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

Perspectives and challenges in developing small molecules targeting purine nucleoside phosphorylase

As a cytosolic enzyme involved in the purine salvage pathway metabolism, purine nucleoside phosphorylase (PNP) plays an important role in a variety of cellular functions but also in immune system, including cell growth, apoptosis and cancer development and progression. Based on its T-cell targeting profile, PNP is a potential target for the treatment of some malignant T-cell proliferative cancers including lymphoma and leukemia, and some specific immunological diseases. Numerous small-molecule PNP inhibitors have been developed so far. However, only Peldesine, Forodesine and Ulodesine have entered clinical trials and exhibited some potential for the treatment of T-cell leukemia and gout. The most recent direction in PNP inhibitor development has been focused on PNP small-molecule inhibitors with better potency, selectivity, and pharmacokinetic property. In this perspective, considering the structure, biological functions, and disease relevance of PNP, we highlight the recent research progress in PNP small-molecule inhibitor development and discuss prospective strategies for designing additional PNP therapeutic agents.

Interactions of 2,6-substituted purines with purine nucleoside phosphorylase from Helicobacter pylori in solution and in the crystal, and the effects of these compounds on cell cultures of this bacterium

Helicobacter pylori represents a global health threat with around 50% of the world population infected. Due to the increasing number of antibiotic-resistant strains, new strategies for eradication of H. pylori are needed. In this study, we suggest purine nucleoside phosphorylase (PNP) as a possible new drug target, by characterising its interactions with 2- and/or 6-substituted purines as well as the effect of these compounds on bacterial growth. Inhibition constants are in the micromolar range, the lowest being that of 6-benzylthio-2-chloropurine. This compound also inhibits H. pylori 26695 growth at the lowest concentration. X-ray structures of the complexes of PNP with the investigated compounds allowed the identification of interactions of inhibitors in the enzyme’s base-binding site and the suggestion of structures that could bind to the enzyme more tightly. Our findings prove the potential of PNP inhibitors in the design of drugs against H. pylori.

Crystallographic snapshots of ligand binding to hexameric purine nucleoside phosphorylase and kinetic studies give insight into the mechanism of catalysis

Purine nucleoside phosphorylase (PNP) catalyses the cleavage of the glycosidic bond of purine nucleosides using phosphate instead of water as a second substrate. PNP from Escherichia coli is a homohexamer, build as a trimer of dimers, and each subunit can be in two conformations, open or closed. This conformational change is induced by the presence of phosphate substrate, and very likely a required step for the catalysis. Closing one active site strongly affects the others, by a yet unclear mechanism and order of events. Kinetic and ligand binding studies show strong negative cooperativity between subunits. Here, for the first time, we managed to monitor the sequence of nucleoside binding to individual subunits in the crystal structures of the wild-type enzyme, showing that first the closed sites, not the open ones, are occupied by the nucleoside. However, two mutations within the active site, Asp204Ala/Arg217Ala, are enough not only to significantly reduce the effectiveness of the enzyme, but also reverse the sequence of the nucleoside binding. In the mutant the open sites, neighbours in a dimer of those in the closed conformation, are occupied as first. This demonstrates how important for the effective catalysis of Escherichia coli PNP is proper subunit cooperation.

Mycoplasma hyorhinis-encoded purine nucleoside phosphorylase: kinetic properties and its effect on the cytostatic potential of purine-based anticancer drugs

A mycoplasma-encoded purine nucleoside phosphorylase (designated PNPHyor) has been cloned and characterized for the first time. Efficient phosphorolysis of natural 6-oxopurine and 6-aminopurine nucleosides was observed, with adenosine the preferred natural substrate (Km = 61 µM). Several cytostatic purine nucleoside analogs proved to be susceptible to PNPHyor-mediated phosphorolysis, and a markedly decreased or increased cytostatic activity was observed in Mycoplasma hyorhinis-infected human breast carcinoma MCF-7 cell cultures (MCF-7.Hyor), depending on the properties of the released purine base. We demonstrated an ∼10-fold loss of cytostatic activity of cladribine in MCF-7.Hyor cells and observed a rapid and complete phosphorolysis of this drug when it was exposed to the supernatant of mycoplasma-infected cells. This conversion (inactivation) could be prevented by a specific PNP inhibitor. These findings correlated well with the high efficiency of PNPHyor-catalyzed phosphorolysis of cladribine to its less toxic base 2-chloroadenine (Km = 80 µM). In contrast, the cytostatic activity of nucleoside analogs carrying a highly toxic purine base and being a substrate for PNPHyor, but not human PNP, was substantially increased in MCF-7.Hyor cells (∼130-fold for fludarabine and ∼45-fold for 6-methylpurine-2'-deoxyriboside). Elimination of the mycoplasma from the tumor cell cultures or selective inhibition of PNPHyor by a PNP inhibitor restored the cytostatic activity of the purine-based nucleoside drugs. Since several studies suggest a high and preferential colonization or association of tumor tissue in cancer patients with different prokaryotes (including mycoplasmas), the data presented here may be of relevance for the optimization of purine nucleoside-based anticancer drug treatment.

Predicting enzyme targets for cancer drugs by profiling human metabolic reactions in NCI-60 cell lines

BACKGROUND

Drugs can influence the whole metabolic system by targeting enzymes which catalyze metabolic reactions. The existence of interactions between drugs and metabolic reactions suggests a potential way to discover drug targets.

RESULTS

In this paper, we present a computational method to predict new targets for approved anti-cancer drugs by exploring drug-reaction interactions. We construct a Drug-Reaction Network to provide a global view of drug-reaction interactions and drug-pathway interactions. The recent reconstruction of the human metabolic network and development of flux analysis approaches make it possible to predict each metabolic reaction's cell line-specific flux state based on the cell line-specific gene expressions. We first profile each reaction by its flux states in NCI-60 cancer cell lines, and then propose a kernel k-nearest neighbor model to predict related metabolic reactions and enzyme targets for approved cancer drugs. We also integrate the target structure data with reaction flux profiles to predict drug targets and the area under curves can reach 0.92.

CONCLUSIONS

The cross validations using the methods with and without metabolic network indicate that the former method is significantly better than the latter. Further experiments show the synergism of reaction flux profiles and target structure for drug target prediction. It also implies the significant contribution of metabolic network to predict drug targets. Finally, we apply our method to predict new reactions and possible enzyme targets for cancer drugs.

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.

Kinetics of phosphorolysis of 3-(beta-D-ribofuranosyl)adenine and 3-(beta-D-ribofuranosyl)hypoxanthine, non-conventional substrates of purine-nucleoside phosphorylase

The properties of two non-conventional substrates of the calf-spleen and Escherichia coli purine nucleoside phosphorylases (PNP), 3-(beta-D-ribofuranosyl)adenine (RibfAde) and 3-(beta-D-ribofuranosyl)hypoxanthine (RibfHyp), are described. In contrast to Ado, RibfAde is a substrate for the mammalian enzyme. With the calf enzyme, the pseudo-first-order rate constants (Vmax/K(m)) for phosphorolysis of RibfAde and RibfHyp are 3% and 13%, respectively, that for phosphorolysis of Ino, while for E. coli PNP the corresponding values are 22% and 30%, respectively. The Michaelis constants (K(m)) for RibfAde were 800 microM (calf PNP) and 150 microM (E. coli PNP). For RibfHyp, the corresponding K(m) values were 220 microM and 260 microM. Two well-characterized inhibitors of calf spleen PNP [9-(2-fluoro-3,4-dihydroxybutyl)guanine] and E. coli PNP (formycin A) were found to inhibit phosphorolysis of RibfAde and RibfHyp with the same inhibition constants as for Ino. Moreover, the inhibition was competitive, which indicates that phosphorolysis of 3-beta-nucleosides occurs at the same active site as for the natural substrate Ino. In particular, the substrate properties of both 3-beta-nucleosides are consistent with their binding to the enzyme in the conformation anti to the imidazole ring about the glycosidic bond, which is superimposable on the structure of natural 9-beta-nucleosides in the conformation anti to the pyrimidine ring. The results are examined in relation to present concepts regarding the binding of substrates and inhibitors at the active site(s) of these enzymes.