Purine nucleosidase

PULSE-CHASE EXPERIMENTS N-END RULE PULSE RADIOLYSIS PURINE NUCLEOSIDASE PURINE NUCLEOSIDE PHOSPHORYLASE PUTRESCINE CARBAMOYLTRANSEERASE PUTRESCINE N-METHYLTRANSEERASE PUTRESCINE OXIDASE Putrescine synthesis,... 

Le Floc h, F. and Lafleuriel, J., The purine nucleosidases of Jerusalem artichoke shoots. Phytochemistry, 20, 2127-2129, 1981. 

Similarly, adenosine is a true inducer of purine nucleosidase. But at a level of 0.5% its repressing effect is great enough to prevent the appearance of enzyme. Use of ribonucleic acid (RNA) or of the phosphate ester of adenosine (AMP) by the fungus leads to a low but continuous supply of adenosine and a resultant high enzyme concentration (Reese, 1968). 

Adachi O, Hours RA, Akakabe Y, Shinagawa E, Ano Y, Yakushi T, Matsushita K (2013) Pentose oxidation by acetic acid bacteria led to a finding of membrane-bound purine nucleosidase. Biosci Biotechnol Biochem 77(5) 1131-1133... 

The oxidizing enzymes involved in the conversion of purines to ureides have been well studied and are described in the next subsection. Little attention has, however, been paid to the hydrolytic enzymes. Conversion of nucleotides to ureides by nodule tissue or cell-fiee extracts or organelle preparations thereof implies that an efficient hydrolytic system is present. A study by Christensen and Jochimsen (1983) identified a 50-fold excess in levels of 5 -nucleotidase in soybean nodules over nodules of Pisum sativum. Similar differences were found for levels of the enzyme in other organs of the two plants. Levels of purine nucleosidase were, however, not significantly different between the two species, although levels in soybean were somewhat higher in all organs. 

AMP to 8.3 mAf for UMP and 20 mA/for XMP. Doremus and Blevins have reported some properties for this enzyme from soybean nodules. Purine nucleosidase (EC 3.2.2.1) was described as having a pH optimum of 7 - 8 and a of 0.3 mM for inosine (Christensen and Jochimsen, 1983). Nucleosidase has been purified from cowpea nodules (Atkins et ai, 1989). The native molecular weight was 160,000 with a subunit molecular weight of 30,600. This enzyme was apparently cytoplasmic and was markedly xanthosine specific. The values for xanthosine and inosine were similar (0.80 and 0.83 mM, respectively). Neither of these enzymes has exhibited any specific relationship to ureide formation. 

Cytokinins, N6-substituted derivatives of adenine, are important in the regulation of many processes in plant tissues. Chism et al. (1984) reported an HPLC assay measuring the hydroxylation of 6-(3-methylbut-2-enylamino)purine (IPA) and 6-(3-methylbut-2-enylamino)-8-hydroxypurine by cytokinin nucleosidases and adenosine nucleosidases. 

Nucleosidase—a hydrolase that breaks nucleosides into a purine or pyrimidine and ribose. 

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. 

Koszalka, G. W. and Krenitsky, T. A. (1979) Nucleosidases from Leishmania donovani. Pyrimidine ribonucleosidase, purine ribonucleosidase, and a novel purine 2 -deoxyribo-nucleosidase. J. Biol. Chem. 254 8185-8193. 

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]. 

Scheme 4. Biosynthesis of the purine alkaloids caffeine and theobromine. Molecular clones have been isolated for all enzymes shown. Abbreviations CS, caffeine synthase MXN, 7-methyIxanthosine nucleosidase MXS, 7-methyIxanthosine synthase.

Figure 6.63 Isoprenoid cytokinin biosynthesis pathway in Arabidopsis. IPT, phosphate-isopentenyl transferase AK, adenosine kinase 1, phosphatase 2,5 -ribonucleotide phosphohydrolase 3, adenosine nucleosidase 4, purine nucleoside phosphorylase 5, zeatin cis-trans isomerase and 6, zeatin reductase.

During digestive processes, nucleoprotein is split into nucleic acids and protein, the latter then being broken down into amino acids. The nucleic acids are attacked by ribonuclease and deoxyribonuclease enzymes to form nucleotides, which are further hydrolysed by nucleotidases to form nucleosides and phosphates. In the intestines these nucleosides are split by nucleosidases into ribose, deoxy-ribose, purine and pyrimidine bases, which later undergo oxidation and decomposition to ammonia, carbon dioxide and water, to be finally expelled as urea. Nucleotide hydrolysis products are conveniently identified and isolated by chromatographic methods (Chapter 14.2). 

Nucleosidase - N-Ribosyl-purine ribohydrolase 3.2.2.1 Small intestine Nucleosides... 

Fig. 29.2 Caffeine is produced from xanthosine derived from four routes (1) inosine-5 -monophosphate (IMP) originating from de novo purine synthesis (de novo route), (2) adenosine released from the 5-adenosyhnethionine (SAM) cycle, (3) the cellular adenosine nucleotide pool (AMP route), and (4) the guanine nucleotide pool (GMP route). Enzymes AMPDA AMP deaminase, APRT adenine phosphorihosyltransferase, AK adenosine kinase, ARN adenosine nucleosidase, GRD guanine deaminase, IMPDH IMP dehydrogenase, 5 NT 5 -nucleotidase...

Bios5mthetic pathways of naturally occurring cytokinins are illustrated in Fig. 29.5. The first step of cytokinin biosynthesis is the formation of A -(A -isopentenyl) adenine nucleotides catalyzed by adenylate isopentenyltransferase (EC 2.5.1.27). In higher plants, A -(A -isopentenyl)adenine riboside 5 -triphosphate or A -(A -isopentenyl)adenine riboside 5 -diphosphate are formed preferentially. In Arabidopsis, A -(A -isopentenyl)adenine nucleotides are converted into fraws-zeatin nucleotides by cytochrome P450 monooxygenases. Bioactive cytokinins are base forms. Cytokinin nucleotides are converted to nucleobases by 5 -nucleotidase and nucleosidase as shown in the conventional purine nucleotide catabolism pathway. However, a novel enzyme, cytokinin nucleoside 5 -monophosphate phosphoribo-hydrolase, named LOG, has recently been identified. Therefore, it is likely that at least two pathways convert inactive nucleotide forms of cytokinin to the active freebase forms that occur in plants [27, 42]. The reverse reactions, the conversion of the active to inactive structures, seem to be catalyzed by adenine phosphoiibosyl-transferase [43] and/or adenosine kinase [44]. In addition, biosynthesis of c/s-zeatin from tRNAs in plants has been demonstrated using Arabidopsis mutants with defective tRNA isopentenyltransferases [45]. 

Investigations on the reversal of nucleosidase activity established that purified nucleosidases from rat liver could synthesize inosine and guanosine from ribose 1-phosphate and the respective purines 89a), This procedure has been extended to the enzymic synthesis of iV-ribosylnicotinamide 89h) and A-2-deoxyribosylhypoxanthine and -azaguanine 89c) from their respective bases and ribose 1-phosphate. Reactions such as these may play a role in the natural synthesis of nucleotides. 

Nucleoside phosphorylase activity is widely distributed, having been reported in yeast, E. coli, L. casei, and other sources. A nucleosidase specific for uridine has been purified several hundredfold from E. coli, using sonic vibration, ammonium sulfate, and gel steps.It does not attack cytidine or the purine nucleosides. 

A pyridine ribosidase from Xanthomonas pruni has been described which splits nicotmamide riboside in preference to purine ribosides (18 ). The relative activity towards nicotinamide riboside and inosine is about 30 to 1. In contrast to this, extracts of SaccJiaromyces cereviaiae promote the hydroljrtic cleavage of nicotinamide riboside and inosine at approximately the same rate (183). A hydrolytic nucleoadase from a strain of Lactobacillus delbruckii has been purified and also shown to have equal activity towards the two nucleosides (184). Xanthomonas pyridine nucleosidase appears to be unusual for bacteria its significance is not clear as yet. 

Although AdoHcy hydrolase is not found in bacteria, AdoHcy is cleaved irreversibly to adenine and S-rlbosyl-L-homocystelne by AdoHcy nucleosidase ". This enzyme has been partially purified from E, aoliy and found to catalyze also the hydrolysis of 5 -methy1-thioadenosine to adenine and methylthiorlbose. The enzyme is inactive towards AdoMet and a variety of other purine and pyrimidine nucleosides. The specific activity of AdoHcy nucleosidase is 1000 x greater than that of AdoHcy hydrolase emphasizing the need to remove AdoHcy, a potent inhibitor of reactions which utilize AdoMet as a substrate. 

Intestinal mucosa. Nucleosidase. Phosphatase. Nucleosides. Hexoses. Purines + sugar. Hezose pho pbate. 

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