CD39/NTPDase-1 activity and expression in normal leukocytes

INTRODUCTION

CD39/NTPDase-1 is a cell surface enzyme expressed on leukocytes and endothelial cells that metabolizes ATP to ADP and AMP. CD39 is expressed on numerous different types of normal leukocytes, but details of its expression have not been determined previously.

METHODS

We examined CD39 expression and activity in leukocytes isolated from healthy volunteers. Expression of CD39 on leukocytes was measured by FACS and activity of CD39 in lymphocytes and neutrophils was determined by an enzymatic radio-TLC assay.

RESULTS

We established that CD39 is expressed on neutrophils, lymphocytes, and monocytes. The enzyme is found on >90% of monocytes, neutrophils, and B-lymphocytes, and 6% of T-lymphocytes and natural killer cells. Per cell density of expression varied, with the highest expression on monocytes and B-lymphocytes. ATPase and ADPase activities were highest on B-lymphocytes, lower on neutrophils, lowest on T-lymphocytes. The ratio of ADPase:ATPase activity was 1.8 for neutrophils and B-lymphocytes and 1.4 for T-lymphocytes. Hypertensive volunteers had lower levels of CD39 on their T-lymphocytes and NK cells. No correlation between age, gender, ethnic background, or cholesterol level and CD39 expression was observed.

CONCLUSIONS

We conclude that CD39 activity and expression are present to varying degrees on all leukocytes types examined. Differences between leukocyte types should be considered when examining CD39 in disease states.

Characterization of an ATP diphosphohydrolase activity (APYRASE, EC 3.6.1.5) in rat blood platelets

In the present report we describe an apyrase (ATP diphosphohydrolase, EC 3.6.1.5) in rat blood platelets. The enzyme hydrolyses almost identically quite different nucleoside di- and triphosphates. The calcium dependence and pH requirement were the same for the hydrolysis of ATP and ADP and the apparent Km values were similar for both Ca(2+)-ATP and Ca(2+)-ADP as substrates. Ca(2+)-ATP and Ca(2+)-ADP hydrolysis could not be attributed to the combined action of different enzymes because adenylate kinase, inorganic pyrophosphatase and nonspecific phosphatases were not detected under our assay conditions. The Ca(2+)-ATPase and Ca(2+)-ADPase activity was insensitive to ATPase, adenylate kinase and alkaline phosphatase classical inhibitors, thus excluding these enzymes as contaminants. The results demonstrate that rat blood platelets contain an ATP diphosphohydrolase involved in the hydrolysis of ATP and ADP which are vasoactive and platelet active adenine nucleotides.

Heightened Expression of CD39 by Regulatory T Lymphocytes Is Associated with Therapeutic Remission in Inflammatory Bowel Disease

BACKGROUND

To evaluate whether changes in expression of CD39 by regulatory T lymphocytes (Treg) impact treatment response in inflammatory bowel disease. To then define the biological role of expression of CD39 on Treg in an animal model of colitis.

METHODS

A prospective study of consecutive patients commencing anti-tumor necrosis factor therapy with infliximab (IFX) or adalimumab (ADA), who were then followed for 12 months. Treatment responses were defined both symptomatically and by endoscopy showing mucosal healing. Peripheral blood Tregs were quantified by flow cytometry. Functional importance of CD39 expression by Treg was determined in an adoptive T-cell transfer model of colitis.

RESULTS

Forty-seven patients (ulcerative colitis, n = 22; Crohn's disease, n = 25) were recruited; 16 patients were complete responders and 13 nonresponders to anti-tumor necrosis factor. CD39 expression by Treg was lower in active inflammatory bowel disease and increased significantly after treatment in responders (CD39Treg/total Treg; 8% at baseline to 22.5% at late time point, P < 0.001). Responders were more likely to have therapeutic drug levels and in multivariate analysis therapeutic drug levels were associated with higher expression of CD39 by FoxP3 Treg and lower frequencies of interleukin 17A expressing cells. Tregs with genetic deletion of CD39 exhibit decrements in potential to suppress intestinal inflammation in a murine (CD45RB) T-cell transfer model of colitis in vivo, when compared with wild-type Treg.

CONCLUSIONS

Increased expression of CD39 by peripheral blood Treg is observed in the setting of clinical and endoscopic remission in inflammatory bowel disease. Deficiency of CD39 expression by Treg can be linked to inability to suppress experimental colitis.

Demonstration of a novel ecto-enzyme on human erythrocytes, capable of degrading ADP and of inhibiting ADP-induced platelet aggregation

The role of ADP as an important inducer of platelet aggregation is generally accepted. Therefore it has been postulated by many authors that the enzymatic removal of extracellular ADP from the circulation is essential to avoid platelet aggregation and thrombus formation. Here we show that erythrocytes essentially contribute to the clearance of ADP. The removal of ADP from suspensions of washed human erythrocytes was due to at least two different activities. One activity, which had already been observed by earlier workers, was identified as adenylate kinase, on the basis of the reaction products and the inhibition by adenosine(5')pentaphospho(5')adenosine (Ap5A). This enzyme was not associated with the cells and was always detectable in cell-free supernatants, indicating that the enzyme had leaked from the cells into the extracellular medium. In contrast, the second activity, which is described here for the first time, was tightly bound to the cells. The activity was not inhibited by Ap5A. The main product of the reaction was AMP, and enzyme activity depended on the presence of divalent cations. The Michaelis constant was about 28 mumol/l. This activity seemed to be an ecto-ADPase. Studies with various inhibitors revealed that degradation of ADP was not due to a non-specific phosphatase. Besides the well known ADPase on the endothelium, the ecto-activity on erythrocytes may play an important part in destroying pro-aggregatory ADP.

Enzymes that hydrolyze adenine nucleotides of patients with hypercholesterolemia and inflammatory processes

The activity of NTPDase (EC 3.6.1.5, apyrase, CD39) was verified in platelets from patients with increasing cholesterol levels. A possible association between cholesterol levels and inflammatory markers, such as oxidized low-density lipoprotein, highly sensitive C-reactive protein and oxidized low-density lipoprotein autoantibodies, was also investigated. Lipid peroxidation was estimated by measurement of thiobarbituric acid reactive substances in serum. The following groups were studied: group I, < 150 mg.dL(-1) cholesterol; group II, 151-200 mg.dL(-1) cholesterol; group III, 201-250 mg.dL(-1) cholesterol; and group IV, > 251 mg.dL(-1) cholesterol. The results demonstrated that both ATP hydrolysis and ADP hydrolysis were enhanced as a function of cholesterol level. Low-density lipoprotein levels increased concomitantly with total cholesterol levels. Triglyceride levels were increased in the groups with total cholesterol above 251 mg.dL(-1). Oxidized low-density lipoprotein levels were elevated in groups II, III, and IV. Highly sensitive C-reactive protein was elevated in the group with cholesterol levels higher than 251 mg.dL(-1). Oxidized low-density lipoprotein autoantibodies were elevated in groups III and IV. Thiobarbituric acid reactive substance content was enhanced as a function of cholesterol level. In summary, hypercholesterolemia is associated with enhancement of inflammatory response, oxidative stress, and ATP and ADP hydrolysis. The increased ATP and ADP hydrolysis in group IV was confirmed by an increase in CD39 expression on its surface. The increase in CD39 activity is possibly related to a compensatory response to the inflammatory and pro-oxidative state associated with hypercholesterolemia.

Role of CD39 in COVID-19 Severity: Dysregulation of Purinergic Signaling and Thromboinflammation

CD39/NTPDase1 has emerged as an important molecule that contributes to maintain inflammatory and coagulatory homeostasis. Various studies have hypothesized the possible role of CD39 in COVID-19 pathophysiology since no confirmatory data shed light in this regard. Therefore, we aimed to quantify CD39 expression on COVID-19 patients exploring its association with severity clinical parameters and ICU admission, while unraveling the role of purinergic signaling on thromboinflammation in COVID-19 patients. We selected a prospective cohort of patients hospitalized due to severe COVID-19 pneumonia (n=75), a historical cohort of Influenza A pneumonia patients (n=18) and sex/age-matched healthy controls (n=30). CD39 was overexpressed in COVID-19 patients’ plasma and immune cell subsets and related to hypoxemia. Plasma soluble form of CD39 (sCD39) was related to length of hospital stay and independently associated with intensive care unit admission (adjusted odds ratio 1.04, 95%CI 1.0-1.08, p=0.038), with a net reclassification index of 0.229 (0.118-0.287; p=0.036). COVID-19 patients showed extracellular accumulation of adenosine nucleotides (ATP and ADP), resulting in systemic inflammation and pro-coagulant state, as a consequence of purinergic pathway dysregulation. Interestingly, we found that COVID-19 plasma caused platelet activation, which was successfully blocked by the P2Y₁₂ receptor inhibitor, ticagrelor. Therefore, sCD39 is suggested as a promising biomarker for COVID-19 severity. As a conclusion, our study indicates that CD39 overexpression in COVID-19 patients could be indicating purinergic signaling dysregulation, which might be at the basis of COVID-19 thromboinflammation disorder.

Salivary gland extracts from the deerfly contain a potent inhibitor of platelet aggregation

Salivary gland extracts of the deerfly contain a potent inhibitor of platelet aggregation, which assists the insect in obtaining a blood meal. The extract prevents platelet aggregation induced by ADP, thrombin, and collagen and inhibits fibrinogen binding to the glycoprotein IIb/IIIa receptor on platelets. The active component in deerfly salivary gland extract appears to be a protein that is comparatively more potent than the disintegrins present in viper venoms. Isolation and characterization of this protein may provide different directions in therapeutics and studies of normal platelet physiology.

Effect of high glucose levels in human platelet NTPDase and 5'-nucleotidase activities

OBJECTIVES

The objective of this work was to evaluate the effect of different glucose levels on the ATP, ADP and AMP hydrolysis in the platelets of diabetic, hypertensive and diabetic/hypertensive participants.

METHODS

The activities of the enzymes NTPDase (ATP and ADP hydrolysis) and 5'-nucleotidase (AMP hydrolysis), and CD39 expression were analyzed in human blood platelets of diabetic (DM-2), hypertensive (HT) and diabetic/hypertensive (DM-2/HT) patients. To evaluate the interference of glucose and fructose in NTPDase and 5'-nucleotidase activities, experiments were performed with glucose, fructose and mannitol concentrations ranging from 5 to 30 mM in platelet-rich plasma (PRP). Pre-incubation times of 10, 120 min and 24h were used.

RESULTS

NTPDase and 5'-nucleotidase activities increased with increasing glucose and fructose concentrations (P<0.001) and the different times of pre-incubation did not interfere in ectonucleotidases activities (P>0.5). NTPDase and 5'-nucleotidase activities demonstrated a positive correlation between serum glucose levels and ATP and ADP hydrolysis in DM-2 and DM-2/HT patients. CD39 expression demonstrated that DM-2, HT and DM-2/HT groups presented a significant increase when compared to the control group (P<0.004).

CONCLUSION

The hydrolysis of adenine nucleotides is enhanced in platelets of patients with diabetes and hypertension. We observed that an increasing glucose concentration had a direct effect on ATP, ADP and AMP hydrolysis. Furthermore, CD39 expression was enhanced in all patients groups, indicating that these enzyme activities are related with diabetes and hypertension.

Lipopolysaccharide alters nucleotidase activities from lymphocytes and serum of rats

ATP exerts a proinflammatory role and induces cytokine release by acting at P2X(7) receptors. The product of ATP hydrolysis is the nucleoside adenosine, an important immunomodulator. The main source of extracellular adenosine is the hydrolysis of extracellular ATP by a group of ecto-enzymes: ENTPDase family, NPP family and ecto-5'-nucleotidase. Considering the role of ATP and adenosine in inflammatory processes, we investigated the effect of lipopolysaccharide on ectonucleotidases activities and expression in lymphocytes from mesenteric lymph nodes and serum of rats, in order to better understand the involvement of extracellular nucleotide hydrolysis in an endotoxemia model. We observed significant changes on nucleotidase activities from lymphocytes and serum of rats after in vitro and in vivo exposure to LPS. In vitro results have shown an increase on nucleotide hydrolysis in lymphocytes and a decrease on the enzyme activity of NPP in blood serum. In vivo, we observed an increase on nucleotide hydrolysis in lymphocytes and a decrease in the hydrolysis of all nucleotides tested in blood serum. After 24 and 48 h of LPS treatment, there was a reduction in NTPDase1, 2, 3 and ecto-5'-nucleotidase transcripts. These results suggest that there is a time-dependent enhancement of extracellular nucleotides metabolism in lymphocytes and blood serum after the induction of an endotoxemic model. The changes observed suggest that these enzymes can act in the regulation of extracellular nucleosides and nucleotides in a model able to trigger inflammatory process.

Expression of Ecto-nucleoside Triphosphate Diphosphohydrolases-2 and -3 in the Enteric Nervous System Affects Inflammation in Experimental Colitis and Crohn's Disease

OBJECTIVE

Recent studies have suggested that the enteric nervous system can modulate gut immunity. Ecto-nucleoside triphosphate diphosphohydrolases [E-NTPDases] regulate purinergic signalling by sequential phosphohydrolysis of pro-inflammatory extracellular adenosine 5'-triphosphate [ATP]. Herein, we test the hypothesis that E-NTPDases modulate gut inflammation via neuro-immune crosstalk.

DESIGN

We determined expression patterns of NTPDase2 and NTPDase3 in murine and human colon. Experimental colitis was induced by dextran sodium sulphate [DSS] in genetically engineered mice deficient in NTPDase2 or NTPDase3. We compared plasma adenosine diphosphatase [ADPase] activity from Crohn's patients and healthy controls, and linked the enzyme activity to Crohn's disease activity.

RESULTS

NTPDase2 and -3 were chiefly expressed in cells of the enteric nervous system in both murine and human colon. When compared with wild type, DSS-induced colitis was exacerbated in Entpd2, and to a lesser extent, Entpd3 null mice as measured by disease activity score and histology, and marked anaemia was seen in both. Colonic macrophages isolated from Entpd2 null mice displayed a pro-inflammatory phenotype compared with wild type. In human plasma, Crohn's patients had decreases in ADPase activity when compared with healthy controls. The drop in ADPase activity was likely associated with changes in NTPDase2 and -3, as suggested by inhibitor studies, and were correlated with Crohn's disease activity.

CONCLUSIONS

NTPDase2 and -3 are ecto-enzymes expressed in the enteric nervous system. Both enzymes confer protection against gut inflammation in experimental colitis and exhibit alterations in Crohn's disease. These observations suggest that purinergic signalling modulated by E-NTPDases governs neuro-immune interactions that are relevant in Crohn's disease.

A direct, rapid, radioassay for adenosine diphosphatase

Radiolabelled adenosine diphosphate [beta 32Pi]ADP was synthesized from radioactive orthophosphate and adenosine-5'-monophosphate with dicyclohexylcarbodiimide. The purified radioactive adenosine diphosphate was used as the substrate in a rapid, direct, one-step assay for adenosine diphosphatase in human neutrophilic leukocyte homogenates. Certain kinetic properties of the enzyme activity have been determined.

Assay, kinetics and properties of plasma adenosine diphosphatase. The relationship to acid and alkaline phosphatase and variations in disease

A rapid radioassay was used to characterise the adenosine diphosphatase (ADPase) activities in human plasma. There was a major peak at pH 9.3, 80% of whose activity was attributable to non-specific alkaline phosphatase, with the remaining 20% probably due to a specific ADPase. There was also a small peak of ADPase activity at pH 4.0. Inhibitor and chromatographic studies showed that whilst much of this activity was attributable to non-specific acid phosphatase, there was a discrete acid ADPase. Assays of plasma ADPase activities in vascular disorders, including myocardial infarction, peripheral vascular disease and diabetes mellitus, reveal no alterations from control values. Activities of alkaline ADPase were elevated in both chronic and acute liver failure. Acid ADPase was also increased in chronic liver disease and it is suggested that alterations in ADPase activities in liver disorders may contribute to the haemostatic problems observed in these patients.

The effect of stress upon hydrolysis adenine nucleotides in blood serum of rats

Alterations of enzyme activities involved in adenine nucleotide hydrolysis have been reported in spinal cord and blood serum after repeated restraint stress. On the other hand, no effect was observed in the spinal cord of rats after acute stress. In the present study, we investigated the effect of acute stress on the hydrolysis of adenine nucleotides in rat blood serum. Adult male Wistar rats were submitted to 1-h restraint stress and were sacrificed at 0, 6, 24 and 48 h. Increased ATP and ADP hydrolysis were observed in the blood serum of stressed rats 24 h after stress (58% and 54%, respectively, when compared to controls). On the other hand, the AMP hydrolysis was increased after 6 h (68% when compared to controls) and at 24 h (94% when compared to controls) after stress. The results suggest that altered activity of soluble enzymes in serum may be a biochemical marker for stress situations.

Characterization of Regulatory T Cells in Preterm and Term Infants

Our study aimed to study regulatory T cells (Tregs) and their expression of CD45RA, HLA-DR, and CD39 in preterm and full-term infants. In an observational study, we used a three-color flow cytometry for determination of Tregs and their expression of CD45RA, HLA-DR, and CD39 in preterm and full-term infants. The percentages of CD4+CD25+highFoxp3+, CD39+ Tregs, HLA-DR+ Tregs and the expression of Foxp3+ in CD4+CD25+highFoxp3 Tregs cells were significantly lower in neonates when compared to healthy adult controls. The levels of naïve resting Tregs (CD45RA+Tregs) were significantly higher in neonates than controls. The percentages of CD4+CD25+highFoxp3+Tregs, total CD4+CD25+ and CD4+CD25+high were significantly higher in preterm infants when compared to the full-term group. Moreover, CD45RA+Tregs were significantly higher in preterm than in term infants. We found significant inverse correlations between the gestational age and the levels of both Tregs (r = - 0.395, p = 0.017) and CD45RA+Tregs (r = - 0.422, p = 0.010). Relative to full-term, the frequencies, and phenotypes of Tregs were affected by prematurity. A larger longitudinal study with a sufficient number of newborns is needed to investigate the Treg pool of term and preterm infants thoroughly and to explore the association between the Treg pool and clinical variables.

ADPase activity in human maternal and cord blood: possible evidence for a placenta-specific vascular protective mechanism

ADPase enzyme activity was assessed in maternal and cord plasma by adding radiolabelled ADP and quantitating the degradation products. Cord plasma had sufficiently greater ADPase activity than the corresponding maternal plasma obtained ante- and post-partum. Thus, residual radiolabelled ADP was 30, 32 and 17% of total radioactivity after 30 min incubation (37 degrees C) in maternal ante-partum, maternal post-partum and cord plasmas, respectively. ADPase may act as a platelet aggregation inhibitor in the placental and fetal circulation.

Acetylsalicylic acid inhibits ATP diphosphohydrolase activity by platelets from adult rats

BACKGROUND AND METHODS

The in vitro effect of the nonsteroidal anti-inflammatory drug, acetylsalicylic acid (ASA), on the extracellular adenine nucleotide hydrolysis by intact rat blood platelets was studied.

RESULTS

Our results demonstrate that aspirin, at final concentrations of 2.0 and 3.0 mM, inhibits ATP extracellular hydrolysis in vitro by approximately 17% and 21%, respectively. Aspirin, at a final concentration of 3.0 mM, also inhibited in vitro extracellular ADP hydrolysis by approximately 41%. The same concentrations of this drug, however, did not alter AMP hydrolysis by intact rat blood platelets under similar assay conditions. The kinetic analysis demonstrated that the inhibition of ADP and ATP hydrolysis by aspirin in rat platelets is of the uncompetitive type.

CONCLUSION

In this study, we demonstrated an inhibitory effect of ASA upon E-NTPDase 3 activity of platelets from adult rats and discussed the significance of our findings.

Free radical-induced inhibition of ATP diphosphohydrolase activity (EC 3.6.1.5) from rat blood platelets

In the present report we demonstrate the in vitro effects of free radicals on an ecto-ATP diphosphohydrolase activity (apyrase, EC 3.6.1.5) from rat blood platelets. Rat blood platelets were exposed to an oxidant-generating system (H2O2/Fe2+/ascorbate) and the ATP diphosphohydrolase activity was inhibited. The enzyme inhibition was prevented by glutathione (GSH) and cysteine but not by trolox as a vitamin E analogue. The TBARS (thiobarbituric acid reactive substances) assay and the determination of sulphydryl groups indicate that the inhibition of the enzyme activity in resting platelets is not related to lipid peroxidation or to oxidation of sulphydryl residues. These results demonstrate the susceptibility of ATP diphosphohydrolase activity from platelets to free radicals and suggest that amino acid residues which are essential for the enzyme function are probably modified.

Regulation of the concentration of free arachidonic acid in homogenates of human platelets

With the intention to study the regulation of the availability of free arachidonic acid through the enzymes of the Lands cycle, we established a model system in homogenates of human platelets. Phospholipase A2, arachidonoyl-CoA synthetase and lysophosphatidyl acyltransferase proved to be simultaneously active and a steady turnover of arachidonic acid was the consequence. EGTA suppressed the deacylating activity that acted on endogenous membrane phospholipids and prevented eicosanoid formation from previously esterified exogenous arachidonoyl-CoA. The reacylating enzymes took part in the control of eicosanoid biosynthesis by re-esterification of liberated arachidonic acid. Blockade of the reacylation by apyrase made arachidonic acid completely available for further metabolization into 12-HETE and thereby induced an increase in the eicosanoid release.

Studies on the kinetic properties and subcellular localization of adenosine diphosphatase activity in human peripheral blood lymphocytes

Using a recently developed radioassay, the conditions for measuring adenosine diphosphatase (ADPase) activity in human lymphocytes were optimized. Kinetic studies showed that the activity was optimal at pH 8.0 and required 0.2 mM MgCl2. The Km of the enzyme for ADP was 0.03 mM. Analytical subcellular fractionation showed that the ADPase activity was distributed between the plasma membrane and mitochondria. Studies with the non-penetrating inhibitor, diazotized sulphanilic acid, indicate that plasma membrane ADPase activity is located on the external aspect of the cell. Polymorphonuclear leucocytes were found to have about three times the ADPase activity. Lymphocytes, therefore, have the ability to generate AMP on their surface which can be further metabolized to adenosine by ecto-5' nucleotidase. A role for this system is discussed.

Properties of vascular ADP-ase

The properties of vascular ADP-ase were investigated in microsomal preparations of rabbit aortic cell homogenates. The enzyme was found to have a Km of 9 microM. Its activity was not affected by substrates for other purine nucleoside phosphatases nor by substrates for non-specific phosphatases. However, ATP and beta-gamma-methylene ATP inhibited the enzyme. None of sodium fluoride, ouabain, neuraminidase affected ADP-ase activity whereas oligomycin potentiated the activity. The enzyme had an SH requirement and a divalent cation requirement.

Assessment of adenosinergic activity of small extracellular vesicles in plasma of cancer patients and healthy donors

The adenosinergic pathway converting endogenous ATP to adenosine (ADO) is a major immunosuppressive pathway in cancer. Emerging data indicate that plasma small extracellular vesicles (sEV) express CD39 and CD73 and produce ADO. Using a noninvasive, highly sensitive newly developed assay, metabolism of N6-etheno-labeled eATP, eADP or eAMP by ecto-nucleotidases on the external surface of sEV was measured using high pressure liquid chromatography with fluorescence detection. Ecto-nucleotidase activity in sEV isolated from plasma of randomly selected cancer patients and healthy donors (HDs) was compared. Relative to sEV of HDs, sEV from the plasma of melanoma patients metabolized eATP to eADP and eAMP to eADO with significantly greater efficiency. Activities of both CD39 and CD73 were elevated, as determined by the use of pharmacologic inhibitors selective for these enzymes. In contrast, metabolic activity of CD39 and CD73 on sEV isolated from plasma of patients with head and neck cancer was comparable to that of HDs, suggesting that the activity of ecto-nucleotidases on sEV may differ depending on the cancer type or cancer stage. The N6-etheno-purine assay measuring contributions of ecto-nucleotidases residing on the surface of sEV to the extracellular ATP to ADO pathway can discriminate cancer patients from HDs, differentiate among different cancer types, and potentially identify patients most likely to benefit from anti-adenosinergic therapy designed to inhibit the adenosine-mediated immune suppression.

Inhibition and kinetic alterations by excess free ATP and ADP of the ATP diphosphohydrolase activity (EC 3.6.1.5) from rat blood platelets

ATP diphosphohydrolase (EC 3.6.1.5) catalyzes the hydrolysis of diphospho- and triphosphonucleosides and is activated by divalent cations. The enzyme described in rat blood platelets hydrolyzes Ca(2+)-ATP and Ca(2+)-ADP with a high affinity for these Ca(2+)-nucleotide complexes as substrates. In the present paper, we demonstrate that free ATP or free ADP induces inhibition and kinetic alterations of the enzyme from rat blood platelets. From these results, we draw conclusions about the binding of free nucleotides to the enzyme and their action as inhibitors with respect to calcium-nucleotide complex.