Purification and characterization of adenosine nucleosidase from barley leaves

Purification of a non-specific nucleoside hydrolase from Alaska pea seeds

A non-specific nucleoside hydrolase has been isolated from germinated Alaska pea seeds. The enzyme catalyzes the hydrolysis of both purines and pyrimidines along with ribo- and deoxyribonucleosides. A purification scheme utilized ammonium sulfate precipitation, ion exchange chromatography and size exclusion chromatography, resulted in 103-fold purification with a recovery of 2.8%. The purified protein has a specific activity of 0.308 μmol/min•mg. The subunit molecular weight was 26103 Da and the enzyme exists as a dimer. The enzyme retains a significant amount of activity over a wide pH range with the maximum activity occurring at a pH of 6.0. The maximum activity was observed with adenosine as the substrate followed by inosine and guanosine, respectively. The Km for adenosine was 184 ± 34 μM and for inosine 283 ± 88 μM. In addition to the nucleoside hydrolase activity, adenosine deaminase activity was seen in the initial extract. Using adenosine as the substrate with the initial extract from the germinated seeds, the products adenine, inosine, and hypoxanthine were identified based on their retention times during reverse phase HPLC.

Purine salvage in plants

Purine bases and nucleosides are produced by turnover of nucleotides and nucleic acids as well as from some cellular metabolic pathways. Adenosine released from the S-adenosyl-L-methionine cycle is linked to many methyltransferase reactions, such as the biosynthesis of caffeine and glycine betaine. Adenine is produced by the methionine cycles, which is related to other biosynthesis pathways, such those for the production of ethylene, nicotianamine and polyamines. These purine compounds are recycled for nucleotide biosynthesis by so-called "salvage pathways". However, the salvage pathways are not merely supplementary routes for nucleotide biosynthesis, but have essential functions in many plant processes. In plants, the major salvage enzymes are adenine phosphoribosyltransferase (EC 2.4.2.7) and adenosine kinase (EC 2.7.1.20). AMP produced by these enzymes is converted to ATP and utilised as an energy source as well as for nucleic acid synthesis. Hypoxanthine, guanine, inosine and guanosine are salvaged to IMP and GMP by hypoxanthine/guanine phosphoribosyltransferase (EC 2.4.2.8) and inosine/guanosine kinase (EC 2.7.1.73). In contrast to de novo purine nucleotide biosynthesis, synthesis by the salvage pathways is extremely favourable, energetically, for cells. In addition, operation of the salvage pathway reduces the intracellular levels of purine bases and nucleosides which inhibit other metabolic reactions. The purine salvage enzymes also catalyse the respective formation of cytokinin ribotides, from cytokinin bases, and cytokinin ribosides. Since cytokinin bases are the active form of cytokinin hormones, these enzymes act to maintain homeostasis of cellular cytokinin bioactivity. This article summarises current knowledge of purine salvage pathways and their possible function in plants and purine salvage activities associated with various physiological phenomena are reviewed.

Structure and function of nucleoside hydrolases from Physcomitrella patens and maize catalyzing the hydrolysis of purine, pyrimidine, and cytokinin ribosides

We present a comprehensive characterization of the nucleoside N-ribohydrolase (NRH) family in two model plants, Physcomitrella patens (PpNRH) and maize (Zea mays; ZmNRH), using in vitro and in planta approaches. We identified two NRH subclasses in the plant kingdom; one preferentially targets the purine ribosides inosine and xanthosine, while the other is more active toward uridine and xanthosine. Both subclasses can hydrolyze plant hormones such as cytokinin ribosides. We also solved the crystal structures of two purine NRHs, PpNRH1 and ZmNRH3. Structural analyses, site-directed mutagenesis experiments, and phylogenetic studies were conducted to identify the residues responsible for the observed differences in substrate specificity between the NRH isoforms. The presence of a tyrosine at position 249 (PpNRH1 numbering) confers high hydrolase activity for purine ribosides, while an aspartate residue in this position confers high activity for uridine. Bud formation is delayed by knocking out single NRH genes in P. patens, and under conditions of nitrogen shortage, PpNRH1-deficient plants cannot salvage adenosine-bound nitrogen. All PpNRH knockout plants display elevated levels of certain purine and pyrimidine ribosides and cytokinins that reflect the substrate preferences of the knocked out enzymes. NRH enzymes thus have functions in cytokinin conversion and activation as well as in purine and pyrimidine metabolism.

Calcium-stimulated guanosine--inosine nucleosidase from yellow lupin (Lupinus luteus)

Guanosine-inosine-preferring nucleoside N-ribohydrolase has been purified to homogeneity from yellow lupin (Lupinus luteus) seeds by ammonium sulfate fractionation, ion-exchange chromatography and gel filtration. The enzyme functions as a monomeric, 80kDa polypeptide, most effectively between pH 4.7 and 5.5. Of various mono- and divalent cations tested, Ca(2+) appeared to stimulate enzyme activity. The nucleosidase was activated 6-fold by 2mM exogenous CaCl(2) or Ca(NO(3))(2), with K(a)=0.5mM (estimated for CaCl(2)). The K(m) values estimated for guanosine and inosine were 2.7+/-0.3 microM. Guanosine was hydrolyzed 12% faster than inosine while adenosine and xanthosine were poor substrates. 2'-Deoxyguanosine, 2'-deoxyinosine, 2'-methylguanosine, pyrimidine nucleosides and 5'-GMP were not hydrolyzed. However, the enzyme efficiently liberated the corresponding bases from synthetic nucleosides, such as 1-methylguanosine, 7-methylguanosine, 1-N(2)-ethenoguanosine and 1-N(2)-isopropenoguanosine, but hydrolyzed poorly the ribosides of 6-methylaminopurine and 2,6-diaminopurine. MnCl(2) or ZnCl(2) inhibited the hydrolysis of guanosine with I(50) approximately 60 microM. Whereas 2'-deoxyguanosine, 2'-methylguanosine, adenosine, as well as guanine were competitive inhibitors of this reaction (K(i) values were 1.5, 3.6, 21 and 9.7 microM, respectively), hypoxanthine was a weaker inhibitor (K(i)=64 microM). Adenine, ribose, 2-deoxyribose, 5'-GMP and pyrimidine nucleosides did not inhibit the enzyme. The guanosine-inosine hydrolase activity occurred in all parts of lupin seedlings and in cotyledons it increased up to 5-fold during seed germination, reaching maximum in the third/fourth day. The lupin nucleosidase has been compared with other nucleosidases.

Transition state analysis of adenosine nucleosidase from yellow lupin (Lupinus luteus)

The transition state of adenosine nucleosidase (EC 3.2.2.7) isolated from yellow lupin (Lupinus luteus) was determined based upon a series of heavy atom kinetic isotope effects. Adenosine labeled with 13C, 2H, and 15N was analyzed by liquid chromatography/electrospray mass spectrometry to determine kinetic isotope effects. Values of 1.024+/-0.004, 1.121+/-0.005, 1.093+/-0.004, 0.993+/-0.006, and 1.028+/-0.005 were found for [1'-13C], [1'-2H], [2'-2H], [5'-2H], and [9-15N] adenosine, respectively. Using a bond order bond energy vibrational analysis, a transition state consisting of a significantly broken C-N bond, formation of an oxocarbenium ion in the ribose ring, a conformation of C3-exo for the ribose ring, and protonation of the heterocyclic base was proposed. This transition state was found to be very similar to the transition state for nucleoside hydrolase, another purine metabolizing enzyme, isolated from Crithidia fasciculata.

Purification and characterisation of adenosine nucleosidase from Coffea arabica young leaves

An adenosine nucleosidase (ANase) (EC 3.2.2.7) was purified from young leaves of Coffea arabica L. cv. Catimor. A sequence of fractionating steps was used starting with ammonium sulphate salting-out, followed by anion exchange, hydrophobic interaction and gel filtration chromatography. The enzyme was purified 5804-fold and a specific activity of 8333 nkat mg-1 protein was measured. The native enzyme is a homodimer with an apparent molecular weight of 72 kDa estimated by gel filtration and each monomer has a molecular weight of 34.6 kDa, estimated by SDS-PAGE. The enzyme showed maximum activity at pH 6.0 in citrate-phosphate buffer (50 mM). The calculated Km is 6.3 microM and Vmax 9.8 nKat.

ATP N-glycosidase - a novel ATP-converting activity from a marine sponge Axinella polypoides

A novel nucleosidase enzymatic activity was discovered in the marine sponge Axinella polypoides. This enzyme, designated as ATP N-glycosidase, converts adenosine-5'-triphosphate into adenine and ribose-5-triphosphate. The crude extract of A. polypoides was capable of hydrolysing 25 micro mol ATP.min-1 per g wet weight of sponge. The catalytic activity of a sponge crude extract per mg total protein is comparable with specific activities of purified plant adenosine and bacterial AMP nucleosidases. The preferred substrate for the novel enzyme is ATP but any compound containing adenosine-5'-diphosphoryl fragment is also cleaved. The biochemical properties (Km, Kip, environmental requirements) of ATP N-glycosidase show similarities with previously described adenine-specific nucleosidases; however, the pattern of its biochemical characteristics does not match with that of any of those enzymes.

Purine metabolism during white spruce somatic embryo development: salvage of adenine, adenosine, and inosine

Contribution of the adenine, adenosine and inosine salvage to the purine nucleotide and nucleic acid biosynthesis during white spruce (Picea glauca) somatic embryo maturation was estimated by in situ assays using [8-(14)C]adenine, [8-(14)C]adenosine and [8-(14)C]inosine. The salvage of adenine and adenosine was high during the initial stages of embryo maturation, characterized by rapid cell proliferation, but it declined upon further embryo development. Inosine salvage activity was always much lower than that observed for adenine and adenosine. Consistent with these results, activities of adenine phosphoribosyltransferase (APRT) and adenosine kinase (AK) measured in the embryo extracts in vitro were much higher than the activity of inosine kinase (IK) during all stages of embryo development. Utilization of adenosine and inosine for nucleotide and nucleic acid synthesis was found to be regulated by the enzymes AK and IK, as the pattern of their activities was very similar to the activity of adenosine and inosine salvage, estimated with exogenously supplied precursors. However, little correlation between salvage of adenine and activity of APRT was found throughout somatic embryo maturation. As no adenosine nucleosidase activity was found in white spruce embryos, adenosine, but not adenine, seems to be the major end product of adenylate catabolism and becomes the predominant substrate for purine salvage in vivo. Thus, adenosine salvage appeared to have the most important role in white spruce embryos. Studies on the metabolic fate of [8-(14)C]adenine and [8-(14)C]adenosine suggest that turnover of adenine nucleotides is rapid, as some of them are utilized for nucleic acid synthesis. In contrast, most of [8-(14)C]inosine taken up by the embryos seems to be directly catabolized by the conventional purine catabolic pathway via ureides in all stages of embryo maturation.

Adenine nucleotide N-glycosidase activity of the A-chain of cinnamomin characterized by 1H-nuclear magnetic resonance

Plant ribosome-inactivating proteins specifically cleave an N-glycosidic bond of a unique adenosine in the largest ribosomal RNA, releasing an adenine from ribosomes of different sources. Here, 1H-nuclear magnetic resonance is used to analyze the enzymatic products of the A-chain of cinnamomin, a type-II ribosome-inactivating protein (RIP) acting on the nucleotides in situ. The enzymatic activities of the RIP on nine nucleotides are compared. Cinnamomin A-chain can cleave the N-glycosidic bond and release an adenine base from adenine nucleotides except 5'-ATP; however, it cannot act on 5'-GMP, 5'-CMP, and 5'-UMP. The A-chain in the mixture of cinnamomin A- and B-chain exhibits higher activity toward adenine nucleotides than the A-chain alone does, suggesting that the B-chain can conformationally stabilize the A-chain. Intact cinnamomin also exhibits lower activity toward adenine nucleotides. However, cinnamomin B-chain and heat-denatured intact cinnamomin cannot hydrolyze all the tested nucleotides. We conclude that hydrolysis of the N-C glycosidic bond of nucleotide compounds by cinnamomin A-chain has a base preference, and the negatively charged phosphate group(s) reduces the recognition ability of the A-chain to adenine nucleotide.

Substrate competition and specificity at the active site of amylopullulanase from Clostridium thermohydrosulfuricum

A highly thermostable pullulanase purified from Clostridium thermohydrosulfuricum strain 39E displayed dual activity with respect to glycosidic bond cleavage. The enzyme cleaved alpha-1,6 bonds in pullulan, while it showed alpha-1,4 activity against malto-oligosaccharides. Kinetic analysis of the purified enzyme in a system which contained both pullulan and amylose as the two competing substrates were used to distinguish the dual specificity of the enzyme from the single substrate specificity known for pullulanases and alpha-amylases.

Rapid assay of cyclic AMP phosphodiesterase and 5'-nucleotidase by means of chromatography on cellulose-nitrate membrane strips

A simple chromatographic procedure with the use of modified cellulose-nitrate membrane strips, 80 x 40 mm, has been worked out for the rapid isotopic assay of cyclic AMP (cAMP) phosphodiesterase (EC 3.1.4.17) and 5'-AMP nucleotidase (EC 3.1.3.5) in crude extracts of various tissues from animals and plants. The assay is based on enzymatic conversion of the product to adenine, a relatively inert compound which, in contrast to cAMP and 5'-AMP, is strongly adsorbed by the cellulose-nitrate membrane. Due to this property rapid separation of adenine from the unconverted substrate (cAMP or 5'-AMP) is possible. Commercial 5'-nucleotidase and easily obtainable crude extract of adenosine nucleosidase from barley leaves are used as coupling enzymes for the phosphodiesterase assay. The assay of phosphodiesterase in 0.5-2 microliter of blood (10(-5) to 4.10(-5) units) has been demonstrated on several examples.

Adenine and adenosine metabolizing enzymes in cell-free extracts from Euglena gracilis

1. Activities of the following enzymes involved in adenine and adenosine metabolism were found in cell-free extracts from Euglena gracilis: acid phosphatase (EC 3.1.3.2), 5'-methylthioadenosine phosphorylase (EC 2.4.2.-), adenine deaminase (EC 3.5.4.2), adenine phosphoribosyltransferase (EC 2.4.2.7) and adenosine kinase (EC 2.7.1.20). 2. The activities occurred both in heterotrophic and photoautotrophic cells and their levels did not change during light-induced chloroplast development. 3. Neither S-adenosylhomocysteinase (EC 3.3.1.1), 5'-methylthioadenosine nucleosidase (EC 3.2.2.9) and nucleoside phosphotransferase (EC 2.7.1.77) nor adenosine degrading enzymes: adenosine deaminase (EC 3.5.4.4), adenosine nucleosidase (EC 3.2.2.7), and purine-nucleoside (adenosine) phosphorylase (EC 2.4.2.1) were found in the Euglena extracts. 4. Comparison of the adenine and adenosine metabolism in Euglena and in other organisms is comprehensively presented. The metabolism in Euglena gracilis differs from that in higher animals and plants.

S-Adenosylhomocysteinase from yellow lupin seeds: stoichiometry and reactions of the enzyme-adenosine complex

Plant (Lupinus luteus) S-adenosylhomocysteinase, an alpha 2 dimer, forms a 1:2 enzyme-adenosine complex. The binding sites for adenosine are not equivalent. Binding of the first molecule of adenosine is fast (k greater than 7 x 10(5) M-1 s-1), whereas the second molecule of adenosine binds in a slow process with a half-life of 5 min. Adenosine in the 1:1 and 1:2 enzyme--substrate complexes reacts slowly (k = 0.05 min-1) to give finally free enzyme, adenine, and ribose. The enzyme does not lose its ability to catalyze the synthesis of S-adenosylhomocysteine during the reactions. The relevance of the data to the catalytic functioning of the plant S-adenosylhomocysteinase is discussed.

5'-Methylthioadenosine nucleosidase. Purification and characterization of the enzyme from Lupinus luteus seeds

5'-Methylthioadenosine nucleosidase (EC 3.2.2.9), the enzyme which catalyzes hydrolytic cleavage of 5'-methylthioadenosine with the formation of adenine and 5'-methylthioribose, has been purified to homogeneity from Lupinus luteus seeds. The nucleosidase has a native molecular weight of 62 000 and consists of two identical subunits, as judged by gel filtration and dodecylsulfate/polyacrylamide gel electrophoresis. The nucleosidase exhibits highest specificity towards the natural substrate with a Km of 4.1 X 10(-7) M for 5'-methylthioadenosine. It does not cleave adenine from S-adenosylhomocysteine. Among the synthetic analogs of 5'-methylthioadenosine tested, eleven compounds appear to be able to substitute as substrates. Furthermore, the enzyme can liberate hypoxanthinine from six inosyl (deaminated) derivatives obtained by enzymatic deamination of 5'-methylthioadenosine and its synthetic analogs. The Km for 5'-methylthioinosine is 55 microM, and the maximal velocity about 50-times lower than for 5'-methylthioadenosine. The reaction catalyzed by the nucleosidase can be inhibited by adenine (Ki = 11 microM), 3-deazaadenine (Ki = 19 microM), and 9-erythro(2-hydroxyl-3-nonyl)adenine (ki = 37 microM).

Purine metabolism in mesophyll protoplasts of tobacco (Nicotiana tabacum) leaves

The overall metabolism of purines was studied in tobacco (Nicotiana tabacum) mesophyll protoplasts. Metabolic pathways were studied by measuring the conversion of radioactive adenine, adenosine, hypoxanthine and guanine into purine ribonucleotides, ribonucleosides, bases and nucleic acid constituents. Adenine was extensively deaminated to hypoxanthine, whereupon it was also converted into AMP and incorporated into nucleic acids. Adenosine was mainly hydrolysed to adenine. Inosinate formed from hypoxanthine was converted into AMP and GMP, which were then catabolized to adenine and guanosine respectively. Guanine was mainly deaminated to xanthine and also incorporated into nucleic acids via GTP. Increased RNA synthesis in the protoplasts resulted in enhanced incorporation of adenine and guanine, but not of hypoxanthine and adenosine, into the nucleic acid fraction. The overall pattern of purine-nucleotide metabolic pathways in protoplasts of tobacco leaf mesophyll is proposed.

Nucleoside phosphotransferase from yellow lupin seedling cotyledons

Nucleoside phosphotransferase (nucleotide:3'-deoxynucleoside 5'-phosphotransferase, EC 2.7.1.77) from yellow lupin seedling cotyledons was purified and the active enzyme consists of a single polypeptide chain, Mr = 72 000 +/- 3000. In transphosphorylation, purine and pyrimidine nucleosides are good phosphate acceptors and 5'-nucleotides are effective phosphate donors. Among 2'- and 3'-nucleotides, only 3'-AMP and 3'-psi MP acted as phosphate donors, and p-nitrophenylphosphate appeared less active in this regard. The purine and pyrimidine bases inhibit transphosphorylation. The Km values determined for the inosine:5'-AMP pair were 400 micrometers for both the compounds. The enzyme showed optimum activity at pH 8.0 in mM Tris-HCl buffer. Antisulfhydryl reagents and EDTA did not affect enzyme activity.

Inosine nucleosidase from Azotobacter vinelandii. Purification and properties

An enzyme catalyzing the hydrolysis of purine nucleosides was found to occur in the extract of Azotobacter vinelandii, strain O, and was highly purified by ammonium sulfate fractionation, DEAE-cellulose chromatography, hydroxylapatite chromatography and gel filtration on Sephadex G-150. A strict substrate specificity of the purified enzyme was shown with respect to the base components. The enzyme specifically attacked the nucleosides without amino groups in the purine moiety: inosine gave the maximum rate of hydrolysis and xanthosine was hydrolyzed to a lesser extent. The pH optimum of inosine hydrolysis was observed from pH 7 to 9, while xanthosine was hydrolyzed maximally at pH 7. The Km values of the enzyme for inosine were 0.65 and 0.85 mM at pH 7.1 and 9.0, respectively, and the value for xanthosine was 1.2 mM at pH 7.1. Several nucleotides inhibited the enzyme: the phosphate portions of the nucleotides were suggested to be responsible for the inhibition by nucleotides. Although the inhibition of the enzyme by nucleotides was apparently non-competitive type with respect to inosine, allosteric (cooperative) binding of the substrate was suggested in the presence of the inhibitor. The physiological significance of the enzyme was discussed in connection with the degradation and salvage pathways of purine nucleotides.

Adenosylhomocysteinase and adenosine nucleosidase activities in Lupinus luteus cotyledons during seed formation and germination

The activities of adenosylhomocysteinase (EC 3.3.1.1) and adenosine nucleosidase (EC 3.2.2.7) were assayed in extracts from yellow lupin (Lupinus luteus L.) cotyledons at different stages of seed formation and seedling development. Adenosylhomocysteinase activity was demonstrated in all the cotyledon extracts examined. Its lowest level was found in the dry seeds and the highest, in 4-day-old seedling cotyledons. Extracts from the cotyledons of maturating seeds, dry seeds, and seedlings up to the second day of growth exhibited no adenosine nucleosidase activity. Adenosine nucleosidase activity appeared in the cotyledons of 2-day-old seedlings and its highest level was reached in 4-to 5-day-old seedlings. There is no inhibitor of adenosine nucleosidase in the maturating and dry yellow lupin seeds. No activator of a possible zymogen form of adenosine nucleosidase from maturating or dry seeds occurs in the growing seedlings.

Adenosylhomocysteinase from yellow lupin seeds. Purification and properties

Adenosylhomocysteinase from yellow lupin seeds (Lupinus luteus) has been purified to homogeneity. Active enzyme, Mr = 110000, consists of two probably identical subunits with Mr = 55000 as judged by gel filtration and dodecyl sulphage/polyacrylamide gel electrophoresis in the presence of 2-mercaptoethanol. The isoelectric point of the enzyme was shown to be 4.9 +/- 0.1. It was demonstrated by disc and pore gradient electrophoresis that the most purified fraction formed multimers. The enzyme shows optimum activity at pH 8.5-9.0. Km values are 2.3 micrometer, 4.6 mM and 12 micrometer for adenosine, DL-homocysteine and S-adenosyl-L-homocysteine, respectively. The energy of activation for S-adenosylhomocysteine synthesis was estimated as 14.4 kcal/mol (60.2 kJ/mol) and temperature coefficient as 2.4. The equilibrium constant for the hydrolysis of S-adenosylhomocysteine amounts to 5 X 10(-7) M. Anti-sulfhydryl reagents such as p-hydroxymercuribenzoate and N-ethylmaleimide acted as irreversible inhibitors. The enzyme exhibits high specificity for homocysteine whereas some of the rare nucleosides tested could substitute for adenosine.