Simultaneous assay of adenosine deaminase and purine nucleoside phosphorylase activity as possible biochemical means to detect non-Hodgkin lymphomas of the oral cavity

Release of soluble and vesicular purine nucleoside phosphorylase from rat astrocytes and microglia induced by pro-inflammatory stimulation with extracellular ATP via P2X7 receptors

Purine nucleoside phosphorylase (PNP), a crucial enzyme in purine metabolism which converts ribonucleosides into purine bases, has mainly been found inside glial cells. Since we recently demonstrated that PNP is released from rat C6 glioma cells, we then wondered whether this occurs in normal brain cells. Using rat primary cultures of microglia, astrocytes and cerebellar granule neurons, we found that in basal condition all these cells constitutively released a metabolically active PNP with Km values very similar to those measured in C6 glioma cells. However, the enzyme expression/release was greater in microglia or astrocytes that in neurons. Moreover, we exposed primary brain cell cultures to pro-inflammatory agents such as lipopolysaccharide (LPS) or ATP alone or in combination. LPS alone caused an increased interleukin-1β (IL-1β) secretion mainly from microglia and no modification in the PNP release, even from neurons in which it enhanced cell death. In contrast, ATP administered alone to glial cells at high micromolar concentrations significantly stimulated the release of PNP within 1 h, an effect not modified by LPS presence, whereas IL-1β secretion was stimulated by ATP only in cells primed for 2 h with LPS. In both cases ATP effect was mediated by P2X₇ receptor (P2X₇R), since it was mimicked by cell exposure to Bz-ATP, an agonist of P2X₇R, and blocked by cell pre-treatment with the P2X₇R antagonist A438079. Interestingly, ATP-induced PNP release from glial cells partly occurred through the secretion of lysosomal vesicles in the extracellular medium. Thus, during inflammatory cerebral events PNP secretion promoted by extracellular ATP accumulation might concur to control extracellular purine signals. Further studies could elucidate whether, in these conditions, a consensual activity of enzymes downstream of PNP in the purine metabolic cascade avoids accumulation of extracellular purine bases that might concur to brain injury by unusual formation of reactive oxygen species.

Plasma purine nucleoside phosphorylase in cancer patients

BACKGROUND

Purine nucleoside phosphorylase (PNP) is the purine salvage enzyme that converts guanosine to guanine and inosine to hypoxanthine.

METHODS

279 samples from patients with differing cancers were collected during treatment at both pre- and post-dose stages for plasma PNP activity and compared with a normal population.

RESULTS

Normal plasma PNP activity was found to be 3.2+/-1.4 U/l (n=55) as compared with the cancer patients (pre-dose 12.3+/-7.4 U/l [n=215] and post-dose 11.2+/-5.9 U/l [n=64]). Levels of plasma PNP did not differ greatly between the different cancer types but were on average four times greater than that found in the reference population.

Adenosine deaminase: functional implications and different classes of inhibitors

Adenosine deaminase (ADA) is an enzyme of the purine metabolism which catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine, respectively. This ubiquitous enzyme has been found in a wide variety of microorganisms, plants, and invertebrates. In addition, it is present in all mammalian cells that play a central role in the differentiation and maturation of the lymphoid system. However, despite a number of studies performed to date, the physiological role played by ADA in the different tissues is not clear. Inherited ADA deficiency causes severe combined immunodeficiency disease (ADA-SCID), in which both B-cell and T-cell development is impaired. ADA-SCID has been the first disorder to be treated by gene therapy, using polyethylene glycol-modified bovine ADA (PEG-ADA). Conversely, there are several diseases in which the level of ADA is above normal. A number of ADA inhibitors have been designed and synthesized, classified as ground-state and transition-state inhibitors. They may be used to mimic the genetic deficiency of the enzyme, in lymphoproliferative disorders or immunosuppressive therapy (i.e., in graft rejection), to potentiate the effect of antileukemic or antiviral nucleosides, and, together with adenosine kinase, to reduce breakdown of adenosine in inflammation, hypertension, and ischemic injury.

Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

Serum adenosine deaminase levels as an index of tumor growth in head and neck malignancy

This study was conducted on 40 biopsy proved patients of head and neck cancers,85% of patients presented with squamous cell carcinoma in various grades of differentiation. When seruma denosine deaminase activity was compared between controls and cases, significant increase was found in the activities (control 51.54 ±12.09 IU/L and cases 106.87 ±29.75 IU/ L). The duration of illness didn't reflect any statistical significance with the adenosine deminase activity. It was 97.59 ±62.93 IU/L in case where duration of illness was 98 ±30.98 IU/L in patient with more than one year of disease. The lymph node showed stronger correlation with adenosine deaminase activity, its level was 83.41±1.41 IU/L in patients with N(3) The rise in serum adenosine deaminase activity was found to be directly related to the stage I disease. It was 57.80 ±4.60 IU/ L in patient with stage I disease while in patients with stage IV had 135.87 ±18.39 IU/L of activity. According to histological grading, highest level was found in patients having squamous cell carcinoma( 113.41 ±32 IU/L). The activity of adenosine deaminase decreases with radiotherapy and after surgery. This may help in assessing the decrease in tumour mass and improvement in patient’s clinical condition.

Adenosine deaminase activity in gastric cancer

Adenosine deaminase activity, the key enzyme of adenosine inactivation, was studied in slices taken endoscopically from gastric cancer and macroscopically unchanged gastric mucosa surrounding the cancer. The activity of the enzyme was measured in mucosa homogenates by determination of ammonia liberated from substrate. It was found that adenosine deaminase activity in neoplastic lesions did not differ significantly from normal mucosa and that the gastric region studied (antrum, corpus) did not have an impact. A significant difference in enzyme activity was noticed between intestinal and diffuse-type gastric carcinoma (according to Lauren's classification); the intestinal type was characterized by lower adenosine deaminase activity than was the diffuse type. Since the activity of adenosine deaminase in gastric cancer did not exhibit significant differences from normal mucosa the diagnostic value of its determination is of less importance.