Purification and Properties of Purine Nucleoside Phosphorylase from Salmonella typhimurium

The comparative analysis of the properties and structures of purine nucleoside phosphorylases from thermophilic bacterium Thermus thermophilus HB27

Two recombinant purine nucleoside phosphorylases from thermophilic bacterium Thermus thermophilus HB27 encoded by genes TT_C1070 (TthPNPI) and TT_C0194 (TthPNPII) were purified and characterized. The comparative analysis of their sequences, molecular weight, enzymes specificity and kinetics of the catalyzed reaction were realized. As a result, it was determined that the TthPNPI is specific to guanosine while the TthPNPII to adenosine. According to the results of the size exclusion chromatography and SAXS study both enzymes are hexameric molecules. Based on the sequence alignment with homologous purine nucleoside phosphorylases (PNPs), Asn was identified as a purine base recognizing residue in the active site of TthPNPI and Asp in TthPNPII. The three-dimensional structure of TthPNPII was solved at 2.5 Å resolution by molecular replacement method using crystals grown in microgravity. Position of phosphate in the active site cavity is located. The possible arrangement of adenosine and guanosine in TthPNPII active site cavity is considered using superposition with the structures of homologous trimeric and hexameric PNPs complexed with corresponding substrates. The peculiarities of oligomeric structure of TthPNPII in comparison with homologous PNPs are described. It is shown that two trimeric molecules of TthPNPII in the asymmetric part of the unit cell are connected by three two-fold axis into a hexamer with 32-point symmetry. This type of hexameric structure of PNP is found for the first time. The interface area between the subunits in trimeric molecule and between the trimers in TthPNPII hexamer is described.Communicated by Ramaswamy H. Sarma.

Towards understanding the E. coli PNP binding mechanism and FRET absence between E. coli PNP and formycin A

The aim of this study is threefold: (1) augmentation of the knowledge of the E. coli PNP binding mechanism; (2) explanation of the previously observed 'lack of FRET' phenomenon and (3) an introduction of the correction (modified method) for FRET efficiency calculation in the PNP-FA complexes. We present fluorescence studies of the two E. coli PNP mutants (F159Y and F159A) with formycin A (FA), that indicate that the aromatic amino acid is indispensable in the nucleotide binding, additional hydroxyl group at position 159 probably enhances the strength of binding and that the amino acids pair 159-160 has a great impact on the spectroscopic properties of the enzyme. The experiments were carried out in hepes and phosphate buffers, at pH7 and 8.3. Two methods, a conventional and a modified one, that utilizes the dissociation constant, for calculations of the energy transfer efficiency (E) and the acceptor-to-donor distance (r) between FA and the Tyr (energy donor) were employed. Total difference spectra were calculated for emission spectra (λ_ex 280nm, 295nm, 305nm and 313nm) for all studied systems. Time-resolved techniques allowed to conclude the existence of a specific structure formed by amino acids at positions 159 and 160. The results showed an unexpected pattern change of FRET in the mutants, when compared to the wild type enzyme and a probable presence of a structure created between 159 and 160 residue, that might influence the binding efficiency. Additionally, we confirmed the indispensable role of the modification of the FRET efficiency (E) calculation on the fraction of enzyme saturation in PNP-FA systems.

Nucleotides, Nucleosides, and Nucleobases

We review literature on the metabolism of ribo- and deoxyribonucleotides, nucleosides, and nucleobases in Escherichia coli and Salmonella,including biosynthesis, degradation, interconversion, and transport. Emphasis is placed on enzymology and regulation of the pathways, at both the level of gene expression and the control of enzyme activity. The paper begins with an overview of the reactions that form and break the N-glycosyl bond, which binds the nucleobase to the ribosyl moiety in nucleotides and nucleosides, and the enzymes involved in the interconversion of the different phosphorylated states of the nucleotides. Next, the de novo pathways for purine and pyrimidine nucleotide biosynthesis are discussed in detail.Finally, the conversion of nucleosides and nucleobases to nucleotides, i.e.,the salvage reactions, are described. The formation of deoxyribonucleotides is discussed, with emphasis on ribonucleotidereductase and pathways involved in fomation of dUMP. At the end, we discuss transport systems for nucleosides and nucleobases and also pathways for breakdown of the nucleobases.

Tumor-targeting Salmonella typhimurium, a natural tool for activation of prodrug 6MePdR and their combination therapy in murine melanoma model

The PNP/6-methylpurine 2'-deoxyriboside (6MePdR) system is an efficient gene-directed enzyme prodrug therapy system with significant antitumor activities. In this system, Escherichia coli purine nucleoside phosphorylase (ePNP) activates nontoxic 6MePdR into potent antitumor drug 6-methylpurine (6MeP). The Salmonella typhimurium PNP (sPNP) gene has a 96-% sequence homology in comparison with ePNP and also has the ability to convert 6MePdR to 6MeP. In this study, we used tumor-targeting S. typhimurium VNP20009 expressing endogenous PNP gene constitutively to activate 6MePdR and a combination treatment of bacteria and prodrug in B16F10 melanoma model. The conversion of 6MePdR to 6MeP by S. typhimurium was analyzed by HPLC and the enzyme activity of sPNP was confirmed by in vitro (tetrazolium-based colorimetric assay) MTT cytotoxicity assay. After systemic administration of VNP20009 to mice, the bacteria largely accumulated and specifically delivered endogenous sPNP in the tumor. In comparison with VNP20009 or 6MePdR treatment alone, combined administration of VNP20009 followed by 6MePdR treatment significantly delayed the growth of B16F10 tumor and increased the CD8(+) T-cell infiltration. In summary, our results demonstrated that the combination therapy of S. typhimurium and prodrug 6MePdR is a promising strategy for cancer therapy.

Proteomic approach for characterization of hop-inducible proteins in Lactobacillus brevis

Resistance to hops is a prerequisite for the capability of lactic acid bacteria to grow in beer and thus cause beer spoilage. Bactericidal hop compounds, mainly iso-alpha-acids, are described as ionophores which exchange H+ for cellular divalent cations, e.g., Mn2+, and thus dissipate ion gradients across the cytoplasmic membrane. The acid stress response of Lactobacillus brevis TMW 1.465 and hop adaptation in its variant L. brevis TMW 1.465A caused changes at the level of metabolism, membrane physiology, and cell wall composition. To identify the basis for these changes, a proteomic approach was taken. The experimental design allowed the discrimination of acid stress and hop stress. A strategy for improved protein identification enabled the identification of 84% of the proteins investigated despite the lack of genome sequence data for this strain. Hop resistance in L. brevis TMW 1.465A implies mechanisms to cope with intracellular acidification, mechanisms for energy generation and economy, genetic information fidelity, and enzyme functionality. Interestingly, the majority of hop-regulated enzymes are described as manganese or divalent cation dependent. Regulation of the manganese level allows fine-tuning of the metabolism, which enables a rapid response to environmental (stress) conditions. The hop stress response indicates adaptations shifting the metabolism into an energy-saving mode by effective substrate conversion and prevention of exhaustive protein de novo synthesis. The findings further demonstrate that hop stress in bacteria not only is associated with proton motive force depletion but obviously implies divalent cation limitation.

Purine nucleoside phosphorylases: properties, functions, and clinical aspects

The ubiquitous purine nucleoside phosphorylases (PNPs) play a key role in the purine salvage pathway, and PNP deficiency in humans leads to an impairment of T-cell function, usually with no apparent effects on B-cell function. This review updates the properties of the enzymes from eukaryotes and a wide range of prokaryotes, including a tentative classification of the enzymes from various sources, based on three-dimensional structures in the solid state, subunit composition, amino acid sequences, and substrate specificities. Attention is drawn to the compelling need of quantitative experimental data on subunit composition in solution, binding constants, and stoichiometry of binding; order of ligand binding and release; and its possible relevance to the complex kinetics exhibited with some substrates. Mutations responsible for PNP deficiency are described, as well as clinical methods, including gene therapy, for corrections of this usually fatal disease. Substrate discrimination between enzymes from different sources is also being profited from for development of tumour-directed gene therapy. Detailed accounts are presented of design of potent inhibitors, largely nucleosides and acyclonucleosides, their phosphates and phosphonates, particularly of the human erythrocyte enzyme, some with Ki values in nanomolar and picomolar range, intended for induction of the immunodeficient state for clinical applications, such as prevention of host-versus-graft response in organ transplantations. Methods of assay of PNP activity are reviewed. Also described are applications of PNP from various sources as tools for the enzymatic synthesis of otherwise inaccessible therapeutic nucleoside analogues, as coupling enzymes for assays of orthophosphate in biological systems in the micromolar and submicromolar ranges, and for coupled assays of other enzyme systems.

Crystal structure of the ternary complex of E. coli purine nucleoside phosphorylase with formycin B, a structural analogue of the substrate inosine, and phosphate (Sulphate) at 2.1 A resolution

The ternary complex of purine nucleoside phosphorylase from E. coli with formycin B and a sulphate or phosphate ion crystallized in the hexagonal space group P6122 with unit cell dimensions a=123.11, c=241.22 A and three monomers per asymmetric unit. The biologically active hexamer is formed through 2-fold crystallographic symmetry, constituting a trimer of dimers. High-resolution X-ray diffraction data were collected using synchrotron radiation (Daresbury, England). The crystal structure was determined by molecular replacement and refined at 2.1 A resolution to an R-value of 0.196. There is one active centre per monomer, composed of residues belonging to two subunits of one dimer. The phosphate binding site is strongly positively charged and consists of three arginine residues (Arg24, Arg87 and Arg43 from a neighbouring subunit), Ser90 and Gly20. It is occupied by a sulphate or phosphate anion, each oxygen atom of which accepts at least two hydrogen bonds or salt-bridges. The sulphate or phosphate anion is also in direct contact with the ribose moiety of formycin B. The ribose binding site is composed of Ser90, Met180, Glu181 and His4, the latter belonging to the neighbouring subunit. The base binding site is exposed to solvent, and the base is unspecifically bound through a chain of water molecules and aromatic-aromatic interactions. In all monomers the nucleosides are in the high syn conformation about the glycosidic bonds with chi in the range 100 to 130 degrees. The architecture of the active centre is in line with the known broad specificity and the kinetic properties of E. coli PNP.

2-Chloro-2'-deoxyadenosine (cladribine) and its analogues are good substrates and potent selective inhibitors of Escherichia coli purine-nucleoside phosphorylase

2-Chloro-2'-deoxyadenosine (CldAdo), a nucleoside that has proven useful in the treatment of several chronic lymphoid malignancies, and its analogue, 2-bromo-2'-deoxyadenosine, are both effective inhibitors of the bacterial (Escherichia coli) purine-nucleoside phosphorylase (PNP), with Ki values of 4.5 microM and 6.3 microM, respectively. The examination of a series of base-modified analogues of CldAdo has shown that several other compounds have similar inhibitor properties, and has indicated that 6-benzyloxy-2-chloro-9-(2'-deoxy-beta-D-ribofuranosyl)purine is the most potent inhibitor with a Ki value of 0.5 microM, competitive with respect to inosine (Ino). CldAdo itself and its base-modified analogues, discounting those substituted at C(8), are also substrates for the E. coli PNP and undergo rapid glycosidic bond cleavage. CldAdo is degraded with substrate efficiency, i.e. Vmax/Km similar to that observed for Ino (130%), although the individual kinetic constants, Km and Vmax, are both approximately an order of magnitude lower than for Ino. All compounds tested are totally inactive as substrates and inhibitors for mammalian (calf spleen) PNP and therefore constitute a new class of potent selective, although cleavable, inhibitors of bacterial phosphorylases. 8-Bromo-2-chloro-2'-deoxyadenosine and 8-thio-2-chloro-2'-deoxyadenosine are the only base-modified CldAdo derivatives showing inhibitory activity against MOLT-3 (acute T-cell leukemia) and U-937 (histiocytic lymphoma) cells and, as shown in this study, are resistant to degradation by E. coli PNP. The above-mentioned results suggest that both analogues could be effective as oral cytotoxic agents that are noncleavable by enteric bacteria.

AdoP assay detection of mycoplasmal contamination in biological media

The AdoP assay has been shown to be reliable in controlling the absence of mycoplasmas in filtered sera. We therefore compared its effectiveness with that of other tests in the detection of animal cell contamination by mycoplasmas.

Synthesis of deoxyribomononucleotides in Mollicutes: dependence on deoxyribose-1-phosphate and PPi

Cell extracts of Acholeplasma laidlawii B-PG9, Acholeplasma morum S2, Mycoplasma capricolum 14, and Mycoplasma gallisepticum S6 were examined for 37 cytoplasmic enzyme activities involved in the salvage and biosynthesis of purines. All of these organisms had adenine phosphoribosyltransferase activity (EC 2.4.2.7) and hypoxanthine phosphoribosyltransferase activity (EC 2.4.2.8). All of these organisms had purine-nucleoside phosphorylase activity (EC 2.4.2.1) in the synthetic direction using ribose-1-phosphate (R-1-P) or deoxyribose-1-phosphate (dR-1-P); this activity generated ribonucleosides or deoxyribonucleosides, respectively. The pyrimidine nucleobase uracil could also be ribosylated by using either R-1-P or dR-1-P as a donor. The synthesis of deoxyribonucleosides from nucleobases and dR-1-P has been reported from only one other procaryote, Escherichia coli (L. A. Mason and J. O. Lampen, J. Biol. Chem. 193:539-547, 1951). The reverse of this phosphorylase reaction is more widely known, and we found such activity in all mollicutes studied. Some Acholeplasma species but not the Mycoplasma species can phosphorylate deoxyribonucleosides to deoxyribomononucleotides by a PPi-dependent deoxyribonucleoside kinase activity, which was first reported in this group for the ribose analogs (V. V. Tryon and J. D. Pollack, Int. J. Syst. Bacteriol. 35:497-501, 1985). This is the first report of PPi-dependent purine deoxyribonucleoside kinase activity. An ATP-dependent purine deoxyribonucleoside kinase activity is known only in salmon milt extracts (H. L. A. Tarr, Can. J. Biochem. 42:1535-1545, 1964). Deoxyribomononucleotidase activity was also found in cytoplasmic extracts of these mollicutes. This is the first report of deoxyribomononucleotidase activity.

Trypanosoma cruzi adenine nucleoside phosphorylase. Purification and substrate specificity

An adenine nucleoside phosphorylase has been partially purified from extracts of epimastigotes of the Peru strain of Trypanosoma cruzi, the causative agent of Chagas' disease. The purification procedure separated this enzyme from the three other nucleoside-cleaving enzymes found in extracts. The adenine nucleoside phosphorylase, which efficiently cleaved 5'-deoxy-5'-methylthioadenosine (MTA), had a particle weight of 68,000 and exhibited a broad pH optimum between pH 6 and 8. In addition to MTA, the purified enzyme cleaved and synthesized adenosine and 2'-deoxyadenosine with high efficiency. This contrasts to the enzyme from S-180 cells which has been reported to cleave adenosine poorly and not to cleave 2'-deoxyadenosine. Several observations suggested that the three substrates, MTA, adenosine and 2'-deoxyadenosine, use a common catalytic site: (a) all served as alternate-substrate inhibitors exhibiting mutually competitive inhibition with Ki values equivalent to their respective Km values, (b) 5'-chloroformycin A exhibited a competitive Ki value of 4 microM with each nucleoside substrate, and (c) the Km value of phosphate derived from initial velocity studies (180 +/- 20 microM) was independent of the nucleoside substrate. Substrate specificity studies in both the synthesis and cleavage direction indicated that the enzyme had a broad specificity for bases and nucleosides. For the synthesis of nucleosides, the enzyme demonstrated a preference for an amino group in the position equivalent to the 6 position of purine. Compounds containing a hydroxyl group in this position were not substrates. Although a hydrogen or methyl group could substitute for a 6-amino group, a marked decrease in substrate efficiency was observed with these compounds. Alterations in the purine ring led to decreases in the maximal velocity values as evidenced by the substrate or nonsubstrate properties of 1-, 3-, and 7-deazaadenine and 4-aminopyrazolo[3,4-d]pyrimidine. The Km values for 5-methylthioribose 1-phosphate, ribose 1-phosphate and 2'-deoxyribose 1-phosphate with adenine serving as acceptor were 21, 150 and 370 microM. For nucleoside cleavage, the T. cruzi enzyme catalyzed the phosphorolysis of a variety of 5'-substituted adenine-containing nucleosides including those possessing 5'-hydrogen-, hydroxyl-, halogeno-, alkylthio-, amino- and azido-moieties. Inclusion of an ionized group in the 5'-position, such as 5'-carboxy-5'-deoxyadenosine or AMP, precluded substrate activity. 3'-Deoxyadenosine, arabinosyladenine and alpha-adenosine did not serve as substrates.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine accumulation in Saccharomyces cerevisiae cultured in medium containing low levels of adenine

By monitoring the in vivo incorporation of low concentrations of radiolabeled adenine into acid-soluble compounds, we observed the unusual accumulation of two nucleosides in Saccharomyces cerevisiae that were previously considered products of nucleotide degradation. Under the culture conditions used in the present study, radiolabeled adenosine was the major acid-soluble intracellular derivative, and radiolabeled inosine was initially detected as the second most prevalent derivative in a mutant lacking adenine aminohydrolase. The use of yeast mutants defective in the conversion of adenine to hypoxanthine or to AMP renders very unlikely the possibility that the presence of adenosine and inosine is attributable to nucleotide degradation. These data can be explained by postulating the existence of two enzyme activities not previously reported in S. cerevisiae. The first of these activities transfers ribose to the purine ring and may be attributable to purine nucleoside phosphorylase (EC 2.4.2.1) or adenosine phosphorylase (EC 2.4.2.-). The second enzyme converts adenosine to inosine and in all likelihood is adenosine aminohydrolase (EC 3.5.4.4).

Purine nucleoside phosphorylase polymorphism in the genus Littorina (Prosobranchia: Mollusca)

Examination of eight Atlantic species of the genus Littorina by starch gel electrophoresis of purine nucleoside phosphorylase revealed extensive polymorphism within the L. saxatilis complex. In this group, four alleles have been identified. Heterozygotes are four banded, and thus, as in vertebrates, the enzyme is likely to be a trimer. Breeding experiments confirmed the genetic interpretation of the phenotype patterns. Where species of the saxatilis complex [L. saxatilis (=L. rudis), L. arcana, L. nigrolineata, L. neglecta] are sympatric, there are sometimes significant allele frequency differences between them. A fifth allele was present at a high frequency in L. obtusata and L. mariae, and L. littorea and L. neritoides each possessed unique alleles. A total of eight alleles was identified. Densitometric scanning of heterozygote patterns pointed to activity differences between alleles and also showed that, while the heterotrimeric bands were never less intense than the homotrimeric bands, the heterotrimeric bands were sometimes less intense than expected. It is not clear whether this represents nonrandom association of subunits, decreased stability of heterotrimers, or simply an artifact of the staining and quantifying process.

Adenosine phosphorylase activity in mycoplasma-free growth media for mammalian cells

Mammalian cells have enzymes that deaminate adenosine to inosine, which can readily be phosphorolysed to hypoxanthine. They do not, however, possess enzymes to form adenine by the cleavage of adenosine. For this reason, the release of adenine from adenosine by mammalian cell cultures has usually been interpreted as indicating the presence of mycoplasma, a frequent microbial contaminant that contains high levels of adenosine phosphorylase. We found that some human lymphoblast cultures free of mycoplasma showed high levels of adenosine cleavage and that this activity resulted from adenosine phosphorylase in the bovine serum used as the culture growth supplement. A survey of 13 serum supplements disclosed that fetal bovine serum (six lots) contains the highest adenosine phosphorylase activity, ranging from 9 to 648 nmol adenine produced per hour per ml serum; newborn calf serum (four lots) has much less activity, ranging from 0 to 5 nmol adenine produced per hour per ml serum; and donor horse serum (three lots) contains no detectable activity. These results suggest that mycoplasma tests dependent on the presence of adenosine phosphorylase or other enzyme activities may give false-positives with cultures containing fetal bovine serum supplements.

Purine nucleoside phosphorylase variation in the brook lamprey, Lampetra planeri (Bloch) (Petromyzone, Agnatha): evidence for a trimeric enzyme structure

Genetic evidence for a trimeric structure for purine nucleoside phosphorylase in the brook lamprey is presented. This enzyme is encoded by a single locus with two alleles segregating at frequencies of 0.98 and 0.02 in a Welsh population. It is suggested that this enzyme is likely to be a trimer in all classes of vertebrates.

Two purine nucleoside phosphorylases in Bacillus subtilis. Purification and some properties of the adenosine-specific phosphorylase

Two purine nucleoside phosphorylases (purine-nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) were purified from vegetative Bacillus subtilis cells. One enzyme, inosine-guanosine phosphorylase, showed great similarity to the homologous enzyme of Bacillus cereus. It appeared to be a tetramer of molecular weight 95 000. The other enzyme, adenosine phosphorylase, was specific for adenosine and deoxyadenosine. The molecular weight of the native enzyme was 153 000 +/- 10% and the molecular weight of the subunits was 25 500 +/- 5%. This indicates a hexameric structure. The adenosine phosphorylase was inactivated by 10(-3) M p-chloromercuribenzoate and protected against this inactivation by phosphate, adenosine and ribose 1-phosphate.

Irreversible inhibition of hypoxanthine phosphoribosyltransferase. Further studies on the specificity of periodate-oxidized GMP

Inactivation of hypoxanthine phosphoribosyltransferase caused by periodate-oxidized GMP is irreversible, even under the conditions of polyacrylamide gel electrophoresis and during affinity chromatography on GMP-Sepharose. Partial binding of the inhibitor to the enzyme protein can be demonstrated on dodecyl sulfate gel electrophoresis: The substrate, phosphoribosyl diphosphate in the presence of Mg2, and the product GMP protect the enzyme against inactivation. Periodate-oxidized GMP, AMP and oxidized purine nucleosides do not influence ribosephosphate pyrophosphokinase, 5'-nucleotidase, purine-nucleoside phosphorylase and guanylate kinase. A variety of other purine nucleosides and nucleotides, tested in their periodateoxidized form, do not lead to a compound comparable or superior to oxidized GMP in its effect on hypoxanthine phosphoribosyltransferase. In an erythrocyte system it is clearly demonstrated that oxidized GMP cannot act across an intact cell membrane.

Adenosine phosphorylase activity in a mutant HEp-2 cell line contaminated with Mycoplasm hyorhinis

Metabolic studies in HEp-2/MP,MIR cells (an adenosine kinase, hypoxanthine phosphoribosyltransferase negative mutant) indicated the presence of adenosine phosphorylase activity. This activity, unknown in established mammalian cell lines, resulted in the glycosidic cleavage of both adenosine and the antiviral drug arabinosyladenine. The activity was observed readily in the presence or absence of the adenosine deaminase inhibitor conformycin. Isopycnic separation of [3H] thymidine-labeled DNA species in CsCl density gradients resulted in the appearance of two distinct peaks. The heavier peak coincided with [14C]thymidine-labeled marker DNA of human origin, whereas the lighter peak was within the range associated with mycoplasmal DNA. Testing by commercial laboratories confirmed the presence of mycoplasma in HEp-2/MP,MIR cells. The contaminant was identified as Mycoplasma hyorhinis, a porcine mycoplasma. Following gamma-irradiation (3000 rads) to block cellular mitosis, the mucoplasma-contaminated HEp-2/MP,MIR cells were cocultivated with mycoplasma-free wild-type HEp-2 cells which did not exhibit adenosine phosphorylase activity. Following serial cocultivation in a medium designed to favor the survival of the wild-type cells, adenosine phosphorylase activity was found in the previously uninfected cells. Studies of this nature emphasize the need for investigators to carefully monitor their cell lines for mycoplasma.

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.

Purine-nucleoside phosphorylase from Salmonella typhimurium and Escherichia coli. Initial velocity kinetics, ligand banding, and reaction mechanism

Purine nucleoside phosphorylase from Salmonella typhimurium has been subjected to kinetic analysis i.e. determination of initial velocity patterns and product inhibition studies. The kinetic results suggest that the enzyme works by a sequential reaction mechanism, where the nucleoside, phosphate, and pentose 1-phosphate are all able to bind to the free enzyme, whereas it appears that the purine base binds after addition of the pentose 1-phosphate. The proposed mechanism is confirmed by substrate binding studies. In addition to the enzyme-substrate complexes suggested by the kinetics, the binding studies revealed a 'dead end' complex, consisting of enzyme, phosphate, and purine base. Similar binding experiments were carried out using the enzyme from Escherichia coli. The results suggest that this enzyme works by an identical reaction mechanism. The binding data are in agreement with the presence of six binding sites per native enzyme molecule, one binding site per subunit, for each ligand. Both enzymes show normal Michaelis-Menten kinetics for their substrates with the exception of phosphate, for which the double-reciprocal plots are concave down. This behaviour is seen in both binding and velocity curves, and most likely is a result of negative cooperativity in the binding of phosphate to the enzyme.

Purine nucleoside phosphorylase from Escherichia coli and Salmonella typhimurium. Purification and some properties

The purine nucleoside phosphorylases from Escherichia coli and from Salmonella typhimurium have been purified to electrophoretic homogeneity and crystallized. Comparative studies revealed that the two enzymes are very much alike. They obey simple Michaelis-Menten kinetics for their substrates with the exception of phosphate for which they show negative cooperativity. Gel filtration on Sephadex G-200 of the native enzymes revealed a molecular weight for both enzymes of 138000 plus or minus 10%. By use of dodecylsulphate gel electrophoresis a subunit molecular weight of 23700 plus or minus 5% was determined, suggesting that both enzymes consist of six subunits of equal molecular weight. When the subunits were partially crosslinked with dimethyl suberimidate before dodecylsulphate electrophoresis six protein bands were observed in agreement with the proposed oligomeric state of the enzyme, consisting of six subunits of equal molecular weight. Analysis of the amino acid composition also indicates that the subunits are identical. 6M guanidinium chloride dissociates the enzymes; association experiments with native and succinylated enzymes suggested that only the hexameric form is active. Both enzymes could be dissociated into subunits by p-chloromercuribenzoate; this dissociation is prevented by the substrates: the nucleosides, the pentose 1-phosphates, and mixtures of phosphate and purine bases.

Genetic analysis of thymidine-resistant and low-thymine-requiring mutants of Escherichia coli K-12 induced by bacteriophage Mu-1

Four genes, dra, tpp, drm, and pup, that specify enzymes involved in the catabolism of nucleosides and deoxynucleosides in Escherichia coli are known to be very closely linked in the order dra-tpp-drm-pup. By infecting cells with the phage Mu-1 and isolating low-thymine-requiring derivatives of a strain lacking thymidylate synthetase and also thymidine-resistant mutants of a dra-strain, it has been possible to select for strains in which Mu-1 is inserted in this gene cluster. Making use of the polar effect of Mu-induced mutations on more distal genes in the same transcriptional unit, evidence is presented that dra and tpp are co-transcribed from a promoter to the left of dra, and drm and pup are co-transcribed from a promotor located between tpp and drm. Residual levels of purine nucleoside phosphorylase in drm- mutants induced by phage Mu seem to indicate that a weak promotor lies between drm and pup. From a strain in which Mu-1 is inserted in drm, a mutant has been isolated that has a deletion extending into tpp. Since this strain lacks thymidylate synthetase, it is unable to grow on minimal medium containing thymine. Mutants isolated from this strain that can grow on minimal medium containing thymine have been shown to have increased levels of the enzyme uridine phosphorylase.