Purification and properties of thymidine phosphorylase from Salmonella typhimurium

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.

The kinetic mechanism of Human Thymidine Phosphorylase - a molecular target for cancer drug development

Human Thymidine Phosphorylase (HTP), also known as the platelet-derived endothelial cell growth factor (PD-ECGF) or gliostatin, catalyzes the reversible phosphorolysis of thymidine (dThd) to thymine and 2-deoxy-α-d-ribose-1-phosphate (2dR1P). HTP is a key enzyme in the pyrimidine salvage pathway involved in dThd homeostasis in cells. HTP is a target for anticancer drug development as its enzymatic activity promotes angiogenesis. Here, we describe cloning, expression, and purification to homogeneity of recombinant TYMP-encoded HTP. Peptide fingerprinting and the molecular mass value of the homogenous protein confirmed its identity as HTP assessed by mass spectrometry. Size exclusion chromatography showed that HTP is a dimer in solution. Kinetic studies revealed that HTP displayed substrate inhibition for dThd. Initial velocity and isothermal titration calorimetry (ITC) studies suggest that HTP catalysis follows a rapid-equilibrium random bi-bi kinetic mechanism. ITC measurements also showed that dThd and Pi binding are favorable processes. The pH-rate profiles indicated that maximal enzyme activity was achieved at low pH values. Functional groups with apparent pK values of 5.2 and 9.0 are involved in dThd binding and groups with pK values of 6.1 and 7.8 are involved in phosphate binding.

A single amino acid limits the substrate specificity of Thermus thermophilus uridine-cytidine kinase to cytidine

The salvage pathways of nucleotide biosynthesis are more diverse and are less well understood as compared with de novo pathways. Uridine-cytidine kinase (UCK) is the rate-limiting enzyme in the pyrimidine-nucleotide salvage pathway. In this study, we have characterized a UCK homologue of Thermus thermophilus HB8 (ttCK) biochemically and structurally. Unlike other UCKs, ttCK had substrate specificity toward only cytidine and showed no inhibition by UTP, suggesting uridine does not bind to ttCK as substrate. Structural analysis revealed that the histidine residue located near the functional group at position 4 of cytidine or uridine in most UCKs is substituted with tyrosine, Tyr93, in ttCK. Replacement of Tyr93 by histidine or glutamine endowed ttCK with phosphorylation activity toward uridine. These results suggested that a single amino acid residue, Tyr93, gives cytidine-limited specificity to ttCK. However, replacement of Tyr93 by Phe or Leu did not change the substrate specificity of ttCK. Therefore, we conclude that a residue at this position is essential for the recognition of uridine by UCK. In addition, thymidine phosphorylase from T. thermophilus HB8 was equally active with thymidine and uridine, which indicates that this protein is the sole enzyme metabolizing uridine in T. Thermophilus HB8. On the basis of these results, we discuss the pyrimidine-salvage pathway in T. thermophilus HB8.

The dual role of thymidine phosphorylase in cancer development and chemotherapy

Thymidine phosphorylase (TP), also known as "platelet-derived endothelial cell growth factor" (PD-ECGF), is an enzyme, which is upregulated in a wide variety of solid tumors including breast and colorectal cancers. TP promotes tumor growth and metastasis by preventing apoptosis and inducing angiogenesis. Elevated levels of TP are associated with tumor aggressiveness and poor prognosis. Therefore, TP inhibitors are synthesized in an attempt to prevent tumor angiogenesis and metastasis. TP is also indispensable for the activation of the extensively used 5-fluorouracil prodrug capecitabine, which is clinically used for the treatment of colon and breast cancer. Clinical trials that combine capecitabine with TP-inducing therapies (such as taxanes or radiotherapy) suggest that increasing TP expression is an adequate strategy to enhance the antitumoral efficacy of capecitabine. Thus, TP plays a dual role in cancer development and therapy: on the one hand, TP inhibitors can abrogate the tumorigenic and metastatic properties of TP; on the other, TP activity is necessary for the activation of several chemotherapeutic drugs. This duality illustrates the complexity of the role of TP in tumor progression and in the clinical response to fluoropyrimidine-based chemotherapy.

A possible role of thymidine phosphorylase expression and 5-fluorouracil increased sensitivity in oropharyngeal cancer patients

Thymidine Pi deoxyribosyltransferase (TP) is an enzyme involved in DNA synthesis up-regulated in tumours and it is also a pro-angiogenic factor. TP cannot activate capecitabine, because capecitabine first needs conversion by carboxylesterase and cytidine deaminase into 5-deoxy-fluorouridine. This compound can be activated by TP to 5-fluorouracil (5-FU). Although TP is not necessary for 5-FU toxicity, experimental data suggest that high levels of TP correlate with an enhanced response to 5-FU therapy. In this study, we have analysed by immunohistochemistry CD34, CD68 and TP positive cells in bioptic samples from 53 patients with T_1–3 N_0–1 M₀ oropharyngeal squamous cell carcinoma (OSC) and from 24 patients with non-dysplastic oropharyngeal leukoplakia (NDOLP). Results showed that the mean of TP-positive cells, CD68 positive macrophages and CD34 positive endothelial cells eval-uated as microvessel density (MVD) was significantly higher in OSC than in NDOLP. Moreover, at a median follow-up of 19 months, patients with TP expression and higher MVD showed a better survival rate as compared to those with low MVD, probably as a consequence of 5-FU-based therapy.We hypothesized a role for TP in oropharyngeal tumourigenesis and 5-FU activation in the adjuvant setting of OSC patients.

Role of each residue in catalysis in the active site of pyrimidine nucleoside phosphorylase from Bacillus subtilis: A hybrid QM/MM study

Pyrimidine nucleoside phosphorylase (PYNP) catalyzes the reversible phosphorolysis of pyrimidines in the nucleotide synthesis salvage pathway. We have built a model of a closed active conformation of the three-dimensional structure of PYNP from Bacillus subtilis. Using docking, molecular dynamics, and hybrid quantum-mechanical/molecular-mechanical methods to study the reaction mechanics between PYNP and a substrate, we identified the role of each residue in the active site during the entire catalytic process. The results indicate that the function of His(82), Arg(169), and Lys(188) is to stabilize the uridine in a high-energy conformation by means of electrostatic interactions and that these residues are involved in catalysis. In addition, the function of Asp(162) is likely to activate Lys(188) for phosphorolytic catalysis through polarization effects.

Thymidine phosphorylase profiles in nonmalignant and malignant pancreatic tissue. Potential therapeutic role of capecitabine on tumoral and endothelial cells and tumor-infiltrating macrophages

The drug capecitabine (CAP) is a thymidine Pi-deoxyribosyltransferase (TP) activated oral fluorpyrimidine that generates 5-fluorouracil (5-FU), preferentially, within tumors. Here, in 38 patients with pancreatic cancer we analyzed immunohistochemical TP expression in pancreatic cancer tissue (PCT) and adjacent nonmalignant pancreatic tissue (ANMPT). In addition, a correlation with the main clinical pathological features was made. Furthermore, TP-positive macrophages (MO) isolated from neoplastic tissue were determined. The mean of TP-positive epithelial cells and endothelial cells in terms of microvessel density was significantly higher in PCT than in ANMPT. Because pancreatic cancer is sensitive to 5-FU, TP-activated oral CAP in tumoral and endothelial cells and tumor infiltrating MO could increase the concentration of 5-FU at tumor site, thus resulting in an enhanced antitumor activity.

The role of thymidine phosphorylase in the pathogenesis of allergic rhinitis

The activity and distribution of thymidine phosphorylase (TP) in the nasal mucosa of patients with nasal allergy was examined and compared with those in healthy subjects. TP activity was analyzed by spectrophotometry and expression was examined by immunoblotting and immunohistochemical staining using a monoclonal antibody specific to TP. The expression level of TP detected by immunoblotting showed a correlation with the activity detected by spectrophotometry. In nasal mucosa obtained from patients with nasal allergy, the level of TP was significantly higher than that from normal subjects. Eosinophils, basal cells in mucosal epithelium and fibroblasts in nasal mucosa obtained from patients with nasal allergy were stained with anti-TP monoclonal antibody. Strong staining of eosinophils present in nasal discharge was observed. The present results indicate that an increased number of TP-expressing cells, especially eosinophils in nasal mucosa, might be associated with the pathogenesis of nasal allergy.

The crystal structure of pyrimidine nucleoside phosphorylase in a closed conformation

BACKGROUND

Pyrimidine nucleoside phosphorylase (PYNP) catalyzes the reversible phosphorolysis of pyrimidines in the nucleotide synthesis salvage pathway. In lower organisms (e.g. Bacillus stearothermophilus) PYNP accepts both thymidine and uridine, whereas in mammalian and other higher organisms it is specific for thymidine (designated thymidine phosphorylase, TP). PYNP shares 40% sequence similarity (and presumably significant structural similarity) with human TP, which has been implicated as a growth factor in tumor angiogenesis. It is thought that TP undergoes a major conformational change upon substrate binding that consequently produces an active conformation.

RESULTS

The crystal structure of PYNP from B. stearothermophilus with the substrate analog pseudouridine in its active site has been solved to 2.1 A resolution. This structure confirms the similarity of PYNP to TP and supports the idea of a closed active conformation, which is the result of rigid body movement of the alpha and alpha/beta domains. The active-site cleft, where the pyrimidine and phosphate substrates bind, is between the two domains. The structure reveals an asymmetric dimer in which one subunit is fully closed and the other is only partially closed.

CONCLUSIONS

The closed conformation of PYNP serves as a good model to better understand the domain movement and overall function of TP. Active-site residues are confirmed and a possible mechanism for substrate binding and subsequent domain movement is suggested. Potent inhibitors of TP might have significant therapeutic value in various chemotherapeutic strategies, and the structure of PYNP should provide valuable insight into the rational design of such inhibitors.

Thymidine phosphorylase, 2-deoxy-D-ribose and angiogenesis

Angiogenesis is the term used to describe the formation of new blood vessels from the existing vasculature. In order to attract new vessels, a tissue must release an endothelial-cell chemoattractant. 2-Deoxy-D-ribose is produced in vivo by the catalytic action of thymidine phosphorylase (TP) on thymidine and has recently been identified as an endothelial-cell chemoattractant and angiogenesis-inducing factor. TP, previously known only for its role in nucleotide salvage, is now known to be angiogenic. TP expression is elevated in many solid tumours and in chronically inflamed tissues, both known areas of active angiogenesis. There is evidence that TP is also involved in physiological angiogenesis such as endometrial angiogenesis during the menstrual cycle. The majority of known endothelial-cell chemoattractants are polypeptides that bind to endothelial-cell-surface receptors. In contrast, 2-deoxy-D-ribose appears to lack a cell-surface receptor. Glucose is another sugar that acts as an endothelial-cell chemoattractant. The migratory activity of glucose is blocked by ouabain. It is possible that 2-deoxy-D-ribose and glucose stimulate endothelial-cell migration via a similar mechanistic pathway.

Structural and theoretical studies suggest domain movement produces an active conformation of thymidine phosphorylase

Two new crystal forms of Escherichia coli thymidine phosphorylase (EC 2.4.2.4) have been found; a monoclinic form (space group P21) and an orthorhombic form (space group I222). These structures have been solved and compared to the previously determined tetragonal form (space group P43212). This comparison provides evidence of domain movement of the alpha (residues 1 to 65, 163 to 193) and alpha/beta (residues 80 to 154, 197 to 440) domains, which is thought to be critical for enzymatic activity by closing the active site cleft. Three hinge regions apparently allow the alpha and alpha/beta-domains to move relative to each other. The monoclinic model is the most open of the three models while the tetragonal model is the most closed. Phosphate binding induces formation of a hydrogen bond between His119 and Gly208, which helps to order the 115 to 120 loop that is disordered prior to phosphate binding. The formation of this hydrogen bond also appears to play a key role in the domain movement. The alpha-domain moves as a rigid body, while the alpha/beta-domain has some non-rigid body movement that is associated with the formation of the His119-Gly208 hydrogen bond. The 8 A distance between the two substrates reported for the tetragonal form indicates that it is probably not in an active conformation. However, the structural data for these two new crystal forms suggest that closing the interdomain cleft around the substrates may generate a functional active site. Molecular modeling and dynamics simulation techniques have been used to generate a hypothetical closed conformation of the enzyme. Analysis of this model suggests several residues of possible catalytic importance. The model explains observed kinetic results and satisfies requirements for efficient enzyme catalysis, most notably through the exclusion of water from the enzyme's active site.

Determination of inorganic phosphate by coupling thymidine phosphorylase and reversed-phase high-performance liquid chromatography: application to tonoplast pyrophosphatase activity

We developed an HPLC method for the measurement of inorganic phosphate using thymidine phosphorylase (EC 2.4.2.4). This enzyme catalyzes the phosphorolysis of thymidine to 2-deoxyribose 1-phosphate and thymine. Thymine release was measured at 265 nm after separation by reverse-phase HPLC. The assay was sensitive enough to detect as little as 10 pmol of Pi. The response to the phosphate concentration was linear from 1 to 100 microM. The value of this method was demonstrated in an analysis of the kinetics of Pi release from PPi in the presence of Catharanthus roseus tonoplast pyrophosphatase.

The activity and expression of thymidine phosphorylase in human solid tumours

Thymidine phosphorylase (dThdPase) is identical to platelet-derived endothelial cell growth factor (PD-ECGF) and has angiogenic activity. Since dThdPase seems to have an important role in angiogenesis of tumours, we measured the activity and expression of dThdPase in various tumours and the adjacent non-neoplastic tissues. We assayed dThdPase activity by spectrophotometric means, and the expression of dThdPase was examined by immunoblotting and by immunohistochemical staining using a monoclonal antibody against dThdPase. In the oesophagus, stomach, colorectum, pancreas, and lung, dThdPase activity in carcinomas was significantly higher (P < 0.05) than that in the adjacent non-neoplastic tissues. The expression level of dThdPase detected by immunoblotting correlated well with the activity of dThdPase. In the oesophagus, stomach, colorectum, gall bladder, pancreas and lung, the proportion of dThdPase-positive tumours was significantly higher (P < 0.05 or 0.01) than that of the dThdPase-positive adjacent normal tissues. In oesophageal, gastric colorectal and lung carcinomas, the proportion of dThdPase positivity in advanced carcinomas was significantly higher (P < 0.01) than that in early carcinomas. Tumour-infiltrative macrophages or lymphocytes in the lymph node, alveolar macrophages and Kupffer cells expressed high levels of dThdPase. The results indicate that dThdPase activity and expression level in many tumours are higher than those in the adjacent non-neoplastic tissues, and that dThdPase may have an important role in the proliferation of these solid tumours.

The expression of thymidine phosphorylase and thrombomodulin in human colorectal carcinomas

Thymidine phosphorylase (dThdPase) is an enzyme involved in pyrimidine nucleoside metabolism. dThdPase activity is increased in several types of malignant tumors. Recently, we demonstrated that dThdPase is identical to platelet-derived endothelial cell growth factor (PD-ECGF) and that dThdPase has angiogenic activity. We measured dThdPase activity and the level of thrombomodulin (TM) as a marker for endothelial cells in colorectal carcinomas and adjacent normal tissues from 21 patients, and in adenomas from 13 patients. The average dThdPase activity of colorectal carcinomas (11.58 +/- 6.30 nmol/100 micrograms protein/h) was significantly higher than that of adenomas (8.57 +/- 4.14 nmol/100 micrograms protein/h) or normal tissues (4.89 +/- 3.16 nmol/100 micrograms protein/h). In immunohistochemical study, the expression of dThdPase was observed more frequently in colorectal carcinomas than in adenomas or normal mucosas. The amount of TM in colorectal carcinomas (8.32 +/- 5.07 ng/100 micrograms protein) was significantly higher than that of adenomas (4.51 +/- 4.49 ng/100 micrograms protein) or normal tissues (3.51 +/- 2.78 ng/100 micrograms protein). dThdPase activity in human colorectal carcinomas, adenomas and normal tissues was significantly correlated with the expression of TM in these tissues. These results indicate that the expression levels of both dThdPase and TM in colorectal carcinomas are higher than those in colorectal adenomas and normal tissues and suggest that dThdPase may be involved in angiogenesis in human colorectal carcinomas, adenomas and normal tissues.

Regulation of collagen synthesis by ascorbic acid: characterization of the role of ascorbate-stimulated lipid peroxidation

Recently, we have described the ability of traditional lipid peroxidation inhibitors to inhibit ascorbate-stimulated collagen synthesis. In order to characterize further this effect, we have tested the ability of known and potential inhibitors of lipid peroxidation for their effects on ascorbate-stimulated collagen synthesis and lipid peroxidation. In our experiments, mannitol, a water soluble antioxidant, had no effect on ascorbate-induced collagen synthesis nor on lipid peroxidation. However, alpha-tocopherol, which is a lipophilic antioxidant, inhibited both effects of ascorbate. Superoxide dismutase, catalase, and their polyethylene glycol conjugate forms did not inhibit the ascorbate-stimulated collagen synthesis or lipid peroxidation. In addition, no effect was seen with the oxygen radical scavengers isopropanol, ethanol, or dimethyl sulfoxide. Two iron chelators, o-phenanthroline and alpha,alpha-dipyridyl, both inhibited ascorbate-induced lipid peroxidation and collagen synthesis, consistent with the previously described iron-dependence of lipid peroxidation by ascorbate. These results support a correlation between collagen synthesis and lipid peroxidation and provide a theory for the mechanism of ascorbic acid regulation of collagen synthesis.

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.

Purification and tissue distribution of human thymidine phosphorylase; high expression in lymphocytes, reticulocytes and tumors

Thymidine phosphorylase (dThdPase) is an enzyme involved in pyrimidine nucleoside metabolism, but little is known about its physiological functions. We purified dThdPase from human placenta and used it for antibody preparation. The purified material appears as a single band at 55,000 dalton on sodium dodecylsulfate-polyacrylamide gel electrophoresis. We obtained a specific antibody raised in rabbits that detected a single polypeptide with a molecular weight of 55,000 dalton in the post nuclear homogenates of several human tissues, on immunoblotting. Using the same technique, dThdPase was highly expressed in the liver, lung, spleen, lymph nodes and peripheral lymphocytes. Immunohistochemical staining revealed that macrophage-like cells contained a much higher amount of dThdPase than parenchymal cells in the liver and lung. dThdPase was found to be highly expressed in T- and B-cell-type malignant lymphoma cells, but low in lymphoblastic and myeloblastic leukemia cells. We also found that carcinomas in the stomach, colon and ovary contained higher amounts of this enzyme than non-neoplastic regions of the tissues. These data suggest that dThdPase plays a role in proliferation and/or differentiation of leukocytes and in cancer proliferation.

Kinetic studies of thymidine phosphorylase from mouse liver

Initial velocity and product inhibition studies of thymidine phosphorylase from mouse liver revealed that the basic reaction mechanism of this enzyme is a rapid equilibrium random bi-bi mechanism with an enzyme-phosphate-thymine dead-end complex. Thymine displayed both substrate inhibition and nonlinear product inhibition, i.e., slope and intercept replots vs. 1/[thymine] were nonlinear, indicating that there is more than one binding site on the enzyme for thymine and that when thymine is bound to one of these sites, the enzyme is inhibited. Furthermore, both thymidine and phosphate showed "cooperative effects" in the presence of thymine at concentrations above 60 microM, suggesting that the enzyme may have multiple interacting allosteric and/or catalytic sites. The deoxyribosyl transferase reaction catalyzed by this enzyme is phosphate-dependent, requires nonstoichiometric amounts of phosphate, and can proceed by an "enzyme-bound" 2-deoxyribose 1-phosphate intermediate. These findings are in accord with the rapid equilibrium random bi-bi mechanism and demonstrate that deoxyribosyl transfer by this enzyme involves an indirect-transfer mechanism. These results strongly suggest that phosphorolysis and deoxyribosyl transfer are catalyzed by the same site on thymidine phosphorylase.

The molecular basis for the differential sensitivity of B and T lymphocytes to growth inhibition by thymidine and 5-fluorouracil

Cultured leukemic lymphocytes originating from patients with T, B and non-T, non-B (null) leukemia were tested for their sensitivity to thymidine and 5-fluorouracil. T cells were found to be 5-7 fold more sensitive to thymidine growth inhibition than B-cells. At 10(-3) M concentration of thymidine, T cells showed a progressive (up to 75%) decline in the populating trypan blue-excluding cells, after 72 h. At this concentration of thymidine B cells showed slight inhibition at 24 and 48 h, then at 72 h the surviving cell level returned almost to the level of unperturbed cells. Thymidine at 10(-5) M concentration, caused 40% cell growth inhibition of T cells, however, at this concentration it had little or no effect on B cells. 5-fluorouracil effects on B and T lymphocytes are opposite to that of thymidine. B cells were on an average 5-7 times more sensitive to 5-FU than T cells. 5-FU at 10(-6) M caused up to 45% inhibition of B-cell growth but at this concentration it had no effect on the growth of T cells. B-, T- and null-lymphocytes sensitivity to thymidine and 5-FU was correlated with the level of the catabolic enzyme thymidine phosphorylase. B cells had, on average, 5-fold more thymidine phosphorylase than T or null cells. Furthermore, the enzyme from the B-cell line (HR1K) chromatographed differently on DEAE-Sephadex than the normal peripheral blood lymphocytes enzyme. The normal enzyme from peripheral blood lymphocytes when adsorbed to DEAE-Sephadex was eluted at a salt concentration of 0.3 M KCI, Enzyme activities of HR1K did not adsorb to the DEAE-Sephadex column but were adsorbed to a phosphocellulose column. Enzyme from normal and leukemic lymphocytes showed similar molecular weights of 130,000 dalton as determined by gel filtration.

Catabolism of thymidine in human blood platelets: purification and properties of thymidine phosphorylase

A pyrimidine nucleoside phosphorylase was partially purified from human blood platelets. The purified enzyme, as well as crude enzyme preparations, catalyses the phosphorolysis of thymidine and deoxyuridine, but not of uridine, and is able to catalyse direct pentosyl transfer from these deoxyribonucleosides to uracil or thymine; this enzyme has the properties of a thymidine phosphorylase. It has a molecular weight of about 110,000 and is composed of two identical subunits; it is phosphate dependent, has a maximal activity at a pH value of 5.7, and an isoelectric point of 4.4. This enzyme was mainly of cytoplasmic origin. Although platelet thymidine phosphorylase could promote the degradation or synthesis of thymidine, intact platelets degraded thymidine but were not able to synthesize thymidine from thymine. Blood platelets may play an important role in the degradation of plasma thymidine.

Purification of thymidine phosphorylase from human amniochorion

Thymidine phosphorylase (thymidine : orthophosphate deoxyribosyltransferase, EC 2.4.2.4) has been purified 1500-fold from extracts of human amniochorion. The purified enzyme catalyzes the phosphorolysis of deoxythymidine and to a lesser extent deoxyuridine but not deoxycytidine nor uridine. Discontinuous gel electrophoresis of the freshly purified enzyme shows a band containing 95% of the stainable protein. Gradient gel electrophoresis resolves the preparations into an active fraction with an apparent molecular weight of about 120 000 and a heavier less active or inactive fraction of about 180 000. Storage of the enzyme results in a decrease of the 120 000 dalton component, a loss in activity, and an apparent increase in the high molecular weight component. Sodium dodecyl sulfate gel electrophoresis shows only a single subunit of about 58 000 daltons which does not change on storage. These data are consistent with an active enzyme dimeric in structure which is capable of being converted to a less active form larger in molecular weight and possibly trimeric or tetrameric in structure.

Inhibition of thymidine phosphorylase in vivo provides a rapid method for switching DNA labeling

Uridine blocks the in vivo conversion of thymine to thymidine in Escherichia coli, thus, one can change DNA labels by labelling first with a thymine label (e.g. 14C) and then, at the time of the change, adding 50 microgram uridine per ml and thymidine (e.g. 3H). The cells immediately start using the thymidine, ignore the thymine for several generations, and are not affected by the uridine.

Catabolism of deoxythymidylate in some trypanosomatids

An initial observation concerning the failure of [3H]thymidine at high specific activity to be incorporated into the DNA of Crithidia fasciculata for more than a brief initial period has been correlated with the presence at high specific activity in the organism of a thymidine phosphorylase activity with an equilibrium in the direction of catabolism. This enzyme degrades thymidine to thymine which is not utilized by the organism. The enzyme has also been shown to be present in a number of other trypanosomatids, including the culture forms of Trypanosoma cruzi, where the specific activity was nearly as high as that in C. fasciculata. Evidence is presented that in C. fasciculata, the culture forms of T. cruzi and possibly other species of trypanosomatid, the thymidine phosphorylae, together with a thymidylate phosphatase, forms a catabolic pathway which degrades thymine nucleotides to thymine, which is then excreted. About 60% of the thymine nucleotides made by organisms appear to be metabolized through the pathway, suggesting that their synthesis is not subject to completely effective regulatory control.