Purine Nucleotidases

A strong catabolism due to purine nucleotidase is not the reason for the high PRT activities, because the level of GflP-nucleotidase is very low. 

Pyrimidine 5 -nucleotidase (P5N) is a unique enzyme that was recognized from studies of families with relatively common hemolytic disorders. The enzyme catalyzes the hydrolytic dephosphorylation of pyrimidine 5 -nucleotides but not purine nucleotides. The role of this enzyme is to eliminate RNA and DNA degradation products from the cytosol during erythroid maturation by conversion of nucleotide monophosphates to diffusible nucleosides. P5N is inhibited by lead, and its activity is considered to be a good indicator of lead exposure (PI). 

P5N has two isozymes, P5N-I (pyrimidine nucleotidase) and P5N-II (deoxyri-bonucleotidase) (H6, P2). P5N-I is active principally with pyrimidine substrates at an optimal neutral pH P5N-II activity occurs with both purine and pyrimidine substrates and was maximal with deoxy analogues at an acidic pH optimum. This enzyme was partially purified from human red blood cells and had a molecular weight of 28,000 (T19). The primary structures of both isozymes have not been... 

In addition to the enzymes that catalyse the formation of nucleotides and polynucleotides, a large number of catabolic systems exist which operate at all levels of the internucleotide pathways. The ribonucleases and deoxyribonucleases that degrade polynucleotides are probably not significantly involved in purine analogue metabolism, but the enzymes which dephosphorylate nucleoside 5 -monophosphates are known to attack analogue nucleotides and may be of some importance to their in vivo activity. Phosphatases of low specificity are abundant in many tissues [38], particularly the intestine [29]. Purified mammalian 5-nucleotidases hydrolyse only the nucleoside 5 monophosphates [28] and... 

Purine nucleotides are degraded by a pathway in which they lose their phosphate through the action of 5 -nucleotidase (Fig. 22-45). Adenylate yields adenosine, which is deaminated to inosine by adenosine deaminase, and inosine is hydrolyzed to hypoxanthine (its purine base) and D-ribose. Hypoxanthine is oxidized successively to xanthine and then uric acid by xanthine oxidase, a flavoenzyme with an atom of molybdenum and four iron-sulfur centers in its prosthetic group. Molecular oxygen is the electron acceptor in this complex reaction. 

After purine nucleotides have been converted to the corresponding nucleosides by 5 -nucleotidases and by phosphatases, inosine and guanosine are readily cleaved to the nucleobase and ribose-1-phosphate by the widely distributed purine nucleoside phosphorylase. The corresponding deoxynucleosides yield deoxyribose- 1-phosphate and base with the phosphorylase from most sources. Adenosine and deoxyadenosine are not attacked by the phosphorylase of mammalian tissue, but much AMP is converted to IMP by an aminohydrolase (deaminase), which is very active in muscle and other tissues (fig. 23.20). An inherited deficiency of purine nucleoside phosphorylase is associated with a deficiency in the cellular type of immunity. 

Fig. 4 Mechanisms involved in the extracellular inactivation of nucleotides (a, b and c) and adenosine (d) and their influence on purine concentration in the P2Y and PI receptor biophases, (a) NT-PDasel hydrolyses ATP and ADP very efficiently, thus preventing their action on P2Y receptors (b) NTPDase2 metabolizes ATP preferentially, allowing an accumulation of ADP and thus favouring activation of P2Yi, 12,13 receptors (c) NTPDase3 hydrolyses both ATP and ADP slowly, giving them time to activate both P2Y2,4 and P2Y 1,12,13 receptors. Formation of adenosine depends on the activity of ecto 5 -nucleotidase (CD73). Adenosine inactivation systems also influence adenosine concentration in the PI receptor biophase (d) the nucleoside transporters take up adenosine adenosine deaminase (ADA) regulates both the concentration of adenosine in the Ai receptor biophase and the functionality of Ai receptors.

Purines and pyrimidines in excess of cellular requirements can be degraded. The extent of degradation depends on the organism. Humans cannot degrade purines beyond uric acid because we lack the enzyme uricase, which splits the purine ring to form allantoin. In humans excess AMP is deaminated to IMP by the action of a specific deaminase. IMP is then hydrolyzed by 5 -nucleotidase to form inosine. Inosine and guanine are oxidized to urate as follows ... 

Escherichia coli B was incubated with 2,6-diaminopurine (XXIV), and 6-amino-2-(methylamino)-9-(5-0-phospho-D-ribosyl)purine (XXV) was isolated from the acid-soluble extract of the cells. 5-Nucleotidase liberated a nucleoside containing D-ribose. Hydrolysis of the nucleoside (or nucleotide) with N hydrochloric acid liberated 6-amino-2-(methylamino)purine, which was identified by paper chromatography and by its ultraviolet absorption spectrum. The chromatographic and ion-exchange behavior of the extract also suggested the presence of either a pyrophosphate or a triphosphate of the 6-amino-2-methylamino-(D-ribosyl)purine. In a similar manner, 2,6-diamino-9-(5-0-phospho-D-ribosyl)purine (XXVI) was isolated and identified, together with its possible pyrophosphate or triphosphate. 2,6-... 

The activity is found in erythrocytes, platelets, and lymphocytes, and determination of its value aids in diagnosis of some blood disorders. In this assay, which can readily be used for purine and pyrimidine 5 - and 3 -nucleotidase activities, the nucleoside monophosphate (the substrate) was separated from the nucleoside (the product) using ion-pair reversed-phase HPLC with a mobile phase of 5% methanol-5 mAf potassium dihydrogen phosphate 0.25 mAf 1-decanesulfonic acid was also added to the mobile phase. The elution was carried out at room temperature and the eluent monitored at 254 nm. 

Catabolism of the nucleotides (Figure 24-3, B) begins with removal of their ribose-linked phosphate, a process catalyzed by purine 5 -nucleotidase. Removal of the ribose moiety of inosine and guanosine by the action of purine-nucleoside phosphorylase forms hypoxanthine and guanine, both of which are converted to xanthme. Xanthine is converted to uric acid through the action of xanthine oxidase. 

Other studies have examined the association between the activity of TPMT and other enzymes in the purine pathway and AZA toxicity. In one study, TPMT, HPRT, 5 -nucleotidase, and purine nucleoside phosphorylase activity in the red blood cells (RBC) of 33 RA patients on AZA (dose of approximately 2 mg/ kg/day) and 66 controls was measured. Fourteen RA patients with low (p = 0.004) and seven patients with intermediate TPMT activity (RR 3.1) developed AZA toxicity when compared to patients with normal TPMT activity [66]. Another study measured TPMT activity in 3 RA patients who had experienced AZA-induced hematologic toxicity and 16 RA patients without AZA toxicity. Two patients with AZA-induced hematologic toxicity were TPMT... 

Pyrimidine catabolism occurs mainly in the liver. In contrast to purine catabolism, pyrimidine catabolism yields highly soluble end products. Pyrimidine nucleotides are converted to nucleosides by 5 -nucleotidase. 

Nucleotidase (5 -N) is an integral glycoprotein of the cellular plasma membrane in a wide range of animal cells. Its functional role is still unclear. Possibilities. include recovery of purines and pyrimidines from the extracellular space, the extracellular formation of neuromodular adenosine from released nucleotidases and non-enzymatic functions related to the interaction of 5 -nucleotidase with compartments of the cytoskel-eton and extracellular matrix (Schoen et al., 1987). 5 -N catalyses the production of adenosine by the hydrolytic cleavage of 5 -nucleotide monophosphates (i.e. adenosine-5 -monophosphate). The development of 5 -N in the cerebellum was studied by Schoen et al. (1987, 1988, 1990). 

Purine nucleotide catabolism is outlined in Figure 15.12. There is some variation in the specific pathways used by different organisms or tissues to degrade AMP. In muscle, for example, AMP is initially converted to IMP by AMP deaminase (also referred to as adenylate aminohydrolase). IMP is subsequently hydrolyzed to inosine by 5 -nucleotidase. In most tissues, however, AMP is hydrolyzed by 5 -nucleotidase to form adenosine. Adenosine is then deaminated by adenosine deaminase (also called adenosine aminohydrolase) to form inosine. 

Several classes of enzyme degrade nucleic acids nucleases, phosphodiesterases, nucleotidases, nuclioside phosphorylases, and nucleosidases. The bases of purine nucleotides are degraded to form the nitrogenous waste product uric acid. 

Approximately 80% of red blood cell purines are in the form of adenosine triphosphate (ATP) with an intracellular concentration estimated to be 2-3 mM. In glucose-deprived or aged red cells there is a progressive decline in the ATP content of the erythrocyte leading to the formation of adenosine diphosphate (ADP) and adenosine monophosphate (AMP) AMP is then dephosphorylated (via 5 -nucleotidase) to... 

Figure 22.7 shows pathways of purine catabolism leading to uric acid. As seen in the figure, AMP and GMP can both be hydrolyzed from their phosphates by nucleotidase, ultimately yielding the bases hypoxanthine and xanthine, respectively. 

Figure 11.1 shows pathways of purine catabolism leading to uric acid. As seen in the figure, AMP and GMP can both be hydrolyzed from their phosphates by nucleotidase, ultimately yielding the bases hypoxanthine and xanthine, respectively. Hypoxanthine is converted to xanthine by xanthine oxidase and xanthine is converted to uric acid, also by xanthine oxidase. In addition, AMP can be degraded first in a deamination to form IMP, which loses its phosphate to become inosine. Inosine, in turn, is converted to hypOxanthine. 

Nucleotidases possessing 3 - and 5 -hydrolase activities are present on the surface of L. donovani and L. mexicana (32). Cell surface 5 -nucleotidases are commonly present on the plasma membrane of mammalian cells and are believed to be involved in purine... 

Gottlieb, M. (1985) Enzyme regulation in a trypanosomatid effect of purine starvation on levels of 3 -nucleotidase activity. Science 111 72 74. 

Nucleotidase [15] followed by adenosine nucleosidase [16] are expected to be the enzymes responsible for the step-by-step conversion of the cytokinin nucleotide to the base iPA. Both of these reactions may proceed also in the opposite direction, and in this case they are catalysed by adenosine phosphorylase (ribosylation of iPA, [17]) and adenosine kinase (phosphorylation of iPAR, [18-20]). These enzymes are common in the mutual conversions of adenine and purine metabolites (reviewed in [21]) and their properties have been summarised by [22]. These enzyme activities seem to be the key for understanding the fate of C-labelled adenine (Ade) and adenosine (Ado) in feeding experiments [summarised by 23]. 

Fig. 41.10. Salvage of bases. The purine bases hypoxanthine and gnanine react with PRPP to form the nucleotides inosine and gnanosine monophosphate, respectively. The enzyme that catalyzes the reaction is hypoxanthine-gnanine phosphoribosyltransferase (HGPRT). Adenine forms AMP in a reaction catalyzed by adenine phosphoribosyltransferase (APRT). Nucleotides are converted to nucleosides by 5 -nucleotidase. Free bases are generated from nncleosides by purine nucleoside phosphorylase. Deamination of the base adenine occurs with AMP and adenosine deaminase. Of the purines, only adenosine can be directly phosphorylated back to a nucleotide, by adenosine kinase.

The degradation of the purine nucleotides (AMP and GMP) occurs mainly in the liver (Fig. 41.19). Salvage enzymes are used for most of these reactions. AMP is first deaminated to produce IMP (AMP deaminase). Then IMP and GMP are dephosphorylated (5 -nucleotidase), and the ribose is cleaved from the base by purine nucleoside phosphorylase. Hypoxanthine, the base produced by cleavage of IMP, is converted by xanthine oxidase to xanthine, and guanine is deaminated by... 

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