Studies on 3'-nucleotidase-nuclease from potato tubers. I. Purification and some properties of the enzyme

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.

Molecular cloning and characterization of a cDNA encoding endonuclease from potato (Solanum tuberosum)

A cDNA, StEN1, encoding a potato (Solanum tuberosum) endonuclease was cloned and sequenced. The nucleotide sequence of this clone contains an open reading frame of 906 nucleotides encoding a protein of 302 amino acids, and with a calculated molecular mass of 34.4kDa and a Pi of 5.6. The deduced StEN1 protein contains a putative signal sequence of 25 amino acid residues. The StEN1 encoded protein shows substantial homology to both plant and fungal endonucleases isolated and cloned from other sources. The highest identity (73%) was observed with AgCEL I from celery, Apium graveolens, ZEN1 from Zinnia elegans (69%) and DSA6 from daylily, Hemerocallis (68%). RT-PCR expression analysis demonstrated that the potato StEN1 gene is constitutively expressed in potato, although minor differences in expression level in different tissues were observed.

Single-strand-specific nucleases

Single-strand-specific nucleases are multifunctional enzymes and widespread in distribution. Their ability to act selectively on single-stranded nucleic acids and single-stranded regions in double-stranded nucleic acids has led to their extensive application as probes for the structural determination of nucleic acids. Intracellularly, they have been implicated in recombination, repair and replication, whereas extracellular enzymes have a role in nutrition. Although more than 30 single-strand-specific nucleases from various sources have been isolated till now, only a few enzymes (S1 nuclease from Aspergillus oryzae, P1 nuclease from Penicillium citrinum and nucleases from Alteromonas espejiana, Neurospora crassa, Ustilago maydis and mung bean) have been characterized to a significant extent. Recently, some of these enzymes have been cloned, their crystal structures solved and their interactions with different substrates have been established. The detection, purification, characteristics, structure-function correlations, biological role and applications of single-strand-specific nucleases are reviewed.

Sugar non-specific endonucleases

Sugar non-specific endonucleases are multifunctional enzymes and are widespread in distribution. Apart from nutrition, they have also been implicated in cellular functions like replication, recombination and repair. Their ability to recognize different DNA structures has also been exploited for the determination of nucleic acid structure. Although more than 30 non-specific endonucleases have been isolated to date, very little information is available regarding their structure-function correlations except that of staphylococcal and Serratia nucleases. However, during the past few years, the primary structure, nature of the active site based on sequence homology, and the probable mechanism of action have been postulated for some of the enzymes. This review describes the purification, characteristics, biological role and applications of sugar non-specific endonucleases.

Single-strand-specific nucleases

Single-strand-specific nucleases, which act on single-stranded nucleic acids and single-stranded regions in double-stranded nucleic acids, are multifunctional enzymes and are ubiquitous in distribution. They find wide application as analytical tools in molecular biology research, although enzymes such as P1 nuclease are also used for production of flavor enhancers such as 5' IMP and 5' GMP. Because these enzymes are mainly used as analytical tools, very little attention was paid to aspects relating to their structure-function relationships. However, during the last few years considerable developments have taken place in this area. Single-strand-specific nucleases, their purification, characteristics, biological role, and applications have been reviewed.

Purification and characterization of cytoplasmic 5'(3')-nucleotidase from rabbit spleen: characteristic differences of the enzyme from the rat spleen nucleotidase

1. A 5'(3')-nucleotidase, which preferably hydrolyzed 3'-dTMP and 3'-dUMP, was highly purified from rabbit spleen cytosol. 2. The enzyme also hydrolyzes 3'-UMP, 5'-dUMP and guanine nucleotides, but does not hydrolyze any adenine nucleotides or cytosine nucleotides. 3. The activity is dependent upon Mg2+, Co2+ or Mn2+; the addition of deoxyinosine stimulates the activity, and the pH optimum for the hydrolysis of 3'-dTMP is 7.0. 4. Although the catalytic properties of the enzyme are similar to the 5'(3')-nucleotidase from rat spleen, these nucleotidases differ in their molecular disposition. 5. The charge state of the rabbit enzyme is slightly basic, and the subunit M(r) is about 27 kDa, while the value of the rat enzyme is 26 kDa. 6. Immunochemical experiments with the use of antibodies against the purified nucleotidase indicate that enzymes from rabbit spleen and from rat spleen are composed of different polypeptides.

3'-nucleotidase activity in procyclic and bloodstream stages of Trypanosoma rhodesiense

Both procyclic and bloodstream-derived trypanosomes of Trypanosoma rhodesiense exhibit a nucleotidase activity which is capable of hydrolyzing 3'-ribonucleotides. Nucleotidase activity in assays employing 5'-nucleotide substrates is not detectable in preparations of either phenotype. The 3'-nucleotidase activity in lysates of procyclic trypanosomes is pelletable at 100,000 g, whereas that activity from bloodstream forms is operationally soluble under the conditions employed. The 3'-nucleotidase activities of both stages share the following characteristics: pH optimum of 8.5, substrate preference for purine ribonucleotides, resistance to commonly used phosphatase inhibitors, including fluoride, tartrate and molybdate ions. Both activities are reversibly inhibited by EDTA and reactivated by Co2+ ions. As visualized by fine-structure cytochemical methods, 3'-nucleotidase activity was distributed over the entire surface of procyclic trypanosomes, but not bloodstream parasites bearing an intact surface coat.