Chemiluminescent determination of adenosine, inosine, and hypoxanthine/xanthine

Inosine enhances the efficacy of immune-checkpoint inhibitors in advanced solid tumors: A randomized, controlled, Phase 2 study

BACKGROUND

This study aimed to evaluate whether inosine enhances the efficacy of immune-checkpoint inhibitors in human malignant solid tumors.

METHODS

This single-center, prospective, randomized, open-label study was conducted, from January 2021 to December 2022, in Beijing Friendship Hospital, Capital Medical University, and participants were randomly assigned (1:1) to either the inosine (trial) or non-inosine (control) group that received inosine (dosage: 0.2 g, three times/day) + PD-1/PD-L1 inhibitor or only PD-1/PD-L1 inhibitor ± targeted ± chemotherapy, respectively. Efficacy was assessed every 6 weeks (i.e., after every two-three treatment cycles). The primary endpoint was the objective response rate (ORR); the secondary endpoints were disease control rate, overall survival (OS), and progression-free survival (PFS). The trial was registered at ClinicalTrials.gov (NCT05809336).

RESULTS

Among the 172 participants with advanced malignant solid tumors, 86 each were assigned to the inosine and non-inosine groups, wherein the median PFS (95% CI) was 7.00 (5.31-8.69) and 4.40 (3.10-5.70) months, respectively (hazard ratio [HR] 0.63; 95% CI 0.44-0.90, p = 0.011), and the ORR was 26.7% and 15.1%, respectively (p = 0.061). In the inosine and non-inosine groups, the median OS was not reached and was 29.67 (95% CI 17.40-41.94) months, respectively (HR 1.05 [95% CI 0.59-1.84], p = 0.874). Compared with the non-inosine group, the median PFS and ORR of the inosine group were significantly prolonged and improved in the multiple exploratory subgroup analyses. The safety analysis showed that Grades 3 and 4 adverse reactions occurred in 25 (29%) and 31 (36%) patients in the inosine and non-inosine groups, respectively, and tended to decrease in the inosine group compared with the non-inosine group.

CONCLUSION

Inosine had a tendency to enhance the efficacy of immune-checkpoint inhibitors and reduced immunotherapy-related adverse reactions.

Simultaneous detection of purine metabolites by membrane modified electrochemical sensors

Purine metabolites are important for metabolic and cellular processes. Deregulation of purinergic signaling leads to pathological accumulation of purine degradation products in extracellular fluids and indicates various diseases. In clinical diagnosis at early stages of related diseases, accurate detection of Uric acid and Xanthine is of high importance. Electrochemical methods are fast, simple, sensitive, more convenient, and cost-effective compared to other analytical methods used in purine metabolites signaling. Electrochemical sensors are able to detect more compounds simultaneously. Modification of a glassy carbon electrode sensor with external protective membranes was used in this study to avoid unwanted signal interferences from analyte matrices. Polyvinyl alcohol, Chitosan, and Nafion membranes were selected for sensor modification to compare the electro-neutral, positive and negative charged setting of the Xanthine and Uric acid detection. All three membrane modified sensors showed adequate stability in the phosphate buffer solution after 5 min of incubation and are thus suitable for simultaneous detection of purine metabolites. The best results in anodic peak current response values were observed using the Nafion membrane modified glassy carbon electrode sensor. The approach reported here can be useful for the detection of purine metabolites from various matrices at early stages of clinical diagnosis.

The clinical application of purine nucleosides as biomarkers of tissue Ischemia and hypoxia in humans in vivo

During periods of ischemia and hypoxia, intracellular adenosine triphosphate stores are rapidly depleted. Its metabolism results in release of purine nucleosides into the systemic circulation. While the potential of purine nucleosides as a biomarker of ischemia has long been recognized, this has been limited by their complex physiological role and inherent instability leading to problematic sampling and prolonged, complex analysis procedures. Purine release has been demonstrated from cerebral tissue in patients undergoing carotid endarterectomy and patients presenting to hospital with stroke and transient ischemic attack. Rises in purine nucleosides have also been demonstrated in patients with angina and myocardial infarction, during systemic hypoxia, exercise, in patients with peripheral arterial disease and during surgery. This article reviews purine nucleoside production in ischemia, the development of purine analysis technology and details results of the studies investigating purine nucleosides as a biomarker of ischemia with suggestions for areas of future research.

Enzymes involved in metabolism of extracellular nucleotides and nucleosides: functional implications and measurement of activities

Extracellular nucleotides and nucleosides mediate diverse signaling effects in virtually all organs and tissues. Most models of purinergic signaling depend on functional interactions between distinct processes, including (i) the release of endogenous ATP and other nucleotides, (ii) triggering of signaling events via a series of nucleotide-selective ligand-gated P2X and metabotropic P2Y receptors as well as adenosine receptors and (iii) ectoenzymatic interconversion of purinergic agonists. The duration and magnitude of purinergic signaling is governed by a network of ectoenzymes, including the enzymes of the nucleoside triphosphate diphosphohydrolase (NTPDase) family, the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, ecto-5'-nucleotidase/CD73, tissue-nonspecific alkaline phosphatase (TNAP), prostatic acid phosphatase (PAP) and other alkaline and acid phosphatases, adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP). Along with "classical" inactivating ectoenzymes, recent data provide evidence for the co-existence of a counteracting ATP-regenerating pathway comprising the enzymes of the adenylate kinase (AK) and nucleoside diphosphate kinase (NDPK/NME/NM23) families and ATP synthase. This review describes recent advances in this field, with special emphasis on purine-converting ectoenzymes as a complex and integrated network regulating purinergic signaling in such (patho)physiological states as immunomodulation, inflammation, tumorigenesis, arterial calcification and other diseases. The second part of this review provides a comprehensive overview and basic principles of major approaches employed for studying purinergic activities, including spectrophotometric Pi-liberating assays, high-performance liquid chromatographic (HPLC) and thin-layer chromatographic (TLC) analyses of purine substrates and metabolites, capillary electrophoresis, bioluminescent, fluorometric and electrochemical enzyme-coupled assays, histochemical staining, and further emphasizes their advantages, drawbacks and suitability for assaying a particular catalytic reaction.

Enzyme-coupled assays for simultaneous detection of nanomolar ATP, ADP, AMP, adenosine, inosine and pyrophosphate concentrations in extracellular fluids

Purinergic signaling cascade includes the release of endogenous ATP and other agonists by chemical and mechanical stimuli, modulation of diverse cellular functions and subsequent ectoenzymatic inactivation. Basal release of extracellular purines and its physiological relevance remain controversial. Here we employed a combination of enzyme-coupled approaches for simultaneous bioluminescent (ATP, ADP, PP(i)) and fluorometric (AMP), adenosine, inosine, hypoxanthine) measurements of ATP and its metabolites without additional manipulations or derivatization of sampled biological fluids. By using these sensing techniques, extracellular purines were determined in various cells and tissues both at resting and pro-inflammatory conditions. The results obtained revealed the ability of endothelial, lymphoid and tumor cells to maintain extracellular ATP, ADP and adenosine at certain characteristic nanomolar levels. By quantifying the amounts of endogenously released and/or exogenously applied purines and their metabolites, these sensing techniques may be applied for evaluating purine-converting pathways on the cell surfaces and also for ex vivo analysis of purine homeostasis in the intact tissues. Furthermore, we provide novel insight into the mechanisms underlying tumorigenic effects of ATP by demonstrating the ability of metastatic prostate carcinoma PC3 and breast cancer MDA-MB-231 cells to maintain PP(i), which derives from extracellular ATP in the course of nucleotide pyrophosphatase/phosphodiesterase reaction. Collectively, the results imply a complex pattern of nucleotide turnover where extracellular ATP, ADP and adenosine are maintained at steady-state levels via conunterbalanced release and inactivation of ATP and other purines, and further suggest the importance of basal agonist release for continuous activation and/or desensitization of purinergic receptors.

Cultured astrocytes do not release adenosine during hypoxic conditions

Recent reports based on a chemiluminescent enzymatic assay for detection of adenosine conclude that cultured astrocytes release adenosine during mildly hypoxic conditions. If so, astrocytes may suppress neural activity in early stages of hypoxia. The aim of this study was to reevaluate the observation using high-performance liquid chromatography (HPLC). The HPLC analysis showed that exposure to 20 or 120 minutes of mild hypoxia failed to increase release of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine from cultured astrocytes. Similar results were obtained using a chemiluminescent enzymatic assay. Moreover, since the chemiluminescent enzymatic assay relies on hydrogen peroxide generation, release of free-radical scavengers from hypoxic cells can interfere with the assay. Accordingly, adenosine added to samples collected from hypoxic cultures could not be detected using the chemiluminescent enzymatic assay. Furthermore, addition of free-radical scavengers sharply reduced the sensitivity of adenosine detection. Conversely, use of a single-step assay inflated measured values due to the inability of the assay to distinguish adenosine and its metabolite inosine. These results show that cultured astrocytes do not release adenosine during mild hypoxia, an observation consistent with their high resistance to hypoxia.

Purinergic modulation of glucose uptake into cultured rat podocytes: effect of diabetic milieu

Extracellular purines act via P1 and P2 receptors on podocytes and may influence on their function. This action may be modified under various (patho)physiological conditions leading to development of podocytopathy. Aim of study was to investigate effects of diabetic milieu, represented by high glucose concentration (HG, 30 mM glucose) on purinergic-induced changes of 2-deoxy-D-glucose (2-DG) uptake and on extracellular purines metabolism in cultured rat podocytes. Basal 2-DG uptake was 2.7-fold enhanced in HG compared to normal glucose concentration, NG (1271 ± 86 vs. 477 ± 37 nmol/h/mg protein, P<0.001). ATP stimulated 2-DG uptake by 44 ± 4% and 29 ± 5% in NG and HG, respectively. ATP analogues, β, γ-methylene ATP and 2-methylthio ATP stimulated 2-DG uptake in range of 18-34% in NG and 16-17% in HG. Benzoylbenzoyl ATP increased 2-DG uptake about 24 ± 2% in NG however, its effect in HG reached 50 ± 1%. The antagonists of P2 receptors (suramin, reactive blue 2, PPADS) decreased basal 2-DG uptake in NG and HG; suramin and reactive blue 2 at average of 15 ± 4% in NG but in HG the effect was in following order: suramin 28 ± 3%; PPADS 20 ± 3% and RB-2 9 ± 0.9%. Extracellular adenosine concentration was higher in HG than in NG (0.48 ± 0.01 vs. 5.05 ± 0.39 μM, P < 0.05), however intracellular ATP content and extracellular ATP concentration were not affected. Neither ecto-ATPase nor ecto-5'-nucleotidase activities were affected in HG. In conclusion, diabetic milieu affects purinergic modulation of glucose transport into podocytes which may play a role in development of diabetic podocytopathy.

Hydrogen peroxide-induced translocation of glycolipid-anchored (c)AMP-hydrolases to lipid droplets mediates inhibition of lipolysis in rat adipocytes

BACKGROUND

The insulin-independent inhibition of lipolysis by palmitate, the anti-diabetic sulphonylurea glimepiride and H2O2 in rat adipocytes involves stimulation of the glycosylphosphatidylinositol (GPI)-specific phospholipase-C (GPI-PLC) and subsequent translocation of the GPI-anchored membrane ectoproteins (GPI-proteins), Gce1 and cluster of differentiation antigen (CD73), from specialized plasma membrane microdomains (DIGs) to cytosolic lipid droplets (LDs). This results in cAMP degradation at the LD surface and failure to activate hormone-sensitive lipase. Reactive oxygen species (ROS) may trigger this sequence of events in response to palmitate and glimepiride.

EXPERIMENTAL APPROACH

The effects of various inhibitors of ROS production on the release of H2O2, GPI anchor cleavage and translocation of the photoaffinity-labelled or metabolically labelled Gce1 and CD73 from DIGs to LD and inhibition of lipolysis by different fatty acids and sulphonylureas were studied with primary rat adipocytes.

KEY RESULTS

Glimepiride and palmitate induced the production of H2O2 via the plasma membrane NADPH oxidase and mitochondrial complexes I and III, respectively. Inhibition of ROS production was accompanied by the loss of (i) GPI-PLC activation, (ii) Gce1 and CD73 translocation and (iii) lipolysis inhibition in response to palmitate and glimepiride. Non-metabolizable fatty acids and the sulphonylurea drug tolbutamide were inactive. NADPH oxidase and GPI-PLC activities colocalized at DIGs were stimulated by glimepiride but not tolbutamide.

CONCLUSIONS AND IMPLICATIONS

The data suggest that ROS mediate GPI-PLC activation at DIGs and subsequent GPI-protein translocation from DIGs to LD in adipocytes which leads to inhibition of lipolysis by palmitate and glimepiride. This insulin-independent anti-lipolytic mechanism may be engaged by future anti-diabetic drugs.

Adenosine released by astrocytes contributes to hypoxia-induced modulation of synaptic transmission

Astrocytes play a critical role in brain homeostasis controlling the local environment in normal as well as in pathological conditions, such as during hypoxic/ischemic insult. Since astrocytes have recently been identified as a source for a wide variety of gliotransmitters that modulate synaptic activity, we investigated whether the hypoxia-induced excitatory synaptic depression might be mediated by adenosine release from astrocytes. We used electrophysiological and Ca2+ imaging techniques in hippocampal slices and transgenic mice, in which ATP released from astrocytes is specifically impaired, as well as chemiluminescent and fluorescence photometric Ca2+ techniques in purified cultured astrocytes. In hippocampal slices, hypoxia induced a transient depression of excitatory synaptic transmission mediated by activation of presynaptic A1 adenosine receptors. The glia-specific metabolic inhibitor fluorocitrate (FC) was as effective as the A1 adenosine receptor antagonist CPT in preventing the hypoxia-induced excitatory synaptic transmission reduction. Furthermore, FC abolished the extracellular adenosine concentration increase during hypoxia in astrocyte cultures. Several lines of evidence suggest that the increase of extracellular adenosine levels during hypoxia does not result from extracellular ATP or cAMP catabolism, and that astrocytes directly release adenosine in response to hypoxia. Adenosine release is negatively modulated by external or internal Ca2+ concentrations. Moreover, adenosine transport inhibitors did not modify the hypoxia-induced effects, suggesting that adenosine was not released by facilitated transport. We conclude that during hypoxia, astrocytes contribute to regulate the excitatory synaptic transmission through the release of adenosine, which acting on A1 adenosine receptors reduces presynaptic transmitter release. Therefore, adenosine release from astrocytes serves as a protective mechanism by down regulating the synaptic activity level during demanding conditions such as transient hypoxia.

Effects of diadenosine polyphosphates on glomerular volume

1. Diadenosine polyphosphates (P(1),P(3)-diadenosine triphosphate, Ap(3)A; P(1),P(4)-diadenosine tetraphosphate, Ap(4)A; and P(1),P(5)-diadenosine pentaphosphate, Ap(5)A) are vasoactive molecules. The experimental model of isolated rat renal glomeruli was used to investigate their effects on glomerular vasculature. We measured the changes of glomerular inulin space (GIS) as a marker of glomeruli contractility. 2. Ap(4)A and Ap(5)A induced concentration- and time-dependent reduction of GIS whereas Ap(3)A had no effect. The effects of Ap(4)A and Ap(5)A (both at 1 microM) were prevented by a nonselective P2 receptor antagonist, that is, suramin (10 microM) and P2Y receptor antagonist - reactive blue 2 (50 microM). However, the antagonist of P1 receptor, that is, theophylline (1 microM) and A(1) receptor 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 10 microM) did not affect the responses of glomeruli to Ap(4)A or Ap(5)A. 3. Ap(3)A, in contrast to Ap(4)A and Ap(5)A, prevented angiotensin II-induced reduction of GIS in a concentration- and time-dependent manner. This effect was partially prevented by suramin and markedly reduced by reactive blue 2 and the specific antagonist of P2Y(1) receptor - MRS 2179 (10 microM). However, theophylline and the specific antagonist of A(2) receptor - 3,7-dimethyl-1-propargylxanthine (DMPX; 10 microM) - did not affect Ap(3)A action. 4. We indicate that diadenosine polyphosphates changed the glomerular volume via activation of P2 receptors. We suggest that extracellular Ap(4)A and Ap(5)A via P2X and P2Y receptors may decrease and Ap(3)A via, at least in part, P2Y(1) receptors may increase filtration surface, which in turn may modify glomerular filtration rate.

Insulin and glucose induced changes in expression level of nucleoside transporters and adenosine transport in rat T lymphocytes

Adenosine is an endogenous agent exerting potent action on the immune system including regulation of lymphocyte functioning. Impaired T lymphocyte functioning is a common feature of diabetes. The aims of this study were to examine the effects of glucose and insulin on nucleoside transporters (NT) expression level and adenosine (Ado) transport in rat T lymphocytes cultured under the defined concentrations of glucose and insulin. Performed experiments revealed that rat T lymphocytes expressed the equilibrative nucleoside transporter type 1 and 2 (rENT1, rENT2) and concentrative nucleoside transporter type 2 (rCNT2). The mRNA levels of rENT2 and rCNT2 were highly dependent on insulin but were not affected by changes in extracellular glucose concentration. Exposition of T cells to 10nM insulin resulted in 73% increase in rENT2 mRNA and 50% decrease in the rCNT2 mRNA level. The level of rENT1 mRNA was sensitive to extracellular glucose concentration but not to insulin. The highest differences among cells cultured in high (20mM) and low (5mM) glucose were observed in equilibrative nitrobenzylthioinosine sensitive adenosine transport, which was lowered by 65% in cells cultured at high glucose. Alterations in adenosine transport were accompanied by changes in adenosine accumulation in the cell. These results indicate that adenosine transport in rat T lymphocytes is independently and differentially regulated by glucose and insulin by means of changes in the nucleoside transporters expression level. Altered adenosine transport has a great impact on its intracellular level. This suggests that under diabetic conditions adenosine action on T lymphocytes might be altered.

The effect of insulin on expression level of nucleoside transporters in diabetic rats

Evidence that the time course of insulin-induced changes in adenosine level in diabetic rats is different from that observed for expression of adenosine kinase prompted us to study the insulin effect on expression of nucleoside transporters in tissues of diabetic rats. RNase protection assay demonstrated that mRNA levels of equilibrative (rENT) and Na+-dependent nucleoside transporters (rCNT) were altered in diabetic tissues. The rENT1 mRNA level with respect to values obtained in age- and sex-matched nondiabetic rats was decreased by 45, 32, and 10% in diabetic heart, liver, and kidney, respectively. The level of rENT2 mRNA was lowered by 40% in diabetic kidney and heart, and by 24% in diabetic liver. Changes in the expression pattern of rCNT1 and rCNT2 in diabetic tissues differed significantly from that observed for rENT. The levels of rCNT1 and rCNT2 mRNA did not change significantly in diabetic kidney. In diabetic heart, the mRNA levels of rCNT1 and rCNT2 increased 1.7- and 2-fold, respectively. Changes in expression of nucleoside transporters were accompanied by alterations in adenosine content. Administration of insulin to diabetic rats resulted in a drop in adenosine concentration in examined tissues and return of the rCNT1, rCNT2, and rENT2 but not rENT1 mRNA levels to values observed in nondiabetic rats. In summary, these data demonstrate that insulin affects expression of nucleoside transporters in a cell-specific manner. We conclude that change in the expression level of the nucleoside transporters occurring in tissues of diabetic rat is an important factor influencing adenosine levels in the cell.

Insulin restores expression of adenosine kinase in streptozotocin-induced diabetes mellitus rats

The activity of adenosine kinase is significantly impaired in tissues of diabetic rat. Changes in the activity of adenosine kinase were accompanied by alterations in its mRNA and protein level. These changes depended on insulin level and were not related to glucose level. During the first 7 h after insulin treatment the level of adenosine kinase mRNA, protein and enzymatic activity in kidneys, liver and heart returned to normal values. The observed relation between insulin and adenosine kinase expression level may suggest that insulin increases the rate of adenosine kinase gene transcription. Decreased activity of adenosine kinase was associated with elevated level of adenosine in diabetic tissues. On the 10th day after the STZ treatment there was a 3.5 and 2-fold increase in adenosine content in heart and liver, respectively. On the other hand, in diabetic kidney adenosine level was elevated only by 20%. Administration of insulin to diabetic rats resulted in a drop of adenosine to the level seen in normal heart and liver whereas, in kidneys the adenosine content was 50% lower than that observed under normal conditions. The time-dependent coarse of changes in adenosine level was different from that observed for adenosine kinase activity, what may suggest that other factors, possibly nucleoside transporters are also important for controlling adenosine level in the cell.

Acute effects of progesterone on glucose metabolism in rat adipocytes: are they modulated by endogenous adenosine?

Progesterone rapidly inhibits glucose oxidation of isolated rat adipocytes. Because this inhibition is triggered by endogenous adenosine, the present study was designed to examine the effect of the steroid on cyclic adenosine monophosphate (cAMP) accumulation, its relation to lipolysis, and the possible participation of adenosine. The results strongly indicate that physiological concentrations of progesterone increase the release of adenosine by isolated adipocytes, with maximal release at the end of a 20-minute incubation. Progesterone decreased both cAMP levels and lipolysis in quiescent adipocytes or in adipocytes stimulated by isoproterenol. The increase of endogenous adenosine may explain the decline of cAMP and glycerol levels observed with progesterone. The effects of the steroid on lipolysis disappeared when adenosine was hydrolyzed by adenosine deaminase (ADA). On the other hand, in the absence of endogenous adenosine, the effect of progesterone on the cAMP level was decreased only in isoproterenol-stimulated cells. The inhibitory effects of progesterone on cAMP and glycerol production seem not to be related directly to the adenosine A1 receptor, for selective A1 receptor antagonists (8-cyclopentyl-1,3-dipropylxanthine [DPCPX] and CP 68,247) did not counteract these effects. However, mechanisms mediated by guanyl nucleotide binding proteins cannot be excluded. The decrease of cAMP and of lipolysis may be related to a stimulation of phosphodiesterases (PDEs). When PDEs I [Ca(2+)-calmodulin-regulated PDE family) were blocked by a selective inhibitor (CP 41,757), the progesterone inhibitory effect persisted, suggesting that PDEs I are not regulated by the steroid. On the other hand, the progesterone effect on cAMP accumulation but not on lipolysis disappeared in the presence of a selective inhibitor of the PDE IV family (cAMP-dependent-specific family). Ro 20.1724. When the specific inhibitor of PDE I or PDE IV was combined with ADA, the progesterone effect on cAMP disappeared. Taken together, these results suggest that the progesterone inhibitory action on cAMP levels was not mediated through A1 receptors or through activation of PDE I, but may be related to PDE IV activities. The progesterone effect on lipolysis seemed not to be directly related to changes in cAMP levels; an effect on PDE III activities in relation with the increase of adenosine release cannot be excluded.

The stimulus-sensitive H2O2-generating system present in human fat-cell plasma membranes is multireceptor-linked and under antagonistic control by hormones and cytokines

Previous work demonstrated that human fat-cells possess a plasma-membrane-bound H2O2-generating system that is activated by insulin. Here we show that this system is under antagonistic control by various hormones and cytokines that typically act through several distinct receptor families. Similarly to insulin, oxytocin and tumour necrosis factor alpha acted as stimulators of NADPH-dependent H2O2 generation, whereas isoprenaline, a beta-adrenergic agonist, had inhibitory effects. Surprisingly, the acidic and basic isoforms of fibroblast growth factor as well as homodimeric platelet-derived growth factor AA and BB had antagonistic stimulatory and inhibitory effects on NADPH-dependent H2O2 generation. The agents tested acted at discrete ligand-specific receptors and their mechanisms of action were membrane-delimited and occurred in the absence of ATP. These findings implied that established pathways of signal transduction, including receptor kinases or second-messenger-dependent protein kinases A and C, were not involved and placed the stimulus-sensitive H2O2-generating system in a position comparable with adenylate cyclase. It was concluded that the stimulus-sensitive H2O2-generating system of human fat-cells meets all criteria of a universal signal-transducing system for hormones and cytokines that may link ligand binding to cell-surface receptors to changes in the intracellular redox equilibrium.

Human fat cells possess a plasma membrane-bound H2O2-generating system that is activated by insulin via a mechanism bypassing the receptor kinase

Insulin caused a transient increase in H2O2 accumulation in human fat cell suspensions that was observed only in the presence of an inhibitor of catalase and heme-containing peroxidases, such as azide, and reached peak levels of 30 microM within 5 min. The cells contained a plasma membrane-bound NADPH oxidase, producing 1 mol H2O2/mol of NADPH oxidation, that was activated on exposure of intact cells to insulin at contrations that are physiologically relevant (0.1-10 nM). The hormone effect was rapid and was due to a selective increase in substrate affinity. The enzyme was magnesium dependent, required a flavine nucleotide for optimal activity, and was most active at pH 5.0-6.5. In contrast to all other hormone- or cytokine-sensitive NADPH oxidases that have been characterized in sufficient detail, the human fat cell oxidase retained its hormone responsiveness after cell disruption, and only Mn2+, but no ATP, was required for a ligand-induced activation in crude plasma membranes. The results demonstrate that insulin utilizes tyrosine kinase-independent pathways for receptor signaling and strongly support the view that H2O2 contributes to the intracellular propagation of the insulin signal.

Application of solid-phase extraction on anion-exchange cartridges to quantify 5'-nucleotidase activity

The enzyme 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) catalyzes a critical reaction in intermediary metabolism, the phosphohydrolysis of nucleoside 5'-monophosphates to their corresponding nucleosides. We have evaluated solid-phase extraction on pre-packed anion-exchange cartridges as a chromatographic technique with which 5'-nucleotidase activity may be detected and quantified. Chromatographic conditions were established whereby substrate nucleotide was rapidly and completely separated from its corresponding nucleoside by solid-phase extraction. Both analytes were recovered quantitatively, without loss or degradation. This chromatographic system was integrated into a discontinuous radiochemical assay for 5'-nucleotidase which enabled both substrate utilization and product formation to be assessed simultaneously. Enzyme reaction samples could be analyzed directly for 5'-nucleotidase activity without any pre-chromatography preparation. The high capacity of the solid-phase cartridges and the inability of 5'-nucleotidase to enter the packing bed during analyte elution facilitated termination of the enzyme reaction by applying the entire reaction mixture to the cartridge. Loaded cartridges could then be stored at 4 degrees C prior to chromatography and subsequently batch-eluted. The excellent resolution between substrate and product in solid-phase extraction and the sensitivity of radioisotopic counting enabled detection/quantification of low tissue levels of 5'-nucleotidase in conjunction with ancillary assays for secondary enzyme reactions with the potential to elicit the artifactual loss of 5'-nucleotidase substrate/product when crude biological preparations are examined for 5'-nucleotidase activity. Our results demonstrate that solid-phase extraction on anion-exchange cartridges with elution solvents of appropriate pH offers several unique advantages for 5'-nucleotidase determination.

Simultaneous determination of inosine, hypoxanthine, xanthine, and uric acid and the effect of metal chelators

We describe a sensitive, reproducible method for the simultaneous determination of the ATP catabolites inosine, hypoxanthine, xanthine, and uric acid in biological samples and organ perfusate using reverse-phase chromatography and multiwavelength detection at 254, 270, and 292 nm. Sample preparation includes precipitating proteins with perchloric acid, neutralizing the sample, passing the supernatant over a polyethyleneimine column, and analyzing the collected fractions by high-performance liquid chromatography. Addition of metal chelators to the perchloric acid resulted in increased values for xanthine, hypoxanthine, and uric acid. The method was sensitive (limit of detection, 0.08 nmol on column; S/N = 4) and linear over the range 0.5-30 microM. Precision and accuracy of the method were evaluated for lung tissue and lung perfusate. Coefficients of variation ranged from 2.8 to 6.1% for perfusate and from 1.7 to 12.6% for tissue. Recoveries for all compounds exceeded 90%. We applied this method to rat lung tissue, lung perfusate, and rat and human blood. Advantages of this method are simultaneous quantitation with excellent sensitivity of all compounds, simplified peak identification by using multiwavelength detection, and improved accuracy by preventing loss of compounds with metal chelators.

Beta-adrenergic stimulation of adenine nucleotide catabolism and purine release in human adipocytes

The effects of beta-adrenergic agonists on ATP utilization and adenine nucleotide breakdown in human adipocytes were examined. The catecholamine-induced increase in cAMP was associated with an enhancement of adenine nucleotide catabolism resulting in an increase in release of inosine and hypoxanthine which can not be reutilized for adenine nucleotide synthesis. Therefore, one-third of total cellular adenine nucleotides were irreversibly lost in the presence of 1 mumol/liter isoproterenol. The catecholamine-induced increase in purine release could be blocked by phosphodiesterase inhibitors, suggesting that cAMP is the main precursor of purines in the presence of beta-adrenergic agonists. However, epinephrine (in the simultaneous presence of the alpha 2-adrenergic blocking agent, yohimbine) and isoproterenol were 10 times more potent in stimulating purine release than in elevating cAMP. In addition, purine release ceased when cAMP was still markedly increased, suggesting a compartmentation of the cyclic nucleotide and/or involvement of the hormone-sensitive, low Km cAMP phosphodiesterase. The results document that white fat cells have an enormous potential for dissipating energy, and demonstrate that the pathway involving cAMP formation and hydrolysis constitutes the principle route of adenine nucleotide catabolism in the presence of beta-adrenergic agonists.

The release of adenosine at the electric organ of Torpedo. A study using a continuous chemiluminescent method

Acetylcholine and ATP are costored and coreleased during synaptic activity at the electric organ of Torpedo. It has been suggested that released ATP is converted to adenosine at the synaptic cleft, and in turn this nucleoside would depress the evoked release of acetylcholine. In the present communication we have used a chemiluminescent reaction that let us to monitor continuously the presence of adenosine in this preparation. The chemiluminescent reaction is based on the conversion of adenosine into uric acid and H2O2 by adenosine deaminase, nucleoside phosphorylase, and xanthine oxidase enzymes. The hydrogen peroxide has been detected by peroxidase-luminol mixture. The reaction has a sensitivity on the picomol range and discerned between Adenosine, AMP, ADP, and ATP. We have developed this technique in the hope of understanding whether adenosine is released during synaptic activity or it comes from the released ATP. We have studied the release or formation of adenosine in fragments of the electric organ and in isolated cholinergic nerve terminals obtained from it. In both conditions we have followed the effect of potassium stimulation upon the detection of adenosine. Potassium stimulation increased the extracellular adenosine either in slices or the synaptosomal fraction of Torpedo electric organ. The presence of alpha, beta-methylene ADP, an inhibitor of 5'-nucleotidase, inhibits the detection of adenosine, suggesting that extracellular adenosine is a consequence of ectocellular dephosphorylation of released ATP.