Purine metabolism in germinating wheat embryos

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.

A case of neofunctionalization of a Putranjiva roxburghii PNP protein to trypsin inhibitor by disruption of PNP-UDP domain through an insert containing inhibitory site

The attainment of new function by a protein is achieved through convergent/divergent evolution. In present work, the sequence analysis of a 34kDa protein from Putranjiva roxburghii, earlier reported as a potent trypsin inhibitor, showed resemblance to some of the wound inducible and vegetative storage proteins. A detailed sequence analysis revealed that these proteins belong to PNP-UDP family. In case of P. roxburghii protein, an approximately 46 residue insert disrupts the PNP domain. Similar disruption of PNP domain is observed in related plant proteins. The characterization of recombinant full length and truncated (without 46 residue insert) forms of P. roxburghii PNP family protein (PRpnp) unraveled that trypsin inhibitory active site is located within the insert. The truncated form containing uninterrupted PNP domain showed strong PNP enzymatic activity where it hydrolyzed the N-glycosidic bond of inosine and guanosine. The full length protein, however, showed weak PNP enzyme activity which may be due to presence of the insert. These results indicate towards the neofunctionalization of PRpnp to a potent trypsin inhibitor through an insert containing inhibitory residue to cater to the needs of plant defense. The similar wound inducible and vegetative storage proteins may have also evolved due to evolutionary needs.

Purine and pyrimidine nucleotide metabolism in higher plants

Purine and pyrimidine nucleotides participate in many biochemical processes in plants. They are building blocks for nucleic acid synthesis, an energy source, precursors for the synthesis of primary products, such as sucrose, polysaccharides, phospholipids, as well as secondary products. Therefore, biosynthesis and metabolism of nucleotides are of fundamental importance in the growth and development of plants. Nucleotides are synthesized both from amino acids and other small molecules via de novo pathways, and from preformed nucleobases and nucleosides by salvage pathways. In this article the biosynthesis, interconversion and degradation of purine and pyrimidine nucleotides in higher plants are reviewed. This description is followed by an examination of physiological aspects of nucleotide metabolism in various areas of growth and organized development in plants, including embryo maturation and germination, in vitro organogenesis, storage organ development and sprouting, leaf senescence, and cultured plant cells. The effects of environmental factors on nucleotide metabolism are also described. This review ends with a brief discussion of molecular studies on nucleotide synthesis and metabolism.

De novo purine synthesis in Arabidopsis thaliana. II. The PUR7 gene encoding 5'-phosphoribosyl-4-(N-succinocarboxamide)-5-aminoimidazole synthetase is expressed in rapidly dividing tissues

The small genome size and excellent genetics of Arabidopsis, as well as the ease with which it is transformed, make it a superb candidate for molecular genetic studies of the purine biosynthetic pathway. Herein we report the isolation, physical characterization, and dissection of the expression patterns of the single gene encoding 5'-phosphoribosyl-4-(N-succinocarboxamide)-5-aminoimidazole synthetase. This enzyme, encoded by the PUR7 gene, catalyzes aspartate addition at the alpha-amino group to the growing purine backbone. The expression of the PUR7 as directed by the 5' region, containing the promoter, mRNA leader, and leader intron, was examined in Arabidopsis using a transgenic reporter system. Our analysis demonstrates that the highest level of purine biosynthesis occurs in mitotically active tissues of the plant. Furthermore, purine biosynthesis appears to be under developmental and hormonal regulation. Inhibition of purine biosynthesis using substrate analogs results in arrested plant development and induction of purine gene expression. Purine nucleotides and their derivatives provide multiple cofactors for a variety of metabolic processes. Our findings begin to identify some of the regulatory mechanisms that affect the production of purine nucleotides in Arabidopsis and may give important insights into nitrogen metabolism in general.

Isolating the Arabidopsis thaliana genes for de novo purine synthesis by suppression of Escherichia coli mutants. I. 5'-Phosphoribosyl-5-aminoimidazole synthetase

We have initiated an investigation of the de novo purine nucleotide biosynthetic pathway in the plant Arabidopsis thaliana. Functional suppression of Escherichia coli auxotrophs allowed the direct isolation of expressed Arabidopsis leaf cDNAs. Using this approach we have successfully suppressed mutants in 4 of the 12 genes in this pathway. One of these cDNA clones, encoding 5'-phosphoribosyl-5-aminoimidazole (AIR) synthetase (PUR5) has been characterized in detail. Analysis of genomic DNA suggests that the Arabidopsis genome contains a single AIR synthetase gene. Analysis of the cDNA sequence and mRNA size suggests that this enzyme activity is encoded by a monofunctional polypeptide, similar to that of bacteria and unlike other eukaryotes. The Arabidopsis AIR synthetase contains a basic hydrophobic transit peptide consistent with transport into chloroplasts. Comparison of both the predicted amino acid and nucleotide sequence from Arabidopsis to those of eight other distant organisms suggests that the plant sequence is more similar to the bacterial sequences than to other eukaryotic sequences. This study provides the groundwork for future investigations into the regulation of de novo purine biosynthesis in plants. Additionally, we have demonstrated that functional suppression of bacterial mutants may provide a useful method for cloning a variety of plant genes.

Positive selection for male-sterile mutants of Arabidopsis lacking adenine phosphoribosyl transferase activity

Three mutants of Arabidopsis thaliana deficient in adenine phosphoribosyl transferase activity were isolated by selecting for germination of seeds on a medium containing 0.1 millimolar 2,6-diaminopurine. In each of the mutants, diaminopurine resistance was due to a recessive nuclear mutation at a locus designated apt. The mutants grow more slowly than wild type, and are male sterile due to abortion of pollen development after the meiotic divisions of the pollen mother cells. The reliability and ease with which the mutants can be selected should afford novel opportunities to investigate purine metabolism, pollen development, and genetic problems which require the ability to select for loss-of-function mutations.

Purine catabolism in plants : purification and some properties of inosine nucleosidase from yellow lupin (lupinus luteus L.) seeds

Inosine nucleosidase (EC 3.2.2.2), the enzyme which hydrolyzes inosine to hypoxanthine and ribose, has been partially purified from Lupinus luteus L. cv. Topaz seeds by extraction of the seed meal with low ionic strength buffer, ammonium sulfate fractionation, and chromatography on aminohexyl-Sepharose, Sephadex G-100, and hydroxyapatite.Molecular weight of the native enzyme is 62,000 as judged by gel filtration. The inosine nucleosidase exhibits optimum activity around pH 8. Energy of activation for inosine hydrolysis estimated from Arrhenius plot is 14.2 kilocalories per mole. The K(m) value computed for inosine is 65 micromolar.AMONG THE INOSINE ANALOGS TESTED, THE FOLLOWING NUCLEOSIDES ARE SUBSTRATES FOR THE LUPIN INOSINE NUCLEOSIDASE: xanthosine, purine riboside (nebularine), 6-mercaptopurine riboside, 8-azainosine, adenosine, and guanosine. The ratio of the velocities measured at 500 micromolar concentration of inosine, adenosine, and guanosine was 100:11:1, respectively. Specificity (V(max)/K(m)) towards adenosine is 48 times lower than that towards inosine.In contrast to the adenosine nucleosidase activity which is absent from lupin seeds and appears in the cotyledons during germination (Guranowski, Pawełkiewicz 1978 Planta 139: 245-247), the inosine nucleosidase is present in both lupin seeds and seedlings.

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.

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.

Purine nucleotide metabolism of germinating soybean embryonic axes

Isolated soybean (Glycine max L. cv. Kent) embyronic axes metabolized [(14)C]glycine to ATP within the 1 hour of imbibition. Radioactivity was not detected in GTP until the 3rd hour. Throughout most of the first 24 hours of germination about 10 to 26 times as much label from [(14)C]glycine appears in ATP as GTP. About five times as much [(14)C]hypoxanthine and [(14)C]inosine was converted into GTP as into ATP in embryonic axes. Two independent pools of IMP appear to be used in purine nucleotide synthesis of soybean axes.

Responses of adenine nucleotides in germinating soybean embryonic axes to exogenously applied adenine and adenosine

The ATP content of soybean (Glycine max [L.] Merr. cv. Kent) axes incubated for 3 hours in 1 mm solutions of adenine and adenosine increased over 100% and 75%, respectively, over axes incubated in water. The increase in ATP was primarily due to the conversion of these purines to nucleotides via the nucleotide salvage pathway. The ATP formed was in a metabolically active pool because label from adenine was incorporated into acid-insoluble material. Adenine also increased the levels of GTP, UTP, and CTP, but not to the extent of the ATP level.

Purification and characterization of adenosine nucleosidase from barley leaves

Adenosine nucleosidase (adenosine ribohydrolase, EC 3.2.2.7) has been purified to a nearly homogeneous state from barley leaves. The enzyme is soluble in concentrated salt solution while it aggregates and precipitates at low ionic strength, factors which enabled a simple purification procedure to be carried out. A molecular weight of 66 000 +/- 3000 was estimated for the native enzyme by gel filtration. In sodium dodecyl sulphate polyacrylamide gel electrophoresis of the most purified fraction a single major band of polypeptide chains, with molecular weight of 33 000, was observed. Thus, the native enzyme seems to be dimer of alpha2 type. The pH optima are 4.7 and 5.4 for citrate and (N-morpholino)ethanesulphonic acid buffers, respectively. Adenine and adenosine protect the enzyme against heat inactivation. The enzyme is resistant to -SH reagents, dithiothreitol inhibits it. The Km for adenosine varied from 0.8 to 2.3 micronM depending on temperature and buffer system. The Km for deoxyadenosine was 120 micronM. Besides adenosine, of several nucleosides tested only adenosine N1-oxide, deoxyadenosine and purine riboside acted as substrates. Adenine as well as its derivatives, including plant hormones (cytokinins), have an inhibitory effect on the enzyme. The Ki values of some modified nucleosides and free bases were determined. The physiological role of adenosine nucleosidase in plants is discussed.

Adenylate metabolism of embryonic axes from deteriorated soybean seeds

RNA and protein syntheses in axes excised from dry soybean (Glycine max L.) seeds at different levels of deterioration were assayed. Low rates of protein synthesis in slightly deteriorated seeds were not due to losses in ribosomal or soluble fraction activities. However, the lowered rates of RNA and protein syntheses of deteriorated seeds were associated with reduced ATP content of the tissues. Adenine and adenosine conversions to ATP were reduced in deteriorated axes, and these reductions were reflected in reduced incorporation of these compounds into RNA.

Content of adenosine phosphates and adenylate energy charge in germinating ponderosa pine seeds

An average of 540 picomoles of total adenosine phosphates was found in the embryo of mature seeds of ponderosa pine (Pinus ponderosa Laws.) and 1140 picomoles in the gametophyte. Adenylate energy charges were 0.44 and 0.26, respectively. After stratification, total adenosine phosphates increased 7-fold and 6-fold in embryo and gametophyte, respectively, and energy charges rose to 0.85 and 0.75. During germination, total adenosine phosphates increased to a 20-fold peak on the 9th day in gametophytic tissue, parallel with the peak of reserve regradation and organellar synthesis, and then decreased. In embryo and seedling, total adenosine phosphates elevated 80-fold with two distinct oscillating increases of AMP and ADP. The oscillating increases occurred before the emergence of radicle and cotyledons during which the highest mitotic index prevailed in all tissues. Energy charges fluctuated between 0.65 at the rapid cell dividing stage to 0.85 at the fully differentiated stage of the seedling, while energy charges remained around 0.75 in the gametophyte. These data indicated that the content of adenosine phosphates of germinating seeds reflects growth, organogenesis, and morphogenesis, and that a compartmentalized energy metabolism must exist in dividing and growing plant cells.