Purification and characterization of uridine and thymidine phosphorylase from Lactobacillus casei

Uridine and thymidine phosphorylases have been purified to homogeneity from crude extracts of Lactobacillus casei. Both enzymes had an apparent molecular mass of about 80 kDa. Uridine phosphorylase consisted of four identical subunits while thymidine phosphorylase was composed of two identical ones. The sequence of 23 amino-acid residues from its N-terminal end was analyzed. Uridine phosphorylase had a Km of 5.0 x 10(-3) M for uridine and 1.24 x 10(-1) M for phosphate, while thymidine phosphorylase had a Km of 1.32 x 10(-1) M for thymidine and 1.0 x 10(-1) M for phosphate. Uridine phosphorylase was equally active with uridine and 5-methyluridine, but had a low activity towards thymidine. Its activity was inhibited competitively by 3-O-methyl-alpha D-glucopyranoside, on the other hand thymidine phosphorylase activity was not affected by this compound. Thymidine phosphorylase showed specificity towards the deoxyribosyl moiety of the substrate. In addition, it required a nonsubstituted pyrimidine moiety or one which was substituted in position 5. The pattern of the double-reciprocal plots of the initial velocities vs. the concentrations of either one of the substrates, and the product inhibition kinetics, indicated that the catalytic mechanism of both enzymatic reactions is sequential rather than Ping-Pong and that the sequence of the addition of the substrates is random (rapid equilibrium). In the case of the uridine phosphorylase-catalyzed reaction, the products are also released randomly, while in the thymidine phosphorylase-catalyzed reaction deoxyribose 1-phosphate is released after thymine.     

Enzymatic activities of uridine and thymidine phosphorylase in normal and cancerous uterine cervical tissues

In this study, the preliminary analyses were conducted of enzymatic activities of uridine phosphorylase (UP) and thymidine phosphorylase (TP) in normal tissues and cancer tissues of the uterine cervix. The study was performed on 27 patients of cervical cancer, treated first in our hospital. Normal cervical tissues obtained from 15 patients undergoing hysterectomy for benign diseases were used as controls. The supernatant of the homogenated cervical tissues and the stroma (5-FU and ribose-1-P or deoxyribose-1-P) were analyzed by high performance liquid chromatography, and then the UP and TP activities calculated. TP activity was significantly greater than UP activity (P < 0.0001). Both UP and TP showed significantly greater activity in cancer tissues than in normal tissues (P < 0.0001). In the TP activity of the cancer tissues, there was no significant difference among the histological types, while the TP activity tended to be significantly higher in the cases with lymph node metastasis. These results showed that the TP-mediated route seemed important as the 5FU metabolic pathway in the uterine cervical tissues, and TP enzymatic activity might be associated with lymph node metastasis.     

The role of thymidine phosphorylase and uridine phosphorylase in (fluoro)pyrimidine metabolism in peripheral blood mononuclear cells

Thymidine phosphorylase (TP) and uridine phosphorylase (UP) catalyze the (in)activation of several fluoropyrimidines, depending on their catalytic activity and substrate specificity. Blood cells are the first compartment exposed to most anticancer agents. The role of white blood cells in causing toxic side effects and catalyzing drug metabolism is generally underestimated. Therefore we determined the contribution of the white blood cell compartment to drug metabolism, and we investigated the activity and substrate specificity of TP and UP for the (fluoro)pyrimidines thymidine (dThd), uridine (Urd), 5'-deoxy-5-fluorouridine (5' dFUrd) and 5-fluorouracil (5FU) in peripheral blood mononuclear cells (PBMC) and undifferentiated monocytes and differentiated monocytes: macrophages and dendritic cells. PBMC had an IC50 of 742 microM exposed to 5'dFUrd, increasing to > 2000 microM when both TP and UP activities were inhibited. Total phosphorolytic activity was higher with dThd than with Urd, 5'dFUrd or 5FU. Using a specific TP inhibitor (TPI) and UP inhibitor (BAU) we concluded that dThd and Urd were preferentially converted by TP and UP, respectively, while 5'dFUrd and 5FU were mainly converted by TP (about 80%) into 5FU and FUrd, respectively. 5FU was effectively incorporated into RNA. dThd conversion into thymine was highest in dendritic cells (52.6 nmol thymine/h/10(6) cells), followed by macrophages (two-fold) and undifferentiated monocytes (eight-fold). TPI prevented dThd conversion almost completely. In conclusion, PBMC were relatively insensitive to 5'dFUrd, and the natural substrates dThd and Urd were preferentially converted by TP and UP, respectively. TP and UP were both responsible for converting 5'dFUrd/5FU into 5FU/FUrd, respectively.     

Abnormalities in uridine homeostatic regulation and pyrimidine nucleotide metabolism as a consequence of the deletion of the uridine phosphorylase gene

We report in the present study the critical role of uridine phosphorylase (UPase) in uridine homeostatic regulation and pyrimidine nucleotide metabolism, employing newly developed UPase-/- mice. Our data demonstrate that the abrogation of UPase activity led to greater than a 6-fold increase in uridine concentrations in plasma, a 5-6-fold increase in lung and gut, and a 2-3-fold increase in liver and kidney, as compared with wild type mice. Urine uridine levels increased 24-fold normal in UPase-/- mice. Uridine half-life and the plasma retention of pharmacological doses of uridine were significantly prolonged. Further, in these UPase-/- mice, abnormal uridine metabolism led to disorders of various nucleotide metabolisms. In the liver, gut, kidney, and lung of UPase-/- mice, total uridine ribonucleotide concentrations increased 2-3 times as compared with control mice. Cytidine ribonucleotides and adenosine and guanosine ribonucleotides also increased, although to a lesser extent, in these organs. Most significant deoxyribonucleotide changes were present in the gut and lung of UPase-/- mice. In these tissues, dTTP concentration increased more than 4-fold normal, and dCTP, dGTP, and dATP concentrations rose 1-2 times normal. In kidney, dTTP concentration increased 2-fold normal, and dCTP and dGTP concentrations rose less than 1-fold normal. In addition, the accumulated uridine in plasma and tissues efficiently reduced 5-fluorouracil host toxicity and altered the anesthetic effect of pentobarbital. These data indicate that UPase is a critical enzyme in the regulation of uridine homeostasis and pyrimidine nucleotide metabolism, and 5-fluorouracil activity.     

Thymidine phosphorylase and uridine phosphorylase of Lactobacillus casei

Abstract Comparison of the whole cell protein profiles of Staphylococcus epidermidis grown in pooled human peritoneal dialysate (HPD) with those of cells grown in nutrient broth (NB) revealed proteins of 27, 39, 45, 54 and 98 kDa which were absent or poorly expressed in NB-grown cells. IgG, but not transferrin, was detected bound to the surface of bacteria grown in HPD. Immunoblotting experiments revealed that IgG antibodies present in pooled HPD recognised staphylococcal protein antigens of 16, 27, 35, 39, 45, 54 and 98 kDa. The 16-, 35- and 39-kDa antigens which were associated with the cytoplasmic membrane were repressed following growth in NB or in HPD supplemented with excess iron.     

Synthesis and evaluation of 6-methylene-bridged uracil derivatives. Part 2: optimization of inhibitors of human thymidine phosphorylase and their selectivity with uridine phosphorylase

A series of novel 6-methylene-bridged uracil derivatives have been optimized for clinical use as the inhibitors of human thymidine phosphorylase (TP). We describe their synthesis and evaluation. Introduction of a guanidino or an amidino group enhanced the in vitro inhibitory activity of TP comparing with formerly reported inhibitor 1. Their selectivity for TP based on uridine phosphorylase inhibitory activity was also evaluated. Compound 2 (TPI) has been selected for clinical evaluation based on its strong TP inhibition and excellent modulation of 2'-deoxy-5-(trifluoromethyl)uridine (F(3)dThd) pharmacokinetics. As a result, TAS-102 (a combination of F(3)dThd and TPI) is currently in phase 1 clinical studies.     

Role of thymidine phosphorylase in Fas-induced apoptosis

Thymidine phosphorylase (TP) has chemotactic and angiogenic activity in vitro, and it promotes tumor growth and inhibits apoptosis in vivo. It plays a key role in the invasiveness and metastasis of TP-expressing solid tumors. KB/TP cells transfected with a TP cDNA have been shown to be resistant to hypoxia-induced apoptosis, suggesting that TP has effects on tumor growth and cell death independent of its effects on angiogenesis. However, the mechanisms of cell death inhibition by TP are unknown. In the present study, we demonstrate that caspase-8 is cleaved in control transfectant KB cells early on during Fas-induced apoptosis. Caspase-8 activation leads to the loss of mitochondrial membrane potential, followed by the release of cytochrome c, the activation of caspase-3, and apoptosis. In contrast, Fas-induced caspase-8 cleavage is inhibited in KB/TP cells, which lead to inhibition of the downstream apoptotic cascade and inhibition of apoptosis. These findings indicate that TP plays an important role in intracellular apoptotic signal transduction in the Fas-induced apoptotic pathway. Therefore, inhibition of TP may suppress the progression of TP-overexpressing solid tumors by inducing apoptosis.     

Altered thymidine metabolism due to defects of thymidine phosphorylase.

Mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) is an autosomal recessive human disease due to mutations in the thymidine phosphorylase (TP) gene. TP enzyme catalyzes the reversible phosphorolysis of thymidine to thymine and 2-deoxy-D-ribose 1-phosphate. We present evidence that thymidine metabolism is altered in MNGIE. TP activities in buffy coats were reduced drastically in all 27 MNGIE patients compared with 19 controls. All MNGIE patients had much higher plasma levels of thymidine than normal individuals and asymptomatic TP mutation carriers. In two patients, the renal clearance of thymidine was approximately 20% that of creatinine, and because hemodialysis demonstrated that thymidine is ultrafiltratable, most of the filtered thymidine is likely to be reabsorbed by the kidney. In vitro, fibroblasts from controls catabolized thymidine in medium; by contrast, MNGIE fibroblasts released thymidine. In MNGIE, severe impairment of TP enzyme activity leads to increased plasma thymidine. In patients who are suspected of having MNGIE, determination of TP activity in buffy coats and thymidine levels in plasma are diagnostic. We hypothesize that excess thymidine alters mitochondrial nucleoside and nucleotide pools leading to impaired mitochondrial DNA replication, repair, or both. Therapies to reduce thymidine levels may be beneficial to MNGIE patients.     

Simple radioassay for uridine phosphorylase and 5'-nucleotidase

A method is described whereby a large number of chromatographic fractions containing protein or enzyme components may be run on a single-starch gel. A multiple punch for making the sample holes, gel slab support, and a gel slicer used to cut several slices from the one gel for staining for the various enzyme or protien constituents are described. An example is given of a run using fractions obtained from a Sephadex G-200 column chromatography run of a herring muscle extract. Densitometry may be used to assay the stained gels. In another example a commercially obtained muscle lactate dehydrogenase preparation was chromatographed on a Sephadex G-200 column. The electrophoretic run of the fractions revealed the presence of both heart-type A and muscle-type B lactate dehydrogenase subunits and showed that the column run had effected a partial separation of the various tetramers.     

Targeted deletion of Gpbar1 protects mice from cholesterol gallstone formation

The Gpbar1 [G-protein-coupled BA (bile acid) receptor 1] is a recently identified cell-surface receptor that can bind and is activated by BAs, but its physiological role is unclear. Using targeted deletion of the Gpbar1 gene in mice, we show that the gene plays a critical role in the maintenance of bile lipid homoeostasis. Mice lacking Gpbar1 expression were viable, developed normally and did not show significant difference in the levels of cholesterol, BAs or any other bile constituents. However, they did not form cholesterol gallstones when fed a cholic acid-containing high-fat diet, and liver-specific gene expression indicated that Gpbar1-deficient mice have altered feedback regulation of BA synthesis. These results suggest that Gpbar1 plays a critical role in the formation of gallstones, possibly via a regulatory mechanism involving the cholesterol 7alpha-hydroxylase pathway.     

Homeostatic control of uridine and the role of uridine phosphorylase: a biological and clinical update

Both a 25-hydroxylation and a 1alpha-hydroxylation are necessary for the conversion of vitamin D(3) into the calcium-regulating hormone 1alpha,25-dihydroxyvitamin D(3). According to current knowledge, the hepatic mitochondrial cytochrome P450 (CYP) 27A and microsomal CYP2D25 are able to catalyze the former bioactivation step. Substantial 25-hydroxylase activity has also been demonstrated in kidney. This paper describes the molecular cloning and characterization of a microsomal vitamin D(3) 25- and 1alpha-hydroxylase in kidney. The enzyme purified from pig kidney and the recombinant enzyme expressed in COS cells catalyzed 25-hydroxylation of vitamin D(3) and 1alpha-hydroxyvitamin D(3) and, in addition, 1alpha-hydroxylation of 25-hydroxyvitamin D(3). The cDNA encodes a protein of 500 amino acids. Both the DNA sequence and the deduced peptide sequence of the renal enzyme are homologous with those of the hepatic vitamin D(3) 25-hydroxylase CYP2D25. Genomic Southern blot analysis suggested the presence of a single gene for CYP2D25 in the pig. Immunohistochemistry experiments indicated that CYP2D25 is expressed almost exclusively in the cells of cortical proximal tubules. The expression of CYP2D25 in kidney, but not in liver, was much higher in the adult pig than in the newborn. These findings indicate a tissue-specific developmental regulation of CYP2D25. The results from the current and previous studies on renal vitamin D hydroxylations imply that CYP2D25 has a biological role in kidney.     

Substrate specificity of Escherichia coli thymidine phosphorylase

Substrate specificity of Escherichia coli thymidine phosphorylase to thymidine derivatives modified at 5' -, 3' -, and 2' ,3' - positions of the sugar moiety was studied. Equilibrium and kinetic constants (K(m), K(I), k(cat)) of the phosphorolysis reaction have been determined for 20 thymidine analogs. The results are compared with X-ray and molecular dynamics data. The most important hydrogen bonds in the enzyme-substrate complex are revealed.     

Targeted deletion of endothelial lipase increases HDL particles with anti-inflammatory properties both in vitro and in vivo

Previous studies have shown that targeted deletion of endothelial lipase (EL) markedly increases the plasma high density lipoprotein cholesterol (HDL-C) level in mice. However, little is known about the functional quality of HDL particles after EL inhibition. Therefore, the present study assessed the functional quality of HDL isolated from EL(-/-) and wild-type (WT) mice. Anti-inflammatory functions of HDL from EL(-/-) and WT mice were evaluated by in vitro assays. The HDL functions such as PON-1 or PAF-AH activities, inhibition of cytokine-induced vascular cell adhesion molecule-1 expression, inhibition of LDL oxidation, and the ability of cholesterol efflux were similar in HDL isolated from WT and EL(-/-) mice. In contrast, the lipopolysaccharide-neutralizing capacity of HDL was significantly higher in EL(-/-) mice than that in WT mice. To evaluate the anti-inflammatory actions of HDL in vivo, lipopolysaccharide-induced systemic inflammation was generated in these mice. EL(-/-) mice showed higher survival rate and lower expression of inflammatory markers than WT mice. Intravenous administration of HDL isolated from EL(-/-) mice significantly improved the mortality after lipopolysaccharide injection in WT mice. In conclusion, targeted disruption of EL increased HDL particles with preserved anti-inflammatory and anti-atherosclerotic functions. Thus, EL inhibition would be a useful strategy to raise 'good' cholesterol in the plasma.     

Key role of uridine kinase and uridine phosphorylase in the homeostatic regulation of purine and pyrimidine salvage in brain

Uridine, the major circulating pyrimidine nucleoside, participating in the regulation of a number of physiological processes, is readily uptaken into mammalian cells. The balance between anabolism and catabolism of intracellular uridine is maintained by uridine kinase, catalyzing the first step of UTP and CTP salvage synthesis, and uridine phosphorylase, catalyzing the first step of uridine degradation to β-alanine in liver. In the present study we report that the two enzymes have an additional role in the homeostatic regulation of purine and pyrimidine metabolism in brain, which relies on the salvage synthesis of nucleotides from preformed nucleosides and nucleobases, rather than on the de novo synthesis from simple precursors. The experiments were performed in rat brain extracts and cultured human astrocytoma cells. The rationale of the reciprocal regulation of purine and pyrimidine salvage synthesis in brain stands (i) on the inhibition exerted by UTP and CTP, the final products of the pyrimidine salvage pathway, on uridine kinase and (ii) on the widely accepted idea that pyrimidine salvage occurs at the nucleoside level (mostly uridine), while purine salvage is a 5-phosphoribosyl-1-pyrophosphate (PRPP)-mediated process, occurring at the nucleobase level. Thus, at relatively low UTP and CTP level, uptaken uridine is mainly anabolized to uridine nucleotides. On the contrary, at relatively high UTP and CTP levels the inhibition of uridine kinase channels uridine towards phosphorolysis. The ribose-1-phosphate is then transformed into PRPP, which is used for purine salvage synthesis.     

Purification and kinetic characterization of uridine phosphorylase from Dictyostelium discoideum

The purification and kinetic characterization of uridine phosphorylase from Dictyostelium discoideum are described. Matrex Green A, a dye-affinity chromatography gel, was used for the purification. The enzyme was specifically eluted from the dye bead matrix with the use of its substrate, uridine, resulting in a purification of 70- to 2000-fold. The enzyme preparation exhibited stoichiometry. For nucleoside phosphorolysis, the K_m values for phosphate and uridine were 0.42 and 0.24 mm, respectively, and the K_i for phosphate was 3.0 mm. For nucleoside synthesis, the K_m values for uracil and ribose 1-phosphate were 0.06 and 0.14 mm, respectively, and the K_i for ribose 1-phosphate was 0.05 mm. An ordered sequential bi:bi mechanism is proposed based on product inhibition studies.     

Stabilization of Escherichia coli uridine phosphorylase by evolution and immobilization

Mutation and immobilization techniques were applied to uridine phosphorylase (UP) from Escherichia coli in order to enhance its thermal stability and hence productivity in a biocatalytic reaction. UP was evolved by iterative saturation mutagenesis. Compared to the wild type enzyme, which had a temperature optimum of 40 °C and a half-life of 9.89 h at 60 °C, the selected mutant had a temperature optimum of 60 °C and a half-life of 17.3 h at 60 °C. Self-immobilization of the native UP as a Spherezyme showed a 3.3 fold increase in thermostability while immobilized mutant enzyme showed a 4.4 fold increase in thermostability when compared to native UP. Combining UP with the purine nucleoside phosphorylase from Bacillus halodurans allows for synthesis of 5-methyluridine (a pharmaceutical intermediate) from guanosine and thymine in a one-pot transglycosylation reaction. Replacing the wild type UP with the mutant allowed for an increase in reaction temperature to 65 °C and increased the reaction productivity from 10 to 31 g l⁻¹ h⁻¹.