the cycle, as well as inefficient salvage of hypoxanthine under these conditions.

Purine and pyrimidine transport and permeation in human erythrocytes

Time courses of the uptake of radiolabeled hypoxanthine, adenine and uracil were measured by rapid kinetic techniques over substrate ranges from 0.02 to 5000 microM in suspensions of human erythrocytes at 25 or 30 degrees C. At concentrations above 25 microM, the rate of intracellular phosphoribosylation of hypoxanthine and adenine was insignificant relative to their rates of entry into the cell and time courses of transmembrane equilibration of the substrates could be measured and analyzed by integrated rate analysis. Hypoxanthine and uracil are transported by simple facilitated carriers with directional symmetry, high capacity and Michaelis-Menten constants of about 0.2 and 5 mM, respectively. Adenine is probably transported by a carrier with similar properties but no saturability was detectable up to a concentration of 5 mM. Cytosine entered the cells much more slowly than the other three nucleobases, and its entry seems not to be mediated by a carrier. The hypoxanthine transporter resembles that of one group of mammalian cell lines, which does not exhibit any overlap with the nucleoside transporter and is resistant to inhibitors of nucleoside transport. Results from studies on the effects of the nucleobases on the influx and countertransport of each other were complex and did not allow unequivocal conclusions as to the number of independent carriers involved. At concentrations below 5 microM, radiolabel from adenine and hypoxanthine accumulated intracellularly to higher than equilibrium levels. Part of this accumulation reflected metabolic trapping, especially when the medium contained 50 mM phosphate. But part was due to an apparent concentrative accumulation of free adenine and hypoxanthine up to 3-fold at medium concentrations much less than 1 microM and when cells were incubated in phosphate-free medium. This concentrative accumulation could be due to the functioning of additional high-affinity, low-capacity, active transport systems for adenine and hypoxanthine, but other factors could be responsible, such as saturable binding to intracellular components.

Nucleoside transporter of pig erythrocytes. Kinetic properties, isolation and reaction with nitrobenzylthioinosine and dipyridamole

Rapid kinetic techniques were used to measure the transport of uridine in pig erythrocytes in zero-trans entry and exit and equilibrium exchange protocols. The kinetic parameters were computed by fitting appropriate integrated rate equations to the time-courses of transmembrane equilibration of radiolabeled uridine. Transport of uridine conformed to the simple carrier model with directional symmetry, but differential mobility of substrate-loaded and empty carrier. At 5 degrees C, the carrier moved about 30-times faster when loaded than when empty. Uridine transport was inhibited in a concentration-dependent manner by nitrobenzylthioinosine and dipyridamole and the inhibition correlated with the binding of the inhibitors to high-affinity binding sites on the cells (Kd about 1 and 10 nM, respectively). Thus, in its kinetic properties, differential mobility when empty and loaded, and sensitivity to inhibition by nitrobenzylthioinosine and dipyridamole, the transporter of pig erythrocytes is very similar to that of human erythrocytes. Also, the total number of high-affinity binding sites for nitrobenzylthioinosine and dipyridamole/cell were similar for the two cell types and the [3H]nitrobenzylthioinosine-labeled carrier of pig erythrocytes, just as that of human red cells, was mainly recovered in the band 4.5 protein fraction of Triton X-100-solubilized membranes. However, sodium dodecylsulfate-polyacrylamide gel electrophoresis of photoaffinity-labeled band 4.5 membrane proteins indicated a slightly higher molecular weight for the transporter from pig than human erythrocytes. We have also confirmed the lack of functional sugar transport in erythrocytes from adult pigs by measuring the uptake of various radiolabeled sugars. But in spite of the lack of functional sugar transport we recovered as much band 4.5 protein from pig as from human erythrocyte membranes.

Interaction of [3H]dipyridamole with the nucleoside transporters of human erythrocytes and cultured animal cells

Equilibrium binding of [3H]dipyridamole identified high-affinity (Kd approximately 10 nM) binding sites on human erythrocytes (approximately 5 X 10(5) sites/cell) and on HeLa cells (approximately 5 X 10(6) sites/cell). The equilibration of dipyridamole with these sites on human erythrocytes was compatible with a second-order process which proceeded at 22 degrees C with a rate constant of about 6 X 10(6) M-1 sec-1. Binding of dipyridamole to these sites correlated kinetically with the inhibition of the equilibrium exchange of 500 microM uridine in these cells and was inhibited in a concentration-dependent manner by nucleosides and other inhibitors of nucleoside transport, such as nitrobenzylthioinosine, dilazep and lidoflazine, but not by hypoxanthine, which is not a substrate for the nucleoside transporter of human erythrocytes. The results indicate that the substrate binding site of the transporter is part of the high-affinity dipyridamole binding site. Bound [3H]dipyridamole became displaced from these sites on human erythrocytes by incubation with an excess of unlabeled dipyridamole or high concentrations of nucleosides and inhibitors of nucleoside transport, but neither by hypoxanthine nor sugars. Dissociation of [3H]dipyridamole behaved as a simple first-order process, but the rate constant was about one order of magnitude lower (about 3 X 10(-3) sec-1) than anticipated for typical ligand-protein binding on the basis of the measured association rate and equilibrium constants. The reason for this discrepancy has not been resolved. No high-affinity dipyridamole binding sites were detected on Novikoff rat hepatoma cells, P388, L1210 and S49 mouse leukemia cells or Chinese hamster ovary cells, and their absence correlated with a greater resistance of nucleoside transport in these cells to inhibition by dipyridamole. All cells expressed considerable low affinity (Kd greater than 0.5 microM) and nonspecific binding of dipyridamole.